Mennig, Stoeckhert Mold-Making Handbook

Mold-Making Handbook

Günter MennigKlaus Stoeckhert

Hanser Publishers, Munich Hanser Publications, Cincinnati

3rd Edition

The Editors:Prof. Dr.-Ing. Günter Mennig,Technische Universität Chemnitz, Institut für Allg. Maschinenbau und Kunststofftechnik, Chemnitz, GermanyDr.-Ing. Klaus Stoeckhert (†)

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Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20, 81631 München, Germany Fax: +49 (89) 98 48 09 www.hanser.de

The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone.While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.

Library of Congress Cataloging-in-Publication Data

Werkzeuge der Kunststoffverarbeitung. English. Mold-making handbook / [edited by] G?nter Mennig, Klaus Stoeckhert. -- 3rd Edition. pages cm ISBN 978-1-56990-446-6 (hardcover) 1. Plastics--Molds. I. Mennig, G?nter. II. Stoeckhert, Klaus, 1913- III. Title. TP1150.W4613 2012 668.4’12--dc23 2012027079

Bibliografische Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über <http://dnb.d-nb.de> abrufbar.

ISBN 978-1-56990-446-6E-Book-ISBN 978-1-56990-550-0

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© Carl Hanser Verlag, Munich 2013 Production Management: Steffen Jörg Coverconcept: Marc Müller-Bremer, www.rebranding.de, MünchenCoverdesign: Stephan RönigkTypeset: Manuela Treindl, FürthPrinted and bound by Kösel, KrugzellPrinted in Germany

About the Book

The book uses European steel grades.

References for the comparison of US and European steel grades are available, for example, from Beuth Verlag (Marks, P., US Steels: A comparison of US and Euro-pean steel grades, German-English (2012), Beuth Verlag, Berlin, www.beuth.de) or at www.key-to-steel.com.

Contributors

Dipl.-Ing. (FH) Christopherus Bader (Chapter 5.2) PRIAMUS SYSTEM TECHNOLOGIES AG, Schaffhausen, Switzerland

Prof. Dr.-Ing. Thomas Bauernhansl (Chapter 1.9) Fraunhofer-Institut für Produktionstechnik und Automatisierung IPA, Stuttgart, Germany

Dr.-Ing. Joachim Berthold (Chapter 1.2) Gummersbach, Germany

André Brandt (Chapter 2.2) HASCO Hasenclever GmbH & Co. KG, Lüdenscheid, Germany

Dipl.-Ing. Otto Eiselen (Chapter 1.4) HECO Maschinen- und Werkzeugbau GmbH, Lohmar, Germany

Alfred Erstling (Chapter 3.2) Wuppertal, Germany

Dipl.-Ing. Thomas Eulenstein (Chapter 2.5 and 5.3) Kunststoff-Institut Lüdenscheid, K.I.M.W. NRW GmbH, Lüdenscheid, Germany

Prof. Dr. Ing. Andreas Gebhardt (Chapter 4.7) CP GmbH, Erkelenz and University of Applied Sciences Aachen, Aachen, Germany

Dipl.-Ing. Josef Gockel (Chapter 2.2) HASCO Hasenclever GmbH & Co. KG, Lüdenscheid, Germany

Ing. Rolf Hentrich (Chapter 1.6 and 4.3) Lahr, Germany

Dipl.-Ing. (FH) Felix Hinken (Chapter 2.1) Schneider Form GmbH, Dettingen/Teck, Germany

Dipl.-Ing. Udo Hinzpeter (Chapter 5.3) Kunststoff -Institut Lüdenscheid; K.I.M.W. NRW GmbH, Lüdenscheid, Germany

VIII Contributors

Dr.-Ing. Frank Hippenstiel (Chapter 3.1) BGH Edelstahl Siegen GmbH, Siegen, Germany

Robert Hofmann (Chapter 1.11) Hofmann Modellbau, Lichtenfels, Germany

Prof. Dipl.-Ing. Peter Karlinger (Chapter 2.1) University of Applied Sciences Rosenheim, Rosenheim, Germany

Dipl.-Ing. (FH) Armin Klotzbücher (Chapter 4.1) KWO Kunststo�eile GmbH, Offenau, Germany

Dr. Ulrich Knipp (Chapter 1.3) Bergisch Gladbach, Germany

Günter Konzilia (Chapter 1.10) z-werkzeugbau-gmbh, Dornbirn, Austria

Stefan Krüth (Chapter 4.6) J. & F. Krüth GmbH, Solingen, Germany

Bernhard Mack (Chapter 4.2) Mack Erodiertechnik, Langenau, Germany

Dr. Udo Maier (Chapter 1.3) Pulheim, Germany

Prof. Dr.-Ing. Peter Mitschang (Chapter 1.8) Institut für Verbundwerkstoffe GmbH, Kaiserslautern, Germany

Dipl.-Ing. Dirk Paulmann (Chapter 2.3) HASCO Hasenclever GmbH & Co. KG, Lüdenscheid, Germany

Dipl.-Ing. Norbert Reuber (Chapter 1.7) Kurtz GmbH, Kreuzwertheim, Germany

Dipl.-Ing. Manfred Sander (Chapter 2.3) Iserlohn, Germany

Dipl.-Ing. (FH) Dietmar Schäffner (Chapter 4.2) Viscoch GmbH, Widnau, Switzerland

Prof. Dr.-Ing. Alois K. Schlarb (Chapter 1.8) University Kaiserslautern, Kaiserslautern, Germany

Prof. Dr.-Ing. Ralf Schledjewski (Chapter 1.8) Montanuniversität Leoben, Leoben, Austria

Prof. Dr.-Ing. Friedhelm Schlößer (Chapter 5.1) KION Group AG, Wiesbaden, Germany

IX Contributors

Edgar Seufert (Chapter 3.3) Schmelzmetall Deutschland GmbH, Steinfeld-Hausen, Germany

Dipl.-Ing. Peter Schwarzmann (Chapter 1.5) Illig Maschinenbau GmbH & Co. KG, Heilbronn, Germany

Claudia Steiner (Chapter 4.4) NOVAPAX Kunststo�echnik Steiner GmbH & Co. KG, Berlin, Germany

Prof. Dr.-Ing. Paul Thienel (Chapter 2.4) University of Applied Sciences Südwestfalen, Iserlohn, Germany

Uwe Thiesen (Chapter 5.4) Plastics Germany GmbH & Co. KG, Lohne, Germany

Peter Vetter (Chapter 4.5) Buderus Edelstahl GmbH, Wetzlar, Germany

Dipl.-Betriebsw. Oliver Wandres (Chapter 1.6) Maus GmbH, Karlsruhe, Germany

Prof. Dipl.-Ing. Peter Wippenbeck (Chapter 1.1) Steinbeis-Innovationszentrum Kunststo�echnik, Aalen, Germany

Klaus Zoller (Chapter 1.9) Freudenberg Sealing Technologies GmbH & Co. KG, Weinheim, Germany

Editor’s Preface to the 3rd edition

When this book was first published in Carl Hanser Verlag in 1965 in German, edited by K. Stoeckhert, a market gap was filled, because a second edition was already required a�er four years, this time in collaboration with H. Domininghaus. The third edition came eleven years later (1980), which already had almost twice the number of pages. It was translated and became the 1 st English edition. Due to the personnel change in the editorship, it then took 15 years until the next edition was published with a slightly altered title in English and German. In addition a Chinese edition was launched for the first time. Again it took some 15 years for the now presented 3rd edition (the 5th in German) to be published.

The present edition is not only updated, but modernized and renewed. This is also indicated by the fact that only six of the “old” authors are still involved, and there are new chapters such as micro injection molds, molds for the rubber industry, or rapid prototyping, while others are no longer applicable. Otherwise, as it was already stated in the preface of the 4th German edition, it still applies that this compilation does not intend to serve as a textbook for the detailed design of an injection mold or to replace the catalogue of a manufacturer of standard mold units, and it is also not the long version of a lecture manuscript either. Rather, the brief description of the basic facts and the latest state of the art for the individual mold types and their manufacture allow a direct comparison in a compact form.

The book still addresses both, the reader who is looking for an introduction to a key area of plastics processing as well as the pronounced specialist to enable quick reading into related technical areas, which can result in ideas for their own work. Each chapter is self-contained; the proposed synergistic effect is achieved especially when the reader not only reads “his” chapter, but is willing to “look outside the box” of his own specialist field.

Chemnitz, April 2013 Günter Mennig

Contents

About the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII

Editor’s Preface to the 3rd edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI

1 Molds for Various Processing Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Injection Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

P. Wippenbeck1.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2 Injection Molding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.3 Design of the Molded Part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.1.4 Basic Mold Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.1.5 Types of Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.5.1 Products without Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.5.2 Products with External Undercuts . . . . . . . . . . . . . . . . . . . . . . . . 131.1.5.3 Product with Internal Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . 171.1.5.4 Products with Internal Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . 191.1.6 Gate Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211.1.6.1 Gate Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211.1.6.2 Solidifying Runner, Remaining at the Molded Part . . . . . . . . . 231.1.6.3 Automatically Separated Runner . . . . . . . . . . . . . . . . . . . . . . . . . . 261.1.6.4 Pass Through Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291.1.6.5 Hotrunner Molds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311.1.6.6 Hotrunner Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341.1.7 Venting of the Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371.1.8 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391.1.9 Special Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411.1.9.1 Stack Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411.1.9.2 Injection-Compression Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431.1.9.3 Multi-Component Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

XIV Contents

1.1.9.4 Outsert Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481.1.9.5 Molds for Thermosets and Elastomers . . . . . . . . . . . . . . . . . . . . . 49

1.2 Compression and Transfer Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52J. Berthold1.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521.2.2 Compression Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531.2.2.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531.2.2.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 551.2.2.3 Components of a Compression Mold . . . . . . . . . . . . . . . . . . . . . . . 561.2.3 Transfer Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561.2.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561.2.3.2 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 571.2.3.3 Structure of a Transfer Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 571.2.4 Making Compression Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581.2.4.1 Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591.2.5 Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591.2.5.1 Types of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601.2.5.1.1 Small-Series Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601.2.5.1.2 Test Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601.2.5.1.3 Standard Mold Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601.2.5.1.4 Conventional Compression Mold . . . . . . . . . . . . . . . . . . . . . . . . . . 621.2.5.2 Structural Mold Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621.2.5.2.1 Positive Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621.2.5.2.2 Positive Mold with Lands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631.2.5.2.3 Multi-Cavity Mold with a Common Loading Chamber. . . . . . . 641.2.5.2.4 Multi-Cavity Mold with Individual Loading Chambers . . . . . . 641.2.5.2.5 Mold with Lateral Core Puller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651.2.5.2.6 Split Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 661.2.5.2.7 Hinged Split-Cavity Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 661.2.5.2.8 Mold with Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671.2.5.2.9 Unscrewing Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671.2.5.3 General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681.2.6 Sheet Molding Compound (SMC)-Molds . . . . . . . . . . . . . . . . . . . 691.2.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 691.2.6.2 Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 701.2.6.2.1 Mold Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 721.2.6.2.2 Ejector Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 731.2.6.2.3 Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741.2.6.2.4 Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751.2.7 GMT/LFT Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 751.2.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

XV Contents

1.2.7.2 Process Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761.2.7.2.1 Pressure Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761.2.7.2.2 Flow Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761.2.7.3 Mold Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761.2.8 Practical Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

1.3 Molds for Polyurethane Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78U. Knipp, U. Maier1.3.1 Products, Processes, Applications, Shrinkage, and Mold

Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791.3.1.1 Material Components, Processing, Applications . . . . . . . . . . . . 791.3.1.2 Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801.3.1.3 Mold Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811.3.2 Molds for Low-Density PUR Foam . . . . . . . . . . . . . . . . . . . . . . . . . 821.3.2.1 Processing Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 831.3.2.1.1 Reaction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 831.3.2.1.2 Internal Pressure in Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 841.3.2.2 Filling Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 851.3.2.2.1 Open-Mold Filling Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 851.3.2.2.2 Closed-Mold Filling Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 861.3.2.3 Venting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 871.3.2.4 Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891.3.2.4.1 Clamping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 911.3.2.4.2 Closing and Opening Mechanism, Demolding Aids . . . . . . . . . 921.3.2.4.3 Fastening of Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 931.3.2.5 Molds for Flexible PUR Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . 931.3.2.6 Molds for Semirigid Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 951.3.2.7 Molds for Rigid PUR Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 961.3.3 Molds for PUR Integral Skin Foams (Self-Skinning Foams) . . 991.3.3.1 Influence of Processing on Mold Design . . . . . . . . . . . . . . . . . . . 991.3.3.1.1 Temperature control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 991.3.3.1.2 Sealing at Parting Line, Ejectors, and Side Cores/Sliders . . 1001.3.3.2 Gating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1011.3.3.3 Venting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1031.3.3.4 Molds for Flexible Integral Skin Foams . . . . . . . . . . . . . . . . . . . 1051.3.3.5 Molds for Semirigid Integral Skin Foams . . . . . . . . . . . . . . . . . 1051.3.3.6 Molds for Rigid Integral Skin Foams . . . . . . . . . . . . . . . . . . . . . 1061.3.4 Molds for Microporous PUR Products . . . . . . . . . . . . . . . . . . . . 1071.3.4.1 Molds for Flexible, Microporous PUR Products . . . . . . . . . . . . 1081.3.4.2 Molds for Tough, Rigid, Microporous PUR (RIM) Products . 1081.3.4.3 Molds for Rigid, Microporous PUR Products . . . . . . . . . . . . . . 1091.3.5 Molds for PUR Casting Systems . . . . . . . . . . . . . . . . . . . . . . . . . . 110

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1.4 Blow Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112O. Eiselen1.4.1 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1121.4.1.1 Different Types of Blow Molding Processes . . . . . . . . . . . . . . . 1121.4.1.2 Extrusion Blow Molding Technology . . . . . . . . . . . . . . . . . . . . . 1131.4.1.2.1 Continuous Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1131.4.1.2.2 Intermittent Parison Generation . . . . . . . . . . . . . . . . . . . . . . . . . 1141.4.1.2.3 Parison Generation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1161.4.1.2.4 Different Blow Up Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1171.4.1.2.5 Special Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1191.4.2 Extrusion Blow Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1201.4.2.1 Mold Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1201.4.2.1.1 Prototype Blow Molds Made from Cast Resin . . . . . . . . . . . . . . 1201.4.2.1.2 Prototype Blow Molds with Metal-Coated Model and

Metal-Filled Cast Resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1201.4.2.1.3 Blow Molds Made from Cast Metals . . . . . . . . . . . . . . . . . . . . . . 1201.4.2.1.4 Milled Prototype Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211.4.2.1.5 Production Blow Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1211.4.2.2 Construction Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1231.4.2.2.1 Alignment of Blow Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1231.4.2.2.2 Cutting Edges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1241.4.2.2.3 Clamp Stops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261.4.2.2.4 Venting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261.4.2.3 Blow Mold Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261.4.2.4 Accessories for Blow Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1291.4.2.5 Integrated Postmolding Processes . . . . . . . . . . . . . . . . . . . . . . . . 1301.4.2.5.1 Postcooling with a Cooling Fixture . . . . . . . . . . . . . . . . . . . . . . . 1301.4.2.5.2 Manufacturing the Finished Product in the Blow Molding

Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1311.4.3 Injection Blow Molding and Dip Blow Molding . . . . . . . . . . . . 1331.4.4 Use of Computers for Blow Molding . . . . . . . . . . . . . . . . . . . . . . 135

1.5 Molds for Thermoforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138P. Schwarzmann1.5.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1381.5.2 Process in Thermoforming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1391.5.3 The Mold and the Format Parts . . . . . . . . . . . . . . . . . . . . . . . . . . 1391.5.4 Positive or Negative Forming? . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411.5.5 Design Guidelines for Thermoforming Molds . . . . . . . . . . . . . 1411.5.5.1 Material Choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1411.5.5.2 Molding Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1441.5.5.3 Draft Angles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

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1.5.5.4 Radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1451.5.5.5 Surface Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471.5.5.6 Assisting Plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471.5.5.7 Venting Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1501.5.5.8 Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1521.5.5.9 Avoidance of Edge Webbing in Positive Molds . . . . . . . . . . . . . 1521.5.5.10 Vacuum Losses when Designing Mold Bottom Wrongly . . . . 1531.5.5.11 Suggestions for Temperature Control of Hot Forming

Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1541.5.6 Substructure of Hot Forming Molds . . . . . . . . . . . . . . . . . . . . . . 1551.5.6.1 Vacuum Forming on a Sheet Processing Machine . . . . . . . . . 1551.5.6.2 Pressure Air Forming with Forming/Punching Mold

with Shear Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1561.5.6.3 Pressure Air Forming with Forming/Punching Mold

with Steel Rule Die . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1571.6 Rotational and Slush Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

O. Wandres, R. Hentrich1.6.1 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1581.6.2 Strength of a Rotomolded Part . . . . . . . . . . . . . . . . . . . . . . . . . . . 1581.6.3 Mold Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1591.6.4 Nomenclature of Rotational Molds . . . . . . . . . . . . . . . . . . . . . . . 1601.6.5 Types of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1611.6.5.1 Prototype Rotational Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1611.6.5.2 Sheet Steel Rotational Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1621.6.5.3 Aluminum Rotational Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1631.6.5.4 Electroplated Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1651.6.6 Mold Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1681.6.6.1 Closing and Clamping of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . 1681.6.6.2 Mold Wall Thickness and Centering . . . . . . . . . . . . . . . . . . . . . . 1691.6.6.3 Mold Surfaces and Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1691.6.7 Mold Peripheral. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1701.6.7.1 Mold Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1701.6.7.2 Non-Permanent Release Agent . . . . . . . . . . . . . . . . . . . . . . . . . . . 1711.6.7.3 Mold Coating (Permanent Release Coatings) . . . . . . . . . . . . . . 1711.6.7.4 Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1721.6.7.5 Other Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1721.6.8 Post-Processing of Rotomolded Plastic Products . . . . . . . . . . . 1721.6.8.1 Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1721.6.8.2 Decoration of Rotomolded Plastic Products . . . . . . . . . . . . . . . . 1721.6.9 Electroplated Mold for the Slush Molding Process . . . . . . . . . 173

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1.7 Molds for Thermoplastic Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179N. Reuber1.7.1 Thermoplastic Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1791.7.2 Conventional Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1821.7.2.1 Procedure Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1821.7.2.1.1 Filling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1831.7.2.1.2 Expanding and Fusing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1841.7.2.1.3 Cooling and Stabilizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1861.7.2.1.4 Demolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1871.7.2.2 Special Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1881.7.2.2.1 Process with Non-Perforated Molds . . . . . . . . . . . . . . . . . . . . . . 1881.7.2.2.2 Low Temperature Horizontal (LTH) Process . . . . . . . . . . . . . . . 1881.7.2.2.3 Transfer Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1891.7.2.2.4 Multiple-Density-Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1891.7.3 Mold Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1901.7.3.1 Essential Requirements on the Mold Construction . . . . . . . . 1901.7.3.2 Mold Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1931.7.3.3 Mold Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1931.7.3.4 Special Mold Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1961.7.3.4.1 Mono-Block Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1961.7.3.4.2 Molds with Adjustable Walls (Gradually or Continuously)

for Insulation Plates and Small Blocks . . . . . . . . . . . . . . . . . . . . 1961.7.3.4.3 Mold for the Thin-Walled Technology . . . . . . . . . . . . . . . . . . . . . 1971.7.4 Block Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1971.7.4.1 Process Discription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1971.7.4.2 Constructive Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

1.8 Molds for Continuous Fibre Reinforced Polymer Composites . . . . . . . . . 200P. Mitschang, R. Schledjewski, A. K. Schlarb1.8.1 General Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2001.8.2 Molds for the Vacuum-Autoclave-Technology . . . . . . . . . . . . . . 2011.8.2.1 General Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2011.8.2.2 Prepreg-Low Pressure-Autoclave Technology . . . . . . . . . . . . . . 2021.8.2.3 Molds for the “Soft Core” Technology . . . . . . . . . . . . . . . . . . . . . 2021.8.2.3.1 Master Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2021.8.2.3.2 Mold Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2031.8.2.3.3 Product Substitute . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2041.8.2.3.4 Elastic Mat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2051.8.2.3.5 Manufacturing of Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2051.8.2.4 Molds for the Hard-Core Technology . . . . . . . . . . . . . . . . . . . . . 2071.8.2.4.1 Mold Construction and Materials . . . . . . . . . . . . . . . . . . . . . . . . 2071.8.2.4.2 Manufacture of Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

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1.8.2.5 Molds for Automated Tape Laying . . . . . . . . . . . . . . . . . . . . . . . . 2081.8.3 Continuous Fiber-Reinforced Thermoplastics . . . . . . . . . . . . . 2091.8.3.1 General Information and Fundamentals of the Processes . . 2091.8.3.2 Molds for Semifinished Part Production . . . . . . . . . . . . . . . . . . 2091.8.3.2.1 General Information and Fundamentals of the Processes . . 2091.8.3.2.2 Molds for Flat Semifinished Plates . . . . . . . . . . . . . . . . . . . . . . . 2101.8.3.2.3 Molds for Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2121.8.3.3 Molds for Forming Technology (Thermoforming) . . . . . . . . . . 2131.8.3.3.1 General Information and Fundamentals of the Processes . . 2131.8.3.3.2 Molds for the Stamp Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2141.8.3.3.3 Molds for the Diaphragm Technology . . . . . . . . . . . . . . . . . . . . . 2171.8.3.3.4 Molds for Sandwich Components . . . . . . . . . . . . . . . . . . . . . . . . . 2181.8.3.3.5 Molds for the Process Step Integration . . . . . . . . . . . . . . . . . . . 2191.8.3.4 Molds for the Welding Technology . . . . . . . . . . . . . . . . . . . . . . . 2201.8.4 Molds for the Resin Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2211.8.4.1 General Information and Fundamentals of the Process . . . . 2211.8.4.2 Molds for the Preform Technology . . . . . . . . . . . . . . . . . . . . . . . 2231.8.4.2.1 Binder-Forming Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2231.8.4.2.2 Stitching Technology Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2251.8.4.3 Molds for Vacuum Assisted Processes . . . . . . . . . . . . . . . . . . . . 2271.8.4.3.1 Molds with a Fixed and Flexible Mold Half. . . . . . . . . . . . . . . . 2271.8.4.3.2 Molds with Two Fixed Mold Halves . . . . . . . . . . . . . . . . . . . . . . 2301.8.4.4 Molds for Pressure-Assisted Processes . . . . . . . . . . . . . . . . . . . 2321.8.4.5 Molds for Hollow Components . . . . . . . . . . . . . . . . . . . . . . . . . . . 2351.8.5 Molds for the Winding Technology . . . . . . . . . . . . . . . . . . . . . . . 2381.8.5.1 General Information and Fundamentals of the Process . . . . 2381.8.5.2 Molds for Rotationally Symmetrical Components . . . . . . . . . . 238

1.9 Molds for Elastomer Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241Th. Bauernhansl, K. Zoller1.9.1 Compression Molding (CM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2421.9.2 Transfer Molding (TM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2441.9.3 Injection Molding (IM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2461.9.4 Additional Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2501.9.4.1 Process Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2501.9.4.2 Gate Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2511.9.5 Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2541.9.5.1 Types of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2551.9.5.2 Mold Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

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1.10 Micro Injection Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258G. Konzilia1.10.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2581.10.1.1 Injection Molding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2581.10.1.2 Molded Part Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2581.10.1.2.1 Cooperation with Customers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2591.10.1.3 Materials for Injection Molded Parts . . . . . . . . . . . . . . . . . . . . . 2601.10.2 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2611.10.2.1 The Micro-injection Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2611.10.2.1.1 Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2611.10.2.1.2 Demolding and Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2621.10.2.1.3 Venting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2631.10.2.1.4 Mold Guiding and Centering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2641.10.2.1.5 Temperature Control and Cooling . . . . . . . . . . . . . . . . . . . . . . . . 2651.10.2.2 Special Procedures and Alternative Processes . . . . . . . . . . . . 2651.10.2.2.1 Variotherm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2651.10.2.2.2 Insertion Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2661.10.2.2.3 Multi-Component and Assembly Injection Molding . . . . . . . . 2661.10.2.2.4 Compression Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . 2671.10.2.2.5 Hot Embossing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2671.10.2.3 Environment and Continuing Processes . . . . . . . . . . . . . . . . . . 2681.10.3 Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2691.10.3.1 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2691.10.3.1.1 Materials for Constructional Parts . . . . . . . . . . . . . . . . . . . . . . . 2691.10.3.1.2 Standard Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2691.10.3.2 Cavity Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2691.10.3.2.1 Material for Cavity Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2701.10.4 Manufacturing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2701.10.4.1 In General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2701.10.4.1.1 Mechanical Manufacturing Technologies . . . . . . . . . . . . . . . . . 2701.10.4.1.2 Alternative Manufacturing Processes . . . . . . . . . . . . . . . . . . . . 2741.10.4.1.3 Surface Treatment and Refining . . . . . . . . . . . . . . . . . . . . . . . . . 2761.10.4.1.4 Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2771.10.5 Injection Molding Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2771.10.6 Mold Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2781.10.7 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

1.11 Prototype, Small and Pre-Series Molds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280R. Hofmann1.11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2801.11.2 Indirect Prototype Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2801.11.2.1 Vacuum Casting Polyurethane (PU) on silicone molds . . . . . 280

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1.11.2.1.1 Vacuum Casting PU Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2801.11.2.1.2 Manufacture of Silicone Molds PU . . . . . . . . . . . . . . . . . . . . . . . 2831.11.2.2 Vacuum Casting Polyamide (PA) through silicone molds . . . 2841.11.2.3 Synthetic Resin Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2871.11.2.3.1 Polyurethane Casting with Synthetic Resin Molds . . . . . . . . . 2871.11.2.3.2 Manufacture of Synthetic Resin Molds . . . . . . . . . . . . . . . . . . . 2871.11.2.4 Manufacture of Synthetic Resin Molds for Injection

Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2901.11.2.5 Molds Manufactured through Generative Manufacturing

Procedures on the Example of LaserCUSING®-Technology . 2921.11.2.6 Aluminum Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2941.11.2.6.1 Manufacture of Aluminum Molds . . . . . . . . . . . . . . . . . . . . . . . . 2941.11.2.6.2 Aluminum Molds with LaserCUSING® Loose Parts . . . . . . . . 2951.11.2.6.3 Aluminum Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

2 Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3012.1 Design Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

P. Karlinger, F. Hinken2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3012.1.1.1 Injection Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3032.1.1.2 Phases of the Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3032.1.1.3 From the Offer to the Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3072.1.1.4 The Design Process in Injection Molds . . . . . . . . . . . . . . . . . . . 3182.1.2 Simulation for Injection Mold Making . . . . . . . . . . . . . . . . . . . . 3232.1.2.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3232.1.2.2 The Types of Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3252.1.2.3 The Flow Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3262.1.2.4 Shrinkage and Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3282.1.2.5 Thermal Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3292.1.2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

2.2 Standardization and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332J. Gockel, A. Brandt2.2.1 Standardization for Injection Molding and Hot Runner

Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3322.2.2 Standards in Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3352.2.2.1 Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3362.2.2.2 Standardized Guide Element in Mold Making . . . . . . . . . . . . . 3412.2.2.3 Standards for Demolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3432.2.2.4 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345

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2.3 Hot and Cold Runner Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349D. Paulmann, M. Sander2.3.1 Advantages of Using the Hot Runner Technology . . . . . . . . . . 3492.3.2 Design of Hot Runner Systems and Hot Halves . . . . . . . . . . . . 3512.3.3 Application Areas and Examples . . . . . . . . . . . . . . . . . . . . . . . . . 3522.3.3.1 Hot Runner Solutions for Packaging Parts, Closures, and

Miscellaneous Polyolefin Applications . . . . . . . . . . . . . . . . . . . . 3532.3.3.2 Hot Runner Solutions for Technical Components . . . . . . . . . . 3562.3.3.3 Hot Runner Solution for Small and Micro Injection Molded

Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3582.3.3.4 Hot Runner Solutions for Multi-Point Gating through

Nozzles and Multi-Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3602.3.3.5 Hot Runner Solutions with Needle Valve . . . . . . . . . . . . . . . . . . 3612.3.4 Hot Runner Manifold Systems, Wired Systems, and Hot

Halves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3672.3.5 Hot Runner Control Technology . . . . . . . . . . . . . . . . . . . . . . . . . . 3702.3.6 Cold Runner Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722.3.6.1 Function and Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722.3.6.2 Processable Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3732.3.6.3 Mold Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3752.3.6.4 Demolding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3752.3.6.5 Mold Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376

2.4 Temperature Control of Injection Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377P. Thienel2.4.1 Tasks and Goals of the Mold Temperature Control . . . . . . . . . 3772.4.2 Influence of Processing Temperatures on the Cooling and

Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3792.4.3 Cavity Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3792.4.4 Influence of Temperature Control on the Molded Part

Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3812.4.5 Requirements for the Temperature Control System . . . . . . . . 3832.4.6 Temperature Control Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . 3832.4.7 Flow Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3852.4.7.1 Series Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3852.4.7.2 Parallel Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3862.4.8 Practical Designs of Conventional Temperature Control

Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3872.4.8.1 Flat Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3872.4.8.2 Temperature Control of Molded Part Corners . . . . . . . . . . . . . 3882.4.8.3 Temperature Control of the Core . . . . . . . . . . . . . . . . . . . . . . . . . 3892.4.8.3.1 Temperature Control Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

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2.4.8.3.2 Separating Plate (Deflection Bar) . . . . . . . . . . . . . . . . . . . . . . . . . 3892.4.8.3.3 Spiral Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3902.4.8.3.4 Heat Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3902.4.8.4 More Conventional Temperature Control Options . . . . . . . . . 3922.4.8.4.1 Circumferential Application Temperature Control . . . . . . . . . 3922.4.8.4.2 Inserts Made from Different Materials . . . . . . . . . . . . . . . . . . . . 3922.4.9 New Temperature Control Technologies . . . . . . . . . . . . . . . . . . 3932.4.9.1 Contour-Depending Temperature Control . . . . . . . . . . . . . . . . . 3932.4.9.1.1 Vacuum Brazing Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3942.4.9.1.2 Selective Laser Sintering (SLS) . . . . . . . . . . . . . . . . . . . . . . . . . . . 3952.4.9.2 CO2 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3952.4.9.2.1 CO2 Temperature Control with Sintered Material . . . . . . . . . . 3962.4.9.2.2 CO2 Temperature Control with Conventional Steel . . . . . . . . . 3972.4.9.3 Dynamic Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 3972.4.10 Thermal Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4002.4.11 Position of the Temperature Sensor for External

Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4022.5 Innovative Mold Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

T. Eulenstein2.5.1 Coating Technology – Design Surfaces through Combined

Surface and Coating Technologies . . . . . . . . . . . . . . . . . . . . . . . . 4052.5.2 Temperature Control Technology –

Inductive Heating of Injection Molds . . . . . . . . . . . . . . . . . . . . . 4102.5.3 Vacuum Technology –

Alternative Possibilities, Optimization of Surfaces . . . . . . . . . 4152.5.4 Mold Technology – Flexible Sealing Elements for the Flash-

and Damage-Free Encapsulation of Inserts . . . . . . . . . . . . . . . . 417

3 Materials for Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4213.1 Plastic Mold Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

F. Hippenstiel3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4213.1.2 Steel making and processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4233.1.2.1 Steelmaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4233.1.2.2 Heat treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4253.1.2.3 Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4273.1.2.4 Surface machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4273.1.2.5 Quality assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4283.1.3 Overview of plastic mold steels . . . . . . . . . . . . . . . . . . . . . . . . . . 4283.1.3.1 Pre-hardened plastic mold steels . . . . . . . . . . . . . . . . . . . . . . . . . 4343.1.3.2 Through-hardening plastic mold steels . . . . . . . . . . . . . . . . . . . 436

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3.1.3.3 Corrosion-resistant plastic mold steels . . . . . . . . . . . . . . . . . . . 4373.1.3.4 Plastic mold steels for case hardening . . . . . . . . . . . . . . . . . . . . 4393.1.3.5 Precipitation hardening plastic molds steels . . . . . . . . . . . . . . 4403.1.3.6 Nitriding steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4413.1.4 Concluding comment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441

3.2 Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442A. Erstling3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4423.2.2 Mold Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4433.2.2.1 Casting Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4443.2.2.2 Wrought Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4443.2.2.3 Mechanical Properties and Design Guidelines . . . . . . . . . . . . 4463.2.2.4 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4483.2.2.5 Friction and Wear Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4493.2.3 Manufacture of Aluminum Molds . . . . . . . . . . . . . . . . . . . . . . . . 4513.2.3.1 Abrasive Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4513.2.3.1.1 Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4513.2.3.1.2 Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4543.2.3.1.3 Electrical Discharge Machining (EDM) or Wire EDM . . . . . . 4543.2.3.1.4 Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4543.2.3.2 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4543.2.3.3 Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4573.2.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458

3.3 Copper Alloys-Nonferrous Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460E. Seufert3.3.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4603.3.1.1 Strength Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4603.3.1.2 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4623.3.2.1 Turning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4643.3.2.2 Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4653.3.2.3 Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4653.3.2.4 Threading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4663.3.2.5 Reaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4663.3.2.6 EDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4673.3.2.7 Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4673.3.3 Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4683.3.3.1 Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4693.3.3.2 Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4693.3.3.3 Structuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4703.3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

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4 Manufacturing and Machining Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4714.1 Mold Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

A. Klotzbücher4.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4714.1.2 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4714.1.2.1 Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4714.1.2.2 Visualizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4724.1.2.3 Cubing Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4724.1.2.4 Stereolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4724.1.3 Data Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4744.1.3.1 Data Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4744.1.3.2 Completion of Product Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4744.1.4 Data Transfer in Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . 4754.1.4.1 Verifying of Data Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4754.1.4.2 Feasibility Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4754.1.5 Feedback/Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4764.1.6 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4774.1.6.1 System Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4774.1.6.2 Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4784.1.7 Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4804.1.7.1 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4804.1.7.2 Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4824.1.7.3 Choice of Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4824.1.8 Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4834.1.8.1 Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4834.1.8.2 Unattended Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4844.1.8.3 Releases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4844.1.9 Dimensional Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4854.1.10 Drilling/Deep Hole Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4854.1.11 Electric Discharge Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4864.1.12 Surface Finishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4874.1.13 Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4874.1.14 Trials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4894.1.15 Optimization Process and Finishing . . . . . . . . . . . . . . . . . . . . . . 489

4.2 Electric Discharge Machining (EDM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490D. Schäffner, B. Mack4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4904.2.2 Physical Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4924.2.3 Tolerances and Key Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4944.2.4 Die-Sinking EDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4944.2.5 Wire-cut EDM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497

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4.2.6 Combined and Special Processes . . . . . . . . . . . . . . . . . . . . . . . . . 5004.3 Galvanized Inserts and Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502

R. Hentrich4.3.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5024.3.2 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5024.3.3 Galvanized Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5044.3.4 Model Materials and Model Design . . . . . . . . . . . . . . . . . . . . . . . 5054.3.5 Clamps and Mounting Brackets . . . . . . . . . . . . . . . . . . . . . . . . . . 5074.3.6 Finishing and Installation of Galvanized Injection Inserts . 5094.3.7 Efficiency and Service Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5104.3.8 Galvanized Molds for Other Plastics Processing Methods . . 5134.3.8.1 Molds for Processing Polyurethane Foam . . . . . . . . . . . . . . . . . 5144.3.8.2 PUR Spray Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5154.3.8.3 Laminating Molds for the Aerospace Industry. . . . . . . . . . . . . 5164.3.9 Negative-Stamping Deep Drawing Process

(In-Mold Graining) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5194.4 Polishing Technology in Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521

C. Steiner4.4.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5214.4.2 Definition of the Term Surface Roughness . . . . . . . . . . . . . . . . 5214.4.3 Systematic Polishing Technique . . . . . . . . . . . . . . . . . . . . . . . . . . 5224.4.4 Polishing Behavior-Influencing Factors . . . . . . . . . . . . . . . . . . . 5244.4.5 Polishing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5244.4.5.1 For Superfinishing (Polishing) Surface Preparatory

Leveling Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5244.4.5.2 Lapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5274.4.5.3 Polish Lapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5304.4.5.4 Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5314.4.6 Ultrasonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5324.4.7 Electric Discharge Machining/Erosion for Brilliant

Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5324.5 Heat Treatment and Surface Finishing Techniques . . . . . . . . . . . . . . . . . . 534

P. Vetter4.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5344.5.2 Heat treatment of plastic mold steels . . . . . . . . . . . . . . . . . . . . . 5344.5.2.1 Hardened and tempered plastic mold steels . . . . . . . . . . . . . . . 5354.5.2.2 Through-hardening steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5364.5.2.3 Corrosion-resistant steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5424.5.2.4 Case-hardening steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5434.5.2.5 Nitriding steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5434.5.2.6 Maraging mold steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543

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4.5.2.7 General recommendations for heat treatment . . . . . . . . . . . . . 5444.5.3 Surface finishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5454.5.3.1 Thermal processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5474.5.3.1.1 Flame hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5484.5.3.1.2 Laser hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5494.5.3.2 Thermo-chemical processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5504.5.3.2.1 Case-hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5504.5.3.2.2 Nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5524.5.3.2.3 Gas nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5534.5.3.2.4 Plasma nitriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5544.5.3.2.5 Boriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5554.5.3.3 Electrochemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5554.5.3.3.1 Hard chrome plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5564.5.3.3.2 Nickel plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5584.5.3.4 Chemical and physical processes. . . . . . . . . . . . . . . . . . . . . . . . . 5604.5.3.4.1 CVD coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5604.5.3.4.2 PACVD coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5614.5.3.4.3 PVD coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5614.5.3.5 Comparing and Selecting Surface Treatment Processes . . . . 564

4.6 Surface Structuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567St. Krüth4.6.1 The Photochemical Etching Technology . . . . . . . . . . . . . . . . . . . 5674.6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5674.6.1.2 Why Structuring? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5684.6.1.3 From the Structure Template to the Film . . . . . . . . . . . . . . . . . 5694.6.2 Requirements on the Mold Surface and Construction . . . . . . 5694.6.2.1 Materials and the Selection of Materials . . . . . . . . . . . . . . . . . . 5704.6.2.1.1 Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5714.6.2.1.2 Aluminum and Other Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 5714.6.2.1.3 Heat Treatment and Surface Refinement. . . . . . . . . . . . . . . . . . 5714.6.2.1.4 Grain Depths and Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5724.6.2.1.5 The Gloss Level in the Mold and in the Molded Part . . . . . . . 5734.6.2.2 Processing Methods and Repair Technology . . . . . . . . . . . . . . 5744.6.2.3 Draft Angles, Open Spaces, and Surface Preparation . . . . . . 5744.6.2.4 Contour Changes by Welding of Inserts . . . . . . . . . . . . . . . . . . . 5754.6.2.5 Contour Changes by Shrinking Inserts . . . . . . . . . . . . . . . . . . . 5764.6.2.6 Structure Hardening, Fiber Orientation, Band-Type

Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5774.6.2.7 Etching Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5784.6.3 Special Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5794.6.3.1 Design Types and Etching Combinations . . . . . . . . . . . . . . . . . 579

XXVIII Contents

4.6.3.2 Limitations of the Processing Technology . . . . . . . . . . . . . . . . . 5794.6.3.3 New Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5804.6.4 The Execution of the Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5814.6.4.1 Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5814.6.4.2 Information about the Grain Area and the Mold . . . . . . . . . . . 5814.6.4.3 Concluding Remark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581

4.7 Rapid Prototyping in Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582A. Gebhardt4.7.1 Rapid Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5824.7.2 Fundamentals of the Generative Manufacturing Processes 5834.7.2.1 Process Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5834.7.2.2 Data Flow and Data Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5844.7.2.3 Properties of Generative Components . . . . . . . . . . . . . . . . . . . . 5844.7.2.4 Definitions for Rapid Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5864.7.3 Generative Processes for Mold Making . . . . . . . . . . . . . . . . . . . 5874.7.3.1 Polymerization-Stereolithography . . . . . . . . . . . . . . . . . . . . . . . . 5884.7.3.2 Sintering and Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5904.7.3.3 Layer-Laminate Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5924.7.3.4 Extrusion Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5944.7.3.4.1 3D Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5964.7.4 Machines for Generative Mold Making . . . . . . . . . . . . . . . . . . . 5984.7.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5994.7.5.1 Prototype Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5994.7.5.2 Direct Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5994.7.6 Delimitation to Non-Generative Manufacturing Processes . 6034.7.7 Names and Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605

5 Ordering and Operation of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6075.1 Molds in the Offer Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 607

F. Schlößer5.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6075.1.2 The Planning of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6085.1.2.1 Adjustment Process of Component and Mold . . . . . . . . . . . . . . 6085.1.2.2 Design of the Mold under Consideration of the Product

Life Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6095.1.2.3 Checklist for the Mold Specification . . . . . . . . . . . . . . . . . . . . . . 6115.1.3 Costing in Mold Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6135.1.3.1 Various Methods for Costing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6135.1.3.2 Simplified Costing in the Bidding and Design Phase . . . . . . . 6145.1.3.2.1 Estimated Value Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6145.1.3.2.2 Reference Value Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614

XXIX Contents

5.1.3.2.3 Cost Element Methodology/Variable Costing . . . . . . . . . . . . . . 6165.1.3.2.4 Detail Calculations/Post-Calculations . . . . . . . . . . . . . . . . . . . . . 6185.1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621

5.2 Setup and Control of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622Ch. Bader5.2.1 Requirements for Effective Quality Assurance . . . . . . . . . . . . 6225.2.2 Mold Sensor Systems Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6225.2.2.1 Mold Cavity Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . 6235.2.2.2 The Measuring Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6235.2.2.3 Cavity Temperature Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6255.2.2.4 Sensor Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6255.2.2.5 Quick Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6265.2.3 Data Acquisition and Electronics . . . . . . . . . . . . . . . . . . . . . . . . . 6275.2.4 Setup and Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6285.2.4.1 Cavity Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6285.2.4.2 The Importance of the Cavity Temperature Curve . . . . . . . . . 6315.2.4.3 Switchover to Holding Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . 6325.2.5 The Process Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6335.2.6 Factory-Wide Networking and Monitoring . . . . . . . . . . . . . . . . 6355.2.7 Real-Time Controls in the Injection Molding Process . . . . . . 6375.2.8 The Control of the Injection Molding Process . . . . . . . . . . . . . 6385.2.9 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641

5.3 Wear on Injection Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642T. Eulenstein, U. Hinzpeter5.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6425.3.2 Tribological Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6425.3.3 Abrasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6465.3.3.1 Forms of Damage on Molds and Hot Runners That Cause

Molded Part Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6465.3.3.2 Corrective Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6495.3.4 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6495.3.4.1 Causes and Forms of Damage on Molds That Cause

Molded Part Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6515.3.4.2 Corrective Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6515.3.5 Abrasive Wear of Mold Elements . . . . . . . . . . . . . . . . . . . . . . . . . 6535.3.5.1 Types of Damage on Mold Elements . . . . . . . . . . . . . . . . . . . . . . 6535.3.5.2 Corrective Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6545.3.6 Outlook and Development Trends . . . . . . . . . . . . . . . . . . . . . . . . 658

XXX Contents

5.4 Maintenance, Storage, Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662U. Thiesen5.4.1 General Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6625.4.2 Maintenance, Wear Supply, Hardness . . . . . . . . . . . . . . . . . . . . 6635.4.3 Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6635.4.3.1 Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6635.4.3.2 Inspection Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6635.4.4 Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6655.4.4.1 Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6655.4.4.2 Leakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6665.4.4.3 Breakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6675.4.4.4 Repair Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6675.4.5 Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6695.4.6 Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6695.4.6.1 Preservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6715.4.6.2 Storage Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6725.4.6.3 Mold Labeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6735.4.6.4 Storage size. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6745.4.7 Maintenance and Servicing Costs . . . . . . . . . . . . . . . . . . . . . . . . 674

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677

1 Molds for Various Processing Methods

 ■ 1.1  Injection Molds

P. Wippenbeck

1.1.1  General

Thermoplastics as well as thermosets and elastomers can be processed with the injection molding process, although for cross-linking polymers modified equipment in machines and adequate molds, which are generally heated to 120 to 180 °C, are needed. Due to the large majority of applications, the following covers primarily molds for thermoplastics, with temperature control between 10 to 120 °C, which can go up to 200 °C for “exotic”, highly heat-resistant thermoplastics.

1.1.2  Injection Molding Process

Injection molding is a typical process for the production of parts molded in large volumes, frequently used in high mass production. The associated mold costs are in comparison to other processes relatively high [1 to 7]. Experts know that the profitability of injection molding starts at 10,000 pieces, in some cases it starts at 3,000 pieces depending on the complexity of the molded part and expenses for the mold. The number of cavities in a mold is mostly under 100. Molds for extreme mass production though have even more “drops” (Figure 1.1).

The percentage of production costs for example, the demand of energy or space decreases, the more cavities are brought into a production unit. However, the fall-out risk increases so, that a lower cavity number has been implemented than technically possible. It is a special advantage that very complex elements of the molded part geometry for example, external and internal undercuts, snap-in noses, hinges, spring elements or internal threads can be produced with high degree of

2 1 Molds for Various Processing Methods

automation. These can be in the weight range of micro-injection molding of a few milligrams up to large parts of more than 50 kg (Figure 1.2).

Injection molding machines are characterized according to their clamping force: From 50 up to 100,000 kN, for example the production of boat bodies or wet cells of pre-fabricated houses. It is typical that machine sizes in the range of 100 to 30,000 kN are manufactured in small series.

On the other hand the injection molds are generally individual items and therefore very valuable production resources. Their availability can be of crucial significance. It is possible that a higher investment is required into the mold than into the machinery itself.

The basic design of an injection molding machine is shown in Figure 1.3. As heart of the machine the heated barrel of the injection unit can be taken containing a

FIGURE 1.1  Mold for bottle- preforms, 192 cavities [8]

FIGURE 1.2  Injection mold for car glazing, injection-compression molding and two-component

technology [9]

31.1 Injection Molds

screw which can rotate and move axially backwards during plasticizing and works as a non-rotating piston when injecting and during holding pressure time.

The function of the clamping unit is to move the one mold-half called “clamp”, “movable” or “ejector” half towards the stationary, “fixed” half and the application of clamping force to seal against the separating force of the cavity pressure.

Relatively high pressures are needed, not under 200 bar, mostly in the range of 300 to 1,200 bar. Due to the involved areas, forces are created which must be taken into consideration when designing the side walls of the mold or the needed clamping force of the machine. Calculations within mold design will take o�en a mean value of 1,000 bar. Such a high pressure level is especially needed because sufficient shrinkage compensation must occur during the compression of the melt.

Pellet melting occurs during the screw rotation, generating energy by the barrel heaters and by shear heat which part mostly predominates. Simultaneously there is a forward melt feed towards the closed nozzle so that the screw presses itself back due to its own feed effect. During the reverse motion the screw has to overcome the frictional resistances of the system. To improve the mixing effect, to increase the amount of shear heat or to reach an enlarged degassing effect an additional reverse resistance the so called “back pressure” will be applied. The rotation is turned off when the given rear position corresponding to the “metering stroke” is reached. The melt volume in front of the screw tip is now ready for the injection process.

Thanks to the check valve on the screw tip (Figure 1.4), representing a standard equipment the screw can work as an injection piston and introduce the melt into the relatively cold mold “cavity” until the cavity is completely wetted called “volu-

c

ab

d

FIGURE 1.3  Injection molding machine in typical 3-platen design with toggle clamping unit,

without mold

a: Ejector platen, b: fixed platen, c: mold height adjustment, d: injection unit with

plasticizing cylinder and shut-off nozzle [10]

4 1 Molds for Various Processing Methods

metric filling”. The type of melt flow is “fountain flow” which means that stationary layers are formed outside in contact with the cold mold wall and the melt has to pass through. (Figure 1.5).

Too intense growth of the stationary layer can bring the filling process to a complete standstill, the melt “freezes off”, for example if the injection speed is too low. In addition, a thin skin layer forms at the flow front which offers a certain self-sealing at tight gaps of the mold. This is the basis of the “ventilation” of cavities in the mold design because the entering melt has to push air out of the cavity. Therefore venting gaps of 10 to 30 μm are possible without flashing, because of the flow front skin thickness of about 10 μm. In this range, it is possible to allow the mold to “breath” so improving the ventilation.

a

b

c

d

e

FIGURE 1.4  Screw barrel

a: Barrel head; b: non-return valve; c: screw position at the end of the plasticizing

process; d: screw flight; e: barrel heaters [9]

a

b

c

FIGURE 1.5  Fountain flow during the filling process with thermoplastic melt

a: Stationary solidified layer; b: low viscous core area, „plastic core“;

c: „flow front skin“ of higher viscosity [11]

51.1 Injection Molds

a b c

d

p

t

FIGURE 1.6  Typical cavity pressure progress

a: Switchover from injection pressure to holding pressure; b: volumetric filling;

c: sealing point; d: start of shrinkage [11]

A�er the volumetric filling, the dynamical phase which is dominated by the “injec-tion pressure” of the filling process moves on to the quasi-static phase. This “holding pressure phase” can on one hand get the melt to compress and on the other hand deliver additional melt during the initial cooling, where the pressure in the cavity remains o�en almost constant. Such a post-supply of melt is possible as long as the gate, the junction of the runner into the cavity is still permeable (not yet “sealed”).

A�er the “sealing point” follows a larger reduction in pressure according to the actual cooling rate. The shrinkage of the molded part starts a�er the complete pressure release, which is mostly connected with a separation of the plastic from the inner wall of the cavity (Figure 1.6).

It is important for the mold design that the shrinkage of an injection molded part is not only dependent on the material type, but also strongly on the processing condi-tions, especially on pressure and temperature during the process as well as on the sealing of the gate. It is therefore justified, that the raw material manufacturer only gives a very rough range of shrinkage, for example 0.5 to 1.5%. Both raw material data and process data are necessary for a more precise shrinkage prognosis leading to a process simulation. The situation is still more complicated by the fact that the shrinkage values are dependent on the direction of molecular and filler orientation. Pressure and temperature influence the specific volume of the injection molding compound and thus the weight of the molded part. This provides an important and relatively easy to determine indication for process constancy: experience shows that 70% of all parameter fluctuations are indicated by the weight of the molded part.

6 1 Molds for Various Processing Methods

The holding pressure time has to be slightly longer than the “sealing time” of the gate to prevent back flow of the compressed mass escaping the cavity. The error of a too short holding time reflects directly on the weight of the molded part. The holding pressure time and the set “cooling time” together provide the physical cooling time due to the significant amount of cooling in the holding pressure phase. Normally, the necessary cycle time is determined by the cooling capacity of the mold and rarely by the parallel process of plasticizing with its “plasticizing rate”.

When the molded part is stable enough to absorb the ejection forces without damage or undue deflection it can be “demolded”.

Depending on the design of the machine nozzle, with or without shut-off mecha-nism, it is time for the injection unit to move back: either just a�er the end of holding pressure in case of a shut-off nozzle, so preventing the smallest amount of heat flow from the nozzle to the cooled mold (Figure 1.7) or a�er the end of the plasticizing process.

At the end of the cooling phase, the clamping force of the machine is released and the mold opens which means that the ejector half moves back. A reliable removal of the injection molded part can be assured by an adequate “break time” or “mold open time”. Automated removal devices which are used increasingly, activate a release signal for the next closing process.

FIGURE 1.7  Timeline of a standard injection molding machine equipped with an open nozzle

or with a shut-off nozzle (dashed lines)

PW: Position of the moving mold half (ejector half); FS: Clamping force;

PD: Position of the injection unit (nozzle); PS: Position of the screw;

FE: Force on the axial drive of the screw; PA: Position of the ejectors;

tn: holding pressure time; tk: cooling time; tp: break time; ta: ejector time [11]

71.1 Injection Molds

Every injection molding machine has a security system which is called “mold protec-tion” to prevent mold damage by an obstacle between the mold halves – a molded part of the previous cycle can also be such an obstacle. This “mold protection” works according to the force transducer principle: A signal is activated by an increased resistance caused by a foreign object. This will stop the clamping motion and the red alarm light will start blinking. During machine set-up the following three mold dependent positions (zero points) have to be transmitted to the machine control:

� Ejector rear position, � Nozzle forward position, contacting the sprue bushing, � Closed mold position corresponding to the depth of the mold.

1.1.3  Design of the Molded Part

The following factors can significantly influence the design of the molded part:

� Flow properties of the melt, � Solidification behaviour, � Pressure transfer in the cooling-down melt, � Molecular and fiber orientation, � Shrinkage and its dependence on process parameters, on the gate position and on the direction of measuring,

� Warpage, which increases remarkably through fiber reinforcement.

When designing, it has to be especially taken into account:

� Demoldability, demolding dra� angle, � Permissibility of weld lines, � Permissibility of marks by gates, ejectors, sliders and splits, � Surface structure, � Required tolerances.

The area around the gate has normally far more molecular orientations than other areas. Additionally, there is o�en an overloading due to a too high melt compression or too long holding pressure. This forms an area very liable to fracture. The practical experience coincides with the fact that two thirds of fracture lines pass through the gate. The gate position is o�en indicated by stress crack lines (Figure 1.8).

8 1 Molds for Various Processing Methods

FIGURE 1.8  Stress cracking in direction of molecular orientation [12]

a

a

b

b

b

FIGURE 1.9  Molded part with very unfavorable position of mass concentrations

a: Position of the tunnel gates, b: mass concentrations which are far from the

gate [11]

Important to consider the following fundamental rules:

� No gates and weld lines in high-stressed areas and edges! � Avoid wall thickness differences. The ideal injection molded part has equal wall thickness.

� If mass concentrations cannot be avoided, they have to be positioned as close as possible to gate. Figure 1.9 shows a negative example.

� Minimize wall thicknesses and only as big as absolutely necessary! � Optimize wall thicknesses, position and number of the gates to result in a uniform and inclusion- free flow front progress. Therefore, it is essential to perform a rheo-logical analysis with finite elements. Several experienced simulation programs for example Cadmould or Moldflow can be used for this application.

� Avoid sharp inside edges. � Use simplifications: O�en a few minor design changes are enough to process without the use of sliders or jaws. This decreases the mold costs drastically.

91.1 Injection Molds

The mechanical and thermal design can also be done using different CAE programs. The integration of cooling channels allows a detection of “hot spots” in the mold. These “hot spots” are determining the cooling time and have to be eliminated (see also chapter 2.3).

The tolerances for plastic molded parts are standardized in DIN 16901. However, in practice, narrower tolerances are demanded [13, 14] and achieved.

1.1.4  Basic Mold Structure

Basic injection molds are made of two halves: Fixed half (o�en abbreviated as A) and Ejector half (o�en abbreviated as B), which are consisting of several plates (Figure 1.10). Due to an easier machinability and interchangeability, the cavity is formed by inserts which are tailored to the surrounding frame plates.

a

b

c

d

e

g

f

h

j

k

l

m

n

o

p

i

FIGURE 1.10  Conventional structure of an injection mold.

a: Locating ring of the fixed half; b: basic plate of the fixed half; c: guide pin;

d: guide bushing; e: locating bushing; f: spacer for the ejector box; g: basic plate

of the ejector half; h: insulation plate (in case of higher mold temperatures);

i: screw locking device, j: main screw k: frame plate of the fixed half containing

the inserts; l: frame plate of the ejector half; m: support plate; n: support bar;

o, p: plates of the ejector traverse [15]

10 1 Molds for Various Processing Methods

1.1.5  Types of Ejection

A�er opening the mold, the molded part usually remains on the ejector half. Under-cuts (lateral breakthroughs), bosses, offset or internal threads require additional moving mold parts.

According to the complexity of demolding, the following categories can be consid-ered:

1 Products without undercuts. The relatively easy “open-close molds” are used.

2 Products with external undercuts (e.g., spools and bobbins, transportation boxes with handles, handled vessels, parts with external threads).

3 Products with internal undercuts (e.g., threaded closures, housings with a retracted edge, parts with snap-in noses.

4 Products with external and internal undercuts (e.g., bumper fascias, camera housing).

1.1.5.1  Products without Undercuts

The molded part in Figure 1.11 (dart) has undercuts in longitudinal direction. With an appropriate arrangement of the parting line one can produce the part without using lateral sliders or splits.

The flat centerings shown in Figure 1.11 and Figure 1.12 replace more and more of the tapered centering elements because of the advantage of earlier centering (e.g., 10 to 50 mm before closing the mold completely). Centering of tapered elements occurs only with contact. Correct positioning allows thermal expansions as well. According to Figure 1.12 (on the right), the flat centerings allow different tempera-tures of both mold halves without any loss of centering effect.

The considerable high effort for the hotrunner-valve gate technology is only eco-nomical for large production numbers. However, the electromagnetic operation of the needle offers a cost-effective solution due to the elimination of tubing. It should be noted that the temperature limit for electromagnets is around 80 °C.

If the effort for hotrunner systems are not justifiable, then a “tunnel gate” (Figure 1.36) which is the most common gate type, is being used. Advantageous is that the automatic separation of the runner from the molded part occurs inside the mold without any additional mechanism.

According to Figure 1.13, the centering ring e on the fixed half cares for an exact centering of the mold in the injection molding machine. The ring a on the ejector half though, provides only an inaccurate fit in the case of loosened screw in the mold fixing clamp h. The guide pins d are just used for pre-centering due to significant clearance and can’t be used for a precise inner centering.

111.1 Injection Molds

FIGURE 1.11  Hotrunner mold “dart”, 32 cavities, with electro magnetically operated shut-off

needles

a: Fixed half partially cut; b: fixed half, opened, c: gating detail; d: ejector plate

guiding; e: cavity inserts; f: flat centering; g: mounting bracket; h: hotrunner

manifold; i: electro magnet for the common operation of four shut-off needles;

j: centered hotrunner sprue bushing [15]

The central ejector pin b is actuated at an adjustable position of the opening move-ment or a�er entry of the robot arm. The ejector pins g which are attached in the ejecting plate are pushing the injection molded part off the core and hand it over to the demolding robot. To avoid bending under the influence of cavity pressure, the ejector box i has to be designed sufficiently stable which means to keep relatively small distances between the support bars k and possibly by means of support pins c.

12 1 Molds for Various Processing Methods

FIGURE 1.12  Inner centering in the parting area using the so called flat centering with

lubricant reservoirs [15]

a

b

d

e

f

c

g

h

i

k

FIGURE 1.13  Outer and inner centering of an injection mold

a: Centering ring on the ejector half; b: central ejector pin; c: support bolt;

d: guide bolt; e: centering ring on the fixed half; f: centering cone

(inner centering); g: ejector pins; h: mold fixing device; i: ejector box [15]

Before the mold closes again, the ejector plate with the ejector pins g is brought back to its initial position. Reaching the ejector backwards position is a standard prerequisite for the initiation of the closing movement. Pushback pins which are touching the parting surface outside the cavity ensure to avoid a cavity damage in case of a defect in the pushback mechanism.

131.1 Injection Molds

1.1.5.2  Products with External Undercuts

External undercuts normally have to be demolded using lateral moving elements. Sliders, splits and core pullers can be used for this application.

The term slider is used when the guide area and the wedge of the locking are not identical or parallel but separated. This sort of lateral movement is advantageous for long moving strokes with a relatively small front surface of the moving element (Figure 1.14).

The inclined bolt as a cost effective and simple drive is used when the moving stroke is not extremely long. The inclined bolt is a pin which is anchored to the fixed half opening and closing the slider in the way of a control cam (Figure 1.15 and Figure 1.16). Occasionally, designs can be found where the sliders are arranged between a middle plate and the fixed half and not as usual in the main parting area (Figure 1.15).

Figure 1.17 shows that the bolt f has a 2 to 3° lower inclination angle g than the wedge angle h. This encourages that the last phase of the movement is done by the wedge surface and the bolt is released in final position.

When opening the mold, the bolt f pushes the slider b so far out until the undercut on the molded part is released. A spring- loaded ball catch d is maintaining the slider in open position thus enabling that the bolt later can enter into the drilling of the slider. The molded part remains on the core side until it is ejected from the ejector pins. When closing the mold, the bolt pushes the slider forward until the wedge surface is taking over the last moving phase and the closing itself.

FIGURE 1.14  Ejector half of a mold for vacuum cleaner housing

a: Slider in open position; b: drilling in the slider for the inclined bolt fixed on the

opposite side; c: guide bar for the slider (source: Bosch-Siemens Hausgeräte

GmbH, Giengen)

14 1 Molds for Various Processing Methods

b

c

f

e d

a

FIGURE 1.15  Injection mold for a container with threaded neck

a: Slider; b: inclined bolt; c: wedge surface; d: middle plate; e: latch for the

stroke limited transport of the middle plate; f: stripper plate with stripper ring [1]

a

a

b

FIGURE 1.16  Mold for small parts with external undercuts

a: Ejector half with T-slots and sliders; b: fixed half with only one bolt per slider

[16]

151.1 Injection Molds

g

h

e

f

a

c

b

d

FIGURE 1.17  Elements of the slider design

a: Molded part; b: slider; c: slider stop; d: ball cage; e: wedge surface; f: inclined

bolt; g: inclination angle of the bolt; h: inclination angle of the wedge surface

[15]

The following experienced rules apply for designing slider molds:

� Arrangement of the inclined bolt and the locking wedge on the fixed half, whereas the slider guidance is on the ejector half of the mold.

� Ensure sufficient guidance length in relation to the slider width: minimum factor of 0.5 or even better 0.7.

� Only one bolt per slider if possible. When using two bolts, what is inevitable for wide sliders, the risk of blocking may be larger.

� Bolt inclination angle should be between 15 to 25°. Chose as small as possible to get a high slider closing force.

� To reach an acceptably short bolt length for the necessary slider stroke (depending on the undercut depth) the bolt inclination angle should be rather bigger than smaller. The optimal design will be a reasonable compromise.

� Wedge surface angles should be 2 to 3° higher than the bolt inclination angle. � Inclined bolt has to be sufficiently dimensioned to withstand the opening force. � Slider should be pressed by the wedge to a fixed stop. Counter acting sliders without a fixed stop can, over the course of time, lead to misalignment.

� Guide surfaces of the sliders have to be lubricated. Therefore, they should not lead directly into the cavity but have to be located with offset.

� Cost effective T-slots: Screwed and bolted precision bars.

16 1 Molds for Various Processing Methods

� Slider should be kept in the open position using a reversible latching mechanism (e.g. using ball catch or slider clip).

� Make sure to have an easy disassembly of the slider, for example using a simple to remove slider clip.

� Tempering the sliders directly which means to provide recesses for elastic coolant hoses.

� The locking wedge has to be sufficiently dimensioned to avoid elastic deforma-tion. Sometimes a counter cone on the ejector half is necessary for support. Such centering surfaces should have an angle of approximately 10° to ensure easy handling and avoid self-locking which can happen at an angle of less than 7°.

� Wedge surface should be provided with an exchangeable hardened plate to prevent wear.

� If a mold needs sliders only on one side, either on the right or on the le�, the oppo-site side should be provided with a sufficient counter support to prevent an offset.

FIGURE 1.18  Mold for transport box with external and internal undercuts which are demolded

using splits [1]

171.1 Injection Molds

Molds with “core pullers” are used for very deep lateral undercuts mostly driven by hydraulic means rather than using pneumatic drive. Hydraulic cylinders are also used for bigger splits. In each case a mechanical locking against the separat-ing forces is necessary.

Split molds (Figure 1.18) are clearly different from slider molds because their guiding surfaces and locking surfaces of the movable parts are identical. Sliders will mostly move at right angles to the mold axis, or slightly oblique, for example in an angle of 80 or 100° instead of 90°, but the moving angle of the splits is 12 to 20°. This gives another usage area which refers to undercuts with relatively shallow depth. Because of the high clamping force due to the small angle, split solutions are predestined for molded parts where the undercut takes a large extent (e.g. external ribbing of transport box).

1.1.5.3  Product with Internal Undercuts

Internal undercuts can be found when the inside of the molded part has projections which will hinder the part to get removed from the mold. Internal splits (Figure 1.19), internal sliders which are externally operated and inclined segments (which are used in collapsing cores, Figure 1.19 on the right) can be used.

a

a

bc

d

FIGURE 1.19  Demolding of internal undercuts

a: Flat centering; b: internal splits; c and on the right: collapsible core;

d: internal sliders which are externally operated [17]

18 1 Molds for Various Processing Methods

FIGURE 1.20  Intake manifold, produced in the lost core process

(source: BAYER AG, Leverkusen)

a b c

FIGURE 1.21  Mold for a 20-liter PE packaging bucket with elastically demoldable outside

undercuts

Step I: Situation a�er opening the mold;

Step II: Stroke of the stripper rings 7 and 8,

to get space for the elastic strip-off;

Step III: Stroke of the stripper ring 7 with elastic strip-off;

Step IV: Demolding of the bucket using compressed air.

a: Centering support of the stripper ring and the side wall; b: pneumatic ejector-

plate; c: heated sprue bushing [1]

191.1 Injection Molds

Molded parts with very complex internal contours, for example with multiple internal undercuts, can’t be produced using the conventional injection molding because the needed complex cores can’t be pulled out. If the cores are made from a low-melting metal alloy – preferably by a die- casting process – and are inserted into the mold, they can be melted out of the finished molded part e.g. using electric induction heating or an oil bath.

Typical molded parts for this technology are intake manifolds made from fiber reinforced PA (Figure 1.20). Remarkable is the fact that the processing temperature of the polymer is around 300 °C where the metal alloy of the core (tin-bismuth) has a melt temperature of 140 °C. But the contact surface will only get a contact temperature of 120 °C caused by the high heat conductivity of the metal. However the high manufacturing costs will limit the application field of the process.

Within narrow limits regarding the geometry and the material it is possible to demold undercuts through an elastic strip-over: Moderate elastic deformation and high elongating capability of the plastic e.g. for the production of lids made from LDPE, may save a complex demolding mechanism.

Using such a demolding procedure, it is important to create first space outside before expanding the molded part (Figure 1.21).

First, the molded part remains on the core a�er opening the mold. Then, stripper rings 7 and 8 are operated until enough space is created for the molded part to expand. In the next phase, the stripper ring 8 remains on place to allow the strip-over process. A so called 2-step ejector system, which is offered by many manufacturers of mold standards, is needed

1.1.5.4  Products with Internal Threads

Threaded holes represent internal undercuts that are usually released by the means of an unscrewing mechanism. In general, a rotation movement as well as a corre-sponding axial movement has to be reached. The axial movement can be executed by the threaded core or by the molded part itself.

Therefore, three different concepts result:

A) Rotating and axial movement of the molded part, for example driven by adaptor collets of a take-off magazine.

B) Rotating cores, axial movement of the molded part using a spring-supported base plate with a twist lock for the molded parts.

C) Rotating and axial returning of the cores, whereas the molded parts are secured with a twist lock in its stationary position.

Only a simple electric motor rotating in one direction is needed for the drive of version B. The implementation of an axial movement in rotating movement is rec-

20 1 Molds for Various Processing Methods

ommended for the returning cores of version C for example using a helical spindle (Figure 1.22) or a gear rack (Figure 1.23). A hydraulic drive has the advantage that in closed position of the mold the molding can be unscrewed which means that the outer contour of the molded part can be used as a twist lock, for example by a grooved exterior surface.

a

b

c

FIGURE 1.22  Unscrewing motion, driven by the mold movement

a: Helical spindle; b: pinion; c: transport nut [15]

d

da b c

FIGURE 1.23  Injection mold with hydraulically driven unscrewing cores

a: Transport nut; b:unscrewing core; c: pinion; d: hydraulically operated gear

rack [18]

211.1 Injection Molds

1.1.6  Gate Technology

1.1.6.1  Gate Design

The gate channel in the injection mold serves to convey the melt coming from the nozzle of the injection molding machine to the cavity with the lowest possible pres-sure and heat loss, in shortest possible time and without thermal degradation. In multi-cavity molds, the melt must be supplied to all gates uniformly [1, 2, 7, 12, 19, and 20].

Another aiming dominates the mold group called “family molds”. These are cavities with different geometries and melt flow distances with a common gate system to reach a synchronous volumetric filling (for a cavity with small flow distance a gate branch with a higher pressure loss has to be attached). This requirement of “total balancing” is based on the principle of equal pressure loss in all flow paths to the end of the cavities.

Only when there are equal cavities, the pressure loss in all gate paths is kept equally achieved by a runner balancing. The most successful balancing is that one with equal runner flow paths, the so called “natural balancing”, which o�en leads to signifi-cantly longer gate paths, greater material losses and to larger molds (Figure 1.24). As an alternative, the different cross-sections of the runner (Figure 1.25) can be balanced, but this is only reliable if the calculation was done using sufficient element fineness in the simulation system and secured process data. In narrow gates the risk remains that also in an unbalanced runner (Figure 1.26) the cavities closest to the central feed are filled last, because of melt stagnation in the gate and consequent freeze-off, in extreme cases.

FIGURE 1.24  Principle of “natural balancing” with equal flow paths

FIGURE 1.25  Balancing of a serial configuration

via the flow cross-sections

FIGURE 1.26  Serial configuration

without balancing

22 1 Molds for Various Processing Methods

a

b

FIGURE 1.27  Double Y manifold of a hotrunner mold

a: Deflection angle of 315°; b: deflection angle of 45° [21]

The distribution shown on Figure 1.27 is reached with relatively low effort: by drill-ings in only one level, causing equal flow paths to the gates but different deflection angles. For high precision requirements, the same deflection angles are created by pre- distributing in a second level.

Using constant boundary conditions, the law for balancing is pretty simple: In a branching, the melt flows in channels with a velocity proportional to the height h of the channel. This results in different flow path lengths f1, f2 at the same time,

�1 1

2 2

f hf h

f = flow path lengthh = height of the flow channel, wall thickness

This still applies when taking the law of “intrinsic viscosity” (Non-Newtonian behavior of the melt, viscosity dependent on the shear rate) into account. Not included, however are the cooling effects, forming of solidified layers in the cooled mold and heat dissipation during the flow process. These influences can only be sufficiently considered in a simulation system by newly determining the conditions from element to element.

The position of the gate on the molded part is largely determining the formation of weld lines (meeting of melt fronts) and air traps. The proceeding of the melt in thicker cross-sections is giving big difficulties when the air trap is not detected until the mold is finished (Figure 1.28).

It is one of the standard services of mold making to optimize the position of the gate already in the design phase using simulation technology. This is done to avoid air traps and to bring weld lines to acceptable areas. The strength of a weld line is also dependent on the process parameters: In the worst case it is 20%, in the best

231.1 Injection Molds

case it is 80% of the strength without flow lines. It is always a week point with the consequence that it shouldn’t be positioned in highly stressed areas.

The position of the gate as well as the type of gate is very important for the mold design and the economic efficiency of the injection molding production. An overview is shown in Figure 1.29.

1.1.6.2  Solidifying Runner, Remaining at the Molded Part

If the injection molding compound solidifies in the feed channel, the “sprue” has to be demolded, either in combination with the molded part or already separated from the molded part, which can be done using a mechanism inside the mold. All of the solidified runners need a correspondingly higher injection volume on the machine side.

FIGURE 1.28  Air trap due to accelerated flow of the melt in the thicker frame.

a: Position of the gate; b: air trap [11]

Combinations

In-Mold Separated

Tunnel-Anguss3-Platten-Anguss

Film-Anguss

Tunnel 3-Plate Mold3-Platen gate

Film gateHeißkanalHotrunner

InsulatedRunner

Ante-ChamberRemaining at theMolded Part

Solidifying Runners(cooling-down with molded part)

Sprue Film Gate

Diaphragm GateRing Gate

Sprue gate

Pass-Through Runners

FIGURE 1.29  Overview of different runner types

24 1 Molds for Various Processing Methods

FIGURE 1.30  Sprue system

a: Sprue bushing; b: centering ring on the fixed half; c: taper angle;

d: smallest diameter

The melting and cooling of the runner volume worsens the energy balance. Even when the runner is recycled, there are additional costs and certain reductions in the property values. If the returning volume portion stays small, the continuous return leads to a constant property level.

Of all gates, the sprue gate (Figure 1.30) offers the least resistance to the incoming melt and permits the holding pressure to act a long time on the molded product. This has advantages but also involves risks: Advantageous is that the injection molding process only needs a relatively low pressure with a very long lasting mass post-supply. Nevertheless there is a risk of strong orientations and overloading in the gate areas. If the holding pressure time is set too short, it could easily lead to uncontrollable mass back flow. Therefore it is recommended to use a profiled holding pressure and to set the holding pressure to “declining ramp” [11].

A slim conical shape with a taper angle (Figure 1.30, position c) minimum 2 to 4° is necessary to facilitate the extraction of the sprue out of bushing a. The minimum diameter d is around 1 to 2 mm. A too big cross- section of the sprue can negatively influence the cooling time. If the sprue bushing (as shown) is being held by the centering ring b on the fixed half, the bushing can be removed out of the machine without disassembling the mold. However the mounting screws of the centering ring have to be strong enough to withstand the mold pressure of 1000 bar (a value which can be used for other load cases in the cavity as well).

The extension of the runner to ring-shaped channels or rectangular cross-sections leads to the gate types seen in Figure 1.31 to Figure 1.33.

Using the diaphragm gate (Figure 1.31), round products with high rotation preci-sion can be produced. A post-machining process is needed to separate the runner.

251.1 Injection Molds

FIGURE 1.31  Diaphragm gate

FIGURE 1.33  Film gate

The ring gate (Figure 1.32) is applied for injecting tubular products, in multi-cavity molds. Advantageous is the possibility to support relatively slim cores on the gate half to avoid displacement or bending. Experience has shown that the entering melt always tends to enlarge a small possibly existing displacement and to bend the cores.

Using a film gate (Figure 1.33) can help the melt to spread out before entering the cavity and to achieve more parallel flow paths. The tendency of equal distributing is supported by a runner design with considerably larger cross- section in com-parison to the relatively narrow gate (e.g. 0.5 to 2 mm high). The film gate can be used whenever flat products with minimal warpage are to be molded. Nevertheless the arrangement illustrated in Figure 1.33, specifically for long parts, leads to an unbalanced load of the clamping unit. It is recommended to have a two cavity mold or to install a detour hotrunner.

A lateral pinpoint gate without separation (Figure 1.34) is used when the molded parts should stay attached to the runner for better handling purposes. In general, solidifying runners should be designed to provide a relatively large cross-section as far as possible before ending as short and tight gate channel.

FIGURE 1.32  Ring gate

26 1 Molds for Various Processing Methods

s

FIGURE 1.34  Pinpoint gate without separation from the molded part, s = 0.5 to 2.0 mm

FIGURE 1.35  Circle cross- section as rheologically favorable channel geometry and

approached circle cross- section (deepened half- round)

Small pressure losses and relatively long holding pressure effects occur when the manifold is designed somewhat thicker than the wall thickness of the molded part and when the cross-section is either exactly circular or almost circular (Figure 1.35).

1.1.6.3  Automatically Separated Runner

Shortly before the cavity the runner of the tunnel gate leads into an angled drilling entering tunnel- like the cavity (Figure 1.36). Therefore, prerequisite for the applica-tion is the existence of a side or facing surface, which will accept the gate. A lateral injection will be necessary if the part has not a convenient aperture.

The separation of the tunnel gate runner shown in Figure 1.36 occurs immediately when opening the mold. The molded part f remains on the core of the ejector half and the cutting edge e remains on the fixed half. The cutting edge shouldn’t be round or dull to guarantee a clean separation. To remove the runner d out of the sprue channel, undercuts c in form of a restraint cone are used. Later these cones will be pushed out by the movement of the ejectors a and b. The tunnel itself repre-

271.1 Injection Molds

sents also an undercut which keeps the gate initially on the nozzle side. Therefore, bending of the runner between these undercuts is involved, but it must not break off, otherwise the runner is blocked for the next shot. If the plastic material is brittle, a sufficient bending length must be provided and it has to be demolded in time when still elastic.

If the molded part provides convenient side walls in the ejector half of the mold the tunnel hole can be positioned in the ejector half (Figure 1.37). In this case the separation of the gate occurs during the ejection of part and runner.

FIGURE 1.37  Tunnel gate positioned in the ejector half of the

mold

a: Cutting edge [22]

a

b

c

d

e f

g

FIGURE 1.36  Tunnel gate at the end of a sub-distributor, fed by a hotrunner nozzle

a, b: Runner ejector; c: restraining cone; d: runner; e: cutting edge;

f: molded part; g: hotrunner nozzle [15]

28 1 Molds for Various Processing Methods

FIGURE 1.38  Curved tunnel gate [22]

With a curved tunnel gate (also called submarine gate or cashew gate, Figure 1.38) it is possible to position the gate on the back of the molded part where the appear-ance is as little affected as by the markings of the ejector pins.

The curved tunnel is a steadily narrowing channel, which can be manufactured using divided inserts or using the laser sintering process. This tunnel is especially subjected to extreme bending stresses. When optimizing, it has to be ensured that no bending stress peak will occur.

Three-plate molds are used, when the product must be gated centrally (Figure 1.39) and an injection via hotrunner is not possible.

For demolding the multi-gate runner, an additional parting surface is needed. The mold now consists not only of “fixed plate” and “ejector plate” but has an additional “middle plate” which is moved by latches and stopped by anchor bolts. The parting surface of the gate opens first.

Using restraining undercuts helps keeping the runner on the fixed half which leads to tearing off the pinpoint gate. Similar to the tunnel, the runner has to be demolded out of its undercuts using elastic deformation.

The additionally needed opening width used for runner demolding can be dis-advantageous. The combination with a “heated sprue” channel decreases in the bulkiness of the runner is therefore not rare. Three- plate mold- gates are used for applications with very small gate distances not achievable by hotrunner nozzles because of their space requirements. A typical example is day light of 7 mm for the individual gating of narrow chambers which are positioned next to each other in molded parts for medical use.

291.1 Injection Molds

a

b

c

d

e

FIGURE 1.39  Three-plate mold with a tear- off pinpoint gate

a: Runner and first opening parting surfac; b: runner-restraint pin;

c: stripper plate for the runner; d: stopp bolt; latch unlocking unit [1]

1.1.6.4  Pass Through Channels

Hotrunners are basically an extension of the machine nozzle into the mold [23]. This definition already explains that they are equipped with an active heating system and with a well-functioning insulation to maintain the flowing capacity of the compound.

Using only insulation to get a pass through runner is surely a fascinating cost cutting method but limited in application: only a short cycle time is allowed and only plastic materials not solidifying too fast. Such alternative solutions deserve consideration for specific applications mainly for cost reasons. A continuous pro-duction should be granted because frequent interruptions and color changes will restrict the applicability considerably.

For single-cavity molds, the machine nozzle can be extended using a heat conduc-tion tip, for example made from copper-beryllium, to conduct the heat directly to the gate (Figures 1.40 and 1.41). In addition, the injection molding compound is used as an insulator because it fills the annular gap around the heat conductive tip.

The ante-chamber principle is used at the nozzle end of most hotrunners. However, it must be remembered that it provides stagnant material which goes through high thermal load in contact with the heat conductive tip and it is not exchanged.

30 1 Molds for Various Processing Methods

When molding thermally sensitive material, it is necessary to replace the volume of the annular gap as far as possible with a “cap” made of thermally stable plastic material (e.g. PEI or PEEK, Figure 1.42). The volume of the ante-chamber itself should be kept as small as possible.

The insulated bushing around the ante-chamber should have its own temperature control system to properly control the heat management in the area around the gate (Figure 1.43).

a

c

b

FIGURE 1.40  Heat conductive tip with multi-hole

nozzle and relatively big ante-chamber

a: Nozzle bore; b: ante-chamber; c: gate [24]

FIGURE 1.41  Minimized ante-

chamber with multi-

hole nozzle tip [24]

a

b

FIGURE 1.42  Ante-chamber with insulation cap

a: Multi-hole nozzle;

b: insulation cap [24]

FIGURE 1.43  Ante-chamber bushing with

integrated temperature

control system [25]

311.1 Injection Molds

a b

FIGURE 1.44  Insulation runner

1: Ejector half; 2: middle plate;

3: fixed half;

a: Auxiliary parting surface;

b: “fluid core” of the manifold

(source: BASF AG, Ludwigshafen)

In some cases, for example fast shot sequence and a wide melting temperature range of the injection molding compound, no metallic thermal conduction is needed. If the cross- section of the “insulation channel” is big enough, 12 to 25 mm diameter, a “fluid center” allows passage over a certain time (Figure 1.44). The insulation channel solidifies in the case of interruption. An auxiliary parting surface is opened to demold the solidified insulation channel.

1.1.6.5  Hotrunner Molds

With real hotrunner molds, the continuous heating compensates for heat losses caused by conduction, convection and radiation Therefore insulation plays a crucial role. The heat capacity is usually dimensioned so that the heat-up time is accept-able which means between 15 to 25 minutes. When analyzing the permanent heat losses, an energy consumption of less than 60% of the installed power can mostly be seen [1 to 7, 11, 19, 20, and 23].In general there are two different ways of heating when using hotrunner molds: internal or external heating.In an internally heated hotrunner system (Figure 1.45), the heating elements are positioned in the center of the runner channel: heating rods or so called “torpedos” with cartridge heaters inside. The melt-conveying annular gap contains, up to about the middle between both boundaries, a layer of solidified molding compound. This layer is responsible for sealing the flowing melt and for self-insulation with relatively little effort. But it is difficult to guarantee a safe mass exchange when redirecting the melt from annular gap to annular gap. In such cases, the color change could become long lasting. To achieve an improvement in the color change process, the temperature of the heating rods is raised by about 20 K temporarily, to develop new material lining in the solidifying zone.The difficulties with color change restrict the field of application of internally heated systems. The following combination can be found more o�en: Externally heated manifold and internal heating in the nozzle area, equivalent to the design of the gate elements as a torpedo or as a thermal conductive tip.

32 1 Molds for Various Processing Methods

FIGURE 1.45  Internally heated hotrunner

a: Heating rod with cartridge heater; b: melt channel; c: torpedo with cartridge

heater [18]

External heating is based on a circular flow cross-section in the center of a heated tube or block. The heating elements should care for maintaining the temperature of the melt which means that the insulation to the outside should get particular attention. In general, the external heating with its high efforts for insulating is more complex. Sealing is difficult as well because the melt doesn’t have any self- sealing at a gap formed between hot walls. In case of mass leakage into insulating air gaps troubles will arise because of a 10 time higher heat transfer.

The main advantage of externally heated hotrunners (Figure 1.46) is a better defined and better controllable mass flow, what can be proved by a color change test.

FIGURE 1.46  Externally heated hotrunner in the standard design with floating manifold

a: Manifold; b: support disk; c: tubular heating elements; d: centering pin;

e: heating coil of the nozzle [24]

331.1 Injection Molds

FIGURE 1.47  Hotrunner with disk springs as nozzle support

a: Disk springs; b: multi-hole nozzle tip [24]

The manifold block with the melt channels is heated by tubular heating elements c. The nozzles, directly heated, directly centered in the gate bushings are connected with the manifold only by pressing (“floating manifold”). The dowels d provide for the centering and the anti-twist protection of the manifold block. A central support b cares for sufficient resistance against the force of the machine nozzle using a mate-rial with low thermal conductivity, for example high-alloyed steel or a titanium alloy. Similar supports on the opposite side of the nozzle provide the clamping effect for the manifold using “pretension”. A value of 0.03 mm for the “oversize” in heated state has proven to be practical. This could mean that there is no oversize in cold state and therefore, no sufficient sealing effect. When using spring-loaded support elements, a larger oversize can be used and even in cold state a certain pretension is given (Figure 1.47).

Besides the “floating” manifold, the so called “drop-in systems” are used more and more. The nozzles are screwed into the manifold providing a precise fit and a perfect sealing. As a second centering exists in the gate area the nozzle will slightly bend under the influence of the unavoidable thermal expansion of the manifold. Therefore, the nozzle should have a sufficient bending length. The advantage of this solution is to deliver the manifold with the nozzle completely wired and piped as a complete system to the moldmaker (Figure 1.48). Another alternative are the so called “hot halves”. Only the cavity plate and inserts have to be added to get a complete fixed mold half.

In comparison to the surrounding components, the manifold is insulated using an all-around air gap. Experience shows that the gap width should be between 8 and 10 mm because this is the optimum concerning the minimum amount of heat loss. Shiny surfaces can minimize the radiation losses of the hot manifold. A nickel coating is recommended to reduce radiation. The amount of radiation heat loss is less than 10%. Measures reducing contact heat promise greater effectiveness.

34 1 Molds for Various Processing Methods

Their percentage is about 80% whereas convection losses cause about 12%: A pro-tection cover has to be included to avoid air dra� at the manifold.

1.1.6.6  Hotrunner Nozzles

To keep the shear stress of the material as small as possible the gate should be designed as big as possible in cross-section. Even a small enlargement of the cross-section results in a considerable improvement because the shear stress is inversely proportional to the power of 3 of the diameter. If the tip of a “multi-hole nozzle” reaches into the gate opening, the marking of the gate will be tolerably small. This solution is very o�en chosen (Figure 1.41). A nozzle, seen in Figure 1.49, allows a lateral hotrunner injection in a multiple mold.

The following options will help choosing the manifold and nozzle type. Mostly used designs are marked by X.

FIGURE 1.49  Hotrunner nozzle for lateral injection

FIGURE 1.48  Hotrunner of “drop-in” type completely wired and piped, nozzles screwed into

the manifold [26]

351.1 Injection Molds

TABLE 1.1  Design variations of hotrunners

Type of runner heating internalexternal X

Type of nozzle heating directindirectinternalexternal

X

XNozzle centering indirect by the manifold

direct by the cavitycentering by cavity and manifold

XX

Gating open orificemulti-hole tipneedle valve with conical tipneedle valve with cylindrical tip

XXX

Needle shut-off nozzles (Figure 1.50) offers clean, smooth gates with relatively large cross-sections and low pressure losses during injection.

FIGURE 1.50  Needle shut-off nozzle with

lateral annular piston [26]

36 1 Molds for Various Processing Methods

FIGURE 1.51  Hotrunner shut-off needle operated by a servomotor [28]

Pneumatic as well as hydraulic drives are used for the movement of the needle. Recently, electromagnets and servomotors are used for this application as well (Figure 1.51).

Besides the advantages of the appearance of the gate, it is of increasing significance that the needle shut-off nozzles can be individually operated. Thus cavities can be equipped with multiple gates and even no weld lines are appearing between the gates because of sequential opening and closing of the nozzles (“cascade filling”). Also “family molds”, with different cavities connected to the same manifold, can be filled more uniformly, using this method. Needle shut-offs with a variable outlet cross-section can create and control (Figure 1.52) an individual melt pressure profile in every cavity.

For very low cavity distances multi-nozzles called “cluster nozzles”(Figure 1.53) are favorable. At the same time, however, it should be noted that the branching of the mass flow should happen in a well heated area and not where the melt is already cooling.

FIGURE 1.52  Melt pressure control system (“Dynamic Feed”) using a conical needle with

position control

C: Controller; H: hydraulic unit; p: actual value of the melt pressure;

Ref: nominal value of the melt pressure [29]

371.1 Injection Molds

FIGURE 1.53  Multi- needle shut- off nozzles with grouped needle drive but individual nozzle

heating [21]

FIGURE 1.54  Nozzles with lateral heater cartridges, minimal cavity distance (for 64 cavities)

for molding “outserts” with fiber reinforced PPS [27]

Heater cartridges are advantageous because of their high heating power per surface which also helps to implement small cavity spaces (Figure 1.54).

1.1.7  Venting of the Cavity

The melt flowing into the cavity, has to displace the present air. With short injection times, especially for thin-walled molded parts (e.g. 0,1 second in extreme cases), an air trap can be formed. This will influence the filling process severely: increase in temperature by air compression and consequently degradation of the plastic, “diesel effect”. Sufficient air escape can be achieved by deep grinding marks on the mold parting line that shouldn’t be too smooth or contaminated, by mold inserts, splits or sliders as well as ejector pins. These regular air gaps are sometimes not sufficient so that additional venting systems are needed.

38 1 Molds for Various Processing Methods

a

FIGURE 1.55  Venting groove for high injection speed, reduction of air resistance by a short

length of the gap a

Vent channels are ground into the mold parting line as shallow channels leading away from the cavity with dimensions of 0.01 to 0.04 mm in depth (Figure 1.55).

Such gaps are small enough to prevent escape of the melt because of a skin (0.01 to 0.02 mm thick) which forms at the flow front of the thermoplastic melt (Figure 1.5, Chapter 1.1.2).

Systems which allow an evacuation of the cavity before injection, are more and more used when serious venting problems occur, thus stabilizing the process. For this purpose the cavity has to be sufficiently sealed (Figure 1.56).

FIGURE 1.56  Sealing of the cavity for applying vacuum before injecting

On the le�: Ejector half with peripheral sealing ring

On the right: Fixed half with vacuum connection above the cavity [15]

391.1 Injection Molds

1.1.8  Temperature Control

The cooling channel system in the mold is of high importance to get a uniform and rapid cooling of the melt. The crystallinity is also strongly influenced when using semi-crystalline plastics. Also the flow resistance caused by the solidifying layers, is dependent on the wall temperature of the mold [1 to 7, 11, and 12].

All cavity inserts as well as the sliders and splits have to be cooled “directly”, which means touched and wetted by the cooling medium (Figure 1.57). This will allow a sufficient heat flow. Each insert gap, even with press fit, would drastically reduce the heat transfer. Location of the cooling channels only in the frame plates (indirect temperature control) belongs to the cheap molds category.

The cycle time is normally determined by the required cooling time. Especially when using fast running injection molded parts, it is important to intensify the heat flow through the cavity walls.

The following measures can be considered:

� High heat transfer coefficient, which means a turbulent flow of a low-viscosity medium. If optional water should be preferred over oil what will make the heat transfer 8 to 10 times better by lower viscosity.

� Mold material with higher thermal conductivity, � High temperature differences, possible use of a supercooled medium, � Keeping the cooling channels deposit free. Anti-adhesive internal coating using nickel has proven its worth [30],

� Short heat conduction path, which means smaller distances between the cooling channels and the cavity.

Bigger temperature differences at the cavity wall, mostly called “temperature vari-ance”, have an adverse effect on the injection molded parts. To keep the temperature variance as low as possible, larger distances from the cooling channels to the cavity wall are necessary. Finally the designer has to find an acceptable compromise.

FIGURE 1.57  Indirectly (on the le�) and directly (on the right) tempered cavity inserts [11]

40 1 Molds for Various Processing Methods

Average temperature, part

= 120.0[C]

[C]

120.0

112,5

105,0

97,50

90,00

FIGURE 1.58  Typical temperature distribution in the molded part with the involvement of

cooling channels [31]

Local temperature differences of the melt have to be considered in designing and dimensioning of the channels: The melt which last entered the cavity is located in the gate area. Therefore, there are higher temperatures (up to 30 K) than at the end of the flow path (Figure 1.58).

Mold filling simulations without the inclusion of cooling channels are incomplete. The designer has to be informed about the “hot spots” which can make the cycle longer and can lead to extraordinary warpage effects.

As a general rule it is recommended to let the cooling water flow in the same direc-tion as the mass which flows into the cavity. The entering cooling water has to be brought first close to the gate area. Multiple cooling channels can be connected par-allely or sequentially. The parallel connection gives small temperature differences, but will need a closed circulating cooling system with its small risk of depositions. Whereas the sequential connection offers better flow safety due to the forced flow principle. As a compromise it is important to keep the number of sequentially con-nected channels as small as possible and to have the parallel connections outside the mold to be able to attach monitoring devices.

Especially in narrow cross- sections it is important to find solutions which will allow a sufficient heat flow. Channels with a cross-section under 30 mm² reach the limit of the available media pressure fast (normally around 10 bar) and require o�en a pressure- increasing system.

Evaporation and condensation offer best heat transfer conditions and are used in the so called “heat pipes”. These are closed pipes filled with an evaporable medium and a wick, minimum diameter 3 mm, which are inserted into a mold drilling of perfect surface. A further improvement of heat transfer can be realized using directly media filled drillings (Figure 1.59).

411.1 Injection Molds

FIGURE 1.59  Core insert, directly filled with a wick and evaporation media, schematically

a: Evaporation area; b: condensation area

A perfect reproduction of mold surfaces is highly dependent on the mold tempera-ture. For example, high gloss levels of optical surfaces can only be obtained having high mold temperatures. This is also true for the reproduction of micro-structures. To still obtain an acceptable cycle time, the mold is o�en run using a profiled tem-perature instead of a constant one: heat-up phase before injecting, cooling phase a�er injecting. Special measures are required to keep the heat capacity and heat-up time of the heated region as low as possible when using such a “variothermal” mold. Special solutions using local inductive heating or temporarily moved-in radiators [32] are also known (see chapter 2.4).

1.1.9  Special Designs

1.1.9.1  Stack Molds

For the production of flat, thin-walled injection molded parts, the required size of the injection molding machine is determined by its clamping force. In contrast, the injection capacity and the opening stroke of the clamping unit are only utilized to an unsatisfactory extend. If the mold cavities are placed behind one another in two parting lines of an injection mold (stack arrangement, Figure 1.60 and Figure 1.61), the number of molded parts per shot can be doubled. To obtain a synchronous opening in both parting lines, rack and pinion (Figure 1.60) or lever mechanisms (Figure 1.61) are used [1 to 6, 20, and 32].

It goes without saying that these molds need an ejector mechanism on both end plates, mostly hydraulically or pneumatically driven.

The hotrunner system is located in the middle plate of the stack mold. It is centrally fed by a heated runner pipe which is also called “snorkel”.

Stack molds are built up to 4 stacks. In this case though, a customized machine has to be provided (Figure 1.63).

Also unequal molded parts can be produced in a stack mold for better utilization of injection and plasticizing capacity. The option “tandem mold” (Figure 1.64) is in this case preferred. The both parting lines of the cavities are sequentially locked and filled.

42 1 Molds for Various Processing Methods

This means that while the cavities of one parting line are in cooling phase, the parts of the other parting line are injected and packaged.

FIGURE 1.60  Stack mold with rack and pinion mechanism for synchronous opening in both

parting lines [33]

FIGURE 1.61  Stack mold with lever mechanism for synchronous opening in both parting lines [24]

431.1 Injection Molds

FIGURE 1.62  Middle plate of a stack mold

a: Centrally located “snorkel”

[24]

FIGURE 1.64  Tandem mold with different molded parts in the parting lines [35]

1.1.9.2  Injection-Compression Molds

The difference between injection-compression and conventional injection molding is that the cavities are not completely closed before or during injection by “breath-ing”. The complete closing is only initiated when the mold is totally or almost filled.That means that an additional volume and a larger flow channel is provided. Therefore, a low filling resistance can be obtained as well as a favorable holding pressure method using the clamping unit as a packaging piston. This results in a more uniform pressure distribution in the cavity, a lower clamping force require-ment, lower residual stresses and fewer orientations. Molded parts, where these properties are of primary importance like for CDs or DVDs as well as high-quality optical parts are almost exclusively produced using the injection- compression molding process [2, 3, 6, 20, and 32].

The injection-compression mold has lateral sealings of plunger type (Figure 1.65) to ensure that the cavity is tightly closed during injection.

FIGURE 1.63 4-Stacks-Injection mold

[34]

44 1 Molds for Various Processing Methods

FIGURE 1.65  Injection-compression mold, schematically

On the le�: Cavity with additional volume due to incomplete clamping or mold

breathing; on the right: reclosed mold a�er the compression stroke;

a: Lateral sealing, plunger type [11]

1.1.9.3  Multi-Component Technology

In the field of multi-component technology, considerable progress has been made over the past decade [36 to 41] which can be seen in the variety of available methods and mold designs (Figure 1.66).

A traditional molded part of that type are the rear lights of a motor vehicle, which are manufactured using horizontally moving rotary molds which have 3 to 4 com-ponents (Figure 1.67).

Overmolded so� components ensure an improvement in haptik (Figure 1.68) or an assembly effort reduction for complicated sealing.

In the sandwich method, the skin component is injected first which forms the solidifying layer in accordance to the “fountain flow” principle, followed by the core component in the “fluid core”, simultaneously displacing the skin component.

Robot Transfer

-Core-Back Process

TurningSliding Mold

Transfer Process

Sandwich -VerfahrenSandwich ProcessOvermolding

Co-Injection

Multi Component Process

Turning Mold Index Plate Spider

FIGURE 1.66  Overview of multi-component process varieties [11]

451.1 Injection Molds

The changeover valve is normally a part of the machinery. Also, modified hotrun-ner needle shut off nozzles are used to coaxially operate the 2nd component or the filling media (Figure 1.69).

Good adhesion compatibility is important in most multi-component technologies. This is obtained due to close chemical similarity. Non-adhesion is needed for movable doll limbs or venting grilles for car interiors.

In the simplest case multi-component parts can be produced through inserting of the preforms into an expanded cavity of a second injection mold. This method of working belongs to the transfer molding process (Figure 1.66). A just injected and not already cooled surface of the preforms creates a better adhesion of the compo-nents. Thus an automated transfer using a robot to transfer the preforms can give additional property benefits.

FIGURE 1.67  Mold with vertical rotary axis for the production of motor vehicle rear lights [36]

FIGURE 1.68  Multi-component injection mold with spider carrier of the parts

a: Protruding spider, ready for the removal of the molded parts using a robot [37]

46 1 Molds for Various Processing Methods

FIGURE 1.69  Hollow needle shut off nozzle

On the le�: Opening position for injection of the skin component

On the right: Closing of both components [27]

a

b c

FIGURE 1.70  Turntable mold with a rotation angle of 120° for removal of the molded parts

during the injection molding cycle in the 3rd station

a: Molding of the preforms; b: injecting the 2nd component; c and on the right:

removal station [37]

Mostly the transfer in the injection mold takes place through rotary motion. To obtain this the entire ejector half can be rotated to transport the preforms into the expanded cavity (Figure 1.70) or, to save moving mass, just a part of the ejector half. If just the front plate of the ejector half is rotating, it is called “index plate”. A “spider” (Figure 1.68) or revolving carriers (Figure 1.71) are used when the preforms can be moved while sitting on cores.

An additional functional station, for example for inserting and assembly processes is also gained by the “cube technology”: The 4-sided middle plate of the mold is turning by a vertical rotary axis.

Some automated assembly processes can be integrated into the injection molding process. This process is called “In-Mold-Assembly” and can be realized with double cubes (Figure 1.73) or with a swivel carrier for the molded parts (Figure 1.74).

471.1 Injection Molds

FIGURE 1.71  Two-component mold for toothbrushes with preform carriers in chain design

and two additional functional stations [38]

FIGURE 1.72  Turning cube technology [39, 40]

FIGURE 1.73  Twin cube mold using the closing movement of the cubes for an assembly

phase, here in “ready to inject” position [40]

48 1 Molds for Various Processing Methods

FIGURE 1.74  In-Mold-Assembly using a rotatable carrier for the molded parts [38]

FIGURE 1.75  Hotrunner system for a two component injection mold [41]

Multi-component molds are mostly, especially with high cavity numbers, equipped with a hotrunner system which must provide a perfect heat separation (Figure 1.75).

1.1.9.4  Outsert Technology

In the outsert technology, functional components out of thermoplastic polymers are injected into pre-punched holes of a metal blank. Therefore, the blank is inserted and fixed between the two halves of the injection mold. Sealing elements made from highly heat-resistant plastics (Figure 1.76) for example PEEK can help to compensate for the considerable manufacturing tolerances of metal parts [1, 3, 15, and 32].

491.1 Injection Molds

FIGURE 1.76  Sealing of a metal insert with highly heat-resistant plastic [15]

1.1.9.5  Molds for Thermosets and Elastomers

Within the injection molding cylinder thermoset plastics are heated-up to a tem-perature between 90 and 110 °C, just enough that they are capable to flow and to be injected into the mold. Additional supply of energy in the heated mold, mostly between 150 and 180 °C, cares for a fast “curing”. Therefore, the molds are nor-mally electrically heated. In contact with the hot mold walls the melt gets very low viscous which means that it can enter into rather small gaps, even into venting, ejector clearance etc. Formation of flashes at the molded part is therefore mostly not preventable. The mold wear is mostly higher than in thermoplastic molds due to the abrasive fillers in the thermoset molding compounds [1 to 3].

Analogous to the hotrunner gate of thermoplastic materials, “cold runner systems” are used for thermosets and elastomers. These systems help to prevent runner losses.

Figure 1.77 shows an injection mold for a thermoset housing. In this case, the sprue bushing 19 is tempered so that the gate can’t cure and the “cold runner” stays injectable. The air gap between the sprue bushing and its surrounding plates 1, 2 maintains thermal insulation; the heating cartridges 24, 25 provide the necessary mold temperature of 180 °C. The thermal insulation plates 10, 11 obstruct the thermal flow to the clamping plates of the injection molding machine.

Injection-compression molds are more and more used for processing thermosets due to the reduction of filler orientation with expanded flow channel (Figure 1.65).

Like thermosets the elastomers will only cross-link in the mold by introducing addi-tional heat. But Thermoplastic Elastomers (TPE) can be processed like thermoplastic materials due to their thermoplastic matrix. The common cross-linked elastomers are highly viscous in the injection unit, with exception of the silicone materials which are processed as liquids (LSR = Liquid Silicone Rubber) in the injection molding process (chapter 2.3 cold runner) using a special injection unit. All elastomers tend to the formation of flashes at the molded parts due to the decreasing viscosity in the

50 1 Molds for Various Processing Methods

heated mold. Therefore, air gaps and clearances should not be larger than 0.01 mm. Vacuum vented molds are in this case of high success.

Elastomer molded parts are too easy to bend which means that the common ejector units for elastomers are only applicable to a limited degree. Adjusted devices are then o�en needed to demold and strip off the molded parts. A more o�en used example for such a device to demold undercuts is the elastic expansion of the molded parts by compressed air.

References

[1] Gastrow, H.: Der Spritzgießwerkzeugbau in 130 Beispielen (P. Unger, Ed.), 5. ed. (1998) Carl Hanser Verlag, Munich

[2] Menges, G., Michaeli, W., Mohren, P.: Anleitung zum Bau von Spritzgießwerkzeugen, 6. ed. (2007) Carl Hanser Verlag, Munich

[3] Johannaber, F., Michaeli, W.: Handbuch Spritzgießen (2001) Carl Hanser Verlag, Munich

FIGURE 1.77  Thermoset mold with cold runner sprue bushing [15]

511.1 Injection Molds

[4] Osswald, T., Turng, T., Gramann, P.: Injection Molding Handbook (2001) Carl Hanser Verlag, Munich

[5] Jaroschek, C.: Spritzgießen für Praktiker (2003) Carl Hanser Verlag, Munich[6] Stitz, S., Keller, W.: Spritzgießtechnik (2004) Carl Hanser Verlag, Munich[7] Beitl, F.: 1000 Tipps zum Spritzgießen, Vol. 2 Spritzgießwerkzeuge (2005) Hüthig

Verlag, Heidelberg[8] N. N.: Company Publication MHT AG, Mold & Hotrunner, Hochheim[9] N. N.: Company Publication ENGEL Austria GmbH, Schwertberg, Austria[10] N. N.: Company Publication Netstal Maschinen AG, Näfels, Switzerland[11] Wippenbeck, P.: Seminar handbook injection molding, Steinbeis-Transferzentrum

Kunststofftechnik (2004) Aalen[12] Wippenbeck, P.: Seminar handbook key technology injection molds, Seminars Kunst-

stoffe (2005) Mannheim[13] Starke, L., Meyer, B.: Toleranzen, Passungen und Oberflächengüte in der Kunststoff-

technik (2004) Carl Hanser Verlag, Munich[14] Kiraz, B.: Computer program TolPro, Steinbeis-Transferzentrum Kunststofftechnik

(2004) Aalen[15] N. N.: Company Publication, HASCO-Hasenclever GmbH, Lüdenscheid[16] N. N.: Company Publication BUCHTER Formenbau AG, Hallau, Switzerland[17] N. N.: Company Publication STRACK NORMA GmbH, Lüdenscheid[18] N. N.: Company Publication DME Normalien GmbH, Lüdenscheid[19] Beaumont, J.: Runner and Gating Design Handbook, 2. ed. (2008) Carl Hanser

Verlag, Munich[20] Rees, H., Catoen, B.: Selecting Injection Molds (2005) Carl Hanser Verlag, Munich[21] N. N.: Company Publication Männer Solutions for Plastics GmbH, Bahlingen[22] N. N.: Company Publication Spritzgießwerkzeugbau BAYER AG, Leverkusen[23] Unger, P.: Heißkanaltechnik (2004) Carl Hanser Verlag, Munich[24] N. N.: Company Publication HUSKY Spritzgieß-Systeme GmbH, Augsburg[25] N. N.: Company Publication Mold-Masters Europa GmbH, Baden-Baden[26] N. N.: Company Publication EWIKON Heißkanalsysteme GmbH, Frankenberg[27] N. N.: Company Publication GÜNTHER Heißkanaltechnik GmbH, Frankenberg[28] N. N.: Company Publication XINTECH Systems AG, Dübendorf, Switzerland[29] N. N.: F Company Publication SYNVENTIVE Molding Solutions GmbH, Bensheim[30] N. N.: Company Publication NovoPlan GmbH, Aalen[31] Kaiser, H.: Seminar manual simulation technology, lecture manuscripts Rheologie,

Hochschule für Technik und Wirtschaft (2005) Aalen[32] Wippenbeck, P.: Tradeshow reports injection molds, Kunststoffe (2004) 12, p. 32 and

Kunststoffe (2006) 1, p. 82[33] N. N.: Company Publication YUDO Germany GmbH, Kierspe[34] N. N.: Company Publication, Stackteck, Brampton, ON, Canada[35] N. N.: Company Publication T/Mould GmbH, Bad Salzuflen[36] N. N.: Company Publication Schefenacker GmbH, Schwaikheim[37] N. N.: Company Publication Wilhelm Weber Formenbau GmbH, Esslingen[38] N. N.: Company Publication Zahoransky Formenbau GmbH, Freiburg[39] N. N.: Company Publication Ferromatik Milacron Maschinenbau GmbH,

Malterdingen[40] N. N.: Company Publication FOBOHA GmbH Formenbau, Haslach[41] N. N.: Company Publication PSG Plastic Service GmbH, Mannheim

52 1 Molds for Various Processing Methods

 ■ 1.2  Compression and Transfer Molds

J. Berthold

1.2.1  Introduction

The compression and transfer molding processes are primarily used for process-ing of thermoset molding compounds in the field of plastics processing (so-called curable molding compounds). These molding compounds can be processed under the exposure of pressure and heat, producing parts with high strength, good chemical resistance, dimensional stability, and deflection temperature under load. Molded parts made from thermoset molding compounds are mainly used in the electronics industry (switch and fuse housings, lamp sockets, power outlets, and switch covers) and in the automotive industry (brake pistons, housing parts, and components of fuel and coolant systems). Although injection and injection compression molding have established themselves in thermoset plastics processing over the years, today there are still many molded parts that can also be produced economically in the compression and transfer molding processes.

Today compression molding of non-free-flowing, mat-like thermosets (SMC) has an established presence in the automotive industry: for example, in the production of mostly large-scale auto body parts (under body paneling, etc.).

The compression molding process is also being used in the thermoplastic sector. However, it is only the processing of fiber-reinforced thermoplastic semifinished products (GMT) and LFT-molding compounds (long-fiber-reinforced thermoplastics) that have significant importance. The disadvantage of thermoplastic compression molding is basically that the necessary heating and cooling of the material nega-tively affects the total cycle time within each cycle.

All of these process variations have one thing in common: The product quality is dependent on the quality of the plastic material being processed, the processing machine, and a great emphasis on the quality of the particular mold being used. In discussing the compression molding process, the term mold is used instead of the term tool.

531.2 Compression and Transfer Molds

1.2.2  Compression Molds

1.2.2.1  General Information

The compression molding process is an economical manufacturing technique for the molded parts production of thermoset molding compounds. This is particularly advantageous for the production of large area, thin-walled molded parts that have a tendency to warp. During the compression molding process, the dosed molding compound, which is partially preheated, tableted, or pre-plasticized, is introduced into an open heated mold. Upon closing of the hot mold, the molding material so�ens, is distributed evenly in the remaining cavity, and compacted under pressure. Subsequently, the molding compound hardens under pressure and temperature into a molded part.

This curing time is dependent on the wall thickness of the molded part, the flow settings of the molding compound, and the degree of preheating.

The compression molding process is as follows:

1. Loading the molding compound into the mold, either manually or by using a loading device (Figure 1.78, upper le�)

2. Closing the mold with pressure buildup, heat and pressure exposure to the molding compound, and intermediate ventilation if necessary (Figure 1.78, upper right)

3. Opening the mold and removing the preform in the ejection position, either manually or by a removal device (Figure 1.78, lower le�)

4. Cleaning of the mold (Figure 1.78, lower right)

The mold can be directly heated by an integrated electric heater with heating ele-ments of different shapes or by a heat transfer fluid that flows through the channels into the mold. The so-called indirect heaters are also used and are characterized by the required heat that is introduced to the mold through heating plates, which are integrated in the press by heat conduction. The molds are made out of heat-resistant steel to ensure that they can be operationally reliable under the required conditions of pressure and temperature.

The mold is an important cost factor in the calculation of a plastic product. Mold modifications are usually associated with high costs and big risks. For proper func-tioning of compression molds, it is important that a technical drawing suitable for production be made before designing the mold. In some cases, it may be necessary to investigate and prove the manufacturability of a part with a prototype mold before manufacturing the production mold.

54 1 Molds for Various Processing Methods

FIGURE 1.78  Compression molding process

(1) Compression mold; upper mold half (male mold or force); (2) press table;

(3) press cylinder; (4) press tie bars; (5) compression mold; lower mold half

(female mold); (6) compression molding brace; (7) ejector cylinder

A�er the preparation of a technical drawing and the documentation of a mold part, the mold design is carried into execution. All of the following considerations should be designed into the mold. Five are particularly important:

1. Stability of the mold assembly

2. Optimal heating

3. Necessary demolding and ejection aids (for example, slide mold, split mold, etc.)

4. Dimensional tolerances

5. Dra�s angles

551.2 Compression and Transfer Molds

In the interest of overall lower costs, these issues should be fully thought through and factored in during the design phase to possibly minimize or avoid reworking and changing of the finished mold.

1.2.2.2  Requirements

The high pressures created during compression molding require the mold to be sufficiently stable. The mold components should only deform elastically to a limited extent. If the deformation in the mold is too much, the mold halves can get jammed when opening or the deformation can result in poor demolding of the molded part. With hardened, galvanized, or coated mold surfaces, there is a danger of the surface tearing, which will result in most cases to the mold becoming useless.

Molds for compression molded parts with high walls can experience high pres-sures on the side walls of the mold and must be well reinforced laterally so that the resulting pressure can be absorbed and deflection prevented.

Since the material cost of a mold – even taking into account the increase of steel prices in recent years – is low in relation to the labor and processing costs, it is o�en possible to achieve adequate rigidity without incurring significant additional expenses. Massively oversized molds offer greater stability and more uniform tem-perature distribution, which positively impacts product quality.

To ensure good release properties of the compression molded parts, the product con-tacting surfaces of a mold, which essentially means the cavities, are polished. Since the mold surface is replicated very accurately on the molded part, a high-quality surface finish is necessary for manufacturing of an optically flawless molded part.

To counteract the very severe wearing effect of molding compounds as they flow under pressure, it is advisable to harden the mold surface or apply a wear-resistant coating. On the other hand, the core of the mold steel should be tough so that slight deformations of the mold under pressure do not cause permanent damage. For com-ponents with molding surfaces, it is best to use high-temperature or heat-treated mold steels with hardened surfaces or a hard chrome plating to protect against wear and chemical attack.

To avoid markings/surface effects on the molded parts in the areas of joint lines, it is desirable that the molding parts of the mold are built up of as few components as possible. If segmenting of the mold cannot be avoided due to complex part geometry, it must be ensured that the mold inserts or segments are installed in the compress-ing direction and not transverse to it.

56 1 Molds for Various Processing Methods

1.2.2.3  Components of a Compression Mold

A mold basically consists of an upper and lower half. In the normal course of events, the lower half is clamped to the press table, and the top part is attached to the press force. The guidance of both mold halves to each other is ensured by appropriately hardened guide elements. Asymmetric part geometry can induce considerable compressive forces into the guide elements. Therefore, they need to be held to a high tolerance of dimensional stability.

The ejection of the compression molded part from the mold o�en requires special mechanisms. Compression molded parts such as shallow bowls or plates are o�en ejected by compressed air, which is required anyway on every machine to clean residual raw material and flash from the mold. Many molded parts require ejector pins or bars for ejection. Due to material shrinkage, the use of ejector pins cannot be avoided in compression molded parts with strong ribbing and breakthroughs. To avoid jamming of the ejector plate, which the ejector pins are attached to, they should always have a separate guide. Thus the premature wear of ejector pins is prevented. Ejector pins are beneficial during the ejection stage of thermoset pro-cessing because they vent the mold.

1.2.3  Transfer Molding

1.2.3.1  General

Thermoset molding compounds can also be processed by the transfer molding process. The dosed molding compound for the transfer molding process is inserted (possibly tableted, preheated, or pre-plasticized) into a heated injection cylinder and then injected into the closed heated mold with the aid of an injection piston and sprues. This process allows for very uniform heating of the material. While the well-preheated material flows poorly during the compression molding process, this material enters the cavity in the transfer molding process comparatively well-plasticized. One advantage of this method over the compression molding process is the fact that potential inserts can easily and gently be surrounded, which reduces wear in the mold cavity. Transfer molding is also advantageous for molded parts where cores have to be inserted into the mold. Because the mold is already closed when the material enters, mold cores and other embedded parts can optimally hold up against unilateral pressure that may arise during the filling process.

The transfer molding process, particularly for parts with thicker walls, allows shorter cycle times than the compression molding process and therefore is more economical. The reason for this is how the material is heated. During the compres-

571.2 Compression and Transfer Molds

sion molding process, the heated mold walls just slowly heat the molding compound from the outside to the inside. In transfer molding, however, a high frictional heat is produced in the material by injection.

1.2.3.2  Requirements

The requirements for transfer molds are essentially similar to those for compres-sion molds. The mold in the injection area has to be adequately stable to keep a constant molding pressure in the injection cylinder, from about 1,000 to 1,800 bar. Since the mold is closed before the injection process, care must be taken to ensure that appropriate vents are provided within the cavity. Ventilation of the mold is not possible in the transfer compression molding process because it would interrupt the actual injection process. The dimensioning of the ventilation locations must be configured in such a way that the reaction gases produced can escape from the material. To avoid clogging the vents, it is necessary to some extent to eject pen-etrated molding compounds together with the finished molded part in every cycle.

1.2.3.3  Structure of a Transfer Mold

Unlike the compression mold, there is no loading chamber in the transfer mold. Instead, it has a centrally arranged heating cylinder with transfer piston. This dis-tinction in the arrangement of the transfer piston is shown below:

� Transfer molding from the top, and � Transfer molding from the bottom.

Figure 1.79 (a) shows the arrangement of the injection cylinder and piston from the top. These molds can be used in a conventional press in which an external force holds the molding surface components together. It must be considered that the entire opening stroke of the press cannot be used.

Figure 1.79 (b) shows the mold for the transfer molding process from the bottom. A press with a separate injection unit is required for molds of this design. Most commonly, these are presses with a hydraulic cylinder that is centrally attached to the mold table. This actuates the transfer piston which is linked to the machine control (transfer press).

The design of the cavity in the molds is the same as for compression molds. However, it gives more freedom for sliders, inserts, mold parting planes, and ejectors. A clean, flat parting line between the mold halves is very important due to the high transfer pressures. Therefore, it is advantageous to machine a 1 to 3 mm deep relief cut that is approximately 5 to 10 mm around the edge of the cavity. This relief cut reaches to the outer edge of the mold so that the clamping force is immediately effective in sealing the area.

58 1 Molds for Various Processing Methods

FIGURE 1.79  Transfer mold

(a) Piston from above

(b) Piston from below

(1) Transfer piston; (2) transfer cylinder; (3) flash opening; (4) external clamping;

(5) upper mold half; (6) lower mold half

1.2.4  Making Compression Molds

For reasons mentioned previously, the molds are made of case hardened steel or a fully coated material with a hardened or refined surface and tough core. All com-ponents that do not have molding surfaces and are exposed to wear are made of steel with strengths from 600 to 700 N/mm2. Hard chrome plating or a corrosion-resistant material has to be selected to protect the finish and the molding surfaces against wear caused by friction and chemical attack.

591.2 Compression and Transfer Molds

The shrinkage of the material being processed is taken into account when dimension-ing the components in the cavity areas. Hard chrome-plated molds, which consist of individual segments, are designed and manufactured so that the individual mold parts are compensated for their chromium layer thickness. A self-contained chromium layer has to remain for any adjustment work. Unplated areas should be avoided in any case to prevent a separation of the hard chromed plating from the mold steel.

1.2.4.1  Machining

The molding surfaces (meaning the cavities), depending on the geometry of the part, can be manufactured by a lathe, milling machine, or copy milling machine. In addition, the electrical discharge machining technique (EDM) can be used for areas that are difficult to machine. The choice of machining method does not only depend on technical aspects but also on economic aspects. Irregular-shaped surfaces in the compression mold are most effectively manufactured with a copy milling machine. This requires a set of negative patterns with the interior and exterior surface geom-etry of the compression molded part. Mold parting lines can be attached to the pattern. The stylus, which controls the milling process, traces the pattern on the copy milling machine. It is possible to incorporate shrinkage coefficients through the machine’s internal calculation operations. Alternatively, one might consider continuous processing using computer-aided design/computer-aided manufactur-ing (CAD/CAM). In this case, the actual geometry of the product to be molded is defined by CAD. The parting lines are also defined in the CAD system. A computer numerical control (CNC) program to control the machine operation has to be written in a CAM so�ware program. This program will operate the machining process. Correction for shrinkage coefficients, surface translations, or copies of the machin-ing programs can now easily be generated. It is also common to use CAD/CAM to manufacture eroding electrodes, with which the actual molding surfaces are formed using EDM (electronic discharge machining). A particular advantage of the EDM is that already hardened mold parts can be machined. The molding surfaces created by machining or EDM are finished by grinding and polishing and, if necessary, are then hard-chrome plated.

1.2.5  Mold Design

The mold design, as already mentioned, has a very important significance. Before starting the actual design, a determination of the type of mold has to be made. Various types of molds are briefly described in the following section.

60 1 Molds for Various Processing Methods

1.2.5.1  Types of Molds

1.2.5.1.1  Small-Series MoldThe simplest type of compression mold is the small- or pilot-series mold which is used to obtain samples of experimental design. The mold has no built-in heater. It is clamped between heating plates that are indirectly heated in the press. The mold is positioned by hand into the press, and a�er the compression process it has to be manually removed and transported to a work table where it can be demolded. For this purpose the mold has adequate side handles. Because this mold is only used for a few compressions, a hardening or plating of the molding surface can be dispensed with. A�er the mold is removed from the press, it is no longer heated and cools on the outside of the press. Thus, it takes longer to make the next compression because of the long heating process on the press. This time loss can be counteracted by using external heating plates on which the two mold halves are placed.

1.2.5.1.2  Test MoldTo ensure proper operation of the compression mold and a reliable molding process – even with very hard flowing molding compounds or complex mold part geom-etries – it may be useful to make a test mold before the actual production of the series mold. Using a test mold can, for example, optimize the flow of the molding compound. The identification of suitable processing parameters gives the processor greater security in the part calculation. In addition, different possible interpreta-tions can be checked. Figure 1.80 shows a test mold for determining processing parameters for the transfer molding process. In this case, the objective is to process hard flowing molding compounds. The transfer piston has (on its cylinder surface, immediately behind the piston surface) an undercut with which the residues are withdrawn from the cylinder.

1.2.5.1.3  Standard Mold UnitFigure 1.81 shows a standard mold unit, which is another type of compression mold. Both mold halves are permanently clamped in the press. The cavities for the base mold are carried out in the form of inserts.

The frame itself contains the guide elements/pins for the upper and lower mold half as well as direct heating in the form of heating elements and a corresponding insulation plate. It is also possible to use indirect heating via heating plates that are integrated into the press. The ejection function is integrated into the mold frame. Only the ejector pins are carried through the mold insert to the cavity. The non-molding surface components of a mold frame are standardized and can be assembled a�er the building block principle.

611.2 Compression and Transfer Molds

FIGURE 1.80  Test mold for transfer molding process

(1) Upper clamping plate; (2) transfer

piston; (3) external clamping; (4) mold

core; (5) transfer cylinder; (6) bypass

channel; (7) mold leader pin; (8) upper

mold half; (9) hand grip; (10) lower

mold half; (11) lower clamping plate

FIGURE 1.81  Mold frame

(1) Mold heating; (2) insulation plate;

(3) guide elements; (4) ejector;

(5) space for molding surface inserts

62 1 Molds for Various Processing Methods

1.2.5.1.4  Conventional Compression MoldDue to the part geometry of a plastic component and the associated press require-ments, it is o�en impossible to use a mold frame. The structure of a so-called conventional compression mold (Figure 1.82) is tailored to the part, and in terms of strength of the mold steel, refining of molding surfaces, as well as heating and guide elements, it is designed for maximum economic efficiency.

FIGURE 1.82  Conventional compression mold

(1) Alignment pin; (2) spacer; (3) hydraulic ejector; (4) insulating plate;

(5) shut-off edge inserts; (6) heating tailored to the product

1.2.5.2  Structural Mold Designs

1.2.5.2.1  Positive MoldA positive mold is designed to have a chamber above the cavity that is large enough to completely hold the charge of the molding compound. The force penetrates this chamber and forms with its molding surface together with the female mold the mold cavity (which means that the force immerses into the female mold).

The penetrated area is called the shut-off region. The circumferential edge that separates the cavity from the loading chamber is called shut-off edge. Figure 1.83 shows a conventional positive mold.

Above the shut-off edge, the remaining space between the force and female mold is extended to demold the parts more easily. Due to the type of process, compression molds have to be operated with an excess of material (small material overdose).

This surplus, which is also called overflow, flows into the remaining cavity between the force and female mold. The mold is equipped with a very small gap in the area of the shut-off (maximum 0.05 mm) to avoid unnecessary flash. This ensures the ideal compression of the molded part and creates a clear separation between the actual preform and the excess molding compound. Figure 1.84 shows several dif-ferent designs.

631.2 Compression and Transfer Molds

FIGURE 1.83  Positive mold

(1) Upper mold half; (2) loading chamber; (3) shut-off edge; (4) lower mold half

FIGURE 1.84  Shut-off edge design

(a) Parting line in molding direction, shut-off region enlarged in parallel direction

(b) Parting line in molding direction, shut-off region enlarged conically

(c) Horizontal parting line, loading chamber enlarged conically

(A) 5 to 10 mm; (B) 0.02 to 0.05 mm; (C) 0.5 mm; (D) 0.5°; (E) 2 to 5 mm

1.2.5.2.2  Positive Mold with LandsIn this mold design (Figure 1.85), the molding surfaces of the female- and force-sided areas are at first horizontally separated by a land which forms a shut-off edge. The shut-off region of the loading chamber is located directly behind this edge.

This mold design is disadvantageous for conventional compression molds because of the severe wear that can result.

FIGURE 1.85  Semipositive Mold with lands (enlarged detail of Figure 1.86)

(1) horizontal parting line; (2) loading chamber; (3) shut-off region; (4) land

64 1 Molds for Various Processing Methods

Due to the horizontal separation, the mold parts under molding pressure in the molding compound run until catastrophic failure and create extremely high com-pression stresses in the mold steel.

1.2.5.2.3  Multi-Cavity Mold with a Common Loading ChamberMulti-cavity molds of this type are mostly designed with lands, or shut-offs, which are located in a common shut-off region or loading chamber. Figure 1.86 shows such a multi-cavity compression mold. The increased need for additional molding compound, which is necessary for filling the shut-off region, is disadvantageous for this design. A higher compression force is additionally necessary to get the molding compounds in the cavities to receive an exact impression of the cavity. These molds are not or only partly suitable for hard flowing compression molding compounds or compounds with a high filler content.

FIGURE 1.86  Multi-cavity mold with lands

(1) Shut-off edge; (2) common

loading chamber; (3) land (shut-off);

(4) upper mold half; (5) lower mold

half

1.2.5.2.4  Multi-Cavity Mold with Individual Loading ChambersThe above-mentioned disadvantages can be avoided with this mold design by using individual loading chambers. Figure 1.87 shows the corresponding mold design. To ensure uniform filling of the molding compound into the cavity, the molding compound has to be dosed either with prefabricated tablets (preforms) or by using a loading device. Therefore, damage on the shut-off edges can be avoided. The loading device has to be designed so that the filling can be done outside the press and that the uniformly dosed material is able to be simultaneously inserted into the individual loading chambers of the multi-cavity mold. Thereby, pressure differences developed by nonuniform dosing in the individual cavities, which can increase mold wear, can be avoided. The variation of the dwell-time lengths of the molding compound in the individual loading chambers affects the quality of the preforms.

651.2 Compression and Transfer Molds

FIGURE 1.87  Multi-cavity mold with individual loading chambers

(1) individual loading chamber

1.2.5.2.5  Mold with Lateral Core PullerMolded parts with lateral breakthroughs, for example drill holes, which cannot be created with a slider, can be produced with a mold using a lateral core puller. The drive mechanism of the core puller can be actuated by hand, indirectly by the press motion or directly using a hydraulic cylinder. Figure 1.88 shows such a mold where the core puller is directly actuated by a hydraulic cylinder. One must be sure to design lateral core pullers so that they close tightly in their end position to avoid flash formation, which can occur through resin penetration. It has to be guaranteed that there is still a slight movement of the core. The design of the core includes the inserts, which fit tightly in the last 3 to 5 mm, with the remaining length being either relieved or tapered.

FIGURE 1.88  Mold with lateral core puller

(1) Lug guidance; (2) spacer; (3) hydraulic ejector; (4) insulation plate;

(5) core puller

66 1 Molds for Various Processing Methods

1.2.5.2.6  Split MoldA split mold (Figure 1.89) has to be conical in design which means that the split cavities are driven or held by a conical ring or frame. The cone angle of the splits depends on the size and shape of the product to be molded. In compression molds this angle should not fall below 15° to avoid extensive wedging, or self-locking, of the mold splits as a result of the force generated during molding. Opening of the mold splits is done by a direct or indirect actuating mechanism, for example, through a core puller unit or an ejector mechanism in the press. It is also possible that the mold splits are actuated by using a hydraulic cylinder. To avoid buoyancy of the splits through pressure, it is important that the stop lands (which restrict the immersion depth of the mold force with increasing pressure) are resting on the mold splits, which gives a minimum of flash in the area of split lines.

FIGURE 1.89  Split-cavity mold

(1) Dovetailed sliders; (2) ejector plate; (3) upper mold half; (4) split cavity;

(5) lower mold half

1.2.5.2.7  Hinged Split-Cavity MoldCompression molded parts with undercuts on the outer surfaces, for example electri-cal switchboxes, are molded very easily in a hinged split-cavity mold (Figure 1.90).

FIGURE 1.90  Hinged split-cavity mold

(1) Movable plates; (2) lower mold half;

(3) ejector plate; (4) upper mold half

671.2 Compression and Transfer Molds

The mold has to be designed so that the four mold plates, which form the side walls, are attached to the base plate of the mold with hinges. A central ejector ejects the molded part a�er curing by moving the molding surface of the mold up, which allows ejecting of undercuts through the tapered design of the outer surfaces of the side walls.

1.2.5.2.8  Mold with InsertsSome technical parts are designed so that metallic inserts such as pins, bushings, sleeves, or reinforcement elements have to be embedded. Figure 1.91 shows such a mold design with an example of a metallic hub to be embedded. During the filling cycle, it is therefore important to ensure that the insert shuts off tightly against the mating mold surfaces and is held firmly in the specific position to prevent shi�ing. Conventional compression molds with loading chambers are less suitable for these parts because shi�ing of inserts cannot be effectively prevented.

FIGURE 1.91  Mold with Insert

(1) Transfer piston; (2) transfer cylinder; (3) flash opening; (4) external clamping;

(5) upper mold half; (6) lower mold half; (7) inserts

1.2.5.2.9  Unscrewing MoldUnscrewing molds are used for the fabrication of molded parts with threads, which are integrated in the plastic material and are not manufactured using inserts. The mold is designed so that the thread-forming cores can be set in rotation with a common driving mechanism which allows demolding of the molded parts without damaging the threads. At first, the part remains on the threaded cores. A�er opening the mold, the cores unscrew and the preform can be released. The unscrewing mechanism can be chain and pinion driven, either manually by a hand crank or by a gear rack. Furthermore, the opening movement of the press (through appropriate kinematics) can be used for creating the unscrewing motion. Figure 1.92 shows the different possible designs of unscrewing molds.

68 1 Molds for Various Processing Methods

FIGURE 1.92  Unscrewing Mold

(1) Gear rack drive, pneumatically or hydraulically actuated; (2) drive with

threaded rod actuated by press motion; (3) cylinder; (4) gear rack; (5) hand

wheel drive, torque transmitted by a chain

1.2.5.3  General Aspects

All of the mold designs show that excess material always forms flash during molding, even though theoretically in a positive mold the complete molding compound is supposed to be converted into the product.

This can be caused by mold tolerances in the area of the shut-off edge, expansion differences between female and male mold, and temperature differences and flow characteristics of the molding compound.

In order to keep flash to a minimum, it is necessary to make the compression mold sufficiently stiff.

For functioning and stability of the mold, it is important to pay attention to steel selection and temperature control as well as to ensure safe demolding of the preform and the corresponding flash.

Horizontal separation using positive molds should be avoided for selecting the design of the shut-off edges. It is preferable to maintain the horizontal separation using transfer molds (if the part geometry allows it).

691.2 Compression and Transfer Molds

1.2.6  Sheet Molding Compound (SMC)-Molds

1.2.6.1  Introduction

Sheet Molding Compounds (SMC) are thermoset polyester sheet molding compounds (also called prepreg) that are mainly processed into molded parts in the compres-sion molding process using pressure and temperature. There are also some very few molded parts that are fabricated in the injection molding process using special equipment for material feeding. The glass-fiber content depends on the load to be withstood, between 20 and 40%, with a glass-fiber length of 25 to 50 mm. The matrix material is mostly a polyester or vinyl ester resin. SMCs can be used for a wide variety of applications and have been a constant in the fields of the building construction, chemical, textile, and electronics industries.

Because of the huge progress in SMC-processing in the past 10 years, the commer-cial vehicle construction and passenger car industry are today the most important markets for SMC molded parts.

SMC processing in connection with the in-mold coating (IMC) process is widely accepted in the fabrication of visible parts that have to be given a paint finish. The alternatives to SMC are still the use of conventional metal materials and partly glass mat reinforced thermoplastics (GMT). A significant advantage of SMC over GMT and long fiber thermoplastic (LFT) is, for example, when a good heat resistance and elevated painting temperature are demanded. In addition, SMC molded parts can be fabricated with excellent surface qualities (Class-A surfaces).

The SMC processing is done in heated steel molds. The blanks of a predetermined size are cut from the prepreg material. A�er removing the protective film, which is located on the upper and lower side of the SMC, the blanks are checked for their initial weight and are inserted into the mold using a certain arrangement. The blanks in the cavity have to be designed so that 60 to 80% of the molded part area is covered. Care must be taken to ensure that the flow distances are kept as equal as possible. The processing temperature for standard materials is around 140 to 155 °C. The processing pressure is determined by the molded part geometry and is around 500 to 1,500 N/cm2.

Basically there is a possibility to mold in metal parts such as bushings, nuts, and other reinforcements.

Feeding of the mold and removal of the molded parts can be done manually or automatically, with the decision dependent on quantity and economics.

The molding operation is done on parallel-controlled high speed presses. Mostly it is recommended to integrate the male mold on the bottom and the female mold on the top (Figure 1.93). This makes loading the blanks and removing the molded part easier. For parts that are required to meet a high dimensional tolerance (because of

70 1 Molds for Various Processing Methods

their area of application) it is recommended to use cooling devices. A�er demolding, the part can be clamped inside the cooling device against the shrinking direction.

Holes and breakthroughs in the molded parts can already have occurred during the forming process in the mold. This results in the flowing material being diverted to form flow lines or flow marks and strong orientations, which can significantly affect the strength of the molded part.

Therefore, for strength reasons it is in many cases useful to create holes, openings, and cutouts in a secondary operation by punching, drilling, or milling. Diamond or carbide-tipped tools have proved to be worthwhile in this kind of process.

1.2.6.2  Mold Design

Whereas aluminum and zinc alloys or low-hardness steels can be used for small-series molds (prototypes and pre-series), the processing of SMC to serial parts is done in heated steel molds.

The mold design of a SMC mold is shown in Figure 1.93 and basically corresponds to the design of the compression molds presented in the previous sections. However, there are additional criteria that have to be considered when designing SMC com-pression molds.

FIGURE 1.93  Schematic representation of a SMC compression mold

(1) Female mold; (2) slider; (3) hydraulic cylinder; (4) guide plates (female mold);

(5) pressure plate; (6) guide plates (force); (7) force; (8) ejecting cylinder;

(9) ejector plate; (10) clamping plate; (11) guide pin; (12) support pillar;

(13) ejector pin; (14) heating channel; (15) return pin; (16) insulating plate;

(17) backing plate; (18) guide pin; (19) shut-off edge; (20) guide pin bushing;

(21) shut-off edge; (22) external insulation; (23) insulation plate; (24) air ejector;

(25) heating channels

711.2 Compression and Transfer Molds

FIGURE 1.94  Force of the mold in shown in Figure 1.93 designated as an insert in the base

plate

The mold steel selection depends on the geometry and the purpose of the molded parts, as well as the quantities to be molded. Steels with strengths of 600 to 700 N/mm2 are normally suitable for the mold base and strengths of 1,000 to 1,100 N/mm2 for the components with molding surfaces.

The following steels are preferably used: 1.1730 (C45W3), 1.2311 (40CrMnMo7), 1.2312 (40CrMnMoS8-6), 1.2710 (45NiCr6), and 1.2738 (40CrMnNiMo8-6-4). For the production of the force, it has to be already considered in the design stage whether it is suitable from a cost standpoint to attach the component with molding surfaces to a retainer or male base plate (Figure 1.94).

The design of the shut-off edges is essentially carried out as shown in Figure 1.84. The height should be 20 to 25 mm and the clearance should be 0.05 mm. Consistent and precise machining of the shut-off edges is a prerequisite for product quality. Tight shut-off edges guarantee a uniform compression of the material during the pressing process and prevent a loss of strength of the product. Too much clearance can lead to leakage of resin as well as glass fiber. This creates larger flash, which increases the amount of rework substantially. In mass productions it is advanta-geous to provide the shut-off region with hardened inserts. If the use of borders in both mold halves is not possible from the design perspective, the incorporation of them into the female mold is recommended (Figure 1.95). If the use of borders is not possible at all, the shut-off edges should be flame hardened on both sides.

FIGURE 1.95  Typical design of the hardened shut-off edge insert

72 1 Molds for Various Processing Methods

This will also influence the selection of the mold steel. It is in each case useful to polish the shut-off edges in the demolding direction. The selection of a favorable shut-off edge for a certain molded part is essentially dependent on the experiences of each SMC processor and mold maker. However, it has to be ensured to avoid pinch-off edges (horizontal lands).

1.2.6.2.1  Mold AlignmentThe alignments of the molds can be divided in two main tasks:

1. Alignment of the mold halves (preguidance) and

2. Alignment for the protection of the shut off-edge.

Guide pins and bushings of adequate dimensioning are generally used for the guidance of mold halves, where the guide pins are mostly used in the male mold. It has to be ensured that the pins are designed a minimum of 5 mm higher than the highest point on the male mold. Guide bolts made out of bronze with integrated solid lubricant are a suitable alternative to guide bolts out of hardened steel.

At the end of the holes into which the bushings are inserted, it is advantageous to introduce openings for easier removal of resin residue and flash. A variation, primarily used for large molds, is mainly to use side-mounted alignment bars and wedges (see Figure 1.83) because the diameters of round guide pins are limited by the manufacturers of the standard mold components. Side-mounted alignment bars and wedges are positioned in the mold center on the x- and y-axis and, in regards of the thermal expansion of the mold, they are in the neutral zone.

The actual guidance, which should keep the shut-off clearance constant, is provided by guide blocks (Figure 1.93, item 4). These are located above the shut-off edge and have the task of absorbing the lateral forces that occur through the yielding phenomenon in curved parts.

This lateral pressure leads with insufficient guidance to excessive wear of the shut-off edges.

In certain circumstances, using molds for flat molded parts or parts with a simple geometry, the use of block guidance can be avoided. The mounting surfaces of the sliding plates have to be an integral part of the mold halves and may not be bolted on. They have to be included into the design so, at latest of 40 to 50 mm before immersion, they will become effective. Guide plates impregnated with solid lubricant and preferably located on the male mold have also proved their worth. Lubricating grooves have to be introduced when sliding plates made out of hardened steel are included on both sides.

731.2 Compression and Transfer Molds

1.2.6.2.2  Ejector MechanismThe position of the ejector pins is regulated by the geometry of the molded part. To prevent markings, it has to be ensured that the pins push against ribs, shoulders, and hidden molded part surfaces.

If permitted by the product geometry, ejector pins with the largest possible diameter (about 20 mm) have to be chosen to avoid impressions on the opposite side or a breakthrough of the molded parts. Ejector pins can also provide ventilation under, for example, tall, free-standing bosses and ribs to avoid gas entrapment. In the case of contoured surfaces, the pins must be secured against rotation.

The ejector plate has to be designed in respect to its stability so that deflection can be eliminated.

Ejector plates of bigger molds are heated to avoid sticking of the ejector pins as a result of thermal expansion.

There are various possibilities for actuating the ejector plates:

1. Ejecting by directly attached hydraulic cylinders,

2. Ejecting by a piston in the press table, and

3. Ejecting using a stripper mechanism in the press.

For the operation with more hydraulic cylinders, it is recommended to use flow divid-ers for a synchronization of the ejector plate. The ejector plate has to be designed so it can reach far enough and can easily and securely remove the molded parts without any damage. Another possibility to remove the molded part out of the mold is to use air ejectors (Figure 1.96). However, they are only effective with flat and smooth products.

air FIGURE 1.96  Design of an air ejector

74 1 Molds for Various Processing Methods

The combination of air ejector in the female mold and ejector pins in the male mold is a proven process. The air ejector has the function to release the vacuum that was formed in the female mold during the process of opening the mold. This helps the molded part to stay on the male mold (the predetermined ejection side).

The ejectors have to be retracted completely before the next compression cycle starts to avoid damage to the mold. It is therefore appropriate to monitor the process using limit switches.

1.2.6.2.3  UndercutsSlide molds are more expensive than other molds because of their complexity in production and are more prone to repairs. Therefore, it is mainly recommended to avoid undercuts in the design of molded parts. Sliders can be used if undercuts cannot be avoided. The inner slider is mostly actuated by using the ejector plate in the male mold (Figure 1.97). Direct actuation with a hydraulic cylinder is almost exclusively used in the female mold. Additional locking slides, which can serve as a support, are used to avoid displacement of the slider during the compression process. This extra step can be avoided by using self-locking hydraulic cylinders for operating the sliders. The design of these cylinders is dependent on the compression force.

In each case, the slider running surfaces have to be hardened and provided with a lubricant. With sliders that are complex and difficult to access, it is recommended to install self-lubricating elements or a central lubrication system. To blow out residual resin residues a�er every compression process, it is recommended to install air channels to prevent the slider from sticking.

Large sliders should be heated separately. In order to assure a high degree of operational reliability and protection from mold damage, sliders have to be secured electrically by limit switches.

FIGURE 1.97  Slider design to demold an undercut

751.2 Compression and Transfer Molds

1.2.6.2.4  HeatingSMC molds are like conventional compression molds, being electrically heated by cartridge heaters or tempering media. The distances of the flow channels have to be designed to uniform temperatures with maximum deviations of ±2 to 3 K guar-anteed. To avoid damaging the shut-off edges as a result of thermal expansion, the male mold has to be heated only a few degrees lower. The position of the heating channels should already be considered carefully in the design stage and should be incorporated into the basic concept. This basically determines the cycle time and the quality of the molded part. For complex molds with inserts, sliders, and ejection systems, a compromise needs to be found to achieve optimal temperature control.

The temperature control of larger molds is divided into multiple, separately con-trollable heating circuits. Large sliders, splits, and inserts are each assigned a separate heating circuit or are connected to the remaining circuits by interconnect-ing elements or to the basic heating system. The other circuits can be monitored using temperature sensors and can be controlled using temperature control equip-ment. The same principle applies to electrically heated molds. In the last couple of years, flexible heating elements that can be three-dimensionally bent by hand have established themselves. These allow the contours of a mold to be heated very flexibly. To reduce losses through heat radiation, it is recommended to provide a pressure-resistant insulation plate to the base plate of the male and female mold and provide the external surface of the molds with an insulation coated with reflector foil.

1.2.7  GMT/LFT Molds

1.2.7.1  Introduction

Plate-shaped GMT is a glass mat reinforced, flat, semifinished thermoplastic, which is mostly based on polypropylene that is processed using compression processes. The glass content depends on the requirements and is between 20 and 40 weight %. GMT components show a high impact resistance and stiffness with comparatively low weight. Molded parts made out of GMT are mostly used in the automobile industry for trim and carrier components in nonvisible areas.

LFT molding compounds have recently increased their visibility and usage in the marketplace. In contrast to GMT, these materials are mostly processed in the so-called plasticizing compression molding process. The molding compound is melted down in an extruder and dosed to the corresponding molded part size, (still in a liquid stage) inserted to the mold, and compressed. Inserting the material can be done by hand, with the aid of a robot equipped with a needle gripper, or by an automated

76 1 Molds for Various Processing Methods

mechanical device. The only compression molding process used in LFT processing is the flow molding process. GMT is in some cases substituted by LFT because of the similar mechanical properties of LFT and GMT molded parts. However, LFT has a more competitive cost structure.

1.2.7.2  Process Technology

1.2.7.2.1  Pressure FormingIn the compression molding process, the semifinished material is cut or punched dependent on the part geometry. Then, the blank is heated, inserted in the mold, and compressed. Due to the precisely fitting blanks, the material does not flow inside the mold and no fiber orientations in the molded parts develop. Due to the comparatively large waste and restricted configuration possibilities of the molded parts, the process is finding less frequent usage because it is only suitable for the processing of low stress parts without struts, ribs, and wall thickness differences.

1.2.7.2.2  Flow MoldingThe blanks to be processed for the flow molding process are smaller than the part contour. Although, they are adjusted based on the weight of the finished part, which means that they are thicker than the finished part. The blanks are typically removed from the magazine per each cycle and are put onto the conveyor belt of a preheating oven. Then, they pass through an infrared-radiated or circulating-air heated oven. The blanks, which are now heated to 190 to 210 °C in the oven, are inserted (using a predetermined inserting schema) into the 40 to 80 °C heated steel mold, either by hand or using a loading device. Compression molding takes place on parallel-controlled quick stroke presses with pressures of about 1,500 to 2,500 N/mm2. Removal is done by hand or automatically.

Cooling down to room temperature is dependent on the complexity of the molded part and is either done on a deposit table, on a conveyor belt, or in a water bath. Holes and other openings have to be done either by material flow during the com-pression molding process or subsequent operations, such as punching, drilling, or milling. The flow molding enables the production of complex molded parts with ribs, undercuts, metal inserts, and laminations and is now mostly used for all of the GMT and LFT applications.

1.2.7.3  Mold Construction

The design of GMT/LFT molds has its origin in the SMC technology and differs in just a few fields. The major differences are basically in the design of the shut-off edges, in temperature control, and dimensioning of the mold. Figure 1.98 shows the shut-off edge of a GMT/LFT mold. The immersion depth has in comparison to

771.2 Compression and Transfer Molds

SMC molds a maximum height of 15 mm, where the first 5 mm are used as a lead in. Determining the depth of immersion is the viscosity of the molten, thermoplastic composite material. The shut-off clearance between the male and the female mold is 0.02 mm, which requires precisely tailored mold guidance. The finishing of the shut-off edges has to be done in the pullout direction which will make deforming of a molding easier. The usage of hardened shut-off edges is not necessary, but it is necessary to flame harden the edges. The temperature control is done in water and can easily be compared to that of an injection mold. In the beginning of the process, the mold has to be heated to a uniform temperature of approximately 60 to 80 °C. In the running production, cooling has to be introduced to conduct the heat out of the molding compounds. To reach a uniform temperature distribution (which is the requirement for high product quality), it is useful to divide the temperature control system into several circuits to be able to control specific areas individually. Separated temperature control circuits should be applied for high cores and larger sliders. Because considerably higher cavity pressures are generated during the molding operation when processing GMT than for SMC, care must be taken to ensure that the mold is designed sufficiently rigid in terms of life span and especially of the part quality. For prototype and preproduction runs, molds made from zinc and aluminum alloys have been successfully used.

1.2.8  Practical Example

The following figures show a typical compression mold for the production of a flat housing component. This is a mold for the production of a front hatch of a truck cab. The material to be molded is a SMC material. The cavity has a hard chrome coating for increasing the wear resistance. The used mold steel is a Buderus 2738 mod. TS (HH) (26MnCrNiMo6.5.4). The integrated ribs and functional elements, which are mount-ing domes in this case, are very well seen in Figure 1.100.

FIGURE 1.98  Typical design of a hardened shut-off edge insert of a GMT-Mold

78 1 Molds for Various Processing Methods

FIGURE 1.99  Upper mold half [Picture Buderus Edelstahl GmbH/Peguform]

FIGURE 1.100  Lower mold half [Picture Buderus Edelstahl GmbH/Peguform]

 ■ 1.3  Molds for Polyurethane Products

U. Knipp, U. Maier

The production of molded parts made from polyurethane basically consists of:

� Raw materials, � Mixing and dosing unit, and � Molding equipment the so-called mold for forming the mold cavity with the curing PUR.

A key function when designing molds (comparable to other polymeric materials) is to include the correct shrinkage values in the oversized cavity dimensions to

791.3 Molds for Polyurethane Products

produce precisely dimensioned molded parts. If there are no opening, clamping, and interlocking devices integrated in the mold, a mold carrier has to take over these functions.

Mold making is as equally diverse and unique as the different raw materials and their applications which are available on the market. For reasons of clarity, it is recom-mended to structure the mold technology into different density areas molded parts. A substructure could be the rigidity of the foam, being flexible, semirigid or rigid.

1.3.1  Products, Processes, Applications, Shrinkage, and Mold Carriers

1.3.1.1  Material Components, Processing, Applications

When polyurethane (PUR) is processed, at least two components (polyisocyanate and polyol) are liquid at the processing temperature and are metered exactly accord-ing to the manufacturer’s data sheets and thoroughly mixed. Metering and mixing can be done as seen in Figure 1.101 using for example an agitator mixing head [1]. The resultant liquid mixture then fills the mold where an exothermic reaction (polyaddition of a polyisocyanate and a polyol) takes place, creating a PUR product.

The PUR foam is created by:

� Carbon dioxide (CO2) released during the reaction, � Evaporation of solvents with low boiling points, and � Expansion of inert gases that are either dissolved or dispersed at least in one of the raw material components.

a

b c

d

e

FIGURE 1.101  Polyurethane (PUR) process,

in principle

(a) Material container; (b) metering

pump; (c) adjustable drive motor;

(d) changeover valve; (e) agitator

mixing head

80 1 Molds for Various Processing Methods

A self-skinning foam product (with a cellular core and almost solid skin layers) is created by superimposing the mold temperature in the areas close to the walls and by the formation of propellant gas during evaporation. Following the exother-mic reaction, the temperatures increase with increasing distance from the mold wall. Microporous reaction injection molding (RIM) plastics are a special group of integral-skin, high-density materials. Solid, non-foamed mixes, can be produced using appropriate measures (such as evacuation of the raw materials, adding H2O binders, and avoiding air access) to prevent the undesired formation of bubbles.

The resultant PUR plastics can be quite different, depending on the raw material com-ponents and the conditions under which they are processed. The chemistry-inclined reader can find information in this literature (e.g., [2, 3]) about the diversity of PUR raw materials, the used additives, fillers and reinforcing materials, as well as process-ing variants. Table 1.2 shows various PUR materials and their typical applications.

TABLE 1.2  Classification of Polyurethanes (PUR), Using Examples of Well-Known Applications

in Dependence of Density and Structure

Structure Linear Partly Linked Linked

Density kg/m3

HardnessType

Flexible Semirigid Rigid

20–50 Homogenous foam

Flexible PUR foam Semirigid foam Rigid foam

Seat cushion Instrument panel Refrigerator insulation

200–700 High density foam

Flexible integral skin Semirigid integral skin Rigid integral skin

Headrest Shoe sole Window frames

1,100–1,200 Solid or microcellular

Elastomers Stiff-elastic material Rigid Plastic

Gasket, bellow Wind spoiler on cars Pipe sleeve, pipe

1.3.1.2  Shrinkage

There are two types of shrinkage: thermal shrinkage and shrinkage caused by the reaction. Shrinkage is > 1.5% for more flexible PUR and > 0.5% for more rigid PUR. Fillers and inserts reduce this shrinkage. Before dimensioning the cavity, the shrinkage data supplied by the raw material suppliers must be considered. In plates, shrinkage is not restricted; however, in e.g. boxlike structures, the amount of shrinkage will be less than indicated for flat pieces because of the restraining effect of the core.

Orientation-dependent, anisotropic shrinkage occurs when fibrous reinforcement materials (in the form of short glass fibers) are used. The shrinkage for shell-shaped, solid molded parts can be determined using the finite element method (FEM) in the design phase [4, 5].

811.3 Molds for Polyurethane Products

1.3.1.3  Mold Carriers

PUR molded parts are manufactured in many different sizes and shapes. Starting with, for example, finger-sized, spring-damping components up to high-volume, heavy-duty vehicle bodywork, they cover almost the entire spectrum. Accordingly, the molds are designed to the size, shape, and function of the part to be molded. For the production process, the molds have to be opened and closed, kept shut (with-stand the internal process pressure), and if necessary, brought into an ergonomic operating position.

There are two different solutions for these types of tasks:

1. Self-locking molds (Figure 1.102) – The term self-locking mold is used when the components move the way as explained above and when they are connected to the mold and form a unit. This is mostly the case with small molds (e.g., glove compartment lids) where the rotating movement is enough to open the mold.

FIGURE 1.102  Self-locking mold for glove compartment lid

(Source: Fa. Frimo Group GmbH, Lotte)

2. Mold carrier (Figure 1.103) – In this case, the mold, which only consists of an upper and a lower mold half, is integrated into the mold carrier. An “easier” mold than in Figure 1.103 (a) can be used because the movement operations can be found inside the mold carrier. The mold design can be implemented more simply as well due to the stability and stiffness of the mold carrier. Therefore, standardized mold carriers can be used to operate other molds as well. Especially for bigger parts, mold carriers offer a lot of options of movement to achieve an individual ergonomic and process position.

The movements of small components are mostly pneumatically driven whereas big parts are hydraulically driven. Recently, electric drives are gaining increasing acceptance because higher speed, less maintenance, and energy consumption can be achieved.

82 1 Molds for Various Processing Methods

FIGURE 1.103  Universal mold carrier with two rotation axes

(a) Initial position, mounting plate is opened to the maximum;

(b) turned by 90°, mounting plate is opened to the minimum;

(c) turned by 90°and swiveled by 90° mounting plate is opened to the maximum

1.3.2  Molds for Low-Density PUR Foam

Flexible, semirigid PUR foam products with a cross section of homogeneous foam structure are produced in relatively lightweight molds. These products are usually very bulky. Typically, the PUR volume (and the volume of the mold) of a cushion for an upholstered furniture, with a weight of 15 kg and a density of 45 kg/m3, has a volume of 300 l. Similar or even larger dimensions are common with products made from PUR foam, such as sandwich panels or freezer cabinets.

Basic prototypes can be produced in relatively simple molds, for example, by using plywood, silicon rubber, or epoxy resins. For industrial, automatic production, such molds are not suitable. Short opening and closing cycles cannot be achieved; also, the low viscosity of the liquid mixture makes sealing difficult.

Because the molds are heated by the exothermic PUR reaction, rapid cycling leads to high mold temperatures that can damage the mold as far as restraint by undercuts will cause stresses. Mold temperatures that increase with time are o�en a reason for visible surface defects and for variations in the physical properties of the product from cycle to cycle. Therefore, even for the production of low-density PUR products, it is essential to use molds that ensure reproduction of the processing parameters and permit rapid, automatic operations.

831.3 Molds for Polyurethane Products

1.3.2.1  Processing Parameters

1.3.2.1.1  Reaction TemperatureThe reaction heat amounts generated in PUR processing (depending on the type) are:

� Flexible, 80 to 100 kJ/kg � Semirigid, 100 to 120 kJ/kg � Rigid, 210 to 250 kJ/kg

Consequently, the reaction heat released during the foaming process increases the temperature measured in the center of the product as follows:

� Flexible, 120 to 140 °C � Semirigid, 110 to 130 °C � Rigid, 140 to 180 °C

The large temperature differences are dependent on the variation possibilities of PUR chemistry, which have a great influence on the exothermic heat produced.

Based on the above-mentioned comments, it becomes clear that manufacturing of high-quality products requires molds which have a perfect thermal design of heating and cooling channel arrangement. Constant mold temperatures are essential for molding parts of uniform quality. The mold temperature especially affects the reac-tion speed of the PUR systems, the filling of the mold, the foaming process, and the planned cycle time. Also, incorrect temperatures can affect post-molding operations and the dimensional accuracy of the product. They can also cause warping of the molded part.

The preferred heating and cooling medium for the temperature control circuits is water (mostly with additives for corrosion and frost protection). Heat transfer oils can be used as heat transfer fluids when it is necessary to come close to boiling temperature of water, or beyond, and therefore pressurized temperature circuits would be necessary.

Copper tubing of at least 10 mm in diameter and 1 mm in wall thickness is formed and placed in a mold before casting the (most commonly used) aluminum alloy. This is preferable to drilling channels in the o�en porous aluminum casting, which can lead to water leakage. Also, the molds are usually very large and of complex geometry, which makes the use of inserted copper tubing more economical.

According to data supplied by plastic manufacturers, the mold temperature for low-density PUR foams (MDI basis) should be between 40 and 50 °C ± 1.5 °C. The so-called hot cure foams (TDI basis) require considerable addition of heat; depend-ing on the PUR system used, temperatures from 140 to 180 °C are needed. Two methods are being used successfully:

84 1 Molds for Various Processing Methods

1. The mold is moved through a tunnel shaped oven. With this method of heating, the mold construction must withstand a short-time exposure to up 240 °C. For this reason, molds are made from welded sheet metal (steel approximately 2.5 mm thick, aluminum approximately 3 to 4 mm thick) or from cast aluminum, which should be as thin as possible to keep the heat capacity down.

2. The mold is heated. In addition to induction heating, the method of heating the mold with liquids is preferred. The mold is equipped with two tubing circuits, one for the heating with oil, and the other with water to cool the mold before the part is removed. Due to fluctuating temperatures, the coupling seals with the supply hoses at the mold and the thermostats require careful consideration.

In cases in which the part has some other material (plastic foil, etc.) on its surface that is then backed up (filled) by PUR foam, the homogeneous mold temperature is also important, and even though this is more difficult to achieve caused by the low heat conduction of the foil. The flow of the PUR mixture and its ability to bond properly with the skin material (foil, etc.) require reasonably uniform tempera-tures (examples include the inliner of freezers and instrument panels). For certain economic reasons such as low productivity, other methods are also possible. In the furniture industry, very limited quantities of large pieces are typically made from flexible or rigid PUR foams. Expensive molds made from metals therefore cannot be justified economically. Instead, molds are made from fiberglass-reinforced unsatu-rated polyesters (UP) or epoxy (EP) resins that are heated before production using radiant heaters or heating chambers. A�er two or three cycles, the mold reaches a state that is acceptable for production. However, this method is not suitable for short demolding cycles and requires greater operating windows than would be given with less expensive PUR mixtures.

1.3.2.1.2  Internal Pressure in MoldsAn important characteristic of every foamed material is the “free rise density”, that is, the density of the foam if it were not limited in its expansion. The density ratio is the ratio of the density achieved within the cavity in the mold, divided by the free rise density. The larger the resistance to flow during filling, the larger must be the density ratio selected to ensure that the mold cavity is fully filled with foam.

With diminishing density, the physical properties (flexibility and thermal insula-tion) improve. Therefore, PUR foams with the lowest possible density are used for filling. Thick-walled, simple shaped molded parts are then manufactured with a density ratio of 1.1 to 1.5, which results in an internal mold pressure of up to 0.5 bar. However, the conditions are usually more difficult. Higher flow resistance caused by narrow wall thicknesses and long flow paths, as well as the increasing usage of CO2 as a blowing agent for the foam, result in the following approximate values for internal pressure:

851.3 Molds for Polyurethane Products

Density (kg/m3) Internal pressure (bar)

Flexible foams 45 < 2.0 (30 psi)

Semirigid foams 70 to 100 < 2.0 (30 psi)

Rigid foams 25 to 80 < 1.0 (15 psi)

For safety reasons, a safety factor of 100% should be used when designing the mold to protect against excessive pressures due to overfilling of the mold.

The above listed pressures are relatively low in comparison to other processing methods (e.g., injection molding). However, the resulting forces, which should not be underestimated, can be quite high in view of the large areas encountered. These areas can easily be more than 1 m2 and should also be considered when the need for easy mold handling, heat management, and low cost require the mold to be as light as possible. The internal pressures are also far exceeded when a high density ratio (for the production of the molded part) is deliberately selected. The expected internal pressure p can be estimated:

density ratio 1p � �

1.3.2.2  Filling Technology

For the production of PUR molded parts of low density, there are two different filling methods: filling while the mold is open and filling the closed mold through a filling opening or through a gap (fan gate).

1.3.2.2.1  Open-Mold Filling MethodWhen the open mold is filled, the lower mold half must be positioned so that the liquid reaction mixture will not overflow at the parting line before the mold is closed. If the position of the mold does not correspond with the curing position itself, the position of the mold has to be changed again between filling and curing. With the new electrically positionable mold carriers this can be done exactly, fast, and in a reproducible fashion. To achieve parts free of defects (i.e., without voids), it is best to guide the mixing head close to the surface of the mold, preferably with a mechanical robot. Manipulation of the large mixing head is greatly facilitated if the upper half of the mold can be swung away for easy access.

When the open cavity is filled, slowly foaming mixtures are used because the mold has to have enough time to close. In the past, the so-called “closing time” was brought down to less than four seconds due to the use of constantly improving motion drives. Neverless, the cure times are longer than the cures time for the closed mold injec-tion method of PUR systems.

86 1 Molds for Various Processing Methods

The advantage of this method is that even a�er completion of the mold and produc-tion area, the feed position of the reaction mixture and therefore the flow paths can still be changed. This can be a substantial improvement in quality regarding entrapped air bubbles.

1.3.2.2.2  Closed-Mold Filling MethodFor the filling in the closed mold, there are two different filling methods: either the reaction mixture is poured through a drilled hole (direct gate) (Figure 1.104) or through a film gate in the parting line of the mold. If the mixing head is removed a�er filling, the drilled hole will be automatically or manually sealed using a plug. It is necessary to ensure an easy removal and cleaning of the gate.

Especially for larger components, the mixing head is attached to the upper mold half for accessibility. The position of the gate should be chosen so the flow path end of all flow paths can be reached at the same time. A position very deep inside the mold is preferable to avoid swirls and shi�ing due to the down-flowing and rising foam as much as possible.

FIGURE 1.104  Mold with direct gate and locking plug (source: Fa. Frimo Group GMBH, Lotte)

TABLE 1.3  Cycle Time Comparison for Filling of a Dash Board Using the Open- and the

Closed-Mold Filling Method

Closed-mold filling method

Open-mold filling method

Removing of the finished part/inserting the foil/carrier 135 s 135 s

Start the robot 0 s 2 s

Dosing time 3 s 4 s

Withdrawal of the Robot 0 s 2 s

Closing time 5 s 5 s

Cure time 60 s 113 s

Total 203 s 261 s

871.3 Molds for Polyurethane Products

Very thin-walled, complex parts, possibly with intricate inserts, require a more accurate weight with less variance of magnitude 1% from shot to shot. For these applications film gates in the parting line are to be preferred.Immediately a�er the gate area, the wall thickness should not increase rapidly because it would lead to the occurrence of flow separations and air inclusions.The used PUR systems in the closed-mold filling method have a shorter cure time, which can lead to a shorter cycle time. Therefore, the number of required molds can be reduced. However, a�er having finished the mold, changing the runner and gate position to produce perfect parts is very expensive.

1.3.2.3  Venting

Air present in the mold before foaming must be able to escape without significant resistance (i.e., with low pressure). Higher pressure drops can lead to undesired higher density in the foamed part. At the beginning of the filling process, the mixture flows to the lowest location of the mold. This area must be tight against leaking, because at this moment, the mixture has a viscosity of 200 to 800 mPas (comparable to that of very light lubricant oil). Air is now pushed upward by the foaming mixture and must be able to escape at the end of each flow path at the highest point of the mold. But there should escape no or less foam.In regards to venting, two types distinguished have to be compared.The one type is the type where the foam comes into direct contact with the mold surfaces. A release agent prevents sticking and allows demolding a�er curing. The venting is done through the parting plane or drilled venting holes. The parting plane has to fit perfectly to only have minimal flashes. Flexible seals are rarely used because of the sticking with the reacting mixture and the chance of damage during demolding process. Therefore, the flexible seals have to be exchanged more o�en. The venting holes (at the highest spots) should be designed as small as possible so only a small amount of foam can leak during curing (which progresses with reaction). So-called “foam brakes”, which are made of filters or foams with open cells are also used. Figure 1.105 shows a pin to fixate the sealing element with open cell structure. The air can escape, and the foam (because of its viscous and increasing elastic properties) seals the porous venting gap. These elements have to be exchanged a�er every cycle. Examples include insulation components of the nonvisible areas of a vehicle.Another solution is the use of valves, which open and close the venting holes depending on the applied pressure [6].The other type of venting is the back foaming of inserted foils or rigid inserts in one or both mold halves. Examples are top-quality interior trim for vehicles, instrument panels, glove boxes, and door panels. A so� skin or foil is inserted into the lower

88 1 Molds for Various Processing Methods

mold half, which will get the so�-touch later on in connection with the foam. A rigid carrier made out of injection-molded plastics, composites or other materials, which gives the required stiffness, is located on the upper mold half. During expansion, the foam replaces the air, which escapes through a gap between foil and carrier at the necessary venting positions of the mold cavity.

As soon as the foam reaches the edge, a seal closes the gap between the foil and rigid carrier so that no foam material can escape (Figure 1.106). A�er closing all seals, pressure build-up leads to the desired foam structure. A�er curing, the seals open the gap and release air and reaction gases present. That way molded parts of high quality can be produced. Inflatable hoses made out of endless-material have proven successful as seals. These are cost-effective and have a life span of up to 10,000 strokes. Critical are areas where the seals have to be interrupted because the mold halves are splitted in several parts. High-quality sealing systems offer a uniform sealing gap in these areas, which can be opened and closed.

TransformerFoam

Film

Closed

seal

Open

seal

Air/G

as

FIGURE 1.106  Inflatable seal in back foaming molds (source: Fa. Frimo Group GmbH, Lotte)

(le�) during filling; (right) a�er closing the sealing

a

b

c

FIGURE 1.105  Venting slot

(a) Sealing with open pores; (b) holding pin; (c) cavity

891.3 Molds for Polyurethane Products

1.3.2.4  Mold Design

As already explained in Section 1.3.1.3, a distinction is made between self-locking molds and molds that can be mounted into a mold carrier.

A foaming mold consists of the following components:

1. Lower mold half (fixed for self-locking molds)

� Moving elements, which allow demolding (flaps, slides) � Demolding aids (ejectors) � Heating and cooling system (drilled or through metal casting inserted pipes, mostly using water as a medium)

2. Upper mold half (for self-locking molds o�en rotatable)

� Gate (for closed-mold filling) � Sliders, if necessary, for demolding of undercuts on the upper side

3. Movement mechanisms for closing and opening (for self-locking molds)

� Foundation (for self-locking molds) � Locking (for self-locking molds) � Centering of upper and lower part � Compressed air and vacuum system, if necessary

Self-locking molds have double acting cylinders with hinged joints for the necessary kinematic movements. The cylinders (5) and (6) in Figure 1.107 ensure a folding and clamping function of the mold front. Figure 1.108 shows the three-dimensional CAD design of such a mold for stationary operations.

1

2 474

5 3 6

FIGURE 1.107  Cross section of a self-locking foaming mold

(1) Lower mold half; (2) upper mold half; (3) front of the mold; (4) slider, flap;

(5) swivel cylinder; (6) clamp cylinder; (7) product

90 1 Molds for Various Processing Methods

FIGURE 1.108  Self-locking mold for stationary use (source: Fa. Frimo Group GmbH, Lotte)

Self-locking molds, as well as foam molds inside the mold carrier, can be arranged stationary or moved circumferentially. Using circumferential units, a distinction is made between round tables (Figure 1.109) and general conveyor systems. Especially when using conveyor systems, the mold carriers are mostly specially designed to fit the needs of the unit. Circumferentially moved units are advantageous for the production of large quantities but also little or no variations. The stationary units are very flexible and have shorter downtime caused by defects or maintenance. The combination of circumferentially moved and stationary manufacturing can be useful to combine both advantages.

FIGURE 1.109  Unit with two round tables (2) and two stationary workplaces (1)

(Source: Fa. Frimo Group GmbH, Lotte)

911.3 Molds for Polyurethane Products

1.3.2.4.1  ClampingThe self-locking mold is held closed using a clamping device to avoid any movement of the upper and lower part. The strength and stiffness of these molds has to be designed to withstand internal pressures at simultaneously low deformations. In all other molds, the mold carrier takes over the function of clamping. A proven, cost-effective solution is to attach compressed-air hoses (fire hoses) below the lower mold half (Figure 1.110 and Figure 1.111), which are closed at the ends using terminal strips. A uniform distribution of force to the mounting surface can be ensured. The tolerances in the locking of the upper and lower part of the mold carrier can also be compensated by using hoses. To ensure a sufficient security against undesired opening of the mold, the pressure in the hoses has to be set to 3 to 5 bar. The mold carrier top has to be sufficiently stiff to keep the deformation as small as possible, so the mold remains tight.

b ba

c

d

e

FIGURE 1.110  Principle of clamping using compressed air hoses

(a) Lower mold half; (b) compressed air hoses; (c) hinge/locking device

mechanism; (d) guiding; (e) molded part

FIGURE 1.111  Compressed air hoses for an optimal distribution of the clamping forces

(source: Fa. Frimo Group GmbH, Lotte)

92 1 Molds for Various Processing Methods

1.3.2.4.2  Closing and Opening Mechanism, Demolding AidsDepending on handling requirements (placing of inserts, application of mold release agents, etc.) and also for reasons connected with the demolding of the part, there are several constructions for the closing and opening mechanism:

1. Swinging around a hinge (preferably for small parts)

2. Parallel opening and closing (preferably for flat parts, no direct intervention by the operator is necessary)

3. Parallel opening and then swinging (preferably for large, complicated parts that require manual work at the mold)

Despite the use of mold release agents, a certain amount of adhesion of the foam in the mold is unavoidable. Also, the linear shrinkage of the foam can hinder the part removal. With complicated shapes, this can lead to shrinkage onto the mold cores. There is also a vacuum between the mold and the molded part that must be overcome.

Demolding is facilitated if the areas vertical to the parting plane have a dra� angle of at least 1.5°. Pneumatic (air) ejectors protect the molded surface but the part could easily get wedged during ejection. Such air ejectors should therefore be used only at the most difficult points. To prevent the sticking of the reaction mixture with the air ejector, the ejector should be designed such that simultaneously with the retraction of piston the compressed air would demold the part (Figure 1.112).

With mechanical ejectors, the designer must consider the low compressive strength of the foam; therefore, the ejectors must have a large surface and must be placed in nonsensitive locations. Several independently driven mechanical ejectors are not recommended, because they would not move simultaneously.

a

b

c

c

FIGURE 1.112  Air ejector principle

(a) mold; (b) ejector housing;

(c) compressed air connections

931.3 Molds for Polyurethane Products

1.3.2.4.3  Fastening of InsertsBecause of the low viscosity of the PUR mixtures, any type of insert can be com-bined with PUR foam. Inserts do not have to be located in the parting line only. Steel reinforcements can be held with permanent magnets and with centering pins (Figure 1.113). Wires for later fastening of cover materials can be wedged into grooves on top of well-rounded, tapered pins and ribs.

1

5

1 2 3

4

a) b)FIGURE 1.113  Fastening of steel inserts

(a) Flat reinforcement (sheet metal); (b) wire insert;

(1) Mold; (2) permanent magnet; (3) locating/centering pin;

(4) sheet metal insert; (5) steel wire insert held in groove

1.3.2.5  Molds for Flexible PUR Foams [7, 8]

For automotive seat cushions, two different PUR mixtures are poured into the open mold, either simultaneously or one shortly a�er the other, to create foams of dif-ferent hardness to produce the so-called “dual-hardness seat cushion”. For comfort reasons, the sides of the seats are stiffer than in the center. At the beginning of filling the mold, the joining of the two reaction mixtures of very low viscosity is prevented by separation walls that create grooves between the sides and the center (Figure 1.114). These grooves are later required for fixing e.g. the textile seat cover.

1 2 1

3

FIGURE 1.114  Principle of mold for “dual-hardness seat cushion”

(1) Reaction mixture 1; (2) Reaction mixture 2; (3) separating wall “pillow pipes”

94 1 Molds for Various Processing Methods

70-90

0,7-1,0

8

a)

2 12

b)

FIGURE 1.115  Venting of molds for PUR foam

(a) Drilled into the mold top; (b) screw-type vent nipple

Vent holes are generally used in the production of automotive seat cushions. Typi-cally, vent holes are located in the flat or slightly convex mold lid at a distance of approximately 70 to 90 mm. Vents are small holes at the end of conical projections. They can be either drilled in the plane or convex lid made from cast aluminum or standard vent nipples screwed into the lid (Figure 1.115). The latter vent has the advantage that it can easily be exchanged for smaller vent holes when materials are changed or in case of damage due to poor handling.

Also, the mold lid can have a thinner wall thickness, which is more economical with regard to energy and mold cost. The vent holes must be sprayed with release agent a�er every cycle. The small projections within the vent must be manually removed to ensure proper venting for the next cycle. Venting in the parting plane should be preferred for economic reasons. The molds (which have these vents) for a bench and a single seat are shown in Figure 1.116.

FIGURE 1.116  Molds for automotive seat cushions:

bench and single seat

(Source: Fa. Hennecke GmbH,

Sankt Augustin)

951.3 Molds for Polyurethane Products

1.3.2.6  Molds for Semirigid Foams

High-quality, foamed automotive interior parts consist of three components: a skin or foil, which gives the molded part attractive optics; a rigid carrier, which gives the molded part its stability; and the foam (which is located in between), which ensures so� touch. The skins are actually pre-formed parts that are manufactured in a mold using PUR (spray, cast or slush skins, as well as polyvinyl chloride [PVC]) or in the thermoforming process using PVC, thermoplastic polyolefin (TPO), or other material. As a carrier, parts made from injection molded polymers or rigid PUR foams of higher density with long fibers or glass mats for reinforcement and reduction of thermal expansion are used. O�hen these parts are also reinforced with natural fibers or fiber mats.The production process starts with inserting the skin into the lower mold half and fixing the carrier to the upper mold half. The mold is brought into an ergonomically optimal position. The manual inserting of the skin has to be carried out with the utmost care to ensure that small radii can be precisely reproduced when foaming. A�erwards, the mold is moved into the foaming position and the mold cavity will be filled. The open as well as the closed mold filling are used. A�er the fast-closing process, the foaming and curing process begin. The air can escape through the circumferential gap between skin and carrier until the point where the seals are inflated (see Section 1.3.2.3). Through opening of the seals, gases can escape a�er the curing process. Finally, the molded part can be removed.

1

2

3

4

5

6

7

8

FIGURE 1.117  Cross section through a back-foaming mold with mixing head and film gate

(1) Mixing head; (2) aluminum casting body; (3) heating/cooling channels;

(4) ejector; (5) cavity; (6) upper mold half; (7) lower mold half;

(8) foldable mold – front side (source: Fa. Frimo Group GmbH, Lotte)

96 1 Molds for Various Processing Methods

FIGURE 1.118  Instrument panel – foaming mold with mold carrier

(source: Fa. Frimo Group GmbH, Lotte)

Figure 1.117 shows a mold for this application. The aluminum casting body has an epoxy resin coating which is simultaneously the cavity. This enables a cost-effective and high-quality mold surface without actually having to machine the surface. The aluminum body has inserted steel pipes with water as the heating and cooling agent. For easy and precise removal, large ejectors are installed in the lower mold half. A vacuum can be applied for fixing the skin to the lower mold half. For an optimum distribution of the vacuum, vacuum rooms, which are connected to the cavity through drilled small holes, are located in the lower mold half.

Figure 1.118 shows a mold carrier unit, mold and controlling device, as well as operating panel. The universal mold carrier is able to get into every necessary position for the different molds. The clamping is controlled using compressed-air hoses in the lower mold half (see Section 1.3.2.4.1).

1.3.2.7  Molds for Rigid PUR Foams

Foaming of refrigerators and freezers, storage water heaters, and vehicle super-structures with PUR rigid foam material is now the global standard.

The reasons for this are:

1. PUR rigid foam materials have the lowest thermal conductivity of all technically feasible insulation materials

2. The long-term adhesion of the PUR foam with the skin layers and therefore the resulting sandwich structures provide the composite with great intrinsic stiffness and partly load-bearing function.

971.3 Molds for Polyurethane Products

a d

b

c

FIGURE 1.119  Refrigerator foaming fixture (source: Fa. Hennecke GmbH, Sankt Augustin)

(a) Core; (b) side wall; (c) transfer axis; (d) front wall

Refrigerators/freezers are manufactured using foaming fixtures (Figure 1.119). The pre-formed housing of a refrigerator consists of a plastic interior container (inliner). The painted sheet steel side walls, the lid, and coated aluminum foils act as a diffusion barrier. For the rear wall, plastic foils, paper and sheet steel are used. This weak, hollow part is inserted into the foaming fixture using transporting systems (toothed belts). The core (mostly made out of cast aluminum) dives into the plastic interior container, either through lowering of the core or rising of the refrigerator housing.

Through demolding, the slightly tilted side wall supports are locked using tapering edges. The upper and lower mold half are mechanically or hydraulically pressed towards the side parts. Using an attached mixing head or manipulator-driven (movable) mixing head, the PUR rigid foam is filled into the now closed cavity. The filling opening is located in a nonvisible area (mostly in the front-facing compres-sor room) so that the reaction mixture has short flow paths and the mixing head is easily accessible.

When venting refrigerators, it is not the foaming fixtures that are vented but the cabinets in which the foam is injected. At the highest spots, vent holes of 2 to 3 mm diameter are drilled. Pieces of fiber fleeces are glued under the opening to avoid that foam will escape.

98 1 Molds for Various Processing Methods

FIGURE 1.120  Spindle drivers (source: Fa. Hennecke GmbH, Sankt Augustin)

Foaming fixtures for refrigerators and freezers are very large and can be adapted to different dimensions. The carrier operates the motion of the core, the four sur-rounding sides, and the movement of the completed housing. Because the pressures are low and the requirements are high for moving the large elements rapidly, syn-chronized screw drives (Figure 1.120) are used in combination with locking tapered wedges, which are especially useful for these mechanisms.

The core (Figure 1.121) as well as the four side elements consists of cast aluminum. The core and side walls are heated or heated and cooled. The side walls are ribbed to keep deformations low at a pressure of around 0.7 bar. The stiffness is calculated so that the deformation stays below 0.5 mm, that is for tightness reasons. Factor of safety against yield should be 2. Air ejectors are used for demolding of the core. Cores for these refrigerators are slightly conical. For fixing the interior container, the cores also have drilled vacuum holes and air ejectors (for demolding the cabinet). If the plastic interior container has undercuts, slides are attached to the support core (Figure 1.122).

In addition to the above described, stationary production of refrigerators, the circulat-ing transport systems as well, as more developed concepts are used [9]. The molds will pass a metering and mixing machine. Filling is done either from top or bottom.

FIGURE 1.121  Support core with a view inside (source: Fa. Hennecke GmbH, Sankt Augustin)

991.3 Molds for Polyurethane Products

a

b

FIGURE 1.122  Movable slide with support core (source: Fa. Hennecke GmbH, Sankt Augustin)

(a) Core in foaming position, slide is retracted

(b) Core in ejecting position, slide is extended

Molds for storage hot water heaters are more simply designed. The steel tank (water reservoir) is the core itself. The external shape is created with a two- or three-pieces foaming fixture. Similar to the production of refrigerators sheatings, the steel tank, the outer support wall, and the mold must be preheated to prevent faults in the foam structure and defects in adhesion to the walls.

Molds for refrigerator truck superstructures are flat, supporting structures. Because of the large size of the panels (up to 20 m2) and despite the low specific pressures created by foaming, there are large reaction forces in the mold support. The ease of handling of the supporting mold parts may not be affected by the necessary stiffenings caused by the large forces.

1.3.3  Molds for PUR Integral Skin Foams (Self-Skinning Foams)

1.3.3.1  Influence of Processing on Mold Design

The basic premises for the molding of a desired integral skin structure (a sand-wich made of solid skins and a microporous core) are filling of the cavity without entrapped air bubbles and faultless venting. This requires the application of the well-known rules for gating and venting, including the proper placing (orientation in space relative to the gravity) of the mold during foaming. The internal pressures during foaming are higher than for low-density PUR foams. Typical values are:

Density (g/cm3) 0.3 0.7

Internal mold pressure (bar) 3.0 7.0

1.3.3.1.1  Temperature controlThe mold temperature is extremely important to ensure the proper formation of the skin layers. According to the information of raw materials suppliers, the tempera-tures must be held within ±1.5 °C. Compared to the molds for foams of low density, the heat of reaction, which has to be removed from the mold, is considerably higher, as seen in Table 1.4.

100 1 Molds for Various Processing Methods

TABLE 1.4  Reaction Heat per Unit of Volume (kJ/m3)

Parameter a: absolute values, Parameter b: value relative to flexible foams = 1

Density (kg/m3)

50 500 1,100a (kJ/m3) b a (kJ/m3) b a (kJ/m3) b

Flexible 5,000 1 50,000 10 110,000 22Semi-rigid 6,25025 1.25 62,500 12.5 137,500 27.5Rigid 12,5005 2.5 125,000 25 275,000 55

The upper values for reaction heat, indicated in Section 1.3.2.1.1, are the base of these calculations. It becomes clear that the molds for rigid PUR foams are exposed to much higher levels than molds for flexible PUR foams. Furthermore integral skin products usually have higher wall thicknesses and simultaneously a lower thermal conductivity than solid or microporous parts. Therefore more heat needs to be transferred to the mold, the production is adequate longer. The mold temperatures usually range between 55 °C and 65 °C. Table 1.4 also points out that the good experience with PUR foams of lower density in molds, made from materials, which have only a moderate or low thermal conductivity cannot be applied to the parts of rigid PUR products in ranges of high densities.

1.3.3.1.2  Sealing at Parting Line, Ejectors, and Side Cores/SlidersThe clamping forces of mold carriers are dimensioned such that the pressure over the whole mounting plate is only 5 to 10 bar. The contacting surfaces between the upper and lower mold half in the parting plane are only a fraction of the mounting plate, which means that the contact pressure in the parting plane is of the factor of surface ratio higher. To increase the contact pressure further to ensure that the mold is held closed, the contact area can be reduced by a stepped parting plane (see Figure 1.123), which will create larger specific pressure on the parting plane. When designing, the surface pressures have to be included.Flat parts with sufficient dra� angle on the side do not require additional demold-ing aids. Flexible and semirigid integral skin foam parts can be ejected using com-pressed air. Rigid PUR integral skin foam products are ejected with mechanically or

a

b

FIGURE 1.123  Stepped parting plane

(a): 6–12 mm (steel), 12–20 mm (aluminum); (b): > 2 mm

1011.3 Molds for Polyurethane Products

hydraulically operated ejectors (Figure 1.124). Ejector pins are guided in hardened steel bushings. The fit is H 7/f 7.

During the first mold operation, the liquid mixture will penetrate the clearance between pin and bushing, filling the grooves. From then on, the foamed material will act as a seal. In a case in which several ejectors are used, cocking can be avoided by mounting all ejectors on a common, guided ejector plate.

The aforementioned groove sealing principle is also used with these elements. For the preferentially used molds out of aluminum, the ejectors, core pullers, and slides are manufactured out of steel, including their bushings, and inserted into the “so�” mold.

1.3.3.2  Gating

Proper design and location of the gate ensures filling of the mold with the reaction mixture without entrapped air bubbles. Air bubbles must be avoided because they increase in size during foaming and rise to the surface caused by buoyancy. They can create pin holes or lentil-shaped air enclosures at or near the surface of the part. This can create rejects or requires costly post-molding finishing work at least.

The high-pressure mixing head supplies the reaction mixture from a circular opening with a velocity of magnitude 10 m/s. In the runner system, the velocity can be reduced and the reaction mixture can be additionally mixed. The liquids in the gate should be directed towards the lower mold half.

Of the many methods of gating, two methods are highlighted, the dam gate with throttle and the direct gate [4, 5, 11]. The dam gate in Table 1.5 shows a proven design to inject the reaction mixture at the parting line. The dimensions in this table are valid for a mixing head with a discharge tube diameter of 10 mm. For other diam-eters, the dimensions have to be adapted, wherefore additional tables are available.

When the mixing head is attached to the mold, it is important that the jet pump effect of the flowing mixture does not suck in air through a leaky joint. Further-more, the reaction mixture should not be injected as a free jet into the mold cavity (Figure 1.125). The flow should at least keep contact to one cavity wall, preferred is the later visible part surface.

FIGURE 1.124  Typical ejector with sealing grooves

102 1 Molds for Various Processing Methods

TABL

E 1.

5  D

imensi

ons

for

Dam

Gate

wit

h T

hro

ttle

[1

1]

Mix

ing

head

di

a met

er

[mm

]

Volu

me

flow

[c

m3 /s

]

Thro

ttle

dim

ensi

on

[mm

]Di

men

sion

s fo

r the

gat

e

[mm

]Di

men

sion

s fo

r the

ga

te

[mm

]

Gate

/fil

ling

velo

city

[m

/s]

Thro

ttle

vel

ocity

[m

/s]

1525

35a

Qb

de

e1f

gh

kL

rR

ls

v

10

200

2.8

21.

65.

07.

515

.010

.10.

60.

510

013

01.

31.

61.

2

325

3.9

2.7

2.2

100

130

1.3

2.6

2.0

450

4.9

3.4

2.7

100

130

1.3

3.6

2.8

575

5.9

4.1

3.2

100

130

1.3

4.6

3.5

700

6.8

4.7

3.7

100

130

1.3

5.6

4.3

1031.3 Molds for Polyurethane Products

FIGURE 1.125  Air pockets created by a cross-sectional jump

Diameter control piston

FIGURE 1.126  Schematic of direct gate (fountain flow)

Direct gates should only be used if the wall thickness of the molded part is low and a visible mark of the mixing head outlet tube on the part surface is acceptable. There will also be no problem with turbulence and air inclusions if the wall thickness does not increase suddenly in the area of the already-reduced flow velocity (Figure 1.126).

The good flow characteristics of PUR systems also permit the use of multi-cavity molds in which the single cavities are filled one a�er the other by first filling the thinner-walled parts. The connecting runners must be dimensioned sufficiently large to prevent throttle effects in the flow (pressure drop, high flow velocities). The parallel filling of several cavities with different geometry requires a balanced flow design for the individual flow paths [11].

1.3.3.3  Venting

The foaming reaction mixture must fill the entire cavity. All air, present in the cavity before the beginning of the filling stage, has to be displaced by the foam, so that the part will not show any molding defects, even in the area of vents.

104 1 Molds for Various Processing Methods

Venting is done as a general rule in the parting plane at the highest location. Fre-quently, the shape of the part will not permit complete venting at the parting plane. In this case, auxiliary parting planes with drag plates will be required (Figure 1.127). This adds considerably to the cost of the mold and requires a higher amount of mold release as well as additional cleaning of the part. The product designer should be made aware of such increased costs at the time the part resp. mold is designed.

The best method of venting is to grind vent slots 10 mm wide and 0.05 mm deep into the parting plane of the lower mold half. These slots connect the cavity with a vent channel that is vented to the outside (Figure 1.128). The pressure drop for such a kind of venting is low, and if required, additional slots can be ground to reduce the pressure drop further during start-up operation. Because the mixture at the end of its flow path is already more viscous, the flat slots prevent that the foam will flow through them. At higher internal pressures small flashes can build up. But they can be easily removed and because of the thickness of only 0.05 mm they will not affect the integral skin.

a)

b)

FIGURE 1.127 Mold with auxiliary

parting plane and drag plate

(a) Mold closed; (b) mold open, position

for the part removal and application of

the mold release

a)

b)

10

4

8

0,05

1

2

FIGURE 1.128: Venting at the parting plane

(a) Section; (b) Top view

(1) slots, 10 mm wide and 0.05 mm deep;

(2) vent channel, 8 × 4 mm or semicircular

1051.3 Molds for Polyurethane Products

1.3.3.4  Molds for Flexible Integral Skin Foams

Molds for flexible, PUR integral skin foams are required in the production of motor cycle seats, sun visors, and simple seat upholstery. These large molds usually consist of two parts. For suitable venting, molds for annular headrests require auxiliary parting planes that can be mounted with hinges on one of the mold halves.These molds are usually made from aluminum, o�en with engraving. For high pro-duction rates and intricate surface configuration, electrodeposited shapes (nickel/copper) backed with cast aluminum are used.For filling, the open-mold filling method is used. A�er filling and closing, the mold is swung into the venting position. Because of the sensitivity of the foam to shocks during foaming, the swinging is usually done hydraulically at low accelerations. Venting is facilitated through the parting plane.

1.3.3.5  Molds for Semirigid Integral Skin Foams

Typical applications for semirigid PUR integral skin foams are steering wheels, armrests, spoilers, beer barrel sheathings, and especially shoe soles. The quality of the mold has to be high because all surfaces of the parts are visible. The methods of gating and venting must be carefully considered. It is advantageous to gate with a fan gate from the bottom and also to vent in the parting plane at the highest point of the mold. These solutions are preferred in the manufacturing of steering wheels and annular armrests (Figure 1.129).Unless it is not possible to use an attached mixing head or open-mold filling never-theless the mold must be filled through a fan gate at the lowest point. Such filling is done like a casting process downwards in the parting plane. The conus, in which the mixing head has to be docked, the runner and fan gate are placed in the parting plane and have to be demolded together with the part.

a

b

FIGURE 1.129  Typical mold for a steering wheel, filling, and venting

(a) Venting = 0.1 to 0.2 mm; (b) gate = 0.1 to 0.3 mm

106 1 Molds for Various Processing Methods

FIGURE 1.130  Sport shoe mold (source: KLÖCKNER DESMA Schuhmaschinen GmbH, Achim)

(a) Head stamp; (b) frame; (c) injection channels (runners); (d) sha�;

(e) swivel joint; (f) last

FIGURE 1.131  Shoe sole production unit

(source: KLÖCKNER DESMA Schuhmaschinen GmbH, Achim)

Simple shoe molds made from cast aluminum or tin alloys are filled from the top in a casting process. In case of the closed mold filling (Figure 1.130), the two-parted frame is closed (for filling) and sealed to the sha�.

The mixer outlet docks onto the gate area and injects the reaction mixture into the molded cavity. A�erwards, the head stamp moves from the bottom in sha� direction until the set point of the wall thickness of inner or walking sole is reached. The head stamp overruns the runner of the gate area and closes the cavity.

An impression of a shoe sole production unit with a rotary round table machine, mounted mold carriers, release-agent spraying unit, and the dosing unit for the PUR reaction mixture, is shown in Figure 1.131.

1.3.3.6  Molds for Rigid Integral Skin Foams

Large size is a characteristic for this class of products. Weights of more than 35 kg, with a density of 0.5 g/cm3 may require mold cavities of 70 l (e.g., housing for music player). The mold out of cast aluminum for a 2 m long deck chair is typical from such possible lightweight construction of a mold (Figure 1.132).

There is a great danger that such large molds will warp if the temperature is not uniformly distributed. The mold carrier is not strong enough to overcome such warping. It is therefore important to control not only the temperature of the actual cavity but of the whole mold to prevent leaks caused by warpage at the parting plane.

1071.3 Molds for Polyurethane Products

FIGURE 1.132  Mold for deck chair (source: Fa. Formenbau Eck GmbH, Rastatt)

e

a

d

c

b

gf

FIGURE 1.133  Schematic of mold for window profile

(a) Upper mold half; (b) lower mold half; (c) locating pins; (d) reinforcement

profile; (e) cooling channels; (f) cavity filled with air; (g) PUR-sheathing

For manufacturing of reinforced PUR profiles (up to 7 m long), metal profiles are used (Figure 1.133). Their alignment is done using locating pins, which are inserted in opening direction of the mold.

The gate is placed at the front side. The reinforcement profile is at its ends closed by plugs. The reaction mixture flows a�er a few mm directly onto the plug at the front side and is then radially deflected.

1.3.4  Molds for Microporous PUR Products

Microporous reaction injection molding (RIM) products are usually thin walled (2 to 5 mm thick) and can be compared to injection molded thermoplastic products. The RIM mold can be built lighter because the internal pressures are smaller: 10 to 20 bar and 50 bar in gate area. It is important to ensure that the mold is well sealed against leaking of the very low-viscosity reaction mixture, which can penetrate into gaps

108 1 Molds for Various Processing Methods

as small as 0.05 mm. Temperature control, gating, and venting are similar to those shown for integral skin foams. The explanations in Sections 1.3.3.1 to 1.3.3.3 can be transferred in consideration of their properties. For microporous RIM systems, the filling pattern can be determined and optimized using computer simulations with respect to gate position, flow lines, etc. [4, 5].

1.3.4.1  Molds for Flexible, Microporous PUR Products

The RIM process (for this polyurethane class) is used in the manufacture of technical products and window glass frames. The closed-mold filling technique is used. The principle of molding seals for vehicle windows is shown in Figure 1.134. The steel mold fits the contour of the circumference of the window. The sealing of the mold is done with silicone rubber. Distance pins are used to properly locate the glass in the mold, or else with the essential window frame geometry to manufacture the seal.

a b c

FIGURE 1.134  Schematic section of a window seal mold

(a) Safety glass; (b) rubber seal; (c) molded PUR seal

1.3.4.2  Molds for Tough, Rigid, Microporous PUR (RIM) Products

Applications for hard RIM products are mainly large automotive parts, such as front and rear bumpers, and rocker panels (see Figure 1.135). The most frequently used materials are polyurethanes reinforced with milled short fibers, which are processed in the so-called RRIM process (Reinforced Reaction Injection Molding). The short fibers increase the stiffness and decrease the thermal expansion in the case of temperature changes. They are abrasive, therefore the molds are made from steel (in small series also aluminum).

During RRIM, fibers will orient in the flow direction reaction mixture. Therefore, the shrinkage [4, 5] in the direction of the flow is less compared to the one at perpen-

1091.3 Molds for Polyurethane Products

dicular to the flow. For this reason, the gate position and the final mold dimensions should be determined by mold filling and shrinkage analysis. Particularly, if the product has ribs and radiator grills, it is important to determine the flow patterns by a mold filling analysis already in the design stage to avoid later problems with insufficient venting and flow lines at undesired locations.

To estimate the flow pressure losses at the gate to the end of the filling cycle, the following formula for microporous RIM and RRIM parts with small changes in wall thickness has been proven successful:

2

212 lp

s t� �

��

� ���

where:�p Flow pressure at the end of a filling cycle in the gate;� mean viscosity over the filling time;�l longest flow path;s mean wall thickness; and�t filling time.

1.3.4.3  Molds for Rigid, Microporous PUR Products

Also molds for rigid, hard microporous products are characterized by their small wall thicknesses (2 to 5 mm). By taking advantage of the good flow properties, these molds can be used for very large parts. For the mold design, also the recom-

b

a

c

d

e

ce

a

FIGURE 1.135  Mold for automotive rocker panel: Length: 2,300 mm, wall thickness: 2 mm

(Source: Fa. Modell- & Formenbau Wilhelm Funke GmbH & Co, Alfeld/Leine)

(le�) picture; (right) cross section;

(a) Upper mold half; (b) fan gate; (c) lower mold half with ejector; (d) mounting

plate with mixing head; (e) swiveling side section for molding undercuts

110 1 Molds for Various Processing Methods

mendations explained in Sections 1.3.3.1 to 1.3.3.3 are valid. The equation for the flow pressure in the previous section can here be used as well.For unfilled mixtures, the integral mold pressures are generally < 10 bar. Therefore, also for these products molds out of aluminum are preferred. For glass fiber mat reinforcements, the pressure may be as high as 50 bar because the glass fiber mats create a higher flow resistance. This is particularly the case for a high percentage of glass and long flow paths. The high filling pressures are the reason, that partially the open-mold filling technique is used. For the necessary mixing head movements and paths a robot is used. Within thin walls, there is less risk of air entrapment compared to integral skin foams.Typical examples for very big molded parts, are body parts of agricultural vehicles like harvesters or choppers. Parts of this type can be 3.6 m × 1.8 m × 0.7 m big and can weigh up to 40 kg.In the future, weights around 100 kg should be possible. Integration of functions is also an outstanding feature for these molded parts. This shows that body parts as well as mounting possibilities for extra equipment (e.g., headlamp housing and opening devices) are state of the art [12].Depending on the complexity of the part, the molds can have a large number of sliders. In Figure 1.136 showing the mold for a harvester roof, eight sliders are necessary in the front area where the deep undercut in the area of the fourth slider needs a very stiff construction. The surface (of the lamp area) located above Slider 4 is etched, and the black-dyed PUR system does not need another coating.

1.3.5  Molds for PUR Casting Systems

PUR casting systems are bubble-free reaction mixtures which will be cured to solid parts. The molds only have to withstand the hydrostatic pressures of the poured mixture and can therefore have very thin walls respectively low stiffness.

1 345

7

6

8

FIGURE 1.136  Mold for harvester roof

(le�) slide in retracted position; middle: slider in extended position;

(right) schematic cross section;

(1–5) slide; (6) lower part of the mold; (7) upper part of the mold; (8) cavity

(source: Fa. PESTEL PUR-Kunststo�echnik, Chemnitz)

1111.3 Molds for Polyurethane Products

Molds for cast elastomers operate at temperature between 90 and 120 °C, according to the data supplied by the raw materials supplier. Variations of mold temperature should not exceed ±1.5 °C.Small molds (mostly those made out of steel) generally have no own heating system. Therefore they are put on a heated casting table. The hot molds are manually operated.The centrifugal casting technique is applied for small series when (for quality reasons) the hot curing PUR elastomers have to be used. The reaction mixture is dosed into the wedge-shaped parting plane and the single cavities are filled through milled runner channels in the parting plane.Molds for polyisocyanurate (PIR) casting systems need a homogeneous temperature control. To achieve optimal material properties, the mold temperature should be around 80 to 90 °C. Injection is done using a direct gate from the deepest location of the mold cavity. If this is not possible, the mold is filled from the top through a runner channel that is located in the parting plane and ends at the deepest location of the gate. Cast aluminum is used for these mostly large molds.

References

[1] Maier, U., Wirtz, H.-G., Fietz, J., Frahm, A., Rüb, T., Polyurethan-Schäumanlagen In Kunststoff-Maschinenführer 4th ed. (2003) Publisher: Friedrich Johannaber, Carl Hanser Verlag, Munich

[2] Uhlig, K., Polyurethan Taschenbuch 3rd ed. (2006) Carl Hanser Verlag, Munich[3] Oertel, G., Polyurethane, Kunststoff-Handbuch 7th ed. (1993) Hanser, Munich[4] Michaeli, W., Brüning, D., Ehbing, H., Simulationsso�ware zur Unterstützung der Poly-

urethan-Fertigung In Polyurethantechnik 1998 (1998) VDI-Verlag GmbH, Düsseldorf[5] Wulf, P., Formfüllberechnungen für dünn- und dickwandige kompakte Formteile aus

Polyurethan In PUR 2002 (2002) VDI-Verlag GmbH, Düsseldorf[6] Freser-Wolzenburg, T., Unterdruckgeregeltes Schäumen von Polyurethan In Polyure-

than 2007 (2007) VDI-Verlag GmbH, Düsseldorf[7] Sulzbach, H. M., Technik der rationellen Großserienfertigung von PUR-Formteilen für

den Automobilbau Polyurethane World Congress (1987) Technomic Publishing Co., Lancaster/Basel, pp. 240–247

[8] Bayer, H. G., Vorrichtung zum Hinterschäumen von mit Textil-Flächengebilden herge-stellten Formteilen, insbesondere Sitz- und Rückenlehnenpolster In Polyurethane World Congress (1987) Technomic Publishing Co., Lancaster/Basel, pp. 574–580

[9] Berthold, J., Flexible Anlagen- und Verfahrenstechnik für die Herstellung von Kühl-möbel. In Polyurethan 2007 (2007) VDI-Verlag GmbH, Düsseldorf

[10] Boden, H., Maier, U., Schulte, K., Polyurethan-Integralschaumstoffe/Technologie der Herstellung In Polyurethane, Kunststoff-Handbuch 7th ed. Oertel, G. (Ed.) (1993) Carl Hanser Verlag, Munich, pp. 356–368

[11] N. N., Angussauslegung und Verteiler in der PUR-Verarbeitung Technical information, Bayer MaterialScience AG, Business Unit Polyurethane (2006) Leverkusen

[12] Pestel, K., Brüning, D., Mit großflächigen Polyurethanteilen auf der ÜberholsPUR In Polyurethan 2005 (2005) VDI-Verlag GmbH, Düsseldorf

112 1 Molds for Various Processing Methods

 ■ 1.4  Blow Molds

O. Eiselen

1.4.1  Process Description

1.4.1.1  Different Types of Blow Molding Processes

Blow molding is a collective term for the manufacture of hollow thermoplastic prod-ucts, characterized by the principle that a preform is blown to its desired shape in a blow mold. Blow molds have their origin in the glass industry.There are two different processes:1. Blowing in the flowable thermoplastic temperature range, and2. Stretch-blowing in the stretchable thermoelastic temperature range.

It is understandable that there is more freedom for the creation of intricate shapes in the thermoplastic temperature range than in the thermoelastic range. However, as a result of biaxial orientation, processing in the thermoelastic range yields products with higher strength, and with better barrier characteristics, transpar-ency, and brilliance.There are two distinct methods of processing:1. Creating a preform and transporting it while still hot to the blow mold (one-stage

process), and2. Reheating a previously made, now cold preform to blow temperature and then

transporting it to the blow mold (two-stage process).

A further distinction essentially refers to the method used to make the preforms. From a number of one-stage methods (I), those three having the largest market share for blown products are listed: � Extrusion blow molding, � Injection blow molding, and � Dip blow molding.

In the two-stage process (II), preforms are made from sheet or tubing or are injec-tion molded. There is no major distinction between the type of blow molds used for the two-stage process and those used for the one-stage process [1].Extrusion blow molding is by far the most important method, with reference to both the size of the product and the amount of plastics processed. With today’s technology, pieces with volumes from 1 cm3 to 10 m3 can be blown. This enormous size range does include packaging, storage, and transport containers, but it mainly

1131.4 Blow Molds

includes articles for household and technical use. Although injection blow molding and dip blow molding use a blow method very similar to that of extrusion blow molding, they are restricted to rather unsophisticated shapes and sizes from 10 ml to 10 l. These methods are used mainly for smaller packaging items or for techni-cal parts with injection molded end portions like e.g. axle boots. The majority are rotationally-symmetrical products.

1.4.1.2  Extrusion Blow Molding Technology

In extrusion blow molding, a tubular preform (parison) is placed into the blow mold, where, with the use of internal pressure, it is shaped into the final product. There are two basic methods for producing the tubular preform, either continuous extrusion or intermittent extrusion.

1.4.1.2.1  Continuous ExtrusionIn continuous extrusion, the parison is carried by the blow mold itself to the blow station (shuttle-type blowing machine; see Figure 1.137). Alternatively, the parison is cut at the extruder, and with a grabbing mechanism, transported to the blow mold (Figure 1.138). The transport can be done by a robot. More than one mold can be serviced this way by one extrusion unit. The method of continuous extrusion (up to moldings of 100 l volume) is limited by the sagging strength of the melted plastic, that is, by the length of the extruded section and the required cycle times, < 120 s. Plastics with higher melt strength allow for longer cycle times than those with lower melt strength.

Continuous extrusion is also advantageous for coextrusion. Different materials are extruded as multi-layer composites to achieve the necessary part properties, for

FIGURE 1.137  Schematic of a shuttle-type blowing machine (manufacturer: Krupp Kautex)

(1) Extruder drive; (2) extruder; (3) extruder head with die; (4) blow and

calibration station; (5) blow mold in blow station; (6) finished part; (7) linkage

to move the clamping mechanism and to shuttle the blow mold from the blow

station to the extruder head; (8) clamp, in tie barless construction; (9) height

adjustment for extruder and head

114 1 Molds for Various Processing Methods

example, oxygen barrier for mayonnaise containers or as a hydrocarbon barrier in plastic fuel tanks.

The constant pressure of the continuous extrusion enables the manufacture of materials with different viscosities and compressibilities into an optimal plastic tube laminate. In the accumulator head system (intermittent parison generation), pressure differences of materials with different viscosities and compressibilities would lead to layer thickness problems.

1.4.1.2.2  Intermittent Parison GenerationWith this method, the extruder also works continuously, but the melt is stored in a tubular piston accumulator (Figure 1.139) from which the plastic is intermittently extruded to produce the tubular preform. Figure 1.140 illustrates an extrusion blow molding machine equipped with an accumulator head as it is used, for example, in the manufacture of plastic fuel tanks, barrels, liners, and other similar large structures. The removal of the finished part is done with grippers and a transfer mechanism. The accumulator heads work best with the first in, first out (FiFo) principle. This is necessary in order to accommodate for the limited thermal stability of the plastic.

Blow molding machines with accumulator heads are used for parts with volumes from 30 l to 10,000 l and with shot weights from 500 g to 250 kg. Accumulator heads are also used for plastics with lower melt strength, such as polyamide, low density polyethylene (LDPE), polystyrene, and polycarbonate, as well as for a large group of engineering plastics (alloys).

FIGURE 1.138  Extrusion blow molding machine with preform grapper

(manufacturer: Krupp Kautex)

1151.4 Blow Molds

FIGURE 1.139  Accumulator extrusion head with tubular piston;

melt distribution is improved by overlapping

melt flow over heart-shaped surfaces, first in,

first out (FiFo) principle

FIGURE 1.140  Schematic of an extrusion blow molding machine with accumulator head;

bridge construction

(1) Clamp in open position; (2) hydraulic power pack; (3) extruder motor;

(4) extruder; (5) accumulator head; (6) supporting frame; (7) supporting

columns

116 1 Molds for Various Processing Methods

1.4.1.2.3  Parison GenerationIn extrusion blow molding, the basic shape is a circular tubing. The parison is formed either in a mandrel head, with torpedo heads or spiral distributor heads [2]. To improve the quality of the parison in mandrel heads, overlapping flow and heart-shaped, milled curves are introduced. With torpedo designs, a simple torpedo support is used for polyvinyl chloride while offset supports are used for polyolefins. The spiral distributor head generates the tubing over spiral grooves with a throttle gap. For an improved operating performance, a heart-shaped curve is introduced as a pre-distributor for material feeding.

The dimensions of the extruded tubing are dependent on the geometry of the extrusion die, the extrusion velocity, the melt temperature, and the type of plastic. Depending on the product, the extrusion die may have to be profiled [3]. A more expensive method is to change the gap between mandrel and die by changing the mandrel position using programmed hydraulic actuators. Hydraulically variable gaps are used mainly in the manufacture of the following products:

� Canisters with recessed handles (from 10 l), � super handling drums (Figure 1.141), and � plastic fuel tanks with vey irregular geometries (Figure 1.142).

Generally, with extrusion blow molding, the extrusion die is designed so that the gap between mandrel and die ring converges smoothly toward the orifice. This is

FIGURE 1.141 Clamp unit with a barrel mold;

the mechanism to change the extrusion slot,

using hydraulic actuators with an adjustable

mandrel, is visible on the extrusion die

FIGURE 1.142:  Plastic fuel tank

1171.4 Blow Molds

necessary to maintain a clean flow of plastic when adjusting the wall thickness of the extruded shape. This applies both to extrusion dies with internal and external cones (Figure 1.143).

The orifice angle of the cone can significantly influence the flow anomalies that are frequently found with high molecular polyethylene (PE). In such cases, changing the melt temperature has little effect. Computer programs are available to calculate the distribution of shear rate in the extrusion flow channel. It is important to ensure that the critical shear rate occurs near the die exit resp. is confined to as narrow a region as possible. With the accumulator head, the effects of flow anomalies can be influenced by the extrusion velocity. To achieve the shortest cycle times with continu-ous extrusion, the plastic usually exits from the orifice at close to critical shear rate.

1.4.1.2.4  Different Blow Up MethodsThree methods of blowing the products are used: injecting blow pin, calibrating blow pin, and blow needle. With the injecting blow pin method, the mandrel enters the mold and forms the neck of the product (Figure 1.144). This motion is assisted by preblowing and takes place with closely controlled speed. The cutting sleeve (D2) is larger than the opening in the striker plate (D1) so that the blow mandrel comes to rest on the striker plate, thereby cutting the plastic. This is important for fully automatic deflashing of the product. With the calibration mandrel method, the blow mold closes on the mandrel. This method is usually used for containers with internal threads or when the opening of the container must be even with the surface of the product.

Blow needles are usually used when the article does not have a large opening such as in toys or flat products (e.g., blown cardboard boxes with a needle diameter of ≤ 5 mm) and when the blowing area is not allowed to be in the mold parting plane (Figure 1.145 and Figure 1.146).

FIGURE 1.143  Different extrusion geometries

(I) Extrusion die with internal cone; (II) Extrusion die with external cone

(1) Guide for mandrel; (2) mandrel; (3) die holder; (4) die ring

118 1 Molds for Various Processing Methods

FIGURE 1.144  Blow pin and blow mold opening

(1) Cutting bushing; (2) blow mandrel point; (3) stripper for neck flash;

(4) preblow tubing; (5) cutting insert; (6) cutting ring (striker plate)

FIGURE 1.145  Different products

made in blow needle process

The needle blow method is also used for squeezed canister handles. A separated blast chamber is necessary so that no filling material can sediment (environmental protection). It is therefore important that the area where the needle enters is less and regularly stretched to ensure clean, reproducible production.

FIGURE 1.146:  Blow mold twister

board with blow

needle

1191.4 Blow Molds

1.4.1.2.5  Special ProceduresIn recent years, special procedures in the area of extrusion blow molding for technical parts established themselves increasingly. Mechanical requirements and requirements for reducing the permeability or fluidic requirements do not permit any pinch areas (weld lines on the blow-molded part). To avoid pinch areas, 3D-extrusion blow molding and suction blow molding were introduced and further developed. In both processes, a blow needle is used for inflation and a second blow needle is used for jetting (airexchange). In 3D blow molding, the thermoplastic pipe (using a blow mold and a robot for inserting) is manipulated so that a pinch area is just seen at the beginning and the end of the article (scrap-free processing) (Figure 1.147 and Figure 1.148). In the suction blow process, the pipe is sucked into the closed mold and is blown inside. Squeeze-off sliders close the preform on the top and the bottom before blowing (Figure 1.149). Therefore, parts without weld lines or lateral pinch area can be manufactured.

FIGURE 1.147  3D mold (lower mold half) for fuel-filler pipes

FIGURE 1.148  Fuel-filler pipe manufactured with the 3D parison manipulation;

Article coextruded in a 6-layer form

FIGURE 1.149  Suction blow mold

120 1 Molds for Various Processing Methods

1.4.2  Extrusion Blow Molds

1.4.2.1  Mold Construction

The construction of the blow mold depends on the geometry and the volume of the product, on the required production, and on the degree of automation. Similar to the selection of the plastic, the construction must be looked from the point of view of productivity, process, and economy. Depending on the number of pieces required, blow molds can be classified into prototype and production molds.

1.4.2.1.1  Prototype Blow Molds Made from Cast ResinA model of the product is placed into the frame. Copper cooling tubing is placed around the model, and the frame is then filled with resin. This method is recom-mended only for a limited production quantity and only for simple shapes. Care must be taken not to exceed the compressive strength of the resin.

Advantages:

� Short-term delivery is possible

Disadvantages:

� Models must be made with shrinkage allowance � Limited production � Productivity is not possible to be optimized

1.4.2.1.2  Prototype Blow Molds with Metal-Coated Model and Metal-Filled Cast Resin

This method is similar to the method described in the earlier section, but the model is covered with metal, and the resin is filled with metal.

Advantages:

� Inexpensive for first visual acceptance of products � Short-term delivery is possible

Disadvantages:

� Models must be made with shrinkage allowance � Changes are virtually impossible � Difficult to optimize productivity

1.4.2.1.3  Blow Molds Made from Cast MetalsA preferred material for the mold is a high-quality zinc alloy. With experienced operators, such metal can be cast virtually free from distortion. With simple cooling, such a mold is inexpensive, satisfies all demands, and does not need profile milling.

1211.4 Blow Molds

Advantages:

� Optimization of the product is possible � Quality of the mold cooling can be customized � Pinch-off edges can be optimized � No upper limit production run � Changes are possible

Disadvantages:

� Requires more times and is therefore more expensive � Complex model work (shrinkage model, division construction, casting model) is necessary

� For accurate parts, the cavity has to be copy milled. This has be taken into con-sideration when casting

1.4.2.1.4  Milled Prototype MoldsThe prototype mold (for technical parts made from aluminum) has established itself in the area of continuously improved CAD/CAM technology (for mold making).

Advantages:

� A mold that is manufactured using data offers a dimensionally stable product, which is important for fit components

� Article and mold changes are faster and more easily realizable (product optimi-zation)

� Short product cycle times because manufacturing can be done using 3D models (no need for intermediate models)

� The mold cooling is slightly worse than the one in production blow molds � The number of parts is not limited � The mold can be used as a replacement for production blow molds

1.4.2.1.5  Production Blow MoldsProduction blow molds are always made from several parts. Figure 1.150 illustrates a typical blow mold for a packaging product. Movable mold sections can also be used to relieve deep undercuts or to compress specific areas of the product (i.e., to create articles without weld lines). Figure 1.151 shows a blow mold for a fuel tank, with various side cores to compress some areas and to release undercuts in the product for ejection. Materials used for extrusion blow molds:

� Steel � Forged aluminum � Zinc alloys

122 1 Molds for Various Processing Methods

FIGURE 1.150  Typical blow mold construction

(1) Alignment dowel; (2) pinch insert; (3) head piece; (4) pinch relief;

(5) cutting ring (stricker plate); (6) backing plate; (7) mold base; (8) cavity;

(9) vent slots; (10) blow mold guide pins; (11) blow mold bottom; (12) cooling

manifold

FIGURE 1.151  Blow mold for plastic fuel tank

(a) Lower portion; (b) Upper portion

1231.4 Blow Molds

� Aluminum cast � Special brasses � Copper alloys � Combination of these materials

Before the most suitable materials can be selected, the cost, product, and process requirements must be checked out. Table 1.6 lists some of these materials used in blow molds. Molds for large volumes (> 60 L) are manufactured using either cast zinc or aluminum. For molds with a volume > 1,000 l, aluminum should be used for weight reasons. The pinch-off edge areas are mostly made from steel (for casting molds or molds from forged aluminum) to make deflashing easier or to automate it and to increase the die life. In the last 5 years, the CNC-milled production blow mold (made from aluminum) has established itself. The aluminum industry has developed new materials for this application (Table 1.6).

TABLE 1.6  Materials for Extrusion Blow Molds

Mold materials Density Specific heat capacity

Thermal conductivity

Modulus of Elasticity

Tensile strength

� Cp � 20°C � Rm

kg/dm3 J/(g · k) W/(m · K) N/mm2 N/mm2

Steel 1.2311 7.7 0.46 36 210 · 103 950–1050

Steel 1.4122 7.7 0.46 30 220 · 103 1000

Steel 1.4301 7.9 0.50 1 5 200 · 103 500–700

Aluminum 3.4365 K, AlZnMgCu 1.5

2.83 0.89 130–160 70 · 103 530

Cast aluminum veral 226 2.76 0.88 110–130 75 · 103 160–200

Zinc Casting Zamak Z430 6.7 0.44 98–105 130 · 103 120–250

Elmedur HA 2.1285 8.8 0.42 209 118 · 103 690–890

Copper 2.0090 8.9 0.39 396 110 · 103 200–260

1.4.2.2  Construction Guidelines

1.4.2.2.1  Alignment of Blow MoldDrill bushings and hardened pins are frequently used. The engagement should not be more than the diameter of the pin (1 × D). This is very important with molds used in shuttle machines so that the parallel guides of the clamp and the mold alignment will not be damaged. The fit between pin and bushing is F7/g6. It is important to clear the bore behind the bushing so that any flash or dirt may be removed easily to ensure undisturbed closing of the mold.

124 1 Molds for Various Processing Methods

1.4.2.2.2  Cutting EdgesDuring mold closing, the pinch-off edges precut the product and the flash. The edges must be designed to achieve a good seam quality. The quality is dependent on the following parameters:

� Closing speed � Wall thickness of parison � Preblow air pressure � Melt temperature � Geometry of the pinch-off edge � Plastic/raw material

Figure 1.152 shows a section through a common design of a pinch-off edge. The area of the pinch relief (X) is frequently ribbed to increase the surface area (flash cooling).

The following guidelines for pinch-off edges apply:

� �

� �

� �

� �

� �

� �

� �

1

2 1 2

1 1 1

2 2 2

1 2 1

2 2 2

2 3 2

0.3 mm 2.5 mm

0.2 0.5

0.4 1.0

1.0 2.0

1.5 6.0

1.0 3.0

1.5 2.5

b

s t s

s t s

s t s

s b s

s b s

s t s

1s average thickness of product wall near pinch-off area (mm)

2s average parison wall thickness in pinch-off area (mm)

b1 width of pinching edge

b2 width of pressure zone

t1 depth of pressure zone

t2 depth of flash relief

t3 distance of ribs

The shape of the pinch-off edge has a significant influence on the clamp force required to precut the flash. Table 1.7 gives typical values of the clamping force (in N/cm of edge), depending on the plastic used.

Pinch-off edges are critical, in particular, for coextruded parts with wall structures of different materials, as well as for products made from Selar® RB (DuPont).

1251.4 Blow Molds

However, for all applications there are suitable geometries for the pinch-off edges with sufficient strength (Figure 1.153). The seams of coextruded products must ensure that the multi-material seam is properly layered and undisturbed [4].

FIGURE 1.152  Section through pinch-off area

(b1) width of pinching edge

(b2) width of pressure zone

(t1) depth of pressure zone

(t2) depth of flash relief

(t3) distance of ribs

X ribbed area of the

pinch relief, to increase

the cooling area of the

flash relief

I cavity

�1, �2 transition angles

TABLE 1.7  Specific Clamp Force for Full Automatic Deflashing (Guide Values)

Raw material Specific clamp force Pspec N/cm pinch-off lengthHDPE 900 ÷ 1800

PP 1200 ÷ 1500

PVC 1200 ÷ 1800

PET 1500 ÷ 2500

FIGURE 1.153  Schematic illustration of various pinch-off edge

geometries

(I) Conventional pinch-off edge; (II) coextrusion

pinch-off edge; (III) pinch-off edge for Selar RB

(1) Flash pocket; (2) cavity

126 1 Molds for Various Processing Methods

1.4.2.2.3  Clamp StopsBlow molds should be provided with stops on surfaces outside the cavity area to prevent damage to the pinch-off edges during setup or when clamping the mold without plastic. These stops protect the mold against excessive clamp force; they also ensure parallel clamping and increase the mold life.

1.4.2.2.4  VentingTo improve the surface appearance, blow molds must have optimal venting. Gener-ally, blow molds are vented at the parting plane, both in longitudinal and at traverse planes. Vents have a depth of between 0.05 and 0.15 mm. To improve venting and facilitate cooling, molds for polyolefins are also sandblasted. The plastic and the size and shape of the product define how fine the sandblast finish has to be. Sometimes, etching of the surfaces is also useful for venting. The surfaces of amorphous, non-colored plastic must be polished to achieve the desired transparency and brilliance of the product. Sandblasted cavities for amorphous plastics (e.g., PC or PET) cause matte surfaces. If the product permits, micro sanded cavity surfaces are preferred because they produce a more homogeneous surface appearance than polished surfaces would.

To improve the article geometry, molds are tempered. Some examples are molds for spoilers made from polycarbonate-acrylonitrile butadiene styrene (PC-ABS) blend, water bottles made from PC, luggage made from polypropylene (PP), and so on. Blow molds using fluorine gas for in-line blowing must be especially well vented. Because of the partial pressure drop and the resulting diffusion behavior of the F2N2 mixture, a certain concentration of F2 is unavoidable. This may result in deposits in and on the blow mold. It may also cause problems at the seams and at the inserts. In molds for F2 blowing, the venting channels should be connected with a vacuum system. The use of slit holes on vent pins in critical areas is also useful for venting [5].

In general, engravings should be vented. This should require that every letter is vented with a hole. The hole size should be ≤ 0.4 mm for high-density PE and ≤ 0.3 for PP. These holes are especially useful for etched surfaces with very small etching depth.

1.4.2.3  Blow Mold Cooling

Figure 1.154 shows cooling methods used in blow molds. Depending on the mold material selected, any of the methods shown can be used in the same mold.

Drilled Cooling ChannelsDrilled cooling channels are used for steel and forged aluminum and for mold inserts. To achieve good cooling in the molds, it is important that the channels are as close to the surface as possible and closely spaced. In aluminum molds, greater distances than in steel are possible because the heat conductivity of aluminum is about five times higher than that of steel (Table 1.6).

1271.4 Blow Molds

Milled Cooling ChannelsMilling is used when intensive cooling by drilling channels is not achievable, for example, in the shoulder portion of blown bottles, recessed bottom parts, neck rings, and in preform molds for stretch blow molding.

In milled cooling channels deflection aids (plugs on flow plates) are used to make sure that the fluid has intensive contact with the channel walls to ensure optimal cooling.

Cast-In Tubing for Mold CastingCopper tubing is used in cast zinc molds. With the use of corner-shaped fittings, the tubing can be laid out closely spaced and in proximity to the cavity wall. Figure 1.155 shows a cooling cage with two cooling circuits for the blow molding of a barrel. The distance between the cooling circuits is 40 mm. The tubing is 10 mm distant from the cavity wall. The cast zinc mold, combined with the copper tubing, has very good heat transfer characteristics for mold cooling.

Aluminum castings require steel tubing; it is not possible to use copper because of the relatively high melt temperature of aluminum. The steel cage is more expensive to make and has inherently poorer heat transfer characteristics.

FIGURE 1.154  Schematic of various cooling methods for blow molds

(I) Drilled cooling channel; (II) milled cooling channel; (III) cast-in cooling tubing

(1) Product; (2) cavity block; (3) cooling channels; (4) cooling tubing

Arrows indicate heat flow

128 1 Molds for Various Processing Methods

FIGURE 1.155  Cooling cage for barrel mold made from copper tubing, with two circuits per

mold half. In the areas near engravings, the cooling channels are offset.

FIGURE 1.156  Stainless steel cooling cage for plastic fuel tank blow mold in which areas of

sliders and mold inserts are circumvented

Figure 1.156 shows a stainless steel tubing cooling cage for a fuel tank mold. The areas of sliders and mold inserts have been circumvented.

To achieve the most economical molding cycles, the cooling circuits must be arranged so that the maximum possible amount of heat can be removed in the shortest time. However, the molding cycle is limited by the heat conductivity of the plastic material itself.

In general, there is a quadratic dependence on the product wall thickness. This is based on the Fourier relationship (a constant for nonstationary cooling processes in which Fo numbers indicate equal cooling capabilities of the article).

k2Fo

a ts�

�

tk cooling times wall thicknessa heat conductivity

1291.4 Blow Molds

The temperature of the coolant is of great importance to the cycle time because of the influence of the dew point in the area of the blow mold. If the coolant temperature in the mold is below the dew point, condensation will be created, and correspond-ing flaws will appear in the surface of the product. The dew point can be reduced if the mold area can be air conditioned (dry air) [6].The size of the cooling channel depends on the size of the product. It is important to achieve turbulent flow with minimum pressure drop. There are other possibilities for reducing the cycle time: � Internal cooling with cooled blow air (cycle reduction up to 15%) � Cooling a�er blowing in special fixtures (cycle reduction up to 50%) � Intensive cooling using an immersing device (under water post-cooling), mostly used for filler pipes and fuel tanks (cycle reduction up to 40%)

� Reduction of other cycle time elements, such as an increase in the dry cycle speed of the machine. In modern machines, this possible area of improvement is usually exhausted. A greater reduction may cause problems with the wear and life of the system.

1.4.2.4  Accessories for Blow Molds

FixturesFor secondary operations a�er blowing, it may be necessary to hold the product precisely in place. In such case, the blow mold is combined with a handling fixture (Figure 1.157). The molded product is inserted in the fixture either by the motion of the mold or with the product transporting grabber.

Blow PinThe blow pin (Figure 1.144) serves to blow the shape, to form the neck, and to remove heat, because up to 40% of the heat can be removed with circulating blow air.

Stripper PlateThe stripper plate (Figure 1.144) is required to strip the flash surrounding the blow pin as it retracts. Without this stripper, part of the flash may remain on the blow pin or fall onto the product and cause trouble in the process.

Parison ToolingThe proper size parison, which is formed by the extrusion die (die, core, and core receiver), determines the geometry of the product. In addition to the geometry of the extrusion die, the actual shape of the parison is affected by the geometry of the formation of the parison, the extrusion velocity, material composition (e.g. master-batch or regrinded material), the processing temperature, and if used, support air or preblowing.

130 1 Molds for Various Processing Methods

FIGURE 1.157  Blow mold with handling fixture and trimming die

X1 Horizontal stroke

X2 Vertical offset

R Swing radius

(I) Trimming station; (II) Blow station; (III) Extrusion head station

(1) Blow mold; (2) fixture; (3) flash trimming die; (4) extrusion head

Adjustment SleeveThe adjustment sleeve is used to protect the blow pin and the blow mold during setup of the mold. The (bronze) sleeve is placed into the blow opening of the closed mold. This sleeve makes it possible to center the blow pin with the blow opening of the mold (radial centering). The axial adjustment is then done with the cylinder stroke.

1.4.2.5  Integrated Postmolding Processes

1.4.2.5.1  Postcooling with a Cooling FixturePostcooling fixtures are used to better utilize expensive blow molding units (Figure 1.158). For this purpose, either a large clamp that can accommodate both the blow mold and the fixture or a separate clamping unit for the postcooling fixture is required. Such a separate clamp must be locked to prevent opening of the fixture by the blow pressure. To reduce the cooling time significantly, this pressure should be > 6 bar. With this method, cooling cycles can be reduced by as

1311.4 Blow Molds

much as 50%. With postcooling systems, other cycle time elements have no undue influence because during these times the temperatures in the product walls are being equalized [7].

Postcooling fixtures are usually constructed with movable portions to ensure that the insertion and removal of the product can be properly achieved. In the fixture in Figure 1.158, the lower mold section is removable.

Preferably, aluminum castings are used with drilled or milled cooling channels. Care must be taken to clear the flash area to ensure that the fixture can close properly. The cavity dimensions must have reduced shrink allowances; that is, the cavity of the fixture must be smaller than the blow cavity.

1.4.2.5.2  Manufacturing the Finished Product in the Blow Molding MachineBecause of rising costs, the processor is forced to fully automate the whole process. It is not enough anymore to just automatically deflash the product. More and more o�en, the cutting, drilling, milling, stamping, and marking operations are integrated into the process (i.e., everything takes place during the blow molding cycle). The products are completed within the blow mold with a minimum of operations. Each operation (step) is directly connected with costs. The following are some of the postblow machining methods:

Postblow Machining in a Separate OperationThe secondary operation can be synchronized with the blowing machine or can be independent of the blow molding cycle. Both solutions are expensive and involve labor costs, and they only make sense for small production quantities, when older blow machines are used, or in cases in which the process is prone to frequent breakdowns.

FIGURE 1.158  Open postcooling fixture for the manufacturing of 32-gallon garbage containers

132 1 Molds for Various Processing Methods

Advantages: � If blowing and secondary operations are not synchronized, they will not affect each other

� Postblow shrinkages (i.e., dimensional variations in the machined product are small)

Disadvantages: � Separate product handling is required � Separate controls are required for the secondary operation � Separate safety provisions are required � When the secondary operation begins, the blown products are already cold. Process variations (shape and dimensions of the product) in the blow process can have a negative effect on positioning the product for secondary operations.

Secondary Operations in the Blow MoldThis technique is limited to cutting and stamping and to in-mold labeling (IML) of packaging products.Advantages: � Quality of cutting is good with so� plastics (low- and medium-density PE, PP, TPE)

� The number of processing steps is reduced by using IML

Disadvantages: � Handling of products and flash, and their separation, are difficult � Changes of location of cut or stamp operations are very costly � Mold changes are very expensive � It is frequently difficult to achieve optimal cooling � IML increases cycle time because of added handling time and poorer cooling

Secondary Operation within Handling FixtureThe handling fixture carries the product into the secondary operation station by either the mold motion or the action of a transporter. There, the flash is first trimmed, and the other operations are carried out, such as drilling, milling, and cutting.Advantages: � Nearly all postmolding operations can be integrated � Cost-effective solutions are possible � This system does not prohibit changes to be made in the product design � Positioning is easily modified � Separation of flash and product is simple

1331.4 Blow Molds

Disadvantages: � The quality of cut requires greater cutting edge clearance than in-mold cutting. This can be improved through optimal positioning of the handling fixture.

Combined Secondary OperationsIt is worthwhile to investigate the advantages of combining any of the above post-molding operations, such as the in-mold methods, the method of using handling fixtures, or the use of separate, independent operations. The advantages and dis-advantages are outlined above. Figure 1.159 shows a blow mold for the complete manufacture of an air duct. Note that the unit is equipped with quick connectors for fast mold change. The machine control is encoded to ensure precise correlation to the mold and the sequential program.

FIGURE 1.159  Blow mold for an air duct, with a handling fixture for secondary operations

1.4.3  Injection Blow Molding and Dip Blow Molding

In the injection blow molding, the injection molded preform is carried on the mold core into the blow mold cavity and is blown. Figure 1.160 shows the index table of an injection blow mold machine, with separate stations. The weight of the product and its wall thickness are based on the shape of the injection mold cavity. The weight of the product can be only minimally adjusted by varying the injection parameters. The material used for the injection mold is steel. To improve the filling process, the mold surface is o�en chemically treated [8].With dip blow molding, the blow pin dips into the dip chamber [9]. A�er the neck ring is filled, the blow pin is pulled back. Axial variations in parison wall thickness can be achieved by programming the motion of the dip chamber piston. Figure 1.161 shows the production of the parison. The parison is then taken over by the blow mold, and the product is blown. A�er that, the blow pin is withdrawn, and the blow process is continued with an auxiliary blow pin.

134 1 Molds for Various Processing Methods

FIGURE 1.160  Injection blow molding machine with injection, blow and ejection station

(Upper le�) work table, turning horizontally in the parting plane

(Upper right) injection mold with parted multi-cavity mold

(Lower le�) blow unit

(Lower right) ejecting and stripper of products

FIGURE 1.161  Principle of parison forming for dip blow molding

(a) Blow pin dips into dip chamber; (b) neck ring sits on top of dip-chamber;

(c) neck is filled; (d) blow pin and dip chamber pin move simultaneously;

(e) dip chamber pin motion ends; (f) knife cut ends forming of the preform

1351.4 Blow Molds

The blow molds for injection blow molding and for dip blow molding can be made from aluminum because there are no cutting edges required (for venting; see Section 1.4.2.1).

1.4.4  Use of Computers for Blow Molding

The use of computers in blow mold design is generally accepted. In the area of product design and for CNC (Computerized Numerical Control) programming, three-dimensional (3D-CAD/CAM) systems are preferred.

In the workshop, 3D data can be transformed into 2D drawings as seen in Figure 1.162 and Figure 1.163. Also, all dimensions can be derived from the 3D model in the mold shop.

The driving force in introducing 3D CAD/CAM systems was the automotive industry. Automobile manufacturers develop their products primarily with 3D techniques. Therefore, continuous collision testing and synchronizing of interfaces with certain peripheral assembly groups are possible. The data is presented to the sub-supplier using appropriate interfaces. This enables a permanent data exchange with the sub-suppliers.

Low wage countries (Far East) force the Western European mold making market to an increase of machined production and automation of the work processes. This is only possible with a clever CAD/CAM application.

The delivered article data is transformed into milling programs from the model makers or used by the mold makers for the mold manufacturing. The direct use of article data in a blow mold requires considerable process engineering experience.

FIGURE 1.162  3D mold in shading view

136 1 Molds for Various Processing Methods

FIGURE 1.163  Drawing of the mold (from Figure 1.162) using the 3D mold data

A 1 : 1 model is used as an intermediate stage. This can be helpful because the process development engineers are mostly inexperienced in blow molding and intermediate article optimizations (for process engineering and economic reasons). The model can be useful for blow molding because:

� The time investment can be reduced, � The production quality can be increased, � The partition process can be easier determined using a 1 : 1 model (optimized mold split surfaces),

� Critical areas in process technology can be easier determined. and � 1 : 1 models can be used for installation tests.

Also useful are simulation programs for the blow molding process [10]. These continuously optimized programs can be used as a form of expert system (e.g., the process engineering process is mathematically written and given to the program user). 3D molds and article data are necessary to use simulation programs.

The 3D article data can easily be used to specify reference points for inspection of the products and to program any measuring machines. The 3D CAD design forces the user to describe the shape of the products in a geometrically exact form. Other-wise, the model making and 3D mold making are not possible.

1371.4 Blow Molds

For packaging products, it is relatively easy to determine the necessary filling volume with 3D design methods and then iteratively calculate the necessary outside dimensions of the product. This minimizes errors in volume and dimension that can cause substantial costs for reworking. Also, the geometry of the article can be easily determined. The expected article mass derives from the wall thickness in the middle and the density of the material.

Advantage of CAD:

� It is possible to describe the product exactly � Errors are eliminated due to data transfer and process transparency increases � Design changes are easily made and the overall effort for major changes is reduced � Any desired values can be extracted for dimensional checking of models, molds, and products

� Quick access to information for all people involved

The higher expenditure of a 3D construction is usually compensated by the cost advantages gained when the models are used.

References

[1] Fritz, H.-G., Systematische Verfahrensdarstellung angewandter Blasformtechnologien In Technologien des Blasformens (1977) VDI-Verlag, Düsseldorf

[2] Eiselen, O., Konzepte für Coextrusions-Blasformanlagen, Kunststoffe 78 (1988) 5, pp. 385–389

[3] Daubenbüchel, W., Qualitätssicherung und -überwachung an Extrusionsblasform-maschinen, Kunststoffe 72 (1982) 5, pp. 250–256

[4] Eiselen, O., Verfahrenstechnik beim Coextrusions-Blasformen, Kunststoffe 78 (1988) 7, pp. 589–591

[5] Kulik, M., Vom Vorformling zum Blasteil In Blasformen von Polypropylen (1980) VDI-Verlag, Düsseldorf

[6] Egle, W., Vermeiden von Schwitzwasser auf Spritzgieß- und Blaswerkzeugen, Kunst-stoffe 76 (1986) 1, pp. 32–34

[7] Groh, M., Variantenreich: Nachkühlen beim Blasformen reduziert die Produktions-dauer deutlich Maschinenmarkt 97 (1991) 9, pp. 46–49

[8] SKZ-Seminar, Erodieren, Polieren und Beschichten von Werkzeugen für die Kunst stoff-verarbeitung (1993)

[9] Technologien des Blasformens (1977) VDI-Verlag, Düsseldorf[10] Sti�ung. Dr. R. H., Simulation des Extrusionsblasformens Blasformen und Extrusions-

werkzeuge May 6th (2005)

138 1 Molds for Various Processing Methods

 ■ 1.5  Molds for Thermoforming

P. Schwarzmann

1.5.1  General Information

In thermoforming, the material (semifinished product) is clamped to the outer edge and stretched a�er heating. The wall thickness of the finished product is given by the stretch ratio between the manufactured surface and the starting surface.The contact with the shaping mold during ejection is mostly single sided. The cooling of the shaped component is done from one side through contact with the mold and on the other side through free or forced air cooling.Figure 1.164 shows the basics of thermoforming (mechanical pre-stretching in combination with ejecting using vacuum). In most cases, the shaped product is separated from the clamping edge. Trimming is either done in the thermoforming unit in a combined shape-punch mold or in a separate following station.In the thermoforming process, waste-free shaping of parts is also possible. For example, the clamping edge stays as a part of the finished drawn part.The finished product and the connected requirements, such as geometry, tolerances, number of parts, stackability, stiffness, and temperature resistance determine the mold design, the material choice, temperature control of the mold, the necessary forming process, and the thermoforming machine needed.The wall thickness distribution of the molded part is mainly determined by the mold and the forming process.The form definition (surface reproduction of the mold contour) is determined by the material temperature during ejection, the temperature of the mold, and the resulting contact pressure between the material and the mold surface [1].

(1) Assisting plug

(2) Mold segment

(3) Air channel

(4) Thermoplastic material (semifinished product)

(5) Air collecting channel

(6) Upper clamp frame

(7) Lower clamp frame

(8) Segment carrier plate

(9) Vacuum connection

FIGURE 1.164  Schematic diagram of thermoforming

1391.5 Molds for Thermoforming

1.5.2  Process in Thermoforming

The basic process in thermoforming is:

a) Clamp the material

� between two clamp edges, � between two mold halves, or � between 1 clamp edge and the mold;

b) Material (semifinished parts) should be heated to forming temperature

� mostly with electric heaters, or � contact heating plates;

c) Pre-forming through

� pneumatic pre-stretching, � mechanical pre-stretching with an assisting plug, or � mechanical pre-stretching with a mold itself;

d) Forming using

� vacuum (vacuum forming), � compressed air (pressure forming), � vacuum and pressure forming, and � partially combined with mechanical stamping, squeezing, calibrating of sur-faces parts;

e) Molded part should be cooled before demolding through

� contact with the mold, and � cold air (cooling air fan and air showers);

f) Demold the molded part, and

g) Following steps, such as punching, stacking, printing, and filling.

In every process step, the mold and the format parts (for the process needed) are always involved.

1.5.3  The Mold and the Format Parts

Every part except the mold segments, which are necessary in a specific forming machine and are replaced a�er every mold exchange, are called format parts.

The mold segment, together with the format parts, comprises the mold package.

140 1 Molds for Various Processing Methods

A mold consists of more parts (depending on the thermoforming unit):

� The mold (so-called “form segment”) on which the part is supposed to be molded. In the case of high order size, it can be a multi-cavity mold if the forming surface and the forming machine will allow it.

� An assisting plug is necessary for the case that the material has to be pre-stretched to improve the wall thickness distribution. The negative mold with the stretch ratio drawing depth to diameter (width) of more than 0.25 always requires the use of an assisting plug. The assisting plug is attached on the machine table, on the opposite side of the mold.

� The clamping edges clamp the material for the forming process (in case the semi-finished part cannot be clamped between the two mold halves during pre-forming). � In roll-fed automatic thermoforming machines, the clamping edges are always format parts, which mean that depending on the size of the form segment, the size of the clamping edge changes.

� For sheet processing machines, almost all machine manufacturers offer adjust-able edge clamps. These are thus machine parts and not format parts.

� A segment carrier plate is necessary in positive molds when the surface of the material (the width of the clamping edge) has to be bigger than the outer contour of the positive form mold. The positive form mold is attached to the segment carrier plate. The segment carrier plate is always positioned (for molding) in the clamping level of the clamping edge.

� The mold substructure is the connection between the mold (segment carrier plate) and the machine table. Every machine manufacturer has its own machine specific requirements to the mold substructure. � Mold substructures are mostly format parts. That means, depending on the size of the form segment, different mold substructures are necessary.

� There are also sheet processing machines (ILLIG) with adjustable mold sub-structure. These are machine parts and not format parts.

� Temperature control plates (cooling plates) are used for indirect cooling of the form segment (if it cannot be tempered directly). � Cooling plates are mostly format parts. � There are also sheet processing machines with coolable substructure. These are machine parts and not format parts.

� All of the format parts, including the ones in other stations of the thermoforming unit (preheating station, punching station, stacking station), are included in the mold package. These are needed to produce with the mold in a specific machine.

1411.5 Molds for Thermoforming

1.5.4  Positive or Negative Forming?

Positive forming (Figure 1.165):

� The outer contour of the mold is shaped like the inner contour of the thermo-formed part.

� Pre-forming is done through pre-stretching with the mold itself or pneumatic pre-stretching (if the machine is equipped well enough).

� In the bottom area, during demolding, demolding and restoring forces act in the same direction in the material.

Negative forming (Figure 1.165):

� The inner contour of the mold is shaped like the outer contour of the molded part. � A mechanical pre-forming has to be done using an assisting plug. � In the bottom area, demolding and restoring forces act in reverse direction in the material.

Note that drawings for thermoforming parts have to be dimensioned to the contact surface of the mold.

Schematics of positiveforming

Schematics of negativeforming

FIGURE 1.165  Positive and negative forming in comparison

(a) thicker area; (b) thinner area; (X) shaping dimension

1.5.5  Design Guidelines for Thermoforming Molds

1.5.5.1  Material Choice

Wood � Maple, beech as massive wood, for molds for sampling; � Plywood as carrier plates, base plates; and � Solid wood (phenolic-resin-bonded beech plywood, “phenolic plywood” as a con-struction material)

Wood molds should not be painted. As a molding aid, so� soap can be applied.

142 1 Molds for Various Processing Methods

Advantages of wood:

� easy and fast machining, � no shrinkage problems, and � sufficient stiffness even for bigger parts.

Disadvantages of wood:

� very bad thermal conductivity (i.e., long cycle times), � not suitable for heating and cooling, � changes in dimension, “wood lives”, � venting bore holes get clogged up very fast, and � only suitable for small series and prototypes.

ResinResin molds are produced in multiple variations:

� The resin compound is filled into the “negative” of a model(Suitable for volume up to 2,000 cm3).The “negative” can be made from the following materials: plastic (e.g., thermo-molded components), gypsum, wood, and silicone rubber.

� Resin surface layer with backfilling or laminate structure(Quality is adequate for sampling).The negative of a model is coated with a resin surface layer; as an alternative a laminate can be applied. Then, a porous stamping mass is stamped in, and depending on the size, backfilled and stabilized.

� Resin-coated cast iron front.First, a core is produced as a carrier for the front resin layer. The carrier is made of a porous stamping mass. The core is then connected to the negative or to the base model. The face casting is the concept of filling the cavity (as seen in the metal casting) with the resin compound.

� Resin molding with resin spray coatingThe resin mold is sprayed with two-component paint. This surface has a “coarse sand-blasted” structure due to spraying, which ensures for good air extraction. This procedure is relatively rare, and only a few firms are proficient in it.

Advatages of resin:

� high reproduction accuracy (high repeatability), � easy and fast to work with, � no shrinkage problems, � mostly sufficient stiffness.

1431.5 Molds for Thermoforming

Disadvantages of resin:

� very bad thermal conductivity (e.g., long cycle times), � not suitable for heating and cooling, � decreasing stiffness with increasing temperature.

AluminumAluminum is the most commonly used material for thermoforming molds. The advantages of aluminum as a mold material are its good thermal conductivity and workability. Specific alloys help meet the stiffness requirements. Modern computer-aided milling techniques allow small series of this material. Large-volume molds are made from aluminum blocks cast under vacuum.

Ceramic precision casting for aluminum molds should only be used when a milling process is not economical. It should not be forgotten that the cast models have to be manufactured in the milling process.

Production of aluminum-ceramic precision casting:

� Manufacturing of a model, mostly out of wood, � Manufacturing of a ceramic casting mold out of a model, � Mold manufacturing through casting of a high-quality aluminum alloy into the ceramic mold.

Advantages of aluminum:

� high reproduction accuracy (high repeatability), � relatively easy and fast to work with, � sufficient stiffness, � excellent thermal conductivity.

Disadvantage of aluminum:

� mostly expensive for prototype manufacture.

SteelSteel is mostly used in a combination of form and punch mold (due to its stiffness and hardness).

Application examples include plates and blanking punches and guide pins.

Special materials for thermoforming moldsAir-permeable plate materialAn air-permeable plate can be an epoxy resin filled with aluminum grit. The advan-tage of this material is that no bore holes have to be drilled. The disadvantage is the low stiffness and the relatively bad thermal conductivity.

144 1 Molds for Various Processing Methods

Nickel-galvanosHigh-quality and also expensive molds can be manufactured using the porous nickel-galvanos. In the special procedures, porous galvanic molds with thicknesses up to 2 mm can be manufactured. These molds are backfilled, and if necessary provided with temperature control tubes.

Copper alloysIn special cases, high-strength copper alloys (e.g., copper-beryllium alloys) are used to achieve very short cooling times.

1.5.5.2  Molding Shrinkage

When manufacturing the mold, the process shrinkage of the thermoplastic material has to be taken onto account.

Values for the process shrinkage of hot forming are similar to the values for injec-tion molding. Values for the process shrinkage from the material manufacturer are always averaged values.

The process shrinkage of a thermoformed part is dependent on:

� forming temperature of the material (residual stresses a�er heating), � production temperature of the mold, � demolding temperature of the drawn parts, � the linear expansion coefficient of the semifinished part (plastic foil or plate), � the linear expansion coefficient of the mold.

When a material is sufficiently heated and the drawn part has everywhere the same demolding temperature, the drawn part will have equal process shrinkage. However, this is a problem because of the different demolding temperatures, mainly because of the different wall thicknesses of the drawn part.

For molded parts with tight dimensioning tolerances, prototype parts using test molds have to be manufactured to determine the process shrinkage in different positions and in different directions. A uniform quality of the material has to be maintained.

Example: drinking cupFor the most part, the edge and the bottom areas are thicker and the side wall thinner. The different demolding temperatures have different process shrinkages. The thicker areas have a higher process shrinkage.

1451.5 Molds for Thermoforming

1.5.5.3  Dra� Angles

Positive MoldA positive molded drawn part shrinks to the mold. The colder the drawn part during demolding, the larger the shrinkage tension following the temperature dependent length changes (from freezing temperature to demolding temperature).

When demolding, the molded part is released from the surface of the mold using compressed air. Then, the mold is removed from the finished part. The larger the dra� angle, the faster it can be demolded.

Dra� angles in positive molds:

� It should not fall below 3°. � 0.4° is still usable in connection with a slow demolding. The difficulty in small dra� angles is to adapt the demolding volume air stream to the demolding speed without deforming the molded part.

� 0° is almost impossible and never possible in large-scale production. Care must be taken to ensure that the demolding temperature of the molded part as well as the temperature of the mold is as close as possible to the solidifying temperature of the plastic material. This guarantees that the drawn part is not shrinking to the mold during demolding.

� Small areas can also be demolded with negative dra� angles (undercuts). During this process, the molded part can be deformed or stretched. When overstressing, traces can be found on the finished part (e.g., stress whitening). (Bigger undercuts can be demolded with the aid of loose parts).

Negative MoldA negative molded drawn part shrinks away from the wall of the mold.

Dra� angled in negative molds:

� 3° is generally recommended. � 0° is not recommended but are realized in rare cases. The difficulty is to adapt the demolding air volume stream to the demolding speed without deforming the drawn part.

� Multi-cavity negative molds behave like positive molds during demolding. Grained surfaces are more difficult to demold than smooth surfaces. The deeper the grain, the larger the dra� angle has to be.

1.5.5.4  Radii

Due to the demolding pressure that occurs during demolding, radii in the negative areas are more difficult to demold than in the positive areas.

146 1 Molds for Various Processing Methods

a) Material prestretched c) Material during prestretching

b) Material during final shaping d) Material during final shaping

FIGURE 1.166  Schematic of the resulting demolding pressure

(le�, a and b) Positive mold; (right, c and d) negative mold

Figure 1.166 shows a schematic of the resulting demolding pressure as contact pressure between the formed material and the surface of the mold. Definitions are in Figure 1.166:

Positive (+) surfaces can be demolded without any problems; these are mold sur-faces on which the restoring force of the material part and the demolding pressures, through the formation of a vacuum, act in the same direction. Radii are easy to shape in (+) areas.

Negative (–) surfaces present more demolding difficulties; these are mold surfaces on which the restoring force of the material and the demolding pressures, through the formation of a vacuum, act in the opposite direction. Radii are harder to shape in (–) areas.

A bad contact in the radius area means:

� bad heat conduction, � deformation due to different demolding temperatures, and � bad reproducibilty.

Reference values for radii (R) in negative areas, for a middle forming definition and for vacuum forming of material with a thickness of more than 1 mm are:

� �1.5 material thicknessR

Minimum radii of 0.2 to 0.5 mm are possible with almost all thermoplastic materials (independent from the thickness of the material in the beginning). Prerequisites

1471.5 Molds for Thermoforming

are a material temperature close to the melting point, a relatively small stretching, and a high mold temperature.

For transforming conditions, height to width ratio of about 1 : 1, the radii have to be demolded when under 1.5 times the thickness of the material by compressed air.

Small radii can easily be realized in areas where demolding and reaction forces add together (e.g., upper edge on positive molds). Radii in areas where demolding and reaction forces work against each other should always be chosen as large as possible.

1.5.5.5  Surface Roughness

The surface roughness influences:

� gliding behavior between mold and material, � air flow to the exhaust between mold surface and material, and � the transparency of crystal-clear parts.

The best transform results can be achieved with slightly rough (e.g., sandblasted) surfaces. Surfaces that are too smooth hinder the air exhaustion between mold and plastic. This leads to air trapping or to circular or wave-like markings. Surfaces that are too rough can lead to agitated mold surfaces and do not allow the plastic to slide easily.

Edges, over which the plastic has to glide when forming, can be roughened in the slide direction of the plastic part. Edges of high positive molds can be polished. Sur-faces can be polished when no air can be trapped. An exception is the manufacture of crystal-clear parts because an air trapping is mostly wanted.

Crystal-clear technical parts are manufactured best using skeleton molds without using vacuum or compressed air. Skeleton molds are molds which are built like a scaffold on which only the contour forming edges are shown.

If the molded part cannot be produced using a skeleton mold, molds are made from poorly conducting materials, like wood and resin, and are o�en coated with a so� textile material (e.g., glove material). If aluminum is used, the mold has to be heated to a high temperature, the surfaces have to polished, and venting bore holes in the visible surfaces should be avoided. Bad venting, with little air trapping between the mold surface and the finished part, guarantees crystal-clear surfaces on the finished product.

1.5.5.6  Assisting Plug

Material requirements for the assisting plug include:

� no quenching of the heated material when contacting, � good sliding and friction behavior to the heated material,

148 1 Molds for Various Processing Methods

� sufficient thermal resistance, � sufficient mechanical stiffness, � good workability (milling, turning, and polishing).

Plug materialsWood is the most widely used assisting plug material in sheet processing machines. The best sliding ability can be achieved with solid wood (e.g., maple wood). It is recommended that one pay attention to the fiber direction, as shown in Figure 1.167. Plywood should not be used for plugs due to the occurrence of markings. A lamina-tion of wooden plugs with a so� material (e.g., glove material) decreases the quench-ing and improves the sliding properties. Wooden plug should be used without any lamination or they can wear really fast.

Hardened felt (felt with deep penetrating wood primer) is more expensive than wood: the laminating with glove material is not necessary anymore.

Synthetic foam is an almost universally used assisting plug material and is mostly used in roller machines.

Resins (e.g., polyurethane resin), talc-filled with the better sliding capabilities, are used especially in vacuum machines, when the appearance of the assisting plug is very complicated.

Metal, mostly aluminum, as an assisting plug material is used for the following applications:

� when the mechanical loading is too big for wood or resin, � when the assisting plug should have the contour of the frame or should work as a hub, and

� when heated metal assisting plugs are just used in specific cases due to the special effort for temperature control and heating.

Polytetrafluoroethylene (PTFE) (e.g., Teflon) can be used in special cases as an assist-ing plug material when the material at transforming temperature is very sticky on the surface.

FIGURE 1.167  Fiber orientation when manufacturing an assisting plug out of wood

(a) motion direction of the plug; (b) fiber orientation (solid wood);

(c) bonding (if necessary)

1491.5 Molds for Thermoforming

Polyoxymethylene (POM) is used as an assisting plug material for crystal-clear molded parts.

Determination of the assisting plug contours for negative moldsThere are no “easy” mathematic formulas for calculating the assisting plug contours.

The first step can be calculated as follows:

� The contour of the assisting plug can be determined from the contour of the mold. � The distance a between the contour of assisting plug and the contour of the mold is calculated.a = 1.5 · material thickness + xx = 1 to 3 mmx = 1 mm for material thickness 0.2 … 1 mmx = 3 mm material thickness > approximately 6 mm

� Assisting plug radii R in the bottom area, see table below. � The contours that are recommended (distances and radii) for upper plug give the initial contour, which can be corrected (if needed) when the mold is retracted.

� For slow-running machines (sheet processing machines), it is enough that the bottom area have the needed contours.

� For fast-running machines (automatic roll machines), the assisting plug should have the entire contour (including the ones on the side walls) so that during the stretching process, the excess air can be released (underneath the drawn part). Therefore, mold specific interfering edges have to be taken into account.

� The general approach in the changing of a plug contour is: � increase the radius R, � decrease the diameter if necessary.

Assisting plug diameter,

mm

Material thickness, mm

< 1 1 2 3 4 5 6 7 8 9 10

10 1 1 1 20 1 2 2.5 3 3 3 30 1.5 2.5 3 4 4 4 4 4 40 2 3 4 4 4 4 4 4 4 4 50 2.5 4 4 5 5 5 5 5 5 5 5 60 3 4 5 5 5 5 5 5 5 5 5–6 70 3.5 5 5 5 5 5 5 5 5 5–6 5–6 80 4 5 5 5 5 5 5 5 5–6 5–6 5–6 90 4.5 5 5 5 5 5 5 5–6 5–6 5–6 5–6100 5 5 5 5 5 5 5–6 5–6 5–6 5–6 5–6

150 1 Molds for Various Processing Methods

Determination of the assisting plug contour for positive moldsIn positive molds, assisting plugs are mainly used:

� to avoid webbing on the edges in the bottom area, and � as a demolding aid to mechanically support the drawn part during demolding (while inserting air).

1.5.5.7  Venting Design

The number and diameter of venting bore holes and the width of the venting slots have to be dimensioned sufficiently large to allow a fast venting (exhaustion). On the other hand, the vents should not leave any markings at the molded product.

Venting bore holesBore holes of 0.4 to 0.5 mm are used for:

� negative molds with very high demolding definition, � surfaces with fine grain pattern, and � PP and PE for pressure molding above the crystalline melting point.

Bore holes of 0.5 to 0.6 mm are used for:

� pressure molding, � PP and PE in vacuum forming, and � sensitive visible surfaces in vacuum forming (e.g., brilliant surfaces or surfaces with high grain pattern.

Bore holes of 0.8 mm are used for:

� vacuum forming.

Bore holes of 1.0 mm are used for:

� thick sheets from 6 mm, but not for PE or PP.

Bore holes of 1.0 to 1.5 mm are used for:

� so� foams.

Slot nozzles with a 6 mm diameter are used for:

� volume extraction.

Slot nozzles with an 8 mm diameter are used for:

� high volume extraction (e.g., in negative molds with deep drawn depth and full forming area).

1511.5 Molds for Thermoforming

Venting slotsSlots of 0.2 to 0.3 mm are used for:

� PE and PP in all material thicknesses, if the mold contact is on the visible side; for other material: up to 0.5 (0.8) mm.

Slots of 0.5 mm:

� common value

Slots of 0.6 to 0.8 mm are used for:

� very fast venting, mainly in molds for fast running automatic roll machines, but not for PE and PP.

For the design of venting cross sections, see Figure 1.168.

(a) (b) (c)

FIGURE 1.168  Design of venting bore holes

(a) venting bore hole perpendicularly to the surface of the mold

(b) counter bore in a “negative radius”

(c) venting channel for lateral venting

The venting bore hole on the surface of the mold d1 should a�er 2 to 4 mm become the larger diameter d2 and the larger width of the venting channel d3. Venting bore holes d1 that are too long (deep) can reduce the venting velocity and are harder to drill.

Design of the venting channel systemThe venting channel system is designed correctly when no accumulation of air (due to cross section narrowing) occurs between the mold surface and the vacuum con-nection. The branching of the venting channel system has to be designed so that the sum of the cross sections of the venting channels increases in air flow direction.

An example of checking the “quality” of venting of a thermoform machine with vacuum forming or vacuum and pressure forming is:

152 1 Molds for Various Processing Methods

In work position of the mold, the vacuum gauge (on a scale of 0 to –1) shall not exceed the value of –0.4 for small molds and the value of –0.3 for large molds, when the vacuum is applied and without inserted material (provided that the thermoforming machines without mold shows a value of –0.2).

1.5.5.8  Cavities

Large cavities in positive molds for vacuum forming should be filled with resin to decrease the venting time. Cavities should be stiffened, depending on the thickness and the stiffness. The surface load due to the vacuum application must be considered.

The cavities of the blowing bells should be maintained as small as possible to decrease the time of reaching the maximum forming pressure and to reduce com-pressed air costs.

1.5.5.9  Avoidance of Edge Webbing in Positive Molds

Figure 1.169 shows how edge webbing can be formed.

(a) (b)

FIGURE 1.169  Schematic of webbing on the lower edges of a positive mold

(a) explanation of the webbing

(b) avoiding webbing

When a positive mold (here in shape of a cube) with the side surface “abcd” dives from the bottom into the hot mold, the clamped material (clamped to the clamp-ing frame ABCD) is stretched into tent shape. The following demolding pressure (vacuum or pressure air) presses the side walls of the tent to the side walls of the mold. During this process, in the perpendicular position, the material is stretched (Mm becomes Mo + om). In the transverse direction, the material is compressed (the length v1w1 becomes v2w2). When the warm material in the area of the lower edge of the mold is overstrained in its viscoelastic behavior, pleats can form.

1531.5 Molds for Thermoforming

Pleat formation can be avoided though the following changes on the mold set (Figure 1.169 (b)): � Increase radii on the mold, � Change sharp edges on the clamping frame to “roundings” (inserts), and � Smaller webbing can also be pressed out using a counter mold (punch). (Larger webbing have to be removed with changing the mold or by pulling.)

1.5.5.10  Vacuum Losses when Designing Mold Bottom Wrongly

Vacuum losses are always the result of leakage; they sometimes result from the mold construction and not from bad seals.A schematic illustration of seals in a sheet processing machine with a fixed format frame and a fixed format base can be seen in Figure 1.170 A(f): � If the fixed format base is too high, the upper clamping frame of the machine is not able to press the cold material against the seal f4 in the lower clamping frame. This means that the height of the fixed format base should not exceed the clamping level (seal f4).

� During application of the forming vacuum, the entire cavity of the material up until the seal f1 of the machine table is evacuated. Due to sealing of the base plate of the fixed format base to the seal f1 of the machine table, the base plate’s stiff-ness has to be sufficiently designed so bending upwards can be avoided (vacuum losses would result). Therefore, the base plate of large fixed format bases must be stiffened as seen in Figure 1.170 A(f).

FIGURE 1.170  Seals in a forming station of a ILLIG platen machine

A(f) Mold with fixed format base, with seals f1, f2, f3, f4, f5;

channel for temperature control Tf is sealed;

B(v) Mold with adjustable base, with seals v1, v2, v3 and (v4);

channel for temperature control Tf is not sealed;

154 1 Molds for Various Processing Methods

A schematic illustration of seals in a sheet processing machine with adjustable base can be seen in Figure 1.170 B(v):

� If the fixed format base is too deep or if the stroke of the machine table is not sufficient, the edge of the segment carrier plate cannot seal against the material (v4). A small distance between segment carrier plate and upper clamping frame should be maintained. In machines with an adjustable clamping frame, sealing against the upper clamping frame is possible (as an alternative).

� During application of the forming vacuum, the entire cavity of the material, up to the seal v1 of the machine table, is evacuated. Due to sealing of the segment carrier plate of the fixed format base to the seal v1 of the adjustable forming base, the segment carrier plate’s stiffness has to be sufficiently designed so bending upwards can be avoided (vacuum losses would result). Therefore, the segment carrier plate of large forming molds must be stiffened as seen in Figure 1.170.

1.5.5.11  Suggestions for Temperature Control of Hot Forming Molds

The mold cost can be justified with the number of parts that have to be produced. If the number of parts to be produced is small (mostly the case when manufacturing technical parts), the cheapest mold possible has to be manufactured.

If wood, resin, and plastic block material cannot be used for stiffnes and quality reasons, an aluminum mold that cannot be heated has to be manufactured. (When forming small quantities, the aluminum mold can be heated to production tem-perature in an oven or using a radiant heating system). The quantity of parts for which the extra cost for heating is profitable can be determined by calculation of the production costs.

In thermoforming molds, water is mostly used as a cooling media. The cooling (water flow) can be calculated with the reduction of the heat volume. In sheet processing machines, part of the heat volume can be reduced using air which is blown onto the molded part with a cooling fan. The reduced heat volume using air can be 50% of the entire heat volume to be reduced.

In hot forming mold, the temperature differences, forward to rewind flow of the cooling water, are mostly from 5 °C up to a maximum of 10 °C.

Only in forming/punching molds with shear cutting, are small differences of the maximum 1.5 °C needed in order to prevent the cutting edge of framing (due to linear expansion of the two mold halves).

Closed cooling circuits are advantageous because corrosion protection agents can be added.

In state-of-the-art production, the cooling circuits in the machine and mold have to be passivated before delivery. This means that a corrosion protection layer is applied.

1551.5 Molds for Thermoforming

During the production, corrosion protection agents have to be added to the cooling medium to protect the mold and the machine from deposition and corrosion.

1.5.6  Substructure of Hot Forming Molds

In the following sections, some design examples and finished products from practice are presented (Figures 1.171–1.173).

1.5.6.1  Vacuum Forming on a Sheet Processing Machine

7 3

Example for a mold

Finished product not cutMold station, front lid opened

Principle Construction1 Upper clamp frame, 2 Drawn part,3 Positive plate, 4 Transport chain,5 Segment carrier plate, 6 Lowerclamp frame, 7 Mold base, 8 Machine table

FIGURE 1.171  Mold construction for a sheet processing machine with adjustable clamping

frame and adjustable base with automated feeding (company pictures Fa. ILLIG)

156 1 Molds for Various Processing Methods

1.5.6.2  Pressure Air Forming with Forming/Punching Mold with Shear Cutting

1

2

3

4

5

6

Mold

Example: Hot formed parts areformed and punched out in the

forming/punching mold

Machine with the forming mold.Mold is tilted in stacking position

Cross section of the mold construction

1 Assisting plug2 Blank holder3 Cutting plate

4 Cutting punch5 Mold insert6 Ejector base

FIGURE 1.172  Pressure air forming with forming/punching mold with shear cutting

(company pictures Fa. ILLIG)

1571.5 Molds for Thermoforming

1.5.6.3  Pressure Air Forming with Forming/Punching Mold with Steel Rule Die

1 Assisting plug2 Blank holder3 Cutting plate

4 Cutting punch5 Mold insert6 Base

Cross section of a forming/punching mold Mold with mold change position

Forming station with mold Example: Parts hot formed andpunched in a forming/punching mold

FIGURE 1.173  Pressure air forming with a forming/punching mold with steel rule die

(company pictures Fa. ILLIG)

References

[1] Schwarzmann, P., Thermoformen in der Praxis, Illig, A. (Hrsg.) (1997) Carl Hanser Verlag, Munich, ISBN 3-446-19153-4

158 1 Molds for Various Processing Methods

 ■ 1.6  Rotational and Slush Molds

O. Wandres, R. Hentrich

1.6.1  Process Description

For the economic manufacture of seamless hollow articles made of plastic materials, the well-known procedures for blow molding and rotational molding can be used. In the origins of rotational molding, PVC or plastisols were used for the manufacture. Since “industrialization” of rotational molding in the 1950s, PE is the most widely used processed material. PE is available in different densities and qualities, dry blended and compounded, ultraviolet-(UV)stabilized, electrically conductive, and phosphorescent. Other common materials are PVC, as well as PP, PA 6, PA 12, and PC. All of the RAL colors are feasible and even colors that imitate natural colors are possible (e.g., stone, terracotta, etc.).

The plastic material is mostly powdery. The dosed material is then inserted into the one- or multi-part mold. With the given part size and surface geometry, the amount of the plastic material determines the wall thickness of the hollow part. The filled and closed rotational mold (using clamping elements) is attached to the mold car-riers, which are attached to the machine and drive elements.

The most important characteristic is that the molds should rotate slowly around two axes at right angles to each other. This rotation takes place during the melt process and during the cooling process. A�er melting the plastic material, in a circulating air oven (up to 350 °C), the mold and product are cooled in a cooling station using a water-air mixture or using cold air.

The cycle time for the manufacture of a rotation molded product can be between five to forty minutes, depending on various factors. Important influencing factors are the type and size of the molds, machine performance, material choice, and wall thickness.

1.6.2  Strength of a Rotomolded Part

Where in other plastic molding processes the outer contour of products is thinner in some spots of the wall, the outer contour and radii of rotomolded products is thicker. Therefore, rotomolded products are very stiff. To increase the stiffness of a rotomolded part, the easiest solution is to increase the weight and the wall thickness.

1591.6 Rotational and Slush Molds

Besides the possibility to fill the plastic hollow room with PUR foam, PE foam can be rotated into the mold as a second layer. A precise dosage of the material mix is necessary for this application (e.g., plastic powders from two materials with different melting point). Therefore, a solid plastic skin is rotated first, followed by activation of the propellant-coated material of the second material (due to heat induction), which then starts the foaming process. Another possibility is to insert the propellant-coated material into the mold a�er rotation of the first plastic mate-rial layer. This can either be done using a manually operable filling opening, or by using a so-called “drop box”. This is a well-insulated container that is attached to the outside of the mold and opens (at a desired time) a passage to the mold to insert the “second shot”.

Increased stability can be achieved through stiffness ribs or through targeted inte-gration of tie points between the individual walls of the plastic product.

1.6.3  Mold Requirements

During production as described above, rotational molds are exposed to many heating/cooling cycles. Typically, molds are heated to a minimum of 300 °C and then cooled to ambient temperature (in each production cycle).

These extreme temperature changes require not only the selection of the proper mold but also, more importantly, a suitable design. In order to heat/cool the mold with a minimum amount of energy, molds must be made as thin-walled as possible and of a good heat-conducting material. Furthermore, the mold closures and the mounting of the mold in the carrier must be designed so that they are fast and safe in handling. It is most important that the leakage between the mold and the closure between the cavities (in multi-cavity molds) is reduced to an absolute minimum. Plastic leaks cause problems if they occur during production: the product thickness can be reduced and the plastic material can bake onto the outside of the hot mold. This burnt crust is difficult to remove and forms heat insulation, thus affecting the heat transfer through the mold wall. This can lead to different wall thicknesses of the plastic products. Furthermore, excessive flash builds up, which increases the finishing costs.

The surface quality and the shape of the mold cavity are transferred to the surface of the molded part. The shape of the article, the position of the possible undercut, and the surface quality, possibly with grained texture, will determine both the choice of the correct molding material and the most suitable production process. Also, the different requirements for processing PE, PP, X-PE, PA, PC, and PVC (e.g., corrosion problems that can occur through hydrogen chloride when manufacturing PVC) should be taken into account in the mold design.

160 1 Molds for Various Processing Methods

1.6.4  Nomenclature of Rotational Molds

The design of a rotational mold can be very diverse due to the general simplicity of these molds. Besides the single cavity mold (in which one product is manufactured in one mold), there are also double or multi-cavity molds (in which two or more products are manufactured in one mold and are mechanically separated) and com-bination molds (in which different products can be manufactured in a convertible rotational mold). Figure 1.174 shows such a combination mold.

This cast aluminum rotational mold can be modified using inserts so that either a BBQ-Donut® Half (Figure 1.175) with steps of invert or a half with motor and umbrella holder can be molded. The mold consists of 10 mold shells.

FIGURE 1.174  BBQ-Donut mold

FIGURE 1.175  BBQ-Donut mold

1611.6 Rotational and Slush Molds

Figure 1.175 shows an assembled finished product (floating grill island – “BBQ-Donut®”). Both donut halves are manufactured in a modified combination mold. The rotational molded halves are made of PE and the dimensions are each 4,000 × 2,000 × 1,200 mm.

A single mold is a mold that is manufactured for only one product. If the demand is higher than the output of a mold, identical molds can be produced. For the production of large quantities, the so-called spider concept (multiple cavities are assembled to a frame) is recommended. Opening and closing of the assembled single molds is done in one step and leads to significant time savings in handling.

The construction of a rotational mold can be done in two parts (out of two mold shells) or depending on the complexity, in 3, 4, or 5 parts (Figure 1.174).

How a mold is parted mostly depends on the demoldability of the plastic material to be molded. This means that the mold has to be parted so that the removal of the shells off the products or the removal of the rotomolded plastic part out of the shell is done without causing damage. More decision-making criteria for the choice of the mold separation (e.g., optical and aesthetic requirements, general handling of the mold halves, etc.) can demand additional molded parts.

1.6.5  Types of Molds

Molds that are needed for rotational production can be manufactured from differ-ent thermally conductive materials (where every design has its advantages and disadvantages). The choice of the best suitable rotational mold is based on technical criteria (size, complexity, precision, surface structure, planned quantities, etc.) and economic factors like costs and production time.

Molds from aluminum (aluminum casting and CNC-milled) or sheet steel are used today for rotational molds. Electroplated molding has established itself for special rotational molds for PVC plastisols. There are also a couple of other materials that can be used in prototype production.

1.6.5.1  Prototype Rotational Molds

Due to the high temperature loads when producing rotomolded plastic parts, it is almost impossible to use cheap prototype molds. A rotational mold has to have a certain thermal conductivity and has to constantly withstand changes in heating and cooling.

In the construction of prototypes, molds are used where the mold shells are manu-factured in the metal spraying procedure. This is only possible for easy and flat contours due to the metal spraying procedure.

162 1 Molds for Various Processing Methods

FIGURE 1.176  Prototype mold with carbon fiber molded shells

The sprayed metal is comparatively porous, which leads to a defective surface quality of the plastic product. In addition, the metal can only withstand the constant tem-perature change to a certain degree; it gets porous a�er a while and tends to crack.

An alternative in prototype molding is the use of carbon fiber shells, which are manufactured in an autoclave. The advantage of this application is that there are almost no restrictions for mold design. The disadvantages are the high production costs and limited lifetime.

Figure 1.176 shows a carbon fiber prototype mold for a 400-liter fuel tank with a simplified frame and screwed form flanges. The attachment of required fixtures like a fuel-level sensor, threaded fittings, inserts, etc. enables the rotation of prototypes in production-based design.

1.6.5.2  Sheet Steel Rotational Molds

Especially for high volume products (storage tanks) and for article contours with a low level of difficulty, molds from sheet steel are used. The wall thickness is mostly between 1.5 and 4 mm. Different sheet steels are welded or soldered. A�er a heat treatment for a stress reduction, the mold fit surfaces have to be refinished for better sealing. The weld and solder lines have to be polished and reground. The quality of the surfaces is dependent on the skills of the tinsmith.

A sheet steel rotational mold for the manufacture of an inspection chamber (diameter 1,000 mm, height 2,200 mm) is shown in Figure 1.177. Before opening of the mold shells, the eight rotating steps, which are located in the contrary to the demolding direction, have to be withdrawn using a hinge system.

1631.6 Rotational and Slush Molds

FIGURE 1.177  Sheet Steel Mold

1.6.5.3  Aluminum Rotational Molds

These molds can be made from CNC-milled or casted aluminum mold shells (a com-bination between both methods is possible as well). A mold made from a combined cast aluminum and CNC-milled shells can be manufactured. In addition, a lot of different materials in a mold (aluminum with steel) can be used for the manufacture of mold shells.

The production steps of both applications differ from each other significantly. The complete construction of the mold shells and the design of the form separation and flanges have to be determined for the production of a machined aluminum mold. The manufacture of a mold begins mostly with the production of a positive model (per sample, drawing, or 3D data).

CNC-machined molds are mostly used when the aluminum block to be machined is very flat, when the production time is critical, or when the necessary tolerances are very high.

For an economical rotational production, it is necessary to machine the mold shells to a certain wall thickness, which means to machine on both sides. This will ensure an equal and fast heat penetration of the mold wall.

Figure 1.178 shows a CNC-machined aluminum rotational mold for a sha� cover (diameter 700 mm) with bent tubular steel frame and hand closures.

The use of mold shells manufactured in an aluminum casting is advantageous for large and deep mold shells or for necessary follow-on molds. For the manufacture of aluminum casting mold shells, the casting molds made from sand or sand/ceramic molds have to be manufactured first. The aluminum casting process, shown in Figure 1.179, is done by hand.

164 1 Molds for Various Processing Methods

FIGURE 1.178  CNC-machined rotational mold

FIGURE 1.179  Aluminum casting

Negative molds are needed when manufacturing such casting molds. These cor-respond to the required mold shells and already have the necessary mold flanges and mold thickness (mostly between 7 and 12 mm).

These negative molds are either produced according to the data or using a posi-tive mold that is manufactured before. Structures added to the positive model, like timber graining, terracotta, or stone structures, can be replicated in the casting mold and the aluminum casting.

Depending on the type of aluminum casting process, there are small differences in the manufacture of the casting mold. The most modern type of manufacturing such

1651.6 Rotational and Slush Molds

casting molds is the CNC milling of unmachined sand parts with CAM data. A very complex construction of the mold shells is needed (as seen in CNC-milled molds). The contours of the mold shells can be CNC milled using CAM data. Figure 1.180 shows a 5-axis CNC processing of a sand casting mold for the manufacture of an alu-minum casting rotational mold for a designer garden or lounge chair. A CNC-milled mold is manufactured in about five to six weeks, whereas the standard production time of a casting mold takes about eight to ten weeks. The size and complexity of a mold are substantial factors in the production time.

1.6.5.4  Electroplated Molds

In electroplated molds, a model is molded to the smallest detail (see Section 4.3). The positive model, which corresponds to the finished article, is therefore the basis for electroplated rotational molds and slush molds. The necessary finesse, such as surface quality, dimensional tolerances, mounting parts have to be determined. For example, it is possible in prosthesis manufacture to use rotational molds to get an exact impression of which the inner contour is an impression of the human skin. This shows the precision of electroplated molds and the art to manufacture sufficient models made from resistant materials.

The requirements of the models are strongly influenced by the rotational process-ing methods, as well as by the inserted component material. For the components that almost always have undercuts, elastic materials (PVC, TPU, and TPO) are used. Additionally, it must be ensured that no mold markings are accepted in the visual area of the component.

FIGURE 1.180  Sand mold CNC processing

166 1 Molds for Various Processing Methods

Therefore, single-parted rotational and slush molds are prerequisite and are provided with openings out of which the elastic components are ejected off. Ejecting compo-nents in electroplated molding is also done through small mold openings. Therefore, model materials are used that can be mechanically or chemically destroyed.

For economical mass production of doll parts and toy animals, multiple identical electroplated molds are necessary. Therefore, the models are done as follows:

First, an original model out of suitable wax is modeled. A�er this step, a so-called master model is manufactured with which individual models (for manufacturing series-production molds) are molded in the rotational oven. Here, thick walled PVC models in a harder setting are very common. Where necessary, the possibly elastic PVC models can be stabilized through coating on the inner side with casting resin and wax. It is important, when manufacturing the original model using this model reproduction process, to take the double material shrinkage into account. It is therefore useful to include extensions (conical connecting pieces, etc.) in the original models and master models, which will be necessary for the clamping system (because for the PVC models, these contours are transferred to the serial mold).

To manufacture prostheses slips, silicone negative models of a human forearm are taken of which models are made using either wax or a specific casting resin. This ensures that fine skin pores and skin structures are exactly transferred to the electroplated mold.

For education in schools, universities, and institutes, scientific and anatomi-cal demonstration objects are needed. These are generally manufactured in the rotational process using harder set PVC. For the model construction, enlarged models (Figure 1.181) or original bone (for skeleton construction) are used. These are embedded in plasticine to be able to model the necessary demolding. Due to anatomical contours, the parting plane is normally waved, is also molded, and is provided with fixations (e.g., tongue and groove), so that the mold elements will fit

FIGURE 1.181  Model for the manufacture of an electroplated mold for the production of

anatomical demonstration objects (e.g., eye)

1671.6 Rotational and Slush Molds

together (unshi�able). Using silicone negative molds, which can be manufactured of two or more parts, the model maker has the opportunity to make models of two or more parts of epoxy resin, which then can be used as models for further process-ing. Using this method, the fine bone structures are also projected onto the inner contour of the electroplated mold.

The respective models are provided with brackets and contact feed units, and a�er cleaning will be coated with a silver coating (with a layer thickness < 1 μ) to make them electrically conductive. During electroplated molding, the silvered model forms the cathode. By using suitable electrolytes in the cathode, as in familiar surface-electroplating, copper and nickel are deposited in the required wall thick-ness. For small and medium-sized electroformed molds, a wall thickness of 2 to 3 mm is ideal; for large molds, depending on the design and surface geometry, wall thicknesses up to 5 mm are used. The electroplating process takes between two and eight weeks depending on the required wall thicknesses and the complexity of the surface geometry. A�er reaching the set points for the wall thickness, the brackets and contact feeds are removed, the model is demolded, and the electroplated mold is a negative image of the positive model. To function properly, fixing elements and clamping devices are attached.

Initially, electroplated rotational molds were only processed in acid copper baths. The advantages of this process are the known thermal conductivity of copper and the easy handling of copper baths that are used at room temperature, therefore allowing for the use of wax models. Because the copper, which is separated in these acid baths, is very so�, the (in the bath) molded electroplated molds can easily be mechanically damaged. Another disadvantage is the danger that fission products using PVC can lead to corrosion. To prevent corrosion, the electroplated molds made from copper are provided with a chemical nickel-plating of the inner contour (also the outer contour if necessary). This chemical nickel-coating is in the micron range and can be neglected in regards to the mold surface. However, the very thin nickel coating has a limited service, so the copper electroplated molds have to be nickel plated from time to time.

To eliminate these difficulties and to meet the requirements of a large-scale produc-tion of the automobile industry, it is now state of the art to manufacture electro-plated molds from sulphamate nickel, which guarantees higher wear and corrosion resistance. During the construction of the sulphamate nickel galvanic molds, it is possible to develop local thickenings (e.g., flange areas of multi-part molds for attachment of clamping elements).

One should note that nickel, in comparison to copper, is a less thermal conductive material. Here, it is possible to manufacture molds that consist of 1 to 2 mm sulpha-mate nickel which will then be reinforced with hard copper. This combination com-bines the advantage that nickel offers, with the good thermal conductivity of copper.

168 1 Molds for Various Processing Methods

1.6.6  Mold Construction

1.6.6.1  Closing and Clamping of Molds

The rotational process is a pressureless process. This means that the mold halves, as in other processing operations (e.g., injection molding), do not need to close with high clamping pressure. They still have to be kept together during the bi-axial rotational motion to prevent material from leaking. Therefore, the mold halves are either clamped or screw-fastened.

One of the more cost-effective options is to use screws with fixed mold flanges. However, a�er every cycle, the screws have to be removed, and a�er filling they will have to be attached again. (An example of a screwed mold flanges is shown in Figure 1.176. The cycle times here are of secondary importance.)

To give the mold halves the necessary stability and uniform clamping, a steel frame is attached above the flanges. Manual clamping devices, pneumatic cylinders, or similar closures are installed to this frame. Furthermore, additional protection for the o�en fragile, thin-walled mold shells is offered. The steel frame can either be made out of bent or welded steel tube as seen in Figure 1.178, or out of flat materials.

The use of pneumatic cylinders for clamping molds is possible. However, exclusive materials have to be used (e.g., seals from Kalrez®) for the cylinders due to typical temperatures in the rotational production, which will then lead to very high produc-tion costs of these special cylinders. Because of the use of such closures, pneumatic cylinders are therefore only suitable for mass production of, for example, planters and children’s toys. An example in Figure 1.182 shows a circular spider construc-tion for eight planters with 700 mm diameter each.

FIGURE 1.182  Spider with planter molds

1691.6 Rotational and Slush Molds

Inside the 3,000-mm big spider, clamping of the single molds is done using special pneumatic cylinders. For an optimal heat transfer, the aluminum molds are provided with so-called Profit Pins™ and a permanent release coating.

1.6.6.2  Mold Wall Thickness and Centering

The wall thicknesses of the aluminum molds are very thick in comparison to the thin-walled sheet steel molds (1.5 to 3 mm). Wall thicknesses between 7 and 15 mm are used for casting molds (depending on the quality of the casting process). In the mold shells that are CNC milled out of an aluminum block material, a standardized wall thickness of 7 mm is established.

In regards to the heat transfer material induced disadvantages of higher wall thick-nesses in comparison to steel sheet, the thermal conductivity is offset by aluminum (for wall thicknesses of electroplated molds; see Section 4.3).

Independent to the material, one must take into consideration that the wall thickness in a mold should be equally distributed. In general, the more equal the wall thickness of the mold, the more equal the wall thickness of the rotomolded product will be.

To additionally improve the thermal absorption of the aluminum casting molds, the aforementioned Profit Pins can be casted on.

On the outer side of the mold, the small conical plug enlarges the mold surface and improves the heat absorption and dissipation of the mold. Profit Pins can be partially attached (for a better heat absorption) in problem areas or fully attached. To shorten the production process, the planters shown in Figure 1.182 are provided with such Profit Pins.

To make it easier to combine the different mold shells together and to achieve best precision, locating elements are included in the forming flanges. In general, these locating elements are provided as steel dowels. Tongue and groove locating elements on the flange are also possible.

1.6.6.3  Mold Surfaces and Changes

Surface structures (in aluminum casting molds) like wood structures, linen struc-tures, and leather grain structures can be taken over into the model. An example of aluminum molds for the production of anatomical visual models made from PVC is shown Figure 1.183.

In comparison to the method of producing the surfaces directly inside the mold, it is also possible to change the metal surfaces by appropriate reworking. The most popular surface treatments are grinding with different grain sizes up to high-gloss polishing and surface treatment through sand blasting and shot peening.

170 1 Molds for Various Processing Methods

The surface structure in aluminum molds is determined with shot peening as a reworking process. This is not possible for steel molds.

Especially in fine surfaces (structure-etched), a preserving and refinement of metal surface through a permanent release coating is recommended.

Due to the comparably simple construction of rotational molds, changes are for the most part simpler to implement than, for example, in injection molds. In general, changes can be done by welding, processing from the existing wall thickness, or by inserting new milled or casted batches.

1.6.7  Mold Peripheral

1.6.7.1  Mold Venting

Through heating in the oven area, the air expands inside the mold. Conversely when cooling, the air contracts inside the mold. For this reason, it is essential to provide the rotational molds with a venting unit because the over- and underpres-sure negatively affect the service life of the mold and the quality of the plastic

FIGURE 1.183  Mold and anatomical visual model

1711.6 Rotational and Slush Molds

product. Such a mold venting can be done using multiple different methods. The most widely used method is a tube made from PTFE which is pushed through the mold wall and reaches deeply into mold interior and enables air exchange. In mold geometries in which the powder in the mold can come into contact with the PTFE tube or in which powder can leak out of the pipe, compressed steel wool or plastic foil can be used as a seal. Alternatively, filter rods or so-called Supavent™ stoppers (products made from temperature resistant silicone) can be used.

1.6.7.2  Non-Permanent Release Agent

The melted plastics in rotational molds have the property of sticking to the mold wall. This behavior is necessary for the production of rotational parts. However, the adhesion should not be high because demolding of the plastic part out of the mold will be difficult or not possible at all. To prevent this, release agents are used. These are applied to the molding surface periodically and give sufficient adhesion and the necessary demolding properties. The mostly water-based, nonpermanent release agents are sprayed into the mold or are manually applied with a rag. The release agent layer becomes thinner a�er every production cycle, so the release agent has to be renewed a�er about 50 to 500 cycles. Depending on the plastic material used (e.g., PVC), the complexity, and mold surface quality (e.g., polished mold surfaces), a release agent is not always required.

1.6.7.3  Mold Coating (Permanent Release Coatings)

Due to better repeatability and economical production, the so-called permanent release coatings have increased in importance over recent years. In general, mold coatings are sprayed onto specially prepared metal surfaces in fine layers and are cured layer by layer. Earlier permanent coatings had very strong demolding properties and were mostly only suitable for the coating of mold cores with strong shrink-on situations. Now, a diversification of mold coatings can be seen.

Today’s available coatings allow a downgrading of the release properties (using PTFE parts) and a targeted influencing of the optics and haptics of plastic products. The gloss grade of surfaces (due to coatings) can be chosen in different nuances, from matt to high gloss.

Along with the design influence, coatings take over diverse technical properties, for example the improvement of the heat transfer of the mold, improved flow properties of the plastic melt, and improved gradient blend. The use of permanent coatings is not only restricted to molding surfaces: mold flanges and the outer side of the mold can also be coated to decrease service and maintenance work.

172 1 Molds for Various Processing Methods

1.6.7.4  Threads

The necessary mounting points in a rotomolded plastic product can be tightly fixed inside the mold as threaded inserts. Around the inserts, the plastic melts and forms a constant bonding. Threaded inserts can be held inside the mold with a screw, which has to be screwed in and out for each cycle. In industry, snap inserts (fixed inserts using spring loaded balls) or magnetic holders have established themselves.

Along with the possibility to mold-in threaded inserts, threads can be shaped directly inside the plastic material. Specific thread formers made from brass, steel, or aluminum are used to shape either internal or external threads. These have to be manually screwed off to allow the demolding of the plastic products.

1.6.7.5  Other Inserts

Besides threaded inserts, other parts can be fixed and molded into the plastic part. A common application, for example, is the molding in of mounting brackets made from metal. In general, inserts made from other materials (e.g., plastic material) can be used. The melt temperature, however, has to be higher than that of the rotomolded plastic material.

1.6.8  Post-Processing of Rotomolded Plastic Products

1.6.8.1  Openings

Besides post-processing of the rotated product (e.g., drilling, milling), a predeter-mined breaking point in the plastic product can be created by introducing a knife edge in the mold. The opening is produced in one stroke in the cooling product. When processing (e.g., with a knife), the edge allows for better guidance.

It is also possible to shape the openings directly in the rotational process. A wide-spread method is the use of PTFE bars and plates. Because of the reduced heat absorption of this material and the strong release properties, no plastic material sinters at such PTFE bars or plates (which are attached to the mold). An opening is affixed by rotatation.

1.6.8.2  Decoration of Rotomolded Plastic Products

Due to the self-releasing properties of almost all materials (which are rotation-ally processed), a subsequent decoration of products with screen printing, paint-ing, and placement of stickers is o�en very difficult. Preparation of the plastic product is possible, but very complex. For some time now, there are specially developed possibilities for decorating plastic products using mold-in-systems.

1731.6 Rotational and Slush Molds

For example, colorful logos are directly applied to the mold surface using carrier foil. The melting plastic material bonds permanently with the foil during the pro-cessing. It is more welded than glued, which means that the applied decorations are scratch resistant and almost indestructible.

1.6.9  Electroplated Mold for the Slush Molding Process

Automotive producers demand the highest standards when it comes to the mate-rial quality and the outer appearance of the interior of automobiles. The designers demand, besides the functional geometry, good optics and haptics, individually specified surface structures with a defined degree of gloss, as well as dual tonality of the components. To reach these requirements, so� materials like PVC, TPU, and TPO are especially used for instrument panels, door windowsills, glove compart-ment lids, and center consoles (Figure 1.184). The manufacture of these parts is exclusively done in electroplated molds, which are designed differently depending on the slush technology.

In comparison to rotational molding, in the slush molding technology, open molds are used. A�er preheating the slush mold, the mold is coupled to a container that is filled with powdered raw material (therefore the internationally used expression “powder slush”). Through rotation and vibration, the raw material is brought into the preheated electroplated mold. A specific, possibly defined amount stays at the mold walls. The powder surplus is brought back into the container. Subsequently, the mold and the container are separated, and in the further process, sintering of the powder to a slush skin is continued. This is followed by cooling of the mold to approximately 40 °C when the slush skin can be removed. The skin is inserted into a foaming mold using a hard set carrier and is bonded with the carrier through back-foaming (Figure 1.185 and Figure 1.186).

FIGURE 1.184  Detail of the grain pattern to be molded

174 1 Molds for Various Processing Methods

FIGURE 1.185  Shaping contour with double electroplated mold to produce two instrument

panels at the same time

FIGURE 1.186  Strong jumps in geometry at the outer contour of a double-sided electroplated

mold, which have to be overcome when electroplating

FIGURE 1.187  A complete electroplating mold to use in systems with temperature control

using thermal oil

1751.6 Rotational and Slush Molds

For heating and cooling, different process systems can be used that need adapted electroplated mold shells:

1. Mold shells from sulphamate nickel with a wall thickness of about 3 to 4 mm, which are heated either with hot air or in a sand bed, where cooling is done through spraying with a water-air mixture or though cold air.

2. Mold shells from sulphamate nickel with a wall thickness of about 4 to 5 mm, soldered onto the temperature control pipes made from steel, so that heating and cooling can be done using heat transfer oils (Figure 1.187).

3. Mold shells from sulphamate nickel with a wall thickness of about 3 to 5 mm, which have a double wall at the back. This double wall is either made from sheet metal or from electroplated mold shells and is oil-sealed bonded to the flanges of the electroplated mold shell. Using specially attached nozzles, the oil is brought through the spacing between the electroplated mold and the double wall to ensure heating and cooling of the mold.

In order to meet the great demands of application technology and the design depart-ment on the construction, surface structures, dimensional stability of the slush mold skins, electroplated molds are made available. The goal is reached by using a complex and expensive model technology. Figure 1.188 shows the principle path from CAD data to the finished electroplated mold shell.

Based on the CAD data, which are given by the customer, the so-called “run-off-surface” is determined and the feasibility is verified between customer, foaming mold manufacturer, and electroplating personnel. Additionally, the sealing surface should be designed to ensure a secure sealing against powder losses (using the elastic seal of the powder box adapter), as well as the flange to include the elec-troplated mold shell in the mold carrier. To keep the material loss, which develops through the run-off surface and the adapter contour, as small as possible and to guarantee the technological feasibility, the construction of these elements must be very carefully investigated.

Using the design and taking into account the shrinkage of the used material and the shrinkages expected in model technology, milling of the original model from tooling material follows as the next process step. A�er fine-tuning of the original model surface, an acceptance through the design department of the automobile plants is done. Particular attention will be paid to fairing lines, radii progression and transitions, as well as recesses and grooves for mounting parts. Once the original model is released, in the next step, the grain structure can be applied. It does so by one of the following methods:

a) The zones that will have to be grained need to be bonded with real leather or grain foil (when milling, the foil or leather thickness have to be taken into con-sideration).

176 1 Molds for Various Processing Methods

Steps for the Production of Electroplated Molds

for the Slush and Spray Technology

Work Sequence 1

Leather covering models

(shrinkage model)

(tooling materials)

Work Sequence 2

Silicone negative model Number I

with support shell

Work Sequences 3 and 4

Mother model (epoxy resin)

Correction and engraving work

including designer acceptance

Work Sequence 5

Silicone negative model number II

with supporting shell

from Work Sequence 2

Work Sequence 6

Bath model

Epoxy resin

Work Sequence 7

Electroplated construction

Wall thickness depending on the process

Work Sequence 8

Electroplated shells

Mounted to the support frame

(system dependent)Support frame

2.5 - 4 mm Ni

Supporting shell Silicone II

Supporting shell Silicone I

* 1) Material and process dependent shrinkage

2) 0.0–0.2% shrinkage for silicone manufacture

L + shrinkage *

Leather or structure foil

FIGURE 1.188  Process steps for the manufacture of models and electroplated molds

1771.6 Rotational and Slush Molds

b) Production of the grain structure in the Flotek-process.

c) For special cases, steel models that were photochemically etched, are used.

Methods (b) and (c) are mostly so-called “technical grains”.

Parallel to milling, a so-called “dummy model” is milled out of a material that is easy to work with (with an offset of 6 to 20 mm to the original model). This dummy model is the basis for the manufacture of a GRP (glass-reinforced plastic) supporting shell, which is stabilized with a laminated steel frame. This supporting frame is dependent on the component geometry (made from one or multiple parts).

The structured model (accepted by design and application technology) is fixed inside the supporting shell and is casted with sufficient silicone (a silicone compatibility test should be done in advance) in the space between the original model and the supporting shell. Preferably, silicone types with a linear shrinkage of < 0.1% are chosen. A�er finishing and examination of the silicone negative, the mother model is created from epoxy resin. The mother model is very important because it is used as a model for mold manufacturing during the entire production process of the specific type of vehicle. The tasks of the gravers is retouching and engraving of leather or film joints as well as re-engraving and incorporating of the smallest radii, which cannot be imaged through the covering of foil or leather. When emblems and labels (e.g., the airbag logo in the grain area) are needed, the mechanical incorporation into the mother model is now carried out.

A�er final acceptance and release through the customer, the so-called “work-silicone” can be casted. Out of this silicone, five to six positive bath models made from epoxy resin can be produced. Then, the work silicone has to be renewed. Because the quality of the bath model determines the quality and dimensional stability of the electroplated mold, another control survey of the CAD data will be necessary. In spite of many process steps, in the area of mounting components and attachments, a tolerance (towards the CAD data) of ±0.2 mm and in the open space areas of ±0.5 mm should be maintained. To meet these tight tolerances, the highest demands are placed on the mold making (Figure 1.189).

The bath model is prepared (as described above), made electrically conductive, and put into a sulphamate nickel bath for an electroplated deposition. Depending on the size, geometry and required wall thickness, the model stays between three to seven weeks inside the electroplated bath including disruptions (to renew auxiliary anodes and insert covers). When reaching the required wall thickness, the process is ended and the mold shell is ground to the outer contour. If necessary, the flange surfaces have to be milled, and fixed bore holes have to be drilled. A�er thermal or chemical demolding of the bath models, the inner contour is cleaned, the mold is measured, values are compared to the CAD data, and a defined surface treatment is applied.

178 1 Molds for Various Processing Methods

Depending on the designer, the surface treatment is done using radiation or pho-tochemical etching (gloss or two gloss).

To achieve economic production of instrument panels and other interior components, the state of technology is to use multi-cavity electroplated molds. It is common that instrument panels are implemented as double molds, and glove compartment lids and similar small parts are implemented as 16-cavity molds. A disadvantage of multi-cavity molds is the possibility of early failure of one single cavity. As an emergency measure, the defective cavity is separated from the mold and a replacement is put into the gap. It is o�en advantageous to use single cavities and to combine these to multi-cavity molds in one frame. Therefore, complex adapters are used (which combine the powder box and the electroplated mold).

It should be remembered that the electroplated molds change dimensionally due to the changes between heating and cooling. This causes extremely high stresses inside the mold. To keep these stresses as small as possible, the electroplated mold should be floating in the mold carrier so they can expand freely. And if in spite of all precautions, stress cracks inside the electroplated mold occur, repairs through laser welding and post-engraving can be done. However, the life span of the electroplated molds is limited due to extremely high thermodynamic stresses.

FIGURE 1.189  Double bath model, prepared to an electroplated construction

1791.7 Molds for Thermoplastic Foams

 ■ 1.7  Molds for Thermoplastic Foams

N. Reuber

1.7.1  Thermoplastic Foams

Thermoplastic foams are closed cell foams that are pre-expanded particles before they are processed to a finished product (Figure 1.190 and Figure 1.191).

The bulk density of the material is generally between 8 and 200 kg/m3. A distinc-tion is made between blow-agent containing and non-blow-agent containing foams (Table 1.8).

In particle foams that contain blow agents, a physical blowing agent (mostly a low-boiling alkane, such as butane or pentane) is stored in a polymer matrix.

FIGURE 1.190  Foam particles

(le�) PS-E; (right) PP-E

FIGURE 1.191  Cross section,

enlargement of a PS-E

foam particle (source:

Nova Chemicals)

180 1 Molds for Various Processing Methods

TABL

E 1.

8  O

verv

iew

of

Therm

opla

stic

Foam

s

Poly

mer

Trad

e Na

me

Man

ufac

ture

rBu

lk D

ensi

ty

[kg/

m3 ]

Appl

icat

ion

Tem

pera

ture

[°C]

Typi

cal A

pplic

atio

nVa

por P

ress

ure

for F

oam

ing

[MPa

]

Tem

pera

ture

fo

r Foa

min

g[°

C]w

ith o

rgan

ic b

low

age

nts

expa

ndab

le

polys

tyre

ne

[EPS

]

Styr

ofoa

m®

BASF

, DE

Mos

tly 1

5–30

, lo

wer a

nd

high

er d

ensi

ties

are

poss

ible

Up to

80 

°CPa

ckag

ing,

ther

mal

insu

latio

n of

bu

ildin

gs0.

08–0

.12

117–

123

EPS

Sunp

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TEP

SNo

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nove

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H

Dylit

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nove

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drin

king

cup

s, g

roce

ry

pack

agin

gGe

dexc

el®

Nova

Inno

vene

, CH

16–5

0M

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r the

Los

t Foa

m m

etho

dSt

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hem

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yroc

hem

, US

Sunc

olor

®

prot

ect

Sunp

or,A

T40

Bicy

cle

helm

ets,

spo

rt h

elm

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sa

fety

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met

s, o

ptic

ally

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h qu

ality

pac

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ngPe

ripor

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EM

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25,

lo

wer a

nd

high

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-tie

s ar

e po

s-si

ble

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latio

n of

bui

ldin

g sp

aces

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ch

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the

grou

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e ro

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vert

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onst

ruct

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, in

the

foun

datio

n of

traffi

c ro

utes

as

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pr

otec

tion

expa

ndab

le

polys

tyre

ne

[EPS

] with

in

frare

d ab

sorb

er

Neop

or®

BASF

, DE

Lowe

st d

ensi

ty

whe

n pr

e-fo

amin

g on

ce:

appr

oxim

atel

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, low

est

dens

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hen

post

-foam

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ab

out 9

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80 

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prov

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all,

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Silve

r Po

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Inno

vene

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11-3

5

1811.7 Molds for Thermoplastic Foams

Poly

mer

Trad

e Na

me

Man

ufac

ture

rBu

lk D

ensi

ty

[kg/

m3 ]

Appl

icat

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Tem

pera

ture

[°C]

Typi

cal A

pplic

atio

nVa

por P

ress

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for F

oam

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[MPa

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Tem

pera

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fo

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C]w

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ic b

low

age

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expa

ndab

le

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ne

copo

lym

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l®EE

FGE

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stic

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0 an

d hi

gher

Up to

90 

°C,

max

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ours

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l 104

 °C

Mix

ture

bet

ween

pol

ysty

rene

and

po

lyph

enyl

eth

er o

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PE, P

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gy a

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r mul

tiple

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0.15

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127

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le

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tern

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nd

the

time

up to

11

0 an

d hi

gher

Bum

per c

ores

, chi

ld s

afet

y se

ats

for c

ars,

box

for t

ool k

it, s

un v

isor

s,

impa

ct p

rote

ctio

n, a

ir du

ct, c

harg

e ca

rrie

r, tra

nspo

rt p

acka

ging

, pa

ckag

ing

with

ther

mal

insu

latio

n fu

nctio

n

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513

9148

ARPR

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P In

tern

atio

nal,

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eran

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Kane

ka, B

ARPR

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oust

ical

pan

els

used

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oise

re

duct

ion

with

stru

ctur

e ca

pabi

lity

of

poro

us p

artic

les,

to im

prov

e ac

ous-

tics

in s

peci

fic fr

eque

ncy

rang

es

TABL

E 1.

8  (c

ontin

ued)

Ove

rvie

w o

f Therm

opla

stic

Foam

s

182 1 Molds for Various Processing Methods

These foams are delivered by the raw material manufacturer in the form of pellets and have to be foamed in so-called pre-expanders into foam particles before being processed to a molded part. The target density of the finished part is already set in pre-expanding. To reach very low bulk densities, the pre-expanding is done multiple times with an intermediate storage between the single expanding steps. A�er a material-dependent, intermediate storage period, the material can be processed to the molded part. During this intermediate storage period, the blow agent can escape partly out of the particles. At the same time, the air diffuses from the environment into the particles. The remaining blow agent content is needed when processing the particle to the molded product to maintain bulking force for the complete filling of the mold cavity.

Foams without propellants are already pre-expanded from the raw material manu-facturer in an autoclave or in an extruder and are then delivered to the processor as particle foams. These foams can also be brought close to the target density (before processing to the molded part) with a special process, pressure loading, and following pre-expanding. The molded part density can be set by mechanical compression of the foam particles in the mold or through so-called counter pressure filling. The expand-ing force, which is needed to completely fill the mold cavity, forms by the expansion of air in the particles. Additional expanding force can be produced by compressed air that acts on the particles for a longer period of time (4 to 12 hours) before process-ing. In this method, which is called pressure loading, air diffuses into the particles. The increased internal pressure causes an expansion of the particles when heating.

Almost all particle foams are available in different color and particle sizes. Dyes for coloring white PS-E particles a�er pre-foaming are also available. A lot of materials are equipped (from the manufacturers) with special properties achieved by deposit-ing additives into the polymer matrix or by coating the granules. The heat transfer can be significantly reduced using embedded graphite or aluminum particles. Examples for changing foam properties through coatings are flame-retardant and anti-static coatings.

Essential requirements for PS-E molded parts in the construction sector, test pro-cedures, and standardization are listed in the EPS white book [1].

1.7.2  Conventional Molding

1.7.2.1  Procedure Description

Shape molding machines (Figure 1.192) are used to manufacture molded parts from thermoplastic particle foam. These basically consist of a mold clamping unit with two steam chambers, clamping surfaces for the mold inside the steam chambers, a filling unit, as well as pipeline and valves, which are needed to guide the process media.

1831.7 Molds for Thermoplastic Foams

FIGURE 1.192  Shape molding machine (Factory picture Kurtz GmbH)

The process is broken down into the following steps:

� Filling the mold cavity with foam particles, � Expanding and fusing of the foam particles, � Cooling and stabilizing of the molded part, and � Demolding of the finished molded part

1.7.2.1.1  FillingThe filling process is the most important step in the molded part production because errors that occur cannot be corrected in the following process, especially inhomogeneities, porosities, imperfections, and badly fused areas of the molded part, which are mostly caused by filling errors. The foam particles are brought into the mold cavity with pneumatic conveyor equipment. The particles are fed (using a tube) out of a storage container (filling container) into the filling injector, which then brings the particles into the mold cavity. Air is used as a conveying medium. The conveying air streams through the perforated wall of the mold. The pressure drop required for conveying can be created in various ways.

When filling without pressure, a vacuum is created in the conveyor tube using a venturi nozzle inside the filling injector orifice. The filling container and the mold are vacuumized resp. vented. Due to the vacuum of the venturi nozzle, the foam particles are sucked out of the filling container and are blown into the mold cavity.

During pressure filling (Figure 1.193), an overpressure in the filling container is built by the pressure differences that occur in the venturi nozzle. This overpres-sure supports the conveying process and leads to shorter filling cycles and much better mold filling.

184 1 Molds for Various Processing Methods

Ventilation

Fill injectorCompressedair tank

FIGURE 1.193  Mold with filling injector and pressure filling container

Counter pressure filling is used for closed cell, so� foam particles (PP-E and PE-E). In counter pressure filling, a back pressure is created inside the mold which leads to an increase in pressure inside the conveyor units. This will compress the particles. The mold is filled with more particles than by pressure filling without counter pres-sure. The molded part weight changes and can be aligned with the target weight by changing the pressure level. The smaller particles (through compression) can be brought into mold areas with a cross section smaller than the one of the initial particle diameter.

In the crack filling process, the mold is not completely closed before the filling process. A�er filling the now enlarged cavity, the mold is closed and the foam particles are mechanically compressed. The mold has to have an inlet area so the foam particles cannot escape through the parting plane of the mold. The advantage of this method is the additional venting of the mold through the gap in the parting plane. In addition, the mold cavity is enlarged perpendicularly to the closing direc-tion through mold areas with small mold wall thicknesses that allow a better filling. With the help of the gap size, the molded part weight can be set, and the filling behavior can be changed. The bigger the gap, the heavier the molded part and the better the filling of the thin-walled molded part areas. Typical gap sizes are 1 to 20 mm, and in extreme cases, for thick-walled molded parts, even up to 80 mm. The disadvantage of this method is the nonuniform density distribution with different wall thicknesses of molded part.

1.7.2.1.2  Expanding and FusingBy steaming with saturated water steam, the foam particles are heated to the fusing temperature. The plastic material becomes soft and formable. At the same time, the blow agent evaporates and the gas in the cells tries to expand.

1851.7 Molds for Thermoplastic Foams

The foam particle expand and (almost completely) fill the gaps, which always exist due to the originally round shape of the particles (Figure 1.194). These are now shaped polyhedrically and are welded at the adjacent surfaces under pressure and temperature influences.

To allow a very fast energy input, steam is used. The steam is condensed at the cold particles and at the mold walls and gives off its evaporation enthalpy. Due to the high reduction in volume during condensation, new steam flows in. This ensures a very efficient and fast heat transfer. Inert gases like air worsen the heat transfer substantially so the steam flow has a particular role to play.

In the traditional foaming process, the mold and the steam chamber are built into the mold and flushed with steam (Figure 1.195) to dissolve the inert gases.

Steam inlet

Steam chamber 1

Molded part

Steam outlet

Steam chamber 2

FIGURE 1.195  Flushing of the steam chamber

FIGURE 1.194  Molded part

(le�) Incompletely filled and fused, with gaps between the foam particles;

On the (right) well filled and fused

186 1 Molds for Various Processing Methods

Steam inlet

Steam chamber 1

Steam outlet

Steam flow(across) throughthe molded part

Steam chamber 2

FIGURE 1.196  Transverse steam flow

In the next steaming step, the so-called cross steaming, the steam is passed through the foam particles (Figure 1.196). Especially when designing the parting planes of the mold, it is necessary to pay particular attention that the opposite mold walls are assigned to different steam chambers so that the steam has to flow through the foam particles using a corresponding pressure difference between the chambers (see also Section 1.7.3.1). By varying the steam pressure and the pressure difference between the chambers, temperature and flow velocity can be regulated. Cross steaming is mostly used alternating from both sides. First, the steam flows from Chamber 1 to Chamber 2 and vice versa. Especially in the first transverse steaming step, particles in the mold cavity can still move [2]. Direction, time, and steaming pressure have a significant impact on distortion, residual stresses, shrinkage behavior and fusion of the foam particles.

In the following autoclave steaming (pressure conservation), the outlets of the steaming chambers are closed and a uniform steaming pressure is built up. In this step, a uniform fusion and surface condition is reached. At the end of the autoclave steaming process, the propellant and cellular gas temperature reach their maximum, and therefore, the maximum expanding force of the particles is reached as well. Depending on the material and steaming operations, foam pressures of up to 2 bar can be reached, which will act on the mold walls as mechanical surface pressures. The molded part and the mold have a temperature of approximately 120 °C (for PS-E) to about 150 °C (for PP-E) at the end of the steaming process.

1.7.2.1.3  Cooling and StabilizingThe molded parts have to be cooled until the foam pressure is mostly released so they can be demolded without any uncontrollable expansion. Typical demolding temperatures are about 80 to 85 °C for PS-E and about 70 to 80 °C for PP-E.

1871.7 Molds for Thermoplastic Foams

Steam chamber 1

Cooling line

Outlet opening

Steam chamber 2

FIGURE 1.197  Cross section of the mold with cooling capacity and indicated spraying

Cooling is done by spraying water on the outside of the mold (Figure 1.197) until a temperature of about 95 °C is reached. The water, which adheres in a thin film on the molded part and mold surface, evaporates by reducing the pressure in the chamber. The water removes the evaporation enthalpy (which is needed for evapora-tion) from the molded part and from the mold. The necessary pressure reduction in the pressure chamber to about 200 mbar absolute pressure is mostly done using vacuum pumps. At the end of this process step, the foam parts are dimensionally stable and can be demolded.

1.7.2.1.4  DemoldingTo remove the molded part, the mold is opened and the molded parts are transferred in a specific way to one of the two mold halves. The handover is mostly done using compressed air or by applying a vacuum to one of the two mold halves. A�er com-plete opening of the mold, the foam parts can be blown off by a short compressed air blast. In most cases, mechanical ejectors (Figure 1.198) are integrated into the mold to make the demolding process more reliable or to reach defined demolding velocities for the handover of the molded parts to a handling unit.

Ejector FIGURE 1.198  Mold half, cross section

with extended ejectors

188 1 Molds for Various Processing Methods

1.7.2.2  Special Procedure

1.7.2.2.1  Process with Non-Perforated Molds

Thin-wall technologyIn the thin-wall technology, mold walls are not perforated. The mold walls are heated and cooled from the outside through small channels or small chambers. The steam, which is needed to fuse the particles, is brought into the foam through small bore holes or needles. Because the cooling water is not directly coming in contact with the foam, the molded parts have a small moisture content. Venting of the mold during filling and steaming is done through a gap in the mold parting plane. This application is mostly used for drinking cups and thin-walled food packaging.

The molds are mostly made from copper and copper-beryllium alloys. The mold walls that come into contact with the foam are mostly polished so that when using very small-grained foam particles (so-called cup grade), very smooth, optically attractive surfaces of the molded part can form.

Mold-skinningThe thermoplastic properties of PS-E and PP-E can be used to produce very smooth, optically and haptically attractive surfaces of the molded parts. Therefore, parts of the mold wall are not perforated and are equipped with separate heating and cooling chambers on the back. With the help of these heating chambers, the foam particles are melted to the mold wall and a thin thermoplastic skin forms at the molded part surface. Heating of these areas is done using water steam [3] or using temperature control devices. The mold areas which are not going to be skinned are equipped with a steam chamber, perforation, and cooling system.

The formed molding surfaces image the mold surfaces very well and can also display very fine structures. Polished mold surfaces can give almost reflecting surfaces, although with small crater-shaped flaws. PS-E forms a slightly brittle skin with a higher penetration resistance that will collapse under further mechanical pressure. The surfaces are not watertight or gastight. PP-E forms smooth, elastic layers that also offer a higher wear resistance. Edges of reusable load carriers can be protected against skidding and wear using such a layer.

1.7.2.2.2  Low Temperature Horizontal (LTH) ProcessThe low temperature horizontal (LTH) process substantially reduces the energy con-sumption in the molded part production by avoiding the cycled heating and cooling of the mold components. The mold temperature is constant due to an insulating coating and a mold temperature control system. Cooling of the mold to demolding temperature is eliminated and cycle advantages in comparison to conventional foaming arise.

1891.7 Molds for Thermoplastic Foams

The necessary molds for this application are basically mono-block molds with integrated steam and temperature control channels, as well as a steam- and wear-resistant coating developed specially for this process.

1.7.2.2.3  Transfer TechnologyIn the transfer technology, filling and fusion of the molded part is done in a hot mold. A�erwards, the uncooled molded part is transferred to the cold mold and is then cooled to demolding temperature. This procedure also reduces the energy consumption because the mold is not cyclically heated and cooled. The disadvantage is that two molds are used at the same time, so higher investment costs occur. The quality of the molded part is also lower than in conventional processes.

1.7.2.2.4  Multiple-Density-ProcessIn this process, different foam properties are partially realized in one molded part. Molded parts with carrying capacities and upholstery properties with a higher and lower density can be foamed. Foam pads for household packaging are provided with a high density in the area of support point of devices (Figure 1.199). Other parts of the foam pads, which should only help with better handling, are produced in cheaper, low foam densities. Core pullers are used to distinguish the separate density areas in one mold from each other.

In the single-stage process, the core pullers consist of thin metal sliders, which divide the single areas. They are filled at the same time with the each defined density.

A�erwards, the sliders are retracted. The foam particles fill the cavity (which is made from the metal sliders) completely and only slightly mix themselves to the density limits. Fusing, stabilizing, and demolding are done as explained before.

Areas with higher

densities

FIGURE 1.199  Foam pad with different densities (company picture Kurtz GmbH)

190 1 Molds for Various Processing Methods

A disadvantage of this process is that all densities are processed with the same process parameters, which limits the maximum density differences in the single areas. Also, not all interface geometries can be implemented.

In the multi-step process, areas of one density are first filled, fused, and stabilized as needed. The areas of other densities are defined by core-pullers and sliders. A�erwards, the core-pullers and sliders are retracted and more densities are filled and fused.

1.7.3  Mold Construction

1.7.3.1  Essential Requirements on the Mold Construction

For foaming molds, the terms core or male side and female side for the mold halves, are used (Figure 1.200).

CoreHood

Foam part

FIGURE 1.200  Cross section of a mold with core side and hood side

When designing the molded parts, care should be taken that the design is foam-appropriate. Significantly different wall thickness should also be avoided as well as thin molded part wall thickness in the size of the particle diameters. The gas permeability of the mold walls is very important for all of the process steps as well as the guidance of the gas flow through the foam particle.

Figure 1.201 shows the gas flow through a mold for a box-shaped molded part. In the mold construction in Figure 1.201 on the le�, the gas flow in cross steaming is defined between both mold chambers (due to pressure differences) and is brought through the box wall and box bottom. In the mold construction in Figure 1.201 on the right, the mold walls in the area of the box wall (area X) are in the same steam chamber.

1911.7 Molds for Thermoplastic Foams

X

X

Mold parting line

FIGURE 1.201  Cross section of a box mold with gas flow

It forms an insufficient steam flow through the foam particles. Fusion of the foam particles in the box wall is significantly worse. More disadvantages of this construc-tion are a poorer filling behavior and mostly longer cycle times.

The positioning of filling gates requires considerable experience because no com-mercial fill simulations for particle foams are currently available. The perforation of the mold walls influences the filling behavior and the fusion of the foam particles. It is common that the pressing of core vents and slot nozzles should be done with a diameter of 8, 10, or 12 mm (see Section 1.7.3.3) in regular intervals of 25 to 40 mm. It is recommended to offset the nozzles on opposite mold surfaces by half a pitch division. In small surfaces and with tight radii, smaller nozzles with a diameter of 4 to 8 mm and small division distances are used. In these cases, it is possible to drill bore holes with a diameter of 0.5 to 1 mm. For large, rectangular-shaped molded parts, especially those made from PP-E and PE-E, slotted sheets and hole screens with very large open surfaces are used. Alternative processes to achieve gas-permeable mold walls are sintered nozzles similar to pneumatic silencers, walls made from a porous material (mostly aluminum), or layering the molds with steel sheet material [4]. Through more or less strong perforation, the gas flow can be influenced so that an optimal filling and fusing result can be reached.

A further basic requirement in mold construction is uniform heating and cooling (on all sides) of the mold and the foam particles. The steam has to be able to flow around the mold without any obstructions. The forming condensate should not accumulate in sinks; it should be able to flow out. Areas form in which condensate and cooling water accumulates especially in horizontal surfaces and between ribs.

192 1 Molds for Various Processing Methods

In these cases, inclines and outlet channels or bore holes have to be drilled. As in all non-cutting production procedures, nonuniform wall thicknesses and material accumulations should be avoided. The nonuniform temperature distribution that therefore results leads to inhomogeneous fusion and surface properties of the molded parts. The design steam pressure is a maximum of 1.5 bar with PS-E and 5 bar with PP-E; for special materials steam pressure up to 7.5 bar. In addition to steam pressure, the compound-dependent foam pressure should be around 1 bar to maximum of 2 bar. Cooling of the mold is mostly done by spraying water onto the mold walls. The necessary cooling system and spray nozzles are installed into the steam chamber. It is essential to keep the spray nozzles a sufficient distance from the mold wall so a sufficiently big spray cone can be formed. Position, quan-tity, number, and flow of the single spray nozzles have to be adapted to the mold to achieve optimum results.Demolding is made easier and substantially simplified through dra� angles of 0.5 to 1°. Wall surfaces parallel to demolding are possible but increase the demolding forces substantially. Care must be taken that the mold is stable and the ejector sur-faces are sufficiently large. Small, local undercuts are possible due to the compress-ibility and the elasticity of the foam. Large and deep undercuts will need core-pullers.To lead the forces at the mold wall to the machine structure, supports at the steam chamber have to be installed. The forces result from foam pressures, demolding pres-sures, and pressure differences during the foaming process. The dra� of the VDMA unit sheet (Verband Deutscher Maschinen- und Anlagenbau – German Engineering Federation), VDMA 24473 [5], gives references and criteria for the interpretation.In most cases, the apparent stresses in the material are mostly not the decisive factor for the necessary wall thickness, but the permissible deformations. Ribbing and supports are good constructive measures to reach a higher stiffness with a low mold weight.The constant temperature change leads to problems when using different materi-als due to different thermal expansions of the components. The large temperature expansion (mostly aluminum material) o�en leads to thermal stresses and relative movement of components that are screwed together.It is especially important not to have a direct contact between counter moving alu-minum parts in areas with relative movement during mold opening and closing (in the inlet area of the core in the hood). The cold welding, which occurs through small friction and movement, leads to grooves and outbreaks. When designing, this can be avoided through sufficient clearance of moving parts, a centering unit, brass and bronze bushings, and guide rails.Specific to each machine, the installation dimensions, centering, attachments, and connections (e.g., cooling water) of the different shape-molding machines must be taken into account.

1931.7 Molds for Thermoplastic Foams

The shrinkage of the molding compound should also be taken into account. Typical values are 0.5 to 0.7% for PS-E and 1 to 2% for PP-E. More accurate values can be obtained from the molding compound manufacturers (e.g., [6] or [7]).

A�er milling, the surfaces of the mold are mostly not further treated, although a coating on a PTFE basis would improve the demolding filling and process. On visible surfaces, substantial efforts are still required to eliminate the unwanted marks and to improve the molded part surface optically and haptically. Measures range from using etched grains with fine venting bore holes, inserted meshs, up to leaving out the venting bore holes on the visual surface and venting of the mold through slits on edges and parting planes.

1.7.3.2  Mold Materials

Suitable materials are materials with a good thermal conductivity and small specific heat capacity. A high corrosion resistance is important due to the usage of water steam as a heating medium and water as a coolant. The design temperature is around 120 °C for PS-E and 150 °C for PP-E. The load swells with more than 100,000 cycle lifetime. In most cases, excellent machinability and good weldability, and occasion-ally polishability, are necessary.

The majority of foaming molds are manufactured from aluminum. The contour shaping mold parts are o�en manufactured as sand cast parts and then machined a�erwards. G-AlSi12 and G-AlSi10Mg are o�en used as cast alloys. The corrosion-resistant alloy G-AlMg3Si is very well polishable. Progressively, molds are machined from semifinished products. CAM, in combination with multi-axis milling machines with high cutting performance, allows a more economic machining production of the molds. Then, aluminum-wrought alloys like AlMg3 and AlMgSi are used. These materials have a high strain with good strength. They are weldable, corrosion-resistant, and very well machinable.

High-alloy steels are only used in special cases due to their significantly worse thermal conductivity and machinability.

1.7.3.3  Mold Equipment

Figure 1.202 shows a typical mold with the necessary equipment.

Filling injectorThe most important equipment part is the filling injector (Figure 1.203). It is needed to transport the foam particles into the molds.

For filling, the spindle sleeve is pneumatically retracted, and the compressed air for the venturi nozzle is turned on. A vacuum is applied to the filling injector and the material hose. This vacuum sucks the foam particles out of the filling container.

194 1 Molds for Various Processing Methods

ClampsMolded part

Mold female sideGuide column Filler plate Foaming pressure

Foaming pressure sensor

Blind flange

Clamping flange

Filling injector

Ejector

Temperature sensor

Thermometer

Steam chamber II

Steam chamber I

Cooling water nozzles

Perforation

Cooling line

Male side

FIGURE 1.202  (top) Picture of a mold (company picture Kurtz GmbH);

(bottom) principle drawing of a mold with typical mold equipment

1951.7 Molds for Thermoplastic Foams

The loading for the air flow is 8 to 10 volume percentile but can be set lower as well. When the mold is filled up to the mouth piece, the impact pressure in the mouth piece becomes so high that the predominant part of the compressed air (from the venturi nozzle) cannot flow through the mold anymore and flows through the material hose into the filling container. This is called automated back blowing. The excess material, which is still in the hose, is brought back into the filling container. As soon as the back blowing starts, filling of the mold is completed and the spindle sleeve of the filling injector can be closed. The filling injectors can take over an ejector function using pneumatic multiple position cylinder. The relatively small surface cross section of the spindle sleeve is disadvantageous because it can cause damages at the foam part.

Cooling line and spray nozzler for cooling waterThe cooling line is mostly a soldered (Cu-tube) or a welded (VA-tube) tube system with screwed in full-cone nozzles. The spray nozzles should have a large taper angle to be able to design the cooling line in the limited space of the steam chamber. The positioning of the nozzles has to be done so that a uniform spray pattern will follow and all surfaces of the mold wall are cooled uniformly. Multiple occupancy of a mold with more molded parts and deep molded parts is necessary to build an antler-like structure.

Temperature sensorTemperature sensors, mostly type Pt100 with spring-loaded pressing device and seal, are used to set the demolding time by the mold temperature.

Actuator Compressed airconnection

Connections forcontrol air

Controlspool

Controlcylinder

MaterialconnectionsVenturi nozzlesPinole

Mouth piece

Filling opening

Seal

FIGURE 1.203  Filling injector

196 1 Molds for Various Processing Methods

Foam pressure measurement systemFoam pressure measurement systems are also used as a criterion to set the demold-ing time. There are pneumatic and electronic foam pressure measurement systems.

EjectorEjectors mostly consist of a pole and a plate to reduce the surface pressure to the foam. They can be driven by using the machine movement using starting strips or plates, bowdens with central operation, or pneumatically, using nonsynchronized individual drives. The resetting is obtained with a spring.

Slotted nozzles or core ventsSlotted nozzles and core vents are available in various diameters. The open cross section of the slotted nozzles is a little bit larger than the one of the core vents. Slotted nozzles are more sensitive during installation and operation.

Core pullersCore pullers have to be constructed specific to the mold. The drive is done using pneumatic cylinders directly or using linkage mechanisms (toggle levers). In special cases, core pullers are hydraulically driven.

1.7.3.4  Special Mold Designs

1.7.3.4.1  Mono-Block MoldsThe so-called mono-block molds are molds with an integrated steam chamber or steam distribution. The steam chambers (with their relatively high volume), which are conventionally integrated into the shape molding machines, are not used in these molds. All of the process media (steam, air, water, vacuum, and condensate) are directly brought into the mold using separate connections. The relatively small volume of the mono-block molds reduces the energy consumption substantially. The mono-block construction provides more advantages when a lot of core pullers and laterally positioned core pullers are needed. Mono-block molds are used for large-scale applications in the lost foam areas (lost models for sand casting) and for energy saving concepts like the LTH process.

1.7.3.4.2  Molds with Adjustable Walls (Gradually or Continuously) for Insulation Plates and Small Blocks

In the building sector, insulation panels made from PS-E are mostly used. Especially, when small water absorption is required, like perimeter insulation plated in the base-ment of buildings, these plates are manufactured on shape molding machines. The frequent changeovers of the machine to other plate thicknesses (depending on the number of common insulation plates) are taken into account by changeable molds.

1971.7 Molds for Thermoplastic Foams

The changeover is done in the simplest version by using insert frames, which require a partial dismantling of the mold, using hydraulic changes with manually inserted spacers up to electromotively adjustable molds with a balance between thicknesses and parallelism deviations (to guarantee maximum precision [8]) within sectors).

1.7.3.4.3  Mold for the Thin-Walled TechnologyThese molds are principally built like mono-block molds (Figure 1.204). Special features are the mostly-eliminated perforation, venting the mold through gaps and slits, steaming through small bore holes and annular gaps, as well as the use of copper and copper-beryllium alloys to reach smaller wall thicknesses with good thermal conductivity and sufficient stiffness.

FIGURE 1.204  Drinking cup mold

(1) Mold plate filling side; (2) mold plate male side; (3) centering unit;

(4) mold parting plane; (5) shaping mold parts hood and core; (6) cavity;

(7) ring gap for foaming steam; (8) filling opening; (9) ring gap for venting;

(10) venting gap; (11) O-ring seal; (12) filling injector; (13) inlet for PS-E foam

particles; (14) inlet for heating steam and cooling water; (15) outlet for the

condensate and cooling water; (16) connection for foaming steam; (17) inlet

for ejector air

1.7.4  Block Molds

1.7.4.1  Process Discription

The plates that are used in the construction sector for thermal insulation are mostly cut from PS-E blocks, which are made in block molds (Figure 1.205). The blocks are mostly rectangular-shaped, in special cases also cylindrical. Cutting the blocks is done using heated wires in cutting units which are mostly operated automatically.

198 1 Molds for Various Processing Methods

The block size is about 1 × 1.2 m with a length of 4 to 8 m. The block mold is mostly nonadjustable. There are systems with one or two adjustable side walls to adjust the size to the end product to be produced. It is common to position the block molds vertically. This saves installation space, and the foamed blocks are mostly transported in standing position and stored in standing position at the temporary storage. A�er filling the mold, the foam particles are fused with water steam and are then stabilized with a vacuum. However, cooling with water is not done.

The foam particles are brought out of large silos to the block mold using blowers and are then blown into the block mold using injectors. Due to the high air volume, no expensive compressed air is used. In vertical block molds, it must be ensured that no segregation of foam particles happens due to the height of the fall and the air flow in the block mold. No areas of higher or lower densities should form. Before steaming, the inert gases are partially removed using a vacuum with 0.4 bar of absolute pres-sure. A flushing of the mold as seen in the mold shaping machine is not common but possible in special cases. The flushing procedure gets rid of residual inert gases and improves the heat transfer from water steam to the foam particles substantially. In comparison to the shape molding machine, in which the steam can only stream into the foam particles out of both steam chambers, the block mold can be steamed from three sides and vented from two sides. Typical steaming schemes are shown in Figure 1.206. Using the different steaming parameters, such as direction, pressure, and time, a homogeneous as possible fusion of the foam particles has to be reached.

FIGURE 1.205  Block mold

(company picture Kurtz GmbH)

1991.7 Molds for Thermoplastic Foams

Version N

Steaming with

one direction of

steam flow

Version A Version B

Vacuum and outlet

Steam

Ste

am V

acu

um

Ste

am

Vacu

um

Ste

am

Vacuum and outlet

Vacuum and outlet

Ste

am

Ste

am

Ste

am

Steaming with

two directions of

steam flow

Steaming with

change in direction

of steam flow

FIGURE 1.206  Steaming scheme in block molds

Stabilizing is done without adding cooling water, only by evaporating the conden-sate by vacuum.

For demolding, one of the side walls is opened and the block is pushed to a roller conveyor using hydraulically driven ejectors. To improve the demolding process, a small dra� angle should be brought into the demolding direction. In block molds with adjustable side walls, mostly no dra� angle need to be applied, but the side walls should be retracted.

1.7.4.2  Constructive Design

A block mold consists mostly of a rectangular-shaped steel weld construction of which one side is built as a door that has to completely release the block side surface to demold the block.

The wall surfaces have to be designed for swelling loads of about 2 bar due to steam and foam pressure. The outer structure absorbs all the forces due to inner pressure and foam. The inner walls, which are in contact with the foam and through which the steam is brought into the foam, are only taking over forces from the foam pressure and consist of wedge wire screens and slotted plates. These are manufactured from high-alloyed, corrosion-resistant steels. Slit widths of 0.5 mm are very common. The steam is brought into the space between the outer carrying structure and wedge wire screens by using a pipe system. The steam has to now be able to distribute evenly to allow a uniform steaming. The movable side wall for demolding mostly consists of hinges as well as a form-fitting, hydraulically-driven closing mechanism.

200 1 Molds for Various Processing Methods

References

[1] EPS White Book, EUMEPS Background Information on Standardization of EPS, EUMEPS, June (2003)

[2] Biedermann, S. et al., Filling the Void in Lost Foam Patterns, Modern Casting, December (2006)

[3] BASF AG, Partikelschaumstoff-Formteile auf der Basis von Olefinpolymerisaten mit verdichteter, glatter Außenhaut und Verfahren zu ihrer Herstellung: DE4308764A1, patent application (1993)

[4] Fagerdala Deutschland GmbH: Formwerkzeug zur Herstellung von Partikelschaumform-teilen: EP 1 448 350 B1, European patent (2002)

[5] VDMA, VDMA 24473, Anforderungen an Werkzeuge zur Verarbeitung von Partikel-schaum, VDMA Einheitsblatt Entwurf, May (2007)

[6] BASF AG, Vorläufige Technische Information Neopolen: P9225K TI KSB/NM 43509. BASF AG, January (1998)

[7] Nova Chemicals, Arcel Tooling and Part Design Guide (2003) Nova Chemicals Corporation[8] Kurtz GmbH, Werkzeug zur Herstellung von Formteilen: DE202004003679U1, utility

model document (2004)

 ■ 1.8  Molds for Continuous Fibre Reinforced Polymer Composites

P. Mitschang, R. Schledjewski, A. K. Schlarb

1.8.1  General Objective

The material class of fiber-reinforced materials has shown a substantial growth in the last couple of years in the high-performance range like aerospace and motor sports, as well as in mechanical and plant engineering, in the automobile industry and in the commercial vehicles market. Important reasons for this growth are the advanta-geous properties of these materials, which are seen in the high specific strength and stiffness as well as a fatigue and corrosion behavior. Fiber-reinforced materials are used as lightweight materials in various shapes for different applications.

When processing fiber-reinforced materials, apart from the group of in situ poly-merized thermoplastics, thermoplastic and thermoset processing have to be dif-ferentiated. The essential distinction is the temperature control and the resulting viscosity of the processing masses, which is very low or water-like in a thermo-set material and significantly higher or honey-like in a thermoplastic material.

2011.8 Molds for Continuous Fibre Reinforced Polymer Composites

The process temperature determines significantly the choice of the mold material, and the material property viscosity determines the sealing system of the molds. Other division characteristics are the fiber length, fiber orientation, and the fiber amount (fiber volume ratio of the product to be produced). The fiber structure determines (through its compression behavior) the necessary resistance or stiff-ness of the mold and also influences the material choice for the mold construction. The different thermal expansion coefficient between mold and component has to be taken into consideration when choosing the material. Fiber-reinforced materi-als show a very different behavior depending on fiber direction and type of fiber (glass or carbon fiber). Finally, specific restrictions for processing procedure and the processing variation have to be considered in mold construction.

1.8.2  Molds for the Vacuum-Autoclave-Technology

1.8.2.1  General Objective

Prepreg processing or autoclave technology is a processing procedure for high component requirements. Prepreg materials are semifinished parts in which the fiber structure (either unidirectional or a fabric) is already impregnated with a thermoset matrix (mostly epoxy resin). The prepreg material is layered on top of each other using a loading profile (laminate structure), compacted in a vacuum bag, and cured in an autoclave using a predetermined temperature and pressure profile.

Two processing procedures are applied: the “so� core technology” and the “hard core technology”. As the name implies, the two processes can be differentiated in the design and in the material of the core molds.

In the “so� core technology”, one mold surface is usually designed to be hard. This hard surface is mostly the mold pocket, which is attached to the outer contour of the mold (Figures 1.207 to 1.210). The second mold surface consists of an elastic mat onto which the autoclave pressure is transmitted (constant vertically) to the component surface during curing. The resilience of the elastic mat contributes to the avoidance of cavities inside the laminate. Residual air traps and moisture, which might still be inside the nonwoven fabric, are sucked out using the applied vacuum. The elastic mat can be stiffened locally to ensure best contour accuracy of the thickening or monolithic profiles that were put on.

In the “hard core technology”, mostly multi-part, hard cores made from aluminum are used as an attachment to the nonwoven component. The pressure onto the component occurs through heat expansion of the core during heating.

A void-free molded part quality can only be achieved if the molds are manufactured very precisely and the prepreg material to be processed has very narrow tolerances.

202 1 Molds for Various Processing Methods

1.8.2.2  Prepreg-Low Pressure-Autoclave Technology

According to a given layering, which determines the fiber orientation and the size of the layers, the single layers are manually or using automated machines (tape laying machines) stacked on top of each other. During the following curing process in the autoclave, a�er a material specific cycle (depending on temperature and pressure), the resin becomes liquid (possible residual air traps are sucked out) and can now distribute itself inside the nonwoven fabric. The resin cures in the curing cycle at a pressure of usually 7.5 bar and at temperatures between 120 to 180 °C. A post-curing a�er removing the component out of the mold, gives the material the required quality properties.

The task of the molds in the autoclave technology is to maintain the geometry of the component (as formed in the nonwoven fabrics) during the curing process. Because the prepreg nonwoven fabric has a 15% higher volume than the cured laminate, the difficulty is to design the mold so that the component geometry is maintained and air tractions and delamination do not lead to reject products.

1.8.2.3  Molds for the “So� Core” Technology

1.8.2.3.1  Master ModelThe master model (mock-up) (Figure 1.207), also master core, defines mostly the surface of the component to be manufactured. From the original core, further molds can be replicated. The core, as well as the following molds, is subject to modifications.

Depending on the application and appearance (slightly or strongly curved struc-tures, complex-shaped) and the molds to be molded, a series of criteria define the material choice for the original model.

Key criteria in this respect include:

� processing time for manufacturing, � long-term behavior in storage, � accuracy, � ease of modification, � stiffness, � operating conditions when molding (pressure, temperature, handling), � processing costs (material costs, processing costs, etc.), and � releasing agent capacity.

Depending upon the specific relevant criteria, the materials cast aluminum, cast steel alloy, ureol, obo wood and Rohacell are considered suitable for original surfaces. In the case of slightly curved contours, a welded aluminum or steel construction shows substantial advantages.

2031.8 Molds for Continuous Fibre Reinforced Polymer Composites

FIGURE 1.207  Manufacturing resource production

(1) Original core NC-milled; (2) CFK mold pocket;

(3) component model; (4) elastic mat

1.8.2.3.2  Mold CavityThe cavity is shaped a�er the master model (Figure 1.207). The cavity (comparable to the original surface) can also be manufactured through machining based on drawings. The disadvantage is that a�er wear or mold duplication, a new, expensive machined part has to be manufactured. The mold cavity can be manufactured from different materials. A series of criteria has to be taken into consideration when choosing the material.

Because the mold cavity (which is located at the smooth surface of the component) in the autoclave is exposed to extreme loads, extended research and development from some companies has been done to meet the requirements of this mold and to extend the operational life.

Besides the lifetime, the following criteria are important:

� manufacturing costs, � stiffness (mostly a backing is added), � releasing agent behaviour, � behavior during heating and cooling, � thermal expansion behavior, � warpage,

204 1 Molds for Various Processing Methods

� hardening temperatures during demolding the master mold (should be as low as possible, in order to protect the master mold), and

� processability.

The new fiber-reinforced epoxy resin systems for mold components are suitable for this task. Especially the resins which cure at low temperatures (40 to 60 °C) make it easier to choose the material for the original surface.

1.8.2.3.3  Product SubstituteThe substitute is needed for the manufacture of the elastic mat. It is laid up segment by segment in the cavity (Figure 1.207 and Figure 1.208), and its shape is identical to the shape of the future product. Depending on the required accuracy, the materials aluminum, carbon-fiber-reinforced polymer [CFRP] or glass-fiber-reinforced polymer [GFRP]) can be used for the substitute. To prevent different thermal expansion between the substitute and the elastic mat during hardening at around 120 °C, it is recommended one uses CFRP.

The elastic mats have only a limited working life (depending on the component geometry and handling method) and must be made repeatedly, depending on the number of products to be made. This must be considered in the design of the substi-tute. In case of an extremely curved skin panel, (e.g., an aircra� fuselage segment), which is reinforced with profiles on the inside, the use of wedges at the side of the stringers has been appropriate. These wedges facilitate the subsequent demolding of the elastic mat and increase its working life.

FIGURE 1.208  Substitute manufacture (stringer simulation = laminate thickness)

(1) Frame simulat; (2) substitute paneling; (3) insert wedge for substitute of

longitudinal joint

2051.8 Molds for Continuous Fibre Reinforced Polymer Composites

1.8.2.3.4  Elastic MatAn elastic mat (Figure 1.207 and Figure 1.209) is used in the production of highly integral, monolithic products in order to support the stiffening elements and to seal against the mold; it is thus the second half of the mold. It consists of a curable material that is more or less elastic (Mosites or Airpad); it is placed in a fluid or kneadable state, above the substitute before hardening. Individual stiffeners made from fiber materials provide the necessary stability for the elastic mat needed to maintain the product geometry.

During hardening of the product, the constancy of the resin-to-fiber ratio and the fiber orientation must be ensured even at temperatures of up to 200 °C, at which the resin of the pre-impregnated layup becomes fluid. Thus, the elastic mat is the negative of the product that contains the reinforcements and stiffeners of the product and which forms the shape of the inner skin. It replaces the sheet, which is usually sufficient for sealing skin panels without reinforcements, during the curing process. Because the layup is about 15% higher than that of the laminate (when cured), the elastic mat must be flexible to permit an even action of autoclave pressure on the product. Only in this way can voids and delamination be prevented, ensuring high-quality products.

FIGURE 1.209  Manufacture of the elastic mat

(1) Vacuum sheet and woven layer; (2) substitute; (3) elastic mat;

(4) sealing strip; (5) cavity

1.8.2.3.5  Manufacturing of ProductsFigure 1.210 shows the individual steps in manufacturing the component. The basic material for making monolithic fiber products are pre-impregnated fiber fabrics or unidirectional fibers. Prepreg material on an epoxy resin base has a limited handling time and can be stored for a longer period of time only at temperatures of about −18 °C.

For ease of processing, it is slightly sticky on both sides and is protected by a thin film. To eliminate air inclusions that may have already formed during the layup process, a vacuum is applied to the layup a�er a certain number of layers.

206 1 Molds for Various Processing Methods

The layup is compressed and strengthened at the same time. Profile reinforcements (e.g., stringers) are separately positioned and shaped. A�er completion, the layup is placed into the mold cavity (see Figure 1.210). Subsequently, the stringers can be positioned (in frozen condition for added rigidity and handling) either on top of the layup or into the recesses of the mat. The elastic mat is now positioned exactly over the layup and carefully sealed at the edges. For ease of demolding, the mold and the elastic mat must be sprayed with a release agent before the prepreg mate-rial is in place.

The product is cured in the autoclave, under pressure, temperature, and the specific cycle time. Therea�er, the product is carefully removed from the mold and trimmed.

In most cases, cleaning with a coarse abrasive cloth is sufficient to remove any resin flash. Figure 1.211 shows a pressure bulkhead for an airbus that is manufactured in an autoclave.

FIGURE 1.210  Product manufacturing

(1) Layup male core; (2) layup; (3) CFRP cavity;

(4) making of stringers; (5) stringer; (6) elastic

mat; (7) component

2071.8 Molds for Continuous Fibre Reinforced Polymer Composites

FIGURE 1.211  Pressure bulkhead made for an Airbus (source: Airbus Deutschland GmbH)

1.8.2.4  Molds for the Hard-Core Technology

1.8.2.4.1  Mold Construction and MaterialsSimilar to the so�-core technology, a mold cavity is also needed here; however, this cavity must be more solid because of the higher forces occurring during both the layup and the curing process in the autoclave. Therefore, the cavity is made from aluminum or steel, milled in one piece without previous master models. The cores (Figure 1.212), usually divided, are made very accurately from aluminum.

FIGURE 1.212  Construction of stiffening elements and arrangement of aluminum cores

(a) Three-part aluminum core; (b) product a�er demolding

1.8.2.4.2  Manufacture of ProductsThe manufacturing process using hard-core technology is described using the example of an aviation structure. Cut blanks are supplied in shapes of skin layers and strips. The layers are manually positioned in the mold; the strips are wound around the cores in a partially automated process. In the case of rectangular module cores of the vertical tail shells, multiple layer strips are wound around cores in one single working process; the strips are manually wound around the module cores of the side wall, and then the strips are folded in an automatic folding fixture (Figure 1.212).

The components are cured and demolded. First, the central part of the three-part module is removed from the product and then the side parts (Figure 1.212).

208 1 Molds for Various Processing Methods

A�er all core parts are removed, the product is taken out of the mold and conveyed to a station for further mechanical finishing. In an automated process, the module cores are cleaned of residual resin and again sprayed with a mold-release agent. The core parts are then assembled again to form a module core and are provided on pallets for the next winding process. The mold cavity is also cleaned and sprayed with mold release.

1.8.2.5  Molds for Automated Tape Laying

In automated tape laying, a distinction should be made between thermoset prepregs and thermoplastic semifinished parts, the so-called tapes.

In thermoset tape laying, the manual “laying” process of the prepreg is automated. This can be done directly inside the mold or through an intermediate step of a flat textile structure (prepreg laminate). The aforementioned conditions apply for the mold design and the mold requirements.

In thermoplastic tape laying, thermoplastic, unidirectional fiber-reinforced, fully impregnated and consolidated modules, the so-called tapes are deposited on a mold. The tapes are typically 0.12 to 1 mm thick and 5 to 300 mm wide. The tapes pass a heating source while depositing and are then melted. In a consolidating roll, the tape is pressed to the mold (which is also heated through a heat source) or the previous laminate layer, is consolidated through the applied pressure, and then cooled. If no in situ consolidation is needed, it can be done in a subsequent autoclave process as well. Tape laying molds for thermoplastic tape laying are mostly open molds. Temperatures of up to 450 °C can be reached due to processing of thermoplastic fiber-reinforced plastic material. Pressures of up to 3 MPa are needed for a con-solidation. The pressures can be locally higher due to the prevailing line contact. Aluminum or steel molds are common. Bonding of the first layer in the beginning of the process and an easy release of the component a�er finishing is very important.

FIGURE 1.213  Mold with a concave and convex surface

2091.8 Molds for Continuous Fibre Reinforced Polymer Composites

Both can be realized with appropriate temperature control. Through temperature control, possible residual stresses can be reduced. Figure 1.213 shows a mold for thermoplastic tape laying with a concave and convex surface.

1.8.3  Continuous Fiber-Reinforced Thermoplastics

1.8.3.1  General Information and Fundamentals of the Processes

This section focuses on continuous fiber-reinforced thermoplastic materials and the molds that are needed for a high-volume-compatible production of these materials. The complete process chain is illustrated in Figure 1.214.

The production of semifinished parts, as the first process step, is dedicated to provide fully impregnated and consolidated fiber-reinforced semifinished plates (so-called organic sheets). Through shi�ing the time consuming impregnating step into the semifinished part manufacture, the high production speeds are increased in the shaping process step (e.g., thermo forming) [1]. The finished components can be bonded to assemblies if needed. The joining techniques of induction or vibration welding (depending on the component) are especially suitable for thermoplastic fiber-reinforced materials.

Semifinished part production FKV semifinished plates

Bonding technology Thermoforming

FIGURE 1.214  Process chain for the processing of continuous fiber-reinforced materials

1.8.3.2  Molds for Semifinished Part Production

1.8.3.2.1  General Information and Fundamentals of the ProcessesThe thermoplastic semifinished part production can be divided into the following processes: impregnating, consolidating, and transitioning to the solid state. During processing, the input materials (fiber and matrix) depend on the process-required control variables Temperature T, time t, and pressure p, which will then determine the resulting properties of the semifinished parts [2].

210 1 Molds for Various Processing Methods

For impregnating, the thermoplastic matrix is heated above its melting temperature so that during infiltration, every reinforced filament is fully wetted with the matrix. Due to high melt viscosities of the thermoplastic matrix systems (100 to 1,000 Pas), the wetting and infiltration process is only possible a�er a parallel application of pressure. In the consolidation phase, the material is cooled under pressure and brought back into a solid state. During the cooling process, the morphology of the thermoplastic matrix forms [2].

1.8.3.2.2  Molds for Flat Semifinished PlatesStatic, semicontinuous systems are used for the manufacture of semifinished parts and components. The complexity of a system is influenced by the maximum process temperatures, the process pressures, as well as the variation possibilities during the process cycle [3].

Depending on the working procedure, the different system types consist of dif-ferent mold concepts and pressure profiles. Static presses can consist of fully or bilaterally closed molds. Semicontinuously working presses will only work with fully or bilaterally open molds because the material flow in the feed direction has to be ensured. Continuously working presses though, consist of circulating bands, which apply the pressure onto the laminated part to be manufactured. Figure 1.215 to Figure 1.217 illustrate schematic mold examples for three different types of systems.

Coupling flange forcylinder connection

Shearing edge

Temperaturemonitoring

Insulation plate

Cooling connection

Temperaturecontrol unit

Parallel guidance

FIGURE 1.215  Mold example for a static system

Ram

Shearing edgeCooling plate

Spacer sleeve

Pressing table

Heating plate

Upper Pressing moldwith in feed chambers

Laminatingdirection

FIGURE 1.216  Mold example for a semicontinuous system (intermittent compression molding)

2111.8 Molds for Continuous Fibre Reinforced Polymer Composites

Heating plate

PressureHeat bridge

Pressure pads

Conveyor belts

Cooling plate

FIGURE 1.217  Mold example for a continuous system (isobaric double belt press technology)

As seen in Figure 1.215 to Figure 1.217, processing of continuous fiber-reinforced thermoplastic materials is done in heated molds. These are mostly manufactured from mold steels for warm processing (so-called hot work steels). Well suitable steels are for example 1.2311, 1.1730, and 1.2710. A useful help for selecting the appropriate mold steel is the so-called Key to Steel (Stahlschlüssel Verlag, Marbach, Germany). Different steels and their composition and suitability are therein defined.

Before designing, a decision has to be made whether the temperature control system of the laminate mold should be integrated into the mold (Figure 1.215) or if temperature control using external heating and cooling plates should be favored (Figure 1.216). Molds with external heating and cooling plates (for temperature control) are more cost-effective as molds with an integrated temperature control system. A mold change can be done faster because equipping or removing of tem-perature control connections does not apply. The cycletime of molds without an integrated temperature control system is higher because heating and cooling is done through heat transfer between the heating plates and the mold and through heat conduction in the mold. The temperature control of the mold can be done with electrical heating cartridges or fluids. For the larger temperature range (25 to > 400 °C), electrical heating cartridges are used. Other heating media are hot air, hot steam, hot water, or oil. The greatest effectiveness (energetic) in temperature control of the mold via fluids can be achieved using oils. A distinction is made between silicone oils and high temperature oils (perfluorinated polyether oils). When using silicone oils, process temperatures of about 200 °C in open circuits (presence of air) and process temperatures of about 250 °C in closed circuits (under the exclusion of air) can be realized. High temperature oils cover a range of up to 250 °C in open systems and up to 350 °C in closed systems. If hot air is used as a heating media, maximum temperatures of over 400 °C can be reached. Hot water and hot steam systems cover a temperature range of 100 to 180 °C. Table 1.9 gives an overview of the temperatures that can be reached with different heat sources.

212 1 Molds for Various Processing Methods

TABLE 1.9  Temperature Ranges of Different Heating Media

Heat Source Energy Source Temperature RangeElectrically tempered plates or molds

Electricity Continuously up to Tmax > 400 °C

Plates or molds which are tempered using fluids

Air and hot air Continuously up to Tmax > 400 °C

Hot water or hot steam 100 to 180 °C

Silicone oils Opened circuit, continuously up to Tmax = 200 °C

Closed circuit, continuously up to Tmax = 250 °C

High-temperature oils Opened circuit, continuously up to Tmax = 250 °C

Closed circuit, continuously up to Tmax = 300 °C

Larger presses mostly consist of parallel guide systems of the ram. If that is not the case, a parallel guide system should be integrated to the mold concept for shearing of the mold. Pressing molds with reduced friction edges, but without their own parallel guidance, have to be operated in presses with a parallel guidance system because the ram can get redirected due to nonsymmetrical load distribution and the shot-off edge can get damaged.

1.8.3.2.3  Molds for ProfilesProfiles can be manufactured with static pressing units (autoclave, upper ram press) as well as with semicontinuous or continuous units. Semicontinuous systems, like intermittent compression molding, enable the manufacture of continuous profiles. A distinction has to be made if the profile is open or closed. Open continuous profiles like U-, and triple V-profiles, can be manufactured with simple molds whereas the manufacture of closed profiles (e.g., double I-profile or rectangular hollow profile) can only be realized with an increased mold complexity. Figure 1.218 illustrates a profile mold for the manufacture of open double-V profiles. This mold consists of

Mold base

plates

Male mold

Female mold

Push mold

Heating area

Cooling area

Cooling

connection

FIGURE 1.218  Profile mold for continuous manufacturing of Double-V profiles on hot presses

for intermittent compression

2131.8 Molds for Continuous Fibre Reinforced Polymer Composites

an integrated heating and cooling zone. In the heating zone, the material is heated (over its melt temperature) and impregnated, and in the following cooling zone it is consolidated using pressure. The feed unit has to be provided with a suitable mold that corresponds to the component contour.

The pressing molds are mostly made from hot work steels. Molds that are integrated into the feed unit can be made from aluminum because the forces that are occurring are significantly lower. The manufacture of hollow profiles (as explained before) can only be realized with major molding expenditures. To be able to manufacture a hollow contour, the material has to be laminated around a core. One has to be mindful to bring the core in a fixed position in front of the system so the core will not be brought through the system itself. Depending on the hollow profile, it is sometimes required to attach lateral pressure cylinders to apply pressure to the side walls. With the vertically working standard cylinder, no pressure can be applied to the side walls of a pressing mold. A sound knowledge of the impregnating and con-solidation behavior of thermoplastic fiber-plastic composites is necessary for a mold construction for the manufacture of profiles by intermittent compression molding.

1.8.3.3  Molds for Forming Technology (Thermoforming)

1.8.3.3.1  General Information and Fundamentals of the ProcessesThe general process of thermoforming is illustrated in Figure 1.219. For the shaping thermoforming process, the processing temperature has to be rised above the melting temperature of the thermoplastic resin. A�er reaching the necessary temperature, the semifinished part is brought into the press for processing using a transporting unit to avoid an early cooling of the laminate surface through con-vection. The stiffness of the heated composite is higher due to the continuous fiber reinforcement (in comparison to processing of nonreinforced plate materials), which makes handling much easier.

Heating of theorganic sheet

Positioning ofthe heated

organic sheet

Thermoformingof the

organic sheet

Cooling andremoval of

the work piece

Edging of thework piece

FIGURE 1.219  Typical thermoforming process of organic sheets

214 1 Molds for Various Processing Methods

The following shaping thermoforming process can be realized through various different mold concepts. However, they all have in common the homogeneous pressure distribution during the thermoforming process to avoid delamination and defects. The realizable degrees of forming are influence by different factors. The draping behavior of the chosen fiber architecture is the biggest determining factor. Furthermore, the angles of the reinforcement architecture can change during ther-moforming. These may become noticeable in strongly deformed areas with material thickening of up to 100%, depending on the matrix content. A draping simulation or experimental determination of material thickening is highly recommended to ensure an optimal mold design for processes with fixed cavities (e.g., metal stamp-ing process).

In the following cooling process, the heat stored inside the organic sheets has to be transferred into the mold to lower the matrix temperature below the solidifica-tion temperature. A�er cooling the component, the press opens and the molded part can be removed. The achievable cycle times of the thermoforming cycle are dependent on the component and the decoupled heating and are well below one minute. Table 1.10 illustrates the processing parameters of several chosen basic polymers. A�er a subsequent punching and seaming process, in which the residu-als are removed, the component is finished. Using a suitable component geometry, trimming can be done using a trimming unit that is integrated in the thermoform-ing cycle (into the mold). All of the following thermoforming processes share these process steps: heating, thermoforming, cooling, and trimming [4, 5].

TABLE 1.10  Processing Parameters of Selected Polymers

Polymer Organic Sheet Temperature [°C]

Mold Temperature [°C]

Thermoforming Pressure [bar]

PP 230 90 20–40

PA 6 270 130 20–40

PA 12 240 100 20–40

PC 300 130 20–40

PET 300 130 20–40

PBT 260 100 20–40

PPS 320 120 20–40

PEEK 360 100 20–40

1.8.3.3.2  Molds for the Stamp FormingStamp forming processes are processes that form the heated organic sheet using exactly defined upper and lower stamps and quick-closing presses. This process enables extremely short cycle times of less than one minute. The stamp forming

2151.8 Molds for Continuous Fibre Reinforced Polymer Composites

process can be divided into the pure metal stamping or the silicone stamping process depending on the type of upper stamp.

In the pure metal stamp forming process, both mold halves are made from metal. In silicone stamp forming, one mold half is mostly made from elastic material (mostly the upper mold half). Table 1.11 shows a comparison between both processes con-cerning the most important features.

All of the processes have in common the defined feeding of the semifinished part under tension during the transforming process to reduce or to totally avoid wrin-kling. This can be done using mechanically or pneumatically operated grippers or through springs (Figure 1.220).

Female mold

Roller downholder

Laminate

Pneumaticcylinder

Rollers

FFFF

Pneumatic downholder

FrameCylinder 1

Laminate

Cylinder 2

FIGURE 1.220  Pneumatic downholder and roller downholder for the introduction of membrane

stresses into the laminate to be formed

TABLE 1.11  Comparison of Stamp Forming Processes

Silicone stamp forming process

Metal stamp forming process

Aluminum Steel

Pressure distribution uniform irregular

Undercuts possible to a limited extend

only possible through slider which are provided to the mold

Cavity flexible inflexible

Mold service live [thermoforming]* 1000 10,000 100,000

Component surface good on one side good very good

Mold costs in comparison low high at the highest

Residual stresses in the component high low

* estimated, depending on the material, degree of forming, and the process requirements

216 1 Molds for Various Processing Methods

The clamping velocity of the press is mostly two-staged. The velocity of 40 mm/s in the beginning of the forming process has to be reduced to 5 mm/s in the last 10 mm of the closing process [5–7]. In the pure metal stamp forming process, the cavity of the closed mold is exactly defined and has to be adapted to the process in areas of high draping and local material thickening. To avoid a premature cooling of the molten matrix, the mold halves are heated, depending on the matrix polymer (see Table 1.10). This constant mold tempering can be done using an electric heater (provided to the mold halves) or through oil or water heating. There are no pressing forces orthogonal to the pressing direction due to the fixed molds. Surfaces vertical to the pressing direction should be avoided or should be constructed with a small dra� angle of 3 to 5°. This will make demolding of the component easier.

Fibers in the fiber reinforced polymer composite (FRPC) can leave marks/irregu-larities on the surface of the molds that can affect the surface of the components as well as the draping, due to a change in the frictional behavior. The mold halves should be manufactured from mold steel to reach a targeted number of more than 5,000 forming processes or high specifications for the quality of the surface finish (Class A surfaces). An additional increase of service life can be reached through an increase of surface hardness (e.g., case hardening, nitrating, etc.). The mold should be wetted with a liquid release agent before the first forming process as well as in regular intervals during the process. This prevents sticking of the molten matrix during forming and facilitates demolding.

When using the silicone stamp forming process, the surface quality of the component can only be positively influenced one-sided through a metal matrix. The advantage of this process (in comparison to metal stamp forming) is the homogeneous applica-tion of pressing force. There are pressing forces orthogonal to the pressing direc-tion (transverse extension of the elastomer) which allow slight undercuts. Another advantage is the lower mold costs because only one mold half has to be machined. The geometry of the silicone stamp is manufactured in the casting process off the lower mold half.

The cavity, which is determined by the semifinished part, has to be made by insert-ing wax plates (of a defined thickness) into the lower mold half. As the starting material for a silicone stamp, silicone or polyurethane casting elastomers can be used. Silicone casting elastomers have a greater heat resistance but also a lower cracking resistance in comparison to polyurethane. The service lives as well as the possible degrees of forming are strongly dependent on the breaking strength and elongation at break of the silicone and the temperatures that occur during forming. Figure 1.221 shows a mold with a lower mold half made from metal and a silicone stamp.

2171.8 Molds for Continuous Fibre Reinforced Polymer Composites

FIGURE 1.221  Forming mold with a lower mold half made from metal and a silicone stamp

1.8.3.3.3  Molds for the Diaphragm TechnologyDiaphragm forming is one of the oldest processing procedures for the manufacture of thin-walled components made from continuous fiber-reinforced thermoplastic materials. The non-isothermal diaphragm forming established itself. It has shorter cycle times than diaphragm forming in an autoclave [8, 9].

In non-isothermal diaphragm forming, the organic sheet is positioned between two super-elastic foils (diaphragms), and a vacuum is applied (Figure 1.222). These diaphragms consist of high temperature resistant elastomers, for example fluor-silicone (FVMQ) or acrylate rubber (ACM), and have elongation values of a couple of hundred percent in specific temperature ranges. The packages to be formed are heated by radiation or conduction over the melt temperature of the polymer and are then transported to the forming station. Depending on how the system is designed, the heating station can be transferred out of the forming station.

Clamping

device

Diaphragms

Semifinished part

Female mold

FIGURE 1.222  Mold concept of the diaphragm process

218 1 Molds for Various Processing Methods

A�erwards, the clamping chamber is closed and the diaphragms work as seals. Below the packages is the mold. Vacuum is applied between the mold and the packages to be transformed. Because the underpressure of a vacuum is not enough to form continuously fiber-reinforced thermoplastic materials, pressure is applied from the upper mold half to the packages to be formed. The diaphragm package is formed through the pressure inside the mold, is cooled down in the heated mold half, and can be demolded a�er falling below the recrystallization temperature.

Because the laminate is not in contact with the mold, the temperature loss (in comparison to the stamp forming process) worsens, which means that the cooling or cycle times increase. The service life of the mold is increased in comparison to the stamp forming process because no fiber marks occur on the mold surface due to the protection of the diaphragms so mold wear is then significantly decreased.

1.8.3.3.4  Molds for Sandwich ComponentsTo produce thermoplastic sandwich components in the thermoforming process, a differentiation has to be made between sandwich forming in multiple steps or the difficult one-step process.

In sandwich forming in multiple steps, the top layers and the core are separately transformed first and are connected in the following step [10]. A minimum of two forming molds and a various number of process steps are necessary due to the geometrical differences of the top layers. Forming should be done in separate metal stamp forming molds due to the necessary component accuracy.

In sandwich forming in one step, the pre-contoured core materials are positioned between the top layers, fixed, and together heated as package. A�er reaching the melting temperature of the matrix polymer of the top layers and the transform temperature of the thermoplastic core, the package is transported into the press and transformed [11]. The core material can only conditionally absorb compression forces in the tempered condition. This has to be taken into account when selecting the right core material.

To manufacture sandwich structures, the silicone stamp forming process as well as the diaphragm forming process cannot be used because the pressure distribution during the forming process cannot be influenced. In the metal stamp forming, the forming pressure can be adapted to the process conditions through the cavity and through bonding the top layers together.

Figure 1.223 shows a metal stamp forming mold for sandwich forming with a clamping frame and a transformed component. Due to the compressibility of the core material, higher density deviations in the top layers can be tolerated, and local thickening can subsequently be integrated into the component concept without any mold modifications.

2191.8 Molds for Continuous Fibre Reinforced Polymer Composites

(a) (b)

FIGURE 1.223  Forming of a sandwich component in the one-step process

(a) Sandwich (GF/PP-PMI Foam-GF/PP) which is clamped into the clamping

frame; (b) component directly a�er forming (in the mold)

1.8.3.3.5  Molds for the Process Step IntegrationThrough the integration of process steps like welding or punching into the forming process, the efficiency of the entire process chain increases significantly. Especially the process-integrated realization of functional and reinforcement elements can be used through an integration of a welding process as a process step of thermoforming. This also requires an integration of the attachments into the forming mold and an adapted process control. When designing such an integrated mold, it is essential to ensure that the functional elements are connected through separate, independently controlled pressure transmitters as slider modules in the mold. Figure 1.224 shows a concept of connecting L-profiles during thermoforming.

The L-profile is inserted into the slider module. To ensure a good connection of the organic sheet during forming, the L-profile has to be separately heated before the forming process on the contact surface to the organic sheet. Here, no complete melting of the L-profile occurs so that a defined process control in terms of welding path regulations is enabled.

Organicsheet

Guidingjaws

Profileholder

Upper mold

L-profile

Cylinder

Lower mold

V

FIGURE 1.224  Concept of realizing the tailored blanks technology and forming component

with a process integrated ribs connection

220 1 Molds for Various Processing Methods

20 mm20 mm

Removablemandrelwith bearings

Length-adjustablearrestor

Spring-loaded lowermold plate

Tripping forthe lowermold plate

Replaceablecavity in theupper mold part

Formingmandrel madefrom PPK

Locally heatedorganic sheet

Fixed strippingnozzle

FIGURE 1.225  Mold concept for joining bearings during the forming process and component

with a bearing seat

Similar mold concepts are also possible for the integration of bearing elements in thermoplastic fiber-reinforced plastic components [12].

As seen in Figure 1.225, the mold movement that is needed for the process can be reproducibly manufactured through a mechanical coupling of individual mold com-ponents. Care must be taken to ensure a suitable material for the laminate is used.

1.8.3.4  Molds for the Welding Technology

The two most important welding processes of fiber-reinforced plastic components are vibration and induction welding.

In vibration welding, joining partners are moved relatively towards one another using pressure with a frequency of 240 Hz. The generation of heat is done through friction.

The importance in vibration welding is the relative movement of the joining partners. This will require precise molds to transfer the vibration frequency. Tolerances of 0.05 mm must be met. Because the heat directly forms in the joining zone, there are no restrictions on the mold materials. Due to reasons of high dimensional stability, mostly aluminum or steel molds are used. The joining partner in the lower mold half is positioned by gravitation. The joining partner in the upper mold half can be fixed by applying a low vacuum. The relative movement between the joining partners is forced by using a machine-specific vibration system. The upper mold half together with the fixed joining partner are parts of the vibration system. Attention should be paid to the specifications of the equipment builder in regards to the maximum mass and to the mass distribution (mold and joining part). Figure 1.226 shows an example of a welding mold for simple overlap joints.

Due to the welding outflow of the melted matrix during the welding process, a thick-ness reduction of the joining partners can be seen. This has to be taken into account when designing the mold, otherwise the upper and lower mold will rub against

2211.8 Molds for Continuous Fibre Reinforced Polymer Composites

each other and can therefore be severely damaged. Pretests are probably necessary for dimensioning the necessary mold recesses. When screwing the molds to the base plates, protective measures must be taken to prevent a vibration-dependent loosening of the screws.

In induction molding, the component to be heated is exposed to a high-frequency oscillating magnetic field. Therefore, eddy currents are induced that heat the com-ponent by loss of resistance. If the joining partners are electrically conductive, for example carbon-fiber-reinforced plastic, the joining partners themselves are heated. If this is not the case, an additional welding material (electrically conductive) is needed. For a targeted and efficient generation of heat, there are specific require-ments on the material to be used.

For this reason, the holding devices for the joining partners should be manufactured from electrically non-conductive materials. Ceramics and glass-fiber-reinforced insu-lation materials can be used. Glass-fiber-reinforced high-performance plastics for sample holding devices offer some advantages due to their high mechanical strength, good thermal and electrical insulation properties, easy mechanical machinability, and good dimensional stability.

Through the use of quick-clamping devices, samples can be positioned into the mold fast and in a reproducible manner. To reach a reproducible consolidation of the molten polymer, temperature-controlled pressing molds are used. The ram and the mold table have to be designed plane-parallel.

1.8.4  Molds for the Resin Injection

1.8.4.1  General Information and Fundamentals of the Process

The resin injection technology is a primary shaping process of processing fiber-reinforced composites. Molds are needed on which the targeted geometrical shape can be exactly reflected on the component. The work involved in production and the operation of the mold is determined by the technical specifications of the compo-

Lower joiningpartner

Gap for the vacuum

Overlapping length of thejoining partners

Oblong holes for exactpositioning

Lowermold half

FIGURE 1.226  Vibration welding mold for simple overlapped plates

222 1 Molds for Various Processing Methods

nent, including the materials to be used and the planned quantities. Their spectrum reaches from one-sided molds that are manufactured by hand from casting resins or composite materials, to heated complex steel molds with chrome-plated, high-gloss polished surfaces for parts in the visible areas of motor vehicles.

The variety of process variations in resin injection also offers more options in the design of the molds. The most important basic functions are similar in all mold concepts. These include compacting of the reinforced semifinished part and con-touring of the components, up to curing and demolding [2, 13, 14].

In the vacuum injection process (Vacuum Assisted Resin Injection [VARI]), which is the most simple resin injection process, molds with a fixed and a flexible mold half and molds with two fixed mold halves are used (Figure 1.227). The textile semifinished part is inserted into the cavity, the mold is closed, and a vacuum is applied. The vacuum ensures that the mold stays closed and the reinforced structure is compacted. A�er opening the inlet, the structure is impregnated by infusing the matrix into the fiber material. A�er curing, the finished component can be removed from the mold.

Another very common process (known since 1950) is the resin transfer molding (RTM) process. Filling the cavity is not done through applying vacuum, but through pressure. The basic production process for a component is similar to the vacuum process but is more demanding in terms of the injection mold. In the pressure-sup-ported injection processes, only mold concepts with fixed mold halves can be used.

In all processes, the feeding time of the cavity can be significantly reduced by using assembled semifinished fiber products (preforms). This increases the economic feasibility for the manufacture of components with complex spatial structure.

VacuumPreform

Mold closes

Finishedcomponent

Dry reinforced fiber

Resin system

FIGURE 1.227  Vacuum injection process

2231.8 Molds for Continuous Fibre Reinforced Polymer Composites

1.8.4.2  Molds for the Preform Technology

A preform or a textile preform is a dry fiber structure that is then impregnated with a matrix and transferred into a consolidated fiber-reinforced plastic component. A preform can also be called a Net Shape preform if the preform is a reinforcing structure (depending on the component geometry). A preform can be a 3D structure (3-dimensional fiber orientation) as well as a 3D geometry (3-dimensional component geometry) and consists mostly of different single parts [2].

In general, two different ways of processing preforms can be differentiated. 3D-textile technology can be used for the direct manufacture of preforms. The use of binder or stitching technology is always based on a textile intermediate product, usually 2-dimensional textiles.

1.8.4.2.1  Binder-Forming Molds

(a) (b)

(c) (d)

FIGURE 1.228  Processes of the binder technology (source Eurocopter Deutschland GmbH)

(a) Molds for the binder-forming technology;

(b) molds with an inserted and draped semifinished fiber part;

(c) mold with a preform under a vacuum membrane to fix the semifinished

part during heating and cooling;

(d) finished preform

224 1 Molds for Various Processing Methods

In the binder-forming technology, draped fiber structures are fixed into the end contour using thermoplastic binders. A�er cutting and laying up the laminate, semifinished parts which are coated with binders, are formed on a pressing mold, heated, and then cooled again. The thermoplastic binder is heated above the melting temperature and bonds the single-fiber bundles of the preform structure when cooling. The single steps are illustrated in Figure 1.228.The molds used in the binder-forming technology can be made from metal as well as plastic or fiber-reinforced plastic. It is important that the material can withstand temperatures of up to 200 °C, which can occur in the heating phase. Aluminum molds are used very o�en because the mechanical requirements are rather modest. When choosing the material, care must be taken to prevent the molten thermoplastic binder from sticking to the mold surface. Release agents can be used, but it has to be ensured that a sufficient compatibility with the following process steps (resin injection) is provided. It is also important to determine the quantity of units before choosing the material. The quantity of 10,000 units to be formed can be reached through appropriate surface treatments (e.g., hard anodized aluminum).When using a vacuum membrane for compacting, the mold is constructed very simply. The shaping preform structure is mostly shaped out of a small block (mostly as a positive). Multiple blocks can be connected in a forming mold. The separation of the blocks should be designed so that the semifinished fiber part cannot move a�er positioning. If this is not possible, a counter mold can be used under the vacuum membrane. Material- or production-dependent compromises can be toler-ated in the accuracy of the pressing mold due to the remaining drapeability of the formed preform.To reach a high quantity of units or a high contour accuracy, the investment in a stamp forming system can be economically feasible. The binder coated semifinished parts are heated in a heating section a�er cutting and laying up the laminate. There, the semifinished part is formed and cooled in a cold preform mold. Because clamping and re-feeding devices are necessary, a preform results. This preform can be geometrically exact but not precisely shaped to the end contour due to the still-needed trimming step. This process is directly comparable to the stamp forming of thermoplastic fiber-reinforced plastics.

Correspondingly, the same mold requirements for the wear resistance should be established. Because low-melting thermoplastics are mostly used as a binder, the thermal load of the mold is not very high. The surface quality of the mold can be significantly reduced as well. Molds from ureol or plywood can be used for proto-types.

2251.8 Molds for Continuous Fibre Reinforced Polymer Composites

1.8.4.2.2  Stitching Technology MoldsAnother process for the manufacture of preforms is the textile finishing technol-ogy. The classic, textile production technology, based on stitching, will be based on flat blanks of a reinforcing semi-finished product or on individual parts, which are already made to final dimensions, which are then assembled to complex 3D geometries with the help of the stitching technique. [15].

Preform molds can be simple draping aids or complex stitching templates, depend-ing on the application. A preform mold has to perform different functions:

� Draping: Two-dimensional tailored reinforcements (TRs) are formed to a 3D geometry using draping aids in preform molds.

� Fixing: Multiple TRs are positioned and fixed using fixing aids in the preform mold. � Guidance: The draped and fixed TRs are guided in the stitching system to be able to perform the necessary stitching operations.

� Adding additional functions, for example the positioning of functional elements (inserts).

Depending on the application, different requirements are set for the preform molds. If a preform is needed that is mainly manufactured by hand, a stitching template or similar aids can be used in the simplest case. For complex but smaller components, an alternative preform mold is recommended that is more specialized to the usage of automated technologies.

The individual TRs that are available a�er the cutting process are assembled (in the following step) to sub-preforms using a stitching template (Figure 1.229). Through this preform assembly step, the single TRs can be exactly brought through the contour of the template and then precisely positioned to one another. This position-ing is fixed through an assembly seam (Figure 1.229). The result of the preform assembly step is a net-shaped preform, which can be easily, precisely, and reproduc-ibly positioned into the resin injection mold.

In all preform molds used in sewing machines, lightweight concepts should be real-ized. Due to the high dynamical work processes in sewing (the material is mostly moved), the mold weight has to be kept very low. The molds require a high degree

Stitching templates for an assemblyof TRs to Sub-Preforms

Slits for the assembly seam Assembly seam

FIGURE 1.229  Stitching assembly of Tailored Reinforcements (TRs) to sub-preforms

226 1 Molds for Various Processing Methods

of precision because the stitching process is very susceptive to disturbances (colli-sion of needle and mold, needle breakages and skipped stitches due to resonating of the mold). Complex stitching templates are also used for the manufacture of complex, uneven preforms. The following illustrates an example of the functioning of a 3D stitching template.

Figure 1.230 shows the exploded view of a mold in which the construction and the procedure can be seen very well. The first part of the preform (sub-preform 1) is inserted into the guiding frame of the base plate. The stitching position can be easily recognized using the sewing slits. The draping mold and the second preform (sub-preform 2) are then inserted into the mold and fixed with the cover plate. At the end, the single sub-preforms are stitched to an assembled preform using a sewing machine (Figure 1.230). The draping mold is removed a�er the stitching process and can then be used again.

The stitching-integrated and fixed load input elements, so-called inserts, should be exactly positioned during the preform process, and relative movements between reinforcing structure and inserts should be avoided. Figure 1.231 shows a stitching template for fixing the inserts. The metallic inserts are fixed and positioned to the threaded rod using spacers.

(a)

Coverring platewith stitching slits

Sub-preform 2

Sub-preform 1

Draping mold

Base plate withguiding frame andstitching slits

(b) (c)

FIGURE 1.230  3D stitching template

(a) Expanded view of a 3D stitching template;

(b) 3D stitching template in the stitching machine;

(c) finished preform

2271.8 Molds for Continuous Fibre Reinforced Polymer Composites

FIGURE 1.231  Stitching template for stitching integration of load input elements

1.8.4.3  Molds for Vacuum Assisted Processes

If a resin injection process (which is exclusively working with the assistance of vacuum) is chosen for the manufacture of FRPC components, the two following concepts can be used for the construction of injection mold.

1.8.4.3.1  Molds with a Fixed and Flexible Mold HalfIn the process with a flexible mold half, also called vacuum bagging, compacting the fibers is only done using ambient air pressure. Because the main task of such molds is the contour shaping of the component, they can be less complex than closed molds with two fixed mold halves.

An advantage of these processes that should not be underestimated is the option to use a distribution medium that can be inserted into the fiber material together with the perforated foil (which has to have good release properties). This ensures a very fast injection of the component (impregnating of the fiber structure). In the simple mold, which is shown in Figure 1.232, the base plate is the fixed mold half and the vacuum foil, which can be elongated to a certain degree, is the flexible mold half.

With this mold concept, complex mold geometries can also be realized. If the base plate is replaced with a contour plate, as seen in Figure 1.233, a component with a defined bottom side and defined side surfaces can be obtained. The use of a dis-tributer medium is here recommended.

Air pressure

Inner vacuum line

Base plate

Seal

Inner vacuum foil

Injection linePerforatedfoil

Distributionmedium

Outer vacuum foil

Outer vacuum line

FIGURE 1.232  Flat mold for the vacuum injection process

228 1 Molds for Various Processing Methods

Air pressure

Inner vacuum line

Seal

Contour plate

Inner vacuum foilInjection lineOuter vacuum foil

Outer vacuum line

FIGURE 1.233  Mold with a defined outer contour for the vacuum injection process

The differential pressure decreases continuously between the injection vacuum and environment due to the inflowing resin, which is under ambient pressure. To maintain a compaction pressure at the end of the injection, a second vacuum, which covers the overall structure between environment and mold surface, has to be applied.

If the complexity of the component increases more and more, it could become necessary to replace the flexible mold half (consisting of a vacuum foil) with an expandable contour skin that is specially customized for the component geometry. Such a skin can be manufactured directly in the contour shaping fixed mold half with the aid of an elastomer. A separate auxiliary mold is therefore normally not necessary. If it is a concave shape, for example the interior of a spherical segment, the wall thickness of the composite to be manufactured has to be considered when manufacturing the skin (wax inserting), because otherwise, the skin would get too big. In the later production process, a wrinkle free component can only be realized when the contour skin is fully stretched and not locally compressed. To ensure this, “place holders” can be applied to the injection mold before the application of the elastomer mass. This place holder has to have at minimum the thickness of the finished component. If no higher temperatures to cure the elastomer mass are needed, self-adhesive wax plates can be used (Figure 1.234). The expandable contour skin and the vacuum foil build together the outer mold half (Figure 1.235).

Contour skinWax plates

FIGURE 1.234  Manufacture of a contour skin in a concave injection mold

2291.8 Molds for Continuous Fibre Reinforced Polymer Composites

FIGURE 1.235  Injection mold, ready for the resin injection

An injection mold with only one fixed half can also be constructed as a multi-part core mold. Figure 1.236 illustrates a modular constructed mold that is manufactured from wood and is coated with a surface sealing. The single parts have sufficient dra� angles to each other so that removing the complete core mold out of the finished hollow component a�er curing the matrix will not be a problem. The mold only forms the inner contour of the component to be manufactured. The outer mold half, is in this case, again a vacuum foil (Figure 1.237).

FIGURE 1.236  Modular constructed injection mold made from surface-sealed wood

230 1 Molds for Various Processing Methods

200 mm

FIGURE 1.237  (le�) Modular core mold in injection position; (right) finished component

1.8.4.3.2  Molds with Two Fixed Mold HalvesClosed RTM molds, as illustrated in Figure 1.238, have to fulfill other important tasks besides contour forming of the component. In this mold from upper and lower mold, the force F has to be applied from the outside over the mold to the reinforcing structure, to compact the reinforcement plies.

Injection position

F

F F

F

Venting Venting

Fi

Fi Fi

F > Fi

FIGURE 1.238  Closed RTM mold

2311.8 Molds for Continuous Fibre Reinforced Polymer Composites

0.00

5.00

10.00

15.00

20.00

25.00

45 50 55 60 65 70

Co

mp

ac

tio

n p

re

ss

ure

[b

ar]

Fiber-volume content [%]

FIGURE 1.239  Compaction pressure for different fiber volume contents (standard fabric)

Especially when higher fiber volume contents are used, very high forces occur, which have to be absorbed by the structure of the injection mold. The compaction pressure is dependent on the fiber volume content, which is necessary for the standard fabrics, and is illustrated in Figure 1.239. In addition, when designing the overall system, the compression and closing force have to be applied by the closing system (closing mechanism or press) and should be maintained in the necessary tolerance range.

The fiber volume content determines the flow resistance of the reinforcing structure in an exponential dependency. This means for the filling time of the mold that the component areas with higher fiber volume content are filled (with matrix) more slowly than areas with a smaller fiber content. A maximum cavity tolerance can be defined through the determination of still tolerable fluctuations of the fiber volume fraction in regards to the production time or the component tolerances. With the given interior pressures, this is another design parameter for the permissible deflection of the mold during the production process.

Because pressure differences of one bar are available as the driving pressure in purely vacuum-supported injections, the reachable flow paths in the mold are very limited. A flow path enlargement or a process time decrease through fast filling can be done by applying an additional injection pressure. This should be included into the mold design as well.

In closed molds made from two or more parts, some basic rules should be observed for the mold design. A dra� angle is needed for demolding. This angle should be 5 to 10 mm high and a minimum of 2 to 5° in vertical surfaces. An optimal sealing of the cavity should be ensured so that no bonding of mold parts through the ther-moset material occurs. A heating and cooling system of the mold should be provided depending on the resin system used and the required cure cycle.

To ensure an easy inserting of the reinforcement structure into the cavity, molds with shearing edges are used (Figure 1.240). Exact positioning of the reinforcement

232 1 Molds for Various Processing Methods

plies into the mold is an advantage which is balanced by higher complexity of the mold and higher mold costs.

In the types of gates, the molds can differ in complexity. The simplest gate type is the pinpoint gate. The resin is directly injected through bore holes (in one or more positions) into the reinforcing textile. Pinpoint gates are mostly located in the component surface but can also be positioned at the edge.

Technically more complicated are line gates. Here, the resin is first distributed over a defined distance along the edge of a component and then flows into the reinforc-ing textile using a film gate. The main advantage of the line gates are the shorter injection times. In general, the gate situation should be chosen so that a verifiable resin flow in the mold occurs. Modern simulation programs to determine the flow front can be very helpful in complex geometries.

The venting of the mold can also be important for the process control. In vacuum-assisted processes, the vacuum pump stays active during the entire injection process and is protected from penetrating matrix material by a resin trap. If a plastic tube is used for this venting line, the moment the resin flows out of the mold can be easily recognized. If more lockable exits of a cavity are used, an online flow path control can also be realized in closed mold concepts. The construction of a closed injection mold with two fixed mold halves is dependent on which effort for the actual application is justified or necessary for the process-related implementa-tion.

1.8.4.4  Molds for Pressure-Assisted Processes

Pressure-assisted resin injection processes are used in large scale productions. Optimized flow rate numbers, high fiber volume contents, and reproducible surface

b)

Freefiber ends

Shearing edge

a)

FIGURE 1.240  (a) Mold without a shearing edge; (b) mold with a shearing edge

2331.8 Molds for Continuous Fibre Reinforced Polymer Composites

topographies characterize the process. The overall system consists of computer-controlled, pressure- or volume-controlled matrix injection units and closed, fixed mold concepts. All of the essential characteristics of a closed mold for a pressure-assisted resin injection process are identical to the characteristics of a closed mold for a vacuum-assisted process. However, the mold for pressure-assisted resin injection has to be designed to be able to absorb higher forces. Here, the compacting force (which is needed to compact the reinforcement fibers) and another force (which is caused by the interior pressure of the matrix) add up to be the resulting overall force that is predominant in the mold. When designing the mold and the clamping device, it is important to ensure that the clamping force is always larger than the predominant maximum overall force in the mold.

The design of the cavity and the mold in output-optimized processes must be measured against the criterion of efficiency. Furthermore, quality requirements and reproducibility should be taken into account. The available mold materials are limited by high pressures, possibly wear-resistant molds, and excellent surface quality. The aggressiveness of the used resin system has to be included into the material choice.

Aluminum, which is mechanically easy to be machined, is suitable for the material choice. The surface quality is adequate. The series production, however, requires a longer service life of the molds. For this reason, high strength steels are used. Their surfaces are hardened, ground, and then polished.

Using as small a number of mold components as possible accelerates the mold changing process and inserting of the preform. All components have to be form-fit or force-fit connected and sealed (Figure 1.241). Strength and stiffness require-ments of the molds are considerably more complex with an increase in component geometry and cannot be realized through the shaping component anymore. There-fore, massively designed mold carriers, which determine the introduction of force, are used.

FIGURE 1.241  RTM mold for the interior structural component of a bus (source: Wolfangel)

234 1 Molds for Various Processing Methods

Mold construction Inserting the preform Pressure injection

Lost core

Partially demolded Component

FIGURE 1.242  Process chain in a pressure-assisted process with a multi-part injection mold

Figure 1.242 illustrates a multi-part injection mold made from aluminum for a pro-totype component. The mold is designed so that all of the parts are sealed against each other and are firmly screwed together. No separate clamping device is needed. The mold is designed under consideration of all forces that occur. During gradual insertion of the semifinished part and gradual closing, reinforcing fibers can be compacted in every necessary direction. Due to the constructive design of the component to be manufactured, undercuts occur and a lost core is used. A�er suc-cessful resin injection and a�er running through the cure cycle, the component is demolded (in individual steps). The screwed connections of the mold are unscrewed in appropriate order and the components are removed. The demolding process is completed with the mechanical removal of the lost core.

With the help of simulation programs, with their input quantities of geometry, fiber architecture, fiber volume content, viscosity patterns, gate varieties, and permeabil-ity, the filling behavior of the cavity can be visualized. The danger of “dry”, unwet areas of the component can especially be seen in pressure-assisted processes with an increasing complexity of components. With this innovative design possibility, gate and venting variations for the pressure injection can be tested without any time or cost intensive test runs.

Reactive matrix systems demand a sensitive temperature control during the injection and curing cycle. Aspects of the heat conductivity and heat capacity, especially for the forming exothermic reaction, should be fully taken into account when designing.

During the injection process, observation sensors are used for the quality assur-ance of the manufacturing process. In the pressure-assisted injection, ultrasound measurements or dielectric measurements can be used for this application. The sensor system has to be included into the mold and has to be designed to withstand loads using a validated analysis.

2351.8 Molds for Continuous Fibre Reinforced Polymer Composites

Possible sink marks and surface defects, which occur through the emerging resin shrinkage, can be achieved using an increased holding pressure with the aid of the injection system or through pressing using the mold carrier (press). In the latter case, the inserted sealing systems have to be able to compensate for the additional distance. In both cases, the increased interior pressures have to be considered when designing the mold.

1.8.4.5  Molds for Hollow Components

The manufacture of hollow components can be done with the aid of different process-ing procedures depending on the component geometry. Here, the aforementioned mold concepts and mold requirements for vacuum and pressure-assisted processes can also retain their validity. New mold requirements arise for the manufacture of the hollow space in the component.

A closed injection mold can be used in the blow molding RTM process. First, a flex-ible, inflatable tube (blank) is integrated into the reinforcing textile structure. The now developed preform is inserted into the mold and the blank is connected to a compressed air connection. A�er closing the mold, pressure that is higher than the subsequent injection pressure is applied to the blank. The blank, together with the contour shaping mold wall, form the cavity. The actual resin injection can be done in the manner described. A�er curing the matrix, the pressure in the blank can be reduced to ambient pressure, and the component can be removed from the injection mold. At the end, the blank is removed from the finished component. If this is not possible due to the component geometry, the blank can stay with the component.

Another possibility for manufacturing hollow bodies is to use a modular constructed core mold. If the component geometry and condition of the preform allow it, this concept can be implemented in a closed RTM mold.

The use of soluble and therefore washable (out of the finished component) core materials (e.g., water-soluble Aquapour™ [16]) offers new possibilities when manu-facturing hollow parts. The material is available in the form of a powder that can be mixed with water to a castable compound that is comparable to the composition of gypsum. As casting molds for the core, a variety of model-making materials are suitable because no significant restrictions apply besides the restrictions on the surface. It can be processed at room temperature. For the manufacture of simple component cores, shapes (Figure 1.243) that are formed from standard profiles can be used. If the core dimension exceeds a certain size, a respective reinforcement has to be provided. Depending on the geometry, a metallic grid structure (for sta-bilization) can be casted into the core. A�er demolding the casting compound from the core mold, a drying cycle in the oven follows. Before a coating for sealing the surface is applied, small imperfections on the core can be eliminated (if necessary)

236 1 Molds for Various Processing Methods

using a compatible filler compound. This surface seal has to prevent matrix from flowing inside the existing pores of the core during the resin injection. Therefore, washing of the core material will not be possible in these areas.

A�er a second, final drying process, the preform can be positioned around the core. The component mold can be fed with this package. The component mold has to be modular in design because the package can only be compacted to the final dimen-sion during closing. A�er the injection and the curing process, the component with the core is removed from the mold (Figure 1.244).

In the last step, the core material is detached from the component blank. If a water-soluble core material was used, this can be done using water under pressure. With this process, longer hollow profiles, as well as geometrically complex cavities with undercuts, can be manufactured.

A similar process (to the principle work-flow) uses a metal that melts at low tempera-tures (< 80 °C) as a core material instead of a water-soluble material. This method is called melt core casting (lost wax casting). Here again, the core is manufactured in a separate casting mold. A�er the resin injection and the following curing process of the matrix, the finished component is heated above the melting point of the metal, enabling the core material to melt.

Another possible process variant is to use lost cores made from foams. The foams should be closed cellular foams because it prevents the foam core from filling with resin during the injection phase. If it is open cellular foam, the surface has to be sealed a�er forming the contour. This can be done by coating with resin or through packing in a foil. All foam materials meeting the requirements for compressive strength (fiber compaction and injection pressure) and for temperature (curing and exothermic reaction) can be used. Table 1.12 shows the thermal and mechanical load limits for typical plastic foams.

FIGURE 1.243  (le�) Casting mold made from standard profiles;

(right) demolded core with an embedded metallic reinforcement

2371.8 Molds for Continuous Fibre Reinforced Polymer Composites

FIGURE 1.244  Molds for hollow bodies:

(le�) mold, ready for the resin injection;

(middle) component with the core a�er demolding;

(right) component a�er detaching the core material

TABLE 1.12  Mechanical Properties of Different Plastic Foams

Density � Tmax E-Modulus E3

Compressive strength �3 Pressure

Shear Modulus G12,G13

Shear Strength R13,R23

[kg/m3] [°C] [MPa] [MPa] [MPa] [MPa]PUR foam 30 100 10 0.2 3 0.2

PVC foam 30 80 20 0.3 13 0.35

60 80 60 0.75 22 0.7

PMI foam 52 215 75 0.8 19 0.8

110 215 180 3.6 50 2.4

238 1 Molds for Various Processing Methods

1.8.5  Molds for the Winding Technology

1.8.5.1  General Information and Fundamentals of the Process

The winding technology is a process for the manufacture of mostly rotationally symmetrical parts such as tubes, rollers, or containers. Geometrically complex, not necessarily rotationally symmetrical components, for example elbows and t-pieces can be manufactured in the winding process. Reinforcing fibers in the form of rovings (e.g., glass, carbon, aramid, and polyester or basalt fibers) are wound around a rotating positive core. Thermoset systems, such as low-viscosity epoxy or polyester resins can be used as the matrix material.

Winding machines, which are used in the conventional wet-winding processes, basically consist of a clamping and turning device for one or more cores, a thread guidance mechanism (turn and tiltable in multiple directions), a resin bath for impregnating the fiber roving, and a bobbin stand for storing the fiber material. In thermoplastic winding, the resin bath is not needed because fully impregnated, unidirectional reinforced tapes are manufactured. Instead, a heating section for heating the material and a consolidating roll for pressing the tape are used.

1.8.5.2  Molds for Rotationally Symmetrical Components

The generally rotationally symmetrical mold, which gives the final component its shape, is called the mandrel. It is either directly or with the aid of special adapters fixed to the driving spindles of the winding system (three jaw chucks). Mandrels can be reusable (depending on the component geometry) or remain in the finished part as so-called “lost cores”.

A suitable material for the mandrel manufacture can be steel or in rare cases alumi-num. Standardized round components or tubes are used for small mandrels which have to be reduced (if necessary) to the required outer diameter, using the turning process. A slight conicity can make the demolding later easier. Larger mandrels for containers and tubes consist of a stable inner structure that is covered with bent steel sheets or with a cover made from composite material. These large mandrels have to be removable to ensure the demolding of the component.

The surfaces of these reusable mandrels need to have a very low roughness to minimize bonding of the wound components and to make demolding easier. The surfaces that are in contact with the material are mostly hard-chromed and ground or polished. Therefore, wound components have a very high surface quality with defined dimensions (diameter) on the inside up to the mandrel. However, the surface and the outer diameter are determined by the chosen layup and the winding process. Depending on the application, it may be necessary to reprocess (grind) or to coat (paint, chip protection layer, flammability, and UV protection) the components.

2391.8 Molds for Continuous Fibre Reinforced Polymer Composites

“Lost cores” in pressure tanks or piping can fulfill additional functions as interior containers (liners). They consist of the necessary media resistance (e.g., corrosion), serve as a diffusion barrier (e.g., hydrogen) or enable a secure connection of pipelines (e.g., valves and fittings). Here, welded containers made from low-corrosion steels or aluminum, as well as blow molded plastic containers made from thermoplastic materials with integrated, metallic connection flanges, can be used. Washable cores are much more rarely used (e.g., component with undercuts). These consist of materials that can be melted out or that dissolve using solvents (e.g., special gypsum, foams).

The geometry of the mandrel is primarily determined by the dimensions of the component to be manufactured. There are restrictions on the surface design. The manufacture of undercuts or concave curvature (li�ing the rovings) is very difficult. Cylindrical winding spindles are preferably used.

Unacceptable deflections can be avoided through a lightweight construction of the mandrel.

When winding cylindrical components, the geodetic line has to be modified in the area of reversal zones. Slipping of the rovings can occur when working with small winding angles. This area of the wound component is therefore flawed and cannot be used. Pins, which fix the fibers, can be used to prevent slipping. Thereby, rejects can be minimized or the effective length can be increased. Likewise, 0°-fiber ori-entations can be realized.

Due to the high adhesion forces of the synthetic resins and the shrinkage of the cured components, the surfaces must be carefully prepared before the reinforcing material can be wound onto the core. The removal of resin residues and other con-taminants can be done manually or with the aid of special cleaning machines for large mandrels. Subsequently, a release agent is applied. Adhesion can be reduced in order to facilitate later removal. Most of the release agent must be applied in several layers. They are then polished away and can be used for several production cycles.

Depending on the materials used, it may be necessary to preheat the mandrels. If the mandrels do not have a built-in heating system (oil or electric), they must be heated (to the required temperature) in an oven.

The mostly expensive mandrels have to be removed from the component a�er curing of the matrix material. Cylindrical mandrels are pulled off or pressed out using hydraulically actuated devices. Large winding cores are constructed so that they can be disassembled inside the component into their individual parts.

For special applications, (e.g., water tanks or reservoirs for the low-pressure area), it may be useful to cut open the component a�er winding a few layers to extract the mandrel. Then, the parts at the cutting edge can be glued again, and the winding container can be prepared for the continuation of the winding process.

240 1 Molds for Various Processing Methods

references

[1] Wöginger, A., Prozesstechnologien zur Herstellung kontinuierlich faserverstärkter thermoplastischer Halbzeuge, Series of publications, volume 41, Prof. Dr.-Ing. Alois Schlarb (Ed.) (2004) Kaiserslautern

[2] Neitzel, M., Mitschang, P., Handbuch Verbundwerkstoffe (2004) Carl Hanser Verlag, Munich

[3] Giehl, S., Mitschang, P., Faserverstärkte Sandwich- und Profilstrukturen in einem Schritt, Kunststoffe (2005) 95, pp. 76–78

[4] Nowacki, J., Schuster, J., Mitschang, P., Neitzel, M., Thermoformen von GFK, Kunst-stoffe 89 (1999) 6, pp. 56–60

[5] Breuer, U. P., Beitrag zur Umformtechnik gewebeverstärkter Thermoplaste (1997) Fort-schrittsberichte, VDI-Verlag, VDI-2/433, Düsseldorf

[6] Jehrke, M., Umformen gewebeverstärkter thermoplastischer Prepregs mit Polypropylen- und Polyamidmatrix im Preßverfahren (1995) PhD Thesis RWTH, Aachen

[7] Scherer, R., Charakterisierung des Zwischenlagenabgleitens beim Thermoformen von Kontinuierlich faserverstärkten Polypropylen-Laminaten (1992) Fortschrittsberichte, VDI-Verlag, VDI-5/288, Düsseldorf

[8] Ziegmann, G., Umformen im Diaphragma-Verfahren In Faserverbundwerkstoffe mit thermoplastischer Matrix. Bartz, W. J. (Ed.) (1997) Expert Verlag, Renningen Malms-heim, pp. 143–160

[9] Pohl, C., Michaeli, W., Automated Diaphragm-Forming-Line for Processing of Thermo-plastic Composites with reduced Cycle Time. Proceedings‚ 43rd International SAMPE Symposium, May 31–June 4 (1998), S. 1979–1991

[10] Mehn, R., GF-Thermoplastverbunde im PKW-Bereich In Moderne Werkstoffe. Bartz, W. J. (Ed.) Expert Verlag 2000, Renningen, pp. 302–324

[11] Breuer, U., Ostgathe, M., Neitzel, M., Manufacturing of All-Thermoplastic Sand-wich Systems by a One-Step Forming Technique Polymer Composites 19 (1998) 3, pp. 275–279

[12] Mitschang, P., Kontinuierlich faserverstärkte Thermoplaste – Neue Werkstoff- und Prozessoptionen. Tagungsband 10. Europäische Automobil-Konferenz “Vision Kunst-stoff-Automobil 2015”, Bad Nauheim, 27th–28th of June (2006)

[13] Parnas, R. S., Liquid Composite Molding (1996) Hanser, Munich[14] Arbeitsgemeinschaft Verstärkte Kunststoffe – Technische Vereinigung e. V. (AVK-TV).

Faserverstärkte Kunststoffe und duroplastische Formmassen, (2004) Frankfurt[15] Mitschang, P., Prozessentwicklung und ganzheitliches Leichtbaukonzept zur

durchgängigen abfallfreien Preform-RTM Fertigung. Pro-Preform RTM-Abschluss-bericht BMBF-Projekt Förderkennzeichen, IVW series of publications volume 46, Prof. Dr.-Ing. Alois K. Schlarb (Ed.) (2004) 02PP2460 Kaiserslautern

[16] Advanced Ceramics Research, Inc. (www.acrtucson.com)

2411.9 Molds for Elastomer Processing

 ■ 1.9  Molds for Elastomer Processing

Th. Bauernhansl, K. Zoller

The processing of elastomers began in 1839 with the discovery of vulcanization by Goodyear. Instead of the non-cross-linked natural rubber, as it was already being used by the Indians of South and Central America, a cross-linked, highly elastic material (rubber) was further developed and was then made available for many, especially technical, applications. Elastomers are indispensable in our modern world. In particular, their almost infinitely adjustable elasticity, thermal stability, high wear resistance, and resistance to media make them indispensable for almost all industrial applications [1].

Depending on the application, different elastomers are used that are mixed accord-ing to a particular formulation of various components in a predefined order. The resulting multi-component system is based on a polymer that determines the core properties (such as low-temperature behavior) of the mixture. Next, plasticizers are used, which can improve the behavior at low temperatures, as well as fillers (e.g., carbon black or silica), which can interfere with the polymer matrix as a filler matrix. Through a careful selection of the fillers, elastomers can be qualified for different applications [2].

Through vulcanization (cross-linking) of the rubber mixture, stable elastomer products of different geometries can be generated. This process, which takes place under pressure and elevated temperature in the molds, is also referred to as shaping. The used cross-linking system determines the processing properties, the chemical structure of the network, and the physical properties of the elastomer. The two most common types of networking are the sulfur and the peroxide cross-linking. A�er vulcanization of the rubber mixture, the elastomer components are cooled outside the mold and processed according to the type of elastomer product or purpose [1].

Detailed information about the various shaping techniques and therefore needed mold techniques are described below. Furthermore, different types of molds as well as the specifics of the design and manufacture of molds for elastomer shaping are described. For more detailed information on the topics of elastomer types, material properties as well as mixing technologies, and testing of elastomeric properties at this point, references are made to the relevant literature [3].

242 1 Molds for Various Processing Methods

1.9.1  Compression Molding (CM)

The compression molding process is the oldest still used process for shaping elasto-mer products. It is used in high-wage countries, mainly for small quantities and for elastomers that cannot be processed using other methods. Due to the high degree of robustness of the entire process system against errors, this process is o�en used in low-wage countries like China or India. Prototypes are o�en manufactured with the CM method, since the costs of the molds are relatively small. Thus, the production of small series is faster and more economical than in other processes.

The basic principle of the CM process is shaping of a solid blank (pre-formed and not yet cured mixture) using pressure and heat. The blanks are dimensioned as precisely as possible in terms of weight or volume. Here, oversizing of up to 5% is possible. Depending on the complexity of the product geometry and precision of the technique used, accurately dosed blanks are used [3].

The blanks are inserted into the open mold, which is heated to about 180 °C (Figure 1.245). The mold is then closed at a pressure of about 150 to 700 bar. The level of applied pressure is dependent on the viscosity of the rubber mixture and the flow resistances within the mold. The elastomer plasticizes through heat and pressure, becomes shapeable and adapts to the geometry of the mold cavity. The excess elastomer is collected in so-called overflow grooves along the parting planes. A�er reaching the defined degree of cross-linking, the mold is opened; the product is demolded and finished if needed.

The duration of cross-linking depends on the volume and wall thickness of the elas-tomer component and the used mixture. It can take several hours for thick-walled components. Upon cooling, the blank shrinks by a few percent. This shrinkage is to be considered when designing the mold. In this case, simulations on the basis of empirical values are used [1].

The characteristics of the CM process are the exact determination of the blank weight, the impression surface design, the overflow groove and the determination of the required clamping pressure.

Blank Lower mold half

Upper mold half

Overflow groove Product

FIGURE 1.245  Opened and closed CM mold

2431.9 Molds for Elastomer Processing

The design of the mold is usually divided into two parts. The two shaping plates are centered by a conical guide. Typically, a mold has up to 100 cavities, which can be “compressed” at once.

Advantages � all common types of elastomers can be processed, � very robust process, � only a little know-how of the operator required, � quickly achieving of process capability, � cost-effective molds and machinery, � simple process, and � great flexibility.

The manufacture of the pressing molds is relatively inexpensive and does not require complicated and expensive shape geometries. Furthermore, the CM process causes the least amount of loads on the elastomer and guarantees gentle processing for difficult product applications.

Disadvantages � long cycle times, � flashing, � preliminary processes (production of the blanks) and as well as finishing (de-flashing required),

� sheathing of insert components (e.g., of metal) with defined elastomer-free sur-faces is difficult or impossible because the mold is still open at the beginning of plasticization and thus the inserted components cannot be fixed in the mold,

� deformation, locking and clamping force are applied by a cylinder; this complicates the optimization of the process parameters,

� automatic operations processes are difficult to illustrate, � process variations are possible due to manual operation, � high blank costs, and � imprecise tolerance position of the parts.

The main disadvantage is the long cycle time of the process. Because the blank is cold when inserted into the mold and then heated, the heating time is longer. In addition, the blanks are pre- and post-processed to remove any flashes. Due to the precise requirements for the weight of the blanks, these are expensive to manufac-ture. The procedure may be robust, but also relatively rough in the design process (not exactly definable flashes), which o�en prevents an automatic operation of the process and will affect the tolerance position of the parts.

244 1 Molds for Various Processing Methods

1.9.2  Transfer Molding (TM)

The transfer molding process is a development of the previously discussed CM process. For the TM method, the same machines (presses) are used, which are also necessary for the CM method. The main difference between the two methods is that plasticization and cross-linking takes place in two separate systems. The TM technique is mainly used for small parts (maximum diameter is 100 mm) and for large quantities. The tolerance of the process is a material hardness of 80 to 85 Shore A.

In the simplest case, a TM mold consists of three main components. The upper and lower parts of the mold are attached to the press platens and the middle section is removable. The upper part of the mold serves as a pressing piston. The middle section accepts the mixture and contains the injection channels in the mold cavities, which are incorporated between the middle and lower part of the mold (Figure 1.246). A tapered guide is used to ensure a proper centering of the mold parts to each other. A typical number is between 1 and 20 cavities (in special applications up to 100 cavities).

A pre-dimensioned blank disc is inserted into the so-called displacement cavity between the upper and lower part of the mold, which is preheated to 170 to 180 °C. Then, the cavities between the middle and lower plate are closed. The mold is now completely closed. Therefore, the upper mold part moves towards the middle plate and acts as a piston, which continuously reduces the volume in the displacement cavity. Through the resulting pressure and temperature, the elastomer is plasti-cized and pressed through the injection holes in the cavities. Pressing through the tight injection bore holes (also called flow channels) leads to further heating of the elastomer, since the heat is introduced into the mixture either through the mold walls or through friction.

When designing a TM mold, the effect of the introduced heat can lead to a prema-ture, and therefore unwanted cross-linking (known as scorch), which should be particularly taken into account.

Mold Gate Residue ProductDisplacementcavity

FIGURE 1.246  TM mold with blank or product and residue

2451.9 Molds for Elastomer Processing

The process is designed in a way that a�er complete filling of the cavities, a residue remains in the displacement cavity to serve as a melt cushion. Therefore, the pres-sure of the melt cushion (piston) is greater than the sum of the internal mold pres-sures of the cavities, because otherwise, the cavities would open. The elastomer in the cavities and in the displacement cavity is then cross-linked. A�er reaching the defined degree of cross-linking, the mold and thus the displacement cavity are opened, and the products (which mostly do not need to be re-worked) can be removed. The geometry of the injection bore holes should be selected in a way that an easy demolding of the products is made possible (i.e., defined target braking points should be generated). The elastomer that is located in the displacement cavity is removed and must be discarded (about 15% of the material consumption). The necessary pressure depends on the viscosity of the mixture at 200 to 400 bar, while the temperature of the mold can be between 150 to 180 °C (mold). The heating times are significantly lower compared to the CM method. It is recommended to preheat the blanks to be inserted to about 50 °C to achieve a further reduction in cycle time.The special features of the process are the design of the injection bore holes (defined separation to ensure ejection of the products), the determination of the required clamping pressure, and the exact geometric design of the displacement cavity in rela-tion to the cavities. In addition, the exact positioning and pressure distribution has to be ensured, as this is the prerequisite for a flash-free manufacture of the products.

Advantages � rework-free parts with proper design, � simple and robust process, � little know-how necessary for the operator, � rapid achieving of process capability, and � upper part of the mold with the piston can be used for multiple molds.

The main advantage of the TM process is the possibility of producing flash-free parts and to manufacture molded parts within narrow tolerances. Thus, the method can be used in particular for the production of small and complex parts.The manufacture of flash-free parts requires an accurate design of the mold in terms of pressure and heat distribution as well as ensuring venting of the cavities. Here, it is important to geometrically design the parting plane of the mold so that the pressure conditions and the surface structure of the separating plane allow an escape of the air present in the cavities, but do not allow the elastomer to escape. Renowned mold makers use special micrographs for each parting line and pay special attention to the design of the surfaces so that surface pressures are not too high, as these can lead to destruction of the microstructure a�er a few strokes, which can then lead to process disturbances (air inclusions, flashes, increased contamination of the mold, etc.).

246 1 Molds for Various Processing Methods

Disadvantages � relatively long cycle times, � preliminary processes are necessary (production of the blank disc), � material waste, � relative costly molds, and � precise guidance of the displacement cavity necessary.

The disadvantages of the TM technique mostly relate to the costs. The mold structure for flash-free products is very complex and expensive to manufacture. Therefore, TM processes are mainly profitable for multi-cavity molds (usually over 20 cavities).

Moreover, not all product geometries are manufacturable and a�er each conven-tional TM production, a residue remains a�er each cycle (cured elastomer from the displacement piston), which has to be disposed (as waste) and can account for up to 70% of the overall material consumption.

1.9.3  Injection Molding (IM)

The injection molding process is the latest and most important process for the manufacture of elastomeric molded parts. It was initially used for years in the plastics industry before it was used in the rubber industry.

The process is very similar to the TM process. Prior to the injection into the closed mold, the raw material is drawn in by the injection unit with the aid of a screw and is transported into a storage space in front of the screw. Using friction and heat conduction of the heated cylinder, the mixture is heated and pre-plasticized. Usually a screw/piston unit is used, which means that with the help of the screw, the pre-plasticized mass is pressed into an injection cylinder. The injection screw/piston presses the elastomer during the injection process through the injection or distribution channels into the closed mold (Figure 1.247). Here too, additional heat is brought into the elastomer mixture using pressure (dissipation) and the geometry of the injection system.

During the cure time, the injection cylinder is again filled (dosed) by the screw. A�er curing, the mold is opened, the component is removed, and the mold is then closed again to start the cycle over again. In addition, a vacuum can be applied to ensure the evacuation of the air that is enclosed in the cavities and thus avoid process disturbances. This may be done through a vacuum chamber (in which the mold is located) or through vacuum channels, which are integrated into the mold. In some applications it is even possible to apply the vacuum via the injection cyl-inder.

2471.9 Molds for Elastomer Processing

Plasticizing screw

Pre-plasticized elastomer

Heating plates withheating rods

Hot runner manifold

Divider

Product

FIGURE 1.247  Closed IM mold with the indicated injection unit

By using a screw/piston unit, the process of plasticizing and injecting can be con-trolled separately and is, therefore, better designed. Moreover, in the IM process, the entire process can be optimized by separate control or regulation of the individual processes. This opens up many possibilities and increases the complexity of this process immensely.

Basically, the IM process is suitable for all sizes of precision parts even in combina-tion with metal components. The manufacture of not completely coated metal sur-faces is possible due to the fully closed mold. Because of the diverse energy input (screw, injection cylinder, distribution channels, etc.) into the elastomer mixture, especially through the separate hydraulic unit during injection, significantly higher pressures (up to 2,500 bar) than in the TM technique can be obtained, and the inlet temperature (about 100 °C) into the respective mold cavity is close to the curing temperature. Additionally, cavities can be filled very quickly. With an optimal process design, this results in extremely short heating times in com-parison to the previously mentioned methods. Ideally, curing takes just as long as plasticizing. In this state, the highest reference temperature at the lowest thermal loads is reached.

Because of the high costs of producing the molds, the high investment costs of injection molding machines, as well as the high effort for the industrialization of the process, the IM process is mainly used for medium to large series. The molds are usually constructed from three plates (Figure 1.248). The air that is displaced during injection and the air that is applied through vacuum can escape through the parting planes.

248 1 Molds for Various Processing Methods

FIGURE 1.248  Mold half of an IM mold with 9 cavities

In the following, the most important process parameters on the basis of the entire cycle are briefly presented:

A – Injection phaseDuring the injection phase, the injection time is adjusted via the injection velocity or via the injection pressure. The injection time is the time needed to fill the cavities of a mold up to 95%. The injection velocity or the injection pressure is selected either to be constant or along a function, depending on the type of control. The working pressure adjusts itself depending on the flow resistance. This flow resistance is dependent on the geometry of the injection system and on the characteristics of the elastomer used. Because of the above mentioned influencing parameters and the thermal design of the entire system, the injection temperature, which results in starting the curing cycle of the elastomer in connection with heat input through the walls of the cavities (mold temperature), adjusts itself in the mold.

B – Holding pressure phaseIn the holding pressure phase, the mixture is fixed into the mold; that is, any ther-mally induced expansions of the elastomer mixture during the initial stage of curing (cured until gate) are reduced or avoided. The holding pressure can be as high as the injection pressure and is applied for a defined period of time (depending on the curing process), the so-called holding pressure time, using a time displacement or pressure control.

C – Dosing phaseA�er completion of the holding pressure phase, a new dosing process in the injec-tion unit starts. During the dosing phase, the elastomer is drawn in using the screw at a defined speed (screw speed) and is plasticized along the cylinder wall. For this purpose, thermal energy through the cylinder walls (cylinder temperature) and

2491.9 Molds for Elastomer Processing

mechanical energy through the screw are inserted into the mixture. The mixture is transported into a storage space (in front of the screw using the reciprocating screw principle) using the screw or into a piston cylinder in case of screw pre-plasticization. The so-called original temperature adjusts itself. The time required for this purpose is referred to as the dosing time.

D – Curing phaseThe curing phase leads to cross-linking of the elastomer mixture in the mold. The duration of cross-linking depends on the product geometry (in particular on the wall thickness) and the mixture used. An optimal setting of the original tem-perature, injection temperature, and mold temperature can shorten the resulting curing time. During the curing process, a defined internal pressure forms, which should be compensated through the holding pressure and the through the closing pressure of the machine. This ensures high quality and flash-free components to be manufactured.

Modern IM processes offer the user numerous, almost confusing settings. Process windows are searched for different times, velocities, pressures, temperatures, forces and paths for the individual phases with respect to their dependencies for each product, the mold used, and the used elastomer mixture. The goal is to produce a product as economically and qualitatively robust as possible. The time required to find optimum process windows is, in spite of technical assistance (e.g., by so-called cure time calculators), still dependent on the experience of the particular user or process engineer and the quality of the resources used (mold, elastomer, machine).

Advantages � short cure and thus cycle times due to the high mixture temperature at the begin-ning of curing,

� no preliminary processes (e.g., production of blanks) necessary, � better possibilities for automation, � conversion of mechanical operation energy into heat for heating the raw material, � good adjustment possibilities, and, therefore, a very versatile application, � rework-free parts are possible, � more uniform mold temperature, � tight tolerance position of the parts, and � complex-shaped parts are possible.

The main advantage of the IM process is its wide variety of applications driven by good adjustment possibilities. This is also the main disadvantage, because o�en only highly skilled technicians are able to industrialize a stable and robust IM process.

250 1 Molds for Various Processing Methods

Another advantage is the well-defined process sequence and the exact reproduc-ibility of the parts, which simplifies automation of the entire process (Figure 1.249). In the area of elastomers, as with the plastic industry, inserting the inserts into the mold and removal of the fully cured product is o�en done by handling systems (e.g., robots).

Disadvantages � relatively high investment costs, � expensive molds due to high precision required, � sensitive process, � high maintenance costs, � not all types of elastomers can be used, � complex process control, and � high demands on the binder systems.

The IM process is very complex to use and requires highly skilled installation engineers. Furthermore, IM processes (particularly the molds) are relatively maintenance-intensive and still not suitable for all types of elastomers.

1.9.4  Additional Processes

1.9.4.1  Process Combinations

In injection compression molding (ICM), the elastomer is injected in a slightly open mold using an injection unit. The mold is then totally closed or closed leaving a minimum gap (1/100 mm). Using this process combination, it is possible, to manu-facture flat precision components that are still attached to a “skin”, such as O-rings, flash-free seals, or membranes.

FIGURE 1.249  Fully automated ejection of the product (bellows) out of a IM mold

2511.9 Molds for Elastomer Processing

During the injection transfer molding process (ITM), the elastomer is introduced into the transfer pot (displacement cavity) using an injection unit. The subsequent process is similar to the TM process. The advantage of the ITM process is in the amount of time saved. Due to higher starting temperatures in the transfer pot, the elastomer mixture is brought faster at higher temperatures into the cavities. This leads to shorter heating times while simultaneously avoiding a pressure related overmolding. However, ITM leads to higher loads in the elastomer mixture and can, therefore, not be used for all applications as a replacement for TM.

If the transfer pot is thermally separated from the mold (i.e., works with a cooled displacement cavity that is insulated on the mold side), it is possible to prevent the elastomer in the displacement cavity from curing and the so-called residual can be used for the next cycle. (Otherwise it has to be disposed of as fully cured, round rubber plate.) The injection nozzles are geometrically designed so that the geometric and the thermal curing point exactly coincide. This ensures that no partially cured material gets into the next cycle and thus the quality of the components can be ensured. This is o�en referred to as cooled transfer pots or transfer molding cold runners (Figure 1.250), which can also be used in the TM process without injection using an injection unit [3].

Cooling channels

Cooled transfer pot

Insulation plate

Heating rods

Product

Heating plate withheating rods

FIGURE 1.250  Principle of a transfer molding cold runner

1.9.4.2  Gate Systems

In the classical IM elastomer processing, a so-called hot runner manifold is used for distributing the mixture from the injection nozzle of the injection unit to the individual cavities. The elastomer, which is located in the hot runner manifolds a�er the filling cavities, cures and is removed a�er opening the mold. To make this possible, the molds are designed as three-platen molds. The hot runner is located in the first parting plane, and the products are located in the second parting plane.

252 1 Molds for Various Processing Methods

The fully cured material in the hot runner is (process-related) waste and must be discarded. The necessary removal of waste o�en complicates process-oriented automation of the entire cycle.

To eliminate this negative influence on the efficiency of the IM process, more and more elastomer processors rely on cold runner manifolds (Figure 1.251). Cold runner manifolds are low-temperature gate areas of a mold, in which the curing of the elastomer mixture is prevented. With a cold runner, the cured waste can be significantly reduced in the manufacture of molded rubber parts since the uncured material, which is located in the cold runners, can be used further for the production of molded parts (also see Section 2.3.2). Depending on the type, distinctions between nozzle cold runners and transfer molding cold runners (see the TM process) are made. In the following, the technique of the nozzle cold runners will be explained in more detail.

Nozzle cold runners are used in injection molding process for the production of rubber parts. The gate runner manifolds run through a cold runner block that is tempered with a cooling medium. The transition into the heated curing part of the injection mold is done through individually cooled nozzle elements. The cooling medium is usually water, mixed with special additives. The thermal separation of the cured part from the cooled gate area is done through an insulating plate. The rubber, which is located in the cold runner block, does not cure and must not be thrown away as waste, but it can be directly used for the production of molded parts in the following injection cycle. Transfer molding cold runners are usually

Cold runner manifold

Cooled cold-runner platewith cooling bore holes

Insulation plate

Cooled nozzle

Gate

Mold plates or cavity

Product

Heating plate withheating rods

FIGURE 1.251  Principle of a cold runner manifold

2531.9 Molds for Elastomer Processing

used with a high number of cavities (up to 120) and rather low volume of molded rubber parts, and nozzle cold runners are better suited for a small number of cavi-ties (2 to 48) and for large-volume molded parts.

Three different types of cold runner manifolds (chosen depending on the applica-tion) can be differentiated:

� drilled cold runners, � milled and split cold runners, and � tube cold runners.

As the name suggests, drilled cold runners are cold runner blocks whose runners are drilled into a solid block of steel. Inserts can be used for the necessary deflec-tions of the runners. The cooling channels for tempering the cold channel can also be realized by deep-drilled holes. The advantage of this design is a robust and cost effective implementation. Due to the complex flow properties of the elastomer, the geometry and the surface of the runners have a high impact on the safety of the process reliability. The goal is to uniformly fill the cavities at high pressure (prefer-ably at injection pressure level). The disadvantage of this is o�en the drilled cold runners. Due to the non-optimal geometry of the runner caused by the production process (drilling and inserts), high pressure drops, temperature variations, and so-called dead water zones occur (areas in the cold runner in which the material accumulates over several cycles and cures). With an increasing number of nozzles or cavities of the cold runner, and with an increasing stiffness or temperature sen-sitivity of the material used, the suitability of the drilled cold runners decreases. Cleaning or maintenance is not always easy due to the closed design.

The split or milled cold runner offers more flexibility in the design of the flow chan-nels. Split runners are milled into two plates of one manifold layer or in three plates of two manifold layers. The plates are then accurately joined together by a screw connection. The cold runners are also introduced into the cold runner block through deep-hole drilling. O�en, the ideal geometry of the runners is determined by simula-tion (depending on the elastomer). Thus, the elastomer can be processed with a low pressure loss and no quality-critical dead water zones. The disadvantages of this type are higher production costs and sealing problems. The plates pressed against each other must be designed and manufactured with great precision or provided with a suitable seal to prevent leakage of the elastomer between the plates. The maintenance and cleaning of the split cold runners is always simpler; however, the work has to be carried out by qualified personnel to ensure correct assembly. Split cold runners are o�en used in quality-critical components as well as in elastomer mixtures, but they are difficult to process and have a large number of nozzles (up to 48 nozzles). Tube cold runners are constructed with the aid of pipes that are inserted into grooves and are connected through corner pieces.

254 1 Molds for Various Processing Methods

FIGURE 1.252  Flash-free product (bellows) that was manufactured with a cold runner manifold

Due to the modular structure of this design, cold runner blocks can be built quickly and cheaply. However, it is not in all variations possible to connect the pipes, and the resulting runner geometry is, therefore, o�en not optimal. The temperature distribution in a tube cold runner can be difficult to adjust. Tube cold runners are o�en used for standard parts and average quality.To minimize material consumption further and to facilitate the automation of the IM processes even further, so-called valve gate cold runner manifolds were developed. These systems can be defined from the cavity with a needle valve. It is (similar to the plastic processing) possible to directly inject a product and to demold without post-processing or loss of material (from the cavity). This is generally only possible if nonfunctional, and design-related surfaces are used for a gate area. Furthermore, the location of the gate has to be taken into consideration, since the complete and flow line free filling of the cavity cannot be ensured from all positions. The move-ment of the needle from open to closed position can be performed pneumatically, hydraulically, or electrically. All common systems on the market have their pros and cons and have to be application-specifically evaluated in a manner similar to that used for the different types of cold runner systems. Hydraulic systems are mostly very large and are considered to be critical with regards to leakage. Pneumatic valve gates are difficult to position and, therefore, take more time, but are smaller and cheaper. Electric drives are heat sensitive and o�en relatively expensive but can guarantee a precise positioning of the needle. As a general principle, valve gate cold runner manifolds should only be used when processing costly elastomers or for higher requirements in the automation area (Figure 1.252).

1.9.5  Mold Making

A�er a review of the procedures in elastomer processing, the following describes the specifics in the manufacture of molds. A distinction is made between test or prototype molds and series molds.

2551.9 Molds for Elastomer Processing

1.9.5.1  Types of Molds

Prototype molds have to be manufactured quickly and as close-to-production as possible, to provide the customers with fast prototype components and to secure processing to be close-to-production. The (single) cavity is generally manufactured from so� material (aluminum or unhardened mold steel). The mold structure is usually not very complex so that changes can be made quickly. Using prototype molds, small batches of up to 1,000 pieces of each product can be manufactured.

Series molds are designed according to the number and life time of products. The number of cavities and the process and degree of automation are defined according to the product specification. The series molds are mostly manufactured from hard-ened steel (up to 65 HRC) and are coated with for an optimization of the demolding process as well as the service life.

1.9.5.2  Mold Development

During the construction of an elastomer mold, the mold is designed first, and the structure has to be determined. The shrinkage will be determined depending on the type of elastomer or the process, based on experience using tables or specific so�ware.

A�er selecting the gating system, which depends on the elastomer used (process-ability, cost), and the product (function, geometry, number, etc.), the injection is defined. Types of injection include direct injection, pin-point injection, or a film gate at the manifold layer (in some cases). The choice depends on the infrastructure, such as the construction of the molding machine.

The article to be produced is virtually placed into the mold, and the dividing planes are defined depending on the part geometry, the manufacturing possibilities, as well as the flow paths of the elastomer and the ventilation possibilities of the cavities. The removability of the part and the type of mold removal is considered as well. Depending on the process and degree of automation, the parts can be demolded by hand, with hand ejectors, ejector pins or cones (attached to the machine ejector), a handling system, or through brushes. Although elastomers are very flexible in the ejection, attention should be paid to undercuts. On the one hand, these should be shown in the mold, and on the other hand, the demolding forces should not cause any cracking (microcracks o�en occur).

The molds are designed depending on rheological and thermodynamic aspects. The necessary or possible pressures should be balanced to ensure uniform filling of the cavities and to define the thermal dwelling and the thermal separation of the mold [1].

256 1 Molds for Various Processing Methods

The guide systems for the plates and maybe for the mold inserts should be deter-mined. Standard guide elements from manufacturers of standard mold units are preferred for reasons of costs. In addition to avoiding a static over-determination in the centering system of the plates and cavities, it is important to ensure a smooth operation at temperatures around 200 °C. If a guide cone is used in cavity-self-centering, special attention should be paid to manufacturability (only as accurate as necessary).For bore holes and sha�s, thermal expansion is very important and should be calculated in the course of construction. The assembly and production-oriented design of the mold should not be overlooked. In principle, it must be ensured that the shaping geometry and the entire mold structure can be designed in a way that always the most cost-effective production process can be applied. Flat parting planes should be provided, standards for plate dimensions for the construction should be used, and the diameter of the cavity should be selected so that little machining is required. Radii and aspect ratios should be chosen with care to avoid costly erosion, and tolerances should only be set at the functionally relevant dimensions.The ventilation technique is of enormous importance for the component quality. Depending on the process and the elastomer, milled vent grooves, eroded surfaces or rough coarse patterns can be used. It is o�en necessary to support venting with vacuum through a vacuum chamber (in which the mold is located), or through vacuum channels in the mold. In the ICM process, the mold is not fully closed and the cavity is only closed a�er the injection and venting of the mold through a gap in the parting plane. The steel is selected in consideration of the particular type of elastomer, according to the following criteria: � number of cycles based on the annual amount, � mold design and geometry, � machining capability of the mold maker, � manufacturing process (e.g., EDM steels without sulfur additives), and � necessary surface treatment.

The selection of an appropriate surface, depending on the elastomer, is also impor-tant for the elastomer mold maker. In addition to the removability of the part and the reduction of contamination tendency, the protection of the mold against aggressive elastomers plays a major role here [2].On the one hand, high chromium steel polished surfaces can be used as a surface protection. On the other hand, hard chrome plating with a thickness of approx. 0.005 mm is used as the most common and least expensive coating for elastomer molds. Currently, a coating based on chromium nitride is increasingly used. In addi-tion to these coatings, numerous other options can be considered, such as ceramic coatings or titanium-based surfaces.

2571.9 Molds for Elastomer Processing

FIGURE 1.253  Measurement

of the mold cavities

The actual manufacturing process is similar to the manufacture of molds for thermoplastics, with the difference that the required accuracy is well above the requirements of thermoplastic molds (Figure 1.253) in the individual functional areas within the mold (e.g., the parting plane). Thus, for example, for specific elas-tomer types, even a 0.001-mm difference in production can lead to the formation of flashes. In the past processes had geometrically undefined cutting edges; cur-rently, processes like micro or HSC milling (Figure 1.254), hard turning, and laser machining are increasingly used.

A�er assembly or adjustment of the elastomer mold, the mold proving takes place. For this purpose, the molds are built into the molding machine, heated to operating temperature, the necessary volume of elastomer is determined, and the elastomer is introduced, cured, and demolded into the mold, depending on the forming technique. When problems arise during filling, venting, or demolding, optimization loops are required. Experienced mold makers can limit this to one to two loops.

An inexpensive elastomer mold is characterized by low life-cycle costs, which means long periods of time between necessary mold cleaning, long service life, and robust process windows at relatively low purchase prices. The selection of the correct process, the optimal rheological and thermal design, the use of suitable steel and the precise manufacture of the relevant (function-determining) dimension, as well as the suitable coating lead to the optimal result in the elastomeric mold manufacture.

References

[1] The Technology Library, Technische Elastomerwerkstoffe (2006) SV Corporate Media[2] Schmitt, W., Kunststoffe und Elastomere in der Kunststo�echnik (1987) Kohlhammer,

Stuttgart[3] Röthemeyer, F., Sommer, F., Kautschuktechnologie (2001) Carl Hanser Verlag, Munich

FIGURE 1.254:  HSC-Milling

of a mold cavity

258 1 Molds for Various Processing Methods

 ■ 1.10  Micro Injection Molds

G. Konzilia

1.10.1  General Information

1.10.1.1  Injection Molding Process

What distinguishes a micro injection process from a “normal” injection molding process? Except for parts or structure sizes, the process is almost identical. O�en, even the same procedures are applied, both in the actual injection molding process as well as in the manufacture of the molds. Several studies (NEXUS Task Force Market Analysis) also point out the current potential as well as future developments. The entire process is very important in microinjection molding. Therefore, the process is also o�en referred to microsystems technology (MST). The coordination of the process includes:

� Molded part, � Injection mold, � Injection molding machine, � Peripherals (e.g., clean room, handling facilities), and � Quality assurance and further steps

should be an integral element of the MST.

This presents a new challenge for designers, project engineers, as well as develop-ment engineers.

1.10.1.2  Molded Part Design

In micro injection molding, two major groups of components can be differentiated:

� Injection molded parts (macro-injection molded parts) with microstructures or areas with microstructures (Figure 1.255). This mostly affects the surfaces.

� Micro injection molding parts, in other words, small parts with low weight (mass in mg) and the smallest dimensions and structures (Figure 1.256).

In either case, different tolerance, surface features, and accuracy may be required. For the micro injection molded parts themselves, the same rules apply (e.g., avoid-ance of sink points, etc.) as for “normal” plastic parts. Experience in micro-mold making and preliminary testing is mostly necessary. Here, the collaboration with the customer is a priority, much more so than in the area of standard shapes.

2591.10 Micro Injection Molds

FIGURE 1.255  Microstructure on a component

(source: z-werkzeugbau-gmbh,

Dornbirn, Austria)

1.10.1.2.1  Cooperation with CustomersIf a project for the micro technology is planned, clarity over the terms between the partners should be determined from the beginning. When one uses the term “micro” for small parts, another might think of nanotechnology. If then pictures of parts are used with the typical matchstick-head scale (Figure 1.257 and Figure 1.258), misunderstandings are usually inevitable.

On the assumption that “the sketches are clear”, the conversation partners assume both sides have the same basic information. Sketches, drawings, and o�en samples are rarely represented in the proper size or to true scale. It is therefore helpful to prepare a sample at the correct size. In many cases, this is the only way to properly see the challenges.

The customer will always, at least mentally, form an image of his product. This image mostly consists of surfaces, structures, distinctive shapes, and much more. In the end, when the product is with the customer, it is simply about whether the product matches this imaginary picture. If this is the case, the project is successful; if not,

FIGURE 1.256  Micro injection

molded parts (source:

z-werkzeugbau-gmbh, A)

FIGURE 1.257 and 1.258  Size relation of a symbolic scale and with measurement for

comparisons (Source: z-werkzeugbau-gmbh, A)

260 1 Molds for Various Processing Methods

striving for compromise starts, which means time and money and also means to always work with a compromise. But even successful projects do not always succeed immediately. They need reworking, improvements, and endurance.

1.10.1.3  Materials for Injection Molded Parts

In addition to a variety of proven materials (Table 1.13), the development of new materials is also sought. Given the small amounts of material, this is done somewhat slowly. It is important to use standard materials that are available on the market.

TABLE 1.13  Materials O�en Used in Micro Technology

Materials Possible ApplicationsThermoplastic MaterialsPC Polycarbonate CD, lenses, medical parts, fluidic partsPMMA Polymethyl methacrylate Lenses, coversPOM Polyoxymethylene Bearing parts, gearsPAI PolyamidimidePEI PolyetherimidePSU Polysulfone Optical PartsPPE Polyphenylene etherPA Polyamide Housing, gearsPEEK Polyetheretherketone Pump parts, bearing partsPET Polyethylene terephthalateLCP Liquid Crystal Polymer Housing, bearing parts, medical devicesPVDF PolyvinylidenfluoridePBT Polybutylene terephthalate HousingPFA Perfluoralkoxy alkanePE Polyethylene FilterPP Polypropylene FilterPS Polystyrene Fluidic partsABS Acrylnitrid-Butadien-Styrene Mechanical, medical componentsPPS Polyphenylensulfide Bearing partsETFE Ethylene/TetrafluorethyleneCOC Cyclicoolefin copolymer Lenses, fluidic partsPSU PolysulfoneSAN Styrene-Acrylnitril Optical parts

Elastomeric MaterialsTPE Thermoplastic Elastomer Seals, deformable componentsTPU Thermoplastic Urethane

Special MaterialsMIM Metal Injection Molding Gears, housing, medical componentsCIM Ceramic Injection Molding Connector, optical fiber linksPDLL Biolactat Medical application

2611.10 Micro Injection Molds

Not only because of the availability; the existing experiences with respect to prop-erties and processing are also important considerations when planning a project.

For all groups of plastics, there are now countless subgroups, variations, mixings (blends), and extensions. The various fillers play an important part.

Small pellet sizes, easy-flowing materials, the smallest grain sizes, and filling mate-rial sizes up to micro grains or microfibers are o�en desired. Theoretically, from a single granule, dozens of microcomponents can be injected. Each individual granule should have a high degree of purity and good homogeneity. Here, all health aspects of micro- and nanoparticles should be considered.

1.10.2  Design

1.10.2.1  The Micro-injection Mold

The task of the designer is to implement the agreements with the customers into production and manufacturing-oriented documents. These include the mold but also, as mentioned earlier, the knowledge about the injection molding process, the necessary environment, and subsequent processes.

An extensive knowledge of available standard parts and special manufacturing and production methods help to produce molds at affordable prices and in a reason-able time frame. Another important task of the designer is the design of the mold inserts. Especially when very tight tolerances are involved, it is important to design the construction so that a�er the first injection experiments and the following measurements, reworking and corrections are possible and yet the dates and costs remain within reasonable limits.

In certain cases it will be necessary to provide for preliminary experiments. Pro-grams for the computer-aided simulation of the micro-injection molding can be helpful in the early stages of part development and mold design. Under certain cir-cumstances, such programs are overwhelmed by the size of the parts or structures. The programs available on the market should therefore be subjected to a detailed evaluation and testing phase.

1.10.2.1.1  GateIn molds for injection molding for micro parts, the gate system (Figure 1.259) is always a challenge. O�en, the volume of the gate system is a multiple of the parts volume. The number of cavities, the material, the injection points, and the size of the injection mold are among the factors that determine the gate. It is important to bring the material undamaged and in the right quantity into the cavity. It may be useful to calculate the required gate volume so that the exposure time of the mate-rial in the screw is taken into account. Therefore, the designer in the MST has to

262 1 Molds for Various Processing Methods

be provided with detailed information about this subject. The removal of the gate system from the component shouldn’t be underestimated in some special cases.

Another challenge is the transition from the gate system into the cavity. In prin-ciple, all variations of gate models of the normal injection mold are applicable. For small structures, however, the cross-sectional size of a gate point may also be cor-respondingly small. In particular, it must be checked whether the ratio of the gate cross section fits the part’s volume. Due to rapid “freezing” of the gate, the holding pressure in injection molding may be underutilized under certain circumstances. Therefore, this may affect the part quality. This experience is specifically made in multi-cavity molds. This results in irregular and different part qualities of the individual cavities. The gate can also be a precision component (in these cases) that has to guarantee the uniformity of the filling, taking into consideration the amount, flow properties, and pressure.

It may be helpful to choose and design the gate so that it can be used for subse-quent operations as a carrier or handling aid. In this case, as mentioned before, it is important that the designer acquire extensive knowledge about the system.

When using hot runner systems, the size and mounting option and the heat man-agement have to be considered. The additional resting time of the material in the feeding system (machine and mold) must be included in the calculations.

1.10.2.1.2  Demolding and EjectionThe knowledge of the standard mold making is o�en underutilized for smallest components or structures. Here it is very good if structures can actually be removed from the mold without leaving residues in the mold. Also, tolerances and produc-tion possibilities should not be underestimated. A well-chosen mold separation is very important. It may be necessary to design the construction so that parts of the nozzle sided mold parts move along when opening the mold and thus ensure that the molded part remains on the ejector side.

FIGURE 1.259  Variety of gates (source: z-werkzeugbau-gmbh, A)

2631.10 Micro Injection Molds

The ejection itself is the next challenge. Through the part geometry, but also by the size, it may be necessary not to use ejectors in the traditional sense. Alternatives such as stripper plates, removal handling device, air valves, and a lot of individual solutions of imaginative designers can therefore be applied here. In normal ejection systems, it should also be considered that o�en only very small ejector diameters are used. To prevent lateral forces from affecting and damaging the delicate compo-nents, the ejector system is supported with ball guides that are free of play. When fixing the guide columns, thermal expansion has to be considered.

1.10.2.1.3  VentingIt is important to include the venting options into the design. The choice of mold inserts, parting plane, venting slits, and ejector variations can be very useful and can solve many injection molding complications. To remove the air and thus allow problem-free filling, blind inserts can also be manufactured. Many times, a problem is easily solved through the cavities in the parting plane. Another variation is the evacuation of the mold. The mold is vented using a vacuum connection. This allows material to be injected quickly, which will directly influence the resulting part quality because the so-called diesel effect can be prevented.

It is important that not only the immediate cavity is vented. Only if the entire mold area, including the ejection system and fixing bore holes, is evacuated for molded parts, it is possible to apply a safe vacuum in the cavity. Outwardly, the system must be protected with seals. The evacuation process also affects the cycle time. Especially in large quantities, the longer cycle time and the necessary energy consumption for the vacuum should be taken into account. It is therefore not recommended to evacu-ate a mold in general. In the following example, the difference of the structure tips of Mold A to Mold B is illustrated (Figure 1.260 and Figure 1.261). It is therefore a good example because these tips are worked into the mold plate. The air is trapped in these tips during injection.

FIGURE 1.260  Structure tips Mold A

(without vacuum),

magnified 100 times

FIGURE 1.261:  Structure tips Mold B

(evacuated), magnified

150 times (source:

z-werkzeugbau-gmbh, A)

264 1 Molds for Various Processing Methods

Porous materials can be used as an additional venting option. To create mold geometry and microstructures in these materials of sufficient quality is a priority. Blocking of the pores by the injected material and the strength or hardness provided by such materials, are crucial for their use. Since these materials are mainly in the experimental stage, it is recommended to carry out a series of experiments for the materials and geometries one intends to use.

1.10.2.1.4  Mold Guiding and CenteringThe guides, as they are known from the traditional molds, are also in principle possible to use. Guide columns and cylindrical fine centering must have play for functional reasons. For the smallest components, it is necessary to determine whether this play is acceptable. In microstructures, a minimal flash in the parting plane can be disturbing and can make the component unusable.

Inclined surfaces and conical surfaces are o�en used in micro molds. In this prin-ciple, the mold halves are positioned to each other when closing the mold. The mold halves tense up to each other if the work was not done with absolute precision (which is not so easy with conical surfaces). When opening the mold, the tension releases, and there is a minimal lateral movement. This movement may be sufficient to damage the molded part or minimally deform it. This can have an impact not only on the dimensional stability but also on the process: o�en the molded part can no longer be removed from the mold.

Flat guides offer a secure solution. They are spaced apart from each other so that two surfaces always guide with four guides; the guiding direction is adapted to

FIGURE 1.262  Guiding principle for micro injection molds (source: z-werkzeugbau-gmbh, A)

2651.10 Micro Injection Molds

the molded part, and the guides remain in operation when opening the mold until the plastic part is completely removed from one mold half (mostly the nozzle side) (Figure 1.262). If tension can be avoided, it is recommended to design one mold half minimally mobile. This centers the mold, which reduces the offset at the parting plane.

1.10.2.1.5  Temperature Control and CoolingIn materials that must be cooled in standard molds, it may be necessary for micro-forms to heat the mold to maintain the ideal mold wall temperature. Size and volume (and hence the mass and the surface) of the mold play a crucial role. The term temperature control is therefore more appropriate than cooling.

Temperature control can be done with liquid media, but also through electrical systems. Here, heating cartridges and heating bands or heating coils are common. Experiments with other materials, and thus new temperature control options such as the resistance heating or induction heating, have at least to some extend found their way into the micro molding process. Development of new materials that allow one to design the heating and mold in one step are in the experimental stage. The control of the temperature conditions through sensors, in addition to the pure temperature control, has proven itself and provides additional, o�en very useful information.

1.10.2.2  Special Procedures and Alternative Processes

1.10.2.2.1  VariothermThe principle of the variotherm process (Figure 1.263) is based on the fact that the mold, at a high mold temperature (in the range of the melt temperature), is filled and the molded part is removed from the mold a�er cooling of the mold.

FIGURE 1.263  Variotherm mold with induction heating (source: z-wekzeugbau-gmbh, A)

266 1 Molds for Various Processing Methods

The heating of the mold or of the mold elements also means additional expenses for the mold production with not only higher maintenance requirements but also higher operation costs due to energy expenditure for heating and subsequent cooling.

Add to that fact the much longer cycle time, which may also be at one minute and higher in optimal conditions. Compared to standard molds, in the micro mold making, the mass of the tempered mold inserts can already be reduced by the size. Advantages are also found due to the possibility of alternative heating options. Thus, besides fluid and electrical heating bands or heating cartridges, resistance heating and induction heating may also be used. Today, external heating systems are used that heat the mold cavity from the outside (when opening the mold). A thermal separation of the “variotherm area” from the residual area of a mold should be included in the design.

The advantage of this principle is that the melt does not solidify during injection due to the high wall temperature, but penetrates into the smallest geometries and can exactly shape these. Typically, vacuum is used as support to prevent air pockets.

1.10.2.2.2  Insertion TechnologyCompared to conventional molds, the size of the inserts and injection molded parts are the real challenge for the designer in micro mold making. The durability of the insert in the injection molding compound a�er removal from the mold and the sub-sequent use must be taken into consideration. This can be achieved by constructive measures with special geometric shapes, but also by surface treatments of the inserts.

1.10.2.2.3  Multi-Component and Assembly Injection MoldingEspecially in micro injection molded parts, the question of further processing always arises. The smallest parts are o�en difficult and cost-intensive to further process. Through the use of multi-component injection molding, such assembly operations can be prevented. In this method, two or more different materials can be injected either simultaneously or sequentially and thereby create a composite part.

This method support different functions, such as housings and seals. Certain char-acteristics must be considered in the selection of material of the injection molded parts. The injection molded parts may either have to form a strong connection or the individual parts have to be mobile to each other. Processing parameters such as mold wall and melt temperatures are also important.

The leading manufacturers of injection molding machines provide this technology for their injection molding machines. Good results can be realized with multiple injection units on one machine or integrated turning units. Although the initial investment is higher, the future savings can justify this technique in many cases. One notable project is the micro-planetary gearhead shown in Figure 1.264. This gearhead is completely finished in three steps in one single mold.

2671.10 Micro Injection Molds

FIGURE 1.264  Multi-component injection molded part (source: Oechsler AG)

1.10.2.2.4  Compression Injection MoldingThe compression injection molding is primarily used in areas where small structures or contours must be precisely shaped, but also for precision parts such as lenses. The material can be pressed as a still liquid mass into structures using an emboss-ing unit. This gives an impression that would otherwise not have been possible. It is also a good possibility to compensate the shrinkage of the material for thick parts (lenses), and thus to achieve safe contour shaping or to prevent sink marks. Injection molding machines can be equipped with an embossing unit. Embossing units can also be integrated directly into an injection mold. The embossing stroke can then be operated pneumatically, electrically, or hydraulically. To have the oppor-tunity to intervene in this process, it makes sense to design the mold so that the compression stroke can be set up and monitored. A simple way is to install a dial indicator on the mold with a connection to the compression stroke. For injection molding machines with this option, the corresponding hardware and so�ware for the measurement and control equipment is available.

1.10.2.2.5  Hot EmbossingThis process is also known as vacuum hot embossing, hot forming, hot stamping, or compression molding. Hot embossing itself has nothing to do with the injection molding process, but is particularly an alternative in the area of the impression of micro and nanostructures. It is applicable for almost all materials that are also used in injection molding process. In this process, a structure is pressed into a thermoplastic material (sheets, films, processed pellets). Presses with high pressing force are needed depending on the size and area of the structure. The temperature of the plastic mate-rial must thereby reach the so�ening temperature of the material so that the plastic material fills the mold insert and the structures make a detailed impression. Also in this process, a vacuum can be used to support the molding accuracy. Through hot embossing, a very high ratio of structure height to structure width (aspect ratio) can be shaped because not as high lateral pressures are applied to the mold structures as in injection molding where the injection pressure can be more than 1,000 bar.

268 1 Molds for Various Processing Methods

Application:

� Micro- and nanostructures in micro-optics, � Holographic security features, � Fluidic Components, � Lattice structures, � Preliminary series, and � Planar structures (prism, reflector parts, and optical structures).

For the production of mold inserts, the same possibilities and processes apply as in injection molds. The structures themselves are usually much easier to make and therefore cheaper. Other advantages are the simpler system and equipment technology and very short setup times – especially to change material, because no conversion and cleaning of machines (screw and nozzle) are required.

One disadvantage is the long cycle times. These can be up to twenty times longer than in injection molding, depending on the material and the embossing depths.

Special procedures:

� Both-sided impression, � Hot embossing of composite layers, and � Micro-thermoforming.

1.10.2.3  Environment and Continuing Processes

The micro injection molding process is similar to the conventional injection molding in terms of environment and continuing processes. The size of the parts has an additional influence on the MST process. Here are some examples:

1. Handling systems that combine several injection molding machines in micro or standard and micro injection combination. The micro injection molded parts are positioned precisely in the mold and placed in a precisely defined position. This circumstance can be used very well. Handling units remove and further transport the parts to testing, inspection, control, packaging, and assembly processes.

2. Control functions for the molded parts within the mold and the injection molding machine can be provided.

3. In addition to continuing processes, the issue of cleanliness is also an important part of the environment. Clean rooms or clean room cabins are used or are man-datory in many fields of the micro injection molding technique.

4. Another issue is static charge because components become contaminated by static charge. For very small components, it is also possible that the parts can no longer be fed in a controlled way to subsequent processes due to the electrostatic charge.

2691.10 Micro Injection Molds

The molds have to be constructively prepared for that. Coating instead of lubrication of guiding systems and ejector systems, abrasion-resistant materials and adapted constructional designs should be mentioned here.

Especially on this topic, it is important to look at the whole system in advance. In order to apply the “microsystems technology” successfully, the overall utilization, total cost, location and many other questions need to be processed and analyzed.

1.10.3  Manufacture

1.10.3.1  Construction

The majority of components of a mold can be covered under mold making by the conventional techniques, especially machining techniques. New technologies in the area of laser sintering application and coating techniques promise additional opportunities with high potential for improvement.

1.10.3.1.1  Materials for Constructional PartsIn accordance with the manufacturing technologies, the usual materials for con-structional parts are still in use today. When selecting the material, heat bridges, insulation capacity, strength, thermal expansion, temperature resistance, and wear properties are taken into account. For new technologies, it is recommended to obtain specifications from the manufacturers or further processing partners. Relevant information may be better obtained through meetings and seminars than from the literature.

1.10.3.1.2  Standard PartsThere are already manufacturers of standard parts who offer the corresponding products. Especially in the field of connections for temperature media, heating ele-ments, guides, and constructional parts, a significant increase in manufacturers can be observed. Guides, as mentioned before, can be designed to float, which means that the offered standard systems will be sufficient. It is also possible to switch to punching standards (which have different accuracies) or custom-made products and high-precision guidance. Even for ejectors with diameters up to 0.2 mm, there are already manufacturers that can produce such dimensions on request at low costs.

1.10.3.2  Cavity Stacks

Cavity stacks are mold parts that are used directly or in the immediate environ-ment with the mold.

270 1 Molds for Various Processing Methods

1.10.3.2.1  Material for Cavity StacksIt is recommended to use commercially available materials that are also used in conventional mold making. The reason is that there are a lot of available steels or powder-metallurgical materials, but also light and non-ferrous metals, which are inexpensive. In addition, there is a great potential to find knowledge and experience in relation to any use and processing of these materials. Dependent upon the part geometry, it may be appropriate to use better quality materials. In the micro range, it is o�en necessary to use materials with a fine microstructure and a homogeneous structure. Large carbides or carbide accumulation, inclusions, and impurities can lead to errors. Errors are usually discovered at the end and usually lead to lavishly produced and expensive mold parts that cannot be used.

Here are some hints that influence the choice of materials:

Machinability, erodibility, the possibility of laser processing, necessary hardness, influence of surface treatments, polishability, possibility of coating, tool materials for the fabrication, structure sizes, strength, wear and abrasion (caused by the injection material).

In the “alternative production methods” for mold inserts, the manufacturing process is o�en determinative of the material. Here, it is necessary to be well informed and thus to also plan the necessary measures. This might be determining the appropri-ate quantity of the mold inserts or what surface coating is needed to prevent wear.

1.10.4  Manufacturing Technologies

1.10.4.1  In General

Machine and mold manufacturers seek to be increasingly active in the field of micro mold making. High-precision machines with the ability to use ever smaller molds are now available from several manufacturers. This will also reestablish the so-called alternative methods. These are not only new technologies, but also processes and manufacturing variants that already exist, such as etching in the electronics industry, which is now also of interest in actual mold making. This is because the typical mold making processes are o�en no longer sufficient to achieve the neces-sary results in the micro range.

1.10.4.1.1  Mechanical Manufacturing TechnologiesImportant common mechanical manufacturing techniques in micro mold making are:

2711.10 Micro Injection Molds

� 3D micro-milling (Figure 1.265)Tools with an up to 30 μm diameterTool material: carbide, diamond, steel, and ceramicMachining of steel, brass, copper, bronze, aluminum, plasticsMachines with up to 5 axes

FIGURE 1.265  Micro milling on a 5-axis machine (source: z-werkzeugbau-gmbh, A)

� Ultra-precision machining (Figure 1.266)Tool material: PCD (polycrystalline diamond), MCD (monocrystalline diamond), diamondProfiling and surface processingDiamond turning

FIGURE 1.266  Profiling with diamond tools (source: z-wekzeugbau-gmbh, A)

272 1 Molds for Various Processing Methods

� Micro-EDM (Figure 1.267)Shape and accuracy is largely dependent on the electrode manufactureElectrode material: graphite, copper, tungsten copper, tungsten carbideAll the mechanical fabrication technologies are used for the manufacture of electrodes

FIGURE 1.267  From right to le�: graphite, copper, tungsten electrodes

(source: z-werkzeugbau-gmbh, A)

� Micro-wire cutting (Figure 1.268)Smallest wire diameter is up to 10 μm2D contours

FIGURE 1.268  Gear contour manufactured with 20 μm wire (source: z-werkzeugbau-gmbh, A)

� Grinding operations (Figure 1.269)Grinding wheel determines the quality and structureSurface GrindingJig GrindingCylindrical grinding

2731.10 Micro Injection Molds

FIGURE 1.269  Pin with a 100 μm diameter, cylindrical grinding process

(source: z-werkzeugbau-gmbh, A)

� Laser processing (Figure 1.270)Various types of laser processingLaser beam build-up welding, selective laser melting,Connecting, polishing, engraving

FIGURE 1.270  Laser-processed mold contour (source: z-werkzeugbau-gmbh, A)

Other methods include ultrasonic-assisted EDM, ultrasonic-assisted micro-planning, ultrasonic machining or micro drilling.

Additional equipment in mold machines is offered on the market and create new processing options such as “erosive turning” or shaping of structures with the help of explosive films. Therefore, new possibilities of designing the mold are offered in the field of microsystems technology. An additional subject for the accuracy to be concerned about is the machine itself and the clamping system for the work piece. In both cases, there are different systems and levels of accuracy.

274 1 Molds for Various Processing Methods

TABLE 1.14  Important Manufacturing Technologies in the Micro Mold Area

Type of processing Precision ±

Radius Radius RT

Surface Ra

Surface VDI

Complexity

Micro milling tool diameter 500 μm

0.005 0.25 0.01 0.40 1

Micro milling tool diameter 100 μm

0.0025 0.05 0.005 0.15 6

Spark erosion 0.01 0.05 0.02 1.12 21 3–4

Micro spark erosion 0.005 0.025 0.0025 0.5 14–15 8–10

Micro spark erosion 0.002 0.01 0.001 0.200.12

6 5

15–20

Wire erosion wire diameter 100 μm

0.005 0.07 0.4 12 2–4

Wire erosion wire diameter 20 μm

0.001 0.015 0.1 4 15–20

A comparison in Table 1.14 shows an overview of mechanical processing technolo-gies. The values shown are approximate. New technologies and developments in the field of machines and molds can now achieve even better results.

The value “COMPLEXITY” is considered a comparable number of different pro-cessing technologies. If the same part is manufactured using a milling cutter with a diameter of 0.5 mm and a corner radius of 0.05 is needed instead of 0.25 mm, a milling cutter with 0.1 mm diameter must be used. As a result, the cost increases by a factor of (complexity) 6. If a part must be ultimately manufactured using EDM, the factor can increase up to 20 [1]. Each of these processing methods has its techni-cal justification. The manufacturing method is the result of the technology discus-sions with the customer and the technical possibilities. Experience and detailed knowledge about possibilities, advantages and disadvantages of the individual processes, and the specific customer requirements are crucial in the selection of the processing method.

Since the expenses vary for the different processing methods, it is important to take this into account and consider it in planning. Not only because of the cost, but also due to the time involved. Of course, the customer requirements are a priority and can therefore justify a very expensive process, if this meets specific criteria in order to manufacture a product, or if the market advantage for the customer is provided.

1.10.4.1.2  Alternative Manufacturing ProcessesThe manufacturing process of form shaping components, which are used especially in standard mold making as well as in micro-mold making, are:

2751.10 Micro Injection Molds

Electroplating processElectro formingIn this process, a “prototype” is manufactured using mechanical micro-fabrication techniques. The actual mold insert is manufactured in an electroplating process. This mold insert can range from several tenths of a millimeter in thickness up to 10 mm. The advantage is that the positive-negative impression can be used and that several mold inserts can be shaped by a prototype.

Electroplated coatingHere an electroplated material is applied to a base body such as steel. The part can then be used directly, or it can also be further processed to achieve specific properties. Such electroplated replications can also be manufactured from plastic parts that are metallically coated.

LIGA technology (lithography, electroplating, molding)In this technique, the microstructure is either directly or through masks incor-porated into a suitable material, which is usually located on a carrier plate using UV lithography, lithography with synchronic radiation, or X-ray lithography. The machined interspaces are then filled with metal by an electroplating process. The actual mold inserts are then manufactured from these metal parts.

All these methods allow the high-precision manufacture of very small structures down to the “nano range”. Also, high aspect ratios (e.g., from web depth to web width) is another advantage as well as high surface qualities. One should consider that the materials used in the injection process, compared to steel, have only a limited life. It should also be noted that not all processes can make an impression of dra� angles. This requires additional demolding possibilities.

Mold generation through silicon-based processesThe mold inserts in silicon (Figure 1.271) are produced by lithographic and etching processes, a technology that has been known for decades in the electronics industry. New in this area are the 3D methods, which make the impression of very small structures possible. In mold making, the fixing and protection of mold inserts is important. The fracture risk is very high.

Laser sintering processWith a laser system, metal powders are welded to form inserts. This process comes from the field of rapid prototyping. Many different materials can be used, and these can be further processed and post-treated. The advantage is certainly the fact that cavities are also formed, which ensures optimal temperature control conditions.

276 1 Molds for Various Processing Methods

FIGURE 1.271  Mold core from silicone (source: z-werkzeugbau-gmbh, A)

Metal application processThis process is the application of metals on a basic body by a fusion process for certain processing technologies but also for temperature control. Non-steel mate-rials can be applied to a massive, heavy-duty steel core, and a�erwards diamond machining can be done to meet the high demands.

This process may have technological overlaps with surface treatments methods. Here, the boundaries are not always separated because similar or identical proce-dures and technologies are used.

1.10.4.1.3  Surface Treatment and RefiningAlso in micro machining, surfaces have a relation to the function of the part, such as roughness and optical requirements, but may also be required for the func-tion of the mold. The optical component may be related to the function (as seen in lenses). For the function of the mold, demoldability may be the reason for the surface treatment. “Smooth” surfaces are better or even necessary for the ejection of small parts.

Polishing o�en cannot be done by hand because the structures do not allow it. Other polishing technologies have to be closely examined to determine what impact they have on the surface and the structures. This may lead to undesirable erosion and rounding of edges and corners. The selection of appropriate materials is a “must”.

The subject of coating is so extensive and widespread that some of these points, which offer an advantage in micro-mold making, should be presented. All types of coatings are applicable, from electroplating processes such as nickel coatings and plastic coatings (e.g., Teflon), or vacuum-technical processes for various hard coatings, to mention the most common. Advantages are:

2771.10 Micro Injection Molds

� Cleanliness through reduced abrasion, friction, and corrosion resistance; � Wear resistance for lubricant-free operation, for example, in the clean room; � Improving the flow properties of the injection material and thus improved mold filling, and thus less warping through uniform cooling in thin-walled parts;

� Improvement of the surfaces, such as roughness, abrasion and adhesion; � Abrasion resistance in filled injection materials and thus increased life span; � Improving the ejection of the molded part by reducing the adhesion of the injec-tion material to the mold;

� Minimal correction in dimension (may be constructively planned, e.g., shrink-age); and

� Reduction of cycle time and maintenance costs.

It is advantageous for coatings, to integrate these into the design and to adjust to the basic materials and the dimensional accuracy or tolerances, but also to the injection materials processed. It is always recommended to contact the appropriate coating specialists.

1.10.4.1.4  Quality AssuranceAt the end of the manufacturing processes is the measurement technology. Start-ing with the measurement technologies, which are integrated in the processing machines, and up to tactile and noncontact measuring instruments, there is a wide range of products. The possibilities that are created from manufacturers of micro molded parts are very different. An adjustment to the existing measurement systems is certainly needed. First steps should be carefully considered and calculated. Mea-suring equipment and measurement methods are very expensive. A good education as well as experience and practice of the staff are necessary. The complete range of requirements cannot always be covered with a measuring device.

To facilitate the decision on the acquisition, a thorough processing of the require-ments and information is needed. It is also entirely appropriate to establish part-nerships with universities, colleges, and institutes. There are o�en more facilities available with the appropriate personnel and necessary know-how.

1.10.5  Injection Molding Machine

Especially in micro mold making, as mentioned before, the injection process is closely linked to mold making. Therefore, this section (at least to some extend) will also be integrated. The requirements for an injection molding machine for the micro injection process are:

278 1 Molds for Various Processing Methods

� Exact settings for low shot weight, � Usability of commercial pellets (size of granules), � Homogeneous melt formation, � Short dwell times of the melt in the plasticizing and injection unit, even at very small injection volumes,

� Exact guiding of the mold holders, � Integrated systems such as clean room, handling, control devices and others, and � O�en, high injection pressures and injection velocities are necessary.

Today, all of the leading manufacturers of injection molding machines offer such systems. For sensitive and delicate structures, the mold protection may require additional facilities. The same applies to queries, sensors and machine controllers. In the field of micro system technology, it is common to offer additional facilities and advanced machine control options.

Also in the immediate environment of the injection molding machine, additional equipment may be necessary. If these are already available together with the machine and are also networked to the control system, this is a competitive advan-tage. Such devices may be clean-room systems, systems against electrostatic charge, vacuum pumps, and handling units for storage in a structured way. The same prin-ciple applies for testing, inspection, and quality assurance.

In the controlled removal of micro components from the injection mold, it is pos-sible to hold and transport the component in the correct position. The possibility of the above-mentioned additional facilities, provide further benefit and security for components and molds, as well as for the entire process.

1.10.6  Mold Maintenance

Mold maintenance depends very strongly on the specific conditions of the mold. It is therefore possible that the same rules apply as for standard molds. It is also o�en necessary to consider the specifics, especially mechanical cleaning methods, which can cause damage in the micro-structured molded parts.

In some cases it is necessary to create cleaning concepts. These can include opportunities for mechanical cleaning, to the use of solvents and cleaning agents, up to very complex systems such as ultrasonic cleaning, laser cleaning, or plasma cleaning. In addition to the appropriate environment for the cleaning and neces-sary protective equipment, the purchase, storage, and disposal of equipment and additives have to be considered.

2791.10 Micro Injection Molds

For maintenance, in addition to production deposits, corrosion, and scaling or pol-lution of the cooling systems of the mold, the additional facilities themselves have to be maintained. Especially with very small molds, the connections are much smaller and thus more vulnerable.

The keeping and storage of the molds or of molding parts requires additional atten-tion and care. Lockable cassettes (in which the mold can be stored) to protect the mold from dust and moisture have been successful. For special materials for mold inserts, it is also necessary to provide protection against air (oxidation and humidity).

1.10.7  Outlook

It will be important in the future to create an open space for education, training, and technology for those people dedicated to the field of micro injection molding systems. The cooperation with universities, colleges, and institutions will also continue to be important. All areas of injection mold making will change quickly and drastically. Some examples include:

� Mechanical machining processes, � Alternative manufacture of molded parts, � New materials for molds but also for micro components themselves, � Temperature control systems, � Cleaning possibilities, and � Machine technology

All of these examples constantly undergo further development and innovation. Personal experience and commitment are additional success parameters in order to move safely in the field of micro and nano.

The “alternative manufacturing processes” (Section 1.10.4.1.2) are already in use today. The ratio compared to conventional production technologies in the mold is still very low, not least because of the higher manufacturing costs and lack of awareness. The future will also require even smaller structures and components in the field of mass production. Therefore, the importance of manufacturing process in the classical mold making will increase greatly.

References

[1] Konzilia, G., Microtechnology: A Question of Detail, Kunststoffe (2004) 6, pp. 27–29

280 1 Molds for Various Processing Methods

 ■ 1.11  Prototype, Small and Pre-Series Molds

R. Hofmann

1.11.1  Introduction

The ever-shorter product life cycles require new materials and methods for produc-tion of prototype, small- and pre-series. Due to the industrial demands for shorter product development cycles and rapid market entry of the product with increas-ing quality requirements, the areas of development, particularly in respect to the design and prototype phase, are facing strongly increased time and cost pressures. The high development and production costs in the conventional production of pro-totypes require new technologies that shorten the development time and improve the competitive situation significantly. It is also important how fast the product is available. The development process is significantly accelerated by the use of prototypes. They serve as design, function, and production trials, and early-on are already involved in the construction, development, and production planning. The rapid availability of models helps to shorten the planning stages and to improve product properties.

1.11.2  Indirect Prototype Molding

1.11.2.1  Vacuum Casting Polyurethane (PU) on silicone molds

1.11.2.1.1  Vacuum Casting PU ProcessVacuum casting is one of the most widely used technologies to quickly and inex-pensively reproduce prototypes in small batches (Figure 1.272). Within days, silicone molds are manufactured through stereolithography, milled, or sintered parts. With the help of vacuum casting units, prototypes with two-component casting resins based on epoxy or polyurethane are used that are, in the cured state, similar to the thermoplastics in terms of flexural strength, impact strength, and heat resistance. With special casting resins, transparent components or two- to three-colored components (Figure 1.273) can be casted. So� parts can be pro-duced in any shore hardness, or in two-component casting, and can be produced on hard parts. The vacuum casting process allows the production of small thin-walled or large parts. The mixing and casting of the resins into the appropriate molds is carried out under a vacuum lower than 1 mbar. Very thin-walled and transparent parts with difficult contours can be accurately filled without bubbles.

2811.11 Prototype, Small and Pre-Series Molds

Since both the casting and the mold chamber are under vacuum, the mold filling can take place very quickly and without counter-pressure through compressed air. Highly viscous resins generate a different vacuum with differential pressure systems in the casting chamber and in the form chamber, so that even highly viscous materials can be casted (Figure 1.274 and Figure 1.275).

The resins are used with pot lives (processing time) of at least one to five minutes. At a mold temperature of 60 °C, the resins are cured in less than an hour and can be removed from the mold (Figure 1.276).

FIGURE 1.272  Prototype of an instrument panel made in a silicone mold

FIGURE 1.273  Two to three component prototype parts

282 1 Molds for Various Processing Methods

FIGURE 1.274  Casting process of a low-viscosity polyurethane casting resin in the casting

chamber

FIGURE 1.275  Silicone mold during the casting process in the mold unit

FIGURE 1.276  Finished part: red-translucent rear lamp

2831.11 Prototype, Small and Pre-Series Molds

Process: molding process on a master model

Type of model: a demonstration sample, design sample, functional model, prototype

Materials: epoxy resin, polyurethane

Maximum part size: 1900 × 900 mm

Number of parts from one mold: 30 to 50 parts

Production time: 2 to 5 days

Accuracy: 1 to 2 (1 = very accurate, 6 = inaccurate)

1.11.2.1.2  Manufacture of Silicone Molds PUFor the manufacture of silicone molds using models in a cast, a master model will be needed. This master model is produced via stereolithography, selective laser sintering, or milling and is then cleaned and treated with a release agent. Then, the parting line is determined with the help of colored tape (Figure 1.277).

The gating and riser system is attached to the part and fixed in a mold box with tape (Figure 1.278). Therea�er, the casting frame with the initial model is filled with the silicone mass (Figure 1.279). These are special impression materials which have a low viscosity and do not create air voids and air pores in the silicon molds.

Due to the low shrinkage factor of the silicone masses (less than 0.1%), the dimen-sional accuracy of the functional models to be casted is ensured. For the subsequent casting process, the mostly transparent silicone is degassed at a pressure below 1 mbar in the vacuum casting unit and is then mixed with the cross-linking agent. During degassing, the silicone can grow up to three times its volume, and this operation is completed when the foaming mixture collapses again. The degassed mixture is then poured into the mold box with as few air bubbles as possible under normal pressure.

FIGURE 1.277  Determination of the parting line with the help of colored tape

284 1 Molds for Various Processing Methods

FIGURE 1.278  Fixing the master model

in an open mold box

FIGURE 1.280  Cutting the two mold

halves open

Subsequently, the mold box with silicone is again degassed for a short period of time in a vacuum oven to remove air pockets caused by the casting process.

The silicone mold cures in the heating oven overnight at 40 °C. A�er 24 hours, the casting frame is removed first and the two mold halves are opened along the adhesive tape with the aid of a spreading forceps and a scalpel at the predetermined separation line (Figure 1.280). Thus, the exact location of the position arrangement in reassembling of the two mold halves is ensured.

1.11.2.2  Vacuum Casting Polyamide (PA) through silicone molds

Nylon vacuum casting is a modification of the standard vacuum casting for manu-facturing molded parts by the casting of PA 6 in molds made from silicone. The silicone molds are produced in the same way as in the vacuum technology; in polyurethane.

FIGURE 1.279:  Casting of the silicone

mass

2851.11 Prototype, Small and Pre-Series Molds

FIGURE 1.281  Prototype part of an elastic plug connection made from polyamide

The difference between standard vacuum casting and nylon casting are the mold temperatures, which are in the temperature range from 140 to 160 °C in polyamide casting. This means that higher quality silicones have to be used for the mold.

The starting components are monomers, which are enriched with additive and catalytic components (practical example in Figure 1.282). These silicones have a higher molding accuracy and an output quantity from 30 to 50 molded parts made from (nylon) PA.

Nylon casting

mixing unit

Silicone Mold

FIGURE 1.282  Image of a nylon casting unit

286 1 Molds for Various Processing Methods

Two-component (nylon) polyamide 6 nylon materials are processed in a nylon module and poured into the silicone mold under vacuum. A�er curing, an accurate impression of the prototype model is developed. Molded parts with undercuts can easily be produced using this method. The casting process is completed a�er about 2 minutes; demolding can be done a�er about 6 minutes. With this process, molded parts of high quality can be obtained in the shortest time possible and at low cost.

(Nylon) PA-6 is a thermoplastic material, and its properties correspond to the origi-nal material which is processed by injection molding. Since the mechanical and thermal properties are similar to those of common thermoplastics, it is possible to implement this process not only for samples or prototypes, but also for production parts. Other features include easy coloring, painting, and electroplated surface treatments. The material is weldable. In addition, the materials are characterized by high impact strength and toughness.

Through the use of flexible silicone compounds for the mold manufacture, strongly undercut castings can also be removed from the mold. The casting under vacuum makes it possible to receive castings free from any holes with great accuracy. Sink marks, which normally develop in injection molded parts due to excessive cross sec-tions, are unknown in this process. Even thin strips of 0.3 mm can be easily casted.

This technique easily allows the application of inserts made from plastic material or metal. Reworking, except for the removal of the gate, is hardly necessary. All materials can be recycled as a thermoplastic. The casting of molded parts on the nylon vacuum casting machine (Figure 1.282) can be done without any problems using a computer-controlled pouring system (SPS). The biggest advantage is the time and cost savings due to short production start-up times.

There are various materials available, such as elastic, strong, and extremely rigid types. They all have their different areas of application. The bending strengths of the materials are 700, 1,000 and 2,000 MPa. The components manufactured in nylon vacuum casting are characterized also by high strength and stiffness, low specific weight, high mechanical strength, good wear resistance, as well as chemical resistance. The maximum outer dimensions of the mold are 900 × 600 × 500 mm. Currently, the maximum weight when casting is 1.5 kg.

Process: molding process on a master model

Type of model: functional model, prototype, small-series part

Materials: polyamide, nylon

Maximum part size: 100 × 900 mm

Number of parts from one mold: 30 to 50 parts

Production time: 2 to 5 days

Accuracy: 3 to 4 (1 = very accurate, 6 = inaccurate)

2871.11 Prototype, Small and Pre-Series Molds

1.11.2.3  Synthetic Resin Molds

Synthetic resin molds provide fully functional and ready-to-install pre-series parts. For pre-tests and testing purposes, small series can be manufactured from the mold. Changes can easily be made to also completely document the development phases of the mold.

1.11.2.3.1  Polyurethane Casting with Synthetic Resin MoldsFast curing two-component polyurethane-based casting compounds are well suited for the manufacture of housings, car exterior and interior trims, and other technical parts in the prototype and small-series production. The time to cast these is about a minute and they can be manufactured in small dosing and mixing units using gear or piston pumps to handle about 15 kg throughput per minute. Only relatively low pressures occur in the mold at the influx of the reaction mixtures so that preferably synthetic resin are suitable design molds, which can be created quickly and at low cost. The two-component polyurethane-based casting compositions are manufac-tured in possibly heatable molds at temperatures of 30 to 40 °C at a demolding time of 10 to 30 minutes (Figure 1.283, Figure 1.284, and Figure 1.285). The good mechanical properties of prototype parts conform to the properties of ABS-PP or elastomers.

1.11.2.3.2  Manufacture of Synthetic Resin MoldsFor the manufacture of synthetic resin molds, CAD data is needed. With this data, the mold separations and gating systems can be constructed. The CAD data is converted to CNC milling technologies and milling can be done, using resin blocks (Figure 1.286 (a)). A�er milling, the surfaces are sanded and painted. The resin block material is treated with a release agent and polished. A casting frame is inserted using the parting plane, which forms the outer contour of the synthetic resin mold. The model is coated with surface resins, optimally free of air bubbles (Figure 1.286 (b)). Subsequently, a coupling layer is applied, which is provided with glass fiber (Figure 1.286 (c)). A�er the appropriate reaction time, the backing material, which is made from epoxy resin and aluminum particles, is compressed in the mold (Figure 1.286 (d)).

Into this mixture of aluminum, copper tubes are inserted, which later enable the temperature control of the synthetic resin molds. An upper top layer generates a straight plane and a clean upper surface of the mold (Figure 1.286 (e)).

A�er 24 hours, the model can be removed from the synthetic resin mold. The future wall thickness of the part to be molded is milled on a CNC milling machine (Figure 1.286 (f)). A�er sanding and painting, both mold halves are treated with a release agent and the two mold halves are mounted together using form blockages or guides (Figure 1.286 (g)).

288 1 Molds for Various Processing Methods

FIGURE 1.283  Clamping the mold

FIGURE 1.284  Casting of the plastic parts

FIGURE 1.285:  Removal of the castings

2891.11 Prototype, Small and Pre-Series Molds

a) b)

c) d)

e) f)

g)

FIGURE 1.286  Individual steps of the manufacture of a synthetic resin mold

(a) Milling of the resin block

(b) Application of the surface resin

(c) Application of glass fiber-filled coupling layer

(d) Compaction of the backing material

(e) Upper top layer of the mold

(f) Milling off the wall thickness

(g) Finished and assembled mold

290 1 Molds for Various Processing Methods

Process: milling and casting methods

Type of model: functional model, prototype, small-series part

Materials: Polyurethane

Maximum part size: up to 2000 mm

Number of parts from one mold: 100 to 300 parts

Production time: 10 to 20 days

Accuracy: 2 to 3 (1 = very accurate, 6 = inaccurate)

1.11.2.4  Manufacture of Synthetic Resin Molds for Injection Molding

Synthetic resin molds are made from “tooling” resins and meet the requirements of injection molding and blow molding. They can reach similar stability to aluminum and provide a high surface quality. They have an extremely high glass transition temperature and high temperature resistance and compressive strength. The shrink-age factor is close to zero (+0.02%). Despite the high content of aluminum (80%), the tooling resins can be excellently casted and guarantee extraordinary impression accuracy. Thanks to the excellent chemical resistance and high material density, the cavity can be polished as needed, and the manufacture of visible parts that must have a high-precision, smooth surface can be enabled. When using tooling resins, mold changes can be done without reducing the surface hardness and compressive strength. The tooling resins allow a rapid and cost-effective manufacture of injection molded and blow molded parts. High-quality and dimensionally accurate parts from all thermoplastics (Figure 1.287 (a–e)) can be expected.

Producing mold inserts by casting resins into a positive model is a principally simple method. Stereolithography parts are especially recommended for a positive model. With the conventional model making technology, a parting plane is modeled before the first mold half can be casted. Casting resins are epoxy or acrylate resins with high aluminum content. This method is especially well suited for less complex components. For intricate internal structures and ribs, metal inserts are recom-mended. So far, experiences have been accumulated for relatively small components. Pre-series of more than 200 pieces could be injection molded, depending on the injection molding material, without signs of wear on the mold.

Metal mold inserts should be used to reach higher quantities as well as to get closer to the original processing conditions. In general, the thermal and mechanical prop-erties of a steel mold are not reached; however, the differences are smaller than in the use of plastic molds, especially depending on the cooling conditions in the mold. Fine-casted or LaserCUSING® in manufactured steel molds can also be used as mold inserts for pressure casting.

2911.11 Prototype, Small and Pre-Series Molds

a)

b) c)

d) e)

FIGURE 1.287  Individual steps of the manufacture of an injection mold

(a) Model made from wood, gypsum, leather, wax, silicone, casting resin

or from any rapid prototyping material

(metal inserts can be manufactured if necessary)

(b) Frame with mounted cooling pipes, casting in of resin

(c) Removing of the casting frame

(d) Heat treatment of the mold in the oven

(e) Finished injection mold

Process: milling and casting methods

Model species: functional model, prototype, small-series part

Materials: thermoplastics, thermosets, elastomers

Maximum part size: 350 × 350 mm

Number of parts from one mold: 50 to 300 parts

Production time: 10 to 20 days

Accuracy: 2 to 4 (1 = very accurate, 6 = inaccurate)

292 1 Molds for Various Processing Methods

1.11.2.5  Molds Manufactured through Generative Manufacturing Procedures on the Example of LaserCUSING®-Technology

The direct manufacture of prototype molds using the rapid prototyping process offers significant advantages (see also Section 4.7). By LaserCUSING® and adapted processes, molds without restriction for prototype and small-series production can be used. Also, highly filled thermoplastics of up to 70% GF can still be manufactured to several thousand injection molded parts on standard injection molding units.

The LaserCUSING® inserts are inserted into a standard mold frame made from steel. The inserts can be produced directly in a few days using different materials, such as aluminum, stainless steel, or hot-work steel (Figure 1.288). If the accuracy of the subsequent injection molded parts is not less than 0.1 mm, the LaserCUSING® insert can be finished with a small amount of manual rework. For more accurate part geometry, it must be manually reworked. The inserts are directly generated in the LaserCUSING® machine on a system mount (e.g., Erowa 3-R). The subsequent rework is possible without much alignment using milling, electroplating, or mea-suring equipment. LaserCUSING® of molds has proven successful for medium-size quantities.

The accuracy and surface quality depend on the rework, which determines the surface finish or the appropriate accuracy either manually or with machines. The delivery time of mold inserts is usually 5 to 10 working days. The cost advantage is dependent on the complexity of the plastic part (Figure 1.289). The more complex the component and the more ribs and contours, which cannot be manufactured by milling and require considerable expenditure of electrodes and eroding time, the more economical the LaserCUSING® technology. The LaserCUSING® system oper-ates unmanned, is fully automatic, and extremely reliable.

It is important to introduce the optimum cooling already into the prototype mold in order to optimize the quality and warpage of the components. The most distinctive feature of the LaserCUSING® technology is that special cooling systems are already constructed in the mold design in 3D (Figure 1.290 (a) and (b)), which can then

FIGURE 1.288  Mold insert for a two-component injection mold manufactured using the

LaserCUSING®

technology, manufacturing time is six work days

2931.11 Prototype, Small and Pre-Series Molds

be placed (in the layered structure) in the mold insert (Figure 1.290 (c)) using the LaserCUSING® process. This special surface cooling type, as described in detail in Figure 1.290, offers optimal cooling, which enables less warpage, fewer sink marks, and a faster injection cycle at critical points of the plastic part.

FIGURE 1.289  Two-component injection molded parts

a)

b) c)

FIGURE 1.290  Surface cooling in the mold

(a) Construction of the surface cooling in the mold insert

(b) Illustration of the surface cooling in a stereolithography model

(c) Mold insert with integrated surface cooling,

manufactured in the LaserCUSING®

technology

294 1 Molds for Various Processing Methods

Procedure: laser melting processModel type: small-series partMaterials: thermoplastics, thermosets, elastomersMaximum part size: 350 × 350 mmNumber of parts of one mold: 100 to 1,000 partsProduction time: 5 to 10 daysAccuracy: 2 to 3 (1 = very accurate, 6 = inaccurate)

1.11.2.6  Aluminum Molds

Aluminum molds are an economical alternative to injection molds for small series. Milling is performed using predefined CAD data. The manufacture is much cheaper and quicker than for molds made from steel. Very simple and highly complex parts can be manufactured from original material. Aluminum molds (Figure 1.291) are preferably used for part series from 1.000 to 10.000, for tensile- and falling tests or other material tests when original parts are needed.

FIGURE 1.291  Aluminum molds on the injection molding machine

(le�) upper mold half; (right) lower mold half

1.11.2.6.1  Manufacture of Aluminum MoldsThe entire mold design is created using CAD data. Here, the optimization of the plastic part is done based on the plastic design, which includes a mold flow analysis.

2951.11 Prototype, Small and Pre-Series Molds

This ensures that the liquid plastic maintains the required flow temperature even into the most remote corners of the cavity. Moreover, even in these early stages of the process, abnormalities can be evaluated and transferred to the subsequent series production. To use the concept of rapid tooling, changing mold inserts are designed in standardized mold frames. These changing mold inserts are manufactured in a high-speed machining (high-speed cutting) process.

Complex geometries can be produced through an electrode at an electric discharge machine. Here, the aluminum mold differs fundamentally from the steel-series pro-duction mold. At great expense, the steel molds are optimized for low cycle times. In aluminum molds such optimizations are not needed. A simple design of the molds is sufficient enough, such as using loose parts or inserts instead of complex slides. The simple design contributes significantly to dramatically minimized production lives of the mold.

1.11.2.6.2  Aluminum Molds with LaserCUSING® Loose PartsIn modern prototyping, different technologies can also be combined well. Thus, hybrid molds made from aluminum can be produced by high speed milling, and loose parts and sliders can be manufactured in LaserCUSING®, which results in a significant saving of time (Figure 1.292 and Figure 1.293). By the use of CAD designs, modern high-speed 5-axis milling machines in combination with the LaserCUSING® process, it is possible to manufacture hybrid molds in a few days.

FIGURE 1.292  Removal of the loose parts

from the injection molded part

1.11.2.6.3  Aluminum MaterialsAluminum is the “key for mold making” for even faster production times (see also Chapter 3.2). The search for cost and time savings requires permanent alterna-tive and innovative approaches. Thus, even in prototype making, an area in which mainly steel was used in the past, the materials are tested for their suitability.

FIGURE 1.293  Loose parts in the

LaserCUSING®

technology

296 1 Molds for Various Processing Methods

The use of easy processable and highly thermally conductive materials enables a significant improvement in solving these problems in the plastics industry. Special aluminum has been developed that is suitable for high mechanical stress and appli-cation in plastics processing.

Thus, aluminum has now gained a new importance for the plastic parts manufac-turer. By the type and quantity of the alloy components, certain properties – mostly the mechanical properties – can be selectively varied. The right combination of mechanical, physical, and chemical properties of this material provides the correct solution a�er a value analysis.In addition to the weight advantage of more than 60% compared to steel and the handling advantages, aluminum’s lighter, faster machining and high conductivity and corrosion resistance argue for its use. This reduces the manufacturing time of the plastic molds, and production costs are reduced by short cycle times for the manufacture of parts.

Properties of aluminum alloysAluminum has a density of 2.8 g/cm3 and a modulus of elasticity of 70,000 N/mm2. With the loads of normal forces (tension, compression, bending), the elastic defor-mation equivalent to the ratio of the modulus of aluminum is therefore three times higher than that of steel. Minimum values for the mechanical properties in mold making of aluminum alloys can mainly be found in the DIN/EN 485.Table 1.15 gives a selection composition and comparison possibilities of the dif-ferent materials. Due to the high core strength, it is now possible to move forward into new application areas.The plates are manufactured in the state T 651. This means that these parts are manufactured in the low-stress and stretched version. The material is delivered in the condition necessary for the processing and should not be subjected to heat treatment. (These materials have proven their performance in the aircra� industry with machined structural parts for heavy-duty chip volumes of over 50%).Considerable time and cost savings for the mold maker is realized by the already mentioned high-speed machining (High Speed Cutting). It is characterized by the following advantages: � large cutting volume, � by a factor of five to ten compared to conventional processing techniques, � increased cut and feed speed, � reduced machining force and temperature levels, � high surface quality (rework is not necessary), and � processing of high-strength aluminum alloys with minimal tool wear can easily be carried out deep into the core of the plate.

2971.11 Prototype, Small and Pre-Series Molds

TABL

E 1.

15  P

ropert

ies

of

Diff

ere

nt

Alu

min

um

Mate

rials

Allo

yM

elt

tem

pera

ture

in

terv

al(°

C) K

Spec

ific

w

eigh

t(k

g/dm

3 )

Heat

ex

pans

ion �

(10–6

K–4

)(2

0/10

0 °C

)

Spec

ific

he

at c

apac

ity

J/(k

g · K

)(0

–100

°C)

Youn

g’s

mod

ulus

E(M

Pa)

Ther

mal

co

nduc

tivity

�

W/(

m ·

K)at

20 

°C

Spec

ific

elec

tric

re

sist

ance

�10

–3 μ

· Ω

· m

at 2

0 °C

2017

A51

0/64

02.

7923

.692

074

,000

134

51

2024

50

2/63

82.

7721

.187

573

,000

151

45

2219

545/

645

2.84

22.5

864

71,0

0012

159

3003

643/

654

2.73

23.2

893

69,0

0015

942

3005

632/

653

2.73

23.2

897

69,0

0016

639

5049

620/

650

2.71

23.7

––

––

5754

590/

645

2.67

23.8

900

70,0

0013

253

5083

574/

638

2.66

24.2

900

71,0

0011

759

6060

615/

655

2.70

23.4

945

69,0

0020

033

6082

570/

645

2.71

23.5

960

69,0

0017

442

7020

604/

645

2.78

23.1

875

71,5

0013

749

7075

477/

635

2.80

23.4

960

72,0

0013

052

7050

490/

635

2.83

23.5

860

71,5

0015

443

298 1 Molds for Various Processing Methods

TABL

E 1.

16  O

verv

iew

of

the Indiv

idual Ty

pes

of

Manufa

ctu

re

Tech

nolo

gyPr

oces

sTy

pe o

f Mod

elM

ater

ials

Max

imum

pa

rt s

izeNu

mbe

r of p

arts

fro

m o

ne m

old

Prod

uctio

n tim

e in

day

sAc

cura

cy1

= ve

ry a

ccur

ate

6 =

inac

cura

te

Silic

one

mol

dPo

lyur

etha

neM

oldi

ng p

roce

ss

thro

ugh

a m

aste

r m

odel

Illus

tratio

n sa

mpl

eDe

sign

sam

ple

Func

tiona

l mod

elPr

otot

ype

Epox

y re

sin

Poly

uret

hane

1,90

0 ×

900

30–5

02–

51–

2

Silic

one

mol

d Po

lyam

ide

Mol

ding

pro

cess

th

roug

h a

mas

ter

mod

el

Func

tiona

l mod

elPr

otot

ype

Smal

l ser

ies

part

Polya

mid

eNy

lon

100

× 90

030

–50

2–5

3–4

Synt

hetic

resi

n m

olds

Inje

ctio

n m

oldi

ng

Mill

ing

and

cast

ing

proc

ess

Func

tiona

l mod

elPr

otot

ype

Smal

l ser

ies

part

Ther

mop

last

icTh

erm

oset

Elas

tom

er

50–3

0010

–20

2–4

Synt

hetic

resi

n m

olds

Mill

ing

and

cast

ing

proc

ess

Func

tiona

l mod

elPr

otot

ype

Smal

l ser

ies

part

Poly

uret

hane

100–

300

10–2

02–

3

Com

posi

tiona

l m

old

thro

ugh

ster

eolit

hogr

aphy

Rapi

d Pr

otot

ypin

g Pr

otot

ype

Smal

l ser

ies

part

Ther

mop

last

icTh

erm

oset

Elas

tom

er

500

× 50

020

–100

5–10

3–5

Mol

d th

roug

h La

serC

USIN

G®La

ser m

eltin

g te

chno

logy

Smal

l ser

ies

part

Ther

mop

last

icTh

erm

oset

Elas

tom

er

350

× 35

010

0–1,

000

5–10

2–3

Alum

inum

mol

dsM

illin

g pr

oces

sSm

all s

erie

s pa

rtTh

erm

opla

stic

Ther

mos

etEl

asto

mer

1,00

0–3,

000

10–2

01–

2

2991.11 Prototype, Small and Pre-Series Molds

The spark and wire erosion is largely known only for steel processing. Aluminum can easily be used for the spark erosion too, with mostly higher removal rates than in steel processing. Furthermore, there is no so-called “white layer” (which is extremely hard for steel), so that any necessary polishing can be reduced to a minimum. The performance increases are striking: roughing can be done six to eight times faster, finishing can be done three to five times faster, and fine finishing can be done at least twice as fast as in steel processing. The subsequent polishing process can be typically reduced to about one-third of the time required when compared to steel.

The relatively low abrasion and wear resistance of aluminum can be compensated by an appropriate surface treatment so that sufficient life times can be achieved. Hard anodizing, chemical nickel plating, chrome plating, and special chemical coatings, which facilitate demolding, have proven their worth.

Process: milling

Model type: small-series part

Materials: thermoplastics, thermosets, elastomers

Maximum part size: up to 2,000 mm

Number of parts of one mold: 1,000 to 3,000 parts

Production time: 10 to 20 days

Accuracy: 1 to 2 (1 = very accurate, 6 = inaccurate)

Finally, Table 1.16 shows an overview of the individual manufacturing processes in the prototype, pre- and small-series technology.

2 Mold Design

 ■ 2.1  Design Process

P. Karlinger, F. Hinken

2.1.1  Introduction

Generally, mold making shows a very broad spectrum of use, and molds are needed for a number of processes in plastics manufacturing. Molds have their special fea-tures in all areas; some of these features can be derived from process engineering, and others partly evolved from history (Figure 2.1).

FIGURE 2.1  Example of a mold for the multi-component technology and the back injection

technology

(le�) Multi-component cube mold for the manufacture of caps

(company picture FOBOHA GmbH)

(right) Mold half of a back injection mold

(company picture Georg Kaufmann AG)

302 2 Mold Design

Inje

ctio

n m

oldi

ng

tech

nolg

y

Clea

n ro

om te

chno

logy

Mul

ti-sh

ell t

echn

olog

y

Met

al in

ject

ion

mol

ding

Cera

mic

inje

ctio

n m

oldi

ng

Long

fibe

r tec

hnoo

gyIn

ject

ion

mol

ding

Rods

hape

d pe

llets

Inm

old

com

poun

ding

Inje

ctio

n m

oldi

ng

Casc

ade

inje

ctio

n m

oldi

ng

Prec

isio

n in

ject

ion

mol

ding

CD in

ject

ion

mol

ding

Mic

ro-in

ject

ion

mol

ding

Inje

ctio

n m

oldi

ng w

ithna

no-s

truc

ture

d su

rfac

es

Foam

ing

Phys

ical

foam

ing

Chem

ical

foam

ing

Flui

d te

chno

logy

Wat

er in

ject

ion

tech

nolo

gy

Gas

inje

ctio

n te

chno

logy

Gas

and

wat

er in

ject

ion

Lost

cor

e te

chno

logy

LSR

inje

ctio

n m

old

(Liq

uid

silic

one

inje

ctio

n m

oldi

ng)

Inse

rt te

chno

logy

Ove

rmol

ding

tech

nolo

gy

MID

Tec

hnol

ogy

Inm

old

deco

ratio

n

Two-

com

pone

nt in

ject

ion

mol

ding

One

-com

pone

nt in

ject

ion

mol

ding

Mul

ti-co

mpo

nent

inje

ctio

n m

oldi

ng

Co-in

ject

ion

Inte

rval

inje

ctio

n m

oldi

ng

Mar

ble

inje

ctio

n

Ove

rmol

ding

Rota

ting

mol

ds

Non

-rot

atin

g m

olds

Tran

sfer

tech

niqu

e

Slid

ing

tabl

e te

chno

logy

Core

with

draw

al p

roce

ss

Cube

tech

nolo

gy

Inse

rt

Inde

x pl

ate

tech

nolo

gy

Rota

ry d

isc

tech

nolo

gy

Hyb

rid te

chno

logy

Org

ano

shee

ts

Met

al in

sert

Mul

ti co

mpo

nent

tech

nolo

gyTh

erm

opla

stic

El

asto

mer

s

Poly

uret

hane

Inm

old

deco

ratio

nFi

lms

Dec

orat

ion

Inm

old

hot

Inm

old

labe

ling

Inse

rt m

oldi

ng

Back

com

pres

sion

Pow

der i

njec

tion

mol

ding

Pres

sing

tech

nolo

gy

Inje

ctio

n co

mp

ress

ion

mol

ding

Back

inje

ctio

n m

oldi

ng

FIGU

RE 2

.2  O

verv

iew

of

specia

l pro

cedure

s in

inje

cti

on m

old

ing

3032.1 Design Process

Injection mold making developed the largest portion of the mold making business, and it has been extensively developed over time. For this reason the notes and processes that are listed below reflect the experience of injection mold makers and should be transferred to the other processes. Certainly shorter processes are common in some areas than in injection mold making. The trim and edgefolding mold should especially be mentioned.

2.1.1.1  Injection Molds

Injection molding technology has developed tremendously over the last few years. The cycle times have shortened significantly through new manufacturing tech-nologies and materials. In addition, the demands on the quality of plastic parts in terms of strength and surface quality have increased significantly. Important to mention is the area of structural components, such as front-end modules in the automotive industry where steel is more o�en replaced by plastic, or the exterior as well as interior applications in the automotive sector where the surface qualities are always in the focus.

Furthermore, the plastics industry and thus mold making, due to the use of special technologies (Figure 2.2), will place an emphasis on the new development. Here, besides the normal problems of the mold, there is also the incorporation of new technologies and the integration of well-founded existing process knowledge into the mold design.

The more complex correlations and the necessary process knowledge cause the mold making industry to see itself less and less as a pure service provider, which designs and manufactures a mold using a drawing, but more and more as part of a project team, which has the task to develop a plastic component.

2.1.1.2  Phases of the Mold Design

The mold is very far ahead in the development chain and contributes enormously to functionality, quality, and efficiency in the mold design. When talking about economy, it is less about mold costs but the achievable cycle time and the integra-tion of additional functionality into a component, which can be demonstrated with the example of assembly injection molding. The hose nozzle in Figure 2.3 was assembled together with the valve body in a separate assembly operation for dif-ferent sizes in different colors. For the newly designed valve, the hose nozzle with the integrated size designation, as well as the movable fixing bar, are injected in the multi-component process.

The classification in a very early stage of the process makes the mold design very difficult because many parameters are o�en not precisely defined yet and can still

304 2 Mold Design

change during the implementation (Figure 2.4). This fact makes it more important to narrow down any details as early as possible or to document outstanding issues in order to incorporate them into the design.

The design process, which is shown in the textbooks, can still be found in its funda-mentals. The design phase is usually divided into three main phases (Figure 2.5).

Medical deviceAssembly of the valu and kink

Optimized medical device for

assembly injection molding

Kink protector

Valve

Size designation

Movablefixing bar

FIGURE 2.3  Integration of the component size designation and the assembly into the injection

mold using the example of a medical component

(Result of an industrial project work at the University of Applied Sciences

Rosenheim for the company Createchnik)

Mold constructionMold making

Material type?Machine?

User?

Customer wish?

Production facility?

Component requirements?

FIGURE 2.4  The basic problems when designing injection molds:

The mold design has to start despite uncertain boundary conditions

3052.1 Design Process

Phas

e 3

Phas

e 2

Phas

e 1

Dim

ensi

onin

g pu

rpos

esMold principleidentification

Rheological mold design

Thermal mold design

Finishing of the construction

Mechanical mold design

Kinematics

Stiffness assessmenttransvers to closing direction

Stiffness assessmentin closing direction

Demolding forcesMass forces

Surface pressure

Energy balance of the entire system

General design oftemperature control

Exact temperature control in segments

Homogeneity control

Filling behavior of the cavityqualitatively

Filling behavior of the cavityquantitatively

Design of the manifoldsystem

Pressure loss in the machine nozzle

FIGURE 2.5  Phases and tasks in mold design

1. Finding the principle: Here, the basic principle has to be determined. There are also discussions about what specific technologies are to be used. The individual design possibilities must be carefully weighed and possibly analyzed with appropriate evaluation matrices. The basic axiom for all stages is to ensure a fully automated process.

2. Dimensioning: The three most important tasks of dimensioning are the mechani-cal design, thermal design, and the rheological design. All three methods should be viewed with the same weighting. Certainly, mechanical dimensioning has a slight predominance because functionality must be ensured in any cases. For all dimensioning, numerical methods, better known as simulations, are available. Although the processes have now reached a very high level of significance and accuracy, they are still very rarely used in mold making.

3. Mold design: In this phase, the designs and guidelines for the plate sizes, the number, location and size of the cooling channels, the kind of gating system, and the number of injections are implemented in detailed drawings. In recent years, the transition from 2D drawings to 3D drawings is nearly complete. The complex processing machines, which are particularly necessary in mold making and through which all free-formed surfaces can be processed, have prompted the mold making industry to convert the design of the entire mold into 3D programs.

The ever-shorter development cycles in all sectors, from the household industry to medical technology and the automotive industry, have also shown their effects in

306 2 Mold Design

mold making. The classic serial development has undergone a change to simultane-ous engineering. By using additional prototypes, the result is secured quickly, and if necessary, countermeasures can be taken (Figure 2.6).

Shorter development cycles in mold making means that “mold development” has to be started without the specified component data. Besides the above-mentioned uncertainties (Figure 2.4), mold making is also affected by the fact that only up to about 90% of the mold geometry is defined at this phase. This may mean that newly added undercuts have to be addressed in the inquiry and specification phase. How the mold making industry faces these challenges is explained in the next chapter.

The ever-shorter development times and also the smaller batch sizes indicate a further change. The plastics industry, in particular injection molding, has its advantages in large quantities (> 105) and is mostly, from an economic perspective, unbeatable in this regard.

In general, the price of the mold was of minor importance in regard to the price of the parts. Today, the advantages of freedom in designing are increasingly important. The cost of the mold suddenly becomes increasingly important with decreasing part quantities. The traditional rapid prototyping processes also present economic diffi-culties for the frequently requested numbers between 1,000 and 50,000 components.

Traditional Approach

Simultaneous Engineering

Combination: Simultaneous Engineering – Rapid Prototyping

Planning

Concept

Draft

Elaboration

Production planning

Prototype

Prototype

Prototype

Savings

Prototype

Prototype

Planning

Concept

Draft

Elaboration

Production planning

Planning

Concept

Draft

Elaboration

FIGURE 2.6  Examples for different design methods and timing during the product development

3072.1 Design Process

Here, the first signs of a “new mold making” can be seen. This new mold making attempts to use the highly automated injection molding technology with its high level of quality through less expensive designs and reduced service life.

2.1.1.3  From the Offer to the Design

Basic ProcessesIn general, the product groups, such as cups, preforms, etc., can be derived from common design structures already found on the market. Nevertheless, one must not slavishly abide by these experiences, but they should be incorporated into the design as an addition. The constant competition also requires a continuous development.

Conveniently, a classification according to the main features of the structure and functions, which are also listed in the Deutsches Institut für Normung (DIN) 16750, has to take place (Figure 2.7).

Distinguishing feature Mold type

Number of mold cavities

Type of gate system

Number of parting planes

Type of demolding

One-cavity mold Multi-cavity mold

Mold with solidifying gate Mold with non-solidifying gate

Two-plate mold Three-plate mold Stack mold

Standard mold

Stripper mold

Slidermold

Splitmold

Unscrewingmold

FIGURE 2.7  Basic division of standard injection molds

In classic mold making, which is certainly still realizable for a variety of plastic parts, the component has to be released before the design process can be started. Figure 2.8 shows the process of methodical and planned designing of injection molds.

In the offer phase before designing, small changes have to be implemented due to suggestions from mold making. The reason for the changes is generally one of the following:

� general elimination of design errors at the component, � mold design and construction are simplified, � injection molding is more robust, � the quality of production is increased, or � integration of additional functions.

308 2 Mold Design

Design of the molded part, requirements, order quantity, injection molding machine

Number ofcavities

Mold design

Position of the cavities

Gate systems

Cooling system

Demolding concept

Mold material

Construction

Shrinkage forecast

Two-plate mold Three-plate mold Stack mold

Pattern distribution Symmetry distribution Linear pattern distribution

Normal Point Film Tunnel Fan Bar Disc

Surface cooling Core cooling Slider cooling

Ejector pins Unscrewing unit Stripper plate AirBushings

Venting

Parting plane Inserts Ejectors Lamellae

Design of themolded part Gate system Processing conditions

FIGURE 2.8  Schematic for methodical and planned designing of injection molds

3092.1 Design Process

Request

Technical clarification

Draft

Offer

Customer dialog

Design draft

Detailed design

Intermediatedocumentation

Design completion

Work preparation

CAM

Production

Assembly

Sampling

Design modifications

ReleaseCompletion

documentation

Customer dialog

FIGURE 2.9  Schematic of the process in mold making

The plastic industry (for molds with large machining quantities) demands to begin the design very early due to the ever-increasing time pressures and the fact that production-related prototypes from the series mold are required very early. The resulting tasks cannot be solved alone by the mold making industry but require a permanent dialogue with the client. An intensive dialogue with the customer is needed until the offer phase.

Concepts for the mold, which are generally already executed in 3D, must be discussed prior to offer submission. Possible changes, especially changes in the component design, which influence the function of the mold, have to be discussed with the customer and integrated in the design dra� (Figure 2.9).

Even a�er placing the order, the dialogue with the customer should not end. Inter-mediate steps should always be included, even when detailing. For complex projects, the release of the design and documentation should generally be discussed with the customer.

The target of mold making should be only to deliver a tested mold. In the meantime, many mold makers have acquired injection molding machines to perform it in house and also to ensure, taking into account the global market, short distances and short production times until final finishing.

Even for large molds, the investment of large injection molding machines (up to 40,000 kN) and 3D measurement systems for the sampling and subsequent com-ponent measurement (Figures 2.10 and 2.11) do not cause any concerns.

310 2 Mold Design

FIGURE 2.10  Example of injection molding machines for the assessment of injection molding

molds (company picture Schneider Form GmbH)

FIGURE 2.11  Measurement of mold component with 3D measuring machines

(company picture Schneider Form GmbH)

3112.1 Design Process

Inquiry and SpecificationThe inquiry in mold making is done, depending on the customers, through a variety of different media and can be divided into the following forms:

� standardized forms that are usually sent to many mold makers; � text document or presentation component with sketches and descriptions of all problem zones such as slides, visible surfaces, grains, etc.; and

� a 3D data model.

Regardless of the type of request, it is recommended for the further process to transfer the inquiry to an internal standardized document that contains all the important questions.

The mold inquiry should also collect project-based and process-specific data in addi-tion to general data, such as customer, component name, and material. Requests about the general topic areas of project management, the sampling position, and batch size of the samples should come just as naturally as the definition of the technical issues, such as the quality of the surfaces, the tempering of individual mold components, or whether the “end-of-arm tool” must be included in the mold.

Figure 2.12 shows a schematic overview of a form for requesting a mold. This document should be further developed and adapted to the needs and changes in the market.

Using the data in the mold inquiry form, the dra� for the offer can be created. The design is still usually implemented in 2D. In rare cases, a design can also be created in 3D provided that the molded part data is available in the required quality. In the near future, there will be an increasing trend toward 3D designs.

Regardless of the process drawing, the dra� should include the following informa-tion:

� building size, � type of steel, � gate system, � slider positions for the release, � control system, � position of the parting plane, � cooling, � ejector position, and � cycle time estimation (if required).

A�er consultation with the customer, a binding offer can be created using the data from the inquiry specification and the dra�.

312 2 Mold Design

Name: Job number:Drawing/Data set: Date:Contact person: Order number:

Part Granules:Shrinkage in %: Required cycle in s:

Injection mold Number of mold cavities:Fully automated Partially automated With removal devices

Slider mold Stack mold Three plate moldMiscellaneous:

Specified time Project start:

Design Design is supplied Yes No

When designing, the following document for the manufacture of the moldwill be available: Completely dimensioned drawings on paper ZSB drawingCAD data 2D/3D solid Data exchange according to CAD guidelines

Language: German EnglishOther

Parts list has to be made by SF using a template. Diagrams for cooling, hydraulic, end switch and functions have to be provided.

Language: German EnglishOther

Manufacture Suitable for the machine

Project monitoringY

es No

Mold construction

Mold frame

Female mold plate

Inserts matrix

Mold plate core side

Inserts core side

Sliders

Cores (cooled)

Intermediate plate

Quick release plate

Matrix mold plateCore side

Movable mold half Nozzle sideMovable mold half Nozzle side

Mold details Thermal insulation panels: Yes NoComments:

Surface Matrix side: grain - High-glossCore side: grain EDM-

Comments:

Graining - Yes No

Mold construction

Am

kolo

yAlb

rom

e

-

-

-

-

-

-

-

--

--

- -

nitri

ded

hard

ened

After finishing, all of the data and documents have to be handed over to the company SF.

(Pay attention to quick clamping). The following rules and guidelines apply as well:terms of purchasing, design guidelines, the check list, machine specification sheets,and other possibly placed requirements.

-

-

pro-vided

San

kyo

Cas

e ha

rden

ed

-

-

-

-

-

Cavity(s) as an insert into the mold frame

--

Dimension (length, width, height)

Cavity(s) directly in the mold plate

-

1. Inspection:

Material

-

Specification Mold Request

If the design is not provided, the following points have to be taken intoconsideration: The drafts have to be approved by SF. The originals haveto have a SF head and have to be delivered when finishing the mold.

An agenda in the MS-Project format has to be presented weekly without a request. The process improvement should be documented via pictures.

-

-

-

-

-

-

-

-

-

-

-

-

Draw polishedSand-blasted

EDMEtched

According to the drawing

Draw polishedEtchedSand-blasted

According to the drawingHigh-gloss

FIGURE 2.12  Example for a form with the most important data for the mold design

3132.1 Design Process

Part demolding Core side

Hot runner system Stäubli Rapid release Valve gate

External Internal Lateral

Cold rod Cold runner

Heated nozzle

For reinforced materials and polyamide, the hot runner nozzle has to be armored:

Yes

Manufacturer:

Type of injection Tunnel gate (exchangeable) Film gate Point gate

Banana gate Comments:

Ejecting Drive: mechanically hydraulically Hook locking unit

1 step 2 step

Rectangle ejector Ejector bush

Stripper Stripper plate

All ejector flush and EDM processed

Drive of the ejector plate: hydraulically mechanically

Venting

Sliders

Number of sliders:

Comments:

Guide elements Main guides: rectangular round Roller guides

Ejector plate with four guides and four pushback pins

Sankyo bushings (L min. 1.5 x Ø) with solid lubricant without solid lubricant

Cooling Ensure intensive cooling! (the predetermined cycle has to be reached)

Minimum amount of cooling cycles: Injection side Ejector side

One separate cycle each around the nozzle and the facing mold half

All of the cores and sliders cooled

Amcolov cores have to be

Cooling mandrels have to be provided with riser tubes

Stäubli Rapid release

Hydraulic cylinder Manufacturer:

End switch For hydraulic cylinder For ejector plate Slider

Mech. End switch which is operated with adjustable cams on both sides

Manufacturer:

Electronic Freely suspended cabled allowed!

Pressure sensor Quantity:

Temperature sensor Quantity:

Date stamps Yes No

Mounting Centrally mounted company Dolezych Transporting bridge

Transport security

Mold sign

Yes No

Initial sampling In the price included are:

Mold acceptance

Part price Part prices included: Yes No

Comments:

Transport The transporting costs of the mold to SF should be separately presented.

Miscellaneous

-

-

Scraper ring

-

Pin ejector DIN 1530 A

-

Ensure sufficient venting. Air trapping which was not predictable, has to be removed afterthe first injection (e.g. additional ejectors).

Fan gate

Manufacturer

-

-

Mold data (with SF requirements) has to be attached on the mold. A functional flow plan(core pulling system) has to be attached.

- Sampling per

-

Parts consist of

Processing of injection molds using the SF checklist. Drawiings in the TIFF G4 format (300 dpi) or pause a set, as well as the fully filled mold card and injection parameter card have to be included when delivering the mold. After delivering, the mold has to be painted and functional and cooling schematics have to be attached.

Number of samplings:

-

-

Manufacturer

-

The mold has to be provided with two transport locks (on machine, operation and back side).Four eye-bolts belong to the delivery contents.

Number of nozzles:

Alternative:

-

Hot runner system: is provided

Injection position

Injection technique Cold runner with demolding using anadditional parting plane

No

Guide bushings of the ejector plate have to immerse into the mold plate (0.2 mm air)

-

alternative:

Manufacturer

Slanted ejector contour adapted

Matrix side

-

-

-

Open systemCascade Control

End switch which is integrated into the cylinder

FIGURE 2.12  Example for a form with the most important data for the mold design (continued)

314 2 Mold Design

Design dra�In contrast to the dra� offer, which is o�en designed in 2D or as sketches, the actual design dra�, which will also be detailed later, is executed in 3D. Essentially, the above-mentioned positions are defined by their dimensions. This means that the separation is fixed, the gate system is dimensionally integrated, and the size and shape of the slider are specified. In addition, the cooling is fundamentally determined.

In this phase, the time has come to integrate the process simulation if necessary. In the case of uncertainties, the position of the injection points and the necessary tempering bore holes channels can be specified.

At the end of the design, it is recommended that one consult with the customer, with the goal of the design release. This is important so that major orders can be placed and the fabrication of the mold does not need to be increased by unneces-sary delivery times.

DetailingToday, the detailing phase is usually fully developed in 3D. This simplifies and accelerates the production process in work preparation and the creation of machine programs (Figure 2.13).

In addition to the technical implementation of the mold, organizational processes should especially be considered when detailing. Worth mentioning in particular are:

� Is a computer-aided design (CAD) version provided? This is especially important when external design capacities have to be involved.

� The file structure for the individual modules and components, which is specified for the customer and in-house, has to be followed.

� Use the correct manufacturer of standard parts, or match the design of the stan-dard parts, which are located in-house.

FIGURE 2.13  Completed 3D design of an injection mold

(company picture Schneider Form GmbH)

3152.1 Design Process

These points are all the more important because in major projects multiple design engineers are working in a team.

Final inspection and documentationAt the end of a proper mold design program, a detailed documentation for the mold is required, in addition to the final acceptance of the mold on the machine and the measuring of the components. This provides the customer with the information needed to put the mold into operation in house and helps with maintenance and to provide necessary information for mold changes.

The documentation should contain the following information:

� A description of mold functions. In addition to the usual components such as slider, hot runner needles, etc., special movements (like those of the index plate) of special molds should also be explained.

� Cooling scheme and connecting diagram for the mold. � An electrical connecting diagram for the mold. � Drawings of the mold. Today it is customary to include the most important deriva-tives of the 3D data in the documentation. Detailed drawings and 3D data are only passed on special request.

� Description of maintenance and maintenance cycles. � Pattern protocol with the most important process data or setting data. � Data sheet about the patterned (sampled) parts. � Steel certificates, if necessary.

Here, as already mentioned in the offer phase, company-specific forms should be used in which the most important data can be entered into the existing forms. This facilitates not only the workload but also ensures a consistent quality standard (Figure 2.14).

316 2 Mold Design

Mold Data Sheet

Order- / Project parameters Customer: Company End customer:

Project: Project number:

Product : Product number:

Product material: {chemical name}, {TRADE NAME}, {Manufaturer}

Mold type: One cavity injection mold

Contact person customer: Sector: Tel.: Fax.:

Contact person internally: Sector: Const.:

NC-processing Production: IPL:

Mold or inventory number (customer)

1. Definition of a Datum Point (DP) at Car-line Cross data (vehicle position) on mesh lines (100 mm pattern)(*DP has to be in or close to the product)

X: mm, Y: mm Z: mm

linear: % 2.Shrinkage of original data incl. mesh around DP

vector dependent X = , Y = , Z = %

Original data (vehicle position) in cavity position (mold position) dislocate and rotate (DP reference point for movement)

cav.-Nr. Ordinate/Plane Step sequence Ordinate/Plane Step sequence Ordinate/Plane Step sequence

Position of car line crossin the mold

Cav. 1 Cav. 2 Cav. 3 Cav. 4

X:X:X:X:

mmmmmmmm

1.2. . .

Y:Y:Y:Y:

mmmmmmmm

1. . . .

Z:Z:Z:Z:

mmmmmmmm

1. . . .

rotated aroundDP (Positive Axis)

Cav. 1 Cav. 2 Cav. 3 Cav. 4

X-axis: ° 1.X-axis: ° .X-axis: ° . X-axis: ° .

Y-axis: ° 1.Y-axis: ° .Y-axis: ° . Y-axis: ° .

Z-axis: ° 1.Z-axis: ° . Z-axis: ° .Z-axis: ° .

3.

mirroredCav. 2 Cav. 3 Cav. 4

Y/Z-plane: 1.Y/Z-plane: .Y/Z-plane: .

X/Z-plane: 1.X/Z-plane: .X/Z-plane: .

X/Y-plane 1.X/Y-plane .X/Y-plane .

4. Safe the file as a work model File name:

Mold dimension X: mm Y: mm Z: mm

Weight fixed side: kg Weight movable side: kg Total weight: kg

Injection molding/pressure casting-machine(s): Krauss Maffei {1300}

Parts lists positions-No. schematic Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos. Pos. - Pos.

FIGURE 2.14  Example for a mold data sheet

3172.1 Design Process

Mold Data Sheet

Page

nr.

File name of the drawingDrawing content

File name of the model Plot file name

Rev.Nr.

Pg. 1

FIGURE 2.14  Example for a mold data sheet (continued)

318 2 Mold Design

2.1.1.4  The Design Process in Injection Molds

In Section 2.1.1.3, the procedure for complex and long-term mold projects associated with high amounts of investment was described. In the following, the procedure for the standard molds is described.

Many of these mold projects are not carried out as a joint project between the client and the mold maker. A�er a contract award, mold making solely takes over the project work up to delivery. Consultations are rather rare. If there are consultations, they are mostly done in the design phase with the goal of cutting the costs or to increase the production safety of the mold.

Based on the project data (essentially the geometry data, the production data and the dates), the design should always be implemented from the inside out (Figure 2.15). This means that the first step is the mold cavity. In further steps, the ejection, the temperature control, and the gating system are optimized. In the last step, the structure is defined. This approach is particularly useful when using a computer-aided design (CAD) program in connection with standard parts, because changes to the size of the mold can only be realized with considerable effort in a late design phase. To resolve this problem, templates (as they are o�en called) can be used, in which standard parts catalogs are stored and through which the structure size may be changed at any time during the design phase.

Step 1: Position of the cavity

Step 2: Cavity area

Step 3: Construction

Dire

ctio

n of

Con

stru

ctio

n

FIGURE 2.15  The mold cavity as a starting point for the mold design

3192.1 Design Process

Principle DimensioningDesign

Realization

Project data

Quantities,

Costs,

Deadlines

Geometry,

Materials,

Tolerances,

Requirements

Production machines

Periphery

Number of

cavities

Cavity position

Parting planes

Position and quantity

Design of the

cavities

Gates,

Sprues

Temperature

control

Demolding

2nd Design of

the cavities

Gates,

Sprues

Temperature

control

Demolding

unit

Mold construction

Guiding,

Centering

Venting

Interface,

Clamping

Mechanical

design

FIGURE 2.16  Structures and phases in the mold design

It is advisable to divide the structure shown in Figure 2.16 in three phases:

Phase 1: At this point the parting plane is determined, the location and number of slides is defined and the mold cavities, the mold insert, and the mold core in its basic form are specified. The number of mold cavities should also be coordinated with the customer, if this is not already given. Finally, in this phase, the position of the mold cavities is defined.

Phase 2: This first step should begin by defining the mold plate thicknesses. This is today still mostly carried out by analytical methods. Programs for the calculation of structural mechanics (finite element method [FEM]) are very rarely used. In a second step, it is advisable to define the basic design criteria and weigh these up against each other. Noteworthy are the ejection system, the gating system, and the temperature control. (See also Figure 2.5.)

320 2 Mold Design

Phase 3: In this phase, the points of Phase 2 are detailed and designed. During Phase 2, work was o�en still done using full-scale sketches. In this phase, work is exclusively done using CAD programs. In the last step, guides, centering, and possibly vents or interchangeable inserts for logos/manufacturer’s stamps are incorporated.

Boundary conditions � Molded Part � Material � Tolerance � Quality � Machine � etc.

Parting planes

Releasing undercuts

Design mold insert and mold core

FIGURE 2.17  Design process for a slider mold, with the example of a small box

with lateral bore holes (in accordance to [1])

3212.1 Design Process

Determine the number of cavities

Determine the position and arrangement of the cavities

Demolding, Optimize gate system and cooling

Complete the construction on the nozzle side

FIGURE 2.17  (continued) Design process for a slider mold, with the example of a small box

with lateral bore holes (in accordance to [1])

322 2 Mold Design

Complete the construction on the ejector side

Insert additional construction elements � Venting � Assembly aids � Clamping

system

FIGURE 2.17  (continued) Design process for a slider mold, with the example of a small box

with lateral bore holes (in accordance to [1])

Figure 2.17 illustrates the design process for a small box-shaped molded part. Here, the systematic design process is very important. It is recommended to follow this procedure step by step. For all other mold types, in particular special molds such as multi-component molds, slight modifications in the sequence are certainly neces-sary but can be quickly adapted to this structure.

Elaborate documentation and samplings with the customer, as previously described, are not so common for simpler projects in mold making. Many customers and mold makers increasingly transition to provide the molds as well as sample parts. With guaranteed cycle time, the process technology is documented as well.

3232.1 Design Process

2.1.2  Simulation for Injection Mold Making

2.1.2.1  General Information

The simulation of injection molding processes has become increasingly widespread in the design of plastic components and injection molds. Investigations, to calcu-late the flow process and the resulting properties are generally grouped under the generic term “rheological design”. A number of process variables can be determined: � filling pattern, � filling pressures, � shear rates, � shear stress, � temperatures at different process times, and � hardened surface layers.

From the calculation results above, significant parameters for the component can be derived in further steps: � shrinkage, � warpage behavior, � closing force, and � orientations.

Generally speaking, calculated results from the filling phase, such as the location of fill lines and air pockets, show the highest acceptance of all calculations.By simulating in advance, quality can be secured and certain errors can be excluded. Simulation may allow: � analysis of expected filling problems, � determination of weldline position and air pockets, � avoidance of material damage, � balancing of gate and molded part, and � improvement of dimensional stability.

In addition to error prevention, economic efficiency can be increased by the simu-lation through: � reduction of mold modifications, � reduction of injection trials, � making shorter development times possible, � material savings on the molded parts, � minimization of cycle times, and � minimization of the required clamping force of the machine.

324 2 Mold Design

Current calculation methods

Shell model Surface model Volume model

T, E, LSR, RIM T, E, LSR, RIM T, E, LSR, RIM

T, E, LSR, RIM T, E, LSR, RIM (T)

Cascade injection molding T T T

GIT T (T) T

Filli

ng p

hase

2-K-Sandwich T (T)

Standard injection molding T, E, LSR, RIM T, E, LSR, RIM T, E, LSR, RIM

T, E, LSR, RIM T, E, LSR, RIM (T)

T T T

GIT T (T) T

T (T)

T, E, LSR, RIM T, E, LSR, RIM T, E, RIM

T, E T, E T, RIM

Cooling system T T T, RIM

Mold and molded part T, E, LSR, RIM

Shrinkage and warpage T T T

T: Thermoplastic LSR: Liquid Silicone ResinE: Elastomer RIM: Reactive Injection Molding

Hol

din

g p

ress

ure/

he

at p

hase

Standard injection molding

Compression injection molding

Compression injection molding

Cascade injection molding

2-K-Sandwich

Fiber orientation

Cooling time

Ther

mal

anal

ysis

FIGURE 2.18  Calculation possibilities with the respective models (in accordance to [2])

Unfortunately, no generell statements can be made on the percent savings, since in practice, simulation for the design of injection molds is not yet extensively used.

In the early days of rheological simulation, only the standard injection molding process of thermoplastics had been considered. Today, there are solutions for elas-tomers, thermosets, liquid silicone resin polymers, and also polymers for the reac-tion injection molding (RIM) process. Simulation programs have been developed for various materials as well as for various processing techniques on the shell model and are available in all these varieties.

Due to the current data structure in CAD, calculations should preferably be done using the surface or volume model. Unfortunately, not all the loading calculation possibilities are currently implemented in the different models (Figure 2.18).

The desire of many developers to work on a single data level will likely remain for a while longer. The current exchange formats provide the developers quite good possibilities to use the full development chain and all modules, if so desired and affordable (Figure 2.19).

3252.1 Design Process

RapidPrototyping Planning

Concept

Idea

Datamodel

MoldShape

"Negative"Master model

"Positive"

SimulationAnimation

3-DDesign

FIGURE 2.19  Possibilities of product development

2.1.2.2  The Types of Models

As already mentioned above, the calculation methods are strongly dependent on the data model. The geometries can be illustrated differently from point, to lines, to surface, to volume models and provide different advantages in the CAD program depending on the objectives (Figure 2.20). Thus, for example, free-form surfaces can be produced and modified on surface models.

With the 3D programs, physical models can certainly be better implemented in the future and will then improve the quality of calculation results. In addition, certainly

FIGURE 2.20  Type of models in CAD systems and networks in simulation systems

326 2 Mold Design

among the existing possibilities of free jet simulation as shown in Figure 2.21, new calculation results will be further developed in the near future. It is important to note that the flow channel thicknesses in the flow simulation (i.e., the wall thick-ness) have a great influence on the calculation results, which make finer meshing of the wall thickness necessary. The resulting high element numbers for the finite-element mesh require higher computing power or longer computing times. This interface problem will certainly influence the further development of the simulation programs of injection molding more than new calculation possibilities.

Both in the mid-plane model and the surface model, the thickness of the components is divided into fixed layers. The computing time rises slightly disproportionately to the number of elements. To reduce the computing time without loss of accuracy, the meshes are dynamically refined; that is, the fine structures are closely meshed. The mesh refinement in 3D meshes is even more important because the component thickness is not divided into layers but displayed through the mesh (Figure 2.22).

2.1.2.3  The Flow Pattern

The basis for the flow pattern is the wave propagation theory as known with light waves or flow waves in physics (Figure 2.23). While in simple models the flow

FIGURE 2.21  Simulation of a free jet [2]

FIGURE 2.22  Volume network with local mesh refinement in critical component contours for

a reduction of the computing time, done with Moldex from SimpaTec GmbH

3272.1 Design Process

pattern still has to be developed through segmentation and the subsequent circle method. The simulation programs especially support the developers for complex components. The programs calculate the flow fronts iteratively with the material models and the geometric and process-related boundary conditions, and graphically prepare the results for the user.

Predictions about the optimum gate position for the avoidance of weld lines in critical areas or to avoid air pockets are already made with very high accuracy. Counter active measures can be taken with wall thickness adjustments or correc-tions to the geometry in the design phase of the mold. As seen in the example in Figure 2.24, the weld line can be brought into the cup area through wall thickness changes in the handle, or the air inclusion can be avoided through a profiled wall thickness on the back of the chute.

Theoretically, the influences of process parameters on the flow pattern can be analyzed. It should always be taken into account, however, that the final set param-eters on the machine are o�en influenced by surface requirements, which cannot be estimated in advance. For this reason, care should be taken in the simulation that the location of weld lines and air inclusions is robust and is dependent on the material and process parameters as little as possible (Figure 2.24).

ExampleWave propagation theory

lNew flow front

Old flow front

Circ

le w

ave

Line

ar w

ave

Any

wav

e

Exec

utio

n w

ith c

onst

ruct

ed fi

ling

lines

3D m

odel

New flow front

Old flow front

l

lFi

lling

pic

ture

with

the

help

of a

sim

ulat

ion

prog

ram

New flow front

Old flow front

l

Des

ign

basi

s

2

1

2

1

ss

ll

=

l 2

s2

s1l1

s1

s2

FIGURE 2.23  Basis for the flow pattern method: the wave propagation theory

328 2 Mold Design

Weld line in the grip Weld line in the cup after wall thickness optimization

Air trapping in a small stack box Small stack box without any air trapping

Mass temperature + 20CInjection velocity + 25%

Weld line in thehandle

Air trapping Venting to theparting plane

Insignificant weld linein the wall

FIGURE 2.24  Examples for the optimization of the weld line and the position of an air

inclusion, calculated with Cadmould®

3D-F

2.1.2.4  Shrinkage and Warpage

Shrinkage and warpage is always a very touchy subject in regards to the mold. The data from the raw material manufacturers on the shrinkage of material is generally very broad and only the main directions of orientation are included. Subsequent changes in the mold can o�en be difficult to realize, especially if too much material has already been removed.

While with simple components such as a disk, the relationship between shrinkage and warpage is still understandable and easily comprehensible without extensive computational effort, this is generally not true for complex components. The simu-lation programs offer a remedy to this and support the developer in the analysis and definition of local shrinkage values and warpage predictions. Influences of the temperature control and the gate position can be developed and minimized in advance, or the measurements in the mold can be derived (Figure 2.25).

In the shrinkage and warpage analysis, the process parameters, in addition to the material data, also play a crucial role. The data for both the filling phase and the holding pressure phase are important for the calculation results. Despite the fact

3292.1 Design Process

that the process parameters cannot be completely defined in advance, because surface criteria and quality characteristics influence the process, the results for homogeneous and nonreinforced filled materials are usually very practical.

The results in fiber-filled materials with high filler content, and especially for long fiber lengths, should be considered more critically. Here, extensive studies from BASF on a model part show that in most cases the warpage direction is defined properly, but the strength of the warpage is not always specified correctly [2]. The growing market shares for long fiber-filled and fiber-filled thermoplastics will require an early remedy for this problem in the future. In joint projects between the raw material manufacturers and the simulation companies, new algorithms are being analyzed [3] (Figure 2.26).

2.1.2.5  Thermal Design

The cycle is about 60% determined by the cooling time. The efficiency of a mold depends considerably on the optimal thermal design (Figure 2.27). The cooling

FIGURE 2.25  Shrinkage and warpage analysis on the example of one-half of the housing [1]

(le�) shrinkage and warpage analysis

(right) averaged degree of fiber orientation

Verzug z [mm]2.200

1.275

0.3500

-0.5750

-1.500

FIGURE 2.26  Warpage phase and the comparison of the component with fiber-reinforced

thermoplastics [3]

War

page

z [m

m]

Z1 Z2 Z3 Z4 Z32 Z31 Z26

Measurement UltradurMoldflow Ultradur®

Z25 Z20 Z19

Middle of the component

Upper borderLower border

0.50

0.40

0.30

0.20

0.00

-0.10

-0.20

-0.30

-0.40

-0.50

Z33 Z30 Z27 Z24 Z21 Z18 Z34 Z29 Z28 Z23 Z22 Z17 Z8 Z7 Z6 Z5

B4520B4520

®

0.10

330 2 Mold Design

concepts described in the literature cannot always be ideally implemented in the design due to necessary compromises. This results in partial areas of the mold which cannot provide optimal tempering. As a result, the cooling time extends, and the cycle is lengthened unnecessarily.

For simple components, constructive circle methods can be applied that are similar to the flow pattern method. However, in complex components with (for example) different rib depths and rib densities in combination with free-form surfaces, the limits of the methods are quickly exhausted, and a design with numerical methods is essential.

Model with cooling

channels

Cooling time Average molded part

temperature

Demolding time [s]

FIGURE 2.28  Models of a lid with integrated temperature control and calculation results,

calculated with Cadmould 3D-F

Closing timeof the mold

Inje

ctio

n tim

e

Residual cooling time

Opening time of the mold Demolding

FQ

HQ

TMQ

LQ

StQKQUQ PlUTM QQQ

StrKLU QQQQ

TMQ Heat flow which is fed and retracted by the heat medium

FQ Heat flow of the hot mold mass

HQ Additional heat flow, e.g. hot runner

UQ Heat flow to the environment

LQ Heat flow which is transferredthrough the heat conduction

KQ Heat flow which is transferredthrough convection

StQ Heat flow which is transferredthrough heat radiation

Holding pressure

Cooling time up to 2/3 of the cycle time

Cycle time

FIGURE 2.27  Cycle diagram and heat flows in injection molding

3312.1 Design Process

For the calculation, the cooling system in addition to the imported molded part is shown in the simulation. The calculation results can show temperatures of the molded part as well as local cooling times (Figure 2.28). It should be noted that the cooling channels from the design dra�s cannot yet be imported easily, but in most cases still need to be redesigned.

2.1.2.6  Summary

The simulation is a good and meaningful support for the development of injection molded parts and should be used in every injection molding and mold making operation because simulation is o�en better than trial and error. The programs offer many opportunities and can provide useful assistance when used properly. The calculations for many types of results, however, depend very strongly on the boundary conditions, which o�en can only be correctly defined by experienced users. Therefore, intensive training should be performed before using a simulation.

If there are still cases where the simulation is used very late to recover from damage, considering including simulation into the design process more and more in the future and it should be adapted to the particular business structure (Figure 2.29).

At present In future

Group 1 The simulation is not used A large, fast-growing group, which uses in expensive and easy to use simulation so�-ware and can also use service if necessary.

Group 2 The simulation is only used now and then as an external service

A slow-growing group, which uses asimulation product which is tailored to their needs and uses services if necessary.

Group 3 A simulation product is used, which is adapted to the conditions within the company and services can be utilized.

FIGURE 2.29  State of the art of simulation [1]

References

[1] Menges, G., Michaeli, W., Mohren, P., Spritzgießwerkzeuge, 6th ed. (2007) Carl Hanser Verlag, Munich

[2] Filz, P., Spritzgießsimulation, lecture manuscript at the University of Applied Sciences Rosenheim (2004)

[3] Bernnat, A., Verzugsberechnung: Zufall oder zuverlässig? (2006) VDI-Spritzgießertagung, Baden Baden

332 2 Mold Design

 ■ 2.2  Standardization and Standards

J. Gockel, A. Brandt

2.2.1  Standardization for Injection Molding and Hot Runner Molds

In the Deutsches Institut für Normung (DIN), or German Institute for Standardiza-tion, the Standards Committee NA 121 “molds and clamping devices” deals with the standardization of components for compression, injection, and pressure casting molds. The standardization work is applied on behalf of the manufacturing indus-try (e.g., standard parts manufacturers) and is developed with the collaboration of interested parties. In this context, standards are standardized components for the above-mentioned molds. Having a large number of standardized components is an advantage for design and mold making.The majority of standards for injection molds are also internationally standardized by the International Organization for Standardization (ISO) and thus attains world-wide validity. ISO is the worldwide federation of national standards institutes (ISO Member Corporation) based in Geneva. Responsibility for the ISO standardization in the field of injection molding is held by the ISO/TC 29, molds, subcommittee SC 8, tools for punching technology and molds for injection molding. The develop-ment for the injection mold is performed in the work group WG 3, where Germany provides the secretariat and the chair.For international understanding, a terminology standard has been developed and published as ISO 12165 “parts of pressing, injection and pressure casting molds – terms and symbols”. There, all of the relevant components are listed with names in English, German, French, and Swedish.In addition, the Tooling and Machining Association International (ISTMA) has developed a technical dictionary [1] that contains terms for punching tools, fixtures, and mold making in 10 languages. It is now available in its third edition. Here, the manufacturers and users of molds have an indispensable aid for day-to-day business in the increased globalization of business relationships.Recurring components for injection molds (Figure 2.30) based on the marked position numbers and the parts list (Table 2.1) are mostly standardized nationally and internationally. In addition, it is o�en necessary to develop work standards, manufacture certain components according to these standards, and offer them to the customer. Company-specific components may not always be standardized, because it makes little sense to standardize individual solutions. In the design drawing, reference is made to standard components. In the parts list (Table 2.1), the names conform to ISO 12165 [2].

3332.2 Standardization and Standards

TABLE 2.1  Parts List

Position no. Product no. Indication Standard 1 K12 Clamping plate, protruding with centering recess DIN 16760-1

2 Z20 Centering sleeve DIN 16759

3 Z11 Guide bushing without centering lug DIN 16716

4 Z71 Eye bolt DIN 580

5 Z00 Guide column, protruding with centering spigot DIN 16761

6 Z1227 Mount housing Connec. accord. DIN 16765

7 Z31 Cylinder head screw with hexagon socket DIN EN ISO 4762

8 Z69 Spring washer DIN 472

9 Z35 Set screw with hexagon socket DIN EN ISO 4026

10 Z011 Guide column ISO 8017

11 Z33 Countersunk screw with hexagon socket DIN EN ISO 10642

12 Z121 Heat insulation plate DIN 16713

13 Z57 Support roller DIN ISO 10073

14 Z41 Ejector pin with cylindrical head DIN ISO 6751

15 Z05 Centering unit DIN ISO 8406

16 Z01 Diagonal pull column DIN ISO 8404

17 Z25 Cylinder pin DIN EN ISO 8734

18 Z81 Connection nipples with free passage DIN 1766-1

19 K100 Centering flange, fixed side DIN ISO 10907-1

… Additional examples of standardized components

Z38 Shoulder screw ISO 7379

Z42 Ejector pin with conical head DIN 1530-3

Z46 Flat ejector pin DIN ISO 8693

Z51 Inlet bushing DIN ISO 10072

Z53 Gate bushing DIN ISO 16915

Z60 Compression spring with round wire cross-section DIN EN 13906-1

It has proved useful, to develop hot sides mold specification sheets for injection molds, pressure casting molds, and hot runner systems and to publish them as DIN or DIN ISO standards.

These sheets should describe the specifications of the molds or hot runner systems with the goal of cost reduction and should help avoid misunderstandings between the contracting parties for the mold request (offer phase) and the mold order. They contain detailed information on material procurement, operating equipment, and structural design of the molds or of hot runner systems to the surfaces of the mold.

334 2 Mold Design

Details about machine-specific data, operating data etc. are also discussed. Specifi-cally, the pertinent standards or standard dra�s are:

E DIN 16764-3 “Mold specification sheets – Part 3: Hot Runner Systems/Hot Halves”

DIN ISO 16916 “Mold specification sheet for injection molds”

E DIN ISO 24233 “Mold specification sheet for pressure casting”

In addition to the substance and terminology standards, another topic has been addressed with DIN 16765 [2] “Electrical connections for hot runner molds and mold heating”. In this standard, the control loops for power and signal lines are defined. It is distinguished by the connections A and B, where connector A controls the control units for molds when the power and signal lines are in separate plugs, and connector B when power and signal lines are available in one plug.

FIGURE 2.30  Design drawing with standardized components

3352.2 Standardization and Standards

The standards are summarized thematically and are available on CD in the 3rd edition of the DIN pocket book 262 “pressing, injection and pressure casting molds”.

2.2.2  Standards in Mold Making

Standards help the manufacturer design and implement the mold in the most simple and economic way. With a diversified range of standard parts (Figures 2.30 and 2.31), i.e. standardized components for plastics processing, simple and fast implementation of mold making is possible.

The standardized components correspond to a defined standard and are manufac-tured in a very narrow tolerance field. The particular advantage is the interchange-ability. Regardless of which component must be replaced due to wear or other damage, it can be requested at on short notice from stock of the relevant standards provider.

World-wide availability ensures maximum service friendliness in the increasingly global plastics processing industry. The standards suppliers have been active in various committees of DIN for many years and endeavour to meet current require-ments of the industry with redefined standards.

The spectrum ranges from pre-defined molds in various panel specifications and steel grades to guide elements, components for marking of plastic components and their traceability, various elements of ejection from simple to complex geometries and special temperature connections, and many other parts for tool and mold making.

FIGURE 2.31  K-/P- and Z-standards (drilled/undrilled plates and accessories)

336 2 Mold Design

The program is completed with a wide variety of hot runner technologies and nozzle series for the melt flow of the injection molding machines directly into the mold cavity.

Programs for the easy creation of CAD drawings through so-called standard modules facilitate the procedure for the design engineer at an early stage. Design times can be reduced through the use of stored 2D or 3D data of the respective standards. A�er completion of the design, parts lists can be generated and sent directly to the standards provider via e-mail or fax. Of course, Internet databases with the appropriate wizards in the compilation of the required product spectrum can be used as well.

The most important standards for tool and mold design are listed and explained below. A complete description of the most common standard parts with all its facets would go beyond the scope at this section.

2.2.2.1  Molds

Standardized molds are available in different mold sizes. The commercially available mold sizes range from 075 × 075 mm to 796 × 996 mm and plate thicknesses from 10 to 196 mm. There are basically two different grades of plates.

The first and most common are the K-plates. These are provided with system bore holes for guiding and centering of the individual plates, as well as thread- or screw- through holes, which are adapted to the size. The classic mold structure consists of the nozzle sided clamping platen, and optionally an intermediate plate or hot runner plate frame, nozzle holder plate, two mold plates, intermediate plate on the ejector side, and the ejecting unit with ejector pins and the clamping plate on the ejector side. The required guide elements such as columns, bushings, and sleeves are adapted to each plate size.

All plates are ground on all sides and are designed with a parallelism of 0.008 to 100. The thickness tolerance is +0.05/0.25 mm. Through this design, the mold maker can directly process and mount the plates without rework. The mold maker focuses on the shaping contour and does not have to deal with the manufacture of the mold frame.

If it is necessary that the prefinished K-plates are not used due to the part geometry, undrilled P-plates with the same range of dimensions can be used. The P-plates are available in several quality grades: plates that are ground on all sides with the same parallelism and tolerances as the K-plates but with a somewhat larger allowance, and pre-ground panels, with even larger allowances and a broader parallelism. This is especially useful for removing larger volumes of metal and subsequent heat treatment.

3372.2 Standardization and Standards

FIGURE 2.32  P-plate with special machining

The thickness measurement in these plates varies depending on the type of mate-rial. A distinction is made between curable and not-curable steels. The steels that are not eligible to be used for a downstream heat process due to their chemical composition require lower allowances than these types of steel that are reprocessed a�er a successful cure. This reduces the finishing time for the mold maker and ensures faster turnaround times in the manufacturing process of the entire mold.

To achieve further improvements in the course of the manufacture of the mold, additional work to the available steel plates is offered by many standard parts manufacturers. The plates are machined (to customer specifications) on modern machining centers (Figure 2.32).

Costly deep-hole drillings as well as the complex milling operations should also be considered. All processing steps up to the mold contour can be obtained from the standard parts manufacturer. Through this extended workbench of the mold maker, optimum quality and precision can be achieved.

Special types of moldsFor a number of applications in the plastics processing industry, it is advisable to use pre-defined molds. These are complete structures, which are for example suit-able for the production of extending components with a parting plane transverse to the mold division, such as bobbins. There are different designs of so-called split molds (Figure 2.33).

These molds are designed so that the mold maker only needs to insert the mold contour and ensure proper cooling in the mold. The split molds are distinguished by the types of split operation. There are split operations using so-called curved kerbs, but the most common type is the angle pins guide.

338 2 Mold Design

FIGURE 2.33  Split mold, ejector version

FIGURE 2.34  Quick change mold system with interchangeable mold inserts

This type can again be divided into an ejector and a stripper design. Then almost every conceivable application is covered.

The structure is manufactured from pre-hardened mold steel such as 1.2312 or from a corrosion-resistant steel 1.2085, while the splits are available in different materials. Here, case-hardening steels and depth-hardened steels such as 1.2343 are available.

For the production of small series, special interchangeable standard molds (Figure 2.34) are offered. These are standard molds with a fixed mold frame and interchangeable mold inserts including the ejectors.

These special types of structures enable the economical manufacture of small batches or prototypes. In pre-defined inserts, mold contours are introduced (and can be rapidly sampled on injection molding machines) through milling, electri-cal discharge machining (EDM), high speed milling, or with more recent methods such as laser sintering. The inserts are available in hot-work steels or in aluminum.

3392.2 Standardization and Standards

Different products can be tested or produced in such a standard mold. This greatly reduces the manufacturing costs for prototypes or small series. However, the areas of application are restricted due to the limited application sizes.

Materials for mold makingToday, the question of what is the right material for mold making cannot be answered as easily as it used to be.

Among the known materials from the DIN standards (see Table 2.2), a number of comparable steels from abroad as well as newly developed steels are now available as an alternative to conventional steels.

Steels not classified in the DIN standards, which include the so-called brand names, are now increasingly appearing on the market. These steels (Table 2.3) are character-ized by outstanding properties, which can be selected by the customer depending on the specified application. This allows the mold maker to precisely adjust the materials to the type and application of the mold.

Today, a customer specifically selects the desired material properties, such as cor-rosion resistance, weldability, and low warpage, according to the type of further heat treatment, coating, or etching needed. Property combinations such as good corrosion resistance, good machinability, and much more can therefore be realized.

The higher cost of materials can soon pay for itself with the various properties of the steels. Thus, the mold steel Toolox 44 with its quenched and tempered delivered hardness of 44 HRC can be used directly as contour steel. Further cost-and time-consuming heat treatment with subsequent post-processing can thus be avoided.

Today, corrosion-resistant materials play a significant role. These materials are characterized by a high chromium content (< 15%). Chrome gives the material poor machining properties, which does not happen with the 1.2099HASCO.M. This steel forms only few carbides due to its low carbon content. Thus, only a small amount of chrome is necessary in order to make the material highly resistant to corrosion. Thanks to its sulfur content, it still has excellent machining properties.

Despite all the new developments, the long-standing DIN steels still play an impor-tant role. O�en, in standard applications, depth-hardened or hot work mold steel is sufficient enough, and the cost of brand-name steel would not pay for itself with the application of the mold. The selection of steels should therefore always be considered from two aspects: technical feasibility and economic benefits.

340 2 Mold Design

TABL

E 2.

2  Lo

ng-S

tandin

g D

IN S

teel Q

ualit

ies

Mol

d No

.1.

1730

1.20

831.

2085

1.21

621.

2311

DIN

Clas

sific

atio

nC4

5WX4

2Cr1

3X3

3CrS

1621

MnC

r540

CrM

nMo7

Com

posi

tion

(%)

�C0

.45

�Si

0.30

�M

n0.7

0

�C0

.42

�Si

0.40

�M

n0.3

0 �

Cr13

.0

�C0

.33

�Si

1.0

�M

n1.0

�Cr

16 �

S0.0

75 �

P0.0

330

�C0

.21

�Si

0.25

�M

n1.2

5 �

Cr1.

20

�C0

.40

�Si

0.30

�M

n1.5

0 �

Cr1.

90 �

Mo0

.20

Tens

ile S

tren

gth

Appr

ox. 6

50 N

/mm

2Ap

prox

. 780

N/m

m2

Appr

ox. 1

,000

N/m

m2

Appr

ox. 7

10 N

/mm

2Ap

prox

. 1,0

80 N

/mm

2

Type

Non-

allo

yed

mol

d st

eel

Corr

osio

n re

sist

ant c

ase

hard

enin

g st

eel

Corr

osio

n re

sist

ant h

eat

treat

ed s

tain

less

ste

elSt

anda

rd c

ase

hard

enin

g st

eel

Heat

trea

ted

mol

d st

eel

Exam

ple

of U

seUn

hard

ened

com

pone

nts

for t

he m

anuf

actu

re o

f m

olds

, too

ls, a

nd ji

gs

Mol

d pl

ates

and

inse

rts

for p

last

ic p

roce

ssin

g,

espe

cial

ly fo

r the

man

u-fa

ctur

e of

che

mic

ally

ag

gres

sive

pla

stic

s

Mol

d pl

ates

for a

ggre

s-si

ve p

last

ics

and

corr

od-

ing

tem

pera

ture

con

trol

med

ia

Mol

d pl

ates

and

inse

rts fo

r pl

astic

s pr

oces

sing

Mol

d pl

ates

and

inse

rts

for p

last

ics

proc

ess-

ing.

Rea

dily

pol

isha

ble.

Ph

oto-

etch

ed a

nd s

truc-

ture

EDM

is p

ossi

ble.

Mat

eria

l No.

1.23

121.

2343

1.23

791.

2764

1.27

67DI

N Cl

assi

ficat

ion

40Cr

MnM

oS86

X38C

rMoV

51X1

55Cr

VMo1

21X1

9NiC

rMo4

X45N

iCrM

o4

Com

posi

tion

(%)

�C0

.40

�Si

0.40

�M

n1.5

0 �

Cr1.

90 �

Mo0

.20

�S0

.07

�C0

.38

�Si

1.05

�M

n0.4

0 �

Cr5.

15 �

Mo1

.25

�V0

.38

�C1

.6 �

Si0.

25 �

Mn0

.3 �

Cr12

.0 �

V1.0

�C0

.19

�Si

0.27

�M

n0.3

0 �

Cr1.

25 �

Mo0

.20

�Ni

4.05

�C0

.45

�Si

0.25

�M

n0.3

0 �

Cr1.

35 �

Mo0

.25

�Ni

4.05

Tens

ile S

tren

gth

Appr

ox. 1

,080

N/m

m2

Appr

ox. 7

00 N

/mm

2Ap

prox

. 855

n/m

m2

Appr

ox. 8

50 N

/mm

2Ap

prox

. 880

N/m

m2

Type

Heat

trea

ted

mol

d st

eel

(goo

d m

achi

nabi

lity)

Stan

dard

hot

wor

king

st

eel

High

ly a

lloye

d ch

rom

e st

eel

Spec

ial c

ase

hard

enin

g st

eel (

high

ly p

olis

habl

e)Sp

ecia

l thr

ough

har

deni

ng

stee

l (hi

ghly

pol

isha

ble)

Exam

ple

of U

seM

old

plat

es fo

r the

m

anuf

actu

re o

f pla

stic

m

ater

ials

, mol

d fra

me

for p

ress

ure,

and

in

ject

ion

mol

ding

.

Mol

d pl

ates

and

inse

rts

for A

l-, M

g-, a

nd Z

n-pr

essu

re c

astin

g, m

old

plat

es a

nd in

sert

s fo

r the

m

anuf

actu

re o

f pla

stic

m

ater

ials

.

Die

plat

es a

nd b

lank

ing

punc

hes

for c

uttin

g an

d pu

nchi

ng.

Mol

d pl

ates

and

inse

rts

for t

he m

anuf

actu

re

of p

last

ic m

ater

ials

, es

peci

ally

for h

igh-

glos

s po

lishi

ng.

Mol

d pl

ates

and

inse

rts

for t

he m

anuf

actu

re

of p

last

ic m

ater

ials

, hi

gh c

ompr

essi

ve a

nd

bend

ing

stre

ngth

, as

well

as h

igh-

glos

s po

lishi

ng.

3412.2 Standardization and Standards

TABLE 2.3  The New Generation of Mold Steels

Material no. – – –Name 1.2099HASCO.M Toolox 33 Toolox 44

Composition (%) � C-0.05% � Si-0.35% � Mn-1.2% � S-0.14% � Cr-12.7%

� C-0.24% � Si-1.10% � Mn-0.80% � Cr-1.20% � Mo-0.35%

� C-0.31% � Si-0.60% � Mn-0.90% � Cr-1.35% � Mo-0.80%

Tensile Strength 1060 N/mm2 300 HB 44 HRC

Type Corrosion resistant plastic mold steel (good machinable)

Tempered mold steel Tempered and hardened mold steel (highly polishable and etchable)

Example of use Mold frame steel Mold frame steel Case hardening steel, mold frame steel

2.2.2.2  Standardized Guide Element in Mold Making

To guarantee perfect production and high precision products in injection molding (in addition to a detailed design of the shape contour), highly accurate guidance of the two mold halves is necessary.

The guide elements offered by the standard parts manufacturers are constantly standardized in accordance with DIN/ISO.

There are different types of guide elements. The main elements are the centering sleeves with which the mold plates are very precisely connected to each other. Furthermore, guide pins and guide sleeves are inserted into the mold plates (Figure 2.35). Guiding geared to the bore holes of the system of the entire mold is guaranteed. The so-called low-maintenance guide elements (Figure 2.36) are widely known.

FIGURE 2.35  3D drawing of a mold structure with guide elements and accessories

342 2 Mold Design

FIGURE 2.36  Low-maintenance guide bushings on guide elements

The guide sleeves are made from a special bronze alloy and fitted with grease lubrication reservoirs. Through the embedded solid lubricants, the guide system is automatically lubricated, and the use of oils or grease is unnecessary. These ele-ments can be used in all applications with mold temperatures below 200 °C. Flat guides are available for the individual design of guide systems. These elements can be adapted to the individual requirements using conventional manufacturing methods such as sawing and milling.

Additional locating units are also included in the family of the guide elements. Depending on the requirements made on the part to be produced, the use of round (Figure 2.37 (a)) or flat centering units (Figure 2.37 (b)) can be necessary. In addition to the existing guide system, these centering units are mounted into the parting plane of the mold. When closing the mold, they are used just before the mold halves touch. The two mold halves are very precisely and accurately moved on top of each other using a conical surface guide.

a) b)

FIGURE 2.37  Centering unit for fixing the mold plates

(a) round; (b) flat

3432.2 Standardization and Standards

2.2.2.3  Standards for Demolding

Nowadays it is essential that an injection molded part has not only wide-ranging functionality but also an impeccable appearance. An increasing number of visible parts must be removed from the mold so that permanent functionality of the mold is guaranteed, but also that no visible marks remain on the part caused by mold release agents, which would have a lasting impact on the appearance.

Standard mold release elements should, therefore be used effectively in the planning and design phase; for example, using a CAD file in a cost-effective way.

For demolding undercuts and internal threads, collapsible cores (Figure 2.38) are used. Two technical solutions are available as a standard mold unit:

� collapsible core with central core, solid center studs, and segment sleeve; and � collapsible core with central core and force guided segments.

In both variants, the moving demolding elements (lamellae) can collapse inward once the central core is withdrawn.

The cores are hardened and ready for installation apart from introduction of the contour or grinding of the necessary part thread. They are tailored to the required demolding method for the construction of the mold and do not need additional devices flanged to the mold.

For some cores, fixed central studs are formed onto the molded part resulting in interruptions occur in the undercut. It is therefore necessary to take care that the part does not become caught in the collapsed core. This can be done with the help of mold release aids (e.g., an air shower).

Many plastic products have to be ejected in several stages due to the molded part geometry. Separation of the product from the gate by gradual removal and ejection from the mold is also common practice.

FIGURE 2.38  Collapsible core for demolding undercuts and threads

344 2 Mold Design

FIGURE 2.39  Two-stage ejector with decentralized mounting for the subdivision of stroke

movements

For these individual strokes, normalized two-stage ejectors (Figure 2.39) are used that can automatically divide the ejection movement of the machine into two indi-vidual strokes.

These precision components are centrally located in pairs or eccentrically positioned. They are actuated by a hydraulic machine ejector, the ejectors of the injection mold, and through the opening movement of the machine. Resetting is performed through the ejector hydraulics of the machine or mechanically through back pressure pins.

Normally, the ejection pin and a sliding sleeve, which are positively connected by segments, drive the first stroke (stroke 1) together. At a defined position, the taper bush strikes and the segments now combine the taper bush and the sliding sleeve with each other and release stroke 2. The initially operated ejector plate is fixed. Both stroke lengths are completely controlled. Also, delayed ejector movement can be implemented without problem.

Latch conveyorsFor molding with multiple parting planes, latch conveyors are required (Figure 2.40). Depending on the mold size and expected load, at least two pieces are attached to the side of the mold. When opening the mold, a mold parting plane is defined as opened. As soon as the positive connection of the latch conveyors is mechanically loosened, the second mold parting plane is released. In addition the previously described latch conveyors that are attached to the mold from the outside circular types (Figure 2.41) that are incorporated within the injection mold are also available.

3452.2 Standardization and Standards

FIGURE 2.40  Latsch locking unit for the molds

with multiple parting planes

2.2.2.4  Temperature Control

Temperature control is a key issue in mold and tool making. A large potential for cycle time savings exists in reducing the cooling time of the part.There are several standards for effective cooling. A wide range of water supply cou-plings (Figure 2.42) and associated valves has become part of today’s standard in mold making. The most common systems have cooling channel diameters of 9, 13, and 19 mm cooling channels. For some time now, a nominal diameter of 5 mm for temperature control of small cores has been used. The rapid-connect couplings are available with or without shut-off valves. Advantages of the open system are the lower pressure drops in the cooling cycle, but a disadvantage is the leakage of the temperature control medium while releasing the couplings. Closed systems can prevent this very well; they close the cooling channel and also prevent the entry of oxygen, resulting in less corrosion.

FIGURE 2.42  Rapid-connect couplings with a shut-off valve, straight and angled

FIGURE 2.41:  Circular latch conveyor for molds

with multiple parting planes

346 2 Mold Design

A wide range of matching tubes round out this basic program. Polyvinyl chloride (PVC) tubes of different color (white/red) are still mainly used. Metal braided Viton® tubing or polytetrafluoroethylene (PTFE) tubing is used for higher temperature applications.

In the temperature control of the mold with oil, corrugated metal hoses, which tolerate high temperatures and aggressive medium oil better, are widely used.

Core cooling and deflection elementsSpiral cores (Figure 2.43) made from plastic or aluminum have been used for a long time for cooling the cores. The temperature control medium is fed via one or two threads to the core and then removed again.

So-called deflection elements (Figure 2.44) or fountain cooling systems are used if the aforementioned elements cannot be used due to insufficient space. The tem-perature control medium is introduced into the mold core using a transverse baffle or tube, and the heat of the injection is removed over a defined sequence.

FIGURE 2.43  Spiral core, single or double-threaded made from aluminum or plastic material

FIGURE 2.44  Deflection elements, preferably for composite mold inserts

3472.2 Standardization and Standards

FIGURE 2.45  Ejector pins hardened from hot work steel

Standardized steel/copper core pins (Figure 2.45) are available for specific heat removal in complex or difficult-to-access areas (e.g., lugs) of the cavity. A stan-dardized program of core pins with a diameter of 2.5 to 12 mm can be used for the targeted removal of heat points. These cores are of composite materials made from a steel sleeve and a copper core, which are connected to each other using a special process (MECOBOND® technology).

The copper core directly removes the heat of the melt, while the plastic cools off very quickly, and the part can be removed.

The advantage in using composite pins is their resistance to abrasive materials and associated wear resistance compared to conventional copper elements.

Corrosion protectionCorrosion is the biggest enemy of effective temperature control. A rust-layer thick-ness of only 0.1 mm in the cooling channel leads to a reduction in cooling capacity.

FIGURE 2.46  Corrosion protection cartridges for the reduction of corrosion and lime scale

formation

348 2 Mold Design

d 3

Z940/...

Z941/...

Z9900/...

118˚

Z81/...

d 5d 4

+ 0,

2+

0,1

FIGURE 2.47  Installation example of a corrosion protection cartridge

For this reason, corrosion-resistant steel is used where possible. Where this is not possible, corrosion protection cartridges (Figure 2.46) can be used. These cartridges are used at the beginning of the cooling circuit (Figure 2.47) and are surrounded by water as a temperature control medium. A galvanic element is created between the corrosion protection cartridge and the mold plate. The cartridge dissolves slowly, and the finest particles of the cartridge remain on the walls of the cooling channel without reducing the cooling efficiency. The protective film reduces or prevents corrosion of the cooling channel.

References

[1] ISTMA, www.istma.com[2] N. N., Press-, Spritzgieß- und Druckgießwerkzeuge. DIN-Taschenbuch, Beuth Verlag

GmbH, Berlin, www.beuth.de

3492.3 Hot and Cold Runner Technology

 ■ 2.3  Hot and Cold Runner Technology

D. Paulmann, M. Sander

2.3.1  Advantages of Using the Hot Runner Technology

Cycle time reduction is the key to lower product costs and increased productivity. When using hot runner systems, action must be taken before solidification of the sub-distributor or the gate. Because the cooling time dominates the cycle time and the wall thickness of the solidifying cross section is squared in the calculation of cooling time, cycle times can be significantly reduced by avoiding “thick gates” and voluminous distribution systems.

Reducing the amount of raw material used directly lowers the cost per item. Therefore, a hot runner application is advisable, especially with expensive high-performance plastics with limited back feeding of the solidified gates in the process and in unfavorable ratios of item weight to gate weight. Figure 2.48 clearly shows that the total volume of the gate can account for a significant part of total shot volume. For small parts, the ratio may be even worse. The expensive raw material is – homogeneously and without thermal damage – kept molten by the hot runner system to the part section. In certain cases, a hot runner system allows a smaller machine with a smaller screw/plasticizing unit to be used. Only the actual volume of product is then processed and injected.

Feeding the solidified gate back into the production process costs money. Gate separation, transport, milling in and cleaning, as well as feeding of the recycled material back into the injection molding process requires machines, people, and time. For molds with hot runner systems, these costs are eliminated.

FIGURE 2.48  Frame switch, attached to the solidifying sub-distributors

350 2 Mold Design

By not using recycled material, the reliability of production and the quality of the manufactured parts increase. Mechanical, optical, and thermal properties of the products are consistently good and do not vary with the purity and the amount of recycled material. Production interruptions due to blocked gates are avoided.

The gates can o�en obstruct the ability to automate injection molding production by requiring manual changes. There are many factors that can obstruct automated production, such as a non-free-falling gate that becomes stuck in the demolding operation, entangled, high-volume gate spiders that need to be regularly removed from the machine and the need for manual removal of the gate from the injection molded part. In such cases hot runners offer an effective remedy.

The required clamping force of the injection molding machine is determined by the pressurized, projected area of molded parts and runners. When the ratio of the gate area to the projected surface of the products is unfavorable, the size of the neces-sary injection molding machine can be determined. In these cases, hot runners can allow production on a smaller, cheaper machine.

Especially in actively closing hot runner systems (needle valve), the possibilities for active process control increase significantly. A gate opening under system pressure leads to more uniform part filling and thus to consistent product quality. With large, open needle valve gates, the transfer of the holding pressure is better than in conventional cold runner gates. Family molds can be actively “balanced”.

The topography and optical quality of the injection point is influenced in a defined way. By controlling the melt temperature up to the gate, component properties can be specifically improved. Shear rates can be reduced, as seen in Figure 2.49.

FIGURE 2.49  Optical fiber with gate and visible shear effects, or directly connected with needle

valve (foreground), in optically perfect quality

3512.3 Hot and Cold Runner Technology

Due to the higher shearing of the material in the gate area of the small tunnel gates, milky areas that affect the product quality occur in the parts. By direct gating of the part using a valve gate hot runner, the flow cross section for the melt can be significantly increased, the shearing can be reduced, and the product quality improves.

The use of the hot runner technology allows only certain component geometries and mold concepts. With hot runner systems, gating positions are achieved that cannot be reached conventionally; the active movement or complete avoidance of fill lines can be implemented by sequential injection; multi-component injection molding can be performed with up to five materials in a mold; high-speed stack molds can be built; film and fabric can be back injected; gas and water injection technology can be implemented; multi-cavity molds can be realized; clean room production can be optimized; and the co-injection technique can be enabled.

2.3.2  Design of Hot Runner Systems and Hot Halves

If the mold is recognized as part of the production process and the components of the mold are seen as a coherent functional unit – which means that its components and functional groups mutually interact – it becomes clear how important the correct design of the hot runner system is, taking into account all the parameters of the product, the mold, the material, and the production process for accurate and efficient production. Hot halves consist of all components and hot runner nozzles, clamping plate, frame plate and nozzle plate, as well as all the associated standard parts and connecting components.

This package is individually conceived, designed, and manufactured for each appli-cation in close consultation with the client. Figure 2.50 shows such a hot half. It consists of ready-to-connect electrical wiring of heaters and temperature sensors, temperature control connectors, and guide and clamping elements, which are coordinated with the mold.

The customer receives a completely wired and ready-for-connection system that only has to be connected with the other mold components. A�er connecting the corresponding control and supply peripherals, reliable production can be started. All electrical components of hot halves are checked for functioning, correct wiring, and zone occupancy before delivery. The result of this verification is documented in a diagnostic protocol by the major manufacturers.

All the relevant dimensions (e.g., inside micrometer of guides and centering, nozzle tip position and height, as well as the installation dimensions) are measured in coordinate measuring machines in the assembled state and are recorded in a test report, as shown in Figure 2.51. The concept of the hot halves allows mold makers

352 2 Mold Design

and injection molding specialists to shorten the design and production times of hot runner molds considerably. Not only the design and construction of the hot runner system, but also of the entire fixed mold half can be taken from the hot runner manufacturer.

Close cooperation between the mold designer and hot runner designer, as early as the planning phase, is essential for a smooth project flow and optimal product quality. Some full-service providers also support the customer in the design, construction, and complete construction of the injection mold, except for the cavity areas.

2.3.3  Application Areas and Examples

It makes sense to structure technically homogeneous building and constructive principles in the hot channel area into areas and fields of application. Each pro-duction segment requires specific solutions and product characteristics. Common classifications of the major hot runner manufacturers differ. For example, some of the most common classifications include systems for electrical applications; plugs and connectors; automotive applications in indoor, outdoor, and technical fields; packaging and closures; cosmetic and optical high-quality products; solutions for the medical industry; and small and micro parts, to name only a few. Examples for technical solution principles for different application areas are described below.

FIGURE 2.50  Hot halves, ready for

connection, 12 cavities,

ready for installation

FIGURE 2.51  Hot halves with the

measurements on the

coordinate measuring machine

to develop the test document

3532.3 Hot and Cold Runner Technology

2.3.3.1  Hot Runner Solutions for Packaging Parts, Closures, and Miscellaneous Polyolefin Applications

Especially for polyolefin applications for injection solutions with limited installation space, extra small cavity spacing and long, slim hot runner nozzles are designed for internal gating in the sealing caps and cosmetics industries. These products enable (due to their compact size) gating positions of plastic parts to be reached that would be impossible with conventional nozzle dimensions. The fitting dia-meter of the front nozzle seal must be minimal and is sometimes only 7 mm. The gating can be positioned tightly next to the cores or domes. The maximum nozzle lengths are up to 200 mm, with installation space for the nozzle sha� o�en less than 14 mm. These dimensions also allow internal gating of caps or closures with cooling and unscrewing equipment on the nozzle side of the mold. The series of hot runner nozzles shown in Figure 2.52 is available in fine length variations as flexibly standard products and can therefore be used flexibly for a variety of applications.

Through minimum head diameters of less than 18 mm, extreme multi-cavity molds with very small mold sizes can be realized. The minimum mold size will no longer depend on the size of the hot runner nozzles. In most compact nozzles, nozzle tips, heaters, and thermocouples are interchangeable a�er pulling the cavity from the parting plane in the flanged design. This simplifies maintenance and minimizes downtime of the mold. In the classic variations, all the components can usually also be replaced. To replace the heater and thermocouples, the nozzle side of the mold must then be disassembled. Thermocouples that are positioned close to the point will guarantee temperature recording near the gate. Nozzle heaters are optimized

FIGURE 2.52  Long, slim nozzle “Compact Shot” for applications from the closure and

packaging area

354 2 Mold Design

in practical trials for each nozzle type and length. Material damage is avoided by a homogeneous temperature profile across the entire nozzle length. The nozzle tips are usually made from a CuCoBe alloy with excellent thermal conductivity and are nickel plated or otherwise coated. Thermal insulation of the cooled cavity area is done via sealing rings made from titanium or special steel with low thermal conductivity.

As an ideal complement to long, slim nozzles, some hot runner manufacturers offer tempered sprue bushings for optimum heat dissipation from the gate area. These sprue bushings are individually designed for each application and manufactured to customer specification. They guarantee optimal results for injection and for instal-lation situations in which heat dissipation is usually a problem. Especially useful are nozzles for processing polyolefin and styrene at a small to medium shot weight range of about 2 to 15 g. Figure 2.53 shows an example of an oil bottle cap made from polypropylene. The injection was done in the center with a long, narrow nozzle within the geometry for the tear ring pull (gate position is marked with arrow). The mold components that are needed for the ejection of the ring limit the available space for the hot runner nozzle considerably. The necessary movements in the nozzle side of the mold increase the mold components’ height significantly and thus require, in addition to the small diameter, a particularly long hot runner nozzle.

Other nozzle concepts for polyolefins focus on the vestige quality, pressure stability, and service friendliness. These nozzles are specifically designed for demanding multi-cavity applications and high-speed packaging molds; they are modular and easily constructed. In these nozzles, the user can put together a very wide range of diameter and length combinations and an extensive range of accessories for the perfect combination of the specific application. The smallest nozzle with a diameter of around 13 mm is suitable for both internal gating, as well as gating with limited space, and is available as a standard with a length of up to 150 mm. The largest fit diameter of the nozzle is approximately 22 mm, with a head diameter of 50 mm.

FIGURE 2.53  Tear-off cap, produced in a mold with 8 cavities with “Compact Shot” nozzles

3552.3 Hot and Cold Runner Technology

The lengths range up to 250 mm. The individual selection of the nozzle tip allows optimum adaptation to the specific requirements of the application. Figure 2.54 shows the nozzle body, heating, thermocouples, nozzle tip, and other optional components used for such a module nozzle.

A wide range of single-hole, three-hole and open tip nozzles, made from TiN-coated CuCoBe alloy, or alternatively from a molybdenum alloy, is available as standard in different tip angles. The optimal tip selection guarantees visually flawless parts and a good gate point and avoids threading and running. Components like titanium insulation rings are available to minimize heat transfer from the nozzle into the cooled cavity as well as pre-chamber inserts with precisely implemented calottes geometry.

Installation in the mold can be facilitated with the help of spacers for optimum height adjustment as well as spacer sleeves for positioning of the pre-chamber and the titanium insulation ring. In order to accelerate color change, the volume of plastics in the calottes area can be minimized by using insulator caps. Another advantage of this nozzle type is consistent service friendliness. The nozzle tip as well as the thermocouple and heater can be replaced from the front, without having to remove the mold from the machine. By an appropriate structural design of the hot halves with cable runs between the nozzle plate and the nozzle-sided cavity plate, each nozzle component can be quickly and easily changed a�er pulling the mold plate. This reduces downtime and increases productivity. Typical applications include beverage caps, cosmetic packaging, or screw caps of any kind. The flip top cap, which is shown in Figure 2.55, is a typical example of this product area, where the durability requirements of the hot runner systems are particularly strict due to the high volume to be produced.

FIGURE 2.54  Modular “Rapid Shot” nozzle for high-speed packaging molds

356 2 Mold Design

FIGURE 2.55  Flip top cap, produced in large quantities (millions)

2.3.3.2  Hot Runner Solutions for Technical Components

Specific design concepts of hot runner nozzles are for processing tasks in the field of high performance plastics. The appropriate nozzle size can be selected from a wide range of different diameter/length combinations, with nozzle head diameters from 16 to 60 mm and lengths from 50 to 300 mm. A wide range of different tip geometries and materials ensure optimum visual appearance and heat conduction at the gate point with high resistance to abrasive wear. Special CuCoBe, tungsten carbide, or molybdenum alloys are selected and used depending on the application.

In general, all tip inserts are modular and interchangeable in these nozzle con-cepts. Simply changing the nozzle tip makes it possible to customize the system to changing processing conditions and plastics or a modification of the gate design. Figure 2.56 shows typical nozzles for processing of engineering plastics and dif-ferent nozzle tip geometries.

FIGURE 2.56  Hot runner nozzle for the manufacture of engineering plastics “Techni Shot”

3572.3 Hot and Cold Runner Technology

FIGURE 2.57  Cross section through a “torpedo nozzle”, shows torpedo and heater position

In process-dependent wear of the tip, the tip can be replaced without dismantling the hot runner system from the mold parting plane. Thermocouples and heaters are also interchangeable. Different types of pre-chamber inserts enable easy instal-lation of the nozzles because the incorporation of the front nozzle into the mold is eliminated. Especially with small diameters and large installation depths, the precise manufacture of this fit area is a considerable challenge for mold makers. By mounting the inserts with a precisely ground fit, installation is considerably easier. Torpedo tips from special hard-metal alloys in combination with a retaining nut made of steel (which simultaneously takes over the sealing function in the front nozzle region) guarantee high resistance to chemical wear, which can especially occur by outgassing during the processing of flame-retardant materials. The cross section in Figure 2.57 shows the installation of such a torpedo tip with a screwed-on pre-chamber. The position of the nozzle heater with considerable heat output in the torpedo area close to the gate is also shown.

Another important feature of nozzles for the manufacture of engineering plastics is the homogeneity of the temperature profile across the nozzle length. Particular value must be placed on this requirement when processing melts with a small process window. In the single-nozzle variant, an optimal temperature profile is achieved by additional heat conduction in the nozzle head.

Thus, all the benefits of the system nozzles can be used for single applications without a hot runner manifold, even if not driven with an adjacent machine nozzle. The nozzle bodies are usually made from special hot work steel. This guarantees a high compressive strength, even for continuous temperatures well above 300 °C. With these nozzle concepts, plastics with filler contents of more than 40% and V0-adjusted materials at injection pressures of 2,000 bar or more can be reliably processed. Figure 2.58 shows a typical representative of the manufactured products family with this nozzle series. High processing temperatures, a narrow process window, and the risk of abrasive and chemical wear of the mold components are characteristic of this type of technical components.

358 2 Mold Design

FIGURE 2.58  The nut for the cable attachment made from PA 66 with 40% glass fiber,

flame protection setting V0

2.3.3.3  Hot Runner Solution for Small and Micro Injection Molded Parts

Interesting hot runner solutions have been specifically developed for multi-cavity, gateless injection of small and micro injection molded parts in a confined space. As a standard system, solutions with 4, 8, and 16 injection points and injection intervals of only 8, 10, or 20 mm are offered. Special gauges are available on request. The minimal installation dimensions of hot runner systems enable the use in molds with outer dimensions of 75 × 75 mm. Therefore, even with mold solutions for mini-injection molding machines such as babyplast, modular, multi-cavity hot runner direct connections can be realized. The outer dimensions of mini hot runner systems, illustrated in Figure 2.59, are only 34 × 32 × 100 mm.

The demand for equal pressure conditions at each gate point and uniform filling behavior of all cavities also applies to these small systems. This is achieved through natural balancing of the entire system. Equal flow length and flow cross sections can be realized through an innovative melt flow system. The melt channels in the manifold are not drilled, but are milled by five-axis machining to the lateral surface of a cylinder rod. In Figure 2.60, the so�, rounded transitions of the melt channels are clearly visible. Material damage through sharp edges or dead corners inside the manifold is therefore avoided.

The flat sealing of the calottes area around the tips compensates the radial thermal expansion of the manifold and avoids stresses by hindering thermal expansion. A minimum injection pressure applied to the surface reduces buoyancy forces. The small material volume in the calottes area facilitates the rapid change of material. Tight tolerances for the tip position and length enable exact positioning of the tip

3592.3 Hot and Cold Runner Technology

in the gate at high temperatures, and thus the smallest gate diameters from about 0.4 mm, which is a basic requirement for minimum gate marks. The tips of these systems are manufactured from CuCoBe or a special molybdenum alloy, which is highly wear resistant with good heat conduction. An adapted heat output through-out the manifold length ensures a homogeneous temperature profile and uniform temperature along the flow path at all tips.

Heat losses by heat conduction into the mold can be minimized by supporting elements made from titanium. Even in mini systems, heating, thermocouples, and tips are interchangeable.

One advantage over conventional systems is the minimized control complexity for multi-cavity applications. Only one control point per manifold is needed. These systems are suitable for processing polyolefin, styrene, and acetal. Applications for other plastics may also be possible a�er consultation with the manufacturer. Basi-cally, shot weights are possible from 0.05 to 2 g per nozzle tip. The product made from polyacetal, which is shown in Figure 2.61, has a product weight of only 0.1 g and is produced in a four-cavity mold on a mini-injection molding machine “babyplast”.

FIGURE 2.61  Miniature part of “spring” from POM (Polyoxymethylene), with molded mini hot

runner system

FIGURE 2.59  Mini-hot runner system

with 16 gate positions

FIGURE 2.60  Five-axis milled manifold

rod for natural balancing

360 2 Mold Design

2.3.3.4  Hot Runner Solutions for Multi-Point Gating through Nozzles and Multi-Nozzles

Multi-gating nozzles are principally used for connecting several parts with a nozzle or for filling a part through several gating points. Open and conventional tips from CuCoBe, tungsten carbide, or molybdenum alloys are available, as are special designs for side gating or with valve gate nozzles and niche applications with adjustable tips. Standard parts are available with two, four, six, and eight nozzles with pitch diameters of about 20 to 50 mm in different lengths of 75 to 95 mm. Besides the obvious advantages of multi-nozzle concepts such as cost-effective design of multi-cavity molds and space-saving construction, a critical point of this concept requires special attention. The greater the pitch circle, the greater the radial thermal expansion that loads any type of tip sealing mechanically to shear. Some multi-nozzle concepts completely abandon tip sealing or accept the shear stresses by hindering thermal expansion in the seals. This however implies either poor material exchange and buoyancy forces or seal problems and a risk of fracture in cyclic loading. A special kind of tip sealing solves these problems by providing an entirely new sealing system for each nozzle tip. It is shown in the background of Figure 2.62.

Each tip is sealed through a sealing cap made of special plastic (MurSeal®) in the injection calotte. This new type of sealing material is not only temperature- and pressure-resistant but also elastic. The processed plastic material fills only the well-washed calotte area. Fast material and color changes can be achieved. Specially designed three-hole tips made from TZM ensure good heat conduction in the gate area. At the same time, this tip material is highly resistant to wear. The area sub-jected to injection pressure is minimized. A lower force acts on the nozzle and the mold insert. Increased service life of the nozzle and mold are the positive effects. Gauge differences between cold and warm dimensions are compensated by the elasticity of the sealing cap material. Even nozzles with larger pitch diameters can

FIGURE 2.62  Multi-nozzle with individual tip sealing

3612.3 Hot and Cold Runner Technology

be mounted in a cold state. The familiar cracking of sealing caps made from brittle materials is a thing of the past. The sealing material of the cap is pressure-resistant and dimensionally stable up to 280 °C. Optimized calottes and tip geometries allow the smallest gate diameters and good optical quality of the gate mark on the plastic part. Multiple gating nozzles are principally suitable, due to the temperature profile and the regulation of several tips over a control circuit, for the manufacture of polyolefin, polystyrene, acrylonitrile butadiene styrene (ABS), and other plastics with a larger process window. Only some engineering plastics can be processed a�er consultation with the hot runner manufacturer.

2.3.3.5  Hot Runner Solutions with Needle Valve

Especially in the hot runner area, needle valve gate technology offers interesting potential for qualitative and technical differentiation and will grow steadily in the coming years. Advantages of the plastic product are minimal gate marks. Therefore, the valve gate technology is ideal for direct gating on visual and used surfaces, which require nearly “invisible” gates. Gate marks are optically concealed. Functional sur-faces of technical parts o�en require “smooth” gates, which do not affect the function of the part or the fit accuracy in the assemblies. Targeted opening and closing of the gate points has other advantages over a conventional hot-channel connection.

Better holding pressure transfer, reduction of shear influences, avoidance of running of the hot runner nozzles, faster mold filling and thus shorter cycle times, and higher productivity are some of these process-related advantages. Some special injection molding processes, such as sequential injection, are only possible using needle valve systems. By sequential gating, weld lines can be avoided or moved into areas where they do not negatively affect the mechanical properties of the component. For large visible parts (e.g., from the automotive sector), weld lines are completely avoided by selective opening and closing of the gates. Gas and water injection as well as foaming processes are only made possible by using needle valve systems. A return flow of the auxiliary medium into the hot runner system is avoided. In family molds, uniform part filling without overloading the part with the smaller product weight is achieved. Injecting multiple components through a gate is enabled by a combination of needle and sleeve.

Needle drives with pneumatic piston actuators that can be incorporated directly into the clamping plate of the mold have proven successful. Standardized designs, as shown in Figure 2.63, can be easily installed, and important functions such as radial compensation and needle adjustment are already integrated into the assembly.

These pistons are driven through air pressure of 6 to 8 bar or through an interme-diate pressure amplifier with higher power requirements. The dimensions start at

362 2 Mold Design

about 40 mm piston diameter, so that sufficient force can be established for closing against the holding pressure. Stroke lengths of 4 to 8 mm permit a wide needle retraction, which allows low-shear filling of the molded part. Temperatures in the drive area of > 100 °C are possible through the use of Viton seals. Special pneumatic control units are used, where the exact time of opening and closing can be adjusted depending on the signals from the injection molding machine controller. For pure production environments, this drive is only of limited suitability because lubricated air is used although it is significantly cleaner than hydraulic drives.

A commonly used variation is the hydraulic needle drive, either directly built into the clamping plate of the mold or installed as a complete drive unit into the hot runner manifold, as shown in Figure 2.64. The drive medium is hydraulic oil from the machine or from a separate hydraulic power unit. The dimensions begin at < 20 mm, since the higher pressures (60 to 80 bar) can also ensure sufficient clamping forces on smaller piston areas. Stroke lengths of more than 8 mm are possible. This also ensures that needles with large diameters can be brought far enough out of the gate area of the nozzle. Temperatures in the drive area must be kept permanently below 80 °C because synthetic oils also lose their properties.

Intensive cooling in the clamping plates of the assembly systems is required. Assembly systems require good thermal isolation between the hot runner mani-fold and the drive unit. Operation is via the injection molding machine or external devices. For pure production environments, hydraulic actuators are of only limited use, since even the smallest oil leak would lead to contamination of clean room environments. A plate drive is frequently used, especially with close cavity spacing and multi-cavity systems. The valve gate nozzles are fixed into a plate package similar to an ejector unit, which is pneumatically or hydraulically moved by a drive unit. Spacing of the needles is determined only by the nozzle size and can thus be very small.

FIGURE 2.63  Pneumatic needle drive with needle guide and valve gate nozzle

3632.3 Hot and Cold Runner Technology

Forces and stroke lengths are dependent on the drive used and can be very flexibly designed. Temperatures in the installation environment of the drive units must be kept within the permissible range through design measures. Particularly important is a rigid structure of the plate drive. Tipping and tilting must be reliably prevented through precise plate guidance. Operation is carried out via the injection molding machine or external devices, depending on the requirements of the actuator unit. Especially with centrally mounted single needle shut-off systems, different types of levers, rods, or interlocking profiles are used for needle actuation. These actuators are usually controlled hydraulically or pneumatically through cylinders that are mounted to the outside of the mold. Control, strokes, and forces are determined by the type of actuation unit. The suitability for clean production environments also depends on the selection of the drive.

The new electric needle drives can be divided into servo motor drives and elec-tromagnetic designs. For servo motors, the rotary motion is turned into an axial stroke motion using spindles. For electromagnets, two solenoid coils move one solenoid armature. Figure 2.65 shows an electromagnetic drive unit connected to a shut-off needle using a hot runner nozzle with needle guide (close to the gate) in the nozzle tip.

FIGURE 2.64  Hydraulic needle drive, mounted on the hot runner manifold

364 2 Mold Design

FIGURE 2.65  Valve gate nozzle with electromagnetic drive

Electromagnetic drives can be installed in spaces from 50 × 50 × 60 mm; servo motors require a larger installation height. Stroke lengths up to 3 mm of electro-magnets require, as previously mentioned, the use of special nozzle tips with needle guides close to the gate. Temperatures in the drive area of these designs must be maintained by cooling (< 80 °C). The control of servo motors requires relatively expensive external control devices, which also offer flexibility. Electromagnets can be operated directly by the control unit of the injection molding machine, or simple transformer equipment can be added for multi-cavity applications. Both systems are ideally suited for clean room applications and for use with all-electric machines. The guide in the manifold block is the usual guide variant, into which the guide element is screwed or inserted into the manifold block.

The position of the needle guide is influenced by the radial thermal expansion of the manifold. The expansion must be taken into account when drilling the guide bore holes. An advantage is the easy insertion and easy replacement in case of wear and the possibility of using standard nozzles. To ensure precise control and secure seals, the guides have to be honed (Rz4). Guides in the nozzle head are either directly incorporated into the nozzle body or are carried out separately in combined sealing and guiding elements. In the design of the connection between the nozzle and manifold as a sliding seat, thermal expansion does not affect the needle position.

The thermal expansion of the manifold block is compensated by the radial clear-ance in the guide. If more precise needle guidance is required to prevent wear in the gate, additional guidance close to the gate should be provided. This design is advantageous to ensure that the needle enters pre-centered into the gate. The pre-centering must be done gently through the design of the transition radius and the angle of the needle and nozzle tip to avoid wear on these components. Premature wear of the gate is thus largely prevented. Through a special internal contour of the nozzle tip, the needle is pre-centered in the pre-stroke before it enters into the gate. Particularly accurate are forced guides for the needle using guide elements or special fits in the nozzle tip or profiled needles.

3652.3 Hot and Cold Runner Technology

One advantage is the accuracy and guidance of the needle, which is extremely close to the gate and therefore causes very little wear on the gate diameter. The melt split-ting occurs either centered or close to the gate. This can result in orientation lines in the product due to certain part geometries and materials. The guide variant is preferably used in molds with high production volumes and parts with small- and medium-sized shot weights.

Also important is the design of the front needle seat in the gate. In the conical needle seat, the outer taper on the front diameter of the needle seals at an inner taper in the gate. An advantage is the simple design of the geometries and the self-centering of the needle in the gate. The disadvantage is the lateral force applied to the gate (wear/outbreaks) as well as the indefinite end position between the drive cylinder and gate. In addition, the gate appearance is uneven due to variations in the pip height of the parts that results from the material displaced from the cone. In the cylindrical needle seat, the outside diameter of the needle seals in the cylindrical area of the gate, as seen in the lower detail of Figure 2.66.

Another advantage is the well-defined end of the needle in the axial position. The gate quality is equally good. Mechanical stress and thus wear of the gate is much lower. The gate is resistant to abrasive wear through the cylindrical area. The system is clearly force-balanced in the end position of the needle. In addition to conical and cylindrical needle seats and gates, there are a variety of special options. The lateral gate pulls the needle completely from the melt channel so that an unimpeded melt flow rises at low pressure loss. The disadvantages are the asymmetric heating and the complex (oval) sealing geometry in the gate. In the annular gap gates, the

d 8

"Y"

"Y"

Z40/...Z41/...Z412/...

Rz 4

Ra

Rb

90˚

(l5)

t1

t2

t3

d 2H

6

d 8

max. 0,3x45˚

A

t4

Z3345/...

1,5

R0,2

R1

40˚

"X"

FIGURE 2.66  Cylindrical needle end

in the gate to the cavity

366 2 Mold Design

needle enters through a hole in the usually rotationally symmetric product (e.g., gearwheels, CDs, DVDs) and releases an annular flow gap when opening. This gate type is good for filling parts with high radial accuracy.

The injection of several components through a nozzle is another special version of the gate design. Through a combination of needle and sleeve in one nozzle (a gate), core retraction solutions, i.e. different layers of material passing through one injec-tion point as well as sandwich procedures, can be realized. In this valve gate variant, melt flow in the nozzle and the construction of the drive units is a particular chal-lenge. The materials to be processed have to be in the same processing window. The simple needle adjustment and easy replacement of the needles in the case of wear are important for commissioning and maintenance of the valve gate systems. Adjustment of the needle in the axial direction should be carried out at operating temperature in order to incorporate the thermal expansion in the needle position. The adjustment is mostly done through a combination of thread and lock nut. Some servo motor-driven systems are able to automatically recreate the needle position in the process.

Thermal expansion differences between the clamping plate, hot runner manifold, and nozzle/needle guide can be compensated by constructive measures in the drive. Needles, which are adjusted in the front section of the part contour, must be secured to prevent twisting during operation. This is done by additional guide ele-ments inside or outside the drive. High needle temperatures lead to “sticking” of the needle and unsightly gate marks. Coatings or structuring of the front needle area can reduce the tendency to stick. The temperature of the plastic, the heat capacity of the needle, the contact area between the needle and the cooled gate, and the contact time in the heat conduction influence the needle temperature.

These parameters can be improved by the design of the needle (e.g., hollow), optimization of the contact area in the front cylindrical portion of the gate (“land length”), and the intensification and prolongation of the cooling cycle time. Electric valve gate systems are especially used for medical products, food packaging, and all applications in which oils and lubricants in the mold can result in problems. Actuation of the drive unit is either electromagnetic or servo motor-driven without any use of oil or lubricated air. Leakage and contamination of the produced parts by the needle drive or its supply facilities are excluded. The maintenance of the mold is now significantly simplified. Contamination of clean room environments due to oil leakage from needle drive systems is no longer a problem. The electromagnetic valve gate can be ideally combined with fully electric injection molding machines. Ease of control and quiet operation with low energy consumption are other benefits.

The installation of electromagnetic systems into the mold is very simple and clearly more cost-effective than installing a conventional needle drive. No supply bore holes have to be drilled into the clamping plate. The control of the electromagnets is very

3672.3 Hot and Cold Runner Technology

simple. By temporarily pending 24-V signals from the injection molding machine, the needle strokes can be triggered. This control technique can be implemented with almost any existing machine control system. Ejector pins can be used as valve needles, which are available as standard parts (cost-effective and ex stock). The use of needle diameters 2, 2.5, and 3 mm is possible. Also, multi-cavity valve gate hot runner systems can be easily built with electromagnetic needle drives (Figure 2.67).

2.3.4  Hot Runner Manifold Systems, Wired Systems, and Hot Halves

When selecting the appropriate hot runner manifold for a specific application, there are a number of factors that determine success or failure. Particularly important is the type of blockage and deflection. The melt flow should be transported from the machine nozzle to the hot runner with shear forces as low as possible and without dead corners and edges. Furthermore, special attention should be paid to the heating of the manifold. A homogeneous temperature profile, which ensures uniform heat input into the melt channels, avoids local overheating and damage to the material. The balancing of the manifold (naturally or rheologically) as well as the design of the melt channel diameter and the surface quality are key factors for successful production. The thermal insulation of the hot manifold in the cold mold and the avoidance of excessive heat losses through conduction, convection, and radiation are solved in different ways. Mechanical stability and long-term pressure resistance are more decisive factors. Below, different solutions from standard deflectors and wired systems to complete hot halves are presented. Standard deflectors, as seen in Figure 2.68, are used in applications where a single eccentric gating must be

FIGURE 2.67  32-cavity mold “dart tip” with electromagnetically driven valve gate system

368 2 Mold Design

achieved by simple means. The manifold blocks are available in standard sizes for gauge ranges of 40 to 180 mm and are usually in stock. Material-preserving deflec-tion of the melt without “dead edges” is achieved by using special steel deflectors, which are used in vertical fits in the manifold.

The rounded melt channels of standard deflectors guarantee gentle material guid-ance without dead corners or edges. This “flow-element technique” also enables a quick color change. Particularly maintenance-friendly is the heating of the manifold using flexible tubular heaters. These freely bendable rectangular heating elements enable, when repairs are needed, the replacement of the heating system on site and guarantee optimal heat transfer in the block. For more complex distributions, especially in multi-cavity systems, care must be taken that the design of the hot runner manifold is individually tailored to each application. Flexibility, production safety, melt guides that treat the material gently, and maintenance-friendliness are key. Technically advanced hot runner manifolds are made from special steel.

With optimum homogeneity of the material and pre-determined machining param-eters, the cleanest surfaces of the mass channel bore holes without grooves and chatter marks are achieved. Stationary material is avoided and a high molded part quality is reproducibly ensured. Abrasive polishing can further improve the surface quality of the melt channels, which may be beneficial in critical and frequent color changes.

Through variable diameter designs of the melt channel bore holes (depending on the specific application), the distribution systems are optimized with respect to the opposing parameters holding time, pressure drop and decompression behavior. Preferably, all systems are naturally balanced, and if necessary, rheological balanced manifolds based on Moldflow calculations are used. Flexible balancing of compli-cated cavity arrangements at several levels is reached through the use of vertically adapted melt deflectors, which can contain multiple manifold levels per deflector. This deflection principle is used in both conventional distribution systems as well as fully-wired systems (Figure 2.69).

FIGURE 2.68  Manifold block, single cavity for an eccentric gating of individual cavities

3692.3 Hot and Cold Runner Technology

FIGURE 2.69  Manifold system with a vertically shrunk deflector and a fully wired system, 8-cavities

The blockage through shrunken steel deflectors guarantees tightness of the system even at very high injection pressures. Modern systems are thermally optimized through air gap insulating and support using high-strength titanium elements with a low coefficient of thermal conductivity. Supports made from ceramic materials are also used, but are evaluated critically in respect to mechanical stability and fatigue strength against shear stresses, which may occur during heating and cooling of the system. Polished aluminum reflector plates reduce the heat loss through radiation. The entire frame of the hot runner system avoids chimney effects in the mold and thus different heat distribution. Completely wired hot runner systems complement the extensive range of installed standard systems, installation systems, and hot halves with one more interesting component. In this system design, the customer receives a complete system, fully wired and piped, but without the nozzle-sided mold plates. The systems are individually designed in close consultation with the client, constructed, and manufactured. The nozzles are either screwed into the manifold or preferably mounted floating on the hot runner. This avoids stresses in the nozzle caused by disabled thermal expansion.

The guarantee for dimensional accuracy and proper electrical wiring lies with the hot runner manufacturer. Furthermore, manufacturers o�en guarantee freedom from leaks between the nozzle and manifold. The main advantage of fully-wired systems is the avoidance of additional connecting work in mold making and easy mold maintenance. By using these systems, time can be saved in mold making while maintaining flexibility of the plate production. Another advantage is the pluggable wiring, which ensure electrical safety.

The highest possible reliability and absolutely trouble-free integration into the injection mold can be achieved with the hot half, which was described in Sec-tion 2.3.2. All the boundary conditions relevant to the hot-runner side are consid-ered by the hot runner manufacturer and accordingly implemented in the complete package.

370 2 Mold Design

FIGURE 2.70  Completely assembled hot halves with 32 cavity connection using multi-nozzles

Figure 2.70 shows two of these turnkey systems, equipped with the multi-nozzle described in Section 2.3.3.4.

2.3.5  Hot Runner Control Technology

Choosing the right control technology can be critical to part quality and productivity of the injection molding production. O�en, special hot runner control technologies have distinct advantages over the injection molding machine control. The market offers a wide range of controllers, from the affordable single-zone controller for single nozzles, to the multi-zone controller with fuzzy logic and touch-screen display on PC-basis. The diverse range of products makes it possible to select the optimal controller for each specific control task and each control section. This compromise between the various control options and the impact on all control parameters, and a simple, practical operation at a low price per control, is o�en not easy to find. Simple controllers are used for the control of standard heaters, heating cartridges, and individual nozzles. Setting the target value is user-friendly and is easily carried out using decade switches or digital input fields. The set point and actual value are displayed in a digital display. These controllers o�en consist of a pre-programmed start up for the gentle heating of the heating elements as well as a 50% reduction in temperature. Wrong polarity or breakage of the temperature sensor is diagnosed and displayed by the controller.

Microprocessor-controlled multi-zone controllers are characterized by the modular design of 1 to 128 control zones in the withdrawable slide-in technique, the simple

3712.3 Hot and Cold Runner Technology

adjustment of set points via safe decade switches or by digital inputs, and a very good price-performance ratio.

Functional operation characteristics, such as explicit fault diagnostics using an easily readable digital display, automatic change to manual mode in case of a breaking of a thermal sensor, standard alarm inputs and output for potential-free connection to the injection molding machine, programmable start up and alarm limits, and programmable temperature reduction, are all part of the standard performance package of modern control devices. Powers of about 3,600 W can be used for each control module. Via existing standard interfaces, the control signals can be received from the injection molding machine for situations such as automatic temperature reduction in the event of an interruption in production. In addition, features such as boost (short-term temperature increase for the start-up or color changes) and temperature reduction in a production downtime can be activated by simply press-ing a button. Even the most demanding control tasks of very fast control sections and hot runner control nozzles with a small mass are reliably mastered with PID controllers. Individual optimization is possible for almost any control situation. Thus, besides so� start, temperature reduction, alarm limits, alarm parameters and other values, a short-term temperature increase (boost function) and various memory settings can be carried out as well. Neutral interfaces for communication with a PC and data acquisition for PPS (production, planning & controlling system) systems are already available today on the market at lower prices. Multi-zone controllers have been established based on industrial PCs, especially for the reliable control of most demanding multi-cavity applications with extremely short cycle times.

These controls represent the reference class of today’s hot runner control systems, depending on the accuracy, functionality, ease of use, and adaptability. All control functions can be easily adapted to the Windows-like user interface. Fault analysis and diagnosis of the hot runner system are possible as well as master-slave con-figurations and loading of drawings or photos into the user-friendly touch-screen display for intuitive adjustment of the nozzles and hot runner block control areas.

Through group controls, it is also o�en possible to define up to four independently programmable groups. This feature is advantageous in two- or multi-component injection molding. For each component, the specific start-up and processing parameters can be stored individually. Thus, for example, the damage of the delicate so� component due to high temperatures or incorrect temperature settings is reliably avoided. A detailed documentation for quality assurance is provided through long-term storage of all process data. High control accuracy and control technology requires the use of high-quality connection and wiring components.

372 2 Mold Design

2.3.6  Cold Runner Systems

2.3.6.1  Function and AdvantagesOn the market, cold runner systems (CRS) are available for the processing of sili-cone rubber (LSR) and other elastomers. The processing of thermosets using CRS is still in development stage.The goal of using CRS is the production of waste-free, high quality, cross-linked molded parts.In contrast to hot runner systems, the material to be processed in the CRS is cooled or specifically tempered. Thus, in CRS, material temperatures of 20 to 80 °C are standard, and the mold is then heated up to 200 °C. By comparison, the (thermo-plastic) material in the hot runner is heated to about 180 to 380 °C and then cooled in the mold.Liquid silicone rubbers have a fluid or paste-like consistency and cure in the cavity by heating and thus cross-linking. It must therefore be ensured that the material to be processed is transported in the cold runner under defined conditions to prevent premature curing before reaching the mold cavities.CRS can be designed as open or closed systems. In open systems, curing of the silicone upto the nozzle tip (or more in the mold half) must be prevented. A defined hot-cold separation requires great experience and is in practice o�en not optimally achievable. A major advantage of open systems, however, is the possible small nozzle distance and the larger cavities with the same mold size. Since the entire nozzle control is omitted an open CRS is considerably smaller and cheaper. These advantages incur both high pressure losses and low shot weights compared to closed CRS systems.A distinction is made between machine- and mold-constrained cold runner systems. Standardized cold runner systems contribute significantly to the economic side. The standard elements are shut-off nozzles optionally equipped with chokes that provide enormous economic and production advantages.A closed CRS is controlled by valve gate nozzles. In Figure 2.71, such a system is shown as a mold concept. In the injection molding machine, the right mold half is connected with the hot plate a�er heating with the CRS via the quick clamping system. The parting plane is then between the two mold halves. Figure 2.72 shows the corresponding CRS. The nozzle control can be hydraulically, pneumatically, or electrically driven. Using the nozzle control, precise hot-cold separation can be achieved at low pressure losses and also at high shot weights. In addition, the indi-vidual nozzles are precisely controllable and can thus influence different material conditions and produce parts of consistent quality.A variety of nozzle types of different lengths are available, and depending on the application and size of the shot volume, different kinds of nozzles can be used.

3732.3 Hot and Cold Runner Technology

For the injection units of injection molding machines, different nozzles are also available depending on the application. Nozzles are mounted onto the plasticizing unit of the machine. Single nozzles with the direct material feeding from the mixing and dosing unit complete the program for the injection of large-volume parts such as insulators.

2.3.6.2  Processable Materials

Silicone rubbers are processed as hard rubber (HTV) that is peroxide cross-linked or liquid rubber (LSR) that is addition cross-linked. The processing temperatures of LSR are about 20 °C and of HTV up to 80 °C; the cross-linking is carried out at temperatures of 160 to 200 °C.

Ejector

Mold halfHot plate

Mold halfHot plate

Cold runnersystem

Rapid clampingsystem

FIGURE 2.71  Mold concept of a LSR mold (company picture DME)

FIGURE 2.72  Cold runner system, 16 cavities (company picture DME)

374 2 Mold Design

Silicone elastomers are heat, aging, and chemical resistant. They are characterized by having unmodified mechanical properties over a wide temperature range from −70 to 200 °C. Standard types are available in a hardness of 20 to 90 Shore A. Electrically conductive types, fluorosilicone types, oil-bleeding, fast cross-linking, and hard modified types are offered.

Silicones are used in the automotive industry and mechanical engineering, electron-ics and electrical engineering, as well as in the medical, sanitary and household sectors, as well as in the food industry. (Figures 2.73 to 2.76 show typical products made from silicone.)

Basically, all types of silicone are processed on conventional injection molding machines in a modified version. Corresponding injection units and cylinder modules to multi-component applications are offered by the manufacturers.

FIGURE 2.73  CRS with baby pacifiers in the mold half

(company picture DME)

FIGURE 2.74  Key caps of an automotive steering wheel, manufactured with CRS

(company picture DME)

3752.3 Hot and Cold Runner Technology

FIGURE 2.75  Valve inserts for coffee vending

machines made from silicone

(company picture DME)

2.3.6.3  Mold Technology

In new mold designs, high mechanical stability has to be ensured. Only this guaran-tees a tight mold at the nozzles. The nozzle head is heavily impacted through constant thermally induced stresses. Robust molds minimize the effect of the occurring force.

Sufficient mechanical stability of the injection mold is necessary to avoid unwanted flash formation within the mold parting surfaces and the exceeding of measurement tolerances. Due to the low viscosity of the silicones, elements with tolerances of more than 0.005 tend to flash formation. Therefore, silicone injection molds are tight molds requiring sufficient ventilation. Through specific measures, this problem can be counteracted too. The application of vacuum technology is a common practice.

Gate systemsSilicones are processed in the injection molding process. The use of cold runner systems as gate systems has proven to be advantageous. The cold runner system has the task to lead the rubber without significant temperature increase up to the manifold or to the part, while avoiding pressure drops and maintaining a uniform pressure distribution. The gate manifold has to be balanced. The connection to the part can be executed as a film gate, ring gate, or pinpoint gate. Tunnel gates are also possible. Due to the good flow properties of silicone, the runners can be dimensioned smaller than in thermoplastics processing.

2.3.6.4  Demolding

The selection of the most effective demolding principle is dependent on the order and is always a fundamental decision. Depending on this choice is whether the parts are damaged during demolding or remain usable. The principal ejector types are:

FIGURE 2.76:  Breathing mask

made from silicone

(company picture DME)

376 2 Mold Design

� round ejectors, � sleeve ejectors, and � flat ejectors.

These ejectors, or combinations, are advantageous because the injection molding machines are designed for them specifically, and the cycle times are kept short. Gate marks and deformations of thin-walled parts have to be taken into consideration with these solutions.In contrast, one could also use the following demolding types: � demolding, either manually or with withdrawal equipment; � demolding by handling and brush units; or � demolding by blowing out in connection with a middle plate.

These demolding types require a simple mold structure and are o�en used in silicone mold making. Their disadvantages are the longer cycle times.In addition to these demolding types, techniques are used like ejecting with air ejec-tors by which the parts are blown out of the mold. Mostly, these valves are designed as mushroom ejectors. This technique is effective but complex. Cylindrical ejectors are usually less suitable due to the tight tolerances required.Because of the high wall adhesion of the molded parts, ejection can be influenced by different mold surfaces. Polished surfaces tend toward strong adhesion due to the silicone; roughened surfaces tend to allow for easier demolding.

2.3.6.5  Mold Temperature Control

Important in practice is a sufficient insulation of the contour mold. To avoid heat loss and keep a stable mold temperature of about 200 °C, the molds are insulated against the cold channel and the machine by insulating plates. Good and practical insulation increases the economic benefits significantly. The cross-linking of the silicone material in the mold is largely determined by a uniform temperature profile in the mold and has a direct impact on the quality of the products.Uniform mold temperature control has a major impact on the quality of parts and a minimum cycle time. Here, various solutions are used. Standard cartridge heaters, band heaters, or hot plates are common. This guarantees a uniform heat distribution within the mold. It is possible to heat the molds with fluids such as oil. Sufficient power has to be ensured. Calculations are done with about 50 watts/kg of material to be heated.High demands on cold runner systems require more reliable temperature control of the cold runner system, especially the nozzles. Water is mainly used as a cooling medium. The advantage is a variable number of cooling circuits to ensure a homoge-neous temperature pattern of the cold runner block, including the mounted nozzles.

3772.4 Temperature Control of Injection Molds

 ■ 2.4  Temperature Control of Injection Molds

P. Thienel

Injection molds are among the most expensive and technically complex equip-ment, with changing demands and high standards of accuracy. They need to work efficiently and produce a good part quality. To achieve this, the injection mold has to be a uniformly working heat exchanger. Particularly with high quality injection molded parts, which have to be manufactured cost-effectively, it is important that the heat, which is supplied to the mold by the plastic, is dissipated at each cross section as quickly and evenly as possible. The type and design of the temperature control system have a decisive influence on the efficiency of the process (cycle time) and on the quality and properties of the parts to be produced. These quantities can be positively influenced with the aid of an effectively chosen temperature control system. The selection, design, and dimensioning of the temperature control system is not an easy task and many mold designers are generally unfamiliar with it and therefore o�en treat it secondary.

An impractical system can lead to surface defects, high residual stresses, warping or even cracking of the molded part, and may also lead to mold damages. In addition, insufficient cooling stresses the efficiency due to long cycle times [1–5].

The term “cooling” should be avoided because tempering of molds is not only about the removal of heat, but also about the specific setting of certain temperature values, which can affect the properties of the part reproducibly.

2.4.1  Tasks and Goals of the Mold Temperature Control

The temperature control of an injection mold has to fulfill two main tasks (Figure 2.77). In compliance with the required quality and part properties as a result of the amount and uniformity of temperature distribution in the mold, the injection mold continues to have the task to evenly dissipate the introduced heat in a very short time. Here, quality and efficiency o�en oppose each other.

In the temperature control, a compromise must be found (Figure 2.78) between the shortest possible cycle time and good part quality. Based on the desire for a short cycle time, the mold should be intensively cooled.

378 2 Mold Design

Technical QualityOf the molded part as a result of the uniformity of the temperaturedistribution in the mold andthe temperature level

Economic cycle timeThrough fast removal of heatout of the cavity which isfilled with melt

FIGURE 2.77  Main tasks of mold temperature control

Temperature control

Molded part quality Cycle time

FIGURE 2.78  Compromise in temperature control of molds [6]

This could affect the quality of the molded part. The focus is on the following objec-tives of mold temperature control [1]:1. The target (specified by the manufacturer of raw materials or from experience

found to be good) cavity temperature should be achieved as precisely as possible.2. The temperatures on the cavity surface should be locally as uniform as possible.

This causes small temperature differences between the cavity wall and tempera-ture control medium on the one hand and between the fluid inlet and outlet on the other hand.

To achieve the above mentioned goals and requirements, in addition to the tradi-tional conventional temperature control, new temperature control techniques and concepts are or will be used in the future:

� contour-dependent temperature control, � vacuum brazing, � diffusion welding, � use of materials with high thermal conductivity, � use of heat-conducting cartridges, � variothermal process control (media and inductive heating) � pulsed temperature control, � core cooling, � high-pressure temperature control, and � CO2 cooling.

3792.4 Temperature Control of Injection Molds

The injection molds are increasingly dimensioned by means of calculations for an optimum temperature control before practical implementation. Very o�en, the system boundary method and the finite element method for complex applications are used.

2.4.2  Influence of Processing Temperatures on the Cooling and Cycle Time

The diagram shown in Figure 2.79 shows the strong influence of the demolding and mold-cavity temperature on the cycle time (cooling time). This makes it clear that the cycle time can be greatly affected by the temperature control. The melt temperature, however, has only little influence. A 1 °C higher cavity temperature and a 1 °C lower demolding temperature increase the cooling time by about 2%; a 1 °C higher melt temperature increases the cooling time by only about 0.3%.

100

80

%

60

40

20

0

-20

Mass temp. 280260240220200°C

11090705030°C

806040200°C

ϑ M

W

E

n

Demolding temp.

Rel

ativ

e co

olin

g tim

e ch

ange

Dtk

,r

Cavity temp.

-40

ϑ

ϑ

ϑE ϑW

ϑM

FIGURE 2.79  Relative cooling time change as a function of mass, cavity, and demolding

temperature

Example: Polystyrene, wall thickness 2 mm [7] (n: current operating point)

2.4.3  Cavity Temperature

The cavity temperature is the temperature that is established during the cycle on the cavity surface. This temperature should not result arbitrarily, but should be deliberately influenced and adjusted to a desired value. It is not a constant value, but is subject to fluctuations in production. A characteristic “saw tooth profile”

380 2 Mold Design

(Figure 2.80) develops during a cycle. The temperature fluctuations are generated by the minimum cavity temperature in an open mold (no molded part) and the maximum wall temperature in the injection phase (contact temperature) in direct contact with the melt.

Since the cavity temperature fluctuates (Figure 2.80), a reference value and the average cavity temperature are given for cooling time calculations. These should be nearly constant. If this is not the case, no molded parts with consistent and reproducible quality can be produced!

TABLE 2.4  Cavity temperatures of thermoplastic plastic materials

Plastic Cavity temperature [°C]

Plastic Cavity temperature [°C]

PE – LD 20–60 PA – GF 60–120PE – HD 20–60 CAB 40–80

PP 20–70 CA 40–80PS 15–50 CP 40–80ABS 40–80 PPO 80–120SAN 40–80 ABS/PC 70–90PVC – hard 30–60 ABS/PA 80–100PVC – so� 30–50 PBTP 60–80PMMA 40–80 PETP 120–140POM 40–120 TPU 20–50PC 80–110 SB 30–70PC – GF 80–130 ASA 40–80PC – HT 100–150 PESU 140–190PA 6 60–100 PUR 20–80PA 66 60–100

Injection Demolding

Time t

Cav

ity t

emp

erat

ure

W

* 5

+ 2

0∞

C

W max

W min

W

WE

tMtZyM

tN

ϑ

ϑ

ϑ

Δ ϑ W

* 5

+ 2

0ºC

ϑ

FIGURE 2.80  Time course of the cavity temperature during a cycle [1]

3812.4 Temperature Control of Injection Molds

The cavity temperature or the average cavity temperature results from the interplay of introduced molded part heat, molding compound, temperature control system layout, temperature control medium, temperature control average temperature, and the cycle time, to name the most important factors. Table 2.4 shows the temperature ranges of the cavity temperature for the most important thermoplastics. For control and proper adjustment, the right adjustment and monitoring of the temperature at the temperature control unit is not enough. The sole decisive factor is the actu-ally measured temperature in the mold. This should be monitored and influenced/controlled with appropriate parameters if necessary.

Generally, a homogeneous wall temperature over the entire mold surface is desirable. Different wall temperatures are justified if certain areas of the part must be higher or lower tempered, for example in mass accumulations or warpage compensation. A completely homogeneous wall temperature, particularly in complex molded parts with spatial expansion, is practically not achievable, but this should always be the desired goal. Impacts from an increased cavity temperature are:

� longer cooling time/cycle time, about 2% per degree (Figure 2.79); � accurate imprinting of the surface (glossy, dull); � lower residual stresses; � uniform structure; � better holding pressure effect; � increased crystallinity; � lower post-shrinkage; � better heat resistance; � fewer orientations; � lower flow resistance; and � longer sealing time.

2.4.4  Influence of Temperature Control on the Molded Part Properties

Surface qualityThe imprint accuracy of the mold surface is strongly dependent on the cavity tem-perature; structured and polished surfaces can be better imprinted with increas-ing cavity temperature (adjustment of structure and gloss). Escaping of fiberglass to the cavity surface (“gray haze”) may be reduced or eliminated through higher cavity temperature.

Weld lines can be reduced by higher cavity temperatures or optically eliminated by special thermal equipment on the mold (Variothermal method [6]).

382 2 Mold Design

Mechanical propertiesThe elastic modulus increases with increasing cavity temperature, whereas the toughness decreases with increasing cavity temperature.

Shrinkage and warpageThe molding shrinkage increases with increasing cavity temperature. Lower cavity temperatures reduce the molding shrinkage, but also increase uncontrollable and undesirable post-shrinkage.

Nonuniform temperature control/heat dissipation can lead to considerable warpage due to shrinkage/residual stresses; this effect can also be used for the correction of the mold shape in a targeted temperature control.

Structural propertiesThe degree of crystallization increases with increasing cavity temperature: preven-tion or reduction of post-crystallization (shrinkage), causes changes in toughness properties. High cavity temperatures provide a more uniform structure in the surface layers.

Residual stressesTension and compressive residual stresses in the molded part decrease with higher cavity temperatures. Tension cracks, primarily on corners, can also be avoided or reduced with higher cavity temperatures.

Dimensional deviationsA nonuniform temperature control can lead to dimensional deviations during pro-duction. With a well-executed temperature control, dimensions can be adjusted, can be held constant, and can be reproduced.

FlowabilityThe flow path of the mass in the mold can be influenced by temperature control: higher temperatures increase the maximum flow path.

DemoldingThe necessary demolding stiffness can be adjusted with the temperature control of the mold. Demolding forces can be influenced by temperature control (shrinkage, stiffness, tendency to stick).

3832.4 Temperature Control of Injection Molds

2.4.5  Requirements for the Temperature Control System

To design a temperature control system for an injection mold, appropriate criteria must be known, fulfilled, or aimed for. They include:

� the desired cavity temperature should be adhered to, � the temperature at the mold cavity surface should be locally as uniform as pos-sible, and

� the cooling time (or cycle time) should be, in compliance with a predetermined quality, as short as possible, and

� a sufficient cooling capacity must be present.

To meet these requirements, the following aspects should be coordinated:

� introduced molded part heat; � heat loss by radiation, convection, and conduction; � cooling or cycle time; � diameter, location, and arrangement of cooling channels; � heating/cooling medium throughput; � pressure drop in the temperature control system; � cooling and pumping power (characteristics) of the temperature control unit; � temperature control (internal or external); and � location and place of a thermal sensor.

Without a computational design, these aspects are not coordinated with each other, and an optimum temperature remains more or less le� to chance.

2.4.6  Temperature Control Channels

The temperature control channel is the simplest and most commonly used tem-perature control element. It can be applied in many shapes, arrangements, and modifications. The cheapest way is to drill round channels. Physically, however, non-round geometries, due to the larger contact surfaces, are more effective. However, round temperature control channels impact the stiffness of the mold plate to a lesser extent.

In general, temperature control channels should be located relatively close to the mold surface, the distance between channels should be as small as possible, and the channel diameter should be as large as possible. However, this is only achievable with restrictions: because the stability of the mold plate must not be compromised,

384 2 Mold Design

the pressure drop as a function of the channel length should not exceed certain limits, and the uniformity of the cavity temperature with increasing distance of the channels to the mold surface improves. In addition, cooling channels cannot always be incorporated where desirable for geometric and technical reasons. These critical areas can be heated by other aids explained later.

Therefore, a compromise for the arrangement of the cooling channels must be found, which can be optimized by calculations.

Since each molded part must be tempered individually, only rough ideas on the arrangement of temperature control channels can be given at this point:

� common channel diameters are between 6 and 14 mm; � multiple cooling channels with smaller diameters are better than few channels with a large diameter;

� long channels with a small diameter lead to high pressure losses, which can lead to problems with temperature control unit (pump curve); and

� diameter and spacing of the channels to each other and to the mold surface have a strong influence on the temperature control errors and the Reynolds number (Re) of the medium (beneficial Re > 2300 → turbulent flow), so a thermal design and numerical verification of the channel layout is wise.

As the temperature control usually takes place via channels, there is no spread (ideally), but a selective temperature control, which leads to local wave-shaped temperature differences of the cavity temperature. The temperature control error j (deviation from the desired temperature) results mainly from the geometrical arrangement of the cooling channels (Figure 2.81) and is given as a percentage value. The temperature control error should not exceed the following [1]:

Semi-crystalline thermoplastics: j = 2.5 to 5%

Amorphous thermoplastics: j = 5 to 10%

60 °C

60 °C

61,54 ºC

71,6 ºC

FIGURE 2.81  Influence of temperature control channels

arrangement on the temperature control error [6]

3852.4 Temperature Control of Injection Molds

It can be calculated using the following relationship:

2.8 ln0.22.4 Bi

BABj

A

� �� � �� �� � � � � �� �

With A, distance to the molded part surface, ≈ 0.8 to 1.5 Band B, distance from cooling channel to cooling channel, ≈ 2.5 to 3.5 DK

TM K

WBi

D�

�

�

Here, the following means:

Bi Biot-number

DK Channel diameter

�TM Heat transfer coefficient of the medium

�W Thermal conductivity of the mold material

The uniformity of the cavity temperature thus improves with larger distance A of the cooling channels to the mold cavity surface and with a smaller distance B from cooling channel to cooling channel (Figure 2.81).

2.4.7  Flow Principle

The temperature control channels in the mold can be flown through in three basic ways discussed below.

2.4.7.1  Series Temperature Control

In this method, a channel leads through the mold areas. This channel has an input and an outlet (Figure 2.82).

FIGURE 2.82  Series temperature control [4]

386 2 Mold Design

AdvantageBlockage of the channel through deposits, corrosion, or foreign objects is detected immediately.

DisadvantageThroughout the channel, the temperature of the medium increases: mold regions have different temperatures.

2.4.7.2  Parallel Temperature Control

In this method, a supplying channel is split into multiple parallel channels or core cooling (Figure 2.83).

AdvantageUniform mold temperature control.

DisadvantagePartial blockages are not noticed.

A third principle can be realized with two possibilities – the counter-flow principle. In this case, the channels are arranged so that cold and hot channel sections run past each other, and temperature differences can almost be canceled out. In many cases, this principle is used for the flat series temperature control (e.g., as a spiral) wherein the spiral is guided from the outside to inside and again from the inside to the outside. The average value between “warm” and “cold” spiral section is always the same.

FIGURE 2.83  Parallel temperature control [4]

3872.4 Temperature Control of Injection Molds

2.4.8  Practical Designs of Conventional Temperature Control Options

2.4.8.1  Flat Temperature Control

Figures 2.84 and 2.85 show two examples for flat temperature control. O-rings are provided as sealing elements in two-layered mold plates with milled-in temperature control channels.

FIGURE 2.84  Flat series temperature control using transverse bore holes and sealing plugs [4]

O-RING

IN

OUT

FIGURE 2.85  Flat series temperature control with milled-in channel as a mold plate insert –

sealing through an O-ring [4]

388 2 Mold Design

2.4.8.2  Temperature Control of Molded Part Corners

The temperature of molded part corners should particularly be taken into account, as the well-known corner warpage can be avoided. In the arrangement of the tem-perature control channels, heat dissipation is inevitably better in the counter sunk area than in the core area. This can be explained by the smaller contact area in the core. This leads to a displacement of the plastic polymer core in the direction of the core (Figure 2.86), wherein the contraction of the shi�ed residual melt causes a distortion of the corner geometry.

For the explanation of the different heat transport in a molded part corner, this area is divided into uniform molding part squares (dashed lines) in Figure 2.86. In the counter sunk area, two cooling channels (d) are arranged for one square (a); in the core area, only one cooling channel (c) is arranged for three squares (b).

This effect can be prevented with a lower core temperature or an optimization of temperature control channel layouts (Figure 2.87).

d

ab b

b

c

Cooling bore holes

Remaining melt

FIGURE 2.86  Temperature control and solidifying of one molding part corner [2]

(a) (b) molding part corner squares (c) (d) cooling channels

a) b) c)

FIGURE 2.87  Possibilities of temperature control of the corner [8]

(a) Separately tempered channel in the molding part corner

(b) Channel which is moved into a corner

(c) Usage of thermally conductive materials in the corner area

(e.g., CuBe, CuCoBe, Al-Bronze)

3892.4 Temperature Control of Injection Molds

2.4.8.3  Temperature Control of the Core

Temperature control of the core is usually carried out using a temperature control finger, which guides the temperature control medium into the core and then removes it again. Larger, rectangular cores can be tempered by several flatly arranged tem-perature control fingers. Some basic examples of the variety of core temperature control possibilities are listed below.

2.4.8.3.1  Temperature Control TubesThe temperature control tube (Figure 2.88) is introduced into a core bore hole and is supplied by a feeding unit. The temperature control medium (e.g., water or air) flows through the tube, exits at the front side, and flows between the core bore hole and the temperature control tube back into the outlet.

FIGURE 2.88  Temperature control tube (source: STRACK Norma GmbH)

2.4.8.3.2  Separating Plate (Deflection Bar)The separation plate (Figure 2.89) is the simplest form of the cooling cores. One plate divides the core bore holes into two chambers, which are connected in the base of the core bore hole. Via the inlet, the temperature control medium can flow into the core bore hole and can then subsequently flow back into the outlet over the second chamber.

FIGURE 2.89  Temperature control with separating plates (source: STRACK Norma GmbH)

390 2 Mold Design

2.4.8.3.3  Spiral CoresA spiral core (see also Figure 2.90) is accurately inserted into the core bore hole, which leads the fluid optimally along the bore hole wall and ensures a good heat transfer. Spiral cores (Figures 2.90 and 2.91) can be single- or double-threaded. In single-threaded spiral cores, the medium is brought into the core through a core bore hole and then flows back through the spiral. The double-threaded spiral core carries the temperature control medium into the core through one spiral segment and back to the outlet through a second spiral segment.

FIGURE 2.90  Single-threaded spiral core (source: HASCO Normalien GmbH)

FIGURE 2.91  Double-threaded spiral core (source: HASCO Normalien GmbH)

2.4.8.3.4  Heat PipeThe heat pipe (the operating principle is shown in Figure 2.92) is a self-contained system that uses the evaporation of a volatile liquid to transport heat. In this case, heat can be transported only by the evaporation/condensation process over the length of the pipe. Heat is only dissipated if the “cold” end of the pipe is cooled by a temperature control channel. The heat pipe is ideal for all the other places where temperature control channels cannot be directly accessed. Using the compact heat pipes, heat can be removed locally and can then be transferred to a geometrically favorable position. The heat pipe is therefore also suitable for tempering of slender cores and other inaccessible areas.

The mounting position of the heat pipe determines the performance. Best per-formance is in the vertical working position with overhead temperature control (cooling) (Figure 2.93).

3912.4 Temperature Control of Injection Molds

Kapillarschich

H¸ lle

Dampf Fl¸ ss igkeit

Kondens ationszoneVerdampferzone

Capillary layer

Sleeve

Steam Liquid

Steaming zone Condensation zone

FIGURE 2.92  Functional principle of a heat pipe [4]

FIGURE 2.93  Optimum mounting position of a heat conductive cartridge with a cooling area

through conventional cooling channels (Source: HASCO Normalien GmbH)

FIGURE 2.94  Heat conductive cartridge in horizontal mounting position

(Source: HASCO Normalien GmbH)

In any case, attention should be paid to the specifications of each manufacturer. The heat dissipation is limited and usually cannot be compared to a liquid temperature control, but it can be many times higher than that of a solid copper rod. It has been proven by studies that the horizontal mounting position (Figure 2.94) cannot physically work. Some manufacturers and users say the opposite. They say it is only possible with appropriately modified systems that allow a natural circulation of evaporating gases despite the horizontal mounting position.

392 2 Mold Design

2.4.8.4  More Conventional Temperature Control Options

2.4.8.4.1  Circumferential Application Temperature ControlCavities can be tempered by encircling channels, which are incorporated into the insert (Figure 2.95). The inserts should be sealed with O-rings. In this construc-tion, the advantage is that the channels are easy to clean, if necessary. In addition, a relatively conformal temperature control is possible.

Inlet of the coolant

Cavity

Outlet of the coolant

Cavity

Cooling channels

O-ringsIncorporated channels(round or rectangular) Minimum 1º draft angle

is recommended

FIGURE 2.95  Circumferential application temperature control (round) [6]

2.4.8.4.2  Inserts Made from Different MaterialsThe different thermal conductivity of metals can be used for temperature control of injection molds. In this way, heat can be removed from critical or difficult to access areas. Mostly, copper elements (beryllium copper) are used and are integrated as a segment into the mold insert.

FIGURE 2.96  Temperature control of critical areas using highly heat conductive inserts

(e.g., CuBe) [3]

3932.4 Temperature Control of Injection Molds

The applications include:

� temperature control of molded part corners (Figure 2.96), � heat dissipation from slim cores and ribs, and � complete inserts made from copper alloys.

However, it should be noted that the heat is only derived not dissipated; for example, the copper elements have to be able to supply heat to a temperature control channel in other areas. The higher thermal conductivity of copper alloys in comparison to steel is limited and not comparable to liquid temperature control.

2.4.9  New Temperature Control Technologies

2.4.9.1  Contour-Depending Temperature Control

Contour-dependent temperature control is very close to the ideal temperature control and develops high potential for optimization in terms of molded part quality and cycle time. Thus, the part quality and the efficiency of the part can be significantly increased.

FIGURE 2.97  Layer structure of a core a�er the CONTURA®

system

(source: Innova Engineering GmbH)

394 2 Mold Design

The temperature differences at the conventional temperature-controlled molded parts are usually over 10 °C. Significant reductions in cycle time (about 2% per 1 °C; it results in 20% and more shorter cycle times) enables the contour-dependent tempera-ture control because the hottest regions of the molded parts are cycle-time dependent.

2.4.9.1.1  Vacuum Brazing TechnologyFor contour-dependent temperature control, the disassembly of the mold insert into single-layer elements is required (Figure 2.97) and into which the temperature control channel is milled in. Later, these layer elements are connected together in a vacuum brazing technology and then result in a uniform temperature control system. Figure 2.98 shows the comparison between a conventional and a contour-dependent temperature control.

FIGURE 2.98  Comparison between conventional (top) and contour-dependent (bottom)

executed temperature controls (source: Innova Engineering GmbH)

3952.4 Temperature Control of Injection Molds

2.4.9.1.2  Selective Laser Sintering (SLS)Another method of adjusting cooling channels to a complex mold is selective laser sintering, which makes it possible to build almost all metallic materials in layers. Single-component metal powder is layer by layer completely melted. A component density of nearly 100% is thereby achieved. High surface quality and hardness is achieved through a special surface treatment process that directly follows the construction process.

Extensive cooling systems can also be realized with the laser sintering process. These systems are manufactured as a network structure under the surface to be cooled (Figure 2.99). Through this, the cooling medium (e.g., water or CO2) can be transported near the surface (2 mm) to achieve a uniform temperature distribution [9].

FIGURE 2.99  Surface cooling through a net structure produced using the laser sintering

process

2.4.9.2  CO2 Temperature Control

The CO2 temperature control allows high temperature-control performance as well as temperature control of slim cores in inaccessible areas and in material accumula-tions. However, this type of temperature control is seen as a complementary facility for problem areas and not as the sole temperature control method. Otherwise this type of temperature control would lead to significant costs.

Functional principleThe mold inserts contain expansion chambers that can be loaded using capillary tubes (� = 0.8 to 1.5 mm) with liquid CO2 (Figure 2.100). There, the liquid expands into the gaseous state. Energy is absorbed through the expansion (evaporation), and thus, temperatures down to –78.9 °C are generated in the expansion chamber that allows a short, strong cooling of the mold areas.

396 2 Mold Design

CO2 CO2

FIGURE 2.100  CO2 temperature control in an injection mold (schematic diagram) [6]

The advantages include:

� more uniform temperatures at the molded part (quality), � tightest areas in the mold can be tempered, � high flexibility in the supply of gas through capillary tubes, � separate temperature control of thick-walled areas is possible, � shorter cycle time through effective temperature control of cycle time-determining areas, and

� CO2 temperature control is also possible in conventional molds made from common steels.

The disadvantages are:

� higher costs due to CO2 consumption, process control, and mold making, and � ice formation on the mold is possible.

2.4.9.2.1  CO2 Temperature Control with Sintered MaterialThe CO2 temperature control can be used in conventional steel molds as well as with inserts made from porous sintered material, which mainly consists of steel (TOOLVAC®). When using sintered steel, the liquid CO2 flows through the capillary tubes into the expansion chamber. In the gaseous state, the CO2 penetrates into the porous material and flows, due to the gas pressure, and is uniformly distributed over the core up to the cavity surface. There, heat can be directly withdrawn from the molded part.

The advantages are:

� optimum temperature control and � high temperature-control performance.

3972.4 Temperature Control of Injection Molds

The disadvantages are:

� poor bending strength (follow the design guidelines), � pores can easily clog (extensive cleaning), and � sintered material is poorly processible and weldable.

2.4.9.2.2  CO2 Temperature Control with Conventional SteelIn the temperature control of mold plates made from conventional steel, CO2 is also passed into an expansion chamber and evaporates. However, the CO2 cannot flow through the steel, i.e. only the surface of the expansion chamber is available for heat transfer. The temperature-control performance, as opposed to applications with a sintered material, is correspondingly lower. Nevertheless, there are two key advantages:

� conventional mold steel can absorb higher bending stresses and � a CO2 temperature control can be subsequently introduced into an existing mold.

2.4.9.3  Dynamic Temperature Control

A dynamic temperature control is a variothermal process control. Here, two inde-pendent temperature cycles with different temperatures (Figure 2.101) are used in the mold.

Partial areas or the entire cavity are brought to a melt temperature level via a first temperature control circuit just before the injection of the molding material. The surrounding regions, such as the mold plates, are always at constant temperature or below the demolding temperature using a second temperature control circuit. Shortly a�er the injection phase, the first circuit is connected with the second circuit, which results in cooling of the hot areas to demolding temperature.

Entformungstemperatur

Sc hmelzetemperaturniveau

Massetemperatur

150 C

250 C

Melt temperature level250 ºC

Melt temperature

Demolding temperature

150 ºCFIGURE 2.101  Variothermal

mold temperature

control [6]

398 2 Mold Design

Mold wall

Werkzeugwand

erstarrte R andschicht

Melt with frontal flow

Solidified surface layer

Melt with frontal flow

Mold wall

FIGURE 2.102  Solidifying surface layer in

conventional temperature

control (top)

in comparison to the

variothermal temperature

control (bottom)

This method substantially improves the properties of the injection molded part. The solidification of the surface layer through freezing effects on the colder mold wall during the filling phase is avoided (Figure 2.102).

Furthermore, the variothermal method is suitable to prevent visible weld lines that arise, for example, when reuniting the melt behind the breakthroughs. A good example is the PROMOLD method [5]. Here, the surface temperature just before the injection is brought to the so�ening temperature of the molding compound by an additional temperature control channel placed directly below the weld line. The visible weld line in Figure 2.103 (le�) developed using a conventional mold. On the right, the weld notch a�er variothermal treatment is no longer visible.

High cavity temperatures favor the reduction of residual stresses, and the molecular surface orientations can be reduced. They also support the molding accuracy of the finest structures (micro-injection molding).

FIGURE 2.103  SEM-images of a weld line [5]

3992.4 Temperature Control of Injection Molds

There are different concepts for the variothermal temperature application:

� using liquid media (water/oil), � using an electrical resistance method, � using a cavity surface temperature control, or � using an inductive mold temperature control [10].

Inductive mold temperature controlThe inductive mold temperature control is based on the principle of the transfor-mational energy transmission through a high-frequency stream flowing through a coil. This creates an alternating electromagnetic field whereby eddy currents develop in the surface of the material to be heated. The currents generate Joule heating because of the specific resistance.

The inductive mold temperature control can be externally or internally applied. The external method is particularly suitable for mass production of micro components. The internal method enables temperature control in large-scale, three-dimensional geometries.

Figure 2.104 shows the typical temperature profile. The inductor heats the area to be heated with the open mold to the level of Tmax, which is for example, above the melt temperature. In the period between the departure motion of the inductor and the closing of the mold, heat losses occur through radiation and conduction. Therefore, Tmax should be chosen high enough so that, at the time of injecting, the required surface temperature prevails. When the melt touches the mold wall, the temperature increases at this point abruptly 10 to 12 °C and then cools down slowly.

ca. 1

0-1

2∞

C

t heating

T start

T in

T max

Heatingphase

E inspritzzeitpunkt

Tem

per

atur

e

Zeit

offenpositioniert

Werkzeugnduktor

geschlossenentfernt

Ab

out

10-1

2 ºC

Injection time

MoldInductor

open closedpositioned removed

Time

FIGURE 2.104  Typical temperature profile on the cavity surface of an external inductor [10, 11]

400 2 Mold Design

2.4.10  Thermal Mold Design

In practice, the calculated thermal mold design is done in two ways:

� FEM calculation and � The system boundary method

In the FEM calculation, so�ware is used that operates on the finite element method. A data model of the mold or a mold part is read into the so�ware. The data model also includes the part contour. Under certain conditions (cavity temperature, melt temperature, etc.), a temperature control layout is designed around the molded part with the help of special so�ware. The computer can then simulate three-dimensional temperature distribution in the mold. If undesirable temperature values occur in certain areas, the previously defined temperature channel layout can be changed. Through the simulation, the temperature control system can be virtually changed until a possibly optimum condition is achieved. The advantage of this technique is that the simulation allows a three-dimensional view of the heat flows and tem-peratures. In addition, this technique is very accurate (but only if the boundary conditions have been practically defined!). The disadvantages are the costs and the need for specialized staff and appropriate so�ware licenses. Therefore, such designs are o�en purchased as a service.

The system boundary method was formulated early on [1] and provides a “manual” calculation of a temperature control system for injection molds. The method includes the basics for the specification of the temperature control system from a practical standpoint. Simplified assumptions are made because a three-dimensional simu-lation of complex molded part and channel geometries is not possible. Neverthe-less, the method is suitable for the thermal design of injection molds and is based on accurate physical relationships. Simple tools/mold geometries can be exactly calculated with the system boundary method. For more complex molds, simpli-fied assumptions are made, or more system boundaries are calculated and then combined. The advantage of the system boundary method is its simple and quick application with sufficient accuracy (which can be achieved through a continuous development of a computer program), the extensive material data collection, and a simplified and clear presentation of the results [12].

Figure 2.105 illustrates a balance of the available heat flow on a complete injection mold. In hot runner gate systems, the balance must also include the heat flow of the supplied electrical heat input.

4012.4 Temperature Control of Injection Molds

W‰rmebilanz

Strahlung

ab- oderzuzuf¸hr endeW‰rme/Zeit(Temperiermedium )

Formteil-w‰rme

Leitung

+ + =

QTM

QTM QFT

QFT

QU 0

QK

QL QL

QStr

Heat balance

Conduction

Molded part heat

Radiant heat

Dissipated orintroduced heat/time(temperature controlmedium)

FIGURE 2.105  Heat flow balance on an injection mold, based on Wübken [1]

�PlQ Heat flow, which brings the hot melt into the mold�TmQ Total heat flow, which is introduced and dissipated through the temperature

control medium�LQ Heat flow to the environment through conduction�StrQ Heat flow to the environment through radiation�KQ Heat flow to the environment through convection�UQ Total heat flow that dissipates into the environment

� � �� � � �U L Str KQ Q Q Q

The main design steps of the system boundary method are [5, 6, and 12]:1. Determining the balance of the system boundary and the observed heat flow.2. Calculating the theoretical cooling time.3. Determining the dissipated heat flow due to the heat flow balance, based on the

introduced heat through the molded part.4. Determining temperature control medium throughput due to the heat to be dis-

sipated.5. Determining diameter, position, and arrangement of cooling channels.6. Calculating the temperature control error (temperature differences on the cavity

surface) and possible correction of the channel layout.7. Calculating the pressure loss in the temperature control system.8. Selecting a suitable temperature control device using a characteristic curve of

the device.

402 2 Mold Design

2.4.11  Position of the Temperature Sensor for External Temperature Control

A constant cavity temperature can be best achieved by an external temperature control. For the temperature controllers used in practice, a temperature sensor (thermocouple) is necessary, which is placed in the right location of the mold. Strong temperature fluctuations close to the cavity occur through the injected plastic, especially at the cavity surface (Figure 2.80). These occur for physical reasons (mold material, shape, molding compound, temperature) and cannot be influenced by the cooling system.

Directly prior to injection, the cavity temperature is minW� . When the hot plastic

melt touches the colder mold surface, a contact temperature maxW� immediately sets

at the border point, which continuously decreases as a result of cooling during the cycle. The contact temperature

maxW� is dependent on the heat penetration ability b of the mold and the molding compound. The following applies for the contact of two bodies (mold/molding compound) at different temperatures:

� �� � �

� � �� �

�min

max

W W M MW p

W M

b bb c

b b

bW Heat penetration ability of the mold material

bM Heat penetration ability of the molding compound

�M Melt temperature

minW� Cavity temperature directly before injecting

A temperature fluctuation ��W results over the cycle time that has the amount

ob max minW W W( )� � �� � � at the cavity wall. This becomes smaller with increasing distance from the mold wall until no temperature difference is noticeable anymore (Figure 2.106).

If the temperature amplitude in a dimensionless presentation, of any distance x from the cavity surface, via the quotient position of the probe lx is applied to the temperature oscillation �, a universal context results, out of which the position of the temperature sensor lx can be determined at a desired temperature amplitude (Figure 2.107).

Practical tests have shown that the temperature amplitude of the measurement position of the temperature sensor for an easy external temperature control should not exceed

xW� = 3 to 4 °C.

4032.4 Temperature Control of Injection Molds

Formnestoberfl‰che K¸ hlkanal

Zeit t

W

W, rb

W, X

lXX

Cavity surface Cooling channel

Time t

ϑ

Δϑ W,rb

Δϑ W,X

FIGURE 2.106  Temperature amplitude at the cavity wall and distance lx as a function of time [5]

= Eindringtiefe einer Temperatur-

schwingung = f (a, tzykl)

a = Temperaturleitfähigkeit

= Penetration depth of a temperature

oscillation = f (a, tcycl)

a = Temperature conductivity

Δϑw,x

Δϑw,ob

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Λ

Λ

ΛIx

FIGURE 2.107  Dimensionless presentation of the distance of the thermocouple from the

cavity surface [5]

404 2 Mold Design

Positioning the thermocoupleBesides the location of the temperature sensor, the positioning of the thermocouple at the right place in the mold is important for the quality of control or regulation. The following criteria should be ensured for the arrangement in the injection mold:

1. Distance from the cavity surface (computational determination).

2. Appropriate location of the sensor is dependent on the geometry and the design of the mold and the arrangement of the cooling channels.

3. The sensor should be in a position in the mold where the temperature plays a critical role of the molded part quality and functionality (e.g., tight tolerance dimensions, warpage areas, or areas with high demands on the mechanical properties).

4. The sensor should not be located behind a cooling channel; otherwise all mold areas are suitable.

References

[1] Wübken, G., Thermisches Verhalten und thermische Auslegung von Spritzgießwerkzeugen (1976) Technisch-wissenscha�licher Bericht, IKV Aachen

[2] Schürmann, E., Abschätzmethoden für die Auslegung von Spritzgießwerkzeugen (1979) PhD Thesis, RWTH Aachen

[3] Zöllner, O., Optimierte Werkzeugtemperierung. Information publication from Fa. Bayer (ATI 1104d), Leverkusen (1997)

[4] Menges, G., Michaeli, W., Mohren, P., Anleitung zum Bau von Spritzgießwerkzeugen. 5. Auflage (1999) Hanser Verlag, Munich

[5] Thienel, P., Lecture notes Kunststoechnik and Werkzeuge für Kunststoffe. Fachhochschule Südwestfalen, Plastic processing laboratory, Iserlohn (2007)

[6] Berghoff, M., Berlin, R., Görlitz, R., Hoster, B., Kürten, A., Kürten, Chr., Schmidt, J., Thienel, P., Training, seminar, and project notes of the ISK GmbH. Iserlohn (2003–2007)

[7] Thienel, P., Der Formfüllvorgang beim Spritzgießen von Thermoplasten. PhD Thesis, RWTH Aachen (1977)

[8] Lappe, U., Gestaltung von Formteilen aus technischen Kunststoffen. VDI Bildungswerk, Düsseldorf (1995)

[9] Rapid Tooling: Firmengemeinscha�sprojekt am Lüdenscheider Kunststoff-Institut (KIMW) (2006, 2007)

[10] Induktive Werkzeugtemperierung: Company joint project of the Lüdenscheider Kunststoff-Institut (KIMW) (2006, 2007)

[11] IKFF-Stuttgart. Project information, (2006)[12] Innovative Temperiertechniken. Company joint project of the Iserlohner Kunststoff-

technologie GmbH (ISK) (2002–2007)

4052.5 Innovative Mold Technologies

 ■ 2.5  Innovative Mold Technologies

T. Eulenstein

Those who want to compete in international competition must keep up. This also specifically applies for the tool and mold making industry in Germany. Innovations are in demand if the local companies want to compete in world markets.In the past, manufacturers were looking for so-called unique characteristics in the field of technical features, so in recent years, the design and thus the surface of plastic parts became more and more the focus. This has drastically shi�ed the feasible technological demands for new design possibilities of plastic molded parts. For this reason, new manufacturing processes have to be developed and existing ones must be improved.

2.5.1  Coating Technology – Design Surfaces through Combined Surface and Coating Technologies

Gloss levels, targeted structure impressions, and design effects on molded parts are increasingly gaining importance in the production process. But how can these refinements be optimized and integrated into production without inefficiency? In earlier years, innovative techniques and functionality mainly influenced the decision to purchase a product. With an increasing adjustment of the technical design features, the design becomes more and more the focus of interest. For many companies, quality requirements in accordance with uniform gloss levels within an assembly, dull and scratch resistant surfaces, and the harmony between differ-ent surfaces such as in a vehicle interior have long been taken for granted. The practice shows that increased quality requirements on the surface are not always immediately achieved because the influential factors on the surface quality are very different and complex. Potential sources of error are not always identified in advance. In practice, the quality differences are only seen during the examination phase of an injection mold. The consequences are time delays through optimizing measures and associated costs (e.g., personnel and mold changes).The causes can be complex and can for example be due to unclear objectives, lack of communication between the development partners, the use of non-suitable testing and evaluation criteria, or through the influence of design and process-engineering techniques. The surface requires an interdisciplinary collaboration and targeted project management approach to guide, use, and supply the various core competen-cies with the required information.

406 2 Mold Design

The mold surface has the greatest influence on the molded part surface and thus on the gloss (dullness) because the plastic reflects a mirror image of the surface contour of the mold.

One reason for the generation of differences in gloss on textured molded part surfaces is the uneven or inadequate replication of the mold contour through the inflowing molten plastic. The higher degree of accurate mold surface replication determined the higher achievable accuracy of the finished molded part. The theoreti-cal maximum dullness of a molded part is therefore reached when the plastic surface has the same topography/roughness of the structure that is brought into the mold.

Gloss differences are caused by the differing replication behaviors of the plastic material on the mold wall. This can be caused by different cooling conditions or by shrinkage differences. Figure 2.108 shows two typical examples of surface errors due to gloss differences.

FIGURE 2.108  Gloss differences on molded plastic parts

By using special coating properties, possibly in combination with topography adjustments to the mold surfaces, gloss levels on molded plastic parts can be influenced. Especially for eroded and grained surfaces, many new possibilities are created through this procedure. Thus, the following improvements and advantages are achieved:

� elimination of surface defects, such as flow lines or gloss differences in areas of wall thickness variation;

� adaptation of gloss levels on visible parts, which consist of various materials in one assembly unit;

� production of matt surfaces; � reduction of cooling times; � generation of paint-like surfaces; and � adjustment of thermoset surfaces.

4072.5 Innovative Mold Technologies

The special properties of surface and coating technologies are used to make molded parts appear duller or glossier to avoid, for example, subsequent painting of the molded parts or to adapt different plastics used in an assembly to the appearance.

The adaptation of the gloss level is o�en required for components that are manufac-tured from different materials using design requirements and which later need to have a uniform surface appearance in a system assembly. O�en painting of the parts is unavoidable. With the use of combined surface treatment processes appropriate levels of gloss can be adjusted in many cases. Thus, painting of the components is no longer necessary. At the same time, the once achieved gloss levels are permanently preserved through coating because the mold surfaces are protected from wear and damage. Another possibility is the elimination of surface defects such as flow lines or gloss differences in the area of wall thickness differences.

Generation of dull or glossy surfacesFigure 2.109 shows reasons that can cause a change in the gloss level.

0. 002 0. 01 0.1 1 40

1

2

(f / Hz ) -1/2

WC/ C

TiAlN

TiCNCr N

0. 002 0. 01 0.1 1 40

1

2

(f / Hz ) -1/2

WC/ C

TiAlN

TiCNCr N

1. 2. 3.

Quelle: Xintech AG

Eroded surface White zone

Hairline crack Inclusions

Cleaned with 1. stepPorosity

Solidifying

Source: Xintech AG Source: Plastics Institute in Lüdenscheid

FIGURE 2.109  Possible causes for changes in the gloss level

1. Pre- and post-treatment of the cavity surfaces

2. Micro roughness of the coating

3. Thermal properties of the coatings

For the development of design surface, a distinction between the pre-and post-treatment of mold cavity surface and pure pre-treatment for coating (e.g., eroded mold surfaces) has to be done. For the design development, processes such as micro-radiation, mat-down treatment, or micro mat can be used. Depending on the technology used, as shown in Figure 2.110, paint-like or very dull surfaces can be established.

Micro-roughnesses of the coatingsFor some coating processes, micro-roughnesses can be formed on the surface during the layer deposition. The reasons can be based on the layer morphology or be caused by the process.

408 2 Mold Design

FIGURE 2.110  Examples for the pre- and post-treatment of the cavity surfaces [1]

Thermal properties of coatingsSeveral parameters have to be considered for accurate reproduction of the mold surface: � the cavity temperature, � the contact temperature, and � the heat insensitivity.

The cavity temperature influences the impression directly. The closer the cavity tem-perature is to the so�ening temperature of amorphous materials or the crystalline melting temperature of semicrystalline thermoplastics, the more accurate can the molding compound replicate the mold surface. On the other hand, the economics of the cycle time must be considered, since a high cavity temperature means very long cycle times (extending to approximately 2% per degree).In injection molding, a timeless contact temperature is established between the cavity and the molten plastic. The cavity temperature varies only slightly (��W < 15 K); however, the surface layer of the molten plastic is spontaneously quenched to much more than 100 K. The molding compound freezes suddenly on the cavity wall. The decisive variable of the contact temperature is the heat sensitivity b. It is calculated by the thermal conductivity �, the density � and the specific heat capacity c.An increase of the contact temperature delays freezing of the plastic surface layers on the mold wall. The melt stream does not freeze before the holding pressure has adjusted the molten plastic of the cavity surface. Through a thin insulating layer on the cavity surface, the desired improvement of the impression behavior can be reached without prolonging the cycle times significantly, since the mold tempera-ture level is only slightly raised. The heat penetrates the insulation layer/solidified material layer very fast due to its low thickness and is therea�er discharged in a manner comparable to the uncoated mold. The average cavity temperature increases

4092.5 Innovative Mold Technologies

only slightly. A precondition is that the heat insensitivity of the hard material layer is less than that of the mold material.

Measurements on coated mold inserts, which were performed by thermal wave analysis, have shown that the heat insensitivity of the individual coatings shows significant differences. Figure 2.111 shows the depth profiles of the heat insensitiv-ity for different mold coatings.

These different thermal properties can be used selectively. They are in particular:

� Production of structured molded parts with a dull surface and the same cavity temperature.

� Production of molded parts with the same gloss level at lower cavity temperature and therefore resulting improvement of economic efficiency by shortening the cooling time and cycle time.

In the example in Figure 2.112, the thermal properties (short-term isolation) of a special titanium aluminum nitride (TiAlN) layer, which was applied in a PVD (physical vapor deposition) process, were used to hide the weld lines and adjust the gloss level in a structure-grained surface of a shi�ing gate cover made from PC (Polycarbonate). Figure 2.113 shows a direct comparison of surfaces that are coated and uncoated.

Through the targeted manipulation of the interactions between the molding com-pound and the mold as well as between individual mold components, increases of the process stability, and thus significant reductions of downtimes of machines and molds, can be achieved. The following improvements can be achieved in addition to the gloss level adaptation through the use of surface and coating technologies:

(f / Hz)

0.002 0.01 0.1 1 40

1

2

1/2

WC/C

TiAlN

TiCNCrN

Invasiveness of heat b

RatioCoating (bb) / Steel (bs)

Coating bb/bs

CrN ~0.6

TiCN ~0.42

TiAlN ~0.38

WC/C ~0.2

Thermal properties of hard coatings

Depth profile of the invasiveness of heat

FIGURE 2.111  Thermal properties of coatings

410 2 Mold Design

FIGURE 2.112  Shi�ing gate cover

� increase of wear resistance, � increase of corrosion resistance, � reduction of demolding forces, � reduction of mold deposits, and � improvement of sliding properties.

These improvement possibilities are not restricted to injection molds. Thus, in addi-tion to the mold cavities for thermoplastic, thermoset, and elastomer processing, also extrusion molds, slides and jaws, ejector systems, melt control systems and return valves are treated with success. Also for the treatable types of mold materi-als, the frontiers have been pushed back in recent years. In addition to typical mold steels, nonferrous metals like aluminum or copper alloys can be optimized through different surface and coating technologies.

Prerequisite for the successful use of these technologies is the selection of a suitable surface treatment process. Here, in addition to the specifications (e.g., mold steel, surface structure, used molding compound, and geometry), the adverse effects of the individual laminates are considered as well.

2.5.2  Temperature Control Technology – Inductive Heating of Injection Molds

O�en, the cavity temperatures in injection molds have to be increased to avoid, for example, molded part defects such as weld lines or to be able to exactly replicate surface microstructures or fine-textured mold surfaces. Here, the mold wall tem-peratures cause long cooling times.

FIGURE 2.113:  Sample plate, half of it coated

(dull)

4112.5 Innovative Mold Technologies

To forego a continuously high cavity temperature, the variothermal process can be used. Two constantly tempered circuits are used that are independent from each other or a combination of electric heating and temperature control. Both processes are characterized by the cavity, which is heated by a circulation up to the time of the injection process and then cooled to the plastic’s demolding temperature by the second circuit. An advantage over the constant high cavity temperature is that the cavity can be heated up to melting temperature of injected plastic material at the time of the injection process. The disadvantages are the long heating and cooling times for the mold. An alternative to the variothermal process might be the induction heating of mold surfaces. Depending on the position of the inductor, near-surface regions of the cavity can be heated quickly. Experiments have shown that based on a preset cavity temperature, a temperature difference of 150 °C within 3 seconds can be realized. For other applications (e.g., the avoidance of visible weld line notches) only temperature increases of 30 to 60 °C during the injection phase are required.

Inductive heating of components is known, for example, from the hardening and forging technology. Furthermore, in the household sector, induction cook tops are available on the market.

How does inductive heat work?If a high-frequency stream flows through an inductor, it generates an alternating magnetic field in an electrical conductor. Within the electrical conductor, eddy cur-rents are induced, which cause a heating in the skin depth area (penetration depth of the eddy currents). The skin depth is also dependent on the frequency and can therefore be influenced. Furthermore, through inductive heating, a non-contact temperature increase in the conductor takes place.

Useful applications for injection molding can be seen everywhere where high cavity temperatures are required due to the component geometry or because of quality requirements on the surface. Examples include thin-wall technology, production of micro components and microstructures, as well as “conventional” molded parts requirements with high standards of surface quality. Examples can be found in the prevention of visible weld lines, the generation of dull gloss impressions on textured surfaces, or in the manufacture of optical components such as lenses, among others.

With respect to the use of this technology in injection molds (Figure 2.114), various advantages can be seen compared to conventional temperature control systems:

1. In contrast to conventional temperature control systems, the heat does not need to be transferred by conduction, but can be transferred without contact with the mold surface.

2. The inductive heating of injection molds offers the possibility to generate high cavity temperatures in a very short time (Figure 2.115, curve 1).

412 2 Mold Design

3. The heat can be introduced very locally. Adjacent areas only experience a slight temperature rise (Figure 2.115, curves 2 and 3).

4. Depending on the position of the inductor in or at the mold, the required cavity temperature can be generated near the surface (skin effect). This offers the advantage that much lower amounts of heat are introduced into the mold. In fluid-based temperature control systems, the heat has to be transferred from the interior of the mold in the cavity direction. Therefore, larger mold parts are accidentally heated as well.

5. Increases in cycle time can, in the best case, be kept low or neutral because the introduced heat quickly dissipates due to the effects described in the third list point (Figure 2.115, curve 1).

FIGURE 2.114  Inductor, which is integrated into the injection mold (schematic diagram)

Temperature profile for a heating time of 1.5 sec

00

20

40

60

80

100

120

140

160

180

Tem

pera

ture

[ºC]

Time [s]

1

2

3

21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180

FIGURE 2.115  Representation of temperature at different measuring points

4132.5 Innovative Mold Technologies

In addition to the listed advantages, other critical points can reveal challenges in the practical implementation of this technique:

1. In poor operating configurations, additional mold areas can be unintentionally heated.

2. Due to the self-heating of the inductors, they must be cooled by water in almost all applications.

In principle, for the inductive heating of injection molds, three different options of arranging the inductors are available. Here, the following technological principles are:

1. use of an external inductor,

2. integration of the inductor into the injection mold in the mold area to be heated. and

3. integration of the inductor into the injection mold across from the mold area to be heated.

The first mentioned variant provides the greatest possible degrees of freedom because technical details of the mold play a minor role. The injection mold does not need to be constructively adjusted, in contrast to the variants 2 and 3, because the inductor may be positioned into the open mold in front of the area to be heated by a handling device for example.

The disadvantage can be that a strong “overheating” is necessary so that the required cavity temperature is present at the time of the molding process. This can cause negative consequences in terms of cycle time and part quality.

Versions 2 and 3 are a much greater challenge for integrating the inductor into the injection mold. On the one hand, it must be ensured that the inductor, together with the necessary connection cables (power and water), can be integrated into the mold, and on the other hand that the heating of the desired mold area through the inductor is very well targeted by suitable measures to avoid heating of unwanted areas. Used in combination with the mold temperature control close to the contour, as shown in Figure 2.116, the full potential of this technique can be used.

On the topic of inductive mold heating, a joint project in collaboration with 20 com-panies with the goal of practical testing and implementation was carried out at the Plastics Institute Lüdenscheid. For this purpose a test mold with an integrated induc-tor was built. Figures 2.117 and 2.118 show sample parts with different surfaces that were heated by induction during the molding process. The effect of the short-term heat input with respect to the surface is clearly visible. Figure 2.118 shows a part of an electroplatable ABS. There are no visible weld line notches behind the breakthroughs. In Figure 2.117, a much duller appearance of the structures in the heated zones can be seen. The outer, non-heated areas appear glossier. The induc-

414 2 Mold Design

tion heating and therefore the improved surface quality of the parts was achieved without an increase in cycle time.

Inductive heating is thus another possibility of temperature control in injection molds. It can always be considered for special requirements or when conventional options are not available or only in combination with significant increases in cycle time, such as in the variothermal process, which is frequently used for the manu-facturing of micro components or microstructures.

Overall, the inductive heating of injection molding can provide a good compromise between quality and economics.

FIGURE 2.116  Inductively heated mold insert with temperature control close to the contour

VDI 30

VDI 33 VDI 24

VDI 27

FIGURE 2.117  Different eroded structure

according to VDI 3400

FIGURE 2.118:  High-gloss polish

4152.5 Innovative Mold Technologies

2.5.3  Vacuum Technology – Alternative Possibilities, Optimization of Surfaces

In many cases, a suitable compromise on the part design is no longer possible to optimally satisfy both requirements due to the ever-increasing demands on the function and surface quality of the plastic parts.In the first stage of development, the function of the part is of foremost importance and is o�en only met through subsequent design changes. At this point, the mold or the assembly is already mainly created, and subsequent changes o�en lead to unfavorable wall thickness ratios, which will negatively affect the surface quality.If some areas are designed to be too thin, they cannot be reliably injected; in thick-walled areas, sink marks develop, and highly polished surfaces are criticized as “irregular”.The cause here is the compressed air, which gets between the polymer melt and forming mold contour during the injection process and thus prevents an accurate impression of the melt at the mold wall.Figure 2.119 shows an “irregular” surface and reveals the previously described effect. Through insufficient ventilation, air is compressed between the mold surface and the melt. This creates a “wavy” component surface, which is perceived as irregular in high-gloss polishes. Figure 2.120 shows an error-free impression due to better ventilation.In order to achieve active ventilation, a test mold was provided with a vacuum interface, and molded parts were made with and without an evacuation of the cavity.

Figure 2.121 shows the graphical presentation of results with and without evacu-ation of the cavity and the corresponding measurement results. The surface is much smoother with the evacuation. A reduction of the median roughness of 26% with a PC/ABS can be seen on the polished surface using a white-light distance measurement.

FIGURE 2.119  Part produced with

insufficient ventilation

FIGURE 2.120:  Part produced with

sufficient ventilation

416 2 Mold Design

Hochglanzfl‰che ohne Evakuierung

Hochglanzfl‰che mit Evakuierung

sRa: 0.154 μm sRa: 0.113 μm

Minimierung der Rauhigkeit um 26 %

High-polished surface withoutevacuation

High-polished surface withevacuation

Decrease in roughness of

FIGURE 2.121  Presentation of results without and with evacuation of the cavity

A project, which was carried out at the Plastics Institute Lüdenscheid for active mold venting shows that the following advantages can be reached by using the vacuum technology:

1. thin-walled areas are reliably injected,

2. lower injection pressure,

3. improved weld line strength,

4. no diesel effect/burner,

5. smaller deposit formation,

6. significant reduction of mold wear through corrosion by hot air,

7. no air bubbles and streaks in the part,

8. higher bonding strength at 2-K compounds, and

9. optimization of surface quality through better impression.

To take full advantage of the vacuum technology, sealing of the mold must be already considered during the design phase. The sealing of the parting plane is usually uncomplicated because a seal like that shown in Figure 2.122 can be used. For sealing the moving mold components, such as ejectors and sliders, one should first try to seal the entire ejector unit. If this is not possible, every moving mold element must be sealed individually.

In order to evacuate the cavity, an interface for the vacuum unit must be created. Figure 2.123 shows the option of using a valve, which was introduced into the mold.

4172.5 Innovative Mold Technologies

FIGURE 2.122  Mold half with integrated

sealing ring

Other possibilities include evacuation through an overflow bean, through the existing venting channels, or through the ejector unit. It is important that the first step is to do a leak test on the mold, and a�er that, all further measures to seal and choose a method of evacuation can be initiated.

2.5.4  Mold Technology – Flexible Sealing Elements for the Flash- and Damage-Free Encapsulation of Inserts

The reliable encapsulation of inserts – without damage and at the same time inexpensively and without flash formation – is possible through the use of flexible sealing elements A 4200 (MurSeal®). Figure 2.124 shows the use of such seals in an example of a wrist strap for car doors.

For a partial encapsulation of metal inserts, corresponding metal parts have to be sealed against the melt flow. In this area, despite exact injection mold dimensions, over-injection and damages on the insert surface can occur. The reasons for the over-injection (flash formation) are the rectangular-shaped stamped parts and the intersection formation of the metallic inserts (ratio of edge pull-in, clean-cut per-centage, fracture surface, and stamping flash). What makes it even more difficult is the formation of the intersection changes during the production period, which makes this area prone to flash formation during encapsulation.

FIGURE 2.123  Valve for an evacuation

418 2 Mold Design

FIGURE 2.124  Application of flexible sealing elements for flash- and damage-free

encapsulation of metal inserts

Another problem is inserts with sensitive surfaces, such as painted surfaces. Here, due to variations in paint thickness in combination with the tolerance variations of the insert, damages of the insert surface o�en occur through the insertion, removal, or closing operation of the mold. In this case, surface damage causes not only visual defects in the hybrid component but can also lead to component failure (corrosion) and/or require extensive manual rework.

State of technologyAccording to the current state of technology, it is only possible to encapsulate metal inserts with thermoplastic materials under series conditions without flash forma-tion or damage in the injection molding process. O�en interchangeable inserts made from copper, aluminum, or another metal are used in the mold. Here, the inserts cannot be individually adapted to the depositors, which will make regular replacement necessary. Another possibility is to compress the inserts in the injec-tion mold. This can temporarily avoid flash formation. In this context, it must be noted, however, that damage to the injection mold and the insert surfaces will occur.

4192.5 Innovative Mold Technologies

Innovation through the application of flexible sealing elementsThrough hands-on basic research, flash formation is controlled, and a damage-free encapsulation of inserts in the injection molding process is reliably and cost-effectively implemented by using flexible sealing elements. Here, flexible sealing elements are permanently mounted into the depositor areas to be sealed as a mold insert in the injection mold. The resulting semifinished plastic can replace a�er machining the conventional sealing area made from steel.

The use of flexible sealing elements has been implemented with success on several series injection molds, primarily in the automotive sector. Here, the material of the flexible sealing elements can be combined with a variety of encapsulation materials. Good experience has occurred with encapsulation materials such as PPS 40% GF, PA6 30% GF, PBT 30% GF, TPE. Particularly suitable is the use of flexible seals for plate thicknesses from about 4 mm. The largest realization took place on a metal depositor with a diameter of 25 mm and a weight of about 5 kg. So far, confirmed numbers are moving in the range of up to 1,000,000 shots, with no visible wear on the sealing element.

The material for the flexible sealing element has been produced in cooperation with the company Murtfeldt Kunststoffe GmbH & Co., Dortmund. The distribution was handed over to Hasco Hasenclever GmbH in April 2006. In these respective com-panies, a practical solution called A 4200 (MurSeal®) from the plastics processing industry is available as a semifinished part.

Design informationIn order to achieve sufficient stability against the cavity pressure and optimum heat dissipation, the sealing element A 4200 (MurSeal®) must be embedded into mold inserts made from mold steel. The attachment of the flexible sealing elements can be done by screwing, clamping, or by a frictional connection using locking pins.

The sealing element should always be inserted into the complete sealing area, which is in the fixed and movable mold half and should, as shown in Figure 2.125, always protrude about 0.1 mm in the parting plane.

FIGURE 2.125  Insert for supporting A 4200 (MurSeal®

) against the cavity pressure

420 2 Mold Design

This protrusion results in compression of the closed mold and, if necessary, should be determined or calculated depending on the geometry of the insert. Due to defor-mations of the flexible sealing element, irregularities at the insert (e.g., punched interfaces or complicated mold geometries) can be reliably sealed against the melt stream in the closed mold.

Savings with flexible sealing elements can not only be achieved through a reduced unit cost but by allowing the user to be able to increase quality standards and minimize rejection rates.

3 Materials for Mold Making

 ■ 3.1  Plastic Mold Steels

F. Hippenstiel

3.1.1  Introduction

Due to its exceptionally advantageous combination of properties that are of critical importance for tools and molds, steel is the principal material used in the plastic mold making industry. There is now a range of custom steels available for mold making, with the steel undergoing specialised processes at steel manufacturers to achieve the service hardness required for global applications. Another distinguish-ing feature of steel as a material is that its functional properties can be adapted to special requirements by further appropriate heat treatment and/or surface finishing.

In addition to the necessary quality features, another relevant criterion for evaluating the usual steel concepts for plastic molds is the efficiency of the mold manufacturing process. Throughout the added value chain through to the finished plastic mold, the following evaluation criteria apply:

� Material cost price, � Processing properties, � Cost of further heat treatment, subsequent hard machining and/or surface treat-ment,

� Logistics costs.

Figure 3.1 summarises possible process steps for manufacturing plastic molds, depending on the steel concept. The shortest and most cost-effective route for mold making is generally making the mold out of pre-hardened plastic mold steel. If a higher service hardness is desired, through-hardening plastic mold steels are pre-machined in the annealed state, then usually vacuum hardened and hard machined

422 3 Materials for Mold Making

to finished dimensions. The process when using plastic mold steels for case harden-ing is similar, using case hardening instead of vacuum hardening. Surface finishing now provides further options as regards production routes in tool and mold making. One classic route is surface finishing (e.g. chromium plating, nitriding, or PVD coating) of a mold made of through-hardening plastic mold steel. In some cases it is now possible to apply certain surface finishing processes to pre-hardened plastic mold steels, depending on the process temperature and tempering resistance.

Service hardness generally means the resistance of a material to the penetration of a body (a defined test body in the case of hardness testing).The hardness can be measured with various items of test equipment, Brinell (HB), Rockwell (HRC) and Vickers (HV) hardness being the most common. Table 3.1. shows a summary based on DIN 50150 [1] for comparing the various hardness values.

A B C D

Pre-hardened

plastic mold

steels (hardness

~31 HRC)

Sawing,

Pre-machining

Finish

machiningMachining Machining Machining

Vacuum

hardening

Vacuum

hardening

Hard

machining

Hard

machining

Hard

machining

Surface

finishing

Case

hardening

Sawing,

Pre-machining

Sawing,

Pre-machining

Sawing,

Pre-machining

Hardened

plastic mold

steels (hardness

~62 HRC)

Surface finished

steels

(700-3500 HV)

Case hardening

steels (hardness

~62 HRC)

FIGURE 3.1  Comparison of the production stages involved in the steel concepts for

manufacturing molds and tools for plastics processing

4233.1 Plastic Mold Steels

TABLE 3.1  Conversion table for hardness values, extract from DIN 50150

Rockwell hardness HRC

Brinell hardness HB

Vickers hardness HV

Tensile strength MPa

25 253 266 854

30 287 302 970

35 329 345 1111

40 373 392 1262

45 424 446 1442

50 488 513 1668

55 566 596 1979

60 698

3.1.2  Steel making and processing

Whereas the quality of steel materials is assessed by testing their technological prop-erties, molds and tools are assessed according to their service life, i.e. according to their performance. Specific research and many years of experience have established the associations between functional properties and processing properties of plastic mold steels, forming the basis for ongoing development of this steel group. But the material properties are only some of the factors affecting service life; other factors include the design of the mold, the type of manufacture, the operating conditions, and the heat treatment and surface treatment.

3.1.2.1  Steelmaking

The classic process route for producing special steels is the electric arc process. Scrap is melted down using electrical energy to first produce crude steel, which then undergoes further treatment and refining in secondary metallurgy in a ladle furnace. In most cases the plastic mold steel is now also deep desulfurized in second-ary metallurgy, and then poured by ingot casting or continuous casting for further processing. In order to achieve the required functional properties and processing properties for reliable further treatment, additional processing and treatment steps then follow, mostly performed entirely within the steel mill. One of the most important steps is metal forming, i.e. converting a billet or raw ingot into useable plastic mold steel – using the hot rolling and forging processes. Much of the steel used for tool and mold making is made from rolled plates up to about 200 mm thick; this dimension represents a technological limit as regards the residual porosities due to solidification (core consistency of the finished product). Larger dimensions are produced in the form of forged steel bar in round, flat and square dimensions.

424 3 Materials for Mold Making

The desired plastic forming is mostly carried out under hydraulic presses, to ensure the required material properties. A�er forming, the steel manufacturer carries out suitable heat treatment, which may be either preliminary heat treatment prior to machining, or final heat treatment in the case of pre-hardened plastic mold steels.

Figure 3.2 shows a diagram of the process route for manufacturing forged square dimensions.

In addition to conventional steelmaking, there are also special metallurgical pro-cesses available that are sometimes also used in plastic mold steel making. These new technological solutions are mostly combined with the established traditional methods of steelmaking and applied metallurgy. Melting steels with high proportions of the alloying elements titanium and aluminum (e.g. in the case of precipitation hardening plastic mold steels) is best carried out in a vacuum induction furnace. Special melting processes such as electroslag remelting (ESR) or vacuum arc remelting (VAR) achieve very fine solidification structures because of the increased local solidification speeds associated with this process; the process also reduces the incidence of non-metallic impurities sometimes introduced from the pre-melt. Powder metallurgical production of high-alloy steels combines several advantageous metallurgical features. This special metallurgical process involves melting and treating the melts in medium-sized induction furnaces. The melt is then atomized in an atomizing chamber in a protective gas atmosphere to form fine-grain globular powder. The powder capsules are filled and sealed gas-tight, then compressed by hot isostatic pressing in an argon atmosphere, to create a completely sealed solid. Such compressed alloys can be hot formed using normal processes, despite their extremely high alloy content. Another special metallurgical process is “spray forming”.

1) Melting in the electric arc furnace

2) Secondary metallurgical treatment

3) Vacuum treatment of the melt

4) Ingot casting

5) Forging 6) Heat treatment, e.g. (pre-) hardening and tempering or annealing

7) Forged plastic mold steel

FIGURE 3.2  Schematic diagram of the main steps in producing plastic mold steels for large

dimensions

4253.1 Plastic Mold Steels

This involves atomizing the melt in a protective gas atmosphere using two oscillat-ing nozzles. The resultant melt droplets are sprayed onto a rotating starting plate, and rapidly cooled. The precipitation of the droplets on the starting plate creates a solid steel bolt that can be further processed as appropriate.

3.1.2.2  Heat treatment

Depending on the objective of heat treatment of plastic mold steels, several pro-cesses are available, some of them very different. In some the state of the material is changed selectively throughout the cross section. The widely differing processes available for this are annealing, normalizing, hardening, tempering, or precipitation hardening. Other processes such as nitriding or case hardening only seek to achieve a change in the surface layer. The characteristic temperature ranges of the various heat treatment processes are given depending on the carbon content, as shown in the iron carbon equilibrium diagram (Figure 3.3). In the case of higher-alloy steels, these temperature ranges shi� according to the change in the transforma-tion temperatures.

The aim of hardening is to create a martensitic to bainitic material state as far as possible, which is characterised by higher hardness. When components and molds have been hardened, they are usually also tempered in order either to match the strength behaviour to the particular stress conditions, to transform residual austenite, or to reduce the risk of cracking in the subsequent grinding process. Tempering at higher temperatures in the range around 600 °C serves in particular to achieve a relationship between strength and toughness that is appropriate for the stresses involved.

The atomic structure of iron as a metal is characterized by the iron atoms being held together by a metallic bond. The bonding force results from the attraction between the positively charged atoms and the negatively charged free electrons of the outer electron shell. In their solid state, metals are generally crystalline in structure, so the spatial arrangement of each atom is fixed. The crystal system of the iron is cubic, and depending on the temperature, there is a body-centred cubic or a face-centred cubic lattice. Steels with a carbon content of up to 0.8 percent by mass have a ferritic-perlitic microstructure in the normalised state; in the case of steels with a higher carbon content, the microstructure consists of perlite and grain boundary cementite. If the steel is heated up to temperatures over 723 °C, its microstructure gradually changes, and austenite occurs. Depending on the speed of cooling, the microstructure transforms into various microstructure components (martensite, bainite, perlite and ferrite) as the material cools down. The suitability of the steels for hardening by transformation in the martensitic range is denoted by hardenability, a distinction being made in this regard between potential hardness increase and penetration hardness. Potential hardness increase refers to the associa-

426 3 Materials for Mold Making

tion between the carbon content and the maximum possible hardness (100 percent martensite). The maximum hardness achievable of up to 0.6 carbon percentage by mass is derived as follows:

� � � �Maximum hardness achievable 35 (50 carbon-% <by mass>) 2 HRC

The transformation behaviour of plastic mold steels are described in time, tempera-ture, transformation (TTT) curves in order to better understand the transformation processes involved in hardening and in austenitizing.

Tem

pera

ture

in º

C1600

1536

1500

A1 1392

1300

1200

1100

1000

A3 911

A2 769

A1 723

700

600

500

400

300

200

100

20

Carbon content % by mass

10 0.5 0.8 1 1.5 2

1600

1500

1400

1300

1200

1100

1147

1000

900

800

700

600

500

400

300

200

100

20

Annealing colors

Yellow-white

Light yellow

Yellow

Yellow-red

Light red

Cherry red

Dark red

Brown-red

Dark brown

Tempering colors

GreyBlue-grey

Light blue

PurpleRedYellow-brownStraw yellow

S + δ

S1493ºC

d

α

γ + α

γ

δ + γδ

d

Diffusion annealing

Coarse grain

annealing

S + γ

2.06

Normalizing, hardening

γ + Fe3CACM

Soft annealing

Stress relieve annealing

Recrystallization annealing

α + Fe3C

MS

Hypoentectoid steel Hypereutectoid steel

Blue

FIGURE 3.3  Extract from the iron carbon equilibrium diagram, including the temperature

ranges for certain heat treatment processes

4273.1 Plastic Mold Steels

Information and recommendations on the heat treatment of plastic mold steels are shown in the relevant material data sheets of the steel manufacturers.

3.1.2.3  Machining

Despite the pace of development in the field of machining, with more powerful machine tools, innovative coating technologies with blanking dies, and the introduc-tion of HSC milling, machining of plastic mold steel is still a major cost factor in tool and mold making. Figure 3.4 illustrates the differences in machinability between the steels in terms of the maximum cutting speed for rough milling as a function of material strength. It should be noted that pre-hardened plastic mold steels in the middle strength range have better machinability than high-alloy through-hardening or corrosion-resistant plastic mold steels. Whether these steels are available in the annealed or pre-hardened state is a subordinate factor as concerns machinability.

220

200

180

160

140

120

100600 700 800 900 1000 1100 1200 1300

Strength N/mm2

Vc in

m/m

in

Pre-hardened steels

Corrosion-resistant steels

Post-roughing hardened steels

FIGURE 3.4  Cutting speed Vc as a function of the strength of the work piece in rough milling

3.1.2.4  Surface machining

The surface finish of plastic mold steels decides their suitability for giving the mold a high-quality surface finish by polishing, high-gloss polishing, etch-graining or coating. The crucial factors are absence of flaws due to manufacturing defects (imperfections) such as cracks, pores or inclusions, a high inclusion rating, and a corresponding microstructure homogeneity. Other important aspects for high-gloss polishing are high material hardness (Figure 3.5), correct gradation of the abrasive and polishing compound particle size distribution, and the skill of the polisher.

The pre-hardened plastic mold steel 1.2312 (40CrMnMoS8-6) is not suitable for molds with high surface requirements or mechanical stresses, because of its high sulphur content. In combination with manganese, the sulphur forms long-chain sulfides that can cause the following problems:

428 3 Materials for Mold Making

100

90

80

70

60

5020 H R C 30 H R C 40 H R C 50 H R C 60 H R C 800 HV

Hardness

Polis

habi

lity

in %

Pre-

hard

ened

ste

els

Post

-rou

ghin

g ha

rden

ed s

teel

s

Coat

ed s

teel

s

Polishable to a limited extent

Polishable

High-gloss polishable

2000 HV20 HRC 30 HRC 40 HRC 50 HRC 60 HRC 800 HV 2000 HV

FIGURE 3.5  Polishability of plastic mold steels as a function of hardness

� Banding due to etching away the steel surface, � Rippled surface with polishing as the so� manganese sulfides dissolve out from the harder steel matrix,

� Conditional or restricted suitability for surface treatment, � Restricted weldability.

3.1.2.5  Quality assurance

In addition to testing the chemical composition and determining the technological properties, non-destructive materials testing is an important aspect with plastic mold steels. For example forgings are generally inspected by ultrasonic testing for internal defects such as residual porosities or non-metallic inclusions. Machined workpieces also undergo surface crack testing using methods such as magnetic testing or dye penetration testing.

3.1.3  Overview of plastic mold steels

Widely differing, and sometimes conflicting, demands are made on plastic mold steels in the course of their processing and use as a material for molds, so there is no single steel grade that can fully satisfy all requirements in terms of quality and economic considerations. The most important requirements are [2]:

� Good machinability, good chip formation, low cutting forces, low tool wear � High thermal conductivity – for example rapid cooling of molds heated up by the injection-molding process

4293.1 Plastic Mold Steels

� Through-hardening – ideally uniform hardness penetration a�er hardening and tempering across the entire cross-section

� High crack- and fracture-resistance – high fracture toughness in the area of notches and critical bending stress

� Corrosion resistance – avoiding rust or corrosion attack � Tempering resistance – suitable for carrying out thermal coating processes � Weldability – possibility of welding in the event of design changes or repairs � Surface hardenability – laser hardening, flame hardening or inductive hardening, � Wear resistance – resistance to mechanical damage and abrasion � (High-gloss) polishability – suitable for producing mirror-finish polishes, � Etch-grainability – suitability for producing faultless surfaces a�er texturing by photo etching techniques

� Chromium plateability – suitability for faultless electrolytic chromium plating � Nitridability – hardness penetration with nitriding without loss of strength in the base material

� PVD coatability – suitability for producing a high standard of surface finish with PVD processes

These requirements inevitably translate into different properties profiles for plastic mold steels. The different steel concepts can be grouped as follows:

� Pre-hardened plastic mold steels for all standard applications (hardness range 29 to 44 HRC)

� Through-hardening plastic mold steels for hardening a�er (pre-)machining, e.g. for molds and tools subject to high wear (hardness range: 44 to 62 HRC)

� Corrosion-resistant plastic mold steels e.g. for PVC processing, pre-hardened to approximately 30 HRC for standard applications, up to 53 HRC for high levels of wear, with hardening a�er (pre-)machining

� Plastic mold steels for case hardening (surface hardness a�er case hardening up to 62 HRC)

� Precipitation hardening plastic mold steels for high-wear applications; hardness a�er precipitation hardening: 55 HRC

� Nitriding steels, especially for extruders (surface hardness a�er nitriding process up to 1000 HV)

Table 3.2 shows the classification of plastic mold steels in terms of the most important features required. The steel materials used in mold and tool making for plastics processing are covered by the standard DIN EN ISO 4957 [3], except for a few more recent developments. The chemical compositions under this standard are summarised in Table 3.3 for the most common plastic mold steels.

430 3 Materials for Mold Making

TABL

E 3.

2  Cl

ass

ificati

on o

f pla

stic

mold

ste

els

accord

ing t

o t

he m

ost

im

port

ant

featu

res

requir

ed

Plas

tic M

old

Stee

ls

(Mat

eria

l Num

ber)

Machinability

Thermal Conductivity

Fracture Toughness

Corrosion-Resistance

Weldability

Wear-Resistance

High-Gloss Polishability

Etch-Grainability

Chrome Plating Ability

Through-Hardenability

Nitridability

PVD Coatability

Thermal Strength

Pre-

hard

ened

Pla

stic

Mol

d St

eels

1.23

11+

+±

– –

+±

++

+±

+±

±1.

2312

+ +

+–

––

––

±–

––

––

±±

–±

1.27

11±

++

– –

++

++

++

+ +

++

++

1.27

38+

+±

– –

+±

++

++

+±

±27

38 m

od. T

S*+

+ +

+–

–+

++

+ +

+ +

+ +

+ +

++

+Th

roug

h-Ha

rden

ing

Plas

tic M

old

Stee

ls1.

2343

±±

+–

–±

+ +

+ +

±+

+±

+ +

+ +

+ +

1.23

79–

– –

– –

– –

–+

+–

––

––

±+

+–

–+

+1.

2767

±±

+ +

– –

++

++

++

++

++

++

±1.

2842

++

±–

––

+ +

–±

+–

++

±Co

rros

ion-

Resi

stan

t Pla

stic

Mol

d St

eels

1.20

83±

– –

±+

++

++

+±

+ +

–±

±+

1.20

85+

+–

––

±+

+–

– –

––

±–

+1.

2316

±–

–+

+ +

++

+ +

±+

++

++

+ +

+ +

Plas

tic M

old

Stee

ls fo

r Cas

e-Ha

rden

ing

1.21

62+

+±

– –

++

++

±+

– –

– –

– –

–1.

2764

+±

+–

–+

+ +

+ +

++

+–

––

––

Prec

ipita

tion

Hard

enin

g Pl

astic

Mol

d St

eels

1.27

09–

––

–+

+±

+ +

+ +

++

+±

+ +

+ +

+

+ +

very

goo

d; +

goo

d; ±

neu

tral;

– c

ondi

tiona

lly s

uita

ble;

– –

uns

uita

ble

* m

odifi

ed 1.

2738

, com

para

ble

with

e.g

. Akt

uell

1200

or 2

738

EHT

Plus

4313.1 Plastic Mold Steels

TABL

E 3.

3  Ch

em

ical com

posi

tion o

f st

andard

ste

el m

ate

rials

in m

old

and t

ool m

akin

g f

or

pla

stic

s pro

cess

ing

Mat

eria

l Nu

mbe

rDI

N-Sp

ecifi

catio

nCh

emic

al C

ompo

sitio

n (in

mas

s %)

CSi

Mn

PS

CrM

oNi

Othe

rsUn

allo

yed

Mol

d St

eel

1.17

30C4

5U0.

42–0

.50

0.15

–0.4

00.

60–0

.80

≤ 0.

030

≤ 0.

030

––

––

Pre-

hard

ened

Pla

stic

Mol

d St

eels

1.23

11*

40Cr

MnM

o70.

35–0

.45

0.20

–0.4

01.

30–1

.60

≤ 0.

035

≤ 0.

035

1.80

–2.1

00.

15–0

.25

––

1.23

1240

CrM

nMoS

8-6

0.35

–0.4

50.

30–0

.50

1.40

–1.6

0≤

0.03

00.

05–0

.10

1.80

–2.0

00.

15–0

.25

––

1.27

11*

54Ni

CrM

oV6

0.50

–0.6

00.

15–0

.35

0.50

–0.8

0≤

0.02

5≤

0.02

50.

60–0

.80

0.25

–0.3

51.

50–1

.80

V =

0.07

–0.1

21.

2738

*40

CrM

nNiM

o8-6

-40.

35–0

.45

0.20

–0.4

01.

30–1

.60

≤ 0.

030

≤ 0.

030

1.80

–2.1

00.

15–0

.25

0.90

–1.2

0–

2738

mod

. TS*

*26

MnC

rNiM

o6-5

-40.

260.

101.

45≤

0.01

5≤

0.00

21.

250.

601.

05V

= 0.

12Th

roug

h-Ha

rden

ing

Plas

tic M

old

Stee

ls1.

2343

*X3

7CrM

oV5-

10.

33–0

.41

0.80

–1.2

00.

25–0

.50

≤ 0.

030

≤ 0.

020

4.80

–5.5

01.

10–1

.50

–V

= 0.

30–0

.50

1.23

79 *

X153

CrM

oV12

1.45

–1.6

00.

10–0

.60

0.20

–0.6

0≤

0.03

0≤

0.03

011

.00–

13.0

00.

70–1

.00

–V

= 0.

70–1

.00

1.27

67 *

45Ni

CrM

o16

0.40

–0.5

00.

10–0

.40

0.20

–0.5

0≤

0.03

0≤

0.03

01.

20–1

.50

0.15

–0.3

53.

80–4

.30

–1.

2842

90M

nCrV

80.

85–0

.95

0.10

–0.4

01.

80–2

.20

≤ 0.

030

≤ 0.

030

0.20

–0.5

0–

–V

= 0.

05–0

.20

Corr

oson

-Res

ista

nt P

last

ic M

old

Stee

ls1.

2083

*X4

0Cr1

40.

36–0

.42

≤ 1.

00≤

1.00

≤ 0.

030

≤ 0.

030

12.5

0–14

.50

––

–1.

2085

X33C

rS16

0.28

–0.3

8≤

1.00

≤ 1.

40≤

0.03

00.

05–0

.10

15.0

0–17

.00

–≤

1.00

–1.

2316

*X3

8CrM

o16

0.33

–0.4

5≤

1.00

≤ 1.

50≤

0.03

0≤

0.03

015

.50–

17.5

00.

80–1

.30

≤ 1.

00–

Plas

tic M

old

Stee

ls fo

r Cas

e-Ha

rden

ing

1.21

62 *

21M

nCr5

0.18

–0.2

40.

15–0

.35

1.10

–1.4

0≤

0.03

0≤

0.03

01.

00–1

.30

––

–1.

2764

*X1

9NiC

rMo4

0.16

–0.2

20.

10–0

.40

0.15

–0.4

5≤

0.03

0≤

0.03

01.

10–1

.40

0.15

–0.2

53.

80–4

.30

–Pr

ecip

itatio

n Ha

rden

ing

Plas

tic M

old

Stee

ls1.

2709

*X3

NiCo

MoT

i8-9

-5≤

0.03

≤ 0.

10≤

0.15

≤ 0.

010

≤ 0.

010

≤ 0.

254.

50–5

.20

17.0

0–19

.00

Co =

8.5

0–10

.00;

Ti

= 0

.80–

1.20

Nitr

idin

g St

eels

1.77

3514

CrM

oV6-

90.

11–0

.17

≤ 0.

250.

80–1

.00

≤ 0.

020

≤ 0.

015

1.25

–1.5

00.

80–1

.00

–V

= 0.

20–0

.30

1.85

1931

CrM

oV9

0.27

–0.3

4≤

0.40

0.40

–0.7

0≤

0.02

5≤

0.03

52.

30–2

.70

0.15

–0.2

5–

V =

0.10

–0.2

01.

8550

34Cr

AlNi

7-10

0.30

–0.3

7≤

0.40

0.40

–0.7

0≤

0.02

5≤

0.03

51.

50–1

.80

0.15

–0.2

50.

85–1

.15

Al =

0.8

0–1.

20

* In

gen

eral

, the

se m

ater

ials

are

use

d ul

tra d

eep

desu

lphu

rized

, i.e

. S ≤

0.0

03 %

. **

Typi

cal a

nalys

is, c

ompa

rabl

e to

e.g

. Akt

uell

1200

or 2

738

EHT

Plus

.

432 3 Materials for Mold Making

TABL

E 3.

4  M

ate

rial chara

cte

rist

ics

and u

se o

f th

e s

teels

com

monly

use

d in m

old

and t

ool m

akin

g f

or

pla

stic

s pro

cess

ing

Mat

eria

l Nu

mbe

rDe

liver

y Co

nditi

onSe

rvic

e Ha

rdne

ssAp

plic

atio

nTh

erm

al E

xpan

sion

Coe

ffici

ent

� (1

0–6/K

)Th

erm

al C

ondu

ctiv

ity�

(W/m

K)20

–100

 °C20

–250

 °C20

–500

 °C20

 °C25

0 °C

500 

°CUn

allo

yed

Mol

d St

eel

1.17

30No

rmal

ized,

max

. 190

HB

max

. 190

HB

Clam

ping

pla

tes,

mol

d fra

mes

with

low

st

ress

11.8

13.2

14.2

41.0

39.0

35.0

Pre-

Hard

ened

Pla

stic

Mol

d St

eels

1.23

11Ha

rden

ed a

nd

tem

pere

d,28

0–32

5 HB

280–

325

HBCo

mpr

essi

on a

nd in

ject

ion

mol

ds w

ith

a ha

rden

ing

and

tem

perin

g th

ickn

ess

of

up to

400

mm

11.6

12.8

14.3

34.0

33.5

33.0

1.23

12Ha

rden

ed a

nd

tem

pere

d,28

0–32

5 HB

280–

325

HBCo

re c

ompo

nent

s fo

r com

pres

sion

and

in

ject

ion

mol

ds, m

old

asse

mbl

ies

11.6

12.8

14.3

34.0

33.5

33.0

1.27

11Ha

rden

ed a

nd

tem

pere

d,

upon

requ

est;

Anne

aled

280–

325

HB;

355–

415

HBm

ax. 2

50 H

B

Com

pres

sion

and

inje

ctio

n m

olds

with

a

high

er m

echa

nica

l and

ther

mal

stre

ss,

cont

our h

arde

ning

is re

com

men

ded

11.0

12.4

13.5

33.0

35.0

33.0

1.27

38Ha

rden

ed a

nd

tem

pere

d,28

0–32

5 HB

280–

325

HBFe

mal

e m

old

for c

ompr

essi

on a

nd

inje

ctio

n m

old,

mol

ds fo

r har

deni

ng a

nd

tem

perin

g th

ickn

ess

dim

ensi

ons

of m

ore

than

400

mm

11.6

12.8

14.3

34.0

33.5

33.0

2738

mod

. TS

*Ha

rden

ed a

nd

tem

pere

d,28

0–32

5 HB

.31

0–35

5 HB

280–

325

HB;

310–

355

HBLa

rge

mol

ds, f

emal

e m

olds

for

com

pres

sion

and

inje

ctio

n m

olds

of

larg

e di

men

sion

s

10.8

12.2

13.9

37.4

41.3

39.8

Thro

ugh-

Hard

enin

g Pl

astic

Mol

d St

eels

1.23

43An

neal

ed,

max

. 230

HB

Inst

alle

d ha

rdne

ss

of a

bout

46–

50 H

RCHi

ghly

-stre

ssed

pla

stic

mol

ds, m

old

inse

rts

with

a h

igh

abra

sive

stre

ss10

.311

.612

.823

.025

.027

.0

1.23

79An

neal

ed,

max

. 255

HB

Inst

alle

d ha

rdne

ss

of a

bout

zu 6

3 HR

CPl

astic

mol

ds a

nd m

old

inse

rts

with

a

part

icul

arly

hig

h re

sist

ance

aga

inst

wea

r 9

.012

.013

.020

.021

.022

.0

1.27

67An

neal

ed,

max

. 260

HB

Inst

alle

d ha

rdne

ss

of a

bout

50–

54 H

RCHi

ghly

-stre

ssed

mol

d, la

rge

mol

ds fo

r bo

dywo

rk p

arts

and

mol

ds w

ith h

igh

surfa

ce re

quire

men

ts

11.0

12.2

13.7

31.0

30.0

32.0

1.28

42An

neal

ed,

max

. 230

HB

Inst

alle

d ha

rdne

ss

of a

bout

57–

62 H

RCSm

all p

last

ic m

olds

with

out s

peci

fic

requ

ire m

ents

on

polis

habi

lity

and

grai

nabi

lity

12.2

13.5

14.7

33.0

32.7

31.8

4333.1 Plastic Mold Steels

TABL

E 3.

4  (c

ontin

ued)

Mate

rial chara

cte

rist

ics

and u

se o

f th

e s

teels

com

monly

use

d in m

old

and t

ool m

akin

g f

or

pla

stic

s pro

cess

ing

Mat

eria

l Nu

mbe

rDe

liver

y Co

nditi

onSe

rvic

e Ha

rdne

ssAp

plic

atio

nTh

erm

al E

xpan

sion

Coe

ffici

ent

� (1

0–6/K

)Th

erm

al C

ondu

ctiv

ity�

(W/m

K)20

–100

 °C20

–250

 °C20

–500

 °C20

 °C25

0 °C

500 

°CCo

rros

ion-

Resi

stan

t Pla

stic

Mol

d St

eels

1.20

83An

neal

ed,

max

. 230

HB

Inst

alle

d ha

rdne

ss

of a

bout

50–

54 H

RCSm

all a

nd m

id-s

ize m

olds

and

mol

d in

sert

s, e

.g.,

for P

VC p

roce

ssin

g11

.012

.513

.523

.024

.025

.0

1.20

85Ha

rden

ed a

nd

tem

pere

d,26

5–31

0 HB

265–

310

HBSm

all a

nd m

id-s

ize m

old

fram

es,

e.g.

, for

mol

ds fo

r PVC

pro

cess

ing

10.0

12.0

13.2

23.0

24.0

25.0

1.23

16Ha

rden

ed a

nd

tem

pere

d,26

5–31

0 HB

265–

310

HBM

old

inse

rts,

slit

and

pro

file

dies

, fem

ale

profi

le m

old,

cal

ibra

tion

mol

ds, b

low

m

olds

, als

o su

itabl

e fo

r mid

-size

and

la

rge

mol

ds

10.4

11.0

12.8

17.4

20.1

22.8

Plas

tic M

old

Stee

ls fo

r Cas

e-Ha

rden

ing

1.21

62An

neal

ed,

max

. 210

HB

Surfa

ce h

ardn

ess

to

62 H

RC,

core

har

dnes

s to

300

HB

Com

pres

sion

and

inje

ctio

n m

olds

with

co

mpr

essi

ve s

tress

and

abr

asive

sur

face

lo

ad a

t the

sam

e tim

e

11.5

13.0

14.4

41.0

40.5

35.0

1.27

64An

neal

ed,

max

. 250

HB

Surfa

ce h

ardn

ess

to

62 H

RC,

core

har

dnes

s to

400

HB

Com

pres

sion

and

inje

ctio

n m

olds

with

hi

gh c

ompr

essi

ve s

tress

and

abr

asive

su

rface

load

at t

he s

ame

time

11.5

12.8

14.0

36.0

37. 0

34.0

Hard

enin

g Pl

astic

Mol

d St

eels

1.27

09So

lutio

n an

neal

ed,

abou

t 300

HB

Aer

har

deni

ng

abou

t 50–

54 H

RCPa

rt in

sert

s an

d sm

all m

olds

with

hig

h to

ughn

ess

requ

irem

ents

10.0

16.0

17.3

12.0

15.0

19.0

Nitr

idin

g St

eels

1.77

35Ha

rden

ed a

nd

tem

pere

d to

265–

310

HB

Surfa

ce h

ardn

ess

of

abou

t 850

HV

aer

ni

tridi

ng

Pref

erab

le fo

r pla

stic

izing

uni

ts,

scre

w c

ylin

ders

and

bar

rel e

xtru

ders

12.1

12.9

14.0

36.9

37.1

34.4

1.85

19Ha

rden

ed a

nd

tem

pere

d to

265–

310

HB

Surfa

ce h

ardn

ess

of

abou

t 800

HV

aer

ni

tridi

ng

Pref

erab

ly fo

r pla

stic

izing

uni

ts,

scre

w c

ylin

ders

and

bar

rel e

xtru

ders

12.1

12.9

14.0

27.4

28.7

27.7

1.85

50Ha

rden

ed a

nd

tem

pere

d to

240–

300

HB

Surfa

ce h

ardn

ess

of a

bout

1,0

00 H

V a

er n

itrid

ing

Pref

erab

ly fo

r pla

stic

izing

uni

ts,

scre

w c

ylin

ders

and

bar

rel e

xtru

ders

12.1

13.0

14.0

27.4

28.7

27.7

* m

odifi

ed 1

.27

38

, com

para

ble

wit

h e

.g., A

ktu

ell

12

00

or

27

38

EH

T P

lus

434 3 Materials for Mold Making

Designing molds for plastics processing requires knowledge of the mechanical properties of the plastic mold steels as supplied, or a�er any further heat treatment, and also physical parameters. To produce dimensionally accurate plastic moldings, the shrinking of the plastic molding has to be taken into account when machining the cavity; expansion of the cavity as it heats up to mold working temperature is another factor in this calculation. The thermal expansion coefficients vary, depend-ing on the alloy content. Table 3.4 accordingly also shows the thermal expansion coefficients of the usual plastic mold steels for the temperature ranges between 20 and 500 °C, relative to normal service hardness. Molds are generally cooled in the process of producing plastic moldings, to cool down the plastic molding compound as quickly as possible. The controlling factors for this are the position and dimen-sioning of the cooling channels, the turbulence and flow speeds of the coolant, and also the thermal conductivity of the plastic mold steel. This depends to a significant extent on the chemical composition of the materials, and is likewise shown in the overview for the technically relevant temperature ranges.

3.1.3.1  Pre-hardened plastic mold steelsThese steels are adjusted to service hardnesses of 29 to 44 HRC, by suitable heat treatment (hardening and tempering) a�er the steelmaking process, depending on the load profile. This material concept was originally intended only for making large-scale molds, but has now become established for all common dimensions, so that now around 90% of all plastic mold steels are supplied pre-hardened. The advantage is significantly reduced production time, since no further heat treatment is necessarily required a�er preliminary machining. It also removes any need for reworking as a result of heat treatment distortion, which cannot be entirely avoided. Significant advances in machining technology, such as HSC milling, mean that pre-hardened steels can now be used without difficulty, and machined economically. For applications involving extreme stress, increased wear protection can be provided by nitriding, chromium plating, nickel plating or PVD coating of the appropriate mold surfaces. These materials can fundamentally be divided into two groups. The first group comprises the materials 1.2311 (40CrMnMo7), 1.2312 (40CrMnMoS8-6), 1.2738 (40CrMnNiMo8-6-4) and 1.2316 (X38CrMo16) with a standard hardness of 29 to 34 HRC; the hardness range for 1.2316 (X38CrMo16) is from 27 to 33 HRC. This covers all standard applications in mold and tool-making that do not involve exceptional wear exposure. The pre-hardened corrosion-resistant material 1.2316 (X38CrMo16) has proved effective for applications involving corrosive stress (PVC and molding compounds with aggressive fillers). The second group comprises the material 1.2711 (54NiCrMoV6) and some more recent developments generally based on the material 1.2738 (40CrMnNiMo8-6-4), mostly used for plastic molds subject to higher wear (surface coated as appropriate), or for extremely large-scale molds. Hardnesses from 34 to 44 HRC are usually set in this group.

4353.1 Plastic Mold Steels

32

30

28

26

24

22250 500 750 1000 1250 1500

Diameter in mm

Recommended values for surface hardness = 31 HRC

1.2311 1.2738 2738 mod. TS

Core

Har

dnes

s in

HRC

FIGURE 3.6  Comparison of the through-hardenability of pre-hardened plastic mold steels

Plastic mold steels with a conventional material concept are restricted by the physical limits, especially in the case of large dimensions. The inadequate through-hardening in the dimension range above 1,000 mm, and the excessively low annealing tem-perature at hardnesses in excess of 31 HRC, which can lead to inhomogeneities with loss of quality in machining, and to deficits in mold durability, illustrate the problems with using steel 1.2738 (40CrMnNiMo8-6-4) for very large dimensions in mold and tool making. Significant improvements can be achieved by moderate change of the chemical composition. Figure 3.6 shows the hardenability of 2738 mod. TS compared to the conventional mold steels 1.2311 (40CrMnMo7) and 1.2738 (40CrMnNiMo8-6-4). A striking aspect here is that the through-hardenability of material 1.2311 (40CrMnMo7) applies only with small cross-sections. With the classic 1.2738 (40CrMnNiMo8-6-4), the through-hardenable cross-section is also a limited to a diameter of approximately 1,000 mm, whereas through-hardenability for cross-sections up to 1,250 mm can be achieved with 2738 mod. TS. This makes it possible to produce ingots for large molds with consistently high hardness throughout, including the core.

Reducing the carbon content makes more intensive hardening and improved through-hardenability possible, especially in the case of large dimensions. The smaller amount of the alloying elements silicon and chromium serve to reduce the segregation that occurs in the case of large ingot weights, and adding molyb-denum and vanadium as an alloy also ensures appropriate through-hardenability combined with increased tempering resistance, yielding higher hardnesses than is possible in the case of the standard steels 1.2311 (40CrMnMo7) and 1.2738

436 3 Materials for Mold Making

(40CrMnNiMo8-6-4). Figure 3.7 illustrates the merits of the new plastic mold steel 2738 mod. TS, which covers most applications for hardnesses up to 38 HRC, due to its chemical composition and resultant properties, helping to significantly simplify the material concept in the case of pre-hardened plastic mold steels.

3.1.3.2  Through-hardening plastic mold steels

Heat treatment (hardening followed by tempering) a�er machining is a major factor determining the properties of through-hardening steels. Steels of this group can be hardened to a service hardness of 46 to 62 HRC because of the alloy compo-sition. The standard through-hardening steels now used are the materials 1.2343 (X37CrMoV5-1), 1.2379 (X153CrMoV12), 1.2767 (45NiCrMo16) and 1.2842 (90MnCrV8). This hardness level is achieved by martensite formation result-ing from abrupt cooling a�er austeniting, and depends on the alloying element content and on the surface/volume ratio of the mold. Appropriate chromium, molybdenum and/or nickel content permits through-hardening of very large cross-sections. Since there can be a danger of fracture in the case of molds with larger cavity depths, because of their inadequate toughness, these steels are particularly suitable for molds with shallow cavities, where high pressure peaks are likely. One exception is the material 1.2767 (45NiCrMo16), which has good toughness properties because of its high nickel content, and is also suitable for larger cavity

44 HRC

38 HRC

34 HRC

29 HRC

0 400 1000 1250 1400

Dimension in mm

Har

dnes

s

Option 2 + 3 = increased alloy content 2738 mod. TS

1.271 1.2711 1.2711

1.2311 1.2738

Option 1 Option 2

Option 1 Option 2 Option 2

Option 3

1.2738 1.2738

FIGURE 3.7  Recent developments in the field of pre-hardened plastic mold steels can help

simplify material selection over a wide range of dimensions and strengths

4373.1 Plastic Mold Steels

depths with its hardness of 50 to 54 HRC. A further field of application for through-hardening steels is mold inserts for plastic molds for producing molded bodies with inserts or sealing areas, where high edge pressure mostly occurs. Hardening is now generally carried out in vacuum heat treatment units, in order to minimise any possible changes in dimension or shape due to the lattice structure changing during quenching.

3.1.3.3  Corrosion-resistant plastic mold steels

Certain operating conditions (e.g. fire retardants based on chlorine or bromine arising in the processing of plastic molding compounds) can make high demands on the corrosion resistance of plastic mold steels. In many cases, molds can be made from the plastic mold steels mentioned above, and be protected against cor-rosion by means of galvanic hard chromium plating or chemical nickel plating, but there can be difficulties in the case of intricate cavities. That is why corrosion-resistant steels are also frequently used. The grades available are material 1.2083 (X40Cr14), 1.2085 (X33CrS16) and 1.2316 (X38CrMo16). These steels are supplied in both the hardened and tempered state, with a service strength of approximately 30 HRC (1.2085 (X33CrS16), 1.2316 (X38CrMo16), and in the so� annealed state 1.2083 (X40Cr14). In the latter case, a high service hardness of about 50 to 54 HRC is achieved by appropriate heat treatment a�er machining. Modified variants of material 1.2316 (X38CrMo16) have now also become available. Essentially, the chemical composition is modified in order to achieve improved homogeneity of the material by minimising delta ferrite formation. This results in some improved functional properties such as corrosion resistance, polishability, etc compared to the standard variant.

Heat treatment – especially the tempering temperature and thus the nature, quan-tity and distribution of the precipitated carbides – has a decisive influence on the corrosion resistance of plastic mold steels. Fine carbide precipitations that arise in the lower tempering temperature range from 400 °C, and merely serve to relieve the martensite, have no negative effect on corrosion resistance. But if corrosion-resistant steels are tempered in the range of the secondary hardness maximum to achieve adequate hardness and toughness, there is a clear loss of corrosion resistance, as Figure 3.8 attempts to illustrate. The maximum corrosion speed or the maximum loss of mass in a boiling test (standard corrosion test) occurs about 50 °C above the secondary hardness maximum, corresponding to a temperature at which the precipitated carbides start to grow. This extracts the chromium from the matrix surrounding the carbides, so that corrosion can occur in the chromium-deprived areas, and carbides released. At higher tempering temperatures, the original matrix chromium content can be re-established by diffusion processes in the environment of the carbides, permitting improved corrosion resistance.

438 3 Materials for Mold Making

50

40

30

20

100 100 200 300 400 500 600 700

Tempering temperature in ºC

Modified variant material 1.2316

Weig

ht

loss in %

* 1

0-1

1.2083

FIGURE 3.8  Corrosion resistance as a function of tempering temperature in the case of

plastic mold steels 1.2083 and 1.2316 (modified)

Somewhat as in the case of pre-hardened plastic mold steels, resulfurized corrosion resistant steel 1.2085 (X33CrS16) has very good machining properties. But there are corresponding disadvantages as regards polishing and etch-graining proper-ties, and limited corrosion resistance, so this plastic mold steel should preferably be used as a core material for corrosion resistant molds, or as a mold frame for corrosion resistant molds.

Corrosion resistant steels generally have adequate wear resistance. This wear resis-tance can be increased by surface treatments such as nitriding or hard chromium plating. Hard chromium plating has proved effective in practice. Nitriding reduces corrosion resistance, so it cannot be recommended; other corrosion-resistant hard-enable steels with even higher carbon content are advisable for these exceptional cases. Effective wear protection is achieved only by incorporating carbides (hard phases). The type, proportion, size and shape of carbide, and the carbide distribu-tion are of decisive importance, as illustrated in Figure 3.9.

But wear resistance and corrosion resistance are fundamentally contrary mate-rial properties, since in the case of corrosion-resistant steels, adequate corrosion protection is to be anticipated only if the chromium content in the matrix is above 12% by mass. Carbon has a high affinity to chromium and removes it from the matrix to form carbide, so an increase in wear resistance is to be anticipated with increased carbon content, but combined with a reduction in corrosion resistance. This situation is exacerbated by the fact that wear processes and corrosion pro-cesses normally occur together. Under certain circumstances there may therefore be corrosive attack, since abrasive wear removes passive layers, which cannot be regenerated quickly enough. Conversely, corrosion can also create wear particles.

4393.1 Plastic Mold Steels

6

4

2

00 1 2 3 4

Carbide size dc in μm

Wea

r re

sist

ance

Wab

-1 x

10

6

4

2

00 5 10 15 20

Carbide size Vk in Vol.-%

Wea

r re

sist

ance

Wab

-1 x

10

1.2316

1.2083

1.2379

1.2343

FIGURE 3.9  Abrasive wear resistance as a function of carbide size and of the carbide content

of the corrosion-resistant plastic mold steels compared to through-hardening

steels

3.1.3.4  Plastic mold steels for case hardening

Case hardening steels have a relatively low carbon content, and are designed for carburizing followed by hardening. A�er this special heat treatment, case harden-ing steels are characterised by a hard, wear-resistant surface, and a tough core. The gradual transition from the high hardness at the surface (between 58 and 62 HRC) to the relatively so� core is advantageous. In order to achieve this high hardness at the surface, the carbon content of case hardening steels (which have a basic carbon content of 0.15 to 0.25% by mass) is increased during heat treatment by carbon releasing media to about 0.8% by mass in the range from 1 to 3 millimetres. Although case hardening has been one of the most important methods for improving component properties for more than 70 years, it has now almost lost its importance for mold and tool making. Now the materials 1.2162 (21MnCr5) and 1.2764 (X19Ni-CrMo4) are mostly used for special applications. One advantage of case hardening steels is their low strength in the annealed state, making these materials excellent for hobbing, which is still an economic production process for smaller cavities or multiple nest molds, but has now almost completely lost importance. In hobbing, a hard, polished cavity sinking punch is pressed into a so� bottom die. The basic idea is that an outer mold, i.e. the cavity sinking punch, can be produced more simply and more accurately than an inner mold in a bottom die.

The changes in dimension and shape due to heat treatment are considered problem-atic for case-hardened molds and tools, so additional expense in finish machining must be anticipated. Suitable over-measure must be allowed for the anticipated changes in dimension and shape, since otherwise it will no longer be possible to make subsequent local changes to the cavity, due to inadequate case hardening

440 3 Materials for Mold Making

depth of the hardened layer. Figure 3.10 shows a comparison of the achievable core strengths of case-hardening steels. This shows that there is inadequate core strength with material 1.2162 (21MnCr5), even with a small diameter.

3.1.3.5  Precipitation hardening plastic molds steels

Precipitation hardening or maraging plastic mold steels are high-alloy, low-carbon steels. The carbon content is about 0.03% by mass, the nickel content is usually about 18% by mass, and the elements cobalt, molybdenum and titanium together amount to about 15% by mass. As it cools from the austenitic range, tough, ductile martensite is formed, from which the alloying elements are precipitated in finely distributed compounds by subsequent tempering, bracing the microstructure and hardening it by “precipitation hardening”. This group of steels, originally developed as high-tensile steels for applications in aerospace, has proved effective for smaller molds with particularly complicated cavities, and for mold cores subject to extreme bending loads. The material 1.2709 (X3NiCoMoTi18-9-5), which is supplied in the solution-treated state with a hardness of about 30 HRC, and is still relatively ame-nable to machining in this state, is used in plastic mold making. A�er an annealing process at about 490 °C (precipitation hardening), the material reaches a hardness of about 55 HRC. An over-measure must be allowed for machining the molds, to allow for the fact that there is a uniform reduction in volume of about 0.1% due to the hardening process. If the molds are subject to heavy wear, additional nitriding is recommended, since the maraging steels form no carbides, which serve to resist wear. Precipitation hardening can be performed in combination with nitriding, because of the similar process temperatures.

1800

1600

1400

1200

1000

800

6000 100

1.2162 oil 1.2167 oil 1.2167 air

200 300 400 500 600

Diameter in mm

Cor

e st

reng

th in

MP

a

FIGURE 3.10  Core strength of case-hardening steels 1.2162 and 1.2767 as a function of

diameter and quenching process

4413.1 Plastic Mold Steels

3.1.3.6  Nitriding steels

In plastics processing, nitriding steels are used mostly for screw feeders in injection molding machine plasticizing units, and as barrel extruders; they are used in mold and tool making only in special cases, such as molds with very thin webs. Because of their good tempering resistance, they are also suitable for processing thermo-setting plastic and duroplast that has to be released at higher mold temperatures. Some special nitriding steels have been developed, even though most steels can be nitrided if they contain the alloying elements chromium, molybdenum, vanadium and aluminium for forming nitrides. Nitriding involves diffusing nitrogen into the surface layer at temperatures between 350 and 580 °C, so that the nitride forma-tion creates a hard, wear-resistant surface. The aluminium alloy material 1.8550 (34CrAlNi7-10) achieves a surface hardness of about 1000 HV with a core strength of 25 to 31 HRC in the hardened and tempered state. The nitriding layer has to be limited in the case of this steel because of the low toughness of the layer, so the aluminium-free nitriding steels 1.7735 (14CrMoV6-9) and 1.8519 (31CrMoV9) are frequently preferred. Although these steels have lower surface hardness a�er nitrid-ing, greater nitriding depths can be achieved. The core strength in the hardened and tempered state is approximately 29 to 34 HRC. These nitriding steels are moreover characterised by higher toughness and better inclusion ratings. All nitriding steels are generally hardened and tempered for core strength a�er steelmaking.

3.1.4  Concluding comment

Because of the importance of mold materials for the quality of the plastic moldings produced, it is unwise to economize on mold materials, especially where precision parts have to be mass produced with frequent production runs. In addition to select-ing the right material, there are also additional surface finishing processes available to achieve specific wear, corrosion and surface properties. Beside the particular material concept used, the engineering design and subsequent maintenance of the tools and molds also have a major effect on the anticipated service life, the surface quality of the plastic moldings, and the form constancy of the molds.

References

[1] DIN 50150, Converting hardness values[2] Hippenstiel, F., Grimm, W., Lubich, V., Vetter, P.: Handbook of Plastic Mold Steels.

Buderus Edelstahl GmbH (2002) Wetzlar[3] DIN EN ISO 4957, Tool steels

442 3 Materials for Mold Making

 ■ 3.2  Aluminum Alloys

A. Erstling

3.2.1  Introduction

In just 100 years, aluminum has gone from being an expensive rarity to one of the most versatile and most widely used metals. It is one of the most important industrial materials and has secured its place over the past decades in almost every industry. The development of new applications for aluminum and the steady growth of its demand are explained by a number of its properties, which provide decisive advantages for both the designer and user.

The most important advantages of aluminum can be summarized as follows:

� light weight, � high thermal conductivity, � good electrical conductivity, � good corrosion behavior, � suitability for many surface treatment methods, � variety of semifinished part supply, � ease in processing, and � recyclability.

Other characteristics of aluminum and its alloys are:

� nontoxicity and wholesomeness, � non-magnetic behavior, and � high reflectivity of the untreated surface, particularly with regard to thermal radiation.

Due to the diversity of its properties and the ability to combine these properties, aluminum is a versatile material wherever weight savings, protection, stability, corrosion resistance, and durability are required.

The density of aluminum is 2.7 kg/dm3. By adding alloying elements with a higher specific gravity, it may rise to about 2.9 kg/dm3. So the density is, in the worst case, still about 65% lower than that of steel and 70% lower than that of copper alloys.

Depending on the alloy composition, degree of deformation, and heat treatment condition, the thermal conductivity of aluminum alloys is 110 to 220 W/mK. It is thus between the thermal conductivity of steel (15 to 40 W/mK) and copper alloys (210 to 320 W/mK), which are introduced into mold making.

4433.2 Aluminum Alloys

The corrosion resistance of aluminum depends on the formation of a stable oxide film with self-healing effect. It is higher the purer the material and the drier the environment. In mold making, measures to prevent corrosion may need to be taken.

The shaping in the machining process can be done with very high processing speeds, which enable time savings of about 50 to 70% in mold production compared to using steel.

Aluminum alloys are available as cast or wrought materials. For large molds, cast weights of about 5 tons and dimensions of 3,000 × 2500 × 1,000 mm per mold half can be produced. Cast blocks of thicknesses up to 1,000 mm are possible. Wrought alloys are available in the form of extruded round rods or profiles and as rolled or forged plates.

The largest plate thickness is 200 mm in stretched and in compressed or forged/compressed execution in high-strength alloys with thickness of 600 mm.

3.2.2  Mold Materials

Wrought materials are mainly used for molds. At lower stresses, and if very high machining volumes are necessary for the production of molds, the use of molded blooms offers economic benefits. Both wrought and cast alloys can be naturally hardened or cold or warm hardened depending on the required properties.

Aluminum has occupied a very important place in the application of “mold plates”. Where for decades only steel dominated, today, high-strength aluminum grades are mainly used for the production of cutting and punching tools and in mold making.

Characteristic features of aluminum plates are:

1. They have four times better thermal conductivity than steel, thus reducing the cycle times for the processing of rubber and plastics while saving energy.

2. They allow a reduction of machining time compared to steel through multiple cutting speeds. The higher speeds provide better performance in addition to better surfaces.

3. Their low net weight enables an easy handling and transportation of molds and devices made of aluminum, combined with short setup times.

4. They afford low residual stress through controlled stretching processes, and thus manufacturing possibilities of warp-free mold plates. A post-treatment, such as hardening and tempering, is not necessary.

5. They have high strengths, which can benefit the lifetime of the molds and devices. The same plate thickness as in steel can be used in a targeted design due to higher mechanical properties.

444 3 Materials for Mold Making

6. They have good corrosion resistance. Therefore, one can do without expensive surface treatments. However, it is possible to chrome plate, nickel plate, or hard anodize aluminum plates a�er machining (electrical discharge machining [EDM]).

3.2.2.1  Casting Materials

The AlSi-alloy types are optimally castable because of the solidified eutectic at a low temperature of 570 °C (at 12.5% Si in the Al-base compound). They are particularly suitable for castings with large wall thickness variations and thin ribs. The alloy is very so� and tends to smear when machining. The corrosion resistance is described as good, if free of copper. The addition of Mg or Cu to the Al-Si alloys causes harden-ability and thus an increase in strength. The three-component alloys are also easy to cast, can also be better machined because of their higher hardness, and can be well protected by anodic oxidation. The copper-containing alloys if unprotected are more susceptible to corrosion than copper-free versions.AlMg alloys are difficult to cast but are more resistant to hydrogen chloride (HCl) and to salty air and seawater. Their machinability is very good. AlCuTi alloys achieve the highest strength of all casting materials by hardening. They too are difficult to cast; therefore appropriate measures should be taken when designing the mold. Cooperation with the manufacturer in the design stage is recommended.

3.2.2.2  Wrought Materials

The AlMg and AlMgMn types are among the nonhardenable grades. With increasing Mg content, strength properties increase with decreasing toughness. AlMgSi alloys are among the group of hardenable materials. The increase in strength is achieved through precipitation of the MgSi phase. A further increase is possible through Si-surplus or Cu and Cr additives. To improve the machinability, Pb can also be alloyed.AlCuMg alloys can, depending on the composition, either cold harden (at room temperature) or warm harden. Both Cu and Mg contribute to an increase of strength properties.For AlZnMg materials, the combination Zn + Mg is responsible for the hardenability.AlZnMgCu alloys achieve the highest strength properties of all hardenable materi-als. Cu increases the strength and reduces the tension sensitivity, so that Zn + Mg contents are possible with a simultaneous addition of higher Cr.All the alloys are cold and warm hardenable but are preferably treated by artificial ageing. They are among the preferred materials in mold making because of their excellent strength properties and good machinability.

Table 3.5 summarizes the DIN/EN designations, material numbers, international registration numbers, as well as the composition of the aluminum alloys suggested for mold making.

4453.2 Aluminum Alloys

TABL

E 3.

5  Cl

ass

ificati

on a

nd c

om

posi

tion o

f se

lecte

d a

lum

inum

mate

rials

for

mold

makin

g

DIN

Abbr

evia

tions

Mat

eria

l nu

mbe

rIn

tern

atio

nal

Regi

stra

tion

Num

ber

Com

posi

tion

(wei

ght %

)

CuCr

FeM

gM

nSi

TiZn

Othe

rs

AlM

g33.

3535

5754

0.1

0.3

0.4

2.6/

3.6

0.5

0.4

0.15

0.2

AlM

g4.5

Mn

3.35

4750

830.

10.

05/0

.25

0.4

4.0/

4.9

0.4/

1.0

0.4

0.15

0.25

AlM

gSi1

3.32

1560

820.

10.

250.

50.

6/1.

20.

4/1.

00.

7/1.

30.

10.

2

AlCu

Mg1

3.13

2520

17A

3.5/

4.5

0.1

0.7

0.4/

1.0

0.4/

1.0

0.2/

0.8

–0.

25Ti

+ Z

r0.2

5

AlZn

4.5M

g13.

4335

7020

0.2

0.1/

0.35

0.4

1.0/

1.4

0.05

/0.5

0.35

–4.

0/5.

0Ti

+

Zr0.

08/0

.25

AlZn

MgC

u0.5

3.43

4570

220.

5/1.

00.

1/0.

30.

52.

6/3.

70.

4/1.

00.

5 –

4.3/

5.2

Ti +

Zr0

.2

AlZn

MgC

u1.5

3.43

6570

751.

2/2.

00.

18/0

.28

0.5

2.1/

2.9

0.3

0.4

0.2

5.1/

6.1

Ti +

Zr0

.25

G-Al

Si12

3.25

81A4

13.0

0.05

–0.

50.

050.

001/

0.4

10.5

/13.

50.

150.

1

G-Al

Si7M

g3.

2371

356.

00.

05 –

0.18

0.25

/0.4

50.

16.

5/7.

50.

001/

0.20

0.07

G-Al

-Si1

0Mg(

Cu)

3.23

83XX

0.3

–

0.6

0.2/

0.5

0.1/

0.4

9/11

0.15

0.3

Ni0.

1

G-Al

Cu4T

i3.

1841

XX4.

5/5.

2–

0.18

–0.

001/

0.50

0.18

0.15

/0.3

0.07

G-Al

Cu4T

iMg

3.13

7122

4.0

4.2/

4.9

–0.

180.

15/0

.30

0.00

1/0.

500.

180.

15/0

.30

0.07

446 3 Materials for Mold Making

3.2.2.3  Mechanical Properties and Design Guidelines

The mechanical properties of wrought materials are standardized in DIN/EN 485-1-3 and the cast alloys in DIN 1725 Paper 2. A comparison of the most important aluminum materials is shown in Figure 3.11. These materials offer the standard properties, as well as the advantage of constant hardness, strength and grain fine-ness, and a lack of tension across the cross section of larger plate thicknesses (up to 200 mm). Despite these advantages, the designer needs to include the material-specific properties in the design of the mold.

If the mass-related strength Rm is included into the ratio of permissible stress, aluminum material is superior to its steel partners. Weight savings in proportion to the density of steel are possible but practical only if the strength can be calculated. If elastic deformations (bending) or buckling stability have to be taken into account, the modulus of elasticity (70,000 N/mm2) influences the calculation significantly.

Despite the necessary ribbing, weight savings of about 50% over steel are possible.

For molds that are exposed to elevated temperatures, the high thermal expansion coefficient (approximately twice as high as that of steel) has to be considered in the design. This is especially true for combinations with steel.

Al Mg 3,F 24

H24

6-25 mm

Condition description according to EN

Values for material thickness

100

200

300

400

500

600

N/mm2

min. Rp 0,2

min. R m

Typical range

New: DIN/EN 485-2

(previously: DIN 1745)

stiff

ness

Al Mg 4,5 Mn,W 28

O/H 111

6-50 mm

Al Mg Si 1,F 28

T 651

12.5-60 mm

Al Cu Mg 1,F 39

T 451

12.5-40 mm

Al Cu Mg 2,F 44

T 351

6-12.5 mm

Al Zn 4,5,Mg 1 F 35

T 651

6-40 mm

Al Zn Mg,Cu 0,5 F 45

T 651

12.5-40 mm

Al Zn Mg,Cu 1,5 F 53

T 651

25-50 mm

FIGURE 3.11  Mechanical properties of aluminum materials

4473.2 Aluminum Alloys

The development of aluminum alloys has also not stood still. Today there are new design materials with core strengths of nearly 600 N/mm2 such as ALUMOLD® 1-500 (see Tables 3.6 (a) and (b)). This is a high-strength aluminum alloy that is specially designed for high mechanical stresses and applications in plastic processing. It is based on a 50-year experience in the production of alloys for aerospace (e.g., for Airbus and Boeing). The mechanical properties remain almost constant over the entire material thickness up into the core.

TABLE 3.6  Properties of the Alloy ALUMOLD

a) Mechanical values

Thickness in mm

Rm (MPa)

Rp 0.2 (MPa)

A 5.65 (%)

Rm (MPa)

Rp 0.2 (MPa)

A 5.65 (%)

Hard-ness HB

76–125 550 500 4 580 530 6 185126–150 540 490 2.5 570 520 4 185151–-200 525 480 1 555 510 2 180201–250 505 460 1 535 490 1.5 180251–300 470 435 0.5 510 470 1.5 175300–400* 450 370 3.0 520 460 9.0401–450* 440 350 3.0 520 460 9.0

Minimum values Typical factory values

(values according to testing standard ALCAN IS 5614/5505A) * T652F = hot rolled, forged, compressed

b) Physical values

Specific weight 2.9 kg/dm3

Thermal expansion coefficient (0–100 °C) 23.7 · 10–6/KThermal conductivity (0–100 °C) 153 W/m · KSpecific heat (0–100 °C) 857 J/kg · KYoung’s modulus 72,000 N/mm2

Poisson’s ratio 0.33Melting range 457–630 °C

FIGURE 3.12  Mold made from ALUMOLD 1-500 for the production of a tail light

448 3 Materials for Mold Making

FIGURE 3.13  Series mold with 4 cavities made from ALUMOLD 1-500 for the production of a

wiper wheel

The plates are made in a low stress, stretched or compressed, or forged design. Such a considerable freedom in terms of “light, strong, and resistant” cannot be achieved with any other material of comparable size (up to 600 mm thickness). Figure 3.12 shows a mold made from ALUMOLD 1-500 for the production of a tail light made from polycarbonate (PC) with a projected quantity of 1,000. Figure 3.13 shows a series mold with four cavities made from ALUMOLD 1-500 for producing a wiper wheel made from polyoxymethylene (POM) with a projected quantity of 1 million.

3.2.2.4  Corrosion

In air, aluminum is covered with a tight, firmly adhering oxide layer that results in excellent corrosion resistance against many organic and inorganic materials. If the layer is damaged or removed by machining or pickling, for example, a new one occurs spontaneously because a tight oxide layer only 0.001 microns thick forms immediately on the surface in the presence of atmospheric oxygen.

The properties of the oxide protective layer depend on the base material and the layer forming conditions (e.g., composition and humidity of the ambient air). Corrosion resistance increases with the purity of the base material. For alloyed grades, it is influenced by the type and concentration of the alloying elements. The natural oxide layer is even resistant to aggressive gases as long as they are dry. High humidity and extreme temperature changes will cause condensation. Salts and gases dissolve in the water droplets and reduce the pH down to 3 or lower into the acidic range. The resulting acids can therefore destroy the protective layer.

Similar to the different possibilities for corrosion attack of unalloyed and low-alloyed steels, aluminum is also exposed to various types of corrosion. The mold maker and users should therefore pay greater attention to the following types of attacks:

4493.2 Aluminum Alloys

� condensation, standstill corrosion, and acid concentrate corrosion; � corrosion in water-conducting systems (cooling or cooling-water circuit); � contact corrosion (galvanic); and � stress corrosion cracking.

All types of corrosion can be controlled by appropriate measures. The simplest solution is to apply a surface coating by hard anodizing or chemical nickel plating. The condensation of a water/acid concentrate can be prevented by appropriate mold maintenance (air drying, neutralizing with an alkaline detergent solution, etc.). The attack in water-conducting systems can be taken into account by the user through the use of inhibitors (corrosion-reducing additives) and/or the installation of closed cooling water circuits with oxygen-poor water.

Mold makers should avoid material combinations of aluminum and copper alloys with a mutual temperature control circuit, unless the metallic cooling bore holes can be protected (e.g., by chemical nickel). Contact corrosion, which can develop by physical contact of aluminum and nobler metals (e.g., copper), can be prevented by electrically insulating separating agents or adhesive.

Practice has shown that combinations of aluminum and stainless Cr or CrNi steels and coated steel sheets (zinc, tin, chemical nickel, and organic coating) are unproblematic.

3.2.2.5  Friction and Wear Resistance

The relatively low wear resistance of aluminum can be adjusted by an appropriate surface treatment so that a sufficient service life can be achieved. Hard anodizing, chemical nickel plating, chrome plating, and special chemical coatings to facilitate molding have proven themselves. Surface coatings and gold TiN coatings can be applied to aluminum as for the machining tools.

For the protection of aluminum parts, a wide range of surface layers stand ready. Price and applicability strongly depend on the application. PVD coatings are less expensive than ion implantation layers but set strict limits on part dimensions and batch sizes. Electroplated hard chromium layers can be used for corrosion protec-tion in combination with other layer types.

Hard anodized layers are partially applied or on all sides to serve both the corrosion and the wear protection.

Hard anodizationHard anodizing is suitable for aluminum alloys. It enables the production of extremely hard surfaces (about 350 HV). In contrast to the coatings, the anodic oxide layer is formed from the metal itself (which is why there are no adhesion problems).

450 3 Materials for Mold Making

The thickness of such layers can reach 120 microns; common layer thicknesses are 50 to 60 microns.

The anodizing layer can be impregnated with lubricants (Polytetrafluoroethylene [PTFE], graphite, MoS2) to significantly reduce the friction coefficient (Table 3.7).

The corresponding values for ALUMOLD are about the same, especially a�er hard-anodizing, which is performed on the finished mold. It must be considered that the hard anodized layer grows to about 50% of its thickness beyond the initial surface and about 50% into the metal. In the development of such molds, specialists for hard anodizing should be consulted.

Other coatingsAluminum and its alloys are suitable, like steel and other metals, for a variety of coatings such as chromium and nickel layers. This can be applied by various methods, but the effects may be different on the base material:

� Electrolytic application at room temperature typically does not change the mechanical properties of aluminum alloys but always requires some precautions: � The thickness of the coating is not always identical on an uneven surface (as is the case in a mold). Tight tolerances on plastic parts must be considered.

� Copper plating is generally required before chromium plating: if a crack is running through the coating, the risk of corrosion is a given.

� Electrolytic nickel layers can be simultaneously applied with other materials (e.g., PTFE, SiC), so that the layers have specific characteristics (low friction coefficient with PTFE, high wear resistance with SiC).

� Coatings that are applied in a vacuum (PVD): All metals and a variety of com-binations (TiN, WC, etc.) may be applied according to this method. However, the temperature and duration of treatment can adversely affect the mechanical properties of aluminum alloys.

TABLE 3.7  Friction Coefficients of Different Surface Finishes (Source: ETCA, headquarters,

Technical Investigation Department of the Ministry of Defense)

Material + surface treatment Friction coefficient7075 blank 0.53

7075 + Hard anodized 0.37

7075 + Hard anodized + PTFE 0.21

7075 + Hard anodized + MoS2 0.18

7075 + Hard anodized + Graphite 0.15

4513.2 Aluminum Alloys

3.2.3  Manufacture of Aluminum Molds

3.2.3.1  Abrasive Procedures

3.2.3.1.1  MachiningDespite its relatively high strength, aluminum can be machined at high cutting speeds. As already stated, aluminum allows feed rates of up to five times that of steel, at minimal tool wear. The complete and timely utilization of machining advantages is possible if appropriate tools tailored to the aluminum are used (Table 3.8). These are sharply ground tools with cutting edges made from high speed steel or hard metal of the milling user’s group K 20 and K 10, in accordance with DIN 4990. To avoid the formation of aluminum abrasion on the cutting surfaces, which can be welded with the passing chips (built-up edge), the cutting surfaces of the tools should be precision machined (fine ground, polished, or lapped). Mineral oil emul-sions, which are also used for aluminum machining, can be used as a coolant. In continuous processing, spray lubrication using synthetic lubricants dissolved in water is recommended. As an alternative, there are now also plant-based biological industrial lubricants. These environmentally friendly, ecotoxicologically harmless and rapidly biodegradable products allow for high efficiency, lower disposal costs, and excellent work place hygiene. A new technology is high-speed machining, HSC technology (Table 3.9; see also Section 4.1). When milling, a reduction of cycle times by a factor of 10 in the mechanical parts manufacture of small series is achieved in prototype manufacturing as well as in die and mold making. To fully exploit these advantages, the concept of the high speed milling machine must be fully adapted to the needs of the new technology. The HSC technology offers a clear rationalization potential for the die and mold maker: electrodes, prototypes, or foaming molds can almost always be completely processed.

Even with injection and die casting molds, the processing times can be shortened, although EDM cannot be completely eliminated.

The main objectives are:

� to achieve short processing time, � to eliminate rework, � to reduce the use of lubricant (dry processing/minimal lubrication), and � to obtain time savings and improved efficiency.

Especially for aluminum, the performance potential of the high-speed technology alloys can be fully utilized.

452 3 Materials for Mold Making

TABL

E 3.

8  St

andard

valu

es

for

cutt

ing g

eom

etr

y and c

utt

ing d

ata

Cut s

izeTu

rnin

gM

illin

gDr

illin

gSw

ing

(c

ircul

ar s

aw)

Cutt

ing

Mat

eria

l

SSHM

SSHM

SSHM

SSHM

Mill

ing

Angl

e �

2)10

to 7

10 to

810

to 6

10 to

812

01)12

01)8

9 to

7

Cutti

ng A

ngle

�2)

40 to

30

24 to

10

25 to

20

20 to

15

35 to

202)

15 to

102)

258

3)�

��

���

���

���

Cutti

ng S

peed

m/m

in10

0 to

20

020

0 to

50

015

0 to

40

025

0 to

70

015

0 to

30

025

0 to

60

030

0 to

80

050

0 to

10

0080

to

100

100

to

140

300

to

500

to 1

500

Feed

Rat

e s

mm

/U o

r mm

/atm

0.2

to

0.5

0.05

to

0.25

0.3

to

0.6

0.05

to

0.1

0.1

to

0.5

0.03

to

0.1

0.1

to

0.6

0.03

to

0.1

0.02

to

0.50

0.06

to

0.30

to 0

.02

to 0

.03

Cutti

ng D

epth

a

mm

to 5

to 0

.5to

5to

0.5

to 6

to 0

.5to

7to

0.5

1) Rel

ief A

ngle

; 2)

Hel

ix A

ngle

; 3)

� =

Rou

ghin

g; ��

= S

moo

thin

g or

Pre

cisi

on M

achi

ning

4533.2 Aluminum Alloys

TABL

E 3.

9  H

SC

Machin

ing

Mat

eria

l

�W

ork

piec

e

�Se

mifi

nish

ed P

art

�

Blan

k

Mac

hini

ng O

pera

tion

Spee

d (1

/min

)M

old �

(m

m)

Cutt

ing

spee

d (m

/min

)Pr

oces

s ty

pe

�Co

nven

tiona

l (co

nv.)

�

High

spe

ed (H

SC)

Cast

St e

el

GG 2

5Bl

ank

cast

ings

Groo

ve m

illin

gDe

burr

ing

24,

000

20,

000

12

6

905

377

HSC

conv

.

Alum

inum

Plat

esGe

nera

ting

a fla

t sur

face

6

,000

160

3016

HSC

case

1

Die-

cast

ing

Profi

le m

illin

g of

oil

groo

ves

Mill

ing

of A

l-wro

ught

allo

ysDe

burr

ing

45,

000

36,

000

30,

000

3

.5 4

0 1

2

495

4524

1131

Conv

.HS

CHS

C

Shee

tsCo

ntou

r mill

ing

Groo

ve m

illin

g 6

0,00

0 6

0,00

0 2

0

437

70 7

54HS

C ca

se 2

Conv

. cas

e 3

Plat

es e

.g.,

airc

ra

inte

gral

com

pone

nts

Mill

ing

of th

in-w

alle

d ba

rs 2

4,00

0 5

037

70HS

C

Extru

ded

Profi

les,

rails

Groo

ve m

illin

g 3

5,00

0

8 8

80Co

nv.

Plas

ticPl

ates

Spec

tacl

e fra

me

Groo

ve m

illin

gCo

ntou

r mill

ing

80,

000

20,

000

2

.4

8 6

03 5

03Co

nv.

Conv

.

Com

posi

te

Mat

eria

lsCi

rcui

t boa

rds

Drill

ing

100,

000

0

.3

94

Conv

.

Fibe

r-rei

nfor

ced

com

posi

te m

ater

ials

Groo

ve m

illin

g 4

0,00

0

5 6

28Co

nv.

Glas

s fib

er-re

info

rced

pla

stic

Groo

ve m

illin

g 6

0,00

0

815

08

Non-

Ferr

ous

Mat

eria

lsGr

aphi

teGr

oove

mill

ing

54,

000

6

1017

Coop

er s

heet

s (C

uSn6

)Cu

t mill

ing

40,

000

30

3770

HSC

454 3 Materials for Mold Making

3.2.3.1.2  GrindingThe disadvantage of the nonmagnetic material during grinding can be compensated for by a trick: the aluminum panels are framed with steel bars and mounted on the magnetic table; then plenty of flushing is done. Alternatively, vacuum clamping systems can be used.

3.2.3.1.3  Electrical Discharge Machining (EDM) or Wire EDMThe application of EDM is mostly known for steel processing only. But aluminum can easily be processed with EDM with even higher (for the most part) removal rates compared to steel. Furthermore, the so-called “white layer” (which is extremely hard for steel) does not form, so that any necessary polishing can be reduced to a minimum. The EDM technologies that are mainly applied in practice are divided into two methods; the planetary-EDM (P-EDM process) and the wire EDM (electrical discharge wire cutting or EDC process).

In this case, the following adjustment rules apply for the pulse generator: adjust pulse power and ignition, depending on the electrode surface and the desired surface finish as for the erosion of steel; however, the pulse duration should be reduced by one notch or the pause duration should be increased by one notch.

The performance increases are astounding. In roughing, it was six to eight times faster, in smoothing, three to five times, and in fine finishing at least twice as fast as in steel processing. The subsequent polishing can typically be reduced to about one-third the time required for polishing steel.

3.2.3.1.4  EtchingDeep etching allows different aesthetic surfaces to be obtained. To achieve an attrac-tive chemical engraving, good structural homogeneity and uniform mechanical properties are required. FORTAL® 7075 and ALUMOLD® can be etched very well due to their good mechanical properties.

3.2.3.2  Welding

Welding processes are predominantly used in mold making to prepare mold cor-rections. Generally, aluminum and aluminum alloys can be welded with all known welding processes. For the applicability of each process in practice, there are, as for all metals, differences that are due to alloying elements, which concern specific physical properties and the efficiency.

According to DIN 8593, welding is part of the “bonding process by material com-bination” and the welding process itself has been defined according to DIN 1910. The term “weldability” is described in DIN 8528 Part 1. It can be very different for a certain material composition in a particular manufacturing or structural state (casting, wrought semifinished part, annealed, cold-hardened, and cured) for the

4553.2 Aluminum Alloys

TABL

E 3.

10  O

verv

iew

and e

valu

ati

on o

f th

e w

eld

ing p

rocess

es

applic

able

for

alu

min

um

Wel

ding

pro

cess

Wro

ught

sem

ifini

shed

par

t m

ade

from

alu

min

um a

nd a

lum

inum

allo

ysCa

stin

gs fr

om th

e al

loys

of

the

type

Al99

,98R

bi

s Al

99Al

Mn

AlM

nMg

AlM

gAl

MgM

nAl

MgS

iAl

MgS

iPb

AlCu

Mg

AlCu

MgP

bAl

ZnM

gAl

ZnM

g-Cu

G-Al

SiG-

AlSi

-Mg

G-Al

SiCu

G-Al

Mg

G-Al

MgS

iG-

AlCu

TiG-

AlZn

Mg

(not

DIN

)

AB

AB

AB

AB

AB

AB

AB

AB

AB

AB

AB

AB

AB

AB

Gas

Wel

ding

22

22

33

–2

–4

–3

–4

44

44

44

44

44

–4

–4

Met

al a

rc W

eldi

ng–

4–

4–

4–

4–

4–

4–

4–

3–

3–

3–

4–

4–

4–

4

WIG

Wel

ding

11

11

11

–1

–4

–2

–4

11

11

22

11

11

–3

–1

MIG

Wel

ding

–1

–1

–1

–1

–4

–1

–4

–1

–1

–2

–1

–1

–3

–1

Plas

ma

Wel

ding

11

01

11

–1

–4

–2

–4

01

01

02

01

01

03

01

Lase

r Wel

ding

11

01

11

11

04

02

04

01

01

02

01

01

03

01

A =

With

out w

eldi

ng fi

ller m

ater

ial

B =

With

wel

ding

fille

r mat

eria

l ¹) E

valu

atio

n fo

r fav

orab

le m

ater

ial t

hick

ness

dep

endi

ng o

n th

e pr

oces

s:

1

= be

st, 4

= w

orst

pos

sibl

e ev

alua

tion

–

= no

t pos

sibl

e du

e to

mat

eria

l or p

roce

ss

0

= Ca

uses

are

stil

l unk

now

n

Valu

es in

bra

cket

s m

ean

that

the

proc

ess

was

test

ed in

the

labo

rato

ry, b

ut w

as n

ot p

ut in

to p

ract

ical

use

.

456 3 Materials for Mold Making

different welding processes. This is especially true for the welding of aluminum with other metals. An overview of the welding processes suitable for aluminum and aluminum alloys for welding wrought semifinished parts or castings from alloys of different types is shown in Table 3.10. The evaluation is here only carried out in terms of feasibility. It is intended to be a guide only and applies to the most favor-able range of thickness and initial state. The variants of the various procedures and the weldable material thickness are referred to in the treatment of the method.

In comparison to steel, a closed, dense oxide layer that encloses the molten metal like a skin forms in aluminum during melting in air. It prevents a bond with the base material. Welding with shielding gas (metal or tungsten inert gas welding) causes the existing oxide layer to break, and the protective gas layer prevents a new layer from forming. In gas or arc welding, fluxes can be used. In gas welding, fluxes are applied by brushing the seam flanks. In arc welding, they are in the electrode shell.

One problem with welding the warm hardening alloys used in mold making is the possible decrease in strength and higher weld crack susceptibility. A drop in strength at different levels is caused by the annealing or uniformity effect in cured alloys. If a self-hardening in the heat-influenced zone does not occur, as in the case of AlZnMg, a reduction of hardness, tensile strength, and yield strength up to values of the cast state occurs.

Laser-beam weldingWith the availability of sophisticated high-power lasers – which are now offered with high beam quality on an industrial scale with output power up to 4 kW for neodymium-doped yttrium aluminum garnet (Nd:YAG) laser and up to 40 kW for the CO2 laser – a great potential for applications opens up for different welding and cutting tasks. The welding of aluminum alloys by laser radiation has become increasingly important in recent years even though it requires that clamping and positioning devices be much more accurate (about a factor of 5 to 10) compared to conventional welding processes. Nevertheless, the process-specific benefits include:

� high-speed machining, � local energy input, � high precision machining, � noncontact and thus force-free machining, � low heat load on the work piece, � high flexibility regarding the machining geometry and choice of material, and � good automation.

The laser beam offers the advantage of precise machining, even in places or on materials that cannot be processed with the conventional welding methods.

4573.2 Aluminum Alloys

In addition, no or only minimal warpage occurs during laser welding due to the low heat input. It offers nearly nonwarping welding through very rapid cooling (in some cases ≤ 1 second), and the heat-affected zone is very narrow. These condi-tions result in a problem-free welding process upto creating the desired change in properties of the material surface.

The laser beam is successfully used in the following processing variants:

� Deposit welding: repairs, alterations, corrections, fonts, DIN symbol, changing the date stamp, and impacts (mini-holes) in polished molds.

� Butt welding: inserts (with chamfer on the work piece and on the insert in order to achieve greater penetration depths). Especially for high-strength aluminum alloys of the 7XXX-series, this method has proven itself in practice.

3.2.3.3  Casting

Aluminum castings can be produced with all conventional casting processes. For mold making, only sand casting and gravity casting (under reservation) can be used because of the relatively low quantities required.Sand casting is particularly well suited for the production of individual molds of very large casting weights but also in shaping machines for medium casting sizes and series. The casting mold is made according to an inexpensive model made from foundry sand. Mold corrections are possible with little effort; undercuts also do not have technical demands.Gravity casting requires larger quantities, mainly because of high mold costs. Casting is done in a permanent mold made from cast iron or steel. A distinction is made between the conventional gravity casting and low pressure gravity casting. In the latter case, the melt is pressed from below into the ingot mold at low pressure. The advantage is a good mold filling, a better structure, and thus good mechani-cal properties of the casting. However, the investment cost of casting machines is relatively high.The casting properties of aluminum melts are characterized by the alloy-typical solidification process, the flow and mold filling ability, the tendency to form voids, and the back feed behavior and hot cracking tendency. Surface oxidation and hydro-gen absorption affect the purity and the porosity of the casting. Different cleaning methods can be used to make an improvement. These include gas flushing with inert or chemically active gases, melt filtration, vacuum treatments, or the use of fluxes and cover fluxes.For sand castings, models made from wood, plastic, and gypsum can be used. For individual castings, polystyrene models have proven themselves due to their easy processing and inexpensive manufacturability. A shrinkage coefficient of 1 to 1.4% has to be taken into account. A casting basin with inserted slag barrier (indirect,

458 3 Materials for Mold Making

uphill casting) is conveniently provided at the sprue cup. Gates and feeders, which are distributed as evenly as possible, allow void-free castings. Chill castings on material accumulation points also cause structural improvements.

The casting molds are made either by hand (better degassing due to lower tapped density) or machine molds. The mold machines facilitate the hard work of the mold manufacturer. They allow constant conditions in the molding sand and thus contribute to the quality assurance. It is useful if caster and mold maker jointly determine the optimal mold design. A reference to casting models and accessories can be found in DIN 1511. A more detailed explanation of the casting should be eliminated because of its limited importance in mold making.

3.2.4  Applications

New techniques now offer the ability to go more quickly from the design, to mold making, to the finished plastic part. The industry increasingly requires producing metal prototype molds faster and cheaper. However, in order to rapidly transform prototypes into products available on the market, an acceleration of the subsequent process chain is necessary. HSC technology can make an important contribution. This technology is particularly suitable for high-strength aluminum alloys, which can be excellently machined. For a variety of different alloys for the particular application, the optimum material is selected in the following areas:

� punching, forming, and progressive composite molds; � pilot series molds, prototype molds; � blow molds, foam molds; � pre- and small-series molds; and � series molds of up to 1 million parts, depending on the plastic material (possibly in coated surface finish).

Figure 3.14 shows the two mold halves of a slide mold made from ALUMOLD 1-500. Here, a children’s go-kart mudguard made from polypropylene (PP) in a quan-tity of 1,000 is manufactured. Figure 3.15 illustrates a mold, also made from ALUMOLD 1-500, together with the part manufactured with it. (Other molds made from aluminum to hold the spreading disk made from PC are also included in the complete finished part.) The mold is high-gloss polished and designed for a minimum quantity of 1,000.

Aluminum is thus the most important material in mold making a�er steel to meet the requirements of faster availability of molds and plastic parts on the market, and not at least because, once produced from raw materials, aluminum is available for generations due to its recyclability.

4593.2 Aluminum Alloys

FIGURE 3.14  Slide mold made from ALUMOLD 1-500 for the production of a mudguard;

Le� lower bottom

FIGURE 3.15  Mold made from ALUMOLD 1-500

and finished product that was

manufactured using the mold

References

[1] Mennig, G. (Ed.) Werkzeuge für die Kunststoffverarbeitung (1995) Hanser, Munich[2] Pechiney, Aluminium für Spritzgusswerkzeuge, 1st ed. (1997) Rhenalu[3] DIN-Taschenbuch 450 – Aluminium 1, 2nd ed. (2005) Beuth Verlag[4] Erstling, A., Einsatz von Aluminium im Werkzeug- und Formenbau, various jour-

nals. 3rd ed. (2001) Technologischer Leitfaden of ALMETamb[5] Aluminium Taschenbuch, 15th ed. (1997) Aluminium-Verlag[6] Schweißen von Aluminium (1997) Aluminium Zentrale

460 3 Materials for Mold Making

 ■ 3.3  Copper Alloys-Nonferrous Metals

E. Seufert

Although steel and aluminum alloys are still the majority of the materials used in plastic mold making, the use of copper-based alloys is steadily on the rise. Due to the economic pressure to reduce cycle times to a minimum, the use of copper alloys with their high thermal conductivities is o�en unavoidable. By using modern tooling technologies, the existing concerns about the machinability of copper alloys are also o�en resolved. In most cases today, the manufacture of a casting blanks can be avoided (usually CuCoBe/CuBe2/CuBe2.7), and the molded part can be processed directly from a thermoformed semifinished part.

In currently used copper alloys, a distinction between two large groups should be made. On the one hand, there are the “bronze” alloys. These are mainly copper alloys with aluminum and tin as the main alloying additives. On the other hand, there are the so-called “low-alloyed copper alloys”, which are usually alloyed with Ni, Cr, and Be at levels of less than 2%.

The bronzes have a much lower thermal conductivity in comparison to the most commonly used copper alloys (CuCoNiBe/CuBe/CuNiSiCr); however, they also have excellent sliding properties, which make them an excellent material both for guide elements as well as for forming mold inserts. Low alloyed copper alloys are always the first choice when it comes to using a material with a high thermal conductivity and good mechanical properties.

3.3.1  Properties

In modern mold making, copper alloys have become indispensable because of their special properties and the many applications that result. Particular attention is paid to the criteria in the following sections in the search for solutions.

3.3.1.1  Strength Properties

The alloys that are mainly used in mold making are mostly precipitation hardening alloys, which means that the ultimate strength (hardness) can be largely influenced by the targeted heat treatment (Table 3.11). The solution-annealed and thus so� material condition, which was mainly used in the past because of better machining properties, has become less important. Semifinished products are mostly delivered in the hardened state and can be machined very well with current tooling technology.

4613.3 Copper Alloys-Nonferrous Metals

They make the subsequent hardening from the mold maker superfluous because it o�en did not lead to the desired results (nonuniform distribution of hardness/warpage of the components). Castings (e.g., made from CuBe2.7 or CuCoNiBe) are at least solution annealed in the foundry or hardened at the request of the customer.The highest strengths are achieved in CuBe alloys. Here, tensile strengths of 1300 N/mm2 (hardness of about 390 HB), with a simultaneous elongation (A5) > 3% of the material CuBe2 can be securely realized. This corresponds well to the strength of a 1.2343 hot work mold steel. The lower compressive strength (com-pared to steel) of copper alloys should be considered. It is advisable to work with Cu-mold inserts for the steel frame or to install steel bars at critical points where large clamping forces must be absorbed.Strengths of this magnitude cannot be reached with beryllium free alloys. Strengths of up to 650 N/mm2 with an elongation (A5) > 10% make these materials appealing for less highly stressed parts because of the lower price.Bronzes with a wide range of different alloys, which are standardized and available on the market, cover a very wide range of strengths. In the selection, a compromise between strength, desired sliding properties, and workability should usually be found. One of the most o�en used aluminum multi bronzes in mold making is CuAl-10Ni5Fe4. It combines a strength of 680 N/mm2 with a very low friction coefficient and is well erodible compared to other copper alloys. Due to the excellent sliding properties, which virtually do not exist for low alloyed copper alloys, as well as the higher thermal conductivity, which is at least three times higher than that of steel, the aluminum bronzes are both used especially as highly loaded guide elements (valve guides/guide bushings) as well as for mold inserts.Besides the above mentioned alloys, there are many other alloys, particularly in the bronze range that can attain very different mechanical properties by varying the alloying elements.

TABLE 3.11  Mechanical properties of commonly used copper alloys

Material Description CuBe2.7 cast

CuBe2 CuCoNiBe CuNiSiCr CuAl10Ni5Fe4

Composition in Weight-% Be 2.7Co 0.5Cu residual

Be 1.9Co + Ni 0.3Cu residual

Co 1.0Ni 1.0Be 0.5Fe < 0.2Cu residual

Ni 2.5Cr 0.4Si 0.7Cu residual

Al 10Ni 5Fe 4Mn < 1.0Cu residual

Density in g/cm3 8.1 8.3 8.9 8.8 7.6Module of Elasticity N/mm2 133,000 135, 000 135,000 140,000 118,000Tensile Strength in N/mm2 1,150 1,250 800 700 680Elongation at Break (A5) in % 1 5 10 12 10Elongation Limit in N/mm2 1,000 1,100 620 570 320Hardness in HB 420 400 250 220 170 to 220

462 3 Materials for Mold Making

However, in mold making, it should be considered that the information on strengths relate to the standard temperature of 20 °C, which is irrelevant for use in plastic injection molds. The strength of the chosen material at ambient temperature is crucial to the mold maker. Here, attention should be paid to the data from the respective material manufacturer.

3.3.1.2  Thermal Properties

The remarkable thermal properties of these materials, such as good thermal con-ductivity, high heat penetration, and high thermal shock resistance, should be particularly mentioned here. From an economic perspective, cycle time reduction is the main argument to justify the use of a mold material that is more expensive than steel. Due to the low thermal conductivity of steel, in many molds with short cycle times, a further optimization by complex cooling channels and the use of large temperature control units is no longer possible. Further efforts quickly lead to very different mold wall temperatures, which are subject to quality problems (sink marks or partial overheating). A design that includes the use of copper alloys (see Figure 3.16) at critical points meets the following different requirements:

� cycle time reduction by up to 30% and thus increased output, � qualitatively perfect products through uniform wall temperature, and � constructively simpler design because conformal cooling channels can be avoided.

Figure 3.16 shows a mold with six cavities for the production of screw caps in which the above mentioned aspects have been consistently implemented.

In addition to the thermal conductivity, the heat transfer of the compound into the material of the mold wall is an important factor. The heat penetration is determined by the thermal conductivity, density, and specific heat capacity. Table 3.12 shows the values of various commonly used mold materials. BeCu alloys show a very good

FIGURE 3.16  Mold with six cavities

4633.3 Copper Alloys-Nonferrous Metals

combination of thermal conductivity and heat penetration. For a complete overview of heat transfer and heat transportation, coatings must also be applied to the mold. Here, the commonly used nickel coatings should be particularly mentioned because their heat penetration ability is much lower than that of copper-based material.

A third outstanding and very important thermal property of copper materials is their thermal shock resistance, especially in molds that work with very high melt temperatures (e.g., polyether ether ketone [PEEK] or polyamide-imides [PAI] up to about 360 °C). The higher this ability, the more “load cycles” of the internal stresses caused by the temperature difference between adjacent areas within a component can be withstood without any damages. This ability of a material to tolerate the rapid switch between heating and cooling is expressed through the R-factor. As shown in Table 3.13, the CuBe alloys are significantly more resistant (compared to steel) to thermal interactions. This resistance results in a longer service life because no hot cracks occur that would lead to failure of the component.

3.3.2  Processablity

Thanks to new mold and machine technologies, processing of the relevant copper alloys for mold making is mostly problem free today. First, the machining process with geometrically defined cutting edge is mostly done with carbide or ceramic cutting edges that cover a wide range of properties with a variety of substrates and coatings. On the other hand, optimized machining is mostly possible through the introduction of computer-aided design and manufacturing (CAD/CAM). In particular, the optimum cut distribution by CAD/CAM systems and the full cooling in com-pletely enclosed computer numerical control (CNC) machines have to be mentioned.

TABLE 3.12  Thermal Properties of Metallic Materials

Material Thermal Conductivity [W/mK] Heat Penetration Ability [Ws/m2 K]CuBe2 160 19,800CuCoNiBe 250 27,700Unalloyed Steel 30 14,000X12CrNi18 8 15 7,700AlZnMg Cu 140 20,100

TABLE 3.13  R-factor of Chosen Metallic Materials

Material R-FactorCuBe2 14,600CuCoNiBe 12,100Unalloyed Steel 7,500X12CrNi18 8 1,500AlZnMgCu 14,250

464 3 Materials for Mold Making

These facts, as well as the tremendous advancement in the field of wire and sinker EDM, are the main reason why machining is done from previously heat-treated semifinished parts due to the usually small quantities in mold making. In addition, semifinished parts are usually more quickly available than the castings previously used, which is a performance-related factor in the strongly reduced cycle times of the molds.

As diverse as the properties of various copper alloys may be, there are some prin-ciples that should be considered in any machining operations:

� One should use full-cooling with an increased percentage of lubricant in the emulsion (about 7 to 8%).

� Sharp tools should be used. � If possible, machining should be done as evenly as possible on all sides to avoid warpage.

The values given in Tables 3.14 to 3.17 for cutting speed and feed rate are average values for stable machining conditions. Of course, these are to be adjusted to the mold geometry for existing individual cases as well as to the machine.

3.3.2.1  Turning

Due to the wide range of copper alloys, which are also recognizable by the strength values, different tools and cutting parameters should be used. If possible, especially when cutting off the solid material, a small core may be le� standing and can be broken off by hand. If the use of HSSE tools is unavoidable, the specified values in Table 3.14 should be greatly reduced and a uniform cooling has to be ensured. It may be necessary to achieve chip braking through feeding interruptions, especially in the less stable alloys and in internal machining.

TABLE 3.14  Reference values for turning copper alloys

CuNiSiCr; CuCoNiBe CuAl10Ni5Fe4; CuBe2; CuBe2.7

Roughing HM: ISO P10–P30; K10–K25Vc: 150–300 m/minf: 0.15–0.4 mmGeometry with protective chamber and chip-breaker

HM: ISO P10–P30; K10–K25Vc: 120–250 m/minf: 0.1–0.4 mmGeometry with protective chamber

Smoothing HM: ISO P10–P30; K10–K25Vc: 100–250 m/minf: 0.08–0.25 mmPositive geometry with chip-breaker

HM: ISO P10–P30; K10–K25Vc: 100–220 m/minf: 0.08–0.2 mmPositive geometry with chip-breaker

Vc: Cutting speed f: Feed rate per revolution HM: Carbide grade

4653.3 Copper Alloys-Nonferrous Metals

3.3.2.2  Milling

In milling, the use of the milling base bodies and cutting plates is recommended (Table 3.15). Because there are usually multiple cutting plates for one base body, both an optimum machining and a reduced tool inventory are available. For opera-tions with sha� or ball milling, a solid milling carbide tool is preferred over a HSSE tool. For milling tools made from HSSE (e.g., form cutter), the given values have to be greatly reduced. Usually there are reference values for different materials from the manufacturer. If there are no values for copper alloys, the tensile strength value should be used for a comparative determination of cutting values. For aluminum bronze alloys with a higher strength, care must be taken that milling is done from the edge into the solid, as this group of materials have a tendency for edge chipping.

TABLE 3.15  Reference values for milling copper alloys

CuNiSiCr; CuCoNiBe CuAl10Ni5Fe4; CuBe2; CuBe2.7Face Milling HM: ISO M30–M40; K30–K40

Vc: 150–180 m/minfz: 0.08–0.15 mmGeometry with protective chamberCooling with emulsion or dry

HM: ISO M30–M40; K30–K40Vc: 140–190 m/minfz: 0.08–0.15 mmGeometry with protective chamberCooling with emulsion or dry

Side Milling HM: ISO M30–M40; K30–K40Vc: 200–240 m/minfz: 0.08–0.12 mmGeometry with protective chamberCooling with emulsion or dry

HM: ISO M30–M40; K30–K40Vc: 160–220 m/minfz: 0.08–0.10 mmGeometry with protective chamberCooling with emulsion or dry

Vc: Cutting speed fz: Feed per tooth

3.3.2.3  Drilling

In modern production, the importance of HSS twist drills has declined because all accessible holes are simultaneously drilled during the milling process on CNC machining centers. The same conditions as those for milling apply (Table 3.16) for the cutting inserts or drill bits used in machining centers.

TABLE 3.16  Reference values for drilling copper alloys

Drill � [mm] CuNiSiCr; CuCoNiBe CuAl10Ni5Fe4; CuBe2; CuBe2.73–6 Vc: 50–100 m/min

f: 0.05–0.1Vc: 30–75 m/minf: 0.03–0.08

6–12 Vc: 50–100 m/minf: 0.1–0.2

Vc: 30–75 m/minf: 0.08–0.15

12–25 Vc: 50–100 m/minf: 0.15–0.30

Vc: 30–60 m/minf: 0.1–0.25

Vc: Cutting speed fz: Feed rate per revolution

466 3 Materials for Mold Making

As far as it is possible, especially in deep bore holes (t > 5 · D), high-pressure internal mold cooling and feed interruptions, similar to turning, are used. Short chips and the high pressure of the cooling fluid facilitate flushing of the bore hole and thus prevent the “seizing” on the drill base body.

For drilling depths above t = 10 · D, as it is o�en the case in cooling channels, very sharp HSS drills (DIN 1869) must be used. To avoid jamming of the drill, it is advantageous that the drill tip is ground 0.1 to 0.3 mm off center for deep bore holes that are not subject to specific tolerances. The application of a cooling water emulsion, with increased concentrate proportion if possible, is advisable.

As with milling, drilling passage holes from both sides is recommended to prevent edge chipping in the aluminum bronzes.

3.3.2.4  Threading

Threading can be done using HSSE taps. Sharp tools with coating (TiCN) and increased oil content to about 8% of the cooling emulsion are recommended. Since the copper materials tend to get the tap jammed, especially when unscrewing the tool, it is recommended to drill the core bore holes 0.1 to 0.25 mm larger, depend-ing on the thread size.

By reducing the required torque, taps with exposed teeth can achieve good results. Thread forming, which can be used in so�er copper alloys, are hardly suitable for the high-strength alloys used in mold making. For the manufacture of threads on suit-able CNC machines, a circular milling in the synchronous method is recommended.

3.3.2.5  Reaming

Spiral-toothed HSSE reamers generally provide good results. An even number of teeth with uneven angular distribution, coupled with a tool shank that is optimized to incorporate hydraulic chucks or CNC high precision chucks allows so� cutting and prevents rattling.

For serial production, spiral-toothed solid carbide or single-blade reamers with guide rails are recommended. For cutting speeds and feeds, the information from the manufacturer should be considered.

TABLE 3.17  Reference Values for Reaming Copper Alloys

� in mm Vc in m/min Feed rate mm/U Allowance mm/� to 10 10–15 0.2–0.5 0.10–0.20

10–15 10–15 0.4–0.6 0.15–0.25

15–25 10–15 0.5–1.0 0.20–0.30

25–30 10–15 0.6–1.1 0.25–0.30

4673.3 Copper Alloys-Nonferrous Metals

It is important not to choose a too small allowance for finishing, otherwise the reamers can get jammed, and a surface hardening occurs at the material. Table 3.17 shows cutting data for HSSE reamers.

3.3.2.6  EDM

Basically, the electrical discharge machining of good heat conducting materials sets demanding requirements on the machine and the processor. Aluminum bronzes are best materials to be eroded due to their lower electrical and thermal conductivity, in comparison to CuBe materials. In practice, higher electrical currents are o�en used, or ampere values, like the ones when eroding steel can be set; at the same time, however, the finishing pass is used to minimize the removal of the erosion electrodes.

For EDM, it has been shown that using electrodes made from very hard graphite or tungsten copper is ideal. Wire EDM is o�en a very good alternative to complex milling operations for unstable work piece conditions. Material thicknesses up to 200 mm can easily be cut in copper alloys.

3.3.2.7  Welding

Although no welded joints are provided in most of the new designs, the question of weldability of copper alloys is o�en of great importance. Corrections of design or manufacturing errors as well as the compensation of wear caused by mold opera-tion are the main applications of the potential welding processes.

Basically, when welding hardenable copper alloys, one has to ensure that the cure temperature is exceeded as little as possible and limited in time. A negative change in structure due to evaporation of alloying elements and grain growth at the weld zone of influence is not completely avoidable. Resulting strength losses in the finished component cannot be eliminated by a new heat treatment because of the warpage occurring during hardening, which would make the entire component unusable.

For all welding procedures, good welding fume extraction must be provided because of health hazards from the metal fumes. The use of pure argon as a shielding gas is recommended.

Aluminum bronzes can be easily welded with the manual welding procedures, MIG/MAG and WIG (alternating current). Both connection welding as well as large build-up welding can easily be implemented with SG-CuAl8Ni2 a�er DIN 1733 filler material.

In CuNiSiCr or BeCu alloys, it is recommended to use CuSi3 or the similar CuBe filler materials. Here, WIG (direct current negative pole) is the preferred welding process. Pre-heating to 200 °C of the components to be welded is recommended due to the high thermal conductivity.

468 3 Materials for Mold Making

FIGURE 3.17  Injection molding nozzle with a combination of materials

To the aforementioned “traditional welding techniques”, laser welding has been added in recent years. Advantages of this method are the very high energy density of the laser and the exact controllability. Especially through the o�en necessary material application at the edges of contour shaping molded parts, the component weakening and the necessary post-processing can be greatly reduced by a very small heat affected zone and very fine applications. By using portable laser equipment and the very clean work methods of laser welding (no spatter and welding fumes), it is possible to directly carry out minor repairs in the clamped mold. Flat reweldings can be realized with systems in which a powdered filler material is introduced into the active zone of the laser using a carrier gas.

Another modern welding method is electron beam welding. In contrast to laser welding, here a vacuum is applied; therefore, the use of electron beam welding by mold makers and mold operators is impossible.

An advantage of this method is that it is possible to combine different mold materials because of the enormous energy density of the electron beam and the evacuated area.

Using laser and/or electron beam welding, components can be produced that combine the (targeted) properties of material (e.g., a mold steel and a highly heat conductive copper base) where needed. This allows many previously structurally desirable but not technically feasible applications of copper-steel composite compo-nents. Figure 3.17 shows an injection molding nozzle on which the highly stressed tip and the thread were manufactured from a Co-base alloy.

3.3.3  Surfaces

Depending on the application, surfaces of molded parts must meet the requirements of optical appearance of the component, wear and corrosion protection, and preven-tion of tribochemical reactions. To achieve these objectives, different possibilities are available for the mold maker. These possibilities are discussed below.

4693.3 Copper Alloys-Nonferrous Metals

3.3.3.1  Polishing

Copper materials can be very well polished in the hardened state. The achievement of a homogeneous, mirror-like surface depends on a particularly fine and very uniform structure. A thermoformed, annealed semifinished part usually has such a structure.

Depending on requirements, a sufficient surface quality is o�en reached a�er only a few steps. However, the previous finishing must be done very carefully. The pre-polishing can be done with felt or sisal polishing wheels (harder qualities) with appropriate polishing pastes made with a grain size of about 10 microns. For the next step, flannel or fiber buffing wheels with a correspondingly finer polish are recommended. If the quality achieved with this method is not yet sufficient for the application, very fine brushes have to be used (e.g., natural hair).

3.3.3.2  Coating

Surface coatings can be applied to all Cu alloys used in mold making to prevent chemical reactions or for wear protection as a preventive measure. A corrosion protection, which would be necessary for example in steel molds operating under tropical climate conditions, can be waived due to good resistance of the oxidation layer that forms on the surface (CuO/BeO). For the various CVD and PVD coatings available on the market, the temperature range in which the coating is applied should be observed to avoid a structure change of the base material. Furthermore, the highly varying hardness of the base material and the coating should be taken into consideration. Negative effects such as cracking and flaking of the coating (eggshell effect) occur when the ductile base material deforms elastically under the extremely hard, brittle coating.

Electroplated or chemical chromium and nickel layers are a good option for surface protection.

Above all, chemical nickel has proven to be a very good coating procedure. The advantages are primarily:

� low temperature during deposition, � different types of coating are possible (wear/nonstick), � wide range of layer thickness (dimensional corrections are feasible), � no edge effect during application (no rework), and � can be used for interior coatings of bore holes.

With chemical nickel, the hardness values can reach up to 60 HRC without heat treatment of the parts a�er coating. Hardness values higher than 70 HRC can be reached with an appropriate heat treatment.

470 3 Materials for Mold Making

3.3.3.3  Structuring

To improve the appearance or feel in mold making, patterns are photochemically applied to the mold parts.

For surface structuring through etching, the copper alloys are treated similar to steel. Only the etching medium and reaction times are adjusted based on the experi-ence of the specialist. If a fine-grained regular structure is available, a uniform and perfect result can be achieved in all copper alloys and bronzes.

3.3.4  Summary

Copper alloys are indispensable in modern mold making due to their outstanding properties. Requirements on the molds (e.g., shortest possible cycle time at very high quality) are mostly impossible without the thermal conductivity, thermal shock resistance, and crack resistance of copper alloys. Another large area of applications in the future will be the combinations of copper and steel. Through targeted welding of steel or other alloys, the excellent thermal properties of the Cu materials are also used in areas in which an application was previously not successful due to the mechanical properties. By using modern manufacturing techniques, molded parts can be produced without significant additional effort, which will permanently provide for higher efficiency and better quality in plastics processing.

4 Manufacturing and Machining Methods

 ■ 4.1  Mold Manufacturing

A. Klotzbücher

4.1.1  Introduction

As in other areas of activity, the manufacturing of molds is subject to constant change; driven by technological changes in design process, mold construction, programming, processing, and assembly. This requires continuous changes in processes and reorganization of the business to new investment and training. The fact that the manufacture of molds is strongly influenced by knowledge, experience, and information remains unchangeable. The complexity of the “manufacture of a mold” process is shown in the following.

4.1.2  Design

4.1.2.1  Development

The beginning of all actions is the design of the product. This activity is very impor-tant for the success of future sales of the product. Look, feel, and functionality need to be closely examined, evaluated, tested, and particularly visualized in the run-up of the product creation process.

When developing a product, the designer can access many resources, but in every case, the product is completely engineered, thus entered into a CAD system. If a cubing model exists, it is o�en by digitizing a surface, or the origin was already an engineered article which was milled into a cubing model, and the product in the classical model was manually modified and reverse engineered by digitalizing.

472 4 Manufacturing and Machining Methods

4.1.2.2  Visualizing

For a long time, it was considered that the engineered product in the CAD system was sufficient enough to evaluate the design. However, the visualization of data in the form of a cubing model, or using stereolithography or a prototype mold which is preceded by a series mold, has proved to be successful.

4.1.2.3  Cubing Model

The cubing model is a wood-like plastic model processed with three-dimensional structure that is adjusted to the optical requirements with pastes and filing tools. Here, the designer can create an overall impression of the product itself, and also an overall design with surrounding cubing structures. This only serves the visual decision-making. Functionalities cannot be tested here.

4.1.2.4  Stereolithography

This new technique has experienced a boom in the early 90s and still maintains its value. With this method, it is possible to produce products that can be function-ally tested on the scale of 1 : 1 to the actual product over the product data set, by program, laser system, and powder irradiation. This method thus serves not only the visual inspection; it can also be used for installation testing. A disadvantage is that this method can only be applied to a certain size, and the product is not produced in the original raw material (see Sections 1.11 and 4.7). In the typical stereolithography process, the programs run mainly independently and unattended through data sets. Figure 4.1 and Figure 4.2 respectively, show an exemplary product

FIGURE 4.1  Stereolithography products; mounting strips and lamp housing of a vehicle tail light

4734.1 Mold Manufacturing

of the manufacturing process in the typical color and before the manual surface finishing process (polishing).

When using a prototype mold, an implementation of the product geometry data into a functional mold takes place. This implementation level allows production of a product in its original size and in the original raw material. Design, feel, functional-ity, and installation can be evaluated close to serial production.

Disadvantages are the high price and the parts’ late availability, since the produc-tion of the mold takes several weeks. Figure 4.3 shows two mold halves made from aluminum with the incorporated mold geometry. In the injection process, on a suitable injection molding machine (Figure 4.4), commercially available plastic granules are introduced into the mold cavity via a hot runner, whereby the molded part is formed.

FIGURE 4.3  Example of an aluminum prototype mold for mounting strips of a lamp housing

FIGURE 4.2  Stereolithography products; lamp housing of a vehicle front light,

detached from the carrier

474 4 Manufacturing and Machining Methods

FIGURE 4.4  Example of an aluminum mold prepared for the try-out on the injection molding

machine

For more complex product identifications, it happens that the described methods for visualization can be applied together, e.g., a cubing model defines the design, the future prototype part defines the functionality, and the complete installation defines a product structure using additional stereolithography parts.

4.1.3  Data Model

4.1.3.1  Data Feedback

The approved and therefore released design of a part must now be brought back into the product record. This can be done, for example, via a mechanical process of the modified design model. In this operation, a sensing device moves over the model geometry, similar to a milling process. Also available are data feedbacks through photographic images, which are then engineered via so�ware.

If the data plane was not le�, i.e. the part was assessed and approved on the PC, the changes are of course directly done through alteration of the data on the PC.

The common denominator of all procedures is that the data is manually changed on the PC a�erward, and qualified personnel with the time to do so have to be provided.

4.1.3.2  Completion of Product Data

Many development agencies are engaged in the previously described processes. In the automotive industry and in most industries, it is common that predefined design studies are completely engineered through these offices. In addition, func-tional aspects are considered, legal and international requirements, specifications,

4754.1 Mold Manufacturing

and other demands are implemented by the customer. This process, which goes on for months, establishes the basis to manufacture the molds that meet the require-ments in mold making.

4.1.4  Data Transfer in Mold Making

4.1.4.1  Verifying of Data Quality

When transitioning data into the area of the preparation of operations equipment, the quality of the data is checked. In the data structure, there are tolerances and tolerance limits, for example, in fillets, surface transitions, density, and volume. The transmission of data could cause a loss of data.

Therefore, in the final design of products, there are tolerance agreements between the companies. If these boundary definitions are not met, this can lead to consid-erable expenses and time loss or the sent data is totally useless. Consequently, a detailed verification of the actual quality in the design offices of the mold manu-facturer is necessary.

4.1.4.2  Feasibility Studies

The qualitatively useful data is initially placed into an intended mold position from the equipment designer (Figure 4.5). An examination is done to see where undercuts are present that must be retracted in the later mold using slides during the mold opening operation. In this study, it is determined whether and how the constructed part can be manufactured and if sufficient dra� angles are present on the surfaces. It can happen that mold manufacturing is not possible for complex designs or only be possible under certain conditions, which would no longer allow a subsequent production at the required quality standards.

point 2

New/Red

Slider running direction

Current/Blue

Mounting flange is

opened 40º in

the y-direction

FIGURE 4.5  Detail of a cross section for a visualization of undercut demolding areas through

product data

476 4 Manufacturing and Machining Methods

4.1.5  Feedback/Communication

If the above mentioned problems are present, communication with the customer is essential to find solutions for the problem (Figure 4.6 to Figure 4.8). For simple issues, the customer can usually maintain the dra� angles, change the radii, and adjust any functions. A correction can also go so far that the intended design has to be revised, as this may be the only way out of the non-producibility of the speci-fications. This worst case scenario has a considerable impact, since the design is a selling point for the product and top executives are concerned with the products’ appearance.

Part data: 27.09.06

Inside-slider

Outside-slider

Outside-slider

Inside-slider

Comer-slider 40mm

Head-slider

Z-sliderRib-slider

9

FIGURE 4.6  Arrangement of slider parts for the ejection of a bumper product from the series mold

Part data: 27.09.06 Point 1

Section breakthrough Bigger radius on the rib(cracking in the steel in cavity)

Make bigger draft angle / bigger breakthroug(cracking in the steel in cavity)

FIGURE 4.7  Communication drawing from a data section as part of a feasibility study

4774.1 Mold Manufacturing

Part data: 27.09.06

NEW

Point 6

Comer slider direction

Connection from wheel arch flange to thedesignradius is not constant - risk ofsinkmarks

Change angle and make radius bigger forbetter demolding

FIGURE 4.8  Communication drawing from a data section for the explanation of undercut areas

4.1.6  Design

4.1.6.1  System Environment

A�er the completion of the testing stages, the equipment designer will go over into the mold design. Here, the designer makes use of appropriate and specially devel-oped so�ware. This allows observation of the product record in three-dimensions, to turn it comprehensively, to create cuts, to show and hide areas, and to model the surfaces. The designer makes use of a warehouse of stored modules, such as screws, washers, plates, cylinders, etc. By creating surface extensions of the product, the mold to be produced is created step by step on the computer.

As further aids, the designer can potentially utilize simulation programs, collision functions, computational modules for strength, etc.

Figure 4.9 shows a typical component of a front bumper, as it is provided by the developer/designer. In a first step, the equipment designer develops the so-called surface extensions as shown in Figure 4.10. In the next few weeks, the final mold parting planes are created (Figure 4.11).

FIGURE 4.9  Product data of a bumper-end product

478 4 Manufacturing and Machining Methods

FIGURE 4.10  Design areas of product data

for the determination of the

mold separating planes

on the example of a bumper

Before all of these steps, the designer has to add a shrinkage allowance to the product data, which means that the product is inflated (enlarged) by a certain percentage value. In a few exceptional cases, the product can also be reduced in size. By this measure, the subsequent percent shrinkage value of the molding compound is compensated in the mold. This degree of shrinkage is calculated by using programs and tests and is usually provided to the mold maker.

Despite many simplifying and useful features in the work of the designer, it is still necessary to have the traditional qualifications and skills. The designer must rec-ognize the requirements and difficulties of a mold and take it into account in his work. If errors occur in their area, it can result in significant costs; if the designer, however, can find simple solutions, the expenses in the mold production can be significantly reduced. In this area of the mold making, an average of 30% of all mold costs can be influenced.

4.1.6.2  Releases

During the development process of a mold design, there will always be release steps. In most cases, the customer, together with the person responsible for the design, will also examine for all required checklist features, specifications, requirements, and appointments on the design object. If the requirements correspond with the implementation, the design may be released to production. Minimum dra� angles on the mold parting planes can be tested with a simulation analysis (Figure 4.12); all media connections, such as water, electrical, and hydraulics are entered in block diagrams (Figure 4.13) to be checked; Figure 4.14 shows a collision analysis with the extended slide mechanism.

htfl‰che unter 5∞Sealing surface under 5º

FIGURE 4.11:  Addendum of mold parting

planes to specified product

data on the example of a

bumper

4794.1 Mold Manufacturing

FIGURE 4.12  Test run on a mold extension area for identifying minimum dra� angles

Mold-Dimension:Installation Height : 1995 mmLength : 2700 mmWidth : 1500 mmHeight : 1995 mm

Elektrokasten Connection Female MoldTemperature Control

Connection Hot RunnerTemperature Control / Hydraulic

Connection CoreSide Temperature Control

Connection Core PartHydraulic Ejector Plate

Electric Box

FIGURE 4.13  Determination of mold-peripheral connections and mold dimensions

FIGURE 4.14  Overall mold assembly on the core side, including all movable slide functions

480 4 Manufacturing and Machining Methods

4.1.7  Programming

4.1.7.1  So�ware

The released data is implemented into milling commands in the field of program-ming. Here, a variety of so�ware can be used. Test steps can also be calculated using the available manuals, so that milling collisions with the workpiece material can be avoided. The program language is then outputted, as shown in Figure 4.15, in a numerical sequence by the computer.An important test function is shown in Figure 4.16. The visual tool diameter is com-putationally navigated above the data body; when colliding with it, the program will issue a warning. Here, it has to be ensured that the block of material to be milled will have exactly the dimensions that the computer model will accept. If necessary, the workpiece to be machined must be measured again, before machining, to make sure that the dimensions are correct.

(Core part change)

(According to information)(NC Office tel. 841)

(End-mill)(Wall thickness = 0.00 Allowance = 0.05 Delivery = 0.70 Swing tip)

FIGURE 4.15  Milling step determination of processing a modified area

4814.1 Mold Manufacturing

FIGURE 4.16  Collision analysis on the PC by setting the milling steps

Figure 4.17 illustrates the operating program of the programmer. The programmer can reach the ideal milling programs in various ways. The menus and tools are constantly updated and automated from the so�ware manufacturers. This guar-antees that programming, and thus the producing, can be done competitively and efficiently in the future.

FIGURE 4.17  Visualized milling movement with a menu bar at the programming PC

482 4 Manufacturing and Machining Methods

4.1.7.2  Strategies

When programming, the milling strategies are already determined. Depending on the quality of the object to be machined (the preprocessing); the allowance, and the required surface quality, the line steps, milling cutter selection, feed and speed are defined.For areas with special requirements, such as sharp corners, overflows must be programmed. For hard-to-access areas, the angular head position should be taken into account. For areas which cannot be milled, the programmer makes the decision whether electrodes and EDM processes have to be provided.Individual milling steps take place as follows:1. Pre-machining to a contour measurement of 10 mm with milling cutter D 160 mm

and 5 mm infeed;2. Machining to 1 mm D 66 mm and 3 mm infeed;3. Work-out corner areas D 52 mm and 2 mm infeed;4. Prefinishing the edges to 0.3 mm D 35 mm and 1.5 mm infeed;5. Prefinishing to 0.3 mm D 35 mm and 1.5 mm infeed;6. Finishing completely to 0.3 mm with D 6 mm and 0.5 mm infeed; and,7. Micro-finishing with 0.00 mm allowance and D 20/12/8/2/1 mm milling cutter

diameter and 0.25 mm infeed.

4.1.7.3  Choice of Machinery

Successful treatment comes with choosing suitable machinery with appropriate programming. For each milling step, other machines may be required to work as economically as possibly. Thus, for rough machining (in roughening), machines with the appropriate power (kW) are required. For large components, large cutter heads are used (Figure 4.18). For finer parts, rather delicate machinery is needed

FIGURE 4.18  Roughing operation FIGURE 4.19:  Finishing operation

4834.1 Mold Manufacturing

that can ensure high speeds and fast feeds. For the finishing, so-called high-speed machines are o�en provided (Figure 4.19) that only remove a few tenths of a mm using high speeds and fast feeds.

4.1.8  Machining

A key element in the production process of the finished mold is the machining process. In principle, all constructive features are first introduced in the mold by machining. Here, the workpiece passes through various stations of different machining operations. For larger and more complex molds, machines are required that are powerful and can withstand dynamic loads depending on the processing requirements. As described above, different types of machines in different sizes are necessary for this.

4.1.8.1  Tooling

The appropriate milling tools particularly contribute to economic machining of workpieces. A wide range of tools are available, e.g., cutter heads with as-large-as-possible infeed, to end mills for the smallest inaccessible areas. These can be applied with replaceable cutters made from alloyed special steel/carbides/ceramic and for example as inserts.

FIGURE 4.20  Tool changer as an example of a typical machine configuration (le�)

and tool changer (right) as a scheme

484 4 Manufacturing and Machining Methods

To also ensure an operation that is mostly unattended, the majority of machines are equipped with tool changing systems (Figure 4.20). The machine takes the prede-termined type of milling cutter from the magazine and independently checks the dimensional suitability of the mill at the beginning and during the processing steps.

4.1.8.2  Unattended Operation

The consequence of the machinery, which is necessary to produce molds, is very high investment costs. Continuous improvement of these machines also requires a constant improvement of the machinery within short amortization periods. Due to this requirement, it is necessary to keep the machine in operation 24-hours a day. Therefore, these machines run in multiple shi�s and in unattended operations, such as on weekends and holidays. These unattended operations require technical conditions on the machines, e.g., tool changer and cutting edge corrections. Machine monitoring can be done via camera. The operator can then visually assess the current state of processing from home through a PC and respond quickly to disturbances.

The possibility of a multi-machine operation and programming at the machine itself is o�en used (Figure 4.21).

FIGURE 4.21  Programming station next to a five-axis milling machine

4.1.8.3  Releases

During and a�er completion of the machining processes, an initial check (self-check for quality) is usually done on the milling machine. Using a probe in the spindle, individual areas can be probed; bore holes can be controlled with plug gauges and critical dimensions can be controlled with gauge blocks. The actual dimensional inspection takes place on a measuring machine.

4854.1 Mold Manufacturing

4.1.9  Dimensional Inspection

The machined workpiece is adjusted using a measuring device (Figure 4.22) in the QA area, which means placed in the data using reference surfaces or pins in X, Y, Z directions. The same data, which has been applied for the milling operation, is also used for workpiece measurement. Through probing of the surfaces, radii, and bore holes, a target/actual comparison is made using the measuring so�ware with which bore hole diameter, radius, and other axis centers is calculated by the so�ware.

FIGURE 4.22  Three-axis measuring machine

The result of the measurement is measurement data sets, which can be printed and visually displayed on plots.

4.1.10  Drilling/Deep Hole Drilling

All molds must either be cooled or heated in the injection process. For this purpose, bore holes in the workpiece are required. These deep holes are placed in the work-piece using the appropriate machine. For more complex mold constructions, nearly every forming component is provided with bore holes.

The introduction is also done through programming based on the design template (Figure 4.23). The importance of a refined temperature control is especially impor-tant in the attainment of short-cycle times of the mold. Due to the current cost pressures, it is absolutely necessary to be able to equally control the temperature in all areas. This is achieved by separating the cooling circuits, through bubble bore holes, with special material of different thermal conductivity and outside the mold, e.g., by pulsed cooling (see also Section 2.4).

486 4 Manufacturing and Machining Methods

FIGURE 4.23  Arrangement of the deep holes in a female component for bumpers,

size 2800 × 800 × 1500 mm (le�),

on the right: core carrier on the deep-hole drill

4.1.11  Electric Discharge Machining

Areas that cannot be processed by milling can be processed in an eroding process. First, electrodes are milled in copper, or more o�en, in graphite. A�er the workpiece to be eroded is adjusted to the processing position on the machine table, the work-piece is flooded with an electrically conducting medium (Figure 4.24). The electrode is supplied to the receiving head. The receiving head moves with the electrode, at a certain distance to the workpiece surface. Thereby material is removed also in three-dimensional direction into the material.

FIGURE 4.24  Eroding on a female mold insert

4874.1 Mold Manufacturing

FIGURE 4.25  Female mold insert a�er finishing the polish

4.1.12  Surface Finishing

A�er these processing steps are completed, surface finishing by polishing to a desired quality is done (Figure 4.25). This process is still done manually today, usually by specially trained professionals. The milled surface is polished with a grinding tool, which means that a coating structure is applied to the milled surface by hand through removal of material. Additionally a direction is given to the milled surface. The guidance factor is done because the ejection of the product from the mold, which was polished in the ejection direction, significantly improves. In general, quality indications are made in “Ra”, e.g., Ra 150 on the product interior (core part) and Ra 320 on the product exterior (shell section).

Even the flow properties of the plastic material in the mold are influenced by pol-ishing quality and alignment. In the case of marks through slider movements, the flow behavior of the plastic material in the mold can be affected on the core side by the polishing roughness so that marks can be reduced or avoided.

All of the demolding properties in the mold also depend on the polishes in ribs and surfaces. Some demolding problems can be solved when the correct grade of polish is selected in each area of the mold.

4.1.13  Assembly

For the manufacture of molds, hundreds of components are required, all of which are refined from raw material through mechanical and manual processes to the state of assembly. Many of these components parallel as they pass through these manufacturing steps on schedule.

At the end of the production chain is the assembly of all of these parts. As a rule, the molds consist of the following: shell side, core side, slide parts, hot runners,

488 4 Manufacturing and Machining Methods

clamping plates, hydraulic electrical equipment, ejector plate, and standard parts. All of the necessary parts can be captured through parts lists and can either be self-made or obtained through purchasing through another company. According to the construction plans, these parts are now connected with each other.

Here, the modern workshop uses the information from computerized terminals, from which all design data can be retrieved, so that a production without drawings can already be implemented. Today, it is also possible to assemble complex molds with little paper information by understanding their function. With these modern program systems, mold data can be cut, cooling channels can be understood, parts can be separated, parts can be assembled and disassembled, and many more fea-tures can be accessed.

According to these construction plans, the molds are assembled piece by piece, whereby, in this area, primarily grinding and fitting, assembling and operating tests are included. Part assemblies are equipped with temperature control systems; the circuits are connected and linked. All components are incorporated into the two main component groups; in the female and male mold.

A�er finishing the assembly of these two groups, the spot-grinding of the two mold halves on spotting presses follows (Figure 4.26). Here, the two mold halves have to be closely spotted to each other, so that during the subsequent injection process, the material remains bound in the mold cavity. Leakage of material is a defect and needs to be dealt with later.

The described steps are accompanied by measurements in the manner that various intermediate steps are examined. Therefore, measurement plans are created and

FIGURE 4.26  Spotting press with installed injection mold

4894.1 Mold Manufacturing

tolerance limits are defined. When deviations are seen, rework has to be done or it has to be newly manufactured.

4.1.14  Trials

A�er assembly, the functional tests on the injection molding machine are performed (Figure 4.27). Here, all relevant features are checked in the process. These include: function mold, part demolding, cycle, temperature, parameterization of the settings, flow patterns, injection pressures/hours/courses, and on the articles, wall thickness, optical designs, completeness, dimensional accuracy on gauges, etc.

When assembling with other parts, further knowledge can be gained for the opti-mization of the components.

A�er painting the sample parts, surfaces can be judged: the shrinkage that is brought into the mold can be checked for accuracy a�er the post-shrinkage and painting process. In short, with the first components from the manufactured mold, there are numerous operations that deal with the comparison between the intended original condition and the actual condition.

FIGURE 4.27  Injection molding machine with installed core part

4.1.15  Optimization Process and Finishing

This phase can last several weeks; with this gain in knowledge, optimization plans are created that have the goal of bringing the mold and the process as closely as possible to the original state. Deviations always exist, so tolerance specifications are required.

This approach phase usually also lasts several weeks, accompanied by further testing on the injection molding machine. In the end, there is a customer-released product, which can be used within the defined characteristics of the production.

490 4 Manufacturing and Machining Methods

 ■ 4.2  Electric Discharge Machining (EDM)

D. Schäffner, B. Mack

4.2.1  Introduction

In 1943, Russian physicist Lazarenko published a paper on the “the inversion of the wearing effect as result of electrical discharge”. He researched the wear of switch contacts and discovered a potential processing method of conducting materials where workpiece erosion was done with the help of the destructive effects of an electric charge. He developed the first beginnings of spark erosion. Since about 1954, the resulting erosion technique, electrical discharge machining (EDM), has constantly improved and is now one of the high-precision machining processes. Employees from this area are among the most sought specialists in mold making, because great knowledge is necessary not only for the procedure, but also for pro-gramming 3D-EDM machines, with ever increasing demands due to the increasingly complex forms.

EDM is thermal erosion for electrically conductive materials, which is based on electrical discharging (sparking) between the tool (electrode) and a conductive work-piece. Therefore, every electrically conductive material can be processed, especially steel and aluminum, in mold making. This applies particularly for very hard materi-als where the limit can be reached with traditional machining processes. Both the workpieces as well as the mold are immersed in a non-conductive liquid medium, the so-called “dielectric fluid” (usually oil or deionized water). This dielectric fluid is necessary for the functionality of EDM.

In principle, the electrode is brought down to below a 0.5 mm distance to the work-piece. An electric voltage is created through ion flow, which forms a gas in the liquid medium. This is ignited by an electric discharge. Here, millions of tiny explosions occur that tear out material from the workpiece. By constantly introducing the electrode to the workpiece, material is removed in various ways.

Although the process of material removal is basically the same in the individual processes, a distinction is made between:

� Die-sinking EDM, � Wire-cut EDM, and � Start hole EDM.

There are also some less important types in manufacturing.

4914.2 Electric Discharge Machining (EDM)

Die-sinking EDMWorkpiece and tool (Figure 4.28) are on the machine, completely immersed in the above-mentioned dielectric fluid. The tool is brought closely to the workpiece using a traversing unit. At the beginning of processing, the tool electrode is briefly and repeatedly li�ed and again brought close to the workpiece in a pulsating method. This is necessary to flush the spaces free of worn out, conductive workpiece par-ticles, and thereby, prevent the contact between electrode and workpiece (short circuit).

Especially for die-sinking, since abrasion on the tool through EDM cannot be avoided, several tools for the processing must be manufactured to accurately create the desired contour at the end.

Wire-cut EDMIn this method, the removal is not generated by a tool electrode, but accomplished by using a spool of non-reusable wire. It should be noted, however, that with this method only through holes, recesses or outer contours can be manufactured.

The wire is passed from top to bottom through the workpiece (Figure 4.29). Starting holes must be available in the inner recesses. Here again, the workpiece is usually completely immersed in a dielectric fluid. In some cases, an intensive flushing may be necessary.

Start hole EDMThis type of EDM is, as mentioned before, for the manufacture of bore holes for later processing by wire-cut EDM (Figure 4.30). This is a variation of die-sinking EDM on special machines. Since the processing of the workpiece is carried out without contact, bore hole diameters of up to 0.2 mm can be created.

FIGURE 4.28  Schematic

representation

of die-sinking

EDM

FIGURE 4.29  Schematic

representation

of wire-cut

EDM

FIGURE 4.30  Schematic

representation

of start hole

EDM

492 4 Manufacturing and Machining Methods

4.2.2  Physical Processes

In electrical discharge machining, the material-removing effect by electric spark dis-charges is being exploited. A single discharge has very low energy content between milli-Joules to Joules, and therefore generates only a small removal. A significant removal rate, VW in units of volume each time unit, can only be achieved when a particularly large number of such discharges can be accomplished in a short time. Discharges with high energy lead to a higher removal rate. The crater overlay on the surface and create a roughness image: high processing energies cause rougher surfaces (roughing) than smaller ones (finishing).The workpiece and the tool are subject to the erosive effect of the electrical dis-charge, so that tool wear even occurs in electrical discharge machining, similar to the processes in machining operations.It is not always physically possible to know whether the tool or workpiece is sub-jected to increased wear; the polarity selection is therefore generally based on empirical tests.The equation for the electric discharge energy is:

e e e eW U I t� � �

We (Joule) = Discharge energyUe (Volt) = Discharge voltageIe (A) = Discharge rateste (s) = Discharge time

A�er applying the electric voltage pulse, the electric field builds up in the working gap. The field strength is at its highest at the shortest distance between the work-piece and the tool. Free charge carriers absorb energy in the field and heat the dielectric fluid, which leads to the formation of gas bubbles (1 in Figure 4.31). This results in an increased energy absorption that leads to the formation of a very small plasma channel at the narrowest point between the tool electrode and workpiece (2 in Figure 4.31). This leads to pressures up to 3,000 bar and temperatures up to 40,000 degrees Kelvin. The electric field collapses, and the conducting plasma channel and the gas bubbles implode (3 in Figure 4.31).Due to the sound pressure, which results from the high temperature and the implo-sion, the material is removed in crater-like manner. The voltage is polarized in such a way so that the ions migrate in the plasma to the workpiece and the electrodes to the tool. In this way, the material removal of the workpiece is usually much higher than the material removal of the tool. The wear on the tool cannot be avoided. A�er roughing and an exchange of the finishing electrode, the processing is continued until the final size is reached. The individual processes in eroding are done a million times and lead to a clean finished surface.

4934.2 Electric Discharge Machining (EDM)

1.

2.

3.

Workpiece

Workpiece

FIGURE 4.31  Procedure of the EDM process

494 4 Manufacturing and Machining Methods

4.2.3  Tolerances and Key Data

Roughing: Ra smaller than 3 μm

Finishing: Ra 0.8 to 3 μm

Micro-finishing: Ra 0.5 to 0.8 μm

Microstructures: Ra 0.5 to 0.8 μm

Wire in wire-cut EDM: calibrated to 1 μm

Wire diameter of MS: 0.20 to 0.25 mm

Wire diameter of tungsten: 0.03 mm

Average removal rate in wire-cut EDM: 400 mm2 (see definition later)

Start-hole EDM: Smallest diameter 0.2 mm

Precision wire-cut EDM: ±2 μm

Precision die-sinking EDM: ±3 μm

Workpiece heights wire-cut EDM 200 to 600 mm (depending on the machine type)

Workpiece heights die-sinking EDM 300 to 3,000 mm (depending on the machine type)

4.2.4  Die-Sinking EDM

In the die-sinking EDM machine in Figure 4.32, the upper tool holder with the stroke-lowering unit, the front door for loading the machine, and a tool changer, are illustrated on the le� of the stroke-lowering unit. A typical processing area is shown in Figure 4.33.

The maximum achievable accuracy in this type of EDM is ±3 μm. The tool electrodes are made from copper or graphite. An advantage for graphite, and thus a decision criterion whether to use this material for the electrode, is the lower specific weight and easier processing. Thus, graphite is used for complicated molds (e.g., speaker grille) or for large electrodes, which can take sizes of up to 3 × 3 × 2 m in the automo-tive field. These are then used in the plastics industry for the production of headlin-ers, fenders, bumpers or garden chairs and large containers such as garbage cans.

In the area of small-sized parts, mostly copper is used. This allows the production of smaller sized parts since copper is much more stable compared to graphite.

Figure 4.34 illustrates a negative image of a copper electrode with an electrode holder and a precision electrode for machining of the workpiece. Dents can be seen, which can leave impressions on the workpiece a�er processing.

4954.2 Electric Discharge Machining (EDM)

FIGURE 4.32  Die-sinking EDM machine

FIGURE 4.33  View into the processing area of a die-sinking EDM machine (le�) and detailed

explanation of the tool electrode

elements shown from top to bottom:

(le�):

– Bellows, behind which is the stroke-lowering unit

– High-voltage unit for generating the necessary voltage energy for processing

– Tool electrode, made of copper (in this case)

– Workpiece, some parts are already pre-eroded

– Clamping system (in this case: simple precision vise)

– Magnetic Table

– Machine filled with dielectric fluid

– Tool electrode holder for automatic switching (le� side)

– Finishing electrode

(right): workpiece with already some pre-eroded areas, clamping system,

magnetic table, and dielectric fluid

496 4 Manufacturing and Machining Methods

FIGURE 4.34  Copper electrode

FIGURE 4.35  Copper electrode

FIGURE 4.37  Pins (le�), which are manufactured through die-sinking EDM,

and the counterpart (right)

FIGURE 4.36:  Workpiece (le�) and

electrode (right) for tooth

manufacturing

4974.2 Electric Discharge Machining (EDM)

The example shown in Figure 4.35 is a tool electrode in the form of a spiral screw. It requires a special controller. The workpiece is clamped vertically and processed in the X or Y direction. Another possibility is to incorporate a spiral screw or a gear tooth in a rotated way into a tool, as shown in the example in Figure 4.36.

This workpiece places high technical demands on the machine and the necessary program. The tool (copper electrode), which is shown on the right, moves not only in the Z-axis (vertical axis) during the processing of the workpiece, but also rotates at a right angle to the depth when lowering. This creates a helical toothing.

Figure 4.37 (on the le�) shows pins which were manufactured with an accuracy of 3 μm. The counterpart (Figure 4.37, on the right) was produced using the wire-cut EDM. In the middle, pins and counterpart are combined.

4.2.5  Wire-cut EDM

Figure 4.38 shows a wire-cut EDM machine. The wire runs from a roll in the top le�, over to several idler pulleys, by a guide (center), through the workpiece, and into the tool carrier below the workpiece into a tension roll.

FIGURE 4.38  Wire-cut EDM (without a workpiece, dielectric fluid and clamping system)

498 4 Manufacturing and Machining Methods

Only continuous workpieces can be processed in wire-cut EDM since the wire needs to be brought from top to bottom (comparable to a band saw). The contour is produced with the tool table. The wire usually runs in the same axis from top to bottom, wherein the tool table is moved in the XY plane.A bore hole is required in order to be able to separate a recess within the workpiece. This can be done in advance on a start-hole drilling machine. This machine can produce bore holes with a minimum diameter of 0.2 mm.The wire runs only once past the workpiece and is collected and recycled next to the machine as recyclable scrap waste. Due to the high precision of thickness with accuracy in the 1/1,000 mm range, the erosion wire cannot be reused. Current strength, throughput speed, choice of wire materials, and feed rate affect the quality of the surface. Thus, pre-cutting (roughing) and post-cutting can be done (if neces-sary) in order to improve accuracy.An accuracy of ±2 μm (contour) can be reached with wire-cut EDM. The wire diameter is calibrated to 1 μm. EDM wires are usually made from brass with a diameter of 0.2 to 0.25 mm (standard). Wires are available for particularly precise operations, such as with a diameter of only 0.03 mm. These are made from tungsten due to the necessary strength.“Cutting speeds” are presented in mm2/min (practical term). Based on a workpiece with a height of 100 mm, a cutting speed of 4 mm2/min results. The machine runs fully automatic a�er it is set up. If the wire tears, contrary to expectation, it will be automatically threaded again through the system at the starting hole, will run up to the termination point at a higher speed, and then continues the cutting work again (as it was programmed).The upper head can be moved in 2D direction with today’s equipment. As a result, the position of the upper and the lower mold can vary. However, this is only pos-sible to a limited extent, and depends, among other conditions, on the thickness of the workpiece.The new facilities and the variety of workpieces mean that the clamping tools are always faced with great challenges. To connect two workpieces together in very tight tolerances, the wire diameter should be taken into account in the individual processing steps. This can be achieved in the machine with the help of very advanced so�ware. Figure 4.39 shows a simple workpiece that is predestined for wire-cut EDM. The contours are achieved by the XY moving direction of the lower workpiece table. Corners are precisely rounded to 2 μm. The bore holes are also produced by wire-cut EDM (starting hole drillings are required). For a larger amount of such workpieces, several plates are stacked on top of each other, are welded together, and simultane-ously eroded. This significantly increases the efficiency of this processing method.A more complex flat part is shown in Figure 4.40. It is a profile drawing plate from the extrusion area.

4994.2 Electric Discharge Machining (EDM)

FIGURE 4.39  Typical workpiece for wire-cut EDM

FIGURE 4.40  Wire-cut eroded plate for the extrusion

FIGURE 4.41  Example for a wire-cut eroded longitudinal contour

(le�): Front picture

(right): Overall length of the workpiece

Figure 4.41 shows the precision for very long components. The contour presented here in the 1/10 mm area was reached at a length of 300 mm. Maximum workpiece lengths depend on the design size of the machine. Standard machines can reach a component height of 350 mm. There are occasionally machines where up to

500 4 Manufacturing and Machining Methods

600 mm of wire can be eroded. However, Figure 4.42 shows miniature parts with a greater wall thickness.

4.2.6  Combined and Special Processes

In combination of die-sinking EDM and wire-cut EDM, high precision inserts can be manufactured for the production of micro-plastic parts. Figure 4.43 shows the complexity and precision of this type of process on the example of micro-stamping. Figure 4.44 illustrates another combination part.

FIGURE 4.43  Example for combination of

die-sinking and wire-cut EDM

The possibility that the upper and lower contour (machine-dependent) can run differ-ently leads to a 3D body. Here, not only the mold table is moved in the XY direction, but also a swinging in the Z direction can be carried out. The contours, however, can only be partially executed. Here, the component height and the pivoting range in Z direction have to be taken into account. An example is shown in Figure 4.45.

FIGURE 4.44:  Combination component

with high precision

FIGURE 4.42  Micro-parts with greater wall thickness

5014.2 Electric Discharge Machining (EDM)

FIGURE 4.45  3D component, manufactured through a different upper and lower contour

FIGURE 4.46  Schematic representation of the manufacture of inclined drillings

A further special process for manufacturing inclined drillings on round surfaces is schematically shown in Figure 4.46. Conventional drilling would be difficult to impossible because the drills would not run straight. Laser processing would be another alternative but the edges would not come out clean.

Although one can now look back on a half of a century of EDM development and application, it is still a young and dynamic process, and its potential is still far from being exhausted, especially in mold making.

502 4 Manufacturing and Machining Methods

 ■ 4.3  Galvanized Inserts and Molds

R. Hentrich

4.3.1  General Information

The development of electroplating is closely connected to the advances and changes in the plastics processing industry. In the Federal Republic of Germany in 1955, the production of small galvanized molds began. By installing the most modern manufacturing facilities, as well as the consistent implementation of the practical experience gained, electroplating has continued to develop steadily and offer a wide production range. This ranges from: the manufacture of galvanized injection mold inserts for producing micro-precision parts; shell molds for the reproduction of real leather structures; the manufacturing of instrument panels; large shapes that are used in the aerospace industry for the manufacturing of lightweight components with an overall length of more than 10 m; and to the automotive industry for the manufacture of special body parts made from carbon fibers.

4.3.2  Process Description

Figure 4.47 schematically shows an example of the construction of a galvanized mold on a micropipette to be produced with injection molding. The galvanized mold insert is molded in a galvanic way a�er a so-called “positive model”, meaning the model

a) b) c) d) e)

Cu

Ni

Plastic Cover

FIGURE 4.47  Principle design of a galvanized mold

(a) positive model made from steel; (b) nickel electroplated deposition; (c) copper

reinforcement; (d) ready to assembly galvanic insert; (e) micropipette-molded part

5034.3 Galvanized Inserts and Molds

corresponds to plastic product to be manufactured (it is positive). Consequently, the corresponding mold cavity and thus the galvanic inserts are negative. To this end, the positive model, which can be made from metallic or nonmetallic materials and must be designed in accordance with the guidelines of the galvanic designer, is prepared for the galvanic process.

There are plastic covers on the reference surface, or at the mounting sha�, and a contact wire that connects to the cathode bar of the electroplating bath. Before it is introduced into the galvanic bath, a pretreatment, which is depending on the model material, is required. Nickel should not stick to the base material, thus, metal models must be passivated before dipping into the electroplating bath. Plastic models, as so-called non-conductors, must be made electrically conductive. This is usually done by chemical silver plating. The thickness of this silver layer is < 1 μ and can generally be neglected in the assessment of the surface.

The prepared model is dipped into the nickel electroplating bath, and in compli-ance with the bath composition, the deposition parameters, the current density, the bath temperature, the pH, and so on; according to all the relevant requirements, the required nickel layer or the combination of nickel plus hard copper is depos-ited. Wall thicknesses of up to 20 mm can be achieved depending on the surface geometry and the intended use. The required wall thickness has great influence on the holding time in the electroplating baths. In order to quickly and economically produce galvanized molds, an early coordination between design and the galvanic designer is necessary to determine the outer contour and the wall thickness and thus establish the timeframe for electroplating.

In injection molds, the mechanical processing of the outer contour is done as long as the galvanic mold is still on the model. A�er outside processing, the external model is pulled out of the galvanic mold with an appropriate pull-off device. The finished mold insert is now ready to be installed into the master mold a�er determining the height dimensions. Here, the main advantage of the galvanic mold production can be seen. All the main processing methods including EDM require milling or remod-eling operations where some difficult-to-control tolerances are taken into account. In electroplating, the material is applied to a positive model in a non-shrinking way. It follows that all the surface structures of the used positive model can be reproduced exactly in the negative mold insert in the micro range. Surface quality and dimensional accuracy of the positive model determine the essential quality of the mold insert and thus of the plastic part to be produced.

Modern plastics and their corresponding processing methods place different require-ments on the mold in terms of temperature, pressure, demolding capabilities, and the design of parting planes, etc. These requirements should be taken into account by the designer when designing the mold. This also means that a high degree of flexibility is necessary when using galvanized inserts.

504 4 Manufacturing and Machining Methods

The manufacturers of galvanized molds, mostly specialists who provide services based on electroplating, can use different special electrolytes that can be found in each manufacturing plant. In most applications, nickel sulfate and nickel sulfamate-solutions are used. Using special additives, such as cobalt and organic compounds, and nickel deposits that are adjusted to the intended use and are stress-free and variable in the hardness, can be deposited. The throwing power, i.e., the ability of galvanic electrolytes, even in complicated geometric contours with elevations and indentations, to apply uniformly thick nickel coatings, was substantially improved in recent years in connection with technical application know-how.

Decomposition products of the above-mentioned organic additives may have an adverse effect in the individual case. Thus, the maximum temperature resistance for hard nickel deposits from sulfate baths is 300 °C. For the galvanic designer, it is therefore important to know for which processing method the electroplating process is required, so that the benefits of nickel, together with the electroplating, can be used for specific applications.

4.3.3  Galvanized Materials

Despite all the differences in the application, design, and execution of galvanized molds, the commonality remains to be the plating nickel material. In mold making, nickel is very important as an alloy component of high-strength and corrosion resis-tant mold steels. Pure nickel is almost never immediately processed to mold parts. Due to its chemical, physical, and mechanical properties, nickel material has been state-of-the-art for the electroplating fabrication of self-supporting components, such as shear sieves for dry shaver, parabolic mirrors, pressure cylinders, filters, complex parts for aerospace and rocket technology, and micro-precision parts, e.g., for medical drives, etc. The possibility is good for the use to repair wear parts in mold making through thick nickel-plating.

In electroplating, all the advantages offered by the nickel material are used for mold making. The most important properties of nickel for the processing of plastic materials are:

Wear resistance and hardnessGenerally speaking, the wear behavior of nickel, depending on the composition and hardness, almost reaches the wear behavior of hard chrome. Usually galvanized mold inserts for injection molding are used with a nickel hardness of 44 to 48 HRC. For other applications, such as in the aviation industry or in the slush technology, sulfamate nickel is used with a hardness of about 25 HRC.

5054.3 Galvanized Inserts and Molds

Corrosion resistanceNickel provides effective protection against air oxidation. Far more important is its resistance to aggressive media, e.g., in processing of PVC.

Demolding capability of plastic partsQualitatively perfect manufactured nickel coatings are ductile, non-porous, and have a perfect surface quality. Plastic parts can be easily removed from the mold in the production process. The high passivity of the nickel layer in the PU processing allows a substantial reduction in the use of release agents and thus satisfies the demand for environmentally friendly production. At the same time, an improved surface reproduction is achieved.

Fidelity of reproductionFine contours of the model surface are accurately transferred to the galvanized mold inserts through electroplating, such as genuine leather grains, skin texture, fine diamond cuts as used in micro-reflectors, high-gloss polish, etc.

4.3.4  Model Materials and Model Design

The decisive influence of the positive model for the successful manufacture of gal-vanized molds has been addressed in the procedure description. To ensure success, the advice and technical requirements of the galvanic designer should already be taken into consideration when planning. For the manufacture of positive models, both metallic and nonmetallic materials can be used: the material must be adapted to the particular application. In practice, the materials listed in Table 4.1 have proven valu-able. When making a selection, it is important to note that galvanized mold inserts with better surface quality and increased dimensional stability can be manufactured using metal models. Another advantage of metal models is that generally and, if necessary, completely identical mold inserts can be used for the galvanic structures.

For undercut and difficult-to-demold parts, thermoplastics, such as polymethyl methacrylate (PMMA) as model material for screws and screw gears up to about 15 mm in diameter, are used. Exceptions are small screws with high accuracy requirements that cannot be cut anymore due to the flexibility of the PMMA mate-rial. In these cases, brass models should be used. These are chemically removed a�er the galvanized construction of the galvanized nickel molds.

Epoxy resins are used when duplicating models. A casting resin or silicone negative mold is manufactured based on a master model. The casting resin-positive models are now casted in these molds. This procedure is also used when the material of the model is unstable towards the bath solution, e.g., wood, gypsum, leather, and others.

506 4 Manufacturing and Machining Methods

TABLE 4.1  Materials for the Manufacture of Positive Models

Material PropertiesMetallic MaterialsDrawn brass e.g. Ms 58, Ms 62

Easy to process, well polishable, sensitive to mechanical stress

Stainless steels e.g. Material Nr. 1.4300 and 1.4306

Not sensitive to mechanical stress, corrosion-resistant, very good surfaces, but hard to process

Non-metallic materialsPMMA Easy to process, very well suitable for contours which are hard to demold

Rotomolded PVC parts Especially suitable for the manufacture of toy-rotational molds; Attention: dimensional instabilities!

Epoxy casting resins Especially suitable for small and mid-size models as well as model reproductions and undercut contours

Epoxy laminating resins Preferred material for mid-size and large models; Attention: stiffening ribs or profile rack frames have to be laminated for stability reasons!

Tooling materials (PUR) The material is easy to process, possibly out of a casted block, especially suitable for large models of the aeronautical industry and in automotive engineering. Microporous surfaces have to be sealed.

In special cases, the so-called direct-coating models are used. These are manu-factured from a special polyurethane tooling material and are used for making impressions of grain structures with a suitable film material. This path is usually chosen for prototype molds to give the designer a better way to evaluate the product.

When selecting casting resins, close attention should be paid to low shrinkage. In addition, the top layer of the positive models should consist of a filler-free, acid-resistant, and non-porous surface resin and should be dyed dark. In the past, models were exclusively created in laminate structures for fabricating large molds for the aerospace and automotive industry, which still takes place today. These models consist of a surface resin and a fiberglass laminate of about 5 to 10 mm thickness in which fiberglass ribs or steel frames, fabricated from steel profiles, are laminated so that a sufficient stiffening of the models is ensured. Today, thanks to modern milling machines, such models are o�en made from tooling materials to save time and costs. Usually, casted polyurethane blank is mounted on steel bars and milled using CAD data, taking into account the shrinkage of the material to be processed. Finally, the surface gets a polyurethane coating and is fine-finished. A�er measuring, these models can then be used directly for the galvanized deposition and guarantee the required accuracy and surface quality. Here, a special characteristic should be noted: tooling material is usually available in sheets and is o�en glued together to form blocks. Since the joints may be noticeable on the nickel surface, it is not advisable to use this procedure. To produce dimensionally accurate models with a good surface quality can be very costly. To reduce the production costs for the model in multi-

5074.3 Galvanized Inserts and Molds

injection molds, duplication methods for models were developed that are particularly suitable for small parts such as pens or lipstick tubes. One method is to manufacture a so-called mother-galvanized mold based on a brass or steel-positive model and then complement it to a small mold. In this mother mold, models are pressed from a special plastic powder under high pressure and constant temperature. These are characterized by high surface quality and dimensional equivalence. The shrinkage is very low. The shrinkage for small parts, up to 10 mm in diameter, is about about 0.03 to 0.05 mm and is taken into account in defining the model dimensions.

The requirements on the surface characteristics of the galvanized molds are very different. In dolls parts and toy animals, satin matte surfaces are desirable. To this end, the most strongly undercut galvanized molds are “sand blasted” with corundum or other abrasives. For textured or smooth surfaces a surface treatment by etching or special blasting with different material mix is necessary.

If the manufacturing practices show that the walls of the part are uneven, a remedy can be found through the installation of auxiliary panels made from sheet metal, through painting with aluminum colors, or through wall thickness modifications of rotational mold. Inserts made from plastic material or steel can be fixed into the walls of the molded parts, but must be preheated to initial temperature of the mold. It is important that the pins, which are necessary for the acceptance, are properly installed so that no plastic can leak at these points during the rotation.

If there are inserts made from magnetizable materials, the fixation can be done using so-called holding magnets.

4.3.5  Clamps and Mounting Brackets

In general, appropriate supports should be provided for fixing the rotary molds to the machine arm. In galvanized molds, support ribs and reception flanges take over this job. In addition to connection, these elements should also prevent disruptions, and are used for the installation of fasteners, as shown in Figure 4.48. In almost all cases, these elements are rigidly attached to the mold walls by soldering to effect the heat transfer. By selecting angle irons, perforated sheets, thin flat irons, and others, the heat capacity of these clamps and supports is kept as low as possible. Also, the attachment takes place on locations where the wall thickening does not disturb.

For large molds, e.g., for instrument panels, rigid solder joints with different thermal expansion behavior could lead to tension, which would result in premature mold failure. To avoid this tension, large molds are floating mounted to the mold carrier of the machine.

For economical and clean work, the closing devices, which are needed for rota-tional molds, must be operated quickly and safely, yet still be tight. For simple

508 4 Manufacturing and Machining Methods

plane machined mold openings, as for toys figures and car armrests, flat aluminum caps are used, which are moved and locked through commercially available quick release. The aluminum caps are adjusted by screws connected to the movable arm of the quick release.

In many cases, additional springs are provided to compensate for small angular deviations. However, these must be temperature-resistant springs because normal springs become fatigued very quickly. For other mold types, screw caps, clamping brackets, or extender clamps can also be selected. In any case, the holding force of the clamping elements has to be adapted to the mold size and the sealing surface. Even if the locking system has been carefully designed and the surface sealing is clean, sealing problems can occur due to poor thermal expansion. Intermediate layers made from PTFE or silicone rubber can help in practice.

In many cases, the closures not only carry out the task of sealing but also serve for shaping. Rotationally symmetric sealing plugs are provided for dolls parts that can seal the mold openings securely via conical sealing surfaces. Bayonet closures in individual molds are advantageous, while series molds are clamped via clamping brackets and spring balance (Figure 4.49). The sealing plugs, which extend with their front part into the galvanized mold, receive appropriate mold contours and can, therefore, develop the connections for the head, arms and legs for doll parts (Figure 4.50).

If problems are expected during the demolding of PVC products with strong under-cuts, the following method can be used: The sealing cap is designed to go through a mandrel, which is extended to the inside so that it forms a recess on the PVC part. This is only sealed by a thin membrane that is easy to protrude through. A hollow probe is inserted into the recess, which is connected to a hose and a three way tap.

A

B

f

Section

Cut A-B

e

a

b

c

d

1-2

FIGURE 4.48  Galvanized molds with closing devices

5094.3 Galvanized Inserts and Molds

To facilitate the demolding, the three-way tap is switched over to vacuum. Should back formation of the contour be necessary a�er demolding, compressed air can be supplied via the third path. Precondition for this is a demolding temperature of 40 to 50 °C.

In the manufacture of balls, a similar recess, which is generated with a mandrel, serves for housing of the inflation valve, which in toy parts, is for the housing of the “squeaker”.

4.3.6  Finishing and Installation of Galvanized Injection Inserts

To meet the accuracy requirements that are placed on injection molds, it can be useful to perform the machining of the outer contour of the galvanized structured mold inserts before the model is pulled out of the galvanized insert.

FIGURE 4.49  Rotational-mold series mold

(a) PVC article (toy); (b) electroformed mold; (c) aluminum bung, conical with

molding piece; (d) support rack; (e) mechanically or pneumatically driven

clamping device; (f) pressure-balancing springs

FIGURE 4.50  Galvanic molds for the manufacture of doll parts made from PVC using

aluminum plugs and individual bayonet closure, as well as PVC sample parts

510 4 Manufacturing and Machining Methods

The raw-galvanized molds are aligned exactly to the clamping sha�s or based on the reference surfaces of the models and are processed on specially designed rigid machine tools. Figure 4.47 illustrates that the mold inserts are typically produced in height with excess. This excess is useful to avoid faintness of the angle, which will be discussed later. A�er external processing and demolding, the contour depth of the mold is processed to the final dimension, depending on the contour elongation of the model. In the mold inserts, which are manufactured in the nickel plus copper combination, these two materials are also present in the parting plane. Cutting speed, feed rate, and cutting depth are adapted to the specific conditions because they are milled or turned under the same machining conditions.

In injection molds, particular attention should be paid to the installation of the galvanized molds into the steel or master molds because the occurring injection pressure cannot be absorbed by the galvanized inserts. For this reason, the galvanized mold inserts must be installed free of play into the master mold. Thus, the occurring injection pressure can be immediately transferred to the surrounding steel molds. Cracks, which would be possible with a fit clearance, can be avoided.

If metal models were used for the electroplating, no reworking of the mold contour is required other than cleaning. When using plastic models, an electrically conduc-tive silver layer has to be applied, which initially remains adhered to the contour surface and needs to be removed chemically or by polishing.

4.3.7  Efficiency and Service Life

The economic advantage of galvanized molds is that positive models for electroplat-ing are easy to manufacture, whereas the mold contours and surface finishes at the negative mold using other production methods are manufactured with more difficulty. In addition to the above-described advantages that nickel offers as a mold material, the following further advantages result from the electroplating:

� High dimensional accuracy because a better control over the manufacture of the positive model is possible,

� no hardening distortion, � when using the combination of nickel plus copper, quick shot sequences can be achieved due to good heat dissipation,

� mold inserts, which are matching to the smallest detail, can be manufactured based on a metal model, and

� there is the possibility to adapt the technological values and properties of nickel to the changed application parameters.

5114.3 Galvanized Inserts and Molds

The possibly attainable lifetimes of the galvanized mold inserts correspond well to the lifetimes of the steel molds. The galvanized mold is o�en superior to the steel mold due to the high corrosion resistance as compared to deposition products, which are created during the processing of PVC. However, a prerequisite is the right fit in properly sized master molds, i.e., they cannot spring back under the injection pressure.

As an alternative to other production processes, the economic and technical advan-tages of electroplating in the area of injection mold making are particularly used for the manufacture of:

� small screws, screw gears, and gears with high precision requirements and small module sizes,

� reflectors, � thin, long, symmetrical or asymmetrical parts such as pens, sleeves, caps, pipette tips, and medical products,

� parts which have a realistic reproduction of genuine leather or wood surfaces, which can only partially be achieved by modern etching techniques, and

� plastic figures with an irregular progress of the parting line.

Electroplating mold inserts for transfer molding of artificial teeth made from acrylic material are also among the molds with an irregular progress of the parting line. These electroplating molds, of which larger quantities are produced annually, have significantly contributed to the introduction of more efficient production methods in this special field.

Like all manufacturing processes, electroplating has its limitations. The advantages listed are facing the following disadvantages:

� The temperature resistance of hard nickel and tungsten copper is only about 300 °C;

� Depth slits with a narrow width cannot be properly molded in a galvanized way; � The production time is dependent on the relatively low deposition rates of the special electrolyte. Hence, the mold designer must not make the walls of the galvanized molds too thick and should consult the galvanic designer if in doubt;

� Galvanized molds made from hard nickel are particularly vulnerable to bending stress.

The successful manufacture of galvanized molds includes the comprehensive knowledge and mastery of the “angular or edge weakness”. This is a hairline crack that results from sharp internal angles, low contact radii, and narrow slits along the bisector of the corresponding surfaces (Figures 4.51 and 4.52). Regardless of the thickness of the galvanized deposit, the faintness of the edges continues through the entire thickness and the galvanized mold breaks due to a low stress along this

512 4 Manufacturing and Machining Methods

hairline crack. This phenomenon is a result of physical laws. It could be influenced by suitable organic additives, but with the residual risk of an undesirable change in the material properties of the nickel layer. Figure 4.51 shows the measure, which eliminates weak edges during the copper construction by mechanical processing and subsequent galvanized construction, thus ensuring the durability of the mold inserts.

When molding narrow slits, the risk of a blowhole formation is added to the angle or edge weakness. The problem can be bypassed by using steel pieces to be galva-nized, which are previously inserted into models.

The electroplating of ejector and slide openings would also cause edge weakness. In order to avoid these problems, it is recommended to subsequently incorporate, mechanically or through spark erosion, bore holes and breaktroughs for sliders, ejectors, etc., (see Figure 4.53). The gate channels should be subsequently manu-factured.

In the past, it was very difficult to repair galvanized molds. The only repair mea-sures were the placement of pins, the time-consuming post-electroplating, or the

FIGURE 4.51  Principle illustration of edge weakness

(a) Edge weakness at sharp internal angles

(b) Edge weakness at a radii transition

(c) Continued edge weakness in the copper structure

(d) Compensation for the edge weakness

a: Model; Ks: Edge weakness

FIGURE 4.52  Edge weakness and the formation of blowholes when molding narrow grooves

(a) model; (b) nickel layer; (c) edge weakness; (d) formation of blowholes;

(e) galvanized steel piece with an undercut of (f)

5134.3 Galvanized Inserts and Molds

fitting of nickel fittings. With the advanced laser welding technology, there is now the possibility to apply nickel material to repair damage and defects through laser welding and then to post-process the contour mechanically.

4.3.8  Galvanized Molds for Other Plastics Processing Methods

The previous discussions refer mainly to the use of galvanized mold inserts in injection molds. Figure 4.54 shows an application example. The extensive field of rotational molding and the so-called slush technology is discussed in detail in Section 1.6.

With these applications, however, only a small area of the use of galvanized molds is illustrated. Currently, galvanized molds for modern fiberglass technology, the polyurethane foam processing, PUR spray technology, as well as for the use as laminating and negative deep-drawing molds (in-mold graining), are not only state of the art, but in some cases enable those technologies.

FIGURE 4.54  Injection molding galvanized mold inserts for impeller and spreading disc

FIGURE 4.53  Galvanized mold with a subsequently inserted slider brakethrough and injection

channel

(a) master mold; (b) side slider; (c) galvanized mold; (d) Cu-layer; (e) Ni-layer;

(f) tunnel gate; (g) gate channel

514 4 Manufacturing and Machining Methods

4.3.8.1  Molds for Processing Polyurethane Foam

The need for molds for the PUR foam processing has steadily increased, in which the security requirements in the automotive industry and the opportunities in the furniture industry have a large share. Cost-efficient machines and molds are important prerequisites for a trouble-free and low cost manufacturing.The processor has the choice between simple and very inexpensive casting resin molds, which are working well for some parts and quantities, and high-quality but expensive steel molds. Molds made from aluminum generally already meet the requirements applicable to the product. Galvanized molds are used when special surfaces such as leather grain and the like are demanded. Also, the experience that less release agent is required for nickel molds and that a better mold release of the polyurethane product is still possible, leads to the manufacture of the mold halves for the visible sides using electroforms. The core sides are then usually manufactured with the mold materials aluminum or steel.Using the galvanized molds, the mold structure is as follows:Temperature control pipes can be placed and positioned using small clips (if neces-sary) on the galvanized mold, which is still located on the positive model, and is executed with a wall thickness of 4 to 6 mm and integrated with undercuts into the small steel inserts. A prefabricated aluminum or steel frame is put over the thus pre-pared mold shell and is fixed in the flange area. Depending on the design principle, aluminum-filled epoxy resins are poured in or are compressed. Even back-filling with special concrete (Densit) has proven to be a very good solution. The mechani-cal transfer of the reference surfaces of the model to the mold takes place before demolding. Thus, the requirements for matching with the other mold half are met.For smaller parts, such as for control buttons (Figure 4.55), thicker-walled galvanized molds are also screwed directly to the mold carriers. Also, smaller galvanized molds can be back-filled with bismuth alloys.

FIGURE 4.55  Galvanized mold inserts integrated into a steel body for foaming control buttons

5154.3 Galvanized Inserts and Molds

4.3.8.2  PUR Spray Molds

Modern automotive interior components have previously been almost exclusively formed in the positive thermoforming process using foil. This process has been largely replaced by the slush-molding process described in Section 1.6, by the nega-tive deep-drawing technology (in-mold graining), and by the patented spray process.

In the spray process, the premixed polyurethane is sprayed with a robot into the tempered, open mold. The mold resides on a shuttle or transfer device and is then transported to the spraying position. During the further transport, the sprayed polyurethane material can react, and a�er completion of the reaction, the slush skin is removed from the mold in the demolding area. The slush skin should usually have a wall thickness of 0.8 to 1.2 mm. Extensive experience in programming of the robot arms and nozzles is required to achieve this. In addition, high demands are placed on the spray molds. Among other things, a constant surface temperature of, e.g., 65 °C ± 2 °C is required (Figures 4.56 and 4.57).

FIGURE 4.56  Complete spray mold for the manufacture of instrument panels

FIGURE 4.57  Sixfold spray mold for the manufacture of glove box covers

516 4 Manufacturing and Machining Methods

About 5 to 6 mm thick galvanized molds are manufactured for the spray technology, which are usually executed in the combination of hard sulfamate nickel plus copper or hard nickel plus sulfamate nickel. Inserts are integrated into the nickel or copper layer, through which a mounting on an aluminum or steel frame can be done. In order to guarantee the required temperature accuracy, the galvanized molds are provided with temperature control pipes on the back or with electric heating. It is advantageous to select multiple heating circuits and to directly lay the pipelines onto the layer of copper, because copper, as a good conductor of heat, provides for an even heat distribution. The heating systems are embedded in a sufficiently thick layer of filled epoxy resin, which is subsequently insulated to prevent heat radiation.

Two-colored slush skins are o�en required. The color separation takes place via a design groove, which is illustrated as a raised rib in the mold cavity. Normally, these design ribs are very narrow but are also designed to be very high and require the entire skill set of the galvanized mold experts when manufacturing. When spray-ing the two-colored slush skin, a mask immerses into the mold cavity and seals the appropriate mold part on the design rib, i.e., the contact pressure of the mask should be absorbed from the narrow design rib. Despite this extreme requirement, the life time of the spray molds is nearly unlimited; the molds are only endangered by collision with the robot system.

As an alternative for the manufacture of slush skins in the spray method, compact galvanized molds are also used for suitable parts for fabricating so-called cast skins. Here, the galvanized molds are manufactured as for the spray process and are mounted into a steel mold holder, adjusted towards the CAD data, provided with temperature control systems, and back-casted. To compensate for deviations, which are unavoidable for a number of model steps like original model, silicone negative, mother model, silicone negative 2, and bath model; the digitization of the negative galvanized mold follows then to manufacture the required core from aluminum or steel in a defined distance of 0.8 to 1.2 mm, which is usually constructed in several parts due to undercuts.

As with the so-called slush process, slush skins, which are manufactured in the spray or casting method, are inserted into a back-foaming mold in a second work step and are back-foamed there with a PU foam and connected with a carrier. When the part geometry allows, an effort should be made to design the mold to allow to spray and then back-foam or back-inject in the same mold.

4.3.8.3  Laminating Molds for the Aerospace Industry

In the modern aircra� industry, composites made from Kevlar or carbon fiber are increasingly used for manufacturing large components. This requires appropriately designed molds to withstand the stresses in an autoclave at temperatures up to

5174.3 Galvanized Inserts and Molds

250 °C and to be vacuum-tight at the same time. In addition to molds made from steel or aluminum, galvanic molds made from sulfamate nickel with integrated steel inserts, via which a mounting on a mobile tubular frame is done, have proved to be very effective (Figures 4.58 and 4.59).

The design of the positive models is of particular importance, because large-sized galvanized molds are o�en used in the aerospace industry, and mold shells of up to 9 m long and 3 m wide are currently manufactured in Germany. Bath models

FIGURE 4.58  Galvanic mold for aircra� engine cowling, mounted on a steel frame

FIGURE 4.59  Galvanic mold with rear section for helicopters cowling when measuring and

adjusting

518 4 Manufacturing and Machining Methods

made from glass fiber reinforced epoxy laminate must be designed to be very stiff, so dimensional changes may not occur during electroplating. The progress of the milling and modeling techniques in combination with the new polyurethane tooling materials has resulted in the fact that the bath models are directly milled from a possible non-porous tooling material using CAD data. This manufacturing process does not only reduce costs but also manufacturing time.

The following advantages of galvanized molds are offered to the design engineer in the aerospace mold making:

� exact shaping and excellent surface finish,

� lightweight and therefore easy handling,

� thin wall thicknesses require a low thermal management.

� the hot air in the autoclave can flow around the mold shells dust free,

� flat galvanized mold shells can be adjusted to the desired accuracy using support frames due to the integration of steel inserts into the galvanized mold or the subsequent welding of threaded bolts,

� the “spring back” behavior of fiber composites, which is o�en occurring on U-shaped components, can be compensated for when post-adjusting galvanized molds to ±5 mm, and

� models made from tooling material can be reused for mold duplication.

The category of laminating molds includes the manufacturing of car body parts, which are normally o�en produced in the SMC or RTM process using galvanized molds. When using galvanized molds, the required component surface can be easily achieved. The good surface quality and cost advantages make it possible to manufac-ture molds made from carbon fiber using galvanized molds for the production of car body parts (Figure 4.60). The galvanized molds are exclusively manufactured from nickel for this application, are equipped with a heating system, allow temperatures of up to 150 °C, are mounted to a steel frame, and are backfilled with a special concrete (Densit). Since these materials have almost the same thermal expansion coefficient, the bimetal effect, which results from the heating and cooling phase, is usually avoided, and the fit of body parts to each other is guaranteed. Of course, the expansion parameters in the model and mold design have to be considered.

Vacuum systems are o�en used in the so-called RTM process. The mold load is therefore extremely low and galvanized molds can be manufactured in a light-weight design, i.e., the galvanized molds with wall thicknesses from 3 to 5 mm are fixed to the tubular steel frame using inserts such as the molds for the aerospace industry. The pipe system for the temperature control is inserted and covered by a 10 to 15 mm thick cast resin laminate layer. This mold manufacturing method is very inexpensive. It was successfully used for the production of car body parts.

5194.3 Galvanized Inserts and Molds

4.3.9  Negative-Stamping Deep Drawing Process (In-Mold Graining)

In addition to the production of interior components such as instrument panels, door panels, and so on, which are manufactured in the slush or spraying process, the fabricating of thermoformed parts is again more important due to cost reasons. When using grained films, which are suctioned over a positive core using a vacuum, the material stretches and the grain loses its conciseness at highly stretched points.

To encounter this unwanted effect, the negative-stamping deep drawing process is used. In this case, smooth films are drawn into a negative contour and in the final phase of the deep-drawing operation, the grain structure, which is located in the mold surface, is transferred to the surface of the deep-drawn part. A vacuum-capable mold is required. Based on Japanese developments, it is now possible to produce micro-porous galvanized molds made from nickel, whose pores are above a diameter range of 0.1 to 0.2 mm. The pores are distributed throughout the entire mold surface and ensure a perfect deep drawing and grain stamping due to their high number without having unwanted marks on part (Figures 4.61 and 4.62).

The galvanized mold is, as usual, mounted on a steel or aluminum frame with a vacuum-compatible resin-based backing material, and can be used on conventional thermoforming machines.

Previously, it was not possible to produce deep drawn parts with different grain struc-tures. Through the new TPN (technical porous nickel) forms technology, it becomes

FIGURE 4.60  Complex contour image of a laminate mold for carbon fiber components

520 4 Manufacturing and Machining Methods

possible to manufacture corresponding nickel molds. Hereby, it is only necessary to create models with different grain structures, stitch seams, and applications, etc.

The negative-stamping deep drawing allows for high production volumes with short cycle times. In addition, deep-drawing molds, which are manufactured using nickel mold shells, can be, in many cases, combined with subsequent work operations, namely:

1. Thermoforming and laminating,

2. Thermoforming and injection molding (or press forming), and

3. Thermoforming and back-foaming PUR.

The development in this area is still ongoing. It can be assumed that new ways will be found to match parts and molds in such a way that the production is inexpensive and still attractive.

FIGURE 4.61  Back of a porous TPN (technical porous nickel) shell with inserted temperature

control pipes

FIGURE 4.62  Negative deep drawing mold for an instrument panel with a view into the cavity

5214.4 Polishing Technology in Mold Making

 ■ 4.4  Polishing Technology in Mold Making

C. Steiner

4.4.1  General Information

Mainly hand-held devices are used in precision machining of surfaces in mold making. In many cases, this still represents a more economical way of working, especially when staff is well trained and the polishing work station is well equipped. In the series production, and for simple surface geometries and low surface quali-ties, however, automated polishing processes can be used and a suitable quality can be achieved

4.4.2  Definition of the Term Surface Roughness

Surface roughness is a term for defining surface qualities. At least two criteria are needed to evaluate the surface of a mold insert. First, the accuracy of the geometry of a surface is assessed, i.e., radii and flatness are checked for accuracy and are measured.

Second, the quality of the polish is assessed, which can be determined by measuring the surface roughness. The polish is o�en diminished by polishing defects such as scratches, holes, pores, pinholes, and the orange skin effect.

The most common characteristics for the surface roughness are:

Ra as the arithmetic mean of the deviations of the roughness from the midline,

Rz is the average from the peak to valley distance of five individual sampling lengths,

Rmax is the largest individual roughness within the evaluation length,

Rt is the largest difference in height between peak and valley of the evaluation length.

The schematic representation of the parameter Rmax is shown in Figure 4.63.

All values can be determined, for example, with the aid of surface roughness test instruments by scanning the surface. (According to the new nomenclature, surface roughness can be defined with the characteristic value Rmax. In order to meet the accuracy requirements in steel mold making, Rmax should be used to its advantage.)

522 4 Manufacturing and Machining Methods

FIGURE 4.63  Characteristic value Rmax

4.4.3  Systematic Polishing Technique

Good surfaces can only be achieved through the application of logically consecutive processes, which always leads to smaller and smaller surface roughness.For example:1. Flat surface through: milling, grinding, lapping, polish lapping, polishing;2. Cylindrical external surfaces through: turning, grinding, lapping, polish lapping,

polishing;3. Cylindrical internal surfaces through: drilling, reaming, fine boring, honing,

lapping, polish lapping, polishing.

The roughness depth ranges are shown in Table 4.2, which can be achieved using various machining processes.Processing steps, as they should be complied for the fine machining of surfaces in mold making are shown in Table 4.3.Table 4.3 shows what the individual processing methods look like using different polishing devices in terms of the shape of the surface to be processed and which qualities can be achieved in each case.Taking out some machining processes, it can be said that the achievable surface qualities are limited for each procedure. The le�-sided wedge-shaped outlet of the bars means that better quality and lower surface roughness can be achieved under special conditions with most procedures. Although the slightest roughness depths can be created with today’s techniques such as erosion or when working with high-frequency spindles, rules of a polishing progress still remain in force. The only consequence: today, in contrast to earlier, a finer granularity is used for starting the precision machining process.It must however be kept in mind that the hardness and microstructure of the steel or metal surfaces to be machined are to be considered, especially if high surface quality has to be achieved.

5234.4 Polishing Technology in Mold Making

TABLE 4.2  Achievable Roughness Depths of Metal-Cutting Tools

Processing Method Roughness Depth Range in μm DIN 4766

0.04

0.06

0.10

0.16

0.25

0.40

0.63

1.00

1.60

2.50

4.00

6.30 10 16 25 40 63 100

160

250

400

630

1000

Longitudinal TurningFacingDrillingGun BoringReamingPeripheral MillingFace MillingFilingRound-Longitudinal GrindingRound-Plane GrindingFlat GrindingPolish GrindingSuperfinishing (honing)Cylindrical LappingFlat LappingPolish LappingPolishingBuffingElectric Discharge Machining

TABLE 4.3  Processing Methods of Precision Machining Mold Surfaces

Equi

pmen

t an

d M

etho

d Processing method Achievable Roughness in Rt in μm 1 μ = 0.001 mm

0.04

0.06

0.1

0.16

0.25

0.4

0.63

1 1.6

2.5

4 6.3

10 16 25 40 63 100

160

250

400

630

1000

ring-

finis

h Grinding Diamond Ring

Lapping Metal Ring

Polish Lapping Plastic Ring

Polishing Cloth Circle Ring

flex-

poli

Short-Stroke-Filing Rippling

Short-Stroke Grinding

Short-Stroke Lapping

Short-Stroke-Polish Lapping

poli-

roto

r Grinding Pins

Plastic Body/Wood

Felt Body

For flat and spherical surfaces

For flat and curved surfaces

For flat and curved surfaces/radii

524 4 Manufacturing and Machining Methods

4.4.4  Polishing Behavior-Influencing Factors

The polishability of various grades of steel is usually dependent on nonmetallic inclusions that negatively affect the quality of the polish due to hardness differences. Especially the hard oxides, but also the carbides which break out while polishing, lead to a poor surface quality. The selection of the steel is thus a decisive factor for the quality of the polish. Therefore, materials which are manufactured in special re-melting and degassing or sintering processes, and thus have good homogeneity, high purity, and low degree of segregation, are increasingly used.

The surface quality improves with increasing material hardness. To facilitate the machining, however, so� annealed steels are used.

In this state, the ferritic structure component is disturbend when processing. The ferrite, which is as so� as copper, sits firmly in the roughness of the blades result-ing in built-up edges; an effect that occurs even with files with added chip spaces.

When machining steel, it should not be forgotten that a processing of the crystals in the structure takes place, i.e., the crystals are cut, shaped, and polished. There-fore, the principle cannot be invalidated that unevenness of a surface increases the longer a so� tool will be used.

Although more accurate surface geometries can be produced with machine pro-cessing, only limited roughness values are attainable. Accordingly, the selection of sequential machining processes has to be made.

4.4.5  Polishing Technologies

With the actual machining process of precision machining, the process steps

� lapping, � polish lapping, and � polishing

should only be started with surface roughness values of about 10 μ Rmax or even better values of 5 μ to 2 μ Rmax.

4.4.5.1  For Superfinishing (Polishing) Surface Preparatory Leveling Technology

Since the roughness values of 5 μ to 2 μ Rmax can o�en not be achieved mechani-cally in mold making, preparatory measures are taken before superfinishing. Such a measure is also the removal of an electrical discharge layer.

The quickest way to flatten a surface is by using a sintered diamond ring and an angle hand piece (Figure 4.64).

5254.4 Polishing Technology in Mold Making

FIGURE 4.64  Grinding process with a sintered diamond ring and an angle hand piece

A diamond ring with a diameter of 8, 12, or 20 mm, depending on the size of the surface to be processed, is guided over the grease-free surface at moderate pres-sure. The tool must be regularly sharpened either when first using a sintered tool, as well as a�er prolonged use. This is best done with 80 to 100 grit sandpaper. Using a diamond ring, a spark-eroded surface of about 10 cm2 can be machined in 1 minute and a roughness of about 5 μ Rmax can be achieved. In a second step, a 2 μ roughness Rmax is reached with some fluid (lapping liquid). For normal surfaces in mold making, no other method would be quicker and not more efficient.

When touching-up or during leveling of copy-milled contours, these rings can even be used on any curved surfaces with great success, as long as the radii are not too small and the rings are gimbaled.

In addition to the initial state of a cavity or area, it is also its geometry that deter-mines the use of the corresponding mold (Figure 4.65).

Depending on the initial state, milling cuttens in high-speed spindels on abrasive stones should be used to level any curved surface. Rubber-bonded abrasive wheels may also be used, particularly when dealing with nonferrous metals. Such rubber-bonded abrasive stones are also popular in glass mold making (Figure 4.66).

Important when working with rotating grinding pins is the absolute concentricity of the tools. The dressing of rotating tools can be performed easily using a diamond dressing file. The speed range should not be too high for rotating tools. A speed range of 15,000 min–1 is o�en already sufficient because at higher speeds depres-sions at the deflection can occur.

When working with rotating grinding pins, milling cutters, and so on, flat surfaces cannot be achieved, for example, that a�er such a step, the surface must then be ground with a sanding file (honing stone) or a ceramic fiber sanding file. Sanding files and ceramic fiber sanding files are well distinguished and can therefore be used to reach inaccessible recesses and complicated cavities.

526 4 Manufacturing and Machining Methods

In addition, so�-bound honing stones quickly adjust to the contour shape without breaking them. The selection of honing stones depends on the material. So�-bound stones are used in annealed steels and hard-bound stones are used in hardened steels. The most common honing stones are the aluminum oxide stones, oxide ceramic grinding files, or SiC stones. Qualities should be preferred, which are used with suspension.

Important when using abrasive files is the sequence of the concerted increasingly finer grain sizes, e.g., 180, 240, 320, 400, 600, 800, 1,000 and 1,200; and that a second operation is running in a transverse direction to the previous line. The working stroke should not be too large, optimally it is a stroke of 1.5 to 3.0 mm and 5,000 to 7,000 strokes/min (Figure 4.67).

If better surface roughness and geometrical accuracy is required, such as for mirror gloss surfaces roughness values of about 0.1 μ Rmax, and for optical purposes when 0.04 μ Rmax is necessary, lapping can be used for normal steel grades to flatten the surface.

FIGURE 4.67  Using a honing stone in processing an outer radius

FIGURE 4.65 Working in small radii

with a grinding pin made from aluminum

oxide and a grinding hand piece

FIGURE 4.66: Working on the inside radius

with a rubber-bonded aluminum oxide

grinding body

5274.4 Polishing Technology in Mold Making

4.4.5.2  Lapping

Metal tools are used in the lapping process (Figure 4.68). These will not slacken and will guide the lapping grain to a precise distance to the work piece surface.

In this process, the lapping grain is rolled over the surface to be processed and must not have cutting effects. Therefore, lapping is done with the addition of fluid without any pressure. The fluid ensures that the grains are not grinding against each other and affects the distance between the grains. From experience in the relevant industry, pressure when lapping should not substantially exceed 300 N/cm2. The polisher must therefore ensure that the tool is only touching the diamond grit and to get it to roll with this movement. In mold making, it o�en happens that a lapping tool is only a few mm2 in dimension so that, based on individual diamond grit tips, pressures of several thousand N/cm2 are applied on microsurfaces. If the lapping pressure is too high, built-up edges are almost inevitable as the result because the diamond granules are pressed into the lapping tool and the stuck grains are cutting. The resulting chips cannot be removed, especially since there is no chip space, such as it must be present at milling machines, drills, reamers, circular saws, etc.

The accuracy depends on the initial roughness depth and on the unevenness, which must be removed. This means that it should be attempted to coat it as fine as pos-sible when processing using machine tools, because no hand-operated lapping and polishing tool can work as accurately and as fast as a machine tool.

Especially during EDM of slots, using an additional finishing electrode can help getting the finest possible surface finish. The speed also depends on the initial roughness depth, because this determines how much material can be removed by a different processing method in order to obtain a surface roughness that can then be smoothed out by the polishing process.

FIGURE 4.68  When using the stroke movement of a file hand piece, a brass body is moved

over the surface with diamond paste adding fluid without pressure

528 4 Manufacturing and Machining Methods

Working Stroke

~ 1000 �m Lapping Tool

Workpiece

100

�m70

�m

FIGURE 4.69  Schematic representation of material removal by loose diamond grit in the die

and using the stroke of a file hand piece

In principle, material can only be removed when the cutting tool or the blades are moving. Assuming, for example, the die of a box should be processed; lapping could be used as a processing method. If the lapping body is moved on the bottom of the die using a specific stroke against the vertical wall, only the lapping grains are working with the tool support surface. In the deflecting area of the stroke movement, the lapping grains are only working temporarily, thus less material is removed. Therefore, the surface is inevitably uneven (Figure 4.69).

For the processing of sharp corners, then square, triangular lapping grains, which are moving in an oscillating motion, are better to work with.

It is important to realize that polishing absolutely cannot be done up to the die wall because the lapping grain cannot roll right up to the wall and thus cannot remove material.

For cases where the sharp impression of corners is very important, the designer must provide for a division of the mold.

The diamond grain is one of the most important tools for polishing. There are three main sources of diamond grain sizes:

� broken natural diamond grain, � mono-crystalline synthetic diamond grain, according to a process developed by General Electric, USA, and

� polycrystalline synthetic diamond grain, using a process developed by DuPont, USA.

The diamond pastes with grain sizes of 40 μ, 30 μ, 15 μ, and 10 μ, which are used for lapping large surface roughnesses, consist of polycrystalline synthetic diamond grain. The grain is suitable for surface roughnesses due to its many small edges, since it guarantees a high removal rate. The mono-crystalline diamond with much better

5294.4 Polishing Technology in Mold Making

smoothing properties should be used for grain sizes of 7 μ, 5 μ, 3 μ, 1 μ, and finer. The broken natural diamond grain is used in pastes, which are used for hard metals.

In diamond pastes, not only is the type of diamond grain important, but also the concentration, and particularly the grain size.

It is important that diamond pastes have a particle size range, which will also include smaller grains. These result in a variation in size and allow the small lapping tool to get on top of the normal grain, without pushing it in front of it (example in Figure 4.70).

Defined roughness values are achieved with each diamond grain (see table).

Diamond grain – Steel qualityBasic rule:50 μ diamond grain → annealed Steel 600 N/mm2 strength = 10 μ Rmax

50 μ diamond grain → hardened Steel 600 N/mm2 strength = 7–8 μ Rmax

30 μ → annealed = 4.8–5.5 μ Rmax /hardened = 3.0 μ Rmax

20 μ → annealed = 3.5–4.0 μ Rmax /hardened = 2.5 μ Rmax

15 μ → annealed = 3.0 μ Rmax /hardened = 0.8–1.0 μ Rmax

10 μ → annealed = 1.0–2.0 μ Rmax /hardened = 0.08–0.09 μ Rmax

7 μ → annealed = 0.07–0.08 μ Rmax /hardened = 0.07–0.08 μ Rmax

From the 7 μ diamond grain, there is no difference in roughness (Rmax) between annealed and hardened material: 3 μ → annealed = 0.06 μ Rmax /hardened = 0.06 μ Rmax

1 μ → annealed = 0.04 μ Rmax /hardened = 0.04 μ Rmax

The diagram shows that it is absolutely necessary to work as quickly as possible to gradually level the surface with increasingly finer grains.

ToolHeight 1

ToolHeight 2

Workpiece

10 �

m6

�m

34 �

m

FIGURE 4.70  Schematic representation of the rolling behavior of the diamond grain and the

variation when using grain mixtures

530 4 Manufacturing and Machining Methods

It is such a big step between the diamond grain of 15 μ, with roughness values from 1 to 3 μ Rmax (depending on the material), and the diamond grain of 7 μ, with values from 0.07 to 0.08 μ Rmax, that it is understandable why intermediate steps should not be skipped or omitted. This applies not only as explained in this example for the level of 10 μ, which is between 15 μ and 7 μ diamond paste.Absolute cleanliness should be maintained between the individual work steps.The cleaning process a�er each grain change should also cover washing hands.The following mnemonic for lapping is incorporated for a better understanding:“When using metal tools in combination with diamond paste and fluid, the method should be called LAPPING.”Through the rolling diamond grain in lapping and the braking particles on the surface, the surfaces in lapping remain matte up to the 1 μ diamond grain. If the surface should, besides the desired accuracy also be glossy, it will be taken care of polish lapping.

4.4.5.3  Polish Lapping

The next processing step is polish lapping where elastic tools such as hard wood, plastic, vulcanized fiber, rubber, etc., are used. In polish lapping, the diamond bodies are pressed into the mold by consciously applying pressure.No lapping liquid is used, that is, it is a dry process. The diamond bodies react elastically and smooth the surface with its negative cutting blades. The material removal is limited and surface roughnesses are only slightly flattened. With this processing method, a good gloss can be achieved even with larger particles. The coarser the grain used, the worse the micro-geometry of the surface, because the crystal structure of steel consists of harder and less hard components. Ferrite phases and nickel components as so� components are removed faster with elastic and so� support tools than the hard components, such as chromium carbide and cementite.

FIGURE 4.71  A gimbaled plastic ring is guided over the surface with pressure and without

dilution fluids using an angle hand piece

5314.4 Polishing Technology in Mold Making

The elastic tool holder avoids the hard components and washes out the so�er parts. The longer elastic polish agent carriers are used; the coarser the crystal structure, the greater the risk of over-polishing, also known as the orange peel effect because of the appearance. Figure 4.71 shows a polish lapping step.

4.4.5.4  Polishing

It would be suggested that the word “polishing” be used as a processing step with very so� polishing tools to obtain glossy surfaces as shown in Figure 4.72.With very so� tools, such as felt, leather, and cloth, even surfaces with higher surface roughness can be polished. The diamond grains in their full size are practically embedded and firmly held in the very so� tools. If only the paste without fluid is used, an almost closed, shiny, black layer of paste, corn, and removed material, which polishes and flattens the workpiece surface, forms on the tool surface.Generally applicable is that the longer a crystalline surface is polished the worse it gets. Polishing should be the shortest step in building a qualitatively good surface, provided that the necessary preparations for polishing were carried out cleanly and conscientiously.A common traditional methods of precision machining has deliberately not been mentioned, namely emery cloth or sandpaper.Emery cloth behaves elastically and is therefore unsuitable for precision machin-ing for the already mentioned reasons. Sandpaper is much more stable. With both, there is a risk that grains, which are not well fixed, will break out and leave deep cracks on the surface to be processed that are due to chipping. For the processing of small edges and narrow slits, sandpaper is an effective tool in a conventional machining method. However, special sandpapers are chosen, which result, together with sanding belt holders and grinding hand piece, in a useful tool. Careful working in the short strokes to avoid built-up edge is essential (Figure 4.73).

FIGURE 4.72  Polishing a radius with

a felt body using a grinding

hand piece

FIGURE 4.73  Sanding belt holder with

waterproof sandpaper for

processing of narrow slits

532 4 Manufacturing and Machining Methods

4.4.6  Ultrasonics

Ultrasonic lapping and polishing machines are available as easily manageable, effective, and thus economical equipment (Figure 4.74) for the superfinishing of small and very small areas, bars, and slits.

The surfaces are prepared in a very short time with sintered diamond files or profiled ceramic fiber files. Then, lapping is done without pressure and then polish lapping to a high gloss finish is achieved using vulcanized fiber and wood (Figure 4.75) with some pressure.

FIGURE 4.74  Work in hard to reach contours

with the ultrasound technique

when using a ceramic fiber file

4.4.7  Electric Discharge Machining/Erosion for Brilliant Surfaces

For superfinishing and mold making, the enormous development of the EDM tech-nology of the past few years has been very helpful. Die-sinking EDM in combina-tion with the planetary technology, the use of graphite electrodes as well as wire cutting with special cutting systems, the use of finishing electrodes and appropriate machine settings result in roughness values of a few μ.

Whether excellent surface quality with all grades of steel is achievable, electrical discharge layer with minimal weld penetrations appears questionable.

The possibility of reducing the EDM layer appears almost achieved. Impurities or alloying elements, which are always present in steel, lead to different melting points in the microstructure of the steel. This results in an uneven surface removal. So it is almost always the carbides which cause very deep penetration points.

FIGURE 4.75:  Polishing a surface with a

wooden rod and diamond

paste when using the

ultrasound technology

5334.4 Polishing Technology in Mold Making

In individual cases, a balance between the utilization of the existing EDM capacity, the cost of producing finishing electrodes, and the time required for superfinishing has to be found.

A good polisher saves the EDM capacity if possible. The polishing workplace should also be considered from a cost perspective. For economical superfinishing in mold making, an employee should be accordingly trained. In addition, a polishing work-place, which is tailored to the needs of the operation, should be available. How a workplace can look can be seen in Figure 4.76.

The equipment needs to enable a fatigue-free operation, and the surfaces to be processed must be brought easily and quickly into the best position. A water supply for the cleaning of mold inserts between the work steps is also included in working economically. If all these conditions are met, high quality demands can be placed on a polisher.

FIGURE 4.76  Completely equipped polishing workplace

534 4 Manufacturing and Machining Methods

 ■ 4.5  Heat Treatment and Surface Finishing Techniques

P. Vetter

4.5.1  Introduction

Competitive pressure and constantly increasing requirements as to the properties of plastic moldings demand innovative high-grade mold concepts. The correct heat treatment and surface treatment strategy can play a decisive role in determining the quality and efficiency of the complete mold in international competition. High-grade plastic mold steels and the numerous surface finishing options available enable tool and mold makers to determine the optimum combination of steel and coating for each project, in joint consultation with the steel maker, surface coating contractor, plastics supplier, and plastics processor, especially in the case of new molds. Since the properties of the mold steel selected are a salient factor when selecting the appropriate surface finishing process – for molds already in service as well as for new molds – this paper first considers the principal plastic mold steels (which are mainly standardized in ISO 4957 [1]), together with the heat treatment that may be required. The surface finishing processes now generally used in tool and mold making are then described and their advantages and disadvantages discussed, to inform selection of the most suitable process.

4.5.2  Heat treatment of plastic mold steels

Continuous improvements in machining technology (HSC milling technology, hard coated indexable inserts, etc.) now make it possible to use pre-hardened mold steels in most cases. This trend is based particularly on the growing time and cost pressure in plastic mold making. Figure 4.77 illustrates how the process route in tool and mold making for plastics processing is shortened by using pre-hardened tool steels.

To meet the special requirements the tools and molds have to satisfy in terms of functional properties such as wear resistance, corrosion resistance, or high hardness combined with adequate toughness, a range of special steels are available tailored to the particular criteria, some of them high-alloyed. Depending on the steel concept, these may require more or less complex heat treatment a�er pre-machining in the annealed state.

5354.5 Heat Treatment and Surface Finishing Techniques

Steel Production Mold Production

Delivery from Stock

Time

Pre-machining

Pre-machining

Stress

Relieving

Stress

Relieving

recom-

mended

Machining

to

Hardening

Allowance

Finish-

machining

Finish-

machining

Hardening

and

Tempering

Surface

Finishing

Surface

Finishing

Save Time and

Money

Through-Hardening Steels

(annealed)

Pre-hardened

FIGURE 4.77  Time and cost saving by using hardened and tempered plastic mold steels

4.5.2.1  Hardened and tempered plastic mold steels

Pre-hardened plastic mold steels are available ex stock up to a surface hardness of 44 HRC, depending on the steel grade, and are machined to the finished dimensions of the mold with no other heat treatment, apart from stress relieving as required. This method saves time and money, and significantly minimizes the risk of failure (change in dimension and shape, or cracking) of a contour-close hardening process. Stress relieving at about 40 °C below the tempering temperature is advisable a�er pre-machining in cases where a large cutting volume and/or complicated mold geometries are involved, thermal surface coatings have been specified, or extensive welding work may be required later. Otherwise internal stress due to machining can lead to increased changes in dimension and shape. Figure 4.78 shows a suitable sequence of operations for the treatment and machining of pre-hardened plastic mold steels.

The alloying concept for hardened and tempered steels is generally matched to the as-delivered hardness. If higher hardness is subsequently required, further heat treatment may be possible. But the steel supplier should be consulted beforehand to determine technical feasibility. Coating technology provides viable alternative solutions for targeted improvement of near-surface mold properties.

536 4 Manufacturing and Machining Methods

0

100

200

300

400

500

600

700

800

Time

Tem

pera

ture

ºC

Slow Cooling

Hardness Testing,

Delivery, Pre-machining

of the Contour

Tempering Temperature,

e.g., 600 ºC

Stress Relieving,

e.g., 560 ºC

Maximum Process Temperature,

Coating/Welding without losing

hardness, e.g. 520 ºC

Finish-machining, Grinding

Allowance if applicable

Slow Cooling Slow Cooling

FIGURE 4.78  Correct temperature profile for steel treatment and machining, illustrated by the

tempering steel 2738 mod. TS (HH)

4.5.2.2  Through-hardening steels

The property of good through-hardenability with a high level of hardness is achieved by alloying elements (carbide formers). This entails higher costs in steel procure-ment compared to lower alloyed steels (alloying elements, additional resources required in steelmaking, dimensional constraint in production). Further disadvan-tages are time-consuming, costly operations (hardening process), and declining thermal conductivity as the alloy content increases. Despite these disadvantages, the ability to achieve very high mold hardnesses offers significant advantages where polishability or wear-resistance is required, especially in combination with thermal surface finishing.

As already illustrated in Figure 4.77 the operation for producing a mold from through-hardening steel differs significantly from that for a mold made of pre-hardening steel. Through-hardening steels are pre-machined in the annealed state, followed by stress relieving. This reduces the stresses induced by rough machining. This is followed by further machining to hardening contour. Because of the possible change in dimension and shape during hardening (depending on factors such as the material, different lattice structures of the microstructure types, mold geometry, mass distribution, and heat treatment process), the necessary production oversize to be allowed must be agreed in advance with the heat treatment contractor. It is also essential to determine a design suitable for heat treatment, in order to prevent

5374.5 Heat Treatment and Surface Finishing Techniques

increased change in shape or even stress crack failures. It is preferable to avoid sharp-edged transitions, replacing them with large radii or chamfered edges as far as possible. Uneven wall thicknesses, large differences in volume, forging surfaces that have not been adequately removed, and coarse surface textures all increase the risk of failure. Further information on this is given in reference material such as the Steel Iron Material Sheet SEW 220 [2].

Figure 4.79 shows the thermal cycle for hardening, tempering, and the possibility of thermal surface coating.

For hardening and subsequent tempering operations, the heat treatment contrac-tor should be guided by the recommendations of the steel maker given in the data sheets for the material concerned. These heat treatment recommendations are derived from the chemical composition of the steel used. Material-specific, continu-ous and isothermal Time-Temperature-Transformation (TTT) diagrams, as shown in Figure 4.80, describe the transformation behaviour of the steel’s microstructure [3].

Mold steels are mainly hardened in vacuum furnaces (Figure 4.81).

The temperature sequence in the vacuum furnace is tracked using thermocouples in the furnace chamber, and a fixed or moveable thermocouple at/in the mold if possible. At least two equalizing stages are required in the heating phase, to avoid stress cracking. This adjusts the temperature gradient between surface and core of the mold. In the case of steels with a hardening temperature ≥ 900 °C, three equal-izing stages are required at about 400, 650 and 850 °C. When the austenitising temperature has been reached, a sufficiently long soaking time (see material data sheets) must be allowed for transformation processes (e.g. dissolving carbides).

Tem

pera

ture

Time

Pre

-mach

inin

g

Fin

ish

-mach

inin

g f

or

Hard

en

ing

600-700 ºC

Hardening

Temperature

Oil/Air

Hot Bath

500-550 ºC

Equalize Temperature

Preheat

Stages

Air

CoolingAir

Cooling

Stress Relieving Preheating, Austenization,

QuenchingTempering

Thermal

Surface

Treatment

Furnance

1

2

3

1

2

FIGURE 4.79  Hardening and tempering of high-alloyed plastic mold steels

538 4 Manufacturing and Machining Methods

Tem

pera

ture

[ºC]

Time [s]

1200

1100

1000

900

800

700

600

500

400

300

200

100

00.1 1 10 100 1000 10^4 10^5 10^6

Austenitising: 1020°C, 10 min.Te

mpe

ratu

re [º

C]

Time [s]

1200

1100

1000

900

800

700

600

500

400

300

200

100

00.1 1 10 100 1000 10^4 10^5 10^6

Austenitising: 1000°C

FIGURE 4.80  Continuous and isothermal time-temperature-transformation diagram of the

steel 1.2343 (X37CrMoV5-1) [4]

5394.5 Heat Treatment and Surface Finishing Techniques

The rate of cooling, which depends on the capacity of the heat exchanger in the vacuum furnace as well as on the gas pressure, is set according to the TTT diagram to achieve a fine martensitic microstructure that is as uniform as possible.

Cooling can then proceed in two different ways, depending on the material. In addition to continuous cooling, there is also the option of “hot bath simulation” (Figure 4.82).

This process simulates the hot bath method of hardening in a salt bath, and has the advantage that temperature equalization between face and core takes place in the austenitic microstructure state (e.g. at about 550 °C in the case of 1.2343

Ofentür

Gasklappe (unten)

Gasklappe (oben) Heizelemente

Wärmetauscher

Kühlventilator

Heißgasumwälzer

Heizkamme r

Gas Flap (top) Heating Elements

Furnace

Gas Flap (bottom)

Hot Zone

Heat Exchanger

Cooling Fan

Convection Fan

FIGURE 4.81  Schematic diagram of a vacuum furnace (Source: Schmetz GmbH)

Tempe

ratur

oberhalb MsMs

max. Gasabschreckdruck

min. Gasabschreckdruck

Ende Warmbadsimulation

Schaltpunkt “Umwälzer langsam ”

“Konvektives Erwärmen”

Zeit

Tem

pera

ture

Time

Max Gas Quenching Pressure

Min Gas Quenching Pressure

"Fan slow" switchpoint

"Convective Heating"

End of Hot Bath Simulation

above MsMs

FIGURE 4.82  Using hot bath simulation in vacuum hardening minimizes changes in dimension

and shape

Surface temperature; Core temperature

540 4 Manufacturing and Machining Methods

(X37CrMoV5-1). This minimizes the stress differential and hence the risk of change in dimension and shape, and the danger of cracking.

With both cooling processes, it is essential to ensure symmetrical gas quenching to avoid unbalanced cooling conditions. The cooling phase should end with an equal-izing stage between 150 and a minimum of 100 °C. Lower temperatures increase the stress state and thus the risk of failure. Equalizing is followed immediately by tempering. Martensite is then transformed from the tetragonal lattice structure into cubic martensite. Internal stresses are minimized, and as the tempering temperature rises, toughness increases, but hardness declines. Depending on the steel grade, there should be at least two tempering cycles with the temperatures as similar as pos-sible. Particularly when hardening high-alloy steels, retaining austenite components may remain in the martensitic microstructure, which should be transformed and tempered to achieve uniform properties by repeated tempering. In the case of certain materials, a�er the hardening process and one tempering process, cryogenic treat-ment followed by at least two tempering cycles may also be advisable, to transform the residual austenite. The level of the tempering temperatures required depends on the chemical composition of the steels, and the hardness desired. Figure 4.83 contrasts the tempering curves of the materials 1.2767 (45NiCrMo16) and 1.2343 (X37CrMoV5-1) by way of example, clearly illustrating the principal difference in the tempering behaviour of both materials. Both steels make it possible to achieve a higher hardness of e.g. 50 HRC, but at different tempering temperatures. Being a hot work mold steel, material 1.2343 (X37CrMoV5-1) has high thermal resistance, permitting subsequent thermal surface coating such as gas nitriding at 510 °C, with no loss of hardness.

Steel 1.2343 (X37CrMoV5-1) also gives the option of tempering at about 540 °C in the “secondary hardening maximum”. Despite the high tempering temperature, high hardness is achieved by the formation and growth of special carbides. The high tempering temperature moreover serves to increase the toughness of the mold. A typical steel grade for this process is the cold work mold steel 1.2379 (X153CrMoV12 or D2). The tempering curve in Figure 4.84 shows the relationship between tempering temperature and austenitising temperature. Tempering in the secondary hardening maximum at 510 to 550 °C becomes possible a�er a high hardening temperature of 1080 °C. The general materials processes in the micro structure during tempering are given in Figure 4.85.

The soaking times required for each individual step of the heat treatment process described depend on the steel concept and the mold design. In practice, heat treat-ment contractors are guided very much by their own experience in controlling fur-nace soaking times based on mold weight, according to wall thicknesses, and more recently o�en also with the aid of moveable thermo-couples attached to or inserted in the molds (drill hole). The steelmaker’s data sheets define recommendations for appropriate soaking times depending on wall thickness, based on Figure 4.86.

5414.5 Heat Treatment and Surface Finishing Techniques

Här

tein

HR

C

Temperatur in °C

0 100 300 400 500 700 80020

25

30

35

40

45

50

55

60

200 600

1.2343

Har

dne

ss in

HR

C

Temperature in ºC

60

55

50

45

40

35

30

25

200 100 200 300 400 500 600 700 800

1.2343

Här

tein

HR

C

Temperatur in °C

0 100 300 400 500 700 80020

25

30

35

40

45

50

55

60

200 600

1.2767

Har

dne

ss in

HR

C

Temperature in ºC

60

55

50

45

40

35

30

25

200 100 200 300 400 500 600 700 800

1.2767

FIGURE 4.83  Tempering curves for the cold work steel 1.2767 (45NiCrMo16) and the hot

work steel 1.2343 (X37CrMoV5-1)

Här

tein

HR

C

Temperatur in °C

0 100 300 400 500 700 80030

35

40

45

50

55

60

65

70

200 600

gehärtet von

980°C 1020°C

1060°C1080°C

Har

dne

ss in

HR

C

Temperature in ºC

Hardened from70

65

60

55

50

45

40

35

300 100 200 300 400 500 600 700 800

980ºC 1020ºC

1060ºC1080ºC

FIGURE 4.84  Interrelationship between the tempering temperature and the austenitizing

temperatures of the example steel 1.2379 (X153CrMoV12)

542 4 Manufacturing and Machining Methods

Alloyed steels with secondary

hardening effect

Unalloyed and alloyed steels

without secondary hardening

effect

Formation and growth of special

carbides (secondary hardening effect)

Spheroidisation and coarsening of Fe3C

Martensite (C )+ ε Ferrite + Fe3C

Retained austenite

decay

Low Bainite

Tempering martensite

Tetragonal

Deposition of ε-carbide

100

1

2

3

4

200 300 400 500 600 700 °C

Tempering Temperature

cubic martensite

Hard

ness

Tem

pering

Sta

ge

FIGURE 4.85  Schematic diagram of tempering curves and microstructure influence

200

160

120

80

40

Zei

t

min

Wanddic ke ( H‰rtungsquerschnitt)

1

2

a

aa

a

a = W anddic ke

00 40 80 120 160 200 240 280mm

Tim

e

Wall Thickness (hardening cross section)

a

aa = Wall Thickness

a a

1

2

200

min

160

120

80

40

00 40 80 120 160 200 240 280mm

FIGURE 4.86  Soaking time at austenitizing temperature as a function of wall thickness for

different material groups

1: High-alloyed mold steels (ledeburitic 12% Cr-Steels)

2: Unalloyed and low alloyed mold steels

4.5.2.3  Corrosion-resistant steels

These plastic mold steels are becoming increasingly important for small and medium mold sizes. As well as avoiding corrosion by aggressive plastics materials such as PVC, these steels reduce the mold maintenance effort required, especially during

5434.5 Heat Treatment and Surface Finishing Techniques

shut-downs. Aesthetic considerations calling for high-quality end products are also contributing to this trend. The resulfurized steel 1.2085 (X33CrS16) is used for mold assemblies and mold frames. Due to the sulphur content this steel is not advisable for surface coatings but efficient to machine. Corrosion-resistant steels such as 1.2316 (X38CrMo16, hardened and tempered to about 30 HRC) or 1.2083 (X40Cr14, produced according to the production sequence described for through-hardening steels to about 52 HRC) can be used for molding inserts, as well as numerous similar special grades. In addition to very good corrosion and wear resistance, the mold steel’s high gloss polishability and hence uniform microstructure and purity, is an important precondition for additional surface finishing.

4.5.2.4  Case-hardening steels

Plastic mold steels for case hardening are distinguished by relatively low carbon content up to 0.2%, and are designed for carburising or carbonitriding. Carbon is enriched by diffusion to about 0.8% into the face centred cubic lattice structure. A�er a hardening process, there is high hardness in the surface layer, whilst the core or substrate remains tough. With case-hardening steels and case-hardening, a “two-phase” material can virtually be simulated. When case-hardening steels are used, the alloying concept must be reconciled with the process selected and the mold sizes required.

4.5.2.5  Nitriding steels

Nitriding steels are used in plastics processing mainly for plasticizing elements (extruder housings and extruder screws). Preconditions for good nitridability are alloying elements such as aluminium, chromium, molybdenum or vanadium, which form wear-inhibiting nitrides in the surface layer with the aid of nitrogen diffusion. The steels used, their alloy composition and properties are given in DIN EN 10085 [6].

4.5.2.6  Maraging mold steels

These nickel alloy special steels, such as 1.2709 (X3NiCoMoTi18-9-5) are used as an excellent alternative material for small molds prone to fracture, or for mold inserts, because of their high toughness potential combined with high hard-ness. But they are not the plastic mold steel of choice, because of their restricted dimensional capability and their high purchase price. These steels are generally machined to finished dimensions in the solution-annealed state (about 31 HRC). The almost distortion-free precipitation hardening enables high hardness combined with high toughness. Precipitation processes involve heating to about 490 °C, increasing hardness to approximately 40 to 55 HRC, depending on the type of steel.

544 4 Manufacturing and Machining Methods

Changes in dimension and shape are minimal, and almost non-directional. A reduc-tion in volume of approximately 0.1% a�er precipitation hardening is to be antici-pated in the case of steel 1.2709 (X3NiCoMoTi18-9-5). Initial solution annealing is recommended for welding repairs or changes to molds that have already been precipitation hardened, followed by renewed precipitation hardening. Renewed precipitation hardening alone may be sufficient, depending on the extent of the welding operation.

4.5.2.7  General recommendations for heat treatment

Unmachined parts or molds premachined to low contour can be heat treated under open atmosphere.

For heat treatment in the machined state, it is necessary to have adequate machin-ing overmeasure to allow for rectification of surface defects and decarburising due to heat treatment, as shown in Figure 4.87. For example DIN 7527 [7] prescribes the machining allowances and permissible deviations for open-die forged steel bar. Surfaces that have been inadequately machined can lead to uneven microstructures, increased change in shape, and cracking during heat treatment.

Parts that have been further premachined can undergo heat treatment with less overmeasure in a protective atmosphere such as inert gas, in a salt bath, or under vacuum, as is now common practice.

Successful heat treatment of mold steel is a very complex subject. Depending on the level of knowledge, the mold manufacturer must rely on the heat treatment contractor’s ability and plant-specific experience.

Practical experience also shows how important it is to discuss at an early stage with qualified hardening shops or with the steel maker (“single source”) the questions of material selection, the finished contour required, and the resultant options for heat treatment and mold design (e.g. avoiding uneven mass distributions and sharp edges, through to the notch effect of number punching; see also SEW 220 [2]).

FIGURE 4.87  Metallographic view of an unmachined surface with scale impressions and

decarburization zones (light)

5454.5 Heat Treatment and Surface Finishing Techniques

FIGURE 4.88  Stress crack formation in the reverse of a mold insert made of material 1.2767

(45NiCrMo16) as a result of inadequate tempering effect and sharp-edged EDM

parts

As an example of the damage that can arise, Figure 4.88 shows the front and rear (part cut-out) of a mold insert made of 1.2767 (45NiCrMo16) hardened to 54 HRC; a�er hardening, annealing and electrical discharge machining (EDM), stress crack-ing occurred as a consequence of inadequate annealing in the EDM wire cut sharp-edged channels. Intensive annealing is necessary to minimize stress and increase toughness, especially for critical mold geometries. The brittle martensitic surfaces arising as a result of electrical discharge machining processes should be removed by reworking, or at least homogenized by stress relieving again a�er EDM.

Every heat treatment project requires meticulous incoming and outgoing inspection at the mold manufacturer and the heat treatment contractor, early defect reporting, clear heat treatment instructions, adherence to material data sheets, and inspec-tion of dimensional accuracy and mold hardness including suitable documentation (furnace charts, hardness reports and dimensional reports). Inadequate communica-tion and high cost and time pressure repeatedly lead to unnecessary mold failure. Using high-quality steels combined with proper mold design and qualified hardening technology is still the way to ensure consistent success in heat treatment.

4.5.3  Surface finishing

“Mold release behaviour”, “wear”, “corrosion” or “maintenance frequency” are just some of the practical everyday issues faced by plastic processing contractors. Surface finishing can make a decisive contribution to optimizing molds in service, and to the efficiency of new molds. The aim is to reliably produce end products of suitable quality over the longest possible production period. There are now numerous classic

546 4 Manufacturing and Machining Methods

and recently developed technologies available for surface finishing. Figure 4.89 shows a taxonomy of current surface treatment processes [8].

Selecting the optimum process involves considering such factors as the steel concept (alloying, purity, homogeneity), heat treatment state, hardness (supporting effect), mold geometry and weight, the plastics compound to be processed (fillers, additives, etc.) and the surface finish required for the end product (degree of gloss, etc). The purpose of coating must also be defined, e.g.

� Protective coating for textured or polished surfaces, � Improved wear resistance (cavity or sliding surfaces), � Increased corrosion resistance, � Minimum mold deposits to extend cleaning cycles, � Improve the flow and filling behaviour for purposes including minimizing injec-tion pressure,

� Optimise mold release behaviour, thus reducing cycle times, � Aesthetic reasons (e.g. creating a more matte surface).

Selecting the surface finishing method should, as mentioned above, be established jointly with all the partners involved in mold production and subsequent plastics processing. With increasing bath or chamber sizes and crane capacities in surface

Surface

Treatment

Processes

Ion plantation

Ion implanting

PVD coating

CVD coating

Plasma CVD coating

Induction hardening

Flame hardening

Electron beam hardening

Impulse hardening

Laser re-melting

Welding-on

Laser hardening

Hard-chrome plating

Nickel-plating

Cadmium platingRolling

Vapour blasting

Burnishing

Compacting

Shot peening

Drum plating

Boriding

Nitriding

Case hardening

Carbon nitriding

Sulfidizing

Laser re-melt alloying

Laser tetanizing

Oxidizing

Spraying

Detonation coating

Thermal

Mechanical

Therm

om

echanic

al

Ele

ctr

ochem

ical

Therm

ochem

ical

Chem

ical /

Phys

ical

FIGURE 4.89  Systematics of the standard surface treatment processes

5474.5 Heat Treatment and Surface Finishing Techniques

finishing plants, there are now coating options available for medium-sized to large tool and mold dimensions. The preconditions for all current processes are bright metal surfaces with no chips, EDM residue (“white layer”), grease, oil, machining emulsions or other mineral oil derivates. To avoid surface defects, flaking or differing layer thicknesses, various cleaning methods are used, some of them very resource-intensive, such as hot degreasing, rinsing, electrolytically assisted activation, etching and scouring, depending on the coating process. Depending on the surface technology and on the requirements in terms of subsequent functional properties, the layer to be treated may need to have different surface roughnesses. Treatment is generally applied to ground, peened or photochemically textured surfaces. The final surface specification should be sample inspected at an early stage with the heat treatment and/or coating contractor and with the end customer, depending also on the plastic molding compound to be processed, preferably using test plates. Any pre-treatment such as welding or partial surface layer hardening must be advised in advance to the treatment contractor. Especially for thermal or thermochemical processes, stress relieving should be performed a�er contour rough machining with adequate oversize, to minimise dimensional and shape change (“stress reliev-ing” the mold as a result of the process temperature). The main surface finishing processes of major importance to mold making are now thermal, thermochemical, electrochemical and physical processes.

4.5.3.1  Thermal processes

Thermal hardening processes generate martensitic surface layers with hardnesses > 50 HRC, by partial heating of the surface to hardness temperature (austenitising) and subsequent cooling, aided mainly by the mold steel’s intrinsic carbon content of at least 0.25%. In practice this is applied selectively to particular areas and mold zones subject to wear, such as parting lines. The microstructure transformation means it is not possible to entirely avoid dimensional and shape changes that depend on component stability. These processes require a bright metal surface, free of impurities (grease, chips, etc.) with high-quality surface finish, with no sharp edges, and without thin, possibly through-hardened wall thicknesses. For the three principal processes “flame hardening”, “laser hardening” and “induction hardening” (which is less commonly used in mold making), the rule is that stress cracking cannot be entirely excluded, depending on the material, the component geometry and the intensity of the process parameters (heating, cooling rate). It is therefore advisable to stress relieve the mold a�er preliminary machining.

Another way of reducing stress conditions during the hardening process is to pre-heat the mold as thoroughly as possible to about 100 to 150 °C using a temperature regulating device and the heating/cooling channels of the mold. This can avoid excessively rapid heating and cooling rates. Excessively abrupt quenching media,

548 4 Manufacturing and Machining Methods

such as water spraying, should be avoided with flame hardening, especially when cooling alloyed steels. The pre-hardened mold steels such as 1.2311 (40CrMnMo7), 1.2738 (40CrMnNiMo8-6-4), 2738mod.TS(HH) or 1.2711 (54NiCrMoV6) actually have adequate hardening receptiveness from self-cooling alone – the thermal energy applied at the periphery dissipates into the colder mold core. Immediately a�er the hardening process, partial heating (temperature depending on the tempering curve, generally between 150 to 300 °C) leads to a tempering effect in the hardened surface layer, thus serving to reduce stress and increase toughness. Many users of these processes have learned from experience that the functionality of the mold does not decisively depend on the theoretical maximum possible surface layer hard-ness. Specifying a hardness about 2 to 4 HRC lower prevents the risk of cracking or flaking, whilst ensuring high quality and long durability of the mold.

Figure 4.90 shows an example of a plastic compression mold made of the pre-hardened plastic mold steel 1.2738 (40CrMnNiMo8-6-4). Cracking occurred during flame hardening of the parting lines due to the absence of pre-heating.

Component zones subject to mechanical load (e.g. radii or edges in the case of wall thickness differences) should be excluded from surface hardening, in order to avoid the danger of fracture.

4.5.3.1.1  Flame hardeningFlame hardening was previously the process most frequently used in mold making, and required great experience. Although there are specialist contractors using automated flame hardening equipment, flame hardening is generally carried out by the mold maker manually using oxyacetylene torches, as shown in Figure 4.91.

Flame hardened zone

FIGURE 4.90  Cracking during flame hardening due to excessively abrupt cooling conditions

5494.5 Heat Treatment and Surface Finishing Techniques

This requires skill and experience in operating the torch (consistent distance and speed of the torch), and monitoring the temperature, usually by eye in a darkened room. The process can be assisted by using torch tip spacers, special-purpose torch nozzles, and thermochrome markers or infrared thermometers for better tempera-ture control. Depending on the steel concept and intensity, it is possible to achieve surface hardness depths of approximately 1 to 5 mm (or deeper depending on the steel type).

4.5.3.1.2  Laser hardeningCompared to manual flame hardening, laser surface hardening (e.g. Nd:YAG laser, diode laser), for example using CAD/CAM machining data, can provide reproduc-ible conditions. Precisely controlled laser hardening, properly performed, provides greater security against possible crack formation. The laser beam can generate a very hard, fine martensitic layer with track widths of approximately 40 mm to a maximum depth of 2 mm [9]. Larger areas can be hardened by linear passes using variable laser track widths (depending on the laser power), as illustrated schemati-cally in Figure 4.92. This technique again requires the experience of a specialist contractor, since the surface quality can be impaired if the tracks are too close together or too far apart [10].

Component

Spurmuster

V

Tracks

FIGURE 4.92  Surface hardening in linear passes

FIGURE 4.91  Flame hardening a mold surface in a wear area

550 4 Manufacturing and Machining Methods

Här

te

Abstand von der Oberfläche

Har

dne

ss

Distance from the Surface

FIGURE 4.93:  Typical hardness profile in the surface layer a�er laser hardening

The hardness transition to the basic material is shown schematically in Figure 4.93. Since this process enables only restricted application of energy, hardness typically drops off relatively rapidly from a penetration depth of about 2 mm.

4.5.3.2  Thermo-chemical processes

Compared to thermal surface hardening methods in which the surface hardness is generated by austenitising and quenching, thermo-chemical processes are character-ized by additional diffusion of carbon (case hardening), nitrogen (nitriding), nitrogen and carbon (nitro carburising) and boron (boriding). These diffusible elements can be added by different treatment media. There are various processes using solids (powder, granulates, pastes, etc.), liquids (salt melts) and gas mixtures, or a plasma generated by means of electrical discharge (“glow discharge”), that puts the gasses used into a reactive state [11].

4.5.3.2.1  Case-hardeningCase-hardening [12] is less commonly used for plastics processing molds. Different processes can be used for surface carburisation at about 900 °C and hardening of case-hardening steels at austenitising temperature (Figure 4.94). Direct hardening is the most economically efficient of the processes indicated, although the coarse grain formation that can arise in the carburisation process cannot be reliably removed. Hardening a�er isothermal transformation achieves a uniformly fine microstruc-ture compared to direct hardening, but is more resource-intensive. Selecting the correct heat treatment sequence should therefore be determined jointly with the heat treatment contractor, taking into account the dimensional accuracy required. There may also be a possibility of pre-hardening [13]. This achieves a microstructure corresponding to the end state by hardening and tempering in the pre-machined state. This can minimise the change in dimension and shape (distortion) to be anticipated in subsequent hardening.

The carbon content in the surface layer and the hardness penetration depth (Eht) can be controlled by the duration, medium and temperature of the carburisation.

5514.5 Heat Treatment and Surface Finishing Techniques

Case-hardening depths of about 1 to 2 mm can be achieved using a moderate car-burising process. Intensive carburising procedures are not advisable, since they can give rise to residual austenite formation or to carbide formation at grain boundaries. There is a danger of thin wall structures cracking in later use due to brittleness.

A certain change in dimension and shape is to be anticipated due to the different microstructure arising from the increase in volume in the surface layer. This must be allowed for in the oversize agreed with the heat treatment contractor. It must also be taken into account in this regard that excessive oversize can lead to insufficient and/or uneven surface layers a�er machining. Carburising and hardening – which can be achieved only partially even with the aid of heat-resistant pastes – are fol-lowed by tempering, generally at about 200 °C.

The materials 1.2162 (21MnCr5) and 1.2764 (X19NiCrMo4) listed in Table 4.4 are case-hardening steels typically used to make plastic molds.

Tem

pera

tur

A KernC3

A RandC1

Aufkohlen

Härten

Anlassen

A KernC3

A RandC1

Aufkohlen

Härten

Anlassen

A KernC3

A RandC1

Aufkohlen

Härten

Anlassen

Isotherme

Umwandlung

A KernC3

A RandC1

Aufkohlen

Härten

Anlassen

Zeit

A

B

C

D

A = Direkthärten

B = Einfachhärten

C = Härten nach isotherm. Umwandlung

D = Doppelhärten

A

B

C

D

Tem

pera

ture

Case Hardening

Case Hardening

Case Hardening

Case Hardening

Time

Hardening

Hardening

Hardening

Hardening

Annealing

Annealing

Annealing

Annealing

Isothermal

Transformation

Core

Core

Core

Core

Surface

Surface

Surface

Surface

A = Direct hardening

B = Single hardening

C = Hardening after isothermal transformation

D = Double hardening

AC3

AC1

AC3

AC3

AC3

AC1

AC1

AC1

FIGURE 4.94  Schematic comparison of the possible time-temperature sequences for case

hardening [14]

552 4 Manufacturing and Machining Methods

TABLE 4.4  Comparison of Case-Hardening Plastic Mold Steels

DIN Designation

Alloy Content

Hardening Medium

Surface Hardness

(HRC)

Core Hardness

(HRC)

Tough-ness

Danger of Distortion

21MnCr5 + Oil 61 30 + +

X19NiCrMo4 ++ AirOil

5761

3545

+++

–+

– low; + normal; ++ high

4.5.3.2.2  NitridingThe purpose of nitriding is to disperse nitrogen into the surface zone of the steel by diffusion, form special nitrides, and thus increase the surface hardness to approximately 700 to 1200 HV depending on the plastic mold steel. In contrast to case-hardening described above, this involves no microstructure transformation at process temperatures between 300 and 550 °C. This minimises the risk of distor-tion. Compressive residual stresses build up in the surface zone, which can lead to plastic deformation in the case of very thin walled molds. On the other hand these compressive residual stresses in combination with the higher hardness increase fatigue resistance. Nitrogenising generally results in a small increase in volume. All steels whose tempering temperature is at least 30 °C higher than the nitriding process temperature are in principle suitable for nitriding treatment; cf. Figure 4.79. If this is ignored, there can be a loss of hardness in the basic material, resulting in an inadequate supporting effect where there is a point load, and to increased change of dimension and shape. The formation of chromium nitrides involved in nitriding corrosion-resistant steels decrease their corrosion resistance.

Depending on the steel’s composition, the process temperatures, and particularly the soaking times, layers of varying depths can be achieved. As the alloy content of the steels increases, the surface hardness achievable increases, and the layer thickness achievable declines. It helps to protect against deformation if the sub-strate is hard enough. Excessively intensive treatment can make the material brittle (nitride formation at the grain boundaries of the steel microstructure), leading to premature mold failure.

The basic structure of a nitriding layer is shown in a metallographic microstructure photograph (Figure 4.95). At the beginning of the coating process, the surface zone is enriched with nitrogen. A ceramic phase (compound layer) of diffused elements and components of the steel material [Fe2-3 N (�-nitride) and Fe4 N (�-nitride) arises. This is very hard and brittle, and serves to improve corrosion resistance, especially in the case of low alloy steels. With increasing saturation and process duration, the compound layer is built up further, and the diffusion layer arises below. The diffusion layer has lower hardness than the compound layer, with much improved toughness.

5534.5 Heat Treatment and Surface Finishing Techniques

Abstand von der Oberfläche

Härte

VZ mikroskopische DZ

härtemeßtechnische DZ

CZ Microscopic DZ

Measurablediffusion zone DZ

Distance from the Surface

Har

dnes

s

100 m

m

FIGURE 4.95  General structure of nitriding layers

CZ: compound zone; DZ: diffusion zone

Higher hardness compared to the basic material is generally achieved by incorpo-rating nitrogen atoms in the crystal lattice, and by formation of special nitrides in the microstructure.

Depending on the type of treatment and the layer thickness, any increase in surface roughness can be rectified by subsequent polishing.

Gas nitriding, the best known process beside salt bath nitriding (Tenifer treatment), and plasma nitriding are the most commonly used nitriding processes in plastic mold making, because of their high reproducibility.

4.5.3.2.3  Gas nitridingGas nitriding is performed in a flow of ammonia gas (NH3) at approximately 500 to 550 °C. The ammonia releases by fission the nitrogen necessary for the nitriding process. The resulting hydrogen is dissipated. Intensive treatments to maximize nitriding hardness depths (Nht) can lead to edge li�ing (Figure 4.96).

554 4 Manufacturing and Machining Methods

DiffusingElement

DiffusingElement

Compound LayerEdge Lifting

Outer Contour afterDiffusion Treatment

Original Contour

Dimensional change fromexpansion in volume dueto diffusion

FIGURE 4.96  Volume increase at the edges a�er gas nitriding

Gas nitriding is a comparatively cost-effective process potentially suitable for all mold dimensions. Even parts with high wear stress such as extruder screws and housings made of nitriding steels are treated using this process, because of the hard compound layer that can be achieved. Where there are more exacting requirements in terms of basic material’s wear resistance, it is also possible to nitride steels 1.2344 (X40CrMoV5-1, 50 HRC) or 1.2379 (X153CrMoV12, 60 HRC, hardened at approximately 1080 °C and tempered at least twice in the secondary hardening maximum; cf. Figures 4.79 and 4.84).

4.5.3.2.4  Plasma nitridingPlasma nitriding is based on glow discharge. Gas containing nitrogen (ammonia or gas blends depending on the desired layer formation) is introduced into an evacuated vacuum furnace. When an electric voltage (approx. 600 to 1000 V) is then applied, ionisation occurs at a partial vacuum of 0.3 to 10 mbar. Positive gas ions encounter the cathodically connected tool with high kinetic energy. The tool heats up, and the nitrogen diffuses into the surface. The duration of the treatment depends on the required layer thickness, and is normally between 10 minutes and 36 hours. Plasma nitriding has some advantages over gas nitriding for plastic mold making. Lower process temperatures of approx. 300 to 550 °C minimise the possible changes in dimension and shape, so there is an advantage in the case of long, slender components such as extruder screws, where distortion is critical. Glow discharge produces a cleaning effect at the steel surface; dirty, passivated surfaces are cleaned, thereby avoiding uneven nitriding depths. Plasma nitrided surfaces can be created with compound layers, but also very o�en without, using precisely adjusted process parameters. This is for example a pre-condition for creating a support layer for subsequent additional surface coating such as electroless nickel or PVD coating.

5554.5 Heat Treatment and Surface Finishing Techniques

4.5.3.2.5  BoridingBoriding is a thermo-chemical process that takes place at a temperature of approx. 850 to 1000 °C under a protective gas (e.g. argon). The boriding medium can be powder, granulate or paste [15]. The aim is to form a single-phase layer of Fe2B. A two-phase structure from a covering layer of FeB can be avoided by selecting appropriate process parameters. Otherwise cracks or flaking can occur due to stress differences. The structure and thus the hardness of the layer also depend on the alloy content of the mold steel. Layer thicknesses of up to 200 μm can be achieved in the case of unalloyed steels. As the alloy content increases, the layer thickness that can be achieved declines to 20 μm. Layer thicknesses of 30 to 60 μm are generally used. Boriding can be performed in the course of a hardening process, depending on the molding dimension and austenitising temperature. If this option is not available, a further hardening and tempering process must be performed a�er boriding. There may be changes in dimension and shape because of the high treatment temperatures. In order to minimise the risk of shape change, the same microstructure states should be set before machining to treatment contour and a�er boriding. Slight volume increases and edge li�ing generally occur. The main advantage of boride layers is the high hardness of approx. 1600 to 2100 HV, which minimises abrasive and adhesive wear; boriding is therefore used for applications such as mold tools in glass processing, but more rarely in plastics processing (e.g. for especially wear-resistant extruder screws).

4.5.3.3  Electrochemical processes

The two electrochemical or chemical processes described below, hard chromium plating and electroless nickel, provide protection against wear and corrosion. They have the considerable operational advantage of a low process temperature (about 60 or 80 °C), so no mold distortion arises. A ground or polished surface is essential for high quality coating, with a surface roughness matching the subsequent coating surface. It is important to note that excessively high surface finishes can also cause mold adhesion. For example in the case of SMC (sheet molding compound) press-ing molds for manufacturing plastic bodywork panels, the abrasive papergrit was reduced from 600 to 400 grit to improve mold release behaviour. It is essential the molds are cleaned beforehand to ensure a surface absolutely free of grease and oxides, in order to avoid unsatisfactory adhesion (spalling etc.).

A combination of both processes is increasingly being used as well. In order to opti-mise corrosion resistance and achieve improved supporting effect, an amorphous layer is first applied, such as a 20 μm thick electroless nickel layer. This prevents possible corrosion formation due to the microcrack structure of chromium layers. Additional chromium plating for thickness of about 20 μm confers the advantage of the wear-resistant chromium layer.

556 4 Manufacturing and Machining Methods

4.5.3.3.1  Hard chrome platingGalvanic hard chrome plating has long been an effective process for different plastics processing molds. The bath sizes and crane capacities now available make the process particularly relevant for large molds made of pre-hardened mold steels such as 2738 mod. TS (HH), that is used in hard chrome plated form for processing glass-fibre reinforced plastics using the SMC or GMT process. In the actual elec-trolytic coating process, a contour-close electrode forms the anode, and the mold to be coated forms the cathode. Direct current flows from the anode to the cathode through baths containing chromium, depositing chromium ions on the workpiece. Layer thicknesses of 8 to 30 μm are normally used in plastic mold making. Thicker layers are possible e.g. for dimensional build-up, but result in rough surfaces, and can be reworked by grinding, lapping or polishing. The hardness of a chromium layer lies in the range of approx. 700 to 1100 HV depending on the depositing conditions.

Since chromium is deposited with very high internal stresses, exceeding the tensile strength of the chromium gives rise to micro-cracks, as shown in the metallographic image in Figure 4.97. These can extend to the basic material surface, and lead to impairment of corrosion resistance (pitting) in the case of thinner layers. In order to achieve optimum corrosion protection, processes have been developed in which several hard chrome layers are superimposed, in order to cover micro-cracks, and prevent contact with the non-corrosion resistant steel surface below. Alternatively, a much more homogenous nickel layer can be applied below the chrome layer, as already mentioned.

In the case of hard chrome plating, a�er preliminary cleaning (electrolytic or hot degreasing, flushing, etc.), further activation in the bath is required to improve layer adhesion. A roughened mold surface finish is achieved by anodic connection. The steel surface is etched, and it is possible for impurities (e.g. sulfides, oxides) in the steel, or residue from previous operations (EDM, polishing, etc.) to be released, and cause perforation.

FIGURE 4.97  Microcracks in the hard chromium layer

5574.5 Heat Treatment and Surface Finishing Techniques

FIGURE 4.98  Enlarged surface defect

in a hard chromium layer due

to scoring in the mold surface

50.0 �m 10 �m

FIGURE 4.100  Faults in chromium layers

In order to avoid surface defects on the chromium layer, which can appear enlarged depending on the flaw and layer thickness (Figure 4.98), anodic connection should be as brief as necessary.

As shown in Figure 4.99, very small flaws can be covered by chromium plating where the layer thickness is sufficient.

For this reason it has proved effective in plastic mold making to apply somewhat thicker layers of about 25 to 30 μm, followed by polishing. This can remove smaller flaws on the chromium layer. But flaws can also arise in the chromium layers due to fluctuating process parameters (e.g. current fluctuations) or impurities in the chromium bath (Figure 4.100).

Major flaws can be repaired e.g. by brush plating (partial, electrochemical coating). Flux densities of uniform strength are also necessary to achieve the most uniform possible coat thicknesses. Since the flux density can differ greatly in geometrically critical areas, it is advisable to take this into account in designing the mold contour (Figure 4.101), and constructing the anode.

Chrome Layer

Inclusion

Material 1.2738

FIGURE 4.99:  Extremely fine inclusion in

the steel surface covered by

chromium layer

558 4 Manufacturing and Machining Methods

Cr Cr

falsch richtig

Cr Cr Cr

Wrong Right

Cr

Cr Cr

Cr Cr

FIGURE 4.101  Building up of hard chrome layers with different coating geometries

Plastic molds are frequently required to produce contour-intensive moldings. This demands sophisticated anode construction, e.g. adaptable in configuration and size on site using perforated sheet or wire. Increased magnetism in the mold can also change the distance from anodes of filigree construction in edge and groove areas, resulting in unequal layer thicknesses.

Excessively thin layers or increased wear (e.g. caused by processing glassfibre-reinforced plastics) can lead to breaching of the layer. Deplating is in practice carried out in such cases. This involves anodically connecting the mold in special deplating solutions. This can entail uneven attack of the substrate. The surface must therefore be ground down sufficiently a�er deplating by a minimum of 0.2 mm to achieve a flawless new chromium layer. This serves to remove both visually detect-able, etched-in structures, also any emplacements in the mold steel.

Chromium hydride decays as the chromium layer is formed, releasing hydrogen. This can diffuse into the substrate, and lead to embrittlement of the steel surface. Heating up the mold (dehydrating [3]) to approximately 200 to 220 °C can make it possible to subsequently reverse this diffusion process. A temperature of 160 °C can be sufficient [16] with longer holding times of several hours. Material embrittlement is generally not rectified, because of the comparatively thin layers involved in mold making, and the possibility of distortion, particularly since hydrogen emplacements can be diffused out of the steel in subsequent mold tempering.

4.5.3.3.2  Nickel platingNickel layers are more homogenous and therefore more corrosion resistant com-pared to microcrack-prone hard chrome layers. Electroless nickel has been adopted in plastic mold making firstly because there is no need for anode construction, and secondly because of the uniform layer thicknesses of up to approximately 200 μm without edge build-up compared to electrochemical nickel.

5594.5 Heat Treatment and Surface Finishing Techniques

Another advantage of electroless nickel in plastic mold making is the ability to deposit a uniform, wear resistant and corrosion resistant layer even in thin grooves, channels and drill holes.

In contrast to the electrolytic process, the nickel ions are reduced with the aid of a chemical reducing agent (sodium phosphinate, NaH2PO2) from an aqueous bath. Other elements can be deposited as well as nickel. By varying the bath composition and temperature, special coating compositions (e.g. phosphorus 2 to 15%) can be achieved with different properties.

If the primary concern is for example corrosion resistance, an amorphous nickel phosphorus layer with a high proportion of phosphorus > 10% is applied, albeit with a relatively low layer hardness of about 52 HRC (approx. 550 HV) [17]. Low phosphorus content leads to formation of crystalline Ni-P layers with reduced cor-rosion resistance at higher hardnesses up to 62 HRC (approx. 750 HV), with the advantage of better wear resistance. A further increase in hardness, in both cases to approximately 72 HRC (approx. 1100 HV), can be achieved by subsequent heat treatment at approximately 300 to 400 °C by precipitation hardening of phospho-rus compounds (Figure 4.102). In this case it is essential to take account of the possible distortion due to heating, and the possibility of a reduction in hardness of the plastic mold steel (tempering resistance). In addition to the nickel coating described, it is possible to achieve nickel dispersion layers with the aid of additional hard components (up to approx. 30%), thus achieving the required specific proper-ties. For example bringing in silicon carbide, diamond particles or boron serves to minimise wear. If the aim is to reduce the coefficient of friction, this can be achieved by adding polytetrafluorethylene particles [PTFE (Teflon)].

Temperature

Har

dnes

s-Vi

cker

s un

it

1000

900

800

700

600

5000 100 200 300 400 500 600 700 800ºC

Duration of HeatTreatment = 1 hour

FIGURE 4.102  Precipitation hardening layer of electroless precipitated nickel layers with Ni3P

(by Colin)

560 4 Manufacturing and Machining Methods

4.5.3.4  Chemical and physical processes

With the progressive advances made in recent years, hard coating using CVD, PACVD and PVD processes is gaining in importance. High wear resistance and very good layer adhesion favour the CVD process (chemical vapour deposition) for machining and forming tools and molds, since this can significantly improve e.g. wear resistance of high alloy mold steels containing carbide, as well as powder met-allurgy steels. For plastics processing molds, the PACVD process (plasma assisted chemical vapour deposition) and the PVD process (physical vapour deposition) provide a promising coating solution because of the comparatively low process temperatures of up to 550 °C, and the large number of coating materials available for almost all current plastic mold steels. A combination of a PVD support layer and a PACVD coating is now technically possible. A large number of different plastic processing molds are today treated with almost flawless shape, dimensional stabil-ity and uniform layer formation. By using the existing mold or an appropriate steel selected in the case of new molds (supporting effect, tempering resistance, etc.) it is important to match the mold manufacturing with the coating process. This has yielded some decisive improvements for the user, such as wear resistance when processing abrasive plastics and their additives, corrosion resistance, improved mold release behaviour, and reduced maintenance and cleaning.

4.5.3.4.1  CVD coatingCVD coating is used to create hard, wear-resistant surfaces with a layer usually 2 to 10 μm thick, by chemical vapour deposition of solids. CVD hard coatings such as titanium nitride (TiN; hardness up to 2300 HV), titanium carbide (TiC; up to 3000 HV), or aluminium oxide (Al2O3; 2100 HV) usually used in multi-layer coatings combined with other hard coatings, reduce mold wear by reducing abrasion and adhesion. This technology is therefore typically used for cutting tools (high-speed steels, hard metal alloys, cermets) and for tools for sheet metal forming (cold work mold steels). In order to keep distortion to a minimum, coating is carried out on the hardened, finish-machined and ideally polished mold. Due to the high process temperatures of 900 to 1000 °C with coating under reduced atmosphere, another hardening process follows. Figure 4.103 shows the production sequence within the thermal cycle for producing a mold with CVD coating.

Steels with high carbon and alloy content (air hardening steels) enable a relatively mild quenching rate, which is desirable for distortion reasons in the case of vacuum hardening. Their high hardness provides a good supporting effect for the CVD layer. A�er coating and hardening, the tempering processes are carried out, partly to optimise dimensional stability. Reworking is possible only to a limited extent by means of polishing, because of the low layer thicknesses.

5614.5 Heat Treatment and Surface Finishing Techniques

Tem

pera

ture

1200

1000

800

600

400

200

0Time

Stressrelieving

HardeningTemperingeach 1 h/20

Tic Coating andHardening

Coo

ling

in F

urna

ce

Fini

sh-m

achi

ning

Austenitizationabout 0.5 min/mm

2. Pre-heating

1. Pre-heating Q

uenc

hing

Soaking

Finish Grindingand Polishing

Air

1. 2.Tempering1. Pre-heating

2. Pre-heating

Coating1-5 h

Que

nchi

ng

FIGURE 4.103  Production sequence for coated molds made of 12% ledeburitic chrome steels

Being a resource-intensive operation entailing risks of changes in shape and dimension, CVD coating has not become established in plastics molding the way PVD coating has.

4.5.3.4.2  PACVD coatingThe PACVD process (plasma assisted chemical vapour deposition) is a refinement of the CVD process, with the advantage of much lower process temperatures below 200 °C. This process is therefore becoming more popular in plastic mold making, since there is almost no risk of distortion, unlike with CVD. The chemical reaction from the gas phase at low temperature is enabled by a strong electric field between the mold and the backplate electrode.

Layer thicknesses of approximately 1 to 3 μm have heretofore been applied mostly in the form of Cr2 N layers in mold contours. Furthermore, WC/C is used for sliding surfaces (sliders), and attractive options for tribologically stressed plastic molds are increasingly emerging using DLC layers (diamond-like carbon) [18]. This metal-free layer made of carbon and hydrogen has very good anti-adhesion and anti-friction properties, in addition to high wear-resistance.

4.5.3.4.3  PVD coatingUnlike CVD, PVD involves depositing hard materials from the chemical vapour by physical means. Materials such as hard alloys and cermets can be PVD coated, as well as steels. Depending on the steel material, its heat treatment state and the hard alloy (target material), lower treatment temperatures in the range of approx.

562 4 Manufacturing and Machining Methods

200 to 550 °C mean the mold can be coated in its finished state, without further heat treatment.

Numerous coating materials and combinations with different layer thicknesses ranging from 2 to 10 μm are used, adapted to the supporting effect and the tempering resitance of the material, and the particular requirements. Figure 4.104 shows the example of a metallographic cross section of a 4 μm thick PVD layer made of TiAlN.

There are various methods for transforming the metallic fractions of the hard alloy compounds (titanium, chromium, aluminium) into the gaseous state. The princi-pal method used is evaporating by electron beam or electric arc (thermal energy) or sputtering (kinetic energy). Particles are transported from the hard material source to the mold (substrate) along the lines of electric flux, with the disadvan-tage that undercuts or drill holes are not necessarily effectively coated. In order to minimise this effect, and achieve uniform layer thicknesses as far as possible even with complex mold geometries, the mold can be rotated whilst using several target sources, depending on the process. Drill holes can however be uniformly coated only to a depth of approximately 1.5 × the borehole diameter. Non-metallic components (such as carbon or nitrogen) can also be introduced into the vacuum chamber in a gaseous form during the precipitation process, and incorporated into the hard coating in a controlled manner. This results in numerous possible coating compositions; cf. Table 4.5.

TABLE 4.5  Examples of Hard Coatings and their Properties

Layer Material

Hardness (HV 0.05)

Layer Thickness* (μm)

Coating Temperature (°C)

Max. Application Temperature (°C)

CrN 1800 4 220–450 650

TiN 2400 3 220–450 600

TiCN 3000 3 450 400

TiAlN 3300 6 450 850

* Typical values, layer thicknesses are application-specific.

FIGURE 4.104  Hard layer made of TiAlN (3300 HV, 4 μm thick) [19]

5634.5 Heat Treatment and Surface Finishing Techniques

When the electrically accelerated particles encounter the substrate, the penetration depth (without diffusion) results from their kinetic energy. This ensures adequate and highly effective layer adhesion. Since the adhesion increases with increasing mold temperature, coating temperatures > 450 °C are advantageous considering the mold steel’s tempering resistance and stability of shape. An optimum substrate temperature is important for establishing a uniform, compact layer structure, as well as adequate particle bombardment. The thicknesses of the very hard layers are relatively small depending on the hard material, mostly 3 to 5 μm. Hard mate-rial layers are therefore reliant on adequate supporting effect by the basic mate-rial. A somewhat thicker CrN or TiAlN layer may be selected for molds made of pre-hardened mold steels with hardnesses of approximately 30 HRC, to prevent the PVD layer indenting. There is also the option of first applying a support layer by plasma nitriding (without a compound layer), or an electroless nickel layer. Repolishing can be carried out in combination with CrN where there are exacting surface finish requirements (mirror finish), since the surface roughness increases with increasing layer thickness. When using pre-hardened plastic mold steels with higher hardnesses, such as 2738 mod. TS (HH) with 33 to 38 HRC or 1.2711 (54NiCrMoV6) with 40 to 44 HRC, in combination with the thicker CrN or TiAlN layers, also provide a further improved supporting effect. Figure 4.105 shows a car door module as a typical application.

FIGURE 4.105  Injection mold cavity made of 2343 ISO-B mod., hardness 50 HRC and TiN

coating (4 μm thick, hardness approx. 2300 HV);

car door module (PP 30% long glass fiber) [19]

564 4 Manufacturing and Machining Methods

4.5.3.5  Comparing and Selecting Surface Treatment Processes

As a result of constant innovation in the field of coating technologies, there are now numerous and extremely diverse processes available, some of which can be usefully applied in combination. The developments increasingly include the possibility of repairing the surface coating, and stripping and reapplying coatings.

Reliable selection of a suitable surface treatment process depends on numerous factors, and should therefore be done on a project-specific basis. This will always require a thorough analysis of the case in hand, considering the cost benefit factor, and due selection of the process in consultation with the steel producer, the mold maker, and if applicable the heat treatment contractor, the surface treatment con-tractor, the plastics manufacturer, and the company processing the plastic.

Figure 4.106 illustrates the connection between wear intensity and surface hardness. It is evident that wear is greatly reduced as hardness increases. This connection and the trend towards processing plastic molding compounds with additives explain the constant growth in the area of coating. The overview of common surface finishing processes in Table 4.6 provides further guidance.

200 200 600 800 1000 1200 1400 1600

Nitrierte- und

verchromte Stähle

Ve

rgü

tete

Stä

hle

Ge

hä

rte

teS

täh

le

12

10

8

6

4

2

0

Wear

Inte

nsity in

m/m

x 1

07

12

10

8

6

4

2

0

200 200 600 800 1000 1200 1400 1600

Surface Hardness in HV

Surface Finishing

Pre

-hard

en

ed

mo

ld s

teels

Hard

en

ed

Ste

els

Nitrided and Chromium

Plated Steels

Case H

ard

en

ed

Ste

els

CVD, PACVD and PVD

Coated Steels

FIGURE 4.106  Relationship between wear and surface hardness (schematic) [20]

5654.5 Heat Treatment and Surface Finishing Techniques

TABL

E 4.

6  Su

mm

ary

of

Com

mon S

urf

ace T

reatm

ents

Trea

tmen

tHa

rdne

ss R

ange

Usua

l Pro

cess

Te

mp.

in °C

Usua

l Lay

er

Thic

knes

s (μ

m)

Repr

oduc

-ib

ility

Dim

ensi

onal

St

abili

tyPr

efer

red

Stee

l Con

cept

1000

2000

3000

Flam

e Ha

rden

ing

*to

500

0+

+Q

uenc

hed

and

tem

pere

d st

eels

Lase

r Har

deni

ng*

to 2

000

+ +

+ +

Que

nche

d an

d te

mpe

red

stee

ls

Case

Har

deni

ng92

0 °C

to 2

000

+ +

+ +

Case

har

deni

ng

Elek

tole

ss N

icke

l80

to 2

0+

++

+Pr

e-ha

rden

ed a

nd

thro

ugh

hard

enin

g st

eels

Hard

Chr

ome

Plat

ing

608–

50+

++

Pre-

hard

ened

and

th

roug

h ha

rden

ing

stee

ls

Gas

Nitri

ding

500–

550

to 5

0+

++

+Pr

e-ha

rden

ed a

nd

thro

ugh

hard

enin

g st

eels

**

Plas

ma

Nitri

ding

300–

550

to 3

0+

++

+Pr

e-ha

rden

ed a

nd

thro

ugh

hard

enin

g st

eels

**

Borid

ing

850–

1000

to 6

0+

++

Pre-

hard

ened

and

th

roug

h ha

rden

ing

stee

ls

CVD/

TiC

950

to 1

0+

++

Thro

ugh

hard

enin

g st

eels

PACV

D/DL

C20

0to

3+

++

+Th

roug

h ha

rden

ing

stee

ls**

PVD/

CrN

220–

450

to 1

0+

++

+Pr

e-ha

rden

ed a

nd

thro

ugh

hard

enin

g st

eels

**

PVD/

TiN

220–

450

3+

++

+Th

roug

h ha

rden

ing

stee

ls**

PVD/

TiCN

450

3+

++

+Th

roug

h ha

rden

ing

stee

ls**

PVD/

TiAl

N45

0to

10

+ +

+ +

Pre-

hard

ened

and

th

roug

h ha

rden

ing

stee

ls**

* Au

sten

izatio

n te

mpe

ratu

re o

f the

ste

el m

ater

ial;

**

Tem

perin

g re

sist

ance

in c

ompa

rison

to th

e pr

oces

s te

mpe

ratu

re m

ust b

e co

nsid

ered

566 4 Manufacturing and Machining Methods

References

[1] DIN EN ISO 4957: Werkzeugstähle. DIN Deutsches Institut für Normung e. V. (2001)[2] SEW 220: Werkzeugstähle, Auswahl von Werkstoffkennwerten und Wärmebehand-

lungsangaben. Stahl-Eisen, in preparation[3] Liedtke, D., Jönsson, R.: Wärmebehandlung – Grundlagen und Anwendungen für

Eisenwerkstoffe (1996) Expert Verlag[4] Sommer, P.: Datenbank StahlWissen – NaviMat 1.0 – 2002[5] Oldewurtel, A. in G. Mennig (ed.): Werkzeuge für die Kunststoffverarbeitung (1995)

Hanser, p. 376.[6] DIN Deutsches Institut für Normung e. V. (2001) DIN EN 10085: Nitrierstähle –

Technische Lieferbedingung[7] DIN 7527 Sheet 6: Schmiedestücke aus Stahl, Bearbeitungszugaben und zulässige

Abweichungen für freiformgeschmiedete Stäbe. DIN Deutsches Institut für Normung e. V. (1975)

[8] Hippenstiel, F., Grimm, W., Lubich, V., Vetter, P.: Handbook of Plastic Mold Steels. Buderus Edelstahl GmbH (2002) Wetzlar

[9] N. N.: Company publication Laser Bearbeitungs- und Beratungszentrum NRW (2005)[10] Hoffmann, F.: Elektronenstrahl- und Laserhärten, Seminar (1999) Lüdenscheid.[11] Ratgeber Verschleißschutz der Arbeitsgemeinscha� Wärmebehandlung und Werk-

stoechnik e. V. (AWT) (1997)[12] Wyss, U.: Grundlagen des Einsatzhärtens. Carl Hanser Verlag, Munich 1990[13] Jönsson, R., Matz, W., Sartorius, K.: Vorvergüten. Techn. Mitteilung Nr. 32, Stahl-

werke RöchlingBurbach GmbH, Völklingen[14] DIN 17022-3: Wärmebehandlung von Eisenwerkstoffen; Verfahren der Wärme-

behandlung; Einsatzhärten. DIN Deutsches Institut für Normung e. V. (1989)[15] N. N.: Company publication Elektroschmelzwerk Kempten (1992)[16] Personal report Schulte und Söhne GmbH, Arnsberg (2007)[17] N. N.: Company publication Messrs NovoPlan, Aalen (2002)[18] Personal report Härte- und Oberflächentechnik GmbH & Co. KG, Nürnberg (2007)[19] N. N.: Oerlikon Balzers Coating GmbH, Bingen (2006)[20] Spies, H.-J.: Erhöhung des Verschleißschutzes von Eisenwerkstoffen durch die

Duplex-Randschichttechnik. Stahl und Eisen 117, Nr. 6 (1997)

5674.6 Surface Structuring

 ■ 4.6  Surface Structuring

St. Krüth

Since the 60s, there has been the technology of mold structuring. First, the chemical engraving, which was only occasionally used, found its place quickly.

To this day, many uniform patterns cannot be brought into injection molds than in any other way. The more important plastic injection molding became, especially in the automotive industry, the more important graining became. Although the technique developed in this area, first-class hand work is still the most important requirement for quality. Figure 4.107 shows a positively etched mold section.

FIGURE 4.107  Positively etched mold section in an instrument panel

4.6.1  The Photochemical Etching Technology

4.6.1.1  Introduction

Essentially, the technique is based on the fact that acid etches the metal away. Combined with the possibility to cover parts of steel with a protective paint, and therefore protect these areas from the acid, a relief on two levels can be produced. The decisive factor is the interaction between metal, the covering of varnish, and the acid. The exposure time determines the depth of etching.

The structure is generated by applying the protective paint in the form of a defined structure image that is a�er a film pattern, as shown in Figure 4.108. This is done through films, which transfer the image as a copied surface. A�er removing the carrier material, the grain is positioned as a grid on the mold structure.

568 4 Manufacturing and Machining Methods

FIGURE 4.108  Mold surface

with applied structure image

The films are only partially elastic and also limited in size, making it necessary in most cases to lay several films together and then to touch up the transitions. All areas of the mold that are not to be grained were already covered with PVC tape and paint, as shown in Figure 4.109.

It is called a multiple etching when a new film is applied to the already etched struc-ture a�er etching and being blasted clean. Through multiple etching, the structure can be refined as much as desired. Each etching creates a new layer and gradually dissolves the previously flat surface into a 3-dimensional structure.

4.6.1.2  Why Structuring?

Structural etching of injection molds pursues a variety of goals. Surface design is definitely by far the most important goal. Molds are structured according to the principle of “a product is only as good as it looks” to give the plastic part an appearance in compliance with the product. A plastic handle becomes a

FIGURE 4.109:  Covered with tape,

not to be a patterned surface

FIGURE 4.110  Examples for plastic parts with etched surfaces

5694.6 Surface Structuring

wooden handle, a lid becomes a leather-covered glove box cover, and a simple box becomes a designer product, in which high-gloss alternates with a fine structure. Figure 4.110 shows some examples.

Structures can decorate, refine, transform, or laminate. Sink marks and weld lines are less noticeable, and ribs or thickening disappears behind the grain in transpar-ent materials.

4.6.1.3  From the Structure Template to the Film

For most grains, which are based on film, the original is in a template, as shown in Figure 4.111. For this purpose, the grain is photographed or scanned with the aid of color and light from the surface of the sample part, which may be a leather film, a wood veneer, a structured sample part, or another object. A film of the required size is created by joining and reproducing. A large graphical skill set is therefore required, because the elements must be connected together so that they a�erwards appear as one surface.

FIGURE 4.111  Templates for leather and wood structures

4.6.2  Requirements on the Mold Surface and Construction

To be able to apply, etch, and then replicate a structure by injection molding, some rules need to be observed.

� All areas on the mold must be accessible for the processor. This means that ribs and narrow sections can only be processed as deep as they are wide.

� A minimum dra� angle is necessary to ensure the removal from the mold. This dra� angle depends on the grain depth, the wall thickness, and the material. Usually, calculations can be done, according to the rule “0.01 mm grain depth requires a 1° dra� angle”. A deviation from this rule should be discussed with the person who is graining. If the article is shrunk onto the grain, a higher dra� angle is definitely required.

570 4 Manufacturing and Machining Methods

FIGURE 4.112  Bumper mold with eye-bolts

� Sliders and inserts, which are also grained due to a homogeneous grain transi-tion, need to be installed and removed for the responsible person who is graining. This is necessary in order to protect the parting surfaces against acid attack with protective coating.

� All parts of the mold should be manufactured from the same steel with the same surface treatment in order to avoid differences in etching. If possible, the fiber-orientation of the steel should also be the same in the installation position. This also applies to different components with the same graining that are assembled together.

� In order to hang the mold on each side, a sufficient number of threaded bore holes must be present, as shown in Figure 4.112 on a bumper mold.

� The necessary polishing quality depends on the selected grain. The finest struc-tures require a 600 finish, most grains only need a grain size of 320, and in very coarse structures, a 180 grain is usually sufficient.

� If a structure is a multiple etching, possibly even provided with round etching steps, it is necessary to pay attention to the entire etching abrasion in addition to the etching depth. This expresses how much material is overall removed. A corresponding undercut is created at the grain limit, which can then lead to demolding difficulties. The wall thickness of the part also increases by this value. Openings in the part are correspondingly smaller, and plugs are larger. This should already be considered in the construction. If the graining has to be carried up to around the radius, the surface behind can be etched without graining. This will also increase the wall thickness in these areas.

4.6.2.1  Materials and the Selection of Materials

The etching process is similar to a corrosion process. Therefore, special acids and etching techniques are required for graining corrosion resistant steels. The steel should be selected together with the steel manufacturer and the graining experts.

5714.6 Surface Structuring

In case of doubt, the manufacture of samples is recommended. Most of the samples are etched out of quenched and tempered steels such as 1.2311 or 1.2738. These materials can reach the closest match to the sample plate.

For the selection of the steel, it is recommended to consider the number of parts to be produced, the complexity of the mold, and the properties of the material to be injected.

4.6.2.1.1  SteelThe material 1.2738 is the material most o�en used for larger molds. For best grain-ing results, this steel was further developed and is now available as 2738 mod. TS. Other quenched and tempered steels are 1.2311, 1.2711, and 1.2316 as a corrosion resistant material. For small molds or as mold inserts with high hardness, through hardened plastic mold steels such as 1.2343, 1.2344, and 1.2767 can be used. 1.2083 is available as a corrosion-resistant variant.

In general, these steels must reach their final microstructure by a heat treatment before the graining. Other steels are also etchable. Here, a consultation or an etching trial on an appropriate material sample is recommend.

Mold steels such as 1.2312, which are manufactured to improve the processability with increased sulfur content, are not suitable for graining. There is a risk that the sulfur is dissolved from the surface by the acid attack during graining. This can result in irregular indentations in the grain pattern, depending on the sulfur distribution and formation.

4.6.2.1.2  Aluminum and Other MaterialsAluminum is generally etchable. The etching behavior is, however, completely dif-ferent from that of steel. Aluminum reacts to acid with a strong outgassing and usually with very high etch rates. In addition, a strong reaction heat develops that heats both material as well as acid, and thus again provides for an increased etch rate. Especially for thin-walled parts, which can pass less heat, it leads to changes in the etching pattern.

Other etchable metals are nickel, copper, brass, silver, nickel silver, zinc, and zamac. Different acids and etch techniques are used so that the etching pattern also looks different.

4.6.2.1.3  Heat Treatment and Surface RefinementMold surfaces can be optimized for subsequent use by a subsequent surface treat-ment. The common methods are nitriding and different coatings.

All of these treatments should only be performed a�er the graining. It is important that the gloss level of the surface can be changed with all coating methods. This change cannot be reversed in most layers. For setting the desired gloss level, it is

572 4 Manufacturing and Machining Methods

possible to take the expected change into account when setting the gloss level of the grain. Preliminary tests are necessary for this purpose.A post-graining, as a part of a modification or repair, is not possible in almost all coatings. The layer must be removed before or will be damaged during the rework so will need to be renewed. This is especially true for nitrided surfaces which are generally not grainable.

4.6.2.1.4  Grain Depths and TolerancesThe character of a grain is defined by the grain pattern and the grain depth. Changes in the grain depth can affect the appearance of a grain. If a structure is defined by the customer, a statement about the desired grain depth should also be included. If nothing is defined, it is the depth of the pattern or the depth of the sample plate. There are different specifications for the grain depth. The most common indica-tion, the grain depth, as shown in decimals in 1/100 mm steps, defines the visual difference between grain peak and grain valley.The Rz value can be determined with surface roughness measurement devices. This value is the average roughness depth value from five sections of the overall test section. This value is only meaningful if the overall test section was chosen to representatively show the grain character in each section. The Rz value is usually larger than the grain depth because it includes the grain tips in the grain peak and the grain valley.With modern measurement devices, as shown Figure 4.113 on the le� (white light interferometer sensor for sensing of surface roughness or grains), the grain can also be scanned and evaluated on a surface. Different results can be generated with this data. In addition to the above-mentioned measured values, the surface Rz value,

FIGURE 4.113  White light interferometry

(le�): measuring device; (right): a measurement

5734.6 Surface Structuring

which shows the surface roughness based on the surface, is included. Figure 4.113 on the right, shows a measured grain area in a 3D image.

All measurement methods have their advantages and disadvantages. It is important that only measurements are comparable using the same measuring devices and parameters. This also applies to the filters used.

Depth tolerances cannot be avoided over a component in the etching technology. The rule of thumb is: a tolerance of ±8% is possible. In areas of grain transitions, a lower tolerance can be agreed.

4.6.2.1.5  The Gloss Level in the Mold and in the Molded PartThe graining specialist can influence the gloss level in the mold. When measuring the gloss level, the reflection of the grained surface is determined, which occurs when light hits a defined angle on the surface. The graining and its roughness produce a certain reflection. The reflection can be reduced by over-etching with microstructures or by sand blasting. Figure 4.114 shows a uniformly grained surface with three different gloss levels and the resulting change in color.

The measured values of the mold cannot make any absolute statement about the gloss level on the molded part. The injection molded plastic and the injection parameters have a major impact on the reproduction accuracy of the surface onto the part. Any loss of the reproduction leads to a change in gloss in favor of the own gloss of the material, which is a typical characteristic. When a material is known as glossy, the gloss can be reduced through a microstructure. It is not possible for the graining specialist to safely incorporate a desired gloss level of the product into the mold.

The specialist can only repeat the gloss level, which is defined in the mold within a very narrow tolerance. Therefore, it may be necessary to optimize the gloss level in the mold a�er sampling in one or two “production loops” up until the product matches the sample.

The tolerance in the gloss level may, depending on the material and mold geometry, not be set too narrow, because the reproduction accuracy also decreases depending on the flow path of the material.

FIGURE 4.114  Effect of a gloss level change on the color effect

574 4 Manufacturing and Machining Methods

4.6.2.2  Processing Methods and Repair Technology

To be able to grain a mold, all surfaces not to be grained have to be covered first a�er a thorough cleaning. This is done with the help of PVC tapes and coatings. For graining, the mold should be delivered without a hot runner and without external add-on components. The grain surface is generally finely blasted, and therea�er the film is applied. This is done with the aid of photo-resist, film and light, or by inserting corresponding grain films in which an acid-resistant, self-adhesive color, which already represents the grain pattern, is laid onto the contour using a carrier film. Individual elements are taken to adjust to the contour. Figure 4.115 shows how these individual parts are connected by hand with a brush and etching needle so they are no longer visible a�er the etching process.

When a portion of the graining must be polished and re-grained due to change or repair reasons, it always means an adjustment to an existing surface that may already be worn. In other etching techniques and acids, the graining specialist must adjust to a partial processing of the surrounding surface. The quality of the processing is always dependent on the preparation and the graining. Considering the welding instructions and polishing up to the radii can help. In any case, the graining specialist should already be involved in the work from the very beginning. In individual cases, it is also advisable to grind out the entire cavity and re-grain it.

FIGURE 4.115  Retouching of a mold

4.6.2.3  Dra� Angles, Open Spaces, and Surface Preparation

A minimum dra� angle is necessary to ensure demolding. This dra� angle depends on the grain depth, the wall thickness, and the material. Falling below the previ-ously mentioned rule of thumb should be discussed with the graining specialist. If the part shrinks onto the grain, a higher dra� angle is definitely required.

5754.6 Surface Structuring

Open spaces, which should be covered for graining, must be clearly marked for the mold delivery. The marking in the mold should be done with the help of a mechanical marking. Spotting color is not sufficient due to the necessary mold cleaning. Spot-ting imprints are only sufficient if the edges are clearly and continuously visible. As far as clear dimensions of recognizable reference edges being possible, this may sufficient, as well as vectorized executions of open spaces. In these cases, the procedure should be discussed in advance with the graining specialist.

The mold surfaces to be grained shall have a polish that is customized to the grain-ing. This polish must completely remove all possible residues from the EDM process and surface hardenings of previous work. To check surfaces and radii in a clean silhouette, sand blasting the surface is recommended.

4.6.2.4  Contour Changes by Welding of Inserts

Welding and graining are generally incompatible. Should it still be necessary to weld, the following rules are important to limit the damage as far as possible. In the case of non-compliance, voids, as shown in Figure 4.116, can result. The vertical lines are band-type formations in the steel. The lower area shows a welding spot with edge zone.

The mold must be brought to temperature before the TIG welding. This depends on the material and should be verified with steel suppliers.

A�er welding, a second heating, just below the annealing temperature, is necessary. Therefore, the hardness increases of the welding transitions are largely equalized.

When selecting the welding electrode, it is absolutely necessary to use a material very similar to alloy. Because the weld metal still changes slightly during welding, a consultation with the supplier of the welding electrode is recommended. It is important to note that a mold can be grained subsequently. Figure 4.117 shows the different behavior of different welding alloys with and without annealing of the workpiece.

FIGURE 4.116  Welded mold insert, which is tested with acid

576 4 Manufacturing and Machining Methods

FIGURE 4.117  Trial welding with (a) and without (b) subsequent annealing treatment

FIGURE 4.118  Trial sample with 6 welding tests

Laser welding is a suitable alternative to TIG welding for small areas. There are only a few edge zones due to the significantly lower energy.

Again, the material selection is extremely important. For every material, there is a “best weld metal”. It is recommended to weld and grain with various test sample materials, as shown in Figure 4.118.

Basically, a welded area is always a repair and visible to the trained eye. Custom-ers should be notified in advance. It is also worthwhile to speak with the graining company before welding and to look for other solutions. Once an area is welded, it cannot be reversed.

4.6.2.5  Contour Changes by Shrinking Inserts

An alternative to welding is the shrinking of inserts. For this, an insert with press-fit diameter, which has strongly shrunk a�er cooling with nitrogen, is pressed into a round bore hole. Once the insert has extended again, it is fixed in the bore hole

5774.6 Surface Structuring

without a gap. It should be noted that the insert is manufactured from the same material and material condition, ideally even from the same batch. The material can mostly be taken outside of the cavity. The shrinking should be done taking the fiber direction into account.

Figure 4.119 to Figure 4.121 show the prepared recess, the shrinking, and the condition a�er shrinking.

If the structure direction of the material was not considered in shrinking, it can lead to a visible area, as shown in Figure 4.122.

4.6.2.6  Structure Hardening, Fiber Orientation, Band-Type Formation

Molds can have imperfect surfaces due to steel and processing a�er graining. These include structure hardenings, which for example, develop when dull cutters heat or compress the surface too hard.

A

y

x

R

FIGURE 4.119  Prepared recess

FIGURE 4.121  Condition

a�er shrinking

x +

1

2/3

x

2/3

y

0,5°

A + λ

FIGURE 4.120:  Insert to be shrunk

FIGURE 4.122:  Insert, which was inserted

into a bore hole and which

was subsequently etched

578 4 Manufacturing and Machining Methods

FIGURE 4.123  Structure compression

through dull cutting tool

During etching, the milling track is characterized through color and through a dif-ferent etching depth on the surface. Figure 4.123 shows such a surface.

Every steel is compressed a�er casting through a forging or rolling process. Here it is stretched in one direction. When etching, this direction may be the fiber orienta-tion in the etching ground. This orientation is hardly recognizable. For components, which have to be assembled together, it should be the same. This should already be considered when purchasing the steel. Figure 4.124 shows the fiber orientation in the etching ground of a mold insert.

A slight segregation may occur during solidification depending on the steel material. This process is physically limited and cannot be avoided altogether, especially for large dimensions. This segregation can be stretched during forging and can lead to banding in the material of the materials 1.2311 and 1.2738, which may be visible during polishing and etching. By selecting suitable etching media, this effect can be reduced, but not avoided. The selection of the acid also depends on the desired grain-ing. Compared to the aforementioned plastic mold steels 1.2311 and 1.2738, these events have not been occurring with the advanced plastic mold steel 2738 mod. TS. This modified steel is therefore particularly recommended for graining.

4.6.2.7  Etching Test

Through an etching test, as shown in Figure 4.125, surface defects can be made visible in advance. The contracted graining company can perform this etching test upon request. Here, problems appear, but not their intensity.

FIGURE 4.124:  Steel structure a�er

etching (1.1730)

5794.6 Surface Structuring

FIGURE 4.125  Band-type formation in the structure of a mold from 1.2311,

made visible by an etching test

4.6.3  Special Processes

4.6.3.1  Design Types and Etching Combinations

The simple etching of a structure is also called semi-matte; the over-etched structure is also called a round-matte. In addition to the general single and multi-etching, there are other etching techniques. To adapt grains as close as possible to leather structures, films of different levels of the same graining are laid on top of each other and are etched with a respective proportional etch depth in multiple etching steps. This multilayer technology, with which pyramid-like surfaces or woven and fabric structures can be generated, is very time consuming and costly, but leads to better results.

4.6.3.2  Limitations of the Processing Technology

Films are used in the etching technology. Although there are also stretchable films, there are limits in the application of geometric structures to three-dimensional surfaces. Structure pictures like pyramids can only be generated in a limited way by etching, described in the area of multilayer technology. Etchings have an unde-fined, nonvertical slope angle, so neither a cylindrical hole, nor a hole with a defined edge angle can be etched. During etching, the acid gets under the grain film. This process is called under-etching, and it has an etch depth of about 66% in the lateral removal. A film line is always attacked from two sides so that the under-etching is at about 132%.

A line disappears accordingly by under-etching when the etching depth reaches about 75% of the line width. The fineness of a structural image automatically limits the maximum etching depth.

580 4 Manufacturing and Machining Methods

4.6.3.3  New Technologies

Currently laser technology is developing. It is possible to scan grains in 3D and to map the data using the CAD data of the mold, as seen in Figure 4.126.

A�er reviewing the part, the structure can be burned into the mold surface with a 5-axis laser. For this purpose, the structure is disassembled in up to 40 layers in order to show the 3-dimensionality of the grain as much as possible.

Figure 4.127 shows a leather surface, which is lasered in steel, and a 0.4 mm deep geometric structure. The penny serves for the size comparison. Combinations of the etching and laser technology are also being tested. The main advantages are an unprecedented level of representation accuracy and the possibility to virtually view the result beforehand. One disadvantage is the still relatively large laser head, coupled with the fact that the laser beam should vertically hit the mold surface.

FIGURE 4.126  3D representation of a mold

FIGURE 4.127  Lasered leather surface and geometric structure

5814.6 Surface Structuring

4.6.4  The Execution of the Order

4.6.4.1  Supply

To get a very accurate supply, the graining specialist requires extensive details. The best would be a molded part, which is marked with the graining, grain direc-tion, grain areas, grain depth reduction, and grain-free areas. Separations must be clearly visible. For this, the mold steel, the mold assembly, and the material to be injected are important.

If no molded part is available, 3D images of the part including dimensions would be helpful as well. Product drawings with grain areas are also possible, however, very complex to interpret with large molds.

A demolding analysis should be provided if the location of the flattening is not determined yet.

4.6.4.2  Information about the Grain Area and the Mold

Again, a marked injection molded part is required that features the graining, grain areas, grain depth reduction, and grain-free areas. Grain-free edges on separation spots must be clearly marked. Figure 4.128 shows an appropriately marked part.

Grain-free areas must be, as described above, marked or otherwise identified. Weld areas, if any, must be marked. The mold steel should be known.

FIGURE 4.128  Molded part with markings for the respective grain depths

4.6.4.3  Concluding Remark

The graining of molds is a cra�smanship that is cra�ed down to the last detail. Well-trained specialists can combine know-how and technology to produce good structures. Each mold with its own shape provides a challenge. Tolerances should therefore be considered as a necessary clearance, in which a structure is moved.

582 4 Manufacturing and Machining Methods

 ■ 4.7  Rapid Prototyping in Mold Making

A. Gebhardt

4.7.1  Rapid Tooling

The term rapid tooling developed in the 90s when the additive manufacturing pro-cesses, which until then were exclusively used for the production of positive plastic prototypes, were also used for the production of negatives, which means cavities, or with a generous interpretation, molds. Today, the term rapid tooling has three related, but different meanings:

1. Strategies for the rapid design of tools, molds, and gages in terms of methodology.

2. All processes, including non-generative, manufacturing processes, and process combinations that allow a faster or cheaper manufacture of tool, mold inserts, molds, and gages, but for much lower production quantities than it is possible with conventional methods of mold making. The term includes all types of molding processes (see also Section 1.11).

3. Generative manufacturing process for the direct production of tools, mold inserts, molds, and gages.

This section deals exclusively with rapid tooling (for a more precise definition, see Section 4.7.2.4) in the sense of generative manufacturing of molds and mold inserts, see Point 3.

Rapid tooling, which translates as “fast tool making” or “automatic tool making”, suggests that entire molds can be built with all generative processes. This applies only to undercut-free open-close molds. For complex production molds, that idea is completely unrealistic. Complex mold components like (preferably chilled) inserts or slides are manufactured generatively. The application of generative processes is therefore closely connected with the mold conception and should already be determined during the planning and design of mold. Rapid tooling is not an automatic mold making process. The application of generative processes is only technically and economically feasible in good coordination with non-generative process steps.

The following section presents the generative processes that play a role in mold making and are discussed regarding the suitability in mold making. The process fundamentals are only discussed as far as it is necessary for understanding the features of the generative manufacturing technology. For a comprehensive descrip-tion, a reference to [1] is made.

5834.7 Rapid Prototyping in Mold Making

A comparison of the characteristics of generative and non-generative processes is helpful to understand the possibilities and limitations of generative manufacturing processes. This discussion is postponed to the end of the section (Section 4.7.6) because the principles of generative manufacturing must first be known.

Generative processes are also used to produce cores and molds for sand casting, lost models, molds for investment casting, and master models for a variety of molding processes, preferably in so� (silicone) molds. Molds for sand and investment cast-ings are not discussed in the context of this book, molding techniques are discussed in Section 1.11.

4.7.2  Fundamentals of the Generative Manufacturing Processes

4.7.2.1  Process Principle

Generative manufactured components are generated in layers. The describing data is mathematically sliced into virtual layers of equal thickness (slicing operation). The resulting contour data of each layer is transformed into a physical layer in a generative production system and is connected with the previous one. The part is therefore created layer by layer from bottom to top (Figure 4.129). The many genera-tive methods available today only differ by the type of layer generation, the method of connecting two adjacent layers, and the building material.

The use of the generative manufacturing principle requires a complete 3-dimen-sional data set of the part to be manufactured. This aspect is not further discussed because 3D CAD designs are routinely used in mold making.

RechnerinterneSchnitt darstellung

RechnerinternesCAD-Modell

PhysischesBauteil

Umsetzung derEinzelquerschnitte

in physische Schichten

Zusammenführungder physischenEinzelschichten

zum Bauteil

+

Generativ eFertigung

Virtuelle Ebene .

Erzeugung dermathematischen

Schichtinformation

Reale oderphysische Ebene

Generierung desphysischen

Bauteils

InternalComputer CAD

InternalComputer Section

+

GenerativeManufacturing

Implementation of theIndividual Cross Section

in Physical

Integration of PhysicalIndividual Layers and

the Component

PhysicalComponent

Virtual Level

Generation ofMathematical Layer

Information

Real or PhysicalLevel

Generation of thePhysical Component

FIGURE 4.129  Principle of generative manufacturing-layer construction principle

584 4 Manufacturing and Machining Methods

4.7.2.2  Data Flow and Data Formats

To generate the building data for generative manufacturing, the 3D CAD data has to be mathematically cut into the same layers, which are subsequently required for production. To do this, independent of the CAD programs, all surfaces of the component are covered with a triangle mesh (triangulation, tessellation) and are therefore clearly defined by the three corners and the normal vector of a triangle. The data set with the triangle information is known as STL dataset (STL = stereo-lithographic interface). The STL-formulation provides a de-facto standard in gen-erative manufacturing. Alternatively, the contour can be generated directly from the CAD (native). The resulting data sets are called contour-oriented or SLC data sets (CLI). Molds made from prefabricated foils, sheets or plates are o�en based on 2D data formats such as HPGL and DXF. Details can be found at [1], there, refer to Section 2.2.2. Figure 4.130 shows the corresponding data flow in generative manufacturing, particularly in the manufacture of molds.

3D-CAD-Modell

RapidPrototypingSoftware

FrontendSoftware

(inklusiveOrientieren,Slicen)

Bauprozess

Prozess-spezifischeNacharbeit

(Post Processing)

generativer

3D native,

STL, CLI / SLC

STL, CLI / SLC,

3D native

VirtuellesProduktmodell

Generative FertigungsanlagePrototyper / Fabrikator

CLI, SLI

SLC

Werkzeug-konstruktion

Randbedingungen desgenerativen Prozesses

VirtualProduct Model

Generative Manufacturing SystemPrototyper / Fabricator

Boundary Conditions of theGenerative Process

3D CADModel

MoldDesign

RapidPrototyping

Software

FrontedSoftware(IncludingOrienting.

Slicing)

GenerativeConstruction

Process

Process-SpecificReworking

(Post-Processing)

3D native,STL, CLI / SLC

STL, CLI / SLC3D native

CLI, SLISLC

FIGURE 4.130  Data flow in generative manufacturing, especially in rapid tooling

4.7.2.3  Properties of Generative Components

The generative building principle brings several advantages:

� Components of virtually any complexity, especially those with undercuts and internal cavities (Figure 4.131), can be manufactured. This property usually is not important for cavities, because the component must be able to be removed from the mold. But it is the basis for an optimized internal structure, for example, for contour-adapted cooling channels (see Fig 1.290, surface cooling, in Section 1.11.2.5).

5854.7 Rapid Prototyping in Mold Making

FIGURE 4.131  Geometric freedom of generative processes using the example of a structure

with internal cavities (source: Trumpf)

� The processes that are most suitable for rapid tooling, especially the sintering and melting process (see Section 4.7.3.2), only use as much material as necessary for the production of the component. Isolated geometries as free-standing domes and the like are therefore both produced faster and with less material consumption than with erosive or machining processes.

� The data sets can be scaled to any size. � The component can be produced in virtually any orientation. This defuses the clamping problem.

� The general standard STL format can be processed by all generative machines on the market.

� Some methods allow the change or alteration of the material during the construc-tion process and therefore the local adaptation of the component properties to the operating conditions (Figure 4.132). The material properties, so-called gradient materials that are continuously varying over the cross section of the component, are possible.

FIGURE 4.132  Mold insert with copper insert and contour-adapted cooling (source: FhG-IWS)

586 4 Manufacturing and Machining Methods

There are also disadvantages:

� Due to the principle, the surfaces are staged. Low layer thicknesses, in metal process down to 0.025 mm (standard is 0.1 mm), reduce the layer effect, but reworking doesn’t become superfluous.

� Local melting tends to thermally induced warpage and local hardness increases. They may be reduced by appropriate exposure strategies and protective gas lines.

� The material palette is constantly growing, but is still very limited. The manufac-turers only offer a few standard materials. If special materials such as titanium or CoCr are offered, the limit is one material per material class.

� The manufacturing process is sensitive to parameter changes. The optimiza-tion and monitoring of critical building parameters is essential for a permanent qualitative result.

� There is a close coupling between the process (machine) and material. The same material delivers a component with altered properties on a different machine.

4.7.2.4  Definitions for Rapid Tooling

Rapid tooling means an application-oriented subset of the generative processes. Figure 4.133 shows the application of the generative principle (Rapid Technol-ogy) on the production of prototypes (Rapid Prototyping, RP) and products (Rapid Manufacturing, RM).

Direct Manufacturing

Direct Tooling

Functional Prototyping

Direct ProcessesRapid Tooling

Indirect ToolingFollow-up

Rap

id T

oolin

gPrototype ToolingBridge Tooling

Ind. Prototype ToolingInd. Bridge Tooling

Generative ManufacturingProcess

Rapid Prototyping

Rapid Manufacturing

Solid ImagingConcept Modeling

Solid ImagingConcept Modeling

Functional Prototyping

Indirect ProcessesNot Generative Tooling Processes

FIGURE 4.133  Structure of the generative manufacturing process technology and its

applications rapid prototyping and rapid manufacturing as well as their

association with prototype tooling, direct tooling, and indirect tooling

5874.7 Rapid Prototyping in Mold Making

Molds can be generatively manufactured with rapid prototyping processes and from rapid prototyping materials (prototype tooling), as well as rapid manufactur-ing techniques and serial materials (direct tooling). Serial materials are materials that are used in serial mold making. In contrast to this, model materials are also frequently used in the generative manufacturing technique.

If molds are manufactured on the basis of rapid prototyping components, but not directly produced in the generative process, it is called indirect tooling or subsequent proceedings. Since this is not a generative manufacturing process, indirect methods are, strictly speaking, no rapid tooling process. However, it has become custom-ary and is mainly supported by marketing that it is called rapid tooling within the meaning of Section 2 of the previous definition (see Section 1.11).

Direct or indirect RT methods for producing molds that enable the production of end products, but sacrifice part quality and particularly the output quantity, are referred to as bridge tooling. This is based on the idea to bridge the gap between prototype and serial mold making.

Figure 4.133 highlights that the term rapid tooling describes a common subset in the sense of applying the generative manufacturing processes, but does not justify its own technology layer.

4.7.3  Generative Processes for Mold Making

The implementation of the generative principle as seen in Figure 4.129 lists five process families. The generative processes are associated in Table 4.7, which under-lie the generative machinery, are available today.

TABLE 4.7  The Five Generative Process Families and the Derived Generative Manufacturing

Processes

1. PolymerizationStereolithography (SL)Polymerprinting

2. Sintering/Melting(Selective) Laser Sintering ((S)LS)Selective Laser Melting (SLM)Selective Mask Sintering (SMS)Electron Beam Melting (EBM)

3. Layer Laminate Manufacturing (LLM)

4. Fused Layer Modeling (FLM)

5. Three Dimensional Printing (3DP)

The abbreviation “M” stands either for “Modeling” or for “Manufacturing” depending on the source.

588 4 Manufacturing and Machining Methods

The processes differ primarily in how and out of which material the layer was produced and contoured, and how it is connected with the previous one (the one below). Some methods require supports, which keep the not yet rigid component during the building process in shape. They do not belong to the CAD data set and need to be generated with specialized programs (automatically) and need to be removed from the component, either manually or with special washing process.

The building material has the biggest impact. It can be initially available as a solid, liquid, or gas. To get a solid layer with defined contours, it is melted and cooled or chemically transformed. The contouring is done simultaneously or sequentially with laser scanners, laser plotters, electron beam systems, projectors, infrared radiators, single or multi-nozzle systems, and extruders.

4.7.3.1  Polymerization-Stereolithography

Process PrincipleLiquid monomer, which is located in a installation space, is polymerized by a laser beam at its surface of the layer contour and is thus solidified into a layer. This layer is simultaneous to the polymerization connected with the layer below. A�er gen-erating a layer, a building platform, which is submerged into the building space, is moved downward. The semifinished component, which is connected to the building platform using supports, is also lowered to the next layer thickness and releases a

FIGURE 4.134  Polymerization processes using the example of laser stereolithography

5894.7 Rapid Prototyping in Mold Making

corresponding volume at its surface. This is filled using the gravity and a coating unit with monomer (re-coating). The process is schematically shown in Figure 4.134. (A 3D animation of the process can be found in [2].)

A�er the building process, the component is fully polymerized in a post-curing cabinet (post curing oven) and is freed from the supports. Process variants use nozzles to apply the material and polymerize with high energy lights or using a DLP projector. They o�en use supports from a thermoplastic wax, which can be washed out.

Advantages and DisadvantagesPolymerization processes deliver the best surface finishes and the finest details. However, the supports must be removed in many processes, and the components are post-cured.

MaterialsPolymerization processes can only process materials that are photosensitive and were therefore, until recently, limited to unfilled plastics. Through the use of filled resins, higher strength and temperature-resistance, and also metal and ceramic components (in perspective), can be manufactured today in a multistage process.

With today’s materials, the properties of important technical plastics such as poly-ester, polyamide, or ABS can be simulated.

Application in Mold MakingPrototype Tooling and Indirect Tooling: polymerization processes provide very precise master models with good surfaces and are therefore suitable for demolding. There-fore, a number of processes for the production of so� or hard molds were developed. The possibilities range from the well-known vacuum casting in silicone molds to metal spraying and counter-casting of polyurethanes or filled epoxy resins, up to complex manufacturing processes of hard mold inserts using the Keltool Course 4 Technology process.

Direct Tooling: Molds (simple open-close) are directly manufactured with polymer-ization. The process is described by the manufacturer 3D Systems as ACES Injec-tion Molding (ACES is a stereolithography building style). They are not suitable as elements of serial production molds.

Machines and ManufacturerLaser stereolithography machines (Viper SLA, Viper Pro) and polymer printer (Invi-sion) produce 3D Systems. Polymer printing or jetting systems (Eden Series) are from Objet, systems with DLP projectors (Perfactory) of Envsiontec.

590 4 Manufacturing and Machining Methods

4.7.3.2  Sintering and Melting

Process PrincipleThermoplastic powder, which is located in a building space, is locally melted on its surface of the layer contour and solidified by heat conduction into the surrounding powder. A laser or an electron beam, which is controlled accordingly to the respec-tive layer contour, supplies the melting energy. Alternatively, a mask with the layer contour is created and illuminated by an infrared source.

A�er generating a layer, the bottom of the building space is moved downward. The entire powder cake is lowered by one layer thickness and releases a corresponding volume on its surface, which is then filled with new powder using a coating unit (re-coating). The process is schematically shown in Figure 4.135 [2].

During the generative production, the component is supported from the surround-ing powder must be removed by light sanding (post-processing). Therea�er it can be used.

FIGURE 4.135  Sintering and melting process using the example of laser sintering

Sintering-MeltingIn the literature, it is discussed whether the described process is called sintering or melting. For practitioners, these discussions are not necessary. Especially for the application in mold making, fully dense components are required, which are usually,

5914.7 Rapid Prototyping in Mold Making

but not exclusively, supplied by procedures that are identified by the manufacturer as the melting process.

Coating or Melting with Powder NozzleA variation of the sintering or melting process in the powder bed is the laser-based generation with the powder nozzle. The powder is, parallel to the laser beam, added in and then melted. The processes are suitable for larger layer thicknesses and are preferably used for repair purposes.

Advantages and DisadvantagesSintering and melting processes can process all the materials that behave as a thermoplastic and are available as a powder. They allow the production of internal cavities. The components do not require support structures. The processes are single-stage processes. The components are directly applicable a�er low rework, if the surface quality is accepted. The relatively rough surface is therefore one of the biggest disadvantages of the procedure. Especially for metal components, the rework becomes complex and also affects the accuracy.

MaterialsComponents made from plastic, metal, and ceramics can be produced with sinter-ing and melting processes.

Plastics: Available are unfilled and filled (with aluminum or glass pellets) semicrys-talline polymers of the type PA 11 or PA 12 (Polyamide) and amorphous polymers of the type of polystyrene (PS).

Ceramics: The material range covers almost the entire spectrum: molds and com-ponents made from Al2O3, SiO2, ZrO2, and SiC, fully sintered from Si3N4 and so-called “graded materials”, such as zircon-reinforced aluminum (ZTA) by application of ZrO2 to an Al2O3 layer or an appropriate substrate [3]. Foundry sands can be directly sintered. They are coated with polymers, so that the polymer shells are in fact sintered. This is basically a plastic process.

Metals: A variety of single and multi-component metal powders are available. The focus is on single component powders because component properties can be achieved that are comparable to the properties of milled or eroded parts. Available are steel, stainless steel, mold steels, CoCr-steels, titanium, and aluminum. Grain sizes down to 20 μm require careful handling. Reactive powders such as aluminum and titanium must be processed in completely enclosed building spaces with inert gas atmosphere and preferably an internal material management. The mechanical-technological properties of the components, especially the hardness, yield strength, and tensile strength, are steadily approaching those that have been manufactured from semifinished products with non-generative processes.

592 4 Manufacturing and Machining Methods

Application in Mold MakingPrototype Tooling and Indirect Tooling: Master models for molding processes can be manufactured with sintering processes. This rarely happens due to poor surface and therefore required extensive rework. For prototype molds, plastic molds can be produced which are then preferably casted.

Direct Tooling: Especially metal components, which are directly used as components for the mold, can be manufactured with sintering and melting processes. With increasing process stability and improved materials, a well-adjusted generative and non-generative manufacturing method is increasingly economically attractive.

Internal hollow spaces can be produced, and hence, the principle of conformal cooling can be implemented in terms of manufacturing technology.

By generating with the powder nozzle, molds can be built that are specifically mixing materials so that an optimum thermal efficiency can be realized. So-called bimetal molds are provided with, e.g., copper zones, which are embedded into the mold material and which are directly brought into the process.

Machines and ManufacturerLaser sintering systems produce 3D Systems (sintering station HiQ-Pro), EOS (EOSINT P, M, and S), Concept Laser (M1 Cusing – Cusing is an acronym for cladding and fusing – M2 cusing, M3 linear), and MCP-HEK (MCP Realizer SLM 250, -100). Concept and MCP describe their products as melting plants. ARCAM (EBM S12) offers an electron beam sintering plant and Speedpart (RP3) offers a plastics manufacturing facility with infrared emitters and a mask. The principle of generating with the powder nozzle can be realized by Optomec (LENS 705, -800R) and Trumpf/POM (DMD 505).

4.7.3.3  Layer-Laminate Process

Process PrincipleThe contour layer is cut out of prefabricated foils, sheets, or plates by using a laser, a knife, or a milling cutter. The contour layer is then joined to the previous layer. The simplest methods work with paper, which is provided with a thermally activatable adhesive. Each layer is glued to the previous layer (the first one to the building platform) and then contoured using a laser. The entire layer material remains in the component. Finally, a block is created from which the component must be released. This is quite a complex manual process despite appropriate relief cuts. Paper-layering processes are fully automatic from the roll or with individual sheets. The best known method, the laminated object manufacturing (LOM) of the company Cubic Technologies, USA, is o�en, but incorrectly, identified with the entire process family.

5934.7 Rapid Prototyping in Mold Making

FIGURE 4.136  Layer-laminate process, on the example of laminated object manufacturing

A variant of the method uses laser cutting or milling machines, and joins the indi-vidual cross sections by means of corresponding bore holes and locating pins. The connection is done via diffusion or ultrasonic welding, mechanical clamping, or gluing. The process is schematically shown in Figure 4.136 (see also [2]).

The component is, during the generative production, supported by the surrounding building materials that have to be removed manually a�er the building process. Paper models must be infiltrated a�er the building process.

Advantages and DisadvantagesLayer laminate methods can process all materials that can be mechanically or ther-mally separated. The materials are generally cheap because commercial qualities are used. The large proportion of waste and the high requirements on the thickness tolerances, however, relativize this advantage again.

MaterialsPreferably components made from paper and plastic, but also metal and ceramics, can be manufactured with the layer laminate process. Sheet products can be pur-

594 4 Manufacturing and Machining Methods

chased as a semifinished part on the free market. This means that all commercial qualities are available.

Application in Mold MakingPrototype Tooling and Indirect Tooling: Layer laminate processes can quickly provide cheap, but not very precise master models, made from paper and plastic. For the replication, they should not have any fine isolated details because of the danger of breaking. They have to be treated thoroughly with respect to their surfaces.

Direct Tooling: So-called lamellar molds are made from steel with the layer laminate processes. They quickly create volume, but must be machined due to the surfaces, and are also very limited in their complexity. For metal parts, spreading film will also be used without reworking.

Machines and ManufacturerCubic Technologies (LOM 1015plus, -2050H), Kinergy (Zippy I, -II), Kira (PLT A3, -A4, Katana), and 3D Systems (LD 3D Printer) provide machinery for the production of components made from paper or plastic films.

Layer milling processes on their own machines are offered by Zimmermann (LMP), and so�ware packages for the application on different, sometimes even on their own machines, is offered by Stratoconception/CharlyRobot (rp2i). Service with proprietary layer milling processes is provided by Weihbrecht and Tschopp Engi-neering.

Solidica has presented a hybrid machine (Formation) for the implementation of the Ultrasonic Consolidation principle, which applies aluminum strip by means of ultrasound and contours a�er several layers with the milling cutter. It produces dense aluminum components.

4.7.3.4  Extrusion Process

Process PrinciplePrefabricated wires or bars made from thermoplastic material are melted in a nozzle and are then applied to the previous layer (the first one on the construction plat-form) in a doughy consistency according to the current contour. The layer is being consolidated by heat conduction into the partially finished component.

A�er generating a layer, the building platform and thus the partially finished com-ponent, which is connected with supports, is moved downwards and the following layer is applied. The process is schematically shown in Figure 4.137 (see also [2]).

A�er the building process, the component has to be exempted from the supports. This is done manually or through special washing techniques.

5954.7 Rapid Prototyping in Mold Making

FIGURE 4.137  Extrusion processes on the example of Fused Deposition Modeling (FDM)

Advantages and DisadvantagesThe process is stable, also in an office environment, depending on the machine, is easy to use and delivers components that are successfully used in a product development. It is limited to plastics. The supports must be removed. The extrusion leaves characteristic traces on the surface, which also depends on the orientation of the component in the building space. For the molding process, relatively complex manual adjustments are therefore required.

MaterialsExtrusion processes can only process thermoplastics. There is however a fairly extensive range available, which covers PP, ABS, and a high temperature plastic (polyphenyl, PPSU). The materials can also be obtained in color.

Application in Mold MakingPrototype Tooling and Indirect Tooling: Extrusion processes are due to the poorer surface quality for all types of molding processes, less suitable than the stereolithog-raphy. Particularly high demands on the thermal or mechanical loading capacity may provide exceptions.

Direct Tooling: Extrusion processes for manufacturing inserts are not suitable for production molds.

596 4 Manufacturing and Machining Methods

Machines and ManufacturersStratasys offers a wide range of extrusion equipment according to the FDM prin-ciple. It includes machines with different building sizes and for different materials (Prodigy/FDM200mc; Vantage i, -X, -S, -SE; titanium, Maxum). Inexpensive machines for fast production in an office environment (dimension BST768, -1200, dimension SST768, -1200, Dimension Elite) are available with the dimension building series.

4.7.3.4.1  3D PrintingProcess PrincipleThe powder, which is located in a installation space, is locally connected on its surface (according to the layer contour) by a sequentially injected binder liquid and thus forms a solid layer. The binder input is done by multi-nozzle pressure heads.

A�er generating a layer, the bottom of the building space is moved downward. The entire powder cake is lowered by one layer thickness and thus releases a corre-sponding volume on its surface, which is then filled with new powder by a coating unit (re-coating). The process is schematically shown in Figure 4.138 (see also [2]).

During the generative production, the component is supported by the surrounding powder, which must be removed by suction a�er the construction process. The fol-lowing step is an infiltration. Prior to that, the binder must be removed, depending on the material.

FIGURE 4.138  3D printing process

5974.7 Rapid Prototyping in Mold Making

Advantages and Disadvantages3D printing process can process all materials, which are available as powders. They allow the production of internal cavities. The components do not require support structures. The metal processes are multistage processes and run at room tem-perature. They require a subsequent oven process and an infiltration with bronze. The relatively rough surface is a disadvantage of the process. Rework will be very complex especially for metal components and also affects the accuracy.

MaterialsComponents made from plastic, metal, and ceramics can be manufactured with 3D printing processes. The standards are gypsum-ceramic and starch powders for illustrative models and lost molds for investment casting.

Metals: Stainless steel powder (comparable X2CrNiMo; 1.4404) and a mold steel (X42Cr13 comparable; 1.2083) that can be brought to a hardness of 54 HRC a�er the oven process are available. Due to the bronze infiltration, the mechanical-techno-logical properties of the materials are generally lower than that of the semifinished products; but the thermal conductivity is higher.

Application in Mold MakingPrototype Tooling and Indirect Tooling: 3D Printing provides fast, cheap but not very precise master models. They should not have isolated fine details for the replica-tion because of the danger of breaking. They have to be re-treated thoroughly with respect to their surfaces.

Direct Tooling: Mold inserts, which may also contain internal cooling channels, are made with 3D printing processes. The bronze infiltration leads to a higher thermal conductivity, which may be advantageous if the tendency to worse mechanical-technological properties is accepted.

Machines and ManufacturerMost manufacturers of 3D printing systems focus on machinery for the produc-tion of components made from plastic (Voxeljet, VX800, VX500), starch, gypsum, ceramics (Z Corp, Z-Printer 310plus, -450, -510), or molding sand (Prometal mold sand line). Prometal provides a metal line with two machines (R1 and R2), which are called Direct Metal Printer. Optionally there is a densification furnace to drive off the binder and for infiltration as well as an unpacking station.

598 4 Manufacturing and Machining Methods

4.7.4  Machines for Generative Mold Making

Since generative machines, as opposed to cutting or eroding machines, are generally less well known, the basic structure of generative machines is only briefly described. A description of actual machinery is referred to in [1] and on the websites of the manufacturers (Section 4.7.7).

Laser Scanner

Fenster zur Prozesskamme r

LaserstrahlAbgeschlossenerBauraum

Beschichter Bauteil

BauzylinderPulvervorrats-zylinder

Closed BuildingSpace

Coater

Powder SupplyCylinder

Laser beamWindow to theProcess chamber

Component

Building Cylinder

FIGURE 4.139  Laser sintering system, principle of a closed building chamber

(Source: Phenix)

FIGURE 4.140  Melting unit

(a) with closed building space RealizerSLM 100 (source: MCP-HEK)

(b) with exchangeable modules M3linear (source: Concept Laser)

5994.7 Rapid Prototyping in Mold Making

The example of the sintering or melting technique, which is particularly important for rapid tooling, is shown in Figure 4.139 with a laser-sintering or -melting unit. The illustrated version is closed and suitable for high-temperature processing of the material in an inert gas, but has all the elements of a sintering or melting unit.

The contouring, using a laser-scanning unit, the building plane, in which the con-touring and layer generation are running simultaneously, and the powder storage and application system are clearly visible.

Figure 4.140 shows a melting unit with a totally completed building space for the processing of reactive powder materials and with interchangeable building modules, which are not only used for generating, but also for engraving and marking.

4.7.5  Examples

4.7.5.1  Prototype Tooling

Two examples for prototype tooling by direct application of generative methods are the AIM-application, Figure 4.141, which can be directly inserted into a mold frame and used on the injection molding machine and the sintered negative of a boot profile (Figure 4.142).

FIGURE 4.141 AIM cavity made from stereo-

lithography material for the application in a frame

(source: 3D Systems)

4.7.5.2  Direct Tooling

In the direct tooling methods, preferably mold cavities, inserts, and sliders are manufactured from mold steel. The generation of the powder nozzle is advanta-geously used for repairs and design changes. Figure 4.143 (a) to (c) shows mold inserts, which were produced with different methods. They mainly have internal cooling channels, free-standing domes, or deep slots. Flexible mold concepts are implemented with exchangeable inserts. An example of generative manufactured inserts for a cockpit mold is shown in Figure 4.144.

FIGURE 4.142: Boot profile

(Direct Pattern) and the related

products; laser sintering,

Polyamide (source: EOS)

600 4 Manufacturing and Machining Methods

a)

b) c)

FIGURE 4.143  Mold inserts, manufactured with different generative processes (partially cut)

(a) Direct-Metal-Printing Process (source: Prometal)

(b) Selective Laser Melting (source: MCP-HEK)

(c) Laser Cusing (source: Concept Laser)

FIGURE 4.144  Exchangeable inserts

for Cockpit molds,

injection molding component

(source: EOS)

Figure 4.145 shows a mold insert for high-volume production of injection molded parts (toothbrush heads). On the le� is a generatively produced component with

FIGURE 4.145:  Mold inserts for the

large-series production

(source: Braun/Trumpf)

6014.7 Rapid Prototyping in Mold Making

start drilling holes for wire EDM, on the right is the fully-finished mold insert. The material is a mold steel (1.2343) with a hardness of HRC 53 (without heat treatment).

A systems approach for the optimal combination of generative and non-generative manufacturing steps shows the ecoMold project. For the non-generative rework, but also for mounting onto the clamping plate, the inserts are directly generated to the clamping elements (Figure 4.146).

Examples for the application of generative processes in the repair and mainte-nance, the texturing (Figure 4.147), which is applied with the powder nozzle, and the construction of a mold insert made from 1.2343 on a prefabricated blank, are made from the same material.

Figure 4.148 shows a component made with the generative process and as a finish-machined insert. Generative processes have made their way into the production of

FIGURE 4.146  EcoMold

Generative construction process (top),

generative and non-generative manufactured mold component (bottom le�);

completed mold half (bottom right) (source: FhG IFAM)

602 4 Manufacturing and Machining Methods

standard parts. Figure 4.149 shows a so-called adjustable or cooling pin with an internal helical cooling channel.

Original Material: 1.2330 Reparaturmaterial: 1.2343Repair Material: 1.2343

FIGURE 4.147  Repair of textured surfaces, DMD (source: Trumpf)

FIGURE 4.148  Mold insert, built on a premade blank (both made from 1.2343).

Generated component (le�), fully finished mold insert (right).

DMD (source: Inno-shape)

Area w

ith A

ir Coolin

g

FIGURE 4.149  Adjustable pins, cut (le�), ready to use (right), laser cusing

(source: Hofmann, Lichtenfels)

6034.7 Rapid Prototyping in Mold Making

The hard work of all manufacturers in the field of direct manufacturing and direct tooling can expect continuously new examples. This is referred to the websites of the manufacturers (Section 4.7.7).

4.7.6  Delimitation to Non-Generative Manufacturing Processes

Non-generative, preferably cutting, and erosive processes are used and generally known in mold making; generative manufacturing processes are less used and known. In view of the above, the characteristics of generative manufacturing process are discussed in the following in contrast to non-generative processes based on the most important factors.

MaterialsNon-generative processes work with materials in the form of semifinished parts, which are selected to match the mechanical properties of the product: product-oriented material selection.Generative processes work with a few materials, which are mainly optimized to the generative process and less for the product. The desired mechanical properties can therefore only be approximately achieved: process-oriented material selection.

ToolsNon-generative processes work with different tools which are optimally adjusted to the respective subtask and custom-built (if necessary), which need to be changed more frequently during the manufacturing process (if necessary): product-oriented tool selection.Generative processes work “without tools”, i.e., they do not use tools that are adjusted to the component. The “tool” is a layer-generating and contouring (shaping) element that will not be changed during the production of a component neither from com-ponent to component: process-oriented tool selection.

Component designNon-generatively manufactured components are typically, due to technical reasons, assembled and joined from several parts or elements that o�en consist of different materials: process-oriented component design.Generative components can be, due to the almost unlimited geometric freedom, optimally adapted to their function, can be functionally integrated designed, and can be built out of one piece. When they are assembled and joined from multiple components, then this is usually not done due to constructive or manufacturing requirements, but because of the limited work space or size to simulation of the production assembly: function-oriented component design.

604 4 Manufacturing and Machining Methods

CAD compatibilityThe CAD models of non-generative manufactured components will be implemented through appropriate programs and processors in processing programs for mold machines. Here, machine-specific issues are included to such an extent, that in general, the execution of a program has to be carried out on the machine (more pre-cisely with the controller) for which it was written: machine-dependent programming.

The data for the production of generative components are derived as standard STL data models from CAD data sets. On the basis of the STL data set, all known generative machines can be controlled. Thus, the selection of the optimum process succeeds without a new generation of data set: component-oriented and machine-independent programming.

PrecisionFor the leading conventional methods, the machine typical precision corresponds to the maximum achievable today. They therefore define the state of the art. Con-ventional processes are today, with regard to machine and mold, mastered so well that a defined accuracy is routinely achieved, which typically corresponds to the machine precision.

Generative processes usually only show this accuracy with regard to the contour and thus are 2-dimensional. Further inaccuracies are mainly added due to the layer principle in the 3rd dimension. Generative processes can therefore usually not reach the state of the art of conventional machines. They are, in the majority, also strongly dependent on the calibration. Optimum results can only be achieved with higher requirements if the model is first assembled and the resulting differences are used to calibrate the machine. Through experience, these calibrations are kept to a minimum.

Influence of manual workPrecise conventional methods, such as high-speed milling or eroding, are creating surfaces that either don’t have to be reworked at all or where the rework is mainly limited to surface effects so no measurable dimensional change is caused by it.

Due to the process, generative components have steps that can be easily leveled manually with relatively so� materials such as plastics. In this way, excellent surface qualities are manually achieved. But dimensional deviations are also generated. This can be problematic if a batch of very precise components is manually reworked by different employees.

From the above, it is clear that non-generative methods in regards to accuracy, mate-rial properties, reproducibility, and speed, depending on the geometry, may have advantages over generative processes. This is particularly true when the geometries

6054.7 Rapid Prototyping in Mold Making

are relatively simple. Conversely, one of the most important requirements for a successful application of generative processes follows:

Generative methods are used to an advantage when the components are very complex and are needed very fast and only in small numbers.

4.7.7  Names and Links

Company Supply Program Homepage3D Systems Sintering and Stereolithography systems www.3dsystems.com

ARCAM Electron Beam Melting (EBM) www.arcam.com

CAMLEM Layer Laminate Process (Metal, Ceramic) www.camlem.com

Charlyrobot Layer Milling Process www.charlyrobot.com

Concept Laser Metal Laser Melting Systems www.concept-laser.de

CP Prototypes made from Plastics and Metal www.cp-gmbh.de

Cubic Technologies Layer Laminate Process www.cubictechnologies.com

Envisiontec Polymerization Process (DLP) www.envisiontec.de

EOS Sintering Systems for Plastics, Metals, and Molding Sand

www.EOS.info

Fockele & Schwarze Metal Laser Melting Systems www.fockeleundschwarze.de

ILT Research and Development www.ilt.fhg.de

Inno-Shape Job shop for Metal Laser Melting www.inno-shape.com

IWS Research and Development www.iws.fhg.de

Kira Layer Laminate Process www.kiracorp.co.jp

MCP-HEK Metal Laser Melting Systems www.mcp-group.de

MK-Technology Impression and investment casting systems www.mk-technology.com

Objet Geometries Polymerization Processes (Printer-Lamp) www.2objet.com

On Demand Manufacturing

Generative Contract Manufacturing www.odm.bz

Optoform Joint Venture of 3D Systems and DSM

Phenix Systems Laser Metal and Ceramic Melting Processes www.phenix-systems.com

POM Generating with the Powder Nozzle www.pomgroup.com

Prometal 3D Printing for Metal and Molding Sand www.prometal-rt.com

RTeJournal Online Magazine for Rapid www.rtejournal.de

Solidica Metal Layer Laminate Process www.solidica.com

Solidscape Wax models for investment casting www.solid-scape.com

Stratasys Extrusion Process (FDM) intl.stratasys.com

Trumpf Laser Generating and Coating www.trumpf.com

Z-Corporation 3D Printing Process (Inkjet) www.zcorp.com

606 4 Manufacturing and Machining Methods

References

[1] Gebhardt, A., Generative Fertigungsverfahren. Rapid Prototyping – Rapid Tooling – Rapid Manufacturing. 3rd ed. (2007) Carl Hanser Verlag, Munich

[2] http://www.rtejournal.de/archiv/index_html/filme[3] Gebhardt, A., Vision Rapid Prototyping. Generative Verfahren zur Herstellung von

Keramikbauteilen. In: Kriegesmann, J. (Ed.) DKG-hand book Technische Keramische Werkstoffe. 96. Complement delivery, Section 3.4.2.3, (2007) HvB-Verlag Deutscher Wirtscha�sdienst, Ellerau

5 Ordering and Operation of Molds

 ■ 5.1  Molds in the Offer Phase

F. Schlößer

5.1.1  Introduction

Plastic injection molds are typically special molds and are essentially individual parts, despite all the standardization efforts of the individual components (stan-dards). However, this aspect alone has an enormous impact on the production costs of such a product. But also the other operational services to be provided in connection with the injection mold (e.g., a constructive, qualitatively perfect design, project planning in agreement with the customer, as well as management and distribution of mold) must be mostly based on a single project and a single customer. A digres-sively effective and balancing cost distribution, as is possible in series production, can mostly not be carried out.

In view of the increasing globalization with simultaneous market saturation and ever shorter innovation and product life cycles, the quotation processing can be a significant competitive advantage when quickly created, accurately fulfilling customer requirements, and technically reliable and commercially correct offers.

The processing of a quotation as an important component in the acquisition process is therefore gaining in importance. In order for a provider to remain competitive, the distribution costs have to be as low as possible through efficient and effective design of supply-related processes. Efficient means in this respect to choose the resources and methods used such that minimum cost (which are in this case, the time of quotation preparation) occurs while meeting customer requirements. Effec-tive means to increase the effectiveness of the supplies, which will result in a very high order volume. For companies in the capital goods industry, which includes mold making, this ratio is o�en below 5% [1].

608 5 Ordering and Operation of Molds

With these facts in mind, this chapter will deal with the planning of molds and therea�er with cost accounting in the mold making process.

5.1.2  The Planning of Molds

5.1.2.1  Adjustment Process of Component and Mold

Two variables basically determine the interrelation between component and mold:

a) the technical specification of the component and

b) the number of parts.

The technical specification of the component covers general technical parameters, such as geometric dimensioning, required tolerances, and material specifications. But it also includes processing and mold-specific data, such as location and shape of the gate, the parting seams, necessary gate and ejector design, and special procedures for reducing the harmful influence of the weld line (if necessary). An important aspect in this early phase of a product development process is to resolve the expectations of customers before the discussion of individual points. The design of the component, its later production, as well as the manufacture of the mold, are quite o�en distributed between three different partners. For the planning in mold making, it is therefore of great importance at what point of development the offer is made and which expectations regarding anticipated active participation in the development process are made from the client. Figure 5.1 shows an example of the sequence of a development.

Lasten-heft

SpecificationBook

Pflichten-heft

TechnicalSpecifications

System-bildung/Konzept-

phase

SystemDevelopment /

ConceptPhase

Prototy pen-freigab e

PrototypeRelease SerieSeries Ersatzteile

After Sales

Spare PartsAfter Sales

Entwick-lungs-phase

DevelopmentPhase

allgemeine, umfassendeForderungenan dasGesamtsystem

Ableitung und Detaillierung einzelner System-leistungen1. Machbarkeits-studie incl. 1. Wirtschaftlich-keitsprüfung

FestschreibungundBeauftragungaller Leistungs-pakete(incl. System- u.

Projektfreigabe)

Funktionsnach-weiseGesamtwirtschaft-lichkeitQualitätsforecast

Product Life Cycle

General,comprehensiverequirement onthe entiresystem

Division anddetailing ofindividualecosystemservices1. Feasibility study included2. Economicfeasibility

Establishing andcommissioning of all range ofactivities(incl. system and project release)

Functionality,economic,quality forecast

FIGURE 5.1  Product life cycle and sequence of a development

6095.1 Molds in the Offer Phase

It becomes obvious that there are two ways to integrate mold making into a project. There is the introduction at the development phase that gives far-reaching oppor-tunities to influence the design of the product and the mold with regard to techni-cal and economic aspects at an early stage. But, this procedure comes with a lot of associated risks and requests in case of upcoming problems the assumption of responsibility by the respective developers.

On the other hand, there is the entrance into the development phase on the basis of already narrowly defined specifications requirements. Especially in technologically and economically more advanced products, the earliest possible coordination with all partners, should already be done in the concept phase.

The number of parts as the second important factor of the interrelation between component and mold influences two important mold dimensioning parameters. On the one hand, the required production capacity (number of units per period), taking into account the component conditions (especially wall thickness and thermody-namic material parameters) and any resulting cycle time, requires a sufficiently high number of cavities. On the other hand, the production volume across the entire product life cycle (i.e., including the expected replacement demand) must be considered at the same time.

5.1.2.2  Design of the Mold under Consideration of the Product Life Cycle

Typical individual design parameters of the main influencing factors in the previ-ous section include, for example:

� quality requirements, � production costs (including setup costs), � material costs and type (including the possibility of reusing of gates), � mold costs, and � machine size.

The design itself should come off from the total amount of a component to be pro-duced.

Figure 5.2 shows the resulting choices for the material of the injection mold and the resulting mold life time, measured on the basis of the qualitatively achievable injection cycles.

As a simplification for a design, an 80 to 85% utilization of the maximum possible mold life should be used. By way of example, this means that with a sales expecta-tion of 800,000 components in the overall life cycle (start of series up to the end of the intended end of spare parts production), the mold should have a total output of about 1 million components. The difference of 200,000 components is a safety margin, which should prevent that an additional mold has to be manufactured at a

610 5 Ordering and Operation of Molds

corresponding delivery commitment for individual parts (in many industries, it is common for up to 15 years a�er the end of the series).

A more accurate design of the mold is obtained by comparing the production costs and the proportionate mold costs per part produced. Figure 5.3 shows the progress of manufacturing costs with increasing number of cavities, but also demonstrates that the cost effects of multiple-cavity molds with increasing number of cavities tend to decrease through a hyperbolic curve.

PE / PP

PS / SAN

ABS / CA

PA 6 / PA 66

PA 11

PA 66 GF

PPO / PMMA

POM

PC

Kunststoff

Number of Injection Cycles

5.000

Prototype

Aluminium Steel

10.000

Small Series

50.000 100.000

Medium-Size

300.000 500.000

Medium-Size

>1.000.000Plastic Material

FIGURE 5.2  Molding compound dependent achievable injection cycles for small- to medium-

sized plastic products, according to [2]

0,0%

20,0%

40,0%

60,0%

80,0%

100,0%

120,0%

0 5 10 15 20

Fachzahl

Fe

rtig

un

gs

ko

ste

n j

e B

au

teil

Man

ufac

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g C

osts

per

Com

pon

ent 120.0 %

100.0 %

80.0 %

60.0 %

40.0 %

20.0 %

0.0 %

Number of Cavities

0 5 10 15 20

FIGURE 5.3  Manufacturing costs depending on the number of cavities

6115.1 Molds in the Offer Phase

0%

50%

100%

150%

200%

250%

0 2 4 6 8 10 12 14 16 18 20

Mold Utilization 100%

Mold Utilization 85%

Number of Cavities

Mold Utilization 50%

Costs of the Mold and Manufacturing per Molded Part

8 10 12 14 16 20

FIGURE 5.4  Overall costs (mold costs percentage and manufacturing costs) for each molding

depending on the number of cavities

In contrast, the absolute cost of the mold itself increases almost linearly with increasing number of cavities. Figure 5.4 illustrates the resulting total cost progress (production cost + mold costs percentage), with respect to a single component.

As starting reference values, the costs of a single mold of about 20,000 EUR and the three-shi� use of a 1,500 kN injection molding cell (cost basis: North Rhine-Westphalia in 2007) are shown among others in Figure 5.4. The curves were further differentiated with respect to the overall utilization level of the mold life cycle. Clearly visible is a resulting optimum of the total costs. Further variations of the curve result for instance by changing the cycle time (in the example in Figure 5.4, a cycle time was based on 15 seconds). The longer this lasts, the more the cost optimum is shi�ed towards a higher number of cavities.

5.1.2.3  Checklist for the Mold Specification

As the figures in the previous section show, there is a direct and inseparable relationship between the mold design and the cost of a molding. Accordingly, it is important to always look at the price of a molding and the associated price of the mold in the inquiry phase and to sufficiently consider the interests of the parties concerned – o�en there are, as already described above, three parties: the buyer of the molding and/or the mold, the plastic injection molding company as its direct supplier, as well as the mold manufacturer. Ideal would be a design according to the above-described economic optimum. But there may well be legitimate reasons to deviate from this. These reasons can be from the customer’s side, which for example puts the emphasis on preliminary and extremely cost-effective mold design (small number of cavities, aluminum mold, etc.) through a high risk of change of its component construction.

612 5 Ordering and Operation of Molds

At the processor, it may be beneficial to seek a higher number of cavities than optimal to provide the necessary capacity without providing any additional injec-tion molding machines (to avoid sudden fixed costs). Furthermore, attention must be paid to qualitative and time-capacity arguments.

To create the basis for decisions (in a transparent way) regarding the mold design for those involved and also for subsequent evaluation in terms of cost elements, a systematic approach in the proposal phase of the use of checklists is provided.

An example of this is, for example, the specification sheet of the internationally used standard DIN ISO 16916 [3]. There are also checklists of professional associa-tions such as the “specifications for the manufacture and calculation of molds” of the Austrian Plastics Clusters [4] and also a large number of published, company-

TABLE 5.1  General Example for Contents of a Checklist

1. Description of the Component

� Designation/application/material number if needed � Quantity requirements (life cycle/annual quantity) � Material � Dimensions

(incl. dimensional volume/component surface/projected surface) � Mass � Complexity and special requirements

2. Supply Range � Design (incl. available data basis) � Specification about minimum shot number/mold material � Releasing process � Delivery and payment conditions � Deadlines (target costs, if necessary)

3. Requirements Mold/Machine/Periphery

� Mold type (e.g., normal, stack and multicomponent molds) � Machine type (injection molding machine) � Connection and periphery systems

4. Mold Construction

� Cavity number and building type (e.g., exchangeable inserts) � Centering and Accessories (sensors, actuators) � Temperature control systems (type oil/water/performance/cooling and

heating strategy, connections) � Slider, spindle, and its drive � Surfaces (closing and nozzle side) and inserts, if necessary � Material specifications

5. Gate Systems � Type/position � Distributor � Cold/hot runner

6. Demolding � Gate separation � Ejector system (incl. ejector, design, and tolerance specifications)

7. Accessories/Miscellaneous

� Engravings (writings/symbols, date stamp) � Mounting aids � Documentation (calculation/drawings/CAD data)

6135.1 Molds in the Offer Phase

specific documents (see also Figure 2.12). Similarly, as described later in connection with the mold calculation, the detail of the checklists may vary depending on the project stage and evaluation focus. Thus, the particular order and the associated specification of the mold user to the mold maker will be as comprehensively as possible to ensure a trouble-free application on the mold maker’s own machinery (attachment compatibility etc.). In the pure bidding phase, however, all cost-related features play a major role. A typical structure for the use in various applications of checklists is shown in Table 5.1.

5.1.3  Costing in Mold Making

5.1.3.1  Various Methods for Costing

At first sight, the economic evaluation of a product seems like a trivial task. Exam-ined in more detail, however, a variety of highly dynamic and complex networked individual factors in the creation of a product costing can be seen. The variations in the utilization of the machinery continuously change the calculation base for the products to be manufactured. In practice, it is therefore always recommended to find a balanced compromise between the level of detail of the calculation and economically reasonable use of time.

This means that development of a product and its mold consists of the following phases:

Phase 1: Request with demands and wishesPhase 2: Basic concept and offerPhase 3: Technical specifications and range of activites = orderPhase 4: Designs and calculationsPhase 5: Detailed DesignPhase 6: Production PlanningPhase 7: Parts manufacturing and assemblyPhase 8: Delivery, acceptance, and payment

to use different calculation methods. This results in three differently oriented and cost-oriented areas of responsibility.

for Phases 1 and 2: Pre-calculations + functional costsfor Phases 3 to 6: Simultaneous costingfor the Phases 7 and 8: Post calculation

For molds in the offering phase (Phases 1 and 2), a reasonably accurate but rapid preliminary costing is thus required (supplemented by specified functional costs, if necessary). In the following, the appropriate costing method for this area should therefore be presented first.

614 5 Ordering and Operation of Molds

5.1.3.2  Simplified Costing in the Bidding and Design Phase

5.1.3.2.1  Estimated Value TechnologyEstimating is the fastest method of determining costs in mold making but also the most tedious to learn. The main criticism of the method is usually the claim that estimating is inaccurate. This, however, does not apply to that extent because the estimator has experience in connection between constructive mold designs on one side and has working knowledge of the correlating economic parameters on the other side. The core of criticism of the estimation procedure lies rather in the people and the lack of transparency for other parties involved. Through some “rules” (see also Ehrlenspiel [5] among others) the problem can be reduced:

1. The more detailed the estimate made (component, assembly, and functional group level), the higher the overall accuracy and transparency (law of large numbers).

2. The accuracy can be increased, if necessary, by a distribution of the total esti-mate to individual, specialized experienced people and through an independent assessment of individual functional units.

3. Estimating should be done separately for the categories material, design, and manufacturing effort and should be documented for a later comparison with the costing. Identified differences must be considered when re-estimating and thus contribute to continuous improvement in accuracy.

Improvements can be provided through a mixture of estimation with pre-existing detailed data (e.g., current material price lists or post-calculations of identical parts or similar parts), with including current cost trends (materials, labor, etc.), which are thus taken into account at the same time. Figure 5.5 shows again the described relationships in graphical representation.

5.1.3.2.2  Reference Value MethodologyIn many cases, a single parameter determines the costs of a product so significantly that it can be used for the total evaluation or, as already described above, at least for parts of a product. It can be referred back to a weight-cost ratio in many sub-areas of the mold calculation. In some cases, weight-cost ratios can be used for the evaluation of the entire mold. A similar procedure is shown in Figure 5.6. The effective bounding volume of the component was chosen as the reference value for determining costs for aluminum prototype molds.

In addition to the effective bounding volume and, for example, the weight of the mold, the surface of a component is suitable as a reference value to costing. In the latter parameter, via the surface the complexity of the components (ribs, domes, etc.) are considered.

6155.1 Molds in the Offer Phase

Number ofEstimated Elements

Estimated Tolerance in %

+50 %

-50 %

10 1001

+20 %

+10 %

-20 %

-10 %

a)

b)

ManufacturingCosts

Known Cost Ratios

Estimated Cost Ratios+ y

- y

x

FIGURE 5.5  Estimating method and post-

calculations

(a) Law of large numbers when

estimating (a�er Bronner [6])

(b) Effect on mixing of estimating

and post-calculations

Hüllvolumen (mm³) Bauteil-Hüllvolumen (cm³) Flächenfaktor Kosten72.110,16 72,11 1,00

Mold Costs on the Basis of an Effective Cost Volume (in cm3)

0 €

5,000 €

10,000 €

15,000 €

20,000 €

25,000 €

0 1000 2000 3000 4000

FIGURE 5.6  Example for mold costing of aluminum prototype molds based on the effective

component bounding volume (all 1-cavity molds)

616 5 Ordering and Operation of Molds

Despite the accuracy, which is achievable with the reference value method, it should be borne in mind that this is both very much dependent on the structural compari-son of the molds and on general market influences on pricing. By comparing the data with post-calculations, external influences can, however, be filtered and the accuracy can still be further improved.

To consider differences in design (e.g., number of cavities, material, etc.), indi-vidual correctional factors can also be used, which leads over to the next method-ology to be described: the cost element methodology, which is also called variable costing.

5.1.3.2.3  Cost Element Methodology/Variable CostingAs already described in the introduction, the prediction accuracy of the reference value methodology is very much dependent on the structural similarity of the data-base, which was set in a cost reference. It is therefore advisable to work with data material, which is categorized with the mold type and individual specification (e.g., according to the checklist in Table 5.1). Greater detail and transparency is obtained by extending the cost-determining parameters. The average cost of distribution in the German plastics mold making industry, shown in Figure 5.7, results, based on production costs (i.e., excluding the independent costs of management, sales and profit of each product).

From these individual parameters, for example, the following basic function of a cost formula for cost analysis of a plastic injection mold can be derived:

� � � � � �� � � �0 0 0

0 0

MC [MP] [Mat] [Con]

[Heat] [CST]

a a b b c c

d d e e

� � �

� �

� � � � � � � � �

� � � � � �

Machining 70%

Construction 10%

Material 10%

Heat Treatment5%

Surface Treatment 5%

FIGURE 5.7  Cost distribution in mold and tool making a�er [2]

6175.1 Molds in the Offer Phase

with:MC = Manufacturing costs including constructionMP = Cost rate of machiningMat = Basic cost rate for purchased materialCon = Cost rate of the constructionHeat = Cost rate of the heat treatmentCST = Cost rate of the surface treatment

with a, b, c, d, and e as weighting factors; a0, b0, c0, d0, and e0 as empirically determined constants; and �, �, �, �, and � as an exponential curve of the cost of individual cost terms. Using the example of the term (MP) of the machining, the principle of value determination is explained based on examples. An example value would be based on the tariff structure of North Rhine-Westphalia in 2007 and an average 2.5-shi� operation at an assumed $ 92/hour as an average for all of the necessary machinery. The corresponding exponential value � describes the specific characteristics of the cost curve of each term. Values < 1 describe declining cost curves and values > 1 progressive cost curves. The value � = 1 results in a linear cost curve; see Figure 5.8 below.

For the selected example term (MP), there is a direct and linear relationship between the amount of volume to be machined and the resulting costs of machine utiliza-tion. Therefore, � = 1.0

The weighting factor b corresponds to the calculated or temporarily estimated time-consuming investment on the machining in the measure of processing unit hours. With b0, cost components that are constant and independent from the actual per-formance parameters can be considered. In this term, this would be for example the setup costs or other fixed costs for processing the order (if applicable packages for

Costs

Cost Factor (e.g. hours)

d = 1

d < 1

d > 1β > 1

β = 1

β < 1

FIGURE 5.8  Typical characteristics of costs

618 5 Ordering and Operation of Molds

production planning, disposition, and similar control efforts). For fixed costs of b0 = € 200 and 145 machining hours, the following would result for this example term:

� �1.0MC[amount MP] € 200 230 h € 75/h € 11.075� � � � .

In an analogous approach, the additional cost function terms are now determined. In addition to manufacturing costs, percentage increases for general management, sales and profit and risk to determine the net offer price have to be added. The fol-lowing applies:

� �NSP MC 1 AdPt DtPt PR� �� � � � �

with:

NSP = Net-sales price (Offer price without a discount)

AdPt = Administration overhead rate in %

DtPt = Distribution (sales) overhead rate in %

PR = Overhead rate for profit and risk in %.

5.1.3.2.4  Detail Calculations/Post-CalculationsThe exact way of the cost determination of injection molds is the detailed calcula-tion. While it was sufficient to determine the costs of the entire mold based on the injection molded part and some essential mold parameters (such as mold type and number of cavities), the detailed calculation requires the complete mold design and a resulting bill of materials. This requires the calculation with an appropriately high time effort in advance. The calculation itself is made up of the calculations of the individual components according to the bill of materials, the subsequent assembly and machining processes, as well as the expenses for the mold design and of the general business operations. The simplified illustrated mold in Figure 5.9 is shown and explained as an example of a detailed calculation method.

The individual components of the chosen example mold (mold 1 … mold n) show the essential calculation part of the detailed calculation in Figure 5.10.

A breakdown of costs into material costs (individual part and total amount of common parts), the time required for the component design and the resulting total design costs, as well as the time required for the manufacture and the resulting costs is recommended for each costing item.

In the column “total”, the cost for the material, design and manufacturing for one part in the bill of materials are added together. In the “Remarks” column, general notes and differentiation with respect to the chosen manufacturing process or the selected machine type can be added. It allows for the calculation, in addition to sum-

6195.1 Molds in the Offer Phase

marized, average cost rates (workshop rate etc.) also the use of detailed individual sets (machine hourly rates or cost center sets). In the present example, a differen-tiation has been made between the cost center for the area of computer numerical control (CNC) machining and electrical discharge machining (EDM).

Depending on the requirements of the company, the data can of course be further refined, where the previously mentioned agreement between required accuracy and the effort of data collection is here also applied.

It is significant that the detailed calculation in addition to the quotation enables a detailed post-costing of individual components of the mold and the construction effort as well as a clear, orderly documentation of this data for the use in new calcu-lations. This results in the reduced subsequent computational effort, while improv-ing the accuracy. The level of detail, which is chosen in the example, also enables the calculation of the influence of material price and wage changes according to the tariff, as it is necessary for the continuous updating and comparative project invoices (cross-country comparisons/variant comparisons).

Parting Plane

Pressure Plate Guide Guide

Mold Plate(nozzle side)

Mold Plate(closing side)

Injection Molded Part

Gate Bushing

Centering Ring(nozzle side)

Centering Ring(closing side)

Mold Insert(nozzle side)

Mold Insert(closing side)

Clamping Plate(nozzle side)

Clamping Plate(closing side)

Ejector Housing

Ejector Base plate

Retaining Spring

Ejector Bar

Ejector Pin

Ejector Plate

Back Pushing

fs 09. 200 7

FIGURE 5.9  Sketch of the example mold for the detailed calculation method

620 5 Ordering and Operation of Molds

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5.1.4■ Summary

The inquiry phase of molds is for the provider as well as the customer, which is occupied with the request and inquiry validation, a very delicate matter in the field of tension between available and economically reasonable time utilization on the one hand and the requirements of transparency and accuracy on the other.

One possible way to “approach” the topic is the use of so-called tiered calculation methods. Based on the analysis of individual cost factors and their impact on the overall effort, a first, in many cases already sufficient, basis for financial evaluation of technical requirements and design parameters is created. With more experience, it is recommended, despite higher time expenditure in the beginning, to create a detailed calculation, company specific for the mold vendors and at least industry-specific to the mold maker for the buyer. In addition to an objective basis for the technical specification and the economic evaluation of a mold (e.g., in the form of a negotiation support), the detailed calculation is also a necessary standard and “calibration instrument” for all types of simplified cost calculation.

references

[1] Müller, F., Aktuelle Methoden, Werkzeuge, und Tendenzen in der Angebotsbearbeitung in der Investitionsgüterindustrie, seminar work FH Merseburg (2004)

[2] Edelstahlwerke, Buderus AG (Ed.) Handbuch der Kunststoffformenstähle, 1st ed. (2002) Wetzlar

[3] DIN ISO 16916; ISO 16916 (2004) Press-, Spritzgieß- und Druckgießwerkzeuge-Spezifikationsblatt für Spritzgießwerkzeuge, Sept (2005) Beuth Verlag, Berlin (2005/2007)

[4] Kunststoffcluster Österreich, Erfa-Gruppe Werkzeug- und Formenbau, Pflichtenheft zur Fertigung und Kalkulation von Formwerkzeugen; www.kunststoff-cluster.at, Linz/Wiener Neustadt Sept (2007)

[5] Ehrlenspiel, K., Kiewert, A., Lindemann, U., Kostengünstig Entwickeln und Konstruieren, 4th ed. (2003) Springer-Verlag/VDI, Berlin

[6] Bronner, A., Angebots- und Projektkalkulation, 2nd ed. (1998) Springer-Verlag/VDI, Berlin

622 5 Ordering and Operation of Molds

 ■ 5.2  Setup and Control of Molds

Ch. Bader

5.2.1  Requirements for Effective Quality Assurance

The term “quality” is relative and depends on the particular application. While in one case it is already sufficient to ensure a complete filling of the molded part, the specifications are in other cases far beyond the objective. The location of weld lines, the strength of the components, the quality of the molded part surface, and even the dimensional accuracy and reproducibility are criteria, which affect the term “quality” significantly throughout production.

To monitor all these quality criteria, to reproduce or even influence them, a require-ment must be met: Process parameters such as the cavity pressure or the cavity temperature must be recorded in the cavity of an injection mold since it is only there that the molded part generation is directly reflected.

Capturing indirect process parameters based on machine signals, such as the hydraulic pressure, facilitates the analysis during the process optimization but does not allow any conclusion on the final part quality. To give a simple example: monitoring of the maximum hydraulic pressure or the melt pressure in the nozzle or hot runner system is ultimately no guarantee that a cavity is actually completely filled. Statistical methods, such as the calculation of short-term capability and process capability (formerly, machine capability), also do not guarantee that the quality of the molded parts is actually within a certain range, as long as it is based on machine signals or parameters measured outside the cavity.

5.2.2  Mold Sensor Systems Overview

The most important quality criteria are reflected in the course of pressure, tem-perature, and specific volume during a molding cycle. While volume changes can be difficult to measure and record, pressures have been measured in the cavity (the so-called “cavity pressure”) in industrial environments for many years. Much more recent is the story of the cavity temperature measurement technique, which has rapidly gained in importance in recent years due to various process and technology advantages. To better understand the individual advantages and disadvantages of the two measuring principles, their sensor properties must first be discussed in more detail. Figure 5.11 shows a standard cavity pressure sensor. Sensors such as this have been used for many years to monitor the injection molding process.

6235.2 Setup and Control of Molds

FIGURE 5.11  Mold cavity pressure sensor

5.2.2.1  Mold Cavity Pressure Sensors

Cavity pressure sensors are a contradiction in themselves: on the one hand, they are highly sensitive measuring instruments; on the other, they are in practice o�en treated no more “respectfully” than bolts, pins, or other mold standards, although a significant investment is made in them.

5.2.2.2  The Measuring Principle

Piezoelectric cavity pressure sensors are suitable for direct use in the cavity due to their physical properties. They are usually installed flush with the mold wall and can be adjusted to the surface by spark erosion or grinding of the sensor front. If installed correctly while maintaining the required drilling tolerances, an error-free measurement of the cavity pressure can be assumed.

If the sensor bore does not correspond to the required tolerances, the sensor front touches the sensor bore in many cases, and the sensor loses its sensitivity. In techni-cal language, this effect is called “force shunt” and can cause measurement errors of up to 30%. For this reason, cavity pressure sensors have recently developed that are initially built precisely into a sleeve and then calibrated in a second step [1]. The actual sensor is thus protected during installation, and measurement errors through the installation are excluded (PRIASAFE™ principle). The determined sensitivity is eventually stored in the sensor body itself as a code, so that no adjustments in the subsequent electronics in the industrial use have to be made [2]. The optimal measuring ranges are determined automatically as soon as the sensor is connected to the electronics (PRIASED™ principle).

624 5 Ordering and Operation of Molds

FIGURE 5.12  The PRIASAFE™ principle

Figure 5.12 describes the advantages of this principle. Standard cavity pressure sensors are shown in the top row where the first one is installed correctly (le�) and the second one touches the sensor bore due to an inclined installation (right). As a consequence, the second sensor loses sensitivity (force shunt). This is reflected in the measured values, which are too low (shown schematically by the two internal pressure curves). The so-called PRIASAFE sensors, which are protected by a sleeve, are shown in the lower row. Both the correctly installed sensor (le�), as well as the sensor with the sleeve touching the sensor bore (right), provide identical, problem-free measurement signals. This principle considerably facilitates the handling of cavity pressure sensors.

Depending on requirement, today, cavity pressure sensors are available with differ-ent dimensions, where not only the diameter of the sensor front but also the size of the sensor body plays an important role during the installation. Basically, the size of cavity pressure sensors should only be chosen as small as necessary, both to keep the installation costs to a minimum and to keep the required tolerances as simple as possible.

In exceptional cases, indirectly measuring cavity pressure sensors are used behind an ejector pin. This setup is only recommended if the space constraints do not allow

6255.2 Setup and Control of Molds

a direct measurement. Disadvantages here are the effect of friction between the ejector pin and the bore, the possible contamination during production, and the fact that the position of the sensor cannot be freely chosen.

5.2.2.3  Cavity Temperature Sensors

As already mentioned, the industrial measurement of the cavity temperature has been systematically enhanced only in recent years. The basis for this advancement are specially designed thermocouples, which also are built into the cavity, just as the cavity pressure sensors, and touch the melt or the molded part later on in the process. In contrast to conventional thermocouples, some series have been optimized that on arrival of the plastic melt they can react in a very short time, and can be used for switching and control operations [3]. The application possibilities of these sensors are also very versatile and effective, and the costs are kept within limits, compared to the cavity pressure sensors.

In principle, it should be noted that the functional principle of cavity pressure sensors and cavity temperature sensors is completely different, so the positioning of the sensors should be chosen differently.

5.2.2.4  Sensor Position

The mold cavity pressure constantly decreases from the gate up to the end of the flow path. This is why the cavity pressure sensor should generally be positioned close to the gate. The cavity pressure signal, in addition to monitoring the process, is primarily used for process optimization. This is where the “largest” signal (ampli-tude) is, so the most information is measured in the gate area.

Depending on the cross section of the molded part and the corresponding flow resistance of the melt, a pressure increase is measured during mold filling. A�er switch over to holding pressure, the melt is compressed, and there is a rapid pressure increase. Because of the pressure transfer in the polymer melt from the flow path end up to the sensor position near the gate, it takes some time for the increase in pressure to be measured, and that is why a cavity pressure-dependent switchover to holding pressure tends to be too late.

A cavity temperature signal, however, is precisely measured when the melt reaches the sensor. In this way, the position of the melt is always known and can be used for control purposes as automatic switch over to holding pressure. In contrast to the cavity pressure sensor, the cavity temperature sensor can be placed where it is needed.

A combined pressure/temperature sensor for industrial application is for exactly these reasons not very useful. Figure 5.13 shows typical cavity temperature sensors,

626 5 Ordering and Operation of Molds

which are mounted front-flush in the cavity. These sensors have been optimized in terms of reaction speed and have a front diameter of only 0.6 mm in extreme cases (Figure 5.13, right).

5.2.2.5  Quick Connectors

O�en, only small series are produced, so the standard mold is o�en le� on the injection molding machine while the mold inserts are exchanged for other small series. In these cases, the cavity pressure and cavity temperature sensors have to be automatically decoupled with the mold insert; otherwise, the entire mold must be disassembled to replace the sensors.

For this purpose, quick connectors are used, which connect and disconnect the measuring leads by simply sliding on [4]. The advantage of this principle is that the position of the connection always has to be at the same spot but not the position of the sensors, which can be freely selected in the mold insert. Figure 5.14 shows both a quick connector for the cavity pressure sensors, as well as a quick connector for cavity temperature sensors. Both quick connectors are automatically disconnected when the mold insert is removed or replaced.

Today, for both the cavity pressure sensors and cavity temperature sensors, fail-safe, industrial multi-channel connector concepts are available to simplify the wiring of the sensors and significantly reduce the costs.

FIGURE 5.13  Industrial cavity temperature sensors

6275.2 Setup and Control of Molds

FIGURE 5.14  Quick Connectors for the mold cavity pressure and mold wall temperature

5.2.3  Data Acquisition and Electronics

In principle, various options are available to detect the measured signals and to use for reject diverters, open and closed-loop controls. In the simplest case, the injection molding machine with the appropriate amplifiers can be ordered or retrofitted for cavity pressure and cavity temperature sensors. The disadvantage here is that the number of signals is usually limited, and that the connected functionality, such as the automatic switch over to holding pressure, depends on the capabilities of the individual machine control. So it may be that in some cases switch over to holding pressure (depending on the process) is possible, but there are o�en no opportunities to optimize this process, such as using programmable time delays.

In general, systems that are machine-external are far less expensive, more flex-ible, more powerful, and are able to intelligently capture and process up to 128 measurement signals (cavity pressure and cavity temperature as well as machine signals). The measured analog signals are analyzed in real time so that a variety of digital switch over signals is available, depending on the programmed monitoring and control functions. These “embedded” systems can be connected to the injection molding machine without an additional PC for monitoring and control purposes, and configured using a web browser. The digital control signals can be used from the simple sorting out of the bad parts up to the melt front-dependent opening and closing of valve gates.

628 5 Ordering and Operation of Molds

The disadvantage of these systems is that the measurement and monitoring signals currently cannot be visualized or saved without an additional PC. For this purpose, according to the state of the art, the measuring data is transferred to a PC via an Ethernet interface with high frequency, where they are managed with the help of appropriate application so�ware. The advantage of this technology is that measuring data can be easily captured, stored and further processed using a simple Ethernet cable and without additional analog-to-digital (A/D) cards.

5.2.4  Setup and Optimization

5.2.4.1  Cavity Pressure

Cavity pressure is still the ideal parameter to optimize the injection molding process. Injection rates as well as holding pressure and holding pressure time can be analyzed and optimized based on the curve progression, as the cavity pressure signal reacts sensitively to any change in the setting data. As seen in Figure 5.15, the curve can be principally divided into the following process phases:

� filling phase, � compression phase, � holding pressure phase, and � cooling phase.

Cavi

ty P

ress

ure

350

300

250

200

150

100

50

00 5 10 15 20

Time

Filling

Compression PhaseHolding Pressure

Cooling PhaseShrinkage

FIGURE 5.15  The different process phases in the injection molding process

6295.2 Setup and Control of Molds

While the cooling phase lasts until the mold opens, the time at which the cavity pressure again reaches atmospheric pressure is of great importance. At this moment, the plastic is released from the sensor front, and the shrinkage of the molded part starts. This time is not only significant for explaining the p-V-T-diagram but is also used in practical situations.

A change of machine parameters principally affects the curve of the cavity pres-sure as follows:

During the injection phase, the plastic melt flows are speed-controlled and fill the cavity. This means, depending on the set injection speed and the set profile of the machine control, the cavity is filled under different conditions. These settings deter-mine how the thickness of the boundary layer is formed in different areas of the molded part, that is whether the melt front is stagnant in certain areas or whether it continues to move continuously at a constant melt front velocity.

The goal here should always be a constant melt front velocity so that the molten plastic material is compressed under the same or similar conditions, regardless of the part geometry, and eventually shrinks. A constant melt front velocity can be theoretically determined by means of a mold filling simulation. Whether this will happen in reality, and to which setting on the machine it corresponds is, to say the least, questionable. The optimization based on the curve progression of the cavity pressure is much more precise and realistic. Regardless of the part geometry, a linear increase of the cavity pressure curve corresponds to a constant (on average) melt front velocity, regardless of changes in wall thickness and geometry along the flow path [5]. Figure 5.16 shows different cavity pressure curves in the same molded part. The injection profile has been optimized so that during the injection phase (i.e., before the switchover to holding pressure) a linear pressure gradient is obtained. In this case, it can be assumed that the melt moves into the cavity at the same speed.

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Time

500

400

300

200

100

01.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9

FIGURE 5.16  Cavity pressure profile during the injection phase

(four pressure sensors in different positions)

630 5 Ordering and Operation of Molds

Cav

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Time

FIGURE 5.17  Cavity pressure with pressure peak

(switch over to holding pressure was too late) and switch over signal

Differing theories discussed primarily in the U.S. and termed “Decoupled Molding” contradict the physical principles of process optimization since it always leads to a stagnation of the melt and thus to undesirable process conditions or to different boundary layer thicknesses (= different shrinkage conditions) due to conscious deceleration of the filling before the switchover to holding pressure.

Another factor in the process optimization is to avoid pressure peaks, which are mostly caused by the fact that injecting is done too quickly, and thus a natural shrinkage process is prevented in favor of the shortest possible cycle time. Here, it is important to distinguish that the pressure gradients in the hydraulic system of the machine or in the nozzle, or even in the hot runner, cannot be compared with the pressure gradient in the cavity, since it is only there that the melt solidifies as a function of pressure, temperature, and volume changes. A cavity pressure curve with peak is shown in Figure 5.17. This curve results from a late switch over to holding pressure. As a consequence, molded parts with internal stresses and exces-sive weight due to high compression are manufactured.

A�er the switch over to holding pressure, the further progression of the cavity pressure curve reflects indirectly the compression and the shrinkage of the molded part, which greatly depend on the process conditions, the materials used, and the part geometry. The optimum settings can either be determined empirically or via a so-called Design of Experiment (DOE), depending on the part’s weight and dimension.

The goal is to keep the optimized pressure profile constant during the production and to permanently monitor it. If the pressure profile changes under the same temperature conditions, it may be assumed that the parts properties have changed as well.

6315.2 Setup and Control of Molds

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400350

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150

10050

450

0-3 2 7 12 17 22

FIGURE 5.18  Cavity pressure profile at different holding pressure times

The optimization of the holding pressure time using several cavity pressure curves (open nozzle) is illustrated in Figure 5.18. Only when the gate is sealed can assump-tions of a minimum holding time be made. In the course of the cavity pressure, this can be seen through a difference of, for example, just 0.1 seconds more or less holding pressure time, which will lead to a slow decrease of the pressure curve and not an abrupt pressure drop.

5.2.4.2  The Importance of the Cavity Temperature Curve

Unlike the cavity pressure, the cavity wall temperature may only be partly used for process optimization. However, the cavity temperature carries process informa-tion that is difficult to obtain by monitoring cavity pressure alone, because such information follows a circuitous path and arrives late or not at all. In addition, the cavity temperature measurement technique is a very effective and affordable option compared to standard quality assurance procedures.

Cavity temperature sensors are usually placed near the flow path end or where they are needed for regulating and control functions such as the sequential control. A cavity temperature sensor near the gate or in combination with a pressure sensor, as already mentioned, makes little sense in the industrial environment, since most procedures are not based on an absolute measurement of temperature, but only evaluate and control or monitor relative information such as temperature changes.

The progress of the cavity temperature shows first (i.e. before the melt arrives at the sensor) a relatively constant value that corresponds to the temperature of the cavity. Even before the actual injection process starts, this temperature holds important information that substantially affects the shrinkage behavior of a molded part. If the value changes during production, different shrinkage behavior and parts properties must be expected. Modern monitoring and control systems automatically monitor this surface temperature for each sensor position and each cavity [6].

632 5 Ordering and Operation of Molds

FIGURE 5.19  Cavity temperature curves for different filling times

Once the melt arrives at the sensor, a temperature rise takes place that can be detected with the aid of intelligent electronics and can be used for various open and closed-loop controls. The basic idea is that a melt with the same viscosity must be at the same time at the same place. If the viscosity of the melt changes, the sensor position is reached sooner or later, so temperature increases occur sooner or later. In this manner, the variation of the viscosity can be indirectly monitored. Figure 5.19 shows the time profile of eight cavity temperature curves, where each signal was measured in a separate cavity (8-cavity mold). It can be clearly seen that the filling time of the first cavity filled ends about 1 second before the last cavity filled, which corresponds to a completely unbalanced filling process of a multi-cavity mold.

5.2.4.3  Switchover to Holding Pressure

Modern controls of an injection molding machine switch from a speed-controlled injection phase to a pressure-controlled holding pressure phase. For this purpose, various methods are available, which differ mainly in terms of their accuracy. For all methods, however, the goal is to switch over the target when the cavity is volu-metrically filled. If the holding pressure switchover is done too early, a momentary drop in pressure usually occurs, and the cavity is filled under holding pressure in an undefined way. If the holding pressure switchover is done too late, the melt is still compressed in the injection phase, which results in pressure peaks, overloads, or so-called residual pressure. Both phenomena usually prevent the production of an optimum molded part.

6335.2 Setup and Control of Molds

However, the switchover method differs quite fundamentally. In machine-dependent switchover methods, for example depending on the screw position, it is an open control loop, which cannot react to process variations. If, for example, the viscosity of the melt changes during production, the machine always switches at the same screw position (i.e. at different process conditions). The quality of the molded parts must inevitably vary.

Quite similar is the switchover to holding pressure above a pressure threshold value with using a cavity pressure sensor. Once the optimal change is determined, it cannot react to viscosity variations. Both methods have the dis advantage in that the optimal switch over point is determined with difficulty, first using of a mold-filling study, and the switch over point then changes in the course of production.

This situation is different with the automatic switchover to holding pressure using a cavity temperature sensor [7]. In this case, the volumetric filling of a cavity is automatically determined by detecting the temperature increase that occurs when the melt reaches the sensor of the flow front end. This temperature increase happens within a few milliseconds. If the melt viscosity changes during production, it is automatically compensated, since in this case, a switchover to holding pressure is only done when the cavity is actually filled. In practice, this method represents a substantial improvement because mold-filling studies are not necessary anymore. In each case, the switchover is done automatically. The injection speeds can also be varied without the need to optimize the switchover point again. Here, the sensor position is relatively variable, since a fine tuning can be performed at any time using delay times. Conclusion: The automatic switchover to holding pressure using a cavity temperature sensor solves the switchover problem in several ways.

5.2.5  The Process Monitoring

Neither statistical methods, which are able to determine dimensions and weight of a molded part in advance with the help of a design of experiment (DOE), nor control methods, which fully automatically readjust and optimize the process, are able to prevent rejects. This means that a so-called zero-defect production (0 ppm) is desirable but impossible to achieve in practice. The goal is not to produce zero defects but to deliver. For this reason, there is no way around monitoring the process.

A false belief of many injection molders is that one can achieve this goal of defect-free parts by monitoring the machine parameters. The machine parameters alone do not guarantee the quality of parts, since in the end these are just setting parameters, which do not enable to make any conclusions about, for example, the shrinkage behaviour of a molded part. In the same way, unfilled or overmolded parts can only be detected and sorted out reliably using the machine parameters.

634 5 Ordering and Operation of Molds

The delivery of defect-free parts can therefore only be ensured by using cavity pressure and cavity temperature sensors in the mold.

The measured values of pressure and temperature indirectly reflect the quality of the molded part. If these variables are reproduced within predefined monitoring windows, constant quality can be assumed.

The most common method of process monitoring is therefore the monitoring of these curves using the following monitoring functions:

� maximum values, � minimum values, � integral values (area under the curves), � cavity temperature, � time course of threshold values (viscosity changes), and � time course of the automatic switchover (viscosity changes).

Figure 5.20 shows a typical cavity pressure curve with upper and lower monitor-ing limits for the maximum pressure. If these limits are exceeded or fall too low, this is detected in real time and displayed as an alarm signal. The integral value of the cavity pressure (area under the curve) is monitored between two limits, and is made available as alarm signals when exceeding or falling below. The integral value indirectly reflects the thermal conditions, where higher temperatures also correspond to a higher integral value and vice versa.

The representation in Figure 5.21 shows the maximum and integral value moni-toring functions by means of a cavity temperature curve. Here, a higher integral value also corresponds to generally higher temperature conditions. A higher inte-gral value of the cavity temperature generally shows that the heat has dissipated poorly.

UpperLimitLowerLimit

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FIGURE 5.20  Monitoring functions in the example of cavity pressure

6355.2 Setup and Control of Molds

Lower Limit

Upper Limit

Upper Limit

Lower Limit

49

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4039

MaximumCavity Temperature

Cavity Temperature Integral

Time1 6 11 16 21

FIGURE 5.21  Monitoring functions in the example of cavity temperature

Upper and lower limits are generally determined for these monitoring functions, which are automatically monitored during the entire production. If one of these limits is exceeded or falls below, alarm signals are triggered in real time, which are for example used for sorting out the defective molded parts. Intelligent electronics and so�ware solutions offer versatile solutions for this, which can be tailored to the particular application.

There is no upper limit for the possibilities of an intelligent process monitoring. From a simple analysis of the maximum values of a pressure signal to the analysis of signal slew rates for specific applications, a whole range of monitoring functions is possible and practical.

In practice, a stored reference curve of the cavity pressure or cavity temperature is frequently used to quickly and easily display changes in the process. Modern moni-toring systems are now also able to clearly and reliably analyze complex processes such as rotary table applications or multi-component processes with the help of digital control signals in real time.

5.2.6  Factory-Wide Networking and Monitoring

While it is o�en necessary in injection molding machines or in the process itself to respond immediately (i.e. virtually in real time), there is a further need to collect, document, and manage the current process monitoring information for each machine on a server. For this purpose, the process monitoring system of each individual pro-duction machine is networked to a server, where a single system then summarizes this information for example on a hall monitor. This system shows the status and progress of individual production orders in an ongoing way so that in case of need (e.g., machine standstill) appropriate actions can be taken immediately.

636 5 Ordering and Operation of Molds

In this case, the high costs of a factory-wide network is only justified if the real process monitoring data of the manufactured molded parts can be used as a basis of cavity pressure and cavity temperature. The collection of pure machine data such as cycle time, however, does not justify the effort.

The principle of a factory-wide network of production machines, however, allows a number of other applications. It is, for example, possible to access the status of a production plant anywhere in the world via the Internet – an aspect that should not be underestimated in the course of globalization. In addition, information about each production order, such as the causes of machine standstill, can be entered and recorded by the employees in production using so-called handheld personal digital assistants (PDA) via wireless local area networks (LAN). This information is finally collected and documented on the central server through the network.

A factory-wide networked production monitoring is part of an overall production planning and provides all information on a database, which is necessary for a comprehensive analysis of the entire production. Thus, the management is able to plan production in an optimal way and at the same time to react accordingly to production failures and disturbances. Figure 5.22 shows the structure of such a system. This is based on a stand-alone web server. Every authorized user can access it using a standard web browser. The process information can be collected without expensive and time-consuming connections via machine host computer interfaces.

Machine Signals

zeuginnendruckeDAQ™ eDAQ™ eDAQ™

zeugwand-eratur

Industrial PC(Web browser)

HandheldWireless LAN(Web browser)

Server

Work StationDesktop(Web browser)

Laptop(Web browser)

Prozessüberwachung

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Prozessregelung

Cavity Pressure

Cavity Temperature

Process Closed-Loop Control

Process Open-Loop Control

Process Monitoring

FIGURE 5.22  Principle of the production monitoring system, Shop Floor Control (SFC)

6375.2 Setup and Control of Molds

5.2.7  Real-Time Controls in the Injection Molding Process

Especially cavity temperature sensors are increasingly used to control the injection molding process. Here, the arrival of the melt front on the sensor is detected in real time and used for switch over operations in real time. In contrast to the cavity pressure measurement, the position of the melt is always known this way and can be optimized with the help of programmable delay times. This allows moving weld lines in a certain direction, and the meeting of the melt (e.g., in sequential molding) can be optimized [8].

Figure 5.23 shows a door sill from the automotive sector that is produced in the sequential molding process. A�er opening the first nozzle (1), the melt reaches the first cavity temperature sensor, which automatically initializes the opening of the second nozzle (2). Following the same principle, the following nozzles (3 and 4) are automatically opened and the melt flow can be optimized with the help of pro-grammable delay times.

In classical sequential molding, the opening and closing of various nozzles is usually path- or time-controlled. In this method, the position of the melt is unknown, which is why an optimization of weld lines or the melt flow is practically impossible.

Other methods based on the cavity pressure are also not suitable for optimizing sequential molding processes, since a pressure threshold, which could be used to open a nozzle, can only be set when a pressure rise has occurred. However, the location of the melt at this moment is unknown because the position depends on the viscosity of the melt. An optimization of weld lines or a targeted manipulation of the melt flow is a function of pressure and is therefore not possible either.

The detection of the melt front using cavity temperature sensors is used in practice to automate a variety of applications.

FIGURE 5.23  Sequential injection molding

638 5 Ordering and Operation of Molds

It is possible to control injection molding compression processes in such a way that the compression process only begins when the melt has reached a certain position.

Moving cores can be selectively controlled using this technique so that they auto-matically open or close, depending on the position of the melt. This is for example used in automatic venting of cavities by only closing a moving core when the cavity is almost completely filled.

In the case of multi-cavity molds or family molds, intelligent electronics recognize when the first cavity and the last cavity have been filled. Depending on the applica-tion, switch over to holding pressure can be done fully automatically in either case.

Special techniques such as internal gas pressure processes can also be automated and controlled this way. A cavity temperature sensor at the end of the flow path recognizes the volumetric filling of the cavity and starts automatically the gas injec-tion. A cavity pressure sensor close to the gate recognizes the arrival of the gas bubble and stops the gas injection. This prevents the gas bubble from developing too far and the gas escaping through the gate.

Regardless of the individual application, melt-front-dependent controls are able to significantly improve the quality of molded parts while considerably simplifying management of the process. A method, which is ultimately reflected in cost savings.

5.2.8  The Control of the Injection Molding Process

The conditions during the production of molded parts are changing all the time. Different batches, varying ambient and cooling water temperatures, and wear of the individual machine components such as non-return valves, make it at least seem unlikely that identical parts can be produced using a single machine setup. The numbers of rejects, which are common in practice, seem to confirm this. In addition, there is also the fact that many injection molds will not run always on the same machine, and the settings are not transferable to different machines.

The goal is therefore not to keep the machine setting constant, but rather to recog-nize process variations and to compensate or readjust as necessary. This requires a closed-control loop, which exists only if the measurements done on the molded part itself show the final effects of the different settings. This is impossible without sensors in the mold cavity. For example, it is impossible to draw conclusions about the shrinkage behavior of a molded part through pressure and temperature readings in the nozzle of an injection molding machine or a hot runner system, since the melt never solidifies in this area and the entire cooling process is not recorded here.

On the other hand, cavity temperature sensors near the end of the flow path deliver a set of information that is ideal for the automatic control of the process.

6395.2 Setup and Control of Molds

If the viscosities in multi-cavity molds differ, the individual cavities are filled dif-ferently, which leads to unfilled parts on the one hand, and to overmolded parts on the other.

The problem of differently filled or unbalanced cavities in multi-cavity hot runner molds as shown in Figure 5.24 is well known in practice.

Differently filled cavities are the result of viscosity-related filling time differences in the individual cavities. Temperature sensors close to the end of the flow path rec-ognize, however, exactly when the melt reaches this position. An intelligent control system evaluates the information and controls the individual nozzle temperatures until the individual filling times and thus the viscosities are identical [9].

Identical filling times eventually cause the individual molded parts to be compressed under the same conditions, which is a prerequisite for a homogeneous distribution of weight. This principle can be used in classical multi-cavity molds as well as in multi-component molds, where each component is controlled separately, as well as in processes with inserts. In any case, the process variations and the resulting reject parts are reduced to a minimum.

Another possibility is to regulate the quality of large molded parts, for example in the automotive sector [10]. For this, the process is first optimized with the help of cavity pressure sensors and is switched over to holding pressure using a cavity temperature sensor near the end of the flow path. This optimized state is saved as a reference with the help of a cavity temperature sensor in the flow area of each hot runner nozzle. An intelligent control system is now able to reach this reference state again, regardless of the injection molding machine that the part is produced on.

FIGURE 5.24  Four-cavity hot runner mold with different filling

(right lower cavity is only partially filled)

640 5 Ordering and Operation of Molds

FIGURE 5.25  Cascade-controlled multi-cavity mold (door sills/auto)

It is also possible to regulate the melt flow of the sequential-controlled multi-cavity mold so that all cavities are filled simultaneously. Figure 5.25 shows the previously described (Figure 5.23) door sill, which is produced in a two-cavity mold in the sequential process. In addition to the automatically controlled sequential injection, an intelligent control system ensures that both cavities are always filled at the same time. If this is no longer the case due to process fluctuations, the appropriate hot runner nozzles are automatically readjusted until a simultaneous filling is again guaranteed.

While the flow behavior of the melt is regulated by the temperature rise, the same sensor measures the cavity temperature before the arrival of the melt. This informa-tion can be used to adjust different mold temperatures by automatically controlling the different cooling circuits of a mold. Identical cavity temperatures are required for large molded parts for an optimal shrinkage behavior of the parts.

Intelligent control systems are not just available for thermoplastics, but also for thermosets and elastomers. Other control systems are used for example for the automatic and melt front-dependent opening and closing of valve gates in liquid silicone injection molding.

6415.2 Setup and Control of Molds

5.2.9  Outlook

The production of injection molded parts is a global business, which could not be more diverse. While small series in the high-tech sector are o�en produced in Central Europe, globalization is driving the mass market to the Far East where the requirements for the production seem very different at first glance. It is, for example, already a reality, that a production mold moves monthly from Thailand to Malaysia, and China, and carries both sensor systems as well as intelligent electronics quasi “on board” with it. Looking at the requirements in more detail, a lot of similarities can quickly be found. The quality requirements are high and reject should be, if possibel, always avoided. These objectives and the associated cost savings can only be achieved by the appropriate technical measures. An intelligent automation ultimately always means a simplification of the process and a reduction in costs.

References

[1] PRIAMUS: Patent specification DE 103 59 975[2] PRIAMUS: Patent specification EP 1 381 829[3] Bader, Ch., The ABC of Mould-Sensor Systems, Kunststoffe international, June (2006)

pp. 114–117[4] PRIAMUS: Patent specification DE 10 2004 043 443[5] Kistler: Patent specification EP 0 897 786[6] Bothur, Ch., Hohe Qualität für hohe Stückzahlen, Plastverarbeiter 56 (2005) No. 7,

pp. 56–57[7] PRIAMUS: Patent specification DE 101 55 162[8] Bader, Ch., Maschinenunabhängig und flexibel, Plastverarbeiter 56 (2005) No. 8,

pp. 34–35[9] Bader Ch., Burkhart, Ch., König E., Controlled Conditions, Kunststoffe international,

July (2004) pp. 58–61[10] Lange, O., Alles im Gleichgewicht, Plastverarbeiter 58, Jahrgang (2007) No. 1,

pp. 54–55

642 5 Ordering and Operation of Molds

 ■ 5.3  Wear on Injection Molds

T. Eulenstein, U. Hinzpeter

5.3.1  Introduction

Wear on mold surfaces and mold elements remains a major problem with substan-tial economic importance in injection molding, since it can significantly impact the service life and maintenance intervals of molds. Undesired effects of wear, such as structural changes on mold surfaces, changes in the closing behavior of no return valves, or fretting of ejectors, lead to loss of functionality of the production measures (molds or injection molding machines), for example, by gradual wear or by spontaneous failure.Here it must be noted that in contrast to the plasticizing unit on which wear can usually be initially tolerated, even the slightest signs of wear on the mold surfaces can lead to surface and part defects. The wear processes are the same in plasticiz-ing units, hot runners, and molds. However, the causes, boundary conditions, and in particular the effects of wear differ significantly from each other.Because of the complex relations, problem-oriented solutions are needed, which take the influences of materials, surfaces and processing parameters, and their interac-tions into account. Operational, structural, manufacturing, and material-technical measures can be used to reduce wear, which lead to an optimization for them alone or for the overall effect when preventing or reducing occurring wear mechanisms.

5.3.2  Tribological Fundamentals

The term tribology (Greek tribein: friction) is defined as the science of friction and its consequences. As an introduced generic term, tribology refers to the industrial use and knowledge in the areas of wear, friction, and lubrication. To better understand wear and its causes and effects, a definition should take place first. According to DIN 50320 [1], the following applies:“Wear is the progressive loss of material from the surface of a solid body, caused by mechanical causes, e.g., contact and relative motion of a solid, liquid or gaseous counter body.”The definition of wear must be understood in the sense that wear and friction are not properties of materials, but system characteristics of the elements involved in the process in combination with the collective stresses. The wear may change with the same components if at least one of the other parameters changes.

6435.3 Wear on Injection Molds

Beanspruchungskollektiv

Struktur des Tribosystems

1

2 3

4

Oberflächenveränderungen(Verschleißerscheinungsform)

Materialverlust(V erschleiß-Meßgröße)

Verschleißkenngrößen

Collective Stresses

Structure of the Tribosystem

Surface changes(Types of wear)

Wear Parameters

Material Losses(Measured variable of wear)

FIGURE 5.26  Tribological systems according to DIN 50320 [1]

According to DIN 50320, the elements of the tribological system consist of a base body, a counter body, intermediate material, and surrounding medium (Figure 5.26).

These elements are transferred to the melt area in plastic processing machines, according to [2], in the following way:

1 Base body = component material

2 Counter body = molding compound or especially filler material of the molding compound

3 Intermediate material = generally molding compounds (melt)

4 Surrounding medium

A clear distinction between intermediate materials and the surrounding medium cannot be met in any case, since neither has to be present.

Furthermore, the following applies:

Load = normal and tangential force, temperature, chemical influences by the surrounding medium

Motion = type of motion and speed

The wear mechanisms that occur can be divided into four groups, which appear individually or in superimposed form (Figure 5.27):

� abrasion (abrasive wear), � adhesion (adhesive wear), � surface fatigue, and � tribooxidation (layer wear).

644 5 Ordering and Operation of Molds

Adhäsion Ausbildung und Trennung von Grenzflächen-

Haftverbindungen (Kaltverschweißungen bis zum „Fresse

Abrasion Materialabtrag durch ritzende und verdrängende Beansp

(Mikrozerspanung)

Oberflächenzerrüttung Ermüdung und Rißbildung durch tribologische

Wechselbeanspruchungen

Tribooxidation Entstehung von Reaktionsprodukten bei chemischer Reaktion von Grundkörper, Gegenkörper und

angrenzendem Medium

Bewegungsrichtung

Adhesion

Formation and separation of interfaces-bonding(cold fusion up to fretting)

Abrasion (material removal through scoring and displacement stress,micro machining)

Surface spalling (fatigue and crack formation through tribologicalfluctuating loads)

Tribo-oxidation

Formation of reaction products during the chemical reaction of base body, counter body, and adjacent medium

Moving direction

According to the definition, corrosion is not a part of wear, since it is not caused by mechanical action, but by chemical reactions, and can therefore also occur in completely stationary systems. Since corrosion increases wear or even makes it possible, it is considered as the fi�h wear mechanism [3]. The main types of cor-rosion are summarized in Figure 5.28. Corrosion is largely the dominant damage mechanism in molds and is therefore discussed in fuller detail below.

SchemeType of Corrosion

Uniform Surface Removal

Pitting

Selective Attack

Intercrystalline

Stre

ss C

orro

sion

Crac

king

Transcrystalline

Mixed

Intercrystalline Attack

FIGURE 5.28  Overview of different types of corrosion [2]

FIGURE 5.27  Wear mechanisms [3]

6455.3 Wear on Injection Molds

Chemical CorrosionIn this type of corrosion, the reaction of metals occurs with the surrounding medium by means of direct exchange of electrons. This includes for example the reaction of a metal with an oxidizing gas, wherein solid reaction layers are formed. Layers arrange themselves at the metal/corrosion product/gas phase interface. Corrosive gases may be, for example, water vapor, oxygen, sulfur, halogens, or carbon dioxide. Well-known examples of corrosion include the material iron when exposed to an oxidizing atmosphere. Depending on the temperature range and partial pressure of oxygen, different structured layers of oxides such as FeO, Fe3O4, and Fe2O3 are formed. An example for chemical corrosion describes the high-temperature corro-sion (T > 570 °C) of steel with the formation of wüstite.

22 Fe O 2 FeO� �

The stability of the oxide in equilibrium with the gas phase and the layer sequence of corrosion products may be calculated on the basis of thermodynamic laws and data. The chemical corrosion, which is taking place at high temperatures and the surrounding atmosphere, is interesting at high processing temperatures of modern high-performance thermoplastics.

Scaling (reactions of materials with gas) of metal parts is a type of corrosion that also occurs in injection molds. Examples are uncooled cores that “turn blue” when acquir-ing a scale layer. This scale layer changes the demolding force among other effects.

Electrolytic CorrosionThese are reactions between metals and electrolytic-conductive media, such as residual moisture or fission products. An exchange of electrons is taking place; the anodic dissolution of the less noble metal and the cathodic electron acceptance by the corrosive medium. Streams flow between the partners; in the metal through electrons and in the attacking agent through ions. According to DIN 50900 (Part 2) [4], the combination of cathodic and anodic reactions in the same electrolyte are defined as corrosion elements. Anode and cathode of a corrosion element can be formed by the following combinations:

� material side, through different metals (contact corrosion) or mold inhomogene-ities (contact, local element);

� electrolyte side, through local differences in concentration of the applied solution (concentration element); and

� under different conditions (e.g., temperature, radiation), both material as well as electrolyte side.

� Electro chemical corrosion occurs o�en in case of metal alloys and metallic mate-rials when fluids play a role – an electrolyt produces usually a surface layer (e.g. the already mentioned rust).

646 5 Ordering and Operation of Molds

Stress Corrosion CrackingThis is one of the most unpleasant forms of corrosion, since it occurs suddenly and can quickly lead to failure of the components. The so-called anodic stress corrosion cracking is caused by the interaction of mechanical tensile stresses of sufficient height, and locally acting anodic dissolution processes. Generally, this type of corrosion originates from cracks and damage in the protective passive top layer of the material.

5.3.3  Abrasion

Materials in which wear during processing increasingly occurs and considered to be a general problem are, for example:

→ POM → PBT → PA 66 → PA 6

→ LCP → PPS → PPE → ABS

Since plastics are used for more specialized problem solutions, their characteristics must be adapted to the specified requirements. They are modified by additives. The disadvantage of additives, however, is that not only the material properties change, but also the processing properties. Therefore, increasing filler content o�en leads to a deterioration of the flow behavior and to increased wear, where type, quantity, structure, and hardness of the fillers play important roles.

In general terms, it can be said that each type of material has its own wear effects. Here, however, it should be noted that in addition to the surface hardness of glass fibers (about 1,400 HV), particular color pigments such as titanium dioxide (about 2,200 HV) can cause a significantly higher wear, because the hardness of hardened mold steels are at about 700 to 900 HV.

In abrasive wear, a distinction of two main processes can be made:

� Anti-body cleavage: A hard anti-body or an anti-body from so� base material with hard inclusions wears out the base body (two-body abrasion).

� Particle cleavage (erosion): Freely moving particles, which are carried in an intermediate medium between the base and counter body cause the wear (three-body abrasion).

5.3.3.1  Forms of Damage on Molds and Hot Runners That Cause Molded Part Defects

Damages to the molds and thereby produced injection molded parts which are caused due to abrasion are shown in Table 5.2.

6475.3 Wear on Injection Molds

TABLE 5.2  Abrasive damage on molds and molded parts

Mold Segments Effects on the Mold Causes Molded Part ErrorsHot runner nozzle Material removal at the nozzle tip Encrustation,

Enlarged tear pointGate Leaching in the gate area Enlarged tear point,

Gloss differenceWeld line areas Material removal in the weld line area Gloss differencesParting plane Edge rounding Flash formationStructures Structure removal Gloss and structure changesPolishes Material removal, leaching Gloss and structure changesDemolding systems Material removal, fretting Flash formationLast filled areas Leaching Gloss and structure changes

High forces have to act on the filler particles, so that fillers can cause abrasion. This can only happen through the molding compound, which is the medium that surrounds the filler. This explains that the abrasion is also determined by:

� processing parameters that affect the occurring shear stresses and shear veloci-ties during the injection phase, in particular the melt temperature and injection speed, and

� the viscosity of the molding compound and the wall thickness in connection with the flow path length.

Likewise, the shape of the filler particles determines the wear effect. Globular par-ticles can roll off, uncritical to wear against the forces. A typical wear phenomenon is the formation of flashes in the region of the separating plane (Figure 5.29), which is caused by rounding of the sealing edge as a result of abrasion.

Further, as shown in Figure 5.30, damage due to wear can occur on components of hot runners, since plastics with very high fiber content are processed more o�en in

FIGURE 5.29  Burr formation

in the area of separation [5]

FIGURE 5.30:  Wear on a

nozzle tip [5, 6]

648 5 Ordering and Operation of Molds

injection molding. Thereby, very high demands are placed on the wear resistance of materials used in the hot runner nozzle. To increase the service life, tempered steels or materials, which are coated with wear resistant layers, are used. However, the adhesive strength of the coatings, which are usually applied by means of physical vapor deposition (PVD) or chemical vapor deposition (CVD) is o�en not sufficient for the application and leads to a failure of the components, without reaching the expected increase in service life.

Unlike steels or copper alloys, molybdenum alloys cannot be hardened by heat treatment due to the lack of lattice transformation. For example, SHN-hardening (Figure 5.31), which is specially developed for molybdenum alloys by an Austrian manufacturer, however, allows the setting of a very high surface hardness, which further lowers the wear rate. Although the SHN process takes place at temperatures above 1,000 °C, the ductile base material remains and leads to the formation of a uniform, about 10 μm thick, adherent diffusion layer (Figure 5.32) with a microhard-ness of up to 2,000 HV 0.001. Ready-to-go components can be hardened without changing the dimensional tolerances.

FIGURE 5.31  SHN-diffusion layer on TZM part

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Diffusion Layer Thickness [μm]

Har

dnes

s H

V 0

.001

0 5 10 15 20 25 30

FIGURE 5.32  SHN-diffusion layer thickness [6]

6495.3 Wear on Injection Molds

5.3.3.2  Corrective Measures

Constructive MeasuresUsing the rheological design, the molds should be dimensioned so that the shear rate in the gate is less than 15,000 1/s, in the sprue less than 5,000 1/s, and in the mold cavity less than 1,000 1/s. In the cavity itself, wall thickness variations, bottlenecks, flow obstructions, sharp corners, the last-filled regions of the mold cavity, and structured surfaces are particularly exposed to wear.

Abrasive effects are created due to dynamic forces formed at the collision of flow fronts where weld lines occur in the molded part. This effect is particularly strong at the end of mold filling, in the so-called “compression shock”. The extent of wear increases with the amount of abrasive fillers and reinforcing materials. The conse-quences are local mold surface roughnesses or washouts.

Air may be enclosed in the cavity through insufficient venting channels in the parting plane and in the area of weld lines. The “compressed air” (diesel effect) can there cause oxidative changes or oxidative-corrosive changes of the mold surface, through cleavage products, which are separated in connection with molding com-pound or created by the diesel effect. Surface defects and flash formations in molded parts and mold contamination in the mold cavity result from this.

Surface and Coating TechnologiesUntil now, practical experience has shown that hard coatings (PVD and CVD coat-ings) provide maximum protection from pure abrasive wear. For steels, which cannot be provided with a hard coating due to the treatment temperature, the wear resis-tance can be increased with thermo or electrochemical methods. This also applies for mixed stresses, such as from abrasion and corrosion.

5.3.4  Corrosion

Corrosion is, as already explained in Section 5.3.2, the material damage caused by chemical or electrochemical reaction with a surrounding medium. The corrosive media can be a gas or a liquid. Electrochemical corrosion is common in metal alloys and metal compounds in which liquid media play a role. Here, an electrolyte acts on a metal and usually produces a top layer. In the case of water on steel, this top layer is rust.

Corrosion occurs as mold contamination in the first stage in injection molds. This may be caused by the presence of chemical additives such as flame retardants or propellants, or by thermally damaged materials. Furthermore, corrosive wear can be caused by the release of fission products during processing of various plastics,

650 5 Ordering and Operation of Molds

such as HCl in polyvinyl chloride (PVC) processing. But corrosion can also be caused by insufficiently cleaned residue from the salt bath hardening. Chemical additives in plastics, such as the previously mentioned flame retardants, can also cause corrosion.

A further problem is the corrosion of cooling channels, which leads to significantly poorer heat transfers between the cooling channel wall and the cooling medium, and negatively affects the thermal characteristics of the mold for an extended period of time (Figure 5.33).

If the volatile products which occur during plastics processing do not contain water or other dissociative substances, any wear in the melt region cannot be traced back to electro-chemical corrosion. Electro-chemical corrosion can only be caused by ions. It must then be a purely chemical corrosion caused by degradation products of the polymers or by the functional groups, even in the molten state, as well as optionally by pigments and other additives.

The volatile products react with the steel surface. The resulting reaction products either remain as an independent compound to the plastic melt or they adhere to the steel surface. Results of SIMS (secondary ion mass spectrometry) studies [7], which are obtained a�er over molding of a steel surface with polypropylene (PP) and styrene-acrylonitrile (SAN) resin showed evidence of residues of deposits or compounds of organic origin on the corroded surface due to the higher intensity of the C+ fragment. It could be a heterogeneous organo-metallic reaction. Further studies confirmed that the glass fibers themselves have no effect on chemical changes. The results of [7] show that a change in the surface and volume composi-tion of the steel may occur by over molding with plastic melt.

Corrosive Wear

At components

In cooling

FIGURE 5.33  Corrosion of components and cooling channels [5]

6515.3 Wear on Injection Molds

The undisturbed surface with its alloy components comes in contact with the reac-tive escapes during over molding with plastic. A connection of steel elements and polymer melt components occurs through interactions. This will cause the destruc-tion of the lattice, for example, in the form of lattice stains and mesh widening. Thereby, a destabilization of the lattice occurs, which leads to a removal of the surface due to abrasion. Then, this process repeats.

No clear distinction between chemical reactions (i.e., corrosion) and abrasion during wear of the metal materials through polymer melt [7] can be made. Low mold con-tamination is the initial effect of corrosion. This is caused by poor mold venting and the plastic used. Here, the choice of the processing temperature of the plastic melt, the right gate geometry, and the mold steel is important.

5.3.4.1  Causes and Forms of Damage on Molds That Cause Molded Part Defects

Damages due to the corrosion of the mold and thus to manufactured molded parts are shown in Table 5.3.

TABLE 5.3  Corrosive Damage of Molds and Molded Parts

Areas Effects on the Mold Caused molded Part Errors

Cavity surface Formation of coatings on the mold surface, matte polishes, pitting

Gloss differences up to the so called “orange peel”

Slider, jaw, and ejector

Fretting of the elements Contaminations on the molded part surface “brown wreaths” around the ejector markings

Cooling channels Decreased flow of the temperature control medium, therefore worse thermal conditions

Warpage, shrinkage differences (cooling time extension)

5.3.4.2  Corrective Measures

Material SelectionFor plastics processing, both alloyed and corrosion resistant and nitriding and hardening steels can be used. In general, temperature ranges from 10 to 200 °C; in exceptional cases, temperatures � 200 °C are required in the injection molding process.

Compared to other manufacturing processes, such as die cast aluminum, the thermal and mechanical stresses are considered to be rather low.

In terms of corrosion resistance of mold steels, these are claimed to be influenced by a whole range of different factors. In addition to the environmental conditions, such

652 5 Ordering and Operation of Molds

as condensation on the mold surface, which can still be solved by simple corrective actions, considerably effort must be made to understand the associated corrosion triggering operations due to the complexity of interaction between manufacturing process and type of additives present in the molding compound.

The most important alloy element of all corrosion resistant steels is chrome. The effect of chrome is based on the formation of a chrome oxide layer, which kineti-cally hinders the oxidation of the underlying iron within a vast range of the adja-cent chemical potential. Above a percentage by weight of twelve percent, chrome is ascribed the function of a passivating effect, which is caused by a thin surface film, which primarily consists of chrome oxides. The formation of this passive layer results in a slight tendency to corrosion when steel comes in contact with water. It must however be noted that not all corrosion-resistant steels have the same passivity against all corrosive media, since especially in the plastics processing industry, not only water is present as corrosion triggering medium, but, as men-tioned before, the additives, which are included in the molding compounds (e.g., flame retardants) act aggressively, and the passive layer is thus not stable towards the corrosion-causing medium.

With increasing chrome content, the passivity towards certain corrosive media basically increases as well. The increasing chrome content negatively affects the attainable hardness. The influence of the tempering temperature on the passivity should also be observed, since this can only be increased through the free chro-mium content. Through higher tempering temperatures, more chrome carbides are excreted in the steel matrix. Therefore, the free chrome content and thus the pas-sivity can be reduced. So, lower chrome contents at lower tempering temperatures lead to better passivity than highly tempered steels with high chromium content. Local inhomogeneities (e.g., carbides and contaminations) reduce the passivity and can then lead to a corrosive attack.

Corrosion resistant steels commonly used for plastics processing are 1.2082, 1.2083, and 1.2316. The requirements decide in detail which steel should be chosen since in addition to the above described factors, other criteria must be considered in the selection of steel. The following criteria are examples for this:

� required hardness and toughness, � coatability of steel, � dimensions of the molds, and � polishability of surfaces.

Surface and Coating TechnologiesBesides the approach to use corrosion-resistant steels, surface coatings offer an additional possibility to achieve good or even better corrosion resistance (Table 5.4).

6535.3 Wear on Injection Molds

Suitable hard coatings can very o�en simultaneously provide further protection against abrasive wear. In terms of corrosion protection, different coatings are ini-tially provided, which can, however, lead to different results in their effect. Further disadvantages associated with the coating possibilities must be checked for each individual case. Thus, hard chrome layers result in an edge structure at corners; however, so-called hard coatings can significantly worsen the removal of the molded part through adhesion effects. The mold geometry to be coated can have decisive influence on the selection of the surface treatment.

Corrosion in the temperature control system can be reduced by the adding corro-sion inhibitors to the tempering medium (water).

5.3.5  Abrasive Wear of Mold Elements

Complex molded part geometries require molds that mostly only allow high quality and efficient production by using slides and jaws. These components, sometimes used in the shaping area, are also subject to different wear conditions. For molded parts that are subsequently decorated with surface and coating technologies and/or glued together to a component group through a joining process, no residues of lubricants should be present on the surface. For this reason, the movable mold ele-ments are only lubricated in the smallest dose using special lubricants or treated with surface and coating technologies, in particular based on carbon, to ensure a lubricant-free operation.

5.3.5.1  Types of Damage on Mold Elements

Fretting or adhesive wear is caused by the formation of a local interfacial adhesion connection and subsequent tearing off of solid bonds, combined with material outbreak and transmission (DIN 50323-2). The surfaces of the friction partners form a tight, adhesive bond in poor lubrication and contact conditions or dry-run.

TABLE 5.4  Corrosion Resistance of Coatings

Surface Treatment Layer Thickness [μ] Corrosion Resistance

Chemical Nickel 1 to 150 Good, very good from 30 μm

Hard Chrome 5 to 1000 Limited to good

Thin Layer Chrome 1 to 10 Limited to good

PVD (CrN) 5 Limited

PVD Composite Layers 35 Good, as far as chemical nickel was applied in advance

654 5 Ordering and Operation of Molds

Surfaces with similar material composition or high compound tendency are particu-larly endangered. The consequences are cold welding, abrasion, scratches, holes, fretting, built-up edges, and even mold breakage (Table 5.5).

5.3.5.2  Corrective Measures

Constructive MeasuresMany moving mold parts, such as ejector elements, sliders, and jaws are subjected to very large stresses. A lack of lubrication leads in many cases to mold breakage, fretting corrosion, and seizing of the mold parts. Beginning in the mold design, basic conditions for optimum operation of the moving mold elements have to be created. In addition to geometric and material-technical design of the sliding partners, an adequate mechanical dimensioning has to be ensured as well. Especially for molds that will be used for the processing of high-temperature materials, the respective thermal conditions have to be considered in the design of ejector systems and slides.

Through inadequately “aligned” guiding, different size areas of the surfaces can be subjected to the contact process due to the relative motion of base and conter body. This can lead to an increased surface pressure and the associated increase in temperature, which can lead to a damage of the sliding elements. An increase in temperature associated with a local shape change can be caused by so-called striking edges that also cause damage to the sliders.

Ejectors with a surface coating can be principally used without lubrication in the mold. This requires an optimal surface, because each ridge may increase the possibil-ity of “fretting” of the ejector. The surface quality or roughness, the micro-geometric shape deviation from the ideal macroscopic geometry of components, is an impor-tant characteristic of technical surfaces. The surface roughness is characterized by the production process and provides a three-dimensional stochastic distribution of “roughness hills” and “roughness valleys”. To ensure optimum conditions for a dry-run, the averaged roughness Rz, which consists of the individual roughness depths of five adjacent measurement paths as an arithmetic mean, should not exceed 2 μm.

For the resistance to abrasion, the so-called wear low state/high state characteristic is particularly important (see Figure II.61 in [3]). Therefore, the wear is only low when the tribologically stressed material is harder than the attacking material. In practice, good results are achieved with a surface hardness > 50 HRC.

TABLE 5.5  The Effect of Abrasive Wear

Areas Effects on the Mold Caused Molded Part ErrorsSlider, Jaw, and Ejector Fretting of the Elements Contamination on the molded part

surface, “brown wreaths” around the ejector markings

6555.3 Wear on Injection Molds

Surface and Coating TechnologiesIncreasing manufacturing requirements demand that it is increasingly necessary to work without lubricants such as greases or oils. To still ensure a manufacturing process without increased frictional resistance and the associated disadvantages, there are a number of sliding surface types that offer many application possibilities through their properties and coating processes. Figure 5.34 presents an overview of the most common sliding coating processes and their variations [8].

Mainly the improvement of the sliding properties of moving mold elements should be mentioned as areas of application of the above illustrated surface and coating technologies (Figure 5.35). PVD coatings based on carbon, tungsten, or molybdenum, chemical nickel-polytetrafluoroethylene (PTFE) layers, or so-called beam treatments

Oils

Greases

Pastes

AuxiliaryMaterials

WS2

MoS2

other

Blasting Process

MoST

WC/C

dlc

TiO/C

other

PVD

Nickel PTFE

PTFE

Gliess-Coat

Other Processes

Sliding Layers

FIGURE 5.34  Types of slide coatings [8]

FIGURE 5.35  Application of slide coatings [9]

656 5 Ordering and Operation of Molds

based on tungsten or molybdenum can here be used. The main application areas to be mentioned are sliders, jaws, and ejector.

PVD coatings provide considerable potential in terms of the sliding behavior. The production of anti-wear and anti-friction coatings through PVD processes has become part of the state of the art. This allows a layer deposition on almost all substrate materials with almost any chemical composition. Metallic layers but also such as carbon, diamond, and diamond-like layers can be produced. The usual layer thick-nesses range from 2 to 6 μm, and can be applied at temperatures of 150 to 500 °C. Low alloy steels can be coated without loss of hardness and good adhesion due to the low coating temperatures.

PVD slide coatings are produced by the physical deposition from the vapor phase. They are generally characterized by a high degree of flexibility in terms of the coating materials, which enable a wide range of layer architectures. They can be used in tightly tolerated mold segments with necessary sliding functions. The PVD technol-ogy allows the application of layers such as PTFE or MoS2 at low layer thicknesses and high bonding strength due to the low process temperature. This means that the PVD layers are more effective with lower layer thickness than thicker layers, which were applied with other methods. The optimal layer thickness is determined by the load of the sliding speed. The greater the load or the speed is, the lower the thickness of the solid lubricant should be. Stress corrosion cracking and hydrogen embrittlement of components made from high-strength steels is also not possible due to the low temperature load [10].

MoS2 is a “lubricating” top layer, which is usually used in combination with multi-layer coatings (also DLC). In all MoS2 layers, it should be noted that the layer is in a crystalline state. The friction coefficient of a crystalline layer is � = 0.04; of an amorphous layer, it is � = 0.4. The crystalline layer can be achieved by the selection of the substrate temperature (> 20 °C). MoS2-containing solid lubricant coatings are at temperatures above 300 °C with Ni-and Co-materials no longer compatible due to possible sulfidation in the grain boundaries, which leads to stress corrosion cracking. Above 350 °C, molybdenum sulfide oxidizes in air to molybdenum trioxide with a friction coefficient of 0.5!

Since MoS2 is sensitive to moisture, the possible applications of this so� solid lubricant can be reduced in certain environments. Through a special PVD process (magnetron sputtering), the tribological properties of the applied layer can, however, be optimized. Co-sputtered metal or carbide (WC/Co) give the so� MoS2 an improved friction and wear property, especially at atmospheric conditions. The metal-stabilized MoS2-Me-layers are usually applied with a thickness of 0.5 to 1.5 μm and are char-acterized by high uniformity and adhesion, even on parts with complex geometry. The coating is also done without warpage and loss of hardness of the parts [10], due to the PVD typical low deposition temperature (< 120 °C).

6575.3 Wear on Injection Molds

PTFE is a thermoplastic. It can be applied to the base materials using well-bonding and nonporous sputtering, so that the friction coefficient of the sputtered layer is comparable with the friction coefficient of the pure PTFE. The friction coefficient depends on the pressure, sliding speed, and layer thickness. The lubricating effect is based on long, unbranched, rigid chains that are deposited on each other to form crystallites. Several crystallites join together to form crystal chains. At first, during friction with a sliding partner, some chains adhere together adhesively, the PTFE is sheared in motion, and the adjacent chains are oriented in the sliding direction. PTFE can only be pressure loaded up to the low cold flow resistance [10].

Graphite has a layer lattice structure, which means the bonds in a layer are strong and the bonds between the individual layers are much lower. This results in two properties:

� a good gliding property through slight displacement of the individual layers and � a high load capacity perpendicular to the layer.

The high pressure loading capacity of graphite increases with increasing tempera-ture. Graphite is both thermally and electrically highly conductive. Graphite needs absorbed water and gas molecules for the good lubricating effect. Figure 5.36 shows the friction coefficient as a function of the cycles for different environments [11].

Coeffi

ent o

f Fric

tion

Cycles

In WaterIn Air

In Oils0.1

0.05

00 2000 4000 6000 8000 10000

FIGURE 5.36  Friction coefficient of graphite in different environments [11]

The process technology for the deposition of WC/C layer systems is based on magnetron sputtering. A coordinated heat treatment is absolutely necessary. Repeated tempering to at least 260 °C is recommended. The surface should be precision machined to achieve a high load carrying capacity of the layer. In terms of a long service life of the coated mold parts, a surface roughness of Rz > 2 μm is desirable.

In Figure 5.37, it is notable that a WC/C coating is built from different coating systems to create optimal coating properties in the form of coat adhesion to the substrate and gliding properties on the surface.

658 5 Ordering and Operation of Molds

WC/C-Layer

WC-Layer

Cr-Interlayer

Base

FIGURE 5.37  Layer structure of a WC/C coating

5.3.6  Outlook and Development Trends

In recent years, an ever-increasing growth rate can be noted when using the surface techniques for molds. Hard, wear-resistant and/or corrosion resistant, smooth-glid-ing, and low-contamination surfaces can be produced through surface and coating technologies, which increase the service life of the treated components, and thus improve the economy enormously. Therefore, service life of injection molds can be increased 3- to 10-times through eliminating damage caused by abrasive wear by applying hard coatings using PVD processes. In another case, the cycle time could be reduced by almost 25% by reduction of the demolding force.

Process stability can be increased and a significant reduction in downtime of machines and molds can be achieved by the targeted influence of the interactions between molding compounds and the mold, as well as between individual mold components. The following improvements can be achieved through carefully planned use of surface and coating technologies:

� increase of the wear resistance, � increase of the corrosion resistance, � reduction of demolding forces, � reduction of mold contamination, � improvement of gliding properties, � protection of polished and structured surfaces, � generation of targeted gloss levels, � improving the quality of molded parts, and � cycle time optimization.

6595.3 Wear on Injection Molds

These improvement possibilities do not only apply to injection molds. In addition to mold cavities for thermoplastics, thermoset and elastomer processing, also extru-sion dies, sliders and jaws, ejector systems, melt control systems, no return valves, and cooling channels are treated with success. Also with regard to the treatable mold materials the limits have been further extended in recent years. In addition to typical mold steels, nonferrous metals such as aluminum or various copper alloys can also be optimized by surface and coating technologies.

A prerequisite for a successful application of these technologies is the selection of a suitable surface treatment process for existing or anticipated problems. Here, the side effects of the individual laminates also have to be considered, in addition to the specifications consisting of mold steel, surface structure, used molding compound, and geometry.

Today, optimization possibilities by surface and layer technologies exist for many applications, particularly in regards to the reduction of abrasive wear on the mold surfaces. However, there is also a variety of new applications for which no suitable coating systems are available on the market to permanently improve the demold-ing properties or to reduce the formation of undesired coatings due to decomposed plastic resin through processing conditions.

The potential of surface and coating technologies is certainly far from exhausted. New developments in this field in the following years are also open for further optimization potential. Especially the area of low adhesive coating systems offers significant potential for reducing demolding forces or reducing deposit formation, which is a very frequently encountered error image in the processing of thermo-plastics. By low molecular weight excretion from the molding compound, deposits can form a�er a few thousand cycles on the mold surfaces that come in contact with the plastic, preferably close to the gate and in the venting area (Figure 5.38) and which prevent an accurate replication of the plastic by the cavity surface. The consequences include topography interference on the molded part surface (Figure 5.39) or gloss differences.

Materials in which deposit formation commonly occurs include polyoxymethylene (POM), polypropylene (PP), polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polysulfone (PSU), polybutylene terephthalate (PBT) and polyethylene (PE). Further endangered materials are with additives such as flame retardants, lubricants, colorants, that are excreted at a high material stress (i.e., high shear stress, high shear rate, and melt temperature that is too high). Materials that are modified to be flame retardant can react chemically at a high material stress. The released products of decomposition can cause surface deposits, and depending on configuration, can also cause corrosion in the mold.

660 5 Ordering and Operation of Molds

FIGURE 5.38  Deposit formation close to the gate [5]

FIGURE 5.39  Rough surfaces through coating [5]

The coating formation is caused by the following:

� high material stress, such as high melt temperatures, shear stress, and shear rate, � high shear in the plasticizing unit (e.g., by a screw speed that is too high), � residence times that are too long, � high shearing in the mold (e.g., due to injection speeds that are too high), � lack of mold venting, � processing of materials with residual moisture that is too high, � incorrect or excessive use of lubricants, � incompatibility between the base material and coloring agent/additive, and � drying times that are too long.

6615.3 Wear on Injection Molds

Furthermore, new coating systems with special thermal properties will comple-ment the market as development trends. There is still need for improvement with the surface and coating systems currently available on the market. Not all coating methods today offer reproducible results in terms of coating quality and de- and re-coatability. Also scaling up the application technology for mold weights of more than 2,500 kg will be a development theme for the future, because not only the size of the molds plays a role, but the coating technique must also be adapted to the mold steels used.

References

[1] DIN 50320, Verschleiß, Begriffe, Systemanalysen von Verschleißvorgängen, Gliederung des Verschleißgebietes, Beuth, Berlin (1979) (standard now no longer extended)

[2] Heinke, G., Tribologische Grundlagen und Schadensgrundformen in Aufbereitungs-anlagen, Verschleißminimierung in der Kunststoffverarbeitung (1991) VDI Verlag

[3] Mennig, G., Lake, M. (Eds.): Verschleißminimierung in der Kunststoffverarbeitung, Phänomene und Schutzmaßnahmen (2007) Hanser, Munich

[4] DIN 50900, Korrosion der Metalle, Part 2 (1984) Beuth, Berlin[5] Störungsratgeber für Oberflächenfehler an thermoplastischen Formteilen Kunststoff-

Institut Lüdenscheid für die mittelständische Wirtscha� NRW GmbH[6] Plansee AG, Reutte/Tirol[7] Braun, D., Brito, H., Beiträge zur Untersuchung der Korrosion an Modellwerkzeug-

stählen für kunststoffverarbeitende Maschinen, Werksto�echnik (1985)[8] N. N., Yearbook Surface Technologies (2000) Giesel Verlag, Isernhagen[9] N. N., Company publication Oerlikon Balzers, Bingen (2001)[10] N. N., University Erlangen, MFK (2002)[11] Simon, H., Thoma, M., Angewandte Oberflächentechnik für metallische Werkzeuge (1989)

Carl Hanser Verlag, Munich

662 5 Ordering and Operation of Molds

 ■ 5.4  Maintenance, Storage, Service

U. Thiesen

5.4.1  General Information

Molds for plastics processing are the most important resource for implementing the design of products with many different requirements. O�en produced uniquely, a mold should sometimes produce millions of identical products in the shortest possible time. It is therefore more o�en a part of complex systems and this linkage does not allow for unplanned downtime, without causing significant costs.

If downtimes occur, then they should take place as scheduled, and maintenance must ensure that the functional status remains. If this condition does not exist for maintenance, it must be quickly restored. For this purpose various measures can be used, such as:

� MaintenanceThe existing wearout reserve of the molds should be preserved by maintenance, or the disassembly should be delayed as long as possible.

� InspectionThis verifies whether the mold is in proper working condition.

� ServiceThis is part of the maintenance and should bring a defective part back in its original state.

� OptimizationAs part of the continuous improvement, the mold is continuously optimized a�er sampling, because a�er only several days of production, the mold “sets”, and the mechanical parts are run-in and show where improvements can and must be done.

With this, mold failure can be prevented. The objective to increase the lifetime, and thus to increase the benefits, will be pursued. The improvement of safety and increased mold availability is associated with this. The number of failures is mini-mized. Thus, the operating processes are optimized and a cost control is achieved.

Molds are frequently part of production lines. Their failure results in significant costs. This can lead to a disrupted delivery to the customer. Therefore, it must be the goal to avoid maintenance, and if this is not possible, to ensure short maintenance times. This begins with a smooth flow of spare parts and ends with the decision of an external or internal maintenance of the molds.

6635.4 Maintenance, Storage, Service

5.4.2  Maintenance, Wear Supply, Hardness

All areas that are in motion or have materials flowing through them are at risk of wear. The mold usually has several areas of wear, which need to be coordinated. These areas are subject to wear because they grind against each other. In the molds, solid, liquid, and even gaseous bodies move relative to each other.

The surface hardness and the hardness depth determine the wear. If large or vast quantities are to be produced, all forming parts should be hardened. Although an increased effort is put into the production of molds, the maintenance costs can be reduced. This applies equally to the surface quality of the surfaces that are moving against each other.

5.4.3  Inspection

5.4.3.1  Time

The inspection should be performed at specified intervals and should be done according to an inspection schedule that is defined for each mold. This may vary depending on load:

� hourly, � in shi�s, � daily, � weekly, � monthly, � based on the order, � based on number of parts, or � can be a combination of time and quantity.

5.4.3.2  Inspection Plan

The inspection plan describes the work steps and documents of the implementation. It begins with a visual inspection of the plastic parts and continues with the evalu-ation of the contact surfaces of moving elements in the mold. Their lubrication and control of the connections of the power supply follows. (See Table 5.6 for example.)

However, the adaptation of the date labeling, which is important for the traceability, is part of the inspection plan. The inspection plan is built on experience and should be adapted to the mold or the requirements of the part to be manufactured.

664 5 Ordering and Operation of Molds

TABLE 5.6  Mold Inspection Plan

Mold Inspection PlanMold Naming 12345

Product Name Side Panels

Part Number 123 987 456.0

Time Daily During Manufacturing

End of the Order

Per number of Parts

Date/Signature

OccupancyVisual Control

Part X

Mold Separation 10,000

Ejector X

Guidance X

Slider X

Core-Pulls X

LeakageTemperature Control X

Hot-Runner 10,000

Cylinder X

LubricationGuide Columns Per Shi�

Slider X

Core-Pulls

Collapsing Cores

CleaningMold Separation X

Venting Inserts 5,000

Temperature Control X

Identification

Wath with Date Stand Type of Order

Quality Status X

OthersCentering Rings X

Ejector Pins X

Energy Supply X

Temperature Control X

Supply X

Gripper X

Insulation X

6655.4 Maintenance, Storage, Service

5.4.4  Repair

The repair may be necessary a�er a routine inspection, as well as unplanned during the production. The facility manager, together with the maintenance team, has to decide whether a created flash, for example, can be reworked in production and safing an interruption of the manufacturing. A mold crater or a broken ejector leads directly to a production disruption and to repair in maintenance or in mold making. The case of the repair should be avoided by prevention. Despite all preven-tion, damages to the mold occur repeatedly. Here, errors due to wear and fatigue of the materials as well as errors caused by the periphery can be distinguished.

The ball catch with flash in Figure 5.40 is formed by an expanding core. The flash elimination in expanding cores is always very complex because it has to be coor-dinated in three dimensions. If the undercuts can be shaped with simple sliders, this solution should be preferably chosen.

FIGURE 5.40  Parting line at a ball catch

5.4.4.1  Wear

Mold edges are removed due to the high stress of the mold steels through fillers, such as glass, minerals, and flame-retardant additives, through high mechanical stress, and due to high temperatures at the end of the flow path (diesel effect). The product is an unfilled part with burn marks. Care must therefore be taken to ensure that venting is possible. This is achieved by introducing 0.01 mm “air” into the mold separation. This is sufficient to allow the air, which is driven by the melt front, to escape from the mold. At the same time, the gap is so small that no flash is formed. The repair turns out to be more or less complex because monomer residues o�en plug the vent, and repair leads to a balance between burn mark and flash.

If standards are used in areas subjected to wear, the maintenance costs are signifi-cantly reduced. In Figure 5.41, sprue bushings and tunnel gate inserts are shown as an example of parts with high stress and associated high wear. They are available from stock, save stocking up in-house, and thereby help reduce costs.

666 5 Ordering and Operation of Molds

FIGURE 5.41  Standards for tunnel gate and sprue bushing [1]

5.4.4.2  Leakage

Fluctuating part quality, incompletely filled parts, and gloss variations indicate defective hot runners. The causes include loose connections or cracks on the hot runner manifold. The mold has to be completely disassembled and any leaked plastic (Figure 5.42) has to be mechanically removed.

FIGURE 5.42  Overmolding of a hot runner

6675.4 Maintenance, Storage, Service

O�en, the entire electrical system has to be replaced. To prevent this, great care must be taken when selecting the hot runner system and the proper installation. Other causes can also be the wear in nozzles. Here, the annular gap must be adjusted or the locking mechanism should be renewed. Potential causes of failure are also defective heater bands and cartridges, as well as failed sensors. Careful selection and assembly helps here to avoid this cause for disruption.

The temperature control medium leaks out of the mold during production. This process should be avoided. This can be done by clamping off the temperature control before production begins. An uninterrupted production will result if no fluid is leaking. But if a leak is noticed, it is necessary to find out where the leak is.

In most cases, there is a defective O-ring. This O-ring has to be exchanged. It is important to check whether the conditions for proper functioning have been complied with. Dimensions and fit, as well as stress and integrity of the seals and sealing surfaces, are checked. If it turns out that a crack in the steel due to high stress (e.g., too closely positioned to the contour) is the cause, the bore hole has to be bushed, the crack welded, or the complete insert has to be renewed (in the worst-case scenario). In special processes such as gas-and water-assisted injection molding, the leaks for these media are mostly in the fit of the nozzles. Care must be taken to ensure that material residues do not enter the locking mechanism, but this can be solved constructively.

5.4.4.3  Breakage

In case of breakage, the cause must be found to exclude recurrence. Sliders are sometimes extremely stressed and have sharp transitions, which are then the basis for breakage. Here it is necessary to redesign the weak points. The sliders are sometimes not at the required position because springs can become fatigued or interrupted lubrication may increase the frictional resistance so that the position cannot be reached.

5.4.4.4  Repair Measures

An action plan exists for each mold (Table 5.7). The series of measures should be derived from the failure modes and effects analysis (FMEA) process. This will ensure that manufacturing considerations are taken into account and experience of previous parts or positive findings from existing subsequent processes are used for the mold production in the run-up during the product design.

668 5 Ordering and Operation of Molds

TABLE 5.7  Action Plan

Repair measuresDefect Position Cause Corrective actionFlash Gate Abrasion Renew bit

Ejector Abrasion Change ejector

Maybe enlarge ejector

Contour Abrasion Laser welding

Die spotting not o.k. Renew die spotting

Gloss level

Grain Too glossy Surface blasting

Surface etching

New surface grain

Leakage Tempering O-ring faulty Renew O-ring

O-ring pyrolysed Use chemical and heat stabilised O -rings

O-ring falsely assembled

Hot runner Sealing face without function Rework sealing face

Wear Exchange worn parts

Annular clearance irregular Adjust mounting position in consideration of the thermal expansion

Gate freeze-off Remove possible thermal bridge

Gas injector Material residue in the locking device

Clean locking device

Breakage Ejector Seizure Renew, ream guidance

Breakage Ensure alignment with ejector plate, allow for guide pillars in the ejector plate

Angular pins

Breakage Ensure end position of the slide bar

Make sure of lubrication

Contour Spalling Laser welding

Harden parting edges

Slide bar Hydraulic cylinder Secure limit switch

Offset Rework final position

Rough running Rework material combination, improve lubrication

Breakage Avoid sharp material transition, allow for radii

Offset Surfaces mold division

Faulty die spotting Renew die spotting, polish contour together with the slide bar

Scouring Laser welding, harden parting edges

Centre selvedge Laser welding

Hot runner Ledging on part Adjust nozzle length in consideration of thermal expansion

6695.4 Maintenance, Storage, Service

5.4.5  Optimization

The molds are mostly produced from external mold manufacturers. This is also where the first samplings will the take place. Here it has to be found out if the mold function is given, the flow pattern corresponds to the calculations, and if the shrinkage has occurred as predicted. Then, the mold goes into production onto the planned series machine and is then retracted under series production conditions (i.e., with fully automatic feed of inserts, removal by linear handling systems, or with programmable robots that introduce foils, varnishes, textiles, or other com-ponents). This usually shows, compared to the first samplings, that the variation of the measurements does not make an efficient process. Then, an optimization needs to be initiated (i.e., the part must be brought to nominal size in the connec-tion and functional areas). Compromises restrict the production parameters, and the customer complaints during series production have to be accepted.

It may also be necessary to make adjustments outside of the plastic component, for example for textiles, films, and inserts etc. Even fluctuating quality of purchased inserts may disturb a fully automated process.

The uneven heat balance in the mold is o�en a reason for distorted parts. Using thermal imaging cameras, which are now affordable, hot spots in the mold, can be seen and valuable support for the optimization, not only for the warpage, is provided. The intensification of the temperature control, by pulsed cooling or sintered mold inserts, for example, o�en leads to significant cycle time reductions.

5.4.6  Storage

Different molds are o�en operated on one manufacturing unit, since the demand for parts does not use the full capacity of the machine. Thus, for the production, there is the decision to disarm the mold a�er the production order and then to transport it into central mold storage (Figure 5.43) or to keep it locally at the production line. In the latter case, the inspection must then be carried out in the production.

The storage in the production is the economical solution that allows small produc-tion batches at low external setup efforts. The molds are stored dry (Figure 5.44) and fire-proofed. Also, it should be organizationally ensured that the molds are maintained at any time when needed for production. This is necessary in order to quickly respond to customer requests.

So that manufacturing knows that a mold is available for the production, it should be marked as maintained and available for use. This can be done by manual label-ing or through an electronic approval of the mold in an ERP (Enterprise Resource Planning) system (Figure 5.45) of the operation. A manual labeling should always

670 5 Ordering and Operation of Molds

FIGURE 5.43  Heavy-duty shelving

with molds

FIGURE 5.44:  Corroded mold due to faulty

storage

FIGURE 5.45  Input mask in an ERP system with information on the next maintenance

6715.4 Maintenance, Storage, Service

be present because then the production operation is, in case of failure of the IT systems, always sure that the mold is available, and that no correction or changes are planned. The labeling can be done by an information carrier or, for example, a color-coded sticker. Reserved spaces can also be defined that can only accept molds that have been released for production. However, conversely, restricted areas for the not-yet repaired molds can be defined.

A mold management can be integrated into an ERP system. Figure 5.45 shows the fixed service intervals. Here, 12,000 parts have to be produced until the next service interval. However, also the history of the mold, the accrued repair costs, etc., are stored and this information can then be retrieved at any time.

5.4.6.1  Preservation

The molds have to be preserved at the end of production. For parts, which get a surface finish a�er the manufacturing process, it has to be ensured that Silicone- and Teflon-free preservatives are used.

Figure 5.46 shows surface defects on a coated part, caused by a preservative. This then leads to adhesion problems and/or wetting errors.

Before storing the molds, they should cool down, so that no condensation occurs on the mold surface and corrosion is thus inhibited. The temperature control channels are to be kept closed in order to prevent exposure to oxygen. Another possibility is that the temperature control bore holes are completely removed from water before the deposition. Corrosion is not only a problem for the temperature control channels. Oil, as a temperature control medium, forms decomposition products that usually accumulate in the tank of the temperature control system. At some point, it gets into the mold and can clogg riser holes in the mold cores. It is therefore not only essen-tial to replace the temperature control medium, but also to clean the heater tank.

FIGURE 5.46  Error by silicone or teflon-containing preservative

672 5 Ordering and Operation of Molds

5.4.6.2  Storage Location

A central mold store allows heavy duty shelving in a small space and takes and up the vast majority of molds. Here, a computer-assisted storage location management is possible.

However, there are molds that occupy such huge proportions that a discussion about heavy-duty shelving can be safely dispensed of. These include molds for garbage bins, bumpers, instrument panels, and others, which can easily weigh > 15 t. Production-related storage is also needed. In a central mold storage (Figure 5.47), the accommodation of molds can be done systematically. Molds for large parts should always be stored close to the ground. Molds that are smaller dimensioned and weigh less are placed in heavy-duty shelving. The storage locations have to be marked so that they can be assigned to the molds. This improves traceability. This is even a prerequisite for an electronic mold storage management.

Cost optimization demands a rethinking of the central mold storage. The result of an internal setup workshop may not always result in a reduction in setup times when the molds are stored on the machine. So it may be that the optimal production batch is a daily production of only a few hours. Therefore, it makes sense to store the molds at the production machines. In future mold management, the production site will be increasingly used as storage space.

FIGURE 5.47  Large mold storage

6735.4 Maintenance, Storage, Service

5.4.6.3  Mold Labeling

The labeling should be clearly visible. The following information may be helpful in mold management (Figure 5.48):

� mold number, � owner, � part description/quality level, � dimensions, � weight, � mold manufacturer, and � year of manufacture.

Other markings, such as information about the course of the cooling circuits, the power supply, or the movements of the slider and core pullers (hydraulic diagrams) facilitate mold handling (Figures 5.49 and 5.50).

Part Name / Number

Mold Number Commission Number

Owner

Size Weight Manufacturer

FIGURE 5.48  Mold labeling

Function SequenceMold Clamping

Closed moldRemove straps for secured transportOpen moldEjector unit

Injection CycleMold is open, ejector unit andinclined slider withdrawal positionEjection unit drives backMold is closedClosing pressure is appliedClosing operationClosing pressure is decreasedMold is openedEjector unit drives upRemove injection molded part

Mold Un-clampingMold is open; ejector unit is in withdrawal positionEjection unit drives backMold is closedAttach straps for secured transportMold is unclamped

Commission number: 188200

Hot-Runner Diagram

Mold Top

Socket Box

Ope

ratin

g Si

de

Sprue Bushing

Commission Number: 188200

FIGURE 5.49  Mold function schedule and hot runner scheme

674 5 Ordering and Operation of Molds

Hydraulic Plan BS

Mold Top

Op

era

ting

Sid

e

Ejector Unit in Withdrawal Ejection Unit in Injection

Commission Number: 186600

FIGURE 5.50  Mold hydraulic plan

5.4.6.4  Storage size

The molds not only include the connections and the periphery, but also the elec-trodes and milling programs. These must be present in order to be able to perform a quick repair in case of damage. It should be specified to the mold supplier how the electrodes are set up, so that a precise position in the mold can be found. Each plastic molded part may include grippers, auxiliary devices such as foil feeding, stentering frames, sprue grippers, vacuum suckers, hot runner nozzles, and gas and water injectors.

5.4.7  Maintenance and Servicing Costs

The costs incurred to keep the molds operational (e.g., in the automotive industry for over six years product cycle, and then 15 years for the supply of spare parts) should not be underestimated. There is optimization work in the beginning of pro-duction. The maintenance costs increase with progressive duration of production. Depending on the output quantity, the annual costs per mold are between 3 and 6% of the mold investment (Figure 5.51). The mold maintenance costs must be obtained through the price of parts. It is therefore important to properly estimate the effort and consider it in the price of parts. Every company has to consider exactly to what extent maintenance and repair is assessed to ensure that the supply of customers is maintained. Molds must be subjected to a risk assessment to definitely prevent damage from the production operation. Experience teaches to put some more effort into the molds, to look at the mold maker and its offerings, its machinery, its experi-ence with similar parts, the same as the communication channels and the distance with the connected time expenditure on improvements and changes.

6755.4 Maintenance, Storage, Service

�

�����

�����

�����

�����

������

������

Annu

al Co

sts

Investment $

Maintenance Costs $

�� ����� ��������� ��� ����� ���������

FIGURE 5.51  Mold maintenance costs, which should be considered in the parts price

Reference

[1] i-Mold Web site, http://www.i-mold.de

Subject Index

Aabrasion 643abrasion resistance 277abrasive file 526abrasive stone 525ABS 361, 595, 659ABS-PP 287accessories 335accumulator 114ACES Injection Molding 589acid 567acid copper bath 167acidic range 448action plan 667adhesion 643adjustable pin 602adjustable wall 196adjustment sleeve 130agitated mold surface 147AIM cavity 599air bubble 86air ejector 73, 92air gap 33, 37, 50air gap insulating 369air oxidation 505air-permeable plate 143air pocket 327air trap 22, 23, 37alarm signal 634AlCuMg alloy 444AlCuTi 444alibrating blow pin 117alignment pin 62alignment wedge 72alkaline detergent solution 449alloy composition 442AlMg alloy 444

AlMgSi alloy 444AlSi alloy 444alternating electromagnetic field 399aluminum 70, 77, 121, 135, 143, 148, 161,

193, 201, 213, 233, 255, 271, 292, 297, 338, 424, 442, 571, 591

aluminum alloy 83, 296aluminum bronzes 467aluminum cast 123aluminum castings 131aluminum-ceramic precision casting 143aluminum-filled epoxy resins 514aluminum molds 294aluminum mold shells 163aluminum multi bronzes 461aluminum oxide stones 526aluminum particles 287aluminum prototype mold 473aluminum-wrought alloy 193AlZnMgCu alloys 444amorphous layer 656amorphous nickel phosphorus layer 559amorphous thermoplastics 384anatomical demonstration objects 166angle hand piece 524angle pins guide 337annealed state 534annealing 425annealing color 426anodic dissolution 645anodic stress corrosion cracking 646ante-chamber 29anti-body cleavage 646anti-static coatings 182arc welding 456argon 467artificial ageing 444

678 Subject Index

aspect ratio 267assembly injection molding 266, 303assembly seam 225assisting plug 138, 147, 149atomizing chamber 424austenite 425austenitising 547autoclave steaming 186autoclave technology 201automated tape laying 208automatic roll machine 149automatic switch over 625, 627automatic venting 638automobile industry 75auxiliary blow pin 133auxiliary parting plane 104

Bback blowing 195back feed behavior 457back-foaming mold 88, 516balancing 21, 22ball catch 13ball guides 263ball milling 465band heaters 376banding 578band-type formations 575barrier characteristics 112base body 643bath model 177beryllium copper 392beryllium free alloy 461biaxial orientation 112bimetal effect 518binder-forming molds 223biolactat 260Biot-number 385bismuth alloys 514blank 69, 76, 242blasting 507blind insert 263block mold 197blowhole formation 512blowing bell 152blow mold 458blow mold cooling 126blow molding 112, 290blow molding RTM process 235blow needle 117

blow pin 129blow up methods 117bobbin stand 238bonding strength 416bore holes 485, 491boriding 555boundary layer thicknesses 630brackets 167brass 123, 271, 498, 505, 571brass body 527breakage 667breakdown of costs 618bridge tooling 587brilliant surface 150, 532broken natural diamond 529bronze 72, 130, 271, 461, 597bronze alloys 460bronze bushings 192bronze infiltration 597brown wreaths 654brush plating 557brush units 376bubble bore holes 485buckling stability 446building parameters 586built-up edge 527, 531bulking force 182buoyancy force 358burn mark 665bushings 72butt welding 457bypass channel 61

CCAD 59, 295, 314, 472CAD/CAM 463CAD drawing 336Cadmould 8CAE program 9calibration 604calibration mandrel 117calibration station 113calottes area 355CAM 59carbide 271, 524, 540carbide formers 536carbide-tipped tool 70carbide tool 465carbon 655carbon content 425, 426

679 Subject Index

carbon fiber prototype mold 162carbon-fiber-reinforced polymer 204carbonitriding 543carburising 543, 551carrier film 574carrier foil 173carrier material 567carrying capacity 657cartridge heater 31, 32, 75, 376cascade-controlled multi-cavity mold 640cascade filling 36case hardened steel 58case hardening 216, 550case-hardening steel 338, 439, 543cashew gate 28cast alloy 443cast aluminum 98, 202cast elastomer 111casting 505casting molds 164casting process 457cast metal 120cast resin 120cast skin 516cast steel alloy 202cast zinc mold 127cathodic electron acceptance 645cavity evacuation 38cavity number 1cavity pressure 3, 622cavity stacks 269cavity temperature 379, 622cavity venting 37cementite 530centering 10, 93, 169, 264, 351centering ring 10centering sleeves 341central ejector 11, 67central mold storage 669centrifugal casting technique 111ceramic 271, 591, 593ceramic fiber file 532ceramic fiber sanding file 525ceramic injection molding 260CFK mold pocket 203changing mold inserts 295characteristics of costs 617checklists 612chemical corrosion 645chemical engraving 454, 567

chemical nickel 469, 653chemical nickel plating 167, 299, 437, 449chemical nickel-polytetrafluoroethylene 655chemical silver plating 503chimney effects 369chip braking 464chipping 531chip space 527chrome 652chrome carbides 652chrome plate 444chrome plating 299, 449chromium carbide 530chromium content 339, 438chromium layer thickness 59chromium nitride 256chromium plating 434circumferential application temperature

control 392clamp 507clamp force 125clamping 168clamping device 91clamping edge 138clamping force 2clamping frame 152clamping plate 361clamping pressure 242clamping unit 3, 182clamp stop 126Class A surface 69, 216cleaning concepts 278cleaning process 530clean room 268cleavage product 649CLI 584closed cell foam 179closed-control loop 638closed cooling circuit 154closed-loop control 627closing 168closing time 85cloth 531cluster nozzle 36CNC 135, 463CNC-milled production 123CNC-milled shell 163CNC milling 287coated mold surfaces 55coating 463, 469

680 Subject Index

coating technologies 649cobalt 504cocking 101CoCr 586CoCr-steels 591coextruded parts 124coextrusion 113co-injection technique 351cold fusion 644cold runner manifold 252cold runner system 49, 372cold welding 192, 654cold work mold steel 540collapsible core 343collective stress 643collision analysis 481collision function 477color change 31, 32color-coded sticker 671color pigment 646combination molds 160combinations of aluminum 449combined pressure/temperature sensor 625common loading chamber 64communication drawing 476complete system 33component height 499components for injection molds 332composite molds 458composite pins 347compressed air 415, 649compressed air hoses 91compression injection molding 267compression molding 52, 242compression shock 649computer simulation 108computing time 326concentration element 645condensate 191condensation 449, 652, 671conductive tip 30conformal cooling 592conformal cooling channels 462conical guide 243conical surface guide 342connected functionality 627connecting component 351connecting diagram 315consolidating roll 208contact corrosion 449

contact feed 167contact temperature 19, 402continuous fiber-reinforced thermoplastic

material 209continuously working press 210contour-adapted cooling 585contour-adapted cooling channels 584contour-close hardening process 535contour data 583contour-dependent temperature control 393contour elongation 510contour plate 227contour steel 339control technique 367convection heat loss 34conveyor belt 76conveyor system 90coolant 129cooled transfer pot 251cooling channel 39, 40cooling circuit 127, 640cooling fixture 130cooling line 195cooling phase 629cooling scheme 315cooling system 292, 308cooling time 6cooling tubing 127cooling water emulsion 466coordinate measuring machine 352copper 167, 188, 271, 494, 516, 571copper alloy 123, 144, 460copper-beryllium alloy 188copper cooling tubing 120copper plating 450copper reinforcement 502copper-steel composite 468copper tube 287copper tubing 83copy milling machine 59core 56, 67, 583core cooling 346core puller 17, 65, 189core strength 447core vents 191corner warpage 388correction 476correctional factor 616corrosion 159, 449, 644, 671corrosion element 645

681 Subject Index

corrosion inhibitor 653corrosion protection 154, 556corrosion protection cartridge 347corrosion resistance 193, 410, 429, 443,

505, 511, 658corrosion-resistant plastic mold steel 437corrosion-resistant steel 338, 542, 570cost control 662cost element methodology 616cost formula 616cost function term 618cost of distribution 616cost ratio 615cost saving 641CO2 temperature control 395counter body 643counter-casting 589counter mold 153, 224counter pressure filling 182, 184counter sunk area 388coupling layer 287cover fluxes 457crack filling process 184crack formation 644crack susceptibility 456crater overlay 492critical shear rate 117cross-linking 241cross-linking polymer 1cross steaming 186crust 159cryogenic treatment 540crystal-clear part 147crystalline layer 656crystallinity 381CuBe alloy 461cube technology 46cubing model 471CuCoBe 356CuCoBe alloy 354cup grade 188cure time calculator 249curing phase 249curing position 85curing process 207curing time 53curved kerb 337curved tunnel gate 28cusing 592cutter heads 483

cutting bushing 118cutting data 452, 467cutting edge 124, 257, 451, 463cutting edge corrections 484cutting geometry 452cutting plate 156, 465cutting sleeve 117cutting speed 464, 510cutting unit 197CVD 469, 648CVD coating 560cycle time 158, 379, 658cycle time reduction 462

DDam Gate 102data acquisition 627data feedback 474data model 325data plane 474data quality 475dead water zone 253decade switch 371decarburising 544decentralized mounting 344decorating plastic product 172Decoupled Molding 630deep bore hole 466deep-drawing operation 519deep etching 454deep hole drilling 485deep slot 599defect-free part 634deflashing 125deflecting area 528deflection aid 127deflection angle 22deflection bar 389deflection element 346degassing 283degradation product 650degrees of forming 214delamination 205demoldability 161, 276demolding 6, 10, 50, 54, 145, 150, 171,

177, 183, 187, 238, 505, 570demolding aid 89, 92demolding air volume stream 145demolding concept 308demolding element 343

682 Subject Index

demolding force 382, 410, 658demolding principle 375demolding procedure 19demolding unit 319density 123, 442deposit formation 659depositor 418deposit table 76deposit welding 457depth-hardened steel 338depth of etching 567design 471design draft 314design drawing 334designer product 569Design of Experiment 630design process 318design rules 8design rules slider molds 15detour hotrunner 25development phase 609dew point 129diamond 70, 271diamond dressing file 525diamond grain 528diamond-like layer 656diamond ring 525diamond turning 271diaphragm forming 217diaphragm gate 24dielectric fluid 490dielectric measurement 234die ring 116diesel effect 37, 263, 416, 649, 665die-sinking ED 532die-sinking EDM 491differential pressure 281diffusion barrier 97diffusion welding 593digital control signal 627digital switch over 627dimensional accuracy 503, 510, 622dimensional inspection 485dimensioning 305dip blow molding 112, 133direct-coating model 506direct cooling 39direct gate 86, 103direct production 582direct tooling 586

disassembly 662discharge energy 492displacement cavity 244displacement stress 644dissipation 246distortion 186distributor head 116DLC 656DLC layer 561DLP projector 589documentation 309, 619dosed molding 53dosing 248dosing unit 373double bath model 178double-threaded spiral core 390downholder 215downtime 662draft angle 92, 145, 192, 231, 569drag plate 104draping 216draping aid 225draping behavior 214drawn brass 506drilled cold runner 253drilled cooling channel 127drilling 465drop box 159drop-in system 33drying cycle 235dry-run 653dullness 406dwell time 278dwell-time length 64DXF 584Dynamic Feed 36

Eeddy currents 221eddy currents develop 399edge chipping 465edge lifting 553edge rounding 647edge weakness 511edge webbing 152EDM 59, 454, 490, 532eectric valve gate system 366effective bounding volume 614effective cost volume 615eggshell effect 469

683 Subject Index

ejecting 166ejecting unit 336ejection 10, 263ejection pin 344ejector box 11ejector elements 654ejector half 9ejector pin 624ejector pins 12, 56, 73, 101, 347, 367ejector plate 73ejector position 311ejectors 96, 376, 654ejector type 375elastic deformation 446elastic mat 205elastic modulus 382elastic strip-off 18elastic strip-over 19elastomer mold 49elastomers 241, 291, 640electrical discharge 490electrical heating cartridge 211electrically conductive 221electrically insulating separating agents

449electrically non-conductive materials 221electrical switchboxes 66electric discharge machining 486, 490electric field 492electric heater 53electric needle drives 363electro-chemical corrosion 650electrochemical process 555electrode material 272electrodes 486electro forming 275electroless nickel 555electrolyt 504, 645electrolytic application 450electrolytic corrosion 645electromagnet 363electromagnetic drive 36electromagnetic system 366electron beam 590electron beam welding 468electronic mold storage management 672electronic parts 52electroplated hard chromium 449electroplated mold 165, 173, 175electroplated molding 161, 167

electroplating 275, 502electroslag remelting 424electrostatic charge 268embossing unit 267emery cloth 531encapsulation material 419encapsulation of insert 417encircling channel 392encrustation 647end mills 483energy balance 24energy consumption 31engraving 126, 599Enterprise Resource Planning 669EPE 181epoxy 280epoxy laminate 518epoxy resin 167, 177, 201, 287epoxy resin coating 96epoxy resins 505, 589EPS 180equipment designer 475erosion 646erosion wire 498erosive turning 273ESR 424estimated value technology 614etch-grainability 429etch-graining 438etching 126, 270, 470, 507, 568etching depth 570etching test 578etching trial 571ethernet interface 628evacuation 415evaporation 40evaporation/condensation process 390evaporation enthalpy 187evaporation of alloying elements 467excess air 149excessive weight 630exchange of electrons 645exothermic heat 83exothermic reaction 79, 234expandable contour skin 228expandable polyethylene 181expandable polyethylene-copolymer 181expandable polymethyl methacrylate 181expandable polypropylene 181expandable polystyrene 180

684 Subject Index

expanding 184expanding core 665explosive films 273exposure time 261, 567external inductor 413external undercuts 10, 13extrusion blow molding 112extrusion process 595

Ffactory-wide network 636failure modes and effects analysis 667fairing lines 175family mold 21, 36, 350, 638fasteners 507fastening of inserts 93fast tool making 582fatigue 644feasibility study 475feedback 476felt 531felt body 523FEM 80female mold 54female side 190FEM calculation 400ferrite phases 530ferritic structure 524fiber-filled coupling layer 289fiberglass laminate 506fiberglass technology 513fiber orientation 148, 578fiber-reinforced materials 200fiber-reinforced thermoplastics 329FiFo principle 114file structure 314filler content 646filling 86filling behaviour 546filling gates 191filling injector 183, 193filling pattern 323filling pressures 110filling simulation 40, 629filling time 632filling unit 182film gate 86, 232, 375final inspection 315final part quality 622fine tuning 633

finishing 494finishing electrode 527, 533first in, first out (FiFo) principle 115fission products 167, 649five-axis milling machine 484fixed half 9fixed stripping nozzle 220fixture 129flame harden 77flame hardened 71flame hardening 548flame retardants 649flash 68, 124, 132, 159, 243, 257, 647, 665flash cooling 124flash formation 375, 417flash-free product 254flashing 4flash pocket 125flash relief 125flat centerings 10flat guides 264, 342flat temperature control 387flexible 83flexible heating elements 75flexible integral skin foams 105flexible mold concepts 599flexible PUR foams 93flexible sealing elements 417flexible tubular heaters 368flex-poli 523floating manifold 32, 33Flotek-process 177flow anomalies 117flow channels 244flow channel thickness 326flow fronts 327flow lines 407flow marks 70flow molding process 76flow path 382, 573, 629flow path length 647flow pattern 669flow pattern method 327flow patterns 109flow resistances 242fluid core 44flux density 557fluxes 456FMEA 667foam 236

685 Subject Index

foam brakes 87foaming molds 190foam molds 458foam pressure 186, 196foam structure 88foil feeding 674Fo number 128force 54force shunt 623forged aluminum 121format frame 153format parts 139forming vacuum 154foundry sands 591fountain cooling system 346fountain flow 4, 44fracture-resistance 429free chrome content 652free-form surface 325free jet 101free jet simulation 326free-standing domes 585freezing effects 398fretting 654fretting of ejectors 642frictional heat 57friction coefficient 450, 656front-flush 626fuel tank 116, 122fully electric injection molding machines

366fume extraction 467functional costs 613functional principle 625functional tests 489Fused Deposition Modeling 595fusing 183, 184fusion 186fuzzy logic 370

Ggalvanic electrolyte 504galvanic element 348galvanic hard chromium plating 437galvanic insert 502galvanized mold 502galvanized mold surface 55gas flushing 457gas injection 361, 638gas nitriding 553

gas permeability 190gate 262gate area 40gate cross-section 21gate design 21gate manifold 375gate position 22, 327gate region 7gate system 308, 311gating 101, 105gear rack 20gear rack drive 68generative manufacturing 582generative manufacturing procedure 292generative mold making 598generative process 582glass fiber 646glass-fiber-reinforced polymer 204gliding behavior 147globular particles 647gloss 381, 406, 530gloss differences 647gloss grad 171gloss level 573GMT 52, 69, 75, 556grabbing mechanism 113gradient materials 585grained surfaces 145, 406grain foil 175grain growth 467graining 567grain pattern 173grain sizes 526grain stamping 519grain structure 175, 506graphite 494, 657gravity casting 457grinding 272, 454, 531grinding pins 523, 525GRP supporting shell 177guide blocks 72guide bolts 72guide columns 263guide elements 56, 256, 336, 341, 460guide pins 72guide plates 72guide rails 192guides 269, 351guiding 264gypsum 142, 291

686 Subject Index

Hhairline crack 511hand crank 67handling fixture 132hand piece 531haptics 171hard anodized aluminum 224hard anodizing 299, 449hard chrome 653hard-chromed 238hard chrome-plated mold 59hard chrome plating 55, 256, 556hard chromium plating 438hard coating 649hard core technology 201hardened felt 148hardened inserts 71hardened mold steels 646hardened mold surface 55hardened plastic mold steel 571hardened steel 255hardening 205, 425, 461hard metal 529hardness 422hardness differences 524hard nickel deposits 504hard oxides 524hard turning 257hard wood 530HCl 650HDPE 125heart-shaped curve 116heat capacity 507heat conduction 39, 359heat conduction tip 29heat conductive cartridge 391heat conductive inserts 392heat conductivity 128heated sprue 28heater cartridge 37heating bands 265heating cartridge 265heating coils 265heating plates 53heat loss 31, 383heat penetration 462heat penetration ability 402heat pipe 40, 390heat points 347heat transfer 39, 40

heat transfer fluids 83heat-treated mold steels 55heat treatment 425, 437, 460, 544heavy-duty shelving 670helical spindle 20helical toothing 497high-alloyed steel 33high-alloy steel 193, 424high chromium steel polished surface 256high-density PE 126high energy light 589high filler content 329high-frequency oscillating magnetic field 221high-frequency spindles 522high-gloss 569high-gloss polish 414high-gloss polishing 427high molecular polyethylene 117high-quality mold surface 96high-speed machine 483high-speed machining 295high speed milling 451high-speed stack mold 351high-strength titanium 369high-temperature corrosion 645high-temperature mold steel 55high temperature oils 211high-temperature processing 599hinged split-cavity mold 66hinge system 162hobbing 439holding magnet 507holding pressure 6, 248holding pressure switchover 632holding pressure time 628hole screens 191hollow components 235hollow needle 46hollow probe 508hollow profiles 213, 236holographic security features 268honing stone 525hood side 190hot bath simulation 539hot-cold separation 372hot cracking tendency 457hot cracks 463hot cure foams 83hot curing PUR elastomers 111hot embossing 267

687 Subject Index

hot forming mold 154hot halves 33, 351hot runner 11, 29, 33, 48, 262, 647hot runner control 370hotrunner externally heated 31hotrunner insulation 29hotrunner internally heated 31hot runner manifold 247, 251hotrunner mold 31hot runner shut-off needle 36hot runner system 333, 349, 351, 667hot runner technology 349hot runner-valve gate 10hot sides 333hot spots 9, 40, 669hot stamping 267hot work steel 211, 292, 338, 357HPGL 584HSC machining 453HSC milling 257, 427HSC milling technology 534HSSE tools 464hybrid component 418hybrid mold 295hydraulic actuators 116hydraulic needle drive 362

Iice formation 396IMC 69IML 132immersing device 129impregnating 227imprinting of the surfac 381impurities 424inclined bolt 13inclined drillings 501index plate 46, 315indirect cooling 39indirect heater 53indirect tooling 586induction heating 265induction welding 220inductive heating 41, 411inductive mold temperature control 399infiltration 210, 596inflatable seal 88ingot mold 457inhibitors 449injecting blow pin 117

injection 248injection blow molding 112, 133injection bore holes 245injection-compression mold 43, 49injection compression molding 2, 250injection mold 332, 511injection molding 1, 290injection molding compression 638injection molding machines 2injection molding process 3, 246injection point 350injection rates 628injection speed 4injection transfer molding process 251In-Mold-Assembly 46, 48in-mold coating 69in-mold graining 513, 519inner centering 10insert frames 197inserts 9, 56, 67, 172, 226, 250, 266, 286,

570, 576, 639inspection 662insulating layer 408insulating plate 376insulation cap 30insulation runner 31integral skin product 100integral value 634intelligent electronics 635interchangeability 335interchangeable inserts 320, 418interchangeable standard molds 338intercrystalline attack 644intermediate material 643intermittent compression molding 210intermittent parison generation 114internal cavities 584, 597internal coating 39internal cooling channels 599internal gas pressure processes 638internal gating 354internal helical cooling channel 602internal hollow spaces 592internal pressure 85internal sliders 17internal stress 540, 630internal thread 19, 117internal undercut 10, 17Internet 636intersection change 417

688 Subject Index

investment casting 583ion flow 490ion implantation 449iron carbon equilibrium 425isobaric double belt press technology 211

Jjaws 654jet pump effect 101jetting 119jetting system 589joining technique 209Joule heating 399

KKeltool Course 4 Technology process 589knife edge 172K-plate 336

Llamellar mold 594laminated object manufacturing 592laminate mold 211laminate structure 201laminating 513laminating mold 516land 63lapping 523lapping grain 528lapping liquid 525lapping pressure 527large-sized galvanized molds 517laser-based generation 591laser beam 588laser beam build-up welding 273laser-beam welding 456laser cusing 600LaserCUSING® 290lasered leather surface 580laser hardening 549laser machining 257laser melting process 294laser processing 273laser sintering 269, 587, 599laser technology 580laser welding 178, 468, 513, 576latch conveyor 344lateral hotrunner injection 34lateral pinch area 119lateral venting 151

late switch over 630lattice stains 651lattice transformation 648layer adhesion 556, 563layer effect 586layer generation 583layer-laminate process 592layer milling process 594layer thickness 586layup 206LDPE 114leaching 647leakage 159, 666leak test 417leather 175, 291, 511, 531leather grains 505length-adjustable arrestor 220LFT 52, 69, 75lifetimes 511LIGA technology 275light sanding 590lightweight 518lightweight material 200linear expansion coefficient 144linear pressure gradient 629line gates 232liquid monomer 588liquid silicone injection molding 640liquid silicone rubber 49, 372lithography with synchronic radiation 275loading chamber 63loading device 53, 64locating element 169locating units 342locking device 91locking pin 419locking system 508locking wedge 16loss of functionality 642lost core 234, 236lost-core process 19lost foam areas 196lost models 583low-alloyed copper alloy 460low-contamination surface 658low density polyethylene 114low-density PUR foam 82lower mold half 54, 63, 89low pressure gravity casting 457low temperature horizontal (LTH) process 188

689 Subject Index

LTH 188lubricant-free operation 653lubricants 450lubrication 74, 654lubrication reservoir 342lug guidance 65

Mmachinability 444machine host computer interface 636machining 427, 483magnetron sputtering 656maintenance 662maintenance interval 642male core 206male mold 54male side 190mandrel 238mandrel head 116manifold 26manifold block 33, 364manifold block, single cavity 368manifold system 369manual labeling 669manufacture of a mold 471manufacturing costs 610maraging mold steels 543maraging plastic mold steels 440marking 55, 599martensite 440martensitic microstructure 539martensitic surface layer 547mass-related strength 446master model 166, 202, 207, 283mat-down treatment 407material costs 618material hardness 244material outbreak 653material removal 530material thickening 216mat-like thermosets 52matt surfaces 406mechanical design 9mechanical ejector 187mechanical energy 249mechanical marking 575mechanical processing technology 274melt channels 367melt core casting 236melt cushion 245

melt filtration 457melt flow 640melt-front-dependent controls 638melt front-dependent opening 640melting 599mesh widening 651metal-coated model 120metal injection molding 260metallic hub 67metal mold insert 290metal slider 189metal spraying 161, 589metal stamp forming 215micro component 399micro-cracks 556micro-EDM 272micro-finishing 482, 494micro-geometric shape deviation 654micro-geometry 530micro-injection mold 261micro injection molded part 358micro injection molding 258micro machining 644micro mat 407micro-plastic parts 500micro-porous galvanized mold 519microporous PUR product 107microporous surface 506micro-precision part 502micro-radiation 407micro-reflectors 505micro roughness 407micro sanded 126micro-stamping 500microstructure 41, 411, 425, 522, 550, 571microstructure transformation 547micro-thermoforming 268micro-wire cutting 272middle plane mesh 325MIG/MAG 467milled cold runner 253milled cooling channel 127milled prototype mold 121milling 465milling command 480milling of the resin block 289milling strategy 482milling track 578miniature parts 500mini-hot runner system 359

690 Subject Index

minimum draft angle 479mirror gloss surface 526mirror-like surface 469mock-up 202model technology 175modular constructed core mold 235modular core mold 230modules 477modulus of elasticity 123mold 336mold alignment 123mold breathing 43mold calculation 614mold carrier 81mold cavity 319mold concept 351mold contamination 649, 658mold core 319mold costs 1, 609mold crater 665mold deposit 410, 546mold design 303mold failure 662mold filling analysis 109mold flanges 171Moldflow 8mold flow analysis 294mold frame 61mold halves 56mold handling 673molding accuracy 398molding shrinkage 144mold inquiry form 311mold insert 319, 460mold-in-systems 172mold labeling 673mold lid 94mold life time 609mold maintenance 278mold maintenance costs 674mold making 335mold management 671mold package 140mold-peripheral connection 479mold plate 341mold protection 7mold release 104molds for sampling 141mold shell 514mold-skinning 188

mold specification 611mold steels 341, 591mold surface 406mold temperature 376mold type 307mold venting gap 4mold wear 49, 218molybdenum 655molybdenum alloy 355, 356, 648molybdenum trioxide 656monitoring signals 628monitoring the process 633mono-block mold 189, 196mono-crystalline diamond 271, 528monolithic products 205MoS2 656MoS2-Me-layers 656mother-galvanized mold 507mother model 177mounting brackets 507mounting position 390movable slide function 479moving cores 638moving mold component 416multi-cavity, gateless injection 358multi-cavity hot runner mold 639multi-cavity mold 21, 64, 103, 140, 160,

353, 632multi-channel connector 626multi-component injection molding 266,

351multi-component metal powder 591multi-component process 303multi-component technology 45multi-etching 579multi-gating nozzle 360multi-hole nozzle 30multi-layer coating 656multi-layer composite 113multi-machine operation 484multi-nozzle pressure head 596multi-nozzles 360multiple-cavity mold 610multiple cutting speed 443multiple-density-process 189multiple etching 568multiple shifts 484multi-step process 190multi-zone controller 370mushroom ejector 376

691 Subject Index

Nnano range 275nanostructures in micro-optics 268narrow slits 531natural balancing 21neck rings 127needle blow method 118needle drives 361needle gripper 75needle shut-off nozzles 35needle valve 350, 361needle valve system 361negative deep-drawing 513negative forming 141negative mold 145, 164negative-stamping deep drawing process

519negotiation support 621net-sales price 618net-shaped preform 223, 225networking 635nickel 167, 571nickel coating 33, 276, 505nickel electroplated deposition 502nickel-galvanos 144nickel hardness 504nickel layer 503nickel plate 444nickel plating 434, 558nickel silver 571nickel sulfamate 504nickel sulfate 504nitrating 216nitriding 434, 552, 571nitriding steels 429, 441, 543non-conductive liquid medium 490non-isothermal diaphragm forming 217non-magnetic behavior 442non-metallic components 562nonmetallic inclusions 524nonporous sputtering 657nonwoven fabric 202normalizing 425nozzle cold runners 252nozzle control 372nozzles 355nozzle sided clamping platen 336number of cavities 610nylon vacuum casting 284

Ooffer phase 607offset 668oil-sealed bonded 175one-stage process 112online flow path control 232open control loop 633open controls 627open molds 208optimal switch over point 633optimization 629, 669optimization process 489orange peel 651orange peel effect 531orange skin effect 521organic sheet 209, 213organo-metallic reaction 650orientation lines 365orientations 70, 323, 381orifice angle 117O-rings 387oscillating motion 528outgassing 571outlet channels 192outserts 37outsert technology 48overall costs 611over-etched structure 579over-etching 573overflow bean 417overflow grooves 242over-injection 417overmeasure 544overmolded parts 633, 639overmolding 44over-polishing 531oxide 645oxide ceramic grinding files 526oxide film 443oxide protective layer 448oxygen-poor water 449

PPA 6 158, 284PA 11 591PA 12 158, 591PACVD coating 561painting 489paint-like surface 406paper-layering process 592

692 Subject Index

parallel connections 40parallel-controlled quick stroke press 76parallel guide system 212parallel temperature control 386parison 113parison forming 134parison tooling 129part design 7partial pressure drop 126particle cleavage 646particle foams 179particle size range 529parting plane 100, 186, 245, 256, 372, 416,

477parts list 332passive layer 652pass through channel 29pattern protocol 315Pb 444PBT 659PC 126, 158, 659PE 150, 158, 659PE-E 184PEEK 48PE foam 159perforated foil 227perforation 191permanent release coatings 171peroxide 241PET 125, 659phase before 307phenolic plywood 141photochemical etching 178, 567photo-resist 574physical blowing agent 179piezoelectric cavity pressure sensors 623pilot series molds 458pinch areas 119pinch-off edge 121, 124pinch relief 125pinpoint gate 25, 232, 375pins 239PIR 111piston 244pitch circle 360pitting 644place holder 228planetary-EDM 454planning of molds 608plasma channel 492

plasma nitriding 554plastic body 523plastic coatings 276plasticizing screw 247plastic mold steels 422, 534plastisols 158plate guidance 363plates, drilled/undrilled 335pleat formation 153pluggable wiring 369plywood 224PMI foam 237PMMA 181, 505pneumatic cylinders 168pneumatic needl 362pneumatic piston actuator 361poli-rotor 523polishability 428, 437polished 55, 126, 238polishing 276, 454, 469, 487, 521polishing defects 521polish lapping 523polyacetal 359polyamide 114polyamide casting 285polyamide-imides 463polycarbonate 114, 448polycarbonate-acrylonitrile butadiene styrene

(PC-ABS) blend 126polycrystalline diamond 271polycrystalline synthetic diamond 528polyester 69polyether ether ketone 463polymerization 588polyolefin 116, 126, 353, 354polyoxymethylene 448polypropylene 75, 126polystyrene 114polyurethane 78, 280, 290, 506, 589polyurethane casting 287polyvinyl chloride 116POM 149, 659porous galvanic molds 144porous material 191, 264porous sintered material 396porous stamping mass 142position of the melt 625position of the parting plane 311positive forming 141positive form mold 140

693 Subject Index

positively etched mold 567positive model 503positive mold 62, 145postblow machining 131post-calculations 618postcooling 130post-crystallization 382post-curing 202post-curing cabinet 589post-electroplating 512post-engraving 178post-graining 572post-shrinkage 381powder bed 591powder box 178powder cake 590, 596powder nozzle 591, 599powder slush 173PP 125, 150, 158, 595, 650, 659PP-E 179P-plates 336PPSU 595pre-calculations 613pre-centered 364precipitated carbides 437precipitation hardening 424, 429, 440, 460,

543, 559precision machining 523pre-defined molds 337pre-dimensioned blank 244pre-expanded particles 179pre-expanders 182prefabricated blank 601prefinishing 482pre-foaming 182preform 45, 64, 112preform assembly step 225preform carriers 47pre-forming 141preform technology 223pre-hardened mold steel 338, 548pre-hardened plastic mold steel 421, 434, 535pre-impregnated fiber fabrics 205pre-machining 482pre-polishing 469prepreg 69prepreg-low pressure-autoclave technology

202prepreg processing 201press forming 520

pressure-assisted resin injection 232pressure-controlled holding pressure 632pressure-dependent switchover 625pressure distribution 43pressure filling 183pressure filling container 184pressure loading 182pressure losses 35pressure peaks 630pressure threshold 633pressure transmitters 219pretension 33pretreatment 503PRIASAFE™ principle 623PRIASED™ principle 623process capability 622process optimization 625process parameters 327, 622process simulation 5, 314process step integration 219process variations 633product costing 613product life cycle 608product transporting grabber 129profiled temperature 41profile milling 120profile mold 212Profit Pins 169programmable delay times 637programming 480project stage 613propellant-coated material 159propellant gas 80propellants 182, 649protective gas layer 456protective gas line 586protective passive top layer 646prototype blow mold 120prototype mold 161, 255, 458, 473, 506prototype molding 280prototype tooling 586, 599prototyping 582PS 591PS-E 179PSU 659PTFE 148, 193, 508PTFE tube 171pulsating method 491pulsed cooling 485, 669punching 214, 219

694 Subject Index

punching standards 269punch mold 143PU processing 505PUR 506PUR casting systems 110PUR foam 79, 159, 237PUR foam processing 514PUR integral skin foams 99PUR spray molds 515pushback pins 12PVC 95, 125, 158, 165, 505, 511, 650PVC foam 237PVC tape 568, 574PVD 409, 429, 434, 449, 450, 469, 648PVD coating 561PVD composite layers 653PVD slide coatings 656p-V-T-diagram 629

Qquality assurance 622quenched steel 571quenching 547quick change mold system 338quick connector 133, 626quick release 508quotation processing 607

Rradiation 178radiation heat loss 33radius 146RAL colors 158rapid-connect couplings 345rapid manufacturing 586rapid prototyping 275, 586rapid tooling 295, 582reaction heat 83reaction mixture 101reaction speed 626reactive matrix systems 234ready-to-install pre-series parts 287real time 627, 634reaming 466reception flanges 507re-coating 590reduced friction edges 212reference curve 635reference value methodology 614reflecting surface 188

reflection 573reflector plate 369Reinforced Reaction Injection Molding 108reinforcement elements 219reinforcement plies 230release agent 171, 206, 216, 224, 239releases 484release steps 478releasing agent 202removal rate 492repair 468, 512, 591, 601, 665replacement demand 609reproducibility 622reproduction accuracy 143residual porosities 423residual pressure 632residual stress 144, 186, 209, 382, 443residue 245resin bath 238resin compound 142resin injection 224resin injection technology 221resin molds 142, 287resin spray coating 142resin transfer molding 222resin trap 232resistance heating 265restoring force 146restraining undercuts 28returning cores 20reweldings 468rework 591rework-free 249rework-free parts 245reworking 560Reynolds number 384R-factor 463rheological analysis 8rheological design 323ribbing 446rigid 83rigid integral skin foam 106rigid PUR foam 96, 100RIM systems 108ring-finish 523ring gate 25, 375risk assessment 674Rohacell 202rotary molds 44rotatable carrier 48

695 Subject Index

rotating core 19rotating tool 525rotationally symmetrical component 238rotational mold 507rotational molding 158, 513rotomolded products 158rough coarse patterns 256roughing 494rough machining 482rough milling 427roughness depth 523, 572round-matte 579round tables 90routine inspection 665RRIM 108RTM 222RT methods 587RTM process 518rubber 241, 530rubber-bonded abrasive wheels 525runner 24runner demolding 28runner separation 26runner types 23run-off-surface 175

Ssagging strength 113salt bath 539salt bath hardening 650sample parts 322sampling 309SAN 650sandblast finish 126sand blasting 575sand casting 457, 583sand casting mold 165sand cast parts 193sanding file 525sandpaper 531sandwich component 218sandwich forming 218sandwich molding 44sandwich structures 96scaling 645scanning 521scorch 244scoring 644screw/piston 246seal 153, 171, 263

sealing 416sealing cap 360sealing effect 33sealing element 87sealing elements 48sealing material 360sealing point 5sealing rings 354sealing surface 175sealing time 6, 381sealing with open pores 88seaming 214seamless hollow articles 158secondary operation 131segment carrier plate 140segregation 578seizing 654Selar® RB 124selective laser melting 273selective laser sintering 283, 395selective mask sintering 587selective opening 361self-hardening 456self-healing effect 443self-locking molds 81self-sealing 4self-skinning foam product 80self-skinning foams 99semi-crystalline thermoplastics 384semifinished fiber products 222semifinished product 138semi-matte 579semirigid 83semirigid integral skin foam 105sensor body 623sensor front 623sensor position 625separation plate 389sequential connection 40sequential control 631sequential gating 361sequential injection 351sequential machining 524sequential molding 637series molds 458series temperature control 385service 662service life 642, 648, 658servo motor 363servomotor drive 36

696 Subject Index

setup time 443shape change 555shape molding machine 182shear cutting 154, 156shearing edge 210, 231shear rate 323, 649shear stress 34, 647sheathing of insert 243sheet molding compound 69sheet processing machine 149shell model 324shell mold 502shielding gas 456SHN-hardening 648Shore A 244, 374short-stroke-polish lapping 523shot peening 170shrinkage 3, 5, 80, 142, 186, 193, 242, 255,

328, 382, 507shrinkage allowance 478shrinkage analysis 109shrinkage behavior 631shrinkage conditions 630shrinkage forecast 308shrinkage prognosis 5shrinking 577shut-off edge 62, 71, 76shut-off needles 11shut-off nozzles 372shut-off region 62shuttle machines 123shuttle-type blowing machine 113SiC stones 526side-mounted alignment bars 72signal slew rates 635silicone 177, 291, 505, 508silicone molds 280silicone molds PU 283silicone negative molds 167silicone oils 211silicone rubber 108, 142, 373silicone stamp 216silver 571silver coating 167silver layer 510simplified costing 614SIMS 650simulation 323simulation programs 136, 232simulation technology 22

simultaneous costing 613simultaneous engineering 306single component powders 591single data level 324single-stage processes 591sink marks 235, 293, 415, 569sintered diamond ring 524sintered mold inserts 669sintered nozzles 191sintered steel 396sintered tool 525sintering 585size of the sensor body 624skeleton molds 147skin depth 411skin effect 412skin layer 4, 96skin texture 505slag barrier 457SLC data 584sleeve 361slicing operation 583slide mechanism 478slide mold 459slider 13, 16, 74, 89, 128, 570, 654slider guidance 15slider modules 219slider parts 476slider positions 311slides 319sliding partners 654sliding properties 410, 460sliding sleeve 344slits 512slot nozzles 150, 191slotted sheets 191slush-molding 515slush molds 165slush skin 95, 173slush technology 173, 504, 513small edges 531small series 287small-series mold 60SMC 52, 69, 518, 555snorkel 41soaking times 540soft core technology 201, 202soft foams 150soft tool 524, 531soft touch 95

697 Subject Index

software 480software solutions 635solenoid armature 363solidified eutectic 444solidifying runner 23, 25solid lubricant 656solid reaction layers 645solid wood 141solution-annealed 460spacer sleeve 210, 355spare parts production 609spark erosion 299, 490spatial expansion 381special machining 337specifications requirements 609specific heat capacity 123, 193specific heat removal 347speed-controlled 629spider 46spider concept 161spindle sleeve 193spiral core 346, 390spiral grooves 116spiral screw 497split cold runners 253split mold 17, 66, 337, 338spot-grinding 488spotting color 575spotting press 488spray cone 192spray forming 424spray nozzles 192spreading film 594spring back behavior 518spring-loaded pressing device 195sprue 23sprue bushing 24sprue gate 24sprue gripper 674sprue system 24sprue taper angle 24squeeze-off sliders 119stack mold 41, 43stagnation 630stainless steel 292, 506, 591, 597stamp forming 214stamp forming system 224stamping flash 417stand-alone web server 636standard cavity pressure sensors 624

standard deflectors 367standard injection molds 307standardization 332standardized components 334standard modules 336standard mold 60standard mold unit 343standard parts 351, 488, 602standards 332standards in mold making 335standard web browser 636start hole EDM 491static pressing units 212stationary layers 4steam chamber 182, 185steel 70, 101, 108, 121, 143, 161, 207, 233,

271, 294, 339, 571, 591steel/copper core pins 347steel deflectors 368steel frame 514steel plates 337steel quality 529steel rule die 157steel tube 168steel tubing 127steel weld construction 199stentering frame 674stepped parting plane 100stereolithography 280, 472, 588stiffeners 205stiffness 98stitching technique 225stitching template 225STL data 604STL dataset 584storage 669storage location 672storage space 249stress corrosion cracking 449, 644, 646stress crack failures 537stress cracks 178stress relieving 535stretchable films 579stretch-blowing 112stretching 152stretching processes 443stretch ratio 138striker plate 117stringer 204, 206stripper for neck flash 118

698 Subject Index

stripper plate 129stripper ring 18stroke length 362stroke movement 344, 528structure hardenings 577structure tips 263structuring 470styrene 354sub-distributor 27submarine gate 28substitute 204suction 596suction blow molding 119sulfamate nickel 504sulfur 241, 571sulfur content 339sulphamate nickel 167, 175super-elastic foils 217superfinishing 524supply bore holes 366supply range 612support 588support core 98supporting frame 177support pin 11support rib 507support structure 591, 597surface coating 449surface cooling 293surface crack testing 428surface defect 406, 578surface design 568surface extension 477surface fatigue 643surface finish 55, 292, 427surface finishing 487, 545surface mesh 325surface model 324surface preparatory leveling technology

524surface quality 381, 405, 503, 591surface replication 406surface reproduction 138surface roughness 147, 521, 654surface-sealed wood 229surface sealing 508surface spalling 644surface structure 169surface structuring 567surface treatment 169, 449, 571

surface treatment process 564surrounding medium 643switchover method 633synthetic foam 148synthetic resin mold 287system boundary method 400system characteristics 642

Ttableted 53tablets 64tailored blanks technology 219Tailored Reinforcement 225tandem mold 43tapered guide 244technical porous nickel 519Teflon 148Teflon-free preservative 671temperature control 154, 265, 345, 383,

384, 514temperature control finger 389temperature control tube 389temperature distribution 40temperature fluctuation 380temperature resistance 511temperature sensors 351temperature variance 39tempered sprue bushing 354tempered steel 571, 648tempering 537, 540, 551tempering of mold 377tempering temperature 437template 318, 569Tenifer treatment 553tensile strength 123tension crack 382tessellation 584test mold 60test run 479textured molded part surface 406texturing 601thermal conductivity 96, 123, 142, 161,

193, 428, 442, 460, 597thermal design 9, 329thermal expansion 10, 204, 364thermal expansion coefficient 201, 446thermal imaging cameras 669thermal insulation 49thermal process 547thermal shock resistance 462

699 Subject Index

thermal surface coating 537thermal wave analysis 409thermo-chemical process 550thermocouple 353, 402, 625thermoelastic range 112thermo forming 209thermoforming 138, 213, 520thermoplastic 200, 380thermoplastic foam 179thermoplastics for micro injection molding

260thermoplastic skin 188thermoplastic wax 589thermoset 52, 200, 372, 640thermoset mold 49thermoset surface 406thin layer chrome 653thin-walled molded part 53thin-walled technology 197thin-wall technology 188threaded insert 172threading 466three-body abrasion 646three-hole tips 360three-plate mold 28through-hardenability 435through-hardening 429through-hardening plastic mold steel 436through-hardening steel 536TiAlN 409TiCN 466tiered calculation method 621TIG welding 575Time-Temperature-Transformation 537tin alloy 106TiN-coated CuCoBe alloy 355TiN coating 449titanium 354, 424, 586, 591titanium alloy 33titanium aluminum nitride 409titanium-based surface 256titanium dioxide 646titanium insulation ring 355tolerance agreement 475tool changer 483tool electrode 491, 494tooling 483tooling material 506tool wear 492topography 406

torpedo heads 116torpedo nozzle 357torpedos 31total balancing 21total shot volume 349touching-up 525toughness 382TPO 95, 165TPU 165traceability 672transfer mold 57transfer molding 45, 52, 244transfer molding cold runner 251transfer piston 57transfer pot 251transfer technology 189transformation behaviour 537transparency 147transverse steam flow 186trials 489trial welding 576triangulation 584tribochemical reaction 468tribological element 643tribologically stressed material 654tribological system 643tribooxidation 643trimming 138, 214trimming station 130T-slots 15TTT 537tube cold runner 254tube system 195tubing 346tubular heater 32tubular preform 114tungsten 498, 655tungsten carbide 356tungsten inert gas welding 456tunnel gate 26, 27, 351, 375turbulent flow 39, 129turning 464turning cube technology 47turntable mold 46twin cube mold 47two-body abrasion 646two-colored slush skins 516two-component casting 280two-component mold 47two-component technology 2

700 Subject Index

two-stage ejector 344two-stage process 112

Uultra-precision machining 271Ultrasonic Consolidation principle 594ultrasonic lapping 532ultrasonic machining 273ultrasonic welding 593unattended operations 484undercut 10, 74, 110, 145, 165, 192, 216,

255, 286, 475, 508, 514, 570under-etching 579under water post-cooling 129uneven heat balance 669unfilled parts 633unidirectional reinforced tapes 238unscrewing 20unscrewing mold 67uphill casting 458upper mold half 54, 63, 89ureol 202, 224UV lithography 275

Vvacuum 96, 138, 193, 217, 246, 509vacuum assisted process 227Vacuum Assisted Resin Injection 222vacuum bag 201vacuum bagging 227vacuum brazing technology 394vacuum casting 589vacuum casting PU process 280vacuum connection 263vacuum forming 150vacuum hot embossing 267vacuum sucker 674vacuum system 518vacuum technology 375, 416vacuum vented 50valve gate cold runner manifold 254valve gate nozzle 360valve gate technology 361vapor pressure 180variable costing 616variothermal mold 41variothermal process 411variothermal process control 397variotherm process 265vectorized executions 575

vent holes 94ventilation 4, 57, 73, 256, 415venting 87, 105, 126, 147, 170, 232, 245,

263, 308, 665venting bore hole 150venting channel system 151venting groove 38venting slot 150vent slot 104venturi nozzle 183vertically adapted melt deflectors 368vestige quality 354vibration welding 220vinyl ester 69viscosity 242, 375, 632, 647visualization 475Viton seals 362voids 85, 205volatile products 650volume mesh 325volume model 324volumetric filling 3, 5, 638vulcanization 241vulcanized fiber 530, 532

Wwall thickness 8, 169warp 53warpage 7, 40, 203, 293, 328, 382, 457, 586warpage behavior 323warp-free mold plates 443warping 83washing process 588waste-free shaping 138water-assisted injection 667water injection 361water-soluble core material 236water-soluble material 236wave propagation theory 326wax 291wax models 167wax plates 216WC/C layer systems 657WC/Co 656wear 58, 360, 545, 665wear low state/high state characteristic 654wear mechanisms 643wear on mold surfaces 642wear resistance 188, 277, 410, 449, 504, 658wear supply 663

701 Subject Index

webbing 150web browser 627wedge angle 13wedges 204wedge-shaped parting plane 111wedge wire screens 199weight-cost ratio 614weight savings 446weldability 429welding 219, 454, 467weld line 22, 119, 327, 361, 381, 398, 409,

569, 637, 649weld line notches 413wetting 210wet-winding process 238white light interferometer 572WIG 467

winding technology 238wire-cut EDM 491wire EDM 454wood 141, 148, 202, 291, 511, 532working stroke 526wrinkling 215wrought alloys 443wüstite 645

XX-ray lithography 275

Zzamac 571zero-defect production 633zinc 77, 571zinc alloy 70, 120, 121