RPT Sheet Metal Forming

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Presented at: ISATA 2000, Dublin, Ireland, Sep. 25-27, 2000Isatapapier_E4.doc/Erstelldatum 28.04.2000 09:53/Speicherd. 22.11.2000 15:41

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EXPERIENCES USING RAPID PROTOTYPING

TECHNIQUES TO MANUFACTURE SHEET METAL

FORMING TOOLS

00SE008

Prof. Dr.-Ing. D. H. Mueller and Dipl.-Ing. H. Mueller,

BIBA (Bremer Institut fr Betriebstechnik und angewandte

Arbeitswissenschaft an der Universitt Bremen, Germany)

AbstractThe automotive industry uses Rapid Prototyping Techniques (RPT) like Stereolithography or

Selective Laser Sintering to produce plastic parts for prototypes faster and cheaper compared to thetechniques used up to now. As a consequence of this process there is a need to have sheet metal

parts also available in earlier product phases. This paper deals with Rapid Prototyping (RP) process

chains to manufacture sheet metal forming tools. It presents a systematic of the approaches,

describes them and presents the results of case studies for selected process chains.

Keywords:

Rapid prototyping, rapid tooling, sheet metal forming tools, layer milling

1 Introduction

By using Rapid Prototyping Techniques (RPT) car manufacturer today produce plastic parts forprototypes faster and even cheaper compared to the techniques used up to now. As well metal parts

made either by RPT combined with metal casting processes or by laser sintering, are inserted earlier

as usual into dummies and tests. As a consequence of this process the automotive industry

increasingly demands to have sheet metal parts also earlier than now at their disposal. This paper

deals with the question whether RP process chains are suitable to manufacture them. The ideas and

results which are presented here have been worked out in the EU funded research project RAPTEC

(BE-20511). A full presentation of the project results is available in the internet,

http://ikppc43.verfahrenstechnik.uni-stuttgart.de:80/raptec/

Sheet metal forming tools are geometrically complex, massive and require a high surface quality.

The material must be stiff and resistant to pressure and wear. According to the current industrial

practise tools for cuttings larger than 250 mm are mainly made from low melting point metals. A

main reason is that these metals can be reused. Head zinc or bismuth-tin alloys are used. The tools

are either made by casting or NC milling or by using both. Tools up to a size of 1000 mm do not

need machining. The final shape of larger tools is produced by NC-milling. The batch-quantity for

prototype parts varies from 1 to 100.

1 Acknowledgement

The work presented in this paper has been supported by the BriteEuram programme of the EU.The authors wish to thank to the project partners and the Directorate Xll Science, Research and

Development of the European Commission.


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2 A systematic of the Rapid Prototyping approaches

The search for new ways to produce sheet metal forming tools already has a long history [Walczyk

et all 94]. The development of manual or automatically configurable tools, consisting of elements

which are movable against each other and which are clamped after positioning is one approach.

Publications of this kind are e.g. [Hardt et all 80], [Finkenstein et all 91] and [Ssmatloch 96]. To

stack sheets is an other approach. [Sepold 94] and [Geiger et all 94] propose to join vertically lasercut laminates. [Walczyk et all 94] describes an approach using vertically stacked and clamped

sheets with profiled edges. [Berger et all 93] describes an approach using a mould made by SLA to

cast a sheet metal forming die out of concrete. Investigations of forming blocks for a rubber pad

forming press applying SLA and resin casting have been published in [Fritz 97] and [Voelkner 98].

To manufacture formed sheets by beat out a scythe is an other research approach.

This paper will now focus on approaches using RPT. As there exist many possible approaches a

systematic structure is necessary in order to give an overview. This is done by table 1. The

approaches are structured by a four level subdivision. The highest level distinguishes between a

direct and an indirect method. The direct method manufactures the sheet metal forming tool

completely or at least the shape giving part of it using RPT. The indirect method utilises a patternmade by RPT as an aid. The steps two and three, which are called generic technology and RP

fabrication method break down the solutions. Thus the direct method distinguishes two generic

technologies that is by RPT and RPT based. The generic technology by RPT itself is broken

down into the fabrication methods extrusion and laminated tool. The actual RP process chains

are allocated on level four. The chapters 4 and 5 will tell more about the processes itself.

1. Method 2. Generic Technology3. RP Fabrication

Method

Extrusion

RPT basedDirect RPT hard

structures

Metal spraying on

RP model

Metal Deposition on

model

Non metal castingResin Casting into

RP mouldEOSINT S

DTM Sandsintering

TSF

Contour Crafting

Electroforming (Ni) + Densit

Epoxy casting

LLCC (Laminated Laser Cut Cavities)

Stratoconception

Conveyed adherent process

Rapid Layer Milling

Most RPT combined with several metal

spraying techniques

e. g. EOSINT M or RapidTool + follow up

processes

Profiled-Edge Lamination method

Casting of low

melting point melals

4. Process/ Process Chains

combined with

sandcasting

Directtool

manufacturing

Metal shell forming on

model

Indirect

tool

manufacturing

Casting of low melting

point melals

By RPT Laminated tool

Contour Crafting

C

C

C

C

Table 1: RP process chains to manufacture sheet metal forming tools (Case study )

3 The case studies

The previously mentioned research project Raptec investigated promising RP process chains by

case studies. Two different testparts were used. Figure 1 shows testpart 1, a B-post lower reinforced

panel:

Size 180 x 125 x 75 mm

Material St 1403

Sheet thickness 1,25 mm

The flangings have a 3 mm radius.

For this part several single sided tools were produced by different RP process chains.

C


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The tools were tested on a rubber pad forming press. The study assessed the quality of the formed

part. The working times and the manufacturing costs were determined. They were compared to the

values of the actual production method used at DaimlerChryler. The cost comparison used in each

case the figures of the different tool manufacturer. That means that the costs are influenced by

company specific factors. The time comparison used working times. They are defined as the times

during which work is actually done on the job as it goes through the system. All values were

collected in the course of a normal industrial production. Therefor a tolerance range of 10 to 15 %must be allowed for. The figures 2 and 3 show these comparisons. In each diagram the reference

process chain NC-milling was put to 100%. The descriptions of the corresponding process chains

refer to these diagrams.

Fig. 1:Testpart 1

100

120

100 100

280

320

0

50

100

150

200

250

300

350

NC milling Sheet Metal

Laminating

Rapid Layer

Milling

(Zimmermann)

Shell version of

Nylon tool

EOSint S and

steel cast.

Solid version of

Nylon tool

Leadtime

[%]

Fig. 2: Comparison of working times

100

38

248

8291

482

0

50

100

150

200

250

300

NC milling Sheet Metal

Laminating

Rapid Layer

Milling

(Zimmermann)

Shell version of

Nylon tool

EOSint S and

steel cast.

Solid version of

Nylon tool

Costs[%]

Fig. 3: Comparison of costs for a single sided tool


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Figure 4 shows the second testpart, a shock absorber panel. It is geometrically complex and has a

high drawing depth. The characteristic values are:

Part size 380 x 490 x 280 mm

Cutting size 830 x 850 mm

Cold rolled St 1403

Sheet thickness 1.5 mm

Weight 2 kg

By means of this part a comparison of the production methods NC-milling of Zamak castings and

Layer Milling was made. The description of the generic technology Laminated tool will refer to

the results.

Fig. 4: Shock absorber panel; source: DaimlerChrysler

4 The direct method of tool manufacturing

The next two chapters deal with the direct or indirect tool manufacturing method respectively.

There is a separate subchapter for each generic technology. Here the RP fabrication methods will

be shortly described and the results of the case studies will be presented. Those RP process chains

for which case studies are available are specially marked in table 1.

4.1 Tool manufacturing direct by RPT

Extrusion and laminating are the two RP fabrication methods which are suitable to manufacture

complete tools. Both can produce large and massive parts.

RP fabrication method Extrusion

However one must realise that the RPT which are in the market today do not fulfil the requirements

which were mentioned in the introduction. Contour Crafting however, which is under developmentat the University of Southern California, has the required properties. It is based on the principle of

extrusion. The key feature of it is the use of two towels, which in effect act as two solid planar

surfaces, to create surfaces on the object being fabricated that are exceptionally smooth and

accurate [Khoshnevis 99]. The main innovation of Contour Crafting compared with existing RPT

are:

Exceptionally smooth and accurate surfaces are created because of the elimination of surfacediscontinuities.

Fabrication of a part is considerably faster because the layer thickness is typically much largerthan layer thickness in other rapid prototyping processes.

A wide variety of materials can be used, including thermosets, thermoplastics, metal andceramic pastes mixed with a binder, and also materials that are not commonly used in rapid

prototyping such as plaster, cement, clay, and concrete.


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R P fabrication method Laminated tool

We know at least five different processes which are based on laminating. They manufacture an

object by cutting, stacking and joining layers of solid materials. Sheet metal forming tools can be

made out of thin sheets which are joined e. g. mechanically or by welding [Sepold 94], [Geiger et

all 94], [Walczyk et all 94].

Laminated too case study

Figure 5 shows a laminated tool for the B-post lower reinforced panel. It consisted of 0.5 mm thick

steel sheets which were joined in the way of a vice. Good parts were produced without any rework

of the edges of the sheets. The laminated tool needed 120% of the working time but only 38 % of

the costs of the reference process.

Fig. 5: Laminated tool; source: DaimlerChrysler

Layer Milling

Layer Milling is an other member of the group Laminated tool. Milling thick layers is an idea

which applies a basic feature of RPT to milling. The generative aspect of this approach is to makethe tool by joining plates and shaping them by milling. The plates are much thicker than the layers

of e.g. SLA or SLS or even laminated tools. The plate thickness may vary from 10 to 100 mm. Thus

massive parts can be made rapidly. Layer milling produces exactly the required shape in the

accuracy we are used to milling. The whole range of materials suitable for machining can be used.

Figure 6 shows the principle.

Concerning the question of the benefit of Layer Milling in comparison to milling a casting or a solid

block one can say:

It is not necessary to produce a casting. The amount of metal removed is higher because always short and stiff tools are applicable

Supposed a fastening joint is applied single plates can be replaced in case of design changes. Different materials can be combined in one tool. E.g. in the area of the die a steel plate may be

used.

Because it is possible to mill close to the final contour, less material has to be removed, which isonly valid in comparison with milling from a solid block. This also means that the safe removal

of remaining plates must be technically solved.

Layer milling has the following disadvantages:

The plates must be joint. There may be a loss of stiffness. Danger for staggered joints (danger for an offset between adjacent plates)Two layer milling machines are offered by the market. The French company Charlyrobot sells amachine called Stratoconception. Figure 7 shows the machine of the company Zimmermann.


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Fig. 6: Principle of Layer Milling; Source: Desk Artes

Fig. 7: Layer Milling machining centre; source: Zimmermann

Layer Milling case studies

Figure 8 shows the layer milled tool for the B post panel. It was manufactured out of a standard

plastic material used for sheet metal forming tools and was produced as fast as the reference tool

with only 82 % of the costs (refer to figures 2 and 3)

Fig. 8: Layer milling tool (material: PU)The second study used the shock absorber panel.. The case study supposed that Layer Milling starts

from Zamak plates which were joined by gluing in the machine. The result of this study with


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respect to manufacturing costs and working time is shown by figure 9. The study found out that

Layer Milling only needed 82 % of the time causing only 84 % of costs. The same hourly rates were

used for calculating the costs. That means that the results is free of company specific factors.

0

20

40

60

80

100

120

Working time Costs

[%]

Layer Milling

Zamak route

Fig. 9: Comparison of working time and costs

4.2 Tool manufacturing based on RPTThis generic technology comprises the RPT using metal and reinforced plastic material. Nylon

powders filled with glass balls are on the market and Stereolithography processes working with

glass fibre filled resins are under development. Currently it is not known to what extend the metal

materials fulfil the requirements which were mentioned in the introduction. But at the beginning

they can be rated as suitable. But because of the low building rate the application is not economic or

limited to small tools respectively.

Laser sintering case study

Figure 10 shows two laser sintered nylon tools for the B-post lower reinforced panel. Thenegative tool to the left was made completely out of nylon, the one to the right consisted of a

backfilled shell. The surface quality of the tools were good, though they were not finished. No

stairsteps were visible.About 25 deep drawings were produced in the negative version. The

maximum pressure was 700 bar. A lubrication was used. At 450 bar a well formded part was

produced, except the 3mm small radius of the flangings. The parts which were made in the positive

tool even had well formed flangings. The comparison of the economically relevant data showed that

the shell version needed the same working time and was 10 % cheaper as the reference process NC

milling ( firgures 2 and 3). The solid version used more than three times of the lead time and caused

five times of the costs.

Fig. 10: Laser sintered tools out of glass filled Nylon


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5 The indirect method of tool manufacturing

This chapter will discuss those process chains which utilise an RP made pattern.

5.1 Tool manufacturing by forming of a metal shell

This generic technology can be split into the two RP fabrication methods Metal spraying on RP

model and Metal deposition on RP model. In both cases a metal shell is produced which isbackfilled afterwards.

It is common to produce sheet metal forming tools by metal spraying. Also processes for spraying

steel are available. But they are hardly industrially applied. The main reason is the warpage or

inner stresses of the shells.

Sheet metal forming tools consisting of backfilled electroformed shells are in operation. The

principle design and an application of such a tool can be seen in figure 11. A model of the tool is

manufactured using RPT. On this model a nickel shell is electroformed. After a sufficient shell

thickness has been reached it is removed from the model and backfilled in order to get the required

stiffness. The application shown by figure 11 used concrete for backfilling.

ElectroformedDensit ToolCast

Fig. 11: Design and application of an electroformed shell

5.2 Tool manufacturing using non metal casting

It is current practise of industry to manufacture resin tools by casting. Supposed that the part is

small and has a complex shape RPT are very useful to produce the models which are needed for

casting. Whereas RPT are to expensive for the production of large tools. This was also verified by a

case study which used testpart 2 of chapter 3. For detailed information please refer to

http://ikppc43.verfahrenstechnik.uni-stuttgart.de:80/raptec/.

5.3 Tool manufacturing using casting of low melting point metalsMost of the prototype sheet metal forming tools for car production are made from castings of low

melting point metals. From the technically point of view the required moulds could be produced by

sandsintering. But this application is not economic because the building rates are fare to low. This is

confirmed by the case study of the B-post lower reinforced panel. The tool made by

sandsintering and steel casting needs 2.8 times of the lead time and 2.5 times of the costs of NC

milling (refer to figures 2 and 3).

Contour Crafting, the process mentioned at the beginning of the paper seem very promising also for

this approach.. It can produce parts which are large and massive and it is the only RPT which shows

a way how smooth freeform surfaces can be made by RPT.


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RPT Sheet Metal Forming