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Table of Contents

1 Introduction to Plastics Processing ..................................................................................................... 1

1.0 Introduction ...................................................................................................................................... 1

1.1 Interactivity Basics ................................................................................................................................ 1

1.2 Thermodynamic Principles Governing Injection Molding..................................................................... 3

1.2.1 Filling ...................................................................................................................................... 4

1.2.2 Holding ...................................................................................................................................... 5

1.2.3 Cooling ...................................................................................................................................... 6

2 Shrinkage and Warpage ....................................................................................................................... 9

2.1 In-Mold Shrinkage ................................................................................................................................ 9

2.1.1 Determination of Shrinkage ...................................................................................................... 10

2.1.2 Molded-in Stress ........................................................................................................................11

2.2 Warpage .................................................................................................................................... 12

2.2.1 Common Causes of Nonuniform Shrinkage ............................................................................. 13

2.2.2 Principles of Minimizing Warpage ........................................................................................... 14

2.3 Post-Mold Shrinkage ........................................................................................................................... 16

3 Causes of Molded-Part Variation: Part Design ............................................................................... 17

3.1 Wall Thickness .................................................................................................................................... 17

3.2 Ribs .................................................................................................................................... 18

3.3 Bosses .................................................................................................................................... 21

3.4 Example of Proper Part Design........................................................................................................... 21

3.5 Other Design Considerations ............................................................................................................... 22

4 Causes of Molded-Part Variation: Material ..................................................................................... 23

4.1 Amorphous and Semicrystalline Resins .............................................................................................. 23

4.1.1 Amorphous Polymers................................................................................................................ 25

4.1.2 Semicrystalline Materials ......................................................................................................... 26

4.2 Effects of Fillers, Reinforcements, Pigments, Time, and Stress.......................................................... 29

4.2.1 Effects of Fillers and Fibers ...................................................................................................... 29

4.2.2 Minimizing the Effects of Fiber Reinforcements ...................................................................... 32

4.2.3 Effects of Pigments ................................................................................................................... 34

4.2.4 The Effects of Time and Stress on Dimensional Stability ........................................................ 36

4.3 Shrinkage Predictions: Using Pressure-Volume-Temperature (PVT) Relationships ........................... 37

4.3.1 PVT System Properties ............................................................................................................. 37

4.3.2 Predicting Mold Shrinkage ....................................................................................................... 43

4.3.3 Predicting Mold Warpage ......................................................................................................... 46

4.3.4 Accuracy of Shrinkage Predictions .......................................................................................... 48

5 Causes of Molded-Part Variation: Mold Design .............................................................................. 51

5.1 Cavity Dimensions and Design Factors .............................................................................................. 51
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5.2 Gate Types .................................................................................................................................... 53

5.2.1 Pin, Pinpoint, Tunnel, and Submarine Gates ............................................................................ 54

5.2.2 Edge and Straight-Edge Gates .................................................................................................. 55

5.2.3 Fan Gates .................................................................................................................................. 55

5.2.4 Film, Diaphragm, and Ring Gates ............................................................................................ 55

5.2.5 Tab Gates .................................................................................................................................. 56

5.2.6 Multiple Gates .......................................................................................................................... 565.3 Gate Location .................................................................................................................................... 57

5.3.1 Side, End, and Center Gates ..................................................................................................... 58

5.3.2 Determining Gate Position ........................................................................................................ 60

5.4 Gate Size .................................................................................................................................... 62

5.5 Gate-Design Systems........................................................................................................................... 62

5.6 Runner Design .................................................................................................................................... 63

5.6.1 Multiple-Cavity Molds ............................................................................................................. 64

5.6.2 Ejection .................................................................................................................................... 70

5.7 Mold-Cooling Design .......................................................................................................................... 70

5.7.1 Cooling Channels ...................................................................................................................... 70

5.7.2 Effects of Corners ..................................................................................................................... 745.7.3 Thickness Variations................................................................................................................. 75

5.7.4 Runnerless Molds ..................................................................................................................... 75

5.7.5 Slides .................................................................................................................................... 75

5.7.6 Venting .................................................................................................................................... 75

5.8 Mold Construction Materials .............................................................................................................. 76

5.9 Prototype Molding with SLA or Similar Type Molds......................................................................... 76

5.10 Pitfalls to Avoid................................................................................................................................. 77

6 Causes of Molded-Part Variation: Processing.................................................................................. 79

6.1 Molding Conditions ............................................................................................................................. 796.2 Injection Melt Temperature ................................................................................................................. 83

6.3 Injection Rate and Pressure ................................................................................................................. 83

6.3.1 Injection Speed.......................................................................................................................... 83

6.3.2 Injection Pressure...................................................................................................................... 84

6.4 Holding Pressure and Time ................................................................................................................. 85

6.4.1 Holding Pressure....................................................................................................................... 85

6.4.2 Holding-Pressure Time ............................................................................................................. 87

6.5 Mold Temperature ............................................................................................................................... 88

6.5.1 Predicting Mold Temperature Effects ....................................................................................... 89

6.5.2 Relationship Between Mold Temperature and Shrinkage ......................................................... 90

6.5.3 Relationship Between Wall Thickness and Shrinkage .............................................................. 90

6.6 Demolding Temperature ...................................................................................................................... 91

6.7 Molded-In Stresses .............................................................................................................................. 92

6.8 Other Molding Processes..................................................................................................................... 93

6.8.1 Lost Core Process ..................................................................................................................... 93

6.8.2 Gas Assist Processing ............................................................................................................... 93

6.8.2.1 Foaming ...................................................................................................................... 94

6.8.2.2 Shrinkage of Gas-Assisted Processes ......................................................................... 95
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7 Factors Affecting Post-Mold Shrinkage and Warpage ................................................................... 97

7.1 Effects of Temperature on Dimensions ............................................................................................... 97

7.2 Effects of Moisture on Dimensions ..................................................................................................... 98

7.3 Creep .................................................................................................................................. 103

8 Controlling Mold and Post-Mold Shrinkage and Warpage .......................................................... 105

8.1 Finding the Cause.............................................................................................................................. 105

8.2 Processing Considerations................................................................................................................. 106

8.2.1 Melt Temperatures and Uniformity ........................................................................................ 106

8.2.2 Mold Temperatures and Uniformity ....................................................................................... 107

8.2.3 Filling, Packing, and Holding Pressures ................................................................................. 108

8.2.4 Filling, Packing, and Holding Times ...................................................................................... 108

8.2.5 Part Temperature at Ejection .................................................................................................. 109

8.2.6 Clamp Tonnage ....................................................................................................................... 109

8.2.7 Post-Mold Fixturing and Annealing ....................................................................................... 109

8.2.8 Special Problems With Thick Walls and Sink Marks ............................................................. 109

8.2.9 Nozzles ...................................................................................................................................110

8.2.10 Excessive or Insufficient Shrinkage ...........................................................................111

8.2.11 Secondary Machining ................................................................................................ 111

8.2.12 Quality Control .......................................................................................................... 111

8.3 Material Considerations .....................................................................................................................112

8.3.1 Filler or Reinforcement Content ..............................................................................................114

8.3.2 Degree of Liquid Absorption ...................................................................................................114

8.3.3 Regrind ...................................................................................................................................114

8.4 Tooling Considerations .......................................................................................................................114

8.4.1 Gate Locations .........................................................................................................................114

8.4.2 Types and Sizes of Gates .........................................................................................................1158.4.3 Runner Systems .......................................................................................................................115

8.4.4 Mold-Cooling Layout ..............................................................................................................116

8.4.5 Tool Tolerances ........................................................................................................................116

8.4.6 Draft Angles ............................................................................................................................117

8.4.7 Ejection-System Design ...........................................................................................................117

8.4.8 Elastic Deformation of a Mold ................................................................................................118

8.4.9 Mold Wear ...............................................................................................................................118

8.4.10 Mold Contamination ...............................................................................................................118

8.4.11 Position Deviations of Movable Mold Components ...............................................................119

8.4.12 Special Issues With Gears ......................................................................................................119

8.5 Part Geometry .................................................................................................................................. 1218.5.1 Overall Part Dimensions ......................................................................................................... 122

8.5.2 Wall Thickness........................................................................................................................ 122

8.5.3 Shrinkage-Restricting Features ............................................................................................... 124

8.6 Controlling Warpage ......................................................................................................................... 124

9 Computer-Aided Analysis ................................................................................................................ 127

9.1 Capabilities of CAA .......................................................................................................................... 127

9.2 Limitations of CAA ........................................................................................................................... 128

9.3 Selecting a CAA Program ................................................................................................................. 129
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9.4 Customer Requirements .................................................................................................................... 131

9.5 Management Tools ............................................................................................................................ 132

9.6 Filling Analysis ................................................................................................................................. 133

9.7 Packing and Holding Simulation ....................................................................................................... 136

9.8 Shrinkage/Warpage Simulation ......................................................................................................... 136

9.9 Cooling Analysis ............................................................................................................................... 136

9.10 Costs .............................................................................................................................. 1379.11 Conclusions .............................................................................................................................. 138

10 Case Studies .................................................................................................................................. 139

10.1 Unexpected Housing Shrink............................................................................................................ 139

10.2 Changing Materials Triggers Warpage ........................................................................................... 140

10.3 Creep in a Water Heater Stand ....................................................................................................... 141

10.4 Oversize Part Injection-Molding Alkyd Thermoset ........................................................................ 141

10.5 Inadequate Baby Dish Mold ........................................................................................................... 142

10.6 Gas Entrapment in Baby Dish Mold............................................................................................... 144

10.7 Warpage in a Molded Spool ............................................................................................................ 14510.8 Daisy-Wheel Breakage ................................................................................................................... 147

10.9 PVC Part-Flashing Problems.......................................................................................................... 147

10.10 Polycarbonate Switch Failure ...................................................................................................... 148

10.11 Square Poker Chip Tray, Inadequate Shot Size ........................................................................... 149

10.12 Problem Ejecting Square Poker Chips ......................................................................................... 149

10.13 Military Cup Material Shrinkage ............................................................................................. 150

10.14 Core-Deflection Problems ............................................................................................................ 151

10.15 Elevator Gib Warpage .................................................................................................................. 152

10.16 Sucker-Rod Guide Brittleness ...................................................................................................... 153

10.17 Bottle-Cap Thread Distortion....................................................................................................... 153

10.18 Plastic Post Creep ........................................................................................................................ 15410.19 Excessive Shrinkage of Glass-Filled Nylon ................................................................................. 154

10.20 Preventing Warpage in Thin Molded Lids.................................................................................... 155

11 Data .................................................................................................................................. 157

11.1 Unfilled Materials ........................................................................................................................... 158

11.2 Effects of Fillers .............................................................................................................................. 159

11.3 Shrinkage vs Various Parameters ................................................................................................... 164

11.3.1 Shrinkage vs Injection Temperature .................................................................................... 164

11.3.2 Shrinkage vs Melt Temperature .......................................................................................... 165

11.3.3 Shrinkage vs Mold Temperature ......................................................................................... 16611.3.4 Shrinkage vs Density (Polyethylene) ................................................................................... 166

11.3.5 Shrinkage vs Holding Pressure ............................................................................................ 167

11.3.6 Shrinkage vs Thickness ....................................................................................................... 169

11.4 Moisture Absorption Curves........................................................................................................... 171

11.5 Pressure Volume Temperature (PVT) Curves................................................................................. 172

11.6 Shrinkage and Warpage of Molded Disks....................................................................................... 186

11.7 Angular Warpage ............................................................................................................................ 188

11.8 General Shrinkage Characteristics for Various Plastics ................................................................. 193
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11.9 Material-Specific Shrinkage Characteristics .................................................................................. 198

11.9.1 Polybutylene Terephthalate (PBT) (Crastin) (DuPont) ..................................................... 198

11.9.2 Polyoxymethylene: Acetal (POM) (Delrin ) (DuPont) ...................................................... 200

11.9.3 Polycarbonate (PC) ............................................................................................................. 203

11.9.4 Polyphenylene Sulfide (Fortron) (PPS) ............................................................................. 204

11.9.5 Thermoplastic Elastomer Ether Ester Block Copolymer (TEEE) (Hytrel) (DuPont) ....... 206

11.9.6 Polypropylene (PP) .............................................................................................................. 20711.9.7 Polyethylene Terephthalate (PET) (Rynite) (DuPont) ....................................................... 215

11.9.8 Polyetherimide (PEI) (Ultem) (GE Plastics) ...................................................................... 215

11.9.9 Liquid Crystal Polymer (LCP) (Zenite) (DuPont)............................................................. 216

11.9.10 Polyamide: Nylon (PA) (Zytel) (DuPont) ....................................................................... 217

Appendix A: Conversion Factors and Equivalents ........................................................................... 221

Appendix B: Abbreviations, Acronyms, and Material Names ....................................................... 223

B.1 Abbreviations and Acronyms ............................................................................................................ 223

B.2 Material Names ................................................................................................................................. 228

Glossary .................................................................................................................................. 233

References and Bibliography ............................................................................................................... 237

References (cited in text) .......................................................................................................................... 237

Bibliography .................................................................................................................................. 239

General References .............................................................................................................................. 239

Index .................................................................................................................................. 241
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