Table of Contents Introduction to Plastics Processing 1.0 Introduction 1.1 Interactivity Basics 1.2 Thermodynamic Principles Governing Injection Molding 1.2.1 Filling 1.2.2 Holding 1.2.3 Cooling Shrinkage and Warpage 2.1 In-Mold Shrinkage 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 Causes of Molded-Part Variation: Part Design 17 3.1 3.2 3.3 3.4 3.5 Wall Thickness 17 Ribs 18 Bosses 21 Example of Proper Part Design 21 Other Design Considerations 22 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 Causes of Molded-Part Variation: Mold Design 51 5.1 Cavity Dimensions and Design Factors 51 © Plastics Design Library Table of Contents viii 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 56 5.3 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 74 5.7.3 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 Causes of Molded-Part Variation: Processing 79 6.1 Molding Conditions 6.2 Injection Melt Temperature 6.3 Injection Rate and Pressure 6.3.1 Injection Speed 6.3.2 Injection Pressure 6.4 Holding Pressure and Time 6.4.1 Holding Pressure 6.4.2 Holding-Pressure Time 6.5 Mold Temperature 6.5.1 Predicting Mold Temperature Effects 6.5.2 Relationship Between Mold Temperature and Shrinkage 6.5.3 Relationship Between Wall Thickness and Shrinkage 6.6 Demolding Temperature 6.7 Molded-In Stresses 6.8 Other Molding Processes 6.8.1 Lost Core Process 6.8.2 Gas Assist Processing 6.8.2.1 Foaming 6.8.2.2 Shrinkage of Gas-Assisted Processes Table of Contents 79 83 83 83 84 85 85 87 88 89 90 90 91 92 93 93 93 94 95 © Plastics Design Library ix 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 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 115 8.4.3 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 121 8.5.1 Overall Part Dimensions 122 8.5.2 Wall Thickness 122 8.5.3 Shrinkage-Restricting Features 124 8.6 Controlling Warpage 124 Computer-Aided Analysis 127 9.1 Capabilities of CAA 127 9.2 Limitations of CAA 128 9.3 Selecting a CAA Program 129 © Plastics Design Library Table of Contents x 9.4 Customer Requirements 9.5 Management Tools 9.6 Filling Analysis 9.7 Packing and Holding Simulation 9.8 Shrinkage/Warpage Simulation 9.9 Cooling Analysis 9.10 Costs 9.11 Conclusions 131 132 133 136 136 136 137 138 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 145 10.8 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 154 10.19 Excessive Shrinkage of Glass-Filled Nylon 154 10.20 Preventing Warpage in Thin Molded Lids 155 11 Data 157 11.1 Unfilled Materials 11.2 Effects of Fillers 11.3 Shrinkage vs Various Parameters 11.3.1 Shrinkage vs Injection Temperature 11.3.2 Shrinkage vs Melt Temperature 11.3.3 Shrinkage vs Mold Temperature 11.3.4 Shrinkage vs Density (Polyethylene) 11.3.5 Shrinkage vs Holding Pressure 11.3.6 Shrinkage vs Thickness 11.4 Moisture Absorption Curves 11.5 Pressure Volume Temperature (PVT) Curves 11.6 Shrinkage and Warpage of Molded Disks 11.7 Angular Warpage 11.8 General Shrinkage Characteristics for Various Plastics Table of Contents 158 159 164 164 165 166 166 167 169 171 172 186 188 193 © Plastics Design Library xi 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) 207 11.9.7 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 © Plastics Design Library Table of Contents