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VII Contents Preface V Introduction and Planning 1.1 Introduction 1.2 Oversimplification 1.2.1 Definitions 1.3 Is Injection Molding the Right Choice for this Product? 1.4 The Injection Molding Machine 1.4.1 The Right Machine for the Mold 1.5 The Injection Mold 1.5.1 What Is an Injection Mold? 1.5.2 Elements of an Injection Mold The Plastic Product 11 2.1 The Product Design 12 2.2 Product Drawings 13 2.2.1 Product Shape: How Can the Product Best Be Molded? 13 2.2.2 Parting Line (P/L) 13 2.2.3 Side Cores 15 2.3 Accuracy and Tolerances Required 18 2.3.1 General and Specific Tolerances 20 2.3.2 Are Special Fits with Matching Products Required? 21 2.3.3 Tolerances for the Filling Volume 21 2.3.4 Stacking of Products and Free Dispensing 22 2.3.5 Mismatch (Deliberate) 23 2.4 Tolerances, Mold Alignment, and Mold Costs 25 2.5 Heat Expansion, Alignment, and Mold Cost 28 2.6 Surface Finish 29 2.6.1 Finish of Molding Surfaces 29 2.6.2 Texturing of Surfaces 30 2.6.3 Fitting Surfaces of Mold Parts 31 2.7 Engravings 31 2.7.1 Engravings Versus Applied Labels 31 2.7.2 Two-Color and Two-Material Engraving 32 2.7.3 Depth of Engravings 32 VIII Contents 2.7.4 Font Style and Size of Artwork 33 2.7.5 Polarity of Engraving 33 2.7.6 Are the Locations Selected for Engraving Practical? 34 2.7.7 Engravings in the Walls and Bottoms of Products 34 2.8 General Appearance of the Product 37 2.8.1 Flatness 37 2.8.2 Sinks and Voids 39 2.8.3 Witness Lines 40 2.8.4 Weld Lines 43 2.8.5 Surface Defects (Flow Marks, Splay, Record Grooves, Haze, Jetting, Hooks, and Ripples) 43 2.8.6 Identification of the Molded Piece 44 2.9 Product Strength Requirements 45 2.9.1 Gate Location to Increase Product Strength 46 2.10 Special Features 46 2.10.1 Holes and Counter Bores for Assembly Screws or Rivets 47 2.10.2 Hinges and Snaps for Assembly 47 Cost Factors Affecting Productivity 49 3.1 Where Will the Mold Be Operated? 49 3.1.1 Condition of Ambient (Shop) Air 49 3.2 Coolant Supply 51 3.2.1 Is the Coolant Supply Large Enough for the Planned Mold? 51 3.2.2 Is the Cooling Water Clean? 51 3.3 Power Supply 52 3.4 Will the Mold Run in a Variety of Machines or a Single Machine? 53 3.5 Is the Mold Planned to Run in a Newly Created Operation? 54 3.6 Projected Requirements 55 3.6.1 Making Prototype or Experimental Molds 55 3.6.2 Production Molds 57 3.7 Forecasting the Cycle Time 59 3.7.1 Type of Plastic Molded 60 3.7.2 Wall Thickness of Product 60 3.7.3 Mold Materials 61 3.7.4 Efficiency of Cooling 62 3.7.5 Venting 66 3.7.6 Effect of Molding Machine on Cycle Time 67 3.7.7 Ejection 85 3.7.8 Ambient Temperatures and Humidity 94 Contents 3.7.9 Comparing Molding Cycles of the Same Product in New Molds 94 3.8 Number of Cavities Required 95 3.8.1 Available Operating Time 95 3.8.2 The Minimum Number of Cavities 96 3.8.3 Machine Hour Cost per Unit Molded 97 3.8.4 Mold Cost per Unit Molded 99 3.8.5 Calculation of the Required Clamp Size 100 3.8.6 Shot Size 102 3.8.7 Plasticizing Capacity 102 3.8.8 Preferred (Practical) Number of Cavities 102 3.8.9 Business Decisions 104 3.8.10 Preliminary Estimate of Product Cost 105 Mold Selection 113 4.1 Selection of an Appropriate Mold 113 4.1.1 Dedicated Mold, Universal Mold Shoe 113 4.1.2 “One-Product” Molds or “Family” Molds? 114 4.1.3 Where to Gate 116 4.1.4 Gate Size and Runner Systems 123 4.1.5 Hot Runner Molds 125 4.1.6 Single Cavity Molds 129 4.1.7 Two and More Cavities, Cold or Hot Runner Molds 135 4.1.8 Single- or Multi-Level Molds? 142 4.1.9 Semi or Fully Automatic Operation? 154 4.1.10 Insert Molding 155 4.2 Summary 158 Mold Cost, Mold Price and Delivery 159 5.1 Mold Cost and Records 159 5.1.1 Spare Parts for the Mold 160 5.1.2 The Basic Elements of the Mold Cost 161 5.1.3 Cost of the Preparation of a Mold Manual 170 5.2 Overhead 170 5.3 Mold Price 171 5.3.1 Risk Factor 173 5.4 Mold Cost Is Absorbed by the Molder 175 5.5 Arriving at Mold Cost and Delivery Time 175 5.5.1 Calculating the Mold Cost 176 5.5.2 Estimating the Mold Cost 176 IX X Contents 5.5.3 “Guesstimating” the Mold Cost 180 5.5.4 “Ball Parking” the Mold Price 180 5.5.5 Mold Price Catalogue 180 5.6 The Quotation 181 5.6.1 Delivery Time 181 5.6.2 Confirmation 182 5.7 In-House Mold Making Operations 183 Warranties, Patents, and Ethical Considerations 185 6.1 Warranties and Guaranties 185 6.1.1 Guaranteed Quality 185 6.1.2 Guaranteed Shrinkage 185 6.1.3 Guaranteed Cycle Time 186 6.1.4 Guaranteed Delivery 186 6.2 Patents and Ethical Considerations 186 6.2.1 Patents 187 6.2.2 Ethical Considerations 189 References 191 Appendix 193 Appendix 1: Suggested Contents of a Mold Manual 193 Appendix 2: Mold Set-up Guide Blank 194 Appendix 3: Example of Light-Weighting a Product and Increasing Productivity 197 Appendix 4: Buying a Mold 199 Appendix 5: Suggested Format of a Confirmation of Order 204 Appendix 6: Molding Properties of Injection-Grade Plastics 208 Appendix 7: General Properties 210 Appendix 8: Mechanical Properties 212 Appendix 9: Thermal Properties 216 Appendix 10: Typical Mold Materials 220 Appendix 11: What Characterizes a Good, High-production Mold? 222 Appendix 12: Advice for the Mold Designer 225 Appendix 13: Surface Finishes 228 Index 229