Selecting Injection Molds

Selecting Injection Molds
Weighing Cost vs Productivity
Herbert Rees, Bruce Catoen
Contents
Preface . V
1 Introduction and Planning . 1
1.1 Introduction . 1
1.2 Oversimplification 3
1.2.1 Definitions 3
1.3 Is Injection Molding the Right Choice for this Product? 4
1.4 The Injection Molding Machine 5
1.4.1 The Right Machine for the Mold 6
1.5 The Injection Mold . 7
1.5.1 What Is an Injection Mold? 7
1.5.2 Elements of an Injection Mold . 9
2 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 . 32VIII 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
3 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 94Contents IX
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
4 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
5 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 176X 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
6 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
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