Handbook of Molded Part Shrinkage and Warpage

PDL Handbook Series

Preface

Acknowledgments

1. Introduction to Injection Molding

1.1 Introduction to Plastics

1.2 Interactivity Basics

1.3 Thermal Principles Governing Injection Molding

References

2. Shrinkage and Warpage

2.1 In-Mold Shrinkage

2.2 Warpage

2.3 Postmold Shrinkage

References

3. Causes of Molded-Part Variation: Part Design

3.1 Wall Thickness

3.2 Ribs

3.3 Bosses

3.4 Example of Proper Part Design

3.5 Other Design Considerations

References

4. Causes of Molded-Part Variation: Material

4.1 Amorphous and Semicrystalline Resins

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

4.3 Shrinkage Predictions: Using PVT Relationships

References

5. Causes of Molded-Part Variation: Mold Design

5.1 Cavity Dimensions and Design Factors

5.2 Gate Types

5.3 Gate Location

5.4 Gate Size

5.5 Gate Design Systems

5.6 Runner Design

5.7 Mold Cooling Design

5.8 Mold Construction Materials

5.9 Prototype Molding with Stereolithography or Similar Type Molds

5.10 Pitfalls to Avoid

References

6. Causes of Molded Part Variation: Processing

6.1 Molding Conditions

6.2 Injection Melt Temperature

6.3 Injection Rate and Pressure

6.4 Holding Pressure and Time

6.5 Mold Temperature

6.6 Demolding Temperature

6.7 Molded-In Stresses

6.8 Other Molding Processes

References

7. Factors Affecting Postmold Shrinkage and Warpage

7.1 Effects of Temperature on Dimensions

7.2 Effects of Moisture on Dimensions

7.3 Creep

References

8. Controlling Mold and Postmold Shrinkage and Warpage

8.1 Finding the Cause

8.2 Processing Considerations

8.3 Material Considerations

8.4 Tooling Considerations

8.5 Part Geometry

8.6 Controlling Warpage

References

9. Computer-Aided Analysis

9.1 Capabilities of CAA

9.2 Limitations of CAA

9.3 Selecting a CAA Program

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

References

10. Case Studies

10.1 Unexpected Housing Shrink

10.2 Changing Materials Triggers Warpage

10.3 Creep in a Water Heater Stand

10.4 Oversize Part Injection-Molding Alkyd Thermoset

10.5 Inadequate Baby Dish Mold

10.6 Gas Entrapment in Baby Dish Mold

10.7 Warpage in a Molded Spool

10.8 Daisy-Wheel Breakage

10.9 PVC Part-Flashing Problems

10.10 Polycarbonate Switch Failure

10.11 Square Poker-Chip Tray: Inadequate Shot Size

10.12 Problem Ejecting Square Poker Chips

10.13 Military Cup Material “Shrinkage”

10.14 Core-Deflection Problems

10.15 Elevator Gib Warpage

10.16 Sucker-Rod Guide Brittleness

10.17 Bottle-Cap Thread Distortion

10.18 Plastic Post Creep

10.19 Excessive Shrinkage of Glass-Filled Nylon

10.20 Preventing Warpage in Thin Molded Lids

10.21 A Printed Circuit Board That Did Not Work

10.22 A Cup with a Handle that Won’t Fill

10.23 Leaking Hot Runner Mold

References

11. Data

11.1 Unfilled Materials

11.2 Effects of Fillers

11.3 Shrinkage vs. Various Parameters

11.4 Moisture Absorption Curves

11.5 Pressure–Volume–Temperature Curves

11.6 Shrinkage and Warpage of Molded Disks

11.7 Angular Warpage

11.8 General Shrinkage Characteristics for Various Plastics

11.9 Material-Specific Shrinkage Characteristics

References

Appendix A. Conversion Factors and Equivalents

Appendix B. Abbreviations, Acronyms, and Material Names

B.1 Abbreviations and Acronyms

B.2 Material Names

Glossary

Index

Color Plates

How easy life would be if only moldings were the same size and shape as the mold. But they never are, as molders, toolmakers, designers and end users know only too well. Shrinkage means that the size is always different; warpage often changes the shape too. The effects are worse for some plastics than others. Why is that? What can you do about it? The Handbook of Molded Part Shrinkage and Warpage is the first and only book to deal specifically with this fundamental problem. Jerry Fischer’s Handbook explains in plain terms why moldings shrink and warp, shows how additives and reinforcements change the picture, sets out the effect of molding process conditions, and explains why you never can have a single ‘correct’ shrinkage value. It goes on to demonstrate how to alleviate the problem through careful design of the molded part and the mold, and by proper material selection. It also examines computer-aided methods of forecasting shrinkage and warpage. And most important of all, the Handbook gives you the data you need to work with.

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• Authoritative and rooted in extensive industrial experience, the expert guidance contained in this handbook offers practical understanding to novices, and new insights to readers already skilled in the art of injection molding and mold making.
• Contains the answers to common problems and detailed advice on how to control mold and post-mold shrinkage and warpage.
• Case Studies illustrate and enrich the text; Data tables provide the empirical data that is essential for success, but hard to come by.

Engineers, scientists and technicians specializing in injection molding of plastic components. Designers of plastic components. Process and product manufacturing control engineers. Product development engineers.