Designing Plastic Parts for Assembly

Contents

Foreword to the Eighth Edition

Preface to the Eighth Edition

Foreword to the First Edition

Preface to the First Edition

Acknowledgments

1 Understanding Plastic Materials

1.1 Basic Resins

1.1.1 Thermoplastics

1.1.2 Thermosets

1.2 Basic Structures

1.2.1 Crystalline

1.2.2 Amorphous

1.2.3 Liquid Crystal Polymer (LCP)

1.2.4 New Polymer Technologies

1.2.4.1 Inherently Conductive Polymers (ICP)

1.2.4.2 Electro-Optic Polymers (EOP)

1.2.4.3 Biopolymers

1.3 Homopolymer vs. Copolymer

1.4 Reinforcements

1.5 Fillers

1.5.1 Glass Spheres

1.5.1.1 Microsphere Properties

1.5.1.2 Compounding

1.5.1.3 Injection Molding

1.5.1.4 Mechanical Properties in Injection-Molded Thermoplastic Applications

1.6 Additives

1.7 Physical Properties

1.7.1 Density and Specific Gravity

1.7.2 Elasticity

1.7.2.1 Case History: Elasticity and Denier

1.7.3 Plasticity

1.7.4 Ductility

1.7.5 Toughness

1.7.6 Brittleness

1.7.7 Notch Sensitivity

1.7.8 Isotropy

1.7.9 Anisotropy

1.7.10 Water Absorption

1.7.11 Mold Shrinkage

1.8 Mechanical Properties

1.8.1 Normal Stress

1.8.2 Normal Strain

1.8.3 Stress-Strain Curve

1.9 Creep

1.9.1 Introduction

1.9.2 Creep Experiments

1.9.3 Creep Curves

1.9.4 Stress-Relaxation

1.10 Impact Properties

1.11 Thermal Properties

1.11.1 Melting Point

1.11.2 Glass Transition Temperature

1.11.3 Heat Deflection Temperature

1.11.4 Coefficient of Thermal Expansion

1.11.5 Thermal Conductivity

1.11.6 Thermal Influence on Mechanical Properties

1.11.7 Case History: Planetary Gear Life Durability

2 Understanding Safety Factors

2.1 What Is a Safety Factor

2.2 Using the Safety Factors

2.2.1 Design Safety Factors

2.2.1.1 Design Static Safety Factor

2.2.1.2 Design Dynamic Safety Factor

2.2.1.3 Design Time-Related Safety Factor

2.2.2 Material Properties Safety Factor

2.2.3 Processing Safety Factors

2.2.4 Operating Condition Safety Factor

3 Strength of Material for Plastics

3.1 Tensile Strength

3.1.1 Proportional Limit

3.1.2 Elastic Stress Limit

3.1.3 Yield Stress

3.1.4 Ultimate Stress

3.2 Compressive Stress

3.3 Shear Stress

3.4 Torsion Stress

3.5 Elongations

3.5.1 Tensile Strain

3.5.2 Compressive Strain

3.5.3 Shear Strain

3.6 True Stress and Strain vs. Engineering Stress and Strain

3.7 Poisson’s Ratio

3.8 Modulus of Elasticity

3.8.1 Young’s Modulus

3.8.2 Tangent Modulus

3.8.3 Secant Modulus

3.8.4 Creep (Apparent) Modulus

3.8.5 Shear Modulus

3.8.6 Flexural Modulus

3.8.7 The Use of Various Moduli

3.9 Stress Relations

3.9.1 Introduction

3.9.2 Experiment

3.9.3 Equivalent Stress

3.9.4 Maximum Normal Stress

3.9.5 Maximum Normal Strain

3.9.6 Maximum Shear Stress

3.9.7 Maximum Deformation Energy

3.10 ABCs of Plastic Part Design

3.10.1 Constant Wall

3.10.2 Fillets

3.10.3 Boss Design

3.10.4 Rib Design

3.10.5 Case History: Ribs

3.11 Conclusions

4 Nonlinear Considerations

4.1 Material Considerations

4.1.1 Linear Material

4.1.2 Nonlinear Materials

4.2 Geometry

4.2.1 Linear Geometry

4.2.2 Nonlinear Geometry

4.3 Finite Element Analysis (FEA)

4.3.1 FEA Method Application

4.3.2 Using FEA Method

4.3.3 Most Common FEA Codes

4.4 Conclusions

5 Welding Techniques for Plastics

5.1 Ultrasonic Welding

5.1.1 Ultrasonic Equipment

5.1.2 Horn Design

5.1.3 Ultrasonic Welding Techniques

5.1.4 Control Methods

5.1.4.1 Common Issues with Welding

5.1.4.2 Joint Design

5.1.4.3 Butt Joint Design

5.1.4.4 Shear Joint Design

5.1.4.5 Torsional Ultrasonic Welding

5.1.4.6 Case History: Welding Dissimilar Polymers

5.2 Ultrasonic (Heat) Staking

5.2.1 Standard Stake Design

5.2.2 Flush Stake Design

5.2.3 Spherical Stake Design

5.2.4 Hollow (Boss) Stake Design

5.2.5 Knurled Stake Design

5.3 Ultrasonic Spot Welding

5.4 Ultrasonic Swaging

5.5 Ultrasonic Stud Welding

5.6 Spin Welding

5.6.1 Process

5.6.2 Equipment

5.6.3 Welding Parameters

5.6.4 Joint Design

5.7 Hot Plate Welding

5.7.1 Process

5.7.2 Joint Design

5.8 Vibration Welding

5.8.1 Process

5.8.2 Equipment

5.8.3 Joint Design

5.8.4 Common Issues with Vibration Welding

5.9 Electromagnetic Welding

5.9.1 Equipment

5.9.2 Process

5.9.3 Joint Design

5.10 Radio Frequency (RF) Welding

5.10.1 Equipment

5.10.2 Process

5.11 Laser Welding

5.11.1 Equipment

5.11.2 Process

5.11.3 Noncontact Welding

5.11.4 Transmission Welding

5.11.5 Intermediate Film & ClearWeld™ Welding

5.11.6 Polymers

5.11.7 Applications

5.12 Conclusion

6 Press Fitting

6.1 Introduction

6.2 Definitions and Notations

6.3 Geometric Definitions

6.4 Safety Factors

6.5 Creep

6.6 Loads

6.7 Press Fit Theory

6.8 Design Algorithm

6.9 Case History: Plastic Shaft and Plastic Hub

6.9.1 Shaft and Hub Made of Different Polymers

6.9.2 Safety Factor Selection

6.9.3 Material Properties

6.9.4 Shaft Material Properties at 23°C

6.9.4.1 Shaft Material Properties at 93°C

6.9.4.2 Creep Curves at 23°C

6.9.4.3 Creep at 93°C

6.9.4.4 Pulley at 23°C

6.9.4.5 Pulley at 93°C

6.9.4.6 Creep, Pulley at 23°C

6.9.4.7 Creep, Pulley at 93°C

6.10 Solutions: Plastic Shaft, Plastic Hub

6.10.1 Case A

6.10.2 Case B

6.10.3 Case C

6.10.4 Case D

6.11 Case History: Metal Ball Bearing and Plastic Hub

6.11.1 Fusible Core Injection Molding

6.11.2 Upper Intake Manifold Background

6.11.3 Design Algorithm

6.11.4 Material Properties

6.11.4.1 CAMPUS

6.11.5 Solution

6.11.5.1 Necessary IF at Ambient Temperature

6.11.5.2 IF Available at 118°C

6.11.5.3 IF Verification at –40°C

6.11.5.4 Verification of Stress Level at –40°C, Time = 0

6.11.5.5 Stress Level at –40°C, Time = 5,000 h

6.11.5.6 Stress Level at 23°C, Time = 5,000 h

6.11.5.7 Stress Level at 118°C, Time = 5,000 h

6.12 Successful Press Fits

6.13 Conclusion

7 Living Hinges

7.1 Introduction

7.2 Classic Design for PP and PE

7.3 Common Living Hinge Design

7.4 Basic Design for Engineering Plastics

7.5 Living Hinge Design Analysis

7.5.1 Elastic Strain Due to Bending

7.5.1.1 Assumptions

7.5.1.2 Geometric Conditions

7.5.1.3 Strain Due to Bending

7.5.1.4 Stress Due to Bending

7.5.1.5 Closing Angle of the Hinge

7.5.1.6 Bending Radius of the Hinge

7.5.2 Plastic Strain Due to Pure Bending

7.5.2.1 Assumptions

7.5.2.2 Strain Due to Bending

7.5.3 Plastic Strain Due to a Mixture of Bending and Tension

7.5.3.1 Tension Strain

7.5.3.2 Bending Strain

7.5.3.3 Neutral Axis Position

7.5.3.4 Hinge Length

7.5.3.5 Elastic Portion of the Hinge Thickness

7.6 Computer Flow Chart

7.6.1 Computer Notations

7.7 Computer Flow Chart Equations

7.8 Example: Case History

7.8.1 World-Class Connector

7.8.1.1 Calculations for the “Right Way” Assembly

7.8.1.2 Calculations for the “Wrong Way” Assembly

7.8.2 Comparison Material

7.8.2.1 “Right Way” Assembly

7.8.2.2 “Wrong Way” Assembly

7.8.3 Ignition Cable Bracket

7.8.3.1 Initial Design

7.8.3.2 Improved Design

7.9 Processing Errors for Living Hinges

7.10 Coined Hinges

7.11 Oil-Can Designs

7.12 Conclusion

7.13 Exercise

8 Snap Fitting

8.1 Introduction

8.2 Material Considerations

8.3 Design Considerations

8.3.1 Safety Factors

8.4 Snap Fit Theory

8.4.1 Notations

8.4.2 Geometric Conditions

8.4.3 Stress/Strain Curve and Formulae

8.4.4 Instantaneous Moment of Inertia

8.4.5 Angle of Deflection

8.4.6 Integral Solution

8.4.7 Equation of Deflection

8.4.8 Integral Solution

8.4.9 Maximum Deflection

8.4.10 Self-Locking Angle

8.5 Case History: One-Way Continuous Beam with Rectangular Cross Section

8.5.1 Geometrical Model

8.6 Annular Snap Fits

8.6.1 Case History: Annular Snap Fit, Rigid Beam with Soft Mating Part

8.6.2 Notations

8.6.3 Geometric Definitions

8.6.4 Material Selections and Properties

8.6.5 Basic Formulas

8.6.6 Angle of Assembly

8.6.7 Case History: Digital Wristwatch

8.7 Torsional Snap Fits

8.7.1 Notations

8.7.2 Basic Formulae

8.7.3 Material Properties

8.7.4 Solution

8.8 Case History: Injection Blow Molded Bottle Assembly

8.9 Tooling

8.10 Case History: Snap Fits That Kill

8.11 Assembly Procedures

8.12 Issues with Snap Fitting

8.13 Serviceability

8.14 Exercise

8.14.1 Solution

8.15 Conclusions

9 Bonding

9.1 Failure Theories

9.2 Surface Energy

9.3 Surface Treatment

9.4 Types of Adhesives

9.5 Advantages and Limitations of Adhesives

9.6 Stress Cracking in Bonded Joints of Adhesives

9.7 Joint Design

9.8 Conclusion

10 In-Mold Assembly

10.1 Overmolding

10.2 In-Mold Assembly

10.3 Joint Design

10.4 Tool Design

10.5 Case Histories: Automotive IMA

10.6 Conclusion

11 Fasteners

11.1 Thread Forming

11.2 Case History: Automotive Undercarriage Splash Shield

11.3 Thread Cutting

11.4 Conclusion

Appendix A: Enforced Displacement

Appendix B: Point Force

Appendix C: Molding Process Data Record

Appendix D: Tool Repair & Inspection Record

References

World Wide Web References Related to Plastic Part Design

About the Author Paul A. Tres

Index