Plastic Part Design for Injection Molding

Preface

Contents

1 Introduction

1.1 Thermoplastic Materials

1.2 Thermosetting Plastic Materials

1.3 Structure-Property Relationships

1.4 Additives for Plastic Materials

1.5 General Characteristics of Plastic Materials

References

2 Manufacturing Considerations for Injection Molded Parts

2.1 Introduction

2.2 Mold Filling Considerations

2.2.1 Gating Considerations

2.2.2 Mold Filling Orientation

2.2.3 Mold Filling Pressure Losses

2.2.4 Flow Leaders, Flow Restrictors and Flow Hesitation

2.3 Weld Lines

2.3.1 Introduction

2.3.2 Types of Weld Lines

2.3.3 Material Considerations

2.3.4 Improving Weld Performance and Appearance

2.4 Shrinkage and Warpage of Injection Molded Parts

2.4.1 Introduction

2.4.2 Part Wall Thickness Variations

2.4.3 Pressure-Volume-Temperature Behavior of Polymers

2.4.4 Linear Mold Shrinkage

2.4.5 Anisotropic Shrinkage and Part Distortion/Warpage

2.5 Cooling and Solidification

2.6 Part Ejection

2.6.1 Introduction

2.6.2 Draft Angles

2.6.3 Effect of Cavity and Core Surface Finish

2.6.4 Esthetic Considerations

2.6.5 Undercuts and Holes

2.6.6 Predicting Part Release Forces

2.7 Other Injection Molding Processes

2.7.1 Gas Assisted Injection Molding

2.7.2 Structural Foam Molding

2.7.3 Co-Injection Molding

2.7.4 Injection-Compression Molding

References

3 The Design Process and Material Selection

3.1 Introduction

3.2 The Plastic Part Design Process

3.3 Test Standards for Design Related Plastic Material Properties

3.4 Mechanical Behavior of Plastic Materials

3.4.1 Introduction

3.4.2 Short-Term Stress-Strain Behavior

3.4.3 Long Term Mechanical Properties: Creep

3.4.4 Long-Term Mechanical Properties: Stress Relaxation

3.5 Impact Resistance of Plastic Materials

3.6 Fatigue Properties

3.7 Thermal Properties of Plastic Materials

3.7.1 Thermal Mechanical Behavior

3.7.2 Deflection Temperature Under Load and the Vicat Temperature

3.7.3 Coefficient of Linear Thermal Expansion

3.7.4 Aging at Elevated Temperatures

3.7.5 Flammability

3.8 Melt Flow Properties

3.9 Sources of Plastic Material Property Data

3.10 Standardized Plastic Material Designations

References

4 Structural Design Considerations

4.1 Introduction

4.2 Design Methodology

4.2.1 Design by Experience

4.2.2 Design by Experimental Approach

4.2.3 Design Using an Analytical Approach

4.3 Quantifying the Design Problem

4.3.1 Simplification of Part Geometry

4.3.2 Stress Concentration

4.3.3 Type of Support

4.3.4 Loading Conditions

4.3.5 Plastic Material Properties

4.3.6 Safety Factors

4.4 Beams

4.4.1 Introduction

4.4.2 Properties of a Plane Area (Beam Cross Sections)

4.4.3 The Use of Reinforcing Ribs to Improve Stiffness

4.4.4 Moment of Inertia for Non-Homogeneous Materials and Structures

4.4.5 Sample Beam Analysis

4.5 Plates

4.5.1 Introduction

4.5.2 Sample Plate Problems

4.5.3 Plate Elements with Non-Uniform Wall Sections

4.6 Shells/Pressure Vessels

4.6.1 Introduction

4.6.2 Thin-Walled Pressure Vessels

4.6.3 Thick-Wall Pressure Vessels

4.7 Torsion

4.7.1 Introduction

4.7.2 Torsion for Circular Bars

4.7.3 Torsion for Non-Circular Bars

4.8 Columns

4.9 Dynamic Loads

4.9.1 Introduction

4.9.2 Fatigue Loading

4.9.3 Impact Loading

References

5 Prototyping and Experimental Stress Analysis

5.1 Prototyping Plastic Parts

5.1.1 Introduction

5.1.2 Machined and Fabricated Plastic Prototypes

5.1.3 Some Rapid Prototyping Technologies

5.1.4 Simulating a Production Quality Appearance on Prototype Parts

5.1.5 Prototype Part Casting Techniques

5.1.6 Prototype Injection Mold Tooling

5.1.7 Low Pressure Structural Foam Prototypes

5.1.8 Coordinate Measuring Machines

5.2 Experimental Stress Analysis

5.2.1 Introduction

5.2.2 Brittle Coatings

5.2.3 Strain Gages

5.2.4 Solvent/Chemical Testing

5.2.5 Photoelastic Testing

5.2.6 Optical Strain Measurement Techniques

References

6 Assembly of Injection Molded Plastic Parts

6.1 Introduction

6.2 Press Fit Assemblies

6.2.1 Introduction

6.2.2 Material Considerations

6.2.3 Design of Press Fit Assemblies

6.3 Snap Joint Assemblies

6.3.1 Introduction

6.3.2 Types of Snap Joints

6.3.3 Molding Cantilever Snaps

6.3.4 Design of Snap Joints

6.4 Mechanical Fasteners

6.4.1 Introduction

6.4.2 Screws

6.5 Welding of Thermoplastics

6.5.1 Introduction

6.5.2 Ultrasonic Welding

6.5.3 Vibration Welding

6.5.4 Spin (Rotational) Welding

6.5.5 Electromagnetic Welding

6.5.6 Resistance Welding

6.5.7 Hot Tool Welding

6.5.8 Hot Gas Welding

6.5.9 Extrusion Welding

6.5.10 Infrared and Laser Welding

6.6 Adhesive Bonding

6.6.1 Introduction

6.6.2 Adhesive Theory

6.6.3 Adhesive Selection

6.7 Solvent Bonding

References

7 Design for Enhanced Recyclability and Sustainability

7.1 Plastic Part Design: Recycling Related Issues

7.2 Designing Thermoplastic Products with Enhanced Recyclability

7.2.1 Design for Existing Recycling Infrastructure

7.2.2 Standard Material Identification and Marking Systems

7.2.3 Minimize Components and Materials of Construction

7.2.4 Multi-Component Product Recycling: Design for Disassembly (Pre-Granulation)

7.2.5 Multi-Component Product Recycling: Design for Easy Separation (Post-Granulation)

7.2.6 Compatible Materials for Commingled Recycling

7.2.7 Use Thermoplastic Formulations that Exhibit Good Property Retention

7.2.8 Use General Purpose Thermoplastics --- Minimize the Use of Specialty Additives

7.2.9 Use Recycling Friendly Labels and Attachments

7.2.10 Avoid Contaminating Surface Coatings

7.2.11 Scrapless Manufacturing Processes

7.3 Design for Enhanced Recyclability Case Studies

7.3.1 Case Study 1: More Recyclable Frozen Juice Concentrate Container

7.3.2 Case Study 2: One Time Use Camera is Returnable for Reuse and Recycling

7.3.3 Case Study 4: Preserve® Consumer Products from Recycled Plastics Returnable for Secondary Recycling

7.4 Using Recycled Thermoplastics for Injection Molded Parts

7.4.1 Reuse of Manufacturing Scrap: Regrind Specification and Practices

7.4.2 General Properties of Recycled Thermoplastics Relative to Virgin Thermoplastics

7.4.3 Recycled Thermoplastic Availability, Quality, and Pricing

7.4.4 Product Standards and Agency Considerations

References

Conversion constants

Subject Index