Design of Injection Molded Plastic Parts

by Christoph Jaroschek

Overview

This book creates a new perspective on the design of plastic parts. In many books there is a strong focus on the material, the material properties, and the calculation or dimensioning. What is often not taken into account is that very many plastic components only have to withstand low loads; in very many applications, the focus is on the actual design. This requires a good understanding of the injection molds that must be built to produce the plastic components. Depending on the design of the injection molded component, these molds become more complex and more prone to failure during production.

The complex process of manufacturing a plastic part becomes holistically understandable as a link is created between the molder, the mold maker, and the part designer. The focus is on injection molds and therefore on thermoplastics. 

Everything that is necessary for the design and manufacture of an injection molded component is presented in a simple, extremely practical manner and limited to the essentials. Many descriptive pictures as well as examples based on the demonstration component "Polyman" facilitate the understanding enormously.

Product Details

ISBN-13:	9781569908938
Publisher:	Hanser Publications
Publication date:	07/28/2022
Pages:	224
Product dimensions:	7.60(w) x 9.49(h) x 0.67(d)
Age Range:	18 Years

About the Author

Christoph Jaroschek is Professor in Mechanical Engineering at the Bielefeld University of Applied Sciences, Germany. Previously, he was for 10 years Head of Application Technology and Process Development at a well-known manufacturer of injection molding machines.

Table of Contents

The Author v

Preface vii

1 Plastic Parts 1

1.1 General Information 1

1.1.1 Comparison of Designs (Conventional vs. Plastic) 2

1.1.2 Special Features of Plastics 4

1.1.2.1 Comparison of the Properties of Plastics and Metals 5

1.1.2.2 Special Mechanical Behavior 6

1.1.3 Reasons for Using Plastics 11

1.2 Design Rules 14

1.2.1 Special Design Features of Injection Molded Parts 16

1.2.1.1 Demoldability 16

1.2.1.2 Flow Path to Wall Thickness Ratio 22

1.2.1.3 Sprue Position 23

1.2.1.4 Avoiding Material Accumulation, Thin Wall Thickness 24

1.2.1.5 Stiffeners 25

1.2.1.6 Dimensional Change due to Temperature Fluctuations 28

1.3 Dimensional Deviations between CAD and Injection Molded Part 28

1.3.1 Shrinkage 28

1.3.2 Warpage 32

1.3.3 Corrective Measures for Dimensional Deviations 33

1.4 Design of Connections 37

1.4.1 Screw Fasteners 38

1.4.2 Snap-Fit Connections 40

1.4.3 Bonding and Welding of Seams 42

1.4.3.1 Adhesive-Bonded Joints 42

1.4.3.2 Welded Joints 44

1.4.3.3 Film Hinges 47

1.5 Tolerances and Dimensions 49

1.6 Sizing 54

2 The Injection Molding Manufacturing Process 57

2.1 The Process and What the Designer Should Know 57

2.1.1 Flow Path Lengths Are Limited 58

2.1.2 Molded Part Area Determines Machine Size 60

2.1.3 Wall Thicknesses Determine the Cooling Time 61

2.1.4 Plastic Shrinks as It Cools 62

2.2 Influence of the Process on Component Properties 63

2.2.1 Weld Lines, Meld Lines 64

2.2.2 Surface Quality 65

2.3 Fiber Orientations Influence the Component Dimensions 67

2.4 Forward-Looking Quality Assurance 69

2.4.1 Sink Marks 69

2.4.2 Jetting 70

2.4.3 Diesel Effect 71

2.4.4 Incomplete Filling, Burr Formation, and Deformation during Demolding 72

2.5 Special Injection Molding Techniques 73

2.5.1 Multi-Component Technology 74

2.5.1.1 General Procedure 75

2.5.1.2 Molding Techniques 76

2.5.1.3 Component Design 79

2.5.2 Fluid Injection Technology (FIT) 85

2.5.2.1 Processes 87

2.5.2.2 Component Design 91

3 Molds 97

3.1 General Tasks and Functions 98

3.2 Manufacture and Costs 100

3.2.1 General Machining 101

3.2.2 Surfaces 103

3.2.2.1 EDM - Electrical Discharge Machining 104

3.2.2.2 Etching 105

3.2.2.3 Laser Texturing 106

3.2.2.4 Ceramic Surfaces 107

3.2.3 Steels 108

3.3 Standard Elements 112

3.4 Melt Feed 116

3.4.1 Cold Runners 118

3.4.1.1 Cavity Layout 119

3.4.1.2 Gating to Cavities 121

3.4.1.3 Demolding of the Runner System 124

3.4.2 Mold with Pre-Chamber Nozzle 126

3.4.3 Insulating Channels 127

3.4.4 Hot Runners 129

3.4.4.1 Internally Heated Systems 131

3.4.4.2 Externally Heated Systems 131

3.4.4.3 Hot Runner Nozzles 132

3.4.4.4 Cascade Technology 135

3.5 Temperature Control 136

3.5.1 Concepts for Temperature Control 141

3.5.1.1 Continuous Flow Temperature Control 141

3.5.1.2 Pulse Cooling/Discontinuous Temperature Control 143

3.5.1.3 Variothermal or Intermittent Temperature Control 143

3.5.2 Implementation 145

3.6 Demolding 149

3.6.1 Straight-Line Demolding in the Axial Direction of the Opening Movement 150

3.6.2 Demolding of Contour Areas That Are Not Parallel with the Opening Movement 154

3.6.3 Demolding of Internal Undercuts 156

3.6.4 Demolding of Internal Threads 158

3.7 Increasing Efficiency with Two Parting Planes 159

3.7.1 Stack Molds 160

3.7.2 Tandem Molds 161

3.7.3 Design Features of Stack and Tandem Molds 165

3.7.4 Hot Runner Technology for Stack and Tandem Molds 168

4 Simulation 171

4.1 Goals of Simulation 173

4.1.1 Filling Simulation (Rheological Simulation) for Good Surfaces 173

4.1.2 Warpage Prediction 175

4.1.3 Heat-Flux Analysis 177

4.1.4 Calculation of Mechanical Stability (Structural Mechanics) 178

4.2 Base Models for the Rheological Simulation 178

4.2.1 Shape Models 179

4.2.2 Calculation Models 183

4.2.3 Material Models 185

4.3 Examples and Calculation Results 186

4.3.1 Filling Behavior 186

4.3.2 Holding Pressure Phase 189

4.3.3 Warpage 191

5 Material Selection 193

5.1 Usual Procedure for Selecting Materials 193

5.1.1 Selection Criterion: Temperature 194

5.1.2 Selection Criterion: Chemical Load 195

5.1.3 Selection Criterion: Mechanical Load 195

5.1.4 Selection Criterion: Special Requirement 197

5.1.5 Databases 198

5.2 Important Characteristic Values 202

5.2.1 Characteristic Temperatures 202

5.2.1.1 Glass Transition Temperature 202

5.2.1.2 Melting Temperature 203

5.2.1.3 Degradation Temperature 203

5.2.2 Heat Deflection Temperature 204

5.2.3 Continuous Service Temperature 206

5.2.4 Young's Modulus and Creep Modulus 208

5.2.5 Temperature Function of Young's Modulus 212

5.3 Limits on Mechanical Design 214

5.3.1 Short-Term Loads 214

5.3.2 Long-Term Loads 215

5.3.3 Estimation of Design Limits Using Reduction Factors 216

Index 219