Robert A. Malloy
Plastic Part Design for Injection Molding
An Introduction
Preface
8
Contents
10
1 Introduction
16
1.1 Thermoplastic Materials
16
1.2 Thermosetting Plastic Materials
17
1.3 Structure-Property Relationships
18
1.4 Additives for Plastic Materials
22
1.5 General Characteristics of Plastic Materials
23
References
28
2 Manufacturing Considerations for Injection Molded Parts
30
2.1 Introduction
30
2.2 Mold Filling Considerations
31
2.2.1 Gating Considerations
31
2.2.2 Mold Filling Orientation
36
2.2.3 Mold Filling Pressure Losses
42
2.2.4 Flow Leaders, Flow Restrictors and Flow Hesitation
54
2.3 Weld Lines
63
2.3.1 Introduction
63
2.3.2 Types of Weld Lines
67
2.3.3 Material Considerations
70
2.3.4 Improving Weld Performance and Appearance
73
2.4 Shrinkage and Warpage of Injection Molded Parts
78
2.4.1 Introduction
78
2.4.2 Part Wall Thickness Variations
80
2.4.3 Pressure-Volume-Temperature Behavior of Polymers
84
2.4.4 Linear Mold Shrinkage
87
2.4.5 Anisotropic Shrinkage and Part Distortion/Warpage
91
2.5 Cooling and Solidification
101
2.6 Part Ejection
104
2.6.1 Introduction
104
2.6.2 Draft Angles
105
2.6.3 Effect of Cavity and Core Surface Finish
110
2.6.4 Esthetic Considerations
114
2.6.5 Undercuts and Holes
115
2.6.6 Predicting Part Release Forces
121
2.7 Other Injection Molding Processes
124
2.7.1 Gas Assisted Injection Molding
124
2.7.2 Structural Foam Molding
131
2.7.3 Co-Injection Molding
137
2.7.4 Injection-Compression Molding
139
References
141
3 The Design Process and Material Selection
146
3.1 Introduction
146
3.2 The Plastic Part Design Process
149
3.3 Test Standards for Design Related Plastic Material Properties
157
3.4 Mechanical Behavior of Plastic Materials
158
3.4.1 Introduction
158
3.4.2 Short-Term Stress-Strain Behavior
159
3.4.3 Long Term Mechanical Properties: Creep
167
3.4.4 Long-Term Mechanical Properties: Stress Relaxation
178
3.5 Impact Resistance of Plastic Materials
181
3.6 Fatigue Properties
185
3.7 Thermal Properties of Plastic Materials
186
3.7.1 Thermal Mechanical Behavior
186
3.7.2 Deflection Temperature Under Load and the Vicat Temperature
189
3.7.3 Coefficient of Linear Thermal Expansion
189
3.7.4 Aging at Elevated Temperatures
191
3.7.5 Flammability
192
3.8 Melt Flow Properties
192
3.9 Sources of Plastic Material Property Data
194
3.10 Standardized Plastic Material Designations
198
References
200
4 Structural Design Considerations
202
4.1 Introduction
202
4.2 Design Methodology
202
4.2.1 Design by Experience
203
4.2.2 Design by Experimental Approach
203
4.2.3 Design Using an Analytical Approach
204
4.3 Quantifying the Design Problem
207
4.3.1 Simplification of Part Geometry
208
4.3.2 Stress Concentration
209
4.3.3 Type of Support
212
4.3.4 Loading Conditions
214
4.3.5 Plastic Material Properties
217
4.3.6 Safety Factors
225
4.4 Beams
229
4.4.1 Introduction
229
4.4.2 Properties of a Plane Area (Beam Cross Sections)
231
4.4.3 The Use of Reinforcing Ribs to Improve Stiffness
238
4.4.4 Moment of Inertia for Non-Homogeneous Materials and Structures
246
4.4.5 Sample Beam Analysis
248
4.5 Plates
263
4.5.1 Introduction
263
4.5.2 Sample Plate Problems
265
4.5.3 Plate Elements with Non-Uniform Wall Sections
273
4.6 Shells/Pressure Vessels
275
4.6.1 Introduction
275
4.6.2 Thin-Walled Pressure Vessels
276
4.6.3 Thick-Wall Pressure Vessels
279
4.7 Torsion
280
4.7.1 Introduction
280
4.7.2 Torsion for Circular Bars
280
4.7.3 Torsion for Non-Circular Bars
284
4.8 Columns
287
4.9 Dynamic Loads
289
4.9.1 Introduction
289
4.9.2 Fatigue Loading
289
4.9.3 Impact Loading
295
References
298
5 Prototyping and Experimental Stress Analysis
300
5.1 Prototyping Plastic Parts
300
5.1.1 Introduction
300
5.1.2 Machined and Fabricated Plastic Prototypes
302
5.1.3 Some Rapid Prototyping Technologies
305
5.1.4 Simulating a Production Quality Appearance on Prototype Parts
318
5.1.5 Prototype Part Casting Techniques
321
5.1.6 Prototype Injection Mold Tooling
326
5.1.7 Low Pressure Structural Foam Prototypes
337
5.1.8 Coordinate Measuring Machines
339
5.2 Experimental Stress Analysis
340
5.2.1 Introduction
340
5.2.2 Brittle Coatings
341
5.2.3 Strain Gages
342
5.2.4 Solvent/Chemical Testing
348
5.2.5 Photoelastic Testing
350
5.2.6 Optical Strain Measurement Techniques
352
References
352
6 Assembly of Injection Molded Plastic Parts
356
6.1 Introduction
356
6.2 Press Fit Assemblies
359
6.2.1 Introduction
359
6.2.2 Material Considerations
360
6.2.3 Design of Press Fit Assemblies
362
6.3 Snap Joint Assemblies
367
6.3.1 Introduction
367
6.3.2 Types of Snap Joints
368
6.3.3 Molding Cantilever Snaps
377
6.3.4 Design of Snap Joints
382
6.4 Mechanical Fasteners
387
6.4.1 Introduction
387
6.4.2 Screws
389
6.5 Welding of Thermoplastics
431
6.5.1 Introduction
431
6.5.2 Ultrasonic Welding
432
6.5.3 Vibration Welding
451
6.5.4 Spin (Rotational) Welding
453
6.5.5 Electromagnetic Welding
457
6.5.6 Resistance Welding
459
6.5.7 Hot Tool Welding
459
6.5.8 Hot Gas Welding
461
6.5.9 Extrusion Welding
464
6.5.10 Infrared and Laser Welding
464
6.6 Adhesive Bonding
478
6.6.1 Introduction
478
6.6.2 Adhesive Theory
481
6.6.3 Adhesive Selection
489
6.7 Solvent Bonding
492
References
493
7 Design for Enhanced Recyclability and Sustainability
498
7.1 Plastic Part Design: Recycling Related Issues
498
7.2 Designing Thermoplastic Products with Enhanced Recyclability
500
7.2.1 Design for Existing Recycling Infrastructure
501
7.2.2 Standard Material Identification and Marking Systems
504
7.2.3 Minimize Components and Materials of Construction
506
7.2.4 Multi-Component Product Recycling: Design for Disassembly (Pre-Granulation)
508
7.2.5 Multi-Component Product Recycling: Design for Easy Separation (Post-Granulation)
513
7.2.6 Compatible Materials for Commingled Recycling
516
7.2.7 Use Thermoplastic Formulations that Exhibit Good Property Retention
519
7.2.8 Use General Purpose Thermoplastics --- Minimize the Use of Specialty Additives
522
7.2.9 Use Recycling Friendly Labels and Attachments
524
7.2.10 Avoid Contaminating Surface Coatings
528
7.2.11 Scrapless Manufacturing Processes
530
7.3 Design for Enhanced Recyclability Case Studies
532
7.3.1 Case Study 1: More Recyclable Frozen Juice Concentrate Container
532
7.3.2 Case Study 2: One Time Use Camera is Returnable for Reuse and Recycling
533
7.3.3 Case Study 4: Preserve® Consumer Products from Recycled Plastics Returnable for Secondary Recycling
535
7.4 Using Recycled Thermoplastics for Injection Molded Parts
536
7.4.1 Reuse of Manufacturing Scrap: Regrind Specification and Practices
537
7.4.2 General Properties of Recycled Thermoplastics Relative to Virgin Thermoplastics
541
7.4.3 Recycled Thermoplastic Availability, Quality, and Pricing
544
7.4.4 Product Standards and Agency Considerations
547
References
548
Conversion constants
549
Subject Index
552
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