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James L. Throne
Understanding Thermoforming
Foreword to the Second Edition
5
Contents
8
1 Introduction to Thermoforming
16
1.1 Brief History
16
1.2 General Markets
18
1.3 Terminology
20
1.4 General Characteristics of Thermoformed Products
21
2 General Forming Concepts
24
2.1 Heating and Bending
24
2.2 Simple Heating and Stretching
26
2.3 One-Step Forming
26
2.3.1 Drape Forming
26
2.3.2 Vacuum Forming
27
2.3.3 Free Forming
27
2.3.4 Non-Uniform Heating
28
2.3.5 Matched Mold Forming
28
2.4 Other One-Step Forming Processes
29
2.4.1 Autoclave Forming
29
2.4.2 Diaphragm Forming
29
2.5 Two-Step Forming
30
2.5.1 Pneumatic Preforming
30
2.5.2 Plug Assisted or Mechanical Preforming
32
2.5.3 Pressure Forming
34
2.5.4 Coining
34
2.6 Three-Step Forming
34
2.7 Twin-Sheet Forming
35
2.7.1 Heavy-Gauge Twin-Sheet Forming
35
2.7.2 Light-Gauge Twin-Sheet Forming
37
2.8 Contact Forming
39
2.9 Thermoforming as a Portion of the Overall Manufacturing Process
40
3 Part Design
42
3.1 Part Design Philosophy
42
3.1.1 Design Protocol
43
3.1.2 Project Protocol
44
3.2 Should This Part Be Thermoformed?
45
3.3 General Parameters Affecting Part Design
46
3.3.1 Shrinkage
46
3.4 General Product Design
49
3.4.1 Corner versus Chamfer
50
3.4.2 Draft Angles
50
3.4.3 Thermal Expansion
51
3.4.4 Dimensional Tolerance
52
3.4.5 Improving Dimensional Tolerance
54
3.5 Part Surface Quality
55
3.6 Trim Line Location
56
3.7 In-Mold Decorating and Labeling
57
3.8 Seal Designs on Twin-Sheet Thermoformed Parts
58
3.9 Some Guidelines to Successful Part Design
59
4 Machinery for the Thick-Gauge Forming Process
64
4.1 Shuttle Press
64
4.1.1 Two-Oven Shuttle Press
65
4.2 Cabinet Press
66
4.3 Rotary Thermoforming Press
66
4.4 The Elements of Heavy-Gauge Machinery
68
4.4.1 Sheet Handling
69
4.4.2 Sheet Clamping
69
4.4.3 Sheet Shuttle or Rotation
69
4.4.4 Oven(s)
69
4.4.5 The Forming Press
70
4.4.6 Pneumatic Prestretching
71
4.4.7 Plug Assist Prestretching
71
4.4.8 Load/Unload Elements
72
4.4.9 Vacuum Box and Vacuum System
72
4.4.10 Pressure Box and Pressurization System
73
4.4.11 Condition Monitors and Process Control
74
4.4.12 Safety Elements
76
5 Machinery for the Light-Gauge Forming Process
78
5.1 Standard Roll-Fed Machine
78
5.2 Contact Heater Machines
80
5.3 Form-Fill-Seal Operation
80
5.4 Elements of Light-Gauge Machinery
82
5.4.1 Sheet Take-off or Unwind Station
82
5.4.2 Pin-Chain and Pin-Chain Rail
82
5.4.3 Oven(s)
83
5.4.4 The Forming Press
84
5.4.5 Plug Assist Prestretching Devices
86
5.4.6 Trim Means
86
5.4.7 In-Machine Stacking Means
87
5.4.8 Vacuum Box and Vacuum System
87
5.4.9 Pressure Box and Pressurization System
88
5.4.10 Trim or Web Take-up Station
89
5.4.11 Condition Monitors and Process Control
89
5.4.12 Safety Features
91
6 Machines for Other Applications
92
6.1 Extrusion-Forming Lines
92
6.1.1 Advantages of Extrusion-Forming Lines for Heavy-Gauge Forming
92
6.1.2 Advantages of Extrusion-Forming Lines for Light-Gauge Forming
93
6.1.3 Disadvantages of Extrusion-Forming Lines for Heavy-Gauge Forming
93
6.1.4 Disadvantages of Extrusion-Forming Lines for Light-Gauge Forming
93
6.1.5 Important Extruder Characteristics in In-Line Forming
94
6.2 Matched Mold Forming Machines
94
6.2.1 Foamed Polymer Machines
95
6.2.2 Composite and Composite Laminate Machines
95
6.3 Wheel Machines
96
6.4 Custom Machines
97
6.5 Twin-Sheet Forming Machines
98
7 Molds and Mold Design
100
7.1 Production Mold Materials
100
7.1.1 Cast Aluminum
100
7.1.2 Machined Aluminum
102
7.1.3 Other Production Mold Materials
102
7.2 Prototype Mold Materials
103
7.2.1 Hardwoods
103
7.2.2 Plaster
103
7.2.3 Medium-Density Fiberboard
104
7.2.4 Syntactic Foam
104
7.2.5 Thermoset Plastics
105
7.2.6 Sprayed Metal
105
7.3 Mold Design Elements
105
7.3.1 Cooling
105
7.3.2 Venting
106
7.3.3 Undercuts
107
7.3.4 Mold Surface Texture
108
7.3.5 Textured Mold or Textured Sheet?
109
7.4 Plug Assist Materials and Designs
109
7.5 Other Mold Features
112
7.5.1 Cavity Isolators or Grids and Perimeter Clamps
112
7.5.2 Pressure Box
112
7.5.3 Draw Box
113
7.5.4 Coining
113
7.5.5 Web Breakers and Chasers
113
7.5.6 Rapid Tool Change
114
7.6 Molds for Matched Mold Forming
115
7.7 Molds for Twin-Sheet Forming
115
8 Methods of Heating Sheet
116
8.1 General Heating Concepts
116
8.1.1 Conduction
116
8.1.2 Convection
118
8.1.3 Radiation
119
8.2 Common Thermoforming Heaters
121
8.2.1 Convection or Hot Fluid Heating
121
8.2.2 Electric Heaters
122
8.2.3 Combustion Heating
125
8.2.4 Contact Heating
127
8.3 Selecting the Proper Heater
128
8.4 A Comparison of Widely Used Heaters
130
8.5 Heating Cycle Time
132
8.5.1 Forming Temperature Range
132
8.5.2 Energy Uptake
133
8.5.3 Polymer Characteristics
134
8.5.4 Geometric Factors
135
8.5.5 The Issue of Sheet Sag
136
8.5.6 Heating Composites, Laminates, and Other Plastics
137
8.5.7 Pattern or Zonal Heating
138
8.5.8 One-Sided versus Two-Sided Heating
139
8.5.9 Heating Cycle Time Prediction
141
8.5.10 Equilibration
141
9 Sheet Stretching and Cooling
144
9.1 Modulus and Stiffness
144
9.2 The Concept of Viscoelasticity
145
9.2.1 Elasticity
145
9.2.2 Viscosity
145
9.2.3 Viscoelasticity
146
9.2.4 Measuring Viscoelastic Properties of Polymers
147
9.3 The Concepts of Stress and Strain
148
9.3.1 The Forming Window
149
9.3.2 Forming Area Diagram
150
9.4 Prestretching
151
9.5 Pressure Forming
152
9.6 The Effect of Sheet Cooling
153
9.6.1 Forming Area Diagrams for Laminates and Composites
153
9.6.2 Differential Stretching against a Mold Surface
153
9.7 Draw Ratios
154
9.7.1 The Usefulness of Draw Ratios
155
9.7.2 Wall Thickness Reduction in Laminates
155
9.8 Part Wall Thickness Prediction
157
9.8.1 Finite Element Analysis
157
9.8.2 FEA Data Input
158
9.8.3 Model Verification
159
9.9 Sheet Sag and Sag Rate
160
9.10 Cooling Against a Mold Surface
161
9.10.1 Light-Gauge Cooling Criteria
162
9.10.2 Heavy-Gauge Cooling Criteria
162
9.10.3 Cooling and Residual Stress
162
9.10.4 Coolant Characteristics
162
9.10.5 Coolant Flow Rate and Temperature Control
163
9.11 Cooling Against the Plug
164
9.12 Heat Removal by Mold and Coolant
164
9.13 Forming Times
165
10 Trimming
168
10.1 The Mechanics of Trimming
169
10.2 Light-Gauge Trimming
170
10.2.1 In-Mold Trimming
170
10.2.2 In-Machine Trimming
173
10.2.3 In-Line Trimming
173
10.2.4 Two-Step Trimming
175
10.3 Prototype Trimming
176
10.4 Heavy-Gauge Trimming
177
10.4.1 In-Plane Trimming
177
10.4.2 Hand Power Tool Trimming
179
10.4.3 Milling or 3-Axis Machines
179
10.4.4 Multi-Axis Routers
180
10.4.5 Robotic Trimmers
180
10.5 The Importance of the Trim Fixture
181
10.5.1 Cutter Design
181
10.6 Trim Tolerance
182
10.7 Other Trimming Concepts
183
10.7.1 Water Jet Cutting
183
10.7.2 Laser Cutting
184
10.7.3 Trimming Foam
184
11 Polymers and Plastics
186
11.1 Polymer Characterization
186
11.1.1 Plastic vs. Polymer
186
11.1.2 Thermoset and Thermoplastic Definitions
187
11.1.3 Crystalline and Amorphous Definitions
187
11.1.4 Homopolymers, Copolymers, Terpolymers, and Blends
188
11.1.5 Additives, Fillers and Reinforcements
188
11.2 The Thermoforming Window
190
11.3 Thermoformable Polymers
191
11.3.1 Polystyrene and Other Styrenics
191
11.3.2 Polyvinyl Chloride and Other Vinyls
192
11.3.3 Acrylics
193
11.3.4 Cellulosics
194
11.3.5 Polycarbonate
194
11.3.6 Polyesters
195
11.3.7 Polyethylene
197
11.3.8 Polypropylene
198
11.3.9 Other Polyolefins
202
11.3.10 Formable Biopolymers
204
11.3.11 Other Formable Polymers
206
11.4 Multilayer Polymers
208
11.5 Foamed Plastics
209
11.5.1 High-Density Foams
209
11.5.2 Low-Density Foams
211
11.6 Thermal Properties
213
11.6.1 Heat Capacity
214
11.6.2 Thermal Conductivity
214
11.6.3 Polymer Density
215
11.6.4 Thermal Diffusivity
215
11.6.5 Thermal Coefficient of Expansion
215
11.6.6 Thermal Properties of Multilayer Structures, and Filled and Reinforced Polymers
216
11.7 Infrared Energy Absorption for Specific Polymers
216
12 Issues of Quality Control
220
12.1 Incoming Sheet Quality
220
12.1.1 What the Thermoformer Needs to Know About the Extrusion Process
220
12.1.2 What the Thermoformer Needs to Know About Quality
225
12.1.3 What the Thermoformer Needs to Know About Regrind
230
12.2 The Role of the Purchase Order
232
12.2.1 Incoming Sheet Quality Evaluation
233
12.3 Production Monitoring
233
12.3.1 Monitoring Temperature
233
12.3.2 Sheet Formability
234
12.3.3 Cutting Surfaces – Microfracture Concerns
236
12.3.4 Finished Part Performance
236
13 Comparison with Other Technologies
238
13.1 Classification of Plastics Molding Technologies
240
13.2 Polymer Material Choices
241
13.3 Other Processing Concerns
241
14 Pragmatic Aspects of Thermoforming
244
14.1 Safety
244
14.2 Thermoforming Machine Set-Up
247
14.2.1 Set-Up for a New Machine
247
14.2.2 Set-Up for a Used Machine
248
14.3 Mold Set-Up
249
14.3.1 New Mold Set-Up
249
14.3.2 Existing Mold Set-Up
249
14.4 Trim Set-Up
250
14.4.1 Heavy-Gauge Trim Set-Up
250
14.4.2 Light-Gauge Trim Set-Up
251
14.5 Maintenance
251
14.5.1 Emergency Maintenance
251
14.5.2 Preventative Maintenance
252
14.6 Troubleshooting Tips
254
14.6.1 Non-Crisis Troubleshooting
254
14.6.2 The Crisis Situation
256
References
258
Recommendations for Further Reading
262
Books
262
Other Sources
262
Historical Reading
263
Appendix: Glossary of Thermoforming Terms
264
Subject Index
272
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