James L. Throne Understanding Thermoforming ISBN-10: 3-446-40796-0 ISBN-13: 978-3-446-40796-1 Table of Contents For further information and order see http://www.hanser.de/978-3-446-40796-1 or contact your bookseller.
Contents Foreword to the Second Edition...V 1 Introduction to Thermoforming... 1 1.1 Brief History... 1 1.2 General Markets... 3 1.3 Terminology... 5 1.4 General Characteristics of Thermoformed Products... 6 2 General Forming Concepts... 9 2.1 Heating and Bending... 9 2.2 Simple Heating and Stretching... 11 2.3 One-Step Forming... 11 2.3.1 Drape Forming... 11 2.3.2 Vacuum Forming... 12 2.3.3 Free Forming... 12 2.3.4 Non-Uniform Heating... 13 2.3.5 Matched Mold Forming... 13 2.4 Other One-Step Forming Processes... 14 2.4.1 Autoclave Forming... 14 2.4.2 Diaphragm Forming... 14 2.5 Two-Step Forming... 15 2.5.1 Pneumatic Preforming... 15 2.5.2 Plug Assisted or Mechanical Preforming... 17 2.5.3 Pressure Forming... 19 2.5.4 Coining... 19 2.6 Three-Step Forming... 19 2.7 Twin-Sheet Forming... 20 2.7.1 Heavy-Gauge Twin-Sheet Forming... 20 2.7.2 Light-Gauge Twin-Sheet Forming... 22 2.8 Contact Forming... 24 2.9 Thermoforming as a Portion of the Overall Manufacturing Process... 25 3 Part Design... 27 3.1 Part Design Philosophy... 27 3.1.1 Design Protocol... 28 3.1.2 Project Protocol... 29 3.2 Should This Part Be Thermoformed?... 30 3.3 General Parameters Affecting Part Design... 31 3.3.1 Shrinkage... 31 3.4 General Product Design... 34 3.4.1 Corner versus Chamfer... 35
VIII Contents 3.4.2 Draft Angles... 35 3.4.3 Thermal Expansion... 36 3.4.4 Dimensional Tolerance... 37 3.4.5 Improving Dimensional Tolerance... 39 3.5 Part Surface Quality... 40 3.6 Trim Line Location... 41 3.7 In-Mold Decorating and Labeling... 42 3.8 Seal Designs on Twin-Sheet Thermoformed Parts... 43 3.9 Some Guidelines to Successful Part Design... 44 4 Machinery for the Thick-Gauge Forming Process... 49 4.1 Shuttle Press... 49 4.1.1 Two-Oven Shuttle Press... 50 4.2 Cabinet Press... 51 4.3 Rotary Thermoforming Press... 51 4.4 The Elements of Heavy-Gauge Machinery... 53 4.4.1 Sheet Handling... 54 4.4.2 Sheet Clamping... 54 4.4.3 Sheet Shuttle or Rotation... 54 4.4.4 Oven(s)... 54 4.4.5 The Forming Press... 55 4.4.6 Pneumatic Prestretching... 56 4.4.7 Plug Assist Prestretching... 56 4.4.8 Load/Unload Elements... 57 4.4.9 Vacuum Box and Vacuum System... 57 4.4.10 Pressure Box and Pressurization System... 58 4.4.11 Condition Monitors and Process Control... 59 4.4.12 Safety Elements... 61 5 Machinery for the Light-Gauge Forming Process... 63 5.1 Standard Roll-Fed Machine... 63 5.2 Contact Heater Machines... 65 5.3 Form-Fill-Seal Operation... 65 5.4 Elements of Light-Gauge Machinery... 67 5.4.1 Sheet Take-off or Unwind Station... 67 5.4.2 Pin-Chain and Pin-Chain Rail... 67 5.4.3 Oven(s)... 68 5.4.4 The Forming Press... 69 5.4.5 Plug Assist Prestretching Devices... 71 5.4.6 Trim Means... 71 5.4.7 In-Machine Stacking Means... 72 5.4.8 Vacuum Box and Vacuum System... 72 5.4.9 Pressure Box and Pressurization System... 73 5.4.10 Trim or Web Take-up Station... 74 5.4.11 Condition Monitors and Process Control... 74 5.4.12 Safety Features... 76
Contents IX 6 Machines for Other Applications... 77 6.1 Extrusion-Forming Lines... 77 6.1.1 Advantages of Extrusion-Forming Lines for Heavy-Gauge Forming... 77 6.1.2 Advantages of Extrusion-Forming Lines for Light-Gauge Forming... 78 6.1.3 Disadvantages of Extrusion-Forming Lines for Heavy-Gauge Forming.. 78 6.1.4 Disadvantages of Extrusion-Forming Lines for Light-Gauge Forming... 78 6.1.5 Important Extruder Characteristics in In-Line Forming... 79 6.2 Matched Mold Forming Machines... 79 6.2.1 Foamed Polymer Machines... 80 6.2.2 Composite and Composite Laminate Machines... 80 6.3 Wheel Machines... 81 6.4 Custom Machines... 82 6.5 Twin-Sheet Forming Machines... 83 7 Molds and Mold Design... 85 7.1 Production Mold Materials... 85 7.1.1 Cast Aluminum... 85 7.1.2 Machined Aluminum... 87 7.1.3 Other Production Mold Materials... 87 7.2 Prototype Mold Materials... 88 7.2.1 Hardwoods... 88 7.2.2 Plaster... 88 7.2.3 Medium-Density Fiberboard... 89 7.2.4 Syntactic Foam... 89 7.2.5 Thermoset Plastics... 90 7.2.6 Sprayed Metal... 90 7.3 Mold Design Elements... 90 7.3.1 Cooling... 90 7.3.2 Venting... 91 7.3.3 Undercuts... 92 7.3.4 Mold Surface Texture... 93 7.3.5 Textured Mold or Textured Sheet?... 94 7.4 Plug Assist Materials and Designs... 94 7.5 Other Mold Features... 97 7.5.1 Cavity Isolators or Grids and Perimeter Clamps... 97 7.5.2 Pressure Box... 97 7.5.3 Draw Box... 98 7.5.4 Coining... 98 7.5.5 Web Breakers and Chasers... 98 7.5.6 Rapid Tool Change... 99 7.6 Molds for Matched Mold Forming... 100 7.7 Molds for Twin-Sheet Forming... 100 8 Methods of Heating Sheet... 101 8.1 General Heating Concepts... 101 8.1.1 Conduction... 101
X Contents 8.1.2 Convection... 103 8.1.3 Radiation... 104 8.1.3.1 Radiant Efficiency... 105 8.2 Common Thermoforming Heaters... 106 8.2.1 Convection or Hot Fluid Heating... 106 8.2.1.1 Hot Air Heating... 106 8.2.1.2 Combustion Gas Heating... 107 8.2.1.3 Hot Liquid Heating... 107 8.2.2 Electric Heaters... 107 8.2.2.1 Round or Rod Heaters... 108 8.2.2.2 Flat Panel Heaters... 109 8.2.3 Combustion Heating... 110 8.2.4 Contact Heating... 112 8.3 Selecting the Proper Heater... 113 8.4 A Comparison of Widely Used Heaters... 115 8.5 Heating Cycle Time... 117 8.5.1 Forming Temperature Range... 117 8.5.2 Energy Uptake... 118 8.5.3 Polymer Characteristics... 119 8.5.4 Geometric Factors... 120 8.5.5 The Issue of Sheet Sag... 121 8.5.6 Heating Composites, Laminates, and Other Plastics... 122 8.5.7 Pattern or Zonal Heating... 123 8.5.8 One-Sided versus Two-Sided Heating... 124 8.5.9 Heating Cycle Time Prediction... 126 8.5.10 Equilibration... 126 9 Sheet Stretching and Cooling... 129 9.1 Modulus and Stiffness... 129 9.2 The Concept of Viscoelasticity... 130 9.2.1 Elasticity... 130 9.2.2 Viscosity... 130 9.2.3 Viscoelasticity... 131 9.2.4 Measuring Viscoelastic Properties of Polymers... 132 9.3 The Concepts of Stress and Strain... 133 9.3.1 The Forming Window... 134 9.3.2 Forming Area Diagram... 135 9.4 Prestretching... 136 9.5 Pressure Forming... 137 9.6 The Effect of Sheet Cooling... 138 9.6.1 Forming Area Diagrams for Laminates and Composites... 138 9.6.2 Differential Stretching against a Mold Surface... 138 9.7 Draw Ratios... 139 9.7.1 The Usefulness of Draw Ratios... 140 9.7.2 Wall Thickness Reduction in Laminates... 140 9.8 Part Wall Thickness Prediction... 142
Contents XI 9.8.1 Finite Element Analysis... 142 9.8.2 FEA Data Input... 143 9.8.3 Model Verification... 144 9.9 Sheet Sag and Sag Rate... 145 9.10 Cooling Against a Mold Surface... 146 9.10.1 Light-Gauge Cooling Criteria... 147 9.10.2 Heavy-Gauge Cooling Criteria... 147 9.10.3 Cooling and Residual Stress... 147 9.10.4 Coolant Characteristics... 147 9.10.5 Coolant Flow Rate and Temperature Control... 148 9.11 Cooling Against the Plug... 149 9.12 Heat Removal by Mold and Coolant... 149 9.13 Forming Times... 150 10 Trimming... 153 10.1 The Mechanics of Trimming... 154 10.2 Light-Gauge Trimming... 155 10.2.1 In-Mold Trimming... 155 10.2.2 In-Machine Trimming... 158 10.2.3 In-Line Trimming... 158 10.2.3.1 Canopy or Horizontal Trimming... 159 10.2.3.2 Flatbed or Vertical Trimming... 160 10.2.4 Two-Step Trimming... 160 10.3 Prototype Trimming... 161 10.4 Heavy-Gauge Trimming... 162 10.4.1 In-Plane Trimming... 162 10.4.2 Hand Power Tool Trimming... 164 10.4.3 Milling or 3-Axis Machines... 164 10.4.4 Multi-Axis Routers... 165 10.4.5 Robotic Trimmers... 165 10.5 The Importance of the Trim Fixture... 166 10.5.1 Cutter Design... 166 10.6 Trim Tolerance... 167 10.7 Other Trimming Concepts... 168 10.7.1 Water Jet Cutting... 168 10.7.2 Laser Cutting... 169 10.7.3 Trimming Foam... 169 11 Polymers and Plastics... 171 11.1 Polymer Characterization... 171 11.1.1 Plastic vs. Polymer... 171 11.1.2 Thermoset and Thermoplastic Definitions... 172 11.1.3 Crystalline and Amorphous Definitions... 172 11.1.4 Homopolymers, Copolymers, Terpolymers, and Blends... 173 11.1.5 Additives, Fillers and Reinforcements... 173 11.2 The Thermoforming Window... 175
XII Contents 11.3 Thermoformable Polymers... 176 11.3.1 Polystyrene and Other Styrenics... 176 11.3.2 Polyvinyl Chloride and Other Vinyls... 177 11.3.3 Acrylics... 178 11.3.4 Cellulosics... 179 11.3.5 Polycarbonate... 179 11.3.6 Polyesters... 180 11.3.7 Polyethylene... 182 11.3.8 Polypropylene... 183 11.3.9 Other Polyolefins... 187 11.3.10 Formable Biopolymers... 189 11.3.11 Other Formable Polymers... 191 11.4 Multilayer Polymers... 193 11.5 Foamed Plastics... 194 11.5.1 High-Density Foams... 194 11.5.2 Low-Density Foams... 196 11.6 Thermal Properties... 198 11.6.1 Heat Capacity... 199 11.6.2 Thermal Conductivity... 199 11.6.3 Polymer Density... 200 11.6.4 Thermal Diffusivity... 200 11.6.5 Thermal Coefficient of Expansion... 200 11.6.6 Thermal Properties of Multilayer Structures, and Filled and Reinforced Polymers... 201 11.7 Infrared Energy Absorption for Specific Polymers... 201 12 Issues of Quality Control... 205 12.1 Incoming Sheet Quality... 205 12.1.1 What the Thermoformer Needs to Know About the Extrusion Process... 205 12.1.2 What the Thermoformer Needs to Know About Quality... 210 12.1.3 What the Thermoformer Needs to Know About Regrind... 215 12.2 The Role of the Purchase Order... 217 12.2.1 Incoming Sheet Quality Evaluation... 218 12.3 Production Monitoring... 218 12.3.1 Monitoring Temperature... 218 12.3.2 Sheet Formability... 219 12.3.3 Cutting Surfaces Microfracture Concerns... 221 12.3.4 Finished Part Performance... 221 13 Comparison with Other Technologies [47, 48]... 223 13.1 Classification of Plastics Molding Technologies... 225 13.2 Polymer Material Choices... 226 13.3 Other Processing Concerns... 226
Contents XIII 14 Pragmatic Aspects of Thermoforming... 229 14.1 Safety... 229 14.2 Thermoforming Machine Set-Up... 232 14.2.1 Set-Up for a New Machine... 232 14.2.2 Set-Up for a Used Machine... 233 14.3 Mold Set-Up... 234 14.3.1 New Mold Set-Up... 234 14.3.2 Existing Mold Set-Up... 234 14.4 Trim Set-Up... 235 14.4.1 Heavy-Gauge Trim Set-Up... 235 14.4.2 Light-Gauge Trim Set-Up... 236 14.5 Maintenance... 236 14.5.1 Emergency Maintenance... 236 14.5.2 Preventative Maintenance... 237 14.6 Troubleshooting Tips... 239 14.6.1 Non-Crisis Troubleshooting... 239 14.6.2 The Crisis Situation... 241 References... 243 Recommendations for Further Reading... 247 Books... 247 Other Sources... 247 Historical Reading... 248 Appendix 1: Glossary of Thermoforming Terms... 249 Subject Index... 257