Technology of Thermoforming

Front Matter......Page 1
Preface......Page 3
Table of Contents......Page 0
78120_00toc......Page 6
1. Thermoforming - Definitions, History, Methods and Equipment......Page 20
1.1 Introduction......Page 21
1.2 History......Page 22
1.3 Markets......Page 23
1.4 Some Definitions......Page 30
Clamping of Thin-Gage Sheet......Page 31
Heating of Thin-Gage Sheet......Page 32
Shaping Heavy-Gage Sheet......Page 33
Depth-of-Draw......Page 34
1.5 Methods of Forming......Page 35
One-Step Forming......Page 36
Two-Step Forming with Prestretching......Page 38
Multi-Step Forming......Page 41
Other Variations......Page 44
Heating Source......Page 49
Forming Platform......Page 51
Vacuum System......Page 52
Process Control......Page 56
1.7 Heavy-Gage Thermoforming Machinery Specifics......Page 58
1.8 Thin-Gage Thermoforming Machinery Specifics......Page 66
1.9 References......Page 72
2. Polymeric Materials......Page 74
2.2 Network Nature of Polymers......Page 75
2.3 Addition and Condensation Polymerization......Page 77
2.5 Molecular Weight and Molecular Weight Distribution......Page 78
2.6 Molecular Weight and Properties......Page 81
2.7 Morphology and Properties......Page 83
2.9 Chain Mobility and Polymer Stiffness......Page 90
2.11 Gas Permeation......Page 92
2.12 Copolymerization......Page 93
Plasticizers......Page 94
Other Additives......Page 96
Fillers and Reinforcing Fibers......Page 97
2.16 Stress-Strain Behavior of Plastics......Page 98
Heat Capacity......Page 102
Thermal Diffusivity......Page 104
2.18 Infrared Spectra......Page 107
2.19 Summary......Page 116
2.20 References......Page 123
3. Heating the Sheet......Page 125
3.2 Energy Absorption by Sheet......Page 126
3.3 Heat Transfer Modes......Page 130
3.4 Incorporating Formability and Time-Dependent Heating......Page 135
3.5 Conduction......Page 141
3.6 Convection Heat Transfer Coefficient......Page 144
The Biot Number......Page 145
Effective Radiation Heat Transfer Coefficient......Page 146
Constant Heat Flux......Page 147
3.7 Radiation Heating......Page 148
Black Body Radiation......Page 149
Gray Body - Emissivity......Page 154
Radiant Heater Efficiency - Constant Heat Flux Application......Page 158
3.8 Real Heaters - Efficiencies......Page 161
Radiative Heat Transfer Coefficient......Page 165
Convection and the Heat Transfer Coefficient......Page 166
Rod Heaters......Page 171
3.9 Long-Term Radiant Heater Efficiencies......Page 172
3.10 Edge Losses - View Factor......Page 173
Local Energy Input......Page 176
Pattern Heating......Page 180
Zone, Zoned or Zonal Heating......Page 182
Heater to Sheet Distance......Page 183
Constant Environmental Temperature Approximation......Page 185
Thin-Gage Approximations - Comments......Page 188
Constant Environmental Temperature......Page 190
The Constant Heat Flux Case......Page 194
The Thickness Effect......Page 196
Summary......Page 197
3.13 Equilibration......Page 198
Convection Heating......Page 199
Constant Heat Flux......Page 201
Computed Equilibration Times......Page 202
The W-L-F Equation......Page 203
3.14 Infrared-Transparent Polymers......Page 204
3.15 Computer-Aided Prediction of Sheet Temperature......Page 210
3.16 Guidelines for Determining Heating Cycles......Page 214
Heavy-Gage Guidelines......Page 215
3.17 References......Page 216
4. Stretching the Sheet......Page 219
4.1 Introduction......Page 220
4.2 The Stretching Concept......Page 224
Standard Tensile Tests......Page 230
Hot Tensile Tests......Page 234
Hot Creep Tests......Page 235
Other Stretching Tests......Page 237
Temperature-Dependent Viscosity for Amorphous Polymers......Page 238
Dynamic Mechanical Testing......Page 244
4.4 Stress-Strain-Rate of Strain - Theory......Page 248
Elasticity - a Rationalization......Page 255
Strain Energy Function......Page 257
The Rivlin Form for the Strain Energy Function......Page 258
The Ogden Form for the Strain Energy Function......Page 261
Viscoelastic Models......Page 262
4.5 Available Stress-Strain Data......Page 265
Sensitivity of Models......Page 270
4.6 The Importance of Polymer Material Properties......Page 271
4.7 Practical Aspects of Stretching......Page 280
Funnel Test......Page 283
4.8 Bursting Conditions......Page 287
4.9 Sheet Sag......Page 289
Initial Sag......Page 290
Tensile Sag......Page 291
The Catenary Sag......Page 292
Parabolic Sag......Page 293
Relating Sag to Hot Sheet Strength......Page 294
Sag - a Comment......Page 299
4.10 References......Page 300
Appendix 4.I: Biaxial Stretching of an Elastic Membrane......Page 305
5. Cooling and Trimming the Part......Page 307
5.2 Overall Cooling Heat Balance......Page 308
5.3 Cooling the Formed Shape......Page 310
5.4 Steady State Heat Balance......Page 311
Interfacial Resistance......Page 312
Shape Factor......Page 314
Convection Heat Transfer Coefficient......Page 316
5.5 Cyclic Heat Balance......Page 325
Cooling Thin Sheet in Ambient Air......Page 326
Quiescent Ambient Air......Page 328
Moving Ambient Air......Page 330
Cooling on Nonmetallic Molds......Page 332
5.6 Transient Heat Transfer during Sheet Cooling on the Mold Surface - Computer Models......Page 338
Interfacial Air......Page 342
5.7 Shrinkage......Page 344
Unconstrained Shrinkage......Page 345
Constrained Shrinkage......Page 349
5.8 Trimming......Page 353
Trimming Heavy-Gage Parts......Page 355
Trimming Thin-Gage Parts......Page 357
5.9 Mechanics of Cutting......Page 358
The Trim Region......Page 360
Registering the Trim Site......Page 361
The Nature of the Cut......Page 362
Mechanical Chipping......Page 364
Multiple-Edged Tool or Toothed Saw Performance......Page 366
Abrasive Cut-Off Wheel......Page 368
Fracture Mechanics in Trimming......Page 371
Nibbling......Page 379
Brittleness, Orientation and Trim Temperature......Page 382
5.10 Steel Rule Die......Page 384
Resharpening......Page 387
Tabbing and Notching......Page 388
5.11 Punch and Die Trimming......Page 389
5.12 Drilling......Page 392
Thermal Cutting......Page 397
Water Jet Cutting......Page 399
5.14 Trimming - a Summary......Page 401
5.15 References......Page 404
6. Thermoforming Molds......Page 407
6.1 Introduction......Page 408
6.2 Prototype Molds......Page 409
Wood......Page 410
Fiberboard......Page 411
Plaster......Page 412
Plastic......Page 414
White Metal......Page 422
6.3 Production Molds......Page 425
Aluminum......Page 426
Steel......Page 427
Other Metals......Page 428
6.4 Mold Coolant Channels......Page 429
Mold Channel Flow......Page 430
Sharp-Edged Orifice......Page 431
6.5 Vent Holes......Page 437
Sizing Vacuum Systems - Steady State......Page 439
Sizing Vacuum Systems - Dynamic......Page 440
Solenoid Valve Flow Resistance......Page 442
Vent Hole Resistance to Flow......Page 443
Vent Hole Diameter......Page 449
Other Types of Vents......Page 453
Vent Hole Placement......Page 456
6.6 Surface Treatments......Page 459
Surface Texture......Page 462
Plug Materials......Page 465
Metal Plugs......Page 467
Plug Design Concepts......Page 471
6.8 Sheet Clamping......Page 474
6.10 Other Aspects of Mold Design......Page 477
Undercuts......Page 478
Encapsulation......Page 479
Moving Elements......Page 480
Stripper Plates/Bars......Page 481
Mold Releases......Page 482
Web Breakers, Catchers and Chasers......Page 483
Moats, Dams and Double Steps......Page 484
Chamfers and Radii......Page 485
Heavy-Gage Sheet......Page 487
Thin-Gage Sheet......Page 489
6.12 References......Page 493
7. Parts Design......Page 496
7.2 Elements of Parts Design......Page 497
Material Testing and Its Relevance to Part Performance......Page 499
Philosophy of Parts Design......Page 502
Minimizing the Amount of Sheet to Be Reground......Page 504
Rules for Part Layout on Heavy-Gage......Page 505
Rules for Multiple Part Layout on Thin-Gage......Page 507
Economics of Buying Sheet of Specific Size......Page 509
7.3 Prototyping as a Justification for Thermoformmg......Page 512
7.4 Draw Ratio......Page 514
Areal Draw Ratio......Page 515
Linear Draw Ratio......Page 520
H:D......Page 522
Rim and Lip Sheet for Female Cavities......Page 525
Preblowing or Inflation - Comments......Page 529
Plug Assist - Comments......Page 533
7.5 Computer-Aided Design in Thermoforming......Page 534
7.6 Wall Thickness Prediction - a Justification......Page 537
Geometric Element Analysis or GEA......Page 540
Finite Element Analysis......Page 548
General Comments on Plug Design......Page 554
Plug Assist Analysis......Page 560
Plug Design - Geometric Element Analysis......Page 563
Plug Design - Finite Element Analysis......Page 566
7.7 Regrind......Page 572
Material Property Deterioration on Regrind......Page 573
Property Value Loss - Experiment and Protocol......Page 577
Cascading 100% Regrind......Page 581
General Tips......Page 583
Process Tips......Page 584
Mold Tips......Page 585
Prestretch Tips......Page 587
Part Design Tips......Page 589
Rim and Edge Designs......Page 597
Design - a Comment......Page 601
7.9 References......Page 602
Appendix 7.I: Draw Ratios for Truncated Cone......Page 605
Appendix 7.II: Mechanical Property Loss in Regrind......Page 607
8. Producing Sheet and Film......Page 610
8.1 Introduction......Page 611
8.2 Forming Thin Films......Page 612
8.3 Forming Sheet......Page 615
Single-Screw Extrusion......Page 616
Filtering the Polymer......Page 619
Flow Improvement Devices......Page 622
Pressure and Temperature in an Extruder......Page 624
Sheet Die Concepts......Page 626
Gage Thickness Monitoring and Control......Page 630
Twin-Screw Extrusion......Page 632
8.4 Roll Stacks......Page 635
8.5 Sheet Trimming......Page 638
8.6 Take-Off and Take-Up Rolls......Page 639
8.7 Residence Time and Residence Time Distribution through Extruder and Die......Page 643
8.8 Drying......Page 646
8.9 Producing Biaxially Oriented Sheet......Page 649
8.10 Multilayer Sheet Formation......Page 652
Coextrusion......Page 653
Lamination......Page 657
8.11 Sheet Quality and Quality Control......Page 661
Orientation......Page 666
Sheet Squareness and Flatness......Page 668
Moisture......Page 669
Sheet Appearance......Page 670
Annoyance Factors......Page 671
Lamination......Page 672
8.12 References......Page 673
9. Newer Thermoforming Technologies......Page 677
9.2 Thermoforming Crystallizing Polyethylene Terephthalate......Page 678
PET Crystallinity......Page 679
Characterizing Polyethylene Terephthalate......Page 681
The Effect of Temperature on Crystallization during Sheet Extrusion......Page 688
Cooling CPET on Chill Rolls......Page 691
Forming the Sheet......Page 695
Cooling the Formed Part......Page 697
9.3 Pressure Forming......Page 699
Thin Gage......Page 704
Heavy Gage......Page 706
9.4 Forming Filled and Reinforced Polymers......Page 709
Heating Multilayer Sheet......Page 717
Forming Multilayer Sheet......Page 721
9.6 Twin-Sheet Thermoforming......Page 726
Simultaneous Twin-Sheet Forming......Page 731
Sequential Twin-Sheet Forming......Page 732
Seal Area - Adhesion......Page 734
Seal Area - Compressive Form......Page 736
Seal Area - Design......Page 737
9.7 Polypropylene Thermoforming......Page 740
Sag Test......Page 741
Modified Polypropylenes......Page 747
Cell Architecture - Actual v. Ideal......Page 752
Radiant Energy Transmission......Page 756
Internal Cell Gas Pressure......Page 761
Forming Window for Foam......Page 763
The Forming Equipment......Page 766
Interdigitation......Page 768
Sealed Air Cushion/Dunnage......Page 770
9.10 References......Page 771
10. Set-Up Protocols, Troubleshooting, and the Economics of Thermoforming......Page 778
10.1 Introduction......Page 779
A New Polymer......Page 783
Setting up a New Mold......Page 788
Dry-Cycling the Mold......Page 791
Checking the Vacuum......Page 792
Attaching Cooling to the Mold......Page 793
The Sheet Delivery System......Page 794
Setting the Oven Conditions......Page 795
Forming Step - Simple Vacuum Forming......Page 796
Activating the Assists......Page 797
General Objectives......Page 798
10.3 Troubleshooting the Forming Process......Page 799
The Energy Audit......Page 814
Cost of Extrusion......Page 822
Competitive Costs of Polymers......Page 824
10.5 General Processing Economics......Page 825
Rules of Thumb......Page 827
Global Production Costs......Page 829
Manufacturing Efficiencies......Page 832
The Learning Curve......Page 837
10.6 Isolated Venture Costs......Page 839
10.7 New Venture Economics......Page 850
Entrepreneurial Risks......Page 856
10.8 The Incremental Operation......Page 862
10.9 Comparative Process Economics......Page 864
10.10 References......Page 874
Appendix A: Abbreviations for Thermoformable Polymers Referred to in Text......Page 875
Appendix B: Typical Conversion Factors Used in Thermoforming (US Customary to Metric, Metric to US Customary)......Page 878
Appendix C: Glossary of Thermoforming Terms......Page 880
Author Index......Page 886
A......Page 893
B......Page 894
C......Page 896
D......Page 902
E......Page 905
F......Page 907
G......Page 913
H......Page 914
I......Page 917
L......Page 918
M......Page 919
N......Page 923
O......Page 924
P......Page 925
Q......Page 936
R......Page 937
S......Page 939
T......Page 945
V......Page 950
W......Page 952
Z......Page 953