Thermoforming of Single and Multilayer Laminates

Thermoforming of Single and Multilayer Laminates
Plastic Films Technologies, Testing, and Applications
Syed Ali Ashter
EMD Millipore Corporation
Table of Contents
Dedication
Preface
Acknowledgments

1. Introduction to Thermoforming
   1.1 History
   1.2 Market and Applications
   References
2. The Thermoforming Process
   2.1 Background
   2.2 Basic Principles of Thermoforming
   2.3 Difference between Plastic Sheets and Laminates
   2.4 Theory of Forming Process
   2.5 Forming Characteristics
   2.6 Machinery
   References
3. Review of Characteristics of Common Plastics for Thermoforming
   3.1 Impact of Main Variables
   References
4. Lamination
   4.1 Why Laminates?
   4.2 Elements of Laminates
   4.3 Typical Commercial Laminates
   4.4 Hot-Roll Lamination
   4.5 Extrusion Lamination
   4.6 Flame Lamination
   4.7 Adhesive Lamination
   References
5. New Developments
   5.1 Heating Technology
   5.2 Trimming Technology
   5.3 Thickness Reduction
   5.4 Pressure Forming
   5.5 Vacuum Forming
   5.6 Twin-Sheet Forming
   5.7 Reinforced-Sheet Forming
   5.8 Multilayer Sheet Forming
   5.9 Biaxial Bulge
   5.10 Biaxial Strain
   5.11 Bulge Test Models
   References
6. Mechanics of Materials
   6.1 Stress
   6.2 Strain
   6.3 Stress Relaxation and Creep
   6.4 Creep and Stress Relaxation Models
   6.5 Peeling
   6.6 Delamination
   References
7. Characterization
   7.1 Mechanical Testing
   7.2 Impact Testing
   7.3 Biaxial Bulge Testing
   7.4 Rheological Testing
   7.5 Differential Scanning Calorimetry (DSC)
   7.6 Color Test
   7.7 Specular Gloss Test
   References
8. Matching Material Characteristics to Commercial Thermoforming
   8.1 Packaging
   8.2 Appliances
   8.3 Bathroom
   8.4 Transportation
   8.5 Sports
   References
9. Safety, Recycling and Environmental Issues of Thermoforming and its Products
   9.1 Safety
   9.2 Safety Guards
   9.3 Recycling
   9.4 The Economics of Recycling
   9.5 Handling of Scrap
   9.6 Contamination
   9.7 Environmental Impact
   References
10. Other Processing Approaches
    10.1 Melt Extrusion
    10.2 Coextrusion
    10.3 Calendering
    10.4 Casting
    10.5 Coating
    References
11. Modeling of Thermoforming: A Literature Review
    11.1 Models
    References
12. Troubleshooting
    12.1 Thermoforming
    12.2 Hot-Roll Lamination
    References
    Index
    Index
    Note: Page numbers followed by “f” and “t” refers to figures and tables
    respectively.
    A
    Accidents, causes of
    clamp, 213
    drape mold/plug, 212
    feeding mechanism, 213
    heater unit, 213
    stacker mechanism, 213
    trimming knife, 213
    Acrylics, 54
    56, 207
    208
    Acrylonitrile-butadiene-styrene
    (ABS)
    in household products,
    198
    screw designs for, 239
    Adhesive-coating methods,
    80t
    Adhesive lamination, 76
    80
    dry bonding, 77, 79f
    solventless, 77, 79f
    UV/EB curing, 77
    wet bonding, 77, 79f
    Advance composite process,
    17
    Amorphous polymers, 19
    ARAMIS optical testing system,
    174
    175, 174f
    Areal draw ratio (ADR), 99,
    100t
    ARGUS optical testing system,
    172
    173, 173f, 174f
    Arrhenius equation, 130
    B
    Bathroom enclosures, 203, 204f
    Biaxial bulge testing, 113
    115,
    114f, 169f, 170
    175,
    171f
    Biaxial strain, 115
    116
    Bioplastics, 226
    227
    Blister packs, 196
    Boltzmann superposition
    principle, 133
    134, 134f
    Buckling induced delamination,
    141
    142
    Bulge test models, 116
    120
    boundary value problem,
    solution to, 118
    119
    energy minimization method,
    119
    120
    spherical membrane equations,
    116
    118, 116f
    C
    Cabinet press, 25
    26
    Calendering, 248
    251, 249f
    basic arrangements, 248
    249,
    249f
    components, 249
    250
    Calrod. See Tubular heaters
    Canoes, 207
    208
    Capillary breakup extensional
    rheometer (CaBER), 181,
    182f
    315Carbon footprint, 225
    defined, 226
    227
    measuring, 225
    226
    Casting, 251
    253
    advantages, 251
    252
    film, 252, 252f
    hot-melt, 252, 253f
    materials, 251t
    pressure, 252
    253
    Cellulose nitrate, 4
    Ceramic heaters,
    91
    92
    Charpy impact test, 165
    168,
    166f, 167f
    Clamping mechanism, 36
    38,
    36f, 37f
    Clamshell packaging, 197,
    197f
    Climbing drum peel test, 135,
    136f
    Coating, 253
    259
    methods, 253
    254
    roll. See roll coating
    stages of, 254
    255, 254f
    Coextrusion, 244
    248, 244f
    components of coextruded
    films, 246
    248
    techniques, 245
    246
    feed-block dies, 245, 245f
    multi-manifold external
    combining dies,
    246
    multi-manifold internal
    combining dies,
    245
    246, 246f
    Color test, 187
    Colorimeter, 187f
    Composite and composite
    laminates, 35
    36
    Compression ratio, 235
    Compressive stress, 124f, 125
    Conduction, 85
    87
    Constant-rate-of-extension (CER)
    tension tester, 163
    164,
    163f
    Constitutive equations, 265
    Contact heaters, 33
    34, 33f
    Contact heating, 85
    advantages of, 86
    87
    Contamination, 224
    225
    Contrast gloss, 188
    189
    Convection, 87
    88
    Convection heat transfer
    coefficient, 87t
    Copper-clad laminates, 69
    Creep, 128
    130
    models, 130
    135
    stress-strain response, 129f
    Creep curve, 154
    155, 155f
    Crystallized polyethylene
    terephthalate (CPET),
    200
    201
    D
    Decorative laminates, 15
    16,
    69
    71
    production of, 16
    Deep drawing, 111, 111f
    Delamination, 140
    143, 142f
    buckling induced, 141
    142
    free-edge, 141
    142
    impact-induced, 141
    142
    modes of laminate failure,
    162, 163f
    test, 162
    163
    Diaphragm forming, 111
    112,
    112f
    Differential scanning calorimetry
    (DSC), 183
    186
    cell, 185f
    316 INDEXcurve, 185f
    setup, 184f
    T
    g, determination of,
    185
    186, 186f
    Dip coating, 258, 259f
    Direct-roll coating, 255, 256f
    Displacement, 126
    127
    Drape vacuum forming, 104
    108
    female mold
    billow plug assist, 108
    billow snap back, 107, 107f
    plug assist, 106
    107, 107f
    male mold
    billow plug, 106, 106f
    snap back, 105
    106, 105f
    Drum heaters, 86
    Dry bond lamination, 66
    Dual station machine, 23
    Dynamic mechanical rheological
    testing (DMRT), 177
    180
    loading modes, 178f
    response curve, 179f
    sinusoidal response, 178f
    E
    Elastic limit, 148
    149
    Electrical power tools, 98
    Electronic packaging, 196
    198
    Elongation, 149
    150
    Environmental impact of scrap,
    225
    227
    Extensional rheometry, 180
    183
    Extruders
    single-screw, 230
    238, 233f
    twin-screw, 240
    242
    types of, 232t
    Extrusion-forming lines, 34, 35f
    Extrusion lamination, 65
    66,
    73
    75, 74f
    Extrusion plastometer, 175
    177,
    176f
    F
    Feed-block dies, 245, 245f
    Filament stretching extensional
    rheometer (FiSER), 182
    Film casting, 252, 252f
    Flame lamination, 75
    76, 76f
    Flat-plate heaters, 86
    Flat-strip heaters, 90
    91, 91f
    Flexographic coating, 258
    259
    Fluoropolymers, 58
    62
    Foam polymer machines, 35
    Free-draw vacuum forming, 108,
    108f
    Free-edge delamination, 141
    142
    G
    Gloss, 188
    contrast, 188
    189
    specular, 188
    Glossmeter, 189
    190, 189f
    Gravure coating, 256
    257, 257f
    H
    Hand-held knives, 97
    98, 97f
    Heat-emitting panel heaters,
    93
    94
    Heat lamps, 94
    Heating technology, 84
    94
    Heavy-gauge machinery,
    elements of, 27
    31
    forming press, 29
    load/unload elements, 30
    loading and unloading sheet,
    30
    ovens for, 28
    29
    plug-assist prestretching, 30
    pneumatic prestretching, 29
    sheet clamping, 28
    sheet handling, 27
    28
    vacuum box and vacuum
    systems, 30
    31, 31f
    INDEX 317Height to dimension ratio
    (HDR), 100
    Helix angle, 234
    235
    High-density polyethylene
    (HDPE), 2
    High-impact polystyrene (HIPS),
    2
    Hooke’s law, 148
    Hot creep test, 154
    156
    Hot-melt casting, 252, 253f
    Hot-roll lamination, 71
    73, 72f
    double, 72
    73, 72t, 73f
    troubleshooting, 310
    314
    Hot tensile test, 150
    154
    specimens used, types of,
    151
    152, 152f
    speed of testing, 153
    154,
    154t
    Hot tubs, 204
    205, 205f
    Household packaging, 198
    201
    cups and bowls, 198
    199,
    199f, 204f
    trays and containers,
    199
    201
    Hydroforming, 110
    111, 111f
    Hyperelastic model, 265
    I
    Impact-induced delamination,
    141
    142
    Impact tests, 164
    170
    In-mold lamination, 16
    17
    Infrared flat quartz heaters, 94
    Izod test, 168
    170, 169f
    K
    Kelvin-Voigt model, 130
    133,
    131f
    Keratin, 3
    4
    Kiss coating, 256, 256f
    Knife-like dies, 95
    96
    L
    Laminates, 15
    16, 66
    67, 67f
    commercial, 68
    71
    elements of, 67
    68
    high-pressure, 16
    orientation of, 68f
    types of, 15
    16
    vs. plastic sheets, 15
    17
    Lamination, 65
    adhesive, 76
    80
    extrusion, 65
    66, 73
    75,
    74f
    flame, 75
    76, 76f
    hot-roll, 71
    73, 72f
    Life Cycle Assessment (LCA),
    225, 226f
    Light-gauge forming process,
    31
    32
    Linear draw ratio (LDR), 100
    Luster, 188
    189
    Luster gloss, 188
    189
    M
    Major strain, 115
    Matched-die molding, 109, 110f
    Matched mold-forming
    machines, 34
    36
    Maxwell unit model, 130
    133,
    131f
    Mayer rod coating, 255, 255f
    Mechanical testing, 148
    164
    Medical packaging, 195
    196
    Meissner extensional rheometer,
    182
    183, 183f
    Melt extrusion, 230
    244, 231f
    Melt flow index, 175
    Melt flow rate (MFR), 175
    177
    Minor strain, 115
    Modeling
    draped-formed composite
    laminates, 266
    267
    318 INDEXinfrared heating system,
    267
    268
    plug-assisted thermoforming,
    268
    269
    Modulus, 148
    149
    Multi-manifold external
    combining dies, 246
    Multi-manifold internal
    combining dies,
    245
    246, 246f
    Multilayer extrusion, 74f
    Multilayer sheet forming,
    112
    113, 113f
    Multiple-station machine, 23
    Munstedt extensional rheometer,
    182
    183, 184f
    N
    NEMA industrial laminates, 69
    Normal strain, 127, 127f
    Normal stress, 125
    Nylon 6, 47
    Nylon 6, 6
    screw designs for, 239
    240
    structure of, 47
    Nylon 6, 10, 47
    Nylon 11, 47
    O
    One-step forming, 2
    Open-resistor heaters, 89
    P
    Packaging, 194
    201
    electronic, 196
    198
    household, 198
    201
    cups and bowls, 198
    199
    trays and containers,
    199
    201
    medical, 195
    196
    Peel energy, 137
    138
    Peel force, 137
    138
    Peel strength, 136
    138
    Peel test, 136
    137, 137f,
    157
    161
    45-degree, 158, 158f
    90-degree, 159
    160, 159f
    180-degree, 160
    161, 160f,
    162f
    advantages of, 137
    Peel theory, 138f
    Peeling, 135
    140, 136f,
    157
    158
    adhesive layer thickness, 139,
    139f
    elastic-plastic peeling layer
    thickness, 139
    140, 140f
    rate and temperature, 140,
    141f
    Plastic pallets, 206
    207
    Plastic recycling. See recycling
    Plastic scrap, 218
    Plastic sheets
    heavy gauge, 5, 22
    thin-gauge, 22
    vs. laminates, 15
    17
    Plug-assist prestretching, 30
    Plug-assist vacuum forming, 199,
    199f, 200f
    Plugs, 30
    Poly methyl methacrylate
    (PMMA), 54
    56
    glass transition temperature,
    55
    in hot tubs, 205
    in refrigerator enclosures, 202
    Polyamides, 46
    50
    addition, 57
    condensation, 57
    mechanical properties of, 50,
    51t
    INDEX 319Polyamides (Continued)
    properties of, 49t
    structures of, 46
    47
    Polycarbonates, 50
    53, 70
    chemical resistance, 52
    53
    chemical structure, 50
    decorative laminates, 70
    in household products, 198
    in refrigerator enclosures, 202
    screw designs for, 239
    Polyesters, 45
    46
    decorative laminates, 69
    70
    glass transition temperature,
    46
    structure of, 45
    Polyethylene, 40
    43, 207
    glass transition temperature,
    41
    in packaging, 194
    screw designs for, 239
    structure, 40
    viscosity, 41
    Polyethylene terephthalate
    (PET), 45, 69
    70
    decorative laminates, 69
    70
    glass transition temperature,
    46
    in household products, 198
    Polyimides, 56
    58
    Polymer of
    cyclohexanedimethanol
    and terephthalic acid
    (PCTA), 201
    Polymers, forming characteristics
    of, 19
    21, 20t
    Polyolefins, 40
    45
    Polypropylene, 43
    45, 70
    chemical resistance of, 70
    71
    decorative laminates, 70
    in household products, 198
    structure, 40
    Polystyrene
    decorative laminates, 71
    in household products, 198
    in refrigerator enclosures, 202
    Polytetrafluoroethylene (PTFE),
    58
    60
    chemical resistance, 59
    mechanical properties, 60t
    structure, 58
    Polyvinyl chloride (PVC),
    70
    71
    decorative laminates, 70
    71
    in hot tubs, 205
    in household products, 198
    Poly(vinylidene fluoride)
    (PVDF), 60
    62
    electrical properties of, 62t
    forms, 60
    61
    mechanical properties of,
    61
    62, 62t
    structure, 60
    Pressure casting, 252
    253
    Pressure forming, 100
    103
    free, 101
    102, 101f, 102f
    pressure box forming, 103
    pressure plate or box forming,
    102
    103, 103f
    Pressure gelation process.
    See Pressure casting
    Product life cycle, 226f
    Proportional limit, 148
    149
    PSA films, 68
    Punch and die, 95
    97, 96f, 97f
    Pyrex glass heaters, 92
    Q
    Quartz heaters, 92
    93
    R
    Radiation, 88
    94
    Recycling, 218
    220
    320 INDEXeconomics of, 220
    223
    primary, 219
    secondary, 219
    tertiary, 219
    220
    Refrigerator enclosures, 202
    Reinforced-sheet forming,
    109
    112
    Reverse-roll coating, 257
    258,
    258f
    Rheological testing, 175
    183
    Rheotens, 180
    181, 181f
    Rigid-form-fill seal operation, 34
    Rod heaters, 89
    90
    Roll coating, 255
    259
    dip coating, 258, 259f
    direct-roll coating, 255, 256f
    flexographic coating, 258
    259
    gravure coating, 256
    257,
    257f
    kiss coating, 256, 256f
    Mayer rod coating, 255, 255f
    reverse-roll coating, 257
    258,
    258f
    Roll-fed thermoforming, 22
    Roll forming, 112, 112f
    Rotary thermoforming press,
    26
    27, 26f
    Rubber-die molding, 110, 110f
    S
    Safety, 212
    213
    Safety checks, 216
    218
    maintenance checks, 217
    218
    operational checks, 217
    Safety guards, 213
    218
    cutting/trimming unit, 216
    discharge, 216
    drape plug/clamp, 216
    drape table movement, 215
    heater bank, 215
    hot surfaces, 216
    manually loaded and/or
    unloaded machines,
    214
    215
    material fee, 215
    stacking unit, 216
    Scrap handling, 223
    224
    Screw designs, 238
    240
    acetal copolymer, 238
    239
    acrylonitrile-butadiene-styrene
    (ABS), 239
    Nylon 6/6, 239
    240
    polycarbonate, 239
    polyethylene, 239
    Screw elements, 242
    244, 243f
    Semi-crystalline polymers, 19
    Shear strain, 127f, 128
    Shear stress, 125, 126f
    Sheen, 188
    Shift factor, 128
    129
    Shuttle press, 23
    25, 24f
    double oven, 24
    25, 25f
    Single-flight, two-stage extrusion
    screw, 236, 237f
    Single-screw extruder, 230
    238,
    233f
    extruder screw, 234
    238, 234f
    mixing section designs,
    236
    238, 238f
    single-flight, two-stage
    extrusion screw, 236,
    237f
    vented extruder, 236, 236f
    Single-station machine, 23
    Specular gloss test, 188
    190
    measuring principle of contrast
    gloss, 189f
    sheen measurement system,
    188, 188f
    SPI Designation Code for
    Plastics, 220, 221t,
    222
    223
    INDEX 321Standard roll-fed machines, 33,
    33f
    Steel-rule die, 95, 96f
    Strain, 126
    128
    defined, 117, 148
    normal, 127, 127f
    shear, 127f, 128
    Stress, 124
    126, 124f
    compressive, 124f, 125
    defined, 124
    125, 148
    normal, 125
    shear, 125, 126f
    Stress relaxation, 128
    130,
    156
    157
    models, 130
    135
    stress-strain response, 129f
    Stress-strain curve, 148
    149,
    149f
    Stress-time curve, 156, 157f
    Strip heaters, 86
    Styrofoams, 198
    T
    T-peel test, 135, 136f
    Tensile stress, 124f, 125
    Thermal conductivity, 85
    Thermal diffusivity, 85
    Thermal lamination, 65
    66
    Thermoforming
    advantages of, 1
    events, timeline of, 6t
    history of, 3
    5
    market and applications, 5
    11
    stages, 261
    Thermoforming process
    analysis of, 18f
    background, 14
    15
    basic principles of, 15
    characteristics, 19
    21, 20t
    equipment, 22
    23
    machinery, 21
    38
    choice of equipment, 21
    22
    heavy-gauge, 23
    27
    light-gauge, 31
    32
    plastic sheets vs. laminates,
    15
    17
    theory of, 17
    19
    Thermoforming products
    in appliances, 7, 9f, 201
    202
    in funerals, 8, 10f
    in horticulture, 8
    11, 11f
    in packaging, 7, 8f, 194
    201
    in recreation, 7, 10f, 207
    208
    in transportation, 7, 9f,
    205
    207
    Thermomechanical model, 263
    Thickness reduction, 98
    100,
    99f
    Time
    temperature superposition,
    134
    135
    Transition element, 243f, 244
    Treloar’s equation, 265
    Trimming technology, 95
    98
    Troubleshooting
    adhesive deposited on heat
    rollers, 311
    laminate improperly loaded,
    311
    top and bottom film webs
    not aligned, 311
    blistering, 313
    314
    excessive heat, 313
    high moisture content, 313
    not enough tension, 313
    uneven pressure, 314
    blisters, 281
    282
    excessive moisture, 281
    heat-sensitive material, 282
    heating sheet too fast,
    281
    282
    heating sheet too hot, 281
    uneven heating of sheet, 282
    322 INDEXblotchy look on textured side
    (amorphous materials),
    289
    excessive heat, 289
    high oven shrinkage, 289
    blotchy look on textured side
    (polyolefins), 289
    poor mold contact, 289
    uneven cooling on mold,
    289
    brittle parts, 308
    improper cooling
    temperature on mold, 308
    mold lubricant incompatible
    with plastic, 308
    part cleaned with harmful
    chemical, 308
    plastic gets heated too
    quickly, 308
    bubbling, 312
    feeding sheet too quickly,
    312
    insufficient tension, 312
    trapped air bubbles, 312
    uneven heating, 312
    bumps, 286
    bump on sheet, 286
    contamination in sheet, 286
    incompatible plastic mixed
    within sheet, 286
    loose debris on mold, 286
    water droplets on hot sheet,
    286
    chill marks, 296
    297
    dirt or debris on mold
    surface, 297
    insufficient draft angle on
    part, 296
    mold temperature too low,
    296
    part radius too small, 296
    plug temperature too low,
    296
    sheet too hot, 296
    sheets too hot, 297
    vacuum holes too large, 297
    vacuum rate too high, 297
    wrong forming approach,
    296
    clamp frame too small, 288
    sheet not flat, 288
    curling, 310
    311
    bottom film roll improperly
    loaded, 311
    speed setting too slow, 310
    tension between top and
    bottom film roll is
    unequal, 310
    tension on top and bottom
    film roll is too loose, 310
    difficulty in part removal from
    mold, 301
    302
    air ejection pressure too
    low, 302
    insufficient draft on male
    mold, 301
    male mold temperature too
    cold, 301
    mold surface too rough for
    draft angle, 301
    undercuts on mold too
    severe, 302
    wrong mold material used
    for required draft angle,
    302
    discoloration, 283
    forming temperature too
    hot, 283
    hot spots in forming oven,
    283
    pigment loading too low,
    283
    INDEX 323Troubleshooting (Continued)
    excessive sheet sag, 304
    305
    poor hot strength, 305
    sheet area too large, 305
    sheet too hot, 304
    flow lines on part, 288
    changing melt index within
    resin, 288
    chattering noise, 288
    die lines on back of sheet,
    288
    lines on plastic sheet, 288
    furrows or lines in part, 309
    die lines in plastic, 309
    hot-roll lamination, 310
    314
    loss of vacuum seal, 302
    303
    clamp frames not adjusted
    properly, 303
    clamp frames too cold, 303
    material not holding in
    clamp frames, 303
    seal edge on mold is bad, 302
    orange peel, 313
    excessive heat, 313
    part-to-part inconsistency,
    289
    290
    bad regrind material in
    sheet, 290
    difficulty in forming using
    specific forming
    approach, 290
    uneven heating, 289
    part warpage, 290
    292
    clamp frames too cold, 290
    de-molding temperature too
    high, 291
    gauge variation in finished
    part, 291
    mold too cold, 290
    part sagging due to part
    overheating, 291
    poor mold design, 291
    poor part design, 292
    uneven part cooling, 290
    parts cracks during service
    life, 307
    part too thin in specific
    areas, 307
    parts formed too cold and
    not stress relieved, 307
    parts not assembled
    properly, 307
    poor part design, 307
    poor selection of materials,
    307
    plastic sheet sticks to plug,
    304
    plug temperature too high,
    304
    wrong material for plug, 304
    poor material distribution or/
    and excessive thinning,
    298
    299
    excessive gauge variation,
    299
    excessive sagging, 299
    mold too cold, 299
    uneven heating, 298
    poor part details, 292
    293
    clamping frame too cold,
    292
    hot strength of material too
    high, 293
    incorrect forming technique,
    293
    insufficient vacuum, 292
    part thick in area, 293
    sheet too cold, 292
    poor part surface finish,
    293
    295
    air trapped between mold
    and formed part, 294
    324 INDEXchill marks, 293
    contaminated sheet, 295
    dirt and debris in
    atmosphere, 294
    dirty mold, 294
    dirty sheet, 294
    draft angles too severe, 294
    dust and dirt in atmosphere,
    295
    mold surface creates too
    much drag, 294
    mold surface too rough, 293
    scratched surface on sheet,
    295
    streaks, 295
    voids, 295
    post-molding shrinkage too
    high, 300
    inadequate cooling, 300
    sheet pulls out of clamp frame,
    285
    clamp frames warped, 285
    clamp pressure too low, 285
    high shrinkage, 285
    sheet tearing during forming,
    303
    304
    plastic sheet too hot before
    forming, 303
    poor material distribution,
    304
    poor mold design, 303
    thin-gauge sheet too cold,
    304
    sheet whitening, 306
    sheet stretched beyond
    material yield point, 306
    sheet too cold, 306
    shiny streaks on part, 300
    hot spots on sheet, 300
    sink marks on formed part,
    300
    301
    inadequate vacuum, 301
    mold surface too smooth,
    301
    poor seal edge on mold, 300
    stress whitening or blushing,
    297
    298
    mold too cold, 298
    part too thin, 298
    poor mold design, 298
    sheet too cold, 297
    sheet too hot, 297
    298
    surface of part is not flat, 308
    flat surface of mold not
    reinforced, 308
    tearing during heating and/or
    forming, 309
    310
    grain texture too deep, 310
    high orientation in sheet,
    309
    mixed regrind in sheet, 309
    partially burnt during
    extrusion, 309
    poor hot strength, 309
    texture separation, 287
    deep furrow lines in pattern,
    287
    draw ratio greater than 3:1,
    287
    texture too deep for initial
    part thickness, 287
    texture washout, 287
    high draw ratio, 287
    sheet heated too hot, 287
    thin corners on formed part,
    299
    300
    poor material distribution,
    299
    sheet is too thin
    sheet temperature too high,
    300
    thinning, 282
    283
    INDEX 325Troubleshooting (Continued)
    excessive moisture, 282
    forming technique, 283
    sheet properties, 283
    sheet too thin, 282
    uneven sheet temperature,
    282
    unsatisfactory adhesion of
    laminate, 311
    312
    heat rollers require cleaning,
    312
    insufficient heat, 311
    laminate improperly loaded,
    311
    laminated items
    unsuitable for adhesion,
    312
    speed setting too fast, 311
    variation in sag levels from
    sheet to sheet, 305
    improper use of regrind,
    305
    varying sheet temperature,
    305
    varying billow height or depth,
    305
    306
    air pressure or vacuum
    volume too great, 306
    uneven sheet temperature,
    305
    306
    vacuum or air leaks, 306
    webbing or bridging,
    283
    285
    high sheet sag, 284
    incorrect sheet temperatures,
    284
    insufficient vacuum, 285
    poor mold design or layout,
    283
    284
    timing billow in snapback
    box, 285
    vacuum rate too fast, 284
    wrong forming technique or
    poor design, 284
    wrinkling, 313
    damp material, 313
    not enough roll tension, 313
    Tubular heaters, 89
    90, 90f
    Twin-screw extruder, 240
    242,
    240f
    co-rotating system, 240, 241f
    counter-rotating system,
    241
    242, 241f
    degree of intermeshing, 241f,
    242
    extruder screws, 242, 242f
    Twin-sheet forming, 108
    109,
    109f
    Two-step forming, 2
    3
    U
    Unclad laminates, 69
    Universal testing machine
    (UTM), 150
    151, 151f
    V
    Vacuum forming, 103
    108, 104f
    advantages of, 2
    drape, 104
    108
    free-draw, 108
    Vented extruder, 236, 236f
    Visual gloss, 188
    189
    W
    Wet bond lamination, 66
    Wheel machines, 36
    William-Landel-Ferry (WLF)
    equation, 128
    129
    Y
    Yield point, 148
    149
    Yield strength, 148
    149
    Young’s modulus, 148

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