Understanding Thermoforming

Transcription

1 James L. Throne Understanding Thermoforming ISBN-10: ISBN-13: Table of Contents For further information and order see or contact your bookseller.

2 Contents Foreword to the Second Edition...V 1 Introduction to Thermoforming Brief History General Markets Terminology General Characteristics of Thermoformed Products General Forming Concepts Heating and Bending Simple Heating and Stretching One-Step Forming Drape Forming Vacuum Forming Free Forming Non-Uniform Heating Matched Mold Forming Other One-Step Forming Processes Autoclave Forming Diaphragm Forming Two-Step Forming Pneumatic Preforming Plug Assisted or Mechanical Preforming Pressure Forming Coining Three-Step Forming Twin-Sheet Forming Heavy-Gauge Twin-Sheet Forming Light-Gauge Twin-Sheet Forming Contact Forming Thermoforming as a Portion of the Overall Manufacturing Process Part Design Part Design Philosophy Design Protocol Project Protocol Should This Part Be Thermoformed? General Parameters Affecting Part Design Shrinkage General Product Design Corner versus Chamfer... 35

3 VIII Contents Draft Angles Thermal Expansion Dimensional Tolerance Improving Dimensional Tolerance Part Surface Quality Trim Line Location In-Mold Decorating and Labeling Seal Designs on Twin-Sheet Thermoformed Parts Some Guidelines to Successful Part Design Machinery for the Thick-Gauge Forming Process Shuttle Press Two-Oven Shuttle Press Cabinet Press Rotary Thermoforming Press The Elements of Heavy-Gauge Machinery Sheet Handling Sheet Clamping Sheet Shuttle or Rotation Oven(s) The Forming Press Pneumatic Prestretching Plug Assist Prestretching Load/Unload Elements Vacuum Box and Vacuum System Pressure Box and Pressurization System Condition Monitors and Process Control Safety Elements Machinery for the Light-Gauge Forming Process Standard Roll-Fed Machine Contact Heater Machines Form-Fill-Seal Operation Elements of Light-Gauge Machinery Sheet Take-off or Unwind Station Pin-Chain and Pin-Chain Rail Oven(s) The Forming Press Plug Assist Prestretching Devices Trim Means In-Machine Stacking Means Vacuum Box and Vacuum System Pressure Box and Pressurization System Trim or Web Take-up Station Condition Monitors and Process Control Safety Features... 76

4 Contents IX 6 Machines for Other Applications Extrusion-Forming Lines Advantages of Extrusion-Forming Lines for Heavy-Gauge Forming Advantages of Extrusion-Forming Lines for Light-Gauge Forming Disadvantages of Extrusion-Forming Lines for Heavy-Gauge Forming Disadvantages of Extrusion-Forming Lines for Light-Gauge Forming Important Extruder Characteristics in In-Line Forming Matched Mold Forming Machines Foamed Polymer Machines Composite and Composite Laminate Machines Wheel Machines Custom Machines Twin-Sheet Forming Machines Molds and Mold Design Production Mold Materials Cast Aluminum Machined Aluminum Other Production Mold Materials Prototype Mold Materials Hardwoods Plaster Medium-Density Fiberboard Syntactic Foam Thermoset Plastics Sprayed Metal Mold Design Elements Cooling Venting Undercuts Mold Surface Texture Textured Mold or Textured Sheet? Plug Assist Materials and Designs Other Mold Features Cavity Isolators or Grids and Perimeter Clamps Pressure Box Draw Box Coining Web Breakers and Chasers Rapid Tool Change Molds for Matched Mold Forming Molds for Twin-Sheet Forming Methods of Heating Sheet General Heating Concepts Conduction

5 X Contents Convection Radiation Radiant Efficiency Common Thermoforming Heaters Convection or Hot Fluid Heating Hot Air Heating Combustion Gas Heating Hot Liquid Heating Electric Heaters Round or Rod Heaters Flat Panel Heaters Combustion Heating Contact Heating Selecting the Proper Heater A Comparison of Widely Used Heaters Heating Cycle Time Forming Temperature Range Energy Uptake Polymer Characteristics Geometric Factors The Issue of Sheet Sag Heating Composites, Laminates, and Other Plastics Pattern or Zonal Heating One-Sided versus Two-Sided Heating Heating Cycle Time Prediction Equilibration Sheet Stretching and Cooling Modulus and Stiffness The Concept of Viscoelasticity Elasticity Viscosity Viscoelasticity Measuring Viscoelastic Properties of Polymers The Concepts of Stress and Strain The Forming Window Forming Area Diagram Prestretching Pressure Forming The Effect of Sheet Cooling Forming Area Diagrams for Laminates and Composites Differential Stretching against a Mold Surface Draw Ratios The Usefulness of Draw Ratios Wall Thickness Reduction in Laminates Part Wall Thickness Prediction

6 Contents XI Finite Element Analysis FEA Data Input Model Verification Sheet Sag and Sag Rate Cooling Against a Mold Surface Light-Gauge Cooling Criteria Heavy-Gauge Cooling Criteria Cooling and Residual Stress Coolant Characteristics Coolant Flow Rate and Temperature Control Cooling Against the Plug Heat Removal by Mold and Coolant Forming Times Trimming The Mechanics of Trimming Light-Gauge Trimming In-Mold Trimming In-Machine Trimming In-Line Trimming Canopy or Horizontal Trimming Flatbed or Vertical Trimming Two-Step Trimming Prototype Trimming Heavy-Gauge Trimming In-Plane Trimming Hand Power Tool Trimming Milling or 3-Axis Machines Multi-Axis Routers Robotic Trimmers The Importance of the Trim Fixture Cutter Design Trim Tolerance Other Trimming Concepts Water Jet Cutting Laser Cutting Trimming Foam Polymers and Plastics Polymer Characterization Plastic vs. Polymer Thermoset and Thermoplastic Definitions Crystalline and Amorphous Definitions Homopolymers, Copolymers, Terpolymers, and Blends Additives, Fillers and Reinforcements The Thermoforming Window

7 XII Contents 11.3 Thermoformable Polymers Polystyrene and Other Styrenics Polyvinyl Chloride and Other Vinyls Acrylics Cellulosics Polycarbonate Polyesters Polyethylene Polypropylene Other Polyolefins Formable Biopolymers Other Formable Polymers Multilayer Polymers Foamed Plastics High-Density Foams Low-Density Foams Thermal Properties Heat Capacity Thermal Conductivity Polymer Density Thermal Diffusivity Thermal Coefficient of Expansion Thermal Properties of Multilayer Structures, and Filled and Reinforced Polymers Infrared Energy Absorption for Specific Polymers Issues of Quality Control Incoming Sheet Quality What the Thermoformer Needs to Know About the Extrusion Process What the Thermoformer Needs to Know About Quality What the Thermoformer Needs to Know About Regrind The Role of the Purchase Order Incoming Sheet Quality Evaluation Production Monitoring Monitoring Temperature Sheet Formability Cutting Surfaces Microfracture Concerns Finished Part Performance Comparison with Other Technologies [47, 48] Classification of Plastics Molding Technologies Polymer Material Choices Other Processing Concerns

8 Contents XIII 14 Pragmatic Aspects of Thermoforming Safety Thermoforming Machine Set-Up Set-Up for a New Machine Set-Up for a Used Machine Mold Set-Up New Mold Set-Up Existing Mold Set-Up Trim Set-Up Heavy-Gauge Trim Set-Up Light-Gauge Trim Set-Up Maintenance Emergency Maintenance Preventative Maintenance Troubleshooting Tips Non-Crisis Troubleshooting The Crisis Situation References Recommendations for Further Reading Books Other Sources Historical Reading Appendix 1: Glossary of Thermoforming Terms Subject Index