Technology of Thermoforming

Preface

Contents

1 Thermoforming—Definitions,History,Methods and Equipment

1.1 Introduction

1.2 History

1.3 Markets

1.4 Some Definitions

Gage

Clamping of Thin-Gage Sheet

Clamping of Heavy-Gage Sheet

Heating of Thin-Gage Sheet

Heating Heavy-Gage Sheet

Shaping Thin-Gage Sheet

Shaping Heavy-Gage Sheet

Trimming the Thin-Gage Sheet

Trimming the Heavy-Gage Sheet

Depth-of-Draw

1.5 Methods of Forming

One-Step Forming

Two-Step Forming with Prestretching

Multi-Step Forming

Other Variations

1.6 Thermoforming Machinery

Heating Source

Forming Platform

Vacuum System

Pressure System

Process Control

Trimming and Cut Parts Handling

1.7 Heavy-Gage Thermoforming Machinery Specifics

1.8 Thin-Gage Thermoforming Machinery Specifics

1.9 References

2 Polymeric Materials

2.1 Introduction

2.2 Network Nature of Polymers

2.3 Addition and Condensation Polymerization

2.4 Aromatic and Aliphatic Polymers

2.5 Molecular Weight and Molecular Weight Distribution

2.6 Molecular Weight and Properties

2.7 Morphology and Properties

2.8 Molecular Orientation

2.9 Chain Mobility and Polymer Stiffness

2.10 Stress-Crack Resistance

2.11 Gas Permeation

2.12 Copolymerization

2.13 Blends

2.14 Adducts

Plasticizers

Other Additives

Fillers and Reinforcing Fibers

2.15 Laminates

2.16 Stress-Strain Behavior of Plastics

2.17 Thermal Properties

Heat Capacity

Thermal Conductivity

Thermal Diffusivity

Thermal Expansion Coefficient

2.18 Infrared Spectra

2.19 Summary

2.20 References

3 Heating the Sheet

3.1 Introduction

3.2 Energy Absorption by Sheet

3.3 Heat Transfer Modes

3.4 Incorporating Formability and Time-Dependent Heating

3.5 Conduction

3.6 Convection Heat Transfer Coefficient

The Biot Number

Effective Radiation Heat Transfer Coefficient

Constant Heat Flux

3.7 Radiation Heating

Black Body Radiation

Gray Body—Emissivity

Radiant Heater Efficiency—Constant Heat Flux Application

3.8 Real Heaters—Efficiencies

Radiative Heat Transfer Coefficient

Convection and the Heat Transfer Coefficient

Rod Heaters

3.9 Long-Term Radiant Heater Efficiencies

3.10 Edge Losses—View Factor

Local Energy Input

Pattern Heating

Zone, Zoned or Zonal Heating

Heater to Sheet Distance

3.11 Thin-Gage Sheet—Approximate Heating Rates

Constant Environmental Temperature Approximation

Constant Heat Flux Approximation

Thin-Gage Approximations—Comments

3.12 Heavy-Gage Sheet—Internal Temperature Control

Constant Environmental Temperature

The Constant Heat Flux Case

The Thickness Effect

Summary

3.13 Equilibration

Convection Heating

Constant Heat Flux

Computed Equilibration Times

The W-L-F Equation

The Arrhenius Equation

Relating Shift Factors to Sheet Thickness

3.14 Infrared-Transparent Polymers

3.15 Computer-Aided Prediction of Sheet Temperature

The Radiant Boundary Condition

3.16 Guidelines for Determining Heating Cycles

The Biot Number

Thin-Gage Guidelines

Heavy-Gage Guidelines

Intermediate-Gage Guidelines

3.17 References

4 Stretching the Sheet

4.1 Introduction

4.2 The Stretching Concept

4.3 Polymer Hot Strength

Standard Tensile Tests

Hot Tensile Tests

Hot Creep Tests

Other Stretching Tests

Temperature-Dependent Viscosity for Amorphous Polymers

Dynamic Mechanical Testing

4.4 Stress-Strain-Rate of Strain—Theory

Elasticity—A Rationalization

Strain Energy Function

The Rivlin Form for the Strain Energy Function

The Ogden Form for the Strain Energy Function

Viscoelastic Models

4.5 Available Stress-Strain Data

Sensitivity of Models

4.6 The Importance of Polymer Material Properties

4.7 Practical Aspects of Stretching

Funnel Test

4.8 Bursting Conditions

4.9 Sheet Sag

Initial Sag

Tensile Sag

The Catenary Sag

Parabolic Sag

Relating Sag to Hot Sheet Strength

Sag—A Comment

4.10 References

Appendix 4.1 Biaxial Stretching of an Elastic Membrane

5 Cooling and Trimming the Part

5.1 Introduction

5.2 Overall Cooling Heat Balance

5.3 Cooling the Formed Shape

5.4 Steady State Heat Balance

Interfacial Resistance

Shape Factor

Convection Heat Transfer Coefficient

5.5 Cyclic Heat Balance

Cooling the Free Surface of the Sheet

Cooling Thin Sheet in Ambient Air

Transient Heat Removal From the Sheet

Quiescent Ambient Air

Moving Ambient Air

Cooling on Nonmetallic Molds

5.6 Transient Heat Transfer During Sheet Cooling on the Mold Surface—Computer Models

Interfacial Air

5.7 Shrinkage

Unconstrained Shrinkage

Constrained Shrinkage

5.8 Trimming

Trimming Heavy-Gage Parts

Trimming Thin-Gage Parts

5.9 Mechanics of Cutting

The Trim Region

Registering the Trim Site

The Nature of the Cut

Fracture Mechanics

Mechanical Chipping

Multiple-Edged Tool or Toothed Saw Performance

Abrasive Cut-Off Wheel

Toothless or Shear and Compression Cutting

Fracture Mechanics in Trimming

Nibbling

Brittleness, Orientation and Trim Temperature

5.10 Steel Rule Die

Resharpening

Tabbing and Notching

5.11 Punch and Die Trimming

Forged and Machined Dies

5.12 Drilling

5.13 Other Cutting Techniques

Thermal Cutting

Water Jet Cutting

5.14 Trimming—A Summary

5.15 References

6 Thermoforming Molds

6.1 Introduction

6.2 Prototype Molds

Wood

Fiberboard

Plaster

Plastic

White Metal

Nickel

6.3 Production Molds

Aluminum

Steel

Other Metals

6.4 Mold Coolant Channels

Mold Channel Flow

Expansion

Contraction

Sharp-Edged Orifice

6.5 Vent Holes

Sizing Vacuum Systems—Steady State

Sizing Vacuum Systems—Dynamic

Solenoid Valve Flow Resistance

Vent Hole Resistance to Flow

Vent Hole Diameter

Other Types of Vents

Vent Hole Placement

6.6 Surface Treatments

Surface Texture

6.7 Plug Design Considerations

Plug Materials

Wood Plugs

Plastic Plugs

Metal Plugs

Plug Design Concepts

6.8 Sheet Clamping

6.9 Sag Bands and Sheet Supports

6.10 Other Aspects of Mold Design

Undercuts

Encapsulation

Moving Elements

Stripper Plates/Bars

Mold Releases

Web Breakers, Catchers and Chasers

Moats, Dams and Double Steps

Chamfers and Radii

Prestretching Restraints

6.11 Efficient Use of Sheet

Heavy-Gage Sheet

Thin-Gage Sheet

6.12 References

7 Parts Design

7.1 Introduction

7.2 Elements of Parts Design

Material Testing and Its Relevance to Part Performance

Philosophy of Parts Design

Minimizing the Amount of Sheet to be Reground

Rules for Part Layout on Heavy-Gage

Rules for Multiple Part Layout on Thin-Gage

Economics of Buying Sheet of Specific Size

7.3 Prototyping as a Justification for Thermoforming

7.4 Draw Ratio

Linear Draw Ratio

H:D

Rim and Lip Sheet for Female Cavities

Draw Ratio Usage—A Rationale

Mechanical Assists—Some Design Features

Preblowing or Inflation—Comments

Plug Assist—Comments

7.5 Computer-Aided Design in Thermoforming

7.6 Wall Thickness Prediction—A Justification

Geometric Element Analysis or GEA

Finite Element Analysis

General Comments on Plug Design

Plug Assist Analysis

Plug Design„Geometric Element Analysis

Plug Design„Finite Element Analysis

7.7 Regrind

Material Property Deterioration on Regrind

Property Value Loss—Experiment and Protocol

Cascading 100% Regrind

7.8 General Guidelines for Part Design

General Tips

Process Tips

Mold Tips

Prestretch Tips

Part Design Tips

Rim and Edge Designs

Design—A Comment

7.9 References

Appendix 7.I Draw Ratios for Truncated Cone

Appendix 7.II Mechanical Property Loss in Regrind

8 Producing Sheet and Film

8.1 Introduction

8.2 Forming Thin Films

8.3 Forming Sheet

Single-Screw Extrusion

Filtering the Polymer

Flow Improvement Devices

Pressure and Temperature in an Extruder

Sheet Die Concepts

Gage Thickness Monitoring and Control

Twin-Screw Extrusion

8.4 Roll Stacks

8.5 Sheet Trimming

8.6 Take-OfT and Take-Up Rolls

8.7 Residence Time and Residence Time Distribution Through Extruder and Die

8.8 Drying

8.9 Producing Biaxially Oriented Sheet

8.10 Multilayer Sheet Formation

Coextrusion

Lamination

8.11 Sheet Quality and Quality Control

Sheet Dimensions

Orientation

Sheet Squareness and Flatness

Moisture

Sheet Appearance

Annoyance Factors

Lamination

8.12 References

9 Newer Thermoforming Technologies

9.1 Introduction

9.2 Thermoforming Crystallizing Polyethylene Terephthalate

PET Crystallinity

CPET Patents

Characterizing Polyethylene Terephthalate

The Effect of Temperature on Crystallization During Sheet Extrusion

Cooling CPET on Chill Rolls

Heating CPET in Roll-Fed Thermoformers

Forming the CPET Sheet

Cooling the CPET Formed Part

Trimming Parts from Web

Troubleshooting CPET Forming

9.3 Pressure Forming

Thin Gage

Heavy Gage

9.4 Forming Filled and Reinforced Polymers

9.5 Laminated Sheet Thermoforming

Heating Multilayer Sheet

Forming Multilayer Sheet

9.6 Twin-Sheet Thermoforming

Simultaneous Twin-Sheet Forming

Sequential Twin-Sheet Forming

Seal Area—Adhesion

Seal Area—Compressive Form

Seal Area—Design

9.7 Polypropylene Thermoforming

Sag Test

Modified Polypropylenes

9.8 Thermoforming Foam Sheet

Cell Architecture—Actual v. Ideal

Radiant Energy Transmission

Internal Cell Gas Pressure

Forming Window for Foam

The Forming Equipment

9.9 Other Forming Technologies

Interdigitation

Sealed Air Cushion/Dunnage

9.10 References

10 Set-Up Protocols, Troubleshooting, and the Economics of Thermoforming

10.1 Introduction

10.2 Setting Up a Thermoforming Machine—Protocols

A New Polymer

Setting Up a New Mold

Setting the Mold Stops

Dry-Cycling the Mold

Checking the Vacuum

Attaching Cooling to the Mold

The Sheet Delivery System

Setting the Oven Conditions

Forming Step—Simple Vacuum Forming

Changing Temperature Conditions

Activating the Assists

Pressure Boxes

General Objectives

10.3 Troubleshooting the Forming Process

10.4 Energy and Materials Cost

The Energy Audit

Cost of Extrusion

Cost of Regrind

Competitive Costs of Polymers

10.5 General Processing Economics

Rules of Thumb

Global Production Costs

Manufacturing Efficiencies

The Learning Curve

10.6 Isolated Venture Costs

10.7 New Venture Economics

Entrepreneurial Risks

10.8 The Incremental Operation

10.9 Comparative Process Economics

10.10 References

Appendix A Abbreviations for Thermoformable Polymers Referred to in Text

Appendix B Typical Conversion Factors Used in Thermoforming (US Customary to Metric,Metric to US Customary)

Appendix C Glossary of Thermoforming Terms

Author Index

Subject Index