Vacuum Forming Thermoplastic Sheet Materials

Introduction

Thermoplastic sheet materials such as acrylic can be softened by heat. When a thermoplastic sheet has been softened, it can be bent, twisted, rolled and stretched. After the thermoplastic has cooled down, it becomes rigid and remains in its new deformed shape. This makes thermoplastic sheet materials suitable for:

• vacuum forming
• compression moulding using a plug and yoke
• line bending

Vacuum Forming Process

The vacuum forming process consists of heating a thermoplastic sheet material until it is soft and then drawing it over a mould or pattern using the power of air pressure.

A pattern is first made using a resistant material such as Medium Density Fibreboard (MDF). The sides of the pattern are chamfered so that the vacuum formed plastic can be pulled off it easily. Small holes may be drilled through the pattern at corners and bends. The small holes enable air to be drawn from all parts of the pattern. The vacuum effect draws the softened plastic tightly around all parts of the pattern.

Complex shapes such as model car bodies may be modelled in clay. Plastic sheet materials may be vacuum formed over a clay pattern whilst the clay is still a little soft. This allows the clay to be pulled and scraped out of the vacuum formed plastic after the vacuum forming operation.

Patterns with "under-cuts" may be made from clay because the soft clay can still be removed from the vacuum formed plastic. Clay patterns can be used once because they are usually damaged or completely destroyed during removal from the vacuum formed plastic.

Vacuum Forming Procedure

A pattern is placed in the vacuum former and lowered into the machine. A thermoplastic sheet material is then clamped on the vacuum former and heated until it softens.

Air pressure is added to stretch the plastic slightly so that the plastic stretches more evenly over the pattern when it is being vacuum formed.

The pattern is raised and the vacuum is quickly applied for a few seconds to pull the plastic around the pattern.

When the plastic vacuum forming has hardened off, the clamps may be released and the vacuum formed plastic removed from the vacuum former. The waste material is trimmed off and the edges of the vacuum forming are smoothed and finished.

Typical Materials Used In Vacuum Forming

• Acrylic PMMA
• Acrylonitrile Butadiene Styrene ABS
• Polyester Copolymer PETG
• Polystyrene PS
• Polycarbonate PC
• Polypropylene PP
• Polyethylene PE
• Polyvinyl Chloride PVC

(Source: British Plastics Federation).

CLEAPSS

Please refer to CLEAPSS, Model Risk Assessments for Design and Technology in Secondary Schools and Colleges, Part 1.062 COSHH Regulations.