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 easy.  Small holes may be drilled through the pattern at corners.   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 only 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 begins to soften.  

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.

Most thermoplastic sheet materials can be vacuum formed, the most common types are listed below:

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