Vac-Forming or Thermo-forming is a process where the component is made from a flat plastic sheet which is heated until soft. It is then stretched over a mold or pattern, then sucked down tightly with a vacuum.

The plastic cools very quickly and retains that shape. Vacuum forming is ideal whenever a "shell" type plastic part with uniform wall thickness is required.

Many illuminated signs are manufactured in this way.

Firstly the required material is clamped on a frame. The frame is then closed to prevent the sheet from moving during the operation. A heater is then located above

the workpiece and the temperature is raised until the sheet is almost molten.

At this time it is lowered onto the mould and air is sucked out which forces the heated sheet to take the shape of the mould.

The thermoformed sheet is then cooled with blowers and removed from the frame.

Variations of the process includes male or female tools, plug assistance, pre-blow, pre-stretch and rarely, airbox pressure assistance. Materials used for vacuum forming become thermoplastic at forming temperature and offer little resistance against deformation.

Unlike the thermo-elastics however, they cannot be satisfactorily de-moulded to a flat state. The majority of tools are manufactured from hardwood, plywood and compressed laminates.

Where long runs are planned, cast aluminium tooling or skeleton steel tooling is used.

Owing to better heat conduction this has an effect on the reduction in the cooling cycle and requires less repair and maintenance.

The preferred material for tooling in vacuum forming is cast aluminium.

There is of course an increase in the cost involved and of lead time for tooling manufacture.