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Vacuum Forming

Sheets of pre-extruded rigid plastic are heated horizontally and sucked down into hollow one-piece tools. After the hot plastic solidifies again, its shape conforms to that of the mold. Trimming is usually necessary to put the part in final form. Vacuum forming is done at atmospheric pressure (14.2 psi). A variation of this process, pressure forming, incorporates the use of a pressure box built around the tool which increases the pressures far beyond atmospheric. Greater surface definition in parts can thus be achieved.

Vacuum forming simply requires the heating of a sheet of plastic until it reaches a forming temperature then sucking it into or around a mold. It has been used widely for products from camper shells to cold drink cups.

Pressure Forming

Pressure forming is a sophisticated version of the vacuum forming process. This process closes the appearance gap with traditional molding techniques. Pressure forming is really an old thermoforming technique which has been used by the thin-gage plastic packaging forming industry for many years. The mold makers start by creating a pattern based upon drawings and specifications of the custom part. From the pattern, these craftsmen produce either a hardwood or aluminum mold. A wooden mold is used mainly for a customer required prototype or a very low volume production run. The aluminum mold is used for a full production run and when the pressure forming process is necessary.

The pressure forming technique provides for forming heavier sheet from 0.093" thick up to 0.375" thick. The technique is accomplished by forcing a hot sheet against a mold, usually female, by introducing compressed air to the back side of the heated sheet. This method will provide as much as 75 psi working on the sheet surface as compared to the 14 psi in vacuum forming.

Pressure Forming uses air pressure as a forming aid to increase the detail on the mold side. Features that could not be achieved by vacuum alone can be molded with pressure forming. The mold can be textured or the part painted to get the desired surface finish. The result is the customer achieves the look and feel of an injection or structural foam molded part at a price close to a vacuum-formed one.