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April 2006
The Thermoforming Process

The thermoforming process

Author:
Posted on: 4/1/2006
Source: Composites Technology

Thermoforming is one of the oldest and simplest plastics forming processes. Baby rattles and teething rings were thermoformed in the 1890s, using one of the earliest cellulose-based plastics. The process saw major growth in the 1930s with the development of the first roll-fed thermoforming machines in Europe.

In the thermoforming process, heat and pressure are used to transform flat sheet thermoplastics (unreinforced or reinforced) into a desired three-dimensional shape. The sheet is preheated using one of three methods: Conduction via contact heating panels or rods; convection heating, using ovens which circulate hot air; and radiant heating achieved with infrared heaters. The preheated sheet is then transferred to a temperature-controlled, pre-heated mold and conformed to its surface until cooled. The final part is trimmed from the sheet, and — a distinct advantage of unreinforced and some chopped glass-reinforced thermoplastics — the trim can be reground, mixed with virgin material and reprocessed.

There are numerous variations of thermoforming, distinguished primarily by the method used to conform the sheet to the mold. These range from simple sheet bending, using a folding machine or jig, to more complex processes, such as vacuum forming and pressure forming, which use negative pressure (vacuum) and/or positive pressure (compressed air). When higher pressures and more traditional compression molding machines are used, the process may be termed matched metal die stamping or rubber block stamping, depending on what type of upper mold surface is employed. Diaphragm forming/diaphragm molding — suitable for simple geometries — is typically a lower pressure process and employs air pressure to form the sheet through a flexible diaphragm. Hollow parts can be produced via twin-sheet forming, a process that positions two preheated sheets between two female molds with matching perimeters or contact surfaces. The twin sheets are drawn into the molds, formed and joined to make a single part using a combination of vacuum and air pressure.

Forming temperatures depend on the specific thermoplastic being used, but typically range from 300°F to 400°F (149° C to 205°C). Forming pressures can range widely. Low-pressure processes may require as little as 10 psi (0.7 bar) while sheet materials competitive with glass mat thermoplastic (GMT) can require pressures as high as 3,000 psi (207 bar). For low-pressure materials, tooling does not have to be steel but, depending on the length of a production run, can be aluminum or epoxy. Wood and even plaster tools can be used for prototyping.

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