Thermoforming is a senes of processes for forming thermoplastic sheet or film over a mold, by means of the application of heat and pressure (Fig. 18 11) In this process, a sheet is heated in an oven to the sag point (softening but not to the melting point) The sheet is then removed from the oven, placed over a mold, and forced against the mold through the ap¬plication of a vacuum. TT* mold is usually at room temperature, hence the shape of the plastic becomes set upon contact with the mold. Because of the low strength of the materi¬als formed, the pressure differential caused by the vacuum is usually sufficient for forming, although air pressure or mechanical means are als0 applied for some ports.
Typical ports made by thermo tormina are advertising signs, refrigerator liners, pack¬aging. appliance housings, and panels for shower stalls. Because thermolormin3 IS a draw¬ing and stretching operation, much like sheet metal forming, the material should exhibit
FIGURE 18.11 Various thermoforming processes tor thermoplastic sheet. These processes are commonly used in making advertising signs, cookie and candy trays, panels for shower stalls, and packaging.