The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature. Then it is stretched over or into a temperature-controlled, single-surface mold.

Cast or machined aluminum is the most common mold material, although epoxy, wood and structural foam tooling are sometime used for low volume production. The sheet is held against the mold surface unit until cooled. The formed part is then trimmed from the sheet. The trimmed material is usually reground, mixed with virgin plastic, and reprocessed into usable sheet [1]. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.

There are two general thermoforming process categories. Sheet thickness less than 1.5 mm (0.060 inches) is usually delivered to the thermoforming press in rolls. Thin gauge roll fed thermoforming applications are dominated by rigid or semirigid disposable packaging. Sheet thickness greater than 3 mm (0.120 inches) is usually delivered to the forming press cut to final dimensions and stacked on pallets. Heavy or thick gauge, cut sheet thermoforming applications are primarily used as permanent structural components. There is a small but growing medium gauge market that forms sheet 1.5 mm to 3 mm in thickness.

Heavy gauge forming depended on convection oven heating of the sheet and draping of the sheet over male, female or paired molds. Aircraft windscreens and machine gun turret windows spurred the advance of heavy gauge forming technology during WWII. Heavy gauge parts are used as cosmetic surfaces on permanent structures such as automobiles, refrigerators, spas and shower enclosures, and electrical and electronic equipment.