Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heate 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 sometimes 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 . There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.