Vacuum forming is a process in which a sheet of plastic is heated and then pulled over a mold and held against the mold by forming a vacuum between the mold and the plastic. It is primarily used for objects which are relatively shallow in depth, although it can be used with deeper molds if the sheet is stretched prior to coming into contact with the mold. Molds can be made of wood, cast or machine aluminum or composite materials.
Vacuum forming is most often used on thermoplastics, including High Impact Polystyrene Sheeting (HIPS) and acrylic. Vacuum formed pieces can be found in a wide variety of everyday items, from ATMS to truck engine covers to transit seats.