A process, sometimes called thermoforming, used to make plastic parts. A plastic sheet is heated until it is soft and starts to sag. A vacuum is then used to pull the plastic down tightly over a mold where it is allowed to cool and harden. It is often used in prop building, but is used for many other things as well. Common plastics that are used are: Acrylonitrile Butadiene Styrene (ABS), Cellulose Acetate Butyrate (CAB), polycarbonate, polyethylene and polystyrene.