The vacuum forming process involves heating a plastic sheet with hot air until soft, draping it over a mold, and using vacuum suction to form the sheet tightly to the mold. Common plastics used are polystyrene, polyethylene, polyvinyl chloride, and acrylic. Vacuum forming allows for low-cost production of prototypes and small runs due to inexpensive equipment and fast mold making. However, additional trimming is usually required and high-volume production requires more complex machinery.