high uniform elongation; otherwise it will neck and fail. Thermoplastics have high capacities for uniform elongation, by virtue of their high strain-rate sensitivity exponents, m. (Sec Section 2.2.7.) The sheets used in thermoforming are made by sheet extrusion. Parts with openings or holes cannot be formed by this process, because the pressure differential cannot be maintained during forming.
Molds for thermoformmg are usually made of aluminum, because high strength is not a requirement. The holes in the molds (to pull a vacuum) are usually less than 0.5 mm (0.02 ๒.), in order not to leave any marks on the formed parts. Tooling is inexpensive. Quality considerations include tears, nonuniform wall thickness, improperly filled molds, and poor part definition (surface details).