Thermoforming involves heating a flat extruded sheet of thermoplastic material until it softens and is stretchable. The hot sheet is then forced against the contours of a mold by vacuum, pressure, mechanical means, or a combination of all three. After cooling, the plastic sheet retains the mold shape and detail.

Thermoforming is most commonly used to produce mass parts for the packaging industry. Thermoforming is also used to produce large parts where the volume is too small to economically injection mold. Tooling costs for thermoforming parts when compared to injection-molded tools are significantly lower.

Optimizing the process window is critical to achieving high quality parts. Temperature distribution, wall thickness distribution, minimum parts thickness, surface finish, and possible material degradation must be considered. The size of the parts is limited by the size of the plastic sheet stock available and the geometry of the thermoforming machine. Draw depth and stretch ratio, draft angles, stiffening details, trim lines, and undercuts are also important considerations for this process.