Thermoforming is a hot-deformation process that molds thermoplastic film or sheets on a mold. The shape of the mold is faithfully copied by suctioning out the air that remains between the film and the surface of the mold under low levels of vacuum (~0.5 mbar of final pressure).
In some vacuum machines, compressed air (at a pressure of up to 10 bar) is injected into a bell closed on the other side of the film to achieve greater fidelity of minute details and more uniform stretching. One of the intrinsic characteristics of the thermoforming process is that new material is never added; the quantity of material is predetermined and only deformed.
To obtain the most uniform stretching possible, suitably shaped counter-molds are often used to improve results at the most difficult points.