Thermoforming or thermoforming, a process in which a flat sheet of thermoplastic material is used to give it the desired shape. The process is widely used in the packaging of consumer products to manufacture large items such as bath tubs, large domes, and other large items such as bathtubs. 
for skylights and internal lining for refrigerators.
Thermoforming consists of two main steps: heating and forming.
Heating is generally performed by electric radiators on one or both sides of the initial plastic sheet, at a distance of approximately 125 mm. The duration of the heating cycle needs to be sufficient to soften the sheet, depending on the polymer, its thickness and color. Forming methods can be classified into three basic categories:
1) vacuum thermoforming.
2) press thermoforming.
3) mechanical thermoforming.
Methods for laminar material forming:
Vacuum thermoforming
The oldest method is vacuum thermoforming (called simply vacuum forming in its
in the early 1950's) in which negative pressure is used to adhere the preheated foil
inside the mold cavity. The process is explained in Figure 2.57 in its most basic form. The holes to make the vacuum in the mold are of the order of 0.8 mm in diameter, so their effects on the surface of the plastic are minor.
Press thermoforming
An alternative to vacuum forming involves positive pressure to force the hot plastic into the mold cavity. This is called pressure thermoforming or blow forming; its advantage over vacuum forming is that higher pressures can be developed, since in the former method this parameter is limited to a theoretical maximum of one atmosphere.
Forming pressures of three to four atmospheres are common. The process sequence is similar to the above, the difference being that the sheet is pressed from above into the mold cavity. Ventilation holes in the mold allow trapped air to escape.
Mechanical thermoforming
The third method, called mechanical thermoforming, uses a pair of dies (positive and negative) that are applied against the hot plastic sheet or foil, forcing it to assume its shape. In the pure mechanical forming method, no vacuum or air pressure is used.
Applications:
Thermoforming is a secondary process, the primary process is the one that produces the sheet or film. Only thermoplastics can be thermoformed since extruded sheets of thermoset polymers or elastomers are cross-linked and cannot be softened by reheating. Common thermoformed plastics are polystyrene, cellulose acetate and cellulose acetate butyrate, ABS, PVC, acrylic (polymethylene acrylate), polyethylene and polypropylene.