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 bathtubs and large domes 
for skylights and interior linings for refrigerators.
Thermoforming consists of two main steps: heating and forming.
Heating is generally performed using electric heaters on one or both sides of the starting plastic sheet, at a distance of approximately 125 mm. The duration of the heating cycle must 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) pressure thermoforming.
3) mechanical thermoforming.
Methods for forming sheet material:
Vacuum Thermoforming
The oldest method is vacuum thermoforming (often referred to simply as vacuum forming in its
(in the early days, in the 1950s) in which negative pressure is used to adhere the preheated sheet
inside the mold cavity. The process is illustrated in Figure 2.57 in its most basic form. The vacuum ports in the mold are approximately 0.8 mm in diameter, so their effects on the plastic surface are minimal.
Pressure Thermoforming
An alternative to vacuum forming involves using positive pressure to force the hot plastic into the mold cavity. This is called pressure thermoforming or blow molding; its advantage over vacuum forming lies in the fact that higher pressures can be achieved, 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 previous one, with the difference being that the sheet is pressed from above into the mold cavity. The vent holes in the mold allow trapped air to escape.
Mechanical thermoforming
The third method, called mechanical thermoforming, uses a pair of molds (positive and negative) that are pressed against the heated plastic sheet, forcing it to take on their shape. In the pure mechanical forming method, neither vacuum nor 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 made of thermosetting polymers or elastomers are cross-linked and cannot be softened by reheating. Common thermoformed plastics include polystyrene, cellulose acetate, cellulose butyrate acetate, ABS, PVC, acrylic (polymethyl methacrylate), polyethylene, and polypropylene.