After the inception of the polymer injection molding process in the 1870s, plastic-forming machines were rapidly developed up until the 1930s.
[2] Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, and stamped to a specific shape in a 2-part mold.
[3] It is then slowly rotated (usually around two perpendicular axes), causing the softened material to disperse and stick to the walls of the mold forming a hollow part.
In order to form an even thickness throughout the part, the mold rotates at all times during the heating phase, and then continues to rotate during the cooling phase to avoid sagging or deformation.
Rotocasting is a variant of the process that uses self-curing or UV-curable resins (as opposed to thermoplastics) in an unheated mold.