The process of heating and molding temperature-sensitive materials is called thermoforming. It uses sheets of a polymer called thermoplastic, which is extruded in varying levels of thickness, depending on its intended purpose. Thermoforming uses several different types of molds and processes in order to achieve the final product.
      History
The Romans used the earliest known method of thermoforming. They imported tortoise shells from Asia, and used the keratin in the thermoforming process. Keratin, which can be heating and molded, was treated with hot oil and used to make eating utensils. The earliest industrial use in contemporary times was in the 1800s, when vacuum thermoforming was developed.
      Types
There are three varieties of thermoforming molds. A convex mold protrudes outward. The outer surface of the convex mold is in the exact shape desired for the inside of the molded plastic product. A concave mold has a cavity in the exact shape desired for the exterior of the plastic product. A combination of the two molds used together creates a finished product that is molded on both sides.
      Process
The sheet of thermoforming material is heated in order to increase pliability. Different levels of heat are used for materials of a different thickness. Once the correct level of heat has been reached, the material is molded by one of three common methods. In vacuum thermoforming, the sheet of material is laid on a concave mold and pulled inward to adhere to the shape of the mold with vacuum pressure. Tiny holes in the mold provide the suction, without affecting the shape of the finished product. Pressure thermoforming occurs when the sheet is laid on a convex mold shape and pressed down on the mold. Finally, mechanical thermoforming uses a machine to press the sheet into a concave mold with a convex mold, pressing the sheet between the two molds.
      Materials
Thermoplastic, also known as thermoforming plastic and thermoformable plastic, is the material used for contemporary thermoforming processes. Thermoplastic is a polymer, or a material that contains large molecules in a repeating pattern. This material changes physical properties based on its temperature. With extreme heat, it melts to a liquid form, and with cool temperatures, it becomes extremely hard. Applying a moderate amount of heat causes the thermoplastic to become flexible so it can be molded.
      Applications
Common products made by thermoforming include bathtubs, shower stalls and boat hulls. Thermoforming is also used to make blister packs, which hold single doses of many medications.