A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing. Ice cubes are a common household item which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times.
Thermoplastics offer versatility and a wide range of applications.
They make up the greatest share of plastics used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and soda bottles. This technique is used mainly for small thin-walled parts ranging up to containers with five gallon capacities. Parison programming can be used to vary the wall thickness. Continuous extrusion also allows the use of heat-sensitive materials due to streamlined flow areas and die designs.