What defines thermoplastic material?

Thermoplastic materials are defined by their ability to undergo repeated cycles of heating and cooling without significant chemical change. When heated, they become soft and pliable, allowing them to be molded into different shapes. Upon cooling, they solidify and retain the new shape. This characteristic is fundamental to thermoplastics, as it enables them to be reshaped any number of times, which is advantageous in various manufacturing processes such as injection molding or extrusion.

The ability to return to a solid state after being heated is what differentiates thermoplastics from thermosetting plastics, which cannot be reshaped once they have solidified. Thus, the defining feature of thermoplastics being able to melt into a liquid state upon heating and solidify upon cooling is key to understanding their applications and behavior.