A plastic which cannot be softened by heat is described as
Answer Details
A plastic which cannot be softened by heat is described as a "thermosetting" plastic. Thermosetting plastics are a type of polymer that irreversibly hardens or "sets" when heated. They become strong and rigid and cannot be melted or reshaped by heating, unlike thermoplastics which can be melted and reshaped multiple times by heating. Examples of thermosetting plastics include Bakelite and melamine.