This category of plastic once solidified during the molding process, cannot be softened back. Due to the reason that the units acquire three-dimensional cross-linked structure with strong covalent bonds for the most part that tends to preserve their strength and structure even on exposure to heating.
If the plastic is exposed to long term heat, it may get charred.
Phenolic resins, amino resins, polyester resins, silicon resins, epoxy resins, Bakelite, melamine and polyurethanes are some of the examples of these kind of plastics.
Bakelite being the poor conductor of heat and electricity is used for making electrical switches and handle of several utensils. Melamine due to its resiliency to fire and tolerance to heat is extensively used to make floor tiles, kitchenware and fabrics used by firemen that can resist fire.