Well I don't know what plastic you are working with, but my guesses are that it is a thermosetting polymer. Basically, picture polymers as chains of molecules that are weakly bonded. In some cases, heating the plastic will push the molecules to create hard covalent bonds between themselves called crosslinks, which causes these plastics to irreversibly harden. You will not be able to melt it, or even reshape it.
thenopebig t1_j4cz6sh wrote
Reply to Why do kitchen bags not melt but harden when exposed to a heat gun ? by Athousandlipsticks
Well I don't know what plastic you are working with, but my guesses are that it is a thermosetting polymer. Basically, picture polymers as chains of molecules that are weakly bonded. In some cases, heating the plastic will push the molecules to create hard covalent bonds between themselves called crosslinks, which causes these plastics to irreversibly harden. You will not be able to melt it, or even reshape it.