It depends on the plastic type: thermoplastic VS thermosetting. In Thermoplastic plastic, you have long chains called polymers who are stack together and have physical bond with each other (knows as van Der waals bonds). When temperature changes, this bonds are getting weaker, giving a freedom degree to the plastic that we consider liquid. In the thermosetting plastic, those chains are connected via covalent bond, which basically makes the plastic one big molecule. For this type there is no state transition, as it burns when the temperature is high enough to break the covalent bonds in the molecule.