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Different polymers have different melting points. Many plastics are shaped by heating them until they are soft, and then molding , pressing, stamping, or forming in some other way. It is hard to do this at home, however, because it is necessary to carefully control the temperature. If the plastics are heated much above their softening points, they can darken and char. Since plastics are poor conductors of heat, it is difficult to get the whole sample hot enough to melt without crisping the outside surface. This is a very significant risk if you try to melt the plastic in a pot. More controlled, lower temperatures are necessary than you can usually get over a stove. If the plastic has a melting point less than the boiling point of water, you could heat it in a water bath or in a double boiler. Most plastics need a higher temperatures, however. Your best bet, if you lack a plastic welding setup (which melts the plastic with a stream of heated nitrogen gas), is to use a hair dryer.
Various melting points of different types of plastic:
The melting point of HDPE (High Density Polyethelyne) is about 130 ºC The melting point of LDPE (Low Density Polyethelyne) is about 110 ºC The melting point of PET (Polyethylene terphthalate) is about 250—260 ºC The melting point of PP (Polypropylene) is about 160—170 ºC The melting point of PS (Polystyrene) is about 70—115 ºC The melting point of PVC (Polyvinyl Chloride) is about 75—90 ºC
decomposes before boiling[]
plastic will decompose before boiling