How To Weld Plastic

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You will need to identify the exact plastic and determine that it is not a thermoset plastic. Then find out the melting point and set your heat gun to something a few degrees above that. You should test with a sample first to be sure that you are melting the plastic and not burning it. Also, be aware that some plastics, notably some PEs will shrink when heated. Those are not good canidates for manual seaming. It is very hard to get consistant results with them using manual techniques.
Another issue to be aware of, creating seams using a heat gun and pressure roller is a VERY slow process.