How to Test for Various Plastics: Plastic welding rods oxidize rapidly under the effects of light heat and oxygen causing the material to lose strength and stiffness and to discolor thus losing its ability to weld. Cleaning the rod by removing this surface layer of impurities with a burr knife insures that the welding rod will weld your adjoining material properly. 1.  A short test weld with the available welding rod. If the welding rod or tape is fixed firmly, the problem is solved. 2. Take a small sliver or shaving of the material to be welded and light it with a match, observe the flame and smell the smoke. On PVC: blackish smoke and acrid smell. On Polyethylene: no smoke, the material drips like a candle and also smells of wax. On Polypropylene: no smoke, the material drips like a candle and smells of burnt oil. On Polyamide: no smoke, pulls to form thread, smells of burnt horn On Polycarbonate: yellowish sooty smoke. Sweetish smell. On ABS: blackish smoke, soot flakes, sweetish smell. Because containers, pipes and foils are available in many different colors, it is not possible to judge by the color to recognize the plastic. However, 50% of goods are made of types of polyethylene which makes recognition easier.