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. Attempt a short test weld with the available welding rod. If the welding rod or tape is fixed firmly, the problem is solved.
2. If the welding test above fails, the material (the substrate) needs to be identified and matched to the welding rod or ribbon. Take a small sliver or shaving of the material to be welded and light it with a match, observe the flame and sniff the smoke. (But not too much!)

PVC: blackish smoke and acrid smell.
Polyethylene: no smoke, the material drips like a candle and smells like wax.
Polypropylene: no smoke, the material drips like a candle and smells like burnt oil.
Polyamide: no smoke, pulls to form thread, smells like burnt horn
Polycarbonate: yellowish sooty smoke and a sweetish smell.
ABS: blackish smoke, soot flakes, sweetish smell.

Because plastics are available in many colors, it is not always possible to recognize the plastic by its color. However, 50% of goods are made of some type of polyethylene, which may shorten the recognition process.