Plastic welding is *real* welding. It’s not like epoxy, crazy glue, or a hot glue gun. I use a hot air torch to heat the parts until they melt (about 450 degrees, depending on the type of plastic) . I use plastic filler rod made from the same type of plastic to join parts or build up missing pieces. The repaired parts are 90% as strong as the original part, if machined to the original dimensions, or potentially stronger if the part is built up or reinforced.
Not all things that look like plastic are plastic though. Fiberglass, Bakolite, some glass impregnated nylons, and some blends are unweldable. I sometimes a product called Fiber Flex to braze some non-weldable plastic or plastic-like parts. However, a brazed part will not be as strong as a welded part.
Unobtainable parts can be worth fixing
This is a driver’s door master switch cover that has been discontinued.
Toyota window switch keeps falling into the door
This is a broken plastic lug on the underside of the cover that holds the switch to the cover. Without the lug, the switch falls into the door panel.
You can only weld like types of plastic (and there are a lot of different types).
Different plastics can’t be welded together. Here are a few types of plastics: PE, PP, ABS, PVC, HDPE, and Nylon. That’s far from an all inclusive list though. The easiest way is to find a mark or recycling symbol on the part. If there is no mark, attempt to weld each type of rod to the part until one sticks and won’t pull off. This part is ABS.
Prep the area to be welded by grinding away the old plastic
Next I grind down the remaining part of the lug with a wide flute carbide die grinder bit, leaving a flat surface. Once I prep the surface, I add plastic filler in small coils, cooling the plastic with water after each coil. This solidifies the plastic already deposited, making it easier to build on.
Build up the broken part
Now the rough shape of the lug has been built.
With some careful die grinding, the lug is returned to its original size.
There was also a crack underneath the lug. The crack is ground out (just as you would when welding metal), then groove is filled. On this job I ground extra deep to reach the center of the lug on the other side. This will make sure the lug is well attached.
Now that all the cracks and damage have been repaired, and new screw holes drilled and tapped, the part is ready for re-assembly.
Here’s the part all repaired. Since all of the damage was in non visible areas, it looks as good as new.