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Welding – Plastic

Plastic welding is *real* welding. It’s not like epoxy, crazy glue, or a hot glue gun. The parts to be joined are heated until they melt (about 450 degrees, depending on the type of plastic) with a hot air torch. Plastic filler rod made from the same type of plastic as the base material is melted into the joint. The result is a part that is about 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. A product called Fiber Flex can be used to braze some non-weldable plastic or plastic-like parts. However, a brazed part will not be as strong as a welded part.

This is a driver’s door master switch cover. That has been discontinued.

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.

The first step in plastic welding is to determine what type of plastic the part is made from: PE, PP, ABS, PVC, HDPE, Nylon, etc.. 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.

Next I grind down the remaining part of the lug with a wide flute carbide die grinder bit, leaving a flat surface. Once the surface is prepped 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.

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.

 

It’s ready to be put back into service. Since all of the damage was in non visible areas, it looks as good as new.