I can weld metal and welding plastic is not much different, but easier in my opinion.
There aren’t many choices on color when picking plastic welding rods, so if the broken parts are to be seen as cosmetics, try to stay on the “unseen” side of the parts. If possible try to create a bevel where the pieces will be joined to create a solid weld.
Picking the right welding filler rod is very important. You can use a polyethylene rod to weld up ABS, but when it cools, the polyethylene will just peel right out and will not bond. Years ago I purchased a 1985 LTD that had a salvage title. It was mechanically sound and safe, but had been dropped on the left side leaving broken parts and a cracked fairing. I welded the fairing from the inside and the cracks on the outside were very hard to see. While trying to weld the dash bezel, I was unable to determine the type of plastic it was made from and the weld would peel out when cooled. At the time I worked in an auto repair shop and one of our customers dealt with plastics daily. He came over to the bike seeing what my problem was and asked for a pocketknife. He scrapped a small portion of the back side of the dash off, burned it with a lighter and had me smell the smoke (probably not the healthiest way to determine the type, but I’m still kicking and fairly healthy). He then took each rod and burned them to match the smell. I used the rod that matched the smell and it worked.
Most plastic welding rods come in different colors to distinguish the type of plastic it is, but the parts to be welded can be hard to determine. Some parts will have the type of plastic molded on them. I have also googled the parts and the description of the part said what it is made from.
Most 4 wheeler fenders are made from polyethylene or polypropylene and I have welded many of those back together after my kids trail rides into the trees.
A lot of the hard plastic parts on GWs are ABS which is the case of the fork cover in the photos. I also had a front rotor cover that the plastic where the bolt holds it break off. I took a stainless washer and welded plastic around it to hold the washer and the bolt rested on the washer.
I use a soldering iron from Radio Shack because it is small and I am able to get into the tight areas. Most plastic welders come with a small “boot” end to heat more area.
As with most parts, there will be paint or in the case of the fork cover, chrome coating. This has to be removed by sanding or a Dremel tool. The plastic will not bond to the paint or chrome. If using a power tool to remove it, use caution, the plastic comes off very quick and you can remove more than needed. If so, more can be filled in from the rod.
I try to salvage any pieces broken off so the original size is still there to work with. I will clean the parts to get to the base plastic and hold the broken piece onto the larger piece and “tack” weld them together with the iron. While tacking on I melt a small groove where the break is and lay the filler rod in the groove, melting off about 1/4” of the rod. Then take the iron and start melting the plastic, welding the new plastic into the old. You have to be sure to melt the old plastic to blend or “weld” the two together for a strong bond. If space allows, I will add more plastic to strengthen the areas where the part originally broke. When they mold parts new, they will use only enough plastic to make the part strong, but not any extra. When making many 1,000s or 100,000s of parts, the manufacturer has to watch costs and any excess plastic becomes several thousand dollars after several thousand parts. When I am repairing the parts, the filler rods are cheap and I will add more plastic for strength if fit is not an issue.
Sometimes on the 4 wheelers, there were fenders with pieces completely missing so I would use whole pieces from old fenders or even plastic bowls or totes to replace the missing pieces. My kids rode many multicolored 4 wheelers.
Use caution when welding plastic. Always have a well ventilated area and know that hot plastic will burn skin and continue to burn until it is pulled off or cools down. Plastic cools fairly quickly, but on skin it’s not quick enough!
If welding plastic is something of interest to anyone, the rods can be purchased at Harbor Freight or some of the home improvement stores. Kits are also available ranging from a few dollars to several hundred dollars depending on what is to be welded. Sometimes it is cheaper and easier to buy a new piece and bolt it on, but I enjoy a challenge and if it’s broke I can’t hurt it.
Below are some photos of the fork cover I recently repaired.
Thanks,
Dave
I just searched a few other threads and didn't see the primer used in them.