Repairing a Cracked Bumper with the Nitrogen Plastic Welder

Let’s assume you have a PP/TPO bumper torn to the edge. This section will take you through the whole process—cleaning, prepping, and welding. This describes the basic repair technique that can be adapted to other speciality repairs on tabs and mounting holes.

Clean the bumper before you grind it. Before you touch the bumper with sandpaper or a die grinder, make sure it's clean first. First, clean the entire bumper by washing with 1020 Scuff Magic soap using a red scuff pad to spread the soap and scuff the plastic. This will put small sanding scratches into the bumper to help further improve adhesion. Rinse the soap off and allow the bumper to dry. Once dry, clean the bumper with Polyvance 1000 Super Prep plastic cleaner or 1001 EcoClean This will remove all solvent-soluble contaminants like silicone, wax, mold release agents, etc. Spray on in a heavy, wet coat, let it sit on the surface for a few seconds, then wipe dry with a clean paper towel before it evaporates. Don’t just soak a rag and wipe it around—that only moves the contamination around on the surface and does not remove it.

Align the Outer (Cosmetic) surface. Often the plastic has been stretched or distorted in the damaged area. Before you weld, get the crack aligned as closely as possible. If the plastic is dented or stretched, heat with a heat gun and push the plastic back into position referring to detailed instructions on Page 4. Once the plastic pieces are lined up, use Polyvance’s 6481-1 or 6485 aluminium tape on the outer (cosmetic) surface. It’s best to line up the outer surface to minimize the need for filler and weld the backside of the crack first.

Prep the backside of the crack. Grind the back side with 50 to 80 grit sandpaper to roughen the surface and remove any paint overspray. Grind the surface flat if it doesn’t need to be smooth on the back. Grind a v-groove if the surface needs to be flat when finished. Grind at a slow speed; if you go too fast the friction heat will melt and smear the plastic. You want to put a good, rough sandscratch in the plastic, not melt it.

Weld the backside of the crack. For maximum strength, use the R02-05 wide ribbon on the backside. This wide ribbon spreads the load over a large area for maximum strength. To do the weld, start by preheating the end of the welding rod and the bumper at the end of the crack. The plastic will start to turn glossy. At this point, push the end of the ribbon rod down into the plastic and bend it toward the welder. Make your pass by melting the bottom surface of the rod and the top surface of the bumper simultaneously, forcing the melted plastics together with steady downward pressure on the welding rod. The proper welding pace is 4 inches to 6 inches (10 cm to 15 cm) per minute. It is most important to melt both the rod and the bumper at the same time.

Weld a "Tee". Upon completing the weld down the crack, it is advisable to make another cross stitch weld at the end of the crack along the edge to reinforce the repair. Use the R02-04 ribbon for the edge reinforcement. Because the orignal extruded structure of the ribbon is not disrupted in the welding process, trying to tear the bumper is like trying to tear the rod in two. As long as the rod is welded solidly to the bumper, this will provide a lot of extra strength.

Prep the front side for welding. After the backside weld is cool, peel the aluminum tape off the front side. Using a 6121-T or 6125 cutter bit in a rotary tool, grind out a v-groove down the crack in the plastic about 1/16” to 3/32” deep. The width of the v-groove should match the kind of rod you want to use on the front side. Most of the time the R01-01 1/8” diameter round rod is the best choice because it is easier to finish being a narrow rod. In this case, grind the v-groove about 1/8” wide. For stressed areas or anywhere you want some extra strength, you may choose to use a ribbon on the frontside. In that case, grind the v-groove a little wider than the width of the ribbon. Sand off the paint immediately adjacent to the v-groove with 80 grit in a DA sander.

Weld the front side of the crack. Using your selected welding rod or ribbon, perform the weld in the prepared v-groove the same way you did on the back side.

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