The latest steps for plastic repairs

July 1, 2020
Today plastic repair still entails a necessary degree of artisanship. Mastering this work is essential to properly bringing a part back to life.

What is the state of “artisanship” in today’s collision repair industry? Just one generation ago, repairers were raising this same question because many felt they were quite literally saying goodbye to an era of repairs. One successful, long-time shop owner described the change to ABRN as such, “We’re no longer craftsmen. We don’t reshape metal. We are simply parts replacers.”

To a great degree, that was the course the industry had taken. Swapping out damaged parts with replacements (often from lower-cost, aftermarket sources) was cheaper and more efficient than restoring the fit and beauty of a piece by reforming the steel. Ironically, at the same time, a new kind of artistry was making its way into repairs with the increase in the use of plastic parts. Reforming/restoring plastic often proved cheaper than replacing an entire part and involved the kind of craftsmanship being lost on the metal side of repairs.

(Photo courtesy of Polyvance) When welding plastic, the goal is welding the rod and the repair area at the same time without “puddling” as one typical does during metal welding.

Today plastic repair still entails a necessary degree of artisanship. Mastering this work is essential to properly bringing a part back to life. Help ensure your shop is up to its next plastic repair with these steps and recommendations provided by Polyvance.

Cleaning, aligning damage, handling deformation and aligning tears

Plastic damage comes in many types and often in combinations. Follow these basic repair rules.

In order to maximize strength any repair, thoroughly clean contaminants from the surface in the damaged area with these steps:

Step 1. Clean both sides of the repair with soap and water. Dry with a clean cloth or compressed air.

Step 2. Spray a dedicated, specialized plastic cleaner onto the surface and wipe off while wet with a clean, lint-free cloth. Wipe in one direction to avoid spreading contaminants back over the clean area.

Distortions

If the plastic is distorted, heat with a heat gun and reshape the distorted area. Always heat the plastic all the way through by holding the heat gun on the area until the opposite side of the plastic is uncomfortable to the touch. Once heated, force the plastic back into position with bumper rollers or other appropriate tool, then cool the area with a damp cloth. Stretched areas can be shrunk with the bumper cold. Keep working until the plastic is smooth, then sand overall with 80 grit to help identify remaining low spots. Push out remaining low spots and repeat the process.

Note that thermoset polyurethanes (PUR, RIM) feature a “memory” that often can cause them to return to their original position if held under a heat lamp or in a heated spray booth.

Tears

If the part is cut or torn to the edge, align the cosmetic surface using aluminum body tape and begin the repair process on the back. By aligning the outer surface, you minimize the amount of filler required to restore the proper profile to the part.

Basics of the nitrogen welding process

Welding with nitrogen involves the coordination of both hands, with one controlling the torch and the other feeding the rod. Do NOT “puddle” the rod as you would in metal welding. Instead, melt the bottom surface of the rod and the top of the bumper at the same time and fuse them together with the downward pressure on the rod. This process creates a stronger repair because it leaves the basic structure of the rod intact.

Remember the five important factors for plastic welding: T.F.A.P.S. - Temperature, Flow, Angle, Pressure, and Speed.

·       Temperature - set the temperature on the nitrogen welder’s dial to the proper setting based on the type(s) of plastic. For example, the PP/TPO setting will generate an air flow of about 700 – 800 degrees Fahrenheit.

·       Flow - set between 2.5 – 4 gallons per minute depending on the plastic thickness; less for thinner plastics, more for thicker.

·       Angle – maintaining 45 degrees between the torch and the bumper is optimum. Aim the stream of hot air a little in front of the rod; for a thick rod like the 06 profile, focus a little more heat on the rod.

·       Pressure - put as much downward pressure on the rod as possible to help the rod fuse with the bumper. Keep a steady downward pressure on the rod and keep the rod moving slowly. Don’t overheat the rod and let it fold over backwards.

·       Speed - weld 4 to 6 inches per minute. This can be difficult with a thin rod like the 03 profile, since the rate is so slow. With thicker rods, like the 06 profile, you may need to go even slower. The important thing is to move steadily while keeping proper downward pressure on the rod while also making sure the bumper and the bottom surface of the rod are both melted before they come together.

(Photo courtesy of Polyvance) The five important factors for plastic welding are T.F.A.P.S. (Temperature, Flow, Angle, Pressure, and Speed). Make sure your equipment settings and processes follow all five.

Repairing a cracked bumper with a nitrogen plastic welder

The repair steps listed here are specifically for a PP/TPO bumper torn to the edge, though the technique can be adapted to other specialty repairs on tabs and mounting holes.

1.     Clean the bumper before you grind it. Start by washing the entire bumper (not simply the repair area) with soap and water.

2.     Use 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.

3.     Rinse the soap off and allow the bumper to dry. Once dry, clean the bumper with a specialty plastic cleaner to remove all solvent-soluble contaminants such as silicone, wax and mold release agents. Apply the cleaner in a heavy, wet coat; let it sit on the surface for a few seconds and then wipe dry with a clean paper towel before it evaporates. Caution: Do not simply soak a rag in cleaner and wipe it around since this only moves the contamination around on the surface without removing it.

4.     Align the outer (cosmetic) surface. Often, plastic has been stretched or distorted in the damaged area.

5.     Before welding, align 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.

6.     Once the plastic pieces are lined up, use aluminum body tape on the outer (cosmetic) surface to minimize the need for filler.

7.     Prepare the back of the crack first. Prep the back with 50 to 80 grit sandpaper to roughen the surface and remove any paint overspray.

(Photo courtesy of Polyvance) Polyvance’s head trainer John Wilburn trains Jeff Bown the head trainer for Chicago-based MSO Crash Champions. Be sure to take advantage of available plastic repair training and build your experience.

8.     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. Always grind at a slow speed since going too fast creates friction heat that will melt and smear the plastic. The goal is creating a good, rough sand scratch in the plastic without melting it.

9.     Weld the back of the crack. For maximum strength, use the R02-05 wide ribbon. This wide ribbon spreads the load over a large area for maximum strength. Start the welding by preheating the end of the welding rod and the bumper at the end of the crack. The plastic will start to turn glossy.

10.  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. Again, the proper welding pace is 4 – 6 inches per minute. Always melt both the rod and the bumper at the same time.

11.  Weld a tee. After completing the weld down the crack, reinforce the repair by making another cross-stitch weld at the end of the crack along the edge using the R02-04 ribbon for the edge reinforcement. (Because the original 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 plenty of extra strength.)

12.  Prep the front side for welding. After the back 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 inches 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 R02-01 1/8 in. 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 in.  wide. For stressed areas or anywhere you want added 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.

13.  Sand off the paint immediately adjacent to the v-groove with 80 grit in a DA sander.

14.  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.

15.  Finish the weld. Once the weld is completely cool, sand with 50-80 grit paper in a grinder or DA. Be careful not to sand too fast since this will just melt and smear the plastic. It’s best to use a sharp, new piece of paper and slow down the sander a bit to keep the plastic from melting.

Sometimes the repair may be finished out and feathered with the welding rod, but most often you’re going to need some filler to fill in the low spots.

As always, be sure to follow all OEM and repair instructions. Because plastic repair requires experience, practice your skills and help train others.

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