When ABRN last talked with some of its most recent Top Shops winners, several mentioned they were introducing nitrogen welding gradually. They were giving their techs the opportunity to examine the technology before fully implementing it into their operations. Considering there is a slight learning curve and that nitrogen welding promises to offer a number of repair solutions that ordinarily might involve a new replacement part, this is the smart, expected strategy. Once these shops become comfortable with the technology, others are sure to follow. Indeed, that's always been the path for most new repair techniques in the collision industry.
For now, shops can turn to a number of other plastic repairs that can be conducted with airless welders and other tools they should already possess. Refer to the following instructions, provided by the Urethane Supply Company, to handle some of your most common plastic repairs.
Proper preparation
Virtually all collision repair tasks begin with cleaning the vehicle or part surface. Nowhere is this more important than plastic work, where the strength of the repair relies on a clean start.
Step 1: Begin by cleaning both sides of the repair area with hot water and soap designed for cleaning plastic.
Step 2: Dry with compressed air or a clean shop cloth
Step 3: Hard-to-remove contaminants such as wax or tar may still remain on the plastic. Thoroughly clean the area again with dedicated plastic cleaner.
Step 4: Using a clean, lint-free cloth, wipe the cleaner off while it is still wet. Be certain to wipe in one direction to avoid spreading any contaminants back over the clean area.
Examine the plastic for any sign of distortion. If there is, reshape the distortion with these steps:
Step 1: Heat the distortion with a heat gun. Note that the plastic must be heated all the way through.
Step 2: Maintain the heat gun on the area until the opposite side of the plastic is uncomfortable to the touch.
Step 3: Use a screwdriver handle or other blunt tool to force the plastic back into position.
Step 4: Use a damp cloth to cool the area. Stretched areas will shrink once the bumper cools.
Step 5: Once the distortion is reshaped and smooth, check your work by sanding it with 80 grit paper, which will help you identify any remaining low spots.
Step 6: Repeat the heating, reforming and cooling process to push out any remaining low spots.
Note that this process can be replaced with another for certain plastics. Thermoset polyurethanes (PUR and RIM) possess a “memory” characteristic that sometimes will allow them to return to their original position if they are held under a heat lamp or in a heated spray booth.
Any cuts or tears in the plastic also must be prepped. Use aluminum body tape on the back of the repair area to align the cosmetic surface. Aligning the outer surface helps minimize the amount of filler required to restore the exact profile of the part.
Repairing automotive urethanes
Repairers need to be cautious when working on plastic parts, particularly bumper covers, constructed of automotive urethane (also called PUR). Automotive urethanes are "thermoset" materials, which means they're created by combining two liquid chemicals come to form a solid. If these urethanes are ever heated, they break down and will not adhere to any repair materials. Technicians therefore can never melt these plastics with a hot air welder.
Shops typically can identify these plastics using identification markings on parts or the manufacturer's documentations. If those aren't available, a technician can test the part using a welder. This is done by first selecting a small area on the back of the part away from the repair. Press a hot welding tip into the area.
If the plastic is an automotive urethane, it will liquefy, bubble and smoke. Also, after it cools, the plastic remains tacky or gummy.
Although these plastics are extremely heat sensitive, they still can be easily repaired using an airless plastic welder. Note that the repair techniques for these plastics more closely resemble brazing rather than a true fusion weld. Use the following steps for performing repairs on parts fabricated from automotive urethanes:
Step 1: V-groove the damaged area (which should already be lined up using aluminum body tape or clamps). Using either a teardrop cutter bit or tapered burr, make the v-groove halfway through the back side of the part.
Step 2: Sand the v-groove with 80 grit or coarser paper to put a “tooth” into the plastic.
Step 3: Sand away the paint in the area surrounding the v-groove and radius the edges of the v-groove for extra strength.
Step 4: Set the temperature setting of the airless plastic welder to “clear” and place the appropriate welding rod in the welder.
Step 5: Holding the welder’s tip slightly away from the surface of the plastic, melt the rod into the v-groove. The rod should come out of the bottom of the welder’s shoe completely melted and clear, never bubbling or discolored. Turn the welder up or down as needed to produce the desired result.
Do not overheat the base material. Simply melt the rod onto the surface.
Also, lay down no more than 2 in. of welding rod into the v-groove at a time. Remove the rod from the welder tip, and before the melted rod has time to cool down, go back over it with the hot welder tip and smooth out the well. You can touch the bumper with the welding tip, but keep the tip moving so you don’t overheat the base material.
Step 6: After the weld on the back side cools, repeat the v-groove and welding process on the
cosmetic side. Be sure to create a v-groove deep enough to penetrate the welding rod on the back side.
Step 7: Using coarse sandpaper, sand the weld to a smooth contour.
Step 8: Since the urethane welding rod will not feather very well, cover it with an appropriate epoxy filler to refinish completely. Sand the weld slightly below flush so that filler can cover the welded area completely.
Repairing torn bolt holes
An airless welder also can repair torn bolt holes in plastic.
Step 1: Use a sanding disc to taper the all plastic around the hole down to a point on both sides.
Step 2: Use aluminum body tape to create a bridge across the torn mounting hole.
Step 3: Melt a urethane welding rod into the area.
Step 4: When finished, drill out the new hole.
Padded dash repair
Padded dashes are common repair areas since they are often damaged in front-end collisions and can crack or break due to exposure to extreme temperatures. Use the following repair instructions:
Step 1: Bevel the edges of the area to be welded with a drum sander on an electric die grinder. Open up a cavity at least 1/4 in. deep in the foam backing and vinyl cover.
Step 2: Sand and bevel the vinyl cover around the cavity to allow for featheredging of filler.
Step 3: Using an appropriate size welding rod and an airless welder, start the weld at the bottom of the cavity. Fill the cavity with melted plastic and spread it out so that it overlaps the edges of the vinyl cover by about 1/4 in.
Step 4: After allowing the weld area to cool, grind the area smooth with a drum sander. Rough up the surrounding area to improve filler adhesion.
Step 5: Mix the padded dash filler and apply with plastic squeegee. Cover an area larger than the weld in order to feather out the repair to a smooth contour.
Step 6: Allow the filler to cure for at least 15 minutes, then sand to a smooth contour. Finish sand the filler with 220 grit paper.
Step 7: The dash will need to be re-textured. Begin by cleaning the area with soap and water. Dry with a clean cloth. Apply a plastic cleaner and wipe dry with a clean cloth.
Step 8: Sand with 180-240 grit sand paper to remove any gloss. Blow the area dust free with clean, dry air.
Step 9: Apply a texture product according to the manufacturer's instructions. The instructions should include a “texturing reference” guide to allow you to match the product to the dash texture.
Note: Do not try to spot retexture. Retexture and blend the leading edge or most visible area of the dash pad all the way across. If there is a noticeable difference in texture, retexture the entire pad.
Step 10: Finish by applying a plastic topcoat. In some cases, a lacquer compatible top coat system will be necessary.
One final note, patience is a virtue when working with plastics. These repairs aren't time or labor intensive but need to be performed with a high degree of care if they are to meet your quality standards.
