Method B, Thermoplastics Airless Fusion Welding
Excluding urethane bumper covers, all bumper covers, most other plastics on automobiles, and nearly everything else made of plastic, are made of thermoplastic materials.
This means they can be melted with the application of heat.
Thermoplastic parts are made by melting pellets of plastic and injecting the melted material into a mold, where it cools and solidifies. This means that thermoplastic parts can be melted.
The most common thermoplastic automobile bumper material is TPO (PP+EPM).
TPO is fast becoming the most popular material for all sorts of interior and under hood plastics as well.
TPOs can be welded using the fusion technique described on this section, but our FiberFlex rod often makes an easier and stronger repair on TPO than airless fusion welding.
If you have the equipment, the strongest way to repair any thermoplastic is with the nitrogen/hot air welding process.
The least common bumper material, Xenoy (PC+PBT), is best repaired using the following thermoplastic fusion technique.
V-Groove Damaged Area
- Line up the outer surface of the tear with aluminum body tape or with clamps.
- V-groove halfway through the part.
- Remove the paint in the V-groove and the area surrounding with coarse sandpaper.
Melt the Rod Together with the Base Material
Set the temperature setting of your airless plastic welder to the setting that’s appropriate for the welding rod you selected in the identification process.
In most cases, the welding rod should melt cleanly and not be discolored (the only exception would be nylon, where the rod should turn a light brown).
- Use the tip of the welder to pre-melt about two inches (5 cm) of plastic inside the v-groove.
- Lay the welder tip on the surface of the plastic and slowly melt the rod into the V-groove. Pull the welder toward you so you can see the welding rod fill the V-groove as you make your pass.
- Lay down no more than 2 inches 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 thoroughly melt the rod together with the base material.
It helps to press into the plastic with the edge of the welder tip to mix the materials, then go back and smooth it out.
Keep the heat on it until you have a good mix between the rod and base material.
V-Groove and Weld Opposite Side
After the weld on the back side cools, repeat the V-groove and welding process on the cosmetic side.
V-groove deep enough to penetrate into the welding rod on the back side.
Grind Weld to a Smooth Contour
If you need to refinish the plastic, grind weld to a smooth contour with coarse sandpaper.
Grind the weld slightly so that filler can cover the welded area completely.
Method C, FiberFlex Universal Rod
- Repairing with FiberFlex Universal Rod
FiberFlex is a unique repair material, it sticks to any plastic substrate.
It is not a true welding rod, but rather a thermoplastic or hot-melt adhesive.
When you do a repair with the FiberFlex, you will actually be using the heat of the welder to apply an adhesive.
FiberFlex has a very strong bond and is reinforced with carbon and glass fibers for outstanding strength.
FiberFlex also incorporates sand fillers, so you can create a good cosmetic repair with it as well.
FiberFlex is a popular way to repair TPOs (aka TEO, PP/EPDM, TSOP), the most common automotive bumper material.
The reason is that there are no two TPOs that are exactly alike.
FiberFlex was formulated to work best on TPO bumper covers, but it can also be used to repair virtually any plastic.
It will stick to urethanes and Xenoy, too.
When you are not sure what type of plastic your part is made of, try FiberFlex.
V-Groove Damaged Area
- Line up the outer surface of the tear with aluminum body tape or with clamps.
- Grind away plastic into the shape of a broad V-groove halfway through the backside of the part using a die grinder with the round cutter bit.
- It is very important to put some “tooth” in the plastic by grinding the V-groove with 50 grit or coarser sandpaper.
Use a low-speed grinder. Grinding at high speed will tend to melt many thermoplastics.
- Using 80 grit in a sander, feather back the paint in the area around the V-groove and radius smoothly into the V-groove.
This will give you a better featheredge when you get ready to sand the FiberFlex.
Melt on the FiberFlex
With the airless welder set to the highest temperature setting, use the Teardrop
Welding Tip to melt the FiberFlex welding rod onto the surface.
Best adhesion is achieved by pre-melting one side of the end of the rod, then flipping the rod over so that the melted portion sticks to the plastic.
Cut the melted part of the ribbon off using the edge of the welder tip and spread the FiberFlex into the V-groove.
Do not attempt to melt the base material together with the FiberFlex.
Repairing with FiberFlex is similar to a brazing process.
V-Groove and Weld Opposite Side
After the FiberFlex on the backside cools (you may force cool with air or water), repeat the V-grooving and welding process on the opposite side.
Build the FiberFlex slightly higher than the surface. FiberFlex is also a sandable filler.
Finish Sand
- After allowing the FiberFlex to cool completely, sand with 80 grit paper in a sander at low speed.
- Progress to finer grits, ending with 320 grit.
- Fill any low spots with more FiberFlex or with a hardset filler.
Method D, Thermoset Urethane Repair
Automotive urethane, or PUR, is a “thermoset” material.
Similar to what happens when mixing body filler and cream hardener, thermoset plastic is formed when two liquid chemicals come together in the mold to form a solid.
The importance of this is that if polyurethane is “melted” the plastic decomposes and prevents adhesion of repair materials.
DO NOT TRY TO MELT URETHANE BUMPER COVERS WITH THE WELDER!
A positive way to identify thermoset urethane is to press a HOT welding tip into the backside of the part.
If it’s a urethane, the plastic will liquefy, bubble and smoke.
(Note: welder must be extremely hot for this to happen).
After the heated area cools off, it will remain gummy or tacky. This is an indication that the heat broke down the chemicals in the plastic.
Thermoset urethanes can be easily repaired with the airless plastic welder.
V-Groove Damaged Area
- Line up the outer surface of the tear with aluminum body tape or with clamps.
- V-groove halfway through the back side of the part with the Teardrop Cutter Bit.
You cannot use a hot tool to melt the v-groove into urethane because it will decompose.
- Scratch the inside of the v-groove with coarse sandpaper (80 grit or coarser) to put “tooth” into the plastic.
Also, remove the paint in the area surrounding the v-groove and radius the edges of the v-groove for extra strength.
Melt the Rod into the V-Groove
- Turn the temperature of your airless plastic welder to the “R01” rod setting. Using the R01 urethane welding rod, the rod should come out of the bottom of the welder’s shoe completely melted and clear, not discolored or bubbling.
Turn your welder up or down as needed, until you get this result.
- Holding the welder’s tip slightly off the surface of the plastic, melt the rod into the V-groove.
Don’t overheat the base material, simply melt the rod onto the surface. Again, you are NOT trying to melt the rod and the base material together; the bumper material is NOT meltable!
- Lay down no more than 2 inches 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 weld.
You can touch the bumper with the welding tip, but keep the tip moving so you don’t overheat the base material.
V-Groove and Weld Opposite Side
After the weld on the back side cools, repeat the V-groove and welding process on the cosmetic side. V-groove deep enough to penetrate the welding rod on the back side.
Grind Weld to a Smooth Contour
Using coarse sandpaper, grind the weld to a smooth contour. The urethane welding rod will not feather very well, so it will need to be covered with 2000 FlexFiller 2 epoxy filler to refinish completely.
Grind the weld slightly below flush so that filler can cover the welded area completely.
Repairing Torn Bolt Holes on Urethanes
- Taper the plastic all around the hole down to a sharp edge on both sides using a coarse sandpaper disc in an angle grinder.
- Use aluminum body tape to create a bridge across the torn mounting hole.
Melt R01 urethane welding rod into the area.
- Drill out a hole when finished.
Steps by Photos
Method E, Two-Part Adhesives and PlastiFix
🔾 Repairing plastics with two-part adhesives
- Clean both sides of the plastic in the damaged area with plastic cleaner.
Fix the front side with clamps or aluminum tape to hold the part together.
- Sand the backside of the area to be repaired with 50 grit sand paper or coarser.
V-groove is not required unless you need to sand the backside flat for cosmetic reasons.
Remove paint in the surrounding area with 80 grit in a DA.
Coarse sand scratches are desirable to maximize the mechanical strength of the bond. Blow dust free with clean, dry compressed air.
- If the material is TEO, TPO or PP, apply Filler Prep or Plastic Magic Adhesion Promoter.
Brush or spray a medium wet coat onto the sanded area and allow to flash off; do not overapply.
- Choose a two-part adhesive system to match the hardness of the substrate per the table:**
- Select a reinforcing method for the backside of the part. For flat areas, it is often easiest to cut a backing plate out of scrap material. For contoured areas, use fiberglass cloth or fiberglass drywall tape.
- Prepare reinforcement by cutting 1 to 3 pieces of glass cloth to cover the damaged area about 2”-4” wide.
If using a backing plate, make sure it extends at least 2” beyond damage in all directions.
Rough grind the side of the backing plate that will be applied to the surface.
- Mix the two-part adhesive according to package directions. Spread a generous amount on the backside with a body spreader. If using a backing plate, press the plate firmly into the adhesive, allowing a small amount of adhesive to squeeze out from the edges.
If using fiberglass cloth, lay the cloth into the adhesive and wet the fibers using a 2042-R Saturation Roller.
Apply more adhesive over the area and embed another layer of fiberglass cloth if desired.
- Once the adhesive on the backside is cured, peel the aluminum tape off the front and grind a V-groove about 1”-2” wide with a die grinder and/or a coarse sanding disc.
Sand scratch the inside of the V-groove coarsely. Round off any sharp edges and feather the paint back with 80 grit in a DA.
- If the material is TEO, TPO, or PP, apply adhesion promoter as was done on the backside.
- Mix the two-part adhesive and apply into the V-groove with a body spreader. Slightly overfill the V-groove so that it can be sanded flush when finished.
- When the adhesive on the front side is fully cured, sand with 80 grit in a DA sander, then progress to finer grits to finish.
🔾 Repairing Plastics with PlastiFix Rigid Plastic Repair Kit
The PlastiFix Rigid Plastic Repair Kit allows you to repair cracks, fill gaps, rebuild tabs, and fix stripped threads on most rigid plastics.
The most unique feature of the PlastiFix Rigid Plastic Repair Kit is the FlexMold flexible molding bar.
The FlexMold bar allows you to replace a broken tab by forming a mold from an undamaged piece, then casting your new part using the PlastiFix acrylic adhesive system.
This system is ideal for ABS, acrylic, polycarbonate and other hard plastics, however it does not work on olefinic plastics like PE, PP, or TEO.
To complete the plastic repair after performing a plastic weld, the part must be refinished.
The following instructions provide an overview of the products and processes required to restore your PP/EPDM or TPO plastic bumper fascia back to its original appearance.
BASHEER ALATAWNEH
Business Development Manager
JoClaims
Amman – Jordan