Welding Polypropylene Bumper Support Brackets

Many bumpers have welded-in bumper reinforcements like this Mazda Miata rear. The reinforcement is held in place by tabs in the bumper that pass through slots in the reinforcement and then are melted over like a mushroom.

In this example, the reinforcement is made of the same material as the bumper itself (polypropylene). Look for the ID symbols molded into the parts to find out what kinds of plastic you are working with. If the reinforcement is a different material than the bumper, you cannot use this repair procedure.

Always wear proper safety gear while working!

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Products Used:

Note: This is a list of the Polyvance products that can be used to make this repair. This list does not include sanders, grinders, or other common tools you will need. If you have any questions, please don't hesitate to call technical support at 800-633-3047.

• 1000 Super Prep Plastic Cleaner (several options available) OR 1001-4 EcoPrep Plastic Cleaner
• 6125 Tapered Burr
• Aluminum Body Tape (several options available)
• Nitrogen Plastic Welder (several options available)
• Polypropylene Welding Rod (several sizes/colors available). *Note: We used polypropylene here because it matched the bumper's plastic. Choose the rod that matches the plastic you need for your repair.

Video Notes:

• English Closed Captions have been provided for this video. Click on the CC button to turn them on or off.
• The length of this video is in no way representative of the actual time required to perform a complete repair and therefore should not be used for estimating purposes.

English Transcript:

Many bumpers have welded-in bumper reinforcements like this Mazda Miata rear. The reinforcement is held in place by tabs in the bumper that pass through slots in the reinforcement and then are melted over like a mushroom. In this example, the reinforcement is made of the same material as the bumper itself. Look for the ID symbols molded into the parts. If the reinforcement is a different material, you cannot use this repair procedure.

The first step after cleaning the plastic will be to remove the broken-off tab from the slot in the reinforcement. Use a utility knife to separate the welded-on tab from the reinforcement, then pop it out. Clean out the rectangular hole to prepare it for welding.

Using a 6125 die grinder burr, v-groove into the bumper where the tabs were. V-groove fairly deep into the bumper so your recreated tab will lock into it very well. V-groove all tab locations that need to be replaced. In the case of this Miata, there are two tabs holding the reinforcement in place. Remove any paint in the vicinity of the v-groove to minimize contamination of the weld.

Using the die grinder burr, clean up the area around the slots in the reinforcement to prepare it for welding. Remove any paint overspray and grind down into the raw material.

Place the reinforcement back in the proper position. Use Polyvance's 6485 aluminum tape to hold the reinforcement in position while the weld is done. In this case, since the bumper is quite thin where we made the v-groove, we'll apply some tape to the outer surface to provide support, then wrap the tape over the top to hold it in position. You don't want the reinforcement to move as you're welding it.

Preheat the area first before welding. Blow the hot nitrogen gas directly through the slot to preheat the bumper. Once the substrate is up to temperature, start to heat the welding rod until it turns translucent, and then begin welding. Push all the rod all the way through the hole and into the base. Notice that the welding torch is turned to direct most of the heat onto the rod, since the base material is already near the melting point. Melt and apply several passes of the welding rod to fill the slot and build up on the top side so you can mushroom the material over the reinforcement to hold it in place.

Finish by smoothing the protruding welding rod and spreading out onto the reinforcement with an airless plastic welder. Once the first weld is completely cool, remove the aluminum tape and reapply on the other side to hold it in place for the next weld.

Weld the second and subsequent tabs just like the first. Preheat the hole and bumper to nearly the melting point by blowing the hot nitrogen gas through the slot. Weld first by pushing the rod all the way through the slot so you have penetration into the bumper. Keep making passes with the welding rod until the slot is filled and you have plastic protruding from the top. Notice that we're using every bit of the welding rod in this repair! Finish by smoothing and shaping with an airless plastic welder.

Allow the welds to cool completely, and you're done. This type of repair would be virtually impossible with an airless plastic welder alone. Polyvance's nitrogen plastic welder makes this repair fast, easy, and strong.

Look to Polyvance for all of your plastic repair product and training needs. We offer I-CAR Industry Training Alliance courses, as well as other hands-on classes in addition to the video resources you can find on our website and on our YouTube channel.