Understanding Urethane Bumpers
Urethane bumpers, also known as thermoset polyurethane (PUR) bumpers, were common on domestic cars from the 1970s to the 1990s. While less prevalent today, they can still be found on some vehicles like the Dodge Challenger. Unlike most modern plastic bumpers made of thermoplastic materials, urethane bumpers are thermoset plastics, meaning they don’t melt when heated.
The Challenge of Repairing Urethane Bumpers
Many technicians mistakenly try to melt the base material when attempting to repair urethane bumpers. This approach breaks down the plastic chemically, resulting in a sticky surface that won’t bond properly. The key is to use the heat of the welder to apply a thermoplastic urethane filler rod, essentially creating an adhesive bond rather than a fusion weld.
Step-by-Step Repair Process
1. Identify the material: Look for a yellow color or PUR/RIM markings. If unsure, test with a hot welding tip on an inconspicuous area.
2. Clean and prepare: Thoroughly clean the damaged area and create a v-groove about halfway through the crack using a cutting burr.
3. Realign and secure: Use aluminum body tape to hold the cracked pieces in alignment during the repair process.
4. Apply the filler rod: Set your airless plastic welder to the “urethane” setting. Melt the thermoplastic urethane rod into the v-groove, working in small sections.
5. Smooth the weld: Before the melted rod cools, use the welder tip to smooth out the application, being careful not to overheat the base material.
6. Repeat on the opposite side: Once cooled, create another v-groove on the opposite side and repeat the welding process.
7. Finish the repair: After cooling, sand the welded area smooth and apply a flexible epoxy filler for a seamless finish.
Tips for Success
Always work in a well-ventilated area when using heat-based repair methods.
Practice on scrap pieces before attempting repairs on a vehicle.
Remember that you’re creating an adhesive bond, not melting the base material.
Be patient and work in small sections for the best results.
By following these steps, you can successfully repair cracks in urethane bumpers, extending their lifespan and saving on replacement costs.