Urethane Supply Company
Plastic Repair Gurus
since 1981

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Urethane weld breaks when bending and pulling


After welding a urethane bumper, the repair comes apart when bending and pulling on the bumper

Possible causes:

It is possible that you heated up the base material too much.


Before explaining the possible cause, it is important to know the distinction between thermoplastics and thermoset plastics.

  • Thermoplastic bumpers are made by injecting heated plastic material into a mold and allowing the heated material to cool and solidify. With thermoplastics, you can "undo" the process by reheating the material. For example, if you wanted to make a bumper out of ice, you could inject "heated" ice (water) into the mold and freeze it. If you wanted to undo the process, you could heat up the ice and turn it into water.
  • Thermoset bumpers are made by injecting different chemicals into the mold where they can react. After the chemical reaction has occured, there is no way to undo the reaction. Urethane bumpers are thermoset plastics.

When you weld urethane, you are not actually performing a true "weld". You are really doing a process similar to brazing or using hot-melt glue. When you heat the base material, you are not really melting the plastic. You are really destroying and burning the plastic. Once the plastic has been burned, it is difficult to get the rod to stick to it.


Grind off any burned plastic with a die-grinder to expose clean unburned plastic. Weld the damaged area using R01-01-03-NT. Be careful not to heat up the base material. Just melt the rod into the V-groove. Do NOT try to melt base material. For more information on welding urethane bumpers, follow the instructions in "The Book".

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