OT, car repair: Do those cheap plastic welders really work?

I think you overpaid for what you got. This would have worked as well or better:

How does that thing work? Does it burn acetylene?

No, its an electric heat gun with the filtered air controlled by the pressure regulator. So you need an air compressor with an air filter and

Works like TIC welding: You hold the furnished plastic rod (3 types of rods furnished with kid) that matches with the bumper material and the tip blows hot air on the rod and damaged area together so the parts bond together. A good plastic bond would be stronger than the mother material - just like good welding and solid wood glued joints.

High learning curve to make it work, just like TIC welding. Playing around with it now.

LOL, this has no structural value - duct tape would been better than this for the bumper! I've repaired mufflers and tailpipes with a wire feed welder but many times as the corrosion is so bad, its just better replace the suckers. BTW, for tailpipes and mufflers, foil works pretty well, the one with sticky back for HVAC. YMMV.

The problem you will have is mating the plastic components. The two materials have to both melt at about the same temperature and the materials themselves have to be compatible, i.e. they will fuse together. I suspect that some plastics may not combine well if at all.

Yike. I've just spent a weekend trying to fix some kind of big plastic storage bin (ok, i have too much time on my hands) which involves figuring out what kind of more or less clear plastic it is, and/or what the melting temp is; neither of which I could do. (my cute little $10 IR thermometer was doing OK on the melting point test until it started to register "HIGH" just above 250 fahrenheit or so, when the plastic was still looking good).