wrote: (clip) I would try using an inert gas to see if it makes any difference. CO2 ought to be inert at the temperatures one uses for plastic. It also sounds as if trying to stir with the rod might help. Probably would look worse, but may have better strength. ^^^^^^^^^^^^^^^^^^^ The welder is designed to be used with an air supply. The "flame" is a stream of hot air which melts the plastic. If I had to do this in a location where I had no air supply, I can see how a small CO2 tank might be substituted, but I doubt that it has anything to do with shielding. There is no evidence that the plastic, as it melts, is oxidizing.
Stirring with the rod: That was my first mistake. The material does ot go liquid, like metal. Once welding temperature is reached, as soon as the rod touches the parent material, it sticks. Then, if you move it, you pull out filaments of plastic, which are no longer part of the weld, and just make a mess.
Thanks for the CO2 idea, though. I may try it to see whether I can dispense with the compressor.