stick welding + burning plastic

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im welding some thin metal, about 0.8mm (get our your calculators), ive had success with it before.im welding some pipe together with plastic in the middle. and im welding it together with a layer of pipe over the top of the joint like so: -=-im finding the welds dont seem to penetrate through the outer pipe to the inner when the plastic lights on fire (take that ozone layer/lungs!). they do when the plastic is not on fire.i know its all a bit of a  but has anyone else noticed this?
Reply:What else are you burning besides plastic?
Reply:Originally Posted by pulserWhat else are you burning besides plastic?
Reply:0.8mm is about 0.030" if I remember correctly.On one of the aluminum extrusions I weld at work (~0.060" wall thickness), there is a urethane strip on the inside wall of one of the areas of the  joint to be welded (mitered outside corner, welded verticle-down).   Any penetration through the joint in that area shoots molten filler wire (using pulse-MIG) straight into the plastic.  When that plastic starts melting, the smoke that comes through the joint expells the sheilding gas, the smoke and plastic contaminate the weld and turn it to muck.   The joint fit-up isnt always that great either, so sometimes there's a gap there to weld accross too...   I weld that section first, before the extrusion gets a chance to heat up, weld it fast, with little penetration and that keeps the plastic from melting and smoking too much so I can weld that area, then I go on and weld the rest of the extrusion properly without issue.   If I weld the plastic section last, it will turn to muck every time.I once tried to fix some cracks in the aluminum muffler for one of my 2-stroke dirt bikes.   I made the mistake of prepping the cracked areas and going ahead and welding on them.   The first weld came out fine, but it heated up the metal (and the oil residue that was inside the muffler) and when I tried to weld up the other cracks in nearby areas, the oil smoke comming up through the cracks turned the welds to muck.   I should have First heated up the whole muffler with a torch and burned out all the oil, Then weld the cracks....If there is oil or plastic smoke comming up through the weld joint on your pipes, sounds like you either need to find a way to keep the smoke from happening, or find another way to join the pipes that doesn't involve high heat and metal fusion....