if you want to "Weld" plastic... get a length of filament and put it in a dremel, use this as your MIG (not TIG) weld. The friction will melt all surfaces enough to "wled" them together. An iron is going to burn, not melt them, and if you do get melting, it will be difficult to control where you are melting. Plastic is NOT solder, it will not quickly melt and solidify. It'll take time. That time will lead to the end of the world. Don't use a solder iron.