Soldering irons, which produce a significant amount of heat, are often used to melt and weld plastics. As such, they are very useful around the household: in instances when glue does not adequately fix a plastic tool, soldering often does. However, soldering plastic can give off dangerous fumes. If you choose to solder plastic, solder it outside, and wear a soldering respirator to protect yourself from the fumes.
Heat up your soldering iron, outside. Put on the soldering respirator, per manufacturer's instructions.
Touch the end of your soldering iron to the area of your plastic that you want to melt. If you are welding two pieces of plastic, place them adjacent to one another, clamp them together, and touch the soldering iron to the groove between the two pieces of plastic, allowing them to melt.
Move the soldering iron over the two pieces of plastic. Add plastic filler to the plastic as you move the iron, to avoid warping.