Plastic welding is an option. Plastic welding follows the same basic principle as metal welding. Add material, apply heat, everything fuses together. With plastic welding, there are lots of different types of plastic. However, plastic welding in the auto industry has been a thing for some time, so it’s more likely you can have success. Nylon is the plastic I read about in use in radiators.
An airless plastic welder is little more than a soldering iron, but I have found advantages to them over a soldering iron. Mostly the tip is very large so heat can be applied over a large area.