Friction Welding Plastic

I bought this Brat from ebay. While it seems complete the body was pretty trashed as you can see in the photos. I wanted to restore and not just replace parts and from the photos you can see the body has major cracks at the front. I could glue or tape the body back together, but the joints are never the same, and I want to use this as a runner so strength was important. Luckily a couple months ago I saw an article on hackaday (search friction welding at www.hackaday.com) about friction welding.

Tamiya hard plastic bodies are made of polystyrene, which is a thermoplastic (so is ABS and polycarbonate), so if you warm them enough, they will soften and eventually melt. Friction welding just involves spinning a tube of the material in a dremel or similar and pushing it against the plastic you want to join. For polystyrene the friction is enough heat and melt the plastic to form a good join. This technique is over 30yrs old and apparently was even part of a car toy from the 70s that you could smash and repair.

You have to move reasonably quickly, otherwise you can blow a hole in plastic, which I did at the base of the windscreen. I first tested using some polystyrene card and used the lowest spin speed on the dremel. The initial attempt I worked the weld too fast and it didn't hold very well. But my second try (after welding both sides) I found the weld to be very strong. Fig 7 is me flexing a test right angle joint without any issues. For maximum strength I would recommend welding both sides.

I was pretty happy with the finished weld, and it seems to be pretty strong. It will need sanding on the outside to smooth off, which I will eventually get around too!

All comments welcome.