What types of plastic did you get friction welding to work on? I got it to work on ABS, but not PETG. I did not try the other filament types.
PLA - black. I had a project where two structural pieces were glued together. I designed a 1mm groove where the parts mated, used an acrylic cement to glue them together and then friction welded the groove. As a final touch, I smoothed the seam with a wood burning iron.
TBH, I never did hear how they turned out structurally, but I did a couple destructive tests and the joint was never the part that failed.
Friction welding will work on any thermoplastic. Chuck some filament in a dremel and try it 
Brook
@Brook_Drumm the PETG that I used at MRRF did not work. It flaked into a sandy mess.
It’s all in the technique… can’t describe. It’s like regular welding- takes s lot of practice