This filament welder allows to join 1.75mm filament without sanding or other post processing.

You will need A PTFE tube and coil spring.
-PTFE tube: Length: 40mm, 1.8mmx2.6mm
-Coil spring i took mine from ballpen

Instruction of use:
Spring keep heat a bit longer, so when you push filament from both sides and heat it by lighter or heat gun in the middle. Snap it between covers from both sides, press tight as you can, and push the filament from one side till joint come out from other. 


WHY DO YOU NEED A SPRING?
When you heat the PTFE tube, it softens. Two filaments are pushed in from both sides in that state, and the tube expands at the butted part. A spring is installed to prevent this expansion. Therefore, when you choose a spring, its inner diameter should be approximately the same as the outer diameter of the tube.