This is a simple tool to allow you to join multiple filament ends into one continuous filament. besides the stl you will need 2 pieces of bowden tube at about 20mm each, or any metal tube, i recommend something to have the inside diameter under 2mm, perfect would be a 1.9 hole. To join two ends, you insert both ends into the welder and aply some heat with a lighter, when starting to melt, push from one side until it enters the join into the tube. wait a few seconds until it cools down, you're done. now you have joined the 2 ends.
If using regular bowden i recommend to use a little bit of sandpaper to reduce the diameter of the weld.
Have fun

Sample video of how it works: