Simple filament welding jig for use with soldering iron

This is a really simple jig that works about as well as the one I spent $9 for online, shown on its left.

By “about as well” I mean that both require a lot of practice to use, and both still fail a lot for me. The welded filament will also probably need to be trimmed or filed to get it through your printer. 

However, I don't see any disadvantage to the one I can make for a few cents worth of plastic, and I might be able to improve it over time. It already leaves a smaller bulge at the join than the one I bought. Obviously, you can't use this one with an open flame. I use a soldering iron, propped up like this. Note: use a better stand than I'm illustrating here, and do not leave it unattended.

The filament ends need to be straightened so they meet precisely inside the jig. Then I heat both ends like this (this image was altered in GIMP to make colors consistent). 

The longer side from the opening (shown with the red filament) has a wider entry hole that tapers down to filament diameter. You actually have a few seconds before the PLA cools, so don't panic. Move the jig away from the iron, press the ends together, and push to the right with a little force so the join moves out through the exit.

You can see that the join is not smooth and will probably not feed. With some practice you can do better than this, and you can also try scaling the jig down by few percent. The opening is 1cm wide and you can fit a nail file inside to smooth it as I did.

Finally, I was able to feed a red, black, white filament into my Flashforge and make the layered block as shown.

I have been driving myself a little crazy trying to weld filaments. In fact, it seems to be less trouble to feed the next color while the print is running, at least if you're just doing two colors. Since the printer pulls as well as pushes, this could give different results, but I have not had any trouble so far.