I tried extruding hotter and colder, tried with the idler wheel tighter and with it looser. I even tried taking the insulation off of the heater barrel in case the filament was melting too far up into the barrel. Some changes made it work worse, but none made it work better. I've been seeing a lot of this:
These are supposed to be dodecahedrons.
I did finally figure out the problem late tonight. I completely disassembled the extruder barrel assembly. It turns out that the PTFE thermal barrier was too tight towards the bottom, and greatly restricting movement of the filament. I cleaned it out using a 1/8" drill bit (held by hand, not in a drill) until the filament could move through it with no resistance. I had to remove some more material from the inside of the thermal barrier after screwing it back in to the heater barrel. Now it works quite a bit better:
See that in the back? It's a complete dodecahedron! Here's a closeup:
The one on the right is sitting on top of a green laser pointer. It glows nicely.
It was exciting to see this part completed. I was surprised at how well bridging the top material over the hollow inside of the shape worked.
I just realized that I don't have any pictures of the completed 3D printer up here yet, so here are a couple. The wire routing still needs some work!