Here you can see my extruder's hot-end. The bottom part is a standard welding tip, which already had a 0.6mm hole in it. I drilled it out to 3mm (well, 1/8" really.) almost to the tip, so there's maybe 1-2mm left for the nozzle. Around that is wrapped the nichrome wire for heating, with a thermistor near the tip. Kapton tape surrounds it all for insulation and to keep it all in place. The welding tip goes through a fender washer and into a nut which keeps it in the washer. The washer has 3 holes drilled for the long bolts (black thread in photo) that attach it to the extruder. The white PTFE insulator sits between the washer and the extruder. I carved out a hex nut-shaped hollow in the PTFE so everything stays centered, and in a pinch I could unscrew the nut using the insulator. Leakage between the insulator and the nut is minimal. (Only noticeable when disassembled.)
The latest change to get things working is I added a bunch of Kapton tape around the washer so it stops acting like a heatsink.
I found that the PLA was cooling too much, (especially when it stopped while moving between layers after the extruder was warm,) and creating a plug in the top half of the welding tip. If I pulled out the filament, the portion between the pinch wheel and in the PTFE was a little stringy (it would stretch as I pulled) but the part below that, in the area of the nut and washer, was a perfect 3mm cylinder, which terminated in the really melted part.
And here's my first success with PLA! It's a small stand for my Droid phone, which holds it in portrait position. It fits a bit tight, but that's because I haven't had a chance to calibrate the extruder settings.
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