Well my rebuilt extruder didn't last long enough to even make a raft!
I calibrated the Z origin yesterday but when I started extruding today the nozzle ploughed into the polypropylene bed. Thinking I had made some mistake I calibrated it again and it did the same thing. The PTFE barrel can no longer slip in the clamp because it is pinned. The heater barrel can not slip out of its thread because I have a metal ring around the PTFE to stop it swelling. What seems to have happened is that the PTFE barrel has elongated.
It has also bent somewhat. The last PTFE barrel did not elongate significantly but it did swell. I can only think that because I have reduced the nozzle aperture from 0.5mm to 0.3mm the pressure has increased. I didn't notice much change in the motor current though.
I am not sure what to do now. I could make a thicker PTFE barrel but I will have to make a new clamp, which means converting my machine back to a milling machine, or I could drill the hole out to 0.5mm.
A 0.5mm hole gave me ~1.2mm filament which I stretched to 0.5mm. The 0.3mm hole gave me 0.77mm filament at the same extrusion volume rate. It still needed stretching to meet my 0.5mm target. Also it has the disadvantage that the maximum size I can extrude without stretching is now 0.77mm rather than 1.2mm. I didn't get chance to find out what effect less stretching had if any.
The sleeving I used is already looking sad so I ordered some PTFE sleeving to replace it.