I have not succeeded yet in getting the stainless steel barrel to extrude easily, so I had a go at improving my aluminium extruder to remove the need for a fan.
A lot of heat is lost from the large flange on the top of the heater barrel and transfers by convection to the heatsink above and by conduction through the bolts.
When I stripped it down I noticed the bolts had loosened.
The PTFE had shrunk lengthways and expanded in diameter and was no longer making a good compression seal.
Although no plastic had escaped it had leaked under the PTFE.
I think it was leaking so slowly that it oxidized where it met the air and went hard, stopping further flow. I hadn't run it very long so it may have escaped eventually.
So PTFE is obviously no good in compression at these temperatures. I replaced it with PEEK, which is a shame because it is about ten times more expensive.
I also replaced the aluminium flange with an M8 x 25mm steel washer insulated from the barrel by a PEEK collar.
Here are the parts, the PEEK section is drilled in situ to get perfect alignment: -
And here it is assembled: -
I put some PTFE plumbing tape over the hot end of the PEEK before pushing it into the aluminium in an attempt to improve the seal.
The heatsink now runs at 80°C without the fan. I would have liked it to be lower but as long as it is below the glass transition of the filament and the clamp it should be OK. I tried insulating the bolts with PEEK washers but that only dropped the temperature by 5°C, so not really worth the effort.
After one heat cycle I noticed the bolts were not as tight as they should be so it looks like PEEK creeps a bit as well. Perhaps glass filled would be better.
It is disappointingly complex with lots of machined parts but it does work very well. The heater power has dropped to 50% from about 80-90% with the fan. ABS filament extrudes manually very easily and even HDPE only requires moderate force. I think actually having the long heatsink preheating the filament to just below the glass transition is probably a benefit.
It is too early to say whether this design will be reliable but other than the PEEK section leaking there isn't anything likely to fail. I don't mind making things that are difficult to make provided I only have to do it once.
I also have a much simpler design in mind that should achieve the same short transition zone.