One step forward, one step back

My jerry-built extruder is blocked.
The coach built drive is still working fine, providing quite a bit of force to the filament - enough to dear big chunks out of it.
The extruder nozzle is blocked.

This happened before, during hand testing. I've restarted and stopped the nozzle several times, and only twice had a jam. Last time, clearing the 0.6mm nozzle hole with a pin did not restart the flow, so it isn't a nozzle blockage. Providing *excessive* force suddenly cleared the blockage and restarted the flow, where it went back to normal.

There is one factor that was the same between blockages.

Both times, immediately before, the filament had been left in the nozzle, and the nozzle had been left hot for several minutes, while testing other stuff. Then the nozzle was turned off and cooled down (end of testing).
The next day, the nozzle was blocked.
On other tests, either the filament was withdrawn while hot, removing the bulk of the plastic, or after extrusion the heater was switched off immediately. Restarting the next day was fine.

I *think* what happens is that the heat travels up the filament, softening and widening it. Over a couple of minutes or so (if it cools immediately) this isn't a problem. If the heat is applied for several minutes, the filament softens further and further up, pressing against the sides of the channel, causing more and more friction. Because my insulator isn't uniform (there is a .3 mm step near the top, and a hole across (see screwdriver drive) this gives plenty of friction - blocking the filament channel. This is why clearing the nozzle doesn't help.

I'd suggest that if you're not using the extruder, or between days, turn the heat off - or better, withdraw the filament an inch or two while hot. My fully assembled extruder worked well, but last time I was testing the axes and trying the sort out a software bug, and I left the extruder hot for 20mins or so while doing so. The next time, it's solidly blocked - no extra force is shifting it. I'll either have to leave it hot for a half-hour or so to see if it softens enough to shift, or disassemble and drill out the stuck filament.

I went out yesterday and bought a small vice (£4.50) and a reamer like Nophead's (£3.00). I'm planning to use the Afghan Lathe technique to make a new extruder insulator with a larger melt chamber (4-5mm?) and a tapered entrance to see if this helps.