My machine is back up and running again after replacing the thermistor and MSP430 micro. I added some high temperature insulation to keep the thermistor wires away from the heater wires in future! I salvaged it from the 10A shunt of an old multimeter that I scrapped.
I should really recalibrate the temperature measurement but after finding temperature is not too critical I can't be bothered at the moment. I might wait until I after I have blown it up again!
In my previous article Equations of Extrusion I put forward a theory for the significance of the Y axis crossing point on this graph, i.e. the minimum filament diameter at zero flow rate, was that it was the size of the hole in the nozzle.
My explanation was that perhaps I had inadvertently opened out the hole in the extruder nozzle by drilling too far from the back. While my extruder was off the machine for its new thermistor I had the opportunity to inspect the end of the nozzle.
As you can see the hole is still the correct size, 0.5mm, so my theory was wrong. My revised explanation for the minimum filament size is that HDPE is so viscous that there is a minimum pressure to make it flow through a small hole, below which it does not flow at all. The minimum diameter is then the hole size plus the die swell at that minimum pressure.
I used a fine stiff wire as a probe to try to get an idea of the depth of the hole. I think it is no more than 1mm, so I am at a loss to explain why I get variable die swell and other people do not. Perhaps my HDPE is different, I know mine is translucent whereas Forrest Higgs' is opaque white.