Extruder thoughts

I've been extruding all weekend so I thought I'd share my thoughts.
NB I'm not quite as organised as Nophead, so no pretty graphs and figures yet.
Friction and barrel size seems to have a big effect on the force. I built two heater barrels to the same design:

One had 3.2mm PEEK hole and 3mm welding nozzle tip. Filament *just* fits into the 3mm heater section (requires a little force).

Heating to 240C allows good extrusion with hand powered force.

One to the same design had 3.5mm PEEK hole, with a short 3.2mm section near the barrel. Nozzle was drilled to 3.2mm and filament fits easily.

Heating to 220C allows good fast extrusion with less force.

I suspect that the half-melted filament expands before melting fully and presses against the sides of the barrel, causing extra friction. I suspect that's why nophead's reaming made so much difference - a little extra space allows the filament to expand as it heats.

I also think my BfB extruder probably does work if the screw thread is adjusted correctly. Nophead has reached >12Kg with the screw extruder, so mine slipping is probably due to the spring/acrylic flexing and allowing the ABS to slide. I'll see if I can tighten up the pressure (maybe use some steel reinforcement) to drive the thread properly. Then I'll have to rebuild the heater barrel.