I used the tapered reamer to open it up to 5mm at the bottom end.
I had to remove and replace this with the heater hot. You can see where ABS has run up the thread and then burnt when it met the air. This seems to seal the thread as long as the initial leak is slow enough. I don't think HDPE would seal in the same way, so I run ABS first when I assemble an extruder.
The taper made a big difference. HDPE pushed with 4.6Kg went from 1.1 mm3 to 5.3 mm3 and the times got more consistent. I think it is beneficial in four ways: -
- It removes the friction of sliding the plug along the wall.
- It increases the bore where the very viscous, just-melted plastic is, reducing the viscous drag by a fourth power.
- It thins the hot end of the insulator making the thermal gradient steeper.
- The wall being thinner and having a bigger surface area will allow more heat flow into the melting plastic.
The performance with HDPE is a bit better than the stainless steel extruder when it was fitted with the same nozzle, but the ABS performance is considerably worse. I can't explain why that would be.
A third variant would be to use a longer PEEK tube with a taper to dispense with the heatsink and hopefully be strong enough without the washer and bolts. I think I will have a look at drive mechanisms for some light relief before coming back to that.
It looks like about 5 Kg force should cover the plastics I have tried so far. I don't think anybody has tried pinch wheel with the slippery plastics (HDPE and PCL) so I will have a go at that.