Sunday 8 March 2009

Taper relief

As tapering the stainless steel insulator made so much difference I went back to my PEEK extruder to try the same thing.

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.
Foolishly I didn't measure any ABS flow rates before I made this mod. ABS extrudes at 4.5 mm3 when pushed with 4.6Kg and only 1.3 mm3 when pushed with 2.3 Kg. This is odd in that the differential between ABS and HDPE is less with this variant.

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.


  1. Slightly offtopic question...

    What are you going to do with all this working extruder heads?

    Did you consider selling/offering it? I would really like to buy/have one!;)

    As you progress, you always builds a newer, yet more perfect version of the extruder, so you will always have more and more extruder heads;)

    Just an idea.

    Best regards,

  2. Well I only actually have one complete extruder (the new year one), which is on HydraRaptor. The other two are only half of an extruder. One of those will end up on my Darwin, the other will be a spare.

    Apart from that I lots of random bits from experiments and extruder that have broken, but currently they do not amount to a complete extruder, sorry.

  3. Ok,no problem. I thought you have a bit more (5).

    Anyway I keep an eye on your blog and when you consider the extruder design final, I will build my own.

    Thank you for your answer.

  4. My pinch wheel, although much different in design, pulls HDPE with any problems.

    that one is from over a year ago. I have yet redesigned it to use a stepper motor and use bearing for the spindle and it works a lot better. I think with your research on tapering the insulator and shortening the heated barrel will bring it up the speed. Thanks for posting your findings.