Thursday, 27 December 2007

Cutting corners

When making solid blocks with 0.5mm HDPE filament I noticed that the corners are not very accurate. The right hand edge of the 20mm cube below shows this effect at its worst :-

The problem is that, although the machine makes a perfect right angle, the filament appears to have a minimum bend radius and so cuts the corner. The amount it cuts the corner seems to vary from layer to layer giving rise to the rough edge.

I think the variation is due to the fact that my extruder spindle is a bit off centre. This causes the torque to go up and down as it rotates, which causes the flexible drive cable to wind up and run down again. This causes speed variations despite the fact that the motor speed is well regulated. At some point I will get rid of the flexible drive.

I expect the fact that I am stretching the filament doesn't help with the corner cutting. I improved it a lot by slowing down the drawing of the outline to 4mm/s and leaving the infill at 16mm/s. Here is the result :-

Still not perfect, another thing to try would be to recognise that there is a minimum corner radius and make the nozzle follow an arc of that radius around the corner. At least that way it might be more uniform.

Here is a close up of the top face taken with a scanner:-

As it goes round the corner the filament has an external radius of about 1.5mm and an internal radius of 0.9mm. As it is 0.6mm wide that is probably not bad. You can also see that the zigzag infill sometimes ends a bit short of the edge, probably also due to corner cutting.

To get sharper corners I expect I need to use a nozzle with a smaller hole, so that the filament can be fine without having to be stretched, but that has the disadvantage of slowing down the extrusion rate for a given pressure.


  1. Good improvement so far. Keep going. \(oo)/.
    Hope to see an accurate object can be printed by the machine in the near future.

  2. Thanks for the encouragement. My aim is to get HDPE accurate enough to make Darwin parts but that might not be possible. I will keep working on it though until a supply of PCL or PLA filament becomes avilable.

  3. Your work is really amazing. I always hoped, you can use the HDPE material, because its really widely available, and cheap. And it looks like you are more and more closer.

    I would prefer heating the extruding object over using an yet another hardly available material. (I don't know CAPA, and the others mentioned. But almost every plastic bottle are PP and HDPE (mostly the cleaning stuffs))

    Did you consider heating the extruded object? (with a simple resistor, or with a infra lamp, or whatever). ?

    You are so close, that maybe just a bit heating would be enough.

    Enough of me;) Keep up the good work!
    I'm constantly checking your site for updates. Keep us posting please!

    And please stick with HDPE....
    Best regards,

  4. PCL and PLA are the two plastics RepRap are concentrating on.

    PCL is polycaprolactone, which is the same as CAPA and PolyMorph. The advantage is low melting point so possibly less shrinking when it cools but I haven't seen any figures. PLA is polylactic acid which theoretically can be made at home from starch crops but it doesn't look easy.

    When I have exhausted software solutions for HDPE I may try a hotplate. If we find a way of recycling HDPE then there is the opportunity to add things which can reduce the shrinkage. Many things to try! Hopefully more people will have working machines soon so the research will accelerate.