Friday, 29 February 2008

Too much pressure?

Well my rebuilt extruder didn't last long enough to even make a raft!

I calibrated the Z origin yesterday but when I started extruding today the nozzle ploughed into the polypropylene bed. Thinking I had made some mistake I calibrated it again and it did the same thing. The PTFE barrel can no longer slip in the clamp because it is pinned. The heater barrel can not slip out of its thread because I have a metal ring around the PTFE to stop it swelling. What seems to have happened is that the PTFE barrel has elongated.

It has also bent somewhat. The last PTFE barrel did not elongate significantly but it did swell. I can only think that because I have reduced the nozzle aperture from 0.5mm to 0.3mm the pressure has increased. I didn't notice much change in the motor current though.

I am not sure what to do now. I could make a thicker PTFE barrel but I will have to make a new clamp, which means converting my machine back to a milling machine, or I could drill the hole out to 0.5mm.

A 0.5mm hole gave me ~1.2mm filament which I stretched to 0.5mm. The 0.3mm hole gave me 0.77mm filament at the same extrusion volume rate. It still needed stretching to meet my 0.5mm target. Also it has the disadvantage that the maximum size I can extrude without stretching is now 0.77mm rather than 1.2mm. I didn't get chance to find out what effect less stretching had if any.

The sleeving I used is already looking sad so I ordered some PTFE sleeving to replace it.


  1. I can't say that I'm surprised about your observations with the PTFE barrel. I used one like yours long ago when I was trying to make an extruder that would take resin powder and make 3 mm filament. I definitely had big time swelling between the pump and the extruder head.

    Had I not had connecting rods between the extruder and the pump I expect that I would have had elongation, too.

    When I did my variation on the Mk II extruder I substituted a 1" cube of PTFE drilled for 3 mm for that fool drilled rod. I could never get a nice even hold drilled in that rod anyway.

  2. One thing I could not figure about your cube solution: don't the connecting rods breach the thermal barrier?

  3. Yeah, they do. You'll notice, however, that the connecting rods are out on the edges of flanges. The edges of the flanges don't get that hot and for sure not much heat gets up those connecting rods before convection carries it off.

    First off, the heating coil is down at the bottom of my rather long, thin extruder barrel. The temperature where the barrel connects with the bottom brass flange rarely exceeds 80C. 120C is the hottest I've ever seen it get, and that was only when I left the extruder barrel on without pumping for about half an hour.

    I wanted the filament to not melt before it got into the extruder barrel so I designed it that way very deliberately. Trying to push melted HDPE through a 3 mm hole in PTFE and into a copper extruder barrel was more than I wanted to cope with.

    The design works pretty well. It's fairly extravagant with energy, but we're not talking about more than about 10 watts max.