Saturday 21 February 2009

If at first you don't succeed ...

Remember this?

It was my last attempt to get a high temperature extruder idea working. ABS jammed in it, so I put it to one side. This morning I made a slight modification and got it to work extremely well.

The filament was getting stuck in the end of the stainless steel tube where it enters the heater block. I removed the PTFE tape from the threaded joint as I thought that may have been insulating it. That made a small improvement. I could push ABS through it by hand, but only just.

I then flared the hole with this tapered reamer so that it has a 5mm inside diameter at the end, tapering back to 3.6mm, which is the internal diameter of the stainless steel tube.

That made all the difference, I can now extrude my oversized ABS very easily and even HDPE only requires moderate force.

I am not sure why it made so much difference. It makes the wall thinner, so the heat from the heater can get to the plastic easier. It also reduces the friction of the plastic against the inside pipe wall because any downwards motion causes the plastic to come away from the wall.

The next step is to connect it to my test rig to get some comparative pressure figures. My feeling is this extrudes more easily than my PEEK version. That may well benefit from a taper as well.


  1. Most excellent, I wonder if the drag/sticking has anything to do with something I observed when peering down a cool side extruder barrel machined out of stainless.

    There are ridges around the walls of the machined out inner hole (3.5mm)

    More so than with a brass barrel, this could be to do with the hardness of the material and its reluctance to be machined.

    I guess a taper through the transition zone would fix this too.

    Hmm having an extruder with a constant cross section does'nt allow for expansion and may therefore create back pressure and enhance dribble. It will be interesting to see if the volume increase towards the extruder end has any effect on the dribble.

  2. I don't think my stainless pipe has any ridges in it. I think though that when you push on ABS past its Tg that it swells and plugs the tube. Only if the tube is very slippery, like with PTFE, do you get away with it. Otherwise there is simply too much friction. The harder you push, the more outwards force, so the frictional force increases to match. A taper removes the friction because the movement is no longer along the pipe wall but away from it instead.

    I think the more molten material the longer it will dribble for, assuming the liquid plastic is compressible. However, it would appear from Adrian's blog post that, with ABS at least, the majority of the ooze is due to overrun of the GM3 and goes away with a stepper. I will be trying that soon.

    The heater in the photo does not have an opened out melt chamber, I don't think a big chamber was any benefit so I am moving away from it. I am currently making a new heater / nozzle / barrel arrangement that only requires drilling and tapping operations, no lathe or milling. It should be good for up to 450C!

  3. Interesting jamming of the ABS is exactly what I have experiencing in my designs.

    If I pull out the ABS fiber and insert a coat hanger; it then extrudes until I reach the bottom of the melt chamber.

  4. Hi Freds,

    Yes sounds like the same problem. Can you add slight taper? My reamer is 3mm to 13mm over 80mm. From that I calculate my taper is from 5mm to 3.6mm over 11.2mm.

    It would seem that the shape of the thermal transition zone has more effect that the extruder exit.

  5. Hard to say I am trying to use off the shelf hardware store parts and I think my zone is fairly narrow.

    If I taper it I might loose the physical attachment strength. But what the heck it doesn't work right now anyway.

    I am using 5/32 brass tubing and I noticed Forest used copper tubing in his, so maybe the greater heat conductivity of copper gives a larger thermal transition zone.