Saturday, 27 December 2008

Simple experiment

Inspired by Demented Chihuahua's extruder work, I repeated his experiment using what was left of my old heater. I mounted it in a 30mm M6 stainless steel washer and clamped that in a vice. I used my 0.3mm aluminium nozzle, which I counter bored with a 0.7mm drill to reduce the depth of the 0.3mm hole to about 1.5mm.

I powered the heater from a bench power supply and adjusted it manually to about the right temperature. Green ABS is handy for this because it changes colour at 260°C so you can tell when it is too hot.

I can extrude filament by pushing it by hand with moderate pressure. It comes out at 0.4mm but I should be able to stretch it back down to 0.3mm without any problems. Even with a 0.5mm nozzle I can stretch it down to 0.3mm, but I lose positional accuracy because the orifice no longer defines exactly where the plastic goes.

Originally the heater was 5mm longer, with the excess protruding beyond the half nut. I found that cutting that piece off made it easier to extrude. It was probably a relatively cool section so the plastic remained very viscous there.

When a new piece of filament is inserted into the heater it extrudes very easily. After a while some plastic flows backwards and builds at the entrance to the heater. That causes considerable extra resistance. I plan to tackle that by having a short section of PTFE at the entrance with a heatsink the other side of it. The steep gradient across the PTFE should freeze the back flow over a short distance and, being super slippery, should allow it to slide back into the heater.

Another thing I tried was forcing out the plastic using the shank of a 1/8" drill bit as a piston. The further the drill got to the end of the heater the less force was needed to push it. That confirms what I had suspected. The force to push the plastic though the long 3.5mm section of the barrel is very significant compared to the force to squeeze it through the short small hole in the nozzle. So the heater needs to be kept as short as possible. Obviously there will be a point where the extrusion rate becomes limited by the rate the plastic melts if it is too short, but I expect that is much shorter than the current set-up.


  1. Ah, that's very nice. I was thinking about using the stub of PTFE I have sitting around but I really wanted to get away from using it all together. What about using fire cement or that cerestil stuff you had as a thermal break?Is it really necessary for the stuff to be slippery at all or just not absorb heat of sufficient temperature to melt the plastic?

    I've updated my extruder pump design to be much nicer looking and easier to assemble and I'm gonna start using it in conjunction with a new heater assembly to see if we can get a whole system made from common parts going.


  2. Cerastil is a good thermal conductor so no good for a thermal break. I have no experience of fire cement. If it is a good thermal insulator then it is not very suitable for covering the heater.

    I think we will always get some semi molten plastic backing up and that expands to the full diameter of the tube, that is why I think it needs to be slippery.

  3. Nop,

    It may be that the support you used in this experiment is acting as a heat sink, thus making the plastic more viscose in that region of the bore, just above the half nut. (Note: thos are called a jam nut in the US, perhaps so as not to confuse with a lathe's half nuts.) Thus, the flow resistance in the larger-diameter region might not be so bad, if there wasn't such a cool region. This is a minor nit; this is a fine experiment. I like the idea of an intermediate diameter drilled in the tip, to keep the smallest bore as short as possible.

    -- Larry