Friday, September 14, 2007


More on the granule extruder

The main problem with the extruder was the conduction of heat up the brass and copper screw auger. I decided to make one from a less conductive material - JB-Weld. Here are the initial results. I cast an ordinary 13 mm drill bit in silicone, took it out, then injected mixed JB Weld from the bottom of the cavity with a syringe (to exclude bubbles):

And here it is (with the original drill) after being cut out of the mould:

I probably won't use a drill as the master finally (it's pitch is too coarse, and you have to turn it anti-clockwise, which tends to loosen things), but this shows that you can cast from virtually any helical shape to get what you want.


I may be wrong but I was under the impression than JBWeld is actually quite a good thermal conductor because it contains steel. I have seen it recommended for sticking CPUs to heatsinks.

Obviously these things are only relative so I expect it conducts less well than steel but would an epoxy filled with something else e.g. glass be better?
My thoughts exactly, nophead.

Concrete might also be better...a mix with lots of sand and no gravel, of course, probably with lots of fibers such as glass or even cellulose.
I thought you had a reprap machine! Just print the screw auger in PE, and use that to cast and mould it again in JBWeld.
That way, you can have closer windings to the end of the auger, like your last one, and even raise the inner diameter (which is how "real" augers for extruders are designed). If I find those old lecture notes from my course in polymer chemistry, I´ll send you some more hints (and citations) on designing them.
Just sketched up something, I´ll put in in the Reprap-Forum, Mechanics.
noph & j:

Yes - JB is a goodish conductor (but nowhere near brass and copper, which is what I was using before). It is convenient for experiments, though. When I get the process sorted I'll probably try this stuff:

maybe with glass fibre chop. Indeed that looks pretty rad (copyright (c) Zach...) for all sorts of casting in RP-made moulds...


Quite right - what we need to do ultimately is to make an RP mould of exactly the right shape; there's no point in doing anything else if you're casting anyway.
karl - just picked up your sketch. Reckon we can do that. The fat final bit I didn't know about, but I can see the point about increasing the shear to homogenize better. However, I intend to do my trick of running the melt out down the centre of the shaft from side-holes, which I think shears it quite a lot, so that may not be needed.

Experiments experiments experiments...
I think you needn´t worry about the heat conductance. V2A has a heat conductance of less than 1/20 of copper, and less than 1/10 of brass.
The JBWeld is going to have an even much lower conductance.
If you are going to fab and/or mould something then you might want to consider a non-traditional auger design - two that come to mind would be to have a conical auger (with the point of the cone down at the hot end of things) - this would result in the heat being dissipated over a larger surface area at the top of the device resulting in lower temperatures.

Another (somewhat wilder) idea would be to make an inside-out auger where you cast something like the negative space of a drill bit (so you have a cylinder with a drill-bit shaped hole inside). The cylinder can then be exposed to the air and be of any size to allow thermal cooling. Making it rotate is going to be kinda tricky though.

You could combine both ideas to make a funnel-shaped inside-out auger.
3d_geek :
I guess a conical design won´t work, as the thick end is going to be quite an efficient cooling device for the hot and small lower end. So you need more heating, resulting in a temperatue gradient between the barrel (which has to be conical to, tricky, because it definitely needs to be made of metal) and the auger, making the plastics decompose on the outside, and stick on the auger.

Could you explain your second proposal more clearly? Is it something like an archimedean screw?
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