Sunday, September 09, 2007


Extruding from granules

I was so impressed by the EVA-granule extruder made by Timothy Nixon and Adrian Tan of the School of Mechanical Engineering, University of Adelaide, Australia for Fab@Home (see this link) that I decided to revisit something Forrest and I did almost two years ago (see the blog for Friday, February 17, 2006), nick extra ideas from the Aussies, and have a go at designing one for RepRap to work with polycaprolactone.

Here it is working:

The advantage is, of course, that all thermoplastic polymers are available in bulk at very low cost in granules - it's the standard form. Getting 3mm filament (which is what RepRap currently uses) is not difficult - many companies will do it - but it does add slightly to material costs.

You can find details on the RepRap wiki here. It needs more work, but it'd definitely be a useful addition.

Very nice. Presumably, you can chop up old parts and waste material into similarly-sized pieces and recycle them.
So you had another crack at the auger extruder that we did last year! Brilliant! Especially the way that you made the auger this time!

About the heat migration. I ran into that. For mine, it didn't seem so much to cause trouble on the auger as it did to heat up the feed hopper and make the CAPA granules stick together there, thus jamming the feed.

I wonder if you made some little outlet vents down by the extruder barrel and then forced air from the top of the extruder downward if that would let you control that upward heat migration a bit better?
Obviously you can just slap a slab of something insulating on top of the JB weld but best results would probably come from force cooling said slab. At the cost of a more complex set of side walls for the mould you should be able to include a few (2-5)small metal tubes (of the sort used in model airplane construction and so forth (talk to the guy at your local hobby shop)) running horizontally through the slab at little or no strength cost. Air can be forced through or allowed to convect.
> Presumably, you can chop up old parts

Yup - indeed a RepRap-made old-part-chopper-upper would be a really useful device...

* Heat migration

I'm thinking of both increasing the barrier of insulating materials and turning the device on its side so the auger is horizontal. After briand's suggestion, I'll add an air path for cooling air from a processor fan. I'll start by running it without, and only add the actual fan if it's needed.
Interesting that the Fab@Home people said they use a PTFE base to prevent it sticking whereas we have the opposite problem. Presumably that is because they are using EVA which is glue.
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"I'm thinking of both increasing the barrier of insulating materials and turning the device on its side so the auger is horizontal."

I ran my auger through a block of PTFE and operated my system both vertically and on it's side. The heat seemed to be migrating up the auger itself. Mind, mine was steel which shouldn't have been as transmissive as your copper one. :-(
I'm going to cast a 13mm drill bit in a plastic cup of silicone, twist it out, and fill the cavity with JB Weld high-temp epoxy. I think that'll make a good low-conductivity high-strength auger.
If I recall correctly, JB Weld might be filled with high-thermal-conductivity material. Have you considered filling epoxy with something like ground glass?
Microspheres might be better than ground glass - see your local chandler.

Do you have any PLA granules? They might not be so affected by the transmitted heat.

Vik :v)
Seems to me like the problem could be solved by having a metal tube containing capa, the bottom of which is above melting temperature, the top of which is above melting temperature, into which you force granules using a pair of opposing gears. The granules drop one by one from a hopper above.

You could establish the temperature gradient along the tube by using cooling fins. You might want a gradual gradient, or you might want a sharp gradient. Hard to say without protoyping.

The bottom of the hopper has a gear moving backwards, which breaks up jams, while allowing single granules to drop into the top of the tube above the gears.

This way the moving parts stay cool, but still can exert a known amount of pressure on the molten capa.
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