Friday, December 19, 2008


Well. That didn't work...

That being this - my attempt to make a granule extruder. It worked perfectly first time, but - after it had cooled and then re-heated for a second run - the plug of solidified polymer wouldn't shift, regardless of how much welly I gave the motor. Nophead had the same problem when he tried this, but I thought my use of PTFE might have fixed it. No such luck.


Back to the drawing board.

It was too big anyway.

This is my new vertical design based - as you can see - on the existing filament extruder drive and motor (M). The nozzle and PTFE arrangement (N) at the bottom are the same as the current extruder too, except the internal diameter is 4mm.
It'll need smaller granules than standard (about 1mm). But we also need to make a recycling shredder anyway, and Forrest has been getting interesting results with ABS precipitation to make powder.

The central rod is M4 and is driven down by the larger gear, forcing granules into the PTFE. The mechanics are essentially the same as the previous design, so we have halved the radius and thus increased the pressure by a factor of 4. That should, I hope, shift the frozen plug and get the thing started.

As before, all the force is taken by the rods (R) to a plate at (P) (not shown). All the pyramid hopper and central arrangement floats on those rods and sees no force, though the extrusion force will be considerable.

To the machine...

Bother! I made this using the hairdryer trick, and you can see the delamination top right. This proves that the hairdryer works really well to prevent this - that was the only corner to grow out of the hairdryer flow, and the only corner not to build smoothly.

Serves me right for leaving it running while I drove into the university for a meeting with eD. Fortunately the part is usable - I'll just cyanoacrylate it in a vice and it'll be good as new.

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When you restart, how long are you preheating your extruder barrel before you try to pull the plunger. Way back when the plunger in my second granule extruder would regularly freeze in the extruder barrel. I'd do a good preheat and then it would extract like a champ.
Would a taper in the PTFE prevent the plug from sticking?
Spam above zapped...

I left it heating up a long time. Because it is a screw drive you can just wind on maximum force, and leave the heater running. I did that for about half an hour.

The taper is a nice idea. Hard to make, mind. (Or maybe not - PTFE is very soft, so one should be able to cut it with almost anything - maybe even a cast PU tool...)
I'm wondering if you could use a drill bit as an auger to advance the plastic, instead of an actuator? Something like ? That's how my mixer's sausage-grinder attachment does it.
Industrial extrusion screws are loaded with LDPE at shutdown. The plastic softens lower temp than others, and it's greasy as it heats and is pushed out by production material on restart. PVC, PP, other materials get too hard for the screw to push and because the heaters are still too hot, they carbonise in the barrels and thermoset.
Might it be better to purge the hot end when you're done printing?
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