Monday, January 24, 2011


Filling in with the fillings

I've recently been experimenting with the loading of various fibres into CAPA and PLA. This started out as simple strengthening for the 3mm PLA that I sell, but the addition of 20 micron x 1mm nickel fibres makes it conductive. Dunno how useful this will be yet but it certainly allows RFI shields to be printed.

Then I hit on a brainwave: short polyacrylonitrile fibres. There are normally used to reinforce synthetic rubber. But they have another use: The feedstock for carbon fibre.

My trick is to mix polyacrylonitrile and similar materials in with PLA or CAPA. This is then turned into filament, which in turn is printed into an object on a RepRap - this tends to align the fibres.

The object is then encased in a ceramic and heated to the vapourisation point of the PLA or CAPA. You then have a ceramic mould stuffed with oriented polyacrylonitrile. Introduce an inert atmosphere and raise the temperature and you have a mould full of carbon fibre. This can be surface-activated in the usual ways, or coated with vapour deposition before being vacuum-encased in epoxy to form a complex, 3D carbon fibre composite object.

Yes, I am looking for sponsorship :)

Vik :v)

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Doesn't PAN need to be held in tension as it pyrolyzes? I worry that what you describe would result in a network of amorphous carbon, which has slumped somewhat from its original shape. Resorcinol-formaldehyde is the typical route to that result...the insulation on the Mars rovers, for example, is a mix of pyrolyzed RF gel and silica gel.

There might be something to using fibers in a partially-pyrolized state, i.e. with some of the hydrogen gone, but all of the nitrogen still there. Fibers in that condition might form a significant amount of graphite-like structure, and might even link directly to neighboring fibers. I think they also wouldn't shrink as much. I think they'd be horribly brittle, but I got the impression that short fibers were the idea to begin with.

It's also worth pointing out that there will be a lot of carbon from the polymer binder, except maybe if you use something that de-polymerizes easily. I know from experience that pure polystyrene (i.e., not ABS) can be removed cleanly a few hundred degrees celsius in flowing inert gas, but it's the exception among polymers.

Is there a particular reason not to use chopped carbon fibers as reinforcement?
You're right, using chopped fibre would be easier from many perspectives. I did indeed wonder if the polymer fibres might interlink and possibly fuse a bit before becoming fully carbonised. I don't know if carbon fibre would be any harder to get through the nozzle?

I've used PLA in investment mouldings in the past. There is no discernible residue after heating to 220C in air. The temperature needs to be raised gradually though to avoid trapped outgassing.
Only 220C? That is not much higher than the temperature I extrude it at.
Well, it might have been 225 :) Possibly as high as 240 'cos our oven is a bit variable.

Could chopped carbon fibre be thrown into a heated paste extruder together with your plastic pellets of choice?

carbon fibre will no doubt wear down a nozzle quicker than most other materials, but the question is if his occurs within hours or with months or years, likely quite dependant on how liquid the matrix is.

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