Sunday, 15 March 2009

Constipated Extruder

My "New Year" extruder, which is the one on HydraRaptor that I use to build things, stopped working while building the first layer of an object. That is the lowest temperature layer, so the plastic is at its most viscous.



I couldn't get it to work again, so I removed the drive and tried pushing the filament by hand. I couldn't shift it. I measured the temperature of the molten plastic with a thermocouple and it was correct, so I deduced that the nozzle must be blocked. I removed the nozzle and when I pushed the filament this came out: -



It is dark and glassy looking. No idea what caused it, but it seemed to have blocked the nozzle. I cleared it out with a drill and reassembled it. I took the opportunity to measure its performance with my "lead kebab" test jig.

Even though this extruder has a 0.3mm nozzle and no taper in the PEEK insulator, it works better than the tapered PEEK extruder with a 0.5mm nozzle.



The most notable difference is that this one has a much bigger heater chamber, so perhaps a smaller heater bore melts the plastic quicker.

I got this interesting graph of flow against force, averaging over five runs of 20mm : -


I think the steep part of the curve is where the flow through the nozzle dominates the force required and the first part is where the plug friction dominates. The point where I operate it is right on the knee of the curve. I suspect adding a taper would straighten it out, but I don't want to strip down my only working extruder to prove that.

So I don't know what caused the blockage, but it is the second time I have had an extruder block, so it goes to show that a detachable nozzle is advisable.
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4 comments:

  1. Matt16 March 2009 at 01:18

    Hey Nophead,

    This finding is interesting. Have you tried measuring the MP/BP of the blockage?

    I would like to know what it's composed of. We could probably use that information to find out what's happening in these blockages and possibly work around it somehow.

    Thanks,
    Matt

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  2. nophead16 March 2009 at 08:53

    I upped the temperature to 260C in an attempt to free it. That may have caused the discoloured / denatured filament. The actual blockage may have just been in the 0.3mm hole in the nozzle. Too small to do any meaningful analysis I think.

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  3. Freds17 March 2009 at 02:39

    That sort of looks like what I have been experiencing with cream color ABS, though I think since I have been shooting for 240c before even starting and then doing my sensing as close to the nozzle as I can get I have been cooking my fiber.

    I think the rule of thumb would be if you are at extruding temperature and just starting out with no flow, cranking up the heat is not going to make it flow.

    The best bet would be to try and clear the nozzel while hot with a simular size drill bit.

    How would you clean this kind of blockage from a welding tip?

    Maybe your graphs (a lot of work I know) should also include temperature variation?

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  4. nophead17 March 2009 at 23:41

    I imagine I will be able to unscrew a welding tip as easy as the standard nozzles. I can usually shift them when cold, if not I heat it.

    Yes force against temperature graphs would be nice. The only comparative measurement I have made so far is HDPE drops from 3.8mm^3 to 2.4mm^3 when the temperature is lowered to 200C from 240C.

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