Thursday, June 24, 2010


Dismantling the Extruder

Last night I took apart Kurzweil's extruder in order to replace the insulator.

First I liberated the whole hot end from the extruder in order to pull the insulator out. I ended up with this (apologies for my crappy cellphone photography):

Now I know when building this extruder James and I pushed the insulator all the way into the nozzle, but check out the plug of ABS that formed between the insulator and the smaller nozzle canal. My hypothesis is that at some point the nozzle got clogged and a little bit of ABS was able to squeeze between the insulator and the nozzle. Then it cooled, and the next time I operated the extruder, that little plug-seed got melted again, and some of the pressure that should have been sending the filament out of the front of the nozzle got put towards increasing that plug with new melted filament. All this is exacerbated by the fact that the insulator just above the nozzle was able to buckle and bulge out.

I was curious what that crazy bulge looked like inside, so we cut it open.

What my badly-focused cellphone picture doesn't show is that there is clear evidence of back-flow up the insulator around the filament, or in the vernacular, extruder FAIL.

The replacement we'll be using for the insulator is a thinner PFTE tube surrounded on the nozzle-side by a PEEK tube and on the other end by a similar tube of what I'm pretty sure is aluminum.

After getting the insulator out, I took the nozzle apart so I could redo it, (1)to ensure proper thermistor placement and (2) so I could use Kapton tape, which is easier to maintain than fire cement, as the rest of this post will reveal. I started by cracking off the fire cement layer by layer with some pliers. The Ni-chrome heating element survived, but the thermistor was too tightly sealed into the hole on the side of the nozzle tip, and I ended up having to just tear the leads off and dig out what was left with a pointy implement. Then I put the nozzle in a cup of acetone for a while to soften the ABS still stick in the very tip, which worked like a charm.

So now we are in the market for a thermistor. I looked all over, but nowhere seems to have all of these qualities at once:

  • 200k ohm
  • shaped like this =O instead of this -[]-
  • operating temperature range high enough for ABS (235-240C°)
  • a response curve compatible with the preexisitng software on Kurzweil
  • being sold in reasonable quantities (no, digikey, I do not want to buy 2500 thermistors!)

Hopefully the fellas at BfB will be able to help out, but in lieu of that, any tips would be greatly appreciated.

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I just bought 5 of the 'official' reprap thermistors friom farnell
(although you specified 200k, these are only 100k).
The temp. profile for this thermistor is available in the reprap firmware, or you can generate your own with a python script somewhere.

Nasty backflow from your extruder!
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