Saturday, April 11, 2009

 

One step forward, one step back

My jerry-built extruder is blocked.
The coach built drive is still working fine, providing quite a bit of force to the filament - enough to tear big chunks out of it.
The extruder is blocked.

This happened before, during hand testing. I've restarted and stopped the nozzle several times, and only twice had a jam. Last time, clearing the 0.6mm nozzle hole with a pin did not restart the flow, so it isn't a nozzle blockage. Providing *excessive* force suddenly cleared the blockage and restarted the flow, where it went back to normal.

There is one factor that was the same between blockages.

Both times, immediately before, the filament had been left in the nozzle, and the nozzle had been left hot for several minutes, while testing other stuff. Then the nozzle was turned off and cooled down (end of testing).
The next day, the nozzle was blocked.
On other tests, either the filament was withdrawn while hot, removing the bulk of the plastic, or after extrusion the heater was switched off immediately. Restarting the next day was fine.

I *think* what happens is that the heat travels up the filament, softening and widening it. Over a couple of minutes or so (if it cools immediately) this isn't a problem. If the heat is applied for several minutes, the filament softens further and further up, pressing against the sides of the channel, causing more and more friction. Because my insulator isn't uniform (there is a .3 mm step near the top, and a hole across (see screwdriver drive) this gives plenty of friction - blocking the filament channel. This is why clearing the nozzle doesn't help.

I'd suggest that if you're not using the extruder, or between days, turn the heat off - or better, withdraw the filament an inch or two while hot. My fully assembled extruder worked well, but last time I was testing the axes and trying the sort out a software bug, and I left the extruder hot for 20mins or so while doing so. The next time, it's solidly blocked - no extra force is shifting it. I'll either have to leave it hot for a half-hour or so to see if it softens enough to shift, or disassemble and drill out the stuck filament.

I went out yesterday and bought a small vice (£4.50) and a reamer like Nophead's (£3.00). I'm planning to use the Afghan Lathe technique to make a new extruder insulator with a larger melt chamber (4-5mm?) and a tapered entrance to see if this helps.

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Comments:
If it's ABS, you might be able to dissolve it in acetone. Using that I freed up the ABS that was jammed in my extruder.
At first I had tried to drill it, and I couldn't recover it from the permanent damage to the PTFE barrel, but most of the ABS was gone.
 
That's a possible loosener. It might also be better if I could remove the nozzle (my current quick and dirty design has fire cement over the threads and nozzle).

I think I might have to bite the bullet and build myself a more traditional removable nozzle
:-)
 
Sounds to me like you might have a gap between the end of the barrel and the bottom of the matching hole in the PTFE spacer.

Remove the thread from the last 2mm of the heater barrel and taper it inside and out before fitting it back in. If the available bit of heater barrel isn't long enough, slightly truncate the PTFE spacer.

Removable nozzles are a good idea!

Vik :v)
 
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