Tuesday, April 15, 2008



Anyone who's putting polycapralactone through their extruder will appreciate what ooze is. It's the four or five second dribble of extrudate that continues to come out of the nozzle after the motor's been switched off. This extra material has the potential to mess up a build, and it does my head in. Simply reversing the motor would help to a point but ramping pressure up and down implies a lag. So what we need is a valve.

Here's my original idea, the ball valve section is actually a cylinder which runs through the nozzle to a lever and solenoid:

It suffers from leak potential and precision machining. It took this to Ian and he had a much better idea... a washing machine style valve:

Much simpler to make than the first idea, and initial tests worked really well. No leakage. Developments to come...

Funny, I came up with an idea very similar to the piano wire plug a couple of weeks ago - if you were to slow down the main feed motor as the wire was fed into the nozzle so as to maintain a constant flow rate of polymer despite the decreasing manifold volume caused by the moving wire, you could have an almost perfect halt to extrusion with the wire face flush with the nozzle.

It's been observed that ideas whose time has come are quite often invented simultaneously in several places - I'd take that as a strong indicator that the wire plug valve is a good one!
An alternative idea I had was to mount a needle on a three leg spider in the end of the threaded barrel. Tightening the nozzle fully would press the needle into the hole, losening it half a turn would let the filament flow.

The piano wire looks simpler but perhaps more prone to leaks.

Another idea was to attach a needle to small magnet and have a solenoid around the outside of the nozzle. Not sure if it would have enough pull to resist the polymer flow though.
Seems like the valve would be open only when extruding, which is also when the motor's running. I wonder if you could eliminate the solenoid and use the motor's torque to open the valve. (i.e. motor running -> valve pulled open, motor stopped -> valve snaps shut)
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