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UMO+ Extruder's Design advantages ?


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  • 3 weeks later...
Posted (edited) · UMO+ Extruder's Design advantages ?

There's an advantage in having the heavy feed-motor + drive system NOT on the print-head, as this makes the head lighter, & the lighter it is, the better it's going to be positioned by the 2 moving 6mm rods. Making the printhead heavier would mean greater amplitude vibrations causing greater errors in positioning the nozzle, more ripples visible on printed sharp corners etc, more overshooting when trying to stop, etc.

 

But this means the motor is separated from the nozzle by whatever is the length of plastic in the clear tube. So that makes for less feed-control when doing things like retracting, or changing the feed speed, as the response will be "spongier". E.g, suppose you're feeding plastic at a steady speed, then you suddenly stop the feed motor. Thanks to the long length of plastic, this length will have been compressed slightly, thanks to the resistance of trying to push molten plastic through the tiny nozzle. So you might stop the feed motor "instantaneously" ok, but the compressed solid plastic rod acts like a compression spring, and will still push a bit more molten plastic through the nozzle, as the compressive force in the rod relaxes thanks to this extra plastic dribble. Whether Cura "knows" about this behaviour and compensates somehow by predicting it, I don't know. It's theoretically possible to do this calculation, but rather tricky I expect.

 

In addition to the rod inside the clear tube compressing during extrusion, the clear Bowden tube will itself stretch a tiny bit, so ass that to the list of "springy" items.

 

Ideally you would have the extruder motor system right on top of the nozzle, using some super-lightweight motor weighing next to nothing - but that's going to cost you £££ maybe. Or if you could make the tablek super-lightweight, you would move that around in X & Y directions - but the table may well be heavier than the nozzle+motor, so that part might suffer from being too springy. All mechanical design is a compromise; you're trying to make the most rigid (inelastic) drive system you can which follows the positional commands as exactly as possible, but using the cheapest & simplest (so hopefully also the most reliable) equipment to do this at the same time!

 

The solution used in UMO seems pretty good to me - the worst you can get with a spongy feed-rod & bowden tube is a bit of excessive extrusion at a few places, and that can always be trimmed off & cleaned up later.

 

At the motor end of the extruder, that original knurled threaded-bolt thing was a disaster in my opinion. All it likes doing is grinding the plastic rod to bits, if anything gets jammed. And all you need to jam it is to have the nozzle pressed down onto the baseplate, which stops the hot plastic flowing. I've completely junked the original extruder, also a slightly-improved diy upgrade which came out soon after the machine itself did. I've replaced it with a system which uses 2 geared, smooth wheels with pulley-V-belt style grooves in, and these 2 geared pulleys are squeezed together with a spring. So now, if I jam my nozzle into the baseplate & the rod can't extrude, the smooth pulleys simply slip like a friction clutch, but there's no grinding or damage whatsoever to the rod. This extruder works far, far better than the original ever did. Here's a closeup of my extruder; it uses the original large wooden gear from the original one, and that's it.

Extruder_lores.thumb.jpg.450ab1cbfa5bc5d5cf5fc9c2e2d281e3.jpg

Edited by andywalter
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