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aroth

UM2 Retraction Test - Can anyone print this?

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I've tried a few times now to print this part with retraction enabled in Cura. What happens is that eventually the filament gets crushed/deformed to the point where it can no longer be advanced, and the print fails. My filament ends up looking like this:

crushed_filament.jpg

If I turn retraction off, the part prints just fine. With retraction on, it never makes it more than about halfway through the print before the filament gets stuck. I'm assuming this is a particularly challenging print as far as retraction is concerned, as Cura shows retractions all over the place:

bracelet_retractions.png

Is anyone able to complete a print of this part with retractions turned on? If so, what settings did you use (both in Cura and for the printer's retraction options)?

Or more generally, is there anything I can do to stop the filament from getting stuck when there are lots of retractions in a print?

And a word of warning: If you try to print this part and it gets stuck early on, you'll end up with hundreds of tiny plastic pieces that aren't attached to each other that need to be scraped off the build plate. They tend to fly everywhere when that happens.

 

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Hmn well I tried a print once, where I had been playing with retraction settings, and it retracted multiple times a second and I got the same result as you. I suspect this may be what you are suffering from. If you do it again you can monitor the frequency of retraction. I guess there must be a frequency above which it starts to chew the filament but no idea what its is.

I know Illuminarti has posted that has has printed models with lots of retraction without any issue.

 

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G10 is the retraction code so if you do (in cygwin on pc, or in unix, or on mac)

grep G10 myprint.gcode | wc

Then the smallest number that wc outputs is the qty of retractions. In windows it's easier to do:

find "G10" myprint.gcode > g10s.txt

Then open g10s.txt and see how many lines are in the file (most text editors tell you what line you are on) or in microsoft word you can ask how many words/lines/characters are in the document (my daughter uses that all the time when writing an english paper).

 

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Yes, I've been testing this print extensively the last few days. On my early Ultimaker², with the original extruder design and stronger spring (with the indicator set to the highest position), it prints fine.

With later printers, that have a different spring, it's a bit more of a challenge. If the spring tension is too weak, the filament gets sanded flat by the constant movement of the extruder knurled sleeve. The print does complete, but because of the wearing away of the filament, you tend to get noticeable under-extrusion (just general, less-extrusion-than-there-should-be issues not the specific 'click-back' periodic failures that we normally refer to as under-extrusion).

If the tension is too high, then the filament gets deformed to the point that it no longer fits properly in the Bowden tube - and I think that's what aroth is showing in his photo. Eventually the filament stops moving, and likely gets gouges dug into it. The plastic may even partially melt in the extruder from the friction of the knurled sleeve on it.

 

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down. the spring thightens and pushes down an L-shaped lever that ends behind the roller and puts more pressure on it.

 

Got it. Tried a print with the spring tightened to the second indicator mark (counting up from the bottom mark) and it failed much more rapidly.

It looks like illuminarti had the right idea; I loosened the spring all the way, and managed to get a successful print of the part with retraction enabled. However, it looks like a side-effect of the loosened spring may be underextrusion. I guess it's necessary to strike some sort of a compromise between extrusion rate and retraction-handling?

 

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Yes... with too little tension the print will complete but there will be some under-extrusion due to slipping. You may also find that the filament gets ground to a flat finish without clear teeth marks in it. At the other end of the spectrum, the filament gets distorted and gouged. In the middle is a sweet spot where the filament just gets clear teeth marks in it, with no abrasion.

 

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