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I'm printing parts for a HEVO (HyperCube Evolution CoreXY), using Colorfabb XT filament. After several successful prints. I'm having a lot of failed prints. They start off pretty well but after ~15 layers the filament starts to string and blob.

Printer details

Model: RepRap i3 
Extruder: E3D 1.75 all metal hotend.
Nozzle: .4
Print temp: 260°C (max. recommended).

Steps I took to troubleshoot

  1. First I thought my nozzle was clogged. But it's not I can push the filament by hand without issues.
  2. Lowered the speed to 35 mm/s.
  3. Disabled retraction / disabled part cooling.
  4. increased the max temp with 10 % to ensure the filament flow

Printed parts that fail:

Top view printed part Note: the two parts on the left are printed in one job

Side view printed part

UPDATE 26/07 Dit a few more prints with PETG to eliminate the moisture problem. Same results.

PETG

PETG2

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    $\begingroup$ Welcome! Very well written and documented first post! Lowering speed and increasing temperature is usually the opposite of what you want. Is the cold end properly cooled? The filament you use is of high quality, my favorite brand actually, their filaments print very well using their settings, you print at the maximum temperature and under the minimum suggested speed. $\endgroup$ – 0scar Jul 11 '18 at 21:41
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    $\begingroup$ could you add nozzle size and layer height $\endgroup$ – profesor79 Jul 12 '18 at 9:29
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This looks a lot like under extrusion caused by heat creep.

Heat creep is when the nozzle temperature 'creeps' up through the filament and makes it melt (a bit) and form a blob (or just widen enough to get stuck) a centimeter or two before the nozzle.

The characteristic is usually that everything works perfectly well for some quite fix amount of time, then there is severe under extrusion.

Solutions:

  • Lower the temperature (yes, as then the heat won't creep as much!)
  • Add a fan cooling down the cooling part of the print head

I have the same print head as you it seems (E3D 1.75 all metal) and I had heat creep when I changed the fan to a less noisy one (but also less effective).

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  • $\begingroup$ Thank you all for response. I will investigate the heat creap. I use the fan that came with the E3D. It's a 7 blade fan of good quality. But maybe I need to clean the cold-end and the fan blades. They catch dust after a while. $\endgroup$ – Toon Jul 12 '18 at 11:16
  • $\begingroup$ I had something like that too, but in my case it was caused by retraction settings. The retraction made the filament get out of the nozzle hole and caused a jam and thus an under extrusion. $\endgroup$ – Petar Petrov Jul 13 '18 at 7:21
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This suggestion may not apply in your specific case (you seem to have looked closely at the problem, and it doesn't really fit the apparent time-dependence) but you might have an issue with the extrusion drive. If your hobbed drive isn't tight on the stepper motor then the 'push' might not be enough to match the force you've tested by hand. This explains one way that the extruder can be more sensitive to small performance variations than you would normally expect (heat build up somehow contributing to those variations).

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  • $\begingroup$ Actually I have. I added extra current to the stepper driver because it was skipping when I first started to print with XT I also added tension to the hobbed gear springs. When it get's stuck the hobbed gear grinds the filament away. So no tension issues here. XT is a tough cookie. $\endgroup$ – Toon Jul 16 '18 at 9:23
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I have a possible cause for my problems. XT is sensitive to moisture (hydrolytical stability) When I extruded 20mm filament manually, the string felt a bit rough. When I looked closer very tiny bubbels where visible. I baked the filament at 60°C for 2 hours.

The baked filament allowed me to print a perfect model.

But after one print the filament is again affected by moisture so I need to build a dry-box and do some more tests before I come to a conclusion.

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  • $\begingroup$ Nice to post an answer, please update when you have more information. The only remarkable thing is that it prints fine in the beginning, and all parts fail at about the same height (or are the parts printed in one job?). $\endgroup$ – 0scar Jul 16 '18 at 10:16
  • $\begingroup$ Forgot to mention that the two parts on the left are printed in one job. $\endgroup$ – Toon Jul 16 '18 at 11:11

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