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Odd Issues With Underextruded Layers

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I have issues with underextruded layers appearing in my prints. It's not the same layer between different objects being printed, but if the same object is beng printed, the underextruded layer will appear in the same place every time. This carries across different slicer settings and scaling of the object, for example the cube in the picture has been printed multiple times at different scales and settings, and also has had STL fixes applied to it, but the underextruded layer near the top always appears. The Z lead screw has been cleaned, and no motors appear to get excessively hot during prints. As you can imagine this has been quite frustrating...any suggestions are welcome!





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Now I am going to guess...


Could the underextrusion *in the cube* be a temperature issue? This appears near a drastic change in printed surface per layer. If the "Minimum time per layer" setting would kick in, flow would be reduced, heat demand would be less (since hot filament stays longer in the nozzle), and thus the temperature would be reduced. If this reduction would overshoot to the point where the filament would not melt enough, it might explain this.


Another guess: is it always the same time into a print (thus not at the same layer, but time) when this happens, at a given printing speed in mm/s? Let's say always after 40 minutes at 50mm/s? In such a case, it could be a feeding problem. After a print is completed, if filament is sitting still too long, then it might gradually get flattened in the feeder due to constant pressure of the feeder wheel on one spot. Thus causing a slow gradual deformation. This might cause wide flat in the filament, which has trouble getting into the nozzle. This flat would have to travel all the way through the bowden tube to get to the nozzle, so at a given flow rate, it should always take the same time. To test for this, at the start of a new print, could you mark the filament with a black alcohol marker pen, just at the entrance of the feeder? And then watch if the problem always occurs when that marked spot reaches the nozzle? If not, then you can exclude this as a cause. If yes, you could further investigate.


The Z-stepper drivers (the chips, not the motor itself) getting too hot could also be an issue. I had that occasionally in the beginning with an UM2. But it showed up rather randomly, and its phenomena were different: the Z-plate would just drop 5mm. It didn't look like this. After reducing motor current from 1200mA to 900mA or so (I don't exactly remember), it never happened again.


But as said, all these are guesses...


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I've been experimenting, and it doesn't appear to be any of the issues mentioned in your first two paragraphs, and the z drivers don't reachn unreasonable temperature, and pointing a fan at them doesn't seem to have had any effect. I don't currently have the ability to change motor currents, annoyingly.


Travel between two points however does appear to be causing issues. Whether or not retraction is on doesn't appear to affect anything, but modifying the object in the pictures attached to remove unnecessarily long travels has significantly reduced the issue, but not removed it. I'm thinking this is because there is still some travel within the confines of the print.



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