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Print head hits/grinds the surface of long, tall prints

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As my PLA prints grow in height, the nozzle increasingly seems to drag across the print surface, making enough physical contact that I can see the print plate actually vibrating.


I know this happens with my UM2s, but I think it also happens with my UM3s. Last night, I observed this phenomenon on the print job photoed below. I was using a UM2 and PLA, with a .8 nozzle, printing a .2 layer height. It prints the wall OK. It seemed to print the structure of the infill OK. However, whenever it did one of those rapid repositioning moves, the vibration and sound of contact were very notiecable. I was observing this around hour 5 of this particular job.


What's happening here? Is the print somehow expanding in size as the job continues? Are there any settings which I should be tweaking to get the nozzle to give the print surface a bit more leeway? Do infill patternshave any bearing on this?



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Is the part lifting off of the build plate?  Is you build plate glass really flat?  One of my UM3 glass build plates has a .007"  (0.18mm) bow in the width (X axis) direction while it's perfectly flat in the front to back (Y axis).  I turned it over so that the high point is in the center, rather than at the edges.  If your nozzle is dragging on the part at the sides / edges, but not in the center, you might try turning the glass plate over?


Just a thought . . .

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The infill tends to be printed too fast and comes out irregular.  Also the vertical harmonic resonance of the bed is around 10 or 20Hz and this can get amplified on moves over the infill pattern.


This is fixed if you slow down the infill moves to 35mm/sec or change the infill density or move the print towards the back where the bed doesn't vibrate as much.


If you want to print fast I recommend thicker layers and wider nozzles over higher print speeds.  Maybe try the variable infill feature - I love this feature.


And the reason it tends to get worse on taller prints is I think this harmonic resonance builds upon itself because if the bed is bouncing the infill pattern is varying in Z by a sine wave and each layer amplifies the problem more and more.

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If you change the % infill it will change the spacing.  I think the problem is a harmonic resonance of the bed.  The input vibration is from the number of times the nozzle hits the infill per second.  That causes the initial up-and-down shaking (but very gentle initially) and if that frequency is close to the harmonic frequency of the bed then the shaking is amplified.


Just a theory.  But if correct then changing the frequency will fix the problem.  So possibly changing the infill % by a large amount (double or half) might help.  Definitely pushing it to the back of the glass should help.


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