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I'm printing stacked Prusa COVID-19 face shields on a modded Ender 3. It works fairly well until the second layer of masks, where we get this weird effect that's kind of like stringing. It looks like the curve is becoming "low poly." I'm sorry that I can't be more specific, but I really don't know how to describe it more than this. Please look at the pictures to get more info.

One of the strangest things is that it worked earlier with the same slicer and the same profile. I am using PrusaSlicer, but I tried it with Cura and it still didn't work. There are a few more problems with the prints: some under-extrusion on the supports, and some spots had little gaps between the layers.

Does anyone have any fixes for any of these problems? I would mainly like to know about the strange kind of stringing because that actually prevents me from stacking prints.

I am printing Overture PETG at 240 on the nozzle and 80 on the bed. This is at a speed of 50 mm per second. I have the Basaraba Innovations direct drive mod, the Creality silent board with TH3D Unified Firmware based on Marlin 1.1.9, and the EZABL Pro.

Please look at the pictures to see what is happening.

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This isn't stringing. It's failure to adhere to the previous layer, so tension in the newly extruded material just makes it form a straight line until it bonds to something again. Are you used to printing PETG? You didn't mention fan settings, but having any cooling fan at all will cause failure of layers to bond. Your print speed is borderline for PETG, especially if you're using thick layers like 0.3 mm which you also didn't mention but looks like might be the case.

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  • $\begingroup$ Yes, I am using PETG at 40% fan speed. I am also using .24 mm layer heights. I will try printing with no fan and see how that goes. Thank you! $\endgroup$
    – 3D Printed Cuber
    Apr 11 '20 at 13:17
  • $\begingroup$ .24 at 50 mm/s is probably a higher volumetric rate than your hotend can keep up with for PETG. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Apr 11 '20 at 13:42
  • $\begingroup$ So what speed and layer height do you recommend? $\endgroup$
    – 3D Printed Cuber
    Apr 11 '20 at 14:27
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    $\begingroup$ My profile for PETG is mostly 40 mm/s except 45 for top/bottom layers, at 0.2 mm layer height. This is for use on an Ender 3 with stock hotend. I had a really hard time not getting sporadic underextrusion and poor layer bonding with anything faster. Supposedly PETG actually likes thick layers though so you might do better keeping 0.24 layer height and scaling down speed proportionally (33 mm/s). Aside from letting the hotend keep up with the needed melt rate, going slower keeps the heat over the newly extruded material longer. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Apr 11 '20 at 15:24
  • $\begingroup$ Thanks, I will try this. $\endgroup$
    – 3D Printed Cuber
    Apr 11 '20 at 18:44
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It turns out that the brand of PETG that I was using was not good for stacking prints. I tried another brand of PETG and it worked perfectly. Thank you for all of the suggestions of what could be the problem. If you are stacking face shields (or any other print), do not use Overture PETG.

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