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I'm trying to get good-quality thin wall prints with PLA on my Creality CR10s Pro V2 using Cura. After hours of calibration and testing, I'm getting quite good results. However, there is one issue I can't figure out. It seems that I'm getting uneven extrusion and blobs when the printer deaccelerates or accelerates. This issue causes problems with rounded corners on my test prints (see photos). In Cura, there is no retraction or anything in the arc, just accelerations. I have experimented with acceleration and jerk without success. I also tried to decrease the print speed from 50 mm/s to 20 mm/s, and it seems to have a minor positive effect. Anyone got any ideas on what values I need to change? Can this be a mechanical problem?

General print settings:

  • Temp: 210 °C
  • Layer: 0.2 mm
  • Infill: 0 %
  • Retraction distance: 3 mm
  • Retraction speed: 60 mm/s
  • Cooling: 80 %
  • Flow: 102.5 %
  • Acceleration: 500 mm/s
  • Jerk: 20 mm/s (Tried values from 5 to 20)

Printed at 50 mm/s:

Printed model with blobs printing errors

Printed at 20 mm/s:

Printed model that has been printed slower but still has blobs printing errors

Cura:

Screenshot of the rendered model in Cura

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3 Answers 3

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Look at the gcode for those curves either manually in a text editor, or with a gcode analyzer tool (e.g. gcode.ws) and see if there's anything strange about them. Cura is notorious for numeric instability problems with high resolution curves, where it will end up generating a few tiny moves (on the order of 0.005 mm) here and there that get truncated to zero motion on one axis and nonzero on the other (due to fixed rounding to 3 decimal places). To the printer firmware's motion planner, these look like sharp corners (because the angle changed severely from the last move), forcing it decelerate and re-accelerate.

The right fix for this is fixing Cura's numerical precision flaws. @piezoid has a WIP branch doing this that might eventually push upstream to improve, but I wouldn't hold my breath. Short of that, there are a couple good workarounds:

  • Check the "Maximum Resolution" and "Maximum Deviation" settings in Cura. You might have to unhide them if they're hidden. The defaults on modern Cura versions are 0.5 and 0.025 which tend to avoid the problem well, but older versions had bad values. (And of course, make sure you're using a recent version.)

  • Lowering the resolution (having it use a coarser triangle mesh) exported from your CAD software tends to avoid the problem.

  • Some people have reported success with the Arc Welder plugin, but I'm not sure this is a good idea, and it depends on your printer having firmware that accepts the arc motion commands.

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If you haven't already, try rotating the object about the Z-axis to give a different orientation with respect to X and Y. May not correct the problem but if the effect changes it may shed some light on the cause.

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It could be that the pressure in your nozzle is too high. I would switch on Coasting in Cura and see what effect that has.

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