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What could cause that?

When linear_advance (v1.5) is "off" (M900 K0) - everything is uniform and smooth, only corners are protruding as expected.

When it is "on", I get these horizontal defects. My calibrated K-value is 2, I am getting same defects with K=1 as well as K=2. Position of defects is repeatable, so it is not random mechanics-caused underextrusion.

My setup uses a BMG Bowden extruder, A4988 drivers for Z and extruder with diode smoothers (X, Y are on TMC2209). I print the PETG with 245 °C, Perimeters at 50 mm/s, Gyroid Infill at 120 mm/s.

After the original posting of the question I tried the following:

  • Removed diode smoothers on extruder and Z - no improvement.

  • It seems defect is somewhat connected to infill. Without infill print is nearly perfect. 50x50 box without infill prints perfectly, so it does not look like outer-wall speed-related. Reduced outer wall to 33 mm/s, infill to 100 mm/s (accelerations are also down 750->500, 1000->750), no improvement. Returned to A4988 on all axis - no improvement. Switched to 1/8 step on extruder (maybe it is step speed-related) - no improvement. Disabled infill before walls - no improvement.

  • Starting shooting time-lapses, and outer perimeter is extremely weird on some layers (see below)

  • Migrated configuration to latest Marlin 2.0. Enabled SQUARE_WAVE_STEPPING - no improvement. Gcode is uploaded here: https://s.14.by/3d_issue.gcode

  • After inspecting larger parts printed before linear_advance (on large 100 mm circular parts) - I also noticed this defect, more rare though.

  • Connected to oversized lab power supply, to ensure there is no power sag during high load. 24V 16A max power consumption. Print defects remain exactly the same though.

enter image description here

3D print showing horizontal defects

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  • 1
    $\begingroup$ Which version of Linear Advance (1.0 or 1.5 - that is, pre-1.1.9 Marlin or >=1.1.9) are you using? The K values look like 1.5, but 2 is very high for normal PLA and even 1 is somewhat high. Did you use the calibration pattern to select a K or just guess? $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Sep 15, 2019 at 0:30
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    $\begingroup$ It looks to me like you have severe underextrusion in the areas where the print head has accelerated to significant speeds, but just on some layers. I suspect the mechanism of the problem is that your extruder gearing or idler is not up to producing the pressure that linear advance with K=1 or K=2 is attempting to get, and the filament slips backward. Can you watch closely and see if that appears to be the case? $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Sep 15, 2019 at 0:54
  • 2
    $\begingroup$ @BarsMonster: What temperature? PETG is hard to move through a nozzle fast even at high temps, and at lower temps I would not be surprised at all if it slipped from too much pressure. K between 1 and 2 does make sense for PETG with a bowden though. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Sep 15, 2019 at 1:01
  • 1
    $\begingroup$ @BarsMonster: OK, temp shouldn't be the problem then. Are you printing it too fast? Otherwise my guess is just that the tension holding the filament is not sufficient. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Sep 15, 2019 at 1:13
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    $\begingroup$ @BarsMonster: I don't think you can actually extrude PETG at 120 mm/s. At 0.2 mm layer height (guess) and 0.4 mm nozzle (guess) that's nearly 10 mm³/s volumetric extrusion. There are likely lots of issues going on that just show up more visibly with LA enabled. $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Sep 15, 2019 at 1:31

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