# Horizontal defects when linear_advance enabled?

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.

• 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? – R.. GitHub STOP HELPING ICE Sep 15 '19 at 0:30
• 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? – R.. GitHub STOP HELPING ICE Sep 15 '19 at 0:54
• @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. – R.. GitHub STOP HELPING ICE Sep 15 '19 at 1:01
• @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. – R.. GitHub STOP HELPING ICE Sep 15 '19 at 1:13
• @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. – R.. GitHub STOP HELPING ICE Sep 15 '19 at 1:31