First of all, I'm quite new to 3D printing. After printing some temperature towers, stringing and bridging tests, I wanted to print the Bechys to see everything working together. I'm getting some strange "bumps" in the outer walls. It looks like for 2-3 layers, the filament gets lifted up.

infill before walls set

infill before walls unset

I first thought, the infill is printed too far in the outer shell, so I unchecked "Infill Before Walls" in Cura. Surprisingly the outcome did not change and the "bumps" where in the exact same spots as before.

I am aware of a part cooling problem, but my stock cooler already sits at 100 %. I don't think, the extruder temperature is too hot, with it being only 200 °C. As you can see, the printer only has a one sided cooling solution with a very small fan conduct.

fan conduct

Print settings:

  • Printer: Anycubic I3 Mega S
  • Slicer: Cura
  • Material: PLA
  • Extruder Temp: 200 °C
  • Bed Temp: 60 °C
  • Print speed: 45 mm/s
  • Travel speed: 100mm/s
  • Layer height: 0.2 mm
  • Layer width: 0.4 mm
  • Nozzle: 0.4 mm
  • Infill: 10 %
  • Retraction distance: 4.5 mm
    Retraction speed: 40 mm/s

So I am suspecting it must have something to do with the way, it is sliced. Does anyone have an idea?

Update: I started the print again with aligned seams. Now you can clearly see the artifact. At the point of the seam the print is warped inwards and upwards.

aligned seams

I did some more prints and tried to increase retraction distance up to 10 mm and retraction speed up to 60 mm/s, but I also did not manage to improve the print. I also tried to enable the "retract at layer change" option and to disable "Z Hop When Retracted" but without success.

I set the travel speed down to 45 mm/s with no change in print quality.

  • $\begingroup$ Please slice with layer change aligned and post an image (20 layers should suffice), so that we can see if this is related to layer changes or not. $\endgroup$
    – Davo
    Commented Jun 18, 2020 at 16:37
  • 2
    $\begingroup$ Hi welcome to 3DPrinting.SE! What can be seen on the bottom photo is that your nozzle is a little too far from the bed, and you aren't using enough part cooling (see the overhanging bow). $\endgroup$
    – 0scar
    Commented Jun 18, 2020 at 22:00
  • $\begingroup$ @0scar I am aware of the part cooling but it already sits at a 100 %. I think I will do an upgrade shortly. I have already done a temperature tower, bridging and stringing tests. The benchy was just to see everything together. How did you determine, the bed being to far away from the nozzle? $\endgroup$
    – mrei
    Commented Jun 19, 2020 at 7:11
  • 1
    $\begingroup$ If the fan is already at 100 %, then you print too hot or the cooling fan doesn't provide enough flow, many print fan ducts constrict airflow, we cannot see what you are using. Your first layer doesn't look that well, there are many channels, but it is hard to tell from the photo. Sometimes 200 °C is just too hot, most PLA can be printed from 185 °C. The last photo looks as if the nozzle has overpressure at the end of the outer wall or oozed out filament at the start of the outer wall.This could be a retraction issue. $\endgroup$
    – 0scar
    Commented Jun 19, 2020 at 8:01
  • $\begingroup$ Thanks for the help so far. I'm going to play around with the retraction again and see if that helps. $\endgroup$
    – mrei
    Commented Jun 19, 2020 at 10:47

2 Answers 2


There are a few issues that could cause this. Roughness in your z-axis lead screw as pointed out by @TheLamestUsername is a leading contender I would say. I'd also extended that answer to include checking the belts for your X and Y axes as well as making sure that the rods the gantry slide on are smooth since the fault appears to be in about the same X-Y location as well in the one image.

However, there are a few alternatives as well. Check/confirm that the material that you're using doesn't have a lot of absorbed moisture in it. This can cause little bits of steam to 'pop' when they're extruded causing some bubbles

Because of how the material is folding at the location in the "updated" image this looks to be an issue with the starting and stopping. All of the layer's problems appear to be happening when the gantry moves up to the next layer. Essentially more material is coming out of the nozzle there (I think) than is what should be. Confirming material diameter with some calipers can help if there's just an offset (not too unlikely I don't think).

If the diameters are dead on I'd suggest reducing the nozzle temperature by a few degrees. The sensors used to measure the temperature aren't usually all that accurate, though their precision is often pretty good. While you may have it set at 200C that might not be the actual temperature. Calibration of the thermal sensors can drift over time, so while it might have been good on the last roll it may have drifted slightly. Or the material might be of slightly different composition. Try lowering the nozzle temp by 2-5 degrees and see if that has any effect.

You may also have a retraction setting that can be modified. Pulling the material back ever so slightly when it finishes the one layer as it moves to the next could alos reduce some of the extra material that's coming out.


The fact that the bumps were in the same spots on multiple occasions points to z-axis problems. Turn your printer off and manually turn the z-axis all the way from bottom to top. If there are any tight spots, there is some z-axis binding. If there are no tight spots, skip to the last paragraph.

Try taking off the z-axis by removing the set screws (pictured below). Remember which side was pointing up for a later step.

enter image description here

Once you do that, clean the screw thoroughly with a brush, cloth, solution, or a combination of those. Put the screw back in the opposite way this time.

If you get no bumps, then it was indeed the z-axis

If the bumps are still there, try lowering your temperature some more(200 is still a little high compared to what I do for PLA), and calibrate your e-steps if you haven't already.


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