So the answer depends how far up this is.
1) Would you say it's on layer 1 or 2? Or is it happening more than 0.5mm from the glass.
2) Is this PLA?
3) How does it look in preview mode? Does it look thinner in this area versus other parts of the circle?
4) How thick is it in CAD? Is it at least 0.8mm wide? It's recommended to have wall widths in CAD be 2X the nozzle size but you can definitely print thinner.
Non PLA materials have lots of splitting issues that are easily avoided.
It could be underextruding
It could be just too thin
1) It is happening also further from the glass.
3) It looks exactly the same along the whole circumference.
4) It is 0.8 in CAD, printing with 0.4mm nozzle.
It seems to be too correlated with location to consider underextrusion. Seems to be something with alignment?
I don't see how it could be alignment related. The X and Y steppers should be getting the exact same commands (same exact X and Y coordinates) and the gantry is doing the same thing each time so it should line up. It's common if you have a little underextrusion or something (maybe the bed is bouncing too much) that the liquid PLA acts like snot (like a liquid rubber band) and takes a "short cut" and prints *inside* the desired location. This is why vertical holes in your parts tend to be 0.5mm smaller.
So the height that this happens has to do with temperature for sure - the air is warmer closer to the bed. And air currents. There may be an air conditioner or a fan creating a particular flow pattern of air even if the fan is very far away.
Also the fan comes on at full speed on the printer around the 5th layer (it increases gradually on each layer change until the 5th layer). So each time it does the nozzle cools a bit and it underextrudes a little. Briefly. But it should only underextrude by maybe 3% or so. Normally negligable.
I would just make the wall thicker - in Cura set "horizontal expansion" to 0.3mm to get that wall from 0.8 thick to instead 1.4mm thick. Actually I would measure the ACTUAL wall thickness with a micrometer. If you desire 0.8 and the wall is 0.6 then you could just set Horizontal expansion to 0.1. Alternatively - slow it way down - print at half speed. Or set flow rate to 110%. It could also be that you are near the end of the spool so friction is high in the bowden and putting on a new spool will fix this issue. There are many things that can lower extrusion by 5% and if you do too many at the same time it can cause problems like the one you see. So those are few ideas: horizontal expansion, slow it down, flow rate to 110% (don't go higher than 110%), put on a new spool, raise nozzle temp by 5 degrees. All of those things will produce thicker walls.
Thanks for your responses!
I am attaching another picture showing this issue which skewed prints. All the prints are always skewed in the same way.
You can see that the walls across the line number 1. are stretched, they do not follow a circular shape, hence, the section is thicker where the line crosses the print. The walls across the 2nd line are just fine. For a bigger print this results in two splits across the first line if circles are being printed.
I am not sure how this could be related to underextrustion or the flow rate.
This is common - usually caused when one of the 4 longs belts slips a tooth. This fix is pretty easy.
So look down from above at your print head - look to see if the two rods look perpendicular. Also push the head around to the 4 sides of the printer and watch the side blocks. Do the 2 moving side blocks reach the end of travel at the same moment? If not your rods are not perpendicular.
To fix, loosen two pulleys - for example two pulleys on one of the belts. (or one on each belt). Note how tight the set screw is by the way. Then adjust so things are perpendicular and tighten the hell out of the two set screws that you had loosened. Very very tight.
Another cause is if the holes through one of your pulleys is not centered. Push the print head around and see if maybe one of the 10 pulleys is wobbling. Make sure to look at all 12. Count as you go. There's 8 on the long belts and 4 on the short belts.
Everything looks fine. What is "split" exactly? You have two cylinders - one inside the other. Quite far apart. Isn't that what you want? How does it look different in PREVIEW mode in Cura?
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