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Printing problem: square bumps instead of a flat surface

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Hello everyone,

I'm quite new to 3D printing...

I received my already build Ultimaker about 2 weeks ago and when I try to print with PLA at 210°C and 50mm/s the external surface of the objects looks like this one, having these square bumps in the place of a theoretical flat surface:


Has anyone any idea on what could be the problem in the origin of this?

This happens only on the top of the objects printed and not on the sides.

The material used is the PLA from UM and the printer is driven with Cura 13.06.4

Thank you in advance for any information that you could provide me in order to put me on the right track :)


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Just to clarify what was said above. Daid, Alex, and Ian are theorizing that what you are seeing is the infill pattern from lower layers showing up on the top layer. If they are correct then Ian and Alex are suggestion you have more "solid" layers so that on each successive layer the pattern gets harder and harder to see. Daid suggests lowering the temp to get less sagging when bridging the infill pattern.

I like Ian's suggestion the best which is to increase the top/bottom thickness. For example 4 solid layers is probably plenty so if you are printing .2mm layers than make top/bottom thickness .8mm and you will get 4 solid layers.

I'm not 100% sure that what you are seeing is related to infill - you can check by looking at the gcode view of your model in cura and seeing if the pattern of infill matches the squares on top.


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Yes, I think that more layers of solid overlay will smooth this out. I've seen this before sometimes. Interestingly, I think it's not that it's sagging into the infill, but bubbling up above it. My theory was that the infill is making relatively airtight chambers under the top surface, and the addition of new layers heats the air, causing it to expand, bubbling the surface up. That's why a slightly lower temp might also help - but mostly I'd go for more layers. I generally go for a top/bottom thickness that is at least 4 to 6 times the layer height.


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