When the top surface of a print is not completely closed or shows bumps, we call this pillowing. This behaviour mainly appears because of wrong print settings and bad cooling. On this page we describe what you can do to get rid of pillowing on your prints.
To create a smooth top surface it’s important to use the a thick enough top on your print. A too thin top thickness will logically lead to a top surface with holes in it. Thus, increasing the Top/Bottom thickness in Cura will help to better close the top surface of your print. We advise to use a Top/Bottom thickness of at least 6 times the layer height to properly close your model if it has a flat top surface. This means for example that you need a Top/Bottom thickness of 0.6 mm when using a 0.1 mm layer height. If the top surface of your print is not properly closed you could increase the Top/Bottom thickness. In general we can say that the thinner the layer height, the more top layers you need. Thinner layers will curl up or break easier, thus more layers are needed to completely close the top.
Besides good settings, it might be even more important to properly cool the plastic at the top surface. Make sure the print head fans are running at the maximum fan speed as set in the cooling settings in Cura. If they don’t, the plastic might not cool properly through which it may hang down a bit or curl up at the places where it crosses the infill lines. This means that an uneven surface is created for the next layer that must be placed on top of it, leading to bumps on the top surface.It seems that especially thinner layers have the tendency to curl up. Therefore it could help to increase the layer height.
Lowering the printing temperature could also help, but ensure that your print doesn’t start under extruding. In this case it might be necessary to lower the print speed as well.