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In order to avoid Pillowing, the top surface must be thick enough so that this could be completely closed by the printer. Leaving a very thin layer for the top will definitely leave holes in it when cooling of the filament will do occur. Thus, increasing Top/Bottom Thickness will provide enough layers for the top. Top/Bottom thickness around the Six times the Layer Height is recommended, For example, If your Layer Height is 0.2 mm then Top/Bottom Thickness setting would be = 6*Layer Height=1.2 mm.
Besides Top/Bottom Thickness Cooling plays important role in preventing Pillowing. Make sure the cooling fans are running at their maximum value thus providing even cooling, Otherwise the plastic may cool at random places at the top surface and thus curl up there giving holes in the top layer.
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