Pillowing: why the top surface has holes and bumps

How pillowing forms

When printing, the top layers are laid over the infill, which is not solid but a sparse lattice. There are gaps between its lines. The first top layers have to stretch across them like a bridge, and if they do not have enough time to cool or there are too few of them, they sag.

The result is a surface that looks like a pillow, which is where the name comes from. It feels uneven to the touch, and sometimes tiny holes show between the bumps, through which the infill can be seen.

What makes it worse

• Sparse infill. The bigger the gaps, the more the top layers sag.
• Few top layers. A thin skin sags more easily than a thick one. See also top and bottom layers.
• Weak cooling. The plastic stays soft for longer and sags.
• A nozzle that is too hot. Overheated plastic is runnier and holds worse over a gap.

What to do about it when ordering

The simplest thing is to choose stronger infill. There are three options in our calculator: light with 10 % infill, standard with 15 % and strong with 40 %. On a larger top surface with denser infill, pillowing is much reduced, because the top layers have something to lie on.

If the top surface is small or will be covered by another part later, you do not need to worry about pillowing. If it is to be visible, such as a logo or a label, choose at least standard strength.

How to avoid it in the design

When designing, try not to have a top surface lying over a large cavity. It helps to split it into smaller areas or to put a rib underneath. If a smooth surface matters, rotate the model so that the visible face is on the side or on the plate.

Smoothing the top surface by ironing is a slicer setting that improves the surface, but it does not replace enough layers and a dense infill.