Pillowing and holes in top layers

The top surface never closes properly, leaving bumps and gaps. The top layers are being printed across gaps in the infill below, and without enough support or cooling they sag and curl instead of bridging cleanly.

Looks like: The top surface has bumps, gaps or open holes instead of coming out flat and solid.

Fixes, in the order worth trying

1 · Try this first

Add more top layers

One or two top layers cannot span sparse infill. The usual guidance is enough top layers to total at least 0.6–0.8 mm of solid thickness — at 0.2 mm layers that means four, not two.

Top Layers: Cura → Shell → 4–5 at 0.2 mm layer height
Top shell layers: OrcaSlicer / PrusaSlicer → Process → Strength → 4–5

2 · Try this first

Raise infill density

The top layers have to bridge the gaps in the infill grid. Below about 15 % those gaps get wide enough that no amount of cooling will stop the sag.

Infill Density: Cura → Infill → 15–25 %
Sparse infill density: OrcaSlicer → Process → Strength → 15–25 %, gyroid or grid

3 · Common cause

Turn cooling up

Plastic spanning a gap needs to solidify almost immediately or it droops. This is the one situation where more fan is unambiguously better, PLA especially.

Fan Speed: Cura → Cooling → 100 % for PLA on top layers
Fan speed: OrcaSlicer → Filament → Cooling → Max the material tolerates

4 · Less common

Enable ironing for a flat finish

After the top is solid, ironing runs the hot nozzle back over it with almost no extrusion to smooth the surface. It does not fix structural pillowing — do the three fixes above first — but it turns a good top surface into a very good one.

Enable Ironing: Cura → Top/Bottom → On, Top Surface Only, flow 10 %
Ironing type: OrcaSlicer → Process → Quality → Top surfaces

Change one thing at a time. If you adjust temperature, retraction and speed together and the problem goes away, you have learned nothing and you will be back here next month. Re-print the same test object after each single change.

Frequently asked questions

How many top layers should I use?

Work in millimetres, not layers. Aim for 0.6–0.8 mm of solid top: that is 3–4 layers at 0.2 mm, but 6–8 at 0.1 mm. Slicer defaults are often stated in layers, which is why halving your layer height and changing nothing else suddenly produces pillowing.

Does infill pattern matter for pillowing?

Somewhat. Gyroid and grid give a more even distribution of support points than large-cell patterns like cubic or lightning, so they bridge better at the same density. Lightning infill in particular saves a lot of plastic but leaves very little under the top surface.

Why is only part of the top surface pillowing?

Almost always the area over the widest infill gaps, or a region where cooling is weakest — often the side away from a single-sided part cooling fan. Compare where the bumps are against where the fan blows.