Pillowing or under-extrusion? The gaps look similar.

Look at WHERE they are: pillowing only affects the top surface (the lid); under-extrusion leaves gaps in walls and perimeters all over. If the sides are perfect, it's pillowing.

Why does it happen more with thin layers?

Because each lid layer is thinner and sags more easily between infill. At 0.1 mm layers you need twice as many top layers for the same 1 mm lid.

3D printing troubleshooting

Pillowing (gappy top surface)

Also known as: the top layer comes out with holes, bumps or won't close.

The symptom

The part's top surface won't close properly: gaps between lines, small craters or bumps where the infill below shows through. The rest of the part looks fine.

Sources for this page: Prusa Knowledge Base · Manufacturer Simplify3D · Technical guide

Causes and fixes, ranked by likelihood

1

Too few top layers: the 'lid' tries to bridge over the infill with too little material.

Fix: Increase top layers until the lid is ≥1 mm thick: at 0.2 mm layers that's 5-6 top layers (fast profiles sometimes ship 3).
Source: Simplify3D — Print Quality Guide: Gaps in Top Layers ↗
2

Infill too sparse: the lid's bridges sag between widely spaced infill walls.

Fix: Raise infill to 15-20% for parts with large lids, or keep the % and add more top layers. For hollow decorative boxes, consider vase mode.
Source: Simplify3D — Print Quality Guide: Gaps in Top Layers ↗
3

Not enough cooling on the lid layers: the sagging plastic doesn't set before the next pass.

Fix: Run the fan at 100% on the top layers (many slicers allow per-layer ranges). With PLA this alone fixes half the cases.
Source: Simplify3D — Print Quality Guide: Gaps in Top Layers ↗

hotend tools that help here

Printer comparator → Printers with stronger part cooling close lids much better: compare it along with flow and speeds.

FAQ

Pillowing or under-extrusion? The gaps look similar.: Look at WHERE they are: pillowing only affects the top surface (the lid); under-extrusion leaves gaps in walls and perimeters all over. If the sides are perfect, it's pillowing.
Why does it happen more with thin layers?: Because each lid layer is thinner and sags more easily between infill. At 0.1 mm layers you need twice as many top layers for the same 1 mm lid.

Sources & references

This page is cross-checked against public manufacturer documentation and reference technical guides:

Manufacturer Prusa Knowledge Base — Print quality issues ↗
Technical guide Simplify3D — Print Quality Guide: Gaps in Top Layers ↗

Still stuck? Back to the quick diagnosis or check which materials your printer can handle.