How to Fix Pillowing and Gaps in Top Layers

Pillowing in 3D printing is a surface defect where the top of a print looks uneven and pillow-shaped, with bumps, gaps and small holes. Anycubic's guide describes the cause: insufficient infill support, inadequate cooling and a weak top layer let the top layers become thin, sag and sometimes collapse. The first layers over the infill have to bridge empty cells, and if they do not cool in time they droop into them.

Pick the likely cause

1. Add top layers

Anycubic says a very thin top layer deforms easily and recommends top thickness of 5 to 6 times the layer height. Its example is a 0.1 mm layer height needing 0.5 to 0.6 mm of top thickness. Applied to a 0.2 mm layer height, the same ratio gives about 1.0 to 1.2 mm, or 5 to 6 layers. Most slicers let you set either top layers or top thickness. For how layer height interacts, see layer height explained.

2. Increase infill density

The top layers need something to rest on. Anycubic says around 20% infill is typically enough for most models, and that a stronger pattern helps. It names concentric and lightning in Cura as patterns that support top layers while using little material. The trade-off is more filament and print time; see infill explained for the options.

3. Improve top layer cooling

Anycubic says to enable print cooling and make sure the fan runs at 100% on the top layer. Prusa's cooling article gives the logic: printing must always happen on an already solidified layer, and plastic over gaps has to freeze quickly, so fans matter. Prusa's slicer lets you set cooling per filament, including a minimum layer time and fan speeds. Check that the part cooling fan actually spins, because a failing fan produces exactly this look.

Fast cooling is not free for every material. Prusa notes that more cooling can cause warping or layer separation with some filaments, so use it where the top layers need it. If layers start to split, see fixing layer separation.

4. Check temperature

Anycubic names temperature as one of the biggest factors. A cooler nozzle gives the plastic less time to stay soft over the gaps. Stay inside the range printed on the spool and change in small steps, rechecking layer bonding after each change.

5. Use ironing on flat tops

Prusa explains that ironing runs a second pass over the top surface in which the hot nozzle flattens curled plastic and fills small holes with a tiny amount of extruded filament. It works at a fixed 45 degree angle to the normal top infill pattern. Prusa lists these downsides: longer print time, a risk of heat creep and clogs on large areas (especially with PLA in hot weather), and edges that become slightly fuzzy. It recommends ironing for flat tops such as nameplates, badges, boxes and lids, and says it is not useful for round or organic shapes.

What does not help

Raising only the flow rate, or cranking the temperature up, tends to make the sag worse, since the plastic stays softer and there is more of it over the gap. Fix the support and cooling first.

A sensible order

1. Raise top layers to 5 or 6 (or to 5 to 6 times the layer height in thickness).
2. Raise infill to about 20% if it is lower.
3. Set the fan to 100% on the top layers.
4. Lower the temperature slightly.
5. Turn on ironing for flat tops that still look rough.

If other surfaces also look off, a related problem is under-extrusion and not pillowing; see fixing under-extrusion.

Gaps deeper inside the part, where the infill meets the walls, have a different fix: gaps between walls and infill.

Confirm the fix

Print a flat box about 40 mm square with 10% infill as a test. If the top is smooth with no holes, repeat on a real part. The numbers here come from maker guides and are starting points, not guarantees.