Holes and dips in the top surface (pillowing): top layers, infill and cooling

How to recognise it

The top surface has small holes, lines that do not close, dips or bulges between the infill lines; this is called pillowing, because the surface puffs up like a pillow between one support and the next. You can often see the infill underneath through the holes.

Prusa explains that top solid infill is essentially bridging over the infill pattern: the first solid lines always sag a little between supports, and the sparser the infill, the longer the gap and the more visible the dip. Holes on sloping surfaces, such as the top of a bust's head, depend instead on the thickness of the vertical shells.

The most common causes

Too few top layers: the following layers cover and level the first lines that sag. Prusa recommends at least 3 top layers and points out that thin layers need more of them to reach the same thickness: for example 3 layers at a 0.3 mm layer height, but 9 at 0.1 mm.

Infill too sparse and too little cooling: with low infill the solid lines have to span longer distances and, if the fan does not cool them immediately, they sag like a bridge. According to Prusa, the main purpose of infill is precisely to support the top layers, and most models print with 10–15%.

Too little material: if extrusion is insufficient (partially clogged nozzle, low flow, wet filament) the lines do not touch and leave gaps across the whole surface, not just over the infill. In that case see the guide on clogged nozzles and under-extrusion.

Checks to do, in order

After each change, print a small part with a large flat top face, changing one setting at a time.

1. Check that the material and printer profiles in the slicer are the right ones.
2. Increase the number of top layers: at least 3, and proportionally more if the layer height is low (Prusa gives 9 layers at 0.1 mm).
3. If the infill is below 10–15%, increase it or make it denser only under the top surface, for example with a modifier in PrusaSlicer.
4. Check the part cooling fan and the air duct: with PLA and PETG, cooling helps the first solid lines not to sag (see the guide on overhangs and bridges).
5. If holes also appear on the walls or across the whole surface, check flow and nozzle: it may be under-extrusion.
6. For holes on sloping surfaces, keep “Ensure vertical shell thickness” enabled in PrusaSlicer.
7. For a more even surface you can choose a different pattern for the visible top layer only (“Top fill pattern” in PrusaSlicer).

Notes from manufacturer documentation

Prusa (PrusaSlicer): top and bottom layers are set in Print settings → Layers and perimeters → Solid layers, where you can also set a minimum thickness; the tooltip below the settings shows the resulting thickness. Prusa recommends at least 3 top layers and suggests a modifier that increases the infill in the last few layers before the solid ones. “Ensure vertical shell thickness” adds solid infill near sloping surfaces to avoid the holes typical of organic models. “Top fill pattern” changes the pattern of the visible top layer only.

Prusa, infill page: the main purpose of infill is to support the top layers, which would otherwise have to bridge over empty space; 10–15% is enough for most models and you will rarely need more than 30%. Cooling page: on bridges and overhangs the plastic has to solidify immediately, and the bridge fan speed should be set higher than for normal printing.

What users report in the forums

On the Prusa forum, a user with a MINI printing PETG at 0.1 mm, with 9 top layers and 15% infill, saw a severe form of pillowing, with irregular shards that only disappeared by adding layers. An experienced user named infill that is too sparse as the most common cause and suggested increasing it, using adaptive infill that gets denser near the top surface, or a height range modifier. The thread reports no confirmed fix: it is an individual experience.

When replacing a part is justified

No: holes in the top surface are fixed with top layers, infill and cooling. A part is needed only if the checks point to an extrusion problem, such as a worn nozzle, or to a faulty part cooling fan.

How to verify the result

Record the model and variant, material, slicer profile and change made. Repeat the same test and compare the result with a photo taken before the adjustment. Keep the change only if it improves the defect without introducing others. Use your model’s instructions for temperatures, calibration and disassembly; this guide is not based on our own hands-on tests.