Quick diagnosis
Pillowing shows up only on the top surface as raised, uneven bumps where the outer skin sags into gaps in the infill underneath it. This differs from general poor surface finish caused by wrong temperature, since the sides and bottom of the same print can look fine while only the top is affected.
1. Add more top layers
Pillowing usually means the top skin does not have enough solid material to fully bridge the gaps in the infill pattern below it. Increase the top layer count, or the top thickness setting if the slicer uses one, until it builds up several layers of solid material rather than one or two. A model sliced with too few top layers leaves each one trying to span the infill gaps on its own, and thin layers sag into those gaps as they cool, which is what produces the pillow-like texture on the finished surface.
2. Increase infill density or change the pattern
The infill grid beneath the top skin acts as its support structure, so wide gaps between infill lines give the top layers more distance to bridge unsupported. Raising infill density shortens that span and gives the top skin more contact points to rest on. The pattern matters too: a fine, evenly spaced grid under the top surface generally gives more consistent support than a pattern with large open cells, especially on wide, flat roofs where the top skin has more area to sag across.
3. Improve cooling on the top surface
The top skin is effectively bridging over open infill cells, and bridges need strong cooling to solidify before they sag. If the fan has been turned down elsewhere in the print to help layer bonding lower down, let the slicer increase cooling specifically for top layers or short bridging spans. Weak cooling here gives the extruded plastic time to droop into the gaps below before it sets, and that sag is what shows up as the sunken, uneven texture on the finished top.
4. Slow down the top layer print speed
Printing top layers at the same speed as infill or inner walls does not give each bridging line enough time to cool and hold its shape before the next pass lands on it. Most slicers allow a separate, slower speed for top or solid infill layers. Reducing this speed lets each line set before more weight is added on top of it, which cuts down on sag into the infill gaps and leaves a smoother, flatter finished surface.
5. Consider ironing for a finishing pass
Ironing runs the nozzle back over the finished top surface at a low flow rate to smooth ridges left by the print. It will not fix a top surface that pillows badly, since a thin remelt pass cannot fill large sunken gaps on its own. Use ironing after fixing the underlying support and cooling issues, as a final polish for appearance rather than a substitute for enough top layers and adequate infill support underneath them.
Pillowing checklist
- Top layer count builds up several layers of solid material.
- Infill density and pattern give the top skin firm support.
- Top surface cooling is strong enough to set bridging lines.
- Top layer print speed is slower than infill speed.
- Ironing is used only as a finishing pass, not a fix on its own.
Weak infill support behind the top skin is related to the same causes behind under-extrusion. If the top surface actually cracks rather than just looking bumpy, see layer separation.