The top surface has small rounded bumps — like a quilt or pillow pattern. Each bump corresponds to a void in the infill below, where the top layer sagged slightly into the gap before the filament solidified. Flat top surfaces should look smooth; pillowing makes them look textured or pockmarked.
Distinguish Pillowing from Top Surface Gaps
Pillowing: Bumps and rounded humps on an otherwise complete surface. The top is closed but uneven.
Top surface gaps: Actual holes or missing sections in the top layer. The infill is visible below.
These look similar in photos but need different fixes. Pillowing is mainly a cooling problem. Top surface gaps are mainly a flow and layer-count problem.
The Infill-Cooling Interaction
The top layers must bridge across infill gaps. Wider infill gaps (lower infill density) = longer unsupported spans = more time for filament to sag before cooling. Inadequate cooling means the filament hasn’t solidified when the next layer arrives, compressing the sag further.
Either make the spans shorter (more infill) or cool faster (more fan) or both.
Fix Sequence
1. Add more top layers. Go from 3–4 to 6–7 top layers. The additional layers progressively bridge and fill, so by the time you reach the final visible layer, the surface is built up enough to be flat. This is the simplest fix and often sufficient alone.
2. Increase cooling. 100% fan for top layers is the target. If you’re already at 100%, the issue is that the fan hardware isn’t moving enough air to the surface. Check that the ducts are aimed at the print surface, not the nozzle.
3. Increase infill to 25% or more. Smaller infill voids mean shorter bridging spans in the top layer. Going from 10% to 20% infill makes a bigger difference to top surface quality than almost any other setting change.
4. Reduce top layer speed to 20–30mm/s. The slower the top layer prints, the more time each deposited line has to cool before the nozzle moves to the next line. If you’re printing top layers at 60–80mm/s (infill speed), slow them down.
5. Switch to monotonic or ironing. Monotonic fill ensures adjacent lines always overlap in the same direction, eliminating the small gaps between alternating passes. Ironing adds a slow pass over the top surface with nearly closed flow, smoothing out minor pillowing.
What Doesn’t Fix It
Adding perimeters — perimeters don’t help with pillowing since pillowing happens in the solid infill area between perimeters. Reducing layer height significantly — pillowing is more affected by infill density and cooling than by layer height.