Fixing holes and gaps in the top surface (pillowing)
are explained briefly when you hover over or tap them.
The top of a print should be a closed, smooth surface. Sometimes it is not: there are small holes, rough spots or bumps that look like a puffed-up pillow. That is where the name pillowing comes from. Usually it is not a defect but a few settings in the slicer.
How do you recognise pillowing?
- Holes or gaps between the lines of the top layer.
- Bumps and bulges on flat surfaces, especially large ones.
- The infill pattern shows through, the grid lines underneath are visible through the top surface.
- The problem occurs only on top, the walls and the underside are fine.
If the walls are also thin and full of holes, under-extrusion is the more likely cause. Start there first.
Why does it happen?
The topmost closed surface is not printed onto a solid base but onto the infill. There is air between the infill lines. The first top layers have to span these gaps, much like a small bridge. They easily sag or tear open in the process. Only the following layers smooth everything out. If there are too few, the uneven structure stays visible.
The causes, sorted by frequency
1. Too few top layers
This is the most common cause. Every additional top layer smooths the surface further. Prusa recommends at least 3 top layers. More important than the number, though, is the total thickness: the Prusa profiles set a minimum top thickness of 0.7 mm.
| Layer height | Top layers for about 0.7 to 1 mm |
|---|---|
| 0.1 mm | 7 to 10 |
| 0.2 mm | 4 to 5 |
| 0.3 mm | 3 to 4 |
Fix: Increase the number of top layers or specify a minimum thickness for the top in the slicer. With low layer heights you need correspondingly more layers. Details on walls and top layers are in Walls and top layers.
2. Too little infill
At very low density the gaps between the infill lines are large. The first top layer has to span long distances and sags more.
Fix: Set the infill to at least 10 to 15 %. The Prusa profiles start at 15 %. If you want to use very little infill on decorative parts, compensate with additional top layers. More in Setting infill.
3. Too little cooling
The first top layer above the infill behaves like a bridge. If it cools too slowly, it sinks between the infill lines and tears open. With PLA this is especially noticeable when the fan is throttled.
Fix: With PLA, leave the part cooling fan at full power for the upper layers. With PETG, ABS and ASA, a little more fan than in the base profile is often enough. More on this in Cooling.
4. Under-extrusion
If material is generally missing, gaps remain on the top surface too. More top layers help only to a limited extent here. Bambu Lab names a slightly too low flow value as the most common reason for even gaps in the topmost layer.
Fix: Run a flow calibration, check the nozzle and extruder. The full troubleshooting is in the under-extrusion article linked above.
5. Top surface printed too fast
If the topmost layer is printed very fast, the lines lie next to each other less cleanly. Many profiles therefore print the top surface more slowly than the infill.
Fix: Reduce the speed for the topmost layer if your profile uses the same value as for infill.
Slicer aids against pillowing
- Additional bridge layer: Some slicers, such as OrcaSlicer, can print an extra bridge layer above the infill. This supports the top layers and reduces pillowing.
- Denser infill only below the top surface: Some slicers automatically generate denser infill below top surfaces.
Ironing in brief
Ironing is an additional pass over the topmost layer. The hot nozzle travels over the surface once more with very little material and melts unevenness smooth. The flow is only about 50 to 70 % of the normal value.
The result is very smooth, matte surfaces. The downside: ironing costs a lot of extra time and only works on flat, horizontal surfaces. It is a finishing touch for a top surface that is already closed. It does not fix real holes caused by too few top layers or under-extrusion.
Quick checklist
- Only the top affected? Otherwise check under-extrusion first.
- Top surface at least 0.7 mm thick in total?
- Infill at least 10 to 15 %?
- Part cooling fan sufficient for the upper layers, full for PLA?
- Flow calibration done?
- For especially smooth surfaces: enable ironing.
Continue with: Printing bridges, because the first top layer above the infill behaves like many small bridges.