Symptom: the top surface of the model, instead of being smooth, shows gaps, small craters and rippled bulges. In the worst case you can see the infill through it, and the lines of the final layer curl upwards in a regular pattern that mirrors the infill grid. The sides of the print, meanwhile, look faultless.
Pillowing comes from the top layers having nothing to rest on. The last few layers are in reality bridges spanning the cells of the infill. Material laid down in mid-air contracts as it cools, so if there is too little of it, or if it cools too slowly, the line lifts and snaps. The problem is therefore geometric rather than a material fault: what decides it is the ratio between the size of the infill cell and the number and thickness of the layers meant to cover it. That is good news, because it can almost always be solved in the slicer alone.
Most common causes and fixes
1. There are too few top layers
The default three layers are only enough with dense infill and a low layer height. The first layer over a void always comes out uneven, the second straightens it, the third smooths it. If there are fewer layers than you need, the last one is still a bridge rather than a surface.
How to check: instead of counting layers, measure thickness. Most slicers let you set the top shell in millimetres — Top shell thickness in OrcaSlicer and Cura. It is worth having it at least roughly equal to the nozzle diameter.
What to do: add layers so that the shell reaches the required thickness at your layer height. At a 0.2 mm layer that usually means five layers, at 0.12 mm correspondingly more. This is the single most effective change and on most prints it is enough on its own.
2. The infill is too sparse to bridge over
The sparser the infill, the larger the cell and the longer the bridge to span. At low values no number of top layers will save the surface, because the first of them sags into the void and ruins the base for the ones that follow.
How to check: look at the slicer preview on the layer where the top shell begins. If the lines run over clearly visible, large cells, increasing the density will help more than anything else.
What to do: raise the infill density or change the pattern to one that gives shorter unsupported spans — gyroid and a triangular lattice work better here than a sparse grid. An alternative for large, hollow models is the option to use denser infill only beneath the top surface, available as Ensure vertical shell thickness or as transition layers.
3. Cooling is insufficient
A bridge will only hold its shape if it sets before it has time to sag. A weak fan, a blocked duct or a shroud that blows past the nozzle instead of underneath it produce exactly this symptom. It also happens that the fan only starts after a few layers and on short models never gets up to speed.
How to check: pause the print above the top surface and use your finger to confirm that the airflow really does land just behind the nozzle.
What to do: set cooling to the maximum specified for the material, and not beyond it. Here is the catch: ABS, ASA and HT PETG have a very limited cooling range, because strong cooling causes them to crack and lift off the bed. With these materials pillowing has to be treated with a thicker shell and denser infill, not with the fan. The ranges are collected in the table below.
4. The layer is too tall relative to the nozzle
A thick layer means a thicker, heavier line spanning the same void. On top of that, a tall layer bonds less well to its neighbours horizontally, so the shell does not form a sealed sheet. The rule of thumb is that layer height should not exceed roughly three quarters of the nozzle diameter.
What to do: reduce the layer height, or, if you care about time, leave it as it is for the model and reduce it only for the top layers, if your slicer allows that. Remember too which nozzle you are printing with: PLA Wood and PLA Star require a 0.6 mm nozzle, and with it the same number of top layers gives a completely different shell thickness.
5. Material is missing or the temperature is too high
A bridge extruded with too thin a line will break even with correct shell settings. Too high a temperature works the other way round: the polymer stays fluid for longer and droops between the infill cells instead of holding tension.
How to check: look at the layer just below the shell. If gaps between lines are already visible there, the source is under-extrusion, and the top surface is merely making it visible.
What to do: calibrate flow first, then drop the nozzle temperature by 5 °C within the material's range. Only at the end reach for ironing of the top surface — it smooths, but it will not repair holes you can see the infill through.
| Porima material | Part cooling | Recommended nozzle | Note for the top surface |
|---|---|---|---|
| PLA | 100% | 0.4 mm | full cooling solves most cases |
| Tough PLA (PLA+) | 100% (required) | 0.4 mm | cooling is also needed on bridges |
| Silk PLA | 80–100% | 0.4 mm | the sheen masks small craters |
| PLA Wood | 80–100% | 0.6 mm | a thicker line needs more top layers |
| PLA/CF | 60–100% | 0.4 mm hardened | detail from 0.4 mm, the shell must be thicker |
| PETG | 100% | 0.4 mm | responds well to full cooling |
| HT PETG | 0–10% | 0.4 mm | cooling will not help, add layers instead |
| ABS | 0–40% | 0.4 mm | enclosed build area, cooling strongly limited |
| ASA | 0–30% | 0.4 mm | as above, denser infill is the rescue |
| PA (nylon) | 30–100%, none for the first 5 layers | 0.4 mm | drying before printing is mandatory |
| TPU Flex 98A | 40–70% | 0.4 mm | print slowly, bridges are difficult regardless |
Source: the official FDM technical data table from the manufacturer, Porima. All 25 materials: print temperature table.
Quick checklist
- Set the top shell thickness in millimetres, not as a number of layers.
- Raise the infill density or change the pattern to gyroid.
- Check that the fan airflow really does reach under the nozzle.
- Increase cooling to the maximum permitted for the material.
- Reduce the layer height below three quarters of the nozzle diameter.
- Calibrate flow if gaps are already visible below the shell.
- Turn ironing on last, purely as a finishing step.
Overriding rule: change one parameter at a time. Shell thickness and infill density changed together will not tell you which one worked.
Which material is most at risk
The hardest cases are where cooling is inherently limited. HT PETG runs with 0–10% fan and ASA with 0–30%, so bridges over the infill cool slowly and sag. With these materials you add layers and infill density instead of fighting with a fan which, in an enclosed chamber, ruins bed adhesion anyway. The opposite pole is PLA and PETG with full cooling — with those, pillowing almost always means the top shell is too thin or the infill too sparse.
A separate case is PA. Nylon absorbs moisture exceptionally readily and needs drying for 8–12 hours at 65–80 °C, mandatory before every print. Wet nylon foams in the nozzle, and a foamed line will not hold a bridge. If pillowing appears only on this material, start with filament drying rather than with settings.
Related problems
A smooth top surface starts with even flow. Porima filaments are produced to a ±0.03 mm diameter tolerance, and we publish the manufacturer's full official print data — you don't have to guess it. Our warehouse is near Poznań, in Swarzędz.
See Porima filaments · Print temperature table · All 3D printing problems