Pillowing: holes and bumps on top surfaces

This guide is for anyone whose prints look fine on the walls but have a pitted, lumpy or see-through top. It covers what pillowing looks like, what causes it, the slicer settings that Prusa, OrcaSlicer, Cura and Simplify3D document for top layers, infill, speed, cooling and ironing, and how PLA, PETG and ABS differ.

What does pillowing look like?

Polymaker describes pillowing as "small dips, holes, or 'pillows' in the very top layers, often with infill lines visible underneath." It says the problem is "most common on large flat areas because each top line has to span a bigger distance between infill roads." Simplify3D calls the same defect gaps in the top layers: the top solid layers "are not completely solid," with "gaps or holes between the extrusions."

Pillowing or something else?

The third row is a different problem. Polymaker's guide to ugly tops says it happens on layers with "a very small surface area," such as the tip of a pyramid, because "there is not enough time for a layer to cool before the next hot layer is laid on top of it." Its fixes are printing more than one object at a time, more fan, and a longer minimum layer time.

Why does pillowing happen?

Most prints are a solid shell around a partly hollow inside. Simplify3D explains that when the slicer prints a fully solid layer over sparse infill, that layer "has to span across the hollow air pockets of your infill," and its extrusions "have a tendency to droop or sag down into the air pocket." Prusa says the same: the top solid infill "is essentially bridging over the infill pattern," so "you'll almost always see some sag in the first few solid infill lines." It adds that "the lower the infill, the longer the bridging distance, and as a result the bigger the sag."

The later top layers are supposed to cover that sag. Polymaker lists three root causes when they don't:

• Too little infill under the top surface, "so the filament sags between infill lines."
• Not enough solid top layers, "so the first few bridging lines never get fully covered."
• Weak bridging from the material and cooling setup, for example ABS or other filaments printed with minimal fan.

Simplify3D adds a fourth: if more top layers and more infill don't close the gaps, "then you likely have an under-extrusion issue," meaning the nozzle "is not extruding as much plastic as the software expects."

Step 1: Add top layers or top thickness

This is the first fix both Simplify3D and Polymaker suggest. Extra top layers sit inside the part. Simplify3D notes they "do not add size to the exterior of your part."

How much is enough depends on layer height, because thin layers add up to a thin shell:

• Simplify3D gives a rule of thumb that the solid top section should be "at least 0.5mm thick," so 0.25 mm layers need at least 2 top layers and 0.1 mm layers may need 5. If you see gaps with 3 top layers, it suggests trying 5. The setting is on the Layer tab of Edit Process Settings.
• Prusa suggests "at least 3 top layers" in PrusaSlicer, and notes that thinner layers need more of them, for example 3 top layers at 0.3 mm but 9 at 0.1 mm. You can also set a minimum shell thickness in the same Layers and perimeters section, and the tooltip shows the resulting top thickness as you change it.
• Polymaker suggests keeping the total top thickness "at least 4 to 6 times the layer height," for example 5 or 6 top layers at 0.2 mm.
• OrcaSlicer has two settings under Strength, Top and Bottom Shells. Shell layers sets the number of solid layers, and OrcaSlicer says it's "usually recommended to have at least 3 shell layers for most prints." Shell thickness raises the layer count automatically when the layers alone would be too thin, which OrcaSlicer says "avoids having too thin a shell when layer height is small."
• Cura has Top Thickness and Top Layers. Cura's description says the thickness "divided by the layer height defines the number of top layers."
• Bambu Studio: Polymaker's guide says to open the Strength tab and raise "Top shell layers" under shells.

These numbers aren't identical, so treat them as starting points. Polymaker suggests checking the result in the slicer's layer preview: "If you still see infill showing through in preview, keep increasing top layers or infill until the top skin looks continuous."

Step 2: Give the top more support from the infill

If extra top layers aren't enough, support the roof better from below.

• Raise infill density. Simplify3D says that with very low infill, such as 10%, the top layers have to print over "very large air gaps." If 30% still shows gaps, it suggests trying 50%. Polymaker suggests "typically 15 to 30% for normal parts, higher for very wide, flat roofs," and says that after adding top layers, more infill "is usually the next most effective change for severe pillowing." For comparison, Prusa says most models print fine at 10 to 15% infill, and "you'll rarely need infill higher than 30%."
• Change the infill pattern. Polymaker says grid or rectilinear "gives straight ribs with clear gaps, which can make pillowing worse on big spans," while gyroid "spreads support more evenly in both directions." It suggests trying gyroid with slightly higher density when you want a smooth top on a large area.
• Add density only near the top. Prusa suggests a modifier mesh that "increases the infill for the last few layers before the solid infill," so you don't pay for dense infill through the whole part. Cura's Gradual Infill Steps setting works the other way round: it halves infill density step by step further below top surfaces, so areas "closer to top surfaces get a higher density."
• Print more, thinner layers on top. Polymaker suggests a smaller layer height if you want "the same total top thickness with more individual layers on top."

Step 3: Slow the top surface and check cooling

The first top layers are bridges, so anything that helps bridging helps pillowing.

Top surface speed

• OrcaSlicer has a separate top surface speed. Its wiki says this "is usually set similar to the outer wall speed," and that "lower speeds help minimize surface defects."
• Cura has Top Surface Skin Speed and Top/Bottom Speed in the Speed section.
• Polymaker says that when you can't use much fan, "slowing down the top solid layers can give the filament more time to stick to the infill and to previous lines instead of drooping into gaps." It also says a slightly lower nozzle temperature on top layers, within the filament's recommended range, can help difficult materials bridge more cleanly.

Cooling

Prusa's cooling guide explains why the fan matters: when printing overhangs or bridges, the plastic needs to "freeze and solidify" in place quickly, or "the filament will sag down." The fan helps, but Prusa warns that with some materials more cooling "could make things actually worse and cause warping or layer separation."

• PrusaSlicer sets cooling per filament profile, with Min and Max fan speed and a separate Bridges fan speed, which Prusa says you'd typically set higher than normal printing.
• OrcaSlicer has an Internal bridges fan speed, which applies "for all internal bridges." Its wiki notes the trade-off: running this lower than your regular fan speed can reduce warping "due to excessive cooling applied over a large surface for a prolonged period of time." So raising it for a better top is a balance against warping on large parts.
• Cura has Fan Speed and Enable Bridge Settings, which Cura says will "detect bridges and modify print speed, flow and fan settings while bridges are printed."

Stay within your filament's recommended cooling. Polymaker's advice is to "tune fan speed and top layer speed to improve bridging while staying within the material's recommendations."

Step 4: Rule out under-extrusion

If you've added top layers and infill and still see gaps, Simplify3D points to under-extrusion. A quick check: look at the walls and infill as well as the top. If lines look thin or gappy everywhere, the top surface is a symptom, not the cause. Our under-extrusion guide covers flow, clogs and wet filament.

Ironing: smoothing a top that is already closed

Ironing makes a second pass over the top layer. Prusa says the hot nozzle "flattens any plastic that might have curled up" and "extrudes a small amount of filament to fill any holes in the top surface." Cura describes it as going over the top surface again "extruding very little material," keeping nozzle pressure up "so that the creases in the surface are filled with material."

Ironing finishes a surface. It doesn't replace enough top layers and infill. If ironing itself leaves grooves or edge ridges, see our ironing problems guide. Points from the slicer docs:

• Where to find it. In PrusaSlicer it's under Print settings, Infill, Ironing (Advanced or Expert mode). OrcaSlicer offers ironing for all top surfaces, the topmost surface only, or all solid layers. Cura has Enable Ironing and Iron Only Highest Layer.
• It's for flat tops. Prusa says ironing suits flat tops such as nameplates, badges, boxes and lids, and is not useful for round objects, figures and organic shapes.
• Flow matters. Prusa says ironing is "very sensitive to accurate extruder calibration." Too little flow leaves "shiny groves," too much drags plastic to the edges. OrcaSlicer says a lower ironing flow gives a smoother finish "but may not cover the surface completely," and a higher one can over-extrude.
• It costs time. Prusa calls increased print time "the main downside," and says edges can come out "a tiny bit fuzzy."
• Pair it with monotonic top infill. Prusa suggests using monotonic infill with ironing "to get the best results," and says Original Prusa profiles already use it for top layers. Cura's Monotonic Top Surface Order "makes flat surfaces look more consistent," at the cost of slightly more time.

Material notes: PLA, PETG and ABS

• PLA bridges well with strong part cooling, according to Polymaker, and "can tolerate lower infill on small roofs." Its example: a small PLA part with good cooling might manage with 5% infill and 3 top layers. Prusa says PLA "irons very nicely" but is the most prone to heat creep during ironing.
• PETG usually has worse bridging and overhang behavior than PLA, according to Prusa. Prusa says cooling PETG helps keep detail, and for maximum toughness suggests turning the fan off. Its default suggestion is the first few layers with no fan, then the fan at half power. If you keep the fan low on PETG for toughness, Polymaker's general advice for printing with little fan applies: more top layers, and slower top solid layers so the filament has more time to stick instead of drooping. For ironing, Prusa says PETG "irons well" but extra filament can build up on the nozzle, darken and stick to the print.
• ABS and ASA are printed with reduced or no fan, which Polymaker says "makes bridging and top surfaces more difficult." Prusa's cooling guide says the setting that keeps the fan at a minimum speed is typically on for most materials except ABS, ASA, PC and some flexibles. Polymaker's example for a larger flat ABS part is something like 25% infill and 5 top layers, and it suggests slower top layers too. Prusa says ASA "irons incredibly well."

A sensible order

1. Confirm it's pillowing: dips, holes or lumps on a flat top, not gaps everywhere.
2. Check the slicer preview. If infill shows through the top, add top layers or top thickness.
3. Raise infill density, or switch from grid to gyroid, or add a denser modifier just under the top.
4. Slow the top surface and check bridge and top cooling, within the filament's limits.
5. If gaps remain, check for under-extrusion.
6. Once the top is closed, turn on ironing if you want it smoother.

Polymaker suggests testing on a flat calibration plate first: print it at your usual settings, then again with more infill, more top layers and a better pattern, and compare before changing your everyday profile.

Print problems: what to check before you buy ↗