Pillowing on top surfaces: shell count, infill support and internal bridge settings

Prusa’s slicer documentation gives the mechanism in two sentences: the top solid infill is essentially bridging over the infill pattern, so the first few solid lines almost always sag, and the lower the infill, the longer the bridge and the bigger the sag ^[1]. If the layers above do not cover that sag, it shows through as rounded bumps between the infill lines or as small holes. OrcaSlicer describes a second route on slanted or curved tops, where unsupported solid infill curls up and causes pillowing ^[8].

Sag, curl or under-extrusion

Look at the top under raking light before changing settings.

• Domes or pinholes that line up with the infill grid below. Classic pillowing from sag. The fixes are shell count, infill support and internal bridges.
• Rough or lifted areas mainly on sloped or curved tops. OrcaSlicer’s curl case, addressed by its internal bridge filtering option ^[8].
• Even gaps between every top line across the whole surface. That is too little material, not sag. Prusa’s extrusion multiplier guide says small gaps between lines mean the flow is too low ^[5]. See under-extrusion and flow rate calibration.

Symptom, likely cause, what to check

Top shell count and thickness

Prusa suggests at least 3 top layers and points out that low layer heights need more of them for the same thickness: 3 layers at 0.3 mm, 9 layers at 0.1 mm. PrusaSlicer also lets you set a minimum thickness next to the layer count, which Prusa calls especially useful with variable layer height ^[1].

OrcaSlicer recommends at least 3 shell layers for most prints and raises the layer count automatically when the count would give a shell thinner than the thickness setting ^[7]. Setting a thickness rather than relying on a fixed count keeps the shell from thinning when you change layer height.

Infill that supports the top

Prusa states that the main purpose of infill is to support the top layers, and that most models print with 10–15% infill ^[2]. Pattern matters as much as density:

• Rectilinear gives double the support for top layers compared with Grid for the same amount of material ^[3].
• Support cubic gets denser only as it approaches a top layer, and Lightning builds a branching structure that gets denser toward top surfaces; both save material without adding strength ^[3].
• In Bambu Studio, the Lightning density setting applies only to the layer directly below the top surface, and Bambu Lab positions it for decorative parts ^[11].

Prusa also suggests a modifier mesh that raises infill density for the last few layers before the top solid infill ^[1]. That targets the problem area without making the whole part heavier.

Internal bridge settings

The first solid layer over sparse infill is an internal bridge, and the newer slicers expose settings for it.

OrcaSlicer ^[8]:

• internal_bridge_flow: raising it above 1.0 can improve strength and the quality of the layers above.
• thick_internal_bridges: prints the bridge at a height equal to the nozzle diameter; the wiki recommends enabling it for internal bridges over sparse infill.
• enable_extra_bridge_layer: an extra bridge layer reduces pillowing on top surfaces, which the wiki says helps most on fast printers where bridge and solid infill speeds differ a lot. The internal bridge layers count toward the top shell count.
• dont_filter_internal_bridges: by default small internal bridges are filtered out; Limited filtering adds bridges on heavily slanted surfaces where low-density infill would otherwise leave solid infill to curl.

Bambu Studio has Internal bridge support thickness, which prints support loops under the edges of internal bridges so they do not extrude over air; Bambu Lab says this improves top surface quality, especially at low sparse infill density, and it needs a sparse infill density above 0 ^[10]. Its Sub-top surface pattern sets the layer under the top surface to Monotonic by default to reduce marks showing through, and does not apply when that layer is a bridge ^[10].

Ironing as a finish

Ironing runs the hot nozzle over the finished top layer at the same height with a small extrusion, which flattens plastic that curled up and fills small holes ^[4]. It is a finishing pass, not structural support.

• PrusaSlicer: Print settings → Infill → Ironing (Advanced or Expert mode). Prusa recommends combining it with Monotonic top infill and warns about longer print time, slightly fuzzy edges and heat creep on large PLA surfaces ^[4].
• OrcaSlicer: ironing_type (top surfaces, topmost surface, all solid layers), ironing_flow as a percentage of normal flow, and ironing_spacing at or below the nozzle diameter; the wiki also warns of heat creep and clogs ^[9].
• Bambu Studio: ironing is off by default. Bambu Lab cautions that the nozzle presses on the top layer, so a poorly adhered part can be knocked off the plate, and that too high an ironing flow leaves over-extrusion on the surface ^[12].

Order of checks

1. Identify sag, curl or flow from the surface pattern.
2. Set at least 3 top layers and a minimum top shell thickness.
3. Raise infill density or use a pattern that densifies under the top, or add a modifier mesh under flat tops.
4. In OrcaSlicer, enable thick internal bridges and an extra internal bridge layer; in Bambu Studio, set internal bridge support thickness.
5. Calibrate flow if gaps are uniform across the surface.
6. Add ironing last, for finish.

Flexible filaments need extra margin because they bridge poorly ^[6]; the TPU guide covers their other settings.