HOW TO FIX PILLOWING ON 3D PRINT TOP SURFACES

Pillowing is when the top surface of your print looks bumpy, wavy, or has small holes instead of being smooth and closed. It gets its name from the way the surface bulges up between infill lines - like the fabric of a pillow puffing between its stitching. Large flat surfaces show it most dramatically, but you'll see it on any horizontal top surface when the conditions are right. Common confusion: Pillowing is sometimes confused with under-extrusion on top layers. The difference - under-extrusion on top layers shows as gaps and missing lines. Pillowing shows as closed but bumpy or lifted surfaces - the plastic is there, it just didn't solidify flat.

Your top surface has to bridge across infill - there's air below it. The plastic lines are essentially tiny bridges that need to solidify before gravity pulls them down into the gaps. Two things have to work together for that to happen: enough material to span the gaps, and fast enough cooling to freeze each line in place. When either fails: 1. Not enough top layers - the single most common cause. Each top layer bridges across the one below. With only 2–3 top layers, the first layers sag into infill gaps and the later ones sag into *those* gaps. You need enough layers for each one to support the next. 2. Infill too sparse - wide infill gaps mean longer bridges for each top layer to cross. Less support = more sag. 3. Cooling fan too slow - plastic stays molten too long and droops before it can solidify flat. 4. Top surface speed too fast - plastic is deposited faster than it can cool and firm up, so each new line disturbs the one before it.

Set top layers to at least 5 layers or a minimum of 0.8 mm total, whichever is larger. Most slicers accept both a count and a minimum thickness - use both. • Cura - Top Layers (+ Top Thickness) • PrusaSlicer - Top solid layers • OrcaSlicer - Top shell layers • BambuStudio - Top shell layers At 0.2 mm layer height, 5 top layers gives you 1.0 mm of top thickness - plenty for most prints. At 0.12 mm layer height, you'd need 7+ top layers to reach the same thickness. Setting both count and minimum thickness ensures the slicer adds enough layers automatically based on your layer height.

Below 15% infill, top layer bridge spans get very long. Raise to at least 15–20%. For large flat tops, 20–25% is more reliable. The relationship between infill density and pillowing is non-linear. Going from 10% to 15% makes a huge difference. Going from 25% to 50% barely matters for pillowing (though it adds weight and print time). If you absolutely need to keep infill low (lightweight prints), compensate with extra top layers - 7 or 8 instead of 5.

PLA: 100%. PETG: 70%. ABS/ASA: 20–30% (they crack with aggressive cooling, but some is needed). Most slicers let you set a separate fan speed for top surfaces specifically. Use it - keep your normal print fan at filament-appropriate levels and crank up the top surface fan to maximize cooling on the surfaces that need it most. If your slicer doesn't have a top-surface-specific fan setting, raise the regular fan speed for the print. The few minutes of higher cooling on top layers is worth the marginally worse layer adhesion on the layers immediately below.

Set top surface speed to 20–30 mm/s separately from your normal print speed. Slower plus full cooling equals flat, solid top. This is one of the few places where slow really matters. The reason: each top surface line is essentially a bridge across the line beside it. Slow movement gives each line time to cool and harden before the next one is deposited next to it. If your slicer has a separate "top solid infill speed" setting, use it. Keep normal speeds high and only slow down the top surface specifically.

Ironing runs the nozzle over the finished top surface at near-zero flow to flatten any remaining bumps. Adds time but produces near-mirror finish. Available in all major slicers. Typical ironing settings: • Ironing speed - 15–25 mm/s • Ironing flow - 5–15% • Ironing inset - 0.2 mm Ironing isn't a fix for pillowing - it's a cosmetic enhancement on top of an already-good top surface. If your top surface is severely pillowed, fix the underlying problem first; ironing won't save it.

Set a default of 5 top layers across all your profiles. Increase minimum top thickness rather than just layer count - it scales with layer height automatically. For functional prints with high infill (40%+), pillowing rarely happens. For low-infill cosmetic prints, always check top settings before printing. Test new filaments with a calibration print that has a flat top - you'll know within one print whether your settings are sufficient.