If you’ve ever looked at a 3D printed lid or nameplate and wondered why the flat top looks matte and ridged instead of smooth, the answer is usually that ironing was off. Ironing is a second pass the nozzle makes over a flat top surface, at the same layer height, extruding almost no plastic, just enough to melt the ridges left by the first pass back down flat.
It sounds like a small trick, and it is, but the results are hard to get any other way on an FDM printer. A part that looked like typical layered plastic can come out with a surface closer to injection molding.
How it actually works
During normal printing, the top layer gets filled in with a solid infill pattern, lines of plastic laid side by side. Even with a fine line width, you get visible ridges where each line meets the next. Ironing runs the nozzle back over that same surface a second time, heated and moving slowly, extruding a small fraction of the normal flow rate. The heat re-melts the surface, and the tiny bit of extra material fills the gaps between the ridges instead of piling on top of them.
PrusaSlicer angles the ironing pass 45 degrees relative to the first infill pass, a detail borrowed from an older slicer called Kisslicer. The idea traces back further than that: a user named Neotko first prototyped the technique in Simplify3D under the name “Neosanding,” running a second infill phase at zero or near-zero extrusion. Ultimaker picked it up for Cura, and Kisslicer’s author found that angling the second pass at 45 degrees instead of running it parallel produced a cleaner result. PrusaSlicer’s implementation is the Kisslicer version.
Settings, slicer by slicer
PrusaSlicer keeps ironing under Print Settings > Infill > Ironing, but only in Advanced or Expert mode; it’s hidden in Simple mode. You get three ironing types. “All top surfaces” irons every flat top plane on the model, useful for stepped shapes with more than one horizontal face. “Topmost surface only” irons just the highest layer, so text embossed into a flat top would get its letters ironed but not the gaps between them. “All solid surfaces” runs ironing on every solid layer in the print, including internal ones, which is really meant for near-transparent prints at 100% infill rather than normal parts. Flow rate and spacing live in the same panel; speed is set separately, under Print Settings > Speed > Ironing.
OrcaSlicer organizes the same idea under Quality > Ironing, with a few more dials than PrusaSlicer exposes. Type mirrors PrusaSlicer’s three options, worded slightly differently: Top Surfaces, Topmost Surface, and All solid layers. Pattern lets you pick concentric or rectilinear ironing lines. Flow controls how much material gets extruded during the pass, as a percentage of normal flow, lower for a smoother but less complete finish, higher for better coverage at the risk of over-extrusion. Line spacing sets the distance between ironing passes and should generally sit at or under your nozzle diameter. Inset keeps the ironing path back from the edges to avoid bulging the perimeter, and angle offset or fixed angle controls the direction of the ironing lines relative to the top infill, which changes how light reflects off the finished surface. Bambu Studio, being an OrcaSlicer fork, uses this same set of controls.
Cura also supports ironing, tucked under Special Modes, though its pattern options and defaults differ from PrusaSlicer and OrcaSlicer. If you’re moving a profile between slicers, don’t assume the ironing settings carry over correctly. Re-check them after a switch.
What you’re trading for a smooth top
Ironing isn’t free. The extra pass adds print time, sometimes a meaningful amount on large flat surfaces, and slicers will show you the added time in the preview before you commit to a print.
The bigger risk is mechanical. Because ironing extrudes so little material so slowly, filament barely moves through the hotend during the pass. On some hotends, that’s slow enough to let heat creep up into the cold zone, softening filament above the melt zone and eventually causing a clog. PLA is the most likely to cause problems here because it has the lowest resistance to heat, and the risk goes up in a warm room or during a summer heatwave. If you’re ironing a large area, watch the first layer or two and be ready to pause if extrusion looks like it’s tapering off.
Edges also lose a little sharpness. The ironing toolpath is planned for minimal extrusion, but the nozzle is still its full physical width, so some material bleeds slightly past the intended edge of the ironed area. On a print with crisp corners, you may notice the top edges look a touch softer than the sides.
Ironing also isn’t a fix for pillowing, the bumpy, dimpled top surface that comes from weak top-layer infill sagging into the layer below. That’s a different problem with a different cause, and ironing a pillowed surface just melts a thin, decent-looking skin over infill that’s still not properly supported underneath. Fix the infill and top layer count first, then iron.
Where ironing helps and where it doesn’t
Flat surfaces are the whole point: lids, nameplates, badges, logos, enclosure panels, anything where the top is meant to sit parallel to the bed and be looked at or touched. It’s also worth turning on for surfaces you plan to glue together, since a flatter surface means a smaller gap and a stronger bond.
Curved, organic, or angled surfaces don’t benefit. Ironing targets flat top layers specifically, so a sloped roof or a rounded figure won’t show much difference with it on. It won’t hurt the print either, beyond the wasted time, since the slicer simply won’t find much flat area to iron.
One alternative worth remembering: if a model can be printed upside down, the surface that sits against the build plate usually comes out smoother than even a well-ironed top layer, without the clog risk. It’s not always possible depending on the geometry, but it’s worth checking before reaching for ironing.
It behaves differently by material
The settings themselves don’t change per material, but results do. PLA irons cleanly but carries the highest heat creep risk of the common filaments. PETG also irons well, though it’s more prone to leaving residue stuck to the nozzle, which can scorch, darken, and eventually transfer onto the print if it builds up. ASA irons unusually well and tends to produce some of the smoothest results of any common material. Wood-filled filaments, at least according to Prusa’s own testing, don’t iron well at all; the particulate content seems to fight the process. Flexible filaments carry the same nozzle-buildup risk as PETG, just worse.
Getting the settings right
There’s no universal ironing recipe because it depends on nozzle diameter, hotend, and material. A reasonable starting point that shows up across slicer communities is a flow rate around 10%, spacing around 0.1mm, and a slow ironing speed, then adjusting from there. Too little flow leaves faint, shiny grooves where the pass missed. Too much drags excess plastic to the edges of the surface. Expect to run a few test prints on flat panels to dial it in for your specific setup, and once you find values that work, they’ll usually hold for that printer and nozzle across most materials.
Pairing ironing with a monotonic top infill pattern, where every infill line runs in the same direction instead of back and forth, tends to give the cleanest base for ironing to work from. It’s a small addition to a small trick, but on the right print, the combination is the difference between “that’s a 3D print” and someone asking how you molded it.