Ironing is a setting in 3D printing slicers that makes the top surface of a print smoother by running the hot nozzle back over the final layer a second time. Instead of depositing a full line of plastic, the nozzle passes across the already-printed surface with little to no new material, remelting and flattening the top layer so the visible ridges between lines nearly disappear. The result is a noticeably glossier, more uniform finish on any upward-facing surface.
How the Ironing Pass Works
After the printer finishes laying down the top layer of a surface normally, it makes an additional pass. During this pass, the nozzle moves slowly across the surface, barely touching the top of the plastic. It extrudes only a tiny fraction of the material it normally would, typically around 10 to 15 percent of the standard flow rate. That small trickle of molten filament fills in the tiny gaps and valleys between print lines, while the heat from the nozzle softens the already-deposited plastic just enough to level it out.
Think of it like smoothing wet concrete with a trowel. The tool doesn’t add much new material. It redistributes what’s already there and presses everything flat. The nozzle’s hot metal tip does essentially the same thing, remelting the peaks of the printed lines and letting the plastic settle into a more even plane as it cools.
What It Does to Surface Quality
The improvement is measurable. Research on ironed surfaces has shown a fivefold reduction in average surface roughness compared to standard top layers, bringing it down to around 1.09 micrometers in controlled tests. Even on more challenging materials like PEEK (a high-performance engineering plastic), ironing reduced roughness by about 26 percent. In practical terms, you go from a surface where you can clearly see and feel each print line to one that looks and feels much closer to an injection-molded part.
The effect is most dramatic on large, flat top surfaces like lids, nameplates, or box tops. On small or complex geometry, the improvement may be less noticeable simply because there’s less visible surface area to smooth.
Settings That Matter
Most slicers (Cura, PrusaSlicer, Creality Print, and others) expose a handful of ironing settings. Getting these right makes the difference between a silky finish and a mess of plastic blobs.
- Flow rate: This controls how much filament the nozzle pushes out during the ironing pass. For PLA, 10% of normal flow is a good starting point. PETG and ABS benefit from slightly higher values around 15%. Too high and you’ll get excess plastic pooling on the surface. Too low and the nozzle drags across dry plastic, leaving scratches.
- Speed: Ironing works best between 10 and 20 mm/s, which is much slower than typical printing speeds. The slower the nozzle moves, the more heat it transfers to the surface. If it moves too fast, it won’t melt the ridges enough to flatten them.
- Line spacing: This is the distance between each ironing pass, usually between 0.15 and 0.25 mm. Tighter spacing gives a more polished result but takes longer. Wider spacing is faster but may leave faint lines visible.
Ironing Patterns
Slicers offer several patterns for how the nozzle traces across the surface during ironing. The most common are zigzag (back and forth in straight lines), rectilinear (similar but with slightly different path planning), and concentric (spiraling inward from the edges). Each leaves a subtly different visual texture on the finished surface. Concentric patterns tend to look best on round or circular parts, while zigzag is faster and works well on rectangular geometry. It’s worth testing a small sample with each pattern, since the differences are visible to the naked eye.
The Tradeoff: Print Time
Ironing adds an extra pass over every top-facing surface, and since it moves slowly, the time cost is real. For a roughly cube-shaped object, expect the total print time to increase by 10 to 20 percent. The actual impact depends on how much top surface area your model has relative to its total volume. A tall, narrow part with a small top might add only a minute or two. A wide, flat enclosure lid could add significantly more.
Because ironing only affects top surfaces, it won’t improve walls, overhangs, or bottom layers. If your print has very little flat area on top, the feature won’t do much for you.
Common Problems and Fixes
The most frequent issue with ironing is over-extrusion on the top surface, which shows up as small ridges, blobs, or a rough texture that’s actually worse than no ironing at all. This almost always means the flow rate is set too high. Drop it by 1 to 2 percent at a time until the surface cleans up.
Thin, wispy strings at the edges of ironed surfaces are another common complaint. These happen when the nozzle travels past the boundary of the top surface and drags a small amount of excess plastic with it. Reducing the flow rate or slightly increasing the ironing speed can help. Some slicers also let you limit ironing to only the topmost surface or to surfaces above a certain size, which reduces the chance of artifacts on small features.
Occasionally the ironing pass can cause slight warping on very thin top layers, because the second pass reheats plastic that has already cooled and contracted. Adding one or two extra top layers in your slicer settings gives the ironing pass a thicker foundation to work with and usually eliminates this.
When Ironing Is Worth Using
Ironing shines on functional prints where the top surface is visible or needs to be smooth for a purpose: enclosures, labels, surfaces that will be painted, or parts that slide against each other. It’s also useful for prints where you want to reduce post-processing. A well-ironed surface can skip the sanding step entirely for many applications.
It’s less useful on prints dominated by curves, organic shapes, or steep angles, since those surfaces are formed by walls rather than top layers. And for prints where speed matters more than aesthetics, the 10 to 20 percent time penalty may not be justified. The setting is easy to toggle on and off per print, so the best approach is to enable it when the top surface matters and leave it off when it doesn’t.