The top surface of a standard FDM print has a characteristic ribbed texture — individual extrusion lines visible as parallel ridges under raking light or when touched. For functional parts, this texture is irrelevant. For display models, cosmetic parts, or anything requiring a smooth top surface for labeling or paint adhesion, it's a problem that historically required sanding to solve. The ironing feature in most modern slicers eliminates the need for sanding on flat or near-flat top surfaces by applying a secondary smoothing pass immediately after the top layer prints.
How Ironing Works
Ironing (also called "smoothing" in some slicers) adds one or more additional nozzle passes over the already-deposited top layer at the same Z height. The nozzle is heated to the normal print temperature, so it remelts the surface of the just-printed layer as it passes over it. The flow rate is set very low — typically 5–15% of normal — so the nozzle doesn't deposit significant additional material but instead uses the small amount extruded plus the nozzle body's thermal mass to press and melt the ridge tops flat.
The result is a surface where individual extrusion line ridges are reflowed into a continuous flat sheet. Under oblique light, the difference is striking: the ironed surface shows a nearly uniform gloss without the parallel shadow patterns of unironed FDM top layers. Dimensionally, ironing reduces surface roughness from a typical Ra of 20–40 μm to Ra 5–12 μm on most PLA surfaces.
The Physics of Why It Works
The key mechanism isn't just the low-flow extrusion — it's the conduction of heat from the nozzle body to the freshly printed surface. The nozzle block is a thermal reservoir held at 200°C (for PLA) moving across a surface that's cooled to perhaps 80–100°C. Contact between the nozzle and the ridge tops conducts heat locally, momentarily softening the ridge material past its glass transition temperature and allowing it to reflow under the minimal pressure the nozzle applies.
This means nozzle material matters more for ironing than for printing. Stainless steel nozzles conduct heat less efficiently than brass, and brass nozzles produce slightly better ironing results at equivalent settings. Hardened steel nozzles, which have even lower thermal conductivity, require increased ironing flow or reduced speed to achieve equivalent surface quality. Copper nozzles (used by some high-performance extruder systems) provide the best thermal contact of common nozzle materials and enable faster ironing passes.
Settings: Speed, Flow, and Line Spacing
Ironing speed: 40–60% of the top surface print speed is the typical range. Faster speeds reduce contact time and produce less effective smoothing; slower speeds over-melt the surface and can cause the nozzle to drag material rather than reflow it. For standard PLA at a 50 mm/s top surface speed, an ironing speed of 20–30 mm/s works well.
Ironing flow: 5–15% is the standard range in Cura and OrcaSlicer. Higher flow rates deposit more material, which can cause the surface to bubble or overfill slightly; lower rates risk inadequate contact pressure. 10% is a reliable starting point. In PrusaSlicer, the "ironing flow" parameter functions slightly differently (it's defined as a fraction of the normal flow, not an absolute percentage of nozzle diameter extrusion), so settings don't transfer directly between slicers.
Line spacing: the distance between ironing passes. 0.1–0.2 mm is typical. Closer spacing improves surface quality but increases pass time significantly — an ironing pass at 0.1 mm spacing over a 100 cm² surface takes considerably longer than at 0.2 mm. For most applications, 0.15 mm provides a good quality-time balance. The ironing pattern (zigzag vs. concentric vs. aligned with top layer lines) affects whether the ironing lines are parallel or perpendicular to the existing top surface texture; most users find perpendicular passes produce better results by crossing the existing ridges rather than running parallel to them.
Which Materials Respond Best
PLA irons beautifully at standard temperatures. The material has a well-defined glass transition (~60°C for standard PLA) that allows clean re-flow without excessive softening. The result is a genuinely glossy flat surface in most colors, with matte blacks and saturated reds requiring slightly higher flow due to pigment loading.
PETG irons acceptably but is more temperamental — PETG has a broader glass transition region and a stronger tendency to string. The ironing pass can leave fine stringing artifacts across the surface if travel moves between ironing lines don't fully retract. Enable travel retraction during ironing in slicers that support this option; reduce ironing flow to 8% rather than 10% to minimize stringing risk.
ABS and ASA iron well in enclosures where the ambient temperature is elevated, keeping the surface warmer during the ironing pass. Open-air ironing of ABS tends to produce inconsistent results because the rapid cooling at the surface reduces reflow quality.
TPU and flexible materials should generally not be ironed — the material deforms under the ironing pressure rather than reflowing, and the pattern left by the nozzle creates a worse texture than the original top surface.
When Not to Use Ironing
Ironing only improves flat or near-flat top surfaces. Curved tops, complex contours, and angled surfaces don't benefit and may be slightly worsened by the additional thermal input. Enabling ironing on an object without large flat top faces is a time penalty with no upside.
Ironing adds 10–30% to total print time for objects with significant flat top area. For parts where only functionality matters, skip it. Reserve it for parts where surface appearance is part of the spec.
