Ironing: How to Get Glassy Flat Top Surfaces

Ironing is the setting that turns a slightly ribbed top surface into a near-glassy one. It’s also the setting people enable globally, wonder why prints take 40% longer, and then blame the slicer. Understanding what it actually does makes the difference.

Ironing re-runs the nozzle across the top solid surface after it’s printed, moving slowly with the heater on and a tiny trickle of filament — just enough to remelt the surface and smear the ridges between adjacent top lines flat. It does nothing for side walls, overhangs, or any surface that isn’t a flat top facing up.

When ironing helps

Ironing earns its time cost on a specific kind of part:

• Large flat top surfaces facing up. Lids, plates, faceplates, name signs, anything where a smooth top is the point.
• Parts where the top is the visible/finished face. Coasters, badges, inlays, control panels.
• Surfaces you’d otherwise sand. Ironing can replace light sanding for a satin-flat finish.

On these, ironing visibly removes the fine corduroy texture left between top-surface extrusion lines and produces a uniform sheen.

When ironing is a waste

• No flat top surface. Spheres, organic models, anything where the top is curved or sloped — ironing literally has nothing to act on.
• Small top areas. The setup/finish moves cost more time than the visual gain on a 10 mm patch.
• Top surface won’t be seen. If the top faces down in use or gets covered, you’re paying a time tax for nothing.
• Functional prototypes. Iterate fast; iron only the final.

Rule of thumb: enable ironing per object or per top-surface, not globally. Most slicers let you scope it; use that.

The four settings that matter

1. Ironing type / pattern

Controls which top surfaces get ironed:

• Top surfaces only — the right default. Irons only the last solid top layer.
• Topmost surface only — irons only the single highest top surface (skips lower step tops).
• All solid surfaces — irons every solid layer including internal ones. Almost never what you want; massive time cost for invisible internal surfaces.

Pattern itself is usually concentric or rectilinear; rectilinear is fine for most parts.

2. Ironing flow (%)

How much filament trickles out during the ironing pass, as a percentage of normal flow. Typical default is around 10–15%.

• Too low (under ~8%): the pass doesn’t fully fill the valleys between lines; texture remains.
• Too high (over ~20%): excess plastic accumulates and the surface gets a smeared, uneven, sometimes wavy finish.
• Sweet spot: ~10% on a well-calibrated printer. Tune ±3% by inspecting a test plate.

This setting interacts directly with flow calibration. If base flow is wrong, ironing amplifies the error. Calibrate flow first — see the flow and temperature calibration guide.

3. Ironing line spacing

The gap between adjacent ironing passes, typically ~0.1 mm (much tighter than normal line spacing). Tighter spacing = smoother result but slower.

• 0.10 mm: default, good finish.
• 0.08 mm: glassier, slower; for showpiece tops.
• 0.15 mm+: faster, but ridges may remain partially visible.

4. Ironing speed

Slow on purpose — commonly 15–30 mm/s. The nozzle needs dwell time to remelt the surface. Too fast and it skims over without melting; too slow and you risk over-melting and discoloration on heat-sensitive filaments. The slicer default is usually well chosen; leave it unless you see scorching or incomplete smoothing.

Slicer specifics

Slicer	Where it lives	Notes
PrusaSlicer	Print Settings → Infill → Ironing	Mature; “Ironing Type” controls scope
OrcaSlicer	Quality → Ironing	Per-object override available
Bambu Studio	Quality → Ironing	Same model as OrcaSlicer
Cura	Search “Ironing” (Top/Bottom)	Separate “Iron only highest layer” toggle; defaults differ from the Prusa family

In all four, the concept is identical; only the menu path and default flow/spacing differ slightly.

Ironing vs the alternatives

Ironing isn’t the only path to a smooth top:

• More top layers + thinner top layer height. A thicker top shell with a thin final layer (0.10–0.12 mm) is already fairly smooth before any ironing.
• “Monotonic” top-surface order. Most modern slicers have a monotonic top-fill option that lays top lines in one consistent direction so the sheen is even. Enable this with or instead of ironing — it removes the patchy look from alternating line directions and costs almost no time.
• Post-processing. Sanding then polishing beats ironing on the very highest-finish parts, but ironing gets you most of the way with zero manual work.

The best practical combination for a showpiece top: monotonic top order + a thin final top layer + ironing at ~10% flow, scoped to that surface only.

A quick recommendation

• Flat-topped display/finished part → enable ironing, ~10% flow, monotonic top order.
• Curved or organic top → don’t bother; ironing can’t act on it.
• Functional part / prototype → skip until the final version.
• Top texture still rough after ironing → check flow calibration first, then tighten line spacing.

Where to go next

For the related question of how top-shell thickness and layer height interact with surface quality, see the core settings guide. For seam quality on the side walls (the other most-visible surface), see seam placement.

For material-specific surface behavior, PrintLabGuide ↗ covers the thermal side, and FDM Desk ↗ covers per-printer tuning.