Ironing Settings in FDM Slicers: When It Helps, When It Causes Pinholes and Stringing

Ironing is the one slicer setting that can take a flat top surface from "visibly 3D printed" to "looks injection molded" — and it's also one of the most commonly misused, because turning it on without understanding what it actually needs tends to produce pinholes, blobs, or a print that suddenly takes twice as long. It deserves its own explanation separate from the general slicer settings guides, because the failure modes are specific and easy to avoid once you know what's happening underneath the checkbox.

What Ironing Actually Does

After your top solid infill layer finishes printing normally — walls, then the solid infill pattern filling the top — ironing adds one more pass over that same surface. On this extra pass, the nozzle travels at a much higher speed while extruding only a small fraction of the normal flow rate, essentially dragging the already-hot nozzle across the just-printed plastic and smearing/reflowing its surface flat. It is not a separate print layer in the Z sense; it happens at the same height, as a finishing pass over material that's already there.

Because this only works on plastic that's already solid and flat underneath it, ironing depends entirely on having a fully solid, properly filled top surface before it runs — which is the root of most ironing problems.

The Settings That Control It

SettingWhat It ControlsTypical Starting Point Ironing flow / flow rateHow much plastic is extruded during the ironing pass, as a percentage of a normal line's flow10-15% for PLA, slightly lower (8-12%) for PETG, which is more prone to stringing at excess flow Ironing speedTravel speed of the nozzle during the ironing pass15-25 mm/s — much slower than normal print speed, since the nozzle is doing thermal smearing, not extrusion-driven deposition Ironing line spacingDistance between adjacent ironing passes0.1-0.2mm — tighter spacing gives a smoother result at the cost of more time Ironing patternConcentric, zigzag, or monotonic infill-style travel across the surfaceMonotonic/zigzag generally gives the most even finish on large flat areas; concentric can leave visible rings on very large surfaces Ironing type (where present)Whether ironing applies to top surfaces only, topmost layer only, or all top surfaces including those between layers (e.g. the floor of a cavity)"Top surfaces" or "topmost only" covers the common case; enable "all top surfaces" only if you specifically need interior flat-top cavities smoothed too

Naming varies by slicer — Cura, PrusaSlicer, OrcaSlicer, and Bambu Studio all expose roughly this same set of parameters under slightly different menu labels, but the underlying mechanism and tuning logic is identical across all of them.

When Ironing Actually Helps

Ironing earns its printing-time cost on parts where the top surface is the point: display bases and plaques, lithophane backs, enclosure lids and front panels that will be seen, flat logos, and any single-color print where a glossy, layer-line-free top surface reads as noticeably more finished. It's also useful on parts destined for painting, since a smoother starting surface needs less primer and sanding to hide layer lines before paint goes on.

When It Causes Problems

The most common ironing failure is pinholing or visible gaps appearing in what should be a solid smeared surface. This almost always traces back to an insufficient top layer count — if your top solid infill doesn't have enough layers to form a genuinely continuous, gap-free surface before the ironing pass runs over it, ironing will expose rather than hide any remaining micro-gaps between infill lines. Four to five top layers at a typical 0.2mm layer height is a reasonable minimum before ironing looks clean; thinner top layer stacks will show pinholes even with perfect ironing settings, because the problem is underneath the ironing pass, not in it.

The second common failure is stringing or small blobs at the edges of the ironed area, usually from flow percentage set too high for the material — PETG and other stringier filaments need a noticeably lower ironing flow than PLA to avoid this, and it's worth running a small flat test square in each material you use regularly rather than assuming one flow setting works everywhere. Finally, ironing meaningfully increases print time on large flat areas since it's an entire extra full-coverage pass at low speed — budget for it on parts where the surface actually matters, and skip it on functional parts where nobody will run a finger across the top.

Quick Tuning Procedure

1. Print a simple 40mm x 40mm x 3mm test square with your normal top layer settings (4-5 top layers minimum) and ironing enabled with the slicer's default ironing values.
2. Inspect the surface for pinholes first — if present, increase top layer count before touching any ironing parameter, since that's the actual cause.
3. If the surface is solid but shows stringing or small raised blobs, lower ironing flow in small increments (2-3% steps) and reprint until clean.
4. If the finish looks acceptable but slightly textured rather than glossy, tighten ironing line spacing before increasing flow — closer passes usually improve finish with less risk of stringing than more plastic per pass.

Ironing is a finishing technique, not a fix for an underlying print quality issue — get top layer count and flow calibration right first, and the extra pass will do exactly what it promises: turn a visibly-printed flat surface into one that looks like it came out of a mold.