Ironing in FDM: When It Creates a Smoother Top Surface—and How to Set It Up

What does ironing do?

Ironing, sometimes called smoothing in the slicer, is an extra toolpath over an already printed, flat top surface. The nozzle moves closely over the surface and lays down a small amount of plastic. The heat and light extrusion can smooth out small ridges and fill minor gaps between top lines. So the feature is not a shortcut to a perfect surface: the result still depends on how well the regular top layers are printed. (help.prusa3d.com)

Ironing works best on visible, flat surfaces such as lids, signs, trays, and the tops of boxes. It generally does little for curved or sloped surfaces, since the toolpath is designed for flat top surfaces. (help.prusa3d.com)

Start with a good top surface

Before turning on ironing, make sure the basic print quality is under control. If the top layer has large holes, uneven extrusion, or obvious gaps, the extra pass may drag plastic across the irregularities instead of hiding them.

So check the following first:

• that the regular top layers look even in the slicer preview and on a test print
• that extrusion is reasonably well calibrated
• that the model has enough top layers to cover the infill beneath the surface
• that the nozzle is clean and the filament feeds evenly

Ironing can improve a top surface that is already working well, but it does not replace fixing basic print problems.

Four settings to test

Names and locations vary between slicers, but most have equivalents of the following controls:

Which surfaces to iron

Choose whether the pass should be applied to all flat top surfaces or only to the model's highest surface. The latter can be practical for models with multiple levels, while ironing all top surfaces can be useful if, for example, a recessed flat surface also needs to be smoothed. Check the preview to make sure the ironing toolpath is applied to the right areas. (help.prusa3d.com)

Flow

Flow controls how much plastic is extruded during the pass. Too little can leave marks or small gaps; too much can cause buildup, smearing, and excess material along the edges. Change one setting at a time and compare small test surfaces in the same material.

Spacing between passes

Spacing determines how closely the nozzle passes over the surface. Closer passes can provide more even coverage, but often increase print time. Prusa's documentation states that, in its implementation, spacing is normally smaller than the nozzle diameter, which means the passes overlap. Exact values and behavior depend on the slicer and profile. (help.prusa3d.com)

Speed

A lower speed can give the nozzle more time to affect the surface, but slower is not automatically better. If the result looks glossy, smeared, or has uneven patches, heat and material may be affecting the surface too much. Test small changes and assess both appearance and print time.

Troubleshoot based on appearance

Marks or pits after the pass: try a small adjustment to the flow and check the extrusion. Too little material during ironing can leave visible marks, while too much can be dragged toward the edges of the surface. (help.prusa3d.com)

Clumps or plastic sticking to the nozzle: clean the nozzle and run a short test print. Materials behave differently; Prusa specifically highlights the risk of PETG sticking to the nozzle during ironing. (help.prusa3d.com)

Soft or less-defined edges: ironing can push a little plastic beyond the boundary of the flat surface. Try disabling the feature for that model or choosing a mode that only treats the highest surface. (help.prusa3d.com)

The surface looks almost the same, but the print takes longer: that's a good reason to leave ironing off. The extra pass increases print time, especially over large surfaces. (help.prusa3d.com)

A simple test plan

Print a small model with a flat top surface in the material you actually use. Save a version without ironing for comparison. Then enable ironing and change only one parameter at a time. Note the profile, material, and changes; this makes it easier to repeat a good result.

Consider starting with a single representative surface rather than enabling ironing globally. Once you have found a combination that works, you can save it in a dedicated profile or use the feature selectively on parts where surface finish matters.

Conclusion

Ironing is a useful finishing tool for flat, visible FDM surfaces—not a universal quality setting. Make sure the top layers are already working well, test on a small surface, and adjust flow, pass spacing, and speed gradually. If the improvement in surface finish does not justify the extra time, a well-tuned regular top surface may be the better choice.