Enough to matter. Ironing the flat top of a 60 mm cube — 3,600 mm² at 0.1 mm line spacing and 20 mm/s — takes 39 minutes.
That is on a part which takes 3.95 hours to print in the first place, so the top surface alone adds about a sixth to the job.
Why it is so slow for so little plastic
Only 360 mm³ of material is extruded during the pass. The flow is 0.2 mm³/s — under a twentieth of a normal print's rate — because ironing is not really extrusion. It is dragging a hot nozzle across an already-printed surface with a trickle of plastic to fill the valleys between lines.
The time comes from the path length. At 0.1 mm spacing, covering 3,600 mm² means travelling thirty-six metres of toolpath, and it is travelled slowly so the surface has time to reflow.
It scales with area, not with volume
This is the thing that makes ironing expensive on the wrong parts. Doubling the top area doubles the ironing time, regardless of how tall or dense the part is.
- A tall part with a small top — ironing is nearly free
- A flat wide part — ironing can approach the print time of the part itself
- A part with several stepped top faces — every one of them is ironed, and the total is the sum
When it is worth it
For a visible flat top face that will not be sanded or painted, ironing produces a finish nothing else on an FDM machine matches. Nameplates, coasters, lids, inlays and box tops are the classic cases.
For anything functional, it is time spent on a surface nobody looks at. And on a part that will be painted, primer and a light sand do the same job for free.
Two settings that reduce the cost
- Raise the line spacing. Going from 0.1 mm to 0.15 mm cuts the path length by a third for a modest quality loss.
- Iron only the top surface, not every horizontal face. Most slicers default to all top surfaces, which quietly irons things you will never see.
Give the print time estimator your top-surface area, your ironing spacing as the line width, and one layer of height — the result is what the pass will add.