How to Use Ironing for Smoother 3D Printed Top Surfaces

What Ironing Actually Does

Ironing is a slicer feature that sends the nozzle back over the finished top layer at the exact same Z-height. It melts a small amount of extra filament into the ridges and micro-gaps left by the normal top-solid pass. The result is a flatter surface that needs less sanding or filler before paint or assembly.

The mechanism is simple: the flat face of the nozzle conducts heat into the plastic while a trickle of fresh material (usually 5 to 15 percent of normal flow) fills the voids. You are not removing material, you are redistributing it.

The Three Settings That Matter

Most slicers expose ironing as three adjustable variables: flow rate, speed, and line spacing. Getting these in the right ballpark matters more than any preset.

Ironing flow rate controls how much extra plastic comes out during the pass. Start around 10 percent for PLA. Below 5 percent, the surface stays rough because the nozzle does not supply enough material to fill the gaps. Above 15 percent, you get raised ridges and blobbing at toolpath turns.

Ironing speed determines how fast the nozzle crosses the surface. Start at 20 mm/s. Faster than 35 mm/s and the nozzle does not transfer enough heat to soften the plastic properly. Slower than 10 mm/s and the plastic can smear or discolor from prolonged heat exposure.

Ironing line spacing sets the distance between adjacent ironing passes. A good starting point is 0.10 mm with a 0.4 mm nozzle, roughly 20 to 30 percent of the nozzle diameter. Tighter spacing gives a smoother finish but adds print time.

Material-Specific Tweaks

PLA is the easiest material to iron. Run it with full cooling fan and the standard 10 to 12 percent flow, 20 to 25 mm/s speed, and 0.10 mm line spacing.

PETG is stickier. Drop the flow to 5 to 8 percent and the speed to 15 to 20 mm/s. Keep cooling at 30 to 50 percent. Too much heat and the filament clings to the nozzle shoulder, leaving strings behind.

ABS and ASA need a stable ambient temperature. Print inside an enclosure with cooling fan at 10 to 20 percent. Use 8 to 10 percent flow and 20 to 25 mm/s speed. Without a warm chamber, the top layer can warp from the extra thermal energy ironing adds.

Calibrate Before You Commit

There is no universal ironing profile. Print a 30 by 30 by 4 mm test tile and run a four-tile array with different flow rates: 6 percent, 9 percent, 12 percent, and 15 percent. Keep speed and line spacing fixed during the test. The winning tile should have a uniform satin finish with no dull patches or raised ridges.

Always run this test when you change filament brands or nozzle size. What works on one machine may drag or blob on another.

When Ironing Is Worth It

Enable ironing for the topmost visible surface only unless you have a specific reason to iron internal flats. It roughly quadruples the number of passes over the top layer, so internal ironing adds hours to long prints without visible benefit. Use it for display models, parts that mate together, or anything getting painted or coated.