Ironing in 3D Printing: Glass-Smooth Top Layers

You print a box with a flat top and it looks fine from a distance — but up close, you can see the infill pattern showing through the top surface, tiny ridges between extrusion lines, and a generally textured appearance that screams "3D printed." Now imagine the same box with a top surface so smooth it looks injection-molded. That is what ironing does.

Ironing is a slicer feature that makes an additional pass over the top surface with the nozzle at very low extrusion, essentially melting and smoothing the existing material. The nozzle acts like a hot iron on fabric — it flattens the ridges, fills the gaps, and creates a dramatically smoother surface.

How Ironing Works

After the normal top layer is printed, the slicer generates an additional toolpath where the nozzle:

1. Moves across the top surface in a tight pattern (much tighter than normal infill lines)
2. Extrudes a tiny amount of filament (5-15% of normal flow) to fill micro-gaps
3. Heats and re-melts the surface slightly, causing the ridges to flow together
4. Does not add significant height — the layer stays the same thickness

The result is a top surface that goes from "clearly 3D printed with visible lines" to "smooth and almost glossy."

How to Enable Ironing

OrcaSlicer / PrusaSlicer / BambuStudio

1. Go to Print Settings > Quality
2. Enable Ironing
3. Select Ironing Type: Top surfaces only (most common)

Key settings:

Ironing: Enabled
Ironing Type: Top surface only
Ironing Flow Rate: 10-15%
Ironing Spacing: 0.1 mm
Ironing Speed: 15-20 mm/s
Ironing Pattern: Rectilinear

Cura

1. Open Print Settings
2. Search for "ironing" or go to Top/Bottom > Ironing
3. Enable Ironing

Key settings:

Enable Ironing: Yes
Ironing Pattern: Zigzag
Ironing Line Spacing: 0.1 mm
Ironing Flow: 10%
Ironing Inset: 0.38 mm (slightly less than nozzle diameter)
Ironing Speed: 15-20 mm/s

Optimal Ironing Settings

Flow Rate (Most Critical Setting)

The ironing flow rate controls how much filament is extruded during the ironing pass. This is the most impactful setting.

• Too low (under 5%): Not enough material to fill gaps. Surface may actually look worse.
• Sweet spot (10-15%): Enough material to fill micro-gaps without creating excess.
• Too high (above 20%): Creates ridges, blobs, and an uneven surface. Defeats the purpose.

Start at 10% and increase to 15% if you see unfilled gaps. According to CNC Kitchen's ironing analysis, 10-15% flow rate produces the best results across most filaments.

Spacing (Line Spacing)

This controls the distance between ironing passes. Tighter spacing means more overlap and smoother results, but takes longer.

• 0.1mm: Excellent smoothness, longer time
• 0.15mm: Good smoothness, moderate time
• 0.2mm: Acceptable smoothness, fastest

0.1mm is recommended for visible surfaces. Use 0.15-0.2mm for functional parts where speed matters more.

Speed

Ironing speed affects how much time the nozzle spends heating and smoothing each area.

• Too fast (30+ mm/s): Not enough time to smooth. Lines still visible.
• Sweet spot (15-20 mm/s): Enough dwell time for a smooth result.
• Too slow (under 10 mm/s): Risk of over-melting and creating a wavy surface. Also takes forever.

Pattern

Rectilinear (back and forth): Most common and usually best. Creates a uniform smoothed surface.

Concentric: Irons in spirals from the outside in. Can be good for round objects but leaves a visible pattern center.

Material-Specific Ironing Tips

PLA

PLA responds best to ironing. Its low melting point and good flow behavior mean the ironing pass smooths it beautifully.

Flow: 10-12%
Speed: 15-20 mm/s
Temperature: Same as printing (no change needed)

Results: Excellent. PLA ironing produces near-glass-smooth surfaces.

PETG

PETG can be ironed but results are less dramatic than PLA. PETG's stringy nature means the ironing pass may drag filament and create slight texture.

Flow: 8-12%
Speed: 15 mm/s
Temperature: Same as printing

Results: Good but not as smooth as PLA. The glossy nature of PETG means ironing has less visual impact.

ABS

ABS irons well, similar to PLA. The higher temperature keeps the material workable during the ironing pass.

Flow: 10-15%
Speed: 15-20 mm/s
Temperature: Same as printing

Results: Very good. ABS smooths nicely and can also be post-processed with acetone vapor for an even glossier finish.

TPU

TPU does not iron well. The flexible material deforms under the nozzle pressure and springs back, negating the smoothing effect. Not recommended.

When to Use Ironing

Use ironing when:

• The top surface is a visible face (display models, enclosures, signs)
• You want a professional, finished appearance without post-processing
• The model has large, flat top surfaces where ironing has the most impact
• You are printing text or logos on the top surface — ironing makes them crisp

Skip ironing when:

• The top surface is not visible in the final assembly
• The model is a quick prototype or test print
• The top surface is small or has complex geometry (ironing has minimal impact on tiny areas)
• Print time is critical — ironing adds 15-30% to total print time for surfaces with large top areas

The Time Cost

Ironing adds time. How much depends on the top surface area:

| Top Surface Area | Approximate Extra Time | |---|---| | Small (20x20mm) | 1-2 minutes | | Medium (100x100mm) | 5-10 minutes | | Large (200x200mm) | 15-30 minutes | | Full build plate | 30-60 minutes |

For a typical model with a moderate top surface, expect 5-15 minutes of additional print time. The per-layer time impact is small because ironing only applies to top surfaces, not every layer.

Ironing vs Other Smoothing Methods

| Method | Quality | Time | Cost | Materials | |---|---|---|---|---| | Ironing | Very good | Moderate | Free (slicer setting) | PLA, ABS, PETG | | Sanding | Excellent | High | Sandpaper ($5) | All | | Acetone vapor | Excellent (glossy) | Moderate | Acetone ($10) | ABS, ASA only | | Filler primer + paint | Excellent | Very high | Primer + paint ($15) | All | | Resin coating | Excellent | Moderate | XTC-3D ($25) | All |

Ironing is the only method that happens during printing, requiring zero post-processing effort. For many makers, the combination of ironing during printing plus light sanding afterward produces professional results with minimal effort.

The XTC-3D Coating can be used in addition to ironing for an even smoother, glossy finish on PLA and ABS parts.

Troubleshooting Ironing Issues

Surface looks worse with ironing enabled

• Flow rate too high. Reduce from 15% to 10% or even 8%. Excess material during ironing creates ridges.
• Speed too fast. Slow to 15 mm/s.
• Nozzle dirty. A nozzle with burnt filament deposits drags debris across the smoothed surface. Clean the nozzle before printing.

Visible lines from the ironing pattern

• Spacing too wide. Reduce ironing spacing from 0.2mm to 0.1mm.
• Flow too low. The gaps between ironing lines are not being filled. Increase flow slightly.

Blobs or zits at ironing start/end points

• This happens where the ironing pass starts and stops. Enable retraction during ironing moves (some slicers handle this automatically).
• Ensure pressure advance / linear advance is properly tuned.

Ironing looks good except at edges

• The ironing inset setting controls how close to the edge the ironing pass goes. If the inset is too large, the edges are not smoothed. Set inset to about 0.3-0.4mm (slightly less than nozzle diameter).
• If the inset is too small, the ironing pass may push material over the edge, creating a lip.

Advanced: Ironing Specific Surfaces Only

In OrcaSlicer and PrusaSlicer, you can control which surfaces get ironed:

• Top surfaces only — irons only the final top surface of the model
• Topmost surface only — irons only the very top of the entire print
• All solid surfaces — irons every top-facing solid surface, including ones below overhangs

For most uses, "Top surfaces only" is the right choice. "All solid surfaces" is useful for models with visible flat areas at multiple heights.

In Cura, you can use per-model settings to enable ironing only on specific parts in a multi-body print.

Ironing with Different Nozzle Sizes

Larger nozzles (0.6mm, 0.8mm) benefit more from ironing because their wider extrusion lines leave more visible ridges on top surfaces. Ironing spacing should be proportional to nozzle size:

• 0.4mm nozzle: 0.1mm spacing
• 0.6mm nozzle: 0.12-0.15mm spacing
• 0.8mm nozzle: 0.15-0.2mm spacing

Use a multi-size nozzle pack to experiment with different nozzle sizes and ironing combinations.

Combining Ironing with Other Quality Settings

For the best possible top surface:

1. Use at least 5-6 top layers (at 0.2mm layer height)
2. Set top infill pattern to Monotonic (ensures consistent line direction)
3. Enable ironing with 10% flow, 0.1mm spacing, 15 mm/s
4. Use 20%+ infill to prevent pillowing (which ironing cannot fix)

The monotonic infill pattern is especially important — it ensures all top-layer lines are laid in the same direction, which ironing then smooths. Without monotonic, alternating line directions create a visible texture that ironing improves but cannot fully eliminate.

As Prusa's ironing guide explains, monotonic infill plus ironing is the gold standard for FDM top surface quality.

Final Thoughts

Ironing is free quality. It is a slicer setting that costs nothing but time, and it transforms top surfaces from obviously 3D printed to surprisingly smooth. Enable it by default for visible models, experiment with flow rate to find your sweet spot, and pair it with monotonic top infill for the best results. For most prints, the 10-15% time increase is well worth the dramatic improvement in surface quality.