Gaps between walls (perimeters): causes and fixes

This guide is for prints with thin slits or see-through lines running between the wall lines, often on thin walls, narrow ribs and text, or along one part of a wall. It covers what Prusa, Bambu Lab, OrcaSlicer, UltiMaker Cura, Simplify3D and Polymaker document about flow, gap fill, thin walls, the Arachne and Classic wall generators, and wall order. Defaults are tied to the slicer version we checked.

Is this the right guide?

Check the slicer preview before you print again. If the gap is there in the preview, it's a slicing setting. If the preview shows the walls touching but the print doesn't, start with flow.

Why do gaps appear between walls?

The makers describe two separate causes.

Not enough plastic. Prusa's extrusion multiplier article says minor under-extrusion "causes small gaps that appear between layer lines or perimeters." Bambu Lab's flow rate article says a flow rate that's too low "causes underextrusion (gaps in the printed lines)."

Walls that don't divide evenly into lines. Simplify3D explains that with a fixed size nozzle, walls "only a few times larger than the nozzle diameter" can leave "a gap in the middle." Its example is a 1.0 mm wall printed with a 0.4 mm extrusion width. The slicer has to do something with the leftover space, and that's where the wall generator and gap fill come in:

• Classic walls use one line width. PrusaSlicer's tooltip says the Classic generator "produces perimeters with constant extrusion width and for very thin areas is used gap-fill." Bambu Studio's tooltip says the same for walls.
• Arachne walls vary in width. Prusa says Arachne "automatically widens or thins perimeters as needed," and that gap infill and the small extrusions "used to fill gaps between perimeters are also significantly reduced."

Step 1: Start from your maker's profile

Reset to the profile your printer maker provides for your printer, nozzle and filament, and print the part again. If you've changed line widths, put them back first. Bambu Lab says "for various diameter nozzles, it is usually recommended not to modify the default line width," and if you do, to keep it between 0.75 and 1.5 times the nozzle diameter.

Step 2: Calibrate flow

If the gaps run between ordinary wall lines across the whole print, check flow before any wall settings.

• Prusa: in its visual method, "if there are visible gaps between layer lines, increase the Extrusion Multiplier value," adjusting by 1–2% at a time. It adds that "microscopic gaps near the perimeters are OK." Prusa says the right value "can even differ from spool to spool."
• Bambu Lab: in its manual flow rate calibration, the under-extruded samples show "gaps between the printed lines." Bambu says to pick the smoothest sample.
• OrcaSlicer: its wiki recommends calibrating the flow ratio "before using a value other than 1" for the surface flow settings. Once Set other flow ratios is on, OrcaSlicer also has separate ratios for outer walls, inner walls and gap fill.
• Cura: Wall Flow, Outer Wall Flow and Inner Wall(s) Flow all inherit the main Flow value of 100% in the 5.13.0 definition.

Our under-extrusion guide covers the causes of low flow, from wet filament to temperature. If you raise flow, don't overshoot. Our over-extrusion guide walks through flow calibration in each slicer.

Step 3: Check gap fill

Gap fill is the short back-and-forth extrusion a slicer uses to fill spaces too narrow for a full line. Make sure it's on and not filtered out.

OrcaSlicer warns against turning wall gap fill off completely. It says you could set Filter out tiny gaps to a very large number, "however this is not advised, as gap fill between perimeters is contributing to the model's strength." For models with too much gap fill between perimeters, it suggests switching "to the arachne wall generator" instead.

Step 4: Handle thin walls

A wall too narrow for two lines is where gaps and missing details show up most.

• PrusaSlicer: by default, every wall consists of outside and inside perimeters. Detect thin walls "lets PrusaSlicer generate a single perimeter that works as both an inside & outside shell," though "walls thinner than a single perimeter extrusion width will likely still be ignored." It's on by default in 2.9.6, but Prusa says it's "automatically disabled" with Arachne, "as it already automatically adjusts for thin walls."
• Bambu Studio: Detect thin wall is under Process, Strength, Walls and is off by default in v02.08.02.61. Bambu's wiki says it can slice details such as small text, but it can make walls overlap, and "the excess filament may stick to the nozzle and then stick to other parts." Very small sections may print as a single unclosed line that is "too fragile." In that version's source, the option is greyed out when the wall generator is Arachne.
• OrcaSlicer: Detect thin wall prints a wall that can't fit two line widths "as a single extrusion line." OrcaSlicer says such walls "may have reduced surface quality and strength because they are not closed loops," and recommends the Arachne generator, which disables the option.
• Cura: Print Thin Walls prints "pieces of the model which are horizontally thinner than the nozzle size." It's on by default in 5.13.0.

Make the wall fit the lines

Two makers suggest changing the geometry or line width instead of the gap settings:

• Prusa: PrusaSlicer shows a recommended thin wall thickness for your perimeter count and layer height. Prusa says that if you "change the wall thickness to this exact value" in CAD, you "eliminate unnecessary perimeter overlap." Its example is 0.86 mm, not 0.90 mm, for two 0.45 mm perimeters at 0.2 mm layers, because neighbouring perimeters slightly overlap. Prusa says this recommendation changes with layer height.
• Simplify3D: for a 1.0 mm wall, a 0.5 mm extrusion width "could achieve a fast and strong print." It says this "works best for parts that have fairly consistent wall thicknesses." The setting is on the Extruders tab.

For very fine features, a smaller nozzle is another option. Bambu says small text that won't slice with Classic walls has "a good chance" of slicing with a smaller nozzle profile. Our nozzle sizes guide covers the trade-offs.

Step 5: Try the other wall generator

PrusaSlicer, Bambu Studio and OrcaSlicer let you choose between Classic and Arachne. Arachne is the default in PrusaSlicer 2.9.6 and Bambu Studio v02.08.02.61.

The makers don't fully agree on which looks better:

• Prusa says Arachne "produces nicer-looking prints with fewer artifacts," and that "for most prints, the Arachne perimeter generator is superior." It adds that "for some functional prints where the accuracy of concave corners is important, the classic perimeter generator may be better."
• Bambu Lab says Classic walls give better surface quality "because it has only one seam per layer." It says Arachne "will generate discontinuous wall paths" in some cases, for example where two inner walls of the same width don't fit, and that discontinuous paths "can affect the print surface quality." It also says Arachne walls don't overlap the way Detect thin wall can.
• OrcaSlicer says Classic "does not vary extrusion width and is ideal for fast, predictable slicing," while Arachne "allows better handling of thin features and smooth transitions between wall counts."

So if you see gaps with Classic walls, try Arachne. If you see broken or uneven walls with Arachne, try Classic. Print the same part both ways with nothing else changed.

Arachne settings that mention gaps

If you use Arachne, a few settings decide how it splits a thin wall. The defaults below are the same in PrusaSlicer 2.9.6 and Bambu Studio v02.08.02.61, where the percentages are of the nozzle diameter:

Cura 5.13.0 has matching settings in Walls: Wall Transitioning Threshold Angle (10°), Wall Transitioning Filter Margin (a quarter of the nozzle size), Minimum Wall Line Width (0.85 times the nozzle size), Minimum Thin Wall Line Width (which takes the Minimum Wall Line Width value), and Minimum Feature Size. Prusa says PrusaSlicer's Arachne was "ported" from Cura.

Bambu Lab's wiki says raising Minimum feature size makes the slicer skip thinner features, and lowering it "allows more detail to be preserved during slicing." Change one of these at a time and check the preview.

Step 6: Check wall order

The order walls print in affects how they press together. The makers describe the trade-offs like this. None of the docs we read name wall order as a direct cause of gaps between walls.

• PrusaSlicer: External perimeters first (Expert mode, off by default in 2.9.6) prints "from the outermost to the innermost." Prusa says it "may help with dimensional accuracy" because "extra filament extruded when printing following perimeters is pressed back away from the outer wall," but "the surface may be slightly less smooth."
• Bambu Studio: Order of walls is under Process, Quality, Advanced, with inner/outer as the default. Bambu says inner/outer gives better overhangs, and outer/inner can "minimize the imperfections and interlayer patterns on the surface," though seams "may be more obvious."
• OrcaSlicer: with Inner/Outer, "the external perimeter is deformed by being squashed to the internal perimeter." It recommends Inner/Outer/Inner "for the best external surface finish and dimensional accuracy," with at least 3 walls.
• Cura: Wall Ordering defaults to Inside To Outside in 5.13.0. Cura says printing outer walls earlier "helps with dimensional accuracy," while printing them later lets them "stack better when overhangs are printed."

Our infill showing through walls guide covers wall order and print-infill-first in more detail.

What if the gap is next to the infill or top skin?

If the gap is between the innermost wall and the infill or top surface, rather than between wall lines, the overlap settings matter more than the wall settings:

• For gaps between walls and sparse infill, see our infill showing through walls guide. It covers infill/wall overlap in each slicer and Simplify3D's advice on speed.
• For pinholes where the top surface meets the walls, see our gaps in top layers guide. It covers skin and top infill overlap.

PrusaSlicer's Fill gaps also covers the space "between the inner most perimeters and infill," so check it's on (Step 3).

A sensible order

1. Look at the slicer preview. If the gap is in the preview, it's a slicing setting. If not, start with flow.
2. Reset to your printer maker's profile and default line widths.
3. Calibrate flow on the spool you're using.
4. Make sure gap fill is on, and not filtered out or set to zero speed.
5. For thin walls, try Arachne or your slicer's thin wall option, or change the wall thickness or line width so the lines fit.
6. Compare Classic and Arachne on the same part.
7. If you use Arachne, adjust one transition or minimum width setting at a time and check the preview.
8. For gaps next to infill or the top surface, check the overlap settings in the linked guides.

Change one setting per print and keep the same test file, so you know what fixed it.

Checklist

• Gap checked in the slicer preview.
• Maker's profile and default line widths restored.
• Flow calibrated for this spool.
• Gap fill on, not filtered out, and gap fill speed above zero.
• Thin walls handled by Arachne, a thin wall option, or a wall thickness that fits the lines.
• Classic and Arachne both tried on the problem part.
• Wall order set back to the profile's value, or changed on purpose.
• Infill and top skin overlap checked if the gap is next to them.

How this guide was checked

This is a research-based guide. We haven't printed test models for it. Every setting name, default and quote comes from the Prusa Research, Bambu Lab, OrcaSlicer, UltiMaker Cura, Simplify3D and Polymaker pages and source files listed above, retrieved on 2026-10-13. Defaults are from the PrusaSlicer 2.9.6 and Bambu Studio v02.08.02.61 source and the Cura 5.13.0 base definition. Printer profiles can set their own values. Read more in our editorial standards.

Print problems: what to check before you buy ↗