Walls and Perimeters

What walls are

Every layer starts with one or more closed outlines that follow the edge of the model. The outermost one is the outer wall, the one you see; the rest are inner walls. Slicers call them walls or perimeters. Inside the walls the slicer fills the space with sparse infill, and at the top and bottom of the part with solid layers (see Top and Bottom Layers).

Walls matter more than most beginners expect. Prusa's layer settings guide says the strength of a model is mostly defined by the number of perimeters, not the infill, and that you should increase perimeters for a stronger print. Prusa's own profiles always use at least two perimeters Maker spec.

Where the settings are called what

What it controls	PrusaSlicer	OrcaSlicer	Bambu Studio	Cura
Number of walls	Perimeters	Wall loops	Wall loops	Wall Line Count (or Wall Thickness)
How wall paths are built	Perimeter generator (Arachne or Classic)	Not confirmed	Wall generator (Classic or Arachne), in Process, Quality	Not confirmed
Single line for thin walls	Detect thin walls	Detect thin wall	Detect Thin Wall	Print Thin Walls
Order of inner and outer walls	External perimeters first	Walls printing order	Not confirmed	Wall Ordering

Not confirmed means we did not find the name in that slicer's documentation; the feature may still exist. Cura's Wall Thickness and Wall Line Count are two views of the same thing: Cura divides the thickness by the wall line width to get the count.

Wall count: the first thing to change

Which way it pushes: more walls make the part stronger and stiffer. OrcaSlicer's documentation lists better layer adhesion, strength and rigidity, and less infill ghosting (infill pattern showing through the outer surface), as the gains.

What it costs: print time and material. Every extra wall is another full loop around every layer.

A detail that surprises people: walls overlap slightly so they bond, so wall thickness is not simply count times line width. Prusa's example: with 0.45 mm perimeters at 0.2 mm layers, two perimeters make a wall of 0.86 mm, not 0.90 mm Maker spec. When you design a part to a wall thickness, check the thickness your slicer actually produces in its preview, or the recommended thin wall thickness PrusaSlicer shows.

OrcaSlicer also has Alternate extra wall, which adds a wall on every other layer so the infill is wedged between walls. Its wiki advises against combining it with Lightning infill or with Ensure vertical shell thickness set to All.

Arachne versus Classic

The wall generator decides how the wall paths are laid out inside each layer.

• Classic uses one constant line width. Bambu Lab's wall generator article explains the advantage: each wall is one continuous closed loop, so there is only one seam per layer and the surface is more even. The drawback is that features narrower than the line width, such as small raised text, can disappear from the slice entirely. Prusa notes Classic may be better for accurate concave corners.
• Arachne varies the line width to fit the shape, so it can print thin features and small details that Classic loses. Prusa lists Arachne as PrusaSlicer's default perimeter generator. The OrcaSlicer wiki warns that on large prints Arachne can produce defects more easily because of reduced wall thickness, and Bambu Lab notes that its discontinuous paths can leave more than one seam.

If small text or a thin rib is missing in the preview, try Arachne first. If a large smooth part shows extra seams or patchy walls, try Classic.

Detect thin walls

With Classic walls, a wall too thin for two lines is normally skipped. Detect thin walls prints it as a single line instead. OrcaSlicer warns that these single lines may have reduced surface quality and strength, because they are not closed loops, and Bambu Lab shows how the reduced spacing can make walls overlap and leave excess plastic on the nozzle. PrusaSlicer disables the option when Arachne is active, and Prusa notes that walls thinner than one extrusion width can still be ignored. If a feature matters, model it at least one line width thick.

Wall order

Most profiles print inner walls first, then the outer wall. Changing the order trades surface quality against overhangs:

• Inner then outer: OrcaSlicer calls this best for overhangs, because the overhanging outer wall can lean on its neighbor. Outer surface quality is slightly reduced.
• Outer then inner (PrusaSlicer's External perimeters first): Prusa says it may improve dimensional accuracy but can slightly reduce surface smoothness. OrcaSlicer adds that Z seams look less consistent.
• Inner, outer, inner (OrcaSlicer): best external finish and dimensional accuracy by OrcaSlicer's account, with poorer overhangs. It needs at least three walls to work.

OrcaSlicer also offers Print infill first, which can help some overhangs but gives a worse outer finish and lets the infill show through.

What to change first

1. Part too weak: add walls. Change wall count before infill density, and check the result in the preview.
2. Small features missing in the preview: switch to Arachne, or turn on Detect thin walls with Classic.
3. Holes and outer dimensions off: try outer-first or inner, outer, inner order, then measure. Leave line widths and flow alone until flow calibration is done.
4. Infill showing through: add a wall before touching anything else.

Change one of these at a time and print a small test, so you know which change did the work. Where the seam on each wall starts is its own topic: see Seam Placement.