Perimeters and Shells

Theoretical Background

The Perimeters (or Wall Line Count) setting in PrusaSlicer defines the number of concentric loops printed to form the vertical walls of a 3D model. In Fused Deposition Modeling (FDM), perimeters represent the structural boundary of the part. Adjusting this setting is one of the most powerful ways to balance mechanical strength, print speed, material consumption, and surface quality.

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Perimeter Mechanics & Extrusion Geometry

To understand how perimeters form a solid wall, we must analyze the cross-section of a single extruded bead. In FDM printing, extruded plastic is squeezed between the nozzle tip and the layer below. Because of this compression, the bead does not have a circular cross-section; instead, it is modeled in PrusaSlicer as a rectangle with semicircular ends (often called an obround or stadium shape).

        Extrusion Width (w)
|<--------------------------------->|
  _______________________________
 /                               \  ^
|              Area (A)           | | Layer Height (h)
 \_______________________________/  v
|<--h-->|<----- w - h ----->|<--h-->|

The cross-sectional area $A$ of a single bead with extrusion width $w$ and layer height $h$ is calculated as:

Quality, Strength, and Print Settings

Perimeter Printing Order

PrusaSlicer allows you to configure the order in which perimeters are printed within a single layer. This has a major impact on surface aesthetics and dimensional tolerance.