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FDM Extrusion Width & Wall Planner

FDM / FFF

In FDM/FFF, the actual deposited bead width is not equal to the nozzle diameter — it depends on layer height, flow rate, and material viscosity. Knowing the true extrusion width lets you plan wall perimeter counts precisely, avoid under-extrusion gaps in thin walls, and ensure structural parts have at least two solid perimeters.

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Extrusion width model

w_e = D_nozzle × k_flow + (π/4 − 1) × h_layer A_target = D_nozzle × h_layer × k_flow
D_nozzle
Nozzle orifice diameter[mm]
k_flow
Flow rate multiplier[1.0 = 100%]
h_layer
Layer height[mm]
A_target
Target extrusion cross-section area[mm²]

Slic3r / PrusaSlicer extrusion-width model. The (π/4 − 1) ≈ −0.215 term accounts for the flattened elliptical cross-section of the deposited bead against the build plate or previous layer. For tall layers (h > 0.5D), the simple model w_e ≈ D × k_flow is sufficiently accurate.

Wall planning rules

Minimum wall

≥ 0.8 × nozzle diameter. Below this, the slicer may skip the perimeter or produce under-extruded walls.

Structural minimum

≥ 2 perimeters (2× extrusion width). Single-perimeter walls have low inter-layer bond strength and are prone to splitting.

Perimeter match

Design wall = n × w_e avoids gap/overlap artefacts. Small mismatches (< ±5%) are absorbed by the slicer.

Layer height

25–75% of nozzle diameter maximises layer adhesion and print reliability.

Sources