Walls vs Infill

Walls (perimeters) usually buy you more functional strength per gram than cranking infill, because bending and impact loads concentrate at the outside “skin.” Use infill mainly to support top surfaces, prevent the shell from buckling, and give screws/inserts something to bite into—then tune based on how your part actually fails.

TL;DR

If a functional part is bending, cracking at edges, or feels flimsy, add walls first (more perimeters or slightly wider line width). Only raise infill when you need better top-surface support, crush resistance, or internal support for fasteners/inserts.

Walls vs InfillTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.Walls carry loadInfill supportsTop/bottom close
A quick visual map of the main decisions behind walls vs infill.

What walls and infill actually do (in the print)

Walls (perimeters) form the outer shell and carry most of the load path around edges, corners, and holes. Infill mainly props up top layers, reduces how easily the shell can dent inward, and helps resist crushing when the entire cross-section is compressed. If you add the same amount of plastic, moving it from the middle (infill) to the outside (walls) usually makes the part stiffer and harder to crack.

Add walls first when you see these needs

  • Bending parts (brackets, arms, clips): stiffness is dominated by the outer shell
  • Cracks that start on the outside surface, especially near corners or along the side wall
  • Edge and corner impacts (dropped parts, snapping clips): tougher shell helps
  • Thin features (ribs, tabs, small brackets) where infill barely fits or doesn’t connect well

Increase infill (or choose a stiffer infill pattern) when these are the real problems

  • Top surfaces sagging/pillowing: infill is the scaffold under the top layers
  • Parts getting crushed/dented inward: more internal support can prevent shell buckling
  • Fasteners/inserts need internal support: more infill and/or local solid regions help
  • Large, boxy parts that “oil-can” (walls flex inward/outward): infill can brace the shell

Practical starting points (then tune by failure mode)

Walls (perimeters)
Start around 3–5 walls for functional parts; add walls if it bends too easily or cracks start on the outside.
Infill %
Start around 10–30%; raise it if the part crushes, dents inward, or top layers need more support.
Top/Bottom thickness
If a top surface feels weak or shows pillowing, add top layers/thickness; infill alone won’t fix a thin top skin.
Line width
A slightly wider extrusion can strengthen walls efficiently (more plastic in the shell with modest time cost).

Failure clues: what to change first

Crack begins at an outside corner or runs along a side wall

Likely cause: Shell is too thin for the load path, or stress concentration at corners

Fix: Add walls; also consider rounding inside corners and improving layer adhesion settings.

Part bends like a spring but doesn’t crack

Likely cause: Not enough shell thickness for stiffness; geometry acts like a beam

Fix: Add walls (and/or add ribs in the model); consider wider line width.

Top surface is bumpy or sagging (pillowing)

Likely cause: Not enough support under top layers

Fix: Increase top thickness/top layers; then increase infill or use a more supportive infill pattern.

Part dents inward or collapses under clamp/screw load

Likely cause: Shell buckling due to weak internal support

Fix: Increase infill and/or add walls; consider adding internal ribs or local solid regions near the load.