Infill Basics

Infill is the internal lattice that supports “roofs” and adds stiffness, but it’s rarely the first or best lever for strength. For most FDM parts, walls and top/bottom thickness improve strength and surface support more efficiently than pushing infill density very high.

TL;DR

Start with 2–4 walls and enough top/bottom layers, then use 10–30% infill to support top surfaces and add stiffness. Only push infill higher when you specifically need crush resistance or internal support you can’t get from more walls or better orientation.

Infill BasicsTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.Density10–15, 15–30, 25–40%PatternGrid, Gyroid, LinesGoalLooks, General, StiffWalls firstStrength lever #1Top supportPrevent roof sagTime/weightRises fast with %
A simple matrix helps beginners choose infill density and pattern based on the part goal and highlights when walls/top-bottom are better levers than high infill.

What infill actually does

Infill is the patterned plastic inside the outer walls. It mainly does three jobs: it supports top layers so they don’t sag over empty space, it increases stiffness by acting like internal ribs, and it reduces material and time compared to printing solid while still filling the part enough to be usable.

What infill does not fix (common misconception)

If a part snaps along layer lines, the limiting factor is usually layer bonding or load direction, not “not enough plastic inside.” In that case, more infill often adds time without solving the failure; better fixes are changing orientation so the load pulls along continuous strands, increasing nozzle temperature within reason, reducing cooling for better bonding, or adding walls.

Key infill settings (what each one changes)

  • Infill density: how much internal volume is filled; print time and weight rise quickly as density increases.
  • Infill pattern: the shape of the lattice; changes stiffness directionality, support under top layers, and print speed.
  • Infill overlap / infill-to-wall connection (name varies): how strongly infill ties into walls; too little can leave a weak gap, too much can cause bulges or rough walls.
  • Infill speed: faster infill saves time but can reduce consistency, especially if the printer struggles with rapid direction changes.
  • Infill order (infill before/after walls, if available): affects how clean outer walls look and how well infill anchors to perimeters.

Beginner starting points (typical FDM)

Decorative parts
10–15% infill, 2–3 walls, enough top layers to close without droop.
General functional parts
15–30% infill, 3–4 walls, increase top/bottom thickness before raising infill a lot.
Stiff brackets / load spreaders
25–40% infill, 4+ walls, consider adding ribs/fillets in CAD for big gains.
Avoid as a default
60–100% infill; usually much slower and not much stronger than more walls.

Fast check in slicer preview (what to look for)

  1. Set walls and top/bottom layers first (this defines the “shell” and roof thickness).
  2. Pick an infill density and pattern that matches the goal (lightweight vs stiff vs strong roof support).
  3. Preview the top surfaces: look underneath large flat roofs; you want frequent infill contact so top lines don’t bridge huge gaps.
  4. Preview the wall interface: infill should regularly touch the inner wall, not leave long unconnected runs.
  5. Recheck time and grams: if it’s too heavy/slow, reduce infill first; if it’s too weak, add walls or re-orient before going above ~40% infill.