Infill and strength: why 100 percent is rarely the right answer
How infill percentage, pattern and wall count interact, why walls matter more than fill, and how to dial in strength without wasting print time.
When a printed part breaks, the first reflex in the slicer is: more infill, somewhere toward a hundred percent. The result is a part that prints three times longer and rarely holds three times as much. The reason lies in how FDM parts take load.
Walls carry the load, not the fill
An FDM part behaves mechanically like a hollow box: the outer walls are continuous, bonded layers that take bending and tensile loads. The infill is a loose lattice inside that braces the walls against denting and distributes forces between them. Four to six wall lines add more strength than doubling the infill percentage, at a fraction of the print time.
What infill actually does
Infill has three jobs: it supports the top layers, it keeps walls from denting under point loads, and it carries compressive loads, for example where a part is bolted down. Pure bending parts are fine at 15 to 25 percent. Parts with bolt faces, clamp seats or bearing seats need more locally, which modifiers can set precisely instead of filling the whole part.
- Bending-loaded brackets: 20 to 30 percent infill with four to six walls.
- Bolted flanges: locally 60 to 80 percent around the bores, the rest low.
- Visible parts without load: 10 to 15 percent is enough for top layers.
- Only hydrostatic pressure or sealing faces justify 100 percent.
The pattern is secondary
Gyroid, grid or lines differ less in practice than the slicer dropdown suggests. Gyroid spreads load evenly in all directions and prints without crossings in the plane, which protects the surface. Grid is stiff in two directions and fast. More important than the pattern is that the layers themselves bond well: temperature, dry filament and an orientation that suits the load decide more than any fill geometry.
How we configure functional parts
For functional parts we start with four walls and 25 percent gyroid. Where bolts or clamps sit, local reinforcements are added. Only if a first article fails in testing do we reinforce the failure point specifically, not everywhere wholesale. That keeps print time low and strength where it is needed.
If you are unsure whether your part will hold the load, send us the STEP file with a short description of the loading. We will propose walls, infill and orientation, and print a test sample on request.