Infill is the most over-tuned setting in FDM. The instinct is to crank density when a part feels weak — but for most load cases, adding perimeters (walls) buys more strength per gram than adding infill. Here's how to spend both wisely.
Recommended infill by use case
| Use case | Infill | Pattern | Notes |
|---|---|---|---|
| Visual / display | 10–15% | Gyroid or grid | Cosmetic only; save time and plastic |
| Standard functional | 20–30% | Gyroid | The sweet spot for most parts |
| Structural / load-bearing | 40–60% | Gyroid or cubic | But add wall perimeters first |
| Compression / potting / threads | 60–100% | Concentric or rectilinear | Solid where inserts or high crush loads live |
Strength scales faster with wall (perimeter) count than with infill density. Before you raise infill, add two perimeters — you'll get more stiffness for less mass and print time.
Patterns, honestly compared
| Pattern | Character | Use it for |
|---|---|---|
| Gyroid | Near-isotropic, strong every direction, fast | The default for functional parts |
| Grid / rectilinear | Fast, strong vertically, weak diagonally | Quick prototypes |
| Cubic / octet | Good 3D strength | Parts loaded from several directions |
| Triangular / honeycomb | Strong in-plane, denser | Flat parts loaded in their plane |
| Concentric | Follows the perimeter, flexible | TPU and flexible parts |
| Lightning | Only supports the top surface | Cosmetic parts — near-zero strength |
Why 100% infill is usually wrong
Solid infill seems like the safe choice and rarely is: it multiplies print time, wastes material, and — because thick solid sections cool unevenly — builds internal stress that warps corners and can crack layers. A 40% gyroid part with four perimeters is stiffer per gram, prints faster, and warps less than the same part at 100%. Reserve near-solid infill for local zones that take crush loads or hold threaded inserts.
For a stiffer part at the same weight, raise wall count and drop infill. Walls carry bending load at the outer fiber, where it matters most; interior infill sits near the neutral axis and does little.
When infill genuinely matters
- Compression: parts squeezed face-to-face need dense infill to resist crushing.
- Top-surface support: too little infill and the top layers sag between cells ("pillowing") — raise infill or add top layers.
- Embedded hardware: heat-set inserts and press fits need solid material around them to bite into.
Infill is one dial in a bigger strength system — orientation, walls, and material do the heavy lifting. The Pro FDM course's strength module shows what actually adds strength, with worked examples.
Take the FDM course Next: orientation & strengthInfill checklist
- Default to 20–30% gyroid; change only with a reason.
- Need more strength? Add perimeters before infill.
- Solid-fill only the local zones with crush loads or inserts.
- Seeing top-surface dimples? Raise infill or add top layers, not global density.
- Never reach for 100% as a strength fix — reorient and rib instead.