Ribs, gussets and fillets
When a part flexes too much, the instinct is to make it thicker. That is almost always the wrong move on an FDM print: thick walls waste filament, trap heat, warp, and take forever. The right move is to add shape — ribs, gussets and fillets — which buy stiffness for a fraction of the material. The walls and perimeters carry the load (see Walls, perimeters and infill); these three details put that load-bearing material where the stress actually is.
Ribs instead of thick walls
A flat panel bends like cardboard. Add a thin rib standing on edge across it and it stiffens dramatically, because stiffness scales with the cube of section depth — a 6 mm-tall rib resists bending far more than 6 mm of extra panel thickness, and uses a tenth of the plastic.
The trick is to size the rib so it prints as clean perimeters with no weak infilled core. Make the rib thickness equal to your wall thickness — two or three line widths, so 0.9–1.35 mm on a 0.4 mm nozzle. A fatter rib just gets a hollow, sparsely-filled centre that adds bulk without stiffness.
| Dimension | Guideline |
|---|---|
| Rib thickness | = wall thickness (~0.9–1.35 mm) |
| Rib height | up to ~3× its thickness; taller, add cross-ribs |
| Rib spacing | ~3–5× wall thickness between ribs |
| Draft on tall ribs | 0.5–1° helps printing and de-warping |
Gussets at brackets and corners
Anywhere two faces meet at a right angle and one of them takes load — an L-bracket, a shelf tab, the base of a hook — the joint is the weak point. A gusset (a triangular web bridging the inside corner) ties the two faces together and stops the joint hinging open.
Keep the gusset the same thickness as the walls so it prints as solid perimeters. A gusset that reaches about two-thirds of the way along each face catches most of the bending; bigger than that buys little. On a wall-mount bracket, two or three gussets across the width beat one tall central one, for the same reason ribs win in a grid.
Fillets stop cracks before they start
A sharp inside corner is a stress concentrator — load piles up at the point and cracks start there, layer line or not. Round it with a fillet and the stress spreads along the curve instead of stabbing into a point. This is the cheapest strength upgrade in CAD: it costs nothing to print and often doubles the load a corner survives.
- Inside corners (re-entrant): always fillet. Even a 1–2 mm radius transforms a crack-prone notch. Where a rib or gusset meets a wall, fillet that join too.
- Outside corners: a small chamfer or fillet helps the print (cleaner first layer, fewer elephant-foot snags) but matters less for strength.
Put the three together and you stiffen a part without bulking it up: ribs to beat bending, gussets to hold corners square, fillets to keep stress from piling into a point. Thin material, smartly shaped, beats thick material every time on an FDM machine.