Fillets vs Sharp Internal Corners

Theory

A sharp internal corner has a theoretical stress concentration factor (Kt) approaching infinity as the corner radius approaches zero. In additive manufacturing this is compounded by the discrete layer structure: an FDM part has a notch at every 90° internal corner where adjacent layer lines terminate abruptly. SLA and SLS parts suffer the same stress concentration even though their microstructure is more homogeneous: the geometric notch effect is process-independent. A fillet radius equal to or larger than the wall thickness distributes the load uniformly and nearly eliminates the notch.

Application

Apply a minimum fillet radius of 0.5× wall thickness on all internal corners. For load-bearing parts (clips, brackets, living hinges) use a radius of 1× to 2× wall thickness. In FDM, large fillets also improve layer continuity: the nozzle traces a curve instead of making an abrupt direction change, which maintains extrusion pressure. External (convex) corners do not require fillets for strength, but chamfering them reduces the risk of layer delamination at sharp outer edges.

Common mistakes

Designers often add fillets to external corners (cosmetic) but forget internal corners (structural). Do not add fillets so large they eat into the functional cavity of the part. In SLS and DMLS, even very small sharp corners (0.1–0.2 mm radius) may appear smooth in the build due to powder resolution, but they remain structural stress risers.

Related terms: Minimum Wall Thickness, First-Layer Chamfer / Elephant Foot Avoidance, Sharp Internal Corner Stress Failure Modes