Fillets: a stronger part and a better look

Why a fillet helps

A fillet is a rounded edge or inside corner. Stress concentrates in a sharp inside corner and that is exactly where a part most often breaks. A fillet spreads it over a larger area.

In 3D printing this matters twice as much, because the part is made of layers and is weaker between layers than along the strand. A fillet where a thin wall meets a thicker one often helps strength more than denser infill.

Where to add a fillet

• Inside corners and transitions. A radius of at least 1 to 2 mm, larger for load-bearing parts.
• Outer edges on the side. Vertical edges that are rounded print cleanly and feel nice in the hand.
• Top edges. A fillet on the top of a part is fine, the layers shrink gradually, so no overhang is created.

Watch the bottom edge

Do not put a fillet on the side that lies on the bed. A rounded bottom edge forms a small overhang and the plastic sags there, or you get what is called elephant foot. Use a 45 degree chamfer instead, which prints cleanly.

If you want to round the model everywhere, turn it so that the rounded edges are not horizontal over empty space, see model orientation.

In CAD and for your order

In CAD software a fillet takes a single command, often called Fillet. When you export to STL or 3MF, the fillet is turned into many small triangles, which is fine. If you export at a coarse resolution, the fillet will look faceted, see polygon mesh.