How should fillets be designed for optimal performance in FDM?

Designing fillets for optimal performance in Fused Deposition Modelling (FDM) involves considering the geometry's structural integrity during the printing process as well as its functionality in the final application. The choice of an outer radius that equals the inner radius plus the wall thickness is particularly important because it ensures a smooth transition between the sections of printed parts. This design minimizes stress concentrations and improves the overall strength of the printed piece.

When fillets have this specific design, they can effectively distribute stress over a larger area, reducing the likelihood of weak points that could lead to failure during use. Additionally, the use of a consistent wall thickness aids in maintaining uniform extrusion flow during the printing process, which helps to avoid common issues such as warping, under-extrusion, or sagging.

Other options such as uniform thickness throughout or varying thickness to save on material might compromise the strength and integrity of the part, whereas sharp angles can create stress concentration points that may weaken the structure, making the part more susceptible to fractures or failures under load. Thus, the selection of a radius that corresponds with the inner radius and wall thickness is a manufactured consideration aiming to achieve optimal quality and durability in FDM printing.