Glossary
Fillet radius — definition
The rounding in internal corners, which both spreads stress in the part and prevents cracks in the tool.
A sharp internal corner does three unhelpful things at once. It concentrates stress in the part and becomes the starting point for fatigue cracks. It creates a local accumulation of metal where solidification finishes last. And it leaves a sharp edge in the steel tool that cracks under thermal load.
Radii fix all three. They cost nothing in casting — the tool is machined anyway — and they make the part stronger, not weaker.
This is the cheapest design change there is, and the one we propose most often in a castability review.