Sharp corners greatly increase stress concentration, which, when high enough, can lead to part failure. Sharp corners often come about in non-obvious places, such as a boss attached to a surface, or a strengthening rib. The radius of sharp corners needs to be watched closely because the stress concentration factor varies with radius for a given thickness. As illustrated in the chart to the left, the stress concentration factor is high for R/T values less than 0.5, but for R/T values over 0.5 the concentration lowers. The stress concentration factor is a multiplier that greatly increases stress. It is recommended that an inside radius be a minimum of one times the thickness.
In addition to reducing stresses, the fillet radius provides a streamlined flow path for the molten plastic, resulting in an easier fill of the mold.