1) Sharp corners greatly increase the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts. Sharp corners can come about in non-obvious places. Examples of this are a boss attached to a surface, or a strengthening rib. These corners need to be radiused just like all other corners. The stress concentration factor varies with radius, for a given thickness.

  2) the stress concentration factor is quite high for R/T values lesss than 0.5. For values of R/T over 0.5 the stress concentration factor gets lower. the stress concentration factor is a multiplier factor, it increases the stress. Actual Stress = Stress Concentration Factor K x Stress Calculated,  this is why it is recommended that inside radiuses be a minimum of 1 x thickness. 

 3) In addition to reducing stresses, fillet radiuses provide streamlined flow paths for the molten plastic resulting in easier fills. typically, at corners, the inside radius is 0.5 x material thickness and the outside radius is 1.5 x material thickness. A bigger radius should be used if part design will allow it.

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