Injection Molding Design Guide

Guide for high quality and cost-effective plastic injection molding.

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Radius
  • 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.

    As can be seen from the above chart, 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.

  • 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.