Two empirical equations for predicting sink mark depth on the surface opposite to a reinforcing rib in an injection-molded plastic part are presented. Extra thermal mass and effective packing time near a rib base are used as the two determining parameters for calculating the sink mark depth. A normalized geometric sink mark index is used as a measure of the extra thermal mass near the rib base, whereas, the time from the end of filling to the time required for the pressure to vanish near the rib base is used as the effective packing time.
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