In injection molding, it is important to know the solidification behavior of the resin in the cavity. However, the solidification behavior has not been experimentally clarified. On the other hand, it is difficult to monitor when the moldings pulls away from the cavity surface at cooling stage. In this study, the resin state in the cavity has been monitored by using an ultrasound. The solidification thickness in the cavity has been measured by the ultrasonic echo. Moreover, the amplitude, the

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... se and the mean velocity o£ the sound propagated through the resin have been also investigated in order to detect the resin state.