Improvement of quarity of a molded part using ultrasonic vibration

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This study describes approaches available for a method of improving the quality of molded plastics by ultrasonic vibration. When a crystalline or semi-crystalline thermoplastic is used as the molded resin, the crystallinity of the molded resin is directly related to its quality, because its shrinkage, mechanical strength, Young's modulus and so on vary according to the crystallinity. In general, the cooling speed varies from place to place in the mold, so uniform crystallinity of the molded resin cannot be obtained. On the other hand, it seems that the crystallinity is accelerated by vibration, since supercooled water (<0ﾟC) becomes suddenly crystallized when it is vibrated. However, the influence of vibration on crystallinity has not been clarified. Therefore, when the mold is forcedly vibrated by ultrasonic vibration, it can be expected that the crystallinity of the molded resin increases due to the vibration, and the quality of the molded resin is improved. In this study, the molded resin is vibrated during filling, packing and cooling stages by ultrasonic vibration. In this case, the resin is vibrated directly by an ejector pin or indirectly by the mold. High density polyethylene (HDPE) is used as the molded material. From these experimental results, it is evident that the crystallinity of the resin molded under vibration is improved in comparison with the non-vibrated molded one and that the cooling speed of the molded resin has not been affected by the vibration when the mold is vibrated.