When a thermoplastic resin is molded in an injection molding machine, the molten resin is packed into a mold under high pressure, so the resin shrinks during cooling down to room temperature in the mold. In general, the pressure in the mold decreases along the flow direction during the hold pressure, thus the pressure does not become uniform in the cavity up to the solid state. Therefore, the molded shape is almost never equal to the cavity shape.
In this study, one part of the mold has been made up of a flexible wall in order to produce very accurately moldings. When a part of the filled molten resin has been solidified in the cavity, the gate has been shut and the flexible wall has been deformed by controlling oil pressure. Consequently, it is clarified that uniform thickness moldings can be obtained by using this flexible mold.

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When a crystalline or semi-crystalline thermoplastic is used as a molded resin, the crystallinity of the molded resin is directly related to its quality, because the part 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 cannot be obtained.
On the other hand, it seems that crystallinity is accelerated by vibration, for example, supercooled water (<0°C) suddenly crystallizes when it is vibrated. However, the influence of vibration on crystallization has not been clarified. When a mold is forcibly vibrated by an ultrasonic vibration, it is possible that the crystallinity of the molded resin may increase due to the vibration, and thus the quality may be improved.
In this study, a molded resin is vibrated during the filling, packing and cooling stages by an 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 conditions is improved in comparison with the non-vibrated molded case and that the cooling speed of the molded resin has not been affected by the vibration when the mold is indirectly vibrated.

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