Density changes when molten resin solidifies. When molten resin solidifies, the volume is usually reduced compared to the molten state. If the entire product shrinks evenly at a fixed rate, the molded product will only become smaller and will have a similar shape. However, if the shrinkage differs in parts, the molded product will be deformed and its shape will change. If this happens, the required shape cannot be obtained simply by adjusting the carving dimensions of the mold.
A familiar example is shown below. As shown in Figure (a), potatoes are deformed when fried in oil. This is due to the distribution of water content in the raw potato. When fried in oil, the moisture is lost. Potatoes have more moisture on the surface and less in the center. When a slice of potato is fried in oil, it shrinks more greatly because there is more water on the outside. This is the result of the volume change depending on the location, which appears as shrinkage deformation, and is considered to be the main cause of the volume shrinkage distribution. On the other hand, as shown in Figure (b), squid is deformed when it is grilled. Although the phenomenon of deformation is the same, the mechanism is different from that of potatoes. In the case of squid, the muscle fibers that make up the muscle are structured in such a way that they can only contract in one direction, but not the other. This means that the muscle fibers determine the direction of contraction, and fiber orientation (fiber direction) is considered to be the main cause of contraction deformation.
In this way, it is believed that the deformation of a product is not caused by some force being applied, but rather by the distribution of partial strain leading to overall deformation. This way of thinking is used as the phenomenon model for the calculation of shrinkage-slip deformation analysis in injection molding CAE (Computer Aided Engineering).
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