The depressions and cavities on the surface of plastic products are called shrinkage, which not only affects the appearance of the product, but also reduces the quality and strength of the finished product. The reason for shrinkage is related to molding technology, mold design and the use of plastics. 1. For the shrinkage rate of different plastic raw materials, refer to the reference data in Table 1. Generally, the raw materials that are easy to shrink are crystalline, such as nylon, pleated rubber and so on. During the injection process of the vertical injection molding machine
, the crystalline plastic is heated into a fluid state, and the molecules are arranged irregularly; when injected into the colder cavity, the plastic molecules are slowly arranged in order to form crystals, and the volume shrinks less than the specified size range , Is the so-called shrinkage. 2. Injection technology In the aspect of vertical injection molding machine injection technology control, shrinkage occurs: insufficient pressure, vertical injection molding machine injection speed is too slow, the burner is too small or the runner is too long and so on. Therefore, when using a vertical injection molding machine, attention must be paid to whether the molding conditions and holding pressure are sufficient to prevent shrinkage. 3. Vertical injection molding machine mold and product design, vertical injection molding machine mold runner design and cooling device, also have a great impact on the finished product. Due to the low heat transfer ability of plastic, the farther and thicker it is from the mold wall, the slower it solidifies and cools. There should be enough plastic to fill the mold cavity so that the screw of the vertical injection molding machine can be injected or pressure-maintained. The plastic will not flow back and reduce the pressure. On the other hand, the water surface cannot be cooled too fast, so as to prevent the semi-solid plastic from blocking the flow path and causing the pressure to drop, causing the finished product to shrink. Different mold flow processes have different shrinkage rates, and the temperature of the melting cylinder is properly controlled. It can prevent the plastic parts from overheating; extending the cycle can ensure that the product has sufficient time to cool down. If the shrinkage problem is properly resolved, the quality of the finished product can be improved. Reduce waste products and improve production efficiency.
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