The 'dent' of the molded part is the local shrinkage caused by the plastic injection mold gate sealing or lack of glue. The dent is generally caused by the increase in local shrinkage of the product caused by the increase in the wall thickness of the molded part. It may appear near the outer sharp corner or at the place where the wall thickness changes suddenly, such as behind a protrusion, a steel bar or a support, but not all of them appear in these places, and sometimes they appear In other places. The root cause of dents is the thermal expansion and contraction of the material. The degree of expansion and contraction of a molded part depends on many factors, among which the properties of the plastic, the temperature range of the
injection molding and the pressure protection pressure in the cavity of the plastic injection mold are very important factors. In addition, the size and shape of injection molded parts, cooling rate and uniformity will also be affected.
The expansion and contraction during the molding of plastic materials is related to the thermal expansion coefficient of the processed plastic. The coefficient of thermal expansion during molding is called 'mold shrinkage'. As the molded part cools and shrinks, the molded part loses close contact with the cooling surface of the mold cavity, and the cooling efficiency decreases. At this time, the molding continues to shrink in the cold zone.
Among them, the molded parts of semi-crystalline plastic materials have a high shrinkage rate, which makes the problem of dents more serious; the molding shrinkage of amorphous materials is low, which will greatly reduce the occurrence of dents.
Molded parts with thick walls have a long cooling time and are more prone to large shrinkage. Therefore, the large thickness is the root cause of the dent. Attention should be paid to the design of injection molded parts, and thick-walled parts should be avoided as much as possible. If the thick wall is uneven, it should be designed to be hollow.
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