In the process of producing products, the dent is the most frequent occurrence of the bad phenomenon, and the plastic injected into the mold undergoes volume shrinkage upon cooling, and the early cooling portion, that is, the surface is first hardened, and bubbles are generated inside. The so-called dent, that is, the portion that is cooled slowly produces a conspicuous concave surface in the direction in which the bubble contracts.
Materials with large shrinkage are also prone to dents. When the molding conditions are to be changed to eliminate the dents, the setting conditions should be set in a direction in which the shrinkage is small. That is, the mold temperature, the barrel temperature are lowered, and the injection pressure is increased, but it should be noted that residual internal stress may be caused thereby.
Since the dent is not conspicuous, if it does not affect the appearance, it is intentionally processed into a corrosive pattern on the mold, such as striate, granular, and the like. Also, if the molding material is HIPS, it is also effective to lower the mold temperature to lower the finish. However, once these methods have dents, it is difficult to repair the polished products.
A material with a large mold shrinkage has a large dent. For example, PE and PP produce dents even with a slight rib.