The problem of shrinkage and concavation is caused by the temperature rise of the mold surface and the cooling capacity. The surface of the newly-formed injection molded part is still soft. The internal shrinkage hole that has not been completely eliminated forms a vacuum, which causes the surface of the injection molded part to be pressed under atmospheric pressure. Internal compression, coupled with the effect of shrinkage force, the shrinkage problem arises. Moreover, the slower the surface hardening speed, the more likely it is to cause dents, such as PP material, and the more easily the shrinkage holes are produced.
Therefore, after the injection molded part is prematurely released, it is appropriately cooled to maintain a certain hardness on the surface of the injection molded part, so that it is less likely to cause shrinkage. However, if the problem of shrinkage is serious, moderate cooling will not be eliminated. It is necessary to adopt a method of freezing water to make the surface of the molded part harden quickly to prevent shrinkage, but internal shrinkage holes will still exist. For materials with a soft surface such as PP, due to the effect of vacuum and shrinkage force, the injection molded parts may have the possibility of shrinkage, but the degree of shrinkage has been greatly reduced.
At the same time as taking the above measures, if the method of extending the glue injection time is used instead of the cooling time, the surface shrinkage and even the internal shrinkage cavity improvement will be better.