Shrinkage of injection molded plastic parts

- Aug 15, 2018-

  The shrinkage of the injection molded plastic parts (external shrinkage and internal shrinkage) is due to the disadvantages of insufficient filler replenishment when the thicker part is cooled. We often encounter situations where the pressure is increased, the water inlet is increased, the injection time is extended, and the shrinkage problem cannot be solved. Among the commonly used raw materials, because of the fast cooling rate, the shrinkage of the PC material is the most difficult to solve, and the shrinkage and shrinkage of the PP material is also difficult to handle. Therefore, when encountering a serious problem of shrinkage of thick parts, it is necessary to adopt some unconventional injection molding techniques, otherwise it will be difficult to solve the problem. In practice production, we explored a set of useful techniques to perfuse the problem of injection molding.

  First of all, under the premise of ensuring that the injection molded plastic parts are not deformed, adopt the method of minimizing the cooling time, so that the injection molded plastic parts can be prematurely released at a high temperature. At this moment, the temperature of the outer layer of the injection molded plastic part is still high, and the skin is not too hardened, so the temperature difference in the surface is relatively small, which is beneficial to the overall shortening, thereby reducing the concentration reduction inside the injection molded plastic part. Since the overall shortening amount of the injection molded plastic part is constant, the more the overall shortening is performed, the smaller the concentration shortening amount is, and the internal shrinkage hole and the outer surface shrinkage degree are thus reduced. The problem of shrinkage and concave is caused by the temperature rise of the mold and the cooling capacity. The appearance of the plastic molded plastic just solidified is still soft (not like the hard surface of the PC after the mold is released, it is easy to shrink the hole), and it is not completely eliminated. Since the internal shrinkage cavity forms a vacuum, the appearance of the injection molded plastic part is tightened inward under the pressure of atmospheric pressure, and together with the shortening force, the shrinkage problem occurs. Moreover, the slower the surface hardening speed, the more likely it is to shrink, such as PP material, and the more likely it is to shrink the hole.

  Therefore, after the plastic injection molded parts are prematurely released, they should be properly cooled, so that the plastic molded parts adhere to a certain hardness, so that they are less likely to shrink. However, if the problem of shrinkage is more serious, moderate cooling will not be eliminated. It is necessary to adopt the method of freezing water to make the plastic parts of the plastic parts harden quickly to avoid shrinkage, but internal shrinkage holes will still exist. For materials with a soft surface such as PP, due to the effect of vacuum and shortening force, the injection molded plastic parts may have a concave or concave shape, but the degree of shrinkage has been greatly reduced.