The shrinkage of the molding material has a great influence on the dimensional accuracy of the plastic parts. If the precision of the molding equipment and the mold is high, but the shrinkage of the molding material is large, it is difficult to ensure the dimensional accuracy of the plastic parts. Under normal circumstances, the greater the shrinkage rate of the molding material, the more difficult it is to ensure the dimensional accuracy of the plastic parts. Therefore, when selecting the molding resin, it is necessary to fully consider the influence of the shrinkage rate after the molding of the raw material on the dimensional accuracy of the plastic part. For raw materials selected, the range of shrinkage can not be greater than the dimensional accuracy of plastic parts.
It should be noted that the shrinkage of various resins varies greatly, and the analysis is based on the degree of crystallinity of the resin. In general, the shrinkage of crystalline and semi-crystalline resins is larger than that of non-crystalline resins, and the range of shrinkage change is also relatively large, and the corresponding shrinkage rate of plastic parts after molding is also relatively large; for crystalline resins, The crystallinity is high, the molecular volume is reduced, the shrinkage of the plastic parts is large, the size of the resin spherulites also affects the shrinkage rate, the spherulites are small, the gaps between the molecules are small, the shrinkage of the plastic parts is small, and the impact strength of the plastic parts is relatively high.