Q - shrinkage (%)
D - the actual size of the mold cavity at room temperature
D1 -- the actual size of the part at room temperature
In calculating the size of the cavity and core, first of all, to determine the selected plastic shrinkage, while taking into account the molding method and process conditions, mold structure, the structure of the part and so on, such as the shrinkage factors mentioned above. In particular, the accurate selection of the shrinkage rate is crucial, and the experienced person may be able to select the right shrinkage rate at one time. This is because the shrinkage rate given for each type of plastic is not a fixed value, but a range of shrinkage rates. For example, polyethylene (PE) shrinkage between 1.5 ~ 4%, according to experience in small and medium-sized plastic parts can be taken 2.5 ~ 3%.
Additionally, the difference between the size of the mold cavity at room temperature and the size of the plastic part after shrinkage and the percentage of the actual size of the part at room temperature is called shrinkage.