After the plastic part is taken out of the mold and cooled to room temperature, the performance of dimensional shrinkage is called shrinkage. Since the shrinkage is not only the thermal expansion and cold contraction of the resin itself, but also related to various forming factors, the shrinkage of plastic parts after forming should be called forming shrinkage.



The formal shrinkage of plastic molding is mainly shown in the following aspects:

(1) The linear size shrinkage of plastic parts is reduced when the plastic parts are cooled to room temperature due to thermal expansion and cold contraction, elastic recovery and plastic deformation during demoulding, etc. Therefore, compensation must be considered in cavity design.

(2) During shrinkage directional forming, the molecules are arranged according to the direction, making the plastic part anisotropic. Along the material flow direction (i.e. parallel direction), the shrinkage is large and the strength is high. In the direction perpendicular to the material flow (i.e. vertical direction), the shrinkage is small and the strength is low. In addition, due to uneven density and filler distribution at various parts of the plastic part during forming, the shrinkage is uneven. Shrinkage difference makes plastic parts prone to warping, deformation and crack, especially in extrusion and injection molding, the directivity is more obvious. Therefore, the shrinkage direction should be considered when designing the mold. The shrinkage rate should be selected according to the shape of the plastic part and the flow direction.