Mold shrinkage mainly in the following aspects:
Line dimensional shrinkage due to thermal expansion and contraction of plastic parts, plastic parts when releasing elastic recovery, plastic deformation causes the release of plastic parts cooled to room temperature and its size reduction, it must be considered to compensate for this cavity design.
Directional shrinkage during molding molecules are arranged in a direction to make plastic parts exhibit anisotropy along the stream direction (ie parallel to the direction of) the shrinkage, high strength, and at right angles to the direction of feed flow (ie, the vertical direction) shrinkage is small, low intensity . In addition, since the molding of plastic parts various parts of the packing density and uneven distribution, it is also uneven shrinkage. Errand shrinkage plastic parts prone to warping, deformation, cracks, especially in extrusion and injection molding is more obvious directionality. Therefore, mold design should be considered directional shrinkage of plastic parts according to the shape of the material flow direction shrinkage rate should be selected.
After the molding of plastic parts shrink, due to the molding pressure, shear stress, anisotropy, uneven density, uneven distribution of fillers, mold temperature unevenness, uneven hardening, plastic deformation and other factors. Causing a series of stress, when the viscous flow state can not disappear, so plastic parts under stress residual stress during molding. After releasing the stress tends to affect the balance and storage conditions, so that the residual stress changes after leaving the plastic parts shrink shrink again called. Usually plastic parts within 10 hours after releasing the greatest change in shape after 24 hours, but in the end to be stable for 30 to 60 days. After contraction than usual thermoplastics thermosetting big, big extrusion and injection molding of plastic than the pressure molding.
Sometimes plastic parts shrink after treatment according to the performance and process requirements, the need for post-forming heat treatment, post-treatment can also cause changes in the size of plastic parts. It should be considered after precision casting shrinkage and shrinkage after treatment errors and compensate the mold design.