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After the plastic part is taken out from the mold and cooled to room temperature, the dimensional shrinkage is called shrinkage. Since shrinkage is not only the thermal expansion and contraction of the resin itself, but also related to various forming factors, the shrinkage of the plastic part after forming should be referred to as forming shrinkage.
Forming shrinkage is mainly manifested in the following aspects:
1. The thread size shrinkage of plastic parts is shrinkage due to thermal expansion and contraction, elastic recovery and plastic deformation of the plastic parts during demoulding, and the size of the plastic parts is reduced after cooling to room temperature. Therefore, the cavity must be compensated for design. .
2. When the shrinkage direction is formed, the molecules are arranged in the direction, so that the plastic parts are anisotropic, and the shrinkage is large and the strength is high along the flow direction (ie, the parallel direction), and the shrinkage is small in the direction perpendicular to the flow direction (ie, the vertical direction). Low strength. In addition, due to the uneven density of the various parts of the plastic part and the uneven distribution of the filler during the forming, the shrinkage is uneven. The difference in shrinkage causes the plastic parts to be warped, deformed, and cracked, especially in extrusion and injection molding. Therefore, when designing the mold, the shrinkage direction should be considered. The shrinkage rate should be selected according to the shape of the plastic part and the direction of the flow.
3. After forming the shrinkage plastic parts, due to factors such as forming pressure, shear stress, anisotropy, uneven density, uneven distribution of filler, uneven mold temperature, uneven hardening, plastic deformation, etc., a series of The effect of stress cannot be completely lost in the viscous flow state, so there is residual stress when the plastic part is formed under stress. After demolding, due to the influence of stress tending balance and storage conditions, the residual stress changes and the re-shrinkage of the plastic part is called post-shrinkage. Generally, the plastic parts change the most within 10 hours after demolding, and basically set after 24 hours, but it is stable for 30-60 days. Generally, the post-shrinkage of thermoplastics is greater than that of thermosets, and extrusion and injection molding are larger than compression molding.
4, post-treatment shrinkage sometimes plastic parts according to performance and process requirements, heat treatment after forming, after processing will also lead to changes in the size of plastic parts. Therefore, the high-precision plastic parts of the mold design should consider the error of post-shrinkage and post-processing shrinkage and compensate.