In production, when the plastic melt is injected into the mold cavity under high temperature and high pressure and molded under pressure, when the temperature drops, the melt cools and solidifies into a plastic part. The size of the plastic part is smaller than the size of the mold cavity, which is called shrink. The main reasons for shrinkage are as follows.
In plastic manufacturing, the cross-sectional dimensions of different mold gates are different. Large gates help increase cavity pressure, extend gate closing time, and facilitate more melt flow into the cavity, so the density of the plastic parts is also greater, thereby reducing the shrinkage rate, otherwise it will increase the shrinkage rate.
Changes in chemical structure during plastic manufacturing. Some plastics change their chemical structure during the molding process. For example, in thermosetting plastics, the resin molecule changes from a linear structure to a body-like structure. The volume of the body-like structure is greater than that of the linear structure, so its total volume shrinks and shrinks.
Thin-walled plastic parts with uniform wall thickness cool faster in the mold cavity, and the shrinkage rate tends to be the smallest after demolding. The longer the time for a thick plastic part with the same wall thickness to cool in the cavity, the greater the shrinkage after demolding. If the thickness of plastic parts is different, there will be a certain degree of shrinkage after demolding. In the case of such a sudden change in wall thickness, the shrinkage rate will also undergo a sudden change, resulting in greater internal stress.
Changes in residual stress. When plastic parts are molded, due to the influence of molding pressure and shear force, anisotropy, uneven mixing of additives and mold temperature, there are residual stresses in the molded plastic parts, and the residual stresses will gradually decrease and redistribute, resulting in plastic parts The re-shrinkage is generally called post-shrinkage.