During 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 that of the mold cavity, which is called shorten. The main reasons for shortening are as follows. When making plastic, the cross-sectional dimensions of different mold gates are different. The large gate helps to increase the cavity pressure, prolong the closing time of the gate, and facilitate more melt flow into the cavity, so the density of the plastic part is also greater, thereby reducing the shortening rate, otherwise it will increase the shortening rate.
Changes in the chemical structure of the plastic mold during the manufacturing process. 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 volumetric mass of the body-like structure is greater than that of the linear structure, so its total volume is shortened, resulting in shortening. Thin-walled plastic parts with uniform wall thickness are cooled faster in the mold cavity, and the shortening 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 shortening after demolding. If the thickness of the plastic part is different, there will be a certain degree of shortening after demolding. In the case of such a sudden change in wall thickness, the shortening rate will also change suddenly, resulting in greater internal stress.
Residual stress changes. 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 become smaller and re-spread, resulting in plastic parts The shortening again is generally called post-shortening.
Post time: Jul-19-2021