What aspects should be paid attention to in the processing of injection molds
In plastic mold manufacturers, 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 plastic parts. The size of plastic parts is smaller than the size of the mold cavity, which is called shortening. The injection molding manufacturers think that the main reasons for the reduction are as follows.
When plastic injection molding is made, the sectional dimensions of different mold gates are different. Large gates help to add cavity pressure, extend the closing time of gates, and facilitate more melt to flow into the cavity. Therefore, the density of plastic parts is larger, thus reducing the shortening rate. Otherwise, the shortening rate will be added.
Change of chemical structure during plastic mold manufacturing. Some plastics change their chemical structure during molding. For example, in thermosetting plastics, the resin molecule changes from linear structure to body structure, and the volume mass of body structure is greater than that of linear structure, so its total volume is shortened, resulting in shortening. The cooling rate of thin-walled plastic parts with uniform wall thickness is faster in the mold cavity, and the shortening rate tends to be smaller after demoulding. The longer the cooling time of thick plastic parts with the same wall thickness in the cavity, the greater the reduction after demoulding. If the thickness of plastic parts is different, they will be shortened to a certain extent after demoulding. In the case of this sudden change in wall thickness, the shortening rate will also have a sudden change, resulting in greater internal stress.
Residual stress modification. During the molding of plastic parts, due to the influence of molding pressure, shear force, anisotropy, uneven mixing of additives and mold temperature, there is residual stress in the plastic parts after molding, and the residual stress will gradually decrease and redistribute, resulting in further shortening of the plastic parts, generally known as post shortening