During production, when plastic melt is injected into the mold cavity under high temperature and pressure and formed 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 shortening. The main reasons for shortening are as follows. When making plastic, the cross-sectional dimensions of different mold gates vary. A large gate helps to add cavity pressure, extend the sealing time of the gate, and facilitate more melt to flow into the cavity, resulting in a higher density of the plastic part and a decrease in shortening rate. Otherwise, shortening rate will be added
Changes in chemical structure during the production process of plastic molds. Some plastics may change their chemical structure during the molding process. For example, in thermosetting plastics, resin molecules change from a linear structure to a bulk structure, where the volume mass of the bulk structure is greater than that of the linear structure, resulting in a shortened total volume. Thin walled plastic parts with uniform wall thickness have a faster cooling rate in the mold cavity, and the shortening rate tends to be minimized after demolding. The longer the cooling time of thick plastic parts with the same wall thickness in the mold cavity, the greater the shortening after demolding. If the thickness of plastic parts is uneven, there will be a certain degree of shortening after demolding. In this case of sudden changes in wall thickness, the shortening rate will also undergo a sudden change, resulting in greater internal stress
Residual stress modification. During the molding of plastic parts, due to the influence of molding pressure and shear force, anisotropy, uneven mixing of additives, and mold temperature, there is residual stress in the formed plastic parts, which gradually decreases and redistributes, leading to further shortening of the plastic parts, commonly known as post shortening
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