What are the key factors affecting the shrinkage of plastics

The characteristic of plastic that undergoes dimensional shrinkage after being removed from the mold and cooled to room temperature is called shrinkage. Because this shrinkage is not only caused by the thermal expansion of the resin itself, but also related to various molding factors, the shrinkage of molded plastic parts is called molding shrinkage.
The shrinkage of plastic parts has a significant impact on product quality, mold design and manufacturing. Therefore, it is necessary to have a deep understanding of the factors that affect the shrinkage rate of plastic parts, generally including the following.
1) The forming pressure of the melt after entering the mold cavity is quite high, which increases the density of the melt. Therefore, the greater the pressure in the mold cavity, the smaller the shrinkage after forming. The shrinkage rate of non crystalline plastic and crystalline plastic decreases in a straight and curved shape with the increase of internal pressure, respectively
2) Injection molding temperature: When plastic changes from solid to liquid, it is caused by heating. During the heating process, the volume of plastic raw materials will expand and the shrinkage rate of the plastic part will increase; On the other hand, due to the increase in temperature, the sealing time of the gate is prolonged, and the amount of melt entering the mold cavity increases, resulting in an increase in density and a decrease in shrinkage rate. The result of both actions is generally that the shrinkage rate decreases with the increase in temperature.
3) The form and shape of the gate directly affect the flow process during mold filling, thus also affecting the size of the shrinkage rate, with the most important being the cross-sectional size of the gate. The large cross-sectional size of the gate helps to increase the cavity pressure and extend the gate closure time, making it easier for more melt to enter the cavity, resulting in a higher density and a lower shrinkage rate.
4) The decrease in mold temperature leads to a faster cooling and solidification rate of the melt on the mold cavity wall, and the solidification layer on the wall rapidly thickens, resulting in a rapid solidification of the plastic part and a decrease in its shrinkage rate.
5) Crystalline plastic with thick wall thickness. The shrinkage rate increases with the increase of the wall thickness of the plastic part, while in non crystalline plastics, the changes in shrinkage rate are different. The shrinkage rate of plastics such as ABS and PC is not affected by the wall thickness; The shrinkage rate of plastics such as PE and SAN increases with the increase of wall thickness; The shrinkage rate of HPVC decreases with the increase of wall thickness.