What are the basic factors that affect the shrinkage of plastics?

1) The pressure of the molten material after entering the mold cavity is quite high, which increases the density of the molten material. Therefore, the greater the pressure inside the cavity, the smaller the shrinkage after molding. The shrinkage rate of amorphous and crystalline plastics 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 plastic parts will increase; On the other hand, due to the increase in temperature, the sealing time of the gate is prolonged, the amount of molten material entering the mold cavity increases, the density increases, and the shrinkage rate decreases. The result of the simultaneous action of these two factors is generally that the shrinkage rate decreases with the increase of temperature.

3) The form of the sprue directly affects the flow process during mold filling, and therefore also affects the size of the shrinkage rate, with the most important being the sprue cross-sectional size. The large cross-sectional size of the sprue helps to increase the cavity pressure and prolong the sprue sealing time, making it easier for more melt to enter the cavity, resulting in higher density and reduced shrinkage rate.

4) Lowering the mold temperature accelerates the cooling and solidification rate of the molten material on the wall of the mold cavity, causing the solidification layer on the wall to rapidly thicken and the plastic part to rapidly solidify into shape, resulting in a decrease in the shrinkage rate of the plastic part.

5) Crystalline plastic with thick plastic walls. The shrinkage rate increases with the increase of plastic part wall thickness, while in amorphous plastics, the change in shrinkage rate is different. The shrinkage rate of plastics such as ABS and PC is not affected by 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 increasing wall thickness.