The dimensional shrinkage of plastic after it is taken out of the mold and cooled to room temperature is called shrinkage. Because this shrinkage is not only caused by the thermal expansion and cold shrinkage of the resin itself, but also related to various molding factors, the shrinkage of plastic parts after molding is called molding shrinkage.

Shrinkage of plastic parts has a great impact on product quality, mold design and manufacturing. Therefore, we should deeply understand the factors that affect the shrinkage of plastic parts. Generally speaking, there are the following points.

1) Molding pressure The pressure of the melt entering the cavity is quite high. This pressure increases the density of the melt. Therefore, the greater the pressure in the cavity, the smaller the shrinkage after molding. With the increase of internal pressure, the shrinkage of amorphous plastics and crystalline plastics decreases linearly and in a curve shape

2) Injection temperature When plastic changes from solid to liquid, it is caused by heating. The volume of plastic raw materials will expand and the shrinkage of plastic parts will increase during the heating process; On the other hand, due to the increase of temperature, the gate closing time is prolonged, the melt entering the cavity is increased, the density is increased, and the shrinkage is reduced. The result of both actions is usually that the shrinkage decreases with the increase of temperature.

3) The gate shape and gate type directly affect the flow process in the filling process, and therefore also affect the shrinkage, mainly the gate section size. The gate with large section size helps to increase the cavity pressure, extend the gate closing time, and help more melt to enter the cavity, so the density is also larger and the shrinkage is reduced.

4) The mold temperature reduces the mold temperature, so that the melt cooling and solidification speed of the mold cavity wall is accelerated, the solidification layer of the wall is rapidly thickened, so that the plastic part can be rapidly solidified and formed, and the shrinkage of the plastic part is reduced.

5) The wall thickness of plastic parts is crystalline plastic. The shrinkage rate increases with the increase of the wall thickness of the plastic part, but the change of the shrinkage rate of the amorphous plastic is different. The shrinkage of ABS, PC and other plastics is not affected by the wall thickness; The shrinkage of PE, San and other plastics increases with the increase of wall thickness; The shrinkage of HPVC decreased with the increase of wall thickness.

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