Shrinkage caused by plastic mold production

Plastic shrinkage refers to the percentage difference between the size of the plastic part at the molding temperature and the size after being removed from the mold and cooled to room temperature. It reflects the degree of shrinkage of the size of the plastic part after it is taken out of the mold and cooled, and the shrinkage rate of different polymer materials is different. In addition, the shrinkage rate of plastic has a great relationship with the shape of the plastic part, the complexity of the internal structure, and whether there are inserts.

There are six conditions that can cause shrinkage when producing plastic molds:

1) Thermal expansion and contraction of plastic. When the plastic mold is formed, the plastic raw material needs to be melted. At this time, the melting temperature reaches 200 to 300 degrees. The plastic raw material expands by heat. When the plastic mold is cooled, the temperature decreases, and its volume is bound to shrink.

2) Changes in chemical structure. For example, during the forming process of thermosets, the resin molecule changes from a linear structure to a body-shaped structure, and the volume mass of the body-shaped structure is larger than that of the linear structure, and its total volume becomes smaller, resulting in shrinkage.

3) Changes in residual stress. When the plastic mold is formed, due to the shearing force of the molding pressure, the anisotropy, the inhomogeneity of the additive mixing, and the mold temperature, there is residual stress in the molded plastic mold, and this residual stress will gradually It becomes smaller and redistributed, and as a result, the plastic mold shrinks again. This shrinkage is generally called post-shrinkage.

4) Gate section size. Different molds have different cross-section sizes. Large gates help increase cavity pressure and prolong gate closing time, facilitating more melt flow into the cavity, so the density of plastic parts is also higher, which causes shrinkage decrease the rate, otherwise it will increase the shrinkage rate.

5) Plastic varieties. Crystalline plastics PP and PA show larger shrinkage after ejecting and a wider range of shrinkage than crystalline plastics PC, PS, ABS. The reasons for the large shrinkage of crystalline plastics are: the influence of the thermal expansion coefficient of the plastic and the condensation of the melt when the crystal structure is formed.

6) Wall thickness of plastic mold. Thin-walled plastic molds with uniform thickness are cooled quickly in the mold cavity, and the shrinkage rate after injecting tends to be the smallest. Thick plastic molds with the same wall thickness will cool in the cavity for a long time, and the shrinkage rate after injecting will be large. If the wall thickness of the plastic mold is thick or thin, there will be different degrees of shrinkage after injecting. At such a sudden change in wall thickness, the shrinkage rate will also change suddenly, and this change will cause greater Internal stress.

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