Factors Affecting Injection Molding Process (Part 1)

During the molding process of thermoplastic plastics, due to the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, and strong molecular orientation, the shrinkage rate is larger compared to thermosetting plastics, with a wider range of shrinkage rates and obvious directionality. In addition, the shrinkage rate after molding, annealing, or humidity control treatment is generally higher than that of thermosetting plastics.

During the molding process of plastic parts, the molten material and the outer layer in contact with the surface of the mold cavity immediately cool down to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic slowly cools down to form a high-density solid layer with large shrinkage. So those with thick walls, slow cooling, and high-density layers will shrink more. In addition, the presence or absence of inserts, as well as the layout and quantity of inserts, directly affect the direction of material flow, density distribution, and shrinkage resistance. Therefore, the characteristics of plastic parts have a significant impact on the size and directionality of shrinkage.

The form, size, and distribution of the feed inlet directly affect the direction of the material flow, density distribution, pressure holding and shrinking effect, and forming time. If the feed inlet is directly fed or has a large cross-section (especially thick cross-section), the shrinkage is small but the directionality is large. If the feed inlet is wide and short, the directionality is small. Those close to the feed inlet or parallel to the direction of the material flow experience greater shrinkage.

The molding conditions include high mold temperature, slow cooling of molten material, high density, and large shrinkage, especially for crystalline materials due to high crystallinity and large volume changes, resulting in even greater shrinkage. The temperature distribution of the mold is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the size and directionality of the shrinkage of each part. In addition, maintaining pressure and time also have a significant impact on contraction, with high pressure and long duration resulting in smaller but more directional contraction.

The injection pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound after demolding is large, so the shrinkage can also be reduced appropriately. The material temperature is high and the shrinkage is large, but the directionality is small. Therefore, adjusting factors such as mold temperature, pressure, injection speed, and cooling time during molding can also appropriately change the shrinkage of plastic parts.