Factors of Thermoplastic Molding Shrinkage

1. There is also a volume change due to crystallization during the molding process of thermoplastics. The internal stress is strong and the residual stress frozen in the plastic part has a strong orientation of macromolecules and other factors. Therefore, compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage rate range is wide, and the directionality is obvious. In addition, the shrinkage after molding, and the shrinkage after annealing or humidity conditioning are generally larger than those of thermosetting plastics.

2. When the plastic part is formed, the molten material contacts the surface of the cavity, and the outer layer immediately cools to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling, and high-density layer thickness will shrink greatly.

In addition, the presence or absence of inserts and the layout and quantity of the inserts directly affect the flow direction, density distribution, and shrinkage resistance, so the characteristics of the plastic parts have a greater impact on the shrinkage size and directionality.

3. Factors such as the form, size, and distribution of the feeding port directly affect the direction of the material flow, the density distribution, the pressure-retaining effect, and the molding time. The direct feeding port and the feeding port with a large cross-section (especially thicker cross-section) have small shrinkage but large directionality, and the wide and short feeding port has small directionality. Close to the feed port or parallel to the direction of the material flow, the shrinkage is large.

4. Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large, especially for the crystalline material, the shrinkage is larger due to the high crystallinity and the large volume change. The mold temperature distribution is also related to the cooling and density uniformity inside and outside the plastic part, which directly affects the shrinkage and direction of each part.

In addition, the holding pressure and time also have a great influence on the contraction, and the contraction is small but the direction is large when the pressure is high and the time is long. 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 appropriately reduced, the material temperature is high, the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time and other factors during molding can also appropriately change the shrinkage of the plastic part.

5. When designing the mold, according to the shrinkage range of various plastics, the wall thickness, and shape of the plastic part, the form, size, and distribution of the feeding port, the shrinkage rate of each part of the plastic part is determined according to experience, and then the cavity size is calculated.

For high-precision plastic parts and when it is difficult to grasp the shrinkage rate, the following methods should be used to design the mold:

The mold trial determines the form, size, and molding conditions of the gating system.

The plastic parts to be post-treated are post-treated to determine the dimensional changes (measurement must be taken after 24 hours after demolding).

Correct the mold according to the actual shrinkage.

Retry the mold and change the process conditions to slightly modify the shrinkage value to meet the requirements of the plastic part.