In the molding process of plastics thermoplastics, due to the volume change of crystallization, the internal stress is strong, the residual stress in the plastic part is large, and the molecular orientation is strong, so the shrinkage rate is larger than that of the thermosetting plastic. The shrinkage rate is wide and the directivity is obvious. In addition, the shrinkage after shrinkage, annealing or humidity control after molding is generally larger than that of thermosetting plastic.
       When the plastic part is formed, the molten material contacts the surface of the cavity and the outer layer is immediately cooled to form a low-density solid outer casing. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with a large shrinkage. Therefore, the wall thickness, the slow cooling, and the high density layer thickness are large.
       In addition, the presence or absence of inserts and insert layouts and quantities directly affect the flow direction, density distribution and shrinkage resistance, so the characteristics of the plastic parts have a great influence on the shrinkage size and directionality.
       The factors such as the form, size and distribution of the feed port directly affect the flow direction, density distribution, pressure-preserving and shrinking action and molding time. The direct feed port and the feed port have a large cross section (especially thicker cross section), but the shrinkage is small but the directivity is large, and the feed port width and the short length are small. The shrinkage is large near the feed port or parallel to the flow direction.
       Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage is large. Especially for the crystallized material, the crystallinity is high and the volume changes greatly, so the shrinkage is larger. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic parts, which directly affects the amount of shrinkage and directionality of each part.
       In addition, maintaining pressure and time also has a large effect on shrinkage. When the pressure is large and the time is long, the shrinkage is small but the directivity is large. The injection pressure is high, the viscosity difference of the melt is small, the interlaminar shear stress is small, and the elastic rebound is large after demolding, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the directivity is small. Therefore, factors such as adjustment of mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of the plastic part.
       According to the shrinkage range of various plastics, the thickness and shape of the plastic part, the size and distribution of the feed port, the shrinkage rate of each part of the plastic part is determined empirically, and then the cavity size is calculated.
       For high-precision plastic parts and difficult to grasp the shrinkage rate, it is generally advisable to design the mold by the following method:
       1. Take a small shrinkage rate for the outer diameter of the plastic part, and take a larger shrinkage rate for the inner diameter to leave room for correction after the test.
       2. The test mold determines the form, size and molding conditions of the gating system.
       3. After the plastic parts to be treated are post-treated to determine the dimensional change (measurement must be made 24 hours after demolding).
       4. Correct the mold according to the actual shrinkage.
       5. Re-test the mold and change the process conditions slightly to correct the shrinkage value to meet the requirements of the plastic parts.