Injection molding is an engineering technique that deals with the transformation of plastics into products that are useful and retain their original properties. However, the shrinkage of plastic parts is susceptible to many factors.
The following factors affect the shrinkage of thermoplastics for injection molding services.
Plastic species
Characteristics of plastic parts
Receiving port
Molding conditions
Thermoplastic molding process due to the existence of crystallization of volume changes, internal stress, frozen in the injection molded parts of residual stress, molecular orientation, and other factors, so compared with thermosetting plastics, the shrinkage rate is larger, a wide range of shrinkage rate, directional obvious, in addition to the molding after the.
During molding, the molten material is immediately cooled to form a low-density solid shell in contact with the outer layer of the cavity surface. Since the difference in thermal conductivity of the plastic, injection-molded to make the inner member is cooled slowly to form a high-density large shrinkage solid layer. Therefore, the wall thickness, slow cooling, the high-density layer is thick shrinkage. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the direction of material flow, density distribution, and shrinkage resistance, so the characteristics of the injection molded parts have a greater impact on the size and direction of shrinkage.
Inlet form, size, distribution of these factors directly affect the direction of material flow, density distribution, pressure-holding and shrinkage effect, and molding time. Direct feed port, feed port cross-section large (especially thicker cross-section) is small shrinkage but directional, feed port wide and short length is small directional. The shrinkage is large for those close to the inlet or parallel to the direction of material flow.
The high temperature of injection mold, slow cooling of molten material, high density, large shrinkage, especially for crystalline material due to high crystallinity, volume change, so the shrinkage is greater. The mold temperature distribution is also related to the cooling and density uniformity inside and outside the injection molded parts, which directly affects the size and direction of the shrinkage of each part. In addition, the holding pressure and time also have a greater impact on the shrinkage, with high pressure and long time, the shrinkage is small but directional. High injection pressure, molten material viscosity difference is small, the interlayer shear stress is small, the elastic jump after demolding, so the shrinkage can also be moderately 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 during molding can also change the shrinkage of injection molded parts.
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