During the molding process of plastic thermoplastics, there is still a volume change due to crystallization, internal stress is strong, residual stress in the plastic parts is frozen, and the molecular orientation is strong. Therefore, compared with thermosetting plastics, the shrinkage is high. , Wide range of shrinkage, obvious direction, In addition, shrinkage after molding, shrinkage after annealing or humidity treatment are generally larger than thermoset plastic.
When the plastic parts are molded, the outer layer of the molten material contacts the cavity surface and immediately cools to form a low-density solid shell. As the thermal conductivity of the plastic is poor, 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, Slow cooling, high density thick layer shrinks.
In addition, whether there are inserts and inserts layout, the number has a direct impact on the material flow direction, density distribution and shrinkage resistance, etc. Therefore, the characteristics of plastic parts have a greater impact on shrinkage size and directionality.
Feeding port form, size, distribution These factors directly affect the direction of the material flow, density distribution, pressure packing action and molding time. Direct feed port, large inlet section (especially thick section) with small shrinkage but direction Larger, smaller feed port width and shorter directionality. Close to the feed port or in parallel with the direction of the material is a large contraction.
Molding conditions The mold temperature is high, the melt cooling is slow, the density is high, and the shrinkage is large. Especially for the crystallization material, the crystallinity is high and the volume change is large, 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. , directly affect the size and direction of the contraction of each part.
In addition, holding pressure and time also have a great impact on shrinkage, and the pressure is large, but when the time is long, the shrinkage is small but the directionality is large. The injection pressure is high, the melt viscosity is small, the interlaminar shear stress is small, and elastic rebound after demolding Large, so the shrinkage can also be reduced, the material temperature is high, shrinkage is large, but the directionality is small. Therefore, when adjusting the mold temperature, pressure, injection speed and cooling time and other factors can also be appropriate to change the shrinkage of plastic parts.
The mold design depends on the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the size and distribution of the feed port, the contraction rate of each part of the plastic parts according to experience, and the size of the cavity.
For high-precision plastic parts and difficult to grasp shrinkage, it is generally advisable to use the following method to design the mold:
1. Take a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to allow room for correction after the trial.
2, try to determine the gating system form, size and molding conditions.
3, post-processing of plastic parts after processing to determine the size of the change (measurement must be 24 hours after demoulding).
4, according to the actual contraction correction mold.
5, retry the mold and change the process conditions appropriately to slightly correct the shrinkage value to meet the plastic parts requirements.