In the forming process of thermoplastic plastics, there are many factors, such as volume change from crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation, etc., so compared with thermosetting plastics, the shrinkage rate is larger, the range of shrinkage rate is wide, and the directionality is obvious. In addition, the shrinkage rate after forming, annealing or humidity regulating treatment is generally higher than that of thermosetting plastics Sex plastic is big.
The outer layer of the melt contacting with the cavity surface is cooled immediately to form a low density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of plastic parts is cooled slowly to form a high density solid layer with large shrinkage. Therefore, those with thick wall, slow cooling and high-density layer will shrink greatly. In addition, whether there are inserts and the layout and quantity of inserts directly affect the direction of material flow, density distribution and shrinkage resistance, so the characteristics of plastic parts have a great influence on the shrinkage size and directionality.
The shape, size and distribution of the feeding port directly affect the direction of the material flow, the density distribution, the effect of pressure maintaining and feeding, and the forming time. If the cross-section of the direct feed inlet and the feed inlet is large (especially those with thick cross-section), the shrinkage is small but the directivity is large; if the width and length of the feed inlet are short, the directivity is small. If it is close to the feed inlet or parallel to the feed flow direction, the shrinkage will be large.
The mold temperature is high, the melt cooling is slow, the density is high, and the shrinkage is large. Especially for the crystalline material, the shrinkage is larger because of the high crystallinity and the large volume change. The distribution of mold temperature is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the shrinkage amount and direction of each part. In addition, holding pressure and time also have a great influence on shrinkage, while those with high pressure and long time have a small shrinkage but a large directionality.
The injection pressure is high, the viscosity difference of the melt is small, the shearing stress between layers is small, and the elastic rebound is large after demoulding, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the direction is small. Therefore, the shrinkage of plastic parts can be changed by adjusting the temperature, pressure, injection speed and cooling time.
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 according to experience, and then the cavity size is calculated. For high-precision plastic parts and when it is difficult to master the shrinkage rate, the following methods should generally be used to design the mold.
① The form, size and forming conditions of the pouring system are determined by the mold test.
② The size change of the plastic parts to be post treated shall be determined after post-treatment (the measurement must be 24 hours after demoulding).
③ Correct the mold according to the actual shrinkage.
④ The shrinkage value can be slightly modified to meet the requirements of the plastic part.
Generally, the fluidity of thermoplastic can be analyzed from a series of indexes, such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (flow length / plastic wall thickness).
Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long screw flow length, small apparent viscosity, high flow ratio, good fluidity. For the plastic of the same product name, the instruction manual must be checked to determine whether its fluidity is suitable for injection molding. According to the requirements of mold design, the fluidity of common plastics can be roughly divided into three categories:
① Good fluidity of PA, PE, PS, PP, CA, poly (4) methyls:
② Fluidity medium polystyrene series resin (such as ABS, as), PMMA, POM, polyphenylene ether;
③ Poor fluidity of PC, rigid PVC, polyphenylene ether, polysulfone, polysulfone, fluoroplastics.
The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:
① The fluidity of PS (especially impact resistant and MFR value higher), PP, PA, PMMA, modified polystyrene (such as ABS, as), PC, Ca and other plastics changes greatly with temperature. For PE, POM, the increase or decrease of temperature has little effect on its fluidity. So the former should adjust the temperature to control the fluidity.
② When the injection pressure increases, the melt will be sheared and the fluidity will increase, especially the PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity.
③ The form, size, layout, cooling system design, flow resistance (such as surface finish, material channel section thickness, cavity shape, exhaust system) and other factors of the mold structure pouring system directly affect the actual fluidity of the molten material in the cavity. If the temperature of the molten material is reduced and the flow resistance is increased, the fluidity will be reduced.
The reasonable structure should be selected according to the fluidity of the plastic used in the mold design. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be controlled to properly adjust the filling situation to meet the molding needs.
Crystalline thermoplastics can be divided into crystalline plastics and non crystalline (also called amorphous) plastics according to their no crystallization phenomenon during condensation. The so-called crystallization phenomenon refers to the phenomenon that when the plastic changes from melting state to condensation, the molecules move independently and are completely in an disordered state, and become molecules stop free movement, according to a slightly fixed position, and there is a tendency to make the molecular arrangement become a normal model.
As the appearance standard to distinguish these two kinds of plastics, it can be determined by the transparency of thick wall plastic parts. Generally, the crystalline material is opaque or translucent (such as POM, etc.), and the amorphous material is transparent (such as PMMA, etc.). However, there are some exceptions, such as poly (4) methyls, which are crystalline plastics, have high transparency, while ABS is amorphous but not transparent.