The injection molding process setting should consider the shrinkage rate, fluidity, crystallization, heat sensitive plastics and easily hydrolyzed plastics, stress cracking and melt cracking, thermal properties and cooling rate, hygroscopic properties and other factors.
NO.1
Percentage of contraction
The form and calculation of thermoplastic molding shrinkage As mentioned earlier, the factors that affect the molding shrinkage of thermoplastics are as follows:
1.1 Plastic varieties
Due to the volume change from crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors in the thermoplastic molding process, the shrinkage rate is larger than that of thermoset plastics, the shrinkage range is wide and the directivity is obvious, and the shrinkage rate after the molding, annealing or humidity treatment is generally larger than that of thermoset plastics.
1.2 Characteristics of plastic parts
When forming, 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 plastics, the inner layer of plastic parts is slowly cooled to form a high-density solid layer with large shrinkage. Therefore, the thickness of the wall, the cooling is slow, and the high-density layer is thick. In addition, there is no insert and insert layout, the number of directly affect the direction of material flow, density distribution and shrinkage resistance, so the characteristics of plastic parts have a greater impact on the shrinkage size and directivity.
1.3 Feed port form, size and distribution
These factors directly affect the direction of material flow, density distribution, pressure holding and shrinkage and forming time. Direct feed port, feed port section is large (especially thick section) is reduced but the direction is large, feed port wide and short length is small direction. Close to the feed port or parallel to the direction of the feed flow, the shrinkage is large.
1.4 Forming conditions
Mold temperature is high, melt material cooling slow, high density, large shrinkage, especially for crystallizing material because of high crystallinity, volume change, so the shrinkage is greater. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic 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 contraction, and the pressure is large and the time is long, but the direction is large.
The injection pressure is high, the melt material viscosity difference is small, the interlayer shear stress is small, the elastic rebound after demoulding is large, so the shrinkage can also be reduced appropriately, the material temperature is high, the shrinkage is large, but the direction is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage of plastic parts.
During
Mold Design, according to the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the feed port form size and distribution, the shrinkage rate of each part of the plastic parts is determined according to experience, and then the cavity size is calculated. For high-precision plastic parts and difficult to grasp the shrinkage rate, it is generally appropriate to use the following methods to design the mold:
① The shrinkage rate of the outer diameter of the plastic part is smaller, and the shrinkage rate of the inner diameter is larger, so as to leave room for correction after the test mold.
② Mold test to determine the form, size and forming conditions of the pouring system.
③ The plastic parts to be post-treated are determined by post-treatment to determine the size change (the measurement must be 24 hours after demoulding).
④ Correct the mold according to the actual shrinkage.
⑤ Re-test the mold and can appropriately change the process conditions slightly modify the shrinkage value to meet the requirements of plastic parts.
NO.2
mobility
The flow size of thermoplastics can generally be analyzed from a series of indices such as molecular weight size, melting index, Archimedes helix flow length, apparent viscosity and flow ratio (process length/plastic part wall thickness).
Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melting index, long spiral flow length, small viscosity, flow ratio is large, the flow is good, the same name of the plastic must check its instructions to determine whether its fluidity is suitable for injection molding. According to the mold design requirements, the fluidity of commonly used plastics can be roughly divided into three categories:
① Good fluidity of
PA, PE, PS, PP, CA, poly (4) methyloptene;
② Medium fluidity polystyrene series resins (such AS ABS, AS), PMMA, POM, polyphenyl ether;
③ Poor fluidity PC, hard PVC, polyphenyl ether, polysulfone, polyaromatic sulfone, fluorine plastics.
The fluidity of various plastics also changes due to various molding factors, and the main influencing factors are as follows:
① Temperature material temperature is high, the fluidity increases, but different plastics are also different, PS (especially impact resistance and MFR value is high), PP, PA, PMMA, modified polystyrene (such AS ABS, AS), PC, CA and other plastics fluidity with the temperature changes greatly. For PE and POM, the temperature increase or decrease has little effect on its fluidity. Therefore, the former adjusts the temperature to control the fluidity when forming.
When the pressure of injection molding increases, the molten material is subjected to large shear effect, and the fluidity also increases, especially PE and POM are more sensitive, so the injection molding pressure is adjusted to control the fluidity.
③ The form, size, layout, cooling system design, molten material flow resistance (such as profile finish, material passage section thickness, cavity shape, exhaust system) and other factors directly affect the actual flow of molten material in the cavity, where the molten material to reduce the temperature, increase the flow resistance, the flow will be reduced.
Mold design should be based on the fluidity of the plastic used, choose a reasonable structure. When forming, it can also control the material temperature, mold temperature, injection pressure, injection speed and other factors to properly adjust the filling situation to meet the molding needs.
NO.3
crystallinity
Thermoplastics can be divided into two categories: crystalline plastics and non-crystalline (also known as amorphous) plastics according to the absence of crystallization when they condense.
The so-called crystallization phenomenon is that when the plastic is melted from the state to condensation, the molecules move independently and are completely in an out-of-order state, becoming a phenomenon in which the molecules stop moving freely, press a slightly fixed position, and have a tendency to make the molecular arrangement a regular model.
As the standard for judging the appearance of these two types of plastics, the transparency of the thick wall plastic parts is determined, the general crystalline material is opaque or translucent (such as POM, etc.), and the amorphous material is transparent (such as PMMA, etc.). However, there are also exceptions, such as poly (4) methyl psilene is a crystalline plastic with high transparency, and ABS is an amorphous material but not transparent.
In the mold design and selection of injection molding machine should pay attention to the following requirements and precautions for crystallized plastics:
① The heat required for the material temperature to rise to the molding temperature is much, and equipment with large plasticizing capacity should be used.
② When cooling back, the heat is released, and it should be fully cooled.
③ The proportion difference between the molten state and the solid state is large, the forming shrinkage is large, and the shrinkage and porosity are easy to occur.
④ Fast cooling, low crystallinity, shrinkage, high transparency. The crystallinity is related to the wall thickness of the plastic parts, and the wall thickness is slow cooling, high crystallinity, large shrinkage and good physical property. Therefore, the mold temperature of the crystalline material should be controlled according to the requirements.
⑤ Significant anisotropy and large internal stress. The uncrystallized molecules after demoulding tend to continue crystallizing, and are in an energy imbalance state, which is prone to deformation and warping.
⑥ The crystallization temperature range is narrow, and it is easy to inject unmelted material into the mold or
Block the feed port.
