According to much experience in plastic resin molding industry, Zhejiang Ceeto Mold Co.,Ltd. know much about how to inject high level plastic products, there are three key points for choosen plastic raw material for our high quality plastic injection molds during molding process.
The factors that affect the shrinkage of thermoplastics are as follows:
1.1 Plastic varieties Thermoplastics in the molding process due to the volume change from crystallization, strong internal stress, large residual stress frozen in the plastic parts, strong molecular orientation and other factors, so compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide, and the direction is obvious. In addition, the shrinkage rate after the shrinkage, annealing or humidification treatment is generally larger than that of thermosetting plastics.
1.2 Characteristics of plastic parts During molding, 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, distribution These factors directly affect the material flow direction, density distribution, pressure holding and shrinkage effect 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 Molding conditions The mold temperature is high, the melt material cooling slow, high density, large shrinkage, especially for the crystallization material is due to high crystallinity, large 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.
Item2- Flow avalibility
a) 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:
Good fluidity PA, PE, PS, PP, CA, poly (4) methyloptene;
Medium fluidity polystyrene series resins (such AS ABS, AS), PMMA, POM, polyphenyl ether;
Poor fluidity PC, hard PVC, polyphenyl ether, polysulfone, polyarylsulfone, fluorine plastics.
b) The fluidity of various plastics also varies due to various molding factors, and the main influencing factors are as follows:
When the material temperature is high, the fluidity increases, but different plastics are also different, and the fluidity of PS(especially the impact resistance type and the MFR value is high), PP, PA, PMMA, modified polystyrene (such AS ABS, AS), PC, CA and other plastics changes greatly with temperature. 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 is also increased, especially PE and POM are more sensitive, so the injection molding pressure is adjusted to control the fluidity.
Mold structure gating system form, size, layout, cooling system design, melt flow resistance (such as mold surface finish, material channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual flow of molten material in the cavity, where the melt 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.
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.
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