Plastic varieties in the molding process of thermoplastic, due to the volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation and other factors, compared with thermosetting plastics, the shrinkage rate is larger, the shrinkage range is wide, and the directionality is obvious. In addition, the shrinkage The shrinkage after annealing or humidity control treatment is generally larger than that of thermosetting plastics.

When the plastic part is molded, 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 plastic, the inner layer of plastic parts cools slowly to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling and high-density layer thickness shrink greatly. In addition, the presence or absence of inserts, the layout and quantity of inserts directly affect the material flow direction, density distribution and shrinkage resistance, so the characteristics of plastic parts have a great impact on the shrinkage size and direction.
The form, size and distribution of feed inlet directly affect the material flow direction, density distribution, pressure maintaining and feeding effect and forming time. If the section of the direct feed port and the feed port is large (especially if the section is thick), the shrinkage is small but the directionality is large, and if the width and length of the feed port are short, the directionality is small. If it is close to the feed port or parallel to the material flow direction, the shrinkage is large.
Molding conditions: high mold temperature, slow cooling of molten material, high density and large shrinkage. Especially for crystalline material, the shrinkage is greater due to high crystallinity and large volume change. The mold temperature distribution is also related to the internal and external cooling of plastic parts and the uniformity of density, which directly affects the size and direction of shrinkage of each part. In addition, holding pressure and time also have a great impact on contraction. Those with high pressure and long time have small contraction but large directionality.
The injection pressure is high, the viscosity difference of molten material is small, the interlayer shear stress is small, and 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 directionality is small. Therefore, the shrinkage of plastic parts can also be appropriately changed by adjusting mold temperature, pressure, injection speed and cooling time.
During mold design, the shrinkage rate of each part of the plastic part shall be determined according to experience according to the shrinkage range of various plastics, wall thickness and shape of plastic parts, form, size and distribution of feed inlet, and then the cavity size shall be calculated. For high-precision plastic parts and when it is difficult to master the shrinkage, the following methods should be used to design the mold:
① The form, size and forming conditions of gating system are determined by mold test.
② The size change of plastic parts to be post treated shall be determined after post-treatment (measurement must be 24 hours after demoulding).
③ Correct the die according to the actual shrinkage.
④ Try the mold again, and change the process conditions appropriately, and slightly modify the shrinkage value to meet the requirements of plastic parts.
The fluidity of thermoplastics can generally be analyzed from a series of indexes such as molecular weight, melt index, Archimedean spiral flow length, apparent viscosity and flow ratio (process length / plastic part wall thickness).
If the molecular weight is small, the molecular weight distribution is wide, the molecular structure regularity is poor, the melt index is high, the screw flow length is long, the apparent viscosity is small, and the flow ratio is large, the fluidity is good. For plastics with the same product name, the instructions must be checked to determine whether their fluidity is suitable for injection molding.According to the mold design requirements, the fluidity of common plastics can be roughly divided into three categories:
① Good fluidity PA, PE, PS, PP, CA, poly (4) methylene:
② Polystyrene series resins with medium fluidity (such as ABS, as), PMMA, POM and polyphenylene ether;
③ Poor fluidity PC, hard PVC, polyphenylene ether, polysulfone, fluoroplastics.
The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows:
① When the temperature is high, the fluidity increases, but different plastics also have differences. The fluidity of PS (especially those with high impact resistance and MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, as), PC, Ca and other plastics changes greatly with temperature. For PE, POM and, the increase or decrease of temperature has little effect on their fluidity. Therefore, the former should adjust the temperature to control the fluidity.
② With the increase of injection pressure, the molten material will be greatly sheared and the fluidity will also increase, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding.
③ Mold structure, gating system form, size, layout, cooling system design, molten material flow resistance (such as surface finish, material channel section thickness, cavity shape, exhaust system) and other factors directly affect the actual fluidity of molten material in the cavity. If the molten material is prompted to reduce the temperature and increase the fluidity resistance, the fluidity will be reduced.
Reasonable structure shall 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 amorphous (also known as amorphous) plastics. The so-called crystallization phenomenon is a phenomenon that the molecules move independently and completely in an disordered state from the melting state to the condensation state of plastics, and become a phenomenon that the molecules stop moving freely, press a slightly fixed position, and have a tendency to make the molecular arrangement become a normal model.
As the appearance standard for judging these two types of plastics, it can be determined by the transparency of thick wall plastic parts of plastics. Generally, crystalline materials are opaque or translucent (such as POM), and amorphous materials are transparent (such as PMMA). However, there are exceptions. For example, poly (4) methylene is a crystalline plastic with high transparency, and ABS is an amorphous material but not transparent.
The following requirements and precautions for crystalline plastics shall be noted during mold design and selection of injection molding machine:
① More heat is required for the material temperature to rise to the forming temperature, so the equipment with large plasticizing capacity should be used.
② The heat released during cooling and recycling is large, so it should be fully cooled.
③ The specific gravity difference between molten state and solid state is large, the molding shrinkage is large, and shrinkage and porosity are easy to occur.
④ Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic part. The wall thickness has the advantages of slow cooling, high crystallinity, large shrinkage and good physical properties. Therefore, the mold temperature of crystalline material must be controlled as required.
⑤ Significant anisotropy and large internal stress. After demoulding, the non crystallized molecules tend to continue to crystallize, are in an energy imbalance state, and are prone to deformation and warpage.
⑥ The crystallization temperature range is narrow, and it is easy to inject unmelted material into the die or block the feed inlet.
Heat sensitivity refers to the tendency of discoloration, degradation and decomposition of some plastics which are more sensitive to heat. When they are heated for a long time at high temperature or the section of the feed inlet is too small and the shear effect is large, the material temperature increases. Plastics with this characteristic are called heat sensitive plastics.
Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polytrichloroethylene fluoride, etc. Thermal sensitive plastics produce monomer, gas, solid and other by-products during decomposition, especially some decomposition gases have stimulation, corrosion or toxicity to human body, equipment and molds. Therefore, attention should be paid to mold design, selection of injection molding machine and molding. Screw injection molding machine should be selected. The section of gating system should be large. The mold and barrel should be chrome plated without angular stagnation. The molding temperature must be strictly controlled and stabilizer must be added to the plastic to weaken its thermal sensitivity.
Even if some plastics (such as PC) contain a small amount of water, they will decompose under high temperature and high pressure. This property is called easy hydrolysis, which must be heated and dried in advance.
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