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Injection molding

Introduction: The injection molding process mainly includes four stages: filling-holding pressure-cooling-demolding. These four stages directly determine the quality of the product. It is a complete continuous process. At the same time, there are many Factors. This article shares the seven influencing factors of the injection molding process:

一:shrinkage rate

The form and calculation of the thermoplastic shrinkage rate is as described above. The factors that affect the thermoplastic shrinkage rate are as follows: 1. Due to the volume change caused by the crystallization of the plastic variety during the thermoplastic molding process, the internal stress is strong and freezes in the plastic parts Compared with freezing and thermosetting plastics, the residual stress is large and the molecular orientation is strong. Therefore, the shrinkage rate is large, the shrinkage rate range is wide, and the directivity is obvious. In addition, the shrinkage after molding, annealing or humidity adjustment is usually greater than that of thermosetting plastics. 2. Characteristics of plastic parts When the molten material contacts the surface of the cavity, the outer layer is immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner layer of the plastic part is slowly cooled to form a high-density solid layer with a large shrinkage rate. Therefore, the wall thickness, slow cooling, and high-density layer thickness are further reduced. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the flow direction, density distribution, and shrinkage resistance. Therefore, the characteristics of plastic parts have a greater impact on shrinkage size and directionality. 3. The form, size and distribution of the feed port directly affect the flow direction of the material, density distribution, pressure holding and shrinking effects and molding time. The direct feed port and the larger feed port section (especially the thicker feed port) have a smaller shrinkage rate, but a greater directionality, and a shorter feed port width and shorter length have less directivity. The side close to the inlet or parallel to the direction of material flow shrinks greatly. 4. Molding conditions The mold temperature is high, the molten material cools slowly, the density is high, and the shrinkage rate is large. Especially for crystalline materials, due to high crystallinity and large volume changes, the shrinkage rate is greater. The temperature distribution of the mold is also related to the internal and external cooling of the plastic parts and the density uniformity, which directly affects the shrinkage and directionality of each part. In addition, maintaining pressure and time also have a great effect on contraction. When the pressure is large and the time is long, the shrinkage is small but the directionality is large. The injection pressure is high, the viscosity difference of the molten material is small, the shear stress between the layers is small, and the elastic rebound is large after demolding. Therefore, the shrinkage rate can also be reduced appropriately. The temperature of the material is high, the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, pressure, injection speed and cooling time during molding can also appropriately change the shrinkage rate of the plastic parts.
According to the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, the size and distribution of the inlet, the shrinkage rate of each part of the plastic parts, the empirical determination, and then the cavity size calculation. For high-precision plastic parts and it is difficult to grasp the shrinkage rate, the mold should generally be designed in the following way: ① For the outer diameter of the plastic part, the shrinkage rate is small, and for the inner diameter, the shrinkage rate is large, so that it is left after the trial Room for correction. ②The trial mold determines the form, size and molding conditions of the pouring system. ③ Post-process the plastic parts to be post-processed to determine the dimensional change (must be measured 24 hours after demolding) ④ Modify the mold according to the actual shrinkage rate. ⑤ Try the mold again, and change the processing conditions appropriately to slightly change the shrinkage value to meet the requirements of plastic parts.
二: liquidity

1. The fluidity of thermoplastics can generally be analyzed from a series of indexes such as molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (process length / plastic wall thickness). Small molecular weight, wide molecular weight distribution, poor molecular structure regularity, high melt index, long spiral flow length, low viscosity, and large flow ratio, good fluidity. For plastics of the same name, the instructions must be checked to determine whether the 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: ①Good fluidity: PA, PE, PS, PP, CA, poly (4) methylpentene; ② Medium fluidity: polystyrene series Resin (such as ABS, AS), PMMA, POM, polyphenylene ether; ③ Poor fluidity: PC, hard PVC, polyphenylene ether, polysulfone, polyarylsulfone, fluoroplastic.
2. The fluidity of various plastics also changes due to various molding factors. The main influencing factors are as follows: ①High temperature material temperature increases fluidity, but different plastics also have their own differences, PS (especially impact-resistant type) And higher MFR value), PP, PA, PMMA, modified polystyrene (such as ABS, AS), PC, CA and other plastics fluidity changes with temperature. For PE, POM, temperature increase or decrease has little effect on its fluidity. Therefore, the former should adjust the temperature to control the fluidity during molding. ②Pressure injection molding pressure increases, the molten material is subject to shearing effect, and the fluidity is also increased, especially PE and POM are more sensitive, so the injection pressure should be adjusted to control the fluidity during molding. ③The form, size, layout, cooling system design, flow resistance of the molten material (such as surface finish, channel thickness, cavity shape, exhaust system) and other factors directly affect the molten material in the cavity Within the actual fluidity, where the molten material is caused to lower the temperature and the fluidity resistance is increased, the fluidity is reduced. When designing the mold, a reasonable structure should be selected according to the fluidity of the plastic used. During molding, the material temperature, mold temperature, injection pressure, injection speed and other factors can also be adjusted to properly adjust the filling conditions to meet the molding needs.

Thermoplastics can be divided into crystalline plastics and non-crystalline (also known as amorphous) plastics according to their lack of crystallization during condensation. The so-called crystallization phenomenon is that when the plastic moves from the molten state to the condensation, the molecules move independently, completely in an unordered state, and the molecules stop moving freely, at a slightly fixed position, and have a tendency to make the molecular arrangement become a regular model. Kind of phenomenon. As the criteria for judging the appearance of these two types of plastics, it can be determined by the transparency of the thick-walled plastic parts. 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) methylpentene is a crystalline plastic with high transparency, and ABS is an amorphous material but is not transparent.
When designing the mold and selecting the injection molding machine, pay attention to the following requirements and precautions for crystalline plastics: ①The amount of heat required to increase the material temperature to the molding temperature requires equipment with large plasticizing capacity. ②The heat released during cooling and recirculation is large, so it must be fully cooled. ③ The difference between the specific gravity of the molten state and the solid state is large, the molding shrinkage is large, and shrinkage and porosity are easy to occur. ④Fast cooling, low crystallinity, small shrinkage and high transparency. The crystallinity is related to the wall thickness of the plastic parts, the wall thickness is slow to cool, the crystallinity is high, the shrinkage is large, and the physical properties are good. Therefore, the crystalline material should control the mold temperature as required. ⑤ Significant anisotropy and large internal stress. Molecules that have not crystallized after demolding have a tendency to continue to crystallize, are in an energy imbalance state, and are prone to deformation and warpage. ⑥The crystallization temperature range is narrow, and it is easy to inject unmelted material into the mold or block the feed port.
四:Thermosensitive plastics and easily hydrolyzed plastics

1. Thermal sensitivity means that some plastics are more sensitive to heat. When heated at high temperature for a long time or the cross section of the feed port is too small, when the shearing effect is large, the temperature of the material increases and it is prone to discoloration, degradation and decomposition The characteristic plastic is called heat-sensitive plastic. Such as rigid PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene, etc. Thermosensitive plastics produce monomers, gases, solids and other by-products when they decompose, especially some of the decomposed gases are irritating, corrosive or ※ to human body, equipment and mold. Therefore, attention should be paid to the design of the mold, the selection of the injection molding machine and the molding. The screw injection molding machine should be selected. The pouring system should have a large cross-section. The mold and the barrel should be chrome plated. Add stabilizer to weaken its heat sensitive performance.
2. Some plastics (such as PC) will decompose under high temperature and high pressure even if they contain a small amount of water. This property is called easy hydrolysis and must be heated and dried in advance.
五: Stress cracking and melt rupture

1. Some plastics are sensitive to stress, and are prone to internal stress and brittleness and cracking during molding. The plastic parts will crack under the action of external forces or solvents. For this reason, in addition to adding additives to the raw materials to improve crack resistance, the raw materials should be dried, and the molding conditions should be selected reasonably to reduce internal stress and increase crack resistance. And should choose a reasonable shape of plastic parts, it is not appropriate to set up inserts and other measures to minimize stress concentration. Mold design should increase the stripping slope, select a reasonable feed port and ejector mechanism, and properly adjust the material temperature, mold temperature, injection pressure and cooling time during molding to avoid demolding when the plastic parts are too cold and brittle. After molding, the plastic parts should also be post-treated to improve crack resistance, eliminate internal stress and prohibit contact with solvents.
2. When the polymer melt with a certain melt flow rate passes through the nozzle hole at a constant temperature and its flow rate exceeds a certain value, an obvious lateral crack on the melt surface is called melt rupture, which damages the appearance and physical properties of the plastic part. Therefore, when selecting a polymer with a high melt flow rate, the nozzle, sprue, and feed port sections should be increased to reduce the injection speed and increase the material temperature.
六: thermal performance and cooling speed

1. Various plastics have different specific heat, thermal conductivity, thermal deformation temperature and other thermal properties. Plasticization with a high specific heat requires large heat, so an injection molding machine with a large plasticization capacity should be used. The cooling time of plastics with high heat deformation temperature can be short, and the demolding is early, but it is necessary to prevent cooling deformation after demolding. Plastics with low thermal conductivity have a slow cooling rate (such as ionic polymers, which have a very slow cooling rate), so they must be sufficiently cooled to enhance the cooling effect of the mold. Hot runner molds are suitable for plastics with low specific heat and high thermal conductivity. Plastics with large specific heat, low thermal conductivity, low heat distortion temperature, and slow cooling rate are not conducive to high-speed molding, and an appropriate injection molding machine and mold cooling must be selected.
2. All kinds of plastics must maintain proper cooling rate according to their type characteristics and shape of plastic parts. Therefore, the mold must be equipped with heating and cooling systems according to the molding requirements to maintain a certain mold temperature. When the material temperature increases the mold temperature, it should be cooled to prevent the plastic parts from deforming after demoulding, shorten the molding cycle, and reduce the crystallinity. When the plastic residual heat is not enough to keep the mold at a certain temperature, the mold should be equipped with a heating system to keep the mold at a certain temperature to control the cooling rate, ensure fluidity, improve the filling conditions, or control the plastic parts to cool slowly Prevent uneven cooling inside and outside of thick-walled plastic parts and improve crystallinity. For good fluidity, large molding area and uneven material temperature, it is sometimes necessary to use heating or cooling alternately or to use local heating and cooling in accordance with the molding of plastic parts. To this end, the mold should be equipped with a corresponding cooling or heating system.
七: Hygroscopicity

There are various additives in plastics to make them have different degrees of affinity for water, so plastics can be roughly divided into two types of moisture absorption, moisture adhesion and non-absorption and moisture adhesion. The water content of the material must be controlled within the allowable range Inside, otherwise the water will become gas or hydrolysis under high temperature and high pressure, which will cause the resin to foam, reduce fluidity, and have poor appearance and mechanical properties. Therefore, hygroscopic plastics must be preheated with appropriate heating methods and specifications as required to prevent re-absorption during use.


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