Injection molding machine injection molding process elements

Date：2016-08-04 10:11:21

First, the injection molding machine injection molding process shrinkage

    Thermoplastic injection molding machine in the form of mold shrinkage and calculations mentioned above, the injection molding machine thermoplastic molding shrinkage factors are as follows:

    Plastic Injection Molding Machine 1.1 Variety thermoplastic molding process because there are changes in the volume of crystallized form from the inside should be strong, freeze in plastic residual stress, strong molecular orientation and other factors, as compared with thermosetting plastics is shrinking rate is high, a wide range of shrinkage, clear direction, additional shrinkage after shrinkage after molding, the humidity or annealing process generally large than thermosets.

    1.2 properties of the molded plastic injection molding machine melt in contact with the outer surface of the cavity immediately cooled to form a low-density solid shell. Due to the poor thermal conductivity of plastic, the inner plastic parts shrink slowly cooled to form large, high-density solid layer. So the wall thickness, slow cooling, high-density layer thickness is large shrinkage. In addition, the presence or absence of the insert and insert layout, the number of directly affect the material flow direction, density and shrinkage resistance size, so characteristic of the plastic parts shrink the size, direction greater impact.

    1.3 the inlet of the injection molding form, size and distribution of these factors directly affect the direction of material flow, density, pressure and molding time feeding action. Direct feed opening, a large inlet cross-section (in particular cross-section thick) small contraction but large directional feed mouth width and a short length of the directional small. From the feed stream or with the direction parallel to the mouth near the large shrinkage.

    1.4 molding conditions Mold temperature is high, the melt is cooled slowly, high density, shrink large, especially due to high crystallinity of the crystalline material, volume changes, so the greater the contraction. Mold temperature distribution and plastic parts inside and outside cooling and density uniformity is also related to a direct impact on the amount of shrinkage of each part size and orientation. In addition, the holding pressure and time of contraction greater impact, pressure, long time small contraction but large directivity. Injection pressure is high, the melt viscosity difference is small, small interlaminar shear stress, elastic rebound after releasing large, the right amount of shrinkage can be reduced, high temperature materials, shrink large, small but directional. Therefore, the adjustment factors at the time of molding temperature, pressure, injection speed and cooling time may be appropriate to change the plastic shrinkage. Mold design according to the contraction of the range of all kinds of plastic, plastic wall thickness, shape, size and form feed inlet distribution, empirically determined plastic parts various parts of the shrinkage cavity size calculated again. For high-precision plastic parts and difficult to grasp shrinkage generally preferable to mold design follows:

    ① plastic injection molding machines take smaller outer diameter shrinkage rate, whichever is greater inner diameter shrinkage rate in order to leave room for correction after the tryout.

    ② tryout gating system to determine the form, size and molding conditions.

    ③ to be treated after the injection molding machine plastic parts processing to determine the size of the change (must be measured after 24 hours after release).

    ④ the actual mold shrinkage correction.

    ⑤ try mold may be suitably changing process conditions slightly amended shrinkage values ​​to meet the requirements of plastic parts.

    Second, the liquidity injection molding machine injection molding process

    2.1 thermoplastics mobility size, generally from molecular weight, melt index, Archimedes spiral flow length, apparent viscosity and flow ratio (flow length / plastic wall thickness) and a series of indices for analysis. Small molecular weight, broad molecular weight distribution, molecular structure regularity poor, high melt index, spiral flow length is long, the performance of low viscosity, the flow ratio of the fluidity like, the same name of the plastic must be checked instructions determine flowability applicability for injection molding. Press die design requirements can be broadly common plastic liquidity into three categories:

    ① good liquidity PA, PE, PS, PP, CA, poly (4) methyl pentene;

    ② liquid medium polystyrene series resin (e.g. ABS, AS), PMMA, POM, polyphenylene ether;

    ③ illiquid PC, hard PVC, polyphenylene oxide, polysulfone, polyaryl sulfone, fluorine plastic.

    2.2 plastic injection molding machine mobility is also due to various factors molding change, the main factors are the following effects:

    ① high temperature material temperature is increased mobility, but also vary in different plastics, PS (in particular, the impact resistance and higher MFR values), PP, PA, PMMA, modified polystyrene (such as ABS, AS) , PC, CA plastic liquidity with larger temperature changes. For PE, POM, the temperature increase or decrease its flowability less affected. Therefore, the former in the molding temperature is preferably adjusted to control liquidity.

    ② pressure injection molding machine injection pressure is increased by the large shear melt, liquidity also increased, especially more sensitive PE, POM, it should be adjusted to control the flow of injection pressure during molding.

    ③ injection molding machine in the form of mold gating system structure, size, layout, design of the cooling system, the melt flow resistance (such as the type of surface finish, feeding channel section thickness, shape of the cavity, the exhaust system) and other factors have a direct impact in the melt the actual flow cavity, where the urge to reduce the melt temperature, increasing the liquidity of the resistance to flow is reduced. Mold design should be based on the use of plastic flow, the choice of rational structure. It can also control the molding material temperature, mold temperature and injection pressure, injection speed and other factors to adjust the fill situation appropriately shaped to meet the needs.

    Third, the crystallinity of the injection molding machine injection molding process

    Thermoplastics its crystallization no condensation phenomenon can be divided into crystalline and non-crystalline plastics (also known as amorphous) plastic two categories.

    Is the so-called crystallisation from the molten state to a plastic condenses, the molecules move independently, completely unordered state, stops the free movement of molecules into, according to a fixed position slightly, so that the molecular arrangement and has a tendency to become a formal model of phenomenon.

    As discriminating these two types of standard visual appearance of plastic transparent plastic thick-walled plastic parts, generally crystalline material is opaque or translucent (such as POM, etc.), the amorphous material is transparent (such as PMMA, etc.). But there are exceptions, such as poly (4) methyl pentene to crystalline plastics there is a high transparency, ABS is an amorphous material but not transparent.

    Mold design and selection of the injection molding machine should pay attention to crystalline plastics have the following requirements and precautions:

    ① material temperature rose to more heat molding temperature required, use the plasticizing capacity of the device.

    ② Press release cooling of heat, to cool sufficiently.

    ③ molten and solid big difference in specific gravity, mold shrinkage large, prone to shrinkage, porosity.

    ④ cooling fast, low crystallinity, shrinkage, high transparency. The degree of crystallinity and plastic wall thickness related to the wall thickness of the slow cooling, high crystallinity, large shrinkage, good physical properties. Therefore, the crystalline material should be required mold temperature must be controlled.

    ⑤ significant anisotropy, internal stress. After the release of the molecule has not crystallized continued tendency to crystallize, as a power imbalance, prone to distortion, warping.

    ⑥ crystallization temperature range is narrow, not prone to melt the end of the injection mold or clogged inlet.

    Fourth, the heat-sensitive plastic injection molding machine injection molding process and is easily hydrolyzed plastic

    4.1 refers to certain heat-sensitive plastic are more sensitive to heat, is heated at a high temperature or longer feed inlet cross-section is too small, large shear, melt temperature increased prone to discoloration, degradation, decomposition tendency of having such characteristic plastic called heat-sensitive plastic. Such as hard PVC, polyvinylidene chloride, vinyl acetate copolymer, POM, polychlorotrifluoroethylene and the like. Heat-sensitive plastic decomposition by-products generated when the monomer, gas, or solid, some decomposition gases especially humans, equipment, molds are stimulated, corrosive or toxic effect. Therefore, mold design, injection molding machines and molding the choice should pay attention to, should be used in screw injection molding machine, injection system should be large cross-section, molds and chrome barrel should not be there * angle hysteresis material, must be strictly control the molding temperature, plastic stabilizer is added, weakening its thermal performance.

    4.2 Some plastic (e.g., PC) containing a small amount of water, even, but at a high temperature, high pressure decomposition also occurs, this performance is called easily hydrolyzable, which must be pre-heated and dried.

    Fifth, the injection molding machine injection molding process and melt fracture stress cracking

    5.1 Some of the stress sensitive plastics, easy to produce internal stress during molding and crisp and easy to crack, plastic parts in the external force or phenomenon that is cracking under the effect of the solvent. To this end, in addition to improving additives in the feed opening crack resistance, the raw material should be noted that drying reasonable choice of molding conditions, to reduce stress and increase crack resistance. And should choose the right shape of plastic parts, should not be set inserts and other measures to minimize the stress concentration. Mold design should increase the stripping slope, a reasonable choice of inlet and ejector mechanism, should be suitably adjusted molding material temperature, mold temperature, injection pressure and cooling time, try to avoid too cold brittle plastic release , molded plastic parts after further processing should improve resistance to cracking, eliminate stress and prohibiting contact with the solvent.

    5.2 When some melting of the polymer melt flow rate, at a constant temperature through the nozzle hole flow rate exceeds a certain value, the apparent melt surface transverse cracks called melt fracture, lossy plastic appearance and physical properties. Therefore, in the selection of high melt flow rate polymers and the like, should be increased nozzle, sprue, inlet cross-section to reduce the injection speed, material temperature increase.

    Sixth, the injection molding machine injection molding process thermal properties and cooling rate

    6.1 There are different kinds of plastic injection molding machine specific heat, thermal conductivity, heat distortion temperature thermal properties. High specific heat plasticized needs heat, should be used plasticizing capacity of the injection molding machine. High heat deflection temperature plastic cooling time can be short, early release, but release to prevent deformation during cooling. Low thermal conductivity plastic cooling rate (such as ionic polymers cooled very slowly), it must be cooled sufficiently to strengthen the mold cooling effect. Hot runner mold for low specific heat, high thermal conductivity of plastics. Large specific heat, low thermal conductivity, low thermal deformation temperature, slow cooling is not conducive to high-speed plastic molding, injection molding machines must use the appropriate and strengthen the mold cooling.

    6.2 plastic injection molding machines of various types according to their characteristics and the shape of the plastic parts, which calls for maintaining proper cooling rate. Therefore, the injection molding machine mold heating and cooling systems must be set up by the molding requirements, in order to maintain a certain mold temperature. When the material temperature should be raised when the mold temperature is cooled to prevent deformation of plastic parts after stripping, shorten the molding cycle, reducing the degree of crystallinity. When the plastic is not sufficient to heat the mold to maintain a certain temperature, the mold should be provided with a heating system so that the mold is maintained at a constant temperature to control the cooling rate to ensure liquidity, improving conditions or for controlling the filling of plastic parts allowed to cool slowly, thick-walled plastic parts inside and outside to prevent uneven cooling and increase the degree of crystallinity and the like. Good liquidity, large forming area, the material temperature unevenness of the press molding of plastic parts required heating or cooling conditions sometimes used interchangeably or local heating and cooling and use. For this reason the mold should be provided with a corresponding cooling or heating system.

    Seven, injection molding machine injection molding process hygroscopic

    Plastic due to a variety of additives to the water there are different degrees of closeness, so the plastic can be divided into absorbent, non-absorbent and adherent water is also difficult to stick two water, the water content of the feed must be controlled within the allowable range within, or in high temperature, high pressure gas into the water or the occurrence of hydrolysis, the resin foaming, poor decreased mobility, appearance and mechanical properties. So hygroscopic plastic must require the use of appropriate methods of heating and preheating specification, when used to prevent further moisture.

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