The correct formulation of the injection process is to ensure good plasticization of the plastic melt, and smooth filling, cooling, and shaping, to produce products of the required quality; the most important process parameters in the injection process are temperature (material temperature, nozzle temperature, mold temperature), pressure (plasticizing pressure, injection pressure, cavity pressure) and the corresponding action time (injection time, guarantee time, cooling time), etc. . Only the main process parameters and their mutual influence are discussed below.
I. Influence of temperature
1. Material temperature
The processing temperature of plastic is controlled by the injection machine barrel. The correct choice of barrel temperature is related to the plasticization quality of the plastic, the principle is to ensure smooth injection molding without causing local degradation of plastic. Usually, the maximum temperature at the end of the barrel should be higher than the flow temperature (or melt temperature) of the plastic, but lower than the decomposition temperature of the plastic.
In addition to strictly controlling the maximum temperature of the barrel of the injection machine, the residence time of the plastic melt in the barrel should also be controlled during production. In determining the barrel temperature, the structural characteristics of the product and the mold should also be considered. When molding thin-walled or complex-shaped products with high flow resistance, increasing the barrel temperature can help improve the flow of the melt.
Usually control the maximum temperature of the nozzle is slightly lower than the maximum temperature of the barrel, to prevent the melt in the nozzle mouth of the flow delay phenomenon.
2. Mold temperature
In the injection molding process, the mold temperature is controlled by the cooling medium (usually water), which determines the cooling rate of the plastic melt. The lower the mold temperature, the faster the cooling rate, the more rapidly the melt temperature decreases, resulting in increased viscosity of the melt, increased injection pressure loss, and in severe cases, even cause insufficient mold filling. As the mold temperature increases, the melt fluidity increases, the required mold filling pressure decreases, and the surface quality of the product improves; however, due to the growth of cooling time, the productivity of the product decreases and the molding shrinkage of the product increases.
For crystalline plastics, increasing the mold temperature can improve the density or structural crystallinity of the product because the higher temperature is conducive to crystallization. At higher mold temperature, the relaxation process of polymer macromolecules in the product is faster, and the molecular orientation effect and internal stress are reduced.
II. the influence of pressure
The pressure in the injection molding process includes plasticizing pressure, injection pressure, and cavity pressure.
Plasticizing pressure, also known as back pressure, is the pressure of the melt on the top of the injection machine screw when the screw is rotating backward and is controlled by adjusting the return resistance of the injection hydraulic cylinder. The plasticizing pressure increases the internal pressure of the melt, which enhances the shearing effect and raises the temperature of the melt because of the shear heating of the plastic. The increase of plasticizing pressure slows down the return speed of the screw, prolongs the healing time of the plastic in the screw, and the plasticizing quality can be improved; however, excessive plasticizing pressure also increases the backflow and leakage of the melt inside the barrel metering chamber, reduces the transport capacity of the melt, decreases the plasticizing volume, increases the power consumption and too high plasticizing pressure will make the shear heat or excessive shear stress, and the melt is prone to degradation.
Injection pressure is the melt pressure generated at the head of the screw during the injection. In the selection of injection pressure, the first consideration should be the injection pressure allowed by the injection molding machine, only in the injection pressure is too low back to lead to insufficient cavity pressure, the melt can not smoothly fill the cavity; conversely, the injection pressure is too large, not only will cause product overflow, but also cause product deformation, and even overload the system.
In the injection process, injection pressure and melt temperature are mutually constrained. When the material temperature is high, the combination of injection pressure and material temperature is required to obtain satisfactory results.
The cavity pressure is the melt pressure generated in the mold cavity after the pressure loss of injection pressure through the nozzle, runner, and gate.
III. Injection molding cycle and injection speed
The time required to complete an injection molding is called the injection molding cycle, which includes the time for material addition, heating, mold filling, pressure holding, cooling, as well as mold opening, mold release, mold closing, and auxiliary operations. In the whole injection molding cycle, injection speed and cooling time have a decisive influence on the performance of the product.
The injection speed mainly affects the flow behavior of the melt in the cavity. Usually, as the injection speed increases, the melt flow rate increases, and the shearing effect is strengthened; the temperature of the melt increases due to shear heating, and the viscosity decreases, so it is beneficial to mold filling. And the strength of the fusion pattern of each part of the product is increased. However, due to the increase of injection speed, the melt may change from layer fluid state to flow, which may cause serious melt injection in the mold and cause the air in the mold can not be discharged, this part of the air is compressed under high pressure and speed heating, which will cause local scorching or decomposition of the product.
In actual production, the injection speed is usually confirmed by the test. Generally, the injection speed is first injected at low pressure and slow speed and then adjusted according to the molding situation of the product.
Modern injection machines have realized multi-stage injection technology, i.e., in one injection process, when the injection machine screw pushes the melt into the mold, it can realize the control of process parameters such as different injection speeds and injection pressures at different positions according to different needs. The multi-stage injection process should be formulated and selected according to different varieties of plastics and different products.
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