Factors affecting the injection process

Date：2011/12/6 17:56:49

Injection process is to ensure the proper development of a good plastic melt plastics and smooth filling, cooling in the setting, in order to meet the requirements to produce quality products; in the injection process, the most important process parameters are temperature (melt temperature, nozzle temperature, mold temperature), pressure (plasticization pressure, injection pressure, cavity pressure) and the corresponding role of the various time (injection time, ensure that time, cooling time) and so on. The following will discuss only the main process parameters and interactions.

First, the influence of temperature

1 material temperature

        Plastic processing temperature is controlled injection machine barrel. The correct choice of barrel temperature related to the plastic quality of plastics, the principle is to ensure a smooth plastic injection molding without causing local degradation. Typically, the barrel end of the maximum temperature should be higher than the plastic flow temperature (or melting temperature), but below the decomposition temperature of the plastic.

        In addition to strictly control the production of injection machine barrel of the maximum temperature, but also control the plastic melt residence time in the barrel. In determining the barrel temperature, should also consider the structural characteristics of products and molds. When molding thin-walled or complex shape of the products, the flow resistance, increased barrel temperature helps to improve the melt flow.
Usually controls the nozzle barrel slightly below the maximum temperature the maximum temperature to prevent the occurrence of melt flow at the nozzle exit delay phenomenon.

(2) mold temperature

        In the injection molding process, mold temperature is the cooling medium (usually water) control, which determines the cooling rate of the plastic melt. The lower mold temperature, the faster the cooling, the melt temperature decreases the more quickly, resulting in melt viscosity, injection pressure loss increases, causing serious and even less filling. With the increase in mold temperature, melt flow increases, the required filling pressure decreases, the surface quality of products improved; but because of the cooling time increased, decreased productivity products, products of the mold shrinkage increases.

        For crystalline plastics, due to higher temperatures conducive to crystallization, so the higher the mold temperature can improve the product structure of the density or crystallinity. In the higher mold temperature products faster relaxation process of polymer molecules, the role of molecular orientation and internal stress will be reduced.

Second, the pressures

        Pressure injection molding process, including the plasticization pressure, injection pressure cell cavity pressure.

        Plasticization pressure, also known as back pressure, refers to the injection screw at the top of the melt in the screw turning back when the pressure is by adjusting the injection resistance of the hydraulic cylinder to control the oil return. Plastics increased pressure within the melt pressure, strengthen the shear effect, due to plastic shear heat, thereby increasing the melt temperature. Plasticizing screw back pressure increases to slow down, extending the plastic in the screw of the heating time, the quality of plastics can be improved; but too much also increases cylinder pressure measurement of plastics melt reflux and indoor leakage, reducing the transmission capacity of the melt, reducing the amount of plastics, an increase of power consumption and high heat plasticization pressure would be cut or shear stress is too large, the melt prone to degradation.

        Injection injection pressure is generated in the screw head melt pressure. In the choice of injection pressure, injection molding machine should be considered first allowed injection pressure, injection pressure is too low only in the back leading to a low pressure cavity, the melt can not be successfully filled cavity; the other hand, injection pressure, not only will result in excess product leakage, deformation will result in products, and even the system overload.

        Injection pressure during injection and melt temperature are mutually conditioned. High temperature materials required injection pressure and melt temperature of the combinations will get satisfactory results.

        Injection molding cavity pressure is the pressure through the nozzle, the pressure loss flow congratulate the gate after the mold cavity melt pressure generated.

Third, the injection molding cycle and the injection speed

        Required to complete a time known as injection molding injection molding cycle, which includes feeding, heating, filling, packing, cooling time and mold, mold, die and auxiliary operations such as closing time. In the injection molding cycle time of injection speed and cooling performance of the products have a decisive influence.

        Melt injection speed mainly affects the flow behavior of Germany in the cavity. Usually increases as the injection speed, melt flow rate increases, shear strengthening; due to shear heating and melt temperature increases, viscosity decreases, so conducive to filling. The fusion of the various parts and products, grain strength be increased. However, the injection velocity increases, which may cause the fluid state to the melt flow from the layer, will cause severe melt in the mold injection mold caused by the air can not be discharged, this part of the air is compressed under high pressure, heating rate, will cause local burning or decomposition products.

        In actual production, injection speed is usually tested to confirm. Generally slow speed of the first low-pressure injection molding products, the situation then adjust injection speed.

        Modern injection machine has achieved multi-injection technique, that is an injection process, when the injection machine screw drive melt into the mold, you can achieve depending on the needs in different locations have different speeds and different injection pressure injection other parameters of the control. Multi-stage injection process should be based on different kinds of plastic and different products for the development and selection.

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