Control of injection molding process conditions of

2022-09-23
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Note of processing technology since this year, the control of plastic process conditions

1. The program control of injection speed

the program control of injection speed is to divide the injection stroke of the screw into 3~4 stages, and use their own appropriate injection speed in each stage. For example, slow down the injection speed when the molten plastic first passes through the gate, use high-speed injection during the mold filling process, and slow down at the end of the mold filling. This method can prevent material overflow, eliminate flow marks and reduce the residual stress of products

when filling the mold at low speed, the flow rate is stable, the size of the product is relatively stable, the fluctuation is small, the internal stress of the product is low, and the internal and external stresses of the product tend to be consistent (for example, immersing a polycarbonate product in carbon tetrachloride, and the product formed by high-speed injection has a tendency to crack, and the product does not crack at low speed). Under relatively slow mold filling conditions, the temperature difference of the material flow, especially the temperature difference between the material before and after the gate, is large, which helps to avoid the occurrence of shrinkage and depression. However, as the mold filling time lasts for a long time, it is easy to make the parts appear delamination and poor bonding weld marks, which not only affects the appearance, but also greatly reduces the intensity of large-scale municipal reconstruction and park construction in all parts of China

during high-speed injection, the material flow speed is fast. When the high-speed mold filling is smooth, the molten material will soon fill the mold cavity, and the material temperature will drop less, and the viscosity will drop less. Lower injection pressure can be used, which is a hot material mold filling situation. High speed mold filling can improve the gloss and smoothness of the parts, eliminate the phenomenon of stitching and delamination, shrink small depressions, uniform color, and ensure fullness for most parts of the parts. However, it is easy to cause the products to get fat and blister, or the products to turn yellow, or even burn and zoom, or cause difficulty in demoulding, or uneven mold filling. For high viscosity plastics, it may lead to melt fracture and cloud spots on the surface of the workpiece

high speed and high pressure injection can be considered in the following cases: (1) plastic viscosity is high, cooling speed is fast, and long process parts adopt low pressure and slow speed, which can not completely fill all corners of the mold cavity; (2) For parts with too thin wall thickness, when the molten material reaches the equilibrium state at the thin wall, it is easy to condense and stay. A high-speed injection must be used to make the molten material enter the mold cavity immediately before a large amount of energy consumption; (3) Plastics reinforced with glass fiber or containing a large amount of filling materials must be injected at high speed and high pressure in order to obtain smooth and uniform parts due to poor fluidity

for high-grade precision products, thick wall parts, parts with large wall thickness changes and thick flanges and ribs, it is best to use multi-stage injection, such as level 2, level 3, level 4 or even level 5

2. Program control of injection pressure

generally, the control of injection pressure is divided into primary injection pressure, secondary injection pressure (pressure maintaining) or more than three injection pressures. Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold and prevent material overflow or shortage. The specific volume of molded products depends on the melt pressure and temperature when the gate is closed in the pressure maintaining stage. If the pressure and temperature are the same every time from pressure maintaining to product cooling, the specific volume of the product will not change. At a constant molding temperature, the most important parameter that determines the size of the product is the holding pressure, and the most important variables that affect the dimensional tolerance of the product are the holding pressure and temperature. For example, after the mold filling is completed, the holding pressure immediately decreases, and when the surface layer forms a certain thickness, the holding pressure rises again. In this way, large products with thick walls can be formed with low mold closing force to eliminate collapse and flash

the holding pressure and speed are usually 50% - 65% of the maximum pressure and speed when plastic is filling the mold cavity, that is, the holding pressure is about 0.6-0.8mpa lower than the injection pressure. Because the pressure holding pressure is lower than the injection pressure, the load of the oil pump is low within a considerable pressure holding time, the service life of the solid oil pump can be extended for samples with different diameters, and the power consumption of the oil pump motor is also reduced

the three-stage pressure injection can not only make the parts fill the mold smoothly, but also prevent the welding line, depression, flash and warping deformation. It is good for the molding of thin-walled parts, small parts with multiple heads, large parts with long process, and even the molding of parts with uneven cavity configuration and less tight clamping

3. Program control of plastic filling amount in the injection mold cavity

a certain amount of measurement is adjusted in advance, so that there is still a small amount of melt (buffer) at the end of the screw near the end of the injection stroke. According to the filling situation in the mold, further apply the injection pressure (secondary or tertiary injection pressure) to supplement a little melt. In this way, the product can be prevented from sagging or the shrinkage of the product can be adjusted

4. Program control of screw back pressure and rotation speed

high back pressure can make the melt obtain strong shear, and low rotation speed will also make the plastic get a long plasticizing time in the barrel. Therefore, at present, the program design control of back pressure and rotating speed at the same time is mostly used. For example, in the whole stroke of screw metering, first high speed and low back pressure, then switch to low speed and high back pressure, then switch to high back pressure and low speed, and finally plasticize at low back pressure and low speed. In this way, most of the pressure of molten material in front of the screw is released, reducing the moment of inertia of the screw, thereby improving the accuracy of screw metering. Too high back pressure often causes the discoloration degree of colorant to increase; The mechanical wear of the barrel screw of the pre plastic mechanism increases; The pre molding cycle is prolonged, and the production efficiency is reduced; The nozzle is prone to salivation and the amount of recycled material increases; Even if the self-locking nozzle is used, if the back pressure is higher than the designed spring locking pressure, fatigue damage will also be caused. Therefore, the back pressure must be adjusted properly

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