Generally, the control of injection pressure is divided into one-time injection pressure, two-time injection pressure (holding pressure) or more than three times of injection pressure control. Whether the time of pressure switching is appropriate is very important to prevent the excessive pressure in the mold, overflow or lack of material, etc. The specific volume of the molded product 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 from pressure maintaining to product cooling, the specific volume of the product will not change. At constant molding temperature, the most important parameter to determine the size of products is holding pressure, and the most important variable to affect the dimensional tolerance of products is holding pressure and temperature. For example: after filling the mold, the pressure maintaining pressure immediately reduces. When the surface layer forms a certain thickness, the pressure maintaining pressure rises again. In this way, the large products with thick wall can be formed by using low closing force to eliminate the collapse and flash.
The holding pressure and velocity are usually 50% ~ 65% of the maximum pressure and velocity when filling the mold cavity, that is, the holding pressure is about 0.6 ~ 0.8MPa lower than the injection pressure. Because the pressure maintaining pressure is lower than the injection pressure, the load of the oil pump is lower during the considerable pressure maintaining time, the service life of the solid oil pump is prolonged, 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 avoid the melting line, depression, flash and warping deformation. For thin-walled parts, multi head small parts, long process large parts of the molding, and even cavity configuration is not too balanced and close the mold is not too tight parts of the molding are good.
Program control of plastic filling amount in injection mold cavity
A certain amount of metering is adjusted in advance, so that a small amount of melt (buffer) remains at the end of the screw near the end of the injection stroke. According to the filling situation in the mold, the injection pressure (secondary or tertiary injection pressure) is further applied to supplement a small amount of melt. In this way, the product depression can be prevented or the shrinkage of the product can be adjusted.
Program control of screw back pressure and speed
High back pressure can make the melted material get strong shear, and low speed can also make the plastic get longer plasticizing time in the barrel. Therefore, at present, the back pressure and rotation speed are controlled by programming at the same time. For example: in the whole stroke of screw metering, first high speed and low back pressure, then switch to lower speed and higher back pressure, then switch to high back pressure and low speed, and finally plasticize under low back pressure and low speed, so that the pressure of molten material in front of screw is largely released, the moment of inertia of screw is reduced, and the accuracy of screw metering is improved. Too high back pressure often results in the increase of discoloration degree of colorant; the increase of mechanical wear of the screw rod of the combined barrel of the pre molding mechanism; the prolongation of the pre molding period and the decrease of production efficiency; 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, it will also cause fatigue damage. Therefore, the back pressure must be adjusted properly.
With the development of technology, it is possible to use computer to control the injection molding process. Nippon Steel Institute n-pacs (microcomputer control system) can achieve four feedback controls (pressure maintaining adjustment, mould pressing adjustment, automatic metering adjustment, resin temperature adjustment) and four process controls (injection speed program control, pressure maintaining inspection, screw speed program control, back pressure program control)