Injection speed of the program control is the injection stroke of the screw is divided into 3 to 4 stages, respectively, in each stage using their respective injection speed.
For example, slowing the injection rate at the beginning of the molten plastic through the gate, high speed injection during the filling process and slowing down at the end of the filling mode prevent flashbacks, elimination of flow marks and reduction of product Residual stress and so on.
Low velocity filling flow rate steady, product size is relatively stable, less volatile, product stress is low, the product tends to be uniform in all directions (such as a polycarbonate parts immersed in carbon tetrachloride, high-speed injection molding Cracking tendency of the workpiece, low-speed non-cracking).
In the slower mold filling conditions, the temperature difference of the stream, especially before and after the gate material temperature difference, helps to avoid shrinkage and depression. However, due to the longer mold filling time makes the workpiece easy to delamination And combined with poor weld marks, not only affect the appearance, but also greatly reduce the mechanical strength.
When high-speed injection, the material flow rate is fast, when the high-speed filling smooth, the melt quickly filled the cavity, the material temperature dropped less viscosity decreased less, you can use a lower injection pressure, is a hot charge Modal potential.High-speed filling mold can improve the gloss and smoothness, eliminating the phenomenon of seam lines and stratification, shrinkage depression is small, uniform color, the larger parts of the parts to ensure fullness.
But prone to fat foaming products or yellow parts, burns and even zoom, or cause demoulding difficulties, or uneven filling phenomenon.For high viscosity plastic may lead to melt fracture, so that parts of the surface of the cloud spot .
The following conditions can be considered using high-speed high-pressure injection:
High viscosity plastic, cooling speed, the use of low-pressure slow-flow parts can not be fully filled all corners of the cavity;
Thin wall thickness of the workpiece, the melt reaches the thin wall at the condensate and stay, you must use a high-speed injection, so that a lot of melt energy consumption immediately before entering the cavity;
Glass fiber reinforced plastic, or containing a larger amount of filler plastic, due to poor fluidity, in order to obtain a smooth and uniform surface parts, you must use high-speed high-pressure injection.
For advanced precision products, thick-walled parts, wall thickness changes and with thick flanges and tendons of the workpiece, the best multi-level injection, such as two, three, four or even five.
Injection pressure setting:
The control of injection pressure is usually divided into one injection pressure, two injection pressure (packing pressure) or more than three injection pressure control .Pressure switching timing is appropriate, to prevent the mold pressure is too high, to prevent flash or lack of material It is important that the specific volume of the molded product depends on the pressure and temperature of the melt at the gate closure during the dwell phase.If the pressure and temperature of each transition from durometer to product cooling are the same, Will not change.
Under constant molding temperature, the most important parameter that determines the size of the product is packing pressure, the most important variables that affect the size tolerance of the product are packing pressure and temperature.For example: packing pressure immediately after packing, When the surface to form a certain thickness, packing pressure and then rise, so you can use low-profile thick-walled molded products, to eliminate collapse pit and flash.
Packing pressure and speed are usually plastic filling cavity when the maximum pressure and speed of 50% to 65%, the packing pressure is lower than the injection pressure of about 0.6 ~ 0.8MPa. Because the packing pressure is lower than the injection pressure, Pressure time, the pump load is low, solid oil pump to extend the service life, while the pump motor power consumption is also reduced.
Three-stage injection can not only make the parts filled smoothly, but also do not appear weld line, depression, flash and warpage deformation.For thin-walled parts, long small pieces, long process molding of large parts, or even type Molding of parts that are not well-balanced in cavity configuration and less well-matched molds is good.
Screw back pressure and speed settings:
High back pressure can make the melt get strong shear, low speed will make the plastic in the barrel to obtain a longer plasticization time.Therefore, more use of the back pressure and speed at the same time programming control.
For example: the first full measurement of the screw stroke high speed, low back pressure, then switch to a lower speed, higher back pressure, and then switch to high back pressure, low speed, and finally low back pressure, low speed plastics, In this way, the pressure of the melt at the front of the screw is largely released, reducing the moment of inertia of the screw, thereby improving the accuracy of screw metering.
Excessive back pressure often leads to the increase of colorant discoloration degree; the mechanical wear of the preform molding machine combined with the barrel screw increases; the pre-molding cycle prolongs and the production efficiency drops; the nozzle is prone to drooling and the amount of regenerated material increases; Nozzle, if the back pressure is higher than the design of the spring blocking pressure, can also cause fatigue damage, so, back pressure must be properly adjusted.