Injection molding press | Injection molding injection and back pressure parameter setting

Dec 09, 2021

Injection molding press

The program control of the injection speed is to divide the injection stroke of the screw into 3~4 stages, and use respective appropriate injection speeds 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 filling process, and slow down the speed at the end of the filling process. Using this method can prevent flashing, eliminate flow marks and reduce residual stress of products.
During low-speed filling, the flow rate is stable, the product size is relatively stable, the fluctuation is small, the internal stress of the product is low, and the internal and external stress of the product tends to be the same (for example, a polycarbonate part is immersed in carbon tetrachloride, and high-speed injection molding is used. The product has a tendency to crack, and it does not crack at low speed).
Under relatively slow mold filling conditions, the temperature difference of the material flow, especially the large temperature difference between the material before and after the gate, helps to avoid the occurrence of shrinkage holes and dents. However, due to the long filling time, it is easy to cause delamination and poor welding of the parts, which not only affects the appearance, but also greatly reduces the mechanical strength.
During high-speed injection, the material flow speed is fast. When the high-speed mold filling is smooth, the melt fills the cavity quickly, the material temperature drops less, and the viscosity drops less. Lower injection pressure can be used, which is a hot material filling. Modal situation. High-speed mold filling can improve the gloss and smoothness of the parts, eliminate the phenomenon of seam lines and delamination, with small shrinkage and concave, uniform color, and can ensure fullness for larger parts of the parts.
However, it is easy to cause the product to become fat, bubbling or yellow, or even burn the zoom, or cause difficulty in demolding, or uneven filling of the mold. For high-viscosity plastics, it may cause melt fracture, causing cloud spots on the surface of the part.

High-speed and high-pressure injection can be considered in the following situations:
The plastic has high viscosity, fast cooling speed, and long process parts adopt low pressure and slow speed, which cannot completely fill all corners of the cavity;
For parts with too thin wall thickness, the molten material is easy to condense and stay at the thin wall. A high-speed injection must be used to make the molten material enter the cavity immediately before a large amount of energy is consumed;
Plastics reinforced with glass fibers, or plastics containing a larger amount of filler materials, have poor fluidity. In order to obtain a smooth and uniform surface, high-speed and high-pressure injection must be used.
For high-end precision products, thick-walled parts, parts with large wall thickness changes and thicker flanges and ribs, it is best to use multi-level injection, such as two-level, three-level, four-level or even five-level.

Setting of injection pressure:
Generally, the control of injection pressure is divided into the control of primary injection pressure, secondary injection pressure (holding pressure), or more than three injection pressures. Whether the timing of pressure switching is appropriate is very important to prevent excessive pressure in the mold, prevent flash or lack of material, etc. The specific volume of the molded product depends on the melt pressure and temperature when the gate is closed during the pressure holding stage. If the pressure and temperature are the same every time when switching from the holding pressure to the product cooling stage, 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 filling the mold, the holding pressure is immediately reduced. When the surface layer is formed to a certain thickness, the holding pressure rises again. In this way, a low clamping force can be used to form large thick-walled products and eliminate craters and flash.
The holding pressure and speed are usually 50% to 65% of the highest pressure and speed when the plastic is filled in the mold cavity, that is, the holding pressure is about 0.6 to 0.8 MPa lower than the injection pressure. Since the holding pressure is lower than the injection pressure, the load of the oil pump is low during a considerable holding 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 part fill the mold smoothly, but also will not appear weld lines, dents, flashing and warping deformation. It is good for the molding of thin-walled parts, small parts with multiple heads, large parts with long processes, and even parts with uneven cavity configuration and tight mold clamping.

Setting of screw back pressure and speed:
The high back pressure can make the melt material get strong shear, and the low speed can also make the plastic get longer plasticizing time in the barrel. Therefore, the control of simultaneous programming of back pressure and speed is used more frequently.
For example: in the screw metering full stroke, firstly 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. In this way, most of the pressure of the molten material in the front of the screw is released, and the rotational inertia of the screw is reduced, thereby improving the accuracy of screw measurement.
Excessive back pressure tends to increase the degree of discoloration of the colorant; the mechanical wear of the pre-plasticizing mechanism and the barrel screw increases; the pre-plasticizing 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 type is used If the back pressure of the nozzle is higher than the designed spring locking pressure, it will also cause fatigue damage. Therefore, the back pressure must be adjusted appropriately.

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