How to reasonably set the process in the filling stage of injection molding?

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 mold filling, and slow down at the end of mold filling. This method can prevent overflow, eliminate flow marks and reduce the residual stress of products.

During low-speed mold 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 stresses of the product tend to be consistent (for example, when a polycarbonate product is immersed in tetrachloride tank, the product molded by high-speed injection has cracking tendency, 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 front and back parts of the gate, helps to avoid the occurrence of shrinkage holes and dents. However, because the mold filling time lasts for a long time, it is easy to cause delamination and poor fusion marks, 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 molten material quickly fills the mold cavity, the material temperature decreases less, and the viscosity decreases 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 parts, eliminate the phenomenon of seam and delamination, small shrinkage depression, uniform color, and ensure fullness for most parts of parts.

However, it is easy to cause products to get fat and blister or parts to turn yellow, 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 parts.

High speed and high pressure injection can be considered in the following cases:

The plastic has high viscosity and fast cooling speed. The long process parts adopt low pressure and slow speed, which can not completely fill all corners of the cavity;

For parts with too thin wall thickness, the molten material is easy to condense and stay when reaching the thin wall. One high-speed injection must be adopted to make the molten material enter the mold cavity immediately before a large amount of energy consumption;

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-precision products, thick wall products, products with large wall thickness changes and thick flanges and ribs, multi-stage injection is adopted, such as level 2, level 3, level 4 or even level 5.

Setting of injection pressure:

Generally, the control of injection pressure is divided into primary injection pressure, secondary injection pressure (pressure holding) or more than three injection pressures. Whether the pressure switching time is appropriate is very important to prevent excessive pressure in the die and overflow or shortage of material. 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 holding to product cooling, the specific volume of the product will not change.

At a constant molding temperature, the important parameter determining the product size is the holding pressure, and the important variables affecting the product size tolerance are the holding pressure and temperature. For example, after the mold filling is completed, the holding pressure decreases immediately. When the surface layer forms a certain thickness, the holding pressure rises again. In this way, large products with thick wall can be formed with low clamping force to eliminate pit collapse and flash.

The holding pressure and speed are usually 50% ~ 65% of the high pressure and speed when the mold cavity is filled with plastic, 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 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 welding line, depression, flash and warping deformation. It is good for the molding of thin-wall parts, multi head small parts, long process large parts, and even the molding of parts with unbalanced cavity configuration and less tight mold closing.

Setting of screw back pressure and speed:

High back pressure can make the melt obtain strong shear, and low speed will also make the plastic get a long plasticizing time in the barrel. Therefore, the program design control of back pressure and speed is often used 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, and then switch to high back pressure and low speed, and plasticize under low back pressure and low speed. In this way, most of the molten material pressure in front of the screw is released, reducing the moment of inertia of the screw rod, so as to improve the accuracy of screw metering.

High back pressure often increases the discoloration degree of colorant; 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, it will also cause fatigue failure. Therefore, the back pressure must be adjusted properly.