How to Set Injection Molding Filling Stage Process?

The program control of injection speed involves dividing the injection stroke of the screw into 3 to 4 stages, with each stage using its own appropriate injection speed.

For example, slowing down the injection speed when the molten plastic first passes through the sprue, using high-speed injection during the filling process, and slowing down the speed again at the end of the filling can prevent overflow, eliminate flow marks, and reduce residual stress in the product.

During low-speed charging, the flow rate is stable, the dimensions of the products are relatively stable with minimal fluctuations, the internal stress of the products is low, and the internal and external stresses tend to be consistent (for example, immersing a polycarbonate component in carbon tetrachloride shows that high-speed injection molded components tend to crack, while low-speed ones do not).

Under slower mold filling conditions, the temperature difference in the material flow, especially the temperature difference before and after the gate, helps to avoid the occurrence of shrinkage cavities and sink marks. However, because the mold filling time is prolonged, it is easy for the product to have delamination and poor weld lines, which not only affects the appearance but also greatly reduces the mechanical strength.

During high-speed injection, the flow velocity is fast. When high-speed mold filling proceeds smoothly, the molten material quickly fills the cavity, resulting in a smaller decrease in material temperature and viscosity. This allows for the use of lower injection pressure and represents a hot material mold filling trend. High-speed mold filling can improve the glossiness and smoothness of the parts, eliminate seam lines and delamination, reduce shrinkage pits, and ensure uniform and consistent color, providing fullness for larger parts of the product.

However, this can easily cause the product to become bloated, blistered, or discolored to yellow, and even result in burning or scorching, making demolding difficult, or causing uneven mold filling. For high-viscosity plastics, it may lead to melt fracture, resulting in a cloudy appearance on the surface of the product.

The following situations may consider using high-speed high-pressure injection:

Plastic with high viscosity and fast cooling speed cannot completely fill every corner of the mold cavity using low pressure and slow speed for long process parts.

For parts with excessively thin wall thickness, the molten material is prone to solidifying and remaining in the thin wall areas. Therefore, it is necessary to use a high-speed injection in one shot, allowing the molten material to enter the cavity before a significant amount of energy is consumed.

Plastics reinforced with fiberglass or containing a large amount of fillers have poor fluidity. To obtain smooth and uniform molded parts, high-speed and high-pressure injection must be used.

For advanced precision products, thick-walled components, parts with significant wall thickness variations, and those with thick flanges and ribs, it is best to use multi-stage injection, such as dual, triple, quadruple, or even quintuple injection.

The control of injection pressure is usually divided into primary injection pressure, secondary injection pressure (holding pressure), or control of more than three stages of injection pressure. The timing of pressure switching is crucial for preventing excessive mold pressure, overflow, or material shortage. The specific volume of the molded product depends on the melt pressure and temperature when the gate is sealed during the holding phase. If the pressure and temperature are consistent every time the holding phase switches to the product cooling phase, the specific volume of the product will not change.

At a constant molding temperature, the most important parameter determining the product size is the holding pressure, and the most important variables affecting product size tolerance are the holding pressure and temperature. For example, after the mold is filled, the holding pressure is immediately reduced. When a certain thickness of the surface layer is formed, the holding pressure is increased again. This approach allows for the molding of large, thick-walled products with low clamping force, eliminating sink marks and flash.

The holding pressure and speed are usually 50% to 65% of the highest pressure and speed when the plastic fills the mold cavity, which means 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 on the oil pump is low during the considerable holding time, thereby extending the service life of the oil pump, and at the same time, the power consumption of the oil pump motor is also reduced.

Three-stage pressure injection can ensure smooth mold filling of the workpiece without the occurrence of weld lines, sink marks, flash, and warpage. It is beneficial for the molding of thin-walled parts, small multi-gate parts, large parts with long flow paths, and even for parts with less balanced cavity configurations and those with less tight mold clamping.

High back pressure can provide strong shear to the melt, while low rotation speed allows the plastic to have a longer plasticizing time within the barrel. Therefore, there is currently a greater use of program design control that simultaneously manages back pressure and rotation speed.

For example: in the full stroke of screw metering, first use 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 perform plastication at low back pressure and low speed. In this way, most of the pressure of the melt at the front of the screw is released, reducing the rotational inertia of the screw, thereby improving the accuracy of screw metering.

Excessive back pressure often leads to an increase in the degree of color change of the coloring agent; it causes greater mechanical wear on the screw of the pre-plasticizing mechanism; the pre-plasticizing cycle is extended, resulting in decreased production efficiency; the nozzle is prone to drooling, and the amount of recycled material increases; even if a self-locking nozzle is used, if the back pressure exceeds the designed spring locking pressure, it can also cause fatigue failure. Therefore, the back pressure must be adjusted appropriately.