Briefly describe the control methods of process conditions in injection molding processing

Program control of injection speed

The program control of injection speed divides the injection stroke of the screw into 3-4 stages, and each stage uses its own appropriate injection speed. For example, slowing down the injection speed when molten plastic first passes through the gate, using high-speed injection during the filling process, and slowing down the speed at the end of the filling process. By adopting this method, overflow can be prevented, flow marks can be eliminated, and residual stress in the product can be reduced.

When filling the mold at low speed, the flow rate is stable, the size of the product is relatively stable, and 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, if a polycarbonate part is immersed in carbon tetrachloride, the part formed by high-speed injection molding has a tendency to crack, and the part formed at low speed does not crack). Under relatively slow filling conditions, the temperature difference of the material flow, especially the large temperature difference before and after the gate, helps to avoid the occurrence of shrinkage and depression. However, due to the prolonged filling time, it is easy for the parts to have delamination and poor fusion marks, which not only affects the appearance but also greatly reduces the mechanical strength.

When injecting at high speed, the material flow speed is fast. When the high-speed mold filling is smooth, the molten material quickly fills the mold cavity, and the temperature and viscosity decrease less. A lower injection pressure can be used, which is a hot material filling situation. High speed mold filling can improve the glossiness and smoothness of the parts, eliminate the phenomenon of seam lines and layering, reduce shrinkage and concavity, ensure uniform color consistency, and ensure fullness for most parts of the parts. But it is easy to cause product weight gain, blistering, yellowing of parts, even burns and charring, or difficulty in demolding, or uneven mold filling. For high viscosity plastics, it is possible to cause melt rupture, resulting in cloud and mist spots on the surface of the parts.

High speed and high pressure injection can be considered in the following situations: (1) Plastic has high viscosity, fast cooling speed, and long process parts cannot fully fill all corners of the mold cavity with low pressure and slow speed; (2) Parts with thin wall thickness are prone to condensation and retention when the molten material reaches the thin-walled area. Therefore, a high-speed injection must be used to consume a large amount of molten material energy before it immediately enters the mold cavity; (3) Plastics reinforced with fiberglass or plastics containing a large amount of filling materials, due to poor fluidity, must be injected at high speed and pressure to obtain a smooth and uniform surface.

For advanced precision products, thick walled parts, parts with large wall thickness variations, and parts with thicker protrusions and ribs, it is best to use multi-stage injection, such as second, third, fourth, or even fifth level.

Program control of injection pressure

The control of injection pressure is usually divided into primary injection pressure, secondary injection pressure (holding pressure), or control of injection pressure for three or more times. Whether the timing of pressure switching is appropriate is crucial for preventing excessive pressure inside the mold, as well as preventing material overflow or shortage. The specific volume of the molded product depends on the pressure and temperature of the molten material when the gate is closed during the pressure holding stage. If the pressure and temperature during each transition from holding pressure to the cooling stage of the product are consistent, then the specific volume of the product will not change.

At a constant molding temperature, the most important parameter that determines the size of a 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 the filling of the mold is completed, the holding pressure immediately decreases. When a certain thickness is formed on the surface, the holding pressure rises again. This can use low clamping force to form thick walled large products, eliminating collapse pits and burrs.

The holding pressure and velocity are usually 50% to 65% of the maximum pressure and velocity when filling the mold cavity with plastic, which means that the holding pressure is approximately 0.6 to 0.8 MPa lower than the injection pressure. Due to the lower holding pressure compared to the injection pressure, during a considerable holding time, the load of the oil pump is low, and the service life of the solid oil pump is extended. At the same time, the power consumption of the oil pump motor is also reduced.

Triple pressure injection can ensure smooth mold filling of the workpiece without any welding lines, dents, burrs, or warping deformation. For the molding of thin-walled parts, multi head small parts, long process large parts, and even for parts with uneven cavity configuration and less tight clamping, it is beneficial.

Program control of plastic filling amount in the injection mold cavity

By pre adjusting a certain amount of measurement, there is still a small amount of molten material (buffer) remaining at the end of the injection stroke near the end of the screw. According to the filling situation in the mold, further injection pressure (second or third injection pressure) is applied to supplement a small amount of molten material. This can prevent product indentation or adjust the shrinkage rate of the product.

Program control of screw back pressure and speed

High back pressure can cause strong shear of the molten material, and low rotational speed can also result in longer plasticization time of the plastic inside the barrel. Therefore, at present, more and more programs are used to control both backpressure and speed simultaneously. For example, during the full stroke of screw metering, high speed and low back pressure are first applied, then switched to lower speed and higher back pressure, and then switched to high back pressure and low speed. Finally, plasticization is carried out at low back pressure and low speed. This releases most of the pressure on the molten material in front of the screw, reduces the rotational inertia of the screw, and improves the accuracy of screw metering. Excessive back pressure often leads to an increase in the degree of discoloration of colorants;

The mechanical wear of the pre molding mechanism and the screw of the machine barrel increases; Prolonged pre molding cycle and decreased production efficiency; The nozzle is prone to salivation, resulting in an increase in the amount of recycled material; Even if a self-locking nozzle is used, if the back pressure is higher than the designed spring locking pressure, it will still cause fatigue damage. So, the back pressure must be adjusted appropriately.