What are the Influencing Factors of Plastic Injection Molding Process?

1. Influence of Temperature

A. Material Temperature

The processing temperature of plastic is controlled by the injection machine barrel. The proper selection of barrel temperature is crucial for ensuring smooth plastic injection molding without causing local degradation of the plastic. Generally, the highest temperature at the end of the barrel should be higher than the plastic's flow temperature (or melting temperature) but lower than its decomposition temperature.

In addition to strictly controlling the maximum temperature of the injection machine barrel, the residence time of the plastic melt in the barrel should also be controlled. When determining the barrel temperature, the structural characteristics of the product and the mold should also be considered. For thin-walled or complex-shaped products, which have high flow resistance, increasing the barrel temperature helps improve the flowability of the melt.

Usually, the maximum temperature of the nozzle is slightly lower than that of the barrel to prevent melt elongation at the nozzle.

B. Mold Temperature

The mold temperature during the plastic injection molding process is controlled by a cooling medium, typically water. It determines the cooling rate of the plastic melt. Lower mold temperatures result in faster cooling of the melt, leading to a rapid decrease in melt temperature, increased melt viscosity, increased injection pressure loss, and, in severe cases, insufficient filling of the mold. As the mold temperature increases, the melt flowability improves, the required filling pressure decreases, and the surface quality of the product improves. However, due to increased cooling time, the productivity of the product decreases, and the molding shrinkage of the product increases.

For crystalline plastics, higher mold temperatures favor crystallization, resulting in higher product density or structural crystallinity. At higher mold temperatures, the relaxation process of polymer macromolecules in the product is faster, and both molecular orientation and internal stress decrease.

2. Influence of Pressure

The pressure in the plastic injection molding process includes plasticizing pressure, injection pressure, and cavity pressure.

Plasticizing pressure, also known as back pressure, refers to the pressure exerted on the melt at the top of the screw when it retracts during screw rotation. It is controlled by adjusting the backflow resistance of the injection hydraulic cylinder. Increasing the plasticizing pressure enhances the internal pressure of the melt and strengthens the shear effect. Due to the shear heating of the plastic, it also raises the melt temperature. Increasing the plasticizing pressure slows down the screw retraction speed, prolongs the heating time of the plastic in the screw, and improves plasticization quality. However, excessive plasticizing pressure increases the backflow and leakage of the melt in the metering chamber of the barrel, reducing its conveying capacity, decreasing plasticization, increasing power consumption, and causing degradation of the melt due to excessive shear heating or shear stress.

Injection pressure refers to the melt pressure generated at the head of the screw during injection. When selecting the injection pressure, the allowable injection pressure of the injection machine should be considered first. Only when the injection pressure is too low and leads to insufficient cavity pressure, causing the melt to fail to fill the cavity smoothly, or when the injection pressure is too high, resulting in product overflow, deformation, or even system overload.

During the injection process, the injection pressure and melt temperature are mutually restrictive. Satisfactory results can be obtained with a combination of high melt temperature and the required injection pressure.

Cavity pressure refers to the melt pressure generated in the mold cavity after the injection pressure experiences pressure loss through the nozzle, runner, and gate.

3. Injection Molding Cycle and Injection Speed

The time required to complete one injection molding cycle is called the injection molding cycle, which includes feeding, heating, filling, holding pressure, cooling time, as well as mold opening, demolding, mold closing, and auxiliary operations. Throughout the injection molding cycle, injection speed and cooling time have a decisive influence on the product's performance.

Injection speed primarily affects the flow behavior of the melt in the mold cavity. Generally, with an increase in injection speed, the melt flow velocity and shear action intensify. The melt temperature increases due to shear heating, leading to decreased viscosity, which is favorable for filling the mold. Moreover, the fusion line strength of different parts of the product also increases. However, increasing the injection speed can potentially transition the melt from a laminar flow state to turbulent flow. In severe cases, this can cause the melt to be ejected from the mold, preventing the escape of air from the mold cavity. This trapped air is compressed and heated at high pressure, leading to local scorching or decomposition of the melt.

4. Conclusion

In summary, the plastic injection molding process is influenced by various factors. Temperature parameters, including material temperature, nozzle temperature, and mold temperature, play a crucial role in plasticization, flowability, cooling, and product quality. Pressure parameters, such as plasticizing pressure, injection pressure, and cavity pressure, affect the melt's behavior and filling of the mold. Additionally, the injection molding cycle and injection speed impact the flow characteristics and performance of the product.

It is important to carefully adjust and optimize these influencing factors to achieve the desired results in the injection molding process.