The correct formulation of injection process is to ensure the good plasticization of plastic melt, and to fill and cool the mold smoothly in order to produce products with satisfactory quality; The most important process parameters in the injection process are temperature (material temperature, nozzle temperature, mold temperature), pressure (plasticizing pressure, injection pressure, cavity pressure) and corresponding action time (injection time, guarantee time, cooling time), etc. Only the main process parameters and their mutual influence will be discussed below.
First, the influence of temperature
1. Material temperature
The processing temperature of plastics is controlled by the barrel of injection machine. The correct choice of barrel temperature is related to the plasticizing quality of plastics, and its principle is to ensure smooth injection molding without causing local degradation of plastics. Generally, the maximum temperature at the end of the barrel should be higher than the flowing temperature (or melting temperature) of plastic, but lower than the decomposition temperature of plastic.
In production, besides strictly controlling the maximum temperature of the barrel of the injection machine, the residence time of the plastic melt in the barrel should also be controlled. When determining the barrel temperature, the structural characteristics of products and dies should also be considered. When forming thin-walled or complex-shaped products, the flow resistance is large, and increasing the temperature of the barrel is helpful to improve the fluidity of the melt.
Usually, the maximum temperature of the nozzle is controlled to be slightly lower than that of the barrel, so as to prevent casting of the melt at the nozzle opening.
2. Mold temperature
In the process of injection molding, the mold temperature is controlled by cooling medium (usually water), which determines the cooling rate of plastic melt. The lower the mold temperature, the faster the cooling speed, and the faster the melt temperature decreases, resulting in the increase of melt viscosity, injection pressure loss and even insufficient mold filling. With the increase of mold temperature, melt fluidity increases, mold filling pressure decreases and product surface quality improves. However, with the increase of cooling time, the productivity of products decreases and the molding shrinkage of products increases.
For crystalline plastics, higher temperature is beneficial to crystallization, so increasing mold temperature can improve the density or structural crystallinity of products. At higher mold temperature, the relaxation process of polymer macromolecules is faster, and the molecular orientation and internal stress will be reduced.
Second, the impact of pressure
The pressures in the injection molding process include plasticizing pressure, injection pressure and cavity pressure.
Plasticizing pressure, also known as back pressure, refers to the pressure on the melt at the top of the screw of the injection machine when the screw rotates backward, which is controlled by adjusting the oil return resistance of the injection hydraulic cylinder. Plasticizing pressure increases the internal pressure of the melt and enhances the shearing effect. The shearing heat of plastic increases the temperature of the melt. The increase of plasticizing pressure slows down the return speed of screw, prolongs the heating time of plastic in screw, and improves the plasticizing quality However, the excessive plasticizing pressure also increases the reverse flow and leakage flow of melt in the metering chamber of the charging barrel, reduces the conveying capacity of melt, reduces the plasticizing amount and increases the power consumption. Moreover, the excessive plasticizing pressure will cause shear heating or excessive shear stress, and the melt is prone to degradation.
Injection pressure refers to the melt pressure generated at the screw head during injection. When choosing injection pressure, the injection pressure allowed by injection molding machine should be considered first. Only when the injection pressure is too low, the cavity pressure is insufficient, and the melt can not fill the cavity smoothly; On the contrary, if the injection pressure is too high, it will not only cause product overflow, but also cause product deformation and even system overload.
Injection pressure and melt temperature are mutually restricted in the injection process. Satisfactory results can only be obtained under the combination of injection pressure and material temperature when the material temperature is high.
Cavity pressure refers to the melt pressure generated in the mold cavity after the injection pressure passes through the pressure loss of nozzle, runner and gate.
Third, injection molding cycle and injection speed
The time required to complete one injection molding is called injection molding cycle, which includes feeding, heating, mold filling, pressure maintaining, cooling time, mold opening, demoulding, mold closing and auxiliary operations. In the whole injection molding cycle, injection speed and cooling time have a decisive influence on the properties of products.
Injection speed mainly affects the flow behavior of melt in the cavity. Generally, with the increase of injection speed, the melt flow rate increases and the shear effect strengthens; Melt temperature increases and viscosity decreases due to shear heating, which is beneficial to mold filling. And the fusion grain strength of each part of the product is also increased. However, with the increase of injection speed, the melt may change from layer fluid state to flow. In severe cases, the melt will be sprayed in the mold and the air in the mold will not be discharged. This air will be heated by the compression speed under high pressure, which will cause local burning or decomposition of the product.
In actual production, the injection speed is usually confirmed by experiments. Generally, the injection is performed at low pressure and slow speed, and then the injection speed is adjusted according to the molding condition of the product.
Modern injection machines have realized multi-stage injection technology, that is, in an injection process, when the screw of the injection machine pushes the melt into the mold, the process parameters such as different injection speeds and different injection pressures at different positions can be controlled according to different needs. The multistage injection process should be drawn up and selected according to different varieties of plastics and different products.
