Die casting injection force
The injection force of a die-cast part is the force exerted by the die-casting machine’s shot cylinder to push the shot piston, hydraulically injecting metal into the mold cavity. It is one of the most critical process parameters in the die-casting process, directly affecting the filling speed and integrity of the molten metal, as well as the density of the die-cast part. The injection force must be determined based on factors such as the die-casting’s structure, size, and alloy type. Excessive or insufficient injection force can adversely affect the quality of the die-cast part. In-depth research on the factors affecting injection force and its calculation methods is crucial for rationally setting die-casting process parameters and ensuring stable production.
The injection force is closely related to the working area of the shot cylinder and the injection pressure. Its calculation formula is: Injection Force = Shot Cylinder Area × Injection Pressure. The shot cylinder area is determined by the specifications of the die-casting machine. Different tonnage die-casting machines are equipped with shot cylinders of varying diameters. For example, a 1000-ton die-casting machine typically has a shot cylinder diameter of 120-160mm. The injection pressure refers to the pressure of the hydraulic oil within the shot cylinder, generally ranging from 5-20MPa. This pressure can be precisely controlled by adjusting the hydraulic system’s pressure valve. In actual production, the injection force should be selected based on the principle of “sufficient but not excessive”: sufficient injection force ensures that the molten metal fills the mold cavity and achieves a dense microstructure; excessive injection force increases the mold load, leading to flash and increased energy consumption. For example, for small zinc alloy die-castings, the injection force is typically 50-100kN; for large aluminum alloy die-castings, the injection force may be 500-1000kN.
The structure and size of the die-cast part are key factors influencing the selection of injection force. Complex die-cast parts often feature narrow runners, deep cavities, or thin walls, creating significant flow resistance during the molten metal filling process and requiring a higher injection force to ensure complete filling. For example, box-shaped castings with multiple elongated holes require approximately 30% higher injection force than simple flat castings. The projected area of the casting is also directly proportional to the injection force. The larger the projected area, the greater the reaction force on the mold cavity during filling, and the higher the required injection force. Generally speaking, the required injection force is 5-10 kN per square centimeter of projected area. For large castings with a projected area of 1000 cm², the required injection force can reach 5000-10000 kN. Furthermore, the wall thickness of the casting also affects the injection force: thin-walled parts require a higher injection force to ensure rapid filling and prevent premature solidification of the molten metal; thick-walled parts can use a lower injection force to reduce internal stress.
The characteristics of the die-casting alloy also significantly affect the injection force requirements. Different alloys vary in fluidity, solidification temperature, and viscosity, resulting in different injection forces required during the filling process. Zinc alloys have good fluidity and low solidification temperatures, requiring relatively low injection forces. Aluminum alloys have lower fluidity but require higher strength, so moderate injection forces are required. Copper alloys have poor fluidity and high solidification temperatures, requiring higher injection forces to ensure effective filling. For example, when die-casting parts with the same structure, copper alloys require 50%-80% higher injection forces than zinc alloys. The alloy’s pouring temperature also affects the injection force: if the temperature is too high, the alloy’s fluidity improves, and the injection force can be appropriately reduced. If the temperature is too low, the alloy’s viscosity increases, increasing flow resistance and requiring a higher injection force. Therefore, when determining the injection force, a comprehensive adjustment should be made based on the alloy type and pouring temperature.
The design of the mold’s runner and cavity has a significant impact on the efficiency of the injection force transmission. A reasonable runner design can reduce the flow resistance of the molten metal and improve the effective utilization of the injection force. For example, main runners and branch runners with circular cross-sections have less flow resistance than those with square or trapezoidal cross-sections, and can achieve a higher filling speed at the same injection force. The length and cross-sectional area of the runner must also be matched. A runner that is too long or too thin will increase pressure loss, requiring a higher injection force to ensure that the molten metal fills the cavity. The surface roughness of the mold cavity also affects the injection force: the smoother the surface, the lower the friction coefficient, the lower the flow resistance of the molten metal, and the lower the required injection force. Therefore, by optimizing the mold design and improving the transmission efficiency of the injection force, the required injection force can be reduced while ensuring filling quality, thereby reducing mold wear and energy consumption.
The dynamic characteristics of the injection force have a significant impact on the quality of die-cast parts. The injection force is not a static, unchanging force; rather, it changes dynamically over time, with its variation curve closely correlated to the injection speed. During the slow-speed injection phase, the injection force slowly increases, pushing the molten metal forward steadily. During the fast-speed injection phase, the injection force rapidly increases, creating a pressure peak that ensures the molten metal quickly fills the mold cavity. During the holding phase, the injection force remains stable to achieve shrinkage compensation. The dynamic response speed of the injection force is also critical. If the injection force cannot be adjusted in time with changes in injection speed, it can lead to unstable molten metal flow, generating eddies and air entrainment. Modern die-casting machines utilize servo-hydraulic systems, which enable rapid response and precise control of the injection force, perfectly matching the dynamic characteristics of the injection force with the filling process, thereby improving the quality and stability of die-cast parts. Optimizing the dynamic characteristics of the injection force can also reduce shock and vibration during the die-casting process, extending the service life of the mold and die-casting machine.
