Die casting speed
Die casting speed refers to the velocity of molten metal flowing through the injection molding system during the die casting process. It comprises two key parameters: injection speed and filling speed. These parameters significantly impact the molding quality, surface finish, and internal structure of die-cast parts. Injection speed refers to the speed of the injection piston, measured in meters per second (m/s); filling speed refers to the speed at which the molten metal enters the mold cavity, also measured in meters per second. Filling speed is closely related to injection speed and is often indirectly controlled through injection speed. In die casting production, properly selecting and controlling the casting speed is crucial for ensuring die-casting quality.
The choice of die-casting speed depends on the die-casting’s structure, size, wall thickness, and alloy type. Thin-walled, complex die-castings require a higher filling speed to prevent the molten metal from prematurely solidifying during the filling process. Typical filling speeds are 5-10 m/s. For example, thin-walled aluminum alloy die-castings, such as mobile phone casings, typically require a filling speed of 8-10 m/s to ensure the molten metal fills the cavity quickly. For thicker-walled, simple die-castings, the filling speed can be lowered to 2-5 m/s. For example, some zinc alloy motor casings can meet the requirements with a filling speed of 3-4 m/s. Different alloys have different filling speed requirements. Aluminum alloys generally require higher filling speeds than zinc alloys because aluminum alloys solidify faster, requiring a faster filling speed to ensure the cavity is fully filled.
During the die-casting process, the injection speed is typically controlled in multiple stages to accommodate the various filling phases. The first stage is a slow injection, with a relatively low speed (typically 0.1-0.3 m/s). This stage aims to smoothly push the molten metal to the front of the die chamber, avoiding splashing and entrainment of gases, while also preventing premature solidification within the chamber. The second stage is a fast injection, with a rapidly increasing speed (typically 1-5 m/s). This allows the molten metal to quickly enter the mold cavity and complete the filling process before solidification. For large or complex die-castings, the injection speed can be increased in the third stage, then appropriately reduced near the end of the filling phase to minimize impact and entrainment. For example, in the die-casting process for an automotive transmission housing, the injection speed is 0.2 m/s in the first stage, increased to 3 m/s in the second stage, and then reduced to 1.5 m/s in the third stage. This speed control ensures efficient filling while minimizing defects.
The impact of die-casting speed on die-casting quality is primarily reflected in the following aspects. Properly increasing the filling speed can increase the fluidity of the molten metal, ensuring sufficient filling of the cavity, reducing defects such as underfilling and cold shuts, and improving the surface quality of the die-casting, making the surface smoother and flatter. However, excessively high filling speeds can increase the impact of the molten metal on the cavity, easily generating eddy currents and air entanglement, resulting in defects such as pores. This can also increase mold wear and shorten mold life. Excessively low filling speeds can cause the molten metal to cool too quickly during the filling process, reducing fluidity and leading to underfilling or cold shuts, loosening the internal structure of the die-casting, and reducing mechanical properties. For example, when the filling speed is less than 2 m/s, cold shuts are very common in the production of thin-walled aluminum alloy die-castings, while filling speeds exceeding 10 m/s can lead to an increase in internal pores in the die-casting.
Controlling the stability of the die-casting speed is crucial to ensuring the consistency of die-casting quality. Modern die-casting machines use advanced servo control systems that can precisely control the injection speed so that the deviation between the actual speed and the set speed does not exceed ±0.1m/s. During the production process, the injection system of the die-casting machine needs to be calibrated and maintained regularly, and the wear of components such as the injection cylinder and injection rod needs to be checked. Severely worn parts should be replaced in a timely manner to ensure the stability of the injection speed. At the same time, it is also necessary to pay attention to the impact of factors such as the temperature of the molten metal and the temperature of the injection chamber on the die-casting speed. When these factors change, the injection speed parameters should be adjusted in a timely manner to ensure the stability of the die-casting quality. For example, when the temperature of the molten metal decreases, its fluidity decreases. At this time, the injection speed needs to be appropriately increased to compensate for the lack of fluidity and ensure that the mold cavity can be filled.