Filling Theory of Metal Die Casting
The filling theory of metal die casting is a scientific study of the process by which molten metal fills the mold cavity under high pressure. It forms the foundation of die-casting process design and mold optimization. This theory, which encompasses multiple disciplines, including fluid mechanics, thermodynamics, and metallurgy, aims to reveal the flow patterns, heat transfer, and solidification behavior of molten metal during the filling process. The quality of the die-casting is directly affected by defects such as underfill, cold shuts, porosity, and inclusions. Therefore, a thorough understanding of the filling theory of metal die casting is crucial for optimizing die-casting process parameters, improving mold design, and ensuring die-casting quality.
The filling process of metal die-casting is essentially a non-steady-state, high-speed flow process. Under the influence of the injection force, the molten metal flows from the pressure chamber through the runner into the mold cavity. Its flow state is affected by a variety of factors. According to the principles of fluid mechanics, the flow of molten metal can be divided into two states: laminar and turbulent. In the laminar state, the molten metal flows smoothly, and the flow layers do not interfere with each other, which facilitates smooth filling of the cavity and reduces air entrapment and oxidation. In contrast, the turbulent state causes the molten metal to flow in a disorderly manner, easily generating eddies and splashing, leading to gas entrapment and the formation of oxide slag, which affects the quality of the die-cast part. Filling theory provides a theoretical basis for controlling the flow state by studying the relationship between parameters such as molten metal flow rate, pressure, and temperature and the flow state. For example, by controlling the injection speed and pressure, the molten metal can maintain a laminar state as much as possible during the filling process, improving filling quality.
The filling pattern of molten metal is an important research topic in filling theory, mainly including three modes: jet filling, full-wall filling, and intermediate filling. Jet filling refers to the molten metal entering the mold cavity in the form of a high-speed jet. The front end of the jet is prone to atomization and oxidation, which is suitable for small, thin-walled, and simple-shaped castings. Full-wall filling is the molten metal entering the mold cavity at a slower speed, gradually rising along the bottom of the cavity, and smoothly filling the entire cavity. It is suitable for large, thick-walled, and complex-shaped castings. This filling mode can effectively reduce air entrapment and oxidation. Intermediate filling is somewhere in between the two, presenting different characteristics depending on the casting structure and process parameters. By analyzing the advantages, disadvantages, and applicable scope of different filling modes, filling theory provides guidance for selecting the appropriate filling mode based on the casting structure, which helps to improve the molding quality of die castings.
Heat transfer and solidification behavior play an important role in the filling process of metal die-casting and are also an important part of filling theory. During the filling process, the molten metal contacts the surface of the mold cavity, and a violent heat exchange occurs. The mold absorbs the heat of the molten metal, resulting in a decrease in the temperature of the molten metal, an increase in viscosity, and a decrease in fluidity. If the molten metal solidifies too early during the filling process, defects such as insufficient pouring or cold shut will occur. Filling theory establishes a heat transfer model by studying the temperature field distribution of the molten metal, the advancement speed of the solidification front, and the temperature changes of the mold, providing theoretical support for optimizing the design of the mold cooling system and the die-casting process parameters. For example, by rationally designing the cooling water channel of the mold and controlling the mold temperature, the solidification rate of the molten metal can be slowed down to ensure a smooth filling process.
With the development of computer technology, numerical simulation has become an important tool for studying metal die-casting filling theory, driving the continuous improvement of filling theory. By establishing a mathematical model and using a computer to numerically simulate the molten metal filling process, the flow trajectory, temperature distribution, and pressure changes of the molten metal can be intuitively displayed, and potential defects such as air holes, cold shuts, and under-casting can be predicted. Numerical simulation technology not only verifies the correctness of traditional filling theory but also discovers some new flow phenomena and patterns, enriching the content of filling theory. For example, simulations have revealed that when molten metal fills a complex cavity, the flow velocity and pressure distribution vary significantly at different locations, requiring optimization of the runner and venting system design to balance these differences. The application of numerical simulation technology enables filling theory to better guide actual production, providing a more scientific and accurate basis for die-casting process optimization and mold design, and promoting technological advancement in the die-casting industry.
