Die casting chamber filling degree
The chamber fill level in a die casting process refers to the ratio of the volume of molten metal within the chamber to the chamber’s effective capacity. It is a key parameter influencing the stability of the injection process and casting quality. A reasonable chamber fill level ensures a stable flow of molten metal during the injection process, reducing air entrapment and oxidation. However, excessive or insufficient chamber fill levels can negatively impact the die casting process. For example, if the chamber fill level is too low (e.g., below 40%), the amount of molten metal in the chamber is small, and the punch’s movement of the molten metal can easily generate splashing and vortexes, leading to the entrainment of large amounts of air into the molten metal and the formation of porosity defects. If the chamber fill level is too high (e.g., above 85%), insufficient space is left in the chamber for the molten metal to escape, potentially leading to overflow and waste during the initial injection phase. This also increases the load on the injection system and affects the stability of the injection speed. Therefore, the chamber fill level should typically be controlled between 50% and 80%, with the specific value adjusted based on the weight of the casting, chamber diameter, and alloy properties.
Setting the chamber filling level is closely related to the weight of the casting and the chamber dimensions. Given a given casting weight, the chamber diameter directly determines the filling level: a chamber diameter that is too large will result in underfill, while a chamber diameter that is too small will result in overfill. Therefore, when designing the mold and selecting the die-casting machine, it is important to calculate the appropriate chamber diameter based on the average weight of the casting to ensure a reasonable filling level. For example, for a 500g aluminum alloy casting, if a chamber with an 80mm diameter and an effective length of 300mm (effective volume approximately 1500cm³) is selected, the molten metal volume will be approximately 500cm³ (aluminum alloy density is approximately 2.7g/cm³), resulting in a filling level of approximately 33%, which is below the acceptable range. In this case, a chamber with a smaller diameter (such as 60mm) should be selected to increase the filling level. However, if a chamber with a 50mm diameter (effective volume approximately 589cm³) is selected, the filling level will be approximately 85%, which is within the acceptable range.
The chamber fill level significantly influences the flow characteristics of molten metal during the injection molding process. When the fill level is optimal, the molten metal forms a continuous, smooth flow front within the chamber. The thrust from the injection punch acts evenly on the molten metal, allowing it to enter the mold cavity at a steady rate, reducing turbulence and air entrainment. When the fill level is too low, the contact area between the punch and the molten metal is small, resulting in uneven thrust distribution. This can easily lead to turbulent flow within the chamber, even causing “flow interruptions,” which can cause significant air entrainment. When the fill level is too high, the molten metal has little buffer space within the chamber, resulting in greater initial impact force, potentially causing it to overflow from the chamber-mold junction. This excess molten metal can also slow the injection speed, compromising the filling effect. For example, in the production of small, thin-walled castings, if the fill level is only 30%, splashing of the molten metal during injection can increase the casting’s porosity by over 20%. However, when the fill level is increased to 60%, the porosity can be reduced to below 5%.
The chamber’s fullness also affects the efficiency of injection pressure transmission. During the injection process, injection pressure must be transmitted to the mold cavity via the molten metal to compensate for shrinkage and compact the casting. When the chamber is properly filled, the molten metal forms a continuous “liquid column” within the chamber, enabling efficient and uniform pressure transmission and ensuring consistent pressure distribution throughout the cavity. When the chamber is too low, a large amount of air is present. The compressibility of air causes hysteresis and loss in pressure transmission, resulting in insufficient pressure at the end of the cavity and affecting the density of the casting. When the chamber is too high, the gas content in the molten metal is low, but the excessive height of the liquid column increases backpressure in the injection system, leading to increased injection pressure loss and similarly affecting pressure transmission efficiency. For example, experimental data shows that when the fill level drops from 50% to 30%, the actual pressure at the end of the cavity drops by approximately 30%, resulting in a significant increase in shrinkage in the thick-walled areas of the casting.
In actual production, controlling the chamber fill level requires a comprehensive consideration of both production flexibility and stability. For companies producing a wide variety of castings in small batches, frequent chamber changes may be necessary to ensure the proper fill level, due to the large variations in casting weights. This increases production setup time and costs. In these cases, an adjustable chamber can be used or the amount of molten metal scooped can be adjusted to dynamically control the fill level. For example, a metered pouring system can precisely control the amount of molten metal entering the chamber each time, ensuring a 50% to 80% fill level for castings of varying weights. Furthermore, computer simulation technology can be used to predict the injection molding process at various fill levels, helping engineers determine the optimal fill level range before production and reducing process debugging time. Through simulation, one die-casting company found that, when producing a series of aluminum alloy housings, maintaining a fill level between 65% and 75% resulted in optimal shot stability, a 12% increase in casting yield, and an 8% reduction in raw material waste. This fill level management approach, based on precise control and simulation optimization, is a key tool for improving die-casting production efficiency and product quality.
