Compression chamber fullness
The chamber fill ratio refers to the ratio of the volume of molten metal within the chamber to the chamber’s effective volume. It is a key parameter in the die-casting process that influences injection molding and die-casting quality, directly impacting the flow of the molten metal, pressure transmission, and gas entrainment. A reasonable chamber fill ratio ensures smooth molten metal flow and effective pressure transmission during the injection process, minimizing defects. Excessive or insufficient chamber fill ratios can negatively impact die-casting quality. The formula for calculating chamber fill ratio is: Fill ratio = (molten metal volume / chamber effective volume ) × 100%, where the chamber effective volume refers to the volume formed from the injection piston’s starting position to the moment the punch contacts the sprue bushing.
The chamber filling level should be determined based on the weight of the die-casting, the chamber diameter, and the characteristics of the die-casting alloy. For small and medium-sized die-castings, the chamber filling level is typically controlled between 40% and 80%, while for large die-castings, the filling level can be increased to 60% to 90%. Different alloys have different filling requirements. Zinc alloys have better fluidity, so the filling level can be controlled between 40% and 70%. Aluminum and magnesium alloys have relatively poor fluidity and are easily oxidized, so the filling level is typically controlled between 60% and 80% to reduce oxidation and air entrainment of the molten metal within the chamber. For example, when producing aluminum alloy automotive wheels, the chamber filling level should be controlled between 70% and 80%. If it is below 60%, the flow of molten metal within the chamber becomes unstable, easily entraining air and causing increased porosity within the die-casting. If it is above 85%, the excess molten metal may overflow the chamber, causing waste and safety hazards.
Underfilling the die casting chamber can negatively impact the die casting process in several ways. When the chamber fill level is below 30%, the amount of molten metal in the die casting chamber is low. During the injection process, the shot piston pushes the molten metal over a longer distance, causing a sharp increase in the metal’s velocity within the chamber. This can lead to turbulence and the entrainment of large amounts of air and scale. These gases and scale enter the die cavity, causing defects such as porosity and inclusions in the die casting. Furthermore, underfilling reduces the energy efficiency of the injection system and the efficiency of pressure transmission. This can lead to insufficient pressure during cavity filling, resulting in poor filling and cold shuts. For example, when the chamber fill level of a zinc alloy die casting is below 30%, the porosity of the die casting can increase by over 50%, and the surface quality can significantly deteriorate, with pitting and cold shut streaks appearing. Furthermore, underfilling increases wear on the shot piston and chamber. The low amount of molten metal prevents effective lubrication and cooling of the injection components, increasing friction.
Overfilling the injection chamber can also lead to a host of problems. When the filling level exceeds 90%, the molten metal in the injection chamber nearly fills the entire chamber. During the injection process, the molten metal easily overflows from the gap between the injection chamber and the injection piston, resulting in metal waste and equipment contamination. The overflowing hot metal also poses a safety hazard. Excessive filling makes it difficult for gases within the injection chamber to escape. These gases are compressed during the injection process and may eventually enter the mold cavity with the molten metal, forming pores. Furthermore, overfilling increases the load on the injection system, as greater force is required to push large amounts of molten metal. Long-term operation can increase wear on components such as the injection cylinder and injection rod, shortening the equipment’s service life. For example, when the injection chamber filling level of an aluminum alloy die-casting reaches 95%, the probability of flashing during the injection process increases by 30%, the porosity defect rate of the die-casting increases significantly, and the failure rate of the injection system also increases significantly.
Controlling the chamber filling level requires precise metering and stable operation. During production, the required volume of molten metal must be accurately calculated based on the weight of the die-cast part. A quantitative pouring system (such as a spoon or automatic pouring machine) controls the amount of molten metal injected into the chamber to ensure the filling level remains within the set range. For automated production lines, sensors monitor the weight or volume of the molten metal in real time, providing feedback to the control system, which automatically adjusts the pouring volume to keep the filling level fluctuation within ±5%. Regular inspection of the chamber for wear is also necessary, as wear increases its effective volume, reducing the actual filling level. For example, if the chamber diameter increases by 0.5mm due to wear, its effective volume increases by approximately 5%. In this case, the pouring volume must be increased accordingly to maintain the set filling level. Precisely controlling the chamber filling level can effectively improve the consistency of die-cast part quality, reduce scrap rates, and extend the service life of the mold and equipment.
