Die-casting pressure is a key process parameter that influences the quality of die-cast parts during the die-casting process. It refers to the force exerted by the working fluid in the die-casting machine’s shot cylinder on the shot piston, which is transmitted to the molten metal through the shot rod, causing the molten metal to fill the mold cavity and form a dense die-cast part. Die-casting pressure can be divided into injection pressure and specific pressure. Injection pressure refers to the total pressure in the shot cylinder, measured in Newtons (N); specific pressure is the ratio of injection pressure to the projected area of the die-cast part in the injection direction, measured in megapascals (MPa). Specific pressure more intuitively reflects the pressure on the molten metal and is a commonly used parameter in actual production.
The choice of injection pressure should be determined based on the structural characteristics of the die-casting, the type of alloy, and the mold conditions. For small, simple die-castings, such as zinc alloy toy parts, a specific pressure of 30-50 MPa is typically selected. For medium-sized, complex aluminum alloy die-castings, such as automobile engine hoods, the specific pressure should be increased to 60-100 MPa. For large, unevenly thick, or high-strength die-castings, such as magnesium alloy automobile wheels, the specific pressure may need to reach 100-150 MPa. Appropriately increasing the specific pressure can increase the fluidity of the molten metal, ensuring sufficient cavity filling, reducing defects such as porosity and shrinkage, and improving the density and mechanical properties of the die-casting. For example, when the specific pressure of an aluminum alloy die-casting is increased from 60 MPa to 80 MPa, its tensile strength can be increased by 5%-10%, and its hardness can also be improved to a certain extent.
During the die-casting process, injection pressure varies in multiple stages, each with its own specific pressure effect. During the slow-shot phase, low pressure exists primarily to push the molten metal smoothly into the cavity, preventing splashing and gas entrainment. The specific pressure during this phase is typically 5-15 MPa. During the fast-shot phase, pressure rapidly increases, allowing the molten metal to fill the mold cavity at a high rate. The specific pressure during this phase is typically 30-80 MPa. The boost phase, the final stage of the injection process, reaches its maximum pressure. This phase compacts the solidifying metal, compensates for volumetric shrinkage, and reduces shrinkage cavities and porosity. The boost pressure is typically 20%-50% higher than that during the fast-shot phase. For example, during the die-casting of an aluminum alloy part, the slow-shot phase has a specific pressure of 10 MPa, the fast-shot phase has a specific pressure of 60 MPa, and the boost pressure reaches 90 MPa. This pressure gradient ensures both efficient filling and improved density of the die-cast part.
Excessive or insufficient die-casting pressure can negatively impact the quality of die-cast parts. If the pressure is too low, the molten metal lacks fluidity, making defects like under-casting and cold shut more likely to occur. This results in low density and poor mechanical properties in the die-casting. Excessive pressure increases the load on the die, accelerating die wear and shortening its lifespan. It can also cause flash on the die-casting, increasing the workload for subsequent cleaning and, in severe cases, cracking. For example, when the specific pressure exceeds 150 MPa, excessive stress can cause cracks in thin-walled aluminum alloy die-castings, particularly at corners and areas with sudden changes in wall thickness. Therefore, in actual production, die trials are necessary to determine the optimal die-casting pressure parameters. These are typically set based on empirical values and then gradually adjusted based on the defects observed in the die-casting until a qualified die-casting is obtained.
The precision of die-casting pressure control is crucial to the stability of die-casting part quality. Modern die-casting machines typically utilize closed-loop control systems to precisely control the injection pressure. Pressure sensors monitor pressure changes during the injection process in real time, compare them with the set value, and promptly adjust the working fluid pressure within the injection cylinder to ensure that the deviation between the actual pressure and the set pressure does not exceed ±2 MPa. Furthermore, the stability of the injection pressure is also related to the performance of the die-casting machine and the status of the hydraulic system. Regular maintenance of the die-casting machine and keeping the hydraulic system clean and in normal working condition help ensure stable die-casting pressure. For example, regularly replacing the hydraulic oil and cleaning the filter can prevent pressure fluctuations caused by hydraulic oil contamination or oil line blockage, thereby ensuring the consistency of die-casting part quality.
