The die-casting specific pressure refers to the pressure exerted by the injection punch on a unit area of molten metal. It is a crucial parameter in the die-casting process and directly affects the density, mechanical properties, and surface quality of the casting. Reasonable determination of the specific pressure requires comprehensive consideration of multiple factors such as alloy type, casting structure, and mold conditions. It is key to ensuring smooth die-casting production and stable product quality. If the specific pressure is too small, the molten metal filling power is insufficient, and defects such as insufficient pouring and cold shut are prone to occur. If the specific pressure is too large, the mold load will increase, leading to problems such as flash and cracks, while also increasing energy consumption. Therefore, accurate determination of the specific pressure requires optimization based on theoretical analysis combined with production practice.
The type of alloy is the primary basis for determining the specific pressure for die casting. Different alloys vary greatly in fluidity, solidification characteristics, and strength, and therefore have varying pressure requirements. Zinc alloys have good fluidity and a low melting point, requiring a relatively low specific pressure, typically between 20 and 60 MPa. For example, when producing zinc alloy toy car shells, a specific pressure of 30 to 40 MPa can meet filling requirements. Aluminum alloys have moderate fluidity, with a specific pressure typically between 30 and 80 MPa. High-silicon aluminum alloys (such as ADC12) have good fluidity, so the specific pressure can be taken at the lower limit, while aluminum-magnesium alloys (such as AM60) require the upper limit to ensure filling. Although magnesium alloys have low density, they have poor fluidity at high temperatures, requiring a specific pressure of 40 to 100 MPa. Copper alloys have high melting points and poor fluidity, requiring the highest specific pressure of all die-casting alloys, typically between 80 and 150 MPa. Some complex copper alloy castings even require a specific pressure exceeding 200 MPa. When a die-casting plant produced brass bathroom accessories, the initial specific pressure was only 100MPa, which caused shrinkage in the castings. After adjusting to 130MPa, the defect rate dropped from 15% to 2%.
The casting structure significantly influences the choice of die-casting pressure ratio, primarily in terms of wall thickness, complexity, and size. Thin-walled castings (less than 2mm thick) experience rapid cooling of the molten metal in the mold cavity, necessitating a higher pressure ratio to accelerate filling. For example, when die-casting a mobile phone midframe (0.8-1.2mm thick) from aluminum alloy, a pressure ratio of 60-80 MPa is required to avoid cold shuts and under-casting. Thick-walled castings (greater than 5mm thick) require a lower pressure ratio. Excessively high pressure ratios can lead to excessive internal stresses and even cracks. For example, the pressure ratio for aluminum alloy die-casting of automotive motor end covers (6-8mm thick) is typically 30-40 MPa. Complex castings with deep cavities or narrow slots create significant resistance to molten metal flow and require a higher pressure ratio to ensure complete filling. For example, a gearbox housing with multiple ribs requires a pressure ratio 20-30% higher than that of a simple part made of the same material. In addition, the selection of specific pressure for large castings also needs to consider pressure loss. Because the molten metal flows a long distance in the mold cavity, the pressure decays significantly, and the specific pressure needs to be appropriately increased. When a company produced a 1.2-meter-long aluminum alloy bumper bracket, the specific pressure was increased from the conventional 50MPa to 65MPa, which solved the problem of insufficient filling at the far end.
Mold design and usage also influence the specific pressure required for die casting. The mold’s runner configuration, venting system, and temperature control directly impact the flow resistance of the molten metal. Using a wide runner and optimally positioned ingates can reduce flow resistance and lower the required specific pressure. For example, increasing the ingate cross-sectional area from 8mm² to 12mm² in a certain mold reduced the specific pressure requirement for aluminum alloy castings from 70MPa to 55MPa while still ensuring good filling. Molds with poor venting prevent gas from escaping the cavity in a timely manner, hindering molten metal filling and requiring a higher specific pressure to overcome gas resistance. However, a properly designed venting system can reduce the specific pressure by 10-15%. Mold temperature is also a crucial factor. Low-temperature molds (e.g., 80-120°C for zinc alloy molds) cause the molten metal to solidify rapidly, requiring a higher specific pressure. High-temperature molds (e.g., 200-250°C for aluminum alloy molds) can reduce the specific pressure requirement. In addition, the degree of mold wear also needs to be considered. The mold that has been used for a long time has increased surface roughness of the cavity and increased flow resistance, so the specific pressure needs to be increased by 5-10% compared to the new mold.
The method for determining die casting pressure ratios typically involves three steps: theoretical calculation, empirical comparison, and trial mold adjustments. Theoretical calculations estimate the initial pressure ratio based on the projected area of the casting, alloy density, and required filling rate. The formula is: pressure ratio = (casting mass × filling rate × resistance coefficient) / ingate area. The resistance coefficient is selected between 0.8 and 1.2, depending on the alloy type and mold complexity. Empirical comparisons refer to established process parameters for similar castings. For example, when producing a casting with a similar structure to an aluminum alloy motor housing, a 50 MPa pressure ratio setting can be directly referenced and fine-tuned based on actual conditions. Trial mold adjustments are crucial for ultimately determining the pressure ratio. By gradually varying the pressure ratio and observing casting defects, the optimal range is found. During trial molds, a medium pressure ratio can be initially used. If underfilling occurs, the pressure ratio can be gradually increased. If flashing occurs, the pressure ratio can be appropriately reduced. One die-casting company has established a pressure ratio parameter database containing thousands of examples of pressure ratio settings for various castings. Combined with trial mold feedback, this database has reduced the time required to determine the pressure ratio for a new casting from the traditional 2-3 days to within one day, significantly improving process development efficiency. At the same time, with the development of intelligent die-casting technology, dynamic optimization of specific pressure can be achieved through real-time monitoring of the injection curve and casting quality, further improving production stability.
