Calculate the required clamping force for die casting machines
Calculating the required clamping force of a die-casting machine is a critical step before die-casting production, directly impacting mold safety and casting quality. Insufficient clamping force can cause the mold parting surface to expand, resulting in flash, burrs, and even damage to the mold. Excessive clamping force increases equipment energy consumption and accelerates mold wear, making accurate clamping force calculation crucial. The clamping force calculation is based on the expansion force generated by the molten metal filling the mold. This expansion force is the force exerted by the molten metal on the mold cavity under high pressure. Its magnitude is closely related to the projected area of the casting and the injection pressure ratio. The calculation formula is: Clamping Force ≥ Projected Area of Casting × Injection Pressure × Safety Factor. The safety factor is typically set between 1.2 and 1.5 to compensate for calculation errors and fluctuations during production, ensuring sufficient margin for clamping force.
Accurately measuring the projected area of a casting is fundamental to calculating clamping force. This refers to the projected area of the casting on the mold parting surface. This includes the projections of all areas in contact with the molten metal, including the casting body, gates, and overflow channels. When measuring, it’s important to note that the projected area of complex castings cannot be simply calculated based on the overall dimensions; it requires precise measurement using CAD software. For example, for an automotive wheel casting with multiple bosses and grooves, the projected area must include the wheel body, gate system, and overflow channels. Omitting the overflow channel area (typically 10-20% of the casting area) will result in an undercalculated clamping force, increasing production risk. For symmetrical castings, measuring half the area and multiplying by two improves measurement efficiency. For irregularly shaped castings, a grid method can be used. Dividing the projected area into several small grids, calculating the area of each grid and summing the results, can reduce the error to within 5%. A die-casting plant once failed to take the gate area into account, resulting in the calculated clamping force being 15% less than the actual requirement. This caused severe flash during production and forced the plant to shut down and replace the die-casting machine with one with greater clamping force, causing construction delays.
The proper selection of shot pressure significantly impacts clamping force calculations. Shot pressure is the pressure per unit area exerted by the injection punch on the molten metal and is determined based on the alloy type, casting structure, and quality requirements. Shot pressures for aluminum alloy die casting typically range from 30-80 MPa, for zinc alloys from 20-60 MPa, and for copper alloys from 80-150 MPa. For thin-walled, complex castings, a higher shot pressure (e.g., 60-80 MPa for thin-walled aluminum alloys) is required to ensure the molten metal fills the mold cavity. For thick-walled, simple castings, a lower shot pressure (e.g., 30-50 MPa for thick-walled aluminum alloys) can be used to reduce die expansion forces. For example, when producing mobile phone casings (wall thickness 0.5-1 mm), an aluminum alloy shot pressure of 70 MPa is required, while for motor end caps (wall thickness 3-5 mm), a shot pressure of 40 MPa is sufficient. The selection of shot pressure also needs to consider mold strength. Excessively high shot pressure will aggravate mold wear. Therefore, the shot pressure should be reduced as much as possible while ensuring the quality of the casting. A die-casting plant reduced the shot pressure of aluminum alloy from 65MPa to 55MPa by optimizing the pouring system, reducing the clamping force requirement by 15% and extending the mold life by 20%.
Determining the safety factor requires comprehensive consideration of various uncertainties in production to ensure sufficient margin in the clamping force. When the casting structure is complex and the projected area calculation is difficult, a larger safety factor (1.4-1.5) should be used; when the casting structure is simple and the projected area measurement is accurate, a smaller safety factor (1.2-1.3) can be used. Furthermore, factors such as mold wear and equipment pressure fluctuations must be considered. For example, if a mold has been used for more than 100,000 cycles, wear on the parting surface may lead to a decrease in sealing performance, and the safety factor should be increased to 1.5. When producing battery casings, a new energy vehicle parts factory used a safety factor of only 1.2 because they failed to account for mold wear after long-term use. Late in production, leakage from the parting surface occurred, forcing the machine to shut down for mold repairs, resulting in production interruptions. Therefore, the safety factor should be selected conservatively, erring on the side of caution to avoid the risk of insufficient clamping force.
In actual production, calculations of clamping force require empirical correction, taking into account special factors such as mold structure and alloy properties. For molds with core-pulling mechanisms, the core-pulling force will have a certain impact on the clamping force, necessitating an appropriate increase in clamping force (typically by 5-10%). For castings using vacuum die-casting, the lower cavity pressure slightly reduces the expansion force during molten metal filling, so the safety factor can be appropriately reduced (to 1.1-1.2). Furthermore, the actual clamping force output of different brands of die-casting machines may vary. The nominal and actual clamping force of some machines can deviate by as much as 5-10%. Therefore, the correction factor provided by the equipment manufacturer should be considered when making calculations. For example, the actual clamping force of a certain brand’s 2000kN die-casting machine is 95% of the nominal value, so it should be calculated as 1900kN. The most reasonable clamping force can only be determined by combining theoretical calculations with empirical corrections. A large die-casting company has established a clamping force calculation database, which includes calculation cases and actual verification data for thousands of castings. The accuracy of clamping force calculations has reached over 98%, effectively ensuring production stability and economy.
