Determination of core pulling force and core pulling distance in die casting
The core-pulling force and core-pulling distance in die-casting are core parameters in the design of core-pulling mechanisms, directly determining the mechanism’s power selection and structural dimensions. Properly determining these two parameters ensures a smooth and reliable core-pulling process and avoids defects such as deformation and strain in die-cast parts. The core-pulling force refers to the force required to extract the core from the die-casting. Its magnitude is related to factors such as the die-casting material, molding area, surface roughness, and demolding angle. The core-pulling distance is the minimum distance the core must be pulled out to ensure it is completely clear of the side holes or undercuts in the die-casting. It is typically the sum of the molding depth and a safety margin. Accurately calculating these two parameters is key to avoiding redundancy or deficiencies in mechanism design and requires a comprehensive determination based on theoretical formulas and practical experience.
The calculation of core-pulling force is based on the clamping force and friction between the die casting and the core. The theoretical calculation formula is: Core-pulling force = (Clamping force of the casting on the core + Friction) / cosθ, where θ is the core draft angle. Clamping force is generated by the solidification shrinkage of the molten metal and is proportional to the molding area, the alloy shrinkage, and the core surface roughness. For example, an aluminum alloy with a shrinkage of 0.8% to 1.2% has a 30% to 50% greater clamping force than a zinc alloy (shrinkage of 0.5% to 0.8%). Friction is related to the core surface condition. Reducing the surface roughness Ra from 3.2μm to 0.8μm can reduce friction by 20% to 30%. A safety factor (1.5-2.0) must be introduced during actual calculations to account for uncertainties such as residual molten metal pressure and die-cast part deformation. For example, for an aluminum alloy die-cast part with a molding area of 100 cm², if the core-pulling force is calculated to be 50 kN, the actual design should be based on 75-100 kN.
The core pulling distance should be determined based on the principle that the core is completely separated from the die casting. The minimum core pulling distance is equal to the maximum depth of the die casting side hole or undercut. For example, if the depth of the die casting undercut is 20mm, the minimum core pulling distance is 20mm. To ensure safety, a safety margin of 5-10mm is required, so the actual core pulling distance is 25-30mm. For complex structures, such as side holes with steps, the core pulling distance should be calculated based on the deepest step depth. If the deepest step depth is 35mm and the safety margin is 8mm, the core pulling distance is 43mm. When there is an interference area between the core and the die casting, the core pulling process should be simulated through 3D modeling to determine the distance required for the core to completely separate from the interference area. For example, if there is a protrusion on the side of the core, the core pulling distance should be greater than the sum of the protrusion height and the step depth. If necessary, a 10%-15% margin can be added to account for dimensional fluctuations.
Different methods are required to calculate the core-pulling force and core-pulling distance for special structures. For threaded cores, the core-pulling force includes the holding force and friction at the thread engagement. The influence of the thread profile angle (generally 60°) must be considered in the calculation. The formula is: Core-pulling force = Holding force / cos (profile angle / 2). Furthermore, the core-pulling distance of a threaded core must be sufficient to fully unscrew the thread: core-pulling distance = thread length + (2-3) thread pitches. For curved cores, the core-pulling force is calculated based on the projected area of the curved surface. The larger the projected area, the greater the core-pulling force. The core-pulling distance is determined based on the maximum protrusion of the curved surface to ensure that all points along the core’s trajectory are free from the die-casting. For combined cores (multiple cores pulling together), the core-pulling force is calculated for each core separately, and the maximum value is used as the total core-pulling force. The core-pulling distances of all cores must be coordinated to avoid interference during the core-pulling process.
Verification and adjustment of core-pulling force and core-pulling distance are crucial steps in design implementation. During mold trials, the actual core-pulling force is measured using a pressure sensor and compared to the theoretically calculated value. If the deviation exceeds 20%, the values of parameters such as the molding area and draft angle must be rechecked to ensure they are reasonable. For example, if the actual core-pulling force is significantly greater than the calculated value, the draft angle may be too small (less than 1°) or the surface roughness of the core exceeds the standard. In this case, the draft angle may need to be appropriately increased or the surface roughness reduced. The core-pulling distance can be adjusted by applying mold release agent to the core and observing the die-casting after core pulling to see if there are any signs of strain. If there are localized signs of strain, the core-pulling distance is insufficient and needs to be increased by 5-10mm before trying again. For molds produced in large quantities, changes in the core-pulling force must be monitored regularly. If the core-pulling force suddenly increases by more than 15%, it may indicate core wear or die-casting sticking. Timely inspection and maintenance are required to ensure that the core-pulling parameters remain within a reasonable range.
