Calculation of core pulling force of die casting mold
Calculating the core-pulling force of a die-casting mold is fundamental to its design. Its purpose is to determine the minimum force required to extract the core, providing a basis for selecting the mechanism type, determining the power source parameters, and verifying the structural strength. The calculation of core-pulling force must consider multiple factors, including the casting’s grip on the core, friction, and the casting’s resistance to deformation. By combining theoretical formulas with empirical corrections, a value close to actual operating conditions is obtained. Accurate calculation is crucial to ensuring reliable mold operation, as it can prevent core jamming and casting damage caused by insufficient core-pulling force, or unwieldy mechanisms and increased energy consumption caused by excessive core-pulling force.
The primary component of core-pulling force is the holding force exerted by the casting on the core. Its magnitude is related to the shrinkage rate of the molten metal, the surface area of the core, and the coefficient of friction between the casting and the core. The theoretical formula for calculating holding force is: Fhold = K × A × σshrink, where K is the safety factor (1.2-1.5), A is the contact area between the core and the casting (mm²), and σshrink is the shrinkage stress of the molten metal (MPa). Shrinkage stress varies across alloys, ranging from approximately 80-120 MPa for aluminum alloys, 60-90 MPa for zinc alloys, and 100-150 MPa for copper alloys. For example, for an aluminum alloy casting with a core contact area of 5000 mm², its holding force Fhold = 1.3 × 5000 × 100 = 650,000 N (650 kN). Therefore, a hydraulic core-pulling mechanism is required to provide sufficient core-pulling force.
Friction is another important component of core-pulling force. It is determined by the holding force and the coefficient of friction. The calculation formula is: F = μ × F, where μ is the coefficient of friction between the casting and the core. The smoother the core surface finish, the smaller the μ value. Typically, the friction coefficient between a steel core with an Ra of 0.8μm and an aluminum alloy casting is 0.15-0.25. If the core surface is nitrided, the μ value can be reduced to 0.1-0.15. For the aluminum alloy casting mentioned above, F = 0.2 × 650,000 = 130,000 N (130 kN). Therefore, the total core-pulling force must include at least the sum of the holding force and friction, or 780 kN (other resistances must be added in the actual calculation).
The deformation resistance of a casting must be calculated individually based on the casting structure. When the core extraction direction is inconsistent with the casting’s rigidity, the casting may undergo elastic deformation, generating additional resistance. For example, when the inner core of a thin-walled cylindrical casting is extracted, the casting will expand radially due to the core-pulling force. The deformation resistance Fchange = E × ε × A, where E is the elastic modulus of the casting material (approximately 70 GPa for aluminum alloys), ε is the elastic strain (typically ≤ 0.001), and A is the cross-sectional area of the deformation zone. For an aluminum alloy cylindrical casting with a diameter of 100 mm and a wall thickness of 5 mm, the deformation resistance Fchange = 70 × 10³ × 0.001 × (3.14 × 100 × 5) ≈ 11,000 N (11 kN), which must be factored into the total core-pulling force.
Correction of core-pulling force must take into account complex factors in actual production, such as core geometry, core-pulling speed, and mold temperature. If the core surface has uneven texture, the core-pulling force should be increased by 20%-30%. When the core-pulling speed exceeds 50 mm/s, the core-pulling force should be increased by 10%-15% due to increased inertia. Excessively high mold temperatures (above 300°C) increase casting shrinkage forces, requiring the core-pulling force to be multiplied by a correction factor of 1.2-1.3. Furthermore, for inclined core-pulling (angle θ with the demolding direction), the core-pulling force should be calculated as F pull = F total / cos θ. For example, if θ = 30°, the core-pulling force should be increased by 15.4% (1/cos 30° ≈ 1.154).
The calculation of the core-pulling force requires final adjustment through mold trials. The theoretically calculated value may differ from the actual core-pulling force, and the parameters must be corrected based on the mold trial results. If the core cannot be pulled or the casting cracks during the mold trial, it indicates insufficient core-pulling force, and the clamping force and friction force must be recalculated to check for any missing deformation resistance. If the core-pulling mechanism operates smoothly but the power source load is excessive, the safety factor may be too high during the calculation and can be appropriately reduced. Only by combining theoretical calculations with mold trials can accurate core-pulling force values be obtained, providing a reliable basis for the optimized design of the core-pulling mechanism.
