Design of die-casting mold ejection mechanism
The design of the die-casting mold ejection mechanism is crucial for ensuring smooth demolding of die-cast parts. Its quality directly impacts the product’s surface quality, dimensional accuracy, and production efficiency. After mold opening, the ejection mechanism must eject the die-casting from the core or cavity with appropriate force and speed while preventing deformation, strain, or cracking. The design must comprehensively consider the die-casting’s material, structural shape, wall thickness distribution, and overall mold layout. The appropriate ejection method, actuator, and power source must be selected to ensure smooth, reliable operation and easy maintenance.
The design of the ejection mechanism must adhere to the fundamental principles of “uniform force, synchronized motion, and precise repositioning.” Uniform force requires that the ejection force be evenly distributed across the die-cast part surface to avoid localized excessive stress and deformation. This is particularly true for thin-walled parts (wall thickness <2mm) and brittle materials (such as zinc alloy). Force distribution can be achieved by increasing the number of ejection points or expanding the ejection area. Synchronized motion requires that all ejection actuators maintain consistent speed and stroke. This can be achieved through rigid connections (such as ejector plates) or synchronized transmission mechanisms (such as rack and pinion). Synchronization error must be controlled within 0.1mm. Precise repositioning is crucial to ensuring the next die-casting cycle proceeds smoothly. Repositioning accuracy must reach 0.05mm to prevent interference between the ejected component and the core or cavity. For example, in the die-casting mold design for an automotive engine hood, the combination of 12 evenly distributed ejector pins and two symmetrical ejector plates ensures smooth demolding of large, thin-walled parts, keeping deformation within 0.5mm.
The selection of ejector actuators must be determined based on the structural characteristics of the die-cast part. Common ejector components include ejector pins, ejector plates, ejector tubes, inclined ejectors, and core-pulling ejector assemblies. Ejector pins are suitable for flat surfaces or shallow recesses. Their diameter must meet strength requirements, calculated using the formula d ≥ √(4F/(π[σ])), where F is the force acting on a single ejector pin and [σ] is the allowable material stress (300 MPa for H13 steel). For example, an ejector pin subjected to a force of 500 N must have a diameter ≥ 0.8 mm, but in practice, a diameter of 1 mm or greater is used. Ejector plates are suitable for large flat-bottomed parts. Their thickness is determined by rigidity requirements, generally 10-20 mm. The surface roughness Ra of the contacting part must be ≤ 0.8 μm to avoid ejection marks. Ejector tubes are suitable for demolding cylindrical or tubular cores. Their inner diameter should be 0.5-1 mm larger than the core diameter to ensure a smooth fit and prevent gap flash. The inclined ejector is used for products with an inner inclined boss. The inclination is consistent with the core pulling angle (10°-25°), and the length must meet the dual requirements of the core pulling distance and the ejection stroke.
The choice of power source must match the ejection force and ejection stroke. Commonly used ejectors include mechanical ejectors, hydraulic ejectors, and pneumatic ejectors. Mechanical ejectors utilize the die-casting machine’s mold opening force to drive the ejector pin. They feature a simple structure and low cost, with an ejection force of 5-50kN, making them suitable for small and medium-sized molds. Hydraulic ejectors are controlled by an independent hydraulic system, with an ejection force of 50-500kN, an adjustable stroke (50-300mm), and a speed of 0.05-0.5m/s. They are suitable for large and complex molds and can achieve segmented ejection (slow first, then fast) to reduce product damage. Pneumatic ejectors have a lower ejection force (≤10kN) and are suitable for small, light, thin-walled parts. Their advantage is a fast response speed (≤0.1s), which facilitates automation. The parameters of the power source must be determined through calculation. The total ejection force Ftotal = K×Fhold, where K is the safety factor (1.5-2.0) and Fhold is the holding force of the casting on the core. Fhold = P×A (P is the specific pressure of the molten metal, and A is the surface area of the core). For example, if the surface area of the core of an aluminum alloy casting is 1000cm² and the specific pressure is 80MPa, then Fhold = 80,000N and Ftotal = 120,000-160,000N. A hydraulic system with an ejection force ≥160kN must be selected.
The guidance and lubrication design of the ejection mechanism are critical to ensuring long-term stable operation. The guide components include guide pins, guide sleeves, and guide plates. The guide pins have a diameter of 10-30mm, with a clearance of 0.01-0.03mm between them. The guide length to ejection stroke ratio should be ≥2:1, ensuring smooth, unbiased movement. The lubrication system requires oil reservoirs located at the guide pins, push rods, and other moving joints, which should be regularly refilled with high-temperature grease (temperature resistant ≥200°C). For automated production lines, an automatic lubrication device can be designed, refilling every 500-1000 molds. Furthermore, the mechanism must include anti-seizure protection, such as a gap monitoring sensor between the push rod and the mold plate. This automatically shuts down the machine when the resistance exceeds a set value (e.g., 1.5 times the normal force) to prevent component damage. This design allows the ejection mechanism to have a service life of over 100,000 molds, meeting the demands of high-volume production.
