Facilitates demoulding and core pulling of die castings
Facilitating die-casting demolding and core pulling is key to improving production efficiency and reducing scrap rates. This requires coordinated optimization of casting design, mold structure, and process parameters to ensure smooth and stable demolding and core pulling, avoiding casting damage or mold failure. A sound design can reduce demolding resistance and simplify core pulling, shortening each production cycle by 5-10 seconds while also reducing mold wear and extending its service life.
Casting design facilitates demolding through simplified structure and slope settings. All surfaces in contact with the mold require adequate draft: the inner surface (cavity) must have a draft of 1° or greater, and the outer surface (core) must have a draft of 0.5° or greater. For castings >100mm in height, the draft should increase by 0.5° for every 50mm increase in height. For a 150mm-tall aluminum alloy cylindrical part, the inner surface draft of 1.5° reduced demolding resistance by 30%. Undercuts and deep cavity structures should be avoided. Undercuts require core pulling or forced demolding, which increases costs and the risk of deformation. One snap-fit part has replaced undercuts with rounded corners, achieving core-free demolding and increasing the pass rate from 90% to 99%. Casting edges should be radiused or chamfered by 0.5-1mm to prevent sharp corners from rubbing against the mold. After chamfering the edges of one flat part, the demolding strain rate dropped from 15% to 1%.
Optimizing the design of core-pulling mechanisms can significantly improve convenience. Mechanical core-pulling mechanisms (such as inclined guide pins) are preferred over hydraulic core-pulling mechanisms. Mechanical core-pulling mechanisms offer fast response and low cost, making them suitable for core-pulling distances less than 50mm. A 30mm core-pulling mechanism on a certain housing reduced its failure rate by 60% after replacing the hydraulic cylinder with an inclined guide pin. The core-pulling direction should be aligned with the demolding direction as much as possible, minimizing the inclined core-pulling angle. Core-pulling is more stable when the angle is ≤15°. For a casting with an inclined hole, reducing the core-pulling angle from 25° to 12° shortened the core-pulling time by 0.8 seconds. Core-pulling mechanisms require limit and guide devices to ensure precise movement. A core-pulling mechanism experienced offset due to a lack of guides. Adding guide pins reduced the positioning error from 0.2mm to 0.05mm.
Adjustment of process parameters has a significant impact on the smoothness of demolding and core pulling. The mold temperature needs to be uniform and stable. Aluminum alloy molds should be controlled at 200-250℃, and zinc alloy molds at 150-180℃. Too low a temperature will increase demolding resistance. A zinc alloy part had difficulty demolding due to a mold temperature of 130℃. The problem was solved after the temperature was raised to 160℃. The injection speed and pressure need to match. High-speed filling (6-8m/s) can ensure that the casting fits tightly into the cavity and reduce mold sticking. By increasing the injection speed, certain complex parts can be cored without sticking. The selection and spraying of release agents are crucial. Water-based release agents are suitable for aluminum alloys, and oil-based release agents are suitable for zinc alloys. The spraying amount needs to be uniform (5-10ml per mold). After a factory adopted an automatic spray system, the uniformity of release agent distribution increased by 40%, and demolding became smoother.
Mold surface treatment can reduce frictional resistance. Polishing the cavity and core surfaces to Ra ≤ 0.8μm lowers the coefficient of friction between the casting and the mold. For one mold, release force decreased by 25% after polishing. Surface coatings (such as TiN and CrN) can further improve wear resistance and lubricity. The friction coefficient of a TiN-coated mold surface decreased from 0.3 to 0.15. Coating the core of one core-pulling mold doubled its service life. The exhaust system must ensure that no gas remains in the core-pulling area. A 0.05-0.1mm deep exhaust groove is installed at the end of the core-pulling part. In one casting, poor exhaust at the core-pulling part caused mold sticking. Adding an exhaust groove eliminated the defect.
Automated auxiliary equipment can improve the reliability of demolding and core pulling. Robotic part removal, coordinated with the core pulling action, allows the casting to be removed instantly upon completion, reducing the time the casting remains in the mold. The introduction of robots on one production line reduced part removal time by 1.2 seconds. A core pulling and demolding force monitoring system provides real-time feedback on force fluctuations, automatically shutting down the machine when the set value (e.g., core pulling force > 50kN) is exceeded, preventing mold damage. One factory, through this monitoring system, promptly detected core pulling anomalies and reduced three major failures. Regular maintenance of the core pulling and demolding mechanisms, including inspection of the guide pins and sliders for wear every 1,000 cycles and replacement of grease every 5,000 cycles, has extended the interval between core pulling failures for one mold from 10,000 to 30,000 cycles through standardized maintenance.
