Die castings facilitate demoulding and core pulling of die castings
Smooth demolding and core pulling of die-cast parts are critical to ensuring production efficiency and product quality. The key lies in reducing demolding resistance and core pulling difficulty through rational structural design and process optimization, thereby avoiding surface damage or deformation of the casting. In die-casting production, problems with demolding and core pulling can not only result in scrapped castings but can also damage the mold and increase production costs. Therefore, ease of demolding and core pulling must be fully considered from the die-casting structural design stage, combined with mold design and process parameter adjustment to form a complete solution.
Properly designed draft angles are essential for smooth demolding. The draft angle refers to the angle between the die-cast part wall and the mold cavity wall, and its magnitude directly affects demolding resistance. Draft angle requirements vary for different casting surfaces: External surfaces, due to their relatively small contact area with the mold cavity, can have a smaller draft angle, typically 0.5°-1°. Internal surfaces (such as those in holes and slots) require a larger draft angle of 1°-3° due to the stronger wrapping force of the mold core. Casting height is also a significant factor in determining draft angle. For castings over 100mm tall, the draft angle should be at least 2° to offset the effects of gravity and friction. For example, the internal bore of an automotive transmission housing, with a depth of 150mm, is designed with a draft angle of 3° to ensure even force distribution during ejection and avoid scratching the internal bore surface.
It is equally important to optimize the structure of the casting to reduce the difficulty of core pulling. The complexity of the core pulling mechanism is closely related to the structure of the casting. Simplifying the shape and number of the core pulling parts can significantly reduce the probability of core pulling failure. For structures that must be core pulled, such as lateral holes, bosses, etc., a shallow cavity design should be used as much as possible to reduce the core pulling stroke. For example, by controlling the height of the lateral boss within 5mm, the core pulling stroke can be shortened to less than 10mm, reducing the load on the core pulling mechanism. At the same time, the corners of the core pulling part need to be designed with fillets with a radius of not less than 1mm to avoid damage to the casting or core due to stress concentration during core pulling. In addition, the core pulling direction should try to avoid the weak parts of the casting to prevent deformation of the casting caused by excessive core pulling force.
The design of the mold’s ejection system has a significant impact on the demoulding effect. The ejection mechanism must ensure that the casting is evenly stressed during the demoulding process to avoid deformation caused by excessive local stress. The ejection points should be distributed on the rigid parts of the casting, such as ribs, flanges, etc. The ejection area should be determined according to the weight and material of the casting. The bearing pressure per square centimeter of the ejection area of aluminum alloy castings should not exceed 5MPa. The movement speed of the ejection mechanism must be stable and controllable. The ejection should be slow in the initial stage to avoid excessive instantaneous impact force, and then it can be accelerated appropriately to improve efficiency. For example, the ejection of a large engine cylinder adopts a multi-point synchronous ejection system. The ejection points are distributed on the ribs at the bottom of the cylinder. The speed gradient is achieved through hydraulic servo control to ensure smooth demoulding of the cylinder.
The selection and commissioning of the core-pulling mechanism must be considered in conjunction with the structural characteristics of the casting. For simple lateral core pulling, an inclined guide column core-pulling mechanism is preferred. This mechanism features a simple structure, low cost, and high reliability, making it suitable for applications with low core-pulling forces (less than 5kN). For applications with larger core-pulling forces or longer core-pulling strokes, a hydraulic core-pulling mechanism can be used. This hydraulic system provides a stable core-pulling force, and the core-pulling speed and stroke can be precisely controlled. For example, the lateral deep-hole core pulling of the rear axle housing of an automobile utilizes a dual-cylinder synchronous hydraulic core-pulling mechanism with a core-pulling force of 20kN and a stroke of 300mm. Displacement sensors monitor the core-pulling position in real time to ensure hole position accuracy. The commissioning of the core-pulling mechanism must ensure that the core-pulling action is coordinated with the mold closing and ejection actions to avoid motion interference.
Optimizing process parameters can further improve the demolding and core pulling effects. Excessively high mold temperature will cause the casting to stick to the cavity, while too low a temperature will increase demolding resistance. The mold temperature needs to be controlled within a reasonable range according to the alloy type: the temperature of aluminum alloy die-casting molds is generally 180-220°C, and that of zinc alloys is 120-160°C. The injection pressure and holding time must also be moderate. Excessive pressure will cause the casting to fit too tightly to the mold, while too long a holding time will increase the shrinkage and tightening force of the casting. For example, when die-casting thin-walled aluminum alloy parts, the injection pressure ratio is controlled at 50-80MPa, and the holding time is 1-2 seconds, which can both ensure the density of the casting and reduce demolding resistance. In addition, regular polishing and lubrication of the mold cavity and core can reduce the surface friction coefficient and reduce wear during demolding and core pulling.
