The process of core pulling on the side of the die-casting die bending pin
The core-pulling process of the die-casting mold’s bent pin side is a dynamic process driven by the mold opening force, in which the core is pulled out in an orderly manner through the cooperation of the bent pin and the slider. It can be divided into four stages: mold closing and locking, mold opening preparation, core-pulling execution, and core-pulling completion. The motion characteristics and stress conditions of each stage vary, requiring precise structural design to ensure smooth transitions between stages and avoid problems such as jamming, impact, or inaccurate positioning. The bent pin’s broken line or arc-shaped structure enables complex core-pulling trajectories, making it particularly suitable for castings requiring multi-directional and variable-angle core pulling. The rationality of its motion process directly determines the molding quality of the casting and the service life of the mold.
The mold closing and locking stage is the initial state of the core pulling mechanism. At this time, the mold is in a closed state, the bent pin is fully inserted into the guide hole of the slider, and the slider is in the forming position under the combined action of the bent pin and the wedging device. The wedging block fits tightly against the inclined surface of the slider, locking the slider to resist the lateral pressure of the molten metal on the core during die casting. At this time, the bent pin mainly plays a positioning role and does not bear the locking force. The core and the cavity form a complete casting molding space, and the guide part of the slider is fully embedded in the guide groove to ensure the accuracy of the core position. During the mold closing stage, it is necessary to ensure that the fitting clearance between the bent pin and the guide hole of the slider is uniform and the contact area of the wedge block meets the standard. Otherwise, it will cause uneven force on the slider, affecting the dimensional accuracy of the casting. For example, the contact area of the wedge block of a certain mold is less than 50% when the mold is closed, resulting in a slight retreat of the slider during die casting and an error of more than 0.1mm in the size of the side hole of the casting.
The mold opening preparation stage begins at the moment when the mold begins to open and ends at the moment before the bending pin drives the slider to start core pulling. At the beginning of mold opening, the fixed mold and the movable mold are separated, and the wedge block first disengages from the slider, releasing the locking constraint on the slider. At this time, the bending pin is still in contact with the slider guide hole, but no lateral driving force is generated. The key to this stage is to ensure that the wedge block acts on the slider before the bending pin, to avoid the wedge block and the slider being in a locked state when the mold is opened, causing the bending pin or the slider to be damaged by excessive force. The disengagement distance between the wedge block and the slider needs to be greater than the initial stroke of the bending pin to start driving the slider, usually set to 1-2mm, which can be achieved by adjusting the installation height of the wedge block. During the mold opening preparation stage, the slider is stationary and the core remains in the molding position, ready for core pulling.
The core pulling execution phase is the core process in which the bending pin drives the slider to complete the core pulling action. When the mold opening stroke reaches the point of action between the bending pin working section and the slider guide hole, the bending pin begins to apply lateral force to the slider, driving the slider along the guide groove, and the core is gradually pulled out of the casting. Depending on the shape of the bending pin, the core pulling trajectory can be divided into straight, broken line, or curved types: the core pulling trajectory of a straight bending pin is single-directional, similar to an inclined guide column; a broken line bending pin can achieve two sections of core pulling in different directions, such as first pulling out 5mm and then pulling up 10mm; a curved bending pin can achieve an arc-shaped trajectory core pulling to meet the demolding requirements of complex curved castings. During the core pulling process, the movement speed of the slider is determined by the mold opening speed and the bending pin angle. The speed of the broken line bending pin will suddenly change at the corner, and a rounded corner transition (R≥5mm) is required to reduce impact and avoid damage to the casting.
The final stage of core pulling occurs when the slider reaches its endpoint and is secured by a positioning device. When the slider reaches the preset core pulling distance, a positioning device (such as a spring-steel ball or stop) locks the slider, preventing it from moving during subsequent mold opening. At this point, the core is completely free of the casting, maintaining a sufficient safety distance (typically ≥5mm) between it and the casting to ensure smooth ejection by the ejection mechanism. After core pulling, the bent pin maintains partial contact with the slider guide hole, preparing for reset during mold closing. The slider’s positioning accuracy must be checked during this stage. Any positioning deviation exceeding 0.1mm can cause interference between the bent pin and the slider guide hole during mold closing, making the positioning device’s reliability crucial. For example, fatigue failure of the spring in the spring-steel ball positioning system in one mold caused the slider to deflect by 0.3mm after core pulling. This allowed the bent pin to collide with the guide hole during mold closing, causing it to bend and damage.
The mold closing and reset stage is the reverse action process of the core pulling mechanism. When the mold is closed, the movable mold moves toward the fixed mold, and the bending pin pushes the slider to move in the opposite direction through the guide hole, gradually returning to the molding position. During the reset process, the movement trajectory of the slider is opposite to that during core pulling. The broken line bending pin must ensure that the slider moves smoothly at the corners without jamming. When the slider approaches the molding position, the wedge block begins to contact the slider and gradually locks the slider until the mold is closed in place. After the reset is completed, the bending pin, slider, and wedge device return to the initial state and wait for the next die-casting cycle. During the reset stage, it is necessary to ensure that the slider is reset in place. This can be controlled by setting a reset limit block. The contact gap between the limit block and the slider is ≤0.02mm to ensure the accurate position of the core. For large sliders, a buffer device (such as a polyurethane pad) can be set on the reset path to reduce the impact when the mold is closed in place.
