Design of delayed core pulling for inclined pins in die casting mould
The delayed core-pulling design of the diagonal pins in die-casting molds is a key technical solution to the demolding sequence issues of complex castings. Through a special structural design, the diagonal pins only begin to drive the core-pulling action after a certain mold opening stroke, thereby coordinating the core-pulling sequence of multiple cores and preventing interference between cores or between cores and the casting. This delayed core-pulling design is primarily used for castings with multiple undercuts, deep cavities, or staggered cores. Conventional synchronous core-pulling for these castings can cause casting deformation or core jamming. Therefore, delayed core-pulling control is required to ensure orderly core-pulling, ensuring production safety and casting quality.
The principle of delayed core pulling is to provide a certain idle stroke between the bevel pin and the slider. This allows the bevel pin to slide relative to the slider during the initial mold opening without driving it. Only after the idle stroke is complete does the bevel pin begin to drive the slider to complete the core pulling. Common structural forms include an extended bevel pin design, a stepped slider guide hole design, and an additional delay block design. The extended bevel pin design incorporates a straight section parallel to the axis at the bevel pin head. The length of the straight section is the delayed stroke. During mold opening, the straight section of the bevel pin slides within the slider guide hole until the bevel contacts the slider, at which point core pulling begins. The stepped slider guide hole design incorporates a larger diameter straight hole at the entrance of the guide hole. The bevel pin head generates no lateral force when sliding within the straight hole, and only drives the slider after entering the smaller diameter bevel hole. The additional delay block design incorporates a block between the slider and the template. This block restricts the slider’s movement during mold opening. Only when the template moves until the block disengages does the slider begin core pulling, driven by the bevel pin.
The delay stroke must be calculated based on the casting structure and core-pulling sequence requirements. Typically, the delay stroke is calculated to ensure that the first core pulled is completely disengaged before the delayed core begins to move. For example, if a casting has two staggered undercuts, core A must be pulled 15mm before core B can begin pulling. Therefore, the delay stroke for core B must meet the mold opening stroke required to pull core A 15mm. Based on the angle α of the inclined pin, the relationship between the mold opening stroke (L) and the core-pulling distance (S) is L = S/sinα. If α = 20°, the mold opening stroke required to pull core A 15mm is L ≈ 15/sin20° ≈ 43.9mm. Therefore, the delay stroke for core B must be no less than 43.9mm, meaning the straight length of the inclined pin or the step length of the guide hole is 45mm. The delay stroke must also take into account mold closing accuracy to avoid insufficient or excessive delay due to assembly errors. Generally, a 1-2mm margin is required.
The structural details of the delayed core pulling mechanism directly impact the delay effect and reliability of the mechanism. The straight section of the inclined pin and the guide hole of the slider must utilize a clearance fit (H8/f7) to ensure smooth sliding of the straight section within the hole without binding. The surface roughness of the straight section must be controlled below Ra0.8μm to reduce sliding friction. The transition between the straight and inclined sections must be rounded ( R2-R3) to avoid stress concentration and scratching the guide hole. For the step design of the slider guide hole, a 15°-30° guide cone should be provided at the step to guide the inclined pin from the straight hole into the inclined hole. The step diameter should be 2-4mm larger than the inclined pin diameter to ensure smooth passage of the inclined pin head. The additional delay stopper should be made of wear-resistant material (such as quenched 45 steel). The contact area between the stopper and the slider should be sufficiently large to prevent localized wear. The stopper must be securely fixed to prevent loosening during mold opening, which could lead to delay failure.
The movement coordination of the delayed core-pulling mechanism must be verified through mold motion simulation to ensure smooth transitions between each stage. During the initial mold opening phase, the delay mechanism must ensure that the slider is stationary and that the inclined pins or blocks are not subjected to abnormal forces. After the delay expires, the slider must start smoothly under the drive of the inclined pins without impact. During the core-pulling process, the slider’s trajectory must maintain a safe distance from other cores to avoid interference. For multiple sets of delayed core-pulling mechanisms, the delay sequence and stroke of each set must be determined through calculation to form an orderly core-pulling sequence. For example, if three sets of cores need to be pulled in the order A→B→C, with A having no delay, B having a delay of 40mm, and C having a delay of 80mm, then different lengths of inclined pin straight sections or blocks must be used to control the process, ensuring that B begins core-pulling at 40mm of mold opening and C at 80mm.
The strength check of the delayed core-pulling design needs to consider the stress characteristics of the delay stage. Although the inclined pin does not drive the core pulling when sliding in the straight section, it may generate lateral force due to the guide hole error. Therefore, the straight section needs to have sufficient bending strength. The slider is constrained by the block during the delay stage. The contact area between the block and the slider needs to be locally reinforced to avoid deformation of the block or depression of the slider. In addition, at the moment the delay ends, the contact between the inclined pin and the slider will generate an impact load. It is necessary to provide a buffer structure (such as a spring washer) on the contact surface or increase the contact area to reduce the impact stress. For molds produced in large quantities, it is also necessary to calculate the wear life of the vulnerable parts of the delay mechanism (such as the inclined pin straight section and the block). If necessary, surface hardening treatment (such as nitriding and chrome plating) should be used to improve wear resistance and ensure the long-term stable operation of the mechanism.
