Design of Slider Positioning Spring in Lateral Core Pulling
In the lateral core-pulling mechanism of an injection mold, the slider positioning spring is a critical component that ensures the slider accurately returns to its original position and maintains a stable position after mold opening. Its design directly impacts mold precision and part quality. After completing the core-pulling action, the slider, guided by the spring, must return to its initial position, ready for the next mold closing. Improper spring design can cause the slider to reset inaccurately, become stuck, or even damage mold components. Therefore, a thorough understanding of the design principles, parameter selection, and installation of the slider positioning spring is crucial for improving mold performance.
Calculating the spring force of the slider positioning spring is a core aspect of the design, and requires comprehensive consideration of the slider’s weight, core pulling resistance, and motion inertia. The spring force must be sufficient to overcome the friction and possible lateral forces experienced by the slider during the reset process, ensuring that the slider can smoothly return to the designated position. Generally, the spring preload should be set to 1.2-1.5 times the total resistance of the slider to reserve a certain safety margin. For example, when the slider weighs 5kg and the friction experienced during core pulling is approximately 10N, the initial spring force should not be less than (5×9.8+10)×1.2≈76.56N. At the same time, the maximum deformation of the spring must be calculated based on its material and diameter to avoid exceeding the elastic limit and causing permanent deformation. Generally, the compression of the spring should not exceed 40% of the free length to ensure stability during long-term use.
The type of spring should be selected in combination with the spatial layout of the mold and the working environment. Commonly used slider positioning springs in molds include cylindrical helical compression springs, rectangular cross-section springs, and disc springs. Cylindrical helical compression springs have a simple structure and low cost, and are suitable for scenarios where there is ample space and low elasticity requirements; rectangular cross-section springs have strong load-bearing capacity and long life, and are suitable for situations where the slider is heavy or the core pulling resistance is high; disc springs have the characteristics of small size and high elasticity, and are suitable for precision molds with limited space. In addition, the choice of spring material is also crucial. High-strength spring steel such as 65Mn or 50CrVA is usually used. These materials have good elastic limit and fatigue strength, and can maintain stable performance in frequent compression-rebound cycles. At the same time, surface anti-corrosion treatment is required to cope with oil and water vapor erosion in the mold working environment.
The spring’s installation method and positioning structure directly impact its performance. The spring should be installed as close to the slider’s center of gravity as possible to avoid unbalanced loading during the slider’s reset, which can lead to uneven force and localized wear. Common installation methods include built-in spring holes and side-mounted springs. Built-in spring holes involve machining a blind hole in the slider into which the spring is inserted. This method offers excellent stability but requires high hole machining precision, with the verticality deviation of the hole controlled within 0.02mm/m. Side-mounted springs are secured to the side of the slider via a spring seat, facilitating installation and replacement, but potentially increasing the slider’s overall size. Additionally, guide posts or sleeves are required at both ends of the spring to prevent lateral bending during compression. The diameter of the guide posts should match the inner diameter of the spring, with a clearance of 0.1-0.2mm to ensure smooth spring movement.
The spring design needs to consider the coordination with other core-pulling mechanisms. For example, when the slider is used in conjunction with the inclined guide pin, the spring’s elastic force needs to be coordinated with the driving force of the inclined guide pin to avoid excessive elastic force causing the inclined guide pin to bear additional load, or too little elastic force causing the slider to be unable to follow the movement of the inclined guide pin in time. In molds with hydraulic or pneumatic core-pulling devices, the spring mainly plays an auxiliary positioning role. At this time, the spring parameters need to be adjusted according to the thrust of the power device to ensure that when the power device fails, the spring can temporarily maintain the position of the slider to prevent accidents. At the same time, the life of the spring needs to match the overall life of the mold. Through simulation calculation of the number of spring fatigue times, its service life is generally required to be no less than 100,000 times. For molds produced in batches, spring materials and structures with longer lifespans should be selected.
In practical applications, the design of the slider positioning spring requires verification through mold trials and continuous optimization. During the mold trials, the slider’s reset accuracy must be observed. Positioning marks can be placed between the slider and the template to measure the deviation after each reset. If the deviation exceeds 0.05mm, the spring’s spring force or installation position must be adjusted. Furthermore, the spring’s deformation after long-term use must be checked, with its free length and spring force changes regularly measured. If the spring force decays by more than 20%, it should be replaced promptly. Furthermore, finite element analysis software can be used to simulate the spring’s stress state, optimizing parameters such as the number of coils and pitch to avoid stress concentration. Only by comprehensively considering mechanical properties, installation space, and collaborative work requirements can a reliable slider positioning spring be designed to ensure the stable operation of the lateral core pulling mechanism.
