Injection molding gear rack tilt side core pulling mechanism
The tilted rack-and-pinion lateral core-pulling mechanism for injection molding is a complex mechanism that utilizes a rack-and-pinion transmission to achieve tilted core pulling. It is suitable for molding parts with inclined concave or convex undercuts. Its core-pulling direction forms a specific angle (typically 10°-45°) with the mold opening and closing directions, meeting the molding requirements of complex plastic parts. Compared with traditional inclined guide pin core-pulling mechanisms, this mechanism offers a longer core-pulling distance, more stable core-pulling force, and higher motion precision. It is widely used in molds for precision plastic parts such as automotive parts and household appliances. A deeper understanding of its structural composition, operating principles, and design key points is crucial for improving the molding quality of complex plastic parts.
The structure of the rack and pinion inclined lateral core pulling mechanism is relatively complex, mainly including components such as the rack core pulling rod, gear, guide slider, drive rack, and positioning device. One end of the rack core pulling rod cooperates with the inclined side concave structure of the plastic part, and the other end is machined with a rack that meshes with the intermediate gear. The gear is mounted on the fixed plate via a bearing, which plays the role of transmitting motion and changing the direction of force. The drive rack is fixed to the movable mold or fixed mold and drives the rack core pulling rod by meshing with the gear during the opening and closing of the mold. The guide slider is connected to the rack core pulling rod and moves along the inclined guide rail to ensure the accuracy of the core pulling direction. The positioning device (such as a spring or a positioning pin) is used to fix the rack core pulling rod in a specified position after the core pulling is completed to prevent it from moving before the mold is closed. In addition, a lubrication device (such as an oil nozzle) is required in the mechanism to reduce wear on the rack and pinion and improve transmission efficiency and service life.
This mechanism operates by using the linear motion of the mold opening and closing to drive a rack-and-pinion transmission, achieving an inclined core-pulling motion. During the mold opening process, the movable mold separates from the fixed mold, and a drive rack fixed to the movable mold moves with it, meshing with the gear and driving its rotation. This rotation drives the rack’s core-pulling rod in an inclined direction, achieving the core-pulling motion, until the rod is completely free of the undercut structure of the plastic part. At this point, a positioning device locks the rod in the end-of-core-pulling position. During the mold closing process, the drive rack moves in the opposite direction with the movable mold, driving the gear in the opposite direction, which in turn drives the rack’s core-pulling rod back in an inclined direction until it returns to the molding position, completing a core-pulling and reset cycle. Compared to inclined guide pin core-pulling mechanisms, the rack-and-pinion transmission has the advantage that the core-pulling force and distance are not limited by the mold opening and closing angles. By increasing the number of teeth or length of the rack-and-pinion, a longer core-pulling distance (up to 300mm or more) can be achieved. Furthermore, the core-pulling process is smooth, and the force transmission efficiency is high (over 90%).
The parameter design of the gear rack is the key to the performance of the mechanism, and needs to be determined comprehensively based on the core pulling force, core pulling distance and installation space. The module of the gear is the core parameter. The larger the module, the stronger the bearing capacity of the gear. The module is usually selected based on the core pulling force. The formula is: module m = (core pulling force × safety factor) / (2 × number of gear teeth × allowable bending stress ) . For example, when the core pulling force is 5000N , the safety factor is 1.5 , the number of gear teeth is 20 , and the allowable bending stress is 200MPa , the module m ≈ ( 5000×1.5 ) / (2×20×200×10⁶)×1000 ≈ 0.94mm . In practice, a gear with a module of 1mm can be selected . The length of the rack needs to be determined based on the core pulling distance and the gear diameter. The effective length of the rack is = Core pulling distance + gear pitch circumference / 2 to ensure that the gear and rack remain in mesh during the core pulling process. Gears and racks are typically made of high-strength alloy structural steel (such as 40Cr), which reaches a hardness of 28-32HRC after quenching and tempering, and the tooth surfaces are quenched to 50-55HRC to ensure sufficient strength and wear resistance.
The design of the guiding and positioning system is crucial to the mechanism’s motion accuracy and stability. The clearance between the guide slider and the inclined guide rail should be controlled within 0.01-0.02mm. The guide rail’s tilt angle must be consistent with the inclination of the undercut of the plastic part, with a deviation of no more than 0.5°. Otherwise, the core pulling direction will deviate, affecting the dimensional accuracy of the plastic part. The guide rail’s length should be greater than the core pulling distance to ensure that the guide slider moves along the guide rail throughout the core pulling process and prevent bending and deformation of the rack core pulling rod. The positioning device must provide sufficient positioning force to prevent the rack core pulling rod from shifting due to vibration or inertia after the core pulling is completed. A combination of springs and locating pins is typically used. The spring’s preload should be sufficient to overcome the weight and inertia of the rack core pulling rod. The clearance between the locating pin and the locating hole should be 0.01-0.02mm, with a positioning accuracy of no more than 0.03mm. Furthermore, a stopper (such as a stopper) should be included in the mechanism to prevent excessive core pulling or incomplete resetting. The position accuracy of the stopper should be controlled within 0.02mm.
The design of this mechanism must consider its coordination with other mold systems and ensure reliability. The rack-and-pinion mechanism should be coordinated with the ejection mechanism to avoid interference during the core pulling and ejection processes. Typically, the core pulling action should be designed to occur before the ejection action, or a sequential control mechanism should be used to achieve orderly movement of the two. The mechanism’s lubrication system requires regular maintenance. High-temperature grease (resistant to temperatures exceeding 150°C) is recommended, and relubrication should be performed every 5,000 molds to reduce wear on the rack and pinion. To improve mechanism reliability, thrust bearings can be installed on the pinion shaft to absorb axial forces and prevent axial movement of the gear. Dust covers can be installed on the rack core pulling rod to prevent plastic melt or impurities from entering the rack and pinion meshing area and affecting transmission accuracy. Furthermore, for large or high-precision rack-and-pinion core pulling mechanisms, finite element analysis is required to simulate the stress conditions of the rack and pinion and optimize structural parameters to avoid premature failure caused by stress concentration. Through reasonable design and fine manufacturing, the rack and pinion inclined lateral core pulling mechanism can meet the molding requirements of complex plastic parts and provide a reliable solution for high-precision, large-draw-length lateral core pulling.
