Key Design Points of Injection Mold Ejection Mechanisms

I. Core Design Principles

1. Uniform Ejection PrincipleEjection force must be evenly distributed on the product contact surface to avoid deformation or damage caused by excessive local stress. During design, ejection points should be reasonably arranged according to product shape, wall thickness and material properties. For large or thin-walled products, the distance between ejection points should not exceed 150 millimeters to ensure balanced force on the product during ejection.
2. Product Protection PrincipleSharp structures should be avoided at the contact parts between the ejection mechanism and the product, and ejection elements fitting the product contact surface should be prioritized. For soft plastics or products with high surface precision requirements, the contact area between ejection elements and the product should be appropriately increased, and ejection pressure should be controlled within the material yield strength range to prevent defects such as ejection whitening and indentation.
3. Mold Adaptation PrincipleThe ejection mechanism must coordinate with the overall mold structure to avoid interference with the gating system and cooling system. The ejection stroke must meet the requirements of complete product demolding, usually 5–10 millimeters longer than the maximum product demolding height, ensuring smooth separation of the product from the mold cavity. Meanwhile, the mechanism must move flexibly without jamming.
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II. Key Component Design Points

1. Ejector Pin DesignEjector pins are the most commonly used ejection elements. Their diameter should be determined based on force conditions, with a common range of 3–20 millimeters. The minimum diameter should not be less than 3 millimeters to prevent bending or fracture during ejection. The end face of the ejector pin must be flat, and the contact area with the product should be polished to a surface roughness of Ra≤0.8μm to avoid scratching the product. For deep-cavity or complex-cavity products, stepped ejector pins can be used to enhance rigidity.
2. Ejector Plate/Tube DesignEjector plates are suitable for large-area, thin-walled or complex-shaped products, enabling comprehensive and uniform ejection. The thickness of the ejector plate must be calculated based on the force-bearing area to ensure sufficient rigidity and prevent deformation during ejection. Ejector tubes are suitable for annular, cylindrical and other hollow products. The inner diameter of the ejector tube must fit the product inner wall, and the outer diameter must match the mold cavity size to ensure uniform force during ejection. A reasonable fit clearance should be reserved to prevent flash.
3. Guidance and Reset DesignThe ejection mechanism must be equipped with guidance devices, usually guide pillar and bushing assemblies, to ensure precise ejection movement and avoid offset interference. The guidance clearance should be controlled within a reasonable range to ensure smooth movement. Reset devices typically use return springs, whose selection should be based on the weight of the ejection mechanism and reset force requirements, ensuring complete reset of the ejection mechanism during mold clamping with a reset accuracy error of no more than 0.1 millimeters.
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III. Adaptability Optimization Design

1. Product Material AdaptationDifferent plastic materials have significant differences in mechanical properties, requiring targeted optimization of ejection design. For crystalline plastics (e.g., PP, PE), which have high post-molding shrinkage rates, the ejection area of the mechanism can be appropriately increased. For rigid and brittle plastics (e.g., PS, PMMA), ejection speed should be reduced and contact area increased to avoid product cracking caused by stress concentration.
2. Product Structure AdaptationFor products with complex structures such as deep cavities, ribs and buckles, auxiliary ejection mechanisms must be installed. For example, deep-cavity products can use a two-stage ejection mechanism to first eject the product a certain distance before complete demolding; products with buckles can use angled lifters to complete buckle demolding during ejection, avoiding buckle damage caused by forced ejection.
3. Production Efficiency AdaptationIn mass production scenarios, the ejection mechanism must balance stability and efficiency. Hydraulic or pneumatic ejection systems can be adopted to achieve adjustable ejection speeds, adapting to the demolding requirements of different products. Meanwhile, ejection limit devices should be installed to ensure precise ejection strokes and avoid product damage or mechanism failure caused by over-ejection.

IV. Technological Trends and Development Directions

1. Intelligent Monitoring OptimizationIntegrating intelligent sensing technology, pressure and displacement sensors are installed at key parts of the ejection mechanism to real-time monitor ejection force and stroke. Ejection parameters are adjusted through data feedback to avoid product defects caused by parameter deviations and improve production stability.
2. Lightweight and High-Precision DevelopmentHigh-strength lightweight materials (e.g., aluminum alloys, carbon fiber composites) are used to manufacture ejection elements, reducing mechanism movement inertia and improving response speed. Meanwhile, precision machining technology enhances the fit accuracy of the ejection mechanism to meet the production requirements of precision injection-molded products.
3. Environmental Protection and Energy Conservation OptimizationOptimize the transmission structure of the ejection mechanism to reduce friction loss and energy consumption; adopt oil-free lubrication technology to avoid product and production environment pollution caused by lubricating oil, in line with the trend of green production.