Injection ejection mechanism guide
The guidance system of the injection molding ejection mechanism is crucial for ensuring smooth demolding of plastic parts and improving production stability. Its function is to guide ejection components (such as the push rod, push tube, and ejector plate) along a fixed trajectory, precisely avoiding jamming, wear, or part deformation caused by uneven force. During operation, the ejection mechanism must withstand both the ejection resistance of the plastic part and its own inertia. Poor guidance can cause problems such as bent push rods and tilted ejector plates, and even damage the mold. For example, a certain automobile bumper mold had excessive clearance (0.2mm) in the ejection mechanism’s guidance, resulting in tilted push plates and uneven force on the push rods, causing them to break. This caused the production line to shut down for four hours, resulting in direct economic losses exceeding 50,000 yuan. Therefore, a scientifically designed ejection mechanism guidance system is crucial for ensuring continuous and stable operation of injection molding production.
The ejector mechanism’s guidance method should be selected based on the mold structure and the characteristics of the plastic part. Small molds (projected area <500 cm²) typically utilize guide pins and bushings. The guide pins have a diameter of 16-25 mm, and the guide bushings are 1.5-2 times the guide pin diameter to ensure stable guidance. Medium-sized molds can adopt a combination of "guide pins + guide blocks." In addition to the guide pins, wear-resistant guide blocks (made of Cr12MoV, HRC55-60 hardness) are installed on both sides of the ejector plate to reduce lateral deflection. Large molds (projected area >2000 cm²) require a four-pin plus central guide pin structure. The central guide pin is 20%-30% larger in diameter than the peripheral guide pins and serves as the primary guide. For example, in a large mold for a washing machine housing, four peripheral guide pins with a diameter of 30 mm and one central guide pin with a diameter of 40 mm are used. Together, these four guide blocks ensure the ejector plate’s motion accuracy is controlled within 0.03 mm, effectively preventing part deformation during ejection.
Proper setting of guide clearance is the core of ejection mechanism guide design. The clearance between the guide pin and guide sleeve is determined according to the mold’s precision level. For precision molds (IT5-IT6), the clearance is 0.01-0.02mm, while for standard molds (IT7-IT8), it is 0.02-0.05mm. Too small a clearance increases motion resistance, while too large a clearance eliminates the guiding function. The clearance between the guide block and the ejector plate should be controlled within 0.03-0.08mm, and a lubrication groove (2mm wide and 0.5mm deep) should be provided on the guide block surface to ensure adequate lubrication. For example, in precision electronic connector molds, the guide pin and guide sleeve utilize an H7/f6 clearance fit (0.015-0.03mm) and are lubricated with high-temperature grease (resistant to temperatures exceeding 200°C). This maintains a stable motion resistance of 50-80N for the ejection mechanism, ensuring precise demolding of micro plastic parts (weighing less than 0.5g).
The material selection and heat treatment of guide components directly impact their lifespan. Guide pins and guide sleeves are typically made of 20CrMnTi carburized steel, which undergoes carburizing and quenching (carburizing depth 0.8-1.2mm , surface hardness HRC 58-62 ) for excellent wear and fatigue resistance. Guide blocks are made of Cr12MoV tool steel, through-hardened (hardness HRC 55-60 ), and then ground to ensure a surface roughness Ra ≤ 0.8μm . For high-production molds (over 1 million molds per year), guide components can be chrome-plated (coating thickness 5-10μm ) to further improve wear resistance. For example, a beverage preform mold, by chrome-plating its guide pins and combining them with MoS₂ solid lubricant, reduced guide component wear from 0.02mm/10,000 molds to 0.005mm/10,000 molds, extending its service life to over 3 million molds.
The installation and commissioning of the guide system must adhere to the principle of precise positioning. When installing the guide post, ensure perpendicularity to the template (≤0.01mm/100mm). This can be corrected using a dial indicator before being pressed into the template. The guide sleeve and push plate must be fitted with a transition fit (H7/m6) to ensure no looseness. During the commissioning phase, the push plate’s parallelism must be tested. A dial indicator is fixed to one side of the fixed mold, the probe contacts the push plate’s surface, and the push plate is manually pushed back and forth. The parallelism error should be ≤0.03mm. The return stroke accuracy of the ejection mechanism must also be tested, ensuring that the gap between the push plate and the template after resetting is ≤0.02mm. For example, during the commissioning of a laptop computer casing mold, laser interferometer testing revealed a push plate parallelism error of 0.05mm. After adjusting the guide block’s shim thickness (adding a 0.02mm shim), the error was reduced to 0.01mm, resolving the warping issue during demolding.
Maintenance of the guide system is an important measure to extend its lifespan. Before daily production, the lubrication condition of the guide components must be checked and special mold grease must be added (1-2 times per shift). Iron and plastic chips must be cleaned from the guide parts weekly to prevent impurities from entering the fitting gap. The guide clearance must be measured monthly, and when the clearance exceeds 50% of the initial value, the guide pins and sleeves must be replaced promptly. For mechanisms that use hydraulic or pneumatic ejection, the sealing condition of the guide components must also be regularly checked to prevent leakage of hydraulic oil or compressed air from contaminating the guide surface. For example, a guide system maintenance log established by an automotive parts company shows that regular maintenance can extend the failure interval of guide components from 3 months to 12 months, significantly reducing mold maintenance costs. With the development of intelligent mold technology, some high-end molds have been installed with guide gap sensors that can monitor gap changes in real time and issue warnings, further improving the reliability of the ejection mechanism’s guidance.
