Precautions for secondary demolding in injection molding
The secondary ejection mechanism in injection molding addresses the issue of parts remaining incompletely released from the mold after the first demolding. It is particularly suitable for parts with deep cavities, thin walls, or internal undercuts. Its design requires a clear sequence of secondary ejection actions to ensure coordinated and seamless operation. The first demolding step typically involves initial ejection, pushing the part out of the cavity a certain distance (5-15mm) to separate it from the cavity surface or loosen the undercut. The second demolding step fully ejects the part, freeing it from the ejection mechanism. This sequence can be controlled using mechanical stops, hydraulic delays, or pneumatic valves. For example, in a secondary ejection mechanism controlled by a mechanical stop, during the first ejection, the ejector plate moves driven by the ejector cylinder of the injection molding machine. When it reaches a set distance (10mm), the stopper contacts the push plate, stopping the ejector plate. The push plate then continues its movement to complete the second demolding step. The distance difference between the two demolding steps is determined based on the part’s structure to ensure the part remains stable after the first demolding step, preventing it from tilting or falling.
The driving force of the secondary ejection mechanism in injection molding must be properly distributed. The first ejection force is typically greater than the second because the first ejection must overcome the clamping force between the part and the cavity and the undercut resistance, while the second ejection only needs to overcome the friction between the part and the ejector mechanism. The driving force calculation must take into account the part’s surface area, shrinkage, and undercut depth. For example, the first ejection force for a deep-cavity part (50mm cavity depth, 100mm diameter) must overcome the clamping force between the part and the cavity wall (approximately 3000N) and possible undercut resistance (approximately 1000N), resulting in a total driving force of at least 4000N. The second ejection force, however, only requires 1000-1500N. For the hydraulically driven secondary demolding mechanism, the pressure of the two demolding operations must be controlled separately by a pressure regulating valve. The first demolding pressure is set to 60%-80% of the maximum ejection pressure of the injection molding machine, and the second is set to 30%-50%. For example, when the ejection pressure of the injection molding machine is 100kN, the first demolding pressure is 70kN and the second is 40kN to avoid deformation of the plastic part or damage to the ejection mechanism due to excessive pressure.
The secondary ejection mechanism for injection molding requires high guiding precision. The ejector components must maintain smooth movement without deflection or binding during both demolding processes. Therefore, independent guiding devices, such as guide pins, guide bushings, and guide rods, are required. The guide pins must be symmetrically arranged, with clearances between the ejector plate and the ejector plate controlled to 0.02-0.04mm, ensuring that the coaxiality error of all components does not exceed 0.05mm during ejection. For example, the secondary ejection mechanism features four 20mm diameter guide pins, located at the four corners of the ejector plate. These pins and guide bushings utilize an H7/g6 fit. The guide bushings are fixed to the moving platen, and the guide pins move with the ejector plate. This high-precision guiding ensures smooth, dual-ejection movement. Furthermore, ejector components, such as the ejector pin and ejector plate, must be securely mounted, utilizing a transition fit (H7/k6) with the ejector plate to prevent vibration during the secondary ejection process that could loosen the components and affect ejection accuracy.
The stroke control of the secondary ejection mechanism during injection molding requires precise control. The distance between the two ejections must be strictly set based on the part size and demolding requirements. The initial ejection distance is typically 1/3-1/2 of the part’s height, ensuring that the part is partially released from the mold cavity while still supported by the ejector mechanism. For example, for a 30mm tall part, the initial ejection distance is 10-15mm. The secondary ejection distance must be greater than the total height of the part to ensure that the part is completely released from the ejector mechanism. For example, for the 30mm tall part mentioned above, the secondary ejection distance is 35-40mm, allowing the part to fall smoothly onto the conveyor below. Stroke control can be achieved using limit screws, stops, or electronic rulers. Electronic rulers offer the highest accuracy, maintaining a stroke error within ±0.1mm and are suitable for secondary ejection of high-precision parts. During debugging, the secondary ejection distance should be gradually tested and parameters optimized through multiple trial molds to avoid incomplete ejection due to insufficient stroke or mechanism collision due to excessive stroke.
During maintenance and commissioning of the secondary ejection mechanism for injection molding, special attention should be paid to wear of moving parts and changes in clearances. Regularly inspect the fit of the guide pins and sleeves, as well as the ejector pins and ejector holes. Replace any wear exceeding 0.1mm to prevent deviation in the ejection motion due to excessive clearance. Regularly lubricate the interface between the first and second ejection mechanisms (such as stops and tie rods) to reduce impact wear and extend service life. During commissioning, manually operate the mechanism first to confirm there is no interference before conducting a no-load test. Observe the coordination of the two ejection movements. Once the movements are stable, proceed with a trial with material. During a trial with material, inspect the ejection performance of the part, paying particular attention to whether the part tilts after the first ejection and whether the second ejection is smooth. If the part becomes stuck in the ejection mechanism, adjust the second ejection speed (typically by 5%-10%) or increase the ejection angle. Through meticulous maintenance and commissioning, the failure rate of the secondary ejection mechanism can be reduced by over 50%, ensuring stable production.
