Injection mold demoulding mechanism (ejection mechanism)
The demolding mechanism (also known as the ejection mechanism) of an injection mold is a key device for smoothly removing plastic parts from the mold cavity or core after molding. The rationality of its design directly affects the surface quality, dimensional accuracy, and degree of production automation of the plastic parts. The basic requirements of the demolding mechanism include: ensuring that the plastic parts are not deformed or damaged, and that the plastic parts have a good appearance after demolding; the ejection action is smooth and reliable, without interference with other mold mechanisms; the structure is simple, easy to manufacture, and easy to maintain and adjust; it can adapt to the needs of automated production and can achieve automatic detachment or ejection of plastic parts. Common types of demolding mechanisms include push rod demolding mechanisms, push tube demolding mechanisms, push plate demolding mechanisms, ejector plate demolding mechanisms, and inclined ejector demolding mechanisms. Different types of demolding mechanisms are suitable for plastic parts with different structures and materials, and they need to be reasonably selected according to the specific situation.
The push rod ejection mechanism is the most widely used demolding mechanism. Its simple structure, ease of manufacture, and reliable operation make it suitable for demolding most small and medium-sized plastic parts. The push rod typically has a circular cross-section, with one end fixed to the push plate and the other end in contact with the part. After mold opening, the push rod’s axial movement ejects the part from the core. The push rod’s diameter should be determined based on the part’s weight and load conditions, typically ranging from 3 to 10 mm. For large parts or areas subject to high loads, a larger diameter push rod or increased number of push rods should be used to avoid bending or part deformation. The placement of the push rods is also crucial. They should be evenly distributed in stress-bearing areas of the part, such as thick walls, near bosses, or at the base of ribs. Avoid placing them in thin-walled areas or on critical surfaces to prevent whitening, deformation, or scratches. Furthermore, the clearance between the push rod and the mold plate should be appropriately controlled, typically about H7/f7, to ensure flexible push rod movement while preventing melt from escaping the gap and generating flash.
The push-tube demolding mechanism is suitable for demolding cylindrical or annular plastic parts. Its characteristic is that the push tube forms a tight fit with the core, evenly pushing the part off the core and preventing deformation due to uneven force. The push tube’s inner diameter has a clearance fit with the core, while its outer diameter has a transition fit with the mold plate, ensuring that the push tube does not deflect radially during movement. The length of the push tube should be determined by the core height, generally 5-10mm longer than the core to ensure complete ejection of the part. The wall thickness of the push tube should not be too thin, otherwise its strength and rigidity will be affected. For push tubes with smaller diameters, a hollow structure can be used to reduce weight and save material. Compared with the push rod demolding mechanism, the push-tube demolding mechanism requires higher manufacturing precision, especially the fit between the push tube, the core, and the mold plate. This requires precision machining to ensure that the push tube does not stick or leak.
The push plate demolding mechanism is suitable for large, thin-walled plastic parts, or those where no ejection marks are allowed on the surface. It uses a push plate that matches the core shape to eject the entire part, offering the advantages of uniform ejection force and stable force on the part. The push plate is typically located at the bottom or side of the core, providing a large contact area with the plastic part, effectively preventing deformation or damage during the demolding process. The push plate’s guiding accuracy is crucial, and guide pins and sleeves are typically required to ensure smooth movement. The clearance with the core should be controlled between 0.05-0.1mm to prevent friction or collision between the push plate and the core. For parts with deep cavities, the push plate’s stroke should be long enough to ensure the part is completely released from the core. The push plate’s reset requires a reset lever or spring, the length of which should be coordinated with the push rod to ensure accurate reset of the push plate.
The demolding of complex plastic parts often requires the use of a combined or specialized demolding mechanism to meet varying demolding requirements. For example, for parts with undercuts or undercuts, a tilted ejector or core-pulling mechanism is required. The tilted ejector simultaneously ejects the part and simultaneously pulls the core sideways, enabling smooth demolding of the undercut area. For large, curved parts, a combination of a top plate ejector and a push rod ejector can be used to ensure sufficient ejection force while preventing part deformation. For parts made of brittle materials (such as polystyrene and polymethyl methacrylate), multiple push rods or a push plate ejector should be used to reduce the ejection force per unit area and prevent part breakage. Furthermore, the ejector mechanism’s operation sequence must be coordinated with the mold opening and closing movements. Typically, after the mold is opened to a certain distance, the ejector mechanism begins operating, driven by the injection molding machine’s ejector unit. After the part is ejected, a reset mechanism returns the ejector mechanism to its initial position during the mold closing process. In short, the design of the demoulding mechanism needs to be optimized according to the structural characteristics, material properties and production requirements of the plastic parts to ensure that the plastic parts can be demoulded efficiently and with high quality, laying a good foundation for subsequent production processes.
