Key points in designing ejector plate demoulding for injection molding
Push-plate demolding in injection molding is a highly efficient demolding method suitable for deep-cavity, thin-walled parts. Using a push-plate that conforms to the inner or outer surface of the part, it pushes the entire part out of the mold cavity during demolding. This method offers advantages such as uniform demolding force and reduced part deformation. Compared to traditional ejector pin demolding, push-plate demolding avoids ejection marks on the part’s surface, making it particularly suitable for products with high aesthetic requirements, such as cosmetic bottles and medical device housings. The push-plate demolding mechanism operates as follows: After mold opening, the push-plate, driven by the demolding mechanism, moves along the mold axis, relative to the cavity or core, ejecting the part from the core or cavity. For example, when producing PET bottle preforms, the push-plate conforms perfectly to the outer surface of the preform, ensuring smooth movement during demolding. This ensures uniform force distribution on the preform, avoids deformation of the bottle neck or wrinkles caused by excessive localized force, and ensures product dimensional accuracy.
The design of the fit between the ejector plate and the mold is crucial for ensuring effective demolding. The clearance between the ejector plate and the mold core must be strictly controlled. Too small a clearance can hinder the ejector plate’s movement or even cause it to jam. Excessive clearance can allow the plastic melt to penetrate the gap during injection, forming flash that affects both the part’s appearance and the ejector plate’s movement. Generally, the clearance should be controlled between 0.01-0.03mm. For parts requiring high precision, the clearance can be reduced to 0.005-0.01mm. The guide mechanism between the ejector plate and the mold plate is equally important. Guide pins and bushings are typically used, with at least four guide pins evenly distributed around the ejector plate. This ensures the ejector plate remains level during movement and prevents deformation of the part caused by deflection. The fit accuracy of the guide pins and bushings is H7/f7, with a surface roughness of Ra ≤ 0.8μm, to ensure smooth movement. In addition, the thickness of the push plate needs to be determined according to the size of the plastic part and the demolding force, generally 1/5-1/3 of the maximum diameter of the plastic part, to ensure that the push plate has sufficient rigidity to prevent bending deformation during the demolding process.
The ejector plate’s stroke must be designed to ensure complete demolding of the part while avoiding interference with other mold components. The minimum ejector plate stroke should be greater than the part’s height or depth, typically 1.2-1.5 times the part’s height, to ensure complete release of the part from the core or cavity. For example, for a deep-cavity part with a height of 50mm, the ejector plate stroke should be set to 60-75mm. Part shrinkage should also be considered when determining the stroke. For plastics with significant shrinkage, such as PE (1.5%-3.6%), the stroke should be increased by 1-2mm to prevent the part from shrinking and clinging to the core, preventing complete ejection. Limiting devices for the ejector plate’s stroke are essential. These can be implemented as stoppers or baffles. Once the ejector plate reaches a set position, the stopper accurately prevents further movement, preventing damage to the mechanism caused by excessive movement. The stopper plate height should be precisely calculated based on the ejector plate’s stroke, with an error within ±0.1mm.
The calculation of the ejector plate’s demolding force and the design of its drive mechanism directly impact demolding reliability. The magnitude of the demolding force is related to factors such as the contact area between the part and the core, the plastic’s friction coefficient, and shrinkage. The calculation formula is: F = K × A × P, where F is the demolding force (N), K is the coefficient (usually 0.8-1.2), A is the contact area between the part and the core (mm²), and P is the clamping force per unit area (MPa, typically 0.5-1.5 MPa). For example, for a PP part with a contact area of 10,000 mm², the demolding force is approximately F = 1.0 × 10,000 × 1.0 = 10,000 N. The drive mechanism must provide sufficient driving force. Common drive methods include hydraulic cylinders and mechanical ejectors. Hydraulic cylinder drive has the advantages of large driving force and adjustable speed. It is suitable for large push plates. The thrust of the cylinder should be greater than 1.5 times the calculated demoulding force. Mechanical ejector drive has a simple structure and low cost. It is suitable for small and medium-sized push plates. The number and diameter of the ejector pins need to be determined according to the distribution of the demoulding force to ensure uniform force.
Detailed design of the ejector plate for mold release is crucial to avoiding defects in plastic parts. The contact surface between the ejector plate and the part must be smooth and flat, with a surface roughness of Ra ≤ 0.8μm, to prevent scratches on the part during the demolding process. For parts with patterns or complex curves, the inner surface of the ejector plate must perfectly match the outer surface of the part, with a fit of at least 95% to ensure even distribution of demolding force. Venting grooves should be provided on the ejector plate to expel air between the plate and the part during demolding, preventing part deformation or demolding difficulties caused by air pressure. The venting grooves should be 0.02-0.05mm deep and 5-10mm wide, evenly distributed around the circumference of the ejector plate. Furthermore, the relative motion between the ejector plate and the core should be greased to reduce friction and wear, thereby extending its service life. For thermoplastics, the ejector plate’s temperature should be consistent with the mold cavity temperature. Cooling water channels can be installed within the ejector plate to prevent dimensional deviations caused by temperature differences. These detailed designs can effectively improve ejector plate demolding stability and part quality.
