Design of the push plate ejection mechanism
The push plate ejection mechanism is a crucial device used in die-casting molds to demold large, thin-walled, or complex-shaped die-castings. Its core design focuses on evenly transmitting the ejection force across the entire end face or large area of the die-casting through the entire push plate, preventing deformation caused by excessive localized force. The design of the push plate ejection mechanism begins with determining the basic dimensions and shape of the push plate. The push plate’s area should be slightly larger than the projected area of the die-casting to ensure sufficient contact area. The thickness of the push plate is calculated based on the weight of the die-casting and the ejection force. Typically, 10-30mm steel plate is used. For large die-castings, the push plate thickness can be increased to 40-50mm to ensure rigidity and prevent bending deformation during the ejection process.
The connection and guidance design between the push plate and other mold components are critical to ensuring smooth operation of the ejection mechanism. The push plate typically contacts the die-cast part through push rods. These push rods must be evenly distributed across the push plate. The number and diameter of the push rods are determined based on the weight and shape of the die-cast part. Generally, one to two push rods with a diameter of 5-8mm are required for every square centimeter of die-cast part projection area to ensure even distribution of ejection force. The push plate typically uses a guide pin and guide bushing structure. Four guide pins are typically located at the four corners of the push plate. The guide pins and guide bushings utilize an H7/H6 clearance fit, controlled within 0.01-0.02mm, to prevent deflection during push plate movement. The guide pins and guide bushings must be made of high-strength alloy steel and heat-treated for wear resistance. The straightness error of the guide pins must be controlled within 0.01mm/m to prevent bending that could cause the push plate to become stuck.
The ejection stroke design of the ejector plate ejection mechanism must meet the requirements for complete demolding of the die-casting. The ejection stroke should be greater than the maximum height or depth of the die-casting, and a margin of 5-10mm is usually required to ensure that the die-casting can be smoothly separated from the core. The calculation of the ejection stroke must comprehensively consider factors such as the structural dimensions of the die-casting, the height of the core, and the opening and closing distance of the mold. For die-castings with deep or complex cavities, the ejection stroke must be increased accordingly. The reset of the ejector plate usually relies on a reset spring or reset rod. The number and specifications of the reset springs must be determined according to the weight of the ejector plate and the reset force requirements to ensure that the ejector plate can be accurately and quickly reset to its initial position. The length of the reset rod must be precisely controlled to ensure that the end face of the reset rod is flush with the parting surface when the mold is closed to avoid affecting the mold closing accuracy.
The material selection and heat treatment of the push plate have a significant impact on its service life and performance. Push plates are typically made of 45 steel or Q235 steel. For more demanding applications, alloy tool steels such as Cr12MoV can be used. 45 steel push plates require quenching and tempering to a hardness of HB220-250 to improve their strength and toughness. Cr12MoV push plates require quenching and low-temperature tempering to a hardness of HRC50-55 to enhance wear resistance. Surface treatment of the push plate is also crucial. Phosphating or anti-rust paint is typically required to prevent rust. Contact areas with the push rod are ground to a surface roughness of Ra1.6μm or less to reduce friction.
The design of the push plate ejection mechanism also needs to consider coordination with other mold systems. When designing the push plate, sufficient clearance should be reserved for components such as the runner and core to prevent interference during the ejection process. The push rod hole on the push plate should be fitted with the push rod using a clearance of H7/f7 to ensure flexible movement of the push rod while preventing excessive clearance that could cause the push plate to wobble. For molds with core pulling mechanisms, the push plate’s ejection stroke must be coordinated with the core pulling action. Typically, the core pulling is performed first, followed by ejection, to avoid additional stress on the die-casting during demolding. Furthermore, the push plate ejection mechanism should be designed to facilitate mold assembly and maintenance. The connection between the push plate and components such as the push rod and guide pins should be removable to facilitate replacement after wear. As die-casting molds continue to grow in size and precision, the design of the push plate ejection mechanism is constantly being optimized. Advanced techniques such as finite element analysis can be used to simulate and analyze the stresses acting on the push plate, further improving its design rationality and reliability.
