Bending part push force and pressing force
The ejector force and presser force of a bent part are important parameters in the design of bending dies, directly affecting the forming quality of the bent part and the service life of the die. The ejector force refers to the force that pushes the workpiece against the punch from below during the bending process. Its function is to ensure that the workpiece fits tightly against the punch during bending to prevent wrinkling and deviation. At the same time, it ejects the workpiece from the die during the return stroke to prevent the workpiece from being retained. The presser force refers to the force that presses the workpiece against the die or press plate from above during the bending process to prevent the workpiece from slipping and wrinkling during bending and to ensure the dimensional accuracy of the bent part. Reasonable calculation and setting of the ejector force and presser force are key to ensuring a stable bending process and improving product quality.
The magnitude of the ejector force depends on the material, thickness, shape, and bend angle of the part being bent. It’s typically proportional to the bending force, generally ranging from 5% to 20% of the bending force. For simple V-shaped bends, the ejector force can be calculated using the formula: Ftip = Ktip × Fbend, where Fbend is the bending force and Ktip is the ejector force coefficient (0.05-0.2). For example, for a V-shaped part with a bending force of 100kN, the ejector force can be 5-20kN. The specific value should be adjusted based on the material’s friction coefficient and the complexity of the part being bent. A higher value should be used for rough surfaces and complex bends. Too little ejector force can result in a loose fit between the workpiece and the punch, creating a gap and causing bending angle errors. Too much ejector force increases the die load, exacerbates punch wear, and can even cause surface damage to the workpiece.
The structure of the ejector assembly depends on the type of bending die. Common options include spring ejector assemblies, rubber ejector assemblies, and cylinder (hydraulic cylinder) ejector assemblies. Spring ejector assemblies offer a simple structure and low cost, making them suitable for small bent parts and small-batch production. The spring stiffness must be selected based on the ejector force, and the compression must meet the ejector stroke requirements. Rubber ejector assemblies increase their ejector force with stroke, making them suitable for medium-sized bent parts. Cylinder or hydraulic ejector assemblies offer stable ejector force, which can be controlled by adjusting air or hydraulic pressure. They are suitable for large bent parts and large-scale production. For example, the bending die for automotive panels utilizes a hydraulic cylinder ejector assembly, which can achieve an ejector force of 50-200kN. Pressure sensors monitor the ejector force in real time to ensure stability and reliability.
The pressing force must be sufficient to prevent the workpiece from slipping and wrinkling during the bending process. It is typically 10%-30% of the bending force and is calculated as: Fpress = Kpress × Fbend, where Kpress is the pressing force coefficient (0.1-0.3). For U-shaped bends and parts with flanges, a higher pressing force is required to prevent wrinkling at the flange. For thin parts (thickness < 1mm), the pressing force should also be appropriately increased to avoid wavy deformation during bending. For example, when bending a 0.5mm thick brass U-shaped part with a bending force of 20kN, a pressing force of 4-6kN is recommended to ensure a smooth and wrinkle-free flange. Too little pressing force can lead to inaccurate workpiece positioning and dimensional deviations after bending. Too much pressing force can increase the overall load on the die, causing wear on the press plate or scratches on the workpiece surface.
The structural design of the press device must ensure uniform pressure. Common types include rigid press plates, elastic press plates, and pneumatic press devices. Rigid press plates are suitable for high-precision bent parts. They utilize bolts or a wedge mechanism to achieve stable pressure, making them suitable for high-volume production. Elastic press plates utilize springs or rubber for pressure, offering a simple structure and suitable for small and medium-sized bent parts. Pneumatic press devices utilize a cylinder to provide pressure, offering adjustable pressure and ease of operation, making them suitable for applications requiring frequent adjustments. The working surface of the press plate must conform well to the workpiece surface, with a surface roughness of Ra ≤ 1.6μm to avoid scratching the workpiece. For irregularly shaped bent parts, the press plate should be contoured to ensure uniform pressure across all areas. For example, for Z-shaped bent parts with flanges, the press plate must conform perfectly to the flange surface to ensure uniform pressure distribution and prevent flange deformation during bending.
The matching relationship between the ejector force and the pressing force is crucial to the bending quality. The two need to work together to ensure that the workpiece neither deflects nor wrinkles during the bending process. For bent parts with complex shapes, the ejector force and the pressing force need to be adjusted through trial molds. Usually, a smaller force is set for trial bending, and the forming of the workpiece is observed before gradually increasing to the appropriate value. For example, if wrinkles are found on one side of the workpiece during the trial bending, it means that the pressing force on that side is insufficient, and the pressing force on the corresponding part needs to be increased; if the bending angles of the workpiece are inconsistent, it may be that the ejector force is uneven, and the spring stiffness or hydraulic pressure of the ejector device needs to be adjusted. In automated bending production, the ejector force and the pressing force can be monitored in real time by sensors, and combined with the PLC control system, automatic adjustment can be achieved to ensure the stability of the quality of the bent parts.
The calculation and setting of ejector and hold-down forces must be considered in conjunction with actual production practices, taking into account factors such as material properties, mold structure, and production batch size. Optimal values are determined through a combination of theoretical calculations and trial mold adjustments. Proper ejector and hold-down forces significantly improve the dimensional accuracy and surface quality of bent parts, reduce mold wear, and extend mold life, making them an essential component of bending process design.
