Springback of bent parts and its prevention method
Springback is a common phenomenon in the bending process caused by the elastic recovery of the material. It refers to the deformation of the workpiece, where the bending angle and radius deviate from the mold’s formed dimensions after the bending force is removed. This phenomenon stems from the simultaneous plastic and elastic deformation of the material during bending. When the external force disappears, the elastic deformation partially recovers, causing the workpiece shape to deviate from the mold design. Springback directly affects the dimensional accuracy and assembly performance of the bent part, especially for parts requiring high precision (such as automotive structural components and electronic connectors). Improper springback control can lead to product failure or assembly difficulties. Therefore, a thorough understanding of the springback mechanism and the implementation of effective prevention measures are core tasks in bending process design.
The mechanism of springback in bent parts is closely related to the mechanical properties of the material. The higher the yield strength and the lower the elastic modulus, the more pronounced the springback. When the material is subjected to a bending moment, the outer fibers experience tensile stress and the inner fibers experience compressive stress. The stress distribution changes linearly, with the stress in the neutral layer reaching zero. After unloading, the material in the tensile stress zone contracts and the compressive stress zone expands, increasing the bend angle and radius. For example, high-strength steel (such as DP600) has a yield strength exceeding 600 MPa, and its springback is 3-5 times that of ordinary low-carbon steel. Designing a mold based on the process parameters of ordinary steel would result in significant angular deviations. Furthermore, the larger the relative bending radius (r/t), the greater the proportion of elastic deformation in the material, and the more pronounced the springback. When r/t > 10, the springback can reach 5°-10°.
The fundamental method for preventing springback in bent parts is die compensation. Based on the estimated springback amount, a pre-set offset angle or shape is set during die design to ensure that the workpiece reaches the designed dimensions after springback. For V-shaped bent parts, the die angle can be reduced by a springback angle Δα. For example, if a 90° workpiece angle is required and a 3° springback is expected, the die angle can be designed to 87°. For U-shaped parts, the die sidewalls can be designed with an inward slope (usually 0.5°-2°) to offset the springback caused by the outward expansion of the sidewalls. The key to die compensation is accurately estimating the springback amount, which requires a combination of material properties, bending parameters, and production experience. For complex parts, the compensation amount can be gradually adjusted through trial molds until the workpiece dimensions meet the required standards. For example, after a trial mold of a U-shaped bracket, the sidewalls expanded 1.5°. The die sidewall slope was adjusted from 1° to 1.8°, and the dimensions met the required standards after another trial mold.
The pressure correction method increases the correction force at the end of the bend, forcing the material to undergo more plastic deformation, reducing elastic recovery and thus reducing springback. This correction force is typically 3-5 times the normal bending force. This can be achieved by increasing the punch stroke or designing a specialized correction structure, such as a raised correction rib at the bottom of the V-die, to apply concentrated pressure to the workpiece bottom at the end of the bend. The pressure correction method is suitable for parts with relatively small bend radii (r/t < 5) and is particularly effective for thick materials (t > 3mm), reducing springback by over 50%. For example, a 5mm thick Q345 steel U-shaped part can reduce angular springback from 4° to 1.5° after applying a correction force of 150kN (compared to a normal bending force of 50kN). However, it should be noted that excessive correction force can cause surface damage to the workpiece and shorten the die life.
For high-strength materials or parts with large springback, process optimization methods are required to prevent springback. Common measures include stretch bending, warm bending, and step-by-step bending. The stretch bending process applies axial tension (typically 30%-50% of the material’s yield strength) to the workpiece while bending, causing the material to bend under tensile stress and reducing elastic recovery. This process is suitable for slender bent parts (such as automotive door frames and elevator guide rails) and can control springback to less than 1°. Warm bending involves heating the material to a certain temperature (such as 200-300°C for aluminum alloys and 600-800°C for high-strength steel) before bending. This reduces the material’s yield strength, increases its plasticity, and significantly reduces springback. For example, the springback of a 6061 aluminum alloy bent part heated to 250°C was reduced from 8° to 2°. Step-by-step bending breaks down the complex bending process into multiple small-angle bends, gradually approaching the target shape. The springback amount is cumulatively reduced after each bending step. It is suitable for complex polygonal parts. For example, a Z-shaped part can be bent into a U shape first and then corrected to a Z shape to reduce the springback error of one-time forming.
Preventing springback in bent parts requires a comprehensive approach tailored to the specific situation. For simple parts, die compensation and pressurization correction are preferred. For complex or high-strength parts, process optimization is combined with CAE simulation software (such as AutoForm and Dynaform) to predict springback and optimize process parameters. For example, a high-strength steel bent part for a new energy vehicle battery housing had a springback of 7°, as determined by Dynaform simulation. After applying 5° die compensation and a stretch-bending process (with a tensile force of 300kN), the actual springback was only 0.8°, meeting the required precision. With the application of new materials and complex parts, springback prevention technology is evolving towards intelligent technology. By monitoring stress and strain during the bending process in real time and dynamically adjusting process parameters, precise springback control is achieved.
