Deformation of sheet metal bending
Sheet metal bending is a critical process in metal plastic processing. Its deformation involves complex phenomena such as plastic flow, stress-strain distribution, and springback. During bending, the sheet metal is subjected to bending moments, creating varying stress states within it. The outer layer experiences tensile stress, while the inner layer experiences compressive stress. At the interface between tension and compression lies a neutral layer with zero stress. As the degree of bending increases, this neutral layer gradually shifts inward. This is because the tensile deformation of the outer layer is greater than the compressive deformation of the inner layer, causing the neutral layer to shift in position. This phenomenon is more pronounced with thicker sheets or smaller bend radii.
The degree of deformation in sheet metal bending is usually expressed as the ratio of the bending radius to the sheet metal thickness, that is, the relative bending radius. The smaller the relative bending radius, the more severe the bending deformation and the more complex the plastic flow of the material. During the deformation process, the outer fibers of the sheet metal will become longer due to stretching. When the tensile stress exceeds the yield limit of the material, the outer layer of the material will undergo plastic deformation; the inner fibers will shorten due to compression and will also undergo plastic deformation. Due to the uneven plastic deformation of the material, the thickness of the sheet metal will change to a certain extent after bending. Usually, the outer layer thickness will be slightly thinner and the inner layer thickness will be slightly thicker. The change in overall thickness is related to the bending radius and the plasticity of the material. Under normal circumstances, the thickness change is small, but the impact of this factor on dimensional accuracy needs to be considered in precision bending parts.
The deformation of sheet metal bending is also accompanied by springback. This is because the elastic deformation of the material during the bending process will recover after unloading, resulting in a deviation between the actual angle and radius of the bent part and the size of the mold. The magnitude of the springback is related to factors such as the mechanical properties of the material, the bending radius, the bending angle, and the thickness of the sheet metal. Generally speaking, the higher the yield strength of the material and the smaller the elastic modulus, the greater the springback; the larger the bending radius and the larger the bending angle, the greater the springback. In order to reduce the impact of springback on the accuracy of the bent part, a compensation method is usually used in mold design. That is, the angle and radius of the mold are adjusted according to the estimated springback amount so that the bent part meets the design requirements after springback. In addition, the bent part can be forced to correct by adding a correction bending process to reduce the springback.
During sheet metal bending, the plastic flow of the material causes cross-sectional distortion of the bent part. For wide sheet metal bending, the strong constraints in the width direction restrict material flow, resulting in minimal cross-sectional distortion and maintaining the original shape. However, for narrow sheet metal bending, the constraints in the width direction are weaker, causing the material to flow to the sides, resulting in significant cross-sectional distortion and a fan-shaped appearance. This cross-sectional distortion affects the dimensional accuracy of the bent part. Therefore, when designing the bending process, it is necessary to select an appropriate mold structure based on the width of the sheet metal. For example, for narrow sheet metal bending, a mold with a side pressure device can be used to restrict the lateral flow of the material and reduce cross-sectional distortion.
The deformation of sheet metal during bending is also closely related to the mold structure and bending process parameters. The mold’s corner radius, die depth, and punch-die gap all affect the material’s deformation process. For example, if the mold’s corner radius is too small, the sheet metal will experience excessive stress concentration at the bend, making it prone to cracking. If the punch-die gap is too large, the sheet metal will rebound and affect bending accuracy. If the gap is too small, the friction between the mold and the sheet metal will increase, increasing the bending force and potentially causing excessive thinning of the sheet metal. Furthermore, process parameters such as bending speed and lubrication conditions also affect deformation. While appropriately increasing the bending speed can reduce springback, excessively high speeds can lead to brittle fracture. Good lubrication can reduce friction, minimize mold wear, and improve the uniformity of material deformation.
In summary, sheet metal bending deformation is a complex process involving the interaction of multiple factors. Its deformation characteristics include neutral layer offset, thickness variation, springback, and cross-sectional distortion. In actual production, it is necessary to fully understand these deformation patterns and rationally design mold structure and process parameters to control the deformation process, improve the quality and precision of bent parts, and meet the application requirements of different products.
