Minimum bending radius of bending die
The minimum bend radius of a bending die refers to the smallest inner bend radius a part can achieve without cracking. It is a crucial parameter in bending process design, directly impacting the part’s forming quality and die life. The minimum bend radius is closely related to factors such as the material’s plasticity, thickness, rolling direction, bending method, and die structure. Properly determining the minimum bend radius can prevent cracking in parts, reduce material consumption, and improve production efficiency. For different materials and process conditions, the minimum bend radius must be determined through testing or empirical data to ensure a safe and reliable bending process.
The plasticity of a material is a key factor influencing the minimum bend radius. Materials with greater plasticity (such as mild steel, brass, and aluminum alloy) have smaller minimum bend radii; materials with less plasticity (such as high-carbon steel, stainless steel, and cast iron) have larger minimum bend radii. Material plasticity can be measured through indicators such as elongation and reduction of area. Materials with an elongation ≥20% are considered highly plastic and can have a minimum bend radius as small as 0.1t (where t is the material thickness). Materials with an elongation ≤10% are considered low plastic and require a minimum bend radius ≥1t. For example, the minimum bend radius for mild steel (30% elongation) is 0.1t, while that for high-carbon steel (10% elongation) is 1t. Forcing a narrow bend radius can cause the outer fibers of the material to tear.
Material thickness and rolling direction have a significant impact on the minimum bending radius. The thicker the material, the larger the minimum bending radius. This is because the elongation of the outer fibers of thick materials is greater, making it easier to exceed the plastic limit of the material. For example, the minimum bending radius of mild steel with a thickness of 1mm is 0.1mm, while the minimum bending radius of mild steel with a thickness of 5mm needs to be increased to 0.5mm. The rolling direction will cause the material to be anisotropic. When bending along the rolling direction, the material has better plasticity and the minimum bending radius can be 20%-30% smaller than that perpendicular to the rolling direction. For example, the minimum bending radius of a brass plate along the rolling direction is 0.3t, while that in the vertical direction is 0.4t. Therefore, during design, the bending direction should be made consistent with the rolling direction as much as possible to reduce the minimum bending radius.
The bending method and mold structure also affect the minimum bending radius. Corrective bending can force the material to produce more complete plastic deformation through a larger correction force, so the minimum bending radius can be 30%-50% smaller than free bending. Bending dies with a pressing device can prevent material slippage and reduce the stretching of the outer fibers, and the minimum bending radius can also be appropriately reduced. For example, the minimum radius of free bending of low-carbon steel is 0.2t, which can be reduced to 0.1t during corrective bending. The mold’s corner radius and surface quality are equally important. The smaller the punch corner radius, the more likely it is to cause stress concentration and cracking. Therefore, the punch corner radius is usually equal to the minimum bending radius of the bent part, and the surface roughness must be ≤0.8μm to reduce additional stress caused by friction.
In actual production, the determination of the minimum bending radius requires combined testing and verification. Samples with different radii are tested to observe whether cracking, fissures, or severe thinning (thinning ≥15% is considered unqualified) occurs, thereby determining the critical minimum radius. For example, a test bend of a 2mm thick stainless steel plate revealed fine cracks at a 1mm radius, but no cracks at a 1.2mm radius. Therefore, the minimum bending radius is determined to be 1.2mm (0.6t). For critical parts, mechanical property testing (such as bending and tensile tests) is also required to ensure that the strength of the bent parts meets the required performance. Furthermore, material pretreatment (such as annealing) can be used to increase material plasticity and reduce the minimum bending radius. For example, after annealing, the minimum bending radius of cold-rolled steel sheets can be reduced from 0.3t to 0.15t.
The application of the minimum bend radius requires comprehensive consideration of the part’s intended use. If the part undergoes subsequent processing (such as electroplating or painting), the bend radius should be appropriately increased to prevent cracking due to stress concentration during processing. If the part is subject to significant loads, sufficient strength must be ensured at the bend radius to prevent fatigue fracture caused by an excessively small radius. For example, the bend radius of automotive structural parts must be ≥0.5t to ensure safety under impact loads. With the introduction of new materials (such as high-strength aluminum alloys and advanced high-strength steels), the determination of the minimum bend radius requires the accumulation of more test data, combined with CAE simulation analysis, to optimize bending process parameters and achieve the minimum bend radius while ensuring quality.
