Double-stage forming using critical pre-bending radius in roll bending of pipe with rectangular cross-section

Bent pipes are widely used in many high-end industries as a structural component. Consequently, pipe roll bending has become an attractive manufacturing technology for forming lightweight products. For industrial applications, bent pipes should be accurately shaped into 2D or 3D shapes without defects. In this study, the behavior of pipes with rectangular cross-sections under roll bending was considered in order to improve product quality. When a pipe product is bent with a large curvature, the pipe cross-section shrinks under the bending moment. In order to minimize defects and improve product quality, this paper proposes double-stage forming to regulate the shrinking load, which causes the axial wrinkles and poor strength, as structural members. In double-stage forming, the bending radius at the pre-bending stage is assumed to be a key parameter to avoid cross-section distortion. In order to determine the pre-bending radius, material behavior was analyzed to figure out the reason for the cross-section shrinkage. The bending stresses on the cross-section of the pipe material subject to bending moments bring about the distribution of inward forces, which induce shrinkage moments on the cross-section. The main idea in this paper is that the cross-section distortion can be minimized by regulating the pre-bending radius at the first stage. The proposed process design was evaluated through nonlinear finite element simulation using a commercial program and experiments.