An Investigation of Wall Curl Reduction Through Post-Stretch Forming

When sheet metals go through drawbeads or die corners, stress differentials are generated across metal thickness. The draw wall will curl up upon release of stamping tools, resulting in so-called wall curl. It is a serious problem in the deep drawings of U-channel type of structures such as rails. Numerical modeling is conducted to investigate a post-stretch forming process for wall curl reduction. In this process a set of lockbeads in the binder activates just before the end of punch stroke, locking the remaining blank in the binder. The continuation of the punch stroke then creates a final increment of pure stretch. It is most effective for deep drawings of U-channel type of structures such as rails. This technique is also known as “post-stretch” or “shape set” in the automotive industry and in the literature. Finite element simulations for a straight channel are conducted in order to understand the wall curl reduction mechanism of the process and to determine its effectiveness. After an examination of deformation profile after drawing and wall curl as a result of springback, various magnitude of post-stretch amount is modeled and their deformation history is analyzed. It is found that a post-stretch strain around 2% almost completely eliminates wall curl. CAE investigations demonstrate that the technique is equally applicable to more complex 3D channel, where a step channel is examined. The effectiveness of this concept is demonstrated by laboratory experiment on the forming of a U-channel. Various implementation techniques for the process in an industrial environment are also suggested, together with a discussion on the associated benefits and costs for production use.