The Study Of Autobody Panel Hemming Process

Hemming process, by folding the edge of outer panel along the inner panel to form one combined component, is widely used to connect two metal plates and obtain smooth edge profile, and also as the last forming process in autobody panel production, have great impact on automobile's quality and appearance. The panels which most used the hemming processes in autobody are doors, hood and so on. With the development of the modern automotive industry, quality and appearance of the car have become increasingly demanding. So it's very important and urgent to study and solve the quality problems encountered in the hemming process of autobody panel.Autobody panel hemming process is a 180-degree bending process of outer panel edge. It usually includes three steps: flanging, pre-hemming and final hemming. According to the geometry of sheet, the hemming process can be divided into four categories: flat surface-straight edge hemming, flat surface-curve edge hemming, curve surface-straight edge hemming and curve surface-curve edge hemming. Hemming process, which is different from simple sheet bending, often results some defects in the formed panels as roll-in and roll-out, springback, recoil of hem contours and warping, wrinkling, cracking within the hemmed outer panel and so on.In this paper, the hemming process, which widely used in automobile industry, was studied by a method of constructing mechanic models combined with FEM simulation results. The warp producing mechanism in the straight edge- plane hemming process was analyzed in detail using this mechanic model. The reason why the warp produced at the position of near the contact point between the out panel and the position surface of hemming die during the pre-hemming process, and the phenomena of warp reducing firstly and then increasing before the position point T and the warp reducing again after point T during the final hemming process were interpreted.Also in this paper, finite element analysis method also was used to study the impact of the flanging die corner Rd, flange height L, the path of pre-hemming, end position of pre-hemming, face angle of pre-hemming punch and final hem force and other process parameters on roll-in, pre-hem angle and warp. The study showed that the calculated results are consistent with the experimental data in the related literatures. And some of the new phenomena and laws appeared in the hemming process were summarized and analyzed in this paper.The works finished in this paper provides a theoretical basis to understand hemming process and further to analyze the reasons of warp producing in the hemming processes, and also can help engineer to set up the hemming process, design hemming die and try-out the hemming die.