The evaluation of springback in 3D stamping and coining processes

An effective predictive technique of the elastic springback in a fully 3D 90 degrees V-punch V-die bending process is presented. This is based on a combined approach in which an explicit finite element code was used to simulate the loading phase of the process whilst an implicit procedure was used to analyse the springback phase. Two different punches with a nose radius of 4 and 8 mm were used. An increase in the springback ratio with the coining load was observed with the lower nose radius. Conversely, an increase in the springback ratio with the coining load up to a peak value (> 1), corresponding to a force of about 20 kN, followed by a decrease in the springback ratio with increasing the coining load, was observed for the larger punch nose radius. This 'negative springback', namely, the increase in the bend angle after unloading, for coining loads higher than about 13 kN, takes place because during coining the bend angle under loading is higher than the desired one. Therefore, in the final stage of the coining, the sheet blank is unbent and after unloading undergoes an inward springback. The comparison between the finite element method (FEM) results and experimental data obtained by laboratory tests carried out under similar conditions shows an excellent agreement. (C) 1998 Elsevier Science S.A. All rights reserved.