Investigation into the characteristics of flexible roll forming with incremental counter forming process to improve shape errors of profiles with variable cross-sections

Abstract

Profiles with variable cross-sections in the directions of width and depth have advantages for practical designs and aesthetic shapes for industrial applications. To meet increasing demands for the profiles, the flexible roll forming process has been developed to fabricate profiles with variable cross-sections. A roll-formed profile with variable cross-sections fabricated by flexible roll forming has shape errors, such as warping and buckling, because of geometrical deviations in transitional zones of the profile between an initial metal strip and a roll-formed profile. To reduce the shape errors, a new process, the incremental counter forming (ICF), has been proposed. The shape errors of roll-formed profiles are reduced by the ICF process. Process parameters of the ICF process have been investigated. As the longitudinal strain at the flange part in the concave zone increases and the longitudinal strain at the flange part in the convex zone decreases by the ICF process, warping of a profile has been improved. When the longitudinal strain in the straight zone reaches a critical limit, the additional longitudinal strain works as an excessive longitudinal strain to worsen the warping of the profile. To design a span length between a forming roll of flexible roll forming and a roll set of the ICF process, the deformation length of flexible roll forming has been analytically predicted. Shape parameters of a profile with variable cross-sections, such as the radius of curvature, the transition angle in transition zones, and the bending angle are investigated to predict the deformation length of a profile during flexible roll forming. The deformation length has been predicted by adapting the energy method to consider the longitudinal deformation at flange parts and the bending deformation at bending parts of a profile. Predicted results have been validated by the FEA results. The results have shown that the proposed method is capable of predicting the deformation length of profiles with variable cross-sections within ranges of reasonable assumptions. The flange part in the convex zone of a profile has been investigated. As the bending angle of the flange part increases stepwise during the flexible roll forming process, the compressive stress at the flange part in the convex zone increases to induce buckling. The transition angle as a shape parameter, in the convex zone of the profile has been investigated to avoid buckling by using analytical approaches combined with FE simulations. A profile with a transition angle which lies within the range without buckling is applied by the ICF process to improve warping over the profile. Finally, applications of profiles with variable cross-sections are designed to verify the effectiveness of the ICF process experimentally. For fabricating profiles with variable cross-sections in the direction of depth, the flexible roll forming process is investigated by FE simulations. Variable cross-sections in depth of profiles are newly designed and studied. The transition angle in transition zones has been investigated regarding the additional shape errors. Two types of shape errors are defined. As the first shape error, deviations between the targeted amount of variable cross-sections in the depth direction and the obtained amount of variable cross-sections are defined. As the second shape error, geometrical deviations between the targeted shape of an edge and the obtained flange edge is measured after flexible roll forming in the depth direction. As the transition angle of the transition zones increases, i.e. the amount of variable cross-sections in the depth direction increases, the results show that the shape errors increase. The concept of the ICF process has been introduced to improve the shape error of profiles with variable cross-sections in the depth direction. Process parameters, such as amount of counter forming of the ICF process, the roll distance in each ICF roll set, and the gap between an upper roll and a bottom roll of the ICF process, are investigated by employing design of experiments (DOE). Initial blanks are designed for optimum blank shapes by applying sensitivity analysis. Until the optimum blank shapes are acquired, sensitivity analysis is iterated to improve the shape error of the flange edge. The FEA results have shown that both methods work to improve the shape errors of the profiles with variable cross-sections in the depth direction. Finally, the study has been shown that the effectiveness of the ICF process is capable of improving the accuracy of the profiles with variable cross-sections in the width and depth directions.