The most commonly used manufacturing method in the production of body panels in the home-appliance industry is bending. Various problems can be encountered in bending caused by both material and process parameters. The dimensional errors are one of the most important of these problems, and they generally originate from springback.
Several methods are introduced to minimize the springback and its effects on final part geometry. These methods are over-bending, bending with coining, bending in tension, reverse bending, etc. The amount of springback after bending must be predicted, and the effects of sheet metal properties and process parameters on springback must be well known to compensate for the springback by over-bending. Through this explanation, it can be said that material properties such as elastic modulus, yield strength, anisotropy, etc., and forming process parameters including die radius, punch-die clearance, etc., affect springback. Springback also depends on the bending method. Air bending, V-die bending, straight flanging, and rotary die bending give different springback characteristics for the same material. The springback problems, particularly its variation, can cause substantial financial losses in the production involving sheet metal forming. The expense of springback problems due to production delays, tool replacement costs, rejected scrap materials, etc. was reported to exceed $50 million annually in the United States automotive industry .
When the applied load is removed after forming the metal sheet, the material tends to recover elastically and return to its original form . Springback can be defined as the elastic recovery that results after getting rid of the bending moment during forming . Besides, springback is the dispersion of the forming stresses in the material, met after the forming dies are removed, and thus residual stresses are encountered [4, 5]. As the induced forming stresses increase, the springback also increases .
Springback is a forming problem where multiple interactions of many variables including mechanical properties, process parameters, and dimensional factors are involved. Process parameters and mechanical properties interact and generate the stress distribution through the sheet thickness that will affect springback . These effects make springback estimation and compensation difficult. Therefore, to make a healthy analysis, factor interactions must be taken into account besides the mean effects .
Independent of the bending type, the amount of springback increases with the ratio of bending radius to sheet material thickness [2, 9, 10]. As the sheet material thickness (t) decreases or the r/t ratio increases, the springback angle monotonically increases [11–13]. In a recent paper by Wang et al. , it was shown that with decreasing r/t ratio, the springback ratio decreases gradually. As a result of the increased thickness, the residual stresses encountered in the bending zone decreases.
Clearance between the punch and die is generally selected 1.1 times the sheet thickness, considering the thickness tolerance of %10. Springback monotonically increases as the clearance increases because the clearance dictates the conforming of the sheet to the die . Ling et al.  stated that this tendency becomes less visible as the die radius increases from 0.5t to 3.0t. With the narrowing of the die clearance, plastic deformation in the bending zone is localized and intensified decreasing the springback .
The elastic modulus is the most influential material property on the springback. A higher elastic modulus leads to smaller elastic deformation at the bending zone, and thus less springback . Since bending is an elastic-plastic deformation yield strength or plastic flow stress is also very influential, because along with Young’s modulus it determines the elastic resilience . Increasing the strain hardening coefficient (n) also increases the elastic strain component in the total bending strain, and thus the springback .
A significant amount of work is published on industrially standard bending methods. Among them, numerous papers focus on straight flanging, some being experimental as well as numerical [12, 15, 19, 20, 21]. Numerical work is mostly on finite element prediction of springback. However, literature on the rotary die bending process is very weak.
The oldest and most common method used in forming refrigerator doors and side panels from sheet metal is straight flanging. The schematic representation of this process is given in Fig. 1. The purpose of this process is to obtain a 90° bent flange mostly. In straight flanging, the punch performs a linear motion similar to the V-die bending and air bending processes. However, unlike the other methods, the bending process takes place around the bending die, not around the punch tip. Throughout the linear movement of the punch, the position of the contact segment between the punch and the sheet changes continuously. This motion continues until bending is complete. Here, the entire movement of the punch on the bending edge can be called wiping or wipe die bending. Critical process parameters are the die (bending) radius (Rd), blank thickness (t), clearance between the punch and die (c), the flange length (Lf), and the blank-holder (pressure pad) force (Fbh).
Another method of bending box-type parts in the home appliance industry is rotary die bending (Fig. 2). In this process, the upper die, called the rocker, replaces the punch. Instead of the linear movement made by a solid punch, the rocker rotates during the downward linear motion. With the help of this rotation, the flange is locally bent around the die shoulder. Similar to straight flanging, sometimes it is possible to use a blank-holder but in general a blank-holder is not used in this process. This simplification is an advantage of the process.
Rotary die bending can be used to decrease the springback of the parts according to straight flanging. Besides, it is more robust to process variation. However, the literature on the rotary die bending process is inadequate. In this article, the springback effects in flanging using a solid punch and a rotary die are experimentally compared on cold-rolled carbon and stainless steel sheets. The factors tested under material variability faced in the industrial environment were the die radius, punch-die clearance, bending axis with respect to rolling, and flange length.