Production of profiles having a variable cross section lengthwise by flexible roll-forming
According to DIN 8586, roll forming is a bending technology with rotating tool motion used to manufacture open and closed profiles in a continuous production process. In order to obtain the desired profile, several stages feeding a split strip of metal through successive pairs of stands as shown in [Figure 1] are needed. It is important to mention, that the quality of the produced parts is depending from the chosen bending angle sequence. The lay-out of a conventional roll forming tool can be done nowadays by commercially available software. In former times a lot of experiences and process know-how especially for the tooling lay-out was necessary.
Compared with other forming processes, classic roll forming offers a number of technical, economical and ecological advantages e.g. high productivity at low tool costs. So far, the process is limited to the production of profiles with constant cross sections in longitudinal direction. Conventional roll forming is not the appropriate technique to produce small lot sizes. Often many kilometers of the same type of profiles are produced until the forming rolls are changed. From the beginning of the tool change till the restart of the production easily one or two shifts can pass by. As a result, the manufacture of customized products with small lot sizes, such as profiles with a variable cross section lengthwise, cannot be performed by conventional roll forming machines. Therefore the PtU Darmstadt developed the so called “flexible roll forming” in order to satisfy customer’s expectations, such as the need for load adapted product geometries, the individualisation of products and low production costs. This new forming technology enables the production of profiles with variable cross sections in an highly efficient and continuous process.
State of the art – research at PtU
A symmetric u-section was defined as first targeted geometry. By adding further conventional roll forming steps, more complex profiles such as hat sections or closed profiles can be achieved. See [Figure 2] for examples. In the meantime the potential of this technique has been realised by other research institutes. The feasibility of the process could be shown by the realisation of several prototypes. During these investigations, first process limits were detected in terms of geometric deflections, like wrinkles.
Currently, the producibility of new target geometries is proven either by time consuming FE-analyses or laboratory tests. Due to the high degree of freedom concerning possible geometries, the verification of any new profile is always a very cost-intensive and time-consuming procedure. [Figure 3] shows the results of a FE-analysis – in this case the distribution of the plastic strains lengthwise in the profile flange.
As mentioned above – the flexibility of this process and the variety of the influencing parameters complicates the lay-out procedure of new target geometries. That is why actual research work is dealing with the interdependences between all the influencing parameters to get more basic knowledge about the flexible roll forming process. Using this basics – the lay out process should be shortened. During these investigations different steel grades, geometric parameters and forming strategies will be considered.
The research project is funded by the German Research Foundation (DFG).