1 Wide steel strips are uncoiled,	2 shaped between pairs of rolls, and passed
3 as continuous profiles to a stamping unit,	4 from which they emerge as finished sills.

And there is another consideration: ever since the introduction of super high-strength steels – which are needed for lightweight construction and good crash behavior – it has become increasingly difficult to produce deep-drawn parts that can achieve the tolerances required in automotive design. In the case of roll forming, the very high degree of dimensional consistency needed is achieved by a quick readjustment of the rolls.

Variable tools

When it came to establishing roll forming as a cost-efficient alternative that conforms to the highest technical standards, Füller and his colleagues faced a major challenge. The technology must be able to produce a large number of complex geometries and variants. In addition to processing super high-strength steels, the researchers also strived to increase the flexibility of the process. The basic principles underlying the mobile stands and their control were worked out by the Daimler specialists in collaboration with scientists from the University of Darmstadt.

Further development is now being carried out in-house and with external industry partners. In the first step, the researchers conducted studies on the internal pilot instal­lation with various wall thicknesses and materials in the same tool set. The tests were so successful that it was possible to carry out a material changeover without delay in the first series application involving the sill for the 212 series (E-Class).

The second stage of development is concerned with flexible, controllable stands that can be used to make variable cross-sections within a part. Although this increases the overall complexity, the controllable stands ensure that the process is extremely flexible and cost-efficient, since one installation can then be used to make profiles with a great variety of shapes. An initial experimental rig has now been started up for prototype tests.

Super high-strength steels

Karl-Heinz Füller’s colleague Günter Asth, head of Development C/E-Class Body-in-White in Sindelfingen, mentions one other “profound” advantage: “The use of super high-strength steels enables us to reduce wall thicknesses and therefore achieve lightweight constructions. This in turn makes it possible to cut the weight by up to 1.5 kilograms per part.”

In addition to examining the issue in terms of process engineering, Daimler researchers are also utilizing various methods of materials research to closely study the materials used. Füller’s colleague Oliver Röcker, for example, developed a simulation of the roll forming processes in all its phases. It not only shows how the shape of a sheet metal blank changes when it passes through the individual stands but also indicates which points are likely to be subjected to critical stresses and strains during the part’s production. Finally, the researchers want to use finite-element method calculations to achieve the most realistic possible simulations of the component properties, so that they can take account of the complex effects these properties have on the part’s subsequent behavior.

Manufacturing-driven design

In addition to the requirements of the automotive designers, the project also takes the needs of the body-in-white and production planners into account. “One objective is to set up the roll forming and joining processes in the most cost-effective way possible with regard to the production of variants and investments in equipment,” explains Detlef Denker, head of Pressed Panel Planning at the Sindelfingen plant. “That will increase the quality of entire components and simultaneously lower costs. The parts can also be immediately used in the body-in-white, which reduces the elaborate logistics processes and avoids further costs.”