Lightweight Design in the Door Outer Panel

Innovative steel products reduce the outer panel weight of planar components such as the doors by up to 33 % — without any loss of oil canning or dent resistance and at very attractive lightweight design costs of just € 2.25/kg. ThyssenKrupp has verified the suitability of the Litecor steel-polymer composite for large-scale production using the example of a side door.

Inexpensive lightweight doors are particularly required in the volume market, as the high costs of aluminum prevent its use here. As a reduced outer panel sheet thickness leads to the loss of oil canning and dent resistance, this has to be compensated. To do this, ThyssenKrupp has studied the Litecor steel-polymer composite in detail, ➊. To enable an assessment of the state of the art, a benchmark involving measurements on eight standard doors was first undertaken. Oil canning and dent resistance were the significant assessment criteria in this, ➋.

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Lightweight Litecor door, sandwich structure 0.25/0.40/0.20 mm

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Door benchmark and comparison with InCar plus reference door and InCar plus Litecor door

The InCar plus doors are oriented towards the best standard doors in the benchmark in terms of oil canning and dent resistance. 67 points at which oil canning is investigated in detail by means of simulation are defined on the InCar plus doors. Oil canning and dent resistance are additionally measured at ten selected points on the test rig. Good correlation between the simulation and test was revealed on the whole.

Door Concept with Litecor

The Litecor steel-polymer composite, consisting of two very thin steel cover sheets and a plastic core, was developed specifically to improve oil canning. The Litecor outer panel reveals performance comparable with that of the reference with a weight reduction of 33 % at the same time, ➌.

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Comparison of oil canning performance at two test points

The forming behavior of various materials under near-production pressing conditions can be determined on the basis of a modular tool which simulates the area of the door handle recess. In addition to Litecor, a conventional bake hardening steel CR210BH and aluminum from the group of 6000 alloys are being studied in the first test series. The styling edges, which are becoming increasingly striking on vehicle doors, can be easily implemented with Litecor. This composite material offers greater design freedom in comparison with aluminum.

Litecor Forming Simulation

For Litecor, forming simulation for the entire manufacturing process chain is extremely important as a basis for manufacturing planning at the customer. New products must be easy to integrate into the customers’ CAx landscape. To achieve this, product-specific solutions which can be implemented on an industrial scale are being developed in close cooperation with software manufacturers. To obtain realistic predictions, the other characteristics of the polymer core layer have to be taken into consideration in the Litecor forming simulation in addition to the characteristics of the steel cover sheets. In the past, this was only possible very inadequately or with extensive modeling effort, and was unsuitable for analyzing the entire process chain. While shell elements for the steel cover sheets and volumetric elements for the polymer core layers are suitable for characterizing the Litecor-specific properties, they are limited to the 1st forming stage, as the software available on the market does not (yet) offer any trimming options for volumetric elements. Subsequent operations such as post-forming, hemming or beading cannot therefore be simulated. A mill bar model which meets the necessary requirements was developed on this basis in close cooperation with software manufacturers. This is now available to the ThyssenKrupp Group. The high forecasting accuracy of the above described method has already been verified on various test and production components and has been used successfully for numerous feasibility analyses, ➍.

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Simulation of the forming capacity of Litecor (left) and actual forming behavior (right)

Following successful forming simulation of the entire Litecor manufacturing chain, further development is now focusing on advanced grid refinement algorithms to reduce computing time. This is the prerequisite for economical use in highly complex processes involving hemming operations, for example.

The usual processes for joining the outer panel to the inner structure are table-top hemming (machine hemming) and roller hemming with a robot. Both hemming processes are state of the art for steel and aluminum materials. Due to the material’s high stiffness, however, special adaptations are required for the Litecor composite to achieve a flawless hemmed joint. ThyssenKrupp has developed a simple and effective solution for table-top hemming: a groove in the bending die which specifically influences the material when bending the outer panel. The groove leads to the formation of a defined, softer area in the inner fibers during bending. This consequently enables longitudinal compensation in the inner fibers during the first hemming step so that the tensile fibers are relieved in the outer hemming process. This avoids cracks. This modification to the hemming tool has no influence on the processing of monolithic metal sheets.

Process consistency and therefore production fitness for hemming Litecor were verified on a newly developed table-top hemming system which is suitable for use in volume production, ➎.

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Mechanical hemming system and process steps

New Table-top Hemming Tool

As well as additional functions in the machine, the focus was primarily on the requirements posed by Litecor during development of the new table-top hemming system. Precise control of the process variables “force” and “position” deserve a special mention here. Besides these technological improvements, the topic of saving energy was also a major objective in further developing the hemming machine. The use of controlled electric drives and extensive omission of the usual pneumatic drives are also making a significant contribution towards reducing the energy requirements and costs of operating equipment. Cycle time optimization below the usual times is also likely.

The doors can be hemmed with constant quality and without cracking within the usual body shop cycle time. Geometries similar to those of conventional steel doors can be produced, and only minor additional adaptations to the hemming system are necessary. Besides the geometric adaptations for the pre-hemming operation, the defined hemming force with holding time in the finish hemming position deserves a mention here.

Roller Hemming with Litecor

However, the method applied for Litecor during roller hemming with the robot is a different one, as the forming processes are carried out only partially and not over on the entire circumference. The task is not to avoid cracking in the outer fibers but to reduce waviness in the closed flange. To achieve this, the roller application points on the component differ from those which are usually used; secondly, a further pass may be necessary for smoothing with the robot. Hemming capability with the robot has been verified on individual prototype parts.

Cost comparison and Possible Applications

In the outer panel, the Litecor steel-polymer composite reduces the weight per door by 1.6 kg with attractive lightweight design costs of € 2.25/kg, ➏ and ➐. In addition, this concept can be implemented virtually without adapting the manufacturing process at the OEM. Only the table-top folding tool requires a minor modification.

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InCar plus reference door and InCar plus Litecor door cost comparison

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InCar plus reference door and InCar plus Litecor door cost and weight comparison

The Litecor composite application studies were undertaken using the example of the door outer panel, but can also be transferred to other outer panel components such as the hood, trunk lid or roof. Litecor is also suitable for components in the body’s inner structure, e.g. in the vehicle floor. Large-scale production of Litecor for inner parts is planned in the medium term. Production for outer panel components is then intended.

Outlook

The use of very thin metal sheets in the outer panel is another option for building especially light steel doors. The necessary reduction of oil canning and dent resistance can be compensated by specifically applying PU-based plastic as a backing.

A spray mixing head specifically applies the reactive mixture onto the surfaces. The hybrid material’s properties can be specifically adjusted by selecting different PU formulations, using additives which promote stiffness and applying a variable number of layers, thus achieving maximum lightweight design with outstanding performance at the same time. The weight reduction in comparison with the reference door will be up to 1.5 kg.

In the manufacturing process, the plastic layer(s) is (are) sprayed on directly after pressing the outer panels to simplify complex handling of the very thin and therefore sensitive components. The PU plastic is hardened and heat-resistant a few seconds after application. These characteristics are of enormous importance during subsequent cataphoretic painting. Studies have proved that the plastic does not contaminate the cataphoretic painting bath and retains its adhesion and stiffening effect at 180 °C for 60 min in the oven.

Practical tests on the dent test rig reveal oil canning resistance comparable with that of the InCar plus reference door. Due to its acoustic advantages, the outer panel, which consists of thin, plastic-backed metal sheets, also has the potential to achieve comparable performance with just a few secondary acoustic measures. As a result, further weight reductions can be achieved. A cost forecast for this concept indicates lightweight design costs of around € 2.0/kg.