Boge Elastmetall manufactures safety-relevant structural components in series from locally reinforced fiber composite material. Using the example of a brake pedal, the automotive supplier presents the development and manufacturing process. The partners are the companies Celanese as material supplier, Cevotec for structural design and M.A.i for automation technology.

UD Tapes in Serial Components
Hybrid designs combine materials with different properties in one component and can lower previous weight and cost limits. This is possible by tuning the component properties to the local requirements in accordance with the principle of using the right material in the right place on component part level. Fiber- reinforced plastics are increasingly penetrating areas of the vehicle that were previously reserved for metals. The proportion of plastics in structural vehicle components is increasing. Economical lightweight designs for relevant piece numbers with short and continuous fiber-reinforced thermoplastics are of particular interest for automobile manufacturers. Continuous fiber-reinforced thermoplastic materials consist of fibers and a matrix. The specific material characteristics of the fiber provides high lightweight design potential, Figure 1, while the thermoplastic matrix leads to short processing times and thus enables high quantities [1].

This results in a hybrid construction of organo sheet and UD tapes for the shell.

A distinction is made between organo sheets and tapes: If fibers are predominantly arranged in one direction and provided in thin layers, they are called unidirectional tapes (UD tapes). If, however, the fiber architecture is multi- dimensional, for example by a warp and weft thread or by sewing different fiber orientations to one another, these are so-called organo sheets. To represent different fiber orientations in the organo sheet, textile technology is used. For example, fibers are woven or laid down in different directions and sewn before they are impregnated with the thermoplastic. In contrast, UD tapes are made directly from fiber rovings. Rovings can be unwound from a bobbin and drawn directly through a thermoplastic melt. This shortened value- added chain has a positive effect on the price of materials and makes the semi-finished product attractive as a UD tape for price- sensitive products.

With increasing piece numbers, compo nent costs approach asymptotically to mate rial costs and reduce the relative share in operating equipment. Thus, the decision for the use of organo sheets or tapes is dependent on the volume scenario, provided the mass throughput in the production of the tape laminates is guaranteed. Because that is the challenge: the process technology in the pro cessing of tapes is more complex than the processing of organo sheets, from individual tapes to tape-laminates. That means the mul tiple laying or stacking of individual UD tapes must take place before forming and consolidation. Accordingly, tapes are only economically viable for large production numbers, since there is not yet a facility or device for the quick stacking of tapes which can be applied in series production. This leads to a major problem when using tapes in series products. Taking this chal lenge and producing serial components based on UD tapes was the main task of the consortium under the leadership of Boge Elastmetall in the development of the mate rial, the product, the manufacturing process and the associated systems engineering. The development was implemented using the example of a brake pedal, Figure 2.

"The tacked-on UD tapes and the organo sheet are heated together"

Approach The wall thickness of the organo sheet is designed according to the present state of the art for the maximum load in the shell. In the surrounding areas, the shell is oversized. Thus, the component includes material, which is paid for, processed and carried along, although it is technically not necessary. It would be better if the fiber composite content could be adapted to the local requirements of the shell, for example by local orientation of the fiber, the local fiber volume content or the local wall thickness. Following this approach, the organo sheet is undersized in such a way that it carries the stresses in a large area and is locally reinforced by the application of load path oriented tapes, Figure 3.

This results in a hybrid construction of organo sheet and UD tapes for the shell, in which the organo sheet corresponds to the major part of the shell volume (mass flow rate) and local, multi-directionally (MD) applied fiber tapes reinforcing the shell at their stress peaks. They are locally oriented to the load paths. This procedure increases the fiber efficiency, reduces the wall thickness and thus the component weight as well as the cycle time during draping and re-consolidation.

The manufacturing process is divided into two stages: First, a tailored blank made of organo sheet and tape is produced, the so-called tailored performance blank. A contoured organo sheet blank serves as a stable base to which the strips of the UD tapes are attached fully automatically according to the load paths. Subsequently, this tailored blank is also fully automatically heated, draped, consolidated and injection molded with an injection molding compound.

Until this technology could be applied in series production, numerous other questions besides the development of an automation technology had to be answered, which can be divided into different development levels, Figure 5. When organo sheets and tapes are processed in a joint method, it is essential that the tapes withstand the static and dynamic loads in all operating conditions, such as temperature and humidity. The tapes must neither delaminate nor tear. For this purpose, a surface-to-surface adhesive bond must be ensured. This requires an appropriately high matrix pressure and a temperature above the melting point within the surface areas at the respective point at which the materials meet. If all process parameters are met, the polymer chains will diffuse across the surface and the gap between the components to be joined will be closed [2]. This micro-level of component development represents an important area of process planning, as it specifies cycle times and molding functions via the necessary pressure and temperature profiles.

The combined process described in [3] is applied together with a recently developed molding technology, where a high matrix pressure is maintained over all tolerance ranges of the semi-finished products and process parameters in order to consolidate all layers without intermediate steps in the draping tool. An upstream interim consolidation does not take place. Thus, the UD tapes, which are only tacked on, and the organo sheet are heated together.

"The overloaded area is laid or stacked with tapes along the main stress directions."

Simulation-based Design
If UD tapes can now be safely applied to organo sheets in a single process step, the question arises as to how many to lay where in the component, in which length, Figure 5. Cevotec answers this question by means of computer-aided simulation. For this, the material properties and parameters are required: In particular, stress-strain curves are determined for the materials under real application conditions such as humidity and temperature. In addition, samples are taken from the injection molding compound and tested in the combination of organo sheets and organo sheets with tapes and compared to simulation results to validate the model.

Another prerequisite is the single-layer simulation of the continuous fiber shell and the ability to optimize the topology of the injection molding compound. At the beginning of the development cycle, a first model without tapes is set up, and the wall thickness of the organo sheet is undersized based on empirical values. Fiber orientations of the organic sheet are approximated by kinematic draping simulation; the rib structure is designed with the help of a topology optimization. The load applied to this blank provides stress level and distribution within the organo sheet. Now, the overloaded area of the continuous fiber shell can be successively laid or stacked with tapes along the dominant main stress directions. Assembled UD tapes change the continuous fiber shell, which affects the shell stiffness. This results in a feedback to the topology optimization and thus to the rib structure. If the economic and mechanical requirements are met in combination of UD tapes, organo sheet and injection molding compound, the iteration in the simulative part of the development is completed. A development of the shell model approximates the contour of the organo sheet as well as the number, position and orientation of the UD tapes.

The equipment of the partner M.A.i in line with the process requirements consists of two plants: a tape-laying (stacking) manufacturing cell, where the UD tapes are cut to size, laid or stacked on the organo sheet and tacked, and a manufacturing cell for processing the tailored performance blanks [3]. The automated laying of the UD tapes is based on the tape feed as rolls, optical image processing with two cameras, the freely programmable length of the individual UD tapes and the tacking by means of laser. Computer-based image processing is becoming more efficient, reliable and affordable. This, in turn, increases the performance and cost-effectiveness of the manufacturing cells based on them. With this development in mind, it can be predicted that the processing of unidirectional tapes is gaining increasingly economic significance. In the first manufacturing cell, in which tapes and organo sheets for tailored performance blanks are combined, two semi- finished products are placed, Figure 6.

Organo sheets are fed to magazines as stacks of contoured blanks; UD tape reinforcements are manufactured in the form of rolls. Both semi-finished products are joined, and a semi-finished product, Figure 5, is created. According to the component design, pieces of UD tapes are cut to size and measured online by a camera system for essential geometrical parameters such as length, width and position to the gripper hand before the piece of tape is picked up by a robot. At the same time, a second camera system monitors the contour and position of the organo sheet. Now, the geometrical data of both tape and organo sheet blank is known and the system calculates the placement position of each tape. Then, the UD tapes placed in these positions are tack welded by a laser at several points. Tack welding by laser, tape orientation or position in connection with tape length and the system for image processing are all freely programmable, which makes this system concept flexible and safe in handling. The number of tack welding points per tape, their position and each individual tacking point can be freely programmed. This allows the system, for example, to tack weld a second tape layer on top of the first two tapes simultaneously with the organo sheet in one operation.

In the second step, the tailored performance blanks are heated above the melting point of the matrix, formed, draped and re-consolidated. This creates a material bond between the tape and the organo sheet and is comparable to the die welding process. The formed part is cooled down to a temperature below the melting point and inserted into the cavity for overmolding. At the same time, the shell reaches a sufficient temperature on the surface area to create a permanent bond when it is in contact with the injection molding compound. After the injection molding process, the finished component can be removed. It will be provided with a number, cooled down, assembled, packaged and delivered.

The free programmability makes this system concept flexible and safe in handling

With increasing volumes, component costs asymptotically approach material costs, which predominantly determine the manufacturing costs. Thus, the decision whether to apply organo sheets or tapes is dependent on the unit price scenario. In cooperation with the partners Celanese, Cevotec and M.A.i, Boge Elastmetall has succeeded in manufacturing UD tape reinforcements for safety components in series production which meet the requirements of the automotive industry. The structure simulation is carried out with a single-layer simulation approach in combination with a kinematic draping simulation and additional modeling of the plastic overmolding after topology optimization. Series production is based on a two-stage process with a flat tape stacking device and subsequent draping, respectively re-consolidation in one process step without intermediate consolidation. In the subsequent process step, it is possible to use the combination process developed by Boge Elastmetall as a basis for fully automated forming, re-consolidation and overmolding with an injection molding compound.