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Hybrid Lightweight Design Concept for the Vehicle Floor By presenting the Integrated Sandwich Floor, Röchling Automotive showcases a vehicle floor concept featuring a hybrid design based on low weight reinforced thermoplastics. The structurally integrated load-bearing component allows weight and space savings and meets highest standards in terms of thermal management, rigidity and acoustics.
Author DIPL.WIRTSCH.ING. MARKUS SATTEL is Head of Product Line Underbody Systems and is Responsible for the Development of New Vehicle Floor Systems at Röchling Automotive SE & Co. KG in Worms (Germany).
Lightweight and Safe Components Reducing vehicle weight plays a key role in the development of future mobility concepts – regardless of the type of propulsion system used. For instance, it is essential to reduce overall weight in order to achieve the optimizations in terms of efficiency and emissions required by increasingly strict regulations for conventionally driven applications. At the same time, an increasing number of functions need to be integrated into the vehicle without impacting its weight – especially in the case of hybrid and electric applications, where overall weight is a determining factor for the driving range. Finally, safety must be kept in mind: If lightweight components are to play structurally significant roles in the vehicle, they must fulfill strict crash
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safety and rigidity requirements. The Integrated Sandwich Floor (ISF) from Röchling Automotive, Figure 1, shows how these high goals can be achieved economically and sensibly utilizing the latest developments in lightweight materials.
Development Cooperation For the reasons stated above, globally active OEMs are interested in solutions in the field of lightweight design which can be used in series production. Accordingly, the basic development of the ISF took place in the research project “Light eBody”, a joint development effort by automakers, suppliers and research facilities around the world that was also sponsored by the German Federal Ministry for Education and Research. The goal for the cooperation was the development of a resource-efficient, lightweight and ready-for-production multi-material car body for electric vehicles. In the field of materials and manufacturing technolog y, Röchling Automotive was responsible for the development of the frontal inner floor.
Innovative Lightweight Materials During normal operation, the car floor has several functions. Firstly, it must reliably protect the body from damage caused by rock
chips etc. and provide optimal acoustic insulation of the vehicle interior from below. Secondly, aerodynamics and thermal insulation are also subject to demanding specifications. Whereas conventional car floor concepts already fulfill these requirements, they are composed of numerous individual parts: carpet, impact protection, damping, foam, anti-drumming foil, sheet metal, anti-rust coating and underbody cover. Röchling’s solution, on the other hand, combines all necessary functions in one component, Figure 2. As a foundation for this innovative concept, the supplier uses low weight reinforced thermoplastics (LWRT) materials that unite glass fiber with a polypropylene matrix. These low-weight thermoplastics offer excellent thermal and acoustic isolation properties and are at the same time much lighter than conventional materials. Moreover, depending on the application, their flexible layer volume and fiber configuration can be individually scaled to provide tailor-made solutions. To realize the goals of the Light eBody project, Röchling Automotive engineers used Stratura, which is based primarily on glass fiber and polypropylene. The resulting component combines a load-bearing inner vehicle floor with a carpet layer for the cabin and delivers outstanding acoustic absorption properties. Additionally, with the help of a specially constructed sealing foil that serves as a moisture barrier, body seal-
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Figure 1
The ISF is based on the LWRT material Stratura and its permutations Stratura Hybrid and versions with an integrated hollow profile (© Röchling Automotive)
ing at floor level is already integrated too. In the project, the multifunctional f loor module was bonded to the car body with a circumferential flange. A process was optimally designed to deliver the necessary shear strength and to allow the use of lightweight materials for structural components. In addition to its thermoacoustic properties, the material’s crash behavior is especially relevant. The fibers contained in parts made from carbon fiber reinforced plastics (CFRP) can break up in a crash situation – a reaction the ISF avoids due to its multi-layer structure. Moreover, the tried and tested sandwich construction absorbs a high portion of the energ y released during a collision. In concrete figures, this means that a Stratura component absorbs as much energy as sheet steel with a thickness of 0.8 mm, Figure 3, at just half the overall weight.
Figure 2
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Optimizing Materials and Broadening the Concept Röchling Automotive systematically expanded the expertise it gained during the Light eBody project with regard to the integration of various functionalities for complex applications in the floor of vehicles – both with conventional and alternative propulsion architectures – and made it usable for series production, consequently, it is possible to adjust the sandwich construction to suit the individual requirements of any application. In order to keep the necessary development process as efficient as possible, an early definition of the goals regarding integrated capabilities and material properties is helpful. Form, structure and material thickness of the vehicle floor component are then optimized according to these predetermined requirements and application-specific conditions. In this way,
Röchling Automotive achieves the bonding of different layers into a multifunctional hybrid lightweight design component.
Lightweight Construction for New Propulsion Architectures On t he basis of Stratura, Röch ling Automotive expanded the material range for the ISF further by developing Statura Hybrid. This solution features scalable physical properties that make it especially desirable for hybrid and pure electric vehicles. Depending on the individual demand, it includes one or several layers of aluminum foil to reinforce the material and, in turn, the resulting component. At the same time, Stratura Hybrid has a high electromagnetic compatibility (EMC), meaning that it provides protection against electromagnetic fields (so-called induction fields), which are
The integrated vehicle floor concept is significantly lighter and thinner compared to conventional components (© Röchling Automotive)
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generated whenever an electrical circuit is opened or closed. In pure electric vehicles in particular, these fields occur naturally and unavoidably when sudden load changes occur, for instance when an emergency stop is initiated at full speed. If no countermeasures are taken, induction fields generate potentially harmful electrical currents in neighboring electric components. Highly conductive materials like silver, copper and gold achieve an almost perfect shielding performance. However, in order to provide a cost-efficient floor solution with the highest possible EMC protection for hybrids and electric vehicles, Röchling Automotive integrates one or more microperforated aluminum layers into Stratura. The result is a high degree of protection – a shielding effect of over 110 dB can be realized with a sheet of aluminum just 0.1 mm thick. In addition to aluminum layers, the ISF can be further individualized and reinforced by integrating hollow profile elements made from aluminum, resulting in greater adaptability to different vehicle design concepts, Figure 4. The highly tunable technical versatility of Stratura and Stratura Hybrid and the possible inclusion of hollow profiles make them significant innovations in the field of lightweight
Figure 3
Compared to sheet steel, Stratura and Stratura Hybrid absorb significantly more crash energy while reducing area density (© Röchling Automotive)
design. In order to gain additional advantages over conventional f loor assemblies from the current development stage, Röchling Automotive engineers are constantly working to integrate as many additional functional components as possible.
Comprehensive Integration of Functionalities After conclusively testing the concept using prototypes and demonstrators, Röchling Automotive currently focuses on the execution of specific vehicle projects. The various material approaches and the use of a sandwich design not only allow excellent thermal and acoustic characteristics to be combined in a single, very lightweight component – they also allow the complete substitution of vehicle floor systems. Compared to conventional f loor modules, therefore, the ISF offers the potential to reduce weight by up to 30 kg, or 50 %. As a result, the integrated car f loor concept meets the industry trend towards ever more clean and fuel-efficient vehicle designs.
Load-bearing Safetyrelevant Component
Figure 4
Integrated aluminum elements reinforce hybrid floor concepts (© Röchling Automotive)
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Designed from the ground up as a structural and load-bearing part of the car body, the ISF sets itself apart from other lightweight design approaches by allowing the use of plastics in an area previously dominated by steel or aluminum. The ISF’s greatest hallmark in this field is its high rigidity, which is realized both
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Figure 5 In drop tower tests, the Stratura specimen’s fibers do not break, and crash energy is absorbed into the component evenly (© Röchling Automotive) through the benefits of its multi-layer construction and the possibility of integrating load-bearing aluminum profiles into the material. Due to this special structure, the energy released in case of a collision is distributed evenly in the component. This enhances the crash absorption potential of the ISF, as no single point is affected in a way that could cause the material to break, Figure 5. As a result, the floor becomes a safety-relevant component that improves the crash resistance of the whole vehicle.
Individual Manufacturing Optimization The ISF’s flexible structure and high plasticity allow the integration of additional technical components such as wiring harnesses or conduits and provide several other benefits. The possibility of adding hollow profiles when using Stratura Hybrid shows one of these advantages: the concept’s individually adjustable geometry. Even in later stages of development, the solution’s properties can be changed to suit different requirements. Thus, Stratura Hybrid offers development engineers a significantly higher degree of flexibility compared to other pressed materials such as sheet molding compounds (SMC), and it is made possible by combining a more detailed
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material setup with Röchling’s so-called Softlofting process. By gently lofting the base materials within the press, this process generates a three-dimensional network of glass fibers bound in polypropylene. This allows mechanical and acoustic parameters to be adjusted individually and improves the acoustic absorption capabilities of the finished component significantly. Readiness of a product for series production requires not only materials and development concepts – it needs realistic lean pro-
was optimized in parallel with the material and structural development.
Application and Further Development Following its holistic approach, Röchling Automotive developed the ISF much further than originally planned during the Light e-Body research project. The integrated vehicle floor concept is ready to save fuel and weight not just in cars with conventional
Components for the powertrain or for a wireless inductive battery loading infrastructure could be added in the future. duction processes too. This is why Röchling Automotive employs a time-saving and cost-efficient continuous molding process for its latest car f loor system. Using the so-called One Shot concept, all materials including (if necessary) the aluminum hollow profiles and microperforated aluminum layers are pressed together simultaneously. This advanced production method
propulsion, but also in vehicles with alternative propulsion architectures from hybrids to pure electric cars. Moreover, in addition to wiring harnesses, it is conceivable that further technical capabilities can be included in the floor system. If the material is modified accordingly, for instance, components for the powertrain or for a wireless inductive battery loading infrastructure
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Figure 6
In addition to the vehicle floor, Stratura Hybrid also shows great potential for use in multifunctional battery housings (© Röchling Automotive)
could be added in the future. The high EMV and crash protection offered by Stratura Hybrid make these possibilities especially interesting from a technical point of view. Another Röchling component already demonstrates the material’s potential in this field: the multifunctional battery housing, Figure 6. This solution keeps the battery of hybrid and electric vehicles safe and replaces the metal housing currently still in widespread use. Stratura Hybrid battery housings not only feature crash protection in the event of an accident: They also reduce both cost and weight while shielding the other electrical components of the vehicle with their high EMC protection. Moreover, thanks to its high potential for functional integration and minimal thickness, the ISF offers additional freedom regarding vehicle design and production. The concept thus opens up new and far-ranging perspectives for the development of clean, efficient and safe cars for tomorrow.
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Summary Röchling’s Integrated Sandwich Floor is a holistic approach to the development of lowweight, crash-resistant lightweight design components. As a hybrid vehicle floor, the ISF is highly flexible and can be structurally adjusted to suit a variety of applications. It consistently utilizes the potential of lightweight design components and shows that they can do much more than just act as substitutes for single components: They can also make complex structures lighter or allow entirely new ones to be built. Developed from the ground up as a multifunctional part, the ISF offers OEMs a space-saving weight-reduced solution for practically all propulsion architectures. In this way, Röch li ng Automotive demonstrates the potential of hybrid floor systems and supports customers in the implementation of lightweight design measures for enhancing efficiency and reducing emissions. |
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