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The smart fortwo tailgate is the first mass-produced application using the E-LFT process (LFT with UD reinforcement). It is also the first LFT component to feature a completely visible grained, moulded-in-colour surface.
(Published on November-December 2008 – JEC Magazine #45)
1- ANDREAS RÜEGG, MECHANICAL ENGINEER ETH, ESORO AG
2- STEFAN ZIEGLER, MECHANICAL ENGINEER ETH, ESORO AG
3- DIEGO JAGGI, CEO OF ESORO AG
4- NORBERT STÖTZNER, MECHANICAL ENGINEER FH, WEBER AUTOMOTIVE GMBH
The tailgate structure of the new smart fortwo (Figure 1) is the first series component to be developed in E-LFT (Endless-fibre-reinforced Long-Fibre Thermoplastics) design. It is in production at Weber Fibertech, Germany, since January 2007.
The E-LFT process made it possible to use a single structural design concept for all versions while fulfilling the higher requirements, such as crash pulse and stiffness. With easier handling and an additional storage box (coupe), customer utility was improved as well. Compared to the previous versions, the component costs were lowered significantly.
The E-LFT process
The new E-LFT process enables economical production of highly loadable structural components in volumes ranging from 30,000 to 300,000 parts per year.
The process is based on the well-established LFT technology . Long-fibre-reinforced thermoplastic (LFT) parts are currently the first choice for large, semi-structural automotive applications. The reasons for this include very efficient production methods, such as direct LFT compounding (LFT-DILC) , and attractive component characteristics, especially in terms of weight, function integration and impact resistance. Structural applications, however, require an LFT composite with unidirectional continuous fibre tape inserts (EF) where the EF zone forms a load-bearing structure [1, 2].
E-LFT components can become a genuine alternative to steel and aluminium in structural applications.
What is new in the E-LFT process is the combination of long fibres (LFT) with complex multi-layered inserts of unidirectional continuous fibres (EF) in a one-step process [3, 4].
LFT moulding compounds have relatively modest mechanical properties, but provide excellent design freedom and enable the economical production of large components. Unidirectional continuous fibre tapes (EF) have excellent mechanical properties and can be inserted three-dimensionally to conform to load paths and component geometry.
LFT components can thus be strategically reinforced with EF inserts. With this combination, the E-LFT process enables the production of complex load-bearing moulded components, widening the application window for such components to include use as substitutes for components previously manufactured from metals.
More or less the same materials are used for EF and LFT, usually glass fibres in a polypropylene matrix . However, other fibres and matrices, e.g. PA, PET, PBT or ABS, can also be processed. In the case of EF, it can make sense to use carbon fibres in order to fulfil very high stiffness specifications.
The cheap raw materials and the one-step process help to keep production costs low. The low specific density and good mechanical properties of the materials make it possible to produce high-quality, lightweight components. The fact that the reinforcements do not cover the surface, but can be inserted locally and as webbing, results in further weight savings.
To satisfy quality and cycle time requirements, the E-LFT process is designed for fully-automated production. The main stages of the production system are: LFT-D processing, EF processing, handling (by robots) and the press (Figure 2). LFT is compounded directly or can be provided as rod-shaped material. The EF processing unit was especially developed for this process. The handling system was also specifically developed for quick transport and accurate positioning of the LFT and EF tapes. A frame-type press with synchronized travel such as those used for LFT processing is used.
The E-LFT process is designed so that the EF tapes are inserted according to load and to conform to component design. Thus, the tapes can be inserted in almost any configuration. This way, the reinforcements (EF) optimally match the forces arising. One of the main process features is the use of multiple EF tape layers. Since most components have to transmit more than two forces, several EF tape inserts are required. The tapes may even criss-cross (Figure 4).
Depending on the requirements, various EF cross-sections may be necessary. The available EF systems can be used to incorporate EF tapes of various thickness, width and, of course, length.
smart fortwo tailgate
The E-LFT process was chosen for the new tailgate for several reasons. For one, the smart fortwo 451 tailgate had to have a higher integration potential and better customer utility, while satisfying much higher rigidity and crashworthiness requirements. While the previous smart fortwo 450 model used a different design for each version, a single structural concept was preferred for the new tailgate.
In the previous version of the coupe, the tailgate consisted of a tubular aluminium frame and an inner part in PP-TV20. There was no flap, and thus no storage space. In the previous cabrio, the tailgate consisted of an ABS-PC inner part with flap. The storage space was used for the roof edges, which were fastened with belts that could retain them in a front crash situation (Figure 5).
The first developmental step was to work out an E-LFT solution for the tailgate.
The tailgate structural concept combines an inner part with storage space and an inside lid. In this E-LFT solution, the roof edges are retained by the inside lid in a crash and no longer require fastening belts. This new concept provides considerable freedom for configuring the storage bin and simplifies roof edge storage. The new solution achieves the following goals:
The structural concept was designed by numerical simulation and validated by appropriate tests. The main emphasis was on the behaviour of the inside lid in a frontal crash: to ensure occupant safety, the lid has to retain the two roof edges (5.2 kg) at 64 km/h with the very high pulse peak typical of compact cars. The concept was designed and tested for room temperature, -30°C and +60°C. Esoro introduced a new test rig where three component configurations can be tested in a single sledge test.
The sledge test results were very good. For all configurations and temperatures, the two roof edges were retained without failure of the inside lid. It was also shown that the simulated behaviour of the structure agreed very well with its actual performance.
Series development and mass production
The concept validation data were used to further develop the E-LFT tailgate, which was numerically optimized in conformance with the stricter crashworthiness requirements currently in force, and tested in sledge and full-vehicle crash tests. The E-LFT tailgate passed both tests and met all other test requirements (e.g. lid endurance for 50,000 load cycles under climatic conditions) and various full-vehicle tests in actual operation.
Production of the E-LFT tailgate started in January 2007 in Weber Fibertech’s newly-built production site in Markdorf, Germany. Weber Fibertech is a division of Weber Automotive, which holds the patent rights of the E-LFT process. The production site is equipped with a 3,600 ton press, an LFT-DILC unit for LFT compounding, an EF processing line and several robots for fully-automated production. At present, 700 tailgates are produced daily.
Structural rigidity was increased by more than 20%, compared to the previous models. The E-LFT cabrio tailgate weighs as much as the earlier cabrio version made from ABS-PC, whereby the projected area was slightly larger and the crashworthiness requirements are also tighter. Compared with earlier coupe versions with an aluminium tube frame and PP TV 20 interior, the E-LFT tailgate with storage bin and closable inner lid has notably enhanced functionality. Last but not least, costs were reduced by more than 10%.
The E-LFT process enables the production of components that go far beyond previous applications of pure LFT or GMT components. Some future E-LFT applications are shown below.
Highly structural component – 2/3 rear seatback
An example of a highly structural component is a 2/3 rear seatback with integrated middle belt joint. The study of a prototype part with actual crash tests is described below. The initial point was to make a 1:1 substitution of a metal rear seatback with identical add-on parts. The requirements were:
Component design normally starts with a topology optimisation, which gives a first indication of where the reinforcements (EF tapes) have to be placed. Based on this data, the E-LFT component can be designed and calculated with implicit (static) and explicit (dynamic) simulation tools such as ANSYS and LSDyna.
To validate the accuracy of the FEM simulations, a static belt test and a frontal crash test (50% male dummy, two 18kg cubes) were performed.
The E-LFT component withstood the static belt test and the stiffness was 94%, very similar to the metal version (100%). The part also passed the frontal crash tests, without splinters, contour cracks or open fractures. The energy was dissipated only with interlaminar cracks. The forward displacement of the rear seatback was within regulatory limits, and the luggage was fully retained.
The weight reduction requirement was fulfilled at over 40% weight saved, compared to the steel version.
The new E-LFT process developed by Esoro on behalf of Weber Fibertech enables economical high-volume production of highly loaded lightweight structural components. This opens up entirely new markets for thermoplastic fibre composite components. 30-40% weight savings can be achieved compared to steel. The first E-LFT component has been in mass production since January 2007. A number of additional E-LFT structural components are under development at Esoro / Weber Fibertech. Additional production lines are planned at the same site. The primary goal of further activities is to promote the potential of the new process in the development departments of OEMs and tier-one suppliers, and to continue with the series-oriented development of projects in the targeted component areas.
 Jaggi, D., Hüsler, D., Ziegler, S., Rüegg, A., Stötzner, N.: Vollautomatischer Produktionsprozess zur Herstellung von grossflächigen Strukturbauteilen als Verbund aus LFT und unidirektionalen EF-Profilen. AVK-TV Tagung, September 2002, Baden-Baden
 Jaggi, D., Stötzner, N., Rüegg, A., Ziegler, S.: Seat structure application – a way to employ E-LFT in the future. EATC Seminar, June 2004, Paris
 Rüegg, A., Stötzner, N., Jaggi, D., Ayer, R.: Seating Structures and other Structural Applications with Locally Unidirectional Reinforced Thermoplastic Composites. SPE Automotive Composites Conference, September 2005, Troy
 Jaggi, D., Stötzner, N., Ziegler, S., Ayer, R.: Endlos- und langfaserverstärkte thermoplastische Bauteile im E-LFT Prozess am Beispiel von crashbelasteten Sitzstrukturen. 7. Europäische Automobil-Konferenz „VISION Kunststoff-Karosserie 2015“, September 2005, Bad Nauheim
 Wulf, B.U., de Cuitis, H., Mettler, A., Zogg, M.: Prozessorientierte Fertigung von Unterbodensystemen in der Fahrzeugindustrie. 4. AVK TV Tagung, October 2001, Baden-Baden
Weber Fibertech is a division of Weber Automotive. A supplier to the automotive industry since 1969, Weber entered into the fibre reinforced plastics industry in 1998. Today, the company is a high-end supplier of motor blocks, crankshafts, entire engines and fibre-reinforced plastic components.
Esoro is an independent engineering company that specializes in innovation and development for industrial customers, focusing on lightweight structural composite technologies and mobility/vehicle design. Esoro has developed the E-LFT process for Weber and is also in charge of E-LFT component design and engineering. The company delivers a full service from conceptual design and FEM (crash) simulation up to the first prototype parts.