Black carbon fibre starts appearing in car bodies
By David Vink
Posted 31 July 2012
The distinctive black of carbon fibre reinforced plastics is now appearing in car body structures. PRW’s sister title, European Plastics News, reviewed CFRP and other lightweighting developments shown by BMW, VW and plastics groups at this year's VDI event.
At the VDI plastics in automotive engineering conference and exhibition in Mannheim, Germany, Dr Jochen Töpker, CFRP vehicle project manager at BMW, said the i3 Megacity and i8 Vehicle designs draw upon experience in Formula 1 monocoque crash performance as well as lightweight components in the K1200R Sport motorcycle.
BMW is using thermosetting epoxide-based resin transfer moulding (RTM) for the i3 CFRP cell, Tšpker said. In addition, recyclates and naturally reoccurring materials reduce the weight of the i3 and i8 car designs by 25% compared to conventional thermoplastic car designs.
Talking about advanced functional multi-materials for future electric vehicles, Timo Göbel, BMW material development manager for bodywork and suspension, said there is a trade-off between HT, IM and HM carbon fibres in fibre tensile strength and Young's modulus, as well as cost and availability. HT fibres show the best balance for bodywork.
CFRP is much more complex than metal. Extruded aluminium and steel are isotropic and homogenous, but CFRP has many properties and characteristics, and offers choices in fabric stacking, orientation angles, 150-600g/m2 "grammage" and various laminate, braiding and recyclate options.
In the i8 "life module", a monolithic CFRP sheet design involves gluing sheets together at the flanges, allowing the company to automate large production series.
CFRP requires new material approval and advanced joining techniques are fundamentally important for future lightweight material mixes. Göbel said process technology improvements offer "opportunities for radical innovation development", but that more research is needed.
He said the i3 and i8 designs are "practically steel-free", adding: "If these set a precedent, we are turning a big wheel. We are trying not to use metal and in the longer term, crash performance will be achieved with a CFRP chassis instead of aluminium."
Krino Bornemann is responsible at Volkswagen in Wolfsburg, Germany, for CFRP technology in low fuel consumption vehicles. At the VDI conference, he discussed a new advanced RTM (aRTM) process to reduce CFRP production costs through 15-20% faster cycle times, which the company is using on the VW XL1 concept car.
Bornemann classified processing by production volume. Pre-impregnated fabric preforms in simple tools offer 60-80min cycle times, suitable for volumes of less than 5,000 vehicles per year. For 50,000-300,000 vehicles per year, he highlighted two processes: RTM with resin injection into dry fabric, in more complex tools, in cycle times of 4-40min; and hot compression moulding of fully consolidated thermoplastic fabrics and inserts (such as organic sheet) in cycle times of 1-3min.
The cell geometry and fibre orientation of the XL1 monocoque CFRP passenger cell shown at the event ensures maximum load bearing and minimum use of additional CFRP components. Tooling and process conditions allow production of up to 24 such monocoques per day.
Future challenges include energy-efficient production of fibres with, for example, flexural modulus of 170,000 N/mm2 and tensile strength of 2,000 N/mm2.
Audi technology network manager Heinrich Timm said in a press briefing that the company's lightweight aluminium and CFRP Audi Space Frame (ASF) saves weight and improves handling, safety, braking energy and acceleration.
Timm said he considers CFRP monocoque design to be relatively expensive and better suited for expensive and "image-driven" cars like the Lamborghini Aventador and MacLaren MPC series.
With Audi's "ultra material" hybrid concepts, lightweight bodywork also leads to weight saving elsewhere in the car, reversing the upward weight spiral, he said. Fibre reinforced plastics and especially CFRP are a "very useful extension to the lightweight materials portfolio", he added.
Timm said for CFRP to break through into volume vehicles the industry needs to find suitable cycle times for large series production and reduce processing costs by 90% compared to 2010 levels. Also, designs need to allow for carbon fibre anisotropy, as longitudinal strength is high, but transverse strength low.
At the VDI event, Dow Automotive displayed hybrid solutions, including CFRP parts produced by KraussMaffei and Cannon Afros which are bonded to metal with Dow's Betaforce PUR and Betamate epoxide resin based adhesives. Dow also presented its latest Voraforce epoxide resin with CFRP curing of less than three minutes.
The company also showed Betafoam polyurethane foam-filled metal cavities, with potential to use 700g of foam injected at six points in car bodywork structures. Dow Automotive is an assembly technology partner in the Light e-Body project, supported by the German federal education and research ministry.
BASF showed a new injection moulding version of the EASI (Energy Absorpton Safety Integrity) crash resistance technology, incorporating steel cord fabric in plastic (also see p36).
BASF also announced the first production part to be made in continuous fibre-reinforced Ultramid PA: a seat shell for the new Opel Astra OPC sport coupe, first shown at the Geneva Auto Show in March.
In the in-mould-forming process, a PA-impregnated continuous glass fibre fabric preform laminate is overmoulded with impact-modified, short glass fibre reinforced PA, creating ribs and part edges. High strength enables thinner wall thickness, cutting the weight of the seat shell by 45%.
BASF also presented a lightweight roof module developed with automotive engineering consultancy EDAG. The highly integrated sandwich structure consists of an Elastolit D PUR foam core, plus CFRP outer layers based on BASF epoxide, PUR or PA resin. The structure is treated with BASF Tinuvin CarboProtect lacquer for UV protection.
The demonstration part has dry CFRP and short fibre reinforced inserts, unidirectional reinforcement and load bearing metal inserts. Weighing 2.9kg, the roof module saves up to 65% in weight versus conventional steel roof modules and more than 35% versus aluminium.
Swiss mouldmaker Georg Kaufmann announced the Lightweight Integrated Process Application (LIPA) project, in which it is working with sensor producer Kistler, infrared equipment producer Krellus, composites processor Quadrant and ASE Industrieautomation.
The LIPA project focuses on lightweight fibre composite design for series production. The process starts with a thermoplastic composite sheet, which is heated and formed into a finished part, which may also be backmoulded to other materials.
The partners expect to install a fully automated LIPA production cell in October.
Severin Sauren, responsible for production, materials and processing at Daimler in Sindelfingen, Germany, described the latest hybrid metal/sheet moulding compound (SMC) bootlid for the Mercedes-Benz SL Roadster. The new design has rationalised from two SMC parts and one metal part to single SMC and metal parts. It provides antennae integration and weight saving in a limited space, while retaining strength for the folding roof.
Sauren said Mercedes-Benz started producing SMC parts with conductive in-mould coating in 2010, so no sanding or grinding is needed before online painting. The next challenge will be lightweight SMC and faster curing, he said. Automated deflashing and innovative tool technologies are at the validation stage.
He said Daimler perceives SMC as the "third bodywork material alongside steel and aluminium" and it plans to expand economically efficient SMC production to other models and component groups.
Lanxess showed two developments: a break pedal which is 50% lighter than a steel pedal, and an airbag housing. Both applications depend on overmoulding of continuous glass fibre reinforced plastic organic sheets for tailored reinforcement.
The airbag housing was designed in co-operation with moulder Takata-Petri in Aschaffenburg, machinery producer KraussMaffei in Munich, Bond-Laminates in Brilon and mouldmaker Christian Karl Siebenwurst in Dietfurt.
In this application, the Lanxess Durethan DP BKV 240 H2.0 grade of PA is overmoulded onto Bond_ Laminates' Tepex Dynalite 102 RG 600 47% reinforced organic sheet material. The airbag housing is 30% lighter than an equivalent made with just PA6, and side wall thickness is reduced from 3-4mm to 0.5-1mm.
At the VDI event, Borealis focused on substituting glass fibre reinforced PA 66 with its Xmod heat stabilised PP material. It says 30% short glass fibre reinforced Xmod GD301FE saves at least 15% in weight, and reduces system costs by 10% lower compared with PA solutions.
Mahle in the UK has used the Xmod material since 2009 for moulding the VW Golf 1.4 and 1.6 litre engine air intake manifold. Ršchling Automotive revealed in May this year that it too is supplying PP manifolds to VW, for 1.0 litre 3-cylinder engines in Europe and 1.6 litre 4-cylinder engines in China.
A more recent PA substitution is the Fiat Bravo 327 air filter system housing: replacing 30% glass fibre reinforced PA66 with Xmod WB300UBB mineral filled and glass fibre reinforced PP has reduced the part's weight by 16-17%.
Harald Hammer, Borealis mobility business unit vice president, told European Plastics News Borealis can also offer PP solutions for pedals, front ends and dashboard carriers to compete with PA-based organic sheet designs.
Recticel Automobilsysteme presented its Compolite prototype door panel, produced as part of the Flanders Drive project in Belgium. Compolite involves spraying PUR onto a sandwich of two glass fibre or natural fibre mats with a foam core which is cured in a press - with or without outer surface decoration. Surface weights of Compolite mouldings are typically 1,300g/m3 for 3.5mm part thickness and 1,100kg/m3 for 2.5mm.
Dr Gérard Liraut, head of polymers and fluids at Renault in Guyancourt, France, made a joint presentation on polycarbonate glazing opportunities with Dr Matteo Terragni, segment manager automotive glazing at Sabic Innovative Plastics' office in Milan, Italy.
Liraut said PC glazing has potential on the R26R car to cut side window weight from 2.2kg to 1.3kg, and backlight weight from 3.6kg to 2.1kg. The Twizy car on display in Mannheim was shown as having high impact PMMA windshield reflectors and fixed roof glazing. The car's side panels are made of thermoformed PC.
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