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Most companies with expertise in producing high-strength, low- weight structural parts in composites have their origins in the aerospace or track racing businesses. For many years the aerospace industry has relied on advanced carbon fibre composites to achieve maximum structural performance at minimum weight. Such components arc typically produced using laborious processes with slow-reacting epoxy resins, and cured under heat and pressure in an autoclave, where cycle times often reach 20 hours or more.

Different production routes are needed for the automotive industry. While injection moulding of glass-filled polymer resins has proved successful for plastic underhood components, there has not been the same success in developing production routes to replace the well established stamping and forging processes used for large- volume structural and semi-structural components.

Design data is another issue. Automotive composites are rarely 'simple' laminates or sandwich structures. Many failure criteria used elsewhere in the composites industry are not applicable. While there is a wealth of information regarding epoxy and epoxy-like systems, automotive composites are more likely to be vinyl ester, isocyanurate or similar systems. Test methods, failure criteria and material models have to be developed for the more ductile, lower Tg resin systems and less directed fibre architectures used to make auto parts.

The auto industry needs the support of the plastics business in the same way that it has traditionally relied on the steel industry (and aluminium industry) to do most of the basic development work. Resin companies such as DSM and DuPont are understandably keen, as are fibre suppliers such as Toray, Owens Corning and Saint Gobain, but business pressures have meant the emphasis is too often on competition rather than cooperation (European Automotive Design, April).

The Mercedes-Benz SLR McLaren features two conically tapering longitudinal members made of CFRP to absorb energy in a crash

In Europe, the USA and the Far East, governments have stepped in to sponsor projects to develop new ways of making high performance composite parts more quickly and economically. The first barrier to success-obtaining less expensive fibres and resins-has proved difficult to overcome even though great strides have been made by the chemical companies, and the capital cost of production equipment remains a serious issue. Nevertheless, progress is being made.

One groundbreaking collaborative project was the DaimlerChrysler initiated 'Triple C' (Compound, Characterization and Consistency), a quality improvement programme to install traceability systems and increase consistency throughout the entire sheet moulding compound (SMC) value chain-to try to reach the surface quality and productivity of steel.

As the industry pursues ever more aggressive weight-reduction goals, however, it appears that carbon fibre rather than glass fibre technology may be required. Glass fibres are still the leading reinforcement fibre in plastic products due to their low cost and good performance and developments such as high-performance short glass fibre reinforced (HPGF) polypropylene by Denmark's Borealis group (EAD, April) have extended the technical envelope. However, market research company Freedonia has recently predicted limited growth for glass mat thermoplastic (GMT) in vehicles, the second largest reinforced plastic use, which they attribute to slow production advances and the lack of significant new applications.

DSM Composite Resins is a specialist in resins for SMC/BMC and has a dedicated competence centre at Ludwigshafen, Germany, for the advancement of SMC technology in the automotive industry. Current programmes include new carbon fibre SMC (CFSMC) formulations that are said to give excellent mouldability, very low part weight (typically 60-70 per cent less than steel) and high stiffness.The focus is weight reduction, quality consistency, cost saving and recycling for automotive customers.

"Today, almost all body/chassis materials research is focused on making carbon fibre composites technically and economically viable for future vehicles," says David Warren, programme manager for Transportation Composite Materials Research at Oak Ridge National Laboratory in the USA. Warren is also closely involved with the US Department of Energy's Automotive Lightweighting Materials and High Strength Weight Reduction Materials programmes.

ATR is ready to start volume production of its latest process for Class A surface carbon-fibre components

Carbon fibre composites offer high W stiffness and low density, but attempts to make them production-worthy involve huge challenges.

"There must be cost reduction or at least cost parity with current steel designs. Fortunately, cost is not simply viewed as material price per pound. Instead, the total system cost is evaluated, accounting for savings in capital equipment, assembly, part count and part finishing. The largest cost reduction effort is aimed at carbon fibre itself. Cost-effective, high-volume processing methods are also under development, using techniques that will be compatible with automotive manufacturing plants and methodologies. Much of our current manufacturing research centres on fibre- reinforced thermosets-thermoplastics work has been stalled, due to concerns about thermal expansion and creep. Significant effort is also going into development of fast, reliable and repeatable, high- volume technologies for joining composites and dissimilar materials- methods that enable composite parts to be removed for repair or replacement.

"The goal of current composite materials research is a mostly carbon fibre composite body-in-white (BIW) which is 60 per cent lighter than comparable steel vehicles. The DoE project's initial design phase is complete, with a proposed structure that is 67 per cent lighter than the steel baseline, with greater bending and torsion stiffness.The design uses 55kg of chopped carbon fibre and 8.2kg of carbon fibre fabric. Currently, tooling is being procured and installed to make the various parts of the BIW,with assembly scheduled for later this year or early 2004."

Already in production is the hood for GM's 2004 Chevrolet Corvette Z06 Commemorative Edition-the first time carbon fibre has been used as original equipment for a painted exterior panel on a North American-produced vehicle. With between 2000 and 3000 components being produced in year one, GM claims this will be the highest production volume of a single carbon fibre component using aerospace autoclave technology.The hoods are produced by MacLean Quality Composites, an operating unit of MacLean Vehicle Systems (MVS).

"We expect this project is the first of many automotive applications for our carbon fibre technology, from body panels to structural components," says Jeff Keller, VP and general manager, plastics and composites for MVS.

The 9.3kg carbon fibre hood is only two-thirds the weight of the standard fibreglass SMC hood. The painted, class-A outer skin is only 1.2 mm thick, similar to stamped sheet metal and about half the thickness of the SMC part, and is claimed to be the thinnest polymer composite skin on a production vehicle.The exterior skin panel is carbon fibre/epoxy prepreg, while the inner structure is a hybrid of carbon fibre SMC and low-density fibreglass SMC.

The MVS process uses cellular manufacturing and a combination of automated and manual techniques to keep the overall investment costs low. Although an autoclave is used, the uni-directional carbon/ epoxy prepreg supplied by Toray Composites is formulated to fully cure in only 10 minutes at 150C.

The prepreg is automatically cut into patterns and oriented in multiple directions to achieve balanced strength and stiffness properties.To achieve the 2004 model year volumes and provide tooling durable enough for years of aftermarket production, Invar nickel/iron alloy was selected for the moulds.

At IAA in Frankfurt last month, Giulio Strambi, technical department manager for Italy's ATR Group, told EAD his company was now ready to move its latest process for Class A surface carbon- fibre components from the R&D lab into volume production. ATR has great experience in structural carbon fibre/epoxy composites, producing chassis and bodywork structures for a variety of Formula racecars and supercars. Recent projects have included the Enzo Ferrari and the Porsche Carrera GT EAD, June).

ATR's prepreg based process uses multi-step curing under pressure. "The key to success is, of course, the control of the curing conditions and these have now been perfected within our R&D company" said Strambi. "We have demonstrated a number of prototype motorcycle and vehicle components with A-class surface finish and are in serious discussions with OEMs. We can at present produce components up to door panel size, and at rates of 5 per hour, or 50 per day. Both size and productivity will be improved if we build a production line-and we do want to sell parts rather than license the technology."

If not carbon?

All-polypropylene composites are being considered for a broad rangeof thermoforming applications and could become formidable competitors to glass-matreinforced thermoplastics (GMT).Amoco Fabrics (a subsidiary of BP) was first to reach market with its Curv composite, and in April, at the JEC composites exhibition in Paris, industrial yarns supplier Lankhorst unveiled its Pure product. Both Curv and Pure consist of PP reinforced with PP tape. Although they are suitable for several processes, they are expected to see most use in thermoforming.

Derek Rilcy is market development manager for Curv: "A 5000- tonnes/year production line was brought on stream in Gronau, Germany in 2002.The plan is to seed the market from here, with capacity to be added when required. Though it has been marketed as a standalone thermoformable sheet, much of the interest in Curv is its application in sandwich structures, with foamed plastics (PP and others) or paperboard as inner materials and Curv on the exterior to provide stiffness or other properties."

Curv sheet is made using hot -compaction, in which the surfaces of highly drawn fibres in the fabric are melted and then recrystallised, forming a matrix that holds the fibres in place. Around 80 per cent of the original fibre properties are maintained in an all-polypropylene sheet that shows strength and stiffness levels similar to GMT-PP, but higher impact strength, abrasion resistance and elongation at break.

DaimlerChrysler has looked at Curv for thermoformed vehicle undershields, and processors are testing it as an insert moulding material for local reinforcement in injection-compression and compression moulded parts.Though Curv melts at about 175C, injection is rapid enough that the Curv insert is not negatively affected and still offers sufficient strength and stiffness that glass-fibre levels can be reduced.

Curv's pricing is grade-dependent, but is usually twice the level of GMT. Amoco says part weight usually can be reduced by 40 to 50 per cent, so Curv can be cost-neutral on a systems basis.

Lankhorst is working with a UK thermoformcr to test the market for automotive applications of its Pure PP tape product. The company is offering test amounts for automotive projects. Lankhorst plans to license the technology for tape manufacturing to ensure global production and availability.

Hans Jacobs, a Lankhorst director, says: "Compression moulding of Pure is feasible, but thermoforming-especially automotive parts-is the target market."

Each Pure tape is wound of three layers of PP, with the outer layers melting at a lower temperature than the inner one.The processing window of the material is between 135C, at which point the outer layers of the tape start to melt, and 180C, when the core layer begins to melt.

"During processing, the outer layers melt enough to facilitate processing, but you retain the strength and e-modulus of the inner layer," says Jildert De Rapper, managing director of Lankhorst. "You have the low density of PP (0.8 compared with 1.3 or so for fibre- reinforced PP) at lower weight, plus higher stiffness and better abrasion resistance." The PP tape, grey or white in colour, is said to cost much less than carbon fibre.

Long fibre reinforced thermoplastic composites (LFRT) are some of the most dynamic and fastest growing materials in the plastics industry. Although they have been around for several decades, advances in technology, the development of inline compounding and increased acceptance in the automotive market have been the major driving forces for their rapid growth-30 per cent per year, according to market surveys by BRG Townsend. LFRT competes with a wide range of materials, including short fibre reinforced engineering plastic, GMT, sheet moulding compound/ bulk moulding compound (SMC/BMC) and metals.

LNP's Verton MFX long glass fibre reinforced polypropylene composite meets higher performance criteria required for the Super Integrated Door Module. This integrates the structural components as well as the functional ones

"This is a fast growing and evolving market," says Robert Constable, business manager at BRG Townscnd."In the past two years we have seen one supplier enter and exit the market and several others have changed ownership-for example, GE's purchase of LNI* SABIC's purchase of StaMax. New applications and technologies continue to be announced and the whole issue of inline compounding versus precompounded pellets needs to be better understood. How have users of in-line compounding addressed the issues of hidden costs such as liability, formulation and process expertise, and quality control?"

Ticona is a leading player in LFRT. Its Celstran LPRT is used by several OEMs, including Jaguar for the door modules and the grill opening reinforcement of the XJ. Metal carrier plates in the door modules were replaced with LPRT in both the front and rear doors, allowing use of modular technology and assembly. The carrier plates in the door modules component measure 1.0 x 0.7m, weigh around 1.7kg, and fit precisely into the shape of the vehicle doors. Functional elements such as internal door openers and speakers are directly built into the door module.

The door module plates are supplied by UK company Automould using a Celstran grade that is a PP matrix reinforced with 30 per cent long glass fibre.The material offers thermal resistance from -40 to +80C, high dimensional stability, good insulation characteristics, and high tensile strength.The grill opening reinforcement for the Jaguar X3 50 is a U-shaped support 1.6m wide that provides for precise positioning of the vehicle's lights, the radiator grill, the bumper and the front wing. The Celstran grade used is a nylon 6/6 matrix reinforced with 50 per cent long glass fibres.

StaMax P long glass fibre PP thermoplastic is used for the front- end module on the Porsche Cayenne and Volkswagen Touareg.These SUVs are nearly double the weight of BMW's Mini Cooper, the car in which a front end module of StaMax P made its debut.

"The decision by the automaker to make StaMax P the material of choice for the front-end module of the Cayenne and Touareg is an industry milestone," says Andrew Hopkins, general manager of OC Automotive, a development partner for StaMax. "OEMs have been reluctant to specify a long glass fibre thermoplastic composite for the structures of heavy vehicles like SUVs. These two automobiles will go a long way to proving to the industry that LPRT offers structural performance in addition to obvious benefits such as weight reduction, cost savings and parts integration."

SymaLITE, from Quadrant Plastic Composites, is a move to breathe new life into GMT composites through adding significantly lighter weight plus inmould decorating capabilities to the existing properties of commercially proven GMT-good energy management, very- high impact strength per unit stiffness and sound damping. GMT composites are not notch sensitive and have a ductile failure mode even at low temperatures, allowing their use in safety-related automotive applications such as door modules, instrument panels and bumpers.

Quadrant says a new mat technology and manufacturing process make it possible to 'tune' the physical and mechanical properties of SymaLITE sheet-form composites on the fly, providing new levels of design and processing flexibility. The sheet materials make use of a patented hybrid glass/PP fleece to give a higher stiffness-to- weight ratio than conventional GMT. Varying the ratio of the various fibres and the way the fleece is subsequently needled allows mechanical and physical properties to be optimised.The glass/PP content can vary from 20 to 60 per cent glass to favour ductility or stiffness.

SymaLITE composites are processed in a slightly different manner than classic GMT-a low-pressure stamping or thermoforming process rather than much higher pressure flow forming via compression moulding.This makes the materials amenable to the use of paint films and polymer skins. Blanks of the composites are heated to a processing temperature of 180-200C via infra-red, hot air or contact ovens. Glass fibres in the fleece have memory (or back force) and try to return to their initial orientation upon heating.This causes the laminate to loft up five to six times its original thickness.

The heated blanks are then moved robotically from the oven to an aluminium tool, where they are subjected to low-pressure forming. Making use of a technique called 'tailored consolidation', during the moulding process the density of SymaLITE composites can be reduced to one-third that of the original laminate, effectively creating a long-fibre reinforced, air-permeated composite part.

Diaphorm says its 'soft moulding' process produces fully finished thermoplastic composite parts that cost as much as 70 per cent less than compression mouldings in short to medium runs.

"Both the part and surface finish are co-moulded in a one-step process that uses inexpensive tooling, low moulding pressures and lightweight equipment," says Bob Miller, Diaphorm Division general manager. "The process fills the gap between hand lay-up and compression moulding and is ideal for structural composites in lot sizes between 1000 and 50,000."

The Pressure Diaphorm process works like compression moulding and part tolerances are similar. It uses a single-sided mould, an oven to melt the resin and a rubber diaphragm to conform that material to the shape of the mould. Current maximum part size is 1.1 x 2.2m, with walls up to 9.5mm thick. Larger partsize capability is under development. Moulded-in finishes include felted, textured, woven print, smooth, high-gloss and Class A surfaces.

"The tooling can cost one quarter that of compression-moulding tooling," says Miller. "Moreover, the low-pressure process requires a much less expensive machine to amortise over the cost of the parts produced."

But amid all this optimism, a cautionary tale. Less than three years ago ConocoPhillips announced the developm\ent of Cevolution, a pitch-based carbon fibre fleece product that would be a cheaper route to reinforcement. Earlier this year, after euro500 million of investment on development and new manufacturing by the company and its predecessors during a 14 year programme, ConocoPhillips pulled the plug. Apparently no-one was interested in the new product after all.

If the use of carbon fibre becomes more widespread, will it retain its cachet as a material exotic enough to provide the finish to a Lamborghini?

Mercedes-Benz SLR McLaren

The bodyshell of the Mercedes-Benz SLR McLaren is made entirely from carbon fibre composite, as are the front and rear structure and the passenger cell, the swing-wing doors and the bonnet. According to M-B, the weight advantage over steel is around 50 per cent. Overall, the primary structure of the SLR McLaren is around 30 per cent lighter than a conventional steel construction.

In addition, on impact, carbon fibres are characterised by four to five times higher energy absorption than steel or aluminium. To exploit these qualities, two 620mm carbon fibre longitudinal members, each weighing 3.4kg, have been inserted in the front structure. A cross member and a horizontal sandwich panel made from carbon fibre composite mean the SLR is the first series-produced car to have a front crash structure made entirely from carbon fibre.

In a collision, the fibres of the carbon fibre composite elements shred from front to rear, absorbing the energy of the impact with constant deceleration. The energy absorption of the longitudinal members can be tuned to meet specific requirements. This fine tuning means the deceleration values result not only in predictable energy absorption behaviour but also in a weight advantage, because this design uses only as much material as is actually needed.

The two conical longitudinal members of the front crash structure are made up of a main body and an internal web, a basic configuration that proved to be the most successful during the four- year development of this SLR component. The aim of the engineers from the Advanced Design department at the Mercedes-Benz Technology Centre in Sindelfingen and of DaimlerChrysler Research was not only an unprecedented degree of passive safety, high rigidity, tremendous strength and as low a weight as possible, but also to draw up manufacturing concepts with a high degree of automation.

Mercedes-Benz adapted traditional manufacturing methods used in the textile sector, such as sewing, knitting, weaving and braiding, for the processing of high-performance carbon fibres. Several patented solutions had to be developed and tested in order to ensure short cycle times and high repeat precision. The manufacture of the complex fibre structure of the longitudinal members takes just 12 minutes.

Mercedes-Benz is also the first car manufacturer to use components manufactured using the Advanced SMC method developed with Menzolit-Fibron and Volkswagen (European Automotive Design, June). The rear shelf has a very complex form with several apertures, yet it is automatically manufactured as a single part. The product was introduced at the JEC Composites Show in April and received the top award in the JEC Awards 'Automotive Transport' category.

"Advanced SMC is reinforced with oriented uni-directional carbon fibres, though some chopped, random glass fibre also is used," said Peter Stachel, R&D director of Menzolit-Fibron's Compounding Division. "The oriented carbon fibres enable manufacturers to tailor the lay-up to specific performance requirements. Moulding cycle times can be as short as three to four minutes."

The British company McLaren Composites manufactures over 50 carbon fibre and fibreglass components for the SLR. Here too familiar processes from the aeronautical industry were adapted and developed. The degree of integration achieved in the manufacture of the bodyshell is remarkable. The entire floor assembly, for example, including all support members and securing elements, is made in one piece.

The roof of the BMW M3 CSL

The CFRP roof of the M3 CSL is the first series application of an automated RTM production process developed over the past two years by BMW. The development team included CFRP specialists at the company's Landshut plant-where the parts are being produced- engineers at BMW's Research and Innovation Centre and the automotive engineers at BMW M GmbH. The roof of is 6kg, more than 50 per cent lighter than a comparable steel roof.

The roof is made a three-stage process. The first step is to place five layers of carbon fibres on top of one another for preforming. A particularly important requirement in this context and one of the most significant innovations in the process - is to ensure that the carbon fibres are properly aligned, come in the right position and have the right structure to give the roof the stability and looks required. The second production step is the resin transfer moulding (RTM) injection process. The preformed, multi-layer carbon fibre mat is placed into an 1800-tonne press for injection of the transparent epoxy resin. The roof hardens in the heated mould and is subsequently removed by a robot. The final stage is a clear-coat paint finish that leaves the tissue structure of the CFRP material on display.

BMW says the degree of automation means the time required for manufacturing the roof is less than one-fifth of the time previously required for similar components. The company is convinced the process will pave the way for economical, highquality series production of carbon-fibre body components.

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Source: European Automotive Design

Copyright Findlay Publications Limited Oct 2003

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