The use of carbon fibre reinforced plastic in the automotive industry is ever growing as new materials and new manufacturing processes become available. Designers are always developing new ways to reduce weight, improve efficiency, and reduce cost; CFRP is giving them the freedom to reconsider the use of thermoplastic composites for both interior and exterior applications.
Any new trends in the automotive industry are assessed in terms of manufacturing cycle times, reduced production cost through automation, decreased environmental impact, improved comfort and safety, improved acoustic performance, and how much of the new material can be recycled. CFRP is being looked at much more seriously by manufacturers in the design of their cars as the cost of oil and raw materials such as steel is continually increasing. The weight-to-performance ratio of CFRP means it is an attractive prospect to manufacturers aiming to reduce the weight and overall performance of a vehicle.
CFRP is widely used throughout the aerospace industry and for lightweight racing cars, but the lack of suitable design processes has meant that the mass production car market has previously been unable to embrace the technology fully. Thermoplastic composites have been used extensively on interior car parts such as sun shades, door panel trim, bumpers and parcel shelves. However, as yet it has always been a significant challenge to develop the processes which can produce structural exterior parts, without taking away from the crash safety standards of the vehicle. The other challenge is to make the use of CFRP economically viable in a large scale production scenario.
Design solutions for high volume production of lightweight parts
There are several methods of manufacturing CFRP and researchers are moving ever closer to developing the techniques to produce lightweight CFRP structures that can be manufactured quickly enough for the mass produced market. CFRP can now be produced to provide high strength parts with a greater stiffness to weight ratio than ever before; it also retains excellent impact performance and dampening properties, which are highly desirable for car manufacturers.
New resin transfer molding processes, composite braiding and continuous laminating are some of the processes being developed which allow designers far greater freedom; enabling them to create larger more robust load-bearing parts and connecting pieces throughout their vehicles.
There is still a distance to travel for researchers and engineers alike to develop the processes which will enable mass production on a large scale. Research shows that in the manufacturing of the VW Golf, over 250,000 parts are produced per year with target manufacturing cycles of one minute per part (1). Engineers at the Fraunhofer Institute for Chemical Technology ICT have developed a process of thermoplastic resin transfer molding (T-RTM) by which they are able to reduce composite part manufacturing cycles to around five minutes (2); so it is clear that further innovation is required, but that the future of lightweight CFRP production on a grand scale is within reach.
Lessons to be learnt from composite application in the aerospace industry
The aerospace industry has long used thermoplastic composites in production of aircraft, and there are lessons that can be taken from aerospace innovation and applied to the automotive industry both in manufacturing and repair procedures. The Boeing 787 depicted below (3) makes greater use of CFRP and other composite materials than any previous Boeing commercial airplane. Almost half of the airframe is constructed from composite materials, which provides a 20% saving in the overall weight.
The weight saving to cost ratio of an airplane is of much greater significance than that of an automobile (4), using a cost to weight trade off chart by Ashby, it is estimated that a reduction of 1kg in weight will result in a saving of 2,900 litres of fuel per year in a single aisle airplane. In his trade off chart this is presented as a saving of $100-500 per kg, compared with an automotive saving of $1-2 per kg. The principle is the same however, in that every kilogram a manufacturer can shave off the design will represent a saving in the cost and efficiency of the car.
The testing and development of composites used in aerospace can give key indicators to the automotive industry. For example, when selecting materials for the production of the 787, Boeing found that where Aluminium handles compression extremely well, it is sensitive to tension loads. Whereas composite materials are very good under tension, but are not as capable of handling compression loads. Boeing has used more composites in the highly tension-loaded areas of the fuselage which should reduce the amount of maintenance due to fatigue when compared with the use of Aluminium.
In fact the use of CFRP and composite materials should reduce maintenance all round as they do not corrode and there is less risk of fatigue than with metal components. While routine maintenance is expected to reduce, non-routine maintenance is also expected to decrease as a result of the durability of composite materials. Downtime for these non-routine repairs can also be reduced by using composite bonded repair techniques; another method of interest to the automotive industry. Repairs are permanent, damage tolerant and do not require an autoclave. Where previously a bonded repair could require around 24 hours of downtime, a composite bonded repair can be completed within an hour, and provides a more aerodynamic and aesthetic finish.
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(4) Ashby, M, Materials Selection in Mechanical Design, 3rd Edition, Elsevier, (2005)