Saving weight is almost as important as saving money in the automotive industry, as it can help to meet stringent new emissions targets. Lou Reade reports.
Car manufacturers, who are keen to slash the weight of their vehicles in order to find room for new features (and to boost fuel efficiency), usually turn to plastics for help.
This is not a new trend. Plastics account for around 20 per cent of the weight of a typical modern car. Fuel tanks and bumpers are two components that are routinely made from plastics, and automotive companies are confident that other parts of the car could follow suit.
Despite this greater use of plastics, cars have actually been getting heavier – by around 15kg per year over the last few decades.
“This now needs to reverse, in order to deliver CO2 reduction targets,” says Patrick Cazuc, EMEA marketing manager for automotive at DuPont Performance Polymers. “The main driver for weight saving is CO2 reduction.”[Page Break]
The CO2 targets are tough, particularly in Europe: current emission levels of 165g per km driven must be reduced to 130g/km by 2015 – and to 95g/km by 2020. Every 100kg of weight reduction brings an approximate 10g reduction in CO2 emissions.
“Lightweighting is one of the primary factors that can help to achieve this reduction,” says Cazuc.
Replacing a large metal part with plastics is the best way to work towards the new emission targets. But plastics can make an incremental difference as well, helping designers to shave off small amounts of weight here and there – which may end up being just as important.
“Any saving, at gramme level, is worth it,” says Cazuc.[Page Break]
Reduction factor
While metal-replacement is the usual route to reduce weight, some techniques are looking to reduce the weight of existing plastic components. Mecaplast, the Monaco-based supplier of plastic automotive components, claims it can cut the weight of interior and exterior trim parts by about one-third using a new injection moulding process.
It is working on a project called Plume, sponsored by the French government, which uses specially designed ‘adjustable’ moulds – and a blowing agent – to reduce the density of moulded polypropylene (PP) components.
“Parts made by this process could lead to a total vehicle weight reduction of 5 to 7kg,” says Elsa Germain, research and innovation engineer at Mecaplast.
Material is injected into a mould that has moving walls. Once the skins of the part have solidified, the walls retract slightly. This reduces pressure in the cavity, causing a chemical blowing agent – which had been dissolved in the melt – to come out of solution in areas that are still molten.
This creates a cellular structure, which fills the newly created space. The foaming process on its own can create a weight reduction of at least 30 per cent, compared to conventional injection moulding.[Page Break]
But this can be improved upon by adjusting the compound formulation. The compounds incorporate new types of reinforcing filler that improve surface quality and reduce part weight (by up to 7 per cent,) while maintaining mechanical properties. Compounds with different fillers will be trialled during the project. The compounds, based on PP impact copolymer, have good flow properties and a melt flow index above 50g/10min.
Mecaplast will run trials of the process on two components: a tailgate interior trim, and exterior beltline mouldings. The beltline mouldings will be produced in two versions – one with a grained surface (that requires no painting) and another that will be paintable.
The project involves a number of French companies, including compound producer Sumika Polymer Compounds (part of Sumitomo Chemical Group), mould-maker Cero, polymer science research laboratory IMP, and Cemef.
The project, which started in September 2012, will run for two years with a total budget of €2.4 million.[Page Break]
Fibre increase
But it’s carbon fibre reinforced plastics (CFRPs) that could have the greatest effect on the automotive industry – and their use is expected to accelerate in future. Consultancy Frost & Sullivan estimates an annual growth rate of more than 30 per cent, taking the market to nearly $100m by 2017 – from its current position of just below $15m. This is despite several factors that will hamper the take-up of the technology.
“We identified four factors that could restrain the market,” says Sandeepan Mondal, senior research analyst at the company. “These are: high cost; long cycle times; a lack of engineering expertise with these materials; and recyclability issues.”
The ‘low modulus’ grades of carbon fibre used by the automotive industry typically cost up to $25/kg – which is lower than the $35/kg for high modulus grades used in the aerospace industry. But the aerospace industry makes larger parts, which leads to better economies of scale.
“There is also a lack of general engineering expertise among automotive OEMs,” he says. “Most have invested in metal assembly plants, and are not keen on making more capital expenditure on a new technology.”
He says that automotive companies are likely to adopt a ‘hybrid’ approach, and use a range of materials – including aluminium, high strength steels and thermoplastic composites – to deliver weight savings.[Page Break]
And while CF materials are most likely to save weight – to help meet future emissions targets – they are difficult to recycle. The End of Life Vehicles (ELV) Directive insists that 85 per cent of all vehicles must be recyclable by 2015. This would be more difficult if CFRP parts were involved, he says.
Mondal estimates that 90 per cent of CFRP automotive applications will use carbon-epoxy composites, with the rest composed of carbon fibres embedded in thermoplastics such as polyamide, polyurethanes and Peek – depending on the application.
“Under-bonnet parts are most likely to use CF-reinforced PA, because epoxy does not have high enough chemical or heat resistance,” says Mondal.
Beyond 2017, he believes that efforts to reduce cycle times for CF parts will continue to improve.
“Many carbon fibre producers are looking to bring cycle times below five minutes,” he says. “This is still long compared with steel, but at the same time the cost of carbon fibre is likely to come down.” [Page Break]
Carbon adoption
The US, whose automotive industry has suffered recently, will be slowest to adopt these measures, he believes. Japan, a global centre of CF production, will be faster, but Europe is leading the charge to adopt CF technologies – with BMW at the front of the queue.
This year, BMW intends to roll out two new mass-production cars – the i3 ‘Megacity’ electric vehicle, and the i8 hybrid ‘supercar’ – that rely heavily on carbon fibre-reinforced plastic (CFRPs). It has been able to do this because it has automated the processes used to make carbon fibre textiles, and to shape them into automotive components.
BMW is using resin transfer moulding (RTM) to reduce production time for each CFRP part to less than 10 minutes – much faster than the 20 to 30 minutes that was needed just a few years ago. The CFRP part in the Megacity car will be the ‘passenger cell’, which sits on top of an aluminium chassis. The CFRP part is half the weight of an equivalent steel part, says the company.
BMW has definite plans to transfer it within the company.
“More and more car projects, starting from 2105 and 2016, will use carbon fibre as a structural element to reduce weight,” says Joerg Pohlman, joint managing director of SGL-Automotive Carbon Fibers – a joint venture between BMW and carbon fibre producer SGL.
BMW’s vehicle concept was one of three automotive-themed winners at this year’s JEC Composites Awards. It was the winner of the Special Prize for its LifeDrive architecture – which was designed and built specifically for electric vehicles.
In the Automotive category itself, ECM of France won for its self-supporting composite structure for a light urban electric vehicle. It was designed and manufactured using thermoset resin and glass fibre reinforcement to replace a traditional steel body-in-white. The prototype meets industrial feasibility criteria and results in 30-40% weight savings. Further development using carbon fibre could improve this further.[Page Break]
The key benefits are: weight reduction at acceptable cost for the automotive industry; reduced vehicle emissions due to reduced car weight; design of vehicle parts with integrated functional aspects; reduced number of parts to build a car; and savings on investment and assembly costs.
In the Thermoplastics category, MVC of Brazil and Arkema of France won for their ‘Sofia Project’, in which a PMMA-based thermoplastic resin formulation is used to make bodies for buses, vans or cars using resin transfer moulding (RTM). The resin has several advantages over traditional thermoset resins, including lower weight, better mechanical properties, and better surface quality. The main body parts will be made of a sandwich structure with a low-density PU core, bonded together with structural adhesives.
Saving weight by specifying new materials is not the only way to meet the tough new emissions standards: greater engine efficiency and reduced rolling resistance of tyres are arguably more important. But the number of methods available means that incremental weight savings can be made across the car – adding up to a substantial overall reduction.[Page Break]
Mainstream aim
A two-year UK research project, ACOMPLICE (Affordable Composites for Lightweight Car Structures), is looking to develop composite materials that could one day be used in mainstream cars.
“The targets we’ve set are vehicles like BMWs or high-end Toyotas,” said Elaine Arnold, collaborative R&T project manager at Umeco and the project’s co-ordinator.
Composites are usually restricted to high-end cars, as they too expensive to be used in mass market models. Arnold says that SMC short fibre materials have been fitted in some cars – for applications like parcel shelves – but “nothing really structural”.
The manufacturing partner in the project is Aston Martin – due mainly to it being a UK-owned car manufacturer, rather than as a producer of mass market vehicles. Another partner is Delta Manufacturing, which will handle design for manufacture of components.
The partners have identified two semi-structural components that will be redesigned in composites to save weight. Arnold says it is too early to reveal details about them.
Automation plays a key role in reducing the cost of composite component manufacture, as does materials development. Umeco is working on fast-curing resins, which would allow cycle times to be reduced. Work on the project will focus on the company’s pre-impregnated DForm material – which, says Arnold, maintains unidirectional fibre orientation while being highly formable.
“It’s not yet optimised for automated manufacturing, but that’s what we’d like to do in the project,” she says.
