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A weight loss programme for automotive designers

02 April 2012

The automotive manufacturing sector’s quest to reduce vehicle weight and fuel consumption without compromising performance and safety has driven a raft of developments in lightweight materials for vehicle structural applications. Bert Suffis reports.

Arguably, the most important innovation in the area of automotive component weight reduction was the development in the late 1980s of expanded polypropylene (EPP). Unlike previous materials whose energy absorption capabilities were questionable, EPP boasted two fundamental physical capabilities which rapidly established it as a ‘reference’ material for low-speed impact protection.

Firstly, its closed cell foam construction means it is isotropic, offering the same consistent performance irrespective of the direction from which impact is received. This represented a stark contrast with injection moulded and rigid polyurethane products, whose performance is traditionally tuned to only a single crash scenario.

Its other advantage is that it can be deformed on multiple occasions but will still return to its original shape following low-speed impact events. These qualities mean that the material not only fulfils, but in many cases exceeds, the exacting performance requirements of governments and the industry, such as passive safety requirements as dictated by the EURO NCAP tests.

During the 1990s, specialist grades, such as porous products and those offering discrete surface resistivity, were developed for the first time and the material was manufactured in a range of colours.

Automotive designers were quick to seize on the opportunities offered by the full range of materials and nowadays it is common to find them used in many parts of a vehicles, with applications ranging from passive safety products such as doors, side impact energy absorbers and door panels, head restraints and seat cores, seating and knee protection.

In some applications, such as seating, total weight reduction of up to 35 per cent over alternative systems is achievable, with greater design flexibility allowing consolidated parts and the ability to complement other materials where necessary. Tooling costs are significantly lower than those for injection moulding, while lightweight products allow the possibility to de-mould negative features without the need for further investment in tooling.

The ease with which this material is moulded, allowing greater freedom of design, has been one of the key developments since its introduction more than two decades ago. Recent material developments and improved tooling have pushed the boundaries of feasible 3D shapes, as well as allowing the creation of smaller and thinner products.

Aesthetically, too, advances continue apace. Forget the ‘beady’ raw material - the latest products can deliver a range of surface appearances, presenting a warm, uniform surface texture. This also plays a key role in reducing component count and weight, not least in ‘secondary’ visible areas where the limited exposure to UV means they do not need to be covered.

The ARPRO ‘sandwich’
The evolution of electric vehicles has accelerated the pace of automotive weight reduction as designers strive to extend battery life. To this end, in 2010, a new ‘sandwich’ technology called inrekor was launched, containing a core of ARPRO, surrounded by bonded skins to deliver exceptional strength to weight performance.

Indeed, inrekor’s structural properties make it suitable for use in vehicle chassis, traditionally one of the greatest contributors to vehicle weight. A campaign is now underway to convince designers, manufacturers and vehicle buyers that the material can deliver the required levels of strength, stability and safety for these applications.

The stiffness and flexural strength of inrekor actually increases exponentially with respect to the thickness of the core, firmly allaying any concerns in this area. Meanwhile, the built-in capability for ‘intelligent’ function integration – insulation, isolation, and air ducting, as well as built-in mounting points - means that a lightweight, cost-effective structure can be created with minimal capital investment. Moreover, fixing the final product is simple too as, unlike some honeycomb products, it can be riveted, bonded, spot-welded or otherwise fastened, depending on the application requirements.
The isotropic nature of the ARPRO core means that any impact energy rapidly dissipates in all directions – a fact the industry seems to have taken in all seriousness, as several highly innovative electric vehicles have now been launched, equipped with an inrekor chassis.

The versatility of the sandwich technology and developments in adhesion methods and products mean that surface skins in a variety of materials can be bonded to the core depending on the application and any aesthetic requirements. Design changes are easily accommodated, with minimal tooling or capital expenditure, as rather than CNC machining of finished sandwich panels, the cores are usually cut with water and the skins with lasers. Both materials are also suitable candidates for traditional pressing or stamping operations.

Among recent examples of successful inrekor applications is a specially engineered fuel tank for Aston Martin Racing. An inrekor shell was created, which not only ensures a tight fit within the vehicle roll cage, but which also meets FIA dimensional requirements for such components. And the speed with which parts can be created from scratch is also amply demonstrated by the 13-week turnaround from initial CAD design to the actual on-vehicle use of a completely novel inrekor chassis for a replica Speedster 356 vehicle.

Bert Suffis is with JSP, the developer and manufacturer of ARPRO EPP

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