Stiff challenge

Plastic components could be made much stronger and stiffer by selectively coating them with a thin layer of metal alloy. DuPont is ready to unveil its Plating on Plastic (Pop) technology at this month’s K2007 plastics exhibition in Germany. It says that components treated using the technique could compete head-on with light metals in terms of mechanical performance. It is the combination of plastic and metal elements that leads to the increased mechanical performance, says DuPont. The plastic has high stiffness, the metal layer has high strength of around 700MPa – more than that of mild steel. The new technique brings the two together. The main target is likely to be the automotive market, where DuPont has already begun to consider applications such as oil catch pans. But it is likely to have more impact as a replacement for parts that are cast or machined from aluminium and magnesium. Examples of aluminium parts that could be targeted by this technique include the cross-car beam – which contributes to bodyshell stiffness, and supports the dashboard – and various complex joints that connect the elements of an aluminium bodyshell. DuPont is not ready to reveal full details of the technology, as it is still making patent applications. But Eureka has interviewed several key players in the company – including Nandan Rao, vice president of technology at DuPont Engineering Polymers. “Imagine standing on a ping-pong ball,” he says. “You will crush it. But by using this technique to coat the ball, it would support your weight – with only a 100 micron layer. It acts like an exoskeleton.” He is keen to stress that the technique goes beyond simple surface treatment. “It’s not overmoulding and it’s not a coating,” he says. “It’s an electrochemical approach to depositing a metal alloy on an activated thermoplastic part.” He also refers to the technique as ‘plastic-metal hybrids’, ‘nano-metal technology’ and by the codename ‘DNM’ – standing for DuPont nano-material. While exact details are still officially under wraps, the process is expected to work like this. A plastic part is injection moulded, using one of up to 10 specially modified grades of nylon. It is then sent back to DuPont for ‘treatment’. The company activates the surface of the part and selectively adds one of a range of metal alloys to the surface. Five alloys will initially be available. DuPont is looking at several different ways of activating the surface and applying the metal alloy – each depending on the geometry of the part. “We intend to create an electrical field that allows deposition,” says Rao. “If there’s a part with one critical area, you would only apply the metal alloy there – where the force is felt.” Design engineers may wonder how they can model such a part. But Clive Robertson, business development manager for high performance engineering polymers – who directly leads this project – says it can be managed. “This technique brings extra freedom – but also added complexity,” he says. “These types of part can be modelled using the latest FEA software, but require the designer to understand where the loads are on the part. We can help to select where you should apply the metal.” The key to this concept is being able to get the high strength metal alloy onto the plastic surface. At high temperatures, the difference in thermal expansion could lead to high strain at the materials interface. “This is a key consideration,” says Robertson. “It’s about correct selection of plastic and alloy – and how the materials are blended.” He stresses that the alloy forms a homogeneous ‘cage’ around the plastic – which also has its own tensile strength. “This alloy is a super-strong material, which has been developed by a partner,” says Robertson. “We think we can use it to get into the range of magnesium and aluminium stiffness.” Its tensile strength of around 700MPa is a long way ahead of mild steel (450MPa) and traditional ‘cosmetic’ surfaces such as chromium. The company has tested a series of cantilevered plastic beams – and compared them with ‘metal-clad’ equivalents. A 4mm thick beam with 100 micron alloy layer, for example, saw a doubling in stiffness. “The thickness of the layer can actually be varied,” says Robertson. “It might be anywhere between 50 and 150 microns to tailor it for a specific application.” At the end of the day, DuPont says the design flexibility of plastics is the main selling point – coupled with the ability to transform its mechanical performance. “You can make very complex parts more easily in plastics than you can in aluminium or magnesium – either by casting or machining,” says Robertson. “I’m not saying this can replace all metals, but it could hit applications that have never been accessible before.” DuPont Closing in on metal Aside from super-strength materials, DuPont is also planning to showcase other innovative materials at K2007 – including a family of thermally conductive plastics. “Many products such as high intensity lamps and flat panel displays need very efficient heat dissipation,” says Nandan Rao. He says that adding filler materials such as boron nitride can produce a conductivity of around 35W/mK – but that commercial grades are likely to be in the range of 10-15W/mK. “If we can combine thermal conductivity and superstructure, that’s the biggest hurdle to making things ‘metal-like’,” he says. “We are close with superstructure – but further away with thermal conductivity.” The breakthrough needed, he says, is to get higher conductivity but using less filler material. POINTERS A thin layer (around 100 microns) of high strength metal alloy can boost the strength and stiffness of plastic components Layer will be applied electrochemically to selected areas of the component Complex automotive components that are currently made in aluminium – such as cross-car beams – are likely to be the main target