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Engineering polymers for aesthetic and functional challenges in the automotive industry

Du Pont Engineering Polymers : 25 October, 2001  (New Product)
The more applications engineering polymers capture in the automotive sector in both interior and exterior applications, the more knowledge system suppliers to this industry acquire and the more ambitious the aims they set themselves in new developments and improvement of existing products. To achieve these aims, system suppliers now collaborate with raw material suppliers such as DuPont at a very early stage. They identify trends in good time so that application development teams can respond rapidly to new challenges with specially tailored materials. Examples of such advanced special grades include electro-magnetic shielding or very easy-flowing Zytel polyamides, abrasion-resistant Delrin polyacetals, electroplatable and dimensionally stable thermoplastic polyesters such as Crastin PBT and Rynite PET, vibration-damping, heat-resistant polyamides and Vespel parts and shapes that enable virtually wear-free shaft bearings to be produced, despite high forces and peripheral speeds.
All-round resistance to electrical, mechanical and corrosive stresses
Zytel polyamides are among the oldest engineering thermoplastics but as a result of continual customer-focused product development they offer a very wide range of speciality grades. Special property combinations are provided by semi-aromatic polyamides such as Zytel HTN. These dimensionally stable, heat and chemical-resistant materials are optimised to handle the combination of thermal and mechanical stresses in the engine compartment, while their molecular and crystalline structure also makes them ideal materials for damping acoustic and mechanical vibrations. They therefore meet the increasing demand for thermoplastics that can maintain high dimensional stability, despite the steep rise in engine compartment service temperatures that has occurred in recent years, thereby ensuring reliable operation of electrical, pneumatic and mechanical under-the-bonnet components. When used as air ducting elements for turbocharged and direct-injection engines, which are inherently noisier, these thermoplastics help achieve a significant reduction in noise levels. With a highly qualified team of Noise, Vibration and Harshness specialists and a fully equipped development laboratory, DuPont is able to carry out simulations and component testing at an early stage to ensure vibration-optimised design. In this way costly correction work on the finished mould is avoided.

A current example of the use of Zytel HTN under the bonnet is the valve block (see pic. 1) developed for car and commercial vehicle diesel engines by the system supplier, Woco, in Bad Soden-Salmünster, Germany. As an electro-pneumatic converter, this component performs the functions of several individual valves, e.g. for controlling the vane adjustment of exhaust-driven turbochargers, exhaust gas recirculation systems and diesel engine shut-off. These valves previously had to be connected to separate pipes because they were positioned at different points in the engine compartment. By integrating a number of electro-pneumatic functions in one housing, the new valve block offers considerable cost-saving potential, since the standard, modular design reduces production, assembly and logistics costs.

In the production of the housing, Woco first insert moulds a punched metal grid, which introduces conductor functions in the finished part. This precomponent is then overmoulded with the housing material in a second step. Both moulding materials used are from the Zytel HTN53 series and combine high heat resistance and dimensional stability with good dielectric properties. For the many different functional parts contained in a valve block, including the coil bobbins, Woco uses a heat-stabilised, glass-fibre- and mineral-reinforced grade of Zytel HTN.

But the applications for Zytel HTN are much wider than this. Grades with very low warpage are suitable for rocker covers and grades with increased resistance to hydrolysis are a preferred material for oil-carrying parts. DuPont also supplies special grades for gas-assist moulding, grades with very good weldability and high bursting resistance for complex intake pipes and grades that are ideal for blow moulding of air duct pipes.

Just as Zytel HTN opens up new horizons for the automotive industry that were previously closed to conventional thermoplastics, so special grades of the high-impact resistant grade Zytel ST point the way to significant cost reductions. With their very good melt flow, they make it possible to use lower mould filling pressures and therefore 'smaller' injection moulding machines or alternatively to achieve significant time savings through faster cycles. They are therefore ideal for highly efficient mass production of components with complex geometry such as clips and cable clamps.

In many cases superior to 'real' metal
In the past, aesthetic considerations, such as the shine of chrome, the matt silvery surface of light metal alloys and universal paintability, often led to metals being used for decorative interior and exterior automotive applications for which their high strength was not required. With special grades of Zytel, mineral-filled Minlon and the thermoplastic polyester Crastin PBT, DuPont offers many different alternatives that are equally suitable for painting and electroplating, have very high dimensional stability and also 'shine'. Applications range from wheel trim and interior or exterior door handles to highly stressed headlamp housings.

For example, the lacquered silver or chrome-plated spoke trim for the sports steering wheels used in the BMW 3 series produced by steering wheel and airbag manufacturer Takata-Petri in Aschaffenburg is injection moulded from Minlon (see pic.2). This readily paintable and electroplatable thermoplastic engineering material helps prevent sink marks in visible areas. It also has ideal deformation and fracture behaviour for this application, ensuring that the wheel spoke trim does not splinter when struck by the opening airbag cover during airbag activation. The grade selected here, Minlon 73M40, also meets the requirement for very high dimensional stability at all temperatures likely to occur in automotive passenger compartments. As a result of its vibration-damping properties, typical of semi-crystalline polymers, the material also helps prevent rattling noises. Finally, the good flow of the material permits fast cycles and therefore very economic large-scale production.

Pointing the way forward for automotive electronics and mechanics
Where the increasing number of comfort functions dictates that electrical, electronic and electromechanical automotive components must be packed tightly together in a very confined space under the bonnet, interactions between the different components and thermal damage due to poor heat removal must both be avoided. With its semi-crystalline thermoplastics, DuPont supplies a versatile portfolio of special grades for such applications. A current example can be found in the new Zytel EMX grades. With their excellent electromagnetic shielding properties, plastic housings made from this material enable components to be tightly packed without influencing each other's functions.

The 'Micro Power Relay Silent' from Tyco Electronics meets the specification for minimal noise generation. Two factors proved crucial in achieving this aim: the novel design principle of the electromechanical component and the special vibration-damping grade of Crastin PBT developed in collaboration with DuPont for the housing. The material also offers good electrical insulation properties and withstands temperatures of over 165°C as well as oil and battery acid. The high rigidity and strength of the newly developed grade enable the housing, despite its thin-walled design, to resist the mechanical stresses imposed as it is fitted onto or detached from the electromechanical component. The housing material's easy-flowing melt cuts production costs and there are no contact-damaging gas emissions from the material. This silent relay has been used as a standard part on the Ford Focus since June 2001.

Another immensely important development is the use of sensors in aggressive environments, such as those used on brakes that come into direct contact with road salt, oils and moisture. To ensure that the sensor housings have extremely good dimensional stability and retain their mechanical properties as fully as possible under these adverse conditions, DuPont has developed Crastin PBT HR. This grade has high resistance to hydrolysis and complies with the requirements of USCAR III, while also providing the excellent electrical insulating properties typical of these thermoplastic polyesters.

Quietly operating, low-wear materials for drives and bearings
For tightly packed components in power-operated drives and positioning elements, DuPont has developed special polyacetal grades that permit a high degree of miniaturisation. These include, for example, the Delrin Low Friction/Low wear grades, which are used for gearwheels in very small drives that operate lubricant-free for the entire service life of the vehicle without any significant wear or appreciable noise (an important requirement for use inside the passenger compartment).

The shafts of the engine shut-off flaps produced by Kolbenschmidt Pierburg operate virtually wear-free in bushings made from abrasion-, corrosion- and heat-resistant Vespel from DuPont. These flaps, which have to endure heat, mechanical forces and corrosive media and are used, for example, in the exhaust gas re-circulation systems for V6 diesel engines produced by the Volkswagen group, interrupt the air supply to the engine when it is switched off and so prevent after-burning. The Vespel bushings, despite small bearing surfaces, can cope with the oscillating mechanical stresses and withstand temperatures of up to around 200 °C continuously as well as corrosive environments such as the sulphuric acid contained in the exhaust gas. DuPont supplies the Vespel bushings to Kolbenschmidt Pierburg as finished parts made to tight tolerances.

Vespel parts and shapes in the form of rollers, bushings, valve seats, seals, piston rings or bearings, can provide solutions for a wide range of difficult design challenges. They are particularly suitable for applications in which a combination of high-temperature resistance, low wear and good mechanical properties is required. In such applications, they increase the reliability of machinery and equipment and can reduce the cost of warranty services, maintenance and repair.

Fluorpolymers for special requirements
When the application requires advanced electrical insulating properties, or maximum durability in extreme temperatures, harsh chemicals and abrasion, environments that destroy nearly all other thermoplastics and elastomers, DuPont Teflon and Tefzel fluoropolymers can provide an adequate answer. And if friction is the problem, Teflon provides the smooth solution for many non- and minimally lubricated mechanical systems. So typical automotive applications of Teflon and Tefzel include powertrain, engine, and brake systems as well as control cable liners and other electrical components, opening the gate for innovative developments.

Cross-disciplinary collaboration pushes back existing limits
DuPont engineering polymers are continually showing that their performance potential is by no means exhausted yet. To drive forward new developments, DuPont has concentrated specialist knowledge in 'competence centres', e.g. for NVH (Noise, Vibration and Harshness, overcoming acoustic and mechanical vibrations), blow moulding of complex parts, laser welding and gas- and water-assist injection moulding In cross-disciplinary collaboration with other divisions of the company, solutions for complex technical challenges are developed all over the world, while basic research partnerships have been established with research institutes throughout Europe and beyond.
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