Materials

10-2001

Design Application

Plastics shine in new advances

Tom Shelley reports on recent advances in plastics and their uses, including plastic silencers for car exhausts and materials that glow all night long

Despite the maturity of the base technologies, plastics continue to advance, both in terms of stretching the capabilities of the materials and using them in ever more ingenious ways.

Two of the most striking, in Eureka's opinion, are a way of using plastics to greatly improve the corrosion resistance of car exhausts while still maintaining their ability to survive gases at high temperatures, and a plastic torch that can glow visibly in the dark for up to 20 hours.

GloTorch

The 'GloTorch' is the brainchild of Stephen Lambert and Craig Baillie of Jameson Technologies, based in Hereford. Their idea was to make a plastic-bodied torch that could readily be located in the dark.

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Plastic additive materials that depend on radioactivity to glow in the dark are very much out of favour, and the developers soon found there was nothing else commercially available with a visible afterglow of more than an hour or two. So, in conjunction with Warwick University, they invented something new. They call their new family of substances 'Dual Glo', because they are excited by both ultraviolet and blue light. Exposure to bright sunlight for 20 seconds is sufficient to charge the material for 12 hours. At the end of this period, the torch body maintains a luminosity of at least 0.3mcd/sq m, the DIN 67510 Part 4:1996 standard for minimum light output to be clearly discernible to the human eye in the dark. A prolonged test by Eureka shows that the material does exactly what it is claimed to do, with no sign of deterioration over several months (more information at http://perso.libertysurf.co.uk/739415/ ).

The GloTorch is available in fluorescent green but the team has now developed similar substances that afterglow in red, white and blue. As well as being a boon to campers, the substance looks to be the answer to the search for a material that will maintain glow in internal car door handles. We understand the material will allow conformance with a relevant, proposed European Union Directive, which until now has not been enforced because of the lack of any way of meeting its requirements. Both the additive materials and the application in the torch are protected by patent applications.

Poly-Zhaust

If the torch is a shining example of what can be done for consumer products, the development of the 'Poly-Zhaust' is an astonishing implementation of plastics in engineering.

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According to DuPont, the idea is the brainchild of South African engineer Fanie Ferreira. Although the idea initially met with widespread scepticism, Ferreira was able to convince Bevan Davis, managing director of Conver-Tek in Germiston near Johannesberg, that the idea was worth pursuing. As a result, they applied for patents and set up a new company, Poly-Zhaust, to assemble products under licence.

The concept incorporates a stainless-steel inner tube encased in a reinforced nylon silencer shell filled with Owens Corning 'Silentex' continuous glass fibres. A special grade of glass reinforced nylon 66 was developed for the shell in partnership with Plastamid, another South African company. The crucial bushings between the steel tubing and nylon shell are made of DuPont's 'Teflon' PTFE product. It was found to be the only material which had the necessary temperature resistance, a low enough coefficient of friction to allow repeated expansion and contraction movements between the steel and nylon, heat barrier properties, machinability and the ability to survive for the 15-year design life.

Tests have now taken place over three years on over 1,000 cars, with engine sizes from 1.3 to 2.5 litres in Europe, Australia, Canada, Iceland, Namibia, South Africa and the United States.

Bevan Davis has said: "Automotive engineers scoffed at the idea because they did not believe that the polymers used in the new development could withstand the heat of a car exhaust system. Conventional exhaust system testing is carried out on static test rigs where heat profiles of 900degC can be reached due to lack of airflow cooling. The results fully support our claims. They demonstrated that the first 'plastic' exhaust system is capable of withstanding temperature extremes from -40degC to 300degC. Fuel savings of between 5 to 10% were recorded on the 1.3 litre cars under test due to the aerodynamic free flow design of the unit, and savings with diesel engines were even more noticeable. And, because the silencer unit is up to 40% smaller in volume than conventional systems, it is not only much lighter, but also intrudes less into boot/petrol/tank/spare wheel space". More information at www.dupont.com and at www.teflon.com .

On the other side of the car engine, air intake manifolds are now being made by a new process. Mark IV Automotive in Orbey, France, is making the manifolds for the US manufactured Ford Ranger and Explorer SUVs. Instead of using lost core technology, two shells of the manifold are produced at the same time on one tool using standard injection moulding principles. The two parts are then assembled by vibration welding.

When assembled, the manifold, coloured black, weighs 3.1kg. Moulded from Rhodia's Technyl A218V35, 35% glass-fibre reinforced polyamide, the finished product is significantly cheaper than its lost core made predecessor. Cost savings are achieved not only by the improvement in process but also by innovations in design. The manifold now incorporates location fixtures for a number of additional components, including the throttle body, the idle valve, the positive crankcase ventilation and the exhaust gas recycling valve, speeding up the subsequent assembly of the engine. Despite the temperature of the exhaust gases, which reach 550degC, there is no damage to the polyamide, even when the proportion of recycled gases combined with fresh air from the filter reaches 15%.

Both Mark IV Automotive and Rhodia Engineering Plastics have carried out many tests on the part. A Rhodia EGR test bench established the optimum temperature distribution between the manifold pipes to ensure better engine performance. Other evaluations included temperature ageing, fatigue measurements after exposure to electrodynamic shaking and pressure loss. At an early stage, the CAD engineers at Rhodia recommended that the stiffness of the final welded manifold should be increased by the incorporation of additional ribs. This enabled the manifold to more easily tolerate the dynamic burst pressure created when an engine backfires. The redesigned part can now withstand pressure of up to 10bar without damage.

Another application where plastics have ventured to supplant metals is in grippers used in wire processing machines for making springs for mattresses and furniture. The grippers are subjected to extreme wear and mechanical forces at elevated temperatures. In a new-generation design, machinery maker Spžhl of St Gallen, Switzerland, has replaced ceramic-coated aluminium grippers with parts injection moulded in a lubricated grade of Victrex PEEK polymer. Both designs perform satisfactorily, but the PEEK grippers moulded by Treff, also in Switzerland, are made in a single process step whereas the coated aluminium grippers require two. PEEK has a melting temperature of 340degC and a heat distortion temperature of 290degC.

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