|Low wear, low friction injection moulded bushings, made of DuPont Vespel TP polyimide, are used to precisely guide the operating rods of EGR poppet valves manufactured by Wahler. Click to enlarge.|
In another example of the increasing use of plastics in the powertrain (earlier post), Gustav Wahler GmbH & Co. KG of Esslingen (Germany) uses bushings made of DuPont Vespel TP polyimide to position and guide the cylindrical operating rods of EGR slip-in valves (EGR = Exhaust Gas Recirculation). These valves are used in four and six cylinder stratified-charged gasoline engines.
Located behind the precatalytic converter and operating at temperatures of up to 220 °C (428 °F), they regulate the quantity of exhaust gases recirculated into the combustion air to reduce NOx emissions. It is during this process that the bushings, made of Vespel TP polyimide, which combines excellent friction and wear performance with a very low thermal expansion coefficient, support the consistently smooth operation of the valves, including their fast and precise opening and closing.
Compared to equivalent components made of metal, the injection moulded Vespel parts provide significant long-term benefits, according to DuPont. They are chemically inert to the residues contained within exhaust gases, whilst no contact corrosion can occur because polyimide is not electrically conductive.
Additional benefits include their significantly lower weight—in part a consequence of the high performance polymer’s considerably lower density than brass or steel. Also the high stiffness and tensile strength of Vespel TP, combined with its high level of formability by injection moulding, allows reduced wall thicknesses for additional design freedom.
Wahler worked closely with DuPont from early on in the design phase for these valves, versions of which are also used in numerous engines from other OEMs.
The mutual development work with DuPont proved to be very efficient, because in this case the material producer is also the supplier of the moulded parts. Because the optimisation of material and design was conducted in collaboration, we were able to fulfil our requirements for the bushing very quickly. Moreover, DuPont took over simulation of the relatively long but thin-walled moulded part’s behaviour when under servicing loads as well as across a broad temperature range from a cold start to full throttle. This helped us to optimise material usage to such an extent that we only require very little installation space, and considerably less than that required for a metal bearing.
—Martin Barth, Development Engineer for gasoline EGR in the valve business unit at Wahler
Wahler worked with DuPont in specifying Vespel TP, principally because this material enables the cost-efficient and precise injection moulding of the bushings in a single shot, without the need for post-machining.
Despite its thermoplastic properties, Vespel TP offers the required levels of temperature resistance and dimensional stability. As a result, tight tolerances are retained across a range of operating conditions and even after more than 3.6 million closing and opening procedures. It provides good acoustic attenuation properties, helping to reduce noise emission from the valves. The low levels of friction on the surface help prevent the formation of grime deposits.
Vespel. Polyimides have been a key class of polymers for DuPont since the 1960s, when Kapton film and Vespel parts and shapes were first commercialized. Based on the selection of the starting materials (monomers), the additives used, and methods of processing, the properties of polyimides can be tailored.Over the years, these tools have been used to expand the Vespel product portfolio from a single polyimide to five different families of products, including Vespel S, TP, ASB, CP and CR.
The Vespel TP polymers are processable as a thermoplastic and are supplied by DuPont as a finished part. They offer a high degree of design freedom, making them highly suitable for the production of moulded parts with enhanced functionality. Vespel TP polymers exhibit high levels of wear, chemical and temperature resistance, making them suitable for the replacement of metal or ceramic parts.
The broad portfolio of TP grades means that a wide range of application specific requirements can be met. The TP grade specified by Wahler for the EGR valve guides is characterised by its very good mechanical properties, its high creep resistance at elevated temperatures and its particularly high impact resistance, which facilitates their press fitting in the bushing carrier.
In April 2009 DuPont introduced Vespel SP-2515 polyimide parts, which can improve dimensional control, reduce component weight and drive cost out in fluid- and motion-control systems. Parts made from Vespel SP-2515 demonstrate a low coefficient of thermal expansion to better match metal, excellent wear properties against aluminum and a low coefficient of friction, all of which can help improve dimensional control, reduce weight and increase component efficiency for lower total system cost.
Parts made of Vespel SP-2515 offer high thermal conductivity—more than 4 times higher than parts made of Vespel SP-21—o help remove heat from critical bearing surfaces, which can lead to cooler running parts and higher PV (pressure x velocity) limits. DuPont Vespel SP-2515 is initially targeted for the global automotive and commercial vehicle industry.
Commercialized in seal rings, which demand low friction, extreme tolerance control over a broad temperature range, and long life, Vespel SP-2515 has demonstrated more consistent leakage control than cast iron and other polymeric alternatives. The ability to ‘fine tune’ leakage rates over a broader temperature range helps deliver an efficient and smooth operating transmission and can also lead to a smaller oil pump, which helps reduce parasitic energy losses.
DuPont Vespel SP-2515 (DuPont 2009)