molded plastic is replacing metal for engine bushings that must withstand temperatures
up to 428F and more than 3.6 million opening and closing procedures.
supplier Gustav Wahler GmbH & Co. of Esslingen, Germany, uses bushings made of DuPont Vespel TP polyimide to position and guide the
cylindrical operating rods of exhaust gas recirculation slip-in valves. Used in four and six cylinder gasoline
engines, they regulate the quantity of exhaust gases recirculated into the
combustion air to reduce nitrogen oxide emissions.
Vespel is thermally resistant, it can carry loads at high temperatures with
very low creep way beyond the reach of most plastics.
thermoplastic properties, Vespel TP offers the required levels of temperature
resistance and dimensional stability," says Martin Barth, a development
engineer in the valve business unit at Wahler. "Because the optimization of
material and design was conducted in collaboration, we were able to fulfill our
requirements for the bushing very quickly. Moreover, DuPont took over
simulation of the relatively long but thin-walled molded part's behavior when
under servicing loads, as well as across a broad temperature range from a cold
start to full throttle. This helped us to optimize material usage to such an
extent that we only require very little installation space, and considerably
less than that required for a metal bearing."
of the Vespel parts compared to metal are numerous. They are chemically inert
to the residues contained within exhaust gases, and no contact corrosion can
occur because polyimide is not electrically conductive. They also weigh less
because of 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 molding, allows reduced wall thicknesses for
additional design freedom.
developed by DuPont in the 1960s and has always been supplied as a part
processed by a proprietary direct forming process or a basic shape through
isostatic molding. Shapes such as rods or sheet are often used for prototype
quantities because tooling is expensive. Parts are often machined to achieve a
developed new polyimide materials for injection molding in the 1990s. Injection
molding removes some of the costly geometric limitations of the previously used
processes. Potential molded-in features include axial tabs on thrust washers,
seal rings with molded in gaps and scarf joints.
molding; however, is not a cakewalk with Vespel TP, whose melt temperatures
vary from a low of about 725F to a high of about 800F. Operating at these
temperatures can be dangerous if proper safety precautions are not followed.
DuPont produces parts on appropriately equipped molding machines. DuPont would
not provide any other details on the injection machines used, nor on their
numbers or location. It's believed that DuPont uses contract manufacturers to
make the parts in the U.S.,
Europe and Asia.
issue with Vespel is its high price. DuPont would not provide any examples of
list prices. The price of a sheet with a thickness of 0.125 inch sold by a
wholesale distributor is $45 per square inch.
The TP grade
specified by Wahler for the valve guides is characterized 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. 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.
properties of polyimides can be tailored based on the selection of the starting
materials (monomers), the additives used and methods of processing.
Wahler employs 1,650 people at its headquarters in Germany,
as well as at its sites in Slovakia,
Brazil and India and representative offices in the U.S. and China. Annual sales are about β¬250 million ($349 million). Wahler
customers include engine and vehicle manufacturers, for both the car and
commercial vehicle sectors, from across the world.
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