Molded Plastic Replaces Metal for High-Heat Engine Bushings

Injection 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.

Auto 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.

Because Vespel is thermally resistant, it can carry loads at high temperatures with very low creep way beyond the reach of most plastics.

Reduced Space

"Despite its 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."

The benefits 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.

Vespel was 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 final shape.

DuPont 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.

Injection 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.

The other 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.

The properties of polyimides can be tailored based on the selection of the starting materials (monomers), the additives used and methods of processing. 

Gustav 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 €250 million ($349 million). Wahler customers include engine and vehicle manufacturers, for both the car and commercial vehicle sectors, from across the world.