Molded Plastic Replaces Metal for High-Heat Engine Bushings

February 2, 2010

3 Min Read
Molded Plastic Replaces Metal for High-Heat Engine Bushings

Injectionmolded plastic is replacing metal for engine bushings that must withstand temperaturesup to 428F and more than 3.6 million opening and closing procedures.

Autosupplier Gustav Wahler GmbH & Co. of Esslingen, Germany, uses bushings made of DuPont Vespel TP polyimide to position and guide thecylindrical operating rods of exhaust gas recirculation slip-in valves.  Used in four and six cylinder gasolineengines, they regulate the quantity of exhaust gases recirculated into thecombustion air to reduce nitrogen oxide emissions.

BecauseVespel is thermally resistant, it can carry loads at high temperatures withvery low creep way beyond the reach of most plastics.

Reduced Space

"Despite itsthermoplastic properties, Vespel TP offers the required levels of temperatureresistance and dimensional stability," says Martin Barth, a developmentengineer in the valve business unit at Wahler. "Because the optimization ofmaterial and design was conducted in collaboration, we were able to fulfill ourrequirements for the bushing very quickly. Moreover, DuPont took oversimulation of the relatively long but thin-walled molded part's behavior whenunder servicing loads, as well as across a broad temperature range from a coldstart to full throttle. This helped us to optimize material usage to such anextent that we only require very little installation space, and considerablyless than that required for a metal bearing."

The benefitsof the Vespel parts compared to metal are numerous. They are chemically inertto the residues contained within exhaust gases, and no contact corrosion canoccur because polyimide is not electrically conductive. They also weigh lessbecause of considerably lower density than brass or steel. Also the highstiffness and tensile strength of Vespel TP, combined with its high level offormability by injection molding, allows reduced wall thicknesses foradditional design freedom.

Vespel wasdeveloped by DuPont in the 1960s and has always been supplied as a partprocessed by a proprietary direct forming process or a basic shape throughisostatic molding. Shapes such as rods or sheet are often used for prototypequantities because tooling is expensive. Parts are often machined to achieve afinal shape.

DuPontdeveloped new polyimide materials for injection molding in the 1990s. Injectionmolding removes some of the costly geometric limitations of the previously usedprocesses. Potential molded-in features include axial tabs on thrust washers,seal rings with molded in gaps and scarf joints.

Injectionmolding; however, is not a cakewalk with Vespel TP, whose melt temperaturesvary from a low of about 725F to a high of about 800F. Operating at thesetemperatures can be dangerous if proper safety precautions are not followed.DuPont produces parts on appropriately equipped molding machines. DuPont wouldnot provide any other details on the injection machines used, nor on theirnumbers or location. It's believed that DuPont uses contract manufacturers tomake the parts in the U.S.,Europe and Asia.

The otherissue with Vespel is its high price. DuPont would not provide any examples oflist prices. The price of a sheet with a thickness of 0.125 inch sold by awholesale distributor is $45 per square inch.

The TP gradespecified by Wahler for the valve guides is characterized by its very goodmechanical properties, its high creep resistance at elevated temperatures andits particularly high impact resistance, which facilitates their press fittingin the bushing carrier. It provides good acoustic attenuation properties,helping to reduce noise emission from the valves. The low levels of friction onthe surface help prevent the formation of grime deposits.

Theproperties of polyimides can be tailored based on the selection of the startingmaterials (monomers), the additives used and methods of processing. 

GustavWahler 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). Wahlercustomers include engine and vehicle manufacturers, for both the car andcommercial vehicle sectors, from across the world.

Low wear, low friction injection molded bushings are used to precisely guide the operating rods of poppet valves. Photo:  DuPont

Molded Plastic Replaces Metal for High-Heat Engine Bushings A

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