Thermostat Housing

PPA material provides chemical resistance, manufacturing economy in conversion from aluminum

Engineers at Chrysler Group selected a grade of AMODEL polyphthalamide (PPA) from Solvay Advanced Polymers, Alpharetta, GA, for the thermostat housing on the 2.4-liter, 4-cylinder engine that is standard on rear-wheel-drive versions of the Jeep Liberty SUV. The housing is mounted in the traditional position on the cylinder head, positioning the thermostat in the hot coolant stream as it exits the engine.

In the engine’s original front-wheel-drive configuration, its thermostat housing was machined from aluminum integral with the intake manifold. When the engine was adapted for a rear-drive configuration, the decision was made to give the new design a plastic intake manifold and a separate thermostat housing . AMODEL AS-1933 HS was selected for the new housing.

“We chose the AMODEL material because of its properties,” says Dipan Patel, senior engineer for the RWD engine group. He points out that the engine uses HOAT-technology, long-life coolant, which has different additives than regular glycol-based coolants that tend to wear away other plastics. However, 33% glass-filled AS-1933 HS features proprietary technology for improved chemical resistance. “This grade is more resistant to the coolant than Nylon 6/6,” says Patel. Test results after immersion in 50/50 glycol/water mixture for 1000 hours at 275°F show tensile strength of 11,000 kpsi, tensile modulus of 1100 kpsi and Izod impact strength of 1 ft-lb/in.

AS-1933 HS was also chosen for its temperature performance. Patel notes that the housing needs to maintain its strength and ductility from -20° to 240°F, with a normal maximum operating temperature of 225°F. With a heat-deflection temperature of 530°F, the material’s performance envelope easily accommodates this operating temperature range. This PPA grade was developed to exceed the automotive industry’s performance requirements for polymeric materials exposed to antifreeze at 226°F, even when tested at 275°F. Other applications have included heater core endcaps, heater hose connectors, and water inlets, outlets and valves.

Where a cast aluminum part would need to be machined to its final configuration, the new thermostat housing is injection-molded as a net shape. Brass inserts for attachment bolts are pressed into the housing’s flange, and threaded steel inserts are molded into the part for attaching a stamped-steel water outlet. The superior mechanical properties of AS-1933 HS at elevated temperatures result in excellent torque retention once the housing is bolted to the cylinder head.

The part is molded by Miniature Precision Components at its facility in Walworth, WI. “Solvay provided technical support and worked with MPC to optimize the molding process,” says Patel, “helping to establish a stable processing window that consistently results in good-quality parts.” Since the resins are hygroscopic and are recommended to contain less than 0.15% moisture when molded, they are shipped in moisture-proof bags. Molding wet resin may result in brittle parts (because of molecular weight reduction), splay marks or nozzle drool. Excessively wet resin is characterized by a foamy extrudate. A mold temperature of 275°F is recommended to ensure full crystallinity in the typical molded part. High crystallinity results in optimum mechanical properties, excellent dimensional stability and good surface appearance. The use of lower mold temperatures can produce parts with lower crystalinity and, consequently, optimal performance may not be achieved. AMODEL resin crystallizes rapidly, leading to faster molding cycles in thick or thin sections, and therefore lower total-piece costs, a significant advantage over materials that require longer molding cycles.

—SG