Thermostat Housing > BMW CONVERTS TO Ryton® PPS FOR CAR ENGINE THERMOSTAT HOLDER
Release 2/28/1996 BMW CONVERTS TO Ryton® PPS FOR CAR ENGINE THERMOSTAT HOLDER
MUNICH, Germany -Bavarian Motor Works (BMW) recently switched to Ryton® polyphenylene sulfide (PPS) for injection molding is its all-in-one car engine thermostat holder assembly. Glass-and-mineral-filled Ryton® BR83B PPS was selected to replace polyamide (nylon) because of Ryton® PPS's ability to withstand ethylene glycol-based antifreeze at high operating temperatures.
The change to Ryton® PPS came when the thermostat assembly was redesigned in 1991. The redesigned part is used in the BMW four-cylinder, two-valve M43 engine. In full commercial production since September 1993, between 600 and 800 units are made daily.
Bavarian Motor Works Gerhard Beier shows where the new, all-in-one engine thermostat assembly (main photo) connects to the engine block. Gerhard and his team created the part, molded from Ryton® PPS, after parts molded from other materials had failed. The Ryton® PPS part is used in the BMW M43 engine.
BMW injection molded the part of polyamide before it was redesigned for automated insertion of the thermostat to reduce assembly time. The new design includes two inverted J-hooks to hold the metal thermostat. The two inverted J-hooks have opposite facing key-ways that allow the thermostat to be placed and then turned to be locked into position. The thermostat spring exerts up to 300 newtons of pressure, holding it firmly in place.
During testing at normal operating temperatures, glycol/water antifreeze moving through the thermostat assembly at approximately 275°F (135°C) caused the polyamide J-hooks of the new design prototypes to fail.
BMW designer, Gerhard Beier, found the nylon material softening at the J-hooks because it becomes hygroscopic in contact with the antifreeze mixture, severely denigrating the actual polymer chains.
"Polyamide easily picks up moisture," explained Beier. "Engine coolant destroyed the amide chain after only 150 hours for standard nylon and 250 hours for aromatic polyamides in our simulated under-the-hood test. We specified at least 500 hours.
"We had used polyamide for several years in the old design that didn't have the J-hooks. One end connected to the radiator; the other, with the thermostat inside, was bolted to the engine block," Beier said. "But we changed the way the thermostat is automatically inserted into the housing and added the hooks to reduce manufacturing time and cost.
"When we discovered that the J-hooks failed when we used polyamide, we molded some test parts from Ryton® PPS using the polyamide prototype mold, low mold temperature and standard Ryton® PPS processing conditions."
"When our first Ryton® PPS prototypes achieved 1000 hours of rated use life without failing, ten times that of nylon 6/6, we stopped the test. And this was not what I would call well-molded Ryton® PPS. It was just a quick test to see if our problem could be solved by using Ryton® PPS. It truly surpassed my expectations," Beier exclaimed.
Ryton® PPS has earned a good reputation as an engineering resin suitable for many automotive applications since its commercial introduction by Phillips Petroleum Company in 1973. Strength, chemical and thermal resistance, long-term dimensional stability, and inherent flame retardance are some of the properties that attract designers.
For the first test prototype, Beier selected Ryton® R-4XT PPS, a toughened glass-filled compound. "We purposely over-engineered the material selection at this point," he explained. "We have now switched to Ryton® BR83B PPS, and are testing Ryton® BR111 PPS."
Ryton® BR83B PPS, a high strength glass-and-mineral filled compound, holds up well in mechanical applications, with a tensile strength of 21.0 ksi, resistance to temperature of more than 500 degrees F, a density of 1.80 g/cc and very low water absorption.
Ryton® BR111 PPS is a new grade of glass-and-mineral filled polyphenylene sulfide which improves on the inherent qualities of Ryton® BR83B PPS with added creep resistance and high torque retention.
Beier said that the original thermostat holder, developed to save space in the engine compartment, was made from aluminum in 1989, then changed to polyamide. The cost saving of 30 percent and weight saving of 40 percent were obtained when using Ryton® PPS compared to the former use of aluminum.
The new assembly looks like a smoker's pipe with the bowl holding the thermostat. The J-hooks extend upwards from this bowl. Precise mold tolerances possible with Ryton® PPS permit the thermostat assembly to operate without leaks. Design specifications require that the holder prevents leakage rates greater than 50 liters an hour under 0.5 bars of pressure at room temperature. The force of the thermostat spring and the smooth bowl edge provided by Ryton® PPS met this goal.
Beier's next cost-cutting step is to further reduce the thermostat holder's wall thickness to 2 millimeters from 2.5 millimeters. "This should not be a problem due to the strength of Ryton® PPS," he said.
BMW engineers also plan to make a dual intake thermostat holder with Ryton® PPS for six-cylinder engines in 1997 car models.
MUNICH, Germany -