Case Studies: Resolving Die-Casting Issues
July 30, 2007
Gentex Respiratory Products (Rancho Cucamonga, CA), designer, developer and manufacturer of aviation oxygen masks and positive pressure breathing systems, wanted to solve porosity issues and reduce scrap rates on a crucial aviation life-support component. The company’s CRU-60/P connector not only mates the pilot’s mask to the oxygen supply, it also has an emergency bailout elbow that connects with an oxygen bottle in the seat, so the pilot is able to breathe if ejected from the aircraft.
For such a critical application, the finished parts have tolerances as tight as 0.003 inch, and they are subjected to a battery of performance tests before being certified as field-ready.
“The CRU-60/P connector assembly undergoes a number of stringent performance tests before it’s sent into the field,” explains Gentex Respiratory Products Manufacturing Manager Bill Kirk. “If the die cast part is too porous, especially in critical areas such as the O-ring groove, it won’t meet the leakage requirements. In today’s high-G, high-altitude aircraft, excess porosity could deprive a pilot of essential oxygen in a combat or emergency situation.”
In an age when most die casters are avoiding short runs of complex parts in favor of larger runs with wider tolerances, Gentex has contracted with Alloy Die Casting (ADC, Buena Park, CA) to manufacture the connector, and has set a target scrap rate of less than 4% rejects after all testing is completed. “Many die cast facilities today focus on high-volume production,” observes ADC Design Engineer Gary Gray. “We’ve become very adept at designing die-cast tools and processes for intricate, close-tolerance parts that other shops might shy away from in low to medium volumes.”
Part DesignAs the industry standard for restraint harness connectors, the CRU-60/P meets all requirements of U.S. Air Force specification MIL-C-38271B. Typically secured to a dovetail mounting plate attached to an airman’s parachute harness, the design delivers positive locking and prevents flailing during an ejection.
The CRU-60/P connects to a conventional three-pin bayonet oxygen mask hose connector. The aircraft supply end incorporates an omni-directional quick disconnect to ensure correct alignment at all times, an advanced fitting design that also warns the pilot when a disconnection has occurred.
Tool DesignTo hold down the part cost, Gray elected to design a family die to cast both components of the connector because they must be assembled together to create a complete unit. “This two-cavity die is a custom-built tool,” he says. “Because of the angles involved on the components, standard tool holders would not work here. So we built a dedicated tool, with a total of five slides.
“We took great pains to design in excellent venting and precise temperature control,” explains Gray. “Because of the porosity restrictions, we really wanted to avoid trapped gas in the casting, so the gates and runners were sized and located with that objective in mind.”
During trial runs, ADC engineers took the first castings into the company’s extensive testing facility, where they used real-time X-rays to evaluate the porosity. “Secondary operations such as milling and grinding have a tendency to make porosity issues more apparent,” Gray adds. “The X-ray inspection helps us spot potential problems with trapped gases, so we can adjust the shot speed and other parameters to force trapped air to the overflow.”
The Gentex CRU-60/P connector is cast from A380 aluminum on a 250-ton Toshiba press, which has automated ladling and extraction, as well as air jets to cool the parts as the conveyor takes them to the trim press. Ingots of raw material are melted in a gas furnace at approximately 1,200 F, and a measured shot is injected at about 8,000 pounds per square inch, producing a gate velocity around 1,200 inches per second.
Alloy Die Casting, a Sanders Industries Co.
- In the die design, ADC engineers took great pains to include venting and precise temperature control to resolve porosity issues.
- ADC was contracted by Gentex to achieve a scrap rate of less than 4%.
- To reduce cost, ADC designed a family die to cast both components of the connector because they must be assembled together to create a complete unit.