Advanced leak testing system secures diecaster's quality.
DuPage Die Casting Co., Niles, Illinois, is a 250-employee company shipping 9000 tons of castings per year. DuPage specializes in producing transmission valve bodies, differential clutch housings, extension housings for transmissions, air conditioning cylinders, engine-mounting brackets and fuel pump rings.
The firm is a Ford Motor Co. Q1 supplier, and a QE and Pentastar award-winning supplier to Chrysler Corp. "We were one of the first diecasters to win Ford's Q1 certification, seven years ago," said George O'Malley, DuPage's manufacturing manager. "Still, we have to prove every day that we're a quality supplier."
The 47-year old company offers a variety of value-added services, including CAD-based casting design and development, and JIT delivery. Its quality system is based on a rigorous SPC program in which each department is considered a "supplier" to the other, accountable for quality standards. One of the standard techniques at DuPage for assuring quality is leak testing its transmission extension housings - a step critical to ensure transmission oil won't leak from the housing once in service.
Leak Testing Evolution
These housings, cast in 380-alloy aluminum and weighing 4.6 lb, are about the size and shape of the orange cones used in road construction. In production at DuPage for several years, every single part sent to the customer must be leak tested. After being trimmed and shotblasted to remove flashing, burrs, sprue remnants and surface contaminants, the parts are brought off-line for leak testing.
"Initially, we used immersion (bubble) testing," O'Malley recalled. "The part was submerged in water and the operator counted the bubbles to determine if the part leaked, and to what degree. That test was reasonably accurate if the operator was alert, but it's a slow process and demands constant attention and a degree of skill to 'read' the bubbles.
"Also, parts get wet, which can delay shipment and even sometimes cause a quality problem," he said. "For example, aluminum can start to corrode if it's wet for an extended period, harming the integrity of the part."
DuPage next tried two automatic leak detectors based on differential pressure testing technology, which relied on a regulator to isolate the casting from the air supply. According to O'Malley, however, "There was no repeatability or reliability, and we found ourselves double and triple-testing. We had a 25-30% re-test rate, and still couldn't be sure if the parts were OK - we ended up scrapping a lot of good parts." Though better than the bubble testing, the method was still fairly time consuming and labor intensive, using two operators to test 500 parts per shift.
Settling on a System
DuPage finally turned to its current computerized leak detection system. Built around Intertech's computerized M-1000 test module with custom software tailored to DuPage's specific requirements, the system features two independent test channels and a computer display screen that automatically shows test results.
The tester uses differential pressure decay testing technology to check the housings to a leakage rate accept/reject level of 3 standard cubic centimeters per minute (SCCM) at 10 pounds per sq in. guage (psig). This procedure pressurizes both the tested part and a nonleaking reference volume reservoir from the same air supply line. Unlike older differential systems that use a regulator on the air source, the part and the reservoir are isolated from the air supply and each other by closing valves, assuring that test results won't he affected by line pressure fluctuations [ILLUSTRATION FOR FIGURE 1 OMITTED].
A transducer then reads the pressure in both the nonleaking reservoir and the test part, and any differential appearing between these pressures indicates a leak in the test part. Calculations automatically convert this changing differential into a measure of leakage rate.
In operation at DuPage, the operator positions the part on a 12-in. diameter circular rubber base, and presses two start buttons simultaneously with both hands. Clamps automatically secure the part to the base to seal the test cavity, and the test is completed in 14 seconds [ILLUSTRATION FOR FIGURE 2 OMITTED]. A new test cycle is begun every 50 seconds.
All test cycle steps proceed under computer control, requiring no operator judgment. The automatic seal check and calibration modes minimize setup time. During testing the monitor provides a real-time display of test pressure, rate of leakage, accept/reject status, as well as a running total of accepted and rejected parts [ILLUSTRATION FOR FIGURE 3 OMITTED]. When a part is tested, the unit automatically stamps a "1" or "2" on it, denoting on which station the test was performed and assuring the customer that every casting has been checked.
"The screen display tells us if the part is within the allowable leak range of 3 SCCM," O'Malley said. "It also feeds data back to us for SPC, allowing us to detect problems early in the production cycle and take quick corrective action."
The display also flags "low pressure faults" that indicate gross leakage (4 SCCM or more). "If we get three gross leakers," O'Malley said, "it tells us something has gone wrong in the casting process, and togo back and check it immediately."
The computer can store up to 100,000 test records per channel for monitor display or disk transfer. Mean, standard deviation and mean [+ or -]3 sigma can be calculated and displayed at any time, and all data can be printed out or downloaded to another computer for further analysis.
DuPage's new system has about a 5% retest rate and tests 750 pieces per shift using one operator. "Its accuracy is far better than both the other testing systems, and we've had a dramatic drop in scrap," O'Malley said.
"Maintenance is low, too," he added. "We're especially impressed with the robber seals that make the tester an airtight container. The edges of the castings are hard on the seals, but ours last three or four months. Between lower maintenance and one less operator, we justified the cost of the system in about a year.
"Diecasting itself is very fast - what we made yesterday, we ship today. This system helps us to be sure testing doesn't become a bottleneck and that what we're testing is good. After the tests, we just put tags on the parts and ship them."
Computerized leak testing has worked so well for DuPage that management is considering adding a similar system to test differential parts.
"This is a very competitive business," O'Malley said. "If our customers find one defect, they let us know; if they find two, we're in their plant explaining why. Many of our customers have their own die casting facilities, and if we don't measure up, they may do it themselves. We can't be satisfied with how good we are today."
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|Title Annotation:||DuPage DieCasting Co.|
|Date:||Oct 1, 1995|
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