Die casting plays role in cost-saving effort for control indicator

  • 04-Aug-2008 03:37 EDT
VIASAT .jpg

Consisting of five parts, the indicator assembly must maintain the close tolerances and smooth finish required to pass the customer’s 100% testing requirements.

While casting about for a way to grant the U.S. Department of Defense’s wish for cost savings associated with an avionics control indicator it was supplying, ViaSat Inc. of Carlsbad, CA, ended up close to home with a company called Alloy Die Casting (ADC) about an hour’s drive north.

ViaSat had been machining all five of the parts individually, maintaining the close tolerances and smooth finish required to pass the customer’s 100% testing requirements. The indicator assembly involves a heat sink; front panel or bezel; access door; and two door pivots. A major part of the challenge for ViaSat was that volumes for the assembly are low—about 2000 parts per year.

“At first, we were concerned that die casting wouldn’t be cost-effective because of the low volumes and high quality requirements,” ViaSat Design Engineer Lon Plourde said. “But ADC showed us how it could be done so efficiently that our investment would pay off after just 600 parts.”

Used in standard military airborne avionics, the assembly is subjected to harsh military environmental requirements and must withstand operating temperatures of -40°C to 55°C, as well as storage temperatures from -62°C to 85°C.  All parts are required to successfully resist 100% humidity and must survive difficult shock and crash loads according to MIL-STD-5400.

The repeatability of the die casting process proved to be an additional benefit for ViaSat. “Our customer is very sensitive to the aesthetics of these parts,” said Plourde. “When you die cast, the result is fairly predictable, and if you run into a problem, it’s typically corrected in the tool and it goes away.”

ViaSat’s Carlsbad location had never specified a die-cast part before, but that did not turn out to be a problem. Plourde and his team worked with ADC tool designers, who made suggestions for making minor changes to improve manufacturability and strength of the assembly while optimizing tool performance and durability.  “ADC provided intuitively sound advice throughout the process,” he said. “They laid out a detailed timetable for each stage, and they delivered on it.”

In addition to the cost and aesthetics, thermal performance was a prime concern. “The design had to dissipate enough heat energy to maintain component temperatures, and we found through testing that we could achieve the thermal properties we needed using A360 aluminum,” Plourde said. The heat sink is a complex design about 5 x 3 in with a series of 25 rectangular “windows” arranged in five columns. Cast on a Toshiba 250-ton press, the 2-in plunger delivers a gate velocity of about 1200 in/s for a 30-ms fill time. “The large gates on this tool help us get a quick fill without a lot of turbulence,” said ADC Design Engineer Gary Gray. “The relatively low inlet velocity also helps minimize tool erosion.”

Having a mass of 1.13 lb after casting, the parts require about five minutes of machining time, during which drilling and tapping takes place. There are nine helicoil inserts in the heat sink, so the cast parts have to be kept flat within +.010 in to accommodate them. “We used ejector pins at virtually every intersection, all the windows and corners, to ensure that the parts came out straight [and] with a minimum of torque that could lead to warping,” said Gray of the tool design.

The heat sink nestles inside the bezel, which measures about 5.75 x 3.35 in. Cast on a press the same size as the one used for the heat sink, the panel requires a 35-ms fill time, yielding about 60 of the 0.25-lb parts each hour.  Because the heat sink is designed to transfer thermal energy away from components inside the indicator and out through the panel, it must make complete surface contact with the bezel for maximum heat transfer. That means the same flatness tolerances had to apply.

“The bezel is the most cosmetic part of the five,” said Gray. “It gets quite a few secondary operations, including trimming, sanding, drilling, and tapping to accommodate switches, controls, and gauges. After hand cleaning, it also receives a chemical film coating for corrosion resistance, and then it’s selectively powder-coated, with key areas masked off for electrical contacts or other design considerations.”

The door is a much smaller part, measuring just 2.5 x <0.75 in and having a cast mass of less than 0.10 lb. With so little metal, the fill time is very short: about 20 ms even at a reduced gate velocity of 1100 in/s on the 135-ton press. “This part gets a small hole at either end,” Gray explained. “It’s just .068 inches in diameter, with a tolerance of +.005 in. That’s pretty small for casting in place with aluminum, but we didn’t want to machine such a small part any more than necessary. There’s also a pair of .025-in holes in the door design, which were just too small to cast. Those we have to drill.” Like the bezel, the door gets a chemical film after machining, then is masked, and is powder-coated.

The two door pivots are a matching pair, originally quoted in the same aluminum alloy as the other components. But Gray and his design team found that they could achieve the fine detail required in this small part by casting it in zinc instead, taking advantage of the material’s superior flow properties and ability to maintain close tolerances.

Because of the hole locations and small part size (<0.75 x 0.50 in and 2.5 oz), ADC casts the door pivots on a Techmire four-slide machine. The technique can produce very thin parts, often .020 in or less, and tolerances of +.001 in are not uncommon. The result is a tightly controlled, high-speed process that can yield thin-walled components so complex they would need to be assembled from two or more pieces if alternative casting methods were used, according to ADC. After casting, the door pivots are finished with chemical film, masking, and powder coating. ADC also provides final assembly of the door pivots, with springs and other components in place, ready for subsequent manufacturing steps in the ViaSat facility.

Alloy Die Casting, a Sanders Industries company, can furnish castings of surface area up to 600 in2, with engineering support, custom tool design and fabrication, CNC machining, and complete turnkey finishing.

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