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"Thinking outside of the box," has become a cliche, but for the Siemens Energy and Automation plant in Nashua, NH, it was a literal requirement. The box in question was the housing for the SIMATIC HawkEye 1600T machine vision camera it was developing. Normal casting or machining methods for producing the "box" was too expensive, so Siemens sought alternatives and found graphite casting using a zinc and aluminum alloy solved the problem.
Designed for quality-control applications, the camera captures images at up to 60 frames per second. With its on-board computer, it can verify that products have been assembled, labeled, or packaged properly in a variety of production applications.
Tom Driscoll, Siemens' hardware engineering manager in charge of designing and producing the HawkEye, said he realized the camera's optics and circuitry were going to be ready weeks before he could get camera housings. He said his dilemma was how to acquire the housing parts.
"The product introduction schedule was the number-one driver," Driscoll said. "We had started with a die-casting company and ran into scheduling problems. It was going to take 14 weeks to get aluminum castings for the camera housing. After consulting several machine shops, we learned that machining the housings from scratch would cost about $250 for each half, $500 for each case. We looked for another solution."
Die casting was too slow; machining was too expensive. After exploring alternatives, Driscoll found Graphicast, Jaffrey, NH, a casting and CNC machining facility that used graphite molds.
"They fit a lot of our requirements, especially our budget. They were fast and had competitive prices."
Better, Stronger, Faster
Graphicast's casting process is suited for low- to medium-annual production runs of 300 to 20,000 parts. The company uses graphite molds to cast parts from ZA-12, a zinc-aluminum alloy that is harder, stronger, and more durable than aluminum, brass, bronze, or plastic. The company offers single-source production capabilities, including in-house design, tooling, net shape casting, machining, and finishing. It offers typical turnaround times of four to six weeks from finished CAD files to production parts.
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Driscoll and his team required the camera have a durable housing. The circuitry within it generated more heat than earlier machine-vision cameras and would need a case capable of conducting and dispersing heat better than an earlier model's extruded-aluminum housing.
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The camera housing also needed a tight seal to withstand dust, humidity, and even production-line washdowns. The team developed a design with two wedge-shaped halves that join along a line that did not transect the holes for the camera's lens, connector ports, or status indicators at the front, back, and top of the housing.
The shape ruled out extrusion as a production method, leaving machining or casting as the alternative.
Driscoll and his colleagues, primarily electrical engineers at Siemens, needed mechanical-engineering advice to optimize the design for the Graphicast casting process.
"Discussing the design with the engineers at Graphicast helped," Driscoll said. "We leveraged their design expertise."
Half the Time
Engineers at Graphicast promised first sample delivery in six weeks, less than half the lead time for die-cast parts.
The result was a case that seals out dust, withstands operating temperatures of 32[degrees] to 122[degrees] F, and is certified watertight for submersions up to 30 minutes. The top and bottom halves are sealed together with an O-ring.
Tooling costs for the graphite mold/ZA-12 process were lower than those for die casting or injection molding. The graphite's properties make it suitable for moldmaking. Its overall stability--with an expansion coefficient less than steel--and low porosity offer molds that hold their shape when filled with molten metal.
Large runners ensure metal is available to fill the mold cavity as the part solidifies, minimizing shrinkage and porosity. Because a graphite mold does not warp or corrode, it has a long shelf life. Also, graphite costs less than tool steel, it requires no heat treating, and its machinability shortens moldmaking.
The graphite mold and ZA-12 casting process yields parts with accuracies and surface finishes better than sand casting and investment casting at a lower cost per part. Unlike sand and investment casting, in which molds are destroyed when the castings are extracted, graphite molds are reusable.
As with the hardened tool steel used for die-casting molds, a graphite mold is machined in two halves and reused. Graphicast uses its proprietary semi-automated machines to fill each mold from the bottom up. This minimizes pour turbulence within the mold, reducing porosity--air inclusions.
Using a process controller to simultaneously control fill rate, cycle time, and temperature, the machines maximize density and minimize porosity.
The ZA-12 alloy's density is about the same as cast iron with a low casting temperature. ZA-12 is spark-proof, so it can be used in hazardous environments. It is non-magnetic, making it useful for electronic shielding.
ZA-12 castings can be produced in volume, with critical-dimension tolerances of [+ or -] 0.003" per inch for the first inch and [+ or -] 0.001" per inch for additional inches. Parts do not require heat treating, and have surface finishes of 63 micro-inches or less. The alloy is as easy to machine as brass or bronze and easier than cast iron or aluminum. In many cases, parts made from the alloy require little or no secondary machining.
Although a ZA-12 casting has a bright, corrosion-resistant finish, it can be chromated, plated, painted, powder-coated, or finished with electro-coated acrylic or epoxy to mimic anodized aluminum.
Because a graphite mold is easy to create and relatively-inexpensive, it can be modified while controlling costs compared to the expense of traditional casting methods.
10 Percent Less than Machining
For Siemens' production runs of a thousand HawkEye housings per year, Graphicast's non-recurring tooling costs were roughly 25 percent that of aluminum die-cast parts. The cost per housing was 10 percent less than machining it.
The final designs for the camera's housing minimized the number and size of the gates--mold cavity inflow points. When a mold is opened and the part removed, these channels must be removed, resulting in rough areas along the edges of the part. To minimize machining, Graphicast designers placed these along edges that were scheduled for machining.
After machining, which included drilling holes for LED status indicator lights and threading holes for assembly screws, Graphicast technicians used a coordinate measuring machine to inspect sample parts.
When a mold is complete, some foundries cast and machine sample parts, then await production, pending customer approval. In contrast, Graphicast offers the option of forgoing samples and starting production runs with guaranteed compliance to the CAD model and tolerances. Another option it offers is providing two first-article samples and preproduction parts.
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The former option eliminates setup and breakdown time, as well as the expense to cast and machine two sample parts. The latter option lets customers save time and money by producing a desired number of pre-production parts for fit and function, beta testing, trade show participation, or other short-run needs.
Siemens took advantage of the waived sample program. Driscoll's team had its first-article samples as well as 50 more pre-production parts four weeks after issuing the purchase order.
The housing halves were machined, chromated, and painted, except for some masked areas where wires would enter the unit. After painting, reference designators--symbols instructing installers which connector cables go into the appropriate sockets--were screen-printed onto each casting. These processes took 10 days. Samples were delivered in less than six weeks, just as Graphicast had promised.
Siemens used the samples to assemble camera prototypes for testing.
"We were happy with the quality of the housings," Driscoll said. "Graphicast helped us achieve our goal of meeting our production schedule and budgets. They listened to our needs, and were responsive." Graphicast
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