Capturing images at up to 60 frames per second, the Siemens SIMATIC HawkEye 1600T is no ordinary camera. With its sophisticated onboard computer, it can verify that products have been assembled, labeled or packaged properly in a variety of production-line/quality-control applications.
During production, Tom Driscoll, the hardware-engineering manager at the Siemens' plant in Nashua, NH, realized the camera's optics and circuitry were going to be ready weeks before the die-cast aluminum housings. Die casting was proving too slow, he decided, and machining each part would be too expensive.
Driscoll then discovered Graphicast Inc., a single-source contract manufacturer with a rapid prototyping and casting process especially suited for low- to medium-volume production runs of 300 to 20,000 parts.
Graphicast promised to have the first samples ready in six weeks, less than half the lead time for die-cast parts. The shop 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.
Primarily electrical engineers, Driscoll and his colleagues needed some mechanical engineering advice from specialists at Graphicast to optimize the housing design for the casting process. “We were able to really leverage their design expertise,” Driscoll recalls. The result of the collaboration is a case that seals out dust, withstands operating temperatures of 32 to 122F and is certified watertight for submersions (such as production-line washdowns) of up to 30 minutes.
The graphite/ZA-12 process is a hedge against uncertainty. Because a graphite mold can be modified quickly and economically, it provides manufacturers with much more flexibility in debugging or improving products while still controlling costs — a major advantage over traditional casting methods. Graphite costs much less than tool steel, it requires no heat treating, and its exceptional machinability dramatically shortens the moldmaking phase. And, because a graphite mold will not warp or corrode, it is reusable.
For Siemens' production runs of 1,000 HawkEye housings per year, Graphicast's non-recurring tooling price was roughly one-fourth the cost of tooling for traditional die-cast aluminum parts. The price per housing was only a tenth that of machining the parts from stock.
Graphicast obtains the best possible casting results by using proprietary automated machines that fill each mold from the bottom up. This minimizes the turbulence of the molten metal, thereby greatly reducing porosity. Using a process controller to simultaneously control fill rate, cycle time and temperature, these machines yield parts of exceptional quality and repeatability at a relatively low cost per part.
Equally important are the attributes of the ZA-12 alloy, a spark-proof, nonmagnetic metal that is ideal for electronic shielding in hazardous environments. ZA-12 parts require no heat treating, and their typical surface finishes of 63 microinches or less are better than finishes from other casting processes. For non-castable features, ZA-12 is machined more easily than cast iron or aluminum.
For quality control, Graphicast technicians measured the critical dimensions of every part and inspected a sampling of the parts more thoroughly with a coordinate measuring machine (CMM). Driscoll's team had its first-article samples, plus 50 more pre-production parts, only four weeks after issuing the purchase order. The housing halves were machined and chromated. They were then painted, except for some masked areas where wires would enter the unit. After painting, “reference designators” — symbols that label sockets for connector cables — were screen-printed onto each casting. These additional steps took 10 more days, and the samples were complete — just shy of the six weeks Graphicast promised.
“We've been very happy with the quality of the housings,” says Driscoll. “Graphicast really helped us meet our production schedule and budget.”