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Graphite Mold Casting is a Better Solution All Around

July 10, 2011
A specialty machine manufacturer cuts cost and saves time with a new component design cast in zincaluminum alloy.
Graphicast uses a proprietary, low-turbulence/ automatic-fill process with graphite molds and a rateand temperature-controlled bottom-filling pouring technique.
The drill support for National Optronics’ 7E Edger machine was formerly assembled from four individually machined pieces (left). Now, Graphicast manufactures the support as a single casting (right), reducing both the cost and time of production.
Graphicast’s Low Turbulence Automatic™ casting machines control fill rate, cycle time, and temperature simultaneously. Because molds are filled from the bottom, these machines minimize the turbulence of the molten metal.
The 7E Edger trims plastic eyeglass lenses to fit different frame styles. In lenses meant for rimless glasses, it drills tiny screw holes. National Optronics worked with Graphicast Inc. to redesign and produce the machine’s drill support structure more quickly and economically as a permanent mold casting from graphite molds.

National Optronics is counting significant time and cost savings as a result of a novel casting process used to manufacture a critical component for one of the lensprocessing machines it builds. The machines are used by optical laboratories and retail stores like Walmart and LensCrafters to finish plastic eyeglass lenses. A part that was, until recently, assembled from four individually machined pieces is being produced more economically now as a single casting that is more rigid and more stable than the original component.

In 2008, as part of an ongoing effort to reduce manufacturing costs, buyer Andy Irvine and his National Optronics colleagues focused on a drill support being made in its machine shop at Charlottesville, VA. “There had to be room for improvement,” Irvine recalled. “To make just one of these steel parts, the shop was machining four sections separately and screwing them together. We figured there had to be a better way to make the part, perhaps even from another material.”

The part supports a drill inside the company’s 7E Edger, an automated 3-axis patternless edger with exceptional accuracy and speed. The machine uses patented dry-cutting technology to trim polycarbonate, Hi-Index, CR-39, and Trivex eyeglass lenses to fit a wide range of frame styles. For rimless eyeglasses, the 7E drills tiny screw holes in the lenses. Its user-friendly software interfaces with related equipment and stores lens programs for each customer; these are programs that allow fast, easy cutting of duplicate lenses if the customer orders a second pair of glasses.

Irvine and National Optronics engineer Steve Hovanic expressed their interest to Kirk Barrett, regional sales manager for Graphicast Inc., a single-source contract manufacturer in Jaffrey, NH. Graphicast has been making various parts for National Optronics for five years, using graphite molds and a low-turbulence process to cast parts from ZA-12, a zinc-aluminum alloy that it explains is harder, stronger, and more durable than aluminum, brass, bronze, or plastic.

Graphicast is a permanent mold caster that uses a proprietary, low-turbulence/automatic-fill process with graphite molds. This rate- and temperature-controlled bottom-filling LTA process produces castings with high density and low porosity, good surface finish, and very consistent dimensions. The foundry also offers in-house design services as well as tooling, net-shape casting, secondary machining, finishing, and CMM part inspection.

Barrett studied the drawings for the four-piece assembly, and suggested National Optronics produce a single casting to function in its place. Then, he quoted a competitive price, according to Irvine. He has a cost-control background as well as technical experience, so it was logical for him to coordinate efforts between National Optronics and Graphicast. After back-and-forth discussions, Graphicast and its customer agreed on a new component design optimized for the casting process.

Graphicast produced a prototype casting, then delivered the first actual drill supports in April 2009. Since then, National Optronics has ordered four to five production runs of the casting each year, with 50 or 60 parts per run.

According to Irvine, the new production method saves National Optronics about $11,000 a year as a result of lower costs for raw materials, machining, and assembly.

“The cast part is a drop-in replacement for a part that used to be quite time-consuming to make, plus it is more rigid, since it is all one piece,” said Irvine. “It’s a better solution all around … It is also a perfect example of how involving a knowledgeable supplier early in the design stage can pay dividends for the manufacturer.”

Graphicast’s casting process is especially well suited for annual production runs of 200 to 20,000 parts. For a typical medium-volume production run, the cost to machine each part from scratch is too high, yet the quantity is too low for high-volume manufacturing methods to be cost-effective. Based on total acquisition cost (cost-per-part times volume plus tooling costs), the graphite mold/ ZA-12 casting process is a lower-cost alternative to CNC machining, diecasting, sand casting, or investment casting, for mediumvolume production runs. Furthermore, the high accuracy and lustrous surface finish of ZA-12 parts virtually eliminate additional finishing steps required, for a much lower total cost per part.

Tooling costs for the graphite mold/ZA-12 process are much lower than for diecasting or injection molding because graphite costs far less than tool steel and requires no heat treating. Graphite’s exceptional machinability shortens the moldmaking phase, too. In fact, frequently a graphite mold can be produced weeks faster than a diecasting mold and at about one-fifth of the cost. Also, because a graphite mold will not warp or corrode, it can be stored indefinitely and reused. Typical turnaround time from finished CAD design to production samples is only four to six weeks.

Graphicast obtains the best possible casting results by using proprietary automated casting machines that fill each mold from the bottom. This minimizes turbulence in 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.

The attributes of the ZA-12 alloy are equally important. It has excellent castability at a relatively low temperature, and because it is spark-proof it can be used in hazardous environments. Since it cannot be magnetized, it is ideal for electronic shielding. ZA- 12 castings have a density approximately the same as cast iron and may include contours, variations in surface elevations, holes, and other precise features.

ZA-12 parts require no heat treating, and their typical surface finishes of 63 microinches or less are better than finishes from many other casting processes. In many cases, ZA-12 parts require little or no machining, but for non-castable features ZA-12 is machined as easily as brass or bronze and more easily than cast iron or aluminum. Although the drill support for the National Optronics 7E Edger needs some secondary machining after it comes out of the mold, it needs far less than was required by the previous method of production.

ZA-12 castings have a bright, corrosion-resistant finish that requires no coating or other preparation. They can be chromated, plated, painted, powder-coated, or finished with electro-coated acrylic or epoxy as necessary.

For OEMs like National Optronics, casting ZA-12 alloy with a graphite mold offers a way to limit the risks associated with product introductions or redesigns. Because a graphite mold can be modified quickly and economically, it provides manufacturers with more flexibility in debugging or improving products while still controlling costs, a major advantage over traditional casting methods. If a part is to be produced in high volumes, any redesign can incur significant costs if it also means redesigning molds for most casting processes.

With a graphite mold, less is at stake, and National Optronics’ financial risk was minimized by starting with a shorter run of ZA-12 parts. If forecasted end-product sales do not materialize, the Graphicast process still yields a low total cost per part at low production volumes. On the other hand, if sales exceed forecasts, the process can be easily scaled up to handle increased volumes.

“Graphicast has been great to work with,” said Irvine. “They were very helpful up front in the design effort, and they have provided valuable technical support at every stage in the process. On the purchasing and production side, working with them has been seamless, like clockwork: We simply place an order and forget it. We never have to worry about it again — they always ship on time.”