When it comes to tooling costs, material helps determine cost, lead time, and the process itself. National Optronics, a Charlottesville, VA-based producer of high-precision machines for cutting and finishing plastic eyeglass lenses, knows this firsthand. The company's machines are used at optical laboratories and stores such as Walmart and LensCrafters and include blockers, tracers, edgers, and surface generators.

Recently, as part of an ongoing effort to reduce manufacturing costs, National Optronics focused on a drill support made at the Charlottesville plant for an edger that trims lenses to fit frame styles.

This effort led the company to consult with Jaffrey, NH-based Graphicast, Inc (graphicast.com), a contract manufacturer that makes various parts for National Optronics using graphite molds to cast parts from ZA-12 zinc-aluminum alloy. Graphicast suggested that the four-piece assembly be produced as a single ZA-12 casting, a lower-cost alternative to CNC machining, die casting, sand casting, and investment casting.

Tooling costs for the graphite mold/ZA-12 process are much lower than those for die casting or injection molding because graphite costs far less than tool steel. Graphite's machinability shortens the moldmaking phase; a graphite mold can often be produced weeks ahead of a die-casting mold, and for about one-fifth the cost. And because a graphite mold will not warp or corrode, it can be stored in­definitely and reused.

Further, the process provides the surface finish and accuracy necessary for elimi­nating additional finishing steps, thereby lowering total cost per part. ZA-12 parts require no heat treating, and their typical surface finishes of 63 micro-inches or less are better than finishes from other casting processes. ZA-12 castings can be produced in volume with critical-dimension tolerances of ±.003" per inch for the first inch and ±.001" per inch for additional inches.

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 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. National Optronics estimates that it is saving about $11,000 a year, resulting from 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," says National Optronics Buyer Andy Irvine. "It's a better solution all around."

Cost-effective response
Under the right conditions, a graphite mold can cast as many as 40,000 parts.

Molds are filled from the bottom, minimizing the turbulence of molten metal within the mold. By simultaneously controlling fill rate, cycle time, and temperature, density is maximized while porosity is minimized, en­suring part quality and re­peatability. Casting release agents, which can spoil a part’s surface finish, are not needed if the mold design has adequate draft (normally 2°) on every surface perpendicular to the parting plane.

The graphite mold/ZA-12 process also offers OEMs a hedge against expensive design modifications. Because graphite is machined eas­ily, graphite molds can be modified quickly and eco­nomically, providing manufacturers with much more flexibility in de­bugging or improving products while still controlling costs — a major advantage over tradi­tional casting methods.

If there's a chance that a part might be redesigned, a high-volume process, such as steel-mold die casting with its large up-front investment, doesn’t make financial sense. With a graphite mold, less is at stake; therefore, the financial risk is minimized by starting with a shorter run of ZA-12 parts.

Casting ZA-12 alloy with a graphite mold is a viable alternative to other production methods in a volatile marketplace, especially for part quantities from 200 to 20,000. In short, it is an effective response to today's demands for reduced costs and shorter lead times for precision metal parts.