DN Staff

August 26, 2010

4 Min Read
Rapid Prototyping Facilitates Rapid Casting

Because the high-precision parts used in communications,healthcare and industrial products must be ready according to assemblers' andmanufacturers' schedules, time-to-market issues have become increasinglysensitive for product and part designers. To help with these issues, moredesigners are looking into a sometimes overlooked casting process that canprovide rapid turnaround, especially for medium-volume production runs (partquantities ranging from 200 to 20,000).

The crux of this process involves the use of graphite moldsto produce parts from ZA-12, a zinc-aluminum alloy that is considered harder,stronger and more durable than aluminum, brass, bronze or plastic. Anexperienced supplier using this process with single-source productioncapabilities including in-house design, rapid prototyping, tooling and moldproduction, casting and machining, can have a typical turnaround time fromfinished CAD design to production samples in four to six weeks.

Rapid Prototyping Facilitates Rapid Casting

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Acquisition& Tooling Costs

For a typical medium-volume production run, the cost tomachine each part from scratch is too high, yet the quantity is often too lowfor high-volume casting methods to be cost-effective. Based on totalacquisition cost (cost-per-part times volume plus tooling costs), the graphitemold/ZA-12 casting process can be a lower-risk alternative to CNC machining,die casting, sand casting and investment casting. Furthermore, the accuracy andsurface finish of ZA-12 parts often eliminate additional finishing stepsrequired with other casting techniques, resulting in a lower total cost perpart.

Tooling costs for the graphite mold/ZA-12 process are muchlower than for die casting or injection molding because graphite costs lessthan tool steel and requires no heat treating. Graphite's exceptional machinabilitydramatically shortens the mold-making phase - a graphite mold can often beproduced faster than a die-casting mold, and for about one-fifth the cost. Andbecause a graphite mold will not warp or corrode, it can be stored indefinitelyand reused.

The best casting results with ZA-12 are obtained withautomated machines that fill each mold from the bottom up to minimize theturbulence of molten metal within the mold, thereby greatly reducing porosity. Processcontrollers should be used to simultaneously control fill rate, cycle time andtemperature, further maximizing the density of the castings.

Alloy Attributes

ZA-12 alloy castings have adensity approximately the same as cast iron and can include contours,variations in surface elevations, holes and other precise features. Surfacefinishes of 63 microinches or less are typical - better than finishes fromother casting processes. Although ZA-12 castings have a bright,corrosion-resistant finish that requires no coating or other preparation, theycan be chromated, plated, painted, powder-coated, or finished withelectro-coated acrylic or epoxy as necessary.

In many cases, ZA-12 parts require little or no machining,but for non-castable features, ZA-12 can be machined like brass or bronze.

Scalable toDemand

Because graphite is machined easily, graphite molds can be modifiedquickly and at relatively low cost, allowing a higher degree of flexibility indebugging or improving products while still controlling cost, which representsan advantage over conventional casting methods. This aspect is increasinglycritical to part designers considering that so many parts must be redesignedafter a short initial production run for reasons varying from a part not performingas expected, a competitor introducing a product with enhanced technology andadditional features, or engineers simply finding a better way to build thedevice.

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Customers who anticipate high-volume production of a part shouldrealize that if the part is redesigned for any reason, a high-volume processsuch as die casting no longer makes financial sense. With a graphite mold, if forecastedend-product sales do not materialize, the process yields a lower total cost perpart. On the other hand, if sales exceed forecasts, the process can be easilyscaled up to handle increased volumes.

Economically, the per-part cost of theGraphite/ZA-12 process is less than other casting methodsonly for production runs of approximately 200 to 20,000 parts.
Forruns below 200 parts, individual machining or machined sand castings might becheaper. For runs of more than 20,000 parts, die casting (from a steelmold) might be cheaper.

Design Assistance and Rapid Prototyping

Use of advanced stereo lithography to produce 3-D plastermodels of a part in a matter of hours will facilitate design and debugging ofany design destined for graphite/ZA-12 castings. Making multiple copies allowsthe model to be reviewed simultaneously by the customer. To resolve potentialproblems prior to moldmaking, sections of a model can be color-coded toindicate where modifications (draft, radii, etc.) are needed.

Any graphite/ZA-12 casting house with rapid manufacturingcapability should also rely on parametric and associative 3-D solid modelingCAD programs. Design changes that result from a review of the plaster modelscan be made on the CAD system, thereby allowing additional models to beproduced overnight to verify that all changes have been made correctly. Whenthe ZA-12 caster receives a green light from the customer, mold-making can begin.

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