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Rapid Prototyping Facilitates Rapid Casting

Article-Rapid Prototyping Facilitates Rapid Casting

Rapid Prototyping Facilitates Rapid Casting

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

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

Rapid Prototyping Facilitates Rapid Casting

Acquisition & Tooling Costs

For a typical medium-volume production run, the cost to machine each part from scratch is too high, yet the quantity is often too low for high-volume casting 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 can be a lower-risk alternative to CNC machining, die casting, sand casting and investment casting. Furthermore, the accuracy and surface finish of ZA-12 parts often eliminate additional finishing steps required with other casting techniques, resulting in a lower total cost per part.

Tooling costs for the graphite mold/ZA-12 process are much lower than for die casting or injection molding because graphite costs less than tool steel and requires no heat treating. Graphite's exceptional machinability dramatically shortens the mold-making phase - a graphite mold can often be produced faster than 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.

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

Alloy Attributes

ZA-12 alloy castings have a density approximately the same as cast iron and can include contours, variations in surface elevations, holes and other precise features. Surface finishes of 63 microinches or less are typical - better than finishes from other casting processes. 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 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 to Demand

Because graphite is machined easily, graphite molds can be modified quickly and at relatively low cost, allowing a higher degree of flexibility in debugging or improving products while still controlling cost, which represents an advantage over conventional casting methods. This aspect is increasingly critical to part designers considering that so many parts must be redesigned after a short initial production run for reasons varying from a part not performing as expected, a competitor introducing a product with enhanced technology and additional features, or engineers simply finding a better way to build the device.

Rapid Prototyping Facilitates Rapid Casting
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Customers who anticipate high-volume production of a part should realize that if the part is redesigned for any reason, a high-volume process such as die casting no longer makes financial sense. With a graphite mold, if forecasted end-product sales do not materialize, the process yields a lower total cost per part. On the other hand, if sales exceed forecasts, the process can be easily scaled up to handle increased volumes.

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

Design Assistance and Rapid Prototyping

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

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

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