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.
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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.
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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.