Casting with graphite molds:
An ideal process for an uncertain economy
The ZA-12/graphite mold process begins with a computer-generated image of the desired part. This image of a part for a medical diagnostic device was created with 3D solid-modeling CAD software. |
A 3D plaster model can be produced as a design proof. Sections of the plaster model can be color-coded to indicate where design modifications are necessary to accommodate the casting process. |
Always a major consideration, time-to-market has become even more important for manufacturers in today’s economy. Whether foreign or domestic, the competition is fierce. In the production of equipment for communications, health care, scientific research, and countless industrial processes, development time can make the difference between a contract won or lost. Because the high-precision parts that go into such equipment must be ready on schedule, more and more manufacturers are investigating an economical yet often overlooked casting process that can provide rapid turnaround, especially for medium-volume production runs.
Dramatically Shorter Time-To-Market
This process makes use of graphite molds to produce parts from ZA-12, a zinc-aluminum alloy that is harder, stronger, and more durable than aluminum, brass, bronze, or plastic. With an experienced supplier that offers single-source production capabilities including in-house design, rapid prototyping, tooling and mold production, casting, and machining, typical turnaround time from finished CAD design to production samples is only four to six weeks. Some shops even offer CMM part inspection and light assembly.
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 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 is a lower-risk alternative to CNC machining, die casting, sand casting, and investment casting. Furthermore, the high accuracy and lustrous surface finish of ZA-12 parts virtually eliminate additional finishing steps required with other casting techniques, for a much 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 far less than tool steel and requires no heat treating. Graphite’s exceptional machinability dramatically shortens the moldmaking phase. In fact, 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.
The best casting results 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 simultaneously control fill rate, cycle time, and temperature, further maximizing the density of the castings.
Exceptional Alloy
Equally important are the attributes of the 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. Typical surface finishes of 63 microinches or less are better than finishes from other casting processes. Although 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.
Design changes are made quickly and easily on the CAD system. |
Cast parts have a bright, corrosion-resistant finish and require no heat treating. |
If desired, parts can be chromated, plated, painted, powder-coated, or finished with electro-coated acrylic or epoxy (shown). |
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. In fact, for those ZA-12 parts that do need drilling, tapping, etc., there are graphite/ZA-12 casting houses that offer high-precision CNC machining under the same roof. Having personnel experienced with ZA-12 perform this work in-house on dedicated machining centers maximizes repeatability while minimizing costs.
A Hedge Against Uncertainty
Because graphite is machined easily, graphite molds can be modified quickly and at relatively low cost, allowing manufacturers a much higher degree of flexibility in debugging or improving products while still controlling costs — a major advantage over conventional casting methods.
These days, parts are often redesigned after a short initial production run. Why so many design changes? Reasons vary. Perhaps the new end product that the part went into did not perform as expected, or a competitor introduced a product with enhanced technology and additional features, or the engineers simply found a better way to build the device. Or perhaps the impetus for the redesign was not functional, but ergonomic or cosmetic.
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, less is at stake, so an equipment manufacturer or a parts vendor can stay flexible. In case 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.
The Importance of Design Assistance and Rapid Prototyping
Any graphite/ZA-12 casting house touting rapid manufacturing capability should offer early and comprehensive design assistance facilitated by state-of-the-art software such as Pro/ENGINEER and Solidworks parametric and associative 3D solid modeling CAD programs.
Advanced stereolithography that produces 3D plaster models of a part in a matter of hours will facilitate design and debugging. Multiple copies allow the model to be reviewed simultaneously by different individuals in the customer’s company. 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 design changes that result from a review of the plaster models can be made quickly and easily on the CAD system. Additional models can be produced overnight to verify that all changes have been made. When the ZA-12 caster receives a green light from the customer, moldmaking begins. Because they generate machine tool “G” code, Pro/MANUFACTURING or other CAM software packages dramatically shorten this stage of the process.
When graphite molds are completed, some casting houses cast 50 to 100 sample parts, and then halt production temporarily pending customer approval. But other shops offer a waived-sample program, which allows the customer to save more time and money by waiving the approval of cast samples in favor of uninterrupted production.
Specially designed equipment (such as the Low Turbulence Automatic casting machine shown here) can control fill rate, cycle time, and temperature simultaneously. Because they fill each mold from the bottom, these machines minimize the turbulence of the molten metal, producing parts of exceptional quality and repeatability.
Once the design is finalized, the graphite mold should be guaranteed for the length of the production run. If mold repair or replacement is necessary, many casting houses require that the customer bear the expense. However, some casters demonstrate confidence in the quality and durability of their molds by covering any repairs or replacement costs through a one-time, up-front tooling charge.
Quality, Speed, Economy, and Flexibility
Casting ZA-12 alloy with a graphite mold offers original equipment manufacturers a viable alternative to other production methods in these uncertain times. Especially for part quantities from 200 to 20,000, the graphite mold/ZA-12 process is as precise as or more precise than other casting methods at a fraction of the cost.
Better casting houses can perform light assembly operations such as inserting bearings, bushings, pems, dowel pins, springs, etc.
The ZA-12/graphite mold process also offers OEMs a hedge against expensive design modifications in a volatile marketplace. Due to unrelenting advances in technology, products tend to “mature” sooner than ever before — a nice way of saying they are nearly obsolete when they come off the assembly line. A commitment to a large die-casting run (above 20,000 parts) can mean a large up-front investment, which turns into a loss if sales fall below expectations. Why not minimize the financial risk by starting with a shorter run of ZA-12 parts? With today’s demands for reduced manufacturing and inventory costs and shorter times to market, the graphite/ZA-12 process is the ideal way to pay as you go, quickly and economically producing high-quality metal parts.
For more information, contact Graphicast, Inc., P.O. Box 430, 36 Knight Street, Jaffrey, NH 03452, TEL: (603) 532-4481, FAX: (603) 532-4261, www.graphicast.com.
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