Cost-conscious molding

June 21, 2007
Permanent graphite molds and a knowledgeable metal-casting house can help designers of zinc-aluminum-alloy parts go from art to part in less than six weeks.

Val Zanchuk
Graphicast Inc.
Jaffrey, N.H.

Edited by Jean M. Hoffman

Most ZA-12 (zinc-aluminum) alloy parts need little or no secondary machining. However, when necessary, a shop with high-speed CNC-machining capabilities can quickly and economically perform boring, drilling, or tapping operations.

The graphite-mold/ZA-12 process begins with a computer-generated image of the part. This image of a part for a medical diagnostic device was created with 3D solid-modeling CAD software. A plaster model (inset) can then be produced as a design proof. Sections of the model can be color coded to indicate where design modifications are needed to accommodate the casting process.

Cast ZA-12 parts have a bright, corrosion-resistant finish and need no heat treating.

Specially designed equipment (such as the Low Turbulence Automatic casting machine shown here) can control fill rate, cycle time, and temperature simultaneously. LTA machines fill each mold from the bottom. This minimizes turbulence of the molten metal inside the mold. The resulting ZA-12 parts are highly repeatable.

Competition these days is fierce whether it is foreign or domestic. Getting a product from concept to production fast can make the difference between a contract won or lost. To help get parts out the door quickly and economically designers are turning to an alternative metal-casting process that uses a permanent graphite mold to rapidly produce 300 to 20,000 parts.

The parts are from ZA-12, a zincaluminum alloy (approximately 11% aluminum) that's harder, stronger, and more durable than brass, bronze, plastic, or aluminum alone. A knowledgeable casting house can use this molding technique to go from art to first article part in six weeks or less.

The cost to machine parts from scratch is too high for typical medium-volume (300 to 20,000 parts) production. Part counts in this range don't have the economics for traditional high-volume casting methods, either. Based on total acquisition cost (cost-per-part volume + tooling costs), the permanent graphite-mold/ZA-12 casting process is an economical alternative to CNC machining and die, sand, or investment casting. And the high accuracy and lustrous surface finish with ZA-12 parts virtually eliminates secondary finishing that other casting techniques need — further reducing overall production times and costs.

Tooling costs less for the graphite-mold/ZA-12 process than for comparable molds used in die casting and injection molding. Graphite costs a lot less than tool steel and needs no heat treating. It easily machines and can dramatically shorten the task of moldmaking. Graphite molds often take weeks less to produce than those for die casting and can be made for about one-fifth the cost.

Compared to typical sand and investment castings, ZA-12 parts will be more accurate and have as good or better surface finishes — all at a much lower part cost. And graphite molds are reusable, unlike sand and investment casting that destroy molds when parts are extracted.

The properties of graphite (a form of carbon) make it a good candidate for molds. It has a coefficient of expansion lower than steel, isn't very porous, and distorts little when filled with molten metal. However, it doesn't conduct heat as well as steel so it takes slightly longer for the part to harden than in die casting. But this is more than offset by exceptional surface finishes, and the ease with which the castings will machine.

A graphite mold is machined in two halves and used continually, similar to hardened tool steels used for die-casting molds. Best casting results come from using the latest semi-automated machines. They fill each mold from the bottom and minimize turbulence of molten metal inside the mold.

Process controllers maximize density and minimize casting porosity by simultaneously controlling fill rates, cycle times, and temperatures. The resulting ZA-12 parts are highly repeatable.

Under the right conditions, a graphite mold can cast as many as 40,000 parts. Casting release agents that can spoil part surface finishes aren't needed in molds with adequate draft (normally 2°) on surfaces perpendicular to the parting plane.

Equally important to the process are ZA-12 alloy properties. The alloy has about the same density as that of cast iron. It easily casts, and its low casting temperature prolongs mold life. ZA-12 is spark-proof, so it can go in hazardous environments. It's also nonmagnetic, making it a candidate for electronic shielding. Zinc is readily available at relatively stable prices, helping ensure the long-term viability of ZA-12 components.

Typically, ZA-12 castings can be produced in volume with critical-dimension tolerances of ±0.003 in./in. for the first inch and ±0.001 in./in. for additional inches. Parts need no heat-treating and surface finishes are typically better than 125 µin. (i.e., as good or better than invest mentor die-cast parts).

Parts have a bright, corrosion-resistant finish that needs no coating or other preparation. But they can be chromated, plated, painted, powder coated, or finished with electrocoated acrylic or epoxy to simulate anodized aluminum.

ZA-12 machines as easily as brass and bronze and more easily than cast iron and aluminum. In many cases, ZA-12 parts need little or no secondary machining. For those that do need boring, drilling, or tapping there are graphite/ZA-12 casting houses that offer high-precision CNC machining under the same roof. It helps to have machine operators experienced with ZA-12 perform this work in-house on dedicated machining centers. This maximizes repeatability while minimizing costs.

As mentioned previously, a graphite mold can be created quickly and at relatively low cost — a major advantage over die casting. By the same token, an existing graphite mold can be modified easily. Obviously, this gives designers more flexibility in debugging or improving products and controlling costs than with traditional casting methods.

Often, parts get redesigned after a short initial production run. In effect, the initial run produces a lot of expensive prototypes. Why so many design changes?

Reasons vary and can come about when the new product doesn't perform as expected. Or when a competitor introduces a product with improved technology or one that costs less. Engineers may also simply find a better way to build the device.

Regardless of the reason, many of the parts in a second-generation device will often need a redesign. For example, a smaller device may need smaller parts; smaller parts require smaller molds. Even designers who anticipate high-volume production often realize a high-volume process like die casting no longer makes financial sense after a redesign.

In contrast, graphite molds are quickly and economically created and modified, so less is at stake. This brings more flexibility. And if sales fall below expectations, there's less cost associated with ZA-12 components compared with the heavy investment in a steel mold for die casting or injection molding.

To help maximize quality and minimize time to market, designers must consider several things. For example, any graphite/ZA-12 contract manufacturer that has rapid-manufacturing capabilities should offer early and comprehensive design assistance. State-of-the-art software such as Pro/Engineer and SolidWorks parametric and associative 3D solidmodeling CAD programs facilitate design modifications that may be necessary later in the process.

Advanced 3D printing technology that produces a plaster model of the part in a few hours will ease design and debugging. This process can produce multiple copies quickly and effortlessly, so several interested parties can review the model. Sections of a model can be color coded to indicate the areas needing such design modifications as draft and radii to accommodate the casting process. Such models and accompanying notes are invaluable for resolving design problems prior to moldmaking.

Any design changes that result from reviewing the plaster models can take place quickly and easily on the CAD system. Additional models can be produced overnight to verify changes. When the ZA-12 caster receives a green light from the customer, mold making begins. Pro/Manufacturing or other CAM software packages generate machine-tool G–code and can dramatically shorten machining time for the mold.

When graphite molds are complete, many shops cast 50 to 100 sample parts, then halt production temporarily pending customer approval. Some casting houses offer waived-sample programs, which let customers save more time and money by waiving the approval of cast samples in favor of uninterrupted production.

With the design finalized, the graphite mold should be guaranteed for the length of the production run. Many casting houses specify that customers bear the expense of any mold repair or replacement. 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. This charge does not cover modifications to a completed mold as from a redesign of the part. But such modifications can typically take place quickly and easily for a modest fee, especially if the shop has high-end CAM software.

Unrelenting advances in technology make products "mature" more quickly than ever before — a nice way of saying they are nearly obsolete when they come off the assembly line. Manufacturers live with the worry that a competitor will undercut them, either with a new design or by moving overseas where labor is dirt cheap.

Casting ZA-12 alloy with a graphite mold offers OEMs an alternative to other production methods in these uncertain times. For part quantities from 300 to 20,000, the graphite-mold/ZA-12 process is as precise or more precise than other casting methods at a fraction of the cost.

The process also offers a hedge against expensive design modifications in a volatile marketplace. A commitment to a large die-casting run (above 20,000 parts) can mean a large up-front investment that turns into a loss if sales are below expectations. The shorter production runs of ZA-12 parts minimize the financial risk.

It is a simple matter to follow up with another run and another if the product is successful. In all, the process is an ideal way to pay as you go.

The graphite mold and tooling for a swivel elbow used on a laboratory equipment stand totaled less than $10,000, including mold samples used for preproduction testing.
ZA-12 casting example: lab-equipment part

The production of a swivel elbow for a laboratory equipment stand illustrates the comprehensive services available from some shops casting ZA-12 (a zinc-aluminum alloy) in graphite molds.

Graphicast Inc. produced the mold and cast the parts. It also provided custom-chromated-steel shafts, assembled the shafts to the castings, and designed reusable packaging to protect the finished parts during shipping and storage.

Pressed for time, the customer ordered tooling for this part before the design was finished. In Pro/Engineer CAD/CAM software, the mold model, design drawings, and CNC G-code are direct derivatives of the part model. As the designers made changes to the part model, the work updated automatically from each new part file. Moldmaking began the day after the final design was approved, reducing the company's lead time by at least four weeks.

Perpendicularity specifications were tight — 0.002 in. to base and 0.008 in. to the opposite face. The mold and tooling for this part totaled less than $10,000, including mold samples used for preproduction testing. Per-unit pricing for the assembled part, including the cost of the shaft and special packaging, was less than $29.

Graphicast Inc., (603) 532-4481,

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