Machine Design

Not your average carbon copy

Graphite permanent molding is a niche process used for building high-precision zinc-aluminum alloy parts.

Jim Tenglin
President Castechnologies Inc.
Attleboro, Mass.

Assortment of parts molded in graphite permanent molds.

Part being removed from graphite permanent mold showing attached runners and gates.

Air-traffic control trackball housing is cast and machined complete.

Designers under the gun to get metal parts out the door routinely select manufacturing processes they're familiar with. This can be problematic, however, for designs under 10 lb produced in short to moderate (500 to 20,000) annual volumes. That's because machining, which serves well during prototyping, is often too costly for production volumes in this range. Conventional casting methods, likewise, may be unsuitable for these annual volumes.

To circumvent this problem, designers sometimes compromise by converting the metal part to plastic. But plastics can fall short in strength and durability. Designers producing parts from aluminum, bronze, brass, and cast iron have an alternative: Graphite permanent molding (GPM).

As one of the "permanent" mold processes, GPM falls into the same category as die casting. The process uses one mold to make many parts. This contrasts to "disposable" mold casting, such as sand or investment, where a mold gets destroyed to extract a single part. The basic graphite-mold design mimics that of die casting. A split or multipiece mold with cavities accepts the molten metal. The metal solidifies in the cavity then the mold separates and the casting ejects. The major difference between the two processes is mold material, metal for die casting, graphite for GPM.

Graphite has many qualities that make good molds. It's extremely stable and doesn't warp, twist, or check when molten metal injects into it. Its lower coefficient of thermal expansion produces parts with high accuracy. The cavity surfaces are also nonwetting, so casting release agents, which can degrade surface finish, aren't needed.

Graphite also stores virtually indefinitely without changing shape, rusting, oxidizing, or deteriorating. Overall, these properties let manufacturers with medium or sporadic volumes take advantage of a process that delivers tight tolerances, cost effectiveness, and repeatability over time.

Moldmaking is critical for GPM. Premium grade billets or blocks of graphite with extremely low porosity and medium-sized grains are required. Graphite is a crystalline material that is quite strong and dimensionally stable. The graphite molds typically weigh upwards of 50 lb or more and can withstand significant stresses. Precision CNC milling cuts cavities into the graphite to exact tolerances. The mold is then polished. The cavity's tight tolerances, coupled with the high thermal conductivity and minimal expansion of the graphite, results in parts with dimensional precision on the order of ±0.005 in. and surface finishes of 125 RMS or better. This makes GPM parts rival those produced in die-casting molds at a fraction of the mold price.

There are design parameters to be considered for GPM. The size of a casting produced in a GPM process is generally less than 163 123 4 in. These dimensions are maximum limits and should not be combined. Casting weight can be ounces to as much as 10 lb. Minimum wall thickness is usually 0.1 in. However, 0.06 in. is possible on small, well-fed areas. A minimum draft of 2° is required on all surfaces perpendicular to the parting line. Corner and edge radii should be a minimum of 0.015 in., preferably 0.032 in. or greater. Interior heavy sections should be avoided unless feeding ribs can be accommodated. Raised lettering and logos are easily reproduced, holes can be cored, and third dimensional slide features parallel to the parting line can be incorporated.

The alloy of choice for GPM is a highly engineered, zinc-aluminum (ZA) material which is approximately 12% aluminum. Designated as ZA-12 the metal offers a compromise between the ZA alloy series ZA-8 (8% Al) and ZA-27 (27% Al). ZA-12 has a low melt temperature of 790°F, machines easily, has excellent pressure tightness, is spark proof, and nonincendiary. It's also nonmagnetic which makes it suitable for electronic shielding.

ZA-12 is often an acceptable option when converting from aluminum, cast iron, steel, brass, and bronze. Its inherent-lubricity makes it a good replacement for bronze in some bearing applications. ZA-12 also has good corrosion resistance in normal atmospheric conditions, in aqueous solutions, and when used with petroleum products. If greater corrosion resistance is required, finishes such as paint, powder coating, and plating are excellent options.

GPM is a niche process. In contrast to die-casting that produces parts much faster in parts-per-minute, GPM production rates are in parts-per-hour. That's because cooling is controlled by the conductive ability of the graphite. This production rate differential equates to a die-cast piece price that is generally 20 to 40% of a GPM cast part. However, a comparable die-casting mold is upwards of 10 times the cost of a graphite mold. Analyzing annual part quantity helps determine whether the reduced piece price offsets high mold costs for die casting.

When annual quantities on the order of 500 to 20,000 pieces are required, GPM is often the process of choice. GPM offers other advantages. One benefit is less lead time (usually three to six weeks) for graphite molds. There's also a flexibility to make engineering changes because it's relatively easy to modify a graphite mold.

At the lower end of the annual quantity range, GPM castings generally become competitive around 300 to 500 parts/yr. Sand or investment castings or sometimes even machining from raw stock may be more attractive for lower quantities. The cost of the mold is often the deciding factor. However, the precision possible with GPM can reduce or eliminate the need for common secondary machining operations that other casting processes need. The result is better part uniformity and reduced labor costs.

Graphite molds can produce highquality castings for upwards of 40,000 cycles or more. One of the keys to this longevity is proper up-front engineering. Still, some of the GPM foundries will eliminate any customer concern by guaranteeing the molds for the life of the project.

Castechnologies Inc.,
(508) 222-2915,

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