Die-Cast Metal Parts Handle Structural, Thermal, Electrical, and Environmental Tasks

Plastics seem to be making inroads into all kinds of parts. So with injection molding becoming more and more common, is metal die casting still a viable production method? Die casting practitioners say yes.

Furthermore, they insist plastics can’t hold a candle to metal castings in terms of mechanical, thermal, electrical, dimensional, and economic performance. But when designers find themselves weighing die casting against plastic injection molding and other processes, a point-by-point comparison can help them pick the right material and manufacturing method.

Parsing properties
Designs often start with primary material requirements like strength, weight, and heat tolerance. Engineers rule out many materials based on these criteria alone. Both die-cast alloys and plastics have a wide range of possible properties. However, common die-castings alloys will have as good or, in many cases, better strength, weight, and heat tolerance than most plastics.

Die-cast parts are stronger than plastic injection moldings with the same dimensions. For instance, aluminum and magnesium die castings are two to three times stronger on a per-pound basis than common injection-molding plastics like acrylonitrile butadiene styrene (ABS) and nylon 6.

The modulus of elasticity for common die-casting alloys ranges from 6,500 to 12,400 ksi. In comparison, common plastics have moduli of only 200 to 400 ksi.

This means designers can create thin-wall die castings that resist stresses capable of deforming plastic-injection-molded parts of the same dimensions. Alternately, a die-cast part can be thinner and lighter than its plastic equivalent and still meet strength requirements.

Typical wall thicknesses for cast designs range from 0.040 to 0.200 in. depending on alloy, part configuration, size, and application. In some small castings, walls can be as thin as 0.020 in. For extremely small zinc parts, miniature die-casting technology can cast still thinner walls.

Stiffness also affects a part’s production considerations. Tooling tolerances are similar for both die castings and plastic-injection-molded parts because both are made in steel dies. However, die-cast parts tend to maintain these tight tolerances with lower scrap rates than plastics because of die-casting alloys’ greater stiffness.

Density is another property engineers may consider, especially for compact, high-efficiency, and lightweight applications. Material strength helps determine if a design made out of die-cast metal can be slimmed down for weight savings. However, in some applications, designers look to add weight without bulk. When designs call for high-density material, zinc is an attractive die-casting choice.

Heating up
One other advantage of die-casting alloys is that they tolerate higher temperatures than most plastics. ABS and nylon 6 melt between 320 and 525°F while zinc melts at 729°F, aluminum at 1099°F, and magnesium at 1105°F.

In many applications, plastic components need protection from the heat coming off motors, microprocessors, transmissions, and other components. Because metal castings operate despite higher temperatures, designers can remove some or all of the heat shielding, cutting weight, part count, and manufacturing costs.

In addition to simply resisting heat, die-cast parts can help manage it. Die-casting alloys have roughly 1,000 times greater thermal conductivities than plastics. This means engineers can cast thermal-management features directly into designs.

For example, die castings excel at handling heat exchange in internal-combustion engines. In electronic components, a die-cast case can keep excess heat from affecting delicate circuits.

Die castings hold an additional benefit for electronics: electromagnetic shielding. Die-casting alloys, with six to nine orders of magnitude greater electrical conductivity than plastics, shield electronic components from outside EMI in cases where plastics can’t.

Show me the data
No matter what properties designers are looking for, it helps if they can work with well-characterized materials. The properties of die-casting alloys have been well documented, so designers and analysts can apply them to CAD models and FEA runs. Design for Die Casting, a manual from the North American Die Casting Association (NADCA), Wheeling, Ill., provides 10 or more physical properties for many die-casting alloys letting designers evaluate and compare the metals.

Designers can also choose from a large list of standard alloys, custom alloys, and alloys currently being developed. As with plastics, the properties of die-casting alloys vary widely and can be tailored to individual applications.

Software helps engineers find the right alloy formula for specific tasks. NADCA, along with Worcester Polytechnic Institute, Worcester, Mass., created i-Select Al software to help designers, product specifiers, and die casters identify alloy chemistries with specific casting properties, such as density, thermal conductivity, ultimate tensile strength, tensile yield strength, ductility, and elasticity.

Production prognosis
With ever-thinner profit margins on parts of all types, a material with the perfect properties might be a no-go if it’s too expensive to process. Many engineers turn to plastic injection molding out of a belief that plastics tooling costs less.

In reality, its price is comparable to die-casting tooling. In fact, some glass-reinforced plastics, such as nylon 6, are harder to process and put more wear on tools than die-casting alloys do. In these cases, it may be less expensive to tool up for die casting.

Both die casting and injection molding are high-speed processes that inject a material into a die to repeatably manufacture complex, thin-walled parts over high-volume production runs.

Die-cast parts are durable and dimensionally stable to close tolerances. Plastic parts, on the other hand, are more prone to warping and surface “sinking” in areas above ribs.

Die-cast parts also resist creep better than plastics. Even engineered glass-filled plastics have elongation factors twice those of aluminum and magnesium alloys. Consequently, the scrap rate for die-cast parts may be lower than those for plastic parts.

Part finishing can represent a major production cost, but it is indispensable because it contributes to the perceived quality of finished parts. Even if a device performs admirably, users will perceive it as substandard if it feels cheap.

Die-cast parts can be manufactured with smooth or textured surfaces, and it usually takes a minimum of effort to prepare them for finishing. Zinc’s high density imparts a feeling of permanence and solidity, and the metal can be finished to look and feel like steel. And aluminum can be anodized, powder coated, or chromed to give it an appearance that appeals to most consumers.

Die casters can also mold detailed lettering and ornamentation into parts. Die-casting alloys’ greater wear resistance means such details last longer on die-cast parts than on plastic ones. Details like these require no secondary finishing operations but send a message of quality to the marketplace.

Going green
Consumers increasingly include a part’s “green” credentials when they assess its quality and value. The amount of postconsumer recycled material in a part, its recyclability, and its carbon footprint all factor into its greenness.

Over 95% of the aluminum die castings manufactured in North America use postconsumer recycled aluminum. And with the production of recycled aluminum alloy requiring less energy than making alloy directly from ore or other methods, parts made from recycled aluminum conserve energy before they leave the shop floor.

In addition, the opportunity to recycle obsolete and worn-out parts is a critical part of “end-of-product-life” planning. More than 85% of the aluminum in a car currently gets reclaimed and recycled, while much of the plastic in a scrapped vehicle is treated as “fluff” and ends up in waste dumps.

Even when plastics are recycled, results are unpredictable. The effects of temperature, time, and the environment can degrade the potential performance of a recycled thermoplastic, while metals can be recycled without adverse effects on their properties.

Both manufacturers and consumers are focusing increasingly on parts’ carbon footprints. And reducing a part’s carbon footprint should cut its manufacturing cost.

A 2008 study compared the carbon footprints of four-cylinder-engine cam covers made with 30% glass-filled nylon, A380 aluminum, and AZ91D magnesium. Making the plastic for the part created 10 times more carbon-equivalent emissions per pound than creating the aluminum.

Magnesium generated much more carbon-equivalent emissions because current technology uses sulfur hexafluoride (SF6), a potent greenhouse gas, as a cover gas to prevent magnesium from oxidizing during die casting.

The U.S. magnesium industry has committed to eliminate SF6 emissions by the end of the year by improving gas-management practices and converting to alternate cover gases. Eliminating SF6 emissions will shrink magnesium parts’ carbon footprints to or smaller than aluminum.

Over the life of the cam cover, the study’s authors estimated aluminum part results in 5 lb less carbon emissions per pound of aluminum than the nylon cover. This is partly due to its light weight and its recyclability. Magnesium will see a similarly low carbon footprint as recycling efforts for it gear up.