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May 7, 2000

4 Min Read
Performance counterbalances cost in switch to magnesium

Editor’s note: Stephen LeBeau, vp of sales and marketing for Thixomat Inc., recently addressed an audience of OEMs, moldmakers, and molders at the company’s first TXM technical conference. He was joined by Stephen C. Erickson, Noranda Magnesium; Tom Pastirik, Hydro Light Metals; and Tom Sweder, Ford Motor Co. Their message? TXM magnesium offers great benefits to designers, and more specifically offers extensive lightweighting opportunities to markets like electronics and automotive. LeBeau presents an overview of the discussion here.

Magnesium is considered by many to be an exotic material. So how could it ever be competitive against the more reasonably priced plastics? Though it may cost more pound for pound, from a performance standpoint, magnesium offers some advantages. It is a material that has high electrical and thermal conductivity, high strength-to-weight and stiffness-to-weight ratios, good ductility, resistance to oils and lubricants, and excellent acceptance of coatings and finishes. When compared to plastics or other metals, these properties make magnesium stand out.

In fact, many applications require the selection of material based on stiffness. And when comparing relative weights and costs of plastics to provide equal stiffness to magnesium, the metal alloy comes out on top (see Table 1). For example, 1 lb of magnesium alloy costs around $1.60 to $1.70/lb, depending on quantity purchased. To achieve a comparable stiffness it would cost around $17.27 to purchase 12.7 lb of high-heat ABS (price based on annual volumes of 300,000 to 500,00 lb).

It Must Be Lighter
Consider that a typical PC enclosure of ABS molded to 2.2-mm wall thickness can be replaced by a magnesium wall thickness of .82 mm (see Table 2), which reduces overall weight by 46 percent and increases tensile strength by 2.4 times.

Many big-name companies—Toshiba, Mitsubishi, NEC, and Panasonic, among others—have discovered magnesium and TXM and are using them to change the definition of the word "portable" in the world of personal computers. Each of these companies is currently using TXM magnesium enclosures in notebook computers, allowing for the design of smaller, lighter personal computers. One model being manufactured, for example, has an overall thickness of 1.8 mm and weighs 1.4 kg.

Inherent Shielding
Though thinner is better, designers of electronic devices also must be sure to guard against electromagnetic and radio frequency interference, which can affect a product’s performance. Because plastics are inherently insulating, they must rely on secondary operations, such as surface modifications, or the internal incorporation of a conductive material, both of which can raise the cost of the final product.

Because of their conductivity, metals are most commonly used as conductors in EMI/RFI shielding applications. In this respect, the weight-saving benefit offered by magnesium cases extends over the entire frequency spectrum. In cases where absorption is the primary shielding mechanism, especially in the spectrum below 1 mHz, aluminum and magnesium are nearly equal on a per-weight basis. However, above the frequency of 1 mHz, which is the portion of the spectrum that affects most commercial applications, magnesium’s ability to produce net-shape thin-wall parts and its lower density give it an advantage over aluminum. Using TXM magnesium, walls can be thinned down to .65 mm.

Keeping It Cool
Another critical design element in the electronic field is keeping components cool, especially in devices using power semiconductors. Designers must maximize heat dissipation through the selection of an appropriate heat sink, and since the thermal conductivity of magnesium alloy is several hundred times that of plastic (see Table 3), the material provides a definite design advantage.

Magnesium can also compete with aluminum in many applications in which thermal management is a requirement. Though aluminum has high thermal conductivity, high thermal diffusivity, and adequately low density, in applications where weight is more critical the use of magnesium is growing significantly. The amount of heat transferred is nearly equal in parts made of aluminum and magnesium. Added benefits are derived, however, by the ability to TXM magnesium into net shapes in applications where aluminum may be limited by design and castability issues and require secondary machining operations.

In the end, TXM magnesium, because of its performance advantages, can be more cost-effective than either plastic or aluminum. Still, as Tom Pastirik pointed out during his presentation, it is important that parts be designed with TXM magnesium in mind. This allows designers to maximize the materials benefits—strength, stiffness, and ductility—while recognizing its limitations—elevated temperature creep and galvanic corrosion.

Editor’s note: Copies of all of the presentations are available from Thixomat.

Contact information
Thixomat Inc.
Ann Arbor, MI
Herbert Pritzker
Phone: (734) 995-5550
Fax: (734) 995-5558
Web: www.thixomat.com

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