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Advances in magnesium molding bode well for processors seeking diversity and business.

Metal fabrication may seem an unlikely ally in a plastics processor's efforts to increase business and hone a competitive edge. But recent developments in magnesium injection molding, coupled with the relative simplicity of the technology, give processors a powerful incentive to add the capability and expand into new markets.

The key advance is a new generation of magnesium alloys that dramatically increase creep resistance in high-heat applications, thus improving their potential for use in automotive underhood components. Many experts consider magnesium parts a good fit for this large-volume market because of their combination of strength and light weight, and for their contribution to noise reduction. Their ability to maintain dimensional tolerance under high heat could be the breakthrough to wider acceptance of the materials.

Some of the new alloys also improve surface finish, an important requirement for another growing application--consumer electronics. Molded (and cast) magnesium has displaced some plastics in products like cell-phone casings, laptop computers, and other portable devices because it offers a better balance of stiffness and thinwall design. Cell-phone providers offer rigid protective casings of magnesium with wall thicknesses down to 0.5 mm. Though finishing still requires extra steps after molding, a better surface speeds the process.

New developments in equipment may also improve the productivity and economy of magnesium molding. These include higher-cavitation molds--four cavities for cell phones, for example, where two-cavity molds have been commonplace, and up to 16 cavities for some general-purpose power-tool components that have typically been produced in four-cavity molds. Suppliers are looking at developing hot-runner systems that replace the hot sprues used in most molds; and equipment makers, though fewer in number than for plastics, are engineering injection machines for specific markets and offering more options in clamp tonnage and controls.

Importantly, magnesium molding is largely exempt from regulatory rules. Unlike a casting operation, a processor does not need a foundry license, which can be difficult to acquire in some locales, or a special facility. Magnesium injection machines are installed as quickly as plastics injection machines and, by most accounts, run as cleanly.

Opportunities for Growth

The use of magnesium is growing by 10% to 15% annually. Magnesium molding is increasing at a higher rate, experts say, though exact numbers are difficult to verify since it remains small compared with competitive processes like magnesium casting and plastics injection. Nevertheless, as Zac Glasford, product manager for one machine supplier, Husky Injection Molding Systems, notes, adding the capability shows current and potential customers the extent of a molder's fabrication expertise. With demand for added work like sub-assembly design and manufacturing on the rise, molders that diversify with different processes and materials improve their value to end-users.

Magnesium injection molding is based on technology developed by Thixomat Inc., of Ann Arbor, Michigan. Semisolid magnesium alloys (as well as aluminum or zinc alloys) are heated and subjected to the shear of a processing screw, which makes them thixotropic and injection-moldable. The technique is called Thixomolding. Processors and equipment suppliers must be licensed by Thixomat to work with the technology, which is distinct from metal injection molding. The number of companies worldwide that have Thixomolding licenses is relatively small and includes two machinery makers, Husky and JSW.

The alloys attracting interest for their creep-resistant properties come from several companies: Advanced Magnesium Technologies (AMT) of North America, an Australian firm with U.S. offices in Livonia, Michigan; Dead Sea Magnesium, an Israeli company with U.S. headquarters in Shelby, Michigan; and Noranda Inc., a mining company in Toronto that's part of Switzerland's Xstrata Group.

Under the Hood

These grades will vie for underhood applications with aluminum and, in some cases, engineering thermoplastics, thermosets, and composites. A key advantage they offer over aluminum and some plastics is light weight. According to figures developed by the Transportation Technology R&D Center at Argonne National Laboratory, magnesium is 36% lighter than aluminum by volume and, when alloyed, has the highest strength-to-weight ratio of any structural metal. It compares favorably with plastics, as well, according to figures from Dead Sea Magnesium, which reports that magnesium alloys have the potential to reduce weight by 30% compared with plastics in dynamic applications and up to 60% in static loads.

AMT offers two products: AM-lite and AM-HP2. Tom Sweder, general manager of AMT North America, says the AM-lite grade provides creep resistance to 130[degrees]C (266[degrees]F). The material also yields good surface properties from Thixomolding and has platability characteristics similar to those of zinc, which improves finishing economy. The AM-HP2 grade is targeted at powertrain applications. It maintains creep resistance to 170[degrees]C (338[degrees]F), within the 300[degrees]F-400[degrees]F range automakers are interested in for these parts, says Ralph Vining, director of engineering at Thixomat. Sweder adds that AM-HP2 maintains good bolt-load retention under high temperatures.

Dead Sea Magnesium also offers two grades: MRI153M and MRI230D. The 153 version yields creep resistance to 135[degrees]C (275[degrees]F), while the 230 grade maintains its properties to 150[degrees]C (302[degrees]F). The 230 grade reportedly provides equivalent tensile yield strength (in MPa) to aluminum at 150[degrees]C, while the 153 version's results are lower. Dead Sea reports that the higher-performing 230D grade is suitable for such parts as a torque converter stator, oil-pump housing, engine block and bracket, timing-chain cover, a lower crankcase bed plate, and a front engine cover. Grade 153 is being targeted at automatic transmission housings, 4WD transfer cases, air-intake manifolds, oil pans, and engine covers.

Vining says Thixomat has tested some of the alloys on parts for the USCAR program, a research consortium of the Big Three automakers. One component was a rear seal carrier, which holds the oil seal that goes around a crankcase. The alloys maintained their creep-resistant properties.

Noranda weighs in with two alloys as well: AJ-52 and AJ-62. Vining says the Noranda alloys hold their strength and dimensional tolerance in the 300[degrees]F-400[degrees]F range. Noranda, in fact, hit the applications jackpot last year, when BMW introduced the six-cylinder, 3.0-L 300i series engine with a magnesium/aluminum alloy crankcase utilizing A J-62 as the magnesium component. Though the block is cast, it demonstrates the performance a magnesium alloy brings to critical automotive applications. The magnesium composite block weighs 22 kg (48 lb) less than a solid aluminum block, generates 30 more horsepower, and consumes 12% less fuel. It also reduces engine noise. BMW has installed the engine in all 3 series and some 5 series vehicles in North America, and in many models worldwide.

In the Plant

In equipment developments, Husky's Glasford says that Husky plans to add a 400-ton model for consumer electronics to its line of three Thixomolding presses. The new machine will be the smallest in the series, the others being 500, 650, and 1000 tons. The machines are based on Husky's Hylectric machine line, hybrid presses with hydro-mechanical clamps, electric screw drives, and accumulators for fast injection.

JSW, meantime, supplies seven Thixomolding machines in its JLM Series. The models are hydraulic and range in size from 83 to 1765 tons (735 to 15,700 kN).

Husky is also developing 2-drop and 4-drop hot runners for magnesium molding. "We've proven the concept and are now releasing products for beta testing," Glasford says, adding that the products could be commercial in a year.

One of the challenges in molding magnesium is the thinwall trend and the move to higher cavitation. "The more cavities, the higher the flow rate of the magnesium has to be," he says. "Whether you have one or eight cavities, the fill rate has to be the same, so there's a need to push more material out in the same amount of time."

Magnesium molding is not a threat to plastics except in some applications where inter-material competition is intense and ongoing, and driven by OEMs seeking the best performance and economy without incurring major new manufacturing costs in the process. Magnesium alloys do provide inherent benefits, of interest to some markets, like EMI/RFI shielding and heat-sink properties, and design flexibility is increasing, notably in such areas as insert molding. Nevertheless, for most plastics processors it will remain an ancillary technology that complements rather than competes with their main areas of expertise.
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Author:Toensmeier, Patrick A.
Publication:Plastics Engineering
Geographic Code:1USA
Date:Dec 1, 2006
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