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THIXOMOLDING, what's that?

THIXOMOLDING, what's that?

Ann Arbor, MI -Injection molding isn't just for plastics anymore. Instead, modified plastic molding machines can successfully turn out strong, lightweight magnesium-alloy parts, thanks to Thixomolding(R)technology from Thixomat Inc.

Initially known only as a niche process for making super-thin enclosures for notebook computers and other small electronic devices, Thixomolding now stands ready to take on injection molding and die-casting in a wide variety of applications. "A lot of people still think small parts' when you say Thixomolding,'" says Ralph Vining, Thixomat's director of engineering, who notes that the vast majority of thixomolded parts weigh in at less than a pound. Yet, machinery innovations now allow Thixomolding to produce parts up to 5 lbs, and new alloy developments promise to open up some demanding automotive transmission and underhood applications.

Thixomolding takes its name from the thixotropic nature of magnesium alloys. When heated to just the right temperature-above the melting point of some alloy components, below that of others-the alloy turns into a slurry with solid and liquid portions. As Thixomat founder and chairman Ray Decker explains, this semi-solid slurry is "shear thinning" and will flow when it's been shaken up a bit. The slurry's thixotropic behavior is what enables this metal molding process to work very much like plastic injection molding: Once the magnesium alloy has been prepared, by chipping, and fed into the molding machine's barrel, heater bands bring the alloy up to temperature. At the same time, the mixing action of the molding machine's rotating screw imparts a fluidity to the thixotropic slurry, allowing it to flow into the mold.

While the machines for Thixomolding and plastic molding share many basic characteristics, Thixomolding machines do sport some important modifications. For one, they have screws and barrels that can accommodate processing temperatures as high as 590C. "That's about 100C lower than die casting but far higher than plastics," Vining notes. The thixo-machines also feature more robust hydraulic systems in order to meet high injection speed requirements (up to 1 m/sec). The final difference: Thixomolding machines also include a system for blanketing the feed throat with argon gas to keep the chipped magnesium from oxidizing.

Car parts and large parts. Demand for strong, lightweight parts at several computer and electronics OEMs-including Sony, Sharp, Toshiba, Panasonic, and NEC-have driven much of Thixomolding's growth so far. And its rise in popularity has been impressive. Decker reports that 50 more Thixomolding systems came on line this year than last year. Thixomat's 40 licensees in North America, Europe, and Asia have more than 140 machines in the field today, up from one licensee and one machine six years ago.

More growth will likely follow if Thixomolding catches on in the automotive world. Over the past two years, thixomolded magnesium alloys, which weigh about 30% less than aluminum, have started turning up in automotive applications. Vining reports that six licensees now have automotive parts in production, including, for example, a formerly-die cast shift cam for the Ford Explorer. The trend toward more automotive parts stands to accelerate, thanks to some new creep-resistant alloys suitable for transmission and underhood applications. According to Decker, traditional magnesium alloys, such as AZ91D, have been kept out of transmission and underhood applications because they have poor creep properties and bolt-retention characteristics above 150C. Thixomat plans to address this problem with a new family of creep-resistant magnesium alloys. Decker expects the new Thixalloy material now in development will double the 120-hr rupture time (at 28 kN load and 175C) that AZ91D has shown in Thixomat's bolt-load-retention studies. Longer term, Thixomat is working on an aluminum Thixalloy.

Large parts represent another Thixomolding opportunity. Decker notes that the largest Thixomolding machine today is the equivalent of a 1,600-ton plastics molding machine. With Thixomolding needing five tons of clamp force per square inch of projected area, that size machine can produce some big parts. Decker reports that the next "big-part" project will be for automotive seat backs, which should go into production sometime this Spring.

One Thixomat licensee in Japan already makes front bezels for a 27-inch Panasonic television set, replacing a less recyclable plastic part. "Magnesium alloys can be re-chipped and molded over and over again," Vining says, noting that recyclability gives Thixomolding an edge over plastics in Europe and Japan, where very tough environmental laws are more stringent.

Designing for Thixomolding. With Thixomolding, processing conditions have strong relationships to design strategy. As Decker tells it, the processing temperature dictates the amount of solids in a given alloy's slurry, and fluidity decreases with higher solids. At the same time, porosity increases as the level of solids decreases. Filling a mold and winding up with a dense part can be a balancing act. A typical thin-wall part, for example, would have solids of roughly 10% for a porosity that is just below 1.5%, according to Decker. That porosity drops to about 0.5% as the solids approach 30%. "Above 30%, you begin to see that the strength begins to taper off," Decker notes.

Beyond the balancing act between mold-filling and porosity, designing for Thixomolding is straightforward. Compared to plastics, Thixomolding alloys have enough strength and stiffness to permit thinner parts. In notebook computers, thixomolded parts typically have nominal wall thicknesses under 1 mm. Decker notes that the Toshiba Libretto even has walls down to 0.7 mm. Vining points out that plastic notebook housings, by contrast, don't get much thinner than 1.2 mm and still meet the OEM's mechanical requirements. Smaller consumer electronics parts, such as mini-disk players, have walls as thin as 0.5 mm. In terms of stiffness, magnesium alloys offer a modulus of elasticity in the neighborhood of 6.5 million psi, compared to 430,000 to 1.16 million psi for unfilled and filled engineering plastics. As a metal, magnesium also has inherent electromagnetic interference (EMI) shielding benefits not offered by plastics.

As for tooling, molds for Thixomolding tools closely resemble injection-molding tools used for equivalent applications. So, just like a tool for thin-wall injection molding, a Thixomolding tool needs to be beefy with extra support pillars and robust steels. So far, Thixomolding licensees have run tools with up to eight cavities, though thin-walls tend to need lower cavitation because magnesium gives up its heat quickly and multi-cavity tools have longer flow-lengths.

With its move into automotive parts and new alloys, Thixomolding will increasingly go head to head not just with injection-molding, but increasingly with die-casting. Vining concedes that die-casting can produce simple, high-volume parts at a lower cost. But for complex parts designed from scratch for Thixomolding, "we have a cost advantage," he says, explaining that Thixomolding can mold in features that have to be machined into die cast parts in expensive secondary operations. The cost scales may tip even further in Thixomolding's favor because of a new hot-runner system from Japan Steel Works. By saving material and runner-trimming operations, the hot runners are predicted to shave as much as 50% from the cost of smaller parts where the runner-to-part ratio is high, Vining says.

Thixomolding from Thixomat Inc. Circle 546

How the processes stack up
Thixomolding Die-casting

Density

98%

95%

Elongation

6%

3%

Minimum wall thickness

0.02 inch

0.05 inch

Tolerance control

NADCA Precision plus or minus 0.002 inch (first inch)

NADCA Standard plus or minus 0.010 (first inch)

Source: Phillips Plastic Corp.


Thixomolded sink beats the heat

Minneapolis, MN-While designing the cooling system for a new wearable PC, called the ViA II, engineers at Logic Product Development had to figure out a way to make the heat sink cost-effective. The finned component works in conjunction with a small, external cooling fan to draw heat away from the CPU's electronics. With no add-ons, the unit, which has an 85C case temperature limit, draws approximately 10W.

Initially, engineers planned on passive cooling only, explains engineer Kevin Johnson, using a machined aluminum heat sink. But after learning that the electronics would remain fully active at all times, calculations showed that the fins on it would need to be three inches tall in order to provide sufficient surface area to dissipate the heat. "That's pretty ridiculous for a product someone is going to wear around their waist," recalls Johnson, conjuring up an image of a gladiator about to do battle. "So at that point, we made a decision to add an external cooling fan, which gave us more leeway in the design of the heat sink."

With the freedom to make the fins shorter or even take a hit on the thermal conductivity, Johnson evaluated both die casting and Thixomolding the heat sink out of magnesium. "What really tipped the scales in favor of Thixomolding was that fact that we could get high density and therefore better conductivity. A die cast part is going to be very spongy. Also, the economics were there." Phillips Plastics Corp.'s Magnesium Injection Molding business unit (Racine, WI) produces the part.

While engineers have not tested this configuration to extreme limits, they have found it to be satisfactory in a wide variety of work settings to date. And, in fact, they are so pleased with the heat sink that they are considering producing the entire case-currently molded out of a polycarbonate ABS alloy--out of thixomolded magnesium, improving the stiffness.

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