Reinventing the wheel through thixomolding.

In the summer of 2007, DatuMag, Mississauga, Ontario, Canada, was approached by an aftermarket supplier of high performance motorcycle parts for help in developing a magnesium wheel. Lightweight wheels deliver better acceleration, shorter stopping distances, less rotating mass and reduced gyroscopic effects.

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Aluminum currently is the aftermarket material of choice due to its strength, weight and affordability. Magnesium alloy wheels, which are 30% lighter than aluminum, are the preferred choice for enthusiasts but more expensive.

DatuMag, founded by Cana-Datum Molds Ltd., Etobicoke, Onatrio, specializes in thixomolding, an emerging technology that presents a more cost-effective way to produce magnesium alloy castings. Similar to plastic injection molding, thixomolding takes advantage of the flow and fill characteristics exhibited by partially molten metals. Chips of magnesium alloy are fed into a heated screw and barrel assembly where an argon shield prevents the material from oxidizing. As the screw rotates inside the barrel, the magnesium is conveyed forward and heated to between 1,040-1,166F (560-630C), where it forms a semi-solid slurry. The semi-solid magnesium alloy is injected into the mold at a high speed. The viscosity of the alloy is reduced due to the material's thixotropic state, and it can therefore flow into the desired form with higher precision than in either casting or forging.

For the motorcycle wheels, the intent was to manufacture and bond the wheel rim to the wheel's center core as a post-casting operation. DatuMag performed flow, fill and strength simulations and provided process control. Toolcam, Mississauga, provided the mold making expertise.

The team encountered several challenges in the development, specifically shot size, ratio of solid to liquid in the slurry, fill times, screw velocity and runner and gate geometry. Compounding these issues was the goal to minimize capital investment by fitting the tool on an existing thixomolding machine rather than a larger one that theoretically would simplify production.

After several simulations, a successful casting design on the existing machine was achieved by progressively adjusting gate sizes and injection speeds, modifying overflows, fine tuning the vacuum system, and adjusting the solid to liquid ratio of the slurry. Modifications to the part model improved the alloy flow by limiting resistance.

However, a new problem arose. The wheel core was showing difficulty in bonding to the rim. The intent was to extrude the rim in a separate process and laser weld the two components together. The structural integrity of this part was important because failures with a magnesium wheel must be more forgiving than those of aluminum or carbon fiber to maximize safety. DatuMag suggested the wheel's inventor, Durrani Racing Components, cast the core and wheel rim together in one shot to solve the issue.

Back at the drawing board, the team came up with a modular design that allowed the core, cavity and slide tip inserts to switch between the front and rear versions of the wheel, both of which utilized a common mold base. The single cast approach dictated the central nozzle and gating geometry be altered. Modifications also were required to the vacuum system, venting system and solid-to-liquid ratio. After nine weeks of development, a single-piece magnesium wheel was able to be cast consistently via thixomolding.

Visit www.datumag.com for more information.