Smaller-but-better permanent magnets

Win Lum
Magnequench Corp.
Anderson, Ind.

Here's a not-so-well-known trend: The cost of high-energy magnets is coming down. The use of neodymium magnets in particular is growing rapidly, allowing manufacturers to reduce costs. Magnequench, for example, expects to reduce the price of NdFeB quenched powders by 7% or better yearly until 2005.

Neodymium-iron-boron (NdFeB or Neo for short) first reached the market about 15 years ago. No other magnet material has a higher energy density. Motors built with Neo magnets can be small, pack a lot of torque, and work efficiently.

Still, Neo magnets have had a reputation for being more expensive than ordinary magnet materials. They found use where the need for compactness and weight savings outweighed the component cost.

Ever more applications have used Neo magnets in the last decade. Suppliers of the material have, over time, implemented process improvements that have reduced its cost. For example, consider a recently introduced magnetic powder called MQP 13-9. It costs $25.50/kg. This compares favorably with earlier rare earths.

The magnetic properties of Neo magnets have improved over time as their costs have dropped. Today, most new products and applications that employ rare earths are designed with Neo magnets. The magnets still cost a bit more than ferrite, but their ability to provide a smaller volume and more torque offers a better total cost.

Moreover, advances in manufacturing processes have let high-energy magnet material take on a wider range of shapes in finished products. The ability to injection-mold high-energy magnets is the most recent development in the magnet industry. Injection-molding capabilities let manufacturers create intricate magnet subassemblies on demand for specific applications. This process allows the magnet to be a part of the structure and not just a component.

Injection molding provides flexibility, allowing the engineer to specify varied, complex shapes and tight tolerances not easily obtained through other manufacturing processes. Injection-molded Neo magnets have made possible innovative designs in industrial, computer storage, and automotive applications. In some cases, the advantages of the new technology were compelling enough to merit redesigning an existing product.

NdFeB processing
It is useful to understand the manufacturing processes now employed to produce Neo magnets. The rare-earth-magnet material can be processed by powder metallurgy (crushing, pressing, and sintering), gas atomization or rapid quenching and pressing. Sumitomo Special Metals Co. Ltd., Japan, holds the patents for the powder-metallurgy process and Magnequench International Inc. holds the patents for the gas atomization and rapid quenching process.

The crushing, pressing, and sintering process consists of pulverizing the NdFeB alloy into fine powder before pressing it in a magnetic field to align the individual particles. The material is then compressed into the desired shape, typically a block. The magnets are sintered at temperatures around 1,100°C and then heat treated at lower temperatures to improve the magnetic properties. Because of shrinkage and warpage during the sintering process, powder-metallurgy magnets must be produced in larger shapes and then ground to their final size.

In the rapid-quenching process, molten NdFeB alloy is ejected onto a chilled, rotating metallic wheel. The alloy cools or quenches as it hits the wheel, rapidly solidifying into ribbonlike flakes, which are crushed to form NdFeB powder. Compared to the powder produced by the metallurgy process, the rapidly quenched powder is considerably coarser and has an ideal microstructure for making a magnet. The powder is used to produce bonded or hot-pressed magnets that are used in various motion-control applications.

Bonded magnets are produced by combining the powdered alloy with an epoxy or thermoset resin and molded through either compression or injection molding techniques. Hot-pressed magnets are produced by cold pressing the powdered alloy to a form and then hot pressing at around 750°C to increase density. This process is remarkably different from sintering because it uses fewer steps and no orienting magnetic field, yet it still provides similar magnetic properties. In either case, bonded or metal magnets can be made to the net final shape, without expensive grinding.

Compact applications
A typical example of a contemporary Neo application comes from a large international appliance manufacturer. One of its products used a 24-Vdc motor employing ferrite arcs in a typical two-pole configuration. The goal was to improve the power density of the motor: more power for the same frame size or a smaller frame size for the same power. The motor could hit an operating temperature as high as 160°C.

Three possible Neo magnet configurations looked promising, using injection or compression-molded magnets. In each case, developers tweaked the magnet's shape to maximize the air-gap field, widened the rotor teeth slightly to accommodate the extra flux, and reshaped the slots a little to retain the winding area.

Magnequench engineers studying the situation found the optimum design to be an injection-molded ring magnet. Injection molded and magnetized in the stator housing, the bonded Neo also cost less to manufacture and assemble because it eliminated the need for magnetizing, gluing and curing the ferrite arcs. All in all, it provided more output power at a total cost that was similar to the old design.