Magnetics Business & Technology - Summer 2015 - (Page 8)

FEATURE ARTICLE Bonded Magnets: A Versatile Class of Permanent Magnets By Dr. John Ormerod, Senior Technology Advisor | Magnet Applications, Inc. A Bunting Magnetics Company Permanent magnets are ubiquitous in modern societies. Devices that use permanent magnets include motors, sensors, actuators and acoustic transducers. These are used in home appliances, speakers, office automation equipment, aerospace, wind turbine generators, medical laboratory diagnostic test equipment and more. It is estimated, for example, that a typical automobile uses up to 120 permanent magnets in windshield wipers, starter motors, seat adjusters, door lock actuators, fuel pumps, sensors and gauges. The development of hybrid electric vehicle and electric vehicle drive technologies has been greatly enhanced by the availability of high performance magnetic materials. Of the many permanent magnet materials, four are still in common use: alnico, ferrite, samarium cobalt (SmCo) and neodymium iron boron (NdFeB). Alnico was invented and commercialized in the early 1940s. Ferrite magnets, also known as ceramic magnets, were first commercialized in 1952. SmCo5 was introduced in 1961 and an improved composition, based on Sm2Co17, was introduced in the early 1970s. The most recently developed material is NdFeB and was first available in 1984. Both the SmCo and NdFeB based materials belong to the family of rare earth magnets. Each magnet material has unique properties that make it more suitable for selected applications than other magnet options. Selection criteria include: magnetic strength, cost, constancy of magnetic output over temperature extremes, corrosion resistance, resistance to demagnetization, and mechanical properties such as density, physical strength or flexibility. Ferrite magnets, while providing less magnetic strength than rare earth magnets, cost far less. Therefore, they are still widely used wherever product cost is a major consideration over magnetic performance. Bonded magnets are an important but often overlooked group of products that magnetic circuit and device designers should consider when choosing the optimum permanent magnet type for their specific application need. In their most basic form bonded magnets consist of two components; a hard magnetic powder and a non-magnetic polymer or rubber binder. The powder may be hard ferrite, NdFeB, SmCo, alnico, or mixtures of two or more magnetic powders known as hybrids. In all cases, the powder properties are optimized through processing and chemistry specifically aimed at utilization in a bonded magnet. The binder that holds the magnetic particles in place can produce either a flexible or rigid magnet. Typical binders for flexible magnets are nitrile rubber and vinyl. Binders for rigid magnets include nylon, PPS, polyester, Teflon and thermoset epoxies. The thermoplastic binders may be formed into sheet via a calendaring or extrusion process or formed into various complex shapes using injection molding. Compression bonding almost exclusively combines isotropic NdFeB powders with a thermoset epoxy binder using a uniaxial room temperature pressing process. A major advantage of bonded magnet processing is near net shape manufacturing requiring zero or minimal finishing operations compared to powder or cast metallurgical processes. In addition value added assemblies can be economically produced in a single operation. Figure 1. What Makes a Magnet a Commercial Success * Flux density (Br) * Energy Product (BHmax) * Resistance to demagnetization (HCJ) * Usable temperature range * Magnetization change with temperature * Demagnetization curve shape * Recoil permeability (minimal - close to one) * Corrosion resistance * Physical strength * Electrical resistivity * Magnetizing field equipment * Available sizes, shapes & manfacturability * Raw material cost and availablity * Tooling investment Figure 2. Bonded Magnet Processes As previously mentioned there are four main processing routes used to manufacture bonded magnets. These processes are calendering, injection molding, extrusion and compression bonding. Calendering is a rolling process for making continuous magnet sheets. It is used for flexible, rubber-based magnets. The granulated compound of ferrite powder and elastomer is fed in the top and through a series of heated rolls. The rolls apply high compressive loads and by applying tension to the strip as it exits the rollers, a continuous roll of several hundred feet can be formed. In some cases mechanical orientation can be achieved due to the applied pressure and utilizing the plate-like shape of the ferrite particles. Typical sheet thicknesses are from 0.012 to 0.250 in. Hard magnetic powders are normally ferrite though some NdFeB and ferrite/NdFeB hybrids are available. Figure 3. 8 Magnetics Business & Technology * Summer 2015

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2015

Editor's Choice
Bonded Magnets: A Versitile Class of Permanent Magnets
Spintronics on Paper: The Whys and Wherefores
Magnets, Materials & Assemblies
Software & Design
Research & Development
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: The Rate Determining Step

Magnetics Business & Technology - Summer 2015