Electromagnets :
7 products
COMPRESSION MAGNETS
The family of PLASTIFORM® brand bonded magnets (injection molded, compression bonded, and flexible) represent one of the more important groups of magnetic materials available today. They can be
tailored to suit a wide range of magnetic and physical strength, size, shape, and temperature characteristics. Compression bonded magnets are well-suited where maximum magnetic strength is required but physical strength can be slightly compromised, and where the profile shape is not complex. Another advantage over injection molded magnets is that tooling costs are relatively low making compression bonded a better choice than injection where volumes are low. With relatively little automation, compression bonded also lends itself well to high volume manufacturing (millions of magnets per year).
MATERIALS
Magnetic powders appropriate for compaction pressing include the many neodymium-iron-boron (NdFeB) grades as well as crushed and powdered samarium cobalt (SmCo). Ferrite is not useful due to the ultra-fine particle size. Combinations of NdFeB grades may also be used to create a specific set of magnetic characteristics.
The NdFeB materials used in this process are isotropic materials which do not require magnetic alignment of the powder during processing. The pressed product is extremely uniform in all directions. Uniformity of final magnetization is totally a function of the magnetizing equipment.
Almost all manufacturers use an epoxy binder. Various other additives are used to improve adhesion to the magnetic powder, corrosion resistance, flowability for improved die-fill, etc. The epoxies are quite resistant to solvents, but may suffer some softening and swelling if exposed for more than a short time. There is also considerable open porosity. To reduce corrosion of the rare earth materials, it may be necessary to apply a coating to the magnet.
TOLERANCES
Magnet dimensions in the X-Y plane are established by the die, by handling out of the die and by cure expansion. They are generally rated at +/-0.003 in. Closer tolerances can be negotiated. Tolerances in the pressing direction (axial length) are more problematic, and depend on many factors including part length, wall thickness, magnet shape and magnetic material. Please consult the factory for specifics.
TEMPERATURE CHARACTERISTICS
The epoxies used for bonded magnets are capable of use at or above 180°C. Maximum use temperature is almost totally a function of two items: 1) the combination of intrinsic coercivity and reversible temperature coeficient which, for any permeance coefficient, causes large irreversible losses at elevated temperatures and 2) degradation of the rare earth powder due to corrosion. Materials are available which perform satisfactorily at and above 165°C. Maximum temperature ratings are available in the catalog. Plastiform technical personnel will be glad to assist in the choice of material for your applications.
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Arnold Magnetic Technologies Groupe - SPS
FLEXIBLE MAGNETS
Spanning the range of applications from advertising specialties (signs and labels) to micro-motor magnets, biomedical, surgical drapes, car ski-rack hold-down systems and automotive sound deadening,
flexible bonded magnets serve us in a multitude of product types and markets. Ferrite flexible magnets have existed as a product for over 25 years. More recently, rare earth flexible bonded material has become available.
Three processes are used for manufacturing:
• an extrusion process which is widely used for
"profile extrusion" (magnets with irregular cross-section),
• calendering or rolling of pelletized compound between two rollers to form a continuous sheet, and
• a combination extruder/calender process.
Each process has advantages and limitations.
PLASTIFORM DIVISION PRODUCT TYPES
The Plastiform Division of Arnold Engineering produces continuous rolls of calendered sheet and strip product in thicknesses of 0.020" to over 0.375" (0.51 mm to over 9.5 mm). Energy product for ferrite materials ranges from 1.0 to 1.6 MGOe (8.0 to 12.7 kJ/m3). The binder choices include nitrile rubber, Nordel and Natsyn. The material is generally vulcanized, but is also available unvulcanized. Rare earth flexible NdFeB magnets are available in higher energy products.
Sheet can be stamped, slit, punched, laminated, or sanded to profile the edges or achieve close thickness tolerance. Magnetized sheet is available in several pole per inch (ppi) configurations from 4 to over 12 ppi, optimized for specific applications. More poles per inch increase the holding strength, but reduce the reach of the field. Fewer poles per inch increase the distance at which the magnet attracts, but reduces the ultimate holding strength. Pole per inch capability is also a function of the thickness of the sheet, thicker sheets having fewer ppi.
FLEXMAG INDUSTRIES PRODUCT TYPES
Flexmag manufactures five primary grades of extruded or calendered product and many more representing customer-required variations. Energy products range from 0.6 to 1.4 MGOe (4.8 to 11.1 kJ/m3). Thickness of extruded sheet is from 0.010" to 0.063" (0.25 to 1.60 mm). Extruded profiles are available in a wide range of shapes. Calendered sheet is also available. For profile extruded and calendered sheet, please contact the plant for size capability details.
Flexmag uses, primarily, a polyethylene binder system. Many variants are available including newer formulations with nitrile rubber. Again, please contact the plant to discuss your specific requirements.
TEMPERATURE CHARACTERISTICS
Both the polyethylene and nitrile rubber binders are capable, with ferrite, of use up to 120°C. Higher temperatures are possible, but with some dimensional change (usually shrinkage) and with loss of flexibility. Testing of nitrile with ferrite has been conducted up to 150°C.
The presence of rare earth metal powder, however, greatly reduces the temperature capability. First, above 80°C the rare earth reacts with the binders causing de-polymerization along with corrosion of the metal. Secondly, the rare earth material is so strong that at high temperatures when the binder is soft, the magnet will self-deform, literally tearing itself apart. It is imperative that flexible rare earth not exceed the recommended use temperature. Below the max recommended temperature, it is a very useful material.
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Arnold Magnetic Technologies Groupe - SPS
INJECTION MOLDED MAGNETS
One of the more important developments in magnetic materials, polymer bonded magnets, have opened a new world of application opportunities. The processing temperatures employed to blend and mold the
material mixture is low enough to allow many different magnet powders to be used alone or in combination in the polymer binder to achieve an unprecedented range of properties.
Another benefit of the injection molding process is that magnet material can be directly molded into, onto, or against other assembly components eliminating subsequent assembly steps. This is called insert injection molding.
Injection molded magnets can be of very simple shapes or very complex. All or only part of the device may be magnetized. It is sometimes cost advantageous to make an entire part out of the magnet material and just magnetize the portion requiring magnetic output.
MATERIALS
Magnetic powders include ferrite, neodymium-iron-boron (NdFeB), and samarium cobalt (SmCo). There are several alloy compositions available of ferrite and NdFeB providing many different magnetic properties. Magnets using combinations of the powders were introduced by Arnold in 1995 and are called "hybrid magnets."
Polymer binders utilized by the Plastiform Division include nylons 6 and 12 and PPS (polyphenylene sulfide). Binder evaluations are a continuous activity in the Development Laboratory, in an effort to improve physical strength, maximum recommended use temperature, improved corrosion resistance for the rare earth magnetic materials, improved dimensional stability in the presence of water and solvents, and greater ease of processing.
TEMPERATURE CHARACTERISTICS
Maximum use temperature is a function of both the magnetic alloy powder and of the binder. For example, ferrite powders in nylon 6 or PPS have tested satisfactorily at 180°C. On the other hand, nylon 12 melts above 170°C and is not recommended for use above 150°C. Magnetic material such as the highest energy NdFeB grade, with intrinsic coercivity of 9,500 Oersteds (760 kA/m), suffers significant irreversible magnetic loss above 120°C, regardless of which binder is used. Maximum use temperatures are listed on each product's specification sheet.
TOLERANCES
An important characteristic of injection molded magnets is that they conform dimensionally to the mold cavity which is precisely machined, the result being close tolerances in the finished product. What little dimensional variability there is results from shrinkage of the polymer during cooling. This depends upon part thickness and shape. Typical tolerances are +/-0.003 in/in. Closer tolerances on critical dimensions can be negotiated.
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Arnold Magnetic Technologies Groupe - SPS
