Custom Molded Magnets
One of the more important developments in magnetic materials is polymer bonded magnets. This has 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.
Magnetic powders include ferrite and neodymium-iron-boron (NdFeB). There are several alloy compositions available of ferrite and NdFeB providing many different magnetic properties. Magnets using combinations of the powders have been developed by Phoenix America Inc. to fit specific application and environmental demands. Polymer binders utilized by the Phoenix America Inc. include nylons 6, 11 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, corrosion resistance for the rare earth magnetic materials, dimensional stability in the presence of water and solvents, and ease of processing.
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 150°C and 180°C respectively. 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.
An important characteristic of injection molded magnets is that they conform dimensionally to the precisely-machined mold cavity, 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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