Magnetizing and Handling

Temperature Effects
 

General Information
Ferrite magnets are sintered permanent magnets composed of Barium or Strontium Ferrite. Ferrite magnets have good resistance to demagnetization and are relatively low cost.

Ferrite magnets are very hard and brittle and require specialized machining techniques. Moreover, they should be machined in an unmagnetized state.

Anisotropic grades are oriented in the manufacturing direction, and must be magnetized in the direction of orientation. Isotropic grades are not oriented and can be magnetized in any direction.

Due to their low cost, Ferrite magnets are used in a wide range of applications, from motors and loudspeakers to toys and crafts, and are the most widely used permanent magnets today.  They are the most common magnet material used in automotive applications today.

Manufacturing Methods

Pressing and sintering involves pressing very fine ferrite powder in a die, and then sintering this pressed magnet. All fully dense Ferrite magnets are produced this way. Ferrite magnets can be wet pressed or dry pressed. Wet pressing yields better magnetic properties, but poorer physical tolerances. Generally, the powder is dry for grade 1 or 5 materials, and wet for grade 8 and higher materials. Sintering involves subjecting the material to high temperatures to fuse the pressed powder together, thus creating a solid material. Magnets produced through this process usually need to have some finish machining, otherwise surface finishes and tolerances are not acceptable. Some manufacturers extrude instead of press wet powder slurry and then sinter the material. This is sometimes done for arc segment shapes, where the arc cross-section is extruded in long lengths, sintered, and then cut to length.

Ferrite is brittle, and prone to chipping and cracking. Special machining techniques must be used to machine this material. The corrosion resistance of Ferrite is considered excellent and no surface treatments are required. 

Injection Molding: Ferrite powder is mixed into a compound and then injection molded in the same way as plastic.

Tooling for this manufacturing process is usually very costly. However, parts produced through this process can have very intricate shapes and tight tolerances. Injection molded ferrite properties are either lower or about the same as grade 1 Ferrite.

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Assemblies

Assemblies using metal or other components and magnets can be fabricated by adhering magnets with adhesives to suit a range of environments, by mechanically fastening magnets, or by a combination of these methods. Due to the relatively brittle nature of these magnet materials, press fits are not recommended.  Echelon Magnetics personnel have many years of experience making assemblies using ferrite magnets.

Magnetizing and Handling

Ferrite magnets require magnetizing fields of about 10 kOe. They can be magnetized with multiple poles on one or both pole surfaces. No special handling precautions are required, except that large blocks of Ferrite magnets are powerful, and care should be taken to ensure that they do not snap towards each other.

Temperature Effects

The curie temperature for ferrite magnets is ~450°C.  Up to about 450°C, changes in magnetization are reversible depending on the operating point of the magnet (permeance coefficient).  If the magnet is exposed to temperatures above this level the magnets will have irreversible demagnetization.  Once the magnets are cooled they may be remagnetized back to their original state.

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