Ferrite Magnets can be isotropic or anisotropic. Isotropic Ferrite Magnets can be magnetised in any direction but have weaker magnetic properties. The Anisotropic Ferrite Magnets have a preferred direction of magnetisation within the structure and have strongest magnetic properties along that direction (they are more powerful than isotropic ferrite magnets).
There are two chemical varieties of ferrite magnet. Strontium ferrite is known by two chemical symbols:- SrFe12O19 or SrO.6Fe2O3. Barium ferrite is also known by two chemical symbols BaFe12O19 or BaO.6Fe2O3.
Ferrite Magnets (Ceramic Magnets) are produced by calcining (at between 1000 to 1350 degrees C) a mixture of iron oxide (Fe2O3) and strontium carbonate (SrCO3) or barium carbonate (BaCO3) to form a metallic oxide. In some grades, other chemicals such as cobalt (Co) and lanthanum (La) are added to improve the magnetic performance. This metallic oxide is then milled to a small particle size (less than a millimetre in size; usually a few microns). Then the process has two main production options depending on the type of magnet required.
SrCO3 + Fe2O3 > SrOFe2O3 + CO2
SrOFe2O3 + 5Fe2O3 > SrO.6(Fe2O3)
BaCO3 + Fe2O3 > BaOFe2O3 + CO2
BaOFe2O3 + 5Fe2O3 > BaO.6(Fe2O3)
The first is to press the dry fine powder in a die which results in an isotropic magnet (e.g. ferrite C1 grade) which has better dimensional tolerances (it will often not require any further machining to final size). The hexagonal crystal structure is random allowing the magnet to be magnetised in any direction afterwards.
The second method involves mixing the fine powder with water to produce a slurry which is then compacted in a die in the presence of an externally applied magnetic field. The external magnetic field helps the hexagonal crystal structure align more perfectly with the magnetic field, improving the magnetic performance (e.g. ferrite C8) – the water in the slurry acts like a lubricant. This results in an anisotropic ferrite magnet with stronger magnetic properties but it will possibly require additional machining stages to give the final dimensions. Sometimes a wet extrusion is performed instead of wet die pressing (to make arcs for example) – the magnet is then cut to required after sintering (sintering is the next stage).
The first method may also have an external magnetic field applied to produce anisotropic magnets as well (e.g. ferrite C5).
The compacted magnets (“green” magnets) are then sintered (at a temperature between 1100-1300 degrees C) to fuse the particles together. If any final machining is performed, it is carried out using diamond cutting tools (wire spark erosion will not work because ferrite is electrically insulating). Quite often the magnetic pole faces are machined/ground to the required finish and the other surfaces are left in an as-sintered state. The magnet is then washed and dried before being magnetised to saturation, inspected and packed for shipping to the customer.
Ferrite magnets made by wet pressing have better magnetic properties but are more likely to have bigger dimensional tolerances. Dry anisotropic hard ferrite magnet has lower magnetic properties than wet anisotropic hard ferrite.
As shown above, the magnets are made with tooling (dies). New shapes may require new tooling and, particularly for the anisotropic magnets, this tooling charge can be considerable. Where existing tools can be modified, this is done to keep costs for tooling as low as possible.
Dimensional tolerances tend to be +/- 0.25mm but +/-3% is also used. It may be possible to produce down to even less than +/-0.25mm but it depends on the grade and size/shape required as to what is achievable. Tighter tolerances cost more as more machining may be required.