High coercivity of ferrite Hcb, small recovery permeability mr, small density (4.5~5.2g/cm3), oxidation resistance and corrosion resistance, demagnetization curve work part is divided into straight line, good dynamic characteristics, no magnetic stability treatment, high material utilization rate, cheap price. The disadvantages are that the remanent Br is low, the magnetic induction temperature coefficient is large, the sintered ferrite is hard and brittle, the machining is difficult, and the electrical resistivity is very high (1012~1014 Mw.cm), can not be used wire cutting or electric discharge machining, only should be used toothless saw or diamond grinder grinding.
Ferrite has two series of isotropy and anisotropy, according to the different molding process, anisotropy ferrite and dry pressure and wet pressure. According to the main component classification, commonly used barium ferrite and strontium ferrite two kinds.
The ferrite powder is not mixed with rubber or plastic, can be made of a variety of plastic ferrites, in order to distinguish with sintered ferrites, this kind of ferrite is called bonded ferrite.
Wet molding and dry molding
Sintered Ferrite is manufactured by presintering (about 1300°C) raw materials into permanent magnet roughs, usually by proportionally batching them. The prefired coarse product has poor performance and is unstable. It needs to be ball milled into powder below the single-domain size, molded under a strong enough external magnetic field, and finally sintered in a short time (1200°C~1280°C).
In ferrite magnetic field forming process, wet pressing forming is to place the slurry after secondary ball grinding directly into the mold. During pressure forming, a strong magnetic field in a certain direction (consistent with the magnetic field direction required for product use) is applied at the same time, so that the easy magnetization axis of single domain grains is aligned along the external magnetic field direction. Pressure type at the same time to use the pump to remove the water, and the upper and lower punch at the gasket to prevent the withdrawal of stock, molding and then dry.
Wet pressing molding because the slurry water, the grain can rotate freely in the molding process, easy to grain orientation, so the magnetic property is good, Br large Hcb high. But when forming to gasket, pumping filter, production efficiency is low. Dry press forming is to add the dry powder without moisture after the secondary ball grinding into the appropriate amount of Binder (Binder), placed in the forming magnetic field to press forming. The magnetic field orientation of dry pressing is slightly poor, and the magnetic property is worse than that of wet pressing.
