Hard Ferrite Magnets
Hard ferrite magnet, made by molding powder mixture of about 80% Fe2O3 and about 20% either BaCo3 or SrO3, is also called ceramic magnet. Along with deepening of research, some additives such as cobalt (Co) and lanthanum (La) are mixed together to improve the magnetic performance. The molded powder metallic green is sintered inside a temperature controlled furnace what is heated with electricity or coal. Though hard ferrite magnet is with low magnetic property, it is still a preferable choice to engineer in their design due to several factors: abundantly available raw material，the cheapest cost among permanent magnet family, low density, excellent stability of chemistry, high maximum working temperature and Curie temperature.
Gathering around our company, there are more than 60 companies have been producing hard ferrite magnet in the same city since year 1983. In the past years they have been aggressively developing new magnet to meet demand of market, continuously innovating technique to enhance efficiency and save cost, strictly execute ISO9001, ISO/TS16949 and 6sigm to improve quality, firmly adhere environmental system ISO14000 for promise of green production. The growth of these companies offers extensive and competitive sources for supplying hard ferrite magnet to our global customer.
Ring type hard ferrite magnet for acoustic application is the primarily marketed product and acts as an important business pillar to our company at the beginning phase. With realization of ?increasing demand for electric motion application, we have been concentrating our works on promotion of arc segment type hard ferrite magnet and have accumulated rich experience on properly producing magnet to minimize noise, maximize performance and optimize other intention of application. Also we have achieved success of developing hard ferrite magnet with irregular structure ( super-small, super-big, super-thin ), complicated geometry and super precision made by wet molding process. Limited by processing difficulty, such kinds of magnet were usually made by dry process and bonded process with shortage of low property and or expensive cost. Now our marketed hard ferrite magnet are extensively assembled inside motor, generator, sensor, loudspeaker, meter, relay, separator and many other applications for defense, automotive, robot, household electric appliance, wireless communication base station and mineral plant …
Strontium Hard Ferrite Magnet & Barium Hard Ferrite Magnet
The formula BaO-6Fe2O3 and SrO-6Fe2O3 describes the chemical composition of barium hard ferrite magnet and strontium hard ferrite magnet. Comparing barium hard ferrite magnet, strontium hard ferrite magnet is with better magnetic performance and higher coercive force. Due to cheaper material cost, barium hard ferrite magnet is still extensively used. Sometimes, mixture of strontium carbonate and barium carbonate are used to manufacture hard ferrite for achieving high magnetic property and cost save. In foodstuff industry, directly contact with barium ferrite magnet should be prevented for safety reason.
Wet Anisotropic, Dry Isotropic & Dry Anisotropic hard ferrite magnet
The molding of hard ferrite magnet is mostly by a press machine equipped with coil what can generate an external magnetic field for getting anisotropic magnet. The material for making anisotropic hard ferrite magnet is usually at wet slurry status so that molecules can be perfectly aligned in molding process. So we call magnets made by such kind process as wet anisotropic hard ferrite magnets what only can be magnetized along pre-orientation. The (BH)max of anisotropic hard ferrite is several times stronger than of isotropic hard ferrite magnet.
The raw material for making isotropic hard ferrite magnets is generally at dry powder status. The molding is by a punch machine what could not apply an external magnetic field on magnet. Thus the subsequently attained magnets are called dry isotropic hard ferrite magnets. Magnetization on isotropic hard ferrite magnet could be done with any preferred orientation and pattern depending on magnetizing yoke.
There is also one type hard ferrite magnet called dry anisotropic hard ferrite magnet. It is made of dry material powder but being oriented by an external magnetic field. The magnetic property of dry anisotropic hard ferrite magnet is lower than wet anisotropic one. Usually the dry and anisotropic process is used to mold magnet with complex structure but higher property than isotropic one.
Ceramic Magnet Shape and Dimensional Tolerance
Hard ferrite magnet can be economically molded with ring, arc segment, rectangular, disc, cylinder, trapezium or 2 combined geometries in one piece. Depending on these basic geometries, sphere, olivary shape, and other irregular geometry are available by further machining process. Slotting in hard ferrite magnet is practicable by pre-design in tooling or diamond grinding process. Hard ferrite magnets with 3S (super-small, super-big and super-thin) structure are also producible by wet molding process.
The dimensional deviation of hard ferrite magnet is controlled within +/-2% before being machined, and could be controlled within +/-0.10mm after being simply ground by diamond tool. Customs tolerance or precise control even +/-0.015mm is achievable, but should be negotiated. In most circumstance, wet anisotropic hard ferrite magnet is supplied with surfaces which parallel to anisotropic orientation un-ground and other surfaces ground. Definitions on tolerance of concentricity, roundness, squareness, perpendicularity and others please refer to MMPA standard No. 0100-00 or DIN17410 or ISO2768.
Tooling of Hard Ferrite Magnet
The economic process for manufacturing hard ferrite magnet at big quantity is by molding with tooling. The tooling for making anisotropic hard ferrite magnet is considerably expensive, but for making isotropic hard ferrite magnet it is cheap. If the required magnet is with same diameter, or same length and width when it is block type, as the current tooling, then we can utilize the ready tooling to mold different thickness/height magnets within permissible range.
In practice, sometimes we slice big block, grind bigger diameter ring/disc and machine arc segment type of close dimension into the required one. This process is helpful to get precise dimension, to save tooling cost when the order quantity is not so big (particularly at prototype phase), and to homogenize weight and flux of each piece product. The shortage of making magnet by machine is very expensive cost.
Anisotropic, Diametrically Oriented Hard Ferrite Magnet.
Ring type anisotropic hard ferrite magnets are mostly used with axial magnetization (parallel to pressing orientation). In the market there are some demands for ring type anisotropic hard ferrite magnet with diametrical magnetization (perpendicular orientation to pressing axis) what is comparatively difficult to be manufactured. This kind of magnet is designed for time-rotors, sensors, stepping motor and pump motor of house hold appliance e.g. washing machines, dishwasher, aquarium and heat-supply system etc. The manufacturing difficulty is the conflict between increasing magnetic force and decreasing product crack ratio. Magnet crack will frequently happen at processes of sintering and injection with shaft. Our engineer successfully broke the bottleneck and got some unique expertise on manufacturing this type magnet after more than 10years research.
The temperature coefficient of remanence of hard ferrite is negative. In contrast with rare earth magnet, the temperature coefficient of intrinsic coercive force of hard ferrite magnet is positive. Increasing temperature will lower remanence of hard ferrite magnet by 0.18%/℃, meanwhile intrinsic coercive force will be increased by about 0.30%/℃. Decreasing environmental temperature will lower the coercive force of hard ferrite magnet. Therefore it is suggested to have components with hard ferrite magnet not working at low temperature condition. The curie temperature of hard ferrite magnet is about 450℃. The recommended working temperature range of hard ferrite magnet is -40℃—250℃. Grain structure change will happen on hard ferrite magnet when the environmental temperature is up to about 800oC. At this temperature, the magnet could not restore original magnet force even by re-magnetization.
Chemical Stability and Coating
In most circumstance, hard ferrite magnet is with excellent stability of chemistry. It is resistant against many chemicals such as alkaline solutions, diluted acids, organic solvents, potassium and sodium hydroxides and brines etc. It can be etched by concentrated organic and inorganic acids such as sulphuric, hydrochloric, phosphoric, hydroflouric and oxalic acids. The etching degree and speed are determined by concentration, temperature and contacting duration. There is no corrosion will happen even it works at damp and hot environment, so it does not need to be coated for protection. For purpose of beauty decoration or surface cleanliness, it could be painted or plated with nickel and gold etc.
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