Product Name:
TrianglepermanentNeodymium Magnet
Before the pressed NdFeB magnet is released it is given a demagnetising pulse to leave it unmagnetised. The compacted magnet is termed a 'green' magnet – it is easy to force to crumble apart and its magnetic performance is not good. The 'green' Neodymium magnet is then sintered to give it its final magnetic properties. The sintering process is carefully monitored (a strict temperature and time profile has to be applied) and occurs in an inert (oxygen free) atmosphere (e.g. argon). If oxygen is present, the resultant oxides destroy the magnetic performance of the NdFeB. The sintering process also causes shrinkage of the magnet as the powder fuses together. The shrinkage gives a magnet close to the required shape but the shrinkage is usually uneven (e.g. a ring may shrink to become an oval). At the end of the sintering process a final rapid quench is applied to rapidly cool the magnet. This is to minimize the unwanted production of 'phases' (in simplified terms, variants of the alloy with poor magnetic properties) that occurs at temperatures below the sintering temperature. A rapid quench maximizes the magnetic performance of NdFeB. Because the sintering process causes an uneven shrinkage, the shape of the Neodymium magnet will not be to the required dimensions.
The next stage is to machine the magnets to the required tolerances. Because machining is required, the Neodymium magnets are made slightly larger when being pressed e.g. larger outer diameter, smaller inner diameter and taller for a ring magnet. Standard magnet dimensional tolerances are +/-0.1mm although +/-0.05mm is achievable at extra cost. The possibility of even tighter tolerances depends on the shape and size of the magnet and may not be achievable. For note, the Neodymium magnet is very hard. Trying to cut holes in NdFeB with a standard drill or carbide tip will blunt the drill bit. Diamond cutting tools (CNC diamond grinding wheels, diamond drills, etc) and wire cutting machines (EDM) have to be used. The NdFeB swarf powder produced during machining needs to be cooled by liquid otherwise it may spontaneously combust. For Neodymium block magnets, there may be cost savings in using much larger magnet blocks made by isostatic pressing and cutting them into smaller Neodymium blocks of the desired size. This is done for speed and for mass production (where enough cutting and grinding machines are present) and is known as "slice and dice". Once the final dimensions for the magnet has been met by machining, the Neodymium magnet is given a protective coating. This is usually a Ni-Cu-Ni coating.
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