NdFeB Magnet Stripping Operation (Strip Casting), Vacuum strip casting furnace rapidly solidifies NdFeB magnet material to create very small grains
Sintered NdFeB magnets manufacture process’s Four Step is Strip Casting, also called NdFeB Magnet Stripping Operation
Alloyed NdFeB is now ready for melting and strip casting. It is heated in a vacuum furnace and a stream of molten metal is forced under pressure onto a cooled drum where it is rapidly cooled at approximately 100,000 degrees-per-second. The high cooling rate produces very small grains of metal that help simplify and enhance the effect of the downstream processing. Also, small grains are an important part of producing high-quality magnets.
What is stripping operation?
For the manufacture process of NdFeB magnet, each step will produce a certain proportion of defective products. The coating process is also existing defective products, without exception. In order to recycle these defective magnets, the magnet manufacturers will strip the coating of defective magnets, then replating them. The simple stripping operation of NdFeB magnet can be divided into chemical stripping and physical stripping, and different types of coating have its own stripping operation solution.
To some degree, the stripping operation will affect the dimension and surface state of the magnet, and later influence its magnetic properties and uniformity.
The flakes of neodym cast alloys are prepared by using the strip casting technique. Then they are put into the HD machine, with vacuum and heat up to 700~800°C. The hydrogen enters the material in the spaces between the atoms and causes the material to expand. The differential expansion generates stress in the ingot and the alloy flakes break down into fine particle.
Vacuum strip casting furnace for Nd-Fe-B permanent magnet
vacuum melting & casting chamber
casting device : tundish and funnel
strip casting device : water cooling roller, roller surface clean & finish turning device.
cooling collection device : water cooling plate and mix mechanism
wind cooling system
medium frequency power
water cooling system
compressed air system
electric control system
It applied for rare earth permanent magnet materials NdFeB, hydrogen storage alloy, nickel-base materials, high temperature alloys melting, rapid quenching and rapid solidification process in vacuum or inert gas atmosphere condition.
Vacuum rapid solidification furnace NdFeB, hydrogen storage alloy, nickel-base materials, high temperature alloys melting
Ultimate vacuum Custom made
Pressure rising rate adjustable
Max temperature Custom made
Temperature uniformity ±3℃ / ±5℃
Heating power Custom made
Temperature control method Intelligent programmable temp controller
Control method PLC auto/manual interlock protection
Max gas charging pressure (absolute pressure) Custom made
Physical stripping of NdFeB magnet
The physical stripping generally means that stripping the coating from the surface of the magnet by the physical method.
For the softer coating, the manufacturer will utilize sandblasting operation to remove it. Compressed air will drive the abrasive particle to impact the surface of the magnet, and the surface will have enough cleanliness and roughness depend on the impact and cutting action of the abrasive particle.1
Both mechanical cutting and grinding process can be applied in the physical stripping of simple geometry shape NdFeB magnet. It should be noted that the direct machining will affect the dimensions of the magnet, and the feed rate must be closely controlled to avoid the embrittlement of magnet.
The difference of expansion character between coating and magnet is also used by magnet manufacturers to strip the coating. The repeated thermal shock treatment will cause the fatigue cracking. The abrasive paper is usually used for the stripping operation of complex shape magnet and halfway stripping magnet.
MACHINING 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”. COATING Once the final dimensions for the magnet has been met by machining, the Neodymium magnet is given a protective coating. This is usually a passivization of the surface or a Ni-Cu-Ni coating. Other coatings may be possible. It is not recommended to use the magnet without any protective layer.
Chemical stripping of NdFeB magnet
Chemical stripping will utilize the chemical reaction to corrode or dissolve the coating of magnet.
The chemical stripping is suitable for small size magnet, and different coating also uses different stripping solution system. The most common coating methods for NdFeB magnet are Nickel-Copper-Nickel coating and Zinc coating. The chemical character of rare earth permanent magnet is more active than coating, hence the chemical reagent and complexing agent need to be introduced to stripping solution system.
Zinc is a kind of amphoteric metal, and it can be reacted under both acid and lye. The strong acid stripping operation is usually using the nitric acid solution to remove the coating at room temperature. The alkaline stripping solution includes sodium hydroxide/potassium hydroxide and sodium nitride, and the temperature of the solution is around 100 deg c.
The stripping process for Nickel-Copper-Nickel coating is much more complex than Zinc coating, especially the primer Nickel layer. The simple formulation of Nickel-Copper-Nickel coating stripping solution is cyanide solution.
Cutting, Machining and Grinding
NdFeB magnets have had a lot of value added by now due to all of the prior steps. Cutting, machining and grinding are performed according to a strict control plan, and waste is minimized by design.
Wire cutting is performed with very fine wire to minimize kerf losses. Machining and grinding are minimized by close controls throughout the previous processes. Waste material is reused and recycled.
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