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How are spherical rare-earth magnets manufactured?


A question came up the other day when playing with Buckyballs of how you manufacture spherical rare-earth magnets.

I'm surprised they could use that name, given the original Buckyball, but these Buckyballs are powerfully magnetic spheres with one side a North pole and the other a South pole which can be stuck together to make 2D and 3D structures.  Sale has been restricted in many places because of people ingesting them and requiring emergency surgery.

I've found information on neodymium magnet production using a sintering process, but can't see how you could produce a perfect Ni/Cu plated sphere with this method as individually machining to a sphere would be excessively laborious and tumbling would be a non-starter because of the attraction between balls. 

EDIT:
The sintering page wasn't the one I thought I was linking, but the one I've linked looks like it may have the answer in die-pressing or isostatic pressing in the aligning field of a solenoid.  Is this how it's done for spherical magnets? (Polishing the coated spheres would still be a problem.)

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In that article you linked to:
http://www.ndfeb-info.com/neodymium_magnets_made.aspx

Near the bottom of the page there is a graphic,
http://www.ndfeb-info.com/images/information/neodymium_magnets_made/diagram01.gif
titled, "Summary of Production of NdFeB magnets" , and that graphic is just a straight flowchart with 5 steps, specifically:
  1. Elements put into furnace to create the alloy.
  2. The alloy is broken down into a very fine powder.
  3. The powder is pressed in the presence of an applied magnetic field (anisotropic magnets).
  4. The "Green" magnet is sintered to give the final magnetic properties and shrinks during the process.
  5. The magnet is machined to its final dimensions, plated, magnetized to saturation, inspected, and then packed to ship out.
The think one of the things that might be confusing is there is actually more than one magnetizing step in the process.

The first magnetizing, the one mentioned in step 3 of the summary, is to produce this so called "anisotropy" in the crystal structure needed, I think, for to make it easier to apply the big, permanent, magnetizing later.

Then there's a demagnetizing step. It's not in the summary, but in the main page the quote is:

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 words in bold are words I think deserve emphasis.

At this point I should mention that phrase "final magnetic properties".   I think the way that should be interpreted is as magnetiz-ability, the ability to become magnetized, but the actual final magnetic field is not present yet.

Thus during the machining, and plating steps, there is NOT a huge magnetic field on the sintered magnet.  Not like huge field you observe on the finished product. 

That's because putting on the big field, the big magnetization, this is the very last manufacturing step

I mean the very last step prior to the more mundane post-manufacturing steps of inspection,  packing, and shipping, and um, getting paid.

Putting on the big field, the final and lasting magnetization, is a big scary deal, as described in this paragraph.  Again the words in bold are my emphasis.

Once plated the Neodymium magnet is then magnetised. The Neodymium magnet is placed in a solenoid coil which is energised to produce a field at least 3 times the value of the magnet’s Hci. It is not unusual for Rare Earth magnets to be 'hit' with a field of 5T. The Rare Earth magnets sometimes have to be physically held in place within the coil otherwise the magnet may react to the applied magnetizing pulse and is propelled out of the coil (a bit like a bullet). The Neodymium magnet, being anisotropic, has a direction of magnetisation locked within its structure. When being magnetised, this direction of magnetisation within the structure aligns with the magnetising field. If the magnet is not aligned with the magnetising field, the magnet will violently spin to align up. It is possible for the magnet to break up / shatter due to the high rotational forces acting on the domains within the magnet. The magnet must be magnetised to saturation to get the maximum performance output. If the magnet is not aligned with the magnetising field, full saturation of Neodymium may not be achieved.


So I think that's your answer.  During the parts of the process where the magnet is being shaped, there is no large magnetic field on the magnet,  and it can be shaped and plated just like any similar lump of metal, and the really big magnetic field gets put on as the very last step in the manufacting.
AndyGadget (author)  Jack A Lopez4 years ago
Thanks for the clarification and expansion.  That explains the whole process.
  The article I initially read and intended to link was nowhere as specific, and I linked that one unintentionally, but didn't appreciate the extra depth in there.  

Is this detail is correct

http://www.powerfulmagnetsby3p.com/how-neodymium-magnets-are-made

uttambarai3 years ago

Thanks for the short details. I had also a quarry about this question and I got me best answers from <a href="http://www.powerfulmagnetsby3p.com/how-neodymium-magnets-are-made">3p Magnets</a>

iceng4 years ago
The spherical magnets are prone to loose their metal plating on the poles
and begin flaking powder after a couple months especially the 3/4" and
larger sizes .
With children the process accelerates.

A
Most magnets are made by pressing the materials into a form. They are then heated and a large electric current is passed through them to magnetize them in the desired orientation. I don't know if they chrome them before or after the magnetizing process. Probably after since the heat and current would discolor the chrome pretty bad.
+1

The balls are formed, solidified, polished, magnetized, then plated.

The milling and polishing is easy before the magnetic field is applied to the alloy.