The pressed green body that leaves the press is fragile — drop it and it would crumble. The particles inside only touch loosely; there is no real bond between them yet. Sintering welds them into a solid magnet, and it works a lot like firing a ceramic pot in a kiln.
How it works, in one sentence
The green body is heated to just below its melting point, the powder particles fuse together where they touch, and the whole block shrinks into a dense, hard magnet.
Like baking, but hotter
Think of baking a cake: the ingredients are mixed and shaped, but they only become a cake in the oven, where heat makes everything bond. In a furnace around 1,000 °C the powder behaves the same way — atoms on the surfaces of neighboring particles migrate and grow together, filling the gaps between grains. The block shrinks noticeably and becomes almost fully dense.
The secret ingredient: a little liquid
NdFeB contains a neodymium-rich phase that melts at a lower temperature than the main magnetic phase. During sintering it turns to liquid and flows between the grains like glue, helping everything pack tight. That is why the finished magnet is dense and strong.
Why temperature control is everything
Too cool, and the magnet stays porous and weak; too hot, and the grains grow too large, which quietly destroys the magnet's resistance to demagnetization. The best window is narrow, so production furnaces hold temperature with great care — often within a few degrees.
What comes out
A dense, hard block — still not magnetic yet; that comes in the next step — but with all its tiny crystals fused together and still pointing the same way.
Read on: heat treatment and aging → · the whole production flow →