Four Types of Sintered NdFeB Forming Processes

Compared with traditional powder metallurgy processes, the molding of sintered NdFeB features two main characteristics: magnetic field orientation and oxidation protection. The molding process essentially determines the geometric shape, size, and orientation of the magnet, and it is a key step in the preparation of sintered NdFeB.

Dry Molding

Through many years of practice and improvement by professionals in the magnetic material industry, dry press molding has become the preferred method for mass production of sintered NdFeB. The magnetic powder is oriented by a magnetic field in a mold cavity of a certain shape, and the pressing process is completed by the die and punch. However, as the density of the green compact increases, the orientation inevitably gets damaged. Dry molding of sintered NdFeB can be divided into two types: parallel pressing and vertical pressing, based on the correspondence between the magnetic field direction and the pressing direction. Vertical pressing is more widely adopted because it causes less damage to the orientation of the sintered NdFeB powder.

Single-Step Molding Process

By increasing the pressure of the molding press, the density of the sintered NdFeB green compact can be increased to more than 4.2g/cm³, thus eliminating the need for isostatic pressing. After pressing with a fully automatic molding press, the green compacts are placed in sintering boxes by a robotic arm, and then transported to a continuous sintering furnace through a sealed channel under the protection of inert gas. This realizes automated production, reduces labor costs, and ensures that the whole process is conducted in a low-oxygen environment, which benefits the stability of the process and the performance of the sintered NdFeB.

Rubber Membrane Isostatic Pressing with Pulse Magnetic Field Molding

After filling the rubber mold with magnetic powder, it is placed inside a metal mold and oriented by a pulse magnetic field. The metal punch compresses the rubber membrane and the magnetic powder. Due to the limitations of the metal mold cavity, the rubber mold expands inward, applying the isostatic pressure to the sintered NdFeB powder sample. Since there is no relative movement between the cavity wall and the powder, the orientation is well preserved.

Near-Net Shaping Process (Single Piece Pressing)

In parallel pressing, the magnetic field orientation and the pressing direction are the same, so the degree of damage to the orientation is much higher than in vertical pressing. Moreover, due to the limitations of the orientation pole size, the pressing area of the product is relatively small. However, parallel pressing can achieve near-net shaping of cylindrical, ring, special-shaped, and single-piece products in one step due to the advantages of feeding and orientation. It offers high precision in pressing, good consistency in magnetic performance, reduces machining allowances, and improves material utilization. However, the single-piece pressing process has higher requirements for the flowability of the sintered NdFeB powder, the press (servo control accuracy, magnetic field size and uniformity, automatic powder feeding, etc.), molds, and sintering processes.