High Performance Ferrite Mass Production Technology

High performance ferrite magnet definition

The high-property permanent magnet must own the following three typical parameters concerning the high magnetic property:

  • High Br:  Br>4200 Gs, Hcj > 3400 Oe
  • High Hcj: Br>3900 Gs, Hcj > 4800 Oe
  • High Br&Hcj: Br > 4100Gs, Hcj > 4100 Oe

It is easy to gain the high-property magnet in the labs but in a mass product it will require high-quality raw materials, equipments, tools and control systems. And another primary condition is the engineers, technical staff and workers should be given a very good training.

This article will brief the second mass production technology of high-property permanent ferrite magnet from the aspects of raw material selection, processing optimization, equipments and tooling molds.

The crucial processing technology in mass production

1. Material selections

Do not try to product high-property magnets with iron-phosphorus materials. Do not use so-called high-quality materials provided by raw material suppliers who are not capable of quality control.

2. Processing optimization

Rough crush: keep the material purity and make the particle size at about 5 um to reduce the powder grinding period, reduce the heat and optimize the particle distribution.

Fine crush: adopt the wetting ball grinding method to get a fine comminution. The proportion of material, ball and water content are crucial. It is reasonable to reduce the raw material properly to make the proportion of the steel balls bigger. And it is necessary to get enough cooling water to prevent the temperature of the ball grinder raising. The material and the size of the steel balls is also very important. The rotating rate of the ball grinder can influence the trace of the steel balls which will directly affect the ball grinding efficiency. The cleanness extent of the material output will have a big influence on the particle distribution in the material slurry. The particle distribution like the PH value can influence the magnetic orientation and magnetic property. Adding an proper amount of water while in ball grinding can improve the ball-grinding efficiency and increase the magnetic orientation limit.

Shape: In the shaping course, the main factors that affect the product magnetic property are magnetic field and product density. So appropriately increasing the charging current and coil circles will be helpful. Adopting magnetizing mold assembly can also make full use of the magnetic field. With a full consideration of the mold strength, tons of the shaping machine and the bearable pressure on the shaped flan when it leaves the mold, it is helpful to increase the shaping stress intensity properly to get a higher magnetic orientation and the completeness of fixed-phase reaction in the sintering process.

Sinter: The sintering is the key processing in the production of ferrite permanent magnet. The magnetic properties will be influenced greatly by all such factors as the oxygen content, the sintering temperature, the length of thermal area, the equality extent of the temperature and the change rate of the profile temperature. If the oxygen is insufficient, the Br and Hcb will be lower and the appearance will have spots in some extents. If the situation is serious, the central overdense locating area or the centre of big and thick products will appear obviously a blank filling layer and the product is fragile; Sintering with high temperature and short thermal area and sintering with low temperature and long thermal area will cause two totally different results about magnetic properties. The equality extent and change rate of the heating temperature will influence the product appearance and Hcj value. Another thing worth attention is that the adjustment on smoke exhausting system will cause a change of the oxygen content in the furnace.

Ingredient proportion: When the technology in each processing procedure is optimized, the ingredient proportion of the materials will play a crucial role. It is not encouraged to use many basic materials to make high-property products because different raw materials have different hardness and contractility which in the ball-grinding period will influence the distribution of granularity in the material slurry and impair the product properties. If an appropriate amount of mixed ingredients is added into, the magnetic properties can get a great improvement. The mixed ingredients should be at least two elements of Ca, Si, Al, La, Cr, Co, Bi. Some elements among them can reduce the sintering temperature, improve the density, saturation, magnetic strength and anisotropic features. They are useful to increase Br value, such as Ca, La, Bi, etc. If the amount added is too small, it would not achieve the expected effect but if the amount added is too much, it would reduce the density of the magnetic flux and cause a loss to Br. The exchange and substituent of Al and Cr can prevent the growth of particulate and therefore can enhance the Hcj value but it is also possible to cause a less sintering density which will lead to an obvious loss to Br value. In order to get an product with high Br and high Hcj, it is important to adjust the La and Co elements. What should be noted is that after the rare metals are added, the magnet will be more reactive on the temperature, being more thermal sensitivity. The amount of the metal element added can be varied with some changing parameters in the ball grinding course.

Equipments and processing molds

The material of the working face of the equipments for rough crush and fine crush is very important. The material must be friction resistant and contain some element useful for ferrite.

With a view to the production cost and efficiency, the trend of mold manufacture is the more cavities the better and using large tunnage pressers to shape. But this concept is wrong when in production of high-property magnets. When a mold has many cavities, it is difficult to maintain a stable compacting period and the magnetic orientation will be uneven. Form the technical view, the ideal number of cavities is to guarantee a consistent qualified rate in the shaping but with some surplus left. The problem is more small pressers are required.

The double-push-slab electric kiln we use produced both by Electronic 48 institute and Beijing 798 factory can meet the demands in a mass production for high-property ferrite permanent magnets. It is necessary to make a proper control on the turnout while sintering the high-property magnets.