What is a bonded magnet?

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What is a bonded magnet?

Bonded magnet is a kind of magnet made by mixing magnetic powder with a certain proportion of binder according to a certain molding process. If the orientation magnetic field is applied in the forming process, the easy magnetization direction of magnetic powder is in the same direction in the magnet, which is called anisotropic bonded magnet, otherwise it is isotropic magnet. There are various forming methods for bonded magnets, including molding, injection molding, extrusion molding and calendering.

Types of bonded magnets

Bonded ferrite, bonded aluminum nickel cobalt, bonded rare earth cobalt and bonded neodymium iron boron are widely used in industry; rubber magnet, plastic magnet, resin magnet and soft metal bonded magnet are named after the adhesive.

Characteristics of bonded magnets

Compared with sintered or cast magnets, the properties of bonded magnets are lower. The maximum magnetic energy product (BH) max of low-cost bonded barium ferrite is 3 kJ / M ~ 3, and that of bonded rare earth cobalt (2-17 type) is as high as 160 kJ / M ~ 3. It is easy to make magnets with high dimensional accuracy and complex shape or assembled parts by bonding process.

Manufacturing process of bonded permanent magnet

Bonded permanent magnet is a kind of magnet made by mixing permanent magnet powder with a certain proportion of binder according to a certain process. According to its final shape, it can be divided into flexible magnet and rigid magnet. According to its production process, it can be divided into four types: rolling molding (also known as roll forming), injection molding, extrusion molding and molding.
Compared with other production methods (casting and sintering), the bonding process has the following characteristics:

  • 1) It can be made into complex shape and integrated with other components;
  • 2) Good consistency of magnetic properties;
  • 3) The appearance size of the magnet is accurate and no subsequent processing is required;
  • 4) Some magnets are isotropic and can be magnetized in any direction.

Bonded permanent magnet material is an important branch of permanent magnet materials. Although its performance is lower than that of sintered magnet, it can be used to prepare small and special-shaped permanent magnet, which is widely used in micro motor, office supplies, automatic control and other fields.
Bonded permanent magnets made of bonded permanent magnets are mainly used in computer (such as: HDD Hard drive spindle motor, CD-ROM disc drive, FDD floppy disk drive spindle motor and FDD stepper motor), automobile, automatic control and other fields. With the development of computer and automobile industry in China and even in the world, the market prospect of the project achievements is very considerable.

Manufacturing process of bonded permanent magnet

  • Calendering is an early bonding method. Its process is as follows: the magnetic powder and binder are mixed evenly in the proportion of about 7:3 (volume ratio), and rolled to the required thickness by two opposite rolling rolls in soft state, and then the product is made by pelletizing. The binder used is NBR and vinyl resin. The product is a flexible magnetic plate with a thickness of 0.3-6 mm, a width of about LM and a length of tens of meters. Generally, ferrite magnetic powder is used. In order to improve the magnetic properties, a small amount of NdFeB magnetic powder can be added. The surface of the magnetic plate does not need to be protected by coating. Generally, the surface is pasted as decoration.
  • Injection molding is evolved from the manufacture of injection molded products. First of all, the magnetic powder and binder are mixed evenly, and then the dried granules are made by mixing and granulating. Then, the particles are sent to the heating chamber with spiral guide rod for heating, injected into the mold for molding, and then the product is obtained after cooling. The binder used is usually nylon 6, polyamide, polyester and PVC, and the amount of binder is 20% ~ 30% (volume percentage). This process can be used to produce various kinds of bonded magnets with complex shapes, and the magnets are rigid. Ferrite, NdFeB and SmCo magnetic powder are commonly used. The latter two kinds of magnetic powder have serious wear on the guide rod and die cavity due to their high hardness, which is one of the most concerned problems at present. Because there is a layer of adhesive film on the surface of magnet, there is no need for surface coating protection.
  • The process of extrusion molding is basically the same as that of injection molding, the only difference is that the heated particles are extruded into the mold through a hole, and the product is also rigid. The binder used is the same as that of injection molding, and the addition amount is about 20% (volume). This process is usually used to produce thin or thin-walled annular magnets which are difficult to be realized by other bonding processes.
  • Compression molding is a bonding method referring to the powder metallurgy process. Firstly, the magnetic powder and the binder are mixed in proportion to make the binder evenly coated on the surface of each magnetic particle particle. After simple granulation and adding a certain amount of additives, the mixed powder is put into the mold to form on the press, and the molding pressure is increased Finally, the product is pressed to 150-7 cm ~ 2 at 120-7 ℃. Generally, the binder used is thermosetting epoxy resin or phenolic resin, with the addition amount of 10% ~ 20% (volume). Due to the small amount of binder added, the magnetic properties of the bonded magnet made by this process are the best, which is the fastest developing process at present. Especially after the appearance of Nd-Fe-B permanent magnet materials, this process has been used almost all bonded NdFeB magnets, and industrialization has been gradually formed. The surface of bonded magnet needs to be protected by coating, generally by cathodic electrophoresis, spraying or other surface protection methods.

Application of bonded magnet

The bonded magnet is mainly used to make micro motor, stepping motor, magnetic seal ring and various toys.

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