What are bonded magnets?

What are bonded magnets?

Bonded magnets appeared in the 1970s. At that time, SmCo had reached commercialization. The market for sintered NdFeB permanent magnets are very good, but it is difficult to precisely process into special shapes, and cracks, breakage, problems such as falling edges and corners are also difficult to assemble, which restricts its application. In order to solve this problem, the permanent magnet is crushed and mixed with plastic, and then molded in a magnetic field. This is probably the most primitive manufacturing method for bonded magnets. Bonded Nd-Fe-B magnets are widely used due to the advantages of low cost, high dimensional accuracy, large freedom of shape, good mechanical strength, and light weight.

Production process of Bonded Nd-Fe-B magnets

Bonded neodymium iron boron is made of permanent magnet powder mixed with adhesive materials such as rubber or hard and lightweight plastics, and directly formed into permanent magnet parts of various shapes according to user requirements.
The preparation of magnetic powder is a key process for processing neodymium iron boron permanent magnets. The performance of the magnetic powder directly affects the magnetic properties of the permanent magnet. The methods for preparing Nd-Fe-B magnetic powder include mechanical crushing method, frit rapid quenching method, HDDR method, gas spray method and mechanical alloying method. At present, the mainstream process adopts the HDDR method, which prepares high-performance rare earth permanent magnet powder through the process of hydrogenation-decomposition-dehydrogenation-recombination: firstly, the alloy is crushed into coarse powder, loaded into a vacuum furnace, and crystallized at a certain temperature. The alloy absorbs hydrogen and undergoes a disproportionation reaction, and then the hydrogen is pumped out to be reconstituted into rare earth permanent magnet powder with extremely small crystal grains. This method can obtain fine crystal grains with an average particle size of 0.3 μm, thereby obtaining magnetic powder with high coercivity.
There are currently 4 processes for press molding of bonded NdFeB: calender molding, injection molding, extrusion molding and compression molding, of which calendering and injection are more mainstream.

  • Calendering is to uniformly mix magnetic powder and binder in a certain volume ratio, roll to the required thickness and then cure to make a finished product. Generally, vinyl resin and nitrile rubber are used as the binder, and the surface of the product needs to be coated protection.
  • Injection molding is to mix magnetic powder with a binder (thermoplastic resin), mix by heating, granulate, dry, then send it to a heating chamber through a spiral guide rod for heating, and inject it into the mold cavity at a certain speed for molding. The finished product, due to the high resin content, can form a protective film on the surface of the magnet. Generally, there is no need for surface anti-corrosion treatment unless there is a higher requirement for the surface anti-corrosion ability.
  • Extrusion molding is basically the same as injection molding, the only difference is that the heated pellets are extruded into the mold through a cavity for continuous molding.
  • Compression molding is to mix the magnetic powder and the binder according to the proportion, granulate and add a certain amount of coupling agent, press and shape in the mold, and solidify at 120°~150° to finally obtain the finished product.

Product performance of bonded NdFeB magnet

With the addition of adhesive, the magnetic performance is lower than that of sintered NdFeB: Bonded NdFeB magnets are used to bond magnetic powder into a large magnet with a binder, and its density is generally only 80% of the theory. The sintered NdFeB magnet is heated at high temperature through a complicated process. Therefore, the bonded NdFeB magnet is weaker than the sintered NdFeB magnet.
The following table is the common product grades and properties of yueci combined with the integration of network resources for calendering and injection bonding NdFeB, for your reference.

Grade Br (Gauss) Hc (Oe) Hci (Oe) (BH)max (MGO) Max Working Temp. ( °C / °F)
BN-8 6,000 – 6,500 4,500 – 5,500 8,000 – 12,000 7 – 9 140 / 275
BN-10 6,500 – 7,000 4,500 – 5,800 8,000 – 12,000 9 -10 110 /  220
BN-12 7,000 – 7,600 5,300 – 6,000 8,000 – 11,000 10 – 12 130 / 260
BN-8H 5,500 – 6,200 5,000 – 6,000 12,000 – 16,000 6 – 9 125 / 250

The national standard for bonded neodymium iron boron permanent magnet materials is GB/T 18880-2012.

  • The product has high precision and large degree of freedom in form: Sintered NdFeB is produced by powder sintering. Generally, only a blank can be produced after sintering, and then through mechanical processing (such as wire cutting, slicing, grinding, etc.) to become various Shaped magnet. Compared with sintered NdFeB, the production process of bonded NdFeB permanent magnets is simpler, without secondary processing, and its products have high dimensional accuracy and no deformation; at the same time, it has a large degree of freedom in form and can be manufactured in various shapes according to actual needs The products, such as long strips, sheets, tubes, rings or other complex shapes, are convenient for mass automated production and have high mechanical strength.
  • Isotropic magnet, easy to magnetize in any direction: Bonded NdFeB is an isotropic magnet with the same magnetism in all directions, so it is convenient to make multi-pole or even countless monolithic magnets, which is usually difficult to achieve for sintered magnets.

Application of bonded NdFeB

The magnetic properties of bonded NdFeB magnets are inferior to that of sintered NdFeB magnets, but because of the convenience of manufacturing multi-stage magnetized ring magnets, the characteristics of excellent performance consistency and uniformity, and easy integration with other metal or plastic parts , Which is much higher than the magnetic properties of bonded ferrite, and is widely used in various types of micro motors and sensor systems. The specific uses of bonded NdFeB can be divided into:

  • Digital product: hard disk drive magnet (HDD)-this is currently the largest application field of bonded NdFeB.
  • Office OA products: drive motors for printers, motors for scanners, synchronous feed motors for copiers (STP), magnetic rollers for laser printers, etc.
  • Automotive motors and magnetic sensor products: including EPS power steering sensor magnets, wiper motors, window shutter motors, seat adjuster motors, etc.
  • Other types of industrial and household motors: mainly include various servo motors, motors for electric tools, air-conditioning refrigeration motors, etc.

Due to the huge difference in magnetic properties and molding, the use of bonded NdFeB and sintered NdFeB does not overlap. Bonded NdFeB is mainly used in the fields of hard disk drive spindle motors and low-power micro-special motors, while sintered NdFeB is more used in fields such as high-power drive motors.
Source: China Permanent Magnet Manufacturer – www.ymagnet.com



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