Preparation of bonded magnets (1) Magnetic powder and binder

In the previous issue, we have learned about isotropic and anisotropic magnetic powders. In this issue, we will talk about the magnetic powders and adhesives of bonded magnets with different processes.

The rare earth permanent magnet alloy used for bonded magnets is a submicron grain alloy powder, which allows people to easily crush the alloy powder to fill and form the desired particle size distribution, and then mix or knead with the binder. Depending on the physical or chemical properties of the adhesive and the differences in forming methods, different mixing and kneading processes are required.

Preparation of bonded magnets (1) Magnetic powder and binder

Preparation of bonded magnets (1) Magnetic powder and binder

Compression formed magnet

The proportion of magnetic powder is 96%~98% (mass fraction), the adhesive is thermosetting resin (usually epoxy) plus curing agent, the resin and curing agent are dissolved in an organic solvent and mixed evenly, then the magnetic powder is added to the solution and fully Stir to disperse the magnetic powder evenly and let it stand in the solution, let the adhesive in the solution form a coating film on the surface of the magnetic powder, then evaporate the organic solvent by heating or natural drying, and then crush the dry hardened magnetic powder to the desired size. Granularity, it can be formed for the next process. In order to improve the filling, orientation and compressibility of the magnetic powder, and increase the mechanical strength of the magnet, it is sometimes necessary to couple the magnetic powder before mixing, or add a lubricant after mixing and crushing.

Injected or extruded magnets

Using thermoplastic resin, to a large extent, the technology of mixing plastic pellets is used. The biggest difference from the plastic industry lies in its high solid filling ratio, and the volume of glass fiber, carbon fiber or other inorganic fillers used for general plastic modification. The ratio is 40%~50%, and the ratio of bonded rare earth permanent magnets may be as high as 80% in order to obtain high performance, so the mechanical structure of the equipment needs to be greatly adjusted, and the length-to-diameter ratio needs to be increased to make the pellets fully Plasticizing, mixing and tempering, but also surface hardening of the mixing part for severe mechanical wear. Considering the oxidation of magnetic powder and resin deterioration that may be caused by high-temperature mixing and subsequent forming processes, as well as the injection/extrusion processing of high-filling ratio pellets, it is necessary to make appropriate choices in terms of resin, various additives, and mixing processes. The most commonly used thermoplastic resin is nylon (PA), and polyphenylene sulfide (PPS) is used for high-temperature-resistant magnets. The former is an excellent engineering plastic widely used. The maximum operating temperature of the magnet is about 120 ° C, while the latter can be used up to 180 ~200℃; Additives include coupling agent, antioxidant, plasticizer, lubricant, etc. These components are fully mixed with magnetic powder before mixing, and then the mixture is sent to single-screw or double-screw for kneading Kneading in the granulator to make pellets for injection or extrusion.

Calendered Magnets

Rubber is used as adhesive, the most commonly used are nitrile rubber and chlorinated polyethylene, if there is a higher temperature resistance requirement, use polyurethane, fully mix the magnetic powder, resin, vulcanizing agent and other additives, and then The rubber is kneaded in an open kneader or a closed kneader, and finally the kneaded agglomerate is broken into a size suitable for the calender. During the kneading and forming process, the rubber is in a viscoelastic state, not like Injection/extrusion magnets are kneaded and formed in the molten state, so the kneading temperature is relatively low, and the anti-oxidation requirements for magnetic powder can be correspondingly lower. Among the polymer materials, there is a kind of thermoplastic elastomer, which shows good elasticity close to that of traditional rubber at room temperature, but at the same time can be processed by plastic or rubber technology, and has been successfully transplanted into calendered permanent magnets. The most commonly used It is chlorinated polyethylene elastomer, which can be used to manufacture bonded magnets with high filling ratio, and can omit the vulcanization process of traditional rubber. The biggest disadvantage of thermoplastic elastomers is that they are not as good as rubber in terms of wear resistance and temperature resistance, but wear resistance requirements are not very prominent for most permanent magnet applications.

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