Manufacturing of Bonded Magnets- Magnetic Powder and Binder
The rare earth permanent magnet alloy used for bonded magnets is a submicron grained alloy powder, which allows people to easily crush the alloy powder to the desired particle size distribution for filling and formation, and then mix or knead it with the binder. According to the difference in the physical or chemical properties of the adhesive and the forming method, different mixing and kneading processes need to be adopted.
Compression molded magnet
The proportion of the magnetic powder is 96%~98% (mass fraction), the adhesive adopts thermosetting resin (usually epoxy) and curing agent, the resin and curing agent are dissolved in an organic solvent and mixed uniformly, and then the magnetic powder is added to the solution and fully Stir to make the magnetic powder uniformly dispersed and stand still in the solution, let the adhesive in the solution form a coating film on the surface of the magnetic powder, then heat or dry naturally to volatilize the organic solvent, and then crush the dried and compacted magnetic powder to the required The particle size can be formed in 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 to add a lubricant after mixing and crushing.
Injection or extrusion molding magnet
The use of thermoplastic resins, to a large extent, extends the technology of mixing plastic particles. The biggest difference from the plastics industry is its high solid filling ratio, and the volume of glass fiber, carbon fiber or other inorganic fillers used for plastic modification. The ratio is 40%~50%. In order to obtain high performance, the ratio of bonded rare earth permanent magnets may be as high as 80%. Therefore, the mechanical structure of the equipment needs to be adjusted greatly, and the aspect ratio needs to be increased to make the pellets sufficient. For plasticizing, mixing and training, the surface hardening treatment of the mixing part should be taken for severe mechanical abrasion. Taking into account the possible oxidation of magnetic powder and resin degradation 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 selections in the compounding of resins, various additives, and the mixing process. The most commonly used thermoplastic resin is nylon (PA), and high-temperature magnets use polyphenylene sulfide (PPS). The former is an excellent engineering plastic with a wide range of applications. The maximum use temperature of the magnet is around 120°C, while the latter can be used up to 180°C. ~200℃; Additives include coupling agents, antioxidants, plasticizers, lubricants, etc., before mixing these components and magnetic powder are thoroughly mixed, and then the mixture is sent to a single spiral or double spiral rod for mixing Kneading is carried out in the granulator to produce granules required for injection or extrusion.
Roll forming magnet
Rubber is used as the adhesive, the most commonly used are cyanide-butadiene rubber and chlorinated polyethylene. If there is a higher temperature resistance requirement, polyurethane is used. The magnetic powder, resin, vulcanizing agent and other additives are fully mixed, and then the Rubber is kneaded in an open kneader or a closed kneader, and finally the kneaded agglomerates are crushed into a size suitable for the calender. During the kneading and forming process, the rubber is in a viscoelastic state, not like The injection/extrusion magnet is kneaded and formed in the molten state, so the kneading temperature is relatively low, and the anti-oxidation requirements for the magnetic powder can be correspondingly lower. There is a type of thermoplastic elastomer in high polymer materials, which exhibits good elasticity close to traditional rubber at room temperature, but at the same time it can be processed by plastic or rubber technology, and has been successfully transplanted into calendered permanent magnets, of which the most commonly used It is chlorinated polyethylene elastomer, which can be used to manufacture bonded magnets with high filling ratio, and the vulcanization process of traditional rubber can be omitted. The biggest disadvantage of thermoplastic elastomers is that they are inferior to rubber in terms of wear resistance and temperature resistance, but the wear resistance requirements are not very prominent for most permanent magnet applications.