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Common problems in the production process of anisotropic bonded magnets

2018-09-18
(1) Degree of orientation of granulated and anisotropic bonded magnets
The magnetic properties of anisotropic bonded magnets depend on both the magnetic properties of the magnetic powder and the degree of orientation of the magnetic powder. Granulation has a crucial influence on the improvement of orientation.
The magnetic powder is dried by wet mixing, and actually a granulation process is completed, and some magnetic powders are bonded together to form a granule. If the warm press forming process is adopted, the viscosity of the epoxy resin binder at a high temperature (above the softening point and below the solidification point) becomes very low. Therefore, the magnetic powder is more easily rotated under the action of a magnetic field, and the magnet can obtain a high degree of orientation. At this time, there is no special requirement for granulation. If the molding process is at room temperature, after the magnetic powder is granulated by the rubber, the acetone has been blown dry, and the binder may have become a state. In a fabricated particle, the easy magnetization direction of the magnetic powder is randomly fixed at different times. The direction. In this case, the degree of orientation of the magnet is not high regardless of the orientation field when the magnet is pressed. One possible solution is to mix and granulate under a magnetic field, so that in a fabricated particle, the direction of easy magnetization of the magnetic powder is fixed in the same direction. When the magnet is pressed, the entire fabricated particles will rotate in the orientation magnetic field, and the direction of easy magnetization of the magnetic powder in each particle is the same, so that the magnet can obtain a higher degree of orientation.
(2) Viscosity and gel time of the binder system
The anisotropic bonded magnet differs from the isotropic bonded magnet in the pressing process in that the direction of easy magnetization of the magnetic powder is aligned along the direction of the oriented magnetic field. Only when a higher degree of orientation is obtained, the magnet exhibits anisotropy and thus has a higher magnetic property. It is known that the rotational torque of the magnetic powder is proportional to the intensity of the orientation magnetic field and the saturation magnetization of the magnetic powder itself, and it is impossible to increase indefinitely. On the other hand, the resistance encountered by the magnetic powder during the rotation is in addition to the inherent magnetostatic interaction force and In addition to the mechanical meshing force caused by the irregular shape, the internal friction force when the epoxy resin binder (the molten state at the room temperature or the molten state under the temperature and pressure process) coated on the surface of the magnetic powder flows. By reducing this internal friction, the magnetic powder can be rotated relatively easily under a limited rotational moment, thereby increasing the degree of orientation.
The parameter characterizing the internal friction during rheology is the viscosity (viscosity is generally the abbreviation of dynamic viscosity, and the dynamic viscosity is the resistance generated when the two areas in the fluid are 1 m2 each, 1 m apart, and the relative moving speed is 1 m/s. It is called dynamic viscosity. Its unit is Pa s (pa sec) or mpa s (mPa s). Viscosity is divided into dynamic viscosity, kinematic viscosity and relative viscosity, which are different. Therefore, it is formulated. When using epoxy resin bonding agents, the viscosity of the system (including epoxy resin, curing agent and necessary additives) requires special attention. According to experience, depending on the strength of the orientation magnetic field, the viscosity of the binder system is preferably from 100 to 100 mPa s (l mPa s = l cP). When the viscosity is lower than 100 mPa? S, the magnetic powder is degummed, that is, the glue will be squeezed out from the gap of the mold; and when the viscosity is higher than 1 000 mPa? S, the rotation of the magnetic powder requires higher torque, that is, strong Orientation magnetic field. Again, it should be noted that the viscosity of the binder system referred to herein refers to the viscosity of the epoxy resin liquid which is liquid at room temperature and the viscosity of the molten epoxy resin in the molten state of the softening point.
Since the epoxy resin is not a Newtonian fluid, the viscosity values measured at a certain temperature are related to the shear stress and shear rate. Therefore, the viscosity values measured by different methods at the same temperature are not the same and are not interchangeable. The viscosity values mentioned here are all measured by a rotary viscometer. The viscometer should be equipped with a water bath or other temperature-controlled heater to measure the viscosity of the molten epoxy glue. Cylindrical rotors are particularly useful for measuring the viscosity of non-Newtonian fluids such as cycloolefin resin fluids. The viscosity of the glue can be measured with a suitable combination of rotor and speed.
In general, it takes a considerable amount of time from the process of "weighing one with a powder and wet mixing with a magnetic powder, one drying and granulating one orientation". It is known that after the curing agent is compounded in the epoxy resin, the reaction starts immediately, and the viscosity of the glue liquid gradually rises with the passage of time. The curing reaction rates of different types of curing agents are different, and the concept of gel time is involved here. By definition, gel time refers to the time during which the thermosetting resin begins to react from the addition of the curing agent (or catalyst) to the loss of fluidity of the liquid binder system. Sometimes for the sake of simplicity, the time when the viscosity is increased to 2 times the initial viscosity is defined as the gel time. It is obvious that the gel time of the epoxy resin binder system used for anisotropic bonded magnets should be long enough to ensure that the viscosity of the glue remains in the range of 100 mPa s to 1 000 mPa s during the magnetic field orientation. Inside. The ideal state diagram of the viscosity and temperature of the binder system with time is given in a temperature and pressure process. As shown in Fig. 9.27, the (a) zone is the time range for optimal magnetic field orientation, and the zone (b) is the time range for optimal pressurization. The reason is that in the (b) zone system, the gel point has passed, the crosslinked network begins to form, and the viscosity increases sharply. At this time, a magnet having a small porosity, a high density, and a high mechanical strength can be obtained by pressurization. In summary, for the design of epoxy resin binders for anisotropic bonded magnets, special consideration must be given to the viscosity and gel time of the binder system. The specific formulation needs to be determined experimentally, and it is considered that the use of latent curing agents or acid anhydride curing agents is two directions that should be worked hard.