What Is a Plastic Magnet?
A plastic magnet refers to a magnet formed by mixing magnetic materials into plastic and molding the mixture.
The magnetic powders used in the mixture include not only ferrite magnets but also neodymium magnets (NdFeB), samarium cobalt magnets (SmCo), and samarium iron nitride magnets (SmFeN). Due to their small and lightweight nature, plastic magnets offer high performance and find applications in various fields.
Manufacturing plastic magnets involves using pellet-shaped (rice-sized granules) plastic resin and a mixture of several additives, similar to the injection molding of resins.
Uses of Plastic Magnet
The primary applications of plastic magnets include various sensors and motors in industrial products and home appliances, as well as magnet rollers (mag rolls) for even toner distribution, pumps, suction goods, audio-visual equipment devices, and more.
They are utilized internally in a wide range of products, and in recent years, they have found application in the drive systems of electric water pumps in hybrid and electric vehicles, EGR valves, gasoline shut-off valves, onboard sensors, and more.
Properties of Plastic Magnet
Advantages
Compared to sintered magnets, plastics offer flexibility with fewer cracks and fractures, and superior dimensional accuracy without the need for grinding compared to unsintered magnets. Plastic magnets allow molding into complex and specialized shapes, surpassing the capabilities of sintered magnets. The production process is minimal, resulting in shorter lead times compared to sintered magnets. Depending on the design, integral molding with metal shafts or plates, and other resin components is possible.
Moreover, unlike sintered magnets, where improvement in dimensional accuracy requires grinding, plastic magnets can achieve enhanced accuracy through cutting. While both plastic and sintered magnets exhibit dimensional variations due to expansion and contraction, the dimensional accuracy after processing is comparable. Although grinding can process magnets collectively, cutting requires individual magnet setups, making grinding more cost-effective.
Disadvantages
For the same ferrite magnets, plastic magnets are several times more expensive than sintered magnets. Additionally, due to the inclusion of resin, plastic magnets have lower magnetic force compared to sintered magnets. Depending on the resin used, changes in dimensions due to absorption of water or expansion/contraction during heating may occur. While there is a specific heat resistance temperature for the magnet itself, for plastic magnets, the heat resistance temperature of the plastic must also be considered.
Considerations such as plastic-specific warping and surface irregularities must be considered based on the shape. Depending on the shape, design, and mold configuration, the generation of burrs, bubbles, variation in magnet density, and cracking may occur. Designing requires knowledge of injection molding, magnetism, and specific knowledge of plastic magnets.
Other Information on Plastic Magnet
1. Types of Resins
Resin types include PA12 (Nylon 12), PA6 (Nylon 6), PPS (Polyphenylene sulfide), and others. Nylon 6 has high water absorption, so it is not combined with rare-earth magnets that may corrode. Since ferrite magnets use iron oxide (iron rust), the magnets themselves do not corrode.
2. Molding Methods
There are two molding methods: injection molding for thermoplastic resins and compression molding for thermosetting resins.
In the case of injection molding, pellets are first dried. Since nylon is hygroscopic, drying prevents hydrolysis of the resin in the molding machine and mold. Using dedicated molding machines and molds for plastic magnets, pellets are melted through heating and compression inside the injection molding machine cylinder. The molten resin containing magnets is poured into the mold in the machine using the machine’s screw. After pouring, the molten resin cools and solidifies in the mold, transferring the mold shape to produce the plastic magnet.
During this series of actions, there are isotropic and anisotropic molding methods, where the magnetic powder in the pellets is molded without magnetic field orientation in the mold and with magnetic field orientation, respectively.
3. Anisotropy and Isotropy
Magnetic polarity is broadly categorized into isotropy, where the orientation within the magnet is not aligned, allowing magnetization in any direction, and anisotropy, where magnetic force can be increased in a specific direction through magnetic field orientation. In terms of production, magnetic force is weaker in anisotropic magnets compared to isotropic magnets. Anisotropic magnets, through magnetic field orientation, can achieve higher magnetic force in a specific direction.
4. Magnetization (Polarization) Direction
For anisotropic magnetic field orientation, the direction of magnetization and polarization methods vary depending on the application and characteristics, such as axial (top-bottom-axis direction), radial (circumferential-radiation direction), and pole anisotropy. For isotropic magnets, magnetic orientation is achieved using a magnetization machine and magnetization yoke. While axial and radial products can be used as molded magnets, many are remagnetized after demagnetization in a radial or axial direction, making them anisotropic.
Regarding pole anisotropy, a magnetic field orientation device with a pre-designed pattern is incorporated into the mold, determining the orientation in the mold. Pole anisotropy is advantageous as plastic magnets are already magnetized according to the pattern, eliminating the need for additional magnetization.