Composite materials are used for the production of plastic-bonded, pressed magnets. They consist of magnetic powder that is embedded in a plastic matrix. Derived from the forming process, a distinction is generally made between two main groups of plastic-bonded magnets: injection-moulded and pressed.
For pressed, plastic-bonded magnets, NdFeB powder is embedded in a thermosetting plastic matrix. Epoxy resin is one of the materials used as plastic. Plastic-bonded NdFeB magnets are axially pressed into moulds. As a result of the very high filling degree of up to 97% NdFeB powder (percent by weight), much higher magnetic values can be achieved compared to plastic-bonded, injection-moulded magnets. Compared to the tools required for injection-moulded magnets, the tools for pressed magnets are less complex and less expensive.
Compared to sintered magnets, much more delicate shapes can be produced with plastic-bonded, pressed magnets. For example, thin-walled rings with Ø 27 x Ø 24 x 30 mm and diameter tolerances of only ±0.05 mm can be produced. Generally, no additional mechanical machining is required. However, if the requirements are particularly high, the magnets can also be ground to more narrow tolerances.
A typical data sheet for a permanent magnetic material contains its key magnetic and mechanical characteristics. The magnetic characteristics are usually measured in accordance with DIN EN 60404-5.
In addition to magnetic values, the data sheet also contains mechanical characteristics such as density, hardness and strength properties.
Epoxy resin is used as plastic matrix in pressed magnets. The maximum operation temperatures depend on magnet materials and shapes. In the case of NdFeB, they can reach up to 160°C.
Due to the low plastics content compared to plastic-bonded, injection-moulded magnets, higher magnetic properties can be achieved when using the same magnetic powder.
The magnetic characteristics of plastic-bonded, pressed magnets vary depending on the magnetic powder being used. The possible maximum operation temperatures vary between +130°C and +160°C.
In case of unfavourable shapes, in particular those with thin wall thicknesses or narrow pole pitches, there can be deviations in the material data.
The chemical resistance of plastic-bonded, pressed magnets is determined by the plastic matrix as well as the magnetic filling material. Pressed magnets feature a plastic content of approx. 10-20 vol.-%, and, compared to injection-moulded magnets, they cannot be produced as dense bodies. While the resin coats the magnetic particles, in corrosive conditions they are offering a greater area of vulnerability than injection-moulded magnets.
In the vast majority of cases, plastic-bonded, pressed magnets are used unprotected. For critical applications, the chemical property or the corrosion resistance can be further improved by adding a plastic coating.
In the past, magnetic rotors were manufactured in multiple successive production processes, which was very costly. Magnetic rings were built and, in a second production step, glued to the shaft. In addition, depending on customer specifications, a burst protection in the form of a stainless steel sleeve was shrunk and glued onto the magnets. Thanks to our new production technology, it is now possible to combine all of these processes into a single step.
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