Whether they are used in engine transmissions and camshafts in the automotive industry or its supplier, as rotary encoders in building services and mechanical engineering, in magnet wheels and ball bearings or pneumatic and hydraulic cylinders: Magnets play an important role in many industrial applications. For example, they help detect and record positions, speeds, rotational speeds, angles and locations. In many cases, plastic-bonded magnets are used for these tasks. They are made by adding a magnetic powder to a thermoplastic base material, such as polyamide (PA) or polyphenylene sulphide (PPS). This compound is then injected straight onto a (generally) metallic carrier.
In some cases, this can cause problems – because the plastic is applied at high temperatures and contracts when it cools down, which causes the material to shrink. This, in turn, is the cause for internal tensions within the magnet and means that it does not optimally stick to the carrier. In addition, different expansion coefficients of magnetic and carrier material can diminish the connection and therefore the functioning of the assembly – especially in the case of applications subject to large temperature swings. While an additional glueing of the magnet seems like a logical solution, this also means additional expenses and does not always lead to an optimal result.
High elasticity and chemical compounds
In these cases, elastomer-bonded magnets are a much better alternative. Here, too, a magnetic powder ensures the desired magnetic effect. However, it is not added to a thermoplastic but rather a synthetic elastomer, such as hydrogenated or non-hydrogenated acrylonitrile butadiene rubber (HNBR, NBR). These materials boast a much greater elasticity than thermoplastics. In addition, the elastomer forms a chemical bond with the carrier material when it is applied. This ensures a particularly reliable and strong bond without internal tensions within the magnet. Furthermore, elastomers feature a high impact resistance and are extremely resistant to many media, such as lubricants and chemicals. Depending on the material, the temperature resistance is up to 180°C.
Various magnetisation types possible
Another advantage of elastomer-bonded magnets is the high homogeneity of the base material. This permits a particularly accurate magnetisation – ideal for applications for which precision is key. Anisotropic magnets can be manufactured with a typical remanence of 230 millitesla. The type of magnetisation used most often is multipolar at the perimeter or sectoral. However, other magnetisation types are possible upon request. The minimum pole width is about one millimetre.
New materials for additional applications
Thanks to the use of elastomer-bonded magnets, MS-Schramberg can significantly expand the scope of application of its products. They are ideal, for example, when thermoplastic magnets cannot be injected into sleeves directly because of the resulting material shrinkage, or when ring magnets, due to the formation of cracks, cannot be injected straight onto the carrier materials but have to be glued on instead. Elastomers are also a functional solution when a high pole pitch accuracy is required or if the expansion coefficients of the carrier material and the magnet differ greatly. In some cases, their use also allows the selection of other carrier materials that make components less expensive to manufacture. Overall, elastomer-bonded magnets offer a significant potential for optimisations in many scenarios.