Injection molded magnets are available using ferrite, neodymium-iron-boron, SmCo and SmFeN, offering thus a wide range of magnetic properties.
Introduction
Injection molded magnets are available using ferrite, neodymium-iron-boron, SmCo and SmFeN, offering thus a wide range of magnetic properties. Binder types include Nylon 6, Nylon 12 and PPS. The different combinations of binders and magnetic alloys offer a wide range of application temperatures from -40°C up to 160°C.
The Injection molding process is particularly well suited to molding complex shapes and thin walled parts. This opens entirely new possibilities in design of magnets and magnetic systems, making thin rings, mechanical non-magnetized details etc. possible. More over, the magnet can be molded over a metallic or polymer component (insert molding) or molded over with a normal plastic, thus manufacturing an entire assembly in one or two process steps.
Manufacturing process
Permanent magnet powder and polymer are first kneaded together to form a compound, which then can be used for injection molding. Injection molding of permanent magnets is basically identical with injection molding of normal plastics. The only difference is that for production of anisotropic magnets a magnetic field is needed in the molding tool. After molding the magnets normally do not need any further operations, except magnetising, before use. In certain cases magnets can be magnetized in the molding tool and separate magnetizing is thus not needed.
Depending on the orientation of the magnet, there are differences in the molding process:
1. Isotropic magnets (no magnetic field in the tool): NdFeB, ferrite
2. Anisotropic (axial, diametral, radial) by electromagnet(s) in the tool: Ferrite, NdFeB, SmCo, SmFeN
3. Polar anisotropy, generated by permanent magnets in the tool: Ferrite
Magnetic properties
Properties of the injection molded magnets depend on the magnetic material and binder used. Properties given in the table 1 and 2 are typical for each grade at the room temperature. The actual properties in a finished magnet may differ from the material properties due to magnet geometry, magnetizing etc. Usually the qeometry of an injection molded magnet do not allow control of the properties by standard methods, like BH-curve tracker. Therefore, it’s important to agree on QA methods.
More information, including demagnetisation curves in various temperatures and physical properties, for each grade can be found in the datasheets (possibility to download). The same information is available in numerical form for magnetic modelling (request for info – e-mail).
Typical magnetic properties of bonded Ferrite magnets grades at 20℃
Fig 2 Typical demagnetisation curves for injection molded NdFeB grade xxx at various temperatures
Typical magnetic properties of bonded NdFeB magnet grades at 20℃
Design considerations
Magnetic properties of bonded magnets are naturally inferior in comparison of fully dense magnets of the same material. The same is true for temperature properties of the magnets. However, the possibility to design magnets with complicated shape, thin walls, multipole magnetizing and functional details more than compensates what is lost in pure magnetic power.
Fig1 Thin walled rings & rotor magnets
Fig2 Rotor magnets overmolded a shaft
Fig3 Rotor magnet with a (non-magnetic) gear wheel
Fig4 Magnetizing patterns possible for bonded magnets
