Injection Molded Magnet
One of the more important developments in magnetic materials is injection molded magnet, which has opened a new world of application opportunities. The processing temperatures employed to blend and mold the material mixture are low enough to allow many different magnet powders to be used alone or in combination in the polymer binder to achieve an unprecedented range of properties.
Another benefit of the injection molding process is that magnet material can be directly molded into, onto, or against other assembly components, eliminating subsequent assembly steps. This is called insert injection molding.
Injection molded magnets can be of very simple shapes or very complex. All or only part of the device may be magnetized. It is sometimes cost advantageous to make an entire part out of the magnet material and just magnetize the portion requiring magnetic output.
Injection molded magnets are available using NeFeB, Ferrite, 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. Moreover, 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.
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:
- Isotropic magnets (no magnetic field in the tool): NdFeB, Ferrite
- Anisotropic (axial, diametral, radial) by electromagnet(s) in the tool: Ferrite, NdFeB, SmCo, SmFeN
- Polar anisotropy, generated by permanent magnets in the tool: Ferrite
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.
An important characteristic of injection molded magnets is that they conform dimensionally to the precisely-machined mold cavity, the result being close tolerances in the finished product. What little dimensional variability there are results from shrinkage of the polymer during cooling. This depends upon part thickness and shape. Typical tolerances are +/-0.003 in/in. Closer tolerances on critical dimensions can be negotiated.