To protect magnet against moisture, dirt, friction or to achieve specific structure/color with magnet, over-molding is needed for magnetic components.
The magnet could be bonded magnet, NdFeB magnet, Ferrite magnet and etc.
The plastic body could be PA, PPS, PP, ABS, POM and etc.
They are widely used in motors ( such as vibration motor, DC motor, micro motor, starter motor, brushless motor, CD-ROM ), sensors, printers, automotive parts and etc.