A method of securing magnetizable inserts within cores of an electric converter includes placing an assembly of rotor cores in a transfer molding press, the magnetizable inserts being disposed within cavities of the rotor cores and the cavities of the rotor cores being in fluid communication, placing a polymer preform proximate a central portion of the assembly of rotor cores, displacing the polymer preform such that the polymer preform changes state and flows radially and then axially through each of the cavities of the rotor cores, removing the assembly of rotor cores from the transfer molding press, and removing polymer waste from the assembly of rotor cores.