Image forming apparatus with image fixing device including an induction heater and a shield located between two sections of a core of the induction heater

An image forming apparatus, comprising:

• an image forming station for transferring a toner image to a sheet;
• and
• a fixing unit including a heating member that at least partially has an arcuate outer surface and a pressing member, the fixing unit being adapted to convey the sheet while sandwiching the sheet between the heating member and the pressing member and to fix the toner image to the sheet by heat at least from the heating member in a conveyance process, wherein the fixing unit includes:
• a coil for generating a magnetic field for induction heating the heating member, the coil being arranged on a virtual arcuate surface extending along, outside and concentric with the arcuate outer surface of the heating member;
• a first core made of a magnetic material and fixedly arranged around the coil to form a magnetic path around the coil;
• a second core made of a magnetic material, arranged between the first core and the heating member along the magnetic path and capable of changing a posture thereof by being rotated about a longitudinal axis intersecting a passing direction of the magnetic field generated by the coil, the second core having a tubular or cylindrical shape centered on the longitudinal axis;
• a shielding member made of a nonmagnetic metal and being curved along the outer surface of the second core to shield the magnetic field generated by the coil;
• and
• a magnetic shielding portion including a rotating mechanism for rotating the second core about the longitudinal axis and thereby changing the posture of the second core between a first posture where the shielding member shields the magnetism and a second posture where the shielding member does not shield the magnetism, wherein
• a relationship of r1≧r2 holds if r1 denotes a radius of curvature of the virtual arcuate surface on which the coil is arranged and r2 denotes a shortest distance from a center of curvature of the arcuate outer surface of the heating member to the outer surface of the shielding member with the second core set in the first posture.