Image forming apparatus with fixing unit having induction heating member and shielding member for controlling induction heating

An image forming apparatus, comprising:

• an image forming unit for transferring a toner image to a sheet;
• and
• a fixing unit including a heating member and a pressing member and adapted to convey the sheet while holding 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 conveying process,
• wherein the fixing unit further includes:
• a coil arranged along an outer surface of the heating member for generating a magnetic field for induction heating the heating member;
• a core arranged to face the heating member with the coil located therebetween in order to form a magnetic path around the coil and made of a magnetic material;
• a shielding member arranged near the magnetic path generated by the coil, including a closed frame portion and made of a nonmagnetic metal, the closed frame portion having a size to permit the core to be relatively inserted therein;
• and
• a magnetic shielding portion for displacing the shielding member between a retracted position for permitting a magnetic flux to pass along a frame surface virtually formed inside the closed frame portion and a shielding position where the core is inserted into the closed frame portion for shielding magnetism by the penetration of a magnetic flux inside the frame surface.

An image forming apparatus, comprising:

• an image forming unit for transferring a toner image to a sheet;
• and
• a fixing unit including a heating member and a pressing member and adapted to convey the sheet while holding 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 conveying process, the heating member including an arcuate part, wherein the fixing unit further includes:
• a coil arranged along an outer surface of the heating member for generating a magnetic field for induction heating the heating member;
• an arch core having an arcuate shape and arranged to face the arcuate part of the heating member with the coil located therebetween in order to form a magnetic path around the coil and made of a magnetic material;
• a shielding member arranged near the magnetic path generated by the coil, the shielding member including a support arranged near one end of the arch core and a closed frame portion fixed to an outer surface of the support and made of a nonmagnetic metal, the closed frame portion having a size to permit the one end of the arch core to be relatively inserted therein;
• and
• a magnetic shielding portion including a drive shaft connected with the support for displacing the shielding member between a retracted position for permitting a magnetic flux to pass along a frame surface virtually formed inside the closed frame portion and a shielding position for shielding magnetism by the penetration of a magnetic flux inside the frame surface.