Fixing device and image forming apparatus including the same

1. A fixing device comprising:

a heating rotating member configured to generate heat by induction heating;

a pressing rotating member that is disposed so as to oppose the heating rotating member;

a fixing nip section that is formed by the heating rotating member and the pressing rotating member, the fixing nip section nipping and transporting a transfer material;

an induction coil disposed apart from and along an outer surface of the heating rotating member, the induction coil being operable for generating magnetic flux for causing the heating rotating member to generate the heat; and

a magnetic member core section forming a magnetic path extending along an inner side of an inner peripheral edge of the induction coil and an outer side of an outer peripheral edge of the induction coil, the magnetic path surrounding the induction coil,

wherein the magnetic member core section includes a first core section, a second core section, a magnetic flux shielding member, and a plurality of third core sections, the first core section opposing the outer surface of the heating rotating member with the induction coil being disposed therebetween, the second core section being disposed beside the first core section and near the inner peripheral edge of the induction coil in a direction in which the magnetic path surrounds the induction coil, the second core section opposing the outer surface of the heating rotating member without the induction coil being disposed therebetween, the magnetic flux shielding member being disposed at a side of an end portion of the first core section opposite to the second core section and separated from the end portion near the outer peripheral edge of the induction coil, the magnetic flux shielding member opposing the outer surface of the heating rotating member without the induction coil being disposed therebetween, the magnetic flux shielding member reducing or blocking the magnetic flux, the plurality of third core sections being disposed at a side of the magnetic flux shielding member facing the end portion of the first core section, the plurality of third core sections opposing the outer surface of the heating rotating member without the induction coil being disposed therebetween, and

wherein at least one of the third core sections is movable to a first position and to a second position, the first position being where an end portion of the at least one of the third core sections that faces the heating rotating member is separated from the outer surface of the heating rotating member by a first distance and opposes the outer surface of the heating rotating member, the second position being where the end portion of the at least one of the third core sections that faces the heating rotating member is separated from the outer surface of the heating rotating member by a second distance and opposes the outer surface of the heating rotating member, the second distance being greater than the first distance.

2. The fixing device according to claim 1 , wherein the plurality of third core sections are disposed side by side in a direction vertical to a transportation direction of the transfer material.

3. The fixing device according to claim 2 , wherein, when the plurality of third core sections are positioned between the first core section and the magnetic flux shielding member, the plurality of third core sections are positioned in contact with or close to the end portion of the first core section.

4. The fixing device according to claim 2 , wherein a plurality of the magnetic flux shielding members corresponding to the plurality of third core sections are provided so as to be annular in correspondence with the plurality of third core sections, and wherein each magnetic flux shielding member has a first surface, a second surface, and an opening, the first surface being a surface facing the end portion of the first core section, the second surface being a surface opposite to the first surface, the opening extending through the first surface and the second surface.

5. The fixing device according to claim 1 , wherein the plurality of third core sections are each positioned at the first position or the second position in accordance with a size of the transfer material.

6. The fixing device according to claim 1 , wherein an end portion of the magnetic flux shielding member facing the heating rotating member is disposed so as to be separated from the outer surface of the heating rotating member by a separation distance,

wherein the first distance is less than or equal to the separation distance, and

wherein the second distance is greater than the separation distance.

7. The fixing device according to claim 1 , wherein the second position includes a 2 A position and a 2 B position, the 2 A position being where part of the magnetic flux shielding member does not overlap the third core sections, the 2 B position being where the magnetic flux shielding member does not overlap the third core sections.

8. The fixing device according to claim 2 , wherein the plurality of third core sections are each disposed at a position corresponding to a size of the transfer material along a direction vertical to the transportation direction of the transfer material.

9. The fixing device according to claim 8 , wherein the plurality of third core sections are each positioned at the first position or the second position in accordance with the size of the transfer material that is transported to the fixing nip section.

10. An image forming apparatus comprising:

an image forming section configured to form a toner image on a transfer material; and

a feed/discharge section configured to supply the transfer material to the image forming section and configured to discharge the transfer material on which the toner image is formed,

wherein the image forming section includes an image carrying member where an electrostatic latent image is formed, a developing device configured to develop the electrostatic latent image to form the toner image, a transfer device configured to transfer the toner image to the transfer material, and a fixing device configured to fix the toner image transferred to the transfer material to the transfer material, and

wherein the fixing device includes

a heating rotating member configured to generate heat by induction heating;

a pressing rotating member that is disposed so as to oppose the heating rotating member;

a fixing nip section that is formed by the heating rotating member and the pressing rotating member, the fixing nip section nipping and transporting a transfer material;

an induction coil disposed apart from and along an outer surface of the heating rotating member, the induction coil being operable for generating magnetic flux for causing the heating rotating member to generate the heat; and

a magnetic member core section forming a magnetic path extending along an inner side of an inner peripheral edge of the induction coil and an outer side of an outer peripheral edge of the induction coil, the magnetic path surrounding the induction coil,

wherein the magnetic member core section includes a first core section, a second core section, a magnetic flux shielding member, and a plurality of third core sections, the first core section opposing the outer surface of the heating rotating member with the induction coil being disposed therebetween, the second core section being disposed beside the first core section and near the inner peripheral edge of the induction coil in a direction in which the magnetic path surrounds the induction coil, the second core section opposing the outer surface of the heating rotating member without the induction coil being disposed therebetween, the magnetic flux shielding member being disposed at a side of an end portion of the first core section opposite to the second core section and separated from the end portion near the outer peripheral edge of the induction coil, the magnetic flux shielding member opposing the outer surface of the heating rotating member without the induction coil being disposed therebetween, the magnetic flux shielding member reducing or blocking the magnetic flux, the plurality of third core sections being disposed at a side of the magnetic flux shielding member facing the end portion of the first core section, the plurality of third core sections opposing the outer surface of the heating rotating member without the induction coil being disposed therebetween, and

wherein at least one of the third core sections is movable to a first position and to a second position, the first position being where an end portion of the at least one of the third core sections that faces the heating rotating member is separated from the outer surface of the heating rotating member by a first distance and opposes the outer surface of the heating rotating member, the second position being where the end portion of the at least one of the third core sections that faces the heating rotating member is separated from the outer surface of the heating rotating member by a second distance and opposes the outer surface of the heating rotating member, the second distance being greater than the first distance.

11. The image forming apparatus according to claim 10 , further comprising a receiving section and a drive controlling section, the receiving section receiving image formation instruction information including size information of the transfer material having the toner image formed thereon by the image forming section, the drive controlling section moving the third core sections to the first position and to the second position.

12. The image forming apparatus according to claim 10 , wherein the plurality of third core sections are disposed side by side in a direction vertical to a transportation direction of the transfer material.

13. The fixing device according to claim 12 , wherein a plurality of the magnetic flux shielding members corresponding to the plurality of third core sections are provided so as to be annular in correspondence with the plurality of third core sections, and wherein each magnetic flux shielding member has a first surface, a second surface, and an opening, the first surface being a surface facing the end portion of the first core section, the second surface being a surface opposite to the first surface, the opening extending through the first surface and the second surface.

14. The image forming apparatus according to claim 10 , wherein the plurality of third core sections are each positioned at the first position or the second position in accordance with a size of the transfer material.

15. The image forming apparatus according to claim 10 , wherein an end portion of the magnetic flux shielding member facing the heating rotating member is disposed so as to be separated from the outer surface of the heating rotating member by a separation distance,

wherein the first distance is less than or equal to the separation distance, and

wherein the second distance is greater than the separation distance.

16. The image forming apparatus according to claim 10 , wherein the second position includes a 2 A position and a 2 B position, the 2 A position being where part of the magnetic flux shielding member does not overlap the third core sections, the 2 B position being where the magnetic flux shielding member does not overlap the third core sections.

17. The image forming apparatus according to claim 12 , wherein the plurality of third core sections are each disposed at a position corresponding to a size of the transfer material along a direction vertical to the transportation direction of the transfer material.

18. The image forming apparatus according to claim 17 , wherein the plurality of third core sections are each positioned at the first position or the second position in accordance with the size of the transfer material that is transported to the fixing nip section.