Fixing device and image forming device

1. A fixing device for fixing an image onto a recording sheet by applying heat and pressure to the recording sheet and the image thereon while the recording sheet is passing through a fixing nip, the fixing device comprising:

a pressure belt movable in a revolving motion;

a rotatable fixing roller; and

a pressure member for pressing an inner circumferential surface of the pressure belt toward the fixing roller, the fixing nip being formed between the fixing roller and the pressure belt while an elastically deformable surface of the fixing roller in rotation is brought into contact with an outer surface of the pressure belt by a pressure applied by the pressure member to the inner circumferential surface of the pressure belt,

the pressure member including:

an elastic pressure member for applying pressures to the fixing roller to cause a pressure distribution in which a greater pressure is applied to a center of the fixing roller than to each of ends thereof, the center and the ends being along an axis direction of the fixing roller; and

a hard pressure member that is disposed parallel to the elastic pressure member on downstream side of the elastic pressure member in a direction of the revolving motion of the pressure belt and applies pressures to the fixing roller to cause a pressure distribution in which a greater pressure is applied to each of the ends of the fixing roller than to the center of the fixing roller, and

a main pressure part that has a supporting surface supporting the elastic pressure member, the supporting surface has a shape in which the supporting surface rises from ends thereof toward a center thereof to form a projection so that the center is closer to the fixing roller than the ends, the center and the ends being along the axis direction of the fixing roller, and the elastic pressure member is in a shape of plane- parallel plates and deforms following the shape of the supporting surface when the elastic pressure member is in the state of not pressing the fixing roller.

2. The fixing device of claim 1 , wherein

an urging force toward the fixing roller is acted on the elastic pressure member and the hard pressure member, and

by the urging force, the elastic pressure member deforms elastically when pressed against the fixing roller and the fixing roller deforms elastically when pressed by the hard pressure member so that the pressures applied by the elastic pressure member and the hard pressure member cause the respective pressure distributions.

3. The fixing device of claim 2 , wherein the main pressure part causes the urging force to act on both of the elastic pressure member and the hard pressure member.

4. The fixing device of claim 3 , wherein

the main pressure part and the hard pressure member have been formed as one unit.

5. The fixing device of claim 3 , wherein

the main pressure part has a flat supporting surface supporting the elastic pressure member, and

the elastic pressure member has a shape of projection so that a center portion thereof is closer to the fixing roller than end portions thereof when the elastic pressure member is in a state of not pressing the fixing roller, wherein the center portion and the end portions are along the axis direction of the fixing roller.

6. The fixing device of claim 1 , wherein

the elastic pressure member has a projection so that a center portion thereof is closer to the fixing roller than end portions thereof when the elastic pressure member is in a state of not pressing the fixing roller, wherein the center portion and the end portions are along the axis direction of the fixing roller, and the elastic pressure member deforms elastically following a shape of an outer circumferential surface of the fixing roller when the elastic pressure member is in a state of pressing the fixing roller.

7. The fixing device of claim 6 , wherein the elastic pressure member has a shape of projection in which the elastic pressure member rises step by step like a staircase from ends thereof toward a center thereof so that the center portion is closer to the fixing roller than to the end portions when the elastic pressure member is in the state of not pressing the fixing roller.

8. The fixing device of claim 6 , wherein the elastic pressure member has a pair of slant surfaces that face toward the fixing roller and rise by degrees from the ends toward the center so that the center portion is closer to the fixing roller than to the end portions when the elastic pressure member is in the state of not pressing the fixing roller.

9. The fixing device of claim 6 , wherein

a difference in height between the center portion and the end portions of the elastic pressure member is in a range from 0.1 mm to 0.6 mm inclusive.

10. An image forming device comprising an imaging forming part for forming an image and transferring the image on a recording sheet and a fixing device for fixing an image onto the recording sheet by applying heat and pressure to the recording sheet and the image thereon while the recording sheet is passing through a fixing nip, the fixing device including:

a pressure belt movable in a revolving motion;

a rotatable fixing roller; and

a pressure member for pressing an inner circumferential surface of the pressure belt toward the fixing roller, the fixing nip being formed between the fixing roller and the pressure belt while an elastically deformable surface of the fixing roller in rotation is brought into contact with an outer surface of the pressurebelt by a pressure applied by the pressure member to the inner circumferential surface of the pressure belt,

the pressure member including:

an elastic pressure member for applying pressures to the fixing roller to cause a pressure distribution in which a greater pressure is applied to a center of the fixing roller than to each of ends thereof, the center and the ends being along an axis direction of the fixing roller; and

a hard pressure member that is disposed parallel to the elastic pressure member on downstream side of the elastic pressure member in a direction of the revolving motion of the pressure belt and applies pressures to the fixing roller to cause a pressure distribution in which a greater pressure is applied to each of the ends of the fixing roller than to the center of the fixing roller, and

a main pressure part that has a supporting surface supporting the elastic pressure member, the supporting surface has a shape in which the supporting surface rises from ends thereof toward a center thereof to form a projection so that the center is closer to the fixing roller than the ends, the center and the ends being along the axis direction of the fixing roller, and the elastic pressure member is in a shape of plane-parallel plates and deforms following the shape of the supporting surface when the elastic pressure member is in the state of not pressing the fixing roller.

11. The image forming device of claim 10 , wherein

an urging force toward the fixing roller is acted on the elastic pressure member and the hard pressure member, and

by the urging force, the elastic pressure member deforms elastically when pressed against the fixing roller and the fixing roller deforms elastically when pressed by the hard pressure member so that the pressures applied by the elastic pressure member and the elastic pressure member cause the respective pressure distributions.

12. The fixing device of claim 11 , wherein the main pressure part causes the urging force to act on both of the elastic pressure member and the hard pressure member.

13. The image forming device of claim 12 , wherein

the main pressure part and the hard pressure member have been formed as one unit.

14. The image forming device of claim 12 , wherein

the main pressure part has a flat supporting surface supporting the elastic pressure member, and

the elastic pressure member has a shape of projection so that a center portion thereof is closer to the fixing roller than end portions thereof when the elastic pressure member is in a state of not pressing the fixing roller, wherein the center portion and the end portions are along the axis direction of the fixing roller.

15. The image forming device of claim 10 , wherein

the elastic pressure member has a projection so that a center portion thereof is closer to the fixing roller than end portions thereof when the elastic pressure member is in a state of not pressing the fixing roller, wherein the center portion and the end portions are along the axis direction of the fixing roller, and the elastic pressure member deforms elastically following a shape of an outer circumferential surface of the fixing roller when the elastic pressure member is in a state of pressing the fixing roller.

16. The fixing device of claim 15 , wherein the elastic pressure member has a shape of projection in which the elastic pressure member rises step by step like a staircase from ends thereof toward a center thereof so that the center portion is closer to the fixing roller than to the end portions when the elastic pressure member is in the state of not pressing the fixing roller.

17. The fixing device of claim 15 , wherein the elastic pressure member has a pair of slant surfaces that face toward the fixing roller and rise by degrees from the ends toward the center so that the center portion is closer to the fixing roller than to the end portions when the elastic pressure member is in the state of not pressing the fixing roller.

18. The image forming device of claim 15 , wherein

a difference in height between the center portion and the end portions of the elastic pressure member is in a range from 0.1 mm to 0.6 mm inclusive.