Image forming apparatus

Abstract

An image forming apparatus includes: a paper conveying portion for conveying a paper; an image forming portion for forming a visible toner image on the paper P being conveyed, with a toner in accordance with input image information; and a fixing unit for fusing and fixing the unfixed toner image formed on the paper to the paper by means of a heat roller and a pressing roller, and is constructed such that the pressing roller is composed of a metal core that forms the core shaft of the pressing roller and an elastic part that covers the outer periphery of the metal core, and the heat capacity at the center portion of the pressing roller with respect to the direction perpendicular to the paper's direction of conveyance is smaller than the heat capacities at both ends of the pressing roller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view showing a configuration of a heat roller and pressing roller which constitute a fixing unit mounted in a conventional image forming apparatus;

FIG. 2 is a schematic sectional view showing the structure of the heat roller and pressing roller;

FIG. 3 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and pressing roller in the image forming apparatus in operation ready mode;

FIG. 4 is an illustrative view showing the heat supply distributions of heaters in the heat roller;

FIG. 5 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and the pressing roller when small-sized paper is fixed by the fixing unit;

FIG. 6 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and the pressing roller when large-sized paper is fixed by the fixing unit;

FIG. 7 is a perspective view showing an overall configuration of an image forming apparatus according to the embodiment of the present invention;

FIG. 8 is a sectional side view showing an internal configuration of the image forming apparatus;

FIG. 9 is a schematic side view showing a heat roller and a pressing roller that constitutes the fixing unit mounted in the image forming apparatus;

FIG. 10 is a schematic sectional view showing the structure of the heat roller and pressing roller;

FIG. 11 is a block diagram showing an electric controller configuration of the image forming apparatus;

FIG. 12 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and pressing roller of the fixing unit in operation ready mode;

FIG. 13 is an illustrative view showing the heat supply distributions of the heaters in the heat roller;

FIG. 14 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and the pressing roller when small-sized paper is fixed by the fixing unit; and

FIG. 15 is an illustrative view showing the temperature distributions on the surfaces of the heat roller and the pressing roller when large-sized paper is fixed by the fixing unit.

Claims

1. An image forming apparatus comprising: a recording medium conveying portion for conveying a recording medium;an image forming portion for forming a visible developer image on the recording medium being conveyed, with a developer in accordance with input image information; and,a fixing portion for fusing and fixing the unfixed developer image formed on the recording medium to the recording medium by means of a heat roller and a pressing roller, characterized in that the pressing roller is composed of a metal core that forms the core shaft of the pressing roller and an elastic part that covers the outer periphery of the metal core, and is constructed so that the heat capacity at the center portion of the pressing roller with respect to the direction perpendicular to the recording medium's direction of conveyance is smaller than the heat capacities at both ends of the pressing roller.

2. An image forming apparatus according to claim 1, wherein the elastic part is formed so that the covering thickness at the position which opposes the center of a recording medium with respect to the direction perpendicular to the recording medium's direction of conveyance is thinner than the covering thickness at the positions that oppose the side edges of the recording medium.

3. An image forming apparatus according to claim 1, wherein the metal core has an approximately cylindrical external shape and is constructed such that the outside diameter of the metal core at the position opposing the center of recording medium with respect to the direction perpendicular to the recording medium's direction of conveyance is greater than the outside diameter at the positions that opposes the side edges of the recording medium.

4. An image forming apparatus according to claim 2, wherein the metal core has an approximately cylindrical external shape and is constructed such that the outside diameter of the metal core at the position opposing the center of recording medium with respect to the direction perpendicular to the recording medium's direction of conveyance is greater than the outside diameter at the positions that opposes the side edges of the recording medium.

5. An image forming apparatus according to claim 2, wherein the elastic part is formed so that the covering thickness varies along the axial direction of the pressing roller in conformity with the designated surface temperature of the heat roller, so as to make the surface temperature of the heat roller during a continuous fixing process by the fixing unit approximately uniform along the axial direction.

6. An image forming apparatus according to claim 3, wherein the elastic part is formed so that the covering thickness varies along the axial direction of the pressing roller in conformity with the designated surface temperature of the heat roller, so as to make the surface temperature of the heat roller during a continuous fixing process by the fixing unit approximately uniform along the axial direction.

7. An image forming apparatus according to claim 4, wherein the elastic part is formed so that the covering thickness varies along the axial direction of the pressing roller in conformity with the designated surface temperature of the heat roller, so as to make the surface temperature of the heat roller during a continuous fixing process by the fixing unit approximately uniform along the axial direction.