JP5376329B2 - Fixing apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device capable of improving separating property of recording medium by improving bending rigidity of the recording medium. <P>SOLUTION: Respective crown sections C1 and D1 and reverse crown sections C2 and D2, in a fixing roller 21 and a press roller 22, are formed by varying the thickness of respective elastic layers 21b and 22b along the axial direction. Ends, on the same side in the respective axial directions of the fixing roller 21 and the press roller 22, are positioned in the axial direction, and also the respective positioned ends and ends on the opposite side are configured to be variable in displacement in the axial direction. Inflection-points passage lines L21 and L22, in at least one of the fixing roller 21 and the press roller 22 in the separated state in which the fixing roller 21 and the press roller 22 are separated from each other, are configured to be in a projected state in the central section in the axial direction. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a fixing device that heats and pressurizes a recording medium to fix an image on the recording medium, and an image forming apparatus including the fixing device.

  2. Description of the Related Art Image forming apparatuses such as copiers, printers, facsimiles, and multi-function machines thereof often use a fixing device that fixes a toner image transferred onto a recording sheet as a recording medium by heat and pressure. Examples of the fixing device include a type in which a pressure roller is pressed against a fixing roller having a heating source such as a heater therein, and a type in which an endless pressure belt is pressed against the fixing roller by a pressing member. These fixing devices pass the recording paper on which the toner image is transferred through a fixing nip formed at a pressure contact portion between the fixing roller and the pressure roller or a pressure contact portion between the fixing roller and the pressure belt. The recording paper is heated and pressurized to fix the toner image on the recording paper.

  FIG. 22 is a simplified diagram of a fixing device having the fixing roller 100 and the pressure roller 200. As shown in FIG. 22, the recording paper P onto which the toner image T has been transferred passes through the fixing nip N between the fixing roller 100 and the pressure roller 200, and then is carried out in the direction of the dashed line a in the figure. It has become. However, since the heat fixing type fixing device as described above melts the toner T on the recording paper P by heat at the time of fixing, the conveyance direction of the recording paper P is caused by the adhesion of the melted toner T to the fixing roller 100. The direction may be changed from the direction of the alternate long and short dash line a toward the fixing roller 100. When the recording paper P is transported to the fixing roller 100 side beyond the boundary indicated by the alternate long and short dash line b due to the adhesive force of the melted toner T, the recording paper P is wound around the fixing roller 100.

  Here, the adhesion force of the melted toner to the fixing roller is F1, and the bending force required to wind the recording paper (the force required to bend the recording paper from the one-dot chain line a to the one-dot chain line b by an angle θ) is F2. Then, when the relationship of F1 <F2 is satisfied, the recording paper can be separated from the fixing roller and carried out.

  Conventionally, in order to easily separate the recording paper from the fixing roller, the toner adhesion force F1 is reduced by adding a release agent such as wax to the toner (Patent Document 1 below). reference). Further, in order to facilitate separation of the recording paper from the fixing roller, as shown by a two-dot chain line in FIG. 22, by reducing the diameter of the fixing roller, the boundary line where the winding occurs is changed from the one-dot chain line b to the one-dot chain line b. There is also a method of increasing the bending force F2 required for winding the recording paper by changing to the position '.

  However, even if the adhesion force of the toner is reduced or the diameter of the fixing roller is reduced as described above, if the recording paper having a small bending rigidity (stiffness) such as a thin paper is used, the recording paper is wound. Since the bending force F2 required for this is reduced, winding around the fixing roller may occur.

  Accordingly, in view of such circumstances, the present invention provides a fixing device capable of improving the separation property of the recording medium by improving the bending rigidity of the recording medium, and an image forming apparatus including the fixing device. It is something to try.

  The invention of claim 1 includes a fixing roller heated by a heating source and a pressure roller disposed in pressure contact with the fixing roller, and the fixing roller and the pressure roller are formed in pressure contact with each other. In the fixing device for fixing an unfixed image on the recording medium to the recording medium by passing the recording medium through the fixing nip, the fixing roller has a crown portion whose outer peripheral surface is formed in a crown shape, and an outer peripheral surface of the fixing roller. Each of the pressure rollers has at least one reverse crown portion formed in a reverse crown shape. The pressure roller includes a crown portion having an outer peripheral surface formed in a crown shape and a reverse roller having an outer peripheral surface formed in a reverse crown shape. At least one crown portion is provided, the crown portion of the fixing roller and the reverse crown portion of the pressure roller are made to correspond to each other, and the reverse clamp of the fixing roller The fixing roller and the pressure roller are brought into pressure contact with each other, the crown portion and the reverse crown portion of each of the fixing roller and the pressure roller, The thickness of the elastic layer of each of the fixing roller and the pressure roller is changed in the axial direction, and the end on the same side in the axial direction of each of the fixing roller and the pressure roller is connected to the shaft. And a fixing device configured to be capable of axially displacing the ends of the fixing roller and the pressure roller opposite to the positioned ends of the fixing roller and the pressure roller. When an imaginary line passing through the inflection point of each of the crown portion and the reverse crown portion in the cross section cut along the axial direction of the pressure roller in the axial direction is referred to as an inflection point passing line, In the separated state in which the fixing roller and the pressure roller are separated from each other, the inflection point passing line of at least one of the fixing roller and the pressure roller is configured to be convex in the central portion in the axial direction. It is a thing.

  According to the first aspect of the present invention, the crown portion of the fixing roller and the reverse crown portion of the pressure roller are made to correspond to each other, and the reverse crown portion of the fixing roller and the crown portion of the pressure roller are made to correspond to each other. Since the roller is in pressure contact, the fixing nip is curved. When the image is fixed, the recording medium enters a curved state due to the recording medium entering the curved fixing nip. In this way, by curving the recording medium, the apparent rigidity of the recording medium can be improved and carried out from the fixing nip. When the recording medium is carried out from the fixing nip, the recording medium is wound around the fixing roller. It becomes possible to suppress sticking.

  Further, by causing the fixing roller and the pressure roller to be in pressure contact with each other so that the crown portion and the reverse crown portion correspond to each other, it is possible to suppress variation in contact pressure at the fixing nip. As a result, it is possible to suppress problems such as occurrence of uneven glossiness in a fixed image, and to realize good image formation.

  According to the first aspect of the present invention, the thickness of the elastic layer of each of the fixing roller and the pressure roller is changed to form the crown portion and the reverse crown portion. Even if an axial deviation occurs in the meantime, variations in contact pressure at the fixing nip can be reduced, and occurrence of uneven glossiness can be suppressed. Furthermore, the fixing roller and the pressure roller are configured so that the end portions on the same side of the fixing roller and the pressure roller are positioned in the axial direction and the end portions on the opposite side can be displaced in the axial direction. Even if it occurs, the fixing roller and the pressure roller can be extended (displaced) to the same end side in the axial direction. As a result, the displacement between the crown portion and the reverse crown portion due to thermal expansion can be reduced, and variations in contact pressure at the fixing nip can be suppressed.

  According to the first aspect of the present invention, since the inflection point passage line of at least one of the fixing roller and the pressure roller is configured to be convex in the central portion in the axial direction, the fixing roller is bent. Even in this case, the difference in the nip width between the center side and the end side can be suppressed, and the occurrence of image defects such as fixing defects and gloss differences can be prevented.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, the amount of protrusion of the inflection point passage line configured to have the convex shape in the separated state is determined by the fixing roller and the pressure roller. It is set to be half the amount of deflection perpendicular to the axial direction when they are brought into pressure contact with each other.

  By setting the protrusion amount of the inflection point passage line as described above, it is possible to suppress the difference in nip width to a value that does not cause image defects.

  According to a third aspect of the present invention, in the fixing device according to the first or second aspect, the top portion of the crown portion and the reverse crown portion of each roller in a separated state in which the fixing roller and the pressure roller are separated from each other. The height difference from the bottom is set to be larger on the end side than on the central side in the axial direction.

  By increasing the height difference between the crown portion and the reverse crown portion in the separated state on the end side rather than on the central portion side in the axial direction, the height difference between the crown portion and the reverse crown portion in the fixing nip is increased on the end side. Can be secured. Thereby, the recording medium can be sufficiently curved on the end side of the roller, and the separation property of the recording medium can be improved.

  According to a fourth aspect of the present invention, in the fixing device according to any one of the first to third aspects, the crown portion and the reverse crown portion of each of the fixing roller and the pressure roller are continuously connected in the axial direction. Is formed.

  By forming the crown portion and the reverse crown portion continuously in the axial direction, it is possible to suppress the occurrence of wrinkles on the recording medium and to ensure the separation of the recording paper.

  According to a fifth aspect of the present invention, in the fixing device according to any one of the first to fourth aspects, the crown portion and the reverse crown portion are arranged such that at least the maximum passage width of the recording medium of the fixing roller and the pressure roller. It is arranged over the part corresponding to the whole.

  By disposing the crown portion and the reverse crown portion over at least a portion of the fixing roller and the pressure roller corresponding to the entire maximum passage width of the recording medium, the recording medium is curved over the entire width direction to fix the fixing nip. And the winding of the recording medium around the fixing roller can be suppressed.

  According to a sixth aspect of the present invention, in the fixing device according to any one of the first to fifth aspects, in a load state in which the fixing roller and the pressure roller are pressed against each other, The height difference from the bottom of the reverse crown is set to 0.16 mm or more and 0.8 mm or less.

  In the above load state in which the fixing roller and the pressure roller are pressed against each other, if the height difference between the top of the crown portion and the bottom of the reverse crown portion in the fixing nip is less than 0.16 mm, the amount of bending of the recording medium in the fixing nip There is a possibility that the apparent rigidity of the recording medium necessary for separating the recording medium satisfactorily cannot be obtained. Therefore, by setting the height difference between the top of the crown portion and the bottom of the reverse crown portion to be 0.16 mm or more, the apparent rigidity of the recording medium is sufficiently secured and the winding of the recording medium around the fixing roller is ensured. Can be suppressed. On the other hand, if the height difference between the top of the crown portion and the bottom of the reverse crown portion in the fixing nip exceeds 0.8 mm in the above load state, the difference in rotational speed between the crown portion and the reverse crown portion increases, and the recording medium is wrinkled. May occur. Therefore, by setting the height difference between the top portion of the crown portion and the bottom portion of the reverse crown portion to be 0.8 mm or less, it is possible to carry out the recording medium without generating wrinkles, and to perform good image formation. it can.

  According to a seventh aspect of the present invention, there is provided a fixing roller heated by a heating source, an endless pressure belt, and a pressing member that presses an inner peripheral surface of the pressure belt and presses the pressure belt against the fixing roller. A fixing device for fixing an unfixed image on the recording medium to the recording medium by passing the recording medium through a fixing nip formed by press-contacting the fixing roller and the pressure belt. Has at least one crown portion whose outer peripheral surface is formed in a crown shape and one reverse crown portion whose outer peripheral surface is formed in a reverse crown shape, and presses the pressure belt of the pressing member. The surface has at least one convex surface portion formed in a convex shape and a concave surface portion formed in a concave shape, and the crown portion of the fixing roller and the concave surface portion of the pressing member are associated with each other. In addition, the reverse crown portion of the fixing roller and the convex portion of the pressing member are disposed in correspondence with each other, the pressing belt is pressed by the pressing member, the fixing roller and the pressing belt And the crown portion and the reverse crown portion of the fixing roller are formed by changing the thickness of the elastic layer of the fixing roller in the axial direction, and the convex surface portion of the pressing member and the reverse crown portion are formed. The concave portion is formed by changing the thickness of the elastic layer of the pressing member over the longitudinal direction, and the end portions on the same side of the fixing roller and the pressing member are positioned in the axial direction or the longitudinal direction. In addition, a fixing device configured to be capable of displacing an end portion of each of the fixing roller and the pressing member opposite to the positioned end portion in an axial direction or a longitudinal direction, the axial direction of the fixing roller being An imaginary line passing through an inflection point of the crown portion and the reverse crown portion in the cross section cut along the axial direction, and the convex surface portion and the concave surface in the cross section cut along the longitudinal direction of the pressing member. An imaginary line that passes through the inflection point with respect to the longitudinal direction is referred to as an inflection point passage line, and in the separated state in which the fixing roller and the pressing member are separated from each other, The inflection point passage line of at least one of the pressing members is configured to have a convex shape at the central portion in the axial direction or the longitudinal direction.

  According to a seventh aspect of the present invention, the crown portion of the fixing roller and the concave surface portion of the pressing member are arranged in correspondence with each other, and the reverse crown portion of the fixing roller and the convex surface portion of the pressing member are arranged in correspondence with each other, thereby fixing the fixing roller. Since the pressure belt and the pressure belt are pressed against each other, the fixing nip is curved. In this case as well, the recording medium can be curved by intruding the recording medium into the fixing nip, and the apparent rigidity of the recording medium can be improved and carried out of the fixing nip as in the first aspect of the invention. be able to. Thereby, when the recording medium is carried out from the fixing nip, it is possible to suppress the recording medium from being wound around the fixing roller.

  According to the seventh aspect of the present invention, the fixing roller and the pressure belt are brought into pressure contact with each other by causing the crown portion and the concave surface portion to correspond and the reverse crown portion and the convex surface portion to correspond to each other. It is possible to suppress variations in contact pressure at the nip. As a result, it is possible to suppress problems such as occurrence of uneven glossiness in a fixed image, and to realize good image formation.

  In the invention of claim 7, since both the fixing roller and the pressing member change the thickness of the elastic layer to form the crown portion, the reverse crown portion, and the convex surface portion and the concave surface portion, Even if an axial deviation occurs between the concave or reverse crown and the convex surface, it is possible to reduce variations in contact pressure at the fixing nip, and to suppress occurrence of uneven glossiness of the image. is there. Furthermore, since the end portions on the same side of the fixing roller and the pressing member are positioned and the end portions on the opposite side can be displaced, even if the fixing roller expands due to thermal expansion, The pressing member can be extended (displaced) to the same end side. As a result, axial displacement between the crown portion and the concave surface portion and between the reverse crown portion and the convex surface portion due to thermal expansion can be reduced, and variation in contact pressure at the fixing nip can be suppressed. it can.

  In the invention of claim 7, the inflection point passing line of at least one of the fixing roller and the pressing member is configured to be convex in the central portion in the axial direction, so that the fixing roller is bent. Even in this case, the difference in the nip width between the center side and the end side can be suppressed, and the occurrence of image defects such as fixing defects and gloss differences can be prevented.

  An eighth aspect of the present invention is an image forming apparatus including the fixing device according to any one of the first to seventh aspects.

  The fixing device according to the present invention can be applied to an image forming apparatus.

  According to the present invention, when the recording medium is carried out of the fixing device, the apparent rigidity of the recording medium can be improved and the recording medium can be prevented from being wound around the fixing roller. Accordingly, it is possible to suppress the occurrence of problems such as jamming of the recording medium, and to provide a highly reliable fixing device and image forming apparatus. Further, the present invention can suppress variations in contact pressure at the fixing nip. Thereby, it is possible to suppress the occurrence of uneven glossiness and the like in the fixed image, and it is possible to maintain good image formation. Further, according to the present invention, it is possible to suppress the difference in the nip width from occurring even when the fixing roller bends during pressure contact. For this reason, even when a low-strength fixing roller is used to shorten the warm-up time, the difference in nip width can be reduced, thereby preventing image defects such as fixing defects and gloss differences. And good image formation can be realized.

1 is a schematic configuration diagram of a color image forming apparatus according to an embodiment of the present invention. FIG. 2 is a perspective view of a fixing device mounted on the color image forming apparatus. FIG. 2 is a cross-sectional view of the fixing device. FIG. 4 is a cross-sectional view of each roller in a state where a fixing roller and a pressure roller provided in the fixing device are separated from each other. FIG. 3 is a cross-sectional view of each roller in a state where the fixing roller and the pressure roller are in pressure contact with each other. It is a figure which shows the attachment structure of a fixing roller and a pressure roller. It is sectional drawing of each roller in the state which spaced apart the fixing roller and pressure roller which concern on other embodiment of this invention. It is sectional drawing of each roller in the state which mutually contacted the fixing roller and pressure roller which concern on the said other embodiment. FIG. 6 is a cross-sectional view of a fixing device according to still another embodiment of the present invention. It is sectional drawing of each member in the state which spaced apart the fixing roller and press member which concern on the said another embodiment. FIG. 6 is a cross-sectional view of each member in a state where a fixing roller and a pressing member according to another embodiment are pressed against each other. It is a figure for demonstrating the measuring method of the apparent rigidity of a recording paper. 6 is a graph showing the relationship between the number of curvatures of the fixing nip and the apparent rigidity of the recording paper. It is a graph which shows the test result which compares the separation property of a recording paper, Comprising: (A) shows a test result in case only one crown part and a reverse crown part are shown, (B) shows a crown part and a reverse crown. The test result in the case of having one part each is shown. It is a graph which shows the state in which the pressure difference has arisen over the axial direction of a roller. It is a figure which shows the state which shifted the fixing roller and pressure roller of this invention to the axial direction. It is a figure which shows the state which shifted the fixing roller and pressure roller of the comparative example to the axial direction. It is a graph which shows the pressure difference with respect to the deviation | shift amount in each of this invention and a comparative example. It is a graph which shows the relationship between a pressure difference and a glossiness difference. It is a graph which shows the glossiness difference with respect to the deviation | shift amount in each of the Example and comparative example of this invention. It is the figure which showed typically the structure of the comparative example and this invention. FIG. 6 is a diagram for explaining an operation in which a recording sheet is wound around a fixing roller.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

FIG. 1 is a schematic configuration diagram of a color image forming apparatus according to an embodiment of the present invention.
As shown in FIG. 1, the color image forming apparatus includes four process units 1Y, 1C, 1M, and 1Bk configured to be detachable from the image forming apparatus main body 50. The process units 1Y, 1C, 1M, and 1Bk have the same configuration except that they contain toners of different colors of yellow, cyan, magenta, and black corresponding to the color separation components of the color image. Therefore, the configuration of one process unit 1Y will be described as an example.

  The process unit 1Y includes a photosensitive member 2 as an electrostatic latent image carrier, a charging device 3 as a charging unit that charges the surface of the photosensitive member 2, and a developing unit that forms a toner image on the surface of the photosensitive member 2. A developing device 4 and a cleaning device 5 as a cleaning means for cleaning the surface of the photoreceptor 2 are provided.

  Above each of the process units 1Y, 1C, 1M, and 1Bk, an exposure device 7 is disposed as an exposure unit that exposes the surface of the photoreceptor 2. On the other hand, an intermediate transfer unit 6 is disposed below each process unit 1Y, 1C, 1M, 1Bk. The intermediate transfer unit 6 has an endless intermediate transfer belt 10. The intermediate transfer belt 10 is stretched around a plurality of rollers 17, 18, and 19, and is configured to be able to run around in the direction of the arrow in the figure. A belt cleaning device 16 is disposed at the left end of the intermediate transfer belt 10 in the figure.

  Four primary transfer rollers 11 as primary transfer means are disposed on the inner peripheral surface of the intermediate transfer belt 10. The photoconductor 2 included in each process unit 1Y, 1C, 1M, and 1Bk is pressed against the outer peripheral surface of the intermediate transfer belt 10 at a position facing the four primary transfer rollers 11 to form a primary transfer nip. A secondary transfer roller 12 as a secondary transfer unit is disposed at a position facing one roller 17 that stretches the intermediate transfer belt 10. The secondary transfer roller 12 is pressed against the outer peripheral surface of the intermediate transfer belt 10 to form a secondary transfer nip.

  A plurality of paper feed cassettes 13 that contain recording paper P as a recording medium are disposed below the image forming apparatus main body 50. Each paper feed cassette 13 is provided with a paper feed roller 14 and the like for carrying out the recording paper P. In the image forming apparatus main body 50, a recording medium conveyance path R (hereinafter referred to as a sheet conveyance path) for guiding the recording sheet P is disposed. In the paper transport path R, a pair of registration rollers 15 is disposed upstream of the secondary transfer roller 12 in the recording medium transport direction A (hereinafter referred to as the paper transport direction). Further, a fixing device 8 for fixing the image on the recording paper P is disposed downstream of the secondary transfer roller 12 in the paper transport direction A. A stock unit 9 for stocking the recording paper P discharged to the outside is provided on the left side of the image forming apparatus main body 50 in the drawing.

The basic operation of the image forming apparatus will be described below with reference to FIG.
When the image forming operation is started, the photosensitive members 2 of the process units 1Y, 1C, 1M, and 1Bk are driven to rotate counterclockwise by a driving device (not shown), and the surface of each photosensitive member 2 is charged by the charging device 3. It is uniformly charged to a predetermined polarity. The surface of each charged photoconductor 2 is irradiated with laser light from the exposure device 7, and an electrostatic latent image is formed on the surface of each photoconductor 2. At this time, the image information to be exposed on each photoconductor 2 is monochromatic image information obtained by separating a desired full-color image into color information of yellow, cyan, magenta, and black. The electrostatic latent image formed on the photoreceptor 2 in this way is supplied with toner by each developing device 4, whereby the electrostatic latent image is visualized as a toner image.

  Further, a constant voltage or a constant current controlled voltage having a polarity opposite to the toner charging polarity is applied to each primary transfer roller 11. As a result, a transfer electric field is formed at the primary transfer nip between each primary transfer roller 11 and each photoconductor 2. Then, the intermediate transfer belt 10 in which the toner images of the respective colors formed on the photoreceptors 2 of the process units 1Y, 1C, 1M, and 1Bk travel in the direction of the arrow in the drawing by the transfer electric field formed in the primary transfer nip. The images are superimposed and transferred sequentially. Thus, a full-color toner image is carried on the intermediate transfer belt 10.

  Residual toner adhering to the surface of each photoconductor 2 after the toner image is transferred is removed from the surface of the photoconductor 2 by each cleaning device 5, and then the surface is subjected to static elimination action by a static elimination device (not shown). Then, the surface potential is initialized to prepare for the next image formation.

  As the paper feed roller 14 rotates, the recording paper P is sent from one of the paper feed cassettes 13 to the paper transport path R. The recording paper P sent to the paper transport path R is timed by the registration roller 15 and sent to the secondary transfer nip between the secondary transfer roller 12 and the roller 17 facing it. At this time, a transfer voltage having a polarity opposite to the toner charging polarity of the toner image on the intermediate transfer belt 10 is applied to the secondary transfer roller 12, thereby forming a transfer electric field in the secondary transfer nip. Then, the toner images on the intermediate transfer belt 10 are collectively transferred onto the recording paper P by the transfer electric field formed in the secondary transfer nip. Further, the toner remaining on the surface of the intermediate transfer belt 10 after image transfer is removed by the belt cleaning device 16. The recording paper P to which the toner image has been transferred is conveyed to the fixing device 8, and the toner image is fixed by applying heat and pressure to the recording paper P. Thereafter, the recording paper P is discharged from the fixing device 8 to the stock unit 9.

  The above description is an image forming operation when a full-color image is formed on the recording medium S. A single color image can be formed using any one of the four process units 1Y, 1C, 1M, and 1Bk. Two or three process units may be used to form a two or three color image.

Next, the basic configuration and basic operation of the fixing device 8 will be described with reference to FIGS.
As shown in FIG. 2, the fixing device 8 includes a fixing roller 21, a pressure roller 22, a guide member 23, and the like. The pressure roller 22 is in pressure contact with the fixing roller 21 at a predetermined pressure, and a fixing nip N is formed at a pressure contact portion where the pressure roller 22 is in pressure contact with each other.

  As shown in FIG. 3, a heater 24 as a heating source is disposed in the fixing roller 21 in the axial direction, and the fixing roller 21 is heated when the heater 24 generates heat. ing. A temperature detection device 25 that detects the surface temperature of the fixing roller 21 is disposed around the fixing roller 21. Based on the surface temperature of the fixing roller 21 detected by the temperature detecting device 25, a control device (not shown) controls the amount of heat generated by the heater 24 so that the surface temperature of the fixing roller 21 becomes a predetermined fixing temperature. It is configured.

  As shown in FIG. 3, the fixing roller 21 includes a cylindrical metal core member 21a, an elastic layer 21b covering the outer peripheral surface of the core member 21a, and an outer peripheral surface of the elastic layer 21b. It is comprised by the mold release layer 21c which coat | covers. Similarly to the fixing roller 21, the pressure roller 22 also has a cylindrical metal core 22a, an elastic layer 22b covering the outer peripheral surface of the core 22a, and an outer periphery of the elastic layer 22b. It is constituted by a release layer 22c that covers the surface. Further, the fixing roller 21 is driven to rotate clockwise in the drawing by a driving means (not shown). On the other hand, the pressure roller 22 is driven to rotate counterclockwise in the figure by the rotational driving of the fixing roller 21.

  When the image is fixed by the fixing device 8 according to the present embodiment, the heater 24 generates heat and the fixing roller 21 is heated. At this time, a control device (not shown) controls the amount of heat generated by the heater 24 based on the temperature detected by the temperature detection device 25 so that the surface temperature of the fixing roller 21 becomes a predetermined temperature (fixing temperature). Then, as shown in FIG. 3, the recording paper P on which the unfixed toner image T is formed is guided along the guide member 23 in the direction of arrow A, and between the rotating fixing roller 21 and the pressure roller 22. In the fixing nip N. As a result, the recording paper P is heated and pressed by the fixing roller 21 and the pressure roller 22, and the unfixed toner image T is fixed on the recording paper P.

Hereinafter, characteristic portions of the fixing device 8 will be described.
As shown in FIG. 2, the fixing roller 21 has a crown portion C1 whose outer peripheral surface is formed in a crown shape and a reverse crown portion C2 whose outer peripheral surface is formed in a reverse crown shape alternately in the axial direction. doing. Similarly, the pressure roller 22 also has a crown portion D1 whose outer peripheral surface is formed in a crown shape and a reverse crown portion D2 whose outer peripheral surface is formed in a reverse crown shape alternately in the axial direction. . As described above, in the present invention, the outer peripheral surfaces of the fixing roller 21 and the pressure roller 22 are formed to be curved so as to wave in the axial direction. The “crown shape” refers to a shape in which the roller diameter decreases toward both ends in the axial direction, and the “reverse crown shape” refers to a shape in which the roller diameter increases toward both ends in the axial direction. I mean. Each outer peripheral surface of the crown portion and the reverse crown portion is formed in a curved shape such as an arc shape or a sin curve shape in the axial direction.

FIG. 4 is a sectional view in the axial direction of each of the rollers 21 and 22 with the fixing roller 21 and the pressure roller 22 being separated from each other.
As shown in FIG. 4, the elastic layers 21 b and 22 b of the fixing roller 21 and the pressure roller 22 are formed so as to repeat thickening and thinning in the axial direction. As described above, the thicknesses of the elastic layers 21b and 22b are changed in the axial direction, and the crown portions C1 and D1 and the reverse crown portions C2 and D2 are formed on the rollers 21 and 22, respectively. The core materials 21a and 22a and the release layers 21c and 22c of the rollers 21 and 22 are formed to have a uniform thickness in the axial direction.

  As shown in FIG. 4, in this embodiment, the crown portion and the reverse crown portion cover at least a portion of the fixing roller 21 and the pressure roller 22 corresponding to the entire maximum passage width W of the recording medium (recording paper). It is arranged. However, the arrangement area of the crown part and the reverse crown part can be a part of the maximum passage width W of the recording medium. Further, the number of the crown portions and the reverse crown portions is not particularly limited as long as one or more crown portions and reverse crown portions are formed on each of the rollers 21 and 22. However, with the fixing roller 21 and the pressure roller 22 facing each other, the number of crown portions and reverse crown portions is adjusted so that one crown portion and the other reverse crown portion always face each other.

  As shown in FIG. 5, in the pressure contact state in which the fixing roller 21 and the pressure roller 22 are in pressure contact with each other, the crown portion C1 of the fixing roller 21 and the reverse crown portion D2 of the pressure roller 22 are made to correspond to each other. The reverse crown portion C2 and the crown portion D1 of the pressure roller 22 are in pressure contact with each other. In this way, the fixing nip N formed between the rollers 21 and 22 is curved so as to wave in the axial direction by pressing the fixing roller 21 and the pressing roller 22 with the protrusions and recesses corresponding to each other. Formed.

  Also, dotted lines L21 and L22 shown in FIG. 4 are cross-sections taken along the axial direction of the fixing roller 21 and the pressure roller 22, and the inflection points of the respective crown portions and reverse crown portions extend in the axial direction. A virtual line that passes through. Hereinafter, these virtual lines L21 and L22 are respectively called inflection point passing lines. In the separated state shown in FIG. 4, the inflection point passage lines L21 and L22 of both the fixing roller 21 and the pressure roller 22 are configured to be convex in the outer diameter direction at the central portion in the axial direction. Yes.

  By the way, in the fixing device of the present invention, in order to shorten the time (warm-up time) from the start of heating of the heater 24 until the fixing roller 21 reaches a predetermined temperature (fixing temperature), the thickness of the fixing roller 21 is reduced. To improve thermal efficiency. However, in the case of such a configuration, the strength decreases as the thickness of each roller 21 becomes thinner. Therefore, when each of the rollers 21 and 22 is brought into a pressure contact state, bending occurs in a direction perpendicular to the axial direction ( (See FIG. 5). Here, since both ends of the pressure roller 22 are pressed toward the fixing roller 21 by an urging means such as a pressure spring (not shown), they are closer to each other on the both ends than the center of each roller 21, 22. Deflection occurs. Then, when the deflection amounts of the rollers 21 and 22 are denoted by reference numerals B21 and B22 in FIG. 5, in this embodiment, the protrusion amounts G21 and G22 in the separated state of the inflection point passage lines L21 and L22 shown in FIG. Is set to be a half value of the deflection amounts B21 and B22.

  In the separated state shown in FIG. 4, the height differences H21 and H22 between the tops of the crown portions of the rollers 21 and 22 and the bottoms of the reverse crown portions adjacent thereto are the same values in the axial direction. However, in the pressure contact state shown in FIG. 5, the pressing force by the urging means acts on both ends of each roller 21 and 22 so that the crushing amount of the elastic layers 21b and 22b is closer to the ends than the center. The height difference H between the top portion of the crown portion and the bottom portion of the reverse crown portion in the fixing nip N becomes smaller on the end side. As described above, in the load state in which the fixing roller 21 and the pressure roller 22 are in pressure contact with each other, the height difference H between the crown portion and the reverse crown portion in the fixing nip N varies in the axial direction, but each height difference H is 0. It is desirable to set it to 16 mm or more and 0.8 mm or less. The reason why it is desirable to set the height difference H in this way will be described in the following description of the operation and effect of the present invention.

FIG. 6 shows a mounting structure of the fixing roller 21 and the pressure roller 22.
As shown in FIG. 6, both end portions of the fixing roller 21 and the pressure roller 22 are attached to and supported by two side plates 71 and 72 disposed at a predetermined interval in the image forming apparatus. In the present embodiment, the left end portions of the fixing roller 21 and the pressure roller 22 in the drawing are positioned in the axial direction, and the right end portions in the respective drawings are configured to be displaceable in the axial direction.

  Specifically, both end portions of the fixing roller 21 and the pressure roller 22 are rotatably supported by bearings 73 a to 73 d such as ball bearings fixed to the two side plates 71 and 72. A bearing 73 a disposed at the left end of the fixing roller 21 in the figure is sandwiched between a stepped portion 74 formed on the outer periphery of the fixing roller 21 and a retaining ring 75. Accordingly, the left end portion of the fixing roller 21 in the figure is positioned so as not to move in the axial direction with respect to the bearing 73a. On the other hand, the right end portion of the fixing roller 21 in the figure is not positioned with respect to the bearing 73b and can be displaced in the axial direction. Similarly, a bearing 73c disposed at the left end of the pressure roller 22 in the figure is sandwiched between a stepped portion 76 formed on the outer periphery of the pressure roller 22 and a retaining ring 77, and the pressure roller 22 is positioned so as not to move in the axial direction with respect to the bearing 73c. On the other hand, the right end portion of the pressure roller 22 in the figure can be displaced in the axial direction with respect to the bearing 73d.

  In this way, the fixing roller 21 and the pressure roller 22 can position the end portions on the same axial side in the axial direction and can displace the end portion on the opposite side to the positioned end portion in the axial direction. It is comprised so that it may become. Further, the positioning end portion or the displaceable end portion may be either end portion as long as it is an end portion on the same side of the fixing roller 21 and the pressure roller 22.

7 and 8 show the configuration of another embodiment of the fixing device according to the present invention.
7 shows a separated state in which the fixing roller 21 and the pressure roller 22 are separated from each other, and FIG. 8 shows a pressure contact state in which they are pressed against each other.

  In this embodiment, unlike the embodiment of the present invention described above, in the separated state shown in FIG. 7, the height difference H21, H22 between the top of the crown portion of each roller 21, 22 and the bottom of the reverse crown portion adjacent thereto is It is comprised so that it may become large toward the both end parts side from the center part side of an axial direction. Other than that, the configuration is the same as that of the embodiment of the present invention, and a description thereof will be omitted.

9 to 11 show the configuration of still another embodiment of the fixing device of the present invention.
First, the basic configuration and basic operation of the fixing device 8 will be described with reference to FIG.
As shown in FIG. 9, the fixing device 8 does not have the pressure roller 22. Instead, the fixing device 8 includes a pressure belt 27 and a pressing member 28. The fixing roller 21 includes a metal core member 21a formed in a cylindrical shape, an elastic layer 21b covering the outer peripheral surface of the core member 21a, and an outer peripheral surface of the elastic layer 21b, as in the above embodiments. It is comprised by the mold release layer 21c to coat | cover. In addition, a heater 24 as a heating element is disposed in the fixing roller 21, and a temperature detection device 25 that detects the surface temperature of the fixing roller 21 is disposed around the fixing roller 21.

  The pressure belt 27 is constituted by an endless belt member made of a polyimide film or the like. The pressing member 28 includes an elastic layer 28a made of silicon rubber or the like and a holder 28b that holds the elastic layer 28a. The elastic layer 28a of the pressing member 28 is disposed so as to contact the inner peripheral surface of the pressure belt 27. In this state, the pressing member 28 is urged toward the fixing roller 21 by an urging means (not shown). . As a result, the pressing member 28 presses the inner peripheral surface of the pressure belt 27 to bring the pressure belt 27 into pressure contact with the fixing roller 21, and a fixing nip N is formed at the pressure contact portion where they are in pressure contact with each other.

  When image fixing processing is performed in this fixing device, the heater 24 generates heat and the fixing roller 21 is heated. At this time, a control device (not shown) controls the amount of heat generated by the heater 24 based on the temperature detected by the temperature detection device 25 so that the surface temperature of the fixing roller 21 becomes a predetermined temperature (fixing temperature). Then, the recording paper P on which the unfixed toner image T is formed is guided in the direction of the arrow A along the guide member 23 and enters the fixing nip N between the rotating fixing roller 21 and the pressure belt 27. . As a result, the recording paper P is heated and pressed by the fixing roller 21 and the pressure belt 27, and the unfixed toner image T is fixed on the recording paper P.

  FIG. 10 shows a separated state in which the fixing roller 21 and the pressing member 28 are separated from each other, and FIG. 11 shows a pressed state in which they are brought into pressure contact with each other. Hereinafter, the configuration of the pressing member 28 will be described in detail with reference to FIGS. 10 and 11. Since the fixing roller 21 according to this embodiment is configured in the same manner as the fixing roller 21 shown in FIG. 4, description of the configuration is omitted.

  The pressing member 28 alternately has a convex surface portion E1 formed in a convex shape and a concave surface portion E2 formed in a concave shape on the pressing surface that presses the pressure belt 27 in the longitudinal direction. The convex surface portion E1 and the concave surface portion E2 are formed by changing the thickness of the elastic layer 28a of the pressing member 28 over the longitudinal direction. In addition, each surface of the convex surface portion E1 and the concave surface portion E2 has a curved shape such as an arc shape or a sin curve shape in the axial direction.

  As shown in FIG. 10, in this embodiment, the convex surface portion E1 and the concave surface portion E2 are disposed over at least a portion of the pressing member 28 corresponding to the entire maximum passage width W of the recording medium (recording paper). Yes. However, the region where the convex surface portion E1 and the concave surface portion E2 are disposed can be a part of the maximum passage width W of the recording medium. Further, the number of the convex surface portion E1 and the concave surface portion E2 is not particularly limited as long as one or more are formed on the pressing member 28. However, in a state where the fixing roller 21 and the pressing member 28 are opposed to each other, the convex portion E1 and the concave portion E2 so that the crown portion C1 and the concave portion E2 always face each other and the reverse crown portion C2 and the convex portion E1 face each other. The number of items is adjusted.

  In the pressure contact state shown in FIG. 11, the fixing roller 21 and the pressing member 28 are pressed against each other by causing the crown portion C1 and the concave surface portion E2 to correspond to each other and the reverse crown portion C2 and the convex surface portion E1 to correspond to each other. Therefore, also in this embodiment, the fixing nip N formed between the fixing roller 21 and the pressure belt 27 is formed to be curved so as to wave in the axial direction.

  In addition, in FIG. 10, in the cross section cut along the longitudinal direction of the pressing member 28, an imaginary line L <b> 28 that passes through the inflection point between the convex surface portion E <b> 1 and the concave surface portion E <b> 2 in the axial direction is represented by the inflection point passage line. The inflection point passing line L28 is configured to be convex toward the fixing roller 21 at the center in the longitudinal direction in a separated state.

  Further, the fixing roller 21 according to the present embodiment is also formed with a thin core material 21a in order to shorten the warm-up time. For this reason, when the fixing roller 21 and the pressing member 28 are brought into pressure contact with each other, the fixing roller 21 bends in a direction orthogonal to the axial direction as in the above-described embodiment. When this bending amount is indicated by reference numeral B21 in FIG. 11, in this embodiment, the protrusion amount G28 in the separated state of the inflection point passage line L28 of the pressing member 28 shown in FIG. 10 is half the value of the bending amount B21. It is set to become.

  Also in this embodiment, in the load state in which the fixing roller 21 and the pressing member 28 are pressed against each other, the height difference between the crown portion and the reverse crown portion (or the convex portion and the concave portion) in the fixing nip N is 0.16 mm. It is desirable to set it to 0.8 mm or less.

  In the separated state shown in FIG. 10, the height difference H21 between the crown portion and the reverse crown portion of the fixing roller 21 and the height difference H28 between the convex portion and the concave portion of the pressing member 28 are the same in the axial direction. Although it is configured to be a value, in the present embodiment as in the embodiment shown in FIG. 7, the height differences H21 and H28 are configured to increase from the center side toward both end sides. May be.

  The fixing roller 21 and the pressing member 28 are not shown in the mounting structure, but the fixing roller 21 and the pressing member 28 are positioned on the same side in the axial direction or the longitudinal direction. The end opposite to the end is configured to be displaceable in the axial direction or the longitudinal direction.

Hereinafter, the operation and effect of the characteristic part of the present invention will be described.
As described above, in the fixing device according to each embodiment of the present invention, the fixing nip is curved so as to wave, so that when the recording paper enters the fixing nip, the recording paper also runs along the fixing nip. Curved to wave. As described above, when the recording sheet is bent, the apparent rigidity of the recording sheet can be greatly improved as compared with the case where the recording sheet is not bent. Thereby, in the fixing device according to the present invention, when the recording paper is carried out from the fixing nip, the apparent rigidity of the recording paper can be improved, and the recording paper can be prevented from being wound around the fixing roller. it can.

  In addition, the present inventor considered that the number of curvatures of the fixing nip affects the apparent rigidity of the recording paper conveyed from the fixing nip, and conducted a test to examine the relationship between them. Hereinafter, the test will be described in detail.

In this test, two types of configurations having the same configuration as the embodiment of the present invention shown in FIG. 4, each having three crown portions and three reverse crown portions, and each having seven each, were prepared. As a conventional type, a fixing device having a fixing roller and a pressure roller having no crown part and reverse crown part was prepared. In each type of the present invention, the amplitude (height) of the crown portion and the reverse crown portion in an unloaded state was set to 0.2 mm. Then, in each fixing device, the apparent rigidity of the various sheets when the various sheets having a basis weight (weight per unit area) of 64 g / m ² , 69 g / m ² , and 90 g / m ² were carried out was measured. .

  A method for measuring the apparent rigidity of the various sheets will be briefly described. First, as shown in FIG. 12, the recording paper P is passed through the fixing nip N between the fixing roller 21 and the pressure roller 22, and the laser light L from the displacement measuring device 70 is applied to the front end of the recording paper P. When irradiated, the conveyance of the recording paper P is stopped. After the stopped recording paper P disappears, the laser beam L is irradiated from the displacement measuring device 70 to the bent recording paper P, and the displacement of the recording paper P is measured. Thereafter, the recording paper P is advanced by a predetermined distance, the recording paper P is again irradiated with the laser light L, and the displacement is measured. Then, the apparent rigidity of the recording paper P is calculated from the obtained displacement of the recording paper P.

FIG. 13 is a graph showing the relationship between the number of curvatures of the fixing nip and the apparent rigidity of various sheets. In FIG. 13, the vertical axis represents the apparent rigidity of the recording paper, and the horizontal axis represents the number of curves of the fixing nip. Here, the number of curvatures of the fixing nip is represented by the number of crown portions and reverse crown portions. That is, when there are no crown part and reverse crown part, the number of curves of the fixing nip is 0, and when there are three crown parts and three reverse crown parts, the number of curves of the fixing nip is 3. Also, in the figure, the triangle plots the measured value of the recording paper with a basis weight of 90 g / m ² , plots the solid square with the measured value of the recording paper with a basis weight of 69 g / m ² , and plots with ●. Is a measured value of a recording paper having a basis weight of 64 g / m ² .

  Referring to the graph of FIG. 13, when the fixing device of the present invention in which the number of curvatures of the fixing nip is 3 or 7 as compared with the case of using the conventional fixing device in which the curvature of the fixing nip is 0, It can be seen that the apparent rigidity of various papers is increased. Further, since the apparent rigidity of the recording sheet is larger when the number of curvatures of the fixing nip is 7, the effect of increasing the apparent rigidity of various sheets as the number of curvatures of the fixing nip increases. It is assumed that there is. The test results shown in FIG. 13 show the effect of improving the apparent rigidity of the recording paper in one embodiment of the present invention, but similar effects can be obtained in other embodiments of the present invention. .

  In addition, as described above, it is preferable that the height difference between the top of the crown portion (or the convex surface portion) and the bottom portion of the reverse crown portion (or the concave surface portion) is 0.16 mm or more in the load state at the time of press contact. This is because if the height difference is less than 0.16 mm, the amount of bending of the recording paper at the fixing nip is reduced, and the apparent rigidity of the recording paper required for separating the recording paper cannot be obtained. The height difference is preferably 0.8 mm or less. When the height difference exceeds 0.8 mm, the rotational speed difference between the crown portion and the reverse crown portion (or the convex surface portion and the concave surface portion) becomes large, and the recording paper. This is because wrinkles may occur. Therefore, by setting the height difference between the crown portion (or convex surface portion) and the reverse crown portion (or concave surface portion) to be not less than 0.16 mm and not more than 0.8 mm, the apparent rigidity of the recording paper is sufficiently obtained. Therefore, it is possible to reliably suppress the winding of the recording paper around the fixing roller, and to suppress the generation of wrinkles on the recording paper and to perform good image formation.

  Further, in order to ensure the separation of the recording paper without generating wrinkles on the recording paper, the crown portion and the reverse crown portion of the fixing roller 21 and the pressure roller 22 are continuously formed in the axial direction. Is preferred. If the crown portion and the reverse crown portion are not formed continuously, such as when the crown portion and the reverse crown portion are disposed at positions separated from each other, wrinkles may occur when the recording paper is conveyed. . For the same reason, in the embodiment of the fixing device using the pressure belt shown in FIG. 9 and the like, the crown portion and the reverse crown portion of the fixing roller 21 are continuously arranged in the axial direction, and the pressing member It is preferable to arrange | position 28 convex surface parts and a concave surface part continuously in a longitudinal direction.

  Further, the present inventor has determined that the recording sheet is separated when the fixing roller and the pressure roller have only one crown portion and one reverse crown portion and one crown portion and one reverse crown portion. The test which compares sex was done. The result is shown in FIG.

  In FIG. 14, (A) shows the test result when only one crown part and one reverse crown part are provided, and (B) shows the test result when one crown part and one reverse crown part are provided. . 14A and 14B, the vertical axis represents the apparent rigidity of the recording paper, and the horizontal axis represents the crown portion in the fixing nip in a state where the fixing roller and the pressure roller are in pressure contact (load state), and It represents the height difference of the reverse crown. Also, in (A) and (B) of the same figure, an alternate long and short dash line α represents a boundary line of apparent rigidity of the recording sheet that divides the case where the recording sheet can be satisfactorily separated from the fixing roller and the case where it cannot. The alternate long and short dash line β indicates a boundary line between the height difference between the crown portion and the reverse crown portion that separates the case where wrinkles may occur on the recording paper and the case where there is no possibility of such wrinkles. That is, when the apparent rigidity of the recording sheet is larger than the boundary line α, the recording sheet can be separated satisfactorily. On the contrary, when the apparent rigidity is smaller than the boundary line α, the recording sheet is separated well. I can't. In addition, when the height difference between the crown portion and the reverse crown portion is larger than the boundary line β, wrinkles are generated on the recording sheet. Conversely, when the height difference is smaller than the boundary line β, wrinkles are not generated on the recording sheet.

  As shown in FIG. 14A, when only one of the crown portion and the reverse crown portion is provided, the apparent rigidity of the recording paper is made larger than the boundary line α so as to separate the recording paper satisfactorily. In this case, the height difference must be set larger than about 1.6 mm, and there is a concern that the recording paper may be wrinkled. On the other hand, when each of the crown portion and the reverse crown portion shown in FIG. 14B is provided, even if the height difference is the same as that shown in FIG. It can be seen that there is a significant improvement. Therefore, in the case of having one crown portion and one reverse crown portion, as shown in FIG. 14B, the apparent rigidity of the recording sheet is set to be larger than the boundary line α and is opposite to the crown portion. The height difference of the crown portion can be set within a range of about 0.72 mm to about 0.8 mm where wrinkles are not generated on the recording paper. Therefore, in the case of having one crown portion and one reverse crown portion, both good separation of the recording paper and good image formation that does not cause wrinkles on the recording paper can be realized.

  As described above, when each of the crown portion and the reverse crown portion is provided, the effect of improving the apparent rigidity of the recording paper is improved as compared with the case where only one crown portion and the reverse crown portion are provided. Remarkably larger. Therefore, the present invention is characterized in that the fixing roller and the pressure roller each have at least one crown portion and one reverse crown portion. Further, as described above, by increasing the number of crown portions and reverse crown portions, the apparent rigidity of the recording paper can be further increased, and the separability can be further improved.

  The test results shown in FIG. 14 show the effect of improving the apparent rigidity of the recording paper in one embodiment of the present invention, but the same effect is achieved in other embodiments of the present invention. Is possible.

  By the way, the fixing roller and the pressure roller described in Japanese Patent Application Laid-Open No. 2005-352297, Japanese Patent Publication No. 7-104636, Japanese Patent No. 3119405, Japanese Patent No. 3267416, etc. already disclosed are either crown portions or It is composed of a roller having a reverse crown portion, and the other is composed of a straight roller. In this case, when the fixing roller and the pressure roller are pressed against each other, a curved surface having a crown shape or an inverted crown shape is pressed against the straight surface, so that a portion with a high contact pressure and a portion with a low contact pressure are generated in the fixing nip. The variation in contact pressure increases over time. As described above, when the contact pressure varies greatly in the fixing nip, when fixing an image, the image portion that has passed through the portion where the contact pressure is high has high glossiness, and the image portion that has passed through the portion where the contact pressure is low is glossy. Therefore, there is a problem that unevenness in glossiness of the image is caused and good image formation cannot be performed.

  On the other hand, the fixing device according to the present invention forms a fixing nip by associating a convex shape including a crown portion or a convex surface portion with a concave shape including a reverse crown portion or a concave surface portion. It is possible to suppress variations in contact pressure. Thereby, it is possible to realize good image formation by suppressing unevenness of glossiness of the image.

  As described above, in the present invention, the variation in pressure in the fixing nip is suppressed by making the crown portion and the reverse crown portion of the fixing roller and the pressure roller correspond to each other. However, when the fixing roller is raised to the fixing temperature, the fixing roller extends in the axial direction due to thermal expansion. The pressure roller also extends in the axial direction by heat from the fixing roller. If the crown portion and the reverse crown portion disposed corresponding to each other are displaced due to the extension of these rollers, the variation in contact pressure at the fixing nip increases.

  FIG. 15 is a graph showing the pressure at the fixing nip when the crown portion and the reverse crown portion of each of the fixing roller and the pressure roller are shifted in the axial direction. When the rollers are pressed in a state where the crown portion and the reverse crown portion are displaced from each other, as shown in FIG. 15, there are a high pressure portion and a low pressure portion from one end side to the other end side in the axial direction. It occurs alternately.

  FIG. 16 shows a state in which the fixing roller 21 and the pressure roller 22 according to the present invention are shifted in the axial direction. As shown in FIG. 16, the distance between the straight core bars 21a and 22a is the same in the axial direction, whereas the thickness of the elastic layers 21b and 22b interposed between the core bars 21a and 22a. The sum is different in the axial direction. Specifically, the sum (t1 + t2) of the thickness of each elastic layer 21b, 22b at the position indicated by the symbol J is the sum of the thickness (t3 + t4) of each elastic layer 21b, 22b at the position indicated by the symbol K. Is also getting bigger. When the fixing roller 21 and the pressure roller 22 are in pressure contact with each other in this state, the compression amount of the elastic layer at the position of the symbol J becomes larger than the compression amount of the elastic layer at the position of the symbol K, and the pressure increases. As described above, in the present invention, when the fixing roller 21 and the pressure roller 22 are displaced in the axial direction and pressed against each other, the pressure varies depending on the sum of the elastic layers, and thus pressure unevenness occurs in the axial direction.

  On the other hand, FIG. 17 shows a state in which the fixing roller 81 and the pressure roller 82 of the comparative example are shifted in the axial direction. In the comparative example shown in FIG. 17, unlike the present invention, the thicknesses of the core bars 81a and 82a of the fixing roller 81 and the pressure roller 82 are changed in the axial direction to form the crown portion and the reverse crown portion. Yes. On the other hand, the elastic layers 81b and 82b and the release layer are formed uniformly in the axial direction. A configuration similar to that of the comparative example is described in, for example, Japanese Patent Laid-Open No. 7-129014.

  Also in the comparative example shown in FIG. 17, when the fixing roller and the pressure roller are displaced in the axial direction and pressed against each other, pressure unevenness occurs in the axial direction. Specifically, as shown in FIG. 17, the sum of the thicknesses of the elastic layers 81b and 82b interposed between the cores 81a and 82a is the same in the axial direction, but at the position indicated by the symbol Q. The distance (d1) between the core bars 81a and 82a is smaller than the distance (d2) between the core bars 81a and 82a at the position indicated by the reference symbol U. For this reason, when the rollers 81 and 82 are brought into pressure contact with each other, the compression amount of the elastic layer at the position of the symbol Q is larger than the compression amount of the elastic layer at the position of the symbol U, and the pressure is increased. As described above, in the comparative example, when the fixing roller 81 and the pressure roller 82 are displaced in the axial direction and pressed against each other, the pressure varies depending on the interval between the core bars, and thus pressure unevenness occurs in the axial direction. .

  As described above with reference to FIGS. 16 and 17, in both the present invention and the comparative example, when the fixing roller and the pressure roller are pressed against each other with the crown portion and the reverse crown portion shifted in the axial direction, The pressure at the fixing nip varies in the axial direction. However, the present inventor has found that the pressure difference of the fixing nip with respect to the axial deviation amount of each roller differs between the present invention and the comparative example.

  FIG. 18 is a graph showing the pressure difference of the fixing nip with respect to the axial deviation amount of the present invention and the comparative example. Here, the “pressure difference” is a difference between a portion where the pressure is highest and a portion where the pressure is the smallest in the axial direction of the roller. Further, the amount of bending of the cored bar in the portion where the pressure difference has occurred is negligible, so it is not considered here.

  As can be seen from FIG. 18, in the present invention indicated by the solid line and the comparative example indicated by the broken line, the pressure difference generated in the present invention is smaller than that in the comparative example even with the same shift amount. Thus, the present invention tends to reduce the pressure difference with respect to the axial displacement amount of each roller as compared with the comparative example.

FIG. 19 is a graph showing the relationship between the pressure difference at the fixing nip and the glossiness difference of the image.
As shown in FIG. 19, it can be seen that the difference in glossiness increases as the pressure difference increases. However, it was found that if the difference in glossiness is 5 or less, it is hardly recognized as image noise in either the high gloss image or the low gloss image. That is, it can be said that a gloss difference of 5 or less is within an allowable range for image quality. Here, the “gloss difference” is a difference in gloss between a portion having the highest glossiness and a portion having the lowest glossiness in one image. Further, in this case, the glossiness was measured using a gloss meter PG-1M manufactured by Nippon Denshoku Industries Co., Ltd. at a measurement angle (incident angle) of 60 °.

  Furthermore, in the present invention and the comparative example described above, the difference in glossiness that occurs when the crown portion and the reverse crown portion are shifted in the axial direction was examined, and the results shown in the graph of FIG. 20 were obtained. In FIG. 20, the solid line indicates the present invention, and the broken line indicates the comparative example. Thus, the present invention has a lower gloss difference than the comparative example. This is because the pressure difference with respect to the amount of deviation tends to be smaller than that of the comparative example (see FIG. 18), and the glossiness difference is considered to be smaller accordingly.

  As described above, in the present invention in which the thickness of the elastic layer is changed, the pressure difference caused by the axial deviation between the rollers can be reduced as compared with the comparative example in which the thickness of the core metal is changed. It is possible to suppress the occurrence of uneven glossiness of the image. Similarly, in the embodiment of the present invention using a pressure belt, both the fixing roller 21 and the pressing member 28 change the thickness of the elastic layer in the axial direction (see FIG. 10). ) The pressure difference of the fixing nip with respect to the axial displacement can be reduced as compared with the case where the thickness of the core of the fixing roller 21 or the holder of the pressing member 28 is changed. Thereby, it is possible to suppress the occurrence of uneven glossiness.

  Further, as shown in FIG. 20, in the present invention, when the axial shift amount is up to about ± 0.5 mm, the difference in glossiness can be suppressed to 5 or less and is not recognized as image noise. Therefore, in the above embodiment of the present invention, the end portions on the same side of the fixing roller and the pressure roller are positioned in the axial direction, and the end portions on the opposite side to the positioned end portions are displaced in the axial direction. The configuration is possible (see FIG. 6). As a result, even if the fixing roller expands due to thermal expansion, the fixing roller and the pressure roller are extended (displaced) to the same end side in the axial direction, so that the positional deviation between the crown portion and the reverse crown portion is reduced. It is possible to suppress the occurrence of variations in the contact pressure at the fixing nip.

For example, when the fixing roller of the present invention having an aluminum cored bar having a linear expansion coefficient of 2.42 × 10 ⁻⁶ / ° C. and a length of 240 mm is heated from 20 ° C. to 180 ° C., the heat causes an axial direction. To 0.933 mm. Further, since the heating roller is also heated to substantially the same temperature as the fixing roller, each roller extends slightly less than 1 mm. In this case, the end of the same side of the fixing roller and the pressure roller is positioned, and the configuration of the present invention that enables the opposite end to be displaced is adopted, so that the axial direction of each roller can be changed. The deviation amount could be 0.5 mm or less.

  As described above, the present invention positions the ends of the fixing roller and the pressure roller on the same side and displaces the opposite ends so that the shafts of the rollers due to thermal expansion can be displaced. Directional deviation can be reduced. This makes it possible to suppress the occurrence of uneven glossiness to a higher degree. Similarly, in the embodiment of the present invention using the pressure belt, the end portions on the same side of the fixing roller 21 and the pressing member 28 are positioned in the axial direction, and the end portion on the opposite side is positioned on the shaft. Since it is configured to be displaceable in the direction, it is possible to reduce misalignment between the fixing roller 21 and the pressing member 28 due to thermal expansion, and to further suppress the occurrence of uneven glossiness.

  In the embodiment of the present invention, the inflection point passage lines L21, L22, L28 of the fixing roller 21, the pressure roller 22, or the pressing member 28 are configured to be convex at the center. Yes. Hereinafter, the reason for such a configuration will be described in comparison with the present invention and a comparative example.

FIG. 21 is a diagram schematically showing the configurations of Comparative Example 1, Comparative Example 2, and the present invention.
FIG. 21 shows a state in which the fixing roller and the pressure roller are separated from each other and a state in which they are brought into pressure contact with each other. Of the two rollers facing each other, the upper side is the fixing roller and the lower side is the pressure roller. Also, the dotted line in the figure shows an inflection point passing line that passes through the inflection points of the crown portion and the reverse crown portion of each roller.

In Comparative Example 1, the core material of the fixing roller and the pressure roller is formed thick, and the inflection point passing line in the separated state is a straight shape parallel to the axial direction.
In Comparative Example 2, the core material of the fixing roller and the pressure roller is formed thin, and the inflection point passing line in the separated state is a straight shape parallel to the axial direction.
In the present invention, the core material of the fixing roller and the pressure roller is formed to be thin, and the inflection point passing line in the separated state is convex in the central portion in the axial direction.

  Specifically, in Comparative Example 1, a core material having an outer diameter of 35 mm and a thickness of 2 mm was used, and in Comparative Example 2 and the present invention, a core material having an outer diameter of 25 mm and a thickness of 1 mm was used. Moreover, the protrusion amount (G21 and G22 shown in FIG. 4) of the inflection point passage line of the present invention was set to 0.08 mm. In all the configurations, the height difference between the crown portion and the reverse crown portion in the separated state (the difference in the height of the swell) was 0.4 mm over the entire axial direction.

  Then, using the comparative examples 1 and 2 and the configuration of the present invention, the deflection of the roller, the nip width in the recording sheet conveyance direction in the fixing nip, and the height difference between the crown portion and the reverse crown portion in the fixing nip (the difference in undulation) Was evaluated. The results are shown in Table 1 below.

  According to Table 1, the amount of deflection of the roller in each configuration is 0.03 mm in the case of the comparative example 1, and 0.16 mm in the case of the comparative example 2 and the present invention. The reason why the deflection amount in Comparative Example 2 and the present invention is larger than that in Comparative Example 1 is that the strength of the roller is reduced by forming the core material thin.

  Further, in Comparative Example 2, there is a large difference in the nip width between the central portion side and the end portion side in the axial direction because each roller is greatly bent and pressed. In this case, as shown in the pressure contact state diagram of Comparative Example 2 in FIG. 21, the end portions of the rollers are bent so as to be close to each other, so that the elastic layer of the rollers is less likely to be crushed on the center side, and the nip width Is smaller on the center side than on the end side. According to Table 1, the nip width on the end side in Comparative Example 2 is 6.7 mm, but the nip width on the center side is 6 mm. As described above, when the difference in the nip width between the central portion side and the end portion side becomes large, there is a problem that image defects such as fixing failure and gloss difference occur. In order to prevent such image defects, it is desirable that the difference in nip width between the center side and the end side be 0.5 mm or less.

  On the other hand, in the present invention, the nip width on the center side and the end side is the same value (6 mm in Table 1) despite the occurrence of bending as in Comparative Example 2. In the case of the present invention, such a result is obtained by making the inflection point passing line convex in the central portion, thereby increasing the amount of collapse of the elastic layer in the central portion as compared with Comparative Example 2, and in the central portion. This is because the variation in the amount of crushing between the side and the end side can be suppressed.

  As described above, according to the present invention, it is possible to suppress the difference in the nip width between the center portion side and the end portion side, and thus it is possible to prevent image defects such as fixing failure and gloss difference. In order to set the difference in nip width to 0.5 mm or less as a value that does not cause the image defect, as shown in Table 1, the amount of protrusion of the inflection point passing line (in Table 1, 0. (08 mm) is preferably set to half of the deflection amount of the roller (0.16 mm in Table 1). In addition, although the result shown in Table 1 shows the effect in one Embodiment of this invention, it can show the same effect also in other embodiment of this invention.

  Table 2 below shows the results of comparing the operation and effect of the embodiment of the present invention shown in FIG. 4 and the embodiment of the present invention shown in FIG. That is, when the height difference between the crown portion and the reverse crown portion in the separated state (swelling height difference) is the same value in the axial direction (configuration shown in FIG. 4), both ends from the central portion side in the axial direction. Compared with the case where it is comprised so that it may become large (structure shown in FIG. 7), the effect | action and effect were compared.

  The present invention 1 in Table 2 has the configuration shown in FIG. 4, and the height difference of the undulation when the rollers are separated from each other is set to 0.4 mm at the center and at the end. On the other hand, the present invention 2 in Table 2 has the configuration shown in FIG. 7, and the difference in the undulation when each roller is separated is 0.4 mm at the center, while the end is set to 0.5 mm. This is different from the present invention 1. Other than that, it is set similarly to the first aspect of the present invention. The present invention 1 in Table 2 employs the same invention as in Table 1.

  According to Table 2, in the first aspect of the present invention, the height difference of the undulation at the fixing nip is 0.33 mm on the center side and 0.32 mm on the end side, and the end side is smaller than the center side. In this case, the degree of curving of the recording sheet on the end side of the fixing nip is reduced, and the separability on the end side is reduced.

  On the other hand, in the second aspect of the present invention, the height difference of the waviness in the fixing nip is 0.33 mm on the center side and 0.40 mm on the end side. It has become. As described above, according to the second aspect of the present invention, the difference in the height of the waviness in the separated state is increased from the central portion side toward the both end portions in the axial direction, thereby ensuring a large height difference in the waviness of the fixing nip on the end portion side. It is possible. Accordingly, the recording sheet can be sufficiently curved on the end side of the roller, and the separation property of the recording sheet can be improved. Similarly, also in the embodiment of the present invention using the pressure belt, by increasing the height difference of the undulation in the separated state between the fixing roller 21 and the pressing member 28 from the central side toward the both end sides, Similar actions and effects can be achieved.

  As described above, according to the present invention, the apparent rigidity of the recording medium can be increased at the fixing nip, and the separation property of the recording medium can be improved. Accordingly, since the recording medium can be prevented from being wound around the fixing roller, it is possible to provide a highly reliable fixing device and image forming apparatus.

  Further, the present invention can suppress variations in contact pressure at the fixing nip. Thereby, it is possible to suppress the occurrence of uneven glossiness and the like in the fixed image, and it is possible to maintain good image formation.

  Further, according to the present invention, it is possible to suppress the difference in the nip width from occurring even when the fixing roller bends during pressure contact. For this reason, even when a low-strength fixing roller is used to shorten the warm-up time, the difference in nip width can be reduced, thereby preventing image defects such as fixing defects and gloss differences. And good image formation can be realized.

  In addition, this invention is not limited to the above-mentioned embodiment, Of course, a various change can be added in the range which does not deviate from the summary of this invention. In the above-described embodiment, the inflection point passage lines of both the fixing roller and the pressure roller (or pressing member) are formed in a convex shape, but either one of the inflection point passage lines is formed in a convex shape. This also makes it possible to reduce the difference in nip width. The fixing device of the present invention can be mounted not only on the color image forming apparatus shown in FIG. 1 but also on a monochrome image forming apparatus, a copying machine, a printer, a facsimile, or a complex machine thereof.

8 Fixing Device 21 Fixing Roller 21a Core Material 21b Elastic Layer 21c Release Layer 22 Pressure Roller 22a Core Material 22b Elastic Layer 22c Release Layer 27 Pressure Belt 28 Pressing Member 28a Elastic Layer 28b Holder B21 Bending Amount B22 Bending Amount C1 Crown Part C2 Reverse crown part D1 Crown part D2 Reverse crown part E1 Convex part E2 Concave part G21 Projection amount G22 Projection amount G28 Projection amount H Height difference H21 Height difference H22 Height difference H28 Height difference L21 Inflection point passage line L22 Inflection point passage Line L28 Inflection point passing line N Fixing nip P Recording paper (recording medium)
W Maximum passing width of recording medium

JP 2004-109697 A

Claims (8)

A fixing roller heated by a heat source; and a pressure roller disposed in pressure contact with the fixing roller, and the recording medium passes through a fixing nip formed by pressure contact between the fixing roller and the pressure roller. By fixing the unfixed image on the recording medium to the recording medium,
The fixing roller has at least one crown portion whose outer peripheral surface is formed in a crown shape and each reverse crown portion whose outer peripheral surface is formed in a reverse crown shape,
The pressure roller has at least one crown portion whose outer peripheral surface is formed in a crown shape and one reverse crown portion whose outer peripheral surface is formed in a reverse crown shape,
The crown portion of the fixing roller and the reverse crown portion of the pressure roller are made to correspond to each other, and the reverse crown portion of the fixing roller and the crown portion of the pressure roller are made to correspond to each other. Press the pressure rollers together,
Forming the crown portion and the reverse crown portion of each of the fixing roller and the pressure roller by changing the thickness of the elastic layer of each of the fixing roller and the pressure roller in the axial direction;
The ends of the fixing roller and the pressure roller on the same axial side are positioned in the axial direction, and the ends of the fixing roller and the pressure roller opposite to the positioned ends. A fixing device configured to be displaceable in the axial direction,
An imaginary line passing through the inflection point of each of the crown portion and the reverse crown portion in the cross section cut along the axial direction of the fixing roller and the pressure roller is an inflection point passing line. When you call it,
In the separated state in which the fixing roller and the pressure roller are separated from each other, the inflection point passage line of at least one of the fixing roller and the pressure roller is formed to be convex in the central portion in the axial direction. A fixing device characterized by comprising.   The amount of protrusion of the inflection point passing line configured to be convex in the separated state is half of the amount of deflection perpendicular to the axial direction when the fixing roller and the pressure roller are brought into pressure contact with each other. The fixing device according to claim 1, which is set as described above.   In the separated state in which the fixing roller and the pressure roller are separated from each other, the height difference between the top of the crown portion and the bottom of the reverse crown portion of each roller is more on the end side than the central portion side in the axial direction. The fixing device according to claim 1, wherein the fixing device is set so as to be large.   4. The fixing device according to claim 1, wherein the crown portion and the reverse crown portion of each of the fixing roller and the pressure roller are continuously formed in the axial direction. 5.   The said crown part and the said reverse crown part are arrange | positioned over the part corresponding to the whole maximum passage width of the recording medium of the said fixing roller and the said pressure roller at least. Fixing device.   In a load state in which the fixing roller and the pressure roller are pressed against each other, a height difference between the top of the crown portion and the bottom of the reverse crown portion in the fixing nip is 0.16 mm or more and 0.8 mm or less. The fixing device according to claim 1, wherein the fixing device is set as follows. A fixing roller heated by a heating source; an endless pressure belt; and a pressing member that presses an inner peripheral surface of the pressure belt to press the pressure belt against the fixing roller. In a fixing device for fixing an unfixed image on the recording medium to the recording medium by passing the recording medium through a fixing nip formed by pressure belts being pressed against each other.
The fixing roller has at least one crown portion whose outer peripheral surface is formed in a crown shape and each reverse crown portion whose outer peripheral surface is formed in a reverse crown shape,
The pressing surface of the pressing member that presses the pressure belt has at least one each of a convex surface portion formed in a convex shape and a concave surface portion formed in a concave shape,
The crown portion of the fixing roller and the concave surface portion of the pressing member are arranged in correspondence with each other, the reverse crown portion of the fixing roller and the convex surface portion of the pressing member are arranged in correspondence with each other, and The pressure belt is pressed by a pressing member, the fixing roller and the pressure belt are pressed against each other;
The crown portion and the reverse crown portion of the fixing roller are formed by changing the thickness of the elastic layer of the fixing roller in the axial direction, and the convex surface portion and the concave surface portion of the pressing member are It is formed by changing the thickness of the elastic layer of the pressing member over the longitudinal direction,
The end portions on the same side of the fixing roller and the pressing member are positioned in the axial direction or the longitudinal direction, and the end portions of the fixing roller and the pressing member opposite to the positioned end portions are positioned. A fixing device configured to be displaceable in an axial direction or a longitudinal direction,
A cross section cut along the longitudinal direction of the pressing member and an imaginary line passing through the inflection point between the crown portion and the reverse crown portion in the cross section cut along the axial direction of the fixing roller. When imaginary lines passing through the inflection points of the convex surface portion and the concave surface portion in the longitudinal direction are respectively called inflection point passage lines,
In the separated state in which the fixing roller and the pressing member are separated from each other, the inflection point passing line of at least one of the fixing roller and the pressing member is convex in the central portion in the axial direction or the longitudinal direction. A fixing device characterized by comprising:   An image forming apparatus comprising the fixing device according to claim 1.

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