Patent Application
20090243212
1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of Related Art
In image forming apparatuses according to the related art, such as a copying machine, a printer, and a facsimile, a toner image formed on a photosensitive drum, which is an image bearing member, provided in an image forming portion is transferred onto a sheet, and the sheet is conveyed to a fixing device. Then, the fixing device heats and presses the sheet to fix the toner image to the sheet, and a sheet conveying apparatus discharges the sheet having the toner image fixed thereto to a sheet discharging and stacking portion.
However, in the image forming apparatus according to the related art, for example, when a roller comes into contact with a surface of the sheet having the toner image formed thereon or the rear surface thereof before the toner image that is heated and fixed at a high temperature is cooled down, the surface property of the contact portion is changed or the contact portion dissipates heat from the sheet earlier than the other portions. Consequently, the color of the toner image is changed, or a so-called roller trace, which is viewed as a brilliance difference, occurs, which results in deterioration of image quality. The roller trace is less likely to be visually viewed from a plain sheet, but is more likely to be viewed from a sheet having a high degree of brilliance, such as a projector sheet, glossy paper, or a glossy film.
In order to prevent deterioration of image quality due to the generation of the roller trace, for example, a structure has been proposed in which a roller having an insulating property is used as a conveying roller (discharge roller) of a sheet conveying apparatus contacted with a toner image that is heated and fixed at a high temperature (see Japanese Patent Laid-Open No. 2004-301291).
Furthermore, in order to prevent deterioration of image quality due to the generation of the roller trace, for example, a structure has been proposed in which a roller having a small contact area with a sheet is used as a roller of a sheet conveying apparatus contacted with a toner image that is heated and fixed at a high temperature (see Japanese Patent Laid-Open No. 2006-145586).
However, in the sheet conveying apparatus according to the related art that uses the roller having an insulating property, the manufacturing cost of the roller (insulating roller) is increased since a material forming the insulator layer is expensive. In addition, the hardness of an insulator forming the insulator layer is low. Therefore, when a skid comes into pressure contact with the roller, the insulator layer is compressed, and an insulating effect is lowered. As a result, a roller trace occurs, and image quality deteriorates. In order to prevent the insulating effect from being lowered, when the urging force of a spring that presses, for example, a skid against the roller, the conveying force (discharge force) of a pair of rollers is lowered. As a result, it is difficult to reliably discharge sheets.
In addition, in the sheet conveying apparatus according to the related art that has a plurality of minute protrusions on the outer circumferential surface thereof, in some cases, a toner parting property deteriorates by the plurality of minute protrusions formed on the outer circumferential surface of the roller. When the toner parting property deteriorates as described above, the following problem arises. When the sheet having the toner image heated and fixed thereto passes through a pair of rollers before the toner image is cooled down, toner on the sheet is peeled off, which results in deterioration of image quality. The peeled toner is likely to adhere to the rollers.
When a contact area between toner and the rollers is small and the heights of contact portions between the minute protrusions and the sheet are not uniform, the minute protrusions formed on the outer circumferential surface of the roller cause high pressure to be locally applied to the sheet. Consequently, toner on the sheet is peeled off, or the sheet is deformed and an uneven portion is formed on the sheet. As a result, image quality deteriorates.
In order to prevent the peeled toner from adhering to the roller, it is considered to form a surface protecting layer on the circumferential surface of the roller. However, when the surface protecting layer is formed, the protrusions on the outer circumferential surface of the roller are gently inclined. As a result, a contact area between toner on the sheet and the roller is increased, and it is difficult to sufficiently prevent the generation of a roller trace.
The present invention provides a sheet conveying apparatus capable of reliably conveying a sheet with a low cost, without deteriorating image quality and an image forming apparatus including the same.
According to an aspect of the invention, there is provided a sheet conveying apparatus for conveying a sheet having an image formed thereon. The sheet conveying apparatus includes: a first rotator; a second rotator that comes into pressure contact with the first rotator and contacts a surface of the sheet having the image formed thereon; and a plurality of ribs that are formed on a circumferential surface of the second rotator and disposed in an axial direction so as to be continuous in a circumferential direction. Among the plurality of ribs, the ribs disposed at both ends of the second rotator in the axial direction have a diameter that is smaller than those of the other ribs.
According to another aspect of the invention, there is provided a sheet conveying apparatus for conveying a sheet having an image formed thereon. The sheet conveying apparatus includes: a first rotator; a second rotator that comes into pressure contact with the first rotator and contacts a surface of the sheet having the image formed thereon; and a plurality of ribs that are formed on a circumferential surface of the second rotator and disposed in an axial direction so as to be continuous in a circumferential direction. Among the plurality of ribs, adjacent ribs have the same diameter, or one of the adjacent ribs close to the center in the axial direction has a diameter that is larger than that of the other rib close to both ends in the axial direction. A height difference between adjacent ribs is 0 μm or more and 60 μm or less, and a height difference between the rib disposed at the center of the second rotator and the ribs disposed at both ends thereof in the axial direction is 20 μm or more and 210 μm or less.
According to the above aspects of the invention, it is possible to reliably convey a sheet with a low cost, without deteriorating image quality.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
Hereinafter, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings.
A laser scanner 2 is provided above the printer body 1A, and an image forming portion 1B is provided below the laser scanner 2. In this embodiment, the image forming portion 1B forms an image using electrophotographic system, and includes four image forming units 3 (3Y, 3M, 3C, and 3K) that form yellow (Y), magenta (M), cyan (C), and black (K) toner images, respectively.
The image forming units 3 have the same structure except for toner colors. In each of the image forming units, a charging unit (not illustrated) uniformly charges a photosensitive drum 4a and the laser scanner 2 radiates light corresponding to an image signal to the photosensitive drum, thereby forming an electrostatic latent image. Then, a development device 4b develops the electrostatic latent image into a toner image, thereby obtaining a visible image.
An endless intermediate transfer belt 5 rotatably contacts each of the photosensitive drums 4a, and the toner images that are formed on the photosensitive drums 4a by the above-mentioned method are transferred onto the intermediate transfer belt 5 so as to overlap each other, thereby forming a color image.
In addition, a sheet feed portion 1C including a feed roller 7 that feeds sheets loaded in a sheet cassette 6 is provided below the printer body 1A. In the sheet feed portion 1C, a sheet in the sheet cassette 6 is fed to a secondary transfer portion, which is a nip portion provided between a secondary transfer roller 8 and the intermediate transfer belt 5, by the feed roller 7 in synchronization with the formation of the toner image.
The toner image on the intermediate transfer belt 5 is transferred to the sheet fed to the secondary transfer portion by the secondary transfer portion. In addition, the sheet having the toner image transferred thereon is conveyed to a fixing device B that is provided above the secondary transfer portion. The fixing device B heats and fixes the sheet to fix the transferred toner image to the sheet. Then, the sheet having the toner image thermally fixed thereto is discharged to a discharge tray 9.
As illustrated in
As illustrated in
The discharge skid 23 is pressed against the discharge roller rubber 22a by an urging unit (not illustrated), and is rotated with the rotation of the discharge roller 22. The discharge skid 23 is rotatably attached to a conveying guide 24 illustrated in
However, as illustrated in
In this embodiment, the discharge roller central rib 22b is provided between the discharge roller rubbers 22a. In addition, the outside diameter of each of the discharge roller central rib 22b and the discharge roller rings 22c is larger than that of the discharge skid rubber 23a, in order to stiffen the sheet. According to this structure, when the sheet is conveyed by the discharge roller 22 and the discharge skid 23 while being nipped therebetween, the sheet is curved in a wave shape in the axial direction, as illustrated in
Next, the structure of the discharge skid 23 according to this embodiment will be described. As illustrated in
In addition, a plurality of ring-shaped minute ribs 23c are provided on the circumferential surface of the skid body 23b at predetermined pitches in the axial direction so as to be continuous in the circumferential direction. The pitches between the minute ribs 23c will be described below.
As illustrated in
In this embodiment, the number of minute ribs 23c is 10, but the invention is not limited thereto. Any number of minute ribs 23c may be used as long as they can have a shape that is similar to the shape of the sheet when the sheet is discharged and be uniformly contacted with the sheet. Of course, the number of minute ribs may increase if the conditions are satisfied. It is preferable that the number of minute ribs 23c increase in order to disperse pressing force against the sheet.
In this embodiment, the outer circumferential surfaces of the minute ribs 23c are formed so as to be aligned with the outer circumferential surface of the skid body 23b. That is, in this embodiment, the circumferential surface of the skid body 23b is formed to have a shape corresponding to the sheet pressed by, for example, the central rib 22b and the discharge roller rings 22c, and the plurality of minute ribs 23c form the circumferential surface of the skid body 23b contacted with the sheet. The minute ribs 23c are formed integrally with the skid body 23b, and are made of POM having high slidability. The minute ribs 23c and the skid body 23b may be separately formed.
As illustrated in
Next, parameters of the minute rib 23c according to this embodiment will be described. As illustrated in
In this embodiment, as illustrated in
In this embodiment, the contact width S of each minute rib 23c with the sheet is about 400 μm, the pitch P between adjacent minute ribs is about 700 μm, the height H of the minute rib 23c from the groove is about 200 μm. In addition, each of the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c is about 20 μm, the height difference D between the minute rib 23c disposed at the center and the minute ribs 23c disposed at both ends is about 110 μm, and the curvature r of the curved line linking the tops of the minute ribs 23c is about R45 degrees.
Next, the threshold values of the parameters of the minute ribs 23c that are experimentally calculated will be described.
First, the contact width of the minute rib 23c with the sheet will be described. When the contact width S is more than 600 μm, in the toner image fixed to the sheet, the width of a portion thereof whose surface property is changed by the minute ribs 23c is increased. As a result, it is difficult to sufficiently prevent deterioration of image quality due to a roller trace generated by the minute ribs 23c.
In contrast, when the contact width S of each minute rib 23c with the sheet is 600 μm or less, a portion of the toner image whose surface property is changed is sufficiently narrowed, and it is possible to prevent deterioration of image quality. Therefore, it is preferable that the contact width S of the minute rib 23c with the sheet is set to a predetermined value of 600 μm or less.
Next, the pitch between the minute ribs 23c will be described. When the pitch P between adjacent minute ribs 23c is 200 μm or less, in the toner image fixed to the sheet, the pitch between portions whose surface properties are changed by the minute ribs 23c is reduced. Therefore, as a result, the same state as that when flat shape without ribs is provided is obtained, and it is difficult to prevent deterioration of image quality due to a roller trace. In addition, when the pitch P between adjacent minute ribs 23c is 200 μm or less and the discharge skid 23 is made of a resin material, it is difficult to form the mold of the discharge skid 23. As a result, it is difficult to ensure mass product.
When the pitch P between adjacent minute ribs 23c is more than 1000 μm, in the toner image fixed to the sheet, the pitch between portions whose surface properties are changed by the minute ribs 23c is excessively large. Therefore, in the toner image, a portion whose surface property is changed and another portion whose surface property is not changed are visible.
In addition, when the pitch P between adjacent minute ribs 23c is more than 1000 μm and the length of a portion of the discharge skid 23 that comes into pressure contact with the discharge roller 22 in the rotating shaft direction is maintained at a predetermined value, the number of minute ribs 23c is reduced, and the conveying force of the sheet is lowered. In this case, when the pressure of the discharge skid 23 against the discharge roller 22 is increased in order to maintain the conveying force, it is difficult to prevent deterioration of image quality due to a roller trace or the peeling of toner.
In contrast, when the pitch P between adjacent minute ribs 23c is 200 μm or more and 1000 μm or less, it is not necessary to increase the conveying force of the sheet, thereby preventing deterioration of image quality. Therefore, it is preferable that the pitch P between adjacent minute ribs is set to a predetermined value in the range of 200 μm or more and 1000 μm or less.
Next, the height of the minute rib 23c from the groove will be described. When the height H of the minute rib 23c from the groove is less than 100 μm, a concave portion of the minute rib 23c is close to the sheet, and the concave portion dissipates the heat of the toner image fixed to the sheet. As a result, a roller trace is generated, and image quality deteriorates.
On the other hand, when the height H of the minute rib from the groove is more than 300 μm, it is difficult to form the mold of the discharge skid 23 which is made of a resin material and in which the pitch between adjacent minute ribs is maintained in the above predetermined range. Therefore, it is difficult to ensure the mass product of the discharge skid 23. For this reason, it is preferable that the height of the minute rib 23c from the groove is set to a predetermined value in the range of 100 μm or more and 300 μm or less.
Next, the height difference between adjacent minute ribs 23c will be described. When the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c are less than 0 μm, that is, when the height relationship is reversed such that the height H of the minute rib 23c from the groove is reduced from the center of the discharge skid 23 to both ends thereof, some of the minute ribs 23c do not contact the sheet. Therefore, the minute ribs 23c that contact the sheet locally apply high pressure to the sheet, and image quality deteriorates due to the peeling of toner or an uneven portion of the sheet.
On the other hand, when the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c are more than 60 μm, there is a large variation in the height H of each minute rib 23c, and some of the minute ribs 23c do not contact the sheet. Therefore, similar to when the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c are less than 0 μm, image quality deteriorates due to the peeling of toner or an uneven portion of the sheet. For this reason, it is preferable that the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c is set to a predetermined value in the range of 0 μm or more and 60 μm or less.
Next, the height difference between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof will be described. If the height difference D between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof is less than 20 μm, the minute ribs disposed at both ends of the discharge skid 23 are strongly contacted with the sheet when the sheet or the discharge skid 23 is inclined. Therefore, the peeling of toner occurs, or an uneven portion is formed on the sheet by the minute ribs disposed at both ends of the discharge skid 23. The uneven portion of the sheet causes image quality to deteriorate.
If the height difference D between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof is more than 210 μm, the curvature of a curved line linking the tops of the minute ribs 23c is reduced, and the minute rib disposed at the center of the discharge skid 23 is strongly contacted with the sheet. Therefore, an uneven portion having the same shape as the curved line linking the tops of the minute ribs 23c is formed on the sheet, which causes image quality to deteriorate. For this reason, it is preferable that the height difference D between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof is set to a predetermined value in the range of 20 μm or more and 210 μm or less.
Next, the correlation between the parameters of the minute ribs 23c will be described.
First, the contact width S of each minute rib 23c with the sheet is not correlated to the other parameters. This is because the width of a portion of the toner image whose surface property is changed depends on only the contact width S of the minute rib 23c with the sheet.
In addition, the height H of the minute rib 23c from the groove is not correlated to the other parameters. This is because the degree of dissipation of heat from the toner image fixed to the sheet by a concave portion of the minute rib 23c depends on only the height H of the minute rib 23c from the groove. When the height H of the minute rib from the groove is more than 300 μm, it is difficult to form the mold of the discharge skid 23 which is made of a resin material and in which the pitch P between adjacent minute ribs is maintained in the above-mentioned predetermined range without depending on parameters other than the height H.
The pitch P between adjacent minute ribs, the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c, and the height difference D between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof are correlated to the evaluation of image quality.
As can be seen from the results illustrated in
The reason is that, when the pitch between adjacent minute ribs is small, the height difference between the minute rib disposed at the center of the discharge skid and the minute ribs disposed at both sides thereof is slightly increased and the curvature of the curved line linking the tops of the minute ribs is excessively small. When the curvature of the curved line is excessively small, an uneven portion having the same shape as the curved line linking the tops of the minute ribs is formed, which causes deteriorate of image quality.
When the pitch P between adjacent minute ribs is 1000 μm and the height difference D between the minute rib disposed at the center of the discharge skid and the minute ribs disposed at both ends thereof is less than 110 μm, image quality deteriorates. The reason is that, when the pitch between adjacent minute ribs is large and the height difference between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof is small, the minute ribs disposed at both ends of the discharge skid are strongly contacted with the sheet. In addition, when the pitch P between adjacent minute ribs is 1000 μm and the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c are less than 20 μm, it is difficult to increase the height difference between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof. As a result, image quality deteriorates.
When the height differences ΔH1 to ΔH4 between adjacent minute ribs 23c are less than 20 μm and the height difference D between the minute rib disposed at the center of the discharge skid and the minute ribs disposed at both ends thereof is 210 μm or more, it is difficult to increase the height difference between the minute rib disposed at the center of the discharge skid and the minute ribs disposed at both ends thereof, regardless of the pitch P between adjacent minute ribs. As a result, image quality deteriorates.
On the other hand, when the pitch P between adjacent minute ribs is 700 μm, it is possible to adopt the parameters of the minute ribs in the wide range, which is most preferable. In particular, it is most preferable that the pitch P between adjacent minute ribs is about 700 μm, the height difference D between the minute rib disposed at the center of the discharge skid and the minute ribs disposed at both ends thereof is about 110 μm, and the height differences ΔH1 to ΔH4 between adjacent minute ribs is about 20 μm.
As described above, in this embodiment, when the sheet is stiffened and when the sheet is pressed and curved by, for example, the central rib 22b and the discharge roller rings 22c, the sheet has the curved shape illustrated in
That is, in this embodiment, the discharge skid 23 is formed such that the diameter thereof is reduced from the center to both ends in the axial direction, and a plurality of minute ribs 23c are formed on the circumferential surface of the discharge skid 23 in the axial direction so as to be continuous in the circumferential direction. In this way, the minute rids 23c come into linear contact with the sheet to discharge it while maintaining conveying force required for the sheet. Therefore, it is possible to minimize the amount of heat dissipated from the sheet by the discharge skid 23.
In this embodiment, a case in which the sheet is stiffened by the central rib 22b or the discharge roller rings 22c is given as an example. However, even when the sheet is not stiffened, the discharge skid 23 that comes into contact with a surface of the sheet having an image formed thereon may have the above-mentioned shape. In this case, it is also possible to prevent deterioration of image quality.
Furthermore, since the minute rib 23c does not have an uneven portion in the direction in which the sheet is discharged, in the toner image fixed to the sheet, a portion whose surface property is changed due to contact can be prevented from being visually viewed, without damaging a toner parting property. In addition, since the discharge skid 23 is formed such that the diameter thereof is reduced from the center to both ends in the axial direction, it is possible to prevent high pressure from being locally generated while reducing a contact area. As a result, it is possible to prevent the generation of a roller trace, the peeling of an image, and the generation of an uneven portion on a sheet with a low cost, as compared to the related art.
Furthermore, a plurality of minute ribs 23c are formed on the circumferential surface of the discharge skid 23 in the axial direction so as to be continuous in the circumferential direction, thereby reliably discharging a sheet with a low cost without deteriorating image quality.
Next, a second embodiment of the invention will be described.
In this embodiment, as illustrated in
In the above-mentioned structure, the height differences ΔH1 and ΔH2 between adjacent minute ribs 23c disposed at the center of the discharge skid 23 is approximately 0 μm. It is preferable that the height differences ΔH1 and ΔH2 is 20 μm or less in order to prevent deterioration of image quality. When the height differences ΔH1 and ΔH2 are 20 μm or less, it is possible to contact all the minute ribs 23c with the sheet, and it is possible to prevent high pressure from being locally applied to the sheet.
The relationship between the height differences ΔH1 and ΔH2 and the evaluation of image quality is the same as that in the first embodiment. In addition, the relationship between the evaluation of image quality and the parameters of the minute ribs 23c, such as the contact width S of the minute rib 23c with the sheet, the pitch P between adjacent minute ribs, the height H of the minute rib 23c from the groove, and the height difference D between the minute rib disposed at the center of the discharge skid 23 and the minute ribs disposed at both ends thereof, is the same as that in the first embodiment.
As described above, six minute ribs disposed at the center of the discharge skid 23 have the same outside diameter, thereby reliably ensuring pressing force against the sheet 11 at the center of the discharge skid 23. In addition, since four minute ribs 23c are formed at both ends of the discharge skid 23 in the axial direction such that their outside diameters are reduced toward both ends of the discharge skid in the axial direction, the shape of the minute ribs 23c corresponding to the shape of the discharged sheet can be obtained, similar to the first embodiment.
In this way, conveying force required to convey the sheet 11 can be obtained. In addition, in the toner image fixed to the sheet 11, it is possible to prevent a portion of the toner image whose surface property is changed due to contact with the discharge skid 23 from being visually viewed.
In the above-described embodiments, a full-color laser beam printer is given as an example of the image forming apparatus, but the invention is not limited thereto. For example, the invention may be applied to a monochrome printer and image forming apparatuses other than the printers, such as a copying machine and a facsimile. In addition, in the above-described embodiments, the minute ribs are formed on the discharge skid 23, but the invention is not limited thereto. The minute ribs 23c may be formed on at least one of the discharge skid 23 and the discharge roller rubber 22a. In the above-described embodiments, the minute rib 23c has a ring shape, but the minute rib 23c may have a screw shape.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2008-092675, filed Mar. 31, 2008, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-092675 | Mar 2008 | JP | national |
