Patent Application
20100278576
1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a printer, a copying machine, or a facsimile, a toner image formed by an image forming unit is transferred onto a sheet, and the sheet with the transferred toner image is discharged to a sheet discharge tray through a sheet discharge roller after the toner image is fixed on the sheet in a fixing unit. There is known a method in which a conveying roller pair for conveying the sheet guides the sheet in between the conveying roller pair including two rollers arranged to be opposed to each other, and the sheet is conveyed while being sandwiched with a frictional force generated between the two rollers. In order to prevent deterioration of performance in stacking sheets on the sheet discharge tray due to a curl of the sheet and insufficient stiffness (i.e., insufficient rigidity) of the sheet, stiffening rollers are often adopted as a sheet discharge roller pair for discharging the sheet from an inside of the apparatus onto the sheet discharge tray. When the sheet passes, the stiffening rollers stiffen the sheet through forming the sheet into a multi-corrugated shape in a width direction orthogonal to a sheet conveying direction. Thus, stacking performance is improved.
However, there are various kinds of sheets, and hence stiffness (rigidity) inherent in the sheets is different in each sheet. As a conventional configuration, there is often adopted a configuration in which a center distance between the two rollers opposed in the sheet discharge roller pair is set to a certain distance. When the center distance is set to the certain distance, the sheet with low stiffness is short of a stiffening amount, and hence the sheet hangs down because the sheet does not have desired stiffness. Consequently, the hanging-down sheet may push forward sheets which have been already discharged onto the sheet discharge tray. In order to cope with this situation, a stiffening force is sometimes strengthened through increasing a corrugating amount of the sheet. However, according to the configuration in which the stiffening force is strengthened, in a case of a sheet with high stiffness such as a thick sheet of paper, stiffening is strongly performed by the sheet discharge roller pair. When the stiffening is performed, due to high stiffness of the sheet, the sheet may strongly come into contact with the sheet discharge roller pair. In this case, of the sheet, on a portion strongly coming into contact with the sheet discharge roller pair and a portion not coming into contact therewith, roller marks, i.e., image density nonuniformity, sometimes occur. The roller marks are, for example, coagulation nonuniformity occurring when the roller comes into contact with the toner image which is not fully cooled, and streaks occurring when a corner of the roller comes into contact with the sheet.
There are the following problems. When the sheet has low stiffness, the stiffening amount is insufficient.
When the sheet has high stiffness, the sheet tends to strongly come into contact with the sheet discharge roller pair, and thus the roller marks are likely to occur. Therefore, it has been a challenge to achieve a balance between a configuration in which stiffening is strongly performed when a sheet with low stiffness such as a thin sheet of paper is fed, and a configuration in which occurrence of the roller marks is reduced through weakly performing stiffening when a sheet with high stiffness such as a thick sheet of paper is fed.
There is proposed a technology of changing a stiffening amount of a sheet according to a kind of the sheet (see Japanese Patent Application Laid-Open No. H03-88672). In the technology described in Japanese Patent Application Laid-Open No. H03-88672, there is provided a plurality of stiffening rollers movable in an axial direction of a sheet discharge roller, and positions of the stiffening rollers are shifted by a motor according to the kind of the sheet to be discharged. Thus, the technology of imparting desired stiffness to the sheet according to the kind of the sheet is proposed.
Further, there is proposed a technology of adjusting stiffening of a sheet with a configuration in which a roller shaft body is movable (see Japanese Patent Application Laid-Open No. 2001-302060). In the technology described in Japanese Patent Application Laid-Open No. 2001-302060, in an upper sheet discharge roller and a lower sheet discharge roller constituting a sheet discharge roller pair, a plurality of star wheels is provided to a side of the upper sheet discharge roller, and rollers with rib are provided to a side of the lower sheet discharge roller. There is obtained positional relation in which outer peripheral portions of the star wheels and outer peripheral portions of the rollers with rib overlap each other when viewed in an axial direction. In the technology, a rotating member with a cam surface (tapered surface) is provided. When a rotating shaft of the lower sheet discharge roller is pressed against the cam surface and the rotating member is rotated by a motor, the lower sheet discharge roller moves in the axial direction in conjunction with the cam surface. Thus, an axial distance between the star wheels of the upper sheet discharge roller and the rollers with rib of the lower sheet discharge roller is changed, a corrugating amount of the sheet is increased or decreased, and desired stiffening is performed on the sheet according to a kind of the sheet.
In the technologies disclosed in the above-mentioned patent publications, the stiffening amount of the sheet is changed according to a material and size of the sheet with use of drive from the motor. However, according to each of the configurations, it costs to provide a mechanism for transmitting rotation of the motor, and a control circuit for controlling the motor. Further, the configuration is complicated, and hence there arises a problem in that large space is required and a size of the apparatus is increased.
Therefore, it is an object of the present invention to provide an image forming apparatus which has a low-cost and simple configuration, and performs proper stiffening on sheets irrespective of kinds of the sheets.
According to the present invention, there is provided an image forming apparatus including: an image forming unit configured to form an image on a sheet; a roller pair configured to discharge the sheet having the image formed thereon, the roller pair which has a first roller including a plurality of first roller bodies arranged at intervals in an axial direction and a second roller including a second roller body arranged between the plurality of first roller bodies in the axial direction; a support portion configured to support the first roller and the second roller so as to be able to increase/decrease a distance between an axis line of the first roller and an axis line of the second roller; an elastic member configured to urge at least one of the first roller and the second roller so as to relatively bring one of the first roller and the second roller close to another of the first roller and the second roller; and a regulating portion configured to regulate, at a position at which the plurality of first roller bodies and the second roller body overlap each other in a direction orthogonal to the axial direction, movement of the at least one of the first roller and the second roller in a direction of increasing the distance between the axis line of the first roller and the axis line of the second roller while the sheet is passing between the first roller and the second roller.
According to the present invention, in a case of the sheet such as a thin sheet of paper, in a state of keeping the distance between the axis line of the first roller and the axis line of the second roller without being affected by stiffness of the sheet, the first roller and the second roller discharge the sheet after performing proper stiffening on the sheet such as the thin sheet of paper. In a case of the sheet such as a thick sheet of paper, the first roller and the second roller function to increase the distance between the axis line of the first roller and the axis line of the second roller against the urging force of the elastic member. However, the regulating portion regulates shaft movement at the position at which the part of the plurality of first roller bodies and the part of the second roller body overlap each other when viewed in the axial direction, and hence reduction of a stiffening force with respect to the sheet is prevented. With such simple configuration, it is possible to discharge the sheet in a state in which proper stiffening is performed on the sheet according to a kind of the sheet.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Hereinafter, with reference to the drawings, an embodiment in which an electrophotographic color laser printer is applied as an example of an image forming apparatus according to the present invention is specifically described. Unless otherwise specified, scope of the present invention should not be construed restrictively in terms of dimensions, materials, and shapes of components, and relative arrangement thereof, which are described in this embodiment.
As illustrated in
The charging devices 3 (3a to 3d) uniformly charge surfaces of the photosensitive drums. The scanner units 4 (4a to 4d) form electrostatic latent images on the photosensitive drums 2 (2a to 2d) by irradiation of laser beam based on image information. The developing devices 5 (5a to 5d) cause toner to adhere onto the electrostatic latent images, to thereby visualize the electrostatic latent images as toner images. The cleaning devices 7 (7a to 7d) remove transfer residual toner remaining on the surfaces of the photosensitive drums 2 (2a to 2d) after the toner images are transferred.
In the color laser printer 1 including a sheet discharge unit 26 for discharging a sheet to the outside of the apparatus, the photosensitive drums 2 (2a to 2d), the charging devices 3 (3a to 3d), the developing devices 5 (5a to 5d), and the cleaning devices 7 (7a to 7d) are integrated into a cartridge unit. In the color laser printer 1, images of different colors (yellow, cyan, magenta, and black colors) are formed by an electrophotographic recording method. Transfer rollers 6 (6a, 6b, 6c, and 6d) are provided inside a transfer conveying belt 8 for conveying a sheet, and sandwich the transfer conveying belt 8 together with the photosensitive drums 2 (2a to 2d). The transfer conveying belt 8 circulates and moves as a sheet conveying unit so as to be held in contact with all the photosensitive drums 2 (2a to 2d) while being opposed thereto. The transfer conveying belt 8 is wound around four rollers, i.e., a drive roller 8a, driven rollers 8b and 8c, and a tension roller 8d. The drive roller 8a is rotationally driven by a drive motor (not shown), and thus rotated in an arrow E direction.
The transfer rollers 6 (6a to 6d) are connected to a transfer bias source (not shown), and a positive charge is applied from the transfer rollers 6 (6a to 6d) to the sheet through the transfer conveying belt 8. Through application of transfer bias, toner images of respective colors with negative polarity, which are formed on the photosensitive drums, are sequentially transferred onto the sheet which is held in contact with the photosensitive drums 2 (2a to 2d) while being held by the transfer conveying belt 8. Consequently, a multi-color image is formed.
The sheet feeding unit 23 includes a cassette feed device 23a for feeding, by a pickup roller (sheet feeding roller) 10, a sheet S contained in a sheet feeding cassette 9 provided in a lowermost portion of the apparatus. Further, the sheet feeding unit 23 includes a multi-feed device 23b for feeding, by a pickup roller 25, the sheet S contained (supported) in a multi-feed tray (not shown). The sheet feeding cassette 9 contains a plurality of sheets S, and is loaded into a bottom of the printer main body. The multi-feed tray is opened during use, and supports a plurality of sheets S.
At the time of image formation, for example, the sheets S contained in the sheet feeding cassette 9 are separated and fed one by one by the pickup roller 10. Then, the separated and fed sheet S is conveyed at predetermined timing to the transfer unit 24 by a registration roller pair 13 for correcting skew feed of the sheet S. When the sheets S are fed from the multi-feed tray (not shown), the sheets S contained in the multi-feed tray are separated and fed one by one by the pickup roller 25. Then, the separated and fed sheet S is conveyed to the transfer unit 24 by the registration roller pair 13.
The transfer unit 24 includes the transfer conveying belt 8 provided to be opposed to the photosensitive drums 2 (2a to 2d). The transfer conveying belt 8 electrostatically absorbs the sheet on its outer peripheral surface opposed to the photosensitive drums 2 (2a to 2d), and circulates and moves so as to bring the sheet into contact with the photosensitive drums 2 (2a to 2d). By being electrostatically absorbed by the circulating and moving transfer conveying belt 8, the sheet S is conveyed to a transfer position by the transfer conveying belt 8, and the toner images formed on the photosensitive drums 2 (2a to 2d) are transferred onto the sheet. As a unit for feeding/conveying the sheets S, provided are the sheet feeding cassette 9, the registration roller pair 13, the fixing device 14 for fixing the toner images, and a sheet discharge roller pair 15 for discharging the sheets S to a sheet discharge tray 16 serving as a sheet stacking unit.
In order to form the toner images on a second side of the sheet S, a duplex flapper 21 is operated so as to guide the sheet S to be switched back to a side of a refeeding roller pair 17. When the sheet S is not conveyed, the duplex flapper 21 waits at a position of guiding the sheet S to the side of the refeeding roller pair 17. When the sheet S is conveyed from the fixing device 14, the sheet S lifts up the duplex flapper 21, and is thus conveyed to a side of the sheet discharge roller pair 15. In order to form the toner images on the second side of the sheet S, each of duplex conveying roller pairs 17, 18, 19 and 20 conveys the sheet S to the image forming unit 22.
An operation of the color laser printer 1 at the time of image formation is described. First, when the laser beam irradiated from the scanner units 4 (4a to 4d) correspondingly to the image information is scanned onto the surfaces of the photosensitive drums 2 (2a to 2d) uniformly charged by the charging devices 3 (3a to 3d), latent images are formed on the surfaces of the photosensitive drums. The latent images are developed by the developing devices 5 (5a to 5d), and thus the toner images of four colors, i.e., yellow, magenta, cyan, and black colors, are formed on the surfaces of the photosensitive drums 2 (2a to 2d).
Concurrently with the toner image forming operation, the sheets S contained in the sheet feeding cassette 9 are picked up by the pickup roller 10. Further, the sheets S contained in the multi-feed tray (not shown) are separated and conveyed one by one by the pickup roller 25. Thereafter, the sheet S fed from the sheet feeding cassette 9 or the multi-feed tray is guided to the registration roller pair 13. Then, the sheet S is conveyed to the transfer unit 24 by the registration roller pair 13 in synchronization with the image forming operation. The sheet S conveyed to the transfer unit 24 is absorbed onto the transfer conveying belt 8, and is conveyed to transfer portions in which the photosensitive drums 2 (2a to 2d) and the transfer conveying belt 8 are held in press-contact with each other.
Due to actions of the transfer rollers 6 (6a to 6d) which are respectively arranged in the transfer portions and applied with a voltage with polarity opposite to the polarity of the toner, onto the sheet S conveyed to the transfer portions, the toner images of respective colors formed on the photosensitive drums 2 (2a to 2d) are sequentially superimposed and transferred onto the sheet. The sheet S on which the toner images of four colors are multiple-transferred is separated from the transfer conveying belt 8, and is conveyed to the fixing device 14.
After the fixing toner images are transferred, the sheet S conveyed to the fixing device 14 is pressurized and heated in the fixing device 14. That is, the fixing device 14 includes a fixing sleeve 14a serving as a heat source, and a pressure roller 14b for applying pressure to the sheet S while being held in press-contact with the fixing sleeve 14a. The sheet S passing the fixing device 14 is conveyed and applied with heat and pressure by the fixing device 14. As a result, a full-color print image is fixed as a permanent image on the sheet. After the full-color print image is fixed as the permanent image, the sheet S is discharged by the sheet discharge roller pair 15, and is stacked onto the sheet discharge tray 16 provided below the sheet discharge roller pair 15.
At the time of double-sided printing, the sheet S, which has the image fixed in the fixing device 14 on its first side, passes the duplex flapper 21, is conveyed to the sheet discharge roller pair 15 of the sheet discharge unit 26, and is conveyed to a side of the sheet discharge tray 16 by the sheet discharge roller pair 15. Then, the sheet discharge roller pair 15 is reversely operated in response to a signal output from a control unit (not shown) before a tail end of the sheet S passes through the sheet discharge roller pair 15. Therefore, while the tail end is in the lead, the sheet S passes the duplex flapper 21, and is conveyed by the duplex conveying roller pairs 17, 18, 19, and 20. The sheet S enters a conveyance path extending from a feed roller 11, and passes the registration roller pair 13 and the image forming unit of respective colors. As a result, image formation is performed on the second side of the sheet. Thereafter, the image is fixed on the second side of the sheet S in the fixing device 14, and is discharged by the sheet discharge roller pair 15 onto the sheet discharge tray 16. Thus, the double-sided printing is completed.
With reference to
The upper sheet discharge roller 31 is supported by the sheet discharge frame 30 through upper roller bearings 33. The lower sheet discharge roller 32 is supported by the sheet discharge frame 30 through lower roller bearings 34 so as to be movable in a direction (up-down direction in
In this embodiment, the pressure spring 35 serving as a compression spring urges the lower roller bearing 34 with a force of 200 gf (≈2 N) applied to each side thereof. As long as the lower sheet discharge roller 32 is urged to the upper sheet discharge roller 31 (or the upper sheet discharge roller 31 is urged to the lower sheet discharge roller 32), as a matter of course, a coil spring, a plate spring, or a spring other than those springs can be used as the pressure spring 35. In this embodiment, all of the roller bodies 31a and 32a overlap each other. However, the present invention is not limited thereto. At least part of the roller bodies 31a and 32a, for example, the roller bodies 31a and 32a close to the center in the axial direction at which roller position regulating members 37 and 38 are located, may overlap each other. In this case, the substantially same effect can be also obtained.
In this embodiment, the lower-roller-position regulating member 37 is provided as a movement regulating portion for regulating the lower sheet discharge roller 32 at a predetermined position when the sheet discharge roller pair 15 moves against the pressure springs 35 in a direction of increasing the center distance. The upper-roller-position regulating members 38 are provided between the roller bodies 31a of the upper sheet discharge roller serving as the first roller, and the lower-roller-position regulating member 37 is provided between the roller bodies 32a of the lower sheet discharge roller 32 serving as the second roller. That is, the upper-roller-position regulating members 38 are arranged between the roller bodies 31a at the center portion in a longitudinal direction of the upper sheet discharge roller shaft 31b of the upper sheet discharge roller 31 (see
In this embodiment, the lower-roller-position regulating member 37 is a movement regulating portion of the present invention for regulating movement of the lower sheet discharge roller 32 at a predetermined position when the lower sheet discharge roller 32 moves against an urging force of the pressure springs 35 to increase the center distance between the upper sheet discharge roller 31 and the lower sheet discharge roller 32. The upper sheet discharge roller 31 may move against the urging force of the pressure springs 35 to increase the center distance between the upper sheet discharge roller 31 and the lower sheet discharge roller 32, and a clearance c set to a side of the lower-roller-position regulating member 37 may be provided to a side of the upper-roller-position regulating members 38. In this case, the upper-roller-position regulating members 38 can be used as the movement regulating portion of the present invention. The upper sheet discharge roller 31 is rotationally driven by a drive source (not shown), and its rotation is transmitted to the lower sheet discharge roller 32 through a lower-sheet-discharge-roller drive gear group 36. At a position opposed to the center portion of the lower sheet discharge roller 32 in the sheet discharge frame 30, the lower-roller-position regulating member 37 is provided.
The lower-roller-position regulating member 37 includes a lower-roller-shaft abutting surface 37a (see
Near an upper center portion of the sheet discharge frame 30, the two upper-roller-position regulating members 38 are arranged so as to be located on both sides of the lower-roller-position regulating member 37 located below the upper-roller-position regulating members 38. In the two upper-roller-position regulating members 38, upper-roller-shaft abutting portions 38a described below are brought into contact with the upper sheet discharge roller shaft 31b in advance, and a clearance like the clearance c provided to the lower-roller-position regulating member 37 is not formed. In the upper-roller-position regulating members 38, a certain clearance is formed between each of upper-roller abutting surfaces 38b to be described below as abutting portions and the upper sheet discharge roller shaft 31b which is not subjected to deflection. The upper-roller-position regulating members 38 have a function (see
The upper-roller-position regulating members 38 have a function (see
With reference to
As illustrated in
The first gear portion 39a and the second gear portion 39b of the main drive gear 39 are identical in pitch diameter, module, and shifting amount. In order to correspond to phase shift of the main drive gear 39, the first gear portion 40a and the second gear portion 40b of the driven gear 40 are also relatively shifted in phase by a 0.5 tooth pitch. This configuration is made to be able to transmit the drive to the lower sheet discharge roller 32 while increasing meshing of gears and securing a retreating amount when the lower sheet discharge roller 32 moves so as to be separated from the upper sheet discharge roller 31.
With reference to
With reference to
As illustrated in
As illustrated in
In order to achieve reduction in size of the apparatus and reduction in cost, and to secure the overlapping amount d when the upper sheet discharge roller 31 and the lower sheet discharge roller 32 are viewed in the axial direction, a relatively thin shaft is used for each of the upper sheet discharge roller 31 and the lower sheet discharge roller 32. For example, in a case where a shaft with a diameter of 4 mm is adopted as each of the upper sheet discharge roller 31 and the lower sheet discharge roller 32, deflection is generated due to an external force, and roller positions are not stabilized. As a result, a stiffening force of the sheet discharge roller pair 15 with respect to the sheet may be reduced more than required. Thus, design needs to be made in consideration therewith.
As illustrated in
The clearance c between the lower-roller-shaft abutting surface 37a of the lower-roller-position regulating member 37 and the lower sheet discharge roller shaft 32b is set to be smaller than the overlapping amount d between the upper sheet discharge roller 31 and the lower sheet discharge roller 32. Thus, even when the lower sheet discharge roller shaft 32b abuts against the lower-roller-shaft abutting surface 37a, the overlapping amount d is secured, and hence it is possible to stiffen the sheet such as the thick sheet of paper with high stiffness to a certain degree. Further, it is possible to secure a conveying force with which the sheet discharge roller pair 15 conveys the sheet. Accordingly, the sheet can be stably discharged by the sheet discharge roller pair 15.
With reference to
As illustrated in
In this embodiment, the lower sheet discharge roller 32 is structured to be able to retreat away from the upper sheet discharge roller 31, and the lower sheet discharge roller 32 is urged toward the upper sheet discharge roller 31 with the pressure springs 35. The lower-roller-position regulating member 37 and the upper-roller-position regulating members 38 are arranged, and hence, when the sheet such as the thin sheet of paper with low stiffness is conveyed, it is possible to keep the center distance between the upper sheet discharge roller 31 and the lower sheet discharge roller 32 without being affected by the stiffness of the sheet. Thus, an amount by which the lower sheet discharge roller 32 retreats away from the upper sheet discharge roller 31 is reduced, and the sheet can be discharged after being subjected to proper stiffening.
When the sheet such as the thick sheet of paper with high stiffness or glossy paper is conveyed, the lower sheet discharge roller 32 moves (retreats) away from the upper sheet discharge roller 31 against the urging force of the pressure springs 35, and the overlapping amount d between the upper sheet discharge roller 31 and the lower sheet discharge roller 32 is reduced. In this case, the movement of the shaft of the lower sheet discharge roller 32 is regulated by the lower-roller-position regulating member 37 in the middle of movement, and hence the stiffening force with respect to the sheet is prevented from being reduced more than required. Thus, contact between the sheet such as the thick sheet of paper with high stiffness and the sheet discharge roller pair 15 is appropriately weakened, and the stiffening force imparted to the sheet by the sheet discharge roller pair 15 is decreased. Therefore, proper stiffening can be performed irrespective of kinds of sheets, and the sheets can be stably stacked onto the sheet discharge tray 16. Further, reduction in occurrence of roller marks can be achieved at low cost and with a simple configuration.
As a material of the roller bodies 31a of the upper sheet discharge roller 31 and a material of the roller bodies 32a of the lower sheet discharge roller 32, foamed silicone rubber is used, and hence substantial contact area is smaller in comparison with solid rubber such as ethylene-propylene-diene-methylene (EPDM) rubber which is conventionally used. Both end portions of each of the roller bodies 31a of the upper sheet discharge roller 31 and both end portions of each of the roller bodies 32a of the lower sheet discharge roller shaft 32b are provided with tapered surfaces, and hence the both end portions are not strongly held in contact with the sheet S. With the roller configuration as described above, it is possible to reduce occurrence of the roller marks. The lower-roller-position regulating member 37 provided for the lower sheet discharge roller 32 and the upper-roller-position regulating members 38 provided for the upper sheet discharge roller 31 function to suppress the deflection generated in the center distance direction of the sheet discharge roller pair 15 and generated upstream in the sheet conveying direction. Thus, even when a diameter of the shaft is reduced, it is possible to regulate a change in roller position caused by the deflection, and to suppress the reduction of the stiffening force with respect to the sheet.
In this embodiment, the pressure springs 35 are provided to the side of the lower sheet discharge roller 32, and the lower sheet discharge roller 32 is movable in the direction perpendicular to the sheet conveying direction. However, it is also possible that the pressure springs 35 are provided to the side of the upper sheet discharge roller 31 and the upper sheet discharge roller 31 is movable in the direction perpendicular to the sheet conveying direction. In this case, the clearance c formed in the lower-roller-position regulating member 37 illustrated in
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. 2009-111008, filed Apr. 30, 2009, which is hereby incorporated by reference herein its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-111008 | Apr 2009 | JP | national |
