Color image forming apparatus

There is described a color image forming apparatus, which makes it possible to prevent the intermediate transfer belt from slacking. The apparatus includes a plurality of image bearing members; an intermediate transfer belt onto which a full color toner image is formed; a driving roller to drive the intermediate transfer belt; a transfer unit to transfer the full color toner image onto a sheet; a cleaning unit to remove residual toner remaining on the intermediate transfer belt; a first tension applying unit, located upstream from the transfer unit in a circulating direction of the intermediate transfer belt, to apply a first tension onto the intermediate transfer belt; and a second tension applying unit, located downstream from the cleaning unit in the circulating direction of the intermediate transfer belt, to apply a second tension onto the intermediate transfer belt.

This application is based on Japanese Patent Application NO. 2006-254088 filed on Sep. 20, 2006 with Japan Patent Office, the entire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a color image forming apparatus provided with a plurality of image bearing members and an intermediate transfer belt.

In recent years, image forming apparatuses employing an electro-photographic method, such as a copier, a printer, a facsimile, etc., have been widely introduced in various office environments. Specifically, a lot of color image forming apparatuses capable of outputting a color image onto a sheet have been widely introduced in the market.

Various kinds of configurations have been employed in such the color image forming apparatuses as mentioned in the above. Recently, among other things, a tandem-type color image forming apparatus, in which unicolor toner images are respectively formed on a plurality of photoreceptor drums (image bearing members) and superimposed with each other on the intermediate transfer belt so as to form a color image on it, has been emerged in the market. According to the tandem-type color image forming apparatus, it is possible to achieve both the high image quality and the high productivity in the production of color image products.

In this connection, the intermediate transfer belt, employed in the tandem-type color image forming apparatus, is threaded on a plurality of supporting rollers so as to make it circulatable along the photoreceptor drums. However, since various constituents, such as the plurality of photoreceptor drums, a transfer roller, etc., contact the intermediate transfer belt, sometimes, a part of the intermediate transfer belt slacks under influence of the load incurred onto the intermediate transfer belt during its circulating action. Once the intermediate transfer belt slacks, fluctuations of the circulating velocity of the intermediate transfer belt would occur and result in a transferring deficiency, etc. This would cause a problem of the reproduced image quality.

Conventionally, to cope with the abovementioned problem, various techniques for preventing the intermediate transfer belt from slacking have been proposed so far.

For instance, Patent Document 1 (Tokkai 2001-318507, Japanese Non-Examined Patent Publication) sets forth a technology, which makes it possible to install the tension rollers for giving a tension to the intermediate transfer belt into the inside space surrounded by the inner circumferential surface of the intermediate transfer belt. According to the above technology, it becomes possible not only to alleviate the slack to be generated on the intermediate transfer belt, but also to conduct the image forming operation well.

However, considerations in the technology set forth in Patent Document 1 are not sufficient as a countermeasure for preventing the intermediate transfer belt from slacking, with respect to a color image forming apparatus in which a driving roller for circulating the intermediate transfer belt, a transfer roller for transferring a color toner image formed on the intermediate transfer belt onto a sheet, etc. are separately driven by the separate motors. Referring to FIG.5throughFIG. 7, the abovementioned point will be detailed in the following.

FIG. 5shows a brief configuration of a conventional color image forming apparatus.

A conventional color image forming apparatus1000is so constituted that unicolor toner images are respectively formed on a plurality of photoreceptor drums1100,1101,1102,1103, and successively, superimposed with each other onto an intermediate transfer belt1200so as to form a full color toner image on it, and then, the full color toner image residing on the intermediate transfer belt1200is transferred onto a sheet by a transfer roller1400.

A driving roller1300drives the intermediate transfer belt1200to circulate it in a direction indicated by an arrow X shown inFIG. 5while supporting the intermediate transfer belt1200. The transfer roller1400serves as a roller for transferring the full color toner image formed on the intermediate transfer belt1200onto the sheet. A cleaning brush1500rotates in a predetermined direction, so as to remove residual toner remaining on the intermediate transfer belt1200.

The driving roller1300is rotated by a driving force transmitted from a driving motor1301, the transfer roller1400is rotated by a driving force transmitted from a driving motor1401and the cleaning brush1500is rotated by a driving force transmitted from a driving motor1501. In other words, the driving roller1300, the transfer roller1400and the cleaning brush1500are driven by the separate motors, respectively.

FIG. 6shows an enlarged view of a configuration in the periphery of the transfer roller1400.

As mentioned in the above, the driving roller1300and the transfer roller1400are rotated by the driving forces transmitted from the separate driving motors1301,1401, respectively. Further, the driving roller1300rotates in a direction indicated by an arrow X1shown inFIG. 6, while the transfer roller1400rotates in a direction indicated by an arrow X2shown inFIG. 6. During the time when the transfer roller1400contacts the intermediate transfer belt1200, since the intermediate transfer belt1200is pulled in its circulating direction by the driving force generated by the transfer roller1400, no slack is generated on the intermediate transfer belt1200. However, the friction force generated between the transfer roller1400and the intermediate transfer belt1200is liable to fluctuate under the influences of the image forming conditions, such as a density of an image to be transferred, environmental conditions, presence or absence of a sheet at a transfer nip portion, etc. Sometimes, this would cause the slack on a portion α of the intermediate transfer belt1200shown inFIG. 6.

FIG. 7shows an enlarged view of a configuration in the periphery of the cleaning brush1500.

The cleaning brush1500rotates in a direction indicated by an arrow X3shown inFIG. 7. Although the cleaning brush1500removes the residual toner remaining on the intermediate transfer belt1200, the friction force generated between the cleaning brush1500and the intermediate transfer belt1200is liable to fluctuate depending on an amount of residual toner remaining on the intermediate transfer belt1200. Accordingly, sometimes, this would cause the slacks on portions β1, β2of the intermediate transfer belt1200shown inFIG. 7, under the influence of the driving force for the cleaning brush1500.

SUMMARY OF THE INVENTION

Accordingly, to overcome the abovementioned drawbacks in conventional color image forming apparatus, at least one of objects of the present invention can be attained by the color image forming apparatuses described as follows.(1) According to a color image forming apparatus reflecting an aspect of the present invention, the color image forming apparatus comprises: a plurality of image bearing members; an intermediate transfer belt onto which unicolor toner images, respectively formed on the plurality of image bearing members, are sequentially transferred so as to form a full color toner image on the intermediate transfer belt; a driving roller to drive the intermediate transfer belt; a driving motor to generate a first driving force to be applied to the driving roller; a transfer unit to transfer the full color toner image formed on the intermediate transfer belt onto a sheet; a transfer motor, disposed separately from the driving motor, to generate a second driving force to be applied to the transfer unit; a cleaning unit to remove residual toner remaining on the intermediate transfer belt; a cleaning motor, disposed separately from the driving motor, to generate a third driving force to be applied to the cleaning unit; a first tension applying unit, located upstream from the transfer unit in a circulating direction of the intermediate transfer belt, to apply a first tension onto the intermediate transfer belt; and a second tension applying unit, located downstream from the cleaning unit in the circulating direction of the intermediate transfer belt, to apply a second tension onto the intermediate transfer belt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1shows a brief configuration of a color image forming apparatus1embodied in the present invention.

The color image forming apparatus1is a tandem-type color image forming apparatus provided with an intermediate transfer belt20.

Four sets of image forming sections10Y,10M,10C,10K for forming unicolor toner images of colors Y (Yellow), M (Magenta), C (Cyan) and K (Black) are disposed on an upper side of the intermediate transfer belt20. The image forming sections10Y,10M,10C,10K are aligned in a line in this sequence from left to right and have the same configuration. Referring to the image forming section10Y as an example, its configuration will be detailed in the following. The image forming section10Y is constituted by a photoreceptor drum11, serving as a image bearing member, a scorotron charger12, an exposing unit13, a developing unit14and a cleaning unit15.

The intermediate transfer belt20is shaped in an endless belt and has a predetermined volume resistivity. A primary transfer electrode16is disposed at a position opposing to the photoreceptor drum11while putting the intermediate transfer belt20between them.

Next, a color image forming method for forming a color image will be detailed in the following.

The photoreceptor drum11is driven to rotate by a main motor (not shown in the drawings) and is charged at a negative voltage (for instance, −800 volts) by the discharging action of the scorotron charger12. Successively, the exposing unit13performs the optical image writing action corresponding to the image data, so as to form an electrostatic latent image on the circumferential surface of the photoreceptor drum11. When the electrostatic latent image formed by the exposing unit13passes through the developing unit14, toner charged at a negative polarity in the developing unit14are attracted and adhered to portions of the electrostatic latent image so as to form a toner image on the circumferential surface of the photoreceptor drum11. Successively, the formed toner image is transferred onto the intermediate transfer belt20, which press-contacts the photoreceptor drum11. Then, the residual toner remaining on the photoreceptor drum11is cleaned by the cleaning unit15. Since the unicolor toner images respectively formed on the image forming sections10Y,10M,10C,10K are sequentially transferred onto the intermediate transfer belt20in such a manner that the unicolor toner images are superimposed with each other, a full color toner image is formed on the intermediate transfer belt20. On the other hand, a sheet, onto which the full color toner image is to be formed, is conveyed into a transfer nip portion by the intermediate transfer belt20and a transfer roller40serving as a transferring section, while the transfer roller40press-pushes the sheet onto the intermediate transfer belt20so as to tightly clip it between them. By applying a bias voltage having a polarity opposite to that of the toner (for instance, +500 volts) onto the transfer roller40, the full color toner image residing on the intermediate transfer belt20is transferred onto the sheet. The sheet having the transferred toner image is further conveyed into a fixing unit (not shown in the drawings). Successively, the residual toner remaining on the intermediate transfer belt20are removed by a belt cleaning unit50serving as a cleaning section.

A driving roller30drives the intermediate transfer belt20to rotate in a direction indicated by an arrow A shown inFIG. 1, while supporting the intermediate transfer belt20. The driving roller30is rotated by a driving force generated by a driving motor31.

The transfer roller40is rotated by a driving force generated by a transfer motor41disposed separately from the driving motor31, while a cleaning brush51included in the belt cleaning unit50is rotated by a driving force generated by a cleaning motor52disposed separately from the driving motor31. In this connection, it is also applicable that, considering the installation space and cost, the same motor is employed as both the transfer motor41and the cleaning motor52.

A tension roller60and a tension roller70, both detailed later, apply tensions to the intermediate transfer belt20.

FIG. 2shows a block diagram of a control system of the color image forming apparatus1, indicating a typical configuration of such the control system.

A CPU (Central Processing Unit)101is coupled to a ROM (Read Only Memory)102, a RAM (Random Access Memory)103, etc., through a system bus112, so as to control overall operations of the color image forming apparatus1. The CPU101reads out various kinds of control programs stored in the ROM102and develops the control programs into the RAM103to control operations of each of the sections. Further, the CPU101executes the control programs developed into the RAM103in order to implements various kinds of processing, and stores the processing results into the RAM103, while displaying them on an operation display section105. Further, the CPU101stores the processing results stored into the RAM103into a predetermined storage.

The ROM102includes a magnetic storage medium, an optical storage medium and/or a semiconductor memory so as to store various kinds of program, data, etc. in advance.

A working area, in which various kinds of data, etc. generated or processed by the various kinds of programs executed by the CPU101are temporarily stored, is created in the RAM103.

An HDD (Hard Disc Drive)104has a function for storing image data acquired by reading an original document image through an image reading section106, outputted image data, etc. The HDD104is constituted by a plurality of metallic discs on each of which a magnetic material is coated or vapor-deposited and which overlap with each other at constant intervals. In the HDD104, read/write operations are achieved by making magnetic heads approach the surfaces of the metallic discs, being rotated at high velocity by a disc driving motor.

The operation display section105makes it possible to input various kinds of settings. For instance, a touch panel is employed in the operation display section105, so that the user can establish conditions for the color printing or the monochromatic printing by inputting instructions through the touch panel of the operation display section105. Further, at the same time, various kinds of information, such as setting information of the network, etc., are displayed on the operation display section105.

The image reading section106optically reads the original document image so as to convert it to electric signals. When reading a color document image, the image reading section106generates image data having RGB 10-bit luminance information for every pixel.

An image processing section107applies image processing to the image data generated by the image reading section106and/or the image data transmitted from a personal computer coupled to the color image forming apparatus1. When the color printing operation is implemented in the color image forming apparatus1, R (Red), G (Green) and B (Blue) image data, generated by the image reading section106, etc., are inputted into a color conversion LUT (Look Up Table), so as to convert the R, G and B image data to Y (Yellow), M (Magenta), C (Cyan) and Bk (Black) image data. Successively, various kinds of image processing, such as a correction of gradation reproducing characteristic, a screen processing of halftone dots referring to a gradation correcting LUT, an edge processing for enhancing narrow lines, etc., are applied to the Y (Yellow), M (Magenta), C (Cyan) and Bk (Black) image data converted in the above.

An image forming section108receives the Y (Yellow), M (Magenta), C (Cyan) and Bk (Black) image data processed by the image processing section107, so as to form a reproduced image on the sheet.

The CPU101controls operations of the driving motor31for driving the driving roller30, the transfer motor41for driving the transfer roller40and the cleaning motor52for driving the cleaning brush51.

Incidentally, when a slack is generated on the intermediate transfer belt20, there would occur a problem of the image quality, such as the transferring deficiency caused by the variation of the rotating velocity of the intermediate transfer belt20. Therefore, it is necessary to prevent the intermediate transfer belt20from slacking.

Initially, the method for preventing the intermediate transfer belt20from generating the slack due to the transfer roller40will be detailed in the following.

FIG. 3shows an enlarged view of the configuration in the periphery of the transfer roller40.

Compared to the transferring operation employing the corona discharging action, the transferring operation conducted by the transfer roller40has such advantageous features that a little amount of ozone is generated and no transfer unevenness, caused by the contamination of the discharging wire, etc., emerges on the reproduced image. The transfer roller40is constituted by an axial body (core metal) made of, for instance, a stainless steel bar, and a roller section, which is made of a resin material, such as a foamed silicon resin, an expanded polyurethane resin or an EMPD resin, and formed on a circumferential surface of the axial body.

The driving roller30for driving the intermediate transfer belt20is rotated in a direction indicated by an arrow A1shown inFIG. 3by the driving force transmitted from the driving motor31. On the other hand, the transfer roller40is rotated in a direction indicated by an arrow A2shown inFIG. 3by the driving force transmitted from the transfer motor41disposed separately from the driving motor31to improve the transferability of the transfer roller40.

During the time when the transfer roller40contacts the intermediate transfer belt20, since the intermediate transfer belt20is pulled in its circulating direction by the driving force generated by the transfer roller40, no slack is generated on the intermediate transfer belt20. However, the friction force generated between the transfer roller40and the intermediate transfer belt20is liable to fluctuate under the influences of the image forming conditions, such as density of an image to be transferred, environmental conditions, presence or absence of the sheet at the transfer nip portion, etc. Sometimes, this would cause the slack on a portion γ1of the intermediate transfer belt20shown inFIG. 3.

Accordingly, to alleviate the abovementioned slack, the tension roller60, serving as an upstream tension adding member, is disposed at a position upstream from the transfer roller40in respect to the circulating direction of the intermediate transfer belt20. The tension roller60applies a tension in a direction indicated by an arrow B1shown inFIG. 3to the intermediate transfer belt20. For instance, a mechanical structure for pushing both ends of an axis of the tension roller60in the direction indicated by the arrow B1by urging springs could be applicable for this purpose. Further, another mechanical structure for urging the both ends of the axis of the tension roller60by a cam would be also applicable for this purpose.

As mentioned in the above, by disposing the tension roller60at the position upstream from the transfer roller40in respect to the circulating direction of the intermediate transfer belt20so as to apply the tension onto the intermediate transfer belt20, the tension roller60can absorb the slack of the intermediate transfer belt20, even if the friction force between the transfer roller40and the intermediate transfer belt20fluctuates. As a result, no slack is generated on the intermediate transfer belt20and it becomes possible to stabilize the image quality of the reproduced image formed on the sheet.

Next, the method for preventing the intermediate transfer belt20from generating the slack due to the cleaning brush51will be detailed in the following.

FIG. 4shows an enlarged view of the configuration in the periphery of the cleaning brush51.

The cleaning brush51is disposed in the belt cleaning unit50so that the residual toner attached onto the intermediate transfer belt20are removed by rotating the cleaning brush51. For instance, the cleaning brush51is formed by filling brush fibers onto a circumferential surface of a rolled core material.

The cleaning brush51is rotated in a direction indicated by an arrow A3shown inFIG. 4by the driving force transmitted from the cleaning motor52disposed separately from the driving motor31to improve the cleanability of the cleaning brush51. Although the cleaning brush51removes the residual toner remaining on the intermediate transfer belt20, the friction force generated between the cleaning brush51and the intermediate transfer belt20is liable to fluctuate depending on an amount of the residual toner remaining on the intermediate transfer belt20. This would cause the slacks on portions γ2and γ3of the intermediate transfer belt20shown inFIG. 4, under the influence of the driving force of the cleaning brush51.

Accordingly, to alleviate the abovementioned slacks, the tension roller70, serving as a downstream tension adding member, is disposed at a position downstream from the cleaning brush51in respect to the circulating direction of the intermediate transfer belt20. The tension roller70applies a tension in a direction indicated by an arrow B2shown inFIG. 4to the intermediate transfer belt20. A mechanical structure being same as that of the tension roller60could be considered. For instance, a mechanical structure for pushing both ends of an axis of the tension roller70in the direction indicated by the arrow B2by urging springs could be applicable for this purpose. Further, another mechanical structure for urging the both ends of the axis of the tension roller70by a cam would be also applicable for this purpose.

As mentioned in the above, by disposing the tension roller70at the position downstream from the cleaning brush51in respect to the circulating direction of the intermediate transfer belt20so as to apply the tension onto the intermediate transfer belt20, the tension roller60can absorb the slacks of the intermediate transfer belt20, even if the friction force between the cleaning brush51and the intermediate transfer belt20fluctuates. As a result, none of slacks are generated on the intermediate transfer belt20and it becomes possible to stabilize the image quality of the reproduced image formed on the sheet.

As detailed in the foregoing by referring toFIG. 3andFIG. 4, by disposing the tension roller60at the position upstream from the transfer roller40and the tension roller70at the position downstream from the cleaning brush51, it becomes possible even for the color image forming apparatus, in which the driving roller30, the transfer roller40, etc. are respectively driven by separate motors, to alleviate the slacks to be generated on the intermediate transfer belt20, resulting in a stability of the reproduced image quality.

Referring to the drawings, the embodiment of the present invention has been detailed in the foregoing. However, the scope of the present invention is not limited to the embodiment aforementioned. Modifications and additions applied to the exemplified embodiment by a skilled person without departing from the spirit and scope of the invention shall be included in the scope of the present invention.

While the preferred embodiments of the present invention have been described using specific term, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit and scope of the appended claims.