JP2011033740A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus obtaining high quality images by suppressing separation from an image carrier of a primary transfer roller due to tension of an intermediate transfer belt and preventing occurrence of primary transfer inferiority. <P>SOLUTION: The color image forming apparatus includes: the endless intermediate transfer belt 8; a plurality of photoreceptor drums 2a-2d arranged along a running direction of the intermediate transfer belt 8; and a plurality of the primary transfer rollers 6a-6d contacted on a downstream side for the shaft centers in the photoreceptor drums 2a-2d through the intermediate transfer belt 8. The color image forming apparatus arranges a pressing member 37 for pressing the downstream side part of each photoreceptor drum 2a-2d of the intermediate transfer belt 8 on the side of each photoreceptor drum 2a-2d in the neighborhood of the downstream side of each photoreceptor drum 2a-2d respectively, and deforms the downstream side part of each photoreceptor drum 2a-2d of the intermediate transfer belt 8 to be nearer to each photoreceptor drum 2a-2d rather than a tangent drawn on contact points to the intermediate transfer belt 8 of each photoreceptor drum 2a-2d. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a color image forming apparatus that forms a color image using an intermediate transfer system.

  In recent years, there has been a strong demand for colorization of image forming apparatuses such as copiers and printers that form images on paper by electrophotography, and color image forming apparatuses have been developed in response to this demand and are already in practical use. Since this color image forming apparatus is required to have a monochrome output speed, a tandem type color image forming apparatus has attracted attention.

  The tandem type color image forming apparatus includes an endless intermediate transfer belt stretched between a plurality of rollers, a plurality of image carriers disposed in tandem along the traveling direction of the intermediate transfer belt, A plurality of primary transfer rollers that are in contact with the image carrier through intermediate transfer belts are provided, and the toner images of the respective colors formed on the respective image carriers are sequentially transferred onto the intermediate transfer belt by the action of the primary transfer roller. Primary transcription. Then, the toner image that is primarily transferred and superimposed on the intermediate transfer belt is secondarily transferred onto the sheet at the secondary transfer portion, and the sheet on which the toner image is transferred is fixed by the fixing device. Later it is discharged out of the device.

  In such a tandem type color image forming apparatus, when the primary transfer of the toner image is performed at the transfer nip portion between the intermediate transfer belt and the image carrier, the intermediate transfer belt and the image carrier immediately before the transfer nip portion It is known that toner image degradation called “transfer scattering” occurs when primary transfer by discharge occurs in the gap. As a means for preventing this, the primary transfer roller is arranged on the downstream side (downstream in the running direction of the intermediate transfer belt) directly below the image carrier, and the intermediate transfer belt and the image carrier are upstream of the region where the electric field is applied. A configuration in which the contact is made is known.

  Incidentally, in Patent Document 1, in order to obtain a good transfer image without a transfer defect by sufficiently securing a transfer nip width that is a contact portion between an image carrier of an tandem type color image forming apparatus and an intermediate transfer belt, There has been proposed a configuration in which a plurality of pressing rollers that press the intermediate transfer belt on the side of a plurality of image carriers are arranged so that a tension portion of the intermediate transfer belt that stretches between the pressing rollers is wound around each image carrier. Yes.

JP 2001-318507 A

  However, when a configuration in which the primary transfer roller is arranged downstream of the image carrier is employed, the force that separates the primary transfer roller from the image carrier due to the tension of the intermediate transfer belt is increased, so that the transfer nip is not effective. The toner image becomes stable, and the possibility of occurrence of transfer failure of the toner image increases. In addition, when the primary transfer roller is pressed to the image carrier side, a configuration is adopted in which both ends of the primary transfer roller are pressed by a spring or the like so that the image area is not pressed to deteriorate the primary transfer roller. When both ends of the transfer roller are pressed by a spring or the like, the primary transfer roller is bent, and the transfer nip at the central portion in the axial direction of the primary transfer roller tends to be particularly unstable.

  In the configuration proposed in Patent Document 1, the separation of the primary transfer roller from the image carrier due to the tension of the intermediate transfer belt can be suppressed. Since there is a difference in state, in the configuration in which the intermediate transfer belt is simply pressed by the pressing roller, there is a possibility that unevenness in image density may occur between the central portion and the end portion in the width direction of the paper.

  The present invention has been made in view of the above problems, and the object of the present invention is to prevent the primary transfer failure from occurring by suppressing the separation of the primary transfer roller from the image carrier due to the tension of the intermediate transfer belt. An object of the present invention is to provide a color image forming apparatus capable of obtaining a high quality image by realizing uniform and good transfer in the axial direction of a transfer roller.

  In order to achieve the above object, an invention according to claim 1 is directed to an endless intermediate transfer belt stretched between a plurality of rollers, and a plurality of image carriers disposed along the traveling direction of the intermediate transfer belt. A color image forming apparatus comprising a plurality of primary transfer rollers in contact with a downstream side with respect to a contact with the intermediate transfer belt on the image carrier via the intermediate transfer belt. A pressing member that presses the downstream portion of each image carrier to each image carrier side is disposed in the vicinity of the downstream side of each image carrier, and the downstream portion of each image carrier of the intermediate transfer belt is set to each image carrier. Is held on a tangential line drawn at the contact point of the intermediate transfer belt or closer to the image carrier side than the tangent line.

  The invention according to claim 2 is characterized in that, in the invention according to claim 1, the pressing member is constituted by a crown-shaped roller having a diameter in the central portion in the axial direction larger than a diameter in the axial end portion.

  According to the first aspect of the present invention, in order to prevent the deterioration of the toner image called “transfer scattering”, the primary transfer roller is placed on the image carrier via the intermediate transfer belt and is arranged downstream of the contact point with the intermediate transfer belt. Even if the structure of contact is adopted, the downstream side portion of the image carrier of the intermediate transfer belt is pressed toward the image carrier by the pressing member disposed in the vicinity of the downstream side of the image carrier, and the image carrier of the intermediate transfer belt In order to hold the downstream portion of the image carrier on the tangent drawn at the contact point of the image carrier to the intermediate transfer belt or closer to the image carrier side than the tangent, Therefore, it is possible to prevent the occurrence of primary transfer failure and to improve the image quality.

  According to the second aspect of the present invention, the pressing member is constituted by a crown-shaped roller having a diameter in the central portion in the axial direction larger than the diameter in the axial end portion. Even when pressed against the body, a uniform transfer nip can be formed in the axial direction of the primary transfer roller to achieve a good primary transfer of the toner image, preventing uneven density and stable high-quality color images Can be obtained.

1 is a side sectional view of a color image forming apparatus according to the present invention. FIG. 3 is a schematic diagram illustrating a configuration of a transfer belt portion of a color image forming apparatus according to the present invention. It is the A section enlarged view of FIG. FIG. 4 is a sectional view taken along line BB in FIG. 3.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a cross-sectional view of a color image forming apparatus according to the present invention. The illustrated color image forming apparatus is a multifunction machine having both a copying function, a printer function, and a facsimile function, and forms color images on both sides of a sheet. It is a device that can do. The color image forming apparatus includes an apparatus main body 100 and an automatic document feeder 50 and an original reading apparatus 60 disposed on the upper part of the apparatus main body 100, and the image forming unit 1 is disposed above the apparatus main body 100. A sheet storage unit 70 is disposed in the lower half of the apparatus main body 100, and a transfer conveyance unit 80 and a duplex conveyance unit 90 are disposed between the image forming unit 1 and the sheet storage unit 70 in the apparatus main body 100. ing.

  The document reading device 60 includes an optical frame 61, and a contact glass (not shown) is provided on the upper surface of the optical frame 61. Further, inside the optical frame 61, a light source 62 that irradiates light to a document (not shown) placed on the contact glass to illuminate the document, and irradiation light reflected by the document in a 90 ° direction. A first optical system unit 64 composed of a first reflection mirror 63 that reflects light, a second reflection mirror 65 that reflects reflected light reflected by the first reflection mirror 63 in a 90 ° direction, and the second reflection A second optical system unit 67 composed of a third reflecting mirror 66 that further reflects the reflected light reflected by the mirror 65 in the 90 ° direction is moved along a guide rail (not shown) in the sub-scanning direction (the horizontal direction in FIG. 1). ) Is movably accommodated.

  Further, an imaging lens 68 and a CCD 69 as an image sensor are accommodated in the optical frame 60A, and the reflected light from the original reflected by the third reflecting mirror 66 is converged by the imaging lens 68. The light enters the CCD 69 and forms an image on the CCD 69.

  The image forming unit 1 forms an image by electrophotography, and includes a magenta image forming unit 1M, a cyan image forming unit 1C, a yellow image forming unit 1Y, and a black image forming unit 1K arranged in tandem. ing.

  Each of the image forming units 1M, 1C, 1Y, and 1K is provided with photosensitive drums 2a, 2b, 2c, and 2d, which are image carriers, and an exposure device 3a is provided around each of the photosensitive drums 2a to 2d. , 3b, 3c, 3d, chargers 4a, 4b, 4c, 4d, developing devices 5a, 5b, 5c, 5d, primary transfer rollers 6a, 6b, 6c, 5d and cleaning devices 7a, 7b, 7c, 7d, respectively. Has been.

  Here, the photosensitive drums 2a to 2d are drum-shaped photosensitive members, and are rotationally driven at a predetermined process speed in a direction indicated by an arrow (counterclockwise) by a drive motor (not shown). The exposure devices 3a to 3d form electrostatic latent images on the photosensitive drums 2a to 2d by irradiating the photosensitive drums 2a to 2d with laser beams corresponding to the image signals of the respective colors. The chargers 4a to 4d uniformly charge the surfaces of the photosensitive drums 2a to 2d to a predetermined potential by a charging bias applied from a charging bias power source (not shown).

  Further, the developing devices 5a to 5d contain magenta (M) toner, cyan (C) toner, yellow (Y) toner, and black (K) toner, respectively, and are formed on the photosensitive drums 2a to 2d. In addition, the toner of each color is attached to each electrostatic latent image to visualize each electrostatic latent image as a toner image of each color.

  The primary transfer rollers 6a to 6d are arranged so as to be in contact with the photosensitive drums 2a to 2d via the endless intermediate transfer belt 8 at each primary transfer portion. Here, the intermediate transfer belt 8 is stretched between the secondary transfer counter roller 9, the driving roller 10, and the plurality of driven rollers 11, 12, 13, and in the direction indicated by the arrow on the lower surface side of each photosensitive drum 2 a to 2 d. The secondary transfer counter roller 9 is disposed so as to be able to run in the (clockwise direction), and can be brought into contact with the secondary transfer roller 14 via the intermediate transfer belt 8 in the secondary transfer portion.

  The paper storage unit 70 includes three detachable paper feed cassettes 71, 72, and 73 in which a plurality of paper sheets are stacked and accommodated, and each paper feed cassette 71 to 73 is in the vicinity of each paper feed cassette. A pick roller 15 for sequentially taking out the sheets in 71 to 73 from the upper one, a feed roller 16 for separating and feeding out the taken sheets one by one, and a retard roller 17 are provided. In the vicinity of the bypass tray 74, a pick roller 15, a feed roller 16, a retard roller 17, and a transport roller 18 are provided as bypass transport means. In FIG. 1, reference numeral 19 denotes a conveyance roller.

  The transfer conveyance unit 80 conveys the sheet fed from the sheet storage unit 70 to the image forming unit 1 at an appropriate timing, and the sheet on which the image is formed in the image forming unit 1 to a paper discharge tray (not shown). A registration roller 20 disposed in a first conveyance path L1 from the sheet storage unit 70 to the secondary transfer unit (the contact portion between the secondary transfer counter roller 9 and the secondary transfer roller 14), A conveyance belt 21, a fixing device 22, a conveyance roller 23, and a sheet discharge roller 24 provided in a second conveyance path L2 from the secondary transfer roller 14 and the secondary transfer unit to a sheet discharge roller 24 described later as a sheet discharge unit. It has. The fixing device 22 includes a fixing roller 22a and a pressure roller 22b that rotate in a pressure contact state.

  The transfer conveyance unit 80 can be attached to and detached from the image forming apparatus main body 100. The transfer conveyance unit 80 branches from the second conveyance path L2, and the reverse roller 25 and the reverse roller 26 of the double-side conveyance unit 90 are branched. A third U-shaped third conveying path L3 is provided, and a conveying roller 27 is provided in the middle. In addition to the third transport path L3, a reverse transport path L3 ′ branches from the second transport path L2 and extends downward in the transfer transport unit 80, and a transport roller 28 is provided in the middle thereof. . A flapper 29 for switching the paper transport direction is rotatably provided at a branch portion between the second transport path L2 and the third transport path L3.

  The double-sided conveyance unit 90 reverses the front and back of the sheet on which the image is formed in the image forming unit 1, and is arranged below the intermediate tray 30 that is continuous with the third conveyance path L3 and the reverse conveyance path L3. In addition, the intermediate tray 30 is provided with the reversing roller 25 capable of forward / reverse rotation and the reversing roller 26 pressed against / separated from the reversing roller 25.

  The double-sided conveyance unit 90 is rotatably provided with a large-diameter refeed roller 32, and small-diameter rollers 33 and 34 are rotatably in contact with the outer periphery of the refeed roller 32. The refeed roller 32 is provided with a one-way clutch (not shown). A first transport path L4 extends from the reversing roller 25 and the reversing roller 26 constituting the reversing means, and the fourth transport path L4 joins the first transport path L1.

  A plurality of transport rollers 35 are provided in the fourth transport path L4 along the paper transport direction. Further, in the vicinity of the refeed roller 32, a flapper 36 for switching the transport direction of the paper once drawn into the intermediate tray 30 is rotatably provided. In the present embodiment, the fourth transport path L4 of the duplex transport unit 90 is provided with two or more paper storage positions where two or more sheets can be stored.

  Next, an image forming operation of the color image forming apparatus having the above configuration will be described.

  For example, a document set on the document tray 51 of the automatic document feeder 50 is automatically conveyed onto a contact glass serving as a document reading unit, and an image of the document is optically read by the document reading device 60. That is, the original document (not shown) placed on the contact glass while the first optical system unit 64 and the second optical system unit 67 move along the guide rail (not shown) in the sub-scanning direction (left-right direction in FIG. 1). , The light emitted from the light source 62 of the first optical system unit 64 toward the document is reflected by the document, and the reflected light is reflected by the first reflection mirror 63 of the first optical system unit 64 in the direction of 90 °. Reflect on. Then, this light is reflected in the 90 ° direction by the second reflecting mirror 65 and the third reflecting mirror 66 of the second optical system unit 67, converged by the imaging lens 68, and then incident on the CCD 69. Read.

  When an image of a document is optically read by the document reading device 60 as described above, in the image forming unit 1, each of the photosensitive drums 2a to 2d in each of the image forming units 1M, 1C, 1Y, and 1K is illustrated in the direction of the arrow. The photosensitive drums 2a to 2d are uniformly driven by the chargers 4a to 4d. The photosensitive drums 2a to 2d are rotated at a predetermined process speed (counterclockwise). The exposure devices 3a to 3d emit laser beams modulated by signals for the respective colors of the image read by the document reading device 60, and irradiate the surfaces of the photosensitive drums 2a to 2d with the laser beams. An electrostatic latent image corresponding to each color image signal is formed on each of the photosensitive drums 2a to 2d.

  First, magenta toner is attached to the electrostatic latent image formed on the photosensitive drum 2a of the magenta image forming unit 1M by the developing device 5a to which a developing bias having the same polarity as the charged polarity of the photosensitive drum 2a is applied. The electrostatic latent image is visualized as a magenta toner image. This magenta toner image is shown in the figure by the action of the primary transfer roller 6a to which a primary transfer bias having a polarity opposite to that of the toner is applied in a primary transfer portion (transfer nip portion) between the photosensitive drum 2a and the primary transfer roller 6a. Primary transfer is performed on the intermediate transfer belt 8 that is rotationally driven in the direction.

  The intermediate transfer belt 8 on which the magenta toner image has been primarily transferred as described above moves to the next cyan image forming unit 1C. Also in the cyan image forming unit 1C, similarly to the above, the cyan toner image formed on the photosensitive drum 2b is transferred to the magenta toner image on the intermediate transfer belt 8 at the primary transfer portion.

  Similarly, yellow and black toners respectively formed on the photosensitive drums 2c and 2d of the yellow and black image forming units 1Y and 1K on the magenta and cyan toner images superimposed and transferred on the intermediate transfer belt 8, respectively. The images are sequentially superimposed on each primary transfer portion, and a full-color toner image is formed on the intermediate transfer belt 8. The transfer residual toner that is not transferred onto the intermediate transfer belt 8 and remains on the photosensitive drums 2a to 2d is removed by the cleaning devices 7a to 7d, and the photosensitive drums 2a to 2d are prepared for the next image formation. .

  On the other hand, the sheets stored in, for example, the sheet feeding cassette 72 of the sheet storage unit 70 are picked up one by one from the uppermost one by the pick roller 15 and are separated and conveyed one by one by the feed roller 16 and the retard roller 17. The roller 19 conveys the first conveyance path L1 to the registration roller 20. In the registration roller 20, after the paper is temporarily in a standby state, the secondary transfer counter roller 9 and the secondary transfer roller 14 come into contact with each other at a predetermined timing synchronized with the toner image on the intermediate transfer belt 8. Is supplied to a secondary transfer portion which is a portion.

  In the secondary transfer portion, the full-color toner image carried on the transfer belt 8 is collectively transferred onto the surface (first surface) of the sheet by the action of the secondary transfer roller 14. The sheet on which the toner image is transferred is conveyed to the fixing device 22 by the conveying belt 21, and is heated and pressed in the process of passing through the fixing nip between the fixing roller 22a and the pressure roller 22b of the fixing device 22. The toner image is fixed.

  Thus, the paper whose toner image has been fixed on the surface by the fixing device 22 is determined by the flapper 29 in the conveying direction when face-up paper discharge (FU paper discharge) is selected. The paper is transported as it is toward the paper discharge roller 24 along the second transport path L2, and is discharged to a paper discharge tray (not shown) with the image surface facing up by the paper discharge roller 24.

  On the other hand, when face-down paper discharge (FD paper discharge) or double-sided image mode is selected, the flapper 29 is operated to switch the paper transport direction, and the paper that has passed through the fixing device 22 has a horizontal U-shape. It is transported to the duplex transport unit 90 through the third transport path L3.

  In the duplex conveying unit 90, the reversing roller 26 stands by in a state of being separated from the reversing roller 25, and the reversing roller 26 is pressed against the reversing roller 25 when a sheet is fed to the duplex conveying unit 90 more than a certain amount. . When the reversing roller 26 is pressed against the reversing roller 25 in this way, the reversing roller 25 is rotated forward and pulled in the direction in which the sheet is fed, and when the sheet is pulled more than a certain level, the reversing roller 25 is reversed. Thus, the sheet is switched back and sent to the reverse conveyance path L3 ′ or the fourth conveyance path L4. That is, when face-down paper discharge is selected, the paper transport direction is switched to the reverse transport path L3 ′ by the flapper 36, and the paper is not shown by the paper discharge roller 24 with the image side down. It is discharged to the paper discharge tray.

  When images are formed on both sides of the paper, the paper transport direction is switched to the direction of the fourth surface transport path L4 by the flapper 36, and the paper is transported in the fourth transport by the refeed roller 32 and the transport roller 35. The image is conveyed along the path L4 and is transferred from the fourth conveyance path L4 to the first conveyance path L1, so that the front and back are reversed and the image is faced down and sent to the registration roller 20. . Thereafter, a toner image is formed on the opposite surface (second surface) of the paper through the same process as described above, and the paper on which the toner image is formed is discharged after the toner image is fixed by the fixing device 22. The paper is discharged to a paper discharge tray (not shown) by the paper roller pair 24, and images are formed on both sides of the paper.

  In the above description, the cassette feeding is described. However, when the manual feeding is performed, the sheets set on the bypass tray 74 are picked up one by one from the uppermost one by the pick roller 15, The sheets are separated one by one by the feed roller 16 and the retard roller 17 and conveyed by the conveyance roller 18 to the registration roller 20 through the first conveyance path L1, and thereafter an image is formed on the sheet through the same process as described above.

  Next, the gist of the present invention will be described with reference to FIGS.

  2 is a schematic diagram showing a configuration of an intermediate transfer belt portion of the color image forming apparatus shown in FIG. 1, FIG. 3 is an enlarged view of a portion A in FIG. 2, and FIG. 4 is a sectional view taken along line BB in FIG.

  In the color image forming apparatus according to the present embodiment, the primary transfer rollers 6a to 6d of the magenta image forming unit 1M, the cyan image forming unit 1C, the yellow image forming unit 1Y, and the black image forming unit 1Y are connected to the intermediate transfer belt 8. The primary transfer rollers 6a to 6d are in contact with the respective photosensitive drums 2a to 2d through a predetermined distance ε (see FIG. 3) with respect to the contacts of the photosensitive drums 2a to 2d to the intermediate transfer belt 8. ) Only on the downstream side (downstream in the running direction of the intermediate transfer belt 8). For example, FIGS. 3 and 4 show the relationship between the photosensitive drum 2a and the intermediate transfer belt 8 in the magenta image forming unit 1M. The primary transfer roller 6a is located downstream (right side in FIG. 2) with respect to the photosensitive drum 2a. The primary transfer roller 6a is in contact with the intermediate transfer belt 8 on the downstream side by ε with respect to the contact point of the photosensitive drum 2a with respect to the intermediate transfer belt 8. Similarly, the other primary transfer rollers 6b to 6d are also arranged offset to the downstream side with respect to the respective photosensitive drums 2b to 2d, and are downstream of the contacts of the photosensitive drums 2b to 2d to the intermediate transfer belt 8. It is in contact with the intermediate transfer belt 8 on the side.

  In the present embodiment, the pressing members 37 that press the downstream portions of the respective photosensitive drums 2a to 2d of the intermediate transfer belt 8 toward the respective photosensitive drums 2a to 2d (upper side in the drawing) are provided to the respective photosensitive drums 2a to 2d. These pressing members 37 are disposed in the vicinity of the downstream side of the photosensitive drums 2 a to 2 d of the intermediate transfer belt 8. The drums 2a to 2d are deformed so as to be closer to the photosensitive drums 2a to 2d (upper side in the drawing) than the tangent drawn at the contact point between the drums 2a to 2d and the intermediate transfer belt 8. Here, as shown in FIG. 4 (only the magenta image forming unit 1M is shown in FIG. 4), the pressing member 37 has a crown shape in which the diameter of the central portion in the axial direction is larger than the diameter of the end portion in the axial direction. It is composed of rollers. Similarly, the primary transfer roller 6a is also constituted by a crown-shaped roller in which the diameter of the central portion in the axial direction is larger than the diameter of the end portion in the axial direction. The other pressing members 37 and the primary transfer rollers 6b to 6d are similarly constituted by crown-shaped rollers.

  In the color image forming apparatus according to the present embodiment, the photosensitive drums 2a to 2d are cylinders having a diameter of 40 mm, and the intermediate transfer belt 8 is a single-layer belt having a thickness of 0.1 mm. The primary transfer rollers 6a to 6 are formed on a metal shaft with a roller having a length of 300 mm and an outer diameter of 16 mm by foamed rubber having an Asker C hardness of 40 °. Each of the primary transfer rollers 6a to 6d is at a position 1.5 mm downstream in the horizontal direction from a perpendicular passing through the center of each of the photosensitive drums 2a to 2d, and one end thereof is about 5.0 N toward the photosensitive drums 2a to 2d by a spring. The photosensitive drums 2a to 2d are pressed in a substantially central direction by force. The pressing member 37 is a roller using a SUS pipe having a length of 300 mm, an outer diameter of 10 mm at the end, a crown of 11 mm at the center, and an inner diameter of 6 mm. The pressing member 37 is positioned 15 mm downstream in the horizontal direction from the perpendicular passing through the centers of the photosensitive drums 2 a to 2 d, and the upper end of each pressing roller is approximately 0 with respect to the upper ends of the primary transfer rollers 6 a to 6 d at the axial end. .5mm higher position.

  Thus, in the present embodiment, the primary transfer rollers 6 a to 6 d are brought into contact with the photosensitive drums 2 a to 2 d via the intermediate transfer belt 8 on the downstream side with respect to the contacts to the intermediate transfer belt 8. Since the configuration is adopted, it is possible to prevent deterioration of the toner image called “transfer scattering” due to discharge in the gap between the intermediate transfer belt 8 and the photosensitive drums 2a to 2d immediately before the transfer nip portion. Even if such a configuration is adopted, the downstream portions of the photosensitive drums 2a to 2d of the intermediate transfer belt 8 are exposed to the photosensitive members by the pressing members 37 disposed in the vicinity of the downstream sides of the photosensitive drums 2a to 2d. The photosensitive drums 2a to 2d are pressed to the side of the photosensitive drums 2a to 2d and the downstream portions of the photosensitive drums 2a to 2d are drawn at the contact points of the photosensitive drums 2a to 2d to the intermediate transfer belt 8. Since it is deformed so as to approach the 2a to 2d side, the primary transfer rollers 6a to 6d can be prevented from being separated from the photosensitive drums 2a to 2d by the tension of the intermediate transfer belt 8, thereby preventing primary transfer failure. The image quality can be improved.

  In the present embodiment, each pressing member 37 is constituted by a crown-shaped roller in which the diameter of the central portion in the axial direction is larger than the diameter of the end portion in the axial direction, so that both axial ends of the primary transfer rollers 6a to 6d are spring-loaded. Even if a configuration in which the photosensitive drums 2a to 2d are pressed by a method such as that described above, a uniform transfer nip can be formed in the axial direction of the primary transfer rollers 6a to 6d to achieve good transfer. It is possible to obtain an effect of preventing generation and stably obtaining a high-quality color image.

  The application of the present invention is not limited to the color image forming apparatus according to the embodiment described above, and the same applies to any other color image forming apparatus employing the intermediate transfer belt system. Of course, it is applicable.

DESCRIPTION OF SYMBOLS 1 Image formation part 1M Magenta image formation unit 1C Cyan image formation unit 1Y Yellow image formation unit 1K Black image formation unit 2a-2d Photosensitive drum (image carrier)
3a to 3d Exposure device 4a to 4d Charger 5a to 5d Developing device 6a to 6d Primary transfer roller 7a to 7d Cleaning device 8 Intermediate transfer belt 9 Secondary transfer counter roller 10 Drive roller 11 to 13 Followed roller 14 Secondary transfer roller 15 Pickup roller 16 Feed roller 17 Retard roller 18, 19 Conveyance roller 20 Registration roller 21 Conveyor belt 22 Fixing device 22a Fixing roller 22b Pressure roller 23 Conveyance roller 24 Discharge roller 25 Inversion roller 26 Inversion roller 27, 28 Conveyance roller 29 Flapper 30 Intermediate tray 32 Refeed roller 33, 34 Roller 35 Transport roller 36 Flapper 37 Press member 50 Automatic document transport device 51 Document tray 60 Document reading device 61 Optical frame 62 Document 63 First reflection mirror 64 First light System unit 65 second reflecting mirror 66 third reflecting mirror 67 second optical system unit 68 imaging lens 69 CCD
70 Paper storage unit 71 to 73 Paper feed cassette 74 Bypass tray 80 Transfer conveyance unit 90 Double-sided conveyance unit 100 Image forming apparatus main body L1 First conveyance path L2 Second conveyance path L3 Third conveyance path L3 ′ Reverse conveyance path L4 Fourth conveyance Road

Claims (2)

An endless intermediate transfer belt stretched between a plurality of rollers, a plurality of image carriers arranged along the running direction of the intermediate transfer belt, and the image carrier via the intermediate transfer belt In a color image forming apparatus provided with a plurality of primary transfer rollers that contact downstream with respect to the axis center,
A pressing member that presses a downstream portion of each image carrier of the intermediate transfer belt toward each image carrier is disposed near the downstream side of each image carrier, and downstream of each image carrier of the intermediate transfer belt. A color image forming apparatus, characterized in that a side portion is held on a tangential line drawn at a contact point between each image carrier and an intermediate transfer belt, or closer to each image carrier side than the tangent line. The color image forming apparatus according to claim 1, wherein the pressing member is constituted by a crown-shaped roller having a diameter at a central portion in an axial direction larger than a diameter at an end portion in the axial direction.

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