JP5495098B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile.

Y, M, and C toner images are sequentially formed from a photosensitive member as a first image carrier that carries a yellow (Y) toner image, a magenta (M) toner image, and a cyan (C) toner image. A color image forming unit as a first image forming unit that includes an intermediate transfer belt as an intermediate transfer body to be transferred and secondarily transfers a toner image on the intermediate transfer belt to a recording paper; and a black (B) color toner image An image forming apparatus including a photoconductor as a second image carrier to be carried and a B image forming unit as a second image forming unit for directly transferring a B color toner image on the photoconductor to a recording sheet. It is known (for example, Patent Documents 1 to 3).
In Patent Documents 2 and 3, a color image forming unit includes a Y-color photoreceptor carrying a Y-color toner image, an M-color photoreceptor carrying an M-color toner image, and a C-color toner image. A tandem type image forming apparatus is described in which Y, M, and C color toner images are sequentially transferred from a supported C color photoreceptor to an intermediate transfer belt.

  This type of image forming apparatus includes a color mode as a first image forming mode for forming a full color image and a monochrome mode as a second image forming mode for forming a monochrome image.

  In the image forming apparatus described in Patent Document 1, in the color mode, after the B-color toner image is directly transferred to the recording paper at the transfer nip, the recording paper is transported to the secondary transfer nip by the transport belt. At the next transfer nip, the three-color toner image on the intermediate transfer belt is superimposed and transferred onto the B-color toner image on the recording paper. On the contrary, in the image forming apparatuses described in Patent Documents 2 and 3, in the color mode, the three-color toner images on the intermediate transfer belt are secondarily transferred onto the recording paper at the secondary transfer nip. Thereafter, the recording paper is directly conveyed to the transfer / transfer nip by the conveyance belt, and the B-color toner image on the B-color photoreceptor is transferred to the three-color toner image on the recording paper in the direct transfer nip.

  The recording paper carrying the full-color toner image formed as described above is separated from the transport belt and transported to the fixing device, where the full-color toner image is fixed and then discharged to the paper discharge tray.

  On the other hand, in the monochrome mode, after the conveyance belt and the intermediate transfer belt are relatively separated from each other, the recording paper is directly conveyed to the transfer nip by the conveyance belt, and the B-color toner image is formed on the recording paper at the direct transfer nip. Is transcribed. Thereafter, the recording paper is separated from the conveyance belt and conveyed to the fixing device, where the B color toner image is fixed, and then discharged to the paper discharge tray.

  In the image forming apparatus described above, if the image forming operation is performed during the separating operation for separating the conveyance belt and the intermediate transfer belt, abnormal images such as bunting may occur due to the influence of vibration during the separating operation. It was. For this reason, conventionally, when the next image after the color image is a monochrome image, when the recording paper on which the color image is formed is discharged to the discharge tray, a separation operation is performed to separate the conveyance belt and the intermediate transfer belt. When the separation operation is completed, the recording paper is directly conveyed to the transfer nip to form a B-color toner image. For this reason, there has been a problem in that productivity is lowered when continuous printing from a color image to a monochrome image.

  As a result of diligent research conducted by the present inventors, it has been found that the vibration generated during the separation operation occurs when the separation operation is completed. Specifically, among the stretching rollers that stretch the intermediate transfer belt, the opposing roller that faces the secondary transfer roller is moved away from the secondary transfer roller, or the stretching belt that stretches the conveyance belt. Among the rollers, a plurality of rollers including the secondary transfer roller are moved away from the intermediate transfer belt, thereby separating the intermediate transfer belt and the conveyance belt. Then, when these rollers reach the separated position, they stop by abutting against the members of the apparatus. An impact generated by abutting against a member of the apparatus at the time of stopping becomes vibration and is transmitted to the paper transport unit. The vibration transmitted to the paper transport unit is transmitted to the B-color photoconductor that is in contact with the transport belt of the paper transport unit and forms a direct rotation nip, and affects the image. At this time, if a B-color image is transferred to a recording medium or a latent image is formed on a B-color photoconductor, an abnormal image such as image bunting occurs.

  The present invention has been made in view of the above problems, and its purpose is to suppress a decrease in productivity when continuously passing paper from the first image forming mode to the second image forming mode, and to detect abnormalities. An object of the present invention is to provide an image forming apparatus capable of suppressing an image.

In order to achieve the above object, the invention of claim 1 includes a first image carrier, first latent image forming means for forming a latent image on the first image carrier, and the first image. A first developing means for developing a latent image on the carrier, an intermediate transfer member to which an image formed on the first image carrier is temporarily transferred, and the first image carrier. A first image forming unit comprising: a primary transfer unit that primarily transfers an image onto the intermediate transfer member; and a secondary transfer unit that secondarily transfers an image transferred onto the intermediate transfer member onto a recording medium. A second image carrier provided on the downstream side of the secondary transfer position where the image is secondarily transferred onto the recording medium from the intermediate transfer member, and the second image carrier on the second image carrier. A second latent image forming unit that forms a latent image on the second image forming unit, a second developing unit that develops the latent image on the second image carrier, and the second image bearing unit. A direct transfer unit that transfers an image directly formed on the body on a recording medium, a second image forming unit including a transfer conveyor belt which is stretched around a plurality of stretching rollers, the secondary A paper conveying means for bringing the transfer conveying belt into contact with the intermediate transfer member at a transfer position to form a secondary transfer nip, and conveying the recording medium so as to pass through the secondary transfer nip by the transfer conveying belt ; Among the plurality of stretching rollers that stretch the transfer conveying belt, at least the most upstream stretching roller disposed at the most upstream in the recording medium conveying direction is moved to separate the transfer conveying belt from the intermediate transfer member. A first image forming mode for forming an image on the recording medium using the means, the first image forming unit, and the second image forming unit; and only the second image forming unit. There are, a mode switching means for switching to the second image forming mode for forming an image on the recording medium,
In the second image forming mode, in the image forming apparatus that forms an image by separating the paper transport unit and the intermediate transfer member, the next image forming mode after the execution of the first image forming mode is In the second image forming mode, after the recording medium on which the image corresponding to the first image forming mode is transferred passes through the secondary transfer nip, the paper conveying means and the intermediate transfer member are moved. A separating operation is started, and a recording medium on which an image corresponding to the second image forming mode is transferred is conveyed toward the direct transfer position during the separating operation, and the second image carrier After transferring the image corresponding to the first image forming mode to the recording medium and before starting to form the latent image of the image corresponding to the second image forming mode on the second image carrier. The paper conveying means and the intermediate transfer member It is one to complete the separation operation to be between the distance during the separating operation completed, to the recording medium conveying direction upstream side end portion of the transfer conveyor belt from the conveying roller pair for conveying the recording medium to the transfer conveyor belt However, when the transfer conveyance belt is in contact with the intermediate transfer member, the uppermost flow tension is set to be the same as the distance from the pair of conveyance rollers to the upstream end of the transfer conveyance belt in the recording medium conveyance direction. The gantry roller is moved .
According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the distance between the sheets is Lp, the distance from the secondary transfer nip to the direct transfer nip is Ln, the time required for the separating operation is T1, and the second The second image carrier surface moving distance from the latent image forming position to the direct transfer nip on the image carrier is L0, the process linear velocity of the second image carrier is Vopc, the process linear velocity of the paper conveying means is Vp, In this case, (Ln / Vp) ≦ T1 ≦ {(Ln + Lp) / Vp} − (L0 / Vopc) is satisfied.
According to a third aspect of the present invention, there is provided a first image carrier, first latent image forming means for forming a latent image on the first image carrier, and a latent image on the first image carrier. A first developing means for developing the image, an intermediate transfer member to which an image formed on the first image carrier is transferred primarily, and from the first image carrier onto the intermediate transfer member. A first image forming unit comprising: a primary transfer unit for primary transfer of an image; and a secondary transfer unit for secondary transfer of an image transferred onto the intermediate transfer member onto a recording medium; A second image carrier provided downstream of the secondary transfer position where the image is secondarily transferred from the recording medium to the recording medium, and a latent image is formed on the second image carrier. A second latent image forming means; a second developing means for developing the latent image on the second image carrier; and an image formed on the second image carrier. A direct transfer means for directly transferring the image onto the recording medium, and using the second image forming unit, the first image forming unit, and the second image forming unit, an image is recorded on the recording medium. An image forming apparatus comprising: a first image forming mode to be formed; and mode switching means for switching to a second image forming mode for forming an image on the recording medium using only the second image forming unit. A transfer conveying belt stretched by a plurality of stretching rollers, and at the secondary transfer position, the transfer conveying belt is brought into contact with an intermediate transfer member to form a secondary transfer nip. First paper conveying means for conveying the recording medium with the transfer conveying belt so as to pass through;
A direct transfer nip is formed in contact with the second image carrier at a direct transfer position at which an image is directly transferred from the second image carrier onto a recording medium, and the direct image transfer nip passes through the direct transfer nip. A second paper conveying means for conveying the recording medium; and a plurality of stretching rollers for stretching the transfer conveying belt, and moving at least the uppermost upstream stretching roller disposed in the uppermost stream of the recording medium conveying, A separation unit configured to separate the transfer conveyance belt from the intermediate transfer member, and the first image formation is performed when the next image formation mode after execution of the first image formation mode is the second image formation mode. After the recording medium on which the image corresponding to the mode is transferred passes through the secondary transfer nip, a separation operation for separating the first paper transport unit and the intermediate transfer member is started, and during the separation operation, The second image forming module The recording medium on which an image corresponding to the de is transferred, there is for transporting to said direct transfer nip, during the separating operation completed, the conveying roller pair for conveying the recording medium to the transfer conveyor belt the distance to the recording medium conveying direction upstream side end portion of the transfer conveyor belt, definitive when the transfer conveyor belt is in contact with the intermediate transfer member, a recording medium conveying direction upstream side end of the transfer conveyor belt from the conveying roller pair The uppermost stream stretching roller is moved so as to be the same as the distance to the section.
According to a fourth aspect of the present invention, in the image forming apparatus of the third aspect , after the image corresponding to the first image forming mode on the second image carrier is transferred to a recording medium, the second The separation operation of separating the paper conveying means and the intermediate transfer member is completed before starting to form the latent image of the image corresponding to the image forming mode on the second image carrier. is there.
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the second image forming unit forms a black image.

According to the first aspect of the present invention, after the recording medium has passed through the secondary transfer nip, the separation operation for separating the paper conveying means and the intermediate transfer member is started, and the second image formation is performed during the separation operation. The recording medium on which the image corresponding to the mode is transferred is conveyed directly to the transfer nip. Thereby, after the recording paper on which the image corresponding to the first image forming mode is formed is discharged out of the apparatus, the separation operation is started, and after the separation operation is finished, the recording medium is directly conveyed toward the transfer nip. Compared to the above, it is possible to suppress a decrease in productivity when the first image forming mode is switched to the second image forming mode to continuously pass the paper.
As described above, since the separation operation is started after the recording medium passes through the secondary transfer nip, as shown in Patent Documents 2 and 3, the transfer nip is directly downstream of the secondary transfer nip in the recording medium conveyance direction. In the formed configuration, the image on the second image carrier corresponding to the first image forming mode is transferred to the recording medium even during the separation operation. However, according to the present invention, since the separation operation is completed after the image corresponding to the first image forming mode on the second image carrier is transferred to the recording medium, the first image on the second image carrier is completed. When the image corresponding to the image forming mode is transferred to the recording medium, vibration upon completion of the separation operation does not occur. Thereby, it is possible to suppress the occurrence of bunting or the like in the image corresponding to the first image forming mode. Further, since the latent image of the image corresponding to the second image forming mode is formed on the second image carrier after the separation operation is completed, the latent image of the image corresponding to the second image forming mode is changed to the second image forming mode. When it is formed on the image carrier, no vibration is generated when the separation operation is completed. Therefore, it is possible to suppress the occurrence of bunting or the like in the image corresponding to the second image forming mode.

According to a third aspect of the present invention, the paper conveying means is a second paper conveying means that contacts the second image carrier and directly forms a transfer nip, and the first paper conveying means that performs a separating operation. Therefore, the vibration at the completion of the separating operation of the second image carrier is suppressed. This will be specifically described below.
The secondary transfer nip and the direct transfer nip are formed, and the paper conveying means for carrying and conveying the recording medium is moved away from the intermediate transfer member so as to pass through the secondary transfer nip and the direct transfer nip. When the separating operation is completed, the paper conveying means becomes a vibration source and vibrates. This vibration of the paper conveying means can be divided into those that propagate directly to the second image carrier through the direct transfer nip and those that propagate indirectly through the housing of the apparatus. Since the housing of the device supports a plurality of units in the device, it has high rigidity. Therefore, vibrations that are indirectly propagated to the second image carrier through the housing of the apparatus are attenuated by the housing, so that even if propagated to the second image carrier, the influence on the image is small. On the other hand, the vibration directly propagating to the second image carrier through the direct transfer nip is not attenuated by the housing or the like, and thus has a great influence on the image. In this way, the vibration at the completion of the separation operation is directly propagated through the direct transfer nip, so that the image corresponding to the first image forming mode on the second image carrier is transferred to the recording medium. When the separation operation is completed, the transfer is disturbed, and bunting or the like occurs in the image corresponding to the first image forming mode. Further, when the latent image corresponding to the second image forming mode is formed on the second image carrier, the second image carrier vibrates when the separation operation is completed, and the latent image is disturbed. As a result, bunting or the like occurs in the image corresponding to the second image forming mode.
On the other hand, in the invention of claim 3 of the present application, as described above, since the second paper conveying means for directly forming the transfer nip is separated from the first paper conveying means for performing the separating operation, it occurs when the separating operation is completed. The vibration of the first paper conveying means does not directly propagate to the second image carrier. Therefore, the vibration at the completion of separation that propagates to the second image carrier is only the vibration that is indirectly propagated through the housing, so the secondary transfer nip and the direct transfer nip are formed by one paper conveying means. As compared with the above, vibration of the second image carrier when the separation operation is completed is suppressed. Therefore, when the image corresponding to the first image forming mode on the second image carrier is transferred to the recording medium, the vibration of the second image carrier is suppressed even if the separation operation is completed. Therefore, the disturbance of the transfer is suppressed, and the bunting of the image corresponding to the first image forming mode can be suppressed. Further, when a latent image corresponding to the second image forming mode is formed on the second image carrier, even when the separation operation is completed, the vibration at the completion of the separation of the second image carrier is generated. Since it is suppressed, the disturbance of the latent image is suppressed, and it is possible to suppress the occurrence of bunting in the image corresponding to the second image forming mode.
Accordingly, after the recording medium has passed through the secondary transfer nip, a separation operation for separating the paper conveying unit from the intermediate transfer member is started, and an image corresponding to the second image forming mode is also performed during the separation operation. Even if the recording medium on which the image is transferred is directly conveyed to the transfer nip, abnormal images such as banding of an image corresponding to the first image forming mode and an image corresponding to the second image forming mode are suppressed. . As a result, it is possible to suppress a decrease in productivity when continuously feeding paper from the first image forming mode to the second image forming mode, and it is possible to suppress abnormal images.

  According to the present invention, it is possible to suppress a decrease in productivity when continuously feeding paper from the first image forming mode to the second image forming mode, and it is possible to suppress abnormal images.

1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment. FIG. 3 is an explanatory diagram for explaining a separation operation of a transfer conveyance belt of the image forming apparatus. FIG. 3 is an enlarged configuration diagram illustrating a B color image forming unit and a paper transport device of the image forming apparatus. 6 is a time chart of a separation operation of the image forming apparatus. FIG. 4 is an explanatory diagram for explaining a change in a distance from a nip of a registration roller pair to a suction nip by a separation operation of the image forming apparatus. FIG. 3 is an explanatory diagram for explaining the separation operation of the intermediate transfer belt of the image forming apparatus. FIG. 4 is an enlarged configuration diagram of a main part of an image forming apparatus according to a second embodiment. The principal part expanded block diagram of the modification of the image forming apparatus.

  The best mode for carrying out the present invention will be described below with reference to the drawings. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description is an example of the best mode of the present invention, and does not limit the scope of the claims.

[Embodiment 1]
The best mode for carrying out the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a first embodiment of an image forming apparatus according to the present invention. The image forming apparatus shown in FIG. 1 includes a color image forming unit 100 as a first image forming unit and a black (hereinafter abbreviated as B) image forming unit 101 as a second image forming unit. .
The color image forming unit 100 includes three image forming units 12Y, 12C, and 12M of yellow, cyan, and magenta (hereinafter abbreviated as Y, C, and M), and an intermediate transfer body that extends in a loop and extends substantially horizontally. The image forming apparatus includes a transfer belt 6, three primary transfer rollers 19Y, 19C, and 19M serving as primary transfer means, a secondary transfer roller 28 serving as a secondary transfer means, and the like. The image forming units 12Y, 12C, and 12M are arranged in series along the intermediate transfer belt 6 in the belt moving direction.

  The intermediate transfer belt 6 is supported by a driving roller 17 and a driven roller 18. Inside the intermediate transfer belt 6, primary transfer rollers 19 Y, 19 C, and 19 M as primary transfer means facing the respective photoreceptors 1. A cleaning counter roller 20 is provided. The primary transfer rollers 19Y, 19C, and 19M are opposed to the photoreceptors 1Y, 1C, and 1M provided in the image forming units 12Y, 12C, and 12M via the intermediate transfer belt 6 and slightly downstream in the rotation direction of the intermediate transfer belt. Is provided. An intermediate transfer cleaning unit 7 for removing residual toner on the intermediate transfer belt 6 is provided outside the intermediate transfer belt 6 so as to face the cleaning counter roller 20.

The secondary transfer roller 28 serving as a secondary transfer means is disposed to face the driving roller 17 of the intermediate transfer belt 6, and a secondary transfer nip is formed by the intermediate transfer belt and the secondary transfer roller via the transfer conveyance belt 8. Is formed. As the secondary transfer roller 28, a volume resistivity of 1 × 10 ⁵ [Ω · cm] to 1 × 10 ¹² [Ω · cm] and a thickness of 2 [mm] is used around a metal roller or a core metal. Examples thereof include those coated with a conductive elastic layer of 10 [mm] and rubber hardness of 10 to 100 degrees. The elastic layer may be foamable.

  On the right side of the intermediate transfer belt 6 in the figure, in a space below the fixing device 10, a paper transport device 15 as a paper transport means is provided. The paper conveying device 15 is disposed so as to intersect substantially vertically with the intermediate transfer belt 6 extending substantially horizontally, and includes a driving roller 25, a tension roller 27, a suction roller 51, a secondary transfer roller 28, and a direct transfer roller 19B. A transfer conveyance belt 8 is provided that is rotatably stretched. A voltage is applied to the suction roller 51 to charge the transfer conveyance belt 8. An entrance roller 51 is provided outside the transfer conveyance belt 8 so as to face the suction roller 50.

  The black image forming unit 101 includes an image forming unit 12B, a direct transfer roller 19B serving as a direct transfer unit, and the like. In the B color image forming unit 12B, the Y, M, and C color image forming units are independently provided downstream of the secondary transfer nip in the recording sheet moving direction, and the toner image of the image forming unit 12B is provided. It is arranged to be transferred directly to the recording paper. More specifically, the B image forming unit 12B is disposed in the vicinity of a substantially vertical conveyance path of the recording paper. A direct transfer roller 19B is provided at a position facing the photoconductor 1B via the transfer conveyance belt 8, and a direct transfer nip is formed by the photoconductor 1B and the direct transfer roller 19B via the transfer conveyance belt 8. Yes.

  Each color image forming unit 12Y, 12M, 12C, 12B shown in FIG. 1 includes photoconductors 1Y, 1C, 1M, and 1B, which are image carriers. Further, around each photoreceptor 1Y, C, M, B, charging devices 2Y, C, M, B and photoreceptors 1Y, C, M, B for charging the surfaces of the photoreceptors 1Y, C, M, B are provided. A developing device 3Y, C, M, B for supplying toner to the latent image on the upper side and a cleaning device 4Y for removing deposits such as toner adhering to the surface of the photoreceptors 1Y, C, M, B M, C, B are provided. In this embodiment, a blade type is used as the cleaning devices 4Y, 4M, 4C, and 4B, but a fur brush roller or a magnetic brush cleaning system may be used. The image forming units 12Y, 12C, 12M, and 12B are process cartridges that are detachable from the main body of the image forming apparatus.

  Above the color image forming unit 100, there is provided an exposure device 5 which is a first latent image forming means and also a second latent image forming means, and is charged with photosensitive members 1Y, 1C, 1M, A latent image is formed on the surface of B by a laser. The exposure apparatus 5 is not limited to the laser system, but may be an LED system or the like.

  The image forming apparatus according to the present embodiment includes a full color mode, which is a first image forming mode for creating a full color image composed of Y, C, M, and B toner images on a recording sheet, and a B toner image only on the recording sheet. And a monochrome mode that is a second image forming mode for creating a monochrome image. These modes are switched based on image data by a control unit of an image forming apparatus (not shown). That is, a control unit (not shown) functions as mode switching means.

First, a full color mode for creating a full color image composed of Y, C, M, and B toner images on a recording sheet will be described.
Original data read by the scanner 16, received data such as a facsimile, or color image information transmitted from a computer is color-separated into each color to form plate data for each color and sent to the exposure device 5. The uniformly charged photoconductors 1Y, 1C, 1M, and 1B are exposed on the image portion by the exposure device 5, and toner images are formed by the developing devices 3Y, 3C, and M, B.

  A high voltage having a polarity opposite to that of the toner is applied to the primary transfer rollers 19Y, 19C, and 19M, and each color toner image on the photoreceptors 1Y, 1C, and 1M is transferred onto the intermediate transfer belt 6 by the electric field generated by this voltage. Are sequentially transferred so as to overlap each other, and a color image composed of three colors Y, M, and C is formed on the intermediate transfer belt 6.

  A recording sheet, which is a recording medium on which an output image is formed, is set in the sheet feeding trays 21 and 22. The recording sheet is fed from the sheet feeding trays 21 and 22 by a calling roller 23a, and the recording sheet is fed out from the sheet feeding tray. The sheet feeding roller 23b and the separation roller 23c separate the sheet, and the separated recording sheet is conveyed toward the registration roller pair 24. The recording paper conveyed toward the registration roller pair 24 abuts against the registration roller pair 24 disposed in the paper conveyance path extending along the vertical direction and the skew is corrected. Sandwiched between. Then, it is sent out further upward at a predetermined timing by the registration roller pair 24. The recording paper fed upward by the registration roller pair 24 is conveyed to the entrance roller 51, the transfer conveyance belt 8, and the suction nip, and is brought into close contact with the transfer conveyance belt 8 by the entrance roller 51. The transfer conveyance belt 8 is charged by the suction roller 50, and the recording paper that is brought into close contact with the transfer conveyance belt 8 by the entrance roller 51 is electrostatically adsorbed to the transfer conveyance belt 8. Then, the recording paper electrostatically attracted to the transfer conveyance belt 8 is conveyed to the secondary transfer nip.

  The color image on the intermediate transfer belt 6 is applied between the secondary transfer roller 28 and the counter roller 17 at a secondary transfer nip formed by the secondary transfer roller 28 and the counter roller 17 via the intermediate transfer belt 6. Is transferred to the recording paper conveyed to the secondary transfer nip by the transfer conveying belt 8. At this time, a high voltage having a polarity opposite to the charging polarity of the toner may be applied to the secondary transfer roller 28, or a voltage having the same polarity as the charging polarity of the toner may be applied to the counter roller 17.

  When a high voltage having a polarity opposite to the charging polarity of the toner is applied to the secondary transfer roller 28, it is possible to use a high-voltage power supply for directly applying a voltage to the transfer roller 19B. Since it is not necessary to provide a dedicated power source for applying a voltage, cost reduction and size reduction of the image forming apparatus can be achieved. On the other hand, when a voltage having the same polarity as the charging polarity of the toner is applied to the opposing roller 17, a voltage is applied to the toner through the intermediate transfer belt 6, so that it is satisfactory even if the recording paper absorbs moisture and the resistance decreases. Transcription becomes possible.

  The recording paper on which the three-color toner image is transferred at the secondary transfer nip is directly conveyed to the transfer nip by the transfer conveyance belt 8, and the black toner image formed on the photoreceptor 1B is directly transferred to the recording paper at the transfer nip. Are directly transferred onto the three-color toner image.

  In this way, the recording paper on which the toner images of Y, C, M, and B are transferred has a curvature of the drive roller 25 that stretches the transfer conveyance belt 8 downstream of the secondary transfer nip in the transfer conveyance belt rotation direction. In the bent portion where the rotation direction of the transfer / conveyance belt 8 is suddenly changed, the curvature of the recording paper is separated from the transfer / conveyance belt 8 by the waist of the recording paper and reaches the fixing device 10, and Y, C, M, and B toners on the recording paper. The image is finally fixed by the fixing device 10, and a color image is formed on the recording paper. After the fixing, the recording paper is transported along the paper discharge path R, and is discharged and stacked on the paper discharge tray 31 by the paper discharge roller pair 30 in a face-down state.

  Next, the monochrome mode will be described. At the time of monochrome image creation in the monochrome mode, the exposure device 5 uses the data of a black image formed based on received data such as a document read by a scanner, a facsimile, or image information transmitted from a computer. The image portion on the photoconductor 1B is exposed, a black toner image is formed by the developing device 3B, the black toner image is directly transferred from the photoconductor 1B onto the recording paper conveyed by the transfer conveyance belt 8, and the black on the recording paper The toner image is fixed on the recording paper by the fixing device 10 to form a monochrome image.

  In the monochrome mode, the contact portion (secondary transfer nip) between the intermediate transfer belt 6 and the transfer conveyance belt 8 is released and separated by a separation mechanism that is a separation means described later. Thereby, even if the image forming units 12Y, 12M and 12 and the intermediate transfer belt 6 are not operated, the image formation of the monochrome image is not affected. Therefore, if the image forming units 12Y, C, M and the intermediate transfer belt 6 are not operated in the monochrome mode, the image forming units 12Y, C, M, the intermediate transfer belt 6 and the like can be prevented from being deteriorated correspondingly, and the image forming unit. There is an advantage that the life of the 12Y, C, M and intermediate transfer belt 6 can be extended.

  The transfer of the black toner image used in this embodiment is a direct transfer onto a recording sheet. This direct transfer reduces the number of components and the laser writing image of the black image exposed from the exposure device 5. There is an advantage that writing can be performed in the same direction as laser writing images of Y, C and M images. Further, the black image formed on the photosensitive member 1B by the black image forming unit is directly transferred onto the recording paper, so that the Y, M, and C images are transferred from the photosensitive member 1B to the recording paper via the intermediate transfer belt. The transfer efficiency is higher than when a black image is transferred. For this reason, transferring the black image directly from the photoreceptor 1B onto the recording paper is more easily transferred onto the photoreceptor 1B at the black image forming portion than transferring the black image from the photoreceptor 1B onto the recording paper via the intermediate transfer belt. It is possible to suppress the consumption of black toner when forming a black image.

  FIG. 2 is a diagram illustrating a separation operation of the transfer conveyance belt 8. In FIG. 2, the moving range of the transfer conveyance belt 8 is exaggerated for easy understanding of the separation operation. The secondary transfer roller 28, the suction roller 50, and the entrance roller 51 are configured to be able to reciprocate between a first position indicated by a dotted line in the figure and a second position indicated by a solid line in the figure. The secondary transfer roller 28, the suction roller 50, and the entrance roller 51 are each urged in the right direction (intermediate transfer belt separation direction) in the figure by urging means (not shown). The secondary transfer roller 28, the suction roller 50, and the entrance roller 51 are each provided with pushing means such as an actuator for pushing in the direction opposite to the urging direction of the urging means. The secondary transfer roller 28, the suction roller 50, and the entrance roller 51 are respectively positioned at the first position indicated by the dotted line in the drawing by the pushing means, thereby forming a secondary transfer nip. In the monochrome mode, the pushing force by the pushing means is released. Then, the secondary transfer roller 28, the entrance roller 51, and the suction roller 50 are moved to the second position indicated by the solid line in the drawing by urging means (not shown). Then, it hits a stopper member (not shown) and stops at the second position by restricting the movement of the secondary transfer roller 28, the entrance roller 51, and the suction roller 50. As a result, the transfer conveyance belt 8 is separated from the intermediate transfer belt 6. Thus, in this embodiment, the contacting / separating mechanism as the contacting / separating means is configured by the urging means and the pushing means.

  Here, the feature point of this embodiment is demonstrated. In the conventional image forming apparatus, the above separating operation starts the separating operation after the recording paper on which the color image is formed is discharged, and when the separating operation is finished, the recording sheet is fed from the paper feed trays 21 and 22. It was being transported. However, this has a problem that the productivity when continuously passing from a color image to a monochrome image is lowered. Therefore, by starting the separation operation after the trailing edge of the recording paper passes through the secondary transfer nip, it is possible to suppress a decrease in productivity when continuously feeding a color image to a monochrome image. However, at the end of the separation operation, that is, when the secondary transfer roller 28, the entrance roller 51, and the suction roller 50 move to the second position, each roller hits a stopper member (not shown), and vibration is generated. If image formation is performed by the image forming unit 12B when this vibration occurs, an abnormal image such as bunting occurs in the black image. Therefore, in this embodiment, the apparatus is configured such that the separation operation is completed when the image forming unit 12B is not forming an image.

As shown in FIG. 3, in this embodiment, the surface movement distance of the photoreceptor 1B from the latent image forming position to the direct transfer nip on the photoreceptor 1B of B is L0, and the distance from the secondary transfer nip to the direct transfer nip is When Ln, the process linear velocity of the photoreceptor 1B is Vopc, the process linear velocity of the transfer conveyance belt 8 is Vp, and the distance between the papers is Lp,
(Ln / Vp) ≦ T1 ≦ {(Ln + Lp) / Vp} − (L0 / Vopc)
It was configured to satisfy the relationship. By satisfying this condition, a latent image corresponding to a monochrome image is formed on the photoreceptor 1B of the image forming unit 12B after the transfer of the black toner image is completed on the recording paper on which the three-color toner images are formed. The separation operation can be terminated.

This will be specifically described below with reference to FIG.
As shown in FIG. 4, the separating operation is started immediately after the recording paper is removed from the secondary transfer nip (labeled intermediate transfer nip in FIG. 4). The time required for the trailing edge of the recording paper that has passed through the secondary transfer nip to pass directly through the transfer nip is Ln / Vp. That is, during this time, when the separation operation is completed ((Ln / Lp)> T1), vibration occurs during transfer of the black toner image onto the three-color toner image on the recording paper, and the vibration propagates to the photoreceptor 1B. Then, the photoreceptor 1B vibrates. When the photosensitive member 1B vibrates, a transfer failure occurs, and an abnormal image such as bunting occurs in the black image near the rear end of the recording paper. Therefore, the separating operation is not completed until the trailing edge of the recording paper that has passed through the secondary transfer nip passes directly through the transfer nip, that is, the transfer is performed directly from the secondary transfer nip so as to satisfy (Ln / Vp) ≦ T1. The length Ln to the nip and the linear velocity Vp of the transfer conveyance belt are set.

  The time from when the trailing edge of the recording paper on which the color image is formed passes through the secondary transfer nip until the leading edge of the recording paper on which the monochrome image is formed directly reaches the transfer nip is as shown in FIG. And (Ln + Lp) / Vp. Further, the time taken from the formation of the latent image formed on the surface of the photoreceptor 1B to the transfer onto the recording paper is (L0 / Vopc), and the leading edge of the monochrome image is transferred to the leading edge of the recording paper. In order to achieve this, it is necessary to start writing before reaching the transfer nip directly (L0 / Vopc). During the writing of the monochrome image, when the separation operation is completed (T1> {(Ln + Lp) / Vp} − (L0 / Vopc)), the vibration at the completion of the separation operation propagates to the photoconductor 1B or the exposure apparatus. Or the latent image is disturbed. As a result, bunting occurs in the monochrome image near the leading edge of the recording paper. Therefore, before the monochrome image writing is started on the photosensitive member 1B, the separation operation is completed, that is, the inter-paper distance Lp, the latent image so as to satisfy T1 ≦ {(Ln + Lp) / Vp} − (L0 / Vopc). An image forming position and the like are set.

  An example will be specifically described. An apparatus in which the process linear velocity of the photosensitive member 1B is Vopc, the process linear velocity Vp of the transfer conveyance belt 8 is 200 [mm / s], and the separation operation time T0 is 1 [s]. think about. When the diameter of the photosensitive member 1B is 30 [mm], considering the arrangement and size of the developing device, the surface movement distance L0 of the photosensitive member 1B from the latent image forming position to the direct transfer nip is about 30 [mm]. Become. Since Ln is about 50 to 100 [mm], the paper spacing Lp is about 130 to 180 [mm]. Therefore, when the process linear velocity is 200 [mm / s], the length of A4 is 25 to 28 [ppm], and the output when a color image is continuously output is 30 [ppm]. It will drop somewhat. However, it is possible to significantly reduce the waiting time when continuously passing from a color image to a monochrome image, as compared with the conventional case where paper feeding is started after the separation operation is completed. In the four-tandem mechanism, in order to separate the Y, M, and C photoconductors from the intermediate transfer belt, it is necessary to move one end side of the intermediate transfer belt greatly, and the separation operation takes time. In the embodiment, since the transfer conveyance belt 8 is merely separated from the intermediate transfer belt 6, the separation operation can be completed in a short time of 1 [s].

Further, during the separation operation, the secondary transfer roller 28, the entrance roller 50, and the suction roller 51 move, and the registration roller pair 24 does not move. Therefore, if the distance from the nip between the registration roller pair 24 to the nip between the entrance roller 51 and the transfer conveyance belt 8 (hereinafter referred to as the suction nip) fluctuates during the separation operation, a monochrome image is recorded on the recording paper. Cannot be formed at the predetermined position.
This will be specifically described. As shown in FIG. 5, the distance L1 from the nip of the registration roller pair 24 to the suction nip when the suction roller 50 and the entrance roller 51 are at the position of the dotted line in the figure, and the suction roller 50 at the position of the solid line in the figure. A distance L2 from the nip of the registration roller pair 24 when there is the entrance roller 51 to the suction nip. When the distances L1 = L2 are the same, the image was transferred to a predetermined position on the recording paper both when the image was formed at the position indicated by the dotted line in the figure and when the image was formed at the position indicated by the solid line in the figure. . However, when image formation is performed at L1−L2 = 3 [mm], the image formed at the position indicated by the solid line in the drawing is downstream in the recording paper conveyance direction as compared with the case where the image formation is performed at the dotted line in the drawing. 3 [mm]. Therefore, when the distance from the nip of the registration roller pair 24 to the suction nip becomes shorter while the recording paper on which the monochrome image is formed moves from the registration roller pair 24 to the suction nip, the trailing edge of the monochrome image May not be formed on the recording paper. Conversely, when the distance is increased, the leading edge margin of the recording paper is narrowed, and there is a possibility that a fixing winding jam will occur in the fixing device 10. For this reason, it is preferable that the distance from the nip of the registration roller pair 24 to the suction nip does not vary during the separation operation. Even if it fluctuates, it is necessary to keep it within a range in which the trailing edge is not formed on the recording paper and the fixing winding jam does not occur.

  In the above description, the intermediate transfer belt 6 and the transfer conveyance belt 8 are separated from each other by moving the transfer conveyance belt 8. However, as shown in FIG. Alternatively, the driven roller 18 and the tension roller 29 may be stretched and the roller facing the secondary transfer roller 28 may be the driven roller 18 so that the driven roller 18 can be moved. When the driven roller 18 moves leftward in the figure, the tension roller 29 moves downward in the figure. As a result, the intermediate transfer belt 6 is separated from the transfer conveyance belt 8. In this way, by configuring the intermediate transfer belt 6 to be separated from the transfer conveyance belt 8, the angle of the recording paper entering the transfer nip directly does not change. As a result, the three-color toner image on the recording paper can be prevented from being disturbed, and a good image can be formed.

On the other hand, when the transfer transfer belt 8 is moved to separate the intermediate transfer belt 6 from the transfer transfer belt 8, the intermediate transfer belt 6 is moved to separate the intermediate transfer belt 6 from the transfer transfer belt 8. Has the following advantages.
That is, in this embodiment, when the recording paper on which the full-color image is formed passes through the secondary transfer nip, the separation operation is started. Therefore, during the separation operation, the transfer on the intermediate transfer belt 6 is performed by the intermediate transfer cleaning unit 7. Residual toner is being removed. In the present embodiment, a cleaning blade as an intermediate transfer cleaning unit is brought into contact with the surface of the intermediate transfer belt to remove residual toner on the intermediate transfer belt. When the intermediate transfer belt 6 is moved away, the driven roller 18 is moved away from the transfer conveyance belt 8 as shown in FIG. When the driven roller 18 is moved, the region from the driven roller 18 to the intermediate transfer cleaning unit 7 of the intermediate transfer belt 6 is bent. This bending is eliminated by the driving force of the driving roller 17 and the tension of the tension roller 29. However, during the bending, the contact angle between the cleaning blade and the surface of the intermediate transfer belt is changed. The transfer residual toner on the transfer belt cannot be removed. Further, the region from the driven roller 18 of the intermediate transfer belt 6 to the intermediate transfer cleaning means 7 is bent, so that the sliding resistance between the cleaning blade and the intermediate transfer belt 6 changes, and the cleaning blade turns up. There is a fear. On the other hand, there is an advantage that such a cleaning defect on the surface of the intermediate transfer belt does not occur by moving the transfer conveyance belt 8 to separate the intermediate transfer belt 6 and the transfer conveyance belt 8 from each other.

[Embodiment 2]
Next, the image forming apparatus of Embodiment 2 will be described. Since the image forming apparatus of the second embodiment also basically has the same configuration as that of the first embodiment, only the differences will be described below.

FIG. 7 is an enlarged configuration diagram of a main part of the image forming apparatus according to the second embodiment.
As shown in the drawing, the image forming apparatus according to the second embodiment forms a secondary transfer nip in contact with the intermediate transfer belt, and carries the first recording paper so as to pass through the secondary transfer nip. The first paper transport device 15A, which is a paper transport means, and the B color photoreceptor 1B form a direct transfer nip, and a second paper transport means for carrying and transporting the recording paper so as to pass directly through the transfer nip. And a second paper transport device 15B.

  The first paper transport device 15A has a first transfer transport belt 8A that is stretched by a drive roller 25A, an adsorption roller 50, and a secondary transfer roller 28. The driving roller 25A, the secondary transfer roller 28, the suction roller 50, and the entrance roller 51 of the first paper transport device 15A are held by a first roller holding member (not shown). The secondary transfer roller 28, the suction roller 50, and the entrance roller 51 are held movably with respect to a first roller holding member (not shown). Similarly to the embodiment, the secondary transfer roller 28, the suction roller 50, and the entrance roller 51 are urged by the urging unit in a direction away from the intermediate transfer belt, and the pushing unit urges the urging unit. It is configured to be pushed in the direction opposite to the urging direction.

  The second paper conveying device 15A has a driving roller 25B, a second transfer conveying belt 8B stretched by a driven roller 29 and a direct transfer roller 19B. The driving roller 25B, the driven roller 29, and the direct transfer The roller 19B is held by a second roller holding member (not shown).

  In the image forming apparatus according to the second embodiment, when the secondary transfer roller 28, the entrance roller 51, and the suction roller 50 move to the second position, each roller hits a stopper portion of a first roller holding member (not shown) The first roller holding member vibrates. When the paper conveying device 15 forms the secondary transfer nip and the direct transfer nip as in the image forming apparatus of the first embodiment, the vibration at the completion of the separation of the paper conveying device 15 is caused by the direct transfer nip. Then, it is directly transmitted to the photoreceptor 1B. However, in the second embodiment, the paper transport device 15 is separated into the first paper transport device that forms the secondary transfer nip and the second paper transport device that directly forms the transfer nip. The vibration at the completion of the separation of 15 does not directly propagate to the photoreceptor 1B through the direct transfer nip. In other words, in the image forming apparatus of this embodiment, the vibration at the completion of the separation of the first paper transport device 15A propagates to the photoreceptor 1B via the housing of the device that supports the first roller holding member (not shown). Will do. Since the casing supports various components and units in the apparatus, the casing has high rigidity. For this reason, the vibration at the time of separation of the first paper transport device is attenuated by the housing. Therefore, the vibration propagating to the photoreceptor 1B is reduced. As a result, the vibration of the photosensitive member 1B at the completion of the separation is reduced as compared with the image forming apparatus of the first embodiment. When the separation operation is completed, or when a latent image is formed on the B-color photosensitive member 1B, an abnormal image such as bunting is generated even if the separation operation is completed. Can be suppressed. Therefore, also in the image forming apparatus of the second embodiment, after the recording paper on which the color image is formed passes through the secondary transfer nip, the separation operation is started, and the recording paper on which the monochrome image is formed during the separation operation. Even if the image is conveyed, it is possible to prevent the image from being disturbed by the vibration during the separation operation. As a result, also in the image forming apparatus according to the second embodiment, it is possible to suppress a decrease in productivity when continuously feeding paper from the full color mode to the monochrome mode, and it is possible to suppress abnormal images.

FIG. 8 is an enlarged configuration diagram of a modification of the image forming apparatus according to the second embodiment.
The modification shown in FIG. 8 has a configuration in which the first paper transport device 15 </ b> A and the second paper transport device 15 </ b> B are connected by the transfer transport belt 8. In the configuration of this modified example, the first paper transport device 15A has an entrance roller 51, a suction roller 50, and a tension roller 27, and these rollers are held by a first roller holding member (not shown). Has been. The second paper transport device 15B includes a driving roller 25, a direct transfer roller 19B, and a driven roller 29, and these rollers are held by a second roller holding member (not shown). In this modification, the transfer / conveying belt 8 is a rubber belt. Since the rubber-like belt has a low volume modulus of elasticity and has a higher vibration damping capability than polyimide or polycarbonate, the vibration of the first paper conveyance device 15A (first roller holding member) is transferred from the transfer conveyance belt 8 to the photoreceptor 1B. Propagation can be suppressed. Also in the configuration of this modified example, when the separation operation is completed, the vibration generated in the first paper conveyance device 15A propagates to the second paper conveyance device 15B through the housing, and directly passes through the transfer nip from the second paper conveyance device. And propagates to the photoreceptor 1B. Further, since the transfer conveyance belt is a rubber belt, the vibration of the first paper conveyance device is suppressed from being propagated to the photoreceptor 1B via the transfer conveyance belt 8. Thereby, when the separation operation is completed, vibration directly propagating from the transfer nip to the photoreceptor 1B is suppressed, and vibration of the photoreceptor 1B during the separation operation is reduced.

  In the image forming apparatus according to the second embodiment, an impact absorbing member such as a damper may be provided in the stopper portion of the first roller holding member to suppress vibration of the first roller holding member when the separation operation is completed.

  In the above description, the example in which the B-color image forming unit 12B is provided downstream of the secondary transfer nip in the recording sheet moving direction has been described. However, the B-color image forming unit 12B is provided upstream of the secondary transfer nip. May be. In this case, in the full color mode, after the B color toner image is transferred directly to the recording paper at the transfer nip, the three color toner image is transferred to the recording paper at the secondary transfer nip. In the above description, the color image unit is a tandem type. However, as described in Patent Document 1, the color image unit may be a revolver type.

As described above, according to the image forming apparatus of the first embodiment, the Y, M, and C color photoreceptors 1Y, 1M, and 1C as the first image carrier and the latent images are formed on the photoreceptors 1Y, 1M, and 1C. Exposure device 5 as first latent image forming means, developing devices 3Y, 3M, 3C as first developing means for developing latent images on the respective photoreceptors 1Y, 1M, 1C, and photoreceptors 1Y, 1M. , 1C, an intermediate transfer belt, which is an intermediate transfer body to which images are primarily transferred, and a primary transfer roller 19Y, which is a primary transfer means for primary transfer of images from the photoreceptors 1Y, 1M, 1C onto the intermediate transfer belt. , 19M, 19C, and a secondary transfer roller as a secondary transfer means for secondary transfer of the image transferred onto the intermediate transfer belt onto a recording paper as a recording medium, a color image forming unit as a first image forming unit It has. A B-color photoreceptor 1B as a second image carrier provided on the upstream side or the downstream side in the recording paper conveyance direction from the secondary transfer position where the image is secondarily transferred from the intermediate transfer belt onto the recording paper; The exposure device 5 that is also a second latent image forming unit that forms a latent image on the photoconductor 1B, the present device 4B that is a second developing unit that develops the latent image on the photoconductor 1B, and the photoconductor There is also provided a B-color image forming unit 101 as a second image forming unit provided with a direct transfer roller 19B as a direct transfer means for directly transferring an image formed on 1B onto a recording sheet. Further, the secondary transfer nip is formed by contact with the intermediate transfer belt at the secondary transfer position, and the photoconductor 1B is contacted at the direct transfer position where the image is directly transferred from the photoconductor 1B onto the recording paper. In addition to forming a direct transfer nip, a paper transport device 15 is provided as a paper transport unit that transports the recording paper so as to pass through the secondary transfer nip and the direct transfer nip. Further, an image is formed on the recording paper using a separation mechanism that is a separation unit that separates the paper transport device 15 and the intermediate transfer belt, the color image forming unit 100, and the black image forming unit 101. A controller serving as a mode switching means for switching between a full color mode as a first image forming mode and a monochrome mode as a second image forming mode for forming an image on the recording paper using only the black image forming unit 101; It has. In the monochrome mode, the paper conveyance device 15 and the intermediate transfer belt 6 are separated from each other to form an image.
In such an image forming apparatus, when the next image forming mode after execution of the full color mode is the monochrome mode, after the recording paper on which the image corresponding to the full color mode is transferred passes through the secondary transfer nip. Then, a separation operation for separating the paper conveyance device 15 and the intermediate transfer belt 6 is started, and the recording paper on which an image corresponding to the monochrome mode is transferred is conveyed toward the direct transfer nip during the separation operation. To do. Then, after the image corresponding to the full color mode on the photoconductor 1B is transferred to the recording paper, and before the latent image of the image corresponding to the monochrome mode is started to be formed on the photoconductor 1B, the paper conveying device 15 and the above The separating operation for separating the intermediate transfer belt 6 is completed.
In the image forming apparatus according to the first embodiment, by having the above-described configuration, the separation operation is started after the end of the full color mode, and the recording sheet is conveyed directly to the transfer nip after the completion of the separation operation. In comparison, it is possible to suppress a decrease in productivity when continuously passing from the full color mode to the monochrome mode, and it is possible to suppress the occurrence of abnormal images such as bunting.

The photosensitive member 1B is provided downstream of the secondary transfer position in the recording medium conveyance direction. The distance between the sheets is Lp, the distance from the secondary transfer nip to the direct transfer nip is Ln, and the separation operation is performed. This time is T1, the second image carrier surface moving distance from the latent image forming position to the direct transfer nip on the second image carrier is L0, the process linear velocity of the second image carrier is Vopc, and the paper conveyance When the process linear velocity of the means is Vp,
(Ln / Vp) ≦ T1 ≦ {(Ln + Lp) / Vp} − (L0 / Vopc)
It was configured to satisfy the relationship.
By configuring in this way, after the image corresponding to the full color mode on the photoconductor 1B is transferred to the recording paper, and before the latent image of the image corresponding to the monochrome mode is started to be formed on the photoconductor 1B. The separating operation for separating the paper conveying device 15 and the intermediate transfer belt 6 from each other can be completed.

  Further, since the separation mechanism separates the paper transport device 15 from the intermediate transfer belt 6, it is possible to satisfactorily remove the transfer residual toner on the intermediate transfer belt.

  Further, the intermediate transfer belt may be separated from the paper conveying device. With this configuration, the angle of entry of the recording paper into the direct transfer nip does not fluctuate during the separation operation, so that the three-color toner image on the recording paper is suppressed from being disturbed.

  In the image forming apparatus according to the second embodiment, the first paper transport unit that forms a secondary transfer nip in contact with the intermediate transfer belt and carries and transports the recording paper so as to pass through the secondary transfer nip. A first paper transport device 15A that contacts the photosensitive member 1B and forms a direct transfer nip, and a second paper transport device 15B that is a second paper transport means for carrying and transporting the recording paper so as to pass directly through the transfer nip. The first paper transport device 15A is separated from the intermediate transfer belt. With this configuration, the vibration generated in the first paper conveyance device 15A when the separation operation is completed is not directly transmitted to the photoreceptor 1B via the direct transfer nip. Therefore, the vibration of the photoreceptor 1B when the separation operation is completed is suppressed, and the separation operation is completed when the toner image on the B-color photoreceptor 1B is transferred onto the three-color toner image on the recording paper. When a latent image is formed on the B-color photoconductor 1B, an abnormal image such as bunting can be suppressed even if the separation operation is completed. Therefore, also in the image forming apparatus of the second embodiment, after the recording paper on which the color image is formed passes through the secondary transfer nip, the separation operation is started, and the recording paper on which the monochrome image is formed during the separation operation. Even if the image is conveyed, it is possible to prevent the image from being disturbed by the vibration during the separation operation. As a result, also in the image forming apparatus according to the second embodiment, it is possible to suppress a decrease in productivity when continuously feeding paper from the full color mode to the monochrome mode, and it is possible to suppress abnormal images.

  Further, by configuring the second image forming unit to form a black image, it is not necessary to drive the first image forming unit in the monochrome mode. Therefore, the lifetime of the first image forming unit can be extended.

1Y, C, M, B: photoconductors 2Y, C, M, B: charging devices 3Y, C, M, B: developing devices 4Y, M, C, B: cleaning device 5: exposure device 6: intermediate transfer belt 7: Intermediate transfer cleaning means 8: Transfer conveyance belt 8A: First transfer conveyance belt 8B: Second transfer conveyance belt 10: Fixing device 12Y, C, M, B: Image forming unit 15: Paper conveyance device 15A: First paper Conveying device 15B: second paper conveying devices 19Y, C, M: primary transfer roller 19B: direct transfer roller 24: registration roller pair 100: color image forming unit 101: black image forming unit

JP 10-55094 A JP 2002-182447 A JP 2006-85138 A

Claims (5)

A first image carrier, first latent image forming means for forming a latent image on the first image carrier, and first developing means for developing the latent image on the first image carrier; An intermediate transfer member to which an image formed on the first image carrier is primarily transferred; and a primary transfer means for primary transfer of the image from the first image carrier to the intermediate transfer member; A secondary transfer means for secondary transfer of the image transferred onto the intermediate transfer body onto a recording medium, and a first image forming unit comprising:
A second image carrier provided downstream of the secondary transfer position where the image is secondarily transferred from the intermediate transfer member onto the recording medium in the recording medium conveyance direction; and the second image carrier on the second image carrier. A second latent image forming means for forming a latent image; a second developing means for developing the latent image on the second image carrier; and an image formed on the second image carrier. A second image forming unit comprising a direct transfer means for transferring directly onto the second image forming unit;
Has a plurality of stretching rollers to stretch transcription conveyor belt, to form a secondary transfer nip the transfer conveyance belt at the secondary transfer position in contact with the intermediate transfer member, the two-by the transfer conveyor belt Paper conveying means for conveying the recording medium so as to pass through the next transfer nip;
Of the plurality of stretching rollers that stretch the transfer conveyance belt, at least the most upstream stretching roller disposed at the most upstream position in the recording medium conveyance direction is moved to separate the transfer conveyance belt from the intermediate transfer member. Spacing means;
Using the first image forming unit and the second image forming unit, a first image forming mode for forming an image on the recording medium, and using only the second image forming unit, the above Mode switching means for switching to a second image forming mode for forming an image on a recording medium,
In the second image forming mode, in the image forming apparatus for forming an image by separating the paper transport unit and the intermediate transfer member,
When the next image forming mode after execution of the first image forming mode is the second image forming mode, the recording medium onto which the image corresponding to the first image forming mode is transferred is the secondary transfer nip. A separation operation for separating the paper conveying means and the intermediate transfer member is started, and a recording medium on which an image corresponding to the second image forming mode is transferred during the separation operation The image is conveyed directly to the transfer position, the image corresponding to the first image forming mode on the second image carrier is transferred to a recording medium, and the image corresponding to the second image forming mode is transferred. Before starting the formation of the latent image on the second image carrier, the separation operation of separating the paper conveying means and the intermediate transfer body is completed,
When the separation operation is completed, the distance from the pair of conveying rollers that convey the recording medium to the transfer conveying belt to the recording medium conveying position of the most upstream stretching roller is determined by the transfer conveying belt contacting the intermediate transfer member. An image forming apparatus , wherein the uppermost stream stretching roller is moved so that the distance from the pair of transport rollers to the recording medium transport position of the uppermost stream stretching roller is the same as the distance between the pair of transport rollers. The image forming apparatus according to claim 1.
The distance between the sheets is Lp, the distance from the secondary transfer nip to the direct transfer nip is Ln, the time required for the separation operation is T1, and the second image from the latent image forming position to the direct transfer nip on the second image carrier. When the carrier surface moving distance is L0, the process linear velocity of the second image carrier is Vopc, and the process linear velocity of the paper conveying means is Vp,
(Ln / Vp) ≦ T1 ≦ {(Ln + Lp) / Vp} − (L0 / Vopc)
An image forming apparatus configured to satisfy the above relationship. A first image carrier, first latent image forming means for forming a latent image on the first image carrier, and first developing means for developing the latent image on the first image carrier; An intermediate transfer member to which an image formed on the first image carrier is primarily transferred; and a primary transfer means for primary transfer of the image from the first image carrier to the intermediate transfer member; A secondary transfer means for secondary transfer of the image transferred onto the intermediate transfer body onto a recording medium, and a first image forming unit comprising:
A second image carrier provided downstream of the secondary transfer position where the image is secondarily transferred from the intermediate transfer member onto the recording medium in the recording medium conveyance direction; and the second image carrier on the second image carrier. A second latent image forming means for forming a latent image; a second developing means for developing the latent image on the second image carrier; and an image formed on the second image carrier. A second image forming unit comprising a direct transfer means for transferring directly onto the second image forming unit;
Using the first image forming unit and the second image forming unit, a first image forming mode for forming an image on the recording medium, and using only the second image forming unit, the above In an image forming apparatus comprising mode switching means for switching to a second image forming mode for forming an image on a recording medium,
It has a transfer conveyance belt stretched by a plurality of stretching rollers, and at the secondary transfer position, the transfer conveyance belt is brought into contact with an intermediate transfer member to form a secondary transfer nip and passes through the secondary transfer nip. First paper conveying means for conveying the recording medium by the transfer conveying belt ,
A direct transfer nip is formed in contact with the second image carrier at a direct transfer position at which an image is directly transferred from the second image carrier onto a recording medium, and the direct image transfer nip passes through the direct transfer nip. A second paper conveying means for conveying the recording medium;
Among the plurality of stretching rollers that stretch the transfer conveyance belt, at least the most upstream stretching roller disposed at the most upstream side of the recording medium conveyance is moved to separate the transfer conveyance belt from the intermediate transfer member. Means and
When the next image forming mode after execution of the first image forming mode is the second image forming mode, the recording medium onto which the image corresponding to the first image forming mode is transferred is the secondary transfer nip. A recording medium on which an image corresponding to the second image forming mode is transferred during the separation operation. Is conveyed toward the direct transfer nip,
When the separation operation is completed, the distance from the pair of conveying rollers that convey the recording medium to the transfer conveying belt to the recording medium conveying position of the most upstream stretching roller is determined by the transfer conveying belt contacting the intermediate transfer member. An image forming apparatus , wherein the uppermost stream stretching roller is moved so that the distance from the pair of transport rollers to the recording medium transport position of the uppermost stream stretching roller is the same as the distance between the pair of transport rollers. The image forming apparatus according to claim 3.
After the image corresponding to the first image formation mode on the second image carrier is transferred to a recording medium, the latent image of the image corresponding to the second image formation mode is transferred to the second image carrier. An image forming apparatus comprising: completing a separating operation for separating the paper conveying unit and the intermediate transfer member before starting to form the sheet. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the second image forming unit forms a black image.

Images