JP4732054B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a fax machine using an electrophotographic system or an electrostatic recording system.

  In recent years, for example, image forming apparatuses using an electrophotographic method have been accelerated in speed, function, and color, and various types of printers, copiers, fax machines, and the like have been put on the market.

  Among these, there is an in-line type image forming apparatus in which a plurality of image forming units that form images of different colors are arranged in series along the transfer direction of the transfer target. Since it is possible to form multiple color images at high speed by sequentially transferring toner images from the electrophotographic photosensitive member (photosensitive member) provided in each image forming means onto the transfer target, it has become the mainstay of recent color printers. It is becoming.

  In-line type image forming apparatuses transfer a transfer material such as paper on a transfer belt (transfer material carrier) as an image carrier, and transfer each color toner image on a photoconductor provided in each image forming unit. There is a direct transfer method in which multiple transfer is sequentially performed on a transfer material. The in-line type image forming apparatus includes an intermediate transfer method in which each color toner image on the photosensitive member of each image forming unit is sequentially transferred in multiple to an intermediate transfer belt (intermediate transfer member) as an image carrier. In this intermediate transfer method, the toner images of a plurality of colors transferred onto the intermediate transfer belt are collectively transferred to a transfer material.

  However, conventionally, in these in-line type image forming apparatuses, generally, when printing a single color image, the image forming means for all colors must be operated. For this reason, even if a single color image such as a black single color (black and white image) is printed, it is not necessary for black image formation. Photosensitive body, developing device and developer of other color image forming means such as yellow, magenta and cyan Are also consumed or deteriorated. For this reason, there has been a problem that the life of the color image forming means that is used infrequently is shortened.

  As one means for solving this problem, Patent Document 1 proposes an image forming apparatus provided with means for separating and contacting the image forming means and the conveying means. According to such a configuration, only the image forming means of the color necessary for image formation and the conveying means need to be brought into contact with each other and operated. Therefore, the operation of the color image forming means unnecessary for image formation can be stopped. That is, for example, when printing a full-color image, the photosensitive member and the transport belt are brought into contact with each other in the image forming units of all colors. When printing a black monochromatic image, the photosensitive member and the conveying belt are brought into contact with only the black image forming unit, and the other image forming units are separated from each other. As described above, by appropriately switching the contact / separation state of the conveyance belt with respect to the photosensitive member, it is possible to extend the life of the image forming unit having a low use frequency.

  In the invention of Patent Document 1, contact / separation between the photoconductor and the transport belt is performed while the transport speed of the photoconductor and the transport belt is substantially the same. In the invention of Patent Document 1, typically, the contact / separation between the photoconductor and the transport belt is performed while the transport belt and the photoconductor are substantially stopped.

  Here, if the switching operation of the contact / separation state between the photosensitive member and the transport belt in the image forming unit is performed in a state where the photosensitive member and the transport belt are driven, the operation switching time is shortened. Reduce the first print time (time from power-on to the end of outputting the first image) and downtime (time during which image output cannot be performed) due to switching between full color mode and single color mode during continuous printing It is desirable from the viewpoint of shortening.

  However, in this case, the drive speed of the photoconductor and the conveyor belt is likely to change temporarily due to load fluctuation when the photoconductor and the conveyor belt are brought into contact / separated. In the conventional image forming apparatus, there is a case where the conveyance belt is locally loosened due to the temporary fluctuation of the driving speed.

  When the conveyor belt is slack, the conveyance of the conveyor belt becomes unstable. For this reason, the transport belt is deviated, the end surface of the transport belt interferes with other members, and is easily damaged by rubbing. In particular, in a usage environment in which full-color images and single-color images are printed alternately, the contact / separation between the conveyance belt and the photoconductor is repeated many times. For this reason, the end of the conveyor belt may be cracked, and the durability of the conveyor belt may be reduced.

The problem described above with respect to the conveyance belt can also occur when the image forming apparatus includes an intermediate transfer body as an image conveyance body.
JP-A-10-207151

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of improving the conveyance stability of an image carrier when the image carrier such as a recording material carrier or an intermediate transfer member is separated from and contacting the image carrier. It is.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides a plurality of image carriers that can rotate and carry a toner image, a belt for transferring toner images on the plurality of image carriers to a transfer material, and the belt. A drive support that is one of a plurality of support members that stretch and rotates the belt , and a tension applying member that is one of a plurality of support members that stretch the belt and applies tension to the belt. And a control means for controlling the moving speed of the image carrier and the moving speed of the belt, wherein the plurality of image carriers all come into contact with the belt to form a plurality of transfer nips. In the first mode in which image formation is performed in the state, and in the second state in which only one of the plurality of image carriers is in contact with the belt to form one transfer nip. A second mode of performing and a first mode In the image forming apparatus in which the image carrier that forms the transfer nip and the belt are in rotational movement when switching between the second mode and the second mode, the driving support is in the rotational movement direction of the belt. The tension applying member is disposed downstream of the driving support in the rotational movement direction of the belt, and is disposed downstream of the transfer nip formed when the second mode is executed . The belt is disposed upstream of the transfer nip formed when executing the mode, and the belt supports the drive from the transfer nip formed when executing the second mode in the rotation direction of the belt. When switching according to the mode in which the first state and the second state are executed, the control means is not stretched between the bodies by the other support body, (| A-B | / A) where the moving speed of the image carrier is controlled to be faster than the moving speed of the image carrier, the moving speed of the image carrier is A, and the moving speed of the belt is B. ) × 100 (%) ≧ 4. The image forming apparatus is characterized in that the control is performed.
According to another aspect of the present invention, a plurality of image carriers capable of rotating and carrying toner images, a belt for transferring toner images on the plurality of image carriers to a transfer material, and the belt A driving support for rotating the belt and one of a plurality of supports for stretching the belt, and applying a tension for applying a tension to the belt and the member, and a control means for controlling the moving speed of the belt and the moving speed of the image bearing member, a first of the plurality image bearing members all form a plurality of the transfer nip in contact with the belt In the first mode in which image formation is performed in the above state, and in the second state in which only one of the plurality of image carriers is in contact with the belt to form one transfer nip. A second mode in which In the image forming apparatus in which the image carrier forming the transfer nip and the belt are rotating while switching between the second mode and the second mode, the driving support is configured to move in the rotational movement direction of the belt. The tension applying member is disposed when the second mode is executed in the rotational movement direction of the belt. The belt is disposed downstream of the transfer nip and upstream of the drive support, and the belt is formed when the second mode is executed from the drive support in the rotational movement direction of the belt. not stretched by other said support between said transfer nip to be, when switching according to the mode for executing the second state into the first state, the The control means controls the moving speed of the belt to be slower than the moving speed of the image carrier, and assumes that the moving speed of the image carrier is A and the moving speed of the belt is B (| A An image forming apparatus is provided that is controlled so that −B | / A) × 100 (%) ≧ 4.

  According to the present invention, it is possible to improve the conveyance stability of the image carrier when the image carrier such as the recording material carrier or the intermediate transfer member is separated from and contacting the image carrier.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
[overall structure]
First, the overall configuration of the image forming apparatus of this embodiment will be described. FIG. 1 is a schematic configuration diagram of an image forming apparatus according to the present exemplary embodiment. The image forming apparatus 100 according to this embodiment is a full-color laser beam printer that can form a full-color image on a transfer material P, for example, a recording sheet, an OHP sheet, or the like by an electrophotographic method in accordance with an image information signal. The image information signal is transmitted from an external apparatus such as a personal computer, an image reading apparatus, or a digital camera that can communicate with the image forming apparatus main body (apparatus main body) by wire or wirelessly.

  The image forming apparatus 100 includes a conveyance belt 21 that is an endless belt-shaped transfer material carrier as an image conveyance body. An image forming station that forms images of magenta (M), cyan (C), yellow (Y), and black (K), respectively, as a plurality of image forming means along the moving direction of the conveying belt 21. There is. These image forming stations (hereinafter simply referred to as “stations”) are juxtaposed as first, second, third, and fourth stations 10M, 10C, 10Y, and 10K.

  In this embodiment, the configurations and operations of the stations 10M, 10C, 10Y, and 10K are substantially the same except that the color of the toner used is different. Accordingly, in the following, when there is no particular need to distinguish, the subscripts M, C, Y, and K given to the reference numerals to indicate that they are elements provided for any color are omitted and are generally described. To do.

  The station 10 is provided with a cylindrical electrophotographic photosensitive member as an image carrier, that is, a photosensitive drum 11. In order along the rotation direction of the photosensitive drum 11, a charging roller 12 as a charging unit, an exposure device 13 as an exposure unit, a developing device 14 as a developing unit, and a cleaning unit (cleaning blade, waste toner container, etc.) as a cleaning unit. 16) is provided. Further, a transfer roller 15 as a transfer unit is disposed on the inner peripheral surface side of the conveyor belt 21 at a position facing each photosensitive drum 11 of each station 10. At the position where the transfer roller 15 is disposed, the conveyance belt 21 can contact the photosensitive drum 11 to form a transfer nip (transfer position) N.

  The photosensitive drums 11 of the stations 10M, 10C, 10Y, and 10K are rotationally driven by photosensitive drum driving motors D1, D2, D3, and D4 as photosensitive member driving units, respectively.

The conveyor belt 21 is a belt-like member that is stretched around a plurality of supports and driven to circulate, and at least one of the supports is a drive support that applies a driving force to the conveyor belt 21. In this embodiment, the conveyor belt 21 is stretched around three axes of a driving roller 22, a driven roller 23, and a tension roller 24 as a support. The drive roller 22 is rotationally driven by a conveyor belt drive motor D5 as a conveyor belt driving unit. That is, the drive roller 22 is a drive support that drives the conveyor belt 21 to circulate. The driven roller 23 is driven by the conveyor belt 21. The tension roller 24 applies tension to the transport belt 21 as a tension applying member . The tension roller 24 is urged by the tension spring 25 in the direction of increasing the distance between the drive roller 22 and the driven roller 23 and is driven by the transport belt 21. The moving directions of the photosensitive drum 11 and the conveyor belt 21 at each transfer position N are substantially the same.

  In this embodiment, the driving roller 22 as a driving support among the plurality of supports on which the conveyance belt 21 is stretched is the most downstream photosensitive drum 11K in the moving direction of the conveyance belt 21 (that is, the most downstream transfer nip). N) It is arranged on the downstream side. Further, in this embodiment, no other belt stretching roller is provided between the most downstream photosensitive drum 11K and the driving roller 22.

  The driving speeds of the photosensitive drum driving motors D1 to D4 and the conveying belt driving motor D5 are independently controlled by the CPU 50 as a control unit. In the present embodiment, the drive motors D1 to D5, the CPU 50, and a driver circuit (not shown) of each drive motor function as speed changing means described later. The driver circuit operates each drive motor D1, D2, D3, D4, D5 in accordance with a control signal from the CPU 50. In this embodiment, the CPU 50 controls the sequence of the operation of each unit in the apparatus in addition to the drive motors D1 to D5 as the central element of the control unit that controls the image forming apparatus 100 in an integrated manner. It is.

  The transfer roller 15 is disposed at a position facing the photosensitive drum 11 of each color station 10 on the inner peripheral surface side of the conveyor belt 21. In this embodiment, the relative position of the photosensitive drum 11 with respect to the apparatus main body is fixed. On the other hand, the transfer roller 15 is disposed so as to be movable in a direction in which the transfer roller 15 contacts or separates from the photosensitive drum 11.

  In this embodiment, at each transfer position N, the photosensitive drum 11 and the conveyor belt 21 are in contact with each other to form a transfer nip, and the photosensitive drum 11 and the conveyor belt 21 are separated from each other. The second state can be switched. That is, the image forming apparatus 100 includes contact / separation mechanisms S1, S2, S3, and S4 as switching means for performing the switching. The contact / separation mechanisms S <b> 1 to S <b> 4 can cause each transfer roller 15 to contact the photosensitive drum 11 with a predetermined pressure via the transport belt 21. Further, the contact / separation mechanisms S <b> 1 to S <b> 4 can move the transfer rollers 15 in the direction of retreating from the photosensitive drum 11 to separate the transfer rollers 15 from the inner peripheral surface of the transport belt 21.

  In this embodiment, only when the transfer roller 15 is in contact with the photosensitive drum 11 via the conveyance belt 21, the transfer nip N is formed at the contact portion between the photosensitive drum 11 and the conveyance belt 11. The transfer roller 15 rotates following the conveyance belt 21 only when it is in contact with the photosensitive drum 11 via the conveyance belt 11.

  The contact / separation mechanisms S1 to S4 include a member that enables the transfer roller 15 to move in a direction to contact or separate from the photosensitive drum 11 by a separation cam or lever, and a motor or solenoid that appropriately drives the member. And the like. As such a contact / separation mechanism itself, a general one can be used.

In this embodiment, the image forming apparatus 100 has a full color mode (first mode) and a black single color mode (second mode) as image forming modes. FIG. 2 shows a contact / separation state between the conveyance belt 21 and the photosensitive drum 11 in the full color mode. FIG. 3 shows the contact / separation state between the conveyance belt 21 and the photosensitive drum 11 in the black monochrome mode.

  In this embodiment, the CPU 50 functions as a control unit for the contact / separation mechanisms S1 to S4, controls the contact / separation mechanisms S1 to S4 according to the image forming mode, and sets each transfer roller 15 to the photosensitive drum 11. It moves to the position where it contacts or separates.

  When the conveyance belt 21 is in contact with the photosensitive drum 11, the transfer roller 15 is formed on the surface of the conveyance belt 21 that is stretched between two axes of the driving roller 22 and the tension roller 24. The photosensitive drum 11 is disposed at a position where the contact surface side is convex. Therefore, the transfer belt 15 is also moved away from the photosensitive drum 11 by moving the transfer roller 15 to a position away from the photosensitive drum 11.

  When forming a full-color image, as shown in FIG. 2, the transfer roller 15 is brought into contact with the photosensitive drum 11 via the transport belt 21 in all of the first to fourth stations 10M, 10C, 10Y, and 10K. . On the other hand, when forming a black monochrome image, as shown in FIG. 3, the conveyance belt 21 and the transfer roller 15 are separated from the photosensitive drum 11 in the first, second, and third stations 10M, 10C, and 10Y. . Then, only the transfer roller 15 of the fourth station 10 </ b> K is brought into contact with the photosensitive drum 11 through the conveyance belt 21.

  The operation of switching the contact / separation state between the conveyance belt 21 and the photosensitive drum 11 by moving the transfer roller 15 in the direction in which the transfer roller 15 contacts or separates from the photosensitive drum 11 is performed between the photosensitive drum 11 and the conveyance belt 21. Each is driven and executed while moving. This is advantageous in shortening the first print time and switching between the full color mode and the black single color mode during continuous printing in a short time.

  In addition, the image forming apparatus 100 is provided with a feeding unit 40 that supplies the transfer material P to the transport belt 21. The feeding unit 40 feeds the transfer material P stored in the feeding cassette 41 to the conveying belt 21 by a feeding roller 42 that is rotationally driven according to an image forming operation.

  Further, a fixing device 30 as a fixing unit is provided downstream of the most downstream station 10K in the conveyance direction of the transfer material P. The fixing device 30 includes a fixing roller having a heat source and a driving roller that presses against the fixing roller. The transfer material P is sandwiched and conveyed between these rollers, so that heat and pressure can be applied to the transfer material P on the transfer material P. The toner is fixed on the transfer material P.

  In this embodiment, a part of the configuration of each station 10 is detachable from the apparatus main body as a process cartridge. Here, as the process cartridge, the photosensitive member and at least one of a charging unit, a developing unit, and a cleaning unit as a processing unit acting on the photosensitive unit are integrally formed into a cartridge and can be attached to and detached from the apparatus main body. There is something to do. In this embodiment, the process cartridge includes at least a photosensitive drum and a cleaning device.

[Image formation process]
Next, an image forming process in the image forming apparatus 100 of the present embodiment will be described.

  When an image formation start signal is input to the CPU 50, rotation of the photosensitive belt 11 of the transport belt 21, the first to fourth stations 10M, 10C, 10Y, and 10K is started.

  The transport belt 21 and the transfer roller 15 are switched to a predetermined contact / separation position with respect to the photosensitive drum 11 according to the image forming mode. That is, in the full color mode, the transfer roller 15 contacts the photosensitive drum 11 via the transport belt 21 in all the stations 10M, 10C, 10Y, and 10K. On the other hand, in the black monochrome mode, the transfer roller 15 is separated from the photosensitive drum 11 in each of the magenta, cyan, and yellow stations 10M, 10C, and 10Y. Then, only the transfer roller 15K of the black station 10K contacts the photosensitive drum 11K via the intermediate transfer belt 30.

  Thereafter, the surface of the photosensitive drum 11 is charged to a desired potential (negative polarity in this embodiment) by the charging roller 12. Image exposure based on image information is performed on the surface of the charged photosensitive drum 11 by the exposure device 13. As a result, an electrostatic image (latent image) is formed on the photosensitive drum 11. Next, the electrostatic image formed on the photosensitive drum 11 is developed by the developing device 14 with toner (toner having negative charging characteristics in this embodiment). As a result, a toner image is formed on the photosensitive drum 11.

  The toner image on the photosensitive drum 11 is electrostatically transferred onto the transfer material P carried on the conveyor belt 21 by the action of the transfer roller 15. At this time, a predetermined voltage (positive voltage in this embodiment) is applied to the transfer roller 15 from a transfer power source as a transfer bias output unit. The transfer material P on the conveyance belt 21 is fed from the feeding unit 40 in timing with the toner image on the photosensitive drum 11 and is carried on the conveyance belt 21.

  In the full color mode, the above-described charging, exposure, development, and transfer processes are similarly repeated in the stations 10M, 10C, 10Y, and 10K for each color. The magenta toner image, the cyan toner image, the yellow toner image, and the black toner image respectively formed on the photosensitive drums 11 of the stations 10M, 10C, 10Y, and 10K are transferred on the conveyance belt 21 and conveyed. The material P is sequentially superimposed and transferred. Thereafter, the transfer material P is separated from the transport belt 21 and transported to the fixing device 30. The toner image on the transfer material P is heated and pressed by the fixing device 30 and fixed on the transfer material P. The transfer material P on which the toner image is fixed is then discharged out of the apparatus, and a series of image forming steps is completed.

  On the other hand, in the black monochrome mode, the above-described charging, exposure, development, and transfer processes are performed only in the black station 10K. Then, the black toner image is transferred to the transfer material P that is carried on the transport belt 21 and transported. Thereafter, as in the full color mode, the toner image is fixed on the transfer material P, and the transfer material P is discharged out of the apparatus. In the black monochrome mode, after the contact / separation operation of the conveying belt 21 and the transfer roller 15 with respect to the photosensitive drum 11, the driving of the magenta, cyan, and yellow stations 10M, 10C, and 10Y is stopped. .

  The toner remaining on the photosensitive drum 11 after the toner image is transferred from the photosensitive drum 11 to the transfer material P is removed from the photosensitive drum 11 by the cleaning device 16 and collected in a waste toner container attached to the cleaning device 16. The

[Drive speed of photosensitive drum and conveyor belt]
Next, the relationship between the driving speeds of the photosensitive drum 11 and the conveyance belt 21 in this embodiment will be described.

-During image formation-
First, the relationship between the driving speeds of the photosensitive drum 11 and the conveyance belt 21 during image formation will be described.

  In this embodiment, in the normal state except during the contact / separation operation of the conveyance belt 21 with respect to the photosensitive drum 11 described later, at least during the image formation, the moving speed (circumferential speed) of the surface of the photosensitive drum 11 and the conveyance belt 21. The moving speed (peripheral speed) of the surface is substantially equal.

However, in order to prevent an image defect in which the transferred toner image is lost or to improve the stability of the conveyance speed of the transfer material P by the conveyance belt 21, the photosensitive drum 11 has a transfer position N at each transfer position N. A slight peripheral speed difference may be provided between the peripheral speed and the peripheral speed of the conveyor belt 21. Here, the ratio C of the circumferential speed difference between the circumferential speed (A) of the photosensitive drum 11 and the circumferential speed (B) of the conveying belt 21 to the circumferential speed (A) of the photosensitive drum 11 is expressed as follows:
C = (B−A) / A × 100 (%) (1)
And

  At least during image formation, the peripheral speed difference ratio C represented by the above formula (1) is preferably 3% or less. If the peripheral speed difference ratio C exceeds 3%, the toner image transferred onto the transfer material P may be excessively expanded with respect to the original image, or the toner may be fused to the photosensitive drum 11 or the conveyance belt 21. It tends to occur.

  When a peripheral speed difference is provided between the photosensitive drum 11 and the conveyor belt 21 during image formation, the peripheral speed of the conveyor belt 21 is normally set relatively higher than the peripheral speed of the photosensitive drum 11. That is, with respect to the peripheral speed difference ratio C expressed by the above formula (1), 0 ≦ C ≦ 3 holds at least during image formation. However, if desired, when a peripheral speed difference is provided between the photosensitive drum 11 and the conveyor belt 21 during image formation, the peripheral speed of the conveyor belt 21 may be relatively slower than the peripheral speed of the photosensitive drum 11. it can. In this case, with respect to the peripheral speed difference ratio C expressed by the above formula (1), −3 ≦ C ≦ 0 holds at least during image formation.

In other words, when the peripheral speed of the photosensitive drum 11 is A and the peripheral speed of the transport belt 21 is B, when transferring the toner image on the photosensitive drum 11,
(| A-B | / A) × 100 (%) ≦ 3
Holds. In this embodiment, as described above, when the toner image on the photosensitive drum 11 is transferred, the peripheral speed of the photosensitive drum 11 and the peripheral speed of the transport belt 21 are substantially equal.

-During contact / separation operation of the conveyor belt-
Next, the relationship between the drive speeds of the photosensitive drum 11 and the conveyance belt 21 during the contact / separation operation of the conveyance belt 21 with respect to the photosensitive drum 11, which is the most characteristic feature of this embodiment, will be described.

  In this embodiment, switching between the first state in which the photosensitive drum 11 and the transport belt 21 are in contact with each other to form the transfer nip N and the second state in which the photosensitive drum 11 and the transport belt 21 are separated from each other. Is performed while the photosensitive drum 11 and the conveyor belt 21 are moving. Further, assuming that the moving speed (peripheral speed) of the photosensitive drum 11 is A and the moving speed (peripheral speed) of the conveying belt 21 is B, the moving speed of the photosensitive drum 11 represented by | A−B | and the movement of the conveying belt 21. The speed difference between the first speed and the second speed is greater than when the toner image is transferred from the photosensitive drum 11 to the transfer material P on the transport belt 21 when switching between the first state and the second state. Is bigger. In this embodiment, when the contact / separation operation of the conveying belt 21 is performed on at least one of the plurality of photosensitive drums 11, the above-described speed difference is provided.

  More specifically, in this embodiment, when the transport belt 21 is brought into contact with or separated from the photosensitive drum 11, the peripheral speed of the transport belt 21 is set to the peripheral speed of the photosensitive drum 11 from the viewpoint of improving the durability of the transport belt 21. To be relatively fast.

  With reference to FIGS. 4 and 5, the state of the transport belt 21 immediately after the contact / separation operation of the transport belt 21 with respect to the photosensitive drum 11 when the photosensitive drum and the transport belt are driven at a constant speed will be described. FIG. 4 shows a state of the transport belt 21 immediately after the state of contact / separation of the transport belt 21 with respect to the photosensitive drum 11 in the black monochrome mode (FIG. 3) to the state in the full color mode (FIG. 2). On the other hand, FIG. 5 shows the state of the transport belt 21 immediately after the contact / separation state of the transport belt 21 with respect to the photosensitive drum 11 in the full color mode (FIG. 2) is changed to the state in the black monochrome mode (FIG. 3). .

  In the image forming apparatus 100 according to the present exemplary embodiment, the contact / separation state of the conveyance belt 21 with respect to the photosensitive drum 11 can be switched between a state in the full color mode and a state in the single color mode. At the time of this switching, the conveying belt 21 is kept in contact with the photosensitive drum 11 in the black station 10K. That is, in this embodiment, when switching between the first state and the second state is performed for at least one of the plurality of photosensitive drums 11, at least one of the other photosensitive drums 11 is selected. The first state is maintained.

  When the conveyance belt 21 is switched from the state shown in FIG. 2 to the state shown in FIG. 3, or conversely, when the state shown in FIG. 3 is changed to the state shown in FIG. 2, the photosensitive drum drive motors D1, D2, D3, D4 and the load torque to the conveyor belt drive motor D5 may fluctuate. Thereby, the drive speed of each drive motor D1-D5 tends to become unstable temporarily. At this time, if the photosensitive drum 11 and the conveyor belt 21 are set to be driven at the same peripheral speed, the peripheral speed of the conveyor belt 21 is set to the peripheral speed of the photosensitive drum 11 due to the speed fluctuation of the drive motors D1 to D5. It becomes relatively fast and slow.

  Here, consider a case where the peripheral speed of the conveyor belt 21 is relatively slower than the peripheral speed of the photosensitive drum 11 due to the speed fluctuations of the drive motors D1 to D5 as described above. In this case, due to the speed difference between the two, the conveyance belt 21 is likely to be slack in the section from the transfer nip N on the most downstream side in the moving direction of the conveyance belt 21 to the drive roller 22. Due to this slackness, the deviation of the conveyor belt 21 is deteriorated, and the end surface of the conveyor belt 21 is easily damaged by rubbing with other members.

  Note that the tension roller 24 passes through the tension roller 24 and the driven roller 23 downstream of the driving roller 22 in the moving direction of the conveying belt 21 and reaches the most upstream transfer nip N in the moving direction of the conveying belt 21. This makes it easier to eliminate slack. For this reason, the deviation of the conveyor belt 21 hardly occurs in this section.

  Here, as a typical example, the state of the conveyance belt 21 when switching between the state illustrated in FIG. 2 and the state illustrated in FIG. 3 has been described. However, the same problem may occur in other modes than this switching mode. When the driving roller 22 is located downstream of the most downstream photosensitive drum 11 in the moving direction of the conveying belt 21, the circumferential speed of the conveying belt 21 is relatively slower than the circumferential speed of the photosensitive drum 11 due to speed fluctuation of the driving motor. The same problem can occur. That is, in the section from the most downstream portion where the conveyance belt 21 is in contact with the photosensitive drum 11 to the drive roller 22 during the contact / separation operation of the conveyance belt 21 with respect to the photosensitive drum 11, the same as above. The slack of the conveyor belt 21 is likely to occur.

  Therefore, in this embodiment, when the conveying belt 21 is brought into contact with or separated from the photosensitive drum 11, the circumferential speed of the conveying belt 21 is increased relative to the circumferential speed of the photosensitive drum 11. Due to this peripheral speed difference, the transfer belt 21 is tensioned between the drive roller 22 and the transfer nip N at the most downstream position (that is, the most downstream portion where the transfer belt 21 is in contact with the photosensitive drum 11 during the switching operation). Occurs and the slack of the conveyor belt 21 is removed. Thereby, the conveyance of the conveyance belt 21 always stable can be realized.

(Experimental example)
Next, Table 1 shows the results of endurance tests in which the peripheral speed difference between the conveyance belt 21 and the photosensitive drum 11 is changed variously and the contact / separation between the conveyance belt 21 and the photosensitive drum 11 is repeated.

  Regarding the peripheral speed difference, the peripheral speed of the photosensitive drum 11 is fixed, and the peripheral speed of the conveyor belt 21 is changed. Two endurance tests were performed when the change was made.

  Further, in the durability test, the number of times that the conveyance belt 21 is applied to the photosensitive drum 11 is equal to the number of times when the full-color image formation and the black single-color image formation are switched every other sheet within the number of passing sheets of the conveyance belt 21 whose life is guaranteed. A contact / separation operation was performed. And the state of the end surface of the conveyance belt 21 after durability was confirmed. The state of the conveyor belt 21 after durability is indicated by three-level evaluation symbols. Regarding the symbols in the table, ◯ indicates no problem, Δ indicates rubbing scratches on the belt end face, but no crack, and X indicates crack on the belt end.

  As can be seen from the results in Table 1, from the viewpoint of extending the life of the conveyor belt 21, the ratio of the peripheral speed difference represented by the above-described formula (1) when the conveyor belt 21 is in contact with / separated from the photosensitive drum 11 C is preferably set to 4% or more.

  Further, it is only necessary that the peripheral speed of the transport belt 21 is relatively higher than the peripheral speed of the photosensitive drum 11, and the peripheral speed of either one or both of the photosensitive drum 11 and the transport belt 21 is changed, so It is sufficient if the ratio can be secured.

  In the present embodiment, the drive motors D1 to D5, the CPU 50, and a driver circuit (not shown) of each drive motor function as speed changing means. That is, in this embodiment, the CPU 50 functions as a control unit for the speed changing unit. Then, the photosensitive drum driving motors D1 to D4 and / or the conveying belt driving motor D5 are controlled according to the contact / separation operation of the conveying belt 21 with respect to the photosensitive drum 11, and the peripheral speed of the conveying belt 21 is set to the photosensitive drum. 11 relatively faster than the peripheral speed.

  Furthermore, the greater the difference in the peripheral speed between the peripheral speed of the photosensitive drum 11 and the peripheral speed of the transport belt 21, the more the damage to the transport belt 21 at the time of contact / separation of the transport belt 21 with respect to the photosensitive drum 11 can be suppressed. However, if this peripheral speed difference is too large, deterioration due to sliding between the surface of the photosensitive drum 11 and the surface of the transport belt 21 cannot be ignored. Therefore, it is desirable that the ratio C of the peripheral speed difference is 200% or less.

  That is, when the contact / separation of the conveyance belt 21 with respect to the photosensitive drum 11 is performed, 4 ≦ C ≦ 200 is satisfied with respect to the ratio C of the peripheral speed difference represented by the above formula (1).

  As described above, according to this embodiment, even if the driving speed of the photosensitive drum 11 or the conveyor belt 21 is temporarily changed due to the load fluctuation when the photosensitive drum 11 and the conveyor belt 21 are brought into contact with or separated from each other. In addition, local slack in the conveyor belt 21 can be greatly suppressed. For this reason, it is possible to always ensure a stable transportability of the transport belt 21 and improve the durability of the transport belt 21. Therefore, in the image forming apparatus 100 provided with a plurality of stations 10, the service life of the stations 10 that are not frequently used (life of the photosensitive drum 11, the developing device 14, or the toner) is extended, and the durability of the transport belt 21 is increased. Can be improved.

  In the present embodiment, the configuration in which the conveyor belt 21 is stretched around three axes has been described, but the number of axes is not limited thereto. For example, in any configuration in which the conveyor belt 21 is stretched on a plurality of axes, such as a configuration stretched on two axes as shown in FIG. 6 and a configuration stretched on four axes as shown in FIG. Embodiments can be applied.

Example 2
Next, another embodiment according to the present invention will be described. The basic configuration of the image forming apparatus of the present embodiment is the same as that of the first embodiment. Therefore, elements having the same or equivalent functions and configurations as those of the first embodiment are denoted by the same reference numerals and detailed description thereof is omitted, and characteristic points of the present embodiment will be described below.

  FIG. 8 shows the contact / separation state of the conveyance belt 21 and the photosensitive drum 11 in the full-color mode in the image forming apparatus 100 of this embodiment, and FIG. 9 shows the contact of the conveyance belt 21 and the photosensitive drum 11 in the black monochrome mode. / Indicates a separated state.

  In this embodiment, the shaft configuration of the conveyor belt 21 is different from that of the first embodiment. That is, in the first exemplary embodiment, the driving roller 22 serving as a driving support among the plurality of supports on which the conveyance belt 21 is stretched is the photosensitive drum 11K on the most downstream side in the conveyance direction of the conveyance belt 21 (that is, the most downstream transfer). It was arranged downstream from the nip N). On the other hand, in this embodiment, the driving roller 22 as a driving support is arranged upstream of the photosensitive drum 11M (that is, the most upstream transfer nip N) in the conveying direction of the conveying belt 21 in the conveying direction. Further, in this embodiment, no other belt stretching roller is provided between the most upstream photosensitive drum 11M and the driving roller 22.

  Next, the relationship between the driving speeds of the photosensitive drum 11 and the conveyance belt 21 in this embodiment will be described below.

  The relationship between the peripheral speed of the transport belt 21 and the peripheral speed of the photosensitive drum 11 in the normal state except when the transport belt 21 contacts / separates from the photosensitive drum 11 is the same as in the first embodiment.

  On the other hand, when the transport belt 21 is brought into contact with or separated from the photosensitive drum 11, in this embodiment, the peripheral speed of the transport belt 21 is set to the peripheral speed of the photosensitive drum 11 from the viewpoint of improving the durability of the transport belt 21. Relatively slow.

  With reference to FIGS. 10 and 11, the state of the transport belt 21 immediately after the contact / separation operation of the transport belt 21 with respect to the photosensitive drum 11 when the photosensitive drum and the transport belt are driven at a constant speed will be described. FIG. 10 shows a state of the transport belt 21 immediately after the state of contact / separation of the transport belt 21 with respect to the photosensitive drum 11 in the black monochrome mode (FIG. 9) is changed to the state in the full color mode (FIG. 8). On the other hand, FIG. 11 shows the state of the transport belt 21 immediately after the contact / separation state of the transport belt 21 with respect to the photosensitive drum 11 in the full color mode (FIG. 8) is changed to the state in the black monochrome mode (FIG. 9). .

  In the image forming apparatus 100 according to the present exemplary embodiment, when the contact / separation state is switched between the state in the full color mode and the state in the single color mode, the black station 10K and the transport belt 21 with respect to the photosensitive drum 11 are switched. Maintain contact.

  When the conveyance belt 21 is switched from the state shown in FIG. 8 to the state shown in FIG. 9, or conversely, from the state shown in FIG. 9 to the state shown in FIG. 8, the photosensitive drum drive motors D1, D2, D3, D4 and the load torque to the conveyor belt drive motor D5 may fluctuate. Thereby, the drive speed of each drive motor D1-D5 tends to become unstable temporarily. At this time, if the photosensitive drum 11 and the conveyor belt 21 are set to be driven at the same peripheral speed, the peripheral speed of the conveyor belt 21 is set to the peripheral speed of the photosensitive drum 11 due to the speed fluctuation of the drive motors D1 to D5. It becomes relatively fast and slow.

  Here, consider a case where the peripheral speed of the conveyor belt 21 is relatively higher than the peripheral speed of the photosensitive drum 11 due to the speed fluctuations of the drive motors D1 to D5 as described above. In this case, due to the speed difference between them, the conveyor belt 21 is likely to be slack in the section from the driving roller 22 to the most upstream transfer nip N in the moving direction of the conveyor belt 21. Due to this slackness, the deviation of the conveyor belt 21 is deteriorated, and the end surface of the conveyor belt 21 is easily damaged by rubbing with other members.

  Note that the tension roller 24 passes through the tension roller 24 and the driven roller 23 upstream of the driving roller 22 in the moving direction of the conveying belt 21 and reaches the most upstream transfer nip N in the moving direction of the conveying belt 21. This makes it easier to eliminate slack. For this reason, the deviation of the conveyor belt 21 hardly occurs in this section.

  Here, as a typical example, the state of the conveyance belt 21 when switching between the state illustrated in FIG. 8 and the state illustrated in FIG. 9 has been described. However, the same problem may occur in other modes than this switching mode. When the driving roller 22 is located upstream of the most upstream photosensitive drum 11 in the moving direction of the conveying belt 21, if the peripheral speed of the conveying belt 21 is relatively higher than the peripheral speed of the photosensitive drum 11 due to speed fluctuation of the driving motor. The same problem can occur. That is, in the section from the upstream side where the conveying belt 21 is in contact with the photosensitive drum 11 to the driving roller 22 during the contact / separation operation of the conveying belt 21 with respect to the photosensitive drum 11, the same as above. The slack of the conveyor belt 21 is likely to occur.

  Therefore, in this embodiment, when the conveying belt 21 is brought into contact with or separated from the photosensitive drum 11, the circumferential speed of the conveying belt 21 is relatively slow with respect to the circumferential speed of the photosensitive drum 11. Due to this peripheral speed difference, the most upstream portion where the conveying belt 21 is in contact with the photosensitive drum 11 during the switching operation (the most upstream transfer nip N in the state shown in FIG. 8; the most downstream transfer in the state shown in FIG. 9). Tension is generated between the nip N) and the driving roller 22, and the slack of the conveying belt 21 is removed. Thereby, the conveyance of the conveyance belt 21 always stable can be realized.

(Experimental example)
Next, Table 2 shows the results of an endurance test in which the peripheral speed difference between the conveyance belt 21 and the photosensitive drum 11 is changed variously and the contact / separation between the conveyance belt 21 and the photosensitive drum 11 is repeated.

  Regarding the peripheral speed difference, the peripheral speed of the photosensitive drum 11 is fixed, and the peripheral speed of the conveyor belt 21 is changed. Two endurance tests were performed when the change was made.

  Further, in the durability test, the number of times that the conveyance belt 21 is applied to the photosensitive drum 11 is equal to the number of times when the full-color image formation and the black single-color image formation are switched every other sheet within the number of passing sheets of the conveyance belt 21 whose life is guaranteed. A contact / separation operation was performed. And the state of the end surface of the conveyance belt 21 after durability was confirmed. The state of the conveyor belt 21 after durability is indicated by three-level evaluation symbols. Regarding the symbols in the table, ◯ indicates no problem, Δ indicates rubbing scratches on the belt end face, but no crack, and X indicates crack on the belt end.

  As can be seen from the results in Table 2, from the viewpoint of extending the life of the conveyor belt 21, the ratio of the peripheral speed difference represented by the above-described formula (1) when the conveyor belt 21 is in contact with / separated from the photosensitive drum 11 C is preferably set to -4% or less.

  Further, it is only necessary that the peripheral speed of the transport belt 21 is relatively slower than the peripheral speed of the photosensitive drum 11, and the peripheral speed of one or both of the photosensitive drum 11 and the transport belt 21 is changed, and the above-described peripheral speed is changed. It suffices if the difference ratio can be secured.

  In the present embodiment, as in the first embodiment, the drive motors D1 to D5, the CPU 50, and a driver circuit (not shown) for each drive motor function as speed changing means. That is, in this embodiment, the CPU 50 functions as a control unit for the speed changing unit. Then, the photosensitive drum driving motors D1 to D4 and / or the conveying belt driving motor D5 are controlled according to the contact / separation operation of the conveying belt 21 with respect to the photosensitive drum 11, and the peripheral speed of the conveying belt 21 is set to the photosensitive drum. 11 relatively faster than the peripheral speed.

  Furthermore, the greater the difference in the peripheral speed between the peripheral speed of the photosensitive drum 11 and the peripheral speed of the transport belt 21, the more the damage to the transport belt 21 at the time of contact / separation of the transport belt 21 with respect to the photosensitive drum 11 can be suppressed. However, if this peripheral speed difference is too large, deterioration due to rubbing between the surface of the photosensitive drum 11 and the surface of the conveyor belt 21 cannot be ignored. Therefore, the peripheral speed difference ratio C is desirably set to -200% or more.

  That is, when the contact / separation of the transport belt 21 with respect to the photosensitive drum 11 is performed, −200 ≦ C ≦ −4 is satisfied with respect to the peripheral speed difference ratio C expressed by the above-described formula (1).

  As described above, according to the present embodiment, as in the first embodiment, local slackening due to load fluctuations when the photosensitive drum 11 and the transport belt 21 are brought into contact / separated is greatly suppressed. can do. That is, even if the driving speed of the photosensitive drum 11 or the conveyance belt 21 is temporarily changed, it is possible to greatly suppress the occurrence of local slack in the conveyance belt 21. For this reason, it is possible to always ensure a stable transportability of the transport belt 21 and improve the durability of the transport belt 21.

  In the present embodiment, the configuration in which the conveyor belt 21 is stretched around three axes has been described. However, as described in the first embodiment, an arbitrary configuration in which the conveyor belt 21 is stretched around a plurality of axes is described. In addition, this embodiment can be applied.

As can be seen from the description of the first and second embodiments, assuming that the peripheral speed of the photosensitive drum 11 is A and the peripheral speed of the conveyor belt 21 is B, the photosensitive drum 11 and the conveyor belt 21 come into contact with each other and the transfer nip. At the time of switching between the first state in which the photosensitive drum 11 and the conveying belt 21 are separated from each other,
(| A−B | / A) × 100 (%) ≧ 4
Is preferably satisfied. Also, usually
(| A-B | / A) × 100 (%) ≦ 200
Is desirable.

  With such a configuration, it is possible to always ensure stable transportability of the transport belt 21 and to improve the durability of the transport belt 21 more effectively.

Example 3
Next, still another embodiment according to the present invention will be described. In this embodiment, the image forming apparatus employs an intermediate transfer method. The present invention is not limited to the direct transfer type image forming apparatus as in the first and second embodiments, but can also be applied to an intermediate transfer type image forming apparatus as in this embodiment.

  FIG. 12 shows a schematic cross section of the main part of the image forming apparatus 200 of this embodiment. Members having functions corresponding to those of the image forming apparatus 100 shown in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  The image forming apparatus 200 according to the present exemplary embodiment includes an intermediate transfer belt 61 that is an endless belt-shaped intermediate transfer member as an image carrier. The intermediate transfer belt 61 is stretched around three axes of the drive roller 22, the tension roller 24, and the secondary transfer counter roller 62. In the image forming apparatus 200 of this embodiment, a primary transfer roller 15 as a primary transfer unit corresponding to the transfer roller 15 in the first embodiment is provided on the inner peripheral side of the intermediate transfer belt 61. The intermediate transfer belt 61 contacts the photosensitive drum 11 at the position of the primary transfer roller 15 to form a primary transfer nip (primary transfer position) N1. Each primary transfer roller 15 is movable in a direction in which the primary transfer roller 15 contacts or separates from the photosensitive drum 11 in the same manner as in the first embodiment. A secondary transfer roller 63 as a secondary transfer unit is disposed so as to contact the secondary transfer counter roller 62. A contact portion between the secondary transfer roller 63 and the intermediate transfer belt 61 forms a secondary transfer nip (secondary transfer position) N2. Further, a belt cleaning device 64 for cleaning the toner remaining on the intermediate transfer belt 61 after the secondary transfer process is provided.

  Next, an image forming process in the image forming apparatus 200 of this embodiment will be described.

  When an image formation start signal is input to the CPU 50, the rotation of the intermediate transfer belt 61 and the photosensitive drums 11 of the first to fourth stations 10M, 10C, 10Y, and 10K is started.

  In the full color mode, the primary transfer roller 15 contacts the photosensitive drum 11 via the intermediate transfer belt 61 in all the stations 10M, 10C, 10Y, and 10K. On the other hand, in the black monochrome mode, the primary transfer roller 15 is separated from the photosensitive drum 11 in the magenta, cyan, and yellow stations 10M, 10C, and 10Y. Only the primary transfer roller 15K of the black station 10K contacts the photosensitive drum 11K via the intermediate transfer belt 61.

  Thereafter, the surface of the photosensitive drum 11 is charged to a desired potential (negative polarity in this embodiment) by the charging roller 12. Image exposure based on image information is performed on the surface of the charged photosensitive drum 11 by the exposure device 13. As a result, an electrostatic image (latent image) is formed on the photosensitive drum 11. Next, the electrostatic image formed on the photosensitive drum 11 is developed by the developing device 14 with toner (toner having negative charging characteristics in this embodiment). As a result, a toner image is formed on the photosensitive drum 11.

  The toner image formed on the photosensitive drum 11 is electrostatically transferred to the intermediate transfer belt 61 by the action of the primary transfer roller 15. At this time, a predetermined voltage (positive voltage in this embodiment) is applied to the primary transfer roller 15 from a primary transfer power source as a primary transfer bias output unit.

  In the full color mode, the above-described charging, exposure, development, and transfer processes are similarly repeated in the stations 10M, 10C, 10Y, and 10K for each color. The magenta toner image, the cyan toner image, the yellow toner image, and the black toner image respectively formed on the photosensitive drums 11 of the stations 10M, 10C, 10Y, and 10K are sequentially superimposed on the intermediate transfer belt 61 and subjected to primary transfer. Is done. Next, the toner images of a plurality of colors on the intermediate transfer belt 61 are secondarily transferred collectively to the transfer material P fed at a predetermined timing by the action of the secondary transfer roller 63. At this time, a predetermined voltage (positive voltage in this embodiment) is applied to the secondary transfer roller 63 by a secondary transfer power source as a secondary transfer bias output unit. Thereafter, the transfer material P is conveyed to the fixing device 30. The toner image on the transfer material P is heated and pressed by the fixing device 30 and fixed on the transfer material P. The transfer material P on which the toner image is fixed is then discharged out of the apparatus, and a series of image forming steps is completed.

  On the other hand, in the black monochrome mode, the above-described charging, exposure, development, and transfer processes are performed only in the black station 10K. The black toner image is primarily transferred onto the intermediate transfer belt 61 and then secondarily transferred onto the transfer material P. Thereafter, as in the full color mode, the toner image is fixed on the transfer material P, and the transfer material P is discharged out of the apparatus. In the black monochrome mode, after the contact / separation operation of the intermediate transfer belt 61 and the primary transfer roller 16 with respect to the photosensitive drum 11, the respective members of the magenta, cyan, and yellow stations 10M, 10C, and 10Y are driven. Stop.

  Incidentally, after the toner image is primarily transferred from the photosensitive drum 11 to the intermediate transfer belt 61, the toner remaining on the photosensitive drum 11 is removed from the photosensitive drum 11 by the cleaning device 16, and a waste toner container attached to the cleaning device 16 is attached. To be recovered. Further, the toner remaining on the intermediate transfer belt 61 after the secondary transfer process is collected by the belt cleaning device 64.

  Similar to the image forming apparatus 100 of the first embodiment, the image forming apparatus 200 of the present embodiment has a full color mode and a black single color mode as image forming modes, and an intermediate transfer belt for the photosensitive drum 11 according to each mode. A contact / separation operation 61 is performed.

  In FIG. 12, the driving roller 22 is disposed on the downstream side of the photosensitive drum 11 </ b> K on the most downstream side in the conveyance direction of the intermediate transfer belt 61 (that is, the primary transfer nip N <b> 1 on the most downstream side). On the other hand, as in the image forming apparatus 100 of the second embodiment, the driving roller 22 as a driving support is the uppermost photosensitive drum 11M in the conveying direction of the conveying belt 21 (that is, the uppermost primary transfer nip N1). You may arrange | position more upstream.

  Also in this embodiment, as in the first or second embodiment, when the intermediate transfer belt 61 is brought into contact with or separated from the photosensitive drum 11, the peripheral speed of the intermediate transfer belt 61 and the peripheral speed of the photosensitive drum 11 are determined. A peripheral speed difference is provided. At this time, similarly to the first or second embodiment, when the driving roller 22 as a driving support is arranged downstream of the most downstream primary transfer nip N1 in the moving direction of the intermediate transfer belt 61, the intermediate transfer belt. 61 is driven relatively faster than the photosensitive drum 11. On the other hand, when the driving roller 22 as a driving support is disposed upstream of the most upstream primary transfer nip N <b> 1 in the moving direction of the intermediate transfer belt 61, the intermediate transfer belt 61 is relative to the photosensitive drum 11. Driven late.

  As described above, the present invention suitably operates also in the image forming apparatus 200 of the intermediate transfer type, and can always ensure stable transportability of the intermediate transfer belt 61 and improve the durability of the intermediate transfer belt 61.

  In this embodiment, the configuration in which the intermediate transfer belt 61 is stretched around three axes has been described. However, the number of axes is not limited to this, and the intermediate transfer belt 61 is stretched around a plurality of axes. This embodiment can be applied.

1 is a schematic cross-sectional view of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a schematic cross-sectional view of a main part illustrating a contact / separation state of a conveyance belt and a photosensitive drum in a full color mode in the image forming apparatus of FIG. 1. FIG. 2 is a schematic cross-sectional view of a main part illustrating a contact / separation state of a conveyance belt and a photosensitive drum in a black monochrome mode in the image forming apparatus of FIG. 1. FIG. 6 is a schematic cross-sectional view of a main part showing a conveyance state of the conveyance belt after the contact / separation operation between the conveyance belt and the photosensitive drum when the photosensitive drum and the conveyance belt are driven at a constant speed. FIG. 6 is a schematic cross-sectional view of a main part showing a conveyance state of the conveyance belt after the contact / separation operation between the conveyance belt and the photosensitive drum when the photosensitive drum and the conveyance belt are driven at a constant speed. It is a cross-sectional schematic diagram of a biaxially stretched conveyor belt to which the present invention can be applied. It is a cross-sectional schematic diagram of the conveyance belt of the 4-axis stretch which can apply this invention. FIG. 10 is a schematic cross-sectional view of a main part illustrating a contact / separation state of a conveyance belt and a photosensitive drum in a full color mode in another embodiment of the image forming apparatus according to the present invention. FIG. 10 is a schematic cross-sectional view of a main part illustrating a contact / separation state of a conveyance belt and a photosensitive drum in a black monochrome mode in another embodiment of the image forming apparatus according to the present invention. FIG. 6 is a schematic cross-sectional view of a main part showing a conveyance state of the conveyance belt after the contact / separation operation between the conveyance belt and the photosensitive drum when the photosensitive drum and the conveyance belt are driven at a constant speed. FIG. 6 is a schematic cross-sectional view of a main part showing a conveyance state of the conveyance belt after the contact / separation operation between the conveyance belt and the photosensitive drum when the photosensitive drum and the conveyance belt are driven at a constant speed. It is a cross-sectional schematic diagram of another embodiment of the image forming apparatus according to the present invention.

Explanation of symbols

11 Photosensitive drum (image carrier)
12 Charging roller (charging means)
13 Exposure device (exposure means)
14 Developing device (Developing means)
15 Cleaning device (cleaning means)
16 Transfer roller (transfer means)
21 Conveyor belt (transfer material carrier, image carrier)
22 Drive roller (drive support)
30 Fixing device (fixing means)
61 Intermediate transfer belt (intermediate transfer member, image carrier)
63 Secondary transfer roller (secondary transfer means)
D1-D5 drive motor S1-S5 contact / separation mechanism

Claims (8)

A plurality of image carriers that can rotate and carry toner images; a belt that transfers toner images on the plurality of image carriers to a transfer material; and a plurality of supports that stretch the belts. A driving support for rotating the belt, a tension applying member for applying tension to the belt, and a movement of the image carrier. A first control unit that controls a speed and a moving speed of the belt, and performs image formation in a first state in which all of the plurality of image carriers are in contact with the belt to form a plurality of transfer nips. And a second mode in which image formation is performed in a second state in which only one of the plurality of image carriers is in contact with the belt to form one transfer nip. It is possible to execute the first mode and the second mode. In the image forming apparatus are each in rotational movement and the belt image-bearing member to form the transfer nip when changing Ri,
The drive support is disposed downstream of the transfer nip formed when the second mode is executed in the rotational movement direction of the belt, and the tension applying member is disposed in the rotational movement direction of the belt. It is disposed downstream of the drive support and upstream of the transfer nip formed when the second mode is executed, and the belt is in the second mode in the rotation direction of the belt. Is not stretched by the other support body between the drive support body from the transfer nip formed when performing
When switching according to the mode for executing the first state and the second state, the control means controls the moving speed of the belt to be faster than the moving speed of the image carrier, and The moving speed of the image carrier is A, and the moving speed of the belt is B.
(| A−B | / A) × 100 (%) ≧ 4
An image forming apparatus that is controlled to satisfy the above. When performing image formation by executing the first mode or the second mode, the control means
(| A-B | / A) × 100 (%) ≦ 3
The image forming apparatus according to claim 1, wherein control is performed so that   The image forming apparatus according to claim 1, wherein the belt is a conveyance belt that carries and conveys a transfer material.   The image forming apparatus according to claim 1, wherein the belt is an intermediate transfer belt to which a toner image is transferred from the image carrier. A plurality of image carriers that can rotate and carry toner images; a belt that transfers toner images on the plurality of image carriers to a transfer material; and a plurality of supports that stretch the belts. A driving support for rotating the belt, a tension applying member for applying tension to the belt, and a movement of the image carrier. A first control unit that controls a speed and a moving speed of the belt, and performs image formation in a first state in which all of the plurality of image carriers are in contact with the belt to form a plurality of transfer nips. And a second mode in which image formation is performed in a second state in which only one of the plurality of image carriers is in contact with the belt to form one transfer nip. It is possible to execute the first mode and the second mode. In the image forming apparatus are each in rotational movement and the belt image-bearing member to form the transfer nip when changing Ri,
The drive support is disposed upstream of the transfer nip formed on the most upstream side when the first mode is executed in the rotational movement direction of the belt, and the tension applying member is rotated by the belt. The belt is disposed downstream of the transfer nip formed when the second mode is executed in the movement direction and upstream of the drive support, and the belt is arranged in the rotational movement direction of the belt. It is not stretched by the other support body between the transfer nip formed when performing the second mode from the drive support body,
When switching according to the mode for executing the first state and the second state, the control means controls the moving speed of the belt to be slower than the moving speed of the image carrier; and The moving speed of the image carrier is A, and the moving speed of the belt is B.
(| A−B | / A) × 100 (%) ≧ 4
An image forming apparatus that is controlled to satisfy the above. When performing image formation by executing the first mode or the second mode, the control means
(| A-B | / A) × 100 (%) ≦ 3
The image forming apparatus according to claim 5, wherein control is performed so that   The image forming apparatus according to claim 5, wherein the belt is a conveyance belt that carries and conveys a transfer material.   The image forming apparatus according to claim 5, wherein the belt is an intermediate transfer belt to which a toner image is transferred from the image carrier.

Images