JP2016091012A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of suppressing squeaking and curling caused by a cleaning blade, with a simple apparatus configuration, in an image forming apparatus capable of forming a color image.SOLUTION: A printer 100 includes a conveyance belt 7 and a plurality of process parts 50K, 50Y, 50M, and 50C forming a toner image. In such a state that the absolute value of the developing voltage of the developing device 4 of the process part 50K located at the most upstream is made higher than that in sheet image formation, a pattern image is formed. The pattern image is transferred onto the conveyance belt 7. Further, when the pattern image passes through the process parts 50Y, 50M, and 50C located on the downstream side from the process part 50K, a transfer voltage or a transfer current having the same polarity as that in the sheet image formation is applied.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus that forms an image by electrophotography. More specifically, the present invention relates to a technology of an image forming apparatus that includes a plurality of photoconductors and can form a color image.

  2. Description of the Related Art Conventionally, in an image forming apparatus having a plurality of photoconductors and capable of forming a color image, a cleaning blade is brought into contact with each photoconductor, and toner remaining on the surface of each photoconductor is scraped off from the surface of each photoconductor. Technology is known. In addition, the toner scraped off from the surface of the photoconductor by the cleaning blade stays at the contact point between the cleaning blade and the photoconductor, reducing the friction generated between the cleaning blade and the photoconductor, and reducing noise and turning. It is known to mitigate.

  As a document disclosing an image forming apparatus provided with a cleaning blade, there is, for example, Patent Document 1. In Patent Document 1, an image forming apparatus in which a cleaning blade is brought into contact with each photoconductor, and an image forming unit corresponding to black forms a pattern image and transfers it to an intermediate transfer belt at a timing other than the printing process, A configuration is disclosed in which an image forming unit corresponding to a color other than black reversely transfers toner of a pattern image from an intermediate transfer belt by a transfer bias having a polarity opposite to that of a printing process.

JP 2012-123357 A

  In an image forming apparatus including a plurality of photoconductors corresponding to each color, when printing is repeated only in a specific image forming unit, toner is supplied onto the photoconductors of the image forming units other than the specific image forming unit for a long period of time. Not. For this reason, in the photoconductor included in the image forming unit other than the specific image forming unit, there is a high possibility that the toner staying at the contact point with the cleaning blade is insufficient, and the noise and turning are generated.

  As in the technique of Patent Document 1 described above, black toner is transferred onto a photoreceptor corresponding to a color other than black by a transfer bias having a polarity opposite to that of the printing process, thereby developing corresponding to a color other than black. The toner can be supplied to the photoconductor corresponding to the color other than black, even if the toner of the color other than black is empty even if the unit is not operated. However, a configuration for applying a transfer bias of reverse polarity is required, and the configuration of the transfer portion becomes complicated.

  The present invention has been made to solve the above-described problems of the prior art. That is, an object of the present invention is to provide an image forming apparatus capable of forming a color image and capable of suppressing noise and turning caused by a cleaning blade with a simple apparatus configuration.

  An image forming apparatus designed to solve this problem includes a belt, a photoconductor, a developing unit that supplies toner to the photoconductor, and a transfer that transfers the toner supplied onto the photoconductor onto the belt. A plurality of image forming units, and a control unit, wherein each of the plurality of image forming units is predetermined in the toner conveyance direction on the belt. The control unit uses a developing unit of a predetermined first image forming unit among the plurality of image forming units to form an image to be printed on a non-sheet. At the time of image formation, a forming process for forming a pattern image on the photoreceptor of the first image forming unit and an absolute value of the developing voltage of the developing unit of the first image forming unit at the time of the forming process are printed on the sheet. The image forming image Development adjustment processing for making the development voltage higher than the absolute value of the development voltage at the time of image formation, and the pattern image formed on the photoreceptor of the first image forming unit using the transfer unit of the first image forming unit , When the first transfer process to be transferred onto the belt and the pattern image conveyed by the belt pass through the transfer part of the second image forming part which is an image forming part different from the first image forming part, And a second transfer process in which a transfer voltage or a transfer current having the same polarity as that at the time of the sheet image formation is applied using the transfer unit of the second image forming unit.

  When the absolute value of the developing voltage is high, the toner easily moves from the developing unit to the photoreceptor, and the amount of toner per unit area increases. When the first image forming unit forms a pattern image with a large amount of toner per unit area, the thickness of the toner layer of the pattern image transferred onto the belt increases. The thicker the toner layer, the smaller the capacitance and the more toner charge. Therefore, in the image forming apparatus disclosed in this specification, after the pattern image is transferred to the belt, the potential difference between the front surface and the back surface of the toner layer of the pattern image tends to increase. In this state, when the pattern image is conveyed to the transfer unit of the second image forming unit and a transfer voltage or transfer current having the same polarity as that of the sheet image formation is applied to the transfer unit of the second image forming unit, the charge of the toner As the amount increases further, the potential difference increases. When the potential difference exceeds a limit value that causes discharge, discharge occurs inside the toner layer, and a part of the toner in the toner layer is reversely charged by the discharge. This reversely charged toner is transferred to the photoreceptor of the second image forming unit. That is, the toner of the first image forming unit is reversely transferred to the photoconductor of the second image forming unit by the transfer voltage or transfer current having the same polarity as that at the time of sheet image formation, and the friction generated between the blade and the photoconductor is reduced. Will be reduced.

  The present specification also includes a belt, a photoconductor, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied on the photoconductor onto the belt, and the photoconductor A plurality of image forming units each including a blade that contacts the surface of the belt, and a control unit, wherein the plurality of image forming units are arranged at predetermined intervals in the toner conveyance direction on the belt. The control unit uses a developing unit of a predetermined first image forming unit among the plurality of image forming units to form a pattern image during non-sheet image formation when an image to be printed on a sheet is not formed. And forming the pattern image formed on the photosensitive member of the first image forming unit using the transfer unit of the first image forming unit, A first transfer process for transferring onto the belt; A transfer adjustment process in which an absolute value of a transfer voltage or a transfer current of the transfer unit of the first image forming unit at the time of the first transfer process is larger than that at the time of sheet image formation for forming an image printed on a sheet; When the pattern image conveyed by the belt passes through a transfer portion of the second image forming portion which is an image forming portion different from the first image forming portion, the transfer portion of the second image forming portion is used to An image forming apparatus is disclosed that performs a second transfer process in which a transfer voltage or a transfer current having the same polarity as that at the time of sheet image formation is applied.

  As described above, when a pattern image having a large amount of toner per unit area is formed, a potential difference between the front surface and the back surface of the toner layer of the pattern image is likely to increase, and a discharge of the toner layer causes a part of the toner layer. Part of the toner is reversely charged. This reversely charged toner is transferred to the photoreceptor of the second image forming unit. Furthermore, as the charge amount of the toner increases, the repulsive force between the toners charged to the same polarity also increases. A part of the toner is transferred to the photosensitive member by the repulsive force between the toners. In other words, the reverse transfer or the repulsive force between the toners causes the toner in the first image forming unit to move to the photoconductor in the second image forming unit, thereby reducing the friction generated between the blade and the photoconductor.

  The present specification also includes a belt, a photoconductor, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied on the photoconductor onto the belt, and the photoconductor A plurality of image forming units each including a blade that contacts the surface of the belt, and a control unit, wherein the plurality of image forming units are arranged at predetermined intervals in the toner conveyance direction on the belt. The control unit uses a developing unit of a predetermined first image forming unit among the plurality of image forming units to form a pattern image during non-sheet image formation when an image to be printed on a sheet is not formed. Is formed on the photosensitive member of the first image forming unit, and the rotation speed ratio of the developing unit of the first image forming unit with respect to the conveying speed of the belt during the forming process is printed on the sheet. Sheet forming image Speed adjustment processing for making the rotational speed ratio larger than that at the time of image formation, and the pattern image formed on the photoconductor of the first image forming unit using the transfer unit of the first image forming unit, the belt When the pattern image conveyed by the first transfer process transferred onto the belt and the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, the second image forming unit An image forming apparatus is disclosed in which a transfer portion of the image forming portion is used to execute a second transfer process in which a transfer voltage or a transfer current having the same polarity as that in the sheet image formation is applied.

  As in the image forming apparatus described above, a pattern image having a large amount of toner per unit area can also be formed by increasing the rotation speed ratio of the developing unit of the first image forming unit with respect to the belt. Therefore, even in the above-described image forming apparatus, the toner of the first image forming unit is reversely transferred to the photosensitive member of the second image forming unit by the transfer voltage or transfer current having the same polarity as that at the time of sheet image formation. As a result, the friction generated between the blade and the photoconductor is reduced.

  Further, the present specification includes a belt, a photoconductor, a charging unit that charges the surface of the photoconductor, a developing unit that supplies toner to the photoconductor, and the toner supplied on the photoconductor. An image forming unit including a transfer unit to be transferred onto a belt and a blade that contacts the surface of the photoconductor, the image forming unit including a plurality of the image forming units and a control unit, and the plurality of image forming units Are arranged at predetermined intervals in the toner conveying direction on the belt, and the control unit uses a developing unit of a predetermined first image forming unit among the plurality of image forming units. , When forming a non-sheet image without forming an image to be printed on a sheet, a forming process for forming a pattern image on the photoreceptor of the first image forming unit, and a first image forming unit at the time of the forming process. The absolute value of the development voltage of the development unit Development adjustment processing for making the image higher than the absolute value of the development voltage at the time of sheet image formation for forming an image to be printed on the first image, and the pattern image formed on the photoconductor of the first image forming unit. A first transfer process for transferring onto the belt using a transfer unit of the image forming unit, and the pattern image conveyed by the belt is an image forming unit different from the first image forming unit When passing through the transfer section of the second image forming section, no transfer voltage or transfer current is applied to the transfer section of the second image forming section, and the photoconductor of the second image forming section is used at the time of sheet image formation. An image forming apparatus is also disclosed which is in a weakly charged state, which is a weakly charged state.

  As described above, when a pattern image having a large amount of toner per unit area is formed, the toner layer of the pattern image transferred onto the belt is thick, and the potential difference between the front and back surfaces of the toner layer of the pattern image is large. Tends to be large. In this state, when the pattern image is conveyed to the transfer unit of the second image forming unit, no transfer voltage or transfer current is applied to the transfer unit and the photoconductor is weakly charged. The surface potential of the toner layer becomes higher than the surface potential. For this reason, a part of the toner is transferred to the photoreceptor of the second image forming unit. That is, the toner is reversely transferred without using a transfer voltage or transfer current having a polarity opposite to that at the time of sheet image formation, and the friction generated between the blade and the photoconductor is reduced.

  Further, the present specification includes a belt, a photoconductor, a charging unit that charges the surface of the photoconductor, a developing unit that supplies toner to the photoconductor, and the toner supplied on the photoconductor. An image forming unit including a transfer unit to be transferred onto a belt and a blade that contacts the surface of the photoconductor, the image forming unit including a plurality of the image forming units and a control unit, and the plurality of image forming units Are arranged at predetermined intervals in the toner conveying direction on the belt, and the control unit uses a developing unit of a predetermined first image forming unit among the plurality of image forming units. Forming a pattern image on the photosensitive member of the first image forming unit and forming the pattern image on the photosensitive member of the first image forming unit when forming a non-sheet image without forming an image to be printed on the sheet; The pattern image that has been processed is the first image. The first transfer process to be transferred onto the belt using the transfer part of the forming part, and the absolute value of the transfer voltage or transfer current of the transfer part of the first image forming part at the time of the first transfer process to the sheet A transfer adjustment process for making the image to be printed larger than when forming a sheet image, and the pattern image conveyed by the belt is a second image forming unit different from the first image forming unit. When passing through the transfer unit of the image forming unit, no transfer voltage or transfer current is applied to the transfer unit of the second image forming unit, and the photosensitive member of the second image forming unit is weaker than that at the time of sheet image formation. An image forming apparatus is disclosed that is in a weakly charged state that is a charged state.

  Further, the present specification includes a belt, a photoconductor, a charging unit that charges the surface of the photoconductor, a developing unit that supplies toner to the photoconductor, and the toner supplied on the photoconductor. An image forming unit including a transfer unit to be transferred onto a belt and a blade that contacts the surface of the photoconductor, the image forming unit including a plurality of the image forming units and a control unit, and the plurality of image forming units Are arranged at predetermined intervals in the toner conveying direction on the belt, and the control unit uses a developing unit of a predetermined first image forming unit among the plurality of image forming units. , When forming a non-sheet image without forming an image to be printed on a sheet, a forming process for forming a pattern image on the photosensitive body of the first image forming unit, and the belt conveyance speed at the time of the forming process Current status of the first image forming unit Speed adjustment processing for making the rotation speed ratio of the portion larger than the rotation speed ratio at the time of sheet image formation for forming an image printed on the sheet, and the pattern image formed on the photosensitive member of the first image forming portion Is transferred to the belt using the transfer unit of the first image forming unit, and the pattern image conveyed by the belt is different from that of the first image forming unit. When passing through the transfer section of the second image forming section which is an image forming section, no transfer voltage or transfer current is applied to the transfer section of the second image forming section, and the photoconductor of the second image forming section is An image forming apparatus is disclosed that is in a weakly charged state that is weaker than that in sheet image formation.

  A control method for realizing the functions of the image forming apparatus, a computer program, and a computer-readable storage medium storing the computer program are also novel and useful.

  According to the present invention, an image forming apparatus capable of forming a color image is realized with a simple apparatus configuration capable of suppressing noise and turning caused by a cleaning blade.

1 is a diagram illustrating a schematic configuration of a printer according to an embodiment. 1 is a block diagram illustrating an electrical configuration of a printer according to an embodiment. It is a figure which shows the outline | summary of the reverse transfer operation | movement of the printer concerning embodiment. It is a figure which shows the list of control of each apparatus of 1st reverse transfer operation | movement. It is a time chart which shows the output of each apparatus of 1st reverse transfer operation | movement. It is a figure which shows the list of control of each apparatus of 2nd reverse transfer operation | movement. It is a time chart which shows the output of each apparatus of 2nd reverse transfer operation | movement. It is a figure which shows the list of control of each apparatus of 3rd reverse transfer operation | movement. It is a time chart which shows the output of each apparatus of the 3rd reverse transfer operation. It is a figure which shows the list of control of each apparatus of 4th reverse transfer operation | movement. It is a time chart which shows the output of each apparatus of the 4th reverse transfer operation. FIG. 10 is a block diagram illustrating an electrical configuration of a printer according to a fourth reverse transfer operation. It is a figure which shows the list of control of each apparatus of 5th reverse transfer operation | movement. It is a time chart which shows the output of each apparatus of the 5th reverse transfer operation. FIG. 10 is a block diagram illustrating an electrical configuration of a printer according to a fifth reverse transfer operation. It is a flowchart which shows the procedure of the job execution process concerning a 1st form. It is a flowchart which shows the procedure of the job execution process concerning a 2nd form.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of an image forming apparatus according to the present invention will be described below in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to an electrophotographic printer capable of color printing.

  As shown in FIG. 1, the printer 100 according to the embodiment forms an image, prints the image on a sheet, a cover 15 that covers the print unit 10, and a sheet that is positioned at the bottom before printing. A paper feed tray 91 and a paper discharge tray 92 which is located on the upper surface and which stores printed sheets. The printer 100 is an example of an image forming apparatus.

  The printing unit 10 includes a process unit 50 that forms a toner image by electrophotography, an exposure device 53 that irradiates the process unit 50 with light, a fixing device 8 that fixes unfixed toner on a sheet, and a process unit 50. A transfer belt 7 for transferring the toner image formed by the transfer belt; a secondary transfer device 55 for transferring the toner image transferred by the transfer belt 7 to the sheet; and a position upstream of the secondary transfer device 55 in the sheet transfer direction. And a sheet sensor 12 that outputs different signals depending on the presence or absence of a sheet.

  In the printer 100, a sheet stored in a paper feed tray 91 located at the bottom passes through a paper feed roller 71, a registration roller 72, a secondary transfer device 55, and a fixing device 8, and passes through a paper discharge roller 76. The transport path 11 (a chain line in FIG. 1) is provided so as to be guided to the upper discharge tray 92.

  The process unit 50 can form a color image, and process units corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (K) are arranged in parallel. Specifically, a process unit 50C that forms an image of C color, a process unit 50M that forms an image of M color, a process unit 50Y that forms an image of Y color, and a process unit that forms an image of K color 50K. The toner images transferred to the sheet are arranged at equal intervals in the order of the process units 50K, 50Y, 50M, and 50C from the upstream side in the conveyance direction on the conveyance belt 7. The process units 50K, 50Y, 50M, and 50C are examples of image forming units.

  The process unit 50K includes a drum-shaped photosensitive member 1, a charging device 2 that uniformly charges the surface of the photosensitive member 1, and a developing device 4 that develops the electrostatic latent image on the photosensitive member 1 with toner. , A transfer device 5 for transferring the toner image on the photosensitive member 1 to the conveying belt 7, a cleaning blade 6 in contact with the surface of the photosensitive member 1, and an LED eraser 61 for removing unnecessary charges on the photosensitive member 1. doing. Further, the developing device 4 includes a toner tank 412 that stores toner, and a developing roller 411 that supplies toner in the toner tank 412 to the photosensitive member 1. In the toner tank 412 of this embodiment, toner having a positive charge polarity is accommodated. The photoreceptor 1 and the transfer device 5 are disposed in contact with the transport belt 7. The photosensitive member 1 is opposed to the transfer device 5 with the conveyance belt 7 interposed therebetween. The other process units 50C, 50M, and 50Y have the same configuration as the process unit 50K.

  Further, each of the process units 50K, 50Y, 50M, and 50C of the printer 100 can move the developing roller 411 to at least two positions, that is, an operation position that comes into contact with the photosensitive member 1 and a non-operation position that is separated from the photosensitive member 1. It has. Each process unit 50K, 50Y, 50M, 50C is supplied with a driving force from a motor serving as a driving source along with the image forming operation when the developing roller 411 is in the operating position, and the developing roller 411 is driven by the driving force. Driven by rotation. On the other hand, when the developing roller 411 is in the non-operating position, the developing roller 411 is not driven to rotate.

  In each of the process units 50K, 50Y, 50M, and 50C, the surface of the photoreceptor 1 is uniformly charged by the charging device 2. Thereafter, the surface of the photoreceptor 1 of each of the process units 50K, 50Y, 50M, and 50C is exposed with light from the exposure device 53, and an electrostatic latent image is formed on the photoreceptor 1. Next, the toner in the toner tank 412 is supplied to the photoreceptor 1 via the developing roller 411 of the developing device 4. Thereby, the electrostatic latent image on the photoreceptor 1 is visualized as a toner image. In each of the process units 50K, 50Y, 50M, and 50C, the formed toner image is transferred onto the surface of the conveyance belt 7 by the transfer device 5. At this time, in color printing, toner images are formed by the process units 50K, 50Y, 50M, and 50C, and the toner images are superimposed on the conveyor belt 7. On the other hand, in monochrome printing, a toner image is formed only by the process unit 50 </ b> K and transferred to the surface of the transport belt 7.

  In the printing unit 10, the sheets placed on the paper feed tray 91 are taken out one by one, and the sheets are conveyed to the conveyance path 11. In the printing unit 10, the toner image transferred to the surface of the conveyance belt 7 is transferred to the sheet by the secondary transfer device 55. Thereafter, in the printing unit 10, the sheet on which the toner image is transferred is conveyed to the fixing device 8, and the toner image is thermally fixed to the sheet. Then, the fixed sheet is discharged to the discharge tray 92.

  In each of the process units 50K, 50Y, 50M, and 50C, after the transfer by the transfer device 5, residual toner that is not transferred and remains on the surface of the photoconductor 1 is scraped off from the surface of the photoconductor 1 by the cleaning blade 6. . A part of the toner scraped off from the surface of the photosensitive member 1 by the cleaning blade 6 stays at a contact portion between the cleaning blade 6 and the photosensitive member 1, and the remaining toner is collected in a waste toner box (not shown). The amount of toner staying at the contact portion between the cleaning blade 6 and the photosensitive member 1 is reduced by being collected in a waste toner box or passing through the cleaning blade 6 as time passes.

  Next, the electrical configuration of the printer 100 will be described. As shown in FIG. 2, the printer 100 includes a controller 30 including a CPU 31, a ROM 32, a RAM 33, and an NVRAM (Non Volatile RAM) 34. The controller 30 communicates with the process units 50K, 50Y, 50M, and 50C, a motor 70 that is a driving source of various rollers, the sheet sensor 12, the secondary transfer device 55, the fixing device 8, and the operation panel 40. It is electrically connected to an interface (communication IF) 36. The controller 30 may be a part of the ASIC, or the controller 30 may include the ASIC. Note that the controller 30 in FIG. 2 is a collective term for hardware used for controlling the printer 100, such as the CPU 31, and does not necessarily represent a single piece of hardware that actually exists in the printer 100.

  The ROM 32 stores various control programs and image processing programs for controlling the printer 100, various settings, initial values, and the like. The RAM 33 is used as a work area from which various control programs are read out or as a storage area for temporarily storing image data sent via various interfaces. The NVRAM 34 is a non-volatile storage means, and is used as a storage area for storing various settings, image data, and the like.

  The CPU 31 executes arithmetic operations for realizing various functions such as an image forming function in the printer 100, and becomes the center of control. The CPU 31 controls each component of the printer 100 while storing the processing result in the RAM 33 or the NVRAM 34 according to the control program read from the ROM 32. The CPU 31 is an example of a control unit. The controller 30 may be an example of a control unit.

  The motor 70 is a drive source for rotating members used for image formation and sheet conveyance, such as the conveyance belt 7 and the paper feed roller 71. The fixing device 8 and the photosensitive member 1, the developing roller 411, and the transfer device 5 of each of the process units 50K, 50Y, 50M, and 50C are also rotationally driven by the motor 70.

  The communication IF 36 is hardware that enables communication with an external device by wire or wireless. The printer 100 receives a print job transmitted from an external device via the communication IF 36.

  The operation panel 40 is hardware that enables messages and various settings to be displayed and various information from the user to be input, and is provided on the exterior of the printer 100. The operation panel 40 includes a touch panel having a display function and an input function, and a button group including various input means such as a numeric keypad, an arrow key, an OK button, and a cancel button.

  The printer 100 also includes an individual power supply circuit that controls the power supply from the power source for each component. 2, the power supply circuit 21K that controls the power supply to the charging device 2K of the process unit 50K, the power supply circuit 51K that controls the power supply of the process unit 50K to the transfer device 5K, and the process unit 50Y A power supply circuit 21Y that controls power supply to the charging device 2Y, a power supply circuit 51Y that controls power supply to the transfer device 5Y of the process unit 50Y, and a power supply to the charging device 2M of the process unit 50M are controlled. A power supply circuit 21M, a power supply circuit 51M that controls power supply to the transfer device 5M of the process unit 50M, a power supply circuit 21C that controls power supply to the charging device 2C of the process unit 50C, and a process unit 50C A power supply circuit 51C that controls power supply to the transfer device 5C is illustrated. In addition, each of the process units 50K, 50Y, 50M, and 50C may include a power supply circuit for the developing device 4 and a power supply circuit for the exposure device 53.

  Next, a reverse transfer operation for supplying K color toner from the process unit 50K to the process units 50Y, 50M, and 50C other than the process unit 50K in the printer 100 will be described with reference to FIG. In FIG. 3, among the process units 50K, 50Y, 50M, and 50C, devices unnecessary for explanation are omitted. The printer 100 performs the reverse transfer operation when forming a non-sheet image on which an image to be printed on a sheet is not formed. For example, when a non-sheet image is formed, the interval between sheets in the print job after completion of the print job is applicable. The process unit 50K is an example of a first image forming unit, and the process units 50Y, 50M, and 50C other than the process unit 50K are examples of a second image forming unit.

  The printer 100 can perform five types of operations as reverse transfer operations. In the following description, the first to fifth reverse transfer operations will be described in order. FIG. 4 shows a list of controls of each device in the first reverse transfer operation. FIG. 5 shows the output of each device in the first reverse transfer operation in time series. The operation within the broken line frame in FIG. 5 relates to the first reverse transfer operation. The printer 100 executes the first reverse transfer operation between sheets during monochrome printing.

  In the first reverse transfer operation, the printer 100 first causes the process unit 50K to form a pattern image (FIG. 3A). The pattern image is, for example, a solid image, the length in the vertical direction that is the rotation direction of the photoconductor 1 is 1 to 10 mm, and the width in the horizontal direction that is perpendicular to the vertical direction is image formation of the photoconductor 1. The maximum width of the region.

  In the pattern image forming process of the first reverse transfer operation, as shown in FIGS. 4 and 5, the rotation speed of the developing roller 411 and the rotation speed of the photosensitive member 1 are set at the time of sheet image formation for forming an image to be printed on a sheet. And the absolute value of the developing voltage applied to the developing roller 411 at the moment of developing the pattern image is made higher than that during sheet image formation. By increasing the absolute value of the development voltage, the amount of toner per unit area supplied onto the photoreceptor 1 during development of the pattern image increases. This pattern image forming process is an example of a development adjustment process and a forming process.

  Thereafter, the printer 100 causes the transfer device 5 to transfer the pattern image formed on the photoreceptor 1 of the process unit 50K onto the transport belt 7 (FIG. 3B). In the pattern image transfer process of the first reverse transfer operation, as shown in FIGS. 4 and 5, the position where the pattern image faces the transfer device 5 of the process unit 50K in accordance with the conveyance of the pattern image on the photosensitive member 1. During the period of passing, the transfer current applied to the transfer device 5 of the process unit 50K is made larger than that during sheet image formation. In the transfer device 5, constant current control is performed with the set transfer current. By increasing the transfer current, the charge amount of the transferred toner increases more than the charge amount of the toner transferred during sheet image formation. In addition, the transfer voltage increases by increasing the transfer current. This pattern image transfer process is an example of a transfer adjustment process and a first transfer process.

  Thereafter, the printer 100 conveys the pattern image transferred onto the conveyance belt 7 to the process unit 50Y, and reversely transfers the pattern image onto the photosensitive member 1 by the transfer device 5 of the process unit 50Y (FIG. 3C). This pattern image reverse transfer process is an example of a second transfer process. Also in this pattern image reverse transfer process, as shown in FIGS. 4 and 5, a transfer current having the same polarity as that at the time of sheet image formation is applied in accordance with the conveyance of the pattern image on the conveyance belt 7. Further, during the period in which the pattern image passes through the position facing the transfer device 5 of the process unit 50Y, the transfer current applied to the transfer device 5 of the process unit 50Y is made larger than that during sheet image formation. By increasing the transfer current, the charge amount of the toner on the conveyance belt 7 further increases than the charge amount of the toner transferred when the sheet image is formed.

  Note that the transfer device 5 may perform constant voltage control instead of constant current control. In this case, during the above-described reverse transfer operation, a transfer voltage having the same polarity as that at the time of sheet image formation and a larger absolute value than that at the time of sheet image formation is applied to the transfer device 5 of each of the process units 50K and 50Y. do it.

  In the first reverse transfer operation, the absolute value of the development voltage is increased, the amount of toner supplied per unit area is increased, and the thickness of the toner layer transferred onto the transport belt 7 is increased. By increasing the thickness of the toner layer, the electrostatic capacity of the toner layer is reduced and the charge amount of the toner is increased. As a result, the potential difference between the surface (referred to as the front surface) of the toner layer transferred onto the conveyor belt 7 that faces the photoreceptor 1 and the surface that contacts the conveyor belt 7 (referred to as the back surface) is large. Become. In the first reverse transfer operation, the transfer current is increased, the toner charge amount is further increased, and the potential difference inside the toner layer is further increased. For these reasons, when the potential difference inside the toner layer exceeds the limit value for generating discharge, a discharge occurs between the front and back surfaces of the toner layer, and a part of the toner in the toner layer is reversely charged by the discharge. . As a result, the reversely charged toner is transferred to the surface of the photoreceptor 1.

  The K-color toner transferred to the photoreceptor 1 of the process unit 50Y by reverse transfer is conveyed by the rotation of the photoreceptor 1 of the process unit 50Y and scraped off from the surface of the photoreceptor 1 by the cleaning blade 6 of the process unit 50Y. As a result, toner is supplied to the contact portion between the cleaning blade 6 of the process unit 50Y and the photosensitive member 1, and the friction generated between the cleaning blade 6 of the process unit 50Y and the photosensitive member 1 is reduced. The squeaking and turning caused by the cleaning blade 6 of the process unit 50Y is suppressed.

  Thereafter, the printer 100 conveys the pattern image remaining on the conveyance belt 7 to the process unit 50M, and in the same way as the process unit 50Y, the transfer device 5 of the process unit 50M applies a part of toner to the photosensitive member 1. Relocate. Further, the printer 100 conveys the pattern image remaining on the conveyance belt 7 to the process unit 50C, and, similar to the process units 50Y and 50M, a part of the pattern image is formed on the photoreceptor 1 by the transfer device 5 of the process unit 50C. Transfer toner. As described above, the process units 50M and 50C downstream of the process unit 50Y also perform the reverse transfer operation similar to that of the process unit 50Y, so that the K-color toner is also applied to the photoreceptors 1 of the process units 50M and 50C. Relocated. As a result, the noise and turning caused by the cleaning blades 6 of the process units 50M and 50C are also suppressed.

  Note that in the first reverse transfer operation, the absolute value of the developing voltage and the transfer current of each process unit are made larger than those at the time of sheet image formation to increase the toner charge amount. The transfer current may be the same as that at the time of sheet image formation. In addition, it is not necessary to increase the transfer current in all the process units, and the transfer current may be the same as that in the sheet image formation in some process units. However, when the transfer current is made larger than that at the time of sheet image formation, discharge easily occurs in the toner layer, and the amount of toner transferred to the photoreceptor 1 increases. Therefore, it is preferable to increase the transfer current in the transfer devices 5 of all the process units 50K, 50Y, 50M, and 50C as compared with the sheet image formation. Further, the absolute value of the developing voltage applied to the developing roller 411 may be increased not only at the moment of developing the pattern image but also until the first reverse transfer operation is completed.

  Next, the second reverse transfer operation will be described with reference to FIGS. FIG. 6 shows a list of controls of each device in the second reverse transfer operation. FIG. 7 shows the output of each device in the second reverse transfer operation in time series. The operation within the broken line frame in FIG. 7 relates to the second reverse transfer operation. The printer 100 executes the second reverse transfer operation when the next print job is not received after the print job is completed. The second reverse transfer operation is the first reverse transfer operation in that the rotational speed of the photosensitive member 1 and the transport speed of the transport belt 7 are lowered, the transfer current is not changed, and the execution timing is after the completion of the print job. And different. In the second reverse transfer operation, the supply of the driving force from the motor to the photosensitive member 1 and the application of the developing voltage to the developing roller 411 are temporarily stopped before the pattern image is formed. In the meantime, the developing rollers 411 of the process units 50Y, 50M, and 50C are moved to the non-operating position.

  In the second reverse transfer operation, the printer 100 first causes the process unit 50K to form a pattern image. In the pattern image forming step of the second reverse transfer operation, as shown in FIGS. 6 and 7, the rotation speed of the developing roller 411 is not changed, and the rotation speed of the photosensitive member 1 is reduced to half that during sheet image formation. It is slower than the rotation speed of the developing roller 411. Then, the peripheral speed ratio, which is the ratio between the rotational speed of the photosensitive member 1 and the rotational speed of the developing roller 411, is doubled when the sheet image is formed. Since the rotational speed of the photosensitive member 1 becomes slower than that during sheet image formation, the amount of toner per unit area supplied onto the photosensitive member 1 at the moment of development of the pattern image increases. Since the peripheral speed of the photosensitive member 1 and the speed of the conveying belt 7 correspond to each other, reducing the rotational speed of the photosensitive member 1 corresponds to reducing the toner conveying speed of the conveying belt 7. Similarly to the first reverse transfer operation, the printer 100 increases the developing voltage applied to the developing roller 411 at the time of developing the pattern image as compared with that at the time of forming the sheet image. This also increases the amount of toner per unit area supplied onto the photoreceptor 1 during development of the pattern image. This pattern image forming process is an example of a speed adjustment process and a forming process.

  Thereafter, the printer 100 transfers the pattern image formed on the photoreceptor 1 of the process unit 50K onto the transport belt 7 by the transfer device 5 of the process unit 50K. In the pattern image transfer process of the second reverse transfer operation, unlike the first reverse transfer operation, as shown in FIGS. 6 and 7, the transfer current is the same as that at the time of sheet image formation. However, in the second reverse transfer operation, as the photosensitive member 1 is decelerated, the conveying speed of the conveying belt 7 is also halved. For this reason, the time required for the transfer is doubled compared to the time when the sheet image is formed, and the charge amount of the pattern image is also increased as compared with the time when the sheet image is formed.

  Thereafter, the printer 100 conveys the pattern image transferred onto the conveyance belt 7 to the process unit 50Y, and reversely transfers the pattern image onto the photoreceptor 1 by the transfer device 5 of the process unit 50Y. In the reverse transfer process of the pattern image, as shown in FIGS. 6 and 7, the transfer current is the same as that at the time of sheet image formation, but the conveyance speed of the conveyance belt 7 is half that at the time of sheet image formation. The charge amount of the toner on the belt 7 is larger than the charge amount of the toner transferred when the sheet image is formed.

  In the second reverse transfer operation, as in the first reverse transfer operation, by increasing the development voltage, the thickness of the toner layer is increased and the potential difference between the front surface and the back surface of the toner layer is increased. Further, in the second reverse transfer operation, the rotation speed of the developing roller 411 is not changed, and the rotation speed of the photosensitive member 1 and the conveyance speed of the conveyance belt 7 are decreased, that is, the rotation speed of the development roller 411 with respect to the conveyance speed of the conveyance belt 7. By increasing the ratio, the toner layer is made thicker and the charge amount of the toner is further increased. For these reasons, discharge occurs between the front and back surfaces of the toner layer, and a part of the toner in the toner layer is reversely charged. As a result, the reversely charged toner is transferred to the surface of the photoreceptor 1 of the process unit 50Y.

  The reversely transferred K-color toner is conveyed by the photoreceptor 1 of the process unit 50Y and scraped off from the surface of the photoreceptor 1 by the cleaning blade 6 of the process unit 50Y, as in the first reverse transfer operation. As a result, toner is supplied to the contact portion between the cleaning blade 6 of the process unit 50Y and the photosensitive member 1, and noise and turning caused by the cleaning blade 6 of the process unit 50Y are suppressed. Similarly, in the process units 50M and 50C, the K-color toner is reversely transferred, and the noise and turning caused by the cleaning blades 6 of the process units 50M and 50C are also suppressed.

  In the second reverse transfer operation, the transfer current is the same as that at the time of sheet image formation. However, the charge amount of the toner may be further increased by increasing the transfer current compared to that at the time of sheet image formation. As the charge amount of the toner increases, electric discharge easily occurs inside the toner layer. Even in that case, it is not necessary to increase the transfer current in all the process units, and the transfer current may be the same as that in the sheet image formation in some process units.

  In the second reverse transfer operation, the photosensitive member 1 and the conveyance belt 7 are decelerated to increase the rotation speed ratio of the developing roller 411 with respect to the conveyance speed of the conveyance belt 7 and increase the amount of toner per unit area. However, the developing roller 411 may be accelerated without changing the speed of the photosensitive member 1 and the conveyor belt 7. Even in this case, the rotation speed ratio of the developing roller 411 with respect to the conveyance speed of the conveyance belt 7 increases, and the amount of toner per unit area increases. However, when the photosensitive member 1 and the conveyor belt 7 are decelerated, damage to the toner and each component of the process unit 50K are less than when the developing roller 411 is accelerated.

  Next, the third reverse transfer operation will be described with reference to FIGS. FIG. 8 shows a list of controls of each device in the third reverse transfer operation. FIG. 9 shows the output of each device in the third reverse transfer operation in time series. The operation within the broken line frame in FIG. 9 relates to the third reverse transfer operation. Similar to the second reverse transfer operation, the printer 100 executes the third reverse transfer operation after the print job is completed and the next print job has not been received. The third reverse transfer operation is different from the second reverse transfer operation in that a transfer current and a charging voltage are not applied in the process units 50Y, 50M, and 50C that perform reverse transfer of K-color toner.

  In the third reverse transfer operation, the printer 100 first causes the process unit 50K to form a pattern image. In the pattern image forming process of the third reverse transfer operation, as shown in FIGS. 8 and 9, the rotation speed of the photosensitive member 1 is changed without changing the rotation speed of the developing roller 411 as in the second reverse transfer operation. The speed is reduced to half that during image formation, and is slower than during sheet image formation. Further, the printer 100 increases the developing voltage applied to the developing roller 411 at the time of developing the pattern image higher than that at the time of forming the sheet image, similarly to the first reverse transfer operation and the second reverse transfer operation. As a result, the amount of toner per unit area supplied onto the photoreceptor 1 during development of the pattern image increases. The charging voltage of the process unit 50K is the same as that at the time of sheet image formation.

  Thereafter, the printer 100 causes the transfer device 5 to transfer the pattern image formed on the photoreceptor 1 of the process unit 50 </ b> K onto the transport belt 7. In the third reverse transfer operation, similarly to the second reverse transfer operation, the transport speed of the transport belt 7 decreases as the rotational speed of the photosensitive member 1 decreases. For this reason, the time required for the transfer is doubled compared to the time when the sheet image is formed, and the charge amount of the pattern image is also increased as compared with the time when the sheet image is formed. Note that the transfer current of the process unit 50K is the same as that during sheet image formation.

  Thereafter, the printer 100 conveys the pattern image transferred onto the conveyance belt 7 to the process unit 50Y, and reversely transfers the pattern image onto the photoreceptor 1 by the transfer device 5 of the process unit 50Y. In the reverse transfer process of the pattern image of the third reverse transfer operation, as shown in FIGS. 8 and 9, the pattern image is transferred to the transfer device 5 of the process unit 50Y at least during the period until the pattern image passes through the transfer device 5 of the process unit 50Y. No transfer current is applied. Further, the charging voltage of the charging device 2 of the process unit 50Y is not applied.

  In the third reverse transfer operation, the process unit 50K halves the rotational speed of the photosensitive member 1 and the transport speed of the transport belt 7, thereby transferring a pattern image having a thick toner layer onto the transport belt 7, At the time of transfer, the charge amount of the toner is increased. Thereby, the potential difference between the front surface and the back surface of the toner layer on the conveyor belt 7 is increased. That is, the surface potential of the toner layer is high. When the toner layer is transported to the process unit 50Y to which no transfer current is applied and is contacted with the weakly charged photoconductor 1 without applying a charging voltage, the surface potential of the toner layer is higher than the surface potential of the photoconductor 1 Therefore, a part of the toner in the toner layer is attracted to the photoreceptor 1 of the process unit 50Y. As a result, the K-color toner is transferred to the surface of the photoreceptor 1 of the process unit 50Y.

  The transferred K-color toner is conveyed by the photoreceptor 1 of the process unit 50Y and scraped off from the surface of the photoreceptor 1 by the cleaning blade 6 of the process unit 50Y, as in the first reverse transfer operation and the second reverse transfer operation. It is done. As a result, toner is supplied to the contact portion between the cleaning blade 6 of the process unit 50Y and the photosensitive member 1, and noise and turning caused by the cleaning blade 6 of the process unit 50Y are suppressed. Similarly, in the process units 50M and 50C, the K-color toner is transferred from the toner layer of the pattern image to the photosensitive member 1, and the noise and turning caused by the cleaning blades 6 of the process units 50M and 50C are also suppressed.

  In the third reverse transfer operation, when the pattern image is formed, the developing voltage is increased and the photosensitive member 1 is decelerated. However, only one of them may be used. However, the surface potential of the toner layer tends to be higher when both are performed. Therefore, it is preferable to implement both. Further, in the third reverse transfer operation, the transfer current of the process unit 50K is the same as that at the time of sheet image formation. However, similarly to the first reverse transfer operation, the transfer current of the process unit 50K at the time of pattern image formation is the sheet image formation. The charge amount of the toner may be increased by increasing the time. In the third reverse transfer operation, the transfer current of the process units 50Y, 50M, and 50C is set to 0. However, a transfer current that is greater than 0 and smaller than that at the time of sheet image formation may be used. When the third reverse transfer operation is performed under constant voltage control, the absolute value of the transfer voltage may be made smaller than that during sheet image formation.

  Further, in the third reverse transfer operation, the charging voltage of the process units 50Y, 50M, and 50C is set to 0 to make a weakly charged state that is weaker than that at the time of sheet image formation. A weakly charged state may be obtained by applying a voltage and exposing at least an area overlapping the pattern image.

  Next, the fourth reverse transfer operation will be described with reference to FIGS. FIG. 10 shows a list of controls of each device in the fourth reverse transfer operation. FIG. 11 shows the output of each device in the fourth reverse transfer operation in time series. The operation within the broken line frame in FIG. 11 relates to the fourth reverse transfer operation. The printer 100 executes the fourth reverse transfer operation when the next print job is not received after the completion of the print job, similarly to the second reverse transfer operation and the third reverse transfer operation. The fourth reverse transfer operation is different from the second reverse transfer operation in that exposure is controlled for each of the process units 50K, 50Y, 50M, and 50C.

  Specifically, in the fourth reverse transfer operation, a pattern image of the same size is formed three times by the process unit 50K. And in each process part 50Y, 50M, 50C, the presence or absence of the exposure to the location facing each pattern image differs. Hereinafter, among the three pattern images, a first pattern image, a second pattern image, and a third pattern image are assumed from the upstream side in the conveyance direction of the toner image transferred to the sheet. In FIG. 10, the control of each device in the reverse transfer operation for the first pattern image is “1 of the fourth reverse transfer operation”, and the control of each device in the reverse transfer operation for the second pattern image is “2 of the fourth reverse transfer operation”. The control of each device in the reverse transfer operation for the third pattern image is indicated as “fourth reverse transfer operation 3”.

  In the fourth reverse transfer operation, the transfer operation and the charging operation in each of the process units 50K, 50Y, 50M, and 50C are made common. Accordingly, the printer 100 does not need to prepare a power supply circuit for the transfer device 5 and a power supply circuit for the charging device 2 for each of the process units 50K, 50Y, 50M, and 50C. As shown in FIG. A power supply circuit 51 for the transfer device 5 and a power supply circuit 21 for the charging device 2 that are common to the process units 50K, 50Y, 50M, and 50C are provided.

  In the fourth reverse transfer operation, the printer 100 first causes the process unit 50K to sequentially form each pattern image. In the pattern image forming process of the fourth reverse transfer operation, similarly to the second reverse transfer operation, the rotational speed of the photoconductor 1 is reduced to half that at the time of sheet image formation and is slower than the rotational speed of the developing roller 411. In addition, as in the first reverse transfer operation and the second reverse transfer operation, the printer 100 increases the developing voltage applied to the developing roller 411 at the time of developing each pattern image as compared with that at the time of sheet image formation. As a result, the amount of toner per unit area supplied onto the photoreceptor 1 during development of each pattern image increases.

  Thereafter, the printer 100 transports each pattern image transferred onto the transport belt 7 to the process unit 50Y, and proceeds to the reverse transfer process. In the reverse transfer process of the pattern image in the fourth reverse transfer operation, as shown in FIGS. 10 and 11, no exposure is performed on an area overlapping the first pattern image in the photoreceptor 1 of the process unit 50Y (FIG. 10). (“4th reverse transfer operation” in FIG. 10), exposure is performed on the region overlapping the second pattern image and the third pattern image (“second reverse transfer operation in FIG. 10”, “fourth reverse transfer operation” in FIG. 10). Operation 3 "). The exposure process of the process unit 50Y is an example of a partial exposure process.

  Specifically, first, the time period from the start of exposure for the first pattern image on the photosensitive member 1 of the process unit 50K to the arrival of the leading edge of the first pattern image at the transfer position of the process unit 50K is time T1. The time T1 is common to all of the process unit 50Y, the process unit 50M, and the process unit 50C. Next, the time from when the leading edge of the first pattern image reaches the transfer position of the process unit 50K to when the leading edge of the first pattern image reaches the transfer position in the process unit 50Y is defined as time T2. The time from the leading edge of the pattern image to the trailing edge of the first pattern image passing through the transfer position in the process unit 50Y is defined as time T3.

  At this time, the exposure is started on the photosensitive member 1 of the process unit 50Y after the time T2 from the start of exposure for the second pattern image on the photosensitive member 1 of the process unit 50K, and the exposure is ended after the time T3. As a result, the leading edge of the second pattern image and the leading edge of the exposed portion exposed on the photoreceptor 1 of the process unit 50Y coincide with each other at the transfer position in the process unit 50Y. In addition, the rear end of the second pattern image and the rear end of the exposed portion exposed on the photosensitive member 1 of the process unit 50Y coincide at the transfer position in the process unit 50Y. Then, the third pattern image is processed in the same manner as the second pattern image. That is, it is possible to perform exposure on a region overlapping the second pattern image and the third pattern image without performing exposure on a region overlapping the first pattern image.

  In the printer 100, since the charging control is performed by the common power supply circuit 21, the transfer of the third pattern image in the process unit 50C is performed at least from the timing when the first pattern image is formed in the process unit 50K. Until passing, the charging voltage is continuously applied to the charging devices 2 of the process units 50K, 50Y, 50M, and 50C. In addition, since transfer control is performed by the common power supply circuit 51, the printer 100 controls the transfer position of the third pattern image in the process unit 50C at least from the timing at which the first pattern image is transferred by the process unit 50K. Until it passes, the transfer voltage is continuously applied to the transfer devices 5 of the process units 50K, 50Y, 50M, and 50C.

  In the fourth reverse transfer operation, in a region where exposure is not performed, similarly to the second reverse transfer operation, the toner reversely charged by the discharge is attracted to the high potential photoconductor 1 and reversely transferred. On the other hand, in the exposed area, the surface potential of the photoconductor 1 is low, and the toner that is reversely charged by the discharge is hardly attracted to the photoconductor 1.

  That is, since the first pattern image is not exposed in the process unit 50Y, a large amount of toner is reversely transferred. Therefore, toner is supplied to the contact portion between the cleaning blade 6 of the process unit 50Y and the photosensitive member 1, and the noise and turning caused by the cleaning blade 6 of the process unit 50Y are suppressed. On the other hand, since the second pattern image and the third pattern image are exposed, a large amount of toner remains on the conveying belt 7. In this state, it is transported to the process unit 50M located downstream of the process unit 50Y.

  Then, the printer 100 does not expose a region of the photosensitive member 1 of the process unit 50M that overlaps the first pattern image and the second pattern image (“fourth reverse transfer operation 1” in FIG. In the fourth reverse transfer operation 2 ”), exposure is performed on an area overlapping with the third pattern image (“ fourth reverse transfer operation 3 ”in FIG. 10).

  In the process unit 50M, since the first pattern image and the second pattern image are not exposed, a large amount of toner is reversely transferred. Among them, particularly the toner of the second pattern image is hardly transferred to the photosensitive member 1 of the process unit 50Y, and thus a large amount of toner is reversely transferred. That is, a large amount of toner is reversely transferred even in the process unit 50M located downstream of the process unit 50Y. On the other hand, since the third pattern image is exposed, a lot of toner remains on the transport belt 7. In this state, the sheet is conveyed to the process unit 50C located downstream of the process unit 50M.

  Then, the printer 100 does not expose all the pattern images for the process unit 50C (“fourth reverse transfer operation 1”, “fourth reverse transfer operation 2”, “fourth reverse transfer operation” in FIG. 10). In the operation 3 "), in the process unit 50C, since each pattern image is not exposed, a lot of toner is reversely transferred. In particular, the toner of the third pattern image is hardly transferred to the photoconductor 1 of the process unit 50Y and the photoconductor 1 of the process unit 50M, and thus a large amount of toner is reversely transferred. That is, a large amount of toner is reversely transferred even in the process unit 50C located on the most downstream side.

  In the fourth reverse transfer operation, reversely charged toner due to discharge is transferred onto the surface of the photoreceptor 1 in the process unit other than the process unit 50K by the same mechanism as in the second reverse transfer operation. Further, the upstream process unit other than the process unit 50K exposes a portion corresponding to a part of the pattern image formed by the fourth reverse transfer operation, thereby performing individual transfer control or charging. The amount of toner to be supplied to the downstream process unit can be secured without performing control.

  In the fourth reverse transfer operation, for example, in the photosensitive member 1 of the process unit 50M, in addition to the second pattern image, the first pattern image that has not been exposed in the upstream process unit 50Y, that is, the toner reversal has already been performed. Although the pattern image used for copying is not exposed, the first pattern image may be exposed. However, even if the pattern image has already been used for reverse transfer of toner, there is a considerable amount of toner that can be reversely transferred, so it is preferable to use it for reverse transfer also in the downstream process unit.

  Next, the fifth reverse transfer operation will be described with reference to FIGS. FIG. 13 shows a list of controls of each device in the fifth reverse transfer operation. FIG. 14 shows the output of each device in the fifth reverse transfer operation in time series. The operation within the broken line frame in FIG. 14 relates to the fifth reverse transfer operation. Similar to the fourth reverse transfer operation, the printer 100 executes the fifth reverse transfer operation after the print job is completed and the next print job has not been received. The fifth reverse transfer operation is an application example of the fourth reverse transfer operation and differs from the fourth reverse transfer operation in that the transfer current is controlled for each of the process units 50Y, 50M, and 50C.

  Specifically, also in the fifth reverse transfer operation, three pattern images of the first pattern image, the second pattern image, and the third pattern image are formed. In FIG. 13, the control of each device in the reverse transfer operation for the first pattern image is “1 of the fifth reverse transfer operation”, and the control of each device in the reverse transfer operation for the second pattern image is “2 of the fifth reverse transfer operation”. The control of each device in the reverse transfer operation for the third pattern image is indicated as “third of the fifth reverse transfer operation”. As shown in FIG. 13, in the fifth reverse transfer operation, whether or not a transfer current is applied to each pattern image in each of the process units 50Y, 50M, and 50C is different.

  In the fifth reverse transfer operation, the charging operations in the process units 50K, 50Y, 50M, and 50C are made common. Therefore, as shown in FIG. 15, the printer 100 is provided with a power supply circuit 21 for the charging device 2 that is common to the process units 50K, 50Y, 50M, and 50C. On the other hand, in order to individually control the transfer operation, a power supply circuit for the transfer device 5 is provided for each of the process units 50K, 50Y, 50M, and 50C.

  First, as the fifth reverse transfer operation, the printer 100 causes the process unit 50K to sequentially form each pattern image, similarly to the fourth reverse transfer operation. Thereafter, the printer 100 transports each pattern image transferred onto the transport belt 7 to the process unit 50Y, and proceeds to the reverse transfer process. In the reverse transfer process of the pattern image in the fifth reverse transfer operation, as shown in FIGS. 13 and 14, the transfer device 5 of the process unit 50Y transfers the same polarity to the first pattern image as in the sheet image formation. A current is applied (“1 of 5th reverse transfer operation” in FIG. 13), and no transfer current is applied to the second pattern image and the third pattern image (“5th reverse transfer operation of FIG. 13”). 2 "," 3 of the fifth reverse transfer operation ").

  In the fifth reverse transfer operation, when a transfer current is applied, similarly to the second reverse transfer operation, a discharge is easily generated between the front surface and the back surface of the toner layer, and the toner reversely charged by the discharge is applied to the photoreceptor 1. Reverse transcription. On the other hand, when no transfer current is applied, the amount of charge of the toner does not increase and discharge is difficult to occur. Therefore, the toner is difficult to be reversely charged and is not easily attracted to the photoreceptor 1.

  That is, in the process unit 50Y, since a transfer current is applied to the first pattern image, a large amount of toner is reversely transferred. Therefore, toner is supplied to the contact portion between the cleaning blade 6 of the process unit 50Y and the photosensitive member 1, and the noise and turning caused by the cleaning blade 6 of the process unit 50Y are suppressed. On the other hand, since no transfer current is applied to the second pattern image and the third pattern image, a large amount of toner remains on the transport belt 7. In this state, it is transported to the process unit 50M located downstream of the process unit 50Y.

  Then, the printer 100 applies a transfer current to the first pattern image and the second pattern image in the photosensitive member 1 of the process unit 50M (“fifth reverse transfer operation 1”, “ The second reverse transfer operation 2 ”), no transfer current is applied to the third pattern image (“ 5th reverse transfer operation 3 ”in FIG. 13).

  In the process unit 50M, since a transfer current is applied to the first pattern image and the second pattern image, a large amount of toner is reversely transferred. Among them, particularly the toner of the second pattern image is hardly transferred to the photosensitive member 1 of the process unit 50Y, and thus a large amount of toner is reversely transferred. That is, as in the fourth reverse transfer operation, a large amount of toner is reversely transferred even in the process unit 50M located downstream of the process unit 50Y. On the other hand, since no transfer current is applied to the third pattern image, a large amount of toner remains on the conveyor belt 7. In this state, the sheet is conveyed to the process unit 50C located downstream of the process unit 50M.

  In the printer 100, the transfer current is applied to all the pattern images in the process unit 50C (“fifth reverse transfer operation 1”, “fifth reverse transfer operation 2”, “fifth reverse rotation” in FIG. 13). 3)), a lot of toner is reversely transferred. In particular, the toner of the third pattern image is hardly transferred to the photoconductor 1 of the process unit 50Y and the photoconductor 1 of the process unit 50M, and thus a large amount of toner is reversely transferred. That is, a large amount of toner is reversely transferred even in the process unit 50C located on the most downstream side.

  In the fifth reverse transfer operation, reversely charged toner due to discharge is transferred onto the surface of the photoreceptor 1 in the process unit other than the process unit 50K by the same mechanism as in the second reverse transfer operation. Further, the upstream process unit other than the process unit 50K does not apply a transfer current to a part corresponding to a part of the pattern image formed by the fifth reverse transfer operation, thereby enabling individual charging. The amount of toner to be supplied to the downstream process unit can be secured without performing control.

  Next, job execution processing will be described as control of the printer 100 for executing the above-described reverse transfer operation. Hereinafter, the operation when the black print mode is set in the printer 100 will be described as a first form, and the operation when the black print mode is not set will be described as a second form.

  The printer 100 has a black printing mode in which printing is performed using only the process unit 50K. Since only the process unit 50K is used in the black print mode, even if an error such as toner shortage occurs in a process unit other than the process unit 50K, an error must occur in the process unit 50K. Can be printed.

  First, job execution processing according to the first embodiment will be described with reference to the flowchart of FIG. The job execution process is executed by the CPU 31 when the printer 100 receives a print job via the operation panel 40 or the communication IF 36.

  In the job execution processing of the first form, the printer 100 first increments the processing number N by 1 (S101). The processing number N is a variable that is incremented by one when printing one sheet, and is stored in the RAM 33 or the NVRAM 34. The processing number N is used to determine whether or not to execute the reverse transfer operation.

  Next, the printer 100 determines whether or not the processing number N exceeds the first threshold (S102). In this embodiment, the first threshold is 99. When the processing number N exceeds the first threshold (S102: YES), the number of continuous printings for only the K color is large, and the toner supplied to the blades 6 of the process units 50Y, 50M, and 50C other than the K color is exhausted. There is a high possibility of doing. Therefore, the printer 100 starts executing a first reverse transfer operation that can be performed between sheets (S110). With the start of the first reverse transfer operation, as shown in FIGS. 4 and 5, the printer 100 increases the development voltage when developing the pattern image and increases the transfer current when transferring the pattern image. Various voltages or currents are adjusted (S111). Since the first reverse transfer operation is performed between sheets without reducing the rotation speed of the photosensitive member 1 or the conveyance speed of the conveyance belt 7, there is little influence on the productivity. After S111, the printer 100 counts down the processing number N by 10 (S112).

  After S112, or when the number N of processes does not exceed the first threshold (S102: NO), the printer 100 executes a printing operation for one sheet (S103). Thereafter, the printer 100 determines whether the job is finished (S104). If the job has not ended (S104: NO), the process proceeds to S101, and the printer 100 performs an operation for printing the next page.

  If the job has ended (S104: YES), the printer 100 determines whether the number of processes N exceeds the second threshold (S121). The second threshold value is smaller than the first threshold value. In this embodiment, the second threshold value is 50. If the number of processes N exceeds the second threshold (S121: YES), it is unlikely that there is not enough toner on the process units 50Y, 50M, and 50C other than the K color at present, but a print job in the near future The possibility of running out of toner is high. Therefore, the printer 100 starts executing the second reverse transfer operation in order to prevent toner shortage in the near future job (S130). With the start of the second reverse transfer operation, the printer 100 increases the developing voltage during pattern image formation and decreases the speed of the photosensitive member 1 and the conveyor belt 7 as shown in FIGS. Adjust (S131).

  In S131, any of the third reverse transfer operation, the fourth reverse transfer operation, and the fifth reverse transfer operation may be performed instead of the second reverse transfer operation. After S131, the printer 100 resets the processing number N to 0 (S132). After S132, or when the number N of processes does not exceed the second threshold (S121: NO), the printer 100 ends the job execution process.

  Next, the job execution process of the second form will be described with reference to the flowchart of FIG. In the description of the job execution process of the second mode, the same processes as those of the first mode are denoted by the same reference numerals and description thereof is omitted.

  In the job execution process of the second form, the printer 100 first determines whether or not monochrome printing is performed (S200). Whether or not monochrome printing can be performed can be determined by a monochrome printing instruction or a determination result of the number of colors of an image. If the printing is monochrome printing (S200: YES), the process proceeds to S101, and the printer 100 performs an operation for printing similar to the first embodiment.

  On the other hand, if the printing is not monochrome printing (S200: NO), since the toner is supplied from each of the process units 50K, 50Y, 50M, and 50C by color printing, the printer 100 counts down the processing number N by 20 (S241). .

  After S241, the printer 100 determines whether or not the processing number N is smaller than 0 (S242). When the processing number N is smaller than 0 (S242: YES), it is very unlikely that the toner on the process units 50Y, 50M, and 50C other than the K color is insufficient, and the color on the photoreceptor 1 is also low. Since the toner is replenished, the printer 100 resets the processing number N to 0 (S243). After S243, the process proceeds to S103, and the printer 100 executes a color printing operation for one sheet.

  On the other hand, when the processing number N is not smaller than 0 (S242: NO), the printer 100 determines whether or not the processing number N is larger than the third threshold value (S251). The third threshold is a value smaller than the first threshold, and is 50 in this embodiment. If the number of processes N is larger than the third threshold (S251: YES), it is unlikely that there is not enough toner on the process units 50Y, 50M, and 50C other than the K color, but only the K color is printed after that. May continue. Therefore, the printer 100 forms a pattern image in each of the process units 50K, 50Y, 50M, and 50C in order to prevent toner shortage in the job (S252). As a result, the toner is supplied onto the photoreceptor 1 of each of the process units 50K, 50Y, 50M, and 50C.

  After S252, the printer 100 resets the processing number N to 0 (S253). After S253, or when the number N of processes does not exceed the third threshold (S251: NO), the process proceeds to S103, and the printer 100 executes a color printing operation for one sheet.

  Further, after the job is completed (S104: YES), when the number of processes N exceeds the second threshold (S121: YES), the printer 100 prevents each toner from running out in the near future. Pattern images are formed in the process units 50K, 50Y, 50M, and 50C, respectively (S231). After S231, the printer 100 resets the number N of processes to 0 (S132), and ends the job execution process.

  As described above in detail, in the printer 100, a pattern image having a large amount of toner per unit area is formed in the most upstream process unit 50K, and discharge is generated inside the toner layer. By this discharge, a part of the toner in the toner layer is reversely charged and transferred to the photoreceptor 1 of the process units 50Y, 50M, and 50C downstream of the process unit 50K. That is, in the printer 100, the toner of the process unit 50K is reversely transferred to the photoreceptor 1 of the other process units 50Y, 50M, and 50C by the transfer voltage or transfer current having the same polarity as that at the time of sheet image formation, and the cleaning blade 6 and the photosensitive member 1 are exposed. Friction generated with the body 1 can be reduced.

  In the first reverse transfer operation, it is not necessary to change the speed of the photosensitive member 1 and the like, and the control is simpler than the second reverse transfer operation and the like, and can be easily performed even during execution of the print job. On the other hand, it is not possible to supply more toner than the toner on the developing roller 411 simply by increasing the developing voltage as in the first reverse transfer operation. However, in the second reverse transfer operation or the like, the rotational speed of the photoreceptor 1 is reduced. Thus, the amount of toner that can be supplied from the developing roller 411 can be increased, and the amount of toner per unit area can be increased more reliably.

  Further, the printer 100 according to the embodiment has a shortage of toner in the process units 50Y, 50M, and 50C as compared to a configuration in which toner is directly supplied from the process units 50Y, 50M, and 50C other than the process unit 50K. Even if it exists, the toner can be supplied to the photosensitive member 1 of the process units 50Y, 50M, and 50C. Further, it is unnecessary for the processing units 50Y, 50M, and 50C to bring the developing roller 411 into contact with the photosensitive member 1 during a job, and the impact associated with the operation of the developing roller 411 is avoided even during a monochrome printing job. it can.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the present invention is not limited to a printer, and can be applied to any apparatus having a color printing function, such as a multifunction machine or a FAX apparatus.

  In the embodiment, the toner image formed by each of the process units 50K, 50Y, 50M, and 50C is once transferred to the transport belt 7, and then the toner image on the transport belt 7 by the secondary transfer device 55. However, the toner image formed by each of the process units 50K, 50Y, 50M, and 50C may be directly transferred to the sheet. However, when performing the fourth reverse transfer operation, the transfer voltage continues to be applied during the operation, and the transfer voltage may become unstable when the sheet is conveyed. The method is preferred.

  In the embodiment, each process unit includes a common exposure apparatus, but each process unit may include an exposure apparatus individually. That is, the exposure apparatus may be included in the process unit. Further, the charging polarity of the toner is not limited to the positive electrode but may be a negative electrode.

  The processing disclosed in the embodiments may be executed by a single CPU, a plurality of CPUs, hardware such as an ASIC, or a combination thereof. Further, the processing disclosed in the embodiment can be realized in various modes such as a recording medium or a method recording a program for executing the processing.

DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Charging device 4 Developing device 5 Transfer device 7 Conveying belt 10 Printing unit 50 Process unit 100 Printer

Claims (20)

Belt,
A plurality of photoconductors, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied onto the photoconductor onto the belt, and a blade that contacts the surface of the photoconductor An image forming unit of
A control unit;
With
The plurality of image forming units include:
Are arranged at predetermined intervals in the toner transport direction on the belt,
The controller is
When forming a non-sheet image in which an image to be printed on a sheet is not formed by using a developing unit of a predetermined first image forming unit among the plurality of image forming units, a pattern image is photosensitive to the first image forming unit. Forming process to form on the body,
A development adjustment process in which the absolute value of the developing voltage of the developing unit of the first image forming unit at the time of the forming process is higher than the absolute value of the developing voltage at the time of forming a sheet image for forming an image printed on the sheet;
A first transfer process for transferring the pattern image formed on the photoconductor of the first image forming unit onto the belt using a transfer unit of the first image forming unit;
When the pattern image conveyed by the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, using the transfer unit of the second image forming unit, A second transfer process for applying a transfer voltage or a transfer current having the same polarity as in the sheet image formation;
An image forming apparatus characterized in that The image forming apparatus according to claim 1,
The controller is
An image forming apparatus, wherein an absolute value of a transfer voltage or a transfer current applied to a transfer unit of the first image forming unit during the first transfer process is made larger than that during the sheet image formation. The image forming apparatus according to claim 1, wherein:
The controller is
An image forming apparatus, wherein an absolute value of a transfer voltage or a transfer current applied to a transfer unit of the second image forming unit during the second transfer process is made larger than that during the sheet image formation. The image forming apparatus according to any one of claims 1 to 3,
The controller is
A speed adjustment process is executed to make a ratio of a rotation speed of the developing unit of the first image forming unit to a toner conveyance speed of the belt at the time of the first transfer process larger than that at the time of the sheet image formation. Image forming apparatus. The image forming apparatus according to claim 4,
The controller is
In the speed adjustment process, the toner conveying speed of the belt during the first transfer process is made slower than that during the sheet image formation. The image forming apparatus according to any one of claims 1 to 5,
An exposure unit that exposes the surface of the photoreceptor in the plurality of image forming units;
The plurality of image forming units each include a charging unit that charges the surface of the photoreceptor.
The controller is
In the forming process, a first pattern image and a second pattern image formed after a lapse of a predetermined time after forming the first pattern image are formed.
Of the surface of the photoconductor of the second image forming unit, an area overlapping with the first pattern image formed by the forming process is charged by the charging unit of the second image forming unit. And without exposure by the exposure unit,
Of the surface of the photoconductor of the second image forming unit, an area overlapping with the second pattern image formed by the forming process is charged by the charging unit of the second image forming unit. A partial exposure process for performing exposure by the exposure unit is performed. The image forming apparatus according to any one of claims 1 to 6,
The plurality of image forming units each include a charging unit that charges the surface of the photoreceptor.
An image forming apparatus, comprising: a power unit for a charging unit that is shared by the charging unit of the first image forming unit and the charging unit of the second image forming unit and controls power supply to each charging unit. In the image forming apparatus according to any one of claims 1 to 7,
An image forming apparatus comprising a power supply unit for a transfer unit that is shared by the transfer unit of the first image forming unit and the transfer unit of the second image forming unit and controls power supply to each transfer unit. The image forming apparatus according to claim 1,
The absolute value of the transfer voltage or transfer current applied to the transfer unit of the second image forming unit during the second transfer process is made smaller than that during the sheet image formation, and the photosensitive member of the second image forming unit. In a weakly charged state, which is a weaker charged state than when the sheet image is formed. The image forming apparatus according to claim 9,
The controller is
A speed adjustment process is executed to make a ratio of a rotation speed of the developing unit of the first image forming unit to a toner conveyance speed of the belt at the time of the first transfer process larger than that at the time of the sheet image formation. Image forming apparatus. The image forming apparatus according to claim 10,
The controller is
In the speed adjustment process, the toner conveying speed of the belt during the first transfer process is made slower than that during the sheet image formation. The image forming apparatus according to claim 1,
The controller is
In the forming process, a first pattern image and a second pattern image formed after a lapse of a predetermined time after forming the first pattern image are formed.
In the second transfer process, a region of the surface of the photoconductor of the second image forming unit that overlaps the first pattern image formed by the forming process has the same polarity as that during the sheet image formation. The transfer voltage or transfer current is applied to a region of the surface of the photoconductor of the second image forming unit that overlaps the second pattern image formed by the forming process. An image forming apparatus characterized by not applying voltage. The image forming apparatus according to claim 12,
The controller is
A speed adjustment process is executed to make a ratio of a rotation speed of the developing unit of the first image forming unit to a toner conveyance speed of the belt at the time of the first transfer process larger than that at the time of the sheet image formation. Image forming apparatus. The image forming apparatus according to any one of claims 1 to 13,
The controller is
An image forming apparatus that executes the forming process on the condition that the number of times of printing using only the first image forming unit is greater than a first threshold value. The image forming apparatus according to claim 14,
The controller is
The forming process is executed on the condition that the number of times of printing using only the first image forming unit is greater than a second threshold value that is smaller than the first threshold value, except during the execution of a print job. An image forming apparatus. Belt,
A plurality of photoconductors, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied onto the photoconductor onto the belt, and a blade that contacts the surface of the photoconductor An image forming unit of
A control unit;
With
The plurality of image forming units include:
Are arranged at predetermined intervals in the toner transport direction on the belt,
The controller is
When forming a non-sheet image in which an image to be printed on a sheet is not formed by using a developing unit of a predetermined first image forming unit among the plurality of image forming units, a pattern image is photosensitive to the first image forming unit. Forming process to form on the body,
A first transfer process for transferring the pattern image formed on the photoconductor of the first image forming unit onto the belt using a transfer unit of the first image forming unit;
A transfer adjustment process in which an absolute value of a transfer voltage or a transfer current of the transfer unit of the first image forming unit at the time of the first transfer process is larger than that at the time of sheet image formation for forming an image printed on a sheet;
When the pattern image conveyed by the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, using the transfer unit of the second image forming unit, A second transfer process for applying a transfer voltage or a transfer current having the same polarity as in the sheet image formation;
An image forming apparatus characterized in that Belt,
A plurality of photoconductors, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied onto the photoconductor onto the belt, and a blade that contacts the surface of the photoconductor An image forming unit of
A control unit;
With
The plurality of image forming units include:
Are arranged at predetermined intervals in the toner transport direction on the belt,
The controller is
When forming a non-sheet image in which an image to be printed on a sheet is not formed by using a developing unit of a predetermined first image forming unit among the plurality of image forming units, a pattern image is photosensitive to the first image forming unit. Forming process to form on the body,
Speed adjustment that makes a rotation speed ratio of the developing unit of the first image forming unit to a conveyance speed of the belt during the forming process larger than a rotation speed ratio at the time of forming a sheet image for forming an image printed on a sheet Processing,
A first transfer process for transferring the pattern image formed on the photoconductor of the first image forming unit onto the belt using a transfer unit of the first image forming unit;
When the pattern image conveyed by the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, using the transfer unit of the second image forming unit, A second transfer process for applying a transfer voltage or a transfer current having the same polarity as in the sheet image formation;
An image forming apparatus characterized in that Belt,
A plurality of photoconductors, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied onto the photoconductor onto the belt, and a blade that contacts the surface of the photoconductor An image forming unit of
A control unit;
With
The plurality of image forming units include:
Are arranged at predetermined intervals in the toner transport direction on the belt,
The controller is
When forming a non-sheet image in which an image to be printed on a sheet is not formed by using a developing unit of a predetermined first image forming unit among the plurality of image forming units, a pattern image is photosensitive to the first image forming unit. Forming process to form on the body,
A development adjustment process in which the absolute value of the developing voltage of the developing unit of the first image forming unit at the time of the forming process is higher than the absolute value of the developing voltage at the time of forming a sheet image for forming an image printed on the sheet;
A first transfer process for transferring the pattern image formed on the photoconductor of the first image forming unit onto the belt using a transfer unit of the first image forming unit;
And
When the pattern image conveyed by the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, a transfer voltage or a voltage is applied to the transfer unit of the second image forming unit. An image forming apparatus, wherein a transfer current is not applied, and the photosensitive member of the second image forming unit is set to a weakly charged state, which is a weaker charged state than when the sheet image is formed. Belt,
A plurality of photoconductors, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied onto the photoconductor onto the belt, and a blade that contacts the surface of the photoconductor An image forming unit of
A control unit;
With
The plurality of image forming units include:
Are arranged at predetermined intervals in the toner transport direction on the belt,
The controller is
When forming a non-sheet image in which an image to be printed on a sheet is not formed by using a developing unit of a predetermined first image forming unit among the plurality of image forming units, a pattern image is photosensitive to the first image forming unit. Forming process to form on the body,
A first transfer process for transferring the pattern image formed on the photoconductor of the first image forming unit onto the belt using a transfer unit of the first image forming unit;
A transfer adjustment process in which an absolute value of a transfer voltage or a transfer current of the transfer unit of the first image forming unit at the time of the first transfer process is larger than that at the time of sheet image formation for forming an image printed on a sheet;
And
When the pattern image conveyed by the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, a transfer voltage or a voltage is applied to the transfer unit of the second image forming unit. An image forming apparatus, wherein a transfer current is not applied, and the photosensitive member of the second image forming unit is set to a weakly charged state, which is a weaker charged state than when the sheet image is formed. Belt,
A plurality of photoconductors, a developing unit that supplies toner to the photoconductor, a transfer unit that transfers the toner supplied onto the photoconductor onto the belt, and a blade that contacts the surface of the photoconductor An image forming unit of
A control unit;
With
The plurality of image forming units include:
Are arranged at predetermined intervals in the toner transport direction on the belt,
The controller is
When forming a non-sheet image in which an image to be printed on a sheet is not formed by using a developing unit of a predetermined first image forming unit among the plurality of image forming units, a pattern image is photosensitive to the first image forming unit. Forming process to form on the body,
A speed adjustment process in which a rotation speed ratio of the developing unit of the first image forming unit to a conveyance speed of the belt at the time of the forming process is larger than a rotation speed ratio at the time of sheet image formation for forming an image printed on a sheet. When,
A first transfer process for transferring the pattern image formed on the photoconductor of the first image forming unit onto the belt using a transfer unit of the first image forming unit;
And
When the pattern image conveyed by the belt passes through a transfer unit of a second image forming unit which is an image forming unit different from the first image forming unit, a transfer voltage or a voltage is applied to the transfer unit of the second image forming unit. An image forming apparatus, wherein a transfer current is not applied, and the photosensitive member of the second image forming unit is set to a weakly charged state, which is a weaker charged state than when the sheet image is formed.

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