Image forming device

Abstract

To provide an image forming apparatus that, in a configuration in which a first photoconductor drum used for printing of a monochrome image is arranged on a downstream side of a second photoconductor drum not used for the printing of the monochrome image, can prevent an attachment from being attached to the first photoconductor drum.SOLUTION: An image forming apparatus 1 comprises: photoconductor drums 41K, 41Y, 41M, 41C; transfer rollers 72K, 72Y, 72M, 72C; a cleaning roller 43Y; a cleaning shaft 44; and a control unit 12. When the image forming apparatus 1 is in a monochrome printing mode, the control unit 12 causes a transfer current It(K) to flow in the transfer roller 72K, applies transfer currents It(M), It(C) smaller than the transfer current It(K) to the transfer rollers 72M, 72C, and causes a transfer current It(Y) larger than the transfer currents It(M), It(C) to flow in the transfer roller 72Y.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to an image forming apparatus. [Background technology]

[0002] Conventionally, there is known an image forming apparatus having multiple photosensitive drums and capable of switching between a multi-color printing mode for printing a multi-color image and a monochrome printing mode for printing a monochrome image. In such an image forming apparatus, the photosensitive drum used for printing the monochrome image may be disposed downstream of the photosensitive drum not used for printing the monochrome image in the sheet transport direction.

[0003] As such an image forming apparatus, there is known an image forming apparatus including a plurality of cleaning rollers provided for a plurality of photosensitive drums, respectively, and a paper dust collecting shaft provided for the most upstream cleaning roller (see Patent Document 1 below). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2010-165000 A Summary of the Invention [Problem to be solved by the invention]

[0005] In an image forming apparatus such as that described in Patent Document 1, when printing a monochrome image, the transfer bias of a transfer roller not used for printing may be reduced. In this case, the amount of paper dust adhering to the most upstream photosensitive drum decreases, and there is a possibility that paper dust may adhere to the photosensitive drum used for printing the monochrome image.

[0006] Therefore, an object of the present disclosure is to provide an image forming device that can suppress adhesion of foreign matter to a first photosensitive drum in a configuration in which a first photosensitive drum used for printing monochrome images is positioned downstream of a second photosensitive drum that is not used for printing monochrome images. [Means for solving the problem]

[0007] (1) The image forming apparatus of the present disclosure is switchable between a multi-color printing mode and a monochrome printing mode. When the image forming apparatus is in the multi-color printing mode, the image forming apparatus is capable of printing a multi-color image on a sheet. When the image forming apparatus is in the monochrome printing mode, the image forming apparatus is capable of printing a monochrome image on a sheet.

[0008] The image forming apparatus includes a drum unit, a transfer unit, and a controller.

[0009] The drum unit has a first photosensitive drum, a second photosensitive drum, a cleaning roller, a cleaning shaft, and a third photosensitive drum. The first photosensitive drum is used for printing monochrome images and multi-color images. The second photosensitive drum is not used for printing monochrome images but for printing multi-color images. The second photosensitive drum is located away from the first photosensitive drum. The cleaning roller cleans the surface of the second photosensitive drum. The cleaning shaft collects the deposit from the cleaning roller. The deposit is deposit that is deposited on the surface of the second photosensitive drum from a sheet and is collected from the surface of the second photosensitive drum by the cleaning roller. The third photosensitive drum is not used for printing monochrome images but for printing multi-color images. The third photosensitive drum is located between the first photosensitive drum and the second photosensitive drum.

[0010] The transfer unit has a belt, a first transfer roller, a second transfer roller, and a third transfer roller. The belt transports a sheet from the second photosensitive drum to the first photosensitive drum. The first transfer roller transfers the toner on the first photosensitive drum to the sheet. The second transfer roller transfers the toner on the second photosensitive drum to the sheet. The third transfer roller transfers the toner on the third photosensitive drum to the sheet.

[0011] When the image forming device is in a monochrome printing mode, the control unit applies a first transfer bias to the first transfer roller, applies a third transfer bias lower than the first transfer bias to the third transfer roller, and applies a second transfer bias higher than the third transfer bias to the second transfer roller.

[0012] According to this configuration, when the image forming apparatus is in a monochrome printing mode, the second transfer bias applied to the second transfer roller can charge the deposits on the sheet and cause them to adhere to the second photosensitive drum.

[0013] Therefore, it is possible to prevent the adherent matter on the sheet from adhering to the first photosensitive drum without adhering to the second photosensitive drum.

[0014] As a result, adhesion of foreign matter to the first photosensitive drum can be suppressed.

[0015] (2) The drum unit may have a storage section that stores the deposits collected by the cleaning shaft.

[0016] (3) The image forming apparatus may further include a first developing cartridge, a second developing cartridge, and a third developing cartridge. The first developing cartridge has a first developing roller. The first developing roller supplies toner to the first photosensitive drum. The second developing cartridge has a second developing roller. The second developing roller supplies toner to the second photosensitive drum. The third developing cartridge has a third developing roller. The third developing roller supplies toner to the third photosensitive drum.

[0017] (4) The image forming apparatus may further include a separation mechanism. The separation mechanism moves the second developing roller from the contact position to the separation position. When the second developing roller is located at the contact position, the second developing roller contacts the second photosensitive drum. When the second developing roller is located at the separation position, the second developing roller is separated from the second photosensitive drum. When the image forming apparatus is in a monochrome printing mode, the separation mechanism may position the second developing roller at the separation position.

[0018] According to this configuration, it is possible to prevent the deposits adhering to the second photosensitive drum from being collected into the second developing cartridge by the second developing roller.

[0019] (5) When the image forming apparatus is in a multi-color printing mode, the separation mechanism may allow the second developing roller to be located at the contact position, and the control unit may apply a first developing bias to the second developing roller. When the image forming apparatus is in a monochrome printing mode, the separation mechanism may allow the second developing roller to be located at the contact position, and the control unit may apply a second developing bias to the second developing roller. The second developing bias is smaller than the first developing bias.

[0020] According to this configuration, the deposits adhering to the second photosensitive drum can be collected into the second developing cartridge by the second developing roller.

[0021] (6) The drum unit has a first charging device, a second charging device, and a third charging device. The first charging device charges the first photosensitive drum. The second charging device charges the second photosensitive drum. The third charging device charges the third photosensitive drum. When the image forming apparatus is in a monochrome printing mode, the control unit may apply a first charging bias to the first charging device, apply a third charging bias lower than the first charging bias to the third charging device, and apply a second charging bias higher than the third charging bias to the second charging device.

[0022] According to this configuration, the surface potential of the second photosensitive drum can be increased to cause the adhering matter on the sheet to adhere to the second photosensitive drum.

[0023] Therefore, it is possible to prevent the adherent matter on the sheet from adhering to the first photosensitive drum without adhering to the second photosensitive drum.

[0024] As a result, adhesion of foreign matter to the first photosensitive drum can be suppressed.

[0025] (7) The drum unit may further include a fourth photosensitive drum. The fourth photosensitive drum is located between the first photosensitive drum and the third photosensitive drum.

[0026] (8) The first photosensitive drum may be supplied with black toner, the second photosensitive drum may be supplied with yellow toner, the third photosensitive drum may be supplied with magenta toner, and the fourth photosensitive drum may be supplied with cyan toner. Effect of the Invention

[0027] According to the image forming apparatus of the present disclosure, in a configuration in which a first photosensitive drum used for printing monochrome images is positioned downstream of a second photosensitive drum not used for printing monochrome images, adhesion of foreign matter to the first photosensitive drum can be suppressed. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of an image forming apparatus. [Diagram 2] FIG. 2 is a block diagram for explaining the details of the image forming apparatus, showing a state in a multi-color printing mode. [Diagram 3] FIG. 3 is the block diagram shown in FIG. 2, illustrating the monochrome printing mode. [Figure 4] FIG. 4 is a table for explaining the state of the image forming apparatus in each of the multi-color printing mode, the monochrome printing mode, and the drum cleaning process. [Diagram 5] FIG. 5 is a block diagram of the configuration shown in FIG. 2, illustrating a state in which the drum cleaning process is being performed. [Figure 6] FIG. 6 is a table for explaining the first modified example. [Figure 7] FIG. 7 is a table for explaining the second modified example. [Figure 8] FIG. 8 is a table for explaining the third modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] 1. Overview of image forming apparatus 1 An outline of an image forming apparatus 1 will be described with reference to FIGS.

[0030] The image forming apparatus 1 shown in FIG. 1 can be switched between a multi-color printing mode (see FIG. 2) and a monochrome printing mode (see FIG. 3). When the image forming apparatus 1 is in the multi-color printing mode, the image forming apparatus 1 can print a multi-color image on a sheet S. When the image forming apparatus 1 is in the monochrome printing mode, the image forming apparatus 1 can print a monochrome image on a sheet S.

[0031] The image forming apparatus 1 includes a main body housing 2, a sheet storage section 3, a drum unit 4, an exposure device 5, a plurality of developing cartridges 6K, 6Y, 6M, and 6C, a transfer unit 7, and a fixing device 8.

[0032] 1.1 Main unit 2 The main body housing 2 accommodates a sheet container 3, a drum unit 4, an exposure device 5, developing cartridges 6K, 6Y, 6M, and 6C, a transfer unit 7, and a fixing device 8.

[0033] 1.2 Sheet storage section 3 The sheet storage unit 3 stores the sheet S. The sheet S in the sheet storage unit 3 is transported from the sheet storage unit 3 toward the photosensitive drum 41Y. The photosensitive drum 41Y will be described later. The sheet S is, for example, a printing paper. The sheet storage unit 3 may be a sheet cassette.

[0034] 1.3 Drum unit 4 The drum unit 4 includes a plurality of photosensitive drums 41K, 41Y, 41M, and 41C, and a plurality of charging devices 42K, 42Y, 42M, and 42C.

[0035] 1.3.1 Photosensitive drums 41K, 41Y, 41M, 41C The photosensitive drums 41K, 41Y, 41M, and 41C are arranged in the order of the photosensitive drum 41Y, the photosensitive drum 41M, the photosensitive drum 41C, and the photosensitive drum 41K in the conveying direction of the sheet S by the belt 71. The belt 71 will be described later. That is, the photosensitive drum 41K is located at the most downstream side among the photosensitive drums 41K, 41Y, 41M, and 41C in the conveying direction of the sheet S by the belt 71. The photosensitive drum 41Y is located at the most upstream side among the photosensitive drums 41K, 41Y, 41M, and 41C in the conveying direction of the sheet S by the belt 71.

[0036] The photosensitive drum 41K is used for printing monochrome images and multi-color images. Black toner is supplied to the photosensitive drum 41K. The photosensitive drum 41K extends in an axial direction. The photosensitive drum 41K is rotatable about a drum axis A1. The drum axis A1 extends in the axial direction. Note that "used for printing" means that the toner for printing is carried. Note that "not used for printing" means that the toner for printing is not carried.

[0037] The photosensitive drum 41Y is not used for printing a monochrome image but for printing a multi-color image. The photosensitive drum 41Y is located away from the photosensitive drum 41K in the transport direction. The transport direction intersects with the axial direction. Preferably, the transport direction is perpendicular to the axial direction. Yellow toner is supplied to the photosensitive drum 41Y.

[0038] The photosensitive drum 41M is not used for printing monochrome images but for printing multi-color images. The photosensitive drum 41M is located between the photosensitive drum 41Y and the photosensitive drum 41K in the transport direction. Magenta toner is supplied to the photosensitive drum 41M.

[0039] The photosensitive drum 41C is not used for printing monochrome images but for printing multi-color images. The photosensitive drum 41C is located between the photosensitive drum 41M and the photosensitive drum 41K in the transport direction. Cyan toner is supplied to the photosensitive drum 41C.

[0040] The description of each of the photosensitive drums 41Y, 41M, and 41C is similar to the description of the photosensitive drum 41K, and therefore, the description of the photosensitive drums 41Y, 41M, and 41C will be omitted.

[0041] 1.3.2 Charging devices 42K, 42Y, 42M, 42C The charging device 42K charges the photosensitive drum 41K. The charging device 42Y charges the photosensitive drum 41Y. The charging device 42M charges the photosensitive drum 41M. The charging device 42C charges the photosensitive drum 41C. In this embodiment, each of the charging devices 42K, 42Y, 42M, and 42C is a scorotron type charging device. Each of the charging devices 42K, 42Y, 42M, and 42C may be a charging roller.

[0042] 1.4 Exposure equipment 5 The exposure device 5 exposes the photosensitive drum 41K charged by the charging device 42K. In this embodiment, the exposure device 5 is a laser scan unit. The exposure device 5 can expose not only the photosensitive drum 41K but also the photosensitive drums 41Y, 41M, and 41C. The exposure device 5 may be an exposure head having an LED array.

[0043] 1.5 Developing cartridges 6K, 6Y, 6M, 6C Each of the developing cartridges 6K, 6Y, 6M, and 6C can be attached to the drum unit 4. The developing cartridge 6K includes a developing housing 61K and a developing roller 62K.

[0044] The developing housing 61K contains black toner. The toner is a non-magnetic one-component toner that can be charged by friction. In this embodiment, the toner is positively charged by friction.

[0045] The developing roller 62K is supported by the developing housing 61K. When the developing cartridge 6K is attached to the drum unit 4 and the drum unit 4 is located at the inner position, the developing roller 62K can supply toner in the developing housing 61K to the photosensitive drum 41K. The developing roller 62K extends in the axial direction. The developing roller 62K is rotatable about the developing shaft A2. The developing shaft A2 extends in the axial direction.

[0046] The description of each of the developing cartridges 6Y, 6M, and 6C is the same as the description of the developing cartridge 6K. That is, the developing cartridge 6Y has a developing roller 62Y. The developing roller 62Y supplies yellow toner to the photosensitive drum 41Y. The developing cartridge 6M has a developing roller 62M. The developing roller 62M supplies magenta toner to the photosensitive drum 41M. The developing cartridge 6C has a developing roller 62C. The developing roller 62C supplies cyan toner to the photosensitive drum 41C.

[0047] 1.6 Transcription unit 7 The transfer unit 7 includes a belt 71 and a plurality of transfer rollers 72K, 72Y, 72M, and 72C.

[0048] The belt 71 comes into contact with the photosensitive drums 41K, 41Y, 41M, and 41C. The belt 71 transports the sheet S from the sheet storage unit 3 toward the fixing device 8. The belt 71 transports the sheet S from the photosensitive drum 41Y toward the photosensitive drum 41K.

[0049] The transfer roller 72K transfers the toner on the photosensitive drum 41K to the sheet S being transported by the belt 71. The transfer roller 72Y transfers the toner on the photosensitive drum 41Y to the sheet S being transported by the belt 71. The transfer roller 72M transfers the toner on the photosensitive drum 41M to the sheet S being transported by the belt 71. The transfer roller 72C transfers the toner on the photosensitive drum 41C to the sheet S being transported by the belt 71.

[0050] 1.7 Fixing device 8 The fixing device 8 applies heat and pressure to the sheet S onto which the toner has been transferred, thereby fixing the toner to the sheet S. The sheet S that has passed through the fixing device 8 is discharged onto the top surface of the main body housing 2.

[0051] 2. Drum unit 4 details Next, the drum unit 4 will be described in detail.

[0052] In the image forming apparatus 1, when the image forming apparatus 1 is in a monochrome printing mode (see FIG. 3), a foreign object may adhere to the photosensitive drum 41Y. When the sheet S is a printing paper, the foreign object may be, for example, paper dust. The paper dust is negatively charged by the transfer current, and therefore adheres to the positively charged surface of the photosensitive drum 41Y.

[0053] The drum unit 4 includes a plurality of cleaning rollers 43K, 43Y, 43M, and 43C, a cleaning shaft 44, and a housing portion 45.

[0054] The cleaning roller 43K cleans the surface of the photosensitive drum 41K. The cleaning roller 43Y cleans the surface of the photosensitive drum 41Y. The cleaning roller 43M cleans the surface of the photosensitive drum 41M. The cleaning roller 43C cleans the surface of the photosensitive drum 41C. The deposits on the sheet S adhere to the surface of the photosensitive drum 41Y from the sheet S and are collected from the surface of the photosensitive drum 41Y by the cleaning roller 43Y.

[0055] The cleaning shaft 44 is provided for the cleaning roller 43Y. In this embodiment, the drum unit 4 does not have cleaning shafts for the cleaning rollers 43M, 43C, and 43K. The cleaning shaft 44 collects deposits from the cleaning roller 43Y.

[0056] The storage section 45 stores the deposits collected by the cleaning shaft 44 .

[0057] 3. Details of Image Forming Apparatus 1 Next, the image forming apparatus 1 will be described in detail with reference to FIGS.

[0058] As shown in FIG. 2, the image forming apparatus 1 includes a motor 10, a separating mechanism 11, and a control unit 12.

[0059] 3.1 Motor 10 The motor 10 is connected to the spacing mechanism 11 via a gear train 101. Power from the motor 10 is transmitted to the spacing mechanism 11 via the gear train 101.

[0060] 3.2 Separation mechanism 11 In this embodiment, when the image forming apparatus 1 is in a multi-color printing mode, the developing roller 62K contacts the photosensitive drum 41K, the developing roller 62Y contacts the photosensitive drum 41Y, the developing roller 62M contacts the photosensitive drum 41M, and the developing roller 62C contacts the photosensitive drum 41C. In this state, all of the photosensitive drums 41K, 41Y, 41M, and 41C are capable of carrying toner for printing. Therefore, all of the photosensitive drums 41K, 41Y, 41M, and 41C are used for printing.

[0061] On the other hand, as shown in FIG. 3, when the image forming apparatus 1 is in a monochrome printing mode, the developing roller 62K contacts the photosensitive drum 41K, the developing roller 62Y separates from the photosensitive drum 41Y, the developing roller 62M separates from the photosensitive drum 41M, and the developing roller 62C separates from the photosensitive drum 41C. In this state, the photosensitive drum 41K can carry toner for printing. Therefore, the photosensitive drum 41K is used for printing. On the other hand, the photosensitive drums 41Y, 41M, and 41C cannot carry toner for printing. Therefore, the photosensitive drums 41Y, 41M, and 41C are not used for printing.

[0062] As shown in Figs. 2 and 3, when the image forming apparatus 1 switches from the multi-color printing mode (see Fig. 2) to the monochrome printing mode (see Fig. 3), the separation mechanism 11 moves each of the developing rollers 62Y, 62M, and 62C from the contact position (see Fig. 2) to the separation position (see Fig. 3) by the power from the motor 10. In detail, the separation mechanism 11 moves the developing cartridge 6Y (see Fig. 1) to move the developing roller 62Y from the contact position to the separation position, moves the developing cartridge 6M (see Fig. 1) to move the developing roller 62M from the contact position to the separation position, and moves the developing cartridge 6C (see Fig. 1) to move the developing roller 62C from the contact position to the separation position. As a result, when the image forming apparatus 1 is in the monochrome printing mode, the separation mechanism 11 positions each of the developing rollers 62Y, 62M, and 62K at the separation position.

[0063] 2, when the developing roller 62Y is located at the contact position, the developing roller 62Y contacts the photosensitive drum 41Y. When the developing roller 62M is located at the contact position, the developing roller 62M contacts the photosensitive drum 41M. When the developing roller 62C is located at the contact position, the developing roller 62C contacts the photosensitive drum 41C.

[0064] 3, when the developing roller 62Y is located at the separated position, the developing roller 62Y is separated from the photosensitive drum 41Y. When the developing roller 62M is located at the separated position, the developing roller 62M is separated from the photosensitive drum 41M. When the developing roller 62C is located at the separated position, the developing roller 62C is separated from the photosensitive drum 41C.

[0065] In addition, when the image forming apparatus 1 switches from the monochrome printing mode (see FIG. 3) to the multi-color printing mode (see FIG. 2), the separation mechanism 11 allows each of the developing rollers 62Y, 62M, and 62C to move from the separation position (see FIG. 3) to the contact position (see FIG. 2).

[0066] 3.3 Control Unit 12 The control unit 12 controls the operation of the image forming apparatus 1. The control unit 12 switches the image forming apparatus 1 between a multi-color printing mode and a monochrome printing mode.

[0067] In detail, as shown in FIG. 2, the control unit 12 is electrically connected to the motor 10, the charging devices 42K, 42Y, 42M, 42C, the cleaning rollers 43K, 43Y, 43M, 43C, the cleaning shaft 44, the transfer rollers 72K, 72Y, 72M, 72C, and the developing rollers 62K, 62Y, 62M, 62C.

[0068] 4. Control of Image Forming Apparatus 1 Next, the control of the image forming apparatus 1 by the control unit 12 will be described with reference to FIGS.

[0069] 4.1 Multi-color printing mode 2, when the image forming apparatus 1 is in the multi-color printing mode, each of the developing rollers 62K, 62Y, 62M, and 62C is located at the contact position. In other words, when the image forming apparatus 1 is in the multi-color printing mode, the separation mechanism 11 allows the developing roller 62Y to be located at the contact position.

[0070] 4, the control unit 12 adjusts each of the developing bias Vb(K) applied to the developing roller 62K, the developing bias Vb(Y) applied to the developing roller 62Y, the developing bias Vb(M) applied to the developing roller 62M, and the developing bias Vb(C) applied to the developing roller 62C to +400 V. That is, the control unit 12 applies a first developing bias (+400 V) to the developing roller 62Y.

[0071] In addition, the control unit 12 adjusts the grid bias Vg(K) generated on the grid of the charging device 42K, the grid bias Vg(Y) generated on the grid of the charging device 42Y, the grid bias Vg(M) generated on the grid of the charging device 42M, and the grid bias Vg(C) generated on the grid of the charging device 42C to +850 V.

[0072] Furthermore, the control unit 12 adjusts the transfer current It(K) flowing between the transfer roller 72K and the photosensitive drum 41K, the transfer current It(Y) flowing between the transfer roller 72Y and the photosensitive drum 41Y, the transfer current It(M) flowing between the transfer roller 72M and the photosensitive drum 41M, and the transfer current It(C) flowing between the transfer roller 72C and the photosensitive drum 41C to a magnitude capable of transferring the toner to the sheet S. Specifically, each of the transfer currents It(K), It(Y), It(M), and It(C) is adjusted to a range of -16 μA to -24 μA.

[0073] In addition, the control unit 12 adjusts the cleaning bias Vc(K) applied to the cleaning roller 43K, the cleaning bias Vc(Y) applied to the cleaning roller 43Y, the cleaning bias Vc(M) applied to the cleaning roller 43M, and the cleaning bias Vc(C) applied to the cleaning roller 43C to -400 V.

[0074] Thereby, the cleaning roller 43K collects the residual toner remaining on the photosensitive drum 41K without being transferred to the sheet S. The cleaning roller 43Y collects the residual toner remaining on the photosensitive drum 41Y without being transferred to the sheet S. The cleaning roller 43M collects the residual toner remaining on the photosensitive drum 41M without being transferred to the sheet S. The cleaning roller 43C collects the residual toner remaining on the photosensitive drum 41C without being transferred to the sheet S. Note that any deposits adhering to the surface of the photosensitive drum 41Y remain on the photosensitive drum 41Y without being collected by the cleaning roller 43Y.

[0075] Furthermore, the control unit 12 adjusts the shaft bias Vs applied to the cleaning shaft 44 to −200 V. This makes the potential of the cleaning roller 43Y lower than that of the cleaning shaft 44. Therefore, the remaining toner collected by the cleaning roller 43Y does not move to the cleaning shaft 44, but is held by the cleaning roller 43Y.

[0076] 4.2 Monochrome Print Mode 2 and 3, when the image forming apparatus 1 is switched from the multi-color printing mode to the monochrome printing mode, the control unit 12 drives the motor 10. Then, the separation mechanism 11 moves each of the developing rollers 62Y, 62M, and 62C from the contact position (see FIG. 2) to the separation position (see FIG. 3) by the power from the motor 10.

[0077] As shown in FIG. 4, the control unit 12 sets each of the developing biases Vb(Y), Vb(M), and Vb(C) to the reference potential (0V) while maintaining the developing bias Vb(K) at +400V.

[0078] Furthermore, the control unit 12 adjusts each of the grid biases Vg(Y), Vg(M), and Vg(C) to +210V while maintaining the grid bias Vg(K) at +850V.

[0079] Furthermore, the control unit 12 adjusts the cleaning bias Vc(K) to -400V. This allows the cleaning roller 43K to collect the residual toner remaining on the photosensitive drum 41K without being transferred to the sheet S. The control unit 12 also adjusts each of the cleaning biases Vc(Y), Vc(M), and Vc(C) to -400V. Furthermore, the control unit 12 adjusts the shaft bias Vs to -200V.

[0080] Here, while maintaining the transfer current It(K) in the range of -16 μA to -24 μA, the control unit 12 reduces the transfer currents It(M) and It(C) to the extent that the sheet S can be attracted to the belt 71. Specifically, the transfer currents It(M) and It(C) are adjusted to the range of -5 μA to -8 μA.

[0081] Furthermore, the control unit 12 makes the transfer current It(Y) larger than the transfer currents It(M) and It(C) in order to negatively charge the deposits on the sheet S. In other words, the transfer current It(Y) is made larger than a current that can attract the sheet S to the belt 71. By negatively charging the deposits on the sheet S, the negatively charged deposits can be attached to the surface of the photosensitive drum 41Y. Furthermore, in a drum cleaning process described later, the deposits attached to the surface of the photosensitive drum 41Y can be easily collected by the cleaning roller 43Y.

[0082] The control unit 12 controls the transfer bias Vt(K) applied to the transfer roller 72K so that the transfer current It(K) becomes the target value. The control unit 12 controls the transfer bias Vt(Y) applied to the transfer roller 72Y so that the transfer current It(Y) becomes the target value. The control unit 12 controls the transfer bias Vt(M) applied to the transfer roller 72M so that the transfer current It(M) becomes the target value. The control unit 12 controls the transfer bias Vt(C) applied to the transfer roller 72C so that the transfer current It(C) becomes the target value.

[0083] That is, when the image forming apparatus 1 is in a monochrome printing mode, the control unit 12 applies a transfer bias Vt(K) to the transfer roller 72K, a transfer bias Vt(Y) to the transfer roller 72Y, a transfer bias Vt(M) to the transfer roller 72M, and a transfer bias Vt(C) to the transfer roller 72C. The transfer bias Vt(M) and the transfer bias Vt(C) are lower than the transfer bias Vt(K). The transfer bias Vt(Y) is higher than the transfer bias Vt(M) and the transfer bias Vt(C).

[0084] 4.3 Drum cleaning process 5, the control unit 12 executes a drum cleaning process at a predetermined timing. In the drum cleaning process, the residual toner collected by the cleaning roller 43K is returned to the photosensitive drum 41K, the residual toner collected by the cleaning roller 43Y is returned to the photosensitive drum 41Y, the residual toner collected by the cleaning roller 43M is returned to the photosensitive drum 41M, the residual toner collected by the cleaning roller 43C is returned to the photosensitive drum 41C, and the deposits on the photosensitive drum 41Y are collected by the cleaning roller 43Y and the cleaning shaft 44.

[0085] The remaining toner returned to the photosensitive drum 41K, the remaining toner returned to the photosensitive drum 41Y, the remaining toner returned to the photosensitive drum 41M, and the remaining toner returned to the photosensitive drum 41C are each transferred to the belt 71 (see FIG. 1) and collected by a belt cleaner that cleans the belt 71. The belt cleaner is not shown.

[0086] 2 and 5, when performing the drum cleaning process, the control unit 12 drives the motor 10. Then, the separation mechanism 11 moves all of the developing rollers 62K, 62Y, 62M, and 62C from the contact position (see FIG. 2) to the separation position (see FIG. 5) by the power from the motor 10.

[0087] As shown in FIG. 5, the control unit 12 sets each of the developing biases Vb(K), Vb(Y), Vb(M) and Vb(C) to the reference potential (0V).

[0088] In addition, the control unit 12 adjusts each of the grid biases Vg(K), Vg(Y), Vg(M), and Vg(C) to +210V.

[0089] Furthermore, the control unit 12 adjusts each of the transfer currents It(K), It(Y), It(M), and It(C) to a value that allows the remaining toner to be transferred to the belt 71. Specifically, the control unit 12 adjusts each of the transfer currents It(K), It(Y), It(M), and It(C) to −10 μA.

[0090] Furthermore, the control unit 12 adjusts the cleaning biases Vc(K), Vc(Y), Vc(M) and Vc(C) to +450V.

[0091] As a result, the surface potential of the photosensitive drum 41K becomes lower than the potential of the cleaning roller 43K. Then, the remaining toner held on the cleaning roller 43K returns to the photosensitive drum 41K. Similarly, the remaining toner held on the cleaning roller 43Y returns to the photosensitive drum 41Y, the remaining toner held on the cleaning roller 43M returns to the photosensitive drum 41M, and the remaining toner held on the cleaning roller 43C returns to the photosensitive drum 41C.

[0092] On the other hand, the deposits adhering to the surface of the photosensitive drum 41Y are negatively charged and are therefore collected from the photosensitive drum 41Y by the cleaning roller 43Y.

[0093] Further, the control unit 12 adjusts the shaft bias Vs to +650 V. This makes the potential of the cleaning shaft 44 higher than the potential of the cleaning roller 43Y. Therefore, the deposits collected by the cleaning roller 43Y are collected from the cleaning roller 43Y onto the cleaning shaft 44. The deposits collected by the cleaning shaft 44 are collected in the storage unit 45 (see FIG. 1). 4. Effects (1) According to image forming apparatus 1, as shown in FIG. 4, when image forming apparatus 1 is in a monochrome printing mode, control unit 12 passes transfer current It(K) through transfer roller 72K, passes transfer currents It(M) and It(C) smaller than transfer current It(K) through transfer rollers 72M and 72C, and passes transfer current It(Y) higher than transfer currents It(M) and It(C) through transfer roller 72Y.

[0094] In other words, a transfer bias Vt(K) is applied to transfer roller 72K, transfer biases Vt(M) and Vt(C) lower than the transfer bias Vt(K) are applied to transfer rollers 72M and 72C, and a transfer bias Vt(Y) higher than the transfer biases Vt(M) and Vt(C) is applied to transfer roller 72Y.

[0095] As a result, as shown in FIG. 3, when the image forming apparatus 1 is in a monochrome printing mode, the transfer bias Vt(Y) applied to the transfer roller 72Y can charge the adhesions on the sheet S and cause them to adhere to the photosensitive drum 41Y.

[0096] Therefore, it is possible to prevent the deposits on the sheet S from adhering to the photosensitive drum 41K instead of to the photosensitive drum 41Y.

[0097] As a result, adhesion of foreign matter to the photosensitive drum 41K can be suppressed.

[0098] (2) According to the image forming apparatus 1, as shown in FIG. 3, when the image forming apparatus 1 is in the monochrome printing mode, the spacing mechanism 11 positions the developing roller 62Y at the spacing position.

[0099] Therefore, it is possible to prevent the deposits adhering to the photosensitive drum 41Y from being collected into the developing cartridge 6Y by the developing roller 62Y.

[0100] 5. Variations Next, a modified example will be described with reference to Figures 6 to 8. In the modified example, the same members and biases as those in the above-described embodiment are denoted by the same reference numerals, and the description thereof will be omitted. (1) As shown in FIG. 6, when the image forming apparatus 1 is in a monochrome printing mode, the control unit 12 may apply to the charging device 42Y a grid bias Vg(Y) higher than the grid biases Vg(M) and Vg(C).

[0101] That is, when the image forming apparatus 1 is in a monochrome printing mode, the control unit 12 may apply a grid bias Vg(K) as a first charging bias to the charging device 42K, a grid bias Vg(Y) as a second charging bias to the charging device 42Y, a grid bias Vg(M) as a third charging bias to the charging device 42M, and a grid bias Vg(C) as a fourth charging bias to the charging device 42C. The grid biases Vg(M) and Vg(C) are lower than the grid bias Vg(K). The grid bias Vg(Y) is higher than the grid biases Vg(M) and Vg(C).

[0102] Specifically, the control unit 12 adjusts the grid bias Vg(Y) generated in the grid of the charging device 42Y to +850V.

[0103] According to this modification, the surface potential of the photosensitive drum 41Y can be increased to cause the deposits on the sheet S to adhere to the photosensitive drum 41Y.

[0104] Therefore, it is possible to prevent the deposits on the sheet S from adhering to the photosensitive drum 41K instead of to the photosensitive drum 41Y.

[0105] As a result, adhesion of foreign matter to the photosensitive drum 41K can be suppressed.

[0106] (2) As shown in FIG. 7, the control unit 12 may apply a grid bias Vg(Y) higher than the grid biases Vg(M) and Vg(C) to the charging device 42Y without making the transfer current It(Y) larger than the transfer currents It(M) and It(C).

[0107] 8, when the image forming apparatus 1 is in a monochrome printing mode, the separation mechanism 11 allows the developing roller 62Y to be located at the contact position, and the control unit 12 makes the transfer current It(Y) larger than the transfer currents It(M) and It(C), applies a grid bias Vg(Y) higher than the grid biases Vg(M) and Vg(C) to the charging device 42Y, and further applies a second developing bias (+250V) to the developing roller 62Y. The second developing bias is smaller than the above-mentioned first developing bias (+400V).

[0108] According to this configuration, when the image forming apparatus 1 is in a monochrome printing mode, the potential of the developing roller 62Y is lower than the surface potential of the photosensitive drum 41Y. On the other hand, the deposits adhering to the surface of the photosensitive drum 41Y are positively charged by the charging device 42Y and therefore move from the photosensitive drum 41Y to the developing roller 62Y.

[0109] As a result, the deposits adhering to the photosensitive drum 41Y can be collected into the developing cartridge 6Y by the developing roller 62Y. [Explanation of symbols]

[0110] 1. Image forming device 4 Drum Unit 6K Developing Cartridge (1st Developing Cartridge) 6Y Developing Cartridge (2nd Developing Cartridge) 6M Developing Cartridge (3rd Developing Cartridge) 7 Transcription Unit 11 Separation mechanism 12 Control section 41K Photosensitive drum (first photosensitive drum) 41Y Photosensitive drum (second photosensitive drum) 41M Photosensitive drum (third photosensitive drum) 41C Photosensitive drum (fourth photosensitive drum) 42K Charging device (1st charging device) 42Y Charging device (second charging device) 42M Charging device (third charging device) 43Y Cleaning roller 44 Cleaning shaft 45 Storage unit 62K Developing roller (1st developing roller) 62Y Developing roller (second developing roller) 62M Developing Roller (3rd Developing Roller) 71 Belt 72K Transfer roller (1st transfer roller) 72Y Transfer roller (second transfer roller) 72M Transfer roller (3rd transfer roller) S Seat

Claims

1. An image forming apparatus capable of switching between a multi-color printing mode capable of printing a multi-color image on a sheet and a monochrome printing mode capable of printing a monochrome image on a sheet, A drum unit comprising: a first photosensitive drum used for printing monochrome images and multi-color images; a second photosensitive drum that is not used for printing a monochrome image but is used for printing a multi-color image, the second photosensitive drum being located apart from the first photosensitive drum; a cleaning roller for cleaning the surface of the second photosensitive drum; a cleaning shaft that collects, from the cleaning roller, the deposit that has adhered to the surface of the second photosensitive drum from the sheet and that has been collected from the surface of the second photosensitive drum by the cleaning roller; a third photosensitive drum that is not used for printing a monochrome image but is used for printing a multi-color image, the third photosensitive drum being located between the first photosensitive drum and the second photosensitive drum; a first charging device for charging the first photosensitive drum; a second charging device for charging the second photosensitive drum; a third charging device for charging the third photosensitive drum; A drum unit having a first developing cartridge having a first developing roller for supplying toner to the first photosensitive drum; a second developing cartridge having a second developing roller for supplying toner to the second photosensitive drum; a third developing cartridge having a third developing roller for supplying toner to the third photosensitive drum; a separation mechanism that moves the second developing roller from a contact position where the second developing roller contacts the second photosensitive drum to a separation position where the second developing roller is separated from the second photosensitive drum, and moves the third developing roller from the contact position where the third developing roller contacts the third photosensitive drum to the separation position where the third developing roller is separated from the third photosensitive drum; A transcription unit comprising: a belt that transports a sheet from the second photosensitive drum toward the first photosensitive drum; a first transfer roller for transferring the toner on the first photosensitive drum to the sheet; a second transfer roller for transferring the toner on the second photosensitive drum to the sheet; a transfer unit having a third transfer roller for transferring the toner on the third photosensitive drum to the sheet; A control unit; Equipped with When the image forming apparatus is in a monochrome printing mode, the spacing mechanism positions the second developing roller and the third developing roller at a spacing position; The control unit is A first charging bias is applied to the first charging device; applying a second charging bias to the second charging device, the second charging bias being smaller than the first charging bias; applying a third charging bias to the third charging device, the third charging bias being smaller than the first charging bias; A first transfer bias is applied to the first transfer roller; applying a third transfer bias to the third transfer roller, the third transfer bias being smaller than the first transfer bias; a second transfer bias greater than the third transfer bias is applied to the second transfer roller.

2. A multi-color printing mode capable of printing a multi-color image on a sheet, and a monochrome printing mode capable of printing a monochrome image on a sheet. An image forming apparatus capable of switching between a printing mode and a A drum unit comprising: a first photosensitive drum used for printing monochrome images and multi-color images; a second photosensitive drum that is not used for printing monochrome images but is used for printing multi-color images, a second photosensitive drum located apart from the first photosensitive drum; a cleaning roller for cleaning the surface of the second photosensitive drum; The deposit adhered to the surface of the second photosensitive drum from the sheet, The deposits collected by the cleaning roller from the surface of the drum are A cleaning shaft to be collected from the a third photosensitive drum that is not used for printing monochrome images but is used for printing multi-color images, a third photosensitive drum located between the first photosensitive drum and the second photosensitive drum; a first charging device for charging the first photosensitive drum; a second charging device for charging the second photosensitive drum; a third charging device for charging the third photosensitive drum; A drum unit having a first developing cartridge having a first developing roller for supplying toner to the first photosensitive drum; a second developing cartridge having a second developing roller for supplying toner to the second photosensitive drum; a third developing cartridge having a third developing roller for supplying toner to the third photosensitive drum; a separation mechanism that moves the second developing roller from a contact position where the second developing roller contacts the second photosensitive drum to a separation position where the second developing roller is separated from the second photosensitive drum, and moves the third developing roller from the contact position where the third developing roller contacts the third photosensitive drum to the separation position where the third developing roller is separated from the third photosensitive drum; A transcription unit comprising: a belt that transports a sheet from the second photosensitive drum toward the first photosensitive drum; a first transfer roller for transferring the toner on the first photosensitive drum to the sheet; a second transfer roller for transferring the toner on the second photosensitive drum to the sheet; a third transfer roller for transferring the toner on the third photosensitive drum to the sheet; and a transcription unit having A control unit; Equipped with When the image forming apparatus is in the monochrome printing mode, the spacing mechanism positions the second developing roller and the third developing roller at a spacing position; The control unit is A first charging bias is applied to the first charging device; applying a third charging bias to the third charging device, the third charging bias being smaller than the first charging bias; applying a second charging bias to the second charging device, the second charging bias being greater than the third charging bias; A first transfer bias is applied to the first transfer roller; applying a third transfer bias to the third transfer roller, the third transfer bias being smaller than the first transfer bias; a second transfer bias greater than the third transfer bias is applied to the second transfer roller.

3. A multi-color printing mode capable of printing a multi-color image on a sheet, and a monochrome printing mode capable of printing a monochrome image on a sheet. An image forming apparatus capable of switching between a printing mode and a A drum unit comprising: a first photosensitive drum used for printing monochrome images and multi-color images; a second photosensitive drum that is not used for printing monochrome images but is used for printing multi-color images, a second photosensitive drum located apart from the first photosensitive drum; a cleaning roller for cleaning the surface of the second photosensitive drum; The deposit adhered to the surface of the second photosensitive drum from the sheet, The deposits collected by the cleaning roller from the surface of the drum are A cleaning shaft to be collected from the a third photosensitive drum that is not used for printing monochrome images but is used for printing multi-color images, a third photosensitive drum located between the first photosensitive drum and the second photosensitive drum; a first charging device for charging the first photosensitive drum; a second charging device for charging the second photosensitive drum; a third charging device for charging the third photosensitive drum; A drum unit having a first developing cartridge having a first developing roller for supplying toner to the first photosensitive drum; a second developing cartridge having a second developing roller for supplying toner to the second photosensitive drum; a third developing cartridge having a third developing roller for supplying toner to the third photosensitive drum; a separation mechanism that moves the second developing roller from a contact position where the second developing roller contacts the second photosensitive drum to a separation position where the second developing roller is separated from the second photosensitive drum, and moves the third developing roller from the contact position where the third developing roller contacts the third photosensitive drum to the separation position where the third developing roller is separated from the third photosensitive drum; A transcription unit comprising: a belt that transports a sheet from the second photosensitive drum toward the first photosensitive drum; a first transfer roller for transferring the toner on the first photosensitive drum to the sheet; a second transfer roller for transferring the toner on the second photosensitive drum to the sheet; a third transfer roller for transferring the toner on the third photosensitive drum to the sheet; and a transcription unit having A control unit; Equipped with When the image forming apparatus is in the multi-color printing mode, the separation mechanism positions the second developing roller at the contact position, the control unit applies a first developing bias to the second developing roller; When the image forming apparatus is in the monochrome printing mode, the separation mechanism positions the second developing roller at the contact position and positions the third developing roller at the separation position; The control unit is applying a second developing bias to the second developing roller, the second developing bias being smaller than the first developing bias; A first charging bias is applied to the first charging device; applying a third charging bias to the third charging device, the third charging bias being smaller than the first charging bias; applying a second charging bias to the second charging device, the second charging bias being greater than the third charging bias and greater than the second developing bias; A first transfer bias is applied to the first transfer roller; applying a third transfer bias to the third transfer roller, the third transfer bias being smaller than the first transfer bias; a second transfer bias greater than the third transfer bias is applied to the second transfer roller.

4. An image forming apparatus capable of switching between a multi-color printing mode capable of printing a multi-color image on a sheet and a monochrome printing mode capable of printing a monochrome image on a sheet, A drum unit comprising: a first photosensitive drum used for printing monochrome images and multi-color images; a second photosensitive drum that is not used for printing a monochrome image but is used for printing a multi-color image, the second photosensitive drum being located apart from the first photosensitive drum; a cleaning roller for cleaning the surface of the second photosensitive drum; a cleaning shaft that collects, from the cleaning roller, the deposit that has adhered to the surface of the second photosensitive drum from the sheet and that has been collected from the surface of the second photosensitive drum by the cleaning roller; a third photosensitive drum that is not used for printing a monochrome image but is used for printing a multi-color image, the third photosensitive drum being located between the first photosensitive drum and the second photosensitive drum; a first charging device for charging the first photosensitive drum; a second charging device for charging the second photosensitive drum; a third charging device for charging the third photosensitive drum; A drum unit having a first developing cartridge having a first developing roller for supplying toner to the first photosensitive drum; a second developing cartridge having a second developing roller for supplying toner to the second photosensitive drum; a third developing cartridge having a third developing roller for supplying toner to the third photosensitive drum; a separation mechanism that moves the second developing roller from a contact position where the second developing roller contacts the second photosensitive drum to a separation position where the second developing roller is separated from the second photosensitive drum, and moves the third developing roller from the contact position where the third developing roller contacts the third photosensitive drum to the separation position where the third developing roller is separated from the third photosensitive drum; A transcription unit comprising: a belt that transports a sheet from the second photosensitive drum toward the first photosensitive drum; a first transfer roller for transferring the toner on the first photosensitive drum to the sheet; a second transfer roller for transferring the toner on the second photosensitive drum to the sheet; a third transfer roller for transferring the toner on the third photosensitive drum to the sheet; A transcription unit having A control unit; Equipped with When the image forming apparatus is in a monochrome printing mode, the spacing mechanism positions the second developing roller and the third developing roller at a spacing position; The control unit is A first charging bias is applied to the first charging device; applying a third charging bias to the third charging device, the third charging bias being smaller than the first charging bias; applying a second charging bias to the second charging device, the second charging bias being greater than the third charging bias; A first transfer bias is applied to the first transfer roller; applying a second transfer bias to the second transfer roller, the second transfer bias being smaller than the first transfer bias; a third transfer bias that is smaller than the first transfer bias is applied to the third transfer roller.

5. When the image forming apparatus is in the monochrome printing mode, the control unit applies a developing bias to the first developing roller, the second charging bias is smaller than the developing bias, 2. The image forming apparatus according to claim 1, wherein the third charging bias is smaller than the developing bias.

6. When the image forming apparatus is in the monochrome printing mode, the control unit applies a developing bias to the first developing roller, the second charging bias is greater than the developing bias; 5. The image forming apparatus according to claim 2, wherein the third charging bias is smaller than the developing bias.

7. The first charging bias, the second charging bias, and the third charging bias are positive polarity, the developing bias is positive; 7. The image forming apparatus according to claim 5, wherein the first transfer bias, the second transfer bias and the third transfer bias are negative.

8. The first charging bias, the second charging bias, and the third charging bias are positive polarity, the first developing bias and the second developing bias have a positive polarity; 4. The image forming apparatus according to claim 3, wherein the first transfer bias, the second transfer bias and the third transfer bias are negative.

9. An image forming apparatus as described in claim 7 or claim 8, wherein when the image forming apparatus is in the monochrome printing mode, the control unit applies a negative cleaning bias to the cleaning roller and applies a negative shaft bias smaller than the cleaning bias to the cleaning shaft.

10. The drum unit includes: The image forming apparatus according to claim 1 , further comprising a storage portion for storing the collected deposits on the cleaning shaft.

11. The drum unit includes: The image forming apparatus according to claim 1 , further comprising a fourth photosensitive drum located between the first photosensitive drum and the third photosensitive drum.

12. The first photosensitive drum is supplied with black toner, Yellow toner is supplied to the second photosensitive drum, The third photosensitive drum is supplied with magenta toner, 12. The image forming apparatus according to claim 11, wherein cyan toner is supplied to the fourth photosensitive drum.

Images