Image forming apparatus

Abstract

To reliably prevent color toner fogging while preventing deterioration of an image carrier.SOLUTION: An image forming apparatus comprises a plurality of image forming units, an intermediate transfer body, primary transfer members, a secondary transfer member, and a control unit. When a transition is made from color printing to a monochrome printing, the control unit stops the drive of developer carriers in color image forming units while driving image carriers in the color image forming units, and performs elapsed time determination processing of determining whether the time elapsed from when the drive of the developer carriers in the color image forming units is stopped reaches fogging resolving time. When determining that the elapsed time reaches the fogging resolving time, the control unit causes a monochrome image forming unit to start formation of a toner image. When performing the elapsed time determination processing, the control unit determines the degree of ease of transfer of toner from the image carriers in the color image forming units to the intermediate transfer body, and sets the fogging resolving time on the basis of the degree.SELECTED DRAWING: Figure 4

Description

[Technical field]

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

[0002] A conventional image forming apparatus includes four image forming units, each corresponding to a different color of cyan, magenta, yellow, and black. The four image forming units form toner images using toners of the corresponding colors, and perform primary transfer onto an intermediate transfer body. The toner images on the intermediate transfer body are secondarily transferred onto a sheet. Such an image forming apparatus is disclosed, for example, in Patent Document 1.

[0003] Each of the four image forming units includes an image carrier. The image carrier is rotatably supported. During image formation, a toner image is formed on the image carrier. Then, the toner image is primarily transferred from the image carrier to an intermediate transfer member. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2002-23451 A Summary of the Invention [Problem to be solved by the invention]

[0005] The conventional image forming apparatus described above is capable of performing color printing and monochrome printing. In color printing, toner images are formed in both the color image forming section using color toners (cyan, magenta, and yellow toners) and the monochrome image forming section using black toner. The toner images of each color are primarily transferred onto the intermediate transfer body so as to be superimposed on each other, and then secondarily transferred onto a sheet. On the other hand, in monochrome printing, the monochrome toner image formed in the monochrome image forming section is primarily transferred onto the intermediate transfer body, and then secondarily transferred onto a sheet.

[0006] When color printing is successively switched to monochrome printing, toner may be transferred from the image carrier of the color image forming section to the intermediate transfer body, and in some cases, color toner fogging may occur, in which color toner adheres to the transfer area of ​​the monochrome toner image and its surroundings.

[0007] In order to prevent such color toner fogging, it is conceivable to drive the image carrier in the color image forming section until the toner is no longer transferred from the image carrier to the intermediate transfer body, and then start monochrome printing. However, in this case, the driving time of the image carrier becomes longer, and the image carrier becomes more susceptible to deterioration.

[0008] The present invention has been made to solve the above problems, and has an object to provide an image forming apparatus that can reliably suppress color toner fogging while suppressing deterioration of an image carrier. [Means for solving the problem]

[0009] In order to achieve the above object, an image forming apparatus according to one aspect of the present invention includes a plurality of image forming units that form toner images using toners of different colors, an intermediate transfer body to which the toner images are primarily transferred, a primary transfer member that primarily transfers the toner image to the intermediate transfer body, a secondary transfer member that secondary transfers the toner image to a sheet, and a control unit. The plurality of image forming units include a color image forming unit that uses color toners and a monochrome image forming unit that uses black toner. Each of the plurality of image forming units has an image carrier that carries a toner image that is primarily transferred to the intermediate transfer body and rotates, and a developer carrier that carries toner and supplies toner to the image carrier while rotating, thereby developing an electrostatic latent image formed on the image carrier into a toner image. When performing color printing, the control unit causes all of the plurality of image forming units to form a toner image. When performing monochrome printing, the control unit causes the monochrome image forming unit to form a toner image without causing the color image forming unit to form a toner image. When continuously shifting from color printing to monochrome printing, the control unit stops driving the developer carrier of the color image forming unit, and performs an elapsed time judgment process to judge whether the elapsed time since stopping the driving of the developer carrier of the color image forming unit has reached the fogging elimination time. When it is determined that the fogging elimination time has been reached, the control unit causes the monochrome image forming unit to start forming a toner image. When performing the elapsed time judgment process, the control unit judges the degree of ease of transfer of toner from the image carrier of the color image forming unit to the intermediate transfer body, and sets the fogging elimination time based on the degree. Effect of the Invention

[0010] In the present invention, color toner fogging is reliably suppressed while deterioration of the image carrier is suppressed. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is a schematic diagram of an image forming unit according to an embodiment. [Diagram 3] 1 is a block diagram of an image forming apparatus according to an embodiment. [Figure 4]5 is a flowchart showing a flow of processing performed by a control unit according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the image forming apparatus 100 of this embodiment will be described with reference to Fig. 1 to Fig. 4. Usually, the image forming apparatus 100 is placed on a substantially flat floor surface FL. The direction perpendicular to the floor surface FL is the up-down direction of the image forming apparatus 100.

[0013] <Configuration of Image Forming Apparatus> As shown in Fig. 1, the image forming apparatus 100 includes a main transport path MP. The image forming apparatus 100 also includes a sheet cassette CA. The sheet cassette CA is detachable from the main body of the image forming apparatus 100. The sheet cassette CA stores sheets S used in a print job. The main transport path MP receives the sheets S from the sheet cassette CA. The main transport path MP runs from a supply position P0 of the sheet S through a transfer position P1 and a fixing position P2 to an output tray ET.

[0014] In a print job, a sheet S in a sheet cassette CA is supplied from a supply position P0 to a main transport path MP, and the sheet S is transported along the main transport path MP. An image is formed using toner. The image is then printed on the sheet S during transport. In other words, a transfer process of the image onto the sheet S during transport is performed at a transfer position P1. A fixing process of the image onto the sheet S is performed at a fixing position P2.

[0015] The image forming apparatus 100 includes an image forming unit 1. There are four image forming units 1. The four image forming units 1 correspond to the colors cyan, magenta, yellow, and black, respectively. Each of the four image forming units 1 forms an image (i.e., a toner image) using toner of the corresponding color. In FIG. 1, the reference numerals of the image forming units 1 are suffixed with "A" or "B" for later explanation.

[0016] The following description focuses on one image forming unit 1 and explains its configuration, but the four image forming units 1 have the same configuration. Therefore, the description of the configurations of the other image forming units 1 will be omitted as the following description is incorporated herein.

[0017] 2, the image forming unit 1 includes a photoconductor drum 11, a charging device 12, an exposure device 13, a developing device 14, and a cleaning device 15. The photoconductor drum 11 corresponds to an "image carrier." For example, the photoconductor drum 11, the charging device 12, the developing device 14, and the cleaning device 15 form a single unit. The exposure device 13 is a separate unit from the single unit.

[0018] The photoconductor drum 11 is supported so as to be rotatable about an axis extending in one direction (a direction perpendicular to the plane of the paper in FIG. 2). The photoconductor drum 11 rotates while carrying a toner image on its outer circumferential surface.

[0019] The charging device 12 has a charging roller 120. The charging roller 120 contacts the outer circumferential surface of the photoreceptor drum 11. The charging roller 120 applies a charging bias to the outer circumferential surface of the photoreceptor drum 11. The charging roller 120 charges the outer circumferential surface of the photoreceptor drum 11.

[0020] The exposure device 13 exposes the outer peripheral surface of the photoconductor drum 11 to light to attenuate the charge. The exposure device 13 exposes the outer peripheral surface of the photoconductor drum 11 to light to form an electrostatic latent image on the outer peripheral surface of the photoconductor drum 11.

[0021] The developing device 14 contains a developer containing toner. The developer is a two-component developer containing toner and a carrier. The developing device 14 also has a developing roller 140. The developing roller 140 corresponds to a "developer carrier." The developing device 14 also has a stirring screw 141.

[0022] The developing roller 140 is supported so as to be rotatable about an axis extending in one direction (a direction perpendicular to the paper surface of FIG. 2). The developing roller 140 carries a developer (i.e., toner) on its outer circumferential surface. A magnetic brush is formed on the outer circumferential surface of the developing roller 140.

[0023] A developing bias is applied to the developing roller 140. The developing roller 140 rotates while bringing a magnetic brush into contact with the outer peripheral surface of the photoconductor drum 11. As a result, toner adheres to the outer peripheral surface of the photoconductor drum 11, and a toner image is formed on the outer peripheral surface of the photoconductor drum 11. In other words, the developing roller 140 supplies toner to the outer peripheral surface of the photoconductor drum 11, and develops the electrostatic latent image formed on the outer peripheral surface of the photoconductor drum 11 into a toner image.

[0024] The stirring screw 141 is supported so as to be rotatable about an axis extending in one direction (a direction perpendicular to the plane of FIG. 2). The stirring screw 141 stirs the developer contained in the developing device 14 by rotating. For example, the stirring screw 141 rotates by a driving force from the same driving source (motor) as the developing roller 140. The stirring screw 141 rotates together with the developing roller 140. When the developing roller 140 stops rotating, the stirring screw 141 also stops rotating.

[0025] The cleaning device 15 removes residual toner on the outer circumferential surface of the photoreceptor drum 11 downstream in the rotation direction of the photoreceptor drum 11 from a contact position between the intermediate transfer belt 2 and the photoreceptor drum 11, which will be described later. The toner that has not been transferred to the intermediate transfer belt 2 remains on the outer circumferential surface of the photoreceptor drum 11.

[0026] As shown in FIG. 1, the image forming apparatus 100 includes an intermediate transfer belt 2. The intermediate transfer belt 2 corresponds to an "intermediate transfer body." The intermediate transfer belt 2 is an endless belt. The intermediate transfer belt 2 is stretched by a plurality of tension rollers including a belt drive roller 20 and rotatably supported. In FIG. 1, the reference numerals of rollers other than the belt drive roller 20 among the plurality of tension rollers are omitted.

[0027] The belt drive roller 20 is connected to a belt motor (not shown). The belt drive roller 20 rotates by power transmitted from the belt motor. The intermediate transfer belt 2 rotates in response to the rotation of the belt drive roller 20.

[0028] The image forming apparatus 100 includes a primary transfer roller 3. The primary transfer roller 3 corresponds to a "primary transfer member." There are four primary transfer rollers 3. One primary transfer roller 3 is assigned to each of the colors cyan, magenta, yellow, and black. Each primary transfer roller 3 is disposed on the inner periphery side of the intermediate transfer belt 2. Each photoconductor drum 11 is disposed on the outer periphery side of the intermediate transfer belt 2. Each primary transfer roller 3 is pressed against the photoconductor drum 11 carrying a toner image of the corresponding color via the intermediate transfer belt 2.

[0029] The image forming apparatus 100 includes a secondary transfer roller 4. The secondary transfer roller 4 corresponds to a "secondary transfer member." The secondary transfer roller 4 is in pressure contact with the outer circumferential surface of the intermediate transfer belt 2 at a transfer position P1. The secondary transfer roller 4 sandwiches the intermediate transfer belt 2 between itself and the belt drive roller 20, and forms a transfer nip between itself and the outer circumferential surface of the intermediate transfer belt 2. This forms the transfer nip at the transfer position P1. The main transport path MP passes through the transfer nip.

[0030] The image forming apparatus 100 includes a belt cleaning unit 200. The belt cleaning unit 200 cleans the outer peripheral surface of the intermediate transfer belt 2. The belt cleaning unit 200 removes toner remaining on the outer peripheral surface of the intermediate transfer belt 2 downstream of the transfer position P1 in the rotation direction of the intermediate transfer belt 2.

[0031] In a print job, the sheet S is transported toward the transfer position P1 (i.e., the transfer nip). The sheet S passes through the transfer nip during the transport. That is, the outer circumferential surface of the intermediate transfer belt 2 contacts the sheet S downstream of the contact position with each photoconductor drum 11 in the belt rotation direction.

[0032] Each image forming unit 1 forms a toner image using toner of a corresponding color. Each primary transfer roller 3 primarily transfers the toner image onto the outer circumferential surface of the intermediate transfer belt 2. The intermediate transfer belt 2 rotates while carrying the toner image. A secondary transfer roller 4 secondarily transfers the toner image onto the sheet S passing through the transfer nip.

[0033] In color printing, toner images are formed in all four image forming units 1. The toner images of each color are sequentially transferred (primary transfer) to the intermediate transfer belt 2 and superimposed on each other. As a result, a full-color toner image is formed on the intermediate transfer belt 2. The full-color toner image is then (secondary transfer) transferred onto the sheet S.

[0034] In monochrome printing, toner images are not formed in the three image forming units 1 that use color toners (cyan, magenta, and yellow toners), but a toner image is formed only in one image forming unit 1 that uses black toner. As a result, a monochrome toner image is formed on the intermediate transfer belt 2. The monochrome toner image is secondarily transferred onto the sheet S.

[0035] The three image forming units 1 that use color toners correspond to "color image forming units." The one image forming unit 1 that uses black toner corresponds to "monochrome image forming unit." In the following explanation, the three image forming units 1 that correspond to "color image forming units" are given the suffix "A" and these three image forming units 1 are sometimes referred to as color image forming unit 1A. Additionally, the one image forming unit 1 that corresponds to the "monochrome image forming unit" is given the suffix "B" and this one image forming unit 1 is sometimes referred to as monochrome image forming unit 1B.

[0036] In addition, some conventional image forming apparatuses are configured to be able to separate the color toner photoconductor drum from the intermediate transfer belt during monochrome printing in order to suppress deterioration of the color toner photoconductor drum. However, this configuration requires a drum separation mechanism for separating the photoconductor drum from the intermediate transfer belt, which complicates the structure and increases costs.

[0037] On the other hand, the image forming apparatus 100 of the present embodiment is not provided with a drum separation mechanism. This configuration can prevent the structure from becoming complicated. That is, it can prevent an increase in costs.

[0038] The image forming apparatus 100 also includes a fixing unit FX. The fixing unit FX includes a heating roller and a pressure roller. The fixing unit FX is disposed at a fixing position P2. The heating roller has a built-in heater. The pressure roller is in pressure contact with the heating roller. The heating roller and the pressure roller are in pressure contact with each other to form a fixing nip at the fixing position P2.

[0039] In a print job, the sheet S passes through the fixing position P2. That is, the sheet S is sandwiched in the fixing nip. The fixing unit FX heats the sheet S as it passes through the fixing position P2. Pressure is applied to the sheet S at the fixing position P2. The fixing unit FX applies heat and pressure to the sheet S to fix the toner image to the sheet S. The sheet S after the fixing process is discharged to an output tray ET.

[0040] The image forming apparatus 100 includes a transport unit, the reference numerals of which are omitted. The transport unit includes a transport roller pair. The transport roller pair includes a pair of rollers. The pair of rollers has a transport nip between the rollers. The transport roller pair rotates to transport the sheet S that has entered the transport nip. The transport unit transports the sheet S along a main transport path MP. The transport unit also transports the sheet S along a double-sided printing transport path DP, which will be described later.

[0041] The image forming apparatus 100 is capable of executing a double-sided print job in which an image is printed on both sides of a sheet S, in addition to a single-sided print job in which an image is printed on only one side of the sheet S. In order to execute a double-sided print job, the image forming apparatus 100 is provided with a double-sided print transport path DP.

[0042] The double-sided printing transport path DP branches off from the main transport path MP at a branching position P3 downstream of the fixing position P2 in the sheet transport direction of the main transport path MP, and merges with the main transport path MP at a merging position P4 upstream of the transfer position P1 in the sheet transport direction of the main transport path MP.

[0043] When the job to be executed is a single-sided print job, the sheet S passes through the transfer nip only once, and a single transfer process is performed on the sheet S while it is passing through the transfer nip. After the first transfer process, the sheet S is discharged directly onto the discharge tray ET.

[0044] When the job to be executed is a double-sided printing job, the sheet S passes through the transfer nip twice in order to perform the transfer process once on each of the front and back sides of the sheet S. Specifically, when the sheet S passes through the transfer nip for the first time, the transfer process is performed on one side of the sheet S. After the first transfer process, the sheet S is switched back after the rear end of the sheet S passes through the branch position P3 and before the sheet S is completely discharged onto the discharge tray ET. As a result, the rear end of the sheet S is drawn into the double-sided printing transport path DP.

[0045] Thereafter, the sheet S is transported along the double-sided printing transport path DP. Then, the sheet S on the double-sided printing transport path DP is returned to the main transport path MP from the junction position P4. The sheet S returned to the main transport path MP is transported along the main transport path MP and passes through the transfer nip again. At this time, the orientation of the front and back surfaces of the sheet S is reversed to the orientation when it passed through the transfer nip the previous time. As a result, when the sheet S passes through the transfer nip for the second time, a transfer process is performed on the other side of the sheet S that is opposite to the one side.

[0046] As shown in FIG. 3, the image forming apparatus 100 includes a control unit 5. The control unit 5 includes processing circuits such as a CPU and an ASIC. The control unit 5 also includes storage devices such as a ROM and a RAM. The control unit 5 controls print jobs executed by the image forming apparatus 100. The control unit 5 controls image formation by the four image forming units 1. The control unit 5 controls the rotation of rotating bodies such as the photoconductor drum 11 and the developing roller 140. In other words, the control unit 5 controls motors that rotate the rotating bodies.

[0047] The image forming apparatus 100 includes a communication unit 101. The communication unit 101 includes a communication circuit, a communication memory, a communication connector, and the like. The communication unit 101 is communicably connected to an external device via a network such as a LAN. An example of the external device is a user terminal. A personal computer (PC), a smartphone, a tablet computer, and the like can serve as the user terminal.

[0048] The control unit 5 communicates with an external device using the communication unit 101. For example, print data of a print job is transmitted from an external device (user terminal) to the image forming apparatus 100. The print data includes image data to be printed in the print job.

[0049] The control unit 5 adds a print job corresponding to the received print data to a queue. The control unit 5 executes the print jobs in the queue in the order in which they are received. If there are multiple print jobs in the queue, the multiple print jobs are executed consecutively.

[0050] The control unit 5 determines whether the print job in the queue is color printing or monochrome printing. When performing color printing, the control unit 5 causes both the color image forming unit 1A and the monochrome image forming unit 1B to form toner images. On the other hand, when performing monochrome printing, the control unit 5 causes the monochrome image forming unit 1B to form a toner image without causing the color image forming unit 1A to form a toner image.

[0051] For example, if one print job is color printing and the next print job is monochrome printing, then color printing and monochrome printing will be performed consecutively in that order, i.e., in some cases, there will be a continuous transition from color printing to monochrome printing.

[0052] The image forming apparatus 100 includes an operation unit 102. The operation unit 102 is an operation panel and includes a touch screen. The operation unit 102 receives settings, instructions, and the like from a user. The operation unit 102 is connected to a control unit 5. The control unit 5 detects the settings, instructions, and the like received by the operation unit 102 from the user.

[0053] The image forming apparatus 100 includes a storage unit 6. Various storage devices such as a flash memory, an HDD, and an SSD can be used as the storage unit 6. The storage unit 6 is connected to the control unit 5. The control unit 5 writes data to the storage unit 6 and reads data from the storage unit 6.

[0054] Image forming apparatus 100 includes a temperature and humidity sensor 7. Temperature and humidity sensor 7 outputs values ​​according to the temperature and humidity inside image forming apparatus 100. Control unit 5 detects the temperature and humidity inside image forming apparatus 100 based on the output of temperature and humidity sensor 7.

[0055] The image forming apparatus 100 includes an image density sensor 8. The image density sensor 8 is a reflective optical sensor having a light emitting section and a light receiving section. The image density sensor 8 irradiates light toward the outer peripheral surface of the intermediate transfer belt 2 and outputs a value according to the amount of light reflected from the outer peripheral surface of the intermediate transfer belt 2. The image density sensor 8 changes its output value depending on whether toner is present or not at its detection position (i.e., the light irradiation position). The image density sensor 8 changes its output value depending on the toner density present at the detection position. The control unit 5 detects the toner density on the intermediate transfer belt 2 based on the output of the image density sensor 8.

[0056] Image forming apparatus 100 also includes a voltage control circuit 50, a charging bias power supply 51, a developing bias power supply 52, and a transfer bias power supply 53. Voltage control circuit 50 is connected to control unit 5. Voltage control circuit 50 controls charging bias power supply 51, developing bias power supply 52, and transfer bias power supply 53 based on a control signal output from control unit 5. That is, control unit 5 controls charging bias power supply 51, developing bias power supply 52, and transfer bias power supply 53.

[0057] The charging bias power supply 51 applies a charging bias to the charging roller 120. During monochrome printing, no image is formed in the color image forming unit 1A. Even in this case, the charging bias is applied to the charging roller 120 of the color image forming unit 1A in the same manner as during image formation.

[0058] The developing bias power supply 52 applies a developing bias to the developing roller 140. During monochrome printing, no image is formed in the color image forming unit 1A, but even in this case, the developing bias is applied to the developing roller 140 of the color image forming unit 1A in the same manner as during image formation.

[0059] The transfer bias power supply 53 applies a transfer bias to each of the primary transfer roller 3 and the secondary transfer roller 4. The transfer bias power supply 53 can selectively apply a transfer bias and a reverse transfer bias to the primary transfer roller 3. The transfer bias has a polarity opposite to that of the toner. The reverse transfer bias has the same polarity as that of the toner.

[0060] <Suppression of color toner fog> In color printing, image formation is performed in both the color image forming unit 1A and the monochrome image forming unit 1B, and so toner is consumed in both the color image forming unit 1A and the monochrome image forming unit 1B. On the other hand, in monochrome printing, toner is consumed only in the monochrome image forming unit 1B, and no toner is consumed in the color image forming unit 1A.

[0061] As a result, if toner is stirred in the developing device 14 of the color image forming unit 1A during monochrome printing, the toner in the developing device 14 of the color image forming unit 1A is likely to deteriorate. Therefore, the driving of the developing roller 140 of the color image forming unit 1A is stopped during monochrome printing. That is, during monochrome printing, the rotation of the developing roller 140 of the color image forming unit 1A is stopped, and the rotation of the stirring screw 141 of the color image forming unit 1A is stopped. This suppresses the deterioration of the toner in the developing device 14 of the color image forming unit 1A.

[0062] Here, when color printing is continuously switched to monochrome printing, the driving of the developing roller 140 of the color image forming unit 1A is stopped while the photoconductor drum 11 of the color image forming unit 1A is still driven. The developing roller 140 of the color image forming unit 1A stops rotating while carrying the toner. Therefore, the photoconductor drum 11 of the color image forming unit 1A rotates while being in contact with the toner on the developing roller 140 whose rotation is stopped. As a result, in the color image forming unit 1A, the toner is easily transferred from the developing roller 140 to the photoconductor drum 11 due to the linear speed difference between the photoconductor drum 11 and the developing roller 140.

[0063] As a result, during monochrome printing, color toner adheres to the intermediate transfer belt 2. Depending on the position of the color toner adhered to the intermediate transfer belt 2, the color toner may adhere to the sheet S onto which only the monochrome toner image is to be transferred. In other words, an image defect called color toner fogging may occur.

[0064] For this reason, when continuously switching from color printing to monochrome printing, it is preferable not to start monochrome printing until almost no toner is transferred from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. This suppresses color toner fogging.

[0065] <Monochrome printing start timing> When color printing is successively switched to monochrome printing, the later the timing at which monochrome printing starts, the longer the unnecessary driving of the photoconductor drum 11 continues. The longer the driving time of the photoconductor drum 11, the more likely it is that the photoconductor drum 11 will deteriorate. For this reason, it is preferable to suppress the unnecessary driving of the photoconductor drum 11.

[0066] When color printing is successively switched to monochrome printing, if monochrome printing is started promptly, unnecessary driving of the photoconductor drum 11 is suppressed. However, if monochrome printing is started too early, color toner fogging may occur.

[0067] Here, as the developing roller 140 is driven (i.e., rotated), the charge amount of the toner in the developing device 14 decreases. The reason for this is that every time the carrier passes through the regulating blade as the developing roller 140 is driven, the coating agent of the carrier for charging the toner deteriorates due to the stress of the passage. When the charge amount of the toner decreases, the electrostatic adhesion force between the toner and the carrier weakens, and adhesion of the toner to the photoconductor drum 11 is promoted.

[0068] For this reason, when color printing is successively switched to monochrome printing, it is predicted that color toner fog is less likely to occur if the driving time of the developing roller 140 of the color image forming unit 1A in color printing before switching to monochrome printing (hereinafter referred to as pre-transition driving time) is short than if it is long. Therefore, it is preferable to advance the start timing of monochrome printing as the pre-transition driving time of the developing roller 140 of the color image forming unit 1A is shorter.

[0069] Furthermore, the ease with which toner adheres to the photoconductor drum 11 varies depending on the temperature and humidity (specifically, the amount of moisture in the air) inside the image forming apparatus 100. The greater the amount of moisture present between the photoconductor drum 11 and the toner, the more the toner is promoted to adhere to the photoconductor drum 11.

[0070] For this reason, when continuously switching from color printing to monochrome printing, it is preferable to change the timing of starting monochrome printing according to the temperature and humidity inside image forming apparatus 100. In an environment where the amount of moisture present between photoconductor drum 11 and toner is predicted to be small in the internal environment of image forming apparatus 100, it is preferable to advance the timing of starting monochrome printing.

[0071] As a result, when color printing is successively switched to monochrome printing, the control unit 5 performs processing according to the flow shown in Fig. 4. The flow shown in Fig. 4 starts when the primary transfer of the toner image from the color image forming unit 1A to the intermediate transfer belt 2 is completed.

[0072] In step #1, the control unit 5 stops the rotation of the developing roller 140 of the color image forming unit 1A while continuing to rotate the photoconductor drum 11 of the color image forming unit 1A. By stopping the rotation of the developing roller 140, the rotation of the stirring screw 141 is also stopped.

[0073] At this time, the same developing bias (e.g., 300 V) as that during color printing is applied to the developing roller 140 of the color image forming unit 1A. Also, the surface potential of the photoconductor drum 11 of the color image forming unit 1A is set to the same as that during color printing (e.g., 450 V). If the developing bias applied to the developing roller 140 of the color image forming unit 1A is the same as that during color printing, the surface potential of the photoconductor drum 11 of the color image forming unit 1A is set to the same as that during color printing.

[0074] This suppresses the transfer of toner from the developing roller 140 to the photoconductor drum 11 in the color image forming unit 1A. In other words, the transfer of color toner, which causes color toner fogging, from the photoconductor drum 11 to the intermediate transfer belt 2 is suppressed. This makes it possible to suppress color toner fogging even if the start timing of color printing is advanced.

[0075] As a modified example, from the viewpoint of energy saving, the developing bias applied to the developing roller 140 of the color image forming unit 1A may be set to 0 V. In this case, it is preferable to vary the surface potential of the photoconductor drum 11 of the color image forming unit 1A by the amount of potential variation of the developing bias. That is, in this case, the surface potential of the photoconductor drum 11 of the color image forming unit 1A is set to 150 V (= 450 V - 300 V).

[0076] As another modified example, in color image forming unit 1A, the transfer of toner from developing roller 140 to photoconductor drum 11 may be promoted. In this case, control unit 5 sets the developing bias applied to developing roller 140 in color image forming unit 1A to be higher than the surface potential of photoconductor drum 11, and accelerates the transfer of toner from developing roller 140 to photoconductor drum 11 by the force of the electric field.

[0077] As a result, in color image forming unit 1A, toner on developing roller 140 is quickly transferred to photoconductor drum 11 and removed by cleaning device 15. Even if color toner that causes color toner fog is transferred to intermediate transfer belt 2, the color toner is removed by belt cleaning unit 200. If color toner that causes color toner fog is quickly removed, color toner fog can be suppressed even if the start timing of color printing is advanced.

[0078] In step #2, the control unit 5 determines the degree of ease with which the toner transfers from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. Then, the control unit 5 sets the fogging elimination time based on the degree of ease with which the toner transfers from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2.

[0079] The fog elimination time is the time required for the color toner that causes color toner fog to reach a state where it does not substantially transfer to the intermediate transfer belt 2. Therefore, in order to suppress color toner fog, it is preferable to start color printing after the fog elimination time has elapsed.

[0080] The control unit 5 shortens the fog elimination time as it becomes more difficult for toner to transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. On the other hand, the control unit 5 lengthens the fog elimination time as it becomes more difficult for toner to transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2.

[0081] As a method for determining the degree of ease of toner transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2, the control unit 5 obtains the pre-transition drive time (i.e., the drive time of the developing roller 140 of the color image forming unit 1A in color printing before transitioning to monochrome printing) based on the drive time information 60. The control unit 5 obtains the pre-transition drive time as the degree of ease of toner transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. The drive time information 60 is stored in the storage unit 6 (see FIG. 3). The drive time information 60 is information relating to the pre-transition drive time.

[0082] Then, the control unit 5 sets the fogging elimination time based on the pre-transition drive time. The control unit 5 shortens the fogging elimination time as the pre-transition drive time is shorter. The control unit 5 lengthens the fogging elimination time as the pre-transition drive time is longer.

[0083] For example, information indicating the correspondence between the pre-transition drive time and the fogging elimination time is stored in advance in the storage unit 6. Based on the information, the control unit 5 recognizes the time corresponding to the pre-transition drive time and sets the recognized time as the fogging elimination time. Note that the manufacturer of the image forming apparatus 100 experimentally determines an appropriate fogging elimination time for each different pre-transition drive time. Then, based on the experimental results, the correspondence between the pre-transition drive time and the fogging elimination time in the information is determined.

[0084] As another method for determining the degree of ease of toner transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2, the control unit 5 determines the temperature and humidity (hereinafter referred to as the temperature and humidity environment) inside the image forming apparatus 100 based on the output of the temperature and humidity sensor 7. The control unit 5 determines the temperature and humidity environment as the degree of ease of toner transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. Note that the amount of moisture present between the photoconductor drum 11 and the toner (hereinafter referred to as the amount of intervening moisture) varies depending on the temperature and humidity environment. Therefore, the degree based on the temperature and humidity environment reflects the amount of intervening moisture.

[0085] The control unit 5 sets the fog elimination time based on the temperature and humidity environment. The control unit 5 shortens the fog elimination time as the amount of intervening moisture determined from the temperature and humidity environment is smaller. The control unit 5 lengthens the fog elimination time as the amount of intervening moisture determined from the temperature and humidity environment is larger.

[0086] For example, information indicating the correspondence between temperature and humidity environments and the fogging elimination time is stored in advance in the storage unit 6. Based on the information, the control unit 5 recognizes the time corresponding to the temperature and humidity environment, and sets the recognized time as the fogging elimination time. Note that the manufacturer of the image forming apparatus 100 experimentally determines an appropriate fogging elimination time for each different temperature and humidity environment. Then, based on the experimental results, the correspondence between the temperature and humidity environment in the information and the fogging elimination time is determined.

[0087] The fog elimination time may be set based only on the pre-shift drive time, or based only on the temperature and humidity environment, or based on both the pre-shift drive time and the temperature and humidity environment.

[0088] In step #3, the control unit 5 judges whether or not the elapsed time since the driving of the developing roller 140 of the color image forming unit 1A was stopped has reached the fogging elimination time (i.e., the time set in step #2). In other words, the control unit 5 performs an elapsed time judgment process. If the control unit 5 determines that the fog elimination time has been reached, the process proceeds to step #4. If the control unit 5 determines that the fog elimination time has not been reached, the process of step #3 is repeated.

[0089] In step #4, the control unit 5 starts monochrome printing as the print job. In other words, the control unit 5 switches the print job from color printing to monochrome printing. At this time, the control unit 5 causes the monochrome image forming unit 1B to start forming a toner image while stopping the driving of the developing roller 140 of the color image forming unit 1A. As a result, only the monochrome toner image is primarily transferred onto the intermediate transfer belt 2, and only the monochrome toner image is secondarily transferred onto the sheet S.

[0090] In this embodiment, the control unit 5 sets the fog elimination time based on the degree of ease of transfer of toner from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. In this setting method, the fog elimination time changes depending on the degree.

[0091] Specifically, in this embodiment, the less likely it is that toner will transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2, the shorter the fog elimination time. This allows the drive time of the photoconductor drum 11 to be kept to a minimum necessary. As a result, deterioration of the photoconductor drum 11 can be suppressed. Even if the fog elimination time is short, toner is less likely to transfer from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2, so color toner fog is suppressed. In other words, color toner fog can be reliably suppressed while suppressing deterioration of the photoconductor drum 11.

[0092] In this embodiment, the fog elimination time increases as the toner easily transfers from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2. This makes it possible to reliably suppress color toner fog even in a state in which the toner easily transfers from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2.

[0093] In this embodiment, the fog elimination time is set based on the driving time (pre-transition driving time) of the developing roller 140 of the color image forming unit 1A in color printing before transitioning to monochrome printing. Alternatively, the fog elimination time is set based on the temperature and humidity (temperature and humidity environment) inside the image forming apparatus 100. This makes it possible to easily set the fog elimination time to an appropriate time according to the degree of ease of transfer of toner from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2.

[0094] Here, after the driving of the developing roller 140 of the color image forming unit 1A is stopped when switching from color printing to monochrome printing, the fog elimination time set in the process of step #2 may be corrected based on the output of the image density sensor 8. In other words, the fog elimination time set in the process of step #2 may be corrected based on the actual transfer state of the color toner onto the intermediate transfer belt 2.

[0095] In the configuration for correcting the fog elimination time, after the driving of the developing roller 140 of the color image forming unit 1A is stopped when switching from color printing to monochrome printing, the control unit 5 detects the density of the color toner (i.e., the toner that causes color toner fog) on ​​the intermediate transfer belt 2 based on the output of the image density sensor 8. Then, the control unit 5 corrects the fog elimination time based on the density of the color toner on the intermediate transfer belt 2.

[0096] For example, the control unit 5 compares the density of the color toner on the intermediate transfer belt 2 with a predetermined threshold value. Then, when the density of the color toner on the intermediate transfer belt 2 is smaller than the threshold value (i.e., when the amount of color toner attached to the intermediate transfer belt 2 is small), the control unit 5 performs a correction to shorten the fogging elimination time to be shorter than the currently set time. Alternatively, when the density of the color toner on the intermediate transfer belt 2 is smaller than the threshold value, it may be determined that the fogging elimination time has elapsed at that point, and color printing may be started. This allows the driving time of the photoconductor drum 11 to be shortened.

[0097] In the configuration in which the fog elimination time is corrected, the control unit 5 controls the transfer bias power supply 53 to apply a transfer bias to the primary transfer roller 3 of the color image forming unit 1A until a predetermined time has elapsed since the driving of the developing roller 140 of the color image forming unit 1A is stopped. Then, the control unit 5 switches the bias applied to the primary transfer roller 3 of the color image forming unit 1A from the transfer bias to the reverse transfer bias after the predetermined time has elapsed. Note that the predetermined time is the total time of the time required for the part of the photosensitive drum 11 facing the developing roller 140 to be displaced to a position facing the intermediate transfer belt 2, and the time required for the width in the sub-scanning direction of the toner image primarily transferred from the photosensitive drum 11 to the intermediate transfer belt 2 to become a width that can be read by the image density sensor 8. This allows the density of the color toner on the intermediate transfer belt 2 to be detected reliably.

[0098] As a modified example, a predetermined reference time may be set as the fogging elimination time without setting the fogging elimination time based on the pre-transition driving time and the temperature and humidity environment. The reference time as the fogging elimination time may then be corrected based on the density of the color toner on the intermediate transfer belt 2 (i.e., the output of the image density sensor 8). Also, at least one of setting the fogging elimination time based on the pre-transition driving time and the temperature and humidity environment may be performed, and the fogging elimination time set thereby may be corrected based on the density of the color toner on the intermediate transfer belt 2.

[0099] As another modified example, the transfer of toner from the photoconductor drum 11 of the color image forming unit 1A to the intermediate transfer belt 2 may be suppressed. In this configuration, after the driving of the developing roller 140 of the color image forming unit 1A is stopped when switching from color printing to monochrome printing, the control unit 5 switches the bias applied to the primary transfer roller 3 of the color image forming unit 1A from the transfer bias to the reverse transfer bias. This makes it possible to suppress the transfer of color toner to the intermediate transfer belt 2. As a result, color toner fog can be suppressed even if the fog elimination time is short.

[0100] The embodiments disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is indicated by the claims, not by the description of the above embodiments, and further includes all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0101] 1 Image forming section 1A Color image forming section 1B Monochrome image forming section 2 Intermediate transfer belt (intermediate transfer body) 3 Primary transfer roller (primary transfer member) 4 Secondary transfer roller (secondary transfer member) 5. Control section 6 Memory section 7 Temperature and Humidity Sensor 8 Image density sensor 11 Photoconductor drum (developer carrier) 53 Transfer bias power supply 60 Operating time information 100 Image forming device 140 Developing roller (developer carrier) S Seat

Claims

1. A plurality of image forming units that form toner images using toners of different colors; an intermediate transfer body onto which the toner image is primarily transferred; a primary transfer member for primarily transferring the toner image onto the intermediate transfer body; a secondary transfer member for secondary transferring the toner image onto a sheet; A control unit, The plurality of image forming units include a color image forming unit using color toner; a monochrome image forming unit using black toner; Each of the plurality of image forming units includes an image carrier that rotates while carrying the toner image to be primarily transferred to the intermediate transfer body; a developer carrier that carries the toner and rotates while supplying the toner to the image carrier, thereby developing an electrostatic latent image formed on the image carrier into the toner image; When performing color printing, the control unit causes all of the image forming units to form the toner images, When performing monochrome printing, the control unit causes the monochrome image forming unit to form the toner image without causing the color image forming unit to form the toner image, When the color printing is successively switched to the monochrome printing, the control unit stops driving the developer carrier of the color image forming unit, and performs an elapsed time determination process to determine whether or not an elapsed time since the driving of the developer carrier of the color image forming unit was stopped has reached a fogging elimination time; When it is determined that the fog eliminating time has elapsed, the control unit causes the monochrome image forming unit to start forming the toner image, When performing the elapsed time judgment process, the control unit judges the degree of ease of transfer of the toner from the image carrier of the color image forming unit to the intermediate transfer body, and sets the fog elimination time based on the degree.

2. a storage unit for storing driving time information relating to a driving time of the developer carrier, 2. The image forming apparatus according to claim 1, wherein when performing the elapsed time determination process, the control unit sets the fogging elimination time based on a driving time of the developer carrier of the color image forming unit in the color printing before switching to the monochrome printing.

3. a temperature and humidity sensor for detecting the temperature and humidity inside the image forming apparatus; The image forming apparatus according to claim 1 , wherein when the elapsed time determination process is performed, the control section sets the fogging elimination time based on an internal temperature and humidity of the image forming apparatus.

4. an image density sensor that detects the density of the toner image on the intermediate transfer body; 2 . The image forming apparatus according to claim 1 , wherein after driving of the developer carrier of the color image forming unit is stopped, the control unit corrects the fogging elimination time based on an output of the image density sensor.

5. a transfer bias power supply capable of selectively applying a transfer bias having a polarity opposite to that of the toner and a reverse transfer bias having the same polarity as that of the toner to the primary transfer member; the control unit controls the transfer bias power supply to apply the transfer bias to the primary transfer member of the color image forming unit until a predetermined time has elapsed since driving of the developer carrier of the color image forming unit has been stopped, and after the predetermined time has elapsed, switches the bias applied to the primary transfer member of the color image forming unit from the transfer bias to the reverse transfer bias; 5. The image forming apparatus according to claim 4, wherein the specified time is the total time of a time required for a portion of the image carrier that was facing the developer carrier to be displaced to a position facing the intermediate transfer body, and a time required for a width in the sub-scanning direction of the toner image that is primarily transferred from the image carrier to the intermediate transfer body to become a width that can be read by the image density sensor.

6. a storage unit for storing driving time information relating to a driving time of the developer carrier; a temperature and humidity sensor for detecting the temperature and humidity inside the image forming apparatus; an image density sensor that detects the density of the toner image on the intermediate transfer body; When performing the elapsed time determination process, the control unit sets the fogging elimination time based on a driving time of the developer carrier of the color image forming unit in the color printing before switching to the monochrome printing and an internal temperature and humidity of the image forming apparatus, 2 . The image forming apparatus according to claim 1 , wherein after driving of the developer carrier of the color image forming unit is stopped, the control unit corrects the fogging elimination time based on an output of the image density sensor.

7. a transfer bias power supply capable of selectively applying a transfer bias having a polarity opposite to that of the toner and a reverse transfer bias having the same polarity as that of the toner to the primary transfer member; 2. The image forming apparatus according to claim 1, wherein after driving of the developer carrier of the color image forming unit is stopped, the control unit controls the transfer bias power supply to switch the bias applied to the primary transfer member of the color image forming unit from the transfer bias to the reverse transfer bias.

Images