Image forming device

Abstract

To suppress the promotion of surface wear of the other latent image carriers even when the other latent image carriers are rotationally driven during a control mode in which image formation is carried out using some of a plurality of latent image carriers.SOLUTION: Provided is an image forming device that includes a plurality of charging means for respectively performing charging processing on a plurality of latent image carriers using a charging bias in which an AC voltage is superimposed on a DC voltage, and control means having a control mode in which image formation is carried out using some of the plurality of latent image carriers. When forming an image in the control mode (Yes in S5), the control means causes the AC voltage of the charging bias to stop (S6), which corresponds to the other latent image carriers that are rotationally driven among the plurality of latent image carriers and which is used by the charging means.SELECTED DRAWING: Figure 7

Description

[Technical field]

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

[0002] Conventionally, there has been known an image forming apparatus that uniformly charges the surfaces of a plurality of rotationally driven latent image carriers by charging the surfaces of the carriers, develops the latent images formed on the carriers with toner, and transfers the resulting toner image onto a recording material to form an image.

[0003] For example, Patent Document 1 discloses an image forming apparatus in which toner images formed on five photoconductor drums corresponding to yellow, magenta, cyan, black, and special colors (such as white and transparent colors) are intermediately transferred to an intermediate transfer belt, and then transferred to paper to form an image. In this image forming apparatus, even in a control mode in which the photoconductor drums for special colors are not used and the remaining four photoconductor drums (some latent image carriers) are used to form an image, the photoconductor drums for special colors (other latent image carriers) are driven to rotate. Summary of the Invention [Problem to be solved by the invention]

[0004] However, when a control mode is performed in which image formation is performed using some of the multiple latent image carriers, when other latent image carriers among the multiple latent image carriers are rotated, surface wear of the other latent image carriers may be accelerated. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides an image forming apparatus for forming an image by transferring a toner image obtained by developing a latent image formed after uniformly charging the surfaces of a plurality of rotationally driven latent image carriers with a toner onto a recording material, the image forming apparatus comprising: a plurality of charging means for charging the plurality of latent image carriers using a charging bias in which an AC voltage is superimposed on a DC voltage; and a control means having a control mode in which an image is formed using some of the plurality of latent image carriers, the control means stopping the AC voltage of the charging bias used by the charging means corresponding to the other rotationally driven latent image carriers out of the plurality of latent image carriers when forming an image in the control mode. the part of the latent image carriers carries a toner image made of at least one color of toner selected from yellow toner, magenta toner, and cyan toner, and does not carry a toner image made of black toner, the other latent image carriers carry a toner image made of special toner, and the device has a unit holding section that selectively and detachably holds a black replacement unit that forms a toner image on the surface of the latent image carrier with the black toner and a special replacement unit that forms a toner image on the surface of the latent image carrier with the special toner. It is characterized by the above. Effect of the Invention

[0006] According to the present invention, even when other latent image carriers among a plurality of latent image carriers are rotationally driven during a control mode in which image formation is performed using some of the latent image carriers, it is possible to suppress the promotion of surface wear of the other latent image carriers. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is an explanatory diagram showing the overall configuration of a printer according to an embodiment. [Diagram 2] 5A and 5B are explanatory diagrams for explaining a contacting and separating operation when a contacting and separating means is provided in the printer. [Diagram 3] 1 is a graph showing a relationship between the film thickness of a photoconductor and the number of carrier particles adhering per unit area. [Figure 4] 11 is a graph showing an appropriate range of toner concentration when an AC charging bias is applied to a photoconductor that is idled and an image is formed (printed). [Diagram 5] 11 is a graph showing an appropriate range of toner concentration when an image is formed (printed) on a photoconductor that is idle driven by stopping the AC voltage and applying a DC charging bias. [Figure 6] FIG. 4 is an explanatory diagram showing the contents of control in the embodiment. [Figure 7] 4 is a flowchart showing a control flow in the embodiment. [Figure 8] FIG. 4 is an explanatory diagram showing the surface potential of a photoconductor after charging in a case where charging is performed using an AC charging bias and in a case where charging is performed using a DC charging bias. [Figure 9] 13A and 13B are diagrams illustrating an example of control in which the value of the DC voltage during the period in which the AC voltage of the charging bias is stopped for a photoconductor being idle-driven is made greater than the value of the DC voltage when the AC charging bias is applied. [Figure 10] 1A and 1B are explanatory diagrams illustrating an example of control in which, when the AC charging bias voltage is stopped for a photoconductor that is being driven idle, the value of the development bias while the charged photoconductor surface passes through the development area is made smaller than the value of the development bias while the photoconductor surface that is charged by the AC charging bias passes through the development area. [Figure 11] 1 is a graph showing the amount of surface wear (reduction) of a photoconductor after 100 hours when the photoconductor is idle driven while changing the frequency of the AC charging bias AC voltage (charging AC bias). [Figure 12] 1 is a graph showing the amount of surface wear (reduction) of a photoconductor after 100 hours (hr) when the photoconductor is idle driven while changing the peak-to-peak voltage Vpp of the AC charging bias (charging AC bias). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment in which the present invention is applied to a color printer (hereinafter referred to as a "printer"), which is an image forming apparatus, will be described with reference to the drawings. Note that the image forming apparatus to which the present invention is applied may be a single copy machine or facsimile other than a printer, or a multifunction machine having at least two of the functions of a printer, copy machine, facsimile, and scanner.

[0009] In this embodiment, the present invention is applied to a four-station image forming apparatus as an example. The image forming apparatus of this embodiment can form a color black visible image on a recording material, the color black visible image being composed of a color toner image composed of at least one of yellow (Y), magenta (M), and cyan (C) color toners and a K toner image composed of K toner.

[0010] The printer of this embodiment includes a unit holding section that detachably holds a K process unit (black replacement unit) including a K developing device that performs development processing with black (K) toner. The unit holding section holds a special replacement unit including a special developing device that performs development processing with special toner that forms an inconspicuous image on a recording material, in place of the K process unit, so that an inconspicuous image can be formed.

[0011] Such special toners are used when embedding additional information in an image. For example, they are used when forming an image that is difficult to recognize by the naked eye (a character image such as "COPY" that cannot be seen by a human even at a glance), called an invisible pattern or background pattern, on a recording material together with a visible image using color toners for the purpose of preventing illegal copying, etc. Also, they are used when forming a code image using a visible image and a code image using an invisible image by overlapping them on a recording material for the purpose of increasing the amount of information in a code image such as a QR code (registered trademark).

[0012] As described below, a difficult-to-see image is an image formed using toner that is more transparent under visible light than normal color toner, and in this embodiment, the image is further processed by irradiating it with infrared light or the like to cause it to emit light, develop color, etc., making it easier to see.

[0013] Examples of special toners include transparent infrared absorbing toners and transparent fluorescent toners that fluoresce when exposed to ultraviolet light, and toners that absorb light outside the visible light range or emit light in the visible light range when exposed to light outside the visible light range. In this embodiment, an example in which infrared absorbing toner is used as the special toner will be described. In the following description, the toner-specific symbols for each component are "Y" for yellow toner (Y toner), "M" for magenta toner (M toner), "C" for cyan toner (C toner), "K" for black toner (K ​​toner), and "IR" for infrared absorbing toner (IR toner). As described above, the special toner is at least a color other than yellow, magenta, cyan, and black, or a transparent toner. It also includes white and metallic. However, as the special toner, a transparent toner (transparent toner) whose color development is suppressed under visible light is preferable. In addition, the colorant content is less than that of normal color toners.

[0014] First, the overall configuration and operation of the printer according to this embodiment will be described. FIG. 1 is an explanatory diagram showing the overall configuration of a printer according to this embodiment. The printer of this embodiment is mainly composed of an image forming unit 1, a transfer unit 2, a recording material supply unit 3, a fixing unit 4, a recording material discharge unit 5, a control unit 30, and an image formation control unit 40.

[0015] The image forming unit 1 is provided with four unit holders for holding four process units 6, which are image forming units serving as replaceable units. Three of the unit holders correspond to the three process units 6Y, 6M, and 6C corresponding to the color toners. The remaining unit holder corresponds to the K process unit 6K and the IR process unit 6IR, and either one of the process units 6K and 6IR is selectively attached and held. Note that FIG. 1 illustrates an example in which the IR process unit 6IR is attached to and held by the unit holder, instead of the K process unit 6K. The process units 6Y, 6M, 6C, 6K, and 6IR have the same configuration, except that the types of toner they use are different.

[0016] Since the printer of this embodiment has four unit holders, it is smaller in size than a printer having five unit holders corresponding to the above-mentioned five process units 6Y, 6M, 6C, 6K, and 6IR. That is, a small printer having four unit holders can simultaneously form a full-color image (color black visible image) using Y, M, C, and K toners, and a full-color image (color visible image) and an IR image (hard-to-see image) using Y, M, C, and IR toners.

[0017] In addition, all process units may be made detachable, and the positions at which the process units are attached (unit holding parts) may be interchanged. In this case, by interchanging the position of the IR process unit, it is possible to appropriately interchange the positional relationship between the IR toner image and each color toner image on the recording material (positional relationship in the toner image stacking direction).

[0018] In this embodiment, each process unit 6Y, 6M, 6C, 6K, 6IR is composed of a photoconductor 7 as a latent image carrier that carries a latent image, a charging roller 8 as a charging means for uniformly charging the surface of the photoconductor 7, a developing device 9 as a developing means for developing the latent image on the photoconductor 7, and a photoconductor cleaning device 10 as a latent image carrier cleaning means for cleaning the surface of the photoconductor 7. At a position facing each photoconductor 7, an exposure device 11 is provided as a latent image forming means for forming a latent image on the surface of the photoconductor 7. In this embodiment, an LED unit is used as the exposure device 11, but a laser beam scanning type using a laser diode may also be used.

[0019] The charging roller 8 of this embodiment is connected to a DC power source that outputs a DC voltage and an AC power source that outputs an AC voltage, and a charging bias in which an AC voltage is superimposed on a DC voltage is applied. Although a charging bias consisting of only a DC voltage may be used as the charging bias, the charging uniformity and stability are higher when a charging bias in which an AC voltage is superimposed on a DC voltage is used. The charging uniformity refers to the ability to charge the surface of the photoconductor 7 evenly, and the charging stability refers to the ability to charge stably to a target charging potential.

[0020] The transfer section 2 is provided with an endless intermediate transfer belt 12 serving as an intermediate transfer body onto which the toner image on the photoreceptor 7 is transferred, a plurality of primary transfer rollers 13 serving as primary transfer means for primarily transferring the image on the photoreceptor 7 onto the intermediate transfer belt 12, a secondary transfer roller 14 serving as secondary transfer means for secondarily transferring the toner image transferred onto the intermediate transfer belt 12 onto a recording material, and a belt cleaning device 17 serving as intermediate transfer body cleaning means for cleaning the surface (outer peripheral surface) of the intermediate transfer belt 12.

[0021] The intermediate transfer belt 12 is stretched around a drive roller 15 and a driven roller 16, and travels (rotates) as the drive roller 15 rotates. Each primary transfer roller 13 is disposed so as to press the intermediate transfer belt 12 against each photoconductor 7. As a result, a primary transfer nip is formed at the contact point between the intermediate transfer belt 12 and each photoconductor 7, where an image on each photoconductor 7 is transferred to the intermediate transfer belt 12. Meanwhile, the secondary transfer roller 14 is disposed so as to contact a portion of the intermediate transfer belt 12 wound around the drive roller 15. At the contact point between the secondary transfer roller 14 and the intermediate transfer belt 12, a secondary transfer nip is formed at the contact point between the intermediate transfer belt 12 and each photoconductor 7, where an image on the intermediate transfer belt 12 is transferred to a recording material.

[0022] The recording material supply unit 3 is provided with a paper feed cassette 18 as a recording material storage unit that stores paper P as a recording material, a paper feed roller 19 as a recording material feeding means that feeds paper P from the paper feed cassette 18, and a timing roller 20 as a recording material conveying means that conveys the paper P fed by the paper feed roller 19 to the secondary transfer nip at a predetermined timing. The recording material may be an OHP sheet, an OHP film, cloth, etc., in addition to paper. In addition to plain paper, the paper includes cardboard, postcards, envelopes, thin paper, coated paper (coated paper, art paper, etc.), textured paper such as Japanese paper, tracing paper, etc.

[0023] The fixing section 4 is provided with a fixing device 21 as a fixing means for fixing an image onto the paper P. The fixing device 21 is mainly composed of a fixing roller 22 as a fixing member that is heated by a heat source such as a heater, and a pressure roller 23 as a pressure member that comes into contact with the fixing roller 22 with a predetermined pressure to form a fixing nip.

[0024] The recording material discharge section 5 is provided with a paper discharge roller 24 as a recording material discharge means for discharging the paper P sent from the fixing device 21 outside the device, and a paper discharge tray 25 as a recording material loading section for loading the paper P discharged by the paper discharge roller 24.

[0025] The control unit 30 performs image processing on input image information from a reading device (scanner), a personal computer, etc., and also controls the entire printer. In addition, the image forming control unit 40, under the control of the control unit 30, controls the image forming operations in each part of the printer (the image forming unit 1, the transfer unit 2, the recording material supply unit 3, the fixing unit 4, the recording material discharge unit 5, etc.).

[0026] In addition to the above-mentioned components, the printer of this embodiment is provided with a container holding member 102 that detachably holds a plurality of toner cartridges 26Y, 26M, 26C, 26K, and 26IR as toner containers that contain toner, which is a powder used in image formation. The container holding member 102 is provided with four toner container holding sections, and a corresponding toner cartridge is attached to and held in each toner container holding section. Three of the four toner container holding sections correspond to the three toner cartridges 26Y, 26M, and 26C that correspond to color toners, respectively. The remaining toner container holding section corresponds to the K toner cartridge 26K and the IR toner cartridge 26IR, and selectively attaches and holds either one of the toner cartridges 26K and 26IR. Note that FIG. 1 illustrates an example in which the IR toner cartridge 26IR is attached to and held in the toner container holding section, instead of the K toner cartridge 26K.

[0027] Each of the toner cartridges 26Y, 26M, 26C, 26K, and 26IR contains the same type (color) of toner as the toner in the developing device 9 of each of the above-mentioned 6Y, 6M, 6C, 6K, and 6IR. Toner cartridges 26 corresponding to the toner of the process units 6 attached to the four unit holding parts are attached to the four toner container holding parts of the container holding member 102. When the toner in the developing device 9 of the attached process unit 6 falls below a predetermined amount, the same type of toner is replenished from the toner cartridge 26 attached to the corresponding toner container holding part.

[0028] The printer of this embodiment is also equipped with a waste toner storage container 27. In this waste toner storage container 27, waste toner collected by the belt cleaning device 17 or the photoconductor cleaning device 10 is stored.

[0029] 1, the printer of this embodiment includes a cover member 101 for opening and closing an upper portion of the device body (image forming device body) 100. The cover member 101 is rotatable up and down about a rotation shaft 103 provided in the device body 100. In addition, below the cover member 101, a container holding member 102 is disposed for detachably holding four toner cartridges 26 in a toner container holding portion. The container holding member 102 is rotatable up and down about another rotation shaft 104 provided in the device body 100.

[0030] 1, when the IR process unit 6IR is attached to the unit holding section, the IR process unit 6IR is disposed on the most downstream side in the running direction of the intermediate transfer belt 12, and the color process units 6Y, 6M, and 6C are disposed upstream of the IR process unit 6IR. As a result, an IR toner image (special toner image) made of IR toner is formed on the recording material closer to the recording material than the color toner images made of Y, M, and C color toners. In other words, on the intermediate transfer belt 12, the Y toner image, the M toner image, the C toner image, and the IR toner image are stacked in this order from the belt side, and on the recording material after secondary transfer, the IR toner image, the C toner image, the M toner image, and the Y toner image are stacked in this order from the recording material side.

[0031] By forming the IR toner image closer to the recording material than the color toner image, the IR toner image is hidden by the color toner image, and it is easy to increase the invisibility of the image that is difficult to see due to the IR toner image. However, the location of the IR process unit 6IR relative to the color process units 6Y, 6M, and 6C can be changed as appropriate. Also, as described above, if the mounting positions of the process units 6Y, 6M, 6C, and 6IR are configured to be interchangeable with each other, the position of the IR process unit can be freely interchanged.

[0032] In addition, in the printer of this embodiment, the image density of each toner is adjusted by adjusting the adhesion amount (toner adhesion amount per unit area) of each of Y, M, C, K, and IR toners. In more detail, a toner adhesion amount detection sensor is provided for detecting the toner adhesion amount of test toner images (multiple toner patches created to have different target densities) of each of Y, M, C, K, and IR toners formed on the intermediate transfer belt 12. Based on the results detected by this toner adhesion amount detection sensor, the image creation conditions (image forming conditions) in each process unit of Y, M, C, K, and IR are adjusted so that the desired toner adheres to the desired density.

[0033] The toner adhesion amount detection sensor of this embodiment may be used commonly for each of the Y, M, C, K, and IR test toner images, or may be used individually for each of the Y, M, C, K, and IR test toner images. The toner adhesion amount detection sensor of this embodiment is an optical image density sensor (optical sensor) that irradiates light onto a test toner image and receives specularly reflected light and diffusely reflected light from the test toner image. For the Y, M, and C color toners, the toner adhesion amount of the test toner image (image density of the test toner image) is detected based on the amount of both specularly reflected light and diffusely reflected light received. For the K toner, the toner adhesion amount of the test toner image (image density of the test toner image) is detected based only on the amount of specularly reflected light received.

[0034] On the other hand, the IR toner of this embodiment becomes an invisible image (an image that is difficult to see, or an image that does not substantially have an absorption peak in the visible light region) after the fixing process. However, since the IR toner is a visible image (an image that can be seen, or an image that substantially has an absorption peak in the visible light region) on the intermediate transfer belt 12 before the fixing process, a toner adhesion amount detection sensor similar to that for C, M, Y, and K can be used. In this embodiment, a common toner adhesion amount detection sensor is used for the K test image and the IR test toner image. However, for the IR toner test toner image, it is possible to realize a more accurate detection of the toner adhesion amount by acquiring both the specular reflection light and the diffuse reflection light and detecting the toner adhesion amount of the test image, rather than acquiring only the specular reflection light and detecting the toner adhesion amount of the test toner image.

[0035] Next, the basic operation of the printer of this embodiment will be described. When the image forming operation starts, each photoconductor 7 is rotated and the surface of each photoconductor 7 is uniformly charged to a predetermined polarity by the charging roller 8. Next, based on input image information from a reading device (scanner), a personal computer, etc., an exposure device 11 irradiates the charged surface of each photoconductor 7 with laser light to form a latent image (electrostatic latent image).

[0036] The latent images formed on each photoconductor 7 are based on monochrome image information obtained by decomposing a desired full-color image into color information of Y, M, and C. In more detail, a color conversion and decomposition table is used to convert and decompose the color information (RGB, YCM, etc.) of the input image information into color information (YMC) for the printer, and the input image information is converted into color information of Y, M, and C to generate decomposed monochrome image information. Then, each exposure device 11 for Y, M, and C forms a latent image of each color on the respective photoconductor 7 based on the image information of each color of Y, M, and C.

[0037] When the K process unit 6K is installed, after generating monochrome image information of Y, M, and C, monochrome image information is further generated by extracting K color information, and the monochrome image information of Y, M, and C is corrected. This process is a process for generating K image information, such as UCR (Under Color Removal), and replaces black or gray image information expressed by overlapping Y, M, and C toners with K image information. The exposure device 11 corresponding to K (shared with the exposure device 11 corresponding to IR) forms a K latent image on the photoconductor 7 of the K process unit 6K based on the K image information.

[0038] In this embodiment, when an image that is difficult to see is formed based on additional information included in the input image information or additional information added by the printer, IR image information is generated from the additional information. The additional information included in the input image information may be information added by an application on the personal computer or information added by a print driver on the personal computer. When the IR process unit 6IR is installed, the exposure device 11 corresponding to IR (shared with the exposure device 11 corresponding to K) forms an IR latent image on the photoconductor 7 of the IR process unit 6IR based on the IR image information.

[0039] When the K process unit 6K is installed, toner is supplied from each developing device 9 to each of the Y, C, M, and K latent images formed on the photoconductor 7, and the latent images are developed into Y, C, M, and K toner images. The toner images on each photoconductor 7 are transferred in sequence onto the intermediate transfer belt 12, which travels around, in a superimposed manner. In detail, when the toner image on the photoconductor 7 reaches the position of the primary transfer nip, the toner image on the photoconductor 7 is transferred in sequence onto the intermediate transfer belt 12 by a transfer electric field formed by applying a predetermined voltage to the primary transfer roller 13. In this way, a full-color toner image (visible image) made of Y, C, M, and K toners is formed on the surface of the intermediate transfer belt 12. The toner on each photoconductor 7 that has not been transferred to the intermediate transfer belt 12 is removed by the photoconductor cleaning device 10.

[0040] On the other hand, when the IR process unit 6IR is installed, toner is supplied from the respective developing devices 9 to the Y, C, M, and IR latent images formed on the photoconductor 7, and the latent images are developed into Y, C, M, and IR toner images. The toner images on each photoconductor 7 are transferred in sequence, superimposed on the intermediate transfer belt 12 that travels around, in the same manner as described above. In this way, a full-color toner image (visible image) made of Y, C, and M and an IR toner image (special toner image) made of IR toner are formed on the surface of the intermediate transfer belt 12. Note that the toner on each photoconductor 7 that has not been transferred to the intermediate transfer belt 12 is removed by the photoconductor cleaning device 10, in the same manner as described above.

[0041] Furthermore, when the image forming operation is started, the paper feed roller 19 rotates and feeds the paper P from the paper feed cassette 18. The transport of the fed paper P is temporarily stopped by the timing roller 20. Thereafter, the rotational drive of the timing roller 20 is started at a predetermined timing, and the paper P is transported to the secondary transfer nip in time with the toner image on the intermediate transfer belt 12 reaching the secondary transfer nip.

[0042] When the paper P is transported to the secondary transfer nip, a predetermined voltage is applied to the secondary transfer roller 14, and a transfer electric field is formed in the secondary transfer nip. The toner images on the intermediate transfer belt 12 are transferred all at once to the paper P by the transfer electric field formed in the secondary transfer nip. At this time, the toner remaining on the intermediate transfer belt 12 is removed by the belt cleaning device 17.

[0043] Thereafter, the paper P is transported to a fixing device 21, where the toner image is heated and pressed by a fixing roller 22 and a pressure roller 23, thereby being fixed to the paper P. Then, the paper P is discharged outside the device by a paper discharge roller 24, and placed on a paper discharge tray 25.

[0044] The above description is of the image forming operation when forming a full-color image, but an image may be formed using any one of the four process units 6Y, 6M, 6C, 6IR (or 6K), or may be formed using any two or three of the process units.

[0045] In addition, a contact / separation means for separating the photoconductors 7 not used in image formation from the intermediate transfer belt 12 may be provided. For example, when a full-color image is formed (full-color mode) in the case where the K process unit 6K is installed, all of the four photoconductors 7Y, 7M, 7C, and 7K are brought into contact with the intermediate transfer belt 12 as shown in FIG. 2(a). On the other hand, when a monochrome image is formed using only the K process unit 6K (K monochrome mode), the three photoconductors 7Y, 7M, and 7C not used in image formation are separated from the intermediate transfer belt 12 as shown in FIG. 2(b). This makes it possible to stop the rotational drive of the photoconductors 7Y, 7M, and 7C not used in image formation, and thus makes it possible to extend the lifespan of these photoconductors 7Y, 7M, and 7C (lifespan of the process units 6Y, 6M, and 6C).

[0046] Next, the lifespan determination of the process unit 6 will be described. The life of the process unit 6, i.e., the replacement time, can be determined by the deterioration state of the developer and the wear state of the photoconductor 7. Specifically, in the case of a two-component developer consisting of toner and carrier, when the deterioration state of the developer and the wear state of the photoconductor 7 progress, it becomes difficult to suppress the adhesion of the carrier to the photoconductor 7 and the scattering of toner even if the image formation conditions (such as the toner concentration of the developer) are adjusted. In this case, it becomes necessary to replace the process unit 6.

[0047] Here, the deterioration state of the two-component developer can be roughly divided into two types: a carrier film scraping mode (low image deterioration mode) and a carrier spent mode (high image deterioration mode).

[0048] The carrier film scraping mode (low image degradation mode) is a degradation state in which the carrier adhesion deteriorates due to the carrier film being scraped off and the carrier resistance decreasing. The lower the toner consumption per unit driving time of the developing device 9, the less toner components adhere to the coating layer on the carrier surface, and the wear of the carrier surface progresses, leading to the progress of carrier film scraping. As the carrier film scraping progresses, the carrier resistance decreases, and the injection of reverse polarity charges into the carrier by the development electric field becomes more likely. As a result, the amount of carrier that moves to the photoconductor 7 side by the development electric field increases, and the carrier adhesion, in which the carrier adheres to the surface of the photoconductor 7, deteriorates. Note that even if the carrier film scraping progresses, the toner charging ability of the carrier does not change significantly, so toner scattering does not worsen.

[0049] Carrier spent mode (high image degradation mode) is a degradation state in which too much toner component adheres to the carrier surface, reducing the carrier's ability to charge toner. The more toner consumed per unit driving time of the developing device 9, the more toner component adheres to the coating layer on the carrier surface, and carrier spent progresses. As carrier spent progresses, the carrier's ability to charge toner decreases, the amount of toner with insufficient charge increases, and toner scattering worsens. Note that as carrier spent progresses, carrier resistance increases, improving carrier adhesion.

[0050] FIG. 3 is a graph showing the relationship between the film thickness of the photoreceptor and the number of carrier particles adhering per unit area. 3, even if the deterioration state of the developer is the same, the more the wear state of the photoconductor 7 progresses and the film thickness of the photoconductor 7 becomes thinner, the worse the carrier adhesion due to the carrier film scraping mode becomes. In other words, the worsening state of carrier adhesion due to the carrier film scraping mode differs depending on the wear state of the photoconductor 7 (photoconductor film thickness), in other words, the margin of carrier adhesion differs depending on the wear state of the photoconductor 7 (photoconductor film thickness).

[0051] Both carrier adhesion due to the carrier film scraping mode and toner scattering due to the carrier spent mode occur depending on the toner concentration of the developer. Specifically, if the toner concentration of the developer is too low, the carrier film scraping progresses and carrier adhesion due to the carrier film scraping mode is likely to occur, and if the toner concentration of the developer is too high, carrier spent progresses and toner scattering due to the carrier spent mode is likely to occur. In other words, the appropriate range of toner concentration is determined so that the upper limit can be kept within an acceptable range to keep toner scattering within an acceptable range, and the lower limit can be kept within an acceptable range to keep carrier adhesion within an acceptable range.

[0052] Since the toner concentration of the developer can be maintained within a certain range by the existing toner concentration control, if the above-mentioned appropriate range of the toner concentration is wider than the range that can be controlled by the toner concentration control, carrier adhesion and toner scattering can be suppressed within an acceptable range. However, as the deterioration state of the developer progresses, carrier adhesion and toner scattering worsen, so the above-mentioned appropriate range of the toner concentration narrows over time and finally becomes narrower than the range that can be controlled by the toner concentration control. When this happens, carrier adhesion and toner scattering cannot be suppressed by the toner concentration control, so replacement of the process unit 6 becomes necessary, and the process unit 6 reaches the end of its life. Therefore, in this embodiment, it is determined that the end of its life has come when the width of the appropriate range of the toner concentration (the width of the upper and lower limits) exceeds a predetermined life determination threshold.

[0053] Incidentally, even if the deterioration state of the developer is the same, the thinner the film thickness of the photoconductor 7 becomes as the wear state of the photoconductor 7 progresses, the worse the carrier adhesion becomes. Therefore, the more the wear state of the photoconductor 7 progresses, the worse the carrier adhesion becomes, the narrower the appropriate range of the toner concentration becomes (the lower limit value rises), and the earlier the time when the appropriate range of the toner concentration becomes narrower than the controllable range of the toner concentration control becomes. As a result, the life of the process unit 6 (photoconductor 7) becomes shorter. In particular, in a control mode in which an image is formed using some of the four photoconductors 7Y, 7M, 7C, and 7K (or 7IR), if the other photoconductors not used in the image formation are rotated, the surface wear of the other photoconductors is accelerated, and the life of the other photoconductors becomes shorter.

[0054] For example, when the K process unit 6K is installed and a monochrome image is formed using only the K process unit 6K (K monochrome mode), the photoconductors 7Y, 7M, and 7C of the other three process units 6Y, 6M, and 6C are rotationally driven (idle driving). In this case, the surface wear of the photoconductors 7Y, 7M, and 7C that are not used for image formation is accelerated by the idle driving, and the lifespan of the three process units 6Y, 6M, and 6C (photoconductors 7Y, 7M, and 7C) is shortened.

[0055] In this case, when the contact / separation means as shown in FIGS. 2(a) and 2(b) is provided, the three photoconductors 7Y, 7M, and 7C that are not used for image formation can be separated from the intermediate transfer belt 12 in the K single color mode. In this case, the rotational driving of the three photoconductors 7Y, 7M, and 7C can be stopped in the K single color mode, so that the promotion of surface wear of the photoconductors 7Y, 7M, and 7C due to the idle driving can be suppressed. However, even in this case, the three photoconductors 7Y, 7M, and 7C may be idle-driven in a state in which they are separated from the intermediate transfer belt 12, such as when performing a toner refresh operation of the three process units 6Y, 6M, and 6C in the K single color mode. In such a case, the surface wear of the photoconductors 7Y, 7M, and 7C due to the idle driving of the three process units 6Y, 6M, and 6C is still promoted, and the lifespan is shortened.

[0056] 2(a) and (b), the photoconductor of the process unit mounted on the unit holding section disposed on the most downstream side in the running direction of the intermediate transfer belt 12 cannot be moved toward or away from the intermediate transfer belt 12. Therefore, in a control mode in which the photoconductor of the process unit mounted on the unit holding section is not used for image formation, surface wear of the photoconductor due to idle driving is accelerated, and the life of the photoconductor is shortened. For example, when the IR process unit 6IR is mounted on the unit holding section, when a full-color image or a monochrome image is formed without using the photoconductor 7IR, the photoconductor 7IR of the process unit 6IR is rotationally driven (idle driving). Therefore, surface wear of the photoconductor 7IR not used for image formation is accelerated due to idle driving, and the life of the process unit 6IR (photoconductor 7IR) is shortened.

[0057] Here, when the photoconductor 7 is idle driven, the charging bias of the charging roller 8 is generally left applied (the developing bias is also left applied). This is to prevent a problem that, when the application of the charging bias of the charging roller 8 is stopped, the toner in the developer of the developing device 9 electrostatically moves to the surface of the photoconductor 7 in an uncharged state in the development area, resulting in wasteful consumption of toner. However, as described above, the charging bias of this embodiment is a charging bias in which an AC voltage is superimposed on a DC voltage (hereinafter referred to as an "AC charging bias"). In this case, it has been found that compared to the case where a charging bias of only a DC voltage (hereinafter referred to as a "DC charging bias") is used, the promotion of surface wear due to the idle driving of the photoconductor is significant, and the life of the process unit (photoconductor) is further shortened.

[0058] FIG. 4 is a graph showing an appropriate range of toner concentration when an AC charging bias is applied to a photoconductor that is idle-driven and an image is formed (printed). FIG. 5 is a graph showing an appropriate range of toner concentration when an image is formed (printed) on a photoconductor that is being idle-driven by stopping the AC voltage and applying a DC charging bias. 4 and 5 show an example in which the photoconductor 7IR not used for image formation is idle driven in a control mode (e.g., full-color mode) in which image formation is performed using three of the four photoconductors 7Y, 7M, 7C, and 7IR. At this time, the driving of the developing device 9IR corresponding to the idle driven photoconductor 7IR is stopped, so the deterioration state of the developer does not progress.

[0059] Comparing the graphs in Fig. 4 and Fig. 5, the upper limit of the appropriate range of toner concentration (the upper limit of toner concentration that can keep toner scattering within an acceptable range) shows roughly the same change over time. However, the lower limit of the appropriate range of toner concentration (the lower limit of toner concentration that can keep carrier adhesion within an acceptable range) shows a time change with a greater increase rate in the graph in Fig. 4 than in the graph in Fig. 5.

[0060] This is believed to be because the use of an AC charging bias including an AC voltage roughens the surface of the photoconductor 7, increasing the coefficient of friction, increasing the amount of wear caused by sliding with a contact member such as a cleaning blade that contacts the surface of the photoconductor 7, and reducing the margin for carrier adhesion. Moreover, the developing device 9 corresponding to the photoconductor 7 during idle driving is usually stopped in order to suppress the accelerated deterioration of the developer, so the amount of toner adhering to the surface of the photoconductor 7 during idle driving is small, and lubrication by the toner is not obtained. Therefore, surface wear of the photoconductor 7 is easily accelerated.

[0061] FIG. 6 is an explanatory diagram showing the contents of control in this embodiment. In this embodiment, in a control mode in which an image is formed using some of the four photoconductors, the AC voltage of the charging bias applied to the charging rollers 8 corresponding to the other photoconductors that are rotated among the four photoconductors is stopped. Note that the example in FIG. 6 is an example in which the IR process unit 6IR is installed.

[0062] Specifically, when the image information is only color image information, three of the four photoconductors 7Y, 7M, 7C, and 7IR are used to form an image, and the photoconductor 7IR is not used. In this case, in this embodiment, as shown in FIG. 6, the AC voltage (charging AC bias) in the charging bias applied to the charging roller 8IR corresponding to the photoconductor 7IR is turned off. As a result, the surface of the photoconductor 7IR is not subjected to the hazard caused by the charging AC bias during the idle driving of the photoconductor 7IR not used for image formation, and thus the roughness of the surface is suppressed. As a result, the promotion of surface wear of the photoconductor 7IR is suppressed, the margin of carrier adhesion is not reduced, and the reduction in the lifespan is suppressed. Note that when switching the bias, a desired bias stabilization time is required.

[0063] At this time, in this embodiment, the DC voltage (charging DC bias) in the charging bias applied to the charging roller 8IR corresponding to the photoconductor 7IR is turned on as shown in Fig. 6. This causes the surface of the photoconductor 7IR to be charged during idle driving, thereby preventing the IR toner of the developing device 9IR from electrostatically moving to the surface of the photoconductor 7IR in the development area and wasting the IR toner.

[0064] The DC voltage of the charging bias (charging DC bias) applied to the charging roller corresponding to the photoconductor that is driven in the idling state does not necessarily have to be turned on, and may be turned off. In this case, the toner of the developing device electrostatically moves and adheres to the surface of the photoconductor in the development area, and the lubricating effect of the toner is expected to suppress the promotion of surface wear of the photoconductor that is driven in the idling state. In addition, the charging DC bias may be turned off for a toner refresh operation that forcibly discharges deteriorated toner from the developing device.

[0065] Also, when the image information is only the information of the special color image, one of the four photoconductors 7Y, 7M, 7C, and 7IR is used to form an image, and the color photoconductors 7Y, 7M, and 7C are not used. In this case, in this embodiment, as shown in FIG. 6, the AC voltage (charging AC bias) in the charging bias applied to the charging rollers 8Y, 8M, and 8C corresponding to the color photoconductors 7Y, 7M, and 7C is turned off. As a result, the surfaces of the color photoconductors 7Y, 7M, and 7C not used for image formation are not subjected to the hazard caused by the charging AC bias during the idle driving of the color photoconductors 7Y, 7M, and 7C, and thus surface roughness is suppressed. Therefore, the promotion of surface wear is suppressed, the margin of carrier adhesion is not reduced, and the reduction in life is suppressed.

[0066] In this embodiment, as shown in FIG. 6, the DC voltage (charging DC bias) in the charging bias applied to the charging rollers corresponding to the color photoconductors 7Y, 7M, and 7C is turned on, but it may be turned off as described above.

[0067] FIG. 7 is a flowchart showing the flow of control in this embodiment. When a print job is input and an image forming operation (printing operation) is started (S1), first, it is determined whether the IR process unit 6IR or the K process unit 6K is attached to the most downstream unit holding section (S2).

[0068] At this time, if it is determined that the K process unit 6K is installed (No in S2), image formation is performed in a normal image formation operation (normal control mode) (S3). Then, when image formation for the input print job is completed (Yes in S8), the image formation operation (printing operation) is stopped (S9).

[0069] If it is determined that the IR process unit 6IR is installed (Yes in S2), image information for the current print job is obtained (S4), and it is determined whether the image information is information for an image that uses all the toners Y, C, M, and IR (IR + color image) (S5).

[0070] In this judgment, if it is judged that the information is an image (IR+color image) that uses all the toners (Yes in S5), image formation is performed in a normal image forming operation (normal control mode) (S3). Then, when the image formation of the input print job is completed (Yes in S8), the image forming operation (printing operation) is stopped (S9).

[0071] On the other hand, if it is determined that the information is not an image (IR+color image) that uses all the toners, that is, the information is an image (color image, monochrome image, etc.) that is formed without using the IR process unit 6IR (No in S5), the following control is performed. That is, for the IR process unit 6IR that is not used for image formation, the photoconductor 7IR is idle driven, the driving of the developing device 9IR is stopped (developing bias is on), and the AC voltage of the charging bias (charging AC bias) of the charging roller 8IR is stopped (S6). Then, while performing this control, an image forming operation (printing operation) is performed (S7), and when the image formation of the input print job is completed (Yes in S8), the image forming operation (printing operation) is stopped (S9).

[0072] FIG. 8 is an explanatory diagram showing the photoconductor surface potential after charging when charging is performed using an AC charging bias and when charging is performed using a DC charging bias. The value of the DC voltage in the AC charging bias is the same as the value of the DC voltage in the DC charging bias.

[0073] As shown in Fig. 8, when a DC charging bias is used, the photoconductor surface potential after charging is lower than when an AC charging bias is used. Therefore, as in this embodiment, when the AC voltage of the charging bias applied to a photoconductor not used for image formation is stopped (while the DC voltage is applied), the photoconductor surface potential after charging is lower than the photoconductor surface potential when used for image formation. Therefore, the background potential (the difference between the photoconductor surface potential after charging and the developing bias) becomes smaller, and toner from the developing device 9 is more likely to adhere to the developing area, which may cause problems such as an increase in the amount of toner wasted or carrier adhesion.

[0074] In order to suppress such a problem, as shown in Fig. 9(a), the value of the DC voltage while the AC voltage of the charging bias is stopped (while the DC charging bias is applied) for the photoconductor being idled may be made larger than the value of the DC voltage when the AC charging bias is applied. This allows the background potential (the difference between the photoconductor surface potential after charging and the developing bias) while the AC voltage is stopped to approach the value when the AC charging bias is applied, as shown in Fig. 9(b). Therefore, compared to the case where the value of the DC voltage while the AC voltage is stopped is kept the same as the value of the DC voltage when the AC charging bias is applied, it is possible to reduce the amount of toner wasted and suppress carrier adhesion.

[0075] The method of suppressing the above-mentioned problems is not limited to the method of increasing the value of the DC voltage while the AC voltage is stopped as described above. For example, as shown in FIG. 10(a), when the AC voltage of the charging bias is stopped for the photoconductor being driven in an idle state, the value of the developing bias when the surface of the photoconductor passes through the developing area after the charging may be made smaller than the value when the surface of the photoconductor passes through the developing area after the charging with the AC charging bias. With this method, as shown in FIG. 10(b), the background potential (the difference between the photoconductor surface potential after charging and the developing bias) while the AC voltage is stopped can be made close to the value when the AC charging bias is applied, and the above-mentioned problems can be suppressed.

[0076] [Modifications] Next, a modification of the control content in this embodiment will be described. In the above-described embodiment, the control is to stop the AC voltage (charging AC bias) of the charging bias applied to the charging roller 8 corresponding to the photoconductor 7 that is not used for image formation and is idle driven. In this modified example, instead of stopping the AC voltage (charging AC bias), control is performed to lower at least one of the frequency and the peak-to-peak voltage of the AC voltage (charging AC bias).

[0077] FIG. 11 is a graph showing the amount of surface wear (reduction) of a photoconductor after 100 hours when the photoconductor is idle driven with the frequency of the AC charging bias AC voltage (charging AC bias) changed. The peak-to-peak voltage Vpp of the charging AC bias is fixed.

[0078] As shown in Fig. 11, the lower the frequency of the AC voltage of the AC charging bias (charging AC bias) applied to the photoconductor 7 that is idle driven, the smaller the amount of surface wear of the photoconductor. Therefore, instead of controlling to stop the AC voltage of the charging bias (charging AC bias) applied to the photoconductor 7 that is idle driven without being used for image formation, controlling to lower the frequency of the charging AC bias while keeping it on can also suppress the promotion of surface wear of the photoconductor.

[0079] FIG. 12 is a graph showing the amount of surface wear (reduction) of the photoconductor after 100 hours (hr) when the photoconductor is idle driven while changing the peak-to-peak voltage Vpp of the AC charging bias (charging AC bias). The frequency of the charging AC bias is fixed.

[0080] 12, it can be seen that the lower the peak-to-peak voltage Vpp of the AC charging bias (charging AC bias) applied to the photoconductor 7 that is idle driven, the smaller the surface wear of the photoconductor. Instead of controlling to stop the AC charging bias (charging AC bias) applied to the photoconductor 7 that is idle driven and not used for image formation, controlling to lower the peak-to-peak voltage Vpp while keeping the charging AC bias on can also suppress the acceleration of surface wear of the photoconductor.

[0081] The above description is merely an example, and each of the following aspects provides unique effects. [First aspect] The first aspect is an image forming apparatus (e.g., a printer) that charges and uniformly charges the surfaces of a plurality of rotationally driven latent image carriers (e.g., photoconductors 7Y, 7C, 7M, and 7IR), develops the formed latent image with toner, and transfers the resulting toner image onto a recording material to form an image. The image forming apparatus has a plurality of charging means (e.g., charging roller 8) that charges the plurality of latent image carriers using a charging bias (e.g., AC charging bias) in which an AC voltage is superimposed on a DC voltage, and a control means (e.g., control unit 30 and image formation control unit 40) that has a control mode in which image formation is performed using some of the plurality of latent image carriers (e.g., photoconductors 7Y, 7C, and 7M), and the control means, when forming an image in the control mode, stops the AC voltage of the charging bias (e.g., charging AC bias) used by the charging means (e.g., charging roller 8IR) corresponding to the other rotationally driven latent image carriers (e.g., photoconductor 7IR) among the plurality of latent image carriers. When a charging process for uniformly charging the surface of a latent image carrier is performed using a charging bias in which an AC voltage is superimposed on a DC voltage, the uniformity and stability of charging are improved compared to when a charging process is performed using a charging bias of only a DC voltage. However, it has been found that when a control mode is performed in which an image is formed using some latent image carriers, if other latent image carriers are rotated while being subjected to a charging process using the charging bias (charging bias in which an AC voltage is superimposed on a DC voltage), the surface wear of the latent image carriers is accelerated. This is thought to be because the surface of the other latent image carriers becomes rough due to the AC voltage, increasing the friction coefficient, and because the other latent image carriers are not used for image formation, the amount of toner attached to the surface is small, and lubrication by the toner is not obtained, resulting in excessive wear. According to this aspect, when an image is formed in the control mode, the AC voltage of the charging bias used by the charging means corresponding to the other latent image carrier that is not used for the image formation and is rotated is stopped. This prevents the surface of the other latent image carrier from becoming rough and the friction coefficient from increasing due to the AC voltage, so that even if the amount of toner adhering to the surface of the other latent image carrier is small, excessive wear can be prevented.

[0082] [Second aspect] The second aspect is an image forming apparatus (e.g., a printer) that performs an image formation by charging and uniformly charging the surfaces of a plurality of rotationally driven latent image carriers (e.g., photoconductors 7Y, 7C, 7M, and 7IR), developing the formed latent image with toner, and transferring the resulting toner image onto a recording material, the image forming apparatus comprising a plurality of charging means (e.g., charging roller 8) that performs the charging process on each of the plurality of latent image carriers using a charging bias (e.g., AC charging bias) in which an AC voltage is superimposed on a DC voltage, and a control means (e.g., control unit 30 and image formation control unit 40) having a control mode in which an image is formed using some of the plurality of latent image carriers (e.g., photoconductors 7Y, 7C, and 7M), and the control means, when forming an image in the control mode, reduces at least one of the frequency and peak-to-peak voltage of the AC voltage of the charging bias used by the charging means (e.g., charging roller 8IR) corresponding to other rotationally driven latent image carriers (e.g., photoconductor 7IR) among the plurality of latent image carriers. In this aspect, when an image is formed in the control mode, at least one of the frequency and the peak-to-peak voltage of the AC voltage of the charging bias used by the charging means corresponding to the other latent image carrier that is not used in the image formation and is rotated is lowered, thereby suppressing the surface of the other latent image carrier from becoming rough and the friction coefficient from increasing due to the AC voltage, and therefore, even if the amount of toner adhering to the surface of the other latent image carrier is small, the occurrence of excessive wear can be suppressed.

[0083] [Third aspect] A third aspect is characterized in that in the first or second aspect, the part of the latent image carriers carries a toner image made of at least one color of toner selected from yellow toner, magenta toner, cyan toner, and black toner, and the other latent image carriers carry a toner image made of a special toner (e.g., IR toner). This makes it possible to prevent excessive wear from occurring on the latent image carrier for the special toner, which is generally used less frequently than the yellow toner, magenta toner, cyan toner, and black toner.

[0084] [Fourth aspect] The fourth aspect is characterized in that, in the third aspect, it has a unit holding portion that selectively and detachably holds a black replacement unit (e.g., K process unit 6K) that forms a toner image on the surface of the latent image carrier with the black toner and a special replacement unit (e.g., IR process unit 6IR) that forms a toner image on the surface of the latent image carrier with the special toner. Since the black replacement unit is usually used for forming color images as well as monochrome images, the photoconductor of the black replacement unit is rotated in any control mode for forming images. Therefore, in a configuration in which a special replacement unit can be attached to the unit holder in place of such a black replacement unit, the special replacement unit, which is generally used less frequently, tends to be driven empty, which tends to cause excessive wear on the latent image carrier for the special toner. According to this aspect, it is possible to suppress the occurrence of excessive wear on the latent image carrier for the special toner in such a case.

[0085] [Fifth aspect] A fifth aspect is characterized in that, in any one of the first to fourth aspects, when forming an image in the control mode, the control means causes the charging means corresponding to the other latent image carrier to charge the surface of the other image carrier using a charging bias including a DC voltage. According to this aspect, when forming an image in the control mode, the background potential when passing through the development area can be secured compared to a case where the charging process using the charging bias including the DC voltage is not performed on the charging means corresponding to the other latent image carrier, and as a result, it is possible to prevent the toner from the developing means from moving to the surface of the other latent image carrier and wasting the toner.

[0086] [Sixth aspect] A sixth aspect is characterized in that, in the fifth aspect, the absolute value of the DC voltage when forming an image in the control mode is greater than the absolute value of the DC voltage of the charging bias used by the charging means corresponding to the other latent image carrier in another control mode in which an image is formed using the other latent image carrier. According to this, it is possible to ensure a larger background potential when the surface of the other production support passes through the development area, and it is possible to further suppress unnecessary toner consumption.

[0087] [Seventh aspect] The seventh aspect is characterized in that in the fifth aspect, the magnitude of the developing bias used by the developing means (e.g., developing device 9) corresponding to the other latent image carrier when forming an image in the control mode is smaller than the magnitude of the developing bias used by the developing means in another control mode in which an image is formed using the other latent image carrier. According to this, it is possible to ensure a larger background potential when the surface of the other production support passes through the development area, and it is possible to further suppress unnecessary toner consumption.

[0088] [Eighth aspect] An eighth aspect is characterized in that, in any one of the first to fourth aspects, the control means stops the DC voltage of the charging bias used by the charging means corresponding to the other latent image carrier when forming an image in the control mode. According to this aspect, when forming an image in the control mode, more toner can be attached to the surface of the other latent image carrier than when the charging means corresponding to the other latent image carrier is charged using a charging bias including a DC voltage. This, for example, provides a lubricating effect of the toner, making it possible to suppress the promotion of surface wear of the latent image carrier that is driven idly. In addition, it is also possible to perform a toner refresh operation that forcibly discharges deteriorated toner from the developing means. [Explanation of symbols]

[0089] 1: Image forming section 2: Transfer section 3: Recording material supply section 4: Fixing section 5: Recording material discharge section 6: Process unit 7: Photoreceptor 8: Charging roller 9: Developing device 11: Exposure equipment 12: Intermediate transfer belt 13: Primary transfer roller 14: Secondary transfer roller 21: Fixing device 25: Paper output tray 26: Toner cartridge 30: Control section 40: Image formation control unit [Prior art documents] [Patent documents]

[0090] [Patent Document 1] JP 2018-194624 A

Claims

1. An image forming apparatus for forming an image by transferring a toner image obtained by developing a latent image formed on a surface of a plurality of rotationally driven latent image carriers by a charging process to uniformly charge the surface of the latent image carriers with a toner onto a recording material, the image forming apparatus comprising: a plurality of charging means for performing the charging process on the plurality of latent image carriers using a charging bias in which an AC voltage is superimposed on a DC voltage; a control means having a control mode in which image formation is performed using some of the latent image carriers among the plurality of latent image carriers, when forming an image in the control mode, the control unit stops the AC voltage of the charging bias used by the charging unit corresponding to the other latent image carriers that are rotated among the plurality of latent image carriers, the part of the latent image carriers carries a toner image made of at least one color of toner selected from yellow toner, magenta toner, and cyan toner, and does not carry a toner image made of black toner, the other latent image carrier carries a toner image made of a special toner, an image forming apparatus characterized by having a unit holding section that selectively and detachably holds a black replacement unit that forms a toner image on the surface of the latent image carrier using the black toner and a special replacement unit that forms a toner image on the surface of the latent image carrier using the special toner.

2. An image forming apparatus for forming an image by transferring a toner image obtained by developing a latent image formed on a surface of a plurality of rotationally driven latent image carriers by a charging process to uniformly charge the surface of the latent image carriers with a toner onto a recording material, the image forming apparatus comprising: a plurality of charging means for performing the charging process on the plurality of latent image carriers using a charging bias in which an AC voltage is superimposed on a DC voltage; a control means having a control mode in which image formation is performed using some of the latent image carriers among the plurality of latent image carriers, when forming an image in the control mode, the control unit reduces at least one of a frequency and a peak-to-peak voltage of an AC voltage of a charging bias used by the charging unit corresponding to another latent image carrier that is rotated among the plurality of latent image carriers, the part of the latent image carriers carries a toner image made of at least one color of toner selected from yellow toner, magenta toner, and cyan toner, and does not carry a toner image made of black toner, the other latent image carrier carries a toner image made of a special toner, an image forming apparatus characterized by having a unit holding section that selectively and detachably holds a black replacement unit that forms a toner image on the surface of the latent image carrier using the black toner and a special replacement unit that forms a toner image on the surface of the latent image carrier using the special toner.

3. In the image forming apparatus according to claim 1 or 2, the control means, when forming an image in the control mode, causes the charging means corresponding to the other latent image carrier to charge the surface of the other latent image carrier using a charging bias including a DC voltage.

4. 4. The image forming apparatus according to claim 3, an absolute value of the DC voltage when forming an image in the control mode is greater than an absolute value of a DC voltage of a charging bias used by the charging means corresponding to the other latent image carrier in another control mode in which an image is formed using the other latent image carrier.

5. 4. The image forming apparatus according to claim 3, an image forming apparatus characterized in that the magnitude of the developing bias used by the developing means corresponding to the other latent image carrier when forming an image in the control mode is smaller than the magnitude of the developing bias used by the developing means in another control mode in which an image is formed using the other latent image carrier.

6. 3. The image forming apparatus according to claim 1, The image forming apparatus according to claim 1, wherein the control means stops a DC voltage of a charging bias used by the charging means corresponding to the other latent image carrier when forming an image in the control mode.

Images