Image forming apparatus

Abstract

To suppress image defects caused by dew condensation.SOLUTION: An image forming apparatus 100 has a control part 210 which acquires a detection result by a detection part 511 for detecting current or voltage to be supplied to a charging member 2 when a surface area of a photoreceptor 1 corresponding to respective recording materials P to which a toner image is transferred with a print job during execution of the print job passes through an abutting part CN formed when the charging member 2 comes into contact with the photoreceptor 1, and executes change control to change at least one among a charging bias, an exposure amount of exposure means 3 that forms the potential of an image part on the surface of the photoreceptor 1 and a developing bias such that a potential difference between the potential of the image part on the surface of the photoreceptor 1 at a development position and the developing bias is within a prescribed range on the basis of a change in the detection result by dew condensation on the surface of the photoreceptor 1.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copier, a facsimile machine, or a multifunction machine having a plurality of functions thereof, which forms an image on a recording material using an electrophotographic method or an electrostatic recording method. [Background technology]

[0002] Conventionally, in an image forming apparatus such as a printer using an electrophotographic method, the surface of a photoconductor is uniformly charged, the charged surface of the photoconductor is exposed to light to form an electrostatic latent image on the photoconductor, and toner is supplied to the electrostatic latent image by a developing member to form a toner image on the photoconductor. The toner image formed on the photoconductor is transferred directly or via an intermediate transfer member to a recording material such as paper, and the toner image is fixed to the recording material to form a print image on the recording material. A rotatable drum-type (cylindrical) photoconductor drum is widely used as the photoconductor.

[0003] In such an image forming apparatus, condensation occurring inside the apparatus may cause a change in image density, resulting in a decrease in image quality.

[0004] Patent Document 1 discloses a configuration in which a condensation detection sequence is executed to detect a condensation state prior to an image formation sequence, and a condensation removal sequence is executed if it is determined that a condensation state exists. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5233223 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the configuration of Patent Document 1, condensation is detected and removed before image formation (before printing), so if condensation gradually progresses during image formation (printing), image defects may occur.

[0007] For example, in an environment where a fan is installed on the ceiling of a room where an image forming apparatus is installed, moisture evaporated from paper as a recording material during fixing may flow into the machine by the wind of the fan, causing condensation in various parts of the machine. In particular, when condensation occurs on the surface of the photosensitive drum, the condensed moisture reduces the electrical resistance of the surface of the photosensitive drum, and an excessive current is supplied during charging, causing the surface potential of the photosensitive drum to become higher than normal. As a result, the potential of the electrostatic latent image formed on the surface of the photosensitive drum by exposure may deviate from the appropriate value, causing image defects such as a decrease in image density (hereinafter also referred to as "low density"). Furthermore, as condensation progresses, it may cause image defects such as toner on the developing member moving to the photosensitive drum due to the condensed moisture (hereinafter also referred to as "fogging"). It is desirable to suppress image defects that occur when condensation gradually progresses during image formation (printing).

[0008] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS Accordingly, an object of the present invention is to suppress image defects caused by condensation. [Means for solving the problem]

[0009] The above object is achieved by the image forming apparatus according to the present invention. In summary, one of the main components of the present invention is an image forming apparatus that includes a rotatable photoconductor, a charging member that contacts the photoconductor to form a contact portion and charges the surface of the photoconductor when a charging bias is applied, an application portion that applies the charging bias to the charging member, an exposure means that exposes the charged surface of the photoconductor to light to form a potential of an image portion on the surface of the photoconductor, a developing member that is applied with a developing bias and supplies toner to the image portion at a development position to form a toner image on the surface of the photoconductor, and a transfer means that transfers the toner image from the surface of the photoconductor to a transfer receiving material, and that executes a print job in which a print image is formed on one or more recording materials in response to one start instruction, and the one or more recording materials are discharged from inside the machine to outside the machine through a recording material discharge port. and a control unit that, during execution of a print job, acquires a detection result by the detection unit when an area of ​​the surface of the photoconductor corresponding to each recording material to which a toner image is transferred in the print job passes through the contact unit, and executes change control to change at least one of the charging bias, the exposure amount of the exposure means that forms the potential of the image portion, and the developing bias based on a change in the detection result due to condensation on the surface of the photoconductor, so that a potential difference between the potential of the image portion at the development position and the developing bias is within a predetermined range.

[0010] Another main configuration of the present invention is a recording medium having a rotatable photoconductor, a charging member that contacts the photoconductor to form a contact portion and charges the surface of the photoconductor when a charging bias is applied, an application portion that applies the charging bias to the charging member, an exposure means that exposes the charged surface of the photoconductor to light to form a potential of an image portion on the surface of the photoconductor, a developing member that is applied with a developing bias and supplies toner to the image portion at a development position to form a toner image on the surface of the photoconductor, and a transfer means that transfers the toner image from the surface of the photoconductor to a transfer receiving body, and a recording medium having a recording medium that is a recording medium that is a recording medium, and a transfer unit that transfers the toner image from the surface of the photoconductor to a transfer receiving body, the recording medium having a recording medium that is a recording medium, and a transfer unit that transfers the toner image from the surface of the photoconductor to a transfer receiving body, the recording medium having a recording medium that is a recording medium, and a transfer unit that transfers the toner image from the surface of the photoconductor to a transfer receiving body, the recording medium having a recording medium that is a recording medium, and a transfer unit that transfers the toner image from the surface of the photoconductor to a transfer receiving body, the recording medium having a recording medium that is a recording medium, and the ... and a control unit that, during execution of the print job, acquires a detection result by the detection unit when an area on the surface of the photoconductor corresponding to each recording material to which a toner image is transferred in the print job passes through the contact unit, and controls the image forming apparatus to execute a condensation removal process to remove condensation on the surface of the photoconductor based on a change in the detection result due to condensation on the surface of the photoconductor. Effect of the Invention

[0011] According to the present invention, image defects caused by condensation are suppressed. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 2 is a schematic cross-sectional view of an image forming unit. [Diagram 3] FIG. 2 is a block diagram showing a control configuration of the image forming apparatus. [Figure 4] FIG. 2 is a schematic diagram of an exposure control unit and a scanner unit. [Diagram 5] FIG. 2 is a schematic diagram of an electric circuit around the photosensitive drum. [Figure 6]FIG. 11 is a chart showing the progress of the electrostatic voltage and the like when no condensation occurs in an example. [Figure 7] FIG. 11 is a chart showing the transition of the charged voltage and the like when condensation is present in a comparative example. [Figure 8] FIG. 11 is a chart showing the progress of the electrostatic voltage and the like when condensation is present in an example. [Figure 9] FIG. 1 is a schematic diagram of an environment in which an image forming apparatus is installed. [Figure 10] 10 is a graph for explaining a change in the surface potential of the photosensitive drum when condensation occurs in a comparative example. FIG. [Figure 11] FIG. 4 is a chart showing the transition of the charging voltage during idling in an embodiment. [Figure 12] FIG. 4 is a flowchart of the operation of a print job in the embodiment. [Figure 13] FIG. 13 is a block diagram showing a control configuration of an image forming apparatus according to a first modified example. [Figure 14] 13 is a chart showing the transition of the charging voltage and the like in Modification 1. FIG. [Figure 15] FIG. 13 is a block diagram showing a control configuration of an image forming apparatus according to a first modified example. [Figure 16] 13 is a chart showing the transition of the charging voltage and the like in Modification 2. FIG. [Figure 17] FIG. 10 is a graph showing a surface potential of a photosensitive drum for explaining another configuration example of the embodiment. [Figure 18] 11 is a chart showing the progress of the charging voltage and the like in Example 2. FIG. [Figure 19] 11 is a chart showing the progress of the charging voltage and the like in Example 2. FIG. [Figure 20] FIG. 11 is a schematic diagram for explaining another example of the configuration of an image forming apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0014] [Example 1] <Overall configuration of image forming apparatus> First, the overall configuration of the image forming apparatus of this embodiment will be described below. Fig. 1 is a schematic cross-sectional view of an image forming apparatus 100 of this embodiment.

[0015] The image forming apparatus 100 of this embodiment is a full-color laser beam printer that employs an in-line system and an intermediate transfer system. The image forming apparatus 100 can form a full-color image on a sheet-shaped recording material P (e.g., recording paper, plastic sheet) according to image information. The image information is input to the image forming apparatus 100 from an external device 199 (FIG. 3) communicably connected to the image forming apparatus 100, for example, an image reading device or a host computer such as a personal computer (host computer in this embodiment).

[0016] The image forming apparatus 100 has four image forming units Sa, Sb, Sc, and Sd as a plurality of image forming units (stations) inside (inside) a housing (main body cover) 120 of the apparatus main body 110. The image forming units Sa, Sb, Sc, and Sd form images of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. In this embodiment, the four image forming units Sa, Sb, Sc, and Sd are arranged in a row in a direction intersecting with the vertical direction.

[0017] In addition, elements having the same or corresponding functions or configurations provided for each color may be generally described by omitting the suffixes a, b, c, and d to indicate that the element is provided for one of the colors. For convenience, the magnitude (high / low) of a current, voltage (electric potential), or electric potential difference refers to the magnitude (high / low) when compared in absolute value unless otherwise specified. In addition, since paper is mainly used as the recording material P in the image forming apparatus 100, the recording material P may be referred to as paper, but the recording material P also includes materials other than paper, or materials including materials other than paper.

[0018] In this embodiment, the image forming section S includes photosensitive drums 1 (1a, 1b, 1c, 1d), charging rollers 2 (2a, 2b, 2c, 2d), scanner units 3 (3a, 3b, 3c, 3d), developing devices 4 (4a, 4b, 4c, 4d), pre-charge exposure devices 6 (6a, 6b, 6c, 6d), etc. Fig. 2 is a schematic cross-sectional view showing one representative image forming section S.

[0019] The photosensitive drum 1, which is a rotatable drum-type photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is driven to rotate in the direction of arrow R1 in FIG. 1 (counterclockwise direction) by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) serving as a driving means. In this embodiment, the photosensitive drum 1 is configured by providing a photosensitive layer and a surface layer, in that order from the bottom, on an aluminum bare tube having a diameter of 20 mm. In this embodiment, the surface layer of the photosensitive drum 1 is a thin film layer having a thickness of 23 μm, formed of polyarylate having a relative dielectric constant of 3.0.

[0020] The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2, which is a roller-type charging member serving as a charging means. In this embodiment, the charging roller 2 is a 1.5 mm thick, 1×10 specific volume resistivity roller mounted on a metal shaft with a diameter of 5.5 mm. 6The charging roller 2 is provided with an elastic layer made of conductive rubber having a resistance of about Ωcm. In this embodiment, the length of the charging roller 2 in the direction of the rotation axis is 230 mm. The charging roller 2 contacts the surface of the photosensitive drum 1 to form a charging nip portion (charging position) CN, which is the contact portion between the photosensitive drum 1 and the charging roller 2. In this embodiment, the charging roller 2 is pressed against the surface of the photosensitive drum 1 at both ends in the direction of the rotation axis of the metal shaft through bearings (not shown) at 300 gf on one side, for a total of 600 gf. As a result, the charging roller 2 rotates in accordance with the rotation of the photosensitive drum 1 while forming a charging nip portion CN with a nip width of about 300 μm in the direction of rotation of the photosensitive drum 1. During charging, a charging voltage (charging bias), which is a DC voltage having the same polarity (negative polarity in this embodiment) as the charging polarity of the photosensitive drum 1, is applied to the charging roller 2 by a charging power source (high voltage power source) 501 (FIG. 3) as a charging voltage application means (charging voltage application unit).

[0021] The surface of the rotating photosensitive drum 1 that has been charged is scanned and exposed by a scanner unit (exposure device) 3 as an exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. A controller 200 (FIG. 3) described later performs calculations based on image information input from a host computer 199 (FIG. 3) and outputs an image signal related to image formation in the image forming apparatus 100. An exposure control unit 204 (FIG. 3) described later controls the scanner unit 3 to irradiate laser light onto an image portion (image area) on the surface of the photosensitive drum 1 based on an image signal output by the controller 200, in response to an instruction from an engine control unit 210 (FIG. 3) described later. In addition, in this embodiment, when an electrostatic latent image is formed by the scanner unit 3 according to the image signal, a non-image portion (non-image area) on the surface of the photosensitive drum 1 is uniformly exposed to a laser light having a light amount lower than that of the laser light irradiated onto the image portion. This stabilizes the potential of the non-image portion of the photosensitive drum 1. Hereinafter, forming an electrostatic latent image by the scanner unit 3 will be distinguished as "exposure (or exposure of image area)" and uniformly exposing a non-image area as "weak exposure (or exposure of non-image area)".

[0022] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device (developing unit) 4 as a developing means, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. The developing device 4 has a developing roller 22 as a developing member (developer carrier) and a developing container 23. The developing container 23 contains toner, which is a non-magnetic one-component developer, as a developer. During development, the developing roller 22 abuts against the surface of the photosensitive drum 1 to form a development nip portion (developing position) DN, which is a contact portion between the photosensitive drum 1 and the developing roller 22. During development, the developing roller 22 is rotated by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) as a driving means. The developing roller 22 carries the toner in the developing container 23 and transports it to the development nip portion DN. During development, a development voltage (development bias), which is a DC voltage of the same polarity (negative in this embodiment) as the charging polarity of the photosensitive drum 1, is applied to the development roller 22 by a development power source (high voltage power source) 502 (FIG. 3) serving as a development voltage application means (development voltage application unit). As a result, development is performed by the potential difference between the surface potential of the image portion on the photosensitive drum 1 and the development voltage (potential of the development roller 22).

[0023] In this embodiment, the image forming apparatus 100 has a contact / separation mechanism 80 (FIG. 3) for contacting and separating the developing roller 22 with respect to the photosensitive drum 1. In this embodiment, when the image forming apparatus 100 is stopped, the contact / separation mechanism 80 separates the developing roller 22 from the photosensitive drum 1. During development, the contact / separation mechanism 80 contacts the developing roller 22 with the photosensitive drum 1. In this embodiment, the developing container 23 is configured to be swingable, and is biased in a direction in which the developing roller 22 contacts the photosensitive drum 1 by a pressure spring, which is a biasing member serving as a biasing means. The contact / separation mechanism 80 is configured to move (rotate) the developing container 23 against the biasing force of the pressure spring, thereby separating the developing roller 22 from the photosensitive drum 1. The contact / separation mechanism 80 is configured to allow the developing container 23 to move (rotate) by the biasing force of the pressure spring, thereby allowing the developing roller 22 to contact the photosensitive drum 1. The contact / separation mechanism 80 is driven by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) as a driving means. The developing roller 22 is rotationally driven when brought into contact with the photosensitive drum 1, and stops rotating when separated from the photosensitive drum 1.

[0024] In this embodiment, the toner is a non-magnetic toner manufactured by a suspension polymerization method, and has a normal charging polarity, which is the main charging polarity during development, of negative polarity, and has a volume average particle size of 7.0 μm. This toner is charged to negative polarity when carried on the developing roller 22. In this embodiment, the developing device 4 performs reversal development. That is, the developing device 4 attaches toner charged to the same polarity (negative polarity in this embodiment) as the charging polarity of the photosensitive drum 1 to a portion (image portion, exposed portion) on the photosensitive drum 1 where the charge has been attenuated by exposure after being uniformly charged. In this embodiment, the charging voltage Vpri applied to the charging roller 2 is -1300V, and the surface potential Vd of the photosensitive drum 1 after charging (and before exposure and weak exposure) (hereinafter also referred to as "drum potential after charging") is approximately -700V. In this embodiment, the surface potential Vdl of the non-image portion on the photosensitive drum 1 formed by weak exposure (hereinafter also referred to as "drum potential after weak exposure") is approximately -500V. In this embodiment, the developing voltage Vdc applied to the developing roller 22 is -300V. In this embodiment, the surface potential (hereinafter also referred to as "drum potential after exposure") VL of the image portion on the photosensitive drum 1 where the charge formed by exposure is attenuated is approximately -100V. The electrostatic latent image on the photosensitive drum 1 is reverse-developed and a toner image is formed on the photosensitive drum 1 by the potential difference (hereinafter also referred to as "development contrast") Vcont between the post-exposure drum potential VL on the photosensitive drum 1 at the development nip portion DN and the developing voltage Vdc (potential of the developing roller 22). Note that the toner on the developing roller 22 is electrically held on the developing roller 22 so as not to move to the non-image portion on the photosensitive drum 1 by the potential difference (hereinafter also referred to as "back contrast") Vback between the developing voltage Vdc (potential of the developing roller 22) at the development nip portion DN and the post-weak exposure drum potential Vdl.

[0025] Opposing the four photosensitive drums 1a, 1b, 1c, and 1d, an intermediate transfer belt 10 is disposed as an intermediate transfer body, which is an endless belt for transferring the toner image on the photosensitive drum 1 to a recording material P. The intermediate transfer belt 10 is stretched around a drive roller 11, a tension roller 12, and a secondary transfer opposing roller 13 as a plurality of support rollers, and stretched with a predetermined tension. The drive roller 11 is driven to rotate by a drive motor provided in a drive unit 90 (FIG. 3) as a drive means. The intermediate transfer belt 10 receives a drive force from the drive roller 11 and rotates (circulates, moves around) in the direction of the arrow R2 in FIG. 1 (clockwise direction). The intermediate transfer belt 10 can rotate while being in contact with each of the four photosensitive drums 1. The support rollers other than the drive roller 11 are driven to rotate with the rotation of the intermediate transfer belt 10.

[0026] In this embodiment, the intermediate transfer belt 10 is an endless belt made of a resin material with added conductive agent to give it electrical conductivity, and has a circumference of 700 mm and a thickness of 70 μm. In this embodiment, the intermediate transfer belt 10 is a multi-layered belt (film) having a base layer and a surface layer provided on the base layer. In this embodiment, an endless polyimide resin mixed with carbon as a conductive agent is used as the material for the base layer of the intermediate transfer belt 10. In this embodiment, the intermediate transfer belt 10 exhibits electronic conductive properties as an electrical characteristic, and is characterized by small fluctuations in electrical resistance value with respect to temperature and humidity in the atmosphere. In this embodiment, a polyimide resin is used as the material for the base layer of the intermediate transfer belt 10, but this is not limited to this, and other materials may be used. The thermoplastic resin constituting the base layer of the intermediate transfer belt 10 may be, for example, polyethylene naphthalate, polyester, polycarbonate, polyarylate, acrylonitrile-butadiene-styrene copolymer (ABS), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVdF), or a mixture of these resins. In this embodiment, carbon is used as the conductive agent contained in the material of the base layer of the intermediate transfer belt 10, but the conductive agent is not limited thereto. For example, conductive metal oxide fine particles may be used. In this embodiment, the outer peripheral surface of the base layer of the intermediate transfer belt 10 is provided with a surface layer of high resistance acrylic resin having a thickness of 0.5 μm to 3 μm (with respect to the numerical range, "to" indicates a range including numerical values ​​before and after the numerical range). By forming the surface layer of the intermediate transfer belt 10 as a high resistance layer, the effect of improving secondary transferability can be obtained.

[0027] On the inner peripheral surface side of the intermediate transfer belt 10, primary transfer rollers 14a, 14b, 14c, and 14d, which are roller-type primary transfer members serving as primary transfer means, are disposed in correspondence with the photosensitive drums 1a, 1b, 1c, and 1d. The primary transfer rollers 14 press the intermediate transfer belt 10 toward the photosensitive drums 1, forming a primary transfer nip portion (primary transfer portion, primary transfer position) N1, which is a contact portion between the photosensitive drum 1 and the intermediate transfer belt 10. In this embodiment, the primary transfer roller 14 is a cylindrical metal roller with a diameter of 6 mm. In this embodiment, the primary transfer roller 14 is made of SUS (stainless steel) with a nickel-plated surface. The primary transfer roller 14 is disposed such that the center of rotation of the primary transfer roller 14 is offset 8 mm downstream in the moving direction of the surface of the intermediate transfer belt 10 from the center of rotation of the photosensitive drum 1 in a cross section perpendicular to the rotation axis direction of the photosensitive drum 1. This results in a configuration in which the intermediate transfer belt 10 is wound around the photosensitive drum 1. The primary transfer roller 14 is disposed at a position raised 1 mm toward the photosensitive drum 1 from a common tangential plane (a substantially horizontal plane in this embodiment) on the side of the multiple photosensitive drums 1 that contact the intermediate transfer belt 10 so as to ensure the amount of winding of the intermediate transfer belt 10 around the photosensitive drum 1. In this embodiment, the primary transfer roller 14 presses the intermediate transfer belt 10 with a force of about 200 gf. The primary transfer roller 14 is rotated following the rotation of the intermediate transfer belt 10.

[0028] The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the rotating intermediate transfer belt 10 as a transfer target by the action of the primary transfer roller 14 at the primary transfer nip N1. During the primary transfer, a primary transfer voltage (primary transfer bias) which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner is applied to the primary transfer roller 14 by a primary transfer voltage power source (high voltage power source) 15 as a primary transfer voltage application means (primary transfer voltage application unit). In this embodiment, the primary transfer bias is +100V. For example, when a full-color image is formed, the above-mentioned process is performed sequentially in the four image forming units Sa, Sb, Sc, and Sd, and the toner images of each color of yellow, magenta, cyan, and black are transferred sequentially so as to be superimposed on the same image forming area (area where a toner image can be formed) on the intermediate transfer belt 10.

[0029] The toner (primary transfer residual toner) remaining on the surface of the photosensitive drum 1 after the primary transfer process is collected in the developing device 4 by the developing roller 22 of the developing device 4. The toner collected in the developing device 4 is reused for development. That is, the primary transfer residual toner remaining on the photosensitive drum 1 after the primary transfer process is charged to the normal charging polarity (negative polarity in this embodiment) when passing through the charging nip portion CN. The primary transfer residual toner charged to the normal charging polarity is transported to the developing nip portion DN while remaining attached to the surface of the photosensitive drum 1. Then, in the developing nip portion DN, the toner on the non-image portion on the photosensitive drum 1 moves onto the developing roller 22 by the electric field due to the potential difference (back contrast Vback) between the drum potential Vdl after weak exposure and the developing voltage Vdc (potential of the developing roller 22) and is collected in the developing device 4. Also, in the developing nip portion DN, the toner on the image portion on the photosensitive drum 1 forms a toner image together with the toner supplied from the developing roller 22 to the photosensitive drum 1, and is transported to the primary transfer nip portion N1.

[0030] In this embodiment, the primary transfer residual toner is collected by the developing device 4, but the present invention is not limited to this embodiment. A drum cleaning device may be provided as a photosensitive member cleaning means in the image forming apparatus 100, and the primary transfer residual toner may be removed (cleaned) from the surface of the photosensitive drum 1 by the drum cleaning device. As the drum cleaning device, a device equipped with a cleaning blade as a cleaning member that is disposed in contact with the surface of the photosensitive drum 1 and scrapes and removes the primary transfer residual toner from the rotating photosensitive drum 1 may be used.

[0031] Also, the surface potential of the photosensitive drum 1 changes non-uniformly due to the primary transfer process. In this embodiment, the image forming section S is provided with a pre-charge exposure device 6 as a charge removing means in order to remove (at least a part of the charge) the surface of the photosensitive drum 1 after the primary transfer process (and before the charging process) and smooth the surface potential of the photosensitive drum 1. The pre-charge exposure device 6 is disposed downstream of the primary transfer nip portion N1 and upstream of the charging nip portion CN in the rotation direction of the photosensitive drum 1, and exposes the surface of the photosensitive drum 1 downstream of the primary transfer nip portion N1 and upstream of the charging nip portion CN in the rotation direction of the photosensitive drum 1. The pre-charge exposure device 6 exposes the surface of the photosensitive drum 1 (hereinafter, also referred to as "pre-charge exposure"). As a result, the surface potential of the photosensitive drum 1 is discharged to a predetermined potential (approximately 0 V in this embodiment).

[0032] A secondary transfer roller 20, which is a roller-type secondary transfer member serving as a secondary transfer means, is disposed at a position facing the secondary transfer opposing roller 13 on the outer peripheral surface side of the intermediate transfer belt 10. The secondary transfer roller 20 is pressed toward the secondary transfer opposing roller 13 and contacts the secondary transfer opposing roller 13 via the intermediate transfer belt 10, forming a secondary transfer nip portion (secondary transfer portion, secondary transfer position) N2, which is a contact portion between the intermediate transfer belt 10 and the secondary transfer roller 13. In this embodiment, the secondary transfer roller 20 contacts the intermediate transfer belt 10 with a pressure of 50 N and rotates in accordance with the rotation of the intermediate transfer belt 10 while forming the secondary transfer nip portion N2. A recording material P such as paper is conveyed while being sandwiched between the intermediate transfer belt 10 and the secondary transfer roller 20 at the secondary transfer nip portion N. In this embodiment, the secondary transfer roller 20 is a nickel-plated steel rod with a diameter of 8 mm and a volume resistivity of 10. 8 A roller with an outer diameter of 18 mm was used, covered with a foam sponge mainly composed of NBR and epichlorohydrin rubber, adjusted to a resistance of Ω·cm and a thickness of 5 mm.

[0033] The toner image formed on the intermediate transfer belt 10 is transferred (secondary transfer) onto a recording material P as a transfer target, which is sandwiched and conveyed between the intermediate transfer belt 10 and the secondary transfer roller 20, at the secondary transfer nip portion N2. During the secondary transfer, a secondary transfer voltage (secondary transfer bias), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer roller 20 by a secondary transfer voltage power source (high voltage power source) 21 as a secondary transfer voltage application means (secondary transfer voltage application portion). In this embodiment, the secondary transfer power source 21 can output in the range of +100V to +5000V. The secondary transfer opposing roller 13 is electrically grounded (connected to a ground potential). The recording material (transfer material, recording medium, media) P is stored in a cassette 51 as a recording material storage portion. The recording material P is sent out from a cassette 51 by a feeding roller 50 as a feeding member, and is transported to the secondary transfer nip portion N2 by a pair of registration rollers 52 as a transport member in synchronization with the toner image on the intermediate transfer belt 10.

[0034] Adherents such as toner (secondary transfer residual toner) remaining on the surface of the intermediate transfer belt 10 after the secondary transfer are removed and collected from the surface of the intermediate transfer belt 10 by a belt cleaning device 16 as an intermediate transfer body cleaning means. The belt cleaning device 16 is disposed downstream of the secondary transfer nip portion N2 and upstream of the primary transfer nip portion N1 (the most upstream primary transfer nip portion N1a) in the rotation direction (movement direction of the surface) of the intermediate transfer belt 10. In this embodiment, the belt cleaning device 16 is disposed at a position facing the secondary transfer opposing roller 16 via the intermediate transfer belt 10. The belt cleaning device 16 has a cleaning blade 17 as a cleaning member disposed in contact with the surface of the intermediate transfer belt 10. The belt cleaning device 16 uses the cleaning blade 17 to scrape off and remove (clean) adherents such as secondary transfer residual toner from the surface of the rotating intermediate transfer belt 10. In this embodiment, the cleaning blade 17 is made of polyurethane rubber, and is in contact with the secondary transfer opposing roller 13 via the intermediate transfer belt 10 at a contact pressure of 85.0 gf / cm.

[0035] The recording material P onto which the toner image has been transferred is transported to a fixing device 30 serving as a fixing means. The fixing device 30 has a fixing roller 31 and a pressure roller 32 that is in pressure contact with the fixing roller 31. The fixing device 30 heats and presses the recording material P while nipping and transporting the recording material P in a fixing nip portion formed by the fixing roller 31 and the pressure roller 32. This causes the toner image on the recording material P to be fixed (melted and fixed) onto the recording material P. For example, when a full-color image is formed, the four color toners on the recording material P are melted, mixed, and fixed onto the recording material P in the fixing device 30.

[0036] In this embodiment, the fixing roller (fixing member) 31 is a roller having an outer diameter of 18 mm, in which an elastic layer of insulating silicone rubber is formed on the outer periphery of a metal tube, and the outer periphery of the elastic layer is further coated with an insulating PFA tube. The fixing roller 31 includes a halogen heater 33 as a heating means. The halogen heater 33 generates heat when a voltage is supplied from a power source (not shown) without contacting the fixing roller 31. In this embodiment, the pressure roller (pressure member) 32 is a roller having an outer diameter of 18 mm, in which an elastic layer of conductive silicone rubber is formed on the outer periphery of a core metal, and the outer periphery of the elastic layer is further coated with a conductive PFA tube. The fixing roller 31 and the pressure roller 32 are pressed with a force of 10 kgf to form a fixing nip portion. The pressure roller 32 is rotated by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) as a driving means. The fixing roller 31 rotates following the rotation of the pressure roller 32. The recording material P is conveyed while being sandwiched between the fixing roller 31 and the pressure roller 32 in the fixing nip portion.

[0037] The recording material P on which the toner image is fixed and a print image is formed is discharged (output) to the outside (outside the machine) of the device body 110 through a paper discharge outlet (recording material discharge outlet) 130 as an opening formed in the housing 120 of the device body 110. This recording material P is loaded on a tray 40 provided on the upper part of the housing 120 of the device body 110.

[0038] In addition, the image forming apparatus 100 can form a monochrome image using only one desired image forming unit S, or can form a multi-color image using only some of the multiple image forming units S.

[0039] In this embodiment, the image forming apparatus 100 is a printer capable of handling A4 size paper and has a process speed of 148 mm / sec. The process speed of the image forming apparatus 100 corresponds to the peripheral speed (surface moving speed) of the photosensitive drum 1 and the intermediate transfer belt 10.

[0040] In this embodiment, in each image forming section S, the photosensitive drum 1 and the charging roller 2 and developing device 4 as process means acting on the photosensitive drum 1 are integrated to form a process cartridge 7 (7a, 7b, 7c, 7d). The process cartridge 7 is detachable from the device body 110 of the image forming apparatus 100 via mounting means such as a mounting guide and a positioning member provided in the device body 110 of the image forming apparatus 100. In this embodiment, the device body 110 is the portion of the image forming apparatus 100 excluding the process cartridges 7.

[0041] <Control configuration> Next, a description will be given of the control configuration of image forming apparatus 100 in this embodiment. Fig. 3 is a block diagram for explaining the control configuration of image forming apparatus 100 in this embodiment.

[0042] The image forming apparatus 100 has an engine control unit 210 as a control unit (control means) that controls the entire apparatus. To the engine control unit 210, for example, a primary transfer control unit 201, a secondary transfer control unit 202, a development control unit 203, an exposure control unit 204, a charging control unit 205, a pre-charging exposure control unit 206, a contact / separation mechanism 80, a drive unit 90, and the like are connected.

[0043] The engine control unit 210 incorporates a CPU circuit unit 150 as an arithmetic processing unit (arithmetic processing means), and a ROM 151 and a RAM 152 as storage units (storage means). The CPU circuit unit 150 comprehensively controls each unit of the image forming apparatus 100, such as a primary transfer control unit 201, a secondary transfer control unit 202, a development control unit 203, an exposure control unit 204, a charging control unit 205, a pre-charging exposure control unit 206, a contact / separation mechanism 80, and a driving unit 90, according to a control program stored in the ROM 151. The ROM 151 also stores information on the setting of control values ​​for each unit, such as an environment table and a paper width / paper thickness correspondence table, which the CPU circuit unit 150 calls and reflects in the control. The RAM 152 temporarily holds control data and detection results of sensors, and is also used as a work area for arithmetic processing associated with the control. The primary transfer control unit 201 controls the primary transfer power supply 15 according to an instruction from the CPU circuit unit 150. The secondary transfer control unit 202 controls the secondary transfer power supply 21 in response to instructions from the CPU circuit unit 150. The development control unit 203 controls the development power supply 502 in response to instructions from the CPU circuit unit 150. The exposure control unit 204 controls the scanner unit 3 in response to instructions from the CPU circuit unit 150. The charging control unit 205 controls the charging power supply 501 in response to instructions from the CPU circuit unit 150. The pre-charging exposure control unit 206 controls the pre-charging exposure device 6 in response to instructions from the CPU circuit unit 150.

[0044] Moreover, the image forming apparatus 100 has a controller 200 as a conversion means (conversion unit). When the controller 200 receives print information (image information) and a print command (start instruction and various setting information) from the host computer 199, it outputs an image signal related to image formation in the image forming apparatus 100. Based on this image signal, the engine control unit 210 (CPU circuit unit 150) controls the above-mentioned respective control units (primary transfer control unit 201, secondary transfer control unit 202, development control unit 203, exposure control unit 204, charging control unit 205, pre-charging exposure control unit 206), etc., to execute a print job.

[0045] Here, a print job is a series of operations that is started by one start instruction and forms and outputs an image on one or more recording materials P. A print job generally includes an image forming process, a pre-rotation process, a paper-interval process in the case of forming an image on multiple recording materials P, and a post-rotation process. The image forming process is a period in which an electrostatic latent image of an image that is actually formed on a recording material P and output is formed, a toner image is formed, and the toner image is primarily transferred, secondary transferred, and fixed, and this period is referred to as the time of image formation (during image formation, during printing). More specifically, the timing of image formation differs depending on the position where each process of charging, exposure, development, primary transfer, secondary transfer, and fixing is performed. The pre-rotation process is a period in which a preparatory operation is performed before the image forming process from when a start instruction is input until the image actually starts to be formed. The paper-interval process is a period corresponding to the period between recording materials P when image formation is performed continuously on multiple recording materials P (during continuous printing). The post-rotation process is a period in which an arrangement operation (preparatory operation) is performed after the image forming process. Non-image formation refers to periods other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multiple-rotation process, which is a preparatory operation when the image forming apparatus 100 is turned on or when it returns from a sleep state.

[0046] The image forming apparatus 100 is also provided with an environment sensor 300 as an environment detection unit that detects the environment, which is at least one of the temperature and humidity inside or outside the image forming apparatus 100. In this embodiment, the environment sensor 300 is configured to have a temperature sensor 301 that detects the temperature inside the image forming apparatus 100 and a humidity sensor 302 that detects the humidity inside the image forming apparatus 100. The environment sensor 300 inputs information (signals) indicating the temperature detected by the temperature sensor 301 and information (signals) indicating the humidity detected by the humidity sensor 302 to the engine control unit 210. The engine control unit 210 (CPU circuit unit 150) acquires temperature and humidity information inside the image forming apparatus 100 based on the detection results of the temperature sensor 301 and the humidity sensor 302 of the environment sensor 300.

[0047] Furthermore, the image forming apparatus 100 is provided with a charging current detection circuit 511, as will be described later.

[0048] In this embodiment, the primary transfer power source 15 is shared by the four image forming units S. Although not shown in the figure, the charging power source 501, the developing power source 502, the contact / separation mechanism 80, and the charging current detection circuit 511 are provided independently for each image forming unit S. The drive unit 90 includes a drive motor as a drive source and a drive transmission member. The drive unit 90 includes a drive motor for driving the photosensitive drum 1, the developing roller 22 of the developing device 4, the intermediate transfer belt 10 (drive roller 11), the contact / separation mechanism 80, and the like. The drive motors for driving these members may be provided independently, or the drive motors for driving at least some of these members may be shared. The drive motors for driving the members for each color may be provided independently, or the drive motors for driving at least some of the members for each color may be shared. In this embodiment, the photosensitive drum 1 and the developing roller 22 of the developing device 4 in each image forming unit S can be rotated and stopped independently.

[0049] FIG. 4 is a block diagram for explaining the control of the scanner unit 3 in this embodiment. The control of weak exposure of the non-image portion (non-image portion exposure) in this embodiment will be explained with reference to FIG. 4. The image signal sent from the controller 200 to the exposure control unit 204 via the engine control unit 210 is a multi-value signal (0 to 255) having 8 bits=256 gradations in the depth direction. When this signal is 0, the laser light is off, when it is 255, it is completely on (maximum light amount), and when it is between 1 and 254, it has an intermediate value between off and completely on. Here, the level of weak exposure can be arbitrarily set according to the level of the multi-value signal. In the following explanation, it is assumed that weak exposure is performed using 32 as the level of this multi-value signal. The image signal "0" corresponding to the non-image portion sent from the controller 200 to the exposure control unit 204 via the engine control unit 210 is converted to an image signal "32" by the image signal conversion circuit 68 of the exposure control unit 204, and the image signals "1" to "255" are compressed and converted to image signals "33" to "255". Thereafter, the image signal converted by the image signal conversion circuit 68 is converted by the frequency modulation circuit 61 of the exposure control unit 204 into a serial signal in the time axis direction, and in this embodiment, is used for pulse width modulation of each dot pulse with a resolution of 600 dots / inch. This signal drives the laser driver 62 of the exposure control unit 204, causing the laser diode 63 of the scanner unit 3 to emit light and emit the laser light L. In the scanner unit 3, this laser light L passes through a correction optical system 67 including a polygon mirror 64, a lens 65, and a folding mirror 66, and is irradiated as a scanning light onto the surface of the photosensitive drum 1. The frequency modulation circuit 61 may be provided on the controller 200 side, away from the laser driver 62.

[0050] FIG. 5 is a schematic diagram of an electric circuit around the photosensitive drum 1. As shown in FIG. 3, the charging control unit 205 is connected to a charging power source 501 and a charging current detection circuit 511 as a charging current detection means (charging current detection unit). The charging current detection circuit 511 detects a charging current Ipri, which is a current flowing through the charging roller 2 (charging power source 501) when a voltage is applied to the charging roller 2 as a contact member that contacts the photosensitive drum 1. The charging current detection circuit 511 inputs information (signal) indicating the value of the detected charging current Ipri to the engine control unit 210. It can also be said that the charging current Ipri is a current flowing between the charging roller 2 and the photosensitive drum 1 when a voltage is applied to the charging roller 2. In this embodiment, as shown in FIG. 5(a), the charging current detection circuit 511 is connected between the charging power source 501 and the charging roller 2. 5(b), the charging current detection circuit 511 may be connected between the photosensitive drum 1 and the ground to detect the current flowing between the photosensitive drum 1 and the ground. Even in this configuration, the current flowing to the photosensitive drum 1 via the charging roller 2, which is one of the contact members that contact the photosensitive drum 1, is larger than the current flowing to the photosensitive drum 1 via the other contact members, so that the charging current can be detected sufficiently according to the desired accuracy.

[0051] <Detection of condensation state> Overview of the control in this embodiment One of the features of this embodiment is that the condensation state around the photosensitive drum 1, particularly on the surface of the photosensitive drum 1, is detected based on the change in the charging current Ipri during image formation (printing) in a print job, and the charging voltage Vpri is controlled based on the detection result. By detecting the condensation state on the surface of the photosensitive drum 1 during image formation and controlling the charging voltage Vpri that progresses during image formation, it is possible to suppress image defects caused by condensation. Here, the above control is described focusing on one image forming unit S, but in this embodiment, the above control is performed in each image forming unit S. Note that here, the control for executing a predetermined process such as control of the charging voltage Vpri by obtaining the detection result of the current or voltage (current in this embodiment) supplied to the contact member when a bias is applied to the contact member (the charging roller 2 in this embodiment) is also simply referred to as detecting (or determining) condensation (or condensation state). The same applies to detecting (or determining) condensation (or condensation state) in the modified examples and other embodiments described later.

[0052] FIG. 6 is a chart showing the transition of (a) charging current Ipri, (b) charging voltage Vpri, and (c) post-charging drum potential Vd during execution of a print job in the image forming apparatus 100 of this embodiment. FIG. 6 is a chart showing a case where normal operation is performed in a situation where moisture generated from paper during fixing does not flow into the machine. FIG. 7 is a chart similar to FIG. 6 in a case where normal operation is performed without the control of this embodiment in a situation where moisture generated from paper during fixing flows into the machine as a comparative example. FIG. 8 is a chart similar to FIG. 6 in a case where the control of this embodiment is performed in a situation where moisture generated from paper during fixing flows into the machine. In FIG. 6, FIG. 7, and FIG. 8, the horizontal axis indicates time (hours), and the vertical axis indicates the direction in which the absolute values ​​of current and voltage (potential) become larger. In the explanation of FIG. 6, FIG. 7, and FIG. 8, the unit of time (hours) is arbitrary, and may be, for example, "msec" or "sec", but description thereof is omitted here. As mentioned above, for convenience, terms such as "increase" and "large" refer to absolute values ​​of current and voltage (potential). In addition, the development voltage Vdc and the post-exposure drum potential VL are also shown in Figs. 7 and 8.

[0053] First, the control of normal operation in a situation where moisture generated from paper during fixing does not flow into the machine will be described with reference to Fig. 6. Next, the problem that arises when normal operation is performed without the control of this embodiment in a situation where moisture generated from paper during fixing flows into the machine will be described with reference to Fig. 7. After that, the control of this embodiment will be described with reference to Fig. 8.

[0054] The operations according to the charts of FIGS. 6, 7 and 8 are controlled by the engine control section 210.

[0055] When there is no water inflow First, with reference to FIG. 6, the control of normal operation in a situation where moisture generated from paper during fixing does not flow into the machine will be described.

[0056] When the controller 200 receives print information and a print command from the host computer 199 at time 0, a print job is started. When the print job is started, the photosensitive drum 1 (and the intermediate transfer belt 10) starts to rotate at time t1. At substantially the same timing (time t1) as the photosensitive drum 1 starts to rotate, the charging power source 501 starts to apply a charging voltage Vpri=-1300V to the charging roller 2 (FIG. 6(b)). With the start of application of the charging voltage Vpri to the charging roller 2 at time t1, a charging current Ipri starts to flow (FIG. 6(a)).

[0057] The numbers or symbols (n, n+1, etc.) attached to the time t indicate the number of sheets of image formation (printing) that have started since the start of the print job. Time t1 is the first sheet, time t n represents the start time of the nth image formation. Here, the start time of the nth image formation (printing) is the timing when the area on the photosensitive drum 1 corresponding to the nth recording material S on which the toner image is formed in the print job reaches the charging nip CN. In addition, in FIG. 6, the time when n=1 is represented as t1, and the time when n≧2 is represented as t2. n , t n+1 In this embodiment, the area on the photosensitive drum 1 corresponding to each recording material P on which a toner image is formed in a print job is assumed to match (have the same size as) the image forming area on the photosensitive drum 1 capable of forming a toner image of the image to be formed on each recording material P.

[0058] The charging current Ipri flowing between the charging roller 2 and the photosensitive drum 1 when the charging voltage Vpri is applied to the charging roller 2 is determined by material parameters (such as the film thickness and relative dielectric constant of the photosensitive drum 1), the nip width (rotation direction of the photosensitive drum 1), length (rotation axis direction of the photosensitive drum 1) of the charging nip portion CN, process speed (circumferential speed of the photosensitive drum 1), surface potential of the photosensitive drum 1 before charging process (and after pre-charging exposure) (hereinafter also referred to as "pre-charging drum potential"), and the applied voltage Vpri to the charging roller 2. Here, parameters other than the pre-charging drum potential and the applied voltage Vpri to the charging roller 2 are inherent to the image forming apparatus 100 (apparatus main body 110 and process cartridge 7), and the above-mentioned parameters are used in this embodiment. In addition, since pre-charging exposure is performed by the pre-charging exposure device 6 in this embodiment, the pre-charging drum potential is always approximately constant (approximately 0 V). Therefore, if the charging voltage Vpri applied to the charging roller 2 during image formation is approximately constant, the charging current Ipri flowing during image formation is also approximately constant. In the configuration of this embodiment, a charging current Ipri of about 9.0 μA always flows during image formation, and the post-charging drum potential Vd during image formation is approximately constant at −700 V (FIG. 6C).

[0059] In this way, if there is no inflow of moisture generated from the paper during fixing into the machine, and if the charging voltage Vpri is approximately constant and the pre-charging drum potential is approximately constant, then the charging current Ipri and the post-charging drum potential Vd will be approximately constant during image formation.

[0060] Issues when moisture ingress occurs Next, referring to FIG. 7, a problem that occurs when moisture generated from the paper during fixing flows into the machine will be described.

[0061] As a specific situation, consider a situation in which a ceiling fan is attached to the ceiling of a room in which image forming apparatus 100 is installed, and wind generated by the ceiling fan constantly blows toward image forming apparatus 100 from above image forming apparatus 100, as shown in Fig. 9. Image forming apparatus 100 shown in Fig. 9 has the same configuration as image forming apparatus 100 of this embodiment shown in Fig. 1. Note that the period from time 0 to time t1 in Fig. 7 is the same as that in Fig. 6, so the description will be omitted, and the period from time t1 onwards will be described.

[0062] Paper, which is mainly used as the recording material P, contains moisture. Therefore, after time t1, when the paper passes through the fixing nip of the fixing device 30 heated to a high temperature, the moisture contained in the paper evaporates. In a situation where no wind is blowing toward the image forming device 100, the evaporated moisture is dispersed around the image forming device 100 and does not affect the inside (inside the machine) of the housing 120 of the device body 110 of the image forming device 100. However, as shown in FIG. 9, when the wind generated by the ceiling fan is blown near the paper discharge port 130 of the image forming device 100, the evaporated moisture flows from the paper discharge port 130 into the machine. As a result, the absolute moisture amount inside the machine becomes higher than the absolute moisture amount outside (outside the machine) of the housing 120 of the device body 110 of the image forming device 100. As the absolute moisture content in the machine increases, the absolute moisture content around the photosensitive drum 1 also increases, and moisture begins to adhere (condense) to the periphery of the photosensitive drum 1, particularly to the surface of the photosensitive drum 1, causing the electrical resistance of the surface of the photosensitive drum 1 to decrease. As a result, as shown in Figs. 7(a) and 7(b), even if the charging voltage Vpri is approximately constant, the charging current Ipri gradually increases with the passage of paper (the recording material P on which an image is formed passes through the conveying path inside the image forming apparatus 100). Then, as shown in Fig. 7(c), with the increase in the charging current Ipri, the post-charging drum potential Vd also increases. Usually, the surface charging of the photosensitive drum 1 by the charging roller 2 is formed by a discharge current from the charging roller 2 to the photosensitive drum 1. Such a discharge occurs in at least one of the minute gaps formed between the charging roller 2 and the photosensitive drum 1 on the upstream side and downstream side of the contact portion between the charging roller 2 and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. However, when condensation occurs on the surface of the photosensitive drum 1, in addition to the discharge current, a current flows directly from the charging roller 2 to the photosensitive drum 1 at the contact portion between the charging roller 2 and the photosensitive drum 1. When the post-charging drum potential Vd rises in this way, the post-exposure drum potential VL also rises, making it impossible to form a desired electrostatic latent image (the potential of the electrostatic latent image deviates from the appropriate value), and narrowing the development contrast Vcont.When the development contrast Vcont, which is the potential difference between the development voltage Vdc and the post-exposure drum potential VL, becomes narrow, the electrical force that moves the toner on the development roller 22 onto the photosensitive drum 1 weakens, which can result in image defects such as low density or missing characters or images.

[0063] FIG. 10 is a graph for explaining the change in the relationship of various potentials (voltages) on the photosensitive drum 1 depending on the presence or absence of condensation. FIG. 10(a) shows the above relationship in a normal state before condensation occurs, and FIG. 10(b) shows the above relationship when condensation occurs. Before the moisture-containing wind flows into the machine, an appropriate development contrast Vcont is ensured (FIG. 10(a)). In contrast, when condensation occurs on the surface of the photosensitive drum 1 due to the moisture-containing wind flowing into the machine, the post-charging drum potential Vd rises, and the post-weak exposure drum potential Vdl and the post-exposure drum potential VL also rise with the rise in the post-charging drum potential Vd (FIG. 10(b)). As a result, the development contrast Vcont becomes narrow, and the electrical force that moves the toner on the development roller 22 onto the photosensitive drum 1 becomes weak, which may cause image defects such as low density or missing characters or images.

[0064] Control of this embodiment Next, the control of this embodiment will be described with reference to FIG.

[0065] One of the features of this embodiment is that change control is executed to change the charging voltage Vpri based on the change in the charging current Ipri during image formation (printing) in a print job. That is, in this embodiment, the image forming apparatus 100 detects the condensation state on the surface of the photosensitive drum 1 by monitoring the change in the charging current Ipri during image formation (printing) in a print job. Then, based on the detection result, change control is executed to change the charging voltage Vpri. Note that the period from time 0 to time t1 in FIG. 8 is the same as in FIG. 6 and FIG. 7, so the description will be omitted, and only the period after time t1 will be described.

[0066] As shown in FIG. 8B, the application of the charging voltage Vpri of −1300 V to the charging roller 2 starts at time t1, and the formation of the first image starts. As described above, if moisture generated from the paper during fixing flows into the machine, the moisture will be absorbed from the paper during the period from time t1 to time t n Even if the charging voltage Vpri is substantially constant, as in the period from 100 to 1500 s, moisture begins to adhere to the surface of the photosensitive drum 1, so the charging current Ipri gradually increases. Accordingly, as shown in FIG. 8(c), the post-charging drum potential Vd increases.

[0067] Since the pre-charge drum potential is determined by the pre-charge exposure performed by the pre-charge exposure device 6, the change in the post-charge drum potential Vd correlates with the change in the charging current Ipri. Therefore, the change in the post-charge drum potential Vd, which is directly linked to the change in the image, i.e., the dew state on the surface of the photosensitive drum 1, can be determined from the change in the charging current Ipri. In addition, image defects such as low density and image defects caused by the change in the post-charge drum potential Vd do not become apparent due to a slight potential fluctuation, but occur when the post-charge drum potential Vd changes significantly to a certain extent or more. Therefore, there is a range in which the image defects are not noticeable with respect to the change in the post-charge drum potential Vd (≒change in the charging current Ipri). In this embodiment, since the image forming apparatus 100 is provided with the pre-charge exposure device 6, the pre-charge drum potential is always approximately constant. Therefore, in this embodiment, the change in the charging current Ipri at any timing can be monitored.

[0068] Therefore, in this embodiment, a threshold value (upper threshold value) Ith is set for the charging current Ipri, the change of which is being monitored, and when the charging current Ipri increases during image formation and exceeds the threshold value Ith, it is determined that condensation has begun to form on the surface of the photosensitive drum 1. The threshold value Ith can be appropriately set as a value before image defects such as low density and image defects as described above exceed the allowable range. Then, in this embodiment, the charging voltage Vpri is changed so as to cancel the change in the charging current Ipri, i.e., the change in the post-charging drum potential Vd. Here, at time t n ~time t n+1(that is, during image formation of the nth sheet from the start of the print job) will be described as an example.

[0069] As shown in FIG. 8(a), at time t n ~time t n+1 If the charging current Ipri exceeds the threshold Ith during this period, it is determined that condensation has begun to form on the surface of the photosensitive drum 1. Then, as shown in FIG. 8B, at the start time t n+1 8(b) and (c), the charging voltage Vpri is controlled to be lowered from the initial value Vd of the charged drum potential Vd (−700V in this embodiment) at the initial time (time t1). In this way, the dew condensation state on the surface of the photosensitive drum 1 is determined from the change in the charging current Ipri, and the charging voltage Vpri is controlled to be lowered so as to cancel the increase in the charged drum potential Vd due to the dew condensation. As a result, the exposed drum potential VL falls within the “image OK range” as shown in FIG. 8(c), and the development contrast Vcont can be appropriately secured.

[0070] In this embodiment, by controlling the charging voltage Vpri in this manner, it is possible to suppress image defects (low density, missing characters, etc.) caused by fluctuations in the surface potential of the photosensitive drum 1 due to condensation.

[0071] In addition, if the surface potential of the photosensitive drum 1 is changed suddenly, there is a possibility that a change in color may occur in the image formation before and after the change. Therefore, when changing the charging voltage Vpri between the previous image formation and the next image formation, it is not preferable to change the charging voltage Vpri more than a certain degree between the previous and next image formations. Therefore, as shown in FIG. 8(b), n+1 ~time t n+2 ), n+2th image (time t n+2 ~time t n+3 ), n+3th photo (time t n+3 ~time t n+4), the charging voltage Vpri is gradually lowered. This makes it possible to reduce color changes between previous and next image formations. The change in charging voltage Vpri between previous and next image formations is preferably less than 100V, and more preferably less than 50V. However, in order to prevent the period until the drum potential Vd is returned to the initial state (state at time t1) after charging from becoming too long, the change in charging voltage Vpri between previous and next image formations is typically set to 10V or more. In this embodiment, n+1 After that, the charging voltage Vpri is lowered in increments of 25 V for each image formed. However, depending on the setting of the threshold Ith, for example, the charging voltage Vpri may be changed all at once to cancel the change in the post-charging drum potential Vd.

[0072] Also, it is not preferable to reduce the charging voltage Vpri indefinitely. Even if the charging voltage Vpri is reduced to set the post-charging drum potential Vd within an appropriate range and suppress image defects such as low density, moisture continues to adhere to the surface of the photosensitive drum 1. Therefore, due to the moisture adhered to the surface of the photosensitive drum 1, there is a possibility that fogging will occur, which is a phenomenon in which toner physically adheres to the surface of the photosensitive drum 1 from the developing roller 22 in contact with the photosensitive drum 1. Furthermore, since the adhesion of the toner to the photosensitive drum 1 also increases due to the influence of moisture, the transfer efficiency may decrease and transfer defects may occur even if the toner is of a normally developed toner image.

[0073] Therefore, in this embodiment, as shown in FIG. 8B, a threshold value (lower limit threshold value) Vth is also set for the charging voltage Vpri, and when the charging voltage Vpri falls below this value, control is performed so that image formation in the print job is temporarily suspended. The threshold value Vth can be appropriately set as a value before the above-mentioned fog and decrease in transfer efficiency exceed the allowable range. In FIG. 8B, as an example, a case where the charging voltage Vpri falls below the threshold value Vth on the n+6th sheet (i.e., when the charging voltage Vpri is lowered after the n+5th sheet image formation, the charging voltage Vpri falls below the threshold value Vth on the n+6th sheet image formation) is considered. In this case, instead of continuing the n+6th sheet image formation, the image formation in the print job is temporarily suspended, and the rotation operation of the photosensitive drum 1 (hereinafter also referred to as "idle rotation") is performed as a condensation removal process (moisture removal operation). The minute moisture attached to the surface of the photosensitive drum 1 is gradually transferred to the intermediate transfer belt 10 by the idling rotation, and the moisture on the surface of the photosensitive drum 1 is removed, and the original state (state at time t1) is restored.

[0074] Figure 11 shows the time t n+6 FIG. 1 is a chart showing the transition of the charging current Ipri, the charging voltage Vpri, and the post-charging drum potential Vd during the process of performing idling and returning to the original state (state at time t1) thereafter. As described above, if the charging voltage Vpri is lowered and image formation for the (n+6)th sheet is continued, the charging voltage Vpri falls below the threshold Vth. n+6From then on, the n+6th image formation is not performed and the idle rotation is performed. At this time, in this embodiment, the charging voltage Vpri is set to the initial value (value at time t1) of -1300V and the idle rotation is performed. In order to return the charging voltage Vpri to the initial value, the charging current Ipri becomes large due to moisture on the surface of the photosensitive drum 1 at the beginning of the idle rotation. However, the charging current Ipri decreases as moisture is gradually removed from the photosensitive drum 1. When the charging current Ipri decreases to approximately 9.0 μA, which is the initial value (value at time t1), and the current value does not change (or becomes sufficiently small), it can be determined that moisture has been removed, so the idle rotation is terminated. Then, if there is a subsequent image formation, the charging voltage Vpri is maintained at the initial value and the image formation is resumed. If there is no subsequent image formation, a predetermined post-rotation operation is performed as described below to end the print job. Here, the charging voltage Vpri during the idle rotation can be accurately detected by returning it to a high value such as the initial value as described above, but it is not limited to this and may be lower than the initial value.

[0075] In the above, the execution time of the idling is the time until the charging current Ipri drops to the initial value and stops changing, but it is not limited to this. For example, the time required for the idling may be predicted, and the idling may be executed for the execution time of that time. For example, the idling time required to sufficiently remove condensation when condensation occurs on the surface of the photosensitive drum 1 to the extent that the charging current Ipri exceeds the threshold value Ith is obtained in advance by an experiment or the like. Then, the idling is executed for the execution time of that time. In this case, the charging voltage does not need to be applied to the charging roller 2 during the idling. The execution time of the idling is not limited to this, but can be, for example, about 5 seconds to 1 minute, typically about 10 seconds to 30 seconds. In other words, the execution time of the idling operation is not limited to this, but can be, for example, the time for the intermediate transfer belt 10 to make about 1 to 20 revolutions, typically about 3 to 10 revolutions.

[0076] Furthermore, when performing the idling rotation, it is preferable to separate the developing roller 22 from the photosensitive drum 1 in order to prevent moisture adhering to the surface of the photosensitive drum 1 from transferring to the developing roller 22 and the developing device 4. When the developing roller 22 is separated from the photosensitive drum 1 during the idling rotation, it is not necessary to rotate the developing roller 22 and apply a developing voltage to the developing roller 22 during the idling rotation. However, for example, when the image forming apparatus 100 is not provided with a mechanism for separating the developing roller 22 from the photosensitive drum 1, the developing roller 22 may be configured to rotate in contact with the photosensitive drum 1 during the idling rotation.

[0077] During idling, it is preferable that the intermediate transfer belt 10 rotates in contact with the photosensitive drum 1 in order to efficiently remove moisture from the surface of the photosensitive drum 1. Moisture transferred to the intermediate transfer belt 10 by idling is finally removed (cleaned) by the intermediate transfer belt cleaning device 16. It is also considered that a part of the moisture is removed by evaporating from the photosensitive drum 1 or the intermediate transfer belt 10 or by adhering to the secondary transfer roller 20. Note that it is not necessary to apply the primary transfer voltage or the secondary transfer voltage during idling, but at least one of the primary transfer voltage and the secondary transfer voltage may be applied.

[0078] Here, when it is determined that idling is necessary in some of the multiple image forming stations S, idling can be performed synchronously in all of the image forming stations S. Also, for example, when the image forming apparatus 100 is provided with a belt contact / separation mechanism capable of separating the intermediate transfer belt 10 from at least one photosensitive drum 1, the following may be performed. That is, for example, in the image forming station S where it is determined that idling is necessary, the intermediate transfer belt 10 is brought into contact with the photosensitive drum 1 to rotate the photosensitive drum 1 and the intermediate transfer belt 10. Meanwhile, in the other image forming stations S, the intermediate transfer belt 10 is separated from the photosensitive drum 1 to stop the photosensitive drum 1.

[0079] By executing the idling rotation in this manner, image defects such as the above-mentioned fogging and transfer defects caused by moisture adhering to the surface of the photosensitive drum 1 can be suppressed.

[0080] FIG. 12 is a flowchart showing an outline of a series of operations of a print job in this embodiment. The operations according to the flowchart shown in FIG. 12 are controlled by the engine control unit 210.

[0081] When the engine control unit 210 starts a print job at time 0 (S11), it starts forming (printing) the n(=1)th image at time t1 (S12). Subsequently, the engine control unit 210 determines whether or not the charging current Ipri flowing during the formation (printing) of the nth image is greater than or equal to a threshold value (upper limit threshold value) Ith (Ipri≧Ith) (S13).

[0082] If the engine control unit 210 determines in S13 that Ipri≧Ith is not satisfied (i.e., Ipri<Ith), it determines whether there is a subsequent image formation (S21). If the engine control unit 210 determines in S21 that there is a subsequent image formation, it sets n=n+1, returns to S12, and executes the formation of the nth image. Also, if the engine control unit 210 determines in S21 that there is no subsequent image formation, it determines whether or not the charging current Ipri has increased during the formation of the images from the 1st to the nth (S22). Then, if the engine control unit 210 determines in S22 that there is no increase in the charging current Ipri, it assumes that there is no dew condensation on the surface of the photosensitive drum 1, executes normal post-rotation (S20), and ends the print job. Also, if the engine control unit 210 determines in S22 that there is an increase in the charging current Ipri, since there is a possibility of dew condensation to some extent, it executes idling rotation (S23) to remove the moisture on the surface of the photosensitive drum 1, and then executes normal post-rotation (S20) to end the print job. Note that the post-rotation in S23 may be executed until the charging current Ipri reaches a predetermined value as described above, or may be executed for a preset predetermined time.

[0083] When the engine control unit 210 determines in S13 that Ipri ≧ Ith, it determines whether there is subsequent image formation (S14). When the engine control unit 210 determines in S14 that there is no subsequent image formation, it performs idling rotation (S23) to remove the moisture on the surface of the photosensitive drum 1, and then performs normal post-rotation (S20) to end the print job. Also, when the engine control unit 210 determines in S14 that there is subsequent image formation, it decreases the set value of the charging voltage Vpri (S15). Subsequently, the engine control unit 210 determines whether the decreased set value of the charging voltage Vpri is lower than the threshold value (lower limit threshold value) Vth (Vpri < Vth) (S16). When the engine control unit 210 determines in S16 that it does not fall below (i.e., Vpri ≧ Vth), it sets n = n + 1 and performs the n-th image formation (S19), and returns to S14. Also, when the engine control unit 210 determines in S16 that it falls below (Vpri < Vth), it performs idling rotation (S17) to remove the moisture on the surface of the photosensitive drum 1. Note that the idling rotation in S17 may be performed until the charging current Ipri reaches a predetermined value as described above, or may be performed for a preset predetermined time. The engine control unit 210 determines whether there is subsequent image formation after the idling rotation in S17 (S18). When there is no subsequent image formation, it performs normal post-rotation (S20) to end the print job. Also, when there is subsequent image formation even after the execution of the idling rotation in S17, since the moisture on the surface of the photosensitive drum 1 has been removed by the idling rotation in S17, it sets n = n + 1 and returns to the n-th image formation in S12 (the charging voltage Vpri is the initial value) to continue the print job.

[0084] As described above, in this embodiment, by monitoring the change in the charging current Ipri during image formation (printing) in the print job, the dew condensation state inside the machine, particularly on the surface of the photosensitive drum 1, is detected, and the detection result is reflected in the control of the charging voltage Vpri. Thereby, image defects caused by dew condensation can be suppressed.

[0085] <Modification Example> Next, a description will be given of a modified example of the embodiment 1. In the image forming apparatus of the modified example, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the embodiment 1 will be given the same reference numerals as those of the embodiment 1 and detailed description will be omitted.

[0086] Variation 1 A first modified example (Modified Example 1) of the first embodiment will be described. In Modified Example 1, the image forming apparatus 100 detects the condensation state on the surface of the photosensitive drum 1 by monitoring a change in the primary transfer current during image formation (printing) in a print job.

[0087] FIG. 13 is a block diagram for explaining the control configuration of the image forming apparatus 100 in the first modification. The control configuration in the first modification shown in FIG. 13 is roughly the same as the control configuration in the first embodiment shown in FIG. 3, but in the first modification, a primary transfer current detection circuit 512 is provided instead of the charging current detection circuit 511. In the first modification, the primary transfer control unit 201 is connected to the primary transfer power source 15 and the primary transfer current detection circuit 512 as a primary transfer current detection means (primary transfer current detection unit). The primary transfer current detection circuit 512 detects the primary transfer current Itr, which is a current flowing through the primary transfer roller 14 (primary transfer power source 15) when a voltage is applied to the primary transfer roller 14 as a contact member that contacts the photosensitive drum 1. The primary transfer current detection circuit 512 inputs information (signal) indicating the value of the detected primary transfer current Itr to the engine control unit 210. The primary transfer current Itr can also be said to be a current that flows from the primary transfer roller 14 through the intermediate transfer belt 10 to the photosensitive drum 1 when a voltage is applied to the primary transfer roller 14.

[0088] Fig. 14 is a chart showing the transitions of (a) primary transfer current Itr, (b) charging voltage Vpri, and (c) post-charging drum potential Vd during execution of a print job when control of modified example 1 is performed in a situation where moisture generated from paper during fixing flows into the machine. In Fig. 14, the horizontal axis indicates time (hours), and the vertical axis indicates the direction in which the absolute values ​​of current and voltage (potential) increase upward. Unless otherwise specified, the same symbols as in Fig. 8 have the same meanings as in Fig. 8. The operation according to the chart in Fig. 14 is controlled by engine control unit 210.

[0089] The primary transfer current Itr flowing from the primary transfer roller 14 to the photosensitive drum 1 through the intermediate transfer belt 10 when the primary transfer voltage Vtr is applied to the primary transfer roller 14 is determined by the difference ΔVtr between the primary transfer voltage (Vtr=+100V in this embodiment) and the surface potential of the photosensitive drum 1 before the primary transfer process (and after weak exposure) (hereinafter also referred to as the "pre-transfer drum potential"), except for parameters specific to the image forming apparatus 100 (apparatus main body 110 and process cartridge 7), such as material parameters (film thickness and relative dielectric constant of the photosensitive drum 1, etc.), the width (rotation direction of the photosensitive drum 1), length (rotation axis direction of the photosensitive drum 1), and process speed (circumferential speed of the photosensitive drum 1). Note that the pre-transfer drum potential is the same as the post-weak exposure drum potential Vdl if it is a non-image portion. Therefore, the following description will be given for the non-image portion.

[0090] As described above, when moisture flows into the machine due to the influence of an external device such as a ceiling fan and the charged drum potential Vd fluctuates, the weak-exposure drum potential Vdl also fluctuates accordingly. As shown in FIG. 14(c), the primary transfer voltage Vtr during image formation is substantially constant, so when moisture flows into the machine due to the influence of an external device such as a ceiling fan and the charged drum potential Vd fluctuates, the difference ΔVtr between the weak-exposure drum potential Vdl and the primary transfer voltage Vtr fluctuates (increases) accordingly. Accordingly, as shown in FIG. 14(a), the primary transfer current Itr also fluctuates. Therefore, the change in the primary transfer current Itr correlates with the change in the weak-exposure drum potential Vdl, and the change in the weak-exposure drum potential Vdl correlates with the change in the charged drum potential Vd.

[0091] Therefore, by monitoring the change in the primary transfer current Itr during image formation (printing) in a print job, it is possible to detect the fluctuation in the post-charging drum potential Vd that leads to image defects, that is, the condensation state on the surface of the photosensitive drum 1. When the condensation state on the surface of the photosensitive drum 1 is detected, various image defects caused by condensation can be suppressed by controlling the charging voltage Vpri so as to cancel the fluctuation in the post-charging drum potential Vd, as shown in FIG. 14(b), in the same manner as in the control of the first embodiment. Also, when monitoring the change in the primary transfer current Itr, it is possible to control to perform idling, as in the present embodiment.

[0092] In addition, when the image forming apparatus 100 is provided with a pre-transfer exposure device as a discharging means for discharging (removing at least a part of the charge) the surface of the photosensitive drum 1 before the primary transfer process (and after weak exposure), the pre-transfer drum potential and the pre-charging drum potential are always substantially constant. Therefore, in this case, it is possible to monitor the change in the primary transfer current at any timing. The detection timing will be described in further detail later.

[0093] In addition, although the configuration for monitoring the change in the primary transfer current has been given as an example of the first modified example, other output values ​​may be used as long as they are correlated with the post-charging drum potential Vd. For example, if the post-charging drum potential Vd can be measured directly, the change in the post-charging drum potential Vd may be used to detect condensation. In this case, for example, as shown in FIG. 20, the image forming apparatus 100 may be provided with a potential sensor 514 as a surface potential detection means (surface potential detection unit) for detecting the surface potential of the photosensitive drum 1 after the charging process by the charging roller 2 (and before exposure and weak exposure). Also, the current flowing between the developing roller 22 and the photosensitive drum 1 may be used to detect condensation. FIG. 15 is a block diagram for explaining the control configuration of the image forming apparatus 100 in this case. The control configuration shown in FIG. 15 is roughly the same as the control configuration in the first embodiment shown in FIG. 3, but in the control configuration shown in FIG. 15, a development current detection circuit 513 is provided instead of the charging current detection circuit 511. In this case, the development control unit 203 is connected to a development power source 502 and a development current detection circuit 513 as a development current detection means (development current detection unit). The development current detection circuit 513 detects a development current that is a current that flows through the development roller 22 (development power source 502) when a voltage is applied to the development roller 22 as a contact member that contacts the photosensitive drum 1. The development current detection circuit 513 inputs information (signal) indicating the value of the detected development current to the engine control unit 210. It can also be said that the development current is a current that flows between the development roller 22 and the photosensitive drum 1 when a voltage is applied to the development roller 22. In addition, a contact member for detecting condensation may be provided other than the charging member, the developing member, and the transfer member, and the current that flows between the contact member and the photosensitive drum 1 may be used to detect condensation. In these cases, other points may be controlled in accordance with the above-mentioned embodiment.

[0094] Here, it is more preferable to use the charging roller 2 as in this embodiment as the contact member for detecting condensation on the surface of the photosensitive drum 1. This is for the following reasons, for example. That is, since the charging bias applied to the charging roller 2 is relatively large, it is easier to detect the change in current (change in drum potential after charging) with higher accuracy. In addition, by disposing the pre-charge exposure device 6, it is easy to make the drum potential before charging approximately constant, and to configure the current detection at any timing. In addition, as shown in FIG. 1, when the charging roller 2 is located closer to the paper discharge port 130 (at a higher position) than other contact members, the start of condensation on the surface of the photosensitive drum 1 can be detected with higher accuracy.

[0095] Variation 2 A second modified example (modified example 2) of the first embodiment will be described. In modified example 2, the image forming apparatus 100 predicts the change in the charging current Ipri during image formation (printing) in a print job, and controls the charging voltage Vpri based on the prediction result. As a result, from time t1 to time t n+1 This cancels any change in the post-charging drum potential Vd from the first image formation to the last image formation in a print job.

[0096] Fig. 16 is a chart showing the transitions of (a) charging current Ipri, (b) charging voltage Vpri, and (c) post-charging drum potential Vd during execution of a print job when control of modified example 2 is performed in a situation where moisture generated from paper during fixing flows into the machine. In Fig. 16, the horizontal axis indicates time (hours), and the vertical axis indicates the direction in which the absolute values ​​of current and voltage (potential) increase upward. Unless otherwise specified, the same symbols as in Fig. 8 have the same meanings as in Fig. 8. The operation according to the chart in Fig. 16 is controlled by engine control unit 210.

[0097] In the second modification, the change in the charging current Ipri is predicted and the charging voltage Vpri is controlled so that the change in the post-charging drum potential Vd is reduced.

[0098] As described above, when moisture has entered the machine, the charging current Ipri increases according to the number of sheets passed when the charging voltage Vpri is substantially constant. Therefore, the charging current Ipri also increases during the period from time t1 to time t2 for the first sheet.

[0099] Therefore, in the second modification, based on the change in the charging current Ipri from time t1 to time t2, it is predicted that the second sheet will also change in the same manner as the predicted transition (dashed line) in Figures 16(a) and (c), and the charging voltage Vpri is changed.

[0100] For example, a correspondence table of the change amount ΔIpri of the charging current Ipri, the change amount ΔVd of the post-charging drum potential Vd, and the offset amount (change amount) ΔVpri of the charging voltage Vpri as shown in Table 1 is obtained in advance and stored in the ROM 151. Then, based on the change amount ΔIpri of the charging current Ipri during the image formation of the first sheet (the period from time t1 to time t2), the offset amount ΔVpri of the charging voltage Vpri during the image formation of the second sheet (the period from time t2 to time t3) is changed. Similarly, for the third sheet and thereafter (after time t3), the offset amount ΔVpri of the charging voltage Vpri is changed by predicting the change amount ΔIpri of the charging current Ipri during the image formation of the image from the change amount ΔIpri of the charging current Ipri during the image formation of the sheet immediately before the image formation.

[0101] [Table 1]

[0102] By carrying out such control, the color tone can be kept substantially constant from the first image formation to the last image formation in a print job.

[0103] Although the control based on the change amount ΔIpri of the charging current Ipri during the image formation of the previous sheet has been described here, the present invention is not limited to this. For example, the offset amount ΔVpri of the charging voltage Vpri can be determined based on the change amount ΔIpri of the charging current Ipri during any period of time prior to the image formation of the previous sheet, and fed back to the control of the charging voltage Vpri.

[0104] Moreover, as can be seen from the explanation of the first modification, in the second modification as well, instead of predicting the change in the charging current Ipri, the change in the transfer current Itr (or the change in the developing current) may be predicted.

[0105] <Other configurations> In this embodiment, the image forming apparatus 100 is provided with the pre-charge exposure device 6, and the pre-charge drum potential is made substantially constant, but this is not limited thereto. If the state of the pre-charge drum potential is known in advance, the pre-charge exposure device 6 may not be provided in the image forming apparatus 100. For example, based on print information from the host computer 199, the controller 200 judges whether there is a blank area (all-white area) without an image area in the entire image forming area in the width direction substantially perpendicular to the moving direction of the surface of the photosensitive drum 1. Then, the controller 200 is configured to send the information to the engine control unit 210. In this case, the change in the charging current Ipri when a portion corresponding to the all-white area (i.e., a portion where the all-white area has undergone the primary transfer process and the pre-charge drum potential has been formed) passes through the charging nip portion CN is monitored, and the condensation state on the surface of the photosensitive drum 1 can be detected. Alternatively, other than the image information, the change in the charging current Ipri of an area on the photosensitive drum 1 corresponding to at least one of the margins at the leading end or trailing end in the conveying direction of the recording material P may be monitored. In this way, the control of this embodiment can be executed even in an image forming apparatus 100 that is not provided with a pre-charge exposure device 6. When the image forming apparatus 100 is not provided with a pre-charge exposure device 6, it is possible to reduce the cost and size of the apparatus. This also applies to the case where the image forming apparatus 100 is not provided with a pre-transfer exposure device in the configuration for detecting the primary transfer current of Modification Example 1. The same also applies to the configuration for detecting the development current.

[0106] In the present embodiment and the modified example, the charging voltage Vpri is controlled when the dew condensation state is detected, but the present invention is not limited to this. When the dew condensation state is detected, the exposure amount (laser light amount: μJ / cm2 ) or the developing voltage Vdc may be controlled. A combination of these may be controlled. Even if the charged drum potential Vd rises due to condensation on the surface of the photosensitive drum 1, if the developing contrast Vcont is appropriately within a predetermined range, it is possible to suppress low-density image defects. FIG. 17 is a graph showing the relationship between various potentials (voltages) related to the photosensitive drum 1 to explain a method for appropriately securing the developing contrast Vcont in a situation where condensation occurs on the surface of the photosensitive drum 1. For example, the exposure amount of the scanner unit 3 can be increased in accordance with the rise in the charged drum potential Vd caused by condensation as shown in FIG. 17(b) compared to the relationship of the potentials (voltages) in normal conditions before the condensation occurs in FIG. 17(a). This makes it possible to control so that the developing contrast Vcont does not change (i.e., so that the exposed drum potential VL does not change). Also, as shown in FIG. 17(c), the developing voltage Vdc can be increased in accordance with the rise in the charged drum potential Vd. This also makes it possible to control so that the developing contrast Vcont does not change. In this way, the low density can be suppressed by controlling the exposure amount of the scanner unit 3 and the development voltage Vdc so that the development contrast Vcont is appropriately within a predetermined range. Note that when the back contrast Vback changes, the micro image quality such as the width of a fine line and the size of a dot changes slightly. Therefore, when a condensation state is detected, it is preferable to control the development voltage Vdc and the exposure amount of the scanner unit 3 as described above, and to control the back contrast Vback to be appropriately within a predetermined range by weak exposure (FIGS. 17(b) and 17(c)). This can also suppress changes in micro image quality. In addition, when controlling to prevent low density by any of the above methods, moisture continues to adhere to the surface of the photosensitive drum 1, so it is preferable to remove moisture from the surface of the photosensitive drum 1 by performing idling before fog or transfer failure occurs. Note that the appropriate ranges of the development contrast Vcont and the back contrast Vback can be appropriately set depending on the configuration of the image forming apparatus 100, and can be, for example, within about ±50V of the target value when there is no condensation.

[0107] In this embodiment, the idling is performed when the charging voltage Vpri falls below the threshold Vth due to the progress of condensation on the surface of the photosensitive drum 1, but the timing of performing the idling is not limited to this. For example, when the charging current Ipri exceeds the threshold Ith, the idling may be performed immediately to remove moisture. That is, in this case, the idling is performed without changing the charging voltage Vpri (or the developing voltage Vdc or the exposure amount of the scanner unit 3). Also, the idling can be performed at any timing to remove moisture as long as the range of the post-charging drum potential Vd where no image defects occur (OK range in FIG. 8(c)) is within the range. Even if the image formation in the print job ends before the charging current Ipri exceeds the threshold Ith in FIG. 8(a), the photosensitive drum 1 may be rotated more than usual (i.e., idling) to remove moisture on the photosensitive drum 1 at the timing of the post-image formation after the image formation in the print job, between sheets, or the pre-image formation before the next print job (see FIG. 12). By performing the idling at an appropriate timing to remove moisture in this way, it is possible to prevent the occurrence of image defects such as fogging and transfer defects. The same applies to the modified examples.

[0108] In addition, in this embodiment and the modified example, the continuous change in the output value of the charging current Ipri and the transfer current Itr is monitored, but the present invention is not limited to this. For example, the average value of the output value during the formation of one image as a predetermined period that can be set arbitrarily may be calculated, and the change in the average value may be monitored. For example, when this average value exceeds a predetermined threshold value, the charging voltage Vpri may be controlled. By calculating the average value in this manner, the capacity of the RAM 152 can be saved.

[0109] In addition, in the present embodiment and the modified example, the charging voltage Vpri is subjected to constant voltage control in which the output value of the power source is feedback-controlled so as to be approximately constant at the target voltage, but the present invention is not limited to this. The charging voltage Vpri may be subjected to constant current control in which the output value of the power source is feedback-controlled so as to be approximately constant at the target current. In this case, the charging voltage Vpri can be used to detect condensation by monitoring the change in voltage. Similarly, in the present embodiment and the modified example, the primary transfer voltage Vtr is subjected to constant voltage control in which the output value of the power source is feedback-controlled so as to be approximately constant at the target voltage, but the present invention is not limited to this. The primary transfer voltage Vtr may be subjected to constant current control in which the output value of the power source is feedback-controlled so as to be approximately constant at the target current. In this case, the primary transfer voltage Vtr can be used to detect condensation by monitoring the change in voltage. The same applies to the development voltage.

[0110] In addition, the dew condensation removal process is not limited to idling. For example, the image forming apparatus 100 may be stopped for a predetermined time until the water droplets on the photosensitive drum 1 and the intermediate transfer belt 10 evaporate and the dew condensation state is sufficiently eliminated as in this embodiment. Stopping the image forming apparatus 100 means stopping the rotation and surface movement of at least the photosensitive drums 1a, 1b, 1c, and 1d, the intermediate transfer belt 10, and the members acting on them (such as the charging roller 2 and the developing roller 22). Therefore, for example, a fan as a blowing means for sucking or exhausting air inside and outside the image forming apparatus 100, a heater as a heating means, a control means, etc. may be operating. In addition, during idling (or the stop of the image forming apparatus 100) as the dew condensation removal process, it is also possible to promote the evaporation of water droplets generated by dew condensation by heating a heating means such as the heater of the fixing device 30, for example, by heating the temperature inside the image forming apparatus 100. The heating means is not limited to the heater of the fixing device 30, and may be, for example, a heater provided for heating any part of the image forming apparatus 100 such as the photosensitive drum 1, or a heater specially provided for eliminating the dew condensation. In addition, during idling (or when the image forming apparatus 100 is stopped) as the dew condensation removal process, it is also possible to promote the discharge of moisture generated by the evaporation of water droplets to the outside of the apparatus by sucking or exhausting air using a fan as a blowing means for sucking or exhausting air inside and outside the image forming apparatus 100.

[0111] In this way, the inside of the machine during image formation (printing) in a print job, particularly the state of condensation on the surface of the photosensitive drum 1, can be detected by detecting the current that flows when a predetermined voltage is applied to the contact member or the voltage that is generated when a predetermined current is supplied to the contact member. The detection result can then be reflected in the control of the charging voltage, etc., and the control of the condensation removal process (idling, etc.). This makes it possible to suppress image defects caused by condensation.

[0112] <Effect verification> In order to confirm the effect of this embodiment, the following test was performed on the configuration of this embodiment and the configuration of the comparative example. In a high temperature and high humidity environment (temperature 30°C / relative humidity 80%), a fan was installed at a position 1.0 m above the image forming apparatus 100. Then, under the condition of fan ON (wind speed 2.0 m / sec) and under the condition of fan OFF (no wind), 30 sheets were continuously passed using XEROX Business 4200 LETTER size (Xerox, product name) as the recording material P. Then, the presence or absence of image defects was confirmed. In addition, after the continuous passing of 30 sheets, the same recording material P was passed after a time of 60 seconds, and the presence or absence of image defects was confirmed. The configurations of this embodiment and the comparative example are as follows.

[0113] In this example: Condensation detection based on charging current Ipri: Yes When condensation reaches a certain level: Activates control of the charged voltage Vpri Executes idle rotation after 30 consecutive sheets Comparative Example: Condensation detection based on charging current Ipri: No When a certain level of condensation occurs: The charged voltage Vpri does not change Execution of idle rotation after 30 consecutive sheets: None

[0114] Here, "detection of condensation state based on charging current Ipri" means judging whether charging current Ipri exceeds threshold value Ith in FIG. 8(a). "When condensation state of a certain level or more occurs" means when charging current Ipri exceeds threshold value Ith. "Perform idling rotation after passing 30 consecutive sheets" means that idling rotation is performed for 20 seconds after passing 30 consecutive sheets. The configuration and operation of image forming apparatus 100 of the comparative example are substantially the same as those of image forming apparatus 100 of this embodiment, except for the above points.

[0115] Table 2 shows the evaluation results of this test. Table 2 shows the test results without a fan (fan OFF) and with a fan (fan ON) for this embodiment and the comparative example. Also, the levels of low density of the first, 10th, 20th, 30th sheets of continuous paper passing and after 60 seconds are shown in four ranks, A, B, C, and D. Here, "A" indicates no low density, "B" indicates allowable but slight low density, "C" indicates unacceptable low density, and "D" indicates unacceptable low density plus missing characters. Ranks A and B were judged to be no problem, and ranks C and D were judged to be image defects. Also, when fog occurs due to condensation on the surface of the photosensitive drum 1, it is indicated as "fog".

[0116] [Table 2]

[0117] In the comparative example, when there is no fan, there is no problem because the surface of the photosensitive drum 1 does not condense during continuous paper feed. However, when there is a fan, moisture contained in the recording material P generated during fixing during continuous paper feed flows into the machine, causing condensation on the surface of the photosensitive drum 1 as the number of sheets fed increases, and low-density image defects occurred from the 20th sheet onwards. Furthermore, even 60 seconds after the continuous paper feed, the surface of the photosensitive drum 1 was cooled by the wind from the fan, causing condensation to progress, resulting in fogging even after 60 seconds of continuous paper feed.

[0118] On the other hand, in this embodiment, the condensation state on the surface of the photosensitive drum 1 during continuous paper feed was detected by the change in the charging current Ipri, and the charging voltage Vpri was controlled before low density occurred. Therefore, low density did not occur during continuous paper feed. In addition, the air containing moisture around the photosensitive drum 1 was also removed from the periphery of the photosensitive drum 1 by the idling rotation after continuous paper feed, so no image defects occurred even after 60 seconds of continuous paper feed. In this way, in this embodiment, image defects caused by condensation were suppressed in this test.

[0119] Thus, in this embodiment, the image forming apparatus 100 has a rotatable photosensitive member (photosensitive drum) 1, a charging member (charging roller) 2 that contacts the photosensitive member 1 to form a contact portion CN and charges the surface of the photosensitive member 1 when a charging bias is applied, an application portion (charging power source) 501 that applies a charging bias to the charging member 2, an exposure means (exposure device) 3 that exposes the charged surface of the photosensitive member 1 to form a potential of an image portion on the surface of the photosensitive member 1, a developing member (developing roller) 22 that is applied with a development bias and supplies toner to the image portion at a development position DN to form a toner image on the surface of the photosensitive member 1, and a transfer means (primary transfer roller) 14 that transfers the toner image from the surface of the photosensitive member 1 to a transferee (intermediate transfer belt) 10, and executes a print job in which a print image is formed on one or more recording materials in response to one start instruction, and the one or more recording materials are discharged from inside the machine to outside the machine through a recording material discharge port (paper discharge port) 130. In this embodiment, the image forming apparatus 100 includes a detection unit (charging current detection circuit) 511 that detects a current or voltage (current in this embodiment) supplied to the charging member 2 when the charging bias is applied to the charging member 2 by the application unit 501, and a control unit (engine control unit) 210 that acquires a detection result by the detection unit 511 when an area of ​​the surface of the photoconductor 1 corresponding to each recording material P to which a toner image is transferred in the print job passes through the contact portion CN during execution of the print job, and executes change control to change at least one of the charging bias, the exposure amount of the exposure means 3 that forms the potential of the image portion, and the developing bias so that the potential difference between the potential of the image portion at the development position DN and the developing bias falls within a predetermined range based on a change in the detection result due to condensation on the surface of the photoconductor 1. In this embodiment, the control unit 210 changes the absolute value of the charging bias to be smaller in the change control. However, the control unit 210 can change the exposure amount to be larger in the change control. Also, the control unit 210 can change the absolute value of the developing bias to be larger in the change control.The exposure unit 3 may expose the surface of the photoconductor 1 to light to form the potential of the image portion on the surface of the photoconductor 1 and form the potential of the non-image portion on the surface of the photoconductor 1. In this case, the control unit 210 may further change the exposure amount of the exposure unit 3 for forming the potential of the non-image portion so that the potential difference between the potential of the non-image portion at the development position DN and the development bias falls within a predetermined range in the above-mentioned variation control. The control unit 210 may execute the above-mentioned variation control when the absolute value of the current or voltage (current in this embodiment) indicated by the above-mentioned detection result exceeds a predetermined threshold value. The control unit 210 may predict the change of the above-mentioned detection result in a second period after the first period based on the change of the above-mentioned detection result acquired in the first period, and execute the above-mentioned variation control based on the result of the prediction. The control unit 210 may execute the above-mentioned variation control based on the change of the average value of the above-mentioned detection result.

[0120] In this embodiment, the control unit 210 further controls the photoconductor 1 to perform a dew condensation removal process to remove dew condensation on the surface of the photoconductor 1 based on the change in the detection result. In this embodiment, the control unit 210 controls the photoconductor 1 to perform a dew condensation removal process when the absolute value of the charging bias is smaller in the change control and the absolute value of the charging bias is lower than a predetermined threshold value. In this embodiment, the control unit 210 controls the drive unit 90 that rotates the photoconductor 1 to perform an idling rotation to rotate the photoconductor 1 without forming a toner image on the surface of the photoconductor 1 as the dew condensation removal process. In this embodiment, the control unit 210 obtains the detection result by the detection unit 511 during the idling rotation, and controls the drive unit 90 to end the idling rotation based on the change in the detection result due to the dew condensation on the surface of the photoconductor 1 being removed. Here, the control unit 210 can obtain the detection result by the detection unit 511 when a portion without an image portion in the entire area in the direction substantially perpendicular to the moving direction of the surface of the photoconductor 1 in the above region passes through the contact portion CN. The above-mentioned portion may be at least one of the leading end or trailing end in the moving direction of the surface of the photoconductor 1 in the above-mentioned region. However, in this embodiment, there is a charge removing means (pre-charging exposure device) 6 that removes charge from the surface of the photoconductor 1 that reaches the contact portion CN, and in this case, the control unit 210 can obtain the detection result by the detection unit 511 at any timing while the above-mentioned region that has been charge removed by the charge removing means 6 passes through the contact portion CN. The image forming apparatus may also be configured to have a detection unit (potential sensor) 514 that detects the potential of the surface of the photoconductor 1 that has been formed by charging the surface of the photoconductor 1 by the charging means (charging roller) 2, and a control unit that obtains the detection result of the detection by the detection unit 514 of the potential of the area of ​​the surface of the photoconductor 1 corresponding to each recording material P to which the toner image is transferred in the print job during the execution of the print job, and controls to execute a dew condensation removal process to remove the dew condensation on the surface of the photoconductor 1 based on a change in the detection result due to dew condensation on the surface of the photoconductor 1.

[0121] As described above, according to this embodiment, image defects caused by condensation can be suppressed.

[0122] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0123] In this embodiment, the condensation state on the surface of the photosensitive drum 1 is detected based on the detection result of the charging current Ipri of the multiple image forming units S. By detecting the condensation state across the multiple image forming units S, it is possible to determine whether the condensation state is localized within the machine or is present throughout the entire machine. Then, it becomes possible to control the charging voltage Vpri and the idling rotation based on the determination result, and the effects of condensation can be suppressed more accurately in accordance with the condensation state within the machine.

[0124] FIG. 18 is a chart showing (a) the charging current Ipri (hereinafter, "Ipri(Y)") in the yellow and black image forming units Sa and Sd, (b) the difference ΔIst between the charging current Ipri(Y) in the yellow image forming unit Sa and the charging current Ipri (hereinafter, "Ipri(K)") in the black image forming unit Sd, and (c) the change in the charging voltage Vpri in the yellow and black image forming units Sa and Sd during the execution of a print job when the control of this embodiment is performed in a situation where moisture generated from the paper during fixing flows into the machine. Hereinafter, the yellow image forming unit Sa will be simply referred to as "yellow Sa", and the black image forming unit Sd will be simply referred to as "black Sd". In FIG. 18(a) and (c), the solid lines indicate the charging current Ipri(Y) and charging voltage Vpri in the yellow Sa, and the dashed lines indicate the charging current Ipri(K) and charging voltage Vpri in the black Sd. In FIG. 18, the horizontal axis indicates time, and the vertical axis indicates the direction in which the absolute values ​​of the current and voltage (potential) increase. nis the time when the nth image formation is started, and represents the timing when the nth image formation is started for the yellow Sa, which is the most upstream in the rotation direction of the intermediate transfer belt 10. n The symbol "'" attached to each of the charts in FIG. 18 indicates the timing when image formation for black Sd is started. Furthermore, unless otherwise specified, the same symbols as those in FIG. 8 have the same meanings as those in FIG. 8. Furthermore, as described in the first embodiment, the change in charging current Ipri correlates with the post-charging drum potential Vd. The operation according to the chart in FIG. 18 is controlled by the engine control unit 210.

[0125] In the image forming apparatus 100 of this embodiment, among the multiple image forming units Sa, Sb, Sc, and Sd, the yellow Sa is closest to the paper discharge outlet 130, and the black Sd is farthest from the paper discharge outlet 130 (FIGS. 1 and 9). When moisture generated from the paper during fixing flows in from the paper discharge outlet 130, the wind becomes stronger the closer to the paper discharge outlet 130, so the image forming unit S closer to the paper discharge outlet 130 is more susceptible to the influence of the wind containing moisture. Therefore, even if the charging voltage Vpri is the same for the yellow Sa and the black Sd, as shown in FIG. 18(a), the charging current Ipri(Y) in the yellow Sa increases faster (with fewer sheets of paper passing) than the charging current Ipri(K) in the black Sd.

[0126] Here, in addition to detecting condensation on the surface of the photosensitive drum 1 based on the charging current Ipri in each image forming unit S as in the first embodiment, the difference in the charging current Ipri between the multiple image forming units S can be calculated to determine the overall condensation state inside the machine. That is, when the result shown in FIG. 18(a) is obtained, it can be seen that the condensation progresses slowly in black Sd and quickly in yellow Sa. When there is a bias in the condensation state between the area close to the paper discharge outlet 130 and the area far from the paper discharge outlet 130, it is estimated that a rapid and strong wind is flowing in from the paper discharge outlet 130. Therefore, there is a possibility that condensation will occur rapidly on the surface of the photosensitive drum 1 of the image forming unit S close to the paper discharge outlet 130, and even during the formation of one image, image defects such as low density may occur in the leading edge (first half) of the image but low density may occur in the trailing edge (second half) of the image.

[0127] Therefore, in this embodiment, a threshold value (upper threshold value) ΔIth is also set for the difference ΔIst of the charging current Ipri. Then, by monitoring whether or not this difference ΔIst of the charging current Ipri exceeds the threshold ΔIth, the influence of wind from outside the machine is predicted, and control of the charging voltage Vpri and dew condensation removal processing (idle rotation, etc.) are performed. Specifically, in addition to monitoring the charging current Ipri in each image forming unit S as shown in Fig. 18(a), changes in the difference ΔIst between the charging current Ipri(Y) in yellow Sa and the charging current Ipri(K) in black Sd are also monitored as shown in Fig. 18(b).

[0128] For example, when the result shown in FIG. 18 is obtained, as shown in FIG. 18(a), at time t n+1 At time t , the charging currents Ipri(Y) and Ipri(K) in the image forming units Sa and Sd have not yet reached the threshold Ith. However, as shown in FIG. 18B, the difference ΔIst in the charging currents Ipri is n+1 At the time t , the threshold value ΔIth is exceeded (i.e., the threshold value ΔIth is exceeded during the nth image formation). In this case, in this embodiment, the charging voltage Vpri is controlled in the image formation of the n+1th sheet and thereafter, triggered by the fact that the difference ΔIst in the charging current Ipri exceeds the threshold value ΔIth. The method of controlling the charging voltage Vpri is the same as in the first embodiment. That is, as shown in FIG. 18(c), the charging voltage Vpri is controlled to be lowered so as to cancel the change in the charging current Ipri (≈change in the post-charging drum potential Vd) in each image forming station S. Here, the timing of lowering the charging voltage Vpri in black Sd is slightly later than that in yellow Sa because the image formation start timing is different between yellow Sa and black Sd, and the control in each image forming station Sa, Sd is substantially the same for yellow Sa and black Sd. The same is true for the other image forming stations S. Also, as in the first embodiment, at time t n+1Thereafter, in addition to controlling the charging voltage Vpri, a dew condensation removal process (such as idling) may be performed at any timing. Here, the amount by which the charging voltage Vpri is lowered may be different for each image forming unit S. Yellow Sa and black Sd are taken as examples. The amount by which the charging voltage Vpri is lowered may be smaller for black Sd than for yellow Sa. This is because black Sd, which is the furthest from the paper discharge outlet 130, is less susceptible to the wind (moisture) flowing into the machine than yellow Sa, which is the closest to the paper discharge outlet 130. The same is true for the other image forming units S, and for example, the amount by which the charging voltage Vpri is lowered may be smaller for image forming units S farther from the paper discharge outlet 130. At this time, for example, for image forming units S, such as black Sd, which are less susceptible to the wind (moisture) flowing into the machine, the charging voltage Vpri may not be changed.

[0129] In this embodiment, the change in the difference between the charging current Ipri(Y) (maximum value) in the yellow Sa closest to the paper discharge outlet 130 and the charging current Ipri(K) (minimum value) in the black Sd farthest from the paper discharge outlet 130 is monitored, but the present invention is not limited to this. It is desirable to monitor the change in the difference in the charging current Ipri between the image forming section S that is more susceptible to the influence of the wind (moisture) flowing into the machine and the image forming section S that is less susceptible to the influence, depending on the configuration of the image forming apparatus 100. Preferably, the change in the difference between the maximum value and the minimum value of the charging current Ipri detected in the multiple image forming sections S is monitored, depending on the configuration of the image forming apparatus 100. In addition, the relationship between the detection results of the charging currents between the multiple image forming sections S is not limited to the difference, and if the magnitude relationship between the two, such as a ratio, can be known, the change can be monitored to perform control similar to that of this embodiment.

[0130] Also, if the charging current Ipri in each image forming unit S increases during continuous paper feed, but the difference ΔIst in the charging current Ipri between the image forming units S is small, condensation is progressing evenly throughout the entire machine. In that case, the air volume and speed flowing in from the paper discharge port 130 may be weak, and it is estimated that condensation is progressing relatively slowly. Therefore, in that case, by controlling the charging voltage Vpri and idling based on the charging current Ipri in each image forming unit S in the same manner as in the first embodiment, it is possible to suppress image defects and increase in downtime.

[0131] In addition, by performing control based on the change in the charging current Ipri in the multiple image forming units S, a wider range of device configurations can be adopted. For example, the control of this embodiment can be performed even in a configuration in which the charging power source 501 is common to the multiple image forming units S. Therefore, it is possible to reduce the cost of the device and to make the device smaller. For example, a case in which the charging power source 501 is common to yellow Sa and black Sd will be further described as an example. In this example, the charging current detection circuit 511 is also provided independently for yellow Sa and black Sd. Note that the charging power source 501 may be common to all of the multiple image forming units S provided in the image forming apparatus 100, and the charging current detection circuit 511 may be provided independently for each image forming unit S.

[0132] FIG. 19 is a chart showing the transition of (a) charging current Ipri(Y) and Ipri(K) in the yellow and black image forming units Sa and Sd, (b) common charging voltage Vpri in the yellow and black image forming units Sa and Sd, and (c) post-charging drum potential Vd in the yellow and black image forming units Sa and Sd during execution of a print job in a situation where moisture generated from the paper during fixing flows into the machine in this case. In this embodiment, yellow Sa is most susceptible to condensation, and black Sd is least susceptible to condensation. Therefore, when the charging power source is shared as described above, by controlling the charging voltage Vpri according to yellow Sa, which is most susceptible to the influence of condensation, the charging voltage Vpri may become too low in black Sd. If the charging voltage Vpri is too low, the post-charging drum potential Vd (drum potential Vdl after weak exposure) becomes too low. As a result, the potential difference (back contrast Vback) between the post-weak exposure drum potential Vdl and the development voltage Vdc (potential of the development roller 22) is small, and toner cannot be held on the development roller 22, which may cause fogging. Therefore, in order to execute idling before the charging voltage Vpri is lowered too much in one image forming unit S, a threshold value (lower limit threshold value) Ith_low is set on the lower limit side of the charging current Ipri as shown in FIG. n+1 At this point, the charging current Ipri(Y) for yellow Sa exceeds the upper limit threshold (upper threshold) Ith, and based on this, control is performed to lower the charging voltage Vpri in the same manner as in the first embodiment (FIG. 19(b)). On the other hand, when the charging current Ipri(K) for black Sd falls below the lower limit threshold Ith_low (the n+4th sheet in the example of FIG. 19), once image formation for the n+4th sheet has been performed, image formation for the n+5th sheet and onward is not continued, but instead idling is performed to remove moisture. This type of control makes it possible to suppress image defects such as fogging.

[0133] Even in the case where the charging power source is shared in this way, image defects such as low density can be suppressed by controlling the charging voltage Vpri appropriately, while image defects such as fog can be suppressed by optimizing the timing of executing the idle rotation. Therefore, the cost of the device can be reduced and the device can be made smaller. Here, the charging voltage Vpri is increased in advance to set the post-charging drum potential Vd higher than that in the above example, and when the charging voltage Vpri is lowered, the background exposure amount (weak exposure amount) can be made different between the image forming units S. For example, in accordance with the above example, when lowering the charging voltage Vpri, the background exposure amount in black Sd can be made smaller than the background exposure amount in yellow Sa, so that the back contrast Vback of yellow Sa and black Sa can be controlled to be equal. This makes it possible to lengthen the period until the charging current Ipri(K) in black Sd falls below the lower limit threshold Ith_low (the period until idle rotation is started) and suppress an increase in downtime. The same applies to the other image forming units S, and for example, the background exposure amount can be made smaller as the image forming unit S is farther from the paper discharge outlet 130.

[0134] Thus, in this embodiment, the image forming apparatus 100 has first and second image forming units Sa and Sd, each of which is equipped with a photoconductor 1, a charging member 2, an exposure means 3, a developing member 22, a transfer means 14, and a detection unit 511, and the control unit 210 acquires a first detection result by the detection unit 511 of the first image forming unit Sa when the image forming area in the first image forming unit Sa passes through the abutment portion CN between the photoconductor 1 and the charging member 2, and acquires a second detection result by the detection unit of the second image forming unit Sd when the image forming area in the second image forming unit Sd passes through the abutment portion CN between the photoconductor 1 and the charging member 2, during execution of a print job, and executes change control to change the charging bias, etc., so that the development contrast in each of the first and second image forming units Sa and Sd is within a predetermined range based on a change in the relationship between the first detection result and the second detection result. In this embodiment, the control unit 210 executes the change control in each of the first and second image forming units Sa and Sd when the difference between the first detection result and the second detection result exceeds a predetermined threshold. Also, in this embodiment, the image forming apparatus 100 has a plurality of image forming units including the first and second image forming units Sa and Sd, and the absolute value of the current or voltage indicated by the first detection result is the maximum value among the absolute values ​​of the current or voltage (current in this embodiment) acquired by the respective detection units 511 of the plurality of image forming units, and the absolute value of the current or voltage (current in this embodiment) indicated by the second detection result is the minimum value among the absolute values ​​of the current or voltage (current in this embodiment) acquired by the respective detection units 511 of the plurality of image forming units. Furthermore, in this embodiment, the charging member 2 of the first image forming unit Sa is located closest to the recording material discharge outlet (paper discharge outlet) 130 among the respective charging members 2 of the multiple image forming units, and the charging member 2 of the second image forming unit Sd is located farthest from the recording material discharge outlet 130 among the respective charging members 2 of the multiple image forming units.

[0135] In addition, the detection units 511 of the first and second image forming units Sa, Sd may be configured to respectively detect the current or voltage supplied to the charging member 2 of the first and second image forming units Sa, Sd when a charging bias is applied to the charging member 2 of the first and second image forming units Sa, Sd by a common application unit (common charging power source) 501 that applies a charging bias to the charging member 2 of the first and second image forming units Sa, Sd. In this case, the control unit 210 can change the absolute value of the charging bias to a smaller value in the above-mentioned change control, and can control to execute the above-mentioned condensation removal process when the absolute value of the current or voltage indicated by the detection result of either of the detection units 511 of the first and second image forming units Sa, Sd falls below a predetermined threshold value. Furthermore, during execution of a print job, the control unit 210 can obtain a first detection result by the detection unit 511 of the first image forming unit Sa when the above-mentioned area passes through the contact part CN in the first image forming unit Sa, and obtain a second detection result by the detection unit 511 of the second image forming unit Sd when the above-mentioned area passes through the contact part CN in the second image forming unit Sd, and based on a change in the relationship between the first detection result and the second detection result, control the first and second image forming units Sa, Sd to execute a condensation removal process for removing condensation on the surface of the photoconductor 1. In this case, the control unit 210 can control the first and second image forming units Sa, Sd to execute the condensation removal process when a difference between the first detection result and the second detection result exceeds a predetermined threshold value.

[0136] As described above, in this embodiment, not only is the condensation state detected based on the charging current Ipri in each image forming unit S, but the bias in the condensation state inside the machine is determined based on the difference in the charging current Ipri between the multiple image forming units S. This makes it possible to more accurately control the charging voltage and the condensation removal process (idle rotation, etc.) in accordance with the condensation state inside the machine, thereby suppressing image defects (low density, missing characters, etc.) caused by fluctuations in the surface potential of the photosensitive drum 1. In addition, it is possible to standardize the power source that applies voltage to contact members such as the charging roller 2, thereby reducing the cost and size of the device.

[0137] Note that the same modifications as those described in the modified examples and other configurations of the first embodiment can also be applied to this embodiment. For example, instead of performing control based on the detection results of the charging current in the multiple image forming stations S, it is possible to perform control based on the detection results of the primary transfer current (or the developing current, or the drum potential after charging) in the multiple image forming stations S.

[0138] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0139] In the above embodiment, the image forming apparatus is capable of forming a full-color image, but the present invention is not limited to such an embodiment. The present invention can also be applied to an image forming apparatus capable of forming only a monochrome (black and white or monochromatic) image. The image forming apparatus may be configured such that a toner image is directly transferred from the photoconductor to a recording material as a transferee by applying a transfer voltage to a transfer member that contacts the photoconductor.

[0140] In addition, the photosensitive member is not limited to a drum-shaped one (photosensitive drum), and may be, for example, an endless belt-shaped one (photosensitive belt), etc. In addition, the intermediate transfer member is not limited to an endless belt, and may be, for example, an intermediate transfer drum that is stretched around a frame and formed into a drum shape.

[0141] In the above embodiment, the developing member is in contact with the photosensitive member to perform development, but it may be disposed opposite the photosensitive member without contacting the photosensitive member to perform development. [Explanation of symbols]

[0142] 1 (1a, 1b, 1c, 1d) Photosensitive drum 2 (2a, 2b, 2c, 2d) Charge roller 3 (3a, 3b, 3c, 3d) Scanner unit (exposure device) 4(4a, 4b, 4c, 4d) Developing device 6 (6a, 6b, 6c, 6d) Pre-charging exposure device 10 Intermediate transfer belt 14 (14a, 14b, 14c, 14d) Primary transfer roller 15 Primary transfer power supply 22 Developing roller 100 Image forming device 501 Charging power source 502 Development power supply 511 Charge current detection circuit P Recording material S (Sa, Sb, Sc, Sd) Process cartridge

Claims

1. A rotatable photoreceptor; a charging member that contacts the photoconductor to form a contact portion and charges the surface of the photoconductor by applying a charging bias; an application unit that applies the charging bias to the charging member; an exposure means for exposing the charged surface of the photoconductor to light to form a potential of an image portion on the surface of the photoconductor; a developing member that is applied with a developing bias and supplies toner to the image portion at a developing position to form a toner image on the surface of the photoconductor; a transfer means for transferring a toner image from the surface of the photoreceptor to a transfer receiving material; having An image forming apparatus which executes a print job in which a print image is formed on one or more recording materials in response to a single start instruction, and the one or more recording materials are discharged from inside the apparatus to outside the apparatus through a recording material discharge port, a detection unit that detects a current or a voltage supplied to the charging member when the charging bias is applied to the charging member by the application unit; a control unit that acquires a detection result by the detection unit when an area of ​​the surface of the photoconductor corresponding to each recording material to which a toner image is transferred in the print job passes through the contact unit during execution of a print job, and executes change control to change at least one of the charging bias, the exposure amount of the exposure means for forming the potential of the image portion, and the developing bias based on a change in the detection result due to condensation on the surface of the photoconductor, so that a potential difference between the potential of the image portion at the development position and the developing bias falls within a predetermined range; An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein the control unit changes the charging bias so as to reduce an absolute value of the charging bias in the change control.

3. 2. The image forming apparatus according to claim 1, wherein the control unit changes the exposure amount so as to increase the exposure amount in the change control.

4. 2. The image forming apparatus according to claim 1, wherein the control unit changes the developing bias so as to increase an absolute value of the developing bias in the change control.

5. the exposure unit exposes the surface of the photoconductor to light to form a potential in the image area on the surface of the photoconductor and a potential in the non-image area on the surface of the photoconductor; The image forming apparatus according to claim 1, characterized in that, in the change control, the control unit further changes the exposure amount of the exposure means for forming the potential of the non-image area so that the potential difference between the potential of the non-image area at the development position and the development bias is within a predetermined range.

6. 2 . The image forming apparatus according to claim 1 , wherein the control unit executes the change control when an absolute value of the current or voltage indicated by the detection result exceeds a predetermined threshold value.

7. The image forming apparatus according to claim 1, characterized in that the control unit predicts a change in the detection result in a second period after the first period based on the change in the detection result obtained in the first period, and executes the change control based on the result of the prediction.

8. The image forming apparatus according to claim 1 , wherein the control unit executes the variation control based on a change in an average value of the detection results.

9. a first image forming unit and a second image forming unit each including the photoconductor, the charging member, the exposure unit, the developing member, the transfer unit, and the detection unit; The image forming apparatus of claim 1, wherein the control unit, during execution of a print job, acquires a first detection result by the detection unit of the first image forming unit when the area passes through the contact portion in the first image forming unit, and acquires a second detection result by the detection unit of the second image forming unit when the area passes through the contact portion in the second image forming unit, and executes the change control in each of the first and second image forming units based on a change in the relationship between the first detection result and the second detection result.

10. 10. The image forming apparatus according to claim 9, wherein the control unit executes the change control in each of the first and second image forming units when a difference between the first detection result and the second detection result exceeds a predetermined threshold value.

11. a plurality of image forming units including the first and second image forming units each including the photoconductor, the charging member, the exposure unit, the developing member, the transfer unit, and the detection unit; 10. The image forming apparatus according to claim 9, wherein the absolute value of the current or voltage indicated by the first detection result is the maximum value among the absolute values ​​of the current or voltage acquired by the detection unit of each of the multiple image forming units, and the absolute value of the current or voltage indicated by the second detection result is the minimum value among the absolute values ​​of the current or voltage acquired by the detection unit of each of the multiple image forming units.

12. 10. The image forming apparatus according to claim 9, wherein the charging member of the first image forming unit is located closest to the recording material discharge outlet among the charging members of each of the multiple image forming units, and the charging member of the second image forming unit is located farthest from the recording material discharge outlet among the charging members of each of the multiple image forming units.

13. 2 . The image forming apparatus according to claim 1 , wherein the control unit further controls to execute a dew condensation removal process for removing dew condensation on the surface of the photoconductor based on the change in the detection result.

14. The image forming apparatus according to claim 13 , wherein the control unit, in the variation control, changes the absolute value of the charging bias to be smaller, and controls so as to execute the condensation removal process when the absolute value of the charging bias falls below a predetermined threshold value.

15. The image forming apparatus according to claim 13, characterized in that the control unit controls a drive unit that drives the rotation of the photosensitive member so as to perform an idling rotation of the photosensitive member without forming a toner image on the surface of the photosensitive member as the condensation removal process.

16. The image forming apparatus according to claim 15, wherein the control unit acquires a detection result from the detection unit while the idling rotation is being performed, and controls the drive unit to terminate the idling rotation based on a change in the detection result due to condensation being removed from the surface of the photosensitive body.

17. a first image forming unit and a second image forming unit each including the photoconductor, the charging member, the exposure unit, the developing member, the transfer unit, and the detection unit; the detection units of the first and second image forming units respectively detect a current or a voltage supplied to the charging members of the first and second image forming units when the charging bias is applied to the charging members of the first and second image forming units by the common application unit which applies the charging bias to the charging members of the first and second image forming units; The image forming apparatus according to claim 13, characterized in that, in the change control, the control unit changes the absolute value of the charging bias to a smaller value, and controls so as to execute the condensation removal process when the absolute value of the current or voltage indicated by the detection result of either of the detection units of the first and second image forming units falls below a predetermined threshold value.

18. A rotatable photoreceptor; a charging member that contacts the photoconductor to form a contact portion and charges the surface of the photoconductor by applying a charging bias; an application unit that applies the charging bias to the charging member; an exposure means for exposing the charged surface of the photoconductor to light to form a potential of an image portion on the surface of the photoconductor; a developing member that is applied with a developing bias and supplies toner to the image portion at a developing position to form a toner image on the surface of the photoconductor; a transfer means for transferring a toner image from the surface of the photoreceptor to a transfer receiving material; having An image forming apparatus which executes a print job in which a print image is formed on one or more recording materials in response to a single start instruction, and the one or more recording materials are discharged from inside the apparatus to outside the apparatus through a recording material discharge port, a detection unit that detects a current or a voltage supplied to the charging member when the charging bias is applied to the charging member by the application unit; a control unit that acquires a detection result by the detection unit when an area of ​​the surface of the photoconductor corresponding to each recording material to which a toner image is transferred in the print job passes through the contact unit during execution of a print job, and controls to execute a condensation removal process for removing condensation on the surface of the photoconductor based on a change in the detection result due to condensation on the surface of the photoconductor; An image forming apparatus comprising:

19. The image forming apparatus according to claim 18, wherein the control unit controls a drive unit that drives the rotation of the photoconductor so as to perform an idling rotation of the photoconductor without forming a toner image on the surface of the photoconductor as the condensation removal process.

20. The image forming apparatus according to claim 19, wherein the control unit acquires a detection result from the detection unit while the idling rotation is being performed, and controls the drive unit to terminate the idling rotation based on a change in the detection result due to condensation being removed from the surface of the photosensitive body.

21. a first image forming unit and a second image forming unit each including the photoconductor, the charging member, the exposure unit, the developing member, the transfer unit, and the detection unit; The image forming apparatus of claim 18, wherein the control unit, during execution of a print job, acquires a first detection result by the detection unit of the first image forming unit when the area passes through the contact portion in the first image forming unit, and acquires a second detection result by the detection unit of the second image forming unit when the area passes through the contact portion in the second image forming unit, and controls each of the first and second image forming units to perform a condensation removal process to remove condensation on the surface of the photosensitive body based on a change in the relationship between the first detection result and the second detection result.

22. The image forming apparatus according to claim 21, wherein the control unit controls the first and second image forming units to perform the condensation removal process when a difference between the first detection result and the second detection result exceeds a predetermined threshold value.

23. The image forming apparatus according to any one of claims 1 to 22, characterized in that the control unit acquires the detection result by the detection unit when a portion without the image portion in the entire area in a direction approximately perpendicular to the movement direction of the surface of the photosensitive body in the region passes through the abutment portion.

24. 24. The image forming apparatus according to claim 23, wherein the portion is at least one of a leading end and a trailing end in a moving direction of the surface of the photoconductor in the region.

25. a charge removing means for removing charge from the surface of the photoconductor that reaches the contact portion, The image forming apparatus according to any one of claims 1 to 22, characterized in that the control unit is capable of acquiring a detection result by the detection unit at any timing while the area neutralized by the neutralization means passes through the contact portion.

26. A rotatable photoreceptor; a charging means for applying a charging bias to the surface of the photoconductor; an exposure means for exposing the charged surface of the photoconductor to light to form a potential of an image portion on the surface of the photoconductor; a developing member that is applied with a developing bias and supplies toner to the image portion at a developing position to form a toner image on the surface of the photoconductor; a transfer means for transferring a toner image from the surface of the photoreceptor to a transfer receiving material; having An image forming apparatus which executes a print job in which a print image is formed on one or more recording materials in response to a single start instruction, and the one or more recording materials are discharged from inside the apparatus to outside the apparatus through a recording material discharge port, a detection unit that detects the potential of the surface of the photoconductor that has been charged by the charging unit; a control unit that acquires a detection result obtained by the detection unit during execution of a print job of an electric potential of an area on the surface of the photoconductor corresponding to each recording material to which a toner image is transferred in the print job, and controls to execute a dew condensation removal process for removing dew condensation on the surface of the photoconductor based on a change in the detection result due to dew condensation on the surface of the photoconductor; An image forming apparatus comprising:

27. The image forming apparatus according to claim 26, wherein the control unit controls a drive unit that drives the rotation of the photoconductor so as to perform idling to rotate the photoconductor without forming a toner image on the surface of the photoconductor as the condensation removal process.

Images