JP2019200282A - Image formation device - Google Patents

Abstract

To provide an image formation device that can sufficiently recover secondary transfer remaining toner, test pattern or the like from an intermediate transfer body, and can suppress image failures due to charging unevenness of the intermediate transfer body.SOLUTION: An image formation device 100 comprises: an image formation unit; an intermediate transfer body; a power source that can selectively apply, to a cleaning member, a first voltage in which a target value of current is a first value and a second voltage in which an absolute value of the target value of current is a second value greater than the first value; and a control unit that, when the voltage to be applied to the cleaning member from the power source is changed from the first voltage to the second voltage over a prescribed period during the execution of continuous image formation for transferring a plurality of toner images to a plurality of recording materials, can control a timing at which the image formation unit forms the toner image so that, in the prescribed period, the toner image to be transferred to the recording material is not transferred to a region of the intermediate transfer body before the region thereof passing through the cleaning member arrives at the cleaning member next.SELECTED DRAWING: Figure 14

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system or an electrostatic recording system.

  2. Description of the Related Art Conventionally, as an electrophotographic image forming apparatus, for example, there is an intermediate transfer type image forming apparatus having an intermediate transfer belt which is a rotatable intermediate transfer member constituted by an endless belt. In an intermediate transfer type image forming apparatus, for example, a toner image formed on a photosensitive drum is primarily transferred from the photosensitive drum to the intermediate transfer belt by applying a primary transfer voltage at a primary transfer portion. The toner image primarily transferred onto the intermediate transfer belt is secondarily transferred from the intermediate transfer belt to a recording material such as paper by applying a secondary transfer voltage at the secondary transfer portion. In addition, in an intermediate transfer type image forming apparatus, a test pattern (detection image, patch image) is formed on an intermediate transfer belt in order to stabilize image density or correct color misregistration. May execute a test mode to detect

  On the surface of the intermediate transfer belt that has passed through the secondary transfer portion, residual secondary transfer toner that has not been transferred to the recording material and an untransferred test pattern are adhered. For this reason, the secondary transfer residual toner and the test pattern attached to the surface of the intermediate transfer belt are electrostatically charged using electrostatic force downstream of the secondary transfer unit and upstream of the primary transfer unit in the rotation direction of the intermediate transfer belt. In some cases, an electrostatic cleaning device for collection is disposed.

  Patent Document 1 discloses that the voltage applied to the electrostatic cleaning device is controlled according to the secondary transfer residual toner attached to the surface of the intermediate transfer belt and the conditions of the test pattern. As a result, the secondary transfer residual toner and the test pattern adhering to the surface of the intermediate transfer belt are sufficiently collected.

JP 2013-57811 A

  However, in the method described in Patent Document 1, the voltage applied to the electrostatic cleaning device and the flowing current change when the secondary transfer residual toner and the test pattern are collected.

  At the time of image formation, the intermediate transfer belt having a constant circumferential thickness rotates at a constant speed. At this time, when the voltage applied to the electrostatic cleaning device or the flowing current changes, the amount of charge transfer per unit time from the electrostatic cleaning device to the intermediate transfer belt changes. As a result, the charge amount per unit area, that is, the charge density, changes depending on the position of the intermediate transfer belt in the rotation direction. As a result, uneven charge density occurs in the rotation direction of the intermediate transfer belt, thereby causing uneven charging in the rotation direction of the intermediate transfer belt.

  As described above, when charging unevenness occurs in the intermediate transfer belt, the transfer electric field and transfer current at the time of transferring the toner image from the image carrier to the intermediate transfer belt in the primary transfer portion change, resulting in uneven transfer efficiency. End up. Then, when nonuniformity occurs in the transfer efficiency, image defects such as nonuniform image density occur.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of sufficiently collecting secondary transfer residual toner, a test pattern, and the like from an intermediate transfer body and suppressing image defects due to uneven charging of the intermediate transfer body. .

  The above object is achieved by the image forming apparatus according to the present invention. In summary, according to the present invention, an image forming unit that forms a toner image, a toner image formed by the image forming unit is transferred by a primary transfer unit, and the toner image is transferred to a recording material by a secondary transfer unit. And a rotatable intermediate transfer member that is transported to contact the intermediate transfer member at a cleaning unit that is downstream of the secondary transfer unit and upstream of the primary transfer unit in the rotational direction of the intermediate transfer member. Applied to the cleaning member for collecting the toner on the intermediate transfer member; a first voltage whose current or voltage target value is a first value; and an absolute value of the current or voltage target value for the cleaning member. A power source that can selectively apply a second voltage that is a second value that is greater than the first value, and the cleaning from the power source during execution of continuous image formation for transferring a plurality of toner images to a plurality of recording materials, respectively. Applied to member When the pressure is switched from the first voltage to the second voltage over a predetermined period, the area of the intermediate transfer member that has passed through the cleaning unit during the predetermined period is recorded in the area before reaching the cleaning unit next time. An image forming apparatus comprising: a control unit capable of controlling a timing at which the image forming unit forms a toner image so that a toner image transferred to the material is not transferred.

  According to the present invention, secondary transfer residual toner, test patterns, and the like can be sufficiently collected from the intermediate transfer member, and image defects due to uneven charging of the intermediate transfer member can be suppressed.

1 is a schematic sectional view of an image forming apparatus. It is a schematic sectional drawing of the vicinity of a belt cleaning apparatus. 3 is a schematic block diagram illustrating a control mode of a main part of the image forming apparatus. FIG. It is a schematic diagram of a test pattern for image density correction. It is the flowchart figure and graph figure for demonstrating image density correction control. It is a schematic diagram of a test pattern for image density correction. It is a graph for explaining image density correction control, and a potential chart. It is a graph for demonstrating image density correction control. It is a graph showing the relationship between cleaning residual toner and cleaning current. It is a graph for demonstrating cleaning current control. FIG. 6 is a graph showing the relationship between the cleaning current and the surface potential of the intermediate transfer belt, and a graph showing the temporal transition of the primary transfer current and the cleaning current. FIG. 6 is a graph showing the relationship between primary transfer current and primary transfer residual toner amount. It is a flowchart figure of control of Example 1. FIG. It is a chart for demonstrating the control which changes an image formation start timing. It is a graph for demonstrating an ATVC control method. FIG. 4 is a graph showing the relationship between the number of image formations and the primary transfer voltage, and a graph showing the relationship between the primary transfer voltage and the electrical resistance of the intermediate transfer belt. It is a flowchart of control of Example 2. FIG. 6 is a graph for explaining a surface potential of an intermediate transfer belt.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

1. Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic sectional view of an image forming apparatus 100 of the present embodiment. The image forming apparatus 100 according to the present exemplary embodiment has a function of a tandem type multifunction peripheral (copier, printer, or facsimile machine) that employs an intermediate transfer method and can form a full-color image using an electrophotographic method. ).

  The image forming apparatus 100 includes first, second, third, and fourth images that form yellow (Y), magenta (M), cyan (C), and black (K) images as a plurality of image forming units, respectively. It has image forming units UY, UM, UC, UK. For elements having the same or corresponding functions or configurations in the respective image forming units UY, UM, UC, UK, Y, M, C, K at the end of the code indicating that they are elements for any color are omitted. And may be described in a comprehensive manner. The image forming unit U includes a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, a drum cleaning device 6 and the like which will be described later.

  The image forming unit U includes a photosensitive drum 1 which is a rotatable drum type (cylindrical) photosensitive member (electrophotographic photosensitive member) as an image carrier. The photosensitive drum 1 is configured by applying an organic photoconductor layer (OPC) to an outer peripheral surface of an aluminum cylinder having a diameter of 80 mm. The photosensitive drum 1 is rotationally driven at a predetermined peripheral speed in the direction of arrow R1 (counterclockwise) in the drawing. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential having a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-type charging member as a charging unit. The charging roller 2 is disposed so as to contact the surface of the photosensitive drum 1. During the charging process, a predetermined charging voltage including a negative DC component is applied to the core of the charging roller 2 from a charging power source (not shown). The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner) 3 as an exposure unit, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. The exposure device 3 projects laser light onto the photosensitive drum 1 via a polygon mirror or the like based on image information (image signal). The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device 4 as developing means, and a toner image (developer image) is formed on the photosensitive drum 1. Is done. During the developing process, a predetermined developing voltage including a negative direct current component is applied from a developing power source (not shown) to a developing roller as a developer carrier provided in the developing device 4. In this embodiment, the exposure portion (image portion) on the photosensitive drum 1 whose absolute value of potential has been lowered by being exposed after being uniformly charged is charged with the same polarity as the charging polarity of the photosensitive drum 1. Toner adheres (reverse development). In this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner at the time of development, is negative.

  An intermediate transfer belt 7 that is a rotatable intermediate transfer member constituted by an endless belt is disposed so as to face the four photosensitive drums 1. The intermediate transfer belt 7 is stretched around a secondary transfer counter roller 21 as a plurality of stretching rollers, a driving roller 22, first and second idler rollers 23 and 24, a tension roller 25, and a pressing roller 26. Has been. The secondary transfer counter roller 21 functions as a counter member (counter electrode) of the secondary transfer roller 8 described later. The driving roller 22 is driven by a motor (not shown) having excellent constant speed, and circulates (rotates) the intermediate transfer belt 7. The intermediate transfer belt 7 is rotated (circulated and moved) at a predetermined peripheral speed (surface moving speed) in the direction of arrow R2 (clockwise) in the drawing when the driving force is transmitted by the driving roller 22. In this embodiment, the intermediate transfer belt 7 rotates at a peripheral speed of 150 to 470 mm / sec. The first and second idler rollers 23 and 24 support the intermediate transfer belt 7 extending along the arrangement direction of the photosensitive drums 1Y, 1M, 1C, and 1K. The tension roller 25 applies a constant tension to the intermediate transfer belt 7. The tension roller 25 is urged from the inner peripheral surface side to the outer peripheral surface side of the intermediate transfer belt 7 by springs (not shown) which are elastic members as urging means at both ends in the rotation axis direction. ing. The pressing roller 26 projects the intermediate transfer belt 7 in the vicinity of the upstream side of the secondary transfer portion T2 described later outward to improve the adhesion between the secondary transfer roller 8 described later and the intermediate transfer belt 7.

  A primary transfer roller 5, which is a roller-type primary transfer member serving as a primary transfer unit, is disposed on the inner peripheral surface side of the intermediate transfer belt 7 corresponding to each photosensitive drum 1. The primary transfer roller 5 is urged toward the photosensitive drum 1 via the intermediate transfer belt 7 to form a primary transfer portion (primary transfer nip) T1 where the photosensitive drum 1 and the intermediate transfer belt 7 are in contact with each other. . As described above, the toner image formed on the photosensitive drum 1 is primarily transferred onto the rotating intermediate transfer belt 7 in the primary transfer portion T1. During the primary transfer process, the primary transfer roller 5 is controlled by a primary transfer power supply (high voltage power supply circuit) 41 at a constant voltage having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment). A primary transfer voltage (primary transfer bias) which is a DC voltage is applied. For example, when forming a full-color image, the toner images of each color Y, M, C, and K formed on each photosensitive drum 1 are superimposed on the intermediate transfer belt 7 in each primary transfer portion T1. The primary transfer is performed sequentially.

  On the outer peripheral surface side of the intermediate transfer belt 7, a secondary transfer roller 8 that is a roller-type secondary transfer member as a secondary transfer unit is disposed at a position facing the secondary transfer counter roller 21. The secondary transfer roller 8 is urged toward the secondary transfer counter roller 21 via the intermediate transfer belt 7, and a secondary transfer portion (secondary transfer nip) where the intermediate transfer belt 7 and the secondary transfer roller 8 come into contact with each other. ) T2 is formed. As described above, the toner image formed on the intermediate transfer belt 7 is a recording material such as paper (recording medium) conveyed between the intermediate transfer belt 7 and the secondary transfer roller 8 in the secondary transfer portion T2. , Sheet) P is secondarily transferred onto P. In this embodiment, the secondary transfer roller 8 has a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) by the secondary transfer power supply (high voltage power supply circuit) 42 in the secondary transfer process. A secondary transfer voltage (secondary transfer bias), which is a DC voltage under constant voltage control, is applied. For example, a secondary transfer voltage of +1 to +7 kV is applied and a secondary transfer current of +40 to +120 μA is passed, whereby the toner image on the intermediate transfer belt 7 is secondarily transferred onto the recording material P.

  The recording material P is sent out one by one from a recording material container (not shown) by a pickup roller (not shown) and is conveyed by a pair of conveying rollers (not shown). Thereafter, the recording material P is conveyed by the registration roller (registration roller pair) 9 to the secondary transfer portion T2 in timing with the toner image on the intermediate transfer belt 7.

  The recording material P to which the toner image has been transferred is conveyed to a fixing device 11 as a fixing unit by a pre-fixing conveying device 10 or the like. The fixing device 11 fixes (melts and adheres) the toner image to the recording material P by heating and pressing the recording material P carrying the unfixed toner image. The recording material P on which the toner image is fixed is discharged (output) to the outside of the main body of the image forming apparatus 100 by a discharge roller pair (not shown) or the like.

  In addition, toner (primary transfer residual toner) that is not transferred onto the intermediate transfer belt 7 and remains on the photosensitive drum 1 during the primary transfer process is removed from the photosensitive drum 1 by the drum cleaning device 6 as a photosensitive member cleaning unit. Removed and recovered. Further, on the outer peripheral surface side of the intermediate transfer belt 7, the intermediate transfer member is located downstream of the secondary transfer portion T 2 in the rotation direction of the intermediate transfer belt 7 and downstream of the primary transfer portion T 1 (the most upstream primary transfer portion T 1 Y). A belt cleaning device 30 is disposed as a cleaning means. The toner remaining on the intermediate transfer belt 7 without being transferred to the recording material P during the secondary transfer process (secondary transfer residual toner) is removed from the intermediate transfer belt 7 by the belt cleaning device 30 and collected. In this embodiment, the belt cleaning device 30 electrostatically collects secondary transfer residual toner on the intermediate transfer belt 7. The belt cleaning device 30 will be described in more detail later.

In this embodiment, the intermediate transfer belt 7 includes a base layer (back layer), an elastic layer (intermediate layer), and a surface layer. The base layer is formed so as to have a thickness of 0.05 to 0.15 mm using a material in which an appropriate amount of carbon black is contained as an antistatic agent in resins such as polyimide and polycarbonate, or various rubbers. The elastic layer is formed so as to have a thickness of 0.1 to 0.500 mm using a material in which an appropriate amount of an ionic conductive agent is contained in various rubbers such as CR rubber, urethane rubber, and silicone rubber. The surface layer is formed of a resin such as urethane resin or fluororesin so that the thickness is 0.0002 to 0.020 mm. In this embodiment, the volume resistivity of the intermediate transfer belt 7 is ^{5 × 10 8 ~1 × 10 14} Ω · cm (23 ℃, RH 50%) is. In this embodiment, the intermediate transfer belt 7 has an MD1 hardness of 60 to 85 ° (23 ° C., 50% RH). In this embodiment, the static friction coefficient of the intermediate transfer belt 7 is 0.15 to 0.6 (23 ° C., 50% RH, type 94i manufactured by HEIDON). However, the intermediate transfer belt 7 may not be composed of a plurality of layers. For example, it is a single layer having a thickness of 0.05 to 0.15 mm made of a material containing an appropriate amount of carbon black as an antistatic agent in resins such as polyimide and polycarbonate, or various rubbers. Also good.

In the present embodiment, the primary transfer roller 5 includes a cored bar (base) and an elastic layer formed of an ion conductive foamed rubber on the outer periphery of the cored bar. In this embodiment, the outer diameter of the primary transfer roller 5 is 15 to 20 mm, and the electrical resistance value of the primary transfer roller 5 is N / N (23 ° C., 50% RH) and measured by applying 2 kV to 1 ×. it is a 10 ⁵ ~1 × 10 ⁸ Ω.

In the present embodiment, the secondary transfer roller 8 includes a cored bar (base) and an elastic layer formed of ion conductive foam rubber on the outer periphery of the cored bar. In this embodiment, the outer diameter of the secondary transfer roller 8 is 20 to 25 mm, and the electrical resistance value of the secondary transfer roller 8 is N / N (23 ° C., 50% RH) measured by applying 2 kV to 1 ×. 10 ⁵ to 1 × 10 ⁸ Ω.

In the present embodiment, the secondary transfer counter roller 21 includes a cored bar (base) and an elastic layer formed of electronically conductive rubber on the outer periphery of the cored bar. In this embodiment, the secondary transfer counter roller 21 has an outer diameter of 20 to 22 mm, an electrical resistance value of N / N (23 ° C., 50% RH), and measurement is performed by applying 50 ^V. 1 × 10 ⁵ to 1 × is 10 ⁸ Ω. In this embodiment, the elastic layer of the secondary transfer counter roller 21 has a thickness of 1.5 to 2.5 mm and a hardness of Asker C hardness of 60 to 80 °. The secondary transfer counter roller 21 is electrically grounded (connected to the ground).

  Further, the image forming apparatus 100 is provided with a toner sensor 12 composed of an optical sensor as toner detection means for detecting a test pattern (detection image, patch image) formed on the intermediate transfer belt 7. . In this embodiment, the toner sensor 12 is on the intermediate transfer belt 7 downstream of the primary transfer portion T1 (the most downstream primary transfer portion T1K) and upstream of the secondary transfer portion T2 in the rotation direction of the intermediate transfer belt 7. It is arranged so that toner can be detected. In the present embodiment, the toner sensor 12 is disposed at a position facing the second idler roller 24 with the intermediate transfer belt 7 interposed therebetween. In this embodiment, a test pattern for correcting image density and a test pattern for correcting color misregistration are formed on the intermediate transfer belt 7 at a predetermined timing, and this test pattern is detected by the toner sensor 12. Then, based on the detection result, the image forming conditions are adjusted to correct the image density, and the image forming timing is adjusted to correct the color misregistration. In this embodiment, when the test pattern passes through the secondary transfer portion T2, the secondary transfer roller 8 is separated from the intermediate transfer belt 7 by a separation mechanism (not shown) as a separation means. As a result, the test pattern is conveyed to the belt cleaning device 30 as untransferred toner.

2. Belt cleaning device (electrostatic cleaning device)
FIG. 2 is a schematic cross-sectional view of the vicinity of the belt cleaning device 30 in the present embodiment. The belt cleaning device 30 in this embodiment is composed of an electrostatic brush cleaning device which is an electrostatic cleaning device. Note that “upstream” and “downstream” with respect to the belt cleaning device 30 and its elements mean upstream and downstream in the rotation direction of the intermediate transfer belt 7, even if not specifically mentioned.

  The belt cleaning device 30 is disposed on the outer peripheral surface side of the intermediate transfer belt 7 at a position facing the drive roller 22 and a first housing 31a disposed downstream of the secondary transfer portion T2 and upstream of the drive roller 22. And a second housing 31b. An upstream cleaning brush (hereinafter also simply referred to as “upstream brush”) 32a, an upstream recovery roller 33a, and an upstream cleaning blade 34a are provided in the first housing 31a. An intermediate cleaning brush (hereinafter also simply referred to as “intermediate brush”) 32b, an intermediate recovery roller 33b, and an intermediate cleaning blade 34b are provided in the second housing 31b. Further, a downstream cleaning brush (hereinafter also simply referred to as “downstream brush”) 32c, a downstream recovery roller 33c, and a downstream cleaning blade 34c are provided in the second housing 31b. A backup roller 27 is disposed at a position facing the upstream brush 32 a via the intermediate transfer belt 7, and the upstream brush 32 a is urged toward the backup roller 27 via the intermediate transfer belt 7. The backup roller 27 is electrically grounded. Further, the backup roller 27 is driven to rotate as the intermediate transfer belt 7 rotates. The contact portion between the upstream brush 32a and the intermediate transfer belt 7 is an upstream cleaning portion (cleaning nip) Fa where the surface of the intermediate transfer belt 7 is cleaned by the upstream brush 32a. Further, the intermediate brush 32 b and the downstream brush 32 c are urged toward the driving roller 22 via the intermediate transfer belt 7. A contact portion between the intermediate brush 32b and the intermediate transfer belt 7 is an intermediate cleaning portion (cleaning nip) Fb where the surface of the intermediate transfer belt 7 is cleaned by the intermediate brush 32b. A contact portion between the downstream brush 32c and the intermediate transfer belt 7 is a downstream cleaning portion (cleaning nip) Fc in which the surface of the intermediate transfer belt 7 is cleaned by the downstream brush 32c. In this embodiment, the plurality of cleaning brushes, the plurality of collection rollers, and the plurality of cleaning blades have the same configuration. Therefore, when the cleaning brush, the collection roller, and the cleaning blade are generally described, a, b, and c at the end of the reference numerals indicating whether they are upstream, intermediate, or downstream are omitted.

The cleaning brush 32 as a cleaning member is composed of a rotatable roller-like conductive fur brush (fur brush roller). The brush fibers of the cleaning brush 32 are composed of carbon dispersed nylon fibers, acrylic fibers or polyester fibers having a volume resistivity of 3 × 10 ⁵ to 1 × 10 ¹³ Ω · cm and a fiber thickness of 2 to 15 denier. ing. And this brush fiber is flocked on a metal roller at a rate of flocking density of 50,000 to 500,000 / inch ² to form a cleaning brush 32. The cleaning brush 32 is disposed so as to maintain an intrusion amount of about 1.0 to 2.0 mm with respect to the intermediate transfer belt 7. The cleaning brush 32 is rotationally driven by a drive motor (not shown) in the direction of the arrow in the figure (a direction moving in the direction opposite to the moving direction of the surface of the intermediate transfer belt 7 at the portion facing the intermediate transfer belt 7). The The cleaning brush 32 is rotationally driven at a peripheral speed of 20 to 80% of the moving speed (conveying speed) of the surface of the intermediate transfer belt 7. As a result, the cleaning brush 32 rubs the surface of the intermediate transfer belt 7.

  The collection roller 33 as a collection member is composed of a rotatable metal roller formed of a metal such as aluminum. The collection roller 33 is disposed so as to maintain an intrusion amount of about 1.5 to 2.5 mm with respect to the cleaning brush 32. The collection roller 33 is rotationally driven by a drive motor (not shown) in the direction of the arrow in the figure (the direction in which the cleaning brush 32 moves in the forward direction at the portion facing the cleaning brush 32). The collection roller 34 is driven to rotate at a peripheral speed equivalent to the peripheral speed of the cleaning brush 32.

  The cleaning blade 34 as a scraping member is disposed in contact with the collection roller 34. The cleaning blade 34 is a plate-like member made of an elastic material such as urethane rubber. The cleaning blade 34 has a thickness of 1.6 to 2.2 mm, an IRHD hardness of 70 to 78 ° (23 ° C., 50% RH), and an intrusion of about 0.5 to 2.0 mm with respect to the collection roller 34. Arranged to keep the amount.

  In this embodiment, the outer diameter of the cleaning brush 32 is 18 mm, and the outer diameter of the collection roller 33 is 13 mm.

  The upstream brush 32 positioned at the uppermost stream in the rotation direction of the intermediate transfer belt 7 has a DC voltage that is positively constant-current controlled by an upstream cleaning power supply (high voltage power supply circuit) 43a via an upstream recovery roller 33. A cleaning voltage is applied. As an example, a cleaning voltage is applied to the upstream brush 32a so that a cleaning current of +20 μA flows.

  The intermediate brush 32b located in the middle of the three cleaning brushes 32 has a cleaning which is a DC voltage controlled by a negative constant current by an intermediate cleaning power supply (high voltage power supply circuit) 43b via an intermediate recovery roller 33b. A voltage is applied. As an example, a cleaning voltage is applied to the intermediate brush 32b so that a cleaning current of −15 μA flows.

  The downstream brush 32c located on the most downstream side in the rotation direction of the intermediate transfer belt 7 has a DC voltage that is positively constant-current controlled by a downstream cleaning power supply (high-voltage power supply circuit) 43c via a downstream collection roller 33c. A cleaning voltage is applied. As an example, a cleaning voltage is applied to the downstream brush 32c so that a cleaning current of +20 μA flows.

  The cleaning current control (setting) will be described in more detail later.

  As described above, by applying a cleaning voltage to the cleaning brush 32, a cleaning electric field suitable for cleaning the secondary transfer residual toner and the test pattern is formed between the intermediate transfer belt 7 and the cleaning brush 32. As a result, the secondary transfer residual toner and the test pattern adhering to the surface of the intermediate transfer belt 7 are attracted to the cleaning brush 32 and removed from the surface of the intermediate transfer belt 7. The toner adsorbed to the cleaning brush 32 is further transferred from the cleaning brush 32 to the surface of the collecting roller 33 by an electric field, and is scraped off from the surface of the collecting roller 33 by the cleaning blade 34. Thus, the toner accommodated in the first and second casings 31a and 31b is conveyed to a recovery toner container (not shown) and recovered.

  In the present embodiment, a positive cleaning voltage is applied to the upstream cleaning portion Fa in most of the secondary transfer residual toner and the toner charged to the regular charge polarity (negative polarity in the present embodiment) in the test pattern. It is collected by the upstream brush 32a. Toner that has passed through the upstream cleaning section Fa (toner having a relatively small negative charge amount or toner charged to positive polarity) is transferred by the intermediate brush 32b to which a negative cleaning voltage is applied in the intermediate cleaning section Fb. To be recovered. The toner that has also passed through the intermediate cleaning unit Fb (often negatively aligned by discharge or charge injection in the intermediate cleaning unit Fb) is applied to the downstream brush 32c to which the positive cleaning voltage is applied in the downstream cleaning unit Fc. It is collected by.

3. Control Mode FIG. 3 is a schematic block diagram showing a control mode of the main part of the image forming apparatus 100 of the present embodiment. The control unit 50 includes a CPU 51 as a control unit that is a central element that performs arithmetic processing, a memory (storage medium) such as a RAM 52 and a ROM 53 as storage units, and the like. A RAM 52, which is a rewritable memory, stores information input to the control unit 50, detected information, calculation results, and the like, and a ROM 53 stores a control program, a previously obtained data table, and the like. The CPU 51 and the memory can transfer and read data from each other.

  An external device 200 such as an image reading device (not shown) provided in the image forming apparatus 100 or a personal computer is connected to the control unit 50. An operation unit (operation panel) 150 provided in the image forming apparatus 100 is connected to the control unit 50. The operation unit 150 includes a display unit that displays various types of information to an operator such as a user or a service person under the control of the control unit 50, and an input unit that allows the operator to input various settings relating to image formation to the control unit 50. And is configured.

  In the present embodiment, the control unit 50 is connected to an upstream cleaning power source 43a, an intermediate cleaning power source 43b, and a downstream cleaning power source 43c. In the present embodiment, the cleaning power sources 43a, 43b, and 43c apply a cleaning voltage, which is a direct current controlled DC voltage, to the brushes 32a, 32b, and 32c via the recovery rollers 33a, 33b, and 33c, respectively. .

  The environmental sensor 13 is connected to the control unit 50. The environmental sensor 13 detects the temperature and humidity in the housing of the image forming apparatus 100. Information on the temperature and humidity detected by the environmental sensor 13 is input to the control unit 50. The environment sensor 13 is an example of an environment detection unit that detects at least one of temperature and humidity inside or outside the image forming apparatus 100.

  Based on image information from the image reading device or the external device 200 and control commands from the operation unit 150 or the external device 200, the control unit 50 comprehensively controls each unit of the image forming apparatus 100 to execute an image forming operation. Let Further, the control unit 50 forms a test pattern on the intermediate transfer belt 7 at a predetermined timing, detects the test pattern with the toner sensor 12, and executes a test mode in which image density correction and color misregistration correction are performed. Further, as will be described in detail later, the control unit 50 executes control for changing the image formation start timing.

  Here, the image forming apparatus 100 executes a job (printing operation), which is a series of operations for forming and outputting an image on a single or a plurality of recording materials P, which is started by one start instruction (printing instruction). To do. In general, a job includes an image forming process, a pre-rotating process, a paper-to-paper process when forming images on a plurality of recording materials P, and a post-rotating process. The image forming process is a period in which electrostatic image formation, toner image formation, toner image primary transfer, and secondary transfer are performed on the recording material P and output. (Formation period) refers to this period. More specifically, the timing at which the image is formed differs depending on the position at which the electrostatic image formation, toner image formation, toner image primary transfer, and secondary transfer steps are performed. The pre-rotation process is a period for performing a preparatory operation before the image forming process from when the start instruction is input until the actual image formation is started. The inter-sheet process is a period corresponding to the interval between the recording material P and the recording material P when image formation is continuously performed on a plurality of recording materials P (continuous image formation). The post-rotation process is a period during which an organizing operation (preparation operation) after the image forming process is performed. The non-image forming period (non-image forming period) is a period other than the image forming period, and is from the above-mentioned pre-rotation process, paper-to-paper process, post-rotation process, and when the image forming apparatus 100 is turned on or in the sleep state. A pre-multi-rotation process that is a preparatory operation at the time of return is included. In this embodiment, image density correction control is executed during non-image formation.

4). Image Density Correction Control In the image forming apparatus 100, the image density may fluctuate due to changes in the environment and time. Therefore, the image forming apparatus 100 according to the present embodiment performs image density correction control for the maximum density (solid black image) in the post-rotation after the Nth image formation is completed during the continuous image forming operation.

  The image density correction control of the developing unit in this embodiment will be described with reference to FIGS. FIG. 4 is a schematic diagram showing the arrangement of test patterns for image density correction. FIG. 5A is a flowchart of image density correction control. FIG. 5B is a graph for explaining the relationship between the signal value of the image density correction control and the potential.

  When the image density correction control is executed, the secondary transfer roller 8 is separated from the intermediate transfer belt 7. Then, as shown in FIG. 5B, a test pattern 77 including pattern elements of densities N1 and N2 is created with the following development contrast V1 and development contrast V2 (S301). The development contrast V1 is a development contrast obtained by reducing 40 V with respect to the development contrast V0 corresponding to the target density stored in the ROM 53 in the previous control. Further, the development contrast V2 is a development contrast obtained by increasing the development contrast V0 corresponding to the target density by 40V. As shown in FIG. 4, the test pattern 77 is formed in a 25 mm × 25 mm square on the photosensitive drum 1 and is primarily transferred onto the intermediate transfer belt 7. The test pattern 77 is formed at a position within the maximum image forming width in a direction substantially orthogonal to the moving direction of the intermediate transfer belt 7. Output values Q1 and Q2 corresponding to the optical densities of the pattern elements of density N1 and N2 are obtained by a light emitting element and a light receiving element (not shown) arranged in the toner sensor 12.

  The output value of the toner sensor 12 is input to the control unit 50 and temporarily stored in the RAM 52 (S302). In this embodiment, data that has been set in advance such that Q = 0 when the image density measured using an X-rite reflection densitometer is 0 and Q = 255 when the image density is 2.0 is stored in the ROM 53 in advance. Stored in The control unit 50 calculates the test pattern 77 from the detection signal values Q1 and Q2 detected by the toner sensor 12 by linearly complementing the development contrast Vt corresponding to the target density signal value Qt at the target density (S303). ).

  For example, it is assumed that the target maximum density is 1.2 and the current development contrast corresponding to this is 200V. Further, when the image forming apparatus 100 is an apparatus in which the density near the solid portion changes by about 0.1 when the development contrast changes by 20V, the pattern image is output at the current contrast of ± 40V. That is, an image pattern is formed with a development contrast of 160 V (V1) with a density of about 1.0 (N1) and a development contrast of 240 V (V2) with a density of about 1.4 (N2), and is detected by the toner sensor 12. . If the detection signals at this time are Q1 = 120 and Q2 = 200, a new development contrast Vt = 193V is determined by linear interpolation from the two detected signal values, and stored in the ROM 53 in the previous control. Further, the value of the development contrast V0 is changed (S304).

  Thereafter, the test pattern 77 is cleaned by the belt cleaning device 30 in an untransferred state, and the image density correction control of the developing unit is completed.

  Next, image density correction control of the image forming unit in the present embodiment will be described with reference to FIGS. 3 and 6 to 8.

  FIG. 6 is a schematic diagram illustrating an example of a test pattern for image density correction. Here, a method of forming the maximum density pattern in each step will be described. First, a preset reference contrast potential Vcont0 is obtained based on the amount of moisture in the air in the image forming apparatus 100 obtained from the output of the environment sensor 13. Here, as shown in FIG. 7A, it is assumed that a contrast potential corresponding to the absolute water content is set in advance. As shown in FIG. 7B, the contrast potential Vcont is a differential voltage between the developing bias Vdc and the surface potential Vl of the exposed photosensitive drum 1, and the maximum density increases as the Vcont increases. ing.

Next, a test pattern including a toner patch in each step is formed with a certain potential width (25 V in this embodiment) around the set reference contrast potential Vcont0. Here, the toner patches 61P to 65P in FIG. 6 will be described as an example. In this embodiment,
61P: Vcont0Y + 50V
62P: Vcont0Y + 25V
63P: Vcont0Y
64P: Vcont0Y-25V
65P: Vcont0Y-50V
A 5-step toner patch having a contrast potential of 5 is formed at a maximum signal value of 255 level.

  Next, the image on which the maximum density test pattern is formed is primarily transferred onto the intermediate transfer belt 7. An output value corresponding to the toner optical density of each of the toner patches 61P to 65P of the maximum density test pattern is obtained by a light emitting element and a light receiving element (not shown) arranged in the toner sensor 12. The controller 50 reads the output value of the toner sensor 12 and captures it as density data.

  Next, the control unit 50 calculates an optimum contrast potential that provides a desired maximum density from the read density data. An example of a method for calculating the optimum contrast potential will be described with reference to FIG. First, assuming that the density data obtained by reading the toner patches 61P to 65P having the maximum density pattern shown in FIG. 6 are 101K to 105K, respectively, the contrast potential VcontK that achieves a desired density from a straight line obtained by linearly approximating them. Is calculated. Then, in the control unit 50, the charging potential of the photosensitive drum 1 and the exposure amount of the exposure device 3 are set so that an optimum contrast potential having a desired maximum density is obtained.

5. Cleaning Current Control Next, cleaning current control (setting) in the present embodiment will be described. Here, image density correction control is taken as an example of the test mode. Further, the operation of the belt cleaning device 30 will be described by taking the most downstream one (downstream brush 32c and related elements thereof) as an example. Are omitted as appropriate.

  The optimum cleaning current value for cleaning the secondary transfer residual toner is different from the optimum cleaning current value for cleaning the test pattern for image density correction. This is because the secondary transfer residual toner and the test pattern have different conditions such as the weight of toner per unit area and the amount of charged charge per unit weight of toner, and the optimum electric field for cleaning each toner. This is because they are different.

  That is, since the test pattern has a large amount of toner unlike the secondary transfer residual toner, the absolute value of the cleaning current necessary for electrostatically sufficiently collecting the test pattern by the cleaning brush 32 is the secondary transfer residual toner. It becomes larger than the case of toner.

  Further, FIG. 9 shows the amount of toner (cleaning residual toner) on the intermediate transfer belt 7 after passing through the belt cleaning device 30 (cleaning unit F) and the cleaning current in the secondary transfer residual toner and the test pattern. FIG. The secondary transfer residual toner is affected by discharge at the secondary transfer portion T2, and the absolute value of the charge amount is reduced. On the other hand, since the toner of the test pattern is not affected by the discharge at the secondary transfer portion T2, the absolute value of the charge amount does not decrease. In this example, the charge amount of the secondary transfer residual toner was −40 (μC / g), and the charge amount of the test pattern toner was −50 (μC / g).

  As shown in FIG. 9, the cleaning residual toner amount decreases as the cleaning current value increases, and when the cleaning current value increases above a certain cleaning current value, the cleaning residual toner amount increases. This is because, under a condition smaller than a certain cleaning current value, as the cleaning current value increases, the force that causes the toner to move by receiving electrostatic force from the intermediate transfer belt 7 to the cleaning brush 32 increases. On the other hand, under a condition where the cleaning current value is larger than a certain cleaning current value, the toner charge amount distribution changes due to the influence of discharge in the cleaning portion F as the cleaning current value increases. Therefore, the amount of toner that moves from the intermediate transfer belt 7 to the cleaning brush 32 by receiving an electrostatic force is reduced. As shown in FIG. 9, the cleaning current value at which the cleaning residual toner is minimized is -40 (μC / g) in the case of the test pattern toner having the charge amount of −50 (μC / g). This is larger than that of the secondary transfer residual toner. This is because as the absolute value of the charge amount of the toner increases, the electric field required for moving the toner from the intermediate transfer belt 7 to the cleaning brush 32 increases.

  Therefore, the image forming apparatus 100 according to the present exemplary embodiment performs control to change the cleaning current value in order to satisfactorily clean the secondary transfer residual toner and the test pattern having different toner charge amounts.

FIG. 10 is a graph for explaining the control of the cleaning current in this embodiment. As shown in FIG. 10, when the secondary transfer residual toner passes through the belt cleaning device 30 (cleaning unit F), the target value of the cleaning current supplied to the cleaning brush 32 via the recovery roller 33 is set to the first target. The current value is set to _Ia1 . On the other hand, when the test pattern passes through the belt cleaning device 30 (cleaning unit F), the target value of the cleaning current supplied to the cleaning brush 32 via the recovery roller 33 is set to the second target current value _Ia2 . The second target current value I _a2 has a larger absolute value than the first target current value I _a1 . In the present embodiment, the first target current value I _a1 is 30 μA, the second target current value I _a2 is 60 μA, the time during which the test pattern passes the cleaning unit F (the cleaning current target value is set to the second target current value I _a2) . Set time) was set to 650 msec.

  By performing such a control and executing a test mode between sheets during continuous image formation, belt cleaning corresponding to the respective conditions of the secondary transfer residual toner and the test pattern while suppressing fluctuations in image density The good cleaning ability of the device 30 can be maintained.

  In this embodiment, the cleaning voltage applied to the cleaning brush 32 is controlled at a constant current. Alternatively, the cleaning voltage applied to the cleaning brush 32 may be controlled at a constant voltage. In this case, the cleaning voltage of the first target voltage value is applied by constant voltage control during the period corresponding to the first target current value in this embodiment. Further, a cleaning voltage having a second target voltage value having an absolute value larger than the first target voltage value is applied during a period corresponding to the second target current value in the present embodiment. Further, the target current value (or target voltage value) of the cleaning voltage is not limited to two types, but may be set to three or more types. Further, for example, the target current value (or target voltage value) of the cleaning voltage may be changed according to the information acquired by detecting the toner image (test pattern or the like) on the intermediate transfer belt 7. Typically, the absolute value of the target current value (or target voltage value) of the cleaning voltage may be increased as the test pattern having a larger amount of toner per unit area is sent to the belt cleaning device 30.

6). Relationship between the cleaning current and the charge amount of the intermediate transfer belt As described above, when the cleaning current value supplied to the cleaning brush 32 via the recovery roller 33 is changed during continuous image formation, the recovery roller 33 changes to the cleaning brush 34. The moving charge density changes. Then, the charge density moving from the cleaning brush 32 to the intermediate transfer belt 7 changes, whereby the charge amount of the intermediate transfer belt 7 changes.

  FIG. 11A is a graph showing the relationship between the cleaning current value and the charging potential of the intermediate transfer belt 7 after passing through the belt cleaning device 30 (cleaning unit F). The charging potential of the intermediate transfer belt 7 is a charging potential at a position where the photosensitive drum 1Y of the first image forming unit UY and the intermediate transfer belt 7 are in contact (primary transfer unit T1Y). In the region of the intermediate transfer belt 7 where the cleaning current value when passing through the cleaning portion F is 30 μA, the charging potential of the intermediate transfer belt 7 was zero. This is because the electric charge moved from the cleaning brush 32 to the intermediate transfer belt 7 is sufficiently attenuated during the movement from the cleaning unit F to the primary transfer unit T1Y. On the other hand, in the region of the intermediate transfer belt 7 where the cleaning current value when passing through the cleaning unit F is 60 μA, the charging potential of the intermediate transfer belt 7 was 1000V. This is because the electric charge moved from the cleaning brush 32 to the intermediate transfer belt 7 during the movement from the cleaning unit F to the primary transfer unit T1Y was not sufficiently attenuated.

7. Relationship between the charge amount of the intermediate transfer belt and the primary transfer current When the charging potential of the intermediate transfer belt 7 changes, the primary transfer efficiency is changed by the change in the primary transfer current in the primary transfer portion T1Y of the first image forming unit UY. Changes, causing uneven image density.

  FIG. 11B is a graph showing the relationship between the cleaning current value and the primary transfer current in the primary transfer portion T1Y of the first image forming portion UY. As described above, the intermediate transfer belt 7 receives a positive charge from the cleaning brush 32 when passing through the cleaning unit F. The region of the intermediate transfer belt 7 that has passed through the cleaning portion F under the condition of the cleaning current of 60 μA passes through the primary transfer portion T1Y in a state where the positive charge is not sufficiently attenuated. This region of the intermediate transfer belt 7 is affected by the electric field at the primary transfer portion T1Y, and moves positive charges that are not attenuated from the intermediate transfer belt 7 to the photosensitive drum 1Y. For this reason, when the region of the intermediate transfer belt 7 passes through the primary transfer portion T1Y, the region of the intermediate transfer belt 7 in which the positive charge is sufficiently attenuated passes through the primary transfer portion T1Y. The next transfer current increases.

8. Effect of Primary Transfer Current and Image Density Unevenness FIG. 12 shows the relationship between the primary transfer electricity in the primary transfer portion T1Y of the first image forming portion UY and the amount of primary transfer residual toner on the photosensitive drum 1Y. Show. The primary transfer residual toner amount on the photosensitive drum 1Y decreases as the primary transfer current value increases to a certain primary transfer current value at which the primary transfer residual toner amount becomes the minimum value. Tend. The primary transfer residual toner amount tends to increase as the primary transfer current value increases from a certain primary transfer current value at which the primary transfer residual toner amount becomes the minimum value. The reason why the primary transfer residual toner amount changes in this way is considered as follows. First, in a region where the primary transfer current value is smaller than the certain primary transfer current value, the amount by which the toner on the photosensitive drum 1Y moves onto the intermediate transfer belt 7 increases as the electric field at the primary transfer portion T1Y increases. It is to do. Further, in a region where the primary transfer current value is larger than the certain primary transfer current value, the discharge between the photosensitive drum 1Y and the intermediate transfer belt 7 becomes stronger as the primary transfer current increases. This is because the charging polarity of the toner on the photosensitive drum 1 changes, and the toner is not transferred from the photosensitive drum 1 to the intermediate transfer belt 7.

  In this embodiment, in order to appropriately move the toner from the photosensitive drum 1 to the intermediate transfer belt 7, the target value (target transfer current value) of the primary transfer current is set to 70 μA. However, as shown in FIG. 11B, when the cleaning current value is changed from 30 μA to 60 μA during continuous image formation, the primary transfer current value is changed from 70 μA to 80 μA.

  As shown in FIG. 12, when the primary transfer current value changes from 70 μA to 80 μA, the amount of primary transfer residual toner on the photosensitive drum 1 changes. For this reason, when an image is formed at the boundary where the primary transfer current value changes, there is an increased risk of in-plane unevenness of image density (image density unevenness).

9. Change in image formation start timing The image forming apparatus 100 according to the present embodiment forms an image while avoiding the region of the intermediate transfer belt 7 in which the charging potential is changed under the influence of the cleaning current during continuous image formation. Control to change the image formation start timing is performed.

  FIG. 13 is a flowchart of control for changing the image formation start timing in this embodiment. FIG. 14 is a chart showing an example of changing the image formation start timing.

  When the job is started (S101) and image formation is started (S102), the control unit 50 applies a cleaning voltage to the cleaning brush 32 (S103). Then, after a predetermined number of images are formed during continuous image formation, a test pattern for image density correction is formed (S104).

Here, when the image formation start timing is not controlled, the time for the region of the intermediate transfer belt 7 where the test pattern has been cleaned immediately passes the primary transfer portion T1Y and the intermediate transfer belt 7 at the primary transfer portion T1Y. There is a possibility that the time for forming a normal image overlaps. In this case, as described above, the risk of image density unevenness increases. More specifically, the region of the intermediate transfer belt 7 where the test pattern has been cleaned is disposed in the cleaning unit F during a period in which a cleaning voltage controlled at a constant current with the second target current value _Ia2 is applied to the cleaning brush 32. This is the area of the intermediate transfer belt 7 that has passed. Here, this region is also referred to as a “cleaning region”. More specifically, the normal image is a target image of a job to be transferred to the recording material P and output from the image forming apparatus 100. Here, the area of the intermediate transfer belt 7 on which the normal image is formed at the primary transfer portion T1Y is also referred to as “image forming area”. The cleaning voltage controlled at a constant current with the second target current value _Ia2 sufficiently includes a region of the intermediate transfer belt 7 on which a test pattern (which may be one or a plurality of patch-like images) is formed. And applied to the cleaning brush 32 for a sufficient time. Therefore, the toner constituting the test pattern is not always placed on the entire area of the intermediate transfer belt 7 to be the “cleaning area”.

  Therefore, in this embodiment, the control unit 50 determines whether or not the “cleaning area” immediately overlaps with the “image forming area” as described above (S105). If the control unit 50 determines that there is an overlap in S105 (“Yes”), the normal image that should have been formed in the “image formation region” that overlaps the “cleaning region” (as a result, the image) The image formation start timing of the subsequent images is also changed (S106). In other words, the area (space between papers) between the normal image that should have been formed in the “image forming area” that overlaps the “cleaning area” and the normal image immediately before that is widened to avoid the “cleaning area”. The “image forming area” in which the normal image is formed is included in. Then, image formation is resumed at the changed image formation start timing (S108). On the other hand, if the control unit 50 determines that there is no overlap in S105 (“No”), the normal image following the test pattern is formed at a predetermined interval (between sheets) without changing the image formation start timing. Then, image formation is resumed (S107). Thereafter, when all the images of the job are formed, the control unit 50 ends the job (S109).

  The process (S105) for determining whether the “cleaning area” and the “image forming area” overlap will be further described. When forming a test pattern between sheets during continuous image formation, the control unit 50 causes the RAM 52 to store the time for forming the test pattern on the intermediate transfer belt 7 (0 msec to 650 msec in FIG. 14). Further, the control unit 50 causes the RAM 52 to store the time (3250 msec to 3900 msec in FIG. 14) for the “cleaning area” to pass through the primary transfer unit T1Y again. More specifically, the time for forming the test pattern on the intermediate transfer belt 7 passes through the primary transfer portion T1Y before the region of the intermediate transfer belt 7 to be the “cleaning region” is conveyed to the cleaning portion F. It is time to do.

  Further, the control unit 50 resumes the formation of the normal image after forming the test pattern. Then, the control unit 50 performs the following when starting the formation of the electrostatic image of the normal image formed after the restart. That is, the RAM 52 stores the timing (T_start) in FIG. 14 for starting transfer of the normal image corresponding to each electrostatic image onto the intermediate transfer belt 7 and the timing (T_end in FIG. 14) for ending.

Then, the control unit 50 determines whether the “image forming region” overlaps with the “cleaning region” based on the timings T_start and T_end and the time when the “cleaning region” passes immediately after the primary transfer unit T1Y. to decide. Specifically, in this embodiment,
T_start <3900, and
T_end> 3250
When the above condition is satisfied, the “image forming area” and the “cleaning area” overlap. That is, the time required for the “cleaning area” to pass immediately after the primary transfer portion T1Y overlaps the time required for transferring the normal image to the intermediate transfer belt 7 at the primary transfer portion T1Y. Accordingly, when the control unit 50 determines that the timing T_start and T_end of the normal image satisfy the inequality condition when starting to form the electrostatic image of the normal image after the restart, The image formation start timing is changed. In this way, control for changing the time for forming the normal image is performed. Specifically, as described below, control is performed to change the time during which the normal image is formed on the intermediate transfer belt 7 outside the time when the “cleaning area” immediately passes the primary transfer portion T1Y. Do.

Next, the process of changing the image formation start timing (S106) will be further described. In this example,
T_start> 3900, or
T_end <3250
If the image formation start timing is changed to satisfy the above condition, the “image formation area” and the “cleaning area” do not overlap. Therefore, the control unit 50 changes the time for forming the electrostatic image by changing the timing for starting the formation of the electrostatic image of the normal image so as to satisfy the condition of the inequality. In this embodiment, the time from when the electrostatic image is formed to when the normal image corresponding to the electrostatic image is transferred to the intermediate transfer belt 7 is 258 msec. That is, the time for starting the transfer of the image to the intermediate transfer belt 7 (T_start), the time for starting the formation of the electrostatic image of the image (U_start), and the transfer of the image to the intermediate transfer belt 7 are completed. The time (T_end) and the time (U_end) when the formation of the electrostatic image of the image ends are:
T_start = U_start + 258,
T_end = U_end + 258
It becomes the relationship.

From the above, in the present embodiment, the formation time of the electrostatic image of the normal image is
U_start> 4158 and U_end <3508
Change to meet the conditions.

  By such control, as shown in FIG. 14, the “image forming area” and the “cleaning area” do not overlap. That is, the time for the “cleaning area” to pass immediately after the primary transfer portion T1Y and the time for transferring the normal image to the intermediate transfer belt 7 at the primary transfer portion T1Y do not overlap. This can reduce the risk of in-plane unevenness of image density (image density unevenness) during continuous image formation.

  In the present embodiment, the description has been made by paying attention to one cleaning brush 32 as a representative, but the target current value (or target voltage value) of the cleaning voltage to be applied to all or some of the plurality of cleaning brushes. ) Can be changed. Then, the image forming timing can be changed so that each “cleaning region” and “image forming region” do not overlap.

  As described above, in the image forming apparatus 100 according to the present exemplary embodiment, the image forming unit U that forms a toner image, and the toner image formed by the image forming unit U are transferred by the primary transfer unit T1. And a rotatable intermediate transfer member 7 that is conveyed for transfer to the recording material P at the next transfer portion T2. Further, the image forming apparatus 100 contacts the intermediate transfer body 7 at the cleaning section F downstream from the secondary transfer section T2 and upstream from the primary transfer section T1 in the rotation direction of the intermediate transfer body 7, and a voltage is applied to the intermediate transfer body 7. A cleaning member 32 that collects toner on the transfer body is provided. Further, the image forming apparatus 100 causes the cleaning member 32 to have a first voltage whose current or voltage target value is the first value and a second value whose absolute value of the current or voltage target value is larger than the first value. A power supply 43 capable of selectively applying the second voltage is provided. The image forming apparatus 100 also includes a control unit 50 that can control the timing at which the image forming unit U forms a toner image. The controller 50 switches the voltage applied from the power source 43 to the cleaning member 32 from the first voltage to the second voltage over a predetermined period during execution of continuous image formation for transferring a plurality of toner images to a plurality of recording materials P, respectively. If so, do the following: That is, the toner image transferred to the recording material P is prevented from being transferred to the area before the area of the intermediate transfer body 7 that has passed the cleaning section F during the predetermined period reaches the cleaning section F next time. In this embodiment, the region of the intermediate transfer member that passes through the cleaning unit during the predetermined period is formed by the image forming unit U, transferred to the intermediate transfer member 7 by the primary transfer unit T1, and then the secondary transfer unit. At T2, a test pattern is formed which is not transferred to the recording material P but is conveyed to the cleaning unit F. In this embodiment, the test pattern is a test pattern for image density correction.

  As described above, according to the present embodiment, image formation is performed while avoiding the region where the charge amount of the intermediate transfer belt 7 has changed due to the change in the current flowing through the intermediate transfer belt 7 in the cleaning unit F. As a result, it is possible to suppress the occurrence of image defects due to uneven charging of the intermediate transfer belt 7.

[Example 2]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Accordingly, in the image forming apparatus according to the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals as those in the first embodiment, and detailed description thereof is omitted. To do.

1. Outline of this embodiment In this embodiment, the electrical resistance of the intermediate transfer belt 7 is detected, and when the electrical resistance of the intermediate transfer belt 7 is larger than a predetermined threshold, the image formation start timing similar to that of the first embodiment is changed. Take control.

2. Active Transfer Voltage Control Control Method In the intermediate transfer type image forming apparatus 100, when the image is primarily transferred to the intermediate transfer belt 7, the resistance of the primary transfer roller 5 and the intermediate transfer belt 7 can cope with endurance fluctuations and environmental fluctuations. Thus, it is desired to perform control for setting an appropriate primary transfer voltage value. As such a control method, an Active Transfer Voltage Control method (hereinafter referred to as “ATVC control”) is known.

  In the ATVC control, several types of voltages controlled at a constant voltage are applied from the primary transfer roller 5 to the photosensitive drum 1 charged during non-image formation, and the primary transfer roller 5 and the intermediate transfer are determined from the current value flowing at that time. The electric resistance of the belt 7 is measured. Then, an appropriate primary transfer voltage value is set based on the measurement result. When the toner image is transferred to the intermediate transfer belt 7, the primary transfer voltage set to an appropriate value is applied to the primary transfer roller 5 with constant voltage control.

  A more specific example will be described. Here, one image forming unit U will be described as an example, but all or some of the plurality of image forming units U can perform the same. First, the photosensitive drum 1 is charged by the charging roller 2. Thereafter, as shown in FIG. 15, predetermined several kinds of voltages (V 1, V 2, V 3) are applied to the primary transfer roller 5, and a current is caused to flow to the photosensitive drum 1 side. At this time, current values (I1, I2, I3) corresponding to the voltage values (V1, V2, V3) are detected by a current detection unit 41a (FIG. 3) as current detection means. The current detector 41a is provided in the primary transfer power supply 41, for example. Then, a target transfer voltage value Vtarget (appropriate value) corresponding to the target transfer current value Itarget is calculated from these detection results, and this voltage value is set as a primary transfer voltage value to be applied to the primary transfer roller 5.

  The timing for setting the primary transfer voltage value by the ATVC control is preferably performed immediately before image formation in order to cope with endurance fluctuations and environmental fluctuations of the electrical resistance of the primary transfer roller 5 and the intermediate transfer belt 7. The primary transfer voltage value may be set independently for each color, or may be set by a certain primary transfer portion T1 of one color.

3. FIG. 16A shows the relationship between the number of image formations (number of prints) in the primary transfer portion T1 and the primary transfer voltage value necessary to obtain the target transfer current value during image formation. Indicates. FIG. 16B shows the relationship between the primary transfer voltage value in the primary transfer portion T1 stored in advance in the ROM 53 at 23 ° C. and 50% RH and the electric resistance (volume resistivity) of the intermediate transfer belt 7. FIG.

In this example, the volume resistivity of the intermediate transfer belt 7 at the initial stage of use (at the start of use) was 1 × 10 ¹⁰ Ω · cm (23 ° C., 50% RH). When the volume resistivity of the intermediate transfer belt 7 is the initial value, the target transfer current value of the primary transfer portion T1 is 40 μA, and the primary transfer voltage value necessary to obtain the target transfer current value is 2500V. is there. The electrical resistance of the intermediate transfer belt 7 increases as the number of image formations increases. Accordingly, as shown in FIG. 16A, when the number of times of image formation at the primary transfer portion T1 increases, the electrical resistance of the intermediate transfer belt 7 increases, and 1 required to obtain a target transfer current value of 40 μA. The next transfer voltage value tends to increase. In FIG. 16A, the primary transfer voltage value when the number of image formations is 600,000 is 4000 V, and the volume resistivity of the intermediate transfer belt 7 at that time is 1 × 10 ¹¹ Ω · cm (23 ° C., 50 % RH).

  In the present embodiment, at the time of continuous image formation, the above-described ATVC control is executed every time the number of continuous image formation reaches 20. As a result of executing the ATVC control, a primary transfer voltage value necessary for obtaining a set target transfer current value (40 μA in this embodiment) is obtained. The primary transfer voltage value necessary for obtaining the target transfer current value is stored in the RAM 52.

Also, based on the relationship between the primary transfer voltage value stored in advance in the ROM 53 shown in FIG. 16B and the electrical resistance of the intermediate transfer belt 7, the intermediate transfer belt is determined from the primary transfer voltage value stored in the RAM 52. 7 can be obtained. From FIG. 16B, the primary transfer voltage value when the volume resistivity is 0.5 × 10 ¹⁰ Ω · cm is 2000 V, and 1 when the volume resistivity is 5.0 × 10 ¹¹ Ω · cm. The next transfer voltage value is 5000V. In this embodiment, based on the linear relationship (Y = 1.65 × 10 ¹⁰ X-3.25 × 10 ¹¹ ) between the primary transfer voltage value and the electric resistance of the intermediate transfer belt 7 shown in FIG. Then, the electric resistance of the intermediate transfer belt 7 corresponding to the primary transfer voltage value obtained by the ATVC control is calculated. For example, when the primary transfer voltage value obtained by the ATVC control is 3000 V, the volume resistivity of the intermediate transfer belt 7 is 1.65 × 10 ¹¹ Ω · cm.

4). Determination of Sequence Change of Image Formation Start Timing FIG. 17 is a flowchart of control for changing the image formation start timing in this embodiment.

  When the job is started (S201), the control unit 50 starts ATVC control of the primary transfer unit T1 (S202). Then, as described above, the control unit 50 obtains the electrical resistance of the intermediate transfer belt 7 based on the ATVC control result (S203), and ends the ATVC control (S204). Next, the control unit 50 determines whether or not the obtained electric resistance (volume resistivity) of the intermediate transfer belt 7 is larger than a predetermined threshold value stored in advance in the ROM 53 (S205).

In this embodiment, as will be described in detail later, when the volume resistivity of the intermediate transfer belt 7 is larger than 1.0 × 10 ¹¹ Ω · cm, the image density unevenness caused by the change in the cleaning current (from 40 μA to 60 μA). The occurrence of was confirmed. Therefore, the threshold value in this embodiment, the volume resistivity is set to 1.0 × ^{10 11} Ω · cm.

  If the control unit 50 determines that the value is larger than the threshold value in S205 (“Yes”), the control unit 50 advances the process to S206 and performs the same processes in S206 to S213 as those in S102 to S109 in FIG. Execute the process. On the other hand, if the control unit 50 determines that the value is equal to or less than the threshold value in S205 ("No"), the process proceeds to S211 so that the image formation start timing is not changed.

  FIG. 18 shows the measurement results of the relationship between the time after the intermediate transfer belt 7 passes the cleaning portion F and the surface potential of the intermediate transfer belt 7 in the primary transfer portion T1 under the conditions where the cleaning current is 40 μA and 60 μA. FIG. In this embodiment, the time from when the intermediate transfer belt 7 passes through the cleaning unit F to when it passes through the most upstream primary transfer unit (primary transfer unit T1Y of the first image forming unit UY) is 3.2. [Sec]. This is obtained as a value obtained by dividing the distance (1120 mm) between the cleaning portion F and the primary transfer most upstream portion by the process speed (350 mm / sec) corresponding to the peripheral speed of the intermediate transfer belt 7.

As shown in FIG. 18, when the cleaning current is 60 μA and the electric resistance of the intermediate transfer belt 7 is 1.0 × 10 ¹⁰ Ω · cm, the intermediate transfer belt 7 passes through the primary transfer portion T1Y (at 3.2 sec). The surface potential of the cleaned area of the intermediate transfer belt 7) is 0V. This is because the intermediate transfer belt 7 is charged by receiving the charge at the cleaning unit F, but the charge is attenuated after the intermediate transfer belt 7 passes through the cleaning unit F and passes through the most upstream part of the primary transfer. Because. When the cleaning current is 60 μA and the electric resistance of the intermediate transfer belt is 1.0 × 10 ¹¹ Ω · cm, the intermediate transfer belt 7 is cleaned when the intermediate transfer belt 7 passes through the primary transfer portion T1Y. The surface potential of the region is 1500V. This is because the charge decay rate from when the intermediate transfer belt 7 passes through the cleaning portion F to when it passes through the primary transfer uppermost stream portion decreases.

On the other hand, as shown in FIG. 18, when the cleaning current is 40 μA and the electric resistance of the intermediate transfer belt 7 is 1.0 × 10 ¹¹ Ω · cm, the intermediate transfer belt when the intermediate transfer belt 7 passes through the primary transfer portion T1Y. The surface potential of the cleaned region 7 becomes 0V. That is, when the electric resistance of the intermediate transfer belt 7 is 1.0 × 10 ¹¹ Ω · cm, the intermediate transfer belt 7 passes through the primary transfer portion T1Y depending on whether the cleaning current is 40 μA or 60 μA. The surface potential of the “cleaning area” differs by 1500V. Therefore, when the electric resistance of the intermediate transfer belt 7 is 1.0 × 10 ¹¹ Ω · cm, charging unevenness of the intermediate transfer belt 7 occurs in the primary transfer portion T1Y due to the change in the cleaning current. When uneven charging occurs in the intermediate transfer belt 7, unevenness occurs in the transfer electric field strength at the primary transfer portion T1Y, unevenness occurs in the transfer efficiency of the toner image, and the risk of occurrence of uneven image density increases.

  Table 1 shows the relationship between the surface potential difference of the intermediate transfer belt 7 and the image density unevenness in the primary transfer portion T1. The evaluation standard of the image density unevenness is indicated by ○ (good) when the image density unevenness cannot be visually confirmed, and x (defect) when the image density unevenness is confirmed.

From Table 1, in the configuration of this example, image density unevenness was confirmed when the surface potential difference of the intermediate transfer belt 7 was 1500 V or more. Therefore, in this embodiment, the threshold value of the electrical resistance of the intermediate transfer belt 7 for determining whether or not to perform control to change the image formation start timing is 1.0 × in order to suppress the occurrence of uneven image density. It was set to 10 ¹¹ Ω · cm.

  That is, in the present embodiment, the control unit 50 can selectively execute the following first mode and second mode. In the first mode, the toner image transferred to the recording material P is transferred to the region before the region of the intermediate transfer body 7 that passes through the cleaning unit F reaches the cleaning unit F during a predetermined period in which the second voltage is applied. This mode is not transferred. The second mode is a mode in which the toner image transferred to the recording material P is transferred to the area before the area reaches the cleaning unit F next time. In the present embodiment, the image forming apparatus 100 includes an acquisition unit (current detection unit) 41 a that acquires information related to the electrical resistance of the intermediate transfer body 7. In this embodiment, the control unit 50 selects the first mode and the second mode based on the information. In particular, in this embodiment, the control unit 50 selects the first mode when the electrical resistance of the intermediate transfer member 7 indicated by the information is smaller than a predetermined threshold value. The second mode is selected when the electrical resistance of the intermediate transfer member 7 indicated by the information is larger than the threshold value. In this embodiment, information on the electrical resistance of the intermediate transfer belt 7 is acquired based on the primary transfer voltage value obtained by the ATVC control. However, the present invention is not limited to such an embodiment. As information regarding the electrical resistance of the intermediate transfer belt 7, an index value correlated with the electrical resistance of the intermediate transfer belt 7 may be acquired. The information regarding the electrical resistance is not limited to the electrical resistance value itself, but may be, for example, a voltage value when a predetermined current is supplied or a current value when a predetermined voltage is applied. If there is a sufficient correlation between the amount of repeated use of the intermediate transfer belt 7 and the change in electrical resistance, information about the amount of use of the intermediate transfer belt 7 (for example, print Number of sheets, driving time, etc.) may be used.

As described above, in this embodiment, the detection result of the electrical resistance of the intermediate transfer belt 7 is larger than 1.0 × 10 ¹¹ Ω · cm, and the “image forming area” and the “cleaning area” overlap. The image formation start timing is changed. As a result, the same effects as those of the first embodiment can be obtained, and a decrease in image productivity can be suppressed by not delaying the image formation start timing when it is not necessary.

[Others]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  In the above-described embodiment, the case where the intermediate transfer member is an intermediate transfer belt stretched around a plurality of stretch rollers has been described. However, the intermediate transfer member is formed by, for example, a sheet stretched on a frame. It may be drum-shaped.

  In the above-described embodiment, the case where the cleaning member is a rotatable fur brush roller has been described. However, the cleaning member is a rotatable roller such as a brush whose position is fixed, a metal roller, or a conductive elastic roller. It may be.

DESCRIPTION OF SYMBOLS 30 Belt cleaning apparatus 31 Housing 32 Cleaning brush 33 Collecting roller 34 Cleaning blade 43 Cleaning power supply 50 Control part

Claims (8)

An image forming unit for forming a toner image;
A rotatable intermediate transfer member for transferring a toner image formed by the image forming unit at a primary transfer unit and conveying the toner image to a recording material at a secondary transfer unit;
In the rotational direction of the intermediate transfer member, the cleaning unit that is downstream from the secondary transfer unit and upstream from the primary transfer unit contacts the intermediate transfer member, and a voltage is applied to collect the toner on the intermediate transfer member. A cleaning member;
For the cleaning member, a first voltage whose target value of current or voltage is a first value and a second voltage whose absolute value of the target value of current or voltage is a second value larger than the first value are selected. Power supply that can be applied automatically,
When a voltage applied from the power source to the cleaning member is switched from the first voltage to the second voltage over a predetermined period during execution of continuous image formation for transferring a plurality of toner images to a plurality of recording materials, The image forming unit is configured to prevent the toner image transferred to the recording material from being transferred to the region before the region of the intermediate transfer member that has passed through the cleaning unit for a predetermined period of time reaches the cleaning unit. A control unit capable of controlling the timing of forming the
An image forming apparatus comprising:   The control unit includes a first mode in which a toner image transferred to a recording material is not transferred to the region before the region reaches the cleaning unit, and before the region reaches the cleaning unit next time. The image forming apparatus according to claim 1, wherein a second mode in which a toner image transferred to a recording material is transferred to the area can be selectively executed.   The acquisition unit that acquires information on the electrical resistance of the intermediate transfer member, and the control unit selects the first mode and the second mode based on the information. The image forming apparatus described.   The control unit selects the first mode when the electric resistance of the intermediate transfer member indicated by the information is smaller than a predetermined threshold value, and when the electric resistance of the intermediate transfer member indicated by the information is larger than the threshold value. The image forming apparatus according to claim 3, wherein the second mode is selected.   An area of the intermediate transfer member that passes through the cleaning unit during the predetermined period is formed by the image forming unit, transferred to the intermediate transfer member by the primary transfer unit, and then recorded by the secondary transfer unit. 5. The image forming apparatus according to claim 1, wherein a test pattern which is not transferred onto a material and is transported to the cleaning unit is formed.   6. The image forming apparatus according to claim 5, wherein the test pattern is a test pattern for image density correction.   The image forming apparatus according to claim 1, wherein the cleaning member is a rotatable roller brush.   The image forming apparatus according to claim 1, wherein the intermediate transfer member is an endless belt.