JP2017203895A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform cleaning of a transfer rotation body by widening an inter-sheet time according to the amount of detected texture contamination.SOLUTION: An image formation device includes toner adhesion amount detection means for detecting the amount of toner adhered to the surfaces of latent image carriers or the surface of an image carrier. A bias application part is configured to be able to apply a cleaning bias for cleaning the toner adhered to a transfer rotation body and a non image part bias to the transfer rotation body and/or a transfer counter rotation body. A control part performs cleaning of the transfer rotation body and/or the transfer counter rotation body by controlling the bias application part during an inter-sheet time from when a recording medium is delivered at a transfer nip part till a next recording medium is delivered. A texture contamination pattern comprising only a non-image part corresponding to a texture part is formed on the surfaces of the latent image carriers, and based on a result of detection of the amount of adhered toner of the texture contamination pattern performed by the toner adhesion amount detection means, the inter-sheet time is widened to perform cleaning of the transfer rotation body and/or the transfer counter rotation body.SELECTED DRAWING: Figure 12

Description

  The present invention relates to an image forming apparatus.

  2. Description of the Related Art A transfer device that uses a transfer member brought into contact with the surface of an image carrier when transferring a toner image formed on the image carrier to a recording material is known. An image forming apparatus equipped with such a transfer device forms a toner image with charged toner on an image carrier, and presses the image carrier and a contact-type transfer member such as a transfer roller or a transfer belt against them. The pressure nip is the transfer position. Then, a recording material such as paper is inserted between them, and a transfer bias having a polarity opposite to that of the toner is applied to the transfer member, and the toner image on the image carrier side is applied to the recording material by the action of the electric field formed thereby. It is configured to transfer.

  The contact transfer method described above can hold the recording material more securely than the transfer method using a known corona discharger, and therefore, the toner image is not easily transferred at the transfer position. Further, since the transfer bias can be a relatively low voltage, the entire apparatus can be made compact and compact, and there are various advantages such as no generation of ozone.

  On the other hand, when the original image is wider than the recording material width (for example, when printing a thick original such as a book), the outer portion of the recording material width is developed to form a toner image wider than the recording material width. Is done. Then, the toner that protrudes from the recording material may adhere to the transfer roller or may be scattered to contaminate each portion in the vicinity of the transfer roller, or the back side of the subsequent recording material may be stained.

  In the direct transfer method, since the background dirt toner on the photoconductor directly contacts the transfer member, the transfer dirt picks up the background dirt toner between the papers, and the dirt accumulates. There is a risk of edge contamination. As a means for avoiding such a situation, a method of mechanically cleaning the toner adhering to the transfer member by a cleaning means such as a blade or a fur brush is known. In addition, when there is no recording material at the transfer position (for example, until the recording material has passed through the transfer position and the next recording material reaches the transfer position), a cleaning bias having the same polarity as the toner is applied to the transfer member. There is known a method of applying and returning toner adhering to a transfer member to the image carrier side.

  In a general transfer roller type image forming apparatus, bias cleaning is performed before or after a job, so that the toner on the transfer roller is returned to the image carrier and the toner on the transfer member is moved to the recording material. It is preventing. However, if continuous printing is performed without bias cleaning during the job, dirt is gradually accumulated on the transfer member between sheets.

  Normally, a part of the dirt toner gradually moves to the recording material, so that it does not become a level of dirt that can be visually confirmed. However, if a mode in which the paper interval is longer than usual is entered, there is a risk that stains will be remarkably accumulated between the long papers, resulting in back stains and edge stains. In addition, when the background stain is worse than usual even when the paper interval is short, the stain on the transfer member is accelerated, and when the dirty toner moves to the recording material, there is a case where the stain becomes a level that can be visually confirmed. .

  In order to solve these problems, the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2005-228561 has a large-size recording material to be passed next due to contamination of the non-sheet-passing portion of the transfer roller when the small-size recording material is continuously increased. For the purpose of preventing backside contamination, a transfer roller is required before changing the recording material size due to the relationship between the length in the conveyance direction of the recording material specified from the size of the recording material that has been passed in advance and the number of sheets passed. Paper corresponding to the cleaning time in which the interval between the last recording material of the first recording material passed through and the first recording material of the next recording material passed is determined. It is disclosed that a recording material to be passed next is passed after cleaning the transfer roller at the changed paper spacing and after cleaning the transfer roller.

  However, in this image forming apparatus, due to the relationship between the length in the conveyance direction of the recording material specified from the size of the recording material to be passed and the number of sheets to be passed, the cleaning required for the transfer roller to be performed before changing the recording material size is required. The cleaning time of the time transfer roller is determined, and the paper interval time is adjusted in accordance with the transfer roller cleaning time. For this reason, when the background dirt is bad, the transfer roller is not sufficiently cleaned, and the back dirt and the edge dirt remain even if cleaning is performed. Further, the paper interval becomes long regardless of the state of contamination of the transfer roller (the paper interval becomes long even when cleaning is not necessary). Therefore, the problem that productivity falls and the lifetime of a unit, a supply, or a part becomes short is not solved.

  Accordingly, an object of the present invention is to detect background stains such as OPC, intermediate transfer belt, and secondary transfer belt, and to increase the time between sheets according to the detected amount of background stains. An object of the present invention is to provide an image forming apparatus for cleaning a body.

The above problems are charged by the latent image carrier, a charging unit for charging the moving surface of the latent image carrier, a charging power source for outputting a charging bias to be supplied to the charging unit, and the charging unit. A latent image writing means for writing a latent image on the surface of the latent image carrier, a developing means for developing the latent image to obtain a toner image, and a toner image carried by being driven to travel in a predetermined direction. The image carrier, the latent image carrier, the transfer rotator that contacts the image carrier to form a transfer nip, the transfer rotator, and / or the transfer via the image carrier. A bias applying unit that applies a transfer bias to a transfer counter rotating member that is in contact with the rotating member and transfers a toner image to a recording medium conveyed to the transfer nip, and a control unit that controls the bias applying unit;
An image forming apparatus comprising: a toner adhesion amount detecting unit configured to detect a toner adhesion amount on the surface of the latent image carrier or the image carrier; and the bias application unit cleans the toner adhered to the transfer rotator. A cleaning bias and a non-image portion bias to be applied to the transfer rotator and / or the transfer counter-rotator, and the control unit starts the next recording medium after the recording medium is sent out in the transfer nip portion. The transfer stain and / or the transfer counter-rotator can be cleaned by controlling the bias application unit during the time between sheets before being fed in, and the background stain consisting only of the non-image portion corresponding to the background portion As a result of forming a pattern on the surface of the latent image carrier and detecting the toner adhesion amount of the background dirt pattern by the toner adhesion amount detection means. And Zui is solved by an image forming apparatus which comprises carrying out the cleaning of the transfer rotary member and / or the transfer counter rotating member to expand the sheet time.

  The image forming apparatus according to the present invention detects background stains such as OPC, intermediate transfer belt, and secondary transfer belt, and performs transfer cleaning by expanding the sheet space according to the detected background stain amount. Accordingly, it is possible to reliably recover the dirt state of the transfer rotator and the transfer counter-rotator, and to reliably prevent the back dirt and the edge surface dirt due to the transfer rotator dirt.

1 is a schematic configuration diagram illustrating a configuration of a printer according to an embodiment of the present invention. FIG. 2 is a configuration diagram illustrating a main configuration of an image forming unit of the printer. FIG. 2 is a structural diagram illustrating a main configuration of an intermediate transfer unit of the printer. It is a block diagram which shows the principal part of the electric circuit of the printer. It is a flowchart which shows the flow of the arithmetic processing in process control. FIG. 6 is a schematic diagram illustrating a patch pattern image on an intermediate transfer belt. FIG. 6 is a characteristic diagram illustrating a relationship between a development potential and a toner adhesion amount. It is a graph for demonstrating development potential and background potential. It is a graph which shows the relationship between the ground potential and the degree of background dirt and carrier adhesion. It is a graph which shows the relationship between the charging potential Vd and the charging bias Vc. It is a schematic sectional drawing of the optical sensor 102 which concerns on one Embodiment of this invention. It is a flowchart of the line dirt detection which concerns on 1st Embodiment. It is an example of a paper interstitial stain detection pattern. Example of an operation for bias cleaning between sheets (Example 1) Example of operation for bias cleaning between sheets (Example 2) Example of operation for inter-paper bias cleaning (Example 3) Example of operation for bias cleaning between sheets (Example 4) It is a schematic diagram of transfer bias cleaning. It is the result of having evaluated the edge surface stain | pollution | contamination with respect to the number of continuous printing. It is a flowchart of line | wire background stain | pollution | contamination detection which concerns on 2nd Embodiment. It is a figure which shows the relationship between a background potential and a background dirt toner amount. It is a figure which shows the relationship between a toner density and a background dirt toner amount. It is a flowchart of the line ground stain detection which concerns on 3rd Embodiment.

(First embodiment)
Hereinafter, an electrophotographic printer will be described as an example of an embodiment of an image forming apparatus to which the present invention is applied. First, the basic configuration of the printer will be described. FIG. 1 is a schematic configuration diagram illustrating a configuration of a printer according to an embodiment of the present invention. As shown in FIG. 1, the printer includes four image forming units 1Y, 1C, 1M for forming images of each color of yellow (Y), cyan (C), magenta (M), and black (K). 1K. Hereinafter, the subscripts Y, C, M, and K of the respective symbols indicate members for yellow, cyan, magenta, and black, respectively. The color order of Y, C, M, and K is not limited to the order shown in FIG. 1, and may be another order of arrangement.

  FIG. 2 is a configuration diagram showing the main configuration of the image forming unit of the printer. As shown in FIG. 2, there are a charging roller 3Y as a charging means, a developing device 4Y as a developing means, a cleaning device 5Y and the like around a drum-shaped photosensitive member 2Y as a latent image carrier provided in the image forming unit 1Y. Is arranged. The charging roller 3Y made of a rubber roller rotates while being in contact with the surface of the photoreceptor 2Y. This printer employs a contact DC charging method in which a DC bias not including an AC component is applied as a charging bias to the charging roller 3Y. It should be noted that other methods such as a contact AC charging roller method and a non-contact charging roller method can be employed for the charging roller 3Y. The charging power source 50 outputs the charging bias.

  In the developing device 4Y, a two-component developer containing yellow toner and a magnetic carrier is accommodated. This two-component developer contains a toner having an average particle diameter of 4.9 to 5.5 μm and a small particle diameter / low resistance carrier having a bridge resistance of 12.1 [Log Ω · cm] or less. The developing device 4Y includes a developing roller 4aY that is a developer carrying member facing the photoreceptor 2Y, a screw that conveys and stirs the developer, a toner concentration sensor, and the like. The developing roller 4aY is composed of a hollow and rotatable sleeve and a magnet roller included so as not to be rotated.

  In the image forming unit 1Y, the photosensitive member 2Y and the charging roller 3Y, the developing device 4Y, and the cleaning device 5Y disposed around the photosensitive member 2Y are configured as a process cartridge that is supported by a common support as one unit. . As a result, the image forming unit 1Y can be attached to and detached from the printer main body, and consumable parts can be replaced at the same time when the lifetime is reached. The other image forming units 1C, 1M, and 1K use cyan toner, magenta toner, and black toner as toners, but the other configurations are the same as those of the Y image forming unit.

  Returning to FIG. 1, the description will be continued. Below the image forming units 1Y, 1C, 1M, and 1K, an optical writing unit 6 serving as a latent image writing unit is disposed. The optical writing unit 6 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and performs optical scanning of the laser light L on the surfaces of the photoreceptors 2Y, 2C, 2M, and 2K of each color based on image data. I do. By this optical scanning, electrostatic latent images for yellow, cyan, magenta, and black are formed on the photoreceptors 2Y, 2C, 2M, and 2K.

  Above the image forming units 1Y, 1C, 1M, and 1K, an intermediate transfer unit that transfers the toner images of the respective colors from the photoreceptors (2Y, 2C, 2M, and 2K) of the respective colors to the recording sheet S via the intermediate transfer belt 7. 8 is arranged. The intermediate transfer belt 7 as an image carrier is endlessly moved in the counterclockwise direction in the figure by the rotational drive of at least one of the rollers while being stretched around a plurality of rollers. The intermediate transfer unit 8 includes, in addition to the intermediate transfer belt 7, primary transfer rollers 9Y, 9C, 9M, and 9K, a cleaning device 10 including a brush roller and a cleaning blade, a secondary transfer backup roller 11, an optical sensor unit 20, and the like. It has.

  The primary transfer rollers 9Y, 9C, 9M, and 9K sandwich the intermediate transfer belt 7 between the photosensitive members of the respective colors. As a result, primary transfer nips for Y, M, C, and K in which the photoreceptors 2Y, 2M, 2C, and 2K contact the front surface of the intermediate transfer belt 7 are formed. The intermediate transfer unit 8 includes a secondary transfer roller 12 positioned on the outer side of the belt loop in the vicinity of the secondary transfer backup roller 11 on the downstream side in the belt movement direction from the image forming unit 1K. The secondary transfer roller 12 is sandwiched between the intermediate transfer belt 7 and the secondary transfer backup roller 11 to form a secondary transfer nip portion. The secondary transfer roller 12 is an example of a transfer rotator, and the secondary transfer backup roller 11 is an example of a transfer counter rotator.

  A fixing unit 13 is disposed above the secondary transfer roller 12. The fixing unit 13 includes a fixing roller and a pressure roller that contact each other while rotating to form a fixing nip. The fixing roller incorporates a halogen heater, and power is supplied from the power source to the heater so that the surface of the fixing roller has a predetermined temperature, and a fixing nip is formed between the fixing roller and the pressure roller.

  At the bottom of the printer main body, paper feed cassettes 14a and 14b, a paper feed roller, a resist roller pair 15 and the like are provided that accommodate a plurality of recording sheets S as recording media on which output images are recorded. Further, a manual feed tray 14c for manually feeding paper from the side is provided on the side of the printer main body. Further, on the right side of the intermediate transfer unit 8 and the fixing unit 13 in the drawing, a double-sided unit 16 is provided for transporting the recording sheet S to the secondary transfer nip again during double-sided printing.

  At the upper part of the printer main body, toner supply containers 17Y, 17C, 17M, and 17K for supplying toner to the developing devices of the image forming units 1Y, 1C, 1M, and 1K are disposed. The printer main body is also provided with a waste toner bottle, a power supply unit, and the like.

  FIG. 3 is a structural diagram showing the main configuration of the intermediate transfer unit. The intermediate transfer belt 7 includes a tension roller 17 (located at the leftmost in FIG. 3) that receives a tensile force by a spring, a secondary transfer backup roller 11, and an inlet roller 18 installed upstream of the belt rotation. It is wound around the shaft. Further, a secondary transfer roller 12 is provided so as to face the secondary transfer backup roller 11 via a belt. Further, a cleaning device 10 is installed on the tension roller 17. Also, a power supply 29 that is a bias application unit capable of applying a secondary transfer bias, a cleaning bias, and a non-image portion bias having a smaller absolute value than the cleaning bias, and a control unit 30 that controls the output of the power supply 29 are set. Yes. The power supply 29 is connected to the secondary transfer backup roller 11, and the secondary transfer bias, the cleaning bias, and the non-image portion bias output from the power supply 29 are applied to the secondary transfer backup roller 11.

  In order to prevent interference between the primary transfer bias and the secondary transfer bias, the secondary transfer roller 12 is grounded directly or via a resistor, and is electrically floated when it is not necessary. In this embodiment, it is directly grounded.

  The secondary transfer backup roller 11 that rotationally drives the intermediate transfer belt 7 in a predetermined direction by a drive motor also serves as a repulsive roller to which a repulsive bias (secondary transfer bias) is applied. A cleaning bias is applied to the repulsive roller during non-transfer. This cleaning bias generates an electrostatic force between the secondary transfer roller 12 and the intermediate transfer belt 7. The toner adhering to the secondary transfer roller 12 is transferred from the secondary transfer roller 12 side to the intermediate transfer belt 7 side by electrostatic force and removed from the secondary transfer roller 12 (hereinafter, this removal operation is referred to as bias cleaning). Call). Instead of the configuration of FIG. 3, bias cleaning may be performed by grounding the repulsive roller (secondary transfer backup roller 11) and connecting the power source to the secondary transfer roller 12.

  The secondary transfer roller 12 as a transfer member is made of a sponge material having a resistance of 7 to 8 Ω and may have a mechanism that can contact and separate from the intermediate transfer belt 7. When the repulsive roller applies a repulsive bias (secondary transfer bias) to the secondary transfer roller 12, the toner image primarily transferred onto the intermediate transfer belt 7 is transferred onto the sheet (secondary transfer). Further, even if the secondary transfer bias is applied as an attractive bias to the secondary transfer roller 12 side, the same effect can be obtained, in which case the secondary transfer bias polarity is reversed. Instead of the secondary transfer roller 12, secondary transfer may be performed using a secondary transfer belt that is also a transfer member.

  Further, a mechanical cleaning mechanism (for example, a cleaning brush, a cleaning blade, a cleaning web, etc.) capable of coming into contact with and separating from the secondary transfer roller 12 may be provided. Of course, even when a cleaning mechanism is provided, bias cleaning is effective.

  In place of the belt transfer method as shown in FIG. 3, a bias transfer can be performed by a direct transfer method in which the toner image on the photosensitive member is directly transferred to a sheet.

  Next, the operation of the printer will be described. First, a predetermined voltage is applied to the charging roller of the charging roller 3Y from the charging power source 50, and the surface of the opposing photoreceptor 2Y is charged. The surface of the photosensitive member 2Y charged to a predetermined potential is scanned with the laser light L based on the image data by the optical writing unit 6, whereby an electrostatic latent image is written on the photosensitive member 2Y. When the surface of the photoconductor 2Y carrying the electrostatic latent image reaches the developing device 4Y as the photoconductor 2Y rotates, the electrostatic latent image on the surface of the photoconductor 2Y is developed by the developing roller 4aY arranged to face the photoconductor 2Y. Y toner is supplied to the image. Thereby, a Y toner image is formed on the surface of the photoreceptor 2Y. An appropriate amount of Y toner is supplied from the toner supply container 17Y into the developing device 4Y according to the output of the toner density sensor.

  Similar operations are performed at predetermined timings in the image forming units 1C, 1M, and 1K. As a result, Y, C, M, and K toner images are formed on the surfaces of the photoreceptors 2Y, 2C, 2M, and 2K. These Y, C, M, and K toner images are primarily transferred to the front surface of the intermediate transfer belt 7 in order at the primary transfer nip for Y, C, M, and K. This primary transfer is performed by applying a voltage having a polarity opposite to that of the toner to the primary transfer rollers 9Y, 9C, 9M, and 9K by a transfer power source.

  The recording sheet S is conveyed from one of the paper feed cassettes 14 a and 14 b or the manual feed tray 14 c and stops once when it reaches the registration roller pair 15. Then, the registration roller pair 15 is rotated in accordance with a predetermined timing to send the recording sheet S toward the secondary transfer nip.

  The Y, C, M, and K toner images superimposed on the intermediate transfer belt 7 are secondarily transferred to the recording sheet S at the secondary transfer nip where the secondary transfer roller 12 and the intermediate transfer belt 7 abut. This secondary transfer is performed by applying a voltage having a polarity opposite to that of the toner to the secondary transfer roller 12 from a secondary transfer power source. After leaving the secondary transfer nip, the recording sheet S is conveyed toward the fixing unit 13 and is sandwiched between the fixing nips. The toner image on the recording sheet S is heated and fixed by heat from the fixing roller at the fixing nip. In the case of single-sided printing, the recording sheet S on which the toner image is fixed is discharged out of the apparatus by the respective transport rollers. In the case of duplex printing, the recording sheet S is conveyed to the duplex unit 16 by each conveyance roller and reversed, and an image is formed on the surface opposite to the surface on which the image has been previously formed as described above. After that, it is discharged out of the machine.

  In this printer, control called process control is performed at a predetermined timing in order to stabilize image quality over time and environmental fluctuations. In the process control process, a Y patch pattern image composed of a plurality of patch-like Y toner images is developed on the photoreceptor 2 </ b> Y and transferred to the intermediate transfer belt 7. Similarly, C, M, and K patch pattern images are formed on the photoreceptors 2C, 2M, and 2K. Then, the toner adhesion amount of each toner image in the patch pattern image is detected by the optical sensor unit 20, and the image forming conditions such as the developing bias Vb are adjusted based on the detection result. The optical sensor unit 20 is an example of a toner adhesion amount detection unit.

  FIG. 4 is a block diagram showing a main part of the electric circuit of the printer. FIG. 5 is a flowchart showing the flow of arithmetic processing in process control. As shown in FIG. 4, the control unit 30 includes image forming units 1Y, 1C, 1M, 1K, an optical writing unit 6, a paper feeding motor 81, a registration motor 82, an intermediate transfer unit 8, an optical sensor unit 20, and the like. Are electrically connected. The control unit 30 includes a CPU 30a that executes arithmetic processing and various programs, and a RAM 30b that stores data. The paper feed motor 81 is a drive source for the paper feed rollers of each paper feed cassette and paper feed tray. The registration motor 82 is a driving source for the registration rollers.

  The optical sensor unit 20 includes a plurality of reflective photosensors arranged at a predetermined interval in the belt width direction of the intermediate transfer belt 7. Each of the reflection type photosensors is configured to output a signal corresponding to the light reflectance of an intermediate transfer belt 7 or a patch-like toner image described later on the intermediate transfer belt 7. Four reflection type photosensors are provided. Three of them capture both the regular reflection light and the diffuse reflection light on the belt surface so that the output according to the Y, M, C toner image and the Y, C, M adhesion toner can be performed. Output according to. The remaining one captures only the specularly reflected light on the belt surface and performs output according to the amount of light so as to perform output according to the K toner image and K adhering toner.

  The control unit 30 performs process control processing at a predetermined timing such as when the main power is turned on, when waiting after a predetermined time has elapsed, or when waiting after outputting a predetermined number of prints or more. Specifically, when this predetermined timing arrives, first, as shown in FIG. 5, environmental information such as the number of sheets to be passed, the printing rate, temperature, and humidity is acquired (step S1). Next, the development characteristics of the image forming units 1Y, 1C, 1M, and 1K are grasped. Specifically, development gamma γ and development start voltage Vk are calculated for each color (step S2). Specifically, this is performed as follows. That is, the photoreceptors 2Y, 2C, 2M, and 2K are uniformly charged while rotating. This charging is different from a uniform value (for example, −700 V) during normal printing as the charging bias Vc, and its absolute value is increased. Electrostatic for the patch-like Y toner image, patch-like C toner image, patch-like M toner image, and patch-like K toner image on the photoreceptors 2Y, 2C, 2M, and 2K by the scanning of the laser light L by the optical writing unit 6. A latent image is formed. By developing them with the developing devices 12Y, 12C, 12M, and 12K, Y, C, M, and K patch pattern images are formed on the photoreceptors 2Y, 2C, 2M, and 2K. During development, the controller 30 gradually increases the absolute value of the developing bias Vb applied to the developing roller (4a) for each color. Both the developing bias Vb and the charging bias Vc are negative DC biases.

  As shown in FIG. 6, the Y, C, M, and K patch pattern images are transferred so as to be aligned in the belt width direction without overlapping on the intermediate transfer belt 7. Specifically, the Y patch pattern image YPP is transferred to one end of the intermediate transfer belt 7 in the width direction. The C patch pattern image CPP is transferred to a position slightly shifted to the center side from the Y patch pattern image in the belt width direction. Further, the M patch pattern image MPP is transferred to the other end portion in the width direction of the intermediate transfer belt 7. The K patch pattern image KPP is transferred to a position slightly shifted to the center side of the K patch pattern image in the belt width direction.

  The optical sensor unit 20 includes a first reflective photosensor 20a, a second reflective photosensor 20b, a third reflective photosensor 20c, and a fourth reflective that detect the light reflection characteristics of the belt at different positions in the belt width direction. A type photosensor 20d is provided. Of these four reflective photosensors, the third reflective photosensor 20c employs a sensor that detects only specularly reflected light so as to detect a change in light reflection characteristics of the belt surface due to adhesion of black toner. doing. On the other hand, other reflection type photosensors detect both regular reflection light and diffuse reflection light so as to detect a change in light reflection characteristics of the belt surface due to adhesion of Y, C, or M toner. Of the type.

  The first reflective photosensor 20a is disposed at a position for detecting the amount of Y toner attached to the patch-like Y toner image of the Y patch pattern image YPP formed at one end in the width direction of the intermediate transfer belt 7. The second reflective photosensor 20b is disposed at a position for detecting the C toner adhesion amount of the patch-like C toner image of the C patch pattern image CPP located near the Y patch pattern image YPP in the belt width direction. ing. The fourth reflective photosensor 20d is disposed at a position for detecting the M toner adhesion amount of the patch-like M toner image of the M patch pattern image MPP formed at the other end in the width direction of the intermediate transfer belt 7. ing. The third reflective photosensor 20c is arranged at a position for detecting the amount of K toner attached to the patch-like K toner image of the K patch pattern image KPP located near the M patch pattern image MPP in the belt width direction. ing. The first reflection type photosensor 20a, the second reflection type photosensor 20b, and the fourth reflection type photosensor 20d each have three colors (Y, C, M) of the toner image other than black. If there is, the toner adhesion amount can be detected.

  The control unit 30 calculates the light reflectance of each color patch-like toner image based on the output signals sequentially sent from the four reflective photosensors of the optical sensor unit 20, and the toner adhesion amount based on the calculation result. Is stored in the RAM 30b. The patch pattern image of each color that has passed through the position facing the optical sensor unit 20 as the intermediate transfer belt 7 travels is cleaned from the belt front surface by the cleaning device 10.

  Next, from the image density data (toner adhesion amount) stored in the RAM 30b and the exposure portion potential (latent image potential) data separately stored in the RAM 150b, a linear approximation formula (Y = a × Vb + b) shown in FIG. Is calculated. In the two-dimensional coordinates in the figure, the x-axis indicates the value obtained by subtracting the developing bias Vb applied at that time from the exposure portion potential Vl, that is, the developing potential (Vl−Vb). The Y axis indicates the toner adhesion amount (y) per unit area. In FIG. 7, data is plotted on the XY plane by the number corresponding to the number of patch-like toner images. Based on the plurality of plotted data, a section on the XY plane for performing linear approximation is determined. Thereafter, within the section, the least square method is performed to obtain a linear approximation formula (y = a × Vb + b). At this time, the development gamma γ and the development start voltage Vk are calculated based on the linear approximation formula. The development gamma γ is calculated as the slope of the linear approximation formula (γ = a), and the development start voltage Vk is calculated as the intersection of the linear approximation formula and the X axis (Vk = −b / a). In this way, the development characteristics of the image forming units 1Y, 1C, 1M, and 1K for the respective colors are calculated (step S2).

  Next, based on the obtained development characteristics, a target value (target charge potential) of the charging potential (background portion potential) Vd, a target value (target exposure portion potential) of the exposure portion potential Vl, and a development bias Vb are obtained. (Step 3). Specifically, the target charging potential and the target exposure portion potential are obtained based on a table in which the relationship between the development gamma γ and the charging potential Vd and the exposure portion potential Vl is determined in advance. Thereby, it is possible to select a target charging potential and a target exposure portion potential suitable for the development gamma γ. Further, the developing bias Vb is obtained as follows. That is, the development potential for obtaining the maximum toner adhesion amount is obtained by the combination of the development gamma γ and the development start voltage Vk, and the development bias Vb capable of obtaining the development potential is obtained. Then, based on the developing bias Vb and the background potential, a target charging potential is obtained. Since the surface of the developing sleeve of the developing roller has substantially the same value as the developing bias Vb, if the surface of the photosensitive member is charged to the target charging potential and appropriately exposed, the target developing potential and background potential are obtained. be able to.

  Next, the control unit 30 determines the charging bias Vc. Specifically, the charging bias Vc at which the target charging potential is obtained varies depending on the wear amount of the surface layer of the photoreceptor, the electrical resistance of the charging roller that is influenced by the environment, and the like. Therefore, the control unit 30 stores an algorithm for obtaining a charging bias Vc capable of obtaining a target charging potential from a combination of the environment (temperature and humidity) and the photosensitive body travel distance. This algorithm is constructed based on previous experiments. Then, a charging bias Vc capable of obtaining a target charging potential is obtained by using an algorithm based on a combination of the temperature / humidity detection result by the environmental sensor 52 and the photosensitive body travel distance stored in the RAM.

  As a property of the developer, the background stain is worse with respect to the initial stage, and conversely, the carrier adhesion (edge carrier adhesion) is worse in the initial stage than with the passage of time. For this reason, the optimum background potential shifts to a larger value with the use of the developer. In general, in a high-temperature and high-humidity environment, the background charge is deteriorated due to a low charge amount of the toner. For this reason, in the image density control according to the present embodiment, the background potential is shifted to an optimal value in the initial / time-lapse + environment.

  Already through experiments, the optimum background potential to bring the background dirt and carrier adhesion below the target has been determined under each condition. For this reason, if there is environmental information such as deterioration of the charging roller or carrier and changes in temperature and humidity, a certain degree of correction is possible. However, there is a possibility that the optimum background potential may fluctuate due to errors from the experiment and unexpected factors. On the other hand, since the development start voltage Vk can be considered as a voltage at which development on the photosensitive member 2 is started, it is considered that the background contamination is deteriorated if there is no background potential equal to or greater than the absolute value of the development start voltage Vk.

  Therefore, as shown in FIG. 5, the control unit 30 determines a target development start voltage Vk ′ after the step S3 (step S4). The target development start voltage Vk 'is linked to the environment information in advance by experiments and is tabulated, and the target development start voltage Vk' is determined by referring to the table from the first acquired environment information. Then, the classification is determined by the amount of difference between the development start voltage Vk and the target development start voltage Vk ′ (step S5). For example, if the development start voltage Vk is +40 V or more away from the target development start voltage Vk ′, the classification is classified as follows: Category 1, less than +40 V + 20 V or more, Category 2; Then, it is specified which section the development start voltage Vk is in, and a correction amount is determined for each section (step S6). Next, the target background potential is calculated by adding the correction amount determined in step S5 to the background potential calculated from the charging potential Vd and the developing bias Vb obtained in step S3. Then, the charging bias Vc is determined so as to obtain this target background potential (step S7).

  FIG. 8 is a graph for explaining the development potential and the background potential. As shown in FIG. 8, the background potential is the difference between the charging potential Vd and the development bias Vb, and acts on the non-image portion (background portion) of the image. When the background potential is low, background contamination is likely to occur. On the other hand, when the background potential is large, carrier adhesion is likely to occur. Therefore, it is necessary to set the background potential to an appropriate value.

  FIG. 9 is a graph showing an example of the relationship between the background potential and the degree of background contamination and carrier adhesion. This example shows an example in which the theoretical value of the background potential is set to 140 [V] by performing process control. The reason why it is expressed as a theoretical value is as follows. That is, as described above, the background potential is determined based on the relationship between the appropriate charging potential Vd and the developing bias Vb by the process control, and the charging bias Vc is determined based on the background potential. However, the charging potential Vd is not always the target charging potential due to the charging bias Vc. This is because the discharge start voltage between the charging roller and the photosensitive member changes due to various factors, and the charging bias Vc for obtaining the same charging potential Vd changes accordingly. In the process control, the environment and the photosensitive body travel distance are taken into consideration when determining the charging bias Vc. However, since it is based on a theoretical algorithm, it is not always the case. In addition, the value of the charging bias Vc for obtaining the same charging potential Vd varies depending on other parameters different from the environment and the photosensitive body travel distance.

  In the example shown in the figure, if the background potential is 140 [V], both background contamination and carrier adhesion can be suppressed. Therefore, the control unit 30 determines the target charging potential so that, for example, a background potential of 140 [V] and a desired development potential are obtained during process control. However, since the value of the charging bias Vc for obtaining the charging potential Vd varies depending on various factors, the target charging potential is not always obtained by the charging bias Vc determined by the process control. In some cases, the actual charging potential Vd may deviate greatly from the target charging potential (140 V in the illustrated example). Then, in the figure, the actual background potential exceeds 170V and carrier adhesion occurs, or the actual background potential falls below 110V and background contamination occurs.

  As described above, the charging bias Vc is applied to the charging roller (for example, 3Y) made of a rubber roller. As shown in FIG. 10, the charging potential Vd of the photoconductor (for example, 2Y) exhibits a characteristic represented by the expression “Vd = a × Vc + b”. a is the slope of the graph shown in FIG. 10, and b is the Vd-axis intercept in the graph, which is a negative value. The Vc axis intercept in the graph is almost the same value as the discharge start voltage between the charging roller and the photosensitive member. Further, the inclination a is approximately 1.

  As described above, the printer employs a contact DC charging method in which a charging bias consisting of only a DC component is applied to the charging roller brought into contact with the photosensitive member. Unlike the method using an AC / DC superimposed bias as a charging bias, the contact DC charging method does not require an AC power source, and thus can reduce the cost. On the other hand, since an alternating electric field is not formed between the charging roller and the photoconductor, the value of the charging bias Vc must be greater than the discharge start voltage shown in the graph of FIG. It is not possible to generate a discharge between them, and the photoreceptor cannot be charged at all. Even if it can be charged, the discharge start voltage varies depending on the environment, the amount of wear on the surface layer of the photoreceptor, the electrical resistance of the charging roller, the amount of dirt, and the like. The charging potential Vd varies. For this reason, it is difficult to stably obtain a desired charging potential Vd as compared with the AC charging method.

  In this embodiment, as shown in FIG. 1, an optical sensor unit 20 that is a toner adhesion amount detection unit is installed facing the intermediate transfer belt to constitute a part of the light quantity control device. FIG. 11 is a schematic sectional view of the optical sensor 102. As shown in the figure, the optical sensor 102 mainly receives a light emitting element 311 as a light emitting means, a regular reflection light receiving element 312 as a first light receiving means for receiving specular reflection light, and diffuse reflection light. And a diffuse reflection light receiving element 313 as a second light receiving means. Light emitted from the light emitting element 311 is emitted toward the surface of the intermediate transfer belt 41. Then, regular reflection light regularly reflected by the surface of the intermediate transfer belt 41 and the toner patch transferred to the surface is received by the regular reflection light receiving element 312 and a voltage corresponding to the amount of received light is output. Further, diffuse reflection light diffusely reflected by the surface of the intermediate transfer belt 41 and the toner patch transferred to the surface is received by the diffuse reflection light receiving element 313, and a voltage corresponding to the amount of received light is output.

  As the light emitting element 311 of the optical sensor 102, a GaAs light emitting diode having a peak emission wavelength of 940 [nm] is used. Further, as the regular reflection light receiving element 312 and the diffuse reflection light receiving element 313, those having a Si phototransistor having a peak spectral sensitivity wavelength of 850 [nm] are used. That is, the optical sensor 102 detects infrared light of 830 [nm] or more with no significant difference in reflectance due to color. By using such an optical sensor, it is possible to detect toner patches of all colors Y, M, C, and K with a single sensor.

  In the present embodiment, such an optical sensor 102 is used to detect the amount of background dirt toner on the intermediate transfer belt between sheets (on the photoconductor in the case of the direct transfer method).

  FIG. 12 is a diagram showing a background stain detection flow. As shown in FIG. 12, when a print command is issued (S0), process control is executed (S1). In the process control, toner density (TC) adjustment is performed as necessary (S2), and then charging bias (Vd), Vl (exposure potential), and Vb (development bias) are calculated (S3). Thereafter, printing is started (S4), and interstitial soiling is detected during printing (S5).

  FIG. 13 shows an example of a pattern for detecting line background contamination. In the figure, the patch position is arranged outside the edge of the paper, but may be arranged in the center of the paper. Further, a plurality of patches may be arranged in the main scanning direction and the average may be taken. In the figure, for convenience, the pattern is shown in a patch shape, but the image forming conditions for development, primary transfer, and secondary transfer are not patch-like because a normal paper-to-paper bias is applied.

  Returning to the flow of FIG. The predicted value g of the back stain obtained from the amount of paper background stain detected by the optical sensor 102 and the time between papers (the time from when the recording medium is sent to the next recording medium at the transfer nip portion) is obtained. When the value is smaller than the certain allowable value S (S6), normal printing is performed without extending the interval between sheets and without performing cleaning. If it is greater than a certain allowable value S (S6), the time required for cleaning the transfer body is calculated (S7), and the paper interval is extended by that time to perform cleaning (S8). Perform printing. Note that the certain allowable value S may be determined by sensory evaluation as the degree of inconspicuous stains on the back surface or edge surface, or may be set freely.

  Next, transfer cleaning according to this embodiment will be described. In addition to the toner image developed on the latent image, on the photosensitive member 2 that is a latent image carrier, there is a non-appropriate charge amount, or background-stained toner adhered to the background portion due to mechanical contact. . It is known that the amount of background dirt toner varies depending on the background potential (see FIG. 9). Further, according to the flowchart shown in FIG. 5, the charging bias correction amount is determined (S6), the correction amount is added to the background potential, and Vc is determined (S7), thereby controlling the amount of background contamination toner to a target value or less. doing.

  However, if the background potential is changed, color variations and line thickness variations occur. Therefore, upper and lower limits are provided for the correction value of the charging bias. In other words, there are cases where the amount of background dirt toner cannot be sufficiently suppressed only by correcting the charging bias.

  For this reason, the photoconductor 2 always has a certain amount of background dirt toner, and in the contact transfer system, such toner moves to the transfer member. In the transfer belt method, it is common to clean such toner with a cleaning blade. However, the transfer roller method or the like does not mechanically clean with a cleaning blade, but it is biased to the photosensitive member with an electric force. There is also a method for returning the background dirt toner. In particular, since the transfer roller system is simple and small in size, it is currently widely used.

  In such a transfer roller system, bias cleaning is generally performed before and after image formation. However, when continuous printing is performed, bias cleaning is not performed midway, and dirt toner accumulates on the transfer roller. There is a possibility that. If dirty toner accumulates, it may move to the transfer paper and cause back stains and edge stains. For this reason, there is also a method of suppressing the occurrence of smearing by performing bias cleaning between the sheets (between the sheets at the time of printing) according to the number of sheets passed during the image forming operation.

  Next, a bias cleaning operation between sheets, which is a feature of the present invention, will be described with reference to Examples 1 to 4 in FIGS. In the figure, an example in which the length of the paper is different is shown for easy understanding as an embodiment of the present invention. In the present invention, a calculation process is performed in which a paper interval is determined and an operation corresponding to the paper interval time is performed.

[Paper interval time] <[Transfer bias cleaning time] + [Judgment threshold A] Condition (1)
In the condition (1), the back stain amount is predicted from the detected background stain amount and the paper interval time, and when the predicted value is larger than a certain allowable value S, the paper interval time is increased and the transfer cleaning is performed. To implement. For the calculation of the predicted value (g) of the back stain, the prediction formula (1) obtained from the relationship between the background stain amount, the paper interval time, and the back stain amount obtained in advance by experiments is used.
[Predicted value of backside stain (g)] = α × [Background stain amount of latent image carrier] × [Time between papers] Prediction formula (1)
Here, α is a coefficient obtained from an experiment in advance for each type of transfer paper. Even with the same background stain amount, the back stain amount may vary depending on the type of transfer paper (or characteristics such as smoothness and whiteness). Good.

  The number of cleanings performed at that time may be changed according to the predicted value of the back dirt. When the amount of back dirt is large, the amount of back dirt is not reduced to a certain allowable value by one cleaning, and therefore, the back dirt after cleaning is improved by performing cleaning several times. When the predicted value (g) of the backside stain is smaller than the certain allowable value S, the transfer cleaning is not performed without increasing the paper interval time.

  By performing the control as described above, the transfer cleaning can be performed as many times as necessary when necessary, so that even when the interval between the papers is shorter than the cleaning time, it is possible to prevent the occurrence of back stains and edge stains. When there is little dirt on the transfer body, cleaning is not performed, and a reduction in productivity can be prevented. In this embodiment, the background stain on the transfer body (OPC, intermediate transfer belt, secondary transfer belt, etc.) is detected, and the transfer cleaning is performed by widening the gap between the sheets according to the detected amount of background stain. By inserting it, it is possible to surely prevent the back and dirt on the transfer roller from becoming dirty. In addition, when the transfer body is not very dirty, the unnecessary motor rotation can be prevented by not performing cleaning, so unnecessary power consumption can be suppressed, and units and supplies (such as developer life and photoconductor life) can be suppressed. It is possible to prevent the life of the battery from being shortened.

[Time between sheets] ≧ [Transfer bias cleaning time] + [Judgment threshold A] Condition (2)
In the condition (2), the back stain amount is predicted from the detected background stain amount and the paper interval time, and if the predicted value (g) is larger than a certain allowable value S, the transfer is performed within the paper interval time. Perform cleaning.

  The determination threshold A is a coefficient that is set on the assumption that the position is shifted during the conveyance of the paper, the bias switching control time, or the like. If this coefficient is too small, the cleaning control is terminated before the paper arrives, and it is assumed that the output of the image portion is not in time. If it is too large, bias cleaning tends to be difficult to enter. There is a need. When a plurality of cleaning operations are performed, the transfer cleaning time is calculated as a combined cleaning time. Each time unit is msec or sec.

  In the image forming operation described in the first embodiment (see FIG. 14), the case where the paper interval time is shorter than the transfer cleaning time and bias cleaning is not performed is shown. This corresponds to the case where the transfer cleaning is not performed without increasing the paper interval time because the predicted value (g) of the back stain is less than the certain allowable value S.

  In the image forming operation described in the second embodiment (see FIG. 15), the time between sheets is increased to the same as the transfer cleaning time, and negative bias cleaning and positive bias cleaning for one rotation of the transfer roller are performed once. The case where it puts one by one (1 set) is shown. This is equivalent to the case where the transfer cleaning is performed with a longer interval between sheets because the predicted value (g) of the back stain is larger than the certain allowable value S.

  In the image forming operation described in the third embodiment (see FIG. 16), the time between sheets is increased to twice the transfer cleaning time, and negative bias cleaning and positive bias cleaning for one rotation of the transfer roller are performed for each two times. The case where it puts in once (2 sets) is shown. This is because the predicted value (g) of the back stain is much larger than the certain allowable value S, so that the back stain is not improved to the permissible value S by one set of cleaning, and the two-set transfer cleaning is performed by extending the paper interval time. This corresponds to the case where

  In the image forming operation described in the fourth embodiment (see FIG. 17), the inter-paper time is longer than the transfer cleaning time, and negative bias cleaning and positive bias cleaning for one rotation of the transfer roller are performed immediately before the next printing. The case where it is inserted once (one set) is shown.

  Again, by carrying out the control as described above (Examples 1 to 4), the transfer cleaning can be performed as many times as necessary when necessary, so even if the interval between the papers is shorter than the cleaning time, The edge surface can be prevented from being stained, and when the transfer body is not very dirty, the cleaning is not performed and the productivity can be prevented from being lowered.

  Further, in the present invention, the background stain on the transfer body (OPC, intermediate transfer belt, secondary transfer belt, etc.) is detected, and the transfer cleaning is performed by widening the gap between the sheets according to the detected amount of background stain. Thus, it is possible to reliably prevent the back surface and the edge surface from being stained by the transfer roller. In addition, when the transfer body is not very dirty, unnecessary cleaning of the motor can be prevented by not performing cleaning, so unnecessary power consumption can be suppressed, and units and supplies (such as developer life and photoconductor life) can be suppressed. It is possible to prevent the life of the battery from being shortened.

  FIG. 18 shows a schematic diagram of transfer bias cleaning. Between the sheets, reversely charged toner or background-stained toner such as reversely charged toner is present on the photosensitive member, and the toner moves to the transfer roller, so that the dirt on the roller accumulates. The diagram called “between paper” shows a transfer roller in which dirty toner is accumulated, and circles marked with + and − show toner of that polarity.

  Next, in the figure described as “transfer cleaning (−)”, positively charged toner attached to the transfer roller is returned to the intermediate transfer belt by applying a negative positive bias to the counter roller of the transfer roller. Can do. Next, in the figure described as “transfer cleaning (+)”, the negatively charged toner can be returned to the intermediate transfer belt by applying a positive bias to the counter roller of the transfer roller. Since the background-stained toner that stains the transfer roller includes bipolar toner, it is possible to more effectively prevent the occurrence of back-side stains and edge-side stains by applying both biases. In Examples 2 to 4, the transfer bias cleaning operation is also performed, and the plastic screening is performed after the minus cleaning.

Further, when the gap between the sheets becomes long as in the fourth embodiment, even if the transfer bias cleaning is performed immediately after entering the gap, the transfer roller may be contaminated again between subsequent sheets. For this reason, as shown in the equation (1), by setting a value of the margin coefficient c and performing a bias cleaning operation before this value, the dirty toner accumulated for a long time between the papers is removed. In the latter half of the cleaning, it is possible to efficiently prevent the occurrence of back dirt and edge dirt.
[Cleaning execution timing] = [Print restart timing]-[Transfer bias cleaning time]-[Margin coefficient c] (2)

  In the embodiment of the present invention, the transfer output other than during the transfer bias cleaning between sheets is set to the plus side. This is intended to suppress the accumulation of dirt of the normally charged toner (minus) on the transfer roller between sheets by setting it to the plus side, but it may be set to 0 μA. By setting it to 0 μA, it may be possible to suppress accumulation of dirt on the transfer roller between sheets. If there is a lot of dirt toner accumulated on the transfer roller, it may not be possible to clean the dirt toner with a limited transfer bias cleaning time. There is also a possibility that it will not be generated.

  Further, by applying a cleaning bias for one or more rounds of the transfer roller, dirt on the entire circumference of the transfer roller can be cleaned. There may be a case where the transfer roller is not completely cleaned by only one rotation. In the third embodiment, the negative electrode bias cleaning and the positive electrode bias cleaning are applied for two rotations of the transfer roller, respectively. When these cleaning bias application times are long, the cleaning effect can be obtained more, but the cleaning time becomes long and the cleaning becomes difficult to be performed, so it is necessary to set an appropriate time. Since the optimum cleaning time varies depending on the configuration of the image forming apparatus, it is necessary to perform optimization for each.

  The accumulation of dirt on the transfer roller due to the inter-paper extension occurs not only in the intermediate transfer method but also in the direct transfer method which is often implemented in a monochrome machine or the like. For this reason, the present invention is not limited only to the intermediate transfer method, and the direct transfer method in which dirt on the transfer roller is more likely to accumulate is more effective.

  Also, bias cleaning is effective even with the transfer belt method, but it is also possible to perform regular cleaning with a cleaning blade. However, since the transfer roller system is small in size and low in cost, bias cleaning is generally used, and the effect of the inter-paper bias cleaning of the present invention can be further obtained in the transfer roller system image forming apparatus.

  When the process line speed changes, the execution bias with respect to the transfer current value changes. Therefore, in order to perform proper cleaning, when controlling at multiple process line speeds, control the cleaning output (current value) for each line speed. It is desirable to do. Table 1 shows examples.

  Since the printing speed of plain paper is the standard speed, and the thick paper has poor fixability, it is necessary to output an image at a lower linear speed. In the examples shown in Table 1, 260 mm / s is set as a standard speed, and 150 mm / s (medium speed) and 75 m / s (low speed) are provided. The transfer CL output has a standard speed of 100%, and medium and low speeds are provided with correction coefficients. The output is determined by transfer CL (260 mm / s) × correction coefficient / 100. Thereby, the cleaning performance is changed by applying the cleaning output more than necessary, and the hazard to the photoconductor can be prevented.

  Also, the transfer bias cleaning can be performed more efficiently by adjusting the cleaning output even when the environment changes. If the output of the transfer bias cleaning is too high or too low, the toner on the transfer roller cannot be effectively collected. These proper cleaning output values are known to shift due to changes in resistance of the transfer member and paper when the environment changes. For this reason, in the embodiment of the present invention, the image forming apparatus is provided with an environmental sensor capable of detecting temperature and humidity, and the absolute humidity is converted based on the detection result. From this absolute humidity value, an appropriate transfer cleaning output value is selected from the threshold value table of Table 2.

  As a result, the transfer roller is more efficiently stained, and problems such as back contamination do not occur.

  FIG. 19 shows the result of evaluation of edge contamination with respect to the continuous print number. The rank of the edge surface stain is a graded ranking of the edge surface stain, and the criterion is rank 2 or higher. Rank 2 is a judgment of a level of dirt that is dirty but usually not noticeable. The rank is a total of 5 levels, and rank 5 indicates a state in which it is not dirty at all.

  As the evaluation conditions, the conditions (temperature 27 degrees, humidity 80%) at which dirt is most likely to occur in the image forming apparatus used for evaluation were selected. The photoconductor background dirt was drawn on a paper at a level where the background dirt was not anxious, and the photoconductor background dirt was at that level. Further, since the transfer roller is likely to get dirty with time and the transfer paper tends to become dirty, a transfer roller that has reached the replacement life is used. The printing speed was 30 cpm.

  In FIG. 19, the condition described as “normal” is an evaluation of the transition of dirt between normal sheets under the condition that cleaning is not performed between sheets. The condition described as “peripheral five-sheet binding” is a condition in which the interval between sheets becomes very long (about 10 sec) depending on the stapling time. The condition described as “Example” is an example of the present invention described above, and is a condition in which bias cleaning of one round of the negative electrode and one round of the positive electrode was performed between the papers.

  From FIG. 19, it can be seen that the bias cleaning does not enter even between the normal sheets, and hence the stain tends to deteriorate with respect to the number of printed sheets. Further, when the stapling operation is performed by the peripheral machine, the gap between the sheets is extended, and it can be seen that the dirt is remarkably deteriorated. As a smearing method, the number of printed sheets that divide the standard is reduced to about 1/6 of the normal number. On the other hand, in the examples of the present invention, the deterioration of the edge surface contamination can be prevented by the inter-sheet cleaning, and no significant deterioration of the edge surface contamination was confirmed regardless of the number of printed sheets.

(Second Embodiment)
By performing the control shown in FIG. 12, it is possible to prevent the back stain and the edge stain, but as long as the predicted value (g) of the back stain continues to exceed a certain allowable value S in S6 of FIG. However, there is a problem in that the productivity is lowered because the transfer cleaning is performed with the sheet spread.

  Therefore, while the paper gap is widened and transfer cleaning is performed, the background potential (difference between the charging bias Vd and the developing bias Vb), which is the difference between the surface potential of the photosensitive member being printed and the developing potential of the developing means, is widened. Execute control to improve the background stains that cause back stains and edge stains.

  According to the control shown in FIG. 20, the background potential to be set is calculated, and the correction amount of the charging bias Vd is determined (S3), so that the predicted value (g) of the back stain is controlled to be equal to or less than a certain allowable value S. Do.

  Here, the background potential to be set may be determined from the relationship between the background potential acquired in advance through experiments and the background dirt. FIG. 21 shows an example of the relationship between the background potential acquired for each toner concentration and the background stain. Although the toner density is higher at the same background potential, the amount of background toner is larger. However, the slope is almost constant regardless of the toner density, so the background potential change amount does not depend on the toner density. You may decide from only.

  Further, for example, it may be determined from the relationship between the background potential actually acquired and the background stain by a detection means such as that disclosed in Patent Document 2. Furthermore, the background potential is changed little by little at the time of passing the paper, and the predicted value (g) of the back stain is gradually changed to a certain allowable value S in the next and subsequent paper background stain detection. Also good. If the background potential is greatly changed at a time, the edge effect changes abruptly, so that the dot size change and the line width change become apparent. Furthermore, since a problem that the color variation becomes conspicuous in a full-color image occurs, a method of changing little by little is effective.

(Third embodiment)
By performing the control shown in FIG. 20, it is possible to prevent the back dirt and the edge face dirt without decreasing the productivity. However, the image quality may be deteriorated. As shown in FIG. 9, when the background potential fluctuates to the larger side, the carrier adhesion deteriorates. Further, as described in the second embodiment, since the edge effect changes, the size of the dot changes and the line width changes. Further, when the size of the dots changes, there arises a problem that the color variation occurs in the full color image.

  FIG. 22 shows an example of the relationship between toner density and background stains. The background toner amount tends to increase because the toner charge decreases as the toner concentration increases. Therefore, in the present embodiment, control is performed to increase the charge amount of the toner by decreasing the toner density during printing while the paper gap is widened and transfer cleaning is performed. Thereby, the background stain | pollution | contamination which is the cause of back stain | pollution | contamination and edge surface contamination can be improved.

  In accordance with the control shown in FIG. 23, the toner density to be set is calculated (S2), and the toner density is gradually lowered between sheets, thereby controlling the predicted value (g) of the back stain to be equal to or less than a certain allowable value S. Do. The method of lowering the toner density may be an image formation of a discharge pattern on the image carrier, or may be carried out by normal image formation with less toner replenishment. The method for improving the background stain by lowering the toner concentration is superior to the method for improving the background stain by increasing the background potential, since the degradation of image quality is small. Of course, the toner density may be lowered in combination with the change of the background potential.

  The present invention has been described in detail above using the embodiment. This embodiment is an example, and can be used with various modifications within a range not departing from the gist. For example, the configurations of the first to third embodiments may be used in combination.

1C, 1K, 1M, 1Y Image forming unit 2C, 2K, 2M, 2Y photoreceptor 3Y charging roller 4Y developing device 4aY developing roller 5Y cleaning device 6 optical writing unit 7 intermediate transfer belt 8 intermediate transfer unit 9C, 9K, 9M, 9Y Primary transfer roller 10 Cleaning device 11 Secondary transfer backup roller 12 Secondary transfer roller 12C, 12K, 12M, 12Y Developing device 13 Fixing unit 14a, 14b Paper feed cassette 14c Manual feed tray 15 Registration roller pair 16 Duplex unit 17 Tension roller 17C , 17K, 17M, 17Y Toner supply container 18 Entrance roller 20 Optical sensor unit 20a First reflective photosensor 20b Second reflective photosensor 20c Third reflective photosensor 20d Fourth reflective photosensor 2 Power supply 30 controller 30a CPU
30b RAM
41 Intermediate transfer belt 50 Charging power supply 52 Environmental sensor 81 Paper feed motor 82 Registration motor 102 Optical sensor 150b RAM
311 Light emitting element 312 Regular reflection light receiving element 313 Diffuse reflection light receiving element S Recording sheet

JP 2012-42641 A Japanese Patent Application Laid-Open No. 2015-087563

Claims (4)

A latent image carrier;
Charging means for charging the moving surface of the latent image carrier;
A charging power source that outputs a charging bias for supplying to the charging means;
Latent image writing means for writing a latent image on the surface of the latent image carrier charged by the charging means;
Developing means for developing the latent image to obtain a toner image;
An image carrier that is driven to travel in a predetermined direction and carries a toner image;
The latent image carrier, or a transfer rotator that forms a transfer nip portion in contact with the image carrier; and
A transfer bias is applied to the transfer rotating member and / or the transfer counter rotating member that is in contact with the transfer rotating member via the image carrier, and the toner image is transferred to the recording medium conveyed to the transfer nip portion. A bias application unit;
A control unit for controlling the bias application unit;
In an image forming apparatus comprising:
A toner adhesion amount detection means for detecting the toner adhesion amount on the surface of the latent image carrier or the image carrier;
The bias applying unit is configured to be able to apply a cleaning bias and a non-image portion bias for cleaning the toner attached to the transfer rotating body to the transfer rotating body and / or the transfer counter rotating body,
The control unit controls the bias applying unit and controls the transfer rotator and / or the transfer during a paper interval from when the recording medium is fed at the transfer nip to when the next recording medium is fed. The counter rotating body can be cleaned,
On the basis of the result of forming a background stain pattern consisting only of a non-image portion corresponding to the background portion on the surface of the latent image carrier and detecting the toner adhesion amount of the background stain pattern by the toner adhesion amount detection means. An image forming apparatus characterized in that the transfer rotator and / or the transfer counter rotator is cleaned for a long time.   The image forming apparatus according to claim 1, further comprising a cleaning blade capable of contacting and separating from the transfer rotator.   Control to widen the difference (background potential) between the surface potential of the latent image carrier and the developing potential of the developing means at the same time as cleaning the transfer rotating member and / or the transfer counter rotating member by extending the time between sheets. The image forming apparatus according to claim 1, wherein the image forming apparatus is performed.   4. The control for lowering the toner density during printing is performed at the same time as cleaning of the transfer rotator and / or the transfer counter rotator is performed with an extended time between sheets. The image forming apparatus described in 1.

Images