JP6690394B2 - Image forming device - Google Patents

Description

  The present invention relates to an image forming apparatus.

  There is known a transfer device of a type that uses a transfer member that is in contact with the surface of an image carrier when transferring a toner image formed on the image carrier to a recording material. An image forming apparatus equipped with such a transfer device forms a toner image with charged toner on an image carrier and presses a contact type transfer member such as a transfer roller or a transfer belt onto the image carrier to press them together. The pressure contact 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 recorded on the recording material by the action of the electric field formed by this. It is configured to transfer.

  The contact transfer method described above can hold the recording material more reliably than the transfer method using a well-known corona discharger, so that the transfer deviation of the toner image at the transfer position is less likely to occur. Further, since the transfer bias requires 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 portion outside the recording material width is developed and a toner image wider than the recording material width is formed. To be done. Then, the toner in the portion protruding 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 stain of the succeeding recording material may occur.

  Further, in the direct transfer method, the background stain toner on the photoconductor comes into direct contact with the transfer member. There is a risk that the edge surface will become dirty. As a means for avoiding such a situation, a method of mechanically cleaning the toner adhering to the transfer member with a cleaning means such as a blade or a fur brush is known. Also, when there is no recording material at the transfer position (such as before the recording material passes through the transfer position and the next recording material reaches the transfer position), a cleaning bias of the same polarity as the toner is applied to the transfer member. There is known a method of applying and returning the toner adhering to the transfer member to the image carrier side.

  In a general transfer roller type image forming apparatus, by applying bias cleaning before or after a job, it is possible to return the dirt toner on the transfer roller to the image carrier and move the dirt on the transfer member to the recording material. It is preventing. However, if continuous printing is performed without bias cleaning during the job, dirt will gradually accumulate on the transfer member between the sheets.

  Normally, a part of the dirty toner gradually moves to the recording material, so that the dirt does not reach a level that can be visually confirmed. However, when entering a mode in which the paper interval is longer than usual, dirt may be remarkably accumulated in the long paper, and back dirt and edge surface dirt may occur. Further, if the background stain is worse than usual even if the paper interval is short, the stain on the transfer member is accelerated, and when the stain toner moves to the recording material, the stain may be a level that can be visually confirmed. .

  In order to solve these problems, in the image forming apparatus of Patent Document 1, a large-sized recording material to be passed next time due to contamination of a non-sheet passing portion of a transfer roller at the time of continuous feeding of small-sized recording material For the purpose of preventing back-side contamination, the transfer roller required before changing the recording material size is necessary due to the relationship between the length of the recording material in the conveying direction, which is specified by the size of the recording material that is passed first, and the number of paper passing. The cleaning time is determined, and the paper corresponding to the cleaning time is determined for the interval between the last recording material of the preceding recording material and the first recording material of the next recording material. It is disclosed that the interval is changed, and the recording material to be passed next is passed after the transfer roller is cleaned at the changed interval.

  However, in this image forming apparatus, due to the relationship between the length of the recording material in the conveying direction, which is specified by the size of the recording material to be passed, and the number of passing sheets, the cleaning required by the transfer roller before changing the recording material size is performed. The cleaning time of the time transfer roller is determined, and the paper interval time is adjusted according to the cleaning time of the transfer roller. Therefore, when the background stain is bad, the transfer roller is not sufficiently cleaned, and even if cleaning is performed, back stains and edge stains remain. Further, the paper interval becomes long regardless of the dirt state of the transfer roller (the paper interval becomes long even when cleaning is not necessary). Therefore, the problems of reduced productivity and shortened life of units, supplies, and components cannot be solved.

Therefore, an object of the present invention is to detect background stains such as an OPC, an intermediate transfer belt, and a secondary transfer belt, and extend the paper interval time according to the detected background stain amount to clean the transfer roller ( transfer rotary member ). An object of the present invention is to provide an image forming apparatus that performs the image forming apparatus.

The above-mentioned problem is to be charged by the latent image carrier, a charging unit that charges the moving surface of the latent image carrier, a charging power source that outputs a charging bias for supplying to the charging unit, and the charging unit. And a latent image writing unit that writes a latent image on the surface of the latent image carrier, a developing unit that develops the latent image to obtain a toner image, and a toner image that is driven to run in a predetermined direction to carry the toner image. Image carrier, the latent image carrier, or a transfer rotator that abuts on the image carrier to form a transfer nip portion, the transfer rotator, and / or the transfer via the image carrier. A bias applying unit that applies a transfer bias to the transfer facing rotating member that abuts against the rotating member to transfer the toner image to the recording medium that is conveyed to the transfer nip portion; and a control unit that controls the bias applying unit,
In the image forming apparatus including: a toner adhesion amount detection unit that detects a toner adhesion amount on the latent image carrier or the surface of the image carrier, and the bias applying unit cleans the toner adhered to the transfer rotator. A cleaning bias and a non-image portion bias that are applied to the transfer rotating body and / or the transfer facing rotating body are configured to be applied, and the control unit controls the next recording medium after the recording medium is delivered at the transfer nip portion. During the paper interval until the sheet is fed, the transfer rotating member can be cleaned by controlling the bias applying unit, and a background stain pattern consisting of only the non-image portion corresponding to the background portion is formed on the latent image carrier. is formed on the surface, the toner adhesion amount of the background contamination pattern based on a result of detection by the toner adhesion amount detection means, before expanding the sheet interval time It is solved by an image forming apparatus which comprises carrying out the cleaning of the transferring member.

The image forming apparatus of the present invention detects background stains on the OPC, the intermediate transfer belt, the secondary transfer belt, and the like, and widens the space between sheets according to the detected background stain amount to perform transfer cleaning. Therefore, it is possible to reliably recover the dirt state of the transfer rotator, and reliably prevent the back dirt and the edge surface dirt due to the dirt of the transfer rotator.

FIG. 1 is a schematic configuration diagram showing a configuration of a printer according to an embodiment of the present invention. FIG. 3 is a configuration diagram showing a main configuration of an image forming unit of the printer. FIG. 3 is a structural diagram showing a main configuration of an intermediate transfer unit of the printer. FIG. 3 is a block diagram showing a main part of an electric circuit of the printer. It is a flowchart which shows the flow of the arithmetic processing in process control. It is a schematic diagram explaining the patch pattern image on the intermediate transfer belt. FIG. 6 is a characteristic diagram showing a relationship between a development potential and a toner adhesion amount. 7 is a graph for explaining development potential and background potential. 7 is a graph showing the relationship between the background potential and the degree of background stains and carrier adhesion. 7 is a graph showing the relationship between charging potential Vd and 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 according to the first embodiment. It is an example of a paper stain detection pattern. Operation example of sheet-to-sheet bias cleaning (Example 1) Operation example of sheet-to-sheet bias cleaning (Example 2) Operation example of inter-sheet bias cleaning (Example 3) Operation example of sheet-to-sheet bias cleaning (Example 4) It is a schematic diagram of transfer bias cleaning. It is the result of evaluation of edge stains on the number of continuously printed sheets. It is a flowchart of the line dirt detection according to the second embodiment. FIG. 4 is a diagram showing a relationship between a background potential and a background stain toner amount. FIG. 6 is a diagram showing a relationship between toner concentration and the amount of background dirt toner. It is a flow chart of line dirt detection concerning a 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 showing a configuration of a printer according to an embodiment of the present invention. As shown in FIG. 1, this printer includes four image forming units 1Y, 1C, and 1M for forming images of yellow (Y), cyan (C), magenta (M), and black (K). Equipped with 1K. Hereinafter, the subscripts Y, C, M, and K of the reference numerals 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 a main configuration of an image forming unit of the printer. As shown in FIG. 2, around the drum-shaped photoconductor 2Y which is a latent image carrier provided in the image forming unit 1Y, a charging roller 3Y which is a charging unit, a developing device 4Y which is a developing unit, a cleaning device 5Y, etc. are provided. Is provided. The charging roller 3Y composed of a rubber roller is adapted to rotate while contacting the surface of the photoconductor 2Y. This printer employs a contact DC charging method in which a DC bias containing no AC component is applied to the charging roller 3Y as a charging bias. It should be noted that the charging roller 3Y can also adopt another method such as a contact AC charging roller method or a non-contact charging roller method. The charging bias is output from the charging power supply 50.

  The developing device 4Y contains a two-component developer containing yellow toner and a magnetic carrier. This two-component developer contains a toner having an average particle size of 4.9 to 5.5 μm and a small particle size / low resistance carrier having a bridge resistance of 12.1 [LogΩ · cm] or less. The developing device 4Y is composed of a developing roller 4aY which is a developer bearing member facing the photoconductor 2Y, a screw which conveys and agitates 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 that is included so as not to rotate with the sleeve.

  The image forming unit 1Y is configured as a process cartridge in which the photoconductor 2Y and the charging roller 3Y, the developing device 4Y, and the cleaning device 5Y arranged around the photoconductor 2Y are 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 body, and consumable parts can be replaced at once when the life of the image forming unit 1Y is reached. The other image forming units 1C, 1M, and 1K use cyan toner, magenta toner, and black toner as toners, but 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 as a latent image writing means is arranged. The optical writing unit 6 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and optically scans the surface of the photoconductors 2Y, 2C, 2M, and 2K of each color with the laser light L based on image data. I do. By this optical scanning, electrostatic latent images for yellow, cyan, magenta, and black are formed on the photoconductors 2Y, 2C, 2M, and 2K.

  Above the image forming units 1Y, 1C, 1M and 1K, an intermediate transfer unit that transfers the toner image of each color from the photoconductor (2Y, 2C, 2M, 2K) of each color onto the recording sheet S via the intermediate transfer belt 7. 8 are arranged. The intermediate transfer belt 7, which is an image carrier, is stretched over a plurality of rollers and is endlessly moved in the counterclockwise direction in the drawing by the rotational driving of at least one of the rollers. The intermediate transfer unit 8 includes, in addition to the intermediate transfer belt 7, primary transfer rollers 9Y, 9C, 9M, 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. Is equipped with.

  The primary transfer rollers 9Y, 9C, 9M, and 9K sandwich the intermediate transfer belt 7 between the photoconductors of the respective colors. As a result, primary transfer nips for Y, M, C, and K are formed in which the photoconductors 2Y, 2M, 2C, and 2K come into contact with the front surface of the intermediate transfer belt 7. The intermediate transfer unit 8 includes a secondary transfer roller 12 located outside the belt loop in the vicinity of the secondary transfer backup roller 11 on the downstream side of the image forming unit 1K in the belt moving direction. The secondary transfer roller 12 sandwiches the intermediate transfer belt 7 with the secondary transfer backup roller 11 to form a secondary transfer nip portion. The secondary transfer roller 12 is an example of a transfer rotating body, and the secondary transfer backup roller 11 is an example of a transfer facing rotating body.

  A fixing unit 13 is arranged above the secondary transfer roller 12. The fixing unit 13 includes a fixing roller and a pressure roller that are in contact with each other while rotating to form a fixing nip. The fixing roller has a halogen heater built therein, and electric power is supplied to the heater from a power source 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 lower part of the printer body, there are arranged paper feed cassettes 14a and 14b for accommodating a plurality of recording sheets S, which are recording media on which output images are recorded, a paper feed roller, a pair of registration rollers 15, and the like. In addition, a manual feed tray 14c for manually feeding paper from the side is provided on the side surface of the printer 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 conveying the recording sheet S again to the secondary transfer nip during double-sided printing.

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

  FIG. 3 is a structural diagram showing a main configuration of the intermediate transfer unit. The intermediate transfer belt 7 includes a tension roller 17 (positioned on the leftmost side in FIG. 3) that receives a tensile force by a spring, a secondary transfer backup roller 11, and an entrance roller 18 installed on the upstream side of the belt rotation. It is wrapped around a shaft. Further, the secondary transfer roller 12 is installed so as to face the secondary transfer backup roller 11 via the belt. Further, the cleaning device 10 is installed on the tension roller 17. Further, a power supply 29, which is a bias application unit capable of applying a secondary transfer bias, a cleaning bias, and a non-image portion bias having an absolute value smaller than that of the cleaning bias, and a control unit 30 for controlling the output of the power source 29 are set. There is. 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.

  The secondary transfer roller 12 is grounded directly or through a resistor in order to prevent interference between the primary transfer bias and the secondary transfer bias, 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 force roller to which a repulsive force bias (secondary transfer bias) is applied. A cleaning bias is applied to the repulsive roller during non-transfer. Due to this cleaning bias, an electrostatic force is generated between the secondary transfer roller 12 and the intermediate transfer belt 7. The toner attached 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 is removed from the secondary transfer roller 12 (hereinafter, this removing operation is referred to as bias cleaning). Call). Instead of the configuration of FIG. 3, the repulsive force roller (secondary transfer backup roller 11) may be grounded and the power source may be connected to the secondary transfer roller 12 to perform bias cleaning.

  The secondary transfer roller 12, which is a transfer member, is made of a sponge material having a resistance of 7 to 8 Ω, and may have a mechanism capable of coming into contact with and separating from the intermediate transfer belt 7. When the repulsive force roller applies a repulsive force 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). The same effect can be obtained by applying the secondary transfer bias as an attractive bias to the secondary transfer roller 12 side. In that case, the secondary transfer bias polarity is reversed. Instead of the secondary transfer roller 12, a secondary transfer belt that is also a transfer member may be used to perform the secondary transfer.

  Further, a mechanical cleaning mechanism (for example, a cleaning brush, a cleaning blade, a cleaning web, etc.) capable of contacting and separating from the secondary transfer roller 12 may be provided. Of course, the bias cleaning is effective even if it has a cleaning mechanism.

  Instead 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 photoconductor is directly transferred to a sheet.

  Next, the operation of the printer will be described. First, a predetermined voltage is applied from the charging power source 50 to the charging roller of the charging roller 3Y to charge the surface of the photoconductor 2Y that faces the charging roller 3Y. The optical writing unit 6 scans the surface of the photoconductor 2Y charged with a predetermined potential with the laser light L based on the image data, thereby writing an electrostatic latent image on the photoconductor 2Y. When the surface of the photoconductor 2Y carrying the electrostatic latent image reaches the developing device 4Y with the rotation of the photoconductor 2Y, the electrostatic latent image on the surface of the photoconductor 2Y is moved by the developing roller 4aY arranged to face the photoconductor 2Y. Y toner is supplied to the image. As a result, a Y toner image is formed on the surface of the photoconductor 2Y. In the developing device 4Y, an appropriate amount of Y toner is replenished from the toner replenishing container 17Y according to the output of the toner concentration sensor.

  The same operation is performed in the image forming units 1C, M, K at a predetermined timing. As a result, Y, C, M, and K toner images are formed on the surfaces of the photoconductors 2Y, 2C, 2M, and 2K. These Y, C, M, and K toner images are primary-transferred by being sequentially superposed on the front surface of the intermediate transfer belt 7 at the Y, C, M, and K primary transfer nips. 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 the transfer power supply.

  The recording sheet S is conveyed from any one of the paper feed cassettes 14a and 14b or the manual feed tray 14c, and is temporarily stopped when reaching the registration roller pair 15. Then, the registration roller pair 15 rotates at a predetermined timing to send the recording sheet S toward the secondary transfer nip.

  The Y, C, M, and K toner images superposed 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 contact each other. This secondary transfer is performed by applying a voltage having a polarity opposite to that of the toner to the secondary transfer roller 12 by the secondary transfer power supply. After leaving the secondary transfer nip, the recording sheet S is conveyed toward the fixing unit 13 and is nipped in the fixing nip. The toner image on the recording sheet S is heated and fixed by heat from the fixing roller at the fixing nip. The recording sheet S on which the toner image has been fixed is ejected to the outside of the apparatus by each conveying roller in the case of single-sided printing. Further, in the case of double-sided printing, the recording sheet S is conveyed to the double-sided unit 16 by each conveying roller and inverted, and the image is formed on the surface opposite to the surface on which the image is previously formed as described above. Is discharged to the outside of the aircraft.

  In this printer, control called process control is executed at a predetermined timing in order to stabilize the image quality with environmental changes and aging. In the process control processing, a Y patch pattern image composed of a plurality of patch-shaped Y toner images is developed on the photoconductor 2Y and transferred to the intermediate transfer belt 7. Further, C, M, K patch pattern images are similarly formed on the photoconductors 2C, 2M, 2K. Then, the amount of toner adhesion of each toner image in those patch pattern images 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 toner adhesion amount detection means.

  FIG. 4 is a block diagram showing a main part of an electric circuit of the printer. Further, FIG. 5 is a flowchart showing a 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 or paper feed tray. Further, the registration motor 82 is a drive source for the registration rollers.

  The optical sensor unit 20 has a plurality of reflective photosensors arranged at predetermined intervals in the belt width direction of the intermediate transfer belt 7. Each reflection type photo sensor is configured to output a signal according to the light reflectance of the intermediate transfer belt 7 and the patch-shaped toner image described later on the intermediate transfer belt 7. Four reflective photosensors are provided. Three of them capture both regular reflection light and diffuse reflection light on the surface of the belt so that output corresponding to Y, M, C toner images and Y, C, M adhered toner can be performed, and the respective light amounts Output according to. The other one captures only the specularly reflected light on the belt surface and outputs according to the amount of the light, like the output according to the K toner image or the K attached toner.

  The control unit 30 executes the process control processing at a predetermined timing, such as when the main power is turned on, when waiting after a predetermined time has elapsed, 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 passed sheets, the printing rate, the temperature and the humidity is acquired (step S1). Next, the respective developing characteristics in the image forming units 1Y, 1C, 1M and 1K are grasped. Specifically, the development gamma γ and the development start voltage Vk are calculated for each color (step S2). Specifically, it is performed as follows. That is, the photoconductors 2Y, 2C, 2M, and 2K are rotated and uniformly charged. Regarding this charging, the absolute value of the charging bias Vc is increased, unlike a uniform value (for example, −700 V) during normal printing. Electrostatics for the patch-shaped Y toner image, patch-shaped C toner image, patch-shaped M toner image, and patch-shaped K toner image are applied to the photoconductors 2Y, 2C, 2M, and 2K by scanning the laser light L by the optical writing unit 6. Form a latent image. By developing them with the developing devices 12Y, 12C, 12M, and 12K, Y, C, M, and K patch pattern images are formed on the photoconductors 2Y, 2C, 2M, and 2K. During development, the control unit 30 gradually increases the absolute value of the development bias Vb applied to the development roller (4a) of each color. The developing bias Vb and the charging bias Vc are both 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 portion of the intermediate transfer belt 7 in the width direction. Further, the C patch pattern image CPP is transferred to a position slightly displaced to the center side from the Y patch pattern image in the belt width direction. The M patch pattern image MPP is transferred to the other end of the intermediate transfer belt 7 in the width direction. Further, the K patch pattern image KPP is transferred to a position slightly displaced to the center side from the K patch pattern image in the belt width direction.

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

  The first reflective photosensor 20a is arranged at a position for detecting the Y toner adhesion amount of the patch-shaped Y toner image of the Y patch pattern image YPP formed at one end of the intermediate transfer belt 7 in the width direction. The second reflection type photo sensor 20b is arranged at a position for detecting the C toner adhesion amount of the patch 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 reflection type photo sensor 20d is arranged at a position for detecting the M toner adhesion amount of the patch M toner image of the M patch pattern image MPP formed on the other end portion of the intermediate transfer belt 7 in the width direction. ing. Further, the third reflection type photo sensor 20c is arranged at a position for detecting the K toner adhesion amount of the patch 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 reflective photosensor 20a, the second reflective photosensor 20b, and the fourth reflective photosensor 20d have three toner image colors other than black (Y, C, and M). If so, the toner adhesion amount can be detected.

  The control unit 30 calculates the light reflectance of the patch-shaped toner image of each color 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. Note that the patch pattern image of each color that has passed the position facing the optical sensor unit 20 as the intermediate transfer belt 7 runs is cleaned by the cleaning device 10 from the front surface of the belt.

  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, the linear approximation formula (Y = a × Vb + b) shown in FIG. To calculate. In the two-dimensional coordinates in the figure, the x-axis shows a value obtained by subtracting the developing bias Vb applied at that time from the exposure portion potential Vl, that is, the developing potential (V1-Vb). The Y axis shows 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-shaped toner images. The section on the XY plane where the linear approximation is performed is determined based on the plurality of plotted data. Then, the least squares method is performed in the section 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 each color are calculated (step S2).

  Next, based on the obtained developing characteristics, the target value of the charging potential (ground potential) Vd (target charging potential), the target value of the exposed portion potential Vl (target exposed portion potential), and the developing bias Vb are determined. (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 set in advance. This makes it possible to select the target charging potential and the 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 that can obtain the development potential is obtained. Then, the target charging potential is obtained based on the developing bias Vb and the background potential. Since the surface of the developing sleeve of the developing roller has almost the same value as the developing bias Vb, if the surface of the photoconductor is charged to a target charging potential and is appropriately exposed, a target developing potential or background potential is obtained. be able to.

  The control unit 30 then determines the charging bias Vc. Specifically, the charging bias Vc at which the target charging potential is obtained changes according to the amount of wear of the surface layer of the photoconductor, the electrical resistance of the charging roller that is affected by the environment, and the like. Therefore, the control unit 30 stores an algorithm for obtaining the charging bias Vc capable of obtaining the target charging potential from the combination of the environment (temperature and humidity) and the photosensitive member travel distance. This algorithm was built based on previous experiments. Then, the charging bias Vc capable of obtaining the target charging potential is obtained by using an algorithm based on the combination of the temperature / humidity detection result of the environment sensor 52 and the photoconductor traveling distance stored in the RAM.

  As a property of the developer, background stain is worse in the initial stage than in the initial stage, and conversely, carrier adhesion (edge carrier attachment) is worse in the initial stage than in the initial stage. Therefore, the optimum background potential shifts to a larger value as the developer is used. Further, generally, in a high temperature and high humidity environment, the amount of electrostatic charge of the toner is low, so that the background stain becomes worse, and conversely, in a low temperature and low humidity environment, carrier adhesion becomes disadvantageous. For this reason, in the image density control according to the present embodiment, the background potential is shifted to an optimum value in the initial / age + time environment.

  The optimum ground potential for keeping the background dirt and carrier adhesion below the target has already been found under each condition by experiments. Therefore, if there is environmental information such as deterioration of the charging roller and the carrier and changes in temperature and humidity, it is possible to make some correction. However, there is a possibility that the optimum background potential may fluctuate due to an error from the experiment and an unexpected factor. On the other hand, since the development start voltage Vk can be considered as the voltage at which the development on the photoconductor 2 is started, it is considered that the background stain is deteriorated unless there is a background potential equal to or higher than the absolute value of the development start voltage Vk.

  Therefore, as shown in FIG. 5, the control unit 30 determines the target development start voltage Vk 'after the step S3 (step S4). The target development start voltage Vk 'has been tabulated in advance by experiment with environment information, and the target development start voltage Vk' is determined by referring to the table from the environmental information acquired first. 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 more than + 40V away from the target development start voltage Vk ', it is divided into category 1, category 2 below + 40V + 20V or above, category 3 below + 20V 0V or above. Then, the division of the development start voltage Vk is specified, and the correction amount is determined for each division (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 developing bias Vb, and acts on the non-image portion (background portion) of the image. When the background potential is small, the background stain is likely to occur, while 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 stain and carrier adhesion. In this example, the theoretical value of the background potential is set to 140 [V] by performing the process control. The reason for expressing it as a theoretical value is for the reason explained below. That is, as described above, the background potential is determined by the process control based on the relationship between the appropriate charging potential Vd and the developing bias Vb, and the charging bias Vc is determined based on the background potential. However, due to the charging bias Vc, the charging potential Vd does not always become the target charging potential. This is because the discharge start voltage between the charging roller and the photoconductor changes due to various factors, which changes the charging bias Vc for obtaining the same charging potential Vd. In the process control, the environment and the photosensitive member travel distance are taken into consideration when determining the charging bias Vc, but this is not always the case because it is based on a theoretical algorithm. Further, the value of the charging bias Vc for obtaining the same charging potential Vd changes depending on the environment and other parameters different from the photosensitive member travel distance.

  In the example shown in the figure, if the background potential is 140 [V], both background stain and carrier adhesion can be suppressed. Therefore, the control unit 30 determines the target charging potential so that a background potential of 140 [V] and a desired developing potential can be obtained during the process control. However, since the value of the charging bias Vc for obtaining the charging potential Vd changes due to 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 significantly from the target charging potential (140V in the illustrated example). Then, in the same figure, the actual background potential exceeds 170V and carrier adhesion occurs, or the actual background potential falls below 110V and scumming 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 the characteristic represented by the formula “Vd = a × Vc + b”. a is the slope of the graph shown in FIG. 10, b is the Vd axis intercept in the graph, and has a negative value. The Vc axis intercept in the graph has almost the same value as the discharge start voltage between the charging roller and the photoconductor. Further, the inclination a becomes almost 1.

  As described above, this printer employs the contact DC charging method in which a charging bias composed of only a DC component is applied to the charging roller that is in contact with the photoconductor. The contact DC charging method does not require an AC power source, unlike the method of using an AC / DC superimposing bias as a charging bias, so that the cost can be reduced. 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 larger than the discharge start voltage shown in the graph of FIG. No discharge can be generated between them and the photoreceptor cannot be charged at all. Even if the charging can be performed, the discharge start voltage varies depending on the environment, the amount of wear of the surface layer of the photoconductor, the electric resistance of the charging roller, the amount of dirt, and the like. Therefore, under the same charging bias Vc condition. The charging potential Vd fluctuates. Therefore, it becomes 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 which is a toner adhesion amount detecting means is installed so as to face the intermediate transfer belt and constitutes a part of a light amount control device. FIG. 11 is a schematic sectional view of the optical sensor 102. As shown in the figure, the optical sensor 102 mainly includes 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 the regular reflection light, and a diffuse reflection light. And a diffuse reflection light receiving element 313 as a second light receiving means for doing so. The light emitted from the light emitting element 311 is emitted toward the surface of the intermediate transfer belt 41. Then, the specular reflection light receiving element 312 receives the specular reflection light specularly reflected by the surface of the intermediate transfer belt 41 or the toner patch transferred to the surface, and outputs a voltage according to the amount of received light. Further, the diffuse reflection light receiving element 313 receives the diffuse reflection light diffused and reflected by the surface of the intermediate transfer belt 41 or the toner patch transferred to the surface, and outputs a voltage according to the amount of received light.

  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. As the specular 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, which has no significant difference in reflectance depending on colors. By using such an optical sensor, one sensor can detect toner patches of all colors of Y, M, C, and K.

  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 detection flow of background dirt. As shown in FIG. 12, when a print command is issued (S0), process control is executed (S1). In the process control, the toner density (TC) is adjusted as needed (S2), and then the charging bias (Vd), Vl (exposure potential), and Vb (developing bias) are calculated (S3). After that, printing is started (S4), and inter-paper background stain detection is performed during printing (S5).

  FIG. 13 is an example of a pattern for detecting line background dirt. In the figure, the patch position is located outside the edge of the sheet, but it may be located in the center of the sheet. Alternatively, a plurality of patches may be arranged in the main scanning direction and the average thereof may be calculated. In the drawing, the pattern is shown as a patch for the sake of convenience, but the image forming conditions for development, primary transfer, and secondary transfer are not in the form of a patch because a normal bias between sheets is applied.

  Returning to the flow of FIG. A predicted value g of backside stain obtained from the amount of background stain on the sheet detected by the optical sensor 102 and the time between sheets (the time from when a recording medium is fed at the transfer nip portion until the next recording medium is fed) is obtained. When it is smaller than the certain allowable value S (S6), the printing is not performed without extending the paper interval, and normal printing is performed. If it is larger 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 for cleaning (S8). Perform printing. The constant allowable value S may be determined by sensory evaluation so that the back surface stain and edge surface stain are inconspicuous, or may be freely set.

  Next, the transfer cleaning of this embodiment will be described. In addition to the toner image developed on the latent image, on the photosensitive member 2 which is the latent image carrier, there is a background stain toner that is not attached to an appropriate charge amount or that is attached to the background portion by 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 correction amount of the charging bias is determined (S6), and the correction amount is added to the background potential to determine Vc (S7), so that the amount of the background stain toner is controlled to be equal to or less than the target value. is doing.

  However, if the background potential is changed, the tint variation and line thickness variation occur. Therefore, upper and lower limits are set for the correction value of the charging bias. In other words, the amount of background stain toner may not be sufficiently suppressed only by correcting the charging bias.

  Therefore, 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, but in a transfer roller method, cleaning is not performed mechanically with the cleaning blade, and a bias is applied to the photosensitive member by an electric force. There is also a method of returning the toner that stains the background. In particular, the transfer roller method has a simple structure and is small in size, and is therefore widely adopted at present.

  In such a transfer roller method, bias cleaning is generally performed before and after image formation.However, in the case of continuous printing, the bias cleaning does not enter in the middle of printing, and dirty toner accumulates on the transfer roller. There is a possibility that it will end up. If dirty toner accumulates, it may move to the transfer paper and cause backside dirt or edge dirt. For this reason, there is also a method of suppressing the generation of stains by inserting bias cleaning between sheets (between sheets at the time of printing) according to the number of sheets passed during the image forming operation.

  Next, the bias cleaning operation between sheets, which is a feature of the present invention, will be described with reference to Examples 1 to 4 of FIGS. In the drawing, as an example of the present invention, an example in which the paper interval is different is shown for the sake of clarity, and the paper interval is long from Examples 1 to 4. In the present invention, arithmetic processing is performed to determine the sheet interval and perform an operation according to the sheet interval time.

-[Paper interval] <[Transfer bias cleaning time] + [Judgment threshold A] ... Condition (1)
In the case of 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 lengthened to perform the transfer cleaning. Carry out. The prediction value (g) of the backside stain is calculated by using the prediction formula (1) obtained from the relationship between the backside stain amount and the amount of background stain, which is obtained in advance by an experiment.
[Predicted value of back stain (g)] = α × [background stain amount of latent image carrier] × [interval time] ... Prediction formula (1)
Here, α is a coefficient obtained from an experiment in advance for each type of transfer paper. Even if the amount of background stain is the same, the amount of back stain may differ depending on the type of transfer paper (or the characteristics such as smoothness and whiteness), so it is necessary to conduct an experiment in advance for each type of transfer paper. Good.

  The number of times of cleaning at that time may be changed according to the predicted value of the back stain. When the back stain amount is large, the back stain amount does not decrease to a certain allowable value in one cleaning, and thus the back stain after cleaning is improved by performing the cleaning a plurality of times. When the predicted value (g) of the back smear 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, it is possible to perform the transfer cleaning as many times as necessary, when necessary, so that it is possible to prevent back stains and edge stains from occurring even when the paper interval is shorter than the cleaning time. If the transfer body is less contaminated, cleaning is not performed and productivity can be prevented from decreasing. Further, in the present embodiment, the background dirt on the transfer body (OPC, intermediate transfer belt, secondary transfer belt, etc.) is detected, and the paper interval is widened for a necessary time according to the detected background dirt amount to perform the transfer cleaning. By inserting it, it is possible to reliably prevent the backside dirt and the edge surface dirt due to the dirt on the transfer roller. In addition, when the transfer body is not dirty, cleaning can not be performed and unnecessary rotation of the motor can be prevented, so wasteful power consumption can be suppressed, and units and supplies (developer life, photoreceptor life, etc.) can be suppressed. It is possible to prevent the life of the product from being shortened.

・ [Paper interval] ≧ [transfer bias cleaning time] + [judgment threshold A] ... Condition (2)
In the case of the condition (2), the back stain amount is predicted from the detected background stain amount and the paper interval time, and when 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 value A is a coefficient that is set assuming that the position of the sheet is misaligned during conveyance, bias switching control time, and the like. If this coefficient is made too small, cleaning control may be terminated before the paper arrives, and the output of the image part may not be in time. If it is made too large, bias cleaning tends to be difficult to enter, so set an appropriate value. 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 time between sheets is shorter than the transfer cleaning time, and the case where the bias cleaning is not performed is shown. This is equivalent to the case where the predicted value (g) of the back stain is less than the constant allowable value S, and thus the transfer cleaning is not performed without increasing the paper interval time.

  In the image forming operation described in the second embodiment (see FIG. 15), the paper interval time is made equal to the transfer cleaning time, and the negative bias cleaning and the positive bias cleaning for one round of the transfer roller are performed once. The following shows the case where each is put (1 set). This is equivalent to the case where transfer cleaning is performed with the paper interval time lengthened because the back stain prediction value (g) is greater than the constant allowable value S.

  In the image forming operation described in the third embodiment (see FIG. 16), the paper interval time is increased to twice the transfer cleaning time, and the negative bias cleaning and the positive bias cleaning for one round of the transfer roller are performed for each two. The case where they are put in each time (2 sets) is shown. This is because the predicted value (g) of the back stain was much larger than the constant allowable value S, so the back stain did not improve up to the allowable value S in one set of cleaning, and the paper interval time was lengthened to make two sets of transfer cleaning. It corresponds to the case where is carried out.

  In the image forming operation described in Embodiment 4 (see FIG. 17), the paper interval time is longer than the transfer cleaning time, and the negative bias cleaning and the positive bias cleaning for one round of the transfer roller are performed immediately before the next printing. The case where each was put once (1 set) is shown.

  Again, by carrying out the above-described control (Examples 1 to 4), the transfer cleaning can be carried out as many times as necessary and when necessary, so that even when the paper interval is shorter than the cleaning time, back stain, It is possible to prevent the occurrence of dirt on the edge surface, and when the dirt on the transfer body is small, cleaning is not required, and a decrease in productivity can be prevented.

  Further, according to the present invention, the background stain on the transfer body (OPC, intermediate transfer belt, secondary transfer belt, etc.) is detected, and the sheet interval is widened for a necessary time according to the detected background stain amount, and the transfer cleaning is performed. As a result, it is possible to reliably prevent the backside dirt and the edge surface dirt due to the dirt on the transfer roller. In addition, when the transfer body is less contaminated, unnecessary rotation of the motor can be prevented because cleaning is not entered, so wasteful power consumption can be suppressed, and units and supplies (developer life, photoreceptor life, etc.) can be suppressed. It is possible to prevent the life of the product from being shortened.

  FIG. 18 shows a schematic diagram of the transfer bias cleaning. Between sheets of paper, the reversely charged toner or the backgroundly soiled toner of the reversely charged toner is present on the photoconductor, and the toner is moved to the transfer roller, whereby the dirt on the roller is accumulated. The drawing called "paper interval" shows the transfer roller on which dirty toner has accumulated, and the circles marked + and-show the toner of that polarity.

  Next, in the figure described as "transfer cleaning (-)", the positively charged toner adhering 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. You can Then, in the drawing 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 stain toner that stains the transfer roller is a bipolar toner, it is possible to more effectively prevent the occurrence of back stain and edge stain by applying both biases. Also in Examples 2 to 4, the transfer bias cleaning operation is included, and the plastic screening is performed after the minus cleaning.

Further, when the sheet interval is long as in the fourth embodiment, even if the transfer bias cleaning is performed immediately after entering the sheet interval, the transfer roller may be soiled again between the subsequent sheets. Therefore, as shown in the equation (1), by setting the value of the margin coefficient c and performing the bias cleaning operation before this value, the dirty toner accumulated during the paper interval for a long time is removed. It is possible to carry out cleaning in the latter half of the above and to efficiently prevent the occurrence of backside dirt and edge surface 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 when the transfer bias between the sheets is cleaned is set to the plus side. This aims at suppressing the accumulation of dirt of the normally charged toner (minus) on the transfer roller between the sheets by setting it to the positive side, but it may be set to 0 μA. By setting it to 0 μA, it may be possible to suppress the accumulation of dirt on the transfer roller between the sheets. If there is a large amount of dirt toner accumulated on the transfer roller, it may not be possible to completely clean the dirt toner within the limited transfer bias cleaning time. There is also a possibility that it will not occur.

  Further, by applying the cleaning bias once or more around the transfer roller, it is possible to clean the dirt on the entire circumference of the transfer roller. There is a case where the transfer roller cannot be completely cleaned with only one rotation. In the third embodiment, the negative bias cleaning and the positive bias cleaning are applied for two rotations of the transfer roller. If the cleaning bias application time is long, the cleaning effect is further obtained, but the cleaning time becomes long and the cleaning is difficult to enter. Therefore, 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 optimize each cleaning time.

  The accumulation of dirt on the transfer roller due to the paper-to-paper extension occurs not only in the intermediate transfer method but also in the direct transfer method which is often used in monochrome machines and the like. Therefore, the present invention is not limited to the intermediate transfer method, and the direct transfer method, in which dirt on the transfer roller is more likely to be accumulated, is more effective.

  Further, although the bias cleaning is effective also in the transfer belt method, it is possible to perform the constant cleaning by the cleaning blade. However, in the transfer roller system, bias cleaning is generally used because it is small in size and low in cost, and the effect of the inter-sheet bias cleaning of the present invention is further obtained in the transfer roller system image forming apparatus.

  When the process linear velocity changes, the execution bias with respect to the transfer current value changes. Therefore, in order to perform proper cleaning, when controlling at multiple process linear velocities, the cleaning output (current value) is controlled for each linear velocity. It is desirable to do. Examples are shown in Table 1.

  Since the printing speed of plain paper is the standard speed, and the fixing property of thick paper is poor, it is necessary to output an image at a lower linear speed. In the examples shown in Table 1, the standard speed is 260 mm / s, 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 correction coefficients are provided for medium speed and low speed, and the output is determined by transfer CL (260 mm / s) × correction coefficient / 100. This makes it possible to prevent the hazard to the photoconductor by changing the cleaning property by applying a cleaning output more than necessary.

  Further, 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. It has been known that these appropriate cleaning output values shift when the environment changes due to changes in resistance of the transfer member and the paper. Therefore, in the embodiment of the present invention, the image forming apparatus is provided with the environment sensor capable of detecting the temperature and the 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 table of Table 2.

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

  FIG. 19 shows the result of evaluation of edge stains on the number of continuously printed sheets. The edge surface dirt rank is a step-by-step ranking of the edge surface dirt, and the standard is rank 2 or higher. Rank 2 is a judgment that the level of dirt is dirty but is usually not noticeable. The rank is 5 levels in all, and the rank 5 shows a state in which there is no stain.

  As the evaluation condition, a condition (temperature 27 ° C., humidity 80%) where contamination is most likely to occur in the image forming apparatus used for the evaluation is selected. As for the background stain on the photoconductor, a threshold was drawn on the paper at a level at which the background stain was not noticeable, and the stain was carried out on the photoconductor on that level. Further, since the transfer roller tends to become dirty as time passes 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 a condition in which cleaning is not performed between paper sheets, and the transition of stains between normal paper sheets is evaluated. The condition described as "peripheral machine 5 sheets bound" is a condition in which the paper interval becomes very long (about 10 seconds) depending on the stapling time. The conditions described as "Example" are the above-described examples of the present invention, and are conditions under which bias cleaning was performed for one cycle of the negative electrode and one cycle of the positive electrode between the sheets.

  It can be seen from FIG. 19 that the bias cleaning is not performed even in the normal paper interval, and thus the stain tends to be deteriorated with respect to the number of printed sheets. Further, when the stapling operation is performed by the peripheral machine, the paper interval is extended, so that it can be seen that the stain is remarkably deteriorated. As a method of smearing, the number of printed sheets that falls below the standard is reduced to about 1/6 of the normal number. On the other hand, in the examples of the present invention, it was possible to prevent the deterioration of the edge surface smear by the paper-to-paper cleaning, and no remarkable deterioration of the edge surface smear was confirmed regardless of the number of printed sheets.

(Second embodiment)
By performing the control shown in FIG. 12, back stains and edge stains can be prevented, but as long as the predicted value (g) of the back stains in S6 of FIG. However, there is a problem in that productivity is reduced because the transfer cleaning is performed by widening the area.

  Therefore, the background potential (difference between the charging bias Vd and the developing bias Vb), which is the difference between the surface potential of the photoconductor during printing and the developing potential of the developing unit, is widened while the sheet interval is widened and the transfer cleaning is performed. Control is performed to improve the background stain that is the cause of 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 backside stain is controlled to be equal to or lower than a certain allowable value S. To do.

  Here, the background potential to be set may be determined from the relationship between the background potential and the background stain acquired in advance by an experiment. FIG. 21 shows an example of the relationship between the background potential and the background stain acquired for each toner concentration. Although the same background potential indicates that the higher the toner concentration, the larger the amount of background stain toner, but since the slope is almost constant regardless of the toner concentration, the background potential change amount does not depend on the toner concentration. You may decide from only.

  Further, for example, it may be determined from the relationship between the background potential and the background stain that are actually acquired by the detection unit as disclosed in Patent Document 2. Further, the background potential is changed little by little when the paper is passed, and it is changed so that the predicted value (g) of the backside stain is gradually approached to a certain allowable value S in the detection of the background stain between the sheets after the next time. Is also good. If the background potential is greatly changed at one time, the edge effect changes abruptly, so that the dot size change and the line width change become apparent. Further, in a full-color image, there is a problem that the variation of the tint becomes conspicuous. Therefore, it is effective to gradually change the tint.

(Third Embodiment)
By carrying out the control shown in FIG. 20, back stain and edge stain can be prevented without lowering the productivity. However, the image quality may deteriorate. As shown in FIG. 9, when the background potential is changed to the larger side, carrier adhesion is deteriorated. Further, as described in the second embodiment, since the edge effect changes, the dot size changes and the line width changes. Further, if the dot size changes, the tint of the full-color image will change.

  FIG. 22 shows an example of the relationship between the toner density and the background stain. The amount of background toner tends to increase as the toner concentration increases, because toner charging decreases. Therefore, in the present embodiment, the control is performed to decrease the toner concentration during printing and increase the toner charge amount while the paper interval is widened and the transfer cleaning is performed. As a result, the background stain, which is the cause of the back stain and the edge stain, can be improved.

  According to the control shown in FIG. 23, the toner density to be set is calculated (S2), and the toner density is gradually decreased between the sheets, so that the predicted value (g) of the back stain is controlled to be equal to or lower than a certain allowable value S. To do. The method of lowering the toner density may be performed by forming an image of a discharge pattern on the image bearing member, or may be performed by normal image formation with less toner replenishment. The method of reducing the toner concentration to improve the background stain is superior to the method of increasing the background potential to improve the background stain with less deterioration in image quality. Of course, the toner concentration may be lowered in combination with the change of the background potential.

  The present invention has been described in detail above using the embodiments. This embodiment is an example, and can be variously modified and used without departing from the scope of the invention. For example, the configurations of the first to third embodiments may be combined and used.

1C, 1K, 1M, 1Y Image forming unit 2C, 2K, 2M, 2Y Photoconductor 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 Double-sided unit 17 Tension roller 17C , 17K, 17M, 17Y Toner supply container 18 Inlet 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 source 52 environment 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 JP, 2005-087563, A

Claims (4)

A latent image carrier,
Charging means for charging the moving surface of the latent image carrier,
A charging power source for outputting a charging bias for supplying to the charging means,
A 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 on which a toner image is carried by being driven so as to travel in a predetermined direction,
A latent image carrier, or a transfer rotary member that contacts the image carrier to form a transfer nip portion;
A transfer bias is applied to the transfer rotary member and / or a transfer counter rotary member that is in contact with the transfer rotary member via the image carrier to transfer a toner image to a recording medium conveyed to the transfer nip portion. A bias applying section,
A control unit for controlling the bias applying unit,
In an image forming apparatus including
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 part bias for cleaning the toner adhering to the transfer rotator to the transfer rotator and / or the transfer counter rotator.
The control unit can control the bias applying unit and perform cleaning of the transfer rotary member during a paper interval time from when a recording medium is fed at the transfer nip portion until when the next recording medium is fed. ,
A background stain pattern formed of only a non-image portion corresponding to the background portion is formed on the surface of the latent image carrier, and based on the result of detecting the toner adhesion amount of the background stain pattern by the toner adhesion amount detection means, the paper An image forming apparatus, characterized in that cleaning of the transfer rotary member is performed with an increased time interval.   The image forming apparatus according to claim 1, further comprising a cleaning blade that can contact and separate from the transfer rotating body. The control is performed to widen the difference between the surface potential of the latent image carrier and the developing potential of the developing means (background potential) at the same time as cleaning the transfer rotator by extending the paper interval time. The image forming apparatus according to 1 or 2. The image forming apparatus according to any one of claims 1 to 3, wherein the time between sheets is extended to perform cleaning of the transfer rotator, and at the same time, control is performed to reduce the toner concentration during printing.

Images