JP5247223B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus for transferring a toner image carried on an image carrier by a transfer unit using a transfer member, and more particularly to control for reducing fogging defects when a toner charge amount is reduced.

  A wide range of image forming apparatuses that transfer a toner image carried on an image carrier (photosensitive member or intermediate transfer member) onto a transfer medium (recording material or intermediate transfer member) at a transfer unit using a transfer member such as a transfer roller. It is used. Further, the density of the control toner image (color patch) formed by the image forming means (exposure device, developing device, etc.) and carried on the image carrier is measured by an optical sensor to determine the operating conditions of the image forming means. An image forming apparatus to be set has been put into practical use. Further, an image forming apparatus that sets a constant voltage to be applied to the transfer portion during image formation according to a current value measured by applying a voltage to the transfer portion during non-image formation has been put into practical use.

  Various proposals have heretofore been made to reduce such image defects called “fogging”, “ground fogging”, and “background smudge”.

  Patent Document 1 discloses that a control toner image is formed by changing a DC voltage of a development voltage in a plurality of stages, and a dark portion potential is detected with respect to a background stain on a white background based on a density measurement result of the control toner image by an optical sensor. Control for optimizing exposure intensity and DC voltage is described.

  In Patent Document 2, the amount of scumming toner on the white background portion of the photoconductor is measured using an optical sensor, and the two-component developer is stirred in the developing device until the scumming toner amount on the white background portion becomes a predetermined value or less. The control to perform is described.

  Patent Document 3 describes a control for measuring the accumulated usage time of the photosensitive member and the developing device and optimizing the charging condition and the developing condition of the photosensitive member according to the accumulated usage time with respect to the background stain on the white background. ing.

  Patent Document 4 describes control for setting the frequency of the alternating voltage of the development voltage in accordance with the density measurement result of the white background portion of the photoreceptor using an optical sensor. Here, the density measurement of the white background portion of the photosensitive member is performed by changing the fog removal contrast Vback, which is a potential difference between the dark portion potential VD of the photosensitive member and the DC voltage Vdc of the developing voltage, in a plurality of stages.

Japanese Unexamined Patent Publication No. 7-98526 Japanese Patent Application Laid-Open No. 11-237772 JP 2003-76075 A JP 2006-259101 A

  Normally, when the charge amount of the toner is reduced, the transfer current necessary for transferring from the image carrier to the recording material or the intermediate transfer member is lowered, so the voltage applied to the transfer portion at the time of image formation is set low. It is desirable.

  However, it has been found that if the voltage applied to the transfer portion during image formation is set low with the toner charge amount reduced, the transfer efficiency of the toner image increases, but the “fogging” image defect deteriorates. (See FIG. 4).

  Therefore, it has been proposed to increase the fog removal contrast Vback, which is the potential difference between the white background potential of the image carrier and the DC voltage of the development voltage. However, since the electrostatic force sufficient to overcome the physical adhesive force cannot be obtained for toner having an extremely small amount of charge that increases as the charge amount of the toner decreases, there is almost no effect.

  It is an object of the present invention to provide an image forming apparatus capable of suppressing fog image defects by reducing the amount of toner transferred from an image carrier to a recording material or a white background portion of an intermediate transfer member when the toner charge amount is reduced. It is aimed.

The image forming apparatus of the present invention includes an image bearing member, an image forming means for forming a toner image on said image bearing member, a toner image of the image bearing member is pressed against the image carrier direction via a transfer medium The image forming apparatus includes a transfer member that transfers to the transfer medium at a transfer portion, and power supply means that applies an electric field to the transfer portion. In the case of a detection unit that detects information that reflects the fog amount that is the amount of toner that adheres to the non-image area, and a condition that the detection result of the detection unit increases from the first fog amount to the second fog amount The electric field applied to the transfer portion while the region of the image carrier on which the toner image transferred to the recording material is carried passes through the transfer portion. Control means for controlling the power supply means so that the absolute value is larger than the electric field applied to the transfer portion .

  In the image forming apparatus of the present invention, the voltage applied to the transfer portion is increased when the charge amount of the toner is reduced, thereby increasing the discharge amount at the transfer portion. Since the toner transferred to the recording material or the intermediate transfer member at the transfer portion is exposed to the discharge of the transfer portion and is charged to a high potential of reverse polarity, it is electrostatically pulled back to the image carrier in response to the electric field of the transfer portion. .

  The transfer efficiency decreases by increasing the voltage applied to the transfer part, but the decrease in image density due to the decrease in the amount of applied toner is more visible than the background smudges that are noticeable due to toner adhering to the white background part of the image. Does not impair image quality.

  Accordingly, when the charge amount of the toner is reduced, the toner transferred from the image carrier to the recording material or the white background portion of the intermediate transfer member can be reduced to suppress the fogged image defect.

  Hereinafter, some embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as long as the voltage applied to the transfer portion is increased when the charge amount of the toner is reduced, another embodiment in which a part or all of the configuration of each embodiment is replaced with the alternative configuration. But it can be done.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general structure and control of the image forming apparatus shown by patent documents 1-4, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted. In addition, the reference symbols in parentheses shown in the configuration names used in the claims are examples for assisting understanding of the invention, and are not intended to limit the configuration to the corresponding members in the embodiments. Absent.

<First Embodiment>
FIG. 1 is an explanatory diagram of a configuration of the image forming apparatus according to the first embodiment.

  As shown in FIG. 1, the image forming apparatus 100 is an intermediate transfer type tandem type in which yellow, magenta, cyan, and black image forming portions Pa, Pb, Pc, and Pd are disposed along an intermediate transfer belt 11. .

The intermediate transfer belt 11 is stretched and supported by a drive roller 13, a tension roller 12, and a secondary transfer inner roller 24, and is driven by the drive roller 13 to rotate in the direction of arrow R2 at a predetermined process speed.

  In the image forming portion Pa, a yellow toner image is formed on the photosensitive drum 1 a and is primarily transferred to the intermediate transfer belt 11. In the image forming unit Pb, a magenta toner image is formed on the photosensitive drum 1 b and is primarily transferred to the yellow toner image on the intermediate transfer belt 11. In the image forming portions Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 1c and 1d, respectively, and similarly, the toner images on the intermediate transfer belt 11 are overlapped with each other and sequentially primary transferred.

  The four-color toner images primarily transferred to the intermediate transfer belt 11 are collectively secondary-transferred to the recording material P fed to the secondary transfer portion T2. The recording material P onto which the toner image has been secondarily transferred at the secondary transfer portion T2 is heated and pressed by the fixing device 27, and the toner image is fixed on the surface, and then discharged to the outside.

  The recording material P drawn from the recording material cassette 20 by the pickup roller 21 is separated one by one by the separation roller 22 and stands by by the registration roller 23. The registration roller 23 feeds the recording material P to the secondary transfer portion T2 in time with the toner image on the intermediate transfer belt 11.

  The electrostatic cleaning device 30 removes transfer residual toner attached to the intermediate transfer belt 11 that has passed through the secondary transfer portion T2. The web cleaning device 40 removes fine particles such as external additives attached to the intermediate transfer belt 11 that has passed through the electrostatic cleaning device 30.

  The image forming portions Pa, Pb, Pc, and Pd are configured in substantially the same manner except that the color of toner used in the developing devices 4a, 4b, 4c, and 4d is different from yellow, magenta, cyan, and black. Hereinafter, the image forming unit Pa will be described with reference to FIG. 2, and the image forming units Pb, Pc, and Pd will be described by replacing the reference symbol a attached to the constituent members with b, c, and d. Shall.

<Image forming means>
FIG. 2 is an explanatory diagram of the configuration of the image forming unit.

  As shown in FIG. 2, a charging roller 2a, an exposure device 3a, a developing device 4a, an optical sensor 14a, a primary transfer roller 5a, and a cleaning device 6a are arranged around the photosensitive drum 1a.

  The photosensitive drum (image carrier) 1a is configured by applying an organic photoconductor layer (OPC) to the outer peripheral surface of an aluminum cylinder. The photosensitive drum 1a is rotatably supported at both ends by a flange, and is rotated in the direction of the arrow R1 by transmitting a driving force from a driving motor (not shown) to one end.

  The charging roller 2a is configured by disposing a conductive layer made of a rubber material on the outer periphery of a conductive metal core disposed at the center, and both ends of the metal core are rotatably supported by a bearing member (not shown). The charging roller 2a is pressed against the surface of the photosensitive drum 1a with a predetermined pressing force, and is driven to rotate as the photosensitive drum 1a rotates in the arrow R1 direction.

  The power source D3 applies a charging voltage obtained by superimposing an AC voltage on a DC voltage to the charging roller 2a, and contacts and charges the surface of the photosensitive drum 1a to a negative and uniform uniform potential.

  The exposure device 3a scans the surface of the photosensitive drum 1a with a laser beam that is ON-OFF modulated in accordance with an image signal obtained by developing the image data, and writes an electrostatic image of the image on the surface of the photosensitive drum 1a.

  The developing device 4a develops an electrostatic image by a two-component developing method using a two-component developer containing a magnetic carrier having a positive charge polarity and a non-magnetic toner having a negative charge polarity. The developing device 4a conveys and circulates the two-component developer in the axial direction while stirring and charging with the stirring screw 4h. In the process of circulating the two-component developer in the developing device 4a, the nonmagnetic toner and the magnetic carrier are frictionally charged with each other, and the nonmagnetic toner is charged with a negative polarity and the magnetic carrier is charged with a positive polarity. , And is carried on the rotating developing sleeve 4s.

  The developing sleeve 4s rotates in the counter direction with respect to the photosensitive drum 1a around a magnet 4j fixedly disposed at the center. In response to the magnetic field of the magnet 4j, the photosensitive drum 1a is rubbed with the tip of the two-component developer carried on the tip of the developing sleeve 4s.

  The power source D4 applies an oscillating voltage obtained by superimposing the AC voltage Vac on the predetermined DC voltage Vdc to the developing sleeve 4s. As a result, only the toner in the two-component developer is electrostatically transferred to the photosensitive drum 1a, and the electrostatic image is developed into a toner image.

  Unused toner is replenished from the toner replenishing tank 7a to the two-component developer in the developing device 4a whose toner density (T / D ratio) has decreased due to consumption of toner through the developing sleeve 4s. The developing device 4a mixes and stirs the supplied unused toner and the existing two-component developer to increase and uniformize the toner concentration (T / D ratio) in the two-component developer.

  The optical sensor 14a, which is an example of a detecting unit, causes infrared light to enter the photosensitive drum 1a at an incident angle of 45 degrees from the infrared light emitting element within the axial plane of the photosensitive drum 1a. Then, the regular reflection light is detected by the light receiving element, and 8-bit binary data corresponding to the reflected light intensity, which is an example of information reflecting the toner charge amount, is output to the control unit 110.

  The primary transfer roller 5a is configured by covering an outer periphery of a conductive metal core with an elastic layer of a conductive sponge material. Both ends of the metal core are rotatably supported, and the intermediate transfer belt 11 rotates in the direction of arrow R2. Along with this, it rotates. Both ends of the primary transfer roller 5a are urged toward the photosensitive drum 1a with a predetermined pressing force, and pressurize the primary transfer portion T1 formed between the intermediate transfer belt 11 and the photosensitive drum 1a.

  The power source D1 applies a positive direct current voltage to the primary transfer roller 5a, negatively charges it, and primarily transfers the toner image or control toner image carried on the photosensitive drum 1a to the intermediate transfer belt 11. .

  The optical sensor 15, which is an example of a detection unit, causes infrared light to be incident on the photosensitive drum 1 a at an incident angle of 45 degrees from the infrared light emitting element within the surface of the intermediate transfer belt 11 in the width direction. Then, the regular reflection light is detected by the light receiving element, and 8-bit binary data corresponding to the reflected light intensity, which is an example of information reflecting the toner charge amount, is output to the control unit 110.

  The secondary transfer roller 25 is configured by covering an outer periphery of a conductive metal core 25a with an elastic layer 25b of a conductive sponge material, and the same arrow as that of the intermediate transfer belt 11 by a driving mechanism (not shown) from one end of the metal core 25a. It is rotationally driven in the R2 direction. Both end portions of the secondary transfer roller 25 are biased toward the secondary transfer inner roller 24 with a predetermined pressing force, and are formed between the intermediate transfer belt 11 and the secondary transfer roller 25. Pressurize the portion T2.

  The power source D2 applies a positive direct current voltage to the secondary transfer roller 25, and negatively charges the toner image carried on the intermediate transfer belt 11 to the recording material (P).

  The electrostatic cleaning device 30 slidably rubs the upstream fur brush 31 to which a negative voltage is applied and the downstream fur brush 34 to which a positive voltage is applied to remove the toner adhering to the intermediate transfer belt 11. Remove electrostatically.

  The fur brushes 31 and 34 are configured by flocking a carbon dispersed nylon fiber having a resistance value of 10 MΩ and a fiber thickness of 6 denier onto a metal roller at a rate of flocking density of 500,000 / inch2. The fur brushes 31 and 34 are arranged in sliding contact with the intermediate transfer belt 11 while maintaining an intrusion amount of about 1.0 mm, and are rotated in the direction of the arrow at a speed of 50 mm / sec by a drive motor (not shown).

  The metal rollers 32 and 35 are made of an aluminum metal roller having a hard anodized surface, and are driven to rotate in the direction of the arrow while maintaining an intrusion amount of about 1.0 mm with respect to the fur brushes 31 and 34.

  The cleaning blades 33 and 36 are made of urethane rubber, and are arranged with an intrusion amount of 1.0 mm with respect to the metal rollers 32 and 35.

  The power source D6 applies a voltage of −700 V to the metal roller 32 to form a potential gradient from the tension roller 12 connected to the ground potential to the metal roller 32 through the intermediate transfer belt 11 and the fur brush 31. As a result, the upstream fur brush 31 attracts the transfer residual toner charged to the positive polarity and attached to the intermediate transfer belt 11 from the intermediate transfer belt 11 and transfers it to the metal roller 32. The transfer residual toner adhering to the metal roller 32 is scraped off by the cleaning blade 33.

  The power source D7 applies a voltage of +700 V to the metal roller 35 to form a potential gradient from the tension roller 12 connected to the ground potential to the metal roller 35 through the intermediate transfer belt 11 and the fur brush 34. As a result, the fur brush 34 on the downstream side attracts the transfer residual toner that is negatively charged and adheres to the intermediate transfer belt 11 from the intermediate transfer belt 11 and transfers it to the metal roller 35. Transfer residual toner adhering to the metal roller 35 is scraped off by the cleaning blade 36.

  The toner with a small charge amount adhering to the intermediate transfer belt 11 is not adsorbed to the fur brush 31, but receives negative charge injection from the fur brush 31 in the process of contacting and separating from the fur brush 31, and is relatively negatively charged. Charged to a high potential. The toner that has passed through the fur brush 31 and has been negatively charged again is attracted to the positively charged fur brush 34 and removed from the intermediate transfer belt 11.

  The external additive released from the toner is rubbed on the surface of the intermediate transfer belt 11 at a portion where pressure is applied, such as the secondary transfer portion T2, and adheres in the form of fine particles. Since the particulate external additive cannot be recovered by the electrostatic cleaning device 30, the web cleaning device 40 is disposed downstream of the electrostatic cleaning device 30, adhered to the fibers of the cleaning web 45, and mechanically recovered.

  The cleaning web 45 is formed in a roll shape with one end fixed to the attachment roller 42, and one end pulled out from the roll is fixed to the take-up roller 43. The cleaning web 45 is loaded into the web cleaning device 40 by winding the intermediate portion around the web roller 44 and setting the attachment roller 42 and the take-up roller 43 on the respective rotation shafts.

  For the cleaning web 45, one or more kinds of fiber materials selected from polyester, acrylic, vinylon, water-soluble vinylon, rayon, nylon, polypropylene, cotton, and the like can be used.

  Since the cleaning web 45 deposits deposits on the fiber structure, apparent cleaning performance is lost when the same portion is used for a long time. Therefore, for every 25 sheets of A4 size recording material, the take-up roller 43 is rotated by 1 pitch, the cleaning web 45 is pulled out by 2 mm from the attachment roller 42, and the contaminated portion is rubbed against the intermediate transfer belt 11 to be adjacent. Replace with a new part. The used portion of the cleaning web 45 is sequentially wound around the take-up roller 43. When the cleaning web 45 on the attachment roller 42 side disappears, the cleaning web 45 is replaced with a new one.

<Control means>
The control unit 110 controls the exposure device 3a to write the electrostatic image of the control toner image on the photosensitive drum 1a with a predetermined exposure intensity corresponding to the toner image having a predetermined density gradation, and then the control is performed by the developing device 4a. The toner image is developed.

  The controller 110 detects the output of the optical sensor 14a and measures the density of the control toner image. Then, the toner replenishing tank 7a is operated so as to change the toner density (T / D ratio) in the two-component developer so that the density of the control toner image as an example of the detection result approaches the predetermined density gradation. .

  The control unit 110 applies a constant voltage set during non-image formation to the transfer units (T1, T2) during image formation. Here, transfer bias setting control (ATVC) executed during non-image formation will be described.

  The transfer bias setting control measures the current value by changing the bias stepwise to the primary transfer roller 5a and the secondary transfer roller 25 when paper is not passed before image formation is performed. Then, the voltage-current measurement results measured at the transfer portions (T1, T2) are interpolated to obtain a voltage value that provides an appropriate current. When a voltage value corresponding to the appropriate current is found, the voltage value is used as a constant voltage for the transfer bias applied to the primary transfer roller 5a and the secondary transfer roller 25 during image formation.

  For the secondary transfer portion T2, a constant voltage obtained by adding a recording material sharing voltage corresponding to the type of recording material set at the time of image formation to a voltage value corresponding to an appropriate current is set.

  The control unit 110 increases the constant voltage of the transfer unit (T1, T2) set in this way by a predetermined width when an image is formed in a state where the charge amount is reduced to a predetermined level.

<Example 1>
FIG. 3 is an explanatory diagram of a change in toner charge amount as the cumulative usage time of the developing device increases, and FIG. 4 is an explanatory diagram of a change in transfer efficiency of toner and fog toner as the cumulative usage time increases. 5 is an explanatory diagram of the behavior of the fog toner in the primary transfer portion, FIG. 6 is an explanatory diagram of the measurement timing of the fog toner density by the optical sensor, and FIG. 7 is the relationship between the difference in reflected light amount and the fog toner density on the photosensitive drum. It is explanatory drawing of. FIG. 8 is a flowchart of control according to the first embodiment.

  As shown in FIG. 3, when the two-component developer is continuously stirred in the developing device 4a, the toner charge amount distribution in the developing device 4a shifts to the low charge side. As the cumulative usage time of the developing device 4a increases, the toner whose charged charge amount is near 0 increases, and the average charge amount also shifts to the low charge side.

  As shown in FIG. 4, the set value of the primary transfer current that maximizes the toner transfer efficiency in the primary transfer portion T1 varies between the initial state where the accumulated use time is short and the post-endurance state where the accumulated use time is long. The target current value for primary transfer is set at a place where the optimal image characteristics can be obtained by always ensuring a utilization efficiency of 90% or more, taking into account the transfer efficiency fluctuation due to the cumulative usage time of the two-component developer and environmental fluctuations. Has been. The transfer efficiency is set as high as possible in calculating the running cost of the image forming apparatus 100.

  On the other hand, the transfer efficiency of the fog toner having a low toner charge amount in the post-endurance state with a long accumulated use time also changes in accordance with the set value of the primary transfer current. In the vicinity of a set value at which optimum image characteristics can be obtained, the charging polarity is reversed by receiving a very strong discharge, and a so-called strong omission state that adheres to the photosensitive drum 1a side is obtained. In the vicinity of a set value at which optimum image characteristics can be obtained, by increasing the set value of the current, the transfer efficiency of the fog toner can be reduced, and conspicuous background stains due to toner adhering to the white background portion of the image can be suppressed. Conversely, if the target current value in the primary transfer is reduced so that optimum image characteristics can be obtained in the post-endurance state with a long cumulative usage time, the fog toner transfer efficiency increases and the toner adheres to the white background of the image. And conspicuous background dirt gets worse.

  Therefore, in the first embodiment, the amount of fog toner on the photosensitive drums 1a, 1b, 1c, and 1d is measured using the optical sensors 14a, 14b, 14c, and 14d. When the fog toner amount, which is an example of the detection result, reaches a predetermined value (threshold value S), the target current value in the primary transfer portion T1 of each photosensitive drum 1a, 1b, 1c, 1d is increased.

  As shown in FIG. 5, when a high current in the transfer direction is applied to the fog toner having a small amount of charged charge, the charge polarity of the fog toner is reversed and returned to the photosensitive drum 1 a without being primarily transferred to the intermediate transfer belt 11. End up. Using this characteristic, the fog toner transferred to the intermediate transfer belt 11 is reduced, and the fog toner adhering to the white background portion of the recording material is reduced at the secondary transfer portion T2.

  The transfer efficiency is reduced by increasing the voltage applied to the primary transfer roller 5a, but the reduction in the fixed image density due to the reduction in the amount of applied toner on the intermediate transfer belt 11 is conspicuous because the toner adheres to the white background portion of the image. The image quality is not impaired as much as the background stain. When the target current value is increased, the transfer efficiency of the entire image is reduced, so that the density of the fixed image tends to decrease. However, the visual influence is less than the deterioration of image quality due to the fog toner.

  In the initial state where the accumulated usage time is short, the transfer bias is set with priority on toner utilization efficiency because of image quality and running cost. However, if there is deterioration in image quality due to fog toner, priority is given to removal of fog toner and transfer bias. Set up.

  The amount of fog toner on the photosensitive drum 1a is measured using the optical sensor 14a. The optical sensor 14a emits light from the light emitting surface of the optical sensor 14a, receives reflected light from the surface of the photosensitive drum 1 on the detection surface, and forms a control toner image (density detection) on the photosensitive drum 1. Read the density of the patch. The density measurement result is fed back to highlight gradation control.

  In the image forming apparatus 100, the process speed is P (mm / sec), and the distance from the nip between the developing sleeve 4s and the photosensitive drum 1a to the detection surface of the optical sensor 14a is L1 (mm). The time from when the photosensitive drum 1a starts to be driven until the developing sleeve 4s starts to rotate is T (sec).

  The control unit 110 of the image forming apparatus 100 includes a CPU (not shown), and the CPU reads the control timing from the control timing chart stored in the backup RAM (not shown) and controls the time T described above. ing.

  As shown in FIG. 6, the optical sensor 14a detects the amount of reflected light V1 from the photosensitive drum 1a within T + L1 / P (sec) after the photosensitive drum 1a starts to be driven. Within T + L1 / P (sec) after the photosensitive drum 1a starts to be driven, the reflected light amount V1 can be detected before the fog toner adhering to the photosensitive drum 1a from the developing sleeve 4s reaches the optical sensor 14a.

  After T + L1 / P (sec) from the start of driving of the photosensitive drum 1a, the fog toner attached to the photosensitive drum 1a from the developing sleeve 4s passes through the optical sensor 14a, so that the reflected light amount V1 is reduced to the reflected light amount V2. . The optical sensor 14a measures the amount of reflected light V2 on the surface of the photosensitive drum 1a under the influence of fog toner after T + L1 / P (sec) after the photosensitive drum 1a starts to be driven.

  The control unit 110 calculates the difference between the reflected light amounts V1 and V2 (V1−V2 = ΔV) as an example of the density difference of the white background portion of the image carrier before and after the image forming unit is operated, and the fog toner amount. Ask for.

  As shown in FIG. 7, the difference ΔV obtained by the optical sensor 14a changes according to the fog density (%) on the photosensitive drum 1a. The fog density (%) was obtained by collecting the fog toner adhered to the white background portion of the photosensitive drum 1a with a transparent adhesive tape and counting the number of toners under a microscope.

  Thereafter, as shown in Table 1, if the difference ΔV is larger than the threshold S determined by the set basis weight of the recording material and the presence or absence of the surface coat, the control unit 110 increases the constant voltage of the primary transfer bias. Shift to a predetermined width. In the case of a recording material with a coat, which is an example of the surface state, the threshold value is set so that the toner to be attached to the surface of the recording material is more conspicuous than the recording material without a coating, which is another example of the surface state. Reduce S. A thin recording material with a small basis weight has a more conspicuous toner adhering to the surface of the recording material than a thick recording material. Therefore, the threshold value S is made small so as to advance the control start time.

  In this case, the shift amount of the primary transfer bias is set to a constant voltage such that a large amount of current of 3 μA flows in the set value of the constant voltage calculated from the result of the transfer bias setting control (ATVC).

  As shown in FIG. 8 with reference to FIG. 2, when the user sets the type of recording material (S11) and starts the job (S12), the photosensitive drum 1a, the intermediate transfer belt 11 and the like are started to be driven (S12). S13).

  The controller 110 uses the optical sensor 14a to measure the reflected light amount V1 of the photosensitive drum 1a excluding the influence of the fog toner (S14), and starts the rotation of the developing sleeve 4s (S15).

  The control unit 110 uses the optical sensor 14a to measure the reflected light amount V2 of the photosensitive drum 1a to which the fog toner has adhered (S16), and if the difference value ΔV between the reflected light amounts V1 and V2 is equal to or less than the threshold S (S17). NO), image formation is executed (S19). However, when the difference value ΔV between the reflected light amounts V1 and V2 exceeds the threshold value S (YES in S17), the constant voltage of the primary transfer bias is shifted to the increased side (S18), and image formation is executed (S19). .

  In the control of the first embodiment, the fog toner density on the photosensitive drum 1a is measured by the optical sensor 14a for detecting the density of the control toner image on the surface of the photosensitive drum 1a. Further, the fog toner on the recording material P is reduced without changing the charging conditions and the developing conditions and without requiring the downtime due to the additional stirring of the two-component developer.

<Example 2>
FIG. 9 is an explanatory diagram of toner transfer efficiency and fog toner transfer efficiency in the secondary transfer portion, and FIG. 10 is a control flowchart of the second embodiment.

  As shown in FIG. 3, when the cumulative usage time of the two-component developer increases, the toner having a charged charge amount near 0 increases in the developing device 4a, and the fog toner adhering to the photosensitive drum 1a increases. .

  As shown in FIG. 9, the set value of the secondary transfer current that maximizes the toner transfer efficiency in the secondary transfer portion T2 changes between the initial state where the cumulative use time is short and the post-endurance state where the cumulative use time is long. . The target current value in the secondary transfer is a place where the optimum image characteristics can be obtained by always ensuring the use efficiency of 90% or more in consideration of the transfer time fluctuation due to the accumulated usage time of the two-component developer and the environmental fluctuation. Is set. The transfer efficiency is set as high as possible in calculating the running cost of the image forming apparatus 100.

  On the other hand, the transfer efficiency of the fog toner, which increases in the post-endurance state where the accumulated usage time is long, also changes according to the set value of the secondary transfer current. Since the fog toner has a small charge amount, the charge polarity is reversed near the higher set value applied to the toner having the normal charge amount and the charge polarity is reversed, and the toner adheres to the photosensitive drum 1a side. It is in a so-called strong omission state.

  For this reason, in the vicinity of a higher set value that is applied to toner having a normal charge amount, the fog toner transfer efficiency is greatly reduced, and it is possible to suppress conspicuous scumming due to toner adhering to a white background portion of an image. Conversely, if the target current value is reduced so that optimum image characteristics can be obtained in accordance with the decrease in the average charged charge amount of the toner, the fog toner transfer efficiency increases, and the toner adheres to the white background portion of the image. The noticeable background stain will worsen.

  Therefore, in Example 2, the amount of fog toner on the photosensitive drums 1a, 1b, 1c, and 1d is measured using the optical sensors 14a, 14b, 14c, and 14d. When the total amount of fog toner reaches a predetermined value (threshold value S), the target current value in the common secondary transfer portion T2 is increased.

  The optical sensor 14a is provided for reading the density of a control toner image (density detection patch) formed on the photosensitive drum 1a.

  As shown in FIG. 6, the optical sensor 14a detects the amount of reflected light V1 from the photosensitive drum 1a within T + L1 / P (sec) after the photosensitive drum 1a starts to be driven. After T + L1 / P (sec) from the start of driving of the photosensitive drum 1a, the fog toner attached to the photosensitive drum 1a from the developing sleeve 4s passes through the optical sensor 14a, so that the reflected light amount V1 decreases to the reflected light amount V2. The optical sensor 14a measures the amount of reflected light V2 on the surface of the photosensitive drum 1a to which the fog toner after T + L1 / P (sec) is attached after the photosensitive drum 1a starts to be driven.

  The control unit 110 calculates the difference between the reflected light amounts V1 and V2 (V1−V2 = ΔV) to obtain the fog toner amount.

  As shown in FIG. 7, the differences ΔVa, ΔVb, ΔVc, ΔVd obtained by the optical sensors 14a, 14b, 14c, 14d change according to the fog density (%) on the photosensitive drums 1a, 1b, 1c, 1d. . The control unit 110 adds the differences ΔVa, ΔVb, ΔVc, ΔVd to obtain a total difference value ΔVaccd. Thereafter, as shown in Table 2, if the total difference value ΔVabcd is larger than the set basis weight of the recording material and the threshold value S determined by the presence or absence of the surface coat, the constant voltage of the secondary transfer bias is increased to a predetermined value. Shift width. In the case of a recording material with a coat, the toner adhering to the surface of the recording material is more conspicuous than the recording material without a coat, so the threshold S is made small so that the control start time is advanced. A thin recording material with a small basis weight has a more conspicuous toner adhering to the surface of the recording material than a thick recording material. Therefore, the threshold value S is made small so as to advance the control start time.

  In this case, the shift amount of the secondary transfer bias is set to a constant voltage so that a large amount of current corresponding to 5 μA flows in the set value of the constant voltage calculated from the result of the transfer bias setting control (ATVC).

  By the way, when a continuous image forming job is performed for a long time, there is a possibility that fog toner increases due to some influence in the middle of the job and the image quality deteriorates. For this reason, in the second embodiment, the reflected light amount V2 is measured and the total difference value ΔVabcd is obtained even when the interval between the sheets on which the developing sleeve 4s is rotating in the middle of the job reaches the detection position of the optical sensor 14a. It keeps updating.

  Even when the total difference value ΔVabcd becomes larger than the threshold S determined by the basis weight of the paper and the presence or absence of the surface coat in the middle of the job, the constant voltage of the secondary transfer bias is increased by a predetermined width to the side as needed. shift. The shift amount of the secondary transfer bias is set so that a large amount of current corresponding to 5 μA flows in the set voltage calculated from the ATVC result.

  As shown in FIG. 10 with reference to FIG. 2, when the user sets the type of recording material (S11) and starts the job (S12), the photosensitive drum 1a, the intermediate transfer belt 11 and the like are started to be driven (S11). S13).

  The controller 110 uses the optical sensor 14a to measure the reflected light amount V1 of the photosensitive drum 1a excluding the influence of the fog toner (S14), and starts the rotation of the developing sleeve 4s (S15).

  The control unit 110 measures the reflected light amount V2 of the photosensitive drum 1a to which the fog toner has adhered using the optical sensor 14a (S16), and if the total difference value ΔVabcd of the reflected light amounts V1 and V2 is equal to or less than the threshold value S (S21). NO), image formation is executed (S23). However, if the total difference value ΔVabcd exceeds the threshold value S (YES in S21), the constant voltage of the secondary transfer bias is shifted to the additional side (S22), and image formation is executed (S23).

  At the timing when the optical sensor 14a is positioned between the sheets of the photosensitive drum 1a during continuous image formation (S24), the control unit 110 uses the optical sensor 14a to calculate the reflected light amount V2 of the photosensitive drum 1a to which the fog toner has adhered. Measure (S25). If the total difference value ΔVabcd is equal to or smaller than the threshold value S (NO in S26), image formation is executed (S28). However, if the total difference value ΔVabcd exceeds the threshold value S (YES in S26), the constant voltage of the secondary transfer bias is shifted to the additional side (S27), and image formation is executed (S28).

  Until the continuous image formation of the job ends (NO in S29), the control in steps S24 to S29 is repeated to complete the image formation (S30).

  In the control of the second embodiment, the fog toner density on the photosensitive drum 1a is measured by the optical sensor 14a for detecting the density of the control toner image on the surface of the photosensitive drum 1a. Further, the fog toner on the recording material P is reduced without changing the charging conditions and the developing conditions and without requiring the downtime due to the additional stirring of the two-component developer.

<Example 3>
FIG. 11 is an explanatory diagram of the measurement timing of the fog toner density by the optical sensor, FIG. 12 is an explanatory diagram of the relationship between the difference in reflected light amount and the fog toner density on the intermediate transfer belt, and FIG. 13 is a control flowchart of the third embodiment. It is.

  In the third embodiment, the fog toner density on the intermediate transfer belt 11 is measured by the optical sensor 15, and if the fog toner density exceeds a predetermined level, a predetermined width shift is performed in a direction to increase the constant voltage of the primary transfer bias. To do.

  The optical sensor 15 emits light from the light emitting surface to the surface of the intermediate transfer belt 11, receives light reflected from the surface of the intermediate transfer belt 11 on the detection surface, and forms a control toner image formed on the intermediate transfer belt 11. Read (Registration Patch). The control unit 110 detects the control toner images primarily transferred from the photosensitive drums 1a, 1b, 1c, and 1d to the intermediate transfer belt 11, and measures the misregistration amounts of the color toner images. Then, color misalignment control is performed to control the exposure devices 3a, 3b, 3c, and 3d so as to cancel out the misregistration amount of each color toner image.

  In Example 3, the distance from the development nip of the most downstream image forming unit Pd to the detection position of the optical sensor 15 is L1 (mm), and the detection of the optical sensor 15 from the development nip of the most upstream image forming unit Pa. The distance to the position is L2 (mm). A time from when the intermediate transfer belt 11 starts to be driven to when the developing sleeve of the most downstream image forming unit Pd starts to rotate is defined as T (sec).

  The control unit 110 reads the control timing from a control timing chart stored in a backup RAM (not shown) and controls these times.

  As shown in FIG. 11, the optical sensor 15 measures the amount of reflected light from the intermediate transfer belt 11 within T + L1 / P (sec) after the intermediate transfer belt 11 starts to be driven. Thus, the amount of reflected light V1 on the surface of the intermediate transfer belt 11 where the fog toner is not transferred from any of the image forming portions Pa, Pb, Pc, and Pd is measured.

  Next, after the intermediate transfer belt 11 starts to be driven, the reflected light amount of the intermediate transfer belt 11 is measured after T + L2 / P (sec). Thus, the amount of reflected light V2 on the surface of the intermediate transfer belt 11 where the fog toner may be transferred from at least one of the image forming portions Pa, Pb, Pc, and Pd is measured. The control unit 110 calculates the difference ΔV between the reflected light amounts V 1 and V 2 and recognizes the state of the fog toner on the intermediate transfer belt 11.

  As shown in FIG. 12, the difference ΔV obtained by the optical sensor 15 changes according to the fog density (%) on the intermediate transfer belt 11. As shown in Table 2, the control unit 110 increases the constant voltage of the primary transfer bias if the difference ΔV is larger than the threshold S determined by the set basis weight of the recording material and the presence or absence of the surface coat. Shift to a predetermined width. The primary transfer bias shift amount is set to a constant voltage so that a large amount of current of 3 μA flows in the set voltage calculated from the ATVC result.

  As shown in FIG. 13 with reference to FIG. 2, when the user sets the type of recording material (S11) and starts the job (S12), the photosensitive drum 1a, the intermediate transfer belt 11 and the like are started to be driven (S11). S13).

  The controller 110 uses the optical sensor 15 to measure the reflected light amount V1 of the intermediate transfer belt 11 excluding the influence of the fog toner (S31), and starts the rotation of the developing sleeve 4s (S32).

  The control unit 110 uses the optical sensor 15 to measure the reflected light amount V2 of the intermediate transfer belt 11 to which the fog toner has adhered (S33), and if the difference ΔV between the reflected light amounts V1 and V2 is equal to or less than the threshold S (S34). NO), image formation is executed (S36). However, if the difference ΔV exceeds the threshold value S (YES in S34), the constant voltage of the primary transfer bias is shifted to the increased side (S35), and image formation is executed (S36).

  In the control of Embodiment 3, the fog toner density on the intermediate transfer belt 11 is measured by an optical sensor 15 for detecting the position of the control toner image on the surface of the intermediate transfer belt 11. Further, the fog toner on the recording material P is reduced without changing the charging conditions and the developing conditions and without requiring the downtime due to the additional stirring of the two-component developer.

<Example 4>
FIG. 14 is a flowchart of control according to the fourth embodiment. In the control of Example 3, when the measurement result of the fog toner amount exceeds a predetermined level, the primary transfer bias applied to the primary transfer portion T1 is shifted by a predetermined width. On the other hand, the control of the fourth embodiment shifts the secondary transfer bias applied to the secondary transfer portion T2 by a predetermined width when the measurement result of the fog toner amount exceeds a predetermined level. Since the other configuration and control are the same as those in the third embodiment, steps in FIG. 14 that are the same as those in the third embodiment are denoted by the same reference numerals as those in FIG.

  As shown in FIG. 9, the set value of the secondary transfer current that maximizes the toner transfer efficiency in the secondary transfer portion T2 changes between the initial state where the cumulative use time is short and the post-endurance state where the cumulative use time is long. .

  On the other hand, the fog toner that increases in the post-endurance state with a long accumulated use time has a small charge amount. For this reason, in the vicinity of a higher set value that is applied to a toner having a normal charge amount, a so-called strong omission state in which the charge polarity is reversed due to a very strong discharge and adheres to the photosensitive drum 1a side. become. Therefore, by raising the secondary transfer bias within a range in which the reduction in transfer efficiency can be tolerated, the transfer efficiency of the fog toner can be reduced, and the background smear due to the toner adhering to the white background portion of the image can be suppressed.

  As shown in FIG. 14 with reference to FIG. 2, the control unit 110 uses the optical sensor 15 to measure the reflected light amount V1 of the intermediate transfer belt 11 excluding the influence of the fog toner (S31), and the developing sleeve. The rotation of 4s is started (S32).

  The control unit 110 uses the optical sensor 15 to measure the reflected light amount V2 of the intermediate transfer belt 11 to which the fog toner has adhered (S33), and if the difference ΔV between the reflected light amounts V1 and V2 is equal to or less than the threshold S (S34). NO), image formation is executed (S36). However, if the difference ΔV exceeds the threshold value S (YES in S34), the constant voltage of the secondary transfer bias is shifted to the increased side (S38), and image formation is executed (S36).

  In the control of the fourth embodiment, the fog toner density on the intermediate transfer belt 11 is measured by the optical sensor 15 for detecting the position of the control toner image on the surface of the intermediate transfer belt 11. Further, the fog toner on the recording material P is reduced without changing the charging conditions and the developing conditions and without requiring the downtime due to the additional stirring of the two-component developer.

Second Embodiment
FIG. 15 is an explanatory diagram of a configuration of the image forming apparatus according to the second embodiment.

  As shown in FIG. 15, the image forming apparatus 200 of the second embodiment is a high-speed monochrome image forming apparatus that transfers a black toner image formed on the photosensitive drum 1 to a recording material P and fixes it by a fixing device (not shown). is there.

  Surrounding the photosensitive drum 1, a charger 2, an exposure device 3, a developing device 4, a pre-transfer charging device 8, a transfer roller 5, an optical sensor 14, a cleaning device 6, and a pre-charge exposure device 9 are arranged.

  The photosensitive drum 1 has a positively charged photosensitive layer formed on the outer peripheral surface of an aluminum cylinder, is rotatably supported, and rotates in the direction of arrow R1.

  The charger 2 generates a corona discharge by applying a positive DC voltage from the power source D3 to the discharge wire, and irradiates the charged particles to charge the surface of the photosensitive drum 1 to a uniform positive dark potential VD. .

  The exposure apparatus 3 scans a scanning beam image data obtained by developing the image data with a laser beam that is ON-OFF modulated by a rotating mirror (not shown), and lowers the surface potential of the charged photosensitive drum 1 to the bright portion potential VL. An electrostatic image is formed.

  The developing device 4 rubs the photosensitive drum 1 with a black one-component developer containing magnetic toner carried on the developing sleeve 4s rotating in the counter direction around the magnet 4j. The toner carried on the developing sleeve 4s is conveyed to the opposing distance between the photosensitive drum 1 and the developing sleeve 4s in a state of being negatively charged by being rubbed by the friction plate 4m.

  The pre-transfer charging device 8 is composed of a corona discharge device in which a negative DC voltage is applied to a discharge wire, and the toner image carried on the photosensitive drum 1 is irradiated with negative charged particles to charge the toner image. Optimize the amount for transfer.

  The power source D4 applies an oscillating voltage obtained by superimposing an AC voltage to a positive DC voltage to the developing sleeve 4s, and causes toner to adhere to the dark portion potential VD portion of the photosensitive drum 1, thereby normally developing the electrostatic image.

  The transfer roller 5 is in pressure contact with the photosensitive drum 1 to form a toner image transfer portion T with respect to the recording material P. The power source D applies a positive voltage to the transfer roller 5 and is charged with a negative polarity, and a toner image carried on the photosensitive drum 1 is superimposed on the photosensitive drum 1 and the recording material is nipped and conveyed by the transfer portion T. Electrostatic transfer to P.

  The optical sensor 14 causes infrared light to be incident on the photosensitive drum 1 at an incident angle of 45 degrees from the infrared light emitting element in the axial plane of the photosensitive drum 1, and the regular reflected light is detected by the light receiving element and reflected. 8-bit binary data corresponding to the light intensity is output to the control unit 110. The optical sensor 14 is provided for reading the density of the control toner image (density detection patch), and is applied to the transfer roller 5 on the photosensitive drum 1 that has passed through the transfer portion T as it is by applying a negative voltage. A control toner image is measured.

  The cleaning device 6 slides the cleaning blade 6 h against the photosensitive drum 1 to remove the transfer residual toner that has passed through the transfer portion T while being carried on the photosensitive drum 1.

  The pre-charging exposure device 9 is composed of an LED array having a light emission wavelength substantially equal to that of the laser beam of the exposure device 3, and uniformly exposes the surface of the photosensitive drum 1 to erase the remainder of the previous electrostatic image.

<Example 5>
FIG. 16 is a flowchart of control according to the fifth embodiment. The control of the fifth embodiment is the same as that of the first embodiment except that the transfer bias applied to the transfer portion T is shifted. Therefore, in FIG. 16, the steps common to the first embodiment are denoted by the same reference numerals as those in FIG.

  As the cumulative usage time of the one-component developer filled in the developing device 4 also increases, the toner with a charged charge amount near 0 increases, and the fog toner that adheres to the photosensitive drum 1 increases with development.

  In the initial state where the cumulative usage time of the developing device 4 is short and the post-endurance state where the cumulative usage time is long, the current setting value at which the toner transfer efficiency in the transfer portion T is maximized changes. The target current value in the transfer portion T is a place where the optimum image characteristics can be obtained by always ensuring the use efficiency of 90% or more in consideration of the accumulated usage time of the one-component developer and the fluctuation of the transfer efficiency due to the environmental fluctuation. Is set.

  On the other hand, the fog toner that increases in the post-endurance state with a long accumulated use time has a small charge amount. For this reason, with a high transfer bias applied to a toner having a normal charge amount, the charging polarity is reversed upon receiving a very strong discharge, and the toner adheres to the photosensitive drum 1 side. Become. Therefore, the transfer bias applied to the transfer portion T is increased within a range where the transfer efficiency can be lowered, thereby reducing the transfer efficiency of the fog toner and suppressing the noticeable background stain due to the toner adhering to the white background portion of the image. it can.

  Therefore, in Example 5, the amount of fog toner on the photosensitive drum 1 is measured using the optical sensor 14. When the measured value of the fog toner amount reaches a predetermined value, the transfer bias applied to the transfer portion T is increased by a predetermined width.

  The procedure for measuring the amount of fog toner using the optical sensor 14 is as described in the first embodiment. The optical sensor 14 measures the amount of reflected light V1 on the surface of the photosensitive drum 1 on which the fog toner is not transferred immediately after the photosensitive drum starts to be driven. Subsequently, the amount of reflected light V2 on the surface of the photosensitive drum 1 on which the fog toner may have come into contact with the rotating developing sleeve 4s is measured.

  The controller 110 recognizes the state of the fog toner on the surface of the photosensitive drum 1 based on the difference ΔV between the reflected light amounts V1 and V2. When the difference ΔV is larger than the threshold value determined by the set paper basis weight and the presence / absence of the surface coat, it is determined whether or not to shift the transfer bias set in advance by the transfer bias setting control (ATVC). decide. Thereby, when the charge amount of the toner is below a predetermined value, the transfer bias sets a constant voltage so that a large amount of current of 3 μA flows in the set voltage calculated from the ATVC result.

  As shown in FIG. 16 with reference to FIG. 15, the control unit 110 measures the reflected light amount V2 of the photosensitive drum 1 to which the fog toner has adhered using the optical sensor 14 (S16). If the difference ΔV between the reflected light amounts V1 and V2 is equal to or less than the threshold value S (NO in S17), image formation is executed (S19). However, if the difference ΔV exceeds the threshold value S (YES in S17), the transfer bias constant voltage is shifted to the increased side (S39), and image formation is executed (S19).

  In the control of the fifth embodiment, the fog toner density on the photosensitive drum 1 is measured by the optical sensor 14 for detecting the density of the control toner image on the surface of the photosensitive drum 1. Further, the fog toner on the recording material P is reduced without changing the charging conditions and the developing conditions and without requiring the downtime due to the additional stirring of the two-component developer.

<Example 6>
The image forming apparatus 200 includes a video count unit 101 that counts print dots of one piece of image data of an image forming job.

  The control unit 110 calculates the image ratio of the printed image from the output of the video count unit 101 which is an example of a detection unit. Then, assuming that the image ratio of the A4 size maximum density entire surface image is 100% and the detection result is that a predetermined number of images having an image ratio of 5% or less continue, the toner in the developing device 4 deteriorates and the charge amount is less than a predetermined level. It is estimated that The transfer bias setting control (ATVC) is set in advance so that the toner charge in the developing device 4 is not transferred to the recording material P even if the fog toner density on the photosensitive drum 1 increases below a predetermined level. Shift in a direction to increase the transfer bias.

<Example 7>
The image forming apparatus 200 includes a sheet number coefficient unit 102 that counts the accumulated number of image formations and a rotation time accumulation unit 103 that counts the accumulated rotation time of the developing sleeve 4s as an example of the accumulated use amount of the image forming use unit. .

  The control unit 110 develops a detection result in which the accumulated number of prints in the sheet number coefficient unit 102, which is an example of the detection unit, exceeds 100,000 in A4 conversion, or the accumulated rotation time of the rotation time accumulation unit 103 exceeds 100 hours. It is estimated that the toner in the apparatus 4 deteriorates and the charge amount is below a predetermined level. The transfer bias setting control (ATVC) is set in advance so that the toner charge in the developing device 4 is not transferred to the recording material P even if the fog toner density on the photosensitive drum 1 increases below a predetermined level. Shift in a direction to increase the transfer bias.

<Example 8>
In recent years, in order to reduce running costs and reduce the frequency of replacement of two-component developer, a developing device that mixes a magnetic carrier with non-magnetic toner to be replenished and discharges deteriorated toner together with the increased amount of two-component developer. It has been put into practical use. In this case, since the accumulated time during which the developing device can be used is greatly extended, carriers that are considerably deteriorated compared to the initial state are accumulated in the developing device. That is, in the developing device, a newly supplied carrier having a high charging performance and a carrier having a deteriorated charging performance greatly deteriorated.

  For this reason, as shown in FIG. 3, when the cumulative usage time of the developing device increases, the toner charge amount distribution becomes wider than in the initial state, and the toner whose charged charge amount is almost zero increases. In this state, even if the agitation time is increased to increase the charged charge amount, the charged charge amount does not increase easily. As a result, the two-component developer is further deteriorated by meaninglessly continuing the agitation.

  As shown in FIG. 2, the image forming apparatus 100 controls the exposure device 3a to write an electrostatic image of the control toner image on the photosensitive drum 1a with a predetermined exposure intensity corresponding to the toner image of a predetermined density gradation. Thereafter, the control toner image is developed by the developing device 4a.

  The controller 110 detects the output of the optical sensor 14a and measures the density of the control toner image. Then, the toner supply tank 7a is operated so that the toner density (T / D ratio) in the two-component developer is changed so that the density of the control toner image approaches the predetermined density gradation.

  Here, when the stay time of the two-component developer in the developing device 4a is extended and the toner is deteriorated by stirring for a long time, the charged charge amount of the toner is reduced and the amount of toner adhering to the equally formed electrostatic image is increased. In this case, since the toner application amount increases and the density of the control toner image increases, the control unit 110 reduces the toner supply amount from the toner supply tank 7a to the developing device 4a, and the toner concentration in the two-component developer. (T / D ratio) is reduced. This is because by reducing the toner in the two-component developer, the chance of friction with the carrier is increased and the charge amount of the toner is increased.

  Accordingly, when the density of the control toner image measured by the optical sensor 14 continues to increase during the continuous image formation, the control unit 110 deteriorates the toner in the developing device 4a and the charge amount falls below a predetermined level. It is estimated that it became. Then, even if the charge amount becomes a predetermined level or less and the fog toner density on the photosensitive drum 1a increases, the primary transfer bias and the secondary that are set in advance by the transfer bias setting control (ATVC) so as not to be transferred to the recording material P. Shift in a direction to increase the transfer bias.

It is explanatory drawing of a structure of the image forming apparatus of 1st Embodiment. It is explanatory drawing of a structure of an image formation part. FIG. 6 is an explanatory diagram of a change in toner charge amount with an increase in accumulated usage time of the developing device. FIG. 10 is an explanatory diagram of a change in transfer efficiency of toner and fog toner as the cumulative usage time increases. FIG. 6 is an explanatory diagram of the behavior of fog toner in a primary transfer portion. It is explanatory drawing of the measurement timing of the fog toner density by an optical sensor. It is explanatory drawing of the relationship between the difference of reflected light quantity and the fog toner density on a photosensitive drum. 3 is a flowchart of control according to the first embodiment. FIG. 6 is an explanatory diagram of toner transfer efficiency and fog toner transfer efficiency in a secondary transfer unit. 6 is a flowchart of control according to the second embodiment. It is explanatory drawing of the measurement timing of the fog toner density by an optical sensor. FIG. 6 is an explanatory diagram of a relationship between a difference in reflected light amount and a fog toner density on an intermediate transfer belt. 10 is a flowchart of control according to the third embodiment. 10 is a flowchart of control according to the fourth embodiment. It is explanatory drawing of a structure of the image forming apparatus of 2nd Embodiment. 10 is a flowchart of control in Embodiment 5.

Explanation of symbols

1a, 1b, 1c, 1d Image carrier (photosensitive drum)
2a, 2b, 2c, 2d Image forming means (charging roller)
3a, 3b, 3c, 3d Image forming means (exposure device)
4a, 4b, 4c, 4d Image forming means (developing device)
5a, 5b, 5c, 5d Transfer member (primary transfer roller)
11 Intermediate transfer belts 14a, 14b, 14c, 14d Detection means (optical sensor)
15 Detection means (optical sensor)
24 Secondary transfer inner roller 25 Secondary transfer roller 30 Electrostatic cleaning device 40 Web cleaning device 101 Video count unit 102 Number coefficient unit 103 Rotation time accumulation unit 108 Operation panel 110 Control unit T1, T2 Transfer unit (primary transfer unit, second transfer unit) (Next transfer part)
Pa, Pb, Pc, Pd Image forming unit

Claims (6)

An image carrier;
Image forming means for forming a toner image on the image carrier;
A transfer member that presses in the direction of the image carrier through a transfer medium and transfers a toner image of the image carrier to the transfer medium at a transfer unit;
In an image forming apparatus comprising: a power supply unit that applies an electric field to the transfer unit.
Detection means for detecting information reflecting the amount of fog is an amount of toner adhering to the non-image portion,
The area of the image carrier on which the toner image transferred to the recording material is carried when the detection result of the detection means is increased from the first fog amount to the second fog amount is passing through the transfer portion. Control means for controlling the power supply means so that the absolute value of the electric field applied to the transfer section is larger than the electric field applied to the transfer section during image formation in the case of the first fog amount. An image forming apparatus. It said detecting means includes an optical sensor for detecting the amount of reflected light before the light irradiated to Kizo carrier,
The control means detects the amount of light of the white portion of the image carrier by the optical sensor,
When the light quantity detected by the optical sensor is a second light quantity that is less than the first light quantity, the electric field applied to the transfer unit during image formation is the transfer part during image formation when the first light quantity is used. the image forming apparatus according to claim 1, wherein the controller controls the power supply unit such that the absolute value becomes larger in the same direction with respect to the electric field to be applied to. The detection means includes an optical sensor that detects a reflected light amount of light irradiated on the image carrier ,
Said control means, the image forming apparatus according to claim 1, the light amount of the white background portion of the image bearing member in the case of less than the threshold value, and controlling said power supply means. The image forming unit includes a developing sleeve that conveys a developer that rotates at a predetermined peripheral speed ratio with respect to the image carrier,
The detecting means detects the cumulative rotation amount of the developing sleeve as information reflecting the fogging amount;
It said control means, the image forming apparatus according to claim 1, wherein the cumulative amount of rotation when exceeding the threshold value, and controlling said power supply means. The control means, the applied to the transfer section the voltage set at the non-image-formed at the time of image formation, wherein the image forming time after the information reflecting the fog amount exceeds the threshold value, predetermined pre SL voltage the image forming apparatus according to claim 1 or 2, characterized in that to increase the width. Toner image according to at least one of the basis weight and the surface state of the recording material to be transferred, according to claim 3 to 5, wherein varying the threshold of the information reflecting the amount of fog for controlling said power supply means The image forming apparatus according to any one of the above.

Images