JP2012203178A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that suppresses generation of discharge products due to charging of toner before transfer.SOLUTION: An image forming apparatus includes: an image holding body 30 that holds a charged toner image; a charger 70 that additionally charges the toner image held by the image holding body 30; a transfer unit 50 that receives application of voltage, and transfers the toner image additionally charged by the charger to a transferred body; and a first control unit 1A that controls the amount of additional charge applied to the toner image by the charger 70 in accordance with the amount of charge of the toner image before the additional charge.

Description

  The present invention relates to an image forming apparatus.

  Japanese Patent Application Laid-Open No. 2005-228561 discloses an image forming unit including an image forming unit that forms a toner image on a photosensitive drum, and a transfer brush that transfers the toner image on the photosensitive drum onto a recording material by applying a transfer electric field. The device is shown. In this image forming apparatus, the amount of current flowing through the transfer brush is detected for a test patch formed on the photosensitive drum, and a toner image to be subsequently formed on the photosensitive drum is detected based on the detected amount of current. The charge amount and transfer electric field are controlled.

  Patent Document 2 discloses an image forming apparatus including a charge applying unit that increases the charge amount of toner at a position between development and transfer. Also, Patent Document 3 and Patent Document 4 also show an image forming apparatus including a device for applying a charge to toner.

JP-A-5-107947 Japanese Patent Laid-Open No. 9-6148 JP 2005-62349 A JP 2008-139597 A

  An object of the present invention is to provide an image forming apparatus in which the generation of discharge products due to charging of toner before transfer is suppressed.

An image forming apparatus according to claim 1 is provided.
An image carrier for holding a charged toner image;
A charger for additionally charging the toner image held on the image carrier;
A transfer unit that receives a voltage and transfers a toner image that has been additionally charged by the charger to a transfer target; and
And a first control unit configured to control an additional charge amount toward the toner image by the charger according to a charge amount of the toner image before the additional charge.

An image forming apparatus according to a second aspect includes a detector that detects an electrical resistance value of the transfer object;
And a second control unit that controls a voltage applied to the transfer device in accordance with the electric resistance value of the transfer target detected by the detector.

According to a third aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of photoconductors that receive each color toner image; and an intermediate that receives the transfer of each color toner image on the plurality of photoconductors in turn from the plurality of photoconductors. A transfer body,
The image carrier is a photoconductor arranged on the most downstream side in the moving direction of the intermediate transfer body among the plurality of photoconductors,
The transfer device is a transfer device for transferring a toner image on the intermediate transfer member to a transfer target.

An image forming apparatus according to claim 4
It has a housing that serves as a reference for potential,
The charger is a corotron having a discharge wire and a shield connected between the housing via an electric resistance having a variable resistance value,
The first control unit controls an additional charge amount toward the toner by adjusting a resistance value of the electric resistance.

  According to the image forming apparatus of the first aspect, compared to the case where the first control unit is not provided, the generation of discharge products caused by charging the toner before transfer is suppressed.

  According to the image forming apparatus of the second aspect, compared to the case where the present configuration is not provided, the transfer failure of the toner image due to the difference in the electrical resistance of the paper is suppressed.

  According to the image forming apparatus of the third aspect, compared to the case where the present configuration is not provided, the transfer failure of the portion of the toner image due to the photoconductor disposed on the most downstream side is suppressed.

  According to the image forming apparatus of the fourth aspect, the discharge amount unevenness is suppressed as compared with the case where the present configuration is not provided.

1 is a configuration diagram illustrating a first embodiment of an image forming apparatus of the present invention. It is a figure which shows the additional charging part and charge amount detection part which are shown in FIG. 3 is a flowchart illustrating an operation in the image forming apparatus illustrated in FIG. 1. It is a figure which shows the additional charging part which concerns on 2nd Embodiment. 10 is a flowchart illustrating an operation in the image forming apparatus according to the second embodiment. It is a figure which shows the additional charging part which concerns on 3rd Embodiment of this invention. 10 is a flowchart illustrating an operation in the image forming apparatus according to the third embodiment. It is a schematic block diagram of the additional charging part which concerns on 4th Embodiment of this invention. It is a schematic block diagram of the additional charging part which concerns on 5th Embodiment of this invention. It is a block diagram which shows the image forming apparatus which is 6th Embodiment of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram showing a first embodiment of an image forming apparatus of the present invention.

  The image forming apparatus 1 shown in FIG. 1 includes image forming units 10Y, 10M, 10C, and 10K, an exposure device 20, an intermediate transfer belt 30, and a control unit 1A. The image forming apparatus 1 is a tandem type in which image forming units 10Y, 10M, 10C, and 10K are arranged in parallel for each color of yellow (Y), magenta (M), cyan (C), and black (K). In addition to being able to print a single color image, it is also possible to print a full color image consisting of four color toner images. The image forming units 10Y, 10M, 10C, and 10K form toner images of each color.

  Since the four image forming units 10Y, 10M, 10C, and 10K have substantially the same configuration, the image forming unit 10Y corresponding to yellow will be described as a representative example. The image forming unit 10Y includes a photoreceptor 11Y, a charger 12Y, a developing device 14Y, a primary transfer device 15Y, and a photoreceptor cleaner 16Y. The photoreceptor 11Y has a cylindrical peripheral surface. The photoconductor 11Y holds an image formed on the peripheral surface and rotates in the direction of an arrow a that is around the axis of the cylinder. The charger 12Y charges the peripheral surface of the photoreceptor 11Y. The developing device 14Y is a device that develops the peripheral surface of the photoconductor 11Y with charged toner after exposure, and includes a developing roll 141 that conveys the charged toner to the peripheral surface of the photoconductor 11Y. In the developing unit 14Y of the present embodiment, high definition is typically achieved by using a small particle diameter toner having an average particle diameter of 2.5 μm or more and 4.5 μm or less.

  The primary transfer unit 15Y transfers the toner image formed on the peripheral surface of the photoreceptor 11Y to the intermediate transfer belt 30. The primary transfer unit 15Y is a roll that rotates with the intermediate transfer belt 30 interposed between the primary transfer unit 15Y and the photoreceptor 11Y. A voltage having a polarity opposite to the charging polarity of the toner is applied to the primary transfer unit 15Y with respect to the photoreceptor 11Y by a power supply device (not shown). The primary transfer unit 15Y transfers the toner on the photoconductor 11Y onto the intermediate transfer belt 30 by applying a transfer electric field to the photoconductor 11Y and the intermediate transfer belt 30. The photoconductor cleaner 16Y cleans the peripheral surface of the photoconductor 11Y after the transfer.

  The exposure device 20 emits laser light based on an image signal supplied from the outside, and scans and exposes the photoconductors of the image forming units 10Y, 10M, 10C, and 10K with the laser light.

  The intermediate transfer belt 30 is an endless belt-like member supported by belt support rolls 31, 32, 33, and 34, and circulates and moves in a direction indicated by an arrow b along a path that passes through the image forming units 10Y to 10K. The intermediate transfer belt 30 has toner images transferred from the image forming units 10Y to 10K of the respective colors, and holds the toner images.

  Further, in the image forming apparatus 1, the secondary transfer device 50, the fixing device 60, the belt cleaner 38, the toner concentration measuring unit 41, the environmental condition sensor 45, the charge amount detecting unit 90, and the additional charging unit 70. In addition, a paper transport unit 80 and a paper container C are also provided.

  The secondary transfer device 50 is a roll that rotates with the intermediate transfer belt 30 and the paper sandwiched between the backup roll 33 that is one of the belt support rolls 31 to 34. A voltage having a polarity opposite to the charging polarity of the toner is applied to the secondary transfer unit 50 by the power supply device 51 to the backup roll 33. The secondary transfer unit 50 applies a transfer electric field to the intermediate transfer belt 30 and the paper, thereby transferring the toner image on the intermediate transfer belt 30 onto the paper. The voltage value of the voltage applied to the backup roll 33 by the power supply device 51 is controlled by the control unit 1A.

  The additional charging unit 70 promotes transfer in the secondary transfer unit 50 by additionally charging the toner image held on the intermediate transfer belt 30.

  The charge amount detection unit 90 detects the charge amount of the toner image held on the intermediate transfer belt 30 before the additional charging unit 70 performs additional charging. The additional charging amount by the additional charging unit 70 is controlled by the control unit 1A according to the charging amount before the additional charging of the toner. The additional charging unit 70 and the charge amount detection unit 90 will be described later.

  The belt cleaner 38 cleans the intermediate transfer belt 30 after the transfer by the secondary transfer unit 50. The belt cleaner 38 cleans the peripheral surface of the intermediate transfer belt 30 by bringing the blade into contact with the intermediate transfer belt 30 and scraping off the toner on the intermediate transfer belt 30.

  The toner density measuring unit 41 detects the density of the toner image formed by the image forming units 10Y, 10M, 10C, and 10K. When a density adjustment toner image (test patch image) is formed by the image forming units 10Y, 10M, 10C, and 10K, the density of the test patch image is detected. Although not shown, the toner concentration measuring unit 41 includes a light emitting unit that irradiates light toward the intermediate transfer belt 30 and an optical sensor that detects light reflected by the intermediate transfer belt 30 and the toner image. . The environmental condition sensor 45 detects the environmental condition of the image forming apparatus 1. The environmental condition sensor 45 is a humidity sensor, for example, and supplies humidity information to the control unit 1A.

  The fixing device 60 includes a heating roll 61 and a pressure roll 62, and heats and pressurizes the toner image by passing the paper on which the toner image before fixing is formed by the secondary transfer device 50. Fix on paper.

  The paper transport unit 80 takes out the paper P from the paper container C and transports the paper P along a paper transport path r that passes through the secondary transfer device 50 and the fixing device 60. The paper transport unit 80 includes a pickup roll 81 that takes out the paper stored in the paper container C, transport rolls 82 and 83 that transport the paper, a registration roll 84 that transports the paper to the secondary transfer device 50, and the paper to the outside. A discharge roll 86 for discharging is provided.

  A resistance detector 42 for measuring the electrical resistance of the conveyed paper is connected to the transport roll 82. More specifically, an electrode that contacts the paper is disposed on each peripheral surface of the transport roll 82, and the resistance detector 42 measures the electrical resistance in the thickness direction of the paper sandwiched by the transport roll 82. Data representing the electrical resistance value measured by the resistance detector 42 is supplied to the control unit 1A. The control unit 1A controls the voltage applied to the secondary transfer device 50 according to the measured electric resistance value.

  Here, the intermediate transfer belt 30 corresponds to an example of the image carrier according to the present invention, and the additional charging unit 70 corresponds to an example of the charging unit according to the present invention. The resistance detector 42 corresponds to an example of a detector according to the present invention, and the secondary transfer device 50 corresponds to an example of a transfer device according to the present invention.

[Basic operation of image forming apparatus]
The basic operation of the image forming apparatus 1 shown in FIG. 1 will be described. In the yellow image forming unit 10Y, the photoreceptor 11Y is rotationally driven in the direction of arrow a, and a charge is applied to the peripheral surface of the photoreceptor 11Y by the charger 12Y. The The same applies to the image forming units 10M, 10C, and 10K corresponding to colors other than yellow. The exposure device 20 irradiates the photoconductors of the image forming units 10M, 10C, and 10K with exposure light corresponding to data corresponding to each color in the image signal. As a representative example, yellow (Y) will be described. The exposure device 20 irradiates the peripheral surface of the photoconductor 11Y with exposure light based on an image signal corresponding to yellow among image signals supplied from the outside. An electrostatic latent image is formed on the peripheral surface of 11Y. The developing device 14Y contains toner, and the developing device 14Y forms a toner image by charging the toner and attaching it to the electrostatic latent image. The photoreceptor 11Y rotates while holding a yellow toner image formed on the peripheral surface. The toner image formed on the peripheral surface of the photoreceptor 11Y is transferred to the intermediate transfer belt 30 by a transfer bias potential applied by the primary transfer unit 15Y between the peripheral surface of the photoreceptor 11Y and the intermediate transfer belt 30. . After transfer, the toner remaining on the photoreceptor 11Y is removed by the photoreceptor cleaner 16Y.

  The intermediate transfer belt 30 is stretched around the support rolls 31 to 34 and cyclically moves in the direction of arrow b. The image forming units 10M, 10C, and 10K of colors other than yellow form toner images of the respective colors in the same manner as the yellow image forming unit 10Y, and the toner transferred to the intermediate transfer belt 30 by the image forming unit 10Y. The toner image of each color is transferred so as to be superimposed on the image. The intermediate transfer belt 30 moves while holding the transferred toner image. The charge amount detection unit 90 detects the charge amount of the toner image transferred onto the intermediate transfer belt 30. The additional charging unit 70 additionally charges the toner image on the intermediate transfer belt 30 under the control of the control unit 1A.

  On the other hand, the paper P in the paper container C is taken out by the pick-up roll 81 and is transported by the transport rolls 82 and 83 and the registration roll 84 in the direction of arrow c toward the secondary transfer device 50 through the paper transport path r. . When the paper P is sandwiched between the transport rolls 82, the electrical resistance is measured by the resistance detector 42. The sheet P is sent to the secondary transfer unit 50 at a timing when the toner image is transferred onto the intermediate transfer belt 30 by the registration roll 84. The secondary transfer device 50 applies a bias potential for transfer between the intermediate transfer belt 30 and the sheet by the voltage supplied from the power supply device 51, and transfers the toner image on the intermediate transfer belt 30 to the sheet. The sheet on which the toner image is transferred by the secondary transfer unit 50 is conveyed to the fixing device 60, and the toner image transferred on the sheet is fixed. In this way, the paper on which the image is formed is discharged onto the discharge table 1C by the discharge roll 86. On the other hand, the toner remaining on the intermediate transfer belt 30 after the transfer by the secondary transfer device 50 is removed by the belt cleaner 38.

  In the image forming units 10Y, 10M, 10C, and 10K, a toner image (test patch image) for density adjustment is formed regardless of the image data. The test patch image is a toner image for comparison with a target density for adjusting the toner density. The patch image is formed by the image forming units 10Y, 10M, 10C, and 10K and the exposure device 20, and is transferred to the intermediate transfer belt 30. The toner density measurement unit 41 detects the density of the patch image formed on the intermediate transfer belt 30, and the control unit 1A adjusts the exposure amount of the exposure device 20 based on the density detected by the toner density measurement unit 41. . The patch image on the intermediate transfer belt 30 is removed by the belt cleaner 38 without being transferred to the paper by the secondary transfer device 50.

[Additional Charging Unit and Charging Amount Detection Unit in First Embodiment]
FIG. 2 is a diagram illustrating the additional charging unit and the charge amount detection unit illustrated in FIG. 1.

  The additional charging unit 70 in this embodiment includes a pair of charging rolls 71 and 72 and a power supply device 73 that rotate with the intermediate transfer belt 30 interposed therebetween. Each of the charging rolls 71 and 72 extends over the entire width of the intermediate transfer belt 30 (direction intersecting the paper surface of FIG. 2). One charging roll 71 functions as a charge applying electrode roll to which the power supply device 73 is connected. The other charging roll 72 arranged on the opposite side of the charging roll 71 with respect to the intermediate transfer belt 30 is connected to the ground (ground) potential and functions as a counter electrode roll. In the configuration in which both of the charging rolls 71 and 72 are brought into contact with the intermediate transfer belt 30 to be charged, toner scattering is less than that in a non-contact charging method such as a corotron.

  Both of the charging rolls 71 and 72 are formed of a material having a degree of conductivity that imparts a charge to the toner image on the intermediate transfer belt 30 by the voltage from the power supply device 73. More specifically, the charging roll 72 that is a counter electrode roll has a cylindrical cored bar 721 made of a metal material and an elastic layer 722 formed on the outer peripheral surface of the cored bar 721. The material of the cored bar 721 is, for example, stainless steel or aluminum. The elastic layer 722 has an Asker C hardness of 20 ° to 60 °. Therefore, the charging roll 72 is elastically deformed along the shape of the intermediate transfer belt 30 by being pressed against the intermediate transfer belt 30, and comes into surface contact over the entire width of the intermediate transfer belt 30. Therefore, uneven charging is suppressed. The material of the elastic layer 722 is, for example, conductive foamed rubber. The foam rubber is, for example, EPDM foam rubber or foam urethane rubber. As the conductive rubber material, for example, any material consisting of epichlorohydrin rubber, EPDM rubber in which carbon particles are dispersed, NBR having an ionic conductive function, and urethane rubber can be employed.

  In this embodiment, the charging roll 71 to which the power supply device 73 is connected also has the same configuration as the charging roll 72 that is a counter electrode roll. That is, it has a metal core 711 and an elastic layer 712 made of the same material as the charging roll 72. A voltage having the same polarity as the charging polarity of the toner, that is, a voltage having a polarity opposite to the voltage of the secondary transfer device 50 is supplied from the power supply device 73 to the charging roll 71. The voltage supplied to the charging roll 71 causes discharge from the charging roll 71 to the intermediate transfer belt 30 and the toner, and the toner on the intermediate transfer belt 30 is additionally charged. The voltage supplied by the power supply device 73 is controlled by the control unit 1A.

  The charge amount detection unit 90 includes a probe 91 disposed in the vicinity of the intermediate transfer belt 30 and a surface potential meter 92 connected to the probe 91. The probe 91 is a thin metal plate extending over the entire width of the intermediate transfer belt 30. As the probe 91, a plurality of metal needles arranged across the entire width of the intermediate transfer belt 30 and connected to each other may be employed. The probe 91 is disposed upstream of the additional charging unit 70 in the moving direction b of the intermediate transfer belt 30 and is disposed close to the surface of the intermediate transfer belt 30 on which the toner image is held.

  The surface potential meter 92 measures the charge amount of the toner image before the additional charging by the additional charging unit 70 on the intermediate transfer belt 30 by measuring the potential of the probe 91. Data representing the charge amount of the toner image measured by the surface potential meter 92 is supplied to the control unit 1A. The control unit 1A controls the voltage that the power supply device 73 supplies to the charging roll 71 according to the measured charge amount of the toner image.

  When the toner images are formed by the image forming units 10Y to 10K shown in FIG. 1, the toner charged in the developing device (for example, the developing device 14Y for Y color) is applied to the photoreceptors 11Y to 11K by Coulomb force. Adhere to. Thereafter, the charged toner is transferred to the intermediate transfer belt 30. In the secondary transfer device 50, a transfer electric field is applied to the intermediate transfer belt 30 and the paper, whereby the charged toner is transferred to the paper by Coulomb force.

  Here, when the toner charge amount and the magnitude of the transfer electric field are less than the size required for complete toner transfer, the toner is not transferred onto the paper in the secondary transfer device 50, and image defects occur. There is a fear. The toner used in the image forming apparatus of the present embodiment has an average particle size of 2.5 μm or more and 4.5 μm or less, and the charge amount per toner particle is larger than that of a toner having a particle size larger than this range. small. In addition, the toner according to the exemplary embodiment is more likely to adhere to each other as compared with the toner having a large particle size. For this reason, there is a higher possibility that toner remains on the intermediate transfer belt 30 and image defects occur. Further, the voltage in the secondary transfer device 50 is also suppressed to such an extent that the discharge products due to the discharge due to excessive voltage are suppressed.

  In the image forming apparatus 1, the toner image held on the intermediate transfer belt 30 before being transferred by the secondary transfer device 50 is additionally charged by the additional charging unit 70. That is, the toner is charged to a larger charge amount with the same polarity as before the additional charging. For this reason, the toner transferred to the sheet by the transfer electric field in the secondary transfer device 50 increases, and image defects are reduced.

  However, when the additional charging unit 70 generates a discharge in the intermediate transfer belt 30, a discharge product accompanying the discharge is generated on the intermediate transfer belt 30. A part of the discharge product on the intermediate transfer belt 30 is removed by the belt cleaner 38 (see FIG. 1), but the rest adheres to the intermediate transfer belt 30 and causes fluctuations in electrical characteristics, causing image defects. It becomes.

  In the image forming apparatus 1, an additional charge amount directed toward the toner image by the additional charging unit 70 is controlled according to the charge amount of the toner image before the additional charge, so that the toner is added compared to the case where the control is not performed. Discharge products generated by charging are reduced.

[Control Operation in First Embodiment]
FIG. 3 is a flowchart for explaining the operation of the image forming apparatus shown in FIG.

  Each unit of the image forming apparatus 1 operates based on the control of the control unit 1A. The operation of each part of the image forming apparatus 1 will be described with reference to FIGS.

  The control unit 1A waits for input of image data from the outside of the image forming apparatus 1 (step S11). When image data is supplied (Y in step S11), the image forming units 10Y to 10K receive image data. A corresponding toner image is formed (step S12). In step S12, the paper transport unit 80 starts transporting the paper P.

  Next, the control unit 1A measures the charge amount of the toner image formed by the image forming units 10Y to 10K and transferred to the intermediate transfer belt 30 (step S12). Specifically, the control unit 1A causes the surface potential meter 92 to measure the potential of the intermediate transfer belt 30 during a period in which the toner image on the intermediate transfer belt 30 passes the position of the probe 91 of the charge amount detection unit 90. An average potential in the toner image is obtained. The charge amount of the toner image can be obtained by the obtained potential.

  Next, the control unit 1A causes the additional charging unit 70 to perform additional charging (step S14). Specifically, during the period in which the toner image on the intermediate transfer belt 30 passes through the position of the additional charging unit 70, the voltage to the charging roll 71 is supplied to the power supply device 73.

  Here, the control unit 1A reduces the additional charge amount in the additional charging unit 70 as the charge amount of the toner image obtained in step S12 increases, and conversely, the charge amount of the toner image obtained in step S12. Is smaller, the additional charge amount in the additional charging unit 70 is increased. Specifically, the control unit 1A lowers the voltage that the power supply device 73 supplies to the charging roll 71 as the average potential obtained in step S12 is larger. Note that the voltage level in this specification means the level of the absolute value of the voltage. For example, “lowering the voltage” means bringing the voltage closer to “0V”. As the voltage of the charging roll 71 decreases, the amount of charge additionally charged to the toner image on the intermediate transfer belt 30 decreases.

  When the original charge amount of the toner image on the intermediate transfer belt 30 is relatively large, most of the toner image is transferred to the paper by the secondary transfer device 50, and therefore there are few image defects. In such a case, in the image forming apparatus 1 of the present embodiment, since the additional charge amount is small, the generation of discharge products due to the additional charge amount is reduced. Further, when the charge amount of the toner image on the intermediate transfer belt 30 is relatively small, the additional charge amount becomes large, and the charge amount is sufficient for the toner image to be transferred to the sheet by the secondary transfer unit 50. Charged. In this way, image defects are suppressed and the generation of discharge products is reduced.

  Next, 1 A of control parts measure the electrical resistance of the paper P conveyed with the conveyance roll 82 (step S16). Specifically, the control unit 1 </ b> A causes the resistance detector 42 to measure the electrical resistance of the paper P sandwiched between the transport rolls 82.

  Next, the control unit 1A applies a transfer voltage to the secondary transfer unit 50 to transfer the toner image onto the sheet (step S16). At this time, the control unit 1A applies a larger transfer voltage to the secondary transfer unit 50 as the electrical resistance measured in step S15 is larger, and conversely, as the measured electrical resistance is smaller, the control unit 1A applies to the secondary transfer unit 50. Apply a small transfer voltage.

  For example, when the paper P is a thick paper, the electric resistance in the thickness direction of the paper P is larger than that of the thin paper. For this reason, the electric field in the space between the sheet P and the intermediate transfer belt 30 where the toner image is located is small due to the voltage applied by the secondary transfer unit 50. In the image forming apparatus 1 of the present embodiment, a larger transfer voltage is applied as the electric resistance of the sheet increases. Therefore, transfer of the toner image is promoted, and image defects are reduced. Further, as the electric resistance of the paper is smaller, a smaller transfer voltage is applied, whereby an electric field for transfer is obtained, and the generation of discharge products by the secondary transfer unit 50 is reduced.

  Finally, the control unit 1A fixes the toner image on the paper P to the fixing device 60 (step S17).

  Here, the control unit 1A that performs the processing of step S13 and step S14 corresponds to an example of the second control unit according to the present invention, and the control unit 1A that performs the processing of step S15 and step S16 refers to the present invention. This corresponds to an example of a second control unit.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. In the following description of the second embodiment, the same reference numerals are given to the same elements as those in the embodiments described so far, and differences from the above-described embodiments will be described.

  FIG. 4 is a diagram illustrating an additional charging unit according to the second embodiment.

  The image forming apparatus according to the second embodiment is different from the image forming apparatus according to the first embodiment shown in FIGS. 1 and 2 in that the charge amount detection unit 90 is removed. The additional charging unit 70 according to the present embodiment has the same configuration as the additional charging unit 70 shown in FIG.

  In the image forming apparatus according to the second embodiment, the control unit 1A estimates the charge amount of the toner image based on the image data without causing the surface potentiometer to measure the potential.

  FIG. 5 is a flowchart for explaining the operation of the image forming apparatus according to the second embodiment.

  In the present embodiment, the average image density calculation in step S213 and the operation of additional charging in step S214 are different from the operation of the first embodiment described with reference to FIG. Hereinafter, a description will be given with reference to FIGS. 4 and 5.

  In the process of calculating the average image density (step S213), the average image density of the toner image formed by the toner image formation in step S12 is calculated. In an image area corresponding to an image on one sheet of paper on the intermediate transfer belt 30, a portion where toner exists and a portion where toner does not exist (that is, a portion where the color of the paper can be seen when the toner image is transferred onto the paper). There is. The average image density is a ratio of a portion where the toner exists in the image area. The charge amount of the toner image corresponds to the total charge amount of toner (particles) constituting the toner image. Therefore, the average image density can be used as an index representing the charge amount of the toner image. The control unit 1A estimates the charge amount of the toner image by calculating the average image density of the images formed in the image forming units 10Y to 10K from the image data.

  Next, the control unit 1A causes the additional charging unit 70 to perform additional charging (step S214). Here, the control unit 1A decreases the additional charge amount in the additional charging unit 70 as the charge amount of the toner image increases. Specifically, the additional charge amount in the additional charging unit 70 is decreased as the average image density value calculated in step S213 is increased.

  In the image forming apparatus 1 according to the present embodiment, when the charge amount of the toner image on the intermediate transfer belt 30 is determined to be large from the average image density, the additional charge amount is small. Production is reduced.

  When forming a color image, an average image density is obtained for each color toner. In the tandem type image forming apparatus according to the present embodiment, the image forming units 10Y, 10M, 10C, and 10K for each color are arranged in parallel. When viewed in a single color, the toner image formed by the image forming unit disposed upstream in the moving direction of the intermediate transfer belt 30 has an image density as compared with the toner image formed by the image forming unit disposed downstream. The toner image tends to have a large charge amount even under the same condition. This is because, for example, the toner image formed by the yellow image forming unit 10Y arranged at the most upstream is transferred to the intermediate transfer belt 30 by the primary transfer unit 15Y of the image forming unit 10Y and then arranged downstream. It is conceivable that the charge amount is added while passing through the primary transfer units in the remaining image forming units 10M, 10C, and 10K. On the other hand, the toner image formed by the black image forming unit 10K disposed at the most downstream has a smaller charge amount than the toner images formed by the other image forming units 10Y to 10C, and the secondary transfer unit 50. It tends to be difficult to be transferred.

  Therefore, the control unit 1A charges the additional charging unit 70 with an additional charge amount corresponding to the average image density only when a toner image is formed only by the most downstream image forming unit 10K, that is, when a monochrome image is formed on a sheet. Let it be done. On the other hand, the control unit 1A is sufficient for transfer when a toner image is formed by the image forming units 10Y to 10C arranged upstream of the most downstream image forming unit 10K, that is, when a color image is formed. It is determined that there is a toner charge amount, and charging by the additional charging unit 70 is stopped.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. In the following description of the third embodiment, the same reference numerals are given to the same elements as those in the embodiments described so far, and differences from the above-described embodiments will be described.

  FIG. 6 is a diagram illustrating an additional charging unit according to the third embodiment of the present invention.

  The additional charging unit 370 of the image forming apparatus in this embodiment is different from the additional charging unit 70 of the second embodiment shown in FIG. The difference is that a resistor 374 is provided. The non-contact charger 371 is a corotron and includes a discharge wire 3711 and a shield 3712 for carrying out discharge. The discharge line 3711 is a metal wire extending over the entire width of the intermediate transfer belt 30. The shield 3712 is a metal plate-like member and surrounds the discharge line 3711 except for a portion opened toward the intermediate transfer belt 30. In this embodiment, the charging roll 72 that is a counter electrode is pressed against the tension of the intermediate transfer belt 30 and is elastically deformed by a reaction force from the intermediate transfer belt 30.

  A power supply device 73 is connected to the discharge line 3711 and a voltage is supplied from the power supply device 73. The power supply device 73 receives control of voltage application or application stop from the control unit 1A. The shield 3712 is connected to the ground potential via the variable resistor 374. If the shield 3712 is directly connected to the ground potential, the discharge amount concentrates at a position where the corona discharge starts (starting discharge) in the long extending discharge line 3711, and discharge unevenness occurs. By connecting the shield 3712 to the ground potential via a resistor, discharge unevenness is reduced. The resistance value of the variable resistor 374 changes under the control of the control unit 1A. The charge amount of the non-contact charger 371 in this embodiment is adjusted by using the resistance value of the variable resistor 374. The smaller the resistance value of the variable resistor 374 is, the more charge discharged from the discharge line 3711 of the non-contact charger 371 is taken into the shield 3712, so the amount of discharge toward the toner image on the intermediate transfer belt 30 is smaller. . That is, the additional charge amount of the toner image by the non-contact charger 371 is small. Conversely, the greater the resistance value of the variable resistor 374, the greater the amount of discharge toward the toner image on the intermediate transfer belt 30. Further, in the non-contact charger 371 that is a corotron, a discharge product or dirt resulting from the discharge adheres to and accumulates on the shield 3712, whereby the resistance value between the discharge line 3711 and the shield 3712 increases.

  FIG. 7 is a flowchart for explaining the operation of the image forming apparatus according to the third embodiment.

  In the present embodiment, the environmental condition detection operation in step S310, the additional charging operation in step S314, and the patch formation operation in step S321 and after are different from the operation of the second embodiment described with reference to FIG. . Hereinafter, description will be made with reference to both FIG. 6 and FIG.

  In the environmental condition detection in step S310, the control unit 1A detects the environmental condition of the image forming apparatus 1 (see FIG. 1). More specifically, the control unit 1 </ b> A obtains humidity data via the environmental condition sensor 45.

  In the additional charging in step S314, the control unit 1A decreases the additional charge amount in the additional charging unit 70 as the average image density value calculated in step S213 is larger, similarly to the process in step S214 in the second embodiment. . The additional charge amount is reduced by reducing the resistance value of the variable resistor 374. When the resistance value of the variable resistor 374 decreases, the amount of electric current (current) flowing from the discharge line 3711 of the non-contact charger 371 to the shield 3712 increases and discharged from the discharge line 3711 toward the toner on the intermediate transfer belt 30. Charge amount (current) decreases. Therefore, the additional charge amount by the non-contact charger 371 is reduced.

  Further, in the additional charging in step S314, the control unit 1A adjusts the resistance value finally set in the variable resistor 374 according to various conditions with respect to the resistance value obtained from the value of the average image density. More specifically, the control unit 1A adjusts according to environmental conditions, the cumulative number of images, and the density of the test patch formed on the intermediate transfer belt 30.

  The environmental condition is humidity data obtained from the environmental condition sensor 45 in the environmental condition detection in step S310. Since the non-contact charger 371 uses corona discharge, the discharge start voltage in the discharge line 3711 decreases as the humidity decreases. When the additional charging is performed at a low voltage, uneven discharge due to the starting discharge at the discharge line 3711 occurs, and the bias of the charge distribution of the additional charging increases. In contrast, the control unit 1A increases the discharge start voltage by reducing the resistance value of the variable resistor 374, and reduces the bias of the charge amount distribution caused by the starting point discharge.

  The cumulative number of images is the number of images formed by the image forming apparatus 1. The control unit 1A counts and uses the cumulative number of sheets on which images are formed. In the non-contact charger 371, as the usage time becomes longer, discharge products and dirt accumulate in the shield 3712, and the resistance value between the discharge line 3711 and the shield 3712 increases. For this reason, it becomes difficult to start the discharge of the charge from the discharge line 3711 toward the shield 3712, while the discharge from the discharge line 3711 toward the toner on the intermediate transfer belt 30 is likely to start. That is, the discharge start voltage in the discharge line 3711 decreases. When the additional charging is performed at a low voltage, uneven discharge due to the starting discharge at the discharge line 3711 occurs, and the bias of the charge distribution of the additional charging increases. On the other hand, the control unit 1A decreases the resistance value of the variable resistor 374, thereby increasing the discharge start voltage and reducing the bias of the charge amount distribution caused by the starting point discharge.

  The density of the test patch is the density of the test patch having a predetermined prescribed image density obtained by the processing in steps S322 and S323, that is, the toner adhesion amount on the intermediate transfer belt 30. Here, the processing of steps S322 and S323 will be described first. After forming the image on the sheet, the control unit 1A, when forming a predetermined number of images (for example, 50 sheets) (Yes in step S321), does not depend on the image data (for example, 50%). Test patches having an image density are formed on the image forming units 10Y to 10K (step S322). Then, the control unit 1 </ b> A measures the density of the test patch formed on the intermediate transfer belt 30 by the toner density measurement unit 41 (see FIG. 1) 9. The density of the test patch represents the toner image forming ability in the image forming units 10Y to 10K. That is, even when a test patch having a prescribed image density is formed, the toner amount of the toner image placed on the intermediate transfer belt 30 depends on the state of the photoconductor, the developing device, and the toner of the image forming units 10Y to 10K. In contrast, the charge amount of the toner image is different. Therefore, when the density of the test patch is smaller than a predetermined reference, the control unit 1A increases the charge amount toward the non-contact charger 371 by increasing the resistance value of the variable resistor 374.

  In this way, in step S314, the control unit 1A obtains a resistance value from the average image density value, and this resistance value is set to the environmental conditions, the cumulative number of images, and the density of the test patch formed on the intermediate transfer belt 30. In accordance with the adjustment, the adjusted resistance value is set in the variable resistor 374. Here, for the adjustment of the resistance value, specifically, an adjustment value obtained by multiplying each of the humidity, the cumulative number of images, and the test patch by a weighting coefficient is added to the resistance value obtained from the average image density value. To be executed. The weighting coefficient is information obtained in advance by a test. In addition to the calculation, it is possible to adopt a configuration in which the relationship between the value of the above condition obtained from the experiment and the adjustment value is stored in association with each other and the corresponding adjustment value is read and set during adjustment.

[Example]
(Humidity change)
Here, an example will be described in which the image formation is performed by changing the resistance value of the variable resistor under various conditions with different humidity and cumulative number of images in the configuration described in the third embodiment.

  First, as Example 1, an image forming apparatus having the configuration described in the third embodiment is placed in an environment with a humidity of 85%, and a new non-contact charger 371 that is not used in image formation is used. The resistance value of the variable resistor 374 was set to 1 MΩ, and an image for evaluation was formed.

  Next, as Comparative Example 1, an image forming apparatus having the same configuration as that of the image forming apparatus of Example 1 was placed in a low humidity environment with a humidity of 50%, and the resistance value of the variable resistor 374 was fixed to 1 MΩ. An image for use was formed.

  Next, as Example 2, an image forming apparatus having the same configuration as the image forming apparatus of Example 1 is placed in a low humidity environment with a humidity of 50%, and the resistance value of the variable resistor 374 is reduced to 0.5 MΩ. An image for evaluation was formed.

(Cumulative number change)
Next, as Comparative Example 2, an image forming apparatus having the same configuration as the image forming apparatus of Example 1 is placed in a low humidity environment with a humidity of 85%, and the resistance value of the variable resistor 374 is fixed to 1 MΩ. An image for evaluation was formed after 10,000 images were formed (cycle number 100k).

  Next, as Example 3, an image forming apparatus having the same configuration as the image forming apparatus of Example 1 was placed in a low-humidity environment with a humidity of 85%, and 100,000 images were formed (cycle number 100k). Then, the resistance value of the variable resistor 374 was lowered to 0.5 MΩ, and an image for evaluation was formed.

  Tables 1 and 2 below show the evaluation results of each example and comparative example.

  In any of Examples 1 and 2 and Comparative Example 1, no defect was recognized to the extent that it was determined as a defect with respect to the image quality standard of the image forming apparatus (evaluation: “◯”). However, in Comparative Example 1 in which the resistance value was fixed in spite of the change in humidity, minute transfer unevenness due to charging unevenness due to starting discharge was observed. In contrast, in Example 2 in which the resistance value of the variable resistor 374 was reduced to 0.5 MΩ, no minute transfer unevenness due to charging unevenness was observed.

  In any of Examples 1 and 3 and Comparative Example 2, no defect was found to the extent that it was determined as a defect with respect to the image quality standard of the image forming apparatus (evaluation: “◯”). However, in Comparative Example 2 in which the resistance value was fixed despite the cumulative number of image formations being 100,000 (100 k), minute transfer unevenness due to charging unevenness due to starting point discharge was observed. On the other hand, in Example 3 in which the resistance value of the variable resistor 374 was reduced to 0.5 MΩ, no minute transfer unevenness due to charging unevenness was observed.

[Fourth Embodiment]
Next, a fourth embodiment of the present invention will be described. In the following description of the fourth embodiment, the same reference numerals are given to the same elements as those in the embodiments described so far, and differences from the above-described embodiments will be described.

  FIG. 8 is a schematic configuration diagram of an additional charging unit according to the fourth embodiment of the present invention.

  The additional charging unit 470 of the image forming apparatus according to the present embodiment is the same as the additional charging unit 370 of the third embodiment shown in FIG. 6 except that the non-contact charger 371 and a pair of charging rolls 72 serving as the counter electrodes are provided. On the other hand, as shown in FIG. 8, two sets of non-contact chargers 471A and 471B and counter electrodes 472A and 472B are provided. Each of the non-contact chargers 471A and 471B has the same configuration as the non-contact charger 371 of the third embodiment. Further, the counter electrodes 472A and 472B have the same configuration as that of the charging roll 72 serving as the counter electrode in the third embodiment.

  Since the additional charging unit according to the fourth embodiment includes two non-contact chargers 471A, the voltage applied to one non-contact charger can be set low, so that scattering of the toner image due to excessive voltage is prevented. be able to.

  In the fourth embodiment, an example in which two non-contact chargers 471A and 471A are provided is described. However, the image forming apparatus of the present invention is not limited to this, and may include three or more chargers.

[Fifth Embodiment]
Next, a fifth embodiment of the present invention will be described. In the description of the fifth embodiment below, the same reference numerals are given to the same elements as those in the embodiments described so far, and differences from the above-described embodiments will be described.

  FIG. 9 is a schematic configuration diagram of an additional charging unit according to the fifth embodiment of the present invention. Part (A) of FIG. 9 is a schematic cross-sectional view of the additional charging unit 570 as viewed in the moving direction of the intermediate transfer belt 30. Part (B) of FIG. 9 is a non-contact charger 571 of the additional charging unit 570. FIG. 3 is a plan view when viewed from the intermediate transfer belt 30 side.

  The additional charging unit 570 shown in FIG. 9 is different from the third embodiment shown in FIG. 6 in the configuration of the non-contact charger 371 and the power supply device 574. The non-contact charger 371 includes a plurality of electrode plates 5712 arranged in a line in the width direction X that intersects the moving direction b of the intermediate transfer belt 30, and a shield 3712. The electrode plate 5712 is arranged over the entire width of the intermediate transfer belt 30. Each of the electrode plates 5712 is a rectangular metal plate and is spaced from each other to such an extent that no short circuit or air breakdown occurs. The shield 5712 is a metal plate-like member and surrounds the electrode plate 5712 except for a portion opened toward the intermediate transfer belt 30.

  The power supply device 574 includes a number of power supplies 574 a corresponding to the number of electrode plates 5712. The power source 574a is connected to the electrode plate 5712, respectively. In each of the power supplies 574a, voltages supplied to the electrode plate 5712 are controlled independently from each other by the control unit 1A.

  The operation of the control unit 1A according to the present embodiment corresponds to the operations of the average density calculation (step S213) and the additional charging (step S214) in the operation of the second embodiment described with reference to the flowchart of FIG. Different. The remaining operation is the same as that shown in the flowchart of FIG. 5 and will be described with reference to FIG.

  The control unit 1A according to the present embodiment, for the toner image formed on the intermediate transfer belt 30, instead of the average density calculation (step S213) in the operation of the second embodiment, Density calculation is performed for each of the divided areas divided in the width direction X. Specifically, the average image density in each of the divided areas divided in accordance with the size of the electrode plate 5712 is calculated based on the image data.

  In the additional charging, the control unit 1A first determines the average image density of each divided region constituting this row at the timing when the first row consisting of the divided regions of the toner image on the intermediate transfer belt 30 passes through the electrode plate 5712. Is supplied to the power source 574a connected to the electrode plate 5712 facing each divided region. Each of the electrode plates 5712 is supplied with a voltage corresponding to the average image density of the opposed divided regions from the power source 574a and discharged. Specifically, a larger voltage is supplied as the average image density in the divided area is smaller. Accordingly, each of the electrode plates 5712 charges the divided regions with a charge amount corresponding to the supplied voltage. That is, each of the electrode plates 5712 charges the divided area with a larger charge amount as the average image density is smaller.

  At the timing when the second row of the divided area of the toner image passes through the electrode plate 5712, the control unit 1A sets each divided area with the charge amount corresponding to the average image density of each divided area forming the next row. Charge. In this way, the control unit 1A charges all the divided areas with a charge amount corresponding to the average image density of each divided area.

  In this embodiment, even when the image density is different in each region of the toner image, such as a toner image of an image having a so-called solid portion and a blank portion, additional charging is performed according to the image density in each region.

  In the present embodiment, an example of a one-dimensional array configuration in which the electrode plates 5712 are arranged in a row has been described. However, the charger according to the present invention may have a plurality of electrode plates arrayed two-dimensionally. . When a plurality of electrode plates are two-dimensionally arranged, a plurality of rows of divided areas of the toner image are charged at a time.

[Sixth Embodiment]
Next, a sixth embodiment of the present invention will be described. In the following description of the sixth embodiment, the same reference numerals are given to the same elements as those in the embodiments described so far, and differences from the above-described embodiments will be described.

  FIG. 10 is a schematic configuration diagram of an image forming apparatus according to the sixth embodiment of the present invention.

  Compared with the image forming apparatus 1 shown in FIG. 1, the image forming apparatus 6 shown in FIG. 10 does not include the additional charging unit 70 and the charge amount detecting unit 90 for the intermediate transfer belt 30. Instead, the image forming apparatus 6 includes an additional charging unit 670 in the image forming unit 10K for black (K) color disposed at the most downstream side in the moving direction b of the intermediate transfer belt 30. The additional charging unit 670 additionally charges a black toner image formed on the black photoconductor 11K disposed on the most downstream side in the moving direction b of the intermediate transfer belt 30 among the four photoconductors 11Y to 11K. The additional charging unit 670 is disposed upstream of the primary transfer unit 15Y in the movement direction in the movement of the surface accompanying the rotation of the K-color photoconductor 11K.

  The specific configuration of the additional charging unit 670 of the present embodiment is the same as the configuration illustrated in FIG. However, the additional charging unit 670 of this embodiment is different in that the photoconductor 11K is arranged instead of the roll 72 and the intermediate transfer belt 30 as counter electrodes.

  The black photoconductor 11K in the present embodiment corresponds to an example of an image carrier according to the present invention, and the additional charging unit 670 corresponds to an example of a charging unit according to the present invention. The primary transfer unit 15K corresponds to an example of a transfer unit according to the present invention.

  As described above, the toner image formed by the yellow image forming unit 10Y disposed at the most upstream in the moving direction b is transferred to the intermediate transfer belt 30 by the primary transfer unit 15Y of the image forming unit 10Y. Thereafter, it is considered that the charge amount is added while passing through the primary transfer units of the image forming units 10M, 10C, and 10K disposed downstream. In contrast, the toner image formed by the black image forming unit 10K disposed at the most downstream has a smaller charge amount than the toner images formed by the other image forming units 10Y to 10C, and the secondary transfer unit. 50 tends to be difficult to transfer.

  According to the image forming apparatus 6 of the present embodiment, additional charging is performed on the black toner component (or the toner image in a black monochrome print) having the smallest charge amount among the toner images transferred to the intermediate transfer belt 30. The Therefore, unnecessary additional charging is prevented from being performed on the toner image sufficient for transfer, and the generation of discharge products is reduced.

  In the sixth embodiment described above, an example of the additional charging unit 670 having the same configuration as that of the charging roll 71 shown in FIG. 4 is described as an example of the charger according to the present invention. However, the present invention is not limited to this, and the charger for additionally charging the photosensitive member may be, for example, the corotron shown in FIG. Also, a plurality of chargers may be provided as shown in FIG. 8, and a configuration having a plurality of electrode plates as shown in FIG.

  In addition, the elements described in each of the above-described embodiments can be applied in combination in other embodiments. For example, the adjustment of the charge amount based on the environmental condition and the patch image described with reference to FIG. 5 can be applied to the first embodiment having the charging roll described with reference to FIG. The configuration for obtaining the charge amount by the probe 91 and the surface potential meter 92 described with reference to FIG. 2 can also be applied to a configuration having a corotron.

  In the third embodiment described above, in the additional charging, the resistance value finally set in the variable resistor 374 by the control unit 1A with respect to the resistance value obtained from the value of the average image density is set as the environmental condition and the cumulative value. An example of adjusting according to the number of images and the density of the test patch has been described. However, the adjustment according to any one or a combination of the environmental conditions, the cumulative number of images, and the test patch density is the same as in the first embodiment, the second embodiment, and the fourth to sixth embodiments. Either can be applied.

  In the present embodiment, the environmental condition detection operation in step S310, the additional charging operation in step S314, and the patch formation operation in step S321 and after are different from the operation of the second embodiment described with reference to FIG. . Hereinafter, description will be made with reference to both FIG. 6 and FIG.

  In the embodiment described above, a tandem color printer is shown as an example of the image forming apparatus. However, the image forming apparatus referred to in the present invention is not limited to this, and may be, for example, a monochrome printer without an intermediate transfer belt.

  In the above-described embodiment, a printer is shown as an example of the image forming apparatus. However, the image forming apparatus referred to in the present invention is not limited to a printer, and may be a copying machine or a facsimile, for example.

1, 6 Image forming apparatuses 10Y, 10M, 10C, 10K Image forming unit 11K Photoconductor 15K Primary transfer device 30 Intermediate transfer belt 41 Toner density measuring unit 42 Resistance detector 45 Environmental condition sensor 50 Secondary transfer device 51 Power supply device 70 370, 470, 570, 670 Additional charging unit 71 Charging roll 712 Elastic layer 371 Non-contact charging unit 374 Variable resistor 371, 471A, 471B, 571 Non-contact charging unit 472A, 472B Counter electrode 82 Transport roll 90 Charging amount detection unit 91 Probe 92 Surface electrometer

Claims (4)

An image carrier for holding a charged toner image;
A charger for additionally charging the toner image held on the image carrier;
A transfer unit that receives a voltage and transfers a toner image subjected to additional charging by the charger to a transfer target; and
An image forming apparatus comprising: a first control unit configured to control an additional charge amount toward the toner image by the charger according to a charge amount of the toner image before the additional charge. A detector for detecting an electric resistance value of the transferred body;
The image forming apparatus according to claim 1, further comprising a second control unit that controls a voltage applied to the transfer device in accordance with an electric resistance value of the transfer target detected by the detector. A plurality of photoconductors that receive the formation of each color toner image, and an intermediate transfer body that receives the transfer of each color toner image on the plurality of photoconductors sequentially from the plurality of photoconductors while rotating.
The image carrier is a photoconductor disposed on the most downstream side in the moving direction of the intermediate transfer body among the plurality of photoconductors,
The image forming apparatus according to claim 1, wherein the transfer unit is a transfer unit that transfers a toner image on the intermediate transfer member to a transfer target. It has a housing that serves as a reference for potential,
The charger is a corotron having a discharge wire and a shield connected between the housing via an electric resistance having a variable resistance value,
4. The device according to claim 1, wherein the first control unit controls an additional charge amount toward the toner by adjusting a resistance value of the electric resistance. 5. Image forming apparatus.

Images