JP2018077382A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can reduce image defects caused by an electric charge remaining on an intermediate transfer body.SOLUTION: In an image forming apparatus A of an intermediate transfer system, a control part 20 has a ghost counter measure mode, when primary transfer is performed only in part of a plurality of primary transfer parts 3, in a primary transfer part 3M arranged on the upstream side in the direction of movement of an intermediate transfer belt 56 with respect to a primary transfer part 3K that performs primary transfer, forming a contrast potential that is a potential difference between a surface potential of a photoreceptor drum 50M and a static eliminating bias applied to a primary transfer roller 54M, in order to eliminate a potential remaining on the intermediate transfer belt 56 formed during secondary transfer. In the ghost countermeasure mode, when the resistance of the intermediate transfer belt 56 detected by resistance detection means is large, the control part increases the contrast potential compared with that when the resistance is small.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer (for example, a laser beam printer, an LED printer).

  In an intermediate transfer type image forming apparatus, a toner image is first formed on the surface of a photosensitive drum, and the formed toner image is primarily transferred to an intermediate transfer member by applying a bias to a primary transfer roller (primary transfer member). Thereafter, the toner image is secondarily transferred to the recording material by applying a bias to the secondary transfer roller (secondary transfer member).

  As the intermediate transfer member, an intermediate transfer belt obtained by molding a resin material into a seamless belt shape is generally used. As the intermediate transfer belt, a conductive material added to a resin material and adjusted to a desired electrical resistivity (hereinafter referred to as resistivity) is used.

  Here, a configuration has been conventionally proposed in which image defects are suppressed by setting the resistivity of the intermediate transfer belt within a predetermined range. For example, Patent Document 1 describes a configuration that suppresses image defects such as scaly patterns and image omission by making the front layer high resistance and the back layer low resistance regarding the layers constituting the intermediate transfer belt. ing.

Japanese Patent Laying-Open No. 2015-41013

  In general, if the resistivity of the intermediate transfer belt is set too low, pre-transfer does not occur because toner scatters in the primary transfer portion formed from the photosensitive drum and the primary transfer roller. It is desirable to set the resistivity as high as possible. On the other hand, when the resistivity of the surface layer of the intermediate transfer belt is set high as in the configuration described in Patent Document 1, after the bias is applied to the secondary transfer roller, the surface potential of the intermediate transfer belt is attenuated by the residual charge. The time required increases and the following problems may arise.

  FIG. 10 is a schematic diagram schematically showing the state of the surface charge on the intermediate transfer belt after the secondary transfer. When the resistivity of the surface of the intermediate transfer belt is high, residual charges are accumulated on the intermediate transfer belt after the secondary transfer as shown in FIG. If the accumulated portion enters the primary transfer portion while the residual charge is accumulated, a part of the toner image on the photosensitive drum is scattered due to the residual charge before the photosensitive drum and the intermediate transfer belt come into contact with each other. The image quality is degraded. Further, if there is unevenness in the residual charge, it causes scattering unevenness, resulting in image unevenness and so-called image failure called ghost.

  Accordingly, the present invention has been made in view of such a current situation, and an object of the present invention is to provide an image forming apparatus capable of reducing image defects due to residual charges of an intermediate transfer member.

  In order to achieve the above object, an image forming apparatus according to the present invention forms toner images on the surfaces of a plurality of photoconductors arranged side by side in the moving direction of the intermediate transfer body, and the formed toner images are transferred to the intermediate transfer body. A charging member for charging the photosensitive member in an image forming apparatus for forming a image by secondary transfer of a toner image from an intermediate transfer member to a recording material by applying a bias to a secondary transfer member. A plurality of primary transfer members made of metal that primarily transfer a toner image from the photosensitive member to the intermediate transfer member to form a plurality of primary transfer portions together with the plurality of photosensitive members; and the primary transfer member and the charging member A control unit that controls an application unit that applies a bias; and a resistance detection unit that detects an electrical resistance of the intermediate transfer member, the control unit including only the primary transfer part of the plurality of primary transfer units. If you do In the primary transfer portion disposed upstream of the primary transfer portion that performs the primary transfer in the moving direction of the intermediate transfer member, the photosensitive member is used to neutralize the surface of the intermediate transfer member charged by the secondary transfer. A mode for forming a contrast potential that is a potential difference between the surface potential of the intermediate transfer member and a bias applied to the primary transfer member, and in this mode, when the electrical resistance of the intermediate transfer member detected by the resistance detection unit is large, the contrast potential is small. The contrast potential is made larger than the case.

  According to the present invention, it is possible to reduce image defects due to residual charges on the intermediate transfer member.

1 is a schematic cross-sectional view of an image forming apparatus. FIG. 3 is a schematic cross-sectional view of a part of the intermediate transfer belt. 2 is a block diagram illustrating a configuration of a control unit of the image forming apparatus. FIG. 6 is a graph showing the relationship between the number of images formed and the residual potential of the intermediate transfer belt. It is a table | surface which shows the result of a ghost generation | occurrence | production experiment. 6 is a graph showing a relationship between a surface potential of an intermediate transfer belt and a bias value when a predetermined bias is applied to the primary transfer roller by constant current control in a state where the intermediate transfer belt is charged. It is a flowchart of a mode selection sequence. It is a flowchart of a mode selection sequence. It is a flowchart of a mode selection sequence. FIG. 6 is a schematic diagram schematically showing a state of surface charge on an intermediate transfer belt after secondary transfer.

(First embodiment)
<Image forming apparatus>
First, the overall configuration of the image forming apparatus according to the first embodiment of the present invention will be described with reference to the drawings together with the operation during image formation. Note that the dimensions, materials, shapes, relative arrangements, and the like of the described components are not intended to limit the scope of the present invention only to those unless otherwise specified.

  The image forming apparatus A transfers intermediate toners (developers) of yellow Y, magenta M, cyan C, and black K to an intermediate transfer belt, and then transfers the image to a sheet to form an image. An electrophotographic color image forming apparatus. In the following description, the members using the toners of the respective colors are denoted by Y, M, C, and K representing the corresponding colors as subscripts, but the structure of each member is different in the color of the toner used. Except for, subscripts are omitted as appropriate unless otherwise necessary.

  As shown in FIG. 1, the image forming apparatus A includes an image forming unit that transfers a toner image onto a sheet as a recording material, a sheet feeding unit that supplies the sheet to the image forming unit, and a toner image fixed on the sheet. A fixing unit.

  The image forming unit includes an image forming unit 80 (80Y, 80M, 80C, 80K) that forms toner images of respective colors, and an intermediate transfer unit 81 including an intermediate transfer belt 56 (intermediate transfer member). The color of the toner image formed by each image forming unit 80 is in the order of yellow Y, magenta M, cyan C, and black K from the upstream in the moving direction of the intermediate transfer belt 56 (arrow R2 direction).

  Each image forming unit 80 includes a photosensitive drum 50 (50Y, 50M, 50C, 50K) that carries a toner image and a charging roller 51 (51Y, 51M, 51C, 51K) as a charging member that charges the photosensitive drum 50. ). That is, the image forming apparatus A includes a plurality of photosensitive drums 50 and is arranged in the order of the photosensitive drums 50Y, 50M, 50C, and 50K in the moving direction of the intermediate transfer belt 56 (the direction of the arrow R2). The image forming unit 80 includes a cleaning blade 55 (55Y, 55M, 55C, and 55K), a laser scanner unit 52 (52Y, 52M, 52C, and 52K), and a developing device 53 (53Y, 53M, 53C, and 53K).

  The intermediate transfer unit 81 includes a plurality of primary transfer rollers 54 (54Y, 54M, 54C, 54K), an intermediate transfer belt 56, a secondary transfer roller 64, a secondary transfer counter roller 62, and a cleaning device 65. The intermediate transfer belt 56 is an endless cylindrical belt, a secondary transfer counter roller 62, an idler roller 61 that supports the intermediate transfer belt 56, a tension roller 60 that applies a constant tension to the intermediate transfer belt 56, and a driving roller. 63 is stretched. Further, the intermediate transfer belt 56 rotates in the direction of the arrow R2 when the driving roller 63 is driven by a motor (not shown) having excellent constant speed and rotates in the direction of the arrow R1 shown in FIG. The belt tension with respect to the tension roller 60 is configured to be about 3 to 12 kgf.

FIG. 2 is a schematic cross-sectional view showing a cross section of a part of the intermediate transfer belt 56. As shown in FIG. 2, the intermediate transfer belt 56 includes two layers, a base layer 56a and a surface layer 56b. In the present embodiment, a polyimide resin or a polyether ether ketone resin is used for the base layer 56a, and a surface layer coat in which a fluororesin is added to an acrylic resin is used for the surface layer 56b. The thickness of the base layer 56a is about 60 to 70 μm, and the thickness of the surface layer 56b is about 5 to 7 μm. The surface resistivity after surface coating was 1 × 10 ¹² to 2 × 10 ¹² Ω / □, and the volume resistivity was 4 × 10 ^{11 to} 6 × 10 ¹¹ Ω · cm. In addition, the measurement of electrical resistance uses Hiresta UP made by Mitsubishi Chemical Corporation as a measuring instrument, URS made by Mitsubishi Chemical Corporation (guard electrode outer diameter Φ17.9 mm) as a measurement probe, and applied as measurement conditions. Charging was performed at a voltage of 100 V for 10 seconds.

  The intermediate transfer belt 56 includes a single layer and another intermediate transfer belt 56 including at least two layers. Polyethylene terephthalate, polycarbonate, polyamide, polyphenylene sulfide, polyethersulfone, polyetheretherketone, polyimide, polyamide, and the like can be used as the resin material for the intermediate transfer belt formed of a single layer.

  A suitable amount of carbon black or metal fine particles as a conductive filler is added to such a resin material to impart semiconductivity, so that the surface resistivity is 1 × 10 9 to 5 × 10 11 Ω / □. . Among these, carbon black is preferable from the viewpoint of mechanical properties. Examples of carbon black include acetylene black, oil furnace black, thermal black, and channel black.

  When the base layer 56a is composed of two or more layers, polyimide, polycarbonate, polyvinylidene fluoride, polyphenylene sulfide, polyethylene, polypropylene, polystyrene, polyamide, polysulfone, or the like can be used. Single resins such as polyarylate, polyethylene terephthalate, polybutylene terephthalate, polyethersulfone, polyethernitrile, ethylene-tetrafluoroethylene copolymer, polyetheretherketone, and the like can be used. Mixtures of these can also be used.

  As the material of the surface layer 56b, melamine resin, urethane resin, alkyd resin, or the like can be used. A single resin such as an acrylic resin (acrylic monomer, acrylic resin prepolymer, dipentaerythritol hexane acrylate), silicon hard coat, or fluorine resin can be used. Moreover, these mixtures, these composite materials, etc. can be used.

  The average thickness of the intermediate transfer belt 56 is 30 to 200 μm, preferably 40 to 150 μm. This is because if the thickness is too thin, defects such as folds and wrinkles are likely to occur during the assembling operation to the image forming apparatus A. It is. Further, the intermediate transfer belt 56 is required to have a performance that does not stretch even when subjected to a tension load for a long period of time and does not wear on the surface due to rubbing by the cleaning device 65.

  The primary transfer roller 54 (primary transfer member) is a metal roller made of a metal such as SUM or SUS, and has a straight shape in the thrust direction and a roller diameter of about 6 to 10 mm. ing. Further, the primary transfer roller 54 is disposed to face each photoconductor drum 50 via an intermediate transfer belt 56, and the primary transfer unit 3 (3 </ b> Y, 3 </ b> Y, 3 </ b> Y, 3 </ b> Y, ”where primary transfer is performed with the photoconductor drum 50. 3M, 3C, 3K). Further, the image forming apparatus A includes a separation mechanism (not shown) that contacts and separates primary transfer rollers 54Y, 54M, and 54C other than the primary transfer roller 54K that transfers a black toner image with respect to the intermediate transfer belt 56. In addition, although this embodiment is provided with the separation mechanism, it is good also as a structure which is not provided with this.

The secondary transfer roller 64 (secondary transfer member) is composed of an elastic layer made of NBR rubber or EPDM rubber containing an ionic conductive agent such as a metal complex, and a cored bar. The cored bar diameter is 12 mm and includes an elastic layer. The roller diameter is 24 mm. The electric resistance is ^{3.0 × 10 7 Ω~5.0 × 10 7} Ω. The secondary transfer roller 64 is disposed on the opposite side of the intermediate transfer belt 56 from the side on which the primary transfer roller 54 is disposed. The secondary transfer counter roller 62 is made of EPDM rubber and has a roller diameter of 20 mm and a rubber thickness of 0.5 mm. The Asker C hardness is set to about 70 °.

  Next, an image forming operation when forming a color image will be described with reference to FIG. First, when the control unit 20 (see FIG. 3) receives an image forming job signal, the sheets P stacked and stored in a paper feed tray (not shown) are conveyed to a registration roller 66 by a pickup roller (not shown). The registration roller 66 conveys the sheet P to the image forming unit after matching the operation and timing of the image forming unit.

  On the other hand, in the image forming unit, first, the surface of the photosensitive drum 50 is charged by the charging roller 51. Thereafter, the laser scanner unit 52 irradiates the surface of the photosensitive drum 50 with laser light in accordance with an image signal transmitted from an external device (not shown) or the like, thereby forming an electrostatic latent image on the surface of the photosensitive drum 50. A toner image is formed on the photosensitive drum 50 by developing the electrostatic latent image with the developing device 53 by attaching toner.

  Thereafter, a primary transfer bias having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 54 to form a contrast potential that is a potential difference between the surface potential of the photosensitive drum 50 and the primary transfer bias. As a result, in the primary transfer unit 3, the toner images formed on the respective photosensitive drums 50 are electrostatically attracted to the intermediate transfer belt 56 and sequentially transferred onto the intermediate transfer belt 56. In this embodiment, the photosensitive drum 50 is negatively charged by the charging roller 51, the normal charging polarity of the toner is negative, and the primary transfer bias is positive.

  Thereafter, the toner image on the intermediate transfer belt 56 is sent to the secondary transfer unit 4 formed by the secondary transfer roller 64 and the secondary transfer counter roller 62 by the movement of the intermediate transfer belt 56 in the direction of the arrow R2. In the secondary transfer unit 4, a positive secondary transfer bias having a polarity opposite to the toner charging polarity is applied to the secondary transfer roller 64, so that the toner image on the intermediate transfer belt 56 is applied to the sheet P on the sheet P. Next transferred.

  Thereafter, the sheet P on which the toner image has been transferred is sent to a fixing device (not shown), heated and pressurized to fix the toner image on the sheet P, and then discharged to a discharge unit (not shown).

  The toner remaining on the photosensitive drum 50 after the primary transfer is scraped off by the cleaning blade 55 and removed. Similarly, the toner remaining on the intermediate transfer belt 56 after the secondary transfer is scraped off and removed by the cleaning device 65.

<Control unit>
Next, the configuration of the control unit of the image forming apparatus A will be described.

  FIG. 3 is a block diagram illustrating a partial configuration of the control system of the image forming apparatus A. As shown in FIG. 3, the image forming apparatus A includes a control unit 20 as a control unit. The control unit 20 includes a ROM 22, a RAM 23, and a calculation unit 24.

  The ROM 22 stores setting values and the like necessary for various controls, and is called by the control unit 20 as necessary. In the RAM 23, various data such as the number of image formations and the image printing rate that change with image formation are temporarily recorded and used for various controls. The calculation unit 24 performs calculations necessary for various controls.

  The controller 20 is connected to a temperature sensor 25 (temperature detection means) for detecting temperature and a humidity sensor 26 (humidity detection means) for detecting humidity. Further, the calculation unit 24 calculates an absolute water amount (absolute humidity) from the detected temperature of the temperature sensor 25 and the detected humidity of the humidity sensor 26 and stores it in the ROM 22.

  The controller 20 is connected with a primary transfer power source 27, a secondary transfer power source 28, and a charging power source 29 as application means. The primary transfer power source 27 is electrically connected to the primary transfer roller 54, the secondary transfer power source 28 is electrically connected to the secondary transfer roller 64, and the charging power source 29 is electrically connected to the charging roller 51. The control unit 20 controls the primary transfer power source 27, the secondary transfer power source 28, and the charging power source 29 to apply a predetermined bias to the primary transfer roller 54, the secondary transfer roller 64, and the charging roller 51.

  The control unit 20 is connected to a current detection unit 21 (current detection means). The current detector 21 detects a current flowing through the intermediate transfer belt 56 via the primary transfer roller 54 when a bias is applied to the primary transfer roller 54 from the primary transfer power supply 27.

<Ghost generation experiment>
Next, the result of a ghost generation experiment performed under several conditions in the image forming apparatus A will be described.

  First, as a first condition, image formation was performed in a monochrome mode in which only a black toner image was transferred to a sheet P to form a monochrome image in a low humidity environment (set temperature: 23 ° C., set humidity: 5%). As a result, a ghost began to occur when about 5,000 images were formed. This can be attributed to the following reasons.

  FIG. 4 is a graph showing the relationship between the number of images formed by the image forming apparatus A and the residual potential that is the surface potential of the intermediate transfer belt 56 after the secondary transfer. This residual potential is obtained by cleaning the surface potential of the intermediate transfer belt 56 in a region that is in contact with the sheet P and a region that is not in contact with the intermediate transfer belt 56 when the intermediate transfer belt 56 passes through the secondary transfer portion 4 during application of the secondary transfer bias. This is a potential measured at a position downstream of the device 65.

  As shown in FIG. 4, the residual potential of the intermediate transfer belt 56 increases as the number of image formations increases in both the area in contact with the sheet P and the area not in contact with it. This is because the residual charge correlates with the resistivity of the intermediate transfer belt 56, and the resistivity of the intermediate transfer belt 56 increases due to wear of the intermediate transfer belt 56 as the number of images formed increases. Since the residual charge increases as the number of image formations increases in this way, a portion where the residual charge is accumulated enters the primary transfer unit 3, and a ghost is generated when about 5,000 images are formed. I think that I started to do. Note that the residual potential in the region not in contact with the sheet P in the secondary transfer unit 4 is higher than that in the region in contact with the sheet P. In the region not in contact with the sheet P, the resistance of the sheet P when the secondary transfer bias is applied. This is because a lot of current flows because there is no current.

  Next, after ghosting began to occur, five sheets of images were continuously formed in the monochrome mode using the A3 size sheet P. As a result, a ghost was generated from the third sheet. In the first and second sheets, the portion of the intermediate transfer belt 56 that has passed the secondary transfer portion 4 where the secondary transfer bias is being applied does not overlap the portion to which the subsequent image is transferred. It seems that it is not received. On the other hand, since the primary transfer is performed near the portion where the residual charge is accumulated after the third sheet, it can be considered that a ghost has appeared.

  As described above, the image defect due to the residual charge is likely to occur as the number of image formations increases, and also when the image formation is continuously performed. Further, when image formation is continuously performed, the timing at which the portion of the intermediate transfer belt 56 that has passed the secondary transfer portion to which the secondary transfer bias is applied overlaps the portion to which the subsequent image is transferred varies depending on the size of the sheet P. . Further, since the amount of residual charge varies depending on the number of image formations, the time required for attenuation of the residual charge also varies. For this reason, when image formation is performed continuously, the number of image formations in which image defects due to residual charges occur differs depending on the size of the sheet P and the total number of image formations of the image forming apparatus A.

Next, as a second condition of the ghost generation experiment, image formation was continuously performed in the monochrome mode by changing the installation environment temperature and humidity of the image forming apparatus A. As a result, as shown in FIG. 5, when the absolute water content in the installation environment was less than 5 g / m ^3, ghost was generated, and when it was 5 g / m ³ or more, no ghost was generated. This is presumably because, in a low humidity environment, the adhesion between the intermediate transfer belt 56 and the toner is small, and the charge amount of the toner is large, so that the toner is easily scattered.

  Regarding the occurrence of ghost, with respect to the moving direction of the intermediate transfer belt 56, the image forming unit for forming a yellow toner image is disposed on the most upstream side, and the image forming unit for forming a black toner image is disposed on the most downstream side. The configuration becomes prominent when the monochrome mode is executed. This is because in the full color mode in which toner images of four colors of yellow, magenta, cyan, and black are transferred to the sheet P, toner scattering may occur during transfer of the yellow toner image, but this is difficult to notice in yellow. . In the full color mode, when the black toner image is transferred, the residual charge on the intermediate transfer belt 56 is discharged by the primary transfer current that flows when the other color toner image is primarily transferred on the upstream side. is there.

<Image formation mode>
Next, an image forming mode included in the image forming apparatus A will be described. The image forming apparatus A has three modes as image forming modes: a monochrome mode, a color mode, and a ghost countermeasure mode.

  The monochrome mode is a mode in which only a black toner image is transferred to the sheet P to form a monochrome image. In the monochrome mode, only the primary transfer roller 54K that transfers the black toner image to the intermediate transfer belt 56 is brought into contact with the intermediate transfer belt 56, and the other primary transfer rollers 54Y, 54M, and 54C are separated by the intermediate transfer belt 56. The image is formed in a state separated from the image.

  The full color mode is a mode in which four color toner images of yellow, magenta, cyan, and black are transferred to the sheet P to form a full color image. In the full color mode, all the primary transfer rollers 54 are brought into contact with the intermediate transfer belt 56 and the toner images of the respective colors are superimposed on the intermediate transfer belt 56 to form an image. In the full color mode, the primary transfer bias value is determined with reference to a table in which the installation environment stored in advance in the ROM 22 is associated with the primary transfer bias, and a constant voltage is applied.

  The ghost countermeasure mode is a mode in which only the black toner image is transferred to the sheet P in the primary transfer portion 3K to form a monochrome image, and is a mode for suppressing image defects due to residual charges. . In the ghost countermeasure mode, a monochrome image is formed. However, as in the full color mode, image formation is performed with all the primary transfer rollers 54 in contact with the intermediate transfer belt 56. First, immediately after the start of the image forming job, a charged potential of −200 V is formed on the photosensitive drum 50M of the primary transfer unit 3M. After that, immediately before the portion of the intermediate transfer belt 56 that has passed the secondary transfer portion 4 to which the secondary transfer bias is being applied enters the primary transfer portion 3M, the same polarity as the secondary transfer bias applied during the secondary transfer. A bias is applied to the primary transfer roller 54M. In this embodiment, since the secondary transfer bias is positive, the bias applied to the primary transfer roller 54M is also positive.

  In this way, in the primary transfer unit 3M on the upstream side of the primary transfer unit 3K to which the image is transferred, the primary transfer roller 54M is charged by the secondary transfer bias by applying a bias having the same polarity as the secondary transfer bias. The surface of the intermediate transfer belt 56 can be neutralized. Accordingly, it is possible to suppress image defects due to residual charges on the intermediate transfer belt 56 in the primary transfer portion 3K to which the image is transferred.

  Further, in the primary transfer portion 3M that performs static elimination, by forming a charging potential having the same polarity as the toner on the photosensitive drum 50M, it is possible to prevent the toner from adhering to the photosensitive drum 50M from the developing device 53M.

  In the ghost countermeasure mode, the time from the start of the image forming job to the output of the first sheet is almost the same as the monochrome mode, and is shorter than the full color mode. In the following description, the bias applied to the primary transfer roller 54M when the ghost countermeasure mode is executed is referred to as a static elimination bias.

<Determination method of static elimination bias>
Next, a method for determining the static elimination bias when executing the ghost countermeasure mode will be described.

  Although the resistance value of the intermediate transfer belt 56 is determined at the design stage of the image forming apparatus A, as described above, the resistance value increases as the number of images formed increases, and accordingly, the residual potential on the surface of the intermediate transfer belt 56 is increased. Will increase. Further, the residual potential of the intermediate transfer belt 56 varies depending on the use environment such as the type of image to be formed and the temperature and humidity. As described above, the charged state due to the residual potential on the surface of the intermediate transfer belt 56 changes every moment. For this reason, in the ghost countermeasure mode, it is necessary to apply an appropriate neutralization bias according to the charging state of the intermediate transfer belt 56.

  Therefore, the charging state of the intermediate transfer belt 56 is determined by the method described below. FIG. 6 is a graph showing the relationship between the surface potential of the intermediate transfer belt 56 and the bias value when a bias is applied to the primary transfer roller 54 with constant current control of 30 μA while the intermediate transfer belt 56 is charged. The surface potential of the intermediate transfer belt 56 was measured using a surface potential meter at the position of the cleaning device 65. As shown in FIG. 6, the bias value applied to the primary transfer roller 54 increases as the surface potential of the intermediate transfer belt 56 increases. That is, it can be seen that the surface potential of the intermediate transfer belt 56 increases as the electrical resistance (hereinafter referred to as resistance) of the intermediate transfer belt 56 increases. Accordingly, a bias is applied to the primary transfer roller 54, and the current flowing through the intermediate transfer belt 56 via the primary transfer roller 54 is detected by the current detection unit 21. The controller 20 detects the resistance of the intermediate transfer belt 56 based on the detected current or the applied bias value. That is, the current detection unit 21 and the control unit 20 constitute a resistance detection unit that detects the resistance of the intermediate transfer belt 56. Based on the detected resistance of the intermediate transfer belt 56, the charging state of the intermediate transfer belt 56 can be indirectly determined.

  The detection of resistance is necessary for detecting a resistance by measuring a current flowing when a constant bias is applied to the primary transfer roller 54 by constant voltage control, and for supplying a constant current by constant current control. Any method of detecting the resistance by measuring the bias value may be used.

  Further, as described above, the resistance value of the intermediate transfer belt 56 varies depending on the use of the image forming apparatus A. On the other hand, when a metal roller is used as the primary transfer roller 54, since the roller itself is a conductor, there is no change in resistance due to environmental fluctuation or durability fluctuation. For this reason, by using a metal roller as the primary transfer roller 54, the resistance value of the intermediate transfer belt 56 can be accurately measured even when there are endurance fluctuations and environmental fluctuations.

  Next, a method for determining the static elimination bias in the ghost countermeasure mode will be described. In the present embodiment, in the initial state (before use) of the image forming apparatus A, a current (hereinafter referred to as a reference current value) that flows through the intermediate transfer belt 56 when a bias is applied to the primary transfer roller 54 by constant voltage control is detected. The value is stored in the ROM 22 in advance. In the ghost countermeasure mode, the bias having the same value as that when the reference current value is detected immediately before the portion of the intermediate transfer belt 56 that has passed the secondary transfer portion 4 to which the secondary transfer bias is being applied enters the primary transfer portion 3Y. Is applied to the primary transfer roller 54Y by constant voltage control. At this time, the current value flowing through the intermediate transfer belt 56 is detected and compared with the reference current value. At this time, it is desirable that the magnitude of the bias applied to the primary transfer roller 54Y is set within a range that does not significantly affect the charging of the intermediate transfer belt 56.

  Here, when the resistance of the intermediate transfer belt 56 increases due to durability or the like, the detected current value becomes smaller than the reference current value as it increases. For this reason, as compared with the reference current value, the absolute value of the static elimination bias value is increased as the amount of decrease in the detected current value increases. For example, a table in which the difference value between the detected current value and the reference current value and the neutralization bias value are associated with each other is stored in the ROM 22 in advance, and in this table, the neutralization bias value increases as the differential value between these currents increases. Set to increase the absolute value. As described above, when the resistance of the intermediate transfer belt 56 is large, an appropriate neutralization bias can be applied according to the residual potential of the intermediate transfer belt 56 by increasing the absolute value of the neutralization bias as compared with the case where the resistance is small.

  It should be noted that different tables can be prepared for the static elimination bias depending on the usage environment such as temperature and humidity. That is, the intermediate transfer belt 56 is more easily charged as it is placed in a low humidity environment. For this reason, it is good also as a structure using the table which sets so that the humidity value may be detected by the humidity sensor 26 and the absolute value of a static elimination bias may be made small, so that the detected humidity is high. Alternatively, a table may be used in which the temperature is detected by the temperature sensor 25 and the absolute value of the static elimination bias is decreased as the detected temperature is higher. With such a configuration, a more appropriate static elimination bias can be applied.

  In this embodiment, although the resistance of the intermediate transfer belt 56 is detected based on the current that flows when a constant bias is applied to the primary transfer roller 54 by constant voltage control, the present invention is not limited to this. In other words, for example, the resistance value of the intermediate transfer belt 56 may be calculated based on the current value detected by the current detection unit 21 and the applied bias and directly compared. Further, a configuration may be employed in which a bias value necessary for flowing a constant current to the intermediate transfer belt 56 by constant current control is measured and compared. In other words, any method may be used as long as the resistance of the intermediate transfer belt 56 is detected, and when the detected resistance value is high, the absolute value of the static elimination bias is made larger than when the resistance value is low. As a result, an appropriate neutralization bias can be applied according to the residual potential of the intermediate transfer belt 56.

  Thus, by detecting the resistance of the intermediate transfer belt 56 and applying an appropriate neutralization bias according to the detection result, the effect of neutralizing residual charges on the intermediate transfer belt 56 can be improved. Accordingly, it is possible to improve the effect of reducing image defects due to the residual charge on the intermediate transfer belt 56.

  The charge removal of the residual charge on the intermediate transfer belt 56 is also performed by charging the surface of the photosensitive drum 50M with the polarity opposite to the polarity of the secondary transfer bias by the charging roller 51M at the same timing as the application of the charge removal bias. Can do. As a result, it is possible to obtain the same effect as the configuration in which the charge removal bias is applied to the primary transfer roller 54M.

  Further, in order to neutralize the surface of the intermediate transfer belt 56, it is possible to apply a neutralization bias to the primary transfer roller 54M and to charge the photosensitive drum 50M. That is, in the primary transfer unit 3Y, a contrast potential for removing the residual potential of the intermediate transfer belt 56 may be formed. Regarding the magnitude of the contrast potential, when the resistance of the intermediate transfer belt 56 detected by the same method as described above is large, the same effect as described above can be obtained by increasing the contrast potential as compared with the case where the resistance is small.

  However, from the viewpoint of the life of the photosensitive drum 50, it is desirable to reduce the absolute value of the charging potential of the photosensitive drum 50. For this reason, the configuration in which the charge is applied by applying a bias to the primary transfer roller 54 is more preferable than the configuration in which the charge is removed by the photosensitive drum 50.

  Further, as another configuration for removing the residual charge, a configuration in which a charge removing unit for removing the charge from the intermediate transfer belt 56 after the secondary transfer is separately provided is also conceivable. However, since it is necessary to separately attach a member for performing static elimination and a high-voltage power supply, the cost is increased and the size of the apparatus is increased. Further, if the high voltage value applied to the neutralization unit is insufficient with respect to the residual charge of the intermediate transfer belt 56, the neutralization is not performed sufficiently, and conversely, if it is excessive, the reverse polarity is charged. Also in this case, an image defect occurs. On the other hand, according to the configuration of the present embodiment, it is possible to eliminate residual charges with an appropriate neutralization bias value without causing an increase in cost and an increase in size of the apparatus.

<Mode selection sequence>
Next, a mode selection sequence for selecting an image forming mode will be described with reference to a flowchart shown in FIG.

  As shown in FIG. 7, first, when the control unit 20 receives an image forming job (S1), it is determined whether or not the mode selected in the image forming job is a full color mode (S2). Here, in the case of the full color mode, the full color mode is started (S8).

  If it is not the full color mode, the control unit 20 next determines whether or not the image forming job is a job for continuously forming images (continuous paper passing job) (S3). If the job is not a continuous paper passing job, an image failure due to the residual charge on the intermediate transfer belt 56 hardly occurs due to the above-described reason, and therefore the ghost countermeasure mode is not executed and the monochrome mode is started (S9).

  On the other hand, in the case of a continuous paper passing job, the control unit 20 next determines whether or not the cumulative number of images formed by the image forming apparatus A is 5,000 or more (S5). Here, when the number of formed images is less than 5,000, an image defect due to residual charges on the intermediate transfer belt 56 hardly occurs due to the above-described reason, so the ghost countermeasure mode is not executed and the monochrome mode is started (S9).

  When the number of formed images is 5,000 or more (predetermined or more), an image defect due to the residual charge on the intermediate transfer belt 56 is likely to occur. For this reason, the control part 20 starts the ghost countermeasure mode (S7).

  In the present embodiment, the ghost countermeasure mode is automatically selected and executed. However, the present invention is not limited to this, and the user may select and execute the ghost countermeasure mode.

Next, the result of the ghost evaluation when the ghost countermeasure mode is executed will be described. After forming 5,000 sheets of images in a low humidity environment (set temperature: 23 ° C., set humidity: 5%), the ghost countermeasure mode is executed, and a sheet P of A3 size having a basis weight of 209 g / m ² is used. Image formation was performed continuously. As a result, it was confirmed that no ghost occurred.

  As described above, according to the present invention, it is possible to reduce image defects such as toner scattering and ghost caused by the residual charge on the intermediate transfer belt 56.

(Second Embodiment)
Next, a second embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. About the part which overlaps with the said 1st Embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  The image forming apparatus A according to the present embodiment is different from the first embodiment in selection conditions for the ghost countermeasure mode. In this embodiment, even when the monochrome mode is executed, all the primary transfer rollers 54 are brought into contact with the intermediate transfer belt 56 without separating the primary transfer roller 54 from the intermediate transfer belt 56 by a separation / contact mechanism (not shown). In this state, image formation is performed. Hereinafter, the mode selection sequence of this embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 8, when the control unit 20 receives an image forming job (S11), it is determined whether or not the mode selected in the image forming job is a full color mode (S12). If the full color mode is selected here, the full color mode is started (S18).

  When not in the full color mode, the control unit 20 applies the preset bias to the primary transfer immediately before the portion that has passed the secondary transfer unit 4 to which the secondary transfer bias is being applied enters the primary transfer unit 3Y. The voltage is applied to the transfer roller 54Y by constant voltage control. At this time, the current value flowing through the intermediate transfer belt 56 is detected, and the resistance value of the intermediate transfer belt 56 is calculated and detected from the detected current value, the resistance value of the primary transfer roller 54Y, and the applied bias (S13). ).

  Next, the detected resistance value is compared with a reference resistance value that is a resistance value of the intermediate transfer belt 56 in the initial state (before use) stored in the ROM 22 in advance. Specifically, it is determined whether or not the detected resistance value is 10% or more higher than the reference resistance value (S14).

  If the detected resistance value is 10% or more higher than the reference resistance value, the ghost countermeasure mode is executed (S15). That is, in the primary transfer unit 3M, a neutralizing bias corresponding to the resistance value of the intermediate transfer belt 56 is applied to the primary transfer roller 54M. Specifically, when the resistance value of the intermediate transfer belt 56 is high, a larger static elimination bias is applied than when the resistance value is low. On the other hand, when the resistance value is not 10% or more higher than the resistance value of the intermediate transfer belt 56 in the initial state, the monochrome mode is started (S17).

  By performing such control, the surface of the intermediate transfer belt 56 can be neutralized effectively as in the first embodiment, and toner scattering, ghost, etc. caused by residual charges on the intermediate transfer belt 56 can be achieved. Image defects can be reduced.

(Third embodiment)
Next, a third embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. About the part which description overlaps with the said 1st Embodiment and 2nd Embodiment, the same drawing and the same code | symbol are attached | subjected and description is abbreviate | omitted.

  As described above, in the intermediate transfer type image forming apparatus, the resistance value of the intermediate transfer belt 56 changes due to endurance fluctuations and environmental fluctuations. Active transfer voltage control (hereinafter referred to as ATVC control) is known as a control for applying a suitable primary transfer bias by preventing the transfer property from being deteriorated due to the resistance fluctuation of the intermediate transfer belt 56.

  In the ATVC control, a bias is applied to the primary transfer roller 54 by constant current control or constant voltage control when the primary transfer roller 54 is not transferred. At this time, the resistance fluctuation of the intermediate transfer belt 56 is detected from the fluctuation of the current value flowing through the intermediate transfer belt 56 or the applied bias value. Based on the calculation result of the voltage value and the current value, bias control is performed so that an appropriate current continues to flow through the primary transfer roller 54 during image formation.

  An example in which constant current control is performed will be described. First, at the time of non-transfer, a bias is applied to the primary transfer roller 54 by constant current control so that the current value flowing through the primary transfer roller 54 becomes several kinds of preset current values, and the bias value at this time is measured. Then, an inclination related to the bias value is calculated based on the measured bias value, and an appropriate value of the primary transfer bias value for flowing a predetermined current during the primary transfer is calculated. At the time of primary transfer, the calculated primary transfer bias value is applied to the primary transfer roller 54 by constant voltage control to perform primary transfer.

  In the non-transfer time, for example, the time from when the driving of the image forming apparatus A in the standby state is started based on the image forming job signal until the leading edge of the first sheet reaches the primary transfer unit 3. This is the case of the previous rotation. Further, in the case where images are continuously formed, there is an interval between sheets, which is an interval from the trailing edge of the sheet P conveyed in advance to the leading edge of the succeeding sheet P.

  In the present embodiment, ATVC control is performed during a paper interval. Further, the ghost countermeasure mode is executed at the timing of performing ATVC control. Hereinafter, the mode selection sequence of the present embodiment will be described with reference to the flowchart shown in FIG.

  As shown in FIG. 9, first, when the control unit 20 receives an image forming job (S21), it is determined whether or not the mode selected in the image forming job is a full color mode (S22). Here, in the case of the full color mode, the full color mode is started (S28).

  If it is not the full color mode, the control unit 20 next determines whether it is time to perform ATVC control (S23). In this embodiment, since the ATVC control is performed every time a predetermined number of images are formed, it is determined here whether or not the number of image formation has reached a predetermined number.

  If it is not time to perform the ATVC control, the monochrome mode is started without executing the ghost countermeasure mode (S27). On the other hand, at the timing of performing ATVC control, the ghost countermeasure mode is executed and ATVC control is executed (S25, S26).

  Specifically, as in the first embodiment, the resistance of the intermediate transfer belt 56 is detected in the primary transfer unit 3Y, and the neutralization bias determined according to the detected resistance is applied in the primary transfer unit 3M. . Thereafter, ATVC control is performed during the interval between sheets in the primary transfer unit 3K that primarily transfers the black toner image.

  By performing such control, as in the first and second embodiments, it is possible to effectively eliminate the charge on the surface of the intermediate transfer belt 56, and toner scattering due to residual charges on the intermediate transfer belt 56. And image defects such as ghosts can be reduced. In addition, since ATVC control can be performed in the ghost countermeasure mode, occurrence of downtime can be suppressed.

20 ... Control section (control means)
21 ... Current detection part (current detection means)
27: Transfer power source (bias applying means)
29: Charging power source (applying means)
50. Photosensitive drum (photosensitive member)
51 ... Charging roller (charging member)
56. Intermediate transfer belt (intermediate transfer member)
54. Primary transfer roller (primary transfer member)
64 ... secondary transfer roller (secondary transfer member)
A: Image forming apparatus

Claims (9)

By forming a toner image on the surface of a plurality of photoconductors arranged side by side in the direction of movement of the intermediate transfer member, primarily transferring the formed toner image to the intermediate transfer member, and applying a bias to the secondary transfer member In an image forming apparatus for forming an image by secondary transfer of a toner image from an intermediate transfer member to a recording material,
A charging member for charging the photoreceptor;
A plurality of primary transfer members made of metal that primarily transfer a toner image from the photosensitive member to the intermediate transfer member and form a plurality of primary transfer portions together with the plurality of photosensitive members;
Control means for controlling application means for applying a bias to the primary transfer member and the charging member;
Resistance detection means for detecting the electrical resistance of the intermediate transfer member;
With
The control means includes
In the case where the primary transfer is performed only in a part of the plurality of primary transfer units, in the primary transfer unit disposed on the upstream side in the moving direction of the intermediate transfer body from the primary transfer unit performing the primary transfer, the secondary transfer unit A mode of forming a contrast potential which is a potential difference between the surface potential of the photoconductor and a bias applied to the primary transfer member in order to neutralize the surface of the intermediate transfer member charged by the transfer;
In this mode, when the electric resistance of the intermediate transfer member detected by the resistance detection unit is large, the contrast potential is made larger than when the electric resistance is small.   When the control means forms the contrast potential, a bias applied to the secondary transfer member in the secondary transfer with respect to the primary transfer member constituting the primary transfer portion disposed on the upstream side, The image forming apparatus according to claim 1, wherein a bias having the same polarity is applied.   When the control means forms the contrast potential, the surface of the photoconductor constituting the primary transfer unit disposed on the upstream side is reverse to the bias applied to the secondary transfer member in the secondary transfer. The image forming apparatus according to claim 1, wherein the image forming apparatus is charged to a polarity.   The image forming apparatus according to claim 1, wherein the control unit executes the mode when the number of formed images is equal to or greater than a predetermined number.   5. The image forming apparatus according to claim 1, wherein the control unit executes the mode when the detected electric resistance of the intermediate transfer member is equal to or greater than a predetermined value. 6. .   6. The mode according to claim 1, wherein the control unit executes the mode when a black toner image is transferred to the recording material to form a monochrome image. 7. Image forming apparatus. Current detecting means for detecting a current flowing through the intermediate transfer member when a bias is applied to the primary transfer member;
The control means applies several types of bias to the primary transfer member that performs the primary transfer during non-transfer during execution of the mode, and the current detection means detects a current value corresponding to the bias value. The image forming apparatus according to claim 1, wherein an appropriate value of a bias to be applied to the primary transfer member when performing the primary transfer is set based on the detection result. Current detecting means for detecting a current flowing through the intermediate transfer member when a bias is applied to the primary transfer member;
The resistance detection unit applies a bias to the primary transfer member to detect a current flowing through the intermediate transfer member by the current detection unit, and based on the detected current or the applied bias, the electric current of the intermediate transfer member is detected. The image forming apparatus according to claim 1, wherein resistance is detected. The plurality of primary transfer portions are composed of at least three or more,
In the execution of the mode, the contrast potential is formed in a primary transfer portion disposed upstream of the primary transfer portion that performs the primary transfer with respect to the moving direction of the intermediate transfer member, and the primary potential for forming the contrast potential is formed. The image forming apparatus according to claim 8, wherein an electrical resistance of the intermediate transfer member is detected in a primary transfer unit disposed upstream of the transfer unit.

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