JP2012123097A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce occurrence of an image defect in a toner image transferred onto a recording medium from an intermediate transfer belt at a secondary transfer position by splashing toner on a rear side in a conveyance direction of the recording medium.SOLUTION: A toner image formed at an image forming part is transferred to an intermediate transfer belt 20, and is transferred to a recording medium from the intermediate transfer belt at a secondary transfer position 29. At the secondary transfer position, a secondary transfer roll 26 is pressed against an opposite roll 23 in contact with an inner periphery surface of the intermediate transfer belt through a secondary transfer belt 28 and the intermediate transfer belt, and a transfer voltage is applied between the secondary transfer roll and the opposite roll. On an upstream side of the secondary transfer position, an application roll 30 is provided so as to be in contact with the inner periphery surface of the intermediate transfer belt, and a voltage having a potential on a charge polarity side of the toner is applied to the secondary transfer roll. Thus, an electric field is generated between the intermediate transfer belt and the secondary transfer roll.

Description

  The present invention relates to an image forming apparatus.

  As an image forming apparatus that forms a visible image by transferring toner to an electrostatic latent image on an image carrier, the toner image on the image carrier is temporarily transferred to an intermediate transfer member, and the toner image on the intermediate transfer member Are widely used. As the intermediate transfer member, an endless intermediate transfer belt that is stretched around a plurality of roll-shaped members such as a drive roll and a support roll is frequently used.

  In such an apparatus, the toner image formed on the image carrier is transferred to the intermediate transfer belt at the primary transfer position, and is conveyed to the secondary transfer position by the circular movement of the intermediate transfer belt. At the secondary transfer position, for example, as described in Patent Document 1, a backup member pressed against the inner peripheral surface of the intermediate transfer belt and pressed from the outside of the intermediate transfer belt to the backup member via the intermediate transfer belt A secondary transfer member is disposed. The recording medium is superimposed on the intermediate transfer belt and sandwiched between the secondary transfer member and the backup member. An electric field is formed between the secondary transfer member and the backup member, and the charged toner is transferred from the intermediate transfer belt onto the recording medium in the electric field.

JP 2010-44329 A

In such an image forming apparatus, when a toner image is transferred onto a recording medium, the toner may scatter to the rear side in the conveyance direction of the recording medium, resulting in an image defect.
An object of the present invention is to reduce the occurrence of image defects in which toner scatters to the rear side in the conveyance direction of a recording medium.

In order to solve the above-mentioned problem, an invention according to claim 1 includes an image forming unit that forms an image with toner, and is supported so as to be capable of circular movement in the direction of the peripheral surface, and the image is primarily transferred onto the outer peripheral surface. An intermediate transfer belt, a secondary transfer member that contacts the outer peripheral surface of the intermediate transfer belt and secondary-transfers the toner image on the intermediate transfer belt to a recording medium, and an inner peripheral surface of the intermediate transfer belt. A contact member provided in contact with the secondary transfer member and in contact with the intermediate transfer belt on the upstream side of the secondary transfer position in the circumferential movement direction of the intermediate transfer belt; And an applying member to which a voltage having a potential on the charging polarity side of the toner is applied.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the voltage applied between the application member and the secondary transfer member is the intermediate transfer belt, the secondary transfer member, and the like. An electric field is formed between the toner and the toner, and the electric field is set so as to restrain the toner from moving in the direction along the peripheral surface of the intermediate transfer belt on the upstream side of the secondary transfer position. .

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the voltage is set to a low voltage or is in an OFF state as the humidity at a place where the image forming apparatus is installed decreases. Shall be assumed.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the first, second, or third aspect, the toner image formed by the image forming unit is in a direction crossing the moving direction of the intermediate transfer belt. A discriminating unit for discriminating whether or not a straight line is included, and when it is determined that the straight line is included, the voltage is set to be applied between the application member and the secondary transfer member; It shall be.

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the smoothness of the recording medium onto which the toner image is transferred, the moving speed of the intermediate transfer belt, and the humidity At least one data is acquired, and based on this, the ON / OFF of the voltage applied between the application member and the secondary transfer member is controlled.

  In the image forming apparatus according to the first aspect, it is possible to reduce the occurrence of image defects in which toner scatters to the rear side in the conveyance direction of the recording medium, as compared with an image forming apparatus that does not include this configuration.

  In the image forming apparatus according to claim 2, compared to an image forming apparatus that does not include this configuration, the toner is scattered between the application member and the secondary transfer member on the rear side in the conveyance direction of the recording medium. It can be restrained by the force acting on the toner within the electric field.

  In the image forming apparatus according to the third aspect, it is possible to reduce a decrease in the sharpness of the image as compared with an image forming apparatus that does not include this configuration.

  In the image forming apparatus according to the fourth aspect, compared to an image forming apparatus that does not include this configuration, the occurrence of the image defect is reduced with respect to an image in which an image defect in which the toner scatters on the rear side is likely to occur. It is possible to achieve both reduction of the sharpness of an image in which this image defect is unlikely to occur.

  In the image forming apparatus according to claim 5, the occurrence of the image defect occurs when the image formation condition is such that an image defect in which the toner scatters to the rear side is more likely to occur than in an image forming apparatus that does not include this configuration. It is possible to simultaneously reduce the decrease in the sharpness of the image when the image defect is less likely to occur.

1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 2 is an enlarged view illustrating a configuration of a position where secondary transfer is performed in the image forming apparatus illustrated in FIG. 1. FIG. 2 is a schematic diagram illustrating a state of an electric field formed at a secondary transfer position of the image forming apparatus illustrated in FIG. 1. FIG. 2 is a diagram illustrating a relationship between a voltage applied to an application member of the image forming apparatus illustrated in FIG. 1 and a frequency at which toner scattering occurs. 1 is a diagram showing the relationship between the voltage applied to these application members and the frequency of toner scattering when one roll is provided as the application member of the image forming apparatus shown in FIG. 1 and two rolls are provided. It is. It is the schematic which shows the image defect which is the subject which this invention tends to solve. It is an enlarged view of the image defect shown in FIG. It is a schematic sectional drawing explaining the cause which the image defect shown in FIG.6 and FIG.7 arises. FIG. 6 is a schematic diagram illustrating a secondary transfer position and a force acting on toner on the upstream side of the secondary transfer position.

Hereinafter, embodiments of the invention according to the present application will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is an enlarged view of a position where the secondary transfer of the image forming apparatus shown in FIG. 1 is performed.
This image forming apparatus is an image forming apparatus that forms a color image using four color toners, and outputs yellow (Y), magenta (M), cyan (C), and black (K) images. It includes electrophotographic image forming units 10Y, 10M, 10C, and 10K, and an intermediate transfer belt 20 facing these units. The intermediate transfer belt 20 is stretched so as to face each of the image forming units 10, and the circumferential surface thereof is driven to rotate. In order from the upstream side in the circumferential movement direction of the intermediate transfer belt 20, an image forming unit 10Y that forms a yellow toner image, an image forming unit 10M that forms a magenta toner image, and an image forming unit 10C that forms a cyan toner image. An image forming unit 10K for forming a black toner image is arranged, and on the downstream side thereof, a secondary transfer member 24 for performing secondary transfer is disposed so as to face the intermediate transfer belt 20. A sheet-like recording medium is fed from the sheet storage portion 8 through the conveyance path 9 to the secondary transfer position 29 where the secondary transfer member 24 faces the intermediate transfer belt 20. On the downstream side of the secondary transfer position 29 in the recording medium conveyance path, a recording medium conveyance device 25 to which the toner image has been transferred and a fixing device 7 that heats and pressurizes the toner image and presses them onto the recording medium are provided. ing. Further, a paper discharge holding unit (not shown) that holds the recording medium with the toner image fixed thereon is provided on the downstream side.

  Each of the image forming units 10 has a photoconductive drum 1 that functions as an image holding body with an electrostatic latent image formed on the surface, and around each photoconductive drum 1, the photoconductive drum 1 is provided. A charging device 2 that charges the surface, a developing device 4 that selectively transfers toner to a latent image formed on the photosensitive drum to form a toner image, and a toner image on the photosensitive drum 1 that is an intermediate transfer belt 20 includes a primary transfer roll 5 that performs primary transfer onto 20 and a cleaning device 6 that removes toner remaining on the photosensitive drum after transfer. Further, an exposure device 3 that generates image light based on an image signal is provided for each of the charged photosensitive drums 1, and the photosensitive drum 1 is irradiated with image light upstream of a position facing the developing device 5. Thus, an electrostatic latent image is written.

  The photoconductor drum 1 is formed by laminating an organic photoconductor layer on the peripheral surface of a metal cylindrical member, and the metal portion is electrically grounded. Moreover, a voltage may be applied.

The charging device 2 includes an electrode wire that is stretched with a space from the peripheral surface of the photosensitive drum 1 that is a member to be charged, and a voltage is applied between the electrode wire and the photosensitive drum 1. The surface of the photosensitive drum 1 is charged by generating corona discharge.
In the present embodiment, the device for charging by corona discharge is used as described above, but a contact or non-contact charging device such as a solid discharger, a roll shape or a blade shape can also be used.

  The exposure device 3 generates flashing laser light based on an image signal, and scans this in the main scanning direction (axial direction) of the photosensitive drum 1 by a polygon mirror. As a result, electrostatic latent images corresponding to the images of the respective colors are formed on the surfaces of the respective photosensitive drums 1.

  The developing device 4 uses a two-component developer containing toner and a magnetic carrier. The developing device 4 includes a developing roll 4a at a position facing the photosensitive drum 1, and two developing rollers 4a are arranged on the circumferential surface of the rotating developing roll 4a. A layer of component developer is formed. Toner is transferred from the peripheral surface of the developing roll 4a onto the photosensitive drum 1 to visualize the latent image. In addition, the toner consumed as a result of image formation is replenished according to the amount consumed.

  The primary transfer roll 5 is formed by coating the outer peripheral surface with a rubber material in which conductive particles are added to a metal core material, and the photosensitive drums 1Y, 1M, and 10K for each of the image forming units 10Y, 10M, 10C, and 10K. It is disposed on the back side of the intermediate transfer belt 20 at a position facing 1C and 1K. Then, a transfer voltage is applied between the primary transfer rolls 5Y, 5M, 5C, and 5K and the photosensitive drums 1Y, 1M, 1C, and 1K, and the intermediate transfer belt 20 that passes through the primary transfer position 19 is opposed to the transfer voltage. The toner image on the photosensitive drum is electrostatically transferred.

  The cleaning device 6 removes toner remaining on the photosensitive drum after transfer by a cleaning blade and a cleaning brush arranged in contact with the peripheral surface of the photosensitive drum 1.

The intermediate transfer belt 20 is formed by endlessly forming a film-like member in which a plurality of layers are overlapped. Each layer has a predetermined surface resistivity, for example, by adding a substance for imparting conductivity such as carbon or an ion conductive substance to a resin material such as polyimide, polyamideimide, polycarbonate, or fluorine resin. Have been adjusted so that.
In the intermediate transfer belt of the present embodiment, the surface resistivity of the inner peripheral surface of the innermost layer (hereinafter referred to as “innermost layer”) among the plurality of layers is the outermost layer (hereinafter referred to as “the innermost layer”). It is adjusted to be smaller than the surface resistivity of the outer peripheral surface of the “outermost layer”. The common logarithm of the surface resistivity of the outermost layer is set to about 11.0 LogΩ / □, and the common logarithm of the surface resistivity of the innermost layer is set to be 1.0 LogΩ / □ or less smaller than the value of the surface resistivity of the outermost layer. Is desirable.

  Such an intermediate transfer belt 20 is stretched around a drive roll 21 that is rotationally driven, an adjustment roll 22 that adjusts the deviation of the intermediate transfer belt 20 in the width direction, and a counter roll 23 as shown in FIG. It moves around in the direction of arrow A.

  The secondary transfer member 24 disposed at a position facing the counter roll 23 and the intermediate transfer belt 20 includes a secondary transfer roll 26, an auxiliary roll 27, and a secondary transfer belt 28 stretched between them. It is what has. The secondary transfer belt 28 is sandwiched between the opposing roll 23 and the secondary transfer roll 26 in a state of being overlapped with the intermediate transfer belt 20, and moves around as the intermediate transfer belt 20 is driven around. It has become. In addition, when a recording medium is sent between the intermediate transfer belt 20 and the secondary transfer belt 28, the recording medium is sandwiched and conveyed. Then, a transfer voltage is applied to form a transfer electric field between the secondary transfer roll 26 and the opposing roll 23. This transfer voltage is set, for example, within a range of −1 kV or more and −6 kV or less, and is applied so that the potential of the opposing roll 23 is on the same polarity side as the toner. That is, when the charging polarity of the toner is negative, the transfer voltage is applied so that the opposing roll 23 has a lower potential than the secondary transfer roll 26.

The secondary transfer roll 26 is obtained by forming an outer peripheral layer 26b of a rubber material to which conductive particles are added on the outer peripheral surface of a metal core material 26a. The outer peripheral layer 26b is preferably composed of a plurality of layers. For example, in the case of a three-layer structure, the outer peripheral layer 26b has a core layer, an intermediate layer, and a coating layer covering the surface. The core layer may be formed of a foamed material such as silicone rubber, urethane rubber, or EPDM to which conductive particles such as carbon black are added, and the intermediate layer may be formed of a non-foamed material of these materials. The coating layer can be formed of a tetrafluoroethylene-hexafluoropropylene copolymer, a perfluoroalkoxy resin, or the like. The volume resistivity of the outer peripheral layer of the secondary transfer roll is desirably 10 ⁷ Ωcm or less.

The facing roll 23 is formed by forming an outer peripheral layer 23b on the outer peripheral surface of a metal core member 23a, and the outer peripheral layer 23b may be either a single layer or a plurality of layers. In the case of a single layer structure, for example, it can be formed of silicone rubber, urethane rubber, EPDM or the like to which conductive particles such as carbon black are added. In the case of a two-layer structure, the outer peripheral surface of the rubber layer is covered with a material having a surface resistivity higher than that of the rubber layer. The value of the surface resistivity of the facing roll 23 is desirably 10 ⁷ Ω / □ or more and 10 ¹¹ Ω / □ or less.

Two application rolls 30 as application members are in contact with the inner peripheral surface of the intermediate transfer belt 20 on the upstream side of the secondary transfer position 29 in the circumferential movement direction of the intermediate transfer belt 20 and can rotate around the axis. It is supported.
These two application rolls 30 a and 30 b are metal rolls, and a voltage is applied between them and the secondary transfer roll 26. This voltage is applied so that the potential of the application roll 30 is on the same polarity side as that of the toner. When the toner to be used has a negative polarity, it is desirable to set it within a range of, for example, -1 kV to -6 kV.

  The application roll 30 is in contact with the intermediate transfer belt 20 and is supported at a position where the intermediate transfer belt 20 stretched between the adjustment roll 22 and the opposing roll 23 is hardly pushed outward. The two application rolls 30a and 30b are in contact with the intermediate transfer belt 20 at positions of 3 cm and 5 cm upstream from the line connecting the centers of the secondary transfer roll 26 and the counter roll 23 along the intermediate transfer belt 20. is doing.

  The fixing device 7 includes a heating roll 7a having a built-in heating source and a pressure roll 7b pressed against the heating roll 7a, and these are in contact with each other to form a nip portion. The recording medium to which the toner image has been transferred is fed into the nip portion and heated and pressurized between the heated heating roll 7a and the pressure roll 7b, and the toner image is pressed onto the recording medium. Has become.

Such an image forming apparatus operates as follows.
When the image forming operation is started, the photosensitive drum 1 is rotationally driven, and the charging device 2 charges the surface of each photosensitive drum 1. The exposure device 3 outputs a flashing laser beam based on the image data, and scans the photosensitive layer on the surface of the photosensitive drum 1. As a result, the potential at the irradiation position of the laser light is attenuated, and a difference in electrostatic potential is generated between the exposed position and the unexposed position, and a so-called electrostatic latent image is formed on the surface of the photosensitive drum 1. Is done.

  The electrostatic latent image formed on the photosensitive drum 1 in this way is conveyed to a developing position facing the developing roll 4a by the rotation of the photosensitive drum 1. When passing through the developing position, the toner adhering to the developing roll 4a is electrostatically transferred to the image portion on the surface of the photosensitive drum, and a toner image is formed.

  At the primary transfer position 19, the photosensitive drum 1 and the primary transfer roll 5 face each other via the intermediate transfer belt 20, and a transfer voltage is applied between them. When the toner image developed on the surface of each photosensitive drum 1 is conveyed to the primary transfer position 19, electrostatic force acts on the toner in an electric field formed between the photosensitive drum 1 and the primary transfer roll 5. Then, the toner image on the surface of the photosensitive drum 1 is transferred to the surface of the intermediate transfer belt 20. At this time, the transfer voltage applied to the primary transfer roll 5 has a polarity (+) opposite to the polarity (−) of the toner.

  In this way, the intermediate transfer belt 20 is sequentially opposed to the respective image forming units 10 and the toner images of the respective colors are superimposed and transferred. The intermediate transfer belt 20 on which the toner images of all the colors have been transferred in multiple turns is moved in the direction of arrow A shown in FIG. 1, the opposing roll 23 in contact with the inner surface of the intermediate transfer belt 20, the secondary transfer member 24, and the like. A toner image is conveyed to the secondary transfer position 29 facing each other.

A recording medium P is fed between the secondary transfer belt 28 and the intermediate transfer belt 20 at a timing at which the toner image on the intermediate transfer belt 20 is conveyed, and an electric field for secondary transfer is transferred to the secondary transfer belt 20. It is formed between the roll 26 and the opposing roll 23. The electrostatic force acts on the toner in the electric field formed at this time, and the toner image on the surface of the intermediate transfer belt 20 is transferred to the recording medium P.
Thereafter, the recording medium P is sent to the fixing device 7 by the transport device 25, and the toner image is heated and pressurized to fix the toner image in which a plurality of colors are superimposed on the surface of the recording medium.
In this way, the recording medium on which the color image has been fixed is conveyed toward the paper discharge holding unit, and a series of color image forming operations is completed.

  When the toner image on the intermediate transfer belt 20 is transferred to the recording medium P in the above process, an electric field A for secondary transfer is provided between the secondary transfer roll 26 and the opposing roll 23 as shown in FIG. Is formed. In addition to the electric field A, an electric field B is generated between the intermediate transfer belt 20 and the secondary transfer roll 26 by applying a voltage between the application roll 30 and the secondary transfer roll 26. Yes. The electric field B generates a potential in the intermediate transfer belt 20 according to the resistance values of the application roll 30, the intermediate transfer belt 20, the recording medium P, the secondary transfer belt 28, and the secondary transfer roll 26. This is caused by a potential difference with the secondary transfer roll 26. The electric field B is an area where the intermediate transfer belt 20 on which the toner image is held and the recording medium P are superimposed on the upstream side of the area where the secondary transfer roll 26 and the opposing roll 23 are strongly pressed against each other. This generates a force to restrain the toner sandwiched between the intermediate transfer belt 20 and the recording medium P. That is, the electric field B acts in a direction penetrating the surface of the intermediate transfer belt 20 and the recording medium P, and restrains the toner from moving in a direction along the surface of the recording medium. This restraining force restrains the toner immediately before entering the area where the secondary transfer roll 26 and the opposing roll 23 are strongly pressed against each other from scattering backward in the traveling direction. Thereby, for example, the occurrence of image defects as shown in FIG. 6 is reduced.

  The image defect shown in FIG. 6 is an image having a plurality of fine lines drawn in a direction crossing the traveling direction of the recording medium, and toner constituting the thin lines is scattered on the rear side in the traveling direction of the recording medium. This scattering occurs at dispersed positions on a plurality of fine lines. When this scattering portion is enlarged, it is as shown in FIG. That is, the toner T moves so as to be blown away in the extremely narrow range on the thin line. Such an image defect has a fine line thickness of about 0.1 to 0.5 mm when the fine line intersects the traveling direction of the recording medium P, particularly when the fine line is at an angle close to a right angle. In this case, it tends to occur when the interval between the thin lines is about 2 to 5 mm.

Such an image defect can be considered to occur as follows.
Before entering the region where the secondary transfer roll 26 and the opposing roll 23 are pressed strongly against each other, the intermediate transfer belt 20 and the recording medium P are overlapped as shown in FIG. Contacts the secondary transfer belt 28. At this time, the toner T on the intermediate transfer belt 20 is sandwiched between the recording medium P, and a space S is formed between the fine line toner T ₁ on the front side and the fine line toner T ₂ on the rear side. . This space S is maintained without being crushed when the pressure contact force F ₁ between the intermediate transfer belt 20 and the secondary transfer roll 26 is small, but the secondary transfer roll 26 and the opposing roll 23 are pressed strongly against each other. when enters the area to be, as shown in FIG. 8 (b), the space S is crushed from the front side by a large pressing force F _2. In an image in which a plurality of thin lines are drawn intersecting the conveyance direction of the recording medium P, particularly in an image in which a plurality of fine lines are drawn at an angle close to a right angle with respect to the conveyance direction of the recording medium P, Since the air in the space S is confined, it becomes difficult to form a discharge path. For this reason, when the space S is crushed from the front side, as shown by an arrow C in FIG. 8B, the toner particle group T ₂ forming a thin line on the rear side where the pressure contact force is weak is blown off by air pressure. Then, the air in the space S is released to the rear side. As a result, it is considered that the toner constituting the rear thin line is scattered to the rear side.

  When such a phenomenon occurs, if there is an electric field B shown in FIG. 8B, that is, an electric field B formed between the intermediate transfer belt 20 and the secondary transfer roll 26, the charged toner is generated. A restraining force acts and resists the force blown away to the rear side.

It can be considered that the force that restrains the toner by the electric field B includes the following action.
In the vicinity of the center position where the opposing roll 23 faces the secondary transfer roll 26, the electric field A acts in the thickness direction of the recording medium P from the secondary transfer roll 26 toward the opposing roll 23, whereas the electric field B acts. In the region, the negative charge existing on the intermediate transfer belt 20 is coarser than the region in which the electric field A acts, and the electric field B is in a direction inclined with respect to the surface of the recording medium as shown in FIG. It is formed. Therefore, the electrostatic force E acting on the negatively charged toner particles is also inclined, and the component Eh of the electrostatic force E in the direction along the surface of the recording medium P as shown in FIG. It acts toward the upstream side in the conveyance direction of the recording medium P. This force resists the force W that tries to blow off the toner particles T downstream.

On the other hand, the electric field B formed between the intermediate transfer belt 20 and the secondary transfer roll 26 is an area immediately before the recording medium P contacts the intermediate transfer belt 20 (area indicated by reference numeral D in FIG. 3). But it works. For this reason, toner transfer may occur before the recording medium P comes into contact with the intermediate transfer belt 20, so that there is a possibility that so-called blur occurs, in which the sharpness of the image is reduced. However, as shown in FIG. 3, the area D immediately before the recording medium P contacts the intermediate transfer belt 20 by contacting the recording medium P with the intermediate transfer belt 20 upstream of the position where the recording medium P contacts the secondary transfer belt 26. Therefore, it is easy to achieve both suppression of image defects in which toner scatters to the rear side and suppression of decrease in image sharpness.
Moreover, the voltage applied between the application roll 30 and the secondary transfer roll 26 can be adjusted to suppress the occurrence of defects that reduce the image sharpness. Further, the intermediate transfer belt 20 is composed of a plurality of layers, and the toner can be obtained by adjusting the surface resistivity of the inner peripheral surface of the innermost layer to be smaller than the surface resistivity of the outer peripheral surface of the outermost layer. It becomes easy to achieve both suppression of image defects scattered on the rear side and suppression of reduction in image sharpness.

  Note that a voltage may be applied between the application roll 30 and the secondary transfer roll 26 in all cases when the toner image is secondarily transferred, but image defects in which the toner scatters backward are also possible. It is also possible to control so that a voltage is applied in a condition where the above is likely to occur. Under the condition that the image defect is unlikely to occur, there is little possibility that an image defect in which the toner scatters backward occurs even when the application of the voltage between the application roll 30 and the secondary transfer roll 26 is stopped. When stopped, the possibility of a reduction in the sharpness of the image due to the application of voltage is reduced.

  For example, it is possible to determine from the image data whether or not the image to be transferred is likely to cause a defect, and when the defect is likely to occur, a voltage can be applied to the application roll 30. As an image in which defects are likely to occur, an image including a plurality of fine lines in a direction perpendicular to the traveling direction of the recording medium, an image including multiple circles or arcs, and the like can be considered. In addition, an image having a fine line with an interval behind the image other than the fine line may have a defect. These images are stored as basic forms, and the image processing apparatus can determine whether or not the images corresponding to these basic forms are included from the image data and control the voltage application to the application roll 30.

In addition, environmental conditions, image forming speed, recording medium used, and the like affect the ease of image defects in which toner scatters backward. When the humidity is high, the charge amount of the toner decreases, and the image defect is likely to occur. When the image forming speed, that is, the moving speed of the intermediate transfer belt is high, the space surrounded by the intermediate transfer belt 20, the secondary transfer belt 28, and the toner T is rapidly crushed compared to when the moving speed is low, and the toner is scattered. It tends to occur. Further, when the recording medium P to be used is a smooth one such as coated paper, the air in the space surrounded by the intermediate transfer belt 20, the secondary transfer belt 28 and the toner T is smaller than that of a rough surface. It is difficult to come off and the image defect is likely to occur. Therefore, it is provided with means for acquiring data on the above conditions, and a voltage is applied between the application roll 30 and the secondary transfer roll 26 based on these data or a combination of these data. You may control whether to apply.

  The voltage value applied between the application roll 30 and the secondary transfer roll 26 can also be controlled based on the above data. For example, a sensor for detecting the humidity at the installation position of the image forming apparatus is provided. Based on the detection result, when the humidity is low, the voltage applied between the application roll 30 and the secondary transfer roll 26 is set low, When the humidity is high, the applied voltage is set higher than when the humidity is low. Thus, it is possible to achieve both the suppression of image defects in which the toner scatters to the rear side in the conveyance direction of the recording medium and the suppression of image defects whose sharpness decreases so as to spread. The experimental results for such an effect will be described later.

  In the embodiment described above, the application roll 30 including two metal rolls is used as the application member. However, a member that can apply a potential to the intermediate transfer belt may be employed in another form. it can. For example, it may be a member that is supported by fixing the position of the surface, and the inner peripheral surface of the intermediate transfer belt that circulates contacts and is rubbed. Further, three or more members may be used side by side, or one member may be contacted. As for the position to be supported, the application member may be disposed so as to press the intermediate transfer belt from the inner side against the intermediate transfer belt stretched on the adjustment roll, the opposing roll, etc., and push the intermediate transfer belt outward. Further, in the moving direction of the peripheral surface of the intermediate transfer belt, the toner between the intermediate transfer belt and the secondary transfer member is upstream of the position where the secondary transfer member and the opposing member are pressed against each other. The position can be changed and set as long as the position can form an electric field that restrains the movement in the direction along the line.

On the other hand, in the above embodiment, the secondary transfer member 24 has the secondary transfer roll 26, the auxiliary roll 27, and the secondary transfer belt 28. However, the secondary transfer member is limited to such a form. The secondary transfer roll may be directly pressed against the intermediate transfer belt or the recording medium. Moreover, the pad-shaped member supported so that it may not rotate may be sufficient.
Furthermore, the present invention is not limited to the above-described embodiments, and can be implemented as other forms within the scope of the present invention.

Next, an experiment conducted for confirming the effect of the present invention will be described.
[Experiment 1]
In this experiment, an image was formed using the image forming apparatus shown in FIGS. 1 and 2, and the relationship between the occurrence frequency of image defects in which toner scatters backward and the voltage applied to the application roll 30 was investigated. It is. In the embodiment shown in FIGS. 1 and 2, the potential of the secondary transfer roll 26 is set to 0 V, and the voltage is applied so that the application roll 30 has a positive or negative potential.
As a typical example in which the image defect in which the toner scatters backward easily occurs, the formed image is a thin line having a line width of 0.3 mm formed at intervals of 3 mm in a direction perpendicular to the moving direction of the recording medium. In addition, this thin line is obtained by superimposing an image using black toner and an image using cyan toner.
This image is formed on the intermediate transfer belt 20, transferred onto the recording medium P at the secondary transfer position, and then fixed. Then, the locations where defects in which the toner scatters backward are counted to obtain evaluation characteristic values.

FIG. 4 is based on the number of defects when the voltage value applied to the application roll 30 is 0 V, and the voltage is applied so that the application roll 30 has a negative potential or a positive potential. The number of occurrences of image defects is shown as a relative value.
As shown in this figure, when a negative potential is applied to the application roll 30, the number of image defects is reduced, and when a positive potential is applied, the number of image defects increases.

[Experiment 2]
In this experiment, of the two application rolls 30a and 30b shown in FIG. 2, the effect when only the downstream roll 30a or only the upstream roll 30b is provided is compared with the case where two rolls are provided. Is. The image to be formed and the evaluation method are the same as in Experiment 1.
As shown in FIG. 5, the voltage is applied so that the application roll 30 has a negative potential in either case where only the downstream roll 30a or only the upstream roll 30b is provided as the application roll. , A tendency to reduce image defects is recognized. However, when only one roll is installed, the contact state between the application roll 30 and the intermediate transfer belt 20 is not stable, and the reduction tendency is more stable than the case where two rolls are used. Not. The contact state is improved by using two rolls as in the embodiment shown in FIGS. 1 and 2, and image defects are stably reduced as compared with the case of using only one roll.

[Experiment 3]
In this experiment, the image forming apparatus shown in FIGS. 1 and 2 is used to form an image by changing the voltage applied to the application roll 30 in a high humidity environment and a low humidity environment. Then, the frequency of occurrence of image defects in which toner scatters backward and the degree of image blur are observed and compared. As a result, as shown in Table 1, when the voltage applied to the application roll 30 is increased in a high humidity environment, the scattering of the toner to the rear decreases. The degree of image blur hardly changes. On the other hand, in a low-humidity environment, the effect of suppressing the scattering of the toner to the rear appears significantly even when the voltage applied to the application roll 30 is low. Further, even if the applied voltage is increased, the effect of suppressing the scattering of the toner does not vary greatly. On the other hand, the degree of bleeding is significantly worsened when the voltage applied to the application roll 30 is increased. As a result of such an experiment, in a high humidity environment, the voltage applied to the application roll 30 is set high to suppress toner scattering, and in a low humidity environment, the voltage is within a range where toner scattering is improved. It can be seen that it is desirable to set the voltage to be low or to turn off the application of voltage to suppress the deterioration of the degree of bleeding.
In this experiment, the grade of bleeding is grade 0 when no bleeding occurs, and grade 5 when bleeding is extremely bad. The degree to which the blur is not so noticeable and is acceptable as an image corresponds to grade 2.

1: photosensitive drum, 2: charging device, 3: exposure device, 4: developing device, 5: primary transfer roll, 6: cleaning device, 7: fixing device, 8: sheet storage unit, 9: transport path,
10: Image forming unit, 19: Primary transfer position,
20: Intermediate transfer belt, 21: Driving roll, 22: Adjustment roll, 23: Opposing roll, 24: Secondary transfer member, 25: Conveying device, 26: Secondary transfer roll, 27: Auxiliary roll, 28: Secondary transfer Belt, 29: secondary transfer position, 30: application roll,
P: recording medium, T: toner, A, B: direction of the electric field, C: direction in which the toner is scattered, D: region immediately before the recording medium P is brought into contact with the intermediate transfer belt 20, F _1, F _2: contact pressure

Claims (5)

An image forming unit for forming an image with toner;
An intermediate transfer belt that is supported so as to be capable of circular movement in the direction of the peripheral surface, and on which the image is primarily transferred onto the outer peripheral surface;
A secondary transfer member that contacts the outer peripheral surface of the intermediate transfer belt and secondary-transfers the toner image on the intermediate transfer belt to a recording medium;
An opposing member that is provided in contact with the inner peripheral surface of the intermediate transfer belt and faces the secondary transfer member;
Application in which a voltage that is in contact with the intermediate transfer belt on the upstream side of the secondary transfer position in the circumferential movement direction of the intermediate transfer belt and has a potential on the charging polarity side of the toner is applied to the secondary transfer member. And an image forming apparatus.   The voltage applied between the application member and the secondary transfer member forms an electric field between the intermediate transfer belt and the secondary transfer member, and the electric field is upstream of the secondary transfer position. The image forming apparatus according to claim 1, wherein the image forming apparatus is set so as to restrain movement of the toner in a direction along a peripheral surface of the intermediate transfer belt.   3. The image formation according to claim 1, wherein the voltage is set to a low voltage or is turned off as the humidity at a place where the image forming apparatus is installed is lowered. apparatus.   When it is determined that the toner image formed by the image forming unit includes a straight line in a direction crossing the moving direction of the intermediate transfer belt, and when it is determined that the toner image includes the straight line, The image forming apparatus according to claim 1, wherein the voltage is set to be applied between the application member and the secondary transfer member. At least one data of the smoothness of the recording medium to which the toner image is transferred, the moving speed of the intermediate transfer belt and the humidity is acquired, and is applied between the application member and the secondary transfer member based on the data. The image forming apparatus according to claim 1, wherein the image forming apparatus controls ON / OFF of the voltage or a voltage value.

Images