JP2005189563A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively prevent transfer dust and peeling discharge in an image forming apparatus by a simple structure and also widen the usable range of the surface resistivity of the back of an intermediate transfer belt. <P>SOLUTION: The image forming apparatus has a transfer nip N formed between an image carrier 40 and an intermediate transfer belt 10, and presses the transfer roller 83 and the voltage application member, such as a conductive brush 85, of a primary transfer device 62 in the place of the transfer nip. The image forming apparatus rotates the image carrier and also runs the intermediate transfer belt while applying voltage by the voltage application member, thereby transferring the toner image on the image carrier to the intermediate transfer belt. In such an image forming apparatus, the electric resistance of the voltage application member is 5×10<SP>6</SP>to 1×10<SP>9</SP>Ω, and the voltage application member is pressed to the image carrier via the intermediate transfer belt in the place A that is the furthest downstream of the transfer nip N. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine thereof. Among them, in particular, an electrophotographic image forming apparatus for transferring a toner image formed on an image carrier via an intermediate transfer belt and recording the image on a transfer material such as a sheet / OHP film, particularly for color image formation. The present invention relates to a suitable image forming apparatus.

  Conventionally, in an image forming apparatus, a toner image is formed on a rotating drum-like or belt-like image carrier by performing charging, writing, and developing, and the toner image is once transferred to an intermediate transfer belt that runs. There is an electrophotographic indirect transfer type in which the toner image on the intermediate transfer belt is primarily transferred and secondarily transferred to a transfer material to be transported and the image is recorded on the transfer material.

  In this type of image forming apparatus, a voltage applying member such as a transfer roller or a conductive brush is used as a primary transfer device, and a transfer bias is applied at a fixing nip position between the image carrier and the intermediate transfer belt, thereby supporting the image. The toner image on the body was primarily transferred to the intermediate transfer belt. This primary transfer naturally needs to be performed stably and with high transfer efficiency in order to obtain good image quality.

  However, the conventional primary transfer apparatus has a problem of causing transfer dust and degrading an image. This transfer dust mainly occurs when the toner moves from the image carrier onto the intermediate transfer belt at a position upstream of the transfer nip.

  In order to solve such a problem, in a conventional primary transfer apparatus, as described in Patent Document 1, for example, by setting a high surface resistivity on the back surface of the intermediate transfer belt, a transfer electric field application range is set. Some narrowing prevents the generation of transfer dust by suppressing the transfer electric field upstream of the transfer nip.

  However, if the surface resistivity on the back surface of the intermediate transfer belt is set high, it becomes difficult for current to flow from the voltage application member through the intermediate transfer belt, and it becomes difficult to supply charges to the downstream position of the transfer nip. When the image carrier and the intermediate transfer belt are separated from each other at the primary transfer position, the potential balance between the front and back surfaces of the intermediate transfer belt is deteriorated, causing peeling discharge and disturbing the image.

  Thus, some conventional primary transfer devices, for example, as described in Japanese Patent Application Laid-Open No. 2004-228561, place the voltage applying member away from the transfer nip to a downstream position. However, even in such a primary transfer device, the voltage application member is not arranged in contact with the transfer nip, so if the surface resistivity on the back surface of the intermediate transfer belt is high, it is difficult to supply charges downstream of the transfer nip. When the image carrier and the intermediate transfer belt are separated from each other, the potential balance between the front and back surfaces of the intermediate transfer belt is deteriorated, causing peeling discharge and disturbing the image.

  As described above, when the surface resistivity on the back surface of the intermediate transfer belt is low, a transfer dust problem occurs. When the surface resistivity is high, a problem of generating discharge traces due to peeling discharge occurs, and the usable range is limited to a narrow range.

  In addition, the electrical resistance value of the intermediate transfer belt, particularly the surface resistivity of the back surface, may vary depending on the tolerance of the intermediate transfer belt. Due to this variation, it has been found that if it falls outside the above-mentioned narrow range, transfer dust and transfer defects of discharge traces occur, and optimal transfer efficiency cannot be obtained under the electrical conditions at the target resistance value. If the transfer efficiency is low, problems such as a decrease in image density occur. In addition, the electrical characteristics change due to overlapping of the image formation and receiving an electrical hazard or a change in the temperature / humidity environment, resulting in the above-mentioned transfer dust and discharge defect.

  In addition, in the color image forming apparatus, since toners of a plurality of colors are transferred in a superimposed manner, the toner image on the intermediate transfer belt may be transferred to the image carrier reversely at the time of transferring the second and subsequent colors. With regard to this reverse transfer, it has been found that if the electrical resistance value of the intermediate transfer belt, particularly the surface resistivity of the back surface, deviates from the target value, the reverse transfer rate may increase under the electrical conditions at the target resistance value. It was. If the reverse transfer rate is high, problems such as a decrease in image density and a change in hue occur. Further, since the amount of waste toner increases, the replacement interval of waste toner bottles is narrowed, and an extra bottle capacity is required.

  As an invention for suppressing a decrease in transfer rate due to a change in the electric resistance of the intermediate transfer belt, there are methods proposed in Patent Document 3 and Patent Document 4, for example. These have detection means for detecting information depending on the electrical resistance value of the intermediate transfer belt, so that transfer conditions can be controlled in accordance with changes in the electrical resistance value of the intermediate transfer belt, and transfer efficiency can be maintained. It can be done.

JP 2003-57995 A Japanese Patent Laid-Open No. 9-152791 JP 2000-147849 A JP 2002-116643 A

  However, these have the problem that the detection means and the control means need to be mounted, which increases the cost.

  Accordingly, a first object of the present invention is to form a transfer nip between the image carrier and the intermediate transfer belt, and to rotate the image carrier while applying a voltage by a voltage application member at the transfer nip position. In an image forming apparatus that travels on an intermediate transfer belt and transfers a toner image on an image carrier onto the intermediate transfer belt, a simple structure can effectively prevent transfer dust and peeling discharge, and the electric power of the intermediate transfer belt By reducing the difference in transfer efficiency due to the difference in characteristics, the range of the surface resistivity on the back surface of the intermediate transfer belt that can be used is widened.

  A second object of the present invention is to widen the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used without obstructing the running of the belt and causing image distortion.

  A third object of the present invention is to widen the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used by accurately pressing the voltage application member to the most downstream position of the nip to more reliably suppress the peeling discharge. It is in.

  A fourth object of the present invention is to more reliably suppress peeling discharge, to widen the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used, and to ensure high image quality.

  A fifth object of the present invention is to more reliably supply charges to the nip most downstream position with a voltage application member, and widen the range of the surface resistivity on the back surface of the intermediate transfer belt that can be used.

  A sixth object of the present invention is to provide an image bearing member when the voltage on the resistance of the voltage application member varies with the environment, changes with time, decreases in transfer performance due to manufacturing tolerances, and when the toner on the intermediate transfer belt passes through the transfer position again. In other words, the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used is widened.

  The seventh object of the present invention is to increase the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used with little transfer dust, good dot reproducibility, and high image quality without abnormal discharge traces. It is in.

  An eighth object of the present invention is to more reliably prevent image defects due to abnormal discharge and widen the range of surface resistivity on the back surface of the intermediate transfer belt that can be used.

  A ninth object of the present invention is to obtain a good transfer characteristic without cost and to widen the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used.

  A tenth object of the present invention is to obtain a good image quality and to widen the range of the surface resistivity of the back surface of the intermediate transfer belt that can be used.

In order to achieve the first object described above, the invention according to claim 1 forms a transfer nip between the image carrier and the intermediate transfer belt, and a voltage is applied by a voltage applying member at the transfer nip position. In the image forming apparatus that travels on the intermediate transfer belt that rotates and moves the image carrier and transfers the toner image on the image carrier to the intermediate transfer belt, the electric resistance of the voltage applying member is 5 × 10 ^6. ˜1 × 10 ⁹ Ω, and the voltage application member is pressed against the image carrier at the most downstream position of the transfer nip with the intermediate transfer belt interposed therebetween.

  According to a second aspect of the present invention, in order to achieve the second object described above, in the image forming apparatus according to the first aspect, the voltage applying member has a roller shape.

  According to a third aspect of the present invention, there is provided the image forming apparatus according to the second aspect, wherein the intermediate transfer belt is in contact with the image carrier in order to achieve the third object. The transfer nip is formed by pressing the roller-shaped voltage applying member against the image carrier with the intermediate transfer belt sandwiched downstream of the position.

  According to a fourth aspect of the present invention, in order to achieve the fourth object described above, in the image forming apparatus according to the second or third aspect, the hardness of the voltage application member having a roller shape is set to 50 or less Asker C hardness. It is characterized by.

  According to a fifth aspect of the present invention, in order to achieve the fifth object described above, in the image forming apparatus according to the first aspect, the voltage application member is a conductive brush.

  According to a sixth aspect of the present invention, in order to achieve the sixth object described above, in the image forming apparatus according to any one of the first to fifth aspects, the voltage application member is subjected to constant current control by a current control means. It is characterized by that.

According to a seventh aspect of the present invention, in order to achieve the seventh object described above, in the image forming apparatus according to any one of the first to sixth aspects, the surface resistivity of the back surface of the intermediate transfer belt is set to 10 ^10. -10 ¹³ Ω / □.

According to an eighth aspect of the present invention, in order to achieve the eighth object described above, in the image forming apparatus according to the seventh aspect, the surface resistivity of the back surface of the intermediate transfer belt is 10 ¹⁰ to 10 ¹² Ω / □. It is characterized by that.

  According to a ninth aspect of the present invention, in order to achieve the ninth object described above, in the image forming apparatus according to any one of the first to eighth aspects, the intermediate transfer belt is formed of a single layer. Features.

  According to a tenth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to ninth aspects, wherein at least one of the downstream side and the upstream side of the voltage applying member is achieved. And a tension roller that presses the intermediate transfer belt against the image carrier to form the transfer nip.

According to the first aspect of the present invention, an image in which a voltage is applied by a voltage applying member at a transfer nip position between the image carrier and the intermediate transfer belt to transfer a toner image on the image carrier to the intermediate transfer belt. In the forming apparatus, the difference in transfer conditions due to the resistance of the intermediate transfer belt can be reduced by setting the electric resistance of the voltage application member to 5 × 10 ⁶ to 1 × 10 ⁹ Ω. In addition, the voltage application member is pressed against the image carrier at the most downstream position of the transfer nip with the intermediate transfer belt interposed therebetween, so that the charge is supplied to the intermediate transfer belt and the intermediate at the most downstream position of the transfer nip. The potential balance between the front and back of the transfer belt can be adjusted, and peeling discharge can be suppressed. By these, the range of the intermediate transfer belt that can be used can be expanded, the image quality can be improved, and the cost can be reduced.

  According to the second aspect of the present invention, since the voltage application member has a roller shape, intermediate transfer is performed by applying a voltage by pressing the roller-shaped voltage application member against the most downstream position of the transfer nip. By supplying electric charges to the belt, the potential balance between the front and back surfaces of the intermediate transfer belt at the most downstream position of the nip can be adjusted, thereby preventing peeling discharge. Further, since the voltage application member has a roller shape, it is possible to prevent image disturbance without hindering the running of the belt.

  According to the third aspect of the present invention, in addition, a voltage application member having a roller shape is provided on the image carrier with the intermediate transfer belt sandwiched downstream of the position where the intermediate transfer belt contacts the image carrier. Since the transfer nip is formed by pressing, the transfer nip is formed by the voltage application member, and the voltage application member can be accurately pressed to the most downstream position of the nip to more reliably suppress the peeling discharge.

  According to the fourth aspect of the present invention, since the hardness of the roller-shaped voltage application member is set to 50 or less Asker C hardness, the voltage application member is crushed and pressed against the intermediate transfer belt. Therefore, the charge can be supplied to the intermediate transfer belt in a wide range, and the charge can be supplied to the most downstream position of the transfer nip more reliably, and the strip discharge can be more reliably suppressed. Further, since the contact area is large, local concentration of the contact pressure can be relaxed, the pressure applied to the toner layer can be dispersed, and the transfer omission due to the pressure can be prevented.

  According to the fifth aspect of the invention, in addition to the effect of the first aspect of the invention described above, the voltage application member is a conductive brush, so that the voltage application member that is a conductive brush is the most downstream of the transfer nip. By applying a voltage by pressing against the position, electric charges are supplied to the intermediate transfer belt, and the potential balance between the front and back surfaces of the intermediate transfer belt at the most downstream position of the nip can be adjusted, thereby preventing the peeling discharge. In addition, since the voltage application member is a brush, a relatively wide range can be pressed against the intermediate transfer belt to more reliably supply charges to the nip most downstream position.

  According to the sixth aspect of the invention, in addition to the above-described effect, the voltage application member is controlled at a constant current by the current control means. Therefore, even when the resistance of the voltage application member fluctuates, a constant transfer electric field is applied to the belt surface. A transfer voltage can be applied so that a good transfer performance can always be obtained. In addition, when the image area is small, the transfer voltage is small, and retransfer to the non-image area can be prevented.

According to the seventh aspect of the invention, in addition to the above-described effects, the surface resistivity of the back surface of the intermediate transfer belt is set to 10 ¹⁰ to 10 ¹³ Ω / □, so that the width of the transfer electric field can be reduced, and the outside of the transfer nip can be reduced. Therefore, it is possible to prevent the transfer of toner in the image and to prevent image defects such as transfer dust. In addition, it is possible to prevent image defects due to abnormal discharge caused by high surface resistivity.

According to the eighth aspect of the invention, in addition to this, the surface resistivity of the back surface of the intermediate transfer belt is set to 10 ¹⁰ to 10 ¹³ Ω / □, so that image defects due to abnormal discharge can be more reliably prevented. .

  According to the ninth aspect of the present invention, in addition to the above-described effects, the intermediate transfer belt is constituted by a single layer, so that the cost can be reduced as compared with the laminated structure, and the charge is stagnated at the interface of the layers. The resulting transfer failure can be prevented.

  According to the tenth aspect of the present invention, in addition to the above-described effect, a transfer nip is formed by pressing the intermediate transfer belt against the image carrier on one or both of the downstream side and the upstream side of the voltage application member. Since the tension roller is provided, the voltage application member is arranged after forming the transfer nip with the tension roller in advance, and the voltage application member can be pressed against the image carrier with a stable pressure, thereby preventing uneven transfer. Can do. In addition, since the contact pressure can be minimized, it is possible to prevent the transfer from being lost due to the pressure.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 12 shows the best mode for carrying out the present invention, and is an overall schematic configuration diagram of a color copying machine.

  In the figure, reference numeral 100 is a copying machine main body, 200 is a paper feed table on which it is placed, 300 is a scanner mounted on the copying machine main body 100, and 400 is an automatic document feeder (ADF) further mounted thereon.

  The copying machine main body 100 is provided with an endless intermediate transfer belt 10 in the center. The intermediate transfer belt 10 is usually composed of several layers, but may be composed of a single layer. When a single layer is used, the cost can be reduced as compared with a laminated structure, and transfer defects due to charge stagnation at the interface between layers can be prevented. In the single-layer structure, when the surface resistivity of the belt is defined, the volume resistivity becomes difficult to define, but by making the voltage application member have resistance, the contribution of the volume resistivity to the transfer performance can be reduced. .

  In the illustrated example, the intermediate transfer belt 10 is wound around one drive roller 14 and two driven rollers 15 and 16 so as to be able to run clockwise in the drawing.

  In this illustrated example, a belt cleaning device 17 is provided on the left side of one driven roller 15 to remove residual toner remaining on the intermediate transfer belt 10 after image transfer and prepare for image formation again.

  Further, on the intermediate transfer belt 10 stretched between the driving roller 14 and one driven roller 15, four monochrome image forming means 18 of black, cyan, magenta, and yellow are arranged side by side along the conveying direction. The tandem image forming device 20 is arranged. An exposure device 21 is further provided on the tandem image forming device 20.

  On the other hand, a secondary transfer device 22 is provided on the opposite side of the intermediate transfer belt 10 from the tandem image forming device 20. In the illustrated example, the secondary transfer device 22 includes a secondary transfer belt 24, which is an endless belt, spanned between two rollers 23, and is pressed against the other driven roller 16 via the intermediate transfer belt 10. And the images on the intermediate transfer belt 10 are collectively transferred onto a transfer material.

  A fixing device 25 for fixing the transfer image on the transfer material is provided beside the secondary transfer device 22. The fixing device 25 is configured by pressing a pressure roller 27 against an endless fixing belt 26. Further, the above-described secondary transfer device 22 is also provided with a transfer material conveyance function for conveying the transfer material after image transfer to the fixing device 25.

  In the illustrated example, under such a secondary transfer device 22 and a fixing device 25, a transfer material that inverts the transfer material to form images on both sides of the transfer material in parallel with the tandem image forming device 20 described above. A reversing device 28 is provided.

  Now, when making a copy using this color copying machine, a document is set on the document table 30 of the automatic document feeder 400. Alternatively, the automatic document feeder 400 is opened, a document is set on the contact glass 32 of the scanner 300, and the document feeder 400 is closed and pressed.

  When a start switch (not shown) is pressed, when a document is set on the automatic document feeder 400, the document is transported and moved onto the contact glass 32, and then the document is set on the contact glass 32. Immediately drives the scanner 300 and travels through the first traveling body 33 and the second traveling body 34. Then, the first traveling body 33 emits light from the light source and further reflects the reflected light from the document surface toward the second traveling body 34 and reflects by the mirror of the second traveling body 34 and passes through the imaging lens 35. The document is placed in the reading sensor 36 and the original content is read.

  When a start switch (not shown) is pressed, the drive roller 14 is rotationally driven by a drive motor (not shown), the driven rollers 15 and 16 are driven to rotate, and the intermediate transfer belt 10 is driven. At the same time, the individual image forming means 18 rotates the image carrier 40 to form black, yellow, magenta, and cyan monochrome images on each image carrier 40. Then, as the intermediate transfer belt 10 travels, the monochrome images are sequentially transferred to form a composite color image on the intermediate transfer belt 10.

  On the other hand, when a start switch (not shown) is pressed, one of the paper feed rollers 42 of the paper feed table 200 is selectively rotated at an appropriate timing, and transferred from one of paper feed cassettes 44 provided in multiple stages to the paper bank 43. The material is fed out, separated one by one by the separation roller 45, put into the paper feed path 46, transported by the transport roller 47, guided to the paper feed path 48 in the copying machine main body 100, and abutted against the registration roller 49 and stopped.

  Alternatively, the transfer roller 50 is rotated to feed the transfer material on the manual feed tray 51, separated one by one by the separation roller 52, put into the manual feed path 53, and abutted against the registration roller 49 and stopped. As the transfer material, paper, an OHP film, or the like is used.

  Then, the registration roller 49 is rotated in synchronization with the composite color image on the intermediate transfer belt 10, the transfer material is fed between the intermediate transfer belt 10 and the secondary transfer device 22, and transferred by the secondary transfer device 22. A color image is formed on the transfer material.

  The transfer material after the image transfer is conveyed by the secondary transfer device 22 and sent to the fixing device 25. The fixing device 25 applies heat and pressure to fix the transferred image, and then the switching material is switched by the switching claw 55 and discharged. The paper is discharged by a roller 56 and stacked on a paper discharge tray 57. Alternatively, it is switched by the switching claw 55 and put into the transfer material reversing device 28, where it is reversed and guided again to the transfer position, and an image is also formed on the back surface.

  On the other hand, the intermediate transfer belt 10 after the image transfer is removed by the belt cleaning device 17 to remove residual toner remaining on the intermediate transfer belt 10 after the image transfer, so that the tandem image forming device 20 can prepare for another image formation.

  In the tandem image forming apparatus 20 described above, each single-color image forming means 18 includes, for example, a charging device 60, a developing device 61, a drum-shaped image carrier 40, as shown in FIG. The image forming apparatus includes a primary transfer device 62, an image carrier cleaning device 63, a charge removal device 64, and the like. In the illustrated example, the image carrier 40 is in the form of a drum in which a photosensitive organic photoreceptor is applied to a base tube made of aluminum or the like to form a photosensitive layer. However, the image carrier 40 may be in the form of an endless belt.

  Although not shown, at least the image carrier 40 is provided, and a process cartridge is formed by all or a part of the portion constituting the monochromatic image forming means 18 so that it can be attached to and detached from the copying machine main body 100 at a time, thereby maintaining maintenance. You may make it improve.

  Of the parts constituting the monochromatic image forming means 18, the charging device 60 is formed in a roller shape in the illustrated example, and charges the image carrier 40 by contacting the image carrier 40 and applying a charging bias.

  The developing device 61 may use a one-component developer. However, in the illustrated example, a two-component developer composed of a magnetic carrier and a nonmagnetic toner is used. The agitation unit 66 conveys the two-component developer while stirring and adheres to the developing sleeve 65, and the developing unit transfers the toner of the two-component developer attached to the developing sleeve 65 to the image carrier 10. 67, and the stirring unit 66 is positioned lower than the developing unit 67.

  The stirring unit 66 is provided with two parallel screws 68. The two screws 68 are partitioned by a partition plate 69 except for both ends. A toner concentration sensor 71 is attached to the developing case 70.

  On the other hand, the developing portion 67 is provided with a developing sleeve 65 facing the image carrier 40 through the opening of the developing case 70, and a magnet 72 is fixedly provided in the developing sleeve 65. Further, a doctor blade 73 is provided with the tip approaching the developing sleeve 65. In the illustrated example, the distance at the closest portion between the doctor blade 73 and the developing sleeve 65 is set to 500 μm.

  Then, the two-component developer is conveyed and circulated while being stirred by the two screws 68 and supplied to the developing sleeve 65. The developer supplied to the developing sleeve 65 is drawn up and held by the magnet 72 to form a magnetic brush on the developing sleeve 65. The magnetic brush is trimmed to an appropriate amount by the doctor blade 73 as the developing sleeve 65 rotates. The developer that has been cut off is returned to the stirring unit 66.

  On the other hand, of the developer on the developing sleeve 65, the toner is transferred to the image carrier 40 by the developing bias voltage applied to the developing sleeve 65, and the electrostatic latent image on the image carrier 40 is visualized. After the visualization, the developer remaining on the developing sleeve 65 leaves the developing sleeve 65 and returns to the stirring unit 66 where there is no magnetic force of the magnet 72. When the toner concentration in the stirring unit 66 becomes light by this repetition, it is detected by the toner concentration sensor 71 and the stirring unit 66 is replenished with toner.

  Next, as will be described later, the primary transfer device 62 is provided with a transfer roller 83 as a voltage application member, and is pressed against the image carrier 40 with the intermediate transfer belt 10 interposed therebetween.

  The image carrier cleaning device 63 is provided with a cleaning blade 75 made of, for example, polyurethane rubber, with the tip pressed against the image carrier 40, and the outer periphery is in contact with the image carrier 40 to indicate a conductive fur brush 76. It can be freely rotated in the direction. Further, a metal electric field roller 77 for applying a bias to the fur brush 76 is rotatably provided in the direction of the arrow, and the tip of the scraper 78 is pressed against the electric field roller 77. Further, a collection screw 79 for collecting the removed toner is provided.

  Then, residual toner on the image carrier 40 is removed by a fur brush 76 that rotates in the counter direction with respect to the image carrier 40. The toner adhering to the fur brush 76 is removed by an electric field roller 77 that applies a bias by rotating in the counter direction with respect to the fur brush 76. The electric field roller 77 is cleaned by a scraper 78. The toner collected by the image carrier cleaning device 63 is brought to one side of the image carrier cleaning device 63 by the collection screw 79 and returned to the developing device 61 by the toner recycling device 80 for reuse.

  The neutralization device 64 is, for example, a lamp, and initializes the surface potential of the image carrier 40 by irradiating light.

  Then, along with the rotation of the image carrier 40, the surface of the image carrier 40 is first uniformly charged by the charging device 60, and then the writing light from the above-described exposure device 21 according to the reading content of the scanner 300 by a laser, LED or the like. An electrostatic latent image is formed on the image carrier 40 by irradiating L.

  Thereafter, toner is attached by the developing device 61 to visualize the electrostatic latent image, and the visible image is transferred onto the intermediate transfer belt 10 by the primary transfer device 62. That is, while applying a voltage by the transfer roller 83 of the primary transfer device 62, the image carrier 40 is rotated and the intermediate transfer belt 10 is run, and the toner image on the image carrier 40 is transferred to the intermediate transfer belt 10. . The surface of the image carrier 40 after the image transfer is cleaned by removing residual toner with an image carrier cleaning device 63, and is neutralized with a static eliminator 64 to prepare for another image formation.

  In the above-described tandem type image forming apparatus, in order to increase the first copy speed, it is common to perform black image formation first, and then image formation of the remaining colors only when the original is in color. Like to do.

In such a configuration, the registration roller 49 is usually used while being grounded, but a bias can be applied to remove paper dust from the transfer material. For example, when a bias is applied using a rubber roller having a diameter of 18 mm and a surface covered with a conductive NBR rubber having a thickness of 1 mm, the volume resistance of the rubber material is about 10 ⁹ Ωcm, and the toner transfer side (surface side) A voltage of about −800 V is applied to the transfer material, and a voltage of about +200 V is applied to the back side of the transfer material. In general, in the intermediate transfer system, paper powder is difficult to move to the image carrier 40, and therefore, there is little need to consider paper powder transfer, and there is no problem even if it is grounded. Further, although a DC bias is generally applied as the applied voltage, it is also possible to apply an AC voltage having a DC offset component in order to charge the transfer material more uniformly.

  As described above, since the surface of the transfer material after passing through the registration roller 49 to which a bias is applied is slightly charged to the negative side, a voltage is applied to the registration roller 49 in the transfer from the intermediate transfer belt 10 to the transfer material. In some cases, the transfer conditions change and the transfer conditions are changed.

  By the way, in the intermediate transfer type image forming apparatus as described above, a discharge trace may appear on the screen and the image quality may be impaired. This discharge trace occurs due to various causes. One of the causes is peeling discharge generated when the image carrier 40 and the intermediate transfer belt 10 are separated in the primary transfer portion. It has been found from the results of repeated experiments by the inventors that the peeling discharge is likely to occur when the potential balance between the front and back surfaces of the intermediate transfer belt 10 is poor. Further, transfer performance may be deteriorated due to a difference in surface resistivity on the back surface of the intermediate transfer belt 10. It has been found from the results of repeated experiments by the inventors that this problem is closely related to the electrical resistance of the transfer roller, which is the primary transfer device 62.

  Hereinafter, a specific configuration for adjusting the potential balance between the front and back surfaces of the intermediate transfer belt 10 and reducing the difference in transferability due to belt resistance will be described for each embodiment.

FIG. 1 is a configuration diagram illustrating a primary transfer portion of the first embodiment.
The primary transfer portion includes an image carrier 40 that rotates counterclockwise, an intermediate transfer belt 10 that travels in the direction indicated by the arrow X along with the rotation of the image carrier 40, and a back surface side of the intermediate transfer belt 10 (the lower surface in FIG. 1). A tension roller 82 that presses the surface side of the intermediate transfer belt 10 against the image carrier 40 to form a fixing nip N by being inscribed in a freely rotating manner and biasing upward in the figure. A bias voltage is applied to the transfer roller 83 that is a voltage application member of the primary transfer device 62 that rotates in the clockwise direction by contacting the back side of the intermediate transfer belt 10 at the most downstream position A, and the transfer roller 83 of the primary transfer device 62. It consists of a transfer bias power supply 84 to be applied. The transfer roller 83 as the voltage application member has an Asker C hardness of 50 or less.

  Here, FIG. 1 shows a case where the tension roller 82 is provided on the downstream side of the transfer roller 83. The tension roller 82 may be provided on the upstream side of the primary transfer device 62, and may be provided on both the upstream side and the downstream side of the primary transfer device 62 to make the fixing nip N as long as possible. Is possible. The length of the fixing nip N is desirably 2 mm or more.

  FIG. 1 shows a case where the transfer roller 83 is pressed at the most downstream position A of the fixing nip N where the image carrier 40 and the intermediate transfer belt 10 are in contact with each other. The pressing at the most downstream position A means that a straight line L connecting the center of the image carrier 40 and the most downstream position A of the fixing nip N is the transfer roller 83 and the intermediate transfer belt 10 as in the transfer roller a in FIG. Passes through a region M in contact with or in contact with the region M.

  In FIG. 2, the transfer roller 83 is changed from the position shown in FIG. 1 to the downstream side or the upstream side in order to examine the occurrence state of the discharge trace caused by the above-described peeling discharge when the position of the transfer roller 83 is changed. The state of each change is shown enlarged. In FIG. 2, the transfer rollers b and c in which the region M is separated from the straight line L by distances Db and Dc along the back surface of the intermediate transfer belt 10 are also shown in the drawing. The sign of the distances Db and Dc is 0 when the straight line L passes through the region M, minus (-) when the region M is downstream (Db), and the region M upstream. Case (Dc) is set to plus (+).

  Further, in Example 1, the state of occurrence of discharge traces when the surface resistivity on the back surface of the intermediate transfer belt 10 is changed is also investigated.

  The intermediate transfer belt 10 uses polyimide as a material, disperses carbon black in a polyamic acid solution, flows the dispersion into a metal drum, dries it, and then removes the film peeled off from the metal drum at a high temperature. A polyimide film was formed by stretching under, and cut into an appropriate size to produce an endless belt made of polyimide resin.

  Film formation is performed by injecting a polymer solution in which carbon black is dispersed into a cylindrical mold in accordance with a general method, and rotating the cylindrical mold while heating at 100 to 200 ° C. to form a film by centrifugal molding. . The film thus obtained was demolded in a semi-cured state and covered with an iron core, and the polyimide transfer reaction was allowed to proceed at 300 to 450 ° C. to be cured to obtain the intermediate transfer belt 10.

At this time, the surface resistivity of the back surface of the belt is 1 × 10 ¹⁰ to 1 × 10 ¹³ Ω / □, preferably 1 × 10 ¹⁰ to 1 × 10 ¹² Ω by changing the carbon amount, the firing temperature, the curing rate, and the like. It adjusted so that it might become / □. The resistance value is a value measured with a Hiresta UP (MCP-HT450) high resistivity meter (manufactured by Mitsubishi Chemical Corporation) in an environment of a temperature of 23 ° C. and a humidity of 50%. The probe used was a URS probe (MCP-HTP14) manufactured by the same company, and the applied voltage was 500V.

In FIG. 3, the values of the distances Db and Dc in FIG. 2 are changed from 0 to 1 (mm) in absolute value, and the surface resistivity of the back surface of the intermediate transfer belt 10 is changed from 1 × 10 ^9.3 Ω / □ to 1. The result of having evaluated the discharge trace generation | occurrence | production situation at the time of changing to x10 < ¹³ > ohm / square and performing image formation is shown. In the figure, ◯ indicates the rank of “not generated”, Δ indicates “slightly generated”, and x indicates “ranked”. The goal is “no occurrence”.

  Referring to FIG. 3, when the surface resistivity of the back surface of the intermediate transfer belt 10 is in a logarithmic range of 9.3 to 13.0, the distance D is 0, that is, the transfer roller 83 is brought into contact with the fixing nip N. It is confirmed that no discharge trace is generated.

  In FIG. 3, when the surface resistivity of the back surface of the intermediate transfer belt 10 is 9.3 logarithmically, no discharge trace is generated even if the distance D is 1 in absolute value. However, if the surface resistivity of the back surface of the intermediate transfer belt 10 is lowered to this region, the electric field generated when the primary transfer bias is applied easily acts on the toner outside the fixing nip N, and transfer dust is deteriorated. There is a risk. On the other hand, even if the surface resistivity is too high, the discharge may be caused by a mechanism different from the discharge, and the discharge trace may disturb the image.

  FIG. 4 shows the results of actual image formation using five types of intermediate transfer belts 10 having different surface resistivity on the back surface, and evaluation of transfer dust and discharge traces on the obtained image. . In the figure, the evaluation of transfer dust is performed with a ranking of 1 to 5, and the level at which the image is satisfactory is ranked 4 or higher. In addition, the change in the discharge trace was performed in three ranks: “No problem”, Δ “Permissible limit”, and × “Unacceptable”. In FIG. 4, the evaluation of the discharge trace is shown below the plot points.

Referring to FIG. 4, the transfer dust can be kept within the allowable limit by setting the surface resistivity of the back surface of the intermediate transfer belt to 1 × 10 ¹⁰ Ω / □ or more. It can also be seen that the discharge trace can be within the allowable limit by setting it to 1 × 10 ¹³ Ω / □ or less. Therefore, by setting the surface resistivity of the back surface of the intermediate transfer belt to 1 × 10 ¹⁰ to 1 × 10 ¹³ Ω / □, it is possible to keep transfer dust and discharge traces within an allowable range. Moreover, if the surface resistivity of the back surface is 1 × 10 ¹⁰ to 1 × 10 ¹² Ω / □, discharge traces are not generated, which is more preferable.

  In the first embodiment, as described above, the hardness of the transfer roller 83 is set to 50 or less in Asker C hardness. Therefore, when the roller is pressed against the intermediate transfer belt 10, the transfer roller 83 is slightly deformed and the intermediate transfer belt. As a result, the area of the contact area M with the belt 10 can be expanded, and charges can be supplied to the intermediate transfer belt 10 over a wide range. For this reason, even if the roller position is slightly shifted due to component tolerance or the like, the charge is reliably supplied to the most downstream position of the fixing nip N, and the peeling discharge can be effectively suppressed. At the same time, since the contact area is increased, the local concentration of the pressing pressure is alleviated and the pressure applied to the toner layer is also dispersed, and the transfer omission due to the concentrated pressure is prevented.

  In addition, since the voltage application member of the primary transfer device 62 has a roller shape, there is no possibility of obstructing the running of the intermediate transfer belt 10, and image disturbance can be prevented. In addition, the number of rollers can be reduced, and the primary transfer device 62 can be downsized. By providing the transfer roller 83 on the downstream side of the fixing nip N, the transfer electric field before the fixing nip N can be suppressed to be small. Therefore, it is possible to effectively prevent the toner from being moved by the transfer electric field before the fixing nip N to become transfer dust and deteriorate the image.

  Further, since the tension roller 82 is disposed on at least one of the downstream side and the upstream side of the transfer roller 83, the transfer roller 83 is disposed after the fixing nip N is formed by the tension roller 82 in advance. The transfer roller 83 can be pressed against the image carrier 40 with a stable pressure to prevent uneven transfer, and the pressing pressure can be kept to a minimum. Omission can be prevented.

Further, in the first embodiment, the transfer performance when the resistance of the transfer roller 83 is changed is also investigated. The transfer roller 83 is formed by forming a semiconductive foam around a metal cored bar. Main foam materials include polycarbonate, fluorine rubber, silicon rubber with conductive particles such as carbon dispersed, and semiconductive rubber such as polyurethane, EPDM, NBR, etc. What was produced so that the resistance between this would be 5 × 10 ⁶ to 1 × 10 ⁹ Ω was used. The resistance of the transfer roller 83 is measured by applying a voltage of 1 kV to the core metal of the transfer roller 83 in a state where a metal roller having a weight of 500 g is placed from above the roller and is in close contact with the transfer roller 83 by its own weight. This was done by calculating from the value of the current flowing through the metal roller.

5 to 8 show the transfer rate from the image carrier 40 to the intermediate transfer belt 10 when image formation is performed by changing the resistance of the transfer roller 83 from 1 × 10 ⁶ Ω to 1 × 10 ⁷ Ω. This is a result of evaluating the reverse transfer rate from the intermediate transfer belt 10 to the image carrier 40.

  In general, if the transfer bias is too low, the transfer is insufficient, and if it is too high, an overtransfer state occurs and the transfer rate decreases. Further, at the time of overlap transfer, if the transfer bias at the time of overlap transfer is increased, the electrical polarity of the toner on the intermediate transfer belt 10 already transferred tends to change, and the reverse transfer rate tends to increase. It is important to make the transfer rate and the reverse transfer rate compatible so that there is no problem in the image.

  The transfer rate is the ratio of the toner adhesion amount after transfer to the input toner adhesion amount at the time of primary transfer. If this ratio is less than 95%, transfer non-uniformity can be visually confirmed on the image. End up.

  The reverse transfer rate is the ratio of the amount of toner that is reversely transferred to the image carrier 40 to the amount of toner that has already been transferred onto the intermediate transfer belt 10. If this ratio is 5% or more, the amount of adhesion decreases and the image density is lowered by reverse transfer up to three times, and a difference from the original image density can be made.

The allowable ranges of the above transfer rate and reverse transfer rate were confirmed by experiments.
5 shows a case where the resistance of the transfer roller 83 is 1 × 10 ⁶ Ω, FIG. 6 shows a case where the resistance is 3 × 10 ⁶ Ω, FIG. 7 shows a case where the resistance is 5 × 10 ⁶ Ω, and FIG. in the case of a ⁷ Omega, the transfer bias and the transfer rate is the relationship reverse transfer rate. The intermediate transfer belt 10 has a back surface resistivity of 1 × 10 ¹⁰ Ω / □ and 1 × 10 ¹³ Ω / □, and the difference is shown. The dotted line frame in the figure indicates the range of transfer bias where the reverse transfer rate is 5% or less in the above two belts, and the solid line frame indicates the range where the transfer rate is 95% or more.

Referring to FIGS. 5 to 8, by setting the resistance of the transfer roller 83 to 5 × 10 ⁶ Ω or more, the surface resistivity of the back surface of the intermediate transfer belt 10 is 1 × 10 ¹⁰ Ω / □ to 1 × 10 ^13. In the range of Ω / □, the transfer bias and reverse transfer rate can be set so that there is no problem in terms of image.

  In addition, by suppressing the reverse transfer rate to 5% or less, the amount of waste toner discarded in the waste toner tank can be reduced. Especially in color high-speed machines, the amount of waste toner is larger than in monochrome machines and low-speed machines, so it is extremely effective in saving space by widening the interval between waste toner bottle replacements and making waste toner bottles smaller. It is valid. Even in the case where the toner recycling that reuses the transfer residual toner is installed, the present invention for suppressing the reverse transfer toner to be small is effective in consideration of the problem of color mixing due to the reverse transfer toner.

On the other hand, if the roller resistance is too high, a leak occurs between the roller core and the intermediate transfer belt, and an abnormality occurs on the image as a trace of discharge. FIG. 9 shows the evaluation results of the discharge trace when the roller resistance is changed. Here, the change in the discharge trace was performed in three ranks, as in FIG. 4, with ◯: no problem, Δ: acceptable limit, and x: unacceptable. From FIG. 9, by setting the roller resistance to 1 × 10 ⁹ Ω or less, discharge traces can be prevented within an allowable range.

Therefore, the transfer roller 83 is pressed against the most downstream position A of the fixing nip N, and the resistance of the transfer roller 83 is set in the range of 5 × 10 ⁶ to 1 × 10 ⁹ Ω, so that the surface of the back surface of the intermediate transfer belt 10 can be obtained. When the resistivity is in the range of 1 × 10 ¹⁰ Ω / □ to 1 × 10 ¹³ Ω / □, transfer performance with no problem in terms of image can always be obtained without changing the transfer condition setting. The range of intermediate transfer belts that can be made can be widened.

  The transfer roller 83 may be controlled at a constant current by a current control unit. By doing so, even when the resistance of the transfer roller 83 fluctuates, a transfer voltage can be applied so that a constant transfer electric field acts on the surface of the intermediate transfer belt 10, and good transfer performance can always be obtained. In addition, when the image area is small, the transfer voltage is small, and retransfer to the non-image area can be prevented.

FIG. 10 is a configuration diagram illustrating a primary transfer portion of the second embodiment.
In the second embodiment, a conductive brush 85 is used as a voltage application member of the primary transfer device 62, the tip is pressed against the most downstream position A of the fixing nip N, and a bias voltage is applied from a transfer bias power source 84. It is what I did. The resistance R is put on the holder of the conductive brush 85, and is set to 5 × 10 ⁶ to 1 × 10 ⁹ Ω together with the resistance of the conductive brush 85. Other configurations are the same as those of the first embodiment shown in FIG.

  Here, the resistance of the conductive brush 85 is measured by applying a voltage of 1 kV to the brush holder in a state where the brush is in contact with the metal member by 1 mm and calculating from the current value flowing through the metal member. went.

With such a configuration, using the intermediate transfer belt 10 having a surface resistivity of 1 × 10 ¹³ Ω / □ on the back surface, image formation is performed when the conductive brush 85 is not provided and when the conductive brush 85 is provided. When the conductive brush 85 was not provided, an unacceptable discharge trace was confirmed. However, when the conductive brush 85 was provided, a good image having no discharge trace could be obtained.

Further, when an intermediate transfer belt having a surface resistivity of 1 × 10 ¹⁰ Ω / □ and 1 × 10 ¹³ Ω / □ on the back surface was used, the relationship between the transfer rate, reverse transfer rate, and transfer bias was evaluated. 1 is obtained (FIGS. 5 to 8), and the surface resistivity of the back surface of the intermediate transfer belt 10 is in the range of 1 × 10 ¹⁰ Ω / □ to 1 × 10 ¹³ Ω / □. It was possible to set the transfer bias so that the transfer rate and the reverse transfer rate did not cause any problem on the image.

  Thus, by using the conductive brush 85 as the voltage application member, a relatively wide range can be brought into sliding contact with the intermediate transfer belt 10, and the charge to the most downstream position A of the fixing nip N can be more reliably performed. And the potential balance between the front and back surfaces of the intermediate transfer belt 10 at the most downstream position A of the fixing nip N is adjusted, and peeling discharge can be greatly suppressed.

FIG. 11 is a configuration diagram illustrating a primary transfer portion of the third embodiment.
In the third embodiment, with the intermediate transfer belt 10 in contact with the image carrier 40, the transfer roller 83 is lifted to the image carrier 40 from the Y direction in the downstream of the contact position, and the intermediate transfer belt 10. Is pushed upward to press the transfer roller 83 as a voltage application member against the image carrier 40 to form a fixing nip N.

  According to such a configuration, the image bearing member is sandwiched between the transfer roller 83 at the position indicated by the phantom line in the drawing and the intermediate transfer belt 10 at the position indicated by the phantom line. The transfer roller 83 is necessarily positioned at the most downstream position A of the fixing nip N. Therefore, charges are reliably supplied to the portion where the intermediate transfer belt 10 is peeled from the image carrier 40, and peeling discharge can be prevented.

1 is a configuration diagram of a primary transfer unit in an image forming apparatus which is an example of the present invention. FIG. It is a figure which shows collectively each state when the installation position of the transfer roller which is the voltage application member is changed to the downstream or the upstream. It is a figure which shows the evaluation result of the discharge trace generation condition for every installation position of a transfer roller. FIG. 6 is a relationship diagram of a surface resistivity on the back surface of an intermediate transfer belt, a transfer dust rank, and a discharge trace occurrence state. FIG. 6 is a relationship diagram between the surface resistivity of the back surface of the intermediate transfer belt, the transfer rate, and the reverse transfer rate when the resistance of the transfer roller is 1 × 10 ⁶ Ω. FIG. 6 is a relationship diagram between the surface resistivity of the back surface of the intermediate transfer belt, the transfer rate, and the reverse transfer rate when the resistance of the transfer roller is 3 × 10 ⁶ Ω. FIG. 6 is a relationship diagram between the surface resistivity of the back surface of the intermediate transfer belt, the transfer rate, and the reverse transfer rate when the resistance of the transfer roller is 5 × 10 ⁶ Ω. FIG. 6 is a relationship diagram between the surface resistivity of the back surface of the intermediate transfer belt, the transfer rate, and the reverse transfer rate when the resistance of the transfer roller is 1 × 10 ⁷ Ω. It is a figure which shows the evaluation result of the discharge trace generation condition for every resistance of a transfer roller. It is a block diagram of the primary transfer part in the image forming apparatus which is another example of this invention. It is a block diagram of the primary transfer part in the image forming apparatus which is another example of this invention. 1 is an overall schematic configuration diagram of a color copying machine according to the present invention. It is an enlarged block diagram of the monochromatic image forming means.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Intermediate transfer belt 40 Image carrier 62 Primary transfer apparatus 82 Tension roller 83 Transfer roller (voltage application member)
85 Conductive brush (voltage application member)
N Transfer nip

Claims (10)

A transfer nip is formed between the image carrier and the intermediate transfer belt, and the image carrier is rotated while the voltage is applied by a voltage applying member at the transfer nip position, and the intermediate transfer belt is moved between the image carrier and the intermediate transfer belt. In an image forming apparatus for transferring a toner image on the image carrier to a transfer belt,
The voltage application member has an electric resistance of 5 × 10 ⁶ to 1 × 10 ⁹ Ω, and the voltage application member is pressed against the image carrier at the most downstream position of the transfer nip with the intermediate transfer belt interposed therebetween. An image forming apparatus.   The image forming apparatus according to claim 1, wherein the voltage application member has a roller shape.   With the intermediate transfer belt in contact with the image carrier, the voltage application member having a roller shape is pressed against the image carrier with the intermediate transfer belt sandwiched downstream of the contact position. The image forming apparatus according to claim 2, wherein a transfer nip is formed.   4. The image forming apparatus according to claim 2, wherein the voltage application member having a roller shape has an Asker C hardness of 50 or less.   The image forming apparatus according to claim 1, wherein the voltage application member is a conductive brush.   The image forming apparatus according to claim 1, wherein the voltage application member is subjected to constant current control by a current control unit. 7. The image forming apparatus according to claim 1, wherein a surface resistivity of the back surface of the intermediate transfer belt is set to 10 ¹⁰ to 10 ¹³ Ω / □. The image forming apparatus according to claim 7, wherein a surface resistivity of the back surface of the intermediate transfer belt is set to 10 ¹⁰ to 10 ¹² Ω / □.   The image forming apparatus according to claim 1, wherein the intermediate transfer belt is formed of a single layer.   The tension roller for pressing the intermediate transfer belt against the image carrier to form the transfer nip is provided on at least one of the downstream side and the upstream side of the voltage application member. The image forming apparatus according to 1.

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