JP2011095580A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve image quality by suppressing a white spot phenomenon without upsizing an apparatus. <P>SOLUTION: A plurality of needle electrodes, to discharge the electricity on recording paper in order to prevent the recording paper which has been just subjected to secondary transfer from sticking to an image carrier (intermediate transfer belt), are divided into: a needle electrodes 55 arranged outside a printable area R; and a needle electrodes 58 arranged inside the printable area R. When a second side of the recording paper P is subjected to the secondary transfer during double-sided printing, a value of separation current to be supplied to the needle electrode 55 is made to be larger than a value of separation current to be supplied to the needle electrode 58. Thus, transfer current to be supplied to a secondary transfer roller is made to flow on the needle electrode 55 side arranged outside the printable area R, whereby transfer voltage is decreased, and the white spot phenomenon is suppressed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus that can be used as a copying machine, a printer, a facsimile, and the like, and more specifically, includes a mechanism for transferring a toner image on an image carrier onto a recording medium such as a recording sheet. The present invention relates to an image forming apparatus.

  In an electrophotographic image forming apparatus such as a copying machine or a printer, a photosensitive drum is widely used as an image carrier. A general image forming operation using the photosensitive drum is as follows.

  That is, the surface of the photosensitive drum is uniformly charged with a predetermined potential by a charging device, and laser light, LED light, or the like is irradiated from the exposure device. As a result, the potential on the surface of the photosensitive drum is partially attenuated, and an electrostatic latent image of the original image is formed on the photosensitive drum. Thereafter, when the surface of the photosensitive drum passes through the developing device, development is performed due to the relationship between the surface potential of the electrostatic latent image formed on the photosensitive drum, the charging of the toner, and the bias voltage applied to the developing device. And a toner image is formed on the photosensitive drum. This toner image is a bias voltage (transfer voltage) applied to the transfer member when the recording medium (for example, recording paper) passes between the photosensitive drum and the transfer member (for example, transfer roller) that are in contact with or close to each other. ) Is electrostatically transferred to the recording medium.

  Here, since the recording medium is charged at the time of transfer, the recording medium sticks to the surface of the photosensitive drum unless part of the charge is removed. In order to prevent this, many image forming apparatuses provide a separation electrode on the downstream side of the transfer member in the direction in which the recording medium moves between the photosensitive drum and the transfer member, and a bias voltage having a polarity opposite to the transfer voltage. By applying (separation voltage) to the separation electrode, a discharge is generated between the recording medium and the separation electrode. This reduces the charge charged on the recording medium and facilitates separation of the recording medium from the photosensitive drum (see Patent Document 1 or 2).

  The above image forming operation is based on a system in which a toner image is directly transferred from a photosensitive drum as an image carrier to a recording medium, and this system is generally employed in an image forming apparatus for monochrome printing.

  On the other hand, a so-called tandem type image forming apparatus for color printing employs a system in which a toner image is once carried on an intermediate transfer member and then transferred from the intermediate transfer member to a recording medium.

  That is, a plurality of image forming units respectively corresponding to a plurality of colors of toner are arranged in a line, and a toner image is formed on a photosensitive drum included in each image forming unit. Subsequently, a bias voltage is applied to a transfer member for primary transfer disposed at a position facing each photoconductor drum, and toner images formed on each photoconductor drum are sequentially superimposed on the intermediate transfer body to be primary. Transcript. Subsequently, a bias voltage (transfer voltage) is applied to a transfer member for secondary transfer, and the toner images primarily transferred onto the intermediate transfer member are collectively transferred to a recording medium.

  Here, since the recording medium is charged during the secondary transfer, the recording medium sticks to the surface of the intermediate transfer member unless part of the charge is removed. In order to prevent this, similarly to the above-described monochrome printing image forming apparatus, a separation electrode is provided on the downstream side of the transfer member for secondary transfer, and discharge is performed by applying a bias voltage (separation voltage) to the separation electrode. As a result, the recording medium is partially neutralized to facilitate separation from the photosensitive drum.

Japanese Patent Laid-Open No. 4-338791 JP-A-11-95564

  In the image forming apparatus described above, when a toner image on an image carrier is transferred to a recording medium having high resistance in a low temperature and low humidity environment, a white spot phenomenon may occur. For example, when a toner image is transferred to a recording medium that has once passed through the fixing unit, such as when a toner image is transferred to the second surface of a recording sheet by double-sided printing, white spot phenomenon tends to occur. is there.

  The white spot phenomenon is a phenomenon in which a minute white spot is generated in an image transferred to a recording medium because a part of toner is not transferred to the recording medium. This white spot phenomenon is considered to occur due to the following causes.

  That is, the transfer voltage is applied to the transfer member by constant current control, and the value of the applied current (transfer current) in the constant current control is set to a predetermined value in order to realize appropriate transfer. For this reason, when the resistance of the recording medium is high, the absolute value of the transfer voltage becomes larger than usual just before or during the passage of the recording medium between the image carrier and the transfer member. As a result, the transfer member or the recording medium is discharged and the discharged toner is uncharged or the charged polarity of the toner is reversed, and the toner is transferred from the image carrier to the recording medium. It will not be transcribed. As a result, when transfer is performed on a recording medium such as a white recording paper, the white recording paper is exposed in a minute portion that has not been transferred, so that the portion becomes apparent as a minute white spot.

  In order to suppress the white spot phenomenon, it is conceivable that the length of the transfer member is made larger than the width of the recording medium, and the absolute value of the transfer voltage is reduced by passing a part of the transfer current outside the sheet passing area. However, when this method is adopted, it is necessary to lengthen the transfer member, so that there is a problem that the image forming apparatus becomes large.

  Further, when amorphous silicon is used as the photosensitive drum, since the electrostatic capacity is larger than that of the organic photosensitive member (OPC), the absolute value of the transfer voltage applied to the transfer member is set to a large value. As a result, the white spot phenomenon is more likely to occur.

  In the case of a tandem color printing image forming apparatus, when the toner on the intermediate transfer member passes between a plurality of photosensitive drums and a plurality of primary transfer transfer members (for example, four locations), Since charging occurs, the absolute value of the transfer voltage applied to the transfer member for secondary transfer is set to a large value. As a result, the white spot phenomenon is more likely to occur.

  The present invention has been made in view of the above-described problems, for example, and an object of the present invention is to provide an image forming apparatus capable of suppressing white spot phenomenon and improving image quality without increasing the size of the apparatus. It is to provide.

  In order to solve the above problem, a first image forming apparatus of the present invention includes an image carrier that carries a toner image by electrostatic force, a transfer member that transfers the toner image from the image carrier to a recording medium, Transfer voltage applying means for applying a transfer voltage to the transfer member to transfer the toner image from the image carrier to the recording medium, and conveying the recording medium between the image carrier and the transfer member. A transfer unit, and arranged side by side from a position corresponding to one end of the transfer member to a position corresponding to the other end on the downstream side of the transfer member in the transfer direction of the recording medium by the transfer unit; A plurality of separation electrodes for discharging between the image carrier or the recording medium in order to remove the charge from the recording medium to which the toner image has been transferred and separate it from the image carrier, and the discharge by the separation electrode. And a separation voltage applying means for applying a separation voltage to the separation electrode, wherein the plurality of separation electrodes are arranged at positions corresponding to at least one end of the transfer member. And a second separation electrode other than the first separation electrode, and when the separation voltage application means applies the separation voltage to the plurality of separation electrodes, the first separation electrode and the second separation electrode A separation current having a different value is supplied to each of the separation electrodes.

  In order to solve the above-described problem, the second image forming apparatus of the present invention is the above-described first image forming apparatus of the present invention, wherein the first separation electrode can print an image on the recording medium. It is characterized by being arranged in a region not corresponding to the region.

  In order to solve the above-mentioned problem, the third image forming apparatus of the present invention is the above-described first or second image forming apparatus of the present invention, wherein the separation voltage applying means is a recording medium for double-sided printing. When transferring the toner image to the first surface, the same separation current is supplied to the first separation electrode and the second separation electrode, and the toner image is transferred to the second surface of the recording medium. Control means for performing control so that the absolute value of the separation current supplied to the first separation electrode is larger than the absolute value of the separation current supplied to the second separation electrode. And

  In order to solve the above-described problem, a fourth image forming apparatus of the present invention regulates interference between the transfer voltage and the separation voltage in any one of the first to third image forming apparatuses of the present invention described above. To provide a partition plate provided between the transfer member and the separation electrode, by removing a part of the partition plate between the transfer member and the first separation electrode, A current path is formed for flowing a part of the transfer current supplied to the transfer member when the transfer voltage is applied from the transfer member to the first separation electrode side.

  In order to solve the above problems, a fifth image forming apparatus of the present invention is the image forming apparatus according to any one of the first to fourth aspects of the present invention described above, wherein the image carrier is formed of a material containing amorphous silicon. It is characterized by comprising a photoconductor.

  According to the present invention, the absolute value of the transfer voltage can be effectively reduced without increasing the size of the apparatus, and the image quality can be improved by suppressing the white spot phenomenon.

It is explanatory drawing which shows the image forming apparatus by embodiment of this invention. FIG. 2 is an explanatory diagram showing an enlarged part of the image forming apparatus in FIG. 1. FIG. 5 is a perspective view showing a secondary transfer roller, a separation unit, and the like arranged in the side unit in a state where the side unit is pulled out. FIG. 4 is an enlarged perspective view illustrating a part of a secondary transfer roller, a separation unit, and the like in FIG. 3. It is sectional drawing which shows the isolation | separation unit seen from the arrow VV direction in FIG. It is a front view which shows two separation electrode bodies and an insulating plate which a separation unit has. It is an exploded view showing two separation electrode bodies and an insulating plate which a separation unit has. It is a front view which shows a partition plate. FIG. 6 is a characteristic diagram showing a relationship between a transfer current density and a transfer voltage in secondary transfer for an image forming apparatus according to a comparative example and an image forming apparatus according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows an image forming apparatus according to an embodiment of the present invention. FIG. 2 is an enlarged view of a part of the image forming apparatus in FIG.

  In FIG. 1, an image forming apparatus 1 is an electrophotographic image forming apparatus that can be used as, for example, a copying machine, a printer, a facsimile, or the like. Specifically, the image forming apparatus 1 is a tandem color printing image forming apparatus.

  On the upper part of the image forming apparatus 1, a document placing glass for placing a document, a document reading device (not shown) for optically reading an image from a document placed on the document placing glass, a document placing A document presser 2 for pressing a document placed on the glass, a document transport device 3 for automatically transporting the document on the document placement glass, an operation panel 4 for a user to operate the image forming apparatus 1, and a later-described operation Thus, a paper discharge tray 5 on which the recording paper P discharged from the inside of the housing 6 after image printing processing is placed is provided.

  In addition, a printing mechanism for printing a document image read by the document reading device or the like on the recording paper P is provided in the housing 6 of the image forming apparatus 1. This printing mechanism includes an image forming unit 11, which will be described later. A plurality of conveyance roller pairs 31, 33, a secondary transfer roller 35, a separation unit 37, a fixing device 40, and the like are provided. Further, a paper feed cassette 7 that accommodates the recording paper P is attached to the lower portion of the housing 6.

  The image forming unit 11 is a mechanism for supporting a toner image on the surface of the intermediate transfer belt 14, and includes a driving roller 12, a driven roller 13, an intermediate transfer belt 14, a plurality of support rollers 15, a plurality of image forming units 16, and a plurality of image forming units 16. A primary transfer roller 17 and the like are provided.

  The driving roller 12 and the driven roller 13 are arranged apart from each other, and the driving roller 12 has an outer diameter of about 30 mm, and is made of, for example, a metal shaft and EPDM rubber (ethylene propylene rubber).

  The intermediate transfer belt 14 includes, for example, an elastic layer formed of NBR rubber (nitrile rubber) and a coating layer formed of PTFE (polytetrafluoroethylene) on a base material formed of PVDF (polyvinylidene fluoride) resin. Is an endless belt having a three-layer structure, and is supported between a driving roller 12 and a driven roller 13 and supported by a plurality of support rollers 15. When focusing on secondary transfer, the intermediate transfer belt 14 corresponds to an image carrier.

  The image forming unit 16 is provided below the intermediate transfer belt 14 and forms, for example, magenta, cyan, yellow, and black toner images on the surface of the intermediate transfer belt 14. These four image forming units 16 are arranged in the order of magenta, cyan, yellow, and black from the upstream side to the downstream side of the intermediate transfer belt 14.

  As shown in FIG. 2, each image forming unit 16 is made of amorphous silicon and is arranged so as to face the surface of the intermediate transfer belt 14, a charging device 22 for charging the photosensitive drum 21, An exposure device 23 that forms an electrostatic latent image on the photosensitive drum 21, a developing device 24 that forms a toner image on the photosensitive drum 21, a cleaning device 25 that removes toner remaining on the photosensitive drum 21 after transfer, and the like It has.

  The primary transfer roller 17 is provided so as to face the photosensitive drums 21 of the four image forming units 16 with the intermediate transfer belt 14 interposed therebetween. Each primary transfer roller 17 has an outer diameter of about 20 mm, an axial length of about 305 mm, is composed of, for example, a metal shaft and an EPDM foam, and has a resistance of about 6.5 logΩ.

  Each of the conveying roller pairs 31 is arranged between the intermediate transfer belt 14 and the secondary transfer roller 35, which are hung on the driving roller 12 via the conveying path 32, and the fixing device. A pair of rollers that are sequentially conveyed to 40 and discharged to the discharge tray 5. On the other hand, each conveyance roller pair 33 is a roller pair that conveys the recording paper P via the conveyance path 34 for duplex printing during duplex printing. The conveyance roller pair 31 and 33 is a specific example of the conveyance unit.

  The secondary transfer roller 35 is a transfer member that transfers the toner image carried on the intermediate transfer belt 14 to the recording paper P, and is provided to face the drive roller 12 with the intermediate transfer belt 14 interposed therebetween. The secondary transfer roller 35 has an outer diameter of about 22 mm, and is made of, for example, a metal shaft and an epichlorohydrin foam having conductivity, and has a resistance of about 7 logΩ.

  Further, the axial length of the secondary transfer roller 35 may be equal to or greater than the maximum sheet width that is the width dimension of the recording sheet P having the maximum width on which the image can be printed by the image forming apparatus 1. Is preferably slightly larger than the maximum paper width. In the present embodiment, the maximum sheet width in the image forming apparatus 1 is, for example, 297 mm (A4 side), and thus the axial length of the secondary transfer roller 35 is, for example, about 306 mm.

  Further, a transfer voltage application circuit 36 is electrically connected to the secondary transfer roller 35. The transfer voltage application circuit 36 is a constant current control circuit for applying a transfer voltage, which is a bias voltage for secondary transfer of the toner image carried on the intermediate transfer belt 14 to the recording paper P, to the secondary transfer roller 35. For example, it is attached to the back side of the housing 6.

  Here, the value of the applied current (transfer current) in the constant current control by the transfer voltage application circuit 36 is set to a predetermined value in order to realize appropriate secondary transfer. Specifically, when a transfer voltage is applied to the secondary transfer roller 35, the current density in the sheet passing region where the recording paper P passes between the intermediate transfer belt 14 on the drive roller 12 side and the secondary transfer roller 35 is The value of the transfer current is set so as to be within a predetermined range.

  The separation unit 37 is provided on the downstream side of the secondary transfer roller 35 in the conveyance direction of the recording paper P (see the arrow in FIG. 2). The separation unit 37 is a device that neutralizes the recording paper P to which the toner image has been transferred and separates the recording paper P from the intermediate transfer belt 14. A separation voltage application circuit 38 for applying a separation voltage is electrically connected to the separation electrode bodies 53 and 56 included in the separation unit 37. A partition plate 39 is provided between the separation unit 37 and the secondary transfer roller 35. The separation unit 37, the separation voltage application circuit 38, and the partition plate 39 will be described in detail later.

  A fixing device 40 in FIG. 1 is a device that is provided on the downstream side of the separation unit 37 and fixes the toner image transferred onto the recording paper P by secondary transfer onto the recording paper P.

  Each conveyance roller pair 33, a part of the conveyance path 34, the secondary transfer roller 35, and the separation unit 37 are incorporated in the side unit 41 as shown in FIG. The base unit side of the side unit 41 is pivotally fixed to the side surface of the housing, and the user can pull out the distal end side of the side unit 41 from the housing 2 to perform maintenance, jam processing, and the like. it can.

  FIG. 3 shows the secondary transfer roller 35, the separation unit 37, and the like incorporated in the side unit 41 with the side unit 41 pulled out. FIG. 4 is an enlarged view of a part of the secondary transfer roller 35, the separation unit 37 and the like in FIG. 5 is a cross-sectional view of the separation unit 37, FIGS. 6 and 7 show the separation electrode bodies 53, 56 and the like included in the separation unit 37, and FIG.

  As shown in FIG. 3 or FIG. 4, the separation unit 37 is disposed at a position close to the secondary transfer roller 35 on the downstream side of the secondary transfer roller 35. As shown in FIG. 5, the separation unit 37 includes an electrode housing 51, an electrode support member 52, separation electrode bodies 53 and 56, an insulating plate 59 and a guide 60.

  The electrode housing 51 is formed of, for example, an insulating resin material, and extends along the secondary transfer roller 35. The electrode housing 51 includes a base portion 51A and a lid portion 51B. In the electrode housing 51, two separation electrode bodies 53 and 56 are interposed via two electrode support members 52 formed of an insulating material. It is supported together with an insulating plate 59 inserted between them.

  As shown in FIG. 6, the separation electrode 53 is made of a conductive material such as metal, and has a long plate-like electrode plate portion 54 extending in parallel with the axial direction of the secondary transfer roller 35 and a plurality of needle-like electrodes 55. It consists of and.

  The electrode plate portion 54 is supported between the electrode support members 52 together with the electrode plate portion 57 and the insulating plate 59 of another separation electrode body 56. Further, the length L1 of the electrode plate portion 54 is substantially equal to the length of the secondary transfer roller 35, and is, for example, about 306 mm in this embodiment.

  Each needle-like electrode 55 is an electrode that discharges the recording paper P from the intermediate transfer belt 14 in order to discharge the recording paper P to which the toner image has been transferred and to separate the recording paper P from the intermediate transfer belt 14. It is formed in a pointed needle shape and protrudes from the electrode plate portion 54 accommodated between the electrode support members 52 toward the intermediate transfer belt 14 hung on the drive roller 12 (see FIG. 2).

  Further, the needle-like electrodes 55 are arranged side by side with a predetermined interval at positions corresponding to both end portions of the secondary transfer roller 35 (the interval varies depending on the location). More specifically, the needle-like electrode 55 is formed in a region that does not correspond to the printable region R (the hatched portion in FIG. 6) that is a region where an image can be printed on the recording paper P having the maximum paper width. Has been placed. The width W of the printable area R is equal to or less than the maximum sheet width, and is about 260 mm in the present embodiment, for example.

  Further, as shown in FIG. 7, the separation electrode body 53 is disposed on the separation electrode body 56 via an insulating plate 59. Therefore, as shown in FIG. 5, each needle electrode 55 of the separation electrode body 53 is closer to the secondary transfer roller 35 than each needle electrode 58 of the separation electrode body 56.

  On the other hand, the separation electrode body 56 is formed of a conductive material such as a metal and includes a long plate-like electrode plate portion 57 extending in parallel with the axial direction of the secondary transfer roller 35 and a plurality of needle-like electrodes 58. .

  The electrode plate portion 57 is supported between the electrode support members 52 together with the electrode plate portion 54 and the insulating plate 59 of the separation electrode body 53.

  Each acicular electrode 58 is an electrode that discharges the recording paper P from the intermediate transfer belt 14 in order to discharge the recording paper P to which the toner image has been transferred and to separate the recording paper P from the intermediate transfer belt 14. It is formed in a pointed needle shape and protrudes from the electrode plate portion 57 accommodated between the electrode support members 52 toward the intermediate transfer belt 14 hung on the drive roller 12 (see FIG. 2).

  The needle-shaped electrodes 58 are arranged side by side with a predetermined interval at a position corresponding to the central portion of the secondary transfer roller 35, that is, a position corresponding to a portion excluding both end portions of the secondary transfer roller 35. Depending on the interval). More specifically, the needle-like electrode 58 is disposed in a region corresponding to the printable region R, and is disposed from one end side to the other end side in the width direction of the printable region R.

  The insulating plate 59 is a member that insulates between the separation electrode body 53 and the separation electrode body 56, is formed in a long plate shape by an insulating material, and the electrode plate portion 54 of the separation electrode body 53 and the electrode of the separation electrode body 56 It is provided between the plate part 57.

  The guide 60 in FIG. 5 is a member that guides the recording paper P that passes in the vicinity of the needle-like electrodes 55 and 58 and prevents the needle-like electrodes 55 and 58 and the recording paper P from contacting each other. As shown in FIG. 3, a plurality of electrode housings 51 are attached to a part of the electrode housing 51 facing the transport passage 32 with a predetermined interval.

  The separation voltage application circuit 38 is a circuit for applying a separation voltage having a polarity opposite to the transfer voltage, which is a bias voltage for discharging by the needle electrodes 55 and 58, to the needle electrodes 55 and 58. For example, it is attached to the back side of the housing 6 together with the transfer voltage application circuit 36.

  The separation voltage application circuit 38 is an application circuit unit 38A that is a constant current circuit for applying a separation voltage to each needle electrode 55 and an application that is a constant current circuit for applying a separation voltage to each needle electrode 58. A circuit unit 38B and a control circuit unit 38C for controlling the application circuit units 38A and 38B are provided. The application circuit unit 38A is electrically connected to the separation electrode body 53, and the application circuit unit 38B is electrically connected to the separation electrode body 56.

  When a separation voltage is applied to each needle electrode 55, 58, the value of the separation current supplied from the application circuit unit 38A to each needle electrode 55 and the application circuit unit 38B are controlled by the control circuit unit 38C. The value of the separation current supplied to the needle electrode 58 can be made different from each other. Specifically, the control circuit unit 38C performs the secondary transfer of the toner image onto the recording paper P during single-sided printing, and the secondary transfer of the toner image onto the first surface of the recording paper P during double-sided printing. The value of the separation current supplied from the application circuit unit 38A to each needle electrode 55 is made to coincide with the value of the separation current supplied from the application circuit unit 38B to each needle electrode 58. On the other hand, when the control circuit 38C performs the secondary transfer of the toner image onto the second surface of the recording paper P during double-sided printing, the absolute value of the separation current supplied from the application circuit 38A to each needle electrode 55. Is made larger than the absolute value of the separation current supplied to each needle-shaped electrode 58 from the application circuit section 38B. For example, the value of the separation current supplied from the application circuit unit 38A to each needle electrode 55 is −20 μA, and the value of the separation current supplied from the application circuit unit 38B to each needle electrode 58 is −10 μA.

  The partition plate 39 is disposed between the secondary transfer roller 35 and the separation unit 37 in order to restrict interference between the transfer voltage and the separation voltage, and is attached to the bottom surface of the base portion 51 </ b> A of the electrode housing 51. As shown in FIG. 8, the partition plate 39 is formed in a long plate shape that extends parallel to the axial direction of the secondary transfer roller 35. In this embodiment, the length L2 of the partition plate 39 is substantially equal to or longer than the axial length of the secondary transfer roller 35, for example, 310 mm.

  In addition, a partition removal portion 39A is formed at both end portions of the partition plate 39 by partially removing the end portion on the side facing the transport passage 32. Each partition removal portion 39A is formed in a portion that does not correspond to the printable region R (the hatched portion in FIG. 8). In the partition plate 39, the portion where the partition removal portion 39A is formed corresponds to the region where the needle-like electrode 55 is disposed, and the portion where the partition removal portion 39A is not formed is the region where the needle-like electrode 58 is disposed. It corresponds.

  As shown in FIG. 4, the end surface of the partition plate 39 on the side facing the transport passage 32 is configured to move from the base 51 </ b> A of the electrode housing 51 toward the transport passage 32, except for the portion where each partition removal portion 39 </ b> A is formed. It extends beyond the tip of the needle electrode 58. As a result, the space formed between the secondary transfer roller 35 and each needle electrode 58 is blocked by the partition plate 39.

  On the other hand, in the portion where the partition removal portion 39 </ b> A is formed, the end surface on the side facing the transport path 32 does not reach the tip of each needle electrode 55. Therefore, the partition plate 39 does not block between the secondary transfer roller 35 and each needle electrode 55. Thereby, a part of the transfer current supplied to the secondary transfer roller 35 when the transfer voltage is applied is transferred from the secondary transfer roller 35 to each needle electrode 55 between the secondary transfer roller 35 and each needle electrode 55. A current path S is formed to flow to the 55 side (a part of the transfer current is discharged between the secondary transfer roller 35 and each needle electrode 55).

  The image forming apparatus 1 having the above configuration operates as follows. That is, in each image forming unit 16, the surface of the photosensitive drum 21 is charged by the charging device 22, and then the original image read by the document reader on the surface of the photosensitive drum 21 by the exposure device 23. An electrostatic latent image corresponding to the image is formed. Subsequently, while the toner is charged by the developing device 24, the toner is attached to the portion of the surface of the photosensitive drum 21 where the electrostatic latent image is formed by electrostatic force to form a toner image on the photosensitive drum 21.

  Subsequently, the toner images formed on the photosensitive drums 21 of the image forming units 16 are sequentially applied to the surface of the intermediate transfer belt 14 passing between the photosensitive drums 21 and the primary transfer rollers 17 by electrostatic force. Transfer in layers (primary transfer). As a result, the intermediate transfer belt 14 is in a state of carrying a color toner image.

  Subsequently, the toner image carried on the intermediate transfer belt 14 is transferred between the intermediate transfer belt 14 on the driving roller 12 side and the secondary transfer roller 35 from the paper feed cassette 7 through the conveyance path 32 by the conveyance roller pair 31. Transfer is performed on the conveyed recording paper P or the like (secondary transfer).

  Subsequently, the toner image secondarily transferred to the recording paper P is fixed to the recording paper P by the fixing device 40, and the recording paper P on which the toner image is fixed is discharged to the paper discharge tray 5.

  In addition, when performing duplex printing, the recording sheet P that has been subjected to fixing processing by the fixing device 40 is passed through the conveyance path 34 and the like so that the printing surface (transfer surface) is reversed, and then again. Then, the sheet is conveyed between the intermediate transfer belt 14 on the driving roller 12 side and the secondary transfer roller 35 to perform secondary transfer, and thereafter, fixing processing is performed by the fixing device 40, and then the sheet is discharged to the sheet discharge tray 5.

  In such an operation of the image forming apparatus 1, when performing secondary transfer, the transfer voltage application circuit 36 applies a transfer voltage having a polarity opposite to that of the toner charging voltage to the secondary transfer roller 35. As a result, the toner formed on the surface of the intermediate transfer belt 14 adheres to the recording paper P, and secondary transfer is realized.

  Since the recording paper P immediately after the secondary transfer is charged by the secondary transfer, it is attracted to the intermediate transfer belt 14 side, but the recording paper P passes between the separation unit 37 and the intermediate transfer belt 14. Therefore, the recording paper P does not stick to the intermediate transfer belt 14.

  That is, when the secondary transfer is performed, the separation voltage application circuit 38 applies a separation voltage having a polarity opposite to the transfer voltage to the needle electrodes 55 and 58 of the separation unit 37. As a result, ion discharge occurs between the recording paper P passing between the separation unit 37 and the intermediate transfer belt 14 and the needle-like electrodes 55 and 58, and as a result, the charge on the recording paper P is reduced. . Accordingly, the recording paper P does not stick to the intermediate transfer belt 14.

  Here, when the secondary transfer is performed, a part of the transfer current supplied to the secondary transfer roller 35 by the transfer voltage application circuit 36 is passed through the current path S formed on both side ends of the partition plate 39. It flows to each needle electrode 55 side. For this reason, in order to secure a current density within the predetermined range in the sheet passing area, the absolute value of the transfer current must be increased in consideration of the current flowing through the current path S. Even if the absolute value of the transfer current is increased, the absolute value of the transfer voltage can be lowered. Accordingly, even when the resistance of the recording paper P passing between the intermediate transfer belt 14 on the drive roller 12 side and the secondary transfer roller 35 is high, an increase in the transfer voltage can be suppressed, and the transfer voltage increases. It is possible to prevent the secondary transfer roller 35 or the recording paper P from being discharged, which can effectively suppress the above-described white spot phenomenon.

  Further, when secondary transfer is performed on the second surface of the recording paper P during double-sided printing, the control circuit portion 38C of the separation voltage application circuit 38 supplies a separation current to each needle electrode 55 from the application circuit portion 38A. Is made larger than the absolute value of the separation current supplied to each needle-like electrode 58 from the application circuit section 38B. Thereby, the amount of transfer current flowing from the secondary transfer roller 35 to each needle electrode 55 side through the current path S can be increased while securing the current density within the predetermined range in the sheet passing area. As a result, the amount of decrease in the absolute value of the transfer voltage can be increased, and the occurrence of the white spot phenomenon on the second surface of the recording paper P can be effectively suppressed.

  That is, in the case of duplex printing, when secondary transfer is performed on the second surface of the recording paper P that has been once fixed, the resistance of the recording paper P is significantly increased due to heating or the like in the fixing processing. Therefore, the occurrence of the white spot phenomenon is particularly worrisome. However, according to the image forming apparatus 1 of the present embodiment, the occurrence of the white spot phenomenon can be effectively suppressed.

  As described above, according to the image forming apparatus 1 according to the present embodiment, the absolute value of the transfer voltage in the secondary transfer while ensuring the current density within a predetermined range in the sheet passing area for realizing appropriate secondary transfer. Even when the resistance of the recording paper P is high, the occurrence of the white spot phenomenon can be effectively suppressed and the quality of the printed image can be improved.

  In particular, a configuration in which a separation current having an absolute value larger than the separation current supplied to each needle electrode 58 disposed in the printable region R is supplied to each needle electrode 55 disposed outside the printable region R; As a result, when secondary transfer is performed on the second surface of the recording paper P during double-sided printing, the resistance of the recording paper P is the resistance of the recording paper P when performing secondary transfer during single-sided printing. Or, even when the resistance of the recording paper P is higher than when the secondary transfer is performed on the first surface during double-sided printing, the absolute value of the transfer voltage is reduced while ensuring the appropriate current density in the paper passing area. This can effectively suppress the occurrence of the white spot phenomenon and improve the quality of the printed image.

  This point will be described more specifically with reference to the graph in FIG. 9 in comparison with the image forming apparatus according to the comparative example and the image forming apparatus 1 according to the embodiment of the present invention. The image forming apparatus according to the comparative example has one separation electrode body extending from one end portion in the axial direction of the secondary transfer roller to the other end portion, and is identical to all the needle-like electrodes formed on the separation electrode body. A configuration in which a separation current having a value is supplied and a space between the secondary transfer roller and each needle electrode is blocked over the entire area in the axial direction of the secondary transfer roller by a partition plate having no partition removal unit Is adopted.

  In the case of the image forming apparatus according to the comparative example, when secondary transfer is performed on the second surface of A4 horizontal size, the current density within the predetermined range in the sheet passing area (for example, −1.2 to −1.3 μA / cm). ), For example, it is necessary to set the transfer current to, for example, -37.5 μA (current density in the paper passing region at this time is −1.26 μA / cm). As a result, the transfer voltage was −1.67 kV and its absolute value was increased, and the occurrence of a white spot phenomenon was observed (see Δ mark on hatching in FIG. 9).

  In contrast, in the case of the image forming apparatus 1 according to the present embodiment, the appropriate current density (for example, −1.26 μA / cm) in the sheet passing area is set when performing secondary transfer on the second surface of A4 horizontal size. In order to ensure the transfer current, it is necessary to set the transfer current to −41.7 μA in consideration of a part of the transfer current (4.2 μA) flowing through the current path S. When a transfer current having this value was actually applied, the absolute value of the transfer voltage was reduced to −1.41 kV, and no white spot phenomenon was observed (see the mark ● on the hatching in FIG. 9). At this time, there was no abnormality in the secondary transfer of the image due to the interference between the transfer voltage and the separation voltage at both ends (current path S) of the secondary transfer roller 35.

  As a result of repeated experiments, it was found that the white spot phenomenon occurs when the absolute value of the transfer voltage exceeds 1.6 kV, as indicated by a two-dot chain line in FIG. In the image forming apparatus 1 according to the embodiment of the present invention, the absolute value of the transfer voltage is smaller than 1.6 kV. From this, it can be seen that the image forming apparatus 1 can effectively suppress the occurrence of the white spot phenomenon.

  Furthermore, the image forming apparatus 1 according to the embodiment of the present invention can obtain the following effects. That is, a configuration in which a plurality of needle electrodes provided in the separation unit 37 are divided into a needle electrode 55 located outside the printable region R and a needle electrode 58 located inside the printable region R, and the partition plate 39 Since the white dot phenomenon is suppressed by the configuration in which the partition removal portion 39A is formed, the secondary transfer roller 35 need not be significantly longer than the maximum sheet width in order to realize the effect. Good. Accordingly, it is possible to prevent the image forming apparatus 1 from being enlarged.

  Further, in the image forming apparatus 1 according to the present embodiment, the material of each photoconductor drum 21 is amorphous silicon having a larger capacitance than that of the organic photoconductor, and the tandem type image forming unit 11 for color printing is employed. is doing. Focusing on these premise structures, it is highly necessary to increase the absolute value of the transfer voltage to be applied to the secondary transfer roller 35, as described above as the problem to be solved by the invention. Is likely to occur. However, according to the image forming apparatus 1 according to the present embodiment, the separation unit 37 including the needle-like electrode 55 outside the printable region R and the needle-like electrode 58 inside the printable region R even under such a structure. The partition plate 39 having the partition removal portion 39A can effectively suppress the occurrence of the white spot phenomenon.

  Further, by reducing the absolute value of the transfer voltage, the durability of the secondary transfer roller 35 can be increased, and the life of the image forming apparatus 1 can be extended.

  The separation unit 37 has a plurality of needle-like electrodes divided into a needle-like electrode 55 located outside the printable region R and a needle-like electrode 58 located inside the printable region R, and is supplied to the needle-like electrode 55. By making the absolute value of the current relatively large and making the absolute value of the separation current supplied to the needle-like electrode 58 relatively small, sufficient neutralization is performed at both ends of the recording paper P to remove the current from the intermediate transfer belt 14. Separation can be performed quickly, and neutralization can be weakened at the center of the recording paper P. As a result, the recording paper P can be placed along the sheet metal (earth) provided at the position opposite to the surface opposite to the transfer surface of the recording paper P after the secondary transfer in the transport path 32. Can smoothly enter the fixing device 40. Therefore, wrinkles and the like can be suppressed from occurring on the recording paper P.

  In the above-described embodiment, the absolute value of the separation current supplied to the needle-like electrode 55 located at both ends of the secondary transfer roller 35 is applied to the needle-like electrode 58 located at the center of the secondary transfer roller 35. The case where the absolute value of the separation current to be supplied is made larger than the absolute value is taken as an example. However, the present invention is not limited to this, and the absolute value of the separation current supplied to the needle electrode located at one end of the secondary transfer roller 35 It is good also as a structure which makes only larger than the absolute value of the isolation | separation current supplied to another acicular electrode. In this case, the needle electrode located at one end of the secondary transfer roller 35 is electrically separated from the other needle electrode.

  In the embodiment described above, the absolute value of the separation current supplied to each acicular electrode 55 is applied to each acicular electrode 58 when performing secondary transfer on the second surface of the recording paper P during duplex printing. Although the case where control is performed so as to be larger than the absolute value of the supplied separation current is taken as an example, the present invention is not limited to this. For example, when the image forming apparatus is used in an extremely low temperature and low humidity environment, depending on the use state of the image forming apparatus, when performing secondary transfer to the first surface of the recording paper P during double-sided printing, Control is performed so that the absolute value of the separation current supplied to each needle electrode 55 is larger than the absolute value of the separation current supplied to each needle electrode 58 even when secondary transfer is performed on the recording paper P in printing. May be performed.

  Further, in the above-described embodiment, there is a case where the current path S through which the transfer current flows from the secondary transfer roller 35 to each needle electrode 55 side is formed by forming the partition removal portions 39A at both end portions of the partition plate 39. As an example, the present invention is not limited to this. By shortening the length of the partition plate 39, it is possible to provide a current path through which a transfer current flows from the secondary transfer roller 35 to each needle electrode 55 side. In this case, the length of the partition plate 39 is not less than the width W of the printable region R.

  In the above-described embodiment, the case where each photoconductor drum 21 is formed from amorphous silicon has been described as an example. However, a photoconductor drum made of an organic photoconductor (OPC) may be employed.

  In the above-described embodiment, the case where the present invention is applied to an image forming apparatus for tandem color printing has been described as an example. However, the present invention is formed on a photosensitive drum like an image forming apparatus for monochrome printing. The present invention can also be applied to an image forming apparatus that directly transfers a toner image onto a recording sheet. In this case, by increasing the absolute value of the separation current supplied to each acicular electrode arranged outside the printable area, the area between the transfer roller and the photosensitive drum in the area corresponding to both side edges of the recording paper. It is also possible to obtain an effect of suppressing the discharge and suppressing the occurrence of damage to the photosensitive drum. In addition, an effect of reducing damage caused to the photosensitive drum by continuous application of the transfer current outside the paper passing area can be obtained.

  In addition, the present invention can be appropriately changed without departing from the gist or idea of the invention that can be read from the claims and the entire specification, and an image forming apparatus that includes such a change is also included in the technical idea of the invention. included.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 11 Tandem type image forming unit 12 Drive roller 14 Intermediate transfer belt (image carrier)
Reference Signs List 16 Image Forming Unit 17 Primary Transfer Roller 21 Photosensitive Drum 22 Charging Device 23 Exposure Device 24 Developing Device 25 Cleaning Device 31, 33 Pair of Conveying Rollers (Conveying Unit)
32, 34 Conveyance path 35 Secondary transfer roller (transfer member)
36 Transfer voltage application circuit (transfer voltage application means)
37 Separation unit 38 Separation voltage application circuit (separation voltage application means)
38A, 38B Application circuit section 38C Control circuit section 39 Partition plate 39A Partition removal section 53, 56 Separation electrode body 54, 57 Electrode plate section 55, 58 Needle-like electrode (separation electrode)
P Recording paper (recording medium)
R Printable area S Current path

Claims (5)

An image carrier that carries a toner image by electrostatic force;
A transfer member for transferring the toner image from the image carrier to a recording medium;
Transfer voltage applying means for applying a transfer voltage to the transfer member in order to transfer the toner image from the image carrier to the recording medium;
Conveying means for conveying the recording medium between the image carrier and the transfer member;
The toner image is arranged side by side from the position corresponding to one end of the transfer member to the position corresponding to the other end on the downstream side of the transfer member in the transport direction of the recording medium by the transport means. A plurality of separation electrodes for discharging between the image carrier and the recording medium in order to remove the charge and separate the transferred recording medium from the image carrier;
Separation voltage application means for applying a separation voltage to the separation electrode in order to perform the discharge by the separation electrode;
The plurality of separation electrodes include a first separation electrode disposed at a position corresponding to at least one end of the transfer member, and a second separation electrode other than the first separation electrode,
The separation voltage applying means supplies separation currents having different values to the first separation electrode and the second separation electrode when the separation voltage is applied to the plurality of separation electrodes. Image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the first separation electrode is disposed in a region not corresponding to a region where an image can be printed on the recording medium.   The separation voltage application means supplies a separation current having the same value to the first separation electrode and the second separation electrode when transferring a toner image to the first surface of the recording medium during double-sided printing. When the toner image is transferred to the second surface of the recording medium, the absolute value of the separation current supplied to the first separation electrode is larger than the absolute value of the separation current supplied to the second separation electrode. The image forming apparatus according to claim 1, further comprising a control unit configured to perform control. A partition plate provided between the transfer member and the separation electrode in order to regulate interference between the transfer voltage and the separation voltage;
By removing a part of the partition plate between the transfer member and the first separation electrode, a part of the transfer current supplied to the transfer member when the transfer voltage is applied is transferred to the transfer member. 4. The image forming apparatus according to claim 1, wherein a current path is formed to flow from the first to the first separation electrode side. 5.   The image forming apparatus according to claim 1, wherein the image carrier includes a photoconductor formed of a material containing amorphous silicon.

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