JP5422339B2 - Image forming apparatus - Google Patents

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 a recording medium (for example, recording paper) is inserted 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 intermediate transfer member.

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 of the transfer member 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 conveying unit; and provided on the downstream side of the transfer member in the conveying direction of the recording medium by the conveying unit, for discharging the recording medium to which the toner image has been transferred and separating the image from the image carrier. A plurality of separation electrodes for discharging between the carrier and the recording medium; separation voltage applying means for applying a separation voltage to the separation electrodes for performing the discharge by the separation electrodes; and the transfer voltage; A partition plate provided between the transfer member and the separation electrode in order to regulate interference with the separation voltage, and between the transfer member and the separation electrode, By removing all or part of the portion corresponding to at least one end of the transfer member in the axial direction, a part of the transfer current supplied to the transfer member when the transfer voltage is applied is separated from the transfer member. A current path for flowing to the electrode side is formed.

  In order to solve the above problems, the second image forming apparatus of the present invention is the first image forming apparatus of the present invention, wherein the current path is the partition plate and an image can be printed on the recording medium. It is characterized in that it is formed in a part that does not correspond to a region.

  In order to solve the above-described problems, a third image forming apparatus of the present invention is arranged on the downstream side of the transfer member and extends along the transfer member in the first or second image forming apparatus of the present invention. The plurality of separation electrodes are supported by the electrode support portion and arranged from a position corresponding to one end portion of the transfer member to a position corresponding to the other end portion. Each separation electrode has a distal end protruding from the electrode support portion toward the image carrier, and is disposed at a position corresponding to at least one end of the transfer member in the axial direction of the plurality of separation electrodes. The electrode is characterized in that the tip side is inclined or bent so as to approach the transfer member as compared with other separation electrodes.

  In order to solve the above-described problems, a fourth image forming apparatus of the present invention is the first or second image forming apparatus of the present invention, wherein the fourth image forming apparatus is disposed on the downstream side of the transfer member and extends along the transfer member. The plurality of separation electrodes are supported by the electrode support portion and arranged from a position corresponding to one end portion of the transfer member to a position corresponding to the other end portion. Each separation electrode has a distal end projecting from the electrode support portion toward the image carrier, and is disposed in a portion of the plurality of separation electrodes that does not correspond to a region where an image can be printed on the recording medium. The electrode is characterized in that the tip side is inclined or bent so as to approach the transfer member as compared with other separation electrodes.

  In order to solve the above problems, according to a fifth image forming apparatus of the present invention, in the image forming apparatus of any one of the first to fourth aspects of the present invention, the image carrier is formed of a material containing amorphous silicon. And a photosensitive member.

  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.

1 is an explanatory diagram illustrating an image forming apparatus according to a first embodiment of the present 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 the separation electrode body which a separation unit has. It is a front view which shows a partition plate. FIG. 6 is a cross-sectional view of a separation unit provided in an image forming apparatus according to a second embodiment of the present invention when viewed from the same direction as FIG. 5. It is a perspective view which shows the one end side of the separation electrode body provided in the separation unit. It is a front view which shows the separation electrode body provided in the separation unit. Regarding the image forming apparatus according to the comparative example, the image forming apparatus according to the first embodiment of the present invention, and the image forming apparatus according to the second embodiment of the present invention, the relationship between the current density of the transfer current and the transfer voltage in the secondary transfer. FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

(First embodiment)
First, a first embodiment of the present invention will be described. FIG. 1 shows an image forming apparatus according to a first 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. 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. The width is preferably slightly larger than the maximum sheet width, 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, a sheet passing region (recording medium passage) through which the recording sheet P passes between the intermediate transfer belt 14 on the drive roller 12 side and the secondary transfer roller 35. The transfer current value is set so that the current density in the region (see FIG. 7) is 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 body 53 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.

  The fixing device 40 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 the 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. FIG. 5 is a cross-sectional view of the separation unit 37, FIG. 6 shows a separation electrode body 53 included in the separation unit 37, and FIG. 7 shows a partition plate 39.

  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 illustrated in FIG. 5, the separation unit 37 includes an electrode housing 51, an electrode support member 52, a separation electrode body 53, and a guide 56.

  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, and a separation electrode body 53 is supported in the electrode housing 51 via two electrode support members 52 formed of an insulating material. The electrode housing 51 is a specific example of an electrode support portion together with the electrode support member 52.

  As shown in FIG. 6, the separation electrode body 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. The length L1 of the separation electrode body 53 (electrode plate portion 54) is equal to the axial length of the secondary transfer roller 35 or shorter than the axial length of the secondary transfer roller 35, and is, for example, 306 mm in this embodiment. The length L1 of the separation electrode body 53 is preferably larger than the maximum sheet width.

  The electrode plate portion 54 is supported between the electrode support members 52 as shown in FIG. A plurality of needle-like electrodes 55 are arranged at a predetermined interval from a position corresponding to one end of the secondary transfer roller 35 to a position corresponding to the other end (the interval varies depending on the location). 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 supported between the electrode support members 52 toward the intermediate transfer belt 14 hung on the drive roller 12 (see FIG. 2).

  A separation voltage application circuit 38 is electrically connected to the separation electrode body 53 as shown in FIG. The separation voltage application circuit 38 is a constant current control circuit for applying to each needle-like electrode 55 a bias voltage for discharging by each needle-like electrode 55 and having a polarity opposite to the transfer voltage. For example, it is attached to the back side of the housing 6 together with the transfer voltage application circuit 36.

  The guide 56 in FIG. 5 is a member that guides the recording paper P passing through the vicinity of each needle electrode 55 and prevents the needle electrodes 55 and the recording paper P from coming into contact with each other, as shown in FIG. In addition, 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 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. 7, the partition plate 39 is formed in a long plate shape that extends parallel to the axial direction of the secondary transfer roller 35. The length L2 of the partition plate 39 in the present embodiment is equal to the length L1 of the separation electrode body 53 or longer than the length L1 of the separation electrode body 53, for example, 310 mm. The length L2 of the partition plate 39 is preferably larger than the maximum sheet width.

  Further, as shown in FIG. 4, the end surface of the partition plate 39 on the side facing the transport passage 32 is formed in a needle shape from the base portion 51A of the electrode housing 51 toward the transport passage 32 side except for a partition removal portion 39A described later. The electrode 55 extends beyond the tip of the electrode 55 and blocks the space formed between the secondary transfer roller 35 and each needle electrode 55.

  However, a part of the partition plate 39 is removed at both side end portions thereof, that is, portions corresponding to the end portions on both sides in the axial direction of the secondary transfer roller 35 in the partition plate 39. A portion 39A is formed. In the region where the partition removal portion 39A is formed, the secondary transfer roller 35 and each needle electrode 55 are not blocked. That is, a part of the transfer current supplied to the secondary transfer roller 35 when the transfer voltage is applied is provided between the secondary transfer roller 35 and each needle electrode 55 in the region where the partition removal portion 39A is formed. A current path S is formed to flow from the secondary transfer roller 35 to each needle electrode 55 side (a part of the transfer current is discharged between the secondary transfer roller 35 and each needle electrode 55).

  Here, as shown in FIG. 7, the partition removing unit 39 </ b> A includes a printable region R (a hatched portion in FIG. 7) that is a region where an image can be printed on the recording paper P having the maximum paper width. It is formed in a non-corresponding area (the width W of the printable area R is equal to or less than the maximum sheet width). That is, in the partition plate 39, the partition removal portion 39A formed at one end and the partition removal portion 39A formed at the other end are separated by a distance equal to or greater than the width W of the printable region R. . Specifically, in the present embodiment, in the partition plate 39, the distance L3 between the partition removal portion 39A formed at one end and the partition removal portion 39A formed at the other end is a printable region R. Is equal to the width W. The arrangement of the partition plate 39 is determined so that both of the partition removal portions 39A do not overlap the printable region R. Further, the length of each partition removal portion 39A in the axial direction of the secondary transfer roller 35 is determined by the length L2 of the partition plate 39, the width W of the printable region R, and the like, but is about 10 mm, for example.

  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, during the secondary transfer, the separation voltage application circuit 38 applies a separation voltage having a polarity opposite to the transfer voltage to the separation electrode body 53 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 each needle electrode 55, 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 passes 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.

  In particular, when performing a secondary transfer on the second surface of the recording paper P that has been subjected to a fixing process once during double-sided printing, the resistance of the recording paper P may be increased, and thus white spot phenomenon occurs. However, according to the image forming apparatus 1 according to the present embodiment including the partition plate 39 on which the partition removal portion 39A is formed, the occurrence of the white spot phenomenon can be effectively suppressed.

  In this regard, an image forming apparatus in which the length L2 of the partition plate 39 is not less than the length L1 of the separation electrode body 53 and the partition removal portion 39A is not formed (hereinafter referred to as “image forming apparatus according to comparative example”). And the image forming apparatus 1 according to the present embodiment will be described in detail. That is, 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), it is necessary to set the transfer current to -37.5 μA. As a result, the absolute value of the transfer voltage was −1.67 kV, and the occurrence of the white spot phenomenon was observed.

  On the other hand, in the case of the image forming apparatus 1 according to the present embodiment, in order to ensure the appropriate current density in the sheet passing area when performing the secondary transfer on the second surface of A4 horizontal size, Considering that a part (3.8 μA) flows through the current path S, it is necessary to set the transfer current to −41.3 μA. When a transfer current of this value was actually applied, the absolute value of the transfer voltage was reduced to −1.53 kV, and no white spot phenomenon was observed.

  In addition, flowing a part of the transfer voltage from the secondary transfer roller 35 to each needle electrode 55 through the current path S causes the transfer voltage and the separation voltage to interfere with each other at both ends of the secondary transfer roller 35. In other words, it has been confirmed that this interference does not cause any abnormality in the secondary transfer of the image.

  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, as in the case of performing the secondary transfer on the second surface of the recording paper P during double-sided printing, it is possible to effectively generate the white spot phenomenon. Therefore, the quality of the printed image can be improved.

  In addition, a current path S is provided by forming a partition removing portion 39A in the partition plate 39, and a part of the transfer current flows from the secondary transfer roller 35 to each needle electrode 55 via the current path S. Therefore, even if the length of the secondary transfer roller is not extended as compared with the conventional image forming apparatus, it is possible to effectively suppress the white spot phenomenon and to prevent the apparatus from becoming large.

  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. 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, by causing a part of the transfer current to escape to the current path S, the occurrence of the white spot phenomenon can be effectively suppressed even under such a structure. .

  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.

  Further, by forming each partition removal portion 39A (current path S) at a position not corresponding to the printable region R, it is possible to prevent abnormality in the secondary transfer of the image due to the influence of the transfer voltage and the separation voltage. can do.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. FIG. 8 shows a cross section of the separation unit provided in the image forming apparatus according to the second embodiment of the present invention. 9 and 10 show the separation electrode bodies provided in the separation unit, respectively.

  The image forming apparatus according to the second embodiment of the present invention is different from the image forming apparatus 1 according to the first embodiment of the present invention in the separation electrode body provided in the separation unit. That is, as shown in FIGS. 8 and 9, the separation electrode body 62 provided in the separation unit 61 in the image forming apparatus according to the second embodiment has the same length as the separation electrode body 53 according to the first embodiment. Although the electrode plate portion 63 and the plurality of needle-like electrodes 64 are formed in the same manner as the separation electrode body 53 according to the first embodiment, the needle-like electrodes 64 disposed at both side ends of the separation electrode body 62 are provided. Compared to the other needle electrodes 64, the tip side is inclined so as to approach the secondary transfer roller 35.

  Specifically, as shown in FIG. 10, the needle-shaped electrode 64 provided in the separation electrode body 62 is disposed at a position corresponding to both axial ends of the secondary transfer roller 35 and is a printable area. Needle-like electrodes 64 arranged at portions not corresponding to R are inclined in a direction in which their tip ends approach the secondary transfer roller 35. In addition, as shown in FIG. 8, the inclination angle α of the needle electrode 64 is preferably about 15 to 45 degrees, and more preferably about 30 degrees.

  With the image forming apparatus according to the second embodiment of the present invention having such a configuration, the secondary transfer roller 35 and the needle-like electrode 64 are close to each other at positions corresponding to both end portions of the secondary transfer roller 35. Therefore, a part of the transfer current can easily flow through each current path S as compared with the image forming apparatus 1 according to the first embodiment of the present invention. Therefore, the absolute value of the transfer voltage in the secondary transfer can be further reduced, and the occurrence of the white spot phenomenon can be more effectively suppressed.

  For example, in the case where double-sided printing is performed by the image forming apparatus according to the second embodiment of the present invention, when the secondary transfer is performed on the second surface of A4 horizontal size, the appropriate current density in the sheet passing area is ensured. In order to achieve this, considering that a part of the transfer current (4.5 μA) flows through the current path S, it is necessary to set the transfer current to −42.0 μA. When a transfer current of this value was actually applied, the absolute value of the transfer voltage was −1.48 kV, and no white spot phenomenon was observed.

  FIG. 11 shows that the current density in the sheet passing region is appropriate for the image forming apparatus according to the comparative example, the image forming apparatus 1 according to the first embodiment of the present invention, and the image forming apparatus according to the second embodiment of the present invention. The transfer voltage in the secondary transfer in the case of a predetermined value (−1.26 μA / cm) is shown.

  In FIG. 11, when the current density in the paper passing region is −1.26 μA / cm, a separation electrode body having a partition plate on which no partition removal portion is formed and having only needle-like electrodes that are not inclined is provided. In the image forming apparatus according to the comparative example, the transfer voltage is −1.67 kV. On the other hand, in the same case, the image formation according to the first embodiment of the present invention has the partition plate 39 in which the partition removal portion 39A is formed and the separation electrode body 53 having only the needle-like electrode 55 that is not inclined. In the apparatus 1, the transfer voltage is −1.53 kV, and the absolute value of the transfer voltage is smaller than the transfer voltage in the image forming apparatus according to the comparative example. On the other hand, in the same case, the second embodiment of the present invention has the partition plate 39 in which the partition removal portion 39A is formed, and the separation electrode body 53 in which the needle-like electrodes 64 disposed on both side end portions are inclined. In the image forming apparatus according to the embodiment, the transfer voltage is −1.48 kV, and the absolute value of the transfer voltage is smaller than that of the image forming apparatus 1 according to the first embodiment of the present invention.

  Thus, it can be seen from FIG. 11 that according to the image forming apparatus 1 according to the first embodiment of the present invention, the absolute value of the transfer voltage can be reduced, and the occurrence of the white spot phenomenon can be effectively suppressed. According to the image forming apparatus of the second embodiment of the present invention, it can be seen that the absolute value of the transfer voltage can be further reduced and the occurrence of the white spot phenomenon can be more effectively suppressed.

  In each of the above-described embodiments, the case where the partition removal portions 39A are respectively formed on both side end portions of the partition plate 39 is taken as an example. However, the partition removal portion 39A is formed only on one end portion of the partition plate 39. Also good. However, by forming the partition removal portions 39A at both end portions of the partition plate 39, the absolute value of the transfer voltage can be easily and effectively reduced, and the occurrence of the white spot phenomenon can be easily and effectively suppressed. can do.

  Further, in each of the above-described embodiments, the partition transfer portions 39A are formed on both side ends of the partition plate 39 having a length equal to or longer than the length of the separation electrode body 53 (62), so that the secondary transfer roller 35 The case where the current path S for passing the transfer current is provided on the side of the needle electrode 55 (64) is taken as an example. It is also possible to provide a current path through which a transfer current flows from the secondary transfer roller 35 to each needle electrode 55 (64) side by shortening. In this case, the length of the partition plate 39 is not less than the width W of the printable region R.

  Further, in each of the above-described embodiments, the case where each photosensitive drum 21 is formed from amorphous silicon is taken as an example, but a photosensitive drum made of an organic photosensitive member (OPC) can also be adopted.

  Moreover, in each embodiment mentioned above, although the case where the partition plate 39 was attached to the base 51A of the electrode housing 51 was given as an example, the partition plate 39 may be formed integrally with the electrode housing 51.

  Further, in each of the above-described embodiments, 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 of a type that directly transfers the toner image to a recording sheet.

  Further, in the above-described second embodiment, the case where the needle electrodes 64 at both end portions of the separation electrode body 62 are tilted is taken as an example, but only one side may be tilted. Alternatively, the needle-like electrode 64 may be bent in the middle or the needle-like electrode 64 may be entirely curved so that the distal end side of the needle-like electrode 64 approaches the secondary transfer roller 35.

  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)
39 Partition plate 39A Partition removal part 51 Electrode housing (electrode support part)
52 Electrode support member 53, 62 Separation electrode body 54, 63 Electrode plate part 55, 64 Needle-like electrode (separation electrode)
P Recording paper (recording medium)
R Printable area S Current path

Claims (4)

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 image carrier or the image carrier is provided on the downstream side of the transfer member in the conveyance direction of the recording medium by the conveyance means, and is used to neutralize and separate the recording medium from which the toner image has been transferred from the image carrier. A plurality of separation electrodes for discharging to and from the recording medium;
Separation voltage applying means for applying a separation voltage to the separation electrode to perform the discharge by the separation electrode;
A partition plate provided between the transfer member and the separation electrode to regulate interference between the transfer voltage and the separation voltage;
By removing all or a part of the partition plate corresponding to at least one end in the axial direction of the transfer member between the transfer member and the separation electrode, the transfer voltage is applied. A current path for allowing a part of the transfer current supplied to the transfer member to flow from the transfer member to the separation electrode side is formed, and the current path is configured to display an image on the recording medium in the partition plate. An image forming apparatus, wherein the image forming apparatus is formed in a portion not corresponding to a printable region . Wherein arranged downstream of the transcription member includes an electrode support portion extending along said transfer member, said plurality of separate electrodes is supported on the electrode support, one end of said transfer member Each separation electrode is disposed from a corresponding position to a position corresponding to the other end portion, and the leading end side of each separation electrode projects from the electrode support portion toward the image carrier, and the transfer member among the plurality of separation electrodes The separation electrode disposed at a position corresponding to at least one end in the axial direction of the first electrode is inclined or bent so that the tip side approaches the transfer member as compared with the other separation electrodes. Item 2. The image forming apparatus according to Item 1. An electrode support portion is provided on the downstream side of the transfer member and extends along the transfer member. The plurality of separation electrodes are supported by the electrode support portion and correspond to one end portion of the transfer member. Each separation electrode is disposed over a position corresponding to the other end portion, and the leading end side of each separation electrode protrudes from the electrode support portion toward the image carrier, and among the plurality of separation electrodes, on the recording medium The separation electrode disposed at a portion not corresponding to the region where the image can be printed is inclined or bent so that the front end side approaches the transfer member as compared with other separation electrodes. the image forming apparatus according to 1. It said image bearing member is an image forming apparatus according to any one of claims 1 to 3, characterized in that it comprises a formed photoreceptor of a material containing amorphous silicon.

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