JP2011095583A - 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: An auxiliary roller 39 is provided on a side opposite to a side on which a secondary transfer roller 35 comes into contact with an intermediate transfer belt 14. During secondary transfer, transfer current is applied not only between the intermediate transfer belt 14 and the secondary transfer roller 35 but also between the secondary transfer roller 35 and both ends of the auxiliary roller 39, whereby transfer voltage is decreased. Moreover, toner adhering to the secondary transfer roller 35 can be removed by the auxiliary roller 39, and also, deflection of the secondary transfer roller 35 can be corrected by the auxiliary roller 39. <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 electrostatically applied by a bias voltage (transfer voltage) applied to the transfer member when a recording medium (for example, recording paper) passes between the photosensitive drum and the transfer member that are in contact with or close to each other. Is transferred to a recording medium.

  Here, the transfer member is pressed against the photosensitive drum by a biasing member such as a spring, for example, whereby the outer peripheral portion of the transfer member is elastically deformed, and a nip portion is formed between the transfer member and the photosensitive drum. . At the time of transfer, the transfer is realized by the recording medium passing through the nip portion between the transfer member and the photosensitive drum. In order to achieve proper transfer of the toner image to the recording medium, the transfer member and the photosensitive drum are in contact with each other by preventing the transfer member from bending or removing the toner image attached to the transfer member. It is desirable to make the nip portion uniform by maintaining a good value. For this reason, an image forming apparatus having a configuration in which a backup roller is provided at a position facing the photosensitive drum across the transfer member and the transfer member is supported by the backup roller or the transfer member is pressed against the photosensitive drum is known. (See Patent Documents 1 to 3).

  The image forming operation is based on a method in which a toner image is directly transferred from a photosensitive drum as an image carrier to a recording medium, and this method is generally adopted 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.

JP-A-6-3976 JP 2000-98769 A JP 2003-76161 A

  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-described problems, a first image forming apparatus according to the present invention is provided between an image carrier that carries a toner image by electrostatic force and the image carrier that is pressed against the image carrier. A transfer member for transferring the toner image onto a recording medium passing through the nip portion, and a transfer voltage applied to the transfer member for transferring the toner image from the image carrier to the recording medium. An image forming apparatus including a transfer voltage applying unit to be applied, comprising: an auxiliary member that contacts the transfer member at a position different from a contact position between the image carrier and the transfer member, wherein the auxiliary member is electrically conductive A base portion formed of a material; and a high resistance layer provided on the base portion, formed of a material having a resistance higher than that of the transfer member, and in contact with the transfer member. .

  In order to solve the above-mentioned problem, the second image forming apparatus of the present invention is the above-described first image forming apparatus of the present invention, wherein the high resistance layer is not in contact with the end of the transfer member. The base is in contact with or in direct proximity to the end of the transfer member.

  In order to solve the above-described problem, the third image forming apparatus of the present invention is the above-described first image forming apparatus of the present invention, wherein the high resistance layer includes the nip portion of the transfer member and the recording medium. Is not in contact with a region outside the recording medium passage region, which is a region through which the recording medium passes, and the base portion is in contact with or directly close to a region outside the recording medium passage region of the transfer member. It is characterized by.

  In order to solve the above-described problem, the fourth image forming apparatus of the present invention is the above-described third image forming apparatus of the present invention, wherein the high resistance layer is included in the recording medium passage region of the transfer member. And is in contact with an area outside the printable area, which is an area in which an image can be printed on the recording medium.

  In order to solve the above-described problem, the fifth image forming apparatus of the present invention is the above-described first image forming apparatus of the present invention, wherein the high resistance layer prints an image on the recording medium of the transfer member. An area outside the printable area, which is a possible area, is not in contact, and the base is in contact with or in direct proximity to an area outside the printable area of the transfer member. To do.

  In order to solve the above-described problems, a sixth image forming apparatus of the present invention is a toner removal for removing toner adhering to the auxiliary member in any of the first to fifth image forming apparatuses of the present invention described above. A member is provided.

  In order to solve the above-described problems, a seventh image forming apparatus according to the present invention is the image forming apparatus according to any one of the first to sixth aspects of the present invention described above, wherein the auxiliary member sandwiches the transfer member. It is arranged at a position facing the image carrier and supports at least a central region of the transfer member.

  In order to solve the above-described problem, an eighth image forming apparatus of the present invention includes the pressing member that presses the auxiliary member against the transfer member in the seventh image forming apparatus of the present invention described above. It is characterized by.

  In order to solve the above-described problems, a ninth image forming apparatus according to the present invention is the image forming apparatus according to any one of the first to eighth 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.

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. 3 is a perspective view showing a secondary transfer roller and the like disposed on the side unit in a state where the side unit is pulled out in the first embodiment of the present invention. FIG. 4 is an enlarged perspective view showing a part of a secondary transfer roller and the like in FIG. 3. FIG. 3 is an explanatory diagram illustrating a driving roller, an intermediate transfer belt, a secondary transfer roller, an auxiliary roller, and the like according to the first embodiment of the present invention. FIG. 6 is an explanatory diagram showing, in an enlarged manner, cross sections of a drive roller, an intermediate transfer belt, a secondary transfer roller, an auxiliary roller, and the like as seen from the direction of arrows VI-VI in FIG. 5. FIG. 6 is a characteristic diagram showing the relationship between the axial position of the secondary transfer roller 35 and the transfer current between the secondary transfer roller and the auxiliary roller in the image forming apparatus according to the first embodiment of the present invention. 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 the image forming apparatus according to the first embodiment of the present invention. It is explanatory drawing which shows the drive roller, intermediate transfer belt, secondary transfer roller, auxiliary roller, etc. in the 2nd Embodiment of this invention. It is explanatory drawing which expands and shows the cross section of a drive roller, an intermediate transfer belt, a secondary transfer roller, an auxiliary roller, etc. seen from the arrow XX direction in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. First, a first embodiment of the present invention will be described. 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. 3 and 4 partially show a state in which a side unit in which a secondary transfer roller or the like is incorporated is pulled out.

  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 electrode 47, a fixing device 49, 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. 49 is a pair of rollers that are sequentially conveyed to 49 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 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 rotates with the movement of the intermediate transfer belt 14 (rotation of the drive roller 12) (see FIG. 6). The secondary transfer roller 35 has an outer diameter of about 22 mm. For example, the transfer roller 35 is formed of a conductive epichlorohydrin foam around a transfer shaft portion 36 formed of a conductive material such as a metal material. The roller peripheral portion 37 is provided, and the resistance is 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, the outer peripheral portion of the secondary transfer roller 35 is elastically deformed by being pressed against the intermediate transfer belt 14 hung on the driving roller 12 by an urging member 45 (see FIG. 5) such as a spring. A nip portion 38 is formed between the transfer belt 14 and the transfer belt 14. The toner image is transferred to the recording paper P by passing through the nip portion 38.

  An auxiliary roller 39 as an auxiliary member and a cleaning member 43 as a toner removing member are provided on the side of the secondary transfer roller 35 opposite to the side in contact with the intermediate transfer belt 14. The auxiliary roller 39 and the cleaning member 43 will be described later.

  The transfer voltage application circuit 46 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 46 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 recording sheet having a maximum sheet width is formed between the intermediate transfer belt 14 on the drive roller 12 side and the secondary transfer roller 35 (nip portion 38). The value of the transfer current is set so that the current density in the sheet passing region R1 (see FIG. 5) through which P passes is within a predetermined range.

  The separation electrode 47 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 electrode 47 is an electrode for discharging the recording paper P to which the toner image is transferred and separating the recording paper P from the intermediate transfer belt 14. As shown in FIG. 3 or FIG. 4, the separation electrode 47 has a plurality of needle-like electrodes 47 </ b> A arranged along the axial direction of the secondary transfer roller 35. The separation electrode 47 is electrically connected to a separation voltage application circuit (not shown) for applying a separation voltage. Further, a partition plate 48 is provided between the separation electrode 47 and the secondary transfer roller 35 to restrict interference between the transfer voltage and the separation voltage. The partition plate 48 is formed in a long plate shape that extends parallel to the axial direction of the secondary transfer roller 35.

  A fixing device 49 in FIG. 1 is a device that is provided on the downstream side of the separation electrode 47 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, the auxiliary roller 39, the separation electrode 47, the partition plate 48, and the like are incorporated in the side unit 50 as shown in FIG. As for the side unit 50, the base end side is rotatably fixed to the side surface of the housing, and as shown in FIG. 3 or FIG. 4, the user can use the front end of the side unit 50 to perform maintenance, jam processing, and the like. The side can be pulled out of the housing 2. The secondary transfer roller 35 is pressed against the intermediate transfer belt 14 on the drive roller 12 side with the side unit 50 closed.

  5 and 6 show the driving roller 12, the intermediate transfer belt 14, the secondary transfer roller 35, the auxiliary roller 39, the urging member 45, and the like. FIG. 7 shows the relationship between the axial position of the secondary transfer roller 35 and the transfer current between the secondary transfer roller 35 and the auxiliary roller 39. The auxiliary roller 39 and the cleaning member 43 will now be described with reference to FIGS.

  As shown in FIG. 5, the auxiliary roller 39 is located at a position different from the contact position between the intermediate transfer belt 14 and the secondary transfer roller 35, specifically, the side in contact with the intermediate transfer belt 14 of the secondary transfer roller 35. Is in contact with the secondary transfer roller 35 at a position on the opposite side. The auxiliary roller 39 suppresses the transfer voltage, removes the toner attached to the secondary transfer roller 35, supports the secondary transfer roller 35, and the like. The auxiliary roller 39 rotates with the rotation of the secondary transfer roller 35 as shown by the arrow in FIG.

  The auxiliary roller 39 is provided on the auxiliary shaft portion 40 serving as a rotation axis of the auxiliary roller 39, an annular auxiliary roller peripheral portion 41 provided on the outer peripheral side of the auxiliary shaft portion 40, and an outer peripheral side of the auxiliary roller peripheral portion 41. And a high resistance coat layer 42 as a high resistance layer. The auxiliary shaft portion 40 and the auxiliary roller peripheral portion 41 are specific examples of the base portion of the auxiliary member.

  The auxiliary roller peripheral portion 41 is formed of a conductive material, and has an outer diameter of about 12 mm, for example. The axial length of the auxiliary roller peripheral portion 41 is preferably substantially equal to or longer than the transfer roller peripheral portion 37 of the secondary transfer roller 35, and is, for example, about 314 mm in this embodiment.

The high resistance coating layer 42 is a thin film having a thickness of, for example, about 5 μm to 10 μm formed of a material having high resistance such as PTFT (polytetrafluoroethylene), and is applied to the outer peripheral surface of the auxiliary roller peripheral portion 41. Has been. The high resistance coat layer 42 has a resistance higher than the resistance of the transfer roller peripheral portion 37, for example, a resistance of about 10 ¹³ ( ¹³ log 10Ω).

  Further, the high resistance coat layer 42 is applied only to the central portion of the auxiliary roller peripheral portion 41. In the region where the high resistance coat layer 42 is applied, the high resistance coat layer 42 and the transfer roller peripheral portion 37 are in contact with each other, and in the region where the high resistance coat layer 42 is not applied, the auxiliary roller peripheral portion 41 is positioned around the transfer roller. It is in direct contact with the part 37. That is, the central portion of the transfer roller peripheral portion 37 is in contact with the high resistance coat layer 42, and both end portions of the transfer roller peripheral portion 37 are in direct contact with the auxiliary roller peripheral portion 41.

  More specifically, the area through which the recording paper P with the maximum paper width passes is defined as the paper area R1 between the intermediate transfer belt 14 and the secondary transfer roller 35 (nip portion 38), and the recording paper P with the maximum paper width. If the area where the image can be printed is the printable area R2, the area where the high-resistance coating layer 42 is formed includes the entire printable area R2 as shown in FIG. Also does not include the axially outer region. In other words, the axial length of the high-resistance coating layer 42 is not less than the width dimension of the printable area R2 and not more than the width dimension of the paper passing area R1. For example, in this embodiment, since the maximum sheet width of the recording sheet P is 297 mm, the width dimension of the sheet passing region R1 is 297 mm, while the printable region R2 is 289 mm, for example. Therefore, the axial length of the high resistance coat layer 42 is not less than 289 mm and not more than 297 mm (for example, 290 mm).

  When a transfer voltage is applied to the transfer shaft portion 36 of the secondary transfer roller 35 by the transfer voltage application circuit 46 during the secondary transfer, a transfer current is generated between the intermediate transfer belt 14 on the drive roller 12 side and the secondary transfer roller 35. Flowing. At this time, the transfer current also flows between the secondary transfer roller 35 and the auxiliary roller 39. However, most of the transfer current flowing between the secondary transfer roller 35 and the auxiliary roller 39 is formed with the high resistance coating layer 42 in the auxiliary roller peripheral portion 41 of the auxiliary roller 39 as shown in FIG. It flows in a region that does not exist, that is, a region corresponding to both ends of the auxiliary roller 39. On the other hand, little or no transfer current flows in the region where the high resistance coating layer 42 is formed in the auxiliary roller peripheral portion 41, particularly in the region corresponding to the central portion of the auxiliary roller 39.

  On the other hand, the cleaning member 43 is a member that removes toner adhering to the auxiliary roller 39, and the cleaning member 43 is formed of, for example, urethane rubber, and a cleaning blade whose tip is in contact with the outer peripheral surface of the auxiliary roller 39, or For example, a fur brush formed of nylon or the like is provided. The auxiliary roller 39 removes the toner adhering to the outer peripheral surface of the secondary transfer roller 35 from the intermediate transfer belt 14 or the like by adhering it to the outer peripheral surface of itself. The cleaning member 43 removes the toner adhering to the outer peripheral surface of the auxiliary roller 39 in this way.

  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 drive roller 12 side and the secondary transfer roller 35 from the paper feed cassette 7 via the conveyance path 32 by the conveyance roller pair 31 ( Transfer is performed on the recording paper P or the like conveyed to the nip portion 38) (secondary transfer).

  Subsequently, the toner image secondarily transferred to the recording paper P is fixed to the recording paper P by the fixing device 49, and the recording paper P on which the toner image is fixed is discharged to the paper discharge tray 5.

  When performing duplex printing, the recording sheet P once fixed by the fixing device 49 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 drive roller 12 side and the secondary transfer roller 35 (nip portion 38) to perform secondary transfer, and then a fixing process is performed by the fixing device 49, and then the paper discharge tray 5 To discharge.

  In such an operation of the image forming apparatus 1, when performing secondary transfer, a transfer voltage application circuit 46 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.

  Here, when a transfer voltage is applied from the transfer voltage application circuit 46 during the secondary transfer, not only between the intermediate transfer belt 14 and the secondary transfer roller 35 but also the secondary transfer roller 35 and the auxiliary roller as described above. A transfer current also flows between both ends of 39. As described above, in order to ensure a current density within the predetermined range in the sheet passing area while flowing a transfer current between the secondary transfer roller 35 and both ends of the auxiliary roller 39, the current flows into the auxiliary roller 39. Although the absolute value of the transfer current has to be increased in consideration of the current component, the absolute value of the transfer voltage can be reduced even if the absolute value of the transfer current is increased in this way. Therefore, 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 absolute value of the transfer voltage can be suppressed, and the absolute value of the transfer voltage can be suppressed. It is possible to prevent the secondary transfer roller 35 or the recording paper P from being discharged due to an increase in the value, thereby effectively suppressing 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 is significantly increased due to heating or the like in the fixing process. Therefore, the occurrence of a white spot phenomenon is particularly worrisome. However, according to the image forming apparatus 1 of the present embodiment, a transfer current is also passed between the secondary transfer roller 35 and both ends of the auxiliary roller 39. As a result, the absolute value of the transfer voltage can be lowered and 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, 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 duplex printing, the absolute value of the transfer voltage can be reduced while ensuring the appropriate current density in the sheet passing region R1. Therefore, it is possible to 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. 8 in comparison with the image forming apparatus according to the comparative example and the image forming apparatus 1 according to the first embodiment of the present invention. The image forming apparatus according to the comparative example does not include the auxiliary roller 39 having the high resistance coat layer 42.

  In the case of the image forming apparatus according to the comparative example, when the secondary transfer is performed on the second surface of A4 horizontal size, the current density within the predetermined range in the sheet passing region R1 (for example, −1.2 to −1.3 μA / In order to secure (cm), it is necessary to set the transfer current to, for example, -37.5 μA (the current density in the sheet passing region R1 at this time is −1.26 μA / cm). As a result, the transfer voltage was −1.67 kV and the absolute value thereof was increased, and the occurrence of a white spot phenomenon was observed (see Δ mark on hatching in FIG. 8).

  On the other hand, 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 region R1 when performing the 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 −67.0 μA in consideration of the transfer current (−29.5 μA) flowing between both ends of the secondary transfer roller 35 and the auxiliary roller 39. There is. When a transfer current having this value was actually applied, the absolute value of the transfer voltage decreased to -1.37 kV, and no white spot phenomenon was observed (see the mark ● on the hatching in FIG. 8).

  As a result of repeated experiments in an environment where the temperature is 10 degrees and the humidity is 15% or the temperature is 32.5 degrees and the humidity is 80%, the absolute value of the transfer voltage exceeds 1.6 kV as shown by a two-dot chain line in FIG. It was found that the white spot phenomenon occurred. In the image forming apparatus 1 according to the embodiment of the present invention, when the current density in the sheet passing region R1 is within the predetermined range (for example, −1.2 to −1.3 μA / cm), the absolute value of the transfer voltage is Less 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, since the white point phenomenon is suppressed by flowing a transfer current between the secondary transfer roller 35 and both ends of the auxiliary roller 39, the secondary transfer roller 35 has a The length need not be much longer than the maximum paper width. 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, even under such a structure, the absolute value of the transfer voltage can be obtained by flowing a transfer current between both ends of the secondary transfer roller 35 and the auxiliary roller 39. And the occurrence of white spot phenomenon can be effectively suppressed.

  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, since the toner attached to the secondary transfer roller 35 is removed by the auxiliary roller 39 and the toner attached to the auxiliary roller 39 is removed by the cleaning member 43, the back surface of the recording paper P or the like at the time of the secondary transfer, etc. It is possible to prevent dirt from adhering to the surface. Further, by removing the toner adhering to the secondary transfer roller 35, the contact or pressing of the secondary transfer roller 35 to the intermediate transfer belt 14 can be stabilized, and the accuracy of the secondary transfer can be improved. The quality of the printed image can be improved.

  Further, on the side opposite to the intermediate transfer belt 14 (drive roller 12) side, the secondary transfer roller 35 can be supported by the auxiliary roller 39 over the entire axial direction (including the central region). The deflection of the central portion of the secondary transfer roller 35 can be corrected. That is, the secondary transfer roller 35 is pressed against the intermediate transfer belt 14 by pressing both end portions of the transfer shaft portion 36 with the urging member 45, but the portion to which the pressing force is applied is the both end portions of the transfer shaft portion 36. Only the center portion of the secondary transfer roller 35 may bend, and the pressing force against the intermediate transfer belt 14 may be weakened at the center portion of the secondary transfer roller 35. In the image forming apparatus 1 according to the present embodiment, the deflection of the secondary transfer roller 35 is corrected by the auxiliary roller 39, whereby the pressing force of the secondary transfer roller 35 against the intermediate transfer belt 14 is uniform over the entire axial direction. In addition, stable secondary transfer can be realized over the entire image.

  Next, a second embodiment of the present invention will be described. 9 and 10 show the drive roller 12, the intermediate transfer belt 14, the secondary transfer roller 35, the urging member 45, the auxiliary roller 61, and the like in the image forming apparatus according to the second embodiment of the present invention.

  As shown in FIGS. 9 and 10, in the image forming apparatus according to the second embodiment of the present invention, the auxiliary roller 61 faces the intermediate transfer belt 14 (drive roller 12) with the secondary transfer roller 35 interposed therebetween. And the entire axial direction of the secondary transfer roller 35 is pressed. That is, as shown in FIG. 10, the auxiliary roller 61 has a rotation center Q1 of the driving roller 12, a rotation center Q2 of the secondary transfer roller 35, and a rotation center Q3 of the auxiliary roller 39 arranged on the same straight line. Has been placed.

  Further, the auxiliary roller portion provided on the outer peripheral side of the auxiliary shaft portion 62 of the auxiliary roller 61 is divided into a left auxiliary roller peripheral portion 63A and a right auxiliary roller peripheral portion 63B, and the left auxiliary roller peripheral portion 63A and the right auxiliary roller peripheral portion. Between the portion 63B, there is provided a pressing member 65 that presses the auxiliary roller 61 against the secondary transfer roller 35 and presses the secondary transfer roller 35 together with the auxiliary roller 61 toward the intermediate transfer belt 14 (drive roller 12). Yes. The pressing member 65 is attached to the auxiliary shaft portion 62 at one end side and an attachment member 65A having a spring receiving portion formed at the other end side, and an auxiliary member such as a spring attached to the spring receiving portion of the attachment member 65A. A force member 65B is provided.

  Further, on the outer peripheral sides of the left auxiliary roller peripheral portion 63A and the right auxiliary roller peripheral portion 63B, a high resistance coat layer 64 substantially the same as the high resistance coat layer 42 in the first embodiment of the present invention described above is applied. ing.

  The cleaning member 67 is a member that removes the toner adhering to the auxiliary roller 61 in substantially the same manner as the cleaning member 43 in the first embodiment of the present invention described above.

  Also with the image forming apparatus according to the second embodiment of the present invention having such a configuration, substantially the same functions and effects as those of the image forming apparatus 1 according to the first embodiment of the present invention described above can be obtained. Further, according to the image forming apparatus of the second embodiment of the present invention, the auxiliary transfer roller 61 can push the secondary transfer roller 35 toward the intermediate transfer belt 14 (drive roller 12) over the entire axial direction. Thus, the deflection of the central portion of the secondary transfer roller 35 can be suppressed or corrected. In particular, by applying a pressing force to the central portion of the auxiliary roller 61 by the pressing member 65, the central portion of the secondary transfer roller 35 can be reliably pressed, and the bending suppression effect of the central portion of the secondary transfer roller 35 can be reduced. The effect of making the pressing force on the intermediate transfer belt 14 uniform over the entire axial direction can be further increased, and the stabilization of the secondary transfer over the entire image can be further improved.

  In the first embodiment described above, the case where the high resistance coating layer 42 is formed by applying a high resistance material to the outer peripheral surface of the auxiliary shaft portion 40 of the auxiliary roller 39 has been described as an example. Not limited to this. Instead of applying the high-resistance coating layer 42, a tube formed of a high-resistance material may be attached to the auxiliary shaft portion 40. The same applies to the high resistance coat layer 64 in the second embodiment.

  In the first embodiment described above, the case where both ends of the auxiliary roller peripheral portion 41 of the auxiliary roller 39 on which the high resistance coat layer 42 is not formed is brought into direct contact with both ends of the secondary transfer roller 35 is taken as an example. Although the present invention is not limited to this, both ends of the auxiliary roller peripheral portion 41 of the auxiliary roller 39 on which the high resistance coat layer 42 is not formed are directly connected to both ends of the secondary transfer roller 35 (high resistance You may make it adjoin (without interposing the coating layer 42).

  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.

  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, the photosensitive drum hits the image carrier.

  The present invention can be changed as appropriate without departing from the spirit or concept of the invention which can be read from the claims and the entire specification, and an image forming apparatus accompanying such a change is also a technical idea of the present invention. include.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 11 Tandem type image forming unit 12 Drive roller 14 Intermediate transfer belt (image carrier)
16 Image forming unit 17 Primary transfer roller 21 Photosensitive drum 22 Charging device 23 Exposure device 24 Development device 25 Cleaning device 35 Secondary transfer roller (transfer member)
36 Transfer shaft part 37 Transfer roller peripheral part (outer peripheral part)
38 Nip part 39, 61 Auxiliary roller (auxiliary member)
40, 62 Auxiliary shaft (base)
41, 63A, 63B Auxiliary roller circumference (base)
42, 64 High resistance coating layer (high resistance layer)
43, 67 Cleaning member 46 Transfer voltage application circuit 65 Press member P Recording paper (recording medium)
R1 Paper passing area (recording medium passing area)
R2 printable area

Claims (9)

An image carrier that carries a toner image by electrostatic force;
A transfer member that forms a nip portion with the image carrier by being pressed against the image carrier and transfers the toner image to a recording medium that passes through the nip portion;
An image forming apparatus comprising 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,
An auxiliary member that contacts the transfer member at a position different from the contact position between the image carrier and the transfer member;
The auxiliary member is
A base formed of a conductive material;
An image forming apparatus comprising: a high resistance layer which is provided on the base and is formed of a material having a resistance higher than that of the transfer member, and which is in contact with the transfer member.   The high resistance layer is not in contact with an end portion of the transfer member, and the base portion is in contact with or in close proximity to the end portion of the transfer member. Image forming apparatus.   The high resistance layer is not in contact with a region outside the recording medium passage region, which is a region through which the recording medium passes, through the nip portion of the transfer member, and the base portion is the portion of the transfer member. The image forming apparatus according to claim 1, wherein the image forming apparatus is in contact with or directly close to an area outside the recording medium passage area.   The high-resistance layer is in contact with an area outside the printable area, which is an area in the recording medium passing area of the transfer member and capable of printing an image on the recording medium. The image forming apparatus according to claim 3.   The high-resistance layer is not in contact with an area outside the printable area, which is an area where an image can be printed on the recording medium, of the transfer member, and the base portion is printable among the transfer member. The image forming apparatus according to claim 1, wherein the image forming apparatus is in contact with or directly close to an area outside the area.   The image forming apparatus according to claim 1, further comprising a toner removing member that removes toner adhering to the auxiliary member.   7. The auxiliary member according to claim 1, wherein the auxiliary member is disposed at a position facing the image carrier with the transfer member interposed therebetween, and supports at least a central region of the transfer member. Image forming apparatus.   The image forming apparatus according to claim 7, further comprising a pressing member that presses the auxiliary member against the transfer member.   9. 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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