JP2011095582A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve image quality by suppressing a white spot phenomenon without upsizing of an apparatus. <P>SOLUTION: A high resistance coat layer 39 is provided between a shaft 36 of a secondary transfer roller 35 and a roller part 37. An area where the high resistance coat layer 39 is provided is set, in a range including the whole of a printable area R2 being an area where an image can be printed on recording paper having maximum paper width and not including an area outside a paper passing area R1 being an area where the recording paper having the maximum paper width passes between an intermediate transfer belt 14 and the secondary transfer roller 35. Thus, when transfer voltage is applied to the shaft 36 of the secondary transfer roller 35, flow of transfer current between the secondary transfer roller 35 and the intermediate transfer belt 14 is controlled, thereby the transfer voltage is decreased. <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. The transfer member is, for example, a transfer roller, and is formed of a metal material. A shaft portion to which a transfer voltage is applied, and an annular roller portion provided on the outer peripheral side of the shaft portion and formed of an elastic conductive resin or the like, (Refer patent document 1).

  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-2005-77541

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

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

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

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

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

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

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

  In order to solve the above problem, a first image forming apparatus of the present invention includes an image carrier that carries a toner image by electrostatic force, a transfer member that transfers the toner image from the image carrier to a recording medium, An image forming apparatus comprising transfer voltage applying means for applying a transfer voltage to the transfer member for transferring the toner image from the image carrier to the recording medium, wherein the transfer member is formed of a conductive material. The image carrier is provided with a shaft portion to which the transfer voltage is applied by the transfer voltage applying means, and a nip portion that is provided on the outer peripheral side of the shaft portion, is formed of a conductive material, and passes the recording medium during transfer. And a high resistance layer provided between the shaft portion and the outer peripheral portion and having a resistance higher than that of the outer peripheral portion, and the high resistance layer includes: Of the shaft part, in the axial direction Characterized in that it is disposed only in a part of the region containing the parts.

  In order to solve the above-described problem, the second image forming apparatus of the present invention is the above-described first image forming apparatus of the present invention, wherein the region where the high resistance layer is disposed prints an image on the recording medium. It includes the entire possible area.

  In order to solve the above-described problems, a third image forming apparatus according to the present invention is the above-described first or second image forming apparatus according to the present invention, wherein the region where the high resistance layer is disposed includes the nip portion. An area outside the axial direction of the shaft portion is not included with respect to an area through which the recording medium passes.

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

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

It is explanatory drawing which shows the image forming apparatus by embodiment of this invention. FIG. 2 is an explanatory diagram showing an enlarged part of the image forming apparatus in FIG. 1. In the embodiment of the present invention, it 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. FIG. 4 is an enlarged perspective view showing a part of a secondary transfer roller and the like in FIG. 3. In the embodiment of the present invention, it is an explanatory view showing a drive roller, an intermediate transfer belt, a secondary transfer roller and the like. FIG. 6 is a characteristic diagram illustrating a relationship between an axial position of a secondary transfer roller and a transfer current in the image forming apparatus according to the 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 an image forming apparatus according to an embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows an image forming apparatus according to an embodiment of the present invention. FIG. 2 is an enlarged view of a part of the image forming apparatus in FIG. 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 conveying roller pairs 31, 33, a secondary transfer roller 35, a separation electrode 43, a fixing device 45, 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 sequentially convey to 45 and discharge 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 during the secondary transfer. The secondary transfer roller 35 has a shaft portion 36 and a roller portion 37 as an outer peripheral portion. Further, a nip portion 38 is formed between the roller portion 37 and the intermediate transfer belt 14 on the driving roller 12 side. Further, a transfer voltage application circuit 42 is electrically connected to the shaft portion 36 of the secondary transfer roller 35. Details of the secondary transfer roller 35 will be described later.

  The transfer voltage application circuit 42 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 42 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 43 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 43 is an electrode for discharging the recording paper P to which the toner image has been transferred and separating the recording paper P from the intermediate transfer belt 14. As shown in FIG. 3 or FIG. 4, the separation electrode 43 has a plurality of needle-like electrodes 43 </ b> A arranged along the axial direction of the secondary transfer roller 35. The separation electrode 43 is electrically connected to a separation voltage application circuit (not shown) for applying a separation voltage. Further, a partition plate 44 for restricting interference between the transfer voltage and the separation voltage is provided between the separation electrode 43 and the secondary transfer roller 35. The partition plate 44 is formed in a long plate shape extending in parallel with the axial direction of the secondary transfer roller 35.

  A fixing device 45 in FIG. 1 is a device that is provided on the downstream side of the separation electrode 43 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 separation electrode 43, the partition plate 44, and the like are incorporated in the side unit 46 as shown in FIG. The side unit 46 has a proximal end fixed to the side surface of the housing so as to be rotatable. As shown in FIG. 3 or FIG. 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 46 closed.

  FIG. 5 shows the drive roller 12, the intermediate transfer belt 14, the secondary transfer roller 35, the urging member 41, and the like. FIG. 5 shows a cross section of only the roller portion 37 of the secondary transfer roller 35. FIG. 6 shows the relationship between the axial position of the secondary transfer roller 35 and the transfer current. The secondary transfer roller 35 will now be described in detail with reference to FIGS. 5 and 6.

  As shown in FIG. 5, the shaft portion 36 of the secondary transfer roller 35 is a shaft made of a conductive material such as metal and having a diameter of about 12 mm, for example. A transfer voltage application circuit 42 is electrically connected to the shaft portion 36, and a transfer voltage is applied by the transfer voltage application circuit 42 at the time of secondary transfer.

  The roller portion 37 of the secondary transfer roller 35 is provided on the outer peripheral side of the shaft portion 36 and is formed in an annular shape from a conductive material such as conductive epichlorohydrin foam. The roller portion 37 has a thickness of, for example, about 5 mm, an outer diameter of, for example, about 22 mm, and has a resistance of, for example, about 7 logΩ.

  Further, the axial length of the roller portion 37 may be equal to or greater than the maximum sheet width that is the width dimension of the recording sheet P that has the maximum width among the recording sheets P on which an image can be printed by the image forming apparatus 1. Preferably, it is 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 roller portion 37 is, for example, about 310 mm.

  Further, the roller portion 37 is pressed against the intermediate transfer belt 14 that is hung on the driving roller 12 by the biasing member 41 such as a spring, so that the outer peripheral portion thereof is elastically deformed, and thereby the intermediate transfer belt 14. A nip portion 38 is formed between the two. The toner image is transferred to the recording paper P by passing through the nip portion 38.

Further, a high resistance coat layer 39 as a high resistance layer is provided between the shaft portion 36 and the roller portion 37. The high resistance coat layer 39 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 shaft portion 36. Yes. The high resistance coat layer 39 has a resistance higher than the resistance of the roller portion 37, specifically a resistance of 10 ¹⁰ (10.0 log10 Ω · cm) or more, for example, about 10 ¹³ .

  The high resistance coat layer 39 is disposed only in a part of the shaft portion 36 including the central portion in the axial direction, and covers the outer peripheral surface of the shaft portion 36 over the entire circumference in the region. Specifically, a region where the recording paper P having the maximum paper width passes is defined as a paper region R1 between the intermediate transfer belt 14 and the secondary transfer roller 35 (nip portion 38). Assuming that the area where the image can be printed is the printable area R2, as shown in FIG. 5, the area where the high-resistance coat layer 39 is disposed includes the entire printable area R2 and is more axial than the paper passing area R1. The region outside the axial direction of the portion 36 is not included. In other words, the axial length of the high resistance coat layer 39 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. Accordingly, the axial length of the high resistance coat layer 39 is not less than 289 mm and not more than 297 mm.

  When a transfer voltage is applied to the shaft portion 36 of the secondary transfer roller 35 by the transfer voltage application circuit 42 during the secondary transfer, a transfer current flows between the intermediate transfer belt 14 on the drive roller 12 side and the secondary transfer roller 35. . At this time, as shown in FIG. 6, the secondary transfer roller 35 not provided with the high resistance coat layer 39 compared to the central region of the secondary transfer roller 35 provided with the high resistance coat layer 39. A large amount of transfer current flows in the regions on the both end sides. That is, more transfer current flows outside the paper passing area R1 than inside the printable area R2.

  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 45, 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 the fixing process by the fixing device 45 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 thereafter, fixing processing is performed by the fixing device 45, and then the discharge tray 5 To discharge.

  In such an operation of the image forming apparatus 1, when performing the secondary transfer, the transfer voltage applying circuit 42 applies a transfer voltage having a polarity opposite to 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 42 at the time of secondary transfer, as described above, compared to the central region of the secondary transfer roller 35 provided with the high resistance coating layer 39, A large amount of transfer current flows in the regions on both ends of the secondary transfer roller 35 where the high resistance coat layer 39 is not provided. That is, as compared with the printable area R2, the high resistance coat layer 39 may be formed in an area outside the paper passing area R1 (the area outside the printable area R2 and in the paper passing area R1. Includes that region), a large amount of transfer current flows.

  As described above, in order to secure the current density within the predetermined range in the sheet passing area while controlling the flow of the transfer current by the high resistance coating layer 39 of the secondary transfer roller 35, the sheet flows out of the sheet passing area R1. 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 the white spot phenomenon is particularly worrisome. However, according to the image forming apparatus 1 of the present embodiment, the absolute value of the transfer voltage is set by controlling the flow of the transfer current by the high resistance coat layer 39. The occurrence of this 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. 7 in comparison with the image forming apparatus according to the comparative example and the image forming apparatus 1 according to the embodiment of the present invention. The image forming apparatus according to the comparative example is not provided with the high resistance coating layer 39.

  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 its absolute value was increased, and the occurrence of a white spot phenomenon was observed (see Δ mark on hatching in FIG. 7).

  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 secure the transfer current, in consideration of the fact that a large amount of transfer current (for example, 10.0 μA) flows in the regions on both ends of the secondary transfer roller 35 by providing the high resistance coating layer 39, the transfer current is −47. Must be set to 5 μA. When a transfer current of this value was actually applied, the transfer voltage was −1.43 kV and its absolute value was small, and no white spot phenomenon was observed (see the mark ● on the hatching in FIG. 7).

  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, the absolute value of the transfer voltage is 1 when the current density in the sheet passing area is within the predetermined range (for example, −1.2 to −1.3 μA / cm). Less than 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 spot phenomenon is suppressed by providing the high-resistance coating layer 39 and controlling the flow of the transfer current, the length of the secondary transfer roller 35 is set to the maximum sheet width in order to realize the effect. It does not have to be much longer than. 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 reduced by providing the high resistance coat layer 39 and controlling the flow of the transfer current. The occurrence of the 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.

  In the above-described embodiment, the high resistance layer provided between the shaft portion 36 and the roller portion 37 of the secondary transfer roller 35 is formed by applying a high resistance material to the outer peripheral surface of the shaft portion 36. Although the case where the resistance coat layer 39 is used is taken as an example, the present invention is not limited to this. The high resistance layer may be a tube formed of a high resistance material, and the tube may be attached to the shaft portion 36.

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

  In the above-described embodiment, the case where the present invention is applied to an image forming apparatus for tandem color printing has been described as an example. However, the present invention is formed on a photosensitive drum like an image forming apparatus for monochrome printing. The present invention can also be applied to an image forming apparatus that directly transfers a toner image onto a recording sheet. In this case, 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 Shaft part 37 Roller part (outer peripheral part)
38 Nip part 39 High resistance coating layer (high resistance layer)
42 Transfer voltage application circuit (transfer voltage application means)
P Recording paper (recording medium)
R1 Paper passing area R2 Printable area

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;
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,
The transfer member is
A shaft portion made of a conductive material and applied with the transfer voltage by the transfer voltage applying means;
An outer peripheral portion provided on the outer peripheral side of the shaft portion, formed of a conductive material, and forming a nip portion between the image carrier and the nip portion through which the recording medium passes during transfer;
A high resistance layer provided between the shaft portion and the outer peripheral portion and having a resistance higher than the resistance of the outer peripheral portion;
The image forming apparatus, wherein the high resistance layer is disposed only in a part of the shaft portion including a central portion in the axial direction.   The image forming apparatus according to claim 1, wherein the region where the high resistance layer is disposed includes an entire region where an image can be printed on the recording medium.   3. The image according to claim 1, wherein the region where the high resistance layer is disposed does not include a region outside the axial direction of the shaft portion with respect to a region where the recording medium passes through the nip portion. Forming equipment.   4. 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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