JP5476750B2 - Transfer device and image forming apparatus - Google Patents

Description

  The present invention transfers an image, such as a toner image, carried on an image carrier such as a photoconductor provided in an image forming apparatus such as a copying machine, a facsimile, or a printer, from the image carrier to a recording medium such as paper. A transfer apparatus, particularly a so-called tandem type image forming apparatus provided with a plurality of such image carriers, which once carries such images carried on a plurality of image carriers and then transfers them to such a recording medium The present invention relates to a transfer device including an intermediate transfer member and such an image forming apparatus including the transfer device.

  2. Description of the Related Art Conventionally, image forming apparatuses such as copiers, facsimiles, and printers include a transfer device that transfers an image such as a toner image carried on an image carrier such as a photoconductor to a recording medium such as paper from the image carrier. There are known image forming apparatuses (see, for example, [Patent Document 1] to [Patent Document 11]). As one type of such an image forming apparatus, an intermediate transfer member such as an intermediate transfer belt that once transfers the image carried on the image carrier and transfers the carried image to a recording medium, and a recording medium facing the intermediate transfer member. 2. Description of the Related Art A so-called intermediate transfer type image forming apparatus having a transfer device provided with a member for carrying or the like is known (for example, see [Patent Document 1] to [Patent Document 10]).

  An intermediate transfer type image forming apparatus has a primary transfer unit that performs primary transfer for transferring an image on the image carrier to the intermediate transfer member, and a structure that is transferred to the intermediate transfer member and carried by the intermediate transfer member. And a secondary transfer portion that performs secondary transfer to transfer the image on the recording medium, and it is required that good transfer is performed in any transfer portion.

The criteria for whether transfer is good include whether the transfer rate is high, whether the pre-transfer performance is good, and whether transfer blur has occurred.
The conditions for satisfying these criteria are closely related to each other, as will be apparent from the description below. It should be noted that the standard regarding the transfer rate and the pre-transfer performance needs to be satisfied in the primary transfer unit and the secondary transfer unit, and the standard regarding the transfer blur mainly transfers the image to the secondary transfer unit, that is, the recording medium. Needs to be met in some cases.

  First, transfer blur will be described. For example, transfer blur includes a member that bends a conveyance path of a recording medium so that the recording medium is bent when the recording medium is conveyed through a transfer unit (for example, [Patent Document 4] ], [Patent Document 8], [Patent Document 10], and [Patent Document 11]), the image is transferred to the recording medium due to the stress applied to the recording medium by the member. It is.

  The transfer rate is a ratio of an image transferred from a transfer source (for example, an image carrier) to a transfer destination (for example, an intermediate transfer body), and is preferably higher. In general, a bias is applied between the transfer source and the transfer destination so as to improve the transfer rate. For example, in primary transfer from an image carrier (transfer source) to an intermediate transfer member (transfer destination). However, if the resistance of the intermediate transfer member is low, the current generated by the bias leaks through the intermediate transfer member and the transfer rate decreases. As a technique for improving the transfer rate, techniques for applying a bias having the same polarity as the charged polarity of the image to the transfer source side have been proposed (for example, [Patent Document 4], [Patent Document 9], and [Patent Document 10]). reference).

  The pre-transfer performance is that the image moves from the transfer source to the transfer destination in the so-called pre-transfer area on the upstream side in the image movement direction in the area where the transfer source and transfer destination are close to each other with a small air gap. This is the performance of whether or not the transfer is favorably performed depending on the smoothness of the movement at the time, and the smoother the movement, the better. For example, when an image is composed of toner particles, each toner particle is charged to approximately the same polarity according to its charging characteristics, so if it repels during such movement, it will scatter to the transfer destination, etc. At the transfer destination, the entire image is blurred. In addition, when a bias is applied between the transfer source and the transfer destination, the bias generates a discharge between the transfer source and the transfer destination in the pre-transfer region, and the toner is skipped when the toner is moved due to the discharge. As a result, an image having a white-out portion is generated at the transfer destination. The pre-transfer performance is lower as more blurred images or images with white spots are generated.

As a technique for improving the pre-transfer performance, as described below, a technique for forming a potential gradient corresponding to the moving direction of the image in the transfer portion so as to weaken the bias in the pre-transfer area (for example, [Patent Document 1 ] And [Patent Document 2] and [Patent Document 5]) and a technique for bringing the transfer source and the transfer destination into close contact (for example, see [Patent Document 3] to [Patent Document 8]).
However, in the former technique, in order to form such a potential gradient, in addition to the restriction that the members constituting the transfer portion must not have high resistance, in order to form such a potential gradient, a potential difference is formed. Since at least two members are required, the configuration in which the member that faces the intermediate transfer member and conveys the recording medium in the secondary transfer portion is simply a roller-shaped member (for example, [Patent Document 3], [Patent Document 3] 4)), there is a restriction that such a potential gradient cannot be formed by this member, and the latter technique is not suitable for the use of the secondary transfer portion because transfer blur may occur when applied to the secondary transfer portion. There is a restriction.

On the other hand, an intermediate transfer type image forming apparatus includes a plurality of such image carriers and a so-called tandem type image forming apparatus (for example, [Patents] Document 1] to [Patent Document 4]) and a so-called one-drum type image having a single image carrier and a plurality of developing devices for carrying images on the single image carrier. There are image forming apparatuses (see, for example, [Patent Document 5] to [Patent Document 9]). In recent years, the former image forming apparatus suitable for this has become widespread due to a demand for high-speed image formation.
Since such a tandem type image forming apparatus includes a plurality of such image carriers and such developing devices and tends to increase in size, the size of the tandem type image forming apparatus can be reduced by reducing the interval between the image carriers. It is illustrated.
However, when the interval between the image carriers is reduced, if the intermediate transfer member is not high resistance, a transfer bias generally applied between each image carrier and the intermediate transfer member leaks through the intermediate transfer member, There is a problem that the transfer rate decreases.

  In view of such circumstances, in the tandem type intermediate transfer type image forming apparatus, while suppressing the increase in size, the transfer rate is high in each transfer portion, the pre-transfer performance is good, and the transfer blur is suppressed. No one has yet been proposed that meets all these requirements.

  For example, in a tandem type intermediate transfer type image forming apparatus, if a technique for forming a potential gradient in the primary transfer portion is adopted for an intermediate transfer body (see, for example, [Patent Document 1] and [Patent Document 2]). Since the intermediate transfer member must not have a high resistance in order to form a potential gradient, the transfer bias is transmitted to the intermediate transfer member when the interval between the image carriers is reduced in order to suppress an increase in the size of the image forming apparatus. However, even if the intermediate transfer member is not of high resistance, the transfer bias is intermediate to prevent the transfer bias from passing through the intermediate transfer member and reducing the leakage transfer rate. If a member for preventing leakage through the transfer body is provided between the image carriers (see, for example, [Patent Document 2]), the image forming apparatus becomes large. In the first place, if a technique for forming a potential gradient in the primary transfer portion is adopted for an intermediate transfer member, at least two members for forming a potential difference are required to form such a potential gradient. There is a possibility that the forming apparatus will be enlarged.

  In addition, as described above, the member that faces the intermediate transfer member and conveys the recording medium in the secondary transfer portion is a roller-shaped member (see, for example, [Patent Document 3] and [Patent Document 4]). However, since this potential gradient cannot be formed by this member, the pre-transfer performance is low. However, the recording medium is brought into close contact with the intermediate transfer member using the roller-shaped member for improving the pre-transfer performance. Then, transfer blur occurs. Note that such a roller-shaped member has a problem that the separation performance of the recording medium from the intermediate transfer member tends to be lower than when a belt-shaped member is used instead. This is because the roller-shaped member has lower adhesion to the recording medium than the belt-shaped member.

  In addition, a technique for forming a potential gradient (see, for example, [Patent Document 5]) adopted in an image forming apparatus of a type different from that of a tandem type intermediate transfer type image forming apparatus is a tandem type intermediate transfer type image forming apparatus. When employed in the primary transfer portion of the forming apparatus, there are the same problems as described above.

  In addition, a configuration in which the image transfer body and the intermediate transfer body are brought into close contact with each other at the primary transfer portion, which is used in an image forming apparatus of a different type from the tandem type intermediate transfer type image forming apparatus (for example, can be improved) , [Patent Document 6]) and a configuration (for example, refer to [Patent Document 6]) in which a potential gradient is formed in the intermediate transfer member at the secondary transfer portion capable of improving the pre-transfer performance. However, there is a problem that the resistance value of the intermediate transfer member suitable for these is incompatible, and this is the same in the image forming apparatus of the tandem type and intermediate transfer type. This also means that in the primary transfer portion, a configuration in which the image carrier and the intermediate transfer body are in close contact with each other and a configuration in which a potential gradient is formed (for example, refer to [Patent Document 7] for the former configuration in particular). Is the same.

  The present invention relates to a transfer device provided in a so-called tandem intermediate transfer type image forming apparatus such as a copying machine, a facsimile machine, or a printer, and the primary transfer unit and the secondary transfer unit while suppressing an increase in size. The transfer rate is high, the pre-transfer performance is good, and transfer blur is suppressed, and an image forming apparatus equipped with the transfer device is provided.

In order to achieve the above object, the invention according to claim 1 is a rotating intermediate transfer belt disposed opposite to a plurality of image carriers, to which images carried on the respective image carriers are transferred, A portion that is disposed opposite the intermediate transfer belt and has a substantially flat surface at a portion that faces the intermediate transfer belt, and an image carried on the intermediate transfer belt is transferred to a recording medium that passes through the flat portion. The intermediate transfer belt is wound around each image carrier in a region downstream of the region opposite to the intermediate transfer belt and each image carrier in the rotation direction of the intermediate transfer belt. A plurality of primary transfer members arranged to face a plurality of image carriers through the intermediate transfer belt, and the intermediate transfer belt wound around the intermediate transfer member at a position facing the flat portion. Same as the statue And a secondary transfer counter member for applying a positive bias to the upstream side of the plane portion in the conveyance direction of the recording medium on the same plane side as the image carried on the intermediate transfer body. A potential gradient forming unit that forms a potential gradient with a large voltage, and the surface resistivity of the intermediate transfer belt on the secondary transfer counter member side is high, and the secondary transfer body has the 2 next transfer opposed member and Ri surface resistivity medium resistance der opposite the secondary transfer member transports the recording medium by the rotation without a belt shape, a secondary transfer belt formed of a material having ion conductivity The potential gradient forming means is in contact with the surface of the secondary transfer belt opposite to the secondary transfer counter member, and the upstream member occupying the upstream side in the transport direction and the downstream member occupying the downstream side. And the upstream side Is located on the upstream side of the facing region between the secondary transfer facing member and the flat surface portion or the facing region in the transport direction, and the downstream member is the facing region between the secondary transfer facing member and the flat surface portion Alternatively, a bias having the same polarity as that of the image carried on the intermediate transfer member is applied to the upstream member, and the downstream member is grounded. Or the upstream member is grounded and a bias having a polarity opposite to that of the image carried on the intermediate transfer member is applied to the downstream member, or the intermediate transfer is applied to the upstream member. A bias having the same polarity as that of the image carried on the body and a bias having a polarity opposite to that of the image carried on the intermediate transfer member are applied to the downstream member. In the transfer device in which the potential gradient is formed .

According to a second aspect of the invention, in the transfer apparatus according to claim 1, wherein said secondary transfer opposing member is applying an image of the same polarity of the bias that is carried on the intermediate transfer body under the constant current control It is characterized by.

According to a third aspect of the present invention, the transfer device according to the first or second aspect and a plurality of image carriers, which are arranged to face the intermediate transfer belt and each of the images carried by the transfer device is transferred to the intermediate transfer belt. An image forming apparatus having a body.

The present invention is arranged to face a plurality of image carriers, and to rotate an intermediate transfer belt to which an image carried on each image carrier is transferred, and to be arranged to face the intermediate transfer belt. A secondary transfer member having a substantially flat surface at a portion facing the intermediate transfer belt, and an image carried on the intermediate transfer belt is transferred to a recording medium passing through the flat surface; and the intermediate transfer A plurality of image carriers are provided via the intermediate transfer belt so that the intermediate transfer belt wraps around each image carrier in a region downstream of the opposed region between the intermediate transfer belt and each image carrier in the belt rotation direction. A plurality of primary transfer members arranged to face the body, and a bias having the same polarity as the image carried on the intermediate transfer belt wrapped around the intermediate transfer belt at a position facing the flat portion Secondary transfer facing And a potential that forms a potential gradient at the same polarity side as the image carried on the intermediate transfer member toward the upstream side in the conveyance direction of the recording medium on the same plane portion. The intermediate transfer belt has a high surface resistivity on the side of the secondary transfer counter member, and the surface resistance of the secondary transfer body on the side opposite to the secondary transfer counter member. rate is Ri medium resistance der, the secondary transfer member transports the recording medium by the rotation without a belt shape, a secondary transfer belt formed of a material having ion conductivity, the potential gradient forming means, An upstream member that occupies the upstream side in the transport direction and a downstream member that occupies the downstream side in contact with the surface opposite to the secondary transfer counter member of the secondary transfer belt; The secondary transfer counter member and the planar portion The downstream member is located upstream of the opposing region in the transport direction or the opposing region in the transport direction, and the downstream member is downstream of the opposing region of the secondary transfer counter member and the flat portion or the counter region in the transport direction. And a bias having the same polarity as that of the image carried on the intermediate transfer member is applied to the upstream member and the downstream member is grounded, or the upstream member is grounded. And a bias having a polarity opposite to that of the image carried on the intermediate transfer member is applied to the downstream member, or a bias having the same polarity as that of the image carried on the intermediate transfer member to the upstream member. And a bias having a polarity opposite to that of the image carried on the intermediate transfer member is applied to the downstream member, whereby the potential gradient is formed. Since the apparatus is in the apparatus, a potential gradient is formed by the upstream member and the downstream member , which is a transfer apparatus used in a so-called tandem type intermediate transfer type image forming apparatus. It is possible to provide a transfer device that has a high transfer rate and good pre-transfer performance in the secondary transfer unit and the secondary transfer unit, and can suppress transfer blur in a recording medium conveyed by the secondary transfer belt. .

  If the secondary transfer counter member applies a bias having the same polarity as the image carried on the intermediate transfer belt under constant current control, the secondary transfer counter member is used in a so-called tandem type intermediate transfer type image forming apparatus. The transfer device is capable of suppressing the increase in size, and the transfer rate is higher in the secondary transfer portion of the primary transfer portion and the secondary transfer portion, the pre-transfer performance is good, and the transfer blur is suppressed. It is possible to provide a transfer device that can perform the transfer.

  The present invention resides in an image forming apparatus having such a transfer device and a plurality of image carriers that are arranged to face the intermediate transfer belt and each of the images carried by the transfer device is transferred to the intermediate transfer belt. In addition, while suppressing the increase in size, the transfer rate is high in the primary transfer portion and the secondary transfer portion, the pre-transfer performance is good, and the transfer blur can be suppressed, so that the increase in size is suppressed. A so-called tandem type intermediate transfer type image forming apparatus capable of image formation can be provided.

1 is a schematic front view of an image forming apparatus to which the present invention is applied. FIG. 2 is a schematic front view showing a configuration around an image carrier provided in the image forming apparatus shown in FIG. 1. FIG. 2 is a schematic front view of a part of a transfer device provided in the image forming apparatus shown in FIG. 1. FIG. 4 is a schematic front view of a part of a transfer device having a configuration different from that of the transfer device shown in FIG. 3. FIG. 5 is a schematic front view of a part of a transfer device having a configuration different from that of the transfer device shown in FIGS. 3 and 4. FIG. 6 is a schematic front view of a part of a transfer device having a different configuration from the transfer device shown in FIGS. 3 to 5. FIG. 7 is a schematic front view of a part of a transfer device having a configuration different from that of the transfer device shown in FIGS. 3 to 6. FIG. 8 is a schematic front view of a part of a transfer device having a configuration different from the transfer device shown in FIGS. 3 to 7. FIG. 9 is a schematic front view of a part of a transfer device having a configuration different from the transfer device illustrated in FIGS. 3 to 8. FIG. 10 is a schematic front view of a part of a transfer device having a different configuration from the transfer device shown in FIGS. 3 to 9.

  FIG. 1 shows an outline of an image forming apparatus to which the present invention is applied. The image forming apparatus 100 is a multifunction peripheral of a copying machine, a printer, and a facsimile machine, and can perform full-color image formation, but other image forming apparatuses, that is, a monochrome machine, a copying machine, and a printer. It may be a single facsimile, or another multi-function machine such as a copier / printer multi-function machine. When used as a printer, the image forming apparatus 100 performs an image forming process based on an image signal corresponding to image information received from the outside. This is the same when the image forming apparatus 100 is used as a facsimile.

  The image forming apparatus 100 forms an image on a sheet-like recording medium that is a sheet-like recording medium such as plain paper generally used for copying, OHP sheets, cardboard, cardboard, cardboard, and envelopes. It is possible. The image forming apparatus 100 is also a double-sided image forming apparatus capable of forming an image on both sides of a transfer sheet as a recording medium as a recording medium.

  The image forming apparatus 100 includes a main body 101 that occupies a central position in the vertical direction and functions as a printer unit, a reading device 21 that is positioned above the main body 101 and that reads a document, and a document on which a document is stacked and loaded. An automatic document feeder 22 which is an automatic document feeder called ADF which is sent out toward 21, and the photosensitive drums 20 </ b> Y, 20 </ b> M, 20 </ b> C, 20 </ b> BK located below the main body 101 and the transfer belt 11. A sheet feeding device 23 serving as a paper feeding device serving as a paper feeding table on which the transfer paper to be conveyed is stacked.

  The image forming apparatus 100 includes a plurality of photosensitive drums 20Y, 20M, 20C, and 20BK that can form images as images corresponding to colors separated into yellow, magenta, cyan, and black, respectively. 1 is a tandem type image forming apparatus that employs a tandem structure in which the two are arranged side by side. The photoconductive drums 20Y, 20M, 20C, and 20BK are OPCs having an outer diameter of 60 mm and have the same diameter, and are intermediate transfers that are endless belts disposed at substantially the center of the inside of the main body 101 of the image forming apparatus 100. They are arranged at equal intervals of 150 mm on the outer peripheral surface side of the transfer belt 11 serving as a body intermediate transfer belt, that is, on the image forming surface side.

  The transfer belt 11 is movable in the arrow A1 direction, which is the clockwise direction in the figure, while facing the respective photosensitive drums 20Y, 20M, 20C, and 20BK. The visible image, that is, the toner image formed on each of the photoconductive drums 20Y, 20M, 20C, and 20BK is superimposed and transferred onto the transfer belt 11 that moves in the direction of the arrow A1, and is then collectively transferred onto the transfer paper. It has become. As described above, the image forming apparatus 100 employs an intermediate transfer method, in other words, an indirect transfer method.

  In the superimposing transfer to the transfer belt 11, the toner images formed on the photosensitive drums 20Y, 20M, 20C, and 20BK are transferred to the same position on the transfer belt 11 while the transfer belt 11 moves in the A1 direction. As described above, a primary transfer member serving as a transfer charger is disposed at a position facing each of the photosensitive drums 20Y, 20M, 20C, and 20BK with the transfer belt 11 interposed therebetween, and is disposed in contact with the inner peripheral surface of the transfer belt 11. By applying voltage by the primary transfer rollers 12Y, 12M, 12C, and 12BK, the timing is shifted from the upstream side toward the downstream side in the A1 direction, that is, the lower positions of the photosensitive drums 20Y, 20M, 20C, and 20BK, that is, Performed at the transfer position.

  The photosensitive drums 20Y, 20M, 20C, and 20BK are arranged in this order from the upstream side in the A1 direction. The photosensitive drums 20Y, 20M, 20C, and 20BK are provided in image stations 60Y, 60M, 60C, and 60BK, respectively, for forming yellow, magenta, cyan, and black images.

  The image forming apparatus 100 is arranged opposite to the image forming unit 60 as an image forming unit constituted by four image stations 60Y, 60M, 60C, and 60BK, and below the respective photosensitive drums 20Y, 20M, 20C, and 20BK. And a transfer device 99 as transfer means for transferring the toner images carried on the photoconductive drums 20Y, 20M, 20C, and 20BK onto transfer paper.

  The image forming apparatus 100 also includes an optical scanning device 8 serving as an exposure device serving as an optical writing unit serving as an optical writing device serving as an optical writing device disposed above the image stations 60Y, 60M, 60C, and 60BK, and a sheet. A pair of registration rollers 13 for feeding the transfer paper conveyed from the feeding device 23 toward the transfer device 99 at a predetermined timing in accordance with the toner image formation timing by the image stations 60Y, 60M, 60C, and 60BK, and the transfer A sensor (not shown) that detects that the leading edge of the paper has reached the registration roller pair 13.

  The image forming apparatus 100 also fixes as a fixing unit of a belt fixing system for transferring the toner image on the transfer belt 11 transferred by the transfer device 99 and feeding the transfer paper and fixing the toner image on the transfer paper. A discharge unit 79 that transports the transfer paper to one of the paths, and includes a discharge path for discharging the fixed transfer paper to the outside of the main body 101 and a reverse path for transporting the transfer paper toward the registration roller pair 13 again. When the paper discharge unit 79 transports the transfer paper having an image formed on one side thereof to the reversing path, the transfer paper is switched back to be reversed and transported again toward the registration roller pair 13. And a duplex unit 96 that is a sheet reversing device as a paper unit.

  The image forming apparatus 100 also includes a paper discharge tray 75 as a stack unit that is disposed outside the main body 101 and stacks transfer sheets on which image formation has been performed, and a manual sheet feeding device disposed on the right side surface of the main body 101 in FIG. 33, an operation panel (not shown) that operates the image forming apparatus 100, and a control unit (not shown) as a control unit that controls the operation of the entire image forming apparatus 100.

  The transfer device 99 is disposed so as to face the transfer belt 11 and is in contact with the transfer belt unit 10 as a primary transfer device as a belt unit that is an intermediate transfer unit including the transfer belt 11. The toner image on the transfer belt 11 is transferred to a transfer paper having a secondary transfer belt 5 as a paper transport belt as a transfer member that rotates endlessly in the E1 direction which is the same direction as the transfer belt 11 at the contact position with the belt 11. And a secondary transfer unit 76 as a secondary transfer device that transfers and conveys the toner to the printer.

  In addition to the transfer belt 11, the transfer belt unit 10 is driven as a primary transfer roller 12Y, 12M, 12C, 12BK and a tension roller 72, a transfer inlet roller 73, and a tension roller around which the transfer belt 11 is wound. The transfer belt 11 is sandwiched between the roller 74 and the drive roller 74, and the transfer belt 11 is moved endlessly in the A1 direction by rotating the drive roller 72 and the intermediate transfer belt cleaning device 14 for cleaning the surface of the transfer belt 11. A belt drive motor (not shown) and a transfer belt unit drive means (not shown) for displacing the transfer belt 11 and the like according to whether image formation is performed with a multicolor image of two or more colors or a monochrome image of only black, that is, a monochrome image And the primary transfer rollers 12Y, 12M, 12C, and 12BK, respectively. It has a high voltage power supply (not shown), biasing the primary transfer roller 12Y of the positive polarity which is the constant current control from the high voltage power supply, 12M, 12C, and a bias control means (not shown) for applying to 12BK.

The transfer belt 11 includes primary transfer rollers 12Y, 12M, 12C, and 12BK, a tension roller 72, a transfer inlet roller 73, and a driving roller in which conductive fine particles are mixed in a hard resin such as polyimide, polyimide amide, or polycarbonate, and wound thereon. The surface resistivity of the back surface which is the surface in contact with 74, that is, the inner surface, is set to a high resistance, specifically, 10 ⁹ to 10 ¹⁰ Ω / □ or more and 10 ¹² to 10 ¹³ Ω / □ or less. The thickness is reduced to 50 to 100 μm.

  The surface resistivity of the transfer belt 11 is measured by using “Hiresta IP (MCP-HT260)” manufactured by Mitsubishi Oil Chemical Co., Ltd. as a measuring device, and contacting the HRS probe with the back surface of the transfer belt 11 as a measuring surface. This is the value when the measurement time is 500 V and the measurement time is 10 seconds. The same applies to the secondary transfer belt 5.

  As will be described later, the transfer belt 11 is arranged in such a manner that it winds around the photoconductive drums 20Y, 20M, 20C, and 20BK with a predetermined width. However, the transfer belt 11 is made of such a hard material and has a small thickness. Further, while maintaining the mechanical durability, the adhesion to the photosensitive drums 20Y, 20M, 20C, and 20BK is improved, and a minute gap with the photosensitive drums 20Y, 20M, 20C, and 20BK is suppressed.

  Due to such resistance, the transfer belt 11 has a current in the thickness direction from the back surface to the surface when a primary transfer bias is applied by the primary transfer rollers 12Y, 12M, 12C, and 12BK, as will be described later. And hardly flows in the A1 direction, and there is no current leakage between the primary transfer rollers 12Y, 12M, 12C, and 12BK. In addition, since the transfer belt 11 is thin, even if there is some resistance unevenness in the A1 direction, the potential difference unevenness is small. In addition, since the current hardly flows in the A1 direction, the constant current control is performed, so that the transfer rate is maintained and the photosensitive drums 20Y, 20M, 20C, and 20BK are connected to each other. And the distance between the primary transfer rollers 12Y, 12M, 12C, and 12BK can be reduced to reduce the size of the image forming apparatus 100.

Although not shown in detail, the primary transfer rollers 12Y, 12M, 12C, and 12BK have a core metal having a diameter of 8 mm to which a high-voltage power supply is connected, and a resistance layer disposed around the core metal. The outer diameter is 16 mm. The resistance layer is made of a copolymer of acrylonitrile butadiene and epichlorohydrin, which has little resistance unevenness depending on the location and has the property of medium resistance, and has a resistance of 10 ⁷ to 10 ⁸ Ω. The resistance layer is formed by foaming in order to give elasticity to the primary transfer rollers 12Y, 12M, 12C, and 12BK, and the hardness is 30 to 50 degrees in Asker C hardness. The resistance of each of the primary transfer rollers 12Y, 12M, 12C, 12BK and other rollers is such that the roller is placed on a metal flat plate, and the high voltage output terminal of the high voltage power supply “Trek COL-A-TROL (Model 610D)” is the roller core. Connect to gold, connect the ground terminal to the metal flat plate, and when the output of this high-voltage power supply is controlled to a constant current of 25 μA, the output voltage displayed on the high-voltage power supply is ohms from the value 10 seconds after the start of output. It is a value obtained by the law of.

  When the image formation is performed with two or more multi-color images, the transfer belt unit driving means places the transfer belt 11 in a state where the transfer belt 11 is stretched substantially horizontally as shown in FIG. When contact is made with 20Y, 20M, 20C, and 20BK and image formation is performed in a monochrome image, the transfer belt 11 is moved away from the photosensitive drums 20Y, 20M, and 20C so that the left side of the figure is displaced downward. It is assumed that the belt 11 is in contact with only the photosensitive drums 20BK. When the image formation is performed with a monochrome image, the operations of the image stations 60Y, 60M, and 60C are stopped, including the rotation of the photosensitive drums 20Y, 20M, and 20C.

  In addition to the secondary transfer belt 5, the secondary transfer unit 76 is a tension roller around which the secondary transfer belt 5 is wound. The secondary transfer belt 5 is sandwiched between a drive roller 15 and a transfer entrance roller 73 that is a backup roller as a secondary transfer counter roller as a counter roller that is provided and pressed via the transfer belt 11. A secondary transfer belt cleaning device 17 as a cleaning device for cleaning the surface, and a power source 83 for applying a negative secondary transfer bias having the same polarity as that of the toner to the transfer entrance roller 73 as shown in FIG. A potential gradient forming means 84 for forming a potential gradient that becomes a large voltage on the negative polarity side toward the upstream side in the E1 direction, and a bias controlled by a constant current from the power source 83 are transferred to the transfer inlet And a bias control means (not shown) for applying the la 73.

  The secondary transfer belt 5 is wound only on the driving roller 15 and the driven roller 16, and no member that slides on the secondary transfer belt 5 is provided. Therefore, the material of the secondary transfer belt 5 does not have to have high wear resistance. However, it is desirable that the secondary transfer belt 5 has a uniform potential gradient formed by the potential gradient forming unit 84. Therefore, the secondary transfer belt 5 is formed of a material having ionic conductivity and little resistance unevenness such as polyvinylidene fluoride and acrylonitrile butadiene.

The secondary transfer belt 5 contacts the opposite side of the transfer entrance roller 73, that is, the driving roller 15 and the driven roller 16 around which the secondary transfer belt 5 is wound so that the potential gradient is formed by the potential gradient forming unit 84. The surface resistivity of the back surface which is the side surface, that is, the inner surface, is set to a medium resistance, specifically, 10 ⁶ to 10 ⁷ Ω / □ or more and 10 ⁹ to 10 ¹⁰ Ω / □ or less.

  The driving roller 15 and the driven roller 16 stretch the secondary transfer belt 5 between them, and the surface of the secondary transfer belt 5 that faces the transfer entrance roller 73 and the portion of the transfer belt 11 wound around the transfer entrance roller 73. Is a plane portion 5a having a substantially planar shape. The driving roller 15 and the driven roller 16 are respectively in the positional relationship in the E1 direction, the upstream roller as an upstream member that is an upstream electrode that occupies the upstream side across the plane portion 5a, and the downstream side that occupies the downstream side It is a downstream roller as a downstream member which is an electrode.

  The transfer inlet roller 73 is wound around the transfer belt 11 at a position facing the flat surface portion 5a, and the same as the charging polarity of the toner carried on the transfer belt 11 by the power supply 83 and the bias control means. It functions as a repulsive roller that generates a repulsive force against the toner by applying a polar bias.

The transfer entrance roller 73 is similar in material and resistance to the primary transfer rollers 12Y, 12M, 12C, and 12BK. Specifically, the transfer entrance roller 73 has a core metal having a diameter of 16 mm to which a power supply 83 is connected, and a resistance layer disposed around the core metal, although the detailed illustration is omitted. The resistance layer is made of acrylonitrile butadiene, epichlorhydrin, or a copolymer thereof having little resistance unevenness depending on the location and having the property of medium resistance, and the resistance is 10 ⁷ to 10 ⁸ Ω. is there. Unlike the other forms described later, this embodiment does not include a roller disposed opposite to the transfer entrance roller 73. Therefore, the resistance layer does not need to be elastic, and thus does not foam. The hardness is 40 to 60 degrees as JIS-A hardness.

  The transfer entrance roller 73 causes the transfer belt 11 to contact the secondary transfer belt 5 at the flat surface portion 5a, and a secondary transfer nip where the transfer belt 11 and the secondary transfer belt 5 come into contact with each other is formed by this contact. Has been. A secondary transfer bias is applied by the transfer inlet roller 73 and a potential gradient forming means is applied to a secondary transfer portion as a transfer portion including a secondary transfer nip, which is a facing region where the transfer entrance roller 73 and the flat portion 5a face each other. By forming a potential gradient by 84, a secondary transfer electric field is formed that electrostatically moves the toner image carried on the transfer belt 11 from the transfer belt 11 side to the drive roller 15 side as described later. . The toner image is secondarily transferred to the transfer paper conveyed between the transfer belt 11 and the secondary transfer roller 5 by the registration roller 13 by a secondary transfer electric field and nip pressure as will be described later.

  As described above, the secondary transfer belt 5 functions as a secondary transfer body on which the toner image carried on the transfer belt 11 is secondarily transferred onto the transfer paper conveyed thereby and passing through the flat surface portion 5a. Functions as a conveyor belt.

  The potential gradient forming unit 84 is connected to the secondary transfer belt 5, the driving roller 15, the driven roller 16, and a power supply 85 that is connected to the driving roller 15 and applies a negative bias having the same polarity as the toner charging polarity. And a power source 86 that is connected to the driven roller 16 and applies a positive polarity bias having a polarity opposite to the charging polarity of the toner to the driven roller 16.

  Therefore, the potential gradient forming unit 84 forms a potential gradient with a large voltage on the negative polarity side, which is the same polarity as the charging polarity of the toner, toward the upstream side in the E1 direction with respect to the flat portion 5a. This potential gradient causes the bias applied by the transfer entrance roller 73 to be canceled in the secondary transfer portion. The secondary transfer bias in the pre-transfer area included in the secondary transfer portion is weakened, and the pre-transfer is performed. Improve performance. Since the transfer entrance roller 73 functions as a repulsive roller at a position facing the flat portion 5a, in combination with the constant current control, the transfer rate is stably secured and the transfer blur is suppressed. Or prevented. Since the potential gradient is formed with respect to the secondary transfer belt 5, there is no contradiction between making the transfer belt 11 high resistance and making the secondary transfer belt 5 medium resistance. .

  Note that the magnitude of the bias in the secondary transfer portion varies depending on the bias value by the power sources 85 and 86, the distance between the power sources 85 and 86 and the secondary transfer portion, the bias value by the power source 83, and the like. As a result, the polarity of the bias changes whether or not it becomes the same polarity as the charging polarity of the toner. However, since the effective transfer current from the transfer belt 11 to the transfer paper does not change because the bias by the power supply 83 is controlled at a constant current, the transfer rate is stabilized as described above.

  The constant current control is advantageous in that the high-voltage power supply and control required for this are simple, inexpensive, and highly stable. However, the present invention is not limited to this, and for example, based on the drive timing of the registration roller pair 13, the transfer paper It is also possible to control the constant current only at the leading edge of the sheet, detect the voltage at the leading edge of the transfer paper, and perform constant voltage control after the leading edge.

  The optical scanning device 8 shown in FIG. 1 is an LD unit, and scans and exposes the surfaces to be scanned formed by the surfaces of the photosensitive drums 20Y, 20M, 20C, and 20BK, thereby forming an electrostatic latent image. A light source (not shown) that emits a laser beam (not shown) as a laser beam based on an image signal, a polygon mirror (not shown) that scans the laser light emitted by the light source by its rotation, and a polygon mirror (not shown) that drives the polygon mirror to rotate It has a polygon motor and a number of optical elements (not shown) such as an f-θ lens (not shown) and a reflection mirror that form an image on the photosensitive drums 20Y, 20M, 20C, and 20B and scan the laser light scanned by the polygon mirror. doing.

  The fixing device 6 includes a heating roller 62 having a heat source therein, a fixing belt 64 wound around the heating roller 62, a fixing roller 65 wound around the fixing belt 64 together with the heating roller 62, and the fixing roller 65. And a pressure roller 63 that presses against the fixing belt 64 and forms a fixing nip as a fixing portion that is a pressing portion. The heating roller 62, the fixing belt 64, and the fixing roller 65 constitute a belt unit in which the fixing belt 64 moves endlessly. The fixing device 6 passes the transfer paper carrying the toner image through the fixing nip, thereby fixing the carried toner image on the surface of the transfer paper by the action of heat and pressure.

  A paper discharge unit 79 transports the fixed transfer paper transported from the fixing device 6 toward the duplex unit 96, a paper discharge roller 98 that discharges the paper to the outside of the main body 101, and a fixed transfer. There is provided a switching claw 94 for switching whether the paper is guided to the paper discharge path with the conveying roller 97 and discharged out of the main body 101 or the paper is guided to the reverse path with the paper discharge roller 98 to enter the duplex unit 96.

  The duplex unit 96 is a tray 92 on which transfer paper having an image formed on one side, which has been conveyed from the paper discharge unit 79, is temporarily stacked, a reverse roller 93 that switches back the transfer paper on the tray 92, and a reverse roller. The sheet feeding roller 95 and the like for feeding the transfer sheet switched back by 93 to the registration roller 13 are provided.

  The sheet feeding device 23 includes a paper bank 26 in which a plurality of sheet feeding cassettes 25 that can store a plurality of transfer sheets in a bundle of sheets are arranged in a vertical direction, and a plurality of transfer sheets stacked on the sheet feeding cassette 25. Among them, a feeding roller 24 as a sheet feeding roller that contacts the upper surface of the uppermost transfer sheet, a separation roller 27 that separates the transfer sheets fed by the feeding roller 24 one by one, a sheet feeding roller 24 and a separation roller And a paper feed path 29 through which the transfer paper fed by the roller 27 passes.

  The paper feed path 29 includes a transport roller 28 that transports the transfer paper fed by the paper feed roller 24 and the separation roller 27 toward the registration roller pair 13, and a registration roller pair 13 that is located at the downstream end in the transport direction of the transfer paper. Are provided so as to continue from the sheet feeding device 23 into the main body 101, and a conveying roller 28 is also provided in a paper feed path 29 in the main body 101.

  In the sheet feeding device 23, the feeding roller 24 is rotated in the counterclockwise direction in the drawing, and the separation roller 27 acts to guide the uppermost transfer paper into the paper feeding path 29, so that the conveying roller 28 The sheet is fed toward the registration roller pair 13 by rotation, and the transferred transfer paper is abutted against the registration roller pair 13 and stopped.

  The manual paper feed device 33 includes a manual feed tray 34 on which transfer paper is stacked, a feed roller 35 as a paper feed roller that contacts the upper surface of the uppermost transfer paper among the transfer papers stacked on the manual feed tray 34, It has a separation roller 36 for separating the transfer paper fed by the feed roller 35 one by one, and a paper sensor for detecting that the transfer paper is placed on the manual feed tray 34.

  In the manual sheet feeding device 33, the feeding roller 35 is driven to rotate in the clockwise direction in the drawing, and the separation roller 36 acts to guide the uppermost transfer sheet into the sheet feeding path 29 on the main body 101 side and to register the sheet. The transfer paper fed to the roller pair 13 is transported against the registration roller pair 13 and stopped.

The reading device 21 includes a contact glass 21a on which a document is placed, a light source (not shown) that irradiates light on the document placed on the contact glass 21a, and a first light source (not shown) that reflects light reflected from the light source. A first traveling body 21b that includes a reflector and travels in the left-right direction in FIG. 1; a second traveling body 21c that includes a second reflector (not shown) that reflects light reflected by the reflector of the first traveling body 21b; An image forming lens 21d for forming an image of light from the second traveling body 21c, a reading sensor 21e for receiving the light passing through the image forming lens 21d and reading the contents of the document are provided.
The automatic document feeder 22 is rotatable, and the contact glass 21a is exposed when the automatic document feeder 22 is rotated upward.

The operation panel is an operation display unit having a liquid crystal display, a start button that functions as a copy start switch for starting copying, and various key buttons such as a numeric keypad for inputting the number of copies. On the operation panel, it is possible to specify whether image formation is performed with a multicolor image or a monochrome image, and it is possible to set a single-sided print mode, which is a mode for forming an image only on one side of a transfer sheet. Yes. The single-sided print mode includes a direct discharge mode, a reverse discharge mode, and a reverse decal discharge mode, and one of these is selected.
The control means includes a CPU, a memory as a storage means, and the like.

  Regarding the image stations 60Y, 60C, 60M, and 60BK, the configuration will be described as a representative of the configuration of the image station 60Y including the photosensitive drum 20Y. Since the configuration of other image stations is substantially the same, in the following description, for the sake of convenience, reference numerals corresponding to the reference numerals assigned to the configuration of the image station including the photosensitive drum 20Y are used for other image stations. A detailed description will be omitted as appropriate for the station configuration, and those with Y, C, M, and K at the end of the reference numerals are used to form yellow, cyan, magenta, and black images, respectively. It is assumed that this is the configuration.

  As shown in FIG. 2, the image station 60Y including the photosensitive drum 20Y has a primary transfer roller 12Y and a cleaning member around the photosensitive drum 20Y along the rotation direction B1 that is counterclockwise in the drawing. It has a cleaning device 40Y as a means, a charging device 30Y as a charging unit as a charging means, and a developing device 50Y as a developing unit as a developing means.

  The photosensitive drum 20Y, the cleaning device 40Y, the charging device 30Y, and the developing device 50Y are integrated to form a process cartridge 95Y. The process cartridge 95Y can be pulled out of the main body 101 along a guide rail (not shown) fixed to the main body 101, can be pushed into the main body 101, and is detachably installed on the main body 101. .

  When the process cartridge 95Y is pushed into the main body 101, it is loaded and positioned at a predetermined position suitable for image formation. Making a process cartridge in this way is very preferable because it can be handled as a replacement part, so that the maintainability is remarkably improved. The process cartridges 95Y, 95M, 95C, and 95BK constitute an image forming unit.

  The process cartridge 95Y includes at least the photosensitive drum 20Y and the developing device 50Y among the photosensitive drum 20Y, the cleaning device 40Y as a drum cleaning device, the charging device 30Y as a developing device as a charging unit, and the developing device 50Y. Are units that are integrated and are detachably installed on the main body 101.

The charging device 30Y includes a charging roller 31Y that rotates following the photosensitive drum 20Y, and a voltage applying unit (not shown) that applies a bias of an alternating current component to a direct current by the charging roller 31Y, and faces the photosensitive drum 20Y. In the charging region, the surface of the photosensitive drum 20Y is neutralized and at the same time, is uniformly charged to a predetermined polarity.
In this embodiment, a contact roller type charging system is employed, but the charging system may be a proximity roller type or a non-contact type.

The primary transfer roller 12Y has a central position in the A1 direction so that the transfer belt 11 wraps around the photosensitive drum 20Y in a region downstream of the transfer belt 11 and the photosensitive drum 20Y in the A1 direction. Is located downstream of the center position of the photoconductor drum 20Y, and is in contact with the photoconductor drum 20Y via the transfer belt 11 so that the center position is directed to the center position of the photoconductor drum 20Y.
The transfer belt 11 has a predetermined width from a position where the transfer belt 11 starts to contact the photosensitive drum 20Y to a position where the transfer belt 11 starts to move away from the photosensitive drum 20Y from the upstream side toward the downstream side in the A1 direction. The photoconductive drum 20Y is wound around and closely contacts. This width is set in a range of 2 to 6 mm by adjusting the arrangement position of the primary transfer roller 12Y.

  A predetermined voltage suitable for primary transfer is applied to the primary transfer roller 12Y by a high voltage power source and a bias control means, and the primary transfer roller 12Y functions as a primary transfer bias roller. It has become. The primary transfer roller 12Y presses the transfer belt 11 from the inner peripheral surface side to the photosensitive drum 20Y to form a primary transfer nip. A primary transfer electric field is formed in the primary transfer portion as a transfer portion including the primary transfer nip, which is a facing region where the primary transfer roller 12Y and the transfer belt 11 face each other, due to the influence of the primary transfer bias. The

  In the case where the primary transfer bias is applied by the primary transfer roller 12Y, in the primary transfer roller 12Y, a current flows through a distance corresponding to the thickness of the resistance layer from the core metal to the surface of the resistance layer. The transfer belt 11 is about 100 times as thick as the transfer belt 11 and the potential difference between the cored bar and the resistance layer surface is increased. Has been reduced.

  As described above, the transfer belt 11 has a predetermined width from a position where the transfer belt 11 starts to contact the photosensitive drum 20Y to a position where the transfer belt 11 starts to separate from the photosensitive drum 20Y from the upstream side toward the downstream side in the A1 direction. In this area, the pre-transfer area does not occur or becomes small in the primary transfer portion where the primary transfer roller 12Y and the transfer belt 11 face each other. Pre-transfer performance is improved. Further, as described above, since the transfer belt 11 has a high resistance, it is possible to reduce the size of the image forming apparatus 100 while maintaining the transfer rate, and the transfer belt 11 has a high resistance. There is no contradiction between making the secondary transfer belt 5 medium resistance in order to form the potential gradient by the potential gradient forming means 84.

  As shown in FIG. 2, the optical scanning device 8 shown in FIG. 1 irradiates a region between the charging region and the development region D on the photosensitive drum 20Y with a laser beam L that is light-modulated according to image information. Then, the surface of the photosensitive drum 20Y charged by the charging device 30Y is exposed to form an electrostatic latent image that is visualized as a yellow toner image by the developing device 50Y.

  The cleaning device 40Y has a cleaning case 43Y having an opening at a portion facing the photoconductor drum 20Y, and scrapes off unnecessary materials such as residual toner, carrier, and paper dust on the photoconductor drum 20Y in contact with the photoconductor drum 20Y. The brush roller 45Y as a rotating brush to be cleaned and the rotating direction B1 of the photosensitive drum 20Y contact the photosensitive drum 20Y at a position downstream of the brush roller 45Y and scrape off unnecessary matter on the photosensitive drum 20Y. And a cleaning blade 41Y as a blade for cleaning.

  The cleaning device 40Y is also rotatably supported by the cleaning case 43Y, and scrapes the brush roller 45Y and the cleaning blade 41Y, and removes unnecessary materials such as waste toner generated by removal to a waste toner tank (not shown). And a discharge screw 42Y constituting a part of a waste toner path (not shown) for transporting the toner.

  The developing device 50Y includes a developing case 55Y having an opening at a portion facing the photoreceptor drum 20Y, and a developer carrying member disposed in close proximity to the photoreceptor drum 20Y so as to face the photoreceptor drum 20Y from the opening. A developing roller 51Y as a body, and a developing blade 52Y as a doctor as an agent regulating member that regulates the developer on the developing roller 51Y to a certain height.

  The developing device 50Y is also disposed below the developing case 55Y so as to face each other, and is driven to rotate in opposite directions to stir the developer and to supply the developer to the developing roller 51Y. The first conveying screw 53Y and the second agitating screw 54Y as supply members, the partition wall 57Y provided between the first conveying screw 53Y and the second conveying screw 54Y, and the first conveying screw partitioned by the partition wall 57Y. 53Y, and a second storage chamber 59Y that constitutes a developer storage section as a developer storage device that stores the second transport screw 54Y.

  The developing device 50Y also includes a toner hopper 80Y that stores yellow toner, and a T sensor as a toner concentration measuring sensor that is provided at the bottom of the second storage chamber 59Y and serves as a toner concentration detecting unit that measures the toner concentration in the developer. And a toner concentration detection sensor 56Y as a toner concentration detection device.

  The developing device 50Y also includes a bias applying unit (not shown) that applies a developing bias of a DC component, a developing driving unit (not shown) that drives the developing roller 51Y, and the first transport screw 53Y and the second transport screw 54Y in opposite directions. (Not shown) for supplying toner to the second storage chamber 59Y from the toner hopper 80Y.

  The developing roller 51Y includes a magnet roller 81Y as a magnetic field generating unit, and a nonmagnetic developing sleeve 82Y that includes the magnet roller 81Y and is driven and moved in the C1 direction, which is the clockwise direction in the drawing, by the developing driving unit. . In this embodiment, the developing roller 51Y is described as a developer carrier, but the developing sleeve 82Y may be a developer carrier.

  Although not shown, the magnet roller 81Y has a plastic roller fixed to the developing case 55Y and a magnet block that is a plurality of magnets forming a plurality of magnetic poles embedded in the plastic roller. A magnetic pole having the polarity shown in FIG. 2 is formed at the position shown in FIG.

  The developing sleeve 82Y is rotatably supported by the developing case 55Y and the magnet roller 81Y. A developing bias of an appropriate size is applied between the developing sleeve 82Y and the photosensitive drum 20Y by a bias applying unit. The gap between the developing sleeve 82Y and the photosensitive drum 20Y, that is, the developing gap in the developing region D, which is a facing region where the developing sleeve 82Y and the photosensitive drum 20Y face each other, is set to be 0.3 ± 0.05 mm. Yes.

  The developing blade 52Y is formed of a SUS material. The gap between the developing sleeve 82Y and the developing blade 52Y, that is, the doctor gap, is set to be 0.5 ± 0.04 mm, so that excess developer is scraped off and an appropriate amount of developer enters the developing area D. It has become.

The developer is a powdery two-component developer containing toner and carrier.
The carrier is a magnetic carrier, and has a core material and a resin coating layer formed on the surface of the core material. The resin coating layer contains conductive particles in which a conductive coating layer comprising a tin dioxide layer and an indium oxide layer containing tin dioxide provided on the tin dioxide layer is provided on the surface of the substrate particles. .

  The normal charging polarity of the toner is negative, that is, negative polarity. The toner is a magnetic toner but may be a non-magnetic toner, and is mainly composed of a binder resin, a release agent, and a colorant. The binder resin contains a hybrid resin having a vinyl polymer and a polyester polymer. The content of the hybrid resin with respect to the content of the release agent is formed to be in the range of 0.5 to 3.

  The toner concentration in the developer is controlled to be within a predetermined range of about 4 to 11% by weight based on detection by the toner concentration detection sensor 56Y and controlled by the control means described below, and the toner is mixed with the carrier. The ratio is always kept at an appropriate value, which contributes to obtaining a high quality image. The control by the control means means that when the toner density decreases as the toner is consumed by the development, and the toner density detection sensor 56Y detects that the toner density falls below the lower limit of the above-mentioned predetermined range, The toner is supplied from the hopper 80Y to the second storage chamber 59Y by the toner supply means.

  The first transport screw 53Y and the second transport screw 54Y are disposed so as to extend in the width direction of the developing roller 51Y, in other words, in the direction perpendicular to the paper surface in FIG. 2, which is the longitudinal direction of the developing roller 51Y.

  The first transport screw 53Y is rotationally driven by the transport driving means, and supplies the developer in the first storage chamber 58Y to the developing roller 51Y while transporting the developer from the back side to the front side in FIG. The developer transported to the vicinity of the end in the first storage chamber 58Y by the first transport screw 53Y enters the second storage chamber 59Y through an opening (not shown) formed in the partition wall 57Y.

  In the second storage chamber 59Y, the second transport screw 54Y is rotationally driven by the transport drive means to transport the developer sent from the first storage chamber 58Y in the opposite direction to the first transport screw 53Y. . At this time, when the toner is replenished from the toner hopper 80Y, the replenished toner is conveyed while being agitated and mixed in the developer. The developer transported to the vicinity of the end of the second storage chamber 59Y by the second transport screw 54Y returns to the first storage chamber 58Y through another opening (not shown) provided in the partition wall 57Y.

  The toner thus supplied is agitated and mixed by the first conveying screw 53Y and the second conveying screw 54Y while being agitated and conveyed with the developer, frictionally charged, and supplied and carried on the developing roller 51Y.

  The developing roller 51Y, whose developer amount is regulated by the developing blade 52Y and whose layer thickness is regulated, is developed in the developing region D by the developing blade 52Y by the developing bias by the rotation and bias applying means. The yellow toner in the developer is electrostatically transferred to the electrostatic latent image formed on the surface of the photosensitive drum 20Y, and the electrostatic latent image is visualized as a yellow toner image. It has become.

The developer that has consumed the yellow toner by the development is returned to the developing device 50Y as the developing roller 51Y rotates.
In this embodiment, the DC component developing bias is applied by the bias applying means. However, the developing bias may be an AC component, or an AC component superimposed on the DC component.

  As described above, in the developing device 50Y, the developer agitated and conveyed by the first conveying screw 53Y and the second conveying screw 54Y is pumped up by the magnetic force of the magnet roller 81Y and carried on the developing sleeve 82Y to the developing region D. The toner in the developer is electrostatically transferred and supplied to the latent image on the photosensitive drum 20Y, and development, that is, the latent image is visualized. The developer that has consumed the toner after development is released from the surface of the developing sleeve 82Y into the first storage chamber 58Y, and is stored in the first storage chamber 58Y and the second storage chamber 59Y by the first transport screw 53Y and the second transport screw 54Y. The cycle in which the developer is stirred and pumped up to the surface of the developing sleeve 82Y is repeated. The magnet block is arranged to repeat such a cycle.

  In the image forming apparatus 100 having such a configuration, when copying, a document is set on the automatic document feeder 22 or the automatic document feeder 22 is rotated upward to place a document on the contact glass 21a. Then, the automatic document feeder 22 is rotated downward to close and hold the document, and the start button on the operation panel is pressed. The document set on the automatic document feeder 22 is, for example, a bundle-shaped sheet document, and the document set on the contact glass 21a is, for example, a single-bound document that is bound in a main shape. When the image forming apparatus 100 is used as a printer, an image forming start operation is performed after image data to be formed is selected and input by an external input device such as a PC connected to the image forming apparatus 100.

  When copying is performed and the original is set on the automatic document feeder 22, the read original is read by the reading device 21 after the set original is fed onto the contact glass 21a. Is placed on the contact glass 21a, the document is read by the reading device 21 when the start button is pressed, and image data is generated.

  Based on the generated image data or the input image data, the image stations 60Y, 60M, 60C, and 60BK having the above-described configuration operate. In parallel with the document reading operation, each device in each of the process cartridges 95Y, 95M, 95C, and 95BK, the transfer belt unit 10, the secondary transfer unit 76, the fixing device 6 and the like start driving.

  In the image station 60Y, the surface of the photosensitive drum 20Y is uniformly charged by the charging device 30Y as it rotates in the B1 direction, and exposure scanning of the modulated and deflected laser light L from the optical scanning device 8 is performed. Thus, the electrostatic latent image corresponding to the yellow color is formed by the attenuation of the potential of the photosensitive drum 20Y in the exposure unit, that is, the irradiation unit, and the electrostatic latent image is developed with the yellow toner by the developing device 50Y. The yellow toner image obtained by the development is primarily transferred to the transfer belt 11 that moves in the A1 direction by the primary transfer electric field and nip pressure formed by the primary transfer roller 12Y, and includes toner remaining after transfer. Unnecessary substances are removed by the cleaning device 40Y, and are subjected to the next charge removal and charging by the charging device 30Y to return to the initial state.

Similarly, the toner images of the respective colors are satisfactorily formed by good development on the other photoconductive drums 20C, 20M, and 20BK, and the formed toner images of the respective colors are A1 by the primary transfer rollers 12C, 12M, and 12BK. Primary transfer is sequentially performed at the same position on the transfer belt 11 moving in the direction.
In this primary transfer, the transfer rate and pre-transferability are good.

A four-color superimposed toner image constituted by toner images superimposed on the transfer belt 11, that is, a four-color toner image, is a position facing the secondary transfer roller 5 as the transfer belt 11 rotates in the A1 direction. It moves to the next transfer nip, is secondarily transferred to the transfer paper, and a full color image is carried on the transfer paper.
In this secondary transfer, transfer rate, pre-transferability, and transfer blur are good.

  The transfer paper conveyed between the transfer belt 11 and the secondary transfer roller 5 is fed out from the corresponding paper feed cassette 25 by one rotation of the sheet feeding device 23 selected by the sheet feeding device 23 and fed. Is fed from the manual feed tray 34 by the rotation of the feed roller 35 of the manual paper feed device 33, or fed from the duplex unit 96 by the paper feed roller 95. At the timing when the tip of the toner image on the transfer belt 11 faces the secondary transfer belt 5 based on the detection signal from the sensor by the registration roller pair 13. It has been sent out. Such a feed operation is started substantially simultaneously with the document reading operation described above.

  When the toner image of all colors is transferred and carried on the transfer paper, it enters the fixing device 6 and passes through the fixing portion between the fixing belt 64 and the pressure roller 63 due to the action of heat and pressure. The carried toner image is fixed, and a color image is formed on the transfer paper. The fixed transfer paper that has passed through the fixing device 6 is stacked on the paper discharge tray 75 via the paper discharge roller 98 or on the duplex unit 96 via the conveyance roller 97 according to the position of the switching claw 94. Enter to prepare for double-sided image formation. On the other hand, the transfer belt 11 that has finished the secondary transfer is cleaned by removing residual toner and the like remaining on the intermediate transfer belt cleaning device 14 to prepare for the next image formation.

  The secondary transfer belt 5 that has finished the secondary transfer is cleaned by removing residual toner remaining on the secondary transfer belt cleaning device 17 to prepare for the next image formation. The secondary transfer belt cleaning device 17 functions even after the occurrence of a jam in the secondary transfer unit, after image formation on an irregular size transfer sheet, and the like. However, instead of the secondary transfer belt cleaning device 17, a toner having a polarity opposite to the toner charging polarity is applied to the transfer entrance roller 73 to reversely transfer the toner or the like adhering to the secondary transfer belt 5 to the transfer belt 11, This may be cleaned by the intermediate transfer belt cleaning device 14.

  Various configuration examples of the potential gradient forming unit 84 will be described with reference to FIGS. In the description of each configuration example, in order to avoid duplication of the configurations and the like that have already been described, the same configurations are denoted by the same reference numerals and the description thereof is omitted as appropriate.

  The potential gradient forming unit 84 shown in FIG. 4 is opposed to the transfer inlet roller 73 via the transfer belt 11 and the secondary transfer belt 5 in addition to the configuration shown in FIG. A pressure roller 87 that rotates following the secondary transfer belt 5 as a pressure member that pressurizes the transfer belt 11 and the secondary transfer belt 5 is provided.

  By disposing the pressure roller 87, the nip of the secondary transfer nip can be achieved without increasing the tension of the secondary transfer belt 5 or urging the transfer entrance roller 73 against the secondary transfer belt 5 with a strong pressure. It is possible to increase the pressure and improve the transfer rate. If the tension of the secondary transfer belt 5 is increased, the life of the secondary transfer belt 5 may be reduced. If the transfer entrance roller 73 is urged to the secondary transfer belt 5 with a strong pressure, the flatness of the flat portion 5a is increased. However, it is avoided that the transfer blur is likely to occur. In order to reduce the nip pressure of the secondary transfer nip, the configuration example shown in FIG. 3 or the configuration example described later with reference to FIG. 10 is preferable.

  In the case where the pressure roller 87 is provided, a position where a line connecting the rotation center of the transfer inlet roller 73 and the rotation center of the pressure roller 87 is substantially perpendicular to the flat portion 5a in order to prevent or suppress transfer blur. Further, it is desirable to dispose the pressure roller 87 to prevent the transfer inlet roller 73, the transfer belt 11 and the secondary transfer belt 5 from being wound.

  When the pressure roller 87 is provided, if the pressure roller 87, the transfer inlet roller 73, the transfer belt 11 and the secondary transfer belt 5 are all rigid, any one of them is slightly uneven. In this case, when the unevenness is positioned at the secondary transfer nip, the transfer paper does not adhere to the transfer belt 11, and a blank image due to the same discharge as the pre-transfer area or an image that is blurred or scattered is obtained. Since it may occur, it is desirable to give elasticity to any of these.

  If any one of the pressure roller 87, the transfer inlet roller 73, the transfer belt 11, and the secondary transfer belt 5 is elastic, such an image is prevented or suppressed. In order to improve the quality, it is desirable to give elasticity to either the transfer entrance roller 73 or the transfer belt 11. However, if the transfer belt 11 is mechanically durable as described above, it is preferable that the transfer entrance roller 73 be elastic.

  Therefore, the transfer entrance roller 73 is most preferably formed of a rubber material, particularly a soft material or a foamed rubber material among rubber materials. A resistance layer is formed of such a rubber material, and its hardness is 40 to 60 degrees as Asker C hardness. In addition to this, when resistance is applied to the transfer entrance roller 73, it is desirable to form the transfer entrance roller 73 using foamed acrylonitrile butadiene, epichlorohydrin, or a copolymer thereof.

The pressurizing member may be constituted by an elastic member such as a plate spring constituted by a metal thin plate or the like instead of a roller, and the nip pressure of the secondary transfer nip may be increased by the elastic force. When a member that applies a load to the belt 5 rubs against the secondary transfer belt 5, the life of the secondary transfer belt 5 may be shortened, so that the pressure member is connected to the secondary transfer belt 5 such as the pressure roller 87. It is desirable to have a configuration with low friction. In this regard, although the above-described material of the secondary transfer belt 5 is excellent in wear resistance, the durability may be improved by mixing conductive fine particles such as carbon that can also be used for resistance adjustment. . This also applies to the configuration example shown in FIG.
When the transfer entrance roller 73 is made elastic, the pressure roller 87 can be formed of a single metal layer.

  In the present configuration example, the pressurizing member is disposed only to perform such pressurization, and therefore, the pressurizing member is floated, or at least the contact position with the secondary transfer belt 5 is made insulative, so that the influence on the transfer electric field is increased. Reduce as much as possible.

  FIG. 5 shows a configuration example in which the driven roller 16 is used as a pressure roller, and FIG. 6 shows a configuration example in which the driving roller 15 is used as a pressure roller. In this way, by using the driven roller 16 or the driving roller 15 as a pressure roller, it is not necessary to separately provide the pressure roller, and the structure is cheaper and smaller than the case where the pressure roller is separately provided. There is an advantage that the function as described above can be obtained by the pressure roller. In this way, in the configuration in which the driven roller 16 or the driving roller 15 is also used as a pressure roller, the roller that also serves as the pressure roller is grounded so that the power source 85 or the power source 86 can be omitted even if the power source 85 or the power source 86 is omitted. It is possible to form a potential gradient in the transfer portion.

  As shown in FIGS. 7 and 8, when the pressure roller 87 is provided separately from the driving roller 15 and the driven roller 16, the power supply 85 or the power supply 86 can be obtained by making the pressure roller 87 conductive and grounding. Can be omitted. When the power supply 86 is omitted and the pressure roller 87 is grounded as shown in FIG. 7, the pressure roller 87 becomes a downstream member, the driven roller 16 floats, or at least contacts the secondary transfer belt 5. The position is made insulative to reduce the influence on the transfer electric field as much as possible. As shown in FIG. 8, when the power supply 85 is omitted and the pressure roller 87 is grounded, the pressure roller 87 becomes an upstream member and the drive roller 15 floats or contacts at least the secondary transfer belt 5. The position is insulative to reduce the influence on the transfer electric field as much as possible.

  As shown in FIGS. 3 to 8, when a bias is applied to the driving roller 15 and the driven roller 16, it is necessary to prevent or suppress leakage of the bias from these rollers in order to stabilize the potential gradient. There is. In particular, when a bias is applied to the drive roller 15, the bias may leak from the drive system of the drive roller 15, and when the bias is strong, the drive system may be affected. It is desirable to make the members such as the coupling and the like insulative. Further, if the hardness of the driving roller 15 among the driving roller 15 and the driven roller 16 is lowered, the moving speed of the secondary transfer belt, that is, the rotational speed may become unstable. Therefore, the height of these rollers is 50 according to JIS-A hardness. It is desirable to make it about or higher.

  As shown in FIGS. 9 and 10, separately from the driving roller 15 and the driven roller 16, conductive upstream members 88 a and 89 a and downstream members 88 b and 89 b connected to a power source 85 and a power source 86 may be provided. If this is done, the number of parts can be increased and the size can be increased, but there is an advantage that the degree of freedom in selecting the hardness of the driving roller 15 and the driven roller 16 and the insulation thereof is increased. In this case as well, one of the upstream side members 88a and 89a and the downstream side members 88b and 89b may be a pressure member and grounded, and the driving roller 15 and the driven roller 16 float or at least secondary transfer. The contact position with the belt 5 is made insulative to reduce the influence on the transfer electric field as much as possible. The upstream member 88a and the downstream member 88b that are in a roller shape may be pressure rollers.

  FIG. 10 shows a configuration example in which the upstream member 89a and the downstream member 89b are provided as brush-like upstream brushes and downstream brushes instead of roller-like rollers such as upstream rollers and downstream rollers. Yes. If the upstream side member and the downstream side member are formed in a roller shape as shown in FIGS. 3 to 9, the distance between these members may have to be relatively large, and the distance from the upstream side member to the pre-transfer area In addition, since the distance from the downstream member to the secondary transfer nip is increased and the bias is attenuated and applied to these pre-transfer areas, the resistance unevenness is present when the secondary transfer belt 5 has resistance unevenness. However, it becomes a destabilizing factor of the applied bias in such a pre-transfer region and the like, and becomes a factor of lowering the transfer rate and pre-transfer performance.

On the other hand, as shown in FIG. 10, when the upstream member and the downstream member are brush-like upstream brush 89a and downstream brush 89b, the interval between them is made narrower than that of the roller. This is advantageous in terms of transfer rate and pre-transfer performance, and if the distance between them is reduced, a sufficient potential gradient is formed even if the applied bias is low, which is efficient. It is also suitable for reducing the nip pressure of the secondary transfer nip. However, the sliding life of the secondary transfer belt 5 by the upstream brush 89a and the downstream brush 89b may reduce the life of the secondary transfer belt 5, and leakage due to the missing brush fibers may easily occur.
One of the upstream member and the downstream member may be a roller-like member, and the other may be a brush-like member.

  In the configuration examples shown in FIGS. 3, 4, 9, and 10, both the upstream electrode and the downstream electrode apply a bias. In the configuration examples shown in FIGS. 5 to 9, the upstream electrode and the downstream electrode are applied. One of them applies a bias, but in any case, an appropriate potential gradient is formed. However, the configuration example shown in FIGS. 3, 4, 5, 7, 9, and 10 in which a bias having the same polarity as the charging polarity of the toner is applied to the upstream electrode, and the upstream electrode is grounded and downstream The configuration example shown in FIGS. 6 and 8 in which a bias having a polarity opposite to the charging polarity of the toner is applied to the side electrode is superior to the former configuration example in terms of the transfer rate and the degree of pre-transfer performance.

  Specifically, in the former configuration example, if the bias is not increased too much, the transfer rate in the pre-transfer area is very low or no transfer occurs at all, and the transfer rate is very good. Discharge in the area is eliminated and the pre-transfer performance is very good, whereas in the latter configuration example, the transfer rate in the pre-transfer area is lowered even if the bias is not increased too much. In addition, the discharge in the pre-transfer area is almost eliminated and the pre-transfer performance is only good.

In the latter configuration example, in addition to applying a bias having a polarity opposite to the charging polarity of the toner to the downstream electrode, the bias application by the transfer entrance roller 73 is performed under constant current control. Therefore, the bias in the secondary transfer portion does not reach the same polarity as the toner, and the bias by the intermediate transfer belt 11 in the pre-transfer region exerts repulsive force on the toner carried on the intermediate transfer belt. This is because the degree of occurrence is smaller than in the former configuration example.
However, when it is desired to lower the bias applied to the transfer entrance roller 73, the latter configuration example may be more effective.

  Regarding the separation property of the transfer paper that has passed through the secondary transfer portion from the transfer belt 11, the transfer paper that has passed through the secondary transfer nip is transported by the secondary transfer belt 5 as in the configuration example excluding FIG. 5. This is better than the configuration example that passes through the secondary transfer unit 76 as in the configuration example shown in FIG. This is because the transfer paper that has passed through the secondary transfer nip is attracted to the transfer belt 11 by electrostatic attraction or non-electrostatic adhesion between the transfer paper 11 and the former configuration. In the example, the secondary transfer belt 5 also causes the transfer paper that has passed through the secondary transfer nip to be attracted to the secondary transfer belt 5 in the same manner as the transfer belt 11, and the transfer belt 11 performs such an operation. This is because the size of the secondary transfer belt 5 that tends to convey the transfer paper linearly tends to be larger between the size and the size of the action of the secondary transfer belt 5.

  However, in the former configuration, the separation property of the transfer paper from the secondary transfer belt 5 can be lowered. In order to improve the separability, it is preferable to reduce the diameter of the driven roller 16 positioned downstream of the secondary transfer portion in the E1 direction. Since there is no restriction on making the driven roller 16 a metal single layer, the separation property is improved if the driven roller 16 is made of a metal single layer for easy diameter reduction, for example, an outer diameter of 20 mm or less.

  In the case of improving the transfer paper separation from the transfer belt 11, the transfer paper 16 is transferred in the same manner as the diameter of the driven roller 16 is reduced in order to improve the transfer paper separation from the secondary transfer belt 5. It is effective to reduce the diameter of the entrance roller 73. However, as already mentioned, it may be effective to give the transfer entrance roller 73 elasticity or resistance. In particular, in order to give elasticity, it is necessary to secure a thickness of the elastic layer of at least 4 to 5 mm. At this time, if the cored bar diameter is set to 12 mm, the outer diameter can be set to 20 mm. However, if the cored bar diameter is set to 12 mm, the pressurizing force is increased when the pressure roller 87 is provided. May lack strength. Then, according to the configuration of the transfer entrance roller 73, the diameter thereof is set to such an extent that the strength is maintained, and the transfer paper that has passed through the secondary transfer nip is transferred to the secondary transfer belt as in the configuration example excluding FIG. It is preferable to reduce the diameter of the driven roller 16 after adopting the configuration of conveying at 5.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments, and the present invention described in the claims is not specifically limited by the above description. Various modifications and changes are possible within the scope of the above.

  For example, the secondary transfer member is not limited to a belt shape and may be a fixed member such as a plate-like member as long as it has a flat portion at a portion facing the intermediate transfer belt. In addition to the secondary transfer member, a member for conveying the recording medium may be provided so as to pass the secondary transfer portion.

  The image carrier is not limited to the one that carries an image made of toner, but may be one that carries an image made of ink. That is, the image forming apparatus may form an image using ink as in an ink jet printer.

  The image forming apparatus may not be a copier, a printer, and a facsimile machine, but may be a single unit thereof, or may be a multi-function machine of another combination such as a copier and printer. .

  The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

5 Secondary transfer member, Secondary transfer belt 5a Flat portion 11 Intermediate transfer belt 12Y, 12M, 12C, 12BK Multiple primary transfer members 15 Upstream member (FIGS. 3 to 7)
16 Downstream member (FIGS. 3 to 6, FIG. 8)
20Y, 20M, 20C, 20BK Multiple image carriers 73 Secondary transfer facing member 84 Potential gradient forming means 87 Upstream member (FIG. 8), downstream member (FIG. 7)
88a, 89a Upstream member 88b, 89b Downstream member 99 Transfer device 100 Image forming device A1 Rotation direction of intermediate transfer belt

JP 2003-215946 A JP 2004-109702 A JP 2004-144916 A JP 2000-098763 A JP 2002-072709 A JP 2004-163534 A Japanese Patent Laid-Open No. 11-24443 JP 2004-184875 A Japanese Patent Laid-Open No. 08-106219 Japanese Patent No. 3697626 JP 2005-314064 A

Claims (3)

A rotating intermediate transfer belt disposed opposite to the plurality of image carriers, to which images carried on the respective image carriers are transferred,
A recording medium disposed opposite to the intermediate transfer belt and having a substantially flat surface at a portion facing the intermediate transfer belt, and an image carried on the intermediate transfer belt passes through the flat portion. A secondary transfer body to be transferred;
A plurality of intermediate transfer belts are wound through the intermediate transfer belt so that the intermediate transfer belt wraps around each image carrier in a region downstream of the opposing region between the intermediate transfer belt and each image carrier in the rotation direction of the intermediate transfer belt. A plurality of primary transfer members disposed to face the image carrier,
A secondary transfer opposing member that applies a bias having the same polarity as the image that is wound around the intermediate transfer belt at a position facing the plane portion and is carried on the intermediate transfer member;
A potential gradient forming means for forming a potential gradient having a large voltage on the same polarity side as the image carried on the intermediate transfer body toward the upstream side in the conveyance direction of the recording medium on the plane portion in the plane portion. And
The surface resistivity of the intermediate transfer belt on the secondary transfer counter member side is high resistance,
Said secondary transfer member, Ri the secondary transfer opposing member opposite medium resistance Der surface resistivity of,
The secondary transfer body is a secondary transfer belt formed of a material having ion conductivity, which forms a belt shape and conveys a recording medium by rotation,
The potential gradient forming means includes an upstream member that occupies the upstream side in the transport direction and a downstream member that occupies the downstream side in contact with the surface of the secondary transfer belt opposite to the secondary transfer counter member. And
The upstream member is located upstream of the opposing region of the secondary transfer opposing member and the flat surface or the opposing region in the transport direction;
The downstream member is located on the downstream side of the opposing region of the secondary transfer opposing member and the flat surface or the opposing region in the transport direction;
A bias having the same polarity as the image carried on the intermediate transfer member is applied to the upstream member, and the downstream member is grounded.
Or
The upstream member is grounded, and a bias having a polarity opposite to that of the image carried on the intermediate transfer member is applied to the downstream member.
Or
A bias having the same polarity as that of the image carried on the intermediate transfer member is applied to the upstream member and a bias having a polarity opposite to that of the image carried on the intermediate transfer member is applied to the downstream member. A transfer device in which the potential gradient is formed . The transfer apparatus according to claim 1, wherein
The transfer apparatus, wherein the secondary transfer counter member applies a bias having the same polarity as that of an image carried on the intermediate transfer member under constant current control . 3. An image forming apparatus comprising: the transfer device according to claim 1 ; and a plurality of image carriers that are arranged to face the intermediate transfer belt and each of the images carried by the transfer device is transferred to the intermediate transfer belt .

Images