Image forming apparatus

Abstract

To provide an image forming apparatus of an intermediate transfer system that can eliminate static electricity on a recording medium without excess and deficiency after toner images are secondarily transferred to the recording medium.SOLUTION: An image forming apparatus comprises a plurality of image forming units, an intermediate transfer belt, primary transfer members, a secondary transfer roller, a counter roller, and a static elimination device. The secondary transfer roller secondarily transfers toner images primarily transferred to the intermediate transfer belt onto a recording medium. The counter roller is brought into pressure contact with the secondary transfer roller with the intermediate transfer belt therebetween to form a secondary transfer nip part. The static elimination device includes static elimination needles that are arranged with their leading ends directed to the downstream side in a conveyance direction of the recording medium and connected to the ground, and a static elimination needle protective cover that has a guide surface facing the recording medium having passed through the secondary transfer nip part and maintains the constant interval between the recording medium and the static elimination needles. The static elimination device eliminates residual electric charges on the recording medium having passed through the secondary transfer nip part.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral thereof using an electrophotographic system, and particularly relates to an image forming apparatus of an intermediate transfer system that secondarily transfers a toner image primarily transferred onto an intermediate transfer member to a recording medium. [Background Art]

[0002] In an image forming apparatus using an electrophotographic system, an electrostatic latent image formed on an image carrier made of a photoreceptor or the like is developed by a developing device and visualized as a toner image. As such an image forming apparatus, an intermediate transfer type image forming apparatus that secondarily transfers a toner image primarily transferred from a photoreceptor onto an intermediate transfer member such as an intermediate transfer belt to a recording medium such as paper is widely used.

[0003] In an intermediate transfer type image forming apparatus, immediately after a secondary transfer nip portion that secondarily transfers the toner image on the intermediate transfer belt to paper, a separator to which a DC voltage and an AC voltage for paper separation are applied, or a static eliminator grounded is arranged. For example, Patent Document 1 discloses an image forming apparatus including: a secondary transfer device that secondarily transfers a toner image on an intermediate transfer belt onto transfer paper; and a static elimination needle for eliminating static electricity from the back surface of the transfer paper that has completed secondary transfer to facilitate separation from the intermediate transfer belt.

[0004] In the configuration of Patent Document 1, the tip end of the static elimination needle is arranged in a nearly perpendicular state with respect to the paper that has passed through the secondary transfer nip portion. The static elimination needle applies electric charge to the paper or removes electric charge from the paper, reduces the electrostatic adsorption force between the intermediate transfer belt and the paper, and separates the paper from the intermediate transfer belt. As a method for separating paper from the intermediate transfer belt, besides the method using a static eliminator, there is also a method using curvature separation achieved by reducing the diameter of a driving roller (opposing roller) of the intermediate transfer belt that faces the secondary transfer roller. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-192898 [Overview of the project] [Problems that the invention aims to solve]

[0006] When the diameter of the drive roller of the intermediate transfer belt, which is opposite the secondary transfer roller, is small, and the hardness of the secondary transfer roller is large, the paper separation direction becomes biased towards the secondary transfer roller. At this time, the peeling discharge that occurs between the intermediate transfer belt and the paper causes the entire paper to become charged with the same polarity as the toner. In this case, if the paper is separated and discharged using a conventional static eliminator, the static elimination efficiency is too high, causing the toner image that has been secondary transferred to the paper to scatter. When static elimination is performed with AC voltage, the scattering of the toner image is less likely to occur, but the entire image tends to become blurred. In addition, the high-voltage power supply required to apply AC voltage to the static eliminator becomes larger, which leads to an increase in the cost of the equipment.

[0007] On the other hand, reducing the diameter of the drive rollers makes it easier for the paper to separate by curvature, thus eliminating the need for a mechanism to separate the paper from the intermediate transfer belt. However, because the paper is transported to the fuser unit with a high charge level, electrostatic offset is more likely to occur during the fuser process. Therefore, it was necessary to discharge the paper appropriately and without excess or deficiency after the toner image had been secondarily transferred.

[0008] In view of the above problems, the present invention aims to provide an image forming apparatus that can completely remove static electricity from a recording medium after the toner image has been secondarily transferred in an intermediate transfer method. [Means for solving the problem]

[0009] To achieve the above objective, the first configuration of the present invention is an image forming apparatus comprising a plurality of image forming units, an intermediate transfer belt, a primary transfer member, a secondary transfer roller, a counter roller, and a static elimination device. The image forming unit includes an image carrier having a photosensitive layer formed on its surface, a charging device that charges the surface of the image carrier to a predetermined surface potential, an exposure device that irradiates the image carrier charged by the charging device with light to form an electrostatic latent image with reduced charge, and a developing device that develops the electrostatic latent image formed on the surface of the image carrier into a toner image. The intermediate transfer belt is endless and arranged adjacent to the image forming unit, and the toner image formed on the surface of the image carrier is primary transferred to its outer surface. The primary transfer member primary transfers the toner image formed on the surface of the image carrier to the intermediate transfer belt. The secondary transfer roller secondary transfers the toner image primary transferred to the intermediate transfer belt onto a recording medium in a secondary transfer nip portion formed between it and the intermediate transfer belt. The counter roller forms a secondary transfer nip portion by being pressed against the secondary transfer roller via the intermediate transfer belt. The static elimination device removes residual charge from the recording medium after it has passed through the secondary transfer nip section. The static elimination device includes static elimination needles and a static elimination needle protective cover. The static elimination needles are arranged in large numbers at regular intervals across the entire width of the recording medium, perpendicular to the transport direction of the recording medium, with their tips facing downstream in the transport direction of the recording medium, and are connected to ground. The static elimination needle protective cover has a guide surface facing the recording medium after it has passed through the secondary transfer nip section, and maintains a constant distance between the recording medium and the static elimination needles. [Effects of the Invention]

[0010] According to the first configuration of the present invention, the static elimination device includes a static elimination needle positioned with its tip toward the downstream side in the transport direction of the recording medium, and a static elimination needle protective cover having a guide surface facing the recording medium that has passed through the secondary transfer nip section. This makes it possible to maintain a constant distance between the recording medium being transported along the guide surface of the static elimination needle protective cover and the static elimination needle, thereby obtaining uniform static elimination performance when eliminating the charge of the recording medium. Therefore, it is possible to effectively suppress the occurrence of uneven static elimination due to excessive static elimination and the occurrence of electrostatic scattering due to insufficient static elimination. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic cross-sectional view showing the overall configuration of the image forming apparatus 100 according to the first embodiment of the present invention. [Figure 2] Partial cross-sectional view of the area around the paper transport path 19 and double-sided transport path 20 in the image forming apparatus 100 of the first embodiment. [Figure 3] Figure 2: Enlarged view of the area around the secondary transfer nip N. [Figure 4] Side view of the static elimination device 31 [Figure 5] This is an enlarged view of the area around the secondary transfer nip portion N of the image forming apparatus 100 of the first embodiment, showing a modified example in which the tip portion 33a of the static elimination needle 33 is inclined in a direction away from the tangent line L passing through the secondary transfer nip portion N. [Figure 6] This is an enlarged view of the area around the secondary transfer nip portion N of the image forming apparatus 100 of the first embodiment, showing a modified example in which the tip portion 33a of the static elimination needle 33 is inclined in a direction approaching the tangent L passing through the secondary transfer nip portion N. [Figure 7] A photograph of a halftone image where image streaks have formed. [Figure 8] A photograph of a halftone image where electrostatic dispersion occurred. [Figure 9] This diagram shows the appropriate range of angles θ1 and θ2 between the tangent L passing through the secondary transfer nip section N and the tip 33a of the static elimination needle 33. [Figure 10] Enlarged view of the area around the secondary transfer nip portion N in the image forming apparatus 100 according to the second embodiment of the present invention. [Modes for carrying out the invention]

[0012] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Inside the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the transport direction (left side in Figure 1). These image forming units Pa to Pd are provided to correspond to images of four different colors (yellow, cyan, magenta, and black), and sequentially form images of yellow, cyan, magenta, and black through the processes of charging, exposure, development, and transfer, respectively.

[0013] Each of these image forming units Pa to Pd is equipped with photoreceptor drums (image carriers) 1a, 1b, 1c, and 1d, which carry visible images (toner images) of each color. Furthermore, an intermediate transfer belt (intermediate transfer body) 8, which rotates counterclockwise in Figure 1 by a belt-driven motor (not shown), is provided adjacent to each image forming unit Pa to Pd. The toner images formed on these photoreceptor drums 1a to 1d are sequentially transferred and superimposed onto the intermediate transfer belt 8, which moves in contact with each photoreceptor drum 1a to 1d. Subsequently, the toner images transferred onto the intermediate transfer belt 8 are secondaryly transferred onto a transfer paper P, which is an example of a recording medium, by a secondary transfer roller 9. Furthermore, the transfer paper P on which the toner images have been secondary transferred is discharged from the main body of the image forming apparatus 100 after the toner images have been fixed in the fixing unit 13. The image forming process for each photoreceptor drum 1a to 1d is performed while the photoreceptor drums 1a to 1d are rotated clockwise in Figure 1.

[0014] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 arranged at the lower part of the main body of the image forming apparatus 100, and is conveyed along the paper conveyance path 19 via the paper feed roller 12 and the registration roller pair 13 to the nip portion between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt having no joints is mainly used. Further, a blade-shaped belt cleaner 25 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is arranged on the downstream side of the secondary transfer roller 9.

[0015] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photoconductor drums 1a to 1d, there are charging devices 2a, 2b, 2c and 2d for charging the photoconductor drums 1a to 1d, an exposure device 5 for exposing image information onto each of the photoconductor drums 1a to 1d, developing devices 3a, 3b, 3c and 3d for forming toner images on the photoconductor drums 1a to 1d, and cleaning devices 7a, 7b, 7c and 7d for removing developer (toner) and the like remaining on the photoconductor drums 1a to 1d.

[0016] When image data is input from a host device such as a personal computer, first, the surfaces of the photoconductor drums 1a to 1d are uniformly charged by the charging devices 2a to 2d. Next, the exposure device 5 irradiates light according to the image data to form an electrostatic latent image corresponding to the image data on each of the photoconductor drums 1a to 1d. The developing devices 3a to 3d are each filled with a predetermined amount of two-component developer containing yellow, cyan, magenta, and black toner, respectively. It should be noted that when the proportion of toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of a toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in the developer is supplied onto the photoconductor drums 1a to 1d by the developing devices 3a to 3d and adheres electrostatically. As a result, a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5 is formed.

[0017] Then, an electric field with a predetermined transfer voltage is applied between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d by the primary transfer rollers 6a to 6d, whereby the yellow, cyan, magenta and black toner images on the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These images are formed in a predetermined positional relationship set in advance. Thereafter, in preparation for subsequent formation of a new electrostatic latent image, toner and the like remaining on the surfaces of the photosensitive drums 1a to 1d after the primary transfer are removed by cleaning devices 7a to 7d.

[0018] The intermediate transfer belt 8 is stretched across an upstream driven roller 10 and a downstream drive roller 11. When the intermediate transfer belt 8 starts rotating counterclockwise as the drive roller 11 is rotated by a belt drive motor (not shown), the transfer paper P is fed from the pair of registration rollers 13 at a predetermined timing to the secondary transfer nip N (see FIG. 2) formed between the drive roller 11 and a secondary transfer roller 9 provided adjacent to the drive roller 11. Then, the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P passing through the secondary transfer nip N.

[0019] The transfer paper P having the toner image secondarily transferred thereon is conveyed to a fixing unit 14. The fixing unit 14 includes a fixing belt 14a and a pressure roller 14b (both see FIG. 2). The fixing belt 14a is heated by a heating device such as a heater or an induction heating unit (not shown). The pressure roller 14b is pressed against the fixing belt 14a to form a fixing nip, and applies a rotational driving force to the fixing belt 14a.

[0020] The transfer paper P conveyed to the fixing unit 14 is heated and pressed by the fixing belt 14a and the pressure roller 14b, so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full-color image is formed. The conveyance direction of the transfer paper P on which the full-color image is formed is sorted by a branching portion 15 branching into a plurality of directions, and the transfer paper P is discharged onto a discharge tray 18 by a pair of discharge rollers 17 as it is (or after being conveyed to a double-sided conveyance path 20 to form images on both sides).

[0021] Figure 2 is a partial cross-sectional view of the area around the paper transport path 19 and the double-sided transport path 20 in the image forming apparatus 100 of the first embodiment. The side cover 21 constitutes the side surface 102 of the image forming apparatus 100 and is rotatably supported by a cover support shaft 21a provided below the main body of the image forming apparatus 100.

[0022] A hook 22 is provided on the side edge of the side cover 21. The hook 22 engages with engagement pins (not shown) provided on the front and rear frames of the image forming apparatus 100 body, thereby holding the side cover 21 in a closed state. The inner surface of the side cover 21 constitutes one of the transport surfaces of the double-sided transport path 20.

[0023] A transport unit 23 is located inside the side cover 21. The transport unit 23 is rotatably supported by the main body of the image forming apparatus 100 around a unit support shaft 23a, and constitutes a part of the transport surface of the double-sided transport path 20 and the paper transport path 19. The double-sided transport path 20 extends vertically along the side 102 of the image forming apparatus 100 between the inner surface of the side cover 21 and the outer surface of the transport unit 23, and is structured to curve in a roughly C-shape before joining the paper transport path 19. On the inner surface of the transport unit 23, one of the rollers 13b constituting the register roller pair 13 and a secondary transfer roller 9 are attached in order from the upstream side (lower side in Figure 2) in the transport direction of the transfer paper P.

[0024] By rotating only the side cover 21 in the opening direction relative to the image forming apparatus 100, the double-sided transport path 20 is exposed over a wide area. Also, by rotating the side cover 21 together with the transport unit 23 in the opening direction, the transport unit 23 is separated from the main body of the image forming apparatus 100, and the paper transport path 19 is exposed over a wide area. On the other hand, by rotating the side cover 21 together with the transport unit 23 in the closing direction, the transport unit 23 comes into contact with the main body of the image forming apparatus 100, and the secondary transfer roller 9 is pressed against the drive roller 11 via the intermediate transfer belt 8.

[0025] A transport guide 30 is positioned in the transport unit 23. The transport guide 30 guides the transfer paper P that has passed through the secondary transfer nip section N in the paper transport path 19 downstream of the secondary transfer roller 9 and directs it to the fixing section 14. An anti-static device 31 is positioned between the secondary transfer roller 9 and the transport guide 30.

[0026] Figure 3 is an enlarged view of the area around the secondary transfer nip section N in Figure 2. Figure 4 is a side view of the static elimination device 31. The static elimination device 31 includes static elimination needles 33 and a static elimination needle protective cover 34. The static elimination needles 33 are arranged in large numbers at regular intervals over the entire width direction (the direction perpendicular to the paper plane in Figure 3) perpendicular to the transport direction of the transfer paper P. The static elimination needles 33 remove residual charge from the transfer paper P passing through the secondary transfer nip section N, suppressing scattering (electrostatic scattering) and electrostatic offset of the toner image secondary transferred onto the transfer paper P. The static elimination needles 33 are connected to the main frame (not shown) of the image forming apparatus 100 and are in a ground (GND) state.

[0027] The static elimination needle protective cover 34 is made of resin and is positioned between the transfer paper P and the static elimination needle 33 after the transfer paper P has passed through the secondary transfer nip section N, thereby maintaining a constant distance between the transfer paper P and the static elimination needle 33. The guide surface (conveying surface) 34a of the static elimination needle protective cover 34 facing the transfer paper P constitutes part of the paper transport path 19 and functions as a transport guide for the transfer paper P passing through the paper transport path 19. Examples of materials for the static elimination needle protective cover 34 include ABS (acrylonitrile-butadiene-styrene) resin and PC (polycarbonate) resin.

[0028] An inclined surface 34b is formed at the end of the static elimination needle protection cover 34 on the side of the secondary transfer nip section N. When the tangent L between the secondary transfer roller 9 and the drive roller 11 passing through the secondary transfer nip section N is drawn, the inclined surface 34b is inclined toward the downstream side of the transport direction of the transfer paper P (upper right direction in Figure 3) and approaches the tangent L. The leading edge of the transfer paper P that has passed through the secondary transfer nip section N is smoothly guided along the inclined surface 34b to the guide surface 34a. This makes it possible to suppress paper jams caused by interference between the transfer paper P and the static elimination device 31.

[0029] The static elimination needle 33 is positioned on the side of the static elimination needle protective cover 34 opposite to the guide surface 34a, with its tip (needle tip) 33a facing downstream in the direction of transport of the transfer paper P. With this configuration, the tip 33a of the static elimination needle 33 does not come into contact with the transfer paper P after it has passed through the secondary transfer nip section N.

[0030] As mentioned above, the separation of the transfer paper P from the intermediate transfer belt 8 depends on the diameter of the drive roller 11 of the intermediate transfer belt 8 that faces the secondary transfer roller 9, and the hardness of the secondary transfer roller 9. Table 1 shows the relationship between the diameter of the drive roller 11, the Asker C hardness of the secondary transfer roller 9, and the separation performance. The separation performance was determined by measuring the discharge angle of the transfer paper P from the secondary transfer nip section N using a laser displacement meter. A × was indicated if the discharge angle was tilted 8° or more toward the intermediate transfer belt 8 side from the ideal discharge angle (tangential L direction), and a ○ was indicated if the tilt toward the intermediate transfer belt 8 side was less than 8°, or if it was tilted toward the secondary transfer roller 9 side.

[0031] [Table 1]

[0032] As shown in Table 1, in order to maintain good separation of the transfer paper P from the intermediate transfer belt 8, it is necessary to set the diameter of the drive roller 11 to 16 mm or less and the Asker C hardness of the secondary transfer roller 9 to 30° or more.

[0033] Furthermore, the occurrence of electrostatic scattering and electrostatic offset also changes depending on the diameter of the drive roller 11 and the Asker C hardness of the secondary transfer roller 9. Table 2 shows the relationship between the diameter of the drive roller 11, the Asker C hardness of the secondary transfer roller 9, and electrostatic scattering and electrostatic offset. Electrostatic scattering and electrostatic offset were observed visually after printing halftone images, with × indicating the presence of electrostatic scattering and electrostatic offset, and ○ indicating the absence of the presence

[0034] [Table 2]

[0035] As shown in Table 2, when the diameter of the drive roller 11 is 16 mm or less and the Asker C hardness of the secondary transfer roller 9 is 30° or more, electrostatic scattering and electrostatic offset occur when the images are examined.

[0036] Next, the overall charge of the transfer paper P was checked under conditions where image defects occurred and under conditions where they did not. As a result, under conditions where image defects occurred, the overall charge of the transfer paper P was +3kV or higher, while under conditions where image defects did not occur, it was +2kV or lower, or negatively charged (-). From this, it is presumed that because the charge polarity of toner is positive (+), when the entire transfer paper P is strongly positively charged (+), the ability to hold the toner of the same polarity (+) weakens, causing electrostatic scattering and electrostatic offset.

[0037] From the above results, it is clear that in order to maintain good separation of the transfer paper P from the intermediate transfer belt 8 while suppressing the occurrence of image defects, it is necessary to properly discharge the charge from the transfer paper P. However, in the conventional configuration in which the tip 33a of the discharge needle 33 is positioned nearly perpendicular to the transfer paper P, the discharge efficiency becomes too high, which can lead to uneven discharge and potentially uneven image density.

[0038] Therefore, in this embodiment, as shown in Figure 3, the tip 33a of the static elimination needle 33 is positioned downstream of the transfer paper P in the transport direction and approximately parallel to the transport direction, and the side of the static elimination needle protective cover 34 opposite to the static elimination needle 33 is used as a guide surface 34a that guides the transport of the transfer paper P. This makes it possible to maintain a constant distance between the transfer paper P and the static elimination needle 33 as they are transported along the guide surface 34a of the static elimination needle protective cover 34, thereby obtaining uniform static elimination performance when eliminating the charge from the transfer paper P.

[0039] The strength of the static elimination effect (amount of static elimination) by the static elimination device 31 can be adjusted by the amount d of the static elimination needle 33 protruding from the guide surface 34a of the static elimination needle protective cover 34 (see Figure 4). The larger the protrusion amount d, the stronger the static elimination effect, and the smaller the protrusion amount d, the weaker the static elimination effect.

[0040] As shown in Figure 4, a sheet member 35 is attached to the guide surface 34a of the static elimination needle protection cover 34. The guide surface 34a may be made of ABS resin or PC resin, which are the materials of the static elimination needle protection cover 34, but it is preferable to attach a sheet member 35, which has a lower coefficient of friction than the static elimination needle protection cover 34, to the guide surface 34a, such as a PTFE (polytetrafluoroethylene) sheet (No. 903 white, manufactured by Nitto) or a UPE (ultra-high molecular weight polyethylene) sheet (No. 440 white, manufactured by Nitto), to improve sliding performance. The coefficient of friction of the sheet member 35 is preferably 0.3 or less.

[0041] This reduces the transport load on the transfer paper P, and suppresses paper jams and wrinkles in the transfer paper P caused by increased transport load due to surface contact between the transfer paper P and the guide surface 34a of the static eliminator needle protection cover 34. The material and friction coefficient of the guide surface 34a of the static eliminator needle protection cover 34 are shown in Table 3. The friction coefficient was measured by sliding the transfer paper (C2 paper, manufactured by Xerox) under a load of 5 [N] and a speed of 20 [mm / s].

[0042] [Table 3]

[0043] Figures 5 and 6 show modified examples in which the tip 33a of the static elimination needle 33 is inclined with respect to the tangent L passing through the secondary transfer nip portion N. Figure 5 shows the state in which the tip 33a of the static elimination needle 33 is inclined away from the tangent L, and Figure 6 shows the state in which the tip 33a of the static elimination needle 33 is inclined towards the tangent L.

[0044] The static elimination needle 33 does not need to be positioned perfectly parallel to the ideal discharge angle (tangential L direction) of the transfer paper P; it may be tilted at a predetermined angle as shown in Figures 5 and 6. Table 4 shows the relationship between the angle of the tip 33a of the static elimination needle 33 and the occurrence of image defects with respect to the ideal discharge angle of the transfer paper P. The tilt angle of the tip 33a of the static elimination needle 33 is distinguished as positive (+) when the tip 33a moves away from the tangential L (θ1 in Figure 5) and negative (-) when the tip 33a moves towards the tangential L (θ2 in Figure 6). Image defects were evaluated by visually observing the halftone image when the diameter of the drive roller 11 was 16 mm and the Asker C hardness of the secondary transfer roller 9 was 40°.

[0045] [Table 4] *1; Image streaks (static elimination needle pitch unevenness), *2; Electrostatic discharge

[0046] As shown in Table 4, no image defects occurred when the inclination angle of the tip portion 33a was between -15° and +30°, indicating that the transfer paper P was properly statically discharged. However, when the inclination angle became smaller than -15° (larger in the negative direction), the static discharge effect became excessive, and image streaks like those shown in Figure 7 appeared at the pitch (spacing) of the static discharge needles 33. On the other hand, when the inclination angle became larger than +30°, the static discharge effect became insufficient, resulting in spotty electrostatic scattering like that shown in Figure 8.

[0047] As shown in Table 4, when the tip 33a of the static elimination needle 33 is positioned away from the tangent L (see Figure 5), it is preferable that the angle θ1 between the tangent L and the tip 33a be 30° or less. Also, when the tip 33a of the static elimination needle 33 is positioned towards the tangent L (see Figure 6), it is preferable that the angle θ2 between the tangent L and the tip 33a be 15° or less. Figure 9 shows the appropriate ranges for the inclination angles θ1 and θ2 between the tangent L passing through the secondary transfer nip portion N and the tip 33a of the static elimination needle 33.

[0048] Figure 10 is an enlarged view of the area around the secondary transfer nip section N in the image forming apparatus 100 according to the second embodiment of the present invention. In this embodiment, the first static elimination device 31a and the second static elimination device 31b are arranged in order from the upstream side along the transport direction of the transfer paper P. The configuration of the first static elimination device 31a and the second static elimination device 31b is the same as that of the static elimination device 31 of the first embodiment shown in Figure 4.

[0049] If the second static eliminator 31b is placed close to the static elimination needle 33 of the first static eliminator 31a, the discharge space between the static elimination needle 33 of the first static eliminator 31a and the transfer paper P will be eliminated, making it impossible to efficiently remove the charge from the transfer paper P. Therefore, as shown in Figure 10, the second static eliminator 31b is placed at a position further from the tangent L than the first static eliminator 31a, thereby ensuring a discharge space between the static elimination needle 33 of the first static eliminator 31a and the transfer paper P.

[0050] In this embodiment, by arranging two static eliminators, a first static eliminator 31a and a second static eliminator 31b, along the transport direction of the transfer paper P, the upstream first static eliminator 31a reduces the charge potential of the transfer paper P from a high potential level to a medium potential level, and the downstream second static eliminator 31b further reduces the charge potential of the transfer paper P from a medium potential level to a low potential level (near zero). That is, by gradually reducing the residual charge of the transfer paper P, it is possible to suppress the generation of image streaks due to a sudden drop in the charge potential of the transfer paper P, while increasing the static elimination efficiency of the transfer paper P and suppressing the generation of electrostatic scattering.

[0051] Furthermore, by changing the protrusion amount d of the static elimination needle 33 in the first static elimination device 31a and the second static elimination device 31b, the static elimination effect of the transfer paper P can be adjusted to more effectively suppress the occurrence of image defects. Table 5 shows the relationship between the protrusion amount of the static elimination needle 33 in the first static elimination device 31a and the second static elimination device 31b, the static elimination effect, and image defects.

[0052] In Table 5, the protrusion amount d of the static elimination needle 33 is the distance between the tip 33a of the static elimination needle 33 and the edge of the guide surface 34a of the static elimination needle protective cover 34, as shown in Figure 4. More specifically, a positive (+) indicates that the tip 33a of the static elimination needle 33 protrudes outward beyond the edge of the guide surface 34a, while a negative (-) indicates that it is retracted inward from the guide surface 34a.

[0053] [Table 5] *1; Image streaks (static elimination needle pitch unevenness), *2; Electrostatic discharge

[0054] As shown in Table 5, in Test Examples 1 to 3, where only the first static elimination device 31a was installed, the amount of static elimination was appropriate when the protrusion of the static elimination needle 33 was 0 mm, and no image defects occurred (Test Example 2). However, when the protrusion of the static elimination needle 33 was -1 mm, electrostatic scattering occurred due to insufficient static elimination (Test Example 1). Also, when the protrusion of the static elimination needle 33 was 1 mm, image streaks occurred due to excessive static elimination (Test Example 3).

[0055] In contrast, in test examples 4 to 12, where the first static elimination device 31a and the second static elimination device 31b were arranged, when the protrusion amount of the static elimination needle 33 of the first static elimination device 31a was -1 mm or 0 mm, the amount of static elimination was appropriate regardless of the protrusion amount of the static elimination needle 33 of the second static elimination device 31a, and no image defects occurred (test examples 4 to 9). However, when the protrusion amount of the static elimination needle 33 of the first static elimination device 31a was 1 mm, image streaks due to excessive static elimination occurred regardless of the protrusion amount of the static elimination needle 33 of the second static elimination device 31a (test examples 10 to 12).

[0056] Furthermore, comparing Test Example 9 and Test Example 10, although the potential of the transfer paper after static discharge was the same at 1.1kV and the amount of static discharged was the same at 4.9kV, no image defects occurred in Test Example 9, whereas image streaks due to excessive static discharge occurred in Test Example 10. This is thought to be because, although the total amount of static discharged by the first static discharger 31a and the second static discharger 31b was the same, the protrusion amount of the static discharge needle 33 of the first static discharger 31a was 1mm in Test Example 10, resulting in a larger amount of static discharge by the first static discharger 31a and causing excessive static discharge. The same applies to Test Example 6 and Test Examples 11 and 12.

[0057] Based on the above results, in the second embodiment in which the first static elimination device 31a and the second static elimination device 31b are arranged, by arranging the first static elimination device 31a so that the protrusion amount of the static elimination needle 33 is 0 or less, the margin (tolerance) of the protrusion amount of the static elimination needle 33 of the second static elimination device 31b can be increased. Therefore, regardless of the positional accuracy of the first static elimination device 31a and the second static elimination device 31b, or the dimensional tolerances of the static elimination needle 33 or the static elimination needle protective cover 34, the transfer paper P can be stably eliminated in stages, and the occurrence of image defects can be effectively suppressed.

[0058] Furthermore, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. For example, in the second embodiment described above, the first static elimination device 31a and the second static elimination device 31b are arranged along the transport direction of the transfer paper P, but it is also possible to arrange three or more static elimination devices 31 along the transport direction of the transfer paper P. Adding static elimination devices 31 does not affect the image, but adding static elimination devices 31 leads to increased costs, so it is preferable to determine the number to install as needed.

[0059] Furthermore, in the above embodiment, the drive roller 11 that drives the intermediate transfer belt 8 is an opposing roller that faces the secondary transfer roller, and the drive roller 11 is pressed against the secondary transfer roller 9 via the intermediate transfer belt 8 to form the secondary transfer nip portion N. However, the opposing roller that faces the secondary transfer roller 9 may be a roller other than the drive roller 11.

[0060] Furthermore, although the above embodiments described an image forming apparatus 100 using a color printer as an example, as shown in Figure 1, the present invention is not limited to color printers and can be applied to intermediate transfer type image forming apparatuses such as color copiers and color multifunction printers. [Industrial applicability]

[0061] This invention is applicable to image forming apparatuses using an intermediate transfer method. By utilizing this invention, it is possible to provide an image forming apparatus that can completely and completely remove static electricity from the recording medium after the toner image has been secondarily transferred. [Explanation of Symbols]

[0062] Pa~Pd Image Forming Unit 1a~1d Photoreceptor drum (image carrier) 6a~6d Primary transfer roller (primary transfer member) 7a~7d Cleaning equipment 8. Intermediate transfer belt 9. Secondary transfer roller 11. Drive roller (opposing roller) 14 Fixing section 19. Paper transport path 23 Conveyor Unit 30 Conveyor Guide 31 Static eliminator 31a 1st static eliminator 31b Second static eliminator 33 Static Eliminator Needle 33a Tip 34 Static Eliminator Needle Protective Cover 34a Guide surface 34b Slope 35 Sheet material 100 Image forming apparatus N Secondary transfer nip section P Transfer paper (recording medium)

Claims

1. An image carrier having a photosensitive layer formed on its surface, A charging device for charging the surface of the image carrier to a predetermined surface potential, An exposure apparatus that irradiates light onto the image carrier charged by the charging device to form an electrostatic latent image with reduced charge, A developing apparatus for developing the electrostatic latent image formed on the surface of the image carrier into a toner image, Multiple image forming units having, An endless intermediate transfer belt is positioned adjacent to the image forming section, and the toner image formed on the surface of the image carrier is first transferred to the outer surface; A primary transfer member that first transfers the toner image formed on the surface of the image carrier to the intermediate transfer belt, A secondary transfer roller is provided in a secondary transfer nip portion formed between the intermediate transfer belt and the secondary transfer belt, which transfers the toner image that was primary transferred to the intermediate transfer belt onto the recording medium. An opposing roller that forms the secondary transfer nip portion by being pressed against the secondary transfer roller via the intermediate transfer belt, A static elimination device for removing residual charge from the recording medium that has passed through the secondary transfer nip section, In an image forming apparatus equipped with, The static elimination device is, Numerous static elimination needles are arranged at regular intervals across the entire width direction perpendicular to the transport direction of the recording medium, with their tips facing downstream in the transport direction of the recording medium, and connected to ground. A static elimination needle protective cover having a guide surface facing the recording medium that has passed through the secondary transfer nip portion, and guiding the recording medium along the guide surface to restrict contact between the recording medium and the static elimination needle, An image forming apparatus characterized by including

2. The image forming apparatus according to claim 1, characterized in that when a tangent is drawn between the secondary transfer roller and the opposing roller passing through the secondary transfer nip portion, the static elimination device is arranged such that the inclination angle of the tip of the static elimination needle with respect to the tangent is 30° or less when the tip is inclined away from the tangent, and 15° or less when the tip is inclined towards the tangent.

3. The image forming apparatus according to claim 2, characterized in that an inclined surface is formed at the end of the static elimination needle protective cover on the secondary transfer nip side, which is inclined toward the tangential toward the downstream side in the transport direction.

4. The image forming apparatus according to claim 1, characterized in that a plurality of the static elimination devices are arranged along the transport direction of the recording medium.

5. The static elimination device comprises a first static elimination device located on the upstream side in the conveying direction and a second static elimination device located on the downstream side. The image forming apparatus according to claim 4, characterized in that the amount of protrusion of the static elimination needle from the static elimination needle protective cover in the first static elimination device is 0 or less.

6. The image forming apparatus according to claim 5, characterized in that when a tangent line is drawn between the secondary transfer roller and the opposing roller passing through the secondary transfer nip section, the second static eliminator is positioned further from the tangent line than the first static eliminator.

7. The image forming apparatus according to any one of claims 1 to 6, characterized in that a sheet member having a lower coefficient of friction than the static elimination needle protective cover is attached to the guide surface.

8. The image forming apparatus according to claim 7, characterized in that the coefficient of friction of the sheet member is 0.3 or less.

9. The image forming apparatus according to any one of claims 1 to 6, characterized in that the outer diameter of the opposing roller is 16 mm or less, and the Asker C hardness of the secondary transfer roller is 30° or more.

Images