JP2006343364A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To focus attention on pressing force to an image bearing member for a black toner and pressing forces to image bearing members for color toners other than the black toner, and to provide an image forming apparatus capable of preventing occurrence of image defects such as voids and white spots. <P>SOLUTION: Pressing force to an image bearing member on which a black toner image is formed is made 1.5-3.0 times the weakest pressing force among pressing forces to a plurality of image bearing members on which color toner images other than the black toner image are formed and the black toner is transferred at the last. The pressing forces to the plurality of image bearing members on which the color toner images other than the black toner image are formed are weakened in the order of transfer of the color toners onto a transfer body. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrophotographic system such as a copying machine, a printer, a facsimile machine, etc., and an image forming apparatus in which pressing force to each image carrier of a transfer medium passing through a transfer position facing a plurality of image carriers is different. About.

  In general, a color image forming apparatus employs a tandem system in which a plurality of photosensitive members are arranged to face a transfer sheet or an intermediate transfer member, and yellow (Y), magenta (M), cyan (C), An image forming unit such as electrostatic latent image formation and development is executed for each color of black (K). Typical methods for tandem color image formation include the “direct transfer method” in which the toner images of each color formed on a plurality of photoconductors are directly transferred onto a transfer paper and transferred onto a plurality of photoconductors. In addition, there is an “intermediate transfer method” in which toner images of respective colors are transferred onto a transfer sheet after being transferred onto an intermediate transfer body. The direct transfer method employs an endless belt to support and convey the transfer paper, and the intermediate transfer method employs an endless belt to receive and carry each color toner image formed on the photoreceptor. It is common to do.

  In the tandem color image forming apparatus, it is important to prevent the occurrence of “color shift” by accurately superimposing the toner images of the respective colors. In addition, if the force with which the transfer paper or intermediate transfer member is pressed against each image carrier by the pressing roller (hereinafter referred to as “pressing force”) is not appropriate depending on the image pattern, “blank”, “white spot” Such as image defects may occur. Here, “missing” means that a specific color toner is partially lost inside and outside the toner image in a line image such as a character part, and often occurs when colors are overlapped. Less likely to occur with a single color. “White spot” refers to a small white spot on an image, and is likely to occur in a low humidity environment when the pressing force is low. Therefore, in order to prevent such image defects, it is necessary to appropriately set the pressing force according to the image pattern.

  As a conventional technique that has been improved in order to solve the problem of the image defect due to the transfer defect described above, the pressing force of the transfer auxiliary member or the transfer member toward the image carrier at each transfer position is different for each station. Depending on the image forming apparatus (for example, refer to Patent Document 1) provided with means for setting the pressing force or the image pattern of the toner image formed on the image bearing member, the pressing force of the transfer member against the image bearing member is changed. An image forming apparatus (for example, see Patent Document 2) provided with a changing pressing force changing means is disclosed. Further, there is an example in which the pressing position of the pressing roller is displaced according to the monochrome mode and the color mode, and the sheet is attracted to the transfer belt by applying an attracting bias in the monochrome mode (see, for example, Patent Document 3). . Further, by setting the pressing force of the pressing means that presses against the plurality of photosensitive drums to be weaker toward the toner image forming portion on the downstream side in the transfer paper moving direction or the downstream side in the intermediate transfer member moving direction, a void occurs. There is an example in which the pressing force of the downstream toner, which is easy to do, is weakened to prevent hollowing out (see, for example, Patent Document 4).

Japanese Patent Laid-Open No. 06-202430 Japanese Patent Laid-Open No. 10-11607 JP 2001-34082 A JP 2002-14515 A

  The present invention has been made in view of the above problems, and further improves the prior art, paying attention to the pressing force of the black toner against the image carrier and the pressing force of the toner of each color except the black toner against the image carrier, An object of the present invention is to provide an image forming apparatus capable of preventing image defects such as voids and white spots.

In order to solve the above problems, the present invention has the following features.
The image forming apparatus of the present invention passes through a plurality of image carriers, a plurality of developing units that form toner images for each color on each of the plurality of image carriers, and a transfer position facing the plurality of image carriers. A plurality of pressing means for pressing the transfer body against the respective image carriers; and a plurality of transfer means for sequentially superimposing and transferring the toner images onto the transfer body at positions where the plurality of image carriers and the transfer body are in contact with each other. In the image forming apparatus, the pressing unit presses the pressing unit against the image carrier on which the black toner image is formed, and the pressing unit presses against the plurality of image carriers on which the toner images of the respective colors excluding the black toner are formed. The pressure is 1.5 to 3.0 times the weakest pressing force among the pressures.
In order to prevent voids, it is better to reduce the pressing force of the second and subsequent toners on the image carrier, but black toner does not often form line images or characters on top of other colors. The pressing force of the pressing unit on the image carrier on which the black toner image is formed need not be as small as the pressing force of the pressing unit on the plurality of image carriers on which the color toner images are formed, and sufficient transfer efficiency can be obtained. is required. Therefore, the black toner can be sufficiently obtained by making the pressing force of the pressing means for the image carrier on which the black toner image is formed larger than the pressing force of the pressing means for the plurality of image carriers on which the color toner images are formed. Transfer. On the other hand, when the pressing force is low, white spots are likely to occur in a low humidity environment. For this reason, by making the pressing force of the pressing means for the image carrier on which the black toner image is formed larger than the pressing force of the pressing means for the plurality of image carriers on which the color toner images are formed, in a low humidity environment. Prevent white spots.

In the image forming apparatus of the present invention, the color toner that is later transferred to the transfer target is the pressing force of the pressing unit on the plurality of image carriers on which the toner images of the respective colors excluding the black toner are formed. It is characterized by weakening.
In order to prevent voids, it is better to reduce the pressing force of the second and subsequent toners against the image carrier. However, if the pressing force is too weak, white spots due to poor adhesion between the image carrier and the transfer medium may occur. It tends to occur. For this reason, by appropriately increasing the pressing force of the color toner first transferred from the image carrier to the transfer target and gradually decreasing the pressing force of the color toner transferred from the second color onward, Securely prevent disconnection.

When the image forming apparatus of the present invention forms a color toner image by superposing a plurality of color toners,
The black toner is transferred last.
When toner is stacked on the transfer target, the transfer target is charged by the upstream transfer bias, and therefore, white spots are more likely to occur in the toner to be transferred later. For this reason, the generation of white spots can be suppressed by increasing the most downstream pressing force. In a color image forming apparatus, an image may be formed with a single black toner, and the use frequency of the black toner is higher than that of other color toners. For this reason, the black toner is transferred last, thereby preventing white spots of the black toner. In addition, the black toner is transferred onto the transfer medium and the distance from the fixing to the fixing is made shorter than other color toners, so that the black toner is improved and the defective image generated between the transfer and fixing is reduced. prevent.

In the image forming apparatus of the present invention, when a toner image is formed with a single color of black toner, the transferred body is not brought into contact with a plurality of image carriers on which toner images of the respective colors excluding the black toner are formed. And
When an image is formed with a single color of black toner, the image carrier of each color toner except for the black toner is not moved, and the transfer target is not in contact with the image carrier of each color toner except for the black toner. This prevents the image carrier of each color toner except the black toner from being worn.

The image forming apparatus of the present invention is characterized in that the transfer target is a transfer material supported on an endless belt and conveyed to a transfer region.
In the image forming apparatus of the present invention, the transfer target is an endless belt intermediate transfer member conveyed to a transfer region.

In the image forming apparatus of the present invention, the average circularity of the toner used in the developing unit is 0.90 to 0.99.
The closer the toner is to a sphere, the more precise the reproducible high-definition image can be formed, and the occurrence of voids can be suppressed. In particular, if a near-spherical toner having an average circularity of 0.90 to 0.99 is used, the occurrence of voids can be suppressed.

  By means for solving the above-mentioned problems, the present invention provides a pressing force of the pressing means for the image carrier on which the black toner image is formed, and a pressing force of the pressing means for the plurality of image carriers on which the color toner images are formed. The black toner is transferred at the end so that the black toner can be sufficiently transferred, and image defects such as voids and white spots can be prevented. Further, by sequentially decreasing the pressing force of the color toner transferred from the second color onward, it is possible to reliably prevent the color toner from being lost.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to FIGS. However, these are merely embodiments, and do not limit the scope of the claims of the present invention.

FIG. 1 is a schematic configuration diagram of a color image forming apparatus. In this embodiment, a direct transfer method using a transfer medium as a transfer sheet is adopted. This image forming apparatus carries each toner image forming unit, optical writing unit 2, paper feed cassettes 3, 4, registration roller pair 5, and transfer paper 100 so as to pass through the transfer position of each toner image forming unit. A transfer unit 6 as a belt driving device having a transfer conveyance belt 60 as a transfer conveyance member to be conveyed, a fixing unit 7 of a belt fixing system, a paper discharge tray 8, a manual feed tray MF, a toner supply container TC, a belt cleaning device 85, and the like. I have. In the space S indicated by a two-dot chain line, a waste toner bottle, a duplex / reversing unit, a power supply unit, and the like (not shown) are provided.
The toner image forming units 1Y, 1M, 1C, and 1K are sequentially arranged from the upstream side in the moving direction of the transfer paper 100 (the direction in which the transfer conveyance belt 60 travels counterclockwise along the arrow A in the drawing). . The toner image forming units 1Y, 1M, 1C, and 1K are arranged so that the rotation axes of the photosensitive drums are parallel to each other and aligned at a predetermined pitch in the moving direction of the transfer paper 100. The toner image forming units 1Y, 1M, 1C, and 1K include photosensitive drums 11Y, 11M, 11C, and 11K as image carriers and developing units, respectively, and yellow (Y), magenta (M), and cyan (C ) And black (K) color images are formed on the photosensitive drums.
The optical writing unit 2 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each photosensitive drum while scanning with laser light based on image data.
The belt cleaning device 85 includes a brush roller and a cleaning blade, and is disposed so as to be in contact with the outer peripheral surface of the transfer / conveying belt 60, and removes deposits such as toner adhering to the transfer / conveying belt 60.
A one-dot chain line in the drawing indicates a conveyance path of the transfer paper 100. The transfer paper 100 transported from the paper feed cassettes 3 and 4 or the manual feed tray MF is transported by transport rollers while being guided by a transport guide (not shown), and is sent to a temporary stop position where the registration roller pair 5 is provided. The transfer paper 100 delivered at a predetermined timing by the registration roller pair 5 is carried on the transfer conveyance belt 60, conveyed to the toner image forming units 1Y, 1M, 1C, and 1K, and passes through the transfer nips. The respective toner images developed on the photosensitive drums 11Y, 11M, 11C, and 11K of the toner image forming portions 1Y, 1M, 1C, and 1K are superimposed on the transfer paper 100 at the respective transfer nips, and transferred electric fields and transfer nip pressures. Is transferred onto the transfer paper 100, and a full-color toner image is formed on the transfer paper 100.
The photosensitive drums 11Y, 11M, 11C, and 11K after the transfer of the toner image are removed from the transfer toner remaining on the surface of the photosensitive drum by the photosensitive member cleaning device, and further discharged to remove the next electrostatic latent image. Prepare for the formation of
After the full color toner image is fixed by the fixing unit 7, the transfer paper 100 on which the full color toner image is formed corresponds to the rotation position of the switching guide G, and the first paper discharge direction B or the second paper discharge. Head in direction C. When the paper is discharged from the first paper discharge direction B onto the paper discharge tray 8, it is stacked in a so-called face-down state with the image surface down. On the other hand, when the paper is discharged in the second paper discharge direction C, it is conveyed toward another post-processing device (such as a sorter or a binding device) (not shown), or for duplex printing through a switchback unit. Is again conveyed to the registration roller pair 5.

FIG. 2 is a schematic view of a direct transfer type transfer unit. In the direct transfer method, an endless belt for supporting and transporting the transfer paper 100 is adopted, and toner images of respective colors formed on a plurality of photosensitive drums are directly transferred onto the transfer paper. The transfer conveyance belt 60 of the present embodiment is a high-resistance endless single-layer belt having a volume resistivity of 10 ⁹ to 10 ¹¹ Ωcm, and the material thereof is PVDF (polyvinylidene fluoride). The transfer conveyance belt 60 is stretched around rollers 61 to 66 so as to pass through the transfer positions that are in contact with and face the photosensitive drums 11Y, 11M, 11C, and 11K of the toner image forming units. Among these rollers, the support roller 61 at the upstream entrance of the transfer sheet 100 in the moving direction is disposed on the outer peripheral surface of the transfer conveyance belt 60 so that the electrostatic adsorption roller 80 to which a predetermined voltage is applied from the power supply 80a faces each other. ing. The transfer paper 100 that has passed between the support roller 61 and the electrostatic adsorption roller 80 is electrostatically adsorbed on the transfer conveyance belt 60. The driving roller 63 is disposed on the inner peripheral surface of the transfer conveyance belt 60 and drives the transfer conveyance belt 60 counterclockwise. The support roller 64 is disposed downstream of the drive roller 63 in a direction of pushing the outer peripheral surface of the transfer conveyance belt, and ensures a winding angle around the drive roller 63. A tension roller 65 that applies tension to the transfer / conveyance belt 60 by a pressing member (spring) 69 is provided on the inner peripheral surface of the transfer / conveyance belt 60 further downstream from the support roller 64.
At each transfer position, transfer bias applying members 67Y, 67M, 67C, and 67K are provided as transfer electric field forming means for forming a transfer electric field so as to be in contact with the back surface of the transfer conveyance belt 60 at a position facing the photosensitive drum. Provided. These are bias rollers provided with a sponge or the like on the outer periphery, and a transfer bias is applied to the roller core from each of the transfer bias power supplies 9Y, 9M, 9C, 9K. Due to the action of the applied transfer bias, a transfer charge is applied to the transfer conveyance belt 60, and a transfer electric field having a predetermined strength is formed between the transfer conveyance belt 60 and the surface of the photosensitive drum at each transfer position. Further, in order to appropriately maintain the contact between the transfer paper 100 and the photosensitive drum in the transfer area and obtain the best transfer nip, the backup roller 68 is provided in the vicinity of the transfer bias applying members 67Y, 67M, 67C, and 67K. In preparation.
The transfer bias applying members 67Y, 67M, and 67C and the backup roller 68 disposed in the vicinity thereof are integrally supported by the swing bracket 93 so as to be rotatable. The swing bracket 93 swings clockwise about the shaft 94 as the cam 96 fixed to the cam shaft 97 rotates in the direction of the arrow. The support roller 61 at the entrance and the suction roller 80 are integrally supported by the entrance roller bracket 90 and rotate clockwise about the shaft 91 as a rotation center. The hole 95 provided in the swing bracket 93 and the pin 92 fixed to the entrance roller bracket 90 are associated with each other, and swing in the clockwise direction in conjunction with the swing bracket 93. The transfer bias applying members 67Y, 67M, and 67C and the backup roller 68 disposed in the vicinity thereof are separated from the photosensitive drums 11Y, 11M, and 11C by the swinging of the brackets 90 and 93 in the clockwise direction, and the support roller 61 The suction roller 80 also moves downward. With such a configuration, contact between the photosensitive drums 11Y, 11M, and 11C for forming a color toner image and the transfer conveyance belt 60 can be avoided when a toner image is formed with a single black toner.
On the other hand, the transfer bias applying member 67K and the backup roller 68 adjacent thereto are rotatably supported by the exit roller bracket 98. The exit roller bracket 98 is swingable clockwise about a shaft 99 that is coaxial with the support roller 62 at the exit downstream of the transfer paper 100 in the moving direction. With such a configuration, when the transfer unit 6 is attached to or detached from the main body, the transfer unit 6 is rotated clockwise by an operation of a handle (not shown) from the photosensitive drum 11K for black toner image formation to the transfer bias applying member. 67K and the backup roller 68 adjacent thereto can be separated.

FIG. 3 and FIG. 4 show an embodiment when an intermediate transfer system using an intermediate transfer belt as a transfer target is employed.
FIG. 3 is a schematic diagram of an intermediate transfer method in the case of forming a color toner image. In the intermediate transfer method, toner images of respective colors formed on a plurality of photosensitive drums are transferred onto an intermediate transfer member in a superimposed manner, and then transferred onto a transfer sheet at a time. In the intermediate transfer system, an endless belt is generally adopted as an intermediate transfer body in order to receive and carry each color toner image formed on each photosensitive drum. In the intermediate transfer system of this embodiment, the pressing force on each photosensitive drum on which a toner image is formed is reduced in the order of the photosensitive drums 11Y, 11M, and 11C for forming color toner images, and is transferred to the intermediate transfer belt. The weaker the color toner, the later. The pressing force on the photosensitive drum 11K on which the black toner image is formed is larger than the pressing force on the photosensitive drum 11C that is the weakest among the pressing forces on the plurality of photosensitive drums on which the color toner images are formed.

  FIG. 4 is a schematic diagram of an intermediate transfer method in the case of forming a toner image with a single black toner. In the intermediate transfer method of the present embodiment, the intermediate transfer belt is not brought into contact with the photosensitive drums 11Y, 11M, and 11C for forming color toner images excluding black toner, and is applied to the photosensitive drum 11K on which a black toner image is formed. Only contact. Further, when an image is formed with a single black toner, the photosensitive drums 11Y, 11M, and 11C for forming color toner images excluding the black toner are stopped to prevent the photosensitive drum from being worn.

FIG. 5 is a graph showing the relationship between the pressing force and the void. ○ is a single color toner, Δ is a second color toner with two colors superimposed, and x is a third color toner with three colors superimposed. In the case of a single color, even if the pressing force is increased, it is difficult for hollows to occur. However, when the pressing force is 0.3 N / cm or more, the hollow is deteriorated, so even a single color cannot be made 0.3 N / cm or more. As the color of two-color superposition, three-color superposition and toner superposition increases, the later transferred toner is more likely to have voids. In the case of two-color superimposition, a void is generated at 0.2 N / cm or more, and in the case of three-color superposition, it is 0.15 N / cm or more. In order to prevent voids, it is better to reduce the pressing force for the second and subsequent colors, but since black toner does not often form line images or characters on top of other colors, a black toner image is formed. The pressing force on the photosensitive drum does not need to be as small as the pressing force on the plurality of photosensitive drums on which the color toner images are formed, and the black toner should be sufficiently transferred. Therefore, it is desirable that the pressing force of the black toner is 1.5 to 3.0 times the pressing force of the third color, which is the weakest pressing force among the color toners. In the actual machine, the pressing force of black toner is set to 0.2 N / cm and the pressing force of the third color of color toner is set to 0.1 N / cm.

FIG. 6 is a graph showing the relationship between the pressing force and the white potty rank in a low humidity environment. ○ is the first color toner transferred first, and x is the toner transferred last. The toner transferred last has white spots when the pressing force is 0.15 N / cm or less. Thus, when viewed in a low-humidity environment where white spots are likely to occur, white spots are more likely to occur when toner is transferred later than toner transferred first. This is because the transfer paper is easily charged in a low-humidity environment, and the transfer paper is charged by the transfer bias on the upstream side, so white spots are more likely to occur in the toner transferred after the downstream side. In order to prevent white spots, it is better to increase the pressing force. For this reason, in this embodiment, the black toner is made to have a low pressing force by making the pressing force of the black toner larger than the pressing force on the plurality of photosensitive drums on which the color toner images are formed, and transferring the black toner last. Prevents white spots in the environment.

FIG. 7 is a graph showing the relationship between the average circularity of the toner and the voids. FIG. 7 is a graph showing the relationship between the average circularity of the toner and the void when the pressing force is 0.1 N / cm. From the graph of FIG. 7, when the average circularity of the toner is less than 0.90, the toner has an irregular shape far from the spherical shape, and therefore, voids are generated. On the other hand, when the average circularity of the toner exceeds 0.99, in a system employing blade cleaning or the like, defective cleaning of the photosensitive drum or transfer belt occurs, and further, the photosensitive drum is contact-charged. The charging roller or the like is contaminated, and charging failure occurs, causing image failure. Therefore, it has been found that the use of a nearly spherical toner having an average circularity of 0.90 to 0.99 can suppress the occurrence of voids. More preferably, the toner having an average circularity of 0.93 to 0.97 and a toner having an average circularity of less than 0.94 is 10% or less.
As described above, it is important that the toner in the present invention has a specific shape and distribution of the shape. The closer the toner is to a spherical shape, the lower the occurrence of voids and the high density with the reproducibility of appropriate density. An image can be formed. As a method for measuring the shape, an optical detection band method is suitable in which a suspension containing particles is passed through an imaging unit detection band on a flat plate, and a particle image is optically detected and analyzed by a CCD camera. is there. By this method, the average circularity is obtained as a value obtained by dividing the perimeter of an equivalent circle having the same projected area by the perimeter of the actual particle.

1 is a schematic configuration diagram of a color image forming apparatus. It is the schematic of the transfer unit of a direct transfer system. FIG. 3 is a schematic diagram of an intermediate transfer method when forming a color toner image. FIG. 3 is a schematic diagram of an intermediate transfer method when a toner image is formed with a black toner single color. It is the graph which showed the relationship between pressing force and a hollow. It is the graph which showed the relationship between the pressing force in a low humidity environment, and white potty rank. 6 is a graph showing the relationship between the average circularity of toner and voids.

Explanation of symbols

1Y, 1M, 1C, 1K Toner image forming unit 2 Optical writing unit 3, 4 Paper feed cassette 5 Registration roller pair 6 Transfer unit 7 Fixing unit 8 Paper discharge trays 9Y, 9M, 9C, 9K Transfer bias power supplies 11Y, 11M, 11C , 11K photosensitive drum 60 transfer conveyance belt 61, 62, 64, 66 support roller 63 drive roller 65 tension roller 67Y, 67M, 67C, 67K transfer bias applying member 68 backup roller 69 pressing member (spring)
80 Electrostatic adsorption roller 85 Belt cleaning device 90 Entrance roller bracket 91 Shaft 92 Pin 93 Swing bracket 94 Shaft 95 Hole
96 cams
97 Camshaft
98 Outlet roller bracket
99 axes
100 Transfer sheet A Belt running direction B First paper discharge direction C Second paper discharge direction G Switching guide S Space MF Manual feed tray TC Toner supply container

Claims (7)

A plurality of image carriers;
A plurality of developing means for forming a toner image for each color on each of a plurality of image carriers;
A plurality of pressing means for pressing a transfer target that passes through a transfer position facing the plurality of image carriers against each of the image carriers;
In an image forming apparatus comprising a plurality of transfer units that sequentially superimpose and transfer a toner image to a transfer target at a position where the plurality of image carriers and the transfer target come into contact with each other.
The pressing force of the pressing unit on the image carrier on which the black toner image is formed is the weakest pressing force among the pressing units of the pressing unit on the plurality of image carriers on which the toner images of the respective colors excluding the black toner are formed. An image forming apparatus characterized in that the image forming apparatus is 1.5 to 3.0 times as large as the image forming apparatus. The pressing force of the pressing unit against a plurality of image carriers on which toner images of each color excluding the black toner are formed is weaker as the color toner is transferred to the transfer target later. Item 2. The image forming apparatus according to Item 1. When forming a color toner image by superimposing multiple color toners,
The image forming apparatus according to claim 1, wherein the black toner is transferred last. When forming a toner image with black toner only,
4. The image forming apparatus according to claim 1, wherein the transfer object is not brought into contact with a plurality of image carriers on which toner images of each color except the black toner are formed. 5.   The image forming apparatus according to claim 1, wherein the transfer target is a transfer material supported on an endless belt and conveyed to a transfer region.   The image forming apparatus according to claim 1, wherein the transfer target is an intermediate transfer member of an endless belt conveyed to a transfer region. 7. The image forming apparatus according to claim 1, wherein the toner used in the developing unit has an average circularity of 0.90 to 0.99.

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