JP2006267681A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can provide an uniformly glossy output image without increasing the size of its main body and degrading gradation reproducibility. <P>SOLUTION: The image forming apparatus includes: an image carrier; an image exposure means for forming an electrostatic latent image corresponding to an image information signal; a developing means which develops the electrostatic latent image formed on the image carrier with developer; a transfer means which electrostatically transfers the toner image formed on the image carrier to a transfer material; a fixing means which fixes an unfixed toner image on the transfer material separated from the image carrier by holding the unfixed toner under heat and pressure; and a pre-transfer exposure means which re-exposes the image carrier upstream of a transfer section in the direction of the movement of the image carrier. The image forming apparatus is provided with a control means which selectively re-exposes the face of the image carrier around an arbitrary toner image on the image carrier by the pre-transfer re-exposure means. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine and a printer. In particular, the present invention relates to a full-color image forming apparatus capable of forming a color image having good glossiness.

  In image forming apparatuses such as printers and copiers that form images by electrophotography, an electrostatic latent image carrier such as a charged photoconductor is laser-exposed to form a latent image. As an image forming apparatus for forming a toner developed image on the electrostatic latent image by a developing unit and transferring it to a transfer material, or a method for forming a multicolor image, a plurality of photosensitive members corresponding to each color, charging and developing There is known a full-color image forming apparatus that has a transfer portion and sequentially superimposes and transfers it onto a transfer material.

  By the way, it is well known that, in general, a printed image by silver salt photography or plate-making printing is excellent in image quality because of a wide color reproduction range, good gradation reproducibility, gloss uniformity and the like. Therefore, it is desired that an image forming apparatus using an electrophotographic method also realizes image formation with an image quality that is not inferior to these silver halide photographs and plate-making printed images. That is, an image processing apparatus using an electrophotographic method is also required to form an image with uniform gloss, such as a silver salt photograph or a plate-making printed image.

  However, in the conventional electrophotographic method, due to the problems described below, it has been difficult to achieve uniform glossiness that is essential in image quality that is not inferior to silver salt photographs and plate-making printed images.

  It is well known that the gloss level is generally determined mainly by the image surface shape. That is, when the image surface is smooth, the gloss is high, and when the image surface is uneven, the light is irregularly reflected by the uneven portions, thereby reducing the gloss.

  In the electrophotographic system, as shown in FIG. 2, generally, for example, density gradation is formed by changing the exposure area per pixel by a known PWM system using a semiconductor laser (hereinafter referred to as “area modulation”). )is doing. As a result, the surface shape of the toner image on the paper varies greatly depending on each density region. As a result, the gloss varies greatly depending on the density region. For example, when glossy paper is used, the relationship between the density and the glossiness is as shown in FIG. 4, and the uniform glossiness is impaired.

  In order to solve the above problems, for example, as disclosed in Patent Document 1 and Patent Document 2, instead of the above-described area modulation when forming density gradation, each pixel includes, for example, a semiconductor. By changing the intensity of exposure using a laser (hereinafter referred to as “brightness modulation”), it is possible to suppress a gloss change depending on the density region. This is because the surface shape of the toner image on the paper becomes substantially uniform depending on each density region as shown in FIG.

  However, in general, when density gradation formation by the above-described luminance modulation is performed, the toner applied amount curve on the photoreceptor with respect to the development contrast potential difference (the difference between the exposure potential V1 and the development potential Vdc) in the development process becomes steep. In particular, it is said that the gradation reproducibility of highlight density is impaired. That is, although uniform gloss is achieved by image formation by luminance modulation, it is difficult to implement because the gradation reproducibility, which is one of the important image performances as photographic image quality, is impaired as a negative effect.

  In order to solve the problem of uniform glossiness, a method for transferring and fixing a transparent toner to a transfer body in addition to a color toner has been proposed as disclosed in, for example, Patent Documents 3 and 4. ing. As a result, the uneven portions generated in the color toner image can be smoothed by being filled with the transparent toner, so that the gloss of each density region becomes substantially uniform.

JP 2002-031921 A JP 2003-186260 A Japanese Patent Laid-Open No. 04-278967 JP 09-200551 A

  However, it goes without saying that a transparent toner image forming portion is necessary to form an image of the transparent toner, and the main body cannot be enlarged.

  The present invention has been made in view of the above problems, and the intended process is to obtain a uniform gloss output image without enlarging the main body and without impairing the gradation reproducibility. Is to provide a simple image forming apparatus.

  In order to achieve the above object, the present invention provides an image carrier, an image exposure unit for forming an electrostatic latent image corresponding to an image information signal, and a developer that uses the electrostatic latent image formed on the image carrier. Developing means for developing the toner image, transfer means for electrostatically transferring the toner image formed on the image carrier to the transfer material, and fixing the unfixed toner image on the transfer material separated from the image carrier by heating and pressing. In an image forming apparatus, comprising: a fixing unit; and a pre-transfer re-exposure unit that re-exposes the image carrier on the upstream side of the transfer unit in the moving direction of the image carrier. Control means for selectively re-exposing the image carrier surface around an arbitrary toner image is provided.

  According to the present invention, the toner image can be made substantially uniform by performing re-exposure before transfer on the surface of the photoreceptor around the arbitrary toner image, and a uniform glossy output image can be obtained. Further, according to the present invention, the apparatus is not increased in size and the gradation reproducibility is not impaired.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

<Embodiment 1>
FIG. 1 is a cross-sectional view showing a configuration of a main part of a full-color image forming apparatus according to Embodiment 1 of the present invention. The full-color image forming apparatus according to this embodiment includes a plurality of image forming units UC (cyan units), UM (magenta unit), UY (yellow unit), and UK (black unit) are provided, and an intermediate transfer belt 20 is disposed vertically through the image forming units UC, UM, UY, UK. Here, the configuration of the image forming unit UC will be described, but since the configurations of the other image forming units UM, UY, UK are the same, description thereof will be omitted.

  In the image forming unit (cyan unit) UC, 10C is a cylindrical photoconductor as a rotating electrostatic latent image carrier, 11C is a primary charger, and the primary charger 11C is connected to the photoconductor 10C. Installed without contact. Reference numeral 12C denotes an image exposure unit including a semiconductor laser. The image exposure unit 12C exposes the photoconductor 10C to the primary charger 11C on the downstream side in the rotation direction of the photoconductor 10C.

  Reference numeral 13C denotes a developing device, which is installed on the further downstream side of the exposure position of the photoconductor 10C so as to be adjacent to the photoconductor 10C. Reference numeral 20 denotes an intermediate transfer belt, and reference numeral 14C denotes a transfer roller, which is installed at the primary transfer position so as to face the photoreceptor 10C with the intermediate transfer belt 20 interposed therebetween. Reference numeral 30 denotes a secondary transfer portion, which is installed so as to face the secondary transfer outer roller 31 with the intermediate transfer belt 20 interposed therebetween.

  Reference numeral 50C denotes a pre-transfer re-exposure section, which is composed of, for example, a plurality of LED element arrays arranged in the axial direction of the photoreceptor 10C. This LED element is controlled by an LED light emission control unit (not shown) so that the LED elements arranged in the axial direction can emit light individually at any timing, and can be exposed to any thrust and circumferential position of the photosensitive drum. It has become. Reference numeral 15 denotes a cleaner for cleaning the transfer residual toner on the photosensitive member, and reference numeral A denotes a fixing device for fixing the toner image mounted on the transfer material by heating and pressing.

  In the image forming apparatus configured as described above, the image forming operation in the full color mode will be described by taking the operation of the unit UC during image formation as an example. Since the same applies to the magenta, cyan, and black single color modes, a description thereof will be omitted.

  First, the photoconductor 10C rotates, for example, having an optical semiconductor layer formed of amorphous silicon on the surface of a conductive substrate. The surface of the photoreceptor 10C is negatively charged uniformly by the primary charger 11C that performs corona discharge, and then exposed by the exposure unit 12C to form an electrostatic latent image corresponding to the original.

  In the present embodiment, when the gradation is formed, the image exposure unit 12C performs so-called area modulation in which the laser oscillation time is changed by known pulse width modulation. The developing device 13C is a known two-component developing system, in which toner and carrier two-component developer are magnetically adsorbed on a sleeve (not shown), and a DC bias or a DC bias and an AC bias are superimposed and applied to the developing sleeve. Thus, the negatively charged toner is caused to fly to the electrostatic latent image portion of the photoreceptor.

  The toner image formed on the surface of the photoreceptor 10C is selectively re-exposed on the photoreceptor surface around the arbitrary toner portion in the pre-transfer re-exposure section 50C. The operation and effect of the pre-transfer re-exposure unit 50C will be described in detail later.

  The toner image on the photoreceptor 7 is transferred to the intermediate transfer belt 20 by the electric field of the transfer roller 14C. The above operation is performed in each of the image forming units UC, UM, UY, UK, and the toner images formed on the respective photoconductors are sequentially transferred to the intermediate transfer belt 20 in a multiple transfer manner. The untransferred toner remaining on each photoconductor is cleaned by the respective color cleaners 15C, 15M, 15Y, and 15K.

  The toner images sequentially transferred onto the intermediate transfer belt 20 in a multiple transfer are collectively transferred to the transfer material 7 fed from the paper feed unit 40 at the secondary transfer unit 30 in accordance with the timing of the image part of the intermediate transfer belt 20. Is done. The transfer material that has received the transfer of the toner image is conveyed to the fixing device A, is heated and fused, and is discharged as a final image onto a discharge tray. The transfer residual toner remaining on the intermediate transfer belt 20 is cleaned by the cleaner 34.

  By the way, in the present embodiment, an object is to achieve uniform gloss in the image plane in all density regions, and by re-exposing the photoreceptor surface around any toner by the above-mentioned pre-transfer re-exposure unit 50C. This is realized by smoothing the toner image on the surface of the photoreceptor.

  Before describing the details, first, the potential distribution of the photoreceptor when the toner image reaches the pre-transfer exposure unit 50C will be briefly described.

  FIG. 5 shows the potential distribution of the photoreceptor when reaching the transfer re-exposure section 50C. At the time of image formation, the surface potential of the corona discharge by the primary charger 11C on the surface of the photoreceptor becomes the charging potential Vd, and the surface potential of the portion exposed to the laser on the exposure unit 12C becomes the exposure potential V1. For example, at an intermediate density, a predetermined area is laser-exposed on each pixel by area gradation, so that the photoreceptor potential distribution has an uneven well-type potential shape.

  Then, the negatively charged toner adheres to the photoconductor due to the development contrast which is the potential difference between the surface potential Vdc and the exposure potential V1 of the developing sleeve of the developing device 13C. At this time, the surface potential of the toner layer on the surface of the photoconductor is The potential is Vt. That is, since the toner image on the surface of the photosensitive member is constrained in the well of the drum potential potential, the uneven image is maintained while including the repulsive force of the negative charge between the toners. The toner unevenness image on the photoreceptor remains in the final image after the secondary transfer process and the fixing process, and as a result, the gloss unevenness is caused by irregular reflection on the surface of the image unevenness.

  The feature of the present invention is to make the toner unevenness image substantially uniform by eliminating the potential potential.

  That is, as shown in FIG. 6, the photosensitive member surface potential Vd is set to the exposure potential V1 by re-exposing the periphery of the toner image on the photosensitive member, that is, the photosensitive member surface portion of the non-toner portion, before transfer. By removing the potential potential of the mold and setting the toner potential Vt to an unstable potential state that protrudes, the coulomb repulsive force between the toner charges is released and dispersed around the toner, thereby making the toner unevenness image substantially uniform. By performing these pre-transfer re-exposure steps in the UY, UM, UC, and UK units in the same manner, the toner surface of the final image after the fixing step is made uniform, and uniform gloss in the image plane can be realized.

  Further, in the present embodiment, the gradation reproduction, which is a harmful effect of the gloss uniformity technique by luminance modulation described in the prior art, is not impaired. In this embodiment, a toner image is primarily formed on an electrostatic latent image formed by area modulation, and for each toner rearrangement at a pre-transfer re-exposure section thereafter, for each pixel on the photoreceptor. The amount of applied toner is the same as that before re-exposure before transfer. That is, in the electrophotographic system, the image density is almost determined by the amount of applied toner, so that even if re-exposure before transfer is performed, a level equivalent to the gradation reproducibility in the conventional area gradation can be realized.

  In this embodiment, the re-exposure is performed by selecting the periphery of the toner image on the photosensitive member, that is, the non-toner portion. However, even if the toner image and the surrounding photosensitive member surface are uniformly re-exposed. A substantially similar effect can be obtained. In this case, the toner potential after the re-exposure and the surrounding photosensitive member potential are the same potential, and the toner unevenness portion is dispersed and substantially uniformed due to the coulomb repulsive force between the toner charges.

  The fixing device has a fixing configuration in which a toner image formed on a transfer material is heated and pressed and conveyed by fixing between a fixing member having an elastic layer of at least 0.5 mm or more and a pressure member. It is desirable to be. This is because if the fixing member does not have an elastic layer during the fixing process, unevenness in gloss (non-uniformity) occurs because the unevenness (texture) of the paper cannot follow the minute toner unevenness image on the paper.

  As described above, by performing pre-transfer re-exposure on the surface of the photosensitive member around the toner image on the photosensitive member, the toner uneven image can be dispersed and substantially uniformed around the toner image without increasing the size of the apparatus. A uniform glossy output image can be obtained without impairing tone reproduction.

<Embodiment 2>
In the first embodiment, the pre-transfer re-exposure is performed on the surface of the photoconductor around the arbitrary toner image, and the effect of the embodiment has been described. However, the pre-transfer re-exposure is performed on the photoconductor surface around the entire toner image. You don't have to.

  For example, an image data recognition mechanism (not shown) for recognizing image data and text data separately from image data, and for image data in which uniform gloss is important, toner of an electrostatic latent image portion corresponding to the image data Of course, it is also possible to perform re-exposure before transfer only around the image to make it uniformly glossy, and control so that re-exposure before transfer is not performed on text data where resolution is important.

1 is a schematic cross-sectional view of an image forming apparatus showing an embodiment of the present invention. It is a figure which shows the relationship of the reflected light with respect to the electrostatic latent image and toner image which were formed by area modulation per pixel in the conventional image forming apparatus. It is a figure which shows the relationship of the reflected light with respect to the electrostatic latent image and toner image which were formed by the luminance modulation per pixel in the conventional image forming apparatus. It is a figure which shows the relationship between the gradation density at the time of performing the electrostatic latent image formed by the area modulation per pixel in the conventional image forming apparatus, and glossiness. FIG. 3 is a potential distribution diagram showing a photoreceptor surface potential before retransfer pre-transfer in Embodiments 1 and 2 of the present invention. FIG. 3 is a potential distribution diagram showing a photoreceptor surface potential after re-exposure before transfer in Embodiments 1 and 2 of the present invention.

Explanation of symbols

UC, UM, UY, UK Image forming unit 10C, 10M, 10Y, 10K Photoreceptor 11C, 11M, 11Y, 11K Primary charger 12C, 12M, 12Y, 12K Image exposure unit 13C, 13M, 13Y, 13K Developing device 14C, 14M, 14Y, 14K transfer roller 15C, 15M, 15Y, 15K cleaner 20 intermediate transfer belt 30 secondary transfer portion 31 secondary transfer outer roller

Claims (4)

An image carrier, an image exposure unit for forming an electrostatic latent image according to an image information signal, a developing unit for developing the electrostatic latent image formed on the image carrier with a developer, and an image carrier A transfer unit that electrostatically transfers the toner image formed on the transfer material to the transfer material, a fixing unit that fixes the unfixed toner image on the transfer material separated from the image carrier by heating and pressing, and an image carrier than the transfer unit. In an image forming apparatus having pre-transfer re-exposure means for re-exposing the image carrier on the upstream side in the movement direction,
An image forming apparatus comprising: control means for selectively re-exposing an image carrier surface around an arbitrary toner image on the image carrier by the pre-transfer re-exposure means.   The image forming apparatus according to claim 1, wherein the image exposure unit performs an area modulation process based on pixel subdivision using a PWM method.   A control having an image data recognition mechanism for recognizing image data and text data from image data, and re-exposing only the image carrier surface around an arbitrary toner image formed by the image data on the image carrier. 3. The image forming apparatus according to claim 1, further comprising means.   The fixing unit heats and fixes the toner image formed on the transfer material between the fixing member using an elastic layer having a thickness of 0.5 mm or more on the surface and the pressure member by heating and pressing. The image forming apparatus according to claim 1, wherein the image forming apparatus is a fixing device.

Images