JP4929697B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, and more particularly to an image forming apparatus that forms an image having a uniform density.

  In recent years, so-called full-color tandem machines have been proposed in image forming apparatuses such as printers, copiers, and facsimile machines for the purpose of forming color images at high speed and high image quality. A typical example of the tandem machine is one in which four image forming units of yellow, magenta, cyan, and black are arranged in parallel to each other. In this image forming apparatus, first, toner images of each color of yellow, magenta, cyan, and black, which are sequentially formed in each image forming unit, are transferred in multiple (primary transfer) onto the intermediate transfer belt. Next, the toner images of the respective colors transferred and superimposed on the intermediate transfer belt are collectively transferred (secondary transfer) from the intermediate transfer belt onto the paper. A full-color or black-and-white (monochrome) image is obtained by fixing the second-transferred toner image on the paper. There is also an image forming apparatus of a type that sequentially transfers toner images of each color formed by each image forming unit directly onto a sheet.

  On the other hand, recently, from the viewpoint of environmental protection, recycled paper obtained by recycling used paper has been increasingly used as paper. Among such recycled papers, for example, there are papers having a low whiteness compared to, for example, high-quality paper produced from fresh pulp. For this reason, for example, when color image formation is performed on paper such as recycled paper using the image forming apparatus, a situation may occur in which the color of the obtained image appears cloudy. Also, some recycled papers have lower smoothness than fine papers. For this reason, when image formation is performed on paper such as recycled paper using the image forming apparatus, the glossiness of the obtained image may change due to the difference in smoothness (surface property) of the paper. is there.

  Therefore, in addition to the image forming units for the respective colors, a white image forming unit for forming a white toner image is provided, and an image with a uniform density (for example, halftone) is formed on the entire surface using the white image forming unit during image formation. Has been proposed (see Patent Document 1). In addition to this image forming unit for each color, this Patent Document 1 further includes a transparent image forming unit for forming a transparent toner image, and the entire surface has a uniform density using this transparent image forming unit during image formation. A technique is also described in which an image is formed and this is used as a paper and an overcoat layer of a toner image on the paper.

JP 2002-99127 A (page 3-5, FIG. 1)

  By the way, in Patent Document 1, an image forming process using a normal electrophotographic method is adopted in an image forming unit that forms a toner image of a special color such as white or transparent. That is, the photosensitive drum is charged, the charged photosensitive drum is exposed to form an electrostatic latent image, and the electrostatic latent image formed on the photosensitive drum is developed with the toner of the special color. The special color toner image developed above is transferred to an intermediate transfer belt.

However, when an image having a uniform density of special colors is formed by such an image forming process, in-plane unevenness occurs in the image. This in-plane non-uniformity is caused by, for example, non-uniform charging due to a plurality of factors such as non-uniform charging of the photosensitive drum in the charging process, non-uniform exposure of the photosensitive drum in the exposure process, non-uniform development in the developing process, and non-uniform transfer in the transfer process. Is. For example, when such in-plane unevenness occurs in a white image formed as a base layer, the base layer unevenness becomes conspicuous, and on the contrary, the appearance deteriorates. Further, for example, when such in-plane unevenness occurs in a transparent image formed as an overcoat layer, uneven glossiness becomes conspicuous, and the appearance also deteriorates.
Such a problem occurs not only in the above-described tandem type image forming apparatus but also in an image forming apparatus employing an electrophotographic method.

  The present invention has been made to solve such technical problems, and an object thereof is to suppress in-plane unevenness in a uniform density image formed together with an image for image.

  For this purpose, an image forming apparatus to which the present invention is applied has a first image forming unit that forms an image on a recording material, and a second image forming unit that forms an image on a recording material, The first image forming unit includes an image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier, and an electrostatic latent image formed on the image carrier by the latent image forming unit. The image forming apparatus includes a developing unit that develops with a profile and a transfer unit that transfers an image developed on the image carrier onto the recording material by the developing unit. The second image forming unit directly develops the recording material with the profile. It is characterized by comprising a developing device.

  In such an image forming apparatus, the second image forming unit uses an image forming material other than yellow, magenta, cyan, and black, or uses a white image forming material or a transparent image forming material. Can be a feature. The first image forming unit can form yellow, magenta, cyan, and black images, and the second image forming unit can form white or transparent images. Further, the recording material is intermediately transferred before the image formed by the first image forming unit and the image formed by the second image forming unit are transferred to the final recording material. It can be characterized by being a transfer body. The latent image forming unit includes a charging unit that charges the image carrier to a predetermined potential, and an exposure unit that forms an electrostatic latent image by selectively exposing the surface of the image carrier charged by the charging unit. It can be characterized by comprising.

  From another point of view, an image forming apparatus to which the present invention is applied includes an image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier, and an image carrier using the latent image forming unit. A developing unit that develops the electrostatic latent image formed on the body with the image forming material, a transfer unit that transfers the image developed on the image carrier by the developing unit to the recording material, and the latent image to the image carrier. First mode for forming an image on a recording material by forming an electrostatic latent image by the image forming unit, developing by the developing unit, and transferring by the transfer unit, and the latent image forming unit for the image carrier And a control unit that performs control in a second mode in which an image is formed on a recording material by performing development by the development unit and transfer by the transfer unit without forming an electrostatic latent image. It is said.

  In such an image forming apparatus, the latent image forming unit selectively exposes the charging unit that charges the image carrier to a predetermined potential and the surface of the image carrier charged by the charging unit. And the control unit causes the exposure unit to perform exposure in the first mode and does not cause the exposure unit to perform exposure in the second mode. In this case, the control unit can perform charging by the charging unit in the first mode, and can not perform charging by the charging unit in the second mode. In the first mode, an image for an image is formed, and in the second mode, an image for an underlayer or an image for an overcoat layer is formed. Further, the control unit may be characterized in that the magnitude of the developing bias applied between the developing unit and the image carrier is different between the first mode and the second mode.

According to the present invention, since the image forming material is directly developed on the recording material by the second image forming unit, the latent image formation which is one of the causes of in-plane unevenness in the second image forming unit. Can be omitted, and in-plane unevenness in an image having a uniform density formed together with the image for image formed in the first image forming unit can be suppressed.
In addition, according to the present invention, an image is formed without forming a latent image, which is one of the causes of in-plane unevenness in the second mode. Therefore, for the image formed in the first mode. In-plane unevenness in a uniform density image formed together with the image can be suppressed.

The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below in detail with reference to the accompanying drawings.
<Embodiment 1>
FIG. 1 is a diagram showing an outline of the image forming apparatus according to the first embodiment. The image forming apparatus according to the present embodiment uses yellow (Y), magenta (M), cyan (C), and black (K) toner images (which are used to form an image toner image (full color image)). In addition to (image), a white (W) toner image can also be formed. The white toner image is used to form a base layer when an image is formed using paper P having a low whiteness such as recycled paper.

  The image forming apparatus includes, for example, a plurality (five in this embodiment) of image forming units 10 (specifically, 10Y (yellow), 10M (magenta), 10C (cyan), 10K) that form each color component toner image. (Black), 10W (white)). In addition, the image forming apparatus includes an intermediate transfer belt 20 that sequentially transfers (primary transfer) and holds each color component toner image formed by each image forming unit 10. The image forming apparatus further includes a secondary transfer device 30 that collectively transfers (secondary transfer) the superimposed image transferred to the intermediate transfer belt 20 onto the paper P. Furthermore, the image forming apparatus includes a fixing device 50 that fixes the second-transferred image on the paper P.

In the present embodiment, the image forming unit 10 includes a yellow image forming unit 10Y (hereinafter referred to as a yellow unit) and a magenta image forming unit 10M (hereinafter described) from the upstream side in the moving direction of the intermediate transfer belt 20. In the following, it is referred to as a magenta unit), a cyan image forming unit 10C (referred to as a cyan unit in the following description), a black image forming unit 10K (referred to as a black unit in the following description), and a white image forming unit 10W (referred to as follows). In the description, they are arranged in the order of white units).
Here, in the image forming unit 10, the yellow unit 10 </ b> Y, the magenta unit 10 </ b> M, the cyan unit 10 </ b> C, and the black unit 10 </ b> K as a first image forming unit that forms an image toner image are used toner (image shape). It has the same structure except for the color of the material. Accordingly, the yellow unit 10Y will be described as an example.

  The yellow unit 10Y includes a photosensitive drum 11 that has a photosensitive layer (not shown) and is rotatably arranged in an arrow A direction. Around the photosensitive drum 11, a charging roll 12, an exposure unit 13, a developing device 14, a primary transfer roll 15, and a drum cleaner 16 are disposed. The photosensitive drum 11 as an image carrier includes a photosensitive layer (not shown) having a negative charging polarity and is grounded. The charging roll 12 as the charging unit charges the photosensitive layer of the photosensitive drum 11 to a predetermined charging potential by applying a predetermined charging bias from a charging power source (not shown). The exposure unit 13 drives a semiconductor laser (not shown) or the like based on an image signal output from an image signal processing unit (not shown). Then, the exposure unit 13 forms an electrostatic latent image by irradiating the charged surface of the photosensitive drum 11 with the output beam Bm via a predetermined optical system. In the present embodiment, the charging roll 12 and the exposure unit 13 constitute a latent image forming unit. The developing unit 14 as a developing unit accommodates corresponding color component toner (for example, yellow toner in the yellow unit 10Y), and develops the electrostatic latent image on the photosensitive drum 11 with this toner. The developing device 14 uses a two-component developer (hereinafter simply referred to as a developer) including a toner having a negative charge polarity and a carrier having magnetism and a positive charge polarity. The toner on the developing roll is transferred to the electrostatic latent image on the photosensitive drum 11 by applying a predetermined developing bias to the developing roll provided in the developing device 14. A primary transfer bias 15 having a polarity opposite to the toner charging polarity (positive polarity in the present embodiment) is applied to the primary transfer roll 15 as a transfer unit, and the toner image on the photosensitive drum 11 is transferred to the intermediate transfer belt. 20 is electrostatically transferred. The drum cleaner 16 removes residues (toner and the like) on the photosensitive drum 11 after the primary transfer. Details of the white unit 10W will be described later.

  The intermediate transfer belt 20 as a recording material or an intermediate transfer member is rotatably supported by a plurality of (five in the present embodiment) support rolls. Of these support rolls, the drive roll 21 stretches the intermediate transfer belt 20 and drives the intermediate transfer belt 20 to rotate. Further, the opposing roll 22 constitutes a transfer surface of the intermediate transfer belt 20 for the image forming units 10Y, 10M, 10C, and 10K together with the drive roll 21. And this counter roll 22 functions also as a counter electrode of the white unit 10W mentioned later. Further, the correction roll 23 is disposed so as to be capable of tilting with the intermediate transfer belt 20 stretched around the intermediate transfer belt 20 and restricting meandering in a direction substantially orthogonal to the conveyance direction of the intermediate transfer belt 20. ). The driven roll 25 stretches the intermediate transfer belt 20 and rotates following the intermediate transfer belt 20 driven by the drive roll 21. The backup roll 24 stretches the intermediate transfer belt 20 and functions as a constituent member of the secondary transfer device 30 described later. Further, a belt cleaner 26 for removing residues (toner and the like) on the intermediate transfer belt 20 after the secondary transfer is disposed at a portion facing the drive roll 21 with the intermediate transfer belt 20 interposed therebetween.

  The secondary transfer device 30 includes a secondary transfer roll 31 disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 20 and a counter electrode of the secondary transfer roll 31 disposed on the back surface side of the intermediate transfer belt 20. And a backup roll 24. A power supply roll 32 that applies a secondary transfer bias having the same polarity as the charging polarity of the toner (negative polarity in the present embodiment) is disposed in contact with the backup roll 24. On the other hand, the secondary transfer roll 31 is grounded.

  The paper transport system includes a paper tray 40, a transport roll 41, a registration roll 42, a transport belt 43, and a discharge roll 44. In the paper transport system, after the paper P stacked on the paper tray 40 is transported by the transport roll 41, the paper P is temporarily stopped by the registration roll 42 and then moved to the secondary transfer position of the secondary transfer device 30 at a predetermined timing. Send in. Further, the sheet P after the secondary transfer is conveyed to the fixing device 50 via the conveying belt 43, and the sheet P discharged from the fixing device 50 is sent out to the outside by the discharge roll 44.

  Next, the configuration of the white unit 10W as the second image forming unit will be described in detail. FIG. 2 is an enlarged cross-sectional view of the white unit 10W. In the present embodiment, unlike the other image forming units 10Y, 10M, 10C, and 10K, the white unit 10W does not include the photosensitive drum 11, the charging roll 12, the exposure unit 13, and the like. That is, the white unit 10 </ b> W includes a developing device 60 as a developing device disposed to face the intermediate transfer belt 20 to be developed, and a facing roll 22 disposed with the intermediate transfer belt 20 interposed therebetween. ing. The counter roll 22 has a function as a counter electrode of the developing device 60 and also has a function of rotating the intermediate transfer belt 20 in a rotatable manner.

  The developing device 60 has a developing housing 61 in which an opening (development opening) is formed at a portion facing the intermediate transfer belt 20. Further, the developing device 60 includes a developing roll 62 disposed facing the opening of the developing housing 61, a first auger 63 disposed in the developing housing 61 on the side close to the developing roll 62, and the developing roll 62. And a trimmer 67 for regulating the thickness of the developer layer attached to the developing roll 62. Here, the developing roll 62 includes a rotatable developing sleeve 65 made of a nonmagnetic metal, and a magnet roll 66 in which a plurality of magnetic poles are arranged and enclosed in the developing sleeve 65. The developing sleeve 65 is made of, for example, a nonmagnetic aluminum pipe, and is driven to rotate while maintaining a predetermined distance from the peripheral surface of the intermediate transfer belt 20. The developing device 60 is similar to the developing device 14 in the yellow unit 10Y and the like, and includes a toner having a negative charge polarity (white toner in this example) and a carrier having magnetism and a positive charge polarity. Contains the drug.

  The first auger 63 and the second auger 64 are both configured by attaching spiral blades around the rotation axis. However, in the first auger 63 and the second auger 64, the blades are formed in opposite directions. As a result, the first auger 63 stirs and conveys the developer in the developing housing 61 toward the front side in the figure, for example, while the second auger 64 causes the developer in the developing housing 61 to move to the back side in the figure, for example Stir and transport toward The developing housing 61 is provided with a partition wall 61a that partitions the first auger 63 and the second auger 64. However, the partition wall 61a does not exist at both ends in the longitudinal direction of the developing housing 61, and as a result. The developer in the developing housing 61 is circulated and conveyed while being stirred in the developing housing 61 by the first auger 63 and the second auger 64. For this reason, the toner having a negative charging polarity is rubbed by being agitated and conveyed together with a carrier having a positive charging polarity and magnetism, and is charged negatively. The trimmer 67 is made of, for example, a metal plate and is fixed to the developing housing 61. A predetermined gap is formed between the developing sleeve 65 and the free end of the trimmer 67 in order to obtain a preferable developer layer thickness.

In the present embodiment, the rotation direction of the developing sleeve 65 is set in the same direction as the rotation direction of the photosensitive drum 11 at a position facing the intermediate transfer belt 20. A developing power source 68 for applying a developing bias is connected to the developing sleeve 65. The developing power source 68 applies a developing bias in which an alternating current (for example, AC 1 kV (peak to peak value)) is superimposed on a direct current (for example, DC-200 V) to the developing sleeve 65. On the other hand, the opposing roll 22 is grounded. In particular, in this example, the magnitude of the developing bias to be applied is set in advance so that the white toner image formed on the intermediate transfer belt 20 has a predetermined density (for example, a density of 50%).
Each of the developing units 14 used in the yellow unit 10Y, the magenta unit 10M, the cyan unit 10C, and the black unit 10K has the white color except that the toner color to be accommodated and the development target is the photosensitive drum 11. The developing device 60 has the same configuration.

  Next, an image forming process by the image forming apparatus will be described with reference to FIGS. Now, when a start switch (not shown) is turned on, a predetermined image forming process is executed. More specifically, for example, when the image forming apparatus is configured as a digital color copying machine, first, a document set on a document table (not shown) is read by a color image reading device. Next, the obtained read signal is converted into a digital image signal by a processing circuit and temporarily stored in a memory. Then, toner images of each color are formed based on the digital image signals of four colors (Y, M, C, K) stored in the memory.

  That is, the yellow unit 10Y, the magenta unit 10M, the cyan unit 10C, and the black unit 10K are driven according to the digital image signals of the respective colors. Next, in each of the yellow unit 10Y, the magenta unit 10M, the cyan unit 10C, and the black unit 10K, a laser corresponding to the digital image signal by the exposure unit 13 is applied to the photosensitive drum 11 that is uniformly charged by the charging roll 12. By irradiating with light, an electrostatic latent image is formed. Then, the electrostatic latent image formed on the photosensitive drum 11 is developed by the developing device 14 to form toner images of each color. In the case where the image forming apparatus is configured as a printer, a toner image of each color may be formed based on a digital image signal input from the outside such as a personal computer.

  Thereafter, the toner images of the respective colors formed on the respective photosensitive drums 11 are formed on the surface of the intermediate transfer belt 20 by the primary transfer roll 15 at the primary transfer position where the photosensitive drum 11 and the intermediate transfer belt 20 are in contact with each other. Primary transfer is performed in the order of magenta, cyan, and black. On the other hand, the toner remaining on the photosensitive drum 11 after the primary transfer is cleaned by the drum cleaner 16.

  The yellow, magenta, cyan, and black toner images transferred and transferred onto the intermediate transfer belt 20 pass through a portion facing the white unit 10W (developing roll 62) as the intermediate transfer belt 20 rotates. . At this time, the developing device 60 of the white unit 10W operates as follows.

  In the developing device 60, the developing sleeve 65, the first auger 63, and the second auger 64 are rotationally driven by a motor (not shown). Then, in the developing housing 61, the developer is stirred and conveyed while being circulated by the first auger 63 and the second auger 64. As a result, the toner is negatively charged by frictional charging with the carrier, and the toner adheres electrostatically to the carrier. When the developer to be agitated and conveyed passes through the developing roll 62 side, a part of the developer is transferred and adhered to the developing sleeve 65 by the magnetic force of a magnetic pole (not shown) provided on the magnet roll 66. Form a layer. The developer layer formed on the developing sleeve 65 is regulated to a predetermined thickness according to the formed gap when passing through the portion facing the trimmer 67 as the developing sleeve 65 rotates. The toner is conveyed to a developing area facing the intermediate transfer belt 20.

  At this time, a predetermined developing bias is applied to the developing sleeve 65 by a developing power source 68. Then, a developing electric field is formed between the developing roll 62 (developing sleeve 65) and the opposed roll 22 that is grounded by the applied developing bias. As a result, among the developer carried on the developing sleeve 65, a predetermined amount of toner (white toner) proportional to the developing electric field is transferred to the intermediate transfer belt 20 side. The white toner is transferred to the entire surface of the intermediate transfer belt 20 in a sheet facing region (a region in contact with a sheet conveyed in the secondary transfer described later). For this reason, the white toner adheres directly to the intermediate transfer belt 20 in the area where the yellow, magenta, cyan, and black toner images do not exist, and in the area where these toner images already exist, the intermediate toner passes through these toners. It indirectly adheres to the transfer belt 20.

  Then, the developer that has passed through the development region and has a reduced amount of toner used for development is further conveyed as the development sleeve 65 rotates. After that, it is peeled off from the developing sleeve 65 by a repulsive magnetic field by a magnetic pole of the same polarity provided adjacent to the inside of the magnet roll 66 and waits for the next development while being stirred and conveyed by the first auger 63 and the second auger 64 again. Become.

The toner images (yellow, magenta, cyan, black, and white) primarily transferred to the intermediate transfer belt 20 in this way are superimposed on the intermediate transfer belt 20 and subjected to secondary transfer as the intermediate transfer belt 20 rotates. Transported to position. On the other hand, the paper P is conveyed to the secondary transfer position at a predetermined timing, and the secondary transfer roll 31 nips the paper P with respect to the backup roll 24.
Then, at the secondary transfer position, the toner image carried on the intermediate transfer belt 20 is secondarily transferred to the paper P by the action of a transfer electric field formed between the secondary transfer roll 31 and the backup roll 24. . At this time, toner images are stacked on the paper P in the order of white, black, cyan, magenta, and yellow from the front side of the paper P.

  The sheet P on which the toner image is transferred is conveyed to the fixing device 50 by the conveyance belt 43. In the fixing device 50, the toner image on the paper P is heated and pressure-fixed, and then sent out to a paper discharge tray (not shown) provided outside the apparatus. On the other hand, the toner remaining on the intermediate transfer belt 20 after the secondary transfer is cleaned by the belt cleaner 26.

  In the case of the present embodiment, on the output paper P, a full-tone halftone white toner image, that is, a ground layer is formed on the paper P. An image toner image composed of yellow, magenta, cyan, and black is further formed on the white toner image. For this reason, for example, even when a low-whiteness paper such as recycled paper is used for the paper P, the white toner image functions as a base layer, so that the situation where the color of the image toner image appears cloudy is prevented. And a good color image can be obtained.

  In this embodiment, when forming a full-tone halftone white toner image, the white toner image is directly developed on the intermediate transfer belt 20 by using the developing device 60 instead of a normal electrophotographic process. Configured. Thereby, in the formation of the white toner image, the number of processes that cause in-plane unevenness such as uneven charging in the charging process and uneven exposure in the exposure process can be reduced. Also, in the normal electrophotographic process, development and transfer are separate processes, whereas in this embodiment, development and transfer can be performed in the same process, so the number of processes that cause in-plane unevenness Can be further reduced. As a result, in the present embodiment, in-plane unevenness in the white toner image that is the underlayer can be suppressed. In other words, this means that the white toner image can be attached almost uniformly to the entire surface of the paper P, and therefore the appearance of the obtained image can be improved.

<Embodiment 2>
FIG. 3 is a diagram showing an outline of the image forming apparatus according to the second embodiment. In addition to the yellow (Y), magenta (M), cyan (C), and black (K) toner images used to form the image toner image, the image forming apparatus according to the present embodiment is transparent. A toner image (T) can also be formed. The transparent toner image is used to form an overcoat layer on the paper P and an image toner image formed on the paper P. The basic configuration of the image forming apparatus according to the present embodiment is substantially the same as that of the first embodiment. Accordingly, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the present embodiment, the image forming unit 10 includes a transparent image forming unit 10T (referred to as a transparent unit in the following description), a yellow unit 10Y, a magenta unit 10M, and a cyan unit from the upstream side in the moving direction of the intermediate transfer belt 20. 10C and black unit 10K are arranged in this order. Here, the yellow unit 10Y, the magenta unit 10M, the cyan unit 10C, and the black unit 10K have the same configurations as those described in the first embodiment. The transparent unit 10T has the same configuration as the white unit 10W shown in FIG. 2 except that the driving roll 21 is used as a counter electrode instead of the counter roll 22. That is, the transparent unit 10T includes the developing device 60 and the drive roll 21. However, the developing device 60 provided in the transparent unit 10T contains a developer including a toner having a negative charge polarity (transparent toner in this example) and a carrier having magnetism and a positive charge polarity. . In the present embodiment, the drive roll 21 is grounded. In this embodiment, the belt cleaner 26 is disposed so as to face the driven roll 25 with the intermediate transfer belt 20 interposed therebetween.

  Next, an image forming process by the image forming apparatus will be described with reference to FIGS. 3 and 2. In this example, when the image forming process is started, the developing device 60 of the transparent unit 10T develops the transparent toner on the intermediate transfer belt 20 prior to the formation of the image toner image. The operation of the developing device 60 at this time is the same as that in the first embodiment. The transparent toner to be developed is transferred over the entire surface of the sheet on the intermediate transfer belt 20 (the area in contact with the sheet conveyed in the secondary transfer described later), and forms a full-tone halftone image. Therefore, the developed transparent toner image adheres directly to the intermediate transfer belt 20.

  The yellow unit 10Y, the magenta unit 10M, the cyan unit 10C, and the black unit 10K perform image formation in accordance with the passage timing of the transparent toner image formed on the intermediate transfer belt 20. That is, in the yellow unit 10Y, the magenta unit 10M, the cyan unit 10C, and the black unit 10K, laser light corresponding to the digital image signal by the exposure unit 13 is applied to the photosensitive drum 11 that is uniformly charged by the charging roll 12, respectively. Is formed to form an electrostatic latent image. Then, the electrostatic latent image formed on the photosensitive drum 11 is developed by the developing device 14 to form toner images of each color.

  Thereafter, the toner images of the respective colors formed on the respective photosensitive drums 11 are formed on the surface of the intermediate transfer belt 20 by the primary transfer roll 15 at the primary transfer position where the photosensitive drum 11 and the intermediate transfer belt 20 are in contact with each other. Primary transfer is performed in the order of magenta, cyan, and black. At this time, the yellow, magenta, cyan, and black toner images are primarily transferred onto the already formed transparent toner images. On the other hand, the toner remaining on the photosensitive drum 11 after the primary transfer is cleaned by the drum cleaner 16.

The toner images (transparent, yellow, magenta, cyan, and black) primarily transferred to the intermediate transfer belt 20 in this way are superimposed on the intermediate transfer belt 20 and subjected to secondary transfer as the intermediate transfer belt 20 rotates. Transported to position. On the other hand, the paper P is conveyed to the secondary transfer position at a predetermined timing, and the secondary transfer roll 31 nips the paper P with respect to the backup roll 24.
Then, at the secondary transfer position, the toner image carried on the intermediate transfer belt 20 is secondarily transferred to the paper P by the action of a transfer electric field formed between the secondary transfer roll 31 and the backup roll 24. . At this time, toner images are stacked on the paper P in the order of black, cyan, magenta, yellow, and transparent from the front side of the paper P.

  The sheet P on which the toner image is transferred is conveyed to the fixing device 50 by the conveyance belt 43. In the fixing device 50, the toner image on the paper P is heated and pressure-fixed, and then sent out to a paper discharge tray (not shown) provided outside the apparatus. On the other hand, the toner remaining on the intermediate transfer belt 20 after the secondary transfer is cleaned by the belt cleaner 26.

  In the case of the present embodiment, on the output paper P, an image toner image composed of yellow, magenta, cyan, and black is formed on the paper P, and further, a transparent full-screen halftone image is formed thereon. The For this reason, the transparent toner image functions as an overcoat layer. Therefore, a predetermined gloss can be obtained from the entire surface halftone transparent toner image, and a good color image can be obtained.

  In this embodiment, when forming a full-tone halftone transparent toner image, the transparent toner image is directly developed on the intermediate transfer belt 20 by using the developing device 60 instead of the normal electrophotographic process. did. This can reduce the number of processes that cause in-plane unevenness such as uneven charging in the charging process and uneven exposure in the exposure process. Also, in the normal electrophotographic process, development and transfer are separate processes, whereas in this embodiment, development and transfer can be performed in the same process, so the number of processes that cause in-plane unevenness Can be further reduced. As a result, in this embodiment, in-plane unevenness in the transparent toner image that is the overcoat layer can be suppressed. In other words, this means that the transparent toner image can be applied almost uniformly to the entire surface of the paper P, and therefore the appearance of the obtained image can be improved.

<Embodiment 3>
FIG. 4 is a diagram showing an outline of the image forming apparatus according to the third embodiment. The image forming apparatus according to the present embodiment is a type of forming a full-color image by sequentially forming toner images of each color on one photosensitive drum 111 and superimposing the toner images of each color on the intermediate transfer belt 120. An image forming apparatus. In the present embodiment, as in the first embodiment, in addition to the yellow (Y), magenta (M), cyan (C), and black (K) toner images, the white (W) toner image is also used. It can be formed. However, in the present embodiment, unlike the first embodiment, for example, when an image is formed on a colored paper P, an image toner image including a white toner image can be formed. ing.

  The image forming apparatus includes an image forming unit 110 that forms toner images of respective colors, and an intermediate transfer belt 120 to which toner images formed by the image forming unit 110 are sequentially primary transferred. In addition, the image forming apparatus includes a secondary transfer device 130 that collectively transfers the toner images transferred onto the intermediate transfer belt 120 onto the paper P. The image forming apparatus further includes a fixing device 150 that fixes the secondary transferred image on the paper P. Furthermore, the image forming apparatus includes a user interface (UI) 160 for receiving instructions from the user and a control unit 170 that controls the operation of the entire image forming apparatus.

  The image forming unit 110 includes a photosensitive drum 111 as a rotatable image carrier. In the image forming unit 110, a charger 112, an exposure unit 113, a rotary developing device 114, a primary transfer roll 115 as a transfer unit, and a drum cleaner 116 are provided around the photosensitive drum 111. The rotary developing device 114 includes a yellow (Y) developing unit 114Y, a magenta (M) developing unit 114M, and a cyan (C) developing unit 114C as developing units mounted on a support that is rotatable about a rotation axis. , A black (K) developing unit 114K and a white (W) developing unit 114W. Each of the developing devices 114Y, 114M, 114C, 114K, and 114W contains a developer containing the corresponding color component toner and carrier. In this embodiment, the latent image forming unit is configured by the charger 112 and the exposure unit 113 as the charging unit.

  The intermediate transfer belt 120 is supported by a plurality of (five in the present embodiment) support rolls so as to be rotatable. Among these support rolls, the drive roll 121 stretches the intermediate transfer belt 120 and drives the intermediate transfer belt 120 to rotate. The opposing roll 122 forms a transfer surface of the intermediate transfer belt 120 with respect to the photosensitive drum 111 together with the drive roll 121. Further, the correction roll 123 stretches the intermediate transfer belt 120 and functions as a steering roll. The driven roll 125 stretches the intermediate transfer belt 120 and rotates following the intermediate transfer belt 120 driven by the drive roll 121. The backup roll 124 stretches the intermediate transfer belt 120 and functions as a constituent member of the secondary transfer device 130 described later. In addition, a belt cleaner 126 for removing residues (toner and the like) on the intermediate transfer belt 120 after the secondary transfer is disposed at a portion facing the drive roll 121 with the intermediate transfer belt 120 interposed therebetween.

  The secondary transfer device 130 includes a secondary transfer roll 131 arranged in pressure contact with the toner image carrying surface side of the intermediate transfer belt 120 and a counter electrode of the secondary transfer roll 131 arranged on the back side of the intermediate transfer belt 120. And a backup roll 124. A power supply roll 132 that applies a secondary transfer bias having the same polarity as the charging polarity of the toner (negative polarity in the present embodiment) is disposed in contact with the backup roll 124. On the other hand, the secondary transfer roll 131 is grounded.

  The paper transport system includes a paper tray 140, a transport roll 141, a registration roll 142, a transport belt 143, and a discharge roll 144. In the paper transport system, the paper P stacked on the paper tray 140 is transported by the transport roll 141, and then temporarily stopped by the registration roll 142, and then moved to the secondary transfer position of the secondary transfer device 130 at a predetermined timing. Send it in. Further, the paper P after the secondary transfer is transported to the fixing device 150 via the transport belt 143, and the paper P discharged from the fixing device 150 is sent out to the outside by the discharge roll 144.

  FIG. 5 is a diagram for explaining a detailed configuration of the image forming unit 110. In the image forming unit 110, the photosensitive drum 111 has a photosensitive layer (not shown) and is grounded. The charger 112 is a so-called corotron charger, and the charger 112 is connected to a charging power source 161 that switches the magnitude of the charging bias applied to the charger 112 and on / off of the charging bias. The exposure unit 113 includes a laser exposure unit, and a laser driver 162 that controls the output beam Bm is connected to the exposure unit 113. In the rotary developing device 114, the rotary developing device 114 is rotated in the direction of arrow C, and the developing devices (yellow developing device 114Y, magenta developing device 114M, cyan) disposed at the developing position facing the photosensitive drum 111 are used. A developing drive motor 163 for driving a developing sleeve, an auger or the like of any one of the developing device 114C, the black developing device 114K, and the white developing device 114W is mounted. The rotary developing device 114 is connected to a developing power source 164 that supplies a developing bias to the developing sleeve of the developing device at the developing position. The developing power source 164 can adjust the magnitude of the developing bias to be applied. The primary transfer roll 115 is connected to a primary transfer power source 165 that switches the magnitude of the primary transfer bias applied to the primary transfer roll 115 and the on / off of the primary transfer bias.

  FIG. 6 shows a control block of the controller 170 that controls the image forming operation. However, FIG. 6 shows only the blocks related to the control of the image forming unit 110. The CPU 171 of the control unit 170 executes processing while appropriately exchanging data with the RAM 173 in accordance with a program stored in the ROM 172. In addition, information indicating the content of the image forming operation is input from the UI 160 to the control unit 170 via the input / output interface 174. Further, the control unit 170 controls the charging power source 161, the laser driver 162, the development driving motor 163, the development power source 164, and the primary transfer power source 165 via the input / output interface 174.

  Now, a basic image forming process by the image forming apparatus will be described. Now, for example, when an image forming instruction is given by the UI 160 or the like, a predetermined image forming process is executed. That is, a toner image is formed by the image forming unit 110, and the formed toner image is primarily transferred to the intermediate transfer belt 120. The image forming unit 110 forms toner images of single colors (for example, only black) to five colors (yellow, magenta, cyan, black, and white) according to the input image signal. Here, when forming a single color image, the toner image primary-transferred to the intermediate transfer belt 120 is immediately secondary-transferred to the paper P, but an image in which a plurality of color toner images are superimposed (for example, a full-color image). ) Is repeated by the image forming unit 110 by the number of colors. For example, when forming a full-color image in which yellow, magenta, cyan, and black toner images are superimposed, yellow, magenta, cyan, and black toner images are sequentially formed in the image forming unit 110. Sequentially transferred to the intermediate transfer belt 120 sequentially. On the other hand, the intermediate transfer belt 120 rotates at the same cycle as the photosensitive drum 111 of the image forming unit 110 while holding the primary transferred toner image, and yellow, magenta and so on are rotated on the intermediate transfer belt 120 for each rotation. , Cyan and black toner images are transferred and overlaid. The secondary transfer roll 131 and the belt cleaner 126 are separated from the intermediate transfer belt 120 until the final color toner image (black in the case of a full-color image) passes.

The toner images (yellow, magenta, cyan, and black) primarily transferred to the intermediate transfer belt 120 in this manner are superimposed on the intermediate transfer belt 120 and moved to the secondary transfer position as the intermediate transfer belt 120 rotates. It is conveyed. On the other hand, the sheet P is conveyed to the secondary transfer position at a predetermined timing, and the secondary transfer roll 131 nips the sheet P with respect to the backup roll 124.
At the secondary transfer position, the toner image carried on the intermediate transfer belt 120 is secondarily transferred to the paper P by the action of a transfer electric field formed between the secondary transfer roll 131 and the backup roll 124. .

  The sheet P on which the toner image is transferred is conveyed to the fixing device 150 by the conveyance belt 143. In the fixing device 150, the toner image on the paper P is heated and pressurized and fixed, and then sent to a paper discharge tray (not shown) provided outside the apparatus. On the other hand, the toner remaining on the intermediate transfer belt 120 after the secondary transfer is cleaned by the belt cleaner 126.

The basic image forming process in the image forming apparatus has been described above. In this embodiment, the image forming unit 110 executes different operations according to the image forming mode instructed via the UI 160. FIG. 7 is a flowchart showing image forming mode selection processing in the present embodiment.
When the image forming process is started, the control unit 170 determines whether selection of a base formation mode is received from the UI 160 (step 101). If the selection of the base formation mode is not accepted, image formation is executed in the normal mode in which the base layer is not formed with the white toner image (step 102). On the other hand, if selection of the base formation mode is accepted in step 101, image formation is executed in the base formation mode in which the base layer is formed with the white toner image (step 103).

Here, FIG. 8 is a flowchart for explaining the operation of the image forming unit 110 in the normal mode shown in step 102 of FIG.
In this process, first, the control unit 170 determines whether or not the image to be formed is a full-color image (step 201). If it is determined that the image to be formed is not a full-color image (in the case of a monochrome image), the process proceeds to step 208 described later. On the other hand, when it is determined in step 201 that the image to be formed is a full-color image, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114 and the photosensitive drum 111. The yellow developing device (Y developing device) 114Y is opposed to the opposing developing position (step 202). Then, the control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a yellow toner image in the first mode. A forming operation is executed (step 203). Details of the first mode will be described later.

  Next, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, so that the magenta developing device (M developing device) 114M faces the developing position facing the photosensitive drum 111. (Step 204). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a magenta toner image in the first mode. A forming operation is executed (step 205).

  Further, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, and causes the cyan developing device (C developing device) 114C to face the developing position facing the photosensitive drum 111. (Step 206). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a cyan toner image in the first mode. A forming operation is executed (step 207).

  Then, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114, and causes the black developing device (K developing device) 114K to face the developing position facing the photosensitive drum 111. (Step 208). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a black toner image in the first mode. A forming operation is executed (step 209).

  Thereafter, the control unit 170 determines whether or not to form a white toner image (step 210). If it is determined that no white toner image is to be formed, the image formation in the normal mode is terminated. On the other hand, when it is determined that a white toner image is to be formed, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary development device 114 to a development position that faces the photosensitive drum 111. The white developing device (W developing device) 114W is confronted (step 211). Then, the control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a white toner image in the first mode. A forming operation is executed (step 212). Then, the image formation in the normal mode is finished.

FIG. 9 is a flowchart for explaining the operation of the image forming unit 110 in the base formation mode shown in step 103 of FIG.
In this process, first, the control unit 170 determines whether or not an image to be formed is a full color image (step 301). If it is determined that the image to be formed is not a full-color image (monochrome image), the process proceeds to step 308 described later. On the other hand, when it is determined in step 301 that the image to be formed is a full-color image, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114 and the photosensitive drum 111. The yellow developing device (Y developing device) 114Y is opposed to the opposing developing position (step 302). Then, the control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a yellow toner image in the first mode. A forming operation is executed (step 303).

  Next, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, so that the magenta developing device (M developing device) 114M faces the developing position facing the photosensitive drum 111. (Step 304). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a magenta toner image in the first mode. A forming operation is executed (step 305).

  Further, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, and causes the cyan developing device (C developing device) 114C to face the developing position facing the photosensitive drum 111. (Step 306). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a cyan toner image in the first mode. A forming operation is executed (step 307).

  Then, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114, and causes the black developing device (K developing device) 114K to face the developing position facing the photosensitive drum 111. (Step 308). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a black toner image in the first mode. A forming operation is executed (step 309).

  After that, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114 so that the white developing device (W developing device) 114W is opposed to the developing position facing the photosensitive drum 111. (Step 310). Then, the control unit 170 outputs drive signals to the charging power supply 161, the laser driver 162, the development drive motor 163, the development power supply 164, and the primary transfer power supply 165, and in the second mode that is different from the first mode, The image forming operation for forming the white toner image is executed (step 311). Then, the image formation in the base formation mode is finished.

  As described above, in the base formation mode, toner images are formed in the order of yellow, magenta, cyan, black, and white. Accordingly, a half-tone white toner image is formed on the intermediate transfer belt 120 on the yellow, magenta, cyan, and black image toner images. When the toner image on the intermediate transfer belt 120 is secondarily transferred to the paper P, a full-color halftone white toner image is formed immediately above the paper P, and the other color toner images are formed on the white toner image. Will be formed. Therefore, the white toner image functions as a base layer.

Now, the first mode shown in FIGS. 8 and 9 will be specifically described. In the first mode, the control unit 170 controls the charging power supply 161, the laser driver 162, the development drive motor 163, the development power supply 164, and the primary transfer power supply 165 as follows.
First, the control unit 170 outputs a drive signal to the charging power source 161. Accordingly, the charging power supply 161 supplies a normal charging bias to the charger 112, and as a result, the photosensitive drum 111 is charged to a normal charging potential (for example, −700V). Further, the controller 170 outputs a drive signal to the laser driver 162. The laser driver 162 controls the output of the laser beam in accordance with the input image signal. As a result, an electrostatic latent image (for example, the background portion potential is −700 V and the image portion potential is displayed on the photosensitive drum 111. -50V) is formed. Further, the control unit 170 outputs drive signals to the development drive motor 163 and the development power supply 164. Along with this, the developing drive motor 163 drives the developing sleeve and auger of the developing device at the developing position, and the developing power source 164 applies a normal developing bias (for example, DC-350V) to the developing sleeve of the developing device at the developing position. (AC1kV (peak-to-peak value) superimposed) is supplied. At this time, the toner is transferred to the portion having the image portion potential on the photosensitive drum 111, while the toner is not transferred to the portion having the background portion potential on the photosensitive drum 111. As a result, a toner image corresponding to the electrostatic latent image is formed on the photosensitive drum 111. Then, the controller 170 outputs a drive signal to the primary transfer power supply 165. Accordingly, the primary transfer power source 165 supplies a normal primary transfer bias (for example, +1 kV) to the primary transfer roll 115. As a result, the toner image on the photosensitive drum 111 is primarily transferred to the intermediate transfer belt 120.

Next, the second mode shown in FIG. 9 will be specifically described. In the second mode, the control unit 170 controls the charging power supply 161, the laser driver 162, the development drive motor 163, the development power supply 164, and the primary transfer power supply 165 as follows.
First, the control unit 170 does not output a drive signal to the charging power source 161. Therefore, the charging power supply 161 does not supply a charging bias to the charger 112, and as a result, the charging potential of the photosensitive drum 111 remains 0V. Further, the control unit 170 does not output a drive signal to the laser driver 162. As a result, the photosensitive drum 111 remains at the charged potential (0 V). Then, the control unit 170 outputs a drive signal to the development drive motor 163 and the development power supply 164. Along with this, the development drive motor 163 drives the development sleeve and auger of the development unit at the development position, and the development power source 164 applies a different development bias (for example, DC) to the development sleeve of the development unit at the development position. Supply AC-1kV (peak-to-peak value) superimposed on -100V). At this time, since all of the photosensitive drum 111 is at 0 V, the toner is uniformly transferred onto the photosensitive drum 111. As a result, a halftone toner image having a uniform density is formed on the photosensitive drum 111. Then, the controller 170 outputs a drive signal to the primary transfer power supply 165. Accordingly, the primary transfer power source 165 supplies a normal primary transfer bias (for example, +1 kV) to the primary transfer roll 115. As a result, the toner image on the photosensitive drum 111 is primarily transferred to the intermediate transfer belt 120.

  As described above, in the present embodiment, in the base formation mode shown in FIG. 9, a full-color halftone white toner image is formed in the second mode. That is, the charging step and the exposure step are omitted, and a white toner image is formed only by the development step and the transfer step. Thereby, in the formation of the white toner image used as the underlayer, the number of processes that cause in-plane unevenness such as uneven charging in the charging process and uneven exposure in the exposure process can be reduced. As a result, in the present embodiment, in-plane unevenness in the white toner image that is the underlayer can be suppressed. In other words, this means that the white toner image can be attached almost uniformly to the entire surface of the paper P, and therefore the appearance of the obtained image can be improved.

  In this embodiment, a white toner image can also be formed by performing a development process and a transfer process after the first mode, that is, the charging process and the exposure process. Therefore, the white toner image can be used for forming a normal image toner image in addition to the formation of the underlayer.

In this embodiment, the charging process is not performed in the second mode. However, the charging process may be performed as long as at least the exposure process is omitted. In this case, however, it is necessary to adjust the density of the white toner image by adjusting the magnitude of the developing bias.
In this embodiment, the white toner image as the underlayer is formed last. However, for example, image formation of a type in which the toner image formed on the photosensitive drum 111 is sequentially transferred onto the paper P is performed. In the case of an apparatus, it is necessary to first form a white toner image as a base layer. The same applies to an image forming apparatus of a type that intermediately transfers a toner image formed on the photosensitive drum 111 twice.

<Embodiment 4>
The present embodiment is almost the same as the third embodiment, but in the third embodiment, white toner that can be used for forming the underlayer is used, whereas in this embodiment, the overcoat layer is formed. The difference is that a transparent toner that can be used is used. Therefore, as shown in parentheses in FIG. 4, the rotary developing device 114 is equipped with a transparent developing device 114T instead of the white developing device 114W. In the present embodiment, the same components as those in the third embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

In the present embodiment, as in the third embodiment, the image forming unit 110 executes different operations depending on the image forming mode instructed via the UI 160. FIG. 10 is a flowchart showing image forming mode selection processing according to the present embodiment.
When the image forming process is started, the control unit 170 determines whether selection of the overcoat formation mode is received from the UI 160 (step 401). Here, when the selection of the overcoat formation mode is not accepted, the image formation is executed in the normal mode in which the overcoat layer is not formed with the transparent toner image (step 402). On the other hand, if selection of the overcoat formation mode is accepted in step 401, image formation is executed in an overcoat formation mode in which an overcoat layer is formed with a transparent toner image (step 403).

Here, FIG. 11 is a flowchart for explaining the operation of the image forming unit 110 in the normal mode shown in step 402 of FIG.
In this process, first, the control unit 170 determines whether or not an image to be formed is a full color image (step 501). If it is determined that the image to be formed is not a full-color image (monochrome image), the process proceeds to step 508 described later. On the other hand, if it is determined in step 501 that the image to be formed is a full-color image, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114 and the photosensitive drum 111. The yellow developing device (Y developing device) 114Y is opposed to the opposing developing position (step 502). Then, the control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a yellow toner image in the first mode. A forming operation is executed (step 503). Details of the first mode will be described later.

  Next, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, so that the magenta developing device (M developing device) 114M faces the developing position facing the photosensitive drum 111. (Step 504). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a magenta toner image in the first mode. A forming operation is executed (step 505).

  Further, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, and causes the cyan developing device (C developing device) 114C to face the developing position facing the photosensitive drum 111. (Step 506). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a cyan toner image in the first mode. A forming operation is executed (step 507).

  Then, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114, and causes the black developing device (K developing device) 114K to face the developing position facing the photosensitive drum 111. (Step 508). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a black toner image in the first mode. A forming operation is executed (step 509). Then, the image formation in the normal mode is finished.

FIG. 12 is a flowchart for explaining the operation of the image forming unit 110 in the overcoat formation mode shown in step 403 of FIG.
In this process, first, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114, and a transparent developing device (T developing device) is placed at the developing position facing the photosensitive drum 111. 114T is confronted (step 601). Then, the controller 170 outputs drive signals to the charging power supply 161, the laser driver 162, the development drive motor 163, the development power supply 164, and the primary transfer power supply 165, and overcoats in a second mode different from the first mode. An image forming operation for forming a transparent toner image as a layer is executed (step 602).

  Next, the control unit 170 determines whether or not the image to be formed is a full color image (step 603). If it is determined that the image to be formed is not a full-color image (monochrome image), the process proceeds to step 610 described later. On the other hand, if it is determined in step 603 that the image to be formed is a full-color image, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114 and the photosensitive drum 111. The yellow developing device (Y developing device) 114Y is opposed to the opposing developing position (step 604). Then, the control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a yellow toner image in the first mode. A forming operation is executed (step 605).

  Next, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, so that the magenta developing device (M developing device) 114M faces the developing position facing the photosensitive drum 111. (Step 606). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a magenta toner image in the first mode. A forming operation is executed (step 607).

  Further, the control unit 170 outputs a drive signal to the developing drive motor 163 to rotate the rotary developing device 114, and causes the cyan developing device (C developing device) 114C to face the developing position facing the photosensitive drum 111. (Step 608). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a cyan toner image in the first mode. A forming operation is executed (step 609).

  Then, the control unit 170 outputs a drive signal to the development drive motor 163 to rotate the rotary developing device 114, and causes the black developing device (K developing device) 114K to face the developing position facing the photosensitive drum 111. (Step 610). The control unit 170 outputs drive signals to the charging power source 161, the laser driver 162, the development drive motor 163, the development power source 164, and the primary transfer power source 165, and an image for forming a black toner image in the first mode. A forming operation is executed (step 611). Then, the image formation in the overcoat formation mode is finished.

  As described above, in the overcoat formation mode, toner images are formed in the order of transparent, yellow, magenta, cyan, and black. Therefore, on the intermediate transfer belt 120, yellow, magenta, cyan, and black image toner images are formed on the entire halftone transparent toner image. When the toner image on the intermediate transfer belt 120 is secondarily transferred to the paper P, a full-surface halftone transparent toner image is formed on the paper P and the yellow, magenta, cyan, and black image toner images. Will be. Therefore, the transparent toner image functions as an overcoat layer.

  Note that the first mode and the second mode shown in FIGS. 11 and 12 are exactly the same as those in the third embodiment. That is, in the first mode, yellow, magenta, cyan, and black toner images are formed by a normal charging process, exposure process, development process, and transfer process. On the other hand, in the second mode, a transparent toner image is formed only by the development process and the transfer process without performing the charging process and the exposure process.

  As described above, in this embodiment, in the overcoat formation mode shown in FIG. 12, a half-tone transparent toner image is formed on the entire surface in the second mode. That is, the charging step and the exposure step are omitted, and a transparent toner image is formed only by the development step and the transfer step. Thereby, in the formation of the transparent toner image used as the underlayer, the number of processes that cause in-plane unevenness such as uneven charging in the charging process and uneven exposure in the exposure process can be reduced. As a result, in this embodiment, in-plane unevenness in the transparent toner image that is the overcoat layer can be suppressed. In other words, this means that the transparent toner image can be applied almost uniformly to the entire surface of the paper P, and therefore the appearance of the obtained image can be improved.

  In this embodiment, the transparent toner image that is the overcoat layer is formed first, but for example, an image of a type in which the toner image formed on the photosensitive drum 111 is sequentially transferred onto the paper P, for example. In the case of a forming apparatus, it is necessary to form a transparent toner image as an overcoat layer last. The same applies to an image forming apparatus of a type in which a toner image formed on the photosensitive drum 111 is intermediately transferred twice.

  In the first to fourth embodiments, examples using white toner or transparent toner have been described. However, the present invention is not limited to this. For example, toners other than yellow, magenta, cyan, and black, which are used for forming a normal full color image, such as light blue and green, can also be used for forming a full-tone halftone.

1 is a diagram illustrating an outline of an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a diagram for explaining a configuration of a white image forming unit (white unit). FIG. 3 is a diagram illustrating an outline of an image forming apparatus according to a second embodiment. 6 is a diagram illustrating an outline of an image forming apparatus according to Embodiment 3. FIG. It is a figure for demonstrating the detailed structure of an image formation unit. It is a figure which shows the control block of a control part. 14 is a flowchart illustrating image forming mode selection processing according to the third embodiment. 10 is a flowchart for explaining the operation of the image forming unit in the normal mode in the third embodiment. 12 is a flowchart for explaining the operation of the image forming unit in the base formation mode in the third embodiment. 15 is a flowchart illustrating image forming mode selection processing according to the fourth embodiment. 15 is a flowchart for explaining the operation of the image forming unit in the normal mode in the fourth embodiment. 15 is a flowchart for explaining the operation of the image forming unit in the overcoat formation mode in the fourth embodiment.

Explanation of symbols

10Y ... Yellow unit, 10M ... Magenta unit, 10C ... Cyan unit, 10K ... Black unit, 10W ... White unit, 10T ... Transparent unit, 11 ... Photosensitive drum, 12 ... Charging roll, 13 ... Exposure unit, 14 ... Developer , 15 ... primary transfer roll, 16 ... drum cleaner, 20 ... intermediate transfer belt, 21 ... drive roll, 22 ... opposite roll, 50 ... fixing device, 60 ... developing device

Claims (2)

A first image forming unit that forms an image on the rotating intermediate transfer member, and a second image forming unit that forms an image on the intermediate transfer member upstream of the first image forming unit in the rotation direction of the intermediate transfer member. The first image forming unit and the first image forming unit on the downstream side in the rotation direction of the intermediate transfer member with respect to the first image forming unit and on the upstream side in the rotation direction of the intermediate transfer member with respect to the second image forming unit. And a secondary transfer unit that transfers the image formed on the intermediate transfer member to the paper by the second image forming unit,
The first image forming unit includes:
An image carrier that is rotatably arranged facing the intermediate transfer member;
A charging unit that charges the image carrier to a predetermined potential;
An exposure unit that forms an electrostatic latent image by selectively exposing the surface of the image carrier charged by the charging unit ;
A developing unit for developing with toner which is colored an electrostatic latent image formed on the image bearing member,
A primary transfer unit that transfers the image developed on the image carrier by the developing unit to the intermediate transfer member;
The second image forming unit includes:
Wherein when viewed from the rotational direction of the intermediate transfer body, wherein disposed opposite to the intermediate transfer member on the upstream side of the primary transfer portion on the downstream side and the first image forming section than the secondary transfer portion, said two After the residual toner remaining on the intermediate transfer body that has passed through the next transfer section is cleaned, and before the image formed by the colored toner is formed on the intermediate transfer body by the first image forming section. And an image development device that directly develops a region of the intermediate transfer member that contacts the sheet passing through the secondary transfer portion with a transparent toner having the same charging polarity as the colored toner. Forming equipment. The first image forming unit includes a yellow image forming unit that forms a yellow image on the intermediate transfer body, a magenta image forming unit for forming an image of magenta on the intermediate transfer member, the cyan on the intermediate transfer member A cyan image forming unit that forms an image, and a black image forming unit that forms a black image on the intermediate transfer member ,
In the developing unit provided in each of the yellow image forming unit, the cyan image forming unit, the magenta image forming unit, and the black image forming unit constituting the first image forming unit, development is performed by a reversal developing method. Is done,
The image forming apparatus according to claim 1 , wherein the developing unit constituting the second image forming unit performs development by a reversal development method .

Images