JP2011118228A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which an image excellent in color reproducibility is formed in a secondary transfer stage. <P>SOLUTION: The image forming apparatus is equipped with: a plurality of image carriers 1Y, 1M, 1C and 1K, on each of which a toner image of color toner is formed; a second image carrier 1W on which a toner image of transparent toner is formed; and an intermediate transfer body 8 to which the toner images of the color toner formed on the plurality of image carriers 1Y, 1M, 1C and 1K respectively are superposed and primarily transferred after the toner image of the transparent toner formed on the second image carrier 1W is primarily transferred. The aggregation degree of the transparent toner is set to be equal to or above a predetermined value, and adhesive amount of the transparent toner primarily transferred to the intermediate transfer body 8 from the second image carrier 1W is set according to the aggregation degree. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine thereof, and more particularly, primary transfer by superimposing a plurality of color toner images on an intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum. The present invention relates to an image forming apparatus.

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a printer, a technique for primary transfer by superimposing a toner image made of a transparent toner together with a toner image made of a plurality of colored toners on an intermediate transfer body such as an intermediate transfer belt or an intermediate transfer drum Is widely known (for example, see Patent Documents 1 and 2).
In the image forming apparatus in Patent Document 1 or the like, five photosensitive drums (image carriers) are arranged in parallel so as to face the intermediate transfer belt (intermediate transfer member). Of these, four photosensitive drums form colored toner images of black (black), yellow, magenta, and cyan, respectively. In addition, a transparent toner image is formed on one photosensitive drum. Then, the transparent toner image and the black, black, yellow, magenta, and cyan colored toner images are transferred onto the intermediate transfer belt (primary transfer). Further, the toner images of a plurality of colors carried on the intermediate transfer belt are transferred (secondary transfer) onto the recording medium as a color image at a position where the intermediate transfer belt and the secondary transfer member (transfer device) are opposed to each other. .

  Here, in the image forming apparatus disclosed in Patent Document 2 or the like, a technique in which a toner image made of transparent toner is primarily transferred to an intermediate transfer body before a toner image made of a plurality of colored toners is primarily transferred to the intermediate transfer body. Is disclosed. Specifically, in order to form an image having excellent smoothness on the recording medium, the image is formed on the intermediate transfer member so that the sum of the thickness of the colored toner image formed on the intermediate transfer member is substantially constant. The thickness of the transparent toner image is controlled for each pixel.

On the other hand, in Patent Document 3 and the like, the image density of a toner image (density detection pattern) formed on an intermediate transfer body is detected by a density detection sensor, and on the image carrier or intermediate transfer based on the detection result. A technique for controlling image forming conditions when a toner image is formed on a body is disclosed.
Patent Document 4 and the like disclose in detail the degree of toner aggregation and a method for measuring the same.

  The conventional image forming apparatus described above is adjusted on the intermediate transfer member because the secondary transfer rate when the color toner image formed on the intermediate transfer member is secondarily transferred onto the recording medium is not sufficiently high. There is a problem that the color toner image is not reproduced on the recording medium as it is.

Thus, the problem that the color reproducibility is deteriorated in the secondary transfer process can be solved to some extent by using the technique disclosed in the above-mentioned Patent Document 2 and the like. That is, before a toner image made of a plurality of colored toners is superimposed on the intermediate transfer member and is primarily transferred, the toner image made of transparent toner is primarily transferred to the intermediate transfer member, so that the intermediate transfer member and the color toner image are transferred. Since the transparent toner image is interposed therebetween, the adhesion of the color toner image to the intermediate transfer member is weakened, and the secondary transfer rate of the color toner image in the secondary transfer process is improved.
However, even in such a case, the problem that the color reproducibility deteriorates in the secondary transfer process has not been completely solved.

  The present invention has been made to solve the above-described problems, and it is an object of the present invention to provide an image forming apparatus in which an image having good color reproducibility is formed in the secondary transfer process.

As a result of repeated researches to solve the above problems, the present inventor has come to know the following matters.
When a toner image made of transparent toner is primarily transferred to the intermediate transfer body before the toner image made of a plurality of colored toners is primarily transferred onto the intermediate transfer body, the coagulation degree is a predetermined value or more. And by optimizing the adhesion amount of the transparent toner on the intermediate transfer member, the secondary transfer rate when the color toner image formed on the intermediate transfer member is secondarily transferred onto the recording medium is sufficient. Can be increased.
Similarly, when a white toner having a color similar to the background of the recording medium is used instead of the transparent toner, the secondary transfer rate of the color toner image can be sufficiently increased.

  The present invention is based on the matters described above. That is, an image forming apparatus according to the first aspect of the present invention includes a plurality of image carriers on which toner images are formed with colored toners, and transparent toner. Or a second image carrier on which a toner image is formed by white toner, the plurality of image carriers and the second image carrier, and formed on the second image carrier. An intermediate transfer body on which a toner image of colored toner formed on each of the plurality of image carriers is primarily transferred after being primarily transferred with a transparent toner or a white toner, and an intermediate transfer body; A secondary transfer member that secondary-transfers the primary-transferred toner image to a recording medium, and determines the degree of aggregation of the transparent toner or the white toner to be equal to or greater than a predetermined value, according to the degree of aggregation. Said It is from the second image bearing member defining a deposition amount of the intermediate transfer member to the primary transfer is the transparent toner or the white toner is.

According to a second aspect of the present invention, there is provided the image forming apparatus according to the first aspect of the invention, wherein the intermediate transfer member is defined so that the aggregation degree of the transparent toner or the white toner is 12% or more. The amount of adhesion of the transparent toner or the white toner is controlled to be 0.45 mg / cm ² or more.

According to a third aspect of the present invention, there is provided the image forming apparatus according to the first aspect, wherein the intermediate transfer member is defined such that the aggregation degree of the transparent toner or the white toner is 25% or more. The amount of adhesion of the transparent toner or the white toner is controlled to be 0.3 to 0.45 mg / cm ² .

According to a fourth aspect of the present invention, there is provided the image forming apparatus according to the first aspect, wherein the intermediate transfer member is defined such that the aggregation degree of the transparent toner or the white toner is 40% or more. The adhesion amount of the transparent toner or the white toner is controlled to be 0.15 to 0.3 mg / cm ² .

  According to a fifth aspect of the present invention, in the image forming apparatus according to any of the first to fourth aspects, the intermediate transfer after the toner images of the colored toners are all primary transferred. And a detecting unit configured to detect an image density of a toner image on the body, and based on a detection result of the detecting unit, toner based on the colored toner that is primarily transferred from the plurality of image carriers onto the intermediate transfer member. The image density of the image is adjusted.

  According to a sixth aspect of the present invention, in the image forming apparatus according to the fifth aspect, the toner on the intermediate transfer member after the toner image of the transparent toner or the white toner is primarily transferred. The image density of the image is detected by the detection means, and the amount of the transparent toner or the white toner that is primarily transferred from the second image carrier onto the intermediate transfer body is also determined based on the detection result. To be adjusted.

  The present invention provides a transparent toner or a white toner when a toner image made of a transparent toner or a white toner is primarily transferred to an intermediate transfer member before the toner image made of a plurality of colored toners is primarily transferred onto the intermediate transfer member. A toner having a cohesion degree of a predetermined value or more is used as the toner, and the adhesion amount of the transparent toner or the white toner on the intermediate transfer member is optimized, so that the color toner image formed on the intermediate transfer member is placed on the recording medium. The secondary transfer rate at the time of secondary transfer can be sufficiently increased. Thereby, it is possible to provide an image forming apparatus in which an image with good color reproducibility is formed in the secondary transfer step.

1 is an overall configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. It is sectional drawing which shows an image formation part. FIG. 3 is a configuration diagram illustrating the vicinity of an intermediate transfer belt. FIG. 10 is a table showing changes in color reproducibility when the aggregation degree of transparent toner and the amount of transparent toner attached on the intermediate transfer belt are varied. FIG. 6 is a schematic diagram illustrating a state of toner carried on an intermediate transfer belt.

Embodiment.
Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

First, the configuration and operation of the entire image forming apparatus will be described with reference to FIGS.
FIG. 1 is a block diagram showing a printer as an image forming apparatus, and FIG. 2 is an enlarged view showing an image forming unit thereof. FIG. 3 is a configuration diagram showing the vicinity of the intermediate transfer belt 8.
As shown in FIG. 1, an intermediate transfer unit 15 (intermediate transfer belt device) is installed in the center of the image forming apparatus main body 100. Further, image forming units 6Y, 6M, 6C, and 6K for colored toners corresponding to the respective colors (yellow, magenta, cyan, and black) so as to face the intermediate transfer belt 8 (intermediate transfer member) of the intermediate transfer unit 15. And an image forming unit 6W for transparent toner corresponding to the transparent toner. Further, a secondary transfer roller 19 as a secondary transfer member is disposed so as to face the intermediate transfer belt 8 of the intermediate transfer unit 15.

  Referring to FIG. 2, an image forming unit 6Y corresponding to yellow includes a photosensitive drum 1Y as an image carrier, a charging unit 4Y disposed around the photosensitive drum 1Y, a developing unit 5Y, and a cleaning unit 2Y. , And a static elimination unit (not shown). Then, an image forming process (charging process, exposure process, developing process, transfer process, cleaning process) is performed on the photosensitive drum 1Y, and a yellow image is formed on the photosensitive drum 1Y.

The other four image forming units 6M, 6C, 6K, and 6W have substantially the same configuration as the image forming unit 6Y corresponding to yellow except that the color of the toner used is different. An image corresponding to the color is formed. Hereinafter, description of the other four image forming units 6M, 6C, 6K, and 6W will be omitted as appropriate, and only the image forming unit 6Y corresponding to yellow will be described.
Here, the transparent toner (contained in the developing unit and the toner container) used in the image forming unit 6W for transparent toner is the color toner used in the image forming units 6Y, 6M, 6C, and 6K for colored toner. In contrast, the toner does not contain a colorant (or contains a transparent colorant), and is a transparent toner.

Referring to FIG. 2, a photosensitive drum 1Y as an image carrier is rotationally driven counterclockwise in FIG. 2 by a drive motor (not shown). Then, the surface of the photosensitive drum 1Y is uniformly charged at the position of the charging unit 4Y (a charging process).
Thereafter, the surface of the photosensitive drum 1Y reaches the irradiation position of the laser beam L emitted from the exposure unit 7, and an electrostatic latent image corresponding to yellow is formed by exposure scanning at this position (in the exposure process). is there.).

Thereafter, the surface of the photosensitive drum 1Y reaches a position facing the developing portion 5Y, and the electrostatic latent image is developed at this position to form a yellow toner image (developing process).
Thereafter, the surface of the photosensitive drum 1Y reaches a position facing the intermediate transfer belt 8 and the transfer roller 9Y (primary transfer roller), and the toner image on the photosensitive drum 1Y is transferred onto the intermediate transfer belt 8 at this position. Transferred (primary transfer process). At this time, a small amount of untransferred toner remains on the photosensitive drum 1Y.

Thereafter, the surface of the photosensitive drum 1Y reaches a position facing the cleaning unit 2Y, and untransferred toner remaining on the photosensitive drum 1Y at this position is collected in the cleaning unit 2Y by the cleaning blade 2a (cleaning). Process.)
Finally, the surface of the photosensitive drum 1Y reaches a position facing a neutralization unit (not shown), and the residual potential on the photosensitive drum 1 is removed at this position.
Thus, a series of image forming processes performed on the photosensitive drum 1Y is completed.

The image forming process described above is performed in the same manner as the yellow image forming unit 6Y in the other image forming units 6M, 6C, 6K, and 6W. That is, the laser light L based on the image information from the exposure unit 7 disposed above the image forming unit is on the photosensitive drums 1M, 1C, 1K, and 1W of the image forming units 6M, 6C, 6K, and 6W. Irradiated towards. Specifically, the exposure unit 7 emits laser light L from a light source, and irradiates the photosensitive drum through a plurality of optical elements while scanning the laser light L with a polygon mirror that is rotationally driven.
Thereafter, the toner images of the respective colors formed on the respective photosensitive drums through the development process are transferred (primary transfer) on the intermediate transfer belt 8 serving as an intermediate transfer member. In this way, a color image is formed on the intermediate transfer belt 8.

  Here, referring to FIG. 1, five image forming units 6Y, 6M, 6C, 6K, and 6W are formed from the upstream side of the intermediate transfer belt 8 in the running direction, the image forming unit 6W for transparent toner, and the yellow image forming unit 6W. The image forming unit 6Y, the image forming unit 6C for cyan toner, the image forming unit 6M for magenta toner, and the image forming unit 6K for black toner are arranged in this order. Therefore, after the toner image of the transparent toner formed on the photosensitive drum 1W (second image carrier) for transparent toner is primarily transferred to the intermediate transfer belt 8, the photosensitive drum for colored toner is transferred. Toner images of colored toner formed on 1Y, 1M, 1C, and 1K (a plurality of image carriers) are superimposed and primarily transferred. Even if the transparent toner image is secondarily transferred onto the recording medium P, since it is transparent, it hardly affects the image quality of the color image due to the colored toner secondarily transferred onto the recording medium P. Absent.

Thereafter, the intermediate transfer belt 8 (intermediate transfer member) onto which the toner images of the respective colors are transferred in a superimposed manner reaches a contact position (nip portion) with the secondary transfer roller 19 as a secondary transfer member. At this position, the secondary transfer opposing roller 12B sandwiches the intermediate transfer belt 8 between the secondary transfer roller 19 and forms a secondary transfer nip portion. A bias (high voltage) having the same polarity as the polarity of the toner is applied to the secondary transfer counter roller 12B, and a bias (high voltage) opposite to the polarity of the toner is applied to the secondary transfer roller 19. The As a result, the color toner image formed on the intermediate transfer belt 8 is transferred (secondary transfer) onto the recording medium P such as transfer paper conveyed to the position of the secondary transfer nip (secondary transfer). It is a transfer process.) At this time, untransferred toner that has not been transferred to the recording medium P remains on the intermediate transfer belt 8.
Thereafter, the intermediate transfer belt 8 reaches the position of the intermediate transfer cleaning unit 10. At this position, the untransferred toner on the intermediate transfer belt 8 is removed.
Thus, a series of transfer processes performed on the intermediate transfer belt 8 is completed.

Here, referring to FIG. 1, the recording medium P transported to the position of the nip portion (secondary transfer nip portion) is fed to the paper feeding portion 26 (or to the side) disposed below the apparatus main body 100. The paper is fed from the paper feed unit provided via the paper feed roller 27, the registration roller pair 28, and the like.
Specifically, a plurality of recording media P such as transfer paper are stored in the paper supply unit 26 in a stacked manner. When the paper feed roller 27 is driven to rotate counterclockwise in FIG. 1, the uppermost recording medium P is fed toward the rollers of the registration roller pair 28.

  The recording medium P transported to the registration roller pair 28 temporarily stops at the position of the roller nip of the registration roller pair 28 whose rotation drive has been stopped. Then, the registration roller pair 28 is rotationally driven in synchronization with the color image on the intermediate transfer belt 8, and the recording medium P is conveyed toward the secondary transfer nip portion. Thus, a desired color image is transferred onto the recording medium P.

Thereafter, the recording medium P on which the color image has been transferred at the position of the secondary transfer nip portion is conveyed to the position of the fixing portion 20. At this position, the color image transferred to the surface is fixed on the recording medium P by heat and pressure generated by the fixing belt and the pressure roller.
Thereafter, the recording medium P is discharged out of the apparatus by a pair of discharge rollers (not shown). The transferred P discharged from the apparatus by the pair of paper discharge rollers is sequentially stacked on the stack unit as an output image.
Thus, a series of image forming processes in the image forming apparatus is completed.

Next, the configuration and operation of the developing unit in the image forming unit will be described in more detail with reference to FIG.
The developing unit 5Y includes a developing roller 51Y that faces the photosensitive drum 1Y, a doctor blade 52Y that faces the developing roller 51Y, two conveying screws 55Y disposed in the developer containing unit, and an opening in the developer containing unit. A toner replenishing path 43Y communicating with the magnetic sensor 56Y, a magnetic sensor 56Y for detecting the toner concentration in the developer, and the like. The developing roller 51Y includes a magnet fixed inside, a sleeve rotating around the magnet, and the like. In the developer accommodating portion, a two-component developer composed of a carrier and toner is accommodated.
The toner supply path 43Y is for appropriately supplying new toner from a toner container (not shown) containing new toner toward the developing device 5Y. Further, the supply of new toner to the developing device 5Y is performed so that the toner density (the ratio of toner in the developer) detected by the magnetic sensor 56Y falls within a predetermined range.

The developing unit 5Y configured as described above operates as follows.
The sleeve of the developing roller 51Y rotates in the direction of the arrow in FIG. The developer carried on the developing roller 51Y by the magnetic field formed by the magnet moves on the developing roller 51Y as the sleeve rotates.
Thereafter, the toner replenished in the developer accommodating portion circulates through the two separated developer accommodating portions while being mixed and stirred together with the developer by the two conveying screws 55Y (movement in the direction perpendicular to the paper surface of FIG. 2). .) The toner in the developer is attracted to the carrier by frictional charging with the carrier, and is carried on the developing roller 51Y together with the carrier by the magnetic force formed on the developing roller 51Y.

  The developer carried on the developing roller 51Y is conveyed in the direction of the arrow in FIG. 2 and reaches the position of the doctor blade 52Y. The developer on the developing roller 51Y is conveyed to a position facing the photosensitive drum 1Y (development area) after the developer amount is made appropriate at this position. The toner is attracted to the latent image formed on the photosensitive drum 1Y by the electric field formed in the development area. Thereafter, the developer remaining on the developing roller 51Y reaches the upper portion of the developer accommodating portion as the sleeve rotates, and is detached from the developing roller 51Y at this position.

Next, the intermediate transfer unit 15 (intermediate transfer belt device) will be described in more detail with reference to FIG.
As shown in FIG. 3, the intermediate transfer unit 15 includes an intermediate transfer belt 8 as an intermediate transfer member, five primary transfer rollers 9Y, 9M, 9C, 9K, and 9W, a driving roller 12A, a secondary transfer counter roller 12B, The tension roller 12C, the correction roller 12D, the intermediate transfer cleaning unit 10, the density detection sensor 65 as a detection unit, and the like. The intermediate transfer belt 8 is stretched and supported by a plurality of roller members 12A to 12D, and is endlessly moved in the direction of the arrow in FIG. 3 by the rotational drive of one roller member (drive roller) 12A.
Photosensitive drums 1Y, 1M, 1C, and 1K as a plurality of image carriers on which toner images with colored toners (yellow, magenta, cyan, and black) are respectively formed, and a second toner image with transparent toner are formed. The photosensitive drum 1 </ b> W as an image carrier is arranged in parallel so as to face the intermediate transfer belt 8.

The five primary transfer rollers 9Y, 9M, 9C, 9K, and 9W respectively sandwich the intermediate transfer belt 8 between the photosensitive drums 1Y, 1M, 1C, 1K, and 1W to form primary transfer nips. Yes. Then, a high voltage (transfer bias) having a polarity opposite to the polarity of the toner is applied to the primary transfer rollers 9Y, 9M, 9C, 9K, and 1W.
The intermediate transfer belt 8 travels in the direction of the arrow and sequentially passes through the primary transfer nips of the primary transfer rollers 9Y, 9M, 9C, 9K, and 9W. In this way, the toner images of the respective colors on the photosensitive drums 1Y, 1M, 1C, 1K, and 1W are primarily transferred onto the intermediate transfer belt 8 (transparent toner image, yellow toner image, cyan toner image, magenta toner). Image and black toner image in this order).
In the transparent toner image forming unit 6W, the transparent toner image is formed in the same range as the colored toner image so that the transparent toner image is reliably formed below the colored toner image on the intermediate transfer belt 8. An image portion may be formed, or an image portion made of a transparent toner image may be formed in the entire image area (image portion and non-image portion of a colored toner image).

In the present embodiment, the intermediate transfer belt 8 as an intermediate transfer member is made of PI (polyimide), PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PC (polycarbonate), or the like. A conductive material such as carbon black is dispersed in a layer or a plurality of layers.
The intermediate transfer belt 8 is adjusted so that the surface resistivity on the back side of the belt in an environment of 23 ° C. and 50% is in the range of 10 ^9.5 to 10 ^11.5 Ω / □. Here, when the surface resistivity on the back side of the intermediate transfer belt 8 is 10 ^9.5 Ω / □ or more, the intermediate transfer belt is transferred from the first power source 61 (constant current power source) via the secondary transfer counter roller 12B. 8, almost all of the current applied to the recording medium P (current having the same polarity as the toner) is used as a transfer current toward the recording medium P and the secondary transfer roller 19, so that the transfer rate in the secondary transfer process is stabilized. On the other hand, when the surface resistivity of the intermediate transfer belt 8 on the back side of the belt is less than 10 ^9.5 Ω / □, the intermediate transfer belt 8 is intermediately supplied from the first power supply unit 61 (constant current power supply) via the secondary transfer counter roller 12B. Since a part of the current applied to the transfer belt 8 leaks from the inner peripheral surface of the intermediate transfer belt 8 to other members in contact with the intermediate transfer belt 8, the transfer toward the recording medium P or the secondary transfer roller 19 is performed. The current decreases, and the transfer rate in the secondary transfer process decreases. On the other hand, when the surface resistivity of the intermediate transfer belt 8 on the back side of the belt exceeds 10 ^11.5 Ω / □, charges remain on the intermediate transfer belt 8 and abnormal images such as discharge trace images tend to occur. Become. If the surface resistivity of the intermediate transfer belt 8 on the back side of the belt exceeds 10 ^11.5 Ω / □, a thin paper having a small basis weight is used as the recording medium P. After the secondary transfer process, the intermediate transfer belt 8 is used. In the process of separating the recording medium P from the recording medium P, it is difficult to separate the recording medium P from the intermediate transfer belt 8. For these reasons, it is preferable to set the surface resistivity of the intermediate transfer belt 8 on the back side of the belt within the above range. The surface resistivity of the intermediate transfer belt 8 was measured using a “HIRESTA IP” and an “HR probe” attached thereto (both manufactured by Mitsubishi Chemical Corporation) as a 10-second value at an applied voltage of 500V. is there.
The intermediate transfer belt 8 has a volume resistivity of about 10 ⁷ to 10 ¹² Ωcm and a thickness of about 80 to 100 μm.
If necessary, a release layer can be coated on the surface of the intermediate transfer belt 8. At that time, materials used for the coating are ETFE (ethylene-tetrafluoroethylene copolymer), PTFE (polytetrafluoroethylene), PVDF (vinylidene fluoride), PEA (perfluoroalkoxy fluororesin), FEP (four A fluororesin such as fluorinated ethylene-hexafluoropropylene copolymer) or PVF (vinyl fluoride) can be used, but is not limited thereto.
Further, as a method for manufacturing the intermediate transfer belt 8, there are a casting method, a centrifugal molding method, and the like, and a step of polishing the surface is performed as necessary.

The drive roller 12A is rotationally driven by a drive motor (not shown). As a result, the intermediate transfer belt 8 travels in a predetermined traveling direction (clockwise in FIG. 3).
Based on the displacement (displacement amount) of the intermediate transfer belt 8 detected by a meandering detection sensor (not shown), the correction roller 12D has one end as a fixed end and the other end moves in the vertical direction ( The rotation axis is inclined). Thereby, the displacement (meandering) of the intermediate transfer belt 8 in the width direction is corrected.
The tension roller 12 </ b> C is in contact with the outer peripheral surface of the intermediate transfer belt 8. An intermediate transfer cleaning unit 10 (cleaning blade) is installed between the secondary transfer counter roller 12B and the tension roller 12C.

The secondary transfer counter roller 12B is in contact with a secondary transfer roller 19 (secondary transfer member) via an intermediate transfer belt 8 as an intermediate transfer member. The secondary transfer counter roller 12B has a middle resistance rubber layer formed on a core metal, and is set to have a resistance value of 10 ⁷ to 10 ^8.5 Ω in an environment of 23 ° C. and 50%.

A secondary transfer roller 19 (secondary transfer unit 30) as a secondary transfer member is slidably moved to and away from the secondary transfer opposing roller 12B by a contact / separation mechanism (not shown) (in the direction of a double arrow in FIG. 3). It is configured to move.)
During normal operation (secondary transfer process), the secondary transfer roller 19 is pressed against the secondary transfer counter roller 12B via the intermediate transfer belt 8 by the contact / separation mechanism (the state shown in FIG. 3). . When the secondary transfer process is not performed (warming up, standby, etc.).
Although not shown, the secondary transfer unit 30 includes a cleaning blade, a lubricant supply brush, a static elimination needle, a paper dust remover in addition to the secondary transfer roller 19 and the second power supply unit 62 in the housing 36. A brush is included so as to contact the secondary transfer roller 19.

The secondary transfer roller 19 as a secondary transfer member is formed by forming a conductive rubber layer made of NBR (nitrile rubber) or the like and having a rubber hardness of about 48 to 58 Hs on a cored bar.
In this embodiment, the secondary transfer roller is used as the secondary transfer member for performing the secondary transfer process. However, a known secondary transfer device using corona discharge may be used as the secondary transfer member. it can.

Here, referring to FIG. 2 and FIG. 3, in this embodiment, all colored toner images (yellow toner image, cyan toner image, magenta toner image, and black toner image) are primarily transferred. A density detection sensor 65 is installed as detection means for detecting the image density of the toner image (color toner image) on the subsequent intermediate transfer belt 8.
Specifically, the density detection sensor 65 (detection means) is a photosensor including a light emitting element and a light receiving element, and is a toner image (a patch pattern formed for density detection) formed on the intermediate transfer belt. The image density of the toner image is detected by receiving the reflected light of the light emitted from the light emitting element toward the light receiving element. One density detection sensor 65 (detection means) is installed on the further downstream side of the black photosensitive drum 1K on the most downstream side with respect to the traveling direction of the intermediate transfer belt 8 so as to face the surface of the intermediate transfer belt 8. ing.

Density detection by the density detection sensor 65 (detection means) is performed at a timing different from that of a normal image forming process (for example, timing of warming up or timing between sheets). The density detection patch patterns (toner images) detected by the density detection sensor 65 are, for example, all toner images (transparent toner image, yellow toner image, cyan toner image, magenta toner image, and black toner image). And a single toner image (a yellow toner image, a cyan toner image, a magenta toner image, and a black toner image) that are separately superimposed on a transparent toner image. And can be used. Then, the image density of these patch patterns is detected by the density detection sensor 65, respectively, and based on the detection result (density information), the photosensitive drums 1Y, 1M, 1C, 1K, 1W are transferred onto the intermediate transfer belt 8. The image density of the color toner image to be primarily transferred and the adhesion amount of the transparent toner image are respectively adjusted.
Specifically, referring to FIG. 2, the result detected by density detection sensor 65 is sent to control unit 80, and based on the result, control unit 80 controls charging power supply 61 (charging potential) and exposure unit 7 ( Exposure potential), development power source 62 (development bias), and transfer power source 63 (primary transfer bias) are adjusted and controlled. Thereby, the image quality such as the image density of the color image formed on the intermediate transfer belt 8 is optimized.

Hereinafter, the configuration and operation characteristic of the image forming apparatus according to the present embodiment will be described in detail.
In the present embodiment, as described above, the transparent toner image formed on the photosensitive drum 1W (second image carrier) for transparent toner is transferred onto the intermediate transfer belt 8 (intermediate transfer member). After the next transfer, the colored toner images respectively formed on the four photosensitive drums 1Y, 1M, 1C, and 1K (a plurality of image carriers) for the colored toner are sequentially superimposed and primarily transferred.

Here, in this embodiment, the degree of aggregation of the transparent toner is determined to be a predetermined value or more, and the primary transfer is performed from the photosensitive drum 1W for the transparent toner onto the intermediate transfer belt 8 according to the degree of aggregation. The amount of transparent toner attached is determined.
Specifically, in this embodiment, the degree of aggregation of the transparent toner is determined to be 12% or more. Then, the amount of transparent toner attached on the intermediate transfer belt 8 (the amount of toner attached per unit area that is primarily transferred onto the intermediate transfer belt 8) is controlled to be 0.45 mg / cm ² or more. is doing.

More specifically, transparent toner formed so as to have a cohesion degree of 12% or more is accommodated in the developing section and toner container of the image forming section 6W for transparent toner. This transparent toner is a polymerized toner in which a wax having a mean particle size of 5.2 μm as a main material and a wax dispersed therein, and silica and titanium oxide are externally added as external additives. Then, referring to FIG. 4, the degree of aggregation is set to 12% or more by adjusting the particle diameter, the amount of wax, and the like of the externally added silica. The degree of aggregation of the transparent toner can be measured in the same manner as described in paragraphs 0093 and 0094 of Patent Document 4 described above.
Then, the charging potential, the exposure potential, the developing bias, the primary transfer bias, and the like in the image forming unit 6W for the transparent toner are set so that the transparent toner adhesion amount on the intermediate transfer belt 8 is 0.45 mg / cm ² or more. Imaging conditions are set.
With such a configuration, the secondary transfer rate when the color toner image (colored toner image) formed on the intermediate transfer belt 8 is secondarily transferred onto the recording medium P can be sufficiently increased (secondary). The transfer rate can approach 100%), and an image with good color reproducibility is formed in the secondary transfer process.

Such a configuration has been created based on the experimental results shown in FIG.
FIG. 4 is a table showing changes in color reproducibility when the degree of aggregation of the transparent toner and the amount of the transparent toner on the intermediate transfer belt 8 are varied.
In the experiment relating to FIG. 4, in the image forming apparatus 100 according to the present embodiment, an intermediate transfer belt is used by using transparent toners (adjusted by varying the silica particle size and the amount of wax) having different aggregation degrees. 8 shows the amount of colored toner remaining on the intermediate transfer belt 8 after the secondary transfer process when the amount of the transparent toner primarily transferred onto the toner 8 is changed. In FIG. 4, “◎” indicates a level at which color reproducibility is the best without any colored toner on the intermediate transfer belt 8 (secondary transfer rate is 100%), and “◯”. Indicates a level with good color reproducibility (a sufficiently acceptable level) even though only a small amount of colored toner is present on the intermediate transfer belt 8, and “Δ” indicates a small amount of colored toner on the intermediate transfer belt 8. This indicates a level at which the color reproducibility slightly decreases (a level that is slightly unacceptable), and “x” indicates a level at which color toner is present on the intermediate transfer belt 8 and the color reproducibility decreases (an unacceptable level). .). In this experiment, high-quality paper having a smoothness of 30 seconds was used as the recording medium P.

From the experimental results shown in FIG. 4, in this embodiment, the aggregation degree of the transparent toner is set to 12% or more, and the adhesion amount of the transparent toner on the intermediate transfer belt 8 is 0.45 mg / cm ² or more. By setting, the secondary transfer rate of the colored toner can be improved to nearly 100% (the result of “◯” or “◎” in FIG. 4 can be obtained). That is, a color image having an image density optimized by image density control using the density detection sensor 65 is reproduced on the recording medium P as it is. By setting the aggregation degree of the transparent toner to 25% or more and setting the adhesion amount of the transparent toner on the intermediate transfer belt 8 to be 0.45 mg / cm ² or more, the above effect can be further ensured. (A result of “◎” in FIG. 4 can be obtained).

Here, in this embodiment, the aggregation degree of the transparent toner is set to 12% or more, and the amount of the transparent toner adhered on the intermediate transfer belt 8 is set to 0.45 mg / cm ² or more.
On the other hand, the aggregation degree of the transparent toner is set to 25% or more, and the adhesion amount of the transparent toner on the intermediate transfer belt 8 is set to be in the range of 0.3 to 0.45 mg / cm ^2. You can also. Even in such a case, from the experimental results of FIG. 4, it is possible to improve the secondary transfer rate of the colored toner to near 100% and obtain good color reproducibility (“◯” or “◎” in FIG. 4). Can get the result.) In such a case, the degree of aggregation of the transparent toner is set to 30% or more, and the amount of the transparent toner adhered on the intermediate transfer belt 8 is set to 0.3 to 0.45 mg / cm ^2. As a result, the above-described effect is more reliably exhibited (the result of “◎” in FIG. 4 can be obtained). Further, in such a case, since the upper limit (0.45 mg / cm ² ) of the adhesion amount of the transparent toner is determined, the consumption amount of the transparent toner can be reduced to some extent.
Further, the degree of aggregation of the transparent toner can be set to 40% or more, and the amount of the transparent toner adhering on the intermediate transfer belt 8 can be set within the range of 0.15 to 0.3 mg / cm ^2. . Even in such a case, from the experimental results of FIG. 4, it is possible to improve the secondary transfer rate of the colored toner to near 100% and obtain good color reproducibility (“◯” or “◎” in FIG. 4). Can get the result.) Further, in such a case, since the upper limit value (0.3 mg / cm ² ) of the adhesion amount of the transparent toner is determined, the consumption amount of the transparent toner can be reduced to some extent.

Hereinafter, the state of the transparent toner TW and the colored toner T carried on the intermediate transfer belt 8 will be described with reference to FIG.
FIG. 5A1 is a schematic diagram showing a state of the transparent toner TW and the colored toner T carried on the intermediate transfer belt 8 before the secondary transfer step when the transparent toner TW having a low aggregation degree is used. FIG. 5A2 shows a state on the intermediate transfer belt 8 after the secondary transfer step when the transparent toner TW having a low degree of aggregation is used (the state of residual toner that has not been transferred onto the recording medium P). ). FIG. 5B1 is a schematic diagram showing a state of the transparent toner TW and the colored toner T carried on the intermediate transfer belt 8 before the secondary transfer process when the transparent toner TW having a high degree of aggregation is used. FIG. 5B2 shows the state on the intermediate transfer belt 8 after the secondary transfer step when the transparent toner TW having a high degree of aggregation is used (the state of the residual toner that has not been transferred onto the recording medium P). It is a schematic diagram showing.

In general, when the degree of aggregation of the toner increases, the non-electrostatic adhesion force between the toner and the intermediate transfer belt 8 increases, so that the secondary transfer rate decreases. However, it can be seen from the experimental results of FIG. 4 that the higher the degree of aggregation of the transparent toner TW, the higher the secondary transfer rate of the colored toner. This is due to the following mechanism.
As shown in FIG. 5B1, when the degree of aggregation of the transparent toner TW is high, the transparent toner TW and the colored toner T are 1 on the intermediate transfer belt 8 in the order in which the color toners TW and T are primarily transferred. The next transfer is carried out, but is laminated on the intermediate transfer belt 8 in that order. Therefore, only the transparent toner TW is in direct contact with the intermediate transfer belt 8. This is because the entire transparent toner TW has a sufficient degree of aggregation even if there are variations in the particle size, charge amount and non-electrostatic adhesion of each color toner. That is, even if the respective color toners are sequentially primary-transferred, the toners of the respective colors are formed on the intermediate transfer belt 8 in the order of the primary transfer without causing the relative replacement of the respective color toners. Therefore, during the secondary transfer process, the toner of each color is secondarily transferred to the recording medium P in the reverse order. As a result, as shown in FIG. 5B2, the toner remaining on the intermediate transfer belt 8 without being subjected to the secondary transfer is only the transparent toner TW that has been in contact with the intermediate transfer belt 8. The amount of the remaining toner is larger than that when the transparent toner TW having a low aggregation degree shown in FIG. When the transparent toner TW having a high degree of aggregation is used as described above, the amount of toner remaining on the intermediate transfer belt 8 without being subjected to secondary transfer is relatively large, but most of the residual toner is the transparent toner TW. . That is, the secondary transfer rate of the colored toner T (color image) is almost 100%.

  On the other hand, as shown in FIG. 5A1, when the aggregation degree of the transparent toner TW is low, the transparent toner TW and the transparent toner TW on the intermediate transfer belt 8 in the order in which the respective color toners TW and T are primarily transferred. The colored toner T is primarily transferred, but is not stacked on the intermediate transfer belt 8 in that order. That is, in the primary transfer process of each color toner, the replacement of each color toner occurs in part, and not only the transparent toner TW directly contacts the intermediate transfer belt 8 but also a part of the color toner T. Also, the intermediate transfer belt 8 comes into direct contact. This is because there is a variation in the primary transfer speed due to variations in the particle size, charge amount and non-electrostatic adhesion of each color toner, and the toner is replaced because the aggregation degree of the transparent toner TW is low. . Further, since the toner of each color is changed during the secondary transfer process, as shown in FIG. 5 (A2), the toner remaining on the intermediate transfer belt 8 without being subjected to the secondary transfer is not limited to the transparent toner TW. The colored toner T is also included. However, the amount of remaining toner is smaller than when the transparent toner TW having a high aggregation degree shown in FIG. 5B2 is used. When the transparent toner TW having a low degree of aggregation is used as described above, although the amount of toner remaining on the intermediate transfer belt 8 without being subjected to secondary transfer is relatively small, the color toner T is included in the residual toner. . That is, although the amount of toner remaining on the intermediate transfer body without being subjected to secondary transfer is relatively small, the colored toner T also remains. Therefore, the secondary transfer rate of the colored toner T (color image) does not reach 100%.

  Also from such a mechanism, when the toner image by the transparent toner TW is primarily transferred to the intermediate transfer belt 8 before the toner image by the plurality of colored toners T is superimposed and primarily transferred to the intermediate transfer belt 8, It can be seen that the degree of aggregation of the transparent toner TW greatly affects the secondary transfer rate of the colored toner T (color image) in the secondary transfer step.

  As described above, in the present exemplary embodiment, the toner image formed of the transparent toner TW is transferred to the intermediate transfer belt 8 (intermediate transfer member) before the toner image formed of the plurality of colored toners T is superimposed and primarily transferred. In the case of primary transfer to the belt 8, the transparent toner TW having a cohesion degree of a predetermined value or more is used and the amount of the transparent toner adhering to the intermediate transfer belt 8 is optimized, so that it is formed on the intermediate transfer belt 8. The secondary transfer rate when the color toner image thus formed is secondarily transferred onto the recording medium P can be sufficiently increased. Thereby, an image with good color reproducibility can be formed in the secondary transfer step.

In this embodiment, white toner can be used instead of transparent toner. The white toner uses a white colorant and has little influence on the density detection of the colored toner image by the density detection sensor 65. Further, since the white toner is hardly secondarily transferred onto the recording medium P in the secondary transfer step, similarly to the transparent toner in the present embodiment, the influence of the colored toner on the image quality of the color image is small.
In the case where the toner image of the white toner is primarily transferred to the intermediate transfer belt 8 before the toner images of the plurality of colored toners T are superimposed and primarily transferred to the intermediate transfer belt 8, the same as in the present embodiment. In addition, by using a white toner having a cohesion degree of a predetermined value or more and optimizing the adhesion amount of the white toner on the intermediate transfer belt 8, substantially the same effect as in the present embodiment can be obtained.

In this embodiment, the intermediate transfer belt 8 is used as an intermediate transfer member, but an intermediate transfer drum can also be used as an intermediate transfer member.
In the embodiment, the secondary transfer roller 19 is used as the secondary transfer member. However, a secondary transfer belt can be used as the secondary transfer member.
In these cases, the same effect as in the present embodiment can be obtained.

  It should be noted that the present invention is not limited to the present embodiment, and it is obvious that the present embodiment can be modified as appropriate within the scope of the technical idea of the present invention, other than suggested in the present embodiment. is there. In addition, the number, position, shape, and the like of the constituent members are not limited to the present embodiment, and the number, position, shape, and the like suitable for implementing the present invention can be achieved.

1Y, 1M, 1C, 1K photosensitive drum (image carrier),
1W photosensitive drum (second image carrier),
Image forming unit for 6Y, 6M, 6C, 6K colored toner,
1W Imager for clear toner,
8 Intermediate transfer belt (intermediate transfer member),
9Y, 9M, 9C, 9K, 9W primary transfer roller,
19 Secondary transfer roller (secondary transfer member),
65 concentration detection sensor (detection means),
100 Image forming apparatus body (apparatus body),
T color toner, TW transparent toner, P recording medium.

JP 2008-225000 A JP-A-10-213942 Japanese Patent No. 3820733 Japanese Patent No. 3909939

Claims (6)

A plurality of image carriers on which toner images of colored toners are respectively formed;
A second image carrier on which a toner image is formed by transparent toner or white toner;
The plurality of image carriers and the second image carrier are opposed to each other, and after the toner images of the transparent toner or white toner formed on the second image carrier are primarily transferred, An intermediate transfer body on which toner images of colored toners respectively formed on the image carrier are primarily transferred and superimposed;
A secondary transfer member for secondary transfer of the toner image primarily transferred to the intermediate transfer member to a recording medium;
With
The degree of aggregation of the transparent toner or the white toner is determined to be equal to or greater than a predetermined value, and the transparent toner that is primarily transferred from the second image carrier onto the intermediate transfer body according to the degree of aggregation. An image forming apparatus characterized in that the amount of white toner attached is determined. The degree of aggregation of the transparent toner or the white toner is determined to be 12% or more, and the adhesion amount of the transparent toner or the white toner on the intermediate transfer member is 0.45 mg / cm ² or more. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled. The degree of aggregation of the transparent toner or the white toner is determined to be 25% or more, and the adhesion amount of the transparent toner or the white toner on the intermediate transfer member is 0.3 to 0.45 mg / cm ² . The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled to be The degree of aggregation of the transparent toner or the white toner is determined to be 40% or more, and the adhesion amount of the transparent toner or the white toner on the intermediate transfer member is 0.15 to 0.3 mg / cm ² . The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled to be A detecting means for detecting the image density of the toner image on the intermediate transfer body after the toner images of the colored toners are all primary transferred;
The image density of the toner image by the colored toner that is primarily transferred from the plurality of image carriers onto the intermediate transfer member is adjusted based on the detection result of the detection unit. The image forming apparatus according to claim 4.   The image density of the toner image on the intermediate transfer body after the toner image with the transparent toner or the white toner is primarily transferred is detected by the detection means, and the second image carrier is based on the detection result. The image forming apparatus according to claim 5, wherein an adhesion amount of the transparent toner or the white toner that is primarily transferred from a body onto the intermediate transfer body is also adjusted.

Images