JP2002328504A - Image forming device and method for controlling image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make black toner density equal both at the time of forming a full color image and at the time of forming a monochromatic image in an image forming device of a tandem type in which plural pairs of image forming stations S of respective colors are disposed along a paper conveyance direction A and deterioration of a formed image due to reverse transfer of toner or the like is prevented by separating a transfer belt 11 from photoreceptors 3c, 3m, 3y at the image forming stations Sc, Sm, Sy of a downstream side which take no part in image formation by lowering an elevating/lowering mechanism 42 by a controller 43 when the monochromatic image is formed by using only an image forming station Sbk of the most upstream side. SOLUTION: Developing bias voltage Vbk of a developing bias source 44bk of the image forming station Sbk is composed by superposing AC voltage on DC voltage. Further when the monochromatic image is formed, the controller 43 lowers the elevating/lowering mechanism 42 and also lowers frequency of the AC voltage of the developing bias voltage Vbk to lower black toner density.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus capable of forming a monochrome image and a multi-color image such as a full-color image, and a control method of the image forming apparatus.
In particular, a plurality of sets of image forming stations of each color are arranged along the sheet conveying direction, and the respective image forming stations sequentially form an image while conveying the sheet in the sheet conveying direction by the sheet conveying carrier. The present invention relates to a so-called tandem-type image forming apparatus and a method of controlling the image forming apparatus.

[0002]

2. Description of the Related Art The above-described full-color image forming apparatus includes a single-drum type in which a photoconductor is rotated several times to use toner colors and a toner image of each color is repeatedly transferred onto a sheet. It is roughly classified into a tandem type in which a plurality of sets of image forming stations of respective colors arranged in the sheet conveying direction sequentially transfer a toner image to a sheet.

The tandem-type image forming apparatus transfers four types of toners for realizing full color onto paper in one process, and therefore has a processing speed almost four times that of the single-drum type. . For this reason, in recent years, tandem image forming apparatuses have become mainstream, in combination with the fact that the internal apparatuses have been miniaturized and have been assembled (unitized) to be relatively inexpensive.

However, in a tandem-type image forming apparatus, even when a monochrome monochromatic image is formed, the paper comes into contact with a photoconductor (image forming station) of another color.
There is a problem in that unnecessary color toner remaining on the photoreceptor adheres to the paper, thereby deteriorating the image quality. Further, there is also a problem that the life of the photosensitive member and the paper transport belt is shortened due to unnecessary contact of the paper transport belt that carries the paper and sequentially transports each image forming station.

A typical prior art for solving such a problem is disclosed in JP-A-6-258914 and JP-A-10-198121. In these prior arts, at the time of forming a monochromatic image, the paper transport belt is separated from a photoreceptor that is not involved in image formation, thereby preventing the mixing of unnecessary color toners, and allowing the paper transport belt and the photoreceptor to move together. Wear of the two due to contact is avoided.

[0006]

The image forming stations for each color have a desirable arrangement order because a slight difference in image quality occurs depending on the arrangement order. In particular, black, which is most often used as a single color, has a high masking effect on other colors, so if an image is formed on the most downstream side, each color image formed up to that point will be masked, resulting in a blackened image. Therefore, in a full-color image requiring a delicate balance, it is desired to form an image on the most upstream side in order to obtain high image quality.

On the other hand, in a tandem-type image forming apparatus, when a full-color image is formed, the unfixed toner once transferred to the sheet at the upstream image forming station (the above-described unit) is transferred to the photosensitive member of the downstream image forming station. There is a problem that a reverse transfer phenomenon occurs, in which a good color image cannot be obtained. That is, the toner image transferred at the uppermost stream first color image forming station is reversely transferred to the photoconductor at the time of subsequent transfer at the second to fourth color image forming stations. Is reversely transferred in the third and fourth color image forming stations, and the toner image transferred in the third color image forming station is reversely transferred in the fourth color image forming station. For this reason, after the transfer of the toner image is completed in the fourth color image forming station,
For example, the adhesion amount of the first color toner to the paper is reduced by several tens% compared to the initial adhesion amount.

The above-mentioned reverse transfer phenomenon occurs when the toner adheres to the photoconductor from the developing roller in accordance with the image potential of the photoconductor (drum or belt) in the image forming process. Not necessarily
Some have weak negative polarity and some have positive polarity, and discharge occurs during the peeling process from the photoreceptor after transfer, and some have positive polarity later. It is said that this phenomenon is caused by a slight return to the photoreceptor each time the transfer charge of positive polarity is received.

For this reason, in the image forming station on the upstream side, in order to obtain an appropriate image quality at the time of forming a full-color image that requires a delicate balance, an image in which toner is excessively attached in anticipation of the reverse transfer phenomenon is provided. The formation conditions are set.

Therefore, when such a method is applied to the above-mentioned conventional technology, if the paper transport belt is separated from the photoreceptor which is not involved in the image formation during the formation of a single color image, the mixing of toner of an unnecessary color Also, instead of preventing the paper transport belt and the photoreceptor from being worn, there is a problem that the above several tens% of the toner to be lost by the reverse transfer phenomenon remains attached to the paper, and the image density becomes high. Occurs.

An object of the present invention is to provide an image forming apparatus and a control method of an image forming apparatus capable of forming an image in a tandem type by partially using an image forming station on an upstream side in a sheet conveying direction. An object of the present invention is to provide an image forming apparatus capable of improving image quality during use and a method of controlling the image forming apparatus.

[0012]

According to the image forming apparatus of the present invention, a plurality of image forming stations centering on a photoreceptor are arranged along a sheet conveying direction, and a sheet is conveyed by a sheet conveying carrier in the sheet conveying direction. The image forming stations are configured so that a multicolor image can be formed by sequentially superimposing and forming an image while conveying the image, and image forming is partially performed using the image forming station on the upstream side in the sheet conveying direction. In an image forming apparatus in which the paper transporting carrier is separated from a downstream image forming station which is not involved in image formation when performing, at least the developing bias source in the upstream image forming station includes an AC component and a DC component. Is configured to be able to generate a developing bias voltage in which the above-described portion is superimposed. To the time of image formation using the image forming station, characterized in that it comprises a frequency switching means for lowering the frequency of the AC component of the developing bias voltage of an image forming station of the upstream side.

According to the above arrangement, a plurality of image forming stations are provided along the sheet conveying direction, and the image forming stations are sequentially superimposed while conveying the sheet in the sheet conveying direction by the sheet conveying carrier. By performing image formation, in a so-called tandem type image forming apparatus configured to be capable of forming a multi-color image such as a full-color image, when performing image formation partially using the upstream image forming station, By separating the paper transport carrier from the downstream image forming station that is not involved in image formation, the downstream image forming station comes into contact with the paper to prevent deterioration of the formed image due to reverse transfer of toner etc. In such a configuration, the partial image forming station is compared with the image forming station using all the image forming stations. To the time of image formation using the image density becomes darker to the reverse transcription does not occur.

Therefore, in the present invention, the developing bias source in at least the upstream image forming station is
When a developing bias voltage in which an AC component is superimposed on a DC component can be generated and a developing bias voltage in which a large-amplitude AC voltage is superimposed on an appropriate level of DC voltage is used, the toner vibrates in the developing space, and the photosensitive member It becomes easy to move to the upper electrostatic latent image, and as the frequency of the AC component increases,
Since the degree becomes large, the frequency switching unit optimizes the toner density by lowering the frequency of the AC component of the upstream image forming station when forming an image using a partial image forming station.

Therefore, the image densities obtained for the image forming stations located on the upstream side are the same in the case of (full-color) image forming using all the image forming stations and in the case of partially (single-color) image forming. The image density does not undesirably increase when the image forming station is partially used, and the image quality when the image forming station is partially used can be improved.

Further, in the image forming apparatus of the present invention, a plurality of image forming stations centering on the photoreceptor are provided along the sheet conveying direction, and the sheet is conveyed in the sheet conveying direction by the sheet conveying carrier. Each of the image forming stations is configured to form a multicolor image by sequentially superimposing and forming an image. When an image is formed by partially using the image forming station on the upstream side in the sheet conveying direction, the image is formed. In an image forming apparatus in which the paper transporting carrier is separated from a downstream image forming station that is not involved in the formation, at least the developing bias source in the upstream image forming station includes a developing component in which an AC component is superimposed on a DC component. A bias voltage can be generated, and all the image forming stations are supplied to the developing bias source of the upstream image forming station. The time of image formation using the down generates a developing bias voltage obtained by superposing an AC component on a DC component,
The image forming apparatus further includes a bias switching unit for generating a developing bias voltage having only a DC component when an image is formed using the partial image forming station.

According to the above arrangement, at the time of image formation using a partial image forming station, the bias switching means optimizes the toner density by using only the DC component as the developing bias voltage of the upstream image forming station. .

Also in this case, the image densities obtained for the image forming stations located on the upstream side can be obtained by using the image forming stations using all the image forming stations (full color) and partially using the (monochromatic) image. The image density can be made equal at the time of forming, and the image density does not undesirably increase when the above-mentioned partial use is performed, and the image quality when the partial image forming station is used can be improved. .

Further, the image forming apparatus of the present invention is characterized in that the image forming station located at the most upstream position is for black toner.

According to the above arrangement, when the image forming station for black toner is located at the uppermost stream, the black toner becomes the lowermost layer at the time of forming a multi-color (full color) image and affects the toner images of other colors. Is minimized, and a preferable multicolor image can be obtained. Further, in general, there is much more opportunity to form a black single-color image than to form a multi-color image. Therefore, by reducing the amount of toner adhering when forming a single-color image as described above, the toner consumption amount is reduced. Can also be reduced.

According to the image forming apparatus of the present invention, in order to solve the above-mentioned problem, a plurality of image forming stations centering on a photosensitive member are arranged along a sheet conveying direction, and the sheet conveying carrier is used for conveying the sheet. In the image forming apparatus which realizes multi-color image formation by sequentially superimposing images formed by the respective image forming stations while transporting the paper in the direction, when forming a single color image, the image forming apparatus at the most upstream side in the paper transport direction is used. Using a station, while performing image formation by separating the paper transport carrier from the downstream image forming station not involved in image formation, by switching the developing bias voltage to a voltage different from that during multicolor image formation, It is characterized by including a control means for adjusting the amount of toner adhering to the photoreceptor.

According to the above arrangement, by changing the developing bias voltage between the time of forming a multi-color image and the time of forming a single-color image, an image can be formed at an optimum image density.

That is, a reverse transfer phenomenon in which unfixed toner once transferred onto a sheet (transfer material) at the most upstream image forming station during multicolor image formation is robbed at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the image forming apparatus of the present invention,
By changing the developing bias voltage, the setting of the initialization charging potential and the image exposure amount is common between the multicolor image forming and the monochromatic image forming, so that the contrast potential (the potential difference between the developing bias voltage and the bright portion potential) is increased. ) Can be changed, and the amount of toner adhering to the photoconductor can be controlled.

More specifically, when forming a multicolor image in which the amount of toner adhered to the photosensitive drum is to be increased, the absolute value of the developing bias voltage is controlled to be large. At the time of forming a monochrome image in which the amount of adhesion is to be reduced, control is performed so that the absolute value of the developing bias voltage is reduced.

As a result, the difference in image density between the image density when forming a single-color image and the image density when forming a multicolor image is eliminated, and the problem that the image density is too high when forming a single-color image is avoided. Can be prevented.

Further, the control means increases the developing bias voltage applied to the most upstream image forming station at the time of forming a multi-color image than the developing bias voltage applied to the image forming station at the time of forming a monochromatic image. Is more preferred.

Thus, the contrast potential can be increased by increasing the developing bias voltage in forming a multicolor image, and the amount of toner adhered to the photoconductor can be increased as compared with the case of forming a single color image. .

Therefore, it is possible to adjust the amount of toner adhering to the photoreceptor optimally at the time of forming a multicolor image and at the time of forming a monochromatic image, respectively. It is possible to eliminate a density difference generated between the image density and the image density.

In order to solve the above-mentioned problems, the image forming apparatus according to the present invention is provided with a plurality of image forming stations centered on a photoreceptor along a sheet conveying direction, and the sheet conveying support is provided by a sheet conveying carrier. In the image forming apparatus which realizes multi-color image formation by sequentially superimposing images formed by the respective image forming stations while transporting the paper in the direction, when forming a single color image, the image forming apparatus at the most upstream side in the paper transport direction is used. The station is used to form an image by separating the paper transporting carrier from a downstream image forming station that is not involved in image formation, and by switching the contrast potential to a potential different from that used for multicolor image formation. It is characterized by having a control means for adjusting the amount of toner attached to the body.

According to the above configuration, by changing the contrast potential, the amount of toner adhering to the photoreceptor is controlled to an optimum amount when forming a multi-color image and when forming a single-color image. It is possible to form an image with an optimum image density.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station during multicolor image formation is taken away at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the image forming apparatus of the present invention,
By changing the amount of toner adhered to the photoreceptor by changing the contrast potential, the amount of toner transferred to the sheet can be changed, and optimal image formation can be performed.

More specifically, when the contrast potential is changed in a direction to increase, if the exposure intensity at the time of recording the electrostatic latent image is the same, the amount of toner adhering to the photosensitive drum increases. Therefore, when the transfer conditions are the same, the amount of toner transferred onto the transfer paper increases, and the image density can be increased. Conversely, when the contrast potential is changed in a decreasing direction, the amount of toner adhering to the photosensitive drum decreases even if the exposure intensity during the recording of the electrostatic latent image is the same. Therefore, when the transfer conditions are the same, the amount of toner transferred onto the transfer paper decreases, and the image density also decreases.

As described above, by changing the contrast potential between the time of forming a multicolor image and the time of forming a single color (monochrome) image, the image density obtained by the most upstream image forming station during the formation of a multicolor image and the single color The difference in density that occurs between the image density obtained by the image forming station on the most upstream side during image formation and the image density obtained by the image forming station on the most upstream side can be eliminated, and the problem that the image density is too high during monochromatic image formation can be prevented.

The contrast potential means a potential difference between a developing bias voltage and an exposed portion potential (bright portion potential).

Further, it is more preferable that the control means makes the contrast potential of the most upstream image forming station at the time of forming a multicolor image higher than the contrast potential at the time of forming a monochromatic image.

Thus, by controlling the contrast potential at the time of forming a multi-color image in an increasing direction as compared with the time of forming a single-color image, it is possible to increase the amount of toner adhering to the photosensitive member.

Therefore, it is possible to adjust the optimal amount of toner adhering to the photoconductor different between the time of forming a multicolor image and the time of forming a single color image. It is possible to eliminate a density difference generated between the density and the density.

Further, it is more preferable that the most upstream image forming station is an image forming station provided with black toner.

Accordingly, the black image forming station, which has a particularly large influence on other colors, is located at the most upstream side, so that the black toner becomes the lowermost layer when forming a multicolor image, and the influence on other colors is reduced. Can be suppressed.

Accordingly, the difference in density between the image density obtained for the black toner image forming station at the time of forming the multicolor image and the image density obtained at the time of forming the black single-color image is eliminated, and the multi-color image is formed. High quality images can be obtained both when forming black monochromatic images.

Even in a multi-color image forming apparatus capable of forming a multi-color image, the most frequently used is a monochromatic (monochrome) image formation. In the image forming apparatus of the present invention, since the control for reducing the amount of toner adhered at the time of forming a single color image is performed, the consumption of black toner can be suppressed as compared with the conventional multicolor image forming apparatus.

According to the control method of the image forming apparatus of the present invention, in order to solve the above-mentioned problem, a plurality of image forming stations centering on the photoreceptor are provided along the sheet conveying direction.
In a control method of an image forming apparatus that realizes multicolor image formation by sequentially superimposing images formed by each image forming station while conveying a sheet in the sheet conveying direction by the sheet conveying carrier, when forming a single color image, Using the most upstream image forming station in the paper transport direction, separating the paper transport carrier from the downstream image forming station that is not involved in image formation, and performing image formation, and changing the developing bias voltage to a multicolor image By switching to a voltage different from that at the time of formation, the amount of toner adhering to the photoconductor is adjusted.

According to the above control method, an image can be formed at an optimum image density by switching the developing bias voltage between the time of forming a multi-color image and the time of forming a single-color image.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station during multicolor image formation is taken away at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the control method of the image forming apparatus of the present invention, by changing the developing bias voltage,
Since the setting of the initializing charging potential and the image exposure amount is common between the multicolor image forming and the monochromatic image forming, the contrast potential (the potential difference between the developing bias voltage and the light portion potential) is changed, The toner adhesion amount can be controlled.

More specifically, when forming a multicolor image in which the amount of toner adhering to the photosensitive drum is to be increased, the absolute value of the developing bias voltage is controlled to be large. At the time of forming a monochrome image in which the amount of adhesion is to be reduced, control is performed so that the absolute value of the developing bias voltage is reduced. As a result, the difference in image density between the image density when forming a single-color image and the image density when forming a multicolor image is eliminated, and it is possible to prevent a problem that the image density is too high when forming a single-color image.

Further, the developing bias voltage applied to the most upstream image forming station during multicolor image formation is
It is more preferable to increase the voltage than the developing bias voltage applied to the image forming station when forming a single-color image.

As a result, the contrast potential can be increased at the time of forming a multi-color image, and the toner adhesion amount can be increased as compared with the case of forming a single-color image. Therefore, it is possible to adjust the optimal amount of toner adhering to the photoreceptor at the time of forming a multicolor image and at the time of forming a single color image, respectively. The image density difference that occurs between the toner colors of the most upstream image forming station and that occurs during the period can be eliminated.

According to the control method of the image forming apparatus of the present invention, in order to solve the above-mentioned problems, a plurality of image forming stations centering on the photoreceptor are provided along the sheet conveying direction.
In a control method of an image forming apparatus that realizes multi-color image formation by sequentially forming images by sequentially superposing each image forming station while conveying a sheet in the sheet conveying direction by the sheet conveying carrier, when forming a single color image, Using the most upstream image forming station in the paper transport direction, separating the paper transport carrier from the downstream image forming station that is not involved in image formation to perform image formation, and increasing the contrast potential to multicolor image formation. It is characterized in that the amount of toner adhering to the photoreceptor is adjusted by switching to a potential different from that at the time.

According to the above-mentioned control method, it is possible to switch to the optimum image forming conditions in each of the multi-color image formation and the single-color image formation.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station at the time of forming a multicolor image is robbed at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the control method of the image forming apparatus of the present invention, by changing the contrast potential,
It is possible to adjust the amount of toner attached to the photoconductor to an optimum amount.

Specifically, when the contrast potential is controlled in the increasing direction, the amount of toner adhering to the photosensitive drum increases even if the exposure intensity during recording of the electrostatic latent image is the same. Therefore, if the transfer conditions are the same, the amount of toner transferred onto the transfer paper increases, and the image density can be increased. On the other hand, when the contrast potential is controlled in the decreasing direction, the amount of toner adhering to the photosensitive drum decreases even if the exposure intensity during recording of the electrostatic latent image is the same. Therefore, if the transfer conditions are the same, the amount of toner transferred onto the transfer paper is reduced, and the image density is also reduced.

Therefore, the difference in density between the image density obtained by the image forming station for forming a single color image after the multicolor image is formed and the image density obtained by one image forming station after forming the single color image is eliminated. In addition, it is possible to prevent a problem that the image density is too high during the formation of a single color image.

Further, it is more preferable that the contrast potential of the image forming station on the most upstream side in forming a multicolor image is higher than the contrast potential in forming a monochromatic image.

This makes it possible to increase the amount of adhered toner at the time of forming a multi-color image as compared with the time of forming a single-color image. Therefore, it is possible to adjust the optimal amount of toner adhering to the photoconductor at the time of forming a multicolor image and at the time of forming a monochromatic image, respectively. A density difference generated between the density and the image density at the time of forming a monochrome image can be eliminated.

[0059]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a longitudinal sectional view schematically showing the entire configuration of an image forming apparatus according to an embodiment of the present invention. This image forming apparatus is realized as a so-called color printer, and forms a color image on a predetermined sheet such as a recording sheet or an OPC film by an electrophotographic method according to image data transmitted from the outside. is there. For this reason, the optical unit 1, the developing device 2, the photoconductor 3,
It comprises a cleaner unit 4, a charger 5, a transfer belt unit 6, a paper feed tray 7, a paper feed unit 8, and a fixing paper discharge unit 9.

The image data handled by the image forming apparatus corresponds to a color image using each color of black (BK), cyan (C), magenta (M), and yellow (Y). Therefore, the optical unit 1 (1b
k, 1c, 1m, 1y) and the developing device 2 (2bk, 2c, 2
m, 2y), photoconductor 3 (3bk, 3c, 3m, 3y),
The four cleaner units 4 (4bk, 4c, 4m, 3y) and the chargers 5 (5bk, 5c, 5m, 5y) are provided so as to form four types of latent images corresponding to the respective colors. Black, c is cyan, m is magenta, and y is yellow, and the four image forming stations Sbk, Sc, Sm, and Sy are arranged in tandem along the sheet conveyance direction indicated by arrow A. . In the following description, the subscripts bk, c, m, and y will be omitted when generically referring to each color.

The optical unit 1 is realized by a so-called laser scanning unit (LSU) having a laser irradiator and a reflecting mirror, and applies a laser beam to the surface of the photoconductor 3 uniformly charged by the charger 5. The irradiation forms an electrostatic latent image according to the image data. Instead of the LSU, which scans the laser light from a single light source in the axial direction of the photoconductor 3, an LED is provided in the axial direction of the photoconductor 3.
An LED head or the like in which elements are arranged may be used.

The developing unit 2 converts the electrostatic latent images formed on the respective photoconductors 3 as described above into the BK,
The image is visualized by toner of each color of C, M, and Y. The cleaner unit 4 removes and collects toner remaining on the surface of the photoconductor 3 after image transfer from development. The charger 5 cleans the surface of the photoconductor 3 by:
As described above, this is for uniformly charging to a predetermined potential, and a charger-type charger and a contact-type charger are used.

The transfer / transport belt unit 6 disposed below the photosensitive member 3 includes a transfer belt 11 and a transfer roller 12.
(12bk, 12c, 12m, 12y), a transfer belt driving roller 13, transfer belt driven rollers 14 to 16, a transfer belt tension roller 17, and a transfer belt cleaner unit 18. Each transfer roller 12,
Transfer belt drive roller 13, transfer belt driven roller 14
The transfer belt tension roller 17 is rotatably supported by the transfer frame 19 inside the transfer / conveyance belt unit 6, and stretches the transfer belt 11 to rotate in the direction of the arrow A.

The transfer belt 11 is in contact with each photoconductor 3 by a transfer roller 12, and a toner image of each color formed on the photoconductor 3 is applied to a sheet adsorbed and conveyed on the transfer belt 11. Are sequentially superimposed and transferred to form a color toner image (multicolor toner image). The transfer belt 11 is formed endlessly using, for example, a film having a thickness of about 100 μm.

The transfer roller 12 is applied with a high voltage ((+) having a polarity opposite to the charging polarity (eg, −) of the toner) for transferring the toner image. The transfer roller 12
For example, a surface of a metal roller such as stainless steel having a diameter of 8 to 10 mm is coated with a conductive elastic material such as EPDM or urethane foam. This conductive elastic material makes it possible to adhere to the sheet and apply a high voltage uniformly.

The transfer belt cleaner unit 18 includes:
Since the toner adhered to the transfer belt 11 from the photoreceptor 3 causes the back surface of the sheet to become dirty, the toner is provided to remove and collect the toner.

The paper feed tray 7 is a tray for storing sheets used for image formation, and is provided below the image forming section of the present image forming apparatus. Further, a paper discharge tray 20 provided at the upper part of the image forming apparatus is a tray for placing sheets on which images have been formed.

The sheet on the paper feed tray 7 is picked up by the pickup roller 21 and sent out from the paper feed path 22 of the paper feed / conveyance unit 8 to the transfer / conveyance belt unit 6 at a predetermined image forming timing via the registration roller 23. You. The sheet on which the transfer has been completed is taken into the fixing / discharge unit 9 from the transfer / conveyance belt unit 6 and the toner is heated and fixed as described below. The paper is discharged onto the paper tray 20 in an inverted state (with the multicolor toner image facing downward).

The fixing and discharging unit 9 includes the heat roller 3
1, pressure roller 32 and release agent application rollers 33, 34
The heat roller 31 and the pressure roller 32 are configured to rotate with a sheet interposed therebetween. The heat roller 31 is controlled by a control unit (not shown) to a predetermined fixing temperature based on an output value of a temperature detector (not shown).
At the same time, the multicolor toner image transferred to the sheet is melted, mixed, and pressed by thermocompression bonding of the sheet, and is heated and fixed to the sheet.

In the present invention, as described above, the four image forming stations Sbk, Sc, Sm, Sy are arranged in the order of black (BK), cyan (C), magenta (M), and yellow (Y). Thus, interference between the colors such as the masking is suppressed, and the image quality in full color is considered. Then, the image forming station S
In order to avoid the reverse transfer phenomenon at the remaining image forming stations Sc, Sm and Sy when forming a monochrome image using only bk, the transfer frame 19 is supported by the support shaft 41 on the image forming station Sbk side. The image forming station Sy is supported so as to be freely swingable and displaceable about a horizontal axis, and is supported by the elevating mechanism 42 so as to be vertically movable.

The elevating mechanism 42 responds to the operator's selection between full-color image formation and monochrome image formation.
The driving of the transfer frame 19 is controlled by the control device 43.
The image forming station Sy side is raised during the full-color image formation, and the transfer belt 11 is
The transfer belt 11 is lowered during the monochrome image formation so that the transfer belt 11 contacts only the photoconductor 3bk and is separated from the remaining photoconductors 3c, 3m, and 3y.

It should be noted that in the present invention, the control device 43 controls the switching of the developing bias Vbk of the developing bias source 44bk. Each developing unit 2bk,
2c, 2m, and 2y have corresponding developing bias sources 44b.
k, 44c, 44m, and 44y are provided, respectively. As shown in FIGS. 2 and 3, each developing bias source 4
4 is a DC voltage source 45, each AC voltage source 46bk1, 4
6bk2; 46c; 46m; and 46y, respectively.

The developing bias source 44bk includes two AC voltage sources 46bk1 and 46bk2 and a changeover switch 47. When the full color image is formed by the control device 43, the AC voltage source 46bk is used as shown in FIG.
bk1 is selected, and at the time of the monochrome image formation, FIG.
The changeover switch 47 is controlled to switch so that the AC voltage source 46bk2 is selected as shown by. Therefore, the DC voltage V0 generated by the DC voltage source 45 is:
At the time of full-color image formation, the AC voltage of the frequency fbk1 by the AC voltage source 46bk1 is superimposed, and at the time of monochrome image formation, the AC voltage of the frequency fbk2 by the AC voltage source 46bk2 is superimposed.

In the remaining developing bias source 44c, an AC voltage having a frequency fc from an AC voltage source 46c is superimposed on the DC voltage V0. In the developing bias source 44m, an AC voltage having a frequency fm from the AC voltage source 46m is added to the DC voltage V0. The DC voltage V0 is superimposed on the developing bias source 44y.
An AC voltage having a frequency fy by the AC voltage source 46y is superimposed on the AC voltage. For example, V0 = −500 V, AC voltage source 46
bk1, 46bk2, the peak voltage Vp−
p is chosen to be 1500V. Fbk1> fc ≧ f
m ≧ fy and fbk1> fbk2, for example, f
bk1 = 4 kHz and fbk2 = 2 kHz.

Note that fc, fm, fy, fbk2 are 0
It may be. In that case, since the AC voltage source is only 46bk1, the configuration of the developing bias source can be simplified.

Further, if the frequency range of the AC component is set to be equal to or more than the absolute value of the peak voltage of the AC voltage, for example, if the peak voltage Vp-p is set to 1500 V, it is desirable that the frequency range of the AC component is also set to 1500 Hz or more. . Thereby, image fog can be suppressed. Also,
When the peak voltage Vp-p of each AC voltage is set to 2000 V or less, it is possible to prevent the generation of white spots in solid black portions. Furthermore, the frequency range of the AC voltage is set to 10
When the frequency is set to be higher than 00 Hz and the peak voltage Vp-p of the AC voltage is set to 500 V or more, the image density can be stabilized.

As described above, according to the present invention, the image forming station S is a tandem type image forming apparatus in which a plurality of image forming stations S are arranged along the sheet transport direction A, and only the image forming station Sbk on the most upstream side is used. When performing image formation, the transfer belt 11 is connected to the photoconductor 3c at the downstream image forming stations Sc, Sm, and Sy that are not involved in image formation.
3m and 3y to prevent the deterioration of the formed image due to the reverse transfer of the toner, etc., in the above-described monochrome image formation, the control device 43 controls all the image forming stations Sb.
Compared with the full-color image formation using k, Sc, Sm, and Sy, the frequency of the AC component in the developing bias voltage Vbk is lowered from fbk1 to fbk2 to optimize the toner density.

Therefore, the image forming station Sb
The density of the black image obtained by k can be made equal between the time of full-color image formation and the time of monochrome image formation, and the image quality at the time of monochrome image formation can be improved.

Further, since the image forming station Sbk located at the uppermost stream is for black toner, the black toner becomes the lowermost layer at the time of full-color image formation, and the influence of masking on the toner images of other colors is minimized. And a preferable multicolor image can be obtained. Also, in general, there are far more opportunities to form a monochrome image than to form a full-color image.Therefore, it is necessary to reduce the amount of toner adhered during the formation of a monochrome image as described above, thereby reducing the amount of toner consumption. Can also.

[Second Embodiment] The following will describe another embodiment of the image forming apparatus of the present invention with reference to FIGS.

Note that the multicolor image forming apparatus of the present embodiment
As in the first embodiment, the transfer material transport belt is configured to be separated from the image forming stations other than the uppermost stream at the time of forming a monochrome image. Further, the multicolor image forming apparatus of the present embodiment controls the toner density by the image forming station arranged at the most upstream side in each of the time of forming a color image and the time of forming a single-color image so that the toner density is optimal.

As shown in FIG. 4, the multicolor image forming apparatus of this embodiment is arranged such that black (Bk), cyan (C), magenta (M), and yellow (Y) are arranged from the upstream side of the transfer material conveying belt 57. , Each image forming station is provided with a plurality of photoreceptor drums 56Bk, 56C, 56M, 56Y rotatably supported, and each of the photoreceptor drums 56Bk, 56C, 56M
56Y is rotationally driven by a driving device in the direction of arrow F in the figure.

As described above, disposing the black image forming station on the most upstream side of the transfer material conveying belt 57 is as follows.
Since black is formed in the lowermost layer, which has the greatest influence on other colors, it is very preferable for achieving good image quality.

The photosensitive drums 56Bk, 56C, 5
The photoconductor drums 56Bk, 56C, 56M, and 5M are sequentially arranged around the 6M and 56Y from the upstream side in the rotation direction.
Charging devices 51Bk, 51C, 51 for uniformly charging 6Y
M · 51Y and photosensitive drums 56Bk / 56C / 56M
Exposure means 52 for exposing the surface of 56Y to form a latent image
Bk, 52C, 52M, and 52Y, developing devices 53Bk, 53C, 53M, and 53Y that develop the latent images to form Bk, C, M, and Y toner images, and a transfer device that transfers the toner images to a transfer material And the photosensitive drums 56Bk and 56
Cleaning devices 55Bk, 55C, 55M, and 55Y for removing toner remaining on the surfaces of C, 56M, and 56Y are arranged.

Further, the multicolor image forming apparatus of this embodiment is provided with a developing bias source 64 for applying a developing bias voltage to the developing device 53Bk, and a control device (control means) 65 controls the developing bias source 64. To control the photoconductor 5
The amount of toner adhering to 6Bk is adjusted. The control method of the developing bias voltage by the control device 65 will be described later in detail.

Further, the transfer device includes a transfer belt 57 which runs while being stretched between a driving pulley 60 and a driven pulley 61, and the transfer belt 57 runs in the direction of arrow G. Then, the sheet P as the transfer material is electrostatically held on the transfer belt and conveyed in the direction of arrow H. Inside the loop of the transfer belt 57, each photosensitive drum 5
The transfer chargers 54Bk / 54C / 54M / 54Y are arranged at positions facing the 6Bk / 56C / 56M / 56Y,
Each of these transfer chargers 54Bk / 54C / 54M / 54Y
Are the photosensitive drums 56Bk, 56C, 56M, 56Y
Is transferred to the toner image. Further, the sheet P passing through the four photosensitive drums on the transfer belt 57 has
The toner image passes through the fixing device 58 and is fixed.

Further, as shown in FIG. 5, the above color electrophotographic apparatus is used for forming a monochrome (monochrome) image of Bk.
The transfer belt 57 is separated from image forming stations other than the uppermost stream.

In the above color electrophotographic apparatus, the amount of toner adhering from the developing device at the most upstream image forming station to the photosensitive drum at the time of full-color image formation (full-color printing) and at the time of single-color image formation (monochrome printing), respectively. Is controlled to realize an appropriate toner density. A method of controlling the developing bias source 64 for applying the developing bias voltage to the developing device 53Bk by the control device 65 for this purpose will be described below with reference to FIG.

FIG. 6 schematically shows the relationship between the surface potential of the photosensitive drum and the developing bias voltage in reversal development. Further, the initialization charging potential V ₀ and the image exposure amount of the photoconductor are common between the time of forming a full-color image and the time of forming a monochromatic (monochrome) image. Therefore, the light potential V _L obtained after image exposure is also common, only the developing bias voltage V _D are different.

In FIG. 6, the developing bias voltage indicated by the one-dot chain line is the developing bias voltage V _D1 at the time of forming a full-color image, and the developing bias voltage indicated by the two-dot chain line is the developing bias voltage at the time of forming a single-color image. The voltage V _D2 .

The image forming apparatus according to the present embodiment includes a control device 6
5, the initializing charging potential V ₀ and the image exposure amount of the photoconductor are set to be common between the time of full-color image formation and the time of single-color image formation, and the developing bias voltage V _D for the photoconductor of the most upstream image forming station is set. By changing the amount in accordance with the image forming mode, it is possible to adjust the optimal amount of toner adhesion.

[0093] That is, in a full-color image forming to be considered a reverse transcription phenomenon, a good single-color image forming is not necessary to consider, in order to change the adhesion amount of toner to the photosensitive member, only the developing bias voltage V _D Is changed, if the setting of the initialization charging potential V ₀ and the image exposure amount is common in both modes, the contrast potential V _C which is the potential difference between the developing bias voltage V _D and the light portion potential _VL is changed. In other words, the amount of toner adhering to the photoconductor can be adjusted.

[0094] Specifically, the larger the absolute value of the developing bias voltage V _D, the exposure strength at the electrostatic latent image recorded becomes much amount of toner deposited on the photosensitive drum in the same. Therefore, if the transfer conditions are the same, the amount of toner transferred on the transfer paper increases, and the image density can be increased. Conversely,
Decreasing the absolute value of the developing bias voltage V _D, the amount of toner exposure intensity during the electrostatic latent image recorded is adhered to the photosensitive drum in the same is reduced. Therefore, if the transfer conditions are the same, the amount of toner transferred onto the transfer paper decreases,
Image density can be reduced.

[0095] Therefore, at the time unnecessary monochromatic image forming consider reverse transcription phenomenon, in order to reduce the amount of toner adhered to the photoreceptor than the full-color image formation, reduced than when the developing bias voltage V _D full-color image formation Thus, it is possible to prevent the problem that the density of the image becomes higher than the desired image density during the formation of the monochrome image.

FIG. 7 is a toner adhesion amount characteristic diagram showing the correlation between the value of the developing bias voltage and the amount of toner adhering to the photoreceptor. The horizontal axis represents the value of the developing bias voltage, and the vertical axis represents the amount of toner adhesion. Is shown. According to this, as shown in FIG. 7, the amount of toner attached to the photoconductor increases in proportion to the amount of increase in the negative potential of the developing bias voltage.

FIG. 8 shows the values of the developing bias voltage and
FIG. 5 is a characteristic diagram showing a correlation with a formed image density, in which a horizontal axis indicates a value of a developing bias voltage and a vertical axis indicates a toner adhesion amount. According to this, the image density increases in proportion to the developing bias voltage.

As described above, according to the multicolor image forming apparatus of the present embodiment, the image density obtained for the image forming station located on the most upstream side of the transfer material transport belt during full color image formation and the transfer material transport belt The density difference between the image density obtained when forming a single-color image and the image density obtained when forming a single-color image is eliminated. Is eliminated, and an image can be formed at a desired image density.

For example, when a full-color image is formed and when a single-color image is formed, the developing bias voltage in the developing device of the image forming station provided on the most upstream side is adjusted as shown in Table 1 below, and the contrast potential V _C is adjusted. change. Incidentally, in both image forming mode, the applied voltage V _g to the charger grid is fixed to V _g = -650 [V].

[0100]

[Table 1]

Note that the bright portion potential V _{L in} Table 1 above corresponds to V _FF shown in FIG.

[0102] From the above table, the single-color mode than in full-color mode, the contrast potential V _C has become 20V small. Due to this potential difference, the amount of toner adhering to the photoconductor is smaller in the single color mode having a small contrast potential than in the full color mode. Therefore, even in the image formation in the single-color mode in which it is not necessary to consider the reverse transfer phenomenon, by reducing the amount of the toner to substantially the same image density as that in the image formation in the full-color mode, the image generated in the single-color mode and the full-color mode Eliminate density differences,
An image can be formed at a desired image density regardless of the image forming mode.

In the multi-color image forming apparatus of the present embodiment, an example has been described in which the amount of toner adhered to the photosensitive member is adjusted between the time of full-color image formation and the time of single-color image formation by paying attention to the bias voltage. . However, the present invention is not limited to this, and the same effect can be obtained by a method of changing the amount of toner adhesion by focusing on the contrast potential. This is because the control of the developing bias voltage performed for adjusting the toner adhesion amount described above leads to directly controlling the contrast potential.
That is, as shown in FIG. 6, when changing the developing bias voltage, the contrast potential is the difference between the developing bias voltage and the light portion potential V _FF, if the light portion potential V _FF is constant, the contrast potential Change. Therefore, even when the contrast potential is directly controlled, the amount of toner adhering to the photoconductor can be controlled in the same manner as when the developing bias voltage is controlled.

Further, in the present embodiment, the example in which the black toner is provided in the image forming station located on the most upstream side of the sheet conveying belt has been described. However, the present invention is not limited to this. Even if the above toner is provided, the same effect as described above can be obtained. But,
Even in an image forming apparatus capable of forming a full-color image as in the present embodiment, by providing the most frequently used black toner in the most upstream image forming station, the black toner can be supplied to other than the most upstream side. It is more preferable than the image forming station in that it can reduce the consumption of black toner at the time of forming a monochromatic image.

Further, in the present embodiment, an example in which the bias voltage or the contrast potential is changed only for the image forming station having the black toner on the uppermost stream side has been described. By providing a control device for controlling the voltage or the contrast potential, an image can be formed with an appropriate image density in the same manner even when forming a monochromatic image other than black, for example, red.

[0106]

As described above, the image forming apparatus according to the present invention is a tandem type image forming apparatus in which a plurality of image forming stations are arranged along the sheet conveying direction, and the image forming station on the upstream side is provided. When forming an image using
In the image forming apparatus in which the sheet conveyance support is separated from a downstream image forming station not involved in image formation to prevent deterioration of a formed image due to toner reverse transfer or the like, the image on the upstream side is prevented. At the time of image formation using the forming stations, the toner density is optimized by lowering the frequency of the AC component in the developing bias voltage as compared to when forming images using all image forming stations.

Therefore, the image density obtained by the upstream image forming station can be made equal between the time of forming an image using all the image forming stations and the time of forming an image partially using the image forming station. The image density does not undesirably increase when partially used, and the image quality when the partial image forming station is used can be improved.

Further, as described above, the image forming apparatus of the present invention is a tandem type image forming apparatus, and when an image is formed using the upstream image forming station, the downstream image forming station which is not involved in the image formation. In the image forming apparatus in which the formed image is prevented from deteriorating due to the reverse transfer of the toner by separating the paper transporting carrier from the image forming apparatus, all the image forming sources are supplied to the developing bias source of the upstream image forming station. When forming an image using a station, a developing bias voltage in which an AC component is superimposed on a DC component is generated, and when forming an image using a partial image forming station, a developing bias voltage including only a DC component is generated.

Therefore, the image density obtained by the image forming station on the upstream side can be made equal between the time of forming an image using all the image forming stations and the time of forming an image partially using the image forming station. The image density does not undesirably increase when partially used, and the image quality when the partial image forming station is used can be improved.

Further, in the image forming apparatus of the present invention, as described above, the image forming station located at the uppermost stream is for black toner.

Therefore, at the time of forming a multicolor image, the black toner becomes the lowermost layer and the influence on other color toner images is minimized, so that a preferable multicolor image can be obtained. Also,
In general, there are far more opportunities to form a black single-color image than to form a multi-color image, so that toner consumption can be reduced.

As described above, the image forming apparatus of the present invention
At the time of monochromatic image formation, the image forming station on the most upstream side in the sheet conveying direction is used, and the image forming station is separated from the image forming station on the downstream side which is not involved in image forming to form an image. By switching the voltage to a different voltage than when forming multicolor images,
The configuration includes a control unit that adjusts the amount of toner attached to the photoconductor.

Therefore, by switching the developing bias voltage between the time of forming a multi-color image and the time of forming a single-color image, it is possible to form an image with an optimum image density.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station during multicolor image formation is taken away at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Thus, according to the image forming apparatus of the present invention,
By changing the developing bias voltage, the setting of the initialization charging potential and the image exposure amount is common between the multicolor image forming and the monochromatic image forming, so that the contrast potential (the potential difference between the developing bias voltage and the bright portion potential) is increased. ) Can be changed, and the amount of toner adhering to the photoconductor can be controlled.

More specifically, when forming a multicolor image in which the amount of toner adhered to the photosensitive drum is to be increased, the absolute value of the developing bias voltage is controlled to be large. At the time of forming a monochrome image in which the amount of adhesion is to be reduced, control is performed so that the absolute value of the developing bias voltage is reduced.

As a result, the difference in image density between the image density at the time of forming a single-color image and the image density at the time of forming a multi-color image is eliminated. Can be prevented.

The control means may increase the developing bias voltage applied to the most upstream image forming station during multicolor image formation to be greater than the developing bias voltage applied to the image forming station during monochromatic image formation. Is more preferred.

Therefore, the contrast potential can be increased by increasing the developing bias voltage during multicolor image formation, and the amount of toner adhered to the photoreceptor can be increased as compared with single color image formation. .

Therefore, it is possible to adjust the amount of toner adhering to the photoreceptor optimally at the time of forming a multicolor image and at the time of forming a monochromatic image, respectively. There is an effect that a density difference generated between the image density and the image density can be eliminated.

As described above, the image forming apparatus of the present invention
At the time of monochromatic image formation, the image forming station on the most upstream side in the sheet conveying direction is used, the image forming station is separated from the image forming station on the downstream side not involved in the image forming, and the image is formed. Is switched to a potential different from that at the time of forming a multicolor image to control the amount of toner adhering to the photoreceptor.

Therefore, by changing the contrast potential, the amount of toner adhering to the photoreceptor is controlled to an optimum amount when forming a multi-color image and when forming a single-color image. There is an effect that an image can be formed at a density.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station during multicolor image formation is robbed at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the image forming apparatus of the present invention,
By changing the amount of toner adhered to the photoreceptor by changing the contrast potential, the amount of toner transferred to the sheet can be changed, and optimal image formation can be performed.

Specifically, when the contrast potential is changed in a direction to increase, if the exposure intensity at the time of recording the electrostatic latent image is the same, the amount of toner adhering to the photosensitive drum increases. Therefore, when the transfer conditions are the same, the amount of toner transferred onto the transfer paper increases, and the image density can be increased. Conversely, when the contrast potential is changed in a decreasing direction, the amount of toner adhering to the photosensitive drum decreases even if the exposure intensity during recording of the electrostatic latent image is the same. Therefore, when the transfer conditions are the same, the amount of toner transferred onto the transfer paper decreases, and the image density also decreases.

As described above, by changing the contrast potential between the time of forming a multicolor image and the time of forming a single color (monochrome) image, the image density obtained by the most upstream image forming station during the formation of a multicolor image and the single color The difference in density that occurs between the image density obtained by the image forming station on the most upstream side during image formation and the image density obtained by the image forming station on the most upstream side can be eliminated, and the problem that the image density is too high during monochromatic image formation can be prevented.

It is more preferable that the control means makes the contrast potential of the most upstream image forming station at the time of forming a multicolor image higher than the contrast potential at the time of forming a monochromatic image.

Therefore, by controlling the contrast potential at the time of forming a multi-color image in a direction of increasing the contrast potential at the time of forming a single-color image, it is possible to increase the amount of toner adhering to the photosensitive member.

Therefore, it is possible to adjust the optimal amount of toner adhering to the photoreceptor at the time of forming a multi-color image and at the time of forming a single-color image. It is possible to eliminate a density difference occurring between the density and the density.

It is more preferable that the uppermost image forming station is an image forming station provided with black toner.

Therefore, since the black image forming station, which particularly affects other colors, is located on the most upstream side,
At the time of forming a multi-color image, the black toner becomes the lowermost layer, so that the effect on other colors can be suppressed.

Accordingly, the density difference between the image density obtained for the black toner image forming station during the formation of the multicolor image and the image density obtained during the formation of the black single-color image is eliminated, and the multi-color image is formed. High quality images can be obtained both when forming black monochromatic images.

Even in a multi-color image forming apparatus capable of forming a multi-color image, the most frequently used is a monochromatic (monochrome) image formation. In the image forming apparatus of the present invention, since the control for reducing the amount of toner adhered at the time of forming a single color image is performed, the consumption of black toner can be suppressed as compared with the conventional multicolor image forming apparatus.

As described above, the control method of the image forming apparatus of the present invention uses the image forming station on the most upstream side in the paper transport direction when forming a monochromatic image, and uses the image forming station on the downstream side not involved in image formation. This method adjusts the amount of toner adhering to the photoconductor by changing the developing bias voltage to a voltage different from that used for multicolor image formation while performing image formation while separating the paper transporting carrier from the station.

Therefore, by switching the developing bias voltage between the time of forming a multicolor image and the time of forming a single-color image, an effect is obtained that an image can be formed at an optimum image density.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station at the time of forming a multicolor image is robbed at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the control method of the image forming apparatus of the present invention, by changing the developing bias voltage,
Since the setting of the initializing charging potential and the image exposure amount is common between the multicolor image forming and the monochromatic image forming, the contrast potential (the potential difference between the developing bias voltage and the light portion potential) is changed, The toner adhesion amount can be controlled.

Specifically, when forming a multicolor image in which it is desired to increase the amount of toner adhering to the photosensitive drum, control is performed so that the absolute value of the developing bias voltage is increased. At the time of forming a monochrome image in which the amount of adhesion is to be reduced, control is performed so that the absolute value of the developing bias voltage is reduced. As a result, the difference in image density between the image density when forming a single-color image and the image density when forming a multicolor image is eliminated, and it is possible to prevent a problem that the image density is too high when forming a single-color image.

The developing bias voltage applied to the most upstream image forming station during multicolor image formation is
It is more preferable to increase the voltage than the developing bias voltage applied to the image forming station when forming a single-color image.

Therefore, the contrast potential can be increased at the time of forming a multi-color image, and the toner adhesion amount can be increased as compared with the case of forming a single-color image. Therefore, it is possible to adjust the optimal amount of toner adhering to the photoreceptor at the time of forming a multicolor image and at the time of forming a single color image, respectively. The image density difference generated with respect to the toner color of the most upstream image forming station which occurs during the period can be eliminated.

As described above, according to the control method of the image forming apparatus of the present invention, when forming a monochromatic image, the image forming station on the most upstream side in the sheet transport direction is used, and the image forming station on the downstream side not involved in image formation is used. This method adjusts the amount of toner adhered to the photoconductor by changing the contrast potential to a potential different from that for forming a multi-color image while forming the image while separating the paper transporting carrier from the station.

Therefore, there is an effect that it is possible to switch to the optimum image forming conditions in each of the multi-color image formation and the single-color image formation.

That is, the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station during multicolor image formation is taken away at the downstream station is considered. When forming a multicolor image, and when forming a single-color image that does not need to consider the reverse transfer phenomenon, since the image is formed by separating the paper transporting carrier from the downstream image forming station that is not involved in image formation. However, if the amount of toner adhered to the photosensitive member is made the same, an optimum image density cannot be obtained.

Therefore, according to the control method of the image forming apparatus of the present invention, by changing the contrast potential,
It is possible to adjust the amount of toner attached to the photoconductor to an optimum amount.

Specifically, when the contrast potential is controlled in the increasing direction, the amount of toner adhering to the photosensitive drum increases even if the exposure intensity during recording of the electrostatic latent image is the same. Therefore, if the transfer conditions are the same, the amount of toner transferred onto the transfer paper increases, and the image density can be increased. On the other hand, when the contrast potential is controlled in the decreasing direction, the amount of toner adhering to the photosensitive drum decreases even if the exposure intensity during recording of the electrostatic latent image is the same. Therefore, if the transfer conditions are the same, the amount of toner transferred onto the transfer paper is reduced, and the image density is also reduced.

Accordingly, it is possible to eliminate a density difference generated between an image density obtained by an image forming station for forming a single color image after forming a multicolor image and an image density obtained by one image forming station after forming a single color image. In addition, it is possible to prevent a problem that the image density is too high during the formation of a single-color image.

It is more preferable that the contrast potential of the most upstream image forming station during multicolor image formation be higher than the contrast potential during single color image formation.

Therefore, in forming a multi-color image, the amount of toner adhesion can be increased more than in forming a single-color image. Therefore, it is possible to adjust the optimal amount of toner adhering to the photoconductor at the time of forming a multicolor image and at the time of forming a monochromatic image, respectively.
This has the effect of eliminating the density difference that occurs between the image density when forming a multicolor image and the image density when forming a single color image.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a longitudinal sectional view for explaining an operation at the time of full-color image formation.

FIG. 3 is a longitudinal sectional view for explaining an operation during monochrome image formation.

FIG. 4 is a cross-sectional view illustrating an internal configuration of an image forming apparatus according to another embodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating a mechanism when a single-color image is formed by the image forming apparatus of FIG. 4;

6 is an explanatory diagram illustrating a surface potential of a photoconductor of the image forming apparatus of FIG. 4;

7 is a graph showing a relationship between a developing bias value and an amount of toner adhering to a photoconductor in the image forming apparatus of FIG. 4;

8 is a graph showing a relationship between a developing bias value and an image density in the image forming apparatus of FIG.

[Explanation of symbols]

1bk, 1c, 1m, 1y Optical unit 2bk, 2c, 2m, 2y Developing unit 3bk, 3c, 3m, 3y Photoconductor 4bk, 4c, 4m, 3y Cleaner unit 5bk, 5c, 5m, 5y Charging unit 6 Conveyor belt unit 7 Paper feed tray 8 paper feed transport unit 9 fixing paper discharge unit 11 transfer belt (paper transport carrier) 12bk, 12c, 12m, 12y transfer roller 18 transfer belt cleaner unit 19 transfer frame 20 paper discharge tray 21 pickup roller 23 registration roller 25 Paper discharge roller 41 Support shaft 42 Elevating mechanism 43 Controller (frequency switching means, bias switching means) 44bk, 44c, 44m, 44y Developing bias source 45 DC voltage source 46bk1, 46bk2; 46c; 46m; 46y
AC voltage source A Paper transport direction Sbk, Sc, Sm, Sy Image forming station 51Bk / 51C / 51M / 51Y Charging device 52Bk / 52C / 52M / 52Y Exposure means 53Bk / 53C / 53M / 53Y Developing device 54Bk / 54C / 54M / 54Y transfer charger 55Bk / 55C / 55M / 55Y cleaning device 56Bk, 56C, 56M, 56Y photoreceptor drum 57 transfer material transport belt 60 drive pulley 61 driven pulley 64 developing bias source 65 control device (control means)

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 15/16 G03G 15/16 21/00 384 21/00 384 (72) Inventor Eiji Saikou Osaka-shi, Osaka 22-22 Nagaikecho, Abeno-ku, Sharp Corporation (72) Inventor Shoji Nakamura 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Masaaki Otsuki Nagaikecho, Abeno-ku, Osaka-shi, Osaka 22-22 Inside Sharp Co., Ltd. Inside Sharp Corporation (72) Inventor Masayasu Narimatsu 22-22 Nagaikecho, Abeno-ku, Osaka City, Osaka Prefecture Inside Sharp Corporation (72) Inventor Kaori Kushita Osaka (22) Inventor Takashi Kitagawa 22-22 Nagaikecho, Abeno-ku, Osaka City, Osaka Prefecture F-term (reference) 2H027 EA05 EB04 EC09 FA28 FA35 2H030 AA03 AA04 AB02 AD07 AD17 BB02 BB23 BB34 BB46 BB53 2H073 AA02 BA04 BA13 BA45 CA02 CA22 2H077 AD02 AD06 DB08 EA11 GA13 GA15 GA17 2H200 FA05 FA09 GA12 GA14 GA16 GA23 GA33 GA46 GA47 GA56 GA57 GA59 GB12 GB22 GB25 GB27 GB27 JA42 HA JB25 LA03 LB02 LB13 MA03 MA08 MB01 MC01

Claims (12)

[Claims] A plurality of image forming stations centered on a photoreceptor are arranged along a sheet conveying direction, and the image forming stations are sequentially overlapped while conveying sheets in the sheet conveying direction by a sheet conveying carrier. A multi-color image can be formed by performing image formation, and when an image is formed by partially using the image forming station on the upstream side in the paper transport direction, a downstream image not involved in the image formation is formed. In the image forming apparatus configured to separate the sheet transporting carrier from the forming station, at least the developing bias source in the upstream image forming station is configured to generate a developing bias voltage in which an AC component is superimposed on a DC component. The image using the partial image forming station is compared with the image forming using all the image forming stations. When formed, the image forming apparatus characterized by comprising a frequency switching means for lowering the frequency of the AC component of the developing bias voltage of an image forming station of the upstream side. 2. A plurality of image forming stations centering on a photoreceptor are arranged along a sheet conveying direction, and the image forming stations are sequentially superimposed while conveying a sheet in the sheet conveying direction by a sheet conveying carrier. A multi-color image can be formed by performing image formation, and when performing image formation by partially using the image forming station on the upstream side in the paper transport direction, a downstream image not involved in image formation is formed. In the image forming apparatus configured to separate the sheet transporting carrier from the forming station, at least the developing bias source in the upstream image forming station is configured to generate a developing bias voltage in which an AC component is superimposed on a DC component. When an image is formed using all the image forming stations as the developing bias source of the upstream image forming station, Image forming means for generating a developing bias voltage in which an alternating current component is superimposed on a flow component and generating a developing bias voltage of only a direct current component during image formation using the partial image forming station. apparatus. 3. The image forming apparatus according to claim 1, wherein the image forming station located at the most upstream position is for black toner. 4. A plurality of image forming stations centering on a photoreceptor are provided along the sheet conveying direction, and each image forming station is formed by each image forming station while conveying a sheet in the sheet conveying direction by a sheet conveying carrier. In an image forming apparatus that realizes multi-color image formation by sequentially superimposing images, at the time of monochromatic image formation, the image forming station on the most upstream side in the paper transport direction is used, and image formation on the downstream side not involved in image formation is performed. While performing image formation by separating the paper transport carrier from the station, and by switching the developing bias voltage to a voltage different from that for multicolor image formation,
An image forming apparatus comprising: a control unit that adjusts an amount of toner attached to a photoconductor. 5. The image forming apparatus according to claim 1, wherein the control means increases a developing bias voltage applied to the most upstream image forming station during multicolor image formation to a developing bias voltage applied to the image forming station during monochromatic image formation. The image forming apparatus according to claim 4, wherein: 6. A plurality of image forming stations centering on a photoreceptor are arranged along a sheet conveying direction, and each image forming station is formed by each image forming station while conveying a sheet in the sheet conveying direction by a sheet conveying carrier. In an image forming apparatus that realizes multi-color image formation by sequentially superimposing images, at the time of monochromatic image formation, the image forming station on the most upstream side in the paper transport direction is used, and image formation on the downstream side not involved in image formation is performed. A control unit is provided for adjusting the amount of toner adhered to the photoconductor by switching the contrast potential to a potential different from that for multicolor image formation while performing image formation by separating the paper transporting carrier from the station. An image forming apparatus comprising: 7. The apparatus according to claim 6, wherein said control means increases the contrast potential of the image forming station at the most upstream side during multicolor image formation than the contrast potential during monochromatic image formation. Image forming device. 8. The image forming apparatus according to claim 4, wherein the image forming station located at the most upstream position is an image forming station provided with black toner. 9. A plurality of image forming stations centered on a photoreceptor are provided along a sheet conveying direction, and each image forming station is formed by each image forming station while conveying a sheet in the sheet conveying direction by a sheet conveying carrier. In the control method of the image forming apparatus which realizes the multi-color image formation by sequentially superimposing images, at the time of monochromatic image formation, the image forming station on the most upstream side in the paper transport direction is used, and the downstream side not involved in image formation is used. While performing image formation by separating the paper transporting carrier from the image forming station, and by switching the developing bias voltage to a voltage different from that for multicolor image formation,
A method for controlling an image forming apparatus, comprising: adjusting a toner adhesion amount to a photoconductor. 10. A developing bias voltage applied to an image forming station on the most upstream side during multicolor image formation is set higher than a developing bias voltage applied to an image forming station during monochromatic image formation. Item 10. A method for controlling an image forming apparatus according to Item 9. 11. A plurality of image forming stations centering on a photoreceptor are arranged along a sheet conveying direction, and the image forming stations are sequentially superimposed while conveying a sheet in the sheet conveying direction by a sheet conveying carrier. In the control method of the image forming apparatus which realizes multi-color image formation by performing image formation by using the image forming station at the most upstream side in the paper transport direction at the time of monochromatic image formation, the downstream side not involved in image formation The image forming station separates the paper transporting carrier from the image forming station to form an image, and switches the contrast potential to a potential different from that for multicolor image formation, thereby adjusting the amount of toner adhering to the photoreceptor. A method for controlling an image forming apparatus. 12. The control of the image forming apparatus according to claim 11, wherein the contrast potential of the most upstream image forming station during multicolor image formation is made higher than the contrast potential during monochromatic image formation. Method.