JP3616589B2 - Tandem color image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a tandem color image forming apparatus, and in particular, a full color mode using image forming means for all colors, a mono color mode using only black and other single colors, and a multi-color mode using only necessary colors in combination. The present invention relates to potential control of image forming means in a tandem color image forming apparatus capable of switching between these modes.
[0002]
[Prior art]
In recent years, tandem color image forming apparatuses have been proposed in order to increase the speed of color copying with the development of color copying machines. This tandem color image forming apparatus has a structure in which a plurality of image forming means are arranged along the sheet conveying direction. Each image forming unit includes a photoconductive drum, a main charging device, an exposure unit, and a developing device. The photoconductive drum is charged by the main charging device, and then exposed based on the image signal, thereby exposing the photoconductive drum on the photoconductive drum. This is an image forming means using an electrophotographic method in which an electrostatic latent image is formed and a toner image is formed by applying toner of a developing device to the electrostatic latent image. Further, a cleaning device is provided as necessary. The tandem color image forming apparatus is disposed at a position opposite to the belt conveying unit that conveys the sheet by adsorbing the sheet to the belt surface by rotating and the photosensitive drum of each image forming unit through the belt. And a transfer unit, which prints a color image by superimposing toner of each color on a sheet. In addition, there is a mechanism that transfers a toner image onto an intermediate transfer belt instead of a sheet that finally obtains an image, and transfers the toner image onto the sheet again.
[0003]
This tandem color image forming apparatus performs full-color printing using four image forming means for yellow toner, cyan toner, magenta toner, and black toner, or uses only a part of the image forming means to provide necessary colors. It is possible to print only. In this case, only the image forming means corresponding to the combination of toner colors forming a desired color is used to form an image. For example, when developing black, only the image forming means for black toner is used, and when developing red, the image forming means for yellow toner and magenta toner are operated, and image forming means for cyan toner and black toner are used. Do not need to work.
[0004]
As described above, the plurality of image forming units may be unnecessary depending on the printing mode. Conventionally, this unnecessary image forming means has an adverse effect on images formed and consumes wasted power. In order to solve these problems, the following prior arts 1 to 3 have been proposed.
[0005]
<Prior Art 1>
In the technique described in Japanese Patent Laid-Open No. 1-96670, a charging unit is energized to a photosensitive member that is not used for image formation in the same manner as in use, and a developing bias is applied to the developing device as in image formation. . In other words, if the image is not printed, the same operation as printing a blank sheet is performed. However, since there is neither a latent image nor a visible image, only an image of a necessary color is printed.
[0006]
<Conventional technology 2>
In the technique described in Japanese Patent Laid-Open No. 63-249164, only the developing device of the image forming means that is not used is stopped. As a result, since the toner does not move on the photosensitive drum, only the necessary color image is printed.
[0007]
<Prior Art 3>
Other than tandem color copiers, tandem color printers, and other image forming devices capable of two-color development, and full-color copiers with an intermediate transfer member, the unused color developing device is mechanically removed from the photosensitive drum. Some of them are separated to prevent the toner from being placed on the photosensitive drum.
[0008]
[Problems to be solved by the invention]
However, in the method of the prior art 1, even if the image forming means is not used, there is no potential difference between the photosensitive drum and the developing device, so the toner moves to the photosensitive drum to some extent, and then the toner contacts the paper. A defect in the transferred image may occur. Further, since the transfer voltage is turned off, there has been a problem in that the toner image transferred to the sheet by the upstream image forming means partially moves to the photosensitive drum.
[0009]
In the prior art 2, the operation of only the developing device is stopped. However, since the main charging device and the transfer unit work as they are, the charged products (ozone and NOX) are generated, which is not good for the environment. Furthermore, since the voltage is applied to the charging means that is not used, power consumption is wasted.
[0010]
In the prior art 3, the mechanism for switching the tandem color image forming means between the printing position and the non-printing position becomes large, which hinders downsizing and cost reduction. Furthermore, the positioning accuracy is also deteriorated.
[0011]
SUMMARY OF THE INVENTION An object of the present invention is to provide a tandem color image forming system having a simple configuration in which image forming means that is not used does not adversely affect an image, is environmentally friendly, and can realize energy saving.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the first and second inventions of the present invention have a plurality of image forming means, and a toner image is primarily transferred from the plurality of image forming means to a sheet or an intermediate transfer member. Image formation having a first mode in which toner images are color-superposed and image formation is performed using all of the image formation means, and a second mode in which image formation is performed without using at least one of the image formation means In the image forming apparatus, the developing device of the image forming unit includes a developing roller, a supply roller, and a stirring unit, and a developer composed of a positively charged toner and a carrier is stirred and mixed and transferred to the supply roller. supply only toner to a reversal development system for forming a toner image by applying toner to the electrostatic latent image formed on the photosensitive drum, the first In the mode, a voltage is applied to the main charging device of each image forming unit to charge the photosensitive drum, and development is performed by applying a bias potential to the developing roller and the supply roller of the developing device, corresponding to each image forming unit. The absolute value of the bias potential of the primary transfer unit is set to be larger on the downstream side in the transport direction, and in the second mode, the image forming unit and the charging unit that perform image formation are in the first mode. The image forming means that does not perform image formation with the same setting as the above, makes the applied voltage of the main charging device smaller than that in the first mode, and the developing roller and supply roller of the developing device have a polarity opposite to that in the first mode. , plus no toner move to the developing roller, the value carrier is not fly developer, and a bias potential positive toner is set at a value that does not move to the photoreceptor surface of the developing roller each grant Furthermore the bias potential of the primary transfer means corresponding to said image forming means uses an image forming apparatus, characterized in that to reduce the absolute value in the bias potential of the same polarity of the transfer means upstream.
[0013]
Further, the fourth invention of the present invention has a plurality of image forming means, and the toner images are color-superposed by primary transfer of the toner images from the plurality of image forming means to a sheet or an intermediate transfer member. In the image forming method having a first mode in which image formation is performed using all the means and a second mode in which image formation is performed without using at least one of the image formation means, the developing device of the image formation means includes: A developing roller, a supply roller, and a stirring means. A developer composed of toner and a carrier is stirred and mixed, delivered to the supply roller, and only the toner is supplied from the supply roller to the developing roller. The toner image is formed by applying to the electrostatic latent image formed above. In the first mode, a voltage is applied to the main charging device of each image forming unit. The photosensitive drum is charged and developed with a bias potential applied to the developing roller and the supply roller of the developing device, and the absolute value of the bias potential of the primary transfer unit corresponding to each image forming unit is larger on the downstream side. An image forming method characterized by being set is used.
[0014]
In the third and fourth aspects of the present invention, in the second mode, a voltage is applied to the main charging device of the image forming means for forming an image to charge the photosensitive drum, and each of the developing roller and the supply roller of the developing device is charged. Development is performed by applying a bias potential, and the absolute value of the bias potential of the primary transfer unit corresponding to each image forming unit is set to be larger on the downstream side, and image forming units that do not perform image formation are those of the main charging device. The applied voltage is made smaller than when image formation is performed, and the positive toner does not move to the development roller with a reverse polarity bias to the case where image formation is performed on the development roller and supply roller of the development device, and the developer carrier does not fly. value and positive toner in the developing roller to impart a potential set at a value that does not move to the photosensitive member surface, further bias potential conveying direction of the transfer means corresponding to said image forming means Using the image forming method characterized by reducing the absolute value in the bias potential of the same polarity of the transfer means of the flow side.
[0015]
[Action]
In the present invention, the surface potential of the photosensitive drum is set to 0 V by turning off the main charging device of the image forming means that is not used. When the developing device is a reversal developing system, the toner is developed on the photosensitive drum when the bias potential of the developing roller is in a normal state (for example, a positive bias potential is applied to the positively charged toner). Therefore, a bias voltage having a polarity opposite to that of the toner is applied so that the toner does not fly. Even if the primary transfer unit is in a normal state (for example, a negative voltage is applied to the positively charged toner), there is no problem. The toner image transferred to the paper is prevented from being taken to the photosensitive drum. Further, although the transfer voltage is minute, the reversely charged toner that is on the photosensitive drum is not transferred. Since the reversely charged toner is collected by the cleaning device, an effect of removing the reversely charged toner of the developing device can be obtained.
[0016]
【Example】
A tandem color printer employing the present invention will be described with reference to FIG. In the figure, the tandem color printer 1 includes a paper feeding means 2, a vertical conveying path 3, a registration roller pair 4, a belt conveying means 5, a first image forming means 6, a second image forming means 7, and a third image. The image forming apparatus includes a forming unit 8, a fourth image forming unit 9, an intermediate transfer unit 10, a secondary transfer unit 11, a fixing unit 12, a discharge conveyance path 13, a discharge tray 14, and the like.
[0017]
The paper feeding means 2 includes a paper feeding cassette 21, a paper stacking means 22, a paper feed roller 23, and the like, and 1 from the top of a plurality of papers stacked on the paper stacking means 22 according to a paper feed signal. Sheets are fed one by one.
[0018]
The registration roller pair 4 has a role of adjusting the leading edge of the conveyed paper and a role of starting the paper at the secondary transfer position in synchronization with the timing of the image.
[0019]
The belt conveying means 5 includes a driving roller 51, a tension roller 52, and a belt member 53 stretched over the two rollers. The belt conveying means 5 is illustrated in order to adsorb paper onto the surface of the belt member 53. There is no belt charging means. The belt member 53 is stretched over the driving roller 51 and the tension roller 52, and rotates and moves at substantially the same speed as the sheet conveyance speed while maintaining an appropriate tension. Since the material is close to the fixing means 11, considering the heat resistance, elasticity, strength, insulation of the inner surface of the belt, toner releasability on the outer surface of the belt, etc., the outer peripheral surface of the endless belt made of polyimide is coated with fluorine. The one given is appropriate. In addition, since the belt must also serve as an elastic layer, silicon rubber or EPDM having a release layer made of a fluororesin such as PTFE may be used.
[0020]
The first to fourth image forming units (6 to 9) are for yellow, cyan, magenta, and black from the left side in the drawing, and are all units having substantially the same configuration. The black image forming means 9, which consumes a large amount, has a replenishment toner container (not shown) having a larger capacity than those of other colors. Here, the first to fourth image forming means (6 to 9) will be described as a representative by using the first image forming means 6 with reference to FIG.
[0021]
The first image forming unit 6 includes a first photosensitive drum 61, a first main charging device 62, a first LPH (LED PRINT HEAD) 63, a first developing device 64, and a first primary transfer unit. 65, a first cleaning device 66, and a first static elimination lamp 67. The unit is assembled into a housing made of resin, and is attached to the main body. The first photosensitive drum 61 uses an amorphous silicon drum, and is charged by the first main charging device 62 so that the dark potential at the developing position is about + 450V. In the first main charging device 62, a thin tungsten wire of 50 to 100 μm is stretched in the longitudinal direction with respect to the drum in a shield case opened toward the first photosensitive drum 61, and a high voltage of about +5 KV is applied thereto. As a result, the first photosensitive drum 61 is positively charged. An electrostatic latent image is formed on the surface of the first photosensitive drum 61 by the first LPH 63 irradiating the charged surface of the first photosensitive drum 61 with a light beam according to image information. The first LPH 63 is used to reduce the size of the unit, but an LSU (LASER SCANNER UNIT) may be used.
[0022]
Here, the first developing device 64 uses a positively charged toner and a developer composed of a carrier with stirring and mixing so as to have a predetermined concentration, and a supply roller 643 having a magnet member therein includes only the toner among the developers. Is moved to the surface of the developing roller 642, and the toner layer formed on the developing roller 642 is applied to the electrostatic latent image of the portion of the first photosensitive drum 61 irradiated with the light beam. It forms a toner image (so-called reversal development system). The supply roller 643 is composed of an aluminum sleeve that rotates and an internal fixed magnet. The developer rubs the development roller 642 as the sleeve rotates, and the bias potential of the supply roller 643 and the bias potential of the development roller 642 Only the toner is delivered by the generated electric field, and a toner layer is formed on the developing roller 642. Since the dark potential (potential when not exposed) of the first photosensitive drum 61 is + 450V and the white plate potential (potential when exposed) is + 50V or less, the developing bias of the developing roller 642 is set to + 350V. Yes. In this embodiment, an alternating electric field is applied in addition to the developing bias in order to increase the developing efficiency. The alternating electric field is preferably about 1 kV of 400 to 2000 HZ. On the other hand, +700 V is applied to the supply roller 642 as a bias potential necessary for delivering the toner to the developing roller 642.
[0023]
The toner image on the photosensitive drum 61 is transferred to the surface of the intermediate transfer belt 10 at the nip with the first primary transfer means 65. In the present embodiment, the first primary transfer means 65 uses a transfer roller, and a high voltage having a polarity opposite to that of the first photosensitive drum 61 is applied thereto. The primary transfer voltage is -1000 V for the first image forming unit, -1100 V for the second image forming unit, and the third voltage, rather than using the same voltage for all of the first to fourth image forming units (6 to 9). It is preferable that the absolute value is gradually increased from upstream to downstream with -1200V for the image forming unit and -1300V for the fourth image forming unit. The toner that has not been transferred on the first photosensitive drum 61 is scraped off by the rubber blade of the first cleaning means 66 in the next process, and the first static elimination lamp is used to reduce the residual potential on the surface and make it uniform. After being neutralized by 67, it is prepared for the next series of processes.
[0024]
The potential setting for image formation in this way is performed by setting the dark potential +450 V, the developing roller bias +350 V, and the primary transfer voltage from −1000 to −1300 V. These values are the characteristics of the photosensitive drum and the toner. The optimum value varies depending on the performance and environment. In this case, since the dark potential, the developing bias potential, and the transfer potential are changed depending on whether the image is formed or not, the output from the first main charging power source 621 is turned ON / OFF in the first main charging device 62. A first main charging SW 622 is provided, and the first developing device 64 has a first developing roller bias switching SW 6423 for switching between an eleventh developing roller bias power source 6421 and a twelfth developing roller bias power source 6422. And a first supply roller bias switching SW 6433 for switching between the eleventh supply roller bias power source 6431 and the twelfth supply roller bias power source 6432, and the first primary transfer means 65 further includes an eleventh supply roller bias switch 6443. A first primary transfer voltage switching SW 653 for switching between the primary transfer power supply 651 and the twelfth primary transfer power supply 652 is provided. To have. Here, in order to make the invention easy to understand, each switch SW is provided. However, in actuality, the developing roller bias voltage that can be controlled so that a plurality of voltages can be output stepwise is used, and the rising of the main charge is corrected. For the primary transfer power supply, one that can control the voltage variably is used.
[0025]
Returning again to FIG. The intermediate transfer body unit 10 includes a driving roller 101, a driven roller 102, a secondary transfer counter electrode roller 103, and an intermediate transfer belt 101 stretched over these rollers. The toner image primarily transferred onto the surface of the intermediate transfer belt 104 is transferred onto the paper at the secondary transfer position. The intermediate transfer belt 104 is made of a polyimide endless belt with a fluorine coating applied in consideration of elasticity, strength, belt inner surface insulation, toner releasability on the outer surface of the belt, and the like. Is appropriate. In addition, since the belt must also serve as an elastic layer, silicon rubber or EPDM having a release layer made of a fluororesin such as PTFE may be used. Further, a belt cleaning unit (not shown) is provided between the downstream side of the secondary transfer position and the primary transfer position of the first image forming unit 6.
[0026]
The secondary transfer unit 11 includes a secondary transfer roller 111, a secondary transfer power supply 1111, a secondary transfer power supply SW1112, and the like. The secondary transfer roller 111 has a voltage of 1 to 2 kV at the time of transfer. Is applied. The toner image on the surface of the intermediate transfer belt 104 is secondarily transferred to the surface of the sheet, and is secondarily transferred to the sheet by the electric field generated by the secondary transfer roller 111 and the counter electrode roller 103.
[0027]
The fixing unit 12 includes a first fixing roller 121, a first fixing heater 1211, a second fixing roller 122, and a second fixing heater 1221. Specifically, the first fixing roller 121 is made of a material having a good thermal conductivity such as aluminum coated with fluorine, and the first fixing roller 121 is controlled to a predetermined temperature required for fixing by incorporating a first fixing heater 1211. . The driving of the first fixing roller 121 may be driven by contact from the second fixing roller 122, but it is preferable that the first fixing roller 121 is engaged with the second fixing roller 122 by meshing with each other with a helical gear or the like in order to eliminate the concern about slip. . The second fixing roller 122 faces and contacts the first fixing roller 121 and is driven by a driving unit (not shown). The fixing roller 122 is made of an aluminum roller having a good thermal conductivity and the periphery is covered with an elastic layer 1222 in order to secure a fixing nip width, but it may be omitted. The elastic layer 1222 may be any heat-resistant rubber, but silicon rubber or the like is suitable. In this embodiment, in order to perform color fixing, a second fixing heater 1221 is incorporated and controlled to a predetermined temperature necessary for fixing. Although not shown, each roller is provided with a claw member for peeling the paper, a roller for cleaning, and a web.
[0028]
Next, the movement of the paper when printing using all normal colors will be described. The sheets fed one by one from the sheet cassette 2 are adjusted at the leading edge by the registration roller pair 4 and delivered to the belt conveying means 5 in synchronization with the timing of the image data. While the sheet is adsorbed to the belt conveying means 5 by static electricity, the color toner images primarily transferred from the image forming means (6-9) to the surface of the intermediate transfer belt 104 by the rotational movement of the belt are batched at the secondary transfer position. Second transfer is performed. The sheet on which the unfixed toner image is transferred is fixed by being sandwiched and pressed by a nip formed by the heat roller 121 and the pressure roller 122 of the fixing unit 12. Here, the paper on which the toner image is fixed is sequentially discharged to the paper discharge tray 14 through the discharge conveyance path 13.
[0029]
Normally, even in color copiers and printers, full color output is rare. Rather than using all image forming means as described above, only black output or black and red (combination of magenta and yellow) There is much output by. When only black is used, the image forming means (6-8) is not used, and when only black and red are output, the second image forming means 7 (cyan) is not used. The first mode of the present invention represents full-color output, and the second mode represents output only in black or when there is an image forming means that is not used when outputting only in red and black. Represents.
[0030]
The operation of the second image forming means 7 that is not used when outputting only black and red, which is a feature of the present invention, will be described with reference to FIGS.
[0031]
An image of yellow toner is primarily transferred onto the surface of the intermediate transfer belt 104 by the first image forming unit 6 and the first primary transfer unit 65, and then the second image forming unit 7 and the second primary transfer unit 65. No image formation or transfer is performed at the nip with the transfer means 75, and the magenta toner is superimposed on the portion where the yellow toner has been transferred at the nip between the third image formation means 8 and the third transfer means 85, so that the red color is obtained. Express. Further, black toner is primarily transferred at the nip between the fourth image forming unit 8 and the fourth transfer unit 85. In this way, only the black and red images are formed on the intermediate transfer belt 104.
[0032]
The setting of the potential of the second image forming means 7 containing cyan toner that does not perform image formation will be described with reference to FIG.
[0033]
In the second main charging device 72, since the second main charging SW 722 is OFF (no generation of ozone from the main charging device at this time), no potential is applied to the surface of the second photosensitive drum 71. First, the surface potential of the second photosensitive drum 71 at the development position is 0V. In the second developing device 74, -300V is applied to the second supply roller 743, and the developing bias applied to the second developing roller is -150V. Therefore, the plus toner mixed in the developer is supplied to the second supply roller. Therefore, the toner layer is not formed on the second developing roller 742. However, the reversely charged toner (negatively charged toner) contained in the second developing device 64 moves to the surface of the second developing roller 742 by an electric repulsive force with the second supply roller. Further, the plus toner remaining on the second developing roller 742 is electrically attracted to the developing roller and does not move to the surface of the photosensitive drum 71. However, the above-described reversely charged toner (negatively charged toner) moves to the surface of the second photosensitive drum 71 by an electric repulsive force with the developing roller. As a result, since the reversely charged toner in the developing device is consumed, a good result is obtained. The bias of each of the second supply roller 743 and the second developing roller 742 is set such that when the surface of the photosensitive drum is 0 V, the plus toner does not move to the developing roller and the developer carrier does not fly. The value is set so that the positive toner of the developing roller does not move to the surface of the photoreceptor. At the nip between the second photosensitive drum 71 and the second primary transfer means 75, no image is placed on the second photosensitive drum 71, so that the toner is not transferred to the paper, but the upstream image forming means already Since the yellow toner transferred to the paper is carried to the second photosensitive drum 71 and collected, the voltage in the second primary transfer is used to attract the toner on the intermediate transfer belt 104 to the belt side. Apply voltage. Since this voltage is not transferred from the photosensitive drum to the paper, even if it is set lower than the normal transfer voltage, it plays a sufficient role, and the generation of ozone may be small. In this embodiment, −500 V is applied, but this value may be set as appropriate according to conditions and experimental results. Also, the negatively charged toner on the second photosensitive drum 72 is cleaned from the second primary transfer means 75 as it is on the photosensitive drum by an electric repulsive force. There is no.
[0034]
Although a positively charged photosensitive drum is used in this embodiment, a negatively charged photosensitive drum may be used in the same manner. In this case, naturally, the setting of the development bias and the transfer voltage is opposite to that of the present embodiment. Further, when a negatively charged photosensitive drum is used, generation of ozone in the main charging device of the image forming means that is not used and NOX generated in the transfer means can be reduced.
[0035]
In this embodiment, the accuracy of color superimposition of the image is considered, and the tandem color image forming apparatus having the intermediate transfer belt has been described. However, the image is directly transferred to the paper by the primary transfer. The case is equally valid.
[0036]
According to the present invention, the surface potential of the photosensitive drum is set to 0 V by turning off the main charging device of the image forming means that is not used.
Here, when the bias potential of the developing device is in a normal state (for example, a positive bias potential is applied to the positively charged toner), the toner is developed on the photosensitive drum, so that the toner does not fly. A bias voltage having a polarity opposite to that of the toner is set so that the positive toner does not move to the developing roller and does not cause the developer carrier to fly .
By doing so, only a minute amount of reversely charged toner (including toner that should be positively charged, including toner that is partially negatively charged) is transferred to the photosensitive drum.
Even if the primary transfer unit is in a normal state (for example, a negative voltage is applied to the positively charged toner), there is no problem. The toner image transferred to the intermediate transfer belt is not taken to the photosensitive drum.
Further, although the transfer voltage is minute, the reversely charged toner that is on the photosensitive drum is not transferred.
Since the reversely charged toner is collected by the cleaning device, an effect of removing the reversely charged toner of the developing device can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a tandem color printer employing the present invention.
FIG. 2 is a schematic diagram of first image forming means of a tandem color printer employing the present invention.
FIG. 3 is a schematic diagram when only black and red are printed by a tandem color printer employing the present invention.
FIG. 4 is a schematic diagram of a second image forming unit that is not used when only black and red are printed by a tandem color printer employing the present invention.
DESCRIPTION OF SYMBOLS 1; Full color printer body 2; Paper feed means 3; Vertical conveyance path 4; Registration roller pair 5; Belt conveyance means 6; First image forming means (yellow)
7; Second image forming means (cyan)
8; Third image forming means (magenta)
9: Fourth image forming means (black)
10; intermediate transfer body unit 11; secondary transfer means 12; fixing means 13; discharge conveyance path 14; discharge tray

Claims (4)

A plurality of image forming units are provided, and the toner images are primarily transferred from the plurality of image forming units to a sheet or an intermediate transfer member to superimpose the color of the toner images, and all the image forming units are used to form an image. In an image forming apparatus having a first mode for performing and a second mode for performing image formation without using at least one of the image forming units,
The developing device of the image forming unit includes a developing roller, a supply roller, and an agitation unit, and a developer composed of a positively charged toner and a carrier is agitated and mixed and delivered to the supply roller. The toner is transferred from the supply roller to the development roller. A reversal development system that forms a toner image by supplying only the toner and applying this toner to the electrostatic latent image formed on the photosensitive drum ,
In the first mode, a voltage is applied to the main charging device of each image forming unit to charge the photosensitive drum, and development is performed by applying a bias potential to the developing roller and the supply roller of the developing device. The absolute value of the bias potential of the primary transfer unit corresponding to the forming unit is set to be larger on the downstream side in the transport direction. In the second mode, the image forming unit and the charging unit that perform image formation are The image forming means that performs the same setting as in the first mode and does not perform image formation makes the applied voltage of the main charging device smaller than that in the first mode, and sets the developing roller and the supply roller of the developing device to the first mode. vias is opposite polarity, the positive toner is not move to the developing roller, the value carrier is not fly the developer, and positive toner in the developing roller is set at a value that does not move to the surface of the photosensitive member Each grant potential, further a bias potential of the primary transfer means corresponding to said image forming means image forming apparatus characterized by reducing the absolute value in the bias potential of the same polarity of the transfer means upstream. 2. The image forming apparatus according to claim 1, wherein in the second mode, a voltage applied to a main charging device of an image forming unit that does not perform image formation is 0V. A plurality of image forming units are provided, and the toner images are primarily transferred from the plurality of image forming units to a sheet or an intermediate transfer member to superimpose the toner images, and image formation is performed using all the image forming units. In an image forming method having a first mode for performing and a second mode for performing image formation without using at least one of the image forming units,
The developing device of the image forming unit includes a developing roller, a supply roller, and an agitation unit. The developer consisting of a positively charged toner and a carrier is agitated and mixed and delivered to the supply roller. The toner is transferred from the supply roller to the development roller. The reversal development system forms a toner image by supplying only the toner and applying this toner to the electrostatic latent image formed on the photosensitive drum. In the first mode, the main charging of each image forming means is performed. A voltage is applied to the apparatus to charge the photosensitive drum, and development is performed by applying a bias potential to each of the developing roller and the supply roller of the developing device, and the absolute value of the bias potential of the primary transfer unit corresponding to each image forming unit In the second mode, when the photosensitive drum is charged by applying a voltage to the main charging device of the image forming means for forming an image, in the second mode, Further, the developing roller and the supply roller of the developing device are each applied with a bias potential for development, and the absolute value of the bias potential of the primary transfer means corresponding to each image forming means is set larger on the downstream side to form an image. In the image forming means that does not perform the image forming, the applied voltage of the main charging device is made smaller than that in the case of performing image formation, and the polarity is opposite to that in the case of image forming on the developing roller and the supply roller of the developing device. The bias potential is set so that the developer carrier does not fly and the plus toner of the developing roller does not move to the surface of the photosensitive member, and the transfer means corresponding to the image forming means An image forming method, wherein the bias potential has the same polarity as the bias potential of the transfer means on the upstream side in the transport direction and has a small absolute value. 4. The image forming method according to claim 3, wherein the voltage applied to the main charging device of the image forming means that does not perform image formation is 0V .

Images