JPH07168455A - Image forming device - Google Patents

Abstract

PURPOSE:To always perform stable transfer by high transfer efficiency even in the case of environmental change or multiple transfer by providing plural transfer means at positions separated from the transfer position of respective image carriers, arranging them in a multiple transfer direction so that their distances may be shorter and supplying common transfer bias voltage to them. CONSTITUTION:Process units 100a to 100d are provided in the carrying direction between holding rollers 13 and 15 on a transfer belt 11. Feed rollers 23a to 23d being the transfer means are provided corresponding to the image carriers 1a to 1d so that their distances may be successively short on a downsntream side from the transfer position of the image carriers 1a to 1d of the process units 100a to 100d, that is, the abutting position of the image carriers 1a to 1d on paper P. The rollers 23a to 23d are connected to the same bias power source 31. Since the change of the transfer electric field imparted to a developer image which is caused by the environmental change becomes gentle, such stable transfer that the range of the proper transfer bias voltage of half tone image is widened is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is such that a plurality of developing devices are used for development, the developed image is transferred to a supplied transfer material, and the transferred image is fixed on the transfer material as a permanent image. The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus in which a transfer device that transfers a developed image onto a transfer material is improved.

[0002]

2. Description of the Related Art As a conventional apparatus for forming a multicolor image, as disclosed in Japanese Patent Application Laid-Open No. 2-105174,
An apparatus is known in which an image is formed by sequentially repeating the formation of a developer image on a photosensitive drum and the transfer of the developer image on a transfer material for each color of the developer. Further, conventionally, a sheet-shaped insulator capable of holding a charge, such as plastic, is used as a transfer material carrier, and this insulator is charged by a corona charger to form a transfer electric field. However, there is a problem that ozone which is toxic to the human body is generated when the transfer electric field is formed in this way.

In order to solve this problem, a transfer device using a semiconductive sheet as a transfer material carrier has been considered. If a semiconductive sheet is used as the transfer material carrier as in this transfer device, the charge can be removed by itself, so that it is not necessary to provide a charge removing means for the transfer material carrier. Further, since no corona charger is used, good transfer can be performed with a low bias voltage of 2 KV or less, and ozone is not generated at all.

However, in this transfer device,
The transfer machine of a halftone image having a small amount of toner is narrow compared to the transfer margin of the toner image of a solid image having a large amount of toner, that is, the range of the transfer bias voltage at which transfer can be performed well. Therefore, when the electrical resistance of the transfer material changes due to environmental changes in temperature and humidity, the transfer efficiency with respect to the transfer bias voltage of the halftone image changes significantly compared to the transfer efficiency of the solid image, and the same transfer bias as the solid image. There is a problem in that the halftone image cannot be satisfactorily transferred with the voltage.

Particularly, in a multicolor image forming apparatus having a plurality of image carriers, when a bias voltage applying member is provided in each transfer portion of the image carrier to perform multiple transfer, a halftone image is formed. Transfer efficiency significantly changes with an increase in the number of transfers. This is because even if the surfaces of multiple image carriers are charged to the same potential, when the same bias voltage is applied and transfer is performed, the transfer paper is charged in the direction of multiple transfer or already transferred to the transfer material. This is because the magnitude of the transfer electric field changes due to the influence of the developer. Therefore, if the same transfer bias voltage is supplied to the plurality of bias voltage applying members, there is a problem that the color reproducibility of the obtained color image is poor and a high quality multicolor image cannot be formed.

Due to such a problem, an environmental sensor for detecting temperature / humidity and a transfer bias power source capable of automatically changing an output voltage have been conventionally required to stably perform good multiple transfer stably. . For this reason, the device becomes complicated and the cost of the device is increased.

On the other hand, in Japanese Patent Laid-Open No. 53-93031, developer images of different colors are formed on a plurality of photoconductors, and these developer images are sequentially transferred onto a transfer paper conveyed by a belt. There is disclosed a multicolor image forming apparatus which obtains a multicolor image. In this multicolor image forming apparatus,
At each transfer position, a transfer roller is arranged in pressure contact with the back surface of the belt so that the belt is pressed against each photoconductor, and a voltage having a polarity opposite to that of the developer is applied to the transfer roller. Then, the voltage applied to each transfer roller is gradually increased according to the transfer order. That is, when sequentially transferring the developer images of the respective colors,
The transfer efficiency is improved by sequentially increasing the voltage applied to the transfer roller for the second color from the first color and the third color from the second color. For example, the bias voltage of 2.5 kV for the first color, 3.0 kV for the second color, and 3.5 kV for the third color are independently applied to each transfer roller.

However, as described above, in order to sequentially apply a high bias voltage to each transfer roller, since each transfer roller is connected to an independent voltage, a plurality of power supplies are required. Therefore, there is a problem in that the power supply device occupies a large installation space in the multicolor image forming apparatus, the apparatus becomes large-sized, and the multicolor image forming apparatus becomes expensive.

[0009]

As described above, in the conventional image forming apparatus, it is difficult to stably perform multiple transfer, and a large number of power supplies are required to perform multiple transfer stably. There is a drawback that the device becomes large and the cost increases.

In view of the above, the present invention eliminates the above-mentioned drawbacks, and in an image forming apparatus for forming an image by sequentially transferring multiple developer images formed on a plurality of image carriers onto a transfer material, it is possible to prevent environmental change and multiple transfer. It is an object of the present invention to provide an image forming apparatus that can always perform stable transfer with high transfer efficiency.

[0011]

To achieve the above object, the image forming apparatus of the present invention comprises a first image forming means for forming a first developer image on a first image carrier, Second image forming means for forming a second developer image on the second image carrier, and the first image carrier and the first image carrier and the second image forming means. And a conveying unit that sequentially conveys the image forming medium to the second image carrier in this order, and a first distance from the transfer position of the first image carrier along the direction in which the image forming medium is conveyed. At a position separated by
A first transfer which is provided so as to face the first image forming unit via the conveying unit and transfers the first developer image formed on the first image carrier to the image forming medium. Means and
The second portion along the direction in which the image forming medium is conveyed.
The second distance smaller than the first distance from the transfer position of the image carrier of
A second developer image formed on the second image carrier, which is provided facing the second image forming unit via the conveying unit at a position separated by Second transfer means for transferring onto the first, first transfer means and second
And a single voltage applying unit for supplying a transfer bias voltage to the transfer unit.

In order to achieve the above object, the image forming apparatus of the present invention includes a first image forming unit for forming a first developer image on the first image carrier and a second image carrier. A second image forming unit that forms a second developer image, and a first image carrier and a second image carrier that are provided so as to face the first image carrier and the second image carrier. A conveyor belt for sequentially conveying the image forming medium to the body, a pair of drive rollers provided to face each other and driving the conveyor belt, and the second transfer roller rather than the transfer position of the first image carrier. Voltage applying means for supplying a transfer bias voltage to the drive roller so that the magnitude of the transfer electric field for transferring an image at the transfer position of the image carrier is increased.

[0013]

According to the present invention, each of the plurality of transfer means is provided at a position distant from the transfer position of each image carrier, and each of the image transfer members is arranged so that the separation distance becomes small along the direction of multiple transfer. It is arranged corresponding to the carrier. The transfer means arranged in this way is connected to a common power source,
A transfer bias voltage is supplied. Therefore, the magnitude of the transfer electric field increases sequentially along the direction of multiple transfer without using a plurality of power sources. Further, since the transfer means is provided apart from the transfer position of each image carrier, the transfer margin is widened so that the proper transfer bias values of the halftone image and the solid image match.

Further, according to the present invention, when the conveying means is driven, different transfer bias voltages are supplied to each of the pair of rollers, so that a larger transfer electric field is formed on the downstream side in the multiple transfer direction. .

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing a first embodiment of a multicolor image forming apparatus of the present invention. In FIG. 1, a photosensitive drum 1a as an image carrier has a cylindrical shape with a diameter of 40 mm and is rotatably provided in the direction of arrow a in the figure.

The following are arranged around the photosensitive drum 1a along the rotation direction. That is, a charging roller 5a that uniformly charges the photosensitive drum 1a is provided in contact with the surface of the photosensitive drum 1a. On the downstream side of the charging roller 5, there is provided an exposure unit 7a that exposes the charged photosensitive drum 1a to form an electrostatic latent image.
Further, downstream of the exposure unit 7a, there is provided a developing unit 9a which contains a black developer and develops the electrostatic latent image formed by the exposure unit 7a with this developer. A transport belt 11 is provided downstream of the developing device 9a as a transport unit that transports the sheet P, which is a transfer material, to the photosensitive drum 1a. The conveyor belt 11 conveys the sheet P to the transfer position of the photosensitive drum 1a so that the developer image formed on the photosensitive drum 1a and the sheet P come into contact with each other.

Further, a cleaning device 17a and a discharging lamp 19a are provided at the transfer position of the photosensitive drum 1a, that is, at the downstream side of the contact position between the photosensitive drum 1a and the sheet P. The cleaning device 17a removes the developer remaining on the photosensitive drum 1a after the transfer of the developer image onto the paper P by scraping it off with the blade 21.
The charge eliminating lamp 19a is for eliminating the charge on the surface of the photosensitive drum 1a after the transfer. One cycle of image formation is completed by the charge removal by the charge removal lamp 19a, and when the image is formed next time, the uncharged photosensitive drum 1a is charged again by the charging roller 5a.

The conveyor belt 11 has a width that is substantially equal to the length of the photosensitive drum 1a in the direction of the rotation axis.
This conveyor belt 11 has the shape of an endless belt,
A holding roller 13 and a holding roller 15 are provided on the upstream and downstream annular portions of the conveyor belt 11, respectively. The conveyor belt 11 is in contact with the holding rollers 11 and 15 along the outer circumferences of the holding rollers 13 and 15 in this annular portion. The distance from the holding roller 13 to the holding roller 15 is about 300 mm. The holding rollers 13 and 15 are provided so as to be rotatable in the directions of arrows i and j, respectively. With the rotation of the holding rollers 13 and 15, the conveyor belt 11 is moved in the direction e in the figure.

A paper feed cassette 25 for accommodating the paper P is provided in the vicinity of the conveyor belt 11. A pickup roller 27 for picking up the sheets P one by one is provided in the sheet feeding cassette 25 so as to be rotatable in the direction of arrow f in the figure. A registration roller pair 29 including an upper roller and a lower roller is rotatably provided on the front side of the transport belt 11 in the transport direction of the paper P taken out by the pickup roller 27. The registration roller pair 29 sends the conveyed paper P to the conveyor belt 11 at a timing so that the front end of the developer image formed on the photosensitive drum 1a comes to the front end of the paper P.

The above-mentioned photosensitive drum 1a and charging roller 5
a, exposure unit 7a, developing unit 9a, cleaning device 17a
The charge removal lamp 19a constitutes a process unit 100a as an image forming means.

On the conveyor belt 11, between the holding roller 13 and the holding roller 15 along the conveying direction, in addition to the process unit 100a, the process units 100b, 100.
c and 100d (hereinafter, process units 100a, 10
0b, 100c and 100d are collectively referred to as a process unit 100). Process unit 10
Each of 0b, 100c, and 100d has the same configuration as the process unit 100a. That is, the photosensitive drums 1b, 1c and 1d (hereinafter, the photosensitive drums 1a, 1
b, 1c and 1d are collectively referred to as the photoconductor drum 1) and are provided substantially at the center of each process unit 100. Around the photosensitive drum 1, a charging roller 5 is provided.
b, 5c and 5d (hereinafter, the charging rollers 5a, 5b, 5c and 5d are collectively referred to as the charging roller 5) are provided. The exposure units 7b, 7c and 7d are provided downstream of the charging roller 5.
(Hereinafter, the exposure units 7a, 7b, 7c and 7d are collectively referred to as the exposure unit 7). Downstream of the exposure unit 7, developing devices 9b, 9c and 9d (hereinafter, developing devices 9a, 9b, 9c) are provided.
And 9d are collectively referred to as a developing device 9), cleaning devices 17b, 17c and 17d (hereinafter referred to as cleaning device 1).
7a, 17b, 17c and 17d are collectively referred to as a cleaning device 17), static elimination lamps 19b, 19c and 1
9d (hereinafter, static elimination lamps 19a, 19b, 19c and 19
The configuration in which d is collectively referred to as a static elimination lamp 19) is similar to that of the process unit 100a. The difference is the developer contained in the developing device. Process unit 100
B contains yellow, magenta in the process unit 100c, and cyan in the process unit 100d.

Paper P conveyed by the conveyor belt 11
Sequentially come into contact with the respective photosensitive drums 1. Power supply rollers 23a, 23b, 23c and 23d as transfer means are provided in the vicinity of the contact position between the paper P and the respective photosensitive drums 1.
(Hereinafter, the power feeding rollers 23a, 23b, 23c, and 23d are collectively referred to as the power feeding roller 23.) are provided corresponding to the respective photosensitive drums 1. That is, the power feeding roller 23
Are provided in contact with the conveyor belt 11 in the vicinity of the transfer position of the corresponding photoconductor drum 1, and are arranged to face the process unit 100a via the conveyor belt 11. The power feeding roller 23 is adapted to rotate in the direction of the arrow g in the figure following the movement of the conveyor belt 11. The power supply rollers 23a, 23b, 23c and 23d are connected to the same bias power supply 31 which is a bias voltage applying means.

An image forming process of the multicolor image forming apparatus having the above-mentioned structure will be described. The rotating photosensitive drum 1a is first uniformly charged by the charging roller 5a.
It is charged to 500V.

This photosensitive drum 1a, which is uniformly charged by the charging roller 5a, is exposed from the exposure unit 7a to, for example, a laser.
The beam is irradiated to form an electrostatic latent image. This electrostatic latent image is developed by the developing device 9a with the developer charged in advance to about −20 μc / g.

On the other hand, the paper P is taken out from the paper feed cassette 25 by the pickup roller 27 and sent to the registration roller pair 29. The registration roller pair 29 feeds the sheet P onto the conveying unit 11 after timing the rotation of the photosensitive drum 1a so that the tip of the developer image comes to the edge of the sheet P.

On the other hand, when the paper P is conveyed, the transfer voltage of, for example, about 2,000 V is applied to the conveyor belt 11 from each of the power supply rollers 23 as the same bias voltage. By applying the bias voltage, a transfer electric field is formed between the photosensitive drum 1 and the semi-conductive transport belt 11. Therefore, first, the developer image on the photoconductor drum 1a is transferred onto the paper P, and the paper P carrying the developer image is conveyed and reaches the photoconductor drum 1b. Photoconductor drum 1b
On the upper side, the process unit 100b forms a developer image at a timing such that multiple transfer occurs on the conveyed paper P. The developer image formed on the photosensitive drum 1b is transferred onto the previously transferred developer image in an overlapping manner. The paper P is further conveyed to the photosensitive drum 1
Similarly, the developer images of the respective colors are transferred to the photosensitive drum 1d and the photosensitive drum 1d.

The sheet P carrying the image thus formed by the multiple transfer is sent from the conveyor belt 11 to the fixing device 33. The fixing device 33 includes a heating roller 35 and a pressure roller 3.
Have 7. The sheet P is heated and pressed by passing between the heating roller 35 and the pressure roller 37 while the developer image is in contact with the heating roller 35. As a result, the developer image is fixed on the paper P. After that, the paper P is discharged onto the paper discharge tray 34.

The above-mentioned transport belt 11 is formed of 140 .mu.m thick polycarbonate in which carbon is uniformly dispersed. The electric resistance value in the thickness direction per 1 cm ^{2 of the} conveyor belt 11 is 10 ¹⁰ Ω · cm ² .
In this embodiment, the electric resistance value of the conveyor belt 11 is expressed by the electric resistance value in the thickness direction per cm ^{2 in} order to ignore the thickness of the belt.

In a multicolor image forming apparatus in which a bias voltage is applied to the conveyor belt 11 to form a transfer electric field, the electric resistance value of the conveyor belt 11 has an important influence on the transfer.
Electric resistance in the thickness direction per 1 cm ² is 10 ⁶ Ω · cm
^{When the value} is smaller than ² , electric charges are applied to the paper P from the conveyor belt 11, and the electric charges are applied to the developer, and the charging polarity of the developer becomes opposite polarity, and the transfer efficiency decreases, which is not preferable. Further, the electrical resistance in the thickness direction per 1 cm ² is more than ¹⁰ 10 Ω · cm ^2, which requires a high transfer bias voltage in order to obtain a sufficient transfer electric field. Further, when the paper P separates from the photosensitive drum 1 after the transfer, peeling discharge occurs, and the charging polarity of the developer becomes opposite.
For this reason, the developer is reversely attached to the photosensitive drum 1 and the transfer efficiency is reduced, which is not preferable.

On the other hand, with respect to the conveyor belt 11, the power feeding roller 23a moves from the transfer position of the photosensitive drum 1a to the paper P.
Of the conveyor belt 11 at a position slightly spaced downstream in the conveyor direction of
Is in contact with. In the first embodiment, the distance between the photosensitive drum 1a and the power feeding roller 23a is set to 12 mm. In FIG. 1, in order to show the feature of the present embodiment, the separation distance is described in a different scale from other devices. Further, this distance of 12 mm is the maximum separation distance for keeping the conveyance belt 11 for conveying the paper P pressed against the photosensitive drum 1a without bending. Feeding roller 2
Similarly, 3b, 23c, and 23d are also separated from the corresponding transfer positions of the photosensitive drums on the downstream side in the transport direction of the paper P. This separation distance is 10 m for the power feeding roller 23b.
m, 8 mm for the power feeding roller 23c, and 6 mm for the power feeding roller 23d. That is, the distance between the photosensitive drum 1 and the corresponding power supply roller 23 in the direction of multiple transfer is set to be successively shorter. The distance is the center of the contact nip between each photosensitive drum 1 and the sheet P, that is, the distance from the transfer position of the photosensitive drum 1 to the contact position between the conveyor belt 11 and the corresponding power supply roller 23. Is.

As shown in FIG. 2, the power supply roller 23 is provided with an elastic layer 39 made of conductive urethane foam on the outer periphery of a metallic core 38, and the core 38 and the surface of the elastic layer 39 are separated from each other. The electric resistance between them is about 10 ⁴ Ω. A bias power supply 31 is connected to the core body 38. Each of the power feeding rollers 23 has a structure as shown in FIG. 2, and each power feeding roller 23 is connected to the power source 31 in parallel. The power supply roller 23 has an appropriate elasticity, and contacts the conveyance belt 11 from the back surface thereof in the direction of the arrow so that the developer image on the photosensitive drum 1 and the paper P contact with each other at an appropriate pressure. It is designed to rotate.

Images were formed by the above-mentioned multicolor image forming apparatus at room temperature and normal humidity. FIG. 3 shows the range of proper transfer bias voltage for a solid image and a halftone image of this image forming apparatus.

In FIG. 3, the horizontal axis represents the proper transfer bias voltage, and the vertical axis represents the number of times of multiple transfer and the type of image. To determine whether or not the transfer was good, the image after fixing had a predetermined image density and was free from transfer defects and transfer defects.

For comparison, the power feeding rollers 23 are all transferred to the transfer positions of the respective photosensitive drums 1, that is, the photosensitive drums 1.
FIG. 4 is a graph showing the proper transfer bias voltage when the sheet P and the sheet P are in contact with each other.

In FIG. 4, in the case of the solid image, the range of the proper transfer bias voltage is wider than that in the halftone image,
As the number of times of multiple transfer increases, the proper transfer bias voltage shifts to the higher side. On the other hand, in the case of a halftone image, the range of the proper transfer bias voltage is narrow, and the range of the proper transfer bias voltage is further narrowed as the number of times of multiple transfer increases.

On the other hand, as shown in FIG.
When 3 is provided at a position where the paper P and the photosensitive drum 1 are in contact with each other, that is, apart from the transfer position, the proper transfer bias voltage is gradually shifted to the higher side as a whole as the number of transfers increases. The range of the proper transfer bias voltage in transfer is wide. Therefore, it can be seen that even with the same transfer bias voltage, it is possible to perform sufficiently stable transfer for all four transfers.
The supply transfer bias voltage in the first embodiment is +
It was set to 2,000V.

The results shown in FIG. 4 indicate that when the respective power supply rollers 23 are provided at the transfer position of the photosensitive drum 1, the transfer characteristics of the solid image and the halftone image become worse as the number of transfers increases. It is shown that good transfer cannot be performed with the same transfer bias voltage in four times of multiple transfer.

In multiple transfer, the proper transfer bias voltage shifts to the higher side as the number of transfers increases.
Because the amount of the developer on the paper P increases as the number of transfers increases, a weak aerial discharge is generated between the surface of the photosensitive drum and the paper P during transfer, and the paper P on the side to which the developer is transferred is transferred. It is considered that the surface of is charged with the opposite polarity to the transfer bias voltage and weakens the transfer electric field.

Further, when the feeding roller is provided at the position where the paper P and the photosensitive drum 1 are in contact with each other, the reason why the range of the proper transfer bias voltage of the halftone image is narrow is thought to be that the change of the transfer electric field is large. To be

On the other hand, when the feeding roller 23 is provided apart from the transfer position of the photosensitive drum 1, the proper transfer bias voltage in each transfer is widened, and the multiple transfer can be performed with the same transfer bias voltage. The reason is as follows.

First, since the feeding roller 23 is provided away from the transfer position of the photosensitive drum 1, the distance of the conveyor belt 11 acting as a resistor for regulating the current is increased. As a result, the change of the transfer electric field applied to the developer image caused by the environmental change or the like is moderated, so that the range of the proper transfer bias voltage of the halftone image is widened.

Further, the power supply roller is provided on the downstream side in the direction of the multiple transfer so as to be sequentially closer to the position where the corresponding photosensitive drum and the sheet P contact each other. By setting the position of the power feeding roller in this manner, the transfer electric field is relatively increased in the downstream of the conveyance belt 11 in the conveyance direction of the paper P,
As the number of transfers increases, the proper transfer bias is prevented from shifting to a higher position.

Image formation was performed by the multicolor image forming apparatus shown in FIG. 1 even when the temperature and humidity were changed. As a result, there was little change in the range of the proper transfer bias voltage in each transfer, and it was possible to perform sufficiently stable transfer even with the same transfer bias voltage in the four transfers.

In addition to the above-mentioned polycarbonate in which carbon is dispersed, the sheet-shaped conveyor belt 11 adjusted to have an appropriate electric resistance in the first embodiment is
A conductive sheet such as carbon may be dispersed in a high resistance derivative sheet such as polyethylene terephthalate, polyimide, or polytetrafluoroethylene (Teflon). Instead of using conductive particles, a polymer film whose electric resistance is adjusted by adjusting the composition may be used. Furthermore, such a polymer film mixed with an ion conductive substance,
Alternatively, a rubber material such as silicon rubber or urethane rubber having a relatively low electric resistance may be used. The position where each of the power supply rollers 23 is brought into contact with the conveyor belt 11 may be provided on the downstream side from the transfer position of the photosensitive drum, as in the above-described embodiment.
It may be provided on the upstream side as shown in FIG. In FIG. 5, the power feeding roller 4 is provided upstream of the transfer position of the photosensitive drum 1.
5 are provided. However, when the power feeding roller is provided separately on the upstream side, the toner may be scattered on the transfer sheet P due to the influence of the transfer electric field before the photoconductor 1 and the transfer sheet P come into contact with each other. Should be separated on the downstream side.

Further, the distance from the contact position between the photosensitive drum 1 and the paper P to the power feeding roller 23 is greatly influenced by the electric resistance of the conveyor belt 11. However, when the same transfer bias voltage is applied to the power feeding roller 23 from the common power source, the distance from the corresponding photosensitive drum 1 is made smaller toward the downstream side of the conveyor belt 11, and the electric field at the downstream side is reduced. Adjust so that it becomes larger.

Any transfer means may be used as long as it has a lower electric resistance than the transfer belt 11 and can press the paper transfer means and the paper P softly and uniformly on the photosensitive drum 1. That is, in addition to the roller-shaped member, a fixed layer 53 is provided on the surface of the core body 51, and a conductive brush 55 made of rayon or the like containing carbon is provided on the fixed layer 53 as shown in FIG. Alternatively, the rotating brush 57 may be used. Alternatively, as shown in FIG. 7, a fixed blade 6 made of an elastic plate material 65 of conductive rubber fixed to a holder 63.
It may be 7. Further, as shown in FIG.
It is also possible to use a fixed brush 77 composed of a conductive brush 75 fixed to.

Next, a second embodiment of the image forming apparatus of the present invention will be described with reference to FIG. Conveyor belt 111
Is made of a polycarbonate resin in which conductive carbon is dispersed as in the first embodiment, and has an electric resistance value of 10 ⁸ Ω · cm ² per cm ^{2 in} the thickness direction. This conveyor belt 111 has a thickness of 140 μm and a width of 260 m.
m, and the peripheral length is 660 mm. The photosensitive drum 101a has a diameter of 40 mm, and an organic photosensitive member is used as its photosensitive material. This photosensitive drum 101
Four a, 101b, 101c, and 101d (hereinafter collectively referred to as photosensitive drums 101) are arranged horizontally, and the conveyor belts 111 are arranged on these photosensitive drums 101.
Are in contact with even pressing force.

The conveyor belt 111 is held and driven by a holding roller 115 made of a conductive rubber as a driving roller and a holding roller 113 made as a tension roller. The application of the transfer voltage and the pressing of the conveyor belt 111 against the photoconductor 101 are performed by the power feeding roller 123a and the power feeding roller 123 made of conductive urethane foam rubber having a diameter of 12 mm.
b, power feeding roller 123c and power feeding roller 123b (hereinafter collectively referred to as power feeding roller 123).

The conveyor belt 111 is driven in the direction of arrow e by the holding roller 115 and moved in an annular shape. The moving speed of the conveyor belt 11a is 50 mm / sec. A charging roller 145, which is made of stainless steel and has a diameter of 8 mm, is disposed at a position in contact with the conveyor belt 111 and facing the holding roller 113 in order to electrostatically attract the paper P to the conveyor belt 11a. Further, the static elimination roller 147 made of stainless steel and having a diameter of 10 mm is used to remove static electricity from the conveyor belt 111.
11 is arranged at a position which contacts 11 and which faces the holding roller 115. A DC power supply 146 for applying a voltage of DC-2 kV is connected to the charging roller 145, and an AC power supply 149 for applying an AC voltage of 3 kVpp, 2 kHz is connected to the discharging roller 147. The charging roller 145 and the discharging roller 147 are not limited to stainless steel, and may be conductive rubber rollers such as urethane and silicon rubber. Further, it may have a brush or blade shape.

Power feeding roller 1 for applying the transfer voltage of the first color
The distance between 23a and the photosensitive drum 101a is 12 mm.
And This distance is equal to the conveyance belt 11 that conveys the paper P.
This is the maximum separation distance at which 1 can be pressed against the photosensitive drum 101a without bending. When the separation distance is longer than this, the distance between the photosensitive drum 101a and the paper P is opened due to the slack of the transport belt 111, and a transfer failure occurs to lower the transfer efficiency. The optimum transfer margin of the apparatus shown in FIG. 9 is as shown in FIG.

When the photosensitive drum 101 and all the power supply rollers 123 are in contact with each other via the conveyor belt, that is, the optimum transfer margin when the separation distance is 0 mm is as shown in FIG. From FIGS. 10 and 11, it is better to increase the electric resistance component due to the conveyor belt to form the transfer electric field by disposing the feeding roller and the photosensitive drum apart from each other, as in the case of the first embodiment. It can be seen that

From FIG. 10, the optimum transfer voltage was determined to be 1,300 V which is the intermediate value of the transfer margin. The power supply roller 123b for applying the transfer voltage of the second color and the photosensitive drum 1 are also provided.
The separation distance from 01b is 10 mm, and the third color is 8 mm,
And the fourth color was 6 mm. The optimum value of this interval is experimentally determined so as to increase the transfer efficiency on the downstream side of the multiplex transfer. For example, the result of measuring the transfer efficiency of the second color is
It shows in FIG. FIG. 12 compares the case of overlapping and transferring the first color and the case of only the second color. The horizontal axis represents the distance between the photosensitive drum 101 and the power supply roller 123, and the vertical axis represents the transfer. Represents efficiency. As a result, the optimum separation distance that satisfies the transfer efficiency in both cases was set to 10 mm. Similarly, for the third color, the transfer efficiency was evaluated and the optimum separation distance was determined when three colors were overlaid and when only one color was used.

Next, a third embodiment of the image forming apparatus of the present invention will be described with reference to FIG. The difference between the first embodiment and this third embodiment is the position where the power feeding roller is arranged. That is, the separation distance between the first color power supply roller 223a and the photoconductor drum 201a is 12 mm, the separation distance between the second color power supply roller 223b and the photoconductor drum 201b is 8 mm, and the third color power supply roller 223c and the photoconductor. The distance between the power supply roller 223a and the power supply roller 2 is 4 mm, and the distance between the power supply roller 223d for the fourth color and the photoconductor drum 201d is 0 mm.
23b, a power feeding roller 223c, and a power feeding roller 223d are arranged. In this case, the most downstream photosensitive drum 201
At the d transfer position, the photosensitive drum 201d and the feeding roller 2
However, since the separation distance is relatively larger on the upstream side in the conveyance direction of the paper P than on the downstream side, a transfer electric field larger on the upstream side in the conveyance direction of the paper P is generated. Since it is formed, stable multiple transfer is performed.

Power feeding roller 2 for applying the transfer voltage of the first color
The distance between 23a and the photosensitive drum 201a is 12 mm.
And This distance is equal to that of the conveyor belt 21 that conveys the paper P.
This is the maximum separation distance at which the photosensitive drum 1a can be pressed without being bent. When the distance is more than this,
Due to the slack of the transport belt 211a, the photosensitive drum 201
The gap between “a” and the sheet P is widened, and a transfer failure occurs, which lowers the transfer efficiency.

Next, a fourth embodiment of the image forming apparatus of the invention will be described. FIG. 14 is a schematic sectional view showing a multicolor image forming apparatus according to the fourth embodiment. The configuration of the multicolor image forming apparatus shown in FIG. 14 is similar to that shown in FIG.
0b, 400c, and 400d are provided on the transport belt 311 along the transport direction. Process unit 40
Reference numeral 0a has a charging roller 305a and an exposure section 307a as a latent image forming means and a developing device 309a as a developing means around the photosensitive drum 301a. Further, the developing device 30
A cleaning device 317a and a discharge lamp 319a are provided downstream of 9a. Charging roller 305b,
305c and 305d (hereinafter, charging rollers 305a, 30
5b, 305c and 305d are collectively referred to as a charging roller 30.
5), the exposure units 307b, 307c and 307d.
(Hereinafter, exposure units 307a, 307b, 307c and 307
d is collectively referred to as an exposure unit 307), a developing device 309b,
309c and 309d (hereinafter, developing units 309a and 309)
b, 309c, and 309d are collectively referred to as a developing device 309), and the photoconductor drums 301b, 301c, and 301d (hereinafter photoconductor drum 3) are similar to the process unit 400a.
01a, 301b, 301c and 301d are collectively referred to as a photosensitive drum 301. ) Are each arranged around. Cleaning devices 317b, 317c, 317d
(Hereinafter, cleaning devices 317a, 317b, 317c
And 317d are collectively referred to as a cleaning device 317), static elimination lamps 319b, 319c and 319d (hereinafter, static elimination lamps 319a, 319b, 319c and 319).
The same applies to the charge-eliminating lamp 319 which is collectively referred to as d.

Photosensitive drum 3 of process unit 400
01 contacts the surface of the paper P carried on the conveyor belt 311. The conveyor belt 311 is made of polycarbonate having a thickness of 140 μm in which carbon is uniformly dispersed. The electric resistance in the thickness direction per 1 cm ^{2 of the} conveyor belt 311 is 10 ⁸ Ωcm ² . The endless conveyor belt 311 has a width dimension substantially equal to the length dimension of the photosensitive drum 301 in the rotation axis direction. This conveyor belt 3
11 is formed in an endless shape, and conductive holding rollers 313 and holding rollers 315, which are holding means for holding the conveying means, are provided on the upstream and downstream annular portions of the conveying belt 311 respectively. . The conveyor belt 311 is in contact with the holding roller 311 and the holding roller 315 along the outer circumferences of the holding roller 313 and the holding roller 315 in this annular portion. The distance from the holding roller 313 to the holding roller 315 is about 300 mm.
The holding rollers 313 and 315 are provided so as to be rotatable in the direction of arrow k and one direction, respectively. Holding roller 313
And the conveyor belt 3 as the holding roller 315 rotates.
11 is moved in the direction of the arrow shown.

In this embodiment, a rotatable auxiliary roller 323a and an auxiliary roller 3 are provided between the holding roller 311 and the holding roller 315 to prevent the slack of the conveyor belt.
23b, an auxiliary roller 323c and an auxiliary roller 323d are provided. These auxiliary rollers are holding rollers 31.
1. By rotating together with the holding roller 315, the transportability of the transport belt 311 does not deteriorate.

The lengths of the holding roller 313 and the holding roller 315 in the rotation axis direction are substantially equal to the length of the photosensitive drum 301 in the rotation axis direction. Further, the holding roller 313 and the holding roller 315 are provided with metal shafts 314 and 316 as rotating shafts penetrating in parallel with the rotating shaft direction of the photosensitive drum 301.

Also in the image forming apparatus shown in FIG. 14, the same image forming process as in the multicolor image forming apparatus shown in FIG. 1 is executed. That is, an electrostatic latent image is formed on each photoconductor drum 301, and the electrostatic latent image is developed by the developing device 309 with the developer of each color. The formed developer images are multiple-transferred onto the paper P conveyed by the conveyor belt 311 and fixed on the paper P by the fixing device 333. After that, the paper P is discharged onto the paper discharge tray 334.

The metal shaft 314 of the holding roller 313 arranged on the upstream side in the sheet conveying direction is the holding roller 3
It is connected to a power source 341 as a first voltage applying means for applying a bias voltage to the line 13. In addition, the metal shaft 31 of the holding roller 315 arranged on the downstream side in the paper transport direction.
Similarly, 6 is connected to a power source 343 as a second voltage applying means for applying a bias voltage to the holding roller 315. Here, the bias voltage applied to the holding roller 315 has a larger absolute value than the bias voltage applied to the holding roller 313. By thus applying the bias voltage to the conductive holding roller 313 and the holding roller 315, the bias voltage is applied to the conveyor belt 311 and the conveyor belt 311 is charged. Therefore, a transfer electric field is formed between the conveyor belt 311 and each of the photoconductor drums 301, and each developer image is sequentially and multiple-transferred onto the paper P by this transfer electric field.

In this fourth embodiment, the holding roller 3
A DC voltage of +1,000 V was applied to 13 and a DC voltage of +2,000 V was applied to the holding roller 315, and the potential difference of the bias voltage between the holding roller 313 and the holding roller 315 was kept constant at 1,000 V.

An image was formed by such an image forming apparatus at room temperature and normal humidity. A holding roller 3 which is good in transferring a solid image and a halftone image as in the above embodiment.
Check the range of proper transfer bias voltage for 13
The result was almost the same as that of the example.

That is, although the proper transfer bias voltage of the holding roller 313 is slightly shifted in the multiple transfer direction, the range of each transfer bias voltage is wide. Therefore, it has been found that stable transfer can be performed with a constant transfer bias voltage for multiple transfer.

The reason for this is that by supplying a bias voltage to the holding rollers 313 and 315, changes in the transfer electric field caused by environmental changes and the like become gentler than when transfer is performed at the transfer positions of the respective photosensitive drums. . For this reason, the range of the proper transfer bias of the halftone image becomes wide.

The holding roller 315 has a holding roller 31.
Since the bias voltage having an absolute value larger than 3 is applied, the proper transfer bias voltage is minimized from shifting to the higher side in the multiple transfer direction.

When the environment of temperature and humidity is changed,
When images were formed by the multicolor image forming apparatus shown in FIG. 14, stable transfer could be performed with a constant transfer bias voltage. In the fourth embodiment, the conveyor belt 311 is made of polyethylene terephthalate, polyimide, in addition to polycarbonate in which carbon is dispersed.
It is also possible to use conductive sheet such as carbon dispersed in a high resistance derivative sheet such as polytetrafluoroethylene (Teflon). Further, such a polymer film mixed with an ion conductive substance, or a silicon rubber or urethane rubber material having a relatively low electric resistance may be used. However, as the transport belt 311, one having an electric resistance of 10 ⁶ to 10 ¹⁰ Ωcm ² in the thickness direction per 1 cm ² is used. The absolute value of the bias voltage applied to the holding roller 313 provided on the upstream side of the conveyor belt 311 and the holding roller 31 provided on the downstream side of the conveyor belt 311.
The difference from the absolute value of the bias voltage applied to 5 is about 1,0
It may be set to 00V. By thus applying the bias voltage from the first voltage applying unit and the second voltage applying unit, the magnitude of the transfer electric field can be increased toward the downstream side of the conveyor belt 111.

Further, the electric resistance in the thickness direction per ¹ cm ^{2 of} the conveyor belt 111 is 10 ⁶ to 10 ¹⁰ Ωcm ² ,
The upstream holding roller 313 and the downstream holding roller 315
When the potential difference between and is about 1,000 V,
It is necessary to apply a bias voltage of 1,000 V or more to the holding roller 313 on the upstream side. On the other hand, it is necessary to apply a bias voltage of 6,000 V or less to the holding roller 315 on the downstream side. If the bias voltage applied to the holding roller 313 on the upstream side is less than 1,000 V, a transfer electric field having sufficient strength cannot be formed in the moving direction of the conveyor belt 311. Further, the holding roller 3 on the downstream side
If the bias voltage applied to 15 is 6,000 V or more, the transport belt 111 may cause dielectric breakdown.

[0068]

As described above, according to the image forming apparatus of the present invention, since the proper transfer bias of the halftone image is widened, stable transfer can be performed in multiple transfer without being affected by environmental changes. .

Further, when performing multiple transfer, the number of transfer power sources can be reduced and the transfer electric field on the downstream side can be strengthened, so that multiple transfer can be stably performed and the apparatus can be downsized. it can.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a first embodiment of an image forming apparatus of the present invention.

FIG. 2 is a schematic diagram showing a transfer unit including a power supply roller in the image forming apparatus shown in FIG.

FIG. 3 is a graph showing an appropriate transfer bias voltage in the first embodiment of the image forming apparatus of the invention.

FIG. 4 is a graph showing a proper transfer bias voltage in an image forming apparatus to which the present invention is not applied, for comparison with FIG.

FIG. 5 is a schematic diagram showing a modification of the installation position of the power feeding roller included in the transfer section.

FIG. 6 is a schematic view showing a modified example of a power feeding roller.

FIG. 7 is a schematic view showing another modified example of the power feeding roller.

FIG. 8 is a schematic view showing still another modified example of the power feeding roller.

FIG. 9 is a schematic view showing a second embodiment of the image forming apparatus of the invention.

FIG. 10 is a graph showing a proper transfer bias voltage in the second embodiment of the image forming apparatus of the invention.

11 is a graph showing a proper transfer bias voltage in an image forming apparatus to which the present invention is not applied, and is a graph for comparison with FIG.

FIG. 12 is a graph showing the relationship between the transfer efficiency and the distance between the photosensitive drum and the power supply roller.

FIG. 13 is a schematic view showing a third embodiment of the image forming apparatus of the invention.

FIG. 14 is a schematic view showing a fourth embodiment of the image forming apparatus of the invention.

[Explanation of symbols]

5a, 5b, 5c, 5d Charging roller 7a, 7b, 7c, 7d Exposure section 9a, 9b, 9c, 9d Developing device 11 Conveying means 23a, 23b, 23c, 23d Power supply roller 31 Power supply 100a, 100b, 100c, 100d Process unit 305a, 305b, 305c, 305d Charging roller 307a, 307b, 307c, 307d Exposure unit 309a, 309b, 309c, 309d Developing device 311 Conveying means 313, 315 Holding roller 341, 343 Power supply 400a, 400b, 400c, 400d Process unit

Claims (7)

[Claims] 1. A first image forming means for forming a first developer image on a first image carrier, and a second image forming means for forming a second developer image on a second image carrier. And a conveying means that is provided so as to face the first image carrier and the second image carrier and sequentially conveys the image-forming medium to the first image carrier and the second image carrier in this order. Means and the first image formation via the conveying means at a position separated from the transfer position of the first image carrier by a first distance along the direction in which the image forming medium is conveyed. A first surface of the first image carrier, which is provided opposite to the first image carrier;
First transfer means for transferring the developer image of the second image onto the image forming medium, and a first distance from the transfer position of the second image carrier along the direction in which the image forming medium is conveyed. The second developer image formed on the second image carrier, which is provided to face the second image forming unit via the conveying unit at a position separated by a second distance which is smaller than the second image forming unit. It has a second transfer means for transferring onto the image forming medium, and a single voltage applying means for supplying a transfer bias voltage to the first transfer means and the second transfer means. Image forming apparatus. 2. A first image forming means for forming a first developer image on a first image carrier, and a second image forming means for forming a second developer image on a second image carrier. And a means for covering the first image carrier and the second image carrier so that the image forming medium may come into contact with the first image carrier and the second image carrier. First image forming medium
And 1 cm ² which is successively conveyed to the second image carrier in this order.
The electrical resistance in the thickness direction is 10 ⁶ to 10 ¹⁰ Ωcm ²
Of the first conveying member and the first image carrier at a position separated from the transfer position of the first image carrier by a first distance along the conveying direction of the image forming medium. The first means is provided in contact with the conveying means so as to face the image forming means.
First transfer means for transferring the first developer image formed on the image carrier onto the image forming medium, and the second image along the direction in which the image forming medium is conveyed. The carrier is provided at a position separated from the transfer position of the carrier by a second distance smaller than the first distance so as to face the second image forming unit through the carrier so as to be in contact with the carrier. A second transfer unit that transfers the second developer image formed on the second image carrier onto the image forming medium, and a transfer bias voltage to the first transfer unit and the second transfer unit. An image forming apparatus comprising: a single voltage application unit for supplying. 3. A first image forming unit for forming a first developer image on a first image carrier, and a second image forming unit for forming a second developer image on a second image carrier. Means, a third image forming means for forming a third developer image on the third image carrier, and a fourth image forming means for forming a fourth developer image on the fourth image carrier. , The first, second, third, and fourth image carriers, which are provided so as to face each other, and the image forming medium is sequentially arranged in this order with respect to the first, second, third, and fourth image carriers. Transporting means for transporting the image forming medium, and a spacing distance from the respective transfer positions of the first, second, third, and fourth image carriers along the transporting direction of the image forming medium, and the spacing distance. A first, a second, a third and a fourth transfer means which are provided so as to become smaller toward the downstream side in the transport direction of the image forming medium; An image forming apparatus comprising: a single voltage applying unit for supplying a transfer bias voltage to the second, third and fourth transfer units. 4. A first image forming means for forming a first developer image on a first image carrier, and a second image forming means for forming a second developer image on a second image carrier. And a conveyor belt that is provided so as to face the first image carrier and the second image carrier and that sequentially conveys the image forming medium to the first image carrier and the second image carrier. , A pair of drive rollers provided to face each other and driving the conveyor belt, and for transferring an image at the transfer position of the second image carrier rather than the transfer position of the first image carrier. So that the transfer electric field of
An image forming apparatus comprising: a voltage applying unit that supplies a transfer bias voltage to the drive roller. 5. A first image forming means for forming a first developer image on a first image carrier, and a second image forming means for forming a second developer image on a second image carrier. And a conveying means for providing an image forming medium to the first image carrier and the second image carrier in this order in order to face the first image carrier and the second image carrier. A belt, first and second drive rollers that are provided to face each other and are spaced apart from each other, and that drive the conveyor belt; a first power supply that applies a transfer bias voltage to the first drive roller; and a first power supply. An image forming apparatus comprising: a second power supply that applies a transfer bias voltage having a larger absolute value than the absolute value of the transfer bias voltage applied from the second drive roller. 6. A first image forming means for forming a first developer image on a first image carrier, and a second image forming means for forming a second developer image on a second image carrier. And a means for covering the first image carrier and the second image carrier so that the image forming medium may come into contact with the first image carrier and the second image carrier. First image forming medium
And 1 cm ² which is successively conveyed to the second image carrier in this order.
The electrical resistance in the thickness direction is 10 ⁶ to 10 ¹⁰ Ωcm ²
And a conveying belt of, which are spaced apart from each other and are in contact with the conveying means,
The magnitude of the transfer electric field for transferring an image at the transfer position of the second image carrier is larger than that at the transfer position of the pair of drive rollers that drive the conveyor belt and the first image carrier. As described above, the image forming apparatus further comprises a voltage applying unit that supplies a transfer bias voltage to the drive roller. 7. A first image forming means for forming a first developer image on a first image carrier, and a second image forming means for forming a second developer image on a second image carrier. Means, a third image forming means for forming a third developer image on the third image carrier, and a fourth image forming means for forming a fourth developer image on the fourth image carrier. , The first, second, third, and fourth image carriers, which are provided so as to face each other, and the image forming medium is sequentially arranged in this order with respect to the first, second, third, and fourth image carriers. And a pair of drive rollers that are provided so as to face each other and are spaced apart from each other, and that drive the conveyor belt, and the magnitude of the transfer electric field at the transfer positions of the first, second, third, and fourth image carriers. Bias voltage is applied to the drive roller so that the value becomes larger toward the downstream side in the transport direction of the image forming medium. First, second, an image forming apparatus; and a voltage applying means for transferring the third and fourth developer image on an image forming medium.