JPH08305183A - Image forming device - Google Patents

Abstract

PURPOSE: To achieve satisfactory transfer by making a contact force between an image-carrier and a medium on which an image is formed proper even on a downstream side, thereby preventing the development of a void phenomenon. CONSTITUTION: Paper, P, which is carried by a carrying belt 11, is brought into contact with photoreceptor drums 3a-3d in order. In the vicinity of the contact positions of the paper P with the photoreceptor drums 3a-3d transfer rolls 23a-23d, as transfer means, are provided corresponding to the photoreceptor drums 3a-3d, respectively. Distances between the transfer rollers 23a-23d and the photoreceptor drums 3a-3d, respectively, are different. The transfer roller 23a is set almost directly below the contact position of the photoreceptor drum 3a with paper P, while the transfer roller 23b is set apart from the contact position of the photoreceptor drum 3b with paper P. By arranging the transfer rollers in such a way that the more the transfer rollers are situated on the downstream side, the more they are apart from the corresponding photoreceptor drums, excessive pressure is hardly caused between the rollers and corresponding drums on the side of downstream.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic color copying machine or a color printer, and more particularly to a developer image formed on a plurality of photoconductors on a transfer material such as paper. The present invention relates to an image forming apparatus that forms an image by multiple transfer.

[0002]

2. Description of the Related Art As a conventional technique, Japanese Patent Laid-Open No. 6-11034
Japanese Patent Publication No. 3 discloses an image forming apparatus using a transfer belt formed of a semiconductive material. In the above publication, a semi-conductive belt is arranged so as to sequentially convey a sheet to each of a plurality of photoconductors on which a developer image is formed. It is described that a transfer roller is provided. When such a semi-conductive belt is used, by applying a bias voltage to the transfer roller, the developer formed on each photoconductor is multi-transferred onto the conveyed paper. The sheet carrying the multiple-transferred developer image is fixed by a fixing device to form a color image.

However, in the image forming apparatus as disclosed in the above publication, a part of the developer image is difficult to transfer, or the developer is attracted to a place where it is more stable and easy to transfer. A transfer failure, which is so-called a hollow portion, in which the central portion of the is not transferred is likely to occur. Such a transfer failure is particularly noticeable when a thin line such as a character is transferred or when it is transferred onto an OHP sheet or the like having a uniform surface and a hard surface. Therefore, when a color image is formed by the conventional image forming apparatus, there is a problem that the image quality is not good.

[0004]

As described above, the conventional image forming apparatus has a drawback that a defective color image cannot be formed due to a defective transfer due to a void. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus capable of removing the above-mentioned drawbacks and forming a high-quality color image with no voids.

[0005]

SUMMARY OF THE INVENTION The inventors of the present invention have found that a hollow image phenomenon that occurs during color image formation is likely to occur during transfer of a developer image on a photoconductor disposed in the latter stage of a plurality of photoconductors. Found. Further, the hollow phenomenon occurs when the pressure between the photoconductor and the transfer roller is too high.

In order to solve the above-mentioned problems, 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 and a second image carrier. On the body,
A second image forming unit that forms a second developer image, and an image forming medium are sequentially conveyed to the first image carrier and the second image carrier, and the first image carrier and the second image carrier are sequentially conveyed. A conveyance unit for sequentially bringing the image forming medium into contact with the second image carrier, and a position in which the first image carrier comes into contact with the image forming medium, along the direction in which the image forming medium is conveyed. First
Is provided at a position separated by a distance of 1 mm so as to face the first image forming unit via the conveying unit.
Direction in which the image forming medium is conveyed from the position where the second image carrier contacts the image forming medium and the first transfer unit that transfers the developer image onto the image forming medium. Is provided at a position spaced apart by a second distance larger than the first distance so as to face the second image forming unit via the transporting unit, and the second developer image is A second transfer unit that transfers the image onto the image forming medium.

In the image forming apparatus of the present invention, the first image forming means for forming the first developer image on the first image carrier and the second developer on the second image carrier. Second forming an image
Image forming means, the first image bearing member and the second image bearing member, and the image forming medium is sequentially conveyed to the first image bearing member and the second image bearing member. Is provided so as to be in contact with the transporting means so as to face the first image forming means via the transporting means, and the first developer image is formed on the image forming target. A first transfer unit that transfers the image onto a medium and a second transfer unit are provided so as to be in contact with the transfer unit so as to face the second image forming unit via the transfer unit. Second transfer means for transferring onto the image forming medium; first urging means for urging the first transfer means to the conveying means by a first urging force; The transfer means of No. 2 is urged to the conveying means by the urging force of the second magnitude smaller than the first magnitude. And a second biasing means that.

Furthermore, the image forming apparatus of the present invention is the first
A first image forming unit for forming a first developer image on the image carrier, a second image forming unit for forming a second developer image on the second image carrier, and The image forming medium is sequentially conveyed to the first image carrier and the second image carrier, and the first image carrier is transferred to the first image carrier.
Conveying means for sequentially bringing the image forming medium into contact with the image carrier and the second image carrier, and first rotating means for rotating the first image carrier at a speed of a first magnitude. The second
Is provided so as to face the first image forming unit via a second rotating unit that rotates the image carrier at a second speed larger than the first size, and the conveying unit. It is provided so as to face the second image forming unit via the first transfer unit that transfers the first developer image onto the image forming medium and the conveying unit, and the second image forming unit.
Second transfer means for transferring the developer image of 1. to the image forming medium.

[0009]

According to the present invention, a plurality of transfer means are provided so as to face the image forming means through the conveying means, and the transfer means is the image carrier when the downstream side in the conveying direction of the image forming medium. It is arranged apart from the contact position between the body and the image forming medium. Since the developer images formed on the respective image carriers are sequentially transferred onto the image forming medium,
The image forming medium is conveyed with the developer image already transferred to the transfer means arranged downstream in the conveying direction of the image forming medium.

In such a case, since the transfer means located downstream is in contact with the conveying means at a great distance from the contact position between the image carrier and the image forming medium, as described above, The pressure between the transfer means does not rise excessively. That is, the abutting force between the image carrier and the image forming medium has an appropriate magnitude even in the downstream side, and by applying the bias voltage to the transfer means, the bias transfer of the developer image does not cause a hollow phenomenon. Done.

Further, according to the present invention, a plurality of transfer means are provided in contact with the conveying means so as to face the image forming means via the conveying means. Downstream along the transport direction,
It is pressed against the conveying means with a small pressing force.

Therefore, even when the image forming medium on which the developer image has been transferred is conveyed to the transfer means located downstream, the image forming medium is excessively pressed against the image carrier by the transfer means. Never. Therefore, good transfer can be performed without causing a hollow defect. Furthermore, according to the present invention, a plurality of transfer means are provided so as to face the image carrier in which a plurality of transfer means are arranged, and the transfer means, and
Among the image carriers, the downstream side in the transport direction of the image forming medium is configured to rotate at a higher speed.

With this configuration, even when the image forming medium on which the developer image has already been transferred is conveyed to the downstream image carrier, the image carrier rotates at high speed. As a result, a frictional force is generated between the image carrier and the image forming medium due to the sliding contact, and the developer image is easily separated from the image carrier. Therefore, the occurrence of the hollow defect is prevented and good transfer is performed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a first image forming apparatus according to the present invention.
It is sectional drawing which shows the Example of. In FIG. 1, a process unit 1a and a process unit 1 which are image forming means.
b, a process unit 1c and a process unit 1d are provided.

Each process unit has a photoconductor drum 3a, a photoconductor drum 3b, a photoconductor drum 3c and a photoconductor drum 3d which are image bearing members, and forms a developer image on these photoconductors. First, the process unit 1a will be described. In FIG. 1, a photosensitive drum 3a as an image bearing member 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 components are arranged around the photosensitive drum 3a along the rotation direction. That is, a charging roller 5a that uniformly charges the photosensitive drum 3a is provided in contact with the surface of the photosensitive drum 3a. An exposure unit 7a for exposing the charged photosensitive drum 3a to form an electrostatic latent image is provided on the downstream side of the charging roller 5a. Further, a developing device 9a that accommodates a black developer and develops the electrostatic latent image formed by the exposure unit 7a with the developer is provided downstream of the exposure unit 7a. A transport belt 11 is provided downstream of the developing device 9a as a transport unit that transports the sheet P, which is an image forming medium, to the photosensitive drum 3a. The conveyance belt 11 conveys the paper P to the photoconductor drum 3a so that the developer image formed on the photoconductor drum 3a and the paper P come into contact with each other.

Further, a cleaning device 17a and a discharge lamp 19a are provided downstream of the contact position between the photosensitive drum 3a and the paper P. Cleaning device 17a
Is to scrape off the developer remaining on the photosensitive drum 3a after the transfer by the blade 21. The charge eliminating lamp 19a is for eliminating the charge on the surface of the photosensitive drum 3a 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 3a is charged again by the charging roller 5a.

The above-mentioned photosensitive drum 3a and charging roller 5
a, exposure unit 7a, developing unit 9a, cleaning device 17a
Further, the process unit 1a is constituted by the static elimination lamp 19a.

The conveyor belt 11 has a width that is substantially equal to the length of the photosensitive drum 3a in the direction of the rotation axis.
This conveyor belt 11 has the shape of an endless belt,
It is carried on a driving roller 13 and a driven roller 15 that move the conveyor belt at a predetermined speed. The distance from the driving roller 13 to the driven roller 15 is about 300 mm. The drive roller 13 and the driven roller 15 are provided so as to be rotatable in the directions indicated by arrows i and j, respectively.
The conveyor belt 11 is moved in the direction of the arrow e in the figure by the driven roller 15 being driven and rotated by the rotation of the drive roller 13.

Here, the material of the conveyor belt will be described. The conveyor belt 11 is made of polyimide having a thickness of 100 μm in which carbon is uniformly dispersed. This conveyor belt has an electric resistance of 10 ¹² Ω · cm and exhibits semiconductivity. On the transport belt 11, between the drive roller 13 and the driven roller 15, along the transport direction of the paper P, the process unit 1a, the process units 1b, 1c and 1 are provided.
d is arranged.

Each of the process units 1b, 1c and 1d has the same structure as the process unit 1a.
That is, the photoconductor drums 3b, 3c and 3d are provided substantially at the center of each process unit. Charging rollers 5b, 5c and 5d are provided around the respective photosensitive drums. The exposure units 7b and 7c are provided downstream of the charging roller.
And 7d are provided. A developing unit 9 is provided downstream of the exposure unit.
b, 9c and 9d, cleaning devices 17b, 17c and 17d, and static elimination lamps 19b, 19c and 19d are provided in the same manner as the process unit 1a. The difference is the developer contained in the developing device. The process unit 1b contains yellow, the process unit 1c contains magenta, and the process unit 1d contains cyan.

Paper P conveyed by the conveyor belt 11
Sequentially come into contact with the respective photosensitive drums. Transfer rollers 23a, 23b, 23c, and 23d as transfer means are provided in the vicinity of the contact positions between the paper P and the respective photoconductor drums corresponding to the respective photoconductor drums. That is, the transfer roller is provided below the corresponding photosensitive drum in contact with the conveyor belt 11 from the rear side and faces the process unit via the conveyor belt 11. These transfer rollers are adapted to rotate in the direction of the arrow g in the figure following the movement of the conveyor belt 11.

The transfer roller 23a is connected to a positive DC power source 25a which is a voltage applying means. Similarly, the transfer roller 23b is connected to the DC power supply 25b, the transfer roller 23c is connected to the DC power supply 25c, and the transfer roller 23d is connected to the DC power supply 25d.
Connected to each.

On the other hand, in FIG. 1, a paper feed cassette 26 for accommodating the paper P is provided on the right front side 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 26 so as to be rotatable in the direction of arrow f in the figure. Pickup roller 2
A pair of registration rollers 29 is rotatably provided on the front side of the transport belt 11 in the transport direction of the sheet P taken out by the sheet 7. The registration roller pair 29 is the photosensitive drum 3a.
The paper P is sent out to the conveyor belt 11 at a predetermined timing with the rotation of. Further, on the conveyor belt 11, a suction roller 3 for electrostatically attracting the paper P to the surface of the conveyor belt 11.
0 is provided so as to face the drive roller 13 via the conveyor belt 11. The suction roller 30 is connected to a negative DC power supply 31.

Further, in FIG. 1, a fixing device 33 for fixing the developer image transferred on the paper P and a paper discharge tray for discharging the paper P fixed by the fixing device are provided on the left front side of the conveyor belt 11. 34 are provided.

The image forming process of the image forming apparatus having the above-mentioned structure will be described. The rotating photoconductor drum 3a is first uniformly moved to about −5 by the charging roller 5a.
It is charged to 00V.

The photosensitive drum 3a uniformly charged by the charging roller 5a is irradiated with a laser beam corresponding to an image signal from the exposure section 7a to form an electrostatic latent image. This electrostatic latent image is previously about -2 by the developing device 9a.
Development is performed with the developer charged to 0 μc / g, and a black developer image is formed.

Developer images of respective colors are formed on the photosensitive drums 3b, 3c, and 3d, similarly to the photosensitive drum 3a. The paper P is taken out from the paper feed cassette 26 by the pickup roller 27, and is sent onto the conveyor belt 11 by the registration roller pair 29. The surface of the paper P supplied onto the conveyor belt 11 has a suction roller 3
It is brought into contact with 0 and is negatively charged by the suction roller 30, and electrostatically adsorbed on the conveyor belt 11 by this charging.

The paper P is conveyed on the conveyor belt 11 toward the photosensitive drums 3a, 3b, 3c and 3d. When the paper P is conveyed, first, as a transfer bias voltage, a transfer voltage of about 1,000 V is applied to the conveyor belt 11 from the transfer roller 23a. By applying the bias voltage, a transfer electric field is formed between the photosensitive drum 3a and the semi-conductive transport belt 11. Therefore, the black developer image on the photosensitive drum 3a is transferred onto the paper P.

The sheet P carrying the developer image is further conveyed and reaches the photosensitive drum 1b. When the paper P reaches the photosensitive drum 1b, the DC power source 25 is applied to the transfer roller 23b.
By applying a bias voltage of about 1,200 V from b, the yellow developer image is transferred over the black developer image. The paper P is further conveyed, and a bias voltage of about 1,400 V is applied to the transfer roller 23c on the photosensitive drum 1c.
When a voltage of about 1,600 V is applied to the transfer roller 23d, the magenta developer image and the cyan developer image are sequentially transferred in a superimposed manner on the already transferred developer image.

The sheet P carrying the image thus formed by the multiple transfer is sent from the conveyor belt 11 to the fixing device 33, and the developer image is fixed on the sheet P. Then paper P
Are discharged onto the discharge tray 34.

Here, the part relating to transfer will be described in more detail. As shown in FIG. 2, in the transfer roller 23a, an elastic layer 39 made of conductive urethane foam is provided on the outer periphery of a metallic core body 38, and the core body 38 and the elastic layer 3 are provided.
The electric resistance with the surface of 9 is about 10 ⁴ Ω.
A power source 25a is connected to the core body 38. Each of the transfer rollers has the same structure as that shown in FIG. Each transfer roller has an appropriate elasticity, and contacts the conveyance belt 11 from its rear surface so that the developer image on the photoconductor drum and the paper P come into contact with each other at an appropriate pressure in the direction g indicated by the arrow. It is designed to rotate.

A perspective view of the transfer roller 23a is shown in FIG.
The transfer roller 23a has the core body 38, and the transfer roller receives a driving force from a motor (not shown) and rotates about the core body 38 as a rotation center axis. The core body 38 is supported by a bearing 43. The bearing 43 is provided with a spring 45 as an urging means, and the spring 45 urges the transfer roller 23 a so as to elastically contact the conveyor belt 11 with an appropriate pressure.

Similarly to the transfer roller 23a, the transfer rollers 23b, 23c and 23d are elastically urged against the conveyor belt 11 in the state shown in FIG. As described above, the transfer rollers 23a to 23d have the same structure, but the positions of the transfer rollers are different.

That is, as shown in FIG. 1 or 4, the distances of the transfer rollers from the corresponding photoconductors are different from each other, and the transfer roller 23a contacts the photosensitive drum 3a and the paper P at the contact position t. However, the transfer roller 23b is arranged at a position separated from the contact position between the photosensitive drum 1b and the paper P.

FIG. 4 is a view for relatively showing the distance that each transfer roller is separated from the corresponding photoconductor, and that the photoconductor drums 3a, 3b, 3c, 3d are arranged at the same position. The distances of the respective transfer rollers from the photoconductor drum in the case of being assumed are shown. The transfer roller 23a is disposed substantially beneath the as described above abutment position t, the transfer roller 23b is spaced a distance B _1, the transfer roller 23c is spaced a distance C ₁ of large size than B _1, the transfer roller 23c is The photosensitive drum is arranged to be separated from the photosensitive drum with a separation distance D ₁ larger than the separation distance C ₁ . That is, the distance from the photoconductor is increased as the transfer roller is arranged downstream in the transport direction of the paper P.

In this embodiment, the distance between the transfer roller 23b and the photosensitive drum 3b is 1.5 cm, and the transfer roller 2
The distance between 3c and the photoconductor drum 3c is set to 3.0 cm, and the distance between the transfer roller 23d and the photoconductor drum 3d is set to 4.5 cm.

By arranging the transfer roller on the downstream side largely apart from the photosensitive drum, it is possible to prevent the pressure between the transfer roller and the photosensitive drum on the downstream side from becoming excessive.

That is, since the transfer roller 23a transfers the developer image onto the paper P for the first time, the conveyor belt 11, the paper P, and the black paper are provided between the photosensitive drum 3a and the transfer roller 23a. The developer image is present. On the other hand, the transfer by the transfer roller 23b transfers the yellow developer image onto the paper P on which the black developer image has already been transferred. Therefore, the transfer between the photosensitive drum 3b and the transfer roller 23b is performed. The conveyance belt 11, the paper P, the black developer image, and the yellow developer image are present between them.

As described above, as the paper P is conveyed to the downstream side, the developer images of the respective colors are multi-transferred. Therefore, between the transfer roller and the photosensitive drum on the downstream side,
Since a large amount of developer exists, for example, the transfer roller 23d
Is urged against the conveyor belt 11 at the same pressure as the transfer roller 23a, the pressure between the transfer roller 23d and the photoconductor drum 1d at the time of transfer is the pressure between the transfer roller 23a and the photoconductor drum 3a. Will be larger than. For this reason, when the transfer roller on the downstream side is biased to the transport belt in the same manner as the transfer roller on the upstream side, the pressure between the transfer roller and the photosensitive drum becomes excessive.

According to the present embodiment, as described above, the downstream transfer roller is farther away from the photosensitive member, so that the pressure between the downstream transfer roller and the photosensitive member can be set to an appropriate value. With the configuration of this embodiment, it is possible to prevent the hollow image phenomenon that occurs during color image formation.

A printing result when a color image is formed by the image forming apparatus shown in FIG. 1 is shown in FIG. 5, and in the image forming apparatus having the same structure as in FIG. 1, the transfer roller is arranged almost directly under the photosensitive member. FIG. 6 shows the printing result when a color image is formed by using.

It can be seen that, in FIG. 5, a clear character with no voids is formed, whereas in FIG. 6, voids have occurred. The direction in which the transfer roller is separated from the photoconductor may be upstream or downstream of the photoconductor with respect to the conveyance direction of the paper P. However, when a transfer roller is provided on the upstream side of the photoconductor, the influence of the electric field formed between the transfer roller and the photoconductor before the transfer paper reaches the photoconductor affects the developer image on the photoconductor. Reach For this reason, the developer on the photoconductor is easily scattered on the transfer paper P, which causes deterioration of image quality. Therefore, it is desirable to dispose the transfer roller on the downstream side of the photoconductor as in this embodiment.

Furthermore, in this embodiment, when a voltage is applied to the transfer roller, an electric field is formed between the transfer roller and the photoconductor, and the developer image on the photoconductor is transferred to the paper P by the action of this electric field. Electrostatically transferred onto. Therefore, when the transfer roller is provided separately from the photosensitive member, the transfer electric field becomes smaller as the distance becomes larger. For this reason, in this embodiment, in order to maintain a constant transfer efficiency even in the downstream side, a larger bias voltage is applied to the downstream transfer roller.

In this embodiment, the transfer roller 23a is provided directly below the photosensitive drum 3a, but the transfer roller 23a may be provided separately from the photosensitive drum 3a as shown in FIG. . When all the transfer rollers are separated from the photoconductor in this way, the separation distance is B ₂ = 2A ₂ ,
C ₂ = 3A ₂ , D ₂ = 4A ₂ may be increased at equal intervals, or B ₂ = 2A ₂ , C ₂ = 3A ₂ , D ₂ =
It does not have to be evenly spaced, such as 5A ₂ .

Further, it is considered after the transfer of the third color that the developer layer, which is laminated by the multiple transfer and increases in thickness, has a great influence on the pressure between the photosensitive member and the transfer roller, as shown in FIG. You may comprise.

That is, the transfer roller 23a and the transfer roller 23
For b, it is separated from the photosensitive member by almost the same distance A ₃ , connected to the same power source 45, and the transfer roller 2
3c, B larger than A ₃ for the transfer roller 24d
It is separated from the photosensitive member by a distance of ₃ and is connected to the same power source 47. Even with such a configuration, it is possible to prevent a hollow image and form an image of a practical level.

The distance for separating the transfer rollers is about 0.1 to 1.0 cm when the volume resistance of the conveyor belt 11 is in the range of 10 ⁹ to 10 ¹³ Ω · cm and the transfer bias voltage is in the range of 500 to 3000 v. Good.

As the material of the conveyor belt 11, in addition to polyimide in which carbon is dispersed, conductive particles such as carbon are dispersed in a high resistance derivative sheet of polyethylene terephthalate, polycarbonate, polytetrafluoroethylene (Teflon) or the like. It may be one. Instead of using conductive particles, a polymer film whose electric resistance is adjusted by adjusting the composition may be used. Further, such a polymer film mixed with an ion conductive substance, or a rubber material such as silicon rubber or urethane rubber having a relatively low electric resistance may be used.

The toner used in the present invention is a polymerized toner having a practical sphericity of 0.9 or more obtained by measuring a projected view of particles by an image analyzer, and a toner having a particle diameter of 7.5 μm is used. did. By using a spherical toner, the fluidity of the toner is improved, the releasability of the toner from the photoconductor is improved, and the toner with a large particle size that does not impair the high image quality is used, Made less susceptible to the effects of electric field irregularities. These conditions also contribute to the prevention of "middle-out".

In this embodiment, as the transfer means,
Although the transfer roller is used, the same effect as that of this embodiment can be obtained even if the brush is a conductive brush. Next, the second embodiment will be described in detail with reference to FIG.

The structure of the image forming apparatus shown in FIG. 9 is almost the same as that of the image forming apparatus shown in FIG. That is, the process units 50a, 50b, 50c and 50d are connected to the conveyor belt 61.
Each of the process units provided on the photoconductor drum 5
It has 1a, 51b, 51c and 51d.

The conveyor belt 61 moves in the direction of the arrow e in the figure by the rotation of the driving roller 65 and the driven roller 67. Transfer rollers 73a, 73b, 73c, 7 are provided below the respective photosensitive drums along the moving direction of the conveyor belt 61.
3d is provided. Each transfer roller is connected to a positive DC bias power source 75a, 75b, 75c, 75d.
1,200 V for b, 1,400 for transfer roller 73c
V and 1,600 V are applied to the transfer roller 73d. The transfer roller is a sheet P conveyed on the conveyor belt 61.
The developer images formed on the respective photoconductor drums are transferred onto the upper surface thereof in a multiple transfer manner.

In FIG. 9, a paper feed cassette 75 for accommodating the paper P is provided on the right side of the conveyor belt 61, and the paper P is taken out from the paper feed cassette 75 by the rotation of the pickup roller 77 and sent to the registration roller pair 79. . The registration roller pair 79 supplies the sheet P onto the conveyor belt 61 at a predetermined timing.

A suction roller 93 for sucking the paper P on the surface of the conveyor belt 61 is provided on the conveyor belt 61.
This suction roller is connected to a negative DC bias power supply 95.

Further, in FIG. 9, a fixing device 83 is provided on the left side of the conveyor belt to fix the developer images transferred on the sheet P onto the sheet P, and the fixed sheet P is discharged onto the sheet discharge tray 84.
The configuration for discharging the same is the same as in FIG. The structure of each transfer roller is the same as that of the transfer roller shown in FIGS.

In the second embodiment, the transfer roller 73
a, 73b, 73c and 73d are all photosensitive drums 51
a, 51b, 51c, and 51d are directly underneath, and the process belts 50a, 50b, 50
c and 50d, respectively. However, the second
In the above embodiment, the elastic force of the spring 45 as the urging means shown in FIG. 2 is changed so that the urging force of each transfer roller to the conveyor belt is different.

Table 1 shows the biasing force of each transfer roller.
As shown in Table 1, when the biasing force of the transfer roller on the downstream side of the sheet P in the conveying direction is smaller, the hollow portion during printing did not occur. However, when all the transfer rollers were urged with the same size, the characters were missing and good printing could not be performed.

[0059]

[Table 1]

The bias of the transfer roller depends on the size of the apparatus, the material, diameter and hardness of the transfer roller, but is 300 to 15
It may be pressed with a force of about 00 g. Also in the second embodiment, it is desirable that the value of the bias voltage applied to each transfer roller is set to be larger for the transfer roller located downstream. In the downstream, a transfer electric field is formed between the transfer roller and the photoconductor in a state where a large amount of the developer is carried on the paper P, and thus the transfer electric field is weakened by the developer on the paper P. Therefore, when the same bias voltage is applied to all the transfer rollers, the transfer efficiency is lowered in the downstream, so that the above configuration is desirable.

Finally, a third embodiment will be described with reference to FIG. The image forming apparatus of the third embodiment is substantially the same as the image forming apparatus shown in FIG. 9, but the transfer rollers of the transfer rollers are configured to have substantially the same biasing force. Then, in the third embodiment, the photosensitive drums 51a, 51a
The peripheral speeds of b, 51c, and 51d are all different, and the photosensitive drum 51a is 48.7 mm / sec.
1b is 50.3 mm / sec, and the photosensitive drum 51c is 5 mm.
1.9 mm / sec, photosensitive drum 51d is 53.5 m
Each rotates at m / sec.

In the third embodiment, the conveyor belt 61 is configured to move at a peripheral speed of 48.7 mm / sec by the rotation of the driving roller 65. However, each photosensitive drum is moved at the peripheral speed. By rotating the paper P
The photosensitive drum located closer to the downstream side in the transport direction of the
It has a large relative speed of 1.

As described above, the downstream side of the photosensitive member convey belt 6
When the relative speed with respect to 1 is increased, a large frictional force is generated between the paper P conveyed on the conveyor belt 61 and the photosensitive drum toward the downstream side. By applying a bias voltage to the transfer roller in the state where such a large frictional force is generated, separation of the developer image on the photosensitive drum is promoted even on the downstream photosensitive drum, and the developer on the paper P is developed. The image is transferred without causing the hollow image phenomenon.

The peripheral speed of the conveyor belt is usually 30 to 200 mm
/ Sec, and the photosensitive drum is preferably rotated at a peripheral speed within a range of about ± 10% of the peripheral speed of the conveyor belt. In the third embodiment, it is desirable that the magnitude of the bias voltage applied to each transfer roller be made larger in the downstream transfer roller, as in the first and second embodiments.

As described above, according to each of the embodiments of the present invention, it is possible to perform good transfer with no void. Further, it goes without saying that all or a combination of the configurations found in the first, second and third embodiments may be combined without departing from the spirit of the present invention. For example, in the second and third embodiments, the transfer rollers are all provided directly below the photoconductor, but as in the first embodiment, the distance between the transfer rollers and the photoconductor is farther downstream. You may make it large.

[0066]

As described above, according to the present invention, it is possible to prevent the occurrence of the hollow defect in the transfer which occurs at the time of forming a color image and to obtain a good color image.

[Brief description of drawings]

FIG. 1 is a sectional view of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a configuration of a transfer roller used in an example of the present invention.

FIG. 3 is a cross-sectional view showing a configuration of a transfer roller used in an example of the present invention.

FIG. 4 is a diagram relatively showing a position of a transfer roller in the present invention.

FIG. 5 is a diagram showing characters printed by the image forming apparatus of the present invention.

FIG. 6 is a diagram showing characters printed by an image forming apparatus of a comparative example.

FIG. 7 is a diagram relatively showing the position of the transfer roller in the present invention.

FIG. 8 is a diagram relatively showing the position of the transfer roller in the present invention.

FIG. 9 is a sectional view showing an image forming apparatus according to a second embodiment of the present invention.

[Explanation of symbols]

 3a, 53a ... Photosensitive drum 11 ... Transport belt 23a, 73a ... Transfer roller 25a, 75a ... Power supply

Claims (6)

[Claims] 1. 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. An image forming unit, an image forming medium is sequentially conveyed to the first image carrier and the second image carrier, and an image forming medium is sequentially conveyed to the first image carrier and the second image carrier. The transporting means to be brought into contact with the first image carrier and the position where the first image bearing body is brought into contact with the image-forming medium at a position separated by a first distance along the direction in which the image-forming medium is transported, A first device which is provided so as to face the first image forming device via a conveying device and transfers the first developer image onto the image forming medium.
And a second distance larger than the first distance along the direction in which the image forming medium is conveyed from the position where the second image carrier contacts the image forming medium. A second transfer unit which is provided at a separated position so as to face the second image forming unit via the conveying unit and transfers the second developer image onto the image forming medium. An image forming apparatus characterized by the above. 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. An image forming unit, an image forming medium is sequentially conveyed to the first image carrier and the second image carrier, and an image forming medium is sequentially conveyed to the first image carrier and the second image carrier. A conveying unit having a volume resistance of 10 ⁹ to 10 ¹³ Ω · cm to be brought into contact, and a direction in which the image forming medium is conveyed from a position where the first image carrier comes into contact with the image forming medium. Is provided at a position spaced apart by a first distance along the line so as to face the first image forming unit via the conveying unit, and transfers the first developer image onto the image forming medium. First
And a second distance larger than the first distance along the direction in which the image forming medium is conveyed, from the position where the second image carrier contacts the image forming medium. A second transfer unit which is provided at a spaced position so as to face the second image forming unit via the transport unit, and which transfers the second developer image onto the image forming medium; An image forming apparatus comprising: a first transfer means and a second transfer means, and a voltage applying means for applying a transfer bias voltage. 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. An image forming unit, an image forming medium is sequentially conveyed to the first image carrier and the second image carrier, and an image forming medium is sequentially conveyed to the first image carrier and the second image carrier. A conveyance unit that is in contact with the image forming medium, and the first developer image is provided in contact with the conveyance unit so as to face the first image forming unit through the conveyance unit. The second transfer image is transferred to the first transfer means, and the transfer means is provided so as to be in contact with the transfer means so as to face the second image forming means. A second transfer unit that transfers the image onto the image forming medium; and a first transfer unit that connects the first transfer unit to the transport unit. A first urging means for urging with a urging force, and a second urging means for urging the second transfer means with a second urging force smaller than the first urging force to the conveying means. An image forming apparatus comprising: a biasing unit. 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. An image forming unit, an image forming medium is sequentially conveyed to the first image carrier and the second image carrier, and an image forming medium is sequentially conveyed to the first image carrier and the second image carrier. A transporting unit having a volume resistance of 10 ⁹ to 10 ¹³ Ω · cm to be brought into contact with, and abutting on the transporting unit so as to face the first image forming unit via the transporting unit. A first transfer unit that transfers the first developer image onto the image forming medium; and a second transfer unit that is opposed to the second image forming unit via the transfer unit. Second transfer means provided in contact with each other to transfer the second developer image onto the image forming medium; A first urging unit that urges the transfer unit to the transport unit with a urging force of a first size, and a second transfer unit that causes the second transfer unit to the transport unit and is smaller than the first size. An image forming device comprising: a second urging unit that urges with a large urging force; and a voltage applying unit that applies a transfer bias voltage to the first transfer unit and the second transfer unit. apparatus. 5. A first image forming unit for forming a first developer image on a rotatable first image carrier, and a second developer image on a rotatable second image carrier. And a second image forming unit for forming an image forming medium, the image forming medium is sequentially conveyed to the first image carrier and the second image carrier, and the first image carrier and the second image carrier are conveyed to the first image carrier and the second image carrier. Conveying means for sequentially contacting the image forming medium, first rotating means for rotating the first image carrier at a speed of a first magnitude, and second image carrier for the first image carrier. A second rotation unit that rotates at a second speed larger than the size, and a second rotation unit that is provided so as to face the first image forming unit via the transport unit, and that covers the first developer image. First transfer means for transferring onto the image forming medium, and so as to face the second image forming means via the conveying means. And a second transfer unit for transferring the second developer image onto the image forming medium. 6. A first image forming unit for forming a first developer image on a rotatable first image carrier, and a second developer image on a rotatable second image carrier. And a second image forming unit for forming an image forming medium, the image forming medium being conveyed to the first image carrier and the second image carrier sequentially, Conveying means for sequentially bringing the image-forming medium into contact with the first image carrier and the second image carrier, and rotating the first image carrier at a speed equal to or higher than the predetermined size. First rotation means for rotating the second image carrier, second rotation means for rotating the second image carrier at a second speed higher than a first size, and the first image formation via the transport means. First transfer means provided to face the means and transferring the first developer image onto the image forming medium, A second transfer unit that is provided so as to face the second image forming unit via the transport unit and transfers the second developer image onto the image forming medium. Image forming apparatus.