Electrophotographic color image forming apparatus having a plurality of transfer rollers

An electrophotographic color image forming apparatus, having: a plurality of photosensitive media; a plurality of transfer rollers respectively facing the plurality of photosensitive media; and a belt disposed between the photosensitive media and the transfer rollers and circulating while forming a plurality of transfer nips. When a length of the belt between two neighboring transfer nips is B and a distance between a pair of neighboring photosensitive media is C, B is larger than C, and when respective virtual lines connecting centers of the pair of photosensitive media and centers of corresponding transfer rollers are TP lines and a virtual line connecting the centers of the pair of neighboring photosensitive media is PP line, a pair of neighboring TP lines do not cross the PP line at right angles, and are slanted toward the same side to cross the PP line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 10-2004-0089698, filed on Nov. 5, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrophotographic color image forming apparatus, and more particularly, to an electrophotographic color image forming apparatus including a plurality of photosensitive media, a belt contacting the photosensitive media, and a plurality of transfer rollers facing the photosensitive media as interposing the belt between them.

2. Description of the Related Art

An electrophotographic color image forming apparatus is an apparatus for printing an image by: scanning light onto a photosensitive medium that is charged to a predetermined potential to form an electrostatic latent image on an outer circumferential surface of the photosensitive medium; developing the electrostatic latent image by injecting a toner, that is, a developing agent, into a visible image of a predetermined color; and transferring the visible images of different colors onto paper and fusing the images on the paper.FIG. 1shows a conventional electrophotographic color image forming apparatus.

Referring toFIG. 1, the conventional electrophotographic color image forming apparatus includes: a first photosensitive medium10and a second photosensitive medium11; a first scanner12and a second scanner13scanning light onto the pair of photosensitive media10and11; and four developers15,16,17, and18receiving toners of four colors, that is, yellow (Y), cyan (C), magenta (M), and black (K) colors to form the color image by overlapping colors. The Y and C developers15and16face the first photosensitive medium10, and the M and K developers17and18face the second photosensitive medium11.

In addition, an intermediate transfer belt20, on which the visible images formed on the photosensitive media10and11are transferred, is supported by a driving roller21and an idle roller22to be circulated, and proceeds while contacting lower portions of the pair of photosensitive media10and11. A first transfer roller25and a second transfer roller26are formed so that transfer nips can be formed on respective contacting areas of the intermediate transfer belt20and the first and second photosensitive media10and11. The first and second transfer rollers25and26respectively face the first and second photosensitive media10and11while interposing the intermediate transfer belt20between the rollers and the photosensitive media.

The color image formed on the intermediate transfer roller20by the overlapped transfer operations is transferred onto a sheet of paper (P) passing through the transfer nip between the intermediate transfer belt20and a third transfer roller30. A transfer backup roller23is positioned adjacent to the third transfer roller30with the intermediate transfer belt20interposed therebetween, so that a transfer nip can be formed between the intermediate transfer belt20and the third transfer roller30.

In the above conventional electrophotographic color image forming apparatus, a pair of photosensitive media10and11and a pair of transfer rollers25and26are pressed with each other. In addition, soft rollers having outer circumferential surfaces formed of elastic material such as rubber are used as the transfer rollers25and26to increase the width of the transfer nip.

Accordingly, pressure between the photosensitive media10and11and the intermediate transfer belt20increases, and irregular impact between the photosensitive media10and11and the intermediate transfer belt20increases, which causes a transfer defect and an image jitter. In addition, since the soft roller is used as the transfer roller, the fabrication costs increase, and it is difficult to maintain size accuracy of elements.

SUMMARY OF THE INVENTION

The present invention provides an electrophotographic color image forming apparatus, in which a transfer nip is formed between a photosensitive medium and a belt circulating while contacting the photosensitive medium without pressing the photosensitive medium and the transfer roller to each other.

According to an aspect of the present invention, there is provided an electrophotographic color image forming apparatus having: a plurality of photosensitive media; a plurality of transfer rollers respectively facing the plurality of photosensitive media; and a belt disposed between the photosensitive media and the transfer rollers and circulating while forming a plurality of transfer nips, wherein when a length of the belt between two neighboring transfer nips is B and a distance between a pair of neighboring photosensitive media is C, B is larger than C, and when respective virtual lines connecting centers of the pair of photosensitive media and centers of corresponding transfer rollers are TP lines and a virtual line connecting the centers of the pair of neighboring photosensitive media is a PP line, a pair of neighboring TP lines do not cross the PP line at right angles, and are slanted toward the same side to cross the PP line.

According to one aspect, crossing angles θ formed by the crossing of the pair of TP lines and the PP line may be the substantially same as each other.

According to one aspect, when a distance between a rotary center of one photosensitive medium and a rotary center of the corresponding transfer roller is L, a diameter of the photosensitive medium is D1, a diameter of the transfer roller is D2, and a thickness of the belt is t, a formula L>((D1+D2)/2)+t>Lsin θ is satisfied.

According to one aspect, the transfer rollers are not made of an elastomer. According to one aspect, the transfer rollers are made of metal.

According to one aspect, apparatus additionally has a belt driving roller driving the belt to be circulated, wherein when a diameter of the belt driving roller is D0, B is substantially the same as a value of πD0.

According to one aspect, a value of B/πD0 satisfies a formula, 0.97≦B/πD0≦1.03.

According to another aspect of the present invention, there is provided an electrophotographic color image forming apparatus having: a plurality of photosensitive media; a plurality of transfer rollers respectively facing the plurality of photosensitive media; and a belt disposed between the photosensitive media and the transfer rollers and circulating while forming a plurality of transfer nips, wherein when a distance between a center of one photosensitive medium and a center of a corresponding transfer roller is L, a diameter of the one photosensitive medium is D1, a diameter of the corresponding transfer roller is D2, a thickness of the belt is t, and a crossing angle formed by crossing a virtual line connecting a center of the one photosensitive medium and a center of the corresponding transfer roller with a virtual line connecting centers of the one photosensitive medium and a neighboring photosensitive medium is θ, a formula L>((D1+D2)/2)+t<Lsin θ is satisfied.

According to one aspect, crossing angles θ formed by crossing respective virtual lines connecting centers of neighboring photosensitive media and centers of corresponding transfer rollers with the virtual line connecting the centers of the neighboring photosensitive media are substantially the same.

According to still another aspect of the present invention, there is provided an electrophotographic color image forming apparatus having: a plurality of photosensitive media; a plurality of transfer rollers respectively facing the plurality of photosensitive media; and a belt disposed between the photosensitive media and the transfer rollers and circulating while forming a plurality of transfer nips, wherein when a length of the belt between two neighboring transfer nips is B and a distance between a pair of photosensitive media is C, B is larger than C, and the transfer rollers are made of metal.

DETAILED DESCRIPTION

Referring toFIG. 2, an electrophotographic color image forming apparatus100includes four developers110,120,130, and140receiving toner in a case101, two optical scanners159and169, and an intermediate transfer belt170.

The developers110,120,130, and140are cartridge types, which are substituted by new ones when the toner, that is, developing agent, is exhausted. The developers110,120,130, and140include the toners of different colors, for example, yellow (Y), cyan (C), magenta (M), and black (K), respectively. Hereinafter, the developer110including the Y color toner is referred to as first developer, the developer120including the C color toner is referred to as second developer, the developer including the M color toner is referred to as third developer, and the developer including the K color toner is referred to as fourth developer. Each of the developers110,120,130, and140includes an agitator, a supplying roller, a developing roller, and a doctor blade to supply the toner to the photosensitive media157and167.

The photosensitive media157and167are cylindrical metallic drums having respective outer circumferential surfaces, on which a photosensitive material layer is formed, and which are exposed by light scanned from the optical scanners159and169to form an electrostatic latent image. The first photosensitive medium157, located at an upper side is charged by a first charging roller155, and contacts the first and second developers110and120to develop images by receiving the Y and C color toners. The second photosensitive medium167, located at a lower side, is charged by a second charging roller165, and contacts the third and fourth developers130and140to develop images by receiving M and K color toners.

The optical scanners159and169are units to scan the lights corresponding to image information onto the photosensitive media157and167, and each optical scanner generally includes a light source formed of laser diode (LD), an optical deflector having a polygonal rotating mirror, and an f-θ lens compensating an aberration of the light that is defectively scanned therein. The first optical scanner159, located at the upper side, scans the light onto the first photosensitive medium157, and the second optical scanner169, located at the lower side, scans the light onto the second photosensitive medium167.

First and second cleaning blades152and162, which remove used-toner that is not transferred on the intermediate transfer belt170by raking the toner from the photosensitive media157and167, are respectively positioned under the first and second photosensitive media157and167. And first and second used-toner storages150and160, storing the removed toner, are also respectively positioned under the first and second photosensitive media157and167.

The intermediate transfer belt170is supported by a belt driving roller171connected to a motor shaft (not shown) to rotate, a transfer backup roller172, that is, an idle roller, and first and second supporting rollers173and174, that is, the idle rollers to circulate in a clockwise direction. The first and second photosensitive media157and167contact the intermediate transfer belt170that proceeds from upward to downward to form intermediate transfer nips (Ni, refer toFIG. 3). First and second transfer rollers175and176that face the first and second photosensitive media157and167as interposing the intermediate belt170therebetween are disposed inside the intermediate transfer belt170. The first and second transfer roller175and176support the intermediate transfer nips Ni to be formed.

A third transfer roller180is disposed under the transfer backup roller172as interposing the intermediate transfer belt170therebetween. The third transfer roller180is separated from the intermediate transfer belt170while the images of four-colors are transferred onto the intermediate transfer belt170, and contacts the intermediate transfer belt170to form a final transfer nip (Nf, refer toFIG. 3) after the full-color image is formed by transferring and overlapping the images of four colors.

In addition, the electrophotographic color image forming apparatus100includes: a fusing apparatus185fusing the color image onto a sheet of paper (P) using heat and pressure; a paper cassette105, on which the paper (P) is loaded; a pickup roller182picking the sheets of paper from the paper cassette105one by one; a paper aligner184aligning and conveying the picked paper (P); and first, second, and third discharging rollers186,187, and188discharging the paper (P), on which the color image is printed, out of the case101.

Hereinafter, processes of printing the color image of the electrophotographic color image forming apparatus100will be described.

The color image information is formed by mixing image information corresponding to the Y, M, C, and K colors. In the present embodiment, the images of four colors are transferred to be overlapped with each other to form the color image, and the color image is transferred onto the paper (P) and fused on the paper to print the color image.

When the light corresponding to the Y color image information is scanned from the first optical scanner159onto the first photosensitive medium157that is charged to be even potential, the resistance at the portion where the light is scanned is reduced, and electric charges (electrons) attached on the outer circumferential surface of the first photosensitive medium157escape. Therefore, electric potential difference is generated between the portion where the light is scanned and the portion where the light is not scanned, and accordingly, an electrostatic latent image is formed on the outer circumferential surface of the first photosensitive medium157that rotates. Here, the Y toner is supplied to the first photosensitive medium157by the first developer to develop the electrostatic latent image into the Y color visible image. In addition, since the first photosensitive medium157rotates, the Y color image is transferred onto the intermediate transfer belt170through the intermediate transfer nip (Ni, refer toFIG. 3).

Similarly, the M color image is transferred onto the intermediate transfer belt170from the second photosensitive medium167and overlaps with the Y color image. After circulating one period, on the intermediate transfer belt170, the C color image is transferred from the first photosensitive medium157and the K color image is transferred from the second photosensitive medium167, and these are overlapped with each other and the Y and M color images to form the color image.

The paper (P) loaded on the paper cassette105is picked by the pickup roller182one by one, is aligned by the paper aligner184, and passes through the final transfer nip (Nf, refer toFIG. 3) between the third transfer roller180and the intermediate transfer belt170, thus the color image is transferred onto the paper (P). The color image is fused onto the paper (P) by the heat and pressure in the fusing apparatus185, and the paper (P) is discharged onto a discharged paper board102out of the case101by the discharging rollers186,187, and188.

Referring toFIG. 3, the intermediate transfer belt170cannot proceed straightly around the pair of intermediate transfer nips (Ni), but is curved by contacting to the outer circumferential surfaces of the photosensitive media157and167. Therefore, if a length of the intermediate transfer belt170between the pair of intermediate transfer nips (Ni) is B, and a distance between the first and second photosensitive media157and167is C, B is longer than C because of the curved portion. In more detail, C is the same as the distance from a rotating center of the first photosensitive medium157to a rotating center of the second photosensitive medium167.

The first and second transfer rollers175and176are slanted, or disposed, toward the corresponding first and second photosensitive media157and167so that the intermediate transfer belt170can proceed in a curve by contacting the outer circumferential surfaces of the first and second photosensitive media157and167. In more detail, if virtual straight lines connecting the rotating centers of the photosensitive media157and167and the rotating centers of corresponding transfer rollers175and176are TP lines191and192, and a virtual straight line connecting the rotating centers of the photosensitive media157and167is PP line194, the pair of TP lines191and192do not cross the PP line194at the right angles, but cross the PP line194slanted downward. In addition, crossing angles θ formed by crossing the TP lines191and192toward the PP line194are substantially the same as each other. Here, substantially the same means that the compared items are the same as each other within a tolerance range.

The transfer rollers175and176are not adhered to the corresponding photosensitive media157and167, and accordingly, the intermediate transfer belt170is not pressed when it passes between the transfer rollers175and176and photosensitive media157and167. The relation will be described using an equation referring toFIG. 4. If a distance between the rotating center of the first photosensitive medium157and the rotating center of corresponding first transfer roller175is L, a diameter of the first photosensitive medium157is D1, a diameter of the first transfer roller175is D2, a thickness of the intermediate transfer belt170is t, and the crossing angle formed by the crossing of the TP line191and the PP line194is θ, L is larger than ((D1+D2)/2)+t and ((D1+D2)/2)+t is larger than Lsin θ. In addition, referring toFIG. 3, the crossing angles θ formed by the crossing of the pair of TP lines191and192and the PP line194are the substantially the same.

According to the above structure, the intermediate transfer belt170can widely contact the first photosensitive medium157along the outer circumferential surface of the first photosensitive medium157even though the intermediate transfer belt170is not pressed by the first transfer roller175, thus the wide intermediate transfer nip (Ni) can be formed and the image transferring efficiency can be improved. In addition, since the first photosensitive medium157is not pressed by the first transfer roller175, irregular impact on the first photosensitive medium157and the intermediate transfer belt170can be reduced, and accordingly, the generation of transfer defect and image jitter can be reduced.FIG. 4shows the relation between the first photosensitive medium157, the first transfer roller175, and the intermediate transfer belt170, however, the relation can be similarly applied to the second photosensitive medium167, the second transfer roller176, and the intermediate transfer belt170.

Since the color image forming apparatus100of the present invention does not have the structure where the transfer nip is formed by the pressure between the transfer roller and the photosensitive medium, the first and second transfer rollers175and176can be formed using hard rollers. Therefore, the rollers can be formed of metal, such as steel, since the fabrication costs of the rollers are low and size controlling can be performed easily, instead of using an elastomer that increases the fabrication costs and cannot maintain the accuracy of sizes.

Referring toFIG. 3, according to one embodiment, if a diameter of the belt driving roller171is D0, a length of circumference of the belt driving roller171is πD0, and the length πD0 is substantially the same as the length B of the intermediate transfer belt170between the pair of intermediate transfer nip (Ni). Therefore, even if impacts occur regularly whenever the belt driving roller171rotates, the effects of the impacts to the alignment of the images of four colors can be restricted, thereby preventing the image jitter from generating. As described above, ‘substantially the same’ means that compared items are the same as each other within a tolerance range. And desirably, B/πD0 is maintained within a range of 0.97˜1.03.

The electrophotographic color image forming apparatus of the present invention has the following effects. The image transferring efficiency can be improved since the transfer nip is wider than that of the conventional art. In addition, since the transfer roller does not press the photosensitive medium, the impact onto the photosensitive medium and the transfer belt can be reduced, thus the generation of the transfer defect and the image jitter can be prevented. Further, since the transfer roller can be the idle roller, an additional driving unit for the transfer roller is not required. Further still, since the hard roller can be used as the transfer roller, the fabrication costs can be reduced, and maintaining the accuracy of the sizes of elements is more readily accomplished.