Electrostatic transfer type liquid electrophotographic printer

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid electrophotographic printer and, more particularly, to an electrostatic transfer type liquid electrophotographic printer adopting a photoreceptor web as a photoreceptor medium.

2. Description of the Related Art

Electrophotographic printers such as laser printers output a desired image by forming a latent electrostatic image on a photoreceptor medium such as a photoreceptor drum or electroreceptor web, and developing the latent electrostatic image with a predetermined color toner. Electrophotographic printers are classified into a dry type or liquid type according to the toner used. For the liquid type printer, which uses an ink containing liquid carrier and solid toner in a predetermined ratio, it is easy to implement a color image with excellent print quality, compared with the dry type printer which uses solid toner. Electrophotographic printers are classified into a press transfer type and electrostatic transfer type according to the toner image transfer manner. To the press transfer type, after drying a toner image, the dried toner image is hot pressed by a transfer roller such that the image is transferred to a printer paper. The electrostatic transfer type printer transfers a toner image to a print paper by electric force.

FIG. 1

shows an example of a conventional electrostatic transfer type liquid electrophotographic printer, which adopts photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d

as photoreceptor media. As shown in

FIG. 1

, this printer has a plurality of image forming units

1

a

,

1

b

,

1

c

and

1

d

for developing and transferring a predetermined color image to a print paper P. For a color printer, the four image forming units

1

a

,

1

b

,

1

c

and

1

d

for a color image development and transfer are arranged in a line in the direction of transferring the print paper P such that toner images are sequentially developed into four colors, yellow (Y), magenta (M), cyan (C), and black (K) to form a multi-color image. Reference numeral

2

denotes a feed belt

2

for feeding the print paper P.

The image forming units

1

a

,

1

b

,

1

c

and

1

d

include photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d

on the surface of which a latent electrostatic image is to be formed, main chargers

20

a

,

20

b

,

20

c

and

20

d

being installed adjacent to the corresponding photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d

to charge the surfaces of the photoreceptor drums

10

a

,

10

b

,

10

c

, and

10

d

to a predetermined potential, and laser scanning units (LSUs)

30

a

,

30

b

,

30

c

and

30

d

which scan light beams onto the surfaces of the respective photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d

to form a latent electrostatic image thereon. Development units

50

a

,

50

b

,

50

c

and

50

d

which develop the latent electrostatic images into toner images with a predetermined color ink are installed below the respective photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d

. Transfer chargers

70

a

,

70

b

,

70

c

and

70

d

which transfer the developed toner images formed on the respective photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d

to a print paper P by electric force are spaced a predetermined distance apart from the surface of the corresponding facing photoreceptor drums

10

a

,

10

b

,

10

c

and

10

d.

The structure of the development units

50

a

,

50

b

,

50

c

and

50

d

will be described with reference to the development unit

50

a

for yellow (Y) toner image (referred to as Y-development unit

50

a

). Referring to

FIG. 2

, a developer roller

51

, a squeeze roller

52

and a setting roller

53

are installed in the Y-development unit

50

a

. An ink supply unit

57

for supplying an ink to the developer roller

51

is installed adjacent to the developer roller

51

. Scrapers

54

,

55

and

56

are attached to the lower portion of the developer roller

51

, the squeeze roller

52

and the setting roller

53

, respectively, to scrap off the ink adhering to the surface of the corresponding rollers.

Development of a Y-toner image by the Y-development unit

50

a

having the configuration above will be described in greater detail. First, as the surface of the photoreceptor drum

10

a

charged to a predetermined potential by a main discharger

20

a

is irradiated by a light beam from the LSU

30

a

, a latent electrostatic image corresponding to the yellow color is formed. The developer roller

51

of the Y-development unit

50

a

rotates counterclockwise while being separated by a predetermined distance from the photoreceptor drum

10

a

. As an ink is supplied to the rotating developer roller

51

from the ink supply unit

57

, the ink is carried to the gap between the photoreceptor drum

10

a

and the developer roller

51

by the rotation of the developer roller

51

. The toner particles of the ink adhere to the latent electrostatic image formed on the photoreceptor drum

10

a

, so that a toner image is formed. At this time, the surface of the developer roller

51

is charged to a predetermined development potential such that the toner selectively adheres to only the latent electrostatic image, not to a non-image region.

The squeeze roller

52

removes excess liquid carrier from the photoreceptor drum

10

a

while being separated by a predetermined distance from the photoreceptor drum

10

a

and rotating clockwise.

The setting roller

53

rotates counterclockwise while being separated by a predetermined distance from the photoreceptor drum

10

a

, and creates an electric field between the photoreceptor drum

10

a

and the setting roller

53

with application of a predetermined voltage. The binding force between toner particles becomes strengthened by the electric field produced between the setting roller

53

and the photoreceptor drum

10

a

. Adhesiveness of the toner image to the photoreceptor drum

10

a

also increases. As a result, although an excessive amount of liquid carrier remains on the surface of the photoreceptor drum

10

a

for a subsequent electrostatic transfer, the shape and location of the toner image can be kept intact.

Once the toner image is set by the setting roller

53

, the toner image is transferred to a print paper P by the electric field produced by the transfer charger

70

a

to which a potential is applied such that the transfer charger

70

a

is charged to the opposite polarity to the toner.

After a Y-toner image is transferred to the print paper P by the Y-image forming unit

1

a

, a magenta (M)-toner image is developed and transferred to the print paper P by the M-image forming unit

1

b.

As previously described, four toner images in Y, M, C and K are sequentially transferred to a predetermined area on the print paper P feed by a feed belt

2

in accordance with the print paper feed rate, so that a color image is printed on the print paper P. Because a large amount of liquid carrier remains on the resulting color image, a drying process is performed by a drying unit (not shown).

The conventional electrostatic transfer type liquid electrophotographic printer having the configuration described above has the following problems. First, since the conventional printer uses four photoreceptor drums as photoreceptor media, each for a particular color toner image, the multi-color toner images on the four photoreceptor drums must be sequentially transferred to a moving print paper with a predetermined time gap. The respective color toner images are separately transferred, and thus it is difficult to accurately transfer each of the color toner images in a particular area on the print paper in accordance with the print paper feed rate. In other words, an accurate registration control on the development and transfer processes performed by each image forming unit is difficult.

Second, four toner image transfer processes are carried out on a print paper feed by a feed belt, so that the print paper contacts the liquid carrier adhering to the surface of the photoreceptor drums four times. As a result, unnecessary consumption of the liquid carrier increases and the wetness of the print paper also increases.

Third, because the squeeze roller removes liquid carrier in a non-contact manner with respect to the photoreceptor drums, the amount of the liquid carrier remaining on the surface of the photoreceptor drums is nonuniform for all the image forming units. As a result, toner image transfer efficiency differs from color to color.

SUMMARY OF THE INVENTION

To address the above limitations, it is an object of the present invention to provide an electrostatic transfer type liquid electrophotographic printer which uses a photoreceptor web circulating around a continuous path as a photoreceptor medium.

To achieve the objective of the present invention, there is provided an electrostatic transfer type liquid electrophotographic printer comprising: a photoreceptor web circulating around a continuous path; a main charger for charging the surface of the photoreceptor web to a predetermined potential; a plurality of laser scanning units (LSUs) for sequentially forming a plurality of latent electrostatic images by scanning a light beam onto the charged surface of the photoreceptor web; a plurality of developer units arranged in series in the circulation direction of the photoreceptor web, for sequentially developing the plurality of latent electrostatic images into multi-color toner images with inks containing a liquid carrier and charged toner, thereby forming overlapping multi-color toner images on the photoreceptor web; a concentration control unit for controlling the concentration of the multi-color toner images to be suitable for electrostatic transfer by adjusting the amount of the liquid carrier applied to the overlapping toner images formed on the photoreceptor web; and an electrostatic transfer unit for forming an electric field between the photoreceptor web and the same and transferring the overlapping toner images formed on the photoreceptor web to a print paper by electric force.

In one embodiment, the concentration control unit may be installed in the last development unit of the plurality of the development units. It is preferable that the concentration control unit may be a concentration control belt rotating by being supported by at least two rollers while being separated by a predetermined distance from the photoreceptor web. Alternatively, the concentration control unit may be a concentration control roller having a diameter two times larger than the diameter of the developer roller, and rotating while being separated by a predetermined distance from the photoreceptor web.

In another embodiment, the concentration control unit may be spatially separated from the plurality of development units. In this case, the concentration control unit may include a carrier reservoir for storing a liquid carrier, and the concentration control belt or concentration control roller as described previously. The concentration control unit may further comprise a carrier supply nozzle for supplying the liquid carrier into the gap between the photoreceptor web and the concentration control belt. The concentration control belt and the concentration control roller allow the liquid carrier supplied into the gap between the photoreceptor web, and the concentration control belt and the concentration control roller to permeate into the toner images formed on the photoreceptor web.

The electrostatic transfer type electrophotographic printer according to the present invention may further comprise a setting roller for setting the shapes of the toner images formed on the photoreceptor web, wherein the surface of the setting roller is charged to a potential having the same polarity as the toner. It is preferable that the setting roller is installed while being separated from the photoreceptor web to the extent that the setting roller does not contact the liquid carrier layer on the photoreceptor web.

As the electrostatic transfer unit, an electrostatic transfer roller rotating in contact with the photoreceptor web, or a transfer charger installed facing to the photoreceptor web while being separated by a predetermined distance from the photoreceptor web may be used. A predetermined voltage, for example, of −900V—2 kV, having an opposite polarity to the toner, is applied to the electrostatic transfer roller and the transfer charger.

It is preferable that the electrostatic transfer type liquid electrophotographic printer further comprises a pre-conditioning unit for cleaning the surface of the photoreceptor web and forming a liquid carrier layer on the surface before development of the toner images.

According to the present invention, a color image can be obtained by sequentially forming multi-color toner images on the surface of the photoreceptor web, such that the toner images overlap each other. The multi-color toner images can be transferred to a print paper P by just one transfer process. Thus, registration in developing and transferring multi-toner images can be easily controlled. Also, wetness of the print paper and liquid carrier consumption decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings, in which:

FIG. 1

is a schematic view showing the structure of an example of a conventional electrostatic transfer type liquid electrophotographic printer;

FIG. 2

is a detailed view of a development unit of

FIG. 1

;

FIG. 3

is a schematic view of an embodiment of an electrostatic transfer type liquid electrophotographic printer according to the present invention;

FIG. 4

is a view of another example of the electrostatic transfer unit of

FIG. 3

;

FIG. 5

is a detailed view of the structure of a development unit of

FIG. 3

;

FIG. 6

is a partial detailed view of the development unit of

FIG. 3

for illustrating the development process in the liquid electrophotographic printer according to the present invention;

FIG. 7A

is a view of the structure of the concentration control unit of

FIG. 3

, and

FIG. 7B

is a detailed view illustrating the function of the concentration control unit of

FIG. 7A

;

FIG. 8A

is a view of another example of the concentration control unit of

FIG. 3

, and

FIG. 8B

is a detailed view illustrating the function of the concentration control unit of

FIG. 8A

;

FIG. 9A

is a view of another embodiment of an electrostatic transfer type liquid electrophotographic printer according to the present invention, and

FIG. 9B

is a detailed view illustrating the function of the concentration control unit of

FIG. 9A

; and

FIG. 10A

is a view of another example of the concentration control unit of

FIG. 9A

, and

FIG. 10B

is a detailed view illustrating the function of the concentration control unit of FIG.

10

A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrostatic transfer type liquid electrophotographic printer according to the present invention now will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

The configuration of an embodiment of an electrostatic transfer type liquid electrophotographic printer according to the present invention is shown in FIG.

3

. As shown in

FIG. 3

, the electrostatic transfer type liquid electrophotographic printer utilizes a photoreceptor web

110

as a photoreceptor medium. The photoreceptor web

110

circulates around a continuous path by being supported by three rollers

111

,

112

and

113

including a driving roller and a steering roller. A main charger

120

is provided adjacent to the photoreceptor web

110

to uniformly charge the photoreceptor web

110

to a predetermined potential.

Laser scanning units (LSUs)

140

a

,

140

b

,

140

c

and

140

d

for scanning light beams onto the charged photoreceptor web

110

to form a latent electrostatic image, and development units

150

a

,

150

b

,

150

c

and

150

d

for developing the latent electrostatic image as a toner image with a predetermined color ink are provided below the photoreceptor web

110

. To form a multi-color image, for example, four color toner images of yellow (Y), magenta (M), cyan (C) and black (B), four LSUs

140

a

,

140

b

,

140

c

and

140

d

, and four development units

150

a

,

150

b

,

150

c

and

150

d

are provided so that four color toner images are sequentially formed, overlapping each other, and developed into a multi-color image. The four development units

150

a

,

150

b

,

150

c

and

150

d

are arranged below the photoreceptor web

110

, in series in a circulation direction of the photoreceptor web

110

. The structure and operation of the development units

150

a

,

150

b

,

150

c

and

150

d

will be described later in greater detail. In a lower portion of the respective development units

150

a

,

150

b

,

150

c

and

150

d

, ink reservoirs

159

a

,

159

b

,

159

c

and

159

d

which contain Y, M, C and K inks, respectively, are provided. In the inks contained in the ink reservoirs

159

a

,

159

b

,

159

c

and

159

d

, toner charged to a predetermined polarity is dispersed in a liquid carrier. The concentration of ink is in the range of 2.0-3%, and preferably, 2.5%. The term “concentration” in this specification refers to the weight percentage of toner with respect to ink or toner image. Although the toner can be charged to positive or negative potential, the description below will be limited to the toner charged to the positive potential. Also, the four color toner images may be developed in the order of Y, C, M and K.

A concentration control unit

160

for controlling the concentration of toner images to be suitable for an electrostatic transfer process, which will be described later, by adjusting the amount of the liquid carrier of the overlapping toner images formed on the photoreceptor web

110

is provided. To form a distinct color image by the electrostatic transfer of the toner images formed on the photoreceptor web

110

to a print paper P, there is a need to control the concentration of the toner images before the transfer process such that the fluidity of the toner increases. As a result of the experiments, more than 99% transfer efficiency is achieved at a concentration of 20-40%. Transfer efficiency means the percentage of the toner images transferred from the photoreceptor web

110

to a print paper P. If the toner concentration exceeds 40%, the electrostatic transfer process cannot be performed smoothly due to reduced fluidity of the toner, thereby lowering transfer efficiency. If the toner concentration is below 20% by weight, i.e., if the liquid carrier content is too high, toner image leaking may occur on the print paper P due to highly increased fluidity of the toner. In addition, it is very likely that the toner images cannot be kept intact before being transferred to a print paper.

Toner images are sequentially developed by the four development units

150

a

,

150

b

,

150

c

and

150

d

on the surface of the photoreceptor web

110

, so that a toner image formed earlier may be washed off during the development by the ink applied thereon to form a toner image of another color. To prevent this washing-off, there is a need to form a toner image formed earlier with a high toner concentration of, for example, 50%. However, it is very likely that filming of the toner image occurs by the high concentration toner images. Filming refers to the formation of a thin gel film caused by aggregation of toner particles in the toner images. The transfer efficiency becomes lower by this filming.

Accordingly, the concentration control unit

160

increases the fluidity of toner by supplying a sufficient amount of light carrier to an early developed toner image, so that filming of the toner image can be prevented. The concentration control unit

160

controls the toner concentration of the overlapping toner images to be in the range of 20-40% for satisfactory electrostatic transfer. The structure and operation of the concentration control unit

160

will be described later.

The toner images developed on the surface of the photoreceptor web

110

, whose toner concentration has been adjusted to be suitable for electrostatic transfer, are transferred to a print paper P by an electrostatic transfer unit. The electrostatic transfer unit forms an electric field between the photoreceptor web

110

and the electrostatic transfer unit such that the toner images formed on the photoreceptor web

110

are transferred to the print paper P by the electric force. As shown in FIG.

3

, an electrostatic transfer roller

170

may be used as the electrostatic transfer unit. The electrostatic transfer roller

170

rotates in a circulation direction of the photoreceptor web

110

while being in contact with the photoreceptor web

110

, and the print paper P is fed between the electrostatic transfer roller

170

and the photoreceptor web

110

. To create an electric field, a predetermined voltage of −900V—2 kV is applied to the electrostatic transfer roller

170

. The electrostatic transfer roller

170

, at least the surface thereof, is formed of a resistive material having a high resistance of 10

8

-10

9

Ω, for example, of urethane rubber. The reason that a voltage having the opposite polarity to the toner is applied to the electrostatic transfer roller

170

is to attract the toner such that a toner image can be transferred to the print paper P.

Alternatively, a transfer charger

270

, as shown in

FIG. 4

, may be used as the electrostatic transfer unit. The transfer charger

270

is disposed facing the photoreceptor web

110

while being separated by a predetermined distance from the surface of the photoreceptor web

110

. A print paper P passes between the transfer charger

270

and the photoreceptor web

110

. A predetermined voltage of −900V—2 kV is applied to the transfer charger

170

. As a result, the toner images on the surface of the photoreceptor web

110

can be transferred to the print paper P, as described previously.

Turning back to

FIG. 3

, a fusing unit

180

for fusing the toner images transferred to the print paper P may be provided at the paper eject side of the electrostatic transfer roller

170

. The fusing unit

180

may include two fusing rollers

181

and

182

rotating in contact with each other. The two fusing rollers

181

and

182

fix the toner images on the print paper P, which passes between the fixing rollers

181

and

182

, by hot pressing. Reference numeral

190

denotes an eraser for removing the remaining latent electrostatic images from the surface of the photoreceptor web

110

.

The electrostatic transfer type liquid electrophotographic printer according to the present invention may further include a pre-conditioning unit

130

for cleaning the photoreceptor web

110

and forming a liquid carrier film on the surface of the photoreceptor web

110

before development of toner images. The pre-conditioning unit

130

includes a pre-conditioning roller

131

rotating in contact with the photoreceptor web

110

, and a pre-conditioning vessel

132

which contains a liquid carrier C to be supplied to the pre-conditioning roller

131

. A lower portion of the preconditioning roller

131

is immersed in the liquid carrier C to allow the liquid carrier to adhere the surface of the pre-conditioning roller

131

. As the pre-conditioning roller

131

rotates, the liquid carrier C contained in the pre-conditioning vessel

131

is transferred to the surface of the photoreceptor web

110

and forms a thin film thereon. As a result, filming of an early formed toner image on the surface of the photoreceptor web

110

can be retarded.

Hereinafter, the development units

150

a

,

150

b

,

150

c

and

150

d

, and the concentration control unit

160

will be described in greater detail. In the embodiment illustrated in

FIG. 3

, the three development units

150

a

,

150

b

and

150

c

, exclusive of the K-development unit

150

d

(a development unit for black (K)), have the same structure. The concentration control unit

160

is installed in the K-development unit

150

d

. The structure of the three development units

150

a

,

150

b

and

150

c

, which are the same, will be described first with reference to the Y-development unit

150

a

(a development unit for yellow) of FIG.

5

.

Referring to

FIG. 5

, three rollers including a developer roller

151

a

, a toner removal roller

152

a

, and a squeeze roller

153

a

are installed in an upper portion of the Y-development unit

150

a

. The electrostatic transfer type liquid electrophotographic printer according to the present invention employs the development system that uses three rollers

151

a

,

152

a

and

153

a

. The developer roller

151

a

makes the toner particles of the ink to adhere to the latent electrostatic images formed in an image region of the photoreceptor web

110

to form toner images. The toner removal roller

152

a

removes the toner adhering to the non-image region of the photoreceptor web

110

. To end this, a predetermined voltage is applied to the toner removal roller

152

a

. This will be described later. The squeeze roller

153

a

presses a portion of the photoreceptor web

110

in which toner images are formed to squeeze excess liquid carrier from the portion, thereby aggregating the toner particles forming the toner images. A relatively high voltage is applied to the squeeze roller

153

a

such that the photoreceptor web

110

can be charged by the squeeze roller

153

a

to a predetermined potential for another color toner image development. To end this, the squeeze roller

153

a

, at least the surface thereof, is formed of a resistive material with a high resistance of 10

5

-10

7

Ω, and preferably, 10

6

Ω, for example, of urethane rubber.

An ink supply nozzle

158

a

is installed adjacent to the developer roller

151

a

. The ink supply nozzle

158

a

supplies the ink contained in the Y-ink reservoir

159

a

(see

FIG. 3

) in the gap between the photoreceptor web

110

and the developer roller

151

a

. A cleaning roller

154

a

rotating in contact with the developer roller

151

a

is installed below the developer roller

151

a

. The cleaning roller

154

a

removes the ink adhering to the surface of the developer roller

151

. A blade

155

a

is disposed underneath the toner removal roller

152

a

while its one end is in contact with the surface of the toner removal roller

152

a

. A blade

156

a

is disposed underneath the squeeze roller

153

a

while its one end is in contact with the surface of the squeeze roller

153

a

. The two blades

155

a

and

156

a

act to remove the ink or liquid carrier adhering to the surface of the toner removal roller

152

a

and the squeeze roller

153

a

, respectively. As the cleaning means, the cleaning roller

154

a

and the blades

155

a

and

156

a

are interchangeable. Both a cleaning roller and a blade may be installed for each of the rollers

151

a

,

152

a

and

153

a.

Development of a latent electrostatic image into a toner image by the Y-development unit

150

a

having the configuration described previously will be described with reference to FIG.

6

. The photoreceptor web

110

is charged by the main charger

120

to a potential (referred to as a charge potential), for example, of 500-600 volts, and preferably, 550 volts, having the same polarity as the toner. The charged surface of the photoreceptor web

110

is irradiated by a light beams from the Y-LSU (LSU for yellow)

140

a

such that a latent electrostatic image corresponding to yellow color is formed. The Y-LSU

140

a

selectively discharges the surface of the photoreceptor web

110

to form a latent electrostatic image, so that a potential V

BY

Of the image region B

1

, in which the latent electrostatic image is formed, drops to about 100 volts or less (referred to as exposure potential), while a potential V

A

of the non-image region A

1

is maintained at the initial charge potential charged by the main charger

120

.

The latent electrostatic image is developed into a Y-toner image by the Y-development unit

150

a

. In particular, as the photoreceptor web

110

passes over the developer roller

151

, Y-toner adheres to the image region B

1

, in which an electrostatic latent image is formed, to form a Y-toner image. As a predetermined voltage is applied to the developer roller

151

a

, the surface of the developer roller

151

a

is charged to a development potential V

D

of about 350 volts. The development potential V

D

of the development roller

151

a

is determined to be lower than the discharge potential (550 V) of the non-image region A

1

, and to be higher than the exposure potential (100 V) of the image region B

1

. It is preferable that differences between the development potential V

D

and each of the charge potential and the exposure potential are 100 volts or more, and preferably, 200 volts or more. As the potential differences become greater, the affinity of toner particles to the photoreceptor web

110

and the developer roller

151

a becomes more apparent. The developer roller

151

a

rotates in the circulation direction of the photoreceptor web

110

while being separated by a development gap G

D

of 150-200 μm from the photoreceptor web

110

. As the ink containing Y-toner of about 2.5% by weight, which is contained in the Y-ink reservoir

159

a

, is supplied by the ink supply nozzle

158

a

, a nip N

D

as a liquid carrier film having about 6-mm width is formed between the photoreceptor web

110

and the developer roller

151

a.

The toner particles of the ink are charged to positive potential and move in the nip N

D

as follows. The exposure potential V

BY

(100 volts) in the image region B

1

of the photoreceptor web

110

is lower than the development potential V

D

(350 volts) of the development roller

151

a

, so that the toner particles move towards the image region B

1

and adheres to the image region B

1

. The charge potential V

A

(500 volts) in the non-image region A

1

is greater than the development potential V

D

(350 volts) of the developer roller

151

a

, so that the toner particles move towards the developer roller

151

a

and adhere to the developer roller

151

a

. In other words, the toner particles selectively adhere to only the image region B

1

charged to a relatively low potential, so that toner images are formed therein. Excess ink and toner particles stuck to the surface of the developer roller

151

a

are removed by the cleaning roller

154

rotating in contact with the developer roller

151

a.

On the image region B

2

corresponding to the image region B

1

passed through the developer roller

151

a

, an ink layer to be a high-concentration toner image is formed and covered with a liquid carrier layer. On the non-image region A

2

, only a liquid carrier layer is formed. In the image region B

2

passed through the developer roller

151

, the potential V

BY

increases to about 160 voltes. The potential V

A

in the non-image region A

2

drops to about 380 volts. It is desirable that no toner remains in the liquid carrier layers passed through the developer roller

151

a

. In actuality, about 0.5% by weight toner remains in the liquid carrier layers. The remaining toner particles are transferred to the M-development unit

150

b

along the photoreceptor web

110

, and mixed with toner of another color. As a result, the M-development unit

150

b

, C-development unit

150

c

, and K-development unit

150

d

, which are sequentially arranged, and the inks for each color are contaminated by the transfer of toner particles. Thus, there is a need to fully remove the toner particles remaining in the liquid carrier layers.

The toner particles remaining in the liquid carrier layers are removed by the toner removal roller

152

a

disposed adjacent to the developer roller

151

a

. As the photoreceptor web

110

passes the toner removal roller

152

a

, toner particles remaining in the liquid carrier layer in the non-image region A

2

are removed, thereby resulting in a toner-free liquid carrier layer in the non-image region A

2

. In particular, the surface of the toner removal roller

152

a

is charged to a toner removal potential V

R

of about 250 volts with application of a predetermined voltage. The toner removal potential V

R

of the toner removal roller

152

a

is determined to be greater than the exposure potential V

BY

(160 volts) in the image region B

2

and lower than the potential V

A

(380 volts) in the non-image region A

2

. As a potential difference in each region becomes greater, it is much easier to remove the toner particles from the liquid carrier layer. The toner removal roller

152

a

is installed with a gap G

R

of about 150-200 μm from the photoreceptor web

110

. A nip N

R

having a width of 3-5-mm is formed between the toner removal roller

152

a

and the photoreceptor web

110

. The width of the nip N

R

may be varied depending on the diameter of the toner removal roller

152

a

and the size of the gap G

R

. Although the toner removal roller

152

a

can rotate in any direction, it is preferable that the toner removal roller

152

rotates in an opposite direction to the circulation direction of the photoreceptor web

110

for easier formation of the nip N

R

.

In the nip N

R

formed between the photoreceptor web

110

and the toner removal roller

152

a

, the toner particles move as follows. In the non-image region A

2

of the photoreceptor web

110

, the potential V

A

(380 volts) is higher than the toner removal potential V

R

(250 volts) of the toner removal roller

152

a

, so that toner particles dispersed in the liquid carrier layer move towards the toner removal roller

152

a

. The potential V

BY

(160 volts) in the image region B

2

is lower than the toner removal potential V

R

(250 volts) of the toner removal roller

152

a

, so that the toner particles move towards the image region B

2

and adhere to a previously formed toner image. As the toner removal roller

152

a

rotates, the toner particles and liquid carrier adhering to the surface of the toner removal roller

152

a

are removed by the blade

155

a.

As described previously, the toner particles existing in the liquid carrier layer on the non-image region A

2

can be almost completely removed by the toner removal roller

152

a

, so that a toner-free liquid carrier remains in the non-image region A

3

of the photoreceptor web

110

passed through the toner removal roller

152

a

. As a result, the problem of toner transfer to the adjacent development unit can be solved.

As the photoreceptor web

110

a

passes the squeeze roller

153

a

, the toner image region of the photoreceptor web

110

a

is pressed by the squeeze roller

153

a

, so that excess liquid carrier is squeezed from the toner image. In particular, the squeeze roller

153

a

rotates in the circulation direction of the photoreceptor web

110

in contact with the photoreceptor web

110

with a compression force, for example, of about 10 kgf. As a result, the liquid carrier covering the toner image in the image region B

3

of the photoreceptor web

110

, and the liquid carrier adhering to the non-image region A

3

are removed such that just an appropriate amount of the liquid carrier remains therein. Once the photoreceptor web

110

passes the squeeze roller

153

a

, a toner image is formed as an ink layer containing about 50% by weight toner in the image region B

3

of the photoreceptor web

110

. The liquid carrier stuck to the surface of the squeeze roller

153

a

is removed by the blade

156

a

and recovered into the Y-ink reservoir

159

a

. The reason that the concentration of the toner image is increased is to protect the toner image from being washed off by the ink applied to the same to form a toner image in another color.

The squeeze roller

153

a

also acts to charge the photoreceptor web

110

again to a predetermined potential to develop a toner image in another color. To this end, a relatively high voltage is applied to the squeeze roller

153

a

such that the surface of the squeeze roller

153

a

is charged to a squeeze potential V

S

of about 800 volts or more, which is higher than the charge potential. Thus, once the photoreceptor web

110

passes the squeeze roller

153

a

, the potential V

A

in the non-image region A

3

of the photoreceptor web

110

and the potential V

BY

in the image region B

3

are equal to or higher than the charge potential, to allow development of a toner image of another color.

Because the surface of the squeeze roller

153

a

is charged to a relatively high potential, a toner image is formed in the image region B

3

by the repulsive force exerted between the squeeze roller

153

a

and the toner particles, and firmly adheres to the image region B

3

with increased binding force of the toner particles. As a result, no thinning of the toner image at its edges occurs by the pressing of the squeeze roller

153

a

. In addition, washing-off of the toner image by an ink applied to form another toner image does not occur, so that the shape and location of the toner image can be maintained intact.

After a Y-toner image is formed through the steps described above, to develop a toner image of magenta (M), the surface of the photoreceptor web

110

is irradiated by a light beam from the M-LSU

140

b

so that a latent electrostatic image corresponding to a M-toner image is formed. This latent electrostatic image has a potential of about 100 volts, and is developed into a M-toner image by the M-development unit

150

b

in the same manner as for the Y-toner image, as described previously. Next, a toner image of cyan (C) is developed by the C-development unit

150

c.

After toner images are developed in three colors including Y, M and C, a black (K) toner image is developed by the K-development unit

150

d

. The concentration of the overlapping toner images previously formed on the photoreceptor web

110

is adjusted to be suitable for electrostatic transfer by the K-development unit

150

d.

Referring to

FIGS. 7A and 7B

, a developer roller

151

d

and an ink supply nozzle

158

d

are installed in an upper portion of the K-development unit

150

d

. The ink supply nozzle

158

d

supplies the ink contained in the K-ink reservoir

159

d

(see

FIG. 3

) in the gap between the photoreceptor web

110

and the developer roller

151

d

. The developer roller

151

d

develops a latent electrostatic image corresponding to K color, which is formed on the photoreceptor web

110

by the K-LSU

140

d

, into the K-toner image with the ink. A cleaning roller

154

d

for removing the ink stuck to the surface of the development roller

151

d

is installed underneath the development roller

151

d.

As the concentration control unit

160

, a concentration control belt

161

circulating by being supported by two rollers

162

and

163

is installed in the K-development unit

150

d

. The concentration control belt

161

is installed while being separated by a gap G

C1

of 50-100 μm from the photoreceptor web

110

. The gap G

C1

is determined to be smaller than the development gap G

D

of 150-200 μm. It is preferable that the traveling direction of the concentration control belt

161

is opposite to that of the photoreceptor web

110

such that the liquid carrier layer C passed through the concentration control belt

161

becomes as thin as possible with uniformity. The distance between the two rollers

162

and

163

is determined such that the nip N

C1

formed between the concentration control belt

161

and the photoreceptor web

110

has a width of 15 mm or more, preferably, of 20-30 mm. The reason that the nip N

C1

is formed in such a wide width is to allow liquid carrier to uniformly permeate into the toner images for a sufficient period of time.

In general, multi-color toner images are formed as two overlapping layers T

1

and T

2

(first and second toner image layers) on the surface of the photoreceptor web

110

through the development process described previously. To implement a full color image, there is a need to mix two or three colors of Y, M, C and K. Usually, a full color image can be implemented by mixing two colors. This is the reason why the two overlapping layers T

1

and T

2

are formed through the development process. The first toner image layer T

1

is first developed on the surface of the photoreceptor web

110

, and the second toner image T

2

is formed on the first toner image layer T

1

. As previously described, the first and second toner image layers T

1

and T

2

formed by the development process have a toner concentration of about 50%. In particular, for the first toner image layer T

1

which undergoes a few cycles of development because it is formed earlier than other layers, the toner concentration of the first toner image layer T

1

might further increase. For an electrostatic transfer of toner images, there is a need to control the toner concentration of the first and second toner image layers T

1

and T

2

, for example, in the range of 20-40% by weight. In particular, a sufficient amount of liquid carrier is required for the first toner image layer T

1

, such that filming of the first image layer T

1

, which is described previously, can be prevented.

The concentration control belt

161

basically performs the following two functions. As the photoreceptor web

110

passes the developer roller

151

d

of the K-development unit

150

d

, a liquid carrier layer C is formed on the second toner image layer T

2

. Because the K-development unit

150

d

has no toner removal roller and squeeze roller, which are included in the other development units, the liquid carrier layer C retains a relatively large amount of liquid carrier. On the other hand, the gap G

C1

between the photoreceptor web

110

and the concentration control belt

161

is smaller than the development gab G

D

, so that excess amount of the liquid carrier is removed for optimum electrostatic transfer as the photoreceptor web

110

passes the concentration control belt

161

. The removed liquid carrier is carried by being stuck to the surface of the concentration removal belt

161

, and is removed by a blade

164

from the surface of the concentration control belt

161

.

In the nip N

C1

formed between the photoreceptor web

110

and the concentration control belt

161

, the liquid carrier remaining on the second toner image layer T

2

permeates into the second and first toner image layers T

2

and T

1

. Because the width of the nip N

C1

is relatively large, the liquid carrier can infiltrate deeply into the first toner image layer T

1

. As a result, the concentration of the first toner image layer T

1

as well as the second toner image layer T

2

becomes lower to 20-40% by weight so that electrostatic transfer can be smoothly performed with increased fluidity of the toner. The concentration of the overlapping toner images formed on the photoreceptor web

110

is uniformly adjusted by the concentration control belt

161

, so that all the color toner images can be transferred with the same efficiency.

A predetermined voltage may be applied to the surface of the concentration control belt

161

so that the surface is charged to a first potential V

C1

. The first potential V

C1

of the concentration control belt

161

is determined to be higher than the potential in the image region of the photoreceptor web

110

passed through the developer roller

151

d

. When the surface of the concentration control belt

161

is charged to a predetermined first potential V

C1

, the toner particles firmly adhere to the surface of the photoreceptor web

110

by a repulsive force exerted between the concentration control belt

161

and the toner particles of the first and second toner image layers T

1

and T

2

. As a result, although the liquid carrier is sufficiently supplied for the concentration adjustment, the shape of the toner images remains intact.

The K-development unit

150

d

may further include a setting roller

169

. The setting roller

169

is spatially separated from the photoreceptor web

110

to the extent that it does not contact the liquid carrier layer C on the photoreceptor web

110

. The surface of the setting roller

169

is charged to a predetermined second potential V

SET

with application of a voltage. The second potential V

SET

of the setting roller

169

is determined to be higher than the potential in the image region of the photoreceptor web passed through the concentration control belt

161

. The setting roller

169

serves to keep the shape and location of the overlapping toner images on the photoreceptor web

110

, thereby increasing the sharpness of the images transferred to a print paper P.

Another example of the concentration control unit of

FIG. 3

is illustrated in

FIGS. 8A and 8B

. Referring to

FIGS. 8A and 8B

, a developer roller

151

d

, an ink supply nozzle

158

d

and a cleaning roller

154

d

are installed in the K-development unit

250

d

. In the present embodiment, as a concentration control unit, a concentration control roller

261

having a relatively large diameter is installed in the K-development unit

250

d

. The concentration controller roller

261

is installed to be capable of rotating while being separated by a gap G

C2

of 50-100 μm from the photoreceptor web

110

. The gap G

C2

is determined to be smaller than the development gap G

D

, as described previously. It is preferable that the concentration control roller

261

rotates in a direction opposite to the circulation direction of the photoreceptor web

110

for the same reason described as in the previous embodiment. It is preferable that the diameter of the concentration control roller

261

is two times larger than that of the developer roller

151

d

. The concentration control roller

261

has a diameter of 50 mm or more, more preferably, of 60-70 mm. The diameter of the concentration control roller

261

is determined such that the nip N

C2

formed between the photoreceptor web

110

and the concentration control roller

261

has a width of 10 mm or more, more preferably, of 15-20 mm. The nip N

C2

having a relative large width allows the liquid carrier to sufficiently and uniformly permeate into the toner images. The surface of the concentration control roller

261

may be charged to a predetermined first potential V

C2

with application of a voltage. Like the K-development unit

150

d

described in the previous embodiment, the setting roller

169

charged to a predetermined second potential V

SET

may be installed in the K-development unit

250

d.

Operation of the concentration control roller

261

is almost the same as the concentration control belt

161

described in the previous embodiment, and thus the operation of the concentration control roller

261

will be described briefly below. As the photoreceptor web

110

passes the developer roller

151

b

of the K-development unit

250

d

, a liquid carrier layer C that contains an excessive amount of liquid carrier is formed on the surface of the second toner image layer T

2

. The excessive amount of the liquid carrier is removed by the concentration control roller

261

such that an appropriate amount of the liquid carrier for optimum electrostatic transfer remains in the liquid carrier layer C. In the nip N

C2

formed between the concentration control roller

251

and the photoreceptor web

110

, the remaining liquid carrier permeates into the second and first toner image layers T

2

and T

1

. The nip N

C2

is wide enough such that the liquid carrier permeates up to the first toner image layer T

1

for a period of time. As a result, the concentration of the first toner image layer T

1

as well as the second toner image layer T

2

is lower to 20-40% by weight with increased fluidity of the toner, so that an optimum electrostatic transfer can be achieved. Since the surface of the concentration control roller

261

is charged to a predetermined first potential V

C2

, the toner particles firmly adhere to the photoreceptor web

110

, so that the shapes of the toner images remain intact through the concentration control process.

FIGS. 9A and 9B

are partial views of an electrostatic transfer type liquid electrophotographic printer according to another preferred embodiment of the present invention. The same elements as those of the previous embodiment of the liquid electrophotographic printer will not provided here. The elements denoted by the same reference numerals as those of the previous embodiment represents the same elements. Referring to

FIGS. 9A and 9B

, a concentration control unit

360

is installed out of the K-development unit

350

d

. Accordingly, like the other development units, the K-development unit

350

d

just develops a toner image. The concentration of the toner image is controlled by the separate concentration control unit

360

.

Three rollers including a developer roller

151

d

, a toner removal roller

152

d

and a squeeze roller

154

d

are installed in an upper portion of the K-development unit

350

d

. An ink supply nozzle

158

d

is disposed adjacent to the development roller

151

d

, and a cleaning roller

153

, which rotates in contact with the developer roller

151

d

, is installed underneath the developer roller

151

d

. Blades

155

d

and

156

d

are provided underneath the toner removal roller

152

d

and the squeeze roller

153

d

, respectively. These elements of the K-development unit

350

d

are the same and perform the same operations as those of the Y-development unit

150

a

described with reference to

FIG. 5

, and thus detailed descriptions thereof will not provided here.

The concentration control unit

360

includes a carrier reservoir

366

for storage of a liquid carrier C, and a concentration control belt

361

circulating by being supported by two rollers

362

and

363

in the carrier reservoir

366

. A blade

364

may be provided underneath the concentration control belt

361

to remove liquid carrier from the surface of the concentration control belt

361

, wherein one end of the blade

364

is in contact with the surface of the concentration control belt

361

. A setting roller

369

discharged to a predetermined potential V

SET

may be installed in the carrier reservoir

366

. The function of the setting roller

369

is the same as the setting roller

169

described in the previous embodiment.

The concentration control belt

361

is installed while being separated by a gap G

C1

of 50-100 μm from the photoreceptor web

110

, and circulates in an opposite direction to the circulation direction of the photoreceptor web

110

. The gap G

C1

is determined to be smaller than the development gap G

D

, for example, in the range of 150-200 μm. The distance between the two rollers

362

and

363

is determined such that the nip N

C1

formed between the concentration control belt

361

and the photoreceptor web

110

has a width of 15 mm or more, preferably, of 20-30 mm. The liquid carrier layer N

C1

with a relatively large width allows the liquid carrier to sufficiently and uniformly permeate into the toner images for a period of time.

During the development process, multi-color toner images are formed as two overlapping layers T

1

and T

2

on the surface of the photoreceptor web

110

. Unlike the previous embodiment, the K-development unit

350

d

includes a toner removal roller

152

d

and a squeeze roller

153

d

, so that excess liquid carrier does not remain on the surface of the second toner image layer T

2

formed on the photoreceptor web

110

passed through the K-development unit

350

d

. To perform an optimum electrostatic transfer process, there is a need to reduce the concentration of the first and second toner image layers T

1

and T

2

having a relatively high toner concentration of about 50% by supplying liquid carrier thereto. To end this, a carrier supply nozzle

365

for supplying liquid carrier in the gap between the photoreceptor web

110

and the concentration control belt

361

is provided. As the liquid carrier is supplied between the photoreceptor web

110

and the concentration control belt

361

, the nip N

C1

is formed between the photoreceptor web

110

and the concentration control belt

361

. In the nip N

C1

, the liquid carrier permeates into the second and first toner image layers T

2

and T

1

for a sufficient period of time. As a result, the concentration of the second toner image layer T

2

as well as the first toner image layer T

1

becomes lower to 20-40% by weight suitable for optimum electrostatic transfer with increased fluidity of the toner. Instead of using the separate carrier supply nozzle

365

, the concentration control belt

361

can be set such that its bottom surface is dipped into the liquid carrier contained in the carrier reservoir

366

. In this case, the liquid carrier adheres to the surface of the concentration control belt

261

and is transferred to the second and first toner images T

2

and T

1

formed on the photoreceptor web

110

.

The surface of the concentration control belt

361

may be charged to a predetermined first potential V

C1

. In this case, the toner particles of the first and second toner image layers T

1

and T

2

strongly adhere to the photoreceptor web

110

, so that even though sufficient liquid carrier is supplied during a concentration control process, the shapes of the toner images remain intact.

FIGS. 10A and 10B

show a modification of the concentration control unit of

FIGS. 9A and 9B

. Referring to

FIGS. 10A and 10B

, the structure of the K-development unit

350

d

is the same as that of FIG.

9

A. The concentration control unit

460

includes a carrier reservoir

466

for storage of a liquid carrier C, and a concentration control roller

461

having a relatively large diameter, which is installed in the carrier reservoir

466

. The concentration control roller

461

is separated from the photoreceptor web

110

by a predetermined gap G

C2

of 50-100 μm. The concentration control roller

461

is installed such that it can rotate in an opposite direction to the circulation direction of the photoreceptor web

110

. The gap G

C2

is determined to be smaller than the development gap G

D

, as described previously. It is preferable that the diameter of the concentration control roller

461

is two times larger than the diameter of the developer roller

151

d

. The concentration control roller

461

has a diameter of 50 mm or more, preferably, of 60-70 mm. The diameter of the concentration control roller

461

is determined such that the nip N

C2

formed between the concentration control roller

461

and the photoreceptor web

110

has a width of 10 mm, preferably, of 15-20 mm. The nip N

C2

with a relatively large width allows the liquid carrier to sufficient and uniformly permeate into the toner images. The surface of the concentration control roller

461

may be charged to a predetermined first potential V

C2

with application of a voltage. Like the previous embodiment, a setting roller

169

charged to a predetermined second potential V

SET

may be installed in the concentration control unit

460

.

Function of the concentration control roller

461

is the same as that of the concentration control belt

361

described in the previous embodiment, and thus a detailed description thereof will not provided here. According to the present embodiment, there is a need to supplement liquid carrier so as to effectively reduce the concentration of the first and second toner image layers T

1

and T

2

for optimum electrostatic transfer. To achieve this, a lower portion of the concentration control roller

461

is dipped into the liquid carrier C contained in the carrier reservoir

466

for continuous supply of the liquid carrier C. As the concentration control roller

461

rotates, the liquid carrier contained in the carrier reservoir

466

forms a nip N

C2

between the concentration control roller

461

and the photoreceptor web

110

. In the nip N

C2

, the liquid carrier C permeates into the second and first toner image layer T

2

and T

1

for a period of time. As a result, the concentration of the first toner image layer T

1

as well as the second toner image layer T

2

is controlled to be suitable for electrostatic transfer of the toner images. As described with reference to

FIG. 9A

, the carrier supply nozzle

461

for supplying liquid carrier in the gap between the photoreceptor web

110

and the concentration control roller

461

may further provided.

As described previously, the electrostatic transfer type liquid electrophotographic printer according to the present invention has the following advantages. First, because a photoreceptor web is used as a photoreceptor medium, multi-color toner images are sequentially formed on the photoreceptor web such that the toner images overlap each other. The multi-color toner images are simultaneously transferred to a print paper P. Thus, it is easy to control registration in developing and transferring the toner images.

Second, since the multi-color toner images are transferred to a print paper by a single transfer process, the print paper contacts the liquid carrier applied on the photoreceptor web just one time, so that wetness of the print paper by the liquid carrier can be minimized. Also, most of the liquid carrier is recovered in each development unit by the squeeze roller rotating in contact with the photoreceptor web, so that consumption of the liquid carrier decreases.

Third, the concentration of the overlapping toner images formed on the photoreceptor web is uniformly controlled by the concentration control unit before a transfer process, the multi-color toner images can be transferred with the same transfer efficiency.

While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Number	Name	Date	Kind
6049684	Nishikawa et al.	Apr 2000	A
6308034	Nakashima et al.	Oct 2001	B1
6418288	Saitoh	Jul 2002	B2

Electrostatic transfer type liquid electrophotographic printer

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

US Referenced Citations (3)