Electrostatic transfer type liquid electrophotographic printer using a continuous photoreceptor web as a photoreceptor medium

An electrostatic transfer type liquid electrophotographic printer which has a photoreceptor web having a charged surface and opposing back surface, at least one exposing unit for forming a latent electrostatic image onto the charged surface of the photoreceptor web, and at least one development unit for developing the latent electrostatic image on the photoreceptor web into a toner image, wherein each development unit has a developer roller, a toner removal roller, and a squeeze roller, and a backup roller corresponding to at least one of the developer roller, toner removal roller, and squeeze roller, and wherein the photoreceptor web is arranged to provide at least 1 degree of contact wrap around at least one of the backup rollers. The printer further includes an electrostatic transfer unit for transferring the toner images formed in each development unit from the photoreceptor web to a print medium by electrostatic force.

TECHNICAL FIELD

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.

BACKGROUND OF THE INVENTION

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 relatively easy to implement a color image with excellent print quality, compared with the dry type printer which uses solid toner. Electrophotographic printers are also generally classified into an adhesive transfer type and electrostatic transfer type according to the toner image transfer manner. To the adhesive transfer type, after drying a toner image, a transfer roller hot presses the dried toner image such that the image is transferred to a printer paper. The electrostatic transfer type printer transfers a toner image to a print paper by electrostatic forces.

FIG. 1shows an example of a conventional electrostatic transfer type liquid electrophotographic printer, which adopts photoreceptor drums10a,10b,10cand10das photoreceptor media. As shown inFIG. 1, this printer has a plurality of image forming units1a,1b,1cand1dfor developing and transferring a predetermined color image to a print paper P. For a color printer, the four image forming units1a,1b,1cand1dfor 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 numeral2denotes a feed belt2for feeding the print paper P.

The image forming units1a,1b,1cand1dinclude photoreceptor drums10a,10b,10cand10don the surface of which a latent electrostatic image is to be formed, main chargers20a,20b,20cand20dadjacent to the corresponding photoreceptor drums10a,10b,10cand10dto charge the surfaces of the photoreceptor drums10a,10b,10c, and10dto a predetermined potential, and laser scanning units (LSUs)30a,30b,30cand30dwhich scan light beams onto the surfaces of the respective photoreceptor drums10a,10b,10cand10dto form a latent electrostatic image thereon. Development units50a,50b,50cand50dthat develop the latent electrostatic images into toner images with a predetermined color ink are installed below the respective photoreceptor drums10a,10b,10cand10d. Transfer chargers70a,70b,70cand70dwhich transfer the developed toner images formed on the respective photoreceptor drums10a,10b,10cand10dto a print paper P by electric force are spaced a predetermined distance apart from the surface of the corresponding facing photoreceptor drums10a,10b,10cand10d.

The structure of the development units50a,50b,50cand50dwill be described with reference to the development unit50afor yellow (Y) toner image (referred to as Y-development unit50a). Referring toFIG. 2, a developer roller51, a squeeze roller52and a setting roller53are installed in the Y-development unit50a. An ink supply unit57for supplying an ink to the developer roller51is installed adjacent to the developer roller51. Scrapers54,55and56are attached to the lower portion of the developer roller51, the squeeze roller52and the setting roller53, respectively, to scrape off the ink adhering to the surface of the corresponding rollers.

Development of a Y-toner image by the Y-development unit50ahaving the configuration above will be described in greater detail. First, as the surface of the photoreceptor drum10acharged to a predetermined potential by the main charger20aand is irradiated by a light beam from the LSU30a, a latent electrostatic image corresponding to the yellow color is formed. The developer roller51of the Y-development unit50arotates counterclockwise while being separated by a predetermined distance from the photoreceptor drum10a. As ink is supplied to the rotating developer roller51from the ink supply unit57, the ink is carried to the gap between the photoreceptor drum10aand the developer roller51by the rotation of the developer roller51. The toner particles of the ink adhere to the latent electrostatic image formed on the photoreceptor drum10a, so that a toner image is formed. At this time, the surface of the developer roller51is 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 roller52removes excess liquid carrier from the photoreceptor drum10awhile being separated by a predetermined distance from the photoreceptor drum10aand rotating clockwise. The setting roller53rotates counterclockwise while being separated by a predetermined distance from the photoreceptor drum10a, and creates an electric field between the photoreceptor drum10aand the setting roller53with application of a predetermined voltage. The binding force between toner particles becomes strengthened by the electric field produced between the setting roller53and the photoreceptor drum10a. Adhesiveness of the toner image to the photoreceptor drum10aalso increases. As a result, although an excessive amount of liquid carrier remains on the surface of the photoreceptor drum10afor 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 roller53, the toner image is transferred to a print paper P by the electric field produced by the transfer charger70ato which a potential is applied such that the transfer charger70ais 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 unit1a, a magenta (M)-toner image is developed and transferred to the print paper P by the M-image forming unit1b. As previously described, four toner images in Y, M, C and K are sequentially transferred to a predetermined area on the print paper P fed by the feed belt2in 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 drawbacks. 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 fed 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. It is therefore desirable to provide an electrostatic transfer type liquid electrophotographic printer for applying multiple colors to print paper that overcomes the drawbacks discussed above.

SUMMARY OF THE INVENTION

In one aspect of this invention, an electrostatic transfer type liquid electrophotographic printer is provided, which generally includes a photoreceptor web, at least one exposing unit, at least one development unit, and an electrostatic transfer unit. More particularly, the electrophotographic printer of the present invention preferably includes a continuous photoreceptor web having a charged surface and an opposing back surface, wherein the web rotates around a printing path. The printer further preferably includes at least one laser scanning unit for scanning a light beam onto the charged surface of the photoreceptor web to form a latent electrostatic image and at least one development unit for developing the latent electrostatic image on the photoreceptor web into a toner image with an ink containing a liquid carrier and charged toner particles, wherein each development unit preferably includes a developer roller, a toner removal roller, a squeeze roller, a developer backup roller, a toner removal backup roller, and a squeeze backup roller. The photoreceptor web is arranged to provide at least 1 degree of contact wrap around at least one of the backup rollers that correspond to the developer roller, the toner removal roller, and the squeeze roller. The electrostatic transfer unit of the electrophotographic printer preferably provides for transferring the toner images formed in each development unit from the photoreceptor web to a print medium by electrostatic force.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the Figures, wherein the components are labeled with like numerals throughout the several Figures, and initially toFIG. 3, one preferred configuration of an electrostatic transfer type liquid electrophotographic printer100is illustrated, in accordance with the present invention. This printer100generally uses a photoreceptor web110circulating around a continuous path as a photoreceptor medium. This configuration includes aspects of the transfer type liquid electrophotographic printer of the type described in United States Patent Application Publication No. 2002/0110390, the entire contents of which are incorporated herein by reference.

As shown inFIG. 3, the electrostatic transfer type liquid electrophotographic printer100utilizes a photoreceptor web or belt110as a photoreceptor medium. The photoreceptor web110is preferably supported by three rollers111,112and113, including a driving roller and a steering roller. In the preferable embodiment, roller111is a driving roller and roller112is a steering roller; however, it is possible that roller112is a driving roller and roller111is a steering roller, or it is also possible that other driving and/or steering rollers are included in the printer configuration. Roller113may be referred to as a transfer backup roller, as this is the roller against which transfer of an image from the photoreceptor web110to a print paper102occurs. This roller113is preferably biased in order to effect electrostatic transfer of an image to print paper102or other medium. The photoreceptor web110circulates or moves around a continuous path or loop that is defined by the outer surfaces of the rollers111,112, and113. The arrows114,115, and116show the direction of rotation of the rollers111,112, and113, respectively, which roller rotation effects the movement of the photoreceptor web110in the direction indicated by arrows260and262. Alternatively, the rollers111,112, and113could all rotate in the opposite direction to cause the photoreceptor web110to move in the opposite direction; however, this would require repositioning of at least some of the other system components relative to the location of other components in the system. Additional or different rollers may be provided within the system, as desired, which would thereby change the path of the photoreceptor web110needed to encircle these rollers in a similar manner to that shown inFIG. 3.

A main charger120is shown adjacent to the photoreceptor web110to uniformly charge the photoreceptor web110to a predetermined potential. As shown, main charger120is located between the rollers113and111so that the photoreceptor web110can be charged to a particular potential before being exposed to the components of the system that provide ink to the photoreceptor web110, as described below. The main charger120is preferably sized and positioned so that it can sufficiently charge the photoreceptor web110to allow an electrostatic image to be formed thereon by at least one development unit. It is possible that additional chargers are provided (not shown), such as before the photoreceptor web110reaches some or all of laser scanning units140a,140b,140cand/or140d. It is also possible that squeeze rollers153a,153b,153cand/or153dof development units150a,150b,150cand/or150ddescribed below are sufficiently biased to charge the photoreceptor web110periodically through the process. In any case, it is preferable that the photoreceptor web110is recharged in the development units150a,150b,150c, and150dafter each color is provided to the photoreceptor web110.

Laser scanning units (LSUs)140a,140b,140cand140dand development units150a,150b,150cand150dare preferably provided below the photoreceptor web110(i.e., to contact the front surface193of the photoreceptor web110) and between the rollers111and112. The LSUs140a,140b,140, and140dare used for scanning light beams onto the charged photoreceptor web110to form a latent electrostatic image, and development units150a,150b,150cand150dare used for developing the latent electrostatic image as a toner image, each with a predetermined color ink. To form a multi-color image, for example, an electrophotographic printer would preferably be provided with four ink reservoirs159a,159b,159c, and159d, each containing one of the ink colors of yellow (Y), magenta (M), cyan (C) and black (B), four LSUs140a,140b,140cand140d, and four development units150a,150b,150cand150d. With these components, toner images of four different colors can be sequentially formed, overlapping or overlying each other, and developed into a multi-color image. As shown, the four development units150a,150b,150cand150dare arranged sequentially below the photoreceptor web110with their respective rollers in a rotation movement or circulation direction of the photoreceptor web110. The structure and operation of the development units150a,150b,150cand150dwill be described later in greater detail.

In a lower portion of the respective development units150a,150b,150cand150d, ink reservoirs159a,159b,159cand159d, which contain Y, M, C and K inks, respectively, are provided. Toner charged to a predetermined polarity is dispersed in a liquid carrier in the inks contained in the ink reservoirs159a,159b,159cand159d. The concentration of ink is preferably in the range of about 2.0–3.0%, and more preferably about 2.5%, where the term “concentration” refers to the weight percentage of toner solids with respect to carrier liquid. Although it is understood that the present invention is equally applicable to systems where the toner is charged to either a positive or negative potential, the description below is directed to the toner being charged to a positive potential. When the toner is charged to a negative potential, the opposite charging of other components and processes described below (that refer to charges of a positive potential) will be used. In addition, the four color toner images may be developed in an order that is different from the preferable order of Y, M, C, and then K, as described above, such as in the order of Y, C, M, and then K, for example.

After the image comprising at least one color is formed on the photoreceptor web110(i.e., the photoreceptor web110has passed by the development units150a,150b,150c, and150d, at least one of which has provided ink to the photoreceptor web110), the image may then be transferred to a piece of paper or other final image receptor. In this configuration, a print paper102is shown adjacent to the roller113for accepting the image from the photoreceptor web110. In many cases, it may be possible to achieve more than 99% transfer efficiency at an ink solids concentration of 20–40%. In other words, the percentage of the toner images transferred from the photoreceptor web110to a print paper102, or the “transfer efficiency”, may be higher than 99% at a concentration of 20–40%. If the toner concentration is relatively high (e.g., exceeds 40% by weight), the electrostatic transfer process may be more difficult to perform due to reduced fluidity of the toner, thereby lowering transfer efficiency. If the toner concentration is relatively low (e.g., below 20% by weight) and the liquid carrier content is too high, toner image leaking may occur on the print paper102due to highly increased fluidity of the toner. In addition, when the toner concentration is relatively low, it is less likely that the toner images can be kept intact before being transferred to a print paper102. If the toner concentration is relatively high (e.g., above 40% by weight), the electrostatic transfer process may become more difficult or impossible; however, it may be possible to successfully transfer the image using adhesive transfer with certain temperatures and pressures, as desired.

The toner images developed on the surface of the photoreceptor web110, whose toner concentration has preferably been adjusted to be suitable for electrostatic transfer, can be transferred to the print paper102by an electrostatic transfer unit. Such an electrostatic transfer unit forms an electric field between the photoreceptor web110and the electrostatic transfer unit so that the toner images formed on the photoreceptor web110are transferred to the print paper102by the electric force. As shown inFIG. 3, an electrostatic transfer roller170may be used as the electrostatic transfer unit. The electrostatic transfer roller170rotates in a rotation direction171while preferably being in contact with the photoreceptor web110when no paper is present, although a gap provided between the electrostatic transfer roller170and photoreceptor web110is possible. When the print paper102is fed between the electrostatic transfer roller170and the photoreceptor web110, the electrostatic transfer roller170will then be in contact with the print paper102. To create an electric field, a predetermined voltage of 900V–2 kV, for example, is preferably applied to the electrostatic transfer roller170. It is noted, however, that the polarity of the transfer voltage is determined based on the polarity of the ink particles. The surface of the electrostatic transfer roller170is preferably formed of a resistive material having a high resistance of 108–109ohms, for example. One possible material from which the electrostatic transfer roller170may be made is conductive urethane rubber, or may include a roller made of multiple materials, such as a roller comprising an inner core made of a material such as steel and having an outer coating of urethane rubber, for example. The reason that a voltage having the opposite polarity to the toner is applied to the electrostatic transfer roller170is to attract the toner such that a toner image can be transferred to the print paper102.

A fusing unit180for fusing the toner images transferred to the print paper102may be provided at the paper eject side of the electrostatic transfer roller170. The fusing unit180may include two or more fusing rollers181and182rotating in opposite directions and in contact with each other until a print paper102or other transfer medium is introduced between the fusing rollers181and182for fusing. The fusing rollers181and182fix the toner images on the print paper102, which passes between the fusing rollers181and182, by hot pressing. The printer100may further include an eraser unit190for removing the remaining latent electrostatic images from the surface of the photoreceptor web110.

Hereinafter, the development units150a,150b,150cand150dwill be described in greater detail. In the embodiment illustrated inFIG. 3, the three development units150a,150band150c, exclusive of the K-development unit150d(a development unit for black (K)), preferably have generally the same structure. A concentration control unit160can optionally be installed in the K-development unit150d, thereby making the structure of this development unit different from the structure of the others. If such a concentration control unit is not used, the structure of the K-development unit150dmay be the same as the other development units150a,150b, and150c, with a single roller replacing the two rollers152dshown in K-development unit150d. The structure of the three development units150a,150band150c, which are preferably the same, will be described first with reference to the Y-development unit150a(a development unit for yellow (Y)) ofFIG. 4.

Referring additionally toFIG. 4, which shows components of the development unit150athat are not shown inFIG. 3(for clarity purposes), three rollers including a developer roller151a, a toner removal roller152a, and a squeeze roller153aare installed in an upper portion of the Y-development unit150a. This embodiment of the electrostatic transfer type liquid electrophotographic printer100according to the present invention employs a development system that preferably uses these three rollers151a,152aand153a. It is contemplated, however, that a different number of rollers and/or rollers having different functions could be used. In this embodiment, the developer roller151ais used to make the toner particles of the ink adhere to the latent electrostatic images formed in an image region of the photoreceptor web110to form toner images. The toner removal roller152ais used to remove the toner adhering to the non-image region of the photoreceptor web110. To this end, a predetermined voltage is preferably applied to the toner removal roller152a, as will be described in further detail below. The squeeze roller153ais used to press a portion of the photoreceptor web110in 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 preferably applied to the squeeze roller153aso that the photoreceptor web110can be charged by the squeeze roller153ato a predetermined potential for another color toner image development. To this end, at least the surface of the squeeze roller153ais preferably formed of a resistive material with a high resistance of 105–107ohms, and more preferably, 106ohms (e.g., urethane rubber).

An ink supply nozzle158ais preferably installed adjacent to the developer roller151a. This ink supply nozzle158asupplies the ink contained in the Y-ink reservoir159a(seeFIG. 3) in the gap between the photoreceptor web110and the developer roller151a. A cleaning roller154arotating in contact with the developer roller151amay be installed below the developer roller151afor removing the ink adhering to the surface of the developer roller151a. A blade155ais preferably disposed underneath the toner removal roller152a, while one of its ends is in contact with the surface of the toner removal roller152a. A blade156ais preferably disposed underneath the squeeze roller153a, while one of its ends is in contact with the surface of the squeeze roller153a. The two blades155aand156aact to remove the ink or liquid carrier adhering to the surface of the toner removal roller152aand the squeeze roller153a, respectively. As the cleaning means, the cleaning roller154aand the blades155aand156aare interchangeable. In other words, either one or both of a cleaning roller and a blade may be installed for each of the rollers151a,152aand153a.

Continuing to refer toFIGS. 3 and 4, each of the developer rollers151a,152a,153ais preferably provided with a corresponding backup roller251a,252a,253a, respectively. The backup rollers251a,252a,253aare positioned to be adjacent to the back side194of the photoreceptor web110, and are positioned to press snugly against the photoreceptor web110, creating a mechanical wrap that is preferably at least about 1 degree around each backup roller251a,252a,253a. This wrap of the photoreceptor web110can be selected and controlled through the positioning of the various rollers in the development units150a,150b,150c, and150dto create a relatively continuous “arc” or curve of the photoreceptor web110from the general area of roller111to the general area of roller112. The arc or curve preferably extends from the first development roller that the photoreceptor web110passes (e.g., roller151a) to the last development roller that the photoreceptor web110passes (e.g., roller153d) of the multiple development units150a,150b,150c, and150d. In addition, while a mechanical wrap of at least about 1 degree on all of the rollers is preferable, the wrap angle may be different with respect to some of the rollers, where the wrap for some of the rollers is above 1 degree and the wrap on other rollers is less than 1 degree, for example. However, the degree of wrap on any of the rollers should be greater than 0 degrees, in accordance with the present invention.

The application of backup rollers in this system that press firmly enough against the backside194of the photoreceptor web110to form such a mechanical wrap around at least some of the backup rollers is advantageous in that the critical gaps between the rollers and photoreceptor web110can be more easily established and maintained. The developer rollers151a,152a,153aand backup rollers251a,252a,253ahave diameters that are chosen with a preferable nip width N1, N2, N3in mind. The pairs (151aand251a;152aand252a) of rollers are each carefully spaced to provide precise gap distances G1and G2between the roller151aand the web110and between the roller152aand the photoreceptor web110, respectively. In particular, the gap G1between the roller151aand the photoreceptor web110is preferably maintained at a certain distance to facilitate electrostatic transfer of charged toner pigment particles to the photoreceptor web110. If this gap G1is too large, a sufficient portion of the toner might not transfer to the photoreceptor web110, thereby causing poor printing quality. If the gap G1is too small, the transfer of toner might transfer to the photoreceptor web110by a different process than electrostatic transfer, which might also cause poor printing quality. Further, the gap G2between the roller152aand the photoreceptor web110is preferably maintained at a certain distance so that the thickness of the toner or “toner patch” can be properly controlled or metered. Thus, if the gap G2is too large, the toner will be thicker than desired and if the gap G2is too small, the toner will be thinner than desired, wherein both thickness variations can detrimentally effect the quality of the toner image that remains on the photoreceptor web110. In any case, the printer100may further include additional cleaning means to remove any residual ink from the photoreceptor web110after transfer of the toner images.

Additionally, the backup roller253aagainst which the squeeze roller153acan press may be selected to be a heavier roller having reduced flexibility, such that an increased force may be uniformly distributed along the nip N3. When the backup roller253ais a heavier roller, the roller may impart a force that is preferably between about 1 kg and 15 kg, and more preferably between about 5 kg and 10 kg. However, in the case of electrostatic transfer processes, the amount of force required in the nip N3to squeeze excess carrier from the image would typically be minimal. Preferably, the pressure across the width of this nip N3is relatively consistent across the entire width of the rollers, and it is further preferable that the amount of pressure applied is adjustable, as desired.

With respect to the rollers of the development unit150d, the provision of backup rollers is similar to that described above relative to the development units150a,150b, and150c, except that when a concentration control unit160is used, each of the two rollers152dof the concentration control unit is preferably provided with its own corresponding backup roller252d. In this way, pressure may be placed on the rollers of the development unit150din the same manner as described for the other development units. Thus, for each roller in each developer unit (151a,152a,153a,151b,152b,153b,151c,152c,153c,151d,152d,153d) there is preferably a corresponding backup roller (251a,252a,253a,251b,252b,253b,251c,252c,253c,251d,252d,253d, respectively) pressed against the backside194of the photoreceptor web10with a mechanical wrap of preferably at least 1 degree around each backup roller. In the embodiment shown, certain pairs of rollers have a carefully selected gap between them (151aand251a;152aand252a;151band251b;152band252b;151cand251c;152cand252c;151dand251d;152dand252d), as described above. Some of the pairs of rollers (e.g.,153aand253a;153band253b;153cand253c;153dand253d) may not have a gap between them. Rather, the squeeze rollers (153a,153b,153c, and/or153d) may actually contact the front surface193of the photoreceptor web110at the same time that the corresponding backup rollers (253a,253b,253c, and/or253d) contact the back surface194of the photoreceptor web110.

Referring also toFIG. 6, a representative roller208of a development unit is shown (which has a similar configuration to two paired rollers in one of the development units of a printer of the present invention), along with its corresponding backup roller202, to illustrate a simplified view of a gap204between two rollers. Roller208is rotatable in a direction214and backup roller202is rotatable in an opposite direction212, as shown. A backup roller, such as roller202, is preferably provided at each nip area, and is preferably positioned to allow at least about 1 degree of mechanical wrap of the photoreceptor web206about its outer surface. This roller202can advantageously maintain the gap204and the contact nip between the development unit roller208and a photoreceptor web206at a predetermined, desirable distance. Thus, it is preferable that the rollers of a printer of the present invention are adjustable to maintain the desired gaps, nip sizes, and/or compression forces between rollers and the photoreceptor web, where such adjustability may either be automatic (as may be controlled by electronic measurements and feedback loops, for example) or be manual (as may be adjusted by manual movement of the rollers when it is determined that the print quality can be improved with a change in the size of the gap, for example). In any given pair of rollers (e.g., a developer roller and its corresponding back-up roller), either one or both of the rollers may be adjustable for maintaining the gap size, where moving the back-up roller will typically also change the wrap of the photoreceptor web around that back-up roller, which may also be desirable in some cases. If such a change in the wrap of the photoreceptor web around a particular back-up roller is not desirable, the other roller (e.g., the developer roller) may instead be moved to adjust the size of the gap.

Development of a latent electrostatic image into a toner image by the Y-development unit150ahaving the configuration described previously will be further described with reference toFIG. 5, which is a magnified illustration of a portion of the development unit150aofFIG. 4. As described above relative toFIG. 3, before the photoreceptor web110reaches the development units150a,150b,150c, and150d, the main charger120charges the photoreceptor web110to a potential (referred to as a charge potential), for example, of 500–900 volts, and preferably, 550–750 volts, and having the same polarity as the toner. The charged surface of the photoreceptor web110is then irradiated by a light beam from the Y-LSU (LSU for yellow)140aso that a latent electrostatic image corresponding to yellow color is formed. The Y-LSU140aselectively discharges the surface of the photoreceptor web110to form a latent electrostatic image, so that a potential of the image region B1, in which the latent electrostatic image is formed, drops to about 100 volts or less (referred to as exposure potential), while a potential of the non-image region A1is maintained at the initial charge potential charged by the main charger120.

The latent electrostatic image is developed into a Y-toner image by the Y-development unit150a. In particular, as the photoreceptor web110passes over the developer roller151a, Y-toner adheres to the image region B1, in which an electrostatic latent image is formed, to form a Y-toner image. As a predetermined voltage is applied to the developer roller151a, the surface of the developer roller151ais charged to a development potential VDof about 350 volts, for example. The development potential VDof the development roller151ais determined to be lower than the charge potential (e.g., 550 V) of the non-image region A1, and to be higher than the exposure potential (e.g., 100 V) of the image region B1. It is preferable that differences between the development potential VDand each of the charge potential and the exposure potential are 100 volts or more, and more preferably, 200 volts or more. As the potential differences become greater, the affinity of toner particles to the photoreceptor web110and the developer roller151abecomes more apparent. The developer roller151arotates in the circulation direction of the photoreceptor web110while being separated by a development gap GD(e.g., 150–200 μm) from the photoreceptor web110. In one example, as an ink contained in the Y-ink reservoir159acontaining Y-toner of about 2.5% solids by weight is supplied by the ink supply nozzle158a, a nip NDas a liquid carrier film having about 6-mm width is formed between the photoreceptor web110and the developer roller151a. It is understood that as the weight percent of toner and other variables are changed, the size of any nips and gaps may differ.

In this example, the toner particles of the ink are preferably charged to positive potential and move in the nip NDas follows. The exposure potential (e.g., 100 volts) in the image region B1of the photoreceptor web110is lower than the development potential (e.g., 350 volts) of the development roller151a, so that the toner particles move toward the image region B1and adhere to the image region B1. The charge potential (e.g., 550 volts) in the non-image region A1is greater than the development potential VD(e.g., 350 volts) of the developer roller151a, so that the toner particles move towards the developer roller151aand adhere to the developer roller151a. In other words, the toner particles selectively adhere to only the image region B1charged to a relatively low potential, so that toner images are formed therein. Excess ink and toner particles stuck to the surface of the developer roller151acan be removed by a cleaning device such as the cleaning roller154arotating in contact with the developer roller151a, as previously described.

On the image region B2corresponding to the image region B1passed through the developer roller151a, an ink layer of a high-concentration toner image is formed and covered with a liquid carrier layer. On the non-image region A2, only a liquid carrier layer is formed. In the image region B2passed through the developer roller151a, the potential increases to about 160 volts, for example. The potential in the non-image region A2would then preferably drop to about 380 volts, for example. It is desirable that no toner remains in the liquid carrier layers passed through the developer roller151a. However, in some situations, some toner (e.g., about 0.5% by weight toner) remains in the liquid carrier layers. The remaining toner particles can be transferred to the M-development unit150balong the photoreceptor web110, and mixed with toner of another color. As a result, the M-development unit150b, C-development unit150c, and K-development unit150d, which are sequentially arranged, and the inks for each color, can be contaminated by the transfer of toner particles. Thus, there is a need to remove the toner particles remaining in the liquid carrier layers to minimize such contamination.

The toner particles remaining in the liquid carrier layers are preferably removed by the toner removal roller152adisposed adjacent to the developer roller151a. As the photoreceptor web110passes the toner removal roller152a, toner particles remaining in the liquid carrier layer in the non-image region A2are removed, thereby resulting in a toner-free liquid carrier layer in the non-image region A2. In particular, the surface of the toner removal roller152ais preferably charged to a toner removal potential VRof about 250 volts, for one example, with application of a predetermined voltage. The toner removal potential VRof the toner removal roller152ais determined to be greater than the exposure potential (e.g., 160 volts) in the image region B2and lower than the potential (e.g., 380 volts) in the non-image region A2. 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 roller152ais installed with a preferable gap GRof about 150–200 μm, for example, from the photoreceptor web110. A nip NRhaving a width of 3 mm to 5 mm, for example, may be formed between the toner removal roller152aand the photoreceptor web110. The width of the nip NRmay be varied depending on the diameter of the toner removal roller152aand the size of the gap GR. It is understood that as the weight percent of toner is varied, the size of any nips may differ. Although the toner removal roller152acan rotate in either direction, it is preferable that the toner removal roller152arotates in an opposite direction from the circulation direction of the photoreceptor web110for easier formation of the nip NR.

In one example, in the nip NRformed between the photoreceptor web110and the toner removal roller152a, the toner particles move as follows. In the non-image region A2of the photoreceptor web110, the potential (e.g., 380 volts) is higher than the toner removal potential VR(e.g., 250 volts) of the toner removal roller152a, so that toner particles dispersed in the liquid carrier layer can move towards the toner removal roller152a. The potential (e.g., 160 volts) in the image region B2is lower than the toner removal potential VR(e.g., 250 volts) of the toner removal roller152a, so that the toner particles move towards the image region B2and adhere to a previously formed toner image. As the toner removal roller152arotates, a removal device, such as the blade155aofFIG. 4, removes the toner particles and liquid carrier adhering to the surface of the toner removal roller152a.

As described previously, the toner particles existing in the liquid carrier layer on the non-image region A2can be almost completely removed by the toner removal roller152a, so that a toner-free liquid carrier remains in the non-image region A3of the photoreceptor web110passed through the toner removal roller152a. As a result, the problem of toner transfer to the adjacent development unit can be lessened.

Next, as the photoreceptor web110advances to the squeeze roller153a, the squeeze roller153apresses the toner image region of the photoreceptor web110, so that excess liquid carrier is squeezed from the toner image. In particular, the squeeze roller153apreferably rotates in the circulation direction of the photoreceptor web110in contact with the photoreceptor web110with a compression force, for example, of about 10 kg. As a result, the liquid carrier covering the toner image in the image region B3of the photoreceptor web110, and the liquid carrier adhering to the non-image region A3are removed so that just an appropriate and desired amount of the liquid carrier remains therein. Once the photoreceptor web110passes the squeeze roller153a, a toner image is formed as an ink layer containing, for example, about 50% by weight toner in the image region B3of the photoreceptor web110. Any liquid carrier stuck to the surface of the squeeze roller153acan be removed by a removal device, such as the blade156aofFIG. 4, and recovered into the Y-ink reservoir159a. The reason that the concentration of the toner image will typically be 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 roller153aalso can act to charge the photoreceptor web110again to a predetermined potential to develop a toner image in another color, such as in the next sequential development unit. To this end, a relatively high voltage may be applied to the squeeze roller153aso that the surface of the squeeze roller153ais charged to a squeeze potential Vsof about 800 volts or more, for example, which is higher than the charge potential. Thus, once the photoreceptor web110passes the squeeze roller153a, the potential in the non-image region A3of the photoreceptor web110and the potential in the image region B3are equal to or higher than the charge potential. This can allow for development of a toner image of another color.

Because the surface of the squeeze roller153ais charged to a relatively high potential, a toner image is formed in the image region B3by the repulsive force exerted between the squeeze roller153aand the toner particles, and firmly adheres to the image region B3with 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 roller153a. In addition, washing-off of the toner image by an ink applied to form another toner image does not typically 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, in order to then develop a toner image of magenta (M), the surface of the photoreceptor web110is preferably irradiated by a light beam from the M-LSU140bso that a latent electrostatic image corresponding to a M-toner image is formed. This latent electrostatic image can have a potential of about 100 volts, for example, and can be developed into a M-toner image by the M-development unit150bin the same manner as for the Y-toner image, as described previously. Then a toner image of cyan (C) can sequentially be developed by the C-development unit150c. This process is facilitated by the toner particles from Y, M, and C inks being selected to be transparent to the exposing wavelength.

After toner images are developed in three colors including yellow (Y), magenta (M) and cyan (C), a black (K) toner image can be developed by the K-development unit150d. The concentration of the overlapping toner images previously formed on the photoreceptor web110can be adjusted to be suitable for electrostatic transfer by the K-development unit150d.

The use of various rollers, particularly backup rollers, in the printer of the present invention is advantageous to maintain important gaps between rollers of the development units and the photoreceptor web or belt. Thus the gap development and maintenance as illustrated and explained relative toFIGS. 4 and 5(both discussed above) is important to this apparatus. The maintenance of the various gaps between rollers at particular distances will affect print quality and image density. Without such backup rollers, it may be difficult to maintain the desired gaps for each nipped area (typically, two per developer unit) over the length of the photoreceptor web110(FIG. 3, between rollers111and112). This is because capillary forces of the liquid ink in the controlled gap (G1and G2inFIG. 4) will act to pull the photoreceptor web110toward the developer roller, such as roller151aofFIG. 4, and toward the toner removal roller, such as roller152aofFIG. 4. If the tension of the photoreceptor web110is increased to resist the capillary force, belt troughing can occur before the capillary force can be overcome and this troughing will prohibit a uniform gap from being maintained.

The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.