ELECTROPHOTOGRAPHIC PRINTER AND PRINTING METHOD

An electrophotographic printer and a printing method are provided that may achieve an improved image quality, while reducing the risk of a print target medium being deformed. The electrophotographic printer includes a first color printing means. The first color printing means includes a toner developing unit, a toner image transfer unit, and an ultraviolet irradiating-drying unit. The toner developing unit adsorbs a liquid toner containing an ultraviolet absorbent for absorption of ultraviolet light to a latent charge image so as to develop a toner image. The toner image transfer unit transfers the toner image onto a print target medium. The ultraviolet irradiating-drying unit dries the transferred toner image on the print target medium by ultraviolet irradiation to obtain a dried toner image.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Japanese Patent Application No. 2017-140285, filed on Jul. 19, 2017. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

TECHNICAL FIELD

This disclosure relates to an electrophotographic printer and a printing method.

DESCRIPTION OF THE BACKGROUND ART

A known example of the conventional printing methods of image printing for target media, such as paper, is electrophotography (for example, Japanese Unexamined Patent Publication No. 2010-076334). The electrophotography refers to a printing method in which an image developed by applying toner to the surface of a photoconductor is transferred to a print target medium.

SUMMARY

In the printing method described in Japanese Unexamined Patent Publication No. 2010-076334, dry electrophotography is employed. Toners conventionally used in dry electrophotographic printing are particulate toners. The particular toners are conventionally produced by pulverizing pigment components. The average particle sizes of such particulate toners are thus relatively large, approximately 5.5 μm, and their particles have shapes with a low degree of sphericity. Hence, improvement of image quality may be difficult with the printing method described in Japanese Unexamined Patent Publication No. 2010-076334.

In the printing method described in Japanese Unexamined Patent Publication No. 2010-076334, offset printing precedes the dry electrophotographic printing. In the offset printing, any region to which ink should not be applied is impregnated with water. In the printing method described in Japanese Unexamined Patent Publication No. 2010-076334 in which the print target medium is thus impregnated with water, some measures are taken to avoid evaporation of water impregnated into the print target medium and to avoid oversupply of thermal energy to the toner, with an aim to reduce the risk of the print target medium being deformed. In case of the occurrence of evaporation of water impregnated into the print target medium and oversupply of thermal energy to the toner, the print target medium may be likely to deform, possibly resulting in a poor image quality. The deformation of the print target medium may lead to other issues, for example, paper jam.

To address these issues of the known art, this disclosure provides an electrophotographic printer and a printing method that may achieve an improved image quality, while reducing the risk of a print target medium being deformed.

To address the issues of the known art and to serve the purpose of this disclosure, an electrophotographic printer is provided that includes: a toner developing unit that adsorbs a liquid toner containing an ultraviolet absorbent for absorption of ultraviolet light to a latent charge image so as to develop a toner image; a toner image transfer unit that transfers the toner image obtained by the toner developing unit onto a print target medium; and an ultraviolet irradiating-drying unit that irradiates the toner image with ultraviolet light so as to dry the toner image transferred on the print target medium.

In this configuration, the electrophotographic printer may be equipped with a plurality of the toner developing units, a plurality of the toner image transfer units, and a plurality of the ultraviolet irradiating-drying units that are respectively provided for a plurality of different colors. In the electrophotographic printer thus further characterized, a plurality of the toner images are sequentially dried per color on the print target medium so as to form a composite dried toner image including the plurality of the toner images dried and having the plurality of different colors on the print target medium.

The electrophotographic printer equipped with a plurality of the toner developing units, a plurality of the toner image transfer units, and a plurality of the ultraviolet irradiating-drying units respectively for a plurality of different colors may further include a fixing device that heats the print target medium on which the composite dried toner image is formed so as to fix the composite dried toner image onto the print target medium.

To address the issues of the known art and to serve the purpose of this disclosure, a printing method is provided that includes a toner developing step of developing a toner image through adsorption of a liquid toner including an ultraviolet absorbent that absorbs ultraviolet light; a toner image transfer step of transferring the toner image obtained in the toner developing step onto a print target medium; and an ultraviolet irradiating-drying step of irradiating the toner image with ultraviolet light so as to dry the toner image transferred on the print target medium. The printing method may include a plurality of the toner developing steps, a plurality of the toner image transfer steps, and a plurality of the ultraviolet irradiating-drying steps respectively for a plurality of different colors. In the printing method, a plurality of the toner images are sequentially dried per color on the print target medium so as to form a composite dried toner image including the plurality of the toner images dried and having the plurality of different colors on the print target medium.

In this configuration, the printing method may further include a fixing step of heating the print target medium on which the composite dried toner image is formed so as to fix the composite dried toner image onto the print target medium.

This disclosure provides an electrophotographic printer and a printing method that may achieve an improved image quality, while reducing the risk of a print target medium being deformed.

DETAILED DESCRIPTION OF EMBODIMENT

Hereinafter, an embodiment of this disclosure are described in detail referring to the accompanying drawings. It should be understood that none of the technical aspects disclosed herein is limited by the embodiment. Structural and technical elements described in the embodiment below may include elements that are replaceable and easily feasible by those skilled in the art and elements that are substantially identical. Further, the structural and technical elements described herein may be suitably combined and if there are a plurality of embodiments, the embodiments may be combined.

Embodiment

FIG. 1is a drawing schematically illustrating a structural overview of an electrophotographic printer10according to an embodiment. As illustrated inFIG. 1, the electrophotographic printer10includes a first color printing means20, a second color printing means30, a third color printing means40, a fourth color printing means50, and a fixing device60. In the electrophotographic printer10, the first color printing means20, second color printing means30, third color printing means40, fourth color printing means50, and fixing device60are arranged in this order from the upstream side toward the downstream side of a transport path on which a print target medium12is transported. These color printing means and the fixing device are disposed so as to face a print target surface of the print target medium12. The print target medium12is transported on a predetermined transport path by a transport device not illustrated in the drawing.

The first color printing means20prints an image of a first color on the print target medium12. The second color printing means30prints an image of a second color on the print target medium12having the first color image printed thereon. The third color printing means40prints an image of a third color on the print target medium12having the first and second color images printed thereon. The fourth color printing means50prints an image of a fourth color on the print target medium12having the first, second, and third color images printed thereon. The fixing device60fixes the images of the first, second, third, and fourth colors, which have been sequentially printed on the print target medium12, onto the print target medium12. Thus, the electrophotographic printer10performs sequential color printing, i.e., prompts the first color printing means20, second color printing means30, third color printing means40, and fourth color printing means50to sequentially print the images of the first, second, third, and fourth colors on the print target medium12, and then prompts the fixing device60to fix the printed images onto the print target medium12so as to obtain a printed matter70.

An exemplified combination of the first, second, third, and fourth colors in the electrophotographic printer10is combination of C (Cyan), M (Magenta), Y (Yellow), and K (Black). The combination of the first, second, third, and fourth colors in the electrophotographic printer10may be a combination of different colors arranged in the order of transparency. The combination of the first, second, third, and fourth colors used in the electrophotographic printer10is not limited to the YMCK combination and may be selected from other color combinations, an example of which may be combination of R (Red), G (Green), and B (Blue).

The first color printing means20is hereinafter described referring toFIG. 1. The first color printing means20is substantially configured similarly to the second color printing means30, third color printing means40, and fourth color printing means50. Therefore, any technical aspects that are distinct among the first color printing means20, second color printing means30, third color printing means40, and fourth color printing means50alone are hereinafter described, while redundant description of other similar or identical technical aspects may be omitted.

As illustrated inFIG. 1, the first color printing means20has an electrophotographic photoconductor22, an electrifier23, an image exposure unit24, a toner developing unit26, a toner image transfer unit28, and an ultraviolet irradiating-drying unit29.

The electrophotographic photoconductor22is a cylindrical drum. The electrophotographic photoconductor22is disposed along a direction parallel to the width direction of the print target medium12, and the axial direction of the electrophotographic photoconductor22is orthogonal to the transport direction of the print target medium12. The outer peripheral surface of the electrophotographic photoconductor22is facing, at a predetermined position, the print target surface of the print target medium12being transported. The electrophotographic photoconductor22is provided with a rotation driver to allow for clockwise rotation, as illustrated inFIG. 1. In the description below, the clockwise direction of the electrophotographic photoconductor22may be referred to as direction of normal rotation, while the counterclockwise direction of the electrophotographic photoconductor22may be referred to as direction of reverse rotation.

At the predetermined position at which the outer peripheral surface of the electrophotographic photoconductor22is facing the print target surface of the print target medium12being transported, the toner image transfer unit28, which will be described later, is disposed at a position opposite to the predetermined position across the print target medium12. The electrifier23, image exposure unit24, and toner developing unit26, which will be described later, are sequentially arranged at positions away from the predetermined position along the direction of normal rotation so as to face the electrophotographic photoconductor22. The electrophotographic photoconductor22rotates and accordingly sequentially and iteratively pass the positions of the electrifier23, image exposure unit24, toner developing unit26, and toner image transfer unit28facing the electrophotographic photoconductor22.

The electrophotographic photoconductor22is electrified by electric discharge. The electrophotographic photoconductor22is allowed to control the amount of electric charges through light photoconductivity. The electrophotographic photoconductor22may be made of, for example, an organic or inorganic photoconductive insulating material.

The electrifier23is disposed so as to face the outer peripheral surface of the electrophotographic photoconductor22at a position more toward the direction of normal rotation than the position at which the electrophotographic photoconductor22is facing the print target medium12. The electrifier23is a member that generates electric discharge. An example of the electrifier23may be an electric discharge wire that generates corona discharge.

The electrifier23generates electric discharge toward the outer peripheral surface of the electrophotographic photoconductor22and thereby electrifies the outer peripheral surface of the electrophotographic photoconductor22. By thus generating electric discharge toward the outer peripheral surface of the electrophotographic photoconductor22, the electrifier23applies electric charges23ato the outer peripheral surface of the electrophotographic photoconductor22. Of the whole outer peripheral surface of the electrophotographic photoconductor22, the electrifier23applies the electric charges23ato a part of the outer peripheral surface more toward the direction of normal rotation than the position of the electrifier23. The polarity of the electric charges23amay be suitably selected in accordance with the electrophotographic photoconductor22actually used.

The image exposure unit24is disposed so as to face the outer peripheral surface of the electrophotographic photoconductor22at a position more toward the direction of normal rotation than the position at which the electrophotographic photoconductor22is facing the electrifier23. A controller, not illustrated in the drawing, is electrically coupled to the image exposure unit24so as to receive, from the controller, data of a first color image part included in full color image data. The image exposure unit24is a member that radiates light for exposure based on the data of the first color image part received from the controller. Examples of the image exposure unit24may include a semiconductor laser and an LED array.

After the electric charges23aare applied to the outer peripheral surface of the electrophotographic photoconductor22by the electrifier23, the image exposure unit24irradiates the outer peripheral surface with light for exposure, and thereby forms a latent charge image24abased on the data of the first color image part on the outer peripheral surface of the electrophotographic photoconductor22. Of the whole outer peripheral surface of the electrophotographic photoconductor22, the image exposure unit24forms the latent charge image24aon a part of the outer peripheral surface more toward the direction of normal rotation than the position of the image exposure unit24.

The toner developing unit26is disposed so as to face the outer peripheral surface of the electrophotographic photoconductor22at a position more toward the direction of normal rotation than the position at which the electrophotographic photoconductor22is facing the image exposure unit24. The toner developing unit26contains, in its inner space, a liquid toner26aof the first color. The toner developing unit26containing the liquid toner26ahas an electrostatic adsorption mechanism that electrostatically adsorbs the liquid toner26ato the outer peripheral surface of the electrophotographic photoconductor22.

The toner developing unit26electrostatically adsorbs the liquid toner26ato the latent charge image24aformed by the image exposure unit24on the outer peripheral surface of the electrophotographic photoconductor22so as to develop a toner image26bof the first color based on the liquid toner26a. In this embodiment, the toner developing unit26develops the toner image26bby reversal development, which is, however, not limited. The toner image26bmay be developed by normal development. The toner developing unit26may apply a direct current bias voltage or an alternate current bias voltage for image development. Of the whole outer peripheral surface of the electrophotographic photoconductor22, the toner developing unit26forms the toner image26bon a part of the outer peripheral surface more toward the direction of normal rotation than the position of the toner developing unit26.

The liquid toner26aincludes a coloring ink that produces the first color, an ultraviolet absorbent, a solvent, and a dispersing agent. In the liquid toner26a, the coloring ink that produces the first color and the ultraviolet absorbent are dispersed in the solvent by the dispersing agent. In the liquid toner26a, solid particles in the coloring ink have an average particle size less than or equal to 2.0 μm, preferably less than or equal to 1.5 μm, more preferably less than or equal to 1.0 μm. As for an ultraviolet absorption index that refers to the ratio of light absorption energy in the ultraviolet absorption band to the whole light absorption energy, the ultraviolet absorption index of the liquid toner26ain one toner layer is greater than or equal to 70%, preferably greater than or equal to 80%, more preferably greater than or equal to 90%.

To adjust the surface tension or viscosity of the liquid toner26a, an adjuster, such as a solvent, may be further added to the liquid toner26a. The liquid toner26amay further include an additive such as silicon oxide powder. A semiconductor or insulator of metal sulfide or metal oxide such as zinc oxide having no large absorption band within the visible light region, i.e., having a band gap greater than or equal to 3.1 eV, may be further added to the liquid toner26ain the form of particles having an average particle size less than or equal to 300 nm, provided that the conversion of absorbed ultraviolet light into visible light or infrared light has a poor conversion efficiency, and most of absorbed ultraviolet light is convertible into heat.

Examples of the coloring ink may include white-colored, cyan (C), magenta (M), yellow (Y), and black (K) inks. The coloring ink may be used in combination with a transparent ink. The transparent ink may be a coloring material having a feature color, for example, a clear ink. The coloring ink is not limited to these examples and may be selected from feature color inks such as red (R), green (G), blue (B), pearl color, and metallic color inks. The coloring ink is not necessarily limited to inks of any particular colors insofar as at least one color or more is thereby producible. As for the coloring ink used in the liquid toner26a, the coloring ink that produces the first color may be singly used or other coloring ink(s) or transparent ink may be used in combination so as to produce the first color.

Examples of the ultraviolet absorbent may include acetopheminone-based ultraviolet absorbents, α-aminoacetophenone-based ultraviolet absorbents, acylphosphine oxide radical-based ultraviolet absorbents, O-acyloxime-based ultraviolet absorbents, titanocene-based ultraviolet absorbents, radical ultraviolet absorbents such as bimolecular reaction ultraviolet absorbents, and cationic ultraviolet absorbents. The ultraviolet absorbent desirably used may be characterized in that light in the visible light region is limitedly absorbable only to such an extent that does not compromise a color(s) produced by the coloring material, and as much light as possible in the ultraviolet region is absorbable. The ultraviolet absorbent may excel in chemical stability and color stability against heat generated by instantaneous heating. The ultraviolet absorbent has an absorption band effective for light in the ultraviolet absorption band having a wavelength less than or equal to 400 nm, preferably less than or equal to 385 nm, more preferably less than or equal to 365 nm.

The solvent may be any one selected from these examples that can be evaporated by heating. The solvent desirably used may be characterized in that light in the visible light region is limitedly absorbable only to such an extent that does not compromise a color(s) produced by the coloring material. Further, the solvent may excel in chemical stability and color stability against heat generated by instantaneous heating.

Examples of the dispersing agents may include polymeric dispersing agents and surfactants having hydrophilic and hydrophobic groups. Examples of the polymeric dispersing agents may include non-aqueous surfactants, aqueous surfactants, and aqueous and non-aqueous surfactants. Examples of the non-aqueous surfactants may include unsaturated polycarboxylic acid and unsaturated polyamide polycarboxylate. Examples of the aqueous surfactants may include polycarboxylic acid alkylamine salt and nonionic surfactants. Examples of the surfactants having hydrophilic and hydrophobic groups may include anionic surfactants, cationic surfactants, nonionic surfactants, and anionic and cationic surfactants. The dispersing agent desirably used may be characterized in that light in the visible light region is limitedly absorbable only to such an extent that does not compromise a color(s) produced by the coloring material. Further, the dispersing agent may excel in chemical stability and color stability against heat generated by instantaneous heating.

The toner image transfer unit28is disposed at a position more toward the direction of normal rotation than the position at which the electrophotographic photoconductor22is facing the toner developing unit26. The toner image transfer unit28is disposed opposite to, on the other side of the print target surface of the print target medium12, the predetermined position at which the outer peripheral surface of the electrophotographic photoconductor22is facing the print target surface of the print target medium12being transported. Thus, the toner image transfer unit28and the electrophotographic photoconductor22are positioned so that the print target medium12is interposed therebetween. The toner image transfer unit28has a voltage applicator that applies voltage whose polarity differs from that of the electrifier23.

The toner image transfer unit28, through voltage application using, for example, voltage generated by corona discharge or voltage applied to a roller, applies an electrostatic force directed toward the print target medium12to the toner image26bformed on the outer peripheral surface of the electrophotographic photoconductor22. The toner image transfer unit28, using the electrostatic force, peels the toner image26boff the outer peripheral surface of the electrophotographic photoconductor22and transfers the toner image26bto the print target surface of the print target medium12so as to form a toner image26cof the first color on the print target surface of the print target medium12. Of the whole outer peripheral surface of the electrophotographic photoconductor22, the toner image transfer unit28peels off the toner image26bfrom a part of the outer peripheral surface more toward the direction of normal rotation than the position of the toner image transfer unit28.

The first color printing means20prompts the toner image transfer unit28to transfer the toner image to the print target medium12directly from the outer peripheral surface of the electrophotographic photoconductor22. The toner image transfer is not necessarily limited to such direct transfer and may be indirect transfer. The toner image may be transferred from the outer peripheral surface of the electrophotographic photoconductor22to a transfer belt or a transfer drum before the transfer to the print target medium12.

The ultraviolet irradiating-drying unit29is disposed so as to face the print target surface of the print target medium12at a position more downstream than the position of the toner image transfer unit28on the transport path of the print target medium12.FIG. 2is a schematic drawing of a structural overview of the ultraviolet irradiating-drying unit29in the electrophotographic printer10according to the embodiment. As illustrated inFIG. 2, the ultraviolet irradiating-drying unit29has ultraviolet emitting diodes29a, a heat release base plate29b, and an ultraviolet reflective plate29c.

The ultraviolet irradiating-drying unit29has three ultraviolet emitting diodes29a. The number of the ultraviolet emitting diodes is not limited to three and may be one, two, or four or more. The ultraviolet emitting diodes29aof the ultraviolet irradiating-drying unit29may be replaced with or may be used in combination with a metal halide lamp or a xenon lamp that emits light including ultraviolet light. All of the ultraviolet emitting diodes29aare facing the print target surface of the print target medium12and radiate ultraviolet light29dtoward the print target surface of the print target medium12. In the ultraviolet irradiating-drying unit29, the ultraviolet emitting diodes29amay be evenly spaced at intervals and directed toward the print target surface of the print target medium12.

The ultraviolet emitting diodes29aradiate the ultraviolet light29dhaving an ultraviolet irradiation band including part of the ultraviolet absorption band of the ultraviolet absorbent in the liquid toner26a. All of the ultraviolet emitting diodes29amay be configured to radiate the ultraviolet light29dhaving an ultraviolet irradiation band of a similar range to the ultraviolet absorption band of the ultraviolet absorbent in the liquid toner26a. All of the ultraviolet emitting diodes29ais preferably configured to radiate the ultraviolet light29dhaving a center wavelength less than or equal to 385 nm, more preferably less than or equal to 365 nm in the ultraviolet irradiation band, depending on a center wavelength in the ultraviolet absorption band of the ultraviolet absorbent.

There is one heat release base plate29bfor the plural ultraviolet emitting diodes29a, and the heat release base plate29bis disposed in contact with the ultraviolet emitting diodes29aon one side of these diodes opposite to their other side directed toward the print target surface of the print target medium12. The heat release base plate29bis made of a ceramic material or a metallic material having high thermal conductivity, and releases heat generated in the ultraviolet emitting diodes29a.

The ultraviolet reflective plate29cis disposed so as to cover, from the outer side of the heat release base plate29b, the ultraviolet emitting diodes29aon one side of these diodes opposite to their other side directed toward the print target surface of the print target medium12. The ultraviolet light29dradiated from the ultraviolet emitting diodes29ain the opposite direction of the print target surface of the print target medium12is reflected by the ultraviolet reflective plate29, so that the ultraviolet light29dis turned around toward the print target surface of the print target medium12.

The ultraviolet irradiating-drying unit29thus configured irradiates the print target surface of the print target medium12having the toner image26cformed thereon by the toner image transfer unit28with the ultraviolet light29d. The ultraviolet irradiating-drying unit29accordingly forms an ultraviolet irradiating region29eto be irradiated with the ultraviolet light29don the print target surface of the print target medium12. The ultraviolet irradiating-drying unit29, using the ultraviolet light29d, heats the ultraviolet absorbent included in the toner image26cpassing through the ultraviolet irradiating region29e, and thereby evaporates the solvent included in the toner image26cto dry the toner image26c, so that a dried toner image26dof the first color is formed on the print target surface of the print target medium12.

The ultraviolet irradiating-drying unit29may irradiate the toner image with the ultraviolet light29dwith an accumulative irradiation energy greater than or equal to 600 mJ/cm2·g and less than or equal to 10000 mJ/cm2·g. The accumulative irradiation energy described herein refers to an integrated energy value of the ultraviolet light29dwith which the toner image26cper unit area is irradiated. The ultraviolet emitting diodes29aof the ultraviolet irradiating-drying unit29may intermittently radiate the ultraviolet light29dat shorter time intervals than time required for the print target medium12to pass through the ultraviolet irradiating region29e.

As illustrated inFIG. 1, the second color printing means30has an electrophotographic photoconductor32, an electrifier33, an image exposure unit34, a toner developing unit36, a toner image transfer unit38, and an ultraviolet irradiating-drying unit39. By generating electric discharge toward the outer peripheral surface of the electrophotographic photoconductor32, the electrifier33applies electric charges33ato the outer peripheral surface of the electrophotographic photoconductor32. After the electric charges33aare applied to the outer peripheral surface of the electrophotographic photoconductor32by the electrifier33, the image exposure unit34irradiates the outer peripheral surface with light for exposure, and thereby forms a latent charge image34abased on data of a second color image part on the outer peripheral surface of the electrophotographic photoconductor32. The toner developing unit36electrostatically adsorbs a liquid toner36ato the latent charge image34aformed by the image exposure unit34on the outer peripheral surface of the electrophotographic photoconductor32so as to develop a toner image36bof the second color based on the liquid toner36a. The toner image transfer unit38peels the second color toner image36boff the outer peripheral surface of the electrophotographic photoconductor32and transfers the toner image36bonto the dried toner image26dof the first color on the print target surface of the print target medium12so as to form a toner image36cof the second color on the dried toner image26dof the first color. The ultraviolet irradiating-drying unit39dries the toner image36cof the second color and thereby forms a dried toner image36dof the second color on the dried toner image26dof the first color.

As illustrated inFIG. 1, the third color printing means40has an electrophotographic photoconductor42, an electrifier43, an image exposure unit44, a toner developing unit46, a toner image transfer unit48, and an ultraviolet irradiating-drying unit49. By generating electric discharge toward the outer peripheral surface of the electrophotographic photoconductor42, the electrifier43applies electric charges43ato the outer peripheral surface of the electrophotographic photoconductor42. After the electric charges43aare applied to the outer peripheral surface of the electrophotographic photoconductor42by the electrifier43, the image exposure unit44irradiates the outer peripheral surface with light for exposure, and thereby forms a latent charge image44abased on data of a third color image part on the outer peripheral surface of the electrophotographic photoconductor42. The toner developing unit46electrostatically adsorbs a liquid toner46ato the latent charge image44aformed by the image exposure unit44on the outer peripheral surface of the electrophotographic photoconductor42so as to develop a toner image46bof the third color based on the liquid toner46a. The toner image transfer unit48peels the third color toner image46boff the outer peripheral surface of the electrophotographic photoconductor42and transfers the toner image46bonto the dried toner image36dof the second color on the print target surface of the print target medium12so as to form a toner image46cof the third color on the dried toner image36dof the second color. The ultraviolet irradiating-drying unit49dries the toner image46cof the third color and thereby forms a dried toner image46dof the third color on the dried toner image36dof the second color.

As illustrated inFIG. 1, the fourth color printing means50has an electrophotographic photoconductor52, an electrifier53, an image exposure unit54, a toner developing unit56, a toner image transfer unit58, and an ultraviolet irradiating-drying unit59. By generating electric discharge toward the outer peripheral surface of the electrophotographic photoconductor52, the electrifier53applies electric charges53ato the outer peripheral surface of the electrophotographic photoconductor52. After the electric charges53aare applied to the outer peripheral surface of the electrophotographic photoconductor52by the electrifier53, the image exposure unit54irradiates the outer peripheral surface with light for exposure, and thereby forms a latent charge image54abased on data of a fourth color image part on the outer peripheral surface of the electrophotographic photoconductor52. The toner developing unit56electrostatically adsorbs a liquid toner56ato the latent charge image54aformed by the image exposure unit54on the outer peripheral surface of the electrophotographic photoconductor52so as to develop a toner image56bof the fourth color based on the liquid toner56a. The toner image transfer unit58peels the fourth color toner image56boff the outer peripheral surface of the electrophotographic photoconductor52and transfers the toner image56bonto the dried toner image46dof the third color on the print target surface of the print target medium12so as to form a toner image56cof the fourth color on the dried toner image46dof the third color. The ultraviolet irradiating-drying unit59dries the toner image56cof the fourth color and thereby forms a dried toner image56dof the fourth color on the dried toner image46dof the third color.

FIG. 3is a schematic drawing of a structural overview of the ultraviolet irradiating-drying unit59and the fixing device60in the electrophotographic printer10according to the embodiment. As illustrated inFIG. 3, the ultraviolet irradiating-drying unit59has ultraviolet emitting diodes59a, a heat release base plate59b, and an ultraviolet reflective plate59c.

After the dried toner image26dof the first color, dried toner image36dof the second color, and dried toner image46dof the third color are formed on the print target surface of the print target medium12and the toner image56cof the fourth color is further formed thereon by the toner image transfer unit58, the ultraviolet irradiating-drying unit59radiates ultraviolet light59dtoward the print target surface of the print target medium12. The ultraviolet irradiating-drying unit59accordingly forms an ultraviolet irradiating region59eto be irradiated with the ultraviolet light59don the print target surface of the print target medium12. The ultraviolet irradiating-drying unit59, using the ultraviolet light59d, heats the ultraviolet absorbent included in the toner image56cpassing through the ultraviolet irradiating region59e, and thereby evaporates the solvent included in the toner image56cto dry the toner image56c, so that the dried toner image56dof the fourth color is formed on the dried toner image46dof the third color previously formed on the print target surface of the print target medium12. After the print target medium12is caused to pass through the ultraviolet irradiating region59eformed by the ultraviolet irradiating-drying unit59, the print target medium12has, on its print target surface sequentially, the different dried toner images formed on one another; dried toner image26dof the first color, dried toner image36dof the second color, dried toner image46dof the third color, and dried toner image56dof the fourth color. On the print target surface of the print target medium12, a composite dried toner image70ais thus formed that sequentially includes the dried toner image26d, dried toner image36d, dried toner image46d, and dried toner image56d.

As illustrated inFIGS. 1 and 3, the fixing device60is disposed at a position more downstream than the position of the ultraviolet irradiating-drying unit59on the transport path of the print target medium12. The fixing device60is a pair of heaters. One of the heaters is disposed in proximity to and facing the print target surface of the print target medium12, while the other heater is disposed in proximity to and facing the other surface of the print target medium12on the opposite side of the print target surface. The fixing device60is not limited to a pair of heaters and may be a suitable one selected from the conventional heating devices. The fixing device60collectively heats the multiple images of the composite dried toner image70apassing through a heating region60aformed by the pair of heaters. In the heating region60a, the fixing device60combines the multiple images of the composite dried toner image70ainto one print image70band fixes the print image70bto the print target surface of the print target medium12. As a result, a printed matter70with the print image70bprinted thereon is obtained.

The fixing device60may use ultraviolet irradiating means similar to the ones used in the ultraviolet irradiating-drying unit29, ultraviolet irradiating-drying unit39, ultraviolet irradiating-drying unit49, and ultraviolet irradiating-drying unit59. In case where the fixing device60is thus configured, ultraviolet light is radiated from the fixing device60and absorbed by the ultraviolet absorbent included in the composite dried toner image70apassing through the heating region60a, which is the ultraviolet irradiating region, formed by the fixing device60, so that the multiple images of the composite dried toner image70aare collectively heated.

FIG. 4is a flow chart of a printing method according to the embodiment. The printing method is an exemplified method of operating the electrophotographic printer10according to the embodiment. This printing method is hereinafter described referring toFIG. 4. As illustrated inFIG. 4, the printing method according to the embodiment includes a single-color printing step (Step S10) and a fixing step (Step S22). The printing method according to the embodiment includes a plurality of single-color printing steps (Step S10) for the number of colors to be printed. In this embodiment, the printing method includes four single-color printing steps. The single-color printing steps (Step S10) each include an electrifying step (Step S12), an exposure step (Step S14), a developing step (Step S16), a transfer step (Step S18), and a drying step (Step S20).

First, the print target medium12is transported on a predetermined transport path by a transport device not illustrated in the drawing. The first color printing means20, second color printing means30, third color printing means40, and fourth color printing means50respectively perform Steps S10one after another with respect to the print target surface of the print target medium12being transported on the predetermined transport path. The description given below mostly focuses on Step S10performed by the first color printing means20, while omitting detailed description of Steps S10by the second color printing means30, third color printing means40, and fourth color printing means50.

By generating electric discharge toward the outer peripheral surface of the electrophotographic photoconductor22, the electrifier23of the first color printing means20electrifies the outer peripheral surface of the electrophotographic photoconductor22and applies the electric charges23ato the outer peripheral surface of the electrophotographic photoconductor22(Step S12).

Subsequent to Step S12, the image exposure unit24of the first color printing means20receives the data of the first color image part included in full color image data from the controller electrically coupled to the image exposure unit24. After the electric charges23aare applied to the outer peripheral surface of the electrophotographic photoconductor22by the electrifier23, the image exposure unit24irradiates the outer peripheral surface with light for exposure based on the data of the first color image part received from the controller, and thereby forms the latent charge image24abased on the data of the first color image part on the outer peripheral surface of the electrophotographic photoconductor22(Step S14).

The toner developing unit26of the first color printing means20contains the liquid toner26aof the first color including the ultraviolet absorbent in its inner space. Subsequent to Step S14, the toner developing unit26electrostatically adsorbs the liquid toner26ato the latent charge image24aformed by the image exposure unit24on the outer peripheral surface of the electrophotographic photoconductor22. As a result of these steps, the toner developing unit26forms the toner image26bof the first color based on the liquid toner26aon the outer peripheral surface of the electrophotographic photoconductor22(Step S16).

Subsequent to Step S16, the toner image transfer unit28of the first color printing means20, through voltage application, applies an electrostatic force directed toward the print target medium12to the toner image26bformed on the outer peripheral surface of the electrophotographic photoconductor22. The toner image transfer unit28, using the electrostatic force, peels the toner image26boff the outer peripheral surface of the electrophotographic photoconductor22and transfers the toner image26bto the print target surface of the print target medium12, so that the toner image26cof the first color is formed on the print target surface of the print target medium12(Step S18).

Subsequent to Step S18, the ultraviolet emitting diodes29ain the ultraviolet irradiating-drying unit29of the first color printing means20irradiate the print target surface of the print target medium12having the toner image26cformed thereon by the toner image transfer unit28with the ultraviolet light29dhaving an ultraviolet irradiation band including part of the ultraviolet absorption band of the ultraviolet absorbent in the liquid toner26a. The ultraviolet irradiating-drying unit29accordingly forms the ultraviolet irradiating region29eto be irradiated with the ultraviolet light29don the print target surface of the print target medium12. The ultraviolet irradiating-drying unit29, using the ultraviolet light29d, heats the ultraviolet absorbent included in the toner image26cpassing through the ultraviolet irradiating region29e, and thereby evaporates the solvent included in the toner image26cto dry the toner image26c, so that the dried toner image26dof the first color is formed on the print target surface of the print target medium12(Step S20).

Thus, the first color printing means20forms the dried toner image26dof the first color on the print target surface of the print target medium12(Step S10). The second color printing means30, by performing processing steps similar to Step S12to Step20described above, forms the dried toner image36dof the second color on the dried toner image26dof the first color previously formed on the print target surface of the print target medium12(Step S10). The third color printing means40, by performing processing steps similar to Step S12to Step20described above, forms the dried toner image46dof the third color on the dried toner image36dof the second color previously formed on the print target surface of the print target medium12(Step S10). The fourth color printing means50, by performing processing steps similar to Step S12to Step20described above, forms the dried toner image56dof the fourth color on the dried toner image46dof the third color previously formed on the print target surface of the print target medium12(Step S10). Thus, the second color printing means30, third color printing means40, and fourth color printing means50similarly perform Steps S10one after another, so that the composite dried toner image70asequentially including the dried toner images26d,36d,46d, and56dis formed on the print surface of the print target medium12.

After Steps S10are completed for all of the colors, the fixing device60collectively heats the multiple images of the composite dried toner image70aformed on the print target surface of the print target medium12passing through the heating region60a. In the heating region60a, the fixing device60combines the multiple images of the composite dried toner image70ainto one print image70band fixes the print image70bto the print target surface of the print target medium12. As a result, the printed matter70with the print image70bprinted thereon is obtained (Step S22).

The electrophotographic printer10and the printing method using the same are configured and characterized as described thus far. The liquid toners26a,36a,46a, and56aeach containing the ultraviolet absorbent for absorption of ultraviolet light are adsorbed to the latent charge images so as to develop the toner images26b,36b,46b, and56b, and these toner images are then transferred onto the print target medium12. The transferred toner images26c,36c,46c, and56care dried by being irradiated with ultraviolet light having an ultraviolet irradiation band including part of the ultraviolet absorption band of the ultraviolet absorbent in the liquid toners26a,36a,46a, and56a. As a result, the dried toner images26d,36d,46d, and56dare formed. In the electrophotographic printer10and the printing method using the same, the printing materials used are the liquid toners26a,36a,46a, and56ain which solid particles of the coloring ink have an average particle size less than or equal to 2.0 μm. Therefore, improvement of image quality may be feasible in contrast to use of, for example, a powder toner containing rather angular particles, i.e., less spherical particles having a relatively large average particle size of approximately 5.5 μm.

In the electrophotographic printer10and the printing method using the same, the printing materials used are the liquid toners26a,36a,46a, and56acharacterized in that, as for an ultraviolet absorption index that refers to the ratio of light absorption energy in the ultraviolet absorption band to the whole light absorption energy, the ultraviolet absorption index in one toner layer is greater than or equal to 70%, and the absorption band is effective for light in the ultraviolet absorption band having a wavelength less than or equal to 400 nm. Further, these liquid toners are irradiated with ultraviolet light from the ultraviolet irradiating-drying units29,39,49, and59having an ultraviolet irradiation band including part of the ultraviolet absorption band of the ultraviolet absorbent in the liquid toners26a,36a,46a, and56a. The electrophotographic printer10and the printing method using the same, therefore, may allow the toner images26c,36c,46c, and56cto readily dry without much heating of the print target medium12that hardly absorbs ultraviolet light. This may reduce the risk of the print target medium12being thermally deformed. The electrophotographic printer10and the printing method using the same may reduce the risk of paper jam and may also reduce the risk of heat-induced discoloration of the toner images26c,36c,46, and56c. The electrophotographic printer10and the printing method using the same may achieve an improved image quality, while reducing the risk of the print target medium12being deformed.

In the electrophotographic printer10and the printing method using the same, the printing materials used are the liquid toners26a,36a,46a, and56ain which solid particles in the coloring inks preferably have an average particle size less than or equal to 1.5 μm, more preferably less than or equal to 1.0 μm. Improvement of image quality, therefore, may be feasible with the electrophotographic printer10and the printing method using the same.

In the electrophotographic printer10and the printing method using the same, the printing material used are the liquid toners26a,36a,46a, and56acharacterized in that, as for an ultraviolet absorption index that refers to the ratio of light absorption energy in the ultraviolet absorption band to the whole light absorption energy, the ultraviolet absorption index in one toner layer may be greater than or equal to 80% or greater than or equal to 90%. The electrophotographic printer10and the printing method using the same, therefore, may allow the toner images26c,36c,46c, and56cto efficiently dry, further reducing the risk of the print target medium12being thermally deformed.

In the electrophotographic printer10and the printing method using the same, the printing materials used are the liquid toners26a,36a,46a, and56acharacterized in that a center wavelength in the ultraviolet absorption band of the ultraviolet absorbent is preferably less than or equal to 385 nm, more preferably less than or equal to 365 nm, and these liquid toners are irradiated with ultraviolet light having a center wavelength in the ultraviolet irradiation band suitable for the liquid toners26a,36a,46a, and56a. The electrophotographic printer10and the printing method using the same, therefore, may allow the toner images26c,36c,46c, and56cto readily dry with less heating of the print target medium12. This may further reduce the risk of the print target medium12being thermally deformed.

In the electrophotographic printer10and the printing method using the same, the ultraviolet irradiating-drying units29,39,49, and59are used, in which the ultraviolet emitting diodes may be evenly spaced at intervals and directed toward the print target surface of the print target medium12. The electrophotographic printer10and the printing method using the same, therefore, may evenly heat and dry the toner images26c,36c,46c, and56c. This may further reduce the risk of the print target medium12being thermally deformed.

In the electrophotographic printer10and the printing method using the same, ultraviolet light radiated with an accumulated irradiation energy greater than or equal to 600 mJ/cm2·g and less than or equal to 10000 mJ/cm2·g is used. The electrophotographic printer10and the printing method using the same, therefore, may allow the toner images26c,36c,46c, and56cto more efficiently dry with less heating of the print target medium12. This may further reduce the risk of the print target medium12being thermally deformed.

In the electrophotographic printer10and the printing method using the same, the ultraviolet irradiating-drying units29,39,49, and59intermittently radiate ultraviolet light at shorter time intervals than time required for the print target medium12to pass through the ultraviolet irradiating region to dry the toner images26c,36c,46c, and56c. The electrophotographic printer10and the printing method using the same, therefore, may avoid overheating of and radiation loss for the toner images26c,36c,46c, and56cand may allow the toner images26c,36c,46c, and56cto more efficiently dry with less heating of the print target medium12. This may further reduce the risk of the print target medium12being thermally deformed.

The electrophotographic printer10and the printing method using the same carries out the steps for electrification, exposure, development, transfer, and drying for each color, allowing the toner images to dry immediately after the transfer. This may prevent the liquid toners26a,36a,46a, and56afrom bleeding on the print target medium12and allow the dried toner images26d,36d,46d, and56dto be adequately flattened. In case where the color printing order, the printing color type(s), and/or the number of printing color types is desirably changed, the electrophotographic printer10and the printing method using the same may facilitate such changes.

The electrophotographic printer10and the printing method using the same heat the composite dried toner image70aat once and thereby fix the print image70bonto the print target medium12. This may provide an integrated multicolor image with a natural color appearance, instead of such an image that appears disorganized in different colors.