IMAGE FORMING APPARATUS

An image forming apparatus includes: an image forming unit that forms an image with a brilliant toner on a recording medium; a medium color determiner that determines whether the recording medium is white or colored; and an image forming controller that controls the image forming unit. When the medium color determiner determines that the recording medium is colored, the image forming controller increases an amount of the brilliant toner per unit area of the image formed on the recording medium as compared to when the recording medium is white.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an electrophotographic image forming apparatus that uses a brilliant toner containing a brilliant pigment for printing.

2. Description of the Related Art

Japanese Patent Application Publication No. 2018-84677 discloses improving the metallic appearance (or brilliance) of images printed by an image forming apparatus using a brilliant toner containing a brilliant pigment, by specifying the brilliant pigment contained in the brilliant toner. However, in some cases, depending on the color of the recording medium, sufficient brilliance cannot be obtained only by specifying the brilliant pigment.

SUMMARY OF THE INVENTION

An aspect of the present invention is intended to provide good brilliance regardless of whether the recording medium is white or colored.

According to an aspect of the present disclosure, there is provided an image forming apparatus including: an image forming unit that forms an image with a brilliant toner on a recording medium; a medium color determiner that determines whether the recording medium is white or colored; and an image forming controller that controls the image forming unit, wherein when the medium color determiner determines that the recording medium is colored, the image forming controller increases an amount of the brilliant toner per unit area of the image formed on the recording medium as compared to when the recording medium is white.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a view illustrating a configuration of main parts of a printer1as an image forming apparatus of an embodiment according to the present disclosure.

The printer1is a color electrophotographic printer of an intermediate transfer system capable of printing five colors of black (K), yellow (Y), magenta (M), cyan (C), and a special color (S). The special color (S) is a special color, such as gold or silver, exhibiting metallic luster, i.e., having brilliance. The special color may be used alone or in combination with the normal colors (i.e., black, yellow, magenta, and cyan) in a superimposed manner. The present embodiment according to the present disclosure describes an example in which the special color is silver.

As illustrated inFIG. 1, a first sheet feeding cassette11stores recording sheets71a(e.g., paper sheets) as recording media stacked therein. A pickup roller31and a pair of sheet feeding rollers12pick up the recording sheets71afrom the first sheet feeding cassette11and sequentially feed them one by one to a conveying path. A pair of conveying rollers13for conveying the recording sheet71aalong the conveying path, a pair of registration rollers14for correcting skew of the recording sheet71a, and a pair of timing rollers15for feeding the recording sheet71ato a secondary transfer portion47at a predetermined time are sequentially disposed downstream of the pair of sheet feeding rollers12in the direction of arrow A, which indicates a convening direction of the recording sheet71a. The first sheet feeding cassette11, pickup roller31, and pair of sheet feeding rollers12constitute a first sheet feeder10.

Also, a second sheet feeder35is provided upstream of the pair of registration rollers14. The second sheet feeder35includes a second sheet feeding cassette36, a pickup roller37, and a pair of sheet feeding rollers38. The second sheet feeding cassette36stores recording sheets71b(e.g., paper sheets) as recording media stacked therein. The pickup roller37and pair of sheet feeding rollers38pick up the recording sheets71bfrom the second sheet feeding cassette36and sequentially feed them one by one to the pair of registration rollers14.

A recording sheet71aor71bis selectively fed to the pair of registration rollers14from the first sheet feeder10and second sheet feeder35. Hereinafter, when the recording sheets71aand71bneed not be distinguished from each other, they will be referred to as recording sheets71.

A developed image forming unit66includes five image drum units (referred to below as ID units)61S,61C,61M,61Y, and61K that respectively form developer images of the special color (S), cyan (C), magenta (M), yellow (Y), and black (K) and five light emitting diode (LED) heads67S,67C,67M,67Y, and67K. When the ID units61S,61C,61M,61Y, and61K need not be distinguished from each other, they will be referred to simply as ID units61. When the LED heads67S,67C,67M,67Y, and67K need not be distinguished from each other, they will be referred to simply as LED heads67.

The five ID units61S to61K are arranged along the direction of arrow B indicating a movement direction in which an intermediate transfer belt44of an intermediate transfer belt unit30(to be described later) moves in an upper portion of the intermediate transfer belt unit30, and are arranged in order from the upstream side in the direction of arrow B. The five LED heads67S to67K are arranged to face the respective ID units61S to61K to illuminate predetermined portions of photosensitive drums136of the ID units61as described later.

InFIG. 1, the X axis is taken in the movement direction in which the intermediate transfer belt44moves in the upper portion of the intermediate transfer belt unit30, the Y axis is taken in a rotation axis direction of the photosensitive drums136, and the Z axis is taken in a direction perpendicular to both the X and Y axes. The X, Y, and Z axes illustrated in the other drawings (to be described later) indicate the same directions. Specifically, the X, Y, and Z axes in each drawing indicate arrangement directions when the part illustrated in the drawing constitutes the printer1illustrated inFIG. 1. Here, it is assumed that the Z axis is oriented in a substantially vertical direction.

Internal configurations of the ID units61are the same, and thus will be described by taking the ID unit61K for black (K) as an example.FIG. 2is a view illustrating the internal configuration of the ID unit61. InFIG. 1, the ID units61are illustrated such that the shape of a developer container112(seeFIG. 2) of the ID unit61S is different from the shapes of developer containers112of the other ID units61.

As illustrated inFIG. 2, the ID unit61is generally constituted by an image forming main portion111, the developer container112, a developer supply portion113, and the LED head67. The ID unit61and parts thereof have sufficient lengths in the Y axis direction corresponding to the length of the recording sheet71in the Y axis direction. Thus, many of the parts are longer in the Y axis direction than in the X and Z axis directions, and formed in shapes elongated in the Y axis direction.

The developer container112contains developer, and is configured to be attachable to and detachable from a main body of the ID unit61. When the developer container112is attached to the main body of the ID unit61, it is attached to the image forming main portion111through the developer supply portion113.

FIG. 3is an external perspective view of the developer container112schematically illustrating an interior of the developer container112with part of an exterior of the developer container112omitted. As illustrated inFIG. 3, the developer container112includes a container housing120extending in the Y axis direction. A storage chamber121, which is a cylindrical space extending in the Y axis direction, is formed in the container housing120. The storage chamber121contains the developer. Hereinafter, the leftward, rightward, forward, rearward, upward, and downward directions may be defined as viewed from the direction of arrow B illustrated inFIG. 3(or the negative side in the X axis direction).

Substantially at a center of a bottom of the storage chamber121in the left-right direction, a supply opening122through which a space in the storage chamber121communicates with the external space is formed, and a shutter123that opens and closes the supply opening122is provided. The shutter123is connected to a lever124, and opens or closes the supply opening122in accordance with rotation of the lever124. The lever124is operated by a user when the developer container112is attached to or detached from the ID unit61.

For example, in a state in which the developer container112is not attached to the ID unit61(seeFIG. 2), the shutter123closes the supply opening122and prevents the developer contained in the storage chamber121from leaking to the outside. When the developer container112is attached to the ID unit61, the lever124is rotated in a predetermined opening direction, thereby moving the shutter123to open the supply opening122.

This makes the space in the storage chamber121communicate with a space in the developer supply portion113, and the developer in the storage chamber121of the developer container112is supplied to the image forming main portion111through the developer supply portion113. Also, when the developer container112is detached from the ID unit61, the lever124is rotated in a predetermined closing direction, thereby moving the shutter123to close the supply opening122.

Also, an agitator125is disposed in the storage chamber121. The agitator125is formed in a shape such that an elongated member is spiraled about an imaginary central axis extending along the left-right direction, and is rotatable about the imaginary central axis in the storage chamber121. An agitator driver126is disposed at an end of the container housing120.

The agitator driver126is connected to the agitator125. When the agitator driver126is supplied with a driving force from a predetermined drive source disposed in a housing2(seeFIG. 1), it transmits the driving force to the agitator125and rotates the agitator125. Thereby, the developer container112can agitate the developer contained in the storage chamber121, and prevent the developer from aggregating and feed the developer to the supply opening122.

The image forming main portion111(seeFIG. 2) includes an image forming housing130, a developer storage space131, a first supply roller132, a second supply roller133, a developing roller134, a developing blade135, the photosensitive drum136, a charging roller137, and a cleaning blade138. The first supply roller132, second supply roller133, developing roller134, photosensitive drum136, and charging roller137are each formed in a cylindrical shape having a central axis extending in the left-right direction and rotatably supported by the image forming housing130.

In the ID unit61S for the special color (S), the developer container112contains a brilliant toner (to be described later) as a developer and is attached to the image forming main portion111through the developer supply portion113.

The developer storage space131contains the developer supplied from the developer container112through the developer supply portion113. The first supply roller132and second supply roller133each include an elastic layer that is formed by conductive urethane rubber foam or the like and forms a periphery of the roller. The developing roller134includes an elastic layer, a conductive surface layer, or the like forming a periphery of the roller. The developing blade135is formed by, for example, a stainless steel sheet having a predetermined thickness, and a part of the developing blade135abuts the periphery of the developing roller134with the developing blade135slightly elastically deformed.

The photosensitive drum136includes a thin-film charge generation layer and a thin-film charge transport layer that are sequentially formed and form a periphery of the drum, and is chargeable. The charging roller137includes a conductive elastic body that forms a periphery of the roller. The periphery of the charging roller137abuts the periphery of the photosensitive drum136. The cleaning blade138is formed by, for example, a thin-plate-shaped resin member, and a part of the cleaning blade138abuts the periphery of the photosensitive drum136with the cleaning blade138slightly elastically deformed.

The LED head67is located above the photosensitive drum136in the image forming main portion111. The LED head67includes multiple light emitting element chips arranged linearly in the left-right direction, and causes light emitting elements of the light emitting element chips to emit light in a light emitting pattern based on an image data signal supplied from an image formation controller205to be described later (seeFIG. 4).

The image forming main portion111is supplied with a driving force from a motor (not illustrated), thereby rotating the first supply roller132, second supply roller133, developing roller134, and charging roller137in the directions of the arrows (clockwise inFIG. 2) and rotating the photosensitive drum136in the direction of the arrow (counterclockwise inFIG. 2). Further, the image forming main portion111applies respective predetermined bias voltages supplied from the image formation controller205(seeFIG. 4), to the first supply roller132, second supply roller133, developing roller134, developing blade135, and charging roller137, thereby charging them.

The first supply roller132and second supply roller133are charged to cause the developer in the developer storage space131to adhere to their peripheries, and are rotated to apply the developer to the periphery of the developing roller134. The developing blade135removes excess developer from the periphery of the developing roller134to form a thin layer of developer on the periphery. The periphery of the developing roller134with the thin layer of developer formed thereon is brought into contact with the periphery of the photosensitive drum136.

The charging roller137abuts the photosensitive drum136while being charged, thereby uniformly charging the periphery of the photosensitive drum136. The LED head67emits light at predetermined time intervals in a light emitting pattern based on an image data signal supplied from the image formation controller205(seeFIG. 4), thereby sequentially exposing the photosensitive drum136. Thereby, an electrostatic latent image is sequentially formed on the periphery of the photosensitive drum136, in the vicinity of an upper end of the photosensitive drum136.

Then, rotation of the photosensitive drum136in the direction of the arrow brings the part with the electrostatic latent image formed thereon into contact with the developing roller134. Thereby, developer adheres to the periphery of the photosensitive drum136based on the electrostatic latent image, thereby forming a developer image based on the image data. Further, rotation of the photosensitive drum136in the direction of the arrow brings the developer image to the vicinity of a lower end of the photosensitive drum136.

As illustrated inFIG. 1, the intermediate transfer belt unit30is disposed below the ID units61in the housing2. The intermediate transfer belt unit30includes a drive roller41that is driven by a drive source (not illustrated), a tension roller43that applies tension to the intermediate transfer belt44, a pair of reverse bending rollers63, a secondary transfer backup roller42that is disposed to face a secondary transfer roller46and constitutes the secondary transfer portion47, and the intermediate transfer belt44that is stretched around these rollers.

The intermediate transfer belt unit30further includes five primary transfer rollers45S,45C,45M,45Y, and45K that are disposed to respectively face the photosensitive drums136of the ID units61S,61C,61M,61Y, and61K. When the primary transfer rollers45S to45K need not be distinguished from each other, they will be referred to simply as primary transfer rollers45. Each primary transfer roller45primarily transfers a developer image formed on the photosensitive drum136facing the primary transfer roller45, onto the intermediate transfer belt44.

The intermediate transfer belt unit30primarily transfers developer images formed by the developed image forming unit66onto the intermediate transfer belt44as described above, and conveys the primarily transferred developer images to the secondary transfer portion47. In the secondary transfer portion47, the secondary transfer roller46secondarily transfers the developer images primarily transferred on the intermediate transfer belt44onto a recording sheet71fed from the pair of timing rollers15.

A fixing unit62includes an upper roller62afor heating that is driven and rotated in the direction of the arrow by a drive source (not illustrated), and a lower roller62bfor pressing that is pressed against and rotated by the upper roller62a. The fixing unit62applies heat and pressure to a developer image on a recording sheet71fed from the secondary transfer portion47to fuse the developer image and fix the fused developer image to the recording sheet71while conveying the recording sheet71at a predetermined conveying speed with the recording sheet71nipped at the nip portion.

A first separator51is set to a discharge position for guiding a recording sheet71discharged from the fixing unit62and conveyed by a pair of discharge rollers16to pairs of discharge rollers17,18,19, and20or a reprinting position for guiding the recording sheet71to a reprinting conveyor32. The pairs of discharge rollers17to20discharge a recording sheet71guided by the first separator51to a face-down stacker72.

The reprinting conveyor32includes a second separator52that determines the path of a recording sheet71guided by the first separator51set at the reprinting position, a pair of forward reverse rollers21that conveys a recording sheet71forward or backward in a switchback manner as needed, a third separator53that determines the path of a recording sheet71, a pair of 2-path conveying rollers22that conveys a recording sheet71to be subjected to 2-path printing, a pair of double-sided printing conveying rollers23that conveys a recording sheet71to be subjected to double-sided printing, pairs of reprinting conveying rollers24,25, and26that reconvey a recording sheet71fed from them to the pair of timing rollers15, and a retreat portion27that temporarily accommodates a recording sheet71in double-sided printing. The reprinting conveyor32may be configured as a unit.

Each of the roller pairs is supplied with power from a conveyance drive motor (not illustrated) through a drive transmission portion (not illustrated), and each of the separators is supplied with power for rotational position setting from a solenoid actuator (not illustrated) through a motion transmission portion.

In the reprinting conveyor32configured as described above, conveyance of a recording sheet71in 2-path printing will be described.

A recording sheet71that has been subjected to the first fixing by the fixing unit62(or the first printing) is guided to a 2-path printing path28by the second separator52set at an introduction position, the third separator53set at a 2-path printing position, and the pair of forward reverse rollers21operating for forward conveyance.

The recording sheet71that has been conveyed to the 2-path printing path28is conveyed in the direction of arrow C by the pair of 2-path conveying rollers22and pairs of reprinting conveying rollers24,25, and26, returns to the pair of timing rollers15such that the surface (or front surface) subjected to the first printing is an upper surface (or a surface to be printed), and is subjected to the second printing (which is performed on the same surface of the same recording sheet). The first separator51is set to the discharge position, and the recording sheet71after the second printing is conveyed by the pairs of discharge rollers16to20and then discharged to the face-down stacker72.

In this embodiment, the ID unit61S containing the brilliant toner is used together with the other ID units61Y,61M,61C, and61K in 1-path printing, in which, for example, developer images are formed by the ID units61S,61C,61M,61Y, and61K, sequentially transferred onto the intermediate transfer belt44in a superimposed manner, and transferred onto a recording sheet71at a time. However, this is not mandatory, and the ID unit61S may be used in 2-path printing, in which, for example, in the first printing, color image printing is performed by forming developer images with the ID units61C,61M,61Y, and61K, sequentially transferring the developer images onto the intermediate transfer belt44in a superimposed manner, and transferring the developer images onto a recording sheet71at a time to form a color image, and in the second printing, special color image printing is performed by forming a brilliant toner image with the ID unit61S and transferring the brilliant toner image onto the color image on the recording sheet71.

Next, conveyance of a recording sheet71in the reprinting conveyor32in double-sided printing will be described.

A recording sheet71that has been subjected to single-sided printing is conveyed into the retreat portion27from its leading edge by the second separator52set at the introduction position, the third separator53set at a double-sided printing position, and the pair of forward reverse rollers21operating for forward conveyance.

When the trailing edge of the recording sheet71passes through the second separator52and the passage of the trailing edge is detected by, for example, the second separator52, the pair of forward reverse rollers21reverses and rotates in a discharge direction while nipping the trailing edge of the recording sheet71and the second separator52is set to a discharge position.

Thereby, after being mostly accommodated in the retreat portion27, the recording sheet71is conveyed backward to a double-sided printing path29, is conveyed in the direction of arrow C by the pair of double-sided printing conveying rollers23and pairs of reprinting conveying rollers24,25, and26to return to the pair of timing rollers15such that the surface (or back surface) that has not yet been subjected to printing is an upper surface (or a surface to be printed), and is subjected to printing on the back surface in the same manner as in the printing on the front surface. The first separator51is set to the discharge position, and the recording sheet71after the double-sided printing is conveyed by the pairs of discharge rollers16to20and then discharged to the face-down stacker72.

FIG. 4is a block diagram illustrating a configuration of main parts of a portion relating to the present disclosure of a system of the printer1of the present embodiment. The following description is made with reference toFIG. 4.

As illustrated inFIG. 4, the printer1includes an image generator201that receives print information from an external host computer250and analyzes the received print information, an engine controller203that controls engine operation, and an interface202that receives information required for engine control from the image generator201and communicates with the engine controller203.

The engine controller203includes a main controller204that provides instructions for an operational process for image formation on the basis of information transmitted from the interface202, the image formation controller205that controls operation for image formation, an image conveyance controller206that controls conveyance of a formed image, a fixing controller207that performs control of a fixing temperature or the like, a sheet conveyance controller208that monitors the position of a recording sheet71and controls conveyance of the recording sheet71, a secondary transfer controller209that performs secondary transfer control, a printing path controller210that controls positional shift of the first to third separators51to53, and a sheet color determiner211that determines the type of color of a recording sheet71.

The image generator201receives print information from the host computer250to generate a print image, and transmits the print image to the engine controller203through the interface202. The main controller204, which is also a printing speed setter, provides instructions, including the printing speed, for an operational process for image formation, to the image formation controller205, image conveyance controller206, fixing controller207, sheet conveyance controller208, secondary transfer controller209, and printing path controller210.

The image formation controller205controls the ID units61, LED heads67, and the like of the developed image forming unit66to form toner images on the photosensitive drums136. The image conveyance controller206controls the intermediate transfer belt unit30to transfer the toner images formed by the image formation controller205onto the intermediate transfer belt44and convey the toner images to the secondary transfer portion47. The sheet conveyance controller208controls conveyance of a recording sheet71by all the roller pairs and the fixing unit62, and the speed of the conveyance.

Also, the image formation controller205is configured to change the amount of brilliant toner of the image formed by the ID unit61S for the special color. The amount can be changed by, for example, controlling the amount of exposure light from the LED head67S, the voltage applied to the developing roller134, the voltages applied to the first and second supply rollers132and133, the voltage applied to the transfer roller45S, and the like. In this embodiment, the image formation controller205changes the amount of brilliant toner of the image formed by the ID unit61S for the special color, on the basis of a determination by the sheet color determiner211or a medium color calculator, as described later.

When a recording sheet71conveyed under control by the sheet conveyance controller208and toner images conveyed under control by the image conveyance controller206reach the secondary transfer portion47, the secondary transfer controller209controls the secondary transfer portion47to secondarily transfer the toner images onto the recording sheet71. The fixing controller207controls the fixing unit62to apply heat and pressure to a toner image on a recording sheet71to fuse the toner image and fix the image to the recording sheet71.

The developed image forming unit66, intermediate transfer belt unit30, and secondary transfer portion47correspond to or constitute an image forming unit (or print engine)80. The image formation controller205, image conveyance controller206, and secondary transfer controller209correspond to or constitute an image forming controller90. The image forming unit80may form an image with the brilliant toner on a recording sheet71. The image forming controller90may control the image forming unit80.

In this embodiment, since the image forming apparatus uses an intermediate transfer system, the intermediate transfer belt unit30is included in the image forming unit. However, in the case of an image forming apparatus using a direct transfer system, since the image forming apparatus includes no intermediate transfer belt unit, a developed image forming unit that forms an image with a brilliant toner and a transfer unit that transfers the image onto a sheet correspond to the image forming unit.

The printing path controller210controls the first to third separators51to53to set the conveyance path of a recording sheet71in 2-path printing and double-sided printing.

The sheet color determiner211as a medium color determiner determines the color of a recording sheet71on which an image is to be formed (or printed). The sheet color determiner211may determine whether the recording sheet71is white or colored. In this embodiment, the sheet color determiner211determines the sheet color on the basis of an operation by a user. Specifically, one or more buttons for sheet color selection are disposed in a user interface (e.g., a printer panel)220, and the sheet color determiner211determines the sheet color by detecting an operation of the buttons by a user for selecting the sheet color. However, it is also possible that a storage221stores a correspondence table (e.g., as shown inFIG. 6) in which the names (or types) of recording media are associated with flop indexes FI0(to be described later) of the recording media, one or more buttons for medium name (or type) selection are disposed in the user interface220, and when the name (or type) of the recording medium is selected with the buttons, a medium color calculator212determines the flop index FI0of the recording medium on the basis of the correspondence table. In this case, the sheet color determiner211may determine the sheet color on the basis of the determined flop index FI0.

It is also possible that a sheet color measurement unit11aas a medium color calculator is provided in the sheet feeding cassette11(seeFIG. 1) and determines the flop index of the recording sheet71, and the sheet color determiner211determines the sheet color on the basis of the determined flop index. It is also possible that the sheet color determiner211determines the sheet color on the basis of the flop index directly input through the user interface220by a user.

The engine controller203may be processing circuitry. For example, the engine controller203may be a processor that executes a program stored in a memory222to provide the above functions of the engine controller203, or may be dedicated hardware.

InFIG. 4, the above system including the engine controller203, user interface220, storage221, and memory222is provided in the printer1. However, part or all of the system may be provided outside the printer1.

Next, production of the brilliant toner for providing brilliance to a printed image contained in the developer container112of the ID unit61S for the special color will be described. Brilliant toner A was produced as follows.

An aqueous medium with an inorganic dispersant dispersed therein was first obtained. Specifically, 920 parts by weight of industrial trisodium phosphate dodecahydrate was mixed with 27000 parts by weight of pure water, and dissolved therein at a liquid temperature of 60° C. Then, the resulting liquid was added with dilute nitric acid for pH adjustment. The resulting liquid was added with an aqueous calcium chloride solution obtained by dissolving 440 parts by weight of industrial calcium chloride anhydride in 4500 parts by weight of pure water, and was high-speed stirred with a Line Mill (manufactured by Primix Corporation) at a rotation speed of 3566 rpm for 34 minutes while being maintained at a liquid temperature of 60° C. Thereby, an aqueous phase containing a suspension stabilizer (or inorganic dispersant) was prepared. Meanwhile, a pigment dispersion oil medium was obtained.

Specifically, a pigment dispersion liquid was prepared by mixing 680 parts by weight of a brilliant pigment (having a volume median size of 5.4 μm) and 30 parts by weight of a charge control agent (BONTRON E-84, manufactured by Orient Chemical Industries Co., Ltd.) with 7430 parts by weight of ethyl acetate. Then, the pigment dispersion liquid was heated to 55° C. and stirred, added with 260 parts by weight of an ester wax (WE-4, manufactured by NOF Corporation) and 2430 parts by weight of polyester resin, and stirred until solid dissolved. Thereby, an oil phase was prepared.

The oil phase was added to the aqueous phase that had been cooled to 55° C., and suspended by stirring for 5 minutes at a rotation speed of 1000 rpm, so that particles were formed. Then, the ethyl acetate was removed by distilling under reduced pressure.

The slurry containing the particles was added with nitric acid so that the pH of the slurry was adjusted to 1.6 or lower, and was stirred. Tricalcium phosphate as a suspension stabilizer was dissolved therein, and the mixture was dehydrated. Then, the dehydrated particles were re-dispersed in pure water, stirred, and water-washed. After that, through dehydration, drying, and classification, toner base particles were produced. The toner base particles were collected by the classification process.

Then, in an external addition process, the collected toner base particles were added and mixed with 1.0 wt % of small silica (AEROSIL RY200, manufactured by Nippon Aerosil Co., Ltd.) and 1.5 wt % of colloidal silica (X-24-9163A, manufactured by Shin-Etsu Chemical Co., Ltd.), so that brilliant toner A having a volume median size of 12.79 μm was obtained.

The volume median size (Dv50) refers to the particle size at which the cumulative volume percentage is 50%. Here, for each of the brilliant pigment and brilliant toner A, the volume median size was measured by using an accurate particle size distribution analyzer (Multisizer 3, manufactured by Beckman Coulter, Inc.) under the following measurement conditions:

Dispersion liquid: a liquid obtained by dissolving NEOGEN S-20F (manufactured by DKS Co., Ltd.) in the above electrolyte and adjusting the concentration to 5%

Multisizer 3 from Beckman Coulter, Inc. is a particle size distribution measurement device based on the Coulter principle. The Coulter principle is a method, called aperture electrical resistance method, of measuring the volume of a particle by passing a constant current through an aperture in an electrolyte solution and measuring a change in the electrical resistance across the aperture when the particle passes through the aperture.

Here, 10 to 20 mg of the measurement sample was added to 5 ml of the dispersion liquid, dispersed with an ultrasonic disperser for 1 minute, added with 25 ml of the electrolyte, dispersed with the ultrasonic disperser for 5 minutes, and passed through a mesh having an opening size of 75 μm to remove aggregates, so that a sample dispersion liquid was prepared. The sample dispersion liquid was added to 100 ml of the electrolyte, and 30000 particles were measured. Then, the volume median size was determined from the volume particle size distribution of the 30000 particles.

As a comparative example, brilliant toner B having a volume median size of 13.45 μm was produced in the same manner as brilliant toner A except that a brilliant pigment having a volume median size of 8.7 μm was used.

With brilliant toners A and B as experimental samples, a brilliance printing experiment was performed on recording media of different colors as described below.

The printing experiment was performed by using an experimental printer (C941dn, manufactured by Oki data Corporation). The configuration of main parts necessary for the printing experiment of the experimental printer is the same as the configuration of the printer1illustrated inFIG. 1. Thus, the printing experiment will be described with reference to the printer1inFIG. 1. In the description of the printing experiment, media referred to as recording sheets71inFIG. 1will be referred to as recording media.

Brilliant toner A was put in the developer container112of the ID unit61S for the special color (S), and a 100% solid image (having a print image density of 100%) was printed with brilliant toner A on each of the following recording media of different colors while the amount (referred to below as the brilliant toner deposition amount) of brilliant toner per unit area of the brilliant toner image formed on the recording medium before fixing by the fixing unit62was adjusted to each value shown inFIG. 7(to be described later). In the printing, the conveyance speed (i.e., printing speed) of the recording medium was 18 ppm (in A4 landscape printing), and the fixing temperature of the fixing unit62was 160° C.

In the brilliance printing experiment, the ID units61other than the ID unit61S were removed from a main body of the printer1and not used.

Similarly, as a comparative example, brilliant toner B was put in the developer container112of the ID unit61S for the special color (S), and the 100% solid image was printed with brilliant toner B on each of the following recording media of different colors while the brilliant toner deposition amount was adjusted to each value shown inFIG. 9(to be described later).

The recording media used in the experiment were

a white paper sheet (OS coated paper W of 127 g/m2, manufactured by Fuji Xerox CO., Ltd.),

a black paper sheet (colored high quality paper of black, heavy paper, and 90 g/m2, manufactured by Hokuetsu Corporation),

a blue paper sheet (colored high quality paper of blue, heavy paper, and 90 g/m2, manufactured by Hokuetsu Corporation), and

a red paper sheet (colored high quality paper of red, heavy paper, and 90 g/m2, manufactured by Hokuetsu Corporation).

The brilliance (or metallic luster) of each of the recording media before printing and the printed 100% solid images was measured by using a variable angle photometer (GC-5000L, manufactured by Nippon Denshoku Industries Co., Ltd.). Specifically, as illustrated inFIG. 5, with the variable angle photometer, the recording medium was illuminated with a light ray C at an angle of 45° relative to the surface of the recording medium, light reflected by the recording medium was received at angles 0°, 30°, and −65° relative to a direction perpendicular to the surface of the recording medium, and lightness indexes L*0, L*30, and L*−65were respectively calculated from the light reception results obtained at 0°, 30°, and −65°. Then, the brilliance of the recording medium or image was determined by calculating a flop index FI by substituting the lightness indexes into the following equation:

For each of the solid images, an increase in brilliance due to printing was evaluated by using a value (referred to here as a print brilliance score) ΔFI obtained by subtracting the flop index FI0of the recording medium before printing from the flop index of the solid image. The greater the print brilliance score ΔFI, the greater the increase in brilliance due to printing. When the score ΔFI was not less than 7.0, the increase in brilliance due to printing was determined to be good.

For comparison between the flop indexes FI0and specular reflectances of the recording media before printing, the specular reflectances (or glosses) of the recording media before printing were measured by using a surface gloss meter (micro-gloss 75°, manufactured by BYK-Gardner).

For white recording media, to determine the differences between the flop indexes FI0and specular reflectances, the flop indexes and specular reflectances (or glosses) of the following recording media, which were not used for the brilliance printing experiment, were measured, and compared:

Excellent Gloss of 128 g/m2, manufactured by Oki data Corporation,

Color Copy of 250 g/m2, manufactured by Mondi, and

Excellent White of 80 g/m2, manufactured by Oki data Corporation.

FIG. 6is a table showing the flop indexes FI0and specular reflectances of the recording media of the respective colors before printing.

FIG. 7is a table showing the print brilliance scores ΔFI obtained by printing the 100% solid image with brilliant toner A on the recording media of the respective colors while setting the brilliant toner deposition amount to each of the multiple values as described above.FIG. 8is a graph obtained by plotting the values ofFIG. 7.

FIG. 9is a table showing the print brilliance scores ΔFI obtained by printing the 100% solid image with brilliant toner B (as the comparative example) on the recording media of the respective colors while setting the brilliant toner deposition amount to each of the multiple values.FIG. 10is a graph obtained by plotting the values ofFIG. 9.

FIG. 6shows that for the white recording media, the highest specular reflectance is 69.8%, the lowest specular reflectance is 8.2%, and the difference is great, whereas the flop indexes FI0of the white recording media are not greatly different and depend on the color. Thus, it is conceivable that the specular reflectance and flop index FI0are completely different parameters.

From the experimental results ofFIG. 6, it is possible to determine that a recording medium is colored when the flop index FI0is not less than 7.0, and that the recording medium is white when the flop index FI0is not greater than 5.6. Thus, the sheet color determiner211(seeFIG. 4) determines that a recording sheet71stored in the sheet feeding cassette11is colored when the flop index of the recording sheet71is not less than 7.0, and that the recording sheet71stored in the sheet feeding cassette11is white when the flop index of the recording sheet71is not greater than 5.6. As described above, the flop index of the recording sheet71is input through the user interface220. Alternatively, when the sheet color measurement unit11ais provided in the sheet feeding cassette11, the flop index of the recording sheet71is determined from information indicating the flop index of the recording sheet71obtained by measurement by the sheet color measurement unit11a.

In this embodiment, the sheet color determiner211determines that a recording medium is white when the flop index of the recording medium is not greater than 5.6, and that the recording medium is colored when the flop index is not less than 7.0. However, the sheet color determiner211may determine whether a recording medium is white or colored, by using a predetermined flop index as a threshold. The predetermined flop index may be, for example, 6.3, which is a middle value between the flop indexes 5.6 and 7.0.

As shown inFIGS. 7 and 8, in the case of using brilliant toner A with brilliant pigment having a small particle size, for the white recording medium, when the brilliant toner deposition amount is not greater than a specific amount, the score ΔFI is stable within a range of 7.0 or higher, and when the brilliant toner deposition amount is greater than the specific amount, there is a tendency that the score ΔFI decreases as the brilliant toner deposition amount increases. On the other hand, for each of the colored recording media of black, blue, and red, when the brilliant toner deposition amount is not less than a specific amount, the score ΔFI is stable within a range of 7.0 or higher, and when the brilliant toner deposition amount is less than the specific amount, there is a tendency that the score ΔFI decreases as the brilliant toner deposition amount decreases.

When the amount of brilliant toner per unit area of a brilliant toner image on a recording medium is small, the space between the brilliant pigment particles is large on the printed surface after fixing, and the recording medium can be seen through the space. Since the white recording medium has a very high reflectance for white light, even when a brilliant toner image is formed on the white recording medium such that the white recording medium can be seen through the space between the brilliant pigment particles, the brilliance is high.

Also, since the flop index of the white recording medium before printing is 4.0 and low, even when a brilliant toner image is formed on the white recording medium with a small brilliant toner deposition amount, the flop index of the brilliant toner image is sufficiently higher than the flop index of the recording medium. On the other hand, as the brilliant toner deposition amount increases, the amount of brilliant pigment increases, the space reduces, and the brilliant pigment particles aggregate, which reduces reflection of illumination light and reduces the brilliance.

For each of the colored recording media of black, blue, and red, when the recording medium is illuminated with white light, it absorbs light other than the light of the color of the recording medium, which reduces the reflected light. Thus, when a brilliant toner image is formed on the recording medium with a small brilliant toner deposition amount such that the recording medium can be seen through the space between the brilliant pigment particles, the brilliance is low. On the other hand, as the brilliant toner deposition amount increases, the pigment aggregation increases, but the space between the brilliant pigment particles decreases, which reduces absorption of white light by the recording medium and increases the brilliance. Also, since the flop index of the recording medium before printing is high, a large amount of brilliant pigment is required to make the flop index of the brilliant toner image sufficiently higher than the flop index of the recording medium.

For brilliant toner B, which was used as a comparative example in the brilliance printing experiment, the particle size of the brilliant pigment is greater than that of brilliant toner A, but the particle size of the toner itself is substantially the same as that of brilliant toner A. Thus, the number of brilliant pigment particles included in a toner particle is less than that of brilliant toner A.

When the amount of brilliant toner per unit area of a brilliant toner image formed on a recording medium with brilliant toner B is the same as the amount of brilliant toner per unit area of a brilliant toner image formed on a recording medium with brilliant toner A, the number of brilliant pigment particles per unit area of the toner image formed on the recording medium with brilliant toner B is less than that of the toner image formed on the recording medium with brilliant toner A. Thus, for brilliant toner B, as shown inFIGS. 9 and 10, in particular in the blue and red recording media, even when the brilliant toner deposition amount is increased, since the recording medium can be seen through the space between the brilliant pigment particles, the brilliance is not sufficiently increased.

In the above example, brilliant toner A containing the brilliant pigment having a volume median size of 5.4 μm was used. However, it is conceivable that brilliant toners containing brilliant pigments having volume median sizes less than 5.4 μm also provide the same effects.

From the above experimental results, in performing printing with a brilliant toner containing a brilliant pigment having an appropriate volume median size (here 5.4 μm), in order to obtain a good score ΔFI (here not less than 7.0), it is preferable to make the brilliant toner deposition amount not greater than a predetermined value (specifically 0.36 mg/cm2) when the recording medium is white, and make the brilliant toner deposition amount not less than a predetermined value (specifically 0.36 mg/cm2) when the recording medium is not white and colored (here black, blue, or red).

Also, it can be seen that there is a tendency that when printing is performed on a colored recording medium, increasing the brilliant toner deposition amount increases the score ΔFI better than when printing is performed on a white recording medium.

For each of the recording media of the respective colors (white, black, blue, and red), an experiment was performed to determine how the flop index of the recording medium after printing varies with the printing speed.

The experiment was performed under the following conditions. Density adjustment parameters for adjusting the brilliant toner deposition amount were fixed. Specifically, the voltage applied to the developing roller134was set at −200 V, and the voltages applied to the first and second supply rollers132and133were set at −360 V. The 100% solid image was printed on the recording medium at different printing speeds.

The printing was performed by the experimental printer (C941dn, manufactured by Oki data Corporation) using brilliant toner A, as with the brilliance printing experiment.

The printing speeds were 18, 22, 27, 32, and 40 ppm (in A4 landscape printing), as shown inFIG. 11. The A4 landscape printing indicates that the recording medium has a size of A4 and is conveyed with its longitudinal direction parallel to its conveyance direction. The flop index of the recording medium after the printing at each printing speed was measured.FIG. 11show the experimental results.FIG. 11also shows the linear speed at which the recording medium is conveyed.

FIG. 11shows that for the white recording medium, the flop index after printing increases as the printing speed decreases, whereas for the colored recording media of black, blue, and red, the flop index after printing varies little with the printing speed.

On the basis of the results of the above experiments, the printer1of the present embodiment performs setting so that when the sheet color determiner211determines that the recording sheet71to be used for printing is colored (or not white), the brilliant toner deposition amount is increased as compared to when printing is performed on a white recording sheet71. The printer1may set the brilliant toner deposition amount to be not less than a predetermined value (here 0.36 mg/cm2) when the recording sheet is determined to be colored (or not white), and set the brilliant toner deposition amount to be less than a predetermined value (here 0.36 mg/cm2) when the recording sheet is determined to be white.

Also, when the sheet color determiner211determines that the recording sheet71to be used for printing is white, by decreasing the printing speed, it is possible to increase (or improve) the flop index after printing. Thus, when printing is performed on a white recording sheet71, the printer1of the present embodiment sets the printing speed to be lower than the printing speed set when printing is performed on a colored recording sheet71, whose flop index varies little with the printing speed. This increases (or improves) the flop index of the white recording sheet71after printing.

The printer1may be configured as follows.

When the sheet color determiner211determines that the recording sheet71is colored, the image forming controller90may increase the amount of the brilliant toner per unit area of the image formed on the recording sheet71as compared to when the recording sheet71is white.

The sheet color determiner211may determine whether the recording sheet71is white or colored, on the basis of the flop index of the recording sheet71.

When the sheet color determiner211determines that the flop index of the recording sheet71is not less than 7.0, the image forming controller90may make the amount of the brilliant toner per unit area of the image not less than a predetermined value.

When the sheet color determiner211determines that the flop index of the recording sheet71is not greater than 5.6, the image forming controller90may make the amount of the brilliant toner per unit area of the image less than a predetermined value.

The flop index of the recording sheet71may be obtained by the medium color calculator11aor212provided in the printer1.

When the sheet color determiner211determines that the recording sheet71is white, the main controller204may decrease the printing speed as compared to when the sheet color determiner211determines that the recording sheet71is colored.

When the recording sheet71is colored, the image forming controller90may increase the amount of the brilliant toner per unit area of the image formed on the recording sheet71as compared to when the recording sheet71is white.

When the recording sheet71is a first medium having a first flop index, the image forming controller90may increase the amount of the brilliant toner per unit area of the image formed on the recording sheet71as compared to when the recording sheet71is a second medium having a second flop index less than the first flop index.

As described above, when performing printing with a brilliant toner, the printer1of the present embodiment can provide good brilliance regardless of whether the recording medium is white or colored.

In the present embodiment, the medium color determiner for determining whether the recording medium is colored or white is provided in the printer. However, as a modification, it is possible that the medium color determiner is provided on a printer driver installed in a personal computer as a host device, the printer driver transmits information indicating the medium color to the printer along with a printing instruction, and the printer changes the brilliant toner deposition amount on the basis of the transmitted information. Specifically, when the recording medium is a first medium that is colored, the image forming controller may increase the brilliant toner deposition amount as compared to when the recording medium is a second medium that is white.

For example, when the information transmitted to the printer indicates that the recording medium is colored, the image forming controller sets the voltage applied to the developing roller134to −290 V and the voltages applied to the first and second supply rollers132and133to −450 V, thereby setting the brilliant toner deposition amount to approximately 0.60 mg/cm2. On the other hand, when the transmitted information indicates that the recording medium is white, the image forming controller sets the voltage applied to the developing roller134to −110 V and the voltages applied to the first and second supply rollers132and133to −270 V, thereby setting the brilliant toner deposition amount to approximately 0.20 mg/cm2.

As a result, when the recording medium is colored, the brilliant toner deposition amount is increased by the image forming controller as compared to when the recording medium is white, and thus a good brilliance can be obtained.

As another modification, it is possible that the medium color determiner is provided on a server capable of communicating with the printer. It is possible that when a type of recording medium is selected through one or more buttons for medium type selection provided in a user interface (e.g., printer panel) of the printer, the printer transmits the selection result to the server, the medium color determiner provided on the server determines the color of the recording medium and transmits the determination result to the printer, and the printer changes the brilliant toner deposition amount on the basis of the transmitted result.

In the above embodiment, the present disclosure has been described by taking a color electrophotographic printer as an example. However, the present disclosure is not limited to this, and applicable to image forming apparatuses, such as copiers, facsimile machines, and multi-function peripherals (MFPs), that form images on recording media by electrophotography. Also, although a color printer has been described, the present disclosure is applicable to monochrome printers.