Image forming apparatus

An image forming apparatus includes a first image forming unit that uses a first toner for a first toner image and a second image forming unit that uses a second toner for a second toner image, and transfers, a controller that controls the first image forming unit and the second image forming unit, having two control modes, a first print mode in which the first toner image and the second toner image are continuously transferred to the transfer medium; and a second print mode in which the first toner image is transferred thereafter, the transfer medium, on which the first toner image is formed, is passed through the second image forming unit without the second image forming unit performing image transfer. The controller performs control in such a manner that an amount of the first toner in the first print mode is greater than that in the second print mode.

CROSS REFERENCE

The present application is related to, claims priority from and incorporates by reference Japanese Patent Application No. 2013-202909, filed on Sep. 30, 2013.

TECHNICAL FIELD

The present invention relates to an image forming apparatus, such as a printer, a copying machine or a facsimile, that uses an electrophotographic method.

BACKGROUND

Conventionally, an image forming apparatus is known that, on a recording medium, forms a transparent toner image above (on a surface side of) or below (on a recording medium side of) a toner image (color toner image) that forms an image.

For example, an image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2010-152209 (for example, see paragraphs [0040]-[0044]) forms a transparent toner image above a color toner image when a recording medium is a glossy sheet, and forms transparent toner images above and below a color toner image when a recording medium is a regular sheet.

In this image forming apparatus, when a transparent toner image is formed above a color toner image, a one-pass print is performed in which a transfer medium is carried only once along an image forming unit. On the other hand, when transparent toner images are formed above and below a color toner image, a two-pass print is performed in which a transfer medium is carried twice along the image forming unit.

However, in the above-described configuration, between the case where the one-pass print is performed and the case where the two-pass print is performed, a difference in an amount of the transparent toner that is transferred to the transfer medium occurs, and as a results, a difference in an amount of the transparent toner that is finally transferred to the recording medium also occurs, and thus there is a possibility that image quality is affected.

In view of the above problem, a purpose of the present invention is to provide an image forming apparatus that allows an amount of a toner (such as a white toner or a transparent toner) that is transferred to a recording medium to be nearly constant and image quality to be improved.

SUMMARY

An image forming apparatus disclosed in the application includes a first image forming unit that uses a first toner to form a first toner image and transfers the first toner image to a transfer medium that moves in a predetermined moving direction; a second image forming unit that is arranged on a downstream side from the first image forming unit in the movement direction of the transfer medium, uses a second toner to form a second toner image, and transfers the second toner image to the transfer medium; the second toner being characterized to be charged easier than the first toner is, a controller that controls the first image forming unit and the second image forming unit so that the first toner image and second toner image are layered on the transfer medium; the controller having two control modes, a first print mode in which the first toner image and the second toner image are continuously transferred to the transfer medium by the first image forming unit and the second image forming unit so that the second toner image is formed on the first toner image above the transfer medium; and a second print mode that includes an operation in which the first toner image is transferred to the transfer medium by the first image forming unit and thereafter, the transfer medium, on which the first toner image is formed, is passed through the second image forming unit without the second image forming unit performing image transfer, wherein the controller performs control in such a manner that an amount of the first toner that is transferred from the first image forming unit in the first print mode is greater than that in the second print mode.

In the present invention, the amount of the first toner that is transferred from the first image forming unit to the transfer medium in the first print mode is more than that in the second print mode. Therefore, even when a portion of the first toner that is transferred to the transfer medium attaches to the second image forming unit when the transfer medium passes through the second image forming unit (reverse transfer), a sufficient amount of the first toner can remain on the transfer medium. As a result, the amount of the first toner on the recording medium can be nearly constant and image quality can be improved.

DETAILED DESCRIPTION OF EMBODIMENTS

First Embodiment

<Configuration of Image Forming Apparatus>

FIG. 1illustrates a configuration of an image forming apparatus according to a first embodiment of the present invention. Here, the image forming apparatus is described as an electrophotographic printer that forms an image using five kinds of toners including white, yellow, cyan, magenta and black toners.

An image forming apparatus1of the present embodiment is provided with five image drum units (hereinafter, referred to as ID units)10W,10Y,10C,10M,10K as image forming units. The ID units10W,10Y,10C,10M,10K are arranged in a row along a movement direction of an intermediate transfer belt9(to be described later) (here from left to right inFIG. 1).

The image forming apparatus1further has a sheet feeding cassette20(medium housing part) that houses a plurality of sheets of recording media (for example, print sheets)23in a loaded state, a sheet feeding roller21(medium feeding part) that feeds out one by one the recording medium23from the sheet feeding cassette20, a feed carrying path101that guides the medium23that is fed out from the sheet feeding cassette20, a carrying roller unit22(medium carrying part) that further carries the recording medium23that is carried along the feed carrying path101, a carrying path102that guides the recording medium23that is carried by the carrying roller unit22, and a medium sensor53(medium detection part) that detects passage of the recording medium23.

The image forming apparatus1further has an intermediate transfer belt9(transfer medium) that is an endless belt arranged in a manner opposing an under side of the ID units10W,10Y,10C,10M,10K. On an inner peripheral side of the intermediate transfer belt9, primary transfer rollers19w,19y,19c,19m,19kthat primarily transfer toner images that are formed by the respective ID units10W,10Y,10C,10M,10K to the intermediate transfer belt9, a belt drive roller25athat drives the intermediate transfer belt9, a driven roller25b, and a secondary transfer backup roller27aare arranged.

On an outer peripheral side of the intermediate transfer belt9, a secondary transfer roller27bis arranged in a manner that the intermediate transfer belt9is sandwiched between the secondary transfer backup roller27aand the secondary transfer roller27b. By the secondary transfer backup roller27aand the secondary transfer roller27b, a secondary transfer part is formed that transfers a toner image from the intermediate transfer belt9to the recording medium23.

The image forming apparatus1is further provided with a fuser unit24that fuses a toner image by applying heat and pressure to the recording medium23to which the toner image has been transferred by the secondary transfer part. The fuser unit24has a heat application roller28and a pressure application roller29. The heat application roller28has thereinside a heat generation body such as a halogen lamp, and applies heat to the recording medium23. The pressure application roller29applies pressure to the recording medium23by sandwiching the recording medium23between the heat application roller28and the pressure application roller29.

The image forming apparatus1is further provided with a selector50(carrying path switching mechanism) that switches a carrying path of the recording medium23that is ejected from the fuser unit24, an ejection carrying path103, and a re-carrying path104. The ejection carrying path103is a carrying path for ejecting the recording medium23, on which a toner image is fused, to outside of the image forming apparatus1. The re-carrying path104is a carrying path for carrying (re-carrying) the recording medium23, on which a toner image is fused, to the carrying roller unit22without inverting a print side of the recording medium23.

The image forming apparatus1is further provided with an ejection roller unit26(medium ejection part) that ejects the recording medium23that is carried thereto along the ejection carrying path103to outside of the apparatus, an ejection tray2on which the recording medium23that is ejected by the ejection roller unit26is placed, and a re-carrying unit51that carries the recording medium23along the re-carrying path104to the carrying roller unit22. The re-carrying unit51has a re-carrying roller unit52that carries the recording medium23along the re-carrying path104.

LED (light emitting diode) heads14w,14y,14c,14m,14kas exposure part are arranged in a manner opposing the ID units10W,10Y,10C,10M,10K. The LED heads14w,14y,14c,14m,14kform electrostatic latent images that correspond to image data of white, yellow, cyan, magenta and black by irradiating photosensitive drums12w,12y,12c,12m,12k(to be described later) of the ID units10W,10Y,10C,10M,10K with light.

The ID units10W,10Y,10C,10M,10K have the photosensitive drums12w,12y,12c,12m,12kas image carriers, charging rollers13w,13y,13c,13m,13kas charging members, development rollers15w,15y,15c,15m,15kas developer carriers, supply rollers17w,17y,17c,17m,17kas supply members, and layer forming blades16w,16y,16c,16m,16kas layer forming members.

The photosensitive drum12wis also referred to as a “first image carrier.” The photosensitive drums12y,12c,12m,12kare also referred to as “second image carriers.” The charging roller13wis also referred to as a “first charging member.” The charging rollers13y,13c,13m,13kare also referred to as “second charging members.” The LED head14wis also referred to as a “first exposure part.” The LED heads14y,14c,14m,14kare also referred to as “second exposure part.”

Further, the development roller15wis also referred to as a “first developer carrier.” The development rollers15y,15c,15m,15kare also referred to as “second developer carriers.” The supply roller17wis also referred to as a “first supply member.” The supply rollers17y,17c,17m,17kare also referred to as “second supply members.” The layer forming blade16wis also referred to as a “first layer forming member.” The layer forming blades16y,16c,16m,16kare also referred to as “second layer forming members.”

The ID units10W,10Y,10C,10M,10K further have toner tanks18w,18y,18c,18m,18kattached as developer containers. The toner tanks18w,18y,18c,18m,18krespectively contain white, yellow, cyan, magenta and black developers for image development. The developer may be a one-component developer composed of a toner or a two-component developer composed of a toner and a carrier.

The photosensitive drums12w,12y,12c,12m,12kare formed by, for example, coating a photosensitive layer on a surface of a cylindrical conductive supporting body. The photosensitive layer is formed by sequentially laminating a blocking layer, a charge generation layer and a charge transportation layer on the surface of the conductive supporting body. Here, the charge transportation layer has a thickness of about 18 μm.

The charging rollers13w,13y,13c,13m,13khave a configuration in which, for example, a semiconductive urethane rubber layer is provided around a metal shaft and a protective film layer of a urethane resin is used to cover around the semiconductive urethane rubber layer. A charging voltage (CH) is applied to each of the charging rollers13w,13y,13c,13m,13k, and surfaces of the photosensitive drums12w,12y,12c,12m,12kare uniformly charged.

The development rollers15w,15y,15c,15m,15khave a configuration in which, for example, an elastic layer is provided around a metal shaft. The elastic layer is formed, for example, using a semiconductive urethane rubber having an Asker C hardness of 70 degree. A development voltage (DB) is applied to each of the development rollers15w,15y,15c,15m,15k, and electrostatic latent images that are formed on the surfaces of the photosensitive drums12w,12y,12c,12m,12kare developed.

The supply rollers17w,17y,17c,17m,17khave a configuration in which, for example, a foam layer is provided around a metal shaft. The foam layer is formed using, for example, a silicone foam having an Asker F hardness of 50 degree. A supply voltage is applied to each of the supply rollers17w,17y,17c,17m,17k, and developers (toners, or toners and carriers) are supplied to the development rollers15w,15y,15c,15m,15k.

The layer forming blades16w,16y,16c,16m,16khave a configuration in which, for example, a substantially rectangular metal member is bent in a direction away from a circumferential surface of a respective one of the development rollers15w,15y,15c,15m,15k, and a bent portion is in contact with a surface of the respective one of the development rollers15w,15y,15c,15m,15k. A layer formation voltage is applied to each of the layer forming blades16w,16y,16c,16m,16k, and a thickness of a developer layer on each of the development rollers15w,15y,15c,15m,15kis regulated. In the present embodiment, the layer formation voltage and the supply voltage are set to be the same and are referred to as a layer formation and supply voltage (SB).

The intermediate transfer belt9is a seamless endless belt that is formed using plastic film. The intermediate transfer belt9has a two-layer structure including a high resistance layer on the outer peripheral side that is formed using a material having a high electrical resistance and a conductive layer (low resistance layer) on the inner peripheral side that is formed using a material having a low electrical resistance. The intermediate transfer belt9is stretched over the belt drive roller25a, the belt driven roller25band the secondary transfer backup roller27aas support members, and an outer peripheral surface (the high resistance layer) of the intermediate transfer belt9is in contact with the surfaces of the photosensitive drums12w,12y,12c,12m,12k.

The high resistance layer of the intermediate transfer belt9is formed, for example, by adding carbon to polyamide-imide so as to have a volume resistivity of 1011-1013Ω·cm. The conductive layer of the intermediate transfer belt9is formed, for example, by rolling an aluminum thin film as a conductive member and bonding or depositing the rolled aluminum thin film onto the high resistance layer so as to have a volume resistivity of 1014Ω·cm or less.

The primary transfer rollers19w,19y,19c,19m,19khave a configuration in which, for example, a foamed elastic layer is provided around a metal shaft. The elastic layer is formed, for example, using an elastic foam rubber having a medium electrical resistance. A primary transfer voltage (TR) is applied to each of the primary transfer rollers19w,19y,19c,19m,19k, and toner images on the photosensitive drums12w,12y,12c,12m,12kare transferred (primarily transferred) to the intermediate transfer belt9.

The secondary transfer roller27bhas a configuration in which a foamed elastic layer is provided around a metal shaft and a covering layer covers around the elastic layer. The elastic layer is formed, for example, using an elastic foam rubber. The covering layer is formed, for example, using a resin tube. A secondary transfer voltage is applied to the secondary transfer roller27b, and the toner images on the intermediate transfer belt9are transferred (secondarily transferred) to the recording medium23.

The image forming apparatus1is further provided with an up-down mechanism70(FIGS. 2A-2C) that allows the respective photosensitive drums12w,12y,12c,12m,12kof the ID units10W,10Y,10C,10M,10K to be brought in contact with or to be spaced apart from the intermediate transfer belt9.

FIGS. 2A-2Cillustrate schematic diagrams illustrating a basic configuration and operations of the up-down mechanism70. The up-down mechanism70(contact and separation mechanism) has an up-down lever71that extends along an arrangement direction of the ID units10W,10Y,10C,10M,10K.

The up-down lever71is positioned on one side in an axial direction of the photosensitive drums (a direction perpendicular to the paper surface) with respect to the ID units10W,10Y,10C,10M,10K. InFIGS. 2A-2C, for convenience of illustration, the up-down lever71is illustrated in a manner overlapping with the ID units10W,10Y,10C,10M,10K.

As illustrated by arrows A and B inFIG. 2A, the up-down lever71is configured to be movable in the arrangement direction of the ID units10W,10Y,10C,10M,10K. Further, on an end part (here, a left end part) of the up-down lever71, a rack77is formed. A pinion gear76that is rotated by an up-down motor75engages with the rack77. Due to rotation of the up-down motor75, the up-down lever71moves in directions indicated by the arrows A and B (here, leftward and rightward directions).

The up-down lever71further has convex-shaped up position regulation parts72w,72y,72c,72m,72kthat push up contact parts11w,11y,11c,11m,11kthat are respectively provided on the ID units10W,10Y,10C,10M,10K to up positions (retreat positions), and a down position regulation part73that holds the ID units10W,10Y,10C,10M,10K at a down position (transferable position).

Here, a top surface (flat surface) of a portion of the up-down lever71excluding the convex portions, which configure the up position regulation parts72w,72y,72c,72m,72k, and the rack77forms the down position regulation part73. Further, two side surfaces of each of the convex portions that configure the up position regulation parts72w,72y,72c,72m,72kare inclined surfaces.

When the up-down lever71is in a position illustrated inFIG. 2A, the up position regulation parts72y,72c,72m,72kare in positions where the contact parts11y,11c,11m,11kof the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are lifted. On the other hand, the up position regulation part72wis not in a position where the contact part11wof the white ID unit10wis lifted. The contact part11wof the ID unit10wis held on the down position regulation part73.

That is, in a state illustrated inFIG. 2A, the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are in up positions, that is, positions (retreat positions) spaced apart upwardly from the intermediate transfer belt9, and the white ID unit10wis in a down position, that is, a position (transferable position) in contact with the intermediate transfer belt9.

On the other hand, when the up-down lever71moves from the state ofFIG. 2Ain the arrow A direction, as illustrated inFIG. 2B, the up position regulation part72wreaches a position where the contact part11wof the white ID unit10wis lifted. In this case, the up position regulation parts72y,72c,72m,72kare away from the positions where the contact parts11y,11c,11m,11kof the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are lifted. Therefore, the contact parts11y,11c,11m,11kof the ID units10Y,10C,10M,10K are held on the down position regulation part73.

That is, in a state illustrated inFIG. 2B, the white ID unit10wis in an up position, that is, a position (retreat position) spaced apart from the intermediate transfer belt9, and the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are in down positions, that is, positions (transferable positions) in contact with the intermediate transfer belt9.

Further, when the up-down lever71moves from the state ofFIG. 2Ain the arrow B direction, as illustrated inFIG. 2C, the up position regulation parts72w,72y,72c,72m,72kare away from the positions where the contact parts11w,11y,11c,11m,11kof the white, yellow, cyan, magenta and black ID units10W,10Y,10C,10M,10K are lifted. Therefore, the contact part11w,11y,11c,11m,11kof the ID units10W,10Y,10C,10M,10K are all held on the down position regulation part73.

That is, in a state illustrated inFIG. 2C, the white, yellow, cyan, magenta and black ID units10W,10Y,10C,10M,10K are all in the down positions, that is, the positions (transferable positions) in contact with the intermediate transfer belt9.

As just described, the up-down mechanism70allows switching between a first state (FIG. 2A) in which the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are spaced apart from the intermediate transfer belt9and the white ID unit10wis in contact with the intermediate transfer belt9, a second state (FIG. 2B) in which the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are in contact with the intermediate transfer belt9and the white ID unit10wis spaced apart from the intermediate transfer belt9, and a third state (FIG. 2C) in which all the ID units10W,10Y,10C,10M,10K are in contact with the intermediate transfer belt9.

The white toner is also referred to as a “first toner.” Further, a white toner image that the ID unit10W transfers onto the intermediate transfer belt9is also referred to as a “first toner image” (or coating toner image). The ID unit10W is also referred to as a “first image forming unit.”

On the other hand, the color (yellow, cyan, magenta and black) toners are also referred to as “second toners.” Further, color toner images that the ID units10Y,10C,10M,10K transfer onto the intermediate transfer belt9are also referred to as “second toner images” (or image-forming toner images). The ID units10Y,10C,10M,10K are also referred to as “second image forming units.”

Next, the toners are described. The white, yellow, cyan, magenta and black toners are configured to contain a polyester resin, a colorant, a charge control agent and a release agent, and further an external additive such as hydrophobic silica is added thereto. Here, toners manufactured using a pulverizing method are used, but it is also possible to use toners manufactured using a commonly known method such as a polymerization method.

A white colorant is made of an inorganic material based (more specifically, metal based) pigment such as a titanium dioxide. It is preferable that the white colorant is opaque. On the other hand, yellow, cyan, magenta and black colorants are made of organic material based pigments. For example, a pigment yellow is used as the yellow colorant; a pigment cyan is used as the cyan colorant; a pigment magenta is used as the magenta colorant; and a carbon black is used as the black colorant. It is preferable that these yellow, cyan, magenta and black colorants are transparent to some extent.

The white toner has an average particle size of for example, 7.0 μm. The yellow, cyan and magenta toners have an average particle size of, for example, 5.6 μm. The black toner has an average particle size of, for example, 5.7 μm.

Further, the white, yellow, cyan, magenta and black toners have a circularity of, for example, 0.950-0.955. The circularity is an indicator of a degree of irregularity of particles. When the circularity is 1.000, the particles are perfectly spherical. As the circularity becomes less than 1.000, shapes of the particles become more irregular.

The circularity of a toner is measured based on the following formula (1) using a “flow type particle image analyzer FPIA-3000” (manufactured by Sysmex Corporation).
circularity=L1/L2  (1)
Here, L1 is a boundary length of a circle having an area that is the same as an area of a projected image of a particle, and L2 is a boundary length of the projected image of the particle. The measured value of the circularity is obtained by dividing a sum of circularities of all measured particles by the number of the all measured particles.

Further, the white toner has a charge amount of for example, −24 μC/g. The yellow toner has a charge amount of for example, −49 μC/g. The cyan toner has a charge amount of for example, −49 μC/g. The magenta toner has a charge amount of for example, −44 μC/g. The black toner has a charge amount of, for example, −55 μC/g.

The charge amount (blow off charge amount) of a toner is measured by stirring by shaking the toner and a carrier. Here, with respect to 0.5 g of the toner, 9.5 g of a ferrite carrier “F-60” manufactured by Powder-Tech Co., Ltd. is mixed. A shaker named “model YS-LD” manufactured by Yayoi Co., Ltd. is used for the shaking. As schematically illustrated inFIG. 3, the mixture of the toner and the carrier is contained in a container61attached to a front end of an arm62of a shaker60. A shaking frequency is 200 times/minute; a shaking angle is 45°; and a shaking amplitude is 800 mm Shaking time is 30 minutes.

After the stirring, a powder charge amount measurement device “TYPE TB-203” manufactured by Kyocera Corporation is used to perform suction for 10 seconds at a blow pressure of 7 kPa and a suction pressure of 4.5 kPa. From a charge amount and a suction amount after 10 seconds, a charge amount per unit weight, Q/M (unit: μC/g), of the toner particles is calculated.

<Control System of Image Forming Apparatus>

Next, a control system of the image forming apparatus1is described.FIG. 4illustrates a block diagram illustrating the control system of the image forming apparatus1. As illustrated inFIG. 4, the image forming apparatus1is provided with a print controller (controller)30that is responsible for controlling overall operation of the image forming apparatus1. The print controller30has a CPU37.

An interface part32that receives a command and print data from a host device31(such as a personal computer) as an information input part, an operation input part33that receives operation input from a user, a memory34as a storing part, and various sensors38are connected to the print controller30. The memory34has a ROM35and RAM36. A print operation program, a print mode setting screen80(FIG. 5A) (to be described later), a recording medium/print mode correspondence table (FIG. 5B) (to be described later), a voltage setting table (FIG. 6) (to be described later) and the like are stored in the ROM35. The various sensors38include a sensor such as the medium sensor53that detects passage of the recording medium23, a temperature and humidity sensor that detects temperature and humidity of the image forming apparatus1, and the like.

Further, development voltage controllers41a,41b, layer formation and supply voltage controllers42a,42b, charging voltage controllers43a,43b, exposure controllers44w,44y,44c,44m,44k, primary transfer controllers45a,45b, a secondary transfer controller46, a motor controller47, a fuser controller48, an up-down controller49, and a switch controller55are connected to the print controller30.

The development voltage controller41acontrols the development voltage (DB) that is applied to the yellow, cyan, magenta and black development rollers15y,15c,15m,15k. The development voltage controller41bcontrols the development voltage (DB) that is applied to the white development roller15w.

The layer formation and supply voltage controller42acontrols the layer formation and supply voltage (SB) that is applied to the yellow, cyan, magenta and black layer forming blades16y,16c,16m,16kand the supply rollers17y,17c,17m,17k. The layer formation and supply voltage controller42bcontrols the layer formation and supply voltage (SB) that is applied to the white layer forming blade16wand the supply roller17w.

The charging voltage controller43acontrols the charging voltage (CH) that is applied to the yellow, cyan, magenta and black charging rollers13y,13c,13m,13k. The charging voltage controller43bcontrols the charging voltage (CH) that is applied to the white charging roller13w.

The exposure controller44w,44y,44c,44m,44krespectively perform drive controls (light emission controls) of the white, yellow, cyan, magenta and black LED heads14w,14y,14c,14m,14k.

The primary transfer controller45acontrols the transfer voltage (TR) that is applied to the yellow, cyan, magenta and black primary transfer rollers19y,19c,19m,19k. The primary transfer controller45bcontrols the transfer voltage (TR) that is applied to the white primary transfer roller19w. The secondary transfer controller46controls the secondary transfer voltage that is applied to the secondary transfer roller27b.

The motor controller47drive-controls a drum drive motor D1, a belt drive motor D2 and a sheet carrying motor D3. The drum drive motor D1 rotationally drives the photosensitive drums12w,12y,12c,12m,12k. The belt drive motor D2 rotationally drives the belt drive roller25athat drives the intermediate transfer belt9.

A plurality of motors for carrying the recording medium23are collectively illustrated inFIG. 4as the sheet carrying motor D3. Specifically, the sheet carrying motor D3 includes a sheet feeding motor that rotationally drives the sheet feeding roller21, a carrying motor that rotationally drives the carrying roller unit22, a secondary transfer motor that rotationally drives the secondary transfer roller27b, a fuser motor that rotationally drives the pressure application roller29of the fuser unit24, a re-carrying motor that rotationally drives the re-carrying roller52, and an ejection motor that rotationally drives the ejection roller unit26; and these motors are respectively controlled by the motor controller47.

The photosensitive drums12w,12y,12c,12m,12kare rotated by the drum drive motor D1 in a counterclockwise direction inFIG. 1. Further, gears are attached to end parts of the shafts of the respective photosensitive drums12w,12y,12c,12m,12k, development rollers15w,15y,15c,15m,15kand supply rollers17w,17y,17c,17m,17k, and the rotations of the photosensitive drums12w,12y,12c,12m,12kare transmitted to the development rollers15w,15y,15c,15m,15kand the supply rollers17w,17y,17c,17m,17k.

The fuser controller48performs heat application control of a heater of the heat application roller28based on a detection temperature of a thermistor that is provided in the fuser unit24and by referring to a temperature setting table.

The up-down controller49performs control to cause the ID units10W,10Y,10C,10M,10K to be brought into contact with or to be spaced apart from the intermediate transfer belt9by causing the up-down lever71of the up-down mechanism70that is illustrated inFIG. 2Ato move in the arrow A direction or the arrow B direction.

The switch controller55switches a position of the selector50and guides the recording medium23to one of the ejection carrying path103and the re-carrying path104.

As the recording medium23, for example, a transfer sheet or a colored regular sheet is used. The transfer sheet is a medium for transferring an image to a T-shirt or the like. After a toner image is fused on a transfer sheet using the image forming apparatus1, by superimposing the transfer sheet on a T-shirt or the like and applying heat of an iron or the like thereto, the toner image is transferred to the T-shirt or the like. Further, the colored regular sheet is a regular sheet of a color other than white, for example, a regular sheet of black, blue or red.

FIG. 5Aillustrates a schematic diagram illustrating the print mode setting screen80of the image forming apparatus1.FIG. 5Billustrates a correspondence table (recording medium/print mode correspondence table) that associates a type of the recording medium23with a print mode. The print mode setting screen80illustrated inFIG. 5Aand the recording medium/print mode correspondence table illustrated inFIG. 5Bare stored in the ROM35(FIG. 4).

The print mode setting screen80has a medium list81(medium selection part) from which a user selects a recording medium to use, an OK button82for confirming and storing a setting selected using the medium list81, and a cancel button83for canceling a setting selected using the medium list81. From the medium list81, one of “regular sheet” and “transfer sheet” can be selected.

In the present embodiment, the print mode is determined based on the type of the recording medium23that is used in the image forming apparatus1. That is, as illustrated in the recording medium/print mode correspondence table ofFIG. 5B, in a case where the regular sheet is used as the recording medium23, a print mode (two-pass print, to be described later) is performed in which a white toner image is formed below a color toner image (on the recording medium23side). In contrast, in a case where the transfer sheet is used as the recording medium23, a print mode (one-pass print, which will be described later) is performed in which a white toner image is formed above a color toner image (on an opposite side of the recording medium23).

In the case where the regular sheet is used, the reason that the white toner image is formed below the color toner image is that, by using the white toner image as a base, influence of a color of the regular sheet is eliminated and an original color of the color toner image is reproduced.

Further, in the case where the transfer sheet is used, the reason that the white toner image is formed above the color toner image is that, in a state in which the toner image is transferred from the transfer sheet to a T-shirt or the like, the white toner image becomes a base.

FIG. 6illustrates a list (voltage setting table) of setting values of the charging voltages CH, the development voltages DB, the layer formation and supply voltages SB and the primary transfer voltages TR of the respective white, yellow, cyan, magenta and black ID units10W,10Y,10C,10M,10K. In the present embodiment, two voltage settings W1, W2 are available for the white ID unit10W. The voltage setting table is stored in the ROM35.

<Operation of Image Forming Apparatus>

FIG. 7illustrates a flow diagram illustrating operations of the image forming apparatus1of the present embodiment. First, the case where the transfer sheet is used as the recording medium23is described.FIGS. 8A-8Cillustrate positional relations between the ID units10W,10Y,10C,10M,10K and the intermediate transfer belt9and respectively correspond to the above-describedFIGS. 2A-2C.

When the image forming apparatus1is started up (S101), the print controller30drives via the up-down controller49the up-down mechanism70, holds the ID units10W,10Y,10C,10M,10K in the position in contact with the intermediate transfer belt9as illustrated inFIG. 8C, and enters a standby state, that is a standby mode (S102).

Next, a user brings up the print mode setting screen80(FIG. 5A) that is stored in the ROM35from the host device31such as a personal computer. Here, it is assumed that the user selects the “transfer sheet” from the medium list81of the print mode setting screen80, confirms the setting using the OK button82, and stores the setting result (S103).

When the recording medium23is selected at S103, the print controller30reads out the recording medium/print mode correspondence table ofFIG. 5Bfrom the ROM35, and determines the print mode based on the type (transfer sheet or regular sheet) of the recording medium23that the user selected (S104).

That is, when the transfer sheet is selected at S103, the print controller30selects the one-pass print (first print mode) in which, on the recording medium23, a white toner image is formed above the color toner image (YES at S104). Further, when the regular sheet is selected at S103, the print controller30selects the two-pass print (second print mode) in which a white toner image is formed below a color toner image (NO at S104).

Here, since the user selects the transfer sheet, the print controller30selects the one-pass print, in which a white toner image is formed above a color toner image (YES at S104), and proceeds to S105.

Then, while the ID units10W,10Y,10C,10M,10K are held in the state ofFIG. 8C, on the intermediate transfer belt9, white, yellow, cyan, magenta and black toner images are sequentially formed, and are transferred to the intermediate transfer belt9.

That is, first, at S105, a white toner image is formed by the ID unit10W and is transferred to the intermediate transfer belt9. Specifically, the belt drive roller25ais rotationally driven by the belt drive motor D2 and the intermediate transfer belt9is driven at a speed of 46 mm/s so that a print speed for the transfer sheet is 10 PPM (Page Per Minutes).

The print controller30further drives the drum drive motor D1 to rotate the photosensitive drum12w, the development roller15wand the supply roller17w. The charging roller13wrotates following the rotation of the photosensitive drum12w.

The print controller30further reads out from the ROM35the voltage setting W2 for one-pass print that is illustrated inFIG. 6, and instructs the charging voltage controller43b, the development voltage controller41b, the layer formation and the supply voltage controller42band the primary transfer controller46, of the ID unit10W, regarding respective set voltages.

The charging voltage CH of −1060 V is applied to the charging roller13wby the charging voltage controller43b. The charging roller13wrotates while in contact with the photosensitive drum12wand uniformly charges the surface of the photosensitive drum12w.

When the surface of the photosensitive drum12wis charged, the print controller30, via the exposure controller44w, causes the LED14wto radiate light and, on the surface of the photosensitive drum12w, an electrostatic latent image based on white image data is formed.

The development voltage DB of −260 V is applied to the development roller15wby the development voltage controller41b. The layer formation and supply voltage SB of −460 V is applied to each of the layer forming blade16wand the supply roller17wby the layer formation and supply voltage controller42b. The supply roller17wsupplies toner to the surface of the development roller15w. The toner supplied on the surface of the development roller15wreceives a shear force due to passing through the layer forming blade16w, and a layer thickness of the toner is regulated and a toner layer having a uniform thickness is formed. The toner on the development roller15wattaches the electrostatic latent image on the photosensitive drum12wand thereby the electrostatic latent image is developed.

The primary transfer voltage TR of +4 kV is applied to the primary transfer roller19wby the primary transfer controller45b. The white toner image that is formed on the surface of the photosensitive drum12wis transferred to the surface of the intermediate transfer belt9by the primary transfer voltage. In this way, the white toner image is transferred to the intermediate transfer belt9.

The intermediate transfer belt9further moves due to the rotation of the drive roller25a, and the toner image on the intermediate transfer belt9moves to a downstream side.

At subsequent S106, the ID units10Y,10C,10M,10K sequentially form yellow, cyan, magenta and black toner images and transfer the toner images to the intermediate transfer belt9.

The formation of the toner images by the ID units10Y,10C,10M,10K is substantially the same as the formation of the white toner image by the ID unit10W. However, voltages of the respective rollers are set based on settings Y, C, M, K in the voltage setting table ofFIG. 6. That is, the charging voltage CH of −1200 V is applied to each of the charging rollers13y,13c,13m,13kby the charging voltage controller43a, and the development voltage DB of −200 V is applied to each of the development rollers15y,15c,15m,15kby the development voltage controller41a. Further, the layer formation and supply voltage SB of −300 V is applied to each of the layer forming blades16y,16c,16m,16kand the supply rollers17y,17c,17m,17kby the layer formation and supply voltage controller42b.

Rotation speeds of the intermediate belt drive motor D2 and the drum drive motor D1 are the same as when the white toner image is formed by the ID unit10W. Further, the primary transfer voltage TR of +4 kV is applied to each of the primary transfer rollers19y,19c,19m,19kby the primary transfer controller45a. As a result, the yellow, cyan, magenta and black toner images are further transferred to the intermediate transfer belt9to which the white toner image has already been transferred.

In this way, when the white, yellow, cyan, magenta and black toner images are formed on the intermediate transfer belt9, the belt drive roller25afurther rotates and the intermediate transfer belt9further moves so that the toner images reaches the secondary transfer part (the secondary transfer backup roller27aand the secondary transfer roller27b). The secondary transfer voltage of +4 kV is applied to the secondary transfer roller27bby the secondary transfer controller46.

Further, using the timing at which the medium sensor53detects the passage of a leading edge of the recording medium23as a reference, the print controller30controls the carrying of the recording medium23by the carrying roller unit22and controls the drive motor (belt drive motor) D2 and the sheet carrying motor D3 in such a manner that the timing at which the recording medium23reaches the secondary transfer part is the same as the timing at which the toner images on the intermediate transfer belt9reaches the secondary transfer part.

Then, when the recording medium23passes through the secondary transfer part, the toner images on the intermediate transfer belt9are transferred to the recording medium23(S107).

As described above, the white, yellow, cyan, magenta and black toner images are laminated in this order on the intermediate transfer belt9. Therefore, at the secondary transfer part, when the toner images are transferred from the intermediate transfer belt9to the recording medium23, the black, magenta, cyan, yellow and white toner image are sequentially laminated on the recording medium23.

Then, the recording medium23to which the toner images have been transferred is carried to the fuser unit24due to the rotation of the secondary transfer roller27b. The heat application roller28of the fuser unit24is heated to 160° C. in advance by the fuser controller48based on the temperature setting table. The recording medium23is heated and pressed by being sandwiched by the heat application roller28and the pressure application roller29, and the toner images are fused on the recording medium23(S108).

The recording medium23on which the toner images are fused is guided by the selector50to the ejection carrying path103, is ejected from an ejection port by the ejection roller unit26, and is placed on the ejection tray2(S118). As a result, the formation of the toner images onto the transfer sheet is completed.

As described above, on the transfer sheet as the recording medium23, the black, magenta, cyan, yellow and white toner images are sequentially formed. That is, the white toner image is formed above the color toner images.

Next, the case where the regular sheet is used as the recording medium23is described with reference to the flow diagram ofFIG. 7.

S101and S102are the same as in the case where the transfer sheet is used. At S103, it is assumed that the user selects the “regular sheet” from the medium list81of the print mode setting screen80, confirms the setting using the OK button82, and stores the setting result.

As subsequent S104, the print controller30reads out the recording medium/print mode correspondence table illustrated inFIG. 5Bfrom the ROM35. Then, based on the correspondence table, the print controller30selects the two-pass print, in which a white toner image is formed below a color toner image (on the recording medium23side) (NO at S104), and proceeds to S109.

At S109, the above-described up-down controller49drives the up-down mechanism70and moves the up-down lever71to the position illustrated inFIG. 2A. As a result, as illustrated inFIG. 8A, the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are spaced apart from the intermediate transfer belt9and only the white ID unit10W is in contact with the intermediate transfer belt9. Voltages are not applied to the respective rollers of the ID units10Y,10C,10M,10K.

At subsequent S110, a white toner image is formed by the ID unit10W and is transferred to the intermediate transfer belt9. That is, the belt drive roller25ais rotationally driven by the drive motor D2 and the intermediate transfer belt9is driven at a speed of 130 mm/s so that a print speed for the regular sheet is 30 PPM.

The print controller30further drives the drum drive motor D1 to rotate the photosensitive drum12w, the development roller15wand the supply roller17w. The charging roller13wrotates following the rotation of the photosensitive drum12w.

The print controller30reads out from the ROM35the voltage setting W2 for two-pass that is illustrated inFIG. 6, and instructs the charging voltage controller43b, the development voltage controller41b, the layer formation and the supply voltage controller42band the primary transfer controller46, of the ID unit10W, regarding respective set voltages.

The charging voltage CH of −1000 V is applied to the charging roller13wby the charging voltage controller43b. The charging roller13wrotates while in contact with the photosensitive drum12wand uniformly charges the surface of the photosensitive drum12w.

When the surface of the photosensitive drum12wis charged, the print controller30, via the exposure controller44w, causes the LED14wto radiate light and, on the surface of the photosensitive drum12w, an electrostatic latent image based on white image data is formed.

The development voltage DB of −200 V is applied to the development roller15wby the development voltage controller41b. The layer formation and supply voltage SB of −400 V is applied to each of the layer forming blade16wand the supply roller17wby the layer formation and supply voltage controller42b. The supply roller17wsupplies toner to the surface of the development roller15w. A layer thickness of the toner supplied on the surface of the development roller15wis regulated due to passing through the layer forming blade16w, and a toner layer having a uniform thickness is formed. The toner on the development roller15wattaches the electrostatic latent image on the photosensitive drum12wand thereby the electrostatic latent image is developed.

The primary transfer voltage TR of +4 kV is applied to the primary transfer roller19wby the primary transfer controller45b. The white toner image that is formed on the surface of the photosensitive drum12wis transferred to the surface of the intermediate transfer belt9by the primary transfer roller19w. In this way, the white toner image is transferred to the intermediate transfer belt9.

The intermediate transfer belt9to which the white toner image has been transferred further moves due to the rotation of the drive roller25a. However, since the ID units10Y,10C,10M,10K are spaced apart from the intermediate transfer belt9, the yellow, cyan, magenta and black toner images are not transferred to the intermediate transfer belt9. Then, the white toner image reaches the secondary transfer part (the secondary transfer backup roller27aand the secondary transfer roller27b).

The secondary transfer voltage of +4 kV is applied to the secondary transfer roller27bby the secondary transfer controller46. Further, at a timing same as the timing at which the toner image on the intermediate transfer belt9reaches the secondary transfer part, the recording medium23that is carried by the carrying roller unit22reaches the secondary transfer part.

The recording medium23passes through the secondary transfer part and thereby the white toner image on the intermediate transfer belt9is transferred to the recording medium23(S111).

The recording medium23to which the toner image has been transferred is carried to the fuser unit24due to the rotation of the secondary transfer roller27b. The heat application roller28of the fuser unit24is heated to 160° C. in advance by the fuser controller48based on the temperature setting table. The recording medium23is heated and pressed by being sandwiched by the heat application roller28and the pressure application roller29, and the toner image is fused on the recording medium23(S112).

Next, re-carrying of the recording medium23is performed (S113). That is, the print controller30, via the switch controller50, switches the position of the selector50to guide the recording medium23on which the toner image has been fused to the re-carrying path104. Further, by the re-carrying roller52of the re-carrying unit51, the recording medium23is carried along the re-carrying path104to the carrying roller22. On the same surface of the recording medium23on which the white toner image is fused, color toner images are continuously formed. Therefore, the re-carrying unit51re-carries the recording medium23without inverting front and back sides of the recording medium23.

At subsequent S114, the up-down controller49drives the up-down mechanism70and moves the up-down lever71to the position illustrated inFIG. 2B. As a result, as illustrated inFIG. 8B, the yellow, cyan, magenta and black ID units10Y,10C,10M,10K are in contact with the intermediate transfer belt9and the white ID unit10W is spaced apart from the intermediate transfer belt9. Voltages are not applied to the respective rollers of the ID unit10W.

At subsequent S115, the ID units10Y,10C,10M,10K sequentially form the yellow, cyan, magenta and black toner images and transfer the toner images to the intermediate transfer belt9.

That is, the belt drive roller25ais rotationally driven by the belt drive motor D2 and the intermediate transfer belt9is driven at a speed of 130 mm/s so that a print speed is 30 PPM. The formation of the toner images by the ID units10Y,10C,10M,10K is performed in a manner same as the above-described S106.

In this way, the yellow, cyan, magenta and black toner images (that is, color toner images) are transferred to the intermediate transfer belt9. Then, due to the rotation of the belt drive roller25a, the intermediate transfer belt9further moves, and the toner images reach the secondary transfer part (the secondary transfer backup roller27aand the secondary transfer roller27b).

The secondary transfer voltage of +4 kV is applied to the secondary transfer roller27bby the secondary transfer controller46. Further, the print controller30causes the recording medium23that is carried via the re-carrying path104to reach the secondary transfer part at a timing same as the timing at which the toner images on the intermediate transfer belt9reach the secondary transfer part.

When the recording medium23passes through the secondary transfer part, the toner images on the intermediate transfer belt9are transferred to the recording medium23(S116).

As described above, the whitetoner image has already been fused on the recording medium23, and the yellow, cyan, magenta and black toner images are laminated in this order on the intermediate transfer belt9. Therefore, at the secondary transfer part, when the toner images are transferred from the intermediate transfer belt9to the recording medium23, the white, black, magenta, cyan and yellow toner images are sequentially laminated on the recording medium23.

The recording medium23to which the toner image has been transferred is carried to the fuser unit24due to the rotation of the secondary transfer roller27b. The heat application roller28of the fuser unit24is heated to 160° C. in advance by the fuser controller48. The recording medium23is heated and pressed by being sandwiched by the heat application roller28and the pressure application roller29, and the toner images are fused on the recording medium23(S117).

The recording medium23on which the toner images are fused is guided by the selector50to the ejection carrying path103, is ejected from an ejection port by the ejection roller unit26, and is placed on the ejection tray2(S118). As a result, the formation of the toner images onto the regular sheet is completed.

As described above, on the regular sheet as the recording medium23, the white, black, magenta, cyan and yellow toner images are sequentially formed. That is, the white toner image is formed below the color toner images.

FIG. 9Aillustrates a state in which a white toner image91and yellow, cyan, magenta and black toner images (color toner image)92are printed on a surface of a regular sheet P1 as the recording medium23.FIG. 9Billustrates a state in which a color toner image92and a white toner image91are printed on a surface of a transfer sheet P2 as the recording medium23.

As illustrated inFIG. 9A, on the regular sheet P1, the white toner image91is formed below the color toner image92. Therefore, for example, even when a blue regular sheet P1 is used, since the white toner image91has a low light transmittance (it is opaque), influence of the color of the regular sheet P1 can be eliminated and the original color of the color toner image92can be reproduced.

Further, as illustrated inFIG. 9B, on the transfer sheet P2, the white toner image91is formed above the color toner image92. Therefore, as illustrated inFIG. 9C, when a user transfers the transfer sheet P2, for example, on a blue T-shirt P3, a white toner layer91is formed on the T-shirt P3 and a color toner image92(reverted image) is formed thereon. Therefore, without being influenced by the color of the T-shirt P3, the original color of the color toner image92can be reproduced.

In the present embodiment, since a white toner image can be formed on both a regular sheet and a transfer sheet, prior to forming a white toner image on a high cost transfer sheet, it is possible to form a white toner image on a regular sheet of a similar color and perform confirmation of the toner image (image). Therefore, it is possible to reduce cost.

<Operation of Image Forming Apparatus>

In the present embodiment, as illustrated inFIG. 6, the different voltage settings W1, W2 are provided for the one-pass print in which a white toner image is formed above a color toner image, and the two-pass print in which a white toner image is formed below a color toner image.

Specifically, during the one-pass print (W1) and during the two-pass print (W2), while a difference (800 V) between the development voltage and the charging voltage is the same and a difference (200 V) between the development voltage and the supply voltage is also the same, the development voltage (−260V, −200V) is different.

This is because, in the one-pass print in which a white toner image and color (yellow, cyan, magenta and black) toner images are continuously transferred to the intermediate transfer belt9, when the white toner image that has been transferred to the intermediate transfer belt9passes through the color ID units10Y,10C,10M,10K, there is a possibility that the white toner on the intermediate transfer belt9is reversely transferred to the photosensitive drums12y,12c,12m,12k.

The “continuously transferred” means that one toner image is formed by one image forming unit on a transfer medium, next, another toner image is formed directly on the one toner image by another image forming unit before the one toner image is transferred to another medium or fused on the medium.

In particular, a white toner is generally a metal based pigment and thus is relatively difficult to be charged as compared to an organic material based pigment such as a color toner. Therefore, the white toner has a relatively weak adhering force with respect to the intermediate transfer belt9and reverse transfer of the white toner to the photosensitive drums12y,12c,12m,12kis likely to occur. When such reverse transfer occurs, an amount of the white toner on the intermediate transfer belt9decreases and as a result, there is a possibility that an amount of the white toner transferred to the recording medium23also decreases.

Therefore, in the present embodiment, in the one-pass print in which a white toner image and a color toner image are continuously transferred, the absolute value of the development voltage is set larger than that in the two-pass print in which a color toner image is transferred after a white toner image is fused. By increasing the absolute value of the development voltage, an adhesion amount of the white toner on the photosensitive drum12wincreases and the amount of the white toner transferred to the intermediate transfer belt9increases. As a result, even taking into account the decrease due to the reverse transfer of the white toner from the intermediate transfer belt9to the photosensitive drum12yand the like, a sufficient amount of the white toner can remain on the intermediate transfer belt9.

Next, difference in brightness of a toner density in a case where the development voltage is changed is described. For example, assume that a white toner of 0.80 mg/cm2is held on the surface of the photosensitive drum12w, and assume that the white toner is primarily transferred from the photosensitive drum12wto the intermediate transfer belt9and further secondarily transferred to a regular sheet (“color high-quality paper (blue)” manufactured by Kishu Paper Co., Ltd.). In a case where it is assumed that the reverse transfer of the white toner does not occur (that is, the case where the ID units10Y,10C,10M,10K are spaced part from the intermediate transfer roller9), brightness on the regular sheet is L*83. This brightness (L*83) is a sufficient brightness.

However, since the toner moves to the photosensitive drum12wfrom the development roller15w, it is necessary to consider development efficiency and a linear speed ratio between the photosensitive drum12wand the development roller15w. The development efficiency indicates a percentage of the toner amount moved from the development roller15wto the photosensitive drum12w.

The white toner amount on the photosensitive drum12wwhen a sufficient brightness (L*83) is obtained in the case where there is no decrease in the white toner amount due to the reverse transfer is 0.80 mg/cm2as described above. When the linear speed ratio between the development roller15wand the photosensitive drum12wis 1.2 times, in order to obtain a white toner amount of 0.80 mg/cm2on the photosensitive drum12w, it is sufficient to have a white toner of 0.66 mg/cm2(=0.80 mg/cm2±1.2) on the development roller15w.

Therefore, even when the development efficiency is 80%, when there is a white toner of 0.83 mg/cm2on the development roller15w, a white toner of 0.80 mg/cm2(≈0.83 mg/cm2×1.2×0.8) on the photosensitive drum12wcan be obtained. In other words, in the case where the reverse transfer does not occur, even when the development efficiency is 80%, when there is a white toner of 0.83 mg/cm2on the development roller15w, a sufficient brightness (L*83) can be obtained.

On the other hand, assuming the decrease in the white toner amount due to the reverse transfer is 0.20 mg/cm2, a white toner of 0.80 mg/cm20.20 mg/cm2=1.00 mg/cm2on the surface of the photosensitive drum12wis required.

Therefore, in the present embodiment, in the print mode (one-pass print) in which the reverse transfer of the white toner occurs, the development voltage that is applied to the development roller15wis changed from −200 V in the case of the two-pass print to −260 V. Since a surface potential of the photosensitive drum12wafter exposure is −50 V, a difference between this drum surface potential and the development voltage (development potential) is 150 V in the case of the two-pass print (development voltage: −200 V) and is 210 V in the case of the one-pass print (development voltage: −260 V). In this way, in the case of the one-pass print, by increasing the difference between the drum surface potential after exposure and the development voltage, the adhesion amount of the white toner onto the photosensitive drum12wcan be increased.

In this way, by increasing the adhesion amount of the white toner onto the photosensitive drum12w, the white toner amount transferred to the intermediate transfer belt9can be increased. As a result, even when taking into account the decrease of the white toner due to the reverse transfer, sufficient white toner amount on the intermediate transfer belt9can be ensured. Therefore, sufficient brightness can be obtained.

FIG. 10illustrates results obtained by performing the one-pass print and the two-pass print and measuring the brightness of the white toner images printed on blue regular sheets. Data A and data B illustrate the measurement results of the brightness of the white toner images in the case where the one-pass print and the two-pass print were performed by applying a voltage of −200 V to the development roller15w. Data C illustrates the measurement result of the brightness of the white toner image in the case where the one-pass print was performed by changing the voltage applied to the development roller15wto −260 V.

The brightness was measured using a “530 SpectroDensitometer” manufactured by X-Rite Incorporated. Further, as the regular sheet, the “color high-quality paper (blue)” manufactured by Kishu Paper Co., Ltd. is used.

As illustrated inFIG. 10, in the case where the development voltage was set to −200 V, while the brightness in the two-pass print was L*83 (data B), in the one-pass print, the brightness was L*78 (data A) and a decrease in density was observed. This is because in the one-pass print, the white toner image and the color toner images are continuously transferred and thus the above-described reverse transfer of the white toner occurs.

On the other hand, in the case where the one-pass print was performed by setting the development voltage to −260 V, the same brightness of L*83 as in the two-pass print was obtained (data C). By increasing the absolute value of the development voltage, the toner amount on the surface of the photosensitive drum12wincreased and the white toner amount that was transferred to the intermediate transfer belt9also increased, and thus, even when taking into account of the decrease in the toner amount due to the reverse transfer, a sufficient white toner amount could be ensured.

In this embodiment, in the case of the one-pass print, the adhesion amount of the white toner onto the photosensitive drum12wis increased by increasing the absolute value of the development voltage to more than that in the case of the two-pass print. However, it is not limited to the development voltage. For example, it is also possible to increase the adhesion amount of the white toner onto the photosensitive drum12wby increasing an exposure amount (light quantity) of the LED head14w. In this case, for example, for a light quantity Lw of the LED head14win the case of the two-pass print, the light quantity of the LED head14win the case of the one-pass print may be set to 1.2×Lw. The light quantity of the LED head14wcan be controlled by, for example, a length of light emitting time of the LED head14w. Further, it is also possible to combinedly change the development voltage and the exposure amount of the LED head.

Further, here, in the case of the two-pass print, as illustrated inFIGS. 8A and 8B, the ID units10are spaced apart from the intermediate transfer belt9. However, the two-pass print may also be performed without having the ID units10spaced apart from the intermediate transfer belt9. However, for example, inFIG. 8A, in a case where formation and transfer of a white toner image are performed without having the color ID units10Y,10C,10M,10K spaced apart from the intermediate transfer belt9, since the photosensitive drums12y,12c,12m,12kof the ID units10Y,10C,10M,10K that do not perform image formation are also in contact with the intermediate transfer belt9, it is necessary to rotate the respective rollers (the charging roller13y-13k, the development roller15y-15k, and the supply roller17y-17k) of the ID units10Y,10C,10M,10K. As a result, in the ID units10Y,10C,10M,10K, in spite of that image formation is not performed, the photosensitive drums12y,12c,12m,12kand the development rollers15y,15c,15m,15kare in contact with each and further, the development rollers15y,15c,15m,15kand the layer forming blades16y,16c,16m,16kare in contact with each other, and thus there is a possibility that, due to wear at contact portions, lives of the ID units are shortened.

Further, in the image forming apparatus1of the present embodiment, the white, yellow, cyan, magenta and black ID units10W,10Y,10C,10M,10K are arranged in a row along the movement direction (from left to right in FIG.1) of the intermediate transfer belt9. However, for example, as illustrated inFIG. 11A, the white ID unit10W may also be arranged on the downstream side of the ID units10Y,10C,10M,10K in the movement direction of the intermediate transfer belt9.

In this case, on the regular sheet, a toner image is formed by the one-pass print. That is, in a state in which the ID units10Y,10C,10M,10K,10W are all in contact with the intermediate transfer belt9, yellow, cyan, magenta, black and white toner images are primarily transferred to the intermediate transfer belt9and further secondarily transferred to the regular sheet and are fused thereon. As a result, on the regular sheet, the white toner image is formed and the color toner images are formed thereabove.

On the other hand, on the transfer sheet, a toner image is formed by the two-pass print. That is, by having the white ID unit10W spaced apart from the intermediate transfer belt9, yellow, cyan, magenta and black toner images are primarily transferred to the intermediate transfer belt9by the ID units10Y,10C,10M,10K and further secondarily transferred to the transfer sheet and are fused thereon. Subsequently, by having the ID units10Y,10C,10M,10K spaced apart from the intermediate transfer belt9and having the white ID unit10W brought into contact with the intermediate transfer belt9, a white toner image is primarily transferred to the intermediate transfer belt9, and is secondarily transferred to the re-carried transfer sheet (on which the color toner images have been fused) and is fused thereon. As a result, on the transfer sheet, the color toner images are formed and the white toner image is formed thereabove.

Further, in the image forming apparatus1of the present embodiment, the toners of five colors including white, yellow, cyan, magenta and black are used. However, it is also possible to use toners of four colors including white, cyan, yellow and magenta. In this case, for example, as illustrated inFIG. 11B, the white, yellow, cyan and magenta ID units10W,10Y,10C,10M may be arranged.

Further, in place of the white toner, a transparent toner for adjusting glossiness may be used. In this case, the ID unit that uses the transparent toner may be arranged, similar to the ID unit10W illustrated inFIG. 1, on the upstream side of the color ID units10Y,10C,10M,10K, or, similar to the ID unit10W illustrated inFIG. 11A, on the downstream side of the ID units10Y,10C,10M,10K.

Further, the ID unit that uses the white toner and the ID unit that uses the transparent toner may also be respectively arranged on the upstream and downstream sides of the color ID units10Y,10C,10M,10K.

Further, it is also possible to arrange the white (transparent) ID unit10W between any adjacent two of the yellow, cyan, magenta and black ID units10Y,10C,10M,10K.

As described above, in the image forming apparatus1of the present embodiment, in the case of the one-pass print in which a white toner image (coating toner image) and a color toner image (image-forming toner image) are continuously transferred, the adhesion amount of the white toner onto the photosensitive drum12wis increased to more than that in the case of the two-pass print. Therefore, even when the reverse transfer of the white toner from the intermediate transfer belt9to the photosensitive drum12yand the like occurs, the white toner amount on the intermediate transfer belt9can be sufficiently ensured. Therefore, the white toner amount transferred to the recording medium23can be nearly constant and stable print quality can be obtained.

Further, by increasing the development voltage that is applied to the development roller15win the case of the one-pass print to more than that in the case of the two-pass print, increase in the adhesion amount of the white toner on the photosensitive drum12win the case of the one-pass print can be achieved. Further, instead of increasing the development voltage that is applied to the development roller15w, increasing the exposure amount of the LED head14wor increasing the supply voltage that is applied to the supply roller17wcan also achieve the effect.

Further, the image forming apparatus1of the present embodiment is provided with the up-down mechanism70that allows the white ID unit10W and the color ID units10Y,10C,10M,10K to be selectively brought into contact with or spaced apart from the intermediate transfer belt9. Therefore, switching between the one-pass print and the two-pass print can be easily performed.

Second Embodiment

Next, a second embodiment of the present invention is described. An image forming apparatus according to the present embodiment has the configuration illustrated inFIG. 1but is different from the image forming apparatus of the first embodiment in that a process corresponding to a POP (Point of Purchase) mode is performed.

FIG. 12illustrates a schematic diagram illustrating a print mode setting screen80of an image forming apparatus1according to the present embodiment. The print mode setting screen80ofFIG. 12is stored in the ROM35(FIG. 4).

The print mode setting screen80has a POP mode check box84(mode selection part) in addition to the medium list81, the OK button82and the cancel button83that are described in the first embodiment. The POP mode is selected in a case where a high density pattern having an area ratio of 70% or more is printed.

When printing an image such as a POP advertisement having a higher image rate than a general document, there is a tendency that, when a normal amount of toner is supplied, the image is printed with a low toner density. Therefore, in the present embodiment, when the POP mode is selected, by increasing the difference between the development voltage and the supply voltage (supply/development voltage difference), the toner supply amount to the photosensitive drums is increased.

FIG. 13illustrates a list (voltage setting table) of setting values of the charging voltages CH, the development voltages DB, the layer formation and supply voltages SB and the primary transfer voltages TR of the respective ID units10W,10Y,10C,10M,10K according to the second embodiment. In the second embodiment, three voltage settings W1, W2, W3 are available for the white ID unit10W. The voltage setting table is stored in the ROM35.

FIG. 14illustrates a flow diagram illustrating operations of the image forming apparatus1according to the present embodiment. Here, differences from the first embodiment are described.

In the present embodiment, after power of the image forming apparatus1is turned on (S101) and the image forming units10W,10Y,10C,10M,10K are moved to the transferable positions (S102), a user brings up, via the host device31, the print mode setting screen80illustrated inFIG. 12from the ROM35.

In the print mode setting screen80illustrated inFIG. 12, the medium list81is used to receive input for the type of the recording medium23(regular sheet/transfer sheet) and the POP mode check box84is used to receive input regarding whether or not to perform printing in the POP mode (S103).

In the present embodiment, when the regular sheet print is selected in the mode setting screen80, the image forming apparatus1operates in the same manner as in the first embodiment (S101-S104, S109-S118).

Here, it is assumed that the user selects the transfer sheet from the medium list81and further checks the POP mode check box84on the print mode setting screen80, confirms the settings using the OK button82, and stores the setting results.

When the transfer sheet is selected from the medium list81of the print mode setting screen80, similar to the first embodiment, the print controller30brings up from the ROM35the recording medium/print mode correspondence table illustrated inFIG. 5B, selects the one-pass print (first print mode) based on the correspondence table, and proceeds to S201.

At S201, whether the POP mode in the print mode setting screen80is selected (that is, whether the POP mode check box84is checked) is judged. When the POP mode is not selected (NO at S201), printing on the transfer sheet is performed in the same manner as in the first embodiment (S105-S108, S118).

Here, as described above, the POP mode is selected (YES at S201) and thus the print controller30proceeds to S202.

At S202, a white toner image is transferred to the intermediate transfer belt9by the ID unit10W. Here, the belt drive roller25ais rotationally driven by the drive motor D2 and the intermediate transfer belt9is driven at a speed of 46 mm/s so that a print speed for the transfer sheet is 10 PPM.

The print controller30drives the drum drive motor D1 to rotate the photosensitive drum12w, the development roller15wand the supply roller17w. The charging roller13wrotates following the rotation of the photosensitive drum12w.

The print controller30further reads out from the ROM35the POP mode voltage setting W3 of the voltage setting table that is illustrated inFIG. 13, and instructs the charging voltage controller43b, the development voltage controller41b, the layer formation and the supply voltage controller42band the primary transfer controller46, of the ID unit10W, regarding respective set voltages.

The charging voltage CH of −1060 V is applied to the charging roller13wby the charging voltage controller43b. The charging roller13wrotates while in contact with the photosensitive drum12wand uniformly charges the surface of the photosensitive drum12w.

When the surface of the photosensitive drum12wis charged, the print controller30, via the exposure controller44w, causes the LED14wto radiate light and, on the surface of the photosensitive drum12w, an electrostatic latent image based on white image data is formed.

The development voltage DB of −260 V is applied to the development roller15wby the development voltage controller41b. The layer formation and supply voltage SB of −500 V is applied to each of the layer forming blade16wand the supply roller17wby the layer formation and supply voltage controller42b. The supply roller17wsupplies toner to the surface of the development roller15w, and the toner supplied to the surface of the development roller15wforms a toner layer having a uniform thickness due to passing through the layer forming blade16w. The toner on the development roller15wattaches the electrostatic latent image on the photosensitive drum12wand thereby the electrostatic latent image is developed.

The primary transfer voltage TR of +4 kV is applied to the primary transfer roller19wby the primary transfer controller45a. The white toner image that is formed on the surface of the photosensitive drum12wis transferred to the surface of the intermediate transfer belt9by the primary transfer roller19w.

The intermediate transfer belt9further moves due to the rotation of the drive roller25a, and the toner image on the intermediate transfer belt9moves to a downstream side.

At subsequent S203, the ID units10Y,10C,10M,10K sequentially form yellow, cyan, magenta and black toner images and transfer the toner images to the intermediate transfer belt9(S203).

The formation of the toner images by the ID units10Y,10C,10M,10K is substantially the same as the formation of the white toner image by the ID unit10W. However, voltages of the respective rollers are set based on the voltage setting table ofFIG. 13. That is, the charging voltage CH of −1200 V is applied to each of the charging rollers13y,13c,13m,13kby the charging voltage controller43a, and the development voltage DB of −200 V is applied to each of the development rollers15y,15c,15m,15kby the development voltage controller41a. Further, the layer formation and supply voltage SB of −300 V is applied to each of the layer forming blades16y,16c,16m,16kand the supply rollers17y,17c,17m,17kby the layer formation and supply voltage controller42b. Further, the primary transfer voltage TR of +4 kV is applied to each of the primary transfer rollers19y,19c,19m,19kby the primary transfer controller45a.

In this way, when the white, yellow, cyan, magenta and black toner images are formed on the intermediate transfer belt9, the belt drive roller25afurther rotates and thereby, the intermediate transfer belt9further moves and, as described in the first embodiment, at the secondary transfer part (the secondary transfer backup roller27aand the secondary transfer roller27b), the toner images are transferred to the transfer sheet as the recording medium23(S204).

Thereafter, as described in the first embodiment, the toner images are fused on the recording medium23(S205), and thereafter, the recording medium23is ejected from the image forming apparatus1(S118).

In the present embodiment, different from the first embodiment, when the POP mode is selected, the supply/development voltage difference (difference between the supply voltage and the development voltage) is switched and is increased from −200 V of the case where the POP mode is not selected to −240 V. The reason for this is as follows.

In the POP mode, a high density pattern having an area ratio of 70% or more is printed. Therefore, there is a tendency that the toner transfer amount decreases from a leading edge toward a trailing edge of the recording medium23.

FIG. 15illustrates measurement results of the brightness at the leading edge and the trailing edge of the recording medium in a case where a solid image having an area rate of 100% was formed on the recording medium (transfer sheet). The brightness was measured using the “530 SpectroDensitometer” manufactured by X-Rite Incorporated.

Data A1 and data A2 inFIG. 15illustrate measurement results of the brightness in a case where a voltage of −260 V is applied to the development roller15wand a voltage of −460 V is applied to the supply roller17w(therefore, the supply/development voltage difference is −200 V), and a solid image was formed by the one-pass print on the recording medium23(transfer sheet). The data A1 illustrates the measurement result of the brightness at the leading edge of the recording medium23in the movement direction and the data A2 illustrates the measurement result of the brightness at the trailing edge of the recording medium23in the movement direction.

Data B1 and data B2 inFIG. 15illustrate measurement results of the brightness in a case where a voltage of −260 V is applied to the development roller15wand a voltage of −500 V is applied to the supply roller17w(therefore, the supply/development voltage difference is −240 V), and a solid image was formed by the one-pass print on the recording medium23(transfer sheet). The data B1 illustrates the measurement result of the brightness at the leading edge of the recording medium23in the movement direction and the data B2 illustrates the measurement result of the brightness at the trailing edge of the recording medium in the movement direction.

As illustrated inFIG. 15, in the where the voltage of −260 V was applied to the development roller15wand the voltage of −460 V was applied to the supply roller17w, the brightness at the leading edge of the recording medium was L*83 (data A1), but the brightness at the trailing edge dropped to L*78 (data A2). This is because, in the case of the one-pass print, as described in the first embodiment, along with increasing the voltage (development voltage) applied to the development roller15wto increase the adhesion amount of the white toner on the photosensitive drum12w, the toner supply from the supply roller17wto the development roller15wbecame insufficient.

In contrast, in the case where the voltage applied to the supply roller17wwas changed to −500 V while the voltage applied to the development roller15wremained at −260 V, the supply/development voltage difference was −240 V. Therefore, the toner supply amount from the supply roller17wto the development roller15wincreased and as a result, the brightness at the trailing edge of the sheet could be increased to L*83 (data B2).

As described in the first embodiment, a white toner of 0.83 mg/cm2is held on the development roller15w.

The white toner amount on the photosensitive drum12wwhen a sufficient brightness (L*83) is obtained in the case where there is no decrease in the white toner amount due to the reverse transfer is 0.80 mg/cm2. When the linear speed ratio between the development roller15wand the photosensitive drum12wis 1.2 times, in order to obtain a white toner amount of 0.80 mg/cm2on the photosensitive drum12w, it is sufficient to have a white toner of 0.66 mg/cm2(=0.80 mg/cm2÷1.2) on the development roller15w.

Therefore, when the reverse transfer does not occur, even when the development efficiency is 80%, when there is a white toner of 0.83 mg/cm2on the development roller15w, a white toner of 0.80 mg/cm2(≈0.83 mg/cm2×1.2×0.8) on the photosensitive drum12wcan be obtained.

On the other hand, assuming the decrease in the white toner amount due to the reverse transfer is 0.20 mg/cm2, a white toner of 0.80 mg/cm20.20 mg/cm2=1.00 mg/cm2on the surface of the photosensitive drum12wis required.

Therefore, in the present embodiment, in the print mode in which the reverse transfer of the white toner occurs, by increasing the absolute value of the supply/development voltage difference from 200 V to 240 V, the white toner amount supplied from the supply roller17wto the development roller15wis increased to 1.04 mg/cm2. As a result, a white toner of 1.00 mg/cm2(≈1.04 mg/cm2×1.2×0.8) on the photosensitive drum12wcan be obtained. That is, even when the decrease due to the reverse transfer is taken into account, a sufficient white toner amount can be ensured and a density decrease at an edge of the recording medium23can be prevented.

Here, the example is described in which, when the POP printing is performed, the supply/development voltage difference is increased to increase the toner amount on the development roller15w. However, the present invention is not limited to this, but can apply anything as far as the toner amount on the development roller15wis increased when print data having a high area ratio is printed.

As described in the above, according to the second embodiment of the present invention, when print data having a high area ratio is printed, the supply/development voltage difference is increased to increase the toner on the development roller. Therefore, even for print data having a high area ratio, high quality print can be obtained.

In the above-described first and second embodiments, in the two-pass print, the intermediate transfer belt9passes twice through the image forming units10W,10Y,10C,10M,10K. However, the intermediate transfer belt9may also pass three or more times through the image forming units10W,10Y,10C,10M,10K.

In the above-described first and second embodiments, the image forming apparatus of an intermediate transfer system is described. However, the present invention may also be applied to an image forming apparatus of a direct transfer system. In the case of the direct transfer system, without using the intermediate transfer belt9, the toner images of the ID units10W,10Y,10C,10M,10K are directly transferred to the recording medium23. Therefore, the recording medium23becomes a “transfer medium.” Further, the present invention may also be applied to an image forming apparatus of a four-cycle system having a single image carrier. The recording medium as the transfer medium includes a transfer sheet that is disclosed in the first embodiment as an embodiment.

Further, the present invention is not limited to a printer, but can also be applied to various image forming apparatuses such as a copying machine, a facsimile, and an MFP (Multi Function Peripheral).