Transfer device and image forming apparatus

A transfer device includes: a first transfer unit that transfers a developer image formed on an image carrier to an intermediate transfer member; a second transfer unit that transfers the developer image on the intermediate transfer member to a recording medium; and a control unit that controls at least one of a transfer pressure and a transfer electric field by the first transfer unit in accordance with a type of the recording medium to which the developer image is transferred.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-247621 filed Sep. 26, 2008.

BACKGROUND

1. Technical Field

The present invention relates to a transfer device and an image forming apparatus.

2. Related Art

In an image forming apparatus, during transfer of a toner image to a recording medium on a surface of which protrusions have been mechanically formed, for example, embossed paper or the like, a transfer electric field at flat portions is lower than a transfer electric field at protrusions.

SUMMARY

A transfer device of an aspect of the present invention includes: a first transfer unit that transfers a developer image formed on an image carrier to an intermediate transfer member; a second transfer unit that transfers the developer image on the intermediate transfer member to a recording medium; and a control unit that controls at least one of a transfer pressure or a transfer electric field by the first transfer unit in accordance with a type of the recording medium to which the developer image is transferred.

DETAILED DESCRIPTION

First Exemplary Embodiment

herebelow, exemplary embodiments of the present invention will be described in detail in accordance with the drawings. Herein, components that are common to each of colors are described with letters representing the colors appended to the reference numerals. InFIG. 2AtoFIG. 4C, the letters representing the colors are omitted for the components that are common to each color.

Firstly, structure of an image forming apparatus10of the present exemplary embodiment is described. As shown inFIG. 1, the image forming apparatus10includes a five-stage tandem-type image forming section12that transfers a toner image (an example of a developer image) of each of colors based on inputted image data to an endless belt-form intermediate transfer belt24, which will be described later, and forms a full-color toner image.

The image forming section12includes electrophotography-system image forming units14L,14Y,14M,14C and14K, which output images of the colors clear (L), yellow (Y), magenta (M), cyan (C) and black (K), in this order from an upstream side of a transporting direction of recording paper P. The image forming units14L to14K are arranged along a direction of movement of the intermediate transfer belt24(shown by arrow B), with predetermined separation distances from one another.

The image forming units14L to14K include photosensitive drums16L to16K, which serve as image carriers. The photosensitive drums16L to16K are structured by layering a photosensitive layer constituted with an organic photoconductive body or the like on a surface (peripheral face) of a cylindrical body made of conductive metal. The photosensitive drums16L to16K are driven to turn at a predetermined process speed in the direction of arrow A in the drawings (the clockwise direction).

This photosensitive layer is a separated-function form, in which a charge generation layer and a charge transport layer are sequentially laminated. The photosensitive layer has the property that ordinarily resistance is high but, when laser beam is illuminated thereon, the resistivity of a portion that is illuminated with laser beam changes.

Charging units18L to18K, exposure devices20L to20K, developing apparatuses22L to22K, the endless belt-form intermediate transfer belt24, first transfer devices25L to25K and cleaning devices28L to28K are disposed around the respective photosensitive drums16L to16K in this order from a rotation direction upstream side. The charging units18L to18K are electrostatic charging devices that uniformly charge the surfaces (peripheral faces) of the photosensitive drums16L to16K to a predetermined potential. The exposure devices20L to20K illuminate laser beams (exposure beams) on the surfaces (peripheral faces) of the uniformly charged photosensitive drums16L to16K in accordance with color-separated image data (image signals), and form electrostatic latent images with the exposure beams. The developing devices22L to22K transfer charged toner (an example of a developing agent) to the electrostatic latent images (i.e., develop the images) to form toner images. The intermediate transfer belt24turnably extends along a path touching against the photosensitive drums16L to16K. The first transfer devices25L to25K are transfer devices that transfer the toner images formed on the photosensitive drums16L to16K to the intermediate transfer belt24. The cleaning devices28L to28K remove transfer residue toner that is left on the surfaces of the photosensitive drums16L to16K after the transfer.

Brush rollers29L to29K are provided at the cleaning devices28L to28K. The brush rollers29L to29K press and contact to the surfaces (peripheral faces) of the photosensitive drums16L to16K, are driven to rotate in the opposite direction to the direction of rotation of the photosensitive drums16L to16K (the direction of arrow A), and scrape off transfer residue toner from the photosensitive drums16L to16K.

The first transfer devices25L to25K are disposed at the inner side of the intermediate transfer belt24, at positions respectively opposing the photosensitive drums16L to16K. The first transfer devices25L to25K are provided with first transfer rollers26L to26K, respectively. The first transfer rollers26L to26K press the intermediate transfer belt24against the photosensitive drums16L to16K. Herein, portions of contact between the photosensitive drums16L to16K and the intermediate transfer belt24that are caused by the first transfer rollers26L to26K serve as first transfer portions (first transfer positions) T1.

The first transfer devices25L to25K are further provided with first transfer bias power supplies60L to60K, respectively, which apply a first transfer bias to the first transfer rollers26L to26K. The first transfer bias power supplies60L to60K are controlled by a control unit30that serves as a control component, and may alter the first transfer biases that are applied to the first transfer rollers26L to26K.

Here, the charging units18L to18K shown in the drawing are formed as roller-form contact chargers, but non-contact chargers may be used, such as scorotrons, solid state chargers or the like.

The intermediate transfer belt24, which serves as an intermediate transfer member, is entrained around the first transfer rollers26L to26K, a driving roller32that is driven to rotate by an unillustrated drive source, a tension roller33that adjusts tension of the intermediate transfer belt24, a backup roller34that is disposed at a second transfer portion (second transfer position) T2, which will be described later, and a driven roller35. The intermediate transfer belt24is driven so as to turn (is circulated) in the direction of arrow B synchronously with rotation of the photosensitive drums16.

In this intermediate transfer belt24, for example, a material for providing conductivity, such as carbon, an ion conduction material or the like, is dispersed in a resin material, such as a polyimide, polyamideimide, polycarbonate, fluorine-based resin or the like.

A second transfer roller36, which serves as a second transfer unit, is provided at a position opposing the backup roller34and sandwiching the intermediate transfer belt24therebetween. The second transfer roller36transfers a toner image on the intermediate transfer belt24onto recording paper P that is being transferred by a transferring mechanism42, which will be described later. A later-described first transferring belt50is entrained around the second transfer roller36. A portion of contact between the second transfer roller36and the intermediate transfer belt24, with the first transferring belt50therebetween, serves as the second transfer portion (second transfer position) T2. In the present exemplary embodiment, the transfer device is constituted by the first transfer devices25L to25K, the second transfer roller36and the control unit30.

The image forming apparatus10is further provided with a toner removal device38and a fixing device40. The toner removal device38removes transfer residue toner that is left on the intermediate transfer belt24after the toner image has been transferred onto the recording paper P by the second transfer roller36. The fixing apparatus40serves as a fixing section that fixes the toner image that has been transferred onto the recording paper P by the second transfer roller36.

The transferring mechanism42is constituted by a pickup roller46, pairs of transferring rollers47, a guide member48, the first transferring belt50, a second transferring belt58, a paper ejection tray (not shown in the drawings) and such like. The pickup roller46transports the recording paper P accommodated in a paper supply tray44one sheet at a time. The pairs of transferring rollers47are plurally provided (four in the illustration) on a transport path of the recording paper P. The guide member48provides the recording paper P to the second transfer portion (second transfer position) T2. The first transferring belt50is entrained around the second transfer roller36and a guide roller52. The second transferring belt58is disposed at the downstream side of the transport path of the recording paper P relative to the first transferring belt50, and is entrained around guide rollers54and56. The paper ejection tray is provided at the downstream side of the fixing device40.

With this structure, the recording paper P accommodated in the paper supply tray44is transferred by the transferring mechanism42to the second transfer portion (second transfer position) T2at which the second transfer roller36(the first transferring belt50) and the backup roller34oppose one another sandwiching the intermediate transfer belt24. The recording paper P is transferred from the second transfer portion (second transfer position) T2to the fixing device40, and is transferred from the fixing device40to the paper ejection tray.

Next, the first transfer devices25L to25K will be described in detail. Here, given that the image forming units14L to14K of the respective colors have substantially the same structure, the first transfer device25Y of the image forming unit14Y will be described in detail as a representative. Upward and downward directions in the descriptions indicate directions for a case in which the photosensitive drums16L to16K illustrated inFIG. 1toFIG. 4are above and the first transfer devices25L to25K are below. In a case in which the photosensitive drums16L to16K and the first transfer devices25L to25K do not have an upward/downward relationship, “above” and “below” may equally be read as “photosensitive drum side” and “first transfer device side”.

As shown inFIG. 2AandFIG. 2B, the first transfer device25Y includes the first transfer roller26Y, the first transfer bias power supply60Y, a housing74Y and urging mechanisms80Y. The first transfer roller26Y includes a roller main body70Y, and an axle portion72Y that extends from the roller main body70Y to the outside at both ends along an axial direction. The first transfer roller26Y is accommodated in the rectangular box-form housing74Y, which is provided at a position opposing the photosensitive drum16Y (below the photosensitive drum in the illustrations), sandwiching the intermediate transfer belt24. An upper face of this housing74Y is open, and the first transfer roller26Y can be moved in and out through this opening.

The urging mechanisms80Y are respectively attached to a floor face76Y at the inner side of the housing74Y, at positions corresponding with the axle portions72Y of the first transfer roller26Y, and urge the first transfer roller26Y upward. The urging mechanisms80Y urge the first transfer roller26Y upward to press the intermediate transfer belt24against the photosensitive drum16Y. The urging force on the first transfer roller26Y may be adjusted. Below, details of the urging mechanisms80Y will be described.

Each urging mechanism80Y is provided with a bearing82Y, a pair of guide rails84Y, a pair of round plates86Y, a base88Y, a first coil spring90Y, a second coil spring92Y and a movement mechanism96Y. The bearing82Y turnably supports the axle portion72Y of the first transfer roller26Y. The guide rails84Y guide vertical direction movements of the bearing82Y. The pair of round plates86Y are attached one to each of the pair of guide rails84Y. The base88Y connects between the pair of round plates86Y. The first coil spring90Y is disposed between the base88Y and the bearing82Y, and urges the bearing82Y upward. The second coil spring92Y is disposed between the base88Y and the floor face76Y, and urges the base88Y upward. The movement mechanism96Y moves the round plates86Y in the vertical direction.

The bearing82Y is formed in a substantially rectangular parallelopiped shape. Two sides of the bearing82Y are inserted into the pair of letter U-like shape guide rails84Y, respectively, and are slidable. Specifically, the bearing82Y is in a state in which the two side face portions thereof fit into the recess portions of the U shapes of the guide rails84Y. A stopper pin is inserted into an end portion of the axle portion72Y. Thus, shifting of the first transfer roller26Y in the axial direction is suppressed.

The guide rails84Y extend in the vertical direction. Rear faces of the guide rails84Y are attached to the pair of round plates86Y, and lower end portions thereof touch against the upper face of the plate-like base88Y. The upper face of the base88Y is substantially parallel with the intermediate transfer belt24.

A portion at one end of the first coil spring90Y is attached to the lower face of the bearing82Y, and a portion at the other end is attached to the upper face of the base88Y. The first coil spring90Y is disposed such that, as viewed in the direction of illustration ofFIG. 2B(the side view), the central axis of the first coil spring90Y coincides with a straight line L1which extends in the diametric direction of the photosensitive drum16Y (here, a straight line passing through the center of the photosensitive drum16Y and the center of the first transfer roller26Y).

A portion at one end of the second coil spring92Y is attached to the lower face of the base88Y, and a portion at the other end is attached to the upper face of the floor face76Y. The second coil spring92Y is disposed such that, as viewed in the direction of illustration ofFIG. 2B(the side view), the central axis coincides with the straight line L1that extends in the diametric direction of the photosensitive drum16Y, similarly to the first coil spring90Y.

The movement mechanism96Y includes circular rod-form protrusions98Y, guide grooves100Y, a stepper motor104Y, an extended shaft106Y and a pair of cams109Y. The protrusions98Y are disposed at the outer faces of the round plates86Y. The guide grooves100Y are disposed at positions of inner wall faces of the housing74Y that correspond with the protrusions98Y, and guide movement of the inserted protrusions98Y in the vertical direction. The stepper motor104Y is attached to an upper face of a pedestal102Y with substantially an inverted L shape, of which one part is fixed to an outer wall face of the housing74Y. The extended shaft106Y is an extension of a rotation shaft of the stepper motor104Y. The cams109Y are provided on the extended shaft106Y and cause the pair of round plates86Y to move in the vertical direction.

Specifically, the stepper motor104Y is joined to the extended shaft106Y via a coupling108Y at the rotation shaft thereof, and is attached to an upper face of the pedestal102Y such that the extended shaft106Y is parallel with axial directions of the pair of round plates86Y, while passing above the pair of round plates86Y. The pair of cams109Y are disposed such that outer peripheral faces thereof abut against outer peripheral faces of the pair of round plates86Y.

Now, an operation will be described in which the first transfer device25Y switches, from a first state which is shown inFIG. 2AandFIG. 2Bto a second state which is shown inFIG. 3AandFIG. 3B, receiving a control signal from the control section30.

When the stepper motor104Y receives a control signal from the control section30for switching the first transfer device25Y from the first state to the second state, the stepper motor104Y starts to turn. Accordingly, the cams109Y turn, and the pair of round plates86Y are pushed downward by the protruding sides of the cams109Y (the sides thereof with a greater distance from the extended shaft106Y). At this time, the base88Y moves together with the pair of round plates86Y, and the second coil spring92Y disposed between the base88Y and the floor face76Y is compressed. At this time, although the base88Y is moving downward, the first coil spring90Y urges the first transfer roller26Y upward, but a force with which the intermediate transfer belt24presses against the photosensitive drum16Y (a transfer pressure) is made lower than when in the first state. Thus, when the first transfer device25Y is in the second state, the pressing force is lower than when in the first state, and an adhesion force F1of toner that is transferred onto the intermediate transfer belt24(a toner image) is also lowered. As shown inFIG. 4A, the adhesion force F1of the toner is stronger at the middle portion than at edge portions.

As shown inFIG. 1, an operation panel64is provided at the image forming apparatus10. When a type of recording paper P on which an image is to be formed is inputted through the operation panel64, the control section30reads particular information corresponding to that type of recording paper P (size, type, basis weight and the like), which is pre-memorized in a built-in non-volatile memory, and implements various kinds of control at the image forming apparatus10. Now, if the type of recording paper P corresponds to an embossed paper EP, at whose surface protrusions have been mechanically processed (formed), the control section30sends a control signal to the stepper motor104Y to put the first transfer device25Y into the second state. When the type of recording paper P corresponds to usual paper, whose surface is smoother than the embossed paper EP, the control section30sends a control signal to the stepper motor104Y to put the first transfer device25Y into the first state.

When the first transfer device25Y is to be switched from the second state to the first state, the stepper motor104Y is turned forward or backward until the state illustrated inFIG. 2AandFIG. 2Bis reached.

Next, operation of the image forming apparatus10is described. Given that the image forming units14L to14K of the respective colors have substantially the same structure, operations for forming a yellow toner image with the image forming unit14Y will be described. Before the yellow toner image is transferred onto the intermediate transfer belt24, a clear toner image has already been transferred onto the intermediate transfer belt24by the image forming unit14L. Here, the type of the recording paper P is usual paper, and the type of the recording paper P has already been inputted through the operation panel64.

First, the surface of the photosensitive drum16Y is uniformly charged to a negative potential by the charging unit18Y. Laser beam is illuminated at the uniformly charged surface of the photosensitive drum16Y by the exposure device20Y, in accordance with image data for yellow that is sent from the control section30. Thus, an electrostatic latent image of a yellow printing pattern is formed at the photosensitive layer of the photosensitive drum16Y.

The electrostatic latent image is an image formed of static electricity on the surface (the photosensitive layer) of the photosensitive drum16Y. In the photosensitive layer, resistivity of portions at which the laser beam is illuminated is lowered, and the charge that has been charged flows to the surface of the photosensitive drum16Y, while charge at portions at which the laser beam is not illuminated is remained. Thus, an electrostatic latent image, which is referred to as a negative latent image, is formed.

The electrostatic latent image formed on the photosensitive drum16Y in this manner is transferred to a predetermined development position by rotation of the photosensitive drum16Y. Then, at the development position, the electrostatic latent image on the photosensitive drum16Y is made into a visible image (a toner image) by the developing device22Y. A yellow toner accommodated inside the developing device22Y includes, for example, at least a yellow colorant and a binding resin, with a volume average particle diameter in the range of 3 μm to 6 μm.

The yellow toner is frictionally charged by agitation within the developing device22Y, and has the same polarity (negative) as the electrostatic charge on the surface of the photosensitive drum16Y. Therefore, when the surface of the photosensitive drum16Y proceeds to pass the developing device22Y, the yellow toner electrostatically adheres only to the latent image portion at which the surface of the photosensitive drum16Y has been discharged, and the latent image is developed with the yellow toner. Thereafter, the photosensitive drum16Y continues to turn, and the yellow toner image developed on the surface thereof is transferred to the first transfer portion (first transfer position) T1.

When the yellow toner image on the surface of the photosensitive drum16Y is transferred to the first transfer portion (first transfer position) T1, the predetermined first transfer bias is applied from the first transfer bias power supply60Y to the first transfer roller26Y, a transfer electric field is formed, and electrostatic force from the photosensitive drum16Y toward the first transfer roller26Y acts on the toner image. Hence, because the first transfer roller26Y is pressing, by the urging mechanism80Y, the intermediate transfer belt24against the photosensitive drum16Y, the yellow toner image on the surface of the photosensitive drum16Y is transferred onto the surface of the intermediate transfer belt24. At this time, the applied first transfer bias is the opposite polarity (positive) to the polarity of the toner (negative), and constant current control at the image forming unit14Y is performed by the control section30.

Transfer residue toner on the surface of the photosensitive drum16Y is cleaned off by the cleaning device28Y. First transfer biases applied to the first transfer rollers26L and26M to26K of the image forming units14L and14M to14K are controlled in the same manner as described above. Thus, the intermediate transfer belt24to which the yellow toner image has been transferred at the image forming unit14Y is sequentially transferred to the image forming units14M to14K of the remaining colors, and the toner images of the respective colors are transferred so as to be superposed (multiple superposedly transferred).

The intermediate transfer belt24that has passed each of the image forming units14L to14K, and had toner images of all the colors multiple superposedly transferred thereon, turns to transfer the images in the direction of arrow B in the drawing, and reaches the second transfer portion (second transfer position) T2. The second transfer portion (second transfer position) T2is configured by the backup roller34, which touches against the inner face (rear face) of the intermediate transfer belt24, and the second transfer roller36(the first transferring belt50) disposed at the side of the image holding face of the intermediate transfer belt24.

Meanwhile, the recording paper P is supplied to between the second transfer roller36(the first transferring belt50) and the intermediate transfer belt24at a predetermined time by the transferring mechanism42, and the predetermined second transfer bias is applied to the second transfer roller36. The second transfer bias that is applied at this time is the opposite polarity (positive) to the polarity of the toner (negative). Electrostatic force from the intermediate transfer belt24toward the recording paper P acts on the toner image, and the toner image on the surface of the intermediate transfer belt24is transferred onto the surface of the recording paper P.

The second transfer bias at this time is determined on the basis of a resistance detected by a resistance detection unit (not illustrated), which detects resistance at the second transfer portion (second transfer position) T2, and controlled with a constant voltage. After at the second transfer portion (second transfer position) T2, the recording paper P is fed into the fixing device40, the toner image is heated and pressured, and the multiplecolor-superposed (multiple superposedly transferred) toner image is fused and permanently fixed to the surface of the recording paper P. Hence, the recording paper P (ordinary paper) for which fixing of a full-color image has been completed is transferred to the ejection tray, and a sequence of full-color image formation operations is complete.

Now, a case in which the type of the recording paper P inputted at the operation panel64is the embossed paper EP will be described. Operations of the respective units of the image forming apparatus10are substantially the same as in the case in which the type of the recording paper P is ordinary paper.

In the case in which the type of the recording paper P inputted at the operation panel64corresponds to the embossed paper EP, the control section30switches the first transfer devices25L to25K from the first state into the second state. Hence, as shown inFIG. 4A, the respective toner images formed on the photosensitive drums16L to16K are transferred onto the intermediate transfer belt24. Here, pressing forces (transfer pressures) on the intermediate transfer belt24at the first transfer portions T1are lower when the first transfer devices25L to25K are in the second state than when in the first state. Therefore, the adhesion force F1of a toner image onto the intermediate transfer belt24is lowered. The reference symbol Lt inFIG. 4AtoFIG. 4Cindicates the clear toner (transparent toner), and the reference symbol Ft indicates the colored toners of yellow, magenta, cyan and black.

Then, at the position of the second transfer portion T2, the transfer electric field by the second transfer roller36acts on the embossed paper EP, and each toner (the toner image) on the intermediate transfer belt24experiences electrostatic force and is drawn toward the embossed paper EP (seeFIG. 4B). At this time, the distances to the intermediate transfer belt24differ in a case of flat portions EP1and in a case of protrusions EP2of the embossed paper EP. Therefore, there is difference in the magnitude of the transfer electric field (a transfer electric field E1at the indentation EP1is weaker than a transfer electric field E2at the protrusion EP2). Thus, there is also difference in the electrostatic force which draws the toner (inFIG. 4B, an electrostatic force acting on the toner at the indentation EP1is F2, an electrostatic force acting on the toner at the protrusion EP2is F2′, and the electrostatic force F2′ is greater than the electrostatic force F2). However, because the adhesion force F1of the toner onto the intermediate transfer belt24is lower than in the case of the first state, as shown inFIG. 4C, the toner is excellently transferred even at the indentation EP1. That is, for the recording medium that has protrusions on the surface, such as the embossed paper EP, the transfer pressure at the first transfer portion T1is lowered and accordingly the adhesion force of the toner onto the intermediate transfer belt24is lowered. Therefore, at the second transfer portion T2, the toner is excellently transferred even to the indentation EP1, and occurrences of image deletion in the outputted image are suppressed. Here, as shown inFIG. 4C, a small amount of the clear toner Lt is left on the intermediate transfer belt24. However, because at least the colored toners are transferred onto the embossed paper EP, color reproduction characteristics of the outputted image are thoroughly assured.

Second Exemplary Embodiment

Next, a second exemplary embodiment of the image forming apparatus of the present invention will be described with reference to the drawings. Members that are the same as in the first exemplary embodiment are assigned the same reference numerals and will not be described.

Herebelow, an image forming apparatus110will be described. Similarly to the first exemplary embodiment, the image forming apparatus110is provided with the image forming units14L to14K of the respective colors. Given that the image forming units14L to14K have substantially the same structure, a first transfer device112Y of the image forming unit14Y is described in detail as a representative here. Of the components illustrated inFIG. 5AandFIG. 5B, letters representing colors are not given for components that are common to the colors. A first state of the first transfer device112Y is illustrated inFIG. 5Aand a second state of the first transfer device112Y is illustrated inFIG. 5B.

As shown inFIG. 5A, the first transfer device112Y includes the first transfer roller26Y, the first transfer bias power supply60Y, a housing114Y and urging mechanisms118Y. The first transfer roller26Y is accommodated in the rectangular box-form housing114Y, which is disposed below the photosensitive drum16Y. The upper face of this housing114Y is open, and the first transfer roller26Y can be moved in and out through this opening.

The urging mechanisms118Y are respectively attached to a floor face116Y at the inner side of the housing114Y at positions corresponding with the axle portions72Y of the first transfer roller26Y, and urge the first transfer roller26Y upward. The urging mechanisms118Y urge the first transfer roller26Y upward to press the intermediate transfer belt24against the photosensitive drum16Y. Below, details of the urging mechanism118Y will be described.

Each urging mechanism118Y is provided with the bearing82Y, a pair of guide rails120Y and a first coil spring122Y. The bearing82Y turnably supports the axle portion72Y of the first transfer roller26Y. The pair of guide rails120Y guide vertical direction movements of the bearing82Y, and are attached to inner faces of the walls of the housing114Y. The first coil spring122Y is disposed between the bearing82Y and the housing114Y, and urges the bearing82Y upward.

The two side faces of the bearing82Y are inserted into the pair of letter U-like shape guide rails120Y, and the bearing82Y is slidable. Specifically, the bearing82Y is in a state in which the two side face portions thereof fit into the recess portions of the U shapes of the guide rails120Y.

The guide rails120Y extend in the vertical direction. Rear faces thereof are attached to inner faces of the walls of the housing114Y, and lower end portions touch against the floor face116Y of the housing114Y.

A portion at one end of the first coil spring122Y is attached to the lower face of the bearing82Y, and a portion at the other end is attached to the floor face116Y of the housing114Y. The first coil spring122Y is disposed such that, as viewed in the direction of illustration ofFIG. 5A(a side view), the central axis of the first coil spring122Y coincides with the straight line L1that extends in the diametric direction of the photosensitive drum16Y (the straight line passing through the center of the photosensitive drum16Y and the center of the first transfer roller26Y).

A portion at one end of an arm152inFIG. 10is attached to a length direction wall face of the housing114Y. A rotation axis of the arm coincides with the rotation axis of the photosensitive drum16Y. This arm is turned by driving force of a motor151inFIG. 10, and drives the housing114Y to turn such that the central axis of the first transfer roller26Y moves along the line of a theoretical circle M1that is concentric with the photosensitive drum16Y, which is shown by a broken line inFIG. 5AandFIG. 5B. The wall face of the housing114Y is turnably supported by the arm, and attitude control is performed such that the central axis of the first coil spring122Y is along the vertical direction.

Now, an operation will be described in which the first transfer device112Y switches, on receiving a control signal from the control section30and the arm rotating, from the first state shown inFIG. 5Ato the second state shown inFIG. 5B. Here, determination of the type of the recording paper P by the control section30is the same as in the first exemplary embodiment.

A control signal from the control section30for switching the first transfer device112Y from the first state to the second state is sent to the driving motor of the arm, then the driving motor turns the arm. Accordingly, the housing114Y moves along the line of the theoretical circle M1.

At this time, a force with which the intermediate transfer belt24presses against the photosensitive drum16Y by the first transfer roller26Y being urged upward (pushing force) is lowered without the compression ratio of the first coil spring122Y that urges the first transfer roller26Y upward being altered. This is because, when the arm turns, the urging direction (central axis direction) of the first coil spring122Y has an intersecting relationship with the straight line L1, and therefore the pressing force by the first coil spring is dispersed. Therefore, the transfer pressure at the first transfer portion T1is lowered, and the adhesion force F1of the toner onto the intermediate transfer belt24is lowered (seeFIG. 4AtoFIG. 4C). Consequently, the same as in the first exemplary embodiment, the toner will be excellently transferred onto the embossed paper EP, and occurrences of image deletion in the outputted image are suppressed.

Third Exemplary Embodiment

Next, a third exemplary embodiment of the image forming apparatus of the present invention will be described with reference to the drawings. Members that are the same as in the second exemplary embodiment are assigned the same reference numerals and will not be described.

Herebelow, an image forming apparatus130will be described. Similarly to the second exemplary embodiment, the image forming apparatus130is provided with the image forming units14L to14K of the respective colors. Given that the image forming units14L to14K have substantially the same structure, a first transfer device132Y of the image forming unit14Y is described in detail as a representative here. Of the components illustrated inFIG. 6, letters representing colors are not given for components that are common to the colors.

At the first transfer device132Y, the first transfer bias that is applied to the first transfer roller26Y by the first transfer bias power supply60Y can be altered. Specifically, when the first transfer bias power supply60Y receives control signals from the control section30, the first transfer bias power supply60Y alters a transfer current flowing between the photosensitive drum16Y and the first transfer roller26Y (the transfer current is detected by an ammeter). The first transfer bias changes in accordance with these changes in the transfer current.

When the type of recording paper P inputted through the operation panel64corresponds to the embossed paper EP, the control section30lowers the transfer current flowing between the photosensitive drum16Y and the first transfer roller26Y relative to a case in which the type of recording paper P is ordinary paper. Therefore, in a case in which the type of the recording paper P is the embossed paper EP, the transfer electric field at the first transfer portion T1is lowered, and the toner electrostatic force transferring the toner onto the intermediate transfer belt24is lowered. As a result, the adhesion force F1of the toner onto the intermediate transfer belt24is lowered. Therefore, similarly to the exemplary embodiments described above, the toner is excellently transferred onto the embossed paper EP, and occurrences of image deletion in the outputted image are suppressed. Furthermore, in compared to the exemplary embodiments described above, the image forming apparatus130of the present exemplary embodiment lowers the adhesion force of the toner onto the intermediate transfer belt24simply by controlling the transfer current rather than using a complicated mechanism.

The exemplary embodiments described above have constitutions in which the type of the recording paper P inputted through the operation panel64is determined by the control section30and the transfer pressure or transfer electric field at the first transfer portion T1is adjusted. However, the present invention is not necessarily limited to these constitutions. Constitutions are also possible in which the type of the recording paper P is read with an optical sensor150(for example, the optical sensor150is provided at the transport path of the recording paper P at the upstream side with respect to the second transfer portion T2), and the type of the recording paper P is determined by the control section and the transfer pressure or transfer electric field at the first transfer portion T1is adjusted. Specifically, before the recording paper P is transferred to the second transfer portion T2, smoothness of the recording paper P is measured with an optical sensor. If the smoothness is equal to or above a reference level, it is determined that the recording paper is ordinary paper, and if lower than the reference level, it is determined that the recording paper P is the embossed paper EP. For this measurement of smoothness by the optical sensor, determination is made by light amount of reflected light. If the light amount of reflected light is large, the smoothness is high, and if the light amount of reflected light is small, the smoothness is low. The reference value of the determination is memorized in a memory of the control section beforehand.

Hereabove, the embodiments have been presented and described as exemplary embodiments of the present invention. However, these embodiments are examples, and many modifications may be applied within a scope not departing from the spirit of the invention. Obviously, the scope of rights to the present invention is not to be limited by these exemplary embodiments.

Next, results of tests performed in order to verify the effects of the present invention will be described. As shown in the table ofFIG. 9, levels of image deletions at flat portions EP1of embossed paper EP are ranked from grade 1 to grade 3.

First Transfer Load

As a first test, a test is carried out in which the first transfer load (pressing force) of the intermediate transfer belt at the first transfer portion T1is altered. Apparatuses used for this test are the image forming apparatus10of the first exemplary embodiment and the image forming apparatus110of the second exemplary embodiment. Results are shown inFIG. 7.

Firstly, in a state in which the first transfer load at the first transfer position T1is 142 mN/cm, a line image of 1.5 mm wide and a 20 mm×20 mm solid image are formed on embossed paper with red in a toner amount of 200% and clear toner in a toner amount of 100%. In this case, transfer characteristics of the solid portion (image) are excellent, but image deletions occur only at positions of the line image that correspond to flat portions. In the flat portions, there are whitened regions, that is, the level is grade 3 (G3).

Then, in a state in which the first transfer load at the first transfer position T1is 49 mN/cm, a line image of 1.5 mm wide and a 20 mm×20 mm solid image are formed on embossed paper with red in a toner amount of 200% and clear toner in a toner amount of 100%. In this case, transfer characteristics of the solid portion are excellent but image deletions occur only at positions of the line image that correspond to flat portions. In the flat portions, there are slightly whitened regions, that is, the level is grade 2 (G2). That is, the level of image deletions in the line image on the embossed paper is improved by lowering the first transfer load.

Finally, in a state in which the first transfer load at the first transfer position T1is 29 mN/cm, a line image of 1.5 mm wide and a 20 mm×20 mm solid image are formed on embossed paper with red in a toner amount of 200% and clear toner in a toner amount of 100%. In this case, transfer characteristics of the solid portion are excellent, the level of image deletions at positions of the line image that correspond to flat portions is a level at which there are slightly pale regions in the flat portions, that is, the level is grade 1 (G1). That is, phenomenon of image deletions in the line image on the embossed paper is improved by lowering the first transfer load to 29 mN/cm.

First Transfer Current

As a second test, a test is carried out in which the first transfer current flowing between the photosensitive drum and the first transfer roller is altered. An apparatus used for this test is the image forming apparatus130of the third exemplary embodiment. Results are shown inFIG. 8.

Firstly, in each of states in which the first transfer currents are 40 μA, 30 μA and 20 μA respectively, a line image of 1.5 mm wide and a 20 mm×20 mm solid image are formed on embossed paper with red in a toner amount of 200% and clear toner in a toner amount of 100%. In this case, transfer characteristics of the solid portion are excellent but image deletions occur only at positions of the line image that correspond to flat portions. In the flat portions, there are whitened regions, that is, the level is grade 3 (G3).

Then, in a state in which the first transfer current is 15 μA, a line image of 1.5 mm wide and a 20 mm×20 mm solid image are formed on embossed paper with red in a toner amount of 200% and clear toner in a toner amount of 100%. In this case, transfer characteristics of the solid portion are excellent and image deletions occur only at positions of the line image that correspond to flat portions. In the flat portions, there are slightly whitened regions, that is, the level is grade 2 (G2). That is, the level of image deletions in the line image on the embossed paper is improved by lowering the first transfer current.

Finally, in a state in which the first transfer current is 10 μA, a line image of 1.5 mm wide and a 20 mm×20 mm solid image are formed on embossed paper with red in a toner amount of 200% and clear toner in a toner amount of 100%. In this case, transfer characteristics of the solid portion are excellent, the level of image deletions at positions of the line image that correspond to flat portions is a level at which there are slightly pale regions in the flat portions, and the level is grade 1 (G1). That is, phenomenon of image deletions in the line image on the embossed paper is improved by lowering the first transfer current to 10 μA.