Image forming apparatus operable in color and monochromatic modes

An image forming apparatus includes a black image forming portion, a color image forming portion, an intermediary transfer member, a transfer portion, and an executing portion capable of executing an operation in a color mode in which a color image is formed using the color image forming portion and the black image forming portion and an operation in a monochromatic mode in which a monochromatic image is formed using only the black image forming portion. The executing portion is capable of forming the monochromatic image in the operation in the monochromatic mode under application of a voltage to the black developing device. The voltage is applied under a voltage condition such that a line width is broader than a line width under a voltage condition inputted to the black developing device when the operation in the color mode is executed.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image forming apparatus, such as a copying machine, a facsimile machine or a printer of an electrophotographic type or an electrostatic recording type.

Conventionally, for example, in the image forming apparatus of the electrophotographic type, there is a tandem type in which toner images formed with different color toners on a plurality of photosensitive members as image bearing members are successively transferred superposedly onto an intermediary transfer member or a recording material carried on a recording material carrying member. The image forming apparatus of the tandem type has an advantage such that speed-up of the image forming apparatus is easy.

The image forming apparatus of the tandem type using the intermediary transfer member will be further described as an example. In the image forming apparatus of this type, the intermediary transfer member contacts the photosensitive members at primary transfer portions where the toner images are transferred from the photosensitive members onto the intermediary transfer member. For that reason, depending on friction and contact pressures, the photosensitive members and the intermediary transfer member are gradually abraded or are changed in surface characteristic in some instances. For that reason, for example, in an image forming apparatus capable of carrying out image formation by an operation in a full-color mode and an operation in a black (single color) mode, the intermediary transfer member is spaced, during the operation in the black mode, from the photosensitive member which is not in the image formation. As a result, a deterioration of the photosensitive members and the intermediary transfer member is suppressed, so that lifetimes of these members can be extended.

On the other hand, in the image forming apparatus of this type, there is a wide variety of print data to be processed, and there arises a problem such that a density of a character image and a line image (particularly a thin line image) is ensured while forming a high-quality color image. In order to solve this problem, Japanese Laid-Open Patent Application 2009-105827 has proposed an image forming apparatus including a thin line command detecting means for detecting a thin line command by a drawing instruction, a thin line color acquiring means for acquiring a color of a thin line, and a thin line width changing means for thickening a thickness (width) of the thin line.

However, in a constitution in which a process for thickening the thin line on the basis of object data or command data of the image, it becomes difficult to discriminate the character image itself and the line image itself, and therefore the density of the character image and the line image (particularly thin line image) cannot be sufficiently ensured in some cases.

For example, depending on a kind of documents, even when the image is the character image or the line image, in some instances, an attribute of the image is treated as an attribute, of decreasing image data, such as a graphic or image attribute, not a character or line attribute. In this case, when the image (data) is discriminated as the object image (data), this image is not discriminated as the character image or the line image, so that the thickening process as described above cannot be carried out.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an image forming apparatus capable of acquiring a density of a character image or a line image of a black (monochromatic) image.

According to an aspect of the present invention, there is provided an image forming apparatus comprising: a black image forming portion including an image bearing member and a black developing device including a black toner; a color image forming portion configured to form an image with a yellow toner, a magenta toner and a cyan toner; an intermediary transfer member capable of carrying a toner image formed at the black image forming portion and a toner image formed at the color image forming portion; a transfer portion configured to transfer the toner image from the intermediary transfer member onto a recording material; and an executing portion capable of executing an operation in a color mode in which a color image is formed on the recording material by using the color image forming portion and the black image forming portion and an operation in a monochromatic mode in which a monochromatic image is formed on the recording material by using only the black image forming portion, wherein the executing portion is capable of forming the monochromatic image in the operation in the monochromatic mode under application of a voltage to the black developing device, the voltage being applied under a voltage condition such that a line width is broader than a line width under a voltage condition inputted to the black developing device when the operation in the color mode is executed.

DESCRIPTION OF EMBODIMENTS

An image forming apparatus according to the present invention will be described with reference to the drawings.

1. General Constitution and Operation of Image Forming Apparatus

FIG. 1is a schematic sectional view of an image forming apparatus100in this embodiment according to the present invention.

The image forming apparatus100in this embodiment is a tandem-type (in-line type) color image forming apparatus employing an intermediary transfer type. The image forming apparatus100includes, as a plurality of image forming portions (stations), first to fourth image forming portions Pa, Pb, Pc and Pd for forming toner images of yellow (Y), magenta (M), cyan (C) and black (Bk), respectively. As regards elements having the same or corresponding function and constitutions in the respective image forming portions Pa, Pb, Pc and Pd, suffixes a, b, c and d representing the elements for associated colors are omitted, and the elements will be collectively described in some instances.FIG. 2is a schematic sectional view of the image forming portion P. In this embodiment, the image forming portion P is constituted by including a photosensitive drum1, a charging roller2, an exposure device3, a developing device4, a primary transfer roller5, a drum cleaning device6, and the like, which are described later.

The photosensitive drum1which is a drum-shaped photosensitive member (electrophotographic photosensitive member) as an image bearing member is rotationally driven in an indicated arrow R1direction (clockwise direction) inFIG. 1at a predetermined peripheral speed (process speed) of 100 mm/sec by a drum driving motor M1(FIG. 10) as a driving means. A surface of the rotating photosensitive drum1is electrically charged uniformly to a predetermined polarity (negative in this embodiment) and a predetermined potential by the charging roller2as a charging means. During a charging step, to the charging roller2, a charging voltage (charging bias) which is a DC voltage of a negative polarity is applied from a charging voltage source (high voltage source circuit) E1. The charging voltage may also be an oscillating voltage in the form of an AC voltage biased with a DC voltage.

The surface of the charged photosensitive drum1is subjected to scanning exposure to light by the exposure device (laser scanner)3as an exposure means, so that an electrostatic latent image (electrostatic image) is formed on the photosensitive drum1. In this embodiment, an electrostatic latent image forming means for forming the electrostatic latent image on each of the plurality of image bearing members is constituted by the charging rollers2a-2dand the exposure devices3a-3d. The electrostatic image formed on the photosensitive drum1is developed by supplying the toner as a developer by the developing device4as a developing means, so that the toner image is formed on the photosensitive drum1. The developing device4includes a developing roller41feeding the toner to an opposing portion (developing portion) to the photosensitive drum1while carrying the toner and includes a developing container42for accommodating the toner. The developing rollers41a-41dare provided correspondingly to the plurality of image bearing members, respectively, and are an example of a plurality of developing members (developer carrying members) for developing the electrostatic latent images on the image bearing members with toners under application of a developing voltage. During development, to each of the developing rollers41, the developing voltage (developing bias), which is an oscillating voltage in the form of an AC voltage biased with a DC voltage, is applied from a developing voltage source (high voltage source circuit) E2. A DC component of the developing voltage is set at a predetermined negative potential between a charge potential and an exposed portion potential of the photosensitive drum1. In this embodiment, the toner negatively charged to the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum1is deposited on an exposed portion of the photosensitive drum1which is lowered in absolute value of the potential by the exposure to light after the photosensitive drum1is charged uniformly. That is, in this embodiment, the charge polarity (normal charge polarity) of the toner during the development is the negative polarity. In this embodiment, as shown in (a) ofFIG. 13, to the respective developing rollers41a-41dof the image forming portions Pa-Pd, the developing voltage is applied from independent developing voltage sources E2a-E2d.

An endless belt-shaped intermediary transfer belt7as an intermediary transfer member is provided opposed to the respective photosensitive drums1of the image forming portions Pa-Pd. The intermediary transfer belt7is an example of a conveying member for carrying and conveying the toner images transferred from the plurality of the image bearing members. The intermediary transfer belt7is extended around a driving roller13, a tension roller12and an idler roller11which are used as a plurality of stretching rollers (supporting rollers), and is stretched with a predetermined tension. In this embodiment, the intermediary transfer belt7is formed with an endless film formed of polyimide, which is an example of a dielectric resin material. The driving roller13is rotationally driven by a belt driving motor M2(FIG. 10) as a driving means, whereby the intermediary transfer belt7is rotated (circulated and moved) in an arrow R2direction (counterclockwise direction) inFIG. 1. In an inner peripheral surface side of the intermediary transfer belt7, primary transfer rollers5a-5dwhich are roller-type primary transfer members as primary transfer means are provided correspondingly to the photosensitive drums1a-1d. Each of the primary transfer rollers5is pressed (urged) against the intermediary transfer belt7toward the photosensitive drum1, so that a primary transfer portion (primary transfer nip) N1is formed where the photosensitive drum1and the intermediary transfer belt7contact each other.

The toner image formed on the rotating photosensitive drum1is transferred (primary-transferred) electrostatically onto the intermediary transfer belt7. During a primary transfer step, to the primary transfer roller5, a primary transfer voltage (primary transfer bias) which is a DC voltage of an opposite polarity to the normal charge polarity of the toner is applied from a primary transfer voltage source (high voltage source circuit) E3. For example, during full-color image formation, the respective color toner images of yellow, magenta, cyan and black formed on the respective photosensitive drums1a-1dare successively transferred superposedly onto the intermediary transfer belt7.

The toner (residual toner) remaining on the surface of the photosensitive drum1without being transferred onto the intermediary transfer belt7during the primary transfer is removed and collected from the surface of the photosensitive drum1by the drum cleaning device6as a photosensitive member cleaning means. The drum cleaning device6scrapes off the residual toner from the surface of the rotating photosensitive drum1by a cleaning blade61as a cleaning member, so that the residual toner is accommodated in a cleaning container62. The toner accommodated in the cleaning container62is fed to a collecting toner container (not shown) by a feeding screw63.

At a position opposing the driving roller13also functioning as a secondary transfer opposite roller on an outer peripheral surface side of the intermediary transfer belt7, a secondary transfer roller14which is a roller-type secondary transfer member as a secondary transfer means is provided. The secondary transfer roller14is pressed (urged) against the intermediary transfer belt7toward the driving roller13and forms a secondary transfer portion (secondary transfer nip) N2where the intermediary transfer belt7and the secondary transfer roller14are in contact with each other.

The toner images formed on the intermediary transfer belt7as described above are transferred (secondary-transferred) electrostatically onto the recording material S, such as paper, nipped and fed at the secondary transfer portion N2by the intermediary transfer belt7and the secondary transfer roller14. During a secondary transfer step, to the secondary transfer roller14, a secondary transfer voltage (secondary transfer bias) which is a DC voltage of an opposite polarity to the normal charge polarity of the toner during primary transfer is applied from a secondary transfer voltage source (high voltage source circuit) E4. The recording material S is accommodated in a recording material accommodating cassette (not shown) and is fed to a registration roller pair15by a feeding and conveying device (not shown) including a feeding roller, a conveying roller and a conveying guide and the like.

The toner (residual toner) remaining on the surface of the intermediary transfer belt without being transferred onto the recording material S during the secondary transfer is removed and collected from the surface of the intermediary transfer belt7by a belt cleaning device9as an intermediary transfer member cleaning means. The belt cleaning device9scrapes off the residual toner from the surface of the rotating intermediary transfer belt7by a cleaning blade91as a cleaning member, and accommodates the residual toner in a belt cleaning container92. The toner accommodated in the belt cleaning container92is fed to a cleaning toner container (not shown) by a feeding member (not shown). The belt cleaning device9is disposed at a position opposing the tension roller12via the intermediary transfer belt7.

The recording material S on which the toner image is transferred is fed to a fixing device16as a fixing means. The recording material S is, after the toner image is fixed (melt-fixed) on the surface thereof by being heated and pressed by the fixing device16, discharged (outputted) to an outside of the apparatus main assembly110of the image forming apparatus100.

In this embodiment, in each of the image forming portions P, the photosensitive drum1, the charging roller2, the developing device4and the drum cleaning device6are integrally assembled with a cartridge container8into a unit, and constitute a process cartridge10detachably mountable to the apparatus main assembly110.

In this embodiment, the intermediary transfer belt7, the stretching rollers11,12and13, the primary transfer rollers5a-5dand the like are supported by a unit frame71(FIG. 4) as a supporting member, and integrally constitute an intermediary transfer unit70detachably mountable to the apparatus main assembly110.

2. Contact and Spacing Mechanism

In this embodiment, the image forming apparatus100is capable of executing the image formation by operations in two image forming modes consisting of a full-color mode (first mode) and a black (monochromatic) mode (second mode). In the operation in the full-color mode, the toner images are formed by the first to fourth image forming portions Pa-Pd, so that a full-color image can be formed. In the operation in the black mode, the toner image is formed only by the fourth image forming portion Pd of the first to fourth image forming portions Pa-Pd, so that a black (monochromatic) image can be formed. In this embodiment, the image forming apparatus100includes a contact and spacing mechanism20(FIG. 4) for causing the intermediary transfer belt7to contact the photosensitive drums1a-1dand the space the intermediary transfer belt7from the photosensitive drum1a-1dby moving the primary transfer rollers5a-5din an approaching direction or a spacing direction with respect to the photosensitive drums1a-1d.

InFIG. 3, (a) to (c) are schematic sectional views showing three contact and spacing states between the intermediary transfer belt7and the photosensitive drums1a-1din this embodiment.

InFIG. 3, (a) shows an “all contact state”. In the “all contact state”, in all of the image forming portions Pa-Pd, the primary transfer rollers5a-5dare pressed against the intermediary transfer belt7toward the photosensitive drums1a-1d, so that the intermediary transfer belt7is contacted to the photosensitive drums1a-1d. In this embodiment, during the operation in the full-color mode, the “all contact state” is formed.

InFIG. 3, (b) shows a “black contact state”. In the “black contact state”, in the first to third image forming portions Pa-Pc, the primary transfer rollers5a-5care moved away from the photosensitive drums1a-1c, so that the intermediary transfer belt7is spaced from the photosensitive drums1a-1c. Further, in the fourth image forming portion Pd, the primary transfer roller5dis pressed against the intermediary transfer belt7toward the photosensitive drum1d, so that the intermediary transfer belt7is contacted to the photosensitive drum1d. In this embodiment, during the operation in the black mode, the “black contact state” is formed, and in addition, the “black contact state” is set at a home position of the intermediary transfer unit70during stand-by of a print job or the like. During the operation in the black mode, in the first to third image forming portions Pa-Pd, the operations of the photosensitive drums1a-1c, the developing devices4a-4c, and the like are stopped. As a result, deterioration due to abrasion or the like of the photosensitive drums1a-1c, the primary transfer rollers5a-5cand the intermediary transfer belt7which are not used in the image formation by the operation in the black mode is suppressed, so that lifetime extension of these members can be realized.

InFIG. 3, (c) shows an “all spaced state”. In the “all spaced state”, in all of the image forming portions Pa-Pd, the primary transfer rollers5a-5dare moved away from the photosensitive drums1a-1d, so that the intermediary transfer belt7is spaced from the photosensitive drums1a-1d. In this embodiment, the “all spaced state” is formed during mounting and demounting of the intermediary transfer unit70with respect to the apparatus main assembly110. As a result, during the mounting and demounting of the intermediary transfer unit70with respect to the apparatus main assembly110, generation or the like of damage such as scars on the photosensitive drums1a-1dand the intermediary transfer belt7due to friction between the photosensitive drums1a-1dand the intermediary transfer belt7can be suppressed.

In this embodiment, the contact and spacing mechanism20is constituted so as to move also the idler roller11in synchronism with movement of the primary transfer roller5dof the fourth image forming portion Pd. As a result, the intermediary transfer belt7is spaced from the photosensitive drums1a-1dof all of the image forming portions Pa-Pd with reliability.

An operation of the contact and spacing mechanism20will be described also with reference toFIGS. 4 to 7.FIG. 4and (a) to (c) ofFIG. 5are schematic sectional views of the intermediary transfer unit70for illustrating the operation of the contact and spacing mechanism20and each showing a part of elements necessary for explanation. InFIG. 6, (a) to (c) are plan views for illustrating operations of a cam27and sliders (movable members)29and30, of the contact and spacing mechanism20, which are described later, and (d) is a perspective view of the cam27. InFIG. 7, (a) and (b) are plan views each showing a primary transfer holder (bearing member)25described later.

As shown inFIG. 4, the contact and spacing mechanism20includes a rotation shaft26supported by the unit frame71rotatably about a rotational axis substantially parallel to rotational axis directions of the primary transfer rollers5a-5dand the stretching rollers11,12and13. The rotation shaft26is connected with a contact and spacing motor M3(FIG. 10), as a driving means (driving source) provided in the apparatus main assembly110, in a state in which the intermediary transfer unit70is mounted in the apparatus main assembly110, so that a driving force is inputted to the rotation shaft26. The cam27is fixed to each of end portions of the rotation shaft26with respect to the rotational axis direction in an inside of the unit frame71.

In this embodiment, the cams27provided at the end portions of the rotation shaft26with respect to the rotational axis direction have a line-symmetrical constitution with respect to a substantially center line of a widthwise direction (substantially perpendicular to a movement direction) of the intermediary transfer belt7. Further, the sliders29and30, holders21and25and the like, which are described later, are provided correspondingly to each of the cams27so as to have a line-symmetrical constitution with respect to the substantially center line of the widthwise direction of the intermediary transfer belt7. Further, the cams27act on the corresponding sliders29and30and holders21and25and the like so as to move these members in the same direction in synchronism with each other. Accordingly, in the following, description will be made by paying attention to one of the cams27and its associated elements.

As shown inFIG. 4, the contact and spacing mechanism20includes a black slider29and a color slider30which are supported by the unit frame71so as to be movable by being engaged with the cam27. By rotating the cam27, each of the black slider29and the color slider30is moved in a left-right direction in the figure. As shown inFIG. 4, the primary transfer rollers5a-5dare rotatably supported by primary transfer holders25a-25dmovably supported by the unit frame71. Further, as shown in (a) and (b) ofFIG. 7, the primary transfer holders25a-25dare provided with projections23a-23d. In this embodiment, the primary transfer holders25aand25bsupporting the primary transfer rollers5afor yellow and5bfor magenta have the same constitution, and the primary transfer holders25aand25dsupporting the primary transfer rollers5cfor cyan and5dfor black have the same constitution. Further, as shown inFIG. 5, the projection23dprovided on the primary transfer holder25dfor black engages with an inclined surface portion24dof the black slider29. The projections23a-23cprovided on the primary transfer holders25a-25cfor yellow, magenta and cyan engage with inclined surface portions24a-24c, respectively, of the color slider30. In this state, by moving the black slider29and the color slider30in the left-right direction in the figure, the primary transfer rollers25a-25dare moved in the approaching direction or the spaced direction with respect to the photosensitive drums1a-1d.

Further, as shown inFIG. 4, the idler roller11is rotatably supported by an idler roller holder (bearing member)21rotatably supported by the unit frame71. The idler roller holder21is pressed by an urging (pressing) spring22from an inside toward an outside of the intermediary transfer belt7and is abutted against a rail (not shown) provided in the apparatus main assembly110, and thus is positioned. When the black slider29is moved, the idler roller holder21is pushed up in a direction from the outside toward the inside of the intermediary transfer belt7by a pushing-up portion28(FIG. 6) provided on the black slider29. As a result, the idler roller11is moved in a direction of being moved away from the photosensitive drums1a-1d.

The contact and spacing mechanism20switches the contact and spacing state between the intermediary transfer belt7and the photosensitive drums1a-1dto three states consisting of the above-described “all contact state”, “black contact state” and “all spaced state” depending on a rotation stop position of the cam27. InFIG. 5, (a), (b) and (c) correspond to the “all contact state”, the “black contact state” and the “all spaced state”, respectively. InFIG. 6, (a), (b) and (c) show positional relationships among the cam27, the black slider29and the color slider30in the states shown in (a), (b) and (c) ofFIG. 15, respectively. The cam27has a cam surface engageable with the black slider29and a cam surface engageable with the color slider30, and is constituted so that each of the black slider29and the color slider30produces different motions every 120°. That is, (a) to (c) ofFIG. 5and (a) to (c) ofFIG. 6show a change in state when the cam27is rotated by angles of 120°. Specifically, (a) ofFIG. 5and (a) ofFIG. 6show a state in which the black slider29and the color slider30are completely moved to the left side in the figures. Further, (b) ofFIG. 5and (b) ofFIG. 6show a state in which the black slider29is completely moved to the left side in the figures and the color slider30is completely moved to the right side in the figures. Further, (c) ofFIG. 5and (d) ofFIG. 6show a state in which the black slider29and the color slider30are completely moved to the right side in the figures. The above-described switching among the three contact and spacing states can be carried out by combining the movement directions with the shapes of the above-described inclined surface portions24a-24dand the shape of the pushing-up portion28.

In this embodiment, the contact and spacing mechanism20is constituted by the contact and spacing motor M3, the rotation shaft26, the cam27, the sliders29and30, the holders21and25, and the like.

3. Control Mode

FIG. 10is a block diagram showing a schematic control mode of a principal part of the image forming apparatus100in this embodiment. In this embodiment, operations of the respective portions of the image forming apparatus100are controlled in an integrated manner by a controller50provided in the apparatus main assembly110. The controller50includes, as main constituent elements, a CPU51as a computation (calculation) control means and a ROM52and a RAM53which are used as storing means. In the apparatus main assembly110of the image forming apparatus100, communication I/F portion80as a communication means is provided. The controller50is connected with an external device200such as a personal computer via the communication I/F portion80. The CPU51reads a necessary program from the ROM52and controls the respective portions of the image forming apparatus100on the basis of data of a print job received through the communication I/F portion80, for example, and thus causes the image forming apparatus100to execute the print job. The image forming apparatus100includes an unshown original reading device and can also execute the print job (copying) on the basis of original image data. The print job is a series of operation sequences, for forming and outputting the image(s) on a single or a plurality of recording materials, started in accordance with a single start instruction.

The apparatus main assembly110of the image forming apparatus100is provided with an operation display portion (operation panel)17. The operation display portion17has a function of an operating portion for inputting various pieces of information on operation setting of image formation to the controller50and a function of a display portion for displaying information for an operator such as a user or a service person. In this embodiment, the operation display portion17is in the form of a touch panel which can be touch-operable by the operator.

In this embodiment, particularly, the controller50not only causes the contact and spacing mechanism20to operate depending on the image forming mode but also effects control of changing setting of the developing voltage applied to the developing roller41dfor black depending on the image forming mode as described later.

4. Operation Sequence of Print Job

With reference toFIG. 8, an operation sequence of the print job will be described. In the figure, “Bk” is the black, “CL” is the color (in this case, refers to the yellow, the magenta and the cyan), “FC” is full-color, “Dr” is the photosensitive drum, and “ITB” is the intermediary transfer belt.

When the controller50receives the print job, the controller50starts the print job (S1), and starts drive of the photosensitive drum1dfor black and the intermediary transfer belt7(S2). Then, the controller50discriminates whether the print job should be performed by the operation in the black mode or the operation in the full-color mode (S3).

In S3, in the case where the controller50discriminated that the print job should be performed by the operation in the full-color mode, the controller50starts drive of the photosensitive drums1a-1cfor color (S4). Then, the controller50causes the contact and spacing mechanism20to move the primary transfer rollers5a-5cfor color, so that the intermediary transfer belt7is contacted to the photosensitive drums1a-1cfor color (S5). Then, the controller50checks whether or not the respective primary transfer rollers5a-5dare disposed at predetermined positions (i.e., are in the “all contact state”), by a position detecting mechanism (not shown) (S6). In S6, in the case where the controller50discriminated that the primary transfer rollers5a-5dare disposed at the predetermined positions, the color slider50starts full-color image formation (S7) and ends the image formation when the images are formed on a designated number of sheets (recording materials) (S8). Thereafter, the controller50causes the contact and spacing mechanism20to space the primary transfer rollers5a-5cfor color from the photosensitive drums1a-1cfor color (S9). Then, the controller50checks whether or not the respective primary transfer rollers5a-5dare disposed at predetermined positions (i.e., in the “black contact state”), by the position detecting mechanism (S10). In S10, in the case where the controller50discriminated that the primary transfer rollers5a-5dare disposed at the predetermined positions, the controller50stops drive of the photosensitive drum1dfor black, the photosensitive drums1a-1cfor color and the intermediary transfer belt7(S11), and then ends the print job (S15).

On the other hand, in S3, in the case where the controller50discriminated that the print job should be performed in the operation in the black mode, the color slider50starts black image formation (S12) and ends the image formation when the images are formed on a designated number of sheets (recording materials) (S13). Thereafter, the controller50stops drive of the photosensitive drum1dfor black color and the intermediary transfer belt7(S14), and then ends the print job (S15).

In this embodiment, as regards the contact and spacing state between the intermediary transfer belt7and the photosensitive drums1a-1d, the position of the “belt contact state” is a home position during standby of the print job (stand-by state). However, the present invention is not limited thereto. For example, the position of the “all spaced state” may also be the home position. Or, the position of the “all contact state” may also be the home position.

When the intermediary transfer unit70is demounted from the apparatus main assembly110, for example, through the operation display portion17, it is possible to notify the controller50of a message to the effect that the intermediary transfer unit70should be demounted (in such a manner that an exchanging button, of the intermediary transfer unit70, displayed on the operation display portion17is pressed). The controller50responds to this notification, so that the controller50can cause the contact and spacing mechanism20to switch the contact and spacing state between the intermediary transfer belt7and the photosensitive drums1a-1dto the “all spaced state”.

5. Change of Setting of Developing Voltage

As described above, for example, in the color image forming apparatus as in this embodiment, there arises a problem such that a density of a character image and a line image (particularly thin line image) is ensured while forming a high-quality color image.

In this embodiment, in order to acquire a stable density of the character image and the line image (particularly the thin line image) irrespective of information on an attribute of the image such as object data, the image forming apparatus100roughly has the following constitution. That is, particularly, during the operation in the black mode, operation setting other than normal operation setting of the image formation (image forming condition) is selectable, so that the density (or width) of a black character image or line image is adjustable depending on a demand of the user or the like. The normal operation setting is a standard operation setting in the case where the operation setting is not changed in accordance with this embodiment. Specifically, particularly, during the operation in the black mode, it can be assumed that a ratio of the character image or the line image is large. For that reason, in this embodiment, a developing property can be adjusted by changing setting of the developing voltage in operation setting particularly during the operation in the black mode relative to that in the normal operation setting. As a result, typically, a necessary constant of the character image or the line image is ensured, so that it is possible to ensure the density of the black character image or the black line image. Further, by employing such a constitution, the density of the character image or the line image can be adjusted depending on the demand of the user or the like while suppressing a fluctuation in uniform image density over an entirety of the image.

With reference toFIG. 9, an outline of an operation sequence of a print job in the case where a change (switching) of setting of the developing voltage for black in the operation in the black mode (in this embodiment, this developing voltage is also referred to as a “black developing voltage”) is designated will be described. A method of designating the change of the setting will be described later. InFIG. 9, processes similar to those inFIG. 8are represented by the same step numbers as inFIG. 8.

When the controller50receives the print job, the controller50starts the print job (S1), and starts drive of the photosensitive drum1dfor black and the intermediary transfer belt7(S2). Then, the controller50discriminates whether the print job should be performed by the operation in the black mode or the operation in the full-color mode (S3).

In S3, in the case where the controller50discriminated that the print job should be performed by the operation in the full-color mode, the controller50executes the processes of S4to S11similarly as in those described with reference toFIG. 8, and ends the print job (S15). The settings of the developing voltages (normal operation settings) for the respective colors of yellow, magenta, cyan and black during the operation in the full-color mode are the same. However, as described later, the setting of the black developing voltage during the operation in the full-color mode can also be made adjustable relative to the normal operation setting as described later.

On the other hand, in S3, in the case where the controller50discriminated that the print job should be performed by the operation in the black mode, the controller50changes the setting of the black developing voltage to that during the operation in the black mode, designated as described later (S21).

Thereafter, the controller50starts black image formation (S12) and ends the image formation when the images are formed on a designated number of sheets (recording materials) (S13). Thereafter, the controller50stops drive of the photosensitive drum1dfor black color and the intermediary transfer belt7(S14), and then ends the print job (S15). In the case where the change of the setting of the black developing voltage during the operation in the black mode relative to the normal operation setting is not designated, the image formation by the operation in the black mode is executed at the normal operation setting.

In this embodiment, the setting of the black developing voltage during the operation in the black mode can be controlled by the operator such as the user or the service person. In this embodiment, the character image and the line image (particularly the thin line image) can be reproduced in a thick state (high density state) and in a thin state (low density state).

In this embodiment, as described above, the developing voltage is the oscillating voltage in the form of the DC voltage (DC component) biased with the AC voltage (AC component). In this embodiment, as regards the setting of the black developing voltage during the operation in the black mode, a peak-to-peak voltage (Vpp) of the AC component can be changed. In general, a developing property (developing power) changes depending on the AC component of the developing voltage. When the Vpp of the AC component of the developing voltage is increased, a potential difference at a light portion increases and therefore the developing property is improved, so that the image with a high density (thickened thin line) can be obtained. On the other hand, when the Vpp of the AC component of the developing voltage is decreased, the image with a low density (thinned thin line) is obtained. For example, as shown in (a) ofFIG. 11, a relationship between the Vpp of the AC component of the developing voltage and the image density can be acquired in advance by an experiment or the like. Specifically, in this embodiment, the Vpp of the AC component (common to all of the colors in this embodiment) of the developing voltage in the normal operation setting is 1.6 Kvpp. With reference to this normal operation setting, the Vpp of the AC component of the developing voltage during the operation in the black mode can be increased and decreased in a predetermined range on a predetermined changing rate (e.g., 100 Vpp) basis. In the case where the density of the character image or the line image is intended to be increased (i.e., in the case where the thin line is intended to be thickened), setting is made so as to increase the Vpp. On the other hand, in the case where the density of the character image or the line image is intended to be decreased (i.e., in the case where the thin line is intended to be thinned), setting is made so as to decrease the Vpp.

The setting of the black developing voltage during the operation in the black mode can be made by changing a frequency of the AC component in place of or in addition to the change of the Vpp. In general, when the frequency is decreased, toner imparting power increases and therefore the developing property is improved, so that the image with a high density (thickened thin line) can be obtained. On the other hand, when the frequency is increased, the image with a low density (thinned thin line) is obtained. For example, as shown in (b) ofFIG. 11, a relationship between the Vpp frequency of the AC component of the developing voltage and the image density can be acquired in advance by an experiment or the like. Specifically, in this embodiment, the frequency of the AC component (common to all of the colors in this embodiment) of the developing voltage in the normal operation setting is 1600 Hz. With reference to this normal operation setting, the frequency of the AC component of the developing voltage during the operation in the black mode can be increased and decreased in a predetermined range on a predetermined changing rate (e.g., 100 Hz) basis. In the case where the density of the character image or the line image is intended to be increased (i.e., in the case where the thin line is intended to be thickened), setting is made so as to increase the Vpp. On the other hand, in the case where the density of the character image or the line image is intended to be decreased (i.e., in the case where the thin line is intended to be thinned), setting is made so as to decrease the Vpp. In the case of enabling the change of both of the Vpp and the frequency, by interrelating the Vpp with the frequency, the setting can be made in advance so that the density of the character image or the line image can be set at a desired density in a predetermined range. The Vpp and the frequency may also be made independently changeable.

Incidentally, as regards the setting of the black developing voltage during the operation in the black mode, in place of or in addition to at least one of the Vpp of the AC component and the frequency, the DC component can be changed. In general, when an absolute value of the DC component of the developing voltage having the normal polarity of the toner is increased, the image with a high density (thickened thin line) can be obtained. On the other hand, when the absolute value of the DC component of the developing voltage having the normal polarity of the toner is decreased, the image with a low density (thinned thin line) is obtained. A relationship between the absolute value of the DC component of the developing voltage and the image density can be acquired in advance by an experiment or the like. In this case, with reference to this normal operation setting, the absolute value of the DC component of the developing voltage of the same polarity as the normal charge polarity of the toner can be increased and decreased in a predetermined range on a predetermined changing rate basis. In the case where the density of the character image or the line image is intended to be increased (i.e., in the case where the thin line is intended to be thickened), setting is made so as to increase the absolute value of the DC component. On the other hand, in the case where the density of the character image or the line image is intended to be decreased (i.e., in the case where the thin line is intended to be thinned), setting is made so as to decrease the absolute value of the DC component. In the case where both of the absolute value of the DC component and at least one of the Vpp and the frequency of the AC component are made changeable, by changing these parameters in interrelation with each other, setting can be made in advance so that the density of the character image or the line image can be set at a desired density in a predetermined range. The absolute value of the DC component and at least one of the Vpp and the frequency of the AC component can be made independently changeable.

Further, in the case where the black developing voltage during the operation in the black mode is changed, in interrelation with this change, a fog-removing voltage (Vback) and laser power (exposure light quantity: light quantity per unit time of light radiation per unit area) of the exposure device3may also be changed. For example, in the case where the Vpp of the AC component is changed, the charge potential of the photosensitive drum1dfor black by the charging roller2dfor black can be changed so as to approach the Vback (dark-portion potential difference) before the change of the Vpp. With the change of the charge potential, in order to ensure a desired light-portion potential difference, the laser power of the exposure device3dfor black can be changed correspondingly.

InFIG. 12, (a) to (c) are schematic views each showing an example of a touch-operable user interface displayed at the operation display portion17. In this embodiment, through the user interface of the operation display portion17of the apparatus main assembly110, the operator such as the user causes the controller50to change the black developing voltage during the operation in the black mode, and is capable of arbitrarily setting a degree of the change. In the case where the operator intends to change the density of the black character image or the black line image during the operation in the black mode, the operator touches a predetermined button (display region) displayed on the operation display portion17, so that a setting screen120capable of selecting various setting items as shown in (a) ofFIG. 12is called up. Further, the operator calls up a density setting screen122as shown in (b) ofFIG. 12by touching a density setting button121displayed on the setting screen120. For example, in the case where the operator intends to increase the density of the black character image or the belt line image (i.e., to thicken the thin line) during the operation in the black mode, by touching an increment button123, the operator can increase a density section125, set in advance on a predetermined increment basis, to a desired density (level). On the other hand, in the case where the operator intends to decrease the density (i.e., to thin the thin line), by touching a decrement button124, the operator can decrease the density section125to a desired density (level). As a result, the operator is capable of variably setting the black image density during the operation in the black mode. Then, when the operator enters the density setting by touching an enter button126, the operation display portion17inputs the density setting, designated by the operation of the operator, to the controller50. In the ROM52of the controller50, setting of the black developing voltage during the operation in the black mode is stored correspondingly to the density setting designated as described above. The controller50stores and holds, in the RAM53, the designated density setting or black developing voltage setting corresponding to the designated density setting. Then, when the image formation is carried out by the operation in the black mode, the controller50causes the image forming apparatus to form an image in the black developing voltage setting corresponding to the designated density setting (FIG. 9).

The image forming mode may also be selected by the operator by touching a mode selecting button127displayed on an initial screen or the like at the operation display portion17as shown in (c) ofFIG. 12, for example.

Further, the selection of the density setting during the operation in the black mode and the selection of the image forming mode may also be carried out, through a driver software of the image forming apparatus100, in the external device200such as a personal computer communicatably connected with the controller50.

Thus, the image forming apparatus100of this embodiment includes the controller50capable of causing the image forming apparatus100to carry out the image formation by the operation in a first mode as the image forming mode and by the operation in a second mode as the image forming mode. The first mode (full-color mode) is the image forming mode in which the toner images are formed on, of the plurality of the image bearing members1, a predetermined image bearing member1dand other image bearing member1a-1cdifferent from the predetermined image bearing member1d. The second mode (black (monochromatic) mode) is the image forming mode in which the toner image is formed on the predetermined image bearing member1dbut is not formed on other image bearing member1a-1c. The image forming apparatus100further includes the following designating portion. The designating portion causes the controller50to change setting of the developing voltage applied to a predetermined developing member41dwhich is a developing member corresponding to the predetermined image bearing member1din the operation in the second mode, relative to setting of the developing voltage applied to the predetermined developing member41din the operation in the first mode. In this embodiment, the designating means is constituted by the operation display portion17, the communication I/F portion80through which the designation from the external device200is inputted to the controller50, and the like. In the case where the designation by the designating means is made, when the image formation is carried out by the operation in the second mode, the setting of the developing voltage applied to the predetermined developing member41dis changed by the controller50relative to the setting of the developing voltage applied to the predetermined developing member41din the operation in the first mode. Further, in this embodiment, the designating means is capable of variably designating a difference of the setting of the developing voltage applied to the predetermined developing member41din the operation in the second mode relative to the setting of the developing voltage applied to the predetermined developing member41din the operation in the first mode. Further, in this embodiment, the image formation by the operation in the second mode is carried out in a state in which the conveying member7contacts the predetermined image bearing member1dand is spaced from other image bearing members1a-1c.

As described above, in this embodiment, during the operation in the black mode in which it is assumed that an image ratio of particularly the character image or the line image is large, the operation setting different from the normal operation setting is selectable. Further, when the image formation is carried out by the operation in the black mode, the image is formed automatically in the selected operation setting. Accordingly, during the operation in the black mode, irrespective of the information on the attribute of the image such as object data, the density (or the width) of the character image or the line image (particularly the thin line) corresponding to the selection of the operator can be obtained.

In this embodiment, as shown in (a) ofFIG. 13, to the respective developing rollers41a-41dof the image forming portions Pa-Pd, the developing voltage is applied from independent developing voltage sources E2a-E2d, respectively. For that reason, not only in the operation in the black mode but also in the operation in the full-color mode, it is possible to select the operation setting different from the normal operation setting. That is, the setting of the black developing voltage set as described above is applicable to both of during the operation in the black mode and during the operation in the full-color mode. In this case, similarly as described above with reference to (a) and (b) ofFIG. 12, it is possible to cause the controller50to change the black developing voltage during the operation in the full-color mode. Or, the controller50may also be caused to change, as the black density setting, the black developing voltage applied in common during both of the operation in the black mode and the operation in the full-color mode.

That is, in this embodiment, the image forming apparatus100may also include another designating means as described below. Another designating means causes the controller50to change the setting of the developing voltage applied to the developing member41din the operation in the first mode relative to the setting of the developing voltage applied to other developing members41a-41ccorresponding to other image bearing members1a-1cin the operation in the first mode. In this embodiment, similarly as in the designating means, another designating means is constituted by the operation display portion17, the communication I/F portion80through which the designation from the external device200is inputted to the controller50, and the like. In the case where the designation by another designating means is made, when the image is formed by the operation in the first mode, the controller50changes the setting of the developing voltage applied to the predetermined developing member41drelative to the setting of the developing voltage applied to other developing members41a-41c.

By employing such a constitution, also the density (or the width) of the black character image and the black line image during the operation in the full-color mode can be adjusted depending on the demand of the user or the like. Accordingly, also during the operation in the full-color mode, irrespective of the information on the image attribute such as the object data, it is possible to obtain the density (or the width) of a stable character image or a stable line image (particularly the thin line) corresponding to the selection of the operator. In order to realize such a constitution, the developing voltage source E2dfor black may only be required to be provided independently of the voltage sources for other colors used during the operation in the full-color mode. Accordingly, for example, as shown in (b) ofFIG. 13, commonality of a developing voltage source E2CL for at least two colors of yellow, magenta and cyan (in the figure, all of yellow, magenta and cyan) may also be realized.

Then, another embodiment of the present invention will be described. A basic constitution and an operation of an image forming apparatus in this embodiment are the same as those in Embodiment 1. Accordingly, in the image forming apparatus in this embodiment, elements having the same or corresponding functions and constitutions as those in Embodiment 1 are represented by the same reference numerals or symbols and will be omitted from description.

In this embodiment, as shown in (c) ofFIG. 13, the developing voltage is applied from a common (the same) developing voltage source E2to the developing rollers41a-41dof all of the image forming portions Pa-Pd. Also in this case, similarly as in Embodiment 1, the setting of the black developing voltage during the operation in the black mode can be arbitrarily changed to setting other than the normal operation setting.

However, during the operation in the full-color mode, only the setting of the black developing voltage cannot be arbitrarily changed to setting other than the normal operation setting. This is because the developing voltage source for black and the developing voltage sources for other colors used in the operation in the full-color mode (in this embodiment, the developing voltage sources for all of the colors) are used in common, and therefore the setting of the black developing voltage has an influence on other colors.

Accordingly, in this embodiment, the setting of the black developing voltage applied from the common developing voltage source E2to the developing roller41dfor black can be changed to the setting other than the normal operation setting only during the operation in the black mode. Further, during the operation in the full-color mode, the common developing voltage source E2is constituted so as to output the developing voltage in setting (normal operation setting) before the change. Typically, during the operation in the full-color mode, in this normal operation setting, the image is formed using all of the image forming portions Pa-Pd.

Here, the following constitution can be employed in order to enable ensuring of the density of particularly the character image or the line image (particularly the thin line) by selecting the operation setting different from the normal operation setting also during the operation in the full-color mode. That is, during the operation in the full-color mode, the black image is formed using the toners of the plurality of colors (in this embodiment, yellow, magenta and cyan) in place of the black toner. In this case, even when the normal setting developing voltage is outputted from the common developing voltage source E2, the density (or the width) of the black image during the operation in the full-color mode is easily ensured. This is because a toner amount per unit area can be increased by forming the black image with the toners of the plurality of colors. In this case, for example, in a setting screen similar to that shown in (a) ofFIG. 12, the controller50can designate that during the operation in the full-color mode, the black image is formed with the toners of the plurality of colors. Further, it is also possible to variably set the black image density during the operation in the full-color mode. For example, the amounts per unit area of the toners of the respective colors may only be required to be changed by changing exposure amounts (laser powers or areas) by the exposure devices3for the respective toner images of the plurality of colors relative to the black toner image portion. In this case, in the density setting screen similar to that shown in (b) ofFIG. 12, the operator can variably set the black image density during the operation in the full-color mode.

That is, in this embodiment, the image forming apparatus100may also include another designating means as described below. In the operation in the second mode, another designating means causes the controller50to cause the image forming apparatus to form the image of the color (black) of the toner image formed on the predetermined image bearing member1din the operation in the first mode by superposing the toner images formed on other image bearing members1a-1c. In this embodiment, similarly as in Embodiment 1, another designating means is constituted by the operation display portion17, the communication I/F portion80through which the designation from the external device200is inputted to the controller50, and the like. In the case where the designation by another designating means is made, during the operation in the second mode, the controller50effects control so that the image of the color (black) of the toner image formed on the predetermined image bearing member1din the operation in the first mode is formed by superposing the toner images formed on the plurality of other image bearing members1a-1c.

By employing such a constitution, during the operation in the black mode and during the operation in the full-color mode, irrespective of the information on the image attribute such as the object data, it is possible to obtain the density (or the width) of a stable character image or a stable line image (particularly the thin line) corresponding to the selection of the operator.

Other Embodiments

The present invention was described based on the specific embodiments mentioned above, but is not limited to the above-mentioned embodiments.

In the above-described embodiments, the present invention was applied to the image forming apparatus of the intermediary transfer type, but is also applicable to an image forming apparatus of a direct transfer type.FIG. 14is a schematic sectional view of a principal part of the image forming apparatus of the direct transfer type. In the image forming apparatus ofFIG. 14, elements having the same or corresponding functions or constitutions as those of the image forming apparatus ofFIG. 1are represented by the same reference numerals or symbols. The image forming apparatus100ofFIG. 14includes an endless belt-shaped recording material carrying belt (conveying belt107) as a recording material carrying member in place of the intermediary transfer belt7in the image forming apparatus ofFIG. 1. The toner images formed on the photosensitive drums1at the respective image forming portions P are transferred at the respective transfer portions N onto the recording material S carried and conveyed by the recording material carrying belt107. Also the image forming apparatus100of the direct transfer type is constituted, similarly as in the case of the image forming apparatus100of the intermediary transfer type, so as to be capable of forming the toner images by the operation in the full-color mode and the operation in the black mode in some instances. Accordingly, by applying the present invention to also the image forming apparatus100of the direct transfer type, it is possible to achieve effects similar to those of the above-described embodiments.

In the above-described embodiments, the operator such as the user arbitrarily designated that the operation setting for black is operation setting different from the normal operation setting. However, the present invention is not limited thereto. Irrespective of the designation by the operator such as the user, the operation setting for black may also be made in advance so as to be different from the normal operation setting. In this case, the designating means and another designating means are constituted by a program or the like constituted to change the setting in advance.

In the above-described embodiments, the black image was able to be reproduced so as to be not only thick (high density) but also thin (low density). However, the present invention is not limited thereto. Typically, the black image may also be adjustable only in a direction of reproducing the black image as the thick image (with the high density).

This application claims the benefit of Japanese Patent Application No. 2016-143421 filed on Jul. 21, 2016, which is hereby incorporated by reference herein in its entirety.