JP2012018335A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of ensuring reduced occurrence of shock jitter and achieving a longer lifetime of an image forming section without increasing complexity or cost of the device.SOLUTION: An image forming device comprises switching means 72 for switching between a first state in which all of a plurality of image carriers 1Y, 1M, 1C, 1K are abutted against an intermediate transfer body 8, and a second state in which only the image carrier 1K corresponding to a specific color is abutted against the intermediate transfer body 8 while the other image carriers 1Y, 1M, 1C are relatively separated from the intermediate transfer body 8. When a specific color mode in which an image is formed only with a specific color is selected, the switching means 72 is controlled to switch a state to the first state when a surface friction coefficient of the intermediate transfer body 8 is equal to or less than a predetermined threshold, and to the second state when the surface friction coefficient of the intermediate transfer body 8 is greater than the predetermined threshold.

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine thereof, and more particularly, image formation in which a plurality of color toner images are transferred to an intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum It relates to the device.

2. Description of the Related Art Conventionally, in image forming apparatuses such as copying machines and printers, a technique for transferring a plurality of color toner images on an intermediate transfer member such as an intermediate transfer belt is widely known (see, for example, Patent Documents 1 to 6). .)
Specifically, four photosensitive drums (image carriers) are arranged side by side so as to face the intermediate transfer belt (intermediate transfer member). On the four photosensitive drums, black, black, yellow, magenta, and cyan toner images are formed. Then, the toner images respectively formed on the four photosensitive drums are transferred (primary transfer) while being superimposed on the intermediate transfer belt. Further, the toner images of a plurality of colors carried on the intermediate transfer belt are transferred (secondary transfer) onto the recording medium as a color image at a position where the intermediate transfer belt and the secondary transfer member face each other.

In such a color image forming apparatus, the color image mode (full color image mode) and the monochrome image mode (monochrome image mode) are arbitrarily switched by the user according to the type of the document (or image information). It is configured to be.
Specifically, for a normal four-color (YMCK) color document (or color image information), toner images of four colors of black, yellow, magenta, and cyan are transferred on an intermediate transfer belt. “Color image mode” is selected. For a monochrome document (or monochrome image information), a “monochrome image mode” in which only a toner image of black toner is transferred onto the intermediate transfer belt is selected.

In addition, when switching to the “monochrome image mode” in this way, a photosensitive drum that is not used for image formation is used in order to extend the life of an image forming portion (YMC image forming portion) that is not used for image formation. Is controlled to be separated from the intermediate transfer belt.
Specifically, when the “color image mode” is selected, all the four photosensitive drums are in contact with the intermediate transfer belt. When the “monochrome image mode” is selected, the three photosensitive drums on which the three-color (YMC) toner images are formed are separated from the intermediate transfer belt to form a black toner image. Only the photosensitive drum is brought into contact with the intermediate transfer belt.

On the other hand, in such an image forming apparatus, when the recording medium enters the secondary transfer nip portion of the intermediate transfer belt, a speed fluctuation occurs in the intermediate transfer belt. This is a phenomenon in which the image subjected to the transfer process is disturbed. Such “shock jitter” is likely to occur because the vibration easily propagates to the intermediate transfer belt in the “monochrome image mode” in which only the photosensitive drum for black is in contact with the intermediate transfer belt. It has become.
In order to reduce the occurrence of such “shock jitter”, Patent Documents 1 and 2 describe that a load is applied to the intermediate transfer belt by bringing a contact member or a shutter member into contact with the intermediate transfer belt in the monochrome image mode. The technology given is disclosed. Patent Document 3 discloses a technique in which tension adjusting means is provided so that the belt tension of the intermediate transfer belt does not fluctuate. Patent Document 4 discloses a technique for generating an assisting force that attenuates an increase in repulsive force on a contact member inscribed in a belt member. Patent Document 5 discloses a technique for controlling a linear speed ratio between a fixing linear speed and a transfer linear speed. Patent Document 6 discloses a technique in which a dynamic vibration absorber made of an elastic body and an inertial body is provided inside the secondary transfer roller.

In the conventional image forming apparatus, when the “monochrome image mode” is selected, only the black photosensitive drum is in contact with the intermediate transfer belt. Compared to the contacted state, vibration was more easily propagated to the intermediate transfer belt, and “shock jitter” was more likely to occur.
In order to suppress the occurrence of such shock jitter, in Patent Documents 1 to 6, various members are added and controlled, but this complicates the configuration of the image forming apparatus and increases the cost. There was a malfunction.

  The present invention has been made to solve the above-described problems, and it is possible to reliably reduce the occurrence of shock jitter without increasing the complexity and cost of the apparatus, and to increase the life of the imaging unit. An object of the present invention is to provide an image forming apparatus.

  According to a first aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image carriers on which toner images of respective colors are formed; and a toner image formed on each of the plurality of image carriers is subjected to primary transfer. The intermediate transfer member, a secondary transfer member that secondarily transfers the toner image primarily transferred onto the intermediate transfer member to a recording medium, and all of the plurality of image carriers are brought into contact with the intermediate transfer member. A first state of the plurality of image carriers, and only the image carrier corresponding to a specific color among the plurality of image carriers is brought into contact with the intermediate transfer member, and the other image carriers are relatively separated from the intermediate transfer member. And a switching unit that switches between the second state and the surface friction coefficient of the intermediate transfer member is less than or equal to a predetermined threshold when a specific color mode that performs image formation only with the specific color is selected. When it becomes the first state The As surface friction coefficient of the intermediate transfer member is in said second state when exceeding the predetermined threshold value, and controls the switching means.

  An image forming apparatus according to a second aspect of the present invention is the image forming apparatus according to the first aspect, wherein the specific color mode is selected and a basis weight of a recording medium to be passed is predetermined. When the surface friction coefficient of the intermediate transfer body is equal to or lower than a predetermined threshold value, the first state is reached, and when the surface friction coefficient of the intermediate transfer body exceeds the predetermined threshold value, the second state is reached. The switching means is controlled so that the state becomes.

  An image forming apparatus according to a third aspect of the present invention is the image forming apparatus according to the first or second aspect, wherein the predetermined threshold value is varied according to a type of a recording medium to be passed. .

  According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, the type of the recording medium is the basis weight or thickness of the recording medium.

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the driving means for driving the intermediate transfer member and the driving torque in the driving means are detected. Torque detecting means, and the switching means is controlled by a surface friction coefficient of the intermediate transfer member obtained based on a detection result of the torque detecting means.

  The image forming apparatus according to a sixth aspect of the present invention is the image forming apparatus according to the fifth aspect, wherein the driving unit is a driving motor, and the torque detecting unit is configured to obtain a current value flowing through the driving motor. It is a means for detection.

  An image forming apparatus according to a seventh aspect of the present invention is the image forming apparatus according to any of the first to fourth aspects, wherein the image forming apparatus is formed on the intermediate transfer member when the specific color mode is performed. An image area ratio detection unit that directly or indirectly detects an image area ratio of a toner image, and the switching unit is configured to determine a surface friction coefficient of the intermediate transfer member obtained based on a detection result of the image area ratio detection unit. It is to be controlled.

  An image forming apparatus according to an eighth aspect of the present invention is the image forming apparatus according to any one of the first to fourth aspects, wherein the intermediate transfer member is new when the specific color mode is selected. The switching means is controlled so as to be in the first state until the cumulative number of recording media passed through the apparatus reaches a predetermined value from the state.

  An image forming apparatus according to a ninth aspect of the present invention is the image forming apparatus according to any one of the first to eighth aspects, wherein the special color is black.

  An image forming apparatus according to a tenth aspect of the present invention is the image forming apparatus according to any one of the first to ninth aspects, wherein the intermediate transfer member is opposed to the plurality of image carriers. The intermediate transfer belt is provided side by side.

  An image forming apparatus according to an eleventh aspect of the present invention is the image forming apparatus according to the tenth aspect, further comprising a plurality of primary transfer members that respectively contact the plurality of image carriers via the intermediate transfer belt. The switching means is a contact / separation mechanism that moves the primary transfer member facing the other image carrier so as to be contactable / separable with respect to the intermediate transfer belt.

  In the present invention, even when the specific color mode is selected, when the surface friction coefficient of the intermediate transfer member is equal to or less than a predetermined threshold value, the plurality of image carriers are all in contact with the intermediate transfer member. is doing. As a result, it is possible to provide an image forming apparatus in which the occurrence of shock jitter can be reliably reduced and the life of the image forming unit can be increased without increasing the complexity and cost of the apparatus.

1 is an overall configuration diagram illustrating an image forming apparatus according to Embodiment 1 of the present invention. It is sectional drawing which shows an image formation part. FIG. 3 is a configuration diagram illustrating the vicinity of an intermediate transfer belt. FIG. 3 is a configuration diagram illustrating an intermediate transfer belt contact / separation mechanism. FIG. 6 is a diagram illustrating a second state in which only the black photosensitive drum is in contact with the intermediate transfer belt. It is a flowchart which shows the control at the time of a monochrome image mode. It is a graph which shows the image rank of a shock jitter when changing the basic weight of a recording medium, and a belt surface friction coefficient. It is a graph which shows the fluctuation | variation of the image rank of a shock jitter and a belt surface friction coefficient when the cumulative number of paper passes increases. 6 is a graph showing a relationship between an image area ratio of a toner image formed on an intermediate transfer belt and a belt surface friction coefficient. It is a table | surface figure which shows the control table used with the control part of the image forming apparatus in Embodiment 2 of this invention. It is a table | surface figure which shows the generation | occurrence | production state of a shock jitter when the basic weight of a recording medium and a belt surface friction coefficient change.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

Embodiment 1 FIG.
The first embodiment of the present invention will be described in detail with reference to FIGS.
First, the configuration and operation of the entire image forming apparatus will be described with reference to FIGS.
FIG. 1 is a block diagram showing a copying machine as an image forming apparatus, and FIG. 2 is an enlarged view showing an image forming unit thereof. FIG. 3 is a configuration diagram showing the vicinity of the intermediate transfer belt 8.
As shown in FIG. 1, an intermediate transfer unit 15 (intermediate transfer belt device) is installed in the center of the image forming apparatus main body 100. Also, the image forming unit 6K for black toner and three colors (yellow, magenta, and cyan) for forming a color image so as to face the intermediate transfer belt 8 (intermediate transfer member) of the intermediate transfer unit 15 are supported. The color toner image forming sections 6Y, 6M, and 6C are arranged in parallel. Further, a secondary transfer roller 19 as a secondary transfer member is disposed so as to face the intermediate transfer belt 8 of the intermediate transfer unit 15.

  Referring to FIG. 2, an image forming unit 6Y corresponding to yellow includes a photosensitive drum 1Y as an image carrier, a charging unit 4Y disposed around the photosensitive drum 1Y, a developing unit 5Y, and a cleaning unit 2Y. , And a static elimination unit (not shown). Then, an image forming process (charging process, exposure process, developing process, transfer process, cleaning process) is performed on the photosensitive drum 1Y, and a yellow image is formed on the photosensitive drum 1Y.

The other three image forming units 6M, 6C, and 6K have substantially the same configuration as that of the image forming unit 6Y corresponding to yellow except that the color of the toner used is different. A corresponding image is formed. Hereinafter, description of the other three image forming units 6M, 6C, and 6K will be omitted as appropriate, and only the image forming unit 6Y corresponding to yellow will be described.
In the first embodiment, black is set as the “specific color”. Then, the photosensitive drum 1K in the image forming unit 6K for black toner is defined as an “image carrier corresponding to a specific color”, and the photosensitive drums 1Y, 1M, 3M in the three image forming units 6Y, 6M, and 6C for color toner are used. Although 1C is referred to as “other image carrier”, the configurations of these photosensitive drums 1Y, 1M, 1C, and 1K are almost the same, and accordingly, these names are appropriately referred to as “photosensitive drum” or “ Description of the configuration and operation of each photosensitive drum is omitted.

Referring to FIG. 2, a photosensitive drum 1Y as an image carrier is rotationally driven counterclockwise in FIG. 2 by a main drive motor (not shown). Then, the surface of the photosensitive drum 1Y is uniformly charged at the position of the charging unit 4Y (a charging process).
Thereafter, the surface of the photosensitive drum 1Y reaches the irradiation position of the laser beam L emitted from the exposure unit 7, and an electrostatic latent image corresponding to yellow is formed by exposure scanning at this position (in the exposure process). is there.).

Thereafter, the surface of the photosensitive drum 1Y reaches a position facing the developing portion 5Y, and the electrostatic latent image is developed at this position to form a yellow toner image (developing process).
Thereafter, the surface of the photosensitive drum 1Y reaches a position facing the intermediate transfer belt 8 and the primary transfer roller 9Y (primary transfer member), and the toner image on the photosensitive drum 1Y serves as an intermediate transfer body at this position. Are transferred onto the intermediate transfer belt 8 (this is a primary transfer step). At this time, a small amount of untransferred toner remains on the photosensitive drum 1Y.

Thereafter, the surface of the photosensitive drum 1Y reaches a position facing the cleaning unit 2Y, and untransferred toner remaining on the photosensitive drum 1Y at this position is collected in the cleaning unit 2Y by the cleaning blade 2a (cleaning). Process.)
Finally, the surface of the photosensitive drum 1Y reaches a position facing a neutralization unit (not shown), and the residual potential on the photosensitive drum 1 is removed at this position.
Thus, a series of image forming processes performed on the photosensitive drum 1Y is completed.

The image forming process described above is performed in the other image forming units 6M, 6C, and 6K in the same manner as the yellow image forming unit 6Y. That is, a laser beam L based on image information is irradiated from the exposure unit 7 disposed above the image forming unit onto the photosensitive drums 1M, 1C, and 1K of the image forming units 6M, 6C, and 6K. Is done. Specifically, the exposure unit 7 emits laser light L from a light source, and irradiates the photosensitive drum through a plurality of optical elements while scanning the laser light L with a polygon mirror that is rotationally driven.
Thereafter, the toner images of the respective colors formed on the respective photosensitive drums through the development process are transferred (primary transfer) on the intermediate transfer belt 8 serving as an intermediate transfer member. Thus, a desired color image is formed on the intermediate transfer belt 8.
In FIG. 1, although not shown in the drawing, above the exposure unit 7 in the image forming apparatus 100, a document transport unit that transports a document and image information of the document transported by the document transport unit are optically displayed. A reading unit and the like for reading are provided.

  Here, referring to FIG. 1, four image forming units 6Y, 6M, 6C, and 6K are formed from yellow toner image forming unit 6Y and magenta toner image forming from the upstream side in the running direction of intermediate transfer belt 8. The image forming unit 6M, the cyan image forming unit 6C, and the black toner image forming unit 6K are arranged in this order. Accordingly, the intermediate transfer belt 8 has a yellow toner image formed on the yellow photosensitive drum 1Y, a magenta toner image formed on the magenta photosensitive drum 1M, and a cyan toner image. A toner image formed of cyan toner formed on the photosensitive drum 1C and a toner image formed of black toner formed on the black photosensitive drum 1K are sequentially superimposed and primarily transferred.

Thereafter, the intermediate transfer belt 8 (intermediate transfer member) onto which the toner images of the respective colors are transferred in a superimposed manner reaches a contact position (secondary transfer nip portion) with the secondary transfer roller 19 as a secondary transfer member. At this position, the secondary transfer opposing roller 12B sandwiches the intermediate transfer belt 8 between the secondary transfer roller 19 and forms a secondary transfer nip portion. A bias (high voltage) having the same polarity as the polarity of the toner is applied to the secondary transfer counter roller 12B, and a bias (high voltage) opposite to the polarity of the toner is applied to the secondary transfer roller 19. The As a result, the color toner image formed on the intermediate transfer belt 8 is transferred (secondary transfer) onto the recording medium P such as transfer paper conveyed to the position of the secondary transfer nip (secondary transfer). It is a transfer process.) At this time, untransferred toner that has not been transferred to the recording medium P remains on the intermediate transfer belt 8.
Thereafter, the intermediate transfer belt 8 reaches the position of the intermediate transfer cleaning unit 10. At this position, the untransferred toner on the intermediate transfer belt 8 is removed.
Thus, a series of transfer processes performed on the intermediate transfer belt 8 is completed.

Here, referring to FIG. 1, the recording medium P transported to the position of the nip portion (secondary transfer nip portion) is fed to the paper feeding portion 26 (or to the side) disposed below the apparatus main body 100. The paper is fed from the paper feed unit provided via the paper feed roller 27, the registration roller pair 28, and the like.
Specifically, a plurality of recording media P such as transfer paper are stored in the paper supply unit 26 in a stacked manner. When the paper feed roller 27 is driven to rotate counterclockwise in FIG. 1, the uppermost recording medium P is fed toward the rollers of the registration roller pair 28.

  The recording medium P transported to the registration roller pair 28 temporarily stops at the position of the roller nip of the registration roller pair 28 whose rotation drive has been stopped. Then, the registration roller pair 28 is rotationally driven in time with the color image on the intermediate transfer belt 8, and the recording medium P is conveyed toward the secondary transfer nip portion. In this way, a desired color image is transferred onto the recording medium P.

Thereafter, the recording medium P on which the color image has been transferred at the position of the secondary transfer nip portion is conveyed to the position of the fixing portion 20. At this position, the color image transferred to the surface is fixed on the recording medium P by heat and pressure generated by the fixing belt and the pressure roller.
Thereafter, the recording medium P is discharged out of the apparatus by a pair of discharge rollers (not shown). The transferred P discharged from the apparatus by the pair of paper discharge rollers is sequentially stacked on the stack unit as an output image.
Thus, a series of image forming processes in the image forming apparatus is completed.

Next, the configuration and operation of the developing unit in the image forming unit will be described in more detail with reference to FIG.
The developing unit 5Y includes a developing roller 51Y that faces the photosensitive drum 1Y, a doctor blade 52Y that faces the developing roller 51Y, two transport screws 55Y disposed in the developer containing unit, and an opening in the developer containing unit. A toner replenishing path 43Y communicating with the magnetic sensor 56Y, a magnetic sensor 56Y for detecting the toner concentration in the developer, and the like. The developing roller 51Y includes a magnet fixed inside, a sleeve rotating around the magnet, and the like. In the developer accommodating portion, a two-component developer composed of a carrier and toner is accommodated.
The toner supply path 43Y is for appropriately supplying new toner from a toner container (not shown) containing new toner toward the developing device 5Y. Further, the supply of new toner to the developing device 5Y is performed so that the toner density (the ratio of toner in the developer) detected by the magnetic sensor 56Y falls within a predetermined range.

The developing unit 5Y configured as described above operates as follows.
The sleeve of the developing roller 51Y rotates in the direction of the arrow in FIG. The developer carried on the developing roller 51Y by the magnetic field formed by the magnet moves on the developing roller 51Y as the sleeve rotates.
Thereafter, the toner replenished in the developer accommodating portion is circulated through the two separated developer accommodating portions while being mixed and stirred together with the developer by the two conveying screws 55Y rotating in the direction of the arrow in FIG. This is a movement in the direction perpendicular to the paper surface of FIG. The toner in the developer is attracted to the carrier by frictional charging with the carrier, and is carried on the developing roller 51Y together with the carrier by the magnetic force formed on the developing roller 51Y.

  The developer carried on the developing roller 51Y is conveyed in the direction of the arrow in FIG. 2 and reaches the position of the doctor blade 52Y. The developer on the developing roller 51Y is conveyed to a position facing the photosensitive drum 1Y (development area) after the developer amount is made appropriate at this position. The toner is attracted to the latent image formed on the photosensitive drum 1Y by the electric field formed in the development area. Thereafter, the developer remaining on the developing roller 51Y reaches the upper portion of the developer accommodating portion as the sleeve rotates, and is detached from the developing roller 51Y at this position.

Next, the intermediate transfer unit 15 (intermediate transfer belt device) will be described in more detail with reference to FIG.
As shown in FIG. 3, the intermediate transfer unit 15 includes an intermediate transfer belt 8 as an intermediate transfer member, four primary transfer rollers 9Y, 9M, 9C, and 9K as primary transfer members, an auxiliary roller 11, and a drive roller 12A. It is composed of a secondary transfer counter roller 12B, a tension roller 12C, a correction roller 12D, an intermediate transfer cleaning unit 10, and the like. The intermediate transfer belt 8 is stretched and supported by a plurality of roller members 11, 12A to 12D, and is endlessly moved in the direction of the arrow in FIG. 3 by the rotational drive of the drive roller 12A.
The photosensitive drums 1Y, 1M, 1C, and 1K as the four image carriers on which toner images of each color (yellow, magenta, cyan, and black) are respectively formed are opposed to the intermediate transfer belt 8 that is an intermediate transfer member ( Are arranged side by side so as to make contact.

The four primary transfer rollers 9Y, 9M, 9C, and 9K respectively sandwich the intermediate transfer belt 8 between the photosensitive drums 1Y, 1M, 1C, and 1K to form primary transfer nip portions. A high voltage (transfer bias) having a polarity opposite to that of the toner is applied to the primary transfer rollers 9Y, 9M, 9C, and 9K.
The intermediate transfer belt 8 travels in the direction of the arrow and sequentially passes through the primary transfer nip portions of the primary transfer rollers 9Y, 9M, 9C, and 9K. In this way, the toner images of the respective colors on the photosensitive drums 1Y, 1M, 1C, and 1K are primary-transferred on the intermediate transfer belt 8 (a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image). In order.)

In the first embodiment, the intermediate transfer belt 8 as an intermediate transfer member is made of PI (polyimide), PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PC (polycarbonate), or the like. It is composed of a single layer or a plurality of layers, and a conductive material such as carbon black is dispersed. The intermediate transfer belt 8 has a volume resistivity of about 10 ⁷ to 10 ¹² Ωcm and a thickness of about 80 to 100 μm.
If necessary, a release layer can be coated on the surface of the intermediate transfer belt 8. At that time, materials used for the coating are ETFE (ethylene-tetrafluoroethylene copolymer), PTFE (polytetrafluoroethylene), PVDF (vinylidene fluoride), PEA (perfluoroalkoxy fluororesin), FEP (four A fluororesin such as fluorinated ethylene-hexafluoropropylene copolymer) or PVF (vinyl fluoride) can be used, but is not limited thereto.
Further, as a method for manufacturing the intermediate transfer belt 8, there are a casting method, a centrifugal molding method, and the like, and a step of polishing the surface is performed as necessary.

The drive roller 12A is rotationally driven by a drive motor 61 (see FIG. 4) as a drive means. Thus, the intermediate transfer belt 8 travels in a predetermined traveling direction (clockwise in FIG. 3) by the frictional force with the driving roller 12A.
Based on the displacement (displacement amount) of the intermediate transfer belt 8 detected by a meandering detection sensor (not shown), the correction roller 12D has one end as a fixed end and the other end moves in the vertical direction ( The rotation axis is inclined). Thereby, the displacement (meandering) of the intermediate transfer belt 8 in the width direction is corrected.
The tension roller 12 </ b> C is in contact with the outer peripheral surface of the intermediate transfer belt 8. An intermediate transfer cleaning unit 10 (cleaning blade) is installed between the secondary transfer counter roller 12B and the tension roller 12C.
The auxiliary roller 11 is in contact with the inner peripheral surface of the intermediate transfer belt 8 at a position upstream of the four photosensitive drums 1Y, 1M, 1C, and 1K (on the upstream side in the running direction of the intermediate transfer belt 8). Yes.

The secondary transfer counter roller 12B as a secondary transfer member is in contact with a secondary transfer roller 19 (secondary transfer member) via an intermediate transfer belt 8 as an intermediate transfer member. The secondary transfer counter roller 12B has a middle resistance rubber layer formed on a core metal, and is set to have a resistance value of 10 ⁷ to 10 ^8.5 Ω in an environment of 23 ° C. and 50%.
The secondary transfer roller 19 as a secondary transfer member is formed by forming a conductive rubber layer made of NBR (nitrile rubber) or the like and having a rubber hardness of about 48 to 58 Hs on a cored bar.
In the first embodiment, the secondary transfer roller 19 is used as the secondary transfer member for performing the secondary transfer process. However, a known secondary transfer device using corona discharge is used as the secondary transfer member. It is also possible to use a known secondary transfer belt as the secondary transfer member.
In the first embodiment, secondary transfer currents having different polarities are applied to two roller members (secondary transfer counter roller 12B and secondary transfer roller 19) sandwiching the intermediate transfer belt 8. The two roller members 12B and 19 are both configured to function as secondary transfer members. On the other hand, a secondary transfer current is applied to only one of the two roller members 12B and 19 (for example, the secondary transfer roller 19), and only one of them functions as a secondary transfer member. It can also be made.

Hereinafter, the configuration and operation characteristic of the image forming apparatus according to the first embodiment will be described in detail.
4 and 5, in the intermediate transfer unit 15 according to the first embodiment, all of the four photosensitive drums 1Y, 1M, 1C, and 1K (image carrier) are in contact with the intermediate transfer belt 8. In the first state, only the photosensitive drum 1K corresponding to black (specific color) among the four photosensitive drums 1Y, 1M, 1C, and 1K is brought into contact with the intermediate transfer belt 8 and the other three photosensitive drums. A contact / separation mechanism 72 is provided as switching means for switching between the second state in which the body drums 1Y, 1M, 1C are relatively separated from the intermediate transfer belt 8.

  Specifically, the contact / separation mechanism 72 as a switching unit contacts the swing arm 72a and the swing arm 72a that rotatably hold the three primary transfer rollers 9Y, 9M, and 9C for color and the auxiliary roller 11. An eccentric cam 72b, a stepping motor 65 that rotationally drives the eccentric cam 72b, and the like are configured. A support shaft 72a1 of the swing arm 72a is rotatably supported on the side plate of the intermediate transfer unit 15. Then, the eccentric cam 72b is rotated by a predetermined angle by the stepping motor 65 controlled by the control unit 80, so that the primary transfer roller 9Y for color with respect to the three photosensitive drums 1Y, 1M, and 1C for color. , 9M, 9C rotate in the direction in which the swing arm 72a moves away (or abuts) (movement from the state of FIG. 4 to the state of FIG. 5 or movement from the state of FIG. 5 to the state of FIG. 4). .) In this way, the relative contact / separation operation of the three photosensitive drums 1Y, 1M, and 1C with respect to the intermediate transfer belt 8 is performed. That is, the contact / separation mechanism 72 (switching unit) causes the four photosensitive drums 1Y, 1M, 1C, and 1K to contact the intermediate transfer belt 8 (the state shown in FIG. 4) and the specific color. The second state (the state shown in FIG. 5) in which only the photosensitive drum 1K corresponding to black as described above is in contact with the intermediate transfer belt 8 is switched.

The contact / separation mechanism 72 (switching means) is controlled so that all of the four photosensitive drums 1Y, 1M, 1C, and 1K are in contact with the intermediate transfer belt 8 (the state shown in FIG. 4). Thus, an image forming process using a toner of four colors (yellow, magenta, cyan, and black) (“color image mode (full color image mode)”) is appropriately performed. Such a “color image mode” is automatically selected by optically sensing a document set on a document conveyance unit (not shown) of the image forming apparatus main body 100, or is not shown by a user. It is manually selected by operating the operation panel. Specifically, when it is determined to perform image formation with four-color toner (image formation of a full-color image) from a document set on the apparatus main body 100, the “color image mode” is automatically selected (or Will be manually selected).
The “color image mode (full color image mode)” is an image mode in which toner images respectively formed on the four photosensitive drums 1Y, 1M, 1C, and 1K are transferred onto the intermediate transfer belt 8 in an overlapping manner.

On the other hand, the contact / separation mechanism 72 is controlled so that only the black photosensitive drum 1K is brought into contact with the intermediate transfer belt 8 and the other three photosensitive drums 1Y, 1M, 1C are separated. In the state (the state shown in FIG. 5), an image forming process using only black toner (“monochrome image mode”) is appropriately performed. Such a “monochrome image mode” is also automatically selected by optically sensing a document set on a document conveying section (not shown) of the image forming apparatus main body 100, or the user does not show a “monochrome image mode”. It is manually selected by operating the operation panel. Specifically, when it is determined that full-color image formation using four-color toner is unnecessary and image formation using only black toner (monochrome image formation) is performed from a document set on the apparatus main body 100. In addition, the “monochrome image mode” is automatically selected (or manually selected).
The “monochrome image mode (monochrome mode)” as the specific color mode is an image mode in which only the toner image formed on the photosensitive drum 1K for black (specific color) is transferred to the intermediate transfer belt 8.

As described above, the unnecessary photosensitive drum in the image forming mode is separated from the intermediate transfer belt 8 in accordance with the selected image mode, thereby reducing the wear deterioration between the unnecessary photosensitive drum and the intermediate transfer belt 8. As a result, the life of the photosensitive drum (image forming unit) and the intermediate transfer belt 8 is extended.
Although illustration is omitted, when no image forming process is performed in the image forming apparatus 100 (at the time of standby or at the end of the printing operation, etc.), all four of the intermediate transfer belt 8 are used. A second contact / separation mechanism that moves the black primary transfer roller 9K so that the photosensitive drums 1Y, 1M, 1C, and 1K are separated can be provided separately from the contact / separation mechanism 72 described above. As a result, the life of the photosensitive drums 1Y, 1M, 1C, 1K, and 1W (image forming units 6Y, 6M, 6C, 6K, and 6W) and the intermediate transfer belt 8 is further extended.

Here, in the image forming apparatus according to the first embodiment, when the “monochrome image mode” as the specific color mode is selected, the surface friction coefficient (belt surface friction coefficient) of the intermediate transfer belt 8 is a predetermined threshold value. (In the first embodiment, it is set to 0.3.) When the following condition is reached, the first state (the state shown in FIG. 4) is entered, and the surface friction coefficient of the intermediate transfer belt 8 is the above-mentioned predetermined value. The contact / separation mechanism 72 (switching means) is controlled by the control unit 80 so that the second state (the state shown in FIG. 5) is reached when the threshold value is exceeded. That is, in the first embodiment, when the “monochrome image mode” is selected, the monochrome image forming process is not performed uniformly in the second state (the state of FIG. 5), but in the middle. When the surface friction coefficient of the transfer belt 8 does not reach a predetermined value (threshold), it is assumed that shock jitter is likely to occur, and the monochrome image forming process is performed in the first state (the state shown in FIG. 4). The occurrence of shock jitter is reduced.
When the “monochrome image mode” is performed in a state where all the four photosensitive drums 1Y, 1M, 1C, and 1K are in contact with the intermediate transfer belt 8, the three image forming units 6Y, 6M, and 6C for color are used. The three primary transfer rollers 9Y, 9M, and 9C for color are not formed on the three photosensitive drums 1Y, 1M, and 1C for color so that toner adhesion to the intermediate transfer belt 8 does not occur. A reverse voltage is applied to each.

As described above, when the “monochrome image mode” is performed, the belt surface friction coefficient of the intermediate transfer belt 8 is low, and only the black photosensitive drum 1 </ b> K is in contact with the intermediate transfer belt 8. As a result, the contact force (holding force) of the intermediate transfer belt 8 with respect to the photosensitive drum 1K is reduced, and the impact when the recording medium P enters the secondary transfer nip portion easily propagates to the intermediate transfer belt 8. In this state, the other three photosensitive drums 1Y, 1M, and 1C are also brought into contact with the intermediate transfer belt 8 to increase the adhesion (holding force) of the intermediate transfer belt 8, so that the secondary transfer is performed. The impact when the recording medium P enters the nip portion is less likely to propagate to the intermediate transfer belt 8. Therefore, the occurrence of shock jitter due to the entry of the recording medium P into the secondary transfer nip can be reliably reduced.
Further, according to the above-described control, when the “monochrome image mode” is performed and the belt surface friction coefficient of the intermediate transfer belt 8 is not low, unnecessary photosensitive drums 1Y, 1M, and 1C are placed in the middle. Since they are separated from the transfer belt 8, unnecessary deterioration of the photosensitive drums 1Y, 1M, and 1C and the intermediate transfer belt 8 is reduced, and the photosensitive drums 1Y, 1M, and 1C (image forming units 6Y, 6M, and 6C) are reduced. 6C) and the intermediate transfer belt 8 have a long life.
That is, it is possible to achieve both suppression of shock jitter generation in the monochrome image mode and extension of the life of the photosensitive drums 1Y, 1M, and 1C (image forming units 6Y, 6M, and 6C) and the intermediate transfer belt 8. Further, according to the first embodiment, in order to suppress the occurrence of shock jitter in the monochrome image mode, the switching control of the contact / separation mechanism 72 (switching means) used in the monochrome image mode is performed without providing a special member. Is doing. Therefore, the occurrence of shock jitter can be reliably reduced without making the image forming apparatus 100 complicated, expensive, and large.

Here, in the first embodiment, a drive motor 61 (drive) that drives the intermediate transfer belt 8 as detection means for detecting the surface friction coefficient of the intermediate transfer belt 8 when performing the control in the monochrome image mode described above. The torque detecting unit 62 (torque detecting means) for detecting the drive torque in the means) is used.
Referring to FIGS. 4 and 5, torque detection unit 62 serving as a torque detection unit detects a current value flowing through drive motor 61 (drive unit) that rotationally drives drive roller 12 </ b> A (intermediate transfer belt 8). Then, the detection result detected by the torque detection unit 62 is sent to the control unit 80, and the surface friction coefficient of the intermediate transfer belt 8 is obtained by the control unit 80 based on the detection result, and based on the result. Control by the contact / separation mechanism 72 (switching means) described above is performed.
Specifically, the intermediate transfer belt that is idly driven in a state where the number of members in contact with the intermediate transfer belt 8 is constant (for example, the first state) immediately before the monochrome image mode is executed. 8 is detected by the torque detector 62. The surface friction coefficient of the intermediate transfer belt 8 obtained based on the detection result (current value) detected by the torque detection unit 62 is set to a predetermined threshold value (0.3 in the first embodiment). .) If it is determined that the following, the monochrome image mode is executed in the first state (the state of FIG. 4). On the other hand, the surface friction coefficient of the intermediate transfer belt 8 obtained based on the detection result (current value) detected by the torque detection unit 62 is set to a predetermined threshold value (0.3 in the first embodiment). If it is determined that it exceeds the upper limit, the monochrome image mode is executed in the second state (the state shown in FIG. 5).

Here, in the first embodiment, the above-described control in the monochrome image mode is performed only when the basis weight of the recording medium P to be passed is a predetermined value or more. That is, when the monochrome image mode (specific color mode) is selected, the basis weight of the recording medium P to be passed is set to a predetermined value (in the first embodiment, 200 g / m ² ). .) In the case of the above, when the surface friction coefficient of the intermediate transfer belt 8 falls below a predetermined threshold value (set to 0.3 in the first embodiment), the first state (FIG. 4). When the surface friction coefficient of the intermediate transfer belt 8 exceeds the predetermined threshold value, the contact / separation mechanism 72 (switching means) is set so that the second state (the state shown in FIG. 5) is obtained. ) Is controlling.
The reason for performing such control is that even when the surface friction coefficient of the intermediate transfer belt 8 is low, if the basis weight of the recording medium P is not so large, the recording medium P enters the secondary transfer nip portion. This is because the impact is relatively small and shock jitter is less likely to occur.

FIG. 7 is a graph showing the image rank of shock jitter when the basis weight of the recording medium P and the belt surface friction coefficient (μ) of the intermediate transfer belt 8 are varied.
FIG. 7 shows (a) a condition using an intermediate transfer belt 8 having a basis weight of 100 g / m ² as a recording medium P and a belt surface friction coefficient (μ) of 0.3 or less. (B) Conditions using an intermediate transfer belt 8 in which a thick paper having a basis weight of 200 g / m ² is passed as the recording medium P and the belt surface friction coefficient (μ) exceeds 0.3, and (c) the recording medium In the condition using an intermediate transfer belt 8 having a basis weight of 200 g / m ² and a belt surface friction coefficient (μ) of 0.3 or less, the first state (FIG. 4 is a result of an experiment in which the degree of occurrence of shock jitter in the output monochrome image is visually determined for the second state (the state of FIG. 5). The “image rank” on the vertical axis in FIG. 7 is a result of stepwise determination of the degree of shock jitter in the range of 0 to 5, and the higher the rank, the better the degree of shock jitter. When the image rank is 3.5 or more, the level of shock jitter is low and is an acceptable level.
As shown in FIG. 7, regardless of the basis weight of the recording medium P, when the surface friction coefficient of the intermediate transfer belt 8 becomes low in the monochrome image mode, shock jitter is likely to occur. It can be seen that shock jitter is less likely to occur by performing the monochrome image mode with the drums 1Y, 1M, 1C, and 1K in contact with the intermediate transfer belt 8 (first state). Further, even when the surface friction coefficient of the intermediate transfer belt 8 is as low as 0.3 or less, the basis weight of the recording medium P is less than 200 g / m ² and is not so large (a recording medium having a thickness of medium thickness or less). It is also seen that shock jitter is less likely to occur when P is used.
In this way, the control for performing the monochrome image mode in the first state is performed only when the thick paper is passed, so that the above-described photosensitive drums 1Y, 1M, and 1C (imaging units 6Y, 6M, and 6C) are performed. In addition, the effect of extending the life of the intermediate transfer belt 8 can be achieved more reliably.

FIG. 6 is a flowchart showing the control in the monochrome image mode described above.
Referring to FIG. 6, when the monochrome image mode is selected (step S1), first, the paper type (basis weight) of the recording medium P to be passed is determined (step S2). Specifically, referring to FIG. 4, the basis weight of the recording medium P to be passed is detected directly or indirectly by the sheet detection unit 68. As the sheet detection unit 68, a unit based on information about the recording medium P input by the user operating an operation panel (not shown) of the apparatus main body 100 can be used. It is also possible to use a paper thickness sensor installed in the transport path leading to the secondary transfer nip portion.
If it is determined in step S2 that the recording medium P to be passed is not thick paper (thick paper having a basis weight of 200 g / m ² or more), the second state (the state shown in FIG. 5). In step S4, the monochrome image mode is executed.

On the other hand, when it is determined in step S2 that the recording medium P to be passed is a thick paper (thick paper having a basis weight of 200 g / m ² or more), it is further detected by the torque detector 62. Based on the result, it is determined whether the surface friction coefficient (μ) of the intermediate transfer belt 8 is equal to or less than a threshold value (0.3) (step S3).
As a result, when it is determined that the surface friction coefficient (μ) of the intermediate transfer belt 8 is not equal to or less than the threshold value (0.3), the monochrome image mode is set in the second state (the state shown in FIG. 5). Execute (step S4). On the other hand, when it is determined that the surface friction coefficient (μ) of the intermediate transfer belt 8 is equal to or less than the threshold (0.3), the first state ( In the state of FIG. 4, the monochrome image mode is executed (step S5).
If the color image mode (full color image mode) is selected (step S11), the color image mode is executed in the first state (the state shown in FIG. 4) (step S5). ).

Here, FIG. 8 shows fluctuations in the image rank of the shock jitter and the belt surface friction coefficient of the intermediate transfer belt 8 when the cumulative number of sheets passed in the image forming apparatus according to the first embodiment. It is a graph to show.
In FIG. 8, the horizontal axis indicates the cumulative number of sheets of recording medium P (the cumulative number of sheets that have been passed through) in the image forming apparatus 100 since the intermediate transfer belt 8 is in a new state. In FIG. 8, the right vertical axis indicates the surface friction coefficient (belt surface friction coefficient) of the intermediate transfer belt 8, and the black squares in the graph indicate the values. In FIG. 8, the left vertical axis indicates the image rank of shock jitter (as described above with reference to FIG. 7), and the ● mark in the graph indicates the value. In the experiment relating to FIG. 8, the contact / separation mechanism 72 is controlled so as to be in the second state (the state shown in FIG. 5), and a monochrome image having an image area ratio of 5% is continuously fed. I did it.
As shown in FIG. 8, the surface friction coefficient of the intermediate transfer belt 8 gradually increases with time, and shock jitter is less likely to occur. This is because filming gradually occurs on the surface of the intermediate transfer belt 8 with the increase in the number of sheets passing over time, and the surface friction coefficient of the intermediate transfer belt 8 increases.
From the experimental results in FIG. 8, the image forming apparatus 100 according to the first embodiment has a shock jitter image rank of 3.5 (allowable) when the surface friction coefficient (μ) of the intermediate transfer belt 8 is 0.3 or less. It can be seen that the level is not reached. Therefore, in the first embodiment, the threshold value of the surface friction coefficient (μ) of the intermediate transfer belt 8 for executing the monochrome image mode in the second state is set to 0.3.

In the first embodiment, a drive motor 61 (drive means) that drives the intermediate transfer belt 8 is used as a detection means for detecting the surface friction coefficient of the intermediate transfer belt 8 when the control in the monochrome image mode is performed. The torque detection unit 62 (torque detection means) for detecting the drive torque in FIG.
On the other hand, as a detecting means for indirectly detecting the surface friction coefficient of the intermediate transfer belt 8 when the control in the monochrome image mode described above is performed, the image forming apparatus 100 has been used since the intermediate transfer belt 8 is in a new state. It is also possible to use a counter 67 (see FIGS. 4 and 5) that counts the cumulative number of recording media P that have been passed through (the cumulative number of passed sheets). That is, when the monochrome image mode (specific color mode) is selected, the cumulative number of recording media P counted by the counter 67 is a predetermined value (when the image forming apparatus 100 according to the first embodiment is used). 8, the contact / separation mechanism 72 (switching means) can be controlled so that the monochrome image mode is performed in the first state until it reaches 500). . Even when such control is performed, the occurrence of shock jitter in the monochrome image mode is suppressed, and the photosensitive drums 1Y, 1M, and 1C (image forming units 6Y, 6M, and 6C) and the intermediate transfer belt 8 are controlled. It is possible to achieve both a longer service life and a longer life.

Further, as a detecting means for indirectly detecting the surface friction coefficient of the intermediate transfer belt 8 when performing the control in the monochrome image mode described above, it is formed on the intermediate transfer belt 8 when the monochrome image mode is performed. It is also possible to use image area ratio detection means for directly or indirectly detecting the image area ratio of the toner image.
Specifically, the image area ratio detection means is formed immediately before the monochrome image mode is executed from the duty in the exposure unit 7 (the lighting rate of the laser light L emitted when forming a monochrome image). A control unit 80 for obtaining the image area ratio of the toner image can be used. Then, when the monochrome image mode (specific color mode) is selected, the surface friction coefficient of the intermediate transfer belt 8 obtained based on the detection result (image area ratio) of the control unit 80 as the image area ratio detection means is used. The contact / separation mechanism 72 (switching means) can be controlled.
FIG. 9 is a graph showing the relationship between the image area ratio (horizontal axis) of the toner image formed on the intermediate transfer belt 8 and the belt surface friction coefficient (vertical axis) of the intermediate transfer belt 8.
As shown in FIG. 9, as the image area ratio increases, the amount of toner interposed between the photosensitive drum and the intermediate transfer belt increases, and the belt surface friction coefficient also increases. From such experimental results, control of the contact / separation mechanism 72 (switching means) in the monochrome image mode as described above becomes useful.
From the experimental results in FIG. 9, when the image area ratio of the toner image formed on the intermediate transfer belt 8 is 3% or less when the monochrome image mode is performed in the image forming apparatus according to the first embodiment. In addition, the monochrome image mode is executed in the first state assuming that the belt surface friction coefficient (μ) is 0.3 or less. In contrast, when the image area ratio of the toner image formed on the intermediate transfer belt 8 exceeds 3% when the noro image mode is performed, the belt surface friction coefficient (μ) exceeds 0.3. As a result, the monochrome image mode is executed in the second state.
Even when such control is performed, the occurrence of shock jitter in the monochrome image mode is suppressed, and the photosensitive drums 1Y, 1M, and 1C (image forming units 6Y, 6M, and 6C) and the intermediate transfer belt 8 are controlled. It is possible to achieve both a longer service life and a longer life.

  As described above, in the first embodiment, even when the monochrome image mode (specific color mode) is selected, the surface friction coefficient of the intermediate transfer belt 8 (intermediate transfer member) is equal to or less than a predetermined threshold value. At this time, the plurality of photosensitive drums 1Y, 1M, 1C, and 1K (image carrier) are controlled so as to contact with the intermediate transfer belt 8. Accordingly, the occurrence of shock jitter can be surely reduced and the life of the image forming unit can be increased without complicating and increasing the cost of the image forming apparatus 100.

Embodiment 2. FIG.
A second embodiment of the present invention will be described in detail with reference to FIGS.
FIG. 10 is a table showing a control table used by the control unit 80 of the image forming apparatus 100 according to the second embodiment. FIG. 11 is a table showing the state of occurrence of shock jitter when the basis weight of the recording medium P and the belt surface friction coefficient of the intermediate transfer belt 8 fluctuate.
In the image forming apparatus according to the second embodiment, the threshold value (the threshold value of the belt surface friction coefficient) in the control in the monochrome image mode is variable according to the type of the recording medium P to be passed. 1 is different.

  In the image forming apparatus 100 according to the second embodiment, as in the first embodiment, all of the four photosensitive drums 1Y, 1M, 1C, and 1K (image carrier) are brought into contact with the intermediate transfer belt 8. In the first state, only the photosensitive drum 1K corresponding to black (specific color) among the four photosensitive drums 1Y, 1M, 1C, and 1K is brought into contact with the intermediate transfer belt 8 and the other three photosensitive drums. A contact / separation mechanism 72 is provided as switching means for switching between the second state in which the body drums 1Y, 1M, 1C are relatively separated from the intermediate transfer belt 8.

  Here, in the second embodiment, in the monochrome image mode (specific color mode), the contact / separation mechanism is selected according to the type of the recording medium P to be passed (the basis weight in the second embodiment). The threshold value for controlling 72 (switching means) (the threshold value for determining whether to perform the monochrome image mode in the first state or the monochrome image mode in the second state) is varied. The

Specifically, referring to FIG. 10, when the basis weight of the recording medium P detected by the paper detection unit 68 (see FIGS. 4 and 5) is 100 g / m ² , the second is always set. In this state (the state shown in FIG. 5), the monochrome image mode is performed.
On the other hand, when the basis weight of the recording medium P detected by the paper detection unit 68 is 200 g / m ² , the belt surface friction coefficient (μ) is based on the detection result detected by the torque detection unit 62. If it is 0.3 or less, the monochrome image mode is performed in the first state (the state shown in FIG. 4), and the belt surface friction coefficient (μ) is based on the detection result detected by the torque detector 62. ) Exceeds 0.3, the monochrome image mode is performed in the second state (the state shown in FIG. 5).
When the basis weight of the recording medium P detected by the paper detection unit 68 is 250 g / m ² , the belt surface friction coefficient (μ) is 0.35 based on the detection result detected by the torque detection unit 62. When it is determined as follows, the monochrome image mode is performed in the first state (the state shown in FIG. 4), and the belt surface friction coefficient (μ) is 0 based on the detection result detected by the torque detection unit 62. When it exceeds .35, the monochrome image mode is performed in the second state (the state shown in FIG. 5).
When the basis weight of the recording medium P detected by the paper detection unit 68 is 300 g / m ² , the belt surface friction coefficient (μ) is 0.4 based on the detection result detected by the torque detection unit 62. When it is determined as follows, the monochrome image mode is performed in the first state (the state shown in FIG. 4), and the belt surface friction coefficient (μ) is 0 based on the detection result detected by the torque detection unit 62. When it exceeds .4, the monochrome image mode is performed in the second state (the state shown in FIG. 5).

Such control is based on the experimental results shown in FIG.
FIG. 11 is a table showing the state of occurrence of shock jitter when the basis weight of the recording medium P and the belt surface friction coefficient (μ) of the intermediate transfer belt 8 vary in the monochrome image mode.
In FIG. 11, “◯” indicates the result that the image rank of the shock jitter described above with reference to FIG. 7 has reached 3.5 or more (allowable level), and “X” indicates the shock jitter previously described with reference to FIG. This shows the result that the image rank of the above does not reach 3.5.
From the experimental results shown in FIG. 11, it can be seen that as the basis weight of the recording medium P increases, the impact when the recording medium P enters the secondary transfer nip portion increases and shock jitter is likely to occur.

  By performing such control, the length of the photosensitive drums 1Y, 1M, and 1C (image forming units 6Y, 6M, and 6C) and the intermediate transfer belt 8 can be more reliably suppressed while generating the shock jitter in the monochrome image mode. Life expectancy can be achieved.

  In the second embodiment, the threshold value in the monochrome image mode control is varied based on the basis weight of the recording medium, but the threshold value in the monochrome image mode control is based on the thickness (paper thickness) of the recording medium. Can also be varied. Specifically, a paper thickness sensor installed in the conveyance path from the paper feed unit 26 to the secondary transfer nip unit is used as a paper detection unit 68. When the paper thickness detected by the paper detection unit 68 increases, a threshold value of the friction coefficient is obtained. Is also controlled to be larger. Even in such a case, the same effect as in the second embodiment can be obtained.

Furthermore, it is possible to limit the type (paper type) of the recording medium P that is susceptible to shock jitter without depending on the basis weight or thickness of the recording medium, and detect that the type of the recording medium P is passed. Only when it is detected by the unit 68, the monochrome image mode can be controlled in the first state.
Even in such a case, the length of the photosensitive drums 1Y, 1M, and 1C (image forming units 6Y, 6M, and 6C) and the intermediate transfer belt 8 can be more reliably suppressed while generating the shock jitter in the monochrome image mode. Life expectancy can be achieved.

  As described above, in the second embodiment, as in the first embodiment, even when the monochrome image mode (specific color mode) is selected, the intermediate transfer belt 8 (intermediate transfer member). When the surface friction coefficient is equal to or less than a predetermined threshold value, the plurality of photosensitive drums 1Y, 1M, 1C, and 1K (image carrier) are controlled so as to all come into contact with the intermediate transfer belt 8. Accordingly, the occurrence of shock jitter can be surely reduced and the life of the image forming unit can be increased without complicating and increasing the cost of the image forming apparatus 100.

In each of the above embodiments, the specific color is black, the specific color mode is the monochrome image mode, and the contact / separation mechanism 72 (switching unit) is controlled when the monochrome image mode (specific color mode) is performed. The present invention was applied. On the other hand, a switching means for performing a specific color mode in which a color other than black (for example, magenta) is used as a specific color and image formation is performed only with the specific color (all of the plurality of photosensitive drums are set in the middle). The present invention is applied to control of switching between a first state in which the transfer belt 8 is brought into contact with and a second state in which only the photosensitive drum of a specific color is brought into contact with the intermediate transfer belt 8. You can also
Even in such a case, the same effects as those of the above-described embodiments can be obtained.

In each of the above embodiments, the intermediate transfer belt 8 is used as an intermediate transfer member, but an intermediate transfer drum can also be used as an intermediate transfer member.
Even in such a case, the same effects as those of the above embodiments can be obtained.

  It should be noted that the present invention is not limited to the above-described embodiments, and within the scope of the technical idea of the present invention, the embodiments can be modified as appropriate in addition to those suggested in the embodiments. Is clear. In addition, the number, position, shape, and the like of the constituent members are not limited to the above embodiments, and can be set to a number, position, shape, and the like that are suitable for carrying out the present invention.

1Y, 1M, 1C, 1K photosensitive drum (image carrier),
6Y, 6M, 6C, 6K imaging unit,
8 Intermediate transfer belt (intermediate transfer member),
9Y, 9M, 9C, 9K primary transfer roller (primary transfer member),
19 Secondary transfer roller (secondary transfer member),
72 Contact / separation mechanism (switching means),
100 Image forming apparatus body (apparatus body), P recording medium.

JP 2009-294312 A JP 2009-282124 A JP 2008-281933 A JP 2009-92799 A JP 2007-139882 A JP 2009-63737 A

Claims (11)

A plurality of image carriers on which toner images of respective colors are respectively formed;
An intermediate transfer member to which toner images respectively formed on the plurality of image carriers are primarily transferred;
A secondary transfer member for secondary transfer of the toner image primarily transferred onto the intermediate transfer member to a recording medium;
A first state in which all of the plurality of image carriers are brought into contact with the intermediate transfer member; and only an image carrier corresponding to a specific color among the plurality of image carriers is brought into contact with the intermediate transfer member. Switching means for switching between the second state in which the other image carrier is relatively separated from the intermediate transfer member;
With
In a case where a specific color mode in which image formation is performed only with the specific color is selected, the surface of the intermediate transfer member is brought into the first state when the surface friction coefficient of the intermediate transfer member becomes a predetermined threshold value or less. An image forming apparatus, wherein the switching unit is controlled so as to be in the second state when a friction coefficient exceeds the predetermined threshold.   When the specific color mode is selected and the basis weight of the recording medium to be passed is equal to or larger than a predetermined value, the surface friction coefficient of the intermediate transfer member is equal to or lower than a predetermined threshold value. 2. The image according to claim 1, wherein the switching unit is controlled so as to be in the second state when the surface friction coefficient of the intermediate transfer member exceeds the predetermined threshold. Forming equipment.   The image forming apparatus according to claim 1, wherein the predetermined threshold value is varied according to a type of a recording medium to be passed.   The image forming apparatus according to claim 3, wherein the type of the recording medium is a basis weight or a thickness of the recording medium. Driving means for driving the intermediate transfer member;
Torque detection means for detecting drive torque in the drive means;
With
5. The image forming apparatus according to claim 1, wherein the switching unit is controlled by a surface friction coefficient of the intermediate transfer member obtained based on a detection result of the torque detection unit. The drive means is a drive motor,
6. The image forming apparatus according to claim 5, wherein the torque detection unit is a unit that detects a current value flowing through the drive motor. An image area ratio detecting means for directly or indirectly detecting an image area ratio of a toner image formed on the intermediate transfer member when the specific color mode is performed;
5. The image formation according to claim 1, wherein the switching unit is controlled by a surface friction coefficient of the intermediate transfer member obtained based on a detection result of the image area ratio detection unit. apparatus.   When the specific color mode is selected, the first transfer state is maintained until the cumulative number of recording media passed through the apparatus reaches a predetermined value after the intermediate transfer member is in a new state. The image forming apparatus according to claim 1, wherein the switching unit is controlled.   The image forming apparatus according to claim 1, wherein the special color is black.   The image forming apparatus according to claim 1, wherein the intermediate transfer body is an intermediate transfer belt arranged in parallel so that the plurality of image carriers are opposed to each other. A plurality of primary transfer members that respectively contact the plurality of image carriers via the intermediate transfer belt;
11. The contact / separation mechanism according to claim 10, wherein the switching unit is a contact / separation mechanism that moves the primary transfer member facing the other image carrier so as to be contactable / separable with respect to the intermediate transfer belt. Image forming apparatus.

Images