JP2012068556A - Transfer device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain change in a distance of a image carrier belt from an image forming part to a transfer nip.SOLUTION: The transfer device includes: a first tension roller winding the image carrier belt, wound around a drive roller, around itself and applies tension T1 to the image carrier belt; a first tension application part having a proportional coefficient K1 and used for generating the tension T1 to be applied to the image carrier belt by the first tension roller; a roller winding the image carrier belt, wound around the first tension roller, around itself; a second tension roller winding the image carrier belt, wound around the roller mentioned right above, around itself and used for applying tension T2 to the image carrier belt; and a second tension application part having a proportional coefficient K2 smaller than the first proportional coefficient K1 and use for generating the tension T2 to be applied to the image carrier belt by the second tension roller.

Description

  The present invention relates to a transfer device that transfers an image formed on an image carrier belt to a recording material, and an image forming apparatus including the transfer device.

  In Patent Document 1, an endless image carrier belt (transfer belt) is wound around a plurality of rollers and rotated, and an image temporarily formed on the surface of the image carrier belt is recorded on a recording material such as paper. An image forming apparatus that forms an image by transferring the image to the printer is described. More specifically, an image forming unit (image forming station) that forms an image (toner image) by developing the latent image formed on the surface of the latent image carrier (photosensitive drum) is opposed to the image carrier belt. Has been placed. The image forming unit forms an image on the surface of the image carrier belt at a position where the image forming unit and the image carrier belt face each other.

  Further, this image is conveyed to the transfer position as the surface of the image carrier belt rotates. At this transfer position, the image carrier belt and the transfer roller are in contact with each other to form a transfer nip. Then, the recording material is conveyed to the transfer nip in accordance with the timing at which the image on the image carrier belt surface is conveyed to the transfer nip. Thus, the image is superimposed on the recording material passing through the transfer nip, and the image is transferred to the recording material.

  At this time, in order to perform image transfer from the image carrier to the recording material with high transfer efficiency, it is desirable to apply a large pressing force to the recording material passing through the transfer nip. However, when a large pressing force is applied, the recording material may adhere to the image carrier belt. In particular, the image forming apparatus disclosed in Patent Document 1 develops a latent image with a liquid developer in which toner is dispersed in a liquid carrier, so that the recording material is likely to stick to the liquid carrier attached to the image carrier belt.

  In order to solve such a problem, for example, it is conceivable to employ the technique described in Patent Document 2. In the image forming apparatus described in Patent Document 2, an openable / closable gripper is provided as a gripping member on a part of a peripheral surface of a cylindrical pressing roller (corresponding to a transfer roller), and the end of the recording material is gripped by the gripper. Thus, the recording material is prevented from sticking to the image bearing member in contact with the pressing roller. Therefore, by applying the configuration of the pressing roller of Patent Document 2 to the transfer roller of Patent Document 1, it is considered that sticking of the recording material to the image carrier belt can be prevented.

JP 2008-122815 A Special Table 2000-508280

  However, the following problems may occur when combining Patent Documents 1 and 2. In other words, the transfer roller passes the recording material through the transfer nip by rotating while gripping the end of the recording material supplied to the transfer nip with the gripping member. At this time, in order to properly superimpose the image on the image carrier belt on the recording material, it is necessary to convey the image to the transfer nip in accordance with the timing when the recording material passes through the transfer nip as the transfer roller rotates. There is.

  Therefore, it is conceivable to control the timing of forming an image on the image carrier belt in accordance with the rotation of the transfer roller. However, the transfer roller to which the gripping member described in Patent Document 2 is attached is not a cylindrical surface with a smooth peripheral surface. For this reason, the pressing force at the transfer nip fluctuates in synchronization with the rotation cycle of the transfer roller. Such fluctuations affect the distance of the image carrier belt from the image forming unit to the transfer nip. As a result, even when the image formation timing on the image carrier belt is controlled in accordance with the rotation of the transfer roller, the timing of conveying the image on the image carrier belt surface to the transfer nip is shifted, and this image is used as a recording material. There was a possibility of improper transfer.

  Such a problem is common when using a transfer roller whose peripheral surface is not a complete cylindrical surface. For example, when using a transfer roller having a concave portion in which a part of the cylindrical peripheral surface is notched. But it can happen. That is, when the image carrier belt faces the cylindrical peripheral surface portion of the transfer roller and contacts the surface thereof, and when the image carrier belt faces the concave portion of the transfer roller and is separated from the transfer roller surface, Since the pressing force applied to the surface of the image carrier belt is greatly different, the distance of the image carrier belt from the image forming portion to the transfer nip fluctuates at the time of switching, and as a result, the image cannot be properly transferred to the recording material. There was a fear.

  The present invention solves the above problems in a transfer apparatus and an image forming apparatus for transferring an image formed on an image carrier belt by an image forming unit to a recording material at a transfer nip by a transfer roller having a concave portion on the peripheral surface. An object of the present invention is to provide a technique for suppressing fluctuations in the distance of an image carrier belt from an image forming unit to a transfer nip.

In order to achieve the above object, the transfer device according to the present invention has an image carrier belt that carries an image, a drive roller that moves the image carrier belt, and a winding roller that moves the image carrier. The image carrier belt is wound around and has a first tension roller for applying a tension T1 to the image carrier belt and a proportional coefficient K1, and is applied to the image carrier belt by the first tension roller. A first tension applying unit for generating a tension T1, a roller for winding an image carrier belt wound around a first tension roller, an image carrier belt wound around the roller, and an image A second tension roller that applies tension T2 to the carrier belt and a tension coefficient K2 that is smaller than the first proportional constant K1 and that is applied to the image carrier belt by the second tension roller. A second tension applying section for generating 2, via the image carrier belt which has a recess in the peripheral surface is characterized by comprising a roller and the transfer roller abutting. Here, K1 and K2 are
K1 = T1 / Δl1
Δl1: From the position of the first tension roller when no tension is applied to the first tension roller from the image carrier belt, when the tension is applied from the image carrier belt, the first tension roller is Displacement amount in the direction of the force acting on the first tension roller due to the tension applied from the image carrier belt to the moved position T1: The first tension roller when the first tension roller moves Δl1 Tension applied to the image carrier belt by K2 = T2 / Δl2
Δl2: From the position of the second tension roller when no tension is applied from the image carrier belt to the second tension roller, when the tension is applied from the image carrier belt, the second tension roller is Displacement amount in the direction of the force acting on the second tension roller due to the tension applied from the image carrier belt to the moved position T2: The second tension roller when the second tension roller moves Δl 2 Is the tension applied to the image carrier belt.

In order to achieve the above object, an image forming apparatus according to the present invention has an image forming unit for forming an image, an image carrier belt to which an image formed by the image forming unit is transferred, and an image transferred. A first roller that wraps the image carrier belt, wraps the drive roller that moves the image carrier, and the image carrier belt that is wound around the drive roller, and applies a tension T1 to the image carrier belt. A tension roller, a first tension applying unit having a proportionality coefficient K1 and generating a tension T1 to be applied to the image carrier belt by the first tension roller, and an image wound around the first tension roller. More than the first proportionality constant K1, a roller for winding the carrier belt, a second tension roller for winding the image carrier belt wound around the roller, and applying a tension T2 to the image carrier belt. Small proportion A second tension applying portion having a number K2 and generating a tension T2 applied to the image carrier belt by the second tension roller; a recess on the peripheral surface; and a roller contact with the roller via the image carrier belt. The image forming apparatus includes: a transfer roller that is in contact with the image forming unit; and a control unit that controls the timing at which the image forming unit forms an image according to the rotation of the transfer roller. Here, K1 and K2 are
K1 = T1 / Δl1
Δl1: From the position of the first tension roller when no tension is applied to the first tension roller from the image carrier belt, when the tension is applied from the image carrier belt, the first tension roller is Displacement amount in the direction of the force acting on the second tension roller due to the tension applied from the image carrier belt to the moved position T1: The first tension roller when the first tension roller moves Δl1 Tension applied to the image carrier belt by K2 = T2 / Δl2
Δl2: From the position of the second tension roller when no tension is applied from the image carrier belt to the second tension roller, when the tension is applied from the image carrier belt, the second tension roller is Displacement amount in the direction of the force acting on the second tension roller due to the tension applied from the image carrier belt to the moved position T2: The second tension roller when the second tension roller moves Δl 2 Is the tension applied to the image carrier belt.

  In the invention thus configured (transfer device, image forming device), the image carrier belt and the transfer roller come into contact with each other to form a transfer nip. Then, the image carried on the image carrier belt is conveyed to the transfer nip via the winding position of the driving roller. Further, between the driving roller and the transfer nip, the first tension roller contacts the image carrier belt, and the second tension roller is positioned at a position sandwiching the first tension roller and the transfer nip. Contact the carrier belt. In addition, first and second tension applying portions that apply tensions T1 and T2 to the image carrier belt via the first and second tension rollers are provided. Specifically, the first tension applying unit applies to the image carrier belt a tension T1 obtained by multiplying the displacement amount Δl1 of the first stretching roller by the proportional coefficient K1, and the second tension applying unit includes the first tension applying unit. A tension T2 obtained by multiplying the displacement amount Δl2 of the tension roller 2 by a proportional coefficient K2 is applied to the image carrier belt. By applying tensions T1 and T2 to the image carrier belt in this way, the image carrier belt is bent to stabilize the distance from the image forming portion to the transfer nip.

  However, in this invention, since the transfer roller has a concave portion on the peripheral surface, the pressing force applied to the image carrier belt is different between when the concave portion faces the image carrier belt and when it is not. Fluctuates. For this reason, the force acting on the first stretching roller from the image carrier belt varies, and the first stretching roller is greatly displaced, and as a result, the distance from the image forming unit to the transfer nip may vary greatly. was there. That is, by applying tensions T1 and T2 to the image carrier belt with the first and second tension rollers, the distance from the image forming unit to the transfer nip is stabilized, but the circumferential surface As a result of using a transfer roller having a recess, the distance from the image forming section to the transfer nip may not be stable as a result.

  On the other hand, in the present invention, the proportionality coefficient K2 for the displacement amount Δl2 of the second stretching roller is set smaller than the proportionality coefficient K1 for the displacement amount Δl1 of the first stretching roller. The reason for this will be described as follows. When the pressing force applied to the image carrier belt by the transfer roller fluctuates, each of the first tension roller and the second tension roller is displaced to maintain the tension applied to the image carrier belt. At this time, if the proportional coefficient K2 with respect to the displacement amount Δl2 of the second stretching roller is set smaller than the proportional coefficient K1 with respect to the displacement amount Δl1 of the first stretching roller, the fluctuation of the pressing force of the transfer roller can be reduced. Thus, while the second tension roller is largely displaced, the displacement of the first tension roller is kept small. Thus, according to the present invention, it is possible to stabilize the distance from the image forming portion to the transfer nip while suppressing the displacement of the first tension roller regardless of the fluctuation of the pressing force of the transfer roller. .

Various specific configurations of the first and second tension applying units that apply such tension to the image carrier belt are conceivable. Therefore, for example, the first tension applying unit includes a first elastic member having a spring constant k1 that supports the first tension roller, and the second tension applying unit supports the second tension roller. A second elastic member having a spring constant k2 is set, and the wrapping angle of the image carrier belt on the first tension roller is θ1, and the wrapping angle of the image carrier belt on the second tension roller is When θ2, the following equation is established: K1 = k1 / {2 · sin (θ1 / 2)}
K2 = k2 / {2 · sin (θ2 / 2)}
You may comprise so that it may have this relationship.

Alternatively, the first tension applying unit includes a first lever that rotates while supporting the first stretching roller at one end, and a first spring constant k3 that is supported at the other end different from one end of the first lever. The second tension applying unit is supported by a second lever that rotates while supporting the second tension roller at one end and the other end different from one end of the second lever. A second spring having a spring constant k4 is provided, the winding angle of the image carrier belt around the first tension roller is θ1, and one end of the first lever is the first lever from the rotation center of the first lever. The length of the virtual perpendicular lowered to the virtual action line of the force applied to the tension roller is La1, from the center of rotation of the first lever, the force of the first spring supported by the other end of the first lever Lb1 is the length of the virtual perpendicular to the virtual action line, θ2 is the wrap angle of the image carrier belt around the second tension roller, and the second lever. From the center of rotation, the length of the imaginary perpendicular line where the one end of the second lever is lowered to the virtual action line of the force applied to the second stretching roller is La2, and from the center of rotation of the second lever, the second lever Lb2 is the length of the virtual perpendicular lowered to the virtual action line of the force of the second spring supported at the other end of the following equation: K1 = (Lb1 / La1) ² · k3 / {2 · sin (θ1 / 2)}
K2 = (Lb2 / La2) ² · k4 / {2 · sin (θ2 / 2)}
You may comprise so that it may have this relationship.

  Such a lever configuration has the following advantages. That is, as the first and second stretching rollers are displaced, the first and second stretching rollers may slide with respect to the image carrier belt. At this time, since the slide resistance between the first and second tension rollers and the image carrier belt is large, the responsiveness of the first and second tension rollers to the external force is deteriorated, and the first and second tension rollers are deteriorated. It is considered that the displacement of the tension roller 2 is not stable, and as a result, the distance from the image forming portion to the transfer nip is slightly unstable. On the other hand, in the lever structure as described above, the first and second tension rollers and the image carrier that are generated when the first and second tension rollers slide with respect to the image carrier belt. The slide resistance between the belts can be kept small. As a result, the distance from the image forming unit to the transfer nip can be further stabilized.

  Further, as described above, in this image forming apparatus, the timing at which the image forming unit forms an image is controlled according to the rotation of the transfer roller. Therefore, in order to execute such control, the image forming unit develops the latent image carrier on which the latent image is formed, the exposure unit that exposes the latent image carrier to form the latent image, and the latent image. A developing unit may be included, and the control unit may be configured to control the timing of exposure by the exposure unit in accordance with the rotation of the transfer roller. In such a configuration, the timing at which the image forming unit forms an image can be controlled simply by adjusting the exposure timing by the exposure device, and the control configuration can be simplified.

  At this time, a detection unit that detects the rotational position of the transfer roller may be provided, and the control unit may be configured to control the timing of exposure by the exposure unit in accordance with information detected by the detection unit.

  Further, the developing unit uses a liquid developer containing toner and a liquid carrier, and the transfer roller is configured such that a concave portion is formed on the peripheral surface and a holding member that holds the recording material is provided in the concave portion. be able to. However, such a configuration has the advantage that the recording material is prevented from adhering to the image carrier belt by the liquid carrier, but the pressing force of the image carrier belt varies as the transfer roller rotates. As a result, the distance from the image forming unit to the transfer nip may be unstable as a result. Therefore, for such a configuration, it is extremely preferable to apply the present invention to stabilize the distance from the image forming unit to the transfer nip.

  In addition, the image forming unit includes a second image forming unit that forms a second image having a color different from that of the image formed by the image forming unit, and the control unit performs timing when the second image forming unit forms the second image. It is particularly preferable to apply the present invention to a configuration in which the control is performed according to the rotation of the transfer roller. That is, in such a configuration, it is required to superimpose images of different colors on each other while suppressing registration deviation. For this purpose, it is preferable to keep the belt tension in the section from the transfer nip to the image forming portion constant in the moving direction of the image carrier belt. In contrast, in the present invention, the second tension roller abuts against the image carrier belt in the corresponding section, and the displacement of the second tension roller is multiplied by a relatively small proportional coefficient K2. A tension T2 is applied. Accordingly, the belt tension in the corresponding section is kept substantially constant, and as a result, a good color image in which registration deviation is suppressed can be formed.

1 is a diagram illustrating a first embodiment of an image forming apparatus according to the present invention. The block diagram which shows the electric constitution of the apparatus of FIG. The perspective view which shows the whole structure of a secondary transfer roller. The figure for demonstrating arrangement | positioning of a tension roller in more detail. The figure explaining the effect | action of a tension roller and the spring which provides force to this. 2 is a timing chart illustrating an operation example of the image forming apparatus in FIG. 1. FIG. 4 is a diagram illustrating a variation amount of a tension roller displacement and an intermediate transfer belt length. FIG. 6 is a diagram illustrating a relationship between a variation in tension of an intermediate transfer belt and registration deviation. The figure which shows the relationship between a spring constant and a resist deviation. FIG. 5 is a diagram illustrating a second embodiment of an image forming apparatus according to the present invention. The figure explaining the effect | action of a tension roller and the mechanism which provides force to this.

First Embodiment FIG. 1 is a diagram showing a first embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the apparatus of FIG. The image forming apparatus 1 includes four image forming stations 2Y (for yellow), 2M (for magenta), 2C (for cyan), and 2K (for black) that form images of different colors. The image forming apparatus 1 includes a color mode in which four color toners of yellow (Y), magenta (M), cyan (C), and black (K) are overlapped to form a color image, and black (K). A monochrome mode in which a monochrome image is formed using only toner can be selectively executed. In this image forming apparatus, when an image forming instruction is given from an external device such as a host computer to a controller 10 having a CPU, a memory, etc., the controller 10 controls each part of the apparatus to execute a predetermined image forming operation, and copy An image corresponding to the image formation command is formed on a sheet-like recording material RM such as paper, transfer paper, paper, and an OHP transparent sheet.

  Each of the image forming stations 2Y, 2M, 2C, and 2K has the same structure and function except for the toner color. Therefore, in FIG. 1, in order to make the drawing easier to see, reference numerals are given only to the components constituting the image forming station 2 </ b> C, and description of reference numerals to be attached to the other image forming stations 2 </ b> Y, 2 </ b> M, and 2 </ b> K is omitted. In the following description, the structure and operation of the image forming station 2C will be described with reference to the reference numerals in FIG. 1, but the structure and operation of the other image forming stations 2Y, 2M, and 2K also differ in toner color. It is the same except that.

  The image forming station 2C is provided with a photosensitive drum 21 on which a cyan toner image is formed. The photosensitive drum 21 is arranged such that its rotation axis is parallel or substantially parallel to the main scanning direction (direction perpendicular to the paper surface of FIG. 1), and at a predetermined speed in the direction of arrow D21 in FIG. Driven by rotation.

  Around the photosensitive drum 21, a charger 22 that is a corona charger that charges the surface of the photosensitive drum 21 to a predetermined potential, and an electrostatic latent image is formed by exposing the surface of the photosensitive drum 21 according to an image signal. An exposure unit 23 for forming the electrostatic latent image, a developing unit 24 for developing the electrostatic latent image as a toner image, a first squeeze unit 25, a second squeeze unit 26, and an intermediate transfer of the toner image to the transfer unit 3. A primary transfer unit for primary transfer to the belt 31, a cleaning unit for cleaning the surface of the photosensitive drum 21 after transfer, and a cleaner blade are arranged in the order of rotation D21 of the photosensitive drum 21 (in FIG. Around).

  The charger 22 does not come into contact with the surface of the photosensitive drum 21, and a conventionally well-known and commonly used corona charger can be used as the charger 22. When a scorotron charger is used as the corona charger, a negative wire current flows through the charge wire of the scorotron charger and a direct current (DC) grid charging bias is applied to the grid. When the photosensitive drum 21 is charged by corona discharge by the charger 22, the surface potential of the photosensitive drum 21 is set to a substantially uniform potential.

  The exposure unit 23 exposes the surface of the photosensitive drum 21 with a light beam in accordance with an image signal given from an external device to form an electrostatic latent image corresponding to the image signal. The exposure unit 23 can be constituted by a light beam from a semiconductor laser that is scanned in the main scanning direction by a polygon mirror, or a line head in which light emitting elements are arranged in the main scanning direction.

  Toner is applied from the developing unit 24 to the electrostatic latent image thus formed, and the electrostatic latent image is developed with the toner. In the developing unit 24 of the image forming apparatus 1, toner development is performed using a liquid developer in which toner is dispersed in a carrier liquid in an approximate weight ratio of about 20%. In this embodiment, a low-concentration (1-2 wt%) and low-viscosity volatile liquid that is generally used in the past and that uses Isopar (trademark: exon) as a carrier liquid (liquid carrier). Solid particles with an average particle size of 1 μm in which a colorant such as a pigment is dispersed in a non-volatile resin at a normal temperature and a high concentration and viscosity, not a developer, an organic solvent, silicon oil, mineral oil or edible oil, etc. A liquid developer having a high viscosity (about 30 to 10000 mPa · s) having a toner solid content concentration of about 20% is used.

  The first squeeze unit 25 and the second squeeze unit 26 are provided with squeeze rollers, respectively. Each squeeze roller comes into contact with the surface of the photosensitive drum 21 to remove excess carrier liquid and fog toner from the toner image. In the present embodiment, the excess carrier liquid and fog toner are removed by the two squeeze portions 25 and 26. However, the number and arrangement of the squeeze portions are not limited to this, for example, one squeeze portion. May be arranged.

  The toner image that has passed through the squeeze portions 25 and 26 is primarily transferred to the intermediate transfer belt 31 by the primary transfer unit. The intermediate transfer belt 31 is an endless image carrier belt that can temporarily carry a toner image on its surface, more specifically, on its outer peripheral surface, and is stretched around a plurality of rollers 32, 33, 34 and 35. ing. Of these, the roller 32 is mechanically connected to the belt drive motor M3, and functions as a belt drive roller for driving the intermediate transfer belt 31 in the direction of the arrow D31 in FIG. As shown in FIG. 2, in this embodiment, a driver 11 is provided to drive the belt drive motor M3. The driver 11 sends a drive signal corresponding to a command pulse given from the controller 10 to the belt drive motor M3. Output. As a result, the belt driving roller 32 rotates at a peripheral speed corresponding to the command pulse, and the surface of the intermediate transfer belt 31 rotates in the direction D31 at a constant speed. Reference numeral E3 in the figure is an encoder attached to the belt drive motor M3, and a signal corresponding to the rotation of the belt drive motor M3 is given to the driver 11, and the driver 11 receiving the signal drives the belt based on the signal. The motor M3 is feedback-controlled.

  As will be described in detail later, among the rollers 32 to 35 around which the intermediate transfer belt 31 is stretched, only the belt driving roller 32 is driven by the motor, and the other rollers 33, 34 and 35 are driving sources. The driven roller does not have

  The primary transfer unit has a primary transfer backup roller 271, and the primary transfer backup roller 271 is disposed to face the photosensitive drum 21 with the intermediate transfer belt 31 interposed therebetween. At the primary transfer position TR1 where the photosensitive drum 21 and the intermediate transfer belt 31 abut, the toner image on the photosensitive drum 21 is transferred to the outer peripheral surface of the intermediate transfer belt 31 (the lower surface at the primary transfer position TR1). Thus, the cyan toner image formed by the image forming station 2 </ b> C is transferred to the intermediate transfer belt 31. Similarly, the toner images are transferred at the other image forming stations 2Y, 2M, and 2K, so that the toner images of the respective colors are sequentially superimposed on the intermediate transfer belt 31 to form a full-color toner image. On the other hand, when a monochrome toner image is formed, the toner image is transferred to the intermediate transfer belt 31 only in the image forming station 2K corresponding to the black color.

  The toner image thus transferred to the intermediate transfer belt 31 is conveyed to the secondary transfer position TR2. At the secondary transfer position TR2, the secondary transfer roller 4 is disposed opposite to the roller 34 around which the intermediate transfer belt 31 is wound, with the intermediate transfer belt 31 interposed therebetween, and the surface of the intermediate transfer belt 31 and the transfer roller 4 are disposed. The surface is in contact with each other to form a transfer nip. That is, the roller 34 functions as a secondary transfer backup roller.

  At the secondary transfer position TR2, the single-color or multi-color toner images formed on the intermediate transfer belt 31 are transferred from the pair of gate rollers 51 to the recording material RM conveyed along the conveyance path PT. In this embodiment, since the toner image is formed by a wet development method in which a toner image is formed using a liquid developer, in order to obtain good transfer characteristics, the intermediate transfer belt 31 is used in the transfer nip. It is desired that the recording material RM is pressed with a high pressing force. Further, since the liquid developer is interposed, there is a high possibility that the recording material RM sticks to the intermediate transfer belt 31 and is jammed. Therefore, in this embodiment, as will be described in detail later, the secondary transfer roller 4 having a grip portion is used.

  The recording material RM on which the toner image has been secondarily transferred is sent from the secondary transfer roller 4 to a fixing unit (not shown) by a conveyance roller 7 provided on the conveyance path PT. In the fixing unit, heat or pressure is applied to the toner image transferred to the recording material RM to fix the toner image on the recording material RM.

  FIG. 3 is a perspective view showing the overall configuration of the secondary transfer roller. As shown in FIGS. 1 and 3, the secondary transfer roller 4 has a roller base 42 provided with a recess 41 formed by cutting out a part of the outer peripheral surface of a cylinder. In this roller base material 42, a rotation shaft 421 that is rotatable in the direction D4 about the rotation axis A4 is disposed in parallel or substantially in parallel with the rotation axis of the secondary transfer backup roller 34, and at both ends of the rotation shaft 421. Side plates 422 and 422 are respectively attached. More specifically, each of these side plates 422 and 422 has a shape in which a notch 422a is provided on a disk-shaped metal plate. As shown in FIG. 3, the notches 422 a and 422 a are attached to the rotating shaft 421 while facing each other and separated by a distance slightly longer than the width of the intermediate transfer belt 31. In this way, the roller base material 42 is formed, which has a drum shape as a whole but has a concave portion 41 extending in parallel or substantially parallel to the rotating shaft 421 on a part of the outer peripheral surface thereof.

  Further, an elastic layer 43 such as rubber or resin is formed on the outer peripheral surface of the roller base material 42, that is, on the surface region of the metal plate surface excluding the region corresponding to the inside of the recess 41. The elastic layer 43 forms a transfer nip NP so as to face the intermediate transfer belt 31 wound around the drive roller 32.

  In addition, a grip 44 for gripping the recording material RM is disposed inside the recess 41. The gripping portion 44 includes a gripper support member 441 erected on the outer peripheral surface of the roller base 42 from the inner bottom portion of the recess 41, and a gripper member 442 that is supported so as to be able to come into contact with and separate from the distal end portion of the gripper support member 441. And have. The gripper member 442 is connected to a gripper driving unit (not shown). In response to the ungrip command from the controller 10, the gripper driving unit is actuated so that the leading end of the gripper member 442 is separated from the leading end of the gripper support member 441, and the recording material RM is gripped and released. On the other hand, when the gripper driving unit is operated in response to a grip command from the controller 10, the leading end of the gripper member 442 moves to the leading end of the gripper support member 441 and grips the recording material RM. In addition, about the structure of the holding part 44, it is not limited to this embodiment, For example, the well-known holding | grip mechanism described in patent document 2 etc. is employable.

  At both ends of the secondary transfer roller 4, support members 46 are attached to the outer surfaces of the side plates 422 so that they can rotate integrally with the roller base material 42. The support member 46 is formed with a planar area corresponding to the recess 41. The transfer roller side contact member 47 is attached to the planar area. In the contact member 47, the base portion 471 is attached to the support member 46, and the contact portion 472 extends from the base portion 471 in the normal direction of the planar region 461, and the distal end portion of the contact portion 472 Extends to the vicinity of the opening side end of the recess 41. That is, when the roller base material 42 is viewed from the end of the rotating shaft 421, the contact member 47 is disposed so as to close the recess 41. Therefore, when the concave portion 41 reaches a position facing the intermediate transfer belt 31 due to the rotation of the secondary transfer roller 4, the contact member 47 contacts the end surface of the secondary transfer backup roller 34. Thereby, the fluctuation | variation of the load torque seen from the motor which rotationally drives the secondary transfer roller 4 can be reduced.

In this embodiment, the opening length (opening width) W41 of the recess 41 along the rotation direction D4 of the roller base material 42 is about 105 mm. When the elastic layer 43 formed in the region excluding the concave portion 41 on the outer peripheral surface of the secondary transfer roller 4 is in a position facing the intermediate transfer belt 31, the elastic layer 43 is pressed against the intermediate transfer belt 31 and the transfer nip NP. Is formed. The length (transfer nip width) Wnp of the transfer nip NP along the rotation direction D4 of the roller base material 42 is about 11 mm,
(Opening width W41 of the recess 41)> (transfer nip width Wnp at the transfer nip NP)
Have the relationship. Therefore, when the concave portion 41 of the secondary transfer roller 4 faces the intermediate transfer belt 31, the transfer nip temporarily disappears.

  The length of the elastic layer 43 along the rotation direction D4 of the roller base material 42 is set to about 495 mm. This is the largest size of the recording material RM that can be used in the apparatus 1 is wound. It is something that can be done. That is, the length of the elastic layer 43 is determined to be longer than the length of the usable recording material having the maximum length along the rotation direction D4 of the roller base material 42.

  A transfer roller drive motor M4 is mechanically connected to the rotation shaft 421 of the secondary transfer roller 4. In the present embodiment, a driver 12 is provided to drive the transfer roller drive motor M4. The driver 12 drives the motor M4 in accordance with a command given from the controller 10 to rotate the secondary transfer roller 4 in the clockwise direction D4 in FIG. 1, that is, in the width direction with respect to the belt moving direction D31. The secondary transfer backup roller 34 is a driven roller having no drive source. By using the secondary transfer backup roller 34 facing the secondary transfer roller 4 driven by the motor as a driven roller, between the secondary transfer roller 4 and the intermediate transfer belt 31 in the transfer nip NP, or with the intermediate transfer belt 31. Slip with the secondary transfer backup roller 34 can be prevented.

  In the present embodiment, an AC servo motor is used as the motor M4, and the position of the AC servo motor can be controlled by the driver 12 or the torque can be controlled. That is, the driver 12 has a position control circuit and a torque control circuit, and can selectively execute position control and torque control. The controller 10 can input a command pulse related to position information, a command torque related to torque information, and a control switching signal to the driver 12.

  2 is an encoder attached to the transfer roller drive motor M4, and a signal corresponding to the rotation of the transfer roller drive motor M4 is given to the driver 12, and the driver 12 that has received the signal is based on the above signal. The motor M4 is feedback-controlled. The sensor 8 is an optical sensor that detects a home position mark 81 (FIG. 1) connected to one end of the rotating shaft 421 of the secondary transfer roller 4. That is, the sensor 8 detects the home position mark 81 that rotates with the secondary transfer roller 4 and outputs a home position signal to the controller 10. The controller 10 can detect the home position signal and grasp the home position and rotation phase of the secondary transfer roller 4.

  Among the rollers around which the intermediate transfer belt 31 is stretched, the rollers 33 and 35 provided at positions sandwiching the transfer nip NP are elastically supported by springs 331 and 351, respectively, of the rotation shafts of the intermediate transfer belt 31. It is a tension roller that adjusts the tension. More specifically, the rotation shaft of the tension roller 33 is elastically supported by a spring 331 (second elastic member) that can expand and contract in a substantially horizontal direction, whereby the tension roller 33 causes the intermediate transfer belt 31 to move. A predetermined amount can be moved in a substantially horizontal direction in the wound state.

  In addition, the rotation axis of the tension roller 35 is a spring 351 that can expand and contract in a direction substantially orthogonal to a virtual plane that is in contact with both the outer peripheral surface of the drive roller 32 and the outer peripheral surface of the secondary transfer backup roller 34. Thus, the tension roller 35 is movable by a predetermined amount in the same direction while the intermediate transfer belt 31 is wound around. In a steady state, the tension roller 35 is urged by a spring 351 so as to push the intermediate transfer belt 31 stretched between the belt driving roller 32 and the secondary transfer backup roller 34 outward. Yes.

  At the secondary transfer position TR2, when the surface of the secondary transfer roller 4 facing the intermediate transfer belt 31 is switched from the elastic layer 43 to the recess 41 with the rotation of the secondary transfer roller 4, the recess is reversed. When switching from 41 to the elastic layer 43, the load torque of the belt drive motor M3 that drives the belt drive roller 32 varies greatly. The fluctuation is particularly large when a high pressing force is applied between the elastic layer 43 and the intermediate transfer belt 31. Due to this, it is conceivable that the speed fluctuation and vibration of the intermediate transfer belt 31 occur, and the tension of the belt 31 changes temporarily.

  However, in this embodiment, the pair of tension rollers 33 and 35 provided so as to sandwich the secondary transfer position TR2 along the stretching direction of the intermediate transfer belt 31 moves the rotation shaft, thereby changing the tension. Acts to counteract For this reason, the speed fluctuation and vibration of the intermediate transfer belt 31 at the secondary transfer position TR2 are prevented from reaching the primary transfer position TR1 corresponding to each image forming station 2Y, 2M, 2C and 2K. The speed fluctuation or vibration of the intermediate transfer belt 31 at the primary transfer position TR1 disturbs the toner image transferred from the image forming station and deteriorates the image quality. It is prevented beforehand.

  Both of the tension rollers 33 and 35 are in contact with the intermediate transfer belt 31 from the inner side of the intermediate transfer belt 31, that is, from the back side opposite to the surface that is the image carrying surface of the intermediate transfer belt 31. The reason is as follows. First, the contact with the opposite side of the image carrying surface causes the tension rollers 33 and 35 to disturb the toner image carried on the intermediate transfer belt 31, or conversely, it is soiled by toner remaining on the intermediate transfer belt 31. There is nothing to do. In order to increase the tension adjustment effect by the tension roller, it is effective to increase the winding angle of the intermediate transfer belt 31, but the tension roller is brought into contact with the image carrying surface and the winding angle is increased. If it is going to be, it is necessary to give the surface of the intermediate transfer belt 31 a large negative curvature, there is a concern about the influence on the toner image, and there is a structural problem. For these reasons, the tension rollers 33 and 35 are in contact with the back surface of the intermediate transfer belt 31.

  In this embodiment, the belt driving roller 32 and the tension roller are arranged so that the surface of the intermediate transfer belt 31 is in a substantially horizontal posture from the downstream of the tension roller 33 to the upstream of the belt driving roller 32 in the belt moving direction D31. 33 is arranged. The primary transfer position TR1 for each image forming station is arranged on the same plane formed by the surface of the intermediate transfer belt 31 (more specifically, the lower surface to which the toner image is transferred). Furthermore, the moving direction of the rotation shaft of the tension roller 33 is also set to be substantially horizontal. For this reason, even if the tension roller 33 moves in order to absorb speed fluctuations and vibrations, the surface of the intermediate transfer belt 31 is kept horizontal, and the influence on image formation can be minimized. Although it is not essential that the surface of the intermediate transfer belt 31 is horizontal in the vicinity of the image forming station, it is desirable that at least the moving direction of the surface of the belt 31 and the moving direction of the tension roller 33 are the same or substantially the same.

  As for the tension roller 35, the belt driving roller 32 is interposed between the tension transfer roller 35 and the primary transfer position TR1, so that the moving direction is not restricted in image formation. Therefore, the intermediate transfer belt 31 is most subject to speed fluctuations and vibrations by moving in a direction substantially perpendicular to a virtual plane that contacts both the outer peripheral surface of the drive roller 32 and the outer peripheral surface of the secondary transfer backup roller 34. It can be effectively reduced.

  In addition, a carrier application roller 381 that contacts the intermediate transfer belt 31 is provided so as to sandwich the intermediate transfer belt 31 between the tension roller 33 and the tension roller 33. Further, a liquid carrier storage unit 382 that supplies the liquid carrier to the carrier application roller 381 is disposed vertically above the carrier application roller 381. The carrier application roller 381 applies the liquid carrier supplied from the liquid carrier storage unit 382 to the intermediate transfer belt 31. The carrier application roller 381 and the liquid carrier reservoir 382 are supported so as to be movable integrally with the tension roller 33.

  In addition, a cleaner unit 39 is provided in the vicinity of the intermediate transfer belt 31 wound around the tension roller 33 so as to be able to come into contact with and separate from the surface of the intermediate transfer belt 31. The cleaner unit 39 cleans the intermediate transfer belt 31 by scraping off the toner remaining on the surface of the intermediate transfer belt 31. The cleaner unit 39 is supported integrally with the rotation shaft of the tension roller 33 supported by the spring 331, and is displaced according to the displacement of the tension roller 33. For this reason, the relative position between the cleaner unit 39 and the intermediate transfer belt 31 does not change.

  FIG. 4 is a diagram for explaining the arrangement of the tension roller in more detail. In this embodiment, a belt driving roller 32 is provided outside the path from the upstream tension roller 35 to the downstream tension roller 33 via the secondary transfer position TR2 in the moving direction D31 of the intermediate transfer belt 31. Yes. Thus, the driving force applied from the transfer roller driving motor M4 to the intermediate transfer belt 31 via the secondary transfer roller 4, and the driving force applied from the belt driving motor M3 to the intermediate transfer belt 31 via the belt driving roller 32. Can be separated by a tension roller to prevent mutual interference between the two.

Here, along the moving direction D31 of the intermediate transfer belt 31, from the secondary transfer position TR2, the primary transfer corresponding to the image forming station 2K located on the most downstream side in the moving direction D31 of the intermediate transfer belt 31 among the image forming stations. The circumferential length of the intermediate transfer belt 31 up to the position TR1k is represented by the symbol L1. Also, the circumference of the intermediate transfer belt 31 from the secondary transfer position TR2 to the primary transfer position TR1y corresponding to the image forming station 2Y that is the most upstream in the moving direction D31 of the intermediate transfer belt 31 in the image forming station Represented by L2. At this time,
L1 <L2 (Formula 1)
The arrangement of the rollers 32 to 35 is determined so that the relationship is established.

  According to such an arrangement, the following operational effects can be obtained. First, between the secondary transfer position TR2 and the primary transfer position TR1k, as an element capable of absorbing the speed fluctuation and vibration of the intermediate transfer belt 31 generated at the secondary transfer position TR2, a tension roller 35 and a belt drive roller 32 is provided. Among these, the tension roller 35 absorbs fluctuations by changing its position. Further, the belt driving roller 32 suppresses the fluctuation by the driving torque of the belt driving motor M3. As described above, since the propagation of the fluctuation from the secondary transfer position TR2 to the primary transfer position TR1k is prevented in two stages of the tension roller 35 and the belt driving roller 32, the peripheral length L1 of the intermediate transfer belt 31 in this portion. Even if the length is short, the influence on the primary transfer position TR1k is almost certainly suppressed.

  On the other hand, only the tension roller 33 is interposed between the secondary transfer position TR2 and the primary transfer position TR1y. Therefore, by increasing the peripheral length L2 of the intermediate transfer belt 31 in this portion, it is possible to further reduce the propagation of fluctuation from the secondary transfer position TR2 to the primary transfer position TR1y due to the plasticity and elasticity of the belt itself. It becomes.

  In such a configuration, the image formed on the surface of the intermediate transfer belt 31 by the image forming stations 2Y, 2M, 2C, and 2K at the primary transfer position TR1 is superimposed on the recording material RM at the transfer nip NP at the secondary transfer position TR2. Match. Therefore, it is required to convey the image from the image forming stations 2Y, 2M, 2C, and 2K to the transfer nip NP in accordance with the timing when the recording material RM passes through the transfer nip NP. Therefore, it is important to stabilize the distance (for example, the distance L1) from the image forming stations 2Y, 2M, 2C, and 2K to the transfer nip NP. In other words, it is preferable to suppress the displacement of the tension roller 35 as small as possible. Become. Therefore, in this embodiment, each mechanism for applying a tension force to the tension rollers 33 and 35 is configured as follows.

FIG. 5 is a view for explaining the action of the tension roller and the spring for applying a force to the tension roller. The force applied to the intermediate transfer belt 31 by the tension roller 35 is
F1 = k1 · x1 (Formula 2)
It is represented by Here, k1 is a spring constant of the spring 351. X1 is determined from the position of the tension roller 35 (dashed line in FIG. 5) when the intermediate transfer belt 31 is not wound and the force from the intermediate transfer roller 31 is not applied (free state). The amount of displacement of the tension roller 35 up to the position of the tension roller 35 (the solid line in FIG. 5) when the intermediate transfer belt 31 is wound is obtained in the direction of the force F1 acting on the tension roller 35 by the deformation of the spring 351. (Displacement amount of tension roller). In other words, the displacement amount x1 in the present invention is “the tension is applied from the image carrier belt from the position of the first tension roller when the first tension roller is not tensioned from the image carrier belt. Corresponds to a displacement amount Δl1 in the direction of the force acting on the first tension roller by the tension applied from the image carrier belt to the position where the first tension roller has moved. Here, assuming that the winding angle of the intermediate transfer belt 31 around the tension roller 35 is θ1, the force F1 and the tension T1 acting on the upstream side and the downstream side of the winding position along the surface of the intermediate transfer belt 31, respectively. Therefore, from the relationship shown in FIG.
2T1 · sin (θ1) = F1 (Formula 3)
Is established. Therefore, from (Expression 2) and (Expression 3),
T1 = (1/2) .k1.x1 / {sin (.theta.1 / 2)} (Formula 4)
Is guided. In other words, the spring 351 is proportional to the displacement amount x1 (= Δl1) of the tension roller 35 expressed by the following equation.
K1 = k1 / {2 · sin (θ1 / 2)} (Formula 5)
Is applied to the intermediate transfer belt 31.

Similarly, when the spring constant of the spring 331 that supports the tension roller 33 is k2, the displacement amount of the tension roller 33 is x2 (= Δl2), and the winding angle of the intermediate transfer belt 31 around the tension roller 33 is θ2, the tension roller The tension T2 of the intermediate transfer belt 31 due to the displacement of 33 is given by the following equation:
T 2 = (1/2) · k 2 · x 2 / {sin (θ 2/2)} (Formula 6)
It is represented by In other words, the spring 331 is proportional to the displacement amount x2 (= Δl2) of the tension roller 33, which is expressed by the following equation.
K2 = k2 / {2 · sin (θ2 / 2)} (Expression 7)
Is applied to the intermediate transfer belt 31.

The condition for the change in tension of the intermediate transfer belt 31 by the two tension rollers to cancel each other is T1 = T2. Further, since the displacement of the tension roller 35 greatly affects the secondary transfer at the secondary transfer position TR2, in order to reduce the influence, the displacement x1 of the tension roller 35 is smaller than the displacement x2 of the tension roller 33. Is desirable. Therefore, in this embodiment, the following formula:
k1 / sin (θ1 / 2)> k2 / sin (θ2 / 2) (Equation 8)
The relationship is satisfied.

When these representative numerical examples are shown,
θ1: 42.7 degrees,
θ2: 168.9 degrees
k1: 1.0 N / mm,
k2: 1.0 N / mm,
It is. At this time,
k1 / sin (θ1 / 2) ≈2.7 N / mm,
k2 / sin (θ2 / 2) ≈1.0 N / mm,
And the relationship of (Equation 8) is satisfied.

  Next, the operation of the image forming apparatus 1 configured as described above will be described with reference to FIG. FIG. 6 is a timing chart showing an operation example of the image forming apparatus of FIG. When the sensor 8 detects the home position of the secondary transfer roller 4, it outputs a home position detection signal to the controller 10. Then, the controller 10 causes the yellow (Y) exposure unit 23 to start exposure of the photosensitive drum 21 when the time Ta has elapsed from the home position detection signal. The latent image formed by this exposure is developed by the yellow (Y) developing unit 24 to form a yellow (Y) image. The yellow (Y) image is primarily transferred to the intermediate transfer belt 31 at the primary transfer position TR1 and conveyed toward the secondary transfer position TR2.

  Further, the controller 10 causes the exposure unit 23 of magenta (M) to start the exposure of the photosensitive drum 21 when the time Tb (> Ta) has passed from the home position detection signal. The latent image formed by this exposure is developed by the magenta (M) developing unit 24 to form a magenta (M) image. The magenta (M) image is primarily transferred to the intermediate transfer belt 31 at the primary transfer position TR1. In addition, this primary transfer is executed in such a manner that the magenta (M) image is superimposed on the yellow (Y) image passing through the primary transfer position TR1 as the intermediate transfer belt 31 moves. The yellow (Y) and magenta (M) images superimposed on each other in this way are conveyed toward the secondary transfer position TR2.

  Further, under the control of the controller 10, the same operation is performed for cyan (C) and black (K), and the images of four colors (Y), (M), (C), and (K) are superimposed on each other. A color image is formed on the surface of the intermediate transfer belt 31 and conveyed to the secondary transfer position TR2. Further, the controller 10 starts driving the gate roller 51 at the time when the time Te has elapsed from the home position detection signal, and starts conveying the recording material RM to the secondary transfer position TR2. Then, the recording material RM starts to pass the secondary transfer position TR2 from the time when the time ΔTe has passed from the start of conveyance of the recording material RM. At the same time as the leading edge of the recording material RM arrives at the secondary transfer position TR2, the leading edge of the color image on the surface of the intermediate transfer belt 31 also arrives at the secondary transfer position TR2. Then, the secondary transfer bias is applied to the secondary transfer position TR2 at the timing when the recording material RM and the color image begin to pass through the secondary transfer position TR2. Thus, the color image is superimposed on the recording material RM.

  Thus, in this embodiment, the exposure start timing by the exposure unit 23 for each color (Y), (M), (C), (K), the conveyance start timing of the recording material RM by the gate roller 51, and the secondary The timing for starting the application of the transfer bias is controlled based on the home position of the secondary transfer roller 4.

  As described above, in this embodiment, the intermediate transfer belt 31 and the secondary transfer roller 4 come into contact with each other to form the transfer nip NP. Then, the color image formed on the intermediate transfer belt 31 by the image forming stations 2Y, 2M, 2K, and 2C is conveyed to the transfer nip NP via the winding position of the drive roller 32. Further, between the driving roller 32 and the transfer nip NP, the tension roller 35 contacts the intermediate transfer belt 31 and the tension roller 33 contacts the intermediate transfer belt 31 at a position between the tension roller 35 and the transfer nip NP. . Then, springs 331 and 351 for applying tensions T1 and T2 to the intermediate transfer belt 31 via the tension rollers 33 and 35, respectively, are provided. Specifically, the spring 351 applies a tension T 1 obtained by multiplying the displacement of the tension roller 35 by a proportional coefficient K 1 to the intermediate transfer belt 31, and the spring 331 applies a proportional coefficient K 2 to the displacement of the tension roller 33. The multiplied tension T2 is applied to the intermediate transfer belt 31. By applying tensions T1 and T2 to the intermediate transfer belt 31 in this way, the intermediate transfer belt 31 is deflected and the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP (the length of the intermediate transfer belt 31). A) is stabilized.

  However, since the secondary transfer roller 4 has a concave portion 41 on the peripheral surface, the pressing force applied to the intermediate transfer belt 31 is different between when the concave portion 41 faces the intermediate transfer belt 31 and when it is not. Pressure fluctuates. Therefore, the force acting on the tension roller 35 from the intermediate transfer belt 31 fluctuates, and the tension roller 35 is greatly displaced. As a result, the distance from the image forming stations 2Y, 2M, 2K, 2C to the transfer nip NP varies greatly. There was a risk of this (FIG. 7).

  Here, FIG. 7 is a graph showing the relationship between the displacement amount of the tension roller and the variation amount of the length of the intermediate transfer belt from the image forming station to the transfer nip. In the drawing, the displacement amount of the tension roller 35 is taken on the horizontal axis, and the error in the length of the intermediate transfer belt 31 from the image forming station to the transfer nip NP is taken on the vertical axis. Further, graphs are shown for the case where the spring constant k1 of the spring 351 is 0.5 (N / mm), 1.0 (N / mm), and 1.5 (N / mm), respectively. From the drawing, it can be seen that the distance from the image forming station to the transfer nip NP (the length to the intermediate transfer belt 31) varies as the tension roller 35 is displaced.

  In other words, the tension rollers 33 and 35 apply tensions T1 and T2 to the intermediate transfer belt 31 to stabilize the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP. By using the transfer roller 4 having the recess 41 on the peripheral surface, the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP may be unstable as a result.

  In contrast, in this embodiment, the proportional coefficient K2 for the displacement amount x2 (= Δl2) of the tension roller 33 is set smaller than the proportionality coefficient K1 for the displacement amount x1 (= Δl1) of the tension roller 35. The reason for this will be described as follows. When the pressing force applied to the intermediate transfer belt 31 by the secondary transfer roller 4 varies, the tension roller 35 and the tension roller 33 are displaced to maintain the tension applied to the intermediate transfer belt 31. At this time, if the proportional coefficient K2 with respect to the displacement of the tension roller 33 is set smaller than the proportional coefficient K1 with respect to the displacement of the tension roller 35, the tension roller 33 is larger than the fluctuation of the pressing force of the secondary transfer roller 4. In contrast to the displacement, the displacement of the tension roller 35 is kept small. Thus, in this embodiment, the distance (intermediate) from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP is suppressed while suppressing the displacement of the tension roller 35 regardless of the variation in the pressing force of the secondary transfer roller 4. The length of the transfer belt 31) can be stabilized.

  In the above embodiment, the timing at which the image forming stations 2Y, 2M, 2K, and 2C form an image on the intermediate transfer belt 31 (primary transfer) is controlled according to the rotation of the secondary transfer roller 4. More specifically, the controller 10 adjusts the start of exposure by the exposure unit 23 according to the rotation of the secondary transfer roller 4, whereby the image forming stations 2 </ b> Y, 2 </ b> M, 2 </ b> K, and 2 </ b> C are attached to the intermediate transfer belt 31. The timing for forming an image is controlled. In such a configuration, the timing at which the image forming stations 2Y, 2M, 2K, and 2C form an image on the intermediate transfer belt 31 can be easily controlled simply by adjusting the start of exposure by the exposure unit 23. Simplification of the configuration can be achieved.

  In the above embodiment, a toner image obtained by developing a latent image with a liquid developer having toner and a liquid carrier is formed on the intermediate transfer belt 31 and arranged in the recess 41 on the peripheral surface of the secondary transfer roller 4. The image on the intermediate transfer belt 31 is transferred to the recording material RM gripped by the grip portion 44. Such a configuration has the advantage that the recording material RM is prevented from sticking to the intermediate transfer belt 31 by the liquid carrier, but the pressing force of the intermediate transfer belt 31 is accompanied by the rotation of the secondary transfer roller 4. The distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP may not be stabilized as a result. Therefore, for such a configuration, it is extremely preferable to apply the present invention to stabilize the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP.

  Further, the present invention is particularly suitable for the configuration in which the images formed by the plurality of image forming stations 2Y, 2M, 2K, and 2C are overlapped by the intermediate transfer belt 31 as in the above embodiment. It becomes. That is, in such a configuration, it is required to superimpose images of different colors on each other while suppressing registration deviation. For this purpose, the belt tension in the section from the transfer nip NP to the image forming stations 2Y, 2M, 2K, and 2C in the moving direction D31 of the intermediate transfer belt 31 is preferably kept constant (FIG. 8).

  Here, FIG. 8 is a diagram showing the relationship between the amount of change in tension of the intermediate transfer belt and the registration deviation. In this figure, the registration shift (horizontal axis) with respect to the tension variation (vertical axis) of the intermediate transfer belt 31 in the section from the transfer nip NP to the image forming stations 2Y, 2M, 2K, and 2C in the moving direction D31 of the intermediate transfer belt 31. ) Is shown in a graph. From this graph, it can be seen that when the amount of variation in the tension of the intermediate transfer belt 31 is large, the registration deviation also increases.

  In contrast, in this embodiment, the tension roller 33 abuts against the intermediate transfer belt 31 in the corresponding section, and a relatively small proportional coefficient K2 is applied to the displacement amount x2 (= Δl2) of the tension roller 33. The multiplied tension T2 is applied. Therefore, the belt tension in the corresponding section is kept substantially constant, and as a result, a good color image in which registration deviation is suppressed can be formed (FIG. 9).

  Here, FIG. 9 is a diagram showing the relationship between the spring constant of the spring 331 that supports the tension roller 33 and the registration deviation. In the drawing, the displacement amount of the tension roller 33 is taken on the horizontal axis, and the resist displacement is taken on the vertical axis. Further, graphs are shown for the case where the spring constant k2 of the spring 331 is 0.5 (N / mm), 1.0 (N / mm), and 1.5 (N / mm), respectively. From this figure, it can be seen that even when the displacement amount of the tension roller 33 is the same, the smaller the value of the spring constant k2, the more the resist displacement is suppressed.

  Incidentally, the intermediate transfer belt 31 itself expands and contracts, and as a result, the speed of the intermediate transfer belt 31 may fluctuate at the primary transfer position TR1 where the image forming stations 2Y, 2M, 2K, and 2C form (transfer) the image. is there. This speed fluctuation can cause an image to not be properly formed on the intermediate transfer belt 31. On the other hand, in the above-described embodiment, the carrier application roller 381 for applying the liquid carrier to the intermediate transfer belt 31 is provided between the secondary transfer position TR2 and the primary transfer position TR1 in the belt moving direction D31. Yes. As a result, even when the intermediate transfer belt 31 expands and contracts, the influence of the expansion and contraction of the intermediate transfer belt 31 is alleviated by the viscous resistance of the liquid carrier applied by the carrier application roller 381, and the speed of the intermediate transfer belt 31 is reduced. Variations can be suppressed. As a result, an image can be appropriately formed (transferred) on the intermediate transfer belt 31.

Second Embodiment FIG. 10 is a diagram showing a second embodiment of an image forming apparatus according to the present invention. The difference between the first embodiment and the second embodiment resides in the arrangement of the tension rollers 33 and 35 and the mechanism that applies force to these rollers 33 and 35. Therefore, the difference will be mainly described below. The common points are denoted by the same reference numerals and the description thereof is omitted.

  The tension roller 33 is not disposed at the position where the tension roller 33 is disposed in the first embodiment, and the driven roller 36 is disposed instead. The tension roller 33 is disposed between the roller 36 and the secondary transfer position TR2, and is in contact with the intermediate transfer belt 31. The mechanism for applying a force to the tension roller 33 includes a lever 332 in addition to the spring 331. The lever 332 is configured to be rotatable with respect to a rotation center C33 located between one end and the other end. The tension roller 33 is rotatably supported at one end of the lever 332, and the other end of the lever 332 is supported by a spring 331. Therefore, the force generated by the spring 331 acts on the tension roller 33 with the rotation center C33 of the lever 332 as a fulcrum.

  The arrangement of the tension roller 35 is the same as that of the first embodiment, but the mechanism for applying force to the roller 35 is different from that of the first embodiment. That is, this mechanism includes a lever 352 in addition to the spring 351. The lever 352 is configured to be rotatable with respect to a rotation center C35 between one end and the other end. The tension roller 35 is rotatably supported at one end of the lever 352 and the other end of the lever 352 is supported by a spring 351. Therefore, the force generated by the spring 351 acts on the tension roller 35 with the rotation center C35 of the lever 352 as a fulcrum.

Each of the mechanisms that applies force to the tension rollers 33 and 35 applies tension to the intermediate transfer belt 31 via the tension rollers 33 and 35. This will be described in detail with reference to FIG. FIG. 11 is a diagram for explaining the action of the tension roller and a mechanism for applying a force to the tension roller. here,
T1: tension of the intermediate transfer belt 31 k3: spring constant of the spring 351 x1: tension in a state where the intermediate transfer belt 31 is not wound and a force from the intermediate transfer roller 31 is not applied (free state) The amount of displacement of the tension roller 35 from the position of the roller 35 (broken line in FIG. 11) to the position of the tension roller 35 (solid line in FIG. 11) when the intermediate transfer belt 31 is wound is determined by the deformation of the spring 351. What is obtained in the direction of the force acting on the tension roller 35 (displacement amount of the tension roller), in other words, in the present invention, “the first tension roller is not subjected to tension from the image carrier belt. The image carrier bell from the position of the first tension roller to the position where the first tension roller moves when tension is applied from the image carrier belt By the tension exerted from acting on the first tension roller direction displacement amount Δl1 force "
θ1: The winding angle of the intermediate transfer belt 31 around the tension roller 35. Furthermore, the length of the (virtual) perpendicular line from the rotation center C35 of the lever 352 to the (virtual) action line of the force applied to the tension roller 35 from one end of the lever 352 is La1, and the spring 351 from the rotation center C35 of the lever 352 When the length of the (virtual) perpendicular drawn to the (virtual) action line of the force is Lb1, the tension T1 is expressed by the following equation: T1 = (1/2) · (Lb1 / La1) ² · k3 · x1 / sin (Θ1 / 2) (Formula 9)
It is represented by In other words, the spring 351 and the lever 352 are proportional to the displacement amount x1 (= Δl1) of the tension roller 35 by the proportional coefficient K1 expressed by the following equation.
K1 = (Lb1 / La1) ² · k3 / {2 · sin (θ1 / 2)} (Equation 10)
Is applied to the intermediate transfer belt 31.

Similarly, the spring constant of the spring 331 is k4, the winding angle of the intermediate transfer belt 31 around the tension roller 33 is θ2, and the force applied to one end of the lever 332 from the rotation center C33 of the lever 332 to the tension roller 33 (virtual). When the length of the (virtual) perpendicular drawn down to the action line is La2 and the length of the (virtual) perpendicular drawn from the rotation center C33 of the lever 332 to the (imaginary) action line of the spring 331 is Lb2, the tension T2 is expressed by the following equation: T2 = (1/2) · (Lb2 / La2) ² · k4 · x2 / sin (θ2 / 2) (Equation 11)
It is represented by In other words, the spring 331 and the lever 332 are proportional to the displacement amount x2 (= Δl2) of the tension roller 33 and represented by the following equation.
K2 = (Lb2 / La2) ² · k4 / {2 · sin (θ2 / 2)} (Equation 12)
Is applied to the intermediate transfer belt 31.

The condition for the change in tension of the intermediate transfer belt 31 by the two tension rollers to cancel each other is T1 = T2. Further, since the displacement of the tension roller 35 greatly affects the secondary transfer at the secondary transfer position TR2, in order to reduce the influence, the displacement x1 of the tension roller 35 is smaller than the displacement x2 of the tension roller 33. Is desirable. Therefore, in this embodiment, the following formula:
(Lb1 / La1) ² · k3 / sin (θ1 / 2)> (Lb2 / La2) ² · k4 / sin (θ2 / 2) (Equation 13)
The relationship is satisfied.

When these representative numerical examples are shown,
θ1: 39.7 degrees
θ2: 21.6 degrees
k3: 1.0 N / mm,
k4: 0.5 N / mm,
La1 = 25mm
La2 = 30mm
Lb1 = 30mm
Lb2 = 30mm
It is. At this time,
(Lb1 / La1) ² · k3 / sin (θ1 / 2) ≈4.2 N / mm,
(Lb2 / La2) ² · k4 / sin (θ2 / 2) ≈2.7 N / mm,
And the relationship of (Equation 13) is satisfied.

  As described above, also in the second embodiment, the proportionality coefficient K2 for the displacement amount x2 (= Δl2) of the tension roller 33 is set smaller than the proportionality coefficient K1 for the displacement amount x1 (= Δl1) of the tension roller 35. . Accordingly, the tension roller 33 is largely displaced with respect to the variation in the pressing force of the secondary transfer roller 4, whereas the displacement of the tension roller 35 is suppressed to be small. In this way, it is possible to stabilize the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP while keeping the displacement of the tension roller 35 small regardless of fluctuations in the pressing force of the secondary transfer roller 4. It has become.

  Moreover, the structure which gives force to the tension rollers 33 and 35 with a lever like this embodiment has the following advantages. That is, the tension rollers 33 and 35 may slide with respect to the intermediate transfer belt 31 as the tension rollers 33 and 35 are displaced. At this time, since the sliding resistance between the tension rollers 33 and 35 and the intermediate transfer belt 31 is large, the responsiveness of the tension rollers 33 and 35 to external force is deteriorated, and the displacement of the tension rollers 33 and 35 is not stable, As a result, the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP may be slightly unstable. On the other hand, in the lever structure as described above, the sliding resistance between the tension rollers 33 and 35 and the intermediate transfer belt 31 generated when the tension rollers 33 and 35 slide with respect to the intermediate transfer belt 31 is reduced. Can be suppressed. As a result, the distance from the image forming stations 2Y, 2M, 2K, and 2C to the transfer nip NP can be further stabilized.

Others As described above, in these embodiments, the intermediate transfer belt 31 corresponds to the “image carrier belt” of the present invention, and the image forming stations 2Y, 2M, 2K, and 2C correspond to the “image forming unit” of the present invention. The belt driving roller 32 corresponds to the “driving roller” of the present invention, the backup roller 34 corresponds to the “roller” of the present invention, and the tension roller 35 corresponds to the “first stretching roller” of the present invention. The tension roller 33 corresponds to the “second stretching roller” of the present invention, the secondary transfer roller 4 corresponds to the “transfer roller” of the present invention, and the controller 10 serves as the “control unit” of the present invention. It corresponds. In the first embodiment, the spring 351 corresponds to the “first tension applying unit” of the present invention, and the spring 331 corresponds to the “second tension applying unit” of the present invention. In the second embodiment, the spring 351 and the lever 352 cooperate to function as the “first tension applying unit” of the present invention, and the spring 331 and the lever 332 cooperate to operate the “second tension” of the present invention. It functions as a “tension applying part”.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in each of the above embodiments, four image forming stations are arranged in a line along the stretching direction of the intermediate transfer belt 31, but the number and arrangement of the image forming stations are not limited to this.

  In each of the above-described embodiments, the image forming apparatus is a so-called liquid development type that uses a developer in which toner is dispersed in a liquid carrier. However, the application target of the present invention is not limited to that type. That is, regardless of the development method, as illustrated in FIG. 1, for an image forming apparatus in general having a structure in which a transfer roller having a surface shape in which a part of a cylindrical peripheral surface is notched is brought into contact with an intermediate transfer belt. Thus, the present invention can be applied.

  Furthermore, the present invention can also be applied to an apparatus including a transfer roller that does not have a gripping mechanism. For example, in an apparatus in which an elastic surface layer is formed by winding a sheet-like elastic body around the transfer roller surface, it is necessary to provide a notch on the outer peripheral surface of the transfer roller in order to fix the end of the sheet. The present invention can also be applied to an apparatus having such a configuration and having no gripping mechanism.

  DESCRIPTION OF SYMBOLS 10 ... Controller, 2Y, 2M, 2C, 2K ... Image forming station, 31 ... Intermediate transfer belt, 32 ... Belt drive roller, 34 ... Secondary transfer backup roller, 35 ... Tension roller, 351 ... Spring, 33, 37 ... Tension Roller, 331, 371 ... Spring, 4 ... Secondary transfer roller, 41 ... Recess, 43 ... Elastic layer, 44 ... Gripping part, M3 ... Belt drive motor, M4 ... Transfer roller drive motor, TR1 ... Primary transfer position, TR2 ... Secondary transfer position

Claims (8)

An image carrier belt carrying an image;
A driving roller for wrapping the image carrier belt and moving the image carrier;
A first stretching roller for winding the image carrier belt wound around the drive roller and applying a tension T1 to the image carrier belt;
A first tension applying unit having a proportional coefficient K1 and generating the tension T1 applied to the image carrier belt by the first tension roller;
A roller for winding the image carrier belt wound around the first tension roller;
A second stretching roller for winding the image carrier belt wound around the roller and applying a tension T2 to the image carrier belt;
A second tension applying unit having a proportional coefficient K2 smaller than the first proportional constant K1, and generating the tension T2 applied to the image carrier belt by the second tension roller;
A transfer roller having a concave portion on its peripheral surface and contacting the roller via the image carrier belt;
A transfer apparatus comprising:
Here, K1 and K2 are
K1 = T1 / Δl1
Δl1: When the tension is applied from the image carrier belt from the position of the first tension roller when no tension is applied to the first tension roller from the image carrier belt, Displacement in the direction of the force acting on the first tension roller due to the tension applied from the image carrier belt to the position where the tension roller has moved T1: the first tension roller has moved Δl1 Sometimes the tension applied by the first tension roller to the image carrier belt K2 = T2 / Δl2
Δl2: From the position of the second tension roller when no tension is applied to the second tension roller from the image carrier belt, the second tension roller when the tension is applied from the image carrier belt. The amount of displacement in the direction of the force acting on the second tension roller due to the tension applied from the image carrier belt to the position where the tension roller has moved T2: the second tension roller has moved Δl2 Sometimes the tension applied by the second tensioning roller to the image carrier belt. An image forming unit for forming an image;
An image carrier belt to which the image formed by the image forming unit is transferred;
A driving roller that moves the image carrier while winding the image carrier belt to which the image is transferred;
A first stretching roller for winding the image carrier belt wound around the drive roller and applying a tension T1 to the image carrier belt;
A first tension applying unit having a proportional coefficient K1 and generating the tension T1 applied to the image carrier belt by the first tension roller;
A roller for winding the image carrier belt wound around the first tension roller;
A second stretching roller for winding the image carrier belt wound around the roller and applying a tension T2 to the image carrier belt;
A second tension applying unit having a proportional coefficient K2 smaller than the first proportional constant K1, and generating the tension T2 applied to the image carrier belt by the second tension roller;
A transfer roller having a concave portion on its peripheral surface and contacting the roller via the image carrier belt;
A control unit that controls the timing at which the image forming unit forms the image according to the rotation of the transfer roller;
An image forming apparatus comprising:
Here, K1 and K2 are
K1 = T1 / Δl1
Δl1: When the tension is applied from the image carrier belt from the position of the first tension roller when no tension is applied to the first tension roller from the image carrier belt, Displacement in the direction of the force acting on the second tension roller due to the tension applied from the image carrier belt to the position where the tension roller has moved T1: the first tension roller has moved Δl1 Sometimes the tension applied by the first tension roller to the image carrier belt K2 = T2 / Δl2
Δl2: From the position of the second tension roller when no tension is applied to the second tension roller from the image carrier belt, the second tension roller when the tension is applied from the image carrier belt. The amount of displacement in the direction of the force acting on the second tension roller due to the tension applied from the image carrier belt to the position where the tension roller has moved T2: the second tension roller has moved Δl2 Sometimes the tension applied by the second tensioning roller to the image carrier belt. The first tension applying unit includes a first elastic member having a spring constant k1 that supports the first tension roller,
The second tension applying unit includes a second elastic member having a spring constant k2 that supports the second tension roller,
When the winding angle of the image carrier belt on the first tension roller is θ1, and the winding angle of the image carrier belt on the second tension roller is θ2, the following equation K1 = k1 / {2 · sin (θ1 / 2)}
K2 = k2 / {2 · sin (θ2 / 2)}
The image forming apparatus according to claim 2, wherein: The first tension applying unit includes a first lever that rotates while supporting the first tension roller at one end, and a spring constant k3 supported at the other end different from the one end of the first lever. A first spring of
The second tension applying unit includes a second lever that rotates while supporting the second tension roller at one end, and a spring constant k4 supported at the other end different from the one end of the second lever. A second spring of
A winding angle of the image carrier belt around the first stretching roller is θ1,
La1 is the length of the imaginary perpendicular line from the rotation center of the first lever to the imaginary action line of the force applied to the first tension roller by the one end of the first lever,
The length of an imaginary perpendicular line drawn from the rotation center of the first lever to the imaginary action line of the force of the first spring supported by the other end of the first lever is Lb1,
A winding angle of the image carrier belt around the second stretching roller is θ2,
La2 is the length of the imaginary perpendicular line from the rotation center of the second lever to the imaginary action line of the force applied to the second tension roller by the one end of the second lever.
When the length of the virtual perpendicular drawn from the rotation center of the second lever to the virtual action line of the force of the second spring supported by the other end of the second lever is Lb2, Formula K1 = (Lb1 / La1) ² · k3 / {2 · sin (θ1 / 2)}
K2 = (Lb2 / La2) ² · k4 / {2 · sin (θ2 / 2)}
The image forming apparatus according to claim 2, wherein: The image forming unit includes a latent image carrier on which a latent image is formed, an exposure unit that exposes the latent image carrier to form the latent image, and a developing unit that develops the latent image,
5. The image forming apparatus according to claim 2, wherein the control unit controls the timing of exposure by the exposure unit in accordance with the rotation of the transfer roller. A detection unit for detecting the rotational position of the transfer roller;
The image forming apparatus according to claim 5, wherein the control unit controls the timing of exposure by the exposure unit in accordance with information detected by the detection unit. The developing unit uses a liquid developer containing toner and a liquid carrier,
The image forming apparatus according to claim 5, wherein the transfer roller has a recess formed in a peripheral surface, and a gripping member that grips a recording material is disposed in the recess. A second image forming unit that forms a second image of a different color from the image formed by the image forming unit;
The image forming apparatus according to claim 2, wherein the control unit controls the timing at which the second image forming unit forms the second image according to the rotation of the transfer roller. .

Images