JP2011053402A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress a speed change of an image carrier and to prevent image disturbance, resulting from the suppression in a transfer device and image forming apparatus that includes the belt-like image carrier and a transfer roller with a notched shape in a portion of its circumference. <P>SOLUTION: In a period of time for which the circumference of a secondary transfer roller with a recess is located at a transfer nip, the rotating speed ω0 of the secondary transfer roller at a first-half period (time t10 to t11) through which a recording material passes is made different from the rotating speed ω1 at a second-half period (time t11 to t12) for which the elastic layer of the surface comes into direct contact with the intermediate transfer belt, after the passage of the recording material. By doing so reset to the initial value T0 can be made, while gradually decreasing the change in the tension of the intermediate transfer belt, which results from the circumferential speed difference between the belt and the secondary transfer roller. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention includes a belt-like image carrier that carries an image, and a transfer roller that forms a transfer nip in contact with the image carrier and has a notch that is wider than the width of the transfer nip. The present invention relates to an image forming apparatus and a method for forming an image using these.

  In the field of image forming technology for forming an image on a recording material such as paper, there is one configured to transfer an image once formed on a belt-shaped image carrier onto the recording material. For example, in the image forming apparatus described in Patent Document 1, an image forming station forms an image on a transfer belt as an image carrier that is stretched around a plurality of rollers, and the transfer belt and the secondary transfer roller are in contact with each other. An image is transferred from the transfer belt to the recording material by passing the recording material through the transfer nip formed in contact therewith.

  In order to perform image transfer from the image carrier to the recording material with high transfer efficiency, it is desirable to apply a high pressing force to the recording material passing through the transfer nip. On the other hand, such a high pressing force may cause the recording material to adhere to the image carrier. For example, the image forming apparatus described in Patent Document 1 is based on a so-called liquid development method in which an electrostatic latent image is visualized by a developer dispersed in a liquid carrier, and a recording material is stuck due to the remaining liquid carrier. Is likely to occur.

  In order to solve such a problem, for example, it is conceivable to employ the technique described in Patent Document 2. In the image device 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 pressure roller (corresponding to a transfer roller), and an end portion of a recording material is gripped by the gripper. Thus, the recording material is prevented from sticking to the intermediate transfer member in contact with the pressing roller. In addition, the intermediate transfer member which is an image carrier in Patent Document 2 is a roller-like rotating body.

Japanese Patent Laid-Open No. 2008-122815 (for example, FIG. 1) Japanese translation of PCT publication No. 2000-508280 (for example, FIG. 2A)

  When considering the combination of Patent Documents 1 and 2 described above, the following problems may occur. That is, in the pressing roller described in Patent Document 2, since the gripping member is attached, the peripheral surface is not a smooth cylindrical surface, and due to this, the pressing force at the transfer nip is caused by the rotation period of the roller. It will fluctuate synchronously. Such fluctuation causes fluctuations in the speed of the belt-shaped image carrier in contact with the roller, and as a result, the image formed on the image carrier by the image forming station is disturbed.

  Such a problem is common in the case of using a transfer roller whose peripheral surface is not a complete cylindrical surface. For example, even when a transfer roller having a shape in which a part of the peripheral surface is cut out is used. That is, when the image carrier is opposed to the cylindrical peripheral surface portion of the transfer roller and is in contact with the surface thereof, and when the image carrier is opposed to the notch portion of the transfer roller and separated from the transfer roller surface, Since the pressing force applied to the surface of the image carrier is greatly different, a large speed fluctuation occurs at the time of switching. This is particularly noticeable when using a transfer roller having a notch having a size larger than the transfer nip width.

  Some embodiments according to the present invention provide an image forming apparatus and a transfer apparatus having a belt-shaped image carrier and a transfer roller having a shape in which a part of the peripheral surface is cut out. It is an object of the present invention to provide a technique capable of suppressing fluctuations in the speed of an image carrier and preventing image distortion resulting therefrom.

  In order to solve the above problems, an image forming apparatus according to the present invention has a belt-like image carrier that carries an image, a drive roller that wraps the image carrier and moves the image carrier, and the drive roller A first drive source that rotates the image carrier, an image forming unit that forms an image on the image carrier, a roller that winds the image carrier, and a winding part between the image carrier and the roller. And forming a transfer nip, and a transfer roller having a notch portion in which the opening width of the peripheral surface along the rotation direction is wider than the nip width of the transfer nip in the rotation direction, and a second that rotates the transfer roller. A drive roller, a tension roller that abuts on the image carrier and applies tension to the image carrier, and a peripheral surface of the transfer roller that is different from the notch contacts the image carrier to form a transfer nip. form The speed at which the rotation speed of the transfer roller varies between when the image carrier and the transfer roller are in contact with each other through the recording material and when the image carrier and the transfer roller are in contact with each other. And a control unit.

  In addition, in order to solve the above-described problem, the image forming method according to the present invention carries an image on a driving roller that is rotationally driven by a first driving source and a belt-shaped image carrier wound around the roller, A transfer nip is formed in contact with the image carrier wound around a roller, and the opening width of the circumferential surface along the rotation direction has a notch that is wider than the nip width of the transfer nip in the rotation direction. A transfer roller that rotates at a first rotation speed to pass the recording material through the transfer nip to transfer the image carried on the image carrier to the recording material, and the recording material is transferred to the transfer nip. After passing, the transfer roller is rotated at a second rotation speed different from the first rotation speed while the image carrier and the transfer roller are brought into contact with each other.

  When a transfer nip is formed by bringing a transfer roller having a notch into contact with the image carrier, the surface of the transfer roller facing the transfer nip is notched with a notch as the transfer roller rotates. Switch periodically with the surface. The load viewed from the image carrier is greatly different when the surface of the transfer roller facing the transfer nip is a notch and when the surface is different from the notch. Specifically, when the peripheral surface of the transfer roller different from the notch is located in the transfer nip, the two are in contact with each other over a relatively large area, so the friction between them is large, and the peripheral speed of both in the transfer nip It is difficult to eliminate the difference completely. Therefore, the load torque exerted on each other increases.

  Such fluctuations in the load torque cause fluctuations in the tension and speed of the image carrier. If the contact between the transfer roller and the image carrier is continued, fluctuations in the tension of the image carrier are also accumulated during that time. For this reason, when the surface of the transfer roller facing the transfer nip is switched from the peripheral surface to the notch, the accumulated tension is released at once. As a result, vibration and speed fluctuations of the image carrier occur, and this affects the quality of an image formed on the image carrier separately.

  Therefore, in the present invention, the rotation speed of the transfer roller during the period in which the transfer roller peripheral surface is located in the transfer nip is not constant, and the rotation speed is set depending on whether or not there is a recording material in the transfer nip during that period. Make it different. When there is a recording material in the transfer nip, the apparent outer diameter of the transfer roller is increased due to the thickness of the recording material, so that a peripheral speed difference from the image carrier occurs. By making it possible to change the rotation speed of the transfer roller, in the present invention, it is possible to gradually relieve the change in tension accumulated in the image carrier due to the speed difference from the image carrier. It is possible to prevent the speed fluctuation caused by being released at once from adversely affecting the image formation.

  Note that the fluctuation of the tension of the image carrier can be absorbed by bringing the tension roller into contact with the image carrier in the short term. However, it is difficult to completely absorb the sudden fluctuation when the transfer roller surface facing the transfer nip switches from the peripheral surface to the notch. By combining the tension roller and the speed adjustment of the transfer roller described above, it is possible to more effectively suppress the speed fluctuation of the image carrier.

  For example, the rotation speed of the transfer roller when the image carrier and the transfer roller are in contact, that is, when there is no recording material in the transfer nip, the recording material exists in the transfer nip and the image carrier and the transfer roller pass through the recording material. The rotational speed of the transfer roller may be slower than when the roller contacts. This is because when the recording material passes through the transfer nip, the effective outer diameter of the transfer roller increases by the thickness of the recording material. Therefore, if the rotation speed of the transfer roller is constant, the peripheral speed increases, and this may cause a speed difference from the image carrier. In such a case, by reducing the rotation speed of the transfer roller when the recording material is not in the transfer nip, the tension variation of the image carrier caused by the peripheral speed difference while the recording material is in the transfer nip can be reduced. Can be relaxed. In particular, it is preferable to set the rotation speed of the transfer roller so that the peripheral speed of the image carrier when there is no recording material in the transfer nip is slower than the peripheral speed of the image carrier. This is because the fluctuation in the tension of the image carrier caused by the increase in the peripheral speed of the transfer roller can be resolved more quickly and reliably.

  On the other hand, for example, when there is a recording material in the transfer nip, the rotation speed of the transfer roller may be constant. If the rotational speed is changed during this period in which the tension fluctuation is not accumulated so much, the movement of the transfer roller and the image carrier can be unstable. For example, if the speed of the transfer roller is changed when the recording material passes through the transfer nip, an image transferred to the recording material is disturbed. Such a problem can be solved by keeping the rotation speed of the transfer roller constant during a period in which the recording material is present in the transfer nip.

  In this case, the rotation speed of the transfer roller when the notch of the transfer roller is located in the transfer nip is the same as the rotation speed of the transfer roller when the image carrier and the transfer roller are in contact with each other via the recording material, or You may make it substantially the same. In this way, contrary to the above, when the surface of the transfer roller facing the transfer nip is switched from the notched portion to the recording material, it is possible to suppress an abrupt change in the tension of the image carrier.

  Further, it is preferable to change the rotation speed of the transfer roller when the recording material passes through the transfer nip. As described above, if the speed of the transfer roller is changed while the recording material passes through the transfer nip, the image transferred to the recording material is disturbed. Therefore, it is desirable to change the rotation speed after the lapse of this period. .

  Further, an information acquisition unit that acquires recording material information regarding the recording material may be further provided, and the rotation speed of the transfer roller may be set based on the recording material information acquired by the information acquisition unit. As described above, when the recording material passes through the transfer nip, the apparent outer diameter of the transfer roller changes and a peripheral speed difference from the image carrier occurs. Such a peripheral speed difference can take different values depending on the state of the recording material, such as thickness and length. Therefore, by setting the rotation speed of the transfer roller based on the recording material information, it is possible to set the speed according to the state of the recording material.

  Therefore, as the recording material information, at least one of information on the thickness of the recording material and information on the length of the recording material along the moving direction of the image carrier at the transfer nip can be used. Both the thickness and length of the recording material affect the amount of tension fluctuation accumulated in the image carrier. By using this information as recording material information, the fluctuation in tension on the image carrier can be further increased. It can be effectively suppressed.

  Here, the information acquisition unit may include a sensor that detects the length of the recording material. The length of the recording material detected by the sensor can be used as recording material information.

  Further, the second drive source may include a servo motor, for example. By rotating and driving the transfer roller with a servo motor, precise speed control is possible and the position of the transfer roller can be controlled, so that the transfer roller and the image on the image carrier in the transfer nip can be aligned. It becomes possible to carry out with high accuracy.

  In addition, a grip portion for gripping a recording material to which an image is transferred can be disposed in a cutout portion of the transfer roller peripheral surface. By providing such a grip portion, it is possible to reliably prevent the recording material from sticking to the image carrier in the transfer nip. In the present invention, fluctuations in the speed of the image carrier due to the notch provided in the transfer roller are prevented. Therefore, by accommodating the gripping part in the notch part, both the disturbance of the image due to the speed fluctuation and the sticking of the recording material to the image carrier can be effectively prevented, and a good quality image can be obtained. It can be efficiently formed on a recording material.

1 is a diagram showing an 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. 2 is a timing chart illustrating an operation example of the image forming apparatus in FIG. 1. FIG. 2 is a first diagram schematically showing the operation of the image forming apparatus in FIG. 1. FIG. 3 is a second diagram schematically showing the operation of the image forming apparatus in FIG. 1. 4 is a timing chart showing an aspect of speed control of the transfer roller according to the present invention. 6 is a timing chart showing the relationship between the state of the recording material and the transfer roller rotation speed. The figure which shows the example of the table which linked | related the thickness of the recording material, and the circumferential speed difference.

  FIG. 1 is a diagram showing an 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, it is not a volatile liquid developer that is generally used in the past and has a low concentration (1-2 wt%) and low viscosity at room temperature using Isopar (trademark: Exon) as a carrier liquid. In a liquid solvent such as an organic solvent, silicon oil, mineral oil or edible oil, solid particles having an average particle diameter of 1 μm in which a colorant such as a pigment is dispersed in a non-volatile resin having a high concentration and high viscosity at room temperature A liquid developer having a high viscosity (about 30 to 10000 mPa · s) is used which is added together with a dispersant and has a toner solid content concentration of about 20%.

  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 belt as an image carrier that can temporarily carry a toner image on its surface, more specifically, on its outer peripheral surface, and is laid over a plurality of rollers 32, 33, 34 and 35. Has been. 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 member 21 and the intermediate transfer belt 31 are in contact, 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.

  A size detection sensor 50 is disposed on the front side of the gate roller 51 on the conveyance path PT of the recording material RM. The size detection sensor 51 outputs a signal corresponding to the presence or absence of the recording material RM at the arrangement position. From the length of time that the size detection sensor 51 detects the presence of the recording material, the controller 10 can grasp the length of the recording material RM fed into the transfer nip in the belt moving direction D31.

  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 conventionally well-known holding 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. Further, the support member 46 is formed with a planar region 461 corresponding to the recess 41. The transfer roller side contact member 47 is attached to the flat area 461, respectively. 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. Reference numeral 8 denotes a phase detection sensor connected to one end of the rotary shaft 421 of the secondary transfer roller 4. A substantially disc-shaped side plate 81 (FIG. 1) having two notches (cuts) provided on the outer peripheral surface is attached to the side surface of the secondary transfer roller 4, and as the secondary transfer roller 4 rotates. The level of the output signal of the phase detection sensor 8 changes every time the notch provided in the side plate 81 passes through the position where the phase detection sensor 8 is disposed. The controller 10 can grasp the rotational phase of the secondary transfer roller 4 from the output of the phase detection sensor 8.

  Further, in this embodiment, an operation panel 16 for receiving an operation instruction input from the user is provided, and the operation instruction input to the operation panel 16 is transmitted to the controller 10 via the interface 15. The interface 15 also has a function of receiving an image forming command given from an external device such as a host computer and passing it to the controller 10.

  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 that can expand and contract in a substantially horizontal direction, so that the tension roller 33 is substantially in a state where the intermediate transfer belt 31 is wound. A predetermined amount can be moved in the horizontal direction.

  In addition, the rotation shaft of the tension roller 35 is elastically supported by a spring 351 that can expand and contract in a direction substantially perpendicular 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. As a result, 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 tension roller 35 provided between the belt driving roller winding position and the secondary transfer position TR2 along the tension direction of the intermediate transfer belt 31 and the primary transfer downstream of the secondary transfer position TR2. The tension roller 33 provided on the upstream side of the transfer position TR1y acts so as to cancel the variation in tension by moving the rotation shaft thereof. 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. 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 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.

  Then, when the cleaner unit 39 is pressed against the surface of the intermediate transfer belt 31 wound around the tension roller 33, the tension roller 33 and the cleaner unit 39 have a braking action on the intermediate transfer belt 31, and the speed fluctuation is reduced. Furthermore, it can suppress effectively.

  Further, the intermediate transfer belt 32 is located downstream of the most downstream primary transfer position TR1k in the moving direction D31 of the intermediate transfer belt 31 and upstream of the position where the tension roller 35 contacts the intermediate transfer belt 31. It is in contact with the belt 31. This is because the displacement of the tension roller 35 is not transmitted to the primary transfer position TR1k. By doing so, the following operation can be obtained. That is, according to such a configuration, the intermediate transfer belt 31 passing through the primary transfer position TR1 corresponding to each image forming station is always pulled by the belt driving roller 32 with a substantially constant torque. For this reason, the intermediate transfer belt 31 does not loosen at each primary transfer position TR1, and the toner image transferred from each image forming station at the primary transfer position TR1 does not get disturbed. Further, since the slack does not occur, the distance between the primary transfer positions TR1 along the circumference of the intermediate transfer belt 31 does not vary. Therefore, it is possible to prevent the positional deviation when the toner images of the respective colors are superimposed.

  Next, the operation of the image forming apparatus 1 configured as described above will be described with reference to FIGS. FIG. 4 is a timing chart showing an operation example of the image forming apparatus of FIG. 5 and 6 are diagrams schematically showing the operation of the image forming apparatus shown in FIG. In this image forming apparatus 1, when an image formation command for forming a color image is given to the controller 10 from an external device such as a host computer, the controller 10 follows a program stored in a memory (not shown). Control each part of the device. First, the belt drive motor M3 and the transfer roller drive motor M4 are operated to drive the intermediate transfer belt 31 and the secondary transfer roller 4, respectively.

  The phase detection sensor 8 (FIG. 2) provided on the secondary transfer roller 4 is formed from a cylindrical peripheral surface in which the surface of the secondary transfer roller 4 facing the intermediate transfer belt 31 at the secondary transfer position TR2 has an elastic layer 43. An H level signal is temporarily output each time when switching to the recess 41 and when switching from the recess 41 to the elastic layer 43. The controller 10 alternately switches the drive control mode of the secondary transfer roller 4 by the driver 12 between position control and torque control based on the change in the signal level. More specifically, the position control is performed when the concave portion 41 of the secondary transfer roller 4 faces the intermediate transfer belt 31, and the torque is generated when the elastic layer 43 faces the intermediate transfer belt 31 and the transfer nip NP is formed. Control is performed. Note that the position of the belt drive motor M3 is always controlled, and the surface of the intermediate transfer belt 31 rotates at a predetermined moving speed.

  When the output of the phase detection sensor 8 changes at the timing tA0 and the secondary transfer roller 4 facing the secondary transfer position TR2 changes from the recess 41 to the elastic layer 43 to form the transfer nip NP, the controller 10 switches the control. The drive control mode by the driver 12 is switched to torque control according to the signal, and the command torque is applied to the driver 12 to torque control the secondary transfer roller 4. Further, with the timing tA0 as an exposure start point, a toner image is formed at each of the image forming stations 2Y, 2M, 2C, and 2K, and the toner image is primarily transferred onto the surface of the intermediate transfer belt 31.

  That is, as shown in FIG. 4, when the time Ta elapses from the timing tA0, latent image formation by the exposure unit 23 is started in the image forming station 2Y based on various signals from the controller 10, and a toner image is formed with yellow toner. The Further, when the time Tb elapses from the start of the yellow exposure, the magenta exposure is started. When the time Tc elapses from the start of the magenta exposure, the cyan exposure is started, and the time Td from the start of the cyan exposure is started. When elapses, black exposure is started. Thus, the toner images of the respective colors are formed and are sequentially superimposed on the intermediate transfer belt 31 to form a full color toner image TI on the surface of the intermediate transfer belt 31.

  While the toner images of the respective colors are formed in this way, the secondary transfer roller 4 further rotates one turn in the rotation direction D4, and the transfer nip NP that has been once eliminated is formed again. When a predetermined time Te elapses from this timing tA1, the controller 10 inputs a command pulse to a driver (not shown) that controls a gate roller drive motor (not shown) connected to the gate roller 51 to operate the gate roller drive motor. Let As a result, conveyance of the recording material RM to the secondary transfer position TR2 is started (FIG. 5A).

  Further, when the secondary transfer roller 4 facing the secondary transfer position TR2 changes to the concave portion 41 and the transfer nip is eliminated, the controller 10 changes the drive control mode by the driver 12 from the torque control by the control switching signal at the timing tB2. While switching to position control, a command pulse is given to the driver 12. As a result, the secondary transfer roller 4 rotates in the rotation direction D4 and moves to a predetermined recording material gripping position (FIG. 5B). Further, the leading end of the gripper member 442 is separated from the leading end of the gripper support member 441, and the preparation for gripping the recording material RM is completed. Then, the leading end of the recording material RM fed by the gate roller 51 enters between the gripper member 442 and the gripper support member 441, and a paper holding operation is started (FIG. 5B). Here, for convenience of explanation, it is referred to as “paper holding operation”, but as described above, the recording material RM is not limited to paper.

  The controller 10 gives a grip command to the gripper driving unit (not shown) at the same time or slightly delayed from the timing tB2. In response to this gripping command, the gripper drive unit operates to move the tip of the gripper member 442 to the tip of the gripper support member 441. As a result, the leading end of the recording material RM is gripped, and the “paper gripping operation” is completed (FIG. 5C). At the time when the “paper holding operation” is completed, the toner image TI is located upstream of the secondary transfer position TR2 in the moving direction D31 of the surface of the intermediate transfer belt 31 as shown in FIG.

  In this way, the recording material RM is conveyed in the rotational direction D4 together with the secondary transfer roller 4 with its leading end held by the holding portion 44. At the timing tA2 when the elastic layer 43 on the surface of the secondary transfer roller 4 reaches the secondary transfer position TR2 and the formation of the transfer nip NP is started, the recording material RM includes the elastic layer 43 of the secondary transfer roller 4 and the intermediate transfer belt. 31 is nipped in a transfer nip NP between the surface 31 and conveyed along with these rotations. Thereby, the secondary transfer of the toner image TI formed on the intermediate transfer belt 31 to the lower surface (front surface) of the recording material RM is started (FIG. 5D). Further, at this timing tA2, the controller 10 switches the drive control mode by the driver 12 to torque control by the control switching signal and gives the command torque to the driver 12 to control the torque of the secondary transfer roller 4.

  The secondary transfer roller 4 rotates in the rotational direction D4 while being torque controlled in this way, and accordingly, the recording material RM passes between the transfer nips NP while the leading end portion is held by the grip portion 44, and the toner image TI. Secondary transfer proceeds (FIG. 6A). When the grip 44 moves to a position near the upstream end (the right end in FIG. 1) of the transport roller 7, the leading end of the recording material held by the grip 44 is an intermediate transfer belt. It has been sufficiently separated from 31 and conveyed to the conveyance entrance of the conveyance roller 7. As shown in FIG. 6B, the controller 10 gives a release command to the gripper driving unit at the timing when the gripping part 44 moves to the vicinity of the upstream end of the transport roller 7, and the tip of the gripper member 442 is moved to the gripper. The recording material RM is released from being separated from the distal end portion of the support member 441. As a result, the leading end of the recording material RM is reliably fed into the transport roller 7 without sticking to the surface of the intermediate transfer belt 31. Then, the color toner image TI is fixed to the recording material RM by the fixing unit disposed behind the transport roller 7. After the release, the leading end side of the recording material is conveyed to the fixing unit side along the conveying path PT, and the trailing end side of the recording material RM is connected to the elastic layer 43 of the secondary transfer roller 4 at the transfer nip NP. The secondary transfer process is executed while being nipped and conveyed by the intermediate transfer belt 31.

  In the operation as described above, it is ideal that the peripheral surface of the secondary transfer roller 4 and the intermediate transfer belt 31 are moved at the same peripheral speed in the transfer nip NP. However, it is difficult to completely match the peripheral speeds of the two. This is because even if the control error can be completely eliminated, the secondary transfer roller 4 is not a mere cylinder but has a concave portion 41 in part, and the secondary transfer roller 4 has a recording material RM. This is because there is a circumstance that the effective outer diameter fluctuates by being wound.

  That is, even when the elastic layer 43 of the secondary transfer roller 4 is positioned at the transfer nip NP, the apparent outer diameter of the transfer roller between the period when the recording material RM passes through the transfer nip NP and the other period. Is different. For this reason, even if the rotational speed of the secondary transfer roller 4 is constant during the period in which the elastic layer 43 of the secondary transfer roller 4 is located in the transfer nip NP, the peripheral speed is not necessarily constant. The peripheral speed difference between the secondary transfer roller 4 and the intermediate transfer belt 31 changes the tension of the intermediate transfer belt 31 in the vicinity of the transfer nip NP. This causes the speed fluctuation of the intermediate transfer belt 31.

  Therefore, in this embodiment, the rotation speed of the secondary transfer roller 4 during the period in which the elastic layer 43 of the secondary transfer roller 4 is located in the transfer nip NP is made different between the first period and the second period, and the intermediate transfer belt. The fluctuation of the tension of 31 is eased. More specifically, among the periods in which the elastic layer 43 of the secondary transfer roller 4 is located in the transfer nip NP, the period in which the recording material RM passes through the transfer nip NP is referred to as “first period”, and the recording material RM. The period in which the elastic layer 43 is in direct contact with the intermediate transfer belt 31 after passing is defined as a “late period”, and the rotational speed of the secondary transfer roller 4 is changed during these periods. If the rotational speed of the secondary transfer roller 4 is changed while the recording material RM passes through the transfer nip NP, the toner image transferred to the recording material RM is disturbed or the conveyance error of the recording material RM occurs. It is desirable not to change the rotational speed of the secondary transfer roller 4 until the transfer nip NP has been passed.

  FIG. 7 is a timing chart showing an aspect of speed control of the transfer roller according to the present invention. However, FIG. 7A is a comparative example in which the present invention is not applied and the rotation speed of the transfer roller is constant. In the following drawings, a region where the recording material RM is wound and a region where the elastic layer 43 is exposed in the surface region of the secondary transfer roller 4 are collectively referred to as a “peripheral surface portion”. Further, in a state where the recording material RM is not wound around the transfer roller 4 and the elastic layer 43 is exposed, the peripheral speeds of the secondary transfer roller 4 (elastic layer 43) and the intermediate transfer belt 31 in the transfer nip NP are equal. It is assumed that the rotational speed ω0 of the secondary transfer roller 4 is set.

  Consider the case where the rotational speed of the secondary transfer roller 4 is maintained at a constant value ω 0 as shown in FIG. While the recording material RM passes through the transfer nip NP (time t10 to t11), the recording wound around the secondary transfer roller 4 as the effective outer diameter of the transfer roller 4 increases by the thickness of the recording material RM. A difference in peripheral speed between the material RM and the intermediate transfer belt 31 occurs, whereby the tension of the intermediate transfer belt 31 gradually increases from the initial value T0. Since such an increase in tension is absorbed by the tension roller 35 (and the tension roller 33) in the short term, it does not immediately lead to fluctuations in the speed of the intermediate transfer belt 31 at the primary transfer position. That is, the increase in tension is accumulated as elastic energy of the springs 351 and 331 that support the tension rollers 35 and 33.

  When the rear end portion of the recording material RM passes through the transfer nip NP at time t11, the elastic layer 43 directly contacts the intermediate transfer belt 31, so the increase in tension stops. However, the increase in tension accumulated while the recording material RM passes through the transfer nip NP remains accumulated in the intermediate transfer belt 31 and the tension roller 35.

  At time t12, when the surface of the secondary transfer roller 4 facing the secondary transfer position TR2 is switched from the elastic layer 43 to the recess 41, the transfer nip NP is temporarily eliminated, so that the tension accumulated in the intermediate transfer belt 31 is increased. The fluctuation amount is released at once and returns to the original value T0. It is difficult for the tension roller 35 to completely absorb such a rapid fluctuation in tension without time delay, and as a result, vibration and speed fluctuation of the intermediate transfer belt 31 occur.

  On the other hand, in this embodiment, as shown in FIG. 7B, at the time t11 when the rear end portion of the recording material RM passes through the transfer nip NP and the elastic layer 43 comes into direct contact with the surface of the intermediate transfer belt 31. The rotational speed of the secondary transfer roller 4 is changed to ω1 (<ω0). The speed ω 1 is preferably a rotational speed at which the peripheral speed at the transfer nip NP on the surface of the recording material RM wound around the transfer roller 4 is slower than the peripheral speed of the intermediate transfer belt 31. As a result, the tension of the intermediate transfer belt 31 that has increased due to the peripheral speed difference gradually decreases. Therefore, a sudden fluctuation in tension as shown in the comparative example of FIG. As a result, in this embodiment, vibrations and speed fluctuations of the intermediate transfer belt 31 due to rapid fluctuations in tension are prevented, and image disturbance at the primary transfer position TR1 is prevented.

At the time t12 when the surface of the secondary transfer roller 4 facing the secondary transfer position TR2 is switched from the elastic layer 43 to the recess 41, the tension of the intermediate transfer belt 31 is preferably returned to the initial value T0. This is because a rapid change in the tension at the time of switching is more reliably prevented. To achieve this, the following formula:
ω1 = {(t11−t10) / (t12−t11)} × ΔV / R + ω0 (Equation 1)
The changed speed ω1 may be set so as to satisfy the above. Here, the symbol R is the radius of the secondary transfer roller 4 including the elastic layer 43. ΔV is (circumferential speed difference) between the intermediate transfer belt 31 generated when the recording material RM is wound around the secondary transfer roller 4 and the secondary transfer roller 4 including the recording material RM. A value is used.

  This peripheral speed difference ΔV is obtained by subtracting the peripheral speed of the secondary transfer roller 4 from the peripheral speed of the intermediate transfer belt 31, and the effective outer diameter of the transfer roller 4 is increased by the recording material RM as described above. Since the peripheral speed increases, the peripheral speed difference ΔV is normally a negative value. Therefore, as indicated by (Equation 1), the rotational speed ω1 of the secondary transfer roller 4 in the latter period of the period in which the elastic layer 43 of the secondary transfer roller 4 is located in the transfer nip NP is rotated in the previous period. It is preferable to make the speed slower than ω0.

  By setting the speed ω 1 in this way, sudden tension fluctuation of the intermediate transfer belt 31 accompanying the rotation of the secondary transfer roller 4 is prevented. Then, since the gentle fluctuation of the tension can be absorbed by the tension roller, the speed fluctuation of the intermediate transfer belt 31 at the primary transfer position TR1 is prevented as a result. After time t12, the rotational speed of the secondary transfer roller 4 is again set to ω0, so that the state of the apparatus returns to the initial state at time t10 described above.

  Further, the following effects can be obtained by making the tension of the intermediate transfer belt 31 return to the initial value T0 every time the secondary transfer roller 4 makes one round. As shown in FIG. 7B, at the times t10 and t13 when the surface of the transfer roller 4 facing the secondary transfer position TR2 is switched from the recess 41 to the elastic layer 43 and the transfer nip NP is formed, the intermediate transfer belt 31 is moved. The tension is a constant value T0. By doing this, the position of the tension roller 35 at the start of the formation of the transfer nip NP also becomes constant. As a result, the circumferential length of the intermediate transfer belt 31 from the primary transfer position TR1 corresponding to each image forming station to the transfer nip NP, That is, the transport distance of each color toner image to the transfer nip NP is always constant. On the other hand, since the leading position of the recording material RM wound around the secondary transfer roller 4 is regulated by the gripper member 442, the leading position of the recording material RM is always the same with respect to the rotational phase of the secondary transfer roller 4. Therefore, by doing as described above, the relationship between the leading position of the recording material RM and the position of the toner image on the intermediate transfer belt 31 can be maintained constant, and the transfer position of the toner image onto the recording material RM can be maintained. Can be kept constant.

Here, in (Equation 1), the value ΔV is a value that varies depending on the thickness of the recording material RM. Further, the value (t11−t10) of the first term on the right side of (Expression 1) is the time for the recording material RM to pass through the transfer nip NP, and the following expression is obtained from the length L along the conveying direction of the recording material RM:
t11−t10 = L / (ω0 × R) (Formula 2)
Is the value represented by Therefore, the rotational speed ω1 of the secondary transfer roller 4 is affected by the thickness and length of the recording material RM.

  FIG. 8 is a timing chart showing the relationship between the state of the recording material and the transfer roller rotation speed. The recording material picked up in FIG. 7B is referred to as “standard paper”. As shown in FIG. 8A, when “thin paper” having a thickness smaller than that of the standard paper is used as the recording material RM, the thickness of the recording material added to the outer diameter of the secondary transfer roller 4 is small. The degree of increase in tension is less than that of standard paper. On the other hand, when the length of the recording material is the same, the time t11 when the recording material RM finishes passing through the transfer nip NP is the same as that of the standard paper. For this reason, the rotational speed ω2 of the secondary transfer roller 4 after passing through the recording material does not need to be greatly changed from the initial value ω0, and is larger than the rotational speed ω1 of the standard paper.

  Further, as shown in FIG. 8B, when the “short paper” whose transport direction length is shorter than the standard paper is used as the recording material RM, the recording material RM is transferred to the transfer nip at time t31 earlier than the standard paper. Exit NP. For this reason, a longer time can be taken to relieve the tension, and therefore the rotational speed ω3 of the secondary transfer roller 4 in this case can be set to a value close to the initial value ω0.

  As described above, when the tension of the intermediate transfer belt 31 is relaxed by changing the rotation speed of the secondary transfer roller 4, information on the thickness and length of the recording material RM is used to obtain the effect more reliably. It is preferable that the rotational speed of the secondary transfer roller 4 is determined based on at least one of these, more preferably both.

  FIG. 9 is a diagram showing an example of a table in which the recording material thickness and the peripheral speed difference are associated with each other. For each of the assumed thicknesses h1, h2,..., H5 of various recording materials, an experiment is performed in advance in which these are wound around the transfer roller 4 and the peripheral speed is measured. Then, from the results, the recording material thicknesses h1, h2,..., H5 and the peripheral speed differences ΔV1, ΔV2,. It is memorized in the controller 10.

  The thickness of the recording material RM fed to the transfer nip NP can be transmitted to the controller 10 by, for example, an instruction input to the operation panel 16 by the user. For example, “plain paper”, “thick paper”, “OHP sheet” or the like can be selected from the operation panel 16 as the type of the recording material RM, and a thickness corresponding to the selected type of recording material is adopted. Good. Further, information for selecting a recording material may be included in an image forming command given from an external device via the interface 15.

  On the other hand, the length L of the recording material RM may be based on information on the size of the recording paper designated from the user via the operation panel 16 or from the external device via the interface 15 in the same manner as described above. Alternatively, it may be based on the output signal of the paper size detection sensor 50. That is, the length L of the recording material RM can be obtained from the length of time during which the paper size detection sensor 50 detects the presence of the recording material on the transport path PT. The rotation speed of the secondary transfer roller 4 is changed after the trailing edge of the recording material RM has passed through the transfer nip NP. Therefore, the trailing edge of the recording material RM has passed the detection position by the paper size detection sensor 50. Even if the rotation speed is calculated after that, a sufficient time is secured until the rotation speed is actually changed.

  The controller 10 applies the peripheral speed difference ΔV and the recording material length L obtained from these information to (Equation 1) and (Equation 2) to change the rotation speed of the secondary transfer roller 4 t11. In addition, the changed rotation speed ω1 can be set. By controlling the transfer roller drive motor M4 based on the timing and the rotational speed set in this way, the speed of the intermediate transfer belt 31 at the primary transfer position TR1 due to the above-described effect, that is, the change in the tension of the intermediate transfer belt 31. The effect of suppressing fluctuations and preventing image disturbance can be made more reliable.

  As described above, in this embodiment, the rotational speed of the secondary transfer roller 4 during the period in which the elastic layer 43 of the secondary transfer roller 4 is located in the transfer nip NP is not constant, and the recording material RM is included in the period. The rotation speed is made different between the first period during which the toner passes through the transfer nip NP and the second period after the recording material passes. By doing so, the fluctuation in the tension of the intermediate transfer belt 31 caused by the difference in peripheral speed between the transfer roller 4 and the intermediate transfer belt 31 is alleviated, and the fluctuation in the speed of the intermediate transfer belt 31 caused by the sudden fluctuation in tension is prevented. Thus, the disturbance of the image formed on the intermediate transfer belt 31 at the primary transfer position TR1 can be prevented.

  In particular, the image transfer from the intermediate transfer belt 31 to the recording material RM can be stably performed by keeping the rotation speed of the secondary transfer roller 4 constant during the previous period when the recording material RM passes through the transfer nip NP. . Further, by changing the rotational speed ω1 in the latter period after the recording material RM has passed through the transfer nip NP to be slower than the rotational speed ω0 in the previous period (ω1 <ω0), the fluctuation of the tension accumulated in the previous period can be changed. The tension is gradually relaxed, and a sudden change in tension when the concave portion 41 of the secondary transfer roller 4 reaches the transfer nip NP is suppressed. Further, by setting the distribution between the first period and the second period and the rotation speed of the secondary transfer roller 4 in the second period based on the above (Formula 1) and (Formula 2), the influence of fluctuations in tension is minimized. Can be suppressed. Further, since the positional relationship between the leading portion of the recording material RM and the toner image on the intermediate transfer belt 31 can be kept constant, the transfer position of the image onto the recording material RM can be stabilized. Thus, in this embodiment, it is possible to stably form an image with good quality at an appropriate position on the recording material RM.

  In this embodiment, the drive roller 32 is wound around the intermediate transfer belt 31 on the downstream side of the primary transfer position TR1 along the moving direction of the intermediate transfer belt 31, and the first tension roller 35 is interposed on the downstream side of the intermediate transfer belt 31. A secondary transfer position TR2 is provided in contact with the transfer belt 31 and further downstream thereof. At the secondary transfer position TR2, the speed fluctuation of the intermediate transfer belt 31 may occur due to the contact with the secondary transfer roller 4 that has the concave portion 41 and is driven to rotate. In this embodiment, the speed fluctuation by the tension roller 35 may occur. Due to the absorption effect and the effect of blocking the speed fluctuation by the driving roller 32, the speed fluctuation is prevented from reaching the primary transfer position TR1. This eliminates the influence of speed fluctuations in image formation on the photoreceptor 21 by the image forming stations 2Y, 2M, 2C, and 2K and image transfer onto the intermediate transfer belt 31 at the primary transfer position TR1. Disturbance of the image on the belt 31 can be prevented.

  In this embodiment, the tension roller 33 is also provided at a position downstream of the secondary transfer position TR2 and upstream of the primary transfer position TR1y corresponding to the most upstream image forming station 2Y. This prevents the influence of speed fluctuation from the secondary transfer position TR2 to the primary transfer position TR1y toward the downstream side, thereby further suppressing image disturbance. Further, since the cleaner unit 39 is pressed against the surface of the intermediate transfer belt 31 wound around the tension roller 33, the tension roller 33 and the cleaner unit 39 have a braking action on the intermediate transfer belt 31, and the speed is increased. The fluctuation can be suppressed more effectively.

  Further, in this embodiment, image formation on the most downstream side of the plurality of image forming stations is further upstream than the tension roller 35 on the upstream side in the moving direction D31 of the intermediate transfer belt 31 among the two tension rollers. A belt driving roller 32 is disposed downstream of the primary transfer position TR1k corresponding to the station 2K. By providing a belt driving roller 32 connected to the belt driving motor M3 and having a small speed fluctuation due to external factors between the tension roller 35 and the primary transfer position TR1k, the speed fluctuation of the intermediate transfer belt 31 at the secondary transfer position TR2 is achieved. And the like can be more effectively suppressed from propagating to the primary transfer position TR1k.

  On the other hand, on the downstream side of the secondary transfer position TR2, the fluctuation absorption effect by the belt driving roller is limited. Therefore, in this embodiment, the circumferential length along the belt path is made longer on the downstream side of the secondary transfer position TR2 to enhance the effect of absorbing fluctuations due to the plasticity and elasticity of the belt itself, so that the image forming station on the most upstream side is provided. Propagation of speed fluctuation or the like to the primary transfer position TR1y corresponding to 2Y is suppressed. Further, by making the displacement direction of the tension roller 33 the same or substantially the same as the stretching direction of the intermediate transfer belt 31 at the primary transfer position TR1y or the like, the displacement of the tension roller 33 is prevented from adversely affecting image formation. The

  As described above, in this embodiment, the intermediate transfer belt 31 functions as the “image carrier” of the present invention, and the image forming stations 2Y, 2M, 2C, and 2K serve as the “image forming unit” of the present invention. It is functioning. In the above embodiment, the belt driving roller 32 functions as the “driving roller” of the present invention, and the belt driving motor M3 functions as the “first driving source” of the present invention. In the above embodiment, the secondary transfer backup roller 34 functions as the “roller” of the present invention.

  In the above embodiment, the secondary transfer roller 4 functions as a “transfer roller” of the present invention, and the concave portion 41 corresponds to a “notch” of the present invention. Further, the transfer roller drive motor M4 functions as a “second drive source” of the present invention, and the controller 10 and the driver 12 function as a “speed control unit” of the present invention. In this embodiment, the tension roller 35 functions as the “tension roller” of the present invention.

  Furthermore, in the above embodiment, both the operation panel 16 and the interface 15 function as an “information acquisition unit” of the present invention. The paper size detection sensor 50 also functions as the “information acquisition unit” of the present invention. In the above embodiment, the information relating to the thickness and length of the recording material given from the external device or the user, the detection output of the paper size detection sensor 50, and the like correspond to the “recording material information” of the present invention. . Further, the rotational speeds ω0 and ω1 of the transfer roller in the above-described embodiment correspond to the “first rotational speed” and the “second rotational speed” of the present invention, respectively.

  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 the above embodiment, the rotational speed of the secondary transfer roller 4 is controlled according to both the length and thickness of the recording material RM. However, these are not essential requirements, and may be either one, for example. . For example, since it is not always necessary to change the rotation speed immediately after passing through the recording material, the distribution between the first period and the second period may be fixed in advance. In this case, it is preferable to determine the distribution according to the length of the longest recording material that can be used. Furthermore, the effect of the present invention can be obtained even if the distribution between the first period and the second period and the rotation speed after the change are determined in advance, such as in an apparatus in which the size and thickness of the recording material are limited.

  In the above-described embodiment, the rotation speed of the secondary transfer roller 4 is decreased after passing through the recording material. However, the present invention is not limited to this and may be increased. Further, the rotational speed may be gradually increased or gradually decreased. In the above embodiment, the peripheral speed difference when the elastic layer 43 of the secondary transfer roller 4 directly contacts the intermediate transfer belt 31 is set to zero. However, in the secondary transfer roller 4 having the concave portion 41 as in the present embodiment, the tension of the intermediate transfer belt 31 is reset for each turn, and therefore it is not always necessary to make the difference in peripheral speed between the two zero. However, before the surface of the secondary transfer roller 4 facing the secondary transfer position TR2 is switched from the elastic layer 43 to the recess 41 in order to prevent a sudden change in tension, the rotational speed of the secondary transfer roller 4 is as described above. It is desirable to change.

  Further, for example, in the above embodiment, 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. For example, the present invention can be applied to an image forming apparatus including only one image forming station.

  In the above embodiment, the intermediate transfer belt 31, the belt driving roller 32, the secondary transfer backup roller 34, the secondary transfer roller 4, the tension rollers 33 and 35, etc. constitute the transfer unit 3 as a unit. In this case, it is not essential for the transfer unit to include a drive source for driving the transfer roller or the drive roller. For example, when the transfer unit is mounted, a motor fixed to the apparatus main body side is connected to the transfer roller or the drive roller. You may be comprised so that it may function as a drive source by engaging with a roller.

  In the above-described embodiment, 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, an image forming apparatus and a transfer apparatus having a structure in which a transfer roller having a surface shape with a part of a cylindrical peripheral surface cut out is brought into contact with an intermediate transfer belt. The present invention can be applied to the whole.

  Furthermore, the present invention can also be applied to an apparatus including a transfer roller that does not have a grip portion. For example, in an apparatus in which the 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 a simple configuration and no gripping portion.

  2Y, 2M, 2C, 2K ... image forming station (image forming unit), 4 ... secondary transfer roller (transfer roller), 10 ... controller (speed control unit), 12 ... driver (speed control unit), 15 ... interface ( (Information acquisition unit), 16 ... operation panel (information acquisition unit), 31 ... intermediate transfer belt (image carrier), 32 ... belt drive roller (drive roller), 34 ... secondary transfer backup roller (roller), 35 ... tension Roller, 41 ... concave portion (notch), 44 ... gripping portion, 50 ... paper size detection sensor (information acquisition portion), M3 ... belt drive motor (first drive source), M4 ... transfer roller drive motor (second) NP ... transfer nip, TR1 ... primary transfer position, TR2 ... secondary transfer position

Claims (8)

A belt-like image carrier for carrying an image;
A driving roller for moving the image carrier by winding the image carrier;
A first drive source for rotationally driving the drive roller;
An image forming unit for forming an image on the image carrier;
A roller around which the image carrier is wound;
A notch portion that forms a transfer nip in contact with a winding portion of the image carrier and the roller, and has a wider opening width on the peripheral surface along the rotation direction than the nip width of the transfer nip in the rotation direction. A transfer roller having
A second drive source for rotationally driving the transfer roller;
A tension roller that contacts the image carrier and applies tension to the image carrier;
During the period in which the peripheral surface of the transfer roller different from the notch is in contact with the image carrier to form a transfer nip, the image carrier and the transfer roller are in contact via a recording material, An image forming apparatus comprising: a speed control unit configured to change a rotation speed of the transfer roller depending on when the image carrier and the transfer roller are in contact with each other.   The rotation speed of the transfer roller when the image carrier and the transfer roller abut is slower than the rotation speed of the transfer roller when the image carrier and the transfer roller abut via the recording material. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein a rotation speed of the transfer roller when the image carrier and the transfer roller are in contact with each other through the recording material is constant or substantially constant.   The rotation speed of the transfer roller when the notch of the transfer roller is located in the transfer nip is the rotation speed of the transfer roller when the image carrier and the transfer roller are in contact with each other via the recording material. The image forming apparatus according to claim 3, wherein the image forming apparatus is the same as or substantially the same.   The image forming apparatus according to claim 1, wherein a rotation speed of the transfer roller is changed when the recording material passes through the transfer nip.   The information acquisition part which acquires the recording material information regarding the said recording material is provided, The said speed control part adjusts the rotational speed of the said transfer roller based on the said recording material information acquired by the previous period information acquisition part. The image forming apparatus described.   The image forming apparatus according to claim 6, wherein the recording material information is information relating to a thickness of the recording material. An image is carried on a driving roller rotated by a first driving source and a belt-like image carrier wound around the roller,
A transfer nip is formed in contact with the image carrier wound around the roller, and a notch portion having a wider opening width on the peripheral surface along the rotation direction than the nip width in the rotation direction of the transfer nip is provided. Then, using a transfer roller that rotates at a first rotation speed, the recording material is passed through the transfer nip, and the image carried on the image carrier is transferred to the recording material,
After the recording material passes through the transfer nip, the transfer roller is rotated at a second rotation speed different from the first rotation speed while the image carrier and the transfer roller are brought into contact with each other. Image forming method.

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