JP2011186168A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus, capable of reducing sudden load changes at the moment when the tip of a paper sheet is carried to a transfer position or when the rear end of the paper sheet exits from the transfer position without reducing transferability, preventing degradation of image quality, and also preventing transfer output failure due to applied voltage increase at a transfer nip interval and preventing image degradation at the tip and rear of the image. <P>SOLUTION: A secondary transfer counter roller 24 is controlled so that an image part transfer current flowing in an image part on an image carrier surface of an intermediate transfer belt 21 should be constant. Before the paper sheet enters a secondary transfer nip, the image carrier surface and the secondary transfer roller 30 are separated and after entry, both are made to contact through the paper sheet. The transfer bias when the paper sheet enters the secondary transfer nip is under constant voltage control. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, a plotter, and a multifunction machine provided with at least one of them.

Conventionally, a transfer member is pushed down by a push-down amount corresponding to the thickness of a recording medium (hereinafter referred to as “paper”), and a gap is provided between the image carrier and the transfer member. There is known one that suppresses the speed fluctuation of the image carrier that occurs when the paper enters the pressure contact portion (Patent Document 1).
In addition, there is a known one that alleviates the impact of paper entry by inserting a gap forming member between the image carrier and the transfer member to form a gap at a position where the image carrier and the transfer member abut. (Patent Document 2).
Proposed a method to mitigate the impact of entering the paper by switching the transfer pressure when the paper tip enters the transfer nip and when the image part is located, lowering the transfer pressure when entering the paper tip, and increasing it at the image part (Patent Document 3).
In addition to the configuration of Patent Document 3, a method has been proposed in which the tip portion transfer current is made higher than the image portion transfer current in order to prevent the image at the tip portion from being insufficiently transferred (Patent Document 4).
In addition, a method in which all transfer biases including the leading and trailing edges of the sheet are controlled at a constant voltage has been proposed (Patent Document 5).

  However, the one disclosed in Patent Document 1 reduces the speed fluctuation at the time of entering the paper by pushing down the transfer member with a push-down amount corresponding to the thickness of the paper to provide a gap between the image carrier and the transfer member. Although it is considered that the effect can be obtained regardless of the thickness of the paper, depending on the conditions, if the transfer member is pushed down to the area where the speed fluctuation at the time of paper entry can be suppressed, the transfer nip pressure will not be sufficient and the transferability will be reduced. On the other hand, it has been found that if the image is pushed down at a distance at which transferability does not deteriorate, an abnormal image is generated due to speed fluctuation at the time of paper entry. That is, it has been found that there is no point where transferability and speed fluctuation suppression are compatible.

According to the technique disclosed in Japanese Patent Application Laid-Open No. H10-228707, the impact of the paper entering can be reduced by inserting a gap forming member between the image carrier and the transfer member. The carrier is brought into contact with the gap forming member with a linear velocity difference, and the image carrier and the gap forming member are scraped, resulting in a problem that a long life cannot be achieved.
Further, as the gap forming member becomes thicker, it is necessary to increase the thickness of the gap forming member accordingly, and the speed fluctuation of the image carrier is generated by the impact at the moment when the gap forming member is inserted. There was a problem of becoming.
Patent Documents 3 and 4 have a configuration that reduces the impact when the paper enters by changing the transfer pressure. However, when the paper has a paper thickness of 300 g / m ² or more, it is affected by the impact. Since image disturbance occurs, it is necessary to provide a gap as in Patent Documents 1 and 2.
Also, as described in Patent Document 4, if the tip current is increased so as to provide a sufficient transfer electric field to the leading edge of the image, when a gap is provided, the applied voltage increases due to the gap and is applied at the leading or trailing edge of the paper. Voltage rises and transfer output abnormality occurs.
Further, if the current leading edge of the image is controlled at a constant current, abnormal discharge occurs at the leading edge of the image, thereby generating an abnormal image such as so-called image dust that causes toner scattering and white spots (discharge trace image) due to discharge. There was a problem.
Further, if all the transfer bias is controlled at a constant voltage as in Patent Document 5, there is a possibility that an abnormal image such as an insufficient image transfer density due to the paper thickness or usage environment may occur.

  The present invention has been made in view of such circumstances, and reduces sudden load fluctuations at the moment when the leading edge of the paper is transported to the transfer position without degrading the transferability or at the moment when the trailing edge of the paper comes out of the transfer position. Image quality deterioration due to “color misregistration” or “dot position misalignment” can be prevented, transfer output abnormality due to increase in applied voltage in the transfer nip gap, and abnormal image at the leading and trailing edges of the image can also be prevented. An object of the present invention is to provide an image forming apparatus that can perform such a process.

  In order to achieve the above object, the invention described in claim 1 is an image carrier having an image carrying surface for carrying an image, and a recording medium provided at a position facing the image carrying surface. An opposing member having a contact surface; a pressurizing unit that applies pressure to a transfer nip between the image carrying surface and the contact surface; and an contacting / separating unit that contacts and separates the image carrying surface and the contact surface. A transfer means for transferring an image on the image bearing surface to a recording medium sandwiched by the transfer nip by applying a transfer bias between the image bearing surface and the contact surface; and a recording medium in the transfer nip. In the image forming apparatus having the recording medium supply means to be supplied, the transfer means is controlled so that an image portion transfer current flowing in the image portion on the image carrying surface is constant, and the contact / separation means is The recording medium supplied from the recording medium supply means is the Before entering the copying nip, the image carrying surface and the contact surface are separated, and after the recording medium enters the transfer nip, the image carrying surface and the contact surface are brought into contact via the recording medium, The pressure applied by the pressing means is applied to the transfer nip, and the transfer bias when the recording medium enters the transfer nip is controlled at a constant voltage.

  The invention according to claim 2 is an image carrying body having an image carrying surface for carrying an image, and a facing member having a contact surface provided at a position facing the image carrying surface and contacting a recording medium, Pressurizing means for applying pressure to a transfer nip between the image carrying surface and the contact surface, contact / separation means for contacting and separating the image carrying surface and the contact surface, and the image carrying surface and the contact A transfer means for transferring an image on the image bearing surface to a recording medium sandwiched between the transfer nips by applying a transfer bias between the recording surface and a recording medium supply means for supplying the recording medium to the transfer nips; In the image forming apparatus, the transfer unit is controlled so that an image portion transfer current flowing in the image portion on the image bearing surface is constant, and the contact / separation unit is supplied from the recording medium supply unit. Before the trailing edge of the recording medium passes through the transfer nip. The image carrying surface and the contact surface are separated from each other, and after the rear end of the recording medium passes through the transfer nip, the image carrying surface and the contact surface are brought into contact with each other. The transfer bias is configured to be applied to the nip, and the transfer bias when the rear end of the recording medium passes through the transfer nip is controlled by a constant voltage.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the contacting / separating means may remove the image carrying surface and the contact surface before the rear end of the recording medium passes through the transfer nip. The transfer bias is configured to be separated, and the transfer bias when the rear end of the recording medium passes through the transfer nip is controlled by a constant voltage.
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, transfer when a recording medium enters the transfer nip and when the recording medium exits the transfer nip. The bias is characterized in that it is switched stepwise from a non-image portion bias or an image portion bias.
According to a fifth aspect of the present invention, in the image forming apparatus according to the fourth aspect, the magnitude of the transfer bias when the recording medium enters the transfer nip and when the recording medium exits the transfer nip is It is between the non-image portion bias and the image portion bias.

According to a sixth aspect of the present invention, in the image forming apparatus according to the fourth or fifth aspect, the transfer bias when the recording medium enters the transfer nip and when the recording medium exits the transfer nip is the image bias. It has the same polarity as the partial bias.
According to a seventh aspect of the present invention, in the image forming apparatus according to any one of the fourth to sixth aspects, the non-image portion bias has a polarity opposite to the image portion bias and is controlled at a constant voltage. It is characterized by.
According to an eighth aspect of the present invention, in the image forming apparatus according to any one of the first to seventh aspects, a distance between the image carrying surface and the contact surface when the contact / separation means rotates a cam. The transfer bias switching is synchronized with the drive of the cam.
According to a ninth aspect of the present invention, in the image forming apparatus according to any one of the first to eighth aspects, the image carrier is configured to superimpose and transfer images on a plurality of juxtaposed photosensitive drums. It is an intermediate transfer member to be produced.

According to the present invention, it is possible to prevent an abnormal image due to the speed fluctuation of the image carrier generated when the leading edge of the recording medium enters the nip, and to prevent a transfer failure due to insufficient nip pressure after the leading edge of the recording medium enters.
In addition, by applying constant voltage control to the transfer bias when the leading edge of the recording medium enters the transfer nip, the applied voltage does not increase in the gap, so that abnormal transfer output can be prevented and current flows in the gap. Therefore, an abnormal image (discharge trace image) at the front end of the image can be prevented.

1 is a schematic configuration diagram of a tandem type color copying machine as an image forming apparatus according to an embodiment of the present invention. The enlarged view showing the configuration of the secondary transfer nip portion is also a schematic diagram. It is sectional drawing which shows the relationship between the structure of a secondary transfer nip site | part, and an approach / separation means. It is a figure which shows the shape and displacement position of a cam. It is a figure which shows the profile of a cam displacement. FIG. 4 is an enlarged schematic diagram of a state immediately before a sheet enters a secondary transfer nip. FIG. 6 is an enlarged schematic diagram showing a state of a secondary transfer nip immediately before a thick paper is entered. FIG. 6 is an enlarged schematic diagram illustrating a state immediately after the thick paper enters the secondary transfer nip. FIG. 6 is an enlarged schematic diagram illustrating a state immediately after the thick paper exits the secondary transfer nip. 6 is a timing chart of paper, image, cam position, and secondary transfer current when two thick papers are passed. 6 is a timing chart of paper, image, cam position, and secondary transfer current when two thick papers are passed. 10 is a timing chart of paper, image, cam position, and secondary transfer current when two thick papers are passed in Comparative Example 1. 10 is a timing chart of paper, image, cam position, and secondary transfer current when two thick papers are passed in Comparative Example 2. It is a figure which shows the image pattern in a paper passing experiment, (a) is a figure which shows a halftone image, (b) is a figure which shows a patch image.

Embodiments of the present invention will be described below with reference to the drawings.
Here, an embodiment in which the present invention is applied to a tandem type color copying machine (hereinafter simply referred to as “copying machine”) as an image forming apparatus will be described.
[Example 1]
FIG. 1 is a schematic configuration diagram showing a copying machine according to the present embodiment. The copier includes a printer unit 100, a paper feeding unit 200, a scanner unit 300 attached on the printer unit 100, and an automatic document feeder (ADF) 400 attached on the scanner unit 300. ing.
The printer unit 100 includes an endless belt-like intermediate transfer belt 21 as an image carrier and an intermediate transfer member. The intermediate transfer belt 21 is wound around the driving roller 22, the driven roller 23, and the secondary transfer counter roller 24 in a posture in which the side view is an inverted triangular shape, and is driven by rotation of the driving roller 22. It can be moved endlessly in the clockwise direction in the figure. Above the intermediate transfer belt 21, there are four image forming units 1C, M, Y, and K for forming C (cyan), M (magenta), Y (yellow), and K (black) toner images. Arranged along the belt moving direction.

The image forming units 1C, M, Y, and K include photoreceptors 2C, M, Y, and K, developing units 3C, M, Y, and K, and photoreceptor cleaning devices 4C, M, Y, and K. Yes. The photoreceptors 2C, M, Y, and K abut on the intermediate transfer belt 21 to form primary transfer nips for C, M, Y, and K, respectively, and are driven in a counterclockwise direction in FIG. It can be driven to rotate. The developing units 3C, M, Y, and K are for developing the electrostatic latent image formed on the photoreceptors 2C, M, Y, and K with C, M, Y, and K toners. Further, the photoconductor cleaning devices 4C, M, Y, and K clean the transfer residual toner attached to the photoconductors 2C, M, Y, and K after passing through the primary transfer nip.
In this printer, a tandem image forming unit 10 is configured by four image forming units 1C, M, Y, and K arranged in the belt moving direction.

In the printer unit 100, an optical writing unit 15 is disposed above the tandem image forming unit 10. The optical writing unit 15 performs an optical writing process by optical scanning on the surfaces of the photoreceptors 2C, M, Y, and K that are rotationally driven counterclockwise in the drawing to form an electrostatic latent image. Is. Prior to the optical writing process, the surfaces of the photoreceptors 2C, M, Y, and K are uniformly charged by the uniform charging means of the image forming units 1C, M, Y, and K, respectively.
The transfer unit 20 including the intermediate transfer belt 21 has primary transfer rollers 25C, M, Y, and K inside the loop of the intermediate transfer belt 21. These primary transfer rollers 25C, M, Y, and K press the intermediate transfer belt 21 toward the photoreceptors 2C, M, Y, and K on the back side of the primary transfer nip for C, M, Y, and K. Yes.

A secondary transfer roller 30 as an opposing member is disposed below the intermediate transfer belt 21. The secondary transfer roller 30 abuts on the intermediate transfer belt 21 around the secondary transfer counter roller 24 from the belt front surface side to form a secondary transfer nip (hereinafter also simply referred to as “nip”). ing. A sheet is fed into the secondary transfer nip at a predetermined timing. Then, the four-color superimposed toner image on the intermediate transfer belt 21 is batch-transferred onto the sheet at a secondary transfer nip as a transfer nip.
The scanner unit 300 reads image information of a document placed on the contact glass 301 by the reading sensor 302 and sends the read image information to the control unit of the printer unit 100. A control unit (not shown) controls a light source such as a laser diode or an LED in the optical writing unit 15 of the printer unit 100 based on the image information received from the scanner unit 300, and lasers for C, M, Y, and K. Writing light is emitted to optically scan the photoreceptors 2C, M, Y, and K. By this optical scanning, electrostatic latent images are formed on the surfaces of the photoreceptors 2C, M, Y, and K, and the latent images are developed into C, M, Y, and K toner images through a predetermined development process.

The paper feed unit 200 includes paper cassettes 202 arranged in multiple stages in the paper bank 201, a paper feed roller 203 that feeds paper from the paper feed cassette 202, and a separation that separates the fed paper and guides it to the paper feed path 204. A roller 205, a conveyance roller 206 that conveys the sheet to the sheet feeding path 99 of the printer unit 100, and the like are provided.
In addition to the paper feed unit 200, manual paper feed is also possible, and the manual feed tray 98 for manual feed and the paper on the manual feed tray 98 are separated one by one toward the manual feed path 97. A separation roller 96 is also provided. In the printer unit 100, the manual paper feed path 97 joins the paper feed path 99.
Near the end of the paper feed path 99, a registration roller pair 95 is disposed. The registration roller pair 95 sandwiches the paper conveyed in the paper feed path 99 between the rollers and then feeds it toward the secondary transfer nip at a predetermined timing.

In the copying machine according to the present embodiment, when a color image is copied, the original is set on the original plate 401 of the ADF 400 or the ADF 400 is opened and the original is set on the contact glass 301 of the scanner unit 300. Press to close the document. Then, a start switch (not shown) is pressed. Then, when the document is set on the ADF 400, the document is conveyed onto the contact glass 301. Thereafter, the scanner unit 300 starts driving, and the first traveling body 303 and the second traveling body 304 start traveling along the document surface.
Then, the light emitted from the light source by the first traveling body 303 is emitted on the document surface, and the obtained reflected light is folded and directed to the second traveling body 304. The folded light is further folded by the mirror of the second traveling body 304 and then enters the reading sensor 302 through the imaging lens 305. Thereby, the content of the original is read.

When the printer unit 100 receives image information from the scanner unit 300, the printer unit 100 feeds a sheet having a size corresponding to the image information to the sheet feeding path 99. Along with this, the drive roller 22 is rotationally driven by a drive motor (not shown) to move the intermediate transfer belt 21 endlessly in the clockwise direction in the drawing.
At the same time, after starting rotation of the photosensitive members 2C, M, Y, and K of the image forming units 1C, M, Y, and K, uniform charging processing, optical writing processing, and the like for the photosensitive members 2C, M, Y, and K are performed. Perform development processing. The C, M, Y, and K toner images formed on the surfaces of the photoreceptors 2C, M, Y, and K by these processes are sequentially overlapped at the C, M, Y, and K primary transfer nips to be intermediate. Primary transfer is performed on the transfer belt 21 to form a four-color superimposed toner image.

In the paper feed unit 200, one of the paper feed rollers 203 is selectively rotated according to the paper size, and paper is sent out from one of the three paper feed cassettes 202. The fed sheets are separated one by one by the separation roller 4205, introduced into the sheet feed path 206, and then sent to the sheet feed path 99 in the printer unit 100 via the transport roller 206. When the manual feed tray 98 is used, the paper feed roller of the tray is driven to rotate, and the paper on the tray is fed to the manual paper feed path 97 while being separated by the separation roller 96 to be near the end of the paper feed path 99. To. In the vicinity of the end of the paper feed path 99, the paper is stopped by abutting the leading edge against the registration roller pair 95.
Thereafter, when the registration roller pair 95 is rotationally driven at a timing that can synchronize with the four-color superimposed toner image on the intermediate transfer belt 21, it is fed into the secondary transfer nip and is in close contact with the four-color superimposed toner image on the belt. . Then, the secondary transfer is performed collectively on the paper due to the influence of the nip pressure and the transfer electric field.

The sheet on which the four-color superimposed toner image is secondarily transferred at the secondary transfer nip is fed into the fixing device 71 by the sheet conveying belt 70. When the fixing device 71 is sandwiched in the fixing nip between the pressure roller 72 and the fixing belt 73, the four-color superimposed toner image is fixed on the surface by pressure or heat treatment.
The paper on which the color image is formed in this manner is stacked on a paper discharge tray 75 outside the apparatus via a discharge roller pair 74.
When an image is also formed on the other side of the sheet, the sheet is discharged from the fixing device 71 and then sent to the sheet reversing device 75 by the path switching by the switching claw 76. Then, after being turned upside down, it is returned to the registration roller pair 95 again, and then goes through the secondary transfer nip and the fixing device 71 again.
After passing through the secondary transfer nip, the belt cleaning device 26 contacts the surface of the intermediate transfer belt 21 before entering the C primary transfer nip where the primary transfer process is the most upstream of the four colors. ing. The belt cleaning device 26 cleans the transfer residual toner adhering to the belt surface.

FIG. 2 is an enlarged schematic view showing the secondary transfer nip and the surrounding configuration in the printer unit 100 of the copying machine according to the present embodiment.
In the figure, a secondary transfer counter roller 24 that partially wraps the belt around its peripheral surface inside the loop of the intermediate transfer belt 21 backs up the deformable intermediate transfer belt 21 on its peripheral surface. It plays the role of maintaining the shape along a certain curvature.
A secondary transfer roller 30 is in contact with the secondary transfer counter roller 24 on the intermediate transfer belt 21 from the belt front surface side to form a secondary transfer nip.
The secondary transfer roller 30 is rotatably held by the roller unit holder 40 via a bearing (not shown). The roller unit holding body 40 is configured to be rotatable about a rotation shaft 40 a disposed so as to take a posture parallel to the rotation axis of the secondary transfer roller 30. When the roller unit holder 40 rotates counterclockwise in the figure around the rotation shaft 40a, the secondary transfer roller 30 held by the roller unit holder 40 is pressed against the intermediate transfer belt 21 and 2 A next transfer nip is formed.
When the roller unit holding body 40 rotates around the rotation shaft 40a in the clockwise direction in the figure, the secondary transfer roller 30 held by the roller unit holding body 40 is separated from the intermediate transfer belt 21.

In the copier according to the present embodiment, the end of the roller unit holding body 40 opposite to the rotating shaft 40a is always urged toward the intermediate transfer belt 21 by the urging coil spring 45 as the pressing means. . The urging coil spring 45 always applies a force to the roller unit holder 40 to rotate counterclockwise in the drawing around the rotation shaft 40a, thereby causing the secondary transfer roller 30 to move to the intermediate transfer belt 21. It is energizing towards.
The secondary transfer roller 30 is rotationally driven in the counterclockwise direction in the figure by transmitting the rotational driving force of a roller driving motor (not shown) via a drive transmission means such as a gear (not shown). These roller drive motors and drive transmission means are also held by the roller unit holder 40 and rotated together with the secondary transfer roller 30 and the roller unit holder 40. The roller unit holder 40 also holds a cleaning blade 39, a solid lubricant 41, a lubricant pushing device 43, and the like.

The toner on the belt adheres to the surface of the secondary transfer roller 30 that is in contact with the front surface of the intermediate transfer belt 21 that carries the toner image. If this adhering toner is left as it is, it will be transferred to the back side of the paper at the secondary transfer nip, and so-called back dirt will be generated. Therefore, in this copying machine, the toner is mechanically removed from the surface of the secondary transfer roller 30 by bringing the edge of the cleaning blade 39 into contact with the surface of the secondary transfer roller 30. In such a configuration, a load impeding the rotation is applied to the secondary transfer roller 30 due to the contact of the cleaning blade 39, so that the secondary transfer roller 30 is driven to rotate along with the intermediate transfer belt 21. I can't.
For this reason, the secondary transfer roller 30 is rotationally driven by the roller driving motor described above.

The lubricant pressing device 43 applies the lubricant powder to the secondary transfer roller 30 by pressing the solid lubricant 41 made of a zinc stearate block or the like against the secondary transfer roller 30 by the biasing coil spring 42. By applying the lubricant in this way, an increase in rotational load due to contact between the cleaning blade 39 and the secondary transfer roller 30 is suppressed. In addition, occurrence of blade edge entrainment is also suppressed.
Instead of pressing the solid lubricant 41 against the secondary transfer roller 30, a rotary application brush for applying the lubricant to the secondary transfer roller 30 while scraping the lubricant from the solid lubricant 41 may be provided.

FIG. 3 is an enlarged cross-sectional view showing a configuration around the secondary transfer nip.
The secondary transfer roller 30 includes a roller portion 31, a first shaft member 32 and a second shaft member 33 that protrude from both end surfaces in the axial direction thereof and extend in the rotational axis direction, and a first idle roller 34 described later. And a second idling roller 35.
The roller portion 31 includes a cylindrical hollow core 31a, an elastic layer 31b made of an elastic material fixed to the peripheral surface thereof, and a surface layer 31c fixed to the peripheral surface thereof.
Examples of the metal constituting the hollow core 31a include stainless steel and aluminum, but are not limited to these materials. The elastic layer 31b is preferably 70 [°] or less in terms of JIS-A hardness. However, since the cleaning blade 39 is in contact with the roller portion 31, various problems are caused if the elastic layer 31b is too soft. Therefore, it is desirable that the elastic layer 31b has a JIS-A hardness of 40 [°] or more. An elastic layer 31b having a JIS-A hardness of about 50 [°] is formed of epichlorohydrin rubber exhibiting a certain degree of conductivity.

As a rubber material exhibiting conductivity, EPDM or Si rubber in which carbon is dispersed, NBR having an ionic conductivity function, urethane rubber, or the like may be used instead of the above-described conductive epichlorohydrin rubber. Since many rubber materials exhibit good chemical affinity for the toner or exhibit a relatively large coefficient of friction, the surface of the elastic layer 31b made of rubber is covered with the surface layer 31c. ing.
As a result, toner adhesion to the surface of the roller unit 31 is suppressed, and the frictional load with the blade is reduced. As a material for the surface layer 31c, a material in which a resistance adjusting material such as carbon or an ionic conductive agent is contained in a fluororesin resin that exhibits a low coefficient of friction and good toner releasability is suitable.
When the secondary transfer roller 30 rotates in contact with the intermediate transfer belt 21, it may have a slight linear velocity difference from the belt. The friction coefficient of the surface layer 31c is adjusted to 0.3 or less so that the belt does not slip due to this linear speed difference.
The intermediate transfer belt 21 is required to be driven at a constant speed in order to transfer the images of each color in a superimposed manner without color misregistration. Therefore, it is necessary to reduce the surface friction resistance of the surface layer 31c of the secondary transfer roller 30. is important.

The secondary transfer roller 30 having such a configuration is biased toward the intermediate transfer belt 21 that is wound around the secondary transfer counter roller 24. The secondary transfer counter roller 24 that is wound around the intermediate transfer belt 21 passes through the roller portion 24b, which is a cylindrical main body portion, and the rotation center of the roller portion 24b in the rotation axis direction, and the roller portion 24b. And a through shaft member 24a that idles on its surface.
The penetrating shaft member 24a is made of metal, and freely rotates the roller portion 24b freely on its peripheral surface. The roller part 24b as the main body part is composed of a drum-shaped hollow cored bar 24c, an elastic layer 24d made of an elastic material fixed on the peripheral surface of the drum-shaped hollow cored bar 24c, and balls press-fitted to both ends in the axial direction of the hollow cored bar 24c And a bearing 24e.
The ball bearing 24e rotates on the penetrating shaft member 24a together with the hollow core bar 24c while supporting the hollow core bar 24c. The elastic layer 24d is press-fitted into the outer peripheral surface of the hollow cored bar 24c.

The through-shaft member 24 a is rotatable by a first bearing 52 fixed to the first side plate 28 of the transfer unit that stretches the intermediate transfer belt 21 and a second ball bearing 53 fixed to the second side plate 29. It is supported. However, most of the time during the print job is stopped without being driven to rotate.
Then, the roller portion 24b that is to be rotated along with the endless movement of the intermediate transfer belt 21 is freely idled on its own peripheral surface.
The elastic layer 24d fixed on the peripheral surface of the hollow core metal 24c is composed of a conductive rubber material whose resistance value is adjusted by adding an ionic conductive agent so as to exhibit a resistance of 7.5 [LogΩ] or more. Has been. The reason why the electric resistance of the elastic layer 24d is adjusted within a predetermined range is that there is no paper interposition in the secondary transfer nip when using a paper having a relatively small size in the roller axial direction such as A5 size. This is because the transfer current is prevented from being concentrated at the position where the belt and the roller are in direct contact with each other. By setting the electric resistance of the elastic layer 24d to a value larger than the resistance of the sheet, it is possible to suppress such transfer current concentration.
In addition, as the conductive rubber material constituting the elastic layer 24d, foamed rubber is used so as to exhibit elasticity of about 40 [°] in Asker-C hardness. By forming the elastic layer 24d with such foamed rubber, the elastic layer 24d is flexibly deformed in the thickness direction in the secondary transfer nip to form a secondary transfer nip having a certain size in the sheet conveying direction. can do.

In this copying machine, as described above, for the convenience of bringing the cleaning blade 39 into contact with the secondary transfer roller 30, it is possible to use an elastic material as the material of the roller portion of the secondary transfer roller 30. Have difficulty. Therefore, instead of the secondary transfer roller 30, the roller portion 24b of the secondary transfer counter roller 24 is elastically deformed.
In the penetrating shaft member 24a of the secondary transfer counter roller 24, the two end regions that are not located in the roller portion 24b in the entire longitudinal direction are respectively abutted against the secondary transfer roller 30. A cam as a member is fixed so as to rotate integrally with the penetrating shaft member 24a.

Specifically, the first cam 50 is fixed to one end region in the longitudinal direction of the penetrating shaft member 24a. In the first cam 50, a cam portion 50a and a true circular roller portion 50b are integrally formed side by side in the axial direction. The first cam 50 is fixed to the penetrating shaft member 24a by screwing the screw 80 penetrating the roller portion 50b into the penetrating shaft member 24a.
A second cam 51 having the same configuration as that of the first cam 50 is fixed to the other end region in the longitudinal direction of the penetrating shaft member 24a.
A drive receiving pulley 54 is fixed to a region outside the second cam 51 in the axial direction of the penetrating shaft member 24a. A detected disk 59 is fixed on the further outside of the drive receiving pulley 54.

On the other hand, the cam drive motor 58 is fixed to the second side plate 29 of the transfer unit, the motor pulley 57 on the cam drive motor 58 shaft is rotated, and is fixed to the above-described through shaft member 24 a via the timing belt 56. The driving force is transmitted to the drive receiving pulley 54. With the above configuration, the penetrating shaft member 24a can be rotated by driving the cam drive motor 58. At this time, even if the penetrating shaft member 24a is rotated, the roller portion 24b can be freely idled on the penetrating shaft member 24a, so that the rotation of the roller portion 24b by the belt 21 is not hindered. .
As the cam drive motor 58, a stepping motor is used so that the motor rotation angle can be freely set without providing a rotation angle detection means such as an encoder.
When the through-shaft member 24a stops rotating at a predetermined rotation angle, the first cam 50 and the second cam 51 respectively abut the secondary transfer roller 30 against the secondary transfer roller 30 and the roller unit. It pushes back against the urging force of the urging coil spring 45 of the holding body.
As a result, the distance between the secondary transfer counter roller 24 and the secondary transfer roller 30 is adjusted by moving the secondary transfer roller 30 away from the secondary transfer counter roller 24 (and thus the intermediate transfer belt 21). To do.

In this configuration, the distance between the secondary transfer counter roller 24 and the secondary transfer roller 30 is adjusted by the first cam 50, the second cam 51, the cam drive motor 58, various gears, the roller unit holder 40 described above, and the like. Distance adjusting means (contact / separation means) is configured.
The secondary transfer counter roller 24 as a rotatable support rotator freely idles the roller portion 24b on a penetrating shaft member 24a penetrated with respect to the cylindrical roller portion 24b.
When the penetrating shaft member 24a rotates, the cams (50, 51) fixed to both ends in the axial direction of the penetrating shaft member 24a rotate together, so that the drive shaft is transmitted to the penetrating shaft member 24a. It is possible to rotate the cams on both ends only by providing the drive transmission mechanism on one end in the axial direction.

In this copying machine, as shown in FIG. 2, the hollow cored bar 31a of the secondary transfer roller 30 is grounded (indicated by GND), while the toner is in contact with the hollow cored bar 24c of the secondary transfer counter roller 24. A secondary transfer bias having the same polarity as that is applied. As a result, a secondary transfer electric field for electrostatically moving the toner from the secondary transfer counter roller 24 side to the secondary transfer roller 30 side is formed in the secondary transfer nip between the two rollers.
As shown in FIG. 3, the first bearing 52 that rotatably receives the metal penetrating shaft member 24 a of the secondary transfer counter roller 30 is composed of a conductive slide bearing. A high voltage power supply 61 that outputs a secondary transfer bias is connected to the conductive first bearing 52. The secondary transfer bias output from the high-voltage power supply 61 is guided to the secondary transfer counter roller 24 via the conductive first bearing 52.
That is, in the secondary transfer counter roller 24, the transfer bias is made of a metal penetrating shaft member 24a, a metal ball bearing 24e, a metal hollow core metal 24c, and a conductive elastic layer 24d in order. It will be transmitted.

The detected disk 59 fixed to one end of the through shaft member 24a has a detected portion 59a that rises in the axial direction at a predetermined position in the rotation direction of the through shaft member 24a. On the other hand, the optical sensor 60 is fixed to the sensor bracket 55 fixed to the second side plate 29 of the transfer unit.
In the process of rotating the through shaft member 24a, when the through shaft member 24a is positioned within a predetermined rotation angle range, the detected portion 59a of the detected disk 59 enters between the light emitting element and the light receiving element of the optical sensor 60. The optical path between the two is blocked.
When the light receiving element of the optical sensor 60 receives light from the light emitting element, it transmits a light receiving signal to a control unit (not shown). The control unit rotates the cam portion of the cam (50, 51) fixed to the penetrating shaft member 24a based on the timing at which the light reception signal from the light receiving element is interrupted or the drive amount of the cam drive motor 58 from that timing. Know the angular position.

As described above, the cam (50, 51) abuts against the secondary transfer roller 30 at a predetermined rotation angle, and the secondary transfer counter roller 24 resists the urging force of the urging coil spring 45 against the secondary transfer roller 30. Push it back in the direction away from it. The amount of pushing back at this time (hereinafter referred to as the amount of pushing down) is determined by the rotational angle position of the cam (50, 51).
Note that the distance between the secondary transfer counter roller 24 and the secondary transfer roller 30 increases as the pressing amount of the secondary transfer roller 30 increases.
In the secondary transfer roller 30, a first idling roller 34 is provided on the first shaft member 32 that rotates integrally with the roller portion 31 so as to be idling. The first idling roller 34 has a donut disk shape whose outer diameter is slightly larger than that of the roller portion 31. And it has a function as a ball bearing itself, and can idle | slide on the surrounding surface of the 1st shaft member 32. FIG.
A second idling roller 35 having the same configuration as the first idling roller 34 is provided on the second shaft member 33 of the secondary transfer roller 30 so as to be idling.

As described above, in the secondary transfer counter roller 24, the cams (50, 51) fixed to the through shaft member 24a come into contact with the idling rollers (34, 35) at a predetermined rotational angle position. .
Specifically, the first cam 50 fixed to one end side of the penetrating shaft member 24 a hits the first idling roller 34 of the secondary transfer roller 24. At the same time, the second cam 51 fixed to the other end side of the through shaft member 24 a abuts against the second idling roller 35 of the secondary transfer roller 24.
The idling rollers (34, 35) abutted against the cams (50, 51) of the secondary transfer counter roller 24 are prevented from rotating along with the abutment, whereby the rotation of the secondary transfer roller 30 is prevented. There is no hindrance. Even if the idling roller (34, 35) stops rotating, the idling roller is a ball bearing, so the shaft member (32, 33) of the secondary transfer roller 30 can rotate freely independently from the idling roller. Because it can be done. By stopping the rotation of the idling rollers (34, 35) in accordance with the abutment of the cams (50, 51), the occurrence of friction between them is avoided, and the belt drive motor and the secondary transfer roller 30 due to the friction are avoided. It is also possible to avoid the torque increase of the drive motor.

4 is a view showing the outer diameter shape of the cam (50, 51) of the secondary transfer counter roller 24, and FIG. 5 is a view showing the cam displacement profile of the cam (50, 51).
The cam (50, 51) has a shape with two stages of displacement, and can be set to four positions of cam positions A, B, C, D depending on the cam rotation angle. Become.
The cam position A and the cam position C have the same outer diameter, the cam position B has a displacement δ = 1 mm (the outer diameter is set to be 1 mm larger than the cam positions A and C), and the cam position D has a displacement. The amount δ is 0.7 mm (the outer diameter is set to be 0.7 mm larger than the cam positions A and C).
When the cam position B and the cam position D are in contact with the idling rollers (34, 35) on the secondary transfer roller shaft, the secondary transfer roller is pushed down, and the distance between the secondary transfer roller and the secondary transfer counter roller is reduced. It was configured so that it could be changed.

At the cam position A and the cam position C, the cam outer diameter is set so as not to perform the pressing operation of the secondary transfer roller.
The cam displacement profile is symmetrical. Specifically, by making the displacement profile from the center of the cam position A to the center of the cam position B and the displacement profile from the center of the cam position B to the center of the cam position C the same, Even if it is rotated, it is configured to be displaced in the same manner.
Similarly, the displacement profile from the cam position C center to the cam position D center and the displacement profile from the cam position D center to the cam position A are made the same shape.

FIG. 6 is an enlarged schematic diagram showing a state of the secondary transfer nip immediately before the plain paper P1 as the paper is entered. In this copying machine, when the plain paper P1 enters the secondary transfer nip, the cams (50, 51) of the secondary transfer counter roller 24 are moved to idle rollers (34) of the secondary transfer roller 30 as shown in the figure. , 35), the rotation of the through-shaft member 24a of the secondary transfer counter roller 24 is stopped at a position where it does not abut (position where the cam position C faces downward).
That is, when the plain paper P1 is used, the secondary transfer roller 30 is not pushed down by the cam (50, 51). This is because the plain paper P1 having a relatively small thickness does not cause a large load fluctuation on the belt or the secondary transfer roller 30 when entering the secondary transfer nip even if the secondary transfer roller 30 is not pushed down.
In FIG. 6, the strict distinction between the solid line and the broken line of the overlapping members in the axial direction is omitted as appropriate (the same applies to other drawings).

7, 8 and 9 are enlarged schematic views showing the state of the secondary transfer nip during the passing of the thick paper P2 as the paper.
7 shows a state of the secondary transfer nip immediately before the thick paper P2 as the paper enters, FIG. 8 shows a state immediately after the thick paper P2 enters the secondary transfer nip, and FIG. 9 shows the state of the thick paper P2 in the secondary transfer nip. It is an expansion schematic diagram which shows the state immediately after leaving.
In this copying machine, when the thick paper P2 is made to enter the secondary transfer nip, the cams (50, 51) of the secondary transfer counter roller 24 are set to idle rollers (34) of the secondary transfer roller 30 as shown in FIG. , 35), the rotation of the penetrating shaft member 24a of the secondary transfer counter roller 24 is stopped at the position where it abuts (cam position B).
That is, when the thick paper P2 is used, the secondary transfer roller 30 is pushed down by the cams (50, 51), and the secondary transfer roller and the secondary transfer counter roller are set to have an interval X.
As described above, the gap X is formed between the secondary transfer roller 30 and the secondary transfer counter roller 24, so that even when the thick paper P2 enters, the belt when entering the secondary transfer nip. In addition, large load fluctuations on the secondary transfer roller 30 do not occur.

However, if the sheet is passed while the secondary transfer roller is pressed down, a sufficient nip pressure for transfer cannot be obtained, and the transferability of the toner image decreases. In particular, the transfer rate of the paper with poor surface smoothness was significantly reduced.
Therefore, in the present invention, immediately after the sheet enters the secondary transfer nip, the cam (50, 51) of the secondary transfer counter roller 24 is moved to the idling roller (34, 35) of the secondary transfer roller 30 as shown in FIG. ), The cam is rotated clockwise and stopped at a position where the cam position C is downward.
This rotation operation is performed immediately after the sheet enters the nip, and must be completed before the toner image reaches the nip.

During image transfer, the cam (50, 51) of the secondary transfer counter roller 24 does not abut against the idling roller (34, 35) of the secondary transfer roller 30 (position where the cam position C is downward). Keep on.
As shown in FIG. 9, after the image on the intermediate transfer belt is transferred, the cam (50, 51) of the secondary transfer counter roller 24 rotates in the reverse direction until the trailing edge of the sheet passes through the secondary transfer nip. (In this embodiment, it is rotated counterclockwise) and stopped so as to be in a position (cam position B) that abuts against the idling roller (34, 35) of the secondary transfer roller 30.
This is because, in the thick paper P2, even when the secondary transfer nip is discharged, a large load fluctuation is generated on the belt and the secondary transfer roller 30, so that the secondary transfer roller is pushed down in the same manner as the front end of the paper. It becomes possible to prevent.
Subsequently, by stopping the cam (so that the cam position B is downward) so that the secondary transfer roller is pushed down until the next sheet leading edge enters the secondary transfer nip, Similar to the first sheet, it is possible to prevent load fluctuations on the belt and the secondary transfer roller that occur when the second sheet enters the secondary transfer nip.
After the leading edge of the second sheet has entered the transfer nip, the operation after the leading edge of the first sheet has been rotated in the reverse direction (counterclockwise in this embodiment), and the leading edge is within the width range. Thus, the cam (50, 51) of the secondary transfer counter roller 24 is rotated at a position where the cam (50, 51) does not abut against the idling roller (34, 35) of the secondary transfer roller 30 (a position where the cam position A faces downward). Let

As shown in FIGS. 4 and 5, by changing the cam moving position at the time of passing the first sheet and passing the second sheet by making the displacement profile of the cam symmetrical, The service life of the cam surface is improved.
If the displacement profile is not symmetric, for example, when (1) and (2) in FIG. 4 are asymmetric, forward and reverse rotation is repeatedly used at the inclination of either (1) or (2). It becomes half of the invention.
Depending on the basis weight of the paper, there may be a case where a large belt speed fluctuation is observed after a while from the leading edge of the sheet. For example, when a sheet having a basis weight of 300 [g / m ² ] is passed at the cam position B (displacement amount δ: 1 mm) in FIGS. 4 and 5, the belt speed does not change from the time of entering the sheet to the time of discharging. On the other hand, at a basis weight of 160 [g / m ² ], there is no belt speed fluctuation at the time of paper entry and discharge, but speed fluctuation occurs when the depression is released.

This is due to the vibration of the secondary transfer roller coming into contact with the secondary transfer counter roller when the secondary transfer roller is released by rotating the cam that was pressing down the secondary transfer roller, and the speed fluctuation of the intermediate transfer belt It is because it is causing.
Therefore, the cam position D (displacement amount δ: 0.7 mm) in which the pressing amount of the secondary transfer roller is reduced is used to suppress the vibration when the pressing is released. However, at a basis weight of 160 [g / m ² ], the belt speed is reduced. While there was no change, the basis weight of 300 [g / m ² ] caused a small amount of push-down, so that speed fluctuation occurred when the paper entered.
Thus, since it is necessary to change the amount of depression depending on the sheet thickness, the copying machine is provided with a thickness information acquisition unit that acquires thickness information of the sheet supplied to the secondary transfer nip.
As the thickness information acquisition means, a thickness detection sensor that actually detects the thickness of the paper conveyed in the paper feed path 99 may be used, or a data input means that receives thickness information data input from an operator is used. May be. Examples of the thickness detection sensor include an optical sensor that detects light transmittance in the thickness direction, and a sensor that detects the amount of roller movement when a sheet is sandwiched between a pair of conveyance rollers.

The control unit which is a part of the distance adjusting unit is configured to adjust the amount of depression of the secondary transfer roller 30 according to the acquisition result by the thickness information acquiring unit.
Specifically, the data storage means such as a ROM in the control unit stores a data table indicating the relationship between the thickness of the paper and the corresponding rotation stop position (agreement with the amount of depression) of the through shaft member 24a. Yes.
The rotation stop position corresponding to the acquisition result of the sheet thickness is specified from the data table, and the through shaft member 24a is rotated to the rotation stop position to determine the stop position of the multistage cam, and then the sheet is secondarily transferred. The control unit is configured to execute a process for entering the nip. Thus, by setting the secondary transfer pressing amount suitable for the thickness of the sheet (setting the cam position), it is possible to suppress both the shock when the sheet leading edge enters and the shock when releasing the pressing.
As to the rotation stop position of the through shaft member 24a, as described above, the timing at which the optical sensor 60 detects the detected portion 59a of the detected disk 59 and the driving of the stepping motor that is the cam drive motor 58 from the timing. The control unit is configured to grasp based on the quantity.

  In this embodiment, the case where the multi-stage cam for adjusting the distance between the secondary transfer roller and the secondary transfer counter roller is provided in the secondary transfer counter roller, but the distance between the secondary transfer roller and the secondary transfer counter roller is shown. Of course, it is possible to adjust the width of the secondary transfer roller by, for example, providing a pressing member (cam) on the roller unit holding body 40 and a pressure means on the secondary transfer counter roller side, It may be configured to adjust the distance between the secondary transfer counter roller and the secondary transfer counter roller.

Next, the relationship between the sheet, the cam position, and the secondary transfer bias will be described.
Figure 10 is a sheet when a thick paper P ₂ and 2 sheets of paper as the paper, image, cam position, is a timing chart of the secondary transfer bias.
In the first exemplary embodiment, the secondary transfer bias is a repulsive transfer method in which constant current control is performed in the image portion and a bias having the same polarity as the toner (negative polarity in the present exemplary embodiment) is applied to the secondary transfer counter roller. Used.
The secondary transfer bias during non-image area and cam rotation is controlled by constant voltage.

When printing of the first sheet is started, the secondary transfer roller comes into contact with the intermediate transfer belt. The cam at that time is at position A in this embodiment. At this time, the secondary transfer bias of the non-image part is applied (+800 V in this embodiment). Next, when the thick paper approaches the secondary transfer nip, the cam starts to move to the position B (displacement amount δ: 1 mm). The secondary transfer bias is switched to the bias at the time of cam rotation in synchronization with the rotation of the cam (+200 V in this embodiment).
When the cam reaches position B, the secondary transfer roller and the secondary transfer counter roller are separated from each other. The cardboard P ₂ is in a separated state is conveyed, reaches the secondary transfer nip, is that is simultaneously cam the paper tip has entered starts forward operation (rotation clockwise in this example).

Next, when the leading edge of the image reaches the nip, the cam moves to position C and stops, the secondary transfer roller and the secondary transfer counter roller are in contact with each other, and the secondary transfer bias is controlled to a constant current control. It is switched to an image portion current (-30 μA in this embodiment).
In this way, the secondary transfer bias (bias at the time of cam rotation) when the secondary transfer roller and the secondary transfer counter roller are separated from each other is controlled at a constant voltage, thereby suppressing an increase in the applied voltage in the gap and causing a machine abnormality. (Transfer output error) is prevented.
In addition, the bias at the time of cam rotation is set to a magnitude between the non-image portion bias and the image portion bias, that is, switching in stages, thereby smoothly switching the transfer bias at the leading edge of the image, due to insufficient transfer current in the image portion. Transfer defects are prevented.
When the recording medium enters the transfer nip and when the recording medium exits from the transfer nip, the size of the transfer bias is between the non-image portion bias and the image portion bias, so that the front and rear end portions of the recording medium are sufficient. A transfer current can be applied, and an abnormal image at the front and rear end portions of the recording medium can be prevented.

Next, printing is performed by applying an image portion transfer current while the secondary transfer roller and the secondary transfer counter roller are in contact with each other, and when the rear end of the image reaches the nip, the cam rotates backward to position B (this embodiment) In the example, it starts to rotate counterclockwise) and starts the separation operation.
In synchronization with the start of the reverse rotation operation, the secondary transfer bias is switched from the image portion bias, which is constant current control, to the bias at the time of cam rotation, which is constant voltage control.
When the trailing edge of the sheet reaches the nip, the cam moves to the position B and stops, and the secondary transfer bias is switched from the bias at the cam rotation to the non-image portion bias in synchronization with the stop of the cam operation.
Also at the rear end of the image, similarly to the front end, the secondary transfer bias is lowered stepwise to suppress an increase in applied voltage in the gap when the secondary transfer roller and the secondary transfer counter roller are separated from each other. This prevents abnormal output.
The occurrence of an instantaneous speed increase of the image carrier due to a sudden load drop when the recording medium leaves the transfer nip can be suppressed, and the transfer bias when the recording medium trailing edge exits the nip is controlled with a constant voltage. As a result, the applied voltage does not increase in the gap, so that abnormal transfer output can be prevented, and no current flows in the gap, so that an abnormal image (discharge trace image) at the leading edge of the image can be prevented.
By switching the transfer bias stepwise, a sufficient transfer bias can be applied to the front and rear end portions of the recording medium, and an abnormal image at the front and rear end portions of the recording medium can be prevented.

Subsequently, the second sheet is conveyed to the nip, and when the leading edge of the sheet reaches the nip, the cam rotates in the opposite direction to the first sheet (in this embodiment, the reverse operation: rotating counterclockwise) In synchronization with this rotation start, the secondary transfer bias is switched to the cam rotation bias.
After that, the second sheet is printed while the secondary transfer roller and the secondary transfer counter roller are brought into contact with and separated from each other by the rotation of the cam, and the secondary transfer bias is switched in synchronization with the cam rotation. Is printed, the cam moves to position C and stops.
At this time, the non-image portion bias is turned off in synchronization with the cam stop and the printing is finished.
As described above, the contact and separation between the secondary transfer roller and the secondary transfer counter roller and the secondary transfer current are switched in stages using the margin from the leading edge of the sheet to the leading edge of the image and the margin from the trailing edge of the image to the trailing edge of the sheet. Thus, an abnormal image due to belt speed fluctuations, an abnormal image generated at the leading and trailing edges of the image, and an increase in the applied voltage in the gap can be suppressed to prevent an output abnormality of the transfer bias.

[Example 2]
The second embodiment is different from the first embodiment in that the bias at the time of cam rotation is the same polarity as the image portion bias.
Since the apparatus configuration is the same as that of the first embodiment, the description thereof will be omitted. Here, only the portions different from those of the first embodiment will be described based on the printing timing of the first sheet with respect to the relationship between the sheet, the cam position, and the secondary transfer current.
11 sheets when 2 sheets of paper cardboard P ₂ as the paper in the second embodiment, an image, cam position, is a timing chart of the secondary transfer current.
When printing of the first sheet is started, the secondary transfer roller comes into contact with the intermediate transfer belt. The cam at that time is at position A in this embodiment. At this time, the secondary transfer bias of the non-image part is applied (+800 V in this embodiment). Next, when the thick paper approaches the secondary transfer nip, the cam starts to move to the position B (displacement amount δ: 1 mm).

The secondary transfer bias is switched to the bias at the cam rotation in synchronization with the cam rotation (-200 V in this embodiment). When the cam reaches position B, the secondary transfer roller and the secondary transfer counter roller are separated from each other. The cardboard P ₂ is in a separated state is conveyed, reaches the secondary transfer nip, is that is simultaneously cam the paper tip has entered starts forward operation (rotation clockwise in this example).
Next, when the leading edge of the image reaches the nip, the cam moves to position C and stops, the secondary transfer roller and the secondary transfer counter roller are in contact with each other, and the secondary transfer bias is controlled to a constant current control. It is switched to an image portion current (-30 μA in this embodiment).
In this way, even when the bias at the time of cam rotation is the same polarity as the bias of the image portion, the constant current control is used for the image portion bias and the constant voltage control is used for the bias at the time of cam rotation. It is possible to suppress an increase in applied voltage and prevent an output abnormality of the transfer current.
In addition, since the bias at the time of cam rotation and the image portion bias are of the same polarity, it is more effective in preventing transfer failure due to insufficient transfer bias at the leading edge of the image.
By making the transfer bias the same polarity as the image part bias, a sufficient transfer current is applied to the front and rear end of the recording medium when switching to the image part bias at the front end and when switching from the image part bias at the rear end. Thus, an abnormal image at the front and rear end of the recording medium can be prevented.

Next, printing is performed by applying an image portion transfer current while the secondary transfer roller and the secondary transfer counter roller are in contact with each other, and when the rear end of the image reaches the nip, the cam rotates backward to position B (this embodiment) In the example, it starts to rotate counterclockwise) and starts the separation operation.
In synchronization with the start of the reverse rotation operation, the secondary transfer bias is switched from the image portion bias, which is constant current control, to the bias at the time of cam rotation, which is constant voltage control.
When the trailing edge of the sheet reaches the nip, the cam moves to the position B and stops, and the secondary transfer bias is switched from the bias at the cam rotation to the non-image portion bias in synchronization with the stop of the cam operation.
Also at the rear end of the image, the bias at the time of cam rotation is set to the same polarity as the bias at the image portion, the constant current control is applied to the image portion bias, and the constant bias control is applied to the bias at the time of cam rotation. Similarly, it is possible to suppress an increase in the applied voltage in the gap and to prevent abnormal output of the transfer current.
Further, since the bias during rotation of the cam and the image portion bias are of the same polarity, it is more effective in preventing transfer failure due to insufficient transfer bias at the rear end of the image.
By setting the non-image portion bias to a polarity opposite to that of the image portion bias, the transfer of the toner adhering to the non-image portion to the secondary transfer body can be suppressed, and the recording medium is transferred when entering the transfer nip. Switching to the transfer bias when exiting the nip can be facilitated.
By switching the transfer bias in synchronization with the cam drive, a sufficient transfer bias can be applied to the front and rear end portions of the recording medium without delay, and an abnormal image at the front and rear end portions of the recording medium can be prevented.
By using constant current control for the transfer bias in the image area, it is possible to prevent abnormal images due to insufficient transfer bias in the image area.

[Comparative Example 1]
Comparative Example 1 uses the constant transfer voltage control for the image transfer portion, the non-image portion, and the cam rotation, which are secondary transfer biases, as compared with the configuration of Embodiment 1.
Since the configuration of the apparatus is the same as that of the first embodiment, a description thereof will be omitted. Here, regarding the relationship between the sheet, the cam position, and the secondary transfer bias, only the portions different from the first embodiment will be described based on the printing timing of the first sheet.
FIG. 12 is a timing chart of a sheet, an image, a cam position, and a secondary transfer bias when _two sheets of thick paper P2 are passed as sheets in Comparative Example 1.
When printing of the first sheet is started, the secondary transfer roller comes into contact with the intermediate transfer belt. The cam at that time is at position A in this comparative example. At this time, the secondary transfer bias of the non-image part is applied (+800 V in this embodiment).
Next, when the thick paper approaches the secondary transfer nip, the cam starts to move to the position B (displacement amount δ: 1 mm). The secondary transfer bias is switched to the bias at the cam rotation in synchronization with the cam rotation (-200 V in this embodiment).

When the cam reaches position B, the secondary transfer roller and the secondary transfer counter roller are separated from each other. The cardboard P ₂ is in a separated state is conveyed, reaches the secondary transfer nip, is that is simultaneously cam the paper tip has entered starts forward operation (rotation clockwise in this example).
Next, when the leading edge of the image reaches the nip, the cam moves to a position C and stops, the secondary transfer roller and the secondary transfer counter roller are in contact with each other, and the image portion bias is switched (this embodiment). Then -1500V).
Next, printing is performed by applying an image portion transfer bias while the secondary transfer roller and the secondary transfer counter roller are in contact with each other, and when the rear end of the image reaches the nip, the cam rotates backward to position B (this embodiment) In the example, it starts to rotate counterclockwise) and starts the separation operation.
In synchronization with the start of the reverse rotation operation, the secondary transfer bias is switched from the image portion bias to the cam rotation bias.
When the trailing edge of the sheet reaches the nip, the cam moves to the position B and stops, and the secondary transfer bias is switched from the bias at the cam rotation to the non-image portion bias in synchronization with the stop of the cam operation.

[Comparative Example 2]
In the second comparative example, the constant transfer current control is performed for the image transfer portion, the non-image portion, and the cam rotation, which are secondary transfer biases.
The configuration of the apparatus is the same as that of Example 1, and the secondary transfer bias in this comparative example is set to image part: -30 μA, non-image part: +30 μA, and cam rotation: -20 μA.
FIG. 13 shows the application timing of the secondary transfer bias, which is the same as that in Comparative Example 1, and is omitted.

[Paper passing experiment]
In each configuration of Examples 1 and 2 and Comparative Examples 1 and 2, a paper passing experiment was performed under the following conditions.
Paper: “Color Copy (basis weight 300g / m ² )” by Mondi
Temperature and humidity: 23 ℃ 50% RH
27 ℃ 80% RH
Image pattern: halftone image (FIG. 14A), patch image (FIG. 14B)
Evaluation item: Abnormal image at the trailing edge of the image (Chilli or white spots due to insufficient transfer bias)
Abnormal image in the image area (density reduction due to insufficient transfer bias)
Abnormal transfer current output

  The results of Paper Passing Experiment 1 are shown in Table 1 (23 ° C 50% RH) and Table 2 (27 ° C 80% RH). “◯” in the table indicates that each abnormality did not occur, “Δ” indicates that there is an abnormality but is at an acceptable level, and “×” indicates that an abnormality has occurred.

In Examples 1 and 2, there was no occurrence of abnormal images or output abnormalities in any environment, whereas in Comparative Example 1, abnormal images in which the image density was reduced in an environment of 23 ° C. and 50% were acceptable. It was a level. However, in a high-temperature and high-humidity environment at 27 ° C. and 80%, a necessary and sufficient transfer bias could not be obtained due to a decrease in resistance of the paper and the transfer roller, and an abnormal image (decrease in image density) occurred.
Further, in Comparative Example 2, dust caused by discharge at the leading and trailing edges of the image occurred, and abnormal transfer output occurred as the applied voltage increased. From the above, the effect of the present invention could be confirmed.

The present invention has been described above based on the embodiments. However, the embodiments of the present invention are merely examples, and do not limit the present invention. Therefore, various improvements and modifications can be made to the present invention without departing from the scope of the present invention.
For example, in the present embodiment, a tandem type copying machine is illustrated, but the present invention is not limited to this, and is also applicable to a four-cycle type copying machine or an intermediate transfer type copying machine having only one image carrier. it can.
Further, although a repulsive transfer method is used, an attractive transfer method may be used.

2 Photosensitive drum 21 Intermediate transfer belt as image carrier 24 Secondary transfer counter roller 30 as transfer means 30 Secondary transfer roller as counter member 45 Energizing coil springs as pressure means 50, 51 Cam 200 Recording medium supply Feeder as a means

JP-A-4-242276 JP 2007-334292 A JP 2007-286382 A JP 2008-122570 A JP 2001-083812 A

Claims (9)

An image carrier having an image carrying surface for carrying an image;
A facing member having a contact surface provided at a position facing the image bearing surface and contacting a recording medium;
Pressurizing means for applying pressure to a transfer nip between the image carrying surface and the contact surface;
Contact / separation means for contacting / separating the image carrying surface and the contact surface;
Transfer means for transferring an image on the image bearing surface to a recording medium sandwiched between the transfer nips by applying a transfer bias between the image bearing surface and the contact surface;
Recording medium supply means for supplying a recording medium to the transfer nip;
In an image forming apparatus having
The transfer means is controlled so that the image portion transfer current flowing in the image portion on the image bearing surface is constant,
The contact / separation means separates the image bearing surface and the contact surface before the recording medium supplied from the recording medium supply means enters the transfer nip, and after the recording medium enters the transfer nip. The image bearing surface and the contact surface are brought into contact with each other through a recording medium, and the pressure by the pressing means is applied to the transfer nip.
An image forming apparatus, wherein a transfer bias when a recording medium enters the transfer nip is controlled at a constant voltage. An image carrier having an image carrying surface for carrying an image;
A facing member having a contact surface provided at a position facing the image bearing surface and contacting a recording medium;
Pressurizing means for applying pressure to a transfer nip between the image carrying surface and the contact surface;
Contact / separation means for contacting / separating the image carrying surface and the contact surface;
Transfer means for transferring an image on the image bearing surface to a recording medium sandwiched between the transfer nips by applying a transfer bias between the image bearing surface and the contact surface;
Recording medium supply means for supplying a recording medium to the transfer nip;
In an image forming apparatus having
The transfer means is controlled so that the image portion transfer current flowing in the image portion on the image bearing surface is constant,
The contacting / separating means separates the image bearing surface and the contact surface before the trailing edge of the recording medium supplied from the recording medium supplying means passes through the transfer nip, and the trailing edge of the recording medium is the transfer medium. After passing through the nip, the image carrying surface and the contact surface are brought into contact with each other, and the pressure by the pressing means is applied to the transfer nip.
An image forming apparatus, wherein a transfer bias when a rear end of a recording medium passes through the transfer nip is controlled at a constant voltage. The image forming apparatus according to claim 1.
The contacting / separating means is configured to separate the image bearing surface and the contact surface before the rear end of the recording medium passes through the transfer nip.
An image forming apparatus, wherein a transfer bias when a rear end of a recording medium passes through the transfer nip is controlled at a constant voltage. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus, wherein a transfer bias when a recording medium enters the transfer nip and when a recording medium exits the transfer nip is switched stepwise from a non-image portion bias or an image portion bias. The image forming apparatus according to claim 4.
The image formation is characterized in that the magnitude of the transfer bias when the recording medium enters the transfer nip and when the recording medium exits the transfer nip is between the non-image portion bias and the image portion bias. apparatus. The image forming apparatus according to claim 4 or 5,
An image forming apparatus, wherein a transfer bias when a recording medium enters the transfer nip and when a recording medium exits the transfer nip has the same polarity as the image portion bias. In the image forming apparatus according to any one of claims 4 to 6,
An image forming apparatus, wherein the non-image portion bias has a polarity opposite to that of the image portion bias and is controlled at a constant voltage. The image forming apparatus according to any one of claims 1 to 7,
The contact / separation means has a configuration for adjusting a distance between the image bearing surface and the contact surface by rotating a cam,
2. An image forming apparatus according to claim 1, wherein the transfer bias switching is synchronized with the drive of the cam. The image forming apparatus according to any one of claims 1 to 8,
The image forming apparatus, wherein the image carrier is an intermediate transfer body on which images on a plurality of juxtaposed photosensitive drums are superimposed and transferred.

Images