JP2015165291A - Transfer device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transfer device that prevents the occurrence of abnormal images resulting from discharge at a transfer nip.SOLUTION: A transfer device 20 includes a transfer member 30 that forms a transfer nip N, and a power source 39 that supplies a transfer bias for transferring a toner image on an image carrier 21 to a recording material S held at the transfer nip. The power source can supply either one of a first transfer bias or a second transfer bias to the transfer nip, and has contact width changing means 60 for extending, when the first transfer bias is supplied, the contact width between the transfer member and the image carrier on the upstream side in a recording material conveyance direction compared with the case where the second transfer bias is supplied; the first transfer bias is a transfer bias in which a bias in a transfer direction to transfer a toner image from an image carrier side to a recording material side and a bias having a polarity opposite to that of the bias in the transfer direction are alternately switched; the second transfer bias is a transfer bias comprising only the bias in a transfer direction to transfer a toner image from the image carrier side to the recording material side.

Description

  The present invention relates to a transfer device and an image forming apparatus for transferring a toner image on a surface of an image carrier to a recording material sandwiched in the nip at a transfer nip formed by contact between an image carrier and a transfer member.

  A transfer member that forms a transfer nip in contact with the toner image carrying surface of the image carrier, and a voltage for transferring the toner image on the image carrier to a recording material sandwiched in the transfer nip, or As an image forming apparatus provided with a power source for outputting a current, the one described in Patent Document 1 is known. In the image forming apparatus described in Patent Document 1, a toner image is formed on the surface of a drum-shaped photoreceptor serving as an image carrier by a known electrophotographic process. The image forming apparatus is an image carrier and an intermediate transfer member, and an intermediate transfer belt arranged in a loop shape is brought into contact with the photosensitive member to form a transfer nip (primary). The toner image is transferred onto the intermediate transfer belt. For the intermediate transfer belt, a secondary transfer roller serving as one transfer member is brought into contact with the front surface carrying the toner image of the intermediate transfer belt. A secondary transfer counter roller serving as a transfer member is disposed in the loop of the intermediate transfer belt, and the intermediate transfer belt is sandwiched between the secondary transfer counter roller and the secondary transfer roller roller so as to transfer the transfer nip (secondary ) Is formed. The secondary transfer counter roller inside the loop is connected to the ground, while the secondary transfer roller outside the loop is applied with a transfer bias in which DC and AC are superimposed from the power source to form a transfer electric field. Yes. For this reason, the toner image on the intermediate transfer belt is transferred to the recording material fed between the secondary transfer counter roller and the secondary transfer roller, that is, into the secondary transfer nip, by the action of the transfer electric field or nip pressure. doing.

Patent Document 1 discloses a technique for superimposing direct current and alternating current as a transfer bias. However, there is no disclosure that an abnormal image is generated due to image blurring or discharge due to the wide contact width between the transfer member and the image carrier on the upstream side in the recording material conveyance direction. That is, no consideration is given to the fact that when an image is transferred to a recording material using a transfer bias in which the voltage or current is switched alternately, if the contact width is narrow, an abnormal image due to discharge is likely to occur.
Further, it does not disclose that an abnormal image is generated due to reverse transfer or discharge of the image depending on the width of the transfer nip. In other words, when transferring an image to a recording material using a transfer bias in which voltage or current is switched alternately, if the transfer nip width is narrow, an abnormal image due to discharge is not easily generated.
An object of the present invention is to prevent the occurrence of abnormal images due to discharge in the transfer nip.

In order to achieve the above object, the present invention provides a transfer member that forms a transfer nip in contact with a toner image carrying surface of an image carrier and a recording material sandwiched in the transfer nip. A power supply for supplying a transfer bias for transferring the toner image on the carrier to the transfer nip, and the power source is configured to transfer a first toner image on the image carrier to a recording material. The transfer bias can be supplied to the transfer nip, and when the power supply supplies the first transfer bias, the power supply supplies the second transfer bias. And a contact width changing means for increasing a contact width between the transfer member and the image carrier on the upstream side in the recording material conveyance direction, and the first transfer bias is configured to transfer the toner image from the image carrier side to the recording material. Transcribe to the side A transfer bias in which a bias in a transfer direction and a bias having a polarity opposite to that in the transfer direction are alternately switched. The second transfer bias is a transfer in which the toner image is transferred from the image carrier side to the recording material side. It is a transfer bias composed of only a directional bias.
In order to achieve the above object, the present invention provides a transfer member that forms a transfer nip in contact with a toner image carrying surface of an image carrier and a recording material sandwiched in the transfer nip. A power supply for supplying a transfer bias for transferring the toner image on the carrier to the transfer nip, and the power source is configured to transfer a first toner image on the image carrier to a recording material. The transfer bias can be supplied to the transfer nip, and when the power supply supplies the first transfer bias, the power supply supplies the second transfer bias. The first transfer bias includes a transfer direction bias for transferring the toner image from the image carrier side to the recording material side, and a transfer direction bias for expanding the width of the transfer nip. Bahia The second transfer bias is a transfer bias composed only of a bias in a transfer direction for transferring the toner image from the image carrier side to the recording material side. It is characterized by.

According to the present invention, when an image is transferred to a recording material using a transfer bias in which voltage or current is switched alternately, the contact width between the transfer member and the image carrier on the upstream side in the recording material conveyance direction by the contact width changing unit. To spread. Accordingly, the transfer bias is applied in a state where the image carrier and the recording material are in contact with each other, so that an abnormal image due to discharge can be prevented.
According to the present invention, the transfer nip width is widened by the nip width changing means when the image is transferred to the recording material using the transfer bias in which the voltage or current is alternately switched. Thereby, the transfer bias is applied in a state where the image carrier and the recording material are in close contact with each other, so that an abnormal image due to discharge can be prevented.

1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 3 is an enlarged view showing a configuration in the vicinity of a secondary transfer nip in the image forming apparatus. FIG. 3 is a schematic diagram illustrating a configuration of a contact width changing unit according to the first embodiment of the present invention and a state of a transfer nip in a DC transfer mode. FIG. 3 is a schematic diagram illustrating a configuration of a contact width changing unit according to the first embodiment of the present invention and a state of a transfer nip in an AC transfer mode. FIG. 6 is a schematic diagram illustrating a configuration of a nip width changing unit according to a second embodiment of the present invention and a state of a transfer nip in a DC transfer mode. FIG. 6 is a schematic diagram illustrating a configuration of a nip width changing unit according to a second embodiment of the present invention and a state of a transfer nip in an AC transfer mode. The schematic diagram which shows the structure of the contact width change means which concerns on the 3rd Embodiment of this invention, and the state of the transfer nip in DC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration of a contact width changing unit according to a third embodiment of the present invention and a state of a transfer nip in an AC transfer mode. The schematic diagram which shows the state of the transfer nip in a DC transfer mode, and the structure of the nip width change means based on the 4th Embodiment of this invention. The schematic diagram which shows the structure of the nip width change means which concerns on the 4th Embodiment of this invention, and the state of the transfer nip in AC transfer mode. The schematic diagram which shows the structure of the modification of the nip width change means based on the 4th Embodiment of this invention, and the state of the transfer nip in DC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration of a modified example of a nip width changing unit according to a fourth embodiment of the present invention and a state of a transfer nip in an AC transfer mode. The schematic diagram which shows the structure of the nip width change means which concerns on the 5th Embodiment of this invention, and the state of the transfer nip in DC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration of a nip width changing unit according to a fifth embodiment of the present invention and a state of a transfer nip in an AC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration according to a sixth embodiment of the present invention and a state of a transfer nip in a DC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration according to a sixth embodiment of the present invention and a state of a transfer nip in an AC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration according to a seventh embodiment of the present invention and a state of a transfer nip in a DC transfer mode. FIG. 10 is a schematic diagram illustrating a configuration according to a seventh embodiment of the present invention and a state of a transfer nip in an AC transfer mode. FIG. 3 is an enlarged view showing a configuration of an example of a secondary transfer nip. The figure explaining the waveform of the secondary transfer bias which consists of a superposition bias. The schematic block diagram which shows the observation experiment apparatus used for experiment. FIG. 4 is an enlarged schematic diagram showing the behavior of toner during transfer in the secondary transfer nip, where (a) shows the behavior of the toner in the initial transfer stage, (b) shows the behavior of the toner in the initial transfer stage, and (c) shows the transfer behavior. The behavior of the toner of the latter stage application is shown respectively. FIG. 6 is a diagram illustrating a waveform of a secondary transfer bias output from a power source in Comparative Example 1. FIG. 4 is a diagram illustrating a waveform of a secondary transfer bias output from a power supply in Embodiment 1. FIG. 6 is a diagram illustrating a waveform of a secondary transfer bias output from a power supply in Embodiment 2. FIG. 10 is a diagram illustrating a waveform of a secondary transfer bias output from a power supply in Embodiment 3. FIG. 10 is a diagram illustrating a waveform of a secondary transfer bias output from a power supply in Embodiment 4. It is a figure which shows the effect of the comparative example 1, and is a figure which shows the image evaluation on a recording material on the conditions of return time 50%. It is a figure which shows the effect of Example 1-2, and is a figure which shows the image evaluation on a recording material on conditions with the return time of 40%. FIG. 10 is a diagram illustrating an effect of Example 3 and illustrating image evaluation on a recording material under a condition of a return time of 16%. FIG. 10 is a diagram illustrating an effect of Example 4 and illustrating image evaluation on a recording material under a condition of a return time of 16%. FIG. 3 is a schematic configuration diagram showing another embodiment of the image forming apparatus according to the present invention. The enlarged view explaining the structure which concerns on the 8th Embodiment of this invention, the state of the recording material in AC transfer mode, and the position of a guide member. The enlarged view explaining the structure which concerns on 8th Embodiment, the state of the recording material in DC transfer mode, and the position of a guide member. (A) is an enlarged view for explaining the configuration of the recording material contact width changing means and the contact state between the recording material and the guide member in the AC transfer mode, and (b) is the configuration of the recording material contact width changing means and in the DC transfer mode. The enlarged view explaining the contact state of a recording material and a guide member. (A)-(c) is a figure explaining the some form of the support structure of a guide member.

Hereinafter, embodiments for carrying out the present invention will be specifically described with reference to the drawings. Note that, in each drawing and each embodiment, the same member or a member having the same function is basically denoted by the same reference numeral, and redundant description is appropriately omitted.
First, the configuration of an image forming apparatus to which the transfer apparatus according to the present invention is applied will be described. FIG. 1 shows a color copying apparatus which is an embodiment of an image forming apparatus according to the present invention and is a tandem indirect transfer type electrophotographic apparatus. The color copying apparatus includes a printer unit 100, a paper feed table 200 serving as a paper feed unit on which the printer unit 100 is placed, a scanner unit 300 serving as an image reading unit attached on the printer unit 100, An automatic document feeder (ADF) 400 mounted on the scanner unit 300 and a control unit 500 are provided.
The image forming apparatus is not limited to the illustrated color copying apparatus, and may be a single printer, a facsimile, or a multifunction machine having a plurality of these functions.

  The printer unit 100 includes an endless belt-like intermediate transfer belt 21 that is an image carrier and an intermediate transfer member. As shown in FIG. 1, the intermediate transfer belt 21 has an inverted triangular loop shape as shown in FIG. 1, and a plurality of rollers, a driving roller 22, a driven roller 23, and a secondary transfer counter roller, which is a transfer member. (Backup roller) 24 is supported around. The intermediate transfer belt 21 is rotated (referred to as rotation) in the clockwise direction (arrow direction) in FIG.

  Above the intermediate transfer belt 21, four image forming units (image forming units) 1C, 1M for forming toner images of C (cyan), M (magenta), Y (yellow), and K (black) are provided. 1Y and 1K are arranged side by side along the moving direction of the intermediate transfer belt 21, and constitute the tandem image forming unit 10. This is an image forming unit that forms a toner image on an image carrier to be described later.

  Each of the image forming units 1C, 1M, 1Y, and 1K includes photosensitive drums 2C, 2M, 2Y, and 2K as image carriers, developing units 3C, 3M, 3Y, and 3K, and photosensitive member cleaning devices 4C, 4M, and so on. 4Y, 4K. The photosensitive drums 2C, 2M, 2Y, and 2K are in contact with the intermediate transfer belt 21 at positions facing the primary transfer rollers 25C, 25M, 25Y, and 25K, respectively, and have primary transfer nips for C, M, Y, and K, respectively. While being formed, it is rotationally driven in the counterclockwise direction (arrow direction) in FIG. 1 by a driving means (not shown).

  The developing units 3C, 3M, 3Y, and 3K develop the electrostatic latent images formed on the photosensitive drums 2C, 2M, 2Y, and 2K with toners of C, M, Y, and K colors. The photoconductor cleaning devices 4C, 4M, 4Y, and 4K clean the transfer residual toner that has adhered to the photoconductor drums 2C, 2M, 2Y, and 2K after passing through the primary transfer nip.

  An optical writing unit 15 is disposed above the tandem image forming unit 10 in the printer unit 100. The optical writing unit 15 forms an electrostatic latent image by performing optical writing processing by scanning with laser light on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K that are rotationally driven in the direction indicated by an arrow. To do. Prior to the optical writing process, the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K are uniformly charged by charging rollers (not shown). In this color copying apparatus, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. More specifically, it is uniformly charged to about −650 [V]. In this embodiment, a charging bias in which an AC voltage is superimposed on a DC voltage is used. The charging roller is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method of bringing a charging member such as a charging roller into contact with or close to the photosensitive member 2K, a charging method using a charging charger may be adopted.

The transfer unit 20 as a transfer device including the intermediate transfer belt 21 includes primary transfer rollers 25C, 25M, 25Y, and 25K inside the loop of the intermediate transfer belt 21. The primary transfer rollers 25C, 25M, 25Y, and 25K press the intermediate transfer belt 21 toward the photosensitive drums 2C, 2M, 2Y, and 2K on the back side of the primary transfer nip for each color of C, M, Y, and K. ing.
The primary transfer rollers 25Y, 25M, 25C, and 25K are constituted by an elastic roller having a metal core and a conductive sponge layer fixed on the surface thereof. The primary transfer rollers 25Y, 25M, 25C, and 25K are arranged so as to occupy positions shifted from the axial centers of the photosensitive drums 2Y, 2M, 2C, and 2K to the downstream side in the belt movement direction. ing. In this color copying apparatus, a primary transfer bias is applied to such primary transfer rollers 25Y, 25M, 25C, and 25K by constant current control. Instead of the primary transfer rollers 25Y, 25M, 25C, and 25K, a transfer charger, a transfer brush, or the like may be employed as the primary transfer member.

Below the intermediate transfer belt 21, a secondary transfer roller 30 serving as a transfer member is rotatably supported by a shaft 30A. The secondary transfer roller 30 is disposed outside the loop of the intermediate transfer belt 21, and is disposed on the inner side of the loop, and is an outer peripheral surface 24a of a secondary transfer counter roller 24 as a transfer member that is rotatably supported by a shaft 24A. The intermediate transfer belt 21 is sandwiched between the outer peripheral surface 30a and the outer peripheral surface 30a. Accordingly, a secondary transfer nip (transfer nip) N is formed in which the front surface 21a of the intermediate transfer belt 21 and the outer peripheral surface 24a of the secondary transfer roller 30 are in contact with each other. In the example shown in FIGS. 1 and 2, the secondary transfer roller 30 is grounded (grounded), whereas the secondary transfer counter roller 24 is connected to a power source 39 and applied with a secondary transfer bias. . As a result, a secondary transfer electric field that electrostatically moves negative polarity toner from the secondary transfer counter roller 24 side to the secondary transfer roller 30 side between the secondary transfer counter roller 24 and the secondary transfer roller 30. Is formed.
Around the secondary transfer roller 30, a cleaning unit 50 for cleaning the secondary transfer roller 30 is disposed. The cleaning unit 50 includes an application roller 51 that contacts the outer peripheral surface 30a of the secondary transfer roller 30, a lubricant 53 that is accommodated in the casing 55 and pressed against the application roller 51 by a spring 52, an application blade 54, and a cleaning roller. 56. Since the outer peripheral surface 30a of the secondary transfer roller 30 is in contact with the front surface 21a of the intermediate transfer belt 21, the toner of the intermediate transfer belt 21 adheres. Therefore, in this cleaning means 50, the lubricant 53 is applied to the outer peripheral surface 30a by the application roller 51, the applied lubricant 53 is extended by the application blade 54, and the toner adhering to the outer peripheral surface 30a is removed by the cleaning roller 56. ing.

  A power source 39 that outputs a secondary transfer bias for transferring the toner image on the intermediate transfer belt 21 to the recording sheet S sandwiched in the secondary transfer nip N has a DC power source and an AC power source. . The power source 39 is configured to output only a superimposed bias obtained by superimposing an AC voltage on a DC voltage and a DC voltage as a secondary transfer bias. In the present embodiment, as shown in FIG. 1, the secondary transfer roller 30 is grounded while a secondary transfer bias is applied to the secondary transfer counter roller 24. In the present embodiment, the power source 39 is configured to output to the secondary transfer counter roller 24 a DC bias and a superimposed bias obtained by superimposing an AC bias on the DC bias. That is, the secondary transfer bias supplied from the power supply 39 is alternately switched between a bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side, and a bias having a polarity opposite to that in the transfer direction. . That is, the power supply 39 supplies the secondary transfer nip N with a transfer bias for transferring the toner image on the intermediate transfer belt 21 to the recording sheet S sandwiched in the secondary transfer nip N. The alternating current component and the direct current component of the superimposed bias output from the power source 39 may be subjected to constant current control or constant voltage control, respectively.

The power supply 39 for the secondary transfer bias has a DC transfer mode that is a first mode for outputting a DC bias only, and a second mode that outputs a superimposed bias in which an AC bias is superimposed on the DC bias. The mode can be switched to the AC transfer mode, and the mode can be switched by turning on / off the output of the AC bias. The superimposed bias in the AC transfer mode is the first transfer bias. The DC bias in the DC transfer mode is the second transfer bias. The first transfer bias is a transfer bias in which a bias in the transfer direction for transferring the toner image from the intermediate transfer belt 21 side to the recording sheet S side and a bias in the opposite direction to the bias in the transfer direction are alternately switched. The second transfer bias is a transfer bias including only a bias in a transfer direction for transferring the toner image from the intermediate transfer belt 21 side to the recording sheet S side.
For example, when the recording sheet S is not a sheet having large surface irregularities such as rough paper but a sheet having small surface irregularities such as plain paper, a shading pattern that follows the uneven pattern does not appear. For this reason, in the DC transfer mode, a secondary transfer bias consisting only of a DC bias is applied. Also, when using a paper with large surface irregularities such as rough paper, the AC transfer mode is set, and a secondary transfer bias with an AC bias superimposed on a DC bias is output. That is, the secondary transfer bias can be switched between the DC transfer mode and the AC transfer mode according to the type of the recording sheet S to be used (the size of the surface irregularities of the recording sheet S). Switching between the DC transfer mode and the AC transfer mode is performed by controlling the power supply 39 by the control unit 500.

  Although there are various forms of supply of the secondary transfer bias to the secondary transfer nip N, the power supply form can supply “DC bias + AC bias” like the power supply 39, or “DC bias” and “AC bias”. ”Can be supplied individually, or“ DC bias + AC bias ”and“ DC bias ”can be switched and supplied by a single power source.

  The scanner unit 300 reads the image information of the document placed on the contact glass 301 with the reading sensor 302, and the control unit 500 that sends the read image information to the control unit 500 of the printer unit 100 receives from the scanner unit 300. Based on the image information, a light source such as a laser diode in the optical writing unit 15 of the printer unit 100 is controlled to emit laser writing light for C, M, Y, K, and the photosensitive drums 2C, 2M, 2Y and 2K are optically scanned. By this optical scanning, electrostatic latent images are formed on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K, and the latent images are developed into toner images of C, M, Y, and K through a predetermined development process. Is done.

  The paper feed table 200 stores recording sheets S in paper feed cassettes 202 arranged in multiple stages in the paper bank 201. The paper feed table 200 is fed to a paper feed roller 203 that feeds the recording sheet S from the paper feed cassette 202, a separation roller 205 that separates the sent recording sheet S and guides it to the paper feed path 204, and a paper feed path 99 of the printer unit 100. A conveyance roller pair 206 for conveying the recording sheet S is provided.

  In addition to the paper feed table 200, manual paper feed is also possible, and the manual feed tray 98 for manual feed and the recording sheets on the manual feed tray 98 are separated one by one toward the manual feed path 97. A separating roller 96 is also provided. In the printer unit 100, the manual paper feed path 97 joins the paper feed path 99. A registration roller pair 95 is disposed near the end of the paper feed path 99. The registration roller pair 95 is a conveying member that sandwiches the recording sheet S conveyed in the sheet feeding path 99 or the manual sheet feeding path 97 between the rollers and then feeds it toward the secondary transfer nip N at a predetermined timing. Function as.

  In order to obtain a color image copy with this color copying apparatus, an original is set on the original table 401 of the ADF 400, or the ADF 400 is opened and an original is set on the contact glass 301 of the scanner unit 300, and the ADF 400 is closed. Hold down the manuscript.

  When a start button (not shown) is pressed, if 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. The light emitted from the light source of the first traveling body 303 is reflected by the document surface, and the reflected light is turned back and directed toward 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. As a result, the document content is read by the reading sensor 302 and a signal processing circuit (not shown), temporarily stored as image information, and sequentially sent to the printer unit 100.

  When the printer unit 100 receives the image information from the scanner unit 300, the printer unit 100 feeds the recording sheet S having a size corresponding to the image information to the paper feed path 99. Along with this, the drive roller 22 is rotationally driven by a drive motor (not shown) to rotate the intermediate transfer belt 21 in the direction of the arrow. At the same time, after each of the image forming units 1C, 1M, 1Y, and 1K of the tandem image forming unit 10 starts rotating in the direction indicated by the arrow, each surface of the image forming units 1C, 1M, 1Y, and 1K is rotated by the charging roller. It is charged like this.

  Then, based on the image information received from the scanner unit 300, optical writing processing is performed by the optical writing unit 15, and electrostatic latent images are formed on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K, respectively. The developing units 3C, 3M, 3Y, and 3K perform development processing using toner of each color. The toner images of C, M, Y, and K colors formed on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K by these processes are sequentially overlapped at the primary transfer nip for C, M, Y, and K. In addition, primary transfer is performed on the intermediate transfer belt 21 to form a full-color toner image in which four colors are superimposed.

In the paper feed table 200, one of the paper feed rollers 203 is selectively rotated so as to feed the recording sheet S having a size corresponding to the document size, and the recording sheet S is fed from one of the three paper feeding cassettes 202. Is sent out. The recording sheet S sent out is separated one by one by the separation roller 205 and introduced into the paper feed path 204, and is then transported by the transport roller pair 206 and sent to the paper feed path 99 in the printer unit 100. .
When the manual feed tray 98 is used, the paper feed roller 94 of the manual feed tray 98 rotates to feed the recording sheet S on the tray, and is sent to the manual feed path 97 while being separated by the separation roller 96. It reaches near the end of the paper path 99. In the vicinity of the end of the paper feed path 99, the recording sheet S stops by abutting the leading edge against the registration roller pair 95.

  Thereafter, when the registration roller pair 95 rotates at a timing that can synchronize with the full-color toner image on the intermediate transfer belt 21, the recording sheet S is fed into the secondary transfer nip N, and the full-color toner image on the intermediate transfer belt 21. Adheres closely to the toner image. Then, the toner image is collectively transferred onto the recording sheet S by the action of the secondary transfer electric field by the nip pressure and the secondary transfer bias. The secondary transfer nip N is a secondary transfer portion formed by the secondary transfer roller 30 facing the secondary transfer counter roller 24 and the intermediate transfer belt 21.

  The recording sheet S on which the full-color toner image is secondarily transferred in the secondary transfer portion is sent into the fixing device 71 by the paper transport belt 70. Then, the toner image is fixed on the surface by the pressurization and the heat treatment by the fixing device 71 being sandwiched in the fixing nip between the pressure roller 72 and the fixing belt 73.

  The recording sheet S on which the color image is formed in this way is sent out by the discharge roller pair 74 and stacked on the discharge tray 75 outside the apparatus. When an image is also formed on the other side of the recording sheet, the recording sheet S is discharged from the fixing device 71 and then sent to the sheet reversing device 77 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, sent to the secondary transfer nip N, the toner image is secondarily transferred to the other surface, and the toner image is again transferred via the fixing device 71. After fixing, it is stacked on the paper discharge tray 75.

After passing through the secondary transfer nip N, a belt cleaning device 26 is placed on the front surface 21a of the intermediate transfer belt 21 before entering the primary transfer nip for cyan where the primary transfer process is the most upstream of the four colors. Are in contact. This belt cleaning device 26 cleans the transfer residual toner adhering to the surface of the intermediate transfer belt 21. In the image forming apparatus according to the present embodiment, a copy is obtained by executing such a series of steps.
FIG. 32 is an image forming apparatus having a configuration similar to that of the image forming apparatus shown in FIG. The difference between the image forming apparatus shown in FIG. 1 and the image forming apparatus shown in FIG. 32 is that FIG. 1 includes an entrance roller 61 as a contact portion, whereas FIG. 32 does not include the entrance roller 61. Become. Other configurations are basically the same, and are given the same reference numerals. Of the embodiments described below, the embodiment provided with the entrance roller 61 is applicable to the image forming apparatus of FIG. 1, and the embodiment not provided with the entrance roller 61 is applied to the image forming apparatus shown in FIG. Is possible

(First embodiment)
Next, a first embodiment of the present invention will be described.
As shown in an enlarged view in FIG. 2, the secondary transfer nip N forms a gap at a position relatively distant from the joining nip n1 (inter-axis position) indicated by a white arrow in FIG. Further, a winding nip in which the intermediate transfer belt 21 is forcibly wound around the secondary transfer counter roller 24 on the upstream side in the belt moving direction (moving direction of the image carrier) with respect to the joining nip n1 (interaxial position). (Hereinafter referred to as “pre-nip”) n2. This forcible winding is performed by contact width changing means 60 that changes the contact width (pre-nip n2) of the secondary transfer nip N in the recording material traveling direction at least on the upstream side in the belt movement direction, which is the recording material entry side. ing. A guide member 38 that guides the recording sheet S to the nip entrance P1 of the secondary transfer nip N is disposed upstream of the secondary transfer nip N in the belt moving direction.

  The contact width changing unit 60 includes an entrance roller 61 as a contact portion and a moving unit 62 that moves the entrance roller 61 so as to increase or decrease the nip width (contact width). The entrance roller 61 is disposed inside the loop of the intermediate transfer belt 21 on the upstream side of the secondary transfer nip N in the belt movement direction of the intermediate transfer belt 21, and the back surface 21b that is a surface that does not carry the toner image of the intermediate transfer belt 21. It is provided so that it may contact. The entrance roller 61 is supported by a side plate (not shown) of the transfer unit 20 so that the entrance roller 61 can move to increase or decrease the nip width. The entrance roller 61 is a metal roller and is grounded.

  In the AC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S by alternately switching between the bias in the transfer direction and the bias having the reverse polarity, the moving unit 62 is set to 2 on the recording material entry side. The entrance roller 61 is configured to move in the direction of increasing the width of the next transfer nip N. The moving means 62 includes a spring 63 as an urging means for urging the entrance roller 61 in a direction to push the intermediate transfer belt 21 toward the outside of the loop, an eccentric cam 64 in contact with the outer peripheral surface of the entrance roller 61, and an eccentric cam. A drive motor 65 is provided as a drive unit that rotationally drives 64. The eccentric cam 64 is provided so as to rotate integrally with a drive shaft 66 that is rotated by a drive motor 65. The eccentric cam 64 has a cam surface on the outer peripheral surface 64b having the maximum length from the center of rotation to the top dead center 64a. The inlet roller 61 and the eccentric cam 64 are pressed against each other by the action of the spring 63. The power source 39 and the drive motor 65 are connected to the control unit 500 via a signal line, and bias switching and driving thereof are controlled.

The control unit 500 includes a computer including an arithmetic circuit and a storage unit, and bias switching timing, driving timing of the driving motor 65, and driving time (rotation angle) are set and stored in advance.
The moving means 62 changes the position of the entrance roller 61 by changing the pressure contact position of the eccentric cam 64 with the entrance roller 61 by the rotational drive of the drive motor 65. Specifically, when the DC transfer mode is set, the control unit 500 moves the top dead center 64a of the eccentric cam 64 to a position away from the surface 61a of the entrance roller 61 as shown in FIG. As shown in FIG. 4, the rotation of the drive motor 65 is controlled so that the top dead center 64 a of the eccentric cam 64 is moved to the surface 61 a of the entrance roller 61. In the present embodiment, the eccentric cam 64 has a home position as a standby position in the DC transfer mode shown in FIG. 3, and rotates 180 degrees in the AC transfer mode to occupy the nip width increasing position shown in FIG. Is arranged. For this reason, in the AC transfer mode, the entrance roller 61 moves downward in the figure as compared with the DC transfer mode, so the intermediate transfer belt 21 is pushed out, and the width of the pre-nip n2 on the upstream side in the belt moving direction is widened. The nip width of the secondary transfer nip N is also widened.

  As in the present embodiment, between the secondary transfer counter roller 24 to which the superimposed bias obtained by superimposing the AC bias on the DC bias is applied as the secondary transfer bias by the power source 39 and the secondary transfer roller 30 that is grounded. The secondary transfer bias flows through the intermediate transfer belt 21. This secondary transfer bias mainly flows at a position facing a path connecting the shafts of both rollers, that is, a joining nip n1 indicated by a white arrow in FIG. Therefore, the secondary transfer of the toner image from the intermediate transfer belt 21 to the recording sheet S is performed at the joining nip n1 that overlaps the position connecting the axes. If a gap is formed between the front surface 21a of the intermediate transfer belt 21 and the secondary transfer roller 30 slightly upstream of the inter-axis position in the belt movement direction, discharge occurs between the gaps. Occur. As a result, the toner in the toner image reaching the belt region before entering the secondary transfer nip (corresponding to the joining nip n1) is scattered and causes transfer dust. Further, when the secondary transfer bias is obtained by superimposing the AC component on the DC component, the AC component is shifted to a predetermined polarity as compared with the case where there is no AC component. The absolute value of the peak increases. For this reason, discharge at the entrance of the secondary transfer nip N easily occurs.

However, in this embodiment, in the AC transfer mode, the entrance roller 61 moves downward in the drawing compared to the DC transfer mode, the intermediate transfer belt 21 is pushed out, and the width of the prenip n2 is widened toward the recording material entry side. It is done. The pre-nip n2 is an area where the secondary transfer roller 30 and the belt are in contact with each other upstream of the belt area of the intermediate transfer belt 21 supported by the secondary transfer counter roller 24 on the back surface. That is, the length of the pre-nip n2 in the secondary transfer nip N can be adjusted by changing the position of the entrance roller 61 located upstream of the secondary transfer nip N in the belt moving direction. In the AC transfer mode, the amount of the pre-nip n2 is increased by lowering the position of the entrance roller 61, and the secondary transfer bias is applied while the intermediate transfer belt 21 and the recording sheet S are in contact with each other. Can be prevented.
On the other hand, in the DC transfer mode, if the pre-nip n2 that is the contact width between the intermediate transfer belt on the recording material entering side and the secondary transfer roller 30 is excessively widened, the linear velocity difference between the intermediate transfer belt 21 and the recording sheet S is increased. This may cause image blurring. In the present embodiment, the pre-nip n2 is widened in the AC transfer mode in which abnormal images due to discharge are likely to occur. Therefore, the occurrence of image blur is minimized due to the difference in linear velocity between the intermediate transfer belt 21 and the recording sheet S. To the limit.
In general, an abnormal image due to transfer blur is more noticeable in the DC transfer mode, whereas an abnormal image due to discharge is more noticeable in the AC transfer mode. For this reason, it is preferable to adjust the pre-nip width in order to avoid abnormal images that become noticeable in each mode.

(Second Embodiment)
Next, a second embodiment of the present invention will be described.
In the second embodiment, the transfer direction bias and the reverse polarity bias are alternately switched to transfer the toner image on the intermediate transfer belt 21 onto the recording sheet S, and the transfer direction bias. In the DC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S without alternately switching the bias and the reverse polarity bias, the hardness of the secondary transfer roller 30 serving as a transfer member is switched. The width of the secondary transfer nip N is widened and changed. The secondary transfer nip N is an area sandwiched between the secondary transfer counter roller 24 and the secondary transfer roller 30.

As a method for switching the hardness of the secondary transfer roller 30, the secondary transfer roller 30 and the secondary transfer roller 30 ′ having different hardnesses are configured to be detachable from the shaft portion 20 </ b> A, and are manually replaced according to the transfer mode. To do.
Alternatively, as shown in FIGS. 5 and 6, two members of a secondary transfer roller 30 and a secondary transfer roller 30 ′ having different hardnesses are arranged in the transfer unit 20, and these secondary transfer roller 30 and the secondary transfer roller are arranged. The width of the transfer nip may be changed by the nip width changing means 60A for switching the position of the roller 30 ′ with the switching means 600. In the present embodiment, the secondary transfer roller 30 ′ is formed of a material whose hardness is lower than that of the material used for the secondary transfer roller 30 so that the surface of the secondary transfer roller 30 is more easily crushed than the secondary transfer roller 30.

  The nip width changing unit 60A in the present embodiment is a support frame 601 that rotatably supports the secondary transfer roller 30 and the secondary transfer roller 30 ′ by the support shafts 20A and 20B, and a drive unit that swings the support frame 601. And a switching means 600 composed of a drive motor 605. The support frame 601 is supported so as to be swingable about a rotation support shaft 602. Reference numeral 606 is a stopper for regulating the position of the support frame 601 in order to hold the secondary transfer roller 30 at the nip forming position shown in FIG. In the present embodiment, the drive motor 605 is connected to the control unit 500 via a signal line, and is configured to control its drive. Specifically, when the control unit 500 enters the DC transfer mode, the front surface of the intermediate transfer belt 21 is positioned at a position where the surface 30a of the secondary transfer roller 30 faces the secondary transfer counter roller 24 as shown in FIG. In the AC transfer mode, the intermediate transfer belt 21 is moved to a position where the surface 30′a of the secondary transfer roller 30 ′ faces the secondary transfer counter roller 24 as shown in FIG. The rotation of the drive motor 605 is controlled so as to move to a position where it is pressed against the front surface 21a.

In this embodiment, the nip forming position where the secondary transfer roller 30 stands by in the DC transfer mode shown in FIG. 5 is set as the home position, and when the AC transfer mode is entered, as shown in FIG. The secondary transfer roller 30 'having low hardness (soft) moves to the nip forming position.
For this reason, in the present embodiment, when the AC transfer mode is set, the secondary transfer roller 30 ′ having a low hardness is moved to the nip formation position by the nip width changing unit 60A, and the secondary transfer roller 30 is retracted from the nip formation position. Therefore, the width of the transfer nip in the AC transfer mode can be widened. That is, by making the positions of the secondary transfer rollers 30 and 30 ′ having different hardnesses variable, it is possible to adjust the length of the transfer nip in the secondary transfer nip N in the recording material conveyance direction. As a result, since the width of the transfer nip can be increased in the AC transfer mode, the secondary transfer bias is applied while the intermediate transfer belt 21 and the recording sheet S are in close contact with each other, thereby preventing abnormal images due to discharge. Can do. Further, by increasing the width of the transfer nip, the toner can sufficiently reciprocate between the intermediate transfer belt 21 and the recording sheet S in the transfer nip. Thereby, the transfer rate can be improved.
On the other hand, in the DC transfer mode, if the width of the transfer nip is too wide, the toner transferred from the intermediate transfer belt 21 to the recording sheet S in the transfer nip is returned to the intermediate transfer belt while passing through the transfer nip ( There is a concern that the transfer rate is reduced). In this embodiment, the width of the transfer nip is widened in the AC transfer mode in which an abnormal image due to discharge is likely to occur, so that a reduction in transfer rate can be minimized.
While the transfer rate drop due to reverse transfer is more noticeable in the DC transfer mode, abnormal images due to discharge are more noticeable in the AC transfer mode, so the transfer nip width is adjusted to avoid abnormal images that become noticeable in each mode. It is preferable.

(Third embodiment)
Next, a third embodiment of the present invention will be described.
In the third embodiment, the transfer direction bias and the reverse polarity bias are alternately switched to transfer the toner image on the intermediate transfer belt 21 onto the recording sheet S, and the transfer direction bias. In the DC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S without alternately switching the bias and the reverse polarity bias, the contact position of the secondary transfer roller 30 with respect to the intermediate transfer belt 21 Is changed by the contact width changing means 60B to widen the width of the pre-nip n2 that becomes the contact width.

  As shown in FIGS. 7 and 8, the nip width changing unit 60B in the present embodiment includes a support frame 610 that rotatably supports the secondary transfer roller 30 by a support 20A, and a moving unit 611 that slides the support frame 610. It is comprised as a contact position change means which has these. The moving means 611 includes an eccentric cam 664 that contacts the support frame 610 and moves its position, and a drive motor 615 as a drive means that rotationally drives the eccentric cam 664. The support frame 610 is slidable in a direction in which the center position connecting the rotation center of the secondary transfer counter roller 24 and the rotation center of the secondary transfer roller 30 moves in parallel. The eccentric cam 664 is provided to rotate integrally with a drive shaft 667 that is rotated by a drive motor 665. The eccentric cam 664 has a cam surface having a maximum distance from the center of rotation to the top dead center 664a on the outer peripheral surface 664b. That is, in the present embodiment, the nip width changing unit 60B changes the position of the secondary transfer roller 30 in a direction to increase or decrease the offset amount of the secondary transfer roller 30 with respect to the recording material conveyance direction.

  In the present embodiment, the drive motor 615 is connected to the control unit 500 via a signal line, and is configured to control its drive. Specifically, when the DC transfer mode is set, the control unit 500 rotates the eccentric cam 664 in a direction to reduce the offset amount, and when the AC transfer mode is set, as shown in FIG. 8, the offset amount is larger than that in the DC transfer mode. The rotation of the drive motor 615 is controlled so that the eccentric cam 664 is rotated in the direction of increasing the rotation. 7 and 8, the offset decreasing direction is an obliquely lower left direction indicated by an arrow a1, and the offset increasing direction is an obliquely upper right direction indicated by an arrow a2 that is 180 degrees different from the offset decreasing direction.

In this embodiment, the eccentric cam 664 has a home position as a standby position in the DC transfer mode shown in FIG. 7, and its phase is moved in the AC transfer mode to occupy the offset increase position shown in FIG. Has been placed.
For this reason, in this embodiment, when the AC transfer mode is set, the eccentric cam 664 is rotated and the secondary transfer roller 30 moves in a direction to increase the offset amount. The width of n2 can be expanded. In the DC transfer mode, the eccentric cam 664 rotates to move in a direction in which the offset amount of the secondary transfer roller 30 decreases, so that the width of the prenip n2 on the recording material entry side in the AC transfer mode is narrowed. It is done. As a result, since the width of the pre-nip n2 can be increased in the AC transfer mode, the secondary transfer bias is applied while the intermediate transfer belt 21 and the recording sheet S are in contact with each other, so that abnormal images due to discharge can be prevented. it can.
On the other hand, in the DC transfer mode, if the pre-nip n2 that is the contact width between the intermediate transfer belt 21 on the recording material entering side and the secondary transfer roller 30 is excessively widened, the linear velocity between the intermediate transfer belt 21 and the recording sheet S is increased. There is concern that image blur may occur due to the difference. In the present embodiment, the pre-nip n2 is widened in the AC transfer mode in which abnormal images due to discharge are likely to occur. Therefore, the occurrence of image blur is minimized due to the difference in linear velocity between the intermediate transfer belt 21 and the recording sheet S. To the limit.
In general, an abnormal image due to transfer blur is more noticeable in the DC transfer mode, whereas an abnormal image due to discharge is more noticeable in the AC transfer mode. For this reason, it is preferable to adjust the pre-nip width in order to avoid abnormal images that become noticeable in each mode.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described.
In the fourth embodiment, the transfer direction bias and the reverse polarity bias are alternately switched to transfer the toner image on the intermediate transfer belt 21 onto the recording sheet S, and the transfer direction bias. In the DC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S without alternately switching the bias and the reverse polarity bias, the pressure applied to the secondary transfer nip N is changed to change the secondary transfer nip N. The width of the transfer nip N is changed.

  In the present embodiment, the secondary transfer roller 30 is given a pressing force toward the intermediate transfer belt 21 that is wound around the secondary transfer counter roller 24 as shown in FIG. That is, the shaft 20 </ b> A that rotatably supports the secondary transfer roller 30 is supported by the roller unit holder 640. The roller unit holding body 640 extends in the longitudinal direction, and one end in the longitudinal direction is supported by a support shaft 642 so as to be swingable by a device fixing portion (not shown). A compression coil spring 643 is interposed between the other end side in the longitudinal direction of the roller unit holder 640 and the device fixing portion. The roller unit holder 640 is pressed by the restoring force of the compression coil spring 643 so as to rotate counterclockwise around the support shaft 42 in FIG. Accordingly, the secondary transfer roller 30 is pressed toward the intermediate transfer belt 21 to apply pressure to the secondary transfer nip N.

In this embodiment, in such a configuration, a nip width changing unit is provided to change the pressure by the secondary transfer nip 30 applied to the secondary transfer nip N in the AC transfer mode and the DC transfer mode.
The nip width changing means may be configured such that the pressure applied to the secondary transfer nip N is higher in the AC transfer mode than in the DC transfer mode, and two methods can be assumed in general.
The first method uses a spring force of the compression coil spring 643 as a spring force that can obtain a nip width necessary in the AC transfer mode, and suppresses a spring force of the compression coil spring 643 applied to the secondary transfer nip N in the DC transfer mode. It is. The nip width changing means 60C in this case is, as shown in FIGS. 9 and 10, an eccentric cam 674 that contacts the upper portion 640a of the roller unit holder 640 and displaces the position, and a drive that rotationally drives the eccentric cam 674. It can be constituted by a pressure changing means constituted by a drive motor 625 as means. The eccentric cam 674 is provided so as to rotate integrally with a drive shaft 677 that is rotated by a drive motor 625. The eccentric cam 674 has a cam surface having a maximum distance from the center of rotation to the top dead center 674a on the outer peripheral surface 674b.

  In the present embodiment, the drive motor 625 is connected to the control unit 500 via a signal line, and is configured to control its drive. Specifically, the control unit 500 rotates the eccentric cam 664 in a direction to suppress the spring force of the compression coil spring 643 when in the DC transfer mode, and when in the AC transfer mode, the compression coil spring 643 as shown in FIG. The rotation of the drive motor 625 is controlled so that the eccentric cam 674 is rotated in a direction to release the suppression of the spring force. 9 and 10, an arrow a <b> 3 indicates a suppression direction of the spring force applied to the secondary transfer nip N, and an arrow a <b> 4 indicates a suppression release direction of the spring force applied to the secondary transfer nip N. That is, in FIG. 9 and FIG. 10, the suppression direction a3 of the spring force applied to the secondary transfer nip N is the direction in which the other end side of the roller unit holder 640 is pushed down, and the suppression of the spring force applied to the secondary transfer nip N is performed. The release direction a4 is a direction in which the other end side of the roller unit holder 640 is lifted.

In the present embodiment, the eccentric cam 674 stands by in the DC transfer mode shown in FIG. 9 as a home position at a position where the spring force is suppressed (pressed position). It is arranged so as to occupy the nip width increasing position shown in FIG.
Therefore, in the present embodiment, when the AC transfer mode is set, the eccentric cam 674 is rotated by the drive motor 625 in a direction to release the suppression of the spring force of the compression coil spring 643 applied to the secondary transfer nip N. The pressure applied to the secondary transfer nip N by the roller 30 can be made stronger than in the DC transfer mode, and the width of the transfer nip N in the AC transfer mode can be widened. In the DC transfer mode, the eccentric cam 674 rotates and the other end side of the roller unit holder 640 is pushed down, so that the pressure applied to the secondary transfer nip N by the secondary transfer roller 30 is higher than that in the AC transfer mode. Weakened.
As described above, since the width of the secondary transfer nip N can be increased in the AC transfer mode, the secondary transfer bias is applied while the intermediate transfer belt 21 and the recording sheet S are in close contact with each other. Can be prevented. Further, by increasing the width of the secondary transfer nip N, the toner can sufficiently reciprocate between the intermediate transfer belt 21 and the recording sheet S in the secondary transfer nip. Thereby, the transfer rate can be improved.
On the other hand, in the DC transfer mode, if the width of the secondary transfer nip N is excessively widened, the toner transferred from the intermediate transfer belt 21 to the recording sheet S in the transfer nip moves toward the intermediate transfer belt while passing through the transfer nip. There is a concern that the transfer rate is lowered (reverse transfer) and the transfer rate is lowered. In this embodiment, the width of the transfer nip is widened in the AC transfer mode in which an abnormal image due to discharge is likely to occur, so that a reduction in transfer rate can be minimized.
While the transfer rate drop due to reverse transfer is more noticeable in the DC transfer mode, abnormal images due to discharge are more noticeable in the AC transfer mode, so the transfer nip width is adjusted to avoid abnormal images that become noticeable in each mode. That is preferred.

  The second method is a method of increasing the pressure applied to the secondary transfer nip N in the AC transfer mode by using the spring force of the compression coil spring 643 as a spring force capable of obtaining a necessary nip width in the DC transfer mode. The nip width changing means 60D in this case is, as shown in FIGS. 11 and 12, an eccentric cam 674 that contacts the lower portion 640b of the roller unit holder 640 and displaces the position, and a drive that rotationally drives the eccentric cam 674. A drive motor 625 as a means can be used. The eccentric cam 674 is provided so as to rotate integrally with a drive shaft 677 that is rotated by a drive motor 625. The eccentric cam 674 has a cam surface having a maximum distance from the center of rotation to the top dead center 674a on the outer peripheral surface 674b.

  Also in this embodiment, the drive motor 625 is connected to the control unit 500 via a signal line, and is configured to control its drive. Specifically, when the DC transfer mode is entered, the control means 500 rotates the eccentric cam 664 in the direction to be applied at the set value without suppressing the spring force of the compression coil spring 643 as shown in FIG. In the transfer mode, as shown in FIG. 12, the rotation of the drive motor 625 is controlled so that the eccentric cam 674 is rotated in a direction in which an auxiliary force is applied to the spring force of the compression coil spring 643. 11 and 12, an arrow a5 indicates a direction in which the pressure applied to the secondary transfer nip N increases, and an arrow a6 indicates a direction in which the pressure applied to the secondary transfer nip N is released. That is, in FIG. 11 and FIG. 12, the increasing direction a5 of the pressure applied to the secondary transfer nip N is the direction in which the other end side of the roller unit holder 640 is pushed up by the eccentric cam 674, and the pressure applied to the secondary transfer nip N The increase release direction a6 is a direction in which the other end side of the roller unit holder 640 is lowered.

In this embodiment, the eccentric cam 674 waits in the DC transfer mode shown in FIG. 11 and sets the position where the pressure applied to the secondary transfer nip N does not increase (lowered position) as the home position, and when the AC transfer mode is entered. The top dead center 674a is moved toward the lower surface 640b of the roller unit holder 640, and is arranged so as to occupy the nip width increasing position shown in FIG.
For this reason, in the present embodiment, when the AC transfer mode is set, the eccentric cam 674 is rotated by the drive motor 625 in the direction of increasing the pressure applied to the secondary transfer nip N by lifting the roller unit holder 640. Therefore, the pressure applied to the secondary transfer nip N by the secondary transfer roller 30 can be made stronger than in the DC transfer mode, and the width of the secondary transfer nip N in the AC transfer mode can be widened. As a result, since the width of the secondary transfer nip N can be increased in the AC transfer mode, the secondary transfer bias is applied in a state where the intermediate transfer belt 21 and the recording sheet S are in close contact with each other. Can be prevented. Further, by increasing the width of the secondary transfer nip, the toner can sufficiently reciprocate between the intermediate transfer belt 21 and the recording sheet S in the secondary transfer nip. Thereby, the transfer rate can be improved.
In the DC transfer mode, the eccentric cam 674 rotates and the other end side of the roller unit holder 640 is lowered, so that the pressure applied to the secondary transfer nip by the secondary transfer roller 30 is weaker than in the AC transfer mode. It is done. In the DC transfer mode, if the width of the secondary transfer nip N is excessively widened, the intermediate transfer is performed while the toner transferred from the intermediate transfer belt 21 to the recording sheet S passes through the secondary transfer nip N in the secondary transfer nip. There is a concern that the belt is returned to the belt side (reverse transfer) and the transfer rate is lowered. In the present embodiment, the width of the secondary transfer nip N is widened in the AC transfer mode in which abnormal images are likely to occur due to discharge, so that a reduction in transfer rate can be minimized.

Even in this case, since the amount of the secondary transfer nip N can be increased in the AC transfer mode, the secondary transfer bias is applied while the intermediate transfer belt 21 and the recording sheet S are in close contact with each other. Images can be prevented.
Further, if the width of the secondary transfer nip N is excessively widened, there is a concern that image blurring may occur due to a difference in linear velocity between the intermediate transfer belt 21 and the recording sheet S. The widening is performed in the AC transfer mode in which abnormal images due to discharge are likely to occur, so that a reduction in transfer rate can be minimized.
While the transfer rate decrease due to reverse transfer is more noticeable in the DC transfer mode, abnormal images due to discharge are more noticeable in the AC transfer mode. Therefore, the width of the secondary transfer nip is avoided in order to avoid abnormal images that become noticeable in each mode. It is very preferable to adjust the value.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described.
In the fifth embodiment, the transfer direction bias and the reverse polarity bias are switched alternately to transfer the toner image on the intermediate transfer belt 21 to the recording sheet S, and the transfer direction bias. In the DC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S without alternately switching the bias and the reverse polarity bias, the pressure applied to the secondary transfer nip N is changed to change the secondary transfer nip N. The width of the transfer nip N is changed.

  In the present embodiment, the secondary transfer roller 30 is given a pressing force toward the intermediate transfer belt 21 that is wound around the secondary transfer counter roller 24 as shown in FIG. That is, the shaft 20 </ b> A that rotatably supports the secondary transfer roller 30 is supported by the roller unit holder 650. The roller unit holder 650 extends in the longitudinal direction, and one end side in the longitudinal direction is supported by an apparatus fixing portion (not shown) by a support shaft 652 so as to be swingable. A compression coil spring 653 is interposed between the lower portion 650b on the other end side in the longitudinal direction of the roller unit holder 650 and the apparatus fixing portion. The roller unit holder 650 is pressed by the restoring force of the compression coil spring 653 so as to rotate counterclockwise around the support shaft 652 in FIG. Accordingly, the secondary transfer roller 30 is pressed toward the intermediate transfer belt 21 to apply pressure to the secondary transfer nip N.

In this embodiment, in such a configuration, a nip width changing unit 60E is provided to change the pressure by the secondary transfer nip 30 applied to the secondary transfer nip N in the AC transfer mode and the DC transfer mode. .
The nip width changing means 60E includes idle rollers 685 and 685 provided on both sides of the secondary transfer roller 30 in the axial direction, cams 684 and 684 provided on both sides of the secondary transfer counter roller 24 in the axial direction, The pressurizing force changing means may include a driving motor 635 as a driving means for driving the cam 684 to rotate. 13 and 14, only one side of the cam 684 and the idling roller 685 is shown.
Each cam 684 has a part 684a formed on the outer peripheral surface 684b that protrudes to the outer diameter side of the outer peripheral surface 24a of the secondary transfer counter roller 24, and can rotate around the shaft 24A so as to have the same phase. It is supported by. These cams 684 are rotationally driven by the drive motor 635 while maintaining the same phase. Each idling roller 685 has a larger diameter than the secondary transfer roller 30 and is always in pressure contact with the outer peripheral surface 684a of each cam 684 by the spring force of the compression coil spring 653.

  In the present embodiment, the drive motor 635 is connected to the control unit 500 via a signal line, and is configured to control its drive. Specifically, the control unit 500 rotates the cam 684 in a direction to suppress the spring force of the compression coil spring 653 when in the DC transfer mode, and when in the AC transfer mode, as shown in FIG. The rotation of the drive motor 635 is controlled so that the eccentric cam 684 is rotated in the direction in which the suppression of the spring force is released. That is, in the DC transfer mode, the controller 500 rotates the cam 684 to a position where the protruding portion 684a faces the idling roller 685, and pushes down the secondary transfer roller 30 to 2 as shown in FIG. When the width of the next transfer nip is narrowed and the AC transfer mode is set, the cam 684 is rotated so that the protruding portion 684a moves away from the idling roller 685, and the spring force of the compression coil spring 653 associated with the secondary transfer roller 30 is The drive motor 635 is controlled to push up the secondary transfer roller 30 to widen the secondary transfer nip width.

  13 and 14, an arrow a7 indicates a direction in which the spring force applied to the secondary transfer nip N is suppressed, and an arrow a8 indicates a direction in which the spring force applied to the secondary transfer nip N is released. That is, in FIG. 13 and FIG. 14, the suppression direction a7 of the spring force applied to the secondary transfer nip N is the direction in which the other end side of the roller unit holder 650 is pushed down, and the suppression of the spring force applied to the secondary transfer nip N is performed. The release direction a8 is a direction in which the other end side of the roller unit holder 650 is lifted.

In the present embodiment, the cam 684 stands by in the DC transfer mode shown in FIG. 13 and sets the position where the spring force is suppressed (pressed position) as the home position, and when in the AC transfer mode, the phase moves upward. Thus, it is arranged so as to occupy the nip width increasing position shown in FIG.
For this reason, in this embodiment, when the AC transfer mode is set, the cam 684 is rotated by the drive motor 635 in a direction to release the suppression of the spring force of the compression coil spring 653 applied to the secondary transfer nip N. Therefore, the pressure applied to the secondary transfer nip N by the secondary transfer roller 30 can be made stronger than in the DC transfer mode, and the width of the secondary transfer nip N in the AC transfer mode can be widened. In the DC transfer mode, the cam 684 rotates and the other end of the roller unit holder 650 is pushed down, so that the pressure applied to the secondary transfer nip N by the secondary transfer roller 30 is weaker than in the AC transfer mode. It is done.

Thus, in the AC transfer mode, the secondary transfer nip N is increased to increase the width of the secondary transfer nip N so that the secondary transfer bias is applied while the intermediate transfer belt 21 and the recording sheet S are in close contact with each other. Can do. Further, by increasing the width of the secondary transfer nip N, the toner can sufficiently reciprocate between the intermediate transfer belt 21 and the recording sheet S in the secondary transfer nip. Thereby, the transfer rate can be improved.
On the other hand, in the DC transfer mode, if the width of the secondary transfer nip is excessively widened, the toner transferred from the intermediate transfer belt 21 to the recording sheet S in the secondary transfer nip passes through the transfer nip and the intermediate transfer belt side. There is a concern that the transfer rate may be lowered due to the reverse (reverse transfer). In the present embodiment, the width of the secondary transfer nip is widened in the AC transfer mode in which abnormal images are likely to occur due to discharge, so that a reduction in transfer rate can be minimized.
While the transfer rate drop due to reverse transfer is more noticeable in the DC transfer mode, abnormal images due to discharge are more noticeable in the AC transfer mode, so the transfer nip width is adjusted to avoid abnormal images that become noticeable in each mode. It is preferable.
In the present embodiment, the cam 684 is disposed on the secondary transfer counter roller 24 and the idling roller 685 is disposed on the secondary transfer roller 30 side. However, these may be reversed.

(Sixth embodiment)
Next, a sixth embodiment of the present invention will be described.
In the sixth embodiment, the transfer direction bias and the reverse polarity bias are alternately switched to transfer the toner image on the intermediate transfer belt 21 to the recording sheet S, and the transfer direction bias. In the case of the DC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S without alternately switching the bias and the reverse polarity bias, the rotational linear velocity of the pair of resist rollers 95 as the conveying member The length of contact between the recording sheet S and the intermediate transfer belt 21 can be changed by changing the rotational linear velocity of the secondary transfer roller 30 as a transfer member with respect to V0.
That is, the sixth embodiment includes a registration roller pair 95 that conveys the recording sheet S toward the secondary transfer nip N, and the second mode when the power source 39 is in the AC transfer mode that supplies the first transfer bias. The recording material contact width changing means 380B for slowing the rotational linear velocity of the secondary transfer roller 30 relative to the rotational linear velocity of the resist roller pair 95 is provided as compared with the DC transfer mode in which the transfer bias is supplied.

Specifically, the rotational linear speed of the secondary transfer roller 30 with respect to the rotational speed of the registration roller pair 95 in the AC transfer mode is set as the rotational line of the secondary transfer roller 30 with respect to the rotational speed of the registration roller pair 95 in the DC transfer mode. I made it slower than the speed.
In the case of the present embodiment in which the contact length of the recording sheet S is adjusted, there is a problem that a line image is scattered by air if the contact length is long. Although this is not directly attributable to the secondary transfer bias, it is a problem that is likely to occur with coated paper with little surface unevenness and difficult to occur with uneven paper. Therefore, when the recording sheet S is coated paper or plain paper, transfer using a secondary transfer bias of only DC is performed as the DC transfer mode, and when the recording sheet S is concavo-convex paper, DC transfer is performed as the AC transfer mode. And transfer using an alternating superimposed bias. That is, when the transfer mode by the secondary transfer bias is controlled to be switched between the DC transfer mode and the AC transfer mode according to the paper type of the recording sheet S used for image transfer, as shown in FIG. Plain paper), the contact length C1 between the intermediate transfer belt 21 and the recording sheet S is reduced. In the AC transfer mode (uneven paper), the contact between the intermediate transfer belt 21 and the recording sheet S as shown in FIG. The controller 500 controls the linear rotation speed of the secondary transfer roller 30 so that the length C2 is longer than the contact length C1.

  Thus, in the AC transfer mode (uneven paper), if the contact length C2 between the intermediate transfer belt 21 and the recording sheet S is longer than the contact length C1 in the DC transfer mode, discharge can be prevented preferentially. it can. Also, in the DC transfer mode, line dust can be preferentially prevented by making the contact length C1 between the intermediate transfer belt 21 and the recording sheet S shorter than the contact length C2.

Next, a control mode of the linear rotation speed of the resist roller pair 95 and the secondary transfer roller 30 in the DC transfer mode and the AC transfer mode will be specifically described.
As shown in FIGS. 15 and 16, in this embodiment, the controller 500 includes a drive motor 735 that rotationally drives the drive roller 22 and a drive motor 736 that serves as a drive unit that rotationally drives the registration roller pair 95. It is connected via a signal line, and its rotation speed (number of rotations) can be controlled. The drive of the drive motor 736 is controlled by the controller 500 so that the recording sheet S is sent out toward the secondary transfer nip N at a fixed timing. In this embodiment, in the DC transfer mode and the AC transfer mode, the registration roller pair 95 is rotationally driven at the same rotation linear velocity.

In the case of the embodiment shown in FIGS. 15 and 16, since the secondary transfer roller 30 is pressed against the intermediate transfer belt 21, the drive roller 22 is rotationally driven by the drive motor 735 so that the intermediate transfer belt 21 is shown in FIG. When moving in the surrounding direction, the movement rotates around the axis 30A. That is, in this embodiment, the rotational linear velocity of the secondary transfer roller 30 is indirectly controlled by adjusting the rotational linear velocity of the intermediate transfer belt 21.
In the present embodiment, the entrance roller 61 urges the intermediate transfer belt 21 to the outside of the belt loop with a constant force that engages the intermediate transfer belt 21 in the transfer mode on the upstream side of the secondary transfer nip in the belt movement direction. For this reason, in this embodiment, the width of the secondary transfer nip N is constant even in either the DC transfer mode or the AC transfer mode.

  In the DC transfer mode, the control unit 500 controls the rotational speed of the drive motor 735 so that the contact length between the recording sheet S and the intermediate transfer belt 21 becomes the rotational linear speed V1 of the secondary transfer roller 30 that is C1. To do. In the AC transfer mode, the controller 500 is configured so that the contact length between the recording sheet S and the intermediate transfer belt 21 becomes the rotational linear velocity V2 of the secondary transfer roller 30 that is C2 that is longer than the contact length C1. The rotational speed of the drive motor 735 is controlled. That is, C1> C2, and the rotational linear velocity V1> the rotational linear velocity V2. Detection of the rotational linear velocities V1 and V2 of the secondary transfer roller 30 is detected from the rotational speed of the drive motor 735 by known speed detection means (not shown) such as a rotary encoder. Detection information (speed information) detected by the speed detection unit is transmitted to the control unit 500. The controller 500 controls the rotation of the drive motor 735 until the detection information from the work degree detection means becomes V1 and V2. That is, in the present embodiment, the recording material contact width changing unit 380B includes the drive motors 735 and 736 and the control unit 500.

  Thus, in the AC transfer mode, when the rotation linear velocity V2 of the secondary transfer roller 30 is made slower than the rotation linear velocity V1 in the DC transfer mode, the registration roller pair 95 is driven to rotate toward the secondary transfer nip N. As shown in FIG. 16, the recording sheet S is bent on the upstream side of the secondary transfer nip. For this reason, in the AC transfer mode, the contact length between the intermediate transfer belt 21 and the recording sheet S on the upstream side of the secondary transfer nip N becomes longer. For example, when the contact length between the intermediate transfer belt 21 and the recording sheet S in the DC transfer mode is C1, and the contact length between the intermediate transfer belt 21 and the recording sheet S in the AC transfer mode is C2, the relationship C1 <C2 is established. .

That is, the recording sheet when entering the secondary transfer nip is set by making the rotational linear velocity V2 of the secondary transfer roller 30 in the AC transfer mode slower than the rotational linear velocity V1 of the secondary transfer roller 30 in the DC transfer mode. The amount of slackness of S can be increased. For this reason, the contact length between the intermediate transfer belt 21 and the transfer sheet S is increased in the portion before the secondary transfer nip by the amount of increase in the slackness of the recording sheet S, so that a discharge image can be prevented.
Further, when the amount of slack of the recording sheet S is increased in the AC transfer mode, a line dust image on the coated paper is likely to occur. This is because, when the recording sheet S enters the secondary transfer nip portion, a phenomenon that the line image is scattered by air occurs. For this reason, when using coated paper that does not easily allow air to escape as the recording sheet S, it is preferable to limit the control to increase the slack amount of the recording sheet S only during AC transfer. Further, in the AC transfer mode, when the recording sheet S used is a concavo-convex paper for the purpose of improving the transferability of the concavo-convex paper, a post-tilt image is not generated.

(Seventh embodiment)
Next, a seventh embodiment of the present invention will be described.
In this embodiment, the secondary transfer roller 30 is not driven to rotate, but is driven to rotate separately from the intermediate transfer belt 21 by using a drive motor 737 as a drive means, and the rotation linear speeds V1 and V2 are controlled. To do. As in the sixth embodiment, the control directionality is the linear rotation speed V2 of the secondary transfer roller 30 with respect to the rotation speed V0 of the registration roller pair 95 in the AC transfer mode, and the registration roller pair 95 in the DC transfer mode. The rotational speed V0 is set to be slower than the linear rotation speed V1 of the secondary transfer roller 30.
That is, when the transfer mode by the secondary transfer bias is controlled to be switched between the DC transfer mode and the AC transfer mode according to the paper type of the recording sheet S used for image transfer, as shown in FIG. Plain paper), the contact length C1 between the intermediate transfer belt 21 and the recording sheet S is reduced. In the AC transfer mode (uneven paper), the contact between the intermediate transfer belt 21 and the recording sheet S as shown in FIG. The controller 500 controls the linear rotation speed of the secondary transfer roller 30 so that the length C2 is longer than the contact length C1.
That is, in the seventh embodiment, when the AC transfer mode has the registration roller pair 95 that conveys the recording sheet S toward the secondary transfer nip N and the power source 39 supplies the first transfer bias, Compared with the DC transfer mode in which the second transfer bias is supplied, the recording material contact width changing unit 380C that slows the rotation linear velocity of the secondary transfer roller 30 with respect to the rotation linear velocity of the resist roller pair 95 is provided. That is, in the present embodiment, the recording material contact width changing unit 380C includes the drive motors 736 and 737 and the control unit 500.

Thus, in the AC transfer mode (uneven paper), if the contact length C2 between the intermediate transfer belt 21 and the recording sheet S is longer than the contact length C1 in the DC transfer mode, discharge can be prevented preferentially. it can. Also, in the DC transfer mode, line dust can be preferentially prevented by making the contact length C1 between the intermediate transfer belt 21 and the recording sheet S shorter than the contact length C2.
As shown in FIGS. 17 and 18, in this embodiment, the control unit 500 includes a drive motor 735 that rotationally drives the drive roller, a drive motor 737 that rotationally drives the secondary transfer roller 30, and a registration roller pair 95. Is connected via a signal line, and the rotation speed (number of rotations) can be controlled. The drive of the drive motor 736 is controlled by the controller 500 so that the recording sheet S is sent out toward the secondary transfer nip N at a fixed timing. In this embodiment, in the DC transfer mode and the AC transfer mode, the registration roller pair 95 is rotationally driven at the same rotation linear velocity.

In the case of the seventh embodiment shown in FIGS. 17 and 18, the secondary transfer roller 30 rotates around the shaft 30A when the drive motor 737 is driven to rotate. Further, the intermediate transfer belt 21 rotates clockwise as the drive motor 735 is driven to rotate and the drive roller 22 rotates. In this embodiment, the rotation of the drive motor 735 and the drive motor 737 is controlled by the control unit 500 so that the linear rotation speeds of the intermediate transfer belt 21 and the secondary transfer roller 30 are the same. Similar to the sixth embodiment, the rotational linear velocities V1 and V2 are detected by the speed detecting means, the rotational speeds of the drive motors 735 and 737 are detected, and the controller 500 until the rotational linear velocities V1 and V2 are obtained. Thus, the drive motors 735 and 737 are driven to rotate.
In the DC transfer mode, the control unit 500 controls the rotational speed of the drive motor 735 so that the contact length between the recording sheet S and the intermediate transfer belt 21 becomes the rotational linear speed V1 of the secondary transfer roller 30 that is C1. To do. In the AC transfer mode, the controller 500 is configured so that the contact length between the recording sheet S and the intermediate transfer belt 21 becomes the rotational linear velocity V2 of the secondary transfer roller 30 that is C2 that is longer than the contact length C1. The rotational speeds of the drive motors 735 and 737 are controlled. That is, C1> C2, and the rotational linear velocity V1> the rotational linear velocity V2.

  Thus, in the AC transfer mode, when the rotation linear velocity V2 of the secondary transfer roller 30 is made slower than the rotation linear velocity V1 in the DC transfer mode, the registration roller pair 95 is driven to rotate toward the secondary transfer nip N. As shown in FIG. 17, the recording sheet S is bent on the upstream side of the secondary transfer nip. For this reason, in the AC transfer mode, the contact length between the intermediate transfer belt 21 and the recording sheet S on the upstream side of the secondary transfer nip N becomes longer. For example, when the contact length between the intermediate transfer belt 21 and the recording sheet S in the DC transfer mode is C1, and the contact length between the intermediate transfer belt 21 and the recording sheet S in the AC transfer mode is C2, the relationship C1 <C2 is established. .

That is, the recording sheet when entering the secondary transfer nip is set by making the rotational linear velocity V2 of the secondary transfer roller 30 in the AC transfer mode slower than the rotational linear velocity V1 of the secondary transfer roller 30 in the DC transfer mode. The amount of slackness of S can be increased. For this reason, the contact length between the intermediate transfer belt 21 and the transfer sheet S is increased in the portion before the secondary transfer nip by the amount of increase in the slackness of the recording sheet S, so that a discharge image can be prevented.
Further, when the amount of slack of the recording sheet S is increased in the AC transfer mode, a line dust image on the coated paper is likely to occur. This is because, when the recording sheet S enters the secondary transfer nip portion, a phenomenon that the line image is scattered by air occurs. For this reason, when using coated paper that does not easily allow air to escape as the recording sheet S, it is preferable to limit the control to increase the slack amount of the recording sheet S only during AC transfer. Further, in the AC transfer mode, when the recording sheet S used is a concavo-convex paper for the purpose of improving the transferability of the concavo-convex paper, a post-tilt image is not generated.

  In the sixth and seventh embodiments, the linear rotational speeds V1 and V2 of the secondary transfer roller 30 in the DC transfer mode and the AC transfer mode are described as linear speed differences. Even if it is controlled by changing the linear velocity ratio (ratio between the rotational linear velocity V1 and the rotational linear velocity V2) of the transfer roller 30, the contact length C2 between the intermediate transfer belt 21 and the recording sheet S in the AC transfer mode is set. It can be made longer than the contact length C1 in the DC transfer mode.

Hereinafter, specific examples of setting values in the sixth and seventh embodiments will be described.
Linear speed difference setting range of secondary transfer roller 30 in DC transfer mode: ± 0.5%
Linear speed difference setting range of the secondary transfer roller 30 in the AC transfer mode: + 0.5% to -1.0%.
Linear speed difference setting example 1: + 0.5% during DC transfer, 0% during AC transfer
Linear speed difference setting example 2: -0.3% during DC transfer, -0.5% during AC transfer
As a method for detecting the rotational linear velocities V1 and V2 of the secondary transfer roller 30 in the sixth and seventh embodiments, for example, a speed detection means such as a rotary encoder can be used without detecting the rotation of the drive motors 735 and 737. Alternatively, the rotation of the secondary transfer roller 30 may be directly detected, and the drive of the drive motors 735 and 737 may be controlled so that the detected value becomes a predetermined rotation linear velocity. Alternatively, when the drive motors 735 and 737 are PWM (Pulse Width Modulation) controllable motors or stepping motors, the drive motor PWM signal or the stepping motor pulse signal corresponding to the rotational speed is monitored, and the value is a predetermined value. The rotational linear velocities V1 and V2 may be controlled by setting so as to be.

Next, the secondary transfer bias including transfer bias and transfer characteristics will be described.
In each of the above-described embodiments, the secondary transfer counter roller 24 is applied with the secondary transfer bias composed of the superimposed bias in which the AC voltage is superimposed on the DC voltage by the power source 39 in the AC transfer mode.
The DC component of the secondary transfer bias has the same value as the time average value (Vave) of the voltage, that is, the time average voltage value (time average value) Vave as the voltage of the DC component. The time average value Vave of the voltage is a value obtained by dividing the integral value over one period of the voltage waveform by the length of one period.
In the present color copying apparatus in which the secondary transfer bias is applied to the secondary transfer counter roller 24 and the secondary transfer roller 30 is grounded, the secondary transfer bias has the same negative polarity as that of the toner. In the transfer nip N, negative polarity toner is electrostatically pushed from the secondary transfer counter roller 24 side to the secondary transfer roller 30 side. As a result, the toner on the intermediate transfer belt 21 is transferred onto the recording sheet S. On the other hand, when the polarity of the superimposed bias is a positive polarity opposite to the toner, the negative polarity toner is directed from the secondary transfer roller 30 side to the secondary transfer counter roller 24 side in the secondary transfer nip N. Pulls electrostatically. As a result, the toner transferred to the recording sheet S is attracted again to the intermediate transfer belt 21 side.

  By the way, if a recording sheet S that is rich in surface irregularities such as Japanese paper is used, it becomes easy to generate a gray pattern in the image according to the surface irregularities. Not only a bias but also a superimposed bias obtained by superimposing a DC bias on an AC bias is applied as a secondary transfer bias.

  However, the present inventors have found through experimentation that in such a configuration, it is easy to generate a plurality of white spots at an image portion formed on a concave portion on the paper surface. Therefore, the present inventors have conducted extensive research on the cause of white spots, and have found the following. That is, FIG. 19 is a conceptual diagram schematically showing an example of the secondary transfer nip N. In the drawing, the intermediate transfer belt 531 is pressed toward the nip forming roller 536 by the secondary transfer counter roller 533 in contact with the back surface thereof. By this pressing, a secondary transfer nip N in which the front surface of the intermediate transfer belt 531 and the nip forming roller 536 are in contact is formed. The toner image on the intermediate transfer belt 531 is secondarily transferred to the recording sheet S sent to the secondary transfer nip N. A secondary transfer bias for secondary transfer of the toner image is applied to one of the two rollers shown in the figure, and the other roller is grounded. It is possible to transfer the toner image to the recording sheet S by applying a transfer bias to either roller, but it is a case where the secondary transfer bias is applied to the secondary transfer counter roller 533 and the toner is used as the toner. A case of using a negative polarity will be described as an example. In this case, in order to move the toner in the secondary transfer nip N from the secondary transfer counter roller 533 side to the nip forming roller 536 side, the time average value of the potential is used as the secondary transfer bias composed of the superimposed bias. Apply a potential that has the same negative polarity as the polarity.

  FIG. 20 is a diagram illustrating an example of a waveform of the secondary transfer bias composed of a superimposed bias applied to the secondary transfer counter roller 533. In the figure, a time average voltage (hereinafter referred to as “time average value”) Vave [V] represents a time average value of the secondary transfer bias. As shown in FIG. 20, the secondary transfer bias composed of the superimposed bias has a sinusoidal shape as shown in FIG. 20, and has a peak value on the return direction side and a peak value on the transfer direction side. Yes. Of these two peak values, the symbol Vt is the peak value on the side (transfer direction side) that moves the toner from the belt side to the nip forming roller 536 side in the secondary transfer nip N. (Hereinafter referred to as “transfer direction peak value Vt”). Also, the symbol Vr is a peak value in the direction of returning the toner from the nip forming roller 536 side to the belt side (return direction side) (hereinafter referred to as a return peak value Vr). Further, instead of the superimposed bias as shown in the figure, it is possible to reciprocate the toner between the belt and the recording material in the secondary transfer nip N even when an AC bias consisting only of an AC component is applied. However, with the AC bias alone, the toner cannot be transferred onto the recording sheet S simply by reciprocating. By applying a superimposed bias including a DC component and setting the time average voltage Vave [V], which is the time average value, to the same negative polarity as that of the toner, recording is performed relatively from the belt side while reciprocating the toner. It can be moved to the recording material side and transferred onto the recording material.

  The present inventors observed the reciprocal movement and found the following. That is, when the application of the secondary transfer bias is started, first, only a very small amount of toner particles existing on the surface of the toner layer on the intermediate transfer belt 531 are detached from the toner layer, and a concave portion on the surface of the recording material is formed. Head in. However, most toner particles in the toner layer remain in the toner layer. The very few toner particles separated from the toner layer enter the recesses on the surface of the recording material and then return to the toner layer from the recesses when the direction of the electric field is reversed. At this time, the returned toner particles collide with the toner particles remaining in the toner layer, and weaken the adhesion of the toner particles to the toner layer (or recording material). Then, when the electric field is next reversed in the direction toward the recording sheet S, more toner particles than in the beginning are detached from the toner layer and directed toward the concave portion on the surface of the recording material. By repeating such a series of behaviors, the number of toner particles that are separated from the toner layer and enter the recesses on the surface of the recording material is gradually increased, and a sufficient amount of toner particles are placed in the recesses. It was found that it was transferred.

  In the configuration in which the toner particles are reciprocally moved in this way, the toner particles that have entered the recesses on the surface of the recording material are sufficient for the toner layer on the belt unless the return peak value Vr shown in FIG. The image density cannot be pulled back to the image, and the image density is insufficient on the concave portion. If the time average value Vave [V] of the secondary transfer bias is not set to a relatively large value, a sufficient amount of toner cannot be transferred to the convex portion on the surface of the recording material, and an image is formed on the convex portion. Insufficient concentration will occur. In order to obtain a sufficient image density at both the convex and concave portions on the surface of the recording material, the maximum value and the minimum value of the voltage are set so that the time average value Vave [V] and the return peak value Vr are respectively large to some extent. It is necessary to set the voltage (hereinafter referred to as “peak-to-peak voltage”) Vpp from the return peak value Vr that is the value width to the transfer direction peak value Vt to a relatively large value. Then, the transfer direction peak value Vt is inevitably set to a relatively large value. The transfer direction peak value Vt corresponds to the maximum potential difference between the grounded nip forming roller 536 and the secondary transfer counter roller 533 to which the secondary transfer bias is applied. Is likely to occur. In particular, it is easy to cause a white spot in an image portion on the concave portion by generating a discharge in a minute gap formed between the intermediate transfer belt and the concave portion on the surface of the recording material. In order to obtain sufficient image density at the convex portions and concave portions on the surface of the recording material, white spots are generated at image portions on the concave portions of the recording material surface by setting the peak-to-peak voltage Vpp to a relatively large value. It turned out that it was easy to do.

Next, observation experiments conducted by the present inventors will be described in detail.
In order to observe the behavior of the toner in the secondary transfer nip N, the present inventors manufactured a special observation experimental device. FIG. 21 is a schematic configuration diagram showing the observation experimental apparatus. This observation experimental apparatus includes a transparent substrate 210, a developing device 231, a Z stage 220, an illumination 241, a microscope 242, a high-speed camera 243, a personal computer 244, and the like. The transparent substrate 210 includes a glass plate 211, a transparent electrode 212 made of ITO (Indium Tin Oxide) formed on the lower surface thereof, and a transparent insulating layer 213 made of a transparent material coated on the transparent electrode 212. doing. The transparent substrate 210 is supported at a predetermined height by substrate support means (not shown). The substrate support means is configured to be movable in the vertical and horizontal directions in FIG. 7 by a moving mechanism (not shown). In the illustrated example, the transparent substrate 210 is positioned on the Z stage 220 on which the metal plate 215 is placed. However, the movement of the substrate support means causes the developing device 231 disposed on the side of the Z stage 220 to move. It is also possible to move directly above. The transparent electrode 212 of the transparent substrate 210 is connected to an electrode fixed to the substrate support means, and this electrode is grounded.

  The developing device 231 has the same configuration as the developing device of the printer according to the embodiment, and includes a screw member 232, a developing roll 233, a doctor blade 234, and the like. The developing roll 233 is rotationally driven with a developing bias applied by the power source 235.

  When the transparent substrate 210 is moved at a predetermined speed to a position directly above the developing device 231 and facing the developing roll 233 via a predetermined gap by the movement of the substrate support means, the toner on the developing roll 233 Is transferred onto the transparent electrode 212 of the transparent substrate 210. As a result, a toner layer 216 having a predetermined thickness is formed on the transparent electrode 212 of the transparent substrate 210. The toner adhesion amount per unit area with respect to the toner layer 216 includes the developer toner density, the toner charge amount, the developing bias value, the gap between the substrate 210 and the developing roll 233, the moving speed of the transparent substrate 210, and the rotation of the developing roll 233. The speed can be adjusted.

  The transparent substrate 210 on which the toner layer 216 is formed is translated to a position facing the recording material 214 attached to the planar metal plate 215 with a conductive adhesive. The metal plate 215 is installed on a substrate 221 provided with a weight sensor (not shown), and the substrate 221 is installed on the Z stage 220. The metal plate 215 is connected to the voltage amplifier 217. A transfer bias composed of a DC voltage and an alternating voltage is input to the voltage amplifier 217 by the waveform generator 218, and a transfer bias amplified by the voltage amplifier 217 is applied to the metal plate 215. When the Z plate 220 is driven and controlled to raise the metal plate 215, the recording material 214 starts to contact the toner layer 216. When the metal plate 215 is further raised, the pressure on the toner layer 216 increases, but the rise of the metal plate 215 is controlled and stopped so that the output from the weight sensor becomes a predetermined value. With the pressure set to a predetermined value, a transfer bias is applied to the metal plate 215 to observe the behavior of the toner. After the observation, the Z stage 220 is driven and controlled, the metal plate 215 is lowered, and the recording material 214 is separated from the transparent substrate 210. Then, the toner layer 216 is transferred onto the recording material 214.

  The behavior of the toner is observed using a microscope 242 and a high-speed camera 243 disposed above the transparent substrate 210. In the transparent substrate 210, the glass plate 211, the transparent electrode 212, and the transparent insulating layer 213 are all made of a transparent material, so that the transparent substrate 210 is below the transparent substrate 210 from above the transparent electrode 210. The behavior of the toner can be observed.

  As the microscope 242, a zoom lens made of KEYENCE zoom lens VH-Z75 was used. As the high-speed camera 243, FASTCAM-MAX 120KC manufactured by Photoron Co. was used. Photolon FASTCAM-MAX 120KC is driven and controlled by a personal computer 244. The microscope 242 and the high-speed camera 243 are supported by camera support means (not shown). This camera support means is configured so that the focus of the microscope 242 can be adjusted.

  The behavior of the toner on the transparent substrate 210 was photographed as follows. That is, first, the illumination light irradiates the observation position of the toner behavior with the illumination 241 to adjust the focus of the microscope 242. Next, a transfer bias is applied to the metal plate 215 to move the toner of the toner layer 216 attached to the lower surface of the transparent substrate 210 toward the recording material 214. The behavior of the toner at this time was photographed with a high-speed camera 243.

Since the observation experimental apparatus shown in FIG. 21 and the printer according to the embodiment have different transfer nip structures for transferring toner to a recording material, the transfer electric field acting on the toner is different even if the transfer bias is the same. . In order to investigate the appropriate observation conditions, we also examined the transfer bias conditions that give good recess density reproducibility even with an observation experimental apparatus. As the recording material 214, what was called FC Japanese paper type “Sazanami” manufactured by NBS Ricoh Co., Ltd. was used. As the toner, a Y toner having an average particle diameter of 6.8 [μm] mixed with a small amount of K toner was used. Since the observation experimental apparatus is configured to apply a transfer bias to the back surface of the recording material (ripple wave), the polarity of the transfer bias capable of transferring the toner to the recording material is opposite to that of the printer according to the embodiment. (Ie, positive polarity). As the AC component of the secondary transfer bias composed of the superimposed bias, one having a sine wave waveform was adopted. The frequency f of the AC component is set to 1000 [Hz], the DC component (corresponding to the time average value Vave in this example) is set to 200 [V], and the peak-to-peak voltage Vpp is set to 1000 [V]. The toner layer 216 was transferred with a toner adhesion amount of 0.4 to 0.5 [mg / cm ² ]. As a result, a sufficient image density could be obtained on the concave portion on the surface of the “ripple”.

  At that time, the microscope 242 was focused on the toner layer 216 on the transparent substrate 210, and the behavior of the toner was photographed. Then, the following phenomenon was observed. That is, the toner particles in the toner layer 216 reciprocate between the transparent substrate 210 and the recording material 214 due to an alternating electric field formed by the alternating current component of the transfer bias. The amount of toner particles to increase.

  Specifically, in the secondary transfer nip, every time one cycle (1 / f) of the AC component of the secondary transfer bias arrives, an alternating electric field acts once to cause the toner particles to become transparent substrate 210 and the recording material. Reciprocates once with 214. In the first period, as shown in FIG. 22A, only the toner particles present on the surface of the toner layer 216 are detached from the layer. Then, after entering the concave portion of the recording material 216, it returns to the toner layer 216 again. At this time, the returned toner particles collide with the toner particles of the toner layer 216, thereby weakening the adhesion between the latter toner particles and the toner layer 216 or the transparent substrate 210. Thereby, in the next period, as shown in FIG. 22B, more toner particles are detached from the toner layer 216 than in the previous period. Then, after entering the concave portion of the recording material 216, it returns to the toner layer 216 again. At this time, the returned toner particles collide with the toner particles still remaining in the toner layer 216, thereby weakening the adhesion between the latter toner particles and the toner layer 216 or the transparent substrate 210. Thereby, in the next one cycle, as shown in FIG. 22C, more toner particles are detached from the toner layer 216 than in the previous one cycle. In this way, the number of toner particles gradually increases each time they reciprocate. Then, it was found that when the nip passage time has elapsed (when the time corresponding to the nip passage time has elapsed in the observation experimental apparatus), a sufficient amount of toner has been transferred into the concave portion of the recording sheet S.

  Next, the DC voltage (corresponding to the time average value Vave in this example) is set to 200 [V], and the bias per cycle is on the plus side and minus side (in this example, the return direction and the transfer direction side). When the behavior of the toner was photographed under the condition that the peak-to-peak voltage value Vpp was set to 800 [V], the following phenomenon was observed. That is, among the toner particles in the toner layer 216, those existing on the surface of the toner layer detach from the layer in the first one cycle and enter the concave portion of the recording sheet S. However, the entering toner particles remained in the recess without going to the toner layer 216 thereafter. Then, when the next one cycle arrived, very few toner particles were newly detached from the toner layer 216 and entered into the recesses of the recording sheet S. Therefore, when the nip passage time has elapsed, only a small amount of toner particles have been transferred into the recesses of the recording sheet S.

  As a result of further observation experiments, the present inventors have found that the toner particles that have entered the concave portion of the recording sheet S from the toner layer 216 in the first cycle can be returned to the toner layer 216 again. It was found that the value Vr depends on the toner adhesion amount per unit area on the transparent substrate 210. That is, as the toner adhesion amount on the transparent substrate 210 increases, the return peak value Vr at which the toner particles in the recesses of the recording material 213 can be pulled back to the toner layer 216 increases.

  In this embodiment, the time average value (Vave) of the voltage in the AC component of the secondary transfer bias is similarly the center voltage value of the maximum value and the minimum value of the AC component (center value of the maximum value and the minimum value) Voff. It is essential to be on the transfer side. In order to realize this, it is necessary to form a waveform in which the area on the return direction side is smaller than the area on the transfer direction side across the center voltage value Voff of the AC component. A time average value is a time average value of a voltage, which is a value obtained by dividing an integrated value over one period of a voltage waveform by the length of one period.

  As one form for achieving this, for example, as shown in FIG. 23, the rising and falling slopes of the voltage on the return direction side are made smaller than the rising and falling slopes of the voltage on the transfer direction side. Conceivable. Further, as a value indicating the relationship between the center voltage value Voff and the time average value Vave of the voltage, the ratio of the area on the return direction side with respect to the center voltage value Voff in the entire AC waveform is set as a return time (duty ratio) [%]. Was set.

Next, an experiment conducted by the present inventors and a secondary transfer bias will be described.
The inventors prepared a print tester having the same configuration as the printer according to the embodiment. Then, various print tests were performed using the print tester with each device configuration set as follows.

The process linear velocity 173 [mm / s], which is the linear velocity of each photoconductor and intermediate transfer belt 21
・ Secondary transfer bias AC component frequency f Frequency is 500 [Hz]
・ Recording sheet S RESAK 66 (trade name) 175kg paper (manufactured by a special paper)
The resac 66 is paper having a degree of unevenness on the paper surface larger than “ripple waves”. The depth of the concave portion on the paper surface is about 100 [μm] at the maximum. A blue solid image obtained by superimposing the M solid image and the C solid image was output to the Rezac 66 under various secondary transfer bias conditions. Then, in the output blue solid image, the evaluation results for various peak-to-peak voltage values Vpp and time average values Vave are shown in FIG. This is shown in FIG.

The test environment was an environment with a temperature of 10 ° C./humidity of 15%.
A power source that serves as a bias application means generates a waveform with a function generator (Yokogawa Electric FG300), amplifies it 1000 times with an amplifier (Trek High Voltage Amplify Model 10/40), and supplies it to the secondary transfer counter roller 533 in FIG. Applied.
[Comparative Example 1]
A conventional sine wave is used as the AC component described in FIG. 20, and FIG. 20 shows a waveform of a comparative example. In Comparative Example 1, the return time (duty ratio) is 50%, and the effect at this time is shown in FIG. At this time, in all peak-to-peak voltage values Vpp and time average values Vave shown in FIG. 21, the center voltage value Voff of the AC component = time average value Vave. In this embodiment, a low duty wave is set when the return time (duty ratio) is less than 50.
[Example 1]
As an alternating current component, the rising and falling slopes of the voltage on the return direction side were made smaller than the rising and falling slopes of the voltage on the transfer direction side. That is, the time in the transfer direction, which is the voltage output time closer to the transfer direction than the center voltage value Voff, is A, and the voltage output time is a value closer to the opposite polarity than the center voltage value Voff in the transfer direction. When the return time is B, A> B is set. The waveform at this time is shown in FIG. The return time is 40% and a low duty wave is used, and the evaluation effect is shown in FIG.

At this time, the peak-to-peak voltage value Vpp of FIG. 29 is 12 kV.
When the time average value Vave of the voltage was −5.4 kV, the center voltage value Voff of the AC component was −4.0 kV.
[Example 2]
As another means for making the waveform in the area of the return direction smaller than the area of the transfer direction across the central voltage value Voff of the AC component, as shown in FIG. There is a method of making the time shorter than the time A on the transfer direction side. By this method, the return time B with respect to the time A on the transfer direction side can be reduced.
Example 3
As an AC component, the return time B was made shorter than the time A on the transfer direction side. The waveform at this time is shown in FIG. The return time is 16% and a low duty wave is used, and the effect is shown in FIG.
Example 4
As an AC component, the return time B is shorter than the time A on the transfer direction side, and the waveform is rounded. The waveform at this time is shown in FIG. The return time is 16% and a low duty wave, and the effect is shown in FIG.

At this time, in FIG. 31, the peak-to-peak voltage value Vpp = 12 kV.
When the voltage average time Vave = −5.4 kV, the center voltage value Voff = −2.4 kV.

  According to the results of such an experiment, even when compared with Comparative Example 1 and Example 1, the secondary voltage is more secondary than the central voltage value Voff which is the central value of the maximum value and the minimum value of the secondary transfer bias as a voltage. Since the time average value Vave of the transfer bias is on the transfer direction side, the range in which transferability to an uneven recording material is established is markedly expanded. Even if various parameters such as various paper types, image patterns, and usage environments change due to the expansion of the establishment range, white spots are obtained while obtaining sufficient image density at the concave and convex portions on the surface of the recording material. Can be suppressed, and a good image can be obtained.

  This is because the time average value Vave is on the transfer direction side with respect to the center voltage value Voff, so that the required return peak value Vr is secured and the transfer direction peak value Vt causing discharge is not increased. Since only the average value Vave can be increased, it is considered that an effect is obtained.

From the results of Examples 1 to 4, since the return time can be further reduced by making the return time shorter than the transfer time, better image quality can be obtained. That is, assuming that the output time of a voltage closer to the transfer direction than the center voltage value Voff is A and the output time of a voltage closer to the opposite polarity to the transfer direction than the center voltage value Voff is B, A> B By setting the output of the power source 39 so as to achieve better image quality.
Therefore, in the configurations of the first to fourth embodiments, if the secondary transfer bias is the waveform of the secondary transfer bias described in the fifth embodiment, that is, the secondary transfer counter roller 24 In the waveform to be supplied, the time average value (Vave) of the voltage is set to the polarity in the transfer direction in which the toner image is transferred from the image carrier side to the recording material side, and the center value of the maximum value and the minimum value of the voltage ( By using a low duty wave set closer to the transfer direction than Voff), it is possible to prevent abnormal images while improving transferability even when the recording sheet S is uneven paper.
(Eighth embodiment)
Next, an eighth embodiment of the present invention will be described with reference to FIGS. 33, 34, and 35. FIG.
In the eighth embodiment, the transfer direction bias and the reverse polarity bias are alternately switched to transfer the toner image on the intermediate transfer belt 21 to the recording sheet S, and the transfer direction bias. The contact width between the recording sheet S and the intermediate transfer belt 21 is changed in the DC transfer mode in which the toner image on the intermediate transfer belt 21 is transferred to the recording sheet S without alternately switching the bias and the reverse polarity bias. Recording material contact width changing means 380 is provided. The recording material contact width changing unit 380 is configured so that the recording sheet S and the recording sheet S are more in the AC transfer mode in which the power supply 39 supplies the first transfer bias than in the DC transfer mode in which the power supply 39 supplies the second transfer bias. The contact width of the intermediate transfer belt 21 with the front surface 21a is increased.
In the present embodiment, the recording material contact width changing unit 380 is disposed upstream of the secondary transfer nip N in the moving direction of the intermediate transfer belt 21 so as to be able to come into contact with the recording sheet S, and to the secondary transfer nip N. The contact width between the recording sheet S and the front surface 21a of the intermediate transfer belt 21 can be changed by changing the position of the guide member 38 that guides the recording sheet S. The guide member 38 is configured by a plate-like member extending in the width direction of the intermediate transfer belt 21, and its leading end 38 a is positioned on the conveyance path of the recording sheet S and is sent to the secondary transfer nip N by the registration roller pair 95. The recording sheet S is arranged so as to be in contact with the recording sheet S.
FIG. 33 shows the position of the guide member 38 by the recording material contact width changing means 380 in the AC transfer mode, and FIG. 34 shows the position of the guide member 38 by the recording material contact width changing means 380 in the DC transfer mode. . FIG. 35 shows the configuration and operation of the recording material contact width changing means 380. (a) shows the state in the AC transfer mode, and (b) shows the state in the DC transfer mode. In FIG. 34, the configuration of the recording material contact width changing means 380 is omitted, and the operation of the guide member 38 is shown.
As shown in FIG. 35, the recording material contact width changing unit 380 moves the guide member 38 in the direction approaching the front surface 21a of the intermediate transfer belt 21 in the AC transfer mode, and in the DC transfer mode. When the power supply supplies a second transfer bias, the guide member 38 is moved away from the front surface 21a of the intermediate transfer belt 21.
The recording material contact width changing unit 380 includes a support frame 381 to which the guide member 38 is attached, and a moving unit 620A that rotates the support frame 381. The support frame 381 is rotatable about a support 382 provided on a side plate (not shown) of the transfer unit 20. To the support frame 381, the base end 38b side opposite to the front end 38a is fixed so that the front end 38a of the guide member 38 is located on the transport path. A long hole 382 extending in the rotation direction is formed in the support frame 381. A pin 384 provided on a side plate (not shown) of the transfer unit 20 is inserted into the long hole 382 to restrict the rotation range of the support frame 381.
The moving means 620A rotates the eccentric cam 385 that moves the position of the support frame 381 (the tip 38a of the guide member 38) by contacting the other end 38b of the guide member 38 attached to the support frame 381. A drive motor 387 is provided as drive means for driving. The eccentric cam 385 is provided so as to rotate integrally with a drive shaft 386 that is rotated by a drive motor 387. The eccentric cam 387 has a cam surface on the outer peripheral surface 385b having a maximum length from the center of rotation to the top dead center 385a.

  In the present embodiment, the drive motor 387 is connected to the control unit 500 via a signal line, and is configured to control its drive. Specifically, when the control unit 500 enters the DC transfer mode, the eccentric cam 385 moves in the direction in which the guide member 38 moves away from the front surface 21a of the intermediate transfer belt 21 as shown in FIG. In the AC transfer mode, as shown in FIG. 35 (a), the drive motor 387 causes the guide member 38 to rotate the eccentric cam 385 in a direction approaching the front surface 21a of the intermediate transfer belt 21. Control the rotation of In the present embodiment, in FIGS. 33, 34, and 35, the direction away from the front surface 21a is indicated by an arrow a9, and the direction approaching the front surface 21a is indicated by an arrow a10.

  In this embodiment, the eccentric cam 385 has a home position as a standby position in the DC transfer mode shown in FIG. 35B, and is rotated in the AC transfer mode to change its phase. It arrange | positions so that the proximity | contact position shown to (a) may be occupied. That is, in the DC transfer mode, the eccentric cam 385 has the top dead center 385a pushed up in contact with the base end 38b of the guide member 38 of the support frame 381, and in the AC transfer mode, the eccentric cam 385 is opposite to the top dead center 385a. The outer peripheral surface 385b rotates so as to contact the base end 38b of the guide member 38. That is, the tip 38a of the guide member 38 occupies a position closer to the intermediate transfer belt 21 in the AC transfer mode than in the DC transfer mode.

  With this configuration, in this embodiment, when the AC transfer mode is set, the eccentric cam 385 is rotated and the position of the tip 38a of the guide member moves in the direction a10 approaching the intermediate transfer belt 21 as shown in FIG. Therefore, the contact width between the recording sheet S and the front surface 21a of the intermediate transfer belt 21 in the AC transfer mode is increased. In the DC transfer mode, the eccentric cam 385 is rotated, and the position of the leading end 38a of the guide member moves in the direction a9 away from the intermediate transfer belt 21, as shown in FIGS. 34 and 35 (b). Therefore, the contact width between the recording sheet S and the front surface 21a of the intermediate transfer belt 21 in the DC transfer mode is shorter than that in the AC transfer mode.

Therefore, in the AC transfer mode, the recording sheet S in the secondary transfer nip is conveyed in a state where the intermediate transfer belt 21 and the recording sheet S are in close contact with each other on the upstream side of the secondary transfer nip N as compared with the DC transfer mode. Therefore, an abnormal image due to discharge can be prevented. Further, if the contact length between the intermediate transfer belt 21 and the recording sheet S, that is, the contact width is too long, image blurring occurs due to a difference in linear velocity between the intermediate transfer belt 21 and the recording sheet S. It is preferable to increase the contact width (contact length) between the recording sheet S and the recording sheet S only in the AC transfer mode in which an abnormal image due to discharge is likely to occur.
Further, as described above, when a low duty wave is applied as the secondary transfer bias, an abnormal image due to discharge is likely to occur. For this reason, when the position of the leading end 38a of the guide member 38 is changed by the recording material contact width changing means 380 so as to increase the contact width between the intermediate transfer belt 21 and the recording sheet S when the low duty wave is used, unevenness is caused. This is preferable because abnormal images due to electric discharge can be prevented while improving the transferability of paper.

In the eighth embodiment, the support frame 381 to which the guide member 38 is attached is rotatably supported by the support shaft 383, and the guide member 38 is rotated so that the tip 38a is positioned in the AC transfer mode and the DC transfer mode. In addition, the position of the tip 38a of the guide member 38 in the AC transfer mode and the DC transfer mode is controlled by restricting the rotation of the support frame 381 by the pin 384 and the long hole 382. However, the recording material contact width changing means is not limited to such a form. A support form of the support frame 381 as shown in FIGS. 36 (a), 36 (b), and 36 (c) may be used.
36A, a plurality of elongated holes 388 and 388 extending in the horizontal direction are formed in the support frame 381 so as to be parallel to each other by changing their positions in the vertical direction perpendicular to the horizontal direction. Pins 384 and 384 provided on a side plate (not shown) are inserted into the long holes 388 and 388 so that the support frame (guide member 381) can be moved in the horizontal direction.
In the support form shown in FIG. 36B, a plurality of elongated holes 389 and 389 extending in the vertical direction are formed in the support frame 381 so as to be parallel to each other by changing the position in the width direction intersecting the vertical direction. Pins 384 and 384 provided on a side plate (not shown) are inserted into the long holes 389 and 389 so that the support frame (guide member 381) can be moved in the vertical direction.
In the support form shown in FIG. 36 (c), a plurality of elongated holes 390, 390 inclined in the support frame 381 are formed so that the positions thereof are located on the same center line, and pins 384, 384 provided on a side plate (not shown). Are inserted into the long holes 390 and 390, and the support frame (guide member 381) can be moved in the inclined direction.
In FIGS. 36A to 36C, the position indicated by the two-dot chain line indicates the position in the DC transfer mode, and the solid line indicates the position in the AC transfer mode. The movement to the position indicated by the solid line and the alternate long and two short dashes line may be performed by an electric moving means such as a cam, a drive motor, or an electromagnetic solenoid, or a screw is provided at each pin 384 portion in a removable manner. The operator may manually change the print mode according to the AC transfer mode and the DC transfer mode.

  In the eighth embodiment, the present invention is applied to the transfer unit 20 including the entrance roller 61. However, the configuration of the eighth embodiment can be applied to any transfer unit including the guide member 38 even if the entrance roller 61 is not provided. Therefore, the configuration of the transfer unit includes the entrance roller 61. It is not limited.

  In the eighth embodiment, the contact width between the recording sheet S and the intermediate transfer belt 21 is set at the upstream side of the secondary transfer nip N by the position of the guide member 38 disposed between the recording sheet S and the intermediate transfer belt 21. However, the recording material contact width changing means may have another form.

  For example, in the first embodiment shown in FIGS. 3 and 4, as the contact width changing means 60, the position of the entrance roller 61 is changed between the AC transfer mode and the DC transfer mode. That is, in the AC transfer mode, the position of the entrance roller 61 is pushed down from the position indicated by the two-dot chain line to the position indicated by the solid line as shown in FIG. 4, and in the DC transfer mode, as shown in FIG. The entrance roller 61 is pushed up from the position indicated by the solid line to the position indicated by the solid line.

  For this reason, it is arranged on the upstream side in the moving direction of the intermediate transfer belt 21, and serves as a contact portion that contacts the back surface 21b opposite to the front surface 21a carrying the toner image of the intermediate transfer belt 21. The moving means 62 of the contact width changing means 60 that moves the entrance roller 61 moves the entrance roller 61 in the direction approaching the recording sheet S in the AC transfer mode for supplying the first transfer bias, and the second transfer bias. In the DC transfer mode in which the recording medium S is supplied, the entrance roller 61 has a function of moving away from the recording sheet S. Therefore, the contact width changing unit 60 can function as the recording material contact width changing unit 380A, and the moving unit 62 can function as the moving unit 62A of the recording material contact width changing unit 380A. In this case, since one unit has two functions as the contact width changing unit 60 and the recording material contact width changing unit 380A, an abnormality caused by discharge is further reduced while reducing the installation space and reducing the size. Images can be prevented.

  In the embodiment described above, the recording material contact width changing means is more effective in the AC transfer mode in which the power source 39 supplies the first transfer bias than in the DC transfer mode in which the power source 39 supplies the second transfer bias. In the above description, the contact width between the recording sheet S and the front surface 21a of the intermediate transfer belt 21 is increased. However, the contact width between the recording sheet S and the front surface 21 a of the intermediate transfer belt 21 can be regarded as the contact time between the recording sheet S and the front surface 21 a of the intermediate transfer belt 21. For this reason, the recording material contact width changing means is more effective when the power supply 39 is in the AC transfer mode in which the first transfer bias is supplied than in the DC transfer mode in which the power supply 39 is in the second transfer bias. It can also be said to be contact time changing means for increasing the contact time between S and the front surface 21a of the intermediate transfer belt 21.

The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments, and the present invention described in the claims is not specifically limited by the above description. Various modifications and changes are possible within the scope of the above.
For example, the image forming apparatus to which the present invention is applied is not limited to the type of image forming apparatus described above, but may be another type of image forming apparatus. In other words, the image forming apparatus to which the present invention is applied may be a copier, a single facsimile, or a complex machine thereof, or a complex machine such as a monochrome machine related thereto.
The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

20 Transfer device 21 Image carrier (intermediate transfer belt)
24 Secondary transfer counter roller (transfer member)
30 Secondary transfer roller (transfer member)
38 Guide member 39 Power supply 60 Contact width changing means 61 Contact portion 62, 62A, 620A Moving means 60A Nip width changing means 60B Contact position changing means (contact width changing means)
60C, 60D, 60E Pressure changing means (nip width changing means)
95 Conveying member (registration roller pair)
380, 380 (A to C) recording material contact width changing unit 500 control unit 600 switching unit 684, 385 cam 685 idling roller N transfer nip (contact width between transfer member and image carrier)
n2 pre-nip (contact width between transfer member and image carrier)
S recording material V linear velocity of conveyance member V1, V2 linear velocity of rotation of transfer member Vave time average value of voltage Voff central value of maximum and minimum values of voltage X transfer bias

Japanese Patent Laid-Open No. 09-14681

Claims (15)

A transfer member that forms a transfer nip in contact with the surface carrying the toner image of the image carrier;
A power supply for supplying a transfer bias for transferring the toner image on the image carrier to the recording material sandwiched in the transfer nip to the transfer nip,
The power supply is capable of supplying either the first transfer bias or the second transfer bias to the transfer nip when transferring the toner image on the image carrier to the recording material.
When the power supply supplies the first transfer bias, the contact width between the transfer member and the image carrier on the upstream side in the recording material conveyance direction is larger than when the power supply supplies the second transfer bias. Having contact width changing means for expanding,
The first transfer bias is a transfer bias in which the bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side and the bias in the opposite polarity to the bias in the transfer direction are alternately switched,
The transfer apparatus according to claim 2, wherein the second transfer bias is a transfer bias including only a bias in a transfer direction for transferring the toner image from the image carrier side to the recording material side. The transfer apparatus according to claim 1, wherein
The contact width changing unit is disposed upstream of the transfer nip in the moving direction of the image carrier, and contacts a surface of the image carrier opposite to the surface carrying the toner image. When,
A transfer device comprising a moving means for moving the contact portion in a direction to increase or decrease the contact width. The transfer apparatus according to claim 1, wherein
The transfer apparatus according to claim 1, wherein the contact width changing means is a contact position changing means for changing a contact position of the transfer member with respect to the image carrier in the moving direction of the image carrier. A transfer member that forms a transfer nip in contact with the surface carrying the toner image of the image carrier;
A power supply for supplying a transfer bias for transferring the toner image on the image carrier to the recording material sandwiched in the transfer nip to the transfer nip,
The power supply is capable of supplying either the first transfer bias or the second transfer bias to the transfer nip when transferring the toner image on the image carrier to the recording material.
When the power supply supplies the first transfer bias, the power supply further includes a nip width changing unit that widens the transfer nip than when the power supply supplies the second transfer bias.
The first transfer bias is a transfer bias in which the bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side and the bias in the opposite polarity to the bias in the transfer direction are alternately switched,
The transfer apparatus according to claim 2, wherein the second transfer bias is a transfer bias including only a bias in a transfer direction for transferring the toner image from the image carrier side to the recording material side. The transfer device according to claim 4, wherein
A plurality of transfer members having different hardnesses from each other;
The transfer apparatus according to claim 1, wherein the nip width changing unit includes a switching unit that switches a transfer member that is in contact with a surface of the image carrier carrying a toner image. The transfer device according to claim 4, wherein
The transfer device according to claim 1, wherein the nip width changing means is a pressure changing means for changing a pressure applied to the transfer nip. The transfer device according to claim 4, wherein
The nip width changing means includes an idling roller arranged coaxially with the transfer member, and a cam that is arranged to face the idling roller and is rotated by a driving means with an outer peripheral surface thereof in contact with the idling roller. And a pressing force changing unit that changes the pressure applied to the transfer nip by rotating the cam. A transfer member that forms a transfer nip in contact with the surface carrying the toner image of the image carrier;
A power supply for supplying a transfer bias to the transfer nip for transferring the toner image on the image carrier to the recording material sandwiched in the transfer nip;
A conveyance member that conveys the recording material toward the transfer nip;
The power supply can supply either the first transfer bias or the second transfer bias to the transfer nip when transferring the toner image on the image carrier to the recording material.
The first transfer bias is a transfer bias in which the bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side and the bias in the opposite polarity to the bias in the transfer direction are alternately switched,
The second transfer bias is a transfer bias including only a bias in a transfer direction for transferring the toner image from the image carrier side to the recording material side,
When the power supply supplies the first transfer bias, the rotation linear velocity of the transfer member relative to the rotation linear velocity of the transport member is made slower than when the power supply supplies the second transfer bias. The transfer device. A transfer member that forms a transfer nip in contact with the surface carrying the toner image of the image carrier;
A power supply for supplying a transfer bias to the transfer nip for transferring the toner image on the image carrier to the recording material sandwiched in the transfer nip;
The power supply can supply either the first transfer bias or the second transfer bias to the transfer nip when transferring the toner image on the image carrier to the recording material.
The first transfer bias is a transfer bias in which the bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side and the bias in the opposite polarity to the bias in the transfer direction are alternately switched,
The second transfer bias is a transfer bias including only a bias in a transfer direction for transferring the toner image from the image carrier side to the recording material side,
Recording material contact width changing means for increasing the contact width between the recording material and the image carrier when the power source supplies the first transfer bias than when the power source supplies the second transfer bias. A transfer device having The transfer device according to claim 9, wherein
The guide member is disposed upstream of the transfer nip in the moving direction of the image carrier so as to be in contact with the recording material, and guides the recording material toward the transfer nip.
When the power supply supplies the first transfer bias, the recording material contact width changing unit moves the guide member in a direction approaching the image carrier side, and the power supply supplies the second transfer bias. And the guide member is moved in a direction away from the image carrier side. The transfer device according to claim 9, wherein
The recording material contact width changing means is disposed on the upstream side in the moving direction of the image carrier, and a contact portion that contacts a surface of the image carrier opposite to the surface carrying the toner image. ,
When the power supply supplies a first transfer bias, the contact portion is moved in a direction approaching the recording material, and when the power supply supplies a second transfer bias, the contact portion is moved to the recording medium. A transfer device comprising a moving means for moving in a direction away from the material. The transfer apparatus according to any one of claims 1 to 11,
The transfer apparatus according to claim 1, wherein the first transfer bias is obtained by superimposing an AC component on a DC component. The transfer apparatus according to any one of claims 1 to 12,
In the first transfer bias, the waveform in which the bias in the transfer direction and the reverse polarity bias are alternately switched is such that the time average value (Vave) of the bias transfers the toner image from the image carrier side to the recording material side. And a low-duty wave that is set closer to the transfer direction than the center value (Voff) of the maximum value and the minimum value of the bias. The transfer device according to any one of claims 1 to 13,
The transfer device according to claim 1, wherein the image carrier has a belt shape.   An image forming apparatus comprising the transfer device according to claim 1.

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