JP2017116671A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve quality of an image when applying an AC transfer bias to transfer a toner image to a recording material.SOLUTION: There is provided an image forming apparatus that comprises: a nip forming member 30 that is in contact with a surface of an image carrier 21 to form a transfer nip N; and transfer bias application means 81 that applies, to the transfer nip, an AC transfer bias as a transfer bias for transferring a toner image on the image carrier to a recording material sandwiched in the transfer nip, the image forming apparatus including adjustment means 87 that adjusts the value of electric resistance on a route of a transfer current flowing in the transfer nip; and control means, such as a control part 60 that controls the adjustment means to adjust the value of electric resistance when the AC transfer bias is applied to the transfer nip according to a predetermined selection condition.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image forming apparatus.

  Conventionally, an AC transfer bias including a DC component and an AC component is applied to a transfer nip for transferring a toner image on an image carrier to a recording material, and the transferability of the toner image onto a recording material such as uneven paper is applied. Image forming apparatuses that improve the image quality are known.

  For example, in Patent Literature 1, a secondary transfer roller is brought into contact with an intermediate transfer belt portion wound around a secondary transfer counter roller to form a secondary transfer nip, and a DC component and an AC component are superimposed. An image forming apparatus that performs secondary transfer by applying a bias (AC transfer bias) to a secondary transfer nip is disclosed. This image forming apparatus executes an AC mode in which a secondary bias is applied by applying a superimposing bias when forming an image on uneven paper, and a direct current consisting only of a direct current component when forming an image on plain paper other than the uneven paper. A DC mode in which secondary transfer is performed by applying a bias is executed.

  Further, the image forming apparatus disclosed in Patent Document 1 is provided with a heater (heating means) for heating the secondary transfer counter roller. This heater lowers the electrical resistance value of the secondary transfer counter roller by heating the secondary transfer counter roller. In this image forming apparatus, when the AC mode is executed, the heater is turned on to perform secondary transfer with the electric resistance value of the secondary transfer counter roller being lowered, and when the DC mode is executed, the heater is turned off. Thus, the secondary transfer is performed without lowering the electrical resistance value of the secondary transfer counter roller. According to Patent Document 1, it is difficult to generate a discharge that is likely to occur at the entrance side of the secondary transfer nip during execution of the AC mode, and dust and white spots due to the occurrence of the discharge can be suppressed.

  However, in a conventional image forming apparatus that transfers a toner image to a recording material by applying an AC transfer bias including a DC component and an AC component, the quality of an image formed by applying the AC transfer bias is insufficient. There was a problem.

  In order to solve the above-described problems, the present invention provides an image carrier that carries a toner image on its surface, a nip forming member that forms a transfer nip in contact with the surface of the image carrier, An image having transfer bias applying means for applying an AC transfer bias including a DC component and an AC component to the transfer nip as a transfer bias for transferring the toner image on the image carrier to the sandwiched recording material In the forming apparatus, an adjusting unit that adjusts an electric resistance value on a route of a transfer current flowing through the transfer nip, and an electric resistance value when the AC transfer bias is applied to the transfer nip is adjusted according to a predetermined selection condition. As described above, it has a control means for controlling the adjusting means.

  According to the present invention, it is possible to improve the image quality when a toner image is transferred to a recording material by applying an AC transfer bias.

1 is an explanatory diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment. FIG. 4 is an explanatory diagram illustrating a peripheral configuration of a secondary transfer unit in the image forming apparatus. It is a perspective view which shows the structure of the heater arrange | positioned around a secondary transfer part. FIG. 3 is a cross-sectional view of a secondary transfer nip N showing a recording material clamping area N1 and a non-recording material clamping area N2. FIG. 2 is a block diagram illustrating a part of a control system of the image forming apparatus. 6 is a flowchart showing a flow of control in an AC mode in Control Example 1. 6 is a flowchart showing a flow of control in an AC mode in Control Example 2. (A) is explanatory drawing which shows typically the surface state of the uneven paper A, (b) is explanatory drawing which shows the surface state of the uneven paper B typically. 10 is a flowchart showing a control flow in an AC mode in Control Example 3. 10 is a flowchart showing a flow of control in an AC mode in Control Example 4. FIG. 10 is an explanatory diagram illustrating a peripheral configuration of a secondary transfer unit according to Modification 1. FIG. 10 is an explanatory diagram illustrating a peripheral configuration of a secondary transfer unit according to Modification 2. FIG. 10 is an explanatory diagram illustrating another peripheral configuration of the secondary transfer unit according to Modification 2. FIG. 10 is an explanatory diagram illustrating a peripheral configuration of a secondary transfer unit according to Modification 3. FIG. 10 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion P when the pressing force is low in Modification Example 4. FIG. 10 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion P when a high pressing force is used in Modification 4. FIG. 10 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion P when a secondary transfer nip is formed by a high hardness secondary transfer roller in Modification 5. FIG. 10 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion P when a secondary transfer nip is formed by a low hardness secondary transfer roller in Modification 5. It is explanatory drawing which shows the waveform of alternating current transfer bias.

Hereinafter, an embodiment in which the present invention is applied to an electrophotographic image forming apparatus will be described.
FIG. 1 is an explanatory diagram showing a schematic configuration of the image forming apparatus of the present embodiment.
The image forming apparatus of the present embodiment is an intermediate transfer type tandem image forming apparatus. The image forming apparatus 1 is mounted on a printer unit 100 that forms a toner image, a paper feed table 200 on which the printer unit 100 is placed, a scanner unit 300 mounted on the printer unit 100, and a scanner unit 300. The ADF 400 is an automatic document feeder.

  The printer unit 100 includes an endless belt-like intermediate transfer belt 21 that is an intermediate transfer member as an image carrier. The intermediate transfer belt 21 is stretched around a driving roller 22, a driven roller 23, and a secondary transfer counter roller (backup roller) 24 with an inverted triangular shape as shown in FIG. 1 when viewed from the side. . The intermediate transfer belt 21 is moved in the clockwise direction in FIG. 1 (in the direction of arrow X in FIG. 1) by the rotation of the drive roller 22.

  Above the intermediate transfer belt 21, four image forming units (image forming units) 1C, 1M, and C for forming C (cyan), M (magenta), Y (yellow), and K (black) toner images are formed. 1Y and 1K are arranged side by side along the movement direction X of the intermediate transfer belt 21. The image forming units 1C, 1M, 1Y, and 1K constitute an image forming unit 10 that forms a toner image.

  Each of the image forming units 1C, 1M, 1Y, and 1K has 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 4K, respectively. 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. Form. The photosensitive drums 2C, 2M, 2Y, and 2K are rotationally driven in a counterclockwise direction (direction indicated by an arrow in FIG. 1) in FIG.

  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 transfer residual toner attached to the photoconductor drums 2C, 2M, 2Y, and 2K after passing through the primary transfer nip.

  An optical writing unit 15 serving as a latent image forming unit is disposed above the image forming unit 10 in the printer unit 100. The optical writing unit 15 performs an optical writing process by scanning with laser light on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K that are rotationally driven to form an electrostatic latent image. Prior to the optical writing process, the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K are uniformly charged by the respective chargers.

  A transfer unit 20 having an intermediate transfer belt 21 is disposed below the image forming unit 10. Primary transfer rollers 25C, 25M, 25Y, and 25K are provided 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. doing.

  The secondary transfer counter roller 24 is located in the lower part inside the intermediate transfer belt 21, and the lower part of the intermediate transfer belt 21 is wound around the secondary transfer counter roller 24. A secondary transfer roller 30 as a nip forming member is pressed against the outer peripheral surface side of the intermediate transfer belt 21 at a portion wound around the secondary transfer counter roller 24. The secondary transfer roller 30 forms a secondary transfer nip N of the secondary transfer portion P by being sandwiched between the secondary transfer counter roller 24 and the intermediate transfer belt 21.

  On the inner peripheral surface side of the intermediate transfer belt 21, a push-down roller 58 is in contact with the upstream side of the secondary transfer nip N in the intermediate transfer belt moving direction. The push-down roller 58 pushes down the intermediate transfer belt 21 toward the secondary transfer roller 30 to forcibly wind it around the secondary transfer roller 30, and the nip width of the secondary transfer nip N (intermediate transfer of the secondary transfer nip N). The length in the belt movement direction is increased. As a result, the entrance of the secondary transfer nip N moves away from the position between the axes of the secondary transfer counter roller 24 and the secondary transfer roller 30, and discharge occurs on the entrance side of the secondary transfer nip N during secondary transfer. Suppressed.

  Further, a guide plate 51 that guides a recording sheet, which is a recording material, toward the entrance of the secondary transfer nip N is disposed upstream of the secondary transfer nip N in the intermediate transfer belt moving direction. The recording sheet sent toward the secondary transfer nip N by the registration roller pair 95 comes into contact with the guide plate 51, so that the surface of the intermediate transfer belt 21 upstream of the secondary transfer nip N in the intermediate transfer belt moving direction. Guided towards For this reason, the recording sheet comes into contact with the surface of the intermediate transfer belt 21 before entering the secondary transfer nip N, so that the recording sheet is prevented from being turned or jammed.

  The secondary transfer roller 30 is in contact with a cleaning blade 52 that mechanically scrapes the adhering toner and a rotating brush 53 as a lubricant application unit. By applying the lubricant by the rotating brush 53, the toner releasability of the secondary transfer roller 30 is improved. The rotating brush 53 is driven to rotate in contact with both the solid lubricant 531 such as a zinc stearate lump and the secondary transfer roller 30, whereby the lubricant powder obtained by scraping from the solid lubricant 531 is secondary. It is applied to the surface of the transfer roller 30. A paper dust removing brush 54 for removing paper dust is in contact with the surface of the secondary transfer roller 30 on the upstream side of the cleaning blade 52 in the rotation direction of the secondary transfer roller.

  The recording sheet is fed to the secondary transfer nip N by a registration roller pair 95 at a predetermined timing, and the toner images superimposed on the four colors on the intermediate transfer belt 21 are collectively put on the recording sheet at the secondary transfer nip N. Transcribed.

  The scanner unit 300 reads image information of a document placed on the contact glass 301 by the reading sensor 302 and sends the read image information to the control unit 60. The control unit 60 having an image formation control function functions to control a light source such as a laser diode in the optical writing unit 15 based on image information received from the scanner unit 300. This optical writing unit 15 emits laser writing light for C, M, Y, and K, and optically scans the photosensitive drums 2C, 2M, 2Y, and 2K. By this optical scanning, electrostatic latent images are formed on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K, and the electrostatic latent images are converted into toner images of C, M, Y, and K through a predetermined development process. Form.

  The paper feed table 200 includes paper cassettes 202 arranged in multiple stages in the paper bank 201. A sheet feed roller 203 that feeds a recording sheet, a separation roller 205 that separates and feeds the sent recording sheet, and a recording sheet is conveyed to the printer unit 100 to a sheet feeding path 204 and an upper sheet feeding path 99 that extend from the sheet feeding cassette 202. A pair of conveying rollers 206 and the like are 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 manual-feed-side separation roller 96 is also provided. In the printer unit 100, the manual paper feed path 97 joins the upper paper feed path 99.

  Near the lower portion of the transfer unit 20, a registration roller pair 95 is provided so as to sandwich the upper and lower sides of the upper paper feed path 99. The registration roller pair 95 stops rotating as soon as the recording sheet fed from the paper feed cassette 202 or the manual feed tray 98 is sandwiched. The registration roller pair 95 resumes rotational driving at the timing when the sandwiched recording sheet is fed in the secondary transfer nip N in synchronization with the four-color superimposed toner image on the intermediate transfer belt 21 to transfer the recording sheet to the secondary transfer nip N. Feed toward nip N. As described above, the registration roller pair 95 sandwiches the recording sheet conveyed in the upper sheet feeding path 99 or the manual sheet feeding path 97 and then feeds it toward the secondary transfer nip N at a predetermined timing.

  When a color image is copied by the image forming apparatus 1, the document is set on the document table 401 of the ADF 400, or the ADF 400 is opened and the document is set on the contact glass 301 of the scanner unit 300. The document set on the contact glass 301 is held on the contact glass 301 by closing the ADF 400.

  When the start button provided on the operation panel 69 shown in FIG. 5 is pressed, when the document is set on the ADF 400, the document is conveyed onto the contact glass 301. Thereafter, the scanner unit 300 starts driving, and the first traveling body 303 and the second traveling body 304 start traveling along the document surface. Light emitted from the light source of the first traveling body 303 is reflected by the original surface, and the reflected light is reflected by the mirror of the second traveling body 304 and then enters the reading sensor 302 through the imaging lens 305. The document content is read by the reading sensor 302 and a signal processing circuit (attached) having a signal processing function of the sensor, temporarily stored as image information, and then sequentially sent to the printer unit 100.

  When the sheet information is received from the scanner unit 300, the sheet feeding table 200 feeds a recording sheet having a size corresponding to the image information to the upper sheet feeding path 99. Accordingly, in the printer unit 100, the driving roller 22 is rotationally driven to move the intermediate transfer belt 21. At the same time, in the printer unit 100, the photosensitive drums 2C, 2M, 2Y, and 2K are rotated in the directions indicated by the arrows to uniformly charge the respective surfaces. Thereafter, optical writing processing by the optical writing unit 15 is performed based on the image information received from the scanner unit 300, and electrostatic latent images are formed on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K, respectively. The electrostatic latent image is developed with each color toner by the developing units 3C, 3M, 3Y, and 3K. The toner images of C, M, Y, and K colors formed on the surfaces of the photosensitive drums 2C, 2M, 2Y, and 2K by the development process are sequentially superimposed at the primary transfer nip for C, M, Y, and K. Thus, the 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 a recording sheet having a size corresponding to the document size, and the recording sheet is sent out from one of the three paper feed cassettes 202. It is. The fed recording sheets are separated one by one by the separation roller 205 and introduced into the paper feed path 204, and then transported by the transport roller pair 206 and sent to the upper paper feed path 99 in the printer unit 100. When using the manual feed tray 98, the manual feed roller 94 of the manual feed tray 98 rotates to feed the recording sheet on the manual feed tray 98, separated by the manual feed separation roller 96, and sent to the manual feed path 97. This reaches the end of the upper paper feed path 99. In the vicinity of the end of the upper paper feed path 99, the recording sheet stops by abutting the leading edge against the registration roller pair 95.

  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 is fed into the secondary transfer nip N to form a full-color toner image on the intermediate transfer belt 21. In close contact. The toner image is collectively transferred onto the recording sheet by the action of the electric field generated by the pressure at the transfer nip and the secondary transfer bias. The secondary transfer nip N is formed by the secondary transfer roller 30 and the portion of the intermediate transfer belt 21 that is wound around the secondary transfer counter roller 24.

  When the recording sheet on which the full-color toner image is secondarily transferred at the secondary transfer nip N passes the secondary transfer nip N, it is separated from the secondary transfer roller 30 and the intermediate transfer belt 21 by the curvature, and is fixed by the paper conveying belt 70. It is sent into the device 71. The recording sheet fed into the fixing device 71 is sandwiched by the fixing device 71 at a fixing nip between the pressure roller 72 and the fixing belt 73, and the toner image is fixed on the surface by heat and pressure.

  The recording sheet on which the color image is formed is sent out by the discharge roller pair 74 and stacked on a discharge tray 75 outside the apparatus. When an image is also formed on the other side of the recording sheet, the recording sheet 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. The recording sheet sent to the sheet reversing device 77 is turned upside down and then returned to the registration roller pair 95 again. Further, the toner is fed into the secondary transfer nip N and the full color toner image is secondarily transferred to the other surface, and the toner image is fixed again via the fixing device 71 and stacked on the paper discharge tray 75.

  After passing through the secondary transfer nip N, the belt cleaning device 26 comes into contact with the surface 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. Yes. The belt cleaning device 26 cleans transfer residual toner adhering to the surface of the intermediate transfer belt 21.

FIG. 2 is an explanatory diagram showing a peripheral configuration of the secondary transfer portion P in the image forming apparatus 1.
The image forming apparatus 1 of the present embodiment includes a secondary transfer power supply 81 that is a transfer bias applying unit for applying a secondary transfer bias to the secondary transfer nip N, and a route of a transfer current flowing through the secondary transfer nip N. A heater 87 as a heating means as an adjusting means for adjusting the electrical resistance value of the toner, and a predetermined selection of an electrical resistance value when a secondary transfer bias including an AC component as a DC component is applied to the secondary transfer nip N A heater drive unit 600b as a control unit for controlling the drive of the heater 87 and a bias control unit 600a as a control unit for controlling the secondary transfer power supply 81 are provided so as to adjust according to the conditions. The heater 87 as the adjusting means is an electric resistance value changing means for changing the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N.

  As shown in FIG. 2, the secondary transfer counter roller 24 includes a cored bar 241 and a conductive NBR rubber layer 242 coated on the surface thereof. The secondary transfer roller 30 also includes a core metal 311 and a conductive NBR rubber layer 312 coated on the surface thereof. In the secondary transfer power supply 81, an AC power supply circuit unit in which a DC power supply 811 and an AC power supply 812 are connected in series and a DC power supply circuit unit including a single DC power supply 813 are arranged in parallel. 88 is configured to be selectively connected to the secondary transfer opposing roller 24.

  The secondary transfer power supply 81 can output a superimposed voltage obtained by superimposing an AC voltage on a DC voltage to the metal core 241. On the other hand, the core metal 311 of the secondary transfer roller 30 is grounded. In the present embodiment, a secondary transfer bias including an AC component as a DC component is applied to the secondary transfer nip N by outputting a superimposed voltage from the secondary transfer power supply 81 to the cored bar 241 of the secondary transfer counter roller 24. Is done. As a result, the secondary transfer electric field that electrostatically moves the negative polarity toner from the secondary transfer counter roller 24 side (intermediate transfer belt 21 side) toward the secondary transfer roller 30 side (recording sheet side) is second. It is formed at and next to the next transfer nip N. The secondary transfer counter roller 24 and the secondary transfer counter roller 24 and the secondary transfer roller 30 have higher resistance values.

  As the secondary transfer power supply 81, a single DC power supply 813 may be used instead of the DC power supply 811 and the AC power supply 812. In this case, the secondary transfer power supply 81 can output a voltage composed only of the DC voltage to the cored bar 241 by outputting the DC voltage only from the DC power supply 811 after turning off the output of the AC power supply 812. Further, the secondary transfer power supply 81 can output a superimposed voltage obtained by superimposing the AC voltage on the DC voltage to the cored bar 241 by outputting the DC voltage from the DC power supply 811 and the AC voltage from the AC power supply 812. is there.

  The heater 87 is disposed to face the secondary transfer counter roller 24 and generates heat for heating the secondary transfer counter roller 24. The heater 87 is provided on the inner peripheral surface side of the intermediate transfer belt 21 and is disposed in the vicinity of the secondary transfer counter roller 24. The heater 87 is connected to an AC heater power supply 84 which is a power supply for driving the heater via a relay 861.

FIG. 3 is a perspective view showing the configuration of the heater 87.
The heater 87 of the present embodiment is configured by a nichrome wire heater supported on the substrate side of the printer unit 100 via an insulating plate 870 and a support plate 871. The heater 87 has a rated voltage of 200 V and a wattage of 9 W, but is not limited to this. The heater 87 of the present embodiment is a flexible planar heating element, and is integrally formed using an aluminum foil having a thickness of 0.05 mm so as to cover the silicon rubber insulating heating element as the heating element 872 from both sides. ing. The increase value of the surface temperature of the heater 87 is 80 ° C. ± 20 ° C. In addition, the code | symbol 873 in a figure shows an insertion switch.

  The heater 87 generates heat when electric power is supplied from the heater power supply 84. The relay 861 is driven and controlled by the heater driving unit 600 b of the control unit 60. The heater driving means 600 b controls the output power (W) of the heater 87 by controlling the heater power supply 84. The drive circuit of the heater 87 is easily and responsively switched by the heater driving means 600b via the relay 861.

  The heater 87 heats the secondary transfer counter roller 24 constituting the image carrier together with the intermediate transfer belt 21, thereby changing the temperature of the secondary transfer counter roller 24, and the electric resistance value of the secondary transfer counter roller 24. Can be changed. Thereby, the electrical resistance value on the route through which the secondary transfer current flows is changed. In particular, in the present embodiment, the heater 87 is disposed so as to efficiently apply heat not only to the secondary transfer counter roller 24 but also to the intermediate transfer belt 21 and the secondary transfer roller 30 that form the secondary transfer nip N. Has been. Therefore, the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N can be greatly changed by the heater 87.

  The image forming apparatus of the present embodiment selects a DC mode that uses a DC transfer bias consisting of only a DC component as a secondary transfer bias, and an AC mode that uses an AC transfer bias including a DC component and an AC component as a secondary transfer bias. To execute image formation. The AC mode is mainly executed when an image is formed on a recording sheet having a rough surface such as Japanese paper (hereinafter referred to as “uneven paper”), and the DC mode is mainly used for recording sheets other than the uneven paper. This process is performed when an image is formed on a recording sheet having a smooth surface. Examples of the uneven paper include, for example, Rezac 66 (trade name) 260KG paper (six-plate continuous quantity) manufactured by Special Paper Industries Co., Ltd., FC Japanese paper type ripple (trade name) manufactured by NBS Ricoh Co., Ltd., embossed paper, There is linen paper. Examples of the recording sheet other than the uneven paper include plain paper such as My Paper (trade name) manufactured by NBS Ricoh Co., Ltd., and coated paper.

  Here, in the AC mode, discharge is more likely to occur on the entrance side of the secondary transfer nip N than in the DC mode. When this discharge occurs, the toner image on the intermediate transfer belt 21 is disturbed by the impact of the discharge, and dust may be generated. Further, a part of the toner constituting the toner image on the intermediate transfer belt 21 is charged to a reverse polarity (in this embodiment, a positive polarity) by discharge, and is not transferred to the recording sheet side in the secondary transfer nip N. White spots in the image may occur.

The reason why such a discharge is likely to occur in the AC mode is considered as follows.
As shown in FIG. 19, the AC transfer bias is generally obtained by superimposing an AC component of the peak-to-peak voltage Vpp on a DC component Voff of the same degree as the DC transfer bias (same value as the average value Vave). A symbol Vt in FIG. 19 indicates a peak value on the side (transfer direction side) in which the toner is moved from the intermediate transfer belt 21 side to the secondary transfer roller 30 side in the secondary transfer nip N. Further, a symbol Vr in FIG. 19 indicates a peak value in the direction of returning the toner from the secondary transfer roller 30 side to the intermediate transfer belt 21 side (return direction side).

  In general, the transferability (transfer rate of the entire toner image) in the secondary transfer is mainly determined by the amount of transfer current flowing through the toner image per unit area in the secondary transfer nip N. Therefore, in the present embodiment, the DC power source 811 is constant-current controlled so that the target transfer current flows through the secondary transfer nip N, and according to various conditions (type of uneven paper, type of image, etc.) The average value Vave of the AC transfer bias, that is, the value of the DC component Voff varies. On the other hand, the toner adhesion amount (recess transferability) to the recesses that greatly affects the image quality of the uneven paper is mainly determined by the return direction peak value Vr. For this reason, the AC power supply 812 is controlled at a constant voltage so that the AC component peak-to-peak voltage Vpp at which the target return direction peak value Vr is obtained can be obtained according to the varying DC component Voff.

  Here, the transfer direction peak value Vt is the maximum potential difference that can be taken between the grounded secondary transfer roller 30 and the secondary transfer counter roller 24 to which the secondary transfer voltage is applied. The absolute value of the transfer direction peak value Vt is the sum of the absolute value of the average value Vave (Voff) and a half of the peak-to-peak voltage Vpp. As the value increases, discharge between the rollers easily occurs. As a result, the portion of the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 and the surface of the recording sheet whose back surface is supported by the secondary transfer roller 30 On the inlet side of the secondary transfer nip N in which a minute gap is formed between the two, a discharge is likely to occur.

  As described above, in the AC mode, discharge is more likely to occur at the entrance side of the secondary transfer nip N than in the DC mode, and image quality degradation such as dust and whiteout is likely to occur. It is necessary to set the DC component Voff of the transfer bias. Therefore, in the AC mode, there is a restriction on increasing the DC component Voff of the AC transfer bias, and the DC power source 811 has a range in which discharge does not occur even if various conditions change and the value of the DC component Voff fluctuates. Therefore, it is necessary to set a target transfer current value. For this reason, depending on conditions such as the type of recording material, the type of image to be formed, and the image forming conditions such as the process linear velocity, a sufficient amount of transfer current can flow through the secondary transfer nip N within a range where no discharge is generated. In some cases, the image density cannot be secured.

  Here, if the electrical resistance value on the route of the transfer current flowing through the secondary transfer nip N is low, the transfer current easily flows through the transfer nip, so that the direct current of the AC transfer bias necessary for obtaining the necessary transfer current is obtained. The component Voff can be reduced, and the number of cases where a sufficient amount of transfer current cannot flow through the secondary transfer nip N within a range where no discharge is generated can be reduced. However, in order to reduce the electric resistance value on the route of the transfer current, the electric resistance values (hereinafter referred to as “secondary transfer roller 30, intermediate transfer belt 21, secondary transfer roller 30, etc.) that form the secondary transfer nip N, etc. The electrical resistance value of the secondary transfer nip N ”is referred to as“ the electrical resistance value of the secondary transfer nip N ”. Depending on the type of uneven paper and the type of image (difference in image area ratio, difference in distribution of toner image adhering part on the recording sheet, etc.), it is sufficient to adjust the DC component and AC component of the AC transfer bias. In some cases, it is not possible to ensure a high image density.

  Referring to FIG. 4 in detail, when the electrical resistance value of the secondary transfer nip N is low, a non-recording material sandwiching area in which the recording sheet in the secondary transfer nip N is not sandwiched (in this embodiment, the recording sheet). The current easily flows in the axially opposite end regions (N2) of the secondary transfer roller 30 that are not in contact with N2. Therefore, the amount of current flowing through the recording material clamping region (in this embodiment, the central region in the axial direction of the secondary transfer roller 30 in contact with the recording sheet) N1 where the recording sheet is sandwiched in the secondary transfer nip N is reduced. The electric resistance value of the recording material sandwiching area N1 is determined depending on the type of recording sheet S (concave paper or the like) sandwiched in the secondary transfer nip N and the type of toner image (image) existing in the secondary transfer nip N (toner adhesion). It depends on the amount). Therefore, depending on the type of recording sheet S (uneven paper, etc.) and the type of image, even if the electrical resistance value of the recording material clamping region N1 becomes relatively high and the total amount of transfer current is sufficient, Many of them flow to the non-recording material sandwiching area N2, and a necessary amount of current does not flow through the recording material sandwiching area N1, and a sufficient image density may not be secured.

  As described above, when the electrical resistance value of the secondary transfer nip N is high, a sufficient amount of transfer current is passed through the secondary transfer nip N within a range where no discharge is generated on the inlet side of the secondary transfer nip N. In some cases, sufficient image density cannot be ensured. On the other hand, when the electrical resistance value of the secondary transfer nip N is low, even if the total amount of the transfer current is sufficient, a necessary amount of current cannot be passed through the recording material sandwiching area N1, and a sufficient image is obtained. There are cases where the concentration cannot be secured. Therefore, in a configuration in which the electrical resistance value of the secondary transfer nip N cannot be changed, there may be cases where sufficient image density cannot be ensured.

  Therefore, in the present embodiment, the electric resistance value of the secondary transfer nip N is changed by controlling the heater 87 by the heater driving unit 600b in accordance with various conditions such as the type of recording sheet and the type of image. Over a wide range of conditions, it is possible to form a high-quality image in which no dust or white spots occur and a sufficient image density is ensured.

FIG. 5 is a block diagram showing a part of the control system of the image forming apparatus 1 shown in FIG.
The control unit 60 includes a CPU 60a as a calculation means, a RAM 60c as a nonvolatile memory, a ROM 60b as a temporary storage means, a flash memory 60d, and the like. Various components and sensors are electrically connected to the control unit 60 that controls the entire image forming apparatus 1 so that they can communicate with each other. In FIG. Is shown.

  Reference numeral 68 denotes temperature / humidity detection means for detecting temperature / humidity, and the control unit 60 may drive-control the heater 87 by the heater driving means 600b based on the detection result of the temperature / humidity detection means 68. Specifically, the output of the heater 87 is increased or turned on at a low temperature at which transfer dust is likely to occur to reduce the electrical resistance value of the secondary transfer counter roller 24, and the output of the heater 87 is decreased or turned off at a high temperature. Thus, useless operation of the heater 87 is prevented.

  The operation panel 69 includes a touch panel, a plurality of key buttons, and the like, and functions as an operation receiving unit that receives a user operation. An image can be displayed on the screen of the touch panel. The screen of the touch panel can display a screen for selecting a recording sheet to be printed, that is, a paper type (paper type). The operation panel 69 receives a user operation through a touch panel and key buttons, and transmits input information to the control unit 60. When the user selects the paper type used for printing from the paper types displayed on the screen of the touch panel, the operation panel 69 transmits the paper type information selected by the user to the control unit 60. The operation panel 69 can also display an image on the touch panel based on a control signal sent from the control unit 60.

Next, an image quality evaluation experiment conducted by the present inventor will be described.
In this experiment, the secondary transfer portion P of Ricoh Pro C5110 manufactured by Ricoh Company is modified so that a secondary transfer voltage can be applied to the secondary transfer counter roller 24 from outside, and test images are printed on three types of uneven paper. The image quality was evaluated. The three types of concavo-convex paper used were Conqueor High White Laid 300 gsm (hereinafter referred to as “concave paper A”), Special Paper Company Rezak 302 gsm (hereinafter referred to as “concave paper B”), and Special Paper Company Rezac 115 gsm (hereinafter referred to as “concave paper A”). "Uneven paper C"). In this experiment, a Trek COR-A-TROL Model 610D was used as the secondary transfer power supply 81, and a constant current controlled -70 μA transfer current was set to flow to the secondary transfer nip N. Regarding the electrical resistance value of the secondary transfer nip N, a transfer current is input from the secondary transfer power supply 81 in a state where no recording sheet exists in the secondary transfer nip N to which the secondary transfer bias is applied, and the output at that time is output. A voltage value was measured, and a value obtained by dividing the output voltage value by the transfer current value was measured as an electric resistance value of the secondary transfer nip N.

  In addition, regarding the image quality evaluation, the concave transferability (toner adhesion amount in the recesses) of various uneven papers A to C and the convex transferability (toner adhesion amount on the protrusions, dust and white) of various uneven papers A to C. Occurrence of omissions and the like) and halftone reproducibility of various uneven papers A to C were evaluated. Specifically, with respect to the concave portion transferability and the convex portion transferability, a solid image is formed on various uneven papers A to C, and the image density of the concave portion in the formed solid image is visually confirmed to transfer the concave portion. In addition to evaluating the property, the convex portion transferability is evaluated by visually confirming the image density of the convex portion and occurrence of dust or white spots in the formed solid image. As for halftone reproducibility, halftone images are formed on various uneven papers A to C, and halftone reproduction is performed by visually confirming the image density and occurrence of dust or white spots in the formed halftone images. Assess sex.

  In this experiment, the image quality was evaluated when the heater 87 was controlled to change the electrical resistance value of the secondary transfer nip N. Table 1 shows the image quality evaluation results of the uneven paper A, Table 2 shows the image quality evaluation results of the uneven paper B, and Table 3 shows the image quality evaluation results of the uneven paper C. If the image quality evaluation result is good (acceptable range), “◯” is indicated. If the image quality evaluation result is not good but is acceptable depending on conditions such as the type of image or user preference (request), “△” is indicated. An unacceptable case was indicated as “x”.

First, as shown in Tables 1 to 3 described above, in this experiment, the electrical resistance value of the secondary transfer nip N that gives good (◯) results for all the evaluation items in all the uneven papers A to C. Did not exist.
Concerning the concave transferability, any of the concavo-convex papers A to C is defective (x) when the electric resistance value of the secondary transfer nip N is the highest, 10.9 logΩ, and the electric resistance value of the secondary transfer nip N is low. There is a tendency for improvement to become lower. Therefore, there is almost no difference in the recess transferability due to the difference in the types of the uneven papers A to C.

  On the other hand, with respect to the convexity transfer property and the halftone reproducibility, there is a difference between the uneven paper B and the uneven paper C having different paper thicknesses. Specifically, when the electrical resistance value of the secondary transfer nip N is high, the thick uneven paper B is good (◯) or acceptable (Δ), but the thin uneven paper C is poor. (X). This is because, when the electrical resistance value of the secondary transfer nip N is high, the secondary transfer bias necessary for flowing the target transfer current becomes large. This is probably because of the discharge. Therefore, in the AC mode, when an image is formed on uneven paper having a thin paper thickness, the electrical resistance value of the secondary transfer nip N is lower than when forming an image on uneven paper having a large paper thickness. It is preferable to control the heater 87.

  Further, regarding the convexity transferability, when the concavo-convex paper B and the concavo-convex paper C having different paper thicknesses are compared, when the electrical resistance value of the secondary transfer nip N is low, the concavo-convex paper C having a thin paper thickness is good (◯ ) Or acceptable (Δ), whereas the uneven paper B having a large paper thickness is acceptable (Δ) or defective (×). In the case of the uneven paper C having a thick paper thickness, the electrical resistance value in the recording material sandwiching area N1 of the secondary transfer nip N increases, so that if the electrical resistance value of the secondary transfer nip N is low, the electrical resistance of the recording material sandwiching area N1. The difference between the value and the electric resistance value of the non-recording material sandwiching region N2 becomes large. Therefore, current flows preferentially to the non-recording material clamping area N2, the amount of current flowing to the recording material clamping area N1 decreases, the transferability of the toner image in the recording material clamping area N1 deteriorates, and image density is insufficient. It is thought that. Therefore, in the AC mode, when an image is formed on a thick uneven paper, the electrical resistance value of the secondary transfer nip N is higher than when an image is formed on a thin uneven paper. It is preferable to control the heater 87.

  From the above, as a specific control, the heater 87 is controlled so that the electrical resistance value of the secondary transfer nip N is in the range of 10.5 logΩ or more and 10.7 logΩ or less for the uneven paper B having a large paper thickness. It is good. For the uneven paper C having a thin paper thickness, the heater 87 is preferably controlled so that the electrical resistance value of the secondary transfer nip N is in the range of 9.9 logΩ to 10.2 logΩ.

[Control example 1]
FIG. 6 is a flowchart illustrating an example of control in the AC mode (hereinafter referred to as “control example 1”).
When a print job is input to the control unit 60, the control unit 60 starts the print job (S1), first obtains paper type information from the operation panel 69 (S2), and in AC mode based on the paper type information. It is selected whether to control in the DC mode (S3). That is, in this control example 1, based on the paper type information indicating the selection result of the paper type used for the printing selected by the user by operating the operation panel 69, the paper type information indicates the uneven paper corresponding to the AC mode. If it is, the AC mode is selected. Otherwise, the DC mode is selected.

  When the DC mode is selected (No in S3), the control unit 60 sets the DC transfer bias as the secondary transfer bias (S11), and starts the image forming operation with the heater 87 turned off (S11). S10). When the secondary transfer bias is a DC transfer bias, even if the electrical resistance value of the secondary transfer nip N is high, discharge on the entrance side of the secondary transfer nip N hardly occurs, and dust and white spots due to discharge are suppressed. The image density can be ensured. Rather, if the electrical resistance value of the secondary transfer nip N is lowered, current tends to flow preferentially to the non-recording material clamping area N2, and there is a possibility that the image density cannot be secured. According to this control example 1, in the DC mode, the heater 87 remains off and the electrical resistance value of the secondary transfer nip N is kept high, so that high image density is achieved while suppressing dust and white spots due to discharge. It is possible to form a high-quality image in which is ensured.

  When the AC mode is selected (Yes in S3), the control unit 60 sets an AC transfer bias as the secondary transfer bias (S4). Based on the paper type information, the type (selection condition) of the recording sheet is determined as to whether the uneven paper used for printing is thick paper or thin paper (S5). When it is determined that the uneven paper is a thick paper (Yes in S5), as described above, it is preferable that the electrical resistance value of the secondary transfer nip N is higher than that of the thin uneven paper. Therefore, the control unit 60 sets the output time of the heater 87 to a relatively short time (S6). Specifically, based on the result of the image quality evaluation experiment described above, the output time of the heater 87 is set so that the electrical resistance value of the secondary transfer nip N is in the range of 10.5 logΩ to 10.7 logΩ. As a result, the temperature of the secondary transfer counter roller 24 becomes relatively low, and the electrical resistance value of the secondary transfer nip N becomes relatively high.

  Conversely, when it is determined that the uneven paper is thin (No in S5), the control unit 60 sets the output time of the heater 87 to a relatively long time (S7). Specifically, the output time of the heater 87 is set so that the electrical resistance value of the secondary transfer nip N is in the range of 9.9 logΩ to 10.2 logΩ from the result of the image quality evaluation experiment described above. As a result, the temperature of the secondary transfer counter roller 24 becomes relatively high, and the electrical resistance value of the secondary transfer nip N becomes relatively low.

  After setting the heater output time in this way, the controller 60 turns on the heater 87 by the heater driving means (S8), and starts the image forming operation when the heater output time has passed the set time (Yes in S9). (S10). In the AC mode in which the secondary transfer bias is an AC transfer bias, if the electrical resistance value of the secondary transfer nip N is high, discharge on the inlet side of the secondary transfer nip N is likely to occur. Since the heater 87 is turned on to reduce the electrical resistance value of the secondary transfer nip N, dust and white spots due to discharge can be suppressed.

  Furthermore, in this control example 1, the output time of the heater is changed to adjust the electrical resistance value of the secondary transfer nip N depending on whether the type of recording sheet (uneven paper) used in the AC mode is thick paper. To do. As a result, in the case of thin paper where dust and white spots are more likely to occur than with thick paper, the electrical resistance value of the secondary transfer nip N is greatly reduced, thereby stabilizing dust and white spots due to discharge. Can be suppressed. On the other hand, in the case of thick paper in which dust and white spots due to discharge are less likely to occur compared to thin paper, the amount of decrease in the electrical resistance value of the secondary transfer nip N is reduced, thereby making it easy to ensure image density. .

  Table 4 below shows the relationship between the output power of the heater 87 and the electrical resistance value of the secondary transfer nip N in the image forming apparatus 1 of the present embodiment. This Table 4 uses the image forming apparatus used in the quality evaluation experiment described above, and is measured in a use environment where the temperature is 10 ° C. and the relative humidity is 15%. Thus, the electrical resistance value of the secondary transfer nip N can be adjusted by the heater output time. In addition, for example, a temperature sensor that detects the temperature of the secondary transfer counter roller 24 heated by the heater 87 may be provided, and the temperature may be controlled to adjust the electrical resistance value of the secondary transfer nip N.

  The relationship between the concavo-convex paper B and the concavo-convex paper C having different paper thicknesses as described above is due to the difference in electrical resistance value between the concavo-convex paper B and the concavo-convex paper C in the secondary transfer nip N. Therefore, this relationship can be similarly considered by the type of image transferred to the concavo-convex paper in the secondary transfer nip N, specifically, the difference in image area ratio (toner adhesion amount per unit area).

  That is, when the amount of toner is small, discharge does not occur and electric discharge easily occurs. Therefore, when the electrical resistance value of the secondary transfer nip N is high, image quality deterioration such as dust and white spots is likely to occur. Partial transferability and halftone reproducibility are likely to deteriorate. Therefore, in the AC mode, when forming an image with a low image area ratio, the heater 87 is controlled so that the electrical resistance value of the secondary transfer nip N is lower than when forming an image with a high image area ratio. Is preferred.

  Similarly, when there is a large amount of toner, the electrical resistance value in the recording material sandwiching area N1 of the secondary transfer nip N increases. Therefore, if the electrical resistance value in the secondary transfer nip N is low, the electrical resistance of the recording material sandwiching area N1. The difference between the value and the electric resistance value of the non-recording material sandwiching area N2 becomes large, and the image density may be insufficient. Therefore, in the AC mode, when forming an image with a high image area ratio, the heater 87 is controlled so that the electrical resistance value of the secondary transfer nip N is higher than when forming an image with a low image area ratio. It is preferable to do this.

[Control example 2]
FIG. 7 is a flowchart showing another example of control in the AC mode (hereinafter referred to as “control example 2”). In the following description, differences from the control example 1 described above will be mainly described.
In this control example 2, when the AC mode is selected (Yes in S3), the control unit 60 selects the type of image (selection condition) formed on the uneven paper used for printing based on the image information related to the print job. ). Specifically, it is determined whether or not the image area ratio of the image formed on the uneven paper used for printing is equal to or greater than a specified value (S12).

  When it is determined that the image area ratio is greater than or equal to the specified value (Yes in S12), as described above, the secondary transfer nip N is greater than when the image area ratio is less than the specified value and the amount of toner is small. A higher electrical resistance value is preferred. Therefore, the control unit 60 sets the output time of the heater 87 to a relatively short time (S6). Conversely, when it is determined that the image area ratio is less than the specified value (No in S12), the control unit 60 sets the output time of the heater 87 to a relatively long time (S7).

  In this control example 2, the output time of the heater is changed to adjust the electrical resistance value of the secondary transfer nip N depending on whether the type of image formed in the AC mode is an image with a high image area ratio. As a result, the electrical resistance value of the secondary transfer nip N in the case of an image with a low image area ratio with a small amount of toner that tends to cause dust and white spots due to discharge compared to an image with a high image area ratio with a large amount of toner. Can be greatly reduced, and dust and white spots due to discharge can be stably suppressed. On the other hand, in the case of an image with a high image area ratio with a large amount of toner that is less likely to cause dust and white spots due to discharge compared to an image with a low image area ratio with a small amount of toner, the electrical resistance value of the secondary transfer nip N The reduction width is reduced, thereby making it easy to ensure the image density.

  Further, in the image quality evaluation experiment described above, when the uneven paper A and the uneven paper B are compared, the basis weight thereof is almost the same, but the image quality evaluation result of the convex portion transferability is slightly different. This can be considered as follows. As shown in FIG. 8, the concavo-convex paper A has a short surface concavo-convex period and a small area of each convex portion. Therefore, since the area where the surface of the intermediate transfer belt 21 and the concavo-convex paper A contact is small, the transfer current hardly flows through the secondary transfer nip N. For this reason, when the electrical resistance value of the secondary transfer nip N is low, current flows preferentially to the non-recording material sandwiching area N2, resulting in insufficient image density. Therefore, even when the thickness of the concavo-convex paper is the same, it is preferable to change the electrical resistance value of the secondary transfer nip N due to the difference in surface concavoconvex (difference in the convex area ratio). Specifically, in the AC mode, when an image is formed on uneven paper having a low convex area ratio, the electrical resistance value of the secondary transfer nip N is greater than when an image is formed on uneven paper having a high convex area ratio. It is preferable to control the heater 87 so as to be high. This also applies to the case where the electrical resistance value of the uneven paper varies depending on the material of the uneven paper.

[Control Example 3]
FIG. 9 is a flowchart showing still another example of control in the AC mode (hereinafter referred to as “control example 3”). In the following description, differences from the control example 1 described above will be mainly described.
In this control example 3, when the AC mode is selected (Yes in S3), the control unit 60 determines whether the uneven paper used for printing has a large convex area based on the paper type information from the operation panel 69. The type (selection condition) of the recording sheet is determined as being small (S13). When it is determined that the convex / concave paper has a small convex area (Yes in S13), as described above, the electric resistance value of the secondary transfer nip N is higher than that of the concave / convex paper having a small convex area. Is preferred. Therefore, the control unit 60 sets the output time of the heater 87 to a relatively short time (S6). On the other hand, when it is determined that the paper is uneven paper having a large convex area (No in S13), the control unit 60 sets the output time of the heater 87 to a relatively long time (S7).

  In this control example 3, the output time of the heater is changed to adjust the electrical resistance value of the secondary transfer nip N depending on whether or not the type of concavo-convex paper formed in the AC mode is a paper with a small convex area. . As a result, in the case of concavo-convex paper with a large convex area where dust and white spots are more likely to occur due to discharge compared to the concavo-convex paper with a small convex area, the electrical resistance value of the secondary transfer nip N is further reduced. Thereby, dust and white spots due to discharge can be stably suppressed. On the other hand, in the case of concavo-convex paper with a small convex area that is less prone to dust and white spots due to discharge compared to a concavo-convex paper with a large convex area, the reduction width of the electrical resistance value of the secondary transfer nip N is reduced. This makes it easy to ensure image density.

  In addition, depending on the type of uneven paper, the type of image (whether it is a photographic image or a character image), and the user's request (use of printed matter, user's preference, etc.), the three evaluation items (recess transferability) in the image quality evaluation result described above. , Convexity transferability, halftone reproducibility) may change in priority (importance). That is, for example, even if the concave portion transferability is somewhat poor, if the convex portion transferability is good, the image quality of the entire image may be good, while the concave portion transferability and the convex portion transferability may be somewhat poor. If the halftone reproducibility is good, the image quality of the entire image may be good. Therefore, for example, even in the case of the concavo-convex paper A, the heater 87 is controlled so that the electrical resistance value of the secondary transfer nip becomes 9.9 logΩ, for example, in the case where the concave transferability is prioritized and improved. In some cases, it is preferable to control the heater 87 so that the electrical resistance value of the secondary transfer nip is 10.7 logΩ in the case where the convex transfer property is prioritized and improved. Further, for example, in the case of the concavo-convex paper C, the heater 87 is controlled so that the electrical resistance value of the secondary transfer nip becomes 10.2 log Ω in the case where the concave portion transfer property and the convex portion transfer property are preferentially improved. In some cases, it is preferable to control the heater 87 so that the electrical resistance value of the secondary transfer nip is 9.9 logΩ in the case where the halftone reproducibility is improved and improved.

[Control Example 4]
FIG. 10 is a flowchart showing still another example of control in the AC mode (hereinafter referred to as “control example 4”). In the following description, differences from the control example 1 described above will be mainly described.
In the fourth control example, the control unit 60 displays not only the paper type information but also priority information indicating the priority mode selected by the user operating the operation panel 69 from the “solid priority mode” and the “halftone priority mode”. Is obtained from the operation panel 69 (S14). When the AC mode is selected (Yes in S3), the control unit 60 selects the “solid priority mode” in which the user prioritizes the image quality of the solid image portion based on the priority information from the operation panel 69. Or, the content (selection condition) of the user operation indicating whether or not the “halftone priority mode” that prioritizes the image quality of the halftone image portion has been selected is determined (S15).

  When the “solid priority mode” is selected (Yes in S15), the control unit 60 sets the output time of the heater 87 to a relatively short time (S6), and the electrical resistance of the secondary transfer nip N is set. Set the value to be relatively high. On the other hand, when the “halftone priority mode” is selected (No in S15), the output time of the heater 87 is set to a relatively long time (S7), and the electrical resistance value of the secondary transfer nip N is relative. Set to lower.

  In this control example 4, the electrical resistance value of the secondary transfer nip N is adjusted by changing the output time of the heater in accordance with the contents of the user operation. Thereby, it is possible to form an image having an image quality according to the user's request.

  As described above, the AC mode in this embodiment is prepared in advance with a plurality of control modes in which the AC transfer bias is applied with the electrical resistance values of the secondary transfer nip N being different from each other. That is, the controller 60 drives the heater so that the electrical resistance value of the secondary transfer nip N becomes a value corresponding to the selection condition by selecting and executing the corresponding control mode according to the input selection condition. The heater 87 is controlled by the means 600b, and an AC transfer bias is applied to the secondary transfer nip N to execute an image forming operation.

  The selection condition is, as in Control Example 1 and Control Example 3 described above, the type of uneven paper (thickness of the uneven paper, The condition may be that the control mode is selected according to the convex area ratio, material, and the like. In this case, for the type of uneven paper, the control mode may be selected based on input information in which the user operates the operation panel 69 to select the type of uneven paper. Alternatively, a recording material type detection unit that determines the type of the uneven paper may be provided in the paper conveyance path in the image forming apparatus 1 or in the paper feed cassette 202, and the control mode may be selected based on the detection result.

  Further, as in the control example 2 described above, the selection condition is that the image types (image area ratio, on the recording material) such that the electrical resistance value of the secondary transfer nip differs when interposed in the secondary transfer nip N. The condition may be that the control mode is selected according to the distribution of the toner image adhering portion. In this case, as the image type, the control mode may be selected based on input information in which the user operates the operation panel 69 to select the image type. Alternatively, the control unit 60 may determine the image type based on the image information, and select the control mode based on the determination result.

  Further, the selection condition may be a condition of selecting a control mode according to a user's request (use of printed matter, user's preference, etc.) as in Control Example 4 described above. In this case, according to the user's request, the control mode may be selected based on input information obtained by the user operating the operation panel 69. For example, the user can select either “solid priority mode” or “halftone priority mode” from the menu of the operation panel 69, and the control mode corresponding to the content selected by the user is selected. Specifically, when using the uneven paper C, when the “halftone priority mode” is selected, a control mode for controlling the heater 87 so that the electrical resistance value of the secondary transfer nip is 9.5 logΩ is set. When the “solid priority mode” is selected, the control mode for controlling the heater 87 is selected and executed so that the electrical resistance value of the secondary transfer nip becomes 10.2 logΩ.

[Modification 1]
Next, a modification of the image forming apparatus according to the present embodiment (hereinafter, this modification is referred to as “modification 1”) will be described.
FIG. 11 is an explanatory diagram showing a peripheral configuration of the secondary transfer portion P in the first modification.
In the first modification, a secondary transfer power supply 81 is connected to the secondary transfer roller 30 and the secondary transfer counter roller 24 is grounded. In the first modification, the heater 87 is disposed so as to face the secondary transfer roller 30. Even in the first modification, the same effects as those of the above-described embodiment can be obtained. In FIG. 11, a DC power supply circuit unit composed of a single DC power supply 813 is omitted. However, in the first modification, the DC power supply circuit unit may be connected to the secondary transfer roller 30. You may connect to the secondary transfer counter roller 24.

[Modification 2]
Next, another modification of the image forming apparatus according to the present embodiment (hereinafter, this modification is referred to as “modification 2”) will be described.
FIG. 12 is an explanatory diagram showing a peripheral configuration of the secondary transfer portion P in the second modification.
In the second modification, the AC power supply 812 is connected to the secondary transfer counter roller 24, and the DC power supply 811 is connected to the secondary transfer roller 30. The ground is on the secondary transfer roller 30 side. Also in the second modification, the same effects as those of the above-described embodiment can be obtained. In FIG. 12, the DC power supply circuit unit including the single DC power supply 813 is omitted, but in the second modification, the DC power supply circuit unit may be connected to the secondary transfer roller 30. You may connect to the secondary transfer counter roller 24.

The heater 87 may be disposed opposite the secondary transfer opposing roller 24 as indicated by a solid line in FIG. 12, or may be disposed opposite the secondary transfer roller 30 as indicated by a two-dot chain line. May be.
Further, the connection mode and the ground mode of the DC power supply 811 and the AC power supply 812 in the secondary transfer power supply 81 may be reversed as shown in FIG.

[Modification 3]
Next, still another modification of the image forming apparatus according to the present embodiment (hereinafter, this modification is referred to as “Modification 3”) will be described.
FIG. 14 is an explanatory diagram showing a peripheral configuration of the secondary transfer portion P in the third modification.
In the third modification, instead of the heating means by the heater 87, the adjusting means for adjusting the electric resistance value of the secondary transfer nip N is replaced by the nip width of the secondary transfer nip N (the intermediate transfer belt of the secondary transfer nip N). A nip width changing means for changing the moving direction length) is used. The nip width changing means as the adjusting means is an electric resistance value changing means for changing the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N. The heating means and the nip width changing means may be used in combination.

  The nip width changing unit of the third modification includes a push-down roller 58 that contacts the inner peripheral surface of the intermediate transfer belt 21 at the upstream side of the secondary transfer nip N in the intermediate transfer belt moving direction, and the inner peripheral surface of the intermediate transfer belt 21. The nip width of the secondary transfer nip N is changed by moving in the contact / separation direction. Specifically, an eccentric cam 581 is disposed on the rotating shaft of the push-down roller 58. The rotation shaft of the eccentric cam 581 is urged by a spring 582 as an urging unit so that the rotation shaft of the push-down roller 58 approaches the inner peripheral surface of the intermediate transfer belt 21. Thus, the push-down roller 58 pushes the intermediate transfer belt 21 toward the secondary transfer roller 30 side by the biasing force of the spring 582 via the eccentric cam 581.

  A cam drive motor 583 for rotating the eccentric cam 581 is connected to the rotation shaft of the eccentric cam 581, and the cam drive motor 583 is controlled by the control unit 60. In the third modification, when the eccentric cam 581 is half-rotated from the state shown by the solid line in FIG. 14 to the state shown by the broken line in FIG. 14, the rotation shaft of the push-down roller 58 is pushed by the outer peripheral surface of the eccentric cam 581. As shown by a broken line in FIG. 14, the intermediate transfer belt 21 is displaced toward the inner peripheral surface side. As a result, the surface of the intermediate transfer belt 21 on the upstream side of the secondary transfer nip N in the intermediate transfer belt movement direction is displaced in a direction approaching the secondary transfer roller 30. As a result, the amount of the intermediate transfer belt 21 wound around the secondary transfer roller 30 increases, and the nip width of the secondary transfer nip N becomes longer.

  As the amount of the intermediate transfer belt 21 wound around the secondary transfer roller 30 increases, a transfer current also flows to a portion of the intermediate transfer belt 21 that is newly wound around the secondary transfer roller 30. As the area through which the transfer current flows increases, the electrical resistance value of the secondary transfer nip N becomes lower. Therefore, the same control as the control for lowering the electrical resistance value of the secondary transfer nip N by heating with the heater 87 in the above-described embodiment. Can be realized by rotational control of the eccentric cam 581.

[Modification 4]
Next, still another modification of the image forming apparatus according to the present embodiment (hereinafter, this modification is referred to as “Modification 4”) will be described.
15 and 16 are explanatory diagrams showing the peripheral configuration of the secondary transfer portion P in the fourth modification.
In the fourth modification, nip pressure changing means for changing the pressure applied to the secondary transfer nip N is used as the adjusting means for adjusting the electrical resistance value of the secondary transfer nip N instead of the heating means by the heater 87. The nip pressure changing means as the adjusting means is an electric resistance value changing means for changing the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N. The heating means, the nip width changing means, and the nip pressure changing means may be used in combination.

  The nip pressure changing means of the fourth modification changes the pressure applied to the secondary transfer nip N by changing the pressing force that presses the secondary transfer roller 30 against the intermediate transfer belt 21 wound around the secondary transfer counter roller 24. To change. Specifically, the shaft 311 of the secondary transfer roller 30 is rotatably supported by the roller unit holder 31. The roller unit holder 31 is supported at its downstream end in the recording sheet conveyance direction so as to be rotatable with respect to the apparatus main body by a support shaft 32. On the other hand, the upstream end of the roller unit holder 31 in the recording sheet conveyance direction is urged downward in the figure by a tension spring 33 fixed to the apparatus main body. Therefore, the roller unit holder 31 is given a biasing force that rotates around the support shaft 32 in the clockwise direction in the drawing by the restoring force of the tension spring 33.

  Further, an eccentric cam 34 is disposed at a position where the roller unit holder 31 to which such an urging force is applied is rotated in the clockwise direction in the drawing. Since the eccentric cam 34 is in contact with the roller unit holding body 31, the roller unit holding body 31 to which the urging force of the tension spring 33 is applied stops at a predetermined rotation position (rotation angle). When the eccentric cam 34 is rotated by the cam drive motor 35, the rotation position of the roller unit holding body 31 changes according to the rotation angle. The roller unit holder 31 supports the secondary transfer roller 30 so that the secondary transfer roller 30 is displaced in a direction in which the secondary transfer roller 30 comes in contact with and separates from the intermediate transfer belt 21 according to a change in the rotation position. Therefore, the pressure applied to the secondary transfer nip N can be changed by displacing the secondary transfer roller 30 by controlling the cam drive motor 35 by the control unit 60 and changing the rotation angle of the eccentric cam 34. .

  When the rotation angle of the eccentric cam 34 is changed from the rotation angle shown in FIG. 15 as shown in FIG. 16, the pressing force of the secondary transfer roller 30 against the intermediate transfer belt 21 increases, and the rubber layer of the secondary transfer roller 30 increases. 312 is deformed more greatly. As a result, the contact width between the surface of the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 and the surface of the secondary transfer roller 30 increases. Since the electrical resistance value of the secondary transfer nip N decreases as the cross-sectional area of the route through which the transfer current flows increases, the electrical resistance value of the secondary transfer nip N decreases as the contact width increases. Therefore, the same control as the control for lowering the electric resistance value of the secondary transfer nip N by heating with the heater 87 in the above-described embodiment can be realized by the rotation control of the eccentric cam 34.

[Modification 5]
Next, still another modification of the image forming apparatus according to the present embodiment (hereinafter, this modification is referred to as “modification 5”) will be described.
17 and 18 are explanatory diagrams showing the peripheral configuration of the secondary transfer portion P in Modification 5.
In the fifth modification, as an adjusting means for adjusting the electrical resistance value of the secondary transfer nip N, a roller switching means for switching between the two secondary transfer rollers 30 and 30 ′ having different hardness instead of the heating means by the heater 87. Is used. The roller switching means as the adjusting means is an electric resistance value changing means for changing the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N. The heating means, nip width changing means, nip pressure changing means, and roller switching means may be used in combination.

  In the roller switching unit of the fifth modification, the shafts 311 and 311 ′ of the two secondary transfer rollers 30 and 30 ′ having different hardnesses are rotatably supported by the roller unit holder 36. The roller unit holder 36 is configured to be swingable around the rotation shaft 37 by the driving force of the drive motor 38. The swing range of the roller unit holder 36 is restricted by the stoppers 39A and 39B.

  Under the control of the control unit 60, when the roller unit holding body 36 swings counterclockwise in the drawing by the driving force of the driving motor 38 and enters the state shown in FIG. 17, the first secondary having relatively high hardness. The transfer roller 30 contacts the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 to form a secondary transfer nip N. On the other hand, under the control of the control unit 60, when the roller unit holding body 36 swings clockwise in the drawing by the driving force of the driving motor 38 and enters the state shown in FIG. The secondary transfer roller 30 ′ abuts on the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 to form a secondary transfer nip N.

  When the secondary transfer nip N is formed by the first secondary transfer roller 30 having a relatively high hardness as shown in FIG. 17 by swinging the roller unit holder 36, the rubber of the secondary transfer roller 30 is formed. The deformation amount of the layer 312 is relatively small. From this state, when the roller unit holder 36 is swung and the secondary transfer nip N is formed by the second secondary transfer roller 30 ′ having a relatively low hardness as shown in FIG. 18, the secondary transfer nip N is formed. The deformation amount of the rubber layer 312 ′ of the transfer roller 30 ′ is relatively large. As a result, the contact width between the surface of the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 and the surface of the secondary transfer roller increases. Since the electrical resistance value of the secondary transfer nip N decreases as the cross-sectional area of the route through which the transfer current flows increases, the electrical resistance value of the secondary transfer nip N decreases as the contact width increases. Therefore, in the above-described embodiment, the same control as the control for lowering the electric resistance value of the secondary transfer nip N by heating with the heater 87 can be realized by the control of the roller switching means.

[Modification 6]
Next, still another modification of the image forming apparatus according to the present embodiment (hereinafter, this modification is referred to as “Modification 6”) will be described.
In the sixth modification, instead of the second secondary transfer roller 30 ′ in the fifth modification described above, a secondary transfer roller 30 ″ having an electric resistance value lower than that of the first secondary transfer roller 30 is used. In the sixth modification, when the roller switching unit is controlled from the state shown in FIG. 17 to the state shown in FIG. 18, the electrical resistance value of the secondary transfer roller forming the secondary transfer nip N becomes low. As a result, the electrical resistance value on the route of the transfer current flowing through the secondary transfer nip is lowered. Accordingly, also in the sixth modification, the same control as the control for lowering the electrical resistance value of the secondary transfer nip N by heating with the heater 87 in the above-described embodiment can be realized by the control of the roller switching unit.

What was demonstrated above is an example, and there exists an effect peculiar for every following aspect.
(Aspect A)
Secondary transfer for forming a transfer nip such as a secondary transfer nip N in contact with the surface of the image carrier such as an intermediate transfer belt 21 and a secondary transfer counter roller 24 that carry a toner image on the surface. A transfer bias such as a secondary transfer bias for transferring a toner image on the image carrier to a recording material such as a recording sheet including a nip forming member such as a roller 30 and an uneven paper sandwiched in the transfer nip. In the image forming apparatus 1 having a transfer bias applying unit such as a secondary transfer power supply 81 that applies an AC transfer bias including a DC component and an AC component to the transfer nip, on the route of the transfer current flowing through the transfer nip. Adjusting means such as a heater 87 and a heater power source 84 or an eccentric cam 581 and a cam drive motor 583 for adjusting the electric resistance value of the AC transfer bias, and the AC transfer bias. To adjust according to a predetermined selection condition the electrical resistance when applied to, and having a control unit such as control unit 60 for controlling the adjusting means.
When an AC transfer bias is applied to transfer a toner image to a recording material, a discharge is generated on the entrance side of the transfer nip, compared to a case where a DC transfer bias consisting of only a DC component is applied to transfer a toner image to the recording material. It is easy to cause image quality degradation such as dust and white spots due to the discharge. This is considered as follows. That is, since the AC transfer bias is such that the AC component is superimposed on the DC component equivalent to the DC transfer bias, a peak value of the AC component (a peak value whose absolute value is larger than the DC component) is applied. At a certain time, the potential difference with respect to the recording material becomes larger than the DC transfer bias. Therefore, at this time, a potential difference equal to or higher than the discharge start voltage tends to occur on the entrance side of the transfer nip where a minute space is formed between the image carrier and the recording material. When this discharge occurs, the toner image on the image carrier is disturbed by the impact of the discharge and dust is generated, or some toner constituting the toner image on the image carrier is charged to a reverse polarity by the discharge. In other words, the image is not transferred at the transfer nip, resulting in white spots in the image. Therefore, when applying an AC transfer bias, it is necessary to set the magnitude of the DC component of the AC transfer bias and the amplitude of the AC component so that no discharge occurs when the peak value of the AC component is applied. Become.
On the other hand, when applying an AC transfer bias, in order to ensure a sufficient image density, it is necessary to pass a sufficient transfer current to the transfer nip, and for this reason, it is required to increase the DC component of the AC transfer bias. There is. However, as described above, increasing the DC component of the AC transfer bias is limited in relation to the occurrence of discharge, so the type of recording material, the type of image to be formed, the imaging conditions such as the process linear velocity, etc. Depending on the conditions, there is a possibility that a sufficient transfer current cannot be passed within the range of the restriction and a sufficient image density cannot be ensured.
On the other hand, when applying an AC transfer bias, the lower the transfer nip resistance value, the easier the transfer current flows through the transfer nip, and the smaller the DC component of the AC transfer bias required to obtain the required transfer current. Can do. However, when the resistance value of the transfer nip is low, the AC transfer bias may be adjusted depending on the type of recording material and the type of image (difference in image area ratio, difference in distribution of toner image adhering portion on the recording material, etc.). However, sufficient image density may not be ensured. Specifically, when the resistance value of the transfer nip is low, current easily flows in a region where the recording material in the transfer nip is not sandwiched (non-recording material sandwiching region N2), and the recording material in the transfer nip is sandwiched. The amount of current flowing through the existing area (recording material clamping area N1) is relatively small. The electrical resistance value of the recording material sandwiching area N1 varies depending on the type of the recording material and the type of image transferred to the recording material. For this reason, depending on the type of recording material and the type of image, even if the electrical resistance value of the recording material sandwiching area N1 becomes too high, the DC component of the AC transfer bias is large and the total amount of transfer current is sufficient. May flow to the non-recording material sandwiching area N2, and a necessary amount of current may not flow through the recording material sandwiching area N1.
In this aspect, the electrical resistance value of the transfer nip can be adjusted by adjusting means for adjusting the electrical resistance value by, for example, changing the electrical resistance value on the route of the transfer current flowing through the transfer nip by the electrical resistance value changing means. it can. As a result, the electrical resistance value when the AC transfer bias is applied to the transfer nip is adjusted by selectively executing a plurality of control modes in which the AC transfer bias is applied with the transfer nip having different electrical resistance values. can do.
As a result, even if the AC transfer bias is adjusted within a range in which the electric resistance value of the transfer nip is too low to generate discharge, a sufficient amount of current does not flow through the recording material sandwiching area N1, and the image density cannot be secured. When the recording material type or the image type is selected, the AC transfer bias can be applied by increasing the electric resistance value of the transfer nip. As a result, it is possible to set an AC transfer bias for such a recording material type or image type so that a sufficient image density can be obtained without generating dust or white spots, and a high quality image can be set. Can be formed.
Also, the type of recording material and image type in which the transfer nip electrical resistance value is too high, and even if the AC transfer bias is adjusted within a range that does not cause discharge, the image density cannot be secured due to insufficient transfer current flowing through the transfer nip. In the case of the image forming conditions, the AC transfer bias can be applied by lowering the electric resistance value of the transfer nip. As a result, even under such conditions, it is possible to set an AC transfer bias that does not generate dust or white spots and can obtain a sufficient image density, and can form a high-quality image. it can.

(Aspect B)
In the aspect A, the predetermined selection condition includes a type of recording material such as a paper type to be used.
According to this, it is possible to form a high-quality image corresponding to the type of recording material.

(Aspect C)
In the aspect B, the control means controls the adjustment means so that the electrical resistance value is adjusted to a higher electrical resistance value as the recording material to be used is thicker.
According to this, it is possible to form a high quality image corresponding to the difference in thickness of the recording material.

(Aspect D)
In any one of the aspects A to C, the predetermined selection condition includes a type of an image to be transferred to a recording material.
According to this, it is possible to form a high-quality image corresponding to the type of image.

(Aspect E)
In the aspect D, the control unit controls the adjustment unit to adjust the electric resistance value to a lower electric resistance value as the image area ratio of the image transferred to the recording material is smaller. .
According to this, it is possible to form a high quality image corresponding to the difference in the image area ratio.

(Aspect F)
In any one of the aspects A to E, the control unit adjusts the AC transfer bias to the transfer nip by the transfer bias applying unit in a state where the electric resistance value is adjusted to a different electric resistance value by the adjusting unit. A user operation for selecting the control mode to be executed by the control means by executing the control mode selected according to the predetermined selection condition from among a plurality of control modes for applying the control The operation receiving means such as an operation panel 69 is received, and the predetermined selection condition includes contents of a user operation received by the operation receiving means.
According to this, it is possible to form an image with quality according to the user's request.

(Aspect G)
In the aspect F, the image processing apparatus includes notification means such as an operation panel 69 that notifies a plurality of image qualities such as a “solid priority mode” and a “halftone priority mode” that can be selected by the user. A user operation for selecting is accepted.
According to this, it is possible to form an image with quality according to the user's request.

(Aspect H)
In any of the above aspects A to G, the adjusting unit adjusts the electric resistance value of the transfer nip by controlling a nip width changing unit such as an eccentric cam 581 that changes the nip width of the transfer nip. It is characterized by including things.
According to this, the electric resistance value on the route of the transfer current flowing through the transfer nip can be adjusted by the control of the nip width changing means.

(Aspect I)
In the aspect H, the adjusting means controls the nip width changing means so that the nip width is widened when the electric resistance value of the transfer nip is lowered.
According to this, the electric resistance value on the route of the transfer current flowing through the transfer nip can be adjusted by the control of the nip width changing means.

(Aspect J)
In any one of the aspects A to I, the adjusting means controls the electric resistance value by controlling a heating means such as a heater 87 that applies heat to at least one of the image carrier and the nip forming member. It is characterized by including what is adjusted.
According to this, the electric resistance value on the route of the transfer current flowing through the transfer nip can be adjusted by controlling the heating means.

(Aspect K)
In the aspect J, the adjusting unit controls the heating unit so that the amount of heat applied by the heating unit increases when the electric resistance value of the transfer nip is lowered.
According to this, a general material can be used as the material of the image carrier and the nip forming member heated by the heating means.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Image forming part 20 Transfer unit 21 Intermediate transfer belt 24 Secondary transfer counter rollers 25C, 25M, 25Y, 25K Primary transfer rollers 30, 30 'Secondary transfer rollers 31, 36 Roller unit holder 32 Support shaft 33 Tension spring 34 Eccentric cam 35 Cam drive motor 37 Rotating shaft 38 Drive motor 39A, 39B Stopper 58 Depressing roller 60 Control unit 69 Operation panel 81 Secondary transfer power source 84 Heater power source 87 Heater 100 Printer unit 200 Paper feed table 300 Scanner unit 581 Eccentric cam 582 Spring 583 Cam drive motor 600a Bias control means 600b Heater drive means 811 DC power supply 812 AC power supply 813 DC power supply

Japanese Patent Laying-Open No. 2015-187723

Claims (11)

An image carrier that carries a toner image on its surface;
A nip forming member that forms a transfer nip in contact with the surface of the image carrier;
Transfer bias application that applies an AC transfer bias including a DC component and an AC component to the transfer nip as a transfer bias for transferring the toner image on the image carrier to the recording material sandwiched in the transfer nip. An image forming apparatus comprising:
Adjusting means for adjusting an electric resistance value on a route of a transfer current flowing through the transfer nip;
An image forming apparatus comprising: a control unit that controls the adjustment unit so that an electric resistance value when the AC transfer bias is applied to the transfer nip is adjusted according to a predetermined selection condition. The image forming apparatus according to claim 1.
The image forming apparatus, wherein the predetermined selection condition includes a type of recording material to be used. The image forming apparatus according to claim 2.
The image forming apparatus according to claim 1, wherein the control unit controls the adjustment unit such that the electrical resistance value is adjusted to a higher electrical resistance value as the recording material to be used is thicker. The image forming apparatus according to any one of claims 1 to 3,
The image forming apparatus according to claim 1, wherein the predetermined selection condition includes a type of an image to be transferred to a recording material. The image forming apparatus according to claim 4.
The image forming apparatus according to claim 1, wherein the control unit controls the adjustment unit so that the electrical resistance value is adjusted to a lower electrical resistance value as the image area ratio of the image transferred to the recording material is smaller. The image forming apparatus according to any one of claims 1 to 5,
The control means includes a plurality of control modes in which the AC transfer bias is applied to the transfer nip by the transfer bias applying means in a state where the electrical resistance value is adjusted to different electrical resistance values by the adjusting means. Executing the control mode selected according to a predetermined selection condition to control the adjusting means;
Operation receiving means for receiving a user operation for selecting a control mode to be executed by the control means;
The image forming apparatus, wherein the predetermined selection condition includes a content of a user operation received by the operation receiving unit. The image forming apparatus according to claim 6.
Informing means for informing a plurality of image quality selectable by the user,
The image forming apparatus, wherein the operation accepting unit accepts a user operation for selecting an image quality. The image forming apparatus according to any one of claims 1 to 7,
The image forming apparatus according to claim 1, wherein the adjusting unit includes a unit that adjusts an electric resistance value of the transfer nip by controlling a nip width changing unit that changes a nip width of the transfer nip. The image forming apparatus according to claim 8.
The image forming apparatus, wherein the adjusting unit controls the nip width changing unit so that the nip width is widened when the electric resistance value of the transfer nip is lowered. The image forming apparatus according to any one of claims 1 to 9,
The image forming apparatus according to claim 1, wherein the adjusting unit includes a unit that adjusts the electric resistance value by controlling a heating unit that applies heat to at least one of the image carrier and the nip forming member. The image forming apparatus according to claim 10.
The image forming apparatus according to claim 1, wherein the adjusting unit controls the heating unit so that the amount of heat applied by the heating unit increases when the electrical resistance value of the transfer nip is lowered.

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