JP6684466B2 - Image forming device - Google Patents

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 to transfer the toner image to the recording material such as uneven paper. An image forming apparatus that improves the image quality is known.

  For example, in Patent Document 1, a secondary transfer roller is contacted with an intermediate transfer belt portion wound around a secondary transfer facing roller to form a secondary transfer nip, and a DC component and an AC component are superposed. An image forming apparatus is disclosed that applies a bias (AC transfer bias) to a secondary transfer nip to perform secondary transfer. In this image forming apparatus, when forming an image on uneven paper, a superimposing bias is applied to execute an AC mode in which secondary transfer is performed.

  In general, an image forming apparatus has an image forming mode in which the surface moving speed of the image carrier is fast (a high speed mode) capable of forming an image of sufficient quality in a short time, and a surface moving of the image carrier capable of forming a higher quality image. Many have an image forming mode with a low speed (low speed mode). However, in an 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, high speed or high image quality is achieved in the high speed mode without degrading the image quality in the low speed mode. Was difficult.

In order to solve the problems described above, the present invention provides an image carrier that carries a toner image on its surface and moves, a nip forming member that abuts the surface of the image carrier to form a transfer nip, and 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 recording material sandwiched in the transfer nip. In an image forming apparatus having: an image forming mode selected in accordance with a predetermined selection condition from a plurality of image forming modes in which the surface moving speeds of the image carrier are different from each other, control means for controlling an image forming operation; Adjusting means for adjusting an electric resistance value on a route of a transfer current flowing through the transfer nip, wherein the control means has at least two of the plurality of image forming modes. Between the image forming modes, the adjusting means is set so that the electric resistance value when the AC transfer bias is applied to the transfer nip becomes smaller as the surface moving speed of the image carrier becomes faster. In addition to controlling, in at least one image forming mode of the plurality of image forming modes, an electric resistance value when the AC transfer bias is applied to the transfer nip according to the type of the image transferred to the recording material. The adjusting means is controlled so as to adjust .

  According to the present invention, it is possible to achieve high speed or high image quality in the high speed mode without deteriorating the image quality in the low speed mode.

FIG. 1 is an explanatory diagram showing a schematic configuration of an image forming apparatus of an embodiment. FIG. 3 is an explanatory diagram showing a peripheral configuration of a secondary transfer unit in the image forming apparatus. It is a perspective view showing a configuration of a heater arranged around the secondary transfer portion. FIG. 6 is a cross-sectional view of a secondary transfer nip N showing a recording material holding area N1 and a non-recording material holding area N2. FIG. 3 is a block diagram showing a part of a control system of the image forming apparatus. It is a selection flow figure which shows an example of the adjustment method of the electrical resistance value of the secondary transfer nip N about uneven | corrugated paper A-C based on the result of the image quality evaluation experiment. It is an explanatory view showing an example of a touch panel screen operated by a user. It is explanatory drawing which shows the other example of the touch-panel screen which a user operates. 7 is a flowchart showing a flow of a control example in the AC mode in the image forming apparatus. FIG. 9 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion in Modification 1. FIG. 11 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion in Modification 2; FIG. 11 is an explanatory diagram showing another peripheral configuration of the secondary transfer portion in Modification 2. FIG. 13 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion in Modification 3; FIG. 16 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion P when a pressing force is low in Modification 4. FIG. 14 is an explanatory diagram showing a peripheral configuration of a secondary transfer portion P when a high pressing force is applied, 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. 16 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 a modified example 5. It is explanatory drawing which shows the waveform of an alternating current transfer bias.

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

  The printer unit 100 includes an endless belt-shaped intermediate transfer belt 21 which 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 in an inverted triangular shape as shown in FIG. 1 when viewed from the side. . The intermediate transfer belt 21 is surface-moved in the clockwise direction (direction of arrow X in FIG. 1) in FIG. 1 by the rotational driving of the drive roller 22.

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

  Each of the image forming units 1C, 1M, 1Y, 1K has a photoconductor drum 2C, 2M, 2Y, 2K, a developing unit 3C, 3M, 3Y, 3K, and a photoconductor cleaning device 4C, 4M, 4Y, 4K. is doing. The photoconductor drums 2C, 2M, 2Y and 2K are in contact with the intermediate transfer belt 21 at the positions facing the primary transfer rollers 25C, 25M, 25Y and 25K, respectively, to form primary transfer nips for C, M, Y and K. Form. The photoconductor drums 2C, 2M, 2Y and 2K are rotationally driven in the counterclockwise direction in FIG. 1 (direction shown by the arrow in FIG. 1) by the driving means.

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

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

  A transfer unit 20 having an intermediate transfer belt 21 is arranged below the image forming unit 10. Inside the loop of the intermediate transfer belt 21, primary transfer rollers 25C, 25M, 25Y, 25K are provided. The primary transfer rollers 25C, 25M, 25Y, 25K press the intermediate transfer belt 21 toward the photoconductor drums 2C, 2M, 2Y, 2K on the back side of the primary transfer nip for each color of C, M, Y, K. is 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 the 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 sandwiching the secondary transfer roller 30 between the secondary transfer roller 30 and the secondary transfer facing roller 24 which face each other.

  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 moving direction of the intermediate transfer belt. The push-down roller 58 pushes down the intermediate transfer belt 21 toward the secondary transfer roller 30 side and forcibly winds the intermediate transfer belt 21 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 moving direction) is increased. As a result, the entrance of the secondary transfer nip N moves away from the axial position between the secondary transfer counter roller 24 and the secondary transfer roller 30, and discharge is generated at the entrance side of the secondary transfer nip N during the secondary transfer. It's 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 moving direction of the intermediate transfer belt. The recording sheet sent toward the secondary transfer nip N by the pair of registration rollers 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. Will be guided to. Therefore, 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 over or jammed.

  The secondary transfer roller 30 is in contact with a cleaning blade 52 that mechanically scrapes the adhered toner and a rotating brush 53 that serves as a lubricant applying 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 comes into contact with both the solid lubricant 531 such as a zinc stearate lump and the secondary transfer roller 30 and is driven to rotate, so that the lubricant powder obtained by scraping off 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.

  A recording sheet is fed to the secondary transfer nip N by a pair of registration rollers 95 at a predetermined timing, and the four color superposed toner images on the intermediate transfer belt 21 are collectively transferred onto the recording sheet at the secondary transfer nip N. Transcribed.

  The scanner unit 300 reads the image information of the document placed on the contact glass 301 with the reading sensor 302, and sends the read image information to the control unit 60 as a control unit. 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 the image information received from the scanner unit 300. The optical writing unit 15 emits C, M, Y, and K laser writing light to optically scan the photoconductor drums 2C, 2M, 2Y, and 2K. By this optical scanning, an electrostatic latent image is formed on the surface of the photoconductor drums 2C, 2M, 2Y, 2K, and the electrostatic latent image undergoes a predetermined developing process to form toner images of respective colors of C, M, Y, K. Form.

  The paper feed table 200 includes paper feed cassettes 202 arranged in multiple stages within a paper bank 201. A paper feed roller 203 that feeds the recording sheet to a paper feed path 204 and an upper paper feed path 99 that extend from the paper feed cassette 202, a separation roller 205 that separates and feeds the sent recording sheet, and a recording sheet is conveyed to the printer unit 100. A transport roller pair 206 and the like are provided.

  Regarding the paper feeding, in addition to the paper feeding table 200, manual feeding is also possible, and the manual feeding tray 98 for manual feeding and the recording sheets on the manual feeding tray 98 are separated one by one toward the manual feeding path 97. A manual feed side separation roller 96 and the like are also provided. In the printer unit 100, the manual paper feed path 97 merges with the upper paper feed path 99.

  A registration roller pair 95 is provided near the lower part of the transfer unit 20 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 it sandwiches the recording sheet fed from the paper feed cassette 202 or the manual feed tray 98. The registration roller pair 95 restarts the rotation driving at the timing of feeding the sandwiched recording sheet in the secondary transfer nip N in synchronism with the four-color superposed toner image on the intermediate transfer belt 21, and secondarily transfers the recording sheet. Send it toward the 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 original 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, it is conveyed onto the contact glass 301. After that, the scanner unit 300 starts driving, and the first traveling body 303 and the second traveling body 304 start traveling along the document surface. The light emitted from the light source of the first traveling body 303 is reflected on the document 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 (provided additionally) having a signal processing function of the sensor, temporarily stored as image information, and then sequentially sent to the printer unit 100.

  Upon receiving the image information from the scanner unit 300, the paper feed table 200 feeds a recording sheet having a size corresponding to the image information to the upper paper feed path 99. Along with this, 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 their respective surfaces. After that, based on the image information received from the scanner unit 300, the optical writing unit 15 performs the optical writing process, and electrostatic latent images are formed on the surfaces of the photoconductor drums 2C, 2M, 2Y, and 2K, respectively. The electrostatic latent image is developed with toner of each color by the developing units 3C, 3M, 3Y, 3K. Toner images of C, M, Y, and K formed on the surface of the photoconductor drums 2C, 2M, 2Y, and 2K by development processing are sequentially superposed at the primary transfer nip for C, M, Y, and K. Is primarily transferred onto the intermediate transfer belt 21 to form a full-color toner image in which four colors are superposed.

  In the paper feeding table 200, one of the paper feeding rollers 203 is selectively rotated so as to feed a recording sheet of a size corresponding to the original size, and the recording sheet is fed from one of the three paper feeding cassettes 202. Be done. The fed recording sheets are separated one by one by the separation roller 205 and introduced into the paper feed path 204, and then are conveyed by the pair of conveyance rollers 206 and are sent to the upper paper feed path 99 in the printer unit 100. When the manual feed tray 98 is used, the manual feed side paper feed roller 94 of the manual feed tray 98 is rotated and the recording sheet on the manual feed tray 98 is fed out, separated by the manual feed side separation roller 96, and fed to the manual feed paper feed path 97. And reaches near the end of the upper paper feed path 99. In the vicinity of the end of the upper sheet feeding path 99, the leading edge of the recording sheet hits the registration roller pair 95 and stops.

  When the registration roller pair 95 rotates at a timing that can be synchronized 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 pressure in the transfer nip and the electric field generated by 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 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 through the secondary transfer nip N, the recording sheet is curvature-separated from the secondary transfer roller 30 and the intermediate transfer belt 21, and is fixed by the sheet conveyance belt 70. It is fed into the device 71. The recording sheet sent into the fixing device 71 is sandwiched by the fixing device 71 in the 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 is stacked on the discharge tray 75 outside the machine. When an image is formed on the other surface of the recording sheet, the recording sheet is discharged from the fixing device 71 and then sent to the sheet reversing device 77 by switching the path 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 full-color toner image is secondarily transferred to the other surface by being sent to the secondary transfer nip N, the toner image is fixed again via the fixing device 71, and the toner image is stacked on the discharge tray 75.

  After passing through the secondary transfer nip N, the belt cleaning device 26 contacts the surface of the intermediate transfer belt 21 before entering the primary transfer nip for cyan, which is the most upstream primary transfer step among the four colors. There is. 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 according to the present exemplary embodiment includes a secondary transfer power supply 81, which 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. Of the heater 87 as a heating unit as an adjusting unit for adjusting the electric resistance value of the second transfer bias and the electric resistance value when the secondary transfer bias including the AC component in the DC component is applied to the secondary transfer nip N. Further, a heater driving means 600b for controlling the driving of the heater 87 and a bias control means 600a for controlling the secondary transfer power source 81 are provided. The heater 87 as an adjusting unit is an electric resistance value changing unit that changes 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 facing roller 24 includes a core metal 241 and a conductive NBR rubber layer 242 coated on the surface of the core metal 241. The secondary transfer roller 30 also includes a core metal 311 and a conductive NBR rubber layer 312 coated on the surface of the core metal 311. The secondary transfer power supply 81 includes an AC power supply circuit section in which a DC power supply 811 and an AC power supply 812 are connected in series, and a DC power supply circuit section including a single DC power supply 813, which are arranged in parallel, and a changeover switch between them. 88, it is configured to be selectively connected to the secondary transfer counter 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 core metal 241. On the other hand, the core metal 311 of the secondary transfer roller 30 is grounded. In the present embodiment, the secondary transfer power source 81 outputs the superimposed voltage to the core metal 241 of the secondary transfer counter roller 24, so that the secondary transfer bias including the AC component in the DC component is applied to the secondary transfer nip N. To be 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) to the secondary transfer roller 30 side (recording sheet side) is generated. It is formed in the next transfer nip N and its vicinity. The secondary transfer counter roller 24 and the secondary transfer roller 30 have a higher resistance value than the secondary transfer counter roller 24.

  As the secondary transfer power supply 81, a power supply which does not include the single DC power supply 813 but includes only the DC power supply 811 and the AC power supply 812 may be used. In this case, the secondary transfer power supply 81 can output a voltage composed of only the DC voltage to the cored bar 241 by turning off the output of the AC power supply 812 and outputting the DC voltage only from the DC power supply 811. Further, the secondary transfer power supply 81 outputs a DC voltage from the DC power supply 811, and outputs an AC voltage from the AC power supply 812, so that a superimposed voltage obtained by superimposing the AC voltage on the DC voltage can be output to the cored bar 241. is there.

  The heater 87 is arranged so as 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 arranged near the secondary transfer counter roller 24. The heater 87 is connected via a relay 861 to an AC heater power supply 84 that is a power supply for driving the heater.

FIG. 3 is a perspective view showing the configuration of the heater 87.
The heater 87 of the present embodiment is composed of a nichrome wire heater supported on the substrate side of the printer unit 100 via an insulating plate 870 and a supporting 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 by using an aluminum foil having a thickness of 0.05 mm and covering the silicon rubber insulating heating element which is the heating element 872 from both sides. ing. The rise value of the surface temperature of the heater 87 is 80 ° C. ± 20 ° C. Note that reference numeral 873 in the drawing denotes a plug-in switch.

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

  The heater 87 heats the secondary transfer counter roller 24 that constitutes 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. As a result, the electric resistance value on the route through which the secondary transfer current flows is changed. In particular, in the present embodiment, the heater 87 is arranged so that heat can be efficiently applied not only to the secondary transfer counter roller 24 but also to the intermediate transfer belt 21 and the secondary transfer roller 30 forming the secondary transfer nip N. Has been done. Therefore, the heater 87 can greatly change the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N.

  The image forming apparatus according to the present exemplary embodiment selects a DC mode in which a DC transfer bias including only a DC component is used as a secondary transfer bias and an AC mode in which an AC transfer bias including a DC component and an AC component is used as a secondary transfer bias. Image formation is performed. The AC mode is mainly executed when an image is formed on a recording sheet (hereinafter referred to as “concavo-convex paper”) such as Japanese paper having rich surface irregularities, and the DC mode is mainly used for recording sheets other than the concavo-convex paper. It is executed when forming an image on a recording sheet having a smooth surface. Examples of the types of textured paper include Resac 66 (trade name) 260KG paper (6th edition continuous quantity) manufactured by Special Paper Manufacturing Co., Ltd., FC Washi type ripple (trade name) manufactured by NBS Ricoh Co., Ltd., embossed paper, There is linen paper. The types of recording sheets other than the textured paper include plain paper such as My Paper (trade name) manufactured by NBS Ricoh Co., Ltd., coated paper, and the like.

  Further, in the present embodiment, the AC mode is provided with a plurality of linear velocity modes (image forming modes) in which the intermediate transfer belt surface movement velocity (process linear velocity) is different from each other. In the present embodiment, there are three types: a high speed AC mode with a process linear velocity of 352.8 mm / sec, a medium speed AC mode with a process linear velocity of 247 mm / sec, and a low speed AC mode with a process linear velocity of 152.8 mm / sec. Although the linear velocity modes are prepared, the process linear velocity and the number of linear velocity modes of each linear velocity mode are appropriately set.

  Here, 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. When this discharge occurs, the toner image on the intermediate transfer belt 21 may be disturbed by the impact of the discharge and dust may occur. In addition, a part of the toner forming the toner image on the intermediate transfer belt 21 is charged to the opposite polarity (positive polarity in this embodiment) due to the discharge, and is not transferred to the recording sheet side at the secondary transfer nip N. Image white spots may occur.

The reason why such discharge is likely to occur in the AC mode is considered as follows.
As shown in FIG. 18, the AC transfer bias is generally obtained by superposing the AC component of the peak-to-peak voltage Vpp on the DC component Voff (the same value as the average value Vave) which is about the same as the DC transfer bias. Reference numeral Vt in FIG. 18 represents a peak value on the side (transfer direction side) that moves the toner from the intermediate transfer belt 21 side to the secondary transfer roller 30 side at the secondary transfer nip N. Further, reference numeral Vr in FIG. 18 indicates a peak value on the side of returning the toner from the secondary transfer roller 30 side to the intermediate transfer belt 21 side (return direction side).

  Generally, the transferability in the secondary transfer (transfer rate of the entire toner image) 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 supply 811 is subjected to constant current control 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 changes. On the other hand, the amount of toner adhered to the concave portion (concave portion transferability), which greatly affects the image quality of the textured paper, is mainly determined by the return direction peak value Vr. Therefore, the AC power supply 812 is subjected to constant voltage control such that the target return-direction peak value Vr becomes the peak-to-peak voltage Vpp of the AC component according to the fluctuating 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 peak value Vt in the transfer direction is the sum of the absolute value of the average value Vave (Voff) and one half of the peak-to-peak voltage Vpp. As the value increases, discharge between the rollers easily occurs, and 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. At the inlet side of the secondary transfer nip N, where a minute gap is formed between the and, the 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 deterioration such as dust and white spots is more 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 supply 811 is in a range in which discharge does not occur even if various conditions change and the value of the DC component Voff changes. , It is necessary to set the target transfer current value. Therefore, a sufficient amount of transfer current cannot be passed to the secondary transfer nip N within a range where discharge is not generated, and there are cases where sufficient image density cannot be secured.

  As described above, in the AC mode of the present embodiment, three types of linear velocity modes having different process linear velocities are prepared, and the control unit 60 selectively executes one of the linear velocity modes to form an image. Take action. As the process linear velocity is higher, the amount of transfer current flowing in the transfer nip needs to be increased in order to maintain the image density. However, in the AC mode, there is a restriction on increasing the DC component Voff of the AC transfer bias as described above. is there. Therefore, in the high-speed AC mode or the medium-speed AC mode in which the process linear velocity is fast, there is a possibility that sufficient image density cannot be secured.

  Here, if the electric 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, and therefore the direct current of the AC transfer bias required to obtain the necessary transfer current is obtained. It is possible to reduce the component Voff. Therefore, even in the high-speed AC mode or the medium-speed AC mode in which it is difficult to secure a sufficient image density, if the electric resistance value on the route of the transfer current flowing through the secondary transfer nip N is low, the range in which discharge is not generated It becomes possible to flow a sufficient amount of transfer current to the secondary transfer nip N, and it becomes easy to secure a sufficient image density.

  However, in order to reduce the electric resistance value on the route of the transfer current, the electric resistance values of the secondary transfer counter roller 24, the intermediate transfer belt 21, the secondary transfer roller 30 and the like that form the secondary transfer nip N (hereinafter referred to as “ "Electrical resistance value of secondary transfer nip N." In the following, "electrical resistance value of secondary transfer nip N" does not include the electric resistance value of the recording sheet.) Depending on the type of uneven paper and the type of image (difference in image area ratio, difference in distribution of toner image adhesion part on recording sheet, etc.), no matter how much the DC component or AC component of the AC transfer bias is adjusted, In some cases, it is not possible to secure a high image density.

  This will be described in detail with reference to FIG. 4. When the electrical resistance value of the secondary transfer nip N is low, the recording sheet in the secondary transfer nip N in which the recording sheet is not sandwiched (in the present 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. Therefore, the amount of current flowing through the recording material sandwiching region (in this embodiment, the central region in the axial direction of the secondary transfer roller 30 in contact with the recording sheet) N1 in which 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 by the type of the recording sheet S (uneven paper, etc.) sandwiched in the secondary transfer nip N and the type of the toner image (image) existing in the secondary transfer nip N (toner adhesion). It depends on the amount). Therefore, depending on the type of the recording sheet S (uneven paper, etc.) or the type of image, the electric resistance value of the recording material sandwiching region N1 becomes relatively high, and even if the total amount of transfer current is sufficient, In most cases, a large amount flows into the non-recording material sandwiching area N2, a necessary amount of current does not flow in the recording material sandwiching area N1, and sufficient image density cannot be secured.

  As described above, in the case where the electric resistance value of the secondary transfer nip N is high, in the range where the discharge side is not generated at the inlet side of the secondary transfer nip N in the high speed AC mode or the medium speed AC mode in which the process linear velocity is fast. In some cases, a sufficient amount of transfer current cannot be passed to the secondary transfer nip N, and sufficient image density cannot be secured. On the other hand, when the electric resistance value of the secondary transfer nip N is low, in the full linear velocity mode including the low speed AC mode, even if the total amount of the transfer current is sufficient, the amount of current required for the recording material sandwiching region N1 is sufficient. In some cases, it is impossible to secure sufficient image density because the image cannot be flowed.

  Therefore, with the configuration in which the electric resistance value of the secondary transfer nip N cannot be changed, it is difficult to secure the image density only in the low-speed AC mode without deteriorating the image quality in the low-speed AC mode. It is not possible to realize image formation in a high-speed AC mode or a medium-speed AC mode on a sheet (thick uneven paper, uneven paper having a high electric resistance value, etc.). Alternatively, in the configuration in which the electric resistance value of the secondary transfer nip N cannot be changed, it is not possible to further speed up the high speed AC mode or the medium speed AC mode without degrading the image quality in the low speed AC mode.

  Therefore, in the present embodiment, in the linear velocity mode in which the process linear velocity is high, the heater driving means 600b uses the heater driving means 600b so that the electric resistance value of the secondary transfer nip N becomes smaller than in the linear velocity mode in which the process linear velocity is low. Control 87. This makes it possible to realize image formation in the high-speed AC mode or the medium-speed AC mode on a recording sheet of the type that has been difficult to secure image density only in the low-speed AC mode.

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

  Reference numeral 68 is a temperature / humidity detecting means for detecting the temperature / humidity, and the controller 60 may drive and control the heater 87 by the heater driving means 600b based on the detection result of the temperature / humidity detecting means 68. Specifically, the output of the heater 87 is increased or turned on to reduce the electric resistance value of the secondary transfer counter roller 24 at a low temperature where transfer dust is likely to occur, and the output of the heater 87 is reduced or turned off at a high temperature. The 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 the recording sheet to be printed, that is, the type of paper (paper type). The operation panel 69 receives a user's operation using a touch panel or key buttons, and transmits the 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 sends 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 was modified so that a secondary transfer voltage could be externally applied to the secondary transfer counter roller 24, and test images were printed on three types of uneven paper. After forming, the image quality was evaluated. The three types of uneven paper used were Resac 130 kg (151 gsm) manufactured by Special Paper Manufacturing Co., Ltd. (hereinafter referred to as “concave paper A”), Resac 175 kg (203 gsm) manufactured by Special Paper Manufacturing Co., Ltd. (hereinafter referred to as “concave paper B”), Conqueor. It is High White Laid 300 gsm (hereinafter referred to as “uneven paper C”). In this experiment, a Trek COR-A-TROL Model 610D was used as the secondary transfer power supply 81, and it was set so that a constant-current-controlled transfer current of −70 μA would flow to the secondary transfer nip N. Regarding the electric resistance value of the secondary transfer nip N, when a recording sheet does not exist in the secondary transfer nip N to which the secondary transfer bias is applied, a transfer current is input from the secondary transfer power supply 81 and output at that time. The voltage value was measured, and the value obtained by dividing the output voltage value by the transfer current value was measured as the electric resistance value of the secondary transfer nip N. The linear velocity mode used in this experiment is the same as in the above-described embodiment, the high-speed AC mode having a process linear velocity of 352.8 mm / sec, the medium linear velocity AC mode having a process linear velocity of 247 mm / sec, and the process linear velocity of 152. There are three types of low speed AC mode of 8 mm / sec.

  In addition, regarding the image quality evaluation, the concave portion transferability of various concave-convex papers A to C (toner adhesion amount in concave portions) and the convex portion transferability of various concave-convex papers A to C (toner adhesion amount on convex portions, dust and white) Two types of image quality evaluations (such as occurrence of omission) were performed. Specifically, a solid image is formed on each of the various concavo-convex papers A to C, the image density of the concave portion in the formed solid image is visually confirmed to evaluate the concave portion transferability, and the formed solid image is formed. The image density of the convex portion and the occurrence of dust or white spots in the inside are visually confirmed to evaluate the transferability of the convex portion. In this experiment, the image quality was evaluated when the heater 87 was controlled to change the electric resistance value of the secondary transfer nip N. Table 1 shows the image quality evaluation results of the textured paper A, Table 2 shows the image quality evaluation results of the textured paper B, and Table 3 shows the image quality evaluation results of the textured paper C. It should be noted that the image quality evaluation result is “⊚” when it is particularly good, and “◯” when it is good (allowable range), but it is not good, but it depends on conditions such as image type and user's preference (request). When there was something that was acceptable, it was marked with “Δ”, and when it was not acceptable, it was marked with “x”.

  First, as shown in Tables 1 to 3 above, in any of the concave-convex papers A to C, the lower the electric resistance value of the secondary transfer nip N, the better the image quality can be obtained with regard to the concave portion transferability. Regarding the transferability of the convex portion, the higher the electric resistance value of the secondary transfer nip N, the better the image quality tends to be obtained. Further, as shown in Tables 1 to 3 above, in any of the uneven papers A to C, the higher the process linear velocity, the more compatible the recess transferability and the protrusion transferability are (the recess transferability and the protrusion transferability). It is difficult for the transferability to be evaluated as “⊚” or “◯”). Especially, in the high-speed AC mode, both the concave-convex transferability and the convex-part transferability are compatible, as is remarkable for the uneven paper A and the uneven paper C. There is no possible range of the electric resistance value of the secondary transfer nip N.

  As shown in Tables 1 to 3, in any of the uneven papers A to C, when the electric resistance value of the secondary transfer nip N is set to 10.7 logΩ and the image formation is performed in the low speed AC mode, the concave portion transferability and It is possible to achieve both the protrusion transferability. Therefore, in the case where the electric resistance value of the secondary transfer nip N cannot be changed, the electric resistance value of the secondary transfer nip N is usually set to about 10.7 logΩ. However, in this case, depending on the type of the corrugated paper, by lowering the electric resistance value of the secondary transfer nip N, the concave portion transferability and It is possible to achieve both the protrusion transferability.

Specifically, in the case of the corrugated paper A, as shown in Table 1, the electric resistance value of the secondary transfer nip N should be in the range of 9.5 logΩ to 9.9 logΩ, which is lower than the set value (10.7 logΩ). For example, both the concave part transferability and the convex part transferability can be achieved even in the medium speed AC mode, and the concave part transferability and the convex part transferability can be compatible even in the high speed AC mode within a lower range of 9.2 logΩ to 9.5 logΩ.
Similarly, in the case of the uneven paper B, as shown in Table 2, if the electric resistance value of the secondary transfer nip N is 10.2 logΩ which is lower than the set value (10.7 logΩ), the concave portion is formed even in the medium speed AC mode. It is possible to achieve both transferability and transferability of convex portions.

  By controlling the heater 87 so that the electric resistance value of the secondary transfer nip N becomes smaller in the linear velocity mode in which the process linear velocity is high as in the present embodiment than in the linear velocity mode in which the process linear velocity is low, Realizes image formation in a high-speed AC mode or a medium-speed AC mode on uneven paper of a type (uneven paper A or uneven paper B) that has been difficult to ensure a sufficient image quality only in the low-speed AC mode. It becomes possible.

FIG. 6 is a selection flow chart showing an example of a method of adjusting the electric resistance value of the secondary transfer nip N for the uneven papers A to C based on the above-described experimental results.
As shown in FIG. 6, when the recording sheet used for image formation is uneven paper A, which is a medium-thick paper having a relatively thin paper thickness, as shown in Table 1 above, the concave portion transferability is obtained in any linear velocity mode. Also, there is an electric resistance value of the secondary transfer nip N that is compatible with the transferability of the convex portion. Therefore, as the selectable linear velocity mode, three types of high-speed AC mode, medium-speed AC mode, and low-speed AC mode are prepared so that image formation can be selectively performed in any of the linear velocity modes. At this time, since the electric resistance value of the secondary transfer nip N capable of achieving both the concave portion transferability and the convex portion transferability is different for each linear velocity mode, the electrical resistance of the secondary transfer nip N corresponding to the selected linear velocity mode is different. A resistance value is selected, and the heater 87 is controlled so that the electric resistance value is obtained. However, when performing in the high-speed AC mode, it is not possible to make the image quality evaluation the best “⊚” for both the concave portion transferability and the convex portion transferability. It is selected whether to give priority to image quality, an electric resistance value of the secondary transfer nip N is selected according to the selected image quality, and the heater 87 is controlled so that the electric resistance value can be obtained.

  When the recording sheet used for image formation is the corrugated paper B which is a relatively thick paper, as shown in Table 2 above, in the high-speed AC mode, the secondary transfer capable of achieving both the concave transfer property and the convex transfer property. There is no electric resistance value of the transfer nip N. Therefore, as the selectable linear velocity mode, two kinds of medium speed AC mode and low speed AC mode are prepared so that image formation can be selectively performed in either of the linear velocity modes. At this time, since the electrical resistance value of the secondary transfer nip N capable of achieving both the concave portion transferability and the convex portion transferability is different between these linear velocity modes, the secondary transfer nip N depending on the selected linear velocity mode. The electric resistance value of is selected, and the heater 87 is controlled so that the electric resistance value is obtained.

  When the recording sheet used for image formation is the corrugated paper C which is an extremely thick paper having a thicker paper thickness, as shown in Table 3 above, in the high speed AC mode and the medium speed AC mode, the concave portion transferability and the convex portion transferability are obtained. There is no electric resistance value of the secondary transfer nip N that can satisfy both. Therefore, as the selectable linear velocity mode, only the low-speed AC mode is prepared. At this time, even in the low-speed AC mode, the image quality cannot be evaluated as “⊚” which is the best for both the concave part transfer property and the convex part transfer property. Is selected, the electric resistance value of the secondary transfer nip N according to the selected image quality is selected, and the heater 87 is controlled so as to obtain the electric resistance value.

  In the selection flow chart shown in FIG. 6, when the recording sheet to be used is the uneven paper A or the uneven paper B, the linear speed mode can be further selected according to the user's instruction operation. The mode may not be selected. For example, the highest speed linear velocity mode in which the electric resistance value of the secondary transfer nip N that can achieve both the concave portion transferability and the convex portion transferability exists is associated with each of the uneven papers A to C, and the control unit 60 When the recording sheet to be used is selected, image formation is performed in the linear velocity mode corresponding to the selected recording sheet. In this case, the selection condition for selecting the linear velocity mode is such that the thicker the recording sheet used, the slower the linear velocity mode is selected.

  The relationship between the uneven papers A to C having different paper thicknesses shown in the selection flow chart of FIG. 6 is due to the difference in the electric resistance value between the uneven papers A to C in the secondary transfer nip N. Therefore, this relationship can be considered in the same manner depending on the type of the image transferred to the uneven paper in the secondary transfer nip N, specifically, the difference in the image area ratio (toner adhesion amount per unit area). Therefore, in the selection flow chart of FIG. 6, instead of selecting which of the concave-convex papers A to C the recording sheet to be used can be replaced by selecting the image area ratio of the image to be formed. That is, it is possible to replace the case of uneven paper A with a low image area ratio, the case of uneven paper B with a medium image area ratio, and the case of uneven paper C with a high image area ratio.

  At this time, if the image area ratio of the image to be formed is a low image area ratio or a medium image area ratio, the linear velocity mode can be selected according to the user's instruction operation, etc. The mode may not be selected. For example, the fastest linear velocity mode in which the electric resistance value of the secondary transfer nip N capable of achieving both the concave portion transferability and the convex portion transferability exists is associated with each of these three types of image area ratios, and the control unit 60 is used. After acquiring the image area ratio of the image to be formed, performs image formation in the linear velocity mode corresponding to the image area ratio. In this case, the selection condition for selecting the linear velocity mode is that the higher the image area ratio of the image to be formed, the slower the linear velocity mode is selected.

  Table 4 below shows an image pattern including a solid image portion having an image area ratio of 100% and a halftone (HT) image portion having an image area ratio of 10% on the uneven paper A in the low-speed AC mode. It shows the image quality evaluation result when formed.

  According to Table 1 above, in the case of the uneven paper A, in the low speed AC mode, the concave transfer property and the convex transfer property are obtained when the electric resistance value of the secondary transfer nip N is 10.2 logΩ or 10.5 logΩ. In both cases, the image quality evaluation can be the best "⊚". On the other hand, regarding the image quality in the convex portion in this case, when comparing the solid image portion having an image area ratio of 100% and the halftone image portion having an image area ratio of 10%, as shown in Table 4, Whether the electric resistance value of the transfer nip N is 10.2 logΩ or 10.5 logΩ, the image quality cannot be evaluated as the best “⊚” for both the solid image portion and the halftone image portion. Therefore, when forming an image in the low-speed AC mode on the concavo-convex paper A, it is further selected which of the solid image portion and the halftone image portion is to be prioritized, and the secondary transfer nip N corresponding to the selected image quality is selected. The electric resistance value may be selected and the heater 87 may be controlled so as to obtain the electric resistance value.

  Further, in the selection flow chart of FIG. 6, the type (paper thickness) of the recording sheet to be used, the linear velocity mode, and the priority image quality are selected in a stepwise manner. Would increase the number of user operation steps. Therefore, for example, a plurality of image forming modes in which the type (paper thickness) of the recording sheet to be used and the linear velocity mode are set are prepared in advance, and these image forming modes are set as shown in FIG. It may be displayed on the screen of the touch panel so that the user can select it.

The “Resac 130 kg productivity priority mode” in FIG. 7 is an image forming mode in which the linear velocity mode is set to the high-speed AC mode and the electric resistance value of the secondary transfer nip N is set to 9.2 logΩ. An image of sufficient quality can be formed at high speed on a medium-thickness uneven paper such as A.
7 is an image forming mode in which the linear velocity mode is set to the low speed AC mode and the electric resistance value of the secondary transfer nip N is set to 10.5 logΩ. It is possible to form a high-quality image at a low speed on such a medium-thickness uneven paper.
The “RESAC 175 kg productivity priority mode” in FIG. 7 is an image forming mode in which the linear velocity mode is set to the medium velocity AC mode and the electric resistance value of the secondary transfer nip N is set to 9.9 logΩ. An image of sufficient quality can be formed at high speed on thick uneven paper such as the paper B.
The “RESAC 175 kg image quality priority mode” in FIG. 7 is an image forming mode in which the linear velocity mode is set to the low speed AC mode and the electric resistance value of the secondary transfer nip N is set to 10.7 logΩ. It is possible to form a high-quality image at low speed on such thick uneven paper.
“High White 300” in FIG. 7 is an image forming mode in which the linear velocity mode is set to the low speed AC mode and the electric resistance value of the secondary transfer nip N is set to 10.9 logΩ, and it is like the uneven paper C. An image of sufficient quality can be formed at a low speed on a very thick uneven paper.

  Further, according to the image quality evaluation experiment described above, the lower the electric resistance value of the secondary transfer nip N, the better the image quality of the concave portion such as the concave portion transferability, and the lower the image quality of the convex portion such as the convex portion transferability. There is a tendency that the higher the electric resistance value of the secondary transfer nip N, the lower the image quality of the concave portion such as the concave portion transferability, and the higher the image quality of the convex portion such as the convex portion transferability. Further, according to the results of Table 4 described above, a lower electric resistance value of the secondary transfer nip N tends to improve the image quality of an image portion (halftone image portion) having a low image area ratio, The higher the electrical resistance value of the transfer nip N, the higher the image area ratio (solid image portion) tends to be.

  According to this tendency, for example, a user adjustment screen as shown in FIG. 8 is displayed on the screen of the touch panel, and the user touches the up and down buttons 701 and 702 on the screen to finely adjust the electric resistance value of the secondary transfer nip N. You may allow it. More specifically, for example, instead of the configuration in which the electrical resistance value of the secondary transfer nip N is selected depending on whether the priority image quality in the selection flow chart of FIG. A user adjustment screen as shown in 8 is displayed on the screen of the touch panel so that the electric resistance value of the secondary transfer nip N can be adjusted in multiple stages. According to this, it is possible to obtain a more satisfactory image quality.

(Example of control)
FIG. 9 is a flowchart showing an example of control in the AC mode in this embodiment.
When the print job is input to the control unit 60, the control unit 60 starts the print job (S1), first acquires the paper type information from the operation panel 69 (S2), and in the AC mode based on the paper type information. The control or the DC mode is selected (S3). That is, in this control example, based on the paper type information indicating the selection result of the paper type used for printing selected by the user operating the operation panel 69, the paper type information indicates the uneven paper corresponding to the AC mode. In some cases, the AC mode is selected, and in other cases, 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 (S4), and then starts the image forming operation while keeping the heater 87 OFF. S11). When the secondary transfer bias is a direct current transfer bias, even if the electric resistance value of the secondary transfer nip N is high, discharge at the inlet side of the secondary transfer nip N is unlikely to occur, and dust and white spots due to discharge are suppressed. At the same time, the image density can be secured. On the contrary, if the electric resistance value of the secondary transfer nip N is lowered, the current tends to flow preferentially to the non-recording material holding region N2, and the image density may not be secured. According to the present control example, in the DC mode, since the heater 87 is kept OFF and the electric resistance value of the secondary transfer nip N is kept high, high image density is achieved while suppressing dust and white spots due to discharge. It is possible to form a secured high quality image.

  When the AC mode is selected (Yes in S3), the control unit 60 sets the AC transfer bias as the secondary transfer bias (S5). Then, the control unit 60 acquires the information of the linear velocity mode and the information of the priority image quality (S6, S7), and based on the paper type information, the linear velocity mode information and the priority image quality information, the selection flow chart shown in FIG. As described above, the electrical resistance value of the secondary transfer nip N is determined, and the heater output time for obtaining the determined electrical resistance value is determined (S8). After that, the control unit 60 turns on the heater 87 by the heater driving means (S9), and when the heater output time exceeds the set time (Yes in S10), starts the image forming operation (S11).

  The information on the linear velocity mode can be acquired as follows, for example. That is, the control unit 60 displays the linear velocity mode that can be selected according to the paper type information on the screen of the touch panel as in the selection flow chart shown in FIG. When the user selects the linear velocity mode used for printing from the linear velocity modes displayed on the screen of the touch panel, the linear velocity mode selected by the user is transmitted from the operation panel 69 to the control unit 60. As a result, the control unit 60 can acquire the linear velocity mode information.

  The information on the priority image quality can be obtained in the same manner as the information on the linear velocity mode. That is, the control unit 60 displays a screen for selecting the priority image quality on the touch panel according to the paper type information and the linear velocity mode, as in the selection flow chart shown in FIG. Then, when the user selects the priority image quality, the priority image quality selected by the user is transmitted from the operation panel 69 to the control unit 60. As a result, the control unit 60 can acquire information on the priority image quality.

  Table 5 below shows the relationship between the output power of the heater 87 and the electric resistance value of the secondary transfer nip N in the image forming apparatus 1 of this embodiment. It should be noted that Table 5 uses the image forming apparatus used in the above-described quality evaluation experiment, and is measured in a use environment in which the temperature is 10 ° C. and the relative humidity is 15%. Thus, the electric 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 electric resistance value of the secondary transfer nip N.

  According to the present control example, with the configuration in which the electric resistance value of the secondary transfer nip N cannot be changed, it is difficult to secure sufficient image quality only in the low-speed AC mode for uneven paper (medium thick paper or thick paper). It is possible to realize image formation in the high speed AC mode and the medium speed AC mode.

[Modification 1]
Next, a modification of the image forming apparatus according to the present exemplary embodiment (hereinafter, this modification will be referred to as “modification 1”) will be described.
FIG. 10 is an explanatory diagram showing a peripheral configuration of the secondary transfer portion P in the first modification.
In the first modification, the secondary transfer power source 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 arranged so as to face the secondary transfer roller 30. Also in the present modification 1, the same operational effects as the above-described embodiment can be obtained. Note that, in FIG. 10, the DC power supply circuit unit including the single DC power supply 813 is omitted, but in the first modification, the DC power supply circuit unit may be connected to the secondary transfer roller 30, It may be connected to the secondary transfer counter roller 24.

[Modification 2]
Next, another modification of the image forming apparatus according to the present exemplary embodiment (hereinafter, this modification will be referred to as “modification 2”) will be described.
FIG. 11 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 facing roller 24, and the DC power supply 811 is connected to the secondary transfer roller 30. The ground is on the side of the secondary transfer roller 30. Also in the second modified example, the same operational effect as that of the above-described embodiment can be obtained. In FIG. 11, the DC power supply circuit unit consisting of 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, It may be connected to the secondary transfer counter roller 24.

The heater 87 may be arranged to face the secondary transfer counter roller 24 as shown by the solid line in FIG. 11, or may be arranged to face the secondary transfer roller 30 as shown by the chain double-dashed 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 exemplary embodiment (hereinafter, this modification will be referred to as “Modification 3”) will be described.
FIG. 13 is an explanatory diagram showing a peripheral configuration of the secondary transfer portion P in the third modification.
In the third modification, as the adjusting means for adjusting the electric resistance value of the secondary transfer nip N, the nip width of the secondary transfer nip N (intermediate transfer belt of the secondary transfer nip N is used instead of the heating means by the heater 87. A nip width changing means for changing the length in the moving direction) 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 together.

  The nip width changing means of the third modification includes a pressing roller 58 that comes into contact with the inner peripheral surface of the intermediate transfer belt 21 upstream of the secondary transfer nip N in the moving direction of the intermediate transfer belt. The nip width of the secondary transfer nip N is changed by moving the nip width of the secondary transfer nip N. Specifically, the eccentric cam 581 is arranged so as to contact the rotation shaft of the push-down roller 58. The rotation shaft of the eccentric cam 581 is biased by a spring 582 as a biasing means so that the rotation shaft of the push-down roller 58 approaches the inner peripheral surface of the intermediate transfer belt 21. As a result, the push-down roller 58 pushes down 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 that rotationally drives 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-turned from the state shown by the solid line in FIG. 13 to the state shown by the broken line in FIG. 13, the rotation shaft of the push-down roller 58 is pushed by the outer peripheral surface of the eccentric cam 581, As shown by the broken line in FIG. 13, 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 moving direction of the intermediate transfer belt 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.

  When the amount of the intermediate transfer belt 21 wound around the secondary transfer roller 30 increases, the transfer current also flows to the portion of the intermediate transfer belt 21 newly wound around the secondary transfer roller 30. When the area through which the transfer current flows increases, the electric resistance value of the secondary transfer nip N decreases, so the same control as the control of heating the heater 87 to lower the electric resistance value of the secondary transfer nip N in the above-described embodiment. Can be realized by controlling the rotation of the eccentric cam 581.

Table 6 below shows the transfer pressure generated in the secondary transfer nip N and the secondary transfer nip N when the eccentric cam 581 is rotationally driven to change the amount of winding of the intermediate transfer belt 21 around the secondary transfer roller 30. It shows the relationship with the electric resistance value. In this way, the electrical resistance value of the secondary transfer nip N can be adjusted by rotationally driving the eccentric cam 581 to change the amount of the intermediate transfer belt 21 wound around the secondary transfer roller 30.

[Modification 4]
Next, still another modification of the image forming apparatus according to the present exemplary embodiment (hereinafter, this modification will be referred to as “modification 4”) will be described.
14 and 15 are explanatory diagrams showing the peripheral configuration of the secondary transfer portion P in the fourth modification.
In the fourth modification, as the adjusting means for adjusting the electric resistance value of the secondary transfer nip N, a nip pressure changing means for changing the pressure applied to the secondary transfer nip N is used 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 together.

  The nip pressure changing means of the fourth modification changes the pressure applied to the secondary transfer nip N by changing the pressing force for pressing 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 rotatably supported with respect to the apparatus main body by a support shaft 32 at the downstream end in the recording sheet conveyance direction. On the other hand, the end of the roller unit holder 31 on the upstream side in the recording sheet conveying direction is urged downward in the drawing by a tension spring 33 fixed to the apparatus body. Therefore, the roller unit holder 31 is given an urging force that rotates in the clockwise direction in the drawing around the support shaft 32 by the restoring force of the tension spring 33.

  An eccentric cam 34 is arranged at a position where the roller unit holder 31 to which such a biasing force is applied is rotated in the clockwise direction in the figure. Since the eccentric cam 34 is in contact with the roller unit holding body 31, the roller unit holding body 31 to which the biasing 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 holder 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 toward and away from the intermediate transfer belt 21 in accordance with the change in the rotational position. Therefore, by controlling the cam drive motor 35 by the control unit 60 to change the rotation angle of the eccentric cam 34, the secondary transfer roller 30 can be displaced and the pressure applied to the secondary transfer nip N can be changed. .

  When the rotation angle of the eccentric cam 34 is changed from the rotation angle shown in FIG. 14 to that shown in FIG. 15, 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 deforms more. 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 electric 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 electric resistance value of the secondary transfer nip N decreases as the contact width increases. Therefore, the same control as that of heating the heater 87 to lower the electric resistance value of the secondary transfer nip N 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 exemplary embodiment (hereinafter, this modification will be referred to as “Modification 5”) will be described.
16 and 17 are explanatory diagrams showing the peripheral configuration of the secondary transfer portion P in the fifth modification.
In the present modification 5, as the adjusting means for adjusting the electric resistance value of the secondary transfer nip N, instead of the heating means by the heater 87, roller switching means for switching between the two secondary transfer rollers 30 and 30 ′ having different hardnesses. To use. 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, the nip width changing means, the nip pressure changing means and the roller switching means may be used together.

  In the roller switching means of the fifth modification, the shafts 311 and 311 'of the two secondary transfer rollers 30 and 30' having different hardness 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 holder 36 swings counterclockwise in the figure by the driving force of the drive motor 38 to reach the state shown in FIG. 16, the first secondary having a relatively high hardness. The transfer roller 30 contacts the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 to form the secondary transfer nip N. On the other hand, under the control of the control unit 60, when the roller unit holder 36 swings clockwise in the figure by the driving force of the drive motor 38 to the state shown in FIG. The secondary transfer roller 30 ′ contacts the intermediate transfer belt 21 wound around the secondary transfer counter roller 24 to form the secondary transfer nip N.

  When the roller unit holder 36 is swung to form the secondary transfer nip N by the first secondary transfer roller 30 having a relatively high hardness as shown in FIG. 16, the rubber of the secondary transfer roller 30 is formed. The amount of deformation of the layer 312 is relatively small. From this state, when the roller unit holder 36 is swung to form the secondary transfer nip N by the second secondary transfer roller 30 ′ having a relatively low hardness as shown in FIG. 17, the secondary transfer nip N is formed. The amount of deformation 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 electric 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 electric resistance value of the secondary transfer nip N decreases as the contact width increases. Therefore, the same control as the control of heating the heater 87 to lower the electric resistance value of the secondary transfer nip N in the above-described embodiment can be realized by the control of the roller switching unit.

[Modification 6]
Next, still another modification of the image forming apparatus according to the present embodiment (hereinafter, this modification will be referred to as “modification 6”) will be described.
In this modified example 6, instead of the second secondary transfer roller 30 ′ in the modified example 5 described above, a secondary transfer roller 30 ″ whose electric resistance value is lower than that of the first secondary transfer roller 30 is used. In Modified Example 6, when the roller switching means is controlled from the state shown in FIG. 16 to the state shown in FIG. 17, the electric resistance value of the secondary transfer roller forming the secondary transfer nip N becomes low. As a result, the electric resistance value on the route of the transfer current flowing through the secondary transfer nip becomes low. Therefore, also in the sixth modification, the same control as that of heating the heater 87 to lower the electric resistance value of the secondary transfer nip N in the above-described embodiment can be realized by the control of the roller switching unit.

What has been described above is an example, and unique effects are obtained for each of the following aspects.
(Aspect A)
An image carrier such as an intermediate transfer belt 21 and a secondary transfer counter roller 24, which carries a toner image on the surface and moves, and a transfer nip such as a secondary transfer nip N, which contacts the surface of the image carrier, are formed. A secondary transfer bias for transferring the toner image on the image carrier to a nip forming member such as a secondary transfer roller 30 and a recording material such as a recording sheet including an uneven paper sandwiched in the transfer nip. In the image forming apparatus having a transfer bias applying unit such as a secondary transfer power supply 81 for applying an AC transfer bias including a DC component and an AC component to the transfer nip as a transfer bias for the image carrier, In the image forming mode selected from a plurality of image forming modes having different speeds according to a predetermined selection condition, a control unit such as a control unit 60 for controlling an image forming operation, and the transfer unit. The heater 87 and the heater power supply 84 for adjusting the electric resistance value on the route of the transfer current flowing through the transfer current, or the adjusting means such as the eccentric cam 581 and the cam drive motor 583, and the control means includes the plurality of image forming modes. Between at least two image forming modes, the electric resistance value when the AC transfer bias is applied to the transfer nip becomes smaller in the image forming mode in which the surface moving speed of the image carrier is higher. And controlling the adjusting means.
When the toner image is transferred to the recording material by applying the AC transfer bias, the discharge is generated at the entrance side of the transfer nip as compared with the case where the toner image is transferred to the recording material by applying the DC transfer bias composed of only the DC component. It is easy to cause deterioration in image quality such as dust and white spots due to the discharge. This is considered as follows. That is, in the AC transfer bias, since the AC component is superimposed on the DC component having the same degree as the DC transfer bias, the peak value of the AC component (the peak value whose absolute value is larger than that of the DC component) is applied. At that time, the potential difference with respect to the recording material becomes larger than the DC transfer bias. Therefore, at this time, a potential difference more than the discharge start voltage is likely to occur at 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 occurs, or a part of the toner forming the toner image on the image carrier is charged to the opposite polarity by the discharge. If they do not, they will not be transferred at the transfer nip, causing white spots in the image. Therefore, when the AC transfer bias is applied, it is necessary to set the magnitude of the DC component and the amplitude of the AC component of the AC transfer bias so that the discharge does not occur at the time when the peak value of the AC component is applied. Become.
On the other hand, when the AC transfer bias is applied, it is necessary to flow a sufficient transfer current to the transfer nip in order to secure a sufficient image density. Therefore, in order to further increase the surface moving speed of the image carrier in the image forming mode (high-speed mode) capable of forming an image of sufficient image quality in a short time, a sufficient transfer current capable of ensuring a sufficient image density can be obtained. Therefore, it is necessary to increase the DC component of the AC transfer bias. However, as described above, increasing the DC component of the AC transfer bias has a restriction in relation to the occurrence of discharge, so that a sufficient transfer current cannot be flown within the range of the restriction, resulting in a sufficient image density. May not be secured.
On the other hand, when an AC transfer bias is applied, the lower the resistance value of the transfer nip, 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. You can Therefore, if the transfer nip has a low resistance value, it is possible to further increase the surface moving speed of the image carrier while ensuring a sufficient image density.
However, if 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, distribution of toner image adhering portion on recording material, etc.). However, it may not be possible to secure a sufficient image density. Specifically, when the resistance value of the transfer nip is low, the current easily flows in the area (non-recording material holding area N2) in which the recording material is not held in the transfer nip, and the recording material in the transfer nip is held. The amount of current flowing through the existing area (recording material sandwiching area N1) is relatively small. The electric resistance value of the recording material sandwiching area N1 changes depending on the type of the recording material and the type of the image transferred to the recording material. Therefore, depending on the type of recording material or the type of image, even if the electric resistance value of the recording material holding region N1 becomes too high and the DC component of the AC transfer bias is large and the total amount of transfer current is sufficient, many May flow to the non-recording material holding area N2, and it may not be possible to flow a required amount of current to the recording material holding area N1.
Therefore, if a structure in which the resistance value of the transfer nip is low is adopted in order to further increase the surface moving speed of the image carrier in the high speed mode, the type of the recording material and the image are recorded in the low speed mode in which a higher quality image is required. Depending on the type, the required amount of current cannot be applied to the recording material sandwiching area N1 and sufficient image density may not be obtained.
In this aspect, when the image forming is performed in the high speed mode in which the surface moving speed of the image carrier is fast, the adjusting means for adjusting the electric resistance value on the route of the transfer current flowing through the transfer nip is used. The electric resistance value of the transfer nip can be made smaller than in the low speed mode in which the surface moving speed is slow. As a result, in the high-speed mode, transfer can be performed in the transfer nip where the resistance value is low, so even if the surface moving speed of the image carrier is increased, a sufficient transfer current can be obtained within a range that does not cause discharge. It is possible to set an AC transfer bias capable of flowing the image, and it is possible to realize higher-speed image formation while maintaining sufficient quality. On the other hand, in the low speed mode, since the transfer can be performed in the transfer nip having a high resistance value, a necessary amount of current may not be applied to the recording material holding region N1 depending on the type of recording material or the type of image. It is possible to avoid a situation in which sufficient image density is not obtained, and it is possible to form a high-quality image.

(Aspect B)
In the aspect A, the predetermined selection condition includes a selection condition of selecting an image forming mode according to a type of recording material to be used (concavo-convex paper A to C, etc.).
According to this, an image of sufficient quality can be formed at a higher speed depending on the type of recording material.

(Aspect C)
In the aspect B, there is an operation acceptance unit such as an operation panel 69 that accepts a user operation for selecting the type of recording material to be used, and the control unit is used according to the user operation accepted by the operation acceptance unit. It is characterized in that the type of recording material is specified.
According to this, the type of recording material can be easily acquired.

(Aspect D)
In the aspect B or C, the type of the recording material includes the thickness of the recording material used, and the predetermined selection condition is that the thicker the recording material used, the slower the surface moving speed of the image carrier. It is characterized by including a selection condition of selecting an image forming mode.
According to this, an image of sufficient quality can be formed at a higher speed depending on the difference in the thickness of the recording material.

(Aspect E)
In any one of the aspects A to D, the control unit uses the recording to be performed in at least one image forming mode (middle speed AC mode, low speed AC mode, etc.) of the plurality of image forming modes. as the thickness of the wood is thick, the so that Do larger electric resistance when the AC transfer bias is applied to the transfer nip, and controlling the adjusting means.
According to this, it is possible to form a high-quality image according to the difference in the thickness of the recording material in the at least one image forming mode.

(Aspect F)
In any one of the aspects A to E, the control unit is configured to, in the at least one image forming mode of the plurality of image forming modes, control the AC transfer bias in accordance with a type of an image transferred to a recording material. The adjusting means is controlled so as to adjust the electric resistance value when the voltage is applied to the transfer nip.
According to this, it is possible to form a high-quality image according to a difference in image type in the at least one image forming mode.

(Aspect G)
In the aspect F, the image type includes an image area ratio of an image to be transferred to a recording material, and the control unit transfers the image to the recording material in at least one image forming mode of the plurality of image forming modes. The adjusting unit is controlled so that the lower the image area ratio of the image to be formed, the smaller the electric resistance value when the AC transfer bias is applied to the transfer nip.
According to this, it is possible to form a high-quality image according to the difference in the image area ratio of the image in the at least one image forming mode.

(Aspect H)
In any one of the aspects A to G, there is an operation acceptance unit such as an operation panel 69 that accepts a user operation for selecting the surface moving speed of the image carrier, and the predetermined selection condition is the operation acceptance unit. The selection condition is that the image forming mode of the surface moving speed corresponding to the user operation received by the user is selected.
According to this, it is possible to perform the image formation at the image forming speed and the image quality according to the user's request.

(Aspect I)
In any one of the above modes A to H, an operation panel 69 or the like that accepts a user operation for selecting an image quality from a plurality of types of image quality (concave image quality and convex image quality, or solid image quality and halftone image quality) The operation control means has an operation acceptance means, and the control means, in at least one image forming mode of the plurality of image forming modes (high-speed AC mode of the uneven paper A, low-speed AC mode of the uneven paper C, etc.), Controlling the adjusting means such that the electric resistance value when the AC transfer bias is applied to the transfer nip becomes the electric resistance value corresponding to the image quality according to the user operation accepted by the operation accepting means. Characterize.
According to this, in the at least one image forming mode, it is possible to form an image having the image quality according to the user's request.

(Aspect J)
In any one of the aspects A to I, the adjusting unit controls the electric resistance value by controlling a heating unit such as a heater 87 that applies heat to at least one of the image carrier and the nip forming member. It is characterized in that it includes one to be 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.

(Aspect L)
In any one of the aspects A to K, 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 controlling the nip width changing means.

(Aspect M)
In the aspect L, the adjusting unit controls the nip width changing unit so that the nip width is expanded 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 controlling the nip width changing means.

1 Image Forming Apparatus 10 Image Forming Section 20 Transfer Unit 21 Intermediate Transfer Belt 24 Secondary Transfer Opposing Rollers 25C, 25M, 25Y, 25K Primary Transfer Rollers 30, 30 'Secondary Transfer Rollers 31, 36 Roller Unit Holder 32 Support Shaft 33 Extension spring 34 Eccentric cam 35 Cam drive motor 37 Rotating shaft 38 Drive motors 39A, 39B Stopper 58 Push-down roller 60 Control unit 69 Operation panel 81 Secondary transfer power supply 84 Heater power supply 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

JP, 2015-187723, A

Claims (13)

An image carrier that carries a toner image on the surface and moves on the surface,
A nip forming member that contacts the surface of the image carrier to form a 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 AC transfer bias including a DC component and an AC component is applied to the transfer nip. And an image forming apparatus having means,
Control means for controlling an image forming operation in an image forming mode selected according to a predetermined selection condition from a plurality of image forming modes in which the surface moving speed of the image carrier is different from each other;
Adjusting means for adjusting the electric resistance value on the route of the transfer current flowing through the transfer nip,
The control means applies the AC transfer bias to the transfer nip in an image forming mode in which the surface moving speed of the image carrier is higher between at least two image forming modes of the plurality of image forming modes. The adjusting means is controlled so that the electric resistance value at the time of recording is reduced , and at least one of the plurality of image forming modes is in the image forming mode, the image forming apparatus is configured to change the image forming apparatus according to the type of the image transferred to the recording material. An image forming apparatus , wherein the adjusting unit is controlled so as to adjust an electric resistance value when an AC transfer bias is applied to the transfer nip .   The image forming apparatus according to claim 1,
  The image type includes the image area ratio of the image transferred to the recording material,
  In the image forming mode of at least one of the plurality of image forming modes, the control unit controls when the AC transfer bias is applied to the transfer nip as the image area ratio of the image transferred to the recording material is lower. An image forming apparatus, wherein the adjusting means is controlled so that an electric resistance value becomes small.   The image forming apparatus according to claim 1,
  It has operation accepting means for accepting a user operation for selecting an image quality from a plurality of types of image quality,
  In the image forming mode of at least one of the plurality of image forming modes, the control unit determines that the electric resistance value when the AC transfer bias is applied to the transfer nip is the user operation accepted by the operation accepting unit. An image forming apparatus, wherein the adjusting means is controlled so that the electric resistance value corresponds to the image quality according to the above.   An image carrier that carries a toner image on the surface and moves on the surface,
  A nip forming member that contacts the surface of the image carrier to form a 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 AC transfer bias including a DC component and an AC component is applied to the transfer nip. And an image forming apparatus having means,
  Control means for controlling an image forming operation in an image forming mode selected according to a predetermined selection condition from a plurality of image forming modes in which the surface moving speed of the image carrier is different from each other;
  Adjusting means for adjusting the electric resistance value on the route of the transfer current flowing through the transfer nip,
  It has operation accepting means for accepting a user operation for selecting an image quality from a plurality of types of image quality,
  The control means applies the AC transfer bias to the transfer nip in an image forming mode in which the surface moving speed of the image carrier is higher between at least two image forming modes of the plurality of image forming modes. When the AC transfer bias is applied to the transfer nip in the image forming mode of at least one of the plurality of image forming modes, the adjusting unit is controlled so that the electric resistance value becomes small. The image forming apparatus is characterized in that the adjusting means is controlled so that the electric resistance value of the electric resistance value becomes an electric resistance value corresponding to the image quality according to the user operation accepted by the operation accepting means. The image forming apparatus according to any one of claims 1 to 4 ,
The image forming apparatus, wherein the predetermined selection condition includes a selection condition of selecting an image forming mode according to the type of recording material used. The image forming apparatus according to claim 5 ,
It has operation acceptance means for accepting a user operation for selecting the type of recording material to be used,
The image forming apparatus, wherein the control unit specifies the type of recording material to be used according to the user operation received by the operation receiving unit. The claims 5 or the image forming apparatus according to 6,
The type of recording material includes the thickness of the recording material used,
The predetermined forming condition includes an selecting condition that an image forming mode in which the surface moving speed of the image carrier is slower is selected as the recording material used is thicker. The image forming apparatus according to any one of claims 1乃Itaru 7,
In at least one image forming mode of the plurality of image forming modes, the control unit has a larger electric resistance value when the AC transfer bias is applied to the transfer nip as the thickness of the recording material used increases. the large Do so that, the image forming apparatus characterized by controlling the adjusting means. The image forming apparatus according to any one of claims 1乃Itaru 8,
And an operation receiving means for receiving a user operation for selecting the surface moving speed of the image carrier,
The image forming apparatus, wherein the predetermined selection condition includes a selection condition of selecting an image forming mode of a surface moving speed corresponding to a user operation accepted by the operation accepting unit. The image forming apparatus according to any one of claims 1 to 9,
The image forming apparatus, 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, wherein 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. The image forming apparatus according to any one of claims 1 to 11,
The image forming apparatus, 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 12,
The image forming apparatus, wherein the adjusting unit controls the nip width changing unit so that the nip width is expanded when the electric resistance value of the transfer nip is lowered.