JP2017194669A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which can transfer a sufficient amount of toner onto a recording sheet while preventing transfer defects of a toner image on the recording sheet.SOLUTION: The image forming apparatus comprises: a nip width change device 60 which changes the width of a transfer nip; a power supply 39 which outputs a transfer bias including an AC component for transferring a toner image on an image carrier onto a recording sheet at the transfer nip; and control means 200 which switches, according to a prescribed condition, between a first mode in which the duty ratio of the transfer bias represented by (T-Tt)/T×100[%} where T is the cycle of the transfer bias and Tt is the time when the transfer bias is closer to the transfer side where the toner image is moved from the image carrier toward the recording sheet, than a time average value of the transfer bias within the cycle T is a first duty ratio and the width of the transfer nip is a first width and a second mode in which the duty ratio of the transfer bias is a second duty ratio lower than the first duty ratio and the width of the transfer nip is a second width wider than the first width.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus.

The image forming apparatus outputs a transfer bias by superimposing a DC voltage that is a DC component and an AC voltage that is an AC component, and causes a transfer current to flow through the transfer nip due to contact between the image carrier and the nip forming member. Some transfer a toner image on the surface of an image carrier onto a recording sheet sandwiched between transfer nips. When a toner image is transferred to a recording sheet using a transfer bias in this way, depending on the degree of unevenness of the recording sheet and the required image density, transfer of the toner image may occur. It is known to control the output of the transfer bias in order to control the current.
For example, in the image forming apparatus described in Patent Document 1, in order to obtain a good image while suppressing generation of white spots while obtaining a sufficient image density at each of the concave and convex portions on the surface of the recording sheet, transfer is performed. The bias is one in which the voltage in the transfer direction for transferring the toner image from the image carrier side to the recording sheet side and the voltage in the opposite direction to the voltage in the transfer direction are alternately switched, and in one cycle of the alternately switched voltage waveform. Where A is the area on the return direction side from the center value Voff of the maximum and minimum values of voltage, and B is the area on the transfer direction side of the center value Voff of the maximum and minimum values of voltage. It is described that the output of the transfer bias from the transfer power source is controlled so as to change the duty ratio, which is a value of (A + B) × 100 [%], so that the duty ratio of the recording sheet to be used becomes smaller.

  An object of the present invention is to provide an image forming apparatus capable of transferring a sufficient amount of toner onto a recording sheet while preventing transfer failure of the toner image on the recording sheet.

  To achieve the above object, an image forming apparatus according to the present invention includes an image carrier, a nip forming member that forms a transfer nip between the image carrier, and a nip width changing device that changes the width of the transfer nip. A power source that outputs a transfer bias including an AC component to transfer the toner image on the image carrier to the recording sheet at the transfer nip, and the duty ratio of the transfer bias is a first duty ratio, and the cycle of the transfer bias Where T is T, and Tt is the time on the transfer side in which the transfer bias moves the toner image from the image carrier to the recording sheet with respect to the time average value of the transfer bias in period T, and (T−Tt) / T X100 [%], the transfer nip width is the first width, and the transfer bias duty ratio is lower than the first duty ratio, and the transfer nip width is the first width. one Is characterized in that a control means for switching the second mode is a wide second width than the width, the response to a predetermined condition.

  An object of the present invention is to provide an image forming apparatus capable of transferring a sufficient amount of toner onto a recording sheet while preventing a transfer failure of the toner image on the recording sheet.

1 is a schematic configuration diagram illustrating a printer that is an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing a main part of an electric circuit of a transfer power source in the printer of FIG. 1 together with a secondary transfer back roller and a secondary transfer nip backing roller. FIG. 2 is a block diagram schematically illustrating a main configuration of a control system in a printer. FIG. 2 is an enlarged sectional view partially showing a transverse section of an intermediate transfer belt which is an image carrier of the printer of FIG. 1. FIG. 3 is an enlarged plan view showing a partially enlarged intermediate transfer belt. The graph which shows the 1st example of the waveform (reverse peak side duty ratio = 50%) of the secondary transfer bias which consists of a superimposed voltage. The graph which shows the 2nd example of the waveform (reverse peak side duty ratio = 50%) of the secondary transfer bias which consists of a superimposed voltage. The graph for demonstrating the reverse peak side duty ratio in the secondary transfer bias shown by FIG. The graph for demonstrating the reverse peak side duty ratio in the secondary transfer bias shown by FIG. The graph which shows an example of the waveform of the secondary transfer bias which made the reverse peak side duty ratio 35%. The graph which shows an example of the waveform in the secondary transfer bias of the low duty ratio whose reverse peak side duty ratio is 30 [%]. The graph which shows an example of the waveform in the secondary transfer bias of the low duty ratio whose reverse peak side duty ratio is 10 [%]. The graph which shows an example of the waveform in the secondary transfer bias of the high duty ratio whose polarity reverses within one period and whose reverse peak side duty ratio is 80 [%]. Secondary transfer with high duty ratio with constant polarity, reverse peak duty ratio of 80 [%], average potential of −4 [kV], and peak-to-peak potential Vpp of 12 [kV] The graph which shows an example of the waveform in a bias. Secondary transfer with high duty ratio with constant polarity, reverse peak duty ratio of 80 [%], average potential of −4 [kV], and peak-to-peak potential Vpp of 10 [kV] The graph which shows an example of the waveform in a bias. High-duty secondary transfer with constant polarity, reverse peak duty ratio of 80 [%], average potential of −4 [kV], and peak-to-peak potential Vpp of 8 [kV] The graph which shows an example of the waveform in a bias. Secondary transfer with high duty ratio with constant polarity, reverse peak duty ratio of 80 [%], average potential of −4 [kV], and peak-to-peak potential Vpp of 6 [kV] The graph which shows an example of the waveform in a bias. 2A and 2B are diagrams illustrating a configuration of a first embodiment of a nip width varying device provided in the printer and a state in a high duty ratio mode. 2A and 2B are diagrams illustrating a configuration of a first embodiment of a nip width varying device provided in the printer and a state in a low duty ratio mode. FIG. 6 is a diagram illustrating a configuration of a modified example of the first embodiment of the nip width varying device provided in the printer and a state in a high duty ratio mode. FIG. 6 is a diagram illustrating a configuration of a modified example of the first embodiment of the nip width varying device provided in the printer and a state in a low duty ratio mode. The figure explaining the structure at the time of high duty ratio mode and the structure of Embodiment 2 of the nip width variable apparatus with which the printer is provided. The figure explaining the structure at the time of low duty ratio mode and the structure of Embodiment 2 of the nip width variable apparatus with which the printer is provided. The figure explaining the structure at the time of high duty ratio mode, and the structure of Embodiment 3 of the nip width variable apparatus with which the printer is provided. FIG. 5 is a diagram illustrating a configuration of a third embodiment of a nip width varying device provided in the printer and a state in a low duty ratio mode. The block diagram explaining the structure of the control system which has an input operation part provided with the recording sheet selection part. The block diagram which shows the paper feed path of the printer which concerns on the modification 1 of 1st embodiment. The block diagram explaining the structure of the modification 2 of 1st embodiment. The block diagram explaining the structure of 2nd embodiment. The block diagram explaining the structure of 4th embodiment.

  Embodiments, modifications, and examples according to the present invention will be sequentially described below with reference to the drawings. In the embodiment, the modification, and the example, the same reference numerals are given to the components having the same function and configuration, and the duplicate description is appropriately omitted. The drawings may be partially omitted or schematically shown in order to facilitate understanding of some configurations. Content common to the plurality of embodiments may be omitted as appropriate or may be described at the same time. In the embodiment for carrying out the invention, the duty ratio may be described as Duty.

  As a method of changing the transfer current amount applied to the recording sheet at the transfer nip, it is possible to cope with changing the linear velocity of the recording sheet in addition to changing the transfer nip width. For example, widening the transfer nip width decreases the linear velocity to increase the recording sheet nip passage time, and decreasing the transfer nip width shortens the recording sheet nip passage time. Can also respond. However, when the linear velocity is lowered, the image quality is secured, but the number of prints per unit time, that is, the productivity is lowered. In consideration of productivity, it is preferable to change the transfer pressure (transfer nip width) as in this embodiment. However, the linear velocity of the recording sheet is not limited to a constant value, and the linear velocity may be changed according to a predetermined condition.

First, the overall configuration of the image forming apparatus will be described, then a general transfer bias by a transfer power source will be described, and then each embodiment will be described in order.
(overall structure)
FIG. 1 is a schematic configuration diagram illustrating an electrophotographic color printer (hereinafter simply referred to as a printer) which is an embodiment of an image forming apparatus to which the present embodiment is applied. The application field of the image forming apparatus is not limited to a printer, but can be applied to a copier, a facsimile machine, a multifunction machine having a copying function, a FAX function, and the like.
In FIG. 1, the printer according to the embodiment includes four toner image forming units 1Y, 1M, 1C, for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images. It has 1K. The printer also includes a transfer unit 30, an optical writing unit 80, a fixing device 90, a feeding cassette 100, a registration roller pair 101, a control unit 200 as control means, and the like. The toner image forming units 1Y, 1M, 1C, and 1K constitute a process cartridge unit and are detachably supported by the printer casing, and can be attached and detached at the time of maintenance and unit replacement.

  The four toner image forming units 1Y, 1M, 1C, and 1K use yellow, magenta, cyan, and black toners of different colors, but the other configurations are the same and are replaced when the lifetime is reached. The toner image forming units 1Y, 1M, 1C, and 1K have a configuration necessary for forming a toner image in an electrophotographic process. In other words, the toner image forming units 1Y, 1M, 1C, and 1K include drum-shaped photosensitive members 2Y, 2M, 2C, and 2K that serve as latent image carriers, drum cleaning devices 3Y, 3M, 3C, and 3K, static eliminators, and charging devices 6Y. , 6M, 6C, 6K, developing devices 8Y, 8M, 8C, 8K, and the like.

  The photoreceptors 2Y, 2M, 2C, and 2K are obtained by forming an organic photosensitive layer on the surface of the drum base, and are driven to rotate in the clockwise direction in the drawing by a driving unit. In the charging process, the charging devices 6Y, 6M, 6C, and 6K are configured so that charging rollers 7Y, 7M, 7C, and 7K, which are charging members to which a charging bias is applied, are in contact with or close to the photoreceptors 2Y, 2M, 2C, and 2K. The surfaces of the photoreceptors 2Y, 2M, 2C, and 2K are uniformly charged by generating a discharge between the charging rollers 7Y, 7M, 7C, and 7K and the photoreceptors 2Y, 2M, 2C, and 2K. In the embodiment, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed. Instead of a method in which a charging member such as a charging roller is brought into contact with or close to the photoreceptor 2K, a method using a charging charger may be adopted.

In the writing process, the optical writing unit 80 uses a laser beam for each color emitted from a laser diode, which is an example of a light source, based on image information sent from an external device such as a personal computer. 2C and 2K surfaces are optically scanned. Electrostatic latent images corresponding to the respective colors are formed on the surfaces of the uniformly charged photoreceptors 2Y, 2M, 2C, and 2K by this optical scanning. The optical writing unit 80 applies each color laser beam L emitted from the light source to each photoconductor via a plurality of optical lenses and mirrors while polarizing the laser beam L in the main scanning direction by a polygon mirror rotated by a polygon motor. Irradiation. As a light source, you may employ | adopt what performs optical writing with the LED light emitted from several LED of the LED array.
The electrostatic latent images of these colors are the developing devices 8Y, 8M, 8C, which use toners that are powders of yellow (Y), magenta (M), cyan (C), and black (K) and are developers. Each toner image is developed in the developing process using 8K.
When the toner image development process is completed in this manner, the printer uses the toner images formed on the photoreceptors 2Y, 2M, 2C, and 2K after the development process as image carriers and intermediate transfer bodies. A primary transfer process for primary transfer onto the intermediate transfer belt 31 is performed.

  The drum cleaning devices 3Y, 3M, 3C, and 3K are well-known devices that clean transfer residual toner adhering to the surfaces of the photoreceptors 2Y, 2M, 2C, and 2K after the primary transfer process (primary transfer nip described later). It is. The static eliminator is a well-known device that neutralizes residual charges on the photoreceptors 2Y, 2M, 2C, and 2K after being cleaned by the drum cleaning devices 3Y, 3M, 3C, and 3K. By this charge removal, the surfaces of the photoreceptors 2Y, 2M, 2C, and 2K are initialized and prepared for the next image formation.

  Below the toner image forming units 1Y, 1M, 1C, and 1K, a transfer unit 30 is disposed as a transfer device that moves the endless intermediate transfer belt 31 endlessly in the counterclockwise direction in the drawing. Yes. In addition to the intermediate transfer belt 31 as an image carrier, the transfer unit 30 includes a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, which are a plurality of rotating bodies. 35K etc. The transfer unit 30 also includes a belt cleaning device 37, a density sensor 40, and the like.

  The intermediate transfer belt 31 is stretched by a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K disposed inside the loop. A drive motor M as a drive source is connected to the drive roller 32. The intermediate transfer belt 31 is moved endlessly in the same direction by the rotational force of the drive roller 32 that is driven to rotate counterclockwise in the figure by the drive motor M. The drive motor M is connected to the control unit 200. The controller 200 rotates the drive motor M at a predetermined speed, thereby driving the intermediate transfer belt 31 to rotate at a predetermined linear speed.

The four primary transfer rollers 35Y, 35M, 35C, and 35K sandwich the intermediate transfer belt 31 that is moved endlessly between the photoreceptors 2Y, 2M, 2C, and 2K. Thus, primary transfer nips for yellow, magenta, cyan, and black, in which the front surface of the intermediate transfer belt 31 and the photoreceptors 2Y, 2M, 2C, and 2K contact each other, are formed in the printer. A primary transfer bias is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K by a primary transfer power source at the timing of primary transfer. As a result, a transfer field for primary transfer is formed between the yellow, magenta, cyan, and black toner images on the photoreceptors 2Y, 2M, 2C, and 2K and the primary transfer rollers 35Y, 35M, 35C, and 35K.
For example, Y toner formed on the surface of the Y photoreceptor 2Y enters the primary transfer nip for Y as the photoreceptor 2Y rotates. Then, the image is primarily transferred from the photoreceptor 2Y to the intermediate transfer belt 31 by the action of the transfer electric field and nip pressure. The intermediate transfer belt 31 on which the Y toner image has been primarily transferred in this way then passes sequentially through the primary transfer nips for magenta, cyan, and black. Then, the magenta, cyan, and black toner images on the photoreceptors 2M, 2C, and 2K are sequentially superimposed and sequentially transferred onto the Y toner image. A four-color superimposed toner image is formed on the intermediate transfer belt 31 by the primary transfer of the superposition in the primary transfer process. In place of the primary transfer rollers 35Y, 35M, 35C, and 35K, a transfer charger, a transfer brush, or the like may be employed.

Below the transfer unit 30, a sheet conveyance unit 38 including a secondary transfer nip backing roller 36, a sheet conveyance belt (generally referred to as a secondary transfer belt or a transfer member) 41, and the like is disposed. ing. The endless sheet conveying belt 41 that is a nip forming member is stretched by a plurality of rollers that are rotating members such as a secondary transfer nip backing roller 36 and a separation roller 42 that are disposed inside the loop. This belt member is rotated in the clockwise direction in the drawing by the rotational drive of the secondary transfer nip backing roller 36. The intermediate transfer belt 31 is in contact with a region around the secondary transfer back roller 33 in the circumferential direction of the intermediate transfer belt 31 by the secondary transfer nip backing roller 36.
The secondary transfer back roller 33 of the transfer unit 30 and the secondary transfer nip backing roller 36 of the sheet transport unit 38 sandwich the intermediate transfer belt 31 and the sheet transport belt 41 between each other. Thus, a secondary transfer nip N is formed as a transfer nip where the front surface of the intermediate transfer belt 31 and the front surface of the sheet conveying belt 41 as a nip forming member abut.
The secondary transfer nip backing roller 36 disposed in the loop of the sheet conveying belt 41 is grounded, whereas the secondary transfer back roller 33 disposed in the loop of the intermediate transfer belt 31 is transferred to the secondary transfer nip backing roller 33. A secondary transfer bias is applied as a transfer bias by a secondary transfer power source 39 as a power source. As a result, a transfer current flows between the secondary transfer back roller 33 and the secondary transfer nip backing roller 36, and negative polarity toner is directed from the secondary transfer back roller 33 side to the secondary transfer nip backing roller 36 side. The secondary transfer electric field to be electrostatically moved is formed in the secondary transfer nip N. In other words, the printer is configured to output a transfer bias from a transfer power source and to cause a transfer current to flow through the transfer nip due to contact between the image carrier and the nip forming member.
Note that a secondary transfer roller may be used as the nip forming member instead of the sheet conveying belt 41, and this may be brought into direct contact with the intermediate transfer belt 31. Further, a secondary transfer bias may be applied to the secondary transfer nip backing roller 36 by the secondary transfer power source 39, and the secondary transfer back roller 33 may be grounded.

Below the transfer unit 30, a feeding cassette 100 that stores a plurality of recording sheets P in a bundle of sheets is disposed. In the feeding cassette 100, a sheet feeding roller 100a is brought into contact with the uppermost recording sheet P of the sheet bundle, and the recording sheet P is directed to the feeding path by being rotationally driven at a predetermined timing. And send it out. A registration roller pair 101 is disposed near the end of the feeding path. The registration roller pair 101 is rotationally driven at a timing at which the recording sheet P fed from the feeding cassette 100 can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip N, and the recording sheet P P is fed toward the secondary transfer nip N.
The four-color superimposed toner image on the intermediate transfer belt 31 brought into intimate contact with the recording sheet P at the secondary transfer nip N is recorded on the recording sheet P by the action of the secondary transfer electric field (transfer current) due to the secondary transfer bias and the nip pressure. A full-color toner image is formed by secondary transfer onto the top. In this way, when the secondary transfer process is performed and the recording sheet P having a full-color toner image formed on the surface thereof passes through the secondary transfer nip N, the curvature is separated from the intermediate transfer belt 31. Further, the recording sheet P after transfer is separated from the sheet conveying belt 41 by the curvature of the separation roller 42 that is wound around the sheet conveying belt 41.

In the present embodiment, the sheet transfer belt 41 is used as the nip forming member, and the secondary transfer nip N is formed by contacting the intermediate transfer belt 31. Instead of such a configuration, the nip forming roller is used as the nip forming member. The secondary transfer nip N may be formed by contacting the roller surface with the intermediate transfer belt 31.
Untransferred toner that has not been transferred to the recording sheet P adheres to the intermediate transfer belt 31 that has passed through the secondary transfer nip N. In the cleaning process, the transfer residual toner is cleaned from the belt surface by a belt cleaning device 37 that is in contact with the front surface of the intermediate transfer belt 31 at a portion facing the cleaning backup roller 34.

  The printer includes a density sensor 40 as toner density detection means. The density sensor 40 is disposed outside the loop of the intermediate transfer belt 31, and is opposed to a portion where the intermediate transfer belt 31 is wound around the drive roller 32 through a predetermined gap. ing. In this state, when the toner image primarily transferred onto the intermediate transfer belt 31 enters a position facing itself, the toner adhesion amount (image density) per unit area of the toner image is detected. The detection output of the density sensor 40 is transmitted to the control unit 200, and it is determined whether it is a solid image or a halftone image.

A fixing device 90 is disposed downstream of the secondary transfer nip N in the sheet conveyance direction indicated by an arrow A. The fixing device 90 forms a fixing nip with a fixing roller 91 containing a heat source and a pressure roller 92 that rotates while contacting with the fixing roller 91, and in the fixing process, recording after secondary transfer is performed. By passing the sheet P through the fixing nip, the toner in the toner image is softened by the action of heat and pressure, and the full color image is fixed on the recording sheet P.
The recording sheet P that has passed through the fixing nip is discharged from the fixing device 90, passes through a post-fixing conveyance path, and is discharged outside the printer.

  In the printer according to the embodiment, when a monochrome image is formed, the posture of the support plate that supports the primary transfer rollers 35 (Y, M, C) for yellow, magenta, and cyan in the transfer unit 30 is set to a solenoid or the like. Change by driving. As a result, the primary transfer rollers 35 (Y, M, C) for yellow, magenta, and cyan are moved away from the photoreceptor 2 (Y, M, C), and the front surface of the intermediate transfer belt 31 is moved to the photoreceptor 2. Separate from (Y, M, C). In this way, the black image forming unit among the four toner image forming units 1 (Y, M, C, K) with the intermediate transfer belt 31 in contact with only the black photoconductor 2K. Only 1K is driven to form a K toner image on the black photoreceptor 2K. The present invention can be applied not only to an image forming apparatus that forms a color image but also to an image forming apparatus that forms a monochrome image alone.

(Configuration of transfer power supply)
FIG. 2 is a block diagram showing the main part of the electric circuit of the secondary transfer power supply 39 together with the secondary transfer back roller 33, the secondary transfer nip backing roller 36, and the like. The secondary transfer power source 39 includes a DC power source 110 and an AC power source 140 configured to be detachable. The secondary transfer power supply 39 is controlled by a control unit 200 as a control unit. The control unit 200 also controls the operation of a nip width varying device described later.
The DC power source 110 outputs a DC voltage, which is a DC component for applying an electrostatic force from the belt side to the recording sheet side in the secondary transfer nip N to the toner on the surface of the intermediate transfer belt 31. It is a power supply. The DC power supply 110 includes a DC output control unit 111, a DC drive unit 112, a DC voltage transformer 113, a DC output detection unit 114, an output abnormality detection unit 115, an electrical connection unit 221, and the like.
The AC power supply 140 is a power supply that outputs an AC voltage that is an AC component for forming an alternating electric field in the secondary transfer nip N. The AC power supply 140 includes an AC output control unit 141, an AC drive unit 142, an AC voltage transformer 143, an AC output detection unit 144, a removal unit 145, an output abnormality detection unit 146, an electrical connection unit 242 and an electrical connection unit 243. Have.

  The control unit 200 controls the DC power supply 110 and the AC power supply 140, and includes a control device having a CPU (CenAal Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. . The DC output control unit 111 receives a DC_PWM signal for controlling the output level of the DC voltage from the control unit 200. Further, the output value of the DC voltage transformer 113 detected by the DC output detection unit 114 is also input to the DC output control unit 111. The DC output control unit 111 performs the following control based on the duty ratio included in the input DC_PWM signal and the output value of the DC voltage transformer 113. That is, the driving of the DC voltage transformer 113 is controlled via the DC drive unit 112 so that the output value of the DC voltage transformer 113 is set to the output value indicated by the DC_PWM signal.

  The DC drive unit 112 drives the DC voltage transformer 113 according to the control from the DC output control unit 111. The DC voltage transformer 113 is driven by the DC drive unit 112 and outputs a negative DC high voltage. When the AC power supply 140 is not connected, the electrical connection portion 221 and the secondary transfer back roller 33 are electrically connected by the harness 301, and the DC voltage transformer 113 is connected via the harness 301. A DC voltage is output (applied) to the secondary transfer back roller 33. On the other hand, when the AC power supply 140 is connected, since the electrical connection portion 221 and the electrical connection portion 242 are electrically connected by the harness 302, the DC voltage transformer 113 is connected to the AC power supply 140 via the harness 302. Output DC voltage.

The DC output detection unit 114 detects the output value of the DC high voltage from the DC voltage transformer 113 and outputs it to the DC output control unit 111. Further, the DC output detection unit 114 outputs the detected output value to the control unit 200 as an FB_DC signal (feedback signal). This is because the control unit 200 controls the duty ratio of the DC_PWM signal so that transferability does not deteriorate due to the environment and load.
In this printer, since the AC power supply 140 can be attached to and detached from the main body of the secondary transfer power supply 39, the impedance of the output path of the high voltage output is determined depending on whether the AC power supply 140 is connected or not. Changes. For this reason, when the DC power supply 110 performs constant voltage control and outputs a DC voltage, the voltage dividing ratio changes due to the impedance in the output path changing according to the presence or absence of the AC power supply 140. Furthermore, since the high voltage applied to the secondary transfer back surface roller 33 changes, the transferability changes depending on whether or not the AC power supply 140 is present.

Therefore, in this printer, the DC power supply 110 performs constant current control to output a DC voltage, and the output voltage is changed according to the presence or absence of the AC power supply 140. As a result, even if the impedance in the output path changes, the high voltage applied to the secondary transfer back roller 33 can be kept constant, and the transferability can be kept constant regardless of the presence or absence of the AC power supply 140. it can. Furthermore, the AC power supply 140 can be attached and detached without changing the value of the DC_PWM signal.
As described above, in this printer, the DC power supply 110 is controlled at a constant current, but the following configuration may be adopted. That is, if the high voltage applied to the secondary transfer back roller 33 can be kept constant by changing the value of the DC_PWM signal when the AC power supply 140 is attached or detached, a configuration in which the DC power supply 110 is controlled at a constant voltage is adopted. May be.

  The output abnormality detection unit 115 is arranged on the output line of the DC power supply 110, and outputs an SC signal indicating an output abnormality such as a leak to the control unit 200 when an output abnormality occurs due to a ground fault of a wire. To do. As a result, it is possible to perform control for stopping the high voltage output from the DC power supply 110 by the control unit 200.

  The AC output control unit 141 receives an AC_PWM signal for controlling the output level of the AC voltage and the output value of the AC voltage transformer 143 detected by the AC output detection unit 144 from the control unit 200. The AC output control unit 141 performs the following control based on the duty ratio of the input AC_PWM signal and the output value of the AC voltage transformer 143. That is, the driving of the AC voltage transformer 143 is controlled via the AC driver 142 so that the output value of the AC voltage transformer 143 becomes the output value indicated by the AC_PWM signal.

  An AC_CLK signal that controls the output frequency of the AC voltage is input to the AC drive unit 142. The AC drive unit 142 drives the AC voltage transformer 143 based on the control from the AC output control unit 141 and the AC_CLK signal. The AC driver 142 drives the AC voltage transformer 143 based on the AC_CLK signal, so that the output waveform generated by the AC voltage transformer 143 can be controlled to an arbitrary frequency indicated by the AC_CLK signal. .

  The AC voltage transformer 143 is driven by the AC drive unit 142 to generate an AC voltage, and generates a superimposed voltage by superimposing the generated AC voltage and a DC high voltage output from the DC voltage transformer 113. When the AC power supply 140 is connected, that is, when the electrical connecting portion 243 and the secondary transfer back roller 33 are electrically connected by the harness 301, the AC voltage transformer 143 generates the superimposed voltage generated by Applied to the secondary transfer back roller 33 via the harness 301. The AC voltage transformer 143 outputs (applies) the DC high voltage output from the DC voltage transformer 113 to the secondary transfer back roller 33 via the harness 301 when no AC voltage is generated. . The voltage (superimposed voltage or DC voltage) output to the secondary transfer back roller 33 is then fed back into the DC power source 110 via the secondary transfer nip backing roller 36.

  The AC output detection unit 144 detects the output value of the AC voltage of the AC voltage transformer 143 and outputs it to the AC output control unit 141. Further, the detected output value is output to the control unit 200 as an FB_AC signal (feedback signal). This is because the control unit 200 controls the duty ratio of the AC_PWM signal so that the transferability does not deteriorate due to the environment or load. The AC power supply 140 performs constant voltage control, but may perform constant current control. Further, the waveform of the AC voltage generated by the AC voltage transformer 143 (AC power supply 140) may be a sine wave, a rectangular wave, or any other waveform. Is adopted. This is because it is possible to further improve the image quality by making the waveform of the alternating voltage a short-pulse rectangular wave.

  Note that the secondary transfer power supply 39 outputs a DC voltage by a constant current control system that adjusts the output voltage value so that the output current value matches a predetermined target current value. Further, the AC voltage is output by a constant voltage control method in which the amplitude is adjusted so that the peak-to-peak value Vpp (see FIG. 6) of the AC component of the secondary transfer bias matches a predetermined target value.

As described in Patent Document 1, if an alternating electric field is formed in the secondary transfer nip using a superimposed transfer voltage as the secondary transfer bias, the toner can be satisfactorily applied to the concave portions on the surface of the recording sheet rich in surface irregularities. It is already known that secondary transfer is possible. It is also known that the principle is as follows. In other words, when a secondary transfer bias consisting of only a DC voltage is used, only a very small number of toner particles constituting the toner image on the intermediate transfer belt in the secondary transfer nip are transferred from the belt surface to the sheet surface. It cannot be transferred into the recess. Even when the secondary transfer bias composed of the superimposed voltage is used, the same applies until the beginning of the AC component of the secondary transfer bias after the toner image enters the secondary transfer nip. Only a very small amount of toner particles can be transferred into the recesses on the sheet surface. However, when the next one cycle elapses, the amount of toner particles transferred from the belt surface into the sheet surface recesses slightly increases.
Specifically, first, in the first half of the next one cycle, when the toner particles in the concave portion on the sheet surface return to the belt surface, the toner particles hit the toner particles that have remained on the belt surface until then. Weakens the adhesion between the toner and other toner particles and the belt surface. In the latter half, the toner particles having weakened adhesion as described above are transferred from the belt surface into the sheet surface recess together with the toner particles returning to the belt surface.
Further, in the next one cycle, the number of toner particles transferred into the concave portion of the sheet surface is further increased by the same phenomenon. In the secondary transfer nip, as the toner particles reciprocate between the belt surface and the sheet surface recess many times, the number of toner particles transferred into the sheet surface recess gradually increases. When the toner image finally passes through the secondary transfer nip, a sufficient amount of toner particles are transferred into the recesses on the sheet surface.
The control unit 200 controls driving of various driving bodies in the printer, receives detection results of various sensors, and executes arithmetic processing. Driving of the toner image forming units 1Y, 1M, 1C, 1K, the optical writing unit 80, the driving motor M of the intermediate transfer belt 31, and the like are controlled by the control unit 200.

Next, a characteristic configuration of the printer according to the embodiment will be described.
FIG. 3 is a block diagram schematically showing the configuration of the control system of the printer, which is functionally described including the secondary transfer power supply 39 shown in FIG.
The printer includes a control unit 200. The control unit 200 outputs various signals based on various information input to the input unit, and controls the operation of the device connected to the output unit. In the present embodiment, as described above, the secondary transfer power supply 39 is connected to a nip width varying device 60 (described later) via a signal line, and the transfer bias output from the secondary transfer power supply 39 is output. And the operation of the nip width varying device 60 is controlled.
In order to transfer the toner image on the intermediate transfer belt 31 to the recording sheet P at the secondary transfer nip N, the secondary transfer power supply 39 applies a voltage obtained by superimposing an AC voltage on the DC voltage to the secondary transfer nip N. The secondary transfer power supply 39 can output a secondary transfer bias obtained by superimposing the AC voltage generated by the AC power supply 140 on the DC voltage generated by the DC power supply 110 and applies it to the secondary transfer back roller 33.

The control unit 200 is connected to the DC power supply 110 and the AC power supply 140 via signal lines. The printer includes a frequency setting unit 201, a duty ratio setting unit 202, an amplitude setting unit 203, and an output setting unit 204. The frequency setting unit 201, the duty ratio setting unit 202, and the amplitude setting unit 203 are connected to the AC power source 140 via a signal line, and the output setting unit 204 is connected to the DC power source 110 via a signal line.
The frequency setting unit 201 changes the frequency of the AC component output from the AC power supply 140. The duty ratio setting unit 202 changes and sets the voltage duty ratio in the direction in which the toner in the AC voltage waveform is transferred to the recording sheet P in the range of 0 to 100%. The frequency setting unit 201 and the duty ratio setting unit 202 are controlled in frequency and duty ratio by AC_CLK shown in FIG.
The amplitude setting unit 203 sets the maximum voltage difference (peak-to-peak value Vpp) of the AC voltage output from the AC power supply 140. The amplitude setting unit 203 is controlled by the AC_PWM signal shown in FIG.
The output setting unit 204 sets the constant voltage of the DC voltage generated by the DC power source 110 according to the type (smooth / unevenness) of the recording sheet P and the image density. The output setting unit 204 is controlled by the DC_PWM signal shown in FIG.
The frequency setting unit 201, the duty ratio setting unit 202, the amplitude setting unit 203, and the output setting unit 204 may be provided in the secondary transfer power source 39, or provided separately from the secondary transfer power source 39 and the control unit 200. The form with which the side is provided may be sufficient. Details of the control performed by the control unit 200 will be described later.

  As the intermediate transfer belt, it is common to use a belt composed only of a hard base material such as a polyimide belt. In the image forming apparatus described in Patent Document 1, the intermediate transfer belt is driven at a linear speed of 280 [mm / s] in order to form an image at an image forming speed for general users. In order to form an image at an ultra-high speed for business users, the present inventors have a configuration using a combination of an intermediate transfer belt consisting only of a belt substrate made of a hard material and a secondary transfer bias consisting of a superimposed voltage. The following experiment was conducted. That is, an experiment in which a test image is secondarily transferred to a surface uneven sheet (trade name: Rezac 66) manufactured by Tokai Paper Co., Ltd. while the intermediate transfer belt 31 is moved endlessly at an extremely high linear velocity of 630 [mm / s]. It is. As a result, despite the use of a superimposed transfer voltage as the secondary transfer bias, the toner cannot be satisfactorily secondary transferred to the surface recesses of the surface uneven sheet, resulting in uneven image density due to surface unevenness. Has caused. In other words, with an intermediate transfer belt consisting only of a hard belt substrate, if an image is formed at an ultra-high speed for business users, even if an alternating electric field is formed in the secondary transfer nip by a secondary transfer bias consisting of a superimposed voltage, The concave portion of the sheet surface causes toner transfer failure.

Accordingly, the present inventors prepared a printer tester equipped with an elastic belt as an intermediate transfer belt, and super-high-speed (process linear velocity = 630 mm) for business users on the surface uneven sheet (Rezak 66). / S) was conducted for secondary transfer. Then, the toner was successfully secondary transferred to the surface recesses of the surface uneven sheet, and the occurrence of uneven image density following the sheet surface unevenness could be reduced. It is considered that the distance between the surface of the intermediate transfer belt and the surface concave portion of the surface uneven sheet is reduced by flexibly changing the elastic layer of the intermediate transfer belt made of the elastic belt in the secondary transfer nip N.
Therefore, it is more preferable to use an intermediate transfer belt 31 made of an elastic belt. The printer according to the present embodiment uses an elastic belt.

  FIG. 4 is an enlarged sectional view partially showing a transverse section of the intermediate transfer belt 31 formed of an elastic belt mounted on the printer according to the embodiment. The intermediate transfer belt 31 has an endless belt-like base layer (a hard belt base) 31a made of a material having a certain degree of flexibility and high rigidity, and has excellent flexibility laminated on the front surface thereof. And an elastic layer 31b made of an elastic material. As shown in FIG. 5, the elastic layer 31 b is an elastic surface layer in which the particles 31 c are dispersed, and the particles 31 c protrude partly from the surface of the elastic layer 31 b. Densely lined up in the belt surface direction. A plurality of protrusions are formed on the belt surface by the plurality of particles 31c. That is, the intermediate transfer belt 31 has an elastic surface layer as the elastic layer 31b, and this elastic surface layer has a plurality of fine projections formed by a plurality of particles (fine particles) 31c dispersed in the material formed on the surface (elastic) of the elastic surface layer. Provided on the surface of the layer 31b.

  Examples of the material of the base layer 31a include a resin in which an electrical resistance adjusting material made of a filler or an additive for adjusting the electrical resistance is dispersed. From the viewpoint of flame retardancy, the resin is preferably, for example, a fluorine-based resin such as PVDF (polyvinylidene fluoride) or EBE (ethylene / tetrafluoroethylene copolymer), a polyimide resin, or a polyamide-imide resin. . Further, from the viewpoint of mechanical strength (high elasticity) and heat resistance, a polyimide resin or a polyamideimide resin is particularly preferable.

  Examples of the electrical resistance adjusting material dispersed in the resin include metal oxides, carbon black, ionic conductive agents, and conductive polymer materials. Examples of the metal oxide include zinc oxide, tin oxide, titanium oxide, zirconium oxide, aluminum oxide, and silicon oxide. In order to improve the dispersibility, the metal oxide previously subjected to surface treatment may be used. Examples of carbon black include ketjen black, furnace black, acetylene black, thermal black, and gas black. Examples of the ionic conductive agent include tetraalkyl ammonium salts, trialkyl benzyl ammonium salts, alkyl sulfonates, and alkyl benzene sulfonates. Alkyl sulfate, glycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene alkylamine, polyoxyethylene fatty acid alcohol ester, alkyl betaine, lithium perchlorate and the like may be used. A mixture of two or more of these ionic conductive agents may be used. The electrical resistance adjusting material to which the present invention can be applied is not limited to those exemplified so far.

  For the coating liquid that is the precursor of the base layer 31a (in which the electrical resistance adjusting material is dispersed in the liquid resin before curing), a dispersion aid, a reinforcing material, a lubricant, and a heat conducting material are used as necessary. An antioxidant or the like may be added. The additive amount of the electric resistance adjusting material contained in the base layer 31a of the seamless belt suitably equipped as the intermediate transfer belt 31 is preferably 1 × 10 8 to 1 × 10 13 [Ω / □] in terms of surface resistance and 1 in terms of volume resistance. The amount is set to × 106 to 1 × 1012 [Ω · cm].

  The thickness of the base layer 31a is not particularly limited and may be appropriately selected depending on the situation, but is preferably 30 μm to 150 μm, more preferably 40 μm to 120 μm, and particularly preferably 50 μm to 80 μm.

  As described above, the elastic layer 31b of the intermediate transfer belt 31 has a plurality of convex shapes formed by a plurality of dispersed particles 31c on the surface. Examples of the elastic material for forming the elastic layer 31b include general-purpose resins, elastomers, and rubbers. In particular, an elastic material excellent in flexibility (elasticity) is preferably used, and an elastomer material or a rubber material is preferable. Examples of the elastomer material include polyester, polyamide, polyether, polyurethane, polyolefin, polystyrene, polyacryl, polydiene, and silicone-modified polycarbonate. A thermoplastic elastomer such as a fluorinated copolymer may be used. Examples of the thermosetting resin include polyurethane, silicone-modified epoxy, and silicone-modified acrylic resins. Examples of the rubber material include isoprene rubber, styrene rubber, butadiene rubber, nitrile rubber, ethylene propylene rubber, butyl rubber, silicone rubber, chloroprene rubber, and acrylic rubber. Furthermore, chlorosulfonated polyethylene, fluorine rubber, urethane rubber, hydrin rubber and the like can also be exemplified. From the materials exemplified so far, it is possible to appropriately select a material capable of obtaining desired performance.

  Among the elastic materials constituting the elastic layer 31b, acrylic rubber is most preferable from the viewpoints of ozone resistance, flexibility, adhesion to particles, imparting flame retardancy, environmental stability, and the like. The acrylic rubber may be generally commercially available and is not limited to a specific product. However, among various crosslinking systems (epoxy groups, active chlorine groups, carboxyl groups) of acrylic rubber, those having a carboxyl group crosslinking system are superior in terms of rubber physical properties (particularly compression set) and processability. It is preferable to select a crosslinking type. As the crosslinking agent used for the carboxyl group-based acrylic rubber, an amine compound is preferable, and a polyvalent amine compound is most preferable. Specific examples of such amine compounds include aliphatic polyvalent amine crosslinking agents and aromatic polyvalent amine crosslinking agents.

  The acrylic rubber used for the elastic layer 31b may be blended with a crosslinking accelerator for the purpose of promoting the crosslinking reaction of the crosslinking agent described above. The type of the crosslinking accelerator is not particularly limited, but it is preferable that the crosslinking accelerator can be used in combination with the polyvalent amine crosslinking agent described above.

  It is preferable to add an electric resistance adjusting material to the acrylic rubber used for the elastic layer 31b. Regarding the amount of addition, the resistance value of the elastic layer 31b is set to a range of 1 × 10 8 to 1 × 10 13 [Ω / □] in terms of surface resistance and 1 × 10 6 to 1 × 10 12 [Ω · cm] in terms of volume resistance. It is preferable to adjust. The layer thickness of the elastic layer 31b is preferably 200 μm to 2 mm, and more preferably 400 μm to 1000 μm.

  The particles 31c dispersed in the elastic material of the elastic layer 31b have an average particle diameter of 100 μm or less, have a true spherical shape, are insoluble in organic solvents, and have a 3% thermal decomposition temperature of 200 ° C. or higher. Some resin particles are used. Although there is no restriction | limiting in particular in the resin material of particle | grains 31c, An acrylic resin, a melamine resin, a polyamide resin, a polyester resin, a silicone resin, a fluororesin, rubber | gum etc. can be illustrated. The base surface of the particles made of these resin materials may be surface treated with a different material. A hard resin may be coated on the surface of spherical base particles made of rubber. Moreover, as a base particle, you may use a hollow thing and a porous thing. It is also possible to use an intermediate transfer belt 31 in which the particles 31c are not dispersed in the elastic layer 31b.

  As shown in FIG. 5, on the surface of the intermediate transfer belt 31, the overlapping of the particles 31 c is hardly observed. The diameter of the cross section of the particle 31c on the surface of the elastic layer 31b is preferably as uniform as possible. Specifically, the distribution width is preferably ± (average particle diameter × 0.5) μm or less. In order to realize such a distribution width, it is preferable to use particle powder having a narrow particle size distribution. However, the elastic layer 31b may be formed by selectively localizing particles 31c having a specific particle size on the surface. If it is formed, a particle powder having a wide particle size distribution may be used.

  It is assumed that the recording sheet P is of a type rich in surface irregularities such as Japanese paper. In this case, in order to satisfactorily secondary transfer the toner to each of the plurality of concave portions on the surface of the recording sheet P and suppress the occurrence of image density unevenness that is uneven on the surface, the elastic layer 31b has a certain degree of flexibility ( It is necessary to adopt a material excellent in elasticity. And when such an elastic layer 31b is employ | adopted, since it will extend immediately if it stretches only with the elastic layer 31b alone, it cannot endure actual use. For this reason, it is an essential condition to provide a base layer 31a that is stiffer than the elastic layer 31b, and to suppress the elongation of the entire belt over a long period of time due to the rigidity of the base layer 31a.

  As described above, in the printer according to the embodiment, the intermediate transfer belt 31 including the elastic belt in which the elastic layer 31b is stacked on the base layer 31a is used. Thereby, even if an image is formed at an ultra-high speed for business users of belt linear velocity (process linear velocity) = 630 [mm / s], transfer of toner into the concave portion of the surface of the concave-convex sheet can be promoted. . It was possible to suppress the occurrence of image density unevenness following the surface unevenness of the uneven sheet.

  FIG. 6 is a graph showing a first example of the waveform of the secondary transfer bias composed of the superimposed voltage output from the secondary transfer power supply 39. The waveform of the secondary transfer bias shown in the figure is a sine wave. The offset voltage Voff is the value of the DC component (DC voltage) of the secondary transfer bias composed of the superimposed voltage. The polarity of the offset voltage Voff in the figure is negative. When the waveform of the secondary transfer bias is a sine wave as shown in the figure, the offset voltage Voff and the time average value Vave, which is an average potential per one cycle (T) of the secondary transfer bias, have the same value. . Therefore, in FIG. 3, the time average value Vave of the transfer bias has a negative polarity.

  In the configuration in which the secondary transfer bias is applied to the core of the secondary transfer back roller (reference numeral 33 in FIG. 1) as in the printer of the embodiment, the polarity of the secondary transfer bias is the same as the normal charging polarity of the toner. The toner in the secondary transfer nip N electrostatically moves in the transfer direction. Specifically, it is electrostatically moved from the surface side of the intermediate transfer belt 31 to the recording sheet surface side in the nip. In addition, when the polarity of the secondary transfer bias is opposite to the normal charging polarity of the toner, the toner in the secondary transfer nip moves electrostatically in the direction opposite to the transfer direction. Specifically, the toner electrostatically moves from the recording sheet surface side toward the surface side of the intermediate transfer belt 31. By setting the time average value Vave to a negative polarity that is the same polarity as the normal charging polarity of the toner, while moving the toner back and forth between the belt surface side and the sheet surface side in the secondary transfer nip, It is moved from the belt surface side toward the sheet surface side. As a result, the toner image on the surface of the intermediate transfer belt 31 can be secondarily transferred onto the surface of the recording sheet P.

In FIG. 6, the transfer peak value Vt is one of two peak values generated within one cycle T of the secondary transfer bias, and the toner is directed from the belt surface side to the sheet surface side in the secondary transfer nip. It is the peak value for the more strongly electrostatic movement. The reverse peak value Vr is a peak value that is not the transfer peak value Vt. In the secondary transfer bias shown in FIG. 6, the reverse peak value Vr is opposite in polarity (plus polarity) from the transfer peak value Vt.
The transfer peak value Vt is closer to the transfer side (minus polarity side) than the time average value Vave. The reverse peak value Vr is a peak value different from the transfer peak value Vt, and is on the opposite side (+ polarity side) from the transfer side as viewed from the time average value Vave.

  The waveform of the secondary transfer bias output from the secondary transfer power supply 39 is not limited to a sine wave as shown in FIG. A secondary transfer bias of a triangular wave or a rectangular wave may be employed. FIG. 7 is a graph showing a second example of the waveform of the secondary transfer bias composed of the superimposed voltage. The waveform of the secondary transfer bias shown in the figure is a rectangular wave. The sine wave of the secondary transfer bias shown in FIG. 6 and the rectangular wave of the secondary transfer bias shown in FIG. 7 both have a reverse peak side duty ratio described later of 50 [%]. In the secondary transfer bias having a waveform having such characteristics, the time average value Vave per cycle (T) is the same value as the offset voltage Voff. That is, the DC component value is the same as the time average value Vave.

FIG. 8 is a graph for explaining the reverse peak duty ratio in the secondary transfer bias shown in FIG. In the figure, the center potential Vc is the center potential at the AC component (AC voltage) peak-to-peak value Vpp of the secondary transfer bias. The peak-to-peak value Vpp is a value from the reverse peak value Vr to the transfer peak value Vt.
In the present embodiment, with respect to the secondary transfer bias composed of the superimposed voltage, the toner on the surface of the intermediate transfer belt 31 is directed from the belt side toward the recording sheet surface side in the nip in the secondary transfer nip in one cycle. A time mainly intended for electrostatic movement is defined as a transfer side time Tt. That is, in one cycle T of the secondary transfer bias, the secondary transfer bias moves the toner image from the intermediate transfer belt 31 to the recording sheet from the time average value Vave (= average potential) (this embodiment). The time on the negative polarity side is defined as the transfer side time Tt. Further, a time different from the transfer side time Tt (remaining time in one cycle) is defined as a reverse transfer side time Tr. In other words, in one cycle T of the secondary transfer bias, the time during which the secondary transfer bias is on the reverse transfer side (on the opposite side to the transfer side, in this embodiment, on the positive polarity side) from the time average value Vave is reversed. This is defined as a copy time Tr. As shown in FIGS. 6, 7, and 8, the reverse transfer side time Tr is a reverse peak with respect to a time average value Vave (= average potential, which is a predetermined reference value) in one cycle. This is the time when the value is on the side of the value Vr. Further, the ratio of the reverse transfer side time Tr in one cycle is defined as the duty ratio of the secondary transfer bias composed of the superimposed voltage. That is, (T−Tt) / T × 100 [%] is defined as the duty.
In other words, the duty ratio refers to the time applied in the direction in which the toner of the toner image is transferred to the recording sheet P from the time average value Vave of the transfer bias, Tr, and the time applied in the opposite direction. A value of Tr / (Tr + Tt) × 100 [%] when Tt is indicated. In the case of the waveforms shown in FIGS. 6, 7 and 8, the duty ratio is 50 [%]. That is, the reverse peak duty ratio in the illustrated waveform is 50 [%].
In the present embodiment, the characteristic that the duty ratio exceeds 50 [%] is defined as a high duty ratio. The characteristic that the duty ratio is less than 50% is defined as a low duty ratio.
FIG. 8 is a graph for explaining the reverse peak duty ratio in the secondary transfer bias shown in FIG. In the figure, the center potential Vc is the center potential at the AC component (AC voltage) peak-to-peak value Vpp of the secondary transfer bias. The peak-to-peak value Vpp is a value from the reverse peak value Vr to the transfer peak value Vt. Further, the reverse transfer side time Tr passes through the reverse peak value Vr from the moment when the value of the secondary transfer bias starts to rise from the central potential Vc toward the reverse peak value Vr within one cycle (T) of the AC component. This is the time to return to the center potential Vc.
In this embodiment, the duty ratio is B, which is applied in the direction in which the toner of the toner image is transferred to the recording sheet P from the time average value Vave of the transfer bias, and is applied in the opposite direction. A value of Tr / (Tr + Tt) × 100 [%] when time is A is shown. That is, the transfer side time Tt is centered through the transfer peak value Vt from the moment when the value of the secondary transfer bias starts to rise from the center potential Vc toward the transfer peak value Vt within one cycle (T) of the AC component. This is the time to return to the potential Vc. Further, the reverse peak side duty ratio is a ratio of the reverse transfer side time Tr in the period T, and is 50 [%] in the case of the illustrated waveform. That is, the reverse peak duty ratio in the illustrated waveform is 50 [%].

  FIG. 9 is a graph for explaining the reverse peak side duty ratio in the secondary transfer bias shown in FIG. Also in the illustrated rectangular wave, the reverse peak side duty ratio as a ratio occupied by the reverse transfer side time Tr in one cycle (T) is 50 [%].

  In order to electrostatically move the toner from the surface side of the intermediate transfer belt 31 to the surface side of the recording sheet in the secondary transfer nip N using a secondary transfer bias with a reverse peak duty ratio = 50 [%] It is necessary to make the absolute value of the peak value Vt larger than the absolute value of the reverse peak value Vr. If the absolute value of the transfer peak value Vt becomes too large, a discharge is generated between the surface of the intermediate transfer belt 31 and the recording sheet (uneven sheet) P in the secondary transfer nip N. This discharge reversely charges the toner particles and remarkably inhibits secondary transfer of the toner particles, thereby generating a lot of white spots in the image and significantly degrading the image quality. Therefore, it is necessary to keep the absolute value of the transfer peak value Vt to a certain value.

  On the other hand, if the absolute value of the reverse peak value Vr is too small, a sufficient amount of toner cannot be transferred to the surface recesses of the recording sheet (uneven sheet) P. Specifically, if the absolute value of the reverse peak value Vr is too small, the toner particles once transferred from the belt surface to the surface concave portion of the recording sheet (concave sheet) in the secondary transfer nip N are transferred to the belt surface. It cannot be returned. As a result, the toner particles returned from the inside of the surface concave portion are hit against other toner particles or toner particles adhering to the belt surface on the surface of the intermediate transfer belt, and the adhesion force cannot be weakened. Since the number of toner particles transferred into the surface recesses of the recording sheet (concave sheet) cannot be increased by simply vibrating the toner particles, the amount of toner transferred into the surface recesses can be reduced. It will make it short.

  One method for increasing the reverse peak value Vr is to increase the peak-to-peak value Vpp of the AC component. However, if the peak-to-peak value Vpp is increased in order to eliminate the shortage of the reverse peak value Vr, the transfer peak value Vt is increased at the same time, so that it becomes easy to cause white spots due to discharge.

  Another method for increasing the reverse peak value Vr is to decrease the offset voltage Voff. However, if the offset voltage Voff is decreased, the average potential Vave is also decreased accordingly. Therefore, the toner cannot be electrostatically moved favorably from the belt surface side to the sheet surface side in the secondary transfer nip N. There is a risk of causing secondary transfer failure.

(First embodiment)
Therefore, in order to transfer a sufficient amount of toner to the concave portion of the surface of the concavo-convex sheet (a concavo-convex sheet having a larger concavo-convex than a smooth sheet described later) as the recording sheet P, the reverse peak duty ratio is 50%. The following is desirable. More desirably, it should be less than 50 [%]. This is because when the reverse peak side duty ratio is larger than 50 [%], white spots or secondary transfer defects due to discharge are likely to occur. Specifically, when the reverse peak duty ratio is made larger than 50 [%], the average potential (time average value) Vave is shifted to the reverse peak side by that amount, and the absolute value is reduced. It becomes easy to cause the next transfer defect. Then, in order to avoid the occurrence of the secondary transfer failure, if the peak-to-peak value Vpp is increased to increase the average potential Vave, the transfer peak value Vt is increased accordingly. White spots are easily generated. Therefore, it is desirable to set the reverse peak duty ratio to a value less than 50 [%].

FIG. 10 is a graph showing an example of a waveform of the secondary transfer bias in which the reverse peak duty ratio is set to 35 [%] smaller than 50 [%]. The reverse peak value Vr of the secondary transfer bias shown in FIG. 10 is the same as the reverse peak value Vr of the secondary transfer bias shown in FIG. Further, the transfer peak value Vt of the secondary transfer bias shown in FIG. 10 is the same as the transfer peak value Vt of the secondary transfer bias shown in FIG. The difference between the secondary transfer bias shown in FIG. 10 and the secondary transfer bias shown in FIG. 7 is only the reverse transfer side time Tr and the transfer side time Tt. In the secondary transfer bias shown in FIG. 7, the reverse transfer side time Tr and the transfer side time Tt are the same, whereas in the secondary transfer bias shown in FIG. 10, the reverse transfer side time Tr is the transfer side time. It is shorter than Tt. More specifically, the reverse transfer side time Tr of the secondary transfer bias shown in FIG. 7 is 50 [%] of the period T, whereas the reverse transfer side time of the secondary transfer bias shown in FIG. Tr is 35 [%] of one cycle (T). That is, the reverse peak side duty ratio of the secondary transfer bias shown in FIG. 10 is 35 [%].
A secondary transfer bias other than the rectangular wave as shown in FIG. 10 can be used. A secondary transfer bias having a trapezoidal waveform having a predetermined time for the voltage to shift from the reverse peak value Vr to the transfer peak value Vt and from the transfer peak value Vt to the reverse peak value Vr may be used. Further, a secondary transfer bias in which a part or the whole of the waveform is rounded may be used. A secondary transfer bias described below may be used other than a rectangular wave.

  In the secondary transfer bias with the reverse peak duty ratio = 50 [%] shown in FIG. 7, the offset voltage Voff and the average potential Vave have the same value as described above. On the other hand, in the secondary transfer bias having the reverse peak side duty ratio of 35 [%] shown in FIG. 10, the average potential Vave is larger than the offset voltage Voff. The peak-to-peak value Vpp is the same for the secondary transfer bias shown in FIG. 7 and the secondary transfer bias shown in FIG. That is, by setting the reverse peak side duty ratio to less than 50 [%], the peak to peak value Vpp, the transfer peak value Vt, and the reverse peak value Vr are compared with the case of setting the reverse peak side duty ratio to 50 [%]. The average potential Vave can be increased without changing. Therefore, compared to the case where the reverse peak duty ratio is 50% or more, the occurrence of secondary transfer failure and white spots can be suppressed.

  Therefore, in the printer according to the embodiment, when the toner image is secondarily transferred to the recording sheet (uneven sheet) P, the polarity is reversed within one cycle (T) as a secondary transfer bias, and The reverse peak side duty ratio is less than 50 [%]. In such a configuration, compared to a configuration using a secondary transfer bias having a reverse peak side duty of 50 [%] or more, it is possible to suppress the occurrence of a secondary transfer failure and white spots due to discharge. Hereinafter, the characteristic that the reverse peak duty ratio is less than 50% is referred to as a low duty ratio (low duty ratio). On the other hand, the characteristic that the reverse peak side duty ratio exceeds 50 [%] is called a high duty ratio (high duty ratio).

  In the secondary transfer bias with a low duty ratio, if the value of the reverse peak side duty ratio is set to be very low, the absolute value of the reverse peak value Vr becomes larger than the absolute value of the transfer peak value Vt. For example, FIG. 11 is a graph showing an example of a waveform in a secondary transfer bias having a low duty ratio with a reverse peak side duty ratio of 30 [%]. FIG. 12 is a graph showing an example of a waveform in the secondary transfer bias having a low duty ratio with a reverse peak side duty ratio of 30 [%]. In these secondary transfer biases, the average potential Vave is the same −4 [kV], and the peak-to-peak value Vpp is the same 12 [kV]. In the secondary transfer bias of FIG. 11 where the reverse peak duty ratio is 30 [%], the absolute value of the reverse peak value Vr (about 4 kV) is smaller than the absolute value of the transfer peak value Vt (about 8 kV). ing. On the other hand, in the secondary transfer bias of FIG. 12 where the reverse peak duty ratio is 10 [%], the absolute value of the reverse peak value Vr (about 7 kV) is larger than the absolute value of the transfer peak value Vt (about 5 kV). It is getting bigger.

In the secondary transfer bias of FIG. 11, the transfer peak value Vt is larger than the reverse peak value Vr (both are absolute values). Therefore, the value is transferred within one cycle (T) T of the secondary transfer bias. When the transfer side time Tt is set to Vt, a discharge is generated in the secondary transfer nip and white spots are easily caused. On the other hand, in the secondary transfer bias of FIG. 12, the reverse peak value is larger than the transfer peak value Vt (both are absolute values). Therefore, in one cycle (T) of the secondary transfer bias, a discharge is generated in the secondary transfer nip at the reverse transfer side time Tr in which the value is set to the reverse peak value Vr, and white spots are easily caused.
Comparing the absolute value (about 8 kV) of the transfer peak value Vt in the secondary transfer bias in FIG. 11 with the absolute value (about 7 kV) of the reverse peak value Vr in the secondary transfer bias in FIG. 12, the latter is smaller. . Further, when the transfer side time Tt in the secondary transfer bias in FIG. 12 is compared with the reverse transfer side time Tr in the secondary transfer bias in FIG. 12, the latter is shorter. That is, the secondary transfer bias of FIG. 12 has a smaller absolute value of the peak value that induces a white point and a shorter duration of the peak value than the secondary transfer bias of FIG. For this reason, the secondary transfer bias of FIG. 12 is less likely to cause white spots due to the discharge than the secondary transfer bias of FIG. Therefore, when the toner image is secondarily transferred onto the concavo-convex sheet using the secondary transfer bias having a low duty ratio, it is desirable that the value of the reverse peak side duty ratio is considerably reduced.
According to the experiments by the present inventors, the reverse peak side duty ratio is preferably set in the range of 8 [%] to 35 [%], and is set in the range of 8 [%] to 17 [%]. Even better results (suppressing the occurrence of white spots) were obtained. However, if the reverse peak side duty ratio is set to a considerably small value, the ratio of the reverse transfer side time Tr in one cycle (T) is considerably reduced. Then, there is a possibility that the toner particles in the surface recess of the transfer sheet P (concave sheet) cannot be returned to the belt surface within the reverse transfer side time Tr. Therefore, in this case, it is necessary to secure a sufficient reverse transfer side time Tr by making the frequency of the AC component relatively low and making one cycle (T) relatively long.

  Next, the present inventors have made a recording sheet P made of coated paper (smooth sheet) excellent in surface smoothness under conditions using a secondary transfer bias having a low duty ratio or a secondary transfer bias consisting of only a DC voltage. In addition, an experiment was conducted in which a halftone image having a lighter density than the solid image was secondarily transferred. Then, a remarkably insufficient image density due to the secondary transfer failure was caused in the halftone image.

  As a result of earnest research on the cause of such secondary transfer failure, the following was found. That is, a halftone image is not covered with toner as in the case of a solid image, and a toner adhering portion constituting a relatively small number of dot groups and a blank portion where no toner is adhering are formed. It is mixed in the image part. When the intermediate transfer belt 31 provided with the elastic layer 31b is used and a smooth sheet having excellent surface smoothness is used as the recording sheet P, the elastic layer 31b is replaced with a small number of dots in the halftone image in the secondary transfer nip N. It is deformed flexibly in accordance with the shape of the small-number dot toner block. Due to this deformation, not only the surface of the minority dot toner lump in the halftone image but also the side surface of the minority dot toner lump is wrapped with the elastic layer 31b. Then, a charge having a polarity opposite to the normal charge polarity is injected from the elastic layer 31b to each toner particle of the small-number-dot toner mass, thereby reducing the charge amount (Q / M) of the toner or reversely charging the toner. . As a result, it was found that the secondary transfer failure of the toner image was caused. In the case where an uneven sheet is used as the recording sheet P, the elastic layer 31b is deformed into an irregular shape according to the unevenness of the sheet surface. Almost nothing. For this reason, it is not likely to cause a secondary transfer failure even on the surface convex portion of the recording sheet (uneven sheet) P.

  Next, the present inventors have replaced the secondary transfer bias with a low duty ratio or only a DC voltage with a high duty ratio (reverse peak side duty ratio = 80%) shown in FIG. Were used, and an image was secondarily transferred to a smooth sheet as the recording sheet P. As the smooth sheet, an OK top coat (128 gsm) manufactured by Oji Paper Co., Ltd. was used (so-called coated paper, which is a smooth sheet). When a black halftone image (2by2) was secondarily transferred to a smooth sheet under the condition of a process linear velocity of 630 [mm / s] under an environment where the temperature / relative humidity was 27 ° C./80%, secondary transfer failure occurred. The black halftone image could be satisfactorily transferred to the smooth sheet without causing any trouble.

  The reason why the secondary transfer property of the halftone image on the smooth sheet can be improved by using the secondary transfer bias having a high duty ratio is considered to be as follows. That is, when the endless moving intermediate transfer belt 31 enters the secondary transfer nip N, charging of the belt portion that has entered the secondary transfer nip N is started by the secondary transfer bias. Then, when the amount of charge exceeds a certain threshold value, the injection of reverse charge into the minority dot toner mass in the halftone image starts. Charging of the belt portion that has entered the secondary transfer nip N occurs mainly within the transfer side time Tt. Therefore, as the transfer side time Tt increases, the amount of reverse charge injected into the minority dot toner mass increases. The secondary transfer bias with a high duty ratio has a shorter transfer-side time Tt than the secondary transfer bias with a low duty ratio. It is considered possible to suppress the occurrence of

  Another experiment conducted by the present inventors also revealed the following. That is, as shown in FIG. 14 to FIG. 17, the high duty ratio of 2 that does not reverse the polarity is higher than the secondary transfer bias of high duty that reverses the polarity within one period (T) as shown in FIG. 13. The use of the secondary transfer bias can improve the secondary transfer property for the smooth sheet.

  In the secondary transfer bias shown in FIG. 13, the voltage polarity is changed to the transfer side time Tt so that the direction of the electric field is reversed in the reverse transfer side time Tr in the direction of returning the toner from the sheet surface side to the belt surface side. The polarity is opposite to the polarity at. The secondary transfer bias and the secondary transfer bias that does not reverse the polarity as shown in FIGS. 14 to 17 have the same reverse peak duty ratio, and the secondary transfer within one cycle (T). A similar secondary transfer property is obtained by making the average potential (Vave), which is also the integrated value of the electric field, the same. Then, it is necessary to make the transfer peak value Vt at the secondary transfer bias in FIG. 13 larger than the transfer peak value Vt at the secondary transfer bias in FIGS. As a result, despite the fact that the reverse peak duty ratio and the average potential Vave are the same, the secondary transfer bias in FIG. 13 has fewer halftone images than the secondary transfer bias in FIGS. Increasing the amount of reverse charge injected into the dot toner mass. In other words, the secondary transfer bias in FIGS. 14 to 17 can reduce the amount of reverse charge injected into the small number of toner dots by making the transfer peak value Vt smaller than the secondary transfer bias in FIG. Since it is possible, the secondary transfer property can be further improved.

  14 to 17, the reverse peak side duty ratio is 80 [%], and the average potential Vave is −4 [kV]. However, the peak-to-peak potentials Vpp are different from each other. In order to achieve the same reverse peak duty ratio and average potential Vave by making the peak-to-peak potentials Vpp different from each other, the values of the offset voltages Voff are made different from each other. The peak-to-peak potential Vpp of the secondary transfer bias in FIGS. 14, 15, 16, and 17 is 12 [kV], 10 [kV], 8 [kV], and 6 [kV]. By setting the peak-to-peak potential Vpp as described above, the transfer peak value Vt of the secondary transfer bias is gradually decreased in the order of the figure numbers of FIGS. 14, 15, 16, and 17. On the other hand, the reverse peak value Vr of the secondary transfer bias is gradually increased in the order of FIG. 14, FIG. 15, FIG. 16, and FIG. Since the polarity of the reverse peak value Vr is the same as the polarity of the transfer peak value Vr, if attention is paid only to the polarity, the reverse charge is applied to the minority dot toner mass of the halftone image even during the reverse transfer side time Tr. There is a risk of injection. However, since any of the reverse peak values Vr in FIGS. 14 to 17 is a relatively low value, in reality, almost no reverse charge is injected into the minority dot toner mass within the reverse transfer side time Tr. Does not happen. Therefore, when the secondary transfer bias of FIG. 17 having the highest transfer peak value Vt at the transfer side time Tt that mainly causes the injection of reverse charges to the small dot toner mass is used, two images of a smooth sheet (particularly, a halftone image) are obtained. Next transferability can be improved most.

14 to 17, the secondary transfer bias consisting only of a DC voltage of −4 [kV], which is the same as the average potential Vave in FIGS. Exceeds threshold of charge injection start. For this reason, injection of reverse charges to a small number of dot clusters is always caused within one period (T), which causes a significant secondary transfer failure.
That is, in the case of forming an image on a smooth sheet, of the two peak values in the secondary transfer bias, the toner image is transferred from the intermediate transfer belt 31 side to the recording sheet P side serving as the nip forming member in the secondary transfer nip N. The secondary transfer power source 39 outputs a transfer bias in which the reverse peak side duty ratio, which is the duty ratio on the side of the peak value Vr opposite to the transfer peak value Vt that is strongly electrostatically moved, exceeds 50%. In general, an image printed by a user includes a photograph or the like, and often includes a light gray or light color image. When such a halftone image is transferred to a smooth sheet, a secondary transfer failure in the output image can be suppressed by using a secondary transfer bias having a high duty ratio.

)
In the image forming apparatus (printer), as the recording sheet P, a high smooth sheet and a low smooth sheet having a surface smoothness (surface smoothness) lower than that of the high smooth sheet are used. The controller 200 has a high smoothing mode (first mode) for transferring the toner image to the high smoothing sheet and a low smoothing mode (second mode) for transferring the toner image to the low smoothing sheet ( Remembered). The high smoothing mode and the low smoothing mode are mode information. Then, in the high smoothing mode, the control unit 200, among the two peak values in the secondary transfer bias, the transfer peak value in which the toner image is more strongly electrostatically moved from the image carrier side to the nip forming member side in the transfer nip. A transfer bias in which the reverse peak side duty ratio, which is the duty ratio on the side of the peak value (Vr) opposite to (Vt), is 50 [%] or more is output. In the smooth mode, the reverse peak side duty ratio is The high smoothing mode (high duty ratio) and the low smoothing mode (low duty ratio) are controlled while controlling the transfer power supply so as to output a transfer bias that is different from the high smoothing mode and is 50% or less. ) And the secondary transfer nip width W are switched so as to control the operation of the nip width varying device described later.
Specifically, in the high smoothing mode, the small nip width is set at a high duty ratio, and in the low smoothing mode, the large nip width is set at a low duty ratio. Then, the control unit 200 outputs a transfer bias with a reverse peak side duty ratio exceeding 50 [%]. In the low smoothing mode, the polarity is reversed within one cycle (T) and the reverse peak side duty ratio is 50 [%]. The secondary transfer power supply 39 may be controlled so as to output a transfer bias less than [%].

In this embodiment, the secondary transfer nip width W is controlled not only by changing the duty ratio, but also by controlling the operation of a nip width varying device, which will be described later, according to the smooth sheet and the concavo-convex sheet as the type of the transfer sheet P. Is variably controlled.
The control unit 200 controls the nip width varying device so as to increase the secondary transfer nip width W in the low duty ratio mode compared to in the high duty ratio mode. By increasing the transfer nip width W in the low duty ratio mode, the secondary transfer bias is applied while the intermediate transfer belt 31 and the recording sheet P are in close contact with each other, so that abnormal images due to discharge can be prevented. In addition, the toner can be sufficiently reciprocated between the intermediate transfer belt 31 and the recording sheet P in the secondary transfer nip N, whereby the transfer rate can be improved.
On the other hand, in the high duty ratio mode, if the secondary transfer nip width W is excessively widened, the toner transferred from the intermediate transfer belt 31 to the recording sheet P in the secondary transfer nip N passes through the secondary transfer nip N. In addition, there is a concern that the transfer rate is lowered by returning to the intermediate transfer belt side (reverse transfer). Therefore, in the present embodiment, the transfer nip width W is reduced in the high duty ratio mode compared to in the low duty ratio mode. A decrease in transfer rate can be minimized.
By adjusting the secondary transfer nip width W, it is possible to prevent the transfer rate from being lowered due to reverse transfer in the high duty ratio mode, while preventing the occurrence of abnormal images due to discharge in the low duty ratio mode. Thereby, generation | occurrence | production of an abnormal image can be suppressed in both modes, respectively.

Next, embodiments of a nip width varying device that changes the transfer nip width will be described sequentially.
(Embodiment 1 of Nip Width Variable Device)
A nip width varying device (nip width changing device) 60 shown in FIG. 18 moves a secondary transfer nip lining roller 36 that is at least one of a plurality of rotating members that stretch the sheet conveying belt 41. By doing so, the secondary transfer nip width W is changed. The image forming apparatus includes a roller unit holder 640 and a compression coil spring 643 as pressure members that press the sheet conveying belt 41 toward the intermediate transfer belt 31 at the secondary transfer nip. The nip width changing device 60 includes a drive motor. The transfer nip width W is changed by changing the pressure applied to the secondary transfer nip using 625, the eccentric cam 674, or the like.
The nip width varying device 60 changes the secondary transfer nip width W by changing the pressure applied to the secondary transfer nip N in the first mode and the second mode. Each of the secondary transfer nip backing roller 36 and the secondary transfer back roller 33 has a cored bar and an elastic layer provided on the cored bar. As the pressure applied to the secondary transfer nip increases, the amount of collapse of the elastic layers of the secondary transfer nip backing roller 36 and the secondary transfer back roller 33 increases, and the secondary transfer nip width W increases (expands). The secondary transfer nip width W is a length in the sheet conveying direction A orthogonal to the width direction of the recording sheet P, and correlates with the passage time of the recording sheet P. In a narrow sense, the secondary transfer nip width W is a width in which the secondary transfer nip backing roller 36 and the secondary transfer back roller 33 are crushed in the secondary transfer portion, but in a broad sense, the intermediate transfer belt 31 and the sheet conveyance. A pre-nip having a width in contact with the belt 41 is included.

  The secondary transfer nip backing roller 36 is applied with a pressing force toward the intermediate transfer belt 31 wound around the secondary transfer back roller 33. That is, the shaft 20 </ b> A that rotatably supports the secondary transfer nip lining roller 36 is supported by the roller unit holder 640. The roller unit holder 640 is supported at one end in the longitudinal direction by a support shaft 642 so as to be swingable on the apparatus fixing portion. A compression coil spring 643 is interposed between the other end side in the longitudinal direction of the roller unit holder 640 and the device fixing portion. The roller unit holder 640 is pressed around the support shaft 642 by the restoring force of the compression coil spring 643 so as to rotate clockwise in FIG. Accordingly, the secondary transfer nip backing roller 36 is pressed toward the intermediate transfer belt 31 to apply pressure to the secondary transfer nip N.

In the printer according to the embodiment, in such a configuration, the nip width for changing the pressure by the secondary transfer nip backing roller 36 applied to the secondary transfer nip N in the low duty ratio mode and the high duty ratio mode. A variable device 60 is provided. The nip width varying device 60 may be configured so that the pressure applied to the secondary transfer nip N is higher in the low duty ratio mode than in the high duty ratio mode, and roughly two methods can be assumed.
The first method uses the spring force of the compression coil spring 643 as a spring force that can obtain the required secondary transfer nip width W in the low duty ratio mode. This is a technique for suppressing the spring force. As shown in FIGS. 18 and 19, the nip width varying device 60 in this case contacts an upper portion 640a of the roller unit holder 640 and displaces the position thereof, and a drive for rotationally driving the eccentric cam 674. It can be constituted by a pressure changing means constituted by a drive motor 625 as means. The eccentric cam 674 is provided so as to rotate integrally with a drive shaft 677 that is rotated by a drive motor 625. The eccentric cam 674 has a cam surface having a maximum distance from the center of rotation to the top dead center 674a on the outer peripheral surface 674b. It is a drive motor 625 in the nip width varying device 60 that is connected to the control unit 200 shown in FIG.

In the embodiment, the drive motor 625 is connected to the control unit 200 via a signal line, and is configured to control the drive thereof. Specifically, the control unit 200 rotates the eccentric cam 674 in a direction to suppress the spring force of the compression coil spring 643 when in the high duty ratio mode, and when in the low duty ratio mode, as shown in FIG. The rotation of the drive motor 625 is controlled so that the eccentric cam 674 is rotated in a direction in which the suppression of the spring force of the coil spring 643 is released. The ROM of the control unit 200 has a low duty ratio mode in which the output waveform of the transfer bias from the secondary transfer power supply 39 is a low duty ratio (less than 50%) and a high duty ratio that is a high duty ratio (50% or more). Ratio mode is set. In the ROM of the control unit 200, the rotation direction and the rotation amount (rotation time) of the drive motor 625 are set in advance in association with the low duty ratio mode and the high duty ratio mode which are transfer modes. Here, in the high duty ratio mode, the secondary transfer nip width W is narrowed, and in the low duty ratio mode, the rotation direction and amount of rotation of the drive motor 625 are increased in a direction to greatly increase the secondary transfer nip width W. Is set.
18 and 19, an arrow a <b> 3 indicates a direction in which the spring force applied to the secondary transfer nip N is suppressed, and an arrow a <b> 4 indicates a direction in which the spring force applied to the secondary transfer nip N is released. That is, in FIG. 18 and FIG. 19, the suppression direction a3 of the spring force applied to the secondary transfer nip N is the direction in which the other end side of the roller unit holder 640 is pushed down. The release direction a4 is a direction in which the other end side of the roller unit holder 640 is lifted.

In this embodiment, the eccentric cam 674 stands by in the high duty ratio mode shown in FIG. 18 as a home position where the spring force is suppressed (pressed position). Is arranged so as to occupy the nip width increasing position shown in FIG.
For this reason, in this embodiment, when the low duty ratio mode is reached, the drive motor 625 is controlled by the control unit 200 so that the eccentric cam 674 cancels the suppression of the spring force of the compression coil spring 643 applied to the secondary transfer nip N. Since it is rotated, the pressure applied to the secondary transfer nip N by the secondary transfer nip backing roller 36 can be made stronger than in the high duty ratio mode, and the secondary transfer nip width W in the low duty ratio mode can be increased. Can be expanded (can be enlarged). In the high duty ratio mode, the eccentric cam 674 rotates and the other end side of the roller unit holder 640 is pushed down, so that the pressure applied to the secondary transfer nip N by the secondary transfer nip backing roller 36 is low. It is weaker than in transfer mode.
As described above, since the secondary transfer nip width W can be increased in the low duty ratio mode, the secondary transfer bias is applied in a state where the intermediate transfer belt 31 and the recording sheet P are in close contact with each other. Can be prevented. Further, by increasing the secondary transfer nip width W, the toner can sufficiently reciprocate between the intermediate transfer belt 31 and the recording sheet P in the secondary transfer nip N, thereby improving the transfer rate. be able to.

On the other hand, in the high duty ratio mode, if the secondary transfer nip width W is excessively widened, the toner transferred from the intermediate transfer belt 31 to the recording sheet P in the secondary transfer nip N passes through the secondary transfer nip N. In addition, there is a concern that the transfer rate is lowered by returning to the intermediate transfer belt side (reverse transfer). In the present embodiment, the secondary transfer nip width W is widened in the low duty ratio mode in which abnormal images due to discharge are likely to occur, so that a decrease in transfer rate can be minimized.
While the transfer rate decrease due to reverse transfer is more noticeable in the high duty ratio mode, the abnormal image due to discharge is more noticeable in the low duty ratio mode. It is preferable to adjust the width W.

  The second method is the reverse of the first method. The spring force of the compression coil spring 643 is a spring force that can obtain the secondary transfer nip width W required in the high duty ratio mode, and the secondary transfer in the low duty ratio mode. This is a technique for increasing the pressure applied to the nip N. In this case, the nip width varying device (nip width changing device) 60A includes, as shown in FIGS. 20 and 21, an eccentric cam 674 that contacts the lower portion 640b of the roller unit holder 640 and displaces the position, and an eccentric cam. 674 and a driving motor 625 as a driving means for rotationally driving the 674. As in the first method, the eccentric cam 674 is provided so as to rotate integrally with a drive shaft 677 that is rotated by a drive motor 625. The eccentric cam 674 has a cam surface having a maximum distance from the center of rotation to the top dead center 674a on the outer peripheral surface 674b.

Also in the present embodiment, the drive motor 625 is connected to the control unit 200 via a signal line, and is configured to control its drive. Specifically, when the high duty ratio mode is reached, the control unit 200 rotates the eccentric cam 664 in the direction in which the compression coil spring 643 is applied with a set value without suppressing the spring force of the compression coil spring 643, as shown in FIG. In the low duty ratio mode, as shown in FIG. 21, the rotation of the drive motor 625 is controlled so as to rotate the eccentric cam 674 in a direction in which an auxiliary force is applied to the spring force of the compression coil spring 643.
20 and 21, an arrow a5 indicates an increasing direction of the pressure applied to the secondary transfer nip N, and an arrow a6 indicates an increasing release direction of the pressure applied to the secondary transfer nip N. That is, in FIG. 20 and FIG. 21, the increasing direction a5 of the pressure applied to the secondary transfer nip N is the direction in which the other end side of the roller unit holder 640 is pushed up by the eccentric cam 674, and the pressure applied to the secondary transfer nip N The increase release direction a6 is a direction in which the other end side of the roller unit holder 640 is lowered.

In this embodiment, the eccentric cam 674 stands by in the high duty ratio mode shown in FIG. 20 and uses the position where the pressure applied to the secondary transfer nip N does not increase (lowered position) as the home position, and in the low duty ratio mode. In this case, the upper fulcrum 674a is moved toward the lower surface 640b of the roller unit holder 640, and is arranged so as to occupy the nip width increasing position shown in FIG.
For this reason, in this embodiment, when the duty ratio is low, the eccentric cam 674 is rotated by the drive motor 625 in a direction that raises the roller unit holder 640 and increases the pressure applied to the secondary transfer nip N.
According to such a configuration, the pressure applied to the secondary transfer nip N by the secondary transfer nip backing roller 36 can be made stronger than in the high duty ratio mode, and the secondary transfer nip width in the low duty ratio mode. W can be expanded. As a result, since the secondary transfer nip width W can be increased in the low duty ratio mode, the toner can sufficiently reciprocate between the intermediate transfer belt 31 and the recording sheet P in the secondary transfer nip. Thereby, the transfer rate can be improved.

(Embodiment 2 of nip width varying device)
Next, a second embodiment of the nip width varying device (nip width changing device) will be described.
In the second embodiment of the nip width varying device shown in FIG. 22, the secondary transfer nip width W is changed by changing the pressure applied to the secondary transfer nip N in the first mode and the second mode.

  In the present embodiment, the secondary transfer nip backing roller 36 is applied with a pressing force toward the intermediate transfer belt 31 that is wound around the secondary transfer back roller 33 as shown in FIG. That is, the shaft 20 </ b> A that rotatably supports the secondary transfer nip backing roller 36 is supported by the roller unit holder 650. The roller unit holder 650 is supported at one end in the longitudinal direction by a support shaft 652 so as to be swingable on the apparatus fixing portion. A compression coil spring 653 is interposed between the lower portion 650b on the other end side in the longitudinal direction of the roller unit holding body 650 and the device fixing portion. The roller unit holding body 650 is pressed by the restoring force of the compression coil spring 653 so as to rotate counterclockwise around the support shaft 652 in FIG. Accordingly, the secondary transfer nip backing roller 36 is pressed toward the intermediate transfer belt 31 to apply pressure to the secondary transfer nip N.

In the present embodiment, in such a configuration, the nip width can be changed to change the pressure by the secondary transfer nip backing roller 36 applied to the secondary transfer nip N in the low duty ratio mode and the high duty ratio mode. A device (nip width changing device) 60B is provided.
The nip width varying device 60B includes idling rollers 685 and 685 provided on both sides in the axial direction of the secondary transfer nip backing roller 36, and cams 684 and 684 provided on both sides in the axial direction of the secondary transfer back roller 33. The pressurizing force changing means can be constituted by a driving motor 635 as a driving means for driving each cam 684 to rotate. 22 and 23, the sheet conveying belt 41 is omitted, and only one side of the cam 684 and the idling roller 685 is shown.
Each cam 684 has a part 684a formed on the outer peripheral surface 684b projecting to the outer diameter side of the outer peripheral surface 33a of the secondary transfer back surface roller 33, and can rotate around the shaft 24A so as to have the same phase. It is supported by. These cams 684 are rotationally driven by the drive motor 635 while maintaining the same phase. Each idling roller 685 has a larger diameter than the secondary transfer nip lining roller 36 and is always in pressure contact with the outer peripheral surface 684a of each cam 684 by the spring force of the compression coil spring 653.

  In the present embodiment, the drive motor 635 is connected to the control unit 200 via a signal line, and is configured to control the drive thereof. Specifically, when the high duty ratio mode is set as the transfer mode, the control unit 200 rotates the cam 684 in a direction to suppress the spring force of the compression coil spring 653, and when in the low duty ratio mode, as shown in FIG. Then, the rotation of the drive motor 635 is controlled so that the eccentric cam 684 is rotated in the direction in which the suppression of the spring force of the compression coil spring 653 is released. That is, in the high duty ratio mode, the control unit 200 rotates the cam 684 to a position where the protruding portion 684a faces the idling roller 685, and as shown in FIG. Is pushed down to narrow the secondary transfer nip width, and in the low duty ratio mode, the cam 684 is rotated so that the protrusion 684a moves away from the idling roller 685, and the secondary transfer nip backing roller 36 is The drive motor 635 is controlled so that the secondary transfer nip backing roller 36 is pushed up by the spring force of the compression coil spring 653 to widen the secondary transfer nip width W.

  22 and 23, the arrow a7 indicates the direction of suppression of the spring force applied to the secondary transfer nip N, and the arrow a8 indicates the direction of release of suppression of the spring force applied to the secondary transfer nip N. That is, in FIG. 22 and FIG. 23, the spring force suppression direction a7 applied to the secondary transfer nip N is a direction in which the other end side of the roller unit holder 650 is pushed down, and the spring force applied to the secondary transfer nip N is suppressed. The release direction a8 is a direction in which the other end side of the roller unit holder 650 is lifted.

In this embodiment, the cam 684 stands by at the position of the high duty ratio mode shown in FIG. 22 and sets the position where the spring force is suppressed (pressed down position) as the home position. It is arranged so as to occupy the nip width increasing position shown in FIG.
For this reason, in this embodiment, when the low duty ratio mode is set, the cam 684 is rotated by the drive motor 635 in a direction to release the suppression of the spring force of the compression coil spring 653 applied to the secondary transfer nip N. Therefore, the pressure applied to the secondary transfer nip N by the secondary transfer nip lining roller 36 can be made stronger than in the high duty ratio mode, and the secondary transfer nip width W in the low duty ratio mode can be widened. . In the high duty ratio mode, the cam 684 rotates and the other end side of the roller unit holder 650 is pushed down, so that the pressure applied to the secondary transfer nip N by the secondary transfer nip backing roller 36 is reduced to the low duty ratio mode. It is weakened compared to the time.

  As described above, by increasing the secondary transfer nip width W in the low duty ratio mode, the toner can sufficiently reciprocate between the intermediate transfer belt 31 and the recording sheet P in the secondary transfer nip N. As a result, the transfer rate can be improved.

On the other hand, in the high duty ratio mode, if the secondary transfer nip width W is excessively widened, the toner transferred from the intermediate transfer belt 31 to the recording sheet P in the secondary transfer nip N passes through the secondary transfer nip N. In addition, there is a concern that the transfer rate is lowered by returning to the intermediate transfer belt (reverse transfer).
However, in this embodiment, the transfer rate decrease due to reverse transfer is more noticeable in the high duty ratio mode, while the abnormal image due to discharge is more noticeable in the low duty ratio mode. In order to avoid this, it is preferable to adjust the secondary transfer nip width W. In the present embodiment, the cam 684 is disposed on the secondary transfer back surface roller 33 and the idling roller 685 is disposed on the secondary transfer nip backing roller 36 side. However, these may be reversed.

(Embodiment 3 of Nip Width Variable Device)
In the third embodiment of the nip width varying device (nip width changing device), the hardness of the secondary transfer nip backing roller 36 serving as a transfer member is switched between the first mode and the second mode to widen the secondary transfer nip width W. To change. The secondary transfer nip N is an area sandwiched between the secondary transfer back surface roller 33 and the secondary transfer nip backing roller 36. This region also includes the prenip.

As a method of switching the hardness of the secondary transfer nip backing roller 36, the secondary transfer nip backing roller 36 and the secondary transfer nip backing roller 36 'having different hardnesses are configured to be detachable from one shaft portion 20A. It is possible to manually change according to the high duty ratio mode and the low duty ratio mode which are transfer modes.
Alternatively, as shown in FIGS. 24 and 25, two members, a secondary transfer nip backing roller 36 and a secondary transfer nip backing roller 36 ′ having different hardnesses, are arranged in the sheet conveying unit 38, and these secondary transfer. The secondary transfer nip width W is changed by a nip width varying device 60C (nip width changing device) for switching the position of the nip backing roller 36 and the secondary transfer nip backing roller 36 'using the switching means 600. Also good. In the present embodiment, the secondary transfer nip backing roller 36 ′ is made of a material whose hardness is lower than that of the material used for the secondary transfer nip backing roller 36 so that the surface of the secondary transfer nip backing roller 36 is more easily crushed than the secondary transfer nip backing roller 36. It is formed with. 24 and 25, the sheet conveying belt 41 is omitted.

The nip width varying device 60C in the present embodiment swings the support frame 601 and the support frame 601 that rotatably supports the secondary transfer nip backing roller 36 and the secondary transfer nip backing roller 36 by the support shafts 20A and 20B. There is provided switching means 600 composed of a drive motor 605 as drive means. The support frame 601 is supported so as to be swingable about a rotation support shaft 602. Reference numeral 606 is a stopper for regulating the position of the support frame 601 in order to hold the secondary transfer nip backing roller 36 at the nip forming position shown in FIG.
In the present embodiment, the drive motor 605 is connected to the control unit 200 via a signal line, and is configured to control its drive. Specifically, when the high duty ratio mode is entered, the control means 200, as shown in FIG. 24, moves the surface of the intermediate transfer belt 31 at a position where the surface 30a of the secondary transfer nip backing roller 36 faces the secondary transfer back roller 33. When it is moved to a position where it is in pressure contact with the front surface 21a and in the low duty ratio mode, the surface 30'a of the secondary transfer nip backing roller 36 'faces the secondary transfer back roller 33 as shown in FIG. Logic for controlling the rotation of the drive motor 605 so as to move to the position where the intermediate transfer belt 31 is pressed against the front surface 31a at the position is stored in advance in the ROM of the control unit 200.

In this embodiment, the nip forming position where the secondary transfer nip backing roller 36 stands by in the high duty ratio mode shown in FIG. 24 is set as the home position, and in the low duty ratio mode, as shown in FIG. The secondary transfer nip backing roller 36 ′ having a lower hardness (softer) than the backing roller 36 moves to the nip forming position.
For this reason, in this embodiment, when the low duty ratio mode is reached, the soft secondary transfer nip lining roller 36 'is moved to the nip forming position by the nip width varying device 60C, and the secondary transfer nip lining roller 36 is moved. In order to retract from the nip forming position, the secondary transfer nip width W can be widened in the low duty ratio mode.
That is, by changing the positions of the secondary transfer nip backing rollers 36 and 36 ′ having different hardnesses, the secondary transfer nip that is the length of the secondary transfer nip N in the secondary transfer nip N in the sheet conveying direction A is obtained. It becomes possible to adjust W. As a result, since the secondary transfer nip width W can be increased in the low duty ratio mode, the toner can sufficiently reciprocate between the intermediate transfer belt 31 and the recording sheet P in the secondary transfer nip N. Thereby, the transfer rate can be improved.

  On the other hand, in the high duty ratio mode, if the secondary transfer nip width W is excessively widened, the intermediate transfer is performed while the toner transferred from the intermediate transfer belt 31 to the recording sheet P in the secondary transfer nip N passes through the transfer nip. There is a concern that the belt is returned to the belt side (reverse transfer) and the transfer rate is lowered. In the present embodiment, in the high duty ratio transfer mode, the transfer rate decrease due to reverse transfer is more remarkable, whereas in the low duty ratio transfer mode, abnormal images due to discharge are more prominent, and thus abnormalities that become noticeable in each mode. In order to avoid an image, it is preferable to adjust the width W of the secondary transfer nip.

Next, the control content by the control unit 200 will be described.
FIG. 26 is a block diagram illustrating a configuration of a control system having an input operation unit 51 including a recording sheet selection unit. As shown in FIG. 26, the input operation unit 501 has a smooth paper button 501a and a concave / convex paper button 501b as a recording sheet selection unit. In the printer according to the embodiment, an instruction for the user to perform the following operation is described in the instruction manual. That is, when a high smooth sheet (smooth sheet) having excellent surface smoothness such as coated paper is set as the recording sheet P in the paper feed cassette (100 in FIG. 1), the smooth paper button 501a is pressed. . On the other hand, when a low smooth sheet (uneven sheet) having poor surface smoothness such as plain paper or Japanese paper is set as the recording sheet P in the sheet cassette, the uneven sheet button 501b is pressed. That is, the input operation unit 501 functions as an information acquisition unit that can acquire the following type information. That is, at least whether the recording sheet P to be a secondary transfer target of the toner image is a high smooth sheet having excellent surface smoothness or a low smooth sheet having a surface smoothness inferior to that of the high smooth sheet. It is information that can be done.
The input operation unit 501 selects whether the recording sheet P is a high smooth sheet or a low smooth sheet, and inputs the selected type information. A signal line is connected to the input side of the control unit 200. Connected through. The input operation unit 501 includes a smooth paper button 501a for selecting a high smooth sheet and a concave / convex paper button 501b for selecting concave / convex paper that is a low smooth sheet. When the smooth paper button 501a is operated, type information that is a high smooth sheet is output as type information, and when the uneven paper button 501b is operated, type information that is uneven paper is output as type information.

The control unit 200 includes a mode determination unit 206 that determines whether the mode is the high smoothing mode or the low smoothing mode based on the type information input by the input operation unit 501. In the ROM of the control unit 200, a high smoothing mode for transferring the toner image to the high smoothing sheet and a low smoothing mode for transferring the toner image to the low smoothing sheet (uneven sheet) are preset. In the present embodiment, the high smoothing mode is a mode for applying a high duty ratio and is in agreement with the high duty ratio mode. The low smooth sheet (uneven sheet) is a mode in which a low duty ratio is applied, and is in agreement with the low duty ratio mode.
That is, the control unit 200 performs the high-smooth mode for secondary transfer of the toner image to the high smooth sheet and the secondary transfer of the toner image to the low smooth sheet based on the information acquisition result by the input operation unit 501. The transfer mode is switched between the low smoothing mode. Specifically, when the smooth paper button 501a is pressed, the transfer mode is set to the high smooth mode. In the high smoothing mode, a secondary transfer power supply 39 is used to apply a secondary transfer bias with a high duty ratio in order to suppress injection of reverse charges into a small number of toner dots when a halftone image is secondarily transferred to a high smoothness sheet. Output. The secondary transfer bias has a characteristic that the polarity is negative and constant (not inverted), and the reverse peak side duty ratio is in the range of 70 [%] to 90 [%].

  On the other hand, when the uneven paper button 501b is pressed, the control unit 200 sets the transfer mode to the low smoothing mode. In the low smoothing mode, a secondary transfer power supply 39 outputs a secondary transfer bias having a low duty ratio in order to secondary transfer a sufficient amount of toner into the surface recesses of the uneven sheet. The secondary transfer bias has the following characteristics. That is, the polarity is reversed within one period (T), the polarity of the average potential Vave and the polarity of the transfer peak value Vt are negative polarities that make the direction of the electric field the transfer direction, and the polarity of the reverse peak value Vr is the direction of the electric field. Is a positive polarity that reverses the direction of transfer. In addition, the reverse peak duty ratio is in the range of 8 [%] to 17 [%].

The control unit 200 switches the secondary transfer nip width W based on the information acquisition result by the input operation unit 501. When the concavo-convex paper button 501b is pressed, the secondary transfer nip width W is narrowed by any of the nip width variable devices described above.
In this embodiment, the secondary transfer nip width W is adjusted only when the concave / convex paper button 501b is pressed. Conversely, when the smooth paper button 501a is pressed, the secondary transfer nip width W is normally set. You may control so that it may become narrower than a width | variety. Alternatively, any one of the above nip width variable devices may be configured to have a nip width that is narrower than the normal nip width when the smooth paper button 501a is pressed.
In other words, in the embodiment, the nip width varying devices 60, 60A, 60B, and 60C described above decrease (narrow) the secondary transfer nip width W as the duty ratio increases, and decrease the secondary transfer nip as the duty ratio decreases. The operation of the controller 200 is controlled so as to increase (widen) the width W. That is, the drive motor 625 of each nip width varying device is configured so that the secondary transfer nip width W is reduced (narrow) in the high duty ratio mode, and is increased (widened) in the low duty ratio mode. , 635, 605 are controlled.

In the high smoothing mode, the control unit 200 electrostatically moves the toner image more strongly from the intermediate transfer belt 31 side to the sheet conveying belt 41 side in the secondary transfer nip among the two peak values in the secondary transfer bias. The secondary transfer bias is output so that the reverse peak side duty ratio, which is the duty ratio on the side of the peak value Vr opposite to the transfer peak value Vt, is 50 [%] or more (in the high duty ratio mode). The secondary transfer power supply 39 is controlled to output a secondary transfer bias having a reverse peak side duty ratio different from that in the high smoothing mode and 50% or less (in the low duty ratio mode). . In this embodiment, the control unit 200 further controls each nip so that the secondary transfer nip width W is switched between the high smoothing mode (in the high duty ratio mode) and the low smoothing mode (in the low duty ratio mode). Controls the operation of the variable width device.
That is, the control unit 200 is configured so that the secondary transfer nip width W is smaller (narrower) in the high smoothing mode (in the high duty ratio mode) than in the low smoothing mode (in the low duty ratio mode). The operation of the nip width varying devices 60, 60A, 60B, 60C is controlled.

  In such a configuration, when a low smooth sheet (uneven sheet) such as plain paper or Japanese paper is used as the recording sheet P, a secondary transfer bias with a low duty ratio is output from the secondary transfer power supply 39, so that There are the following effects. That is, in the secondary transfer nip, a sufficient amount of toner is transferred into the surface recess of the recording sheet P by favorably reciprocating the toner particles between the belt surface and the surface recess of the recording sheet P. Further, it is possible to suppress the occurrence of uneven image density that is uneven on the surface. Further, by setting the reverse peak side duty ratio to a low duty ratio, generation of white spots due to discharge can be suppressed.

  On the other hand, when a highly smooth sheet (smooth sheet) such as coated paper is used as the recording sheet P, a secondary transfer bias having a high duty ratio is output from the secondary transfer power supply 39, so that the following operational effects are obtained. Play. That is, by setting the reverse peak side duty ratio to a high duty ratio, injection of reverse charges to the minority dot toner group of the halftone image is suppressed, and the secondary transfer property of the halftone image to the smooth sheet is improved. Thereby, the occurrence of insufficient image density of the halftone image can be suppressed, and the occurrence of secondary transfer failure in the output image can be suppressed.

  Although an example in which the secondary transfer bias having a high duty ratio is used in the high smoothing mode and the secondary transfer bias having a low duty ratio is used in the low smoothing mode has been described, the following may be used. That is, a secondary transfer bias having a reverse peak side duty ratio of 50% in the high smoothing mode may be used, while a secondary transfer bias having a low duty ratio in the low smoothing mode may be used. Alternatively, a secondary transfer bias having a high duty ratio in the high smoothing mode and a secondary transfer bias having a reverse peak side duty ratio of 50% in the low smoothing mode may be used.

[Variation 1 of the first embodiment]
The printer according to the first modification of the first embodiment does not include the smooth paper button 501a and the uneven paper button 501b in the input operation unit 501. And it is not the specification into which the information of the surface smoothness of the recording sheet P is input by the user. Instead, a sheet type detection sensor 502 is provided as a smoothness detecting means for detecting the surface smoothness (unevenness) of the recording sheet P. The sheet type detection sensor 502 uses a well-known reflective optical sensor that irradiates the recording sheet P with detection light and detects the type (unevenness degree) of the recording sheet P from the reflectance.
FIG. 27 is a configuration diagram illustrating a paper feed path of a printer according to the first modification. The paper feed path guides the recording sheet P sandwiched between the first guide plate 503 and the second guide plate 504 to the registration nip of the registration roller pair 101. The first guide plate 503 is provided with a through hole, and the smoothness detection sensor 502 is fitted into the through hole. The smoothness detection sensor 502 irradiates the light emitted from the light emitting element toward the recording sheet P in the paper feed path, and receives the specularly reflected light regularly reflected on the surface of the recording sheet P by the light receiving element. The amount of specular reflection obtained on the surface of a smooth sheet such as coated paper is greater than the amount of specular reflection obtained on the surface of an uneven sheet such as Japanese paper.

  The smoothness detection sensor 502 is electrically connected to the input side of the control unit 200 via a signal line. The control unit 200 calibrates the smoothness detection sensor 502 when the apparatus is started immediately after the main power of the printer is turned on. Specifically, in a state where the light emitting element is turned on and the light from the light emitting element is reflected on the surface of the white second guide plate 504, the light emission amount (supply voltage) of the light emitting element is obtained so as to obtain a predetermined regular reflection light amount. ). The supply voltage value at this time is stored in the storage circuit, and thereafter, when the regularity detection light amount on the surface of the recording sheet P is detected by the smoothness detection sensor 502, the same value as the supply voltage value stored in the storage circuit. Is supplied to the light emitting element.

  When the print job is started, the recording sheet P sent out from the paper feed cassette 100 at a predetermined timing is abutted against the registration nip of the registration roller pair 101 that is not driven for skew correction, and is temporarily conveyed. Stopped. At this time, it faces the smoothness detection sensor 502 in the paper feed path. In this state, the control unit 200 detects the amount of regular reflection light obtained on the sheet surface by the smoothness detection sensor 502. When the detection result exceeds a predetermined threshold, the mode determination unit 206 determines that the recording sheet P is a smooth sheet, and performs the above-described high smoothing mode. On the other hand, when the amount of specular reflection does not exceed the predetermined threshold, the mode determination unit 206 determines that the recording sheet P is an uneven sheet, and the above-described low smoothing mode is performed. That is, the mode determination unit 206 also has a function of determining the high smoothing mode (in the high duty ratio mode) or the low smoothing mode (in the low duty ratio mode) based on the detection result of the sheet type detection sensor 502.

  In such a configuration, whether the recording sheet P conveyed to the secondary transfer nip N is a smooth sheet (highly smooth sheet) or a concavo-convex sheet (low smooth sheet) is not determined by a user operation. It can be acquired automatically to improve user operability.

[Modification 2 of the first embodiment]
FIG. 28 is a block diagram illustrating an electric circuit of the input operation unit 501 of the printer according to the second modification of the first embodiment. Unlike the embodiment, the input operation unit 501 does not have a smooth paper button or an uneven paper button. Instead, it has a menu key 501c, an up key 501d, a down key 501e, an enter key 501f, a display 501g, and the like.

  When the menu key 501c is pressed by the user, the control unit 200 displays a menu screen on the display 501g. The user operates the up key 501d or the down key 501e to select the menu by pressing the enter key 501f with the cursor positioned on the desired menu among a plurality of menus displayed on the menu screen. be able to. When the “sheet type input” menu is selected by a user key operation, the control unit 200 displays a sheet brand list on the display 501g. The user can select the same brand as the recording sheet set in the paper feed cassette 100 among a plurality of brands displayed in the brand list by operating the up key 501d and the down key 501e. Since the brand and the surface smoothness of the brand on the recording sheet P have a one-to-one relationship, the brand can function as information indicating the surface smoothness. The menu key 501c, the up key 501d, the down key 501e, and the enter key 501f constitute a brand input unit.

The control unit 200 stores a data table in which a brand is associated with a numerical value of a reverse peak duty ratio in a ROM that functions as a data storage circuit. As for the numerical value, a value of a high duty ratio is set for the brand of the smooth sheet, while a value of a low duty ratio is set for the brand of the uneven sheet. Furthermore, as for the brand of the concavo-convex sheet, the value of the reverse peak side duty ratio is smaller as the brand has a higher degree of ruggedness on the sheet surface.
When a brand is selected by the user's menu operation, the control unit 200 specifies the numerical value of the reverse peak side duty ratio corresponding to the brand from the data table. Then, the result is transmitted to the control unit 200. When the numerical value of the reverse peak side duty ratio is sent, the control unit 200 causes the secondary transfer power supply 39 to output a secondary transfer bias having the same reverse peak side duty ratio as the numerical value in subsequent printing. Thereby, when the brand of the smooth sheet is selected, the high smoothing mode is performed, while when the brand of the uneven sheet is selected, the low smoothing mode is performed.
That is, on the input side to the control unit 200, an up key 501d, a down key 501e, and the like are connected via a signal line as a brand input unit for inputting brand information of the recording sheet P. Then, the control unit 200 outputs a transfer bias having a lower reverse peak duty ratio in the low smoothing mode as the brand information input from the upper key 501d and the lower key 501e becomes a brand having a higher degree of unevenness on the sheet surface. A function of controlling the secondary transfer power supply 39 is provided.

In such a configuration, in the low smoothing mode, the secondary transfer efficiency of the halftone image with respect to the surface convex portion of the concavo-convex sheet can be improved, or the toner with respect to the surface concave portion can be improved as compared with the case where the value of the reverse peak side duty ratio is constant. The amount of transfer can be increased. Specifically, in the concavo-convex sheet, as the degree of the concavo-convex surface of the sheet decreases, the area of the surface convex portion increases, and reverse charge injection to a small number of toner dots of a halftone image easily occurs in the surface convex portion. . On the other hand, as the degree of the sheet surface unevenness increases, the size and depth of the surface recesses increase, and the transfer of toner to the surface recesses easily occurs. Therefore, the value of the reverse peak side duty ratio is decreased as the degree of unevenness of the sheet surface increases. As a result, a sufficient amount of toner can be transferred to the recesses on the surface even if the unevenness of the sheet surface is relatively large. In addition, the halftone image portion can be satisfactorily transferred to the surface convex portion even if the surface unevenness of the sheet is relatively small.
In addition, as an index indicating the degree of unevenness of the sheet surface, it is possible to use the maximum unevenness drop. Moreover, “SURFCOM 1400D” manufactured by Tokyo Seimitsu Co., Ltd. can be exemplified as a commercial machine of a measuring apparatus capable of measuring the maximum unevenness drop. With this measuring apparatus, five locations to be examined regions are randomly selected from the entire surface based on an image obtained by photographing the recording sheet surface with a microscope. For each location, the maximum section height Pt (JIS B 0601: 2001) of the section curve is measured under the condition of an evaluation length of 20 [mm] and a reference length of 20 [mm]. Then, among the obtained five maximum cross-sectional heights Pt, the top three average values are obtained. The above processing is performed on each of the leading end portion, the central portion, and the trailing end portion of the recording sheet P, and a further average of each average value is obtained as the maximum unevenness drop. The recording sheet P whose maximum unevenness drop (= specific information) is, for example, 50 [μm] or more is defined as an uneven sheet (low smooth sheet), and the recording sheet P less than 50 [μm] is a smooth sheet (high smooth sheet). A surface uneven sheet may be used.

  As described above, the image forming apparatus (printer) according to this embodiment includes the intermediate transfer belt 31, the secondary transfer belt 41 that forms the secondary transfer nip between the intermediate transfer belt 31, and the secondary transfer nip. A nip width changing device 60 for changing the width W of the toner image, and a secondary transfer power source for outputting a secondary transfer bias including an AC component for transferring the toner image on the intermediate transfer belt 31 to the recording sheet P at the secondary transfer nip. 39. Further, the first mode in which the duty ratio of the secondary transfer bias is the first duty ratio and the width of the secondary transfer bias is the first width, and the duty ratio of the secondary transfer bias is greater than the first duty ratio. And a control unit 200 that switches the second mode in which the secondary duty nip is a second width wider than the first width according to the type of the recording sheet. The control unit 200 executes the first mode when the recording sheet P is a smooth sheet, and executes the second mode when the recording sheet P is a concavo-convex sheet having larger concavo-convexity than the smooth sheet. Here, the first duty ratio is a high duty ratio higher than 50 [%], and the second duty ratio is a low duty ratio lower than 50 [%]. According to the image forming apparatus of the present embodiment, it is possible to improve the transfer rate to the concavo-convex sheet and to suppress the occurrence of abnormal images due to discharge on the smooth sheet. A sufficient amount of toner can be transferred onto the recording sheet P while preventing transfer failure of the toner image on the recording sheet P.

[Second Embodiment]
As shown in FIG. 29, the image forming apparatus according to the present embodiment includes a halftone image priority mode that prioritizes the image quality of a halftone image over the image quality of a solid image among the images included in the toner image, and a halftone. A mode selection unit 507 is provided for selecting a mode from a solid image priority mode that prioritizes the image quality of the solid image over the image quality of the image.
The control unit 200 executes the first mode (high duty ratio mode) when the mode selected by the mode selection unit 507 is the halftone image priority mode, and the mode selected by the mode selection unit 507 is a solid image priority. If the mode is selected, the second mode (low duty ratio mode) is executed. Here, as in the first embodiment, the duty ratio in the second mode is lower than the duty ratio in the first mode, and the width W of the secondary transfer nip in the second mode is the width of the secondary transfer nip in the first mode. Wider than. The method for controlling the duty ratio and the method for changing the secondary transfer nip width W are the same as in the first embodiment.
In the present embodiment, the duty ratio and the transfer nip width are changed according to whether the mode selected by the mode selection unit 507 is the halftone image priority mode or the solid image priority mode.
The present inventors conducted an experiment to transfer an image to plain paper (for example, Hammer Mill color copy digital). As a result, it was found that a transfer failure due to reverse charge injection occurred in the halftone image portion, while a transfer failure called blurring occurred in the solid image portion. The phenomenon of transfer failure in a halftone image is the same as that described in the first embodiment. The blur in the solid image is the density unevenness of the image that occurs following small irregularities on the recording sheet. This density unevenness is considered to be caused by a difference in strength of the transfer electric field between the concave and convex portions on the surface of the recording sheet. In order to suppress these transfer defects, in the present embodiment, the first mode and the second mode are switched as follows.

  A mode selection unit 507 is connected to the input side of the control unit 200. The mode selection unit 507 selects a halftone image priority mode that prioritizes the image quality of the halftone image over the image quality of the solid image among the images included in the toner image, and the image quality of the solid image over the image quality of the halftone image. This is an operation unit for the user to select a mode from among the priority solid image priority modes. The mode selection unit 507 includes a halftone image priority button 507a that prioritizes the image quality of the halftone image, and a solid image priority button 507b that prioritizes the image quality of the solid image. A mode determination unit (determination unit) 206 of the control unit 200 determines whether the mode type selected by the user in the mode selection unit 507 is the halftone image priority mode or the solid image priority mode.

The ROM of the control unit 200 stores the set values of the secondary transfer bias and the transfer nip width when the halftone image priority mode is selected, and the secondary transfer bias and the transfer nip when the solid image priority mode is selected. The set value of the width is stored (set) in advance.
When the halftone image priority mode is selected, the control unit 200 outputs the secondary transfer bias having a high duty ratio exceeding 50%, and when the solid image priority mode is selected, the control unit 200 outputs the duty ratio. The secondary transfer power supply 39 is controlled so as to output a secondary transfer bias with a low duty ratio exceeding 50 [%].
Further, the control unit 200 controls the nip width variable device (nip nip) described above so that the secondary transfer nip width W is smaller when the halftone image priority mode is selected than when the solid image priority mode is selected. The operation of the width changing device 60, 60A, 60B, 60C is controlled.
That is, the control unit 200 determines the mode selected by the mode selection unit 507 and executes either the first mode or the second mode according to the selected mode.
When giving priority to the image quality of the halftone image, the control unit 200 executes the first mode, reduces the transfer nip width W, and uses a high duty ratio bias as shown in FIGS. 13 to 17 as the secondary transfer bias. . This suppresses the injection of reverse charges into the minority dot toner group constituting the halftone image at the transfer nip, and improves the secondary transferability of the halftone image on the recording sheet P. As a result, it is possible to suppress the occurrence of insufficient image density of the halftone image, that is, transfer failure.

On the other hand, when giving priority to the image quality of the solid image, the control unit 200 executes the second mode, increases the transfer nip width W, and applies a low duty ratio bias as shown in FIGS. 10 to 12 as the secondary transfer bias. Use. By using a bias with a low duty ratio, the toner particles reciprocate in the recesses of the recording sheet P (plain paper), and each time the number of reciprocations increases, the surface is transferred from the surface of the intermediate transfer belt 31 to the recesses of the recording sheet P. The number of toner particles to be increased increases. As a result, a sufficient amount of toner can be transferred into the concave portion of the surface of the recording sheet P, and unevenness in the density of the image that occurs following the small irregularities on the recording sheet P can be suppressed. In addition, by using a bias with a low duty ratio, it is possible to suppress the occurrence of white spots due to discharge as compared with the case of using a bias with a non-low duty ratio. Further, by making the transfer nip width W larger than when the first mode is executed, the number of reciprocating movements of the toner particles in the secondary transfer nip can be increased, and image density unevenness, i.e., generation of blur, is further suppressed. be able to.
According to the second embodiment, the user can obtain an image of desired quality by a simple method of selecting a mode with the mode selection unit 507. That is, an image having a desired quality can be obtained by a simple method out of an image that gives higher priority to the image quality of the solid image portion or an image that gives higher priority to the image quality of the halftone image portion.

Although an example in which the secondary transfer bias having a high duty ratio is used in the first mode and the secondary transfer bias having a low duty ratio is used in the second mode has been described, the following may be employed. That is, a secondary transfer bias having a reverse peak side duty ratio of 50% in the first mode may be used, while a secondary transfer bias having a low duty ratio may be used in the second mode. Alternatively, a secondary transfer bias having a high duty ratio may be used in the first mode, while a secondary transfer bias having a reverse peak side duty ratio of 50 [%] may be used in the second mode.
Instead of the configuration in which either the first mode or the second mode is executed according to the mode selected using the mode selection unit 507, the control unit 200 calculates the density of the image to be output, and the density It is good also as a structure which performs either 1st mode or 2nd mode according to. The control unit 200 switches between the first mode and the second mode according to the density of the toner image. That is, the control unit 200 executes the first mode when the image density (the image area ratio or the amount of toner included in the image) is less than a predetermined value, and the image density is greater than or equal to the predetermined value. If so, execute the second mode. According to such a configuration, it is possible to suppress the occurrence of transfer failure by a simple method. The density of the image may be calculated for each recording sheet P to which the image is transferred, and either the first mode or the second mode may be executed. In this case, the secondary transfer power source 39 and the nip width varying device 60 can be easily controlled. On the other hand, one recording sheet P is divided into a plurality of areas in the conveyance direction, the density of the image transferred to each area is calculated, and either the first mode or the second mode is executed for each area. May be. In this case, the occurrence of transfer failure can be more reliably suppressed.

[Experimental result]
Tables 1-1 to 1-5 shown below show the results of printing experiments conducted by the present inventors.
The results of experiments conducted to show the effects of Embodiments 1 and 2 are shown below.
-HammerMill color copy digital was used as a recording sheet.
The image forming speed (linear speed) was 352.8 mm / sec.
As the secondary transfer bias, a high duty ratio with a duty ratio of 85% and a low duty with a duty ratio of 12% were used.
As the transfer nip width, the high duty ratio was large: about 4.5 mm and small: about 3 mm.
Table 1-1 shows the image quality for each type of secondary transfer bias in the HammerMill color copy digital.

The Hammer Mill color copy digital used in Table 1-1 is a general copy sheet, but still, the deterioration of the halftone transfer rate of the blur and the smooth portion may not be allowed. Even in such a recording sheet, the half-tone transfer property is remarkably improved under the condition that a high duty ratio is applied and the secondary transfer nip width is small.
On the other hand, the blur (particularly the blur of a solid image) is improved by applying a low duty ratio, and is less likely to occur under conditions of a large secondary transfer nip width.

  In Tables 1-2 to 1-5, Rezac 66 is a concavo-convex sheet manufactured by Tokai Paper Co., Ltd., and the depth of the surface recesses increases as the basis weight increases. That is, the magnitude relationship between the depths of the surface recesses is 260 kg> 215 kg> 175 kg. Moreover, the OK top coat (128 gsm) is a surface coat sheet (smooth sheet) manufactured by Oji Paper Co., Ltd.

  In this printing experiment, a blue solid image obtained by superimposing M and C is secondarily transferred to the uneven sheet, while a K halftone image (2by2) is secondarily transferred to the smooth sheet. Transcribed. In Tables 1-3 to 1-5, the numerical value [%] described in the smooth paper column indicates a halftone transfer rate.

  In Table 1-2 to Table 1-5, the recess transferability is the transferability of the toner to the surface recess of the uneven sheet. The recess transferability was evaluated based on the image quality in the surface recess of the uneven sheet. Evaluation is performed in five stages from rank 1 to rank 5, and rank 5 is the best result. Specifically, Rank 5 is a case where a sufficient amount of toner is transferred to the surface recesses and the image quality hardly changes between the surface recesses and the surface projections. Further, in the two to three recesses having the largest depth among the plurality of recesses existing on the sheet surface, the transfer amount of one of the M toner and the C toner is slightly smaller than that of the other. The case where a bad color taste was recognized was set to rank 4. In addition, in the two to three concave portions having the largest depth, a case where a white-out portion is recognized is ranked 3. Further, a case where white spots are scattered in a plurality of concave portions is ranked 2 (not generated in the experimental results of Tables 1-1 to 1-4). In addition, in the case where almost all of the recesses have white spots, the rank is 1 (not generated in the experimental results of Tables 1-2 to 1-5).

  The halftone transfer rate is a transfer rate of a K halftone image onto a smooth sheet, and was measured as follows. First, when the halftone image was primarily transferred onto the intermediate transfer belt 31, the test machine was stopped, the K toner of the halftone image on the intermediate transfer belt 31 was collected by vacuum, and the weight was measured as the total weight. Next, a halftone image was primarily transferred onto the intermediate transfer belt 31 under exactly the same conditions as before, and then immediately transferred onto a smooth sheet. Immediately after this, the test machine was stopped, the transfer residual toner adhering to the intermediate transfer belt 31 was collected by vacuum, and the weight was measured as the transfer residual amount. Then, a solution of “(total weight−remaining transfer amount) / total weight × 100” was obtained as a halftone transfer rate.

  The micro rubber hardness (micro hardness) is a value obtained by measuring the hardness of a belt piece cut from the intermediate transfer belt 31 with a micro rubber hardness meter MD-1 manufactured by Kobunshi Keiki Co., Ltd. In an environment of 23 ° C./50%, the hardness was measured based on the pushing depth of the push needle while the push piece was pressed against the belt piece with a predetermined pressure to deform the belt piece.

  In all of Experiment Nos. 1 to 5, when a solid image was secondarily transferred to the concavo-convex sheet, a secondary transfer bias having a low duty ratio superimposed voltage was used. In Experiment Nos. 1 to 4, when a halftone image was secondarily transferred to a smooth sheet, a secondary transfer bias having a high duty ratio superposed voltage that does not reverse the polarity was used. In contrast, in Experiment No. 5, when a halftone image was secondarily transferred to a smooth sheet, unlike the embodiment, a secondary transfer bias consisting of only a DC voltage was used.

  As can be seen from the results of Experiment Nos. 1 to 4, the higher the elasticity of the intermediate transfer belt 31 (the lower the hardness), the more the concave transferability of the concavo-convex sheet can be improved. On the other hand, the higher the elasticity, the lower the HT transfer rate (halftone transfer rate). From the balance between the concave transferability and the halftone transfer rate, the micro rubber hardness of the intermediate transfer belt 31 is preferably less than 100, and more preferably in the range of 50-80.

As can be seen from Experiment No. 5, when a halftone image is secondarily transferred to a smooth sheet, the use of only a DC voltage as the secondary transfer bias significantly reduces the halftone transfer rate ( 10%). This is because a reverse charge is injected into a small number of toner dots of a halftone image in the secondary transfer nip.

Next, the experiment was performed under the condition that the secondary transfer nip width was changed.
Tables 1-3 show the results of Experiment Nos. 1 and 8. In Experiment No. 8, a secondary transfer bias (superimposed voltage) having the same low duty ratio as that in Experiment No. 1 was applied to widen the secondary transfer nip width. Increasing the transfer nip width improved the uneven paper transferability rank and decreased the transfer rate on smooth paper.

Tables 1-4 show the results of Experiment Nos. 2, 9, and 10. In Experiment Nos. 9 and 10, a secondary transfer bias (superimposed voltage) having the same low duty ratio as in Experiment No. 2 was applied to change the secondary transfer nip width.
In Experiment No. 9, when the secondary transfer nip width was narrowed, the rank of the uneven paper transfer property was lowered and the transfer property on smooth paper was improved.
In Experiment No. 10, when the secondary transfer nip width was widened, the rank of the uneven paper transfer property was improved, and the transfer rate on smooth paper was reduced.

  Tables 1-5 show the results of Experiment Nos. 3 and 11. In Experiment No. 11, a secondary transfer bias (superimposed voltage) having the same low duty ratio as in Experiment No. 3 was applied to change the secondary transfer nip. In Experiment No. 11, when the secondary transfer nip width was narrowed, the rank of the uneven paper transfer property was lowered, and the transfer property on smooth paper was improved.

  Next, a printer according to a modified example in which a part of the configuration of the printer according to the embodiment is replaced with another configuration, and a printer according to an example in which a more characteristic configuration is added to the printer according to the embodiment will be described. . Unless otherwise specified, the configuration of the printer according to the modification and the configuration of the printer according to the example are the same as those in the embodiment.

[Third embodiment]
Next, the third embodiment and the experimental results according to the embodiment will be described. The inventors of the present invention conducted an experiment to print a test image under conditions where the frequency, the peak-to-peak value Vpp, and the DC voltage value (current target value for constant current control) were different for the secondary transfer bias. In the low smoothing mode, a K solid image was secondarily transferred to the Rezac 66, which is an uneven sheet. In the high smoothing mode, a blue halftone image (2by2) was secondarily transferred to an OK topcoat (128 gsm) which is a smooth sheet. The results of this experiment are shown in Table 2 below.

  As shown in Table 2, in both Experiment No. 6 and Experiment No. 7, the intermediate transfer belt 31 having a micro rubber hardness of 80 is used. As can be seen from Experiment No. 1 shown in Table 1, such an intermediate transfer belt 31 can exhibit good recess transferability. However, depending on the characteristics of the secondary transfer bias, there is a possibility that the recess transferability may be reduced.

In the experiments Nos. 6 and 12, the peak-to-peak potential Vpp = 5 [kV], the frequency = 1.4 [kHz], and the DC component current target value = −80 [μA] are set. did. The reverse peak side duty ratio was set to 13 [%] in the low smooth mode (uneven paper) and 80 [%] in the high smooth mode (smooth paper).
In Experiment No. 12, the secondary transfer nip width was set wider than in Experiment No. 6.

In the experiments of Experiment Nos. 7 and 13, the peak-to-peak potential Vpp = 12 [kV], the frequency = 0.8 [kHz], and the DC target current value = −100 [μA]. The reverse peak side duty ratio was set to 13 [%] in the low smooth mode (uneven paper) and 80 [%] in the high smooth mode (smooth paper).
In Experiment No. 13, the secondary transfer nip width was set narrower than in Experiment No. 7.

  Paying attention to the low smooth mode (uneven paper), the transferability of the recesses was better in Experiment No. 7 than in Experiment No. 6. This is due to the following two reasons. The first reason is the difference in frequency. Under the condition of a low duty ratio, the reverse transfer side time Tr is shorter than the transfer side time Tt, and therefore the return time of the toner particles from the surface concave portion of the concavo-convex sheet toward the belt surface tends to be insufficient. The tendency becomes more remarkable as the frequency increases. In Experiment No. 6, since the frequency is higher than in Experiment No. 7, even if the reverse peak side duty ratio is 13 [%], which is the same as Experiment No. 7, the reverse transfer side time Tr is shorter than in Experiment No. 7. As a result, the amount of toner particles that cannot sufficiently return to the belt surface from within the concave portion of the surface of the concavo-convex sheet is increased, and the action of weakening the adhesion of the toner particles on the belt surface is reduced. It got worse.

The second reason is the difference in peak-to-peak value Vpp. Under the condition of a low duty ratio, unless the peak-to-peak value Vpp is set to a large value to some extent, the reverse peak value Vr is insufficient, and the toner particles in the concave portion of the surface of the concavo-convex sheet cannot be satisfactorily returned to the belt surface. In Experiment No. 6, the peak-to-peak value Vpp is smaller than in Experiment No. 7, and the value of the return peak value Vr is insufficient.
Further, in the low smooth mode, as can be seen from comparison between Experiments 6 and 12, and 7 and 13, it can be seen that when the secondary transfer nip width is increased, the transfer property is improved, and when it is decreased, the transfer property is inferior.

  On the other hand, focusing on the high smooth mode (smooth paper), the test number 6 was better than the test number 7 in terms of halftone transferability. This is due to the following two reasons. The first reason is the difference between the peak-to-peak value Vpp and the current target value of the DC component. In Experiment No. 6, the peak-to-peak value Vpp is less than half of Experiment No. 7. In addition, the current target value of the DC component is lower than the experiment number 7. As a result, in Experiment No. 6, since the transfer peak value Vt is smaller than in Experiment No. 7, it is difficult for reverse charge injection to the toner. In addition, unlike Experiment Number 7, Experiment No. 6 has a negative polarity and does not change, which makes reverse charge injection less likely to occur.

  The second reason is the difference in frequency. As described above, the reverse charge is injected into the toner in the high smoothing mode within the transfer side time Tt. At the transfer side time Tt, the amount of reverse charge injected into the toner per unit time increases with time, and the value reaches saturation after a certain period of time. For this reason, when the transfer side time Tt becomes relatively short, the amount of reverse charge injected into the toner in the process of passing through the secondary transfer nip becomes smaller than when the transfer side time Tt is relatively long. Since the frequency is higher in Experiment No. 6 than in Experiment No. 7, the transfer-side time Tt is shorter than in Experiment No. 7 even though the reverse peak side duty ratio is the same 80%. Yes. For this reason, the amount of reverse charges injected into the toner is smaller than in Experiment No. 7.

  From the above experimental results, it is desirable to increase the frequency in the high smoothing mode compared to the low smoothing mode. Thereby, the halftone transferability can be improved in the high smoothing mode, and the recess transferability can be improved in the low smoothing mode.

In the high smoothing mode, it is desirable to lower the peak-to-peak value Vpp compared to the low smoothing mode. Thereby, the halftone transferability can be improved in the high smoothing mode, and the recess transferability can be improved in the low smoothing mode.
Further, in the high smoothing mode, as can be seen from comparison between Experiments 6 and 12, and 7 and 13, it can be seen that if the secondary transfer nip width is reduced, the transfer property is improved, and if it is reduced, the transfer property is inferior.

  Note that if the transfer peak value Vt and the reverse peak value Vr are too large, a discharge is generated between the belt surface and the sheet surface in the secondary transfer nip, resulting in many white spots. However, in the low smooth mode (uneven paper), the transfer peak value Vt and the reverse peak value Vr are necessary because the toner particles need to reciprocate favorably between the belt surface and the surface recess of the uneven sheet in the secondary transfer nip. Needs to be a certain large value. Even if a slight white spot is thereby generated, the image quality is improved as compared with the case where good concave transferability cannot be obtained. On the other hand, in the high smoothing mode (smooth paper), since it is not necessary to reverse the polarity of the secondary transfer bias and reciprocate the toner, it is necessary to use a secondary transfer bias having a high duty ratio that does not reverse the polarity. Is possible. Thereby, in addition to further improving the halftone transferability, the transfer peak value Vt (which is larger than the reverse peak value Vr because the polarity is not reversed) can be lowered to suppress the occurrence of white spots. In order not to reverse the polarity, the peak-to-peak value Vpp and the current target value of the direct current component may be made smaller than the secondary transfer bias having a low duty ratio.

Therefore, the control unit 200 applies the secondary transfer power supply in the high smoothing mode to the secondary transfer power source that has a higher frequency, a lower peak-to-peak value Vp, and a larger DC voltage value (current target value) than the low smoothing mode. 39 is configured to output.
Further, the reverse peak value Vr of the secondary transfer bias having a high duty ratio in the first mode is on the transfer side (minus side in the present embodiment) than the reverse peak value Vr of the secondary transfer bias having a low duty ratio in the second mode. It is preferable that it exists in. Thereby, in the first mode (high smoothing mode), the generation of white spots can be suppressed and the halftone transferability can be improved. In the second mode (low smoothing mode), by increasing the reverse peak value Vr to the side opposite to the transfer side (in the present embodiment, the plus side), a sufficient amount of toner can be applied to the surface recesses of the uneven sheet. It is possible to transfer, and the recess transferability can be improved. Or generation | occurrence | production of a blur can be suppressed.

In each of the above embodiments and examples, so-called plain paper or surface-coated paper is used as an example of the smooth sheet, but the smooth sheet is not limited to this. A sheet having irregularities on the surface but having a relatively small degree of irregularities may be handled as a smooth sheet. As an example of such a sheet, a basis weight of CLASSIC Linear-Solar White (manufactured by Neenah Paper) is 90 [gsm] or 118 [gsm], and a basis weight of CLASSIC CREST-Solar White (manufactured by Neenah Paper) is 90. [Gsm] or 104 [gsm], Rezac 66 (made by Tokushu Tokai Paper Co., Ltd.) having a basis weight of 118 [gsm], and the like. Instead of the above embodiments and examples, the image forming apparatus may be configured as follows. That is, when the recording sheet is a sheet having a relatively small degree of unevenness, the toner image is transferred from the image carrier to the recording nip by the transfer bias having a duty ratio higher than 50% in the transfer nip having the first width. Transfer to On the other hand, when the recording sheet is a concavo-convex sheet such as Rezac 66 175 kg (a sheet having a larger degree of concavo-convexity than the above-mentioned sheet), the duty ratio is a transfer nip having a second width wider than the first width. The toner image is transferred from the image carrier to the recording sheet with a transfer bias lower than 50%.
Moreover, you may comprise as follows. That is, when the recording sheet is a sheet having a relatively small degree of unevenness, the toner image is recorded from the image carrier with a transfer bias having a duty ratio higher than 50% in the transfer nip to which the first pressure is applied. Transfer to sheet. On the other hand, when the recording sheet is a concavo-convex sheet such as Rezac 66 175 kg (a sheet having a larger degree of concavo-convexity than the above-described sheet), a transfer nip to which a second pressure greater than the first pressure is applied, The toner image is transferred from the image carrier to the recording sheet with a transfer bias having a duty ratio lower than 50%.

[Fourth embodiment]
In the image forming apparatus of the fourth embodiment, the input operation unit 501 includes a mode selection unit 509 that arbitrarily selects a high duty ratio mode and a low duty ratio mode, as shown in FIG. And the input operation unit 501 are connected via a signal line. The input operation unit 501 described above selects a high smoothing sheet (smooth sheet) and a low smoothing sheet (uneven sheet). A high duty ratio mode setting key 509a and a low duty ratio mode setting key 509b for arbitrarily selecting a ratio mode by an equipment operator (user) are provided. The control unit 200 sets (executes) the first mode when the high duty ratio mode setting key 509a is selected, and sets (executes) the second mode when the low duty ratio mode setting key 509b is selected. To do. The mode determination unit 206 of the control unit 200 also has a function of determining the high duty ratio mode or the low duty ratio mode based on the mode information selected by the mode selection unit 509. The control unit 200 outputs a secondary transfer bias with a high duty ratio from the secondary transfer power supply 39 in the first mode, and outputs a secondary transfer bias with a low duty ratio from the secondary transfer power supply 39 in the second mode. Further, the control unit 200 controls the operation of the nip width varying devices 60, 60A, 60B, and 60C so that the secondary transfer nip width W is smaller (narrower) in the first mode than in the second mode. . According to the present embodiment, the user of the image forming apparatus can select an image of a desired quality by a relatively simple operation of selecting a mode from the high duty ratio mode and the low duty ratio mode by the mode selection unit 509. Can get.
As described above, the image forming apparatus according to each embodiment switches the transfer bias duty ratio and the transfer nip width according to the type of recording sheet, the density of the transferred image (the density of the toner image), and the like. Thereby, the quality of an image can be improved while maintaining productivity.

What was demonstrated above is an example, and there exists an effect peculiar for every following aspect.
[Aspect A]
In the aspect A, a transfer bias (for example, a secondary transfer bias) composed of a superimposed voltage obtained by superimposing a DC voltage and an AC voltage is output from a transfer power source (for example, a secondary transfer power source 39), and an image carrier (for example, an intermediate transfer belt). 31) and a toner on the surface of the image carrier while a transfer current (for example, a secondary transfer current) flows through a transfer nip (for example, a secondary transfer nip) due to contact between the nip forming member (for example, the sheet conveying belt 41). In an image forming apparatus (for example, a printer) that transfers an image to a recording sheet sandwiched between the transfer nips, an information acquisition unit (for example, an input operation unit 501 or the like) that acquires information on the surface smoothness of the recording sheet to which a toner image is to be transferred. A toner image formed on a highly smooth sheet having excellent surface smoothness based on the information acquisition result obtained by the smoothness detection sensor 502) and the information acquisition unit; The transfer mode is switched between a high smoothing mode for transferring and a low smoothing mode for transferring a toner image to a low smoothing sheet that is inferior in surface smoothness to the high smoothing sheet. Of the two peak values in the bias, the reverse peak which is the duty ratio on the side of the peak value opposite to the transfer peak value in which the toner is more strongly electrostatically moved from the image carrier side to the nip forming member side in the transfer nip. While the transfer bias having a side duty ratio of 50 [%] or more is output from the transfer power supply, the reverse peak side duty ratio in the low smoothing mode is different from that in the high smoothing mode and is 50 [%]. Control means (for example, control unit 200) for outputting the transfer bias described below from the transfer power supply is provided.

At the same time, the information acquisition unit (for example, the input operation unit 501 and the smoothness detection sensor 502) that acquires information about the surface smoothness of the recording sheet to which the toner image is to be transferred, and the information acquisition result by the information acquisition unit Based on the above, a high smoothing mode for transferring a toner image to a highly smooth sheet having excellent surface smoothness, and a low smoothing for transferring a toner image to a low smoothing sheet having a surface smoothness inferior to that of the high smoothing sheet. The secondary transfer nip width is switched depending on the mode.
In the high smoothing mode, transfer is executed with the normal nip width, and in the low smoothing mode, the secondary transfer nip backing roller 36 is moved upward to increase the nip width, thereby increasing the alternating electric field in the nip width. The number of vibrations can be increased, the number of reciprocations of the toner can be increased, and the toner transferability to the concave portion of the recording sheet can be improved.

  In such a configuration, if a flexible image carrier is used, the adhesion between the surface of the image carrier and the surface of the low smooth sheet in the transfer nip is improved, and the surface of the image carrier and the surface of the low smooth sheet are recessed. It is possible to reduce the distance from the bottom surface of. By reducing the distance in this way, it is possible to transfer the toner satisfactorily even to the surface recesses of the low smooth sheet even when printing is performed at a high speed enough to meet the demands for business use. However, if the peak-to-peak value Vpp of the AC voltage of the transfer bias is set too high in order to achieve good transfer, a large number of discharges occur in the transfer nip, and a large number of white spots due to the discharge appear in the image. Will occur. On the other hand, if a highly smooth sheet having excellent surface smoothness is used as the recording sheet, a reverse charge is injected into the toner in the transfer nip, which tends to cause a transfer failure of the toner image.

  Therefore, in aspect A, for a low smooth sheet, a toner image is generated using a transfer bias having a reverse peak side duty ratio different from that when a high smooth sheet is used and 50% or less. Transcript. This makes it possible to transfer a sufficient amount of toner to the surface recesses with a transfer bias having a smaller peak-to-peak value than when the reverse peak duty ratio is larger than 50 [%]. Therefore, it is possible to suppress the transfer failure of the toner to the concave surface of the low smooth sheet and the generation of white spots in the image. On the other hand, for the high smooth sheet, the reverse peak side duty ratio is a value different from the case where the low smooth sheet is used and 50% or more is used. As a result, compared with the case where the reverse peak duty ratio is less than 50 [%], the injection of reverse charges to the toner in the transfer nip is suppressed, thereby suppressing the occurrence of transfer failure of the toner image on the highly smooth sheet. Can do.

  As described above, in aspect A, high-speed printing is achieved, toner transfer failure to the surface recesses in the low smooth sheet and generation of white spots in the image are suppressed, and toner image transfer failure to the high smooth sheet is suppressed. Can be suppressed.

[Aspect B]
In the aspect B, in the high smoothing mode, the transfer bias having a reverse peak side duty ratio exceeding 50% is output from the transfer power source in the high smoothing mode, while the polarity is reversed within one cycle in the low smoothing mode. Further, the transfer bias whose reverse peak side duty ratio is less than 50 [%] is output from the transfer power source, and the secondary transfer nip backing roller 36 is moved to an upper position so as to widen the secondary transfer nip width. The control means is configured to implement the above.
With such a configuration, the following operational effects can be achieved as compared with the case where either the high smoothing mode or the low smoothing mode uses a reverse peak side duty ratio of 50 [%]. That is, it is possible to improve the transferability of the toner to the surface recess of the low smooth sheet, and to further suppress the transfer failure of the toner image to the high smooth sheet.

[Aspect C]
In the aspect C, in the high smoothing mode, the aspect C performs the control of outputting a transfer bias having a reverse peak side duty ratio in a range of 70 [%] to 90 [%] from the transfer power supply. The control means is configured. With such a configuration, it is possible to more reliably suppress the occurrence of defective transfer of the toner image to the highly smooth sheet.

[Aspect D]
In the aspect D, in the aspect B or C, in the low smoothing mode, control is performed to output a transfer bias whose reverse peak side duty ratio is in the range of 8 [%] to 35 [%] from the transfer power supply. The control means is configured. In such a configuration, a sufficient amount of toner can be reliably transferred to the surface recess of the low smooth sheet.

[Aspect E]
In aspect E, in the aspect D, the control means is configured to perform control to output a transfer bias having a reverse peak side duty ratio of 17% or less from the transfer power source in the low smoothing mode. It is characterized by. With this configuration, it is possible to further improve the toner transferability with respect to the concave surface of the low smooth sheet.

[Aspect F]
Aspect F is characterized in that in any one of the aspects B to E, the control means is configured to perform control to output a transfer bias that does not change polarity from the transfer power supply in the high smoothing mode. Is. With such a configuration, it is possible to more reliably suppress the occurrence of secondary transfer failure on a highly smooth sheet as compared with the case where the polarity of the secondary transfer bias is reversed within one cycle.

[Aspect G]
Aspect G is a multilayer structure in which an elastic layer (for example, elastic layer 31b) that is more elastic than the base layer (for example, base layer 31a) is laminated as the image carrier in any one of aspects A to F. It is characterized by using the thing. In such a configuration, the elastic layer is flexibly deformed in the transfer nip, so that the transferability of the toner to the surface recesses of the low smooth sheet can be improved, and high-speed printing that can meet the demand for business use can be achieved. it can.

[Aspect H]
Aspect H is characterized in that, in aspect G, the image bearing member having a micro rubber hardness of 50 to 80 is used. With such a configuration, the elastic layer can be deformed in a favorable and flexible manner in the transfer nip so as to conform to the shape of the toner mass.

[Aspect I]
Aspect I uses a smoothness detection means (for example, smoothness detection sensor 502) for detecting the surface smoothness of the recording sheet as the information acquisition means in any one of aspects A to H, and is detected by the smoothness detection means. On the basis of the result, the control means is configured to perform control to switch the transfer mode between the high smoothing mode and the low smoothing mode. In such a configuration, whether the recording sheet to which the toner image is to be transferred is a high smooth sheet or a low smooth sheet is automatically acquired regardless of the user's operation to improve operability. Can do.

[Aspect J]
Aspect J is characterized in that, in any of Aspects A to H, as the information acquisition means, an input operation means (for example, input operation unit 501) by which a user performs an input operation of the information is used. . In such a configuration, it is possible to acquire whether the recording sheet to which the toner image is to be transferred is a high smooth sheet or a low smooth sheet by a user operation.

[Aspect K]
In the aspect K, in the aspect J, a dedicated input unit (for example, the smooth paper button 501a) for inputting information indicating that the recording sheet is the high smooth sheet and information indicating the low smooth seed are input. The input operation means is provided with a dedicated input unit (for example, a concavo-convex paper button 501b). In such a configuration, it is possible to improve the user's input operability by providing a dedicated input unit.

[Aspect L]
Aspect L is the aspect J, wherein the input operation means is provided with a brand input unit capable of inputting brand information of the recording sheet. In the low smoothing mode, the brand information input to the brand input unit is the sheet surface. The control means is configured to perform control to output a transfer bias having a lower reverse peak duty ratio from the transfer power supply as the brand having a higher degree of unevenness. In such a configuration, a sufficient amount of toner can be transferred to the concave portion of the surface even in the case of the concave and convex sheet having a relatively large degree of surface irregularity in the low smoothing mode. In addition, the halftone image portion can be satisfactorily transferred to the surface protrusion even if the surface unevenness of the sheet is relatively small.

[Aspect M]
In the aspect M, in the aspect A to L, in the high smoothing mode, the control unit is configured to perform control to output a transfer bias having a higher frequency than the low smoothing mode from the transfer power source. It is characterized by that. With such a configuration, it is possible to improve the transferability of the halftone image in the high smoothing mode and to improve the toner transferability with respect to the surface recess of the low smoothing sheet in the low smoothing mode.

[Aspect N]
Aspect N is a control according to any one of aspects A to M, wherein in the high smoothing mode, a transfer bias composed of the superimposed voltage having a peak-to-peak value lower than that in the low smoothing mode is output from the transfer power supply As described above, the control means is configured. In such a configuration, in such a configuration, it is possible to improve the transferability of the halftone image in the high smoothing mode and to improve the toner transferability to the surface recess of the low smoothing sheet in the low smoothing mode.

[Aspect O]
Aspect O is a control according to any one of the aspects A to N, wherein in the high smoothing mode, a transfer bias composed of the superposed voltage obtained by superimposing the DC voltage having a value larger than that in the low smoothing mode is output from the transfer power supply. The control means is configured to implement the above. With such a configuration, it is possible to improve the transferability of the halftone image in the high smoothing mode and to improve the toner transferability with respect to the surface recess of the low smoothing sheet in the low smoothing mode. In addition, the occurrence of white spots can be suppressed in the high smoothing mode.

[Aspect P]
Aspect P is characterized in that, in aspect G or H, an elastic surface layer is provided as the elastic layer, and a plurality of microprojections made of a plurality of fine particles dispersed in the material of the elastic surface layer are provided on the surface of the elastic surface layer. It is what. In this configuration, due to the presence of particles on the surface of the elastic layer, the contact area between the surface of the elastic layer and the toner in the transfer nip is reduced, thereby improving the toner releasability from the surface of the image carrier and improving the transfer efficiency. Can be made.

[Aspect Q]
In the aspect Q, a transfer bias composed of a superimposed voltage obtained by superimposing a DC voltage and an AC voltage is output from a transfer power source, and a transfer current is supplied to the transfer nip formed by the contact between the image carrier and the nip forming member, and the image is In the image forming method for transferring the toner image on the surface of the carrier onto the recording sheet sandwiched between the transfer nips, whether the recording sheet to which the toner image is transferred is at least a highly smooth sheet having excellent surface smoothness. Or acquiring information capable of grasping whether the sheet is a low smooth sheet having a surface smoothness inferior to that of the high smooth sheet, and a toner image on the high smooth sheet based on the acquisition result of the information. Performing a step of switching a transfer mode between a high smoothing mode for transferring and a low smoothing mode for transferring a toner image to the low smooth sheet; In the smooth mode, a transfer bias having a reverse peak side duty ratio of 50% or more is output from the transfer power supply, while in the low smooth mode, the reverse peak side duty ratio is different from that in the high smooth mode. In addition, a transfer bias of 50 [%] or less is output from the transfer power supply. In such a configuration, as in the case A, a high amount of toner is transferred to the surface concave portion of the low smooth sheet having poor surface smoothness while achieving high-speed printing, and the high smooth sheet having excellent surface smoothness is transferred. The occurrence of toner image transfer failure can be suppressed.

[Aspect R]
In mode R, the secondary transfer nip width adjusting roller is manually moved by the user. A manual lever as a manual operation portion is attached to the secondary transfer nip width adjusting roller, and the roller position can be changed by a cam. The position of the nip width adjusting roller can be moved in the direction of widening the nip width by operating the manual lever. When a user uses a low-smooth sheet with poor surface smoothness, the surface of the low-smooth sheet with poor surface smoothness is inferior while changing the nip width by operating the manual lever to achieve high speed printing On the other hand, a sufficient amount of toner is transferred, and the occurrence of transfer failure of the toner image on the high smooth sheet having excellent surface smoothness can be suppressed.

[Aspect S]
In mode S, a transfer bias including an AC component is output from a transfer power source, and a toner image on the surface of the image carrier is transferred while a transfer current flows through a transfer nip formed by contact between the image carrier and the nip forming member. In the image forming apparatus for forming an image by transferring to a recording sheet sandwiched between the transfer nips, the transfer power source is applied in a direction to transfer the toner image to the recording sheet rather than a time average value of a transfer bias. The duty ratio, which is a value of A / (A + B) × 100 [%], can be changed, where B is the time of application and A (+) is the time applied in the opposite direction, and the transfer nip width The nip width varying device can change the transfer nip width as the duty ratio is higher, and the transfer nip width is smaller as the duty ratio is lower. Assumes Kusuru configuration,
According to the density of the toner image transferred to the recording sheet, the transfer nip width and the duty ratio are controlled by controlling the transfer bias output from the transfer power source and the nip width varying device (for example, 60 to 60C). It has a control means to variably control.

[Aspect T]
Aspect T is a density of the toner image according to aspect S, in which the density of the toner image is a halftone image density and a solid image density in which a toner adhesion amount per unit area is larger than the halftone image density. A halftone mode for transferring a toner image having a halftone image density, and a full solid mode for transferring the toner image having a solid image density to the recording sheet; and the control means in the halftone mode, Of the two peak values in the transfer bias, the duty on the side of the peak value opposite to the transfer peak value in which the toner image is more strongly electrostatically moved from the image carrier side to the nip forming member side in the transfer nip. A transfer bias with a reverse peak side duty ratio of 50 [%] or more is output, and in the all-solid mode, the reverse bias The transfer power source is controlled so as to output a transfer bias having a peak-side duty ratio different from that of the high smoothing mode and 50% or less, and the halftone mode and the all-solid mode are controlled. The operation of the variable nip width device is controlled so that the transfer nip width (for example, the secondary transfer nip width W) is switched.

[Aspect U]
Aspect U is characterized in that, in aspect S, the control means controls the operation of the variable nip width device in the halftone mode so that the transfer nip width is smaller than in the full solid mode. Yes.
[Aspect V]
In aspect V, in aspect S or T, in the halftone mode, the control means outputs a transfer bias with a reverse peak side duty ratio exceeding 50%, and in the all solid mode, the polarity is within one cycle. The transfer power supply is controlled so as to output a transfer bias that is reversed and the reverse peak side duty ratio is less than 50 [%].

In an ultrahigh-speed image forming apparatus using an elastic belt as the intermediate transfer belt 31 as an image carrier, there are the following problems depending on the unevenness condition and image density (image pattern) of the recording sheet.
1. Image density: transfer failure during halftone image When an intermediate transfer belt 31 provided with an elastic layer is used to transfer a halftone image by applying an AC transfer bias or a DC bias with a low duty ratio as a secondary transfer bias, Significant image density shortage may occur. In particular, when a high smooth sheet having excellent surface smoothness is used, the lack of image density is remarkable. Here, the duty ratio is a duty ratio on the side of the peak value opposite to the transfer peak value in the AC waveform in which the toner is electrostatically moved more strongly from the image carrier side to the nip forming member side in the transfer nip. A certain duty ratio on the reverse peak side means a ratio expressed by [%] of the application time on the reverse peak side in one cycle of the AC waveform.
That is, the AC bias with a low duty ratio is an AC bias whose application time on the reverse peak side is less than 50%. On the contrary, the AC bias having a high duty ratio is an AC bias having an application time on the reverse peak side of 50% or more.
2. Inferior transfer of the concave portion of the surface of the recording sheet Even in a recording sheet called plain paper, there are smooth paper and paper with poor smoothness. The concave portion of the non-smooth paper has an air gap with the toner layer on the intermediate transfer belt as compared with the convex portion, and the transfer electric field strength is lowered, so that the toner is hardly transferred onto such a recording sheet. In contrast to plain paper, in the case of a recording sheet with conspicuous unevenness such as “Rezak paper” with concavities and convexities in design, the image density in the concavities is insufficient.
In order to improve the above 1 and 2 in the same image forming apparatus, it is necessary to change the optimum AC bias condition and nip width condition.
When the toner image is a halftone image, a toner adhering portion constituting a relatively small number of dot groups and a blank portion where no toner is adhering are mixed in the image portion. Since the elastic layer is easily deformed, the elastic layer wraps the toner of the minority dot group not only from the surface but also from the side surface in the secondary transfer nip N, and causes the toner to inject a charge having a polarity opposite to the normal charging polarity. The charge amount (Q / M) of the toner is reduced.

For this reason, when the AC / secondary transfer bias having a high duty ratio is applied, charging of the intermediate transfer belt 31 is first started in the secondary transfer nip by the secondary transfer bias. Then, when the amount of charge exceeds a certain threshold value, the injection of reverse charge into the minority dot toner mass in the halftone image starts.
Charging of the belt portion that has entered the secondary transfer nip mainly occurs within the transfer side time: Tt. Therefore, as the transfer side time Tt increases, the amount of reverse charge injected into the minority dot toner mass increases.
The secondary transfer bias with a high duty ratio has a shorter transfer-side time Tt than the secondary transfer bias with a low duty ratio. It is thought that the occurrence of Further, the above-mentioned halftone transfer failure is more remarkable on highly smooth paper.
Therefore, as in the mode S, by shortening the transfer nip width, the nip passage time is shortened, the transfer side time: Tt is also shortened, the amount of reverse charge injection is reduced, and the AC bias with a high duty ratio is reduced. The occurrence of transfer defects can be further suppressed as compared with the case where only is applied.

When the toner image on the intermediate transfer belt is secondarily transferred to a recording sheet rich in surface irregularities, it is necessary to reciprocate the toner between the surface of the intermediate transfer belt and the concave portion of the recording sheet surface in the transfer nip. . As the number of reciprocations of the toner increases, the amount of toner transferred from the surface of the intermediate transfer belt toward the concave portion increases, and therefore the number of reciprocation needs to be a certain number or more. If the secondary transfer nip width is narrow in an ultra-high-speed image forming apparatus, it is necessary to increase the AC frequency in order to increase the number of reciprocating movements. Since the AC peak voltage is required, if the frequency is increased, the waveform is broken and the necessary peak voltage cannot be output from the power source. In order to replace it with a transfer power supply that does not break the waveform, a considerably expensive power supply is required.
For this reason, by changing the transfer nip width as necessary (long or short) as in the mode S, the nip passage time becomes long, and even when an extremely high-speed image is formed when the recording sheet is uneven, The number of reciprocations can be maintained at a fixed number, and the occurrence of secondary transfer defects in the recesses can be suppressed.

The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments, and the present invention described in the claims is not specifically limited by the above description. Various modifications and changes are possible within the scope of the above.
In each of the above-described embodiments, the so-called intermediate transfer type image forming apparatus using the intermediate transfer body has been described. However, the so-called direct transfer type image forming apparatus that directly transfers an image from the photosensitive member to the recording sheet P may be used. Good.
In each of the above embodiments, the image carrier (intermediate transfer belt 31) having an elastic layer is used, but an image carrier having no elastic layer may be used. Even when a belt without an elastic layer is used, it is included in the halftone image due to the pressure applied to the secondary transfer nip, the elastic deformation of the roller (secondary transfer back roller 33) that supports the belt, etc. Dot toner may be wrapped around the belt. According to the above-described embodiment, the transfer defect of the halftone image can be prevented by using the secondary transfer bias having a high duty ratio. Even when the intermediate transfer belt 31 having no elastic layer is used, the secondary transfer property on the concavo-convex sheet can be secured by using the secondary transfer bias having a low duty ratio. Alternatively, it is possible to suppress the blur of the solid image.
The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

31 Intermediate transfer belt (image carrier)
31a Base layer 31b Elastic layer 31c Particle (fine particle)
39 Secondary transfer power supply
41 Sheet conveying belt (nip forming member)
60 Nip width variable device (nip width changing device)
60A-60C Nip width variable device (nip width changing device)
200 Control unit (control means)
501 Input operation unit (information acquisition means, input operation means)
501a Smooth paper button (input section)
501b Uneven paper button (input part)
502 Smoothness detection sensor (smoothness detection means)
501c to 501f Brand input unit 206 Mode determination unit 509 Mode selection unit N Transfer nip P Recording sheet Vpp Maximum voltage difference Vave Time average value Vt Transfer peak value Vr Reverse peak value W Secondary transfer nip width (transfer nip width)

JP 2014-77981 A

Claims (29)

An image carrier;
A nip forming member that forms a transfer nip with the image carrier;
A nip width changing device for changing the width of the transfer nip;
A power supply that outputs a transfer bias including an AC component to transfer the toner image on the image carrier to a recording sheet at the transfer nip;
When the period of the transfer bias is T, and the period of the transfer bias in which the transfer bias moves the toner image from the image carrier to the recording sheet is Tt in the period T. A first mode in which a duty ratio of the transfer bias expressed by (T−Tt) / T × 100 [%] is a first duty ratio and a width of the transfer nip is a first width; and the transfer bias A second mode in which the duty ratio is a second duty ratio lower than the first duty ratio and the width of the transfer nip is a second width wider than the first width according to a predetermined condition. An image forming apparatus comprising: a switching control unit. The image forming apparatus according to claim 1.
The image forming apparatus, wherein the first duty ratio is higher than 50 [%] and the second duty ratio is lower than 50 [%]. The image forming apparatus according to claim 1.
The image forming apparatus according to claim 1, wherein the predetermined condition is a type of the recording sheet. The image forming apparatus according to claim 3.
The control means executes the first mode when the recording sheet is a smooth sheet, and executes the second mode when the recording sheet is an uneven sheet having larger unevenness than the smooth sheet. An image forming apparatus. The image forming apparatus according to claim 4.
The image forming apparatus, wherein the first duty ratio is higher than 50 [%] and the second duty ratio is lower than 50 [%]. The image forming apparatus according to claim 4 or 5,
When the recording sheet is a concavo-convex sheet, the control unit controls the power source so as to output the transfer bias having a duty ratio of 8 [%] to 35 [%]. apparatus. The image forming apparatus according to claim 6.
When the recording sheet is a concavo-convex sheet, the control unit controls the power supply so as to output the transfer bias having a duty ratio of 17% or less. The image forming apparatus according to any one of claims 4 to 7,
When the recording sheet is a smooth sheet, the control unit controls the power supply so as to output the transfer bias having a duty ratio of 70 [%] to 90 [%]. Forming equipment. The image forming apparatus according to any one of claims 4 to 8,
When the recording sheet is a concavo-convex sheet, the control unit controls the power source so as to output the transfer bias whose polarity is alternately switched. The image forming apparatus according to any one of claims 4 to 9,
When the recording sheet is a smooth sheet, the control unit controls the power supply so as to output the transfer bias having a constant polarity. The image forming apparatus according to claim 4,
An input operation unit for inputting information on whether the type of the recording sheet is a smooth sheet or an uneven sheet;
An image forming apparatus comprising: a determination unit that determines whether the recording sheet is a smooth sheet or an uneven sheet based on the information input by the input operation unit. The image forming apparatus according to claim 4,
Smoothness detecting means for detecting the surface smoothness of the recording sheet;
An image forming apparatus comprising: a determination unit that determines whether the recording sheet is a smooth sheet or an uneven sheet based on a detection result of the smoothness detection unit. The image forming apparatus according to claim 4,
A brand input unit for inputting brand information of the recording sheet;
An image forming apparatus comprising: a determination unit that determines whether the sheet recording sheet is a smooth sheet or an uneven sheet based on the brand information. The image forming apparatus according to claim 13.
When the recording sheet is a concavo-convex sheet, the control unit outputs the transfer bias having a lower duty ratio as the brand input to the brand input unit is a brand having a higher degree of ruggedness. An image forming apparatus that controls a power source. The image forming apparatus according to claim 1.
The mode is selected from a halftone image priority mode that prioritizes the image quality of the halftone image over the image quality of the solid image, and a solid image priority mode that prioritizes the image quality of the solid image over the image quality of the halftone image. With a mode selector,
The control means executes the first mode when the mode selected by the mode selection unit is the halftone image priority mode, and the mode selected by the mode selection unit is the solid image priority mode. In this case, the image forming apparatus executes the second mode. The image forming apparatus according to claim 15.
The image forming apparatus, wherein the first duty ratio is higher than 50 [%] and the second duty ratio is lower than 50 [%]. The image forming apparatus according to claim 1.
The image forming apparatus, wherein the control unit switches between the first mode and the second mode in accordance with a density of a toner image transferred to the recording sheet. The image forming apparatus according to claim 1 or 2,
A mode selection unit for selecting one of the first mode and the second mode;
The image forming apparatus according to claim 1, wherein the control unit executes one of the first mode and the second mode based on mode information selected by the mode selection unit. The image forming apparatus according to claim 1,
The image forming apparatus according to claim 1, wherein a frequency of the transfer bias in the first mode is higher than the frequency in the second mode. The image forming apparatus according to claim 1,
An image forming apparatus, wherein a peak-to-peak value of the AC component of the transfer bias in the first mode is smaller than the peak-to-peak value in the second mode. The image forming apparatus according to any one of claims 1 to 20,
The transfer bias is a bias in which a DC component is superimposed on the AC component,
The image forming apparatus according to claim 1, wherein an absolute value of the direct current component in the first mode is larger than an absolute value of the direct current component in the second mode. The image forming apparatus according to any one of claims 1 to 21,
The transfer bias has a transfer peak value on the transfer side with respect to the time average value, and an inverse peak value different from the transfer peak value,
The reverse peak value in the first mode is on the transfer side with respect to the reverse peak value in the second mode. The image forming apparatus according to any one of claims 1 to 22,
A pressure member that presses the nip forming member toward the image carrier at the transfer nip;
The image forming apparatus, wherein the nip width changing device changes the transfer nip width by changing a pressure applied to the transfer nip by the pressure member. The image forming apparatus according to any one of claims 1 to 22,
The nip forming member is a belt member stretched around a plurality of rotating members,
The image forming apparatus, wherein the nip width changing device changes the transfer nip width by moving at least one rotating member of the plurality of rotating members. 25. The image forming apparatus according to claim 1, wherein:
The image carrier has a multilayer structure in which a base layer and an elastic layer that is more elastic than the base layer are laminated on the base layer. The image forming apparatus according to claim 25.
The image forming apparatus, wherein the elastic layer has a micro rubber hardness of 50 to 80. The image forming apparatus according to claim 25 or 26.
The image carrier has an elastic surface layer as the elastic layer,
2. The image forming apparatus according to claim 1, wherein the elastic surface layer is provided with a plurality of minute protrusions made of a plurality of fine particles dispersed in the material on the surface of the elastic surface layer. When the recording sheet is a smooth sheet, in the transfer nip to which the first pressure is applied, the transfer bias period is T, and the transfer bias has a toner image larger than the time average value of the transfer bias in the period T. With the transfer bias having a duty ratio expressed by (T−Tt) / T × 100 [%] higher than 50 [%], where Tt is the time on the transfer side to move from the carrier to the recording sheet Transferring the toner image from the image carrier to the recording sheet;
When the recording sheet is a concavo-convex sheet having larger concavoconvexities than a smooth sheet, the duty ratio is lower than 50% in the transfer nip to which a second pressure larger than the first pressure is applied. A transfer method comprising transferring a toner image from the image carrier to the recording sheet by a transfer bias. When the recording sheet is a smooth sheet, in the transfer nip having the first width, the transfer bias period is T, and the transfer bias is within the period T, and the toner image is more than the time average value of the transfer bias. A toner image is generated by the transfer bias having a duty ratio expressed by (T−Tt) / T × 100 [%] higher than 50 [%] when Tt is a time on the transfer side to be moved from the recording sheet to the recording sheet. Is transferred from the image carrier to the recording sheet,
When the recording sheet is a concavo-convex sheet having larger concavo-convexities than a smooth sheet, the transfer bias having a duty ratio lower than 50% in the transfer nip having a second width wider than the first width. And transferring the toner image from the image carrier to the recording sheet.

Images