JP2001305888A - Transfer device and image forming device - Google Patents

Abstract

(57) [Summary] An intermediate transfer belt 241 is formed by an indirect application method.
The present invention provides an image forming apparatus capable of reducing instability in primary transfer performance due to a decrease in effective transfer current (It-Ia) in an image forming apparatus that applies a primary transfer current to the image forming apparatus. A difference between a value of an output current (It) from a primary transfer bias power supply (248) and a value of a feedback current (Ia) fed back from an intermediate transfer belt (241) to a primary transfer bias power supply (248) via an earth roller (246) is kept constant. The primary transfer bias control unit (not shown) of the primary transfer bias power supply 248 is configured to execute the differential constant current control that maintains the current.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transfer method in which a visible image is transferred to an intermediate transfer member by a contact transfer method in an image forming process performed by a copying machine, a facsimile, a printer, etc., and then transferred to another transfer member. The present invention relates to an apparatus and an image forming apparatus including the same.

[0002]

2. Description of the Related Art Conventionally, in this type of image forming apparatus, there has been a problem that the transfer performance at a transfer nip formed by the pressure contact between an image carrier and an intermediate transfer member becomes unstable. When the transfer performance in the transfer nip becomes unstable, there occur problems such as uneven density of the transferred image and unstable density of the entire transferred image.
Particularly, in a color image forming apparatus in which black, yellow, magenta, and cyan color images formed on an image carrier are superimposed and transferred on an intermediate transfer body to form a full-color image, the density gradation of each color image is full-color. Since the color tone of the image is greatly affected, the instability of the transfer performance lowers the color reproducibility.

[0003] The electrical resistance of the intermediate transfer member greatly contributes to the instability of the transfer performance. Specifically, the electrical resistance of the intermediate transfer member varies in its initial value for each product, changes with time due to deterioration of the intermediate transfer member, and changes in the environment such as temperature and humidity. Or change. For this reason, the electrical resistance of the intermediate transfer member varies depending on the use state and use environment. When a transfer bias of a constant voltage is applied to such an intermediate transfer member, a current value flowing through the intermediate transfer member varies depending on the electric resistance value at the time of application, so that the intensity of the transfer electric field formed in the transfer nip changes. Resulting in. Then, the transfer performance becomes unstable due to this change.

As an image forming apparatus capable of suppressing such instability in transfer performance, there is known an image forming apparatus which performs so-called constant current control for maintaining a constant value of a transfer current applied to an intermediate transfer member. In this image forming apparatus, a stable transfer electric field is formed in the transfer nip by applying a constant transfer current to the intermediate transfer body regardless of its electrical resistance value, thereby destabilizing the transfer performance. Can be reduced.

[0005]

However, even when such a constant current control is performed, the transfer performance may become unstable. In particular, when the transfer current is applied to the intermediate transfer belt as the intermediate transfer body by the indirect application method, the transfer performance tends to be unstable. The indirect application method is a method in which a transfer current is applied to the back surface (surface opposite to the transfer surface) of the intermediate transfer belt at a position different from the transfer nip. In such an indirect application method, when the electrical resistance value on the back surface of the intermediate transfer belt is greatly reduced due to environmental fluctuations, the transfer current is more likely to flow in the belt circumferential direction on the back surface than in the belt thickness direction. When it becomes easy to flow like this,
Despite applying a constant transfer current to the intermediate transfer belt, the transfer current transmitted to the belt circumferential direction on the back side of the intermediate transfer belt and flowing into the ground roller increases, and the transfer current in the belt thickness direction increases. And the effective transfer current that reaches the contact surface with the image carrier decreases. And
The transfer performance is reduced due to the decrease in the effective transfer current. On the other hand, when the electrical resistance value on the back surface of the intermediate transfer belt rises to the original value due to environmental fluctuations, the amount of the effective transfer current increases and the transfer performance is restored to the original value. The transfer performance becomes unstable due to the increase and decrease of the effective transfer current due to the above-mentioned environmental fluctuation.

The present invention has been made in view of the above background, and an object of the present invention is to provide a transfer device for applying a transfer current to an intermediate transfer member and an image forming apparatus provided with the same. An object of the present invention is to provide a transfer device and an image forming apparatus that can reduce the instability of transfer performance due to increase and decrease.

[0007]

In order to achieve the above object, an object of the present invention is to provide a method for transferring a visible image carried on an image carrier of an image forming apparatus to a transfer surface of an intermediate transfer member. An intermediate transfer member for moving the intermediate transfer member to an intermediate transfer position, which is a contact position with the image carrier, and then moving the intermediate transfer member to a re-transfer position to transfer the visible image to another transfer member A transfer device comprising: a transfer unit; a transfer current applying unit that applies a transfer current for intermediate transfer to the intermediate transfer body; and an output current control unit that controls a value of an output current It from the transfer current applying unit. The output current control means is configured to control the output current It to maintain a constant value of the effective transfer current reaching the contact position from the transfer current application position on the intermediate transfer member. Characterized by Than it is.

In this transfer device, regardless of the electric resistance of the intermediate transfer member, the value of the effective transfer current that reaches from the application position to the intermediate transfer member to the contact position between the intermediate transfer member and the image carrier is determined. Keep constant.

According to a second aspect of the present invention, in the transfer device of the first aspect, an intermediate transfer belt is provided as the intermediate transfer body, and a ground member that contacts the intermediate transfer belt and guides the transfer current to ground is provided. The transfer current applying means and the ground member are configured to apply the transfer current to the intermediate transfer belt by an indirect application method.

In this transfer apparatus, an intermediate transfer belt having excellent flexibility which can be flexibly deformed according to the shape of an image carrier is provided as an intermediate transfer member. In such a transfer device, by adjusting the relative position between a belt stretching member such as a stretching roller for stretching and supporting the intermediate transfer belt and the image carrier, the contact between the intermediate transfer body and the image carrier is adjusted. Thus, the transfer nip width (nip length in the circumferential direction of the belt) formed can be freely adjusted. Therefore, it is possible to increase the transfer nip width and improve the transfer performance as compared with a transfer device having an intermediate transfer roller in which a transfer nip width that can be formed is limited by hardness as an intermediate transfer member. Further, in this transfer device, a transfer current applied from the back surface of the intermediate transfer belt opposite to the transfer surface is guided from the ground member that contacts the back surface to the ground. With this configuration, it is possible for the belt stretching member that contacts the back surface of the intermediate transfer belt to exhibit functions as a transfer current applying member and a ground member. Further, if the transfer current is applied downstream of the transfer nip in the belt moving direction and is guided to the ground upstream of the transfer nip, the potential of the intermediate transfer belt portion upstream of the transfer nip is reduced. Thus, the occurrence of so-called pre-transfer can be suppressed. The term “pre-transfer” means that a visible image is electrostatically jumped from the image carrier by an electric charge of the intermediate transfer body and transferred to the intermediate transfer body at a position upstream of the transfer nip in the moving direction of the intermediate transfer body. It is a phenomenon that does. When such pre-transfer occurs, the transfer position of the visible image on the intermediate transfer member deviates from the original position, and the image is disturbed.

According to a third aspect of the present invention, in the transfer apparatus of the second aspect, a ground current value detecting means for detecting a value of a ground current Ia flowing into the ground member is provided.
The output current control means is configured to maintain the value of the effective transfer current constant by controlling the output current It so as to make "a" constant.

In this transfer device, the difference between the value of the output current It from the transfer current applying means and the value of the ground current Ia flowing from the intermediate transfer belt to the ground member is kept constant. Controls the value of It. This difference does not flow from the position where the transfer current (output current It) is applied to the intermediate transfer belt to the ground member,
The value is substantially equal to the value of the effective transfer current that reaches the contact surface of the intermediate transfer belt with the image carrier. In such a configuration, without providing a means for directly detecting the value of the effective transfer current flowing from the intermediate transfer belt to the image carrier,
This value can be indirectly detected and kept constant.

Conventionally, the paper transport belt is rotated while being stretched and supported by a transfer bias roller and an earth roller, and the transfer paper fed from the paper feeder onto the paper transport belt is transferred to the paper transport belt and the image forming apparatus. In a transfer device that conveys to a transfer nip formed by contact with a carrier, a so-called difference constant current control for controlling a difference between an output current value to a transfer bias roller and an earth current value flowing into an earth roller to be constant is performed. Things are known. However,
This transfer device has an effective position at the contact position between the paper transport belt and the image carrier, that is, at the final transfer position where the visible image on the image carrier is transferred to the transfer paper as the final transfer member in the image forming process. This is to keep the transfer current constant. Therefore, the present transfer device that maintains a constant effective transfer current at an intermediate transfer position where a visible image is transferred intermediately to an intermediate transfer member in an image forming process is different from the conventional transfer device.

By the way, the present inventor has conducted intensive research and
Transfer current is applied to the intermediate transfer belt by the indirect application method.
In the transfer device, the belt is used as an intermediate transfer belt.
There is a deviation of the volume resistivity in the thickness direction
The surface resistance of the back side is 1 × 10 ⁷Use something less than
And the transfer current applied to the back surface flows almost in the belt thickness direction.
Without flowing the belt in the circumferential direction, leading to the ground.
And found. Also, the surface resistance of the back surface is 1 × 10¹¹
If the transfer current is larger than
To the contact position between the transfer surface of the intermediate transfer belt and the image carrier
I find it difficult. Therefore, the back surface
Resistance value is 1 × 10⁷Intermediate transfer belt less than
10¹¹If the intermediate transfer belt exceeds
It becomes difficult to apply a transfer current by the contact application method.

According to a fourth aspect of the present invention, there is provided the transfer device according to the third aspect, wherein the intermediate transfer belt has a deviation in volume resistivity in a belt thickness direction, and is opposite to the transfer surface. The surface resistance value of the back surface of 1 × 10 ⁷ to 1 × 1
0 ¹¹ [[Ω / □] is provided.

In this transfer apparatus, the surface resistance of the back surface of the intermediate transfer belt is set to 1 × 10 ⁷ to 1 × 10 ^7
11 [[Ω / □] is provided so that an effective transfer current is generated at the intermediate transfer position.
Good transfer performance is exhibited.

According to a fifth aspect of the present invention, there is provided a visible image forming means for forming a visible image on an image carrier, and a transfer device for transferring the visible image to an intermediate transfer member and then to another transfer member. An image forming apparatus comprising:
It is characterized in that two, three or four transfer devices are provided.

In this image forming apparatus, the value of the effective transfer current value is kept constant irrespective of the electric resistance value of the intermediate transfer member by the same operation as the transfer device of the first aspect.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to an electrophotographic color copying machine (hereinafter referred to as a color copying machine) as an image forming apparatus will be described below. First,
A basic configuration of the color copying machine according to the present embodiment will be described. FIG. 1 is a schematic configuration diagram of the color copying machine. In FIG. 1, a color copying machine includes a color image reading device (hereinafter, referred to as a color scanner) 1, a color image recording device (hereinafter, referred to as a color printer) 2, a paper feeding bank 3, and the like.
And so on.

The color scanner 1 forms an image of the document 4 on the contact glass 101 on a color sensor 105 via an illumination lamp 102, mirror groups 103a, 103b, 103c, and a lens 104, thereby forming a color image of the document 4. Information is, for example, Red: Red, Green: Green, Blu
e: Each color separation light of blue (hereinafter, referred to as R, G, B) is read and converted into an electric image signal. This color copier simultaneously reads three color images obtained by color-separating the image of the document 4 by using the R, G, B color separation means and the color sensor 105 composed of a photoelectric conversion element such as a CCD. And, R, G, B obtained by this color scanner 1
The color conversion processing is performed by an image processing unit (not shown) based on the color separation image signal intensity level of Black: Black (hereinafter, Bk).
), Cyan: cyan (hereinafter referred to as C), Ma
genta: Magenta (hereinafter referred to as M), Yellow
w: Color image data of yellow (hereinafter, referred to as Y) is obtained.

The operation of the color scanner 1 for obtaining the Bk, C, M, Y color image data is as follows. That is, first, the illumination lamp 102 and the mirror group 103
The original 4 is scanned in the leftward direction of the arrow in the figure in synchronization with the operation of the color printer 2 to be described later by an optical system including a, 103b, and 103c to obtain one color image data for each scan. .

The color printer 2 includes a photosensitive drum 200 as an image carrier, a writing optical unit 220,
Secondary transfer device 260, paper transport device 270, fixing device 28
0 and so on. Further, around the photosensitive drum 200, a photosensitive member cleaning device 201 and a static elimination lamp 20 are provided.
2, a charger 203, a revolver developing unit 230, a reference image density sensor 204, a primary transfer device 240, and the like.

The writing optical unit 220 includes a semiconductor laser 221, a laser emission drive control unit (not shown), a polygon mirror 222 and a rotation motor 223,
The F / θ lens 224, the reflection mirror 225, and the like convert the color image data from the color scanner 1 into an optical signal on the photosensitive drum 200 and perform optical writing corresponding to the image of the original 4. The photoconductor drum 200 carries an electrostatic latent image by the optical writing.

The revolver developing unit 230 has a Bk
Developing device 231K, C developing device 231C, M developing device 231
The M and Y developing units 231Y and a revolver rotation drive unit (not shown) for rotating the developing units 231 for each color in the counterclockwise direction indicated by an arrow are included. Then, in the standby state of the copying machine main body, among these four developing devices 231, Bk developing device 2
31K is stopped at a rotation angle such that it is located at a developing position which is a position facing the photosensitive drum 200.

The developing units 231 for each color sequentially move to the developing position by rotating the revolver developing unit 230 counterclockwise in the figure. A developer accommodating section (not shown) for accommodating a developer containing a toner of a corresponding color and a ferrite carrier, and a stirring paddle (not shown) rotating to pump up and agitate the developer are provided in the developing devices 231 of the respective colors. A developing sleeve or the like that rotates while the spikes of the developer carried on the surface contact the photosensitive drum 200 is provided. The developer in the developing device 231 for each color is
The toner is carried on the developing sleeve while being charged to a negative polarity by the stirring with the stirring paddle. A developing bias in which an AC voltage Vac is superimposed on a negative DC voltage Vdc is applied to the developing sleeve by a developing bias power supply (not shown).

In this color copying machine, when a copy operation is started by pressing a copy start key (not shown) or the like, the color scanner 1 starts reading Bk color image data. The photosensitive drum 200 is rotated counterclockwise in the figure by a driving unit (not shown), and the intermediate transfer belt 241 is rotated clockwise by the rotation of the driving roller 242.

The surface of the photoreceptor drum 200 which has started to rotate is uniformly charged to a predetermined negative potential by corona discharge at a position facing the charger 203. Then, raster exposure based on the Bk color image signal is performed by the writing optical unit 220. The portion subjected to the raster exposure loses a charge proportional to the exposure amount, and carries a Bk electrostatic latent image.

The Bk developing device 231K, which is located at the developing position, has its front end portion in the developing area (the area facing the developing sleeve and the photosensitive drum 200) so that the Bk electrostatic latent image can be developed from the front end portion. ), The rotation drive of the developing sleeve is started. When the Bk electrostatic latent image passes through the developing area with the rotation of the photosensitive drum 200, the Bk toner in the developer carried on the developing sleeve of the Bk developing device 231K is electrostatically attached. I'm sullen.
On the other hand, the Bk toner does not adhere to the non-latent image portion of the photosensitive drum 200, that is, to the non-exposed portion. As a result of these,
On the surface of the photosensitive drum 200 that has passed the development area,
A Bk toner image having the same shape as the Bk electrostatic latent image is formed.

The Bk toner image developed on the photosensitive drum 200 moves to the primary transfer nip as the photosensitive drum 200 rotates, and rotates at a constant speed while contacting the photosensitive drum 200. The image is transferred onto the transfer surface of the transfer belt 241. After passing through the primary transfer nip, the surface of the photosensitive drum 200 is
After the residual toner is cleaned by 1, the charge is removed at a position facing the discharge lamp 202. Then, the toner is uniformly charged again at a position facing the charger 203.

When the development process of the Bk toner image proceeds to some extent, reading of the C image data by the color scanner 1 starts at a predetermined timing, and the photosensitive drum 200 is written by the laser beam writing based on the C image data.
Starts to form a C electrostatic latent image on the surface. Then, after the trailing end of the Bk electrostatic latent image on the photosensitive drum 200 has passed through the developing area, and before the leading end of the C electrostatic latent image reaches the developing area, the revolver developing unit 230 is activated. About 90 °
Rotate. With this rotation, the C developing device 231C moves to the developing position, starts developing the C electrostatic latent image with the C toner, and forms a C toner image on the photosensitive drum 200. Thereafter, similarly, an M toner image and a Y toner image are formed on the photosensitive drum 200, and these are superposed and transferred on the intermediate transfer belt 241 by the primary transfer nip, and all the toner images are transferred. At this point, a full-color toner image is formed on the intermediate transfer belt 241.

FIG. 2 is a configuration diagram showing a configuration around the photosensitive drum 200 of the color printer 2. 2, the primary transfer device 240 includes a plurality of rollers 242,
243, 244, 245, 246, 247 and an intermediate transfer belt 241 stretched over them. The primary transfer bias power supply 248 and the intermediate transfer belt 24
The device also includes a static eliminator 249 for neutralizing 1, a cleaning grade 250 for cleaning the intermediate transfer belt 241, a lubricant mass 251, a lubricant application brush 252, and the like.

The intermediate transfer belt 241 is configured to exhibit conductive electrical characteristics, and is urged toward the photosensitive drum 200 by a primary transfer bias roller and an earth roller 246 described later. Between the two rollers, the photosensitive drum 200 is brought into contact with the photosensitive drum 200 to serve as an intermediate transfer position.
A next transfer nip is formed.

Of the rollers that stretch the conductive intermediate transfer belt 241, 242 is a drive roller that is driven to rotate clockwise in the figure by drive means (not shown), and the rotation thereof causes the intermediate transfer belt 241 to rotate. Is rotated clockwise in the figure.

Reference numeral 243 denotes a tension roller for adjusting the tension force of the intermediate transfer belt 241 by adjusting the relative position in the color copying machine main body by a position adjusting mechanism (not shown).

Reference numeral 247 denotes a primary transfer bias roller which constitutes a transfer current applying means together with a primary transfer bias power supply 248 connected thereto. The primary transfer bias power supply 248 includes a primary transfer bias control unit (not shown) as an output current control unit, and thereby outputs an output current to the primary transfer bias roller 247 of the toner image on the intermediate transfer belt 241. Control to a value corresponding to the number of superpositions. The primary transfer current output from the primary transfer bias power supply 248 to the primary transfer bias roller 247 is applied to the back surface of the intermediate transfer belt 241.

Reference numeral 246 denotes an earth roller serving as an earth member, and a primary transfer bias roller 247
1 applied to the back surface of the intermediate transfer belt 241 from the
The next transfer current is returned to the transfer bias power supply 248, and is finally led to the ground.

Reference numeral 244 denotes an opposing roller disposed so as to oppose a secondary transfer bias roller of a secondary transfer device described later.

Reference numeral 256 denotes a backup roller for interposing the intermediate transfer belt 241 between the primary transfer nip and the cleaning blade 250 provided on the upstream side in the belt rotation direction to back up the belt cleaning. I have. However, cleaning blade 2
Reference numeral 50 denotes the intermediate transfer belt 2 by a contact / separation mechanism (not shown).
The intermediate transfer belt 241 is held away from the contact position with the intermediate transfer belt 241 while the intermediate transfer belt 241 holding the toner image passes therethrough.

The lubricant mass 251 and the lubricant application brush 2
Numerals 52 are disposed so as to be located between the backup roller 256 and the earth roller 246, respectively. The lubricant application brush 252 is positioned between
While polishing the lubricant mass 251 such as zinc stearate, the lubricant fine particles obtained by polishing are transferred to the intermediate transfer belt 2.
41 is applied to the transfer surface. However, these lubricant aggregates 25
1. The lubricant application brush 252 is moved by a moving mechanism (not shown), and the lubricant application brush 252 performs the intermediate transfer while the intermediate transfer belt 241 holding the toner image passes therethrough. Belt 241
Has been evacuated from the contact position.

As shown in FIG. 2, the primary transfer bias roller 247 is disposed so as to be in contact with the back surface of the intermediate transfer belt 241 downstream of the primary transfer nip in the belt rotation direction. A primary transfer current is supplied to the back surface downstream of the transfer nip. Rollers 242, 243, 244, 24 other than the primary transfer bias roller 247
5 and 246 are connected to the ground, respectively. Among these rollers, the ground roller 246 is disposed at a position closest to the primary transfer bias roller 247.
Therefore, the primary transfer current applied from the primary bias roller 247 to the back surface of the intermediate transfer belt 241 flows in the direction of the earth roller 246 as a whole. As described above, the primary transfer process of this color copying machine employs an indirect application method in which the primary transfer bias is applied downstream of the primary transfer nip, and the primary transfer current is guided to the ground upstream. ing.

B sequentially developed on the photosensitive drum 200
When the k, C, M, and Y toner images pass through the primary transfer nip, the photosensitive drum 200 and the intermediate transfer belt 24
Due to the synergistic effect of the pressing force with the first transfer current and the primary transfer current, the image is superposedly transferred onto the intermediate transfer belt 241 physically and electrostatically. Then, a full-color toner image is formed on the intermediate transfer belt 241 by superimposing and transferring the toner images of all four colors.

Below the primary transfer device 240, a secondary transfer device 260 is provided. This secondary transfer device 2
60 includes three rollers 262, 263, and 264, and a belt 261 stretched around these rollers. Also, 2
Next transfer bias roller 269, Secondary transfer bias power supply 2
69, a transfer paper charge removal charger 266, a belt charge removal charger 267, a cleaning blade 268, and the like.

Of the three rollers that stretch the belt 261, 263 is a driving roller that is driven to rotate counterclockwise in the figure by driving means (not shown). Rotate clockwise.

Further, the belt 261 is sandwiched between the cleaning blade 264 and the cleaning blade 268,
It is a backup roller that backs up the cleaning of the printer.

Reference numeral 262 denotes a driven roller that slides on and follows the belt 261.

The driven roller 262 and the secondary transfer bias roller 269 can be moved up and down in the figure by a moving mechanism (not shown). When these two rollers move to the upper limit, the belt 261 and the intermediate transfer belt 241 come into contact with each other, and the secondary transfer roller 269 is moved via the two belts (the intermediate transfer belt 241 and the belt 261). This contact causes the opposing roller 244
A secondary transfer nip as a retransfer position is formed between the secondary transfer bias roller 265 and the secondary transfer bias roller 265.

The secondary transfer bias power supply 269 includes a secondary transfer bias controller (not shown), and controls the output current to the secondary transfer bias roller 265 to a predetermined value. The secondary transfer current output from the secondary transfer bias power supply 269 to the secondary transfer bias roller 265 is applied to the back surface of the belt 261.

In the present color copying machine, the secondary transfer bias roller 265 and the driven roller are used between the standby state and the start of the superposition transfer of the third color (M) toner image in the primary transfer nip. 262 are each moved to the lower limit (the state shown by the two-dot line). Then, the third color overlapping transfer is started, and the opposing roller 244 and the secondary transfer bias roller 265 are attached to the rear end portion of the three color (Bk, C, M) superposed toner images on the intermediate transfer belt 241. Then, the secondary transfer bias roller 265 and the driven roller 262 are respectively moved to the upper limits. Then, the secondary transfer nip is formed by this movement. The full-color toner image obtained by superimposing the fourth color (Y) toner image in the primary transfer nip is conveyed to the secondary transfer nip as the intermediate transfer belt 241 rotates.

On the other hand, the paper feed bank 3 includes a manual feed tray 310 for storing OHP sheets and thick paper, a back side print paper cassette 320, paper feed cassettes 330, 340, and 3
50, a plurality of paper transport roller pairs, registration roller pairs 302
When the printing operation of the color printer is started, the paper feed rollers 311, 321, 331, 311 are provided.
41, 351 is activated. By this operation, the manual feed tray 310 or the cassettes 320, 33
0, 340, 350, the transfer paper P is discharged,
The sheet is conveyed to the registration roller pair 302 via the conveyance roller pair.

The registration roller pair 302 sends the conveyed transfer paper P onto the belt 261 at a timing at which the transfer paper P can be superimposed on the full-color toner image on the intermediate transfer belt 241 at the secondary transfer nip. The transfer paper P sent in this way passes through the secondary transfer nip while the full-color toner image on the intermediate transfer belt 241 is collectively transferred by the secondary transfer nip.
Then, after passing through a position facing the transfer sheet static elimination charger 266, the static electricity is eliminated, and then supplied onto a paper transport belt described later.

The belt 26 that has passed through the secondary transfer nip
The first portion passes through a position facing the belt static elimination charger 267 and is neutralized, and then the cleaning blade 26
8 cleans the residual toner adhering to the surface.

On the left side of the secondary transfer device 260 in FIG. 1 (FIG. 1), a paper transport belt 270 that is driven to rotate counterclockwise while being stretched by two stretching rollers is disposed. Then, the transfer paper P received from the belt 261 of the secondary transfer device 260 is sent out into the fixing device 280.

The fixing device 280 includes a pressure roller 281
And a heating roller 282. The transfer paper P sent from the paper transport belt 270 is sandwiched between the two rollers to fuse and fix the full-color toner image on the transfer paper P. The transfer paper P on which the full-color image has been fixed in this way is stacked face-up on a copy tray outside the copying machine via a pair of discharge rollers 290.

When a repeat copy for copying images of a plurality of originals 4 is performed, the operation of the color scanner 1
The optical writing process on the photoconductor drum 200 is performed at a predetermined timing following the developing process of the fourth sheet (Y) of the first sheet.
A reading process for the first color (Bk) of the sheet is performed. Also,
The primary transfer is performed on the portion of the intermediate transfer belt 241 that has completed the batch transfer process of the first full-color toner image and has completed the cleaning process by the cleaning blade 250 and the lubricant application process by the lubricant application brush 252. ,
The second Bk toner image is primarily transferred from the photosensitive drum 200.

The above is the copy mode for obtaining a four-color full-color copy. In the case of the three-color copy mode and the two-color copy mode, the same steps are performed for the designated color and the number of times. In the case of the single-color copy mode, the developing unit 23 of the predetermined color
The developing step, the primary transfer step, and the secondary transfer step are performed with 1 kept at the developing position.

Next, the characteristic configuration of the present color printer will be described. FIG. 3 is a perspective view showing the primary transfer bias roller 247 and the ground roller 246. In the figure, the primary transfer bias roller 247
Has a roller core 247a made of a conductive material such as metal and protruding from both ends of the roller. The roller core 247a is rotatably supported by bearings 253 and 254. The inside of the latter bearing 254 is subjected to a conductive process, and is connected to the above-described primary transfer bias power source (not shown). In addition, the earth roller 2
Reference numeral 46 also includes a roller core 246a made of a conductive material and protruding from both ends of the roller. The roller core 246a is rotatably supported by bearings 255 and 256. The inside of the bearing 256 is also connected to the above-described primary transfer bias power source (not shown) which has been subjected to a conductive process.

FIG. 4 is an enlarged sectional view showing a part of the intermediate transfer belt 241. In the figure, an intermediate transfer belt 241 includes a lower layer 241b formed of conductive rubber or the like,
And a coating layer 241a coated thereon, and the coating layer 241 side is a transfer surface. In the intermediate transfer belt 241 having such a configuration, as shown in FIG. 5, the back surface which is the side surface of the lower layer 241b contacts the primary transfer bias roller 247 and the earth roller 246. The current It output from the primary transfer bias power supply 248 is transmitted through the intermediate transfer belt 241 in the circumferential direction, and is fed back from the ground roller 246 to the ground of the primary transfer bias power supply 248 as a feedback current Ia. The photosensitive drum 200 and the intermediate transfer belt 241
Or the effective transfer current flowing into the contact surface with the contact. Of these currents, the one that contributes to the primary transfer of the toner image is the effective transfer current.

FIG. 6 is a schematic diagram showing the electrical resistance at each position of the intermediate transfer belt 241. In FIG. 6, R
1 and R2 are lower layers 241 of the intermediate transfer belt 241 respectively.
b indicates the electrical resistance in the circumferential direction, and R3 indicates the electrical resistance in the thickness direction of the coating layer 241a.
The electrical resistance of the intermediate transfer belt 241 at the portion between the primary transfer bias roller 247 and the ground roller 246 is R1 + R2. Further, the electrical resistance at the portion between the primary transfer bias roller 247 and the photosensitive drum 200 is R1 + R3.

In the conventional copying machine, the value of the primary transfer current (It) output from the primary transfer bias power supply is controlled to be constant (constant current control). FIG. 7 shows the required solid transfer voltage value [V] in the conventional copying machine and the intermediate transfer belt 241.
4 is a graph showing the relationship between the back surface resistivity [Ω / □] and the back surface resistivity. The solid transfer required voltage is a voltage value of a primary transfer bias necessary for transferring the toner image having the highest density to the intermediate transfer belt 241. As shown in the drawing, in the conventional copying machine, the current flowing through the intermediate transfer belt 241 decreases as the back surface resistivity (volume resistance of the lower layer 241b) of the intermediate transfer belt increases as the environment becomes hotter and humid. Therefore, a higher voltage primary transfer bias is applied.

FIG. 8 shows an effective transfer current value [μA] required for solid transfer in a conventional copying machine and an intermediate transfer belt 241.
4 is a graph showing the relationship between the back surface resistivity [Ω / □] and the back surface resistivity. As shown in the drawing, the effective transfer current value required for the solid transfer is constant regardless of the back surface resistivity [Ω / □] of the intermediate transfer belt 241. Here, R1, R2, and R3 shown in FIG. 6 decrease with increasing temperature and humidity in the environment, but the amount of decrease in R2 is R
When the amount exceeds 3, the feedback current Ia increases while the effective transfer current (It-Ia) decreases. In a conventional copying machine, such a decrease in the effective transfer current causes the effective transfer current value to fall below 20 [μA] in FIG. 8 to cause primary transfer failure due to insufficient primary transfer performance.

Therefore, the color copying machine of this embodiment is
The primary transfer bias control unit (not shown) of the secondary transfer bias power supply 248 is configured to control the value of the output current It from the primary transfer bias power supply 248 so as to satisfy the following conditional expression (1). ing.

## EQU1 ## The value of the output current It-the value of the feedback current Ia = constant (however, It-Ia ≧ the effective transfer current value required for solid transfer)

As shown in FIG. 6, the value of the effective transfer current is almost the same as the solution (It-Ia) of the equation (1). Therefore, in the present color copier, the intermediate transfer belt 24 is controlled not only by simply controlling the value of the output current It but also by controlling the difference between the output current It and the value of the feedback current Ia to be constant.
Regardless of the electrical resistance value of 1, the value of the effective transfer current value can be kept constant.

FIG. 9 is a graph showing an example of the relationship between the intermediate transfer belt current [mA] and the back surface resistivity [Ω / □] of the intermediate transfer belt 241 in the present color copying machine. In FIG. 9, the back surface resistivity of the intermediate transfer belt 241 is 1 × 1.
0 9 below the ^[Ω / □], the rate of increase in the feedback current Ia is rapidly increased. However, the critical point of the back surface resistivity where such a sharp rise is recognized is the primary transfer bias roller 2
47 and the primary transfer nip, the ground roller 2
It differs depending on the distance between the first transfer nip 46 and the primary transfer nip.

The inventor of the present invention has conducted intensive studies and has conducted an indirect application of the intermediate transfer belt 2 as in this color copier.
When an intermediate transfer belt having a back surface resistivity of less than 1 × 10 ⁷ [Ω / □] is used for supplying the primary transfer bias to the primary transfer bias 41, most of the output current It is used as the feedback current Ia and the primary transfer bias is used. It has been found that feedback is made to the power supply 248. Further, the back surface resistivity is 1 × 10 ¹¹
It has been found that when an intermediate transfer belt exceeding [Ω / □] is used, it is difficult to flow the effective transfer current to the contact surface between the photosensitive drum 200 of the primary transfer nip and the intermediate transfer belt 241. Therefore, it is desirable to use the intermediate transfer belt 241 having a back surface resistivity of 1 × 10 ⁷ to 1 × 10 ¹¹ [Ω / □]. In addition, regarding the back surface resistivity, not only the intermediate transfer belt 241 having a two-layer structure of the lower layer 241b and the cover layer 241b, but also the intermediate transfer belt having a multilayer structure of three or more layers or a single-layer structure of only one layer. The same is true.

In the above embodiment, the color copying machine detects the value of the feedback current Ia in addition to the value of the output current It, and calculates the difference between the two currents to indirectly determine the value of the effective transfer current. Was explained. However, the present invention is also applicable to an image forming apparatus that detects the value of the effective transfer current flowing from the intermediate transfer belt 241 to the photosensitive drum 200 by another method, such as a unit that detects the value of the effective transfer current. Needless to say.

[0066]

According to the first, second, third or fourth aspect of the present invention, the value of the effective transfer current value is kept constant in the image forming apparatus such as a printer regardless of the electric resistance value of the intermediate transfer member. Therefore, there is an excellent effect that the instability of the transfer performance due to the increase and decrease of the effective transfer current can be reduced.

In particular, according to the invention of claim 2, there is an excellent effect that the transfer nip width can be widened and the transfer performance can be improved as compared with the case where an intermediate transfer roller is provided as an intermediate transfer member. In addition, since the belt tension member can function as a transfer current applying member and an earth member, it is possible to avoid an increase in cost due to newly providing the transfer current applying member and the earth member. Has an excellent effect. Further, there is an excellent effect that the potential of the intermediate transfer belt on the upstream side of the transfer nip can be lowered to suppress the occurrence of pre-transfer.

According to the third aspect of the present invention, the value of the effective transfer current flowing from the intermediate transfer belt to the image carrier can be detected indirectly without providing a means for directly detecting the value. There is an excellent effect that it can be kept constant.

In particular, according to the fourth aspect of the present invention, there is an excellent effect that an effective transfer current can be reliably generated at the intermediate transfer position and good transfer performance can be exhibited.

According to the fifth aspect of the present invention, the value of the effective transfer current value is kept constant irrespective of the electric resistance value of the intermediate transfer member, so that the transfer performance is destabilized due to the increase and decrease of the effective transfer current. There is an excellent effect that can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a color copying machine according to an embodiment.

FIG. 2 is a configuration diagram showing a configuration around a photosensitive drum of the color copying machine.

FIG. 3 shows a primary transfer bias roller of the color copying machine;
FIG. 4 is a perspective view showing an earth roller.

FIG. 4 is an enlarged sectional view showing a part of an intermediate transfer belt of the color copying machine.

FIG. 5 is a schematic view showing the vicinity of a position where a primary transfer bias is applied on the intermediate transfer belt.

FIG. 6 is a schematic diagram showing electric resistance at each position of the intermediate transfer belt.

FIG. 7 shows a required voltage value [V] for solid transfer in a conventional copying machine.
4 is a graph showing the relationship between the resistance and the back surface resistivity [Ω / □] of the intermediate transfer belt.

FIG. 8 shows an effective transfer current value [μA] required for solid transfer in a conventional copying machine and a back surface resistivity [Ω /
Graph showing the relationship with [□].

FIG. 9 shows an intermediate transfer belt current [mA] in the color copying machine according to the embodiment and a back surface resistivity [Ω /
A graph showing an example of the relationship with [□].

[Explanation of symbols]

 Reference Signs List 1 color scanner 2 color printer 200 photosensitive drum (image carrier) 220 writing optical unit 230 revolver developing unit 240 primary transfer device 241 intermediate transfer belt (intermediate transfer member) 247 primary transfer bias roller 248 primary transfer bias power supply 260 Secondary transfer device 270 Paper transport device 280 Fixing device 3 Paper feed bank 3

Claims (5)

[Claims] The intermediate transfer member is moved to an intermediate transfer position, which is a contact position with the image carrier, so that a visible image carried on the image carrier of the image forming apparatus is transferred to a transfer surface of the intermediate transfer member. Moving the intermediate transfer member to a re-transfer position to transfer the visible image to another transfer member after the transfer, and applying a transfer current for intermediate transfer to the intermediate transfer member. In a transfer device including a current application unit and an output current control unit that controls a value of an output current It from the transfer current application unit, the transfer device reaches the contact position from the application position of the transfer current on the intermediate transfer body. A transfer apparatus, wherein the output current control means is configured to control the output current It so as to maintain the value of the effective transfer current constant. 2. The transfer device according to claim 1, further comprising: an intermediate transfer belt provided as the intermediate transfer member; and a grounding member that contacts the intermediate transfer belt and guides the transfer current to ground. A transfer device, wherein the transfer current applying means and the ground member are configured to apply the voltage to the intermediate transfer belt by an application method. 3. The transfer device according to claim 2, further comprising ground current value detecting means for detecting a value of a ground current Ia flowing into the ground member, wherein the output current It is set to keep "It-Ia" constant. Wherein the output current control means is configured to maintain the value of the effective transfer current constant. 4. The transfer device according to claim 3, wherein the intermediate transfer belt has a deviation in volume specific resistance in a belt thickness direction, and a surface resistance value of a back surface opposite to the transfer surface. A transfer device comprising a device adjusted to a range of 1 × 10 ⁷ to 1 × 10 ¹¹ [Ω / □]. 5. An image forming apparatus comprising: a visible image forming means for forming a visible image on an image carrier; and a transfer device for transferring the visible image to an intermediate transfer member and then to another transfer member. 5. An image forming apparatus, comprising: the transfer device according to claim 1, 2, 3, or 4 as the transfer device.