JPH0973242A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably perform an excellent image forming, by preventing an excessive current from being made to flow between a secondary transfer roll and the backup roll. SOLUTION: This image forming device is provided with a semiconductive back up roll 5 for supporting an intermediate transfer body 2 from a rear face side on the position opposite to the secondary transfer roll 4 for secondary transferring the toner image to a recording medium 11 held in contact with a surface of an intermediate transfer body 2 to which the toner image is firstly transferred from an latent image carrier 1 for carrying the toner image corresponding to the image information, the conductive roll 6 arranged while held in contact with the back up roll, a transfer voltage impressing circuit 10 for impressing the transfer voltage between the secondary transfer roll 4 and the back up roll 5, the transfer voltage arithmetic circuit 8 for deciding the transfer voltage impressed to the secondary transfer roll 4 corresponding to the detecting signal of a resistance detecting circuit 7 for detecting the resistance value of the back up roll 5 at the retiring time of the secondary roll 4, and the transfer voltage control circuit 9 for controlling the transfer voltage impressing circuit 10 corresponding to the arithmetic output of the transfer voltage arithmetic circuit 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser printer, and more particularly, to an unfixed toner image formed on a latent image carrier via an intermediate transfer member on a sheet or the like. The present invention relates to an image forming apparatus that transfers a recording medium.

[0002]

2. Description of the Related Art A so-called electrophotographic electronic copying machine using an intermediate transfer member or a laser printer uses a toner to form an electrostatic latent image or the like according to an image signal formed on a latent image carrier such as a photosensitive drum. The toner image on the intermediate transfer body is secondarily transferred onto a recording medium, and then a copy image or a print image is obtained.

Conventionally, as this type of image forming apparatus, those described in JP-B-49-209 and JP-A-62-206567 are known. FIG. 7 is a schematic diagram illustrating a schematic structure of an electronic copying machine as an example of an image forming apparatus using an intermediate transfer member, in which 30 is a document scanning unit and 3 is a document scanning unit.
1 is a document table, 32, 33 and 34 are mirrors, 35 is an imaging lens, 36 is a photoelectric converter such as CCD, 40 is an image processing unit, 41 is a laser, 42 is an imaging optical system, and 43 is scanning optics. A system, 44 is a mirror, 50 is an image forming control unit, and 60 is an image forming unit.

The image forming unit 60 includes a photosensitive drum 1, which is a latent image carrier, an intermediate transfer member 2, a primary transfer unit 3, a secondary transfer roll 4, a backup roll 5, a tray 15 for accommodating a recording medium 11, a recording medium. A feed roller 16 for taking out the medium 11 one by one, a sending roller 17 for feeding the taken-out recording medium between the intermediate transfer body 2 and the secondary transfer roller 4, and residual toner on the intermediate transfer body 2 after the secondary transfer The cleaner 18 for removing the recording medium 11 after the secondary transfer is used as the intermediate transfer member 2
It is composed of a peeling claw 19 for peeling the recording medium 11 from the recording medium 11, and a conveyance belt 20 for conveying the peeled recording medium 11 to the fixing device 21.

In FIG. 1, the original document placed on the original document table 31 includes mirrors 32, 33 and 3 which constitute the original document scanning section 30.
4. An optical system including the imaging lens 35 is used for scanning, and a photoelectric converter 36 such as a CCD reads it as an image signal. The illumination light source is not shown. The read image signal is subjected to required processing in the image processing unit 40 and given to the laser 41. The laser 41 emits laser light modulated by the processed image signal.

The emitted laser light is focused on the imaging optical system 42,
The photosensitive drum 1 is irradiated with the scanning optical system 43 and the mirror 44. The surface of the photoconductor drum 1 is uniformly charged by a charger 13, and the surface of the photoconductor drum 1 is scanned with a laser beam modulated by an image signal to form an image on the surface of the photoconductor drum 1. A corresponding electrostatic latent image is formed.
The electrostatic latent image reaches the developing device 14 by the rotation of the photosensitive drum 1,
Toner is developed.

The photosensitive drum 1 carrying the toner image further rotates to reach the primary transfer portion where the primary transfer device 13 is arranged, and the toner image is primarily transferred to the intermediate transfer member 2. The toner image primarily transferred to the intermediate transfer body 2 reaches the secondary transfer site where the secondary transfer roll 4 is arranged by the movement of the intermediate transfer body 2. In the case of a color copying machine, the above-mentioned latent image formation to primary transfer of the toner image are repeated for the required colors (generally, black: BK, yellow: Y, magenta: M, cyan: C) for intermediate transfer. A color toner image is formed by superposing multicolor toners on the body 2.

At this time, the secondary transfer roll 4, the peeling claw 19 and the cleaner 18 are retracted from the intermediate transfer member 2 until the transfer of the final toner image is completed. When the intermediate transfer body 2 to which the required toner image is primarily transferred reaches the position of the secondary transfer roll 4, the recording medium 11 taken out from the tray 15 by the feed roller 16 and sent out by the delivery roller 16 is the intermediate transfer body. 2 and the secondary transfer roll 4.

At this time, the toner image on the intermediate transfer body 2 is secondarily transferred onto the recording medium 11 by the transfer voltage applied between the secondary transfer roll 4 and the intermediate transfer body 2. The recording medium 11 on which the toner image is secondarily transferred is separated by the peeling claw 19 from the intermediate transfer member 2.
The sheet is separated from the sheet and is sent to the fixing device 21 by the conveyor belt 20. The fixing device 21 heats / pressurizes the recording medium 11 when passing between the pair of fixing rolls to fix the toner image,
Discharge outside the machine.

The image forming unit 60 is an image forming process control unit 50.
Controlled by. FIG. 8 is a schematic diagram for explaining a main configuration of the image forming apparatus. The same reference numerals as those in FIG. 7 correspond to the same portions, 10 is a transfer voltage applying device, 14a is a black developing device, and 14b is a yellow developing device. , 14c are magenta developing units, and 14d are Sian developing units.

In FIG. 1, an unfixed toner image T formed on a latent image carrier 1 such as a photosensitive drum is primary-transferred to a belt-shaped intermediate transfer member 2 by a primary transfer device 3, and this unfixed toner is further transferred. The image T is secondarily transferred from the intermediate transfer member 1 to the recording medium 11 by the secondary transfer roll 4 to form a desired image on the recording medium 11. Black (BK), Yellow (Y), Magenta (M) as shown
And developing devices 14a to 1 corresponding to cyan (C) colors
In a color image forming apparatus including 4d, which forms a full-color image by superposing unfixed toner images of respective colors,
An unfixed toner image of each color formed every one rotation of the latent image carrier 1 is superposed on the intermediate transfer body 2, and the composite image is collectively transferred to the recording medium 11.

In the conventional color image forming apparatus, since the unfixed toner images of respective colors are directly transferred onto the recording medium 11 by multiple transfer, the thickness and surface characteristics of the recording medium and the conveyance characteristics of the recording medium with respect to the latent image carrier are conveyed. The quality of a color image formed on a recording medium is affected by many factors such as the above.
However, in the color image forming apparatus using the intermediate transfer body 2 as described above, since the composite toner image that has already been multi-transferred is transferred onto the recording medium 11, the above unstable factors can be eliminated. Further, there is an advantage that it is possible to effectively prevent the occurrence of image disturbance and color shift at the time of multiple transfer.

Then, in the conventional image forming apparatus having such a structure, from the latent image carrier 1 to the intermediate transfer member 2,
Alternatively, the transfer of the unfixed toner image T from the intermediate transfer body 2 to the recording medium 11 is performed by an electrostatic transfer method, and a corona discharger (primary transfer device 3) provided at the back side of the transfer target at each transfer position. ) Or a bias roll (secondary transfer roll 4) is applied with a voltage having a polarity opposite to that of the toner, whereby the unfixed toner image T is transferred from the latent image carrier 1 to the intermediate transfer member 2 or the intermediate transfer member 2 is transferred. 2 is transferred to the recording medium 11.

[0014]

However, in such a conventional image forming apparatus, when the above-mentioned secondary transfer roll is used as the secondary transfer means for the unfixed toner image on the intermediate transfer member 2, The following problems occur. That is, since the conductive backup roll 5 forming the counter electrode of the secondary transfer roll 4 is disposed at a position facing the secondary transfer roll 4 with the intermediate transfer body 2 interposed therebetween, for example, a small-sized recording medium. When the toner image is transferred onto the recording medium 11, if the secondary transfer roll 4 protruding from the recording medium 11 comes into direct contact with the semi-conductive intermediate transfer body 2, the secondary transfer roll 4 and the backup roll 5 become excessively large. A large current will flow.

Therefore, there is a problem that a sufficient transfer electric field cannot be formed when a toner image is transferred onto a small-sized recording medium, resulting in defective transfer. In addition, at this time, an excessively large current flows, so that the intermediate transfer member is easily damaged. This problem can be prevented by providing the secondary transfer electric field forming portion with a large resistance value that makes it difficult for an excessive current to flow.

As a conventional technique proposed in view of these problems, there is a transfer roller mechanism disclosed in JP-A-1-288880. With this conventional transfer roller mechanism,
While covering the backup roll with a dielectric film to prevent current from flowing into the backup roll, a back electrode is provided on the back side of the intermediate transfer body adjacent to the backup roll to utilize the creeping resistance of the intermediate transfer body. This prevents the generation of excessive current.

However, in the means disclosed in the above publication, when a material having a high resistance is selected as the material of the intermediate transfer body, a large voltage drop occurs in the intermediate transfer body portion between the bias roll and the back electrode. There was a new problem that a transfer electric field having a sufficient strength could not be obtained. Usually, in this type of image forming apparatus, a corona discharger is used as the primary transfer device to transfer the toner image from the latent image carrier to the intermediate transfer body.However, if the resistance of the intermediate transfer body is low, the corona discharger is used. The applied charge conducts along the surface of the intermediate transfer body, and the toner image jumps from the latent image carrier to the intermediate transfer body before the primary transfer position, that is, in the area where the latent image carrier and the intermediate transfer body are not in close contact with each other. As a result, the image quality is deteriorated due to so-called early transfer.

Although the use of an intermediate transfer member having a high resistance is an effective means for solving such a problem of early transfer, the transfer roller mechanism described in the above publication has an intermediate having a high resistance for the above reason. There was a difficulty in using the transfer body. It is also possible to reduce the voltage drop by bringing the back electrode closer to the secondary transfer roll, but the backup roll has a diameter of 25 mm or more, which reduces the voltage drop at the intermediate transfer member. The back electrode could not be brought close to the bias roll to a negligible extent.

When the secondary transfer roll is formed of a high resistance member, the resistance is apt to change due to stains caused by toner or paper dust, which is not a good idea. As described above, it is best to form the backup roll with a high resistance member, but it is necessary to solve the problems associated with it. Two different technical measures have been proposed for this purpose.

The first technical means is a latent image carrier on which a toner image corresponding to image information is formed, an intermediate transfer member on which an unfixed toner image is primarily transferred from the latent image carrier, and an intermediate between these intermediate transfer members. Image formation including a secondary transfer roll that secondarily transfers the unfixed toner image from the transfer body to the recording medium, and a backup roll that supports the intermediate transfer body from the back side at a position facing the secondary transfer roll. In the apparatus, the backup roll is formed by coating a conductive roll with a semiconductive or insulating thin layer, and grounding the conductive roll.

The second technical means is a latent image carrier on which a toner image is formed according to image information, and an intermediate transfer member on which an unfixed toner image is primarily transferred from the latent image carrier.
This intermediate transfer body, a secondary transfer roll that secondarily transfers the unfixed toner image from the intermediate transfer body to the recording medium, and a backup that supports the intermediate transfer body from the back side at a position facing the secondary transfer roll. In an image forming apparatus composed of a roll, the insulating roll is formed as the backup roll by being covered with a semiconductive thin layer, and the semiconductive thin layer is grounded.

According to these technical means, the conductive roll is covered with a semi-conductive or insulating thin layer, and the conductive roll is grounded to form a backup roll.
Alternatively, since the insulating roll is covered with a semi-conductive thin layer and the semi-conductive thin layer is grounded to form a backup roll, even if the secondary transfer roll comes into direct contact with the intermediate transfer body, it is excessive. No current flows from the bias roll to the backup roll.

For this reason, a stable transfer electric field can be formed for recording media of various sizes, and damage to the intermediate transfer member (belt) can be prevented in advance. Further, because the resistance of the backup roll itself prevents the generation of an excessive current, it is possible to select a material having a high resistance as the material of the intermediate transfer member and prevent the deterioration of the image quality due to the early transfer of the toner image. it can.

However, since the backup roll resistance changes due to variations in manufacturing, environmental changes, deterioration during long-term use, etc., when the voltage applied between both rolls is constant, the amount of voltage drop due to the backup roll resistance is Due to the change, the transfer electric field may change and stable transfer may not be possible. In consideration of such problems, a method is known in which a secondary transfer roll and a backup roll are brought into contact with each other via an intermediate transfer member, and the resistance of the secondary transfer roll and the intermediate transfer member is detected to determine the voltage during the secondary transfer. Has been.

FIG. 9 is a schematic diagram for explaining a conventional structure for controlling the secondary transfer voltage by detecting the resistance of the secondary transfer portion. In the figure, 6a is an electrode roll, 7 is a resistance detection circuit, 9 is a transfer voltage control circuit, and 10 is a transfer voltage application circuit. The secondary transfer roll 4 and the backup roll 5 are contacted via the intermediate transfer body 2. The electrode roll 6 is provided on the back surface of the intermediate transfer member 2 at a position close to the backup roll 5.
Contact a.

Then, the resistance value of the both is detected by the resistance detection circuit 7 from the current i flowing from the secondary transfer roll 4 through the intermediate transfer material 2 by the voltage applied from the transfer voltage application circuit 10.
The transfer voltage control circuit 9 controls the transfer voltage output from the transfer voltage applying circuit 10 according to the detected value. As a result, the optimum secondary transfer voltage can be obtained. However, in these methods, the secondary transfer roll 4 is used before the secondary transfer.
Is brought into direct contact with the surface of the intermediate transfer body 2 without passing through the paper, and there is a drawback that accurate resistance detection cannot be performed due to dirt on the back surface of the transfer paper due to toner dirt on the secondary transfer roll or toner dirt on the secondary transfer roll. Have

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus for transferring a toner image from an intermediate transfer member to a recording medium using a secondary transfer roll. An object of the present invention is to provide an image forming apparatus capable of preventing an excessive current from flowing between the secondary transfer roll and the backup roll and stably obtaining a high quality image.

[0028]

In order to achieve the above object, the present invention provides a latent image carrier on which a toner image corresponding to image information is formed, and an unfixed toner image is primarily formed on the latent image carrier. An intermediate transfer member to be transferred, and a secondary transfer roll that secondarily transfers the unfixed toner image from the intermediate transfer member to a recording medium,
In an image forming apparatus composed of a semi-conductive backup roll that supports the intermediate transfer member from the back side at a position facing the secondary transfer roll, the resistance of the backup roll is detected, and the detected resistance of the backup roll is detected. It is characterized in that the optimum transfer voltage according to the above is determined and this is applied as the secondary transfer voltage.

That is, as shown in FIG.
According to the first aspect of the present invention, a latent image carrier 1 on which a toner image corresponding to image information is formed, and an unfixed toner image is primarily transferred from the latent image carrier 1 by the primary transfer device 3. The transfer body 2, a secondary transfer roll 4 that comes into contact with the surface of the intermediate transfer body 2 and secondarily transfers the unfixed toner image onto the recording medium 11, and the transfer roller 2 is located at a position facing the secondary transfer roll 4. Between the semi-conductive backup roll 5 that supports the intermediate transfer member 2 from the back side, the conductive roll 6 that is arranged in contact with the backup roll 5, and the secondary transfer roll 4 and the backup roll 5. A transfer voltage applying circuit 10 for applying a transfer voltage to the secondary transfer roll 4, a resistance detecting circuit 7 for detecting a resistance value of the backup roll 5 when the secondary transfer roll 4 is retracted, and a resistance detecting circuit 7 for detecting the resistance value of the backup roll 5 Secondary transfer low A transfer voltage operation circuit 8 for determining the transfer voltage applied to the transfer voltage control circuit 4 and a transfer voltage control circuit 9 for controlling the transfer voltage application circuit 10 so as to output a transfer voltage according to the operation output of the transfer voltage operation circuit 8. It is characterized by having.

A second aspect of the present invention is characterized in that the transfer voltage applying circuit 10 of the first aspect of the present invention includes a voltage applying roll that makes contact with the backup roll 4 to rotate. Further, a third invention according to claim 3 is the transfer voltage applying circuit 10 according to the first invention.
The transfer voltage output of 1 is connected to the secondary transfer roll 4.

Further, the fourth invention according to claim 4 is as follows.
1st or 3rd invention WHEREIN: The opening / closing switch which isolate | separates the said electroconductive roll 6 from the said resistance detection circuit 7 at the time of the said secondary transfer is provided. A fifth aspect of the present invention is the second aspect of the first aspect, wherein the secondary transfer roll 4 is retracted from the intermediate transfer body 2 at least when the resistance value of the backup roll 5 is detected. It is characterized by having a mechanism.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION With the above-described structure of the present invention, the resistance value of the backup roll 5 which is installed on the back surface of the intermediate transfer member and has little toner stain is detected, and the optimum transfer voltage is determined based on the detection result. By using this as the secondary transfer voltage, the resistance can be detected with high accuracy, and the optimum transfer voltage can be determined, so that a high-quality image can be stably obtained, and the secondary transfer roll is used as the intermediate transfer member 2. It is possible to avoid contamination of the back surface of the recording medium without contaminating the recording medium.

That is, in the structure of the first invention, the latent image carrier 1 carries a toner image corresponding to image information. The primary transfer device 3 primarily transfers the unfixed toner image from the latent image carrier 1 to the intermediate transfer member 2. Secondary transfer roll 4
Touches the surface of the intermediate transfer member 2 to secondarily transfer the unfixed toner image onto the recording medium 11.

The semi-conductive backup roll 5 supports the intermediate transfer member 2 from the back side at a position facing the secondary transfer roll 4, and the conductive roll 6 is arranged in contact with the backup roll 5. A transfer voltage application circuit 10 applies a transfer voltage between the secondary transfer roll 4 and the backup roll 5. The resistance detection circuit 7 detects the resistance value of the backup roll 5 when the secondary transfer roll 4 is retracted, and the transfer voltage calculation circuit 8 is detected according to the detection signal.
Determines the transfer voltage applied to the secondary transfer roll 4.

The transfer voltage control circuit 9 controls the transfer voltage application circuit 10 to output a transfer voltage according to the operation output of the transfer voltage operation circuit 8. Further, in the configuration of the second invention, the voltage application roll provided in the transfer voltage application circuit 10 of the first invention contacts the backup roll 4 and rotates to apply the transfer voltage.

Further, in the configuration of the third invention,
The transfer voltage applying circuit 10 in the first invention connects the transfer voltage output to the secondary transfer roll 4. Further, in the configuration of the fourth aspect of the invention, in the first or third aspect of the invention, the open / close switch disconnects the conductive roll 6 from the resistance detection circuit 7 during the secondary transfer.

Then, in the configuration of the fifth invention,
The secondary transfer roll retraction mechanism provided in the secondary transfer roll 4 in the first aspect of the invention separates the secondary transfer roll 4 from the intermediate transfer body 2 at least when the resistance value of the backup roll 5 is detected.

[0038]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 2 is a schematic view of a main part for explaining a schematic configuration of a color electrophotographic copying machine to which the present invention is applied. In the figure, reference numeral 1 is a photosensitive drum which is a latent image carrier, and the surface of which is rotated by an arrow in the direction of the arrow, and an electrostatic latent image corresponding to image information is formed by the well-known electrophotographic process described in FIG. An image is formed.

Around the photosensitive drum 1, developing devices 14a, 14 corresponding to each color of black (BK), yellow (Y), magenta (M) and cyan (C) are provided.
A toner developing device 14 including b, 14c, and 14d is provided, and a toner image T is formed by developing the electrostatic latent image formed on the photosensitive drum 1 with any one developing device. ing.

Therefore, if the electrostatic latent image written on the photosensitive drum 1 corresponds to the image information of yellow, this electrostatic latent image is transferred to the toner by the developing device 14b containing the yellow (Y) toner. After development, a yellow unfixed toner image is formed on the photosensitive drum 1. Reference numeral 2 is a belt-shaped intermediate transfer member arranged so as to be brought into contact with the surface of the photosensitive drum 1, and is stretched by a plurality of rolls and rotated in the arrow direction.

The unfixed toner image T formed on the photosensitive drum 1 is transferred from the photosensitive drum 1 to the intermediate transfer member 2 at a primary transfer site where the photosensitive drum 1 and the intermediate transfer member 2 are in contact with each other.
Is transferred to the surface of At this primary transfer portion, a corona discharger is provided as the primary transfer device 3 on the back surface side of the intermediate transfer member 2. By applying a voltage having a polarity opposite to the charging polarity of the toner to this corona discharger, The unfixed toner image on the photosensitive drum 1 is electrostatically attracted to the intermediate transfer member 2 and transferred.

In the case of forming a monochromatic image, the intermediate transfer member 2
The unfixed toner image T primarily transferred onto the recording medium 11 is immediately secondarily transferred onto the recording medium 11. However, in the case of forming a color image in which toner images of a plurality of colors are superposed, The steps of forming the toner image and the primary transfer of the toner image are repeated for the number of colors. For example, in the case of forming a full-color image in which toner images of four colors are superposed, unfixed toner images T of black, yellow, magenta, and cyan are formed on the photosensitive drum 1 for each rotation, and these undetermined toner images T are not determined. The deposited toner images T are primarily transferred to the intermediate transfer body 2 in sequence.

On the other hand, the intermediate transfer member 2 rotates in the same cycle as the photosensitive drum 1 while holding the black unfixed toner image T which is primarily transferred first, and the intermediate transfer member 2 rotates once on the intermediate transfer member 2. The yellow, magenta, and cyan color unfixed toner images T are superposed on the black unfixed toner image T for each transfer. The unfixed toner image T primarily transferred to the intermediate transfer body 2 in this manner is transported to the secondary transfer portion facing the transport path of the recording medium 11 as the intermediate transfer body 2 rotates.

At the secondary transfer site, the secondary transfer roll 4
Is in contact with the intermediate transfer member 2 and is discharged from the tray 15 by the feed roller 16 at a predetermined timing.
1 is sandwiched between the secondary transfer roll 4 and the intermediate transfer body 2. On the back surface side of the intermediate transfer member 2 at the secondary transfer portion, a semi-conductive backup roll 5 which is an opposite electrode of the secondary transfer roll 4 is provided.

A conductive voltage applying roll (contact roll) 12 and a conductive roll 6 for resistance detection are in contact with the surface of the backup roll 5. Then, the transfer circuit 26 applies a specified potential difference between the voltage application roll 12 and the conductive roll 6 for resistance detection, detects the current flowing at that time, and determines the optimum preset value based on the detected current value. By applying the secondary transfer voltage to the voltage application roll 12, the unfixed toner image T carried on the intermediate transfer body 2 is electrostatically transferred to the recording medium 11 at the secondary transfer portion.

Recording medium 1 on which an unfixed toner image is transferred
1 is peeled off from the intermediate transfer body 2 by the peeling claw 9 and is sent to the fixing device 21 by the conveying belt 20 to fix the toner image. On the other hand, the cleaner 18 removes residual toner from the intermediate transfer member 2 after the secondary transfer of the unfixed toner image is completed. The cleaner 18 has a cleaning brush 18 a and a cleaning blade 18 b, and removes the toner remaining on the intermediate transfer body 2. In addition, this cleaner is not limited to the above configuration,
It may be provided with other known means.

The secondary transfer roll 4, the peeling claw 9 and the cleaner 18 are arranged so as to come into contact with and separate from the intermediate transfer body 2, and when a color image is formed, the final color of the unfixed color is not fixed. These members are retracted from the intermediate transfer body 2 until the toner image is primarily transferred to the intermediate transfer body 2. In the present embodiment, the intermediate transfer member 2 is formed to have a thickness of 0.1 mm, for example, by adding an appropriate amount of an antistatic agent such as carbon black to a resin such as acrylic, vinyl chloride, polyester, polycarbonate, polyimide or various rubbers. The volume resistivity is adjusted to 10 ⁶ to 10 ¹⁴ Ω · cm.

Regarding the volume resistivity of the intermediate transfer member 2, an HR probe (manufactured by Mitsubishi Yuka, inner electrode diameter 16 mm, ring electrode inner diameter 30 mm) was connected to a high resistance resistivity meter (Mitsubishi Yuka, Hiresta IP). , 500V on the front and back of the intermediate transfer body 2
The voltage is applied and the value after 1 minute is measured. The secondary transfer roller 4 is formed of carbon black silicone foam which an antistatic agent is contained an appropriate amount of the volume resistivity is adjusted to 10 ² 1~10 ⁶ Ω · cm, such as various rubbers such as EPDM rubber ing. The reason why the resistance of the secondary transfer roll 4 is made lower than that of the backup roll 5 is as described above.

The volume resistivity ρV of the secondary transfer roll 4
Is a 1 inch (2.54 cm) wide copper tape wound in the circumferential direction of the secondary transfer roll, a voltage of 50 V is applied between the core metal of the secondary transfer roll and the copper tape, and the current after 1 minute Value I
Read and calculate by the following formula. ρV = 2πW · V / I · l n (r 2 / r 1) where, l _n is natural logarithm, r ₁ is the core metal roll diameter, r ₂ is the outer diameter of the transfer roller, W = 2.54 cm Figure 3 FIG. 4 is an explanatory diagram of a configuration example of a backup roll and a transfer circuit in the present embodiment. 5a is an insulating material, 5b is a semiconductive thin film, 5c is a conductive core material, 7 is a resistance detection circuit, and 8 Is a transfer voltage calculation circuit, 9 is a transfer voltage control circuit, 10 is a transfer voltage application circuit, and 22 is an open / close switch.

In the figure, the backup roll 5 is constructed by covering the surface of an insulating material 5a with a semiconductive thin film 5b. The thin film 5b is 10 to 20.
It is formed to a thickness of 0 μm and its surface resistivity ρS is 10 ⁷
Ω / □ or more. Further, the voltage application roll 12 is in contact with the thin film 5b at a distance of 20 to 40 mm from the contact position with the intermediate transfer member 2 in the circumferential direction, and at this position, the thin film 5b is biased. It can be done.

The surface resistivity of the backup roll 5 is 1 in the circumferential direction of the backup roll 5 when the two metal rolls having a diameter of 15 mm, which are longer than the backup roll 5, are used.
Contact them at a distance of 0 mm, and apply a voltage of 50 between the two metal rolls.
The current value I after 1 minute from the application of V is read and calculated by the following formula. ρS = LV / gI However, L is a backup roll length (cm), g is a distance (cm) between two metal rolls. In FIG. 3, the optimum transfer voltage is set as follows.

When detecting the resistance value of the backup roll 5, the open switch 22 is closed to transfer the transfer voltage applying circuit 1.
A predetermined voltage from 0 is applied to the backup roll 5 via the voltage application roll 12. At this time, the secondary transfer roll 4 is retracted from the intermediate transfer body 2. A current flows from the voltage application roll 12 through the creeping surface of the backup roll 5 and the conductive roll 6, and this current is input from the open / close switch 22 to the resistance detection circuit 7.

The resistance detection circuit 7 detects the input current value and supplies the detected value to the transfer voltage calculation circuit 8. The optimum transfer voltage for the resistance value of the backup roll is stored in advance in the transfer voltage calculation circuit 8 as a table, and the optimum transfer voltage is calculated based on this table. The calculated optimum transfer voltage data is given to the transfer voltage control circuit 9, and the transfer voltage control circuit 9 controls the transfer voltage output of the transfer voltage applying circuit 10 based on the data to control the transfer voltage output from the voltage applying roll 12 to the backup roll 5. Apply to.

In this example, -500V is applied to the backup roll 5 before the secondary transfer, and the current i flowing from the conductive roll 6 to the resistance detection circuit 7 is detected. From this current value i, pS = LV / Gi The resistance value of the backup roll 5 is calculated, and the optimum secondary transfer voltage obtained from the relationship shown in FIG. 4 is determined.

Here, L is the length (cm) of the backup roll 5 in the longitudinal direction, and G is the distance (cm) between the conductive roll 6 and the core material 5. As a result, an optimum transfer voltage is applied between the backup roll 5 and the secondary transfer roll 4 during the secondary transfer. FIG. 4 is an explanatory diagram of an example of a table of optimum transfer voltages for backup roll resistance values stored in the transfer voltage calculation circuit.

The figure shows the resistance value (lo
gSR and SR indicate the range of the optimum transfer voltage (V) with respect to the backup roll surface resistance value) and the bias roll resistance, and the lower limit a overcomes the adhesive force of the toner to the intermediate transfer member to the recording medium. A transfer voltage that can generate a transfer electric field necessary for transfer is shown. The upper limit b is such that the transfer electric field becomes too high and Paschen discharge is generated between the intermediate transfer body and the recording medium. Indicates the transfer voltage that causes a decrease.

From this relationship, it is understood that the higher the resistance value of the backup roll (including the resistance of the secondary transfer roll), the higher the optimum transfer voltage. The data of this relation is stored as a table in the form of digital or analog in the transfer voltage arithmetic circuit and the above-mentioned arithmetic operation is performed. FIG. 5 is a flow chart for explaining the transfer procedure in this embodiment.

In the transfer process, the main controller of the image forming apparatus sets whether the resistance value of the backup roll is measured for each transfer process or when the image forming job is started. First, when the image forming process is started and the backup roll resistance detection mode is selected (Yes in S-1), the open / close switch 22 shown in FIG.
Is closed (S-2), and a predetermined voltage is applied from the transfer voltage applying circuit 10 (S-3).

The resistance detection circuit 7 detects the current flowing along the surface of the backup roll in response to the application of this voltage, and the transfer voltage calculation circuit 8 calculates the optimum transfer voltage based on the detected current value (S-4). , The optimum transfer voltage is determined based on the calculation result. Next, the open / close switch 22 is opened (S-5) to disconnect the resistance detection circuit 7 from the backup roll 5.

When the image forming process progresses, the toner image is primarily transferred to the secondary transfer roll, and the toner image is secondarily transferred to the recording medium fed between the intermediate transfer body and the secondary transfer roll. The above-mentioned optimum transfer voltage is applied between the roll and the secondary transfer roll (S-6), and the secondary transfer process is executed (S-7). Then, if there is another image forming job, the process returns to S-6, and if the job is completed, the process ends.

In S-1, if the resistance detection mode is not set ((No in S-1), the process proceeds to S-6 to execute the secondary transfer process. Fig. 6 shows the backup roll and transfer circuit in this embodiment. It is explanatory drawing of the other structural example of,
The backup roll 5 is formed by coating the surface of a conductor 5d with a semiconductive thin film 5e.
The conductive roll 6 is placed in contact with e.

A transfer circuit 26 is installed between the conductive roll 6 and the conductive core material 5c.
The optimum transfer voltage is calculated based on the current value flowing between the core material 5c and the core material 5c. As the conductor 5d, a rubber roller or a metal roll in which conductive carbon is dispersed is used.

On the other hand, as the thin film 5e, PV is used.
A material whose resistance is controlled by dispersing an appropriate amount of an antistatic agent such as carbon black in dF, polyester film, PFA, or acrylic is used, and its volume resistivity is 10 ⁸ Ω · cm or more. The thinner the thin film 5e is, the closer the conductor 5d is to the secondary transfer roll 4, so that a sufficient transfer electric field can be obtained even if the transfer voltage applied to the secondary transfer roll 4 is low. You can But,
In the present example, the thickness is set to 10 μm to 100 μm because of the problem of pinholes and manufacturing stability.

Further, the thin film 5e exhibits the same effect as thinning as the dielectric constant is higher.
F (dielectric constant 8) was used. In this case, current flows in the thickness direction of the thin film 5e covering the conductor 5d,
In addition, since the thin film 5e is formed to be extremely thin in order to obtain a sufficient transfer electric field at a low voltage, it is necessary to set the volume efficiency of the thin film 5e to be large in order to prevent the occurrence of overcurrent. Occurs.

However, in general, most of the semiconductive materials have a characteristic that volume efficiency is significantly reduced in a high electric field. Therefore, in the constitution of the backup roll 5, the material of the thin film 5e and the secondary film 5e are used. There is a concern that the voltage applied to the transfer roll 4 cannot be freely selected. On the other hand, in the example shown in FIG. 3, since the current flows in the creeping direction of the thin film 5b covering the insulating material 5a, the voltage application roll 12 and the thin film 5b
If the contact point with the thin layer film 5b is sufficiently distant from the contact point between the thin layer film 5b and the intermediate transfer body 2, it is not necessary to set the volumetric low efficiency of the thin layer film 5e so large.

Therefore, there is no fear of the above-mentioned problems relating to the example shown in FIG. 6, and the voltage applied between the material of the thin film 5e and the secondary transfer roll 4 can be freely selected. The resistance value from the contact position between the secondary transfer roll 4 and the intermediate transfer member 2 to the grounding position between the thin film 5e and the conductive roll is 10 ⁷ Ω for the following reason.
It is preferable that it is above.

That is, in this type of image forming apparatus, the current capacity of the power source is set in consideration of safety when a human body touches the apparatus and prevention of accident such as ignition due to jam of a recording medium (so-called paper jam). It is limited to a few mA or less, and when the secondary transfer roll comes into direct contact with the intermediate transfer member and an overcurrent flows, the current limiter of the power supply operates and the power supply voltage drops,
It is configured to prevent continuous energization of overcurrent.

Further, in such a structure, a hole such as a scratch or a (pinhole) is generated in the intermediate transfer member, and the current is concentrated in that portion, so that the intermediate transfer member is greatly damaged or ignited. It is also effective for prevention. Therefore, when transferring to a small-sized recording medium or when there is no recording medium, if the magnitude of the overcurrent is 100 μA or less per unit length,
The current limiter does not work and the power supply voltage does not suddenly drop, and it is possible to prevent transfer failure and damage to the intermediate transfer member.

For this reason, since the transfer voltage is about 1000 V in this configuration example, the volume resistance from the contact position between the backup roll and the intermediate transfer member to the ground position of the conductive roll is set to a unit length (1 cm). ) Per 10 ⁷
It is preferable to set it to Ω or more. The resistance-detecting conductive roll 6 that contacts the backup roll 5 is made of metal or conductive rubber having a volume resistivity of 10 ⁶ Ω · cm or less.

Although the conductive roll is used as the resistance detecting electrode here, it is needless to say that the conductive roll is not limited to this and, for example, a conductive brush or a film-shaped conductive member can be applied. Now, in the example shown in FIG. 6, a predetermined potential difference is provided between the conductor 5d forming the backup roll 5 and the conductive roll 6 for resistance detection, and the resistance is calculated by current detection. That is, at the time of resistance detection, the open / close switch 22 is closed to apply a predetermined voltage, for example, 500 V, from the transfer voltage applying circuit 10 to the core material 5c of the backup roll 5. At this time, the secondary transfer roll 4 is retracted from the intermediate transfer body 2.

The resistance detection circuit 7 detects the resistance value of the backup roll 5 based on the value of the current flowing from the backup roll 5 to the conductive roll 6 in response to the application of the voltage.
The optimum transfer voltage corresponding to the detected resistance value is calculated by the transfer voltage calculation circuit 8. The transfer voltage calculation circuit 8 has the same structure as in FIG.
The table as shown in FIG. 4 is stored in the same manner as described above.

The result calculated by the transfer voltage calculation circuit 8 is given to the transfer voltage control circuit 9, and the transfer voltage control circuit 9 controls the transfer voltage application circuit 10 so that the optimum transfer voltage can be applied to the backup roll 5. To During the secondary transfer, the secondary transfer roll 4 is pressed against the intermediate transfer body 2 via the recording medium, and the transfer voltage applying circuit 10 applies the optimum transfer voltage between the backup roll 5 and the secondary transfer roll 4. A predetermined secondary transfer is executed.

As described above, in this example, the conductive roll 6 is used for both the application of the secondary transfer voltage and the application of the potential difference during the resistance measurement, thereby simplifying the structure. Furthermore, in this example, resistance detection is performed prior to the secondary transfer, and when the secondary transfer voltage is applied, the conductive roll 6 that is the resistance detection roll is floated, that is, separated from the backup roll 5 so that the conductive roll is transferred during the secondary transfer. The unnecessary current from 6 is prevented from flowing out, so that a power source having a small capacity can be used.

It is also possible to make resistance detection during secondary transfer possible by shortening the length of the conductive roll 6 for resistance detection and arranging it outside the image area of the backup roll 5. As described above, according to the present embodiment, the resistance can be detected immediately before the secondary transfer, as compared with the case where the resistance is detected by contacting the bias roll with the intermediate transfer member during the standby or the pre-cycle before the transfer process. The optimum transfer voltage is determined according to the change in the resistance value of the backup roll due to the environment and the change over time, so that a high quality image can be formed and the problem such as toner stain on the recording medium is avoided.

[0075]

As described above, according to the present invention,
Even if the bias roll that applies the transfer voltage, that is, the secondary transfer roll, comes into direct contact with the intermediate transfer member, an excessive current does not flow to the intermediate transfer member. Therefore, a stable transfer electric field should be formed for recording media of various sizes. As a result, it is possible to prevent transfer failure and form a good recorded image, and it is possible to prevent damage to the belt constituting the intermediate transfer member.

Further, the resistance value of the backup roll can be accurately detected immediately before the secondary transfer, and the optimum transfer voltage according to the resistance value of the backup roll can be applied. It is possible to provide an image forming apparatus capable of stably forming a high-quality image even if it fluctuates due to time variations, environmental changes, deterioration due to long-term use, and the like.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating a main configuration of an image forming apparatus according to the present invention.

FIG. 2 is a schematic view of a main part for explaining a schematic configuration of a color electrophotographic copying machine to which the present invention is applied.

FIG. 3 is an explanatory diagram of a configuration example of a backup roll and a transfer circuit in this embodiment.

FIG. 4 is an explanatory diagram of an example of a table of optimum transfer voltages with respect to backup roll resistance values stored in a transfer voltage calculation circuit.

FIG. 5 is a flow chart illustrating a transfer procedure in this example.

FIG. 6 is an explanatory diagram of another configuration example of the backup roll and the transfer circuit in the present embodiment.

FIG. 7 is a schematic diagram illustrating a schematic structure of an electronic copying machine as an example of an image forming apparatus using an intermediate transfer member.

FIG. 8 is a schematic diagram illustrating a configuration of a main part of the image forming apparatus illustrated in FIG.

FIG. 9 is a schematic diagram illustrating a conventional configuration for controlling a secondary transfer voltage by detecting resistance of a secondary transfer unit.

[Explanation of symbols]

1 ...- Latent image carrier (photosensitive drum), 2 ...- Intermediate transfer member, 3 ...- Primary transfer device, 4 ...- Secondary transfer roll, 5 ... Backup roll , 6 ... Conductive roll for resistance detection, 7 ... Resistance detection circuit, 8
.... Transfer voltage calculation circuit, 9 ... Transfer voltage control circuit, 10 ... Transfer voltage application circuit, 11 ... Recording medium, 12 ... Voltage application roll, 13 ...・ Charging device, 14 ・ ・ ・ ・ Developing device, 15 ・ ・ ・ ・ Tray, 16 ・
... Feed rollers, 17 ... Feed rollers,
18 ... Cleaner, 19 ... Peeling nail, 20 ...
... Conveyor belt, 21 ... Fixer, 22 ...
Open / close switch, 23 ... Secondary transfer roll retracting mechanism,
24 ... Drum cleaner, 25 ... Static eliminator,
26 ... Transferring circuit, 30 Document scanning unit, 31
.... Document placing table, 40 ... Image processing unit, 50 ...
・ ・ ・ Imaging process control unit, 60 ・ ・ ・ Imaging unit.

Claims (5)

[Claims] 1. A latent image carrier on which a toner image corresponding to image information is formed, an intermediate transfer member on which a toner image is primarily transferred from the latent image carrier by a primary transfer device, and a surface of the intermediate transfer member. A secondary transfer roll that abuts against the secondary transfer roll to transfer the toner image onto a recording medium; a semi-conductive backup roll that supports the intermediate transfer member from the back side at a position facing the secondary transfer roll; A conductive roll arranged in contact with a backup roll, a transfer voltage application circuit for applying a transfer voltage between the secondary transfer roll and the backup roll, and a backup roll of the backup roll when the secondary transfer roll is retracted. A resistance detection circuit that detects the resistance value,
A transfer voltage calculation circuit that determines a transfer voltage applied to the secondary transfer roll according to a detection signal of the resistance detection circuit;
An image forming apparatus comprising: a transfer voltage control circuit that controls the transfer voltage application circuit so as to output a transfer voltage according to a calculation output of the transfer voltage calculation circuit. 2. The image forming apparatus according to claim 1, wherein the transfer voltage applying circuit includes a voltage applying roll that rotates in contact with the backup roll. 3. The image forming apparatus according to claim 1, wherein the transfer voltage output of the transfer voltage applying circuit is connected to the secondary transfer roll. 4. The image forming apparatus according to claim 1, further comprising an open / close switch that separates the conductive roll from the resistance detection circuit 7 during the secondary transfer. 5. The image forming apparatus according to claim 1, further comprising a secondary transfer roll retracting mechanism that retracts the secondary transfer roll from the intermediate transfer body at least when the resistance value of the backup roll is detected.