JP2004144916A - Transfer device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the desired image density is not obtained even though the same transfer bias is applied, when a bias whose polarity is opposite to that of the toner is simply applied on a contact roller arranged upstream a transfer nip, as for an image forming apparatus using an intermediate transfer belt, having a secondary transfer process, and wherein, a secondary transfer bias is applied from behind the belt. <P>SOLUTION: When a toner image has a negative polarity, a power source 16 for applying a counter roller current as a negative polarity transfer bias on a transfer counter roller 7d, and a power source 17 for applying a contact roller current as a positive polarity electric field on the contact roller 7r are prepared as constant current power sources, and a transfer roller 11 and also a transfer entrance guide plate 19 are grounded. By setting the value of the sum of the counter roller current value and the contact roller current value including the polarity to be a fixed value equal to or exceeding a certain value capable of obtaining the desired image density, the image of satisfactory density is obtained. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
[Industrial applications]
The present invention relates to an image forming apparatus, and more particularly, to a secondary transfer device that transfers a toner image from an intermediate transfer member to a recording medium.
[0002]
[Prior art]
The constant current control is often used for the transfer because the transfer current can be kept constant without being affected by the resistance fluctuation of the resistor in the current path.
FIG. 4 is a diagram illustrating an example of an image forming apparatus having a secondary transfer device.
In the figure, reference numeral 1 denotes a photosensitive drum, 2 denotes a charging device, 3 denotes an exposure device, 4 denotes a developing device, 5 denotes a primary transfer device, 6 denotes an intermediate transfer belt as an intermediate transfer member, 7 denotes a roller, and 8 denotes cleaning. Reference numeral 9 denotes a registration roller, 10 denotes a recording medium, 11 denotes a secondary transfer roller, 12 denotes a recording medium discharging device, 13 denotes a recording medium transport belt, 14 denotes a fixing device, and 15 denotes a paper discharge tray.
FIG. 4 shows an example of a tandem-type image forming apparatus. However, even in an image forming apparatus having another configuration such as a revolver type, the following problems are the same as long as the configuration uses an intermediate transfer member. The solution is similar.
[0003]
In transferring the toner image on the intermediate transfer belt 6 to the recording medium 10 supplied from the registration roller 9, a method of applying a constant current to the secondary transfer roller 11 is as follows. There is a merit that is not affected by the resistance value fluctuation of No. 11. However, when the resistance value of the recording medium 10 becomes low due to a change in environmental conditions or the like, current leaks through the recording medium 10 to a member in contact with the recording medium 10 other than the intermediate transfer belt 6, which contributes to the formation of the essential transfer electric field. Since the transfer current is reduced, the transfer electric field is reduced, and the transfer rate is likely to be insufficient.
[0004]
There is a secondary transfer device that applies a bias having the same polarity as the toner image on the intermediate transfer member to the back surface of the intermediate transfer member to transfer the toner image from the intermediate transfer member to a recording medium (for example, see Japanese Patent Application No. 2001-330505). . In such a transfer device, the transfer electric field does not decrease even if the resistance of the recording medium is low, and stable transfer properties can be obtained.
When a constant current is applied from the inside of the intermediate transfer body, even if a current leaks to a member that comes into contact with the recording medium other than the intermediate transfer body through the recording medium, it is after the transfer electric field is formed, so even if the resistance of the recording medium is low. This is because the transfer electric field is not reduced, and stable transfer properties can be obtained.
[0005]
A space through which the recording medium can pass is a contact unit provided in front of the secondary transfer nip for bringing the intermediate transfer belt into close contact with the secondary transfer roller, and an intermediate transfer belt surface stretched on the secondary transfer opposing roller. There is a transfer device which is regulated by a transfer entrance guide plate arranged at a distance from the surface of the intermediate transfer belt and applies an electric field having a polarity opposite to that of the toner image to the rear surface of the intermediate transfer belt from the above-mentioned close contact means (for example, Application 2002-220874.).
[0006]
In an apparatus provided with a contact unit for bringing the recording medium into close contact with the intermediate transfer belt immediately upstream of the secondary transfer nip portion, since the transfer entrance guide plate and the member of the contact unit come close to each other, a part of the toner on the intermediate transfer belt is Are easily scattered on the transfer entrance guide plate.
The amount of scattered toner is small, and is not an amount that causes a problem in an image forming operation for one sheet. However, scattered toner accumulates on the transfer entrance guide plate in an area where the narrow paper does not pass while the narrow paper is repeatedly imaged. Then, when the next wide paper is passed, the paper passes through the area where the scattered toner is accumulated, and there is a problem that the edge and back side of the paper passing through this area are stained.
[0007]
Therefore, in the above-mentioned Japanese Patent Application No. 2002-220874, the toner on the surface of the intermediate transfer belt is attracted to the intermediate transfer belt by using a method of applying a bias having a polarity opposite to that of the toner to a contact member that comes into contact with the intermediate transfer belt from the back. An electric field is generated to prevent toner on the intermediate transfer belt from scattering on the transfer entrance guide plate.
However, in the above-mentioned Japanese Patent Application No. 2002-220874, in order to prevent the transfer current from flowing into the close contact member, a transfer bias is applied from the recording medium side with a polarity opposite to that of the toner, and as described above, the resistance of the recording medium becomes low. Then, there remains a problem that a desired transfer electric field cannot be obtained.
[0008]
[Problems to be solved by the invention]
In a secondary transfer device for transferring a toner image from an intermediate transfer member to a recording medium by applying a bias having the same polarity as that of the toner image on the intermediate transfer member to the back surface of the intermediate transfer member, the toner is reversely applied to the back surface of the intermediate transfer belt from a contact member. When a polarity bias is simply applied, a desired image density may not be obtained even when the same transfer bias is applied.
[0009]
[Means for Solving the Problems]
In the first aspect, a primary transfer step of transferring a toner image charged to a desired polarity from an image carrier to an intermediate transfer body, and a secondary transfer step of transferring the toner image from the intermediate transfer body to a recording medium And a transfer entrance guide plate facing the surface of the intermediate transfer body on which the toner image is placed before the secondary transfer step, the transfer entrance comprising: A voltage applying means for applying a voltage having a polarity opposite to that of the toner image is provided on the rear surface of the intermediate transfer body near the guide plate, and the transfer entrance guide plate is grounded.
[0010]
According to a second aspect of the present invention, in the transfer device according to the first aspect, the transfer entrance guide plate is arranged at a predetermined distance from the surface of the intermediate transfer belt before a secondary transfer nip. Features.
According to a third aspect of the present invention, in the transfer device according to the first or second aspect, the voltage applying unit includes a contact unit that contacts a back surface of the intermediate transfer body, and a power supply that applies a voltage to the contact unit. It is characterized by the following.
According to a fourth aspect of the present invention, in the transfer device according to the third aspect, the contact unit is a roller that rotates with the intermediate transfer belt.
[0011]
According to a fifth aspect of the present invention, in the transfer device according to any one of the first to fourth aspects, the secondary transfer step includes a transfer contacting a surface of the intermediate transfer member on a side on which the toner image is placed. A bias of the same polarity as that of the toner image on the intermediate transfer member is applied to a transfer counter electrode provided on the back surface of the intermediate transfer member in opposition to the transfer roller, and recording is performed from the intermediate transfer member. A step of transferring the toner image to a medium.
According to a sixth aspect of the present invention, in the transfer device according to the fifth aspect, the transfer roller is grounded.
[0012]
According to a seventh aspect of the present invention, in the transfer device according to any one of the fifth to sixth aspects, a current value having the same polarity as the toner image applied to the transfer counter electrode and the toner applied to the adhesion unit are provided. The present invention is characterized in that control is performed such that the sum of the current value having the polarity opposite to that of the image, including the polarity, becomes a constant value.
According to an eighth aspect of the present invention, in the transfer device according to the seventh aspect, the predetermined value is a value equal to or more than a current value necessary to form a desired image density.
[0013]
According to a ninth aspect of the present invention, in the transfer device according to any one of the fifth to eighth aspects, a power supply for applying the transfer bias to the transfer counter electrode and a power supply for applying an electric field to the contact unit are both constant. It is a current power supply.
According to a tenth aspect, there is provided an image forming apparatus including the transfer device according to any one of the first to ninth aspects.
[0014]
【Example】
Hereinafter, the present invention will be described with reference to Examples.
FIG. 1 is an enlarged view near the secondary transfer portion.
In the figure, reference numeral 7d denotes a transfer opposing roller as a transfer opposing electrode, 7r denotes a contact roller as a contact means, 16 denotes a constant current power supply for transfer, 17 denotes a constant current power supply for preventing dust, and 18, 19 denote transfer entrance guide plates. Are respectively shown. 3 have the same functions.
[0015]
Hereinafter, the description will be made assuming that the intermediate transfer member is a belt, but a drum type intermediate transfer member may be used in an image forming apparatus other than a tandem type. The present invention can be similarly implemented with such an intermediate transfer member.
It is assumed that the toner image on the intermediate transfer belt 6 has a negative polarity.
The transfer facing roller 7d is made of stainless steel, and a negative polarity current is supplied from a high-voltage transfer constant current power supply 16. The secondary transfer roller 11 is grounded, and forms a secondary transfer nip with the transfer opposing roller 7d. A bias applied to the transfer opposing roller 7d from the transfer constant current power supply 16 and having the same polarity as the toner image acts to separate the negative polarity toner from the intermediate transfer belt 6, and the transfer entrance guide plates 18 and 19 are moved here. The secondary transfer operation is performed on the recording medium supplied through the above.
[0016]
The transfer entrance guide plates 18 and 19 are electrically connected to each other, and are located upstream in the moving direction of the intermediate transfer belt 6. The transfer entrance guide plate 19 is spaced from the surface of the intermediate transfer belt 6 by a predetermined distance such that recording media of various thicknesses can pass with a margin. The transfer entrance guide plate 19 is arranged near the secondary transfer roller 11 and is grounded. The transfer entrance guide plate 19 is preferably closer to the secondary transfer roller 11 for smooth supply of the recording medium. However, since both are grounded, the transfer entrance guide plate is set by the transfer bias. 19 also makes it easy for toner to scatter. Therefore, it is necessary to keep the transfer entrance guide plate 19 apart to such an extent that it is not affected by the transfer bias.
[0017]
Since the recording medium 10 is supplied from a registration roller 9 (not shown) and reaches the transfer nip, the distance between the two must be smaller than the length of the shortest recording medium used. Since the transfer entrance guide plate 19 is located between the registration roller 9 and the transfer nip, the distance between the transfer nip and the transfer entrance guide plate 19 must be smaller than that.
[0018]
The contact roller 7r is made of stainless steel, is provided on the back surface of the intermediate transfer belt 6, that is, on the side on which the toner image is not placed, and is disposed near the transfer entrance guide plate 19. The contact roller 7r is configured to be brought into contact with the intermediate transfer belt 6 by being brought into contact with the intermediate transfer belt 6, and a positive current is supplied from a high-voltage constant current source 17 for preventing dust.
Conventionally, there has been an example in which a fixing member is used as a close contact means. However, since the fixing member slides on the back surface of the intermediate transfer belt, the back surface of the belt material is scraped, thereby shortening the life of the belt. By using a roller, the problem of scraping does not occur.
[0019]
An electric field applied between the transfer entrance guide plate 19 and the close contact roller 7r has an effect of pressing the negative polarity toner toward the intermediate transfer belt 6 side. Therefore, the toner on the intermediate transfer belt 6 does not scatter on the transfer entrance guide plate 19.
This position is a place where the adhesion of the recording medium is not good because the intermediate transfer belt 6 is not pressed between the secondary transfer roller 11 and the opposing roller 7d. There is no occurrence of a so-called pre-transfer dust image or a blurred image due to pre-transfer.
[0020]
The reason why both the power supply for supplying the current to the contact roller 7r and the power supply for supplying the current to the transfer opposing roller 7d are set to the constant current control is that the sum of the two current values, or the difference in absolute value because the polarities are actually different. This is because it was found that the transfer rate was determined.
The actual value of the contact roller current supplied to the contact roller 7r and the value of the opposing roller current supplied to the transfer opposing roller 7d were actually changed by the apparatus shown in FIG.
[0021]
FIG. 2 is a graph showing the relationship between the roller current value and the measured value of the image density.
In the figure, the horizontal axis is the sum of the opposing roller current value and the contact roller current value, and the unit is μA. The vertical axis is the image density. The graph shows the contact roller current value as three parameters of 0, 8, and 16 μA.
That is, when the contact roller current value is 0 μA, the opposing roller current value is changed from −16 μA to 0 μA, and when the contact roller current value is 8 μA, the opposing roller current value is changed from −24 μA to −8 μA. Assuming that the roller current value is 16 μA, the opposing roller current value is changed from −32 μA to −16 μA. As a result, the value of the sum including the polarities of both current values changes from -16 μA to 0 μA. In the figure, -16 μA is on the right side of the horizontal axis in consideration of the visibility of the display.
[0022]
The contact roller current value was set in three steps from 0 to 16 μA as described above, and the opposing roller current was varied. As a result, the transfer rate from the image density to the opposing roller current alone was uncertain. However, with respect to the sum of the opposing roller current value and the contact roller current value including the polarity, the transfer rate was substantially the same between the parameters. Of the negative polarity current values supplied to the intermediate transfer belt 6 from the transfer opposing roller 7d, the positive current value supplied to the intermediate transfer belt 6 from the close contact roller 7r is canceled, and the remaining current value is effectively used for transfer. It is considered that it contributes.
[0023]
Here, assuming that the desired image density is 1.5 or more, and preferably, the image density is a density at which the image density is almost flat on the graph, the current value actually contributing to the transfer in this case is about −8 μA or more, preferably If the absolute value is larger than 12 μA, a desired image density can be obtained. In order to obtain the current value, the current value escaping from the intermediate transfer belt 6 to the close contact roller 7r should be added to the opposing roller current value.
[0024]
Even when the power supply for supplying the current to the contact roller 7r and the power supply for supplying the current to the transfer opposing roller 7d are not both controlled by the constant current, the value of the current flowing through both is detected and the polarity is included in the same manner as described above. If the sum of the two current values can be controlled to a desired constant value, a desired image density can be obtained.
Alternatively, a current value flowing through the transfer roller 11 may be detected, and any one of the above power supplies may be controlled so that the value becomes a desired constant value.
FIG. 3 is a conceptual diagram for controlling a transfer bias power supply by detecting a current value flowing through the transfer roller.
[0025]
In the above description, the case where the toner is negatively charged has been described. However, even when the toner is positively charged, since the respective polarities may be reversed, the same can be applied.
[0026]
【The invention's effect】
According to the present invention, good transfer is performed even when the resistance value of the recording medium becomes low due to a change in environmental conditions, and it is possible to prevent a dust image due to pre-transfer occurring before the secondary transfer and to provide a transfer entrance guide plate. Also, dust of the toner can be prevented.
Until now, if the distance of the recording medium in close contact with the intermediate transfer belt is long, the recording medium once in contact with the intermediate transfer belt cannot maintain the contact state up to the transfer nip, and if contact failure occurs halfway, transfer dust tends to occur. However, according to the present invention, not only does the toner on the intermediate transfer belt be prevented from scattering on the transfer entrance guide plate, but also the generation of transfer dust irrespective of the state of contact of the recording medium with the intermediate transfer belt. Can be prevented.
Further, since the back surface of the intermediate transfer belt is not abraded by the contact roller, it has been possible to achieve both a longer life of the intermediate transfer belt and a reduction in transfer dust, which were difficult to achieve in the past.
[Brief description of the drawings]
FIG. 1 is an enlarged view near a secondary transfer portion.
FIG. 2 is a graph showing a relationship between a roller current value and an actually measured image density value.
FIG. 3 is a conceptual diagram for controlling a transfer bias power supply by detecting a current value flowing through a transfer roller.
FIG. 4 is a diagram illustrating an example of an image forming apparatus having a secondary transfer device.
[Explanation of symbols]
6 Intermediate Transfer Belt 7d Transfer Opposite Roller 7r Adhesive Roller 10 Recording Medium 11 Transfer Roller 16 Constant Current Power Supply for Transfer 17 Constant Current Power Supply for Preventing Dust 19 Transfer Entrance Guide Plate

Claims (10)

An image forming apparatus comprising: a primary transfer step of transferring a toner image charged to a desired polarity from an image carrier to an intermediate transfer body; and a secondary transfer step of transferring the toner image from the intermediate transfer body to a recording medium. A transfer entrance guide plate facing the surface of the intermediate transfer body on which the toner image is mounted, before the secondary transfer step, and near the transfer entrance guide plate, A transfer device, comprising: a voltage applying means for applying a voltage having a polarity opposite to that of the toner image to a back surface of the intermediate transfer member, wherein the transfer entrance guide plate is grounded. 2. The transfer device according to claim 1, wherein the transfer entrance guide plate is disposed at a predetermined distance from the surface of the intermediate transfer belt before a secondary transfer nip. 3. 3. The transfer device according to claim 1, wherein the voltage applying unit includes a contact unit that contacts a back surface of the intermediate transfer member, and a power supply that applies a voltage to the contact unit. . 4. The transfer device according to claim 3, wherein the contact unit is a roller that rotates with the intermediate transfer belt. 5. The transfer device according to claim 1, wherein the secondary transfer step includes a transfer roller that comes into contact with a surface of the intermediate transfer body on which the toner image is placed, and wherein the transfer is performed. 6. A bias having the same polarity as that of the toner image on the intermediate transfer member is applied to a transfer counter electrode provided on the back surface of the intermediate transfer member facing the roller, and the toner image is transferred from the intermediate transfer member to a recording medium. A transfer device. 6. The transfer device according to claim 5, wherein the transfer roller is grounded. 7. The transfer device according to claim 5, wherein a current value applied to the transfer counter electrode has the same polarity as the toner image, and a current value applied to the adhesion unit has a polarity opposite to the toner image. A transfer device for controlling the sum including the polarity of the transfer device to a constant value. 8. The transfer device according to claim 7, wherein the fixed value is a value equal to or more than a current value required to form a desired image density. 9. The transfer apparatus according to claim 5, wherein a power supply for applying the transfer bias to the transfer counter electrode and a power supply for applying an electric field to the contact unit are both constant current power supplies. Transfer device. An image forming apparatus comprising the transfer device according to any one of claims 1 to 9.

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