JPH08160776A - Image forming device - Google Patents

Abstract

PURPOSE: To provide an image forming device which prevents offset of a transfer material by preventing specific deterioration of an image carrier and also completely preventing transfer of toner to a transfer roller used in a transfer process. CONSTITUTION: A transfer device 2 used for transferring a toner image on the image carrier 1 is equipped with: the transfer roller 2A which is comprised of a conductive, elastic member and can come into contact with the image carrier and to which, when the toner image is being transferred, a transfer bias 3A having an opposite polarity to the toner is applied and, when transfer is not being carried out, a reverse transfer bias 3B having the same polarity as the toner is applied; and a power source 3 which is electrically connected to the transfer roller and applies each bias at a constant current. Thus, by causing the constant current to flow, even in case the resistance of the transfer roller 2A changes, an increase in the electrification voltage of the image carrier, which is caused by separation discharge, is suppressed to reduce decitaticization load, and the specific deterioration of the image carrier and transfer of the toner to the transfer roller can be prevented.

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,
More specifically, it relates to a transfer device in an image forming apparatus using an electrophotographic process.

[0002]

2. Description of the Related Art Generally, an image forming apparatus using an electrophotographic system includes an electrophotographic copying machine, a printer or a facsimile machine. In electrophotographic image formation, an electrostatic latent image is formed on a uniformly charged photoreceptor by exposure or optical writing, and this electrostatic latent image is formed into a two-component mixture of magnetic carrier and toner. A toner image obtained by processing a visible image with a toner using a system developer is transferred onto a transfer paper or the like to obtain a copy.
Ideally, all of the toner used for the visible image processing is transferred to the transfer paper, but in reality, part of it may remain on the photoconductor. Conventionally, a cleaning step has been provided to remove such residual toner, and in this step, the residual toner is mechanically scraped off using a blade or a brush member that is in contact with the surface of the photoconductor. It was

As a structure used in a transfer process in which a toner image is transferred onto a transfer paper, there is provided a structure provided with a transfer roller for applying a transfer bias to the transfer paper while abutting on the surface of the photoconductor and nipping and transferring the transfer paper. is there.

The transfer roller is made of a conductive elastic material,
Although it is always in contact with the photoconductor, it is also in contact with the non-image region where there is no toner image in addition to the toner image formation region carried on the photoconductor because of the constant contact. . Therefore, the toner attached to the non-image area of the photoconductor is transferred to the transfer roller by the transfer bias. As a result, when the roller surface comes into contact with the back surface of the transfer paper that is conveyed next because it is nipped and transferred by the transfer roller, the toner that has transferred and adhered to the transfer roller surface is transferred to the back surface of the transfer paper, This leads to stains on the back of the transfer paper.

In order to prevent such a problem from occurring, a method has been proposed in which the bias applied to the transfer roller is switched to enable self-cleaning of the transfer roller. As a general self-cleaning method, when a toner image carried on a photoconductor is transferred onto a transfer paper, a transfer bias having a polarity opposite to that of the toner is applied and a photoconductor carrying no toner image is provided. When the peripheral surface of the sheet and the transfer roller are opposed to each other, a reverse transfer bias having the same polarity as that of the toner and the reverse polarity of the transfer bias is applied (for example, Japanese Patent Laid-Open No. 51-9840 and Japanese Patent Laid-Open No. 3840/1983). -248181, JP-A-4-120577). FIG. 4 shows a structure for performing the transfer and cleaning process disclosed in the above publication, and FIG. 5 is a schematic diagram for explaining the transfer and cleaning process with the structure shown in FIG. In FIG. 4, a drum-shaped photosensitive member (photosensitive drum) A is uniformly charged to a negative polarity by a charging device (not shown), and an electrostatic latent image corresponding to an original image is obtained by an exposure device or a writing device (not shown). Is formed. In the electrostatic latent image, the electric charge in the portion irradiated with light is dissipated, and the electric charge in the other portion remains. The electrostatic latent image carried on the photoconductor drum A is subjected to visual image processing by supplying toner charged with a polarity opposite to that of the electrostatic latent image from the developing device to form a toner image. The toner image is transferred to the transfer paper B at the transfer section. The transfer portion is provided with a transfer roller C that is in contact with the photosensitive drum A and is rotatable. The transfer roller C is connected to a transfer bias circuit D, which applies a bias having a polarity opposite to that of the toner, and a reverse transfer bias circuit E, which applies a bias having the same polarity as that of the toner, through a switch F so as to switch electric paths. There is. When the toner image is transferred, the transfer roller C has an electric path connected to the transfer bias circuit D, so that the transfer roller C shown in FIG.
As shown in (A), a transfer bias having a polarity opposite to that of the toner is applied to transfer the toner image onto the transfer paper B. When the image forming area corresponding to the area where the toner image is carried on the photosensitive drum A ends facing the transfer roller C,
The electric path of the transfer roller C is switched to the reverse transfer bias circuit E. When the transfer roller C is applied with a bias having the same polarity as the toner, the transfer roller C repels the toner transferred to the transfer roller C or the toner which is about to transfer to the photoconductor drum A, as shown in FIG. 5B. Reverse transcription is performed.

According to this method, when the toner attached to the non-image portion of the photosensitive drum A is transferred to the transfer roller C or is about to be transferred, the toner is repelled from the transfer roller C and the photosensitive drum Since the toner is reversely transferred to A, the transfer of the toner to the transfer roller C is prevented, and the situation in which the back surface of the transfer paper B conveyed next is soiled is prevented.

Further, as a method for suppressing the adhesion of toner to the transfer roller, when the non-image portion of the photoconductor is opposed,
A method of applying a bias lower than the transfer bias (for example, JP-A-2-39181 and JP-A-3-3918).
No. 2) or the transfer bias applied state of the transfer roller 1
A method of applying a reverse transfer bias application state for more than the time required for one rotation of the transfer roller after application of the time required for rotation or more (for example, Japanese Patent Laid-Open No. 3-69978), or to a transfer roller at the time of non-transfer. A method of applying a cleaning bias having a polarity opposite to that of the toner with a constant current (see, for example, Japanese Unexamined Patent Publication No.
-237477).

[0008]

By the way, the photosensitive member, which is an image carrier, and the transfer member transfer the toner image on the photosensitive member while nipping and transferring a transfer material such as transfer paper. , The facing distance between them is extremely small. Therefore, when a high electric field is formed by applying a high voltage transfer bias to the transfer member, peeling discharge according to Paschen's law is likely to occur and the photoconductor may be charged.

Normally, the charging potential of the photosensitive member is determined by the transfer bias current value and the amount of accumulated charge depending on the moving speed of the photosensitive member. As an example, when the transfer bias current is -20 μA, When the moving speed is relatively low, the charging potential of the photoconductor becomes about 500 V at the background part corresponding to the non-image part of the photoconductor. If the reverse transfer bias current in the transfer member is set to the same value as the transfer bias current with reference to the charged potential of the background part, the reverse transfer of toner from the transfer member to the photoconductor can be performed, but the background part It has been confirmed by experiments that the charging potential at +350 to 400 V reaches.

Moreover, since the reverse transfer bias in the transfer member has a polarity opposite to the charging polarity of the photoconductor, as is apparent from FIG. 5B, charge injection from the transfer member to the photoconductor is promoted. It becomes easy to be charged and the charging potential of the photoconductor rises. The charge on the photoconductor is removed before the next image formation, but depending on the material of the photoconductor, the charge may not be removed. As a structure for static elimination, photo static elimination using a lamp is performed due to the characteristics of the photoconductive layer used as a photoconductor, but the polarity of the electric charges for which this static elimination is effective is negative. However, in recent years, when an organic material such as OPC is used as the photosensitive layer, photo-erasing may not be performed if the positive charge remains.
In particular, as described above, if the positive electrode charges due to a high voltage are charged and remain, it is impossible to erase all the residual charges by one photo-elimination. For this reason, the surface potential set in the next charging step is not uniform, the potential of the electrostatic latent image changes, which adversely affects the image density at the time of development and causes electrostatic fatigue of the photoconductor. It could be

In addition to the reasons described above, the reason why the charging of the photosensitive member is likely to occur is a change in current due to a change in resistance of the transfer member. The resistance of the transfer member changes depending on the temperature and humidity in the usage environment. For this reason, when the resistance of the transfer member is reduced, a current easily flows, which easily causes charge injection into the photoconductor, and promotes charging of the photoconductor. As a result, the uniform charging in the next charging step is hindered, the potential of the electrostatic latent image is locally changed, and the image density upon development is adversely affected.

Therefore, it is conceivable to set a transfer bias potential at which the photosensitive member is not charged, but in this case, there is a possibility that the original purpose of cleaning the transfer member may not be achieved.

Switching between the transfer bias and the reverse transfer bias on the transfer member is usually performed on the basis of the passage of the leading and trailing ends in the moving direction of the transfer material, but when such a switching operation is performed. As shown in FIG. 6, there is a delay in the response speed until each bias current is reached. Normally, when the bias current is switched, the bias current (voltage) that has been set so far is turned off to make the actual output zero, and then the bias current (voltage) is switched again. Therefore, there is a time lag during the switching transition period. When a specific time lag was obtained by an experiment, it was found that the time required to switch the bias current to reach the set value was about 50 ms. For this reason, the transfer bias current (voltage) is zero at the time when the trailing edge of the toner image passes the position of the transfer roller because the device is operating even before the switched bias current (voltage) is reached. The toner adhering to the photoconductor may be transferred to the transfer roller before it becomes, and the transfer paper that is conveyed next may stain the back surface. In addition, at the leading edge of the toner image, the toner is repelled by the reverse transfer bias (voltage) until the reverse transfer bias current (voltage) becomes zero, and a part of the image to be transferred is lost. Will be lost.

In view of the above problems in the conventional image forming apparatus, an object of the present invention is to prevent the characteristic deterioration of the image carrier and to completely prevent the transfer of the toner to the transfer roller used in the transfer step. An object of the present invention is to provide an image forming apparatus capable of preventing the backside of a material from being soiled.

Another object of the present invention is to provide an image forming apparatus which does not cause a situation in which toner is transferred to a transfer roller and a part of an image to be transferred is lost when the bias of the transfer roller is switched. To provide.

[0016]

In order to achieve this object, an invention according to claim 1 is an image carrier and a transfer device for transferring a toner image visualized on the photoconductor to a transfer material. In the image forming apparatus provided with, the transfer device is made of a conductive elastic member, is capable of contacting the photoconductor,
An electric transfer bias is applied to the transfer roller to which a transfer bias having a polarity opposite to that of the toner is applied when the toner image is transferred, and to which a reverse transfer bias having the same polarity as that of the toner is applied when the toner image is not transferred. And a power supply that is electrically connected to each other and applies each bias with a constant current.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the power source has a constant current intensity ratio of a transfer bias applied during transfer and a reverse transfer bias applied during non-transfer (reverse transfer). Bias current / transfer bias current) is set to have a relationship of 0.8 to 0.1 in absolute value.

According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the power source changes the current at the switching timing from the transfer bias to the reverse transfer bias and at the switching timing from the reverse transfer bias to the transfer bias. The feature is that it is set at a time that allows for the time required for rising.

According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, the power supply of the image carrier is 100 mm / sec or more as the switching timing from the transfer bias to the reverse transfer bias. In this case, the rear end position in the moving direction of the transfer material is set at a time corresponding to a position which precedes the position facing the transfer roller by at least 5 mm in advance, and as the switching time from the reverse transfer bias to the transfer bias, It is characterized in that the front end position in the moving direction of the transfer material is set in advance at a time corresponding to a position at least 5 mm ahead of the position facing the transfer roller to the front side.

[0020]

According to the present invention, the bias current is applied to the transfer roller at a constant current.

According to the second aspect of the invention, the constant current intensity ratio of the transfer bias and the reverse transfer bias (reverse transfer bias current / transfer bias current) is 0.8 to 0.
1 relationship is set.

According to the third and fourth aspects of the present invention, the timing of switching from the transfer bias to the reverse transfer bias and the timing of switching from the transfer bias to the reverse transfer bias are set at timings in which the time required for the rise of each bias current is expected. It

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the embodiments shown in the drawings.

FIG. 1 is a schematic view for explaining a main structure of an image forming apparatus according to an embodiment of the present invention. In FIG. 1, a photoconductor that is an image carrier is composed of a drum, and the photoconductor drum 1 can be rotated in a direction of an arrow shown by a drive source such as a stepping motor (not shown). The photosensitive drum 1 has a photosensitive layer made of an organic material such as OPC on the surface thereof, and the photosensitive layer is uniformly charged to a negative electrode by a charging device (not shown). An electrostatic latent image is formed on the uniformly charged photosensitive drum 1 by an exposure device or a writing device (not shown), and the electrostatic latent image is subjected to a visible image processing by a developing device (not shown) to form a toner image.

The toner image carried on the photosensitive drum 1 is electrostatically transferred by the transfer device 2 to the transfer paper S fed from the paper feeding device (not shown).

The transfer device 2 is provided with a transfer roller 2A that can be brought into contact with the photosensitive drum 1, and the transfer roller 2A can be rotated following the photosensitive drum 1. A power source 3 is connected to the transfer roller 2A, and a transfer bias circuit 3A and a reverse transfer bias circuit 3B are provided in the power source 3, and these circuits are arranged in an electric path from the transfer roller 2A. It can be switched by the switch 4. Transfer bias circuit 3
A is a circuit that applies a bias voltage having a polarity opposite to that of the toner carried on the photosensitive drum 1 to the transfer roller 2A, and a reverse transfer bias circuit 3B is a bias voltage having the same polarity as the toner. Is a circuit for applying Each of the circuits 3A and 3B provided in the power supply 3 is composed of a constant current circuit so that the current value applied to the transfer roller 2A does not change even if the resistance of the transfer roller 2A changes. Has become. As a result, even if the resistance value of the transfer roller 2A changes depending on the temperature / humidity of the usage environment, or if peeling discharge occurs between the photoconductor drum 1 and the transfer roller 2A, the charge injection amount in the photoconductor drum 1 Is prevented from becoming large, so that the photosensitive drum 1 is less likely to be charged. Therefore, by making it difficult for the charging voltage of the photosensitive drum 1 to rise, it is possible to reduce the load at the time of photo-electrification.

In this embodiment, the constant current ratio between the transfer bias current and the reverse transfer bias current is set so that the reverse transfer bias becomes smaller, and the absolute value of the reverse transfer bias / transfer bias is 1. And preferably less than 0.
It is set to 8 to 0.1. When the ratio is set to be smaller than the upper limit value of 0.8, the charging potential of the photosensitive drum 1 is −3 as shown in FIG.
It has been confirmed by an experiment that the voltage becomes 00 V or less, and that a value that can be eliminated by optical static elimination is obtained. Further, when the value is set to be smaller than the lower limit value of 0.1, the value of the current flowing through the transfer roller 2A becomes too low to obtain the reverse transfer bias voltage sufficient to cause the repulsion of the toner. Have been confirmed by.

The reason why such a reverse transfer bias current can be made smaller than the transfer bias current is that the transfer paper S exists when the transfer roller 2A is in contact with the non-image portion corresponding to the background portion of the photosensitive drum 1. Since the current flows more easily because it is not performed, the reverse transfer bias voltage can be easily obtained even if the current is reduced.

The operation of the switch 4 is controlled by the controller 5. The control unit 5 is a member provided for sequence control of the overall image forming process, and is related to the present embodiment, and includes a rotation driving unit of the photosensitive drum 1 and a paper feeding device (not shown). An encoder 7 for calculating the amount of rotation of the motor 6 that constitutes the drive unit is connected to the input side,
An actuator (not shown) for switching the switch 4 is connected to the output side. The control unit 5 sets the switching timing of the bias circuit with respect to the leading end and the trailing end in the moving direction of the transfer sheet in consideration of a time lag occurring until the switched bias current is reached when the bias circuit is switched. It is like this. Therefore, in the control unit 5, the front and rear end positions of the toner image on the photosensitive drum 1 and the front and rear ends of the transfer paper fed from the paper feeding device are controlled by the rotation amount of the motor 7 input from the encoder 6. Index the rear end position,
The switching timing of the switch 4 is determined. In this embodiment, the rotation speed of the photosensitive drum 1 is 100 to 200 mm /
In the case of sec, when the time lag is 50 ms as a reference, the next switching timing is set in consideration of the fact that the movement amount of the photosensitive drum 1 after switching the switch 4 reaches 5 to 10 mm. Has been done. FIG.
4 is a timing chart for explaining the switching timing of the switch 4, and in the figure, when the rotation speed of the photosensitive drum 1 is 100 mm / sec or more, the switching timing from the transfer bias to the reverse transfer bias (in FIG. 3, ,
(C) Displayed as switching signal)
The rear end position in the moving direction of the transfer sheet S is the transfer roller 2A.
From the position (nip portion) facing to the front side is set in advance at a position corresponding to a position which precedes the front side by at least 5 mm, and as the timing of switching from the reverse transfer bias to the transfer bias, the tip position in the moving direction of the transfer material is It is set in advance at a time corresponding to a position at least 5 mm ahead of the position (nip portion) facing the roller to the front side. Thus, when the leading edge or the trailing edge of the transfer sheet S in the moving direction faces the transfer roller 2A, a bias current (voltage) corresponding to the switched bias current (voltage) is applied to the transfer roller 2A. be able to.

Since this embodiment has the above-mentioned configuration,
When the photosensitive drum 1 carrying the toner image faces the transfer device 2, the bias circuit of the power source 3 connected to the transfer roller 2A of the transfer device 2 and scattered is switched to the transfer bias circuit 3A. At this time, as shown in FIG. 3, the switching timing is such that the leading end of the transfer sheet S in the moving direction is the transfer roller 2
The switch 4 is switched at a timing corresponding to a position 5 mm ahead of the position facing A, and the transfer bias circuit 3A and the transfer roller 2A are connected to apply a transfer bias. Accordingly, when the front end of the toner image on the photosensitive drum 1 faces the transfer roller 2A, the transfer sheet S
Has arrived, and the toner image is transferred by the transfer bias. In this case, the bias current is gradually reduced by switching the switch 4, but since the current of the polarity necessary for the transfer is flowing, the transfer is not hindered.

When the transfer roller 2A comes into contact with the background portion of the photosensitive drum 1 when the toner image on the photosensitive drum 1 has been transferred to the transfer sheet S, the bias circuit of the power source 3 is switched to the reverse transfer bias circuit 3B. To be At this time, the switching timing is, as shown in FIG. 3, 5 mm from the position where the rear end of the transfer sheet S in the moving direction faces the transfer roller 2A.
The switch 4 is set at a time corresponding to the preceding position, and the reverse transfer bias circuit 3B and the transfer roller 2 are set.
A is connected to A and a reverse transfer bias is applied. In this case, since the reverse transfer bias current is made smaller than the transfer bias current, the amount of charges injected into the photosensitive drum 1 when peeling discharge occurs is reduced, so that the charging voltage of the photosensitive drum 1 is reduced. Is low, and the burden of optical static elimination in the static elimination step is reduced. Moreover, even if the resistance of the transfer roller 2A changes depending on the temperature and humidity of the use environment, a constant current is applied, so that the increase in the amount of current to the photoconductor drum 1 is suppressed, so that the photoconductor drum 1 becomes useless. The rise in the charging voltage is suppressed. In this case, the bias current gradually decreases due to the switching of the switch 4, but since a current having the same polarity as the toner and the polarity necessary for repulsing the toner flows, the self-cleaning of the transfer roller 2A is impeded. There is no such thing.

[0032]

As described above, according to the first aspect of the invention, since the bias current is applied to the transfer roller at a constant current, the resistance of the transfer roller changes depending on the temperature and humidity of the operating environment. In this case, the current flowing through the image carrier does not change, so it is possible to suppress an increase in the charging voltage when the image carrier is charged by peeling discharge, thereby reducing the burden of the charge removal process for the image carrier. As a result, the characteristic deterioration of the image carrier can be prevented, and the backside of the transfer material due to the transfer of the toner to the transfer roller can be prevented.

According to the second aspect of the invention, the constant current intensity ratio of the transfer bias and the reverse transfer bias (reverse transfer bias current / transfer bias current) is 0.8 to 100 in absolute value.
Since the relationship of 0.1 is set, it is possible to make the current flow to the image carrier constant and suppress an increase in the charging voltage of the image carrier. As a result, it is possible to reduce the load of the charge removal process on the image carrier, prevent the characteristic deterioration of the image carrier, and prevent the back stain of the transfer material due to the transfer of the toner to the transfer roller.

According to the third and fourth aspects of the present invention, the timing of switching from the transfer bias to the reverse transfer bias and the timing of switching from the transfer bias to the reverse transfer bias take into account the time required for the rise of each bias current. Since the setting is made, it is possible to prevent the transfer of toner to the transfer roller and the occurrence of a situation where a part of the transferred image is lost.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a surface potential of an image carrier for explaining an operation of a main part of the image forming apparatus shown in FIG.

FIG. 3 is a timing chart for explaining an operation of a main part of the image forming apparatus shown in FIG.

FIG. 4 is a schematic diagram illustrating an example of a main configuration of an image forming apparatus.

5A and 5B are schematic diagrams for explaining a bias action performed in the main part configuration shown in FIG. 4, in which FIG. 5A shows a transfer bias action, and FIG. 5B shows a reverse transfer bias action. ing.

FIG. 6 is a current characteristic diagram for explaining a problem that occurs when a conventional bias is switched.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Image carrier 2 Transfer device 2A Transfer roller 3 Power supply 3A Transfer bias circuit 3B Reverse transfer bias circuit 4 Switch 5 Control unit 6 Encoder 7 Motor which is a drive source for feeding the image carrier and transfer material

Claims (4)

[Claims] 1. An image forming apparatus comprising an image bearing member and a transfer device for transferring a toner image visualized on the photosensitive member onto a transfer material, wherein the transfer device comprises a conductive elastic member. Therefore, a transfer bias having a polarity opposite to the polarity of the toner is applied when the toner image is transferred, and a reverse transfer bias having the same polarity as the polarity of the toner when the toner image is not transferred. And an electric power source electrically connected to the transfer roller for applying each of the biases at a constant current. 2. The image forming apparatus according to claim 1, wherein the power source supplies a constant current intensity ratio of a transfer bias applied during transfer and a reverse transfer bias applied during non-transfer (reverse transfer bias current / transfer bias current). ) Is set to have a relationship of 0.8 to 0.1 in absolute value. 3. The image forming apparatus according to claim 2, wherein the power source allows for the time required for the current to rise at the switching timing from the transfer bias to the reverse transfer bias and at the switching timing from the reverse transfer bias to the transfer bias, respectively. The image forming apparatus is characterized in that it is set at another time. 4. The image forming apparatus according to claim 3, wherein the power supply has a moving speed of the image carrier of 100 mm / s as a switching timing from the transfer bias to the reverse transfer bias.
When it is ec or more, the rear end position in the moving direction of the transfer material is set at a time corresponding to a position which is preceded by at least 5 mm ahead of the position facing the transfer roller, and the reverse transfer bias is switched to the transfer bias. The image forming apparatus is characterized in that the timing is set such that the front end position in the moving direction of the transfer material corresponds to a position at least 5 mm ahead of the position facing the transfer roller to the front side.