JP2016224267A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that ensures stable transferability even when performing image formation using a sheet with a small width, and prevents a toner from scattering inside the apparatus during removal of an attached toner.SOLUTION: An image forming apparatus A comprises a CPU 201 that controls a transfer roller 5 in a toner attachment mode of attaching a toner to a non-paper feed area in a direction orthogonal to a sheet conveyance direction of the transfer roller 5 and a cleaning mode of removing the toner attached to the transfer roller 5 from the transfer roller 5 by applying a predetermined bias. When executing the toner attachment mode, the CPU 201 determines a time to execute the cleaning mode and the value of the bias according to the amount of toner to be attached to the transfer roller 5.SELECTED DRAWING: Figure 5

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a facsimile machine that forms an image on a recording medium using, for example, an electrophotographic image forming system.

  The image forming apparatus performs image formation by transferring a toner image formed on a photosensitive drum as an image carrier onto a sheet by a transfer unit such as a transfer roller that contacts the photosensitive drum to form a transfer nip. Here, when the transfer bias applied to the transfer roller is controlled at a constant current when the toner image is transferred to the sheet, the transferability may change depending on the size of the sheet used for image formation.

  For example, as shown in FIG. 10A, when the size of a sheet used for image formation is large, an area where a sheet is not passed in the sheet width direction orthogonal to the sheet conveying direction on the transfer roller (hereinafter referred to as a non-sheet passing area). ) Is small. In this case, when a transfer current of, for example, 11 μA is caused to flow by constant current control, the transfer current flows evenly to each region divided into five equal parts in the sheet width direction by approximately 2.2 μA, and the charge necessary for transferring the toner image is obtained. Provided for sheets. If a current of 2 μA flows in each region of the sheet, it is assumed that the charge necessary for transferring the toner image is supplied to the sheet.

  On the other hand, as shown in FIG. 10B, when the size of the sheet used for image formation is small, the width of the sheet in the direction orthogonal to the sheet conveyance direction (hereinafter referred to as the sheet width) is orthogonal to the sheet conveyance direction of the transfer roller. Since there is a large difference with the width in the direction in which the sheet is transferred, the non-sheet passing area is large on the transfer roller. When the non-sheet passing area is large as described above, when a transfer bias is applied, the current selectively flows into the non-sheet passing area on the transfer roller where the transfer roller and the photosensitive drum are in direct contact. This is because the resistance value of the portion where the photosensitive drum and the sheet overlap is higher than the electrical resistance value of the photosensitive drum (hereinafter simply referred to as the resistance value), so that a current flows in the direction of the photosensitive drum having a lower resistance value. This is because it is easy. In such a case, for example, when 11 μA is passed as the transfer current, a large amount of current (for example, 4 μA) flows in the contact portion where the photosensitive drum and the transfer roller are in direct contact, and flows in each region of the sheet having a high resistance value. The current is reduced (eg 1 μA). In this case, the charge necessary for transferring the toner image is not sufficiently supplied to the sheet, the force for electrostatically holding the toner image on the sheet is reduced, and the toner image may be disturbed to cause transfer failure. .

  Therefore, methods for preventing such transfer defects have been proposed. In Patent Document 1, toner as a resistor is attached to a sheet non-sheet passing region on a photosensitive drum, and image formation is performed with the toner remaining attached. As a result, the toner becomes a resistor in the transfer nip, the current flowing directly to the photosensitive drum when transferring to a small size sheet is suppressed, and the electric charge necessary for transferring the toner image is secured. However, in this method, since the toner as the resistor is collected and removed from the photosensitive drum when passing through the cleaner, a toner image in the sheet non-sheet passing area is required for each sheet, which causes a problem of toner consumption. In addition, if the toner is to be reused to avoid toner consumption, a mechanism dedicated to reuse is also required. For this reason, the inventors have proposed a configuration in which toner as a resistor is attached to a sheet non-sheet passing region on the transfer roller and the toner as a resistor is continuously maintained on the transfer roller during continuous printing. Yes. As a result, the difference in resistance value between the paper passing area and the non-paper passing area on the transfer roller is reduced, so that it is possible to prevent an excessive current from flowing into the photosensitive drum when the transfer bias is applied. Toner consumption as a resistor can be suppressed.

JP 9-325624 A

  Here, when image formation is performed in a state where toner is adhered to the non-sheet passing area of the transfer roller, a large amount of adhered toner adhered as a resistor to the non-sheet passing area on the transfer roller after the image forming process. Exists.

  As a method for removing (cleaning) the adhered toner on the transfer roller, a cleaning bias having the same polarity as the adhered toner is applied to the transfer roller, thereby causing the toner to electrostatically adhere to the photosensitive drum at the transfer nip. Thereafter, as the photosensitive drum rotates, the adhering toner moves toward the cleaner device that collects and cleans residual toner on the surface of the photosensitive drum, and the rake sheet of the cleaner device is pushed up so that the photosensitive drum and the cleaner device face each other. Move to the area. Thereafter, the residual toner is scraped off from the surface of the photosensitive drum by a cleaning blade provided in the cleaner device. The collected toner collected by the cleaner device is conveyed in the longitudinal direction of the cleaner device by a collected toner conveying screw and conveyed to a collected toner container outside the cleaner device.

  However, in such a situation where the toner adhering to the transfer roller is very much on the transfer roller, if the same cleaning bias is applied to the transfer roller as in the case where the toner does not adhere to the non-sheet passing area, the transfer roller will sensitize the transfer roller. The amount of toner that moves to the body drum is very large. In such a situation, when the rake sheet of the cleaner device is pushed up, as shown in FIG. 11, more toner than the amount that can be discharged from the cleaner device is supplied to the cleaner device. For this reason, the toner may spill out from the gap between the scooping sheets of the cleaner device, and the toner may be scattered in the apparatus main body.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image that prevents toner from scattering into the apparatus main body during cleaning of adhered toner, even when stable transferability is ensured when an image is formed using a sheet having a small sheet width. A forming apparatus is provided.

  In order to achieve this object, an image forming apparatus according to the present invention is an image forming apparatus that forms an image by transferring a toner image carried on an image carrier onto a sheet by a transfer member. A toner adhesion mode in which toner adheres to a non-sheet passing region in a direction perpendicular to the sheet conveying direction of the transfer member, and a toner attached to the transfer member by applying a bias of a predetermined bias value to the transfer member. And a control unit that controls a cleaning mode for cleaning from the transfer member, and the control unit performs the cleaning according to the amount of toner that adheres to the transfer member when the toner adhesion mode is executed. Determining a mode.

  According to the present invention, the transferability can be ensured regardless of the sheet size by attaching the toner as a resistor to the non-sheet passing area on the transfer roller.

  Further, when cleaning the toner adhered on the transfer roller after execution of the toner adhesion mode, the cleaning bias is determined according to the amount of toner adhering to the transfer roller. Accordingly, it is possible to suppress the amount of the adhered toner moving from the transfer roller to the photosensitive drum, and it is possible to prevent the adhered toner spilled from the rake sheet from scattering into the apparatus main body.

1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment of the present invention. 1 is a block diagram illustrating a system configuration of an image forming apparatus according to a first embodiment of the present invention. 3 is a flowchart of control during image formation according to the first embodiment of the present invention. FIG. 6 is an explanatory diagram illustrating a state of a transfer roller when a toner adhesion mode is executed. It is a sequence diagram of cleaning bias application at the time of cleaning mode execution concerning a 1st embodiment of the present invention. 6 is a graph for explaining a relationship between a cleaning bias and a residual toner amount on a photosensitive drum after execution of a toner adhesion mode. 6 is a graph for explaining the relationship between the number of images formed after execution of a toner adhesion mode and the amount of toner adhered on a transfer roller. It is a flowchart of the control at the time of image formation which concerns on 2nd Embodiment of this invention. It is a figure explaining the structure of a transfer roller cleaning blade. It is a figure explaining a prior art. It is a figure explaining the subject of a prior art.

(First embodiment)
<Image forming apparatus>
Hereinafter, the overall configuration of the image forming apparatus A according to the first embodiment of the present invention will be described with reference to the drawings together with the operation during image formation.

  As shown in FIG. 1, the image forming apparatus A includes an image forming unit that transfers a toner image to a sheet, a sheet feeding unit that supplies the sheet to the image forming unit, and a fixing unit that fixes the toner image on the sheet. An electrophotographic printer provided.

  The image forming unit includes a photosensitive drum 1 (image carrier), a charging roller 2, a laser scanner unit 3, a developing device 4, a transfer roller 5 (transfer member), a static elimination needle 19, a cleaner device 14, and the like.

  When the image is formed, when the CPU 201 (control unit) shown in FIG. 2 issues a print signal, the sheets stacked and stored in the paper feed cassette (sheet stacking unit) 10 by the feeding roller 9 and the transport roller 8 are transferred to the image forming unit. Sent out.

  On the other hand, in the image forming unit, a charging bias is applied to the charging roller 2 from a high-voltage power supply (not shown), so that the surface of the photosensitive drum 1 in contact with the charging roller 2 is charged. Then, the laser scanner unit 3 irradiates the photosensitive drum 1 with the laser light L. As a result, the potential of the photosensitive drum 1 is partially reduced, and an electrostatic latent image corresponding to image information is formed on the surface of the photosensitive drum 1.

  In the developing region, a developing bias is applied to the developing sleeve 18, whereby toner is attached to the electrostatic latent image on the photosensitive drum 1 to form a toner image. The toner image formed on the photoconductive drum 1 is sent to a transfer nip formed between the photoconductive drum 1 and the transfer roller 5 in contact with the photoconductive drum 1. When the toner image arrives at the transfer nip, a transfer bias having a polarity opposite to that of the toner is used for the transfer roller 5, and in this embodiment, a negative polarity toner is used. Therefore, a positive polarity transfer bias is applied and the toner image is transferred to the sheet. Is done. In this embodiment, the transfer roller 5 is described as a urethane roller having an Asker C hardness of 20 degrees to 40 degrees, but may be an ion conductive roller or an electronic conductive type roller.

  Thereafter, the sheet on which the toner image has been transferred is sent to the fixing device 11 where it is heated and pressurized to fix the toner image on the sheet. Thereafter, the sheet is conveyed by the discharge roller 12 and discharged to the discharge unit.

  Further, after the image forming process, a cleaning mode is executed, and a toner having the same polarity as that of the toner, that is, a negative polarity cleaning bias is applied to the transfer roller 5 to move the toner adhered on the transfer roller 5 to the photosensitive drum 1. . Thereafter, the toner that has moved to the photosensitive drum 1 moves in the direction of the cleaner device 14 (toner accommodating member) as the photosensitive drum 1 rotates.

  The cleaner device 14 is adjacent to the photosensitive drum 1 and is formed of a cleaning blade 7, a scoop sheet 13, and a toner containing wall 15. The adhering toner moved in the direction of the cleaner device 14 with the rotation of the photosensitive drum 1 then moves up into the cleaner device 14 by pushing up the rake sheet 13 which is the entrance of the toner when entering the cleaner device 14. . Thereafter, the toner is scraped off from the surface of the photosensitive drum 1 by the cleaning blade 7 and stored in the cleaner device 14.

  Further, the scooping sheet 13 is pushed up when the adhering toner is accommodated, and again comes into contact with the photosensitive drum 1 due to rigidity, and the collected toner collected by the cleaner device 14 spills down and scatters outside the cleaner device 14. To prevent that.

<Control unit>
Next, the configuration of the control unit of the image forming apparatus A according to the present embodiment will be described with reference to the drawings.

  As shown in FIG. 2, the control unit includes a CPU 201. Further, the CPU 201 includes an environmental sensor 202 for detecting temperature and humidity, a memory 203, a sheet width detection sensor 206 (sheet width detection means) disposed in the sheet feeding cassette 10 for stacking and storing sheets, and a sheet passing number counter 207 ( Etc.) are connected. The sheet width detection sensor 206 detects sheet size information, for example, size information such as letter size, A4, and B5, and stores them in the memory 203. The size information may be configured such that the user inputs the sheet size from an operation panel (not shown) without using the sheet width detection sensor 206. Further, the number of image formations is stored in the memory 203 by the sheet passing number counter 207.

  Further, the transfer high-voltage power supply 204 is connected to the transfer roller 5 and applies a transfer bias having a reverse polarity to the toner when forming an image and a cleaning bias having the same polarity as that of the toner when cleaning the transfer roller 5 in accordance with an instruction from the CPU 201. To do. This transfer bias is controlled at a constant current based on the temperature and humidity detected by the environment sensor 202, and is controlled so that the current flowing through the photosensitive drum 1 becomes a constant value. The cleaning bias is controlled at a constant voltage. The applied transfer bias value and cleaning bias value are detected by the transfer voltage detection circuit 205 detecting the voltage at the time of bias application.

<Control during image formation>
Next, control during image formation of the image forming apparatus A according to the present embodiment will be described in detail. When the sheet width of a sheet used for image formation is small, the image forming apparatus A according to the present embodiment executes a toner attachment mode in which toner is attached to a non-sheet passing area on the transfer roller 5 by a method described later. Thereby, the difference in resistance value between the sheet passing area and the non-sheet passing area on the transfer roller 5 can be reduced, and the charge necessary for transfer can be supplied to the sheet.

  In addition, when the toner adhesion mode is executed, a large amount of toner as a resistor adheres to the non-sheet passing area on the transfer roller 5 after the image forming process. When a cleaning bias similar to that in a normal state (when the toner adhesion mode is not performed) is applied in such a state, the amount of adhered toner moving from the transfer roller 5 to the photosensitive drum 1 becomes very large. When a large amount of toner moved to the photosensitive drum 1 moves toward the cleaner device 14, as shown in FIG. 11, when the rake sheet 13 is pushed up, a part of the toner spills from the rake sheet, and the toner is removed from the device. There is a risk of scattering into the body.

  Therefore, in the image forming apparatus A according to the present embodiment, when the cleaning mode of the transfer roller is executed after the toner adhesion mode is executed, the image forming apparatus A applies a lower cleaning bias than when the toner adhesion mode is not executed. Toner that moves to the body drum 1 is suppressed. As a result, the amount of adhering toner moving from the transfer roller 5 to the photosensitive drum 1 can be suppressed, and the toner can be prevented from being scraped off by the scooping sheet 13 and scattered in the apparatus.

  Hereinafter, the toner adhesion mode will be described first. Thereafter, the cleaning mode will be described.

<Toner adhesion mode>
In the image forming apparatus A according to the present embodiment, as shown in the flowchart of FIG. 3, first, when an image formation instruction is given, the sheet width detection sensor 206 detects or sheet size information set by the user, for example, letter size. , A5, B4, etc. are stored in the memory 203 (S1). Thereafter, the CPU 201 determines whether the sheet size information stored in the memory 203 is larger than a predetermined size set in advance or below a predetermined size (S2). Here, in this embodiment, the predetermined size refers to a letter size. Therefore, in this step, it is determined whether the size of the sheet used for image formation is larger than the letter size or smaller than the letter size.

  Next, when the size of the sheet used for image formation is larger than the letter size, normal image formation processing is performed without executing the toner adhesion mode (S3). In this case, the non-sheet passing area is small on the transfer roller 5, and the transfer bias does not flow excessively from the transfer roller 5 to the photosensitive drum 1. Therefore, since the charge necessary for transferring the toner image is supplied to the sheet, it is not necessary to attach the toner to the non-sheet passing area on the transfer roller 5.

  On the other hand, when the size of the sheet used for image formation is equal to or smaller than the letter size, it means that the non-sheet passing area on the transfer roller 5 is large. When a transfer bias is applied to the transfer roller 5 when the non-sheet passing area is large, the resistance value of the portion where the photosensitive drum 1 and the sheet overlap is higher than the resistance value of the photosensitive drum, and thus the current has a low resistance value. The transfer roller 5 and the photosensitive drum 1 selectively flow into a portion where they are in direct contact. For this reason, the current flowing through the sheet is reduced, the electric charge necessary for transferring the toner image is not sufficiently supplied to the sheet, and the force for electrostatically holding the toner image on the sheet is reduced. There is a possibility of causing a transfer defect due to disorder.

  Therefore, a toner as a resistor is attached to the transfer roller 5 in a difference region between the width of the transfer roller 5 that is a non-sheet passing region in the direction perpendicular to the sheet conveyance direction and the sheet width of the sheet used for image formation. The toner adhesion mode to be executed is executed (S4).

  As a specific process, a laser beam L is irradiated from the laser scanner unit 3 to an area corresponding to the difference area on the surface of the photosensitive drum 1. As a result, the potential of the area corresponding to the difference area on the surface of the photosensitive drum 1 is lowered, and an electrostatic latent image is formed on the surface of the photosensitive drum 1. Next, by applying a developing bias to the developing sleeve 18, toner is attached to the electrostatic latent image to form a toner image. Thereafter, a transfer bias having a polarity opposite to that of the toner is applied to the transfer roller 5 so that the toner image is attached to the transfer roller 5 as shown in FIG.

  Thereafter, the image forming process is executed in a state where the toner as the resistor is attached to the non-sheet passing area on the transfer roller 5 by executing the toner attaching mode (S5). As a result, the difference in resistance value between the paper passing area and the non-paper passing area on the transfer roller 5 is reduced, so that it is possible to prevent an excessive current from flowing into the photosensitive drum 1 when a transfer bias is applied. Accordingly, the charge necessary for transferring the toner image is sufficiently supplied to the sheet, and stable transferability can be obtained.

<Cleaning mode>
Next, a method for cleaning (removing) the toner attached to the transfer roller 5 after the image forming process by executing the cleaning mode will be described.

  In the image forming apparatus A according to the present embodiment, the first cleaning mode and the second cleaning mode are stored in the memory 203 as cleaning modes, and cleaning is performed using these modes properly according to the situation. Here, the second cleaning mode is a cleaning mode in which the cleaning power is stronger than the first cleaning mode (the bias value of the cleaning bias is large).

  Further, the cleaning force is a force for removing the adhered toner from the transfer roller 5. Since the image forming apparatus A according to the present embodiment uses the negative polarity toner as described above, a negative polarity cleaning bias having the same polarity as the toner is applied to the transfer roller 5 to move the adhered toner to the photosensitive drum 1. Let Therefore, in this embodiment, the cleaning power increases as the cleaning bias value increases in the same polarity direction, and the CPU 201 (bias control unit) changes the cleaning bias in each cleaning mode by changing the bias value of the cleaning bias. Let In the present embodiment, the cleaning bias value when the first cleaning mode is executed is -0.5 kV, and the cleaning bias value when the second cleaning mode is executed is -2 kV.

  First, as shown in the flowchart of FIG. 3, when the sheet used for image formation is larger than the letter size and the image forming process is performed without executing the toner adhesion mode, the second cleaning mode having a strong cleaning power is executed. (S7). Specifically, as shown in FIG. 5A, the -2 kV cleaning bias is continuously applied while the transfer roller 5 makes two revolutions. As a result, the toner on the transfer roller 5 moves to the photosensitive drum 1, and the moved toner moves toward the cleaner device 14 as the photosensitive drum 1 rotates, and pushes up the rake sheet 13 to move into the cleaner device 14. And is scraped off by the cleaning blade 7.

  This is because when the image forming process is performed without executing the toner adhesion mode, the toner that adheres to the transfer roller 5 after the image forming process is only the toner that has not been completely transferred to the sheet existing in the sheet passing area. Accordingly, even when the second cleaning mode having a strong cleaning power is executed, only a small amount of toner moves to the photosensitive drum 1, and if it is a small amount of toner, the toner collides with the rake sheet 13 and the toner enters the apparatus. This is because they are not scattered.

  On the other hand, when the size of the sheet used for image formation is equal to or smaller than the letter size and the toner adhesion mode is executed, the toner adhered as a resistor in the non-sheet passing area on the transfer roller 5 after the image formation processing is performed. There is a large amount. In this case, when the same cleaning bias of −2 kV is applied to the transfer roller 5, a large amount of adhered toner moves from the transfer roller 5 to the photosensitive drum 1.

  FIG. 6 is a graph showing the amount of adhered toner that moves from the transfer roller 5 to the photosensitive drum 1 when a cleaning bias is applied after the toner adhesion mode is executed. In this graph, the horizontal axis represents the value of the cleaning bias, and the vertical axis represents the amount of toner adhering to the photosensitive drum 1. Here, for example, when −2 kV is applied to the transfer roller 5 as in the second cleaning mode, 500 mg of adhered toner moves to the photosensitive drum 1. On the other hand, when −0.5 kV is applied, the movement amount of the adhered toner can be suppressed to 100 mg. In addition, when the amount of adhering toner at the time of entering the cleaner device 14 exceeds 200 mg, the adhering toner is scraped off the scooping sheet 13 and scattered in the apparatus.

  Therefore, in the image forming apparatus A according to the present embodiment, when the size of the sheet used for image formation is equal to or smaller than the letter size and the toner adhesion mode is executed, the cleaning power is smaller than that in the second cleaning mode (cleaning bias value). The first cleaning mode is executed. Specifically, the cleaning bias value for the first round of the transfer roller is set to -0.5 kV, and the second and subsequent rounds are set to -2 kV (S6).

  As a result, the amount of adhering toner cleaned from the transfer roller 5 at a time can be suppressed to a small amount. Specifically, an amount of toner that can pass through the scooping sheet 13 can be cleaned as the toner cleaning amount. For this reason, the toner can be prevented from being scraped off by the scooping sheet 13 and scattered in the apparatus. In addition, by increasing the cleaning bias value after the second round, the first round in which a large amount of toner adheres to the transfer roller 5 is cleaned with a weak cleaning force, and the toner is scattered in the apparatus. In the second and subsequent rounds when the toner adhering on the transfer roller 5 is reduced to some extent, the cleaning process can be performed more rapidly while cleaning the adhering toner more reliably.

  In addition, when the toner adhesion mode is executed, the amount of toner adhering to the transfer roller 5 after the image forming process is larger than usual. On the other hand, the cleaning power in the first cleaning mode executed at this time is weaker than the cleaning power in the second cleaning mode. Therefore, when the first cleaning mode is executed, it is preferable that the cleaning mode execution time is made longer than when the first cleaning mode is not executed. For example, as shown in FIG. 5B, the cleaning mode for one rotation of the transfer roller 5 is lengthened. Thereby, even when the first cleaning mode having a weak cleaning power is executed, the toner adhered on the transfer roller 5 can be more reliably cleaned (removed).

(Second Embodiment)
Next, a second embodiment of the image forming apparatus A according to the present invention will be described. The same parts as those in the first embodiment will be denoted by the same reference numerals and the description thereof will be omitted.

  Similarly to the first embodiment, the image forming apparatus A according to the present embodiment executes the toner adhesion mode when the sheet used for image formation is smaller than the letter size, and applies toner to the non-sheet passing area on the transfer roller 5. Then, the image forming process is performed.

  Here, when performing the image forming process, the photosensitive drum 1 and the transfer roller 5 rotate while being in contact with each other. For this reason, when the image forming process is performed after the execution of the toner adhesion mode, the toner in the non-sheet passing area on the transfer roller 5 rubs against the photosensitive drum 1 as the both rotate, and non-electrostatically the photosensitive member. Move to drum 1. Since the amount of toner movement increases in accordance with the amount of rotation of both, the amount of toner movement increases as the number of images formed increases. In other words, as shown in FIG. 7, as the number of images formed increases, the amount of toner that adheres to the non-sheet passing area on the transfer roller 5 decreases as the number of images formed increases. In this embodiment, when the number of formed images reaches 20, the amount of toner adhered on the transfer roller 5 after the image forming process is 100 mg. In this case, even if the second cleaning mode is executed from the beginning when the cleaning mode is executed, the adhered toner does not collide with the rake sheet 13 and scatter in the apparatus.

  In view of this, in the image forming apparatus A according to the present embodiment, even when the sheet used for image formation is smaller than the letter size, the number of image formations after the toner attachment mode is performed until the cleaning process is performed is predetermined. When the number is greater than or equal to the number, the second cleaning mode is executed without executing the first cleaning mode. Hereinafter, this control will be described with reference to the drawings.

  As shown in the flowchart of FIG. 8, the image forming apparatus A according to the present embodiment detects the size information of the sheet used for image formation until the end of the image forming process. Processing similar to that of apparatus A is performed (S101 to S105). If the size of the sheet used for image formation is larger than the letter size, the second cleaning mode is executed in the same manner as in the control of the image forming apparatus A according to the first embodiment (S109).

  On the other hand, when the sheet used for image formation is a letter size or less, the image forming apparatus A according to the present embodiment passes the number of image formation sheets from the execution of the toner adhesion mode to the execution of the cleaning mode. (S106). Thereafter, it is determined whether or not the number of formed images is equal to or greater than a predetermined number (S107). In the present embodiment, the predetermined number is set to 20.

  Here, when the number of image formations from the execution of the toner adhesion mode to the execution of the cleaning process is equal to or greater than a predetermined number (20 or more in the present embodiment), the transfer roller 5 is not in operation during the image formation process as described above. Most of the toner adhering to the paper passing area moves to the photosensitive drum 1 non-electrostatically. For this reason, there is little adhering toner on the transfer roller 5 after the image forming process, and in this case, it is not necessary to execute the first cleaning mode having a weak cleaning power. Therefore, in such a case, the second cleaning mode is executed from the beginning, and a cleaning bias of −2 kV is applied to the transfer roller 5 (S109).

  Note that, as described above, when the number of formed images exceeds a predetermined number, most of the adhered toner moves to the photosensitive drum 1, so that the resistance value of the non-sheet passing region of the transfer roller 5 decreases. Accordingly, in this case, the toner is again attached to the non-sheet passing area of the transfer roller 5. For this reason, it is the number of image formations after the execution of the last toner adhesion mode that determines whether or not the number of image formations is a predetermined number or more.

  By such control, the cleaning can be performed only in the second cleaning mode, so that the execution time of the cleaning mode can be shortened and the cleaning performance is improved because the cleaning is performed with a strong cleaning power.

  On the other hand, when the number of images formed between the execution of the toner adhesion mode and the execution of the cleaning mode is less than 20, a large amount of toner still remains in the non-sheet passing area on the transfer roller 5 after the image formation processing. It is in an attached state. Therefore, in this case, the first cleaning mode is executed (S108).

  The control in the first and second embodiments is performed not only in the transfer roller type image forming apparatus but also in the case of an intermediate transfer belt (intermediate transfer body) type image forming apparatus. A similar effect can be obtained. The cleaning bias can be controlled not only by constant voltage control but also by constant current control.

  In addition, a transfer roller cleaning blade 16 that comes into contact with the transfer roller 5 is provided without moving the toner adhering on the transfer roller 5 to the photosensitive drum 1, and the transfer roller cleaning blade 16 scrapes off the adhering toner. Is also possible. Thereby, the adhering toner does not collide with the rake sheet 13, and the toner can be prevented from scattering in the apparatus.

  Further, as shown in FIG. 9, a contact pressure variable member 17 that changes the contact pressure between the transfer roller cleaning blade 16 and the transfer roller 5 in each region in the direction orthogonal to the rotation direction of the transfer roller 5 is provided. It is also possible to adopt a configuration in which the contact pressure in the region corresponding to the non-sheet passing region on the transfer roller 5 is increased and the contact pressure in the region corresponding to the sheet passing region is decreased. As a result, it is possible to increase the cleaning force for the non-sheet passing area on the transfer roller 5 to which a large amount of toner adheres, and the transfer roller 5 can be cleaned more efficiently.

  In addition, control for determining various cleaning modes according to the toner amount of toner adhering to the non-sheet passing area on the transfer roller 5 when the toner adhesion mode is executed, as well as whether or not the toner adhesion mode is executed. Good. Here, as a method of detecting the amount of toner adhering to the non-sheet passing region, the toner amount can be detected based on the size information of the sheet used for image formation. Or it is good also as a structure which provides a sensor separately and detects a toner amount.

  In this case, the contents of the cleaning mode to be determined include control for determining the cleaning mode execution time (cleaning time) and the bias value of the cleaning bias in accordance with the amount of toner adhering to the transfer roller. For example, the first cleaning mode is executed when the toner adhesion amount is a predetermined value or more, and the second cleaning mode is executed when the toner adhesion amount is a predetermined value or less.

  Thereby, when the toner adhesion amount in the non-sheet passing area of the transfer roller 5 is large, the first cleaning mode is executed, and the amount of the adhered toner cleaned at once from the transfer roller 5 can be suppressed to a small amount. . For this reason, the toner can be prevented from being scraped off by the scooping sheet 13 and scattered in the apparatus. In addition, by increasing the cleaning bias value after the second round, the first round in which a large amount of toner adheres to the transfer roller 5 is cleaned with a weak cleaning force, and the toner is scattered in the apparatus. In the second and subsequent rounds when the toner adhering on the transfer roller 5 is reduced to some extent, the cleaning process can be performed more rapidly while cleaning the adhering toner more reliably.

  Even when the toner adhesion mode is executed, when the adhesion amount is small, the second cleaning mode is executed, so that cleaning can be performed in a shorter time. Further, since cleaning is performed with a stronger cleaning power, the cleaning property is also improved.

  Even in this case, it is predicted that the amount of toner adhering to the non-sheet passing region of the transfer roller 5 is reduced when a predetermined number of images are formed after the execution of the toner adhesion mode. For this reason, control may be performed to determine whether or not to determine or execute the cleaning mode according to the amount of toner that adheres to the non-sheet passing area of the transfer roller, based on the number of images formed after the toner adhesion mode is executed. . Thus, the cleaning mode can be determined or executed only when necessary, and the cleaning process can be performed more efficiently.

DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum 2 ... Charging roller 3 ... Laser scanner unit 4 ... Developing device 5 ... Transfer roller 6 ... Separating device 7 ... Cleaning blade 8 ... Conveying roller 9 ... Feeding roller 10 ... Paper feed cassette 11 ... Fixing device 12 ... Discharge roller 13 ... scoop sheet 14 ... cleaner device 15 ... toner accommodating wall 16 ... transfer roller cleaning blade 17 ... contact pressure variable member 18 ... developing sleeve 19 ... static elimination needle 201 ... CPU
DESCRIPTION OF SYMBOLS 202 ... Environmental sensor 203 ... Memory 204 ... Transfer high voltage power supply 205 ... Transfer voltage detection circuit 206 ... Sheet width detection sensor 207 ... Paper passing number counter

Claims (5)

In an image forming apparatus for forming an image by transferring a toner image carried on an image carrier onto a sheet by a transfer member,
The transfer member is configured to apply toner to a non-sheet passing region in a direction orthogonal to the sheet conveying direction of the transfer member, and to apply a bias having a predetermined bias value to the transfer member. A control mode for controlling a cleaning mode for cleaning toner adhering to the member from the transfer member;
Have
The image forming apparatus according to claim 1, wherein when the toner adhesion mode is executed, the control unit determines the cleaning mode according to an amount of toner adhering to the transfer member.   The image forming apparatus according to claim 1, wherein the control unit determines a cleaning time according to an amount of toner adhering to the transfer member.   The image forming apparatus according to claim 1, wherein the control unit gradually changes the bias value to a larger value. The transfer member is a rotating member having a rotation axis in a direction orthogonal to the sheet conveying direction,
The image forming apparatus according to claim 3, wherein the control unit changes the bias value to a larger value after at least one transfer member makes a round after applying the bias value.   2. The control unit according to claim 1, wherein the control unit determines whether to execute the cleaning mode according to the amount of toner attached to the transfer member, based on the number of images formed after the toner attachment mode is executed. The image forming apparatus according to claim 4.

Images