JP2006330110A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To remove foreign matter such as paper powder transferred from a recording material such as a paper sheet to a transfer member and depositing thereon. <P>SOLUTION: A toner band TB is formed on an intermediate transfer belt 20 in a non-image region in the downstream side of an image region corresponding to the final sheet in one job. While the toner band TB on the intermediate transfer belt 20 passes through a secondary transfer nip N, a cleaning bias by superposing a reverse transfer bias on an AC bias is applied between a secondary transfer roll 31 and a backup roll 25. By applying the cleaning bias, paper powder PD (calcium carbonate CC, fiber F) depositing on the secondary transfer roll 31 follows the toner to be transferred to the intermediate transfer belt 20. This method is effective when the secondary transfer roll 31 is made of a foamed body and has a great number of pores 31a on its surface. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser printer, and more particularly to an image forming apparatus that transfers a toner image on an image carrier onto a recording material using a contact transfer method.

  As a conventional image forming apparatus, each color component toner image developed on a photosensitive drum by a developing device is primarily transferred onto an intermediate transfer belt sequentially, and the superimposed toner image superimposed on the intermediate transfer belt is secondarily transferred onto a sheet. What is transcribed is known. Here, as the secondary transfer device for performing the secondary transfer, for example, a secondary transfer roll disposed in contact with the toner image carrying surface of the intermediate transfer belt, and a secondary transfer roll disposed so as to sandwich the intermediate transfer belt. The thing provided with the backup roll is known. In this secondary transfer apparatus, a predetermined secondary transfer bias is applied between the secondary transfer roll and the backup roll, so that the toner image on the intermediate transfer belt is electrostatically transferred to the paper.

  In such a secondary transfer device, the intermediate transfer belt and the secondary transfer roll are in direct contact while the sheet does not pass through the secondary transfer device. Then, during this time, the toner adhering to the intermediate transfer belt may be transferred to the secondary transfer roll side. When toner adheres to the secondary transfer roll in this way, when the next sheet passes through the secondary transfer device, this toner is transferred from the secondary transfer roll to the back side of the image carrying surface of the sheet, This will cause the back side of the paper to become dirty.

Therefore, for example, a fluorine coat layer with good releasability is formed on the surface of the secondary transfer roll, and a cleaning blade for cleaning the surface of the secondary transfer roll is provided so that the toner adhered to the surface of the secondary transfer roll There is a technique that scrapes off the surface (see Patent Document 1).
However, the cleaning blade may not be attached depending on the layout of the image forming apparatus. Therefore, a transfer bias is applied to transfer the toner on the intermediate transfer belt to the paper side while the paper is passing through the secondary transfer device, and the transfer bias is applied while the paper is not passing through the secondary transfer device. There is also a technique of alternately applying a reverse transfer bias having a polarity opposite to that of the transfer bias (see Patent Document 2).

JP 2000-181251 (page 5-8, FIG. 2, FIG. 8) Japanese Patent Laid-Open No. 3-69978 (page 3, FIG. 2)

  On the other hand, in this type of image forming apparatus, the sheet and the secondary transfer roll are in direct contact while the sheet passes through the secondary transfer apparatus. Then, the paper dust attached to the surface of the paper is transferred and attached to the secondary transfer roll side. When paper dust adheres to the secondary transfer roll in this manner, the resistance value of the secondary transfer roll gradually increases, resulting in a secondary transfer failure.

In response to such a problem, if the cleaning blade described in Patent Document 1 is provided, it may be possible to remove the paper dust from the secondary transfer roll. However, if the cleaning blade cannot be attached as described above, it cannot be dealt with.
In addition, when the method described in Patent Document 2 is used, it is possible to remove the toner attached to the secondary transfer roll, but it is not possible to remove the paper dust attached to the secondary transfer roll. It was very difficult.
Furthermore, when the secondary transfer roll is formed of a foam, a large number of holes are formed by foaming on the surface of the secondary transfer roll. And when such a hole is clogged with paper dust, even if it uses the cleaning blade of the said patent document 1, the removal will become very difficult.

The present invention has been made to solve such technical problems, and an object of the present invention is to remove foreign matters such as paper dust transferred and attached to a transfer member from a recording material such as paper. .
Another object is to provide an image forming apparatus capable of maintaining stable transfer performance over a long period of time.

  For this purpose, an image forming apparatus to which the present invention is applied has a recording material sandwiched between an image carrier that carries and conveys toner and a transfer nip formed between the image carrier and the image carrier on the image carrier. A transfer member that transfers toner to a recording material, and when no recording material is present in the transfer nip, the toner is supplied to the transfer nip by the image carrier, and an AC bias is applied between the image carrier and the transfer member. And a control unit that executes a cleaning operation.

  In this image forming apparatus, when executing the cleaning operation, the control unit can apply a reverse transfer bias for transferring the toner adhering to the transfer member to the side of the image carrier, superimposed on the AC bias. In addition, the control unit, after executing the cleaning operation, a transfer bias for transferring the toner attached to the image carrier to the transfer member and a reverse transfer bias for transferring the toner attached to the transfer member to the image carrier. Can be applied alternately. Further, the control unit can execute the cleaning operation before the first recording material in one job enters the transfer nip or after the last recording material in one job passes through the transfer nip.

  From another viewpoint, the image forming apparatus to which the present invention is applied includes a toner image forming unit that forms a toner image on the image carrier, and a toner image formed on the image carrier by the toner image forming unit. A transfer portion that transfers to a recording material, the transfer portion being pressed against the image carrier, and a nip forming member that forms a transfer nip for allowing the recording material to pass between the image carrier and the transfer nip. When the recording material passes, a transfer power source that supplies a transfer bias for transferring toner on the image carrier to the recording material between the image bearing member and the nip forming member, and the recording material does not pass through the transfer nip. An AC power source for supplying an AC bias between the image carrier and the nip forming member when the toner band formed on the image carrier by the toner image forming unit passes through the transfer nip. Yes.

  In this image forming apparatus, when the toner band passes through the transfer nip, a reverse transfer bias for supplying a reverse transfer bias for transferring the toner on the nip forming member to the image carrier between the image carrier and the nip forming member. A copying power source can be further included. In this case, the transfer power source and the reverse transfer power source can be connected to the image carrier side, and the AC power source can be connected to the nip forming member side. Further, the recording material further includes a separating member for separating the recording materials conveyed to the transfer nip one by one, and the toner image forming unit has a toner band in a predetermined region of the image carrier corresponding to a portion of the recording material that contacts the separating member. Can be formed. Furthermore, the nip forming member can be made of a foam. Furthermore, a conductive and grounded grounding member can be disposed in contact with the nip forming member.

  In the present invention, “when no recording material is present in the transfer nip” means “a predetermined period selected from a period in which no recording material is present in the transfer nip”. It does not mean that there is always no such thing.

  According to the present invention, when the recording material is not present in the transfer nip, the toner is supplied to the transfer nip and the AC bias is applied. Therefore, the paper powder transferred and adhered from the recording material to the transfer member or the nip forming member. Etc. can be removed.

The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below in detail with reference to the accompanying drawings.
<Embodiment 1>
FIG. 1 is a diagram illustrating an outline of an image forming apparatus to which the exemplary embodiment is applied. In this image forming apparatus, a plurality (four in this embodiment) of image forming units 10 (specifically, 10K, 10Y, and 10M) in which each color component toner image is formed in the main body 1 by, for example, electrophotography. , 10C), an intermediate transfer belt 20 for sequentially transferring (primary transfer) holding the respective color component toner images formed by the respective image forming units 10 is provided. The image forming apparatus also includes a secondary transfer device 30 that collectively transfers (secondary transfer) the superimposed image transferred to the intermediate transfer belt 20 to the paper P, and a fixing device that fixes the secondary transferred image on the paper P. 50. Furthermore, the main body 1 includes an IPS (Image Processing System) 80 that performs predetermined image processing on image data input from an image reading device (IIT) (not shown), a personal computer (PC) (not shown), or the like. . The main body 1 is provided with a control unit 100 that controls the entire image forming operation.

  Here, the image forming unit 10 of each color component that functions as a toner image forming unit includes a photosensitive drum 11 and a charging device 12 that charges the photosensitive drum 11 to a predetermined potential. Further, the image forming unit 10 includes a laser exposure device 13 that writes an electrostatic latent image on a charged photosensitive drum 11, and a developing device 14 that develops the electrostatic latent image on the photosensitive drum 11 containing toner of each color component. Have. Further, the image forming unit 10 includes a primary transfer roll 15 that transfers the toner image carried on the photosensitive drum 11 to the intermediate transfer belt 20, and a drum cleaner 16 that removes residues on the photosensitive drum 11 after the primary transfer. It has.

  In the present embodiment, the photosensitive drum 11 is formed by forming an organic photosensitive layer on the surface of a metal thin cylindrical drum, and the organic photosensitive layer is made of a material having a negative charging polarity. Further, the charging device 12 charges the organic photosensitive layer of the photosensitive drum 11 to a negative polarity by applying a negative polarity bias to the charging roll. Further, the laser exposure device 13 irradiates light on the charged photosensitive drum 11 at a position where a toner image is to be formed, thereby forming an electrostatic latent image. The development by the developing device 14 is performed by a reversal development method. Therefore, the toner used in the developing device 14 is of a negative polarity charging type, and the toner is charged to a negative polarity. In this embodiment, the developing device 14 employs a two-component developing system using a developer containing toner and carrier. Further, a primary transfer bias having a polarity (positive polarity) opposite to the charging polarity of the toner is applied to the primary transfer roll 15, and the toner image on the photosensitive drum 11 is transferred to the intermediate transfer belt 20. I am letting. Further, the drum cleaner 16 has a cleaning blade disposed in pressure contact with the rotation direction of the photosensitive drum 11 to scrape residual toner adhering to the photosensitive drum 11.

The intermediate transfer belt 20 as an image carrier is stretched around a plurality of (six in this embodiment) rolls. These rolls include a drive roll 21 that drives the intermediate transfer belt 20, driven rolls 22, 23, and 26 that stretch the intermediate transfer belt 20, and correction that regulates meandering in a direction substantially perpendicular to the conveyance direction of the intermediate transfer belt 20 There is a roll 24 and a backup roll 25 of a secondary transfer device 30 described later. A belt cleaner 27 that removes residues on the intermediate transfer belt 20 after the secondary transfer is disposed on the intermediate transfer belt 20 with the drive roll 21 interposed therebetween. The belt cleaner 27 has a cleaning blade disposed in pressure contact with the rotation direction of the intermediate transfer belt 20 in the counter direction, and scrapes off residual toner adhering to the intermediate transfer belt 20.
The secondary transfer device 30 includes a secondary transfer roll 31 serving as a transfer member disposed in pressure contact with the toner image carrying surface side of the intermediate transfer belt 20 and a secondary transfer roll disposed on the back surface side of the intermediate transfer belt 20. And a backup roll 25 forming 31 counter electrodes.

  The paper transport system includes a paper tray 40 that stores paper P as a recording material, a pick-up roll 41 that picks up the paper P accumulated in the paper tray 40 at a predetermined timing, and transports the paper P to a transport path 44. Are provided with a transporting section 43 that transports the paper P that has been fed out in step 1 and the paper P that has been transported one by one by the separating section 42. Further, a registration roll (registration roll) 44 for sending the paper P to the secondary transfer device 30 at a predetermined timing is disposed on the downstream side of the transport roll 43 in the paper transport direction. Here, the webbing unit 42 includes a feed roll 42a as a webbing member that rotates in the conveyance direction of the paper P, and a retard roll 42b as a webbing member that rotates in the direction opposite to the stop or paper P conveyance direction. ing. Further, a belt conveyance device 70 for conveying the paper P after the secondary transfer to the fixing device 50 is provided in the conveyance path 45 on the downstream side in the paper conveyance direction from the secondary transfer device 30.

Further, the fixing device 50 includes a heating roll 51 having a heating source therein and rotatably arranged. In addition, the fixing device 50 includes a pressure roll 52 that is disposed in pressure contact with the heating roll 51 and rotates following the heating roll 51. Here, the heating roll 51 is controlled so as to be in a predetermined temperature range by a temperature adjusting unit (not shown).
A discharge path 46 for transporting the paper P toward the upper side is provided downstream of the fixing device 50 in the paper transport direction. A plurality of transport rolls 47 are disposed in the discharge path 46. A paper discharge tray 2 for discharging the image-formed paper P is formed on the downstream side of the discharge path 46 and in the upper part of the main body 1.

  In the present embodiment, a paper reversing conveyance mechanism 60 is provided that reverses the sheet P that has been fixed on one side by the fixing device 50 and returns it to the secondary transfer device 30 when the duplex mode is selected. The sheet reverse conveyance mechanism 60 is provided with a branch path 61 that branches downward with respect to the discharge path 46 from the fixing device 50. The branch path 61 further extends a reverse path 62 downward, and this A return path 63 returning from the reverse path 62 to the transport path 45 before the registration roll 44 is connected in communication. In addition, an appropriate number of transport rolls 64 are provided in these paths as necessary. A gate 65 is provided on the exit side of the fixing device 50 to switch the conveyance direction of the paper P after fixing to the discharge path 46 or the branch path 61. A gate 66 for switching the transport direction of the paper P is provided. Further, a switchback roll 67 is attached to the reversing path 62 so as to be rotatable forward and backward.

  Next, a basic image forming process of the image forming apparatus according to the present embodiment will be described. Image data output from an image reading device (IIT) (not shown), a personal computer (PC) (not shown), or the like is input to an image forming apparatus as shown in FIG. In the image forming apparatus, after predetermined image processing is performed in the IPS 80, an image forming operation is performed by the image forming unit 10 or the like. In IPS 80, the input reflectance data is subjected to predetermined image processing such as shading correction, position shift correction, brightness / color space conversion, gamma correction, frame deletion, color editing, moving editing, and other various image editing. Is done. The image data subjected to the image processing is converted into color material gradation data of four colors of black (K), yellow (Y), magenta (M), and cyan (C), and is output to the laser exposure device 13. .

  The laser exposure device 13 irradiates the photosensitive drums 11 of the image forming units 10K, 10Y, 10M, and 10C with, for example, an exposure beam emitted from a semiconductor laser according to the input color material gradation data. . The surface of the photosensitive drum 11 of the image forming units 10K, 10Y, 10M, and 10C is charged to, for example, −550 V by the charging device 12, and then the surface is scanned and exposed by the laser exposure device 13 to form an electrostatic latent image. Is done. The electrostatic latent image includes a latent image forming region (for example, having a charged potential of −50 V) in which optical writing is performed by the laser exposure device 13 and a non-latent image forming region (for example, in which optical writing is not performed). Having a charging potential of -550V).

  In the developing device 14, a developing bias in which an alternating current having a peak-to-peak of 1 kV is superimposed on a direct current −350 V is applied to a developing sleeve (not shown). For this reason, the latent image forming region (−50 V) formed on the photosensitive drum 11 has a positive potential (300 V) relative to the developing sleeve. On the other hand, the non-image forming region (−550V) formed on the photosensitive drum 11 has a negative potential (−200V) relative to the developing sleeve (−350V). Therefore, the toner on the developing sleeve that is negatively charged is electrostatically transferred to the latent image forming region on the photosensitive drum 11. As a result, the electrostatic latent image formed on the photosensitive drum 11 is transferred to the black (K), yellow (Y), and magenta (M) by the developing device 14 of each of the image forming units 10K, 10Y, 10M, and 10C. ) And cyan (C) toner images.

  The toner images formed on the photosensitive drums 11 of the image forming units 10K, 10Y, 10M, and 10C are transferred onto the intermediate transfer belt 20 at the primary transfer portion where the photosensitive drums 11 and the intermediate transfer belt 20 are in contact with each other. Transcribed. More specifically, in the primary transfer portion, a primary transfer bias is applied to the base material of the intermediate transfer belt 20 by the primary transfer roll 15 with a reverse polarity (positive polarity in the present embodiment) of the toner, The unfixed toner image is sequentially superimposed on the surface of the intermediate transfer belt 20 to perform primary transfer. The unfixed toner image primarily transferred in this way is conveyed to the secondary transfer device 30 as the intermediate transfer belt 20 rotates.

  On the other hand, in the paper transport system, the pickup roll 41 rotates in accordance with the image formation timing, and the paper P is supplied from the paper tray 40. The paper P supplied by the pick-up roll 41 is rolled up by the separation unit 42, and only one paper P is conveyed downstream. Then, the paper P is transported along the transport path 45 by the transport roll 43 and reaches the secondary transfer device 30. Before reaching the secondary transfer device 30, the paper P is temporarily stopped by the registration roll 44, and the registration roll 44 rotates in accordance with the movement timing of the intermediate transfer belt 20 carrying the toner image as described above. The position of the paper P and the position of the toner image are aligned.

  In the secondary transfer device 30, the secondary transfer roll 31 is pressed against the backup roll 25 via the intermediate transfer belt 20. At this time, the sheet P conveyed at the same timing is sandwiched between the intermediate transfer belt 20 and the secondary transfer roll 31. At this time, when a voltage having the same polarity as the toner charging polarity (negative polarity in this embodiment) is applied to the backup roll 25, a transfer electric field is formed with the grounded secondary transfer roll 31 as the counter electrode. . The unfixed toner image carried on the intermediate transfer belt 20 is electrostatically transferred onto the paper P at the secondary transfer position pressed by the secondary transfer roll 31 and the backup roll 25.

  Thereafter, the sheet P on which the toner image is electrostatically transferred is peeled off from the intermediate transfer belt 20 and then conveyed to the fixing device 50 by the belt conveying device 70. The unfixed toner image on the paper P conveyed to the fixing device 50 is fixed on the paper P by being subjected to a fixing process by heat and pressure by the fixing device 50. Thereafter, the paper P on which the fixed image is formed is directed toward the discharge path 46 by the gate 65, and the paper P is discharged to the paper discharge tray 2. On the other hand, after the transfer onto the paper P is completed, the residual toner remaining on the intermediate transfer belt 20 is conveyed to a portion facing the belt cleaner 27 as the intermediate transfer belt 20 rotates, and is intermediated by the belt cleaner 27. It is removed from the transfer belt 20.

  When images are formed on both sides of the paper P, the leading edge of the paper P that has passed through the fixing device 50 enters the branch path 61 by the gate 65, and is conveyed to the reversing path 62 by the gate 66 after being conveyed through the branch path 61. enter in. In the reverse path 62, the paper P is once transported toward the back side by the switchback roll 67, and then temporarily stops at a timing immediately after the rear end of the paper P passes through the gate 66, and then switched back at a predetermined timing. By rotating the roll 67 in the reverse direction, the roll 67 is conveyed in the reverse direction. At that time, the paper P enters the return path 63 by the gate 66. The paper P conveyed on the return path 63 is returned to the conveyance path 44 by the conveyance roll 64. At this time, the paper P is in a state where the front and back sides are reversed, unlike when the paper P is first in the transport path 45. This time, the unfixed toner image is electrostatically transferred onto the back surface of the paper P, fixed by the fixing device 50, and then discharged to the paper discharge tray 2 through the discharge path 45.

Next, the secondary transfer device 30 as a transfer unit and its operation will be described in detail. FIG. 2 is a diagram for explaining the configuration of the secondary transfer apparatus 30 in the present embodiment.
In the secondary transfer device 30, the secondary transfer roll 31 that functions as a transfer member or a nip forming member is made of foamed polyurethane as a foam. For example, a 1 kV voltage is applied by pressing against a metal surface with a load of 1 kg. Resistance is 7.2 logΩ. On the other hand, the backup roll 25 is made of a conductive rubber roll formed of epichlorohydrin rubber, and has a resistance of 7.5 logΩ when a voltage of 1 kV is applied to the metal surface with a 1 kg load.
The intermediate transfer belt 20 is made of a polyimide resin containing an appropriate amount of a conductive agent such as carbon black, and has a thickness of 80 μm and a volume resistivity of 10 log Ωcm. In addition to the polyimide resin, for example, a resin such as polyamide, polyester, polypropylene, polyethylene terephthalate, or various rubbers can be used as the intermediate transfer belt 20.

  The secondary transfer device 30 also includes a power supply unit 90 that supplies a secondary transfer bias (transfer bias) and a reverse transfer bias to the backup roll 25. The power supply unit 90 includes a negative DC power supply (transfer power supply) 91 that supplies a negative voltage and a positive DC power supply (reverse transfer power supply) 92 that supplies a positive voltage. Further, the secondary transfer device 30 switches between the negative DC power supply 91 and the positive DC power supply 92, and further includes a first changeover switch 93 that stops power supply from the negative DC power supply 91 or the positive DC power supply 92. I have. Furthermore, the secondary transfer device 30 has an AC power supply 94 for supplying an AC bias to the secondary transfer roll 31. The secondary transfer device 30 includes a second changeover switch 95 that switches between the AC power supply 94 and the ground, and disconnects the AC power supply 94 or the ground.

FIG. 3 is a diagram illustrating functional blocks of the control unit 100. The control unit 100 controls the entire image forming apparatus as described above. FIG. 3 shows only functional blocks related to the secondary transfer apparatus 30 (see FIG. 2).
The CPU 101 of the control unit 100 executes processing while appropriately exchanging data with the RAM 103 in accordance with a program stored in the ROM 102. In addition, image formation information (specifically, information on image formation timing) from the IPS 80 is input to the control unit 100 via the input / output interface 104. The control unit 100 controls the IPS 80, the first changeover switch 93, and the second changeover switch 95 via the input / output interface 104.

Now, the supply and switching operations of the secondary transfer bias and the charging bias in the secondary transfer device 30 will be described with reference to the timing chart shown in FIG. In the following description, an area in the intermediate transfer belt 20 where a toner image for transfer onto the paper P can be formed is referred to as an image area (image area). Further, with respect to the movement direction of the intermediate transfer belt 20, an area provided between the image areas and an area behind the last image area for the last sheet P in one job are defined as non-image areas (interimage areas). Area).
The control unit 100 calculates the time when the image area on the intermediate transfer belt 20 reaches the secondary transfer device 30 based on the image formation timing information input from the IPS 80. Then, the control unit 100 turns off the first changeover switch 93 before the image area on the intermediate transfer belt 20 reaches the secondary transfer apparatus 30, that is, while the non-image area passes through the secondary transfer apparatus 30. Set to. As a result, no positive or negative bias is supplied to the backup roll 25. At this time, the control unit 100 sets the second changeover switch 95 to the ground side, and the AC bias applied to the secondary transfer roll 31 is OFF (0).

  Next, the control unit 100 connects the first changeover switch 93 to the negative DC power supply 91 side in accordance with the timing at which the leading edge of the image area on the intermediate transfer belt 20 enters the secondary transfer device 30. . Further, the control unit 100 maintains the state in which the second changeover switch 95 is set to the ground side even while the moving direction tip of the image area on the intermediate transfer belt 20 exists in the secondary transfer device 30. As a result, a negative secondary transfer bias (-2700 V) is supplied from the negative DC power supply 91 to the backup roll 25, and the backup roll 25 to which the secondary transfer bias is supplied is grounded. A secondary transfer electric field is formed between the secondary transfer roll 31. The toner (negatively charged) on the intermediate transfer belt 20 is secondarily transferred from the intermediate transfer belt 20 to the paper P by the secondary transfer electric field thus formed.

  Then, the control unit 100 forms a toner band in a non-image area existing downstream of the image area corresponding to the last sheet P in one job based on the image formation timing information input from the IPS 80. A signal is output to the IPS 80. In the IPS 80, immediately after the image area corresponding to the last sheet P in one job, a toner band (toner image) having a density of about 20% over the entire width direction (direction orthogonal to the moving direction) of the intermediate transfer belt 20 is formed. A write signal to be formed is output to the laser exposure device 13. Next, the electrostatic latent image for the toner band formed on the photosensitive drum 11 by the laser exposure device 13 is developed by the developing device 14 and then primarily transferred onto the intermediate transfer belt 20 by the primary transfer roll 15. It is conveyed to the secondary transfer device 30. In this example, the IPS 80 outputs a write signal only to the laser exposure device 13 of the yellow image forming unit 10Y, and a yellow 20% density toner band is formed on the intermediate transfer belt 20. .

  Next, the control unit 100 performs the first switching in accordance with the timing at which the rear end in the moving direction of the image area corresponding to the last sheet P of one job on the intermediate transfer belt 20 passes through the secondary transfer device 30. The switch 93 is set to the positive DC power source 92 side. As a result, a positive reverse transfer bias (+800 V) is supplied to the backup roll 25 from the positive DC power supply 92. At the same time, the control unit 100 switches the second changeover switch 95 to the side connected to the AC power supply 94. As a result, the secondary transfer roll 31 is supplied with an AC bias having a frequency of 2 kHz and a peak-to-peak voltage Vpp of 1.5 kV. Then, a cleaning bias in which a DC reverse transfer bias is superimposed on an AC bias is supplied between the backup roll 25 and the secondary transfer roll 31. In the meantime, a cleaning electric field is formed by the cleaning bias. The toner band formed on the intermediate transfer belt 20 passes through the cleaning electric field. The paper powder transferred and adhered from the paper P to the secondary transfer roll 31 by the cleaning electric field formed in this way and the toner band formed on the intermediate transfer belt 20 is transferred from the secondary transfer roll 31 to the intermediate transfer belt 20. Transition to. The supply of the toner band to the secondary transfer device 30 and the application of an AC bias are referred to as a cleaning operation. Further, the reason why the paper dust attached to the secondary transfer roll 31 by this cleaning operation is transferred to the intermediate transfer belt 20 will be described later.

  Further, after the toner band on the intermediate transfer belt 20 has passed through the secondary transfer device 30, the control unit 100 sets the first changeover switch 93 to the negative DC power supply 91 side, the positive DC power supply 92 side, and the negative switch. Switching to the DC power supply 91 side sequentially. On the other hand, the control unit 100 sets the second changeover switch 95 to OFF immediately after the toner band on the intermediate transfer belt 20 passes through the secondary transfer device 30. As a result, toner, paper dust, and the like adhering to the secondary transfer roll 31 for some reason are more reliably transferred to the intermediate transfer belt 20. Then, the control unit 100 sets the first changeover switch 93 to OFF while the non-image area on the intermediate transfer belt 20 passes through the secondary transfer device 30, and ends the series of operations.

  FIG. 5 shows the results of investigating the relationship between the discharge current value applied to charge calcium carbonate and fibers, which are used as the paper dust of general paper P, and the charge, and the charged charges. Calcium carbonate is a material often used as a filler used to increase the smoothness, whiteness, basis weight, etc. of the paper P, and is widely used particularly for neutral paper. From the figure, it is understood that the calcium carbonate and the fiber have a positive charging polarity.

  FIG. 6 schematically shows the behavior of the toner band TB (toner T) and the paper dust PD when the toner band TB on the intermediate transfer belt 20 passes between the secondary transfer roll 31 and the backup roll 25. FIG. In the present embodiment, as described above, the secondary transfer roll 31 is made of foamed polyurethane, and innumerable holes 31 a are formed on the surface of the secondary transfer roll 31 by foaming. The hole 31a is filled with paper powder PD. The paper powder PD is transferred from the paper P being conveyed. The paper powder PD contains the above-described calcium carbonate CC and fiber F as main components.

  Now, the cleaning operation will be specifically described. First, the toner T constituting the toner band TB formed in the non-image area on the intermediate transfer belt 20 is subjected to the secondary transfer in which the intermediate transfer belt 20 and the secondary transfer roll 31 come into contact with the movement of the intermediate transfer belt 20. It is conveyed to a nip (transfer nip) N. At this time, a cleaning bias in which a DC reverse transfer bias is superimposed on an AC bias is supplied between the secondary transfer roll 31 and the backup roll 25 as described above, and the secondary transfer nip N is cleaned. An electric field is formed. Of the supplied cleaning bias, the AC bias has a function of vibrating the toner T and the paper dust PD present in the secondary transfer nip N. Accordingly, a part of the toner T that is negatively charged by application of an AC bias enters the hole 31a of the secondary transfer roll 31, and the paper powder PD (calcium carbonate CC, fiber that exists in the hole 31a and is charged positively. F) is adsorbed electrostatically. Further, the adhesion of the paper powder PD to the holes 31a of the secondary transfer roll 31 is also reduced by the application of the AC bias. On the other hand, of the supplied cleaning bias, the reverse transfer bias is the intermediate transfer belt 20 that uses toner (negatively charged) in the secondary transfer nip N and toner T that has entered the hole 31a of the secondary transfer roll 31. It has a function to pull in to the side. Accordingly, the toner T present on the intermediate transfer belt 20 in the secondary transfer nip N is attracted to the intermediate transfer belt 20 side, and the toner T that has entered the hole 31a of the secondary transfer roll 31 is reversely transferred to the reverse transfer bias. Is applied again to the intermediate transfer belt 20 side. At this time, the toner T that has entered the hole 31a of the secondary transfer roll 31 is transferred to the intermediate transfer belt 20 together with the adhering paper powder PD. As a result, the paper powder PD is removed from the secondary transfer roll 31 (hole 31a) that has passed through the secondary transfer nip N. The toner T and paper dust PD on the intermediate transfer belt 20 that has passed through the secondary transfer nip N are scraped off and removed by a cleaning blade provided on the belt cleaner 27 (see FIG. 1). At this time, since the density of the toner T forming the toner band TB is at most about 20%, the toner T and the paper powder PD are removed without imposing a burden on the cleaning blade. Note that the length of the toner band TB and the application time of the cleaning bias can be set as appropriate, but at least during the secondary transfer roll 31 rotating once, and further during the secondary transfer roll 31 rotating several times. Preferably, it is done.

  As shown in FIG. 4, the AC bias is then turned off, and the secondary transfer bias and the reverse transfer bias are alternately applied. As a result, for example, toner T or paper powder PD, which has been transferred to the secondary transfer roll 31 side by being charged to a polarity opposite to the normal charging polarity (positive in this example) is moved to the intermediate transfer belt 20 side. Can be made. These are also removed by the belt cleaner 27 (see FIG. 1).

As described above, in this embodiment, the toner band TB composed of toner of a predetermined density (20% in this example) is formed in the non-image area on the intermediate transfer belt 20 and supplied to the secondary transfer device 30. I did it. Then, while the toner band TB passes through the secondary transfer nip N of the secondary transfer device 30, a cleaning bias in which the reverse transfer bias is superimposed on the AC bias is applied. Thereby, paper dust PD adhering to the secondary transfer roll 31 can be transferred to the secondary transfer roll 31 side. The toner T and paper dust PD transferred and adhered to the intermediate transfer belt 20 can be removed by the belt cleaner 27 (see FIG. 1). In particular, in the present embodiment, since the secondary transfer roll 31 is made of foamed polyurethane, innumerable holes 31a are formed on the surface, and the paper powder PD is easily clogged in the holes 31a. In the present embodiment, by adopting such a method, it is possible to take out and remove the paper dust PD clogged in the hole 31a of the secondary transfer roll 31. Further, not only the paper powder PD but also other foreign matters that are positively charged and adhere to the secondary transfer roll 31 are transferred from the secondary transfer roll 31 to the intermediate transfer belt 20 side in the same manner as the paper powder PD. Can do.
By removing paper dust PD and other foreign matters from the secondary transfer roll 31 in this way, it is possible to suppress a change in resistance of the secondary transfer roll 31 with time. In addition, since the change in resistance with time of the secondary transfer roll 31 can be suppressed, it is possible to maintain stable secondary transfer performance over a long period of time.

In this embodiment, after the application of the AC bias to the secondary transfer nip N is stopped, the secondary transfer bias and the reverse transfer bias are alternately switched and supplied to the secondary transfer nip N. Thereby, the toner T adhering to the secondary transfer roll 31 and the paper powder PD remaining on the secondary transfer roll 31 can be more reliably transferred to the intermediate transfer belt 20 side. Therefore, it is possible to suppress the occurrence of a phenomenon in which the toner attached to the secondary transfer roll 31 is reversely transferred from the secondary transfer roll 31 to the back surface of the paper P when the secondary transfer bias is applied. It becomes possible to prevent the back surface contamination.
Furthermore, in the present embodiment, the power supply unit 90 for supplying the secondary transfer bias and the reverse transfer bias to the backup roll 25 can be connected, and the AC for supplying the AC bias to the secondary transfer roll 31. The power supply 94 can be connected. As a result, the wiring can be easily handled, and the layout of each member and the wiring can be simplified.

  In this embodiment, the toner band TB is formed and the cleaning bias is applied at the end of one job. However, the present invention is not limited to this. For example, it is performed before the start of one job. Also good. Alternatively, the number of printed sheets output from the secondary transfer device 30 may be counted, and the toner band TB may be formed and the cleaning bias may be applied every time the number of printed sheets reaches a predetermined number.

<Embodiment 2>
Although the present embodiment is substantially the same as the first embodiment, the toner band TB is not formed over the entire region in the direction (width direction) perpendicular to the moving direction of the intermediate transfer belt 20, but instead of the secondary transfer roll 31. The paper powder PD is intensively formed in a region corresponding to a portion where the paper powder PD is likely to adhere. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  FIG. 7 is a view for explaining the positional relationship between the separating portion 42 and the secondary transfer roll 31 in the image forming apparatus shown in FIG. In the winding portion 42, the feed roll 42a (see FIG. 1) has a shaft 421 that is rotatably arranged, and roll members 422 and 423 attached to the shaft 421. On the other hand, the retard roll 42b (see FIG. 1) has a shaft 424 that is fixedly disposed, and roll members 425 and 426 that are attached to the shaft 424. The roll member 422 of the feed roll 42a abuts on the roll member 425 of the retard roll 42b, and the roll member 423 of the feed roll 42a abuts on the roll member 426 of the retard roll 42b. As described above, the separation unit 42 feeds the paper P one by one by the feed roll 42a that rotates in the transport direction of the paper P and the retard roll 42b that rotates in the direction opposite to the transport direction of the paper P or stops. Ugly. For this reason, the paper P that has passed through the webbing portion 42 is rubbed most at the opposing portion between the roll member 422 and the roll member 425 and the opposing portion between the roll member 423 and the roll member 426. As a result, the paper P A large number of paper dusts PD will adhere to the corresponding part of. The paper powder PD adhering to these parts adheres to the predetermined part B of the secondary transfer roll 31 in a concentrated manner, and only the resistance value at the predetermined part B increases remarkably. Then, since the secondary transfer bias supplied to the secondary transfer roll 31 is constant, the secondary transfer condition is changed between the portion B where the paper powder PD is likely to adhere and the other portion A, resulting in a streak shape. Transfer defects will occur.

  In order to suppress such a problem, it is effective to form the toner band TB over the entire width direction of the intermediate transfer belt 20 as in the above-described first embodiment. It will occur. Therefore, in the present embodiment, as shown in FIG. 8, the toner band TB (specifically, the first toner band TB1 and the second toner band are applied only to the region corresponding to the portion B where the paper dust PD is likely to adhere. TB2) was formed. The formation timing of the first toner band TB1 and the second toner band TB2, the application timing of the cleaning bias to the secondary transfer nip N, and the like are the same as in the first embodiment as shown in FIG. It is.

Here, FIG. 9 shows the relationship between the number of prints and the resistance change ΔR in the secondary transfer roll 31 when the presence or absence of the cleaning operation is used as a parameter. Here, the resistance change ΔR is the difference between the resistance value at the site B and the resistance value at the site A of the secondary transfer roll 31 shown in FIG. In addition, resistance is calculated | required from the electric current value when each site | part A and B is contact | abutted with a 15-g load with respect to the electrode of width 5mm, and the voltage of 1 kV is applied.
As is clear from FIG. 9, the resistance change ΔR in the secondary transfer roll 31 is reduced by cleaning, that is, the formation of the first toner band TB1 and the second toner band TB2 and the application of the cleaning bias. It is understood that the resistance value with respect to the axial direction of the secondary transfer roll 31 is maintained substantially constant. On the other hand, it is understood that the resistance change ΔR increases exponentially as the number of printed sheets increases when cleaning is not performed. From the above results, the effectiveness of performing the cleaning operation can be confirmed.

  Also in the present embodiment, the paper dust PD attached to the secondary transfer roll 31 can be removed as in the first embodiment. In particular, in the present embodiment, the toner band TB (first toner band TB1, first toner band TB1) is concentrated on a portion (intermediate transfer belt 20) corresponding to the portion B where the paper dust PD is likely to adhere due to the whirling operation of the whirling portion 42. Since the second toner band TB2) is formed, the amount of toner T required to create the toner band TB can be reduced. Accordingly, it is possible to reduce the toner T that is wasted and to use the toner T effectively.

For example, as shown in FIG. 10, in the secondary transfer device 30, a conductive roll 96 as a grounding member that is grounded to the secondary transfer roll 31 can be disposed in contact therewith. Here, the conductive roll 96 is made of stainless steel, for example, and is arranged to rotate following the rotation of the secondary transfer roll 31. As in the first and second embodiments, the toner band TB is formed on the intermediate transfer belt 20, and when the toner band TB formed on the intermediate transfer belt 20 passes through the secondary transfer device 30, The next transfer device 30 is supplied with a cleaning bias in which a reverse transfer bias is superimposed on an AC bias.
At that time, since a part of the supplied cleaning bias flows into the conductive roll 96, the paper powder PD is also vibrated between the secondary transfer roll 31 and the conductive roll 96. As a result, the paper powder PD is easily taken out from the secondary transfer roll 31 in the secondary transfer nip N. That is, by attaching the conductive roll 96, taking out of the paper powder PD is assisted, and the paper powder PD can be more easily removed from the secondary transfer roll 31.

  In the first and second embodiments, the secondary transfer device 30 in the intermediate transfer type image forming apparatus has been described as an example. However, the present invention is not limited to this. The present invention can be similarly applied to an image forming apparatus provided with a transfer roll arranged in pressure contact.

1 is a diagram illustrating an overall configuration of an image forming apparatus to which the exemplary embodiment is applied. It is a figure for demonstrating the structure of a secondary transfer apparatus. It is a functional block diagram of a control part. 4 is a timing chart for explaining switching between a secondary transfer bias and an AC bias in the first embodiment. It is a graph for demonstrating the charging polarity of the calcium carbonate which is a main component of paper powder, and a fiber. It is a schematic diagram for demonstrating the behavior of paper dust when a cleaning bias is applied. It is a figure explaining the relationship between the structure of the feed roll in a winding part, and the site | part which paper dust tends to adhere in a secondary transfer roll. 9 is a timing chart for explaining switching between a secondary transfer bias and an AC bias in the second embodiment. FIG. 6 is a graph showing the relationship between the number of prints and resistance change in a secondary transfer roll when toner band formation and whether or not a cleaning bias is applied are used as parameters. FIG. 6 is a diagram for explaining the configuration of another secondary transfer apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Main body, 10 (10K, 10Y, 10M, 10C) ... Image forming unit, 11 ... Photosensitive drum, 12 ... Charging device, 13 ... Laser exposure device, 14 ... Developing device, 15 ... Primary transfer roll, 16 ... Drum Cleaner, 20 ... Intermediate transfer belt, 25 ... Backup roll, 27 ... Belt cleaner, 30 ... Secondary transfer device, 31 ... Secondary transfer roll, 31a ... Hole, 42 ... Scoop, 80 ... IPS, 90 ... Power supply unit, 91 ... Negative DC power supply, 92 ... Positive DC power supply, 93 ... First changeover switch, 94 ... AC power supply, 95 ... Second changeover switch, 96 ... Conductive roll, 100 ... Control unit, T ... Toner, TB ... toner band, PD ... paper dust, N ... secondary transfer nip

Claims (10)

An image carrier for carrying and conveying toner;
A transfer member for sandwiching a recording material in a transfer nip formed between the image carrier and transferring the toner on the image carrier to the recording material;
When the recording material is not present in the transfer nip, a cleaning operation is performed in which toner is supplied to the transfer nip by the image carrier and an AC bias is applied between the image carrier and the transfer member. And an image forming apparatus.   The control unit applies a reverse transfer bias for transferring the toner adhering to the transfer member to the image carrier side in a superimposed manner on the AC bias when the cleaning operation is executed. The image forming apparatus according to claim 1.   The control unit executes the cleaning operation, and then transfers the transfer bias for transferring the toner attached to the image carrier to the transfer member and the toner attached to the transfer member to the image carrier. The image forming apparatus according to claim 1, wherein the reverse transfer bias is alternately applied.   The control unit executes the cleaning operation before the first recording material in one job enters the transfer nip or after the last recording material in one job passes through the transfer nip. The image forming apparatus according to 1. A toner image forming unit for forming a toner image on the image carrier;
A transfer unit that transfers the toner image formed on the image carrier by the toner image forming unit to a recording material;
The transfer portion is
A nip forming member that is disposed in pressure contact with the image carrier and forms a transfer nip for passing a recording material between the image carrier and the image carrier;
A transfer power supply for supplying a transfer bias for transferring toner on the image carrier to the recording material between the image carrier and the nip forming member when the recording material passes through the transfer nip; ,
When the recording material does not pass through the transfer nip and when the toner band formed on the image carrier by the toner image forming unit passes through the transfer nip, the image carrier and the nip forming member An image forming apparatus including an AC power source for supplying an AC bias between the two.   When the toner band passes through the transfer nip, reverse rotation is applied to supply a reverse transfer bias for transferring the toner on the nip forming member to the image carrier between the image carrier and the nip forming member. 6. The image forming apparatus according to claim 5, further comprising a copying power source.   The image forming apparatus according to claim 6, wherein the transfer power source and the reverse transfer power source are connected to the image carrier side, and the AC power source is connected to the nip forming member side. Further comprising a separating member for separating the recording materials conveyed to the transfer nip one by one,
The image forming apparatus according to claim 5, wherein the toner image forming unit forms the toner band in a predetermined region of the image carrier corresponding to a portion of the recording material that contacts with the separating member.   6. The image forming apparatus according to claim 5, wherein the nip forming member is made of a foam.   6. The image forming apparatus according to claim 5, wherein a grounding member having conductivity and grounded is disposed in contact with the nip forming member.

Images