JP4354222B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus capable of transferring a toner image onto both sides of a recording medium with a single sheet passing.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, image forming apparatuses such as copying machines, printers, facsimiles, and the like that can form a toner image on both sides of a recording medium have a toner image on one side of the recording medium by conveying the recording medium. A method of transferring a toner image to the other surface of the recording medium by transferring the image, reversing the recording medium and conveying it again is widely spread.
[0003]
There is also known an image forming apparatus that can transfer a toner image onto both sides of a recording medium by passing the recording medium once (see, for example, Patent Documents 1 and 2).
[0004]
[Patent Document 1]
Japanese Patent No. 2906538
[Patent Document 2]
JP 2000-105513 A
[0005]
[Problems to be solved by the invention]
In the image forming apparatus described in Patent Document 1, the polarity of one of the two toner images formed on the photoconductor (the front and back surfaces of the recording medium) is switched. To switch the polarity of the toner image, polarity switching means such as a reverse polarity corona charging means must be provided, and the structure is complicated.
[0006]
In the image forming apparatus described in Patent Document 2, two types of toners having different polarities are prepared, a toner image on one side is formed with one toner, and a toner image on the other side is formed with a polarity. The other toner is different. For this reason, two developing devices for storing two types of toners having different polarities are necessary, and the structure is complicated.
[0007]
An object of the present invention is to realize an image forming apparatus capable of transferring a toner image onto both sides of a recording medium by passing the recording medium once, and to improve the quality of the formed image. It is.
[0008]
[Means for Solving the Problems]
  The image forming apparatus according to claim 1 is a first image carrier that moves while carrying a toner image on an image carrying surface; a second image carrier that carries and moves a toner image on an image carrying surface; A contact area where the image bearing surface of the first image carrier and the image bearing surface of the second image carrier are brought into contact with each other, and a recording medium on which a toner image is transferred is conveyed between both surfaces; Two conductive elements disposed on the upstream side and the downstream side in the moving direction of the first image carrier in contact with the back surface of the image carrier surface of the first image carrier in the contact area. A roller and two rollers disposed on the upstream side and the downstream side in the moving direction of the second image carrier in contact with the back surface of the image carrier surface of the second image carrier in the contact area. Two conductive rollers and a toner image formed on the recording medium provided downstream of the contact area in the recording medium conveyance direction. And a recording medium conveying means provided between the contact area and the fixing means for holding the recording medium and conveying the recording medium toward the fixing means. The two conductive rollers on the second image carrier side include an upstream nip portion and a downstream nip portion that sandwich the first image carrier and the second image carrier. And a pair of the conductive rollers forming the upstream nip portion and a pair of the conductive rollers forming the downstream nip portion, one of the conductive rollers is applied with a transfer current. And the other conductive roller is an earth roller, and the transfer current application off timing to the two application rollers on the upstream side and the downstream side is as follows:Before the recording medium enters the fixing means;The recording medium is held by the recording medium conveying means, and the rear end of the recording medium passes through the downstream nip portion.
[0009]
Accordingly, the first image carrier having the toner image carried on the image carrying surface and the second image carrier having the toner image carried on the image carrying surface move, and the two image carrying surfaces carrying the toner image are in contact with each other. The recording medium is conveyed between them at the timing of facing each other in the contact area, and a transfer current is applied to the application roller that forms the upstream nip portion and the application roller that forms the downstream nip portion. The toner image on the first image carrier and the toner image on the second image carrier are transferred to both sides of the recording medium while the paper is passed through the contact area once.
[0010]
  This double-sided transfer mechanism has a contact area where the image bearing surfaces of the first image carrier and the second image carrier are brought into contact with each other, and the first image carrier side and the second image carrier side in the contact area. Two conductive rollers are disposed on the upstream side and the upstream side and the downstream side to sandwich the first image carrier and the second image carrier to form an upstream nip portion and a downstream nip portion. In each of the pair of conductive rollers forming the side nip portion and the pair of conductive rollers forming the downstream nip portion, one of the conductive rollers is used as an application roller to which a transfer current is applied. It is a ground roller and has a simple structure.
  When transferring to both sides of the recording medium, the leading end of the recording medium is fixed after the trailing end of the recording medium has passed through the downstream nip.Means sideIf application of the transfer current is turned off before reaching the nip portion of the image, there is no electrostatic adhesion between the image carrier and the recording medium, and fixingmeansTransport of the recording medium to the head becomes unstable, causing jams and the like.
  On the other hand, the application current off timing of the transfer current to the two application rollers on the upstream side and the downstream side isBefore the recording medium enters the fixing means,Since the recording medium is held by the recording medium conveying means and the rear end of the recording medium has passed through the downstream nip portion, the application of the transfer current is turned off, and the static force between the image carrier and the recording medium is obtained. Even after the electrical contact is lost, the recording medium is stably conveyed to the fixing unit by the recording medium conveying unit.
  In addition, when the recording medium is transferred onto both sides, the recording medium is applied to the upstream application roller after the trailing edge of the recording medium has passed through the upstream nip portion and before the trailing edge of the recording medium has passed the downstream nip portion. When application of the transfer current is turned off, the behavior of the transfer current flowing from the downstream application roller changes at the moment when the transfer current is turned off, and the load characteristics change, and as a result, the transfer current applied to the downstream application roller changes. The density change occurs in the toner image transferred to the recording medium by the action.
  On the other hand, the toner image on both sides of the recording medium can be obtained by setting the transfer current application off timing to the two application rollers on the upstream side and the downstream side after the trailing edge of the recording medium passes the downstream nip portion. Transfer to the downstream application roller that occurs when the transfer current application to the upstream application roller is turned off while the toner image is being transferred downstream. There is no change in the behavior of the applied transfer current, the change in the density of the toner image transferred to the recording medium caused by the change in behavior is prevented, and the quality of the image formed on the recording medium is improved.
[0011]
According to a second aspect of the present invention, in the image forming apparatus of the first aspect,
The on-timing of the transfer current applied to the two application rollers on the upstream side and the downstream side is before or before the leading edge of the recording medium reaches the upstream nip portion.
When transferring the recording medium to both sides, when the leading edge of the recording medium is conveyed to the position of the upstream nip portion, the transfer current application to the upstream application roller is turned on to start the transfer of one toner image. After that, when the leading edge of the recording medium is conveyed to the position of the downstream nip portion, when the transfer current application to the downstream application roller is turned on and the transfer of the other toner image is started, the transfer to the downstream application roller is started. At the moment when the transfer current application is turned on, the behavior of the transfer current flowing from the upstream application roller changes to change the load characteristics. As a result, the recording medium is operated by the transfer current applied to the upstream application roller. A change in density occurs in the toner image transferred to the toner image.
[0012]
On the other hand, by setting the transfer current application on timing to the two application rollers on the upstream side and the downstream side when the leading edge of the recording medium reaches the upstream nip portion or earlier, the recording current is applied to both sides of the recording medium. When the toner image is transferred reliably and the transfer current is applied to the application roller on the downstream side at a timing after the transfer current is applied to the application roller on the upstream side and the transfer on the upstream side is started. There is no change in the behavior of the transfer current applied to the applied roller on the upstream side, and the change in the density of the toner image transferred to the recording medium caused by this behavior change is prevented and formed on the recording medium. This improves the quality of images.
[0013]
Claim3The described invention is claimed.2In the image forming apparatus described above, the application current on-timing of the transfer current to the two application rollers on the upstream side and the downstream side is the same.
[0014]
Therefore, it is not necessary to adjust the timing for applying the transfer current to the two application rollers, and the mechanism for applying the transfer current is simplified.
[0020]
The invention according to claim 4 is the claim of claim1, 2 orIn the image forming apparatus according to 3, the transfer current application off timing to the two application rollers on the upstream side and the downstream side is the same.
[0021]
Therefore, it is not necessary to adjust the timing for applying and applying the transfer current to the two application rollers, and the mechanism for applying the transfer current is simplified.
[0022]
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, a single power source for applying a transfer current to the two application rollers on the upstream side and the downstream side is provided. .
[0023]
Therefore, since there is one power source, the cost can be reduced.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
The present inventionThe fruitThe embodiment will be described with reference to FIGS. FIG. 1 is a sectional view schematically showing a color printer as an image forming apparatus.
[0030]
The color printer 100 is a so-called tandem type color printer in which four photosensitive drums 1 are arranged in parallel in the approximate center of the apparatus. As shown in FIG. 2, a cleaning device 2, a charge removal device 3, a charging device 4, and a developing device 5 are disposed around each photosensitive drum 1 to constitute an image forming unit 6. The configuration of each image forming unit 6 is the same, and only the color of the toner stored in the developing device 5 is different. Below the four image forming units 6, a first intermediate transfer belt 7 as a first image carrier is provided, and the four photosensitive drums 1 are arranged in contact along the upper side of the first intermediate transfer belt 7. Has been. An exposure device 8 is disposed above the image forming unit 6.
[0031]
The developing device 5 in each image forming unit 6 stores toners of cyan, magenta, yellow, and black, respectively, and applies toner of each color to the electrostatic latent image formed on the photosensitive drum 1.
[0032]
Between the charging device 4 and the developing device 5 is a writing position, and the photosensitive drum 1 is irradiated with the laser light L emitted from the exposure device 8. The exposure device 8 is a known laser system, and in this embodiment, the optical information corresponding to the color of the color-separated and developed toner is applied to the surface of the uniformly charged photoreceptor drum 1 as a latent image. Irradiate as Instead of the exposure device 8, an exposure device comprising an LED array and an imaging means can be used. A transfer roller 9 and a backing roller 10 are disposed so as to face the photosensitive drum 1 with the first intermediate transfer belt 7 interposed therebetween. The toner image formed on the photosensitive drum 1 is transferred to the first intermediate transfer belt 7 by the action of the transfer roller 9.
[0033]
Cyan, magenta, yellow, and black toner images formed on the photosensitive drum 1 of each image forming unit 6 are sequentially transferred onto the first intermediate transfer belt 7 and transferred onto the first intermediate transfer belt 7. An image is formed. When a monochrome image is formed, the toner image is formed only by the image forming unit 6 that handles black toner, and the monochrome image is transferred onto the first intermediate transfer belt 7.
[0034]
The first intermediate transfer belt 7 is stretched and supported by four rotating rollers 11, 12, 13, and 14. The outer peripheral surface is an image carrying surface 7a carrying a toner image, and moves in a counterclockwise direction as shown by an arrow in the figure. It is possible. A backing roller 15 is provided on the left side of the rotating roller 14 in the belt loop which is the back surface of the image carrying surface 7 a of the first intermediate transfer belt 7. A belt cleaning device 16 is disposed outside the belt loop so as to face the backing roller 15.
[0035]
A two-stage sheet cassette 17 is provided at a lower position of the color printer 100. A recording medium S is stored in each paper feed cassette 17, and the recording medium S is fed one by one by a paper feed roller 18 in order from the highest one, and is sent toward a registration roller 19.
[0036]
A second intermediate transfer belt 20 as a second image carrier is disposed on the right side of the first intermediate transfer belt 7. The second intermediate transfer belt 20 is stretched and supported on the rotating rollers 21, 22, 23, 24, and 25 so as to be movable in the clockwise direction as shown in the figure. The outer surface of the second intermediate transfer belt 20 is an image carrying surface 20a that carries a toner image.
[0037]
The first intermediate transfer belt 7 and the second intermediate transfer belt 20 have a contact area 26 where a part of the image carrying surface 7a abuts a part of the image carrying surface 20a. In the contact area 26, the rotation rollers 13 and 14 are in contact with the back surface of the first intermediate transfer belt 7, and the rotation rollers 24 and 25 are in contact with the back surface of the second intermediate transfer belt 20. These rotating rollers 13, 14, 24, 25 are conductive rollers, and the rotating rollers (hereinafter also referred to as conductive rollers if necessary) 13, 14 are along the moving direction of the first intermediate transfer belt 7. Arranged on the upstream side and the downstream side, rotating rollers (hereinafter also referred to as conductive rollers if necessary) 24 and 25 are arranged on the upstream side and the downstream side along the moving direction of the second intermediate transfer belt 20. ing. The upstream conductive rollers 13 and 24 sandwich the first intermediate transfer belt 7 and the second intermediate transfer belt 20 to form the upstream nip portion 27, and the downstream conductive rollers 14 and 25 are the first intermediate rollers. The downstream nip portion 28 is formed by sandwiching the transfer belt 7 and the second intermediate transfer belt 20. The conductive roller 13 and the conductive roller 14, and the conductive roller 24 and the conductive roller 25 preferably have an outer peripheral surface interval of 5 mm to 200 mm, more preferably 10 mm to 100 mm.
[0038]
The abutting area 26 is an area where the recording medium S is fed by the registration roller 19, and the image carrying surface 7 a of the first intermediate transfer belt 7 carrying the toner image and the second intermediate transfer belt carrying the toner image. When the recording medium S is sent to the contact area at the timing when the 20 image bearing surfaces 20a face each other in the contact area 26, the toner images are transferred to both surfaces of the recording medium S. In the contact area 26, the image carrying surface 7 a and the image carrying surface 20 a can be separated as necessary, and the second intermediate transfer belt 20 is swung around the axis of the rotating roller 21 as a rotation center. To be separated.
[0039]
A belt cleaning device 29 is disposed at a lower position outside the loop of the second intermediate transfer belt 20. The belt cleaning device 29 includes a cleaning blade inside, and wipes off unnecessary toner and paper powder remaining on the surface of the second intermediate transfer belt 20.
[0040]
A fixing device 30 is provided above the second intermediate transfer belt 20. The fixing device 30 includes a fixing roller and a pressure roller each heated by a heating element. The recording medium S after fixing is discharged by the discharge roller 31 and stacked on the discharge tray 32.
[0041]
FIG. 3 is an enlarged front view showing the structure around the contact area 26. Of the pair of conductive rollers 13 and 24 forming the upstream nip 27, one conductive roller 13 is an application roller to which a transfer current is applied, and the other conductive roller 24 is a grounded ground roller. Has been. In the pair of conductive rollers 14 and 25 forming the downstream nip portion 28, one conductive roller 25 is an application roller to which a transfer current is applied, and the other conductive roller 15 is a ground roller. A power source 33 for applying a transfer current is connected to the application rollers 13 and 25.
[0042]
On the entrance side of the fixing device 30 along the conveyance direction of the recording medium S that has passed through the contact area 26, a spur 34 as a nip portion on the fixing device 30 side that holds the recording medium S that has passed through the contact area 26. Is provided.
[0043]
Here, the color printer 100 is provided with a control unit (not shown) that controls each unit according to a program stored in advance according to detection results of various sensors, input data, and the like. The on / off control and the on / off control of the registration roller 19 are performed by control of a control unit based on a detection result from a sensor (not shown) that detects conveyance of the recording medium S.
[0044]
The operation of double-sided printing in which the color toner image is transferred onto both sides of the recording medium S in one pass through the color printer 100 configured as described above will be described.
[0045]
First, each color toner image created by each image forming unit 6 is sequentially transferred from the photosensitive drum 1 to the first intermediate transfer belt 7 by the action of the transfer roller 9, and the color toner image is formed on the first intermediate transfer belt 7. It is formed. It is assumed that the toner forming this color toner image is negatively charged. The color toner image formed on the first intermediate transfer belt 7 is subjected to a second intermediate transfer by applying a negative transfer current from the power source 33 to the application roller 13 at a timing when the color toner image passes through the contact area 26. Transferred to the belt 20. When the color toner image transferred to the second intermediate transfer belt 20 is transferred to the recording medium S, it becomes a first side image (back side image). The color toner image (first surface image) transferred to the second intermediate transfer belt 20 moves as the second intermediate transfer belt 20 rotates.
[0046]
When the transfer of the first surface image from the first intermediate transfer belt 7 to the second intermediate transfer belt 20 is performed, the image forming unit 6 generates the second surface image (image transferred to the front side of the recording medium S). The toner image on the photosensitive drum 1 of each image forming unit 6 is sequentially transferred onto the first intermediate transfer belt 7 by the action of the transfer roller 9, and the color toner image ( A second surface image) is formed. The color toner image (first surface image) on the second intermediate transfer belt 20 and the color toner image (second surface image) on the first intermediate transfer belt 7 are the first intermediate transfer belt 7 and the second intermediate transfer belt. 20 is carried toward the abutment region 26 as it rotates.
[0047]
At the timing when the color toner image (first surface image) on the second intermediate transfer belt 20 and the color toner image (second surface image) on the first intermediate transfer belt 7 are conveyed to the contact area 26, the registration roller 19. The recording medium S sent out by this enters the contact area 26. Then, the color toner image (first surface image) on the second intermediate transfer belt 20 is transferred to one surface of the recording medium S fed into the contact area 26, and the first intermediate is transferred to the other surface of the recording medium S. The color toner image (second surface image) on the transfer belt 7 is transferred.
[0048]
Here, the transfer process of the first surface image and the second surface image onto both surfaces of the recording medium S will be described in detail with reference to the timing chart of FIG.
[0049]
At the timing when the color toner image (first surface image) on the second intermediate transfer belt 20 and the color toner image (second surface image) on the first intermediate transfer belt 7 are conveyed to the contact area 26, the registration roller 19. When the leading edge of the recording medium S sent out reaches the upstream nip portion 27 in the contact area 26 (transfer IN), the power supply 33 is turned on at that timing, and the upstream application roller 13 and the downstream application roller 25. In addition, a transfer current of −30 μA is applied simultaneously. The transfer OUT at the leading end of the recording medium S is a timing at which the leading end image of the recording medium S passes through the downstream nip portion 28.
[0050]
By applying a transfer current to the application roller 13 on the upstream side, a color toner image (second surface image) on the first intermediate transfer belt 7 by a repulsive action between the negatively charged toner and the negative charge of the transfer current. ) Is transferred to one surface of the recording medium S. At this time, the transfer current applied to the application roller 13 flows to the first and second intermediate transfer belts 7 and 20, and most of the current flows to the ground roller 24. In addition, when a transfer current is applied to the application roller 25 on the downstream side, a color toner image (first image) on the second intermediate transfer belt 20 is generated by the repulsive action between the negatively charged toner and the negative charge of the transfer current. Surface image) is transferred to the other surface of the recording medium S. At this time, the transfer current applied to the application roller 25 flows to the second and first intermediate transfer belts 20 and 7, and most of the current flows to the ground roller 14.
[0051]
Thereafter, when the rear end of the recording medium S passes through the downstream nip portion 28 (transfer OUT), the power source 33 is turned off at that timing, and the application of the transfer current to the application rollers 13 and 25 is simultaneously completed. Note that the front end of the recording medium S is held by the spur 34 before the rear end of the recording medium S passes the downstream nip portion 28 (spur IN). Therefore, even after the application of the transfer current is turned off and the electrostatic contact between the second intermediate transfer belt 20 and the recording medium S is lost, the conveyance of the recording medium S to the fixing device 30 is performed by the operation of the spur 34. Is performed stably.
[0052]
Reference configuration example 1Will be described with reference to FIGS. The same parts as those described in FIGS. 1 to 4 are denoted by the same reference numerals, and description thereof is also omitted (the following description is omitted).Reference configuration exampleBut the same).
[0053]
BookReference configuration exampleThe external configuration differs from the first embodiment in that the spur 34 is not provided. FIG. 6 shows a case where an image is formed on a small size recording medium S, and FIG. 7 shows a case where an image is formed on a large size recording medium S.
[0054]
The transfer process of the first surface image and the second surface image onto both sides of the small-sized recording medium S will be described in detail with reference to the timing chart of FIG.
[0055]
At the timing when the color toner image (first surface image) on the second intermediate transfer belt 20 and the color toner image (second surface image) on the first intermediate transfer belt 7 are conveyed to the contact area 26, the registration roller 19. When the leading edge of the recording medium S sent out reaches the upstream nip portion 27 in the contact area 26 (transfer IN), the power supply 33 is turned on at that timing, and the upstream application roller 13 and the downstream application roller 25. In addition, a transfer current of −30 μA is applied simultaneously.
[0056]
By applying a transfer current to the upstream application roller 13, a color toner image (second surface image) on the first intermediate transfer belt 7 is recorded as in the case described in the first embodiment. Transferred to one side of the medium S. Further, by applying a transfer current to the application roller 25 on the downstream side, the color toner image (first surface image) on the second intermediate transfer belt 20 is the same as in the case described in the first embodiment. Is transferred to the other surface of the recording medium S.
[0057]
Thereafter, even when the rear end of the recording medium S passes through the downstream nip portion 28 (transfer OUT), application of the transfer current to the application rollers 13 and 25 is maintained. When the leading edge of the recording medium S reaches the nip portion 30a on the fixing device 30 side (fixing IN), the power source 33 is turned off at the arrival timing, and the application of the transfer current to the application rollers 13 and 25 is completed simultaneously. To do. For this reason, even after the trailing edge of the recording medium S has passed through the downstream nip portion 28, until the leading edge of the recording medium S reaches the nip portion 30a on the fixing device 30 side, the application rollers 13 and 25 are not affected. By maintaining the application of the transfer current on, the recording medium S whose rear end has passed through the downstream nip portion 28 is maintained in an electrostatic contact state with the second intermediate transfer belt 20, and the recording medium is maintained. S is stably conveyed to the fixing device 30.
[0058]
The transfer process of the first side image and the second side image onto both sides of the large size recording medium S will be described in detail with reference to the timing chart of FIG.
[0059]
At the timing when the color toner image (first surface image) on the second intermediate transfer belt 20 and the color toner image (second surface image) on the first intermediate transfer belt 7 are conveyed to the contact area 26, the registration roller 19. When the leading edge of the recording medium S sent out reaches the upstream nip portion 27 in the contact area 26 (transfer IN), the power supply 33 is turned on at that timing, and the upstream application roller 13 and the downstream application roller 25. In addition, a transfer current of −30 μA is applied simultaneously.
[0060]
By applying a transfer current to the upstream application roller 13, a color toner image (second surface image) on the first intermediate transfer belt 7 is recorded as in the case described in the first embodiment. Transferred to one side of the medium S. Further, by applying a transfer current to the application roller 25 on the downstream side, the color toner image (first surface image) on the second intermediate transfer belt 20 is the same as in the case described in the first embodiment. Is transferred to the other surface of the recording medium S.
[0061]
Thereafter, even if the leading end of the recording medium S reaches the nip portion 30a on the fixing device 30 side (fixing IN), the transfer to the application rollers 13 and 25 is performed until the trailing end of the recording medium S passes the downstream nip portion 28. At the timing when the current application state is maintained and the trailing edge of the recording medium S passes through the downstream nip portion 28 (transfer OUT), the application of the transfer current to the application rollers 13 and 25 ends simultaneously. Therefore, the application of the transfer current to the application rollers 13 and 25 is not turned off during the transfer of the toner image on the downstream nip portion 28 side, and the toner image on both surfaces of the recording medium S is not turned off. Transfer is performed reliably.
[0062]
Reference configuration example 2Will be described with reference to FIGS. BookReference configuration exampleThen, of the pair of conductive rollers 13 and 24 forming the upstream nip 27, the conductive roller 24 on the second intermediate transfer belt 20 side is an application roller to which a transfer current is applied, and the other conductive roller. Reference numeral 13 denotes a grounded ground roller. In the pair of conductive rollers 14 and 25 forming the downstream nip portion 28, one conductive roller 25 is an application roller to which a transfer current is applied, and the other conductive roller 15 is a ground roller. A power supply 35 that applies a positive transfer current is connected to the upstream application roller 24, and a power supply 36 that applies a negative transfer current is connected to the downstream application roller 25.
[0063]
The transfer process of the first side image and the second side image onto both sides of the recording medium S will be described in detail with reference to the timing chart of FIG.
[0064]
At the timing when the color toner image (first surface image) on the second intermediate transfer belt 20 and the color toner image (second surface image) on the first intermediate transfer belt 7 are conveyed to the contact area 26, the registration roller 19. When the leading edge of the recording medium S sent out reaches the upstream nip portion 27 in the contact area 26 (transfer IN), the power sources 35 and 36 are turned on at that timing, and +50 μA transfer is applied to the upstream application roller 24. A current is applied, and a transfer current of −30 μA is applied to the application roller 25 on the downstream side.
[0065]
By applying a positive transfer current to the application roller 24 on the upstream side, a color toner image (second image) on the first intermediate transfer belt 7 is attracted by the negatively charged toner and the positive charge of the transfer current. Surface image) is transferred to one surface of the recording medium S. At this time, the transfer current applied to the application roller 24 flows to the second and first intermediate transfer belts 20 and 7, and most of the current flows to the ground roller 13. In addition, when a negative transfer current is applied to the application roller 25 on the downstream side, a color toner image (second toner image) on the second intermediate transfer belt 20 is generated by the repulsive action between the negatively charged toner and the negative charge of the transfer current. The first surface image) is transferred to the other surface of the recording medium S. At this time, the transfer current applied to the application roller 25 flows to the second and first intermediate transfer belts 20 and 7, and most of the current flows to the ground roller 14.
[0066]
Thereafter, when the trailing edge of the recording medium S passes through the downstream nip portion 28 (transfer OUT), the power source 35 is turned off even before the leading edge of the recording medium S reaches the nip portion 30a on the fixing device 30 side. Then, the application of the transfer current to the application roller 24 is completed. Thereafter, when the leading edge of the recording medium S reaches the nip portion 30a of the fixing device 30 (fixing IN), the power source 36 is turned off, and the application of the transfer current to the application roller 25 is completed.
[0067]
For this reason, even after the trailing edge of the recording medium S has passed through the downstream nip portion 28, the transfer current to the application roller 25 remains until the leading edge of the recording medium S reaches the nip portion 30a on the fixing device 30 side. Is maintained on, the recording medium S whose rear end has passed through the downstream nip portion 28 is maintained in an electrostatic contact state with the second intermediate transfer belt 20, and the recording medium S It is stably conveyed to the fixing device 30. Further, the power source 35 is turned off after the rear end of the recording medium S passes through the downstream nip portion 28 and until the front end of the recording medium S enters the nip portion 30a on the fixing device 30 side. Since only 36 is ON, the power consumption is reduced, and the application time of the transfer current to the application roller 25 is shortened, so that the deterioration of the parts due to the application of the transfer current is prevented, and the application roller 25 and other parts. Improves durability.
[0068]
【The invention's effect】
  According to the image forming apparatus of the first aspect of the invention, the transfer current application off timing to the two application rollers on the upstream side and the downstream side is:Before the recording medium enters the fixing means,After the recording medium is held by the recording medium conveying means and the rear end of the recording medium passes through the downstream nip portion, the application of the transfer current is turned off and the electrostatic force between the image carrier and the recording medium is The recording medium can be stably conveyed to the fixing means by the recording medium conveying means even after the close contact is lost. In addition, when transferring to both sides of the recording medium, the transfer current application off timing to the two application rollers on the upstream side and the downstream side is after the trailing end of the recording medium has passed the downstream nip part, There is no change in the behavior of the transfer current applied to the downstream application roller that occurs when the transfer current application to the upstream application roller is turned off during the transfer of the toner image on the downstream side. The change in the density of the toner image transferred to the recording medium caused by this behavior change can be prevented, and the quality of the image formed on the recording medium can be improved.
  According to the second aspect of the present invention, when the recording medium is transferred to both sides, the leading end of the recording medium reaches the upstream nip portion when the transfer current is applied to the two application rollers on the upstream side and the downstream side. This occurs when the transfer current is applied to the downstream application roller at a timing after the transfer current is applied to the upstream application roller and the upstream transfer is started. There is no change in the behavior of the transfer current applied to the upstream application roller, and the change in the density of the toner image transferred to the recording medium caused by this change in behavior can be prevented and formed on the recording medium. It is possible to improve the quality of the image to be displayed.
[0069]
Claim3According to the described invention, the claims2In the image forming apparatus described above, since the transfer current application ON timing to the two application rollers on the upstream side and the downstream side is the same, there is no need to adjust the timing for applying the transfer current to the two application rollers. The mechanism for applying the transfer current can be simplified.
[0071]
According to the invention of claim 4, the claim of claim1, 2 orIn the image forming apparatus described in item 3, since the application current application OFF timing for the two application rollers on the upstream side and the downstream side is the same, it is necessary to adjust the timing for applying the transfer current to the two application rollers. In addition, the mechanism for applying the transfer current can be simplified.
[0072]
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, a single power source for applying a transfer current to the two application rollers on the upstream side and the downstream side is provided. Therefore, the cost can be reduced because there is only one power source.
[Brief description of the drawings]
FIG. 1 shows the present invention.The fruit1 is a cross-sectional view illustrating an outline of a color printer according to an embodiment.
FIG. 2 is an enlarged front view showing a configuration of an image forming unit.
FIG. 3 is an enlarged front view showing a structure around a contact area.
FIG. 4 is a timing chart for explaining a transfer process of a first surface image and a second surface image onto both sides of a recording medium.
[Figure 5]Reference configuration example 1Enlarge the area around the contact area in color printers
FIG.
FIG. 6 is a timing chart for explaining a transfer process of a first surface image and a second surface image onto both surfaces of a small size recording medium S;
FIG. 7 is a timing chart for explaining a transfer process of a first side image and a second side image on both sides of a large size recording medium S;
[Fig. 8]Reference configuration example 2Enlarge the area around the contact area in color printers
FIG.
FIG. 9 is a timing chart for explaining a transfer process of the first surface image and the second surface image onto both surfaces of the recording medium.

Claims (5)

A first image carrier that carries and moves a toner image on an image carrying surface;
A second image carrier that carries and moves a toner image on an image carrying surface;
A contact area where the image bearing surface of the first image carrier and the image bearing surface of the second image carrier are brought into contact with each other, and a recording medium on which a toner image is transferred is conveyed between both surfaces;
Two conductive elements disposed on the upstream side and the downstream side in the moving direction of the first image carrier in contact with the back surface of the image carrier surface of the first image carrier in the contact area. Laura,
Two conductive elements disposed on the upstream side and the downstream side in the moving direction of the second image carrier in contact with the back surface of the image carrier surface of the second image carrier in the contact area. Laura,
A fixing unit provided on the downstream side in the recording medium conveyance direction from the contact area and fixing the toner image on the recording medium;
A recording medium conveying unit provided between the contact area and the fixing unit and holding the recording medium and conveying the recording medium toward the fixing unit;
The two conductive rollers on the first image carrier side and the two conductive rollers on the second image carrier side sandwich the first image carrier and the second image carrier and upstream Forming a nip and a downstream nip,
In each of the pair of conductive rollers forming the upstream nip portion and the pair of conductive rollers forming the downstream nip portion, one of the conductive rollers is an application roller to which a transfer current is applied. And the other conductive roller is an earth roller,
The transfer current application off timing to the two application rollers on the upstream side and the downstream side is before the recording medium enters the fixing unit, the recording medium is held by the recording medium conveying unit, and An image forming apparatus, wherein the rear end of the recording medium passes through the downstream nip portion. The image forming apparatus according to claim 1.
2. An image forming apparatus according to claim 1, wherein the on-timing of the transfer current applied to the two application rollers on the upstream side and the downstream side is before or before the leading edge of the recording medium reaches the upstream nip portion. The image forming apparatus according to claim 2.
Images forming device you wherein the application on the timing of the transfer current for two of the application roller between the upstream and downstream sides are simultaneous. The image forming apparatus according to claim 1, 2 or 3.
2. An image forming apparatus according to claim 1, wherein the application current transfer-off timings for the two application rollers on the upstream side and the downstream side are the same. The image forming apparatus according to claim 1, 2, 3, or 4.
An image forming apparatus, comprising: one power source for applying a transfer current to the two application rollers on the upstream side and the downstream side.

Images