JP2006030268A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus where the smearing of an image is prevented by avoiding the loop part of paper from coming into contact with the fixing roller of a fixing device. <P>SOLUTION: The image forming apparatus 10 is equipped with a photoreceptor 12 on the outer surface of which a toner image is formed and which is rotary driven, a transfer roller 14 transferring the toner image formed on the photoreceptor 12 to the paper, the fixing device 18 provided on the downstream side of the transfer roller in a paper conveying direction and having a pair of pressure roller 20 and fixing roller 22, and a penetration guide 26 provided between the transfer roller 14 and the fixing device 18 and guiding the paper to a nip part 24 formed by a pair of pressure roller 20 and fixing roller 22. The circumferential speed of the photoreceptor 12 is set to be higher than that of the pressure roller 20 of the fixing device 18, and the penetration guide 26 member moves to a retreat position separated from a nip center line 28 from a guide position crossing with the nip center line 28 after the paper is caught at the nip part 24. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, and a facsimile, and more particularly to an image forming apparatus including a guide member that guides a sheet after transfer to a fixing device.

  Conventionally, in an electrophotographic image forming apparatus, a toner image formed on the surface of a rotating image carrier is transferred onto a sheet by a transfer unit in the process of conveying the sheet, and then the sheet is rotated by a fixing device. When passing through the nip portion of the body pair, the toner image is heated and melted and fixed on the paper.

  In such an image forming apparatus, when the peripheral speed of the rotating body of the fixing device is faster than the peripheral speed of the image carrier, the paper is transferred when the leading edge of the paper during image transfer is caught in the rotating body pair of the fixing device. As a result of being pulled, image displacement occurs at the transfer portion. In order to prevent this image shift, there is an image forming apparatus in which the peripheral speed of the rotating body of the fixing device is set slower than the peripheral speed of the image carrier.

JP-A-11-272020 JP-A-8-190291

  However, in the image forming apparatus in which the peripheral speed of the rotating body of the fixing device is set slower than the peripheral speed of the image carrier, the amount of paper fed from the transfer unit is larger than the amount of paper fed from the fixing device. Loop-like deflection occurs in the sheet on the upstream side in the sheet conveyance direction. When the paper loop amount becomes large, the surface of the paper on which the unfixed toner image is formed contacts the rotating body of the fixing device, and as shown in FIG. There was a problem that sometimes occurred.

In order to solve the above problems, the present invention
An image carrier that can be rotationally driven and has a toner image formed on the outer peripheral surface;
Transfer means for transferring the toner image formed on the image carrier to paper;
A fixing device provided on the downstream side of the transfer unit in the paper transport direction and having a pair of rotating bodies;
An image forming apparatus comprising: a guide member provided between the transfer unit and the fixing device and guiding a sheet to a nip portion formed by the pair of rotating bodies;
The peripheral speed of the image carrier is set to be higher than the peripheral speed of the rotating body of the fixing device, and the guide member is located at a position that intersects the nip center line after the sheet is caught in the nip portion. To the position away from the nip center line.

  In the image forming apparatus having the above-described configuration, the sheet moves along the nip center line by the strength of its own waist by moving the guide member to the retracted position away from the nip center line after the sheet is caught in the nip portion. As a result, the loop portion of the sheet moves in a direction away from one rotating body of the fixing device. As a result, it is possible to avoid the loop portion of the sheet from coming into contact with one rotating body of the fixing device, and to prevent image bleeding.

  In the image forming apparatus of the present invention, the detection unit is disposed between the transfer unit and the fixing device and detects the loop amount of the paper, and the loop amount of the paper is reduced based on the output from the detection unit. And a control means for controlling the rotation speed of the rotating body of the fixing device.

  In this way, if the control of reducing the loop amount of the paper by controlling the rotational speed of the rotating body of the fixing device based on the output of the loop amount detecting means is performed, the paper loop portion is rotated on one side of the fixing device. It is possible to more reliably prevent blurring of an image generated by contact with the body.

  The image forming apparatus according to the present invention may further include a determination unit that determines that the sheet is caught in the nip portion, and a drive unit that moves the guide member based on a determination result by the determination unit. Good. In this case, the driving means may comprise an eccentric cam.

  Furthermore, in the image forming apparatus of the present invention, the timing for moving the guide member may be changeable according to the type of paper.

  As described above, according to the image forming apparatus of the present invention, it is possible to prevent the sheet loop portion from coming into contact with one rotating body of the fixing device, and to prevent image blurring.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 shows a main part of an image forming apparatus 10 according to an embodiment of the present invention. The image forming apparatus 10 includes a drum-shaped photoconductor 12 that is an image carrier. The photoreceptor 12 can be driven to rotate in the direction of arrow A. Around the photosensitive member 12, a charging device, an exposure device, a developing device, and the like (not shown) are arranged, and a toner image is formed on the outer peripheral surface of the photosensitive member 12 by these devices.

  A transfer roller 14 as a transfer unit is disposed in contact with the photoconductor 12, and a contact portion between the photoconductor 12 and the transfer roller 14 is a transfer unit 13. The transfer roller 14 rotates in the arrow B direction as the photoconductor 12 rotates. Further, a sheet conveyed from a sheet feeding cassette (not shown) is guided to a sheet feeding guide 16 from the lower side in the drawing and is passed through the transfer unit 13 between the photosensitive member 12 and the transfer roller 14. At this time, the toner image formed on the photosensitive member 12 is transferred onto the sheet by the electric action of the transfer roller 14.

  The sheet is conveyed from the bottom to the top in the figure, and a fixing device 18 is provided on the downstream side of the transfer roller 14 in the sheet conveying direction. The fixing device 18 includes a pressure roller 20 and a fixing roller 22 that are pressed against each other in parallel. These rollers 20 and 22 constitute a pair of rotating bodies. A nip portion 24 is formed between the pressure roller 20 and the fixing roller 22. The fixing roller 22 includes a heater 25, and the fixing roller 22 is heated to a predetermined fixing temperature (for example, 190 ° C.) by the heater 25. The pressure roller 20 is driven to rotate in the direction of arrow C by a motor (not shown), and the fixing roller 22 is driven to rotate in the direction of arrow D as the pressure roller 20 rotates. Yes. Here, the peripheral speed of the photosensitive member 12 is set to be faster than the peripheral speed of the pressure roller 20.

  A rush guide 26 as a guide member is provided between the transfer roller 14 and the fixing device 18. The rush guide 26 is used to guide the sheet having a toner image formed on the surface thereof after passing through the transfer portion 13 to rush into the nip portion 24 of the fixing device 18.

  The rush guide 26 has a guide position indicated by a solid line in the drawing that intersects a nip center line (vertical line in the present embodiment) 28 that is a straight line passing through the nip portion 24 along the sheet conveyance direction, and a right side from the nip center line 28. It is movable between a retracted position indicated by a dotted line in the figure that is separated downward. For example, a solenoid, a cam, a link mechanism, or the like is used as the driving means for moving the entry guide 26 between the guide position and the retracted position. When the entry guide 26 is moved suddenly, the drive guide 26 is moved along the entry guide 26. Therefore, it is preferable to use an eccentric cam whose driving operation is relatively gradual as the driving unit, because unfixed toner on the moving paper may be scattered to deteriorate the image.

  A conveyance guide 30 is disposed between the entry guide 26 and the transfer roller 14. The conveyance guide 30 is for guiding the sheet that has passed through the transfer unit 13 toward the entry guide 26.

  A separation claw 32 for preventing the paper from being wound around the fixing roller 22 is disposed in contact with the outer peripheral surface of the fixing roller 22 of the fixing device 18. Disposed above the fixing device 18 are discharge guides 34 and 36 for guiding the paper that has passed through the fixing device 18 to a paper discharge port (not shown).

Next, the operation of the image forming apparatus 10 having the above-described configuration will be described.
In the image forming apparatus 10, the photosensitive member 12 is rotationally driven in the direction of arrow A, and accordingly, the transfer roller 14 is rotated in the direction of arrow B. In the fixing device 18, the pressure roller 20 is rotationally driven in the direction of arrow C, and the fixing roller 22 is rotated in the direction of arrow D along with this, and the fixing roller 22 is heated to a predetermined fixing temperature by the heater 25. It is heated.

  In this state, a toner image is formed on the outer peripheral surface of the photoconductor 12, and the toner image is transferred by the transfer roller 13 to the sheet sent from the paper feed cassette by the electric action of the transfer roller 14.

  The sheet on which the toner image has been transferred is conveyed upward while being guided along the conveyance guide 30, and the upper end of the sheet P is guided by the rush guide 26 as shown in FIG. Rush into 24.

  Then, when the toner image is fixed by being heated by the fixing roller 22 when passing through the nip portion 24, the paper P is conveyed while being guided by the paper discharge guides 34 and 36 and is discharged from the image forming apparatus 10 (not shown). It is discharged to the tray.

  In the image forming apparatus 10 of the present embodiment, the peripheral speed of the photoconductor 12 is set faster than the peripheral speeds of the pressure roller 20 and the fixing roller 22 of the fixing device 18. For this reason, since the paper feed amount from the transfer unit 13 is larger than the paper feed amount of the fixing device 18, the paper P is fixed to the fixing roller on the upstream side (that is, the lower side) in the paper transport direction of the fixing device 18 as shown in FIG. The loop portion L is formed to bend toward the 22 side. The loop portion L gradually increases, and in some cases, the surface of the paper P on which the unfixed toner image is formed contacts the fixing roller 22, and as shown in FIG. The image may be blurred.

  The reason why the above-described image blur occurs on both side edges of the paper P is that the fixing roller 22 is generally in an inverted crown shape to prevent the occurrence of paper wrinkles in the nip portion 24. Since the nip portion 24 is designed to apply a force that pulls the paper P toward both ends of the roller, the pulling force causes the paper P to have a central portion as a ridge line in the width direction orthogonal to the paper transport direction. This is considered to be because the sheet P is curved so as to be concave toward the fixing roller 22, so that both side edges in the width direction of the paper P are particularly easy to contact the fixing roller 22.

  Further, as a result of detailed examination by the inventors of the present application, the paper P that has entered the nip portion 24 is in a state of being pushed toward the fixing roller 22 by the rush guide 26 disposed in the vicinity of the nip portion 24. It has been found that the loop portion L is easily formed on the paper P.

  Therefore, in the image forming apparatus 10 of the present embodiment, as shown in FIG. 4, after the sheet P is bitten in the nip portion 24 of the fixing device 18, the rush guide 26 is moved from the guide position that intersects the nip center line 28. Then, it is moved from the nip center line 28 to the retreat position away from the pressure roller 20 side. In this way, after the paper P is caught in the nip portion 24, the entry guide 26 moves to the retracted position away from the nip center line 28, so that the paper P moves to the nip center line 28 by the strength of its own waist. Accordingly, the loop portion L of the paper P moves in a direction away from the fixing roller 22 of the fixing device 18. As a result, the loop portion L of the paper P can be prevented from coming into contact with the fixing roller 22, and the occurrence of image blur can be prevented.

  Here, the timing at which the entry guide 26 is retracted is determined based on a determination result of a determination unit that determines that the sheet P is caught in the nip portion 24. This determination means includes, for example, a sensor that also functions as a jam detection function provided in the paper conveyance path and a timer that starts counting upon receiving an output from the sensor, and the count value reaches a preset value. It is determined that the paper P has been caught in the nip portion 24. Based on the determination result of the determination means, the entry guide 26 may be retracted immediately, or the entry guide 26 may be retracted after a predetermined time has elapsed after the sheet P is caught in the nip portion 24. Good.

  Further, the timing at which the entry guide 26 is retracted may be changed according to the type of the paper P. For example, the strength of the paper P differs depending on whether the paper P is thin or thick, how the loop portion L is formed, and the return force of the paper P when the entry guide 26 is retracted (the center of the nip). Therefore, it is preferable to retract the entry guide 26 at a faster timing as the paper P is thinner.

Next, an image forming apparatus 11 according to another embodiment will be described with reference to FIG.
The image forming apparatus 11 of the present embodiment is different from the image forming apparatus 10 in that a loop amount sensor (detecting means) 40 is disposed between the transfer roller 14 and the fixing device 18 and detects the loop amount of the paper P. And a control unit (control means) 42 for controlling the rotation speed of the pressure roller 20 of the fixing device 18 so as to reduce the loop amount of the paper P based on the output from the loop amount sensor 40. Other configurations are the same as those of the image forming apparatus 10.

  Specifically, in the image forming apparatus 11, when the loop amount sensor 40 detects that the loop amount of the paper P is greater than a predetermined value, the control unit 42 determines the number of rotations of the motor 44 that drives the pressure roller 20. The rotational speed of the pressure roller 20 is increased from the normal speed for a predetermined time. As a result, the sheet feeding amount of the fixing device 18 is increased, so that the loop portion L of the sheet P is reduced. As described above, the control for reducing the loop portion L of the paper P is performed in conjunction with the retreating movement of the entry guide 26, so that the image bleeding generated when the toner image surface of the paper P contacts the fixing roller 22 can be more reliably performed. Can be prevented.

  In the fixing device 18 of the image forming apparatus described above, the pressure roller 20 and the fixing roller 22 constitute a pair of rotating bodies. However, the present invention is not limited to this, and the pressure roller and an endless fixing belt are used. A pair of rotating bodies may be configured.

  In the image forming apparatus, the case where the nip center line 28 is a vertical line has been described. However, the present invention can be applied even if the nip center line is a straight line in various directions. It may be a horizontal line.

  Furthermore, the present invention is applicable to both monochrome and full-color image forming apparatuses.

1 is a main part configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 4 is a diagram illustrating a state where a sheet has entered a nip portion of a fixing device. FIG. 6 is a diagram illustrating a state in which a loop is formed on a sheet below the fixing device. The figure which shows the state which retreated the rush guide. FIG. 5 is a configuration diagram of a main part of an image forming apparatus according to another embodiment of the present invention. The figure which shows the state which the image blurring generate | occur | produced in the both-sides edge part of the paper.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 11 ... Image forming apparatus 12 ... Photoconductor (image carrier)
14: Transfer roller (transfer means)
18: Fixing device 20: Pressure roller (rotating body)
22: Fixing roller (rotating body)
24 ... Nip part 25 ... Heater 26 ... Entry guide (guide member)
28 ... Nip center line 40 ... Nip amount sensor (detection means)
42... Control unit (control means)

Claims (5)

An image carrier that can be rotationally driven and has a toner image formed on the outer peripheral surface;
Transfer means for transferring the toner image formed on the image carrier to paper;
A fixing device provided on the downstream side of the transfer unit in the paper transport direction and having a pair of rotating bodies;
An image forming apparatus comprising: a guide member provided between the transfer unit and the fixing device and guiding a sheet to a nip portion formed by the pair of rotating bodies;
The peripheral speed of the image carrier is set to be higher than the peripheral speed of the rotating body of the fixing device, and the guide member intersects the nip center line after the sheet is caught in the nip portion. The image forming apparatus is moved to a position away from the nip center line.   A detecting unit that is disposed between the transfer unit and the fixing device and detects a loop amount of the paper; and a rotating body of the fixing device so as to reduce the loop amount of the paper based on an output from the detecting unit. The image forming apparatus according to claim 1, further comprising a control unit that controls a rotation speed.   3. The apparatus according to claim 1, further comprising: a determination unit that determines that a sheet is caught in the nip portion; and a drive unit that moves the guide member based on a determination result by the determination unit. The image forming apparatus described.   The image forming apparatus according to claim 3, wherein the driving unit includes an eccentric cam.   The image forming apparatus according to claim 1, wherein the timing for moving the guide member can be changed according to the type of paper.

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