JP2010026351A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which prevents image defects due to discharge occurring in the vicinity of a transfer nip part and prevents also image defects such as color misregistration. <P>SOLUTION: The image forming apparatus includes: an image carrier 5 on which an image is formed by a developer; a transfer member 6 which is provided to face the image carrier and transfers the developer image formed on the image carrier to a recording medium 17; and moving means 21K and 22K which move the transfer member in at least a medium conveyance direction. The transfer member 6 is moved in at least the medium conveyance direction, so that a space between the recording medium 17 and the image carrier 5 immediately before the transfer member 6 can be reduced to prevent image defects due to discharge. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic system in which a developer image such as toner is transferred to a recording medium by a predetermined electric field.

  An electrophotographic printer charges a photosensitive drum, writes a latent image corresponding to a printed image on the charged photosensitive drum, develops a toner image on the latent image, and applies the toner image to an electric field having a polarity opposite to that of the toner. Transferred to a recording medium. At that time, an image defect may occur due to the discharge generated in the vicinity of the transfer nip portion. A method for preventing the discharge by using a recording medium guide to prevent the discharge is known (for example, Patent Document 1). reference.).

JP 2007-148464 A

  When the recording medium guide is used, the recording medium is deflected toward the image carrier before it reaches the transfer section and comes into contact with the image carrier, so that the Coulomb force from the transfer member is applied to the developer on the image carrier. Even if added, there is an advantage that the developer is not scattered in the air. However, when the recording medium is conveyed along the recording medium guide, the recording medium cannot be conveyed straight because it is bent toward the image carrier side. In a printer, image defects such as color misregistration may occur.

  SUMMARY OF THE INVENTION In view of the above technical problem, an object of the present invention is to provide an image forming apparatus that prevents image defects due to discharge that occurs in the vicinity of a transfer nip and also prevents image defects such as color misregistration.

  In order to solve the above technical problem, an image forming apparatus of the present invention is provided with an image carrier on which an image is formed by a developer, and the image carrier, and is formed on the image carrier. A transfer member that transfers the developer to a recording medium; and a moving unit that moves the transfer member at least in the medium conveyance direction. According to another image forming apparatus of the present invention, an image carrier on which an image is formed by a developer, and the developer formed on the image carrier that is provided to face the image carrier are recorded. The image forming apparatus includes a transfer member that transfers to a medium, and a moving unit that moves the transfer member in at least a medium conveying direction. The moving unit moves the transfer member according to the thickness of the recording medium.

  According to the image forming apparatus of the present invention, since the transfer member is moved at least in the medium conveyance direction, the gap between the recording medium and the image carrier immediately before the transfer member can be reduced. In addition, it is possible to prevent image defects due to discharge immediately before the transfer member. In addition, since a member that bends the conveyance direction of the recording medium, such as a recording medium guide member, is not used, image defects such as color misregistration can be prevented even when multicolor printing or the like is performed. .

  A preferred embodiment of an image forming apparatus of the present invention will be described with reference to the drawings.

[First embodiment]
The image forming apparatus according to the first embodiment is an electrophotographic multicolor image forming apparatus, which is a developer of four colors of black (K), yellow (Y), magenta (M), and cyan (C). This is an example of a color printer that obtains a color image by superposing and printing a certain toner. FIG. 2 is a schematic diagram showing the configuration of the black toner transfer roller of the image forming apparatus 1. Since yellow, magenta, and cyan have the same configuration and overlap, the transfer of the black toner is performed. The roller will be described, and the other colors will be omitted.

  As shown in FIG. 1, an image forming apparatus 1 according to this embodiment includes photosensitive drums 5K, 5Y, 5M, and 5C (hereinafter collectively referred to as photosensitive drums 5) as image carriers for supporting images. Chargers 2K, 2Y, 2M, and 2C (hereinafter collectively referred to as charger 2) for negatively charging the photosensitive drum 5, and exposure units 3K, 3Y, and 3M for writing an electrostatic latent image on the photosensitive drum 5. 3C (hereinafter collectively referred to as exposure unit 3), and developing units 4K, 4Y, 4M, and 4C (hereinafter referred to as “exposure unit 3”) for visualizing the electrostatic latent image on the photosensitive drum 5 with a negatively charged toner as a developer. Are collectively referred to as a developing device 4) for four colors of black, yellow, magenta, and cyan, respectively.

  Transfer rollers 6K, 6Y, 6M, and 6C (hereinafter referred to as transfer rollers 6) that function as transfer members for transferring a toner image from the photosensitive drum 5 to a recording medium such as paper are endless belt-like transfer. It is disposed in the belt 7 and is pressed against the photosensitive drum 5 with a predetermined pressure from below the photosensitive drum 5 with the transfer belt 7 interposed therebetween. The transfer roller 6 is connected to a transfer power source (not shown).

  The transfer belt 7 is supported by tension by a drive roller 8 disposed on the downstream side in the medium conveyance direction and an idle roller 9 disposed on the upstream side in the medium conveyance direction, and the photosensitive drum 5 and the transfer belt 7 are in contact with each other. The surface is arranged so as to be substantially horizontal. A recording medium 17 such as paper or an OHP sheet of a predetermined size is set on a support plate member 18 in a tray (not shown), and the support plate member 18 is pushed up by a spring 19 so that the recording medium 17 is moved to the hopping roller 14. The recording medium 17 is pressed and sent to the conveyance path one by one.

  A conveyance path of the recording medium 17 is indicated by a broken line in FIG. A first transport roller pair 11, a second transport roller pair 12, and a writing sensor 15 are arranged along the transport path of the recording medium 17. Further, a fixing device 10 for fixing the transferred toner image on the recording medium 17, a discharge sensor 16 for discharging the recording medium 17, and a discharge roller 13 are disposed downstream of the transfer belt 7 in the medium conveyance direction. Then, the recording medium 17 on which a predetermined image is fixed by the fixing device 10 is sent to the upper discharge portion of the image forming apparatus 1.

  The image forming apparatus 1 is controlled by the print control unit 20. The print control unit 20 is an apparatus that controls the image forming apparatus 1 in an integrated manner, and inputs and outputs signals to and from the exposure device 3, the developing device 4, the fixing device 10, the writing sensor 15, and the discharge sensor 16. It is connected. The print control unit 20 includes a microprocessor, a ROM, a RAM, an input / output port, a timer, and the like. The printing control unit 20 is configured to control various driving devices, and is a photosensitive drum 5, a hopping roller 14, a first conveying roller 11, a second conveying roller 12, a drive roller 8, and a fixing device, which are movable rollers. 10, the discharge roller 13 is connected to a drive device (not shown) constituted by a motor or the like, and each drive device is connected to and controlled by the print control unit 20.

  Next, the configuration of the transfer roller will be described with reference to FIG. FIG. 2 shows the configuration of a black toner transfer roller. The other color transfer rollers have the same configuration, and the other color transfer rollers are omitted for the sake of simplicity.

  The transfer roller 6K is supported at both ends of the shaft 32 of the transfer roller 6K in a state where it can be rotated by the transfer roller support portion 21K, and is unaffected by springs 33 mounted on the housing 34 of the transfer roller support portion 21K. The transfer belt 7 is pressed against the photosensitive drum 5K. Further, the transfer roller support portion 21K is mechanically connected to the transfer roller moving portion 22K via a rack gear 31 and the like, and is configured to be movable in parallel with the recording medium conveyance direction. The transfer roller moving unit 22K is an actuator, and uses, for example, a method of converting the rotational motion of the stepping motor into a horizontal motion by a stepping motor (not shown) and a rack and pinion. By controlling the rotation direction and the number of steps of the stepping motor, the position of the transfer roller support portion 21K can be controlled to an arbitrary position. The movement range of the transfer roller support portion 21K is configured such that the center of the transfer roller 6K can move in the recording medium conveyance direction rather than immediately below the center of the photosensitive drum 5K. In this example, the transfer roller 6K can be moved by a distance indicated by L3 in the recording medium conveyance direction from a position where the center of the transfer roller 6K is located immediately below the center of the photosensitive drum 5K. For example, the transfer roller 6K moves to a position of 1 mm. Is possible. The transfer roller moving unit 22K is connected to the print control unit 20, and the transfer roller moving unit 22K is operated by an instruction from the print control unit 20 to move the transfer roller support unit 21K, that is, the transfer roller 6K to a desired position. Let The transfer roller moving unit 22K may be configured by a stepping motor and a rack and pinion, or by combining an elastic member such as a spring and a solenoid.

  Next, the operation of this embodiment will be described with reference to FIGS. The operation of the image forming apparatus 1 is controlled by the print control unit 20. For example, the print control unit 20 processes a print instruction image, transports the recording medium 17, charges control, exposure control, and development control in accordance with a print instruction from a host device such as a personal computer connected to the image forming apparatus 1. Then, transfer control and fixing control are performed, and as a result, printing is performed on the recording medium 17.

  First, the printing operation will be described step by step. When the print control unit 20 receives a print instruction signal from a host device such as a personal computer connected to the image forming apparatus 1, the print control unit 20 starts controlling the fixing device 10 to fix the image. The vessel 10 is controlled to be heated to a usable temperature. When the fixing device 10 reaches a usable temperature, the printing control unit 20 controls the driving device to rotate the photosensitive drum 5 and accelerates the linear velocity until the linear velocity reaches the conveying speed of the recording medium 17 during printing. At the same time, the drive device is controlled to start the rotation of the drive roller 8, the transfer belt 7 is rotated by the drive roller 8, and the linear velocity of the transfer belt 7 is accelerated until the recording medium 17 is conveyed at the time of printing. .

  At the timing when the photosensitive drum 5 starts to rotate, the print control unit 20 controls the charger 2 to apply a negative voltage to the photosensitive drum 5 to charge the photosensitive drum 5 to, for example, −600V. The print control unit 20 controls the developing device 4 to be in a developable state at a timing when the portion of the photosensitive drum 5 charged to −600 V passes through the developing area of the developing device 4 by rotation. For example, in the case of a contact development method using a developing roller, a developing state can be achieved by applying a voltage of −200 V to a developing roller to which negatively charged toner adheres. When the portion of the photosensitive drum 5 charged to −600 V reaches the transfer nip portion by rotation, the printing process is ready for printing.

  Next, the printing control unit 20 rotates the hopping roller 14 with a driving device, and separates and transports the recording media 17 one by one. The separated recording medium 17 is transported to the second transport roller 12 through the first transport roller 11 by the print control unit 20 controlling the driving device, and the second transport roller 12, the transfer belt 7, and the like. The writing sensor 15 disposed between the two detects a timing at which the leading edge of the recording medium 17 passes the position of the writing sensor 15 and notifies the print controller 20 of the timing.

  The print control unit 20 separates the image to be printed on the recording medium 17 into the respective colors of black, yellow, magenta, and cyan from the print instruction signal received from the host device, and further, the exposure unit 3 Conversion into image data writable as an image is performed. Further, the recording medium 17 is conveyed onto the transfer belt 7 after passing through the writing sensor 15.

  The print control unit 20 detects the timing at which the writing sensor 15 detects the leading edge of the recording medium 17, the distances L1K, L1Y, L1M, and L1C between the writing sensor 15 and the transfer nip of each color, and the exposure on the photosensitive drum 5. A predetermined electrostatic latent image is controlled by controlling the exposure device 3 at a timing at which an image to be printed is transferred to a desired position on the recording medium 17 according to a distance L2 from the place to the transfer place and a conveyance speed of the recording medium 17. Is written on each photosensitive drum 5.

  The exposure device 3 is charged to −600 V by using, for example, an LED head that emits light according to image data, and irradiating a portion of the photosensitive drum 5 where a toner image is to be obtained with spot light having a pitch of 600 dpi, for example. The surface of the photosensitive drum 5 is at a potential of about −50 V and forms an electrostatic latent image. The peripheral surface of the photosensitive drum 5 on which the electrostatic latent image is written by the exposure device 3 is moved to the position of the developing device 4 by the rotation operation, and is exposed to a portion of the photosensitive drum 5 where the absolute value of the surface potential is reduced. A toner image, which is a visible image, is developed by the negatively charged toner that is a developer from the developing device 4 adhering to the peripheral surface due to the Coulomb force. Further, the peripheral surface of the photosensitive drum 5 to which the developed toner image adheres is conveyed to the transfer nip portion by a rotating operation.

  A transfer voltage for printing is applied to the transfer roller 6 during a period in which the recording medium 17 is in the transfer nip portion. For example, a positive voltage having a polarity opposite to that of the 1500V to 3000V toner is applied. The toner on the photosensitive drum 5 receives a coulomb force by a positive voltage applied to the transfer roller 6 at the transfer nip portion, and is transferred onto the recording medium 17. These operations are sequentially performed for black, yellow, magenta, and cyan, and toner images of these colors are sequentially transferred onto the recording medium 17.

  The recording medium 17 on which the toner has been developed and transferred is then conveyed to the fixing device 10. In the recording medium 17 conveyed to the fixing device 10, the toner on the recording medium 17 is melted by the heat and pressure of the fixing device 10, and the toner image is fixed on the recording medium 17.

  The recording medium 17 that has passed through the fixing device 10 is detected by the discharge sensor 16 to pass through the rear end of the recording medium 17 and is rotated by the discharge rollers 13 at the timing when the rollers are discharged out of the image forming apparatus 1. The charging devices 2K, 2Y, 2M, and 2C, the developing devices 4K, 4Y, 4M, and 4C, and the transfer power supply (not shown) are stopped to finish the printing. Further, when the passage timing of the recording medium 17 is abnormal, a measure such as stopping the operation of the image forming apparatus 1 is performed. Abnormality detection of the passage timing is performed in the same manner by the recording sensor 17 presence / absence detection operation by the writing sensor 15.

  Next, the position control of the transfer roller 6K in the image forming apparatus according to the present embodiment will be described using an example of a black toner transfer roller with reference to FIGS. The same control is performed for the other colors, but redundant description is omitted for simplicity. FIG. 3 is a movement timing chart of the transfer roller support portion of the first embodiment. FIG. 4 shows the position of the transfer roller when the sheet is not passed according to the first embodiment. FIG. 5 shows the position of the transfer roller when the sheet is passed in the first embodiment.

  In this embodiment, the transfer roller 6K can move substantially parallel to the medium conveyance direction, and the movement is performed by power from the transfer roller moving unit 22K. The transfer roller moving unit 22K is an actuator, and moves the transfer roller 6K in parallel with the medium conveyance direction by moving the transfer roller support unit 21K connected by, for example, a rack-and-pinion method according to an instruction from the print control unit 20. Move.

  The position of the transfer roller 6K is controlled to the position shown in FIG. 4 (when paper is not passed) and the position shown in FIG. 5 (when paper is passed). The position of the transfer roller 6K shown in FIG. 4 is the position of the transfer roller 6K when no paper is passed, that is, when the recording medium 17 is not in the transfer nip portion. In this embodiment, when the sheet is not fed, the position of the transfer roller 6K is controlled so that the center of the transfer roller 6K is directly below the center axis of the photosensitive drum 5K. The position of the transfer roller 6K shown in FIG. 5 is the position of the transfer roller 6K when the sheet is passed, that is, when the recording medium 17 is in the transfer nip portion. In this embodiment, when the sheet is passed, the position of the transfer roller 6K is controlled so that the center of the transfer roller 6K is at a distance L3, for example, about 1 mm in the recording medium conveyance direction from directly below the central axis of the photosensitive drum 5K. .

  The timing for moving the transfer roller 6K will be described with reference to FIG. The print control unit 20 receives a signal from the writing sensor 15 during printing and detects the timing of the leading edge and the trailing edge of the recording medium 17. The time from when the writing sensor 15 detects the leading edge of the recording medium 17 until it reaches the position of the black transfer roller 6K is as follows. Since the distance from the writing sensor 15 to the black transfer roller position is L1K (mm), when the printing speed is SP (mm / S), the distance is L1K / SP (S).

  The print control unit 20 instructs the transfer roller moving unit 22K after L1K / SP (S) after the writing sensor 15 detects the leading end of the recording medium, that is, when the leading end of the recording medium 17 reaches the transferring unit. The transfer roller moving unit 22K starts moving the transfer roller support unit 21K. The transfer roller moving unit 22K controls the moving speed of the transfer roller support unit 21K to be accelerated, constant speed, and decelerated. From the position of the transfer roller when not passing as shown in FIG. 4, the transfer roller when passing as shown in FIG. Move to position. The transfer roller position at the time of non-sheet passing and the transfer roller position at the time of sheet passing are in a positional relationship moved in parallel with the medium transport direction by L3.

  As shown in FIG. 3, the transfer roller support portion 21K moves after (L1K + M) / SP (S) after the writing sensor 15 detects the leading edge of the recording medium, that is, M (mm) from the leading edge of the recording medium 17. The process is completed until the position reaches the transfer nip portion. This distance M (mm) is a margin distance in which an image is not written on the leading end of the recording medium 17. The moving transfer roller 6K moves while the recording medium 17 is held between the transfer nips. At this time, the speed of the transfer roller support portion 21K is controlled so that the moving speed of the transfer roller support portion 21K is too fast to overtake the leading edge of the recording medium 17 and the photosensitive drum 5K and the transfer belt 7 are not in direct contact with each other. To do.

  Next, the print control unit 20 instructs the transfer roller moving unit 22K after (L1K-M) / SP (S) after the trailing edge of the recording medium 17 is detected by the writing sensor 15, and the transfer roller support unit 21K. That is, the transfer roller 6K is moved and moved from the transfer roller position at the time of passing paper shown in FIG. 5 to the transfer roller position at the time of non-passing time shown in FIG. Here, the distance M is a margin distance at which the toner image provided at the rear end portion of the recording medium 17 is not drawn, like the previous distance M. Similar to the leading end of the recording medium, the moving transfer roller 6K moves while the recording medium 17 is sandwiched between the transfer nips.

  By operating in this way, when the toner image is transferred to the recording medium 17, the transfer roller 6K moves L3 in the medium conveyance direction from just below the photosensitive drum 5K in FIG. When transfer is performed and the recording medium 17 is not in the transfer portion, the transfer roller 6K is moved directly below the photosensitive drum 5K in FIG. The movement timing is synchronized with the passage timing of the blank portion of the print area of the recording medium 17 and is performed so as not to hinder the image transfer at all.

  Explaining the control of the transfer voltage, as shown in FIG. 3, the printing control unit 20 is (L1K + M) / SP (S) after the writing sensor 15 detects the leading edge of the recording medium, that is, from the leading edge of the recording medium 17. A transfer voltage is applied at the timing when the position of M (mm) reaches the transfer nip portion. Therefore, the transfer voltage is applied only after the transfer roller position at the time of paper passing in FIG. As shown in FIG. 3, the transfer voltage OFF timing is L1K / SP (S) after the start sensor 15 detects the trailing edge of the recording medium, that is, the position at a distance Mmm from the trailing edge of the recording medium 17. The transfer voltage is turned off at the timing of reaching the part.

  Next, the effect of the difference in the position of the transfer roller 6K in FIGS. 4 and 5 on the transfer process will be described. As a premise, the transfer roller 6K is made of an elastic material such as foam rubber, and the transfer belt 7 is stretched flat under tension. Therefore, the transfer roller 6K is formed of the transfer belt 7 as shown in FIGS. It is deformed on the lower side.

  A gap is created by the curvature of the photosensitive drum 5K before and after the nip portion where the photosensitive drum 5K is in contact with the transfer belt 7 in the medium conveyance direction. The transfer roller 6K comes into contact with the transfer belt 7 below the gap. When a voltage is applied to the transfer roller 6K, an electric field is also applied to the gap immediately above where the transfer roller 6K is in contact with the transfer belt 7. Discharge occurs depending on the distance of the gap and the transfer electric field. If discharge occurs, the photosensitive drum 5K and the toner on the photosensitive drum 5K are positively charged. The positively charged toner cannot be transferred by the transfer electric field at the transfer nip portion, resulting in an image defect. For example, when transfer is performed on the recording medium 17 at the position of the transfer roller 6K when no paper is passed in FIG. 4, discharge occurs in the gap immediately above the transfer roller 6K in contact with the transfer belt 7 before the transfer nip, The toner is reversely charged and image defects are likely to occur.

  Next, at the position of the transfer roller at the time of paper passing in FIG. 5, the center position of the transfer roller 6K is adjusted to a position shifted by a distance L3 in the medium transport direction from directly below the center axis of the photosensitive drum 5K. As a result, the positional relationship between the gap between the photosensitive drum 5K and the recording medium 17 and the transfer roller 6K below the transfer belt 7 also changes, and the transfer roller 6K before the transfer nip is directly above the transfer belt 7. The gap between the recording medium 17 and the photosensitive drum 5K is reduced. Instead, the gap between the recording medium 17 and the photosensitive drum 5K is enlarged at the portion where the transfer roller 6K is in contact with the transfer belt 7 after the transfer nip portion. At such a position of the transfer roller, when a voltage is applied to the transfer roller 6K, a portion that may be discharged in the gap between the recording medium 17 and the photosensitive drum 5K before the transfer nip is reduced. As a result, it is possible to reduce discharge before the transfer nip and prevent image defects due to discharge.

  Although there is a tendency to increase the portion where there is a possibility of discharging in the portion after the transfer nip, there is no influence to the extent that an image defect occurs. This is because the toner is more negatively charged even when the toner transferred to the recording medium 17 is discharged, and the toner and the recording medium are more strongly attracted and adhered to each other by the Coulomb force.

  However, when a transfer voltage is applied without a recording medium at the transfer roller position at the time of paper passing in FIG. 5, a discharge occurs in the gap after the transfer nip, and the photosensitive drum 5K is charged positively. The surface of the photosensitive drum 5K that has been charged is not completely restored even after recharging by the charger, and may be seen as an image defect after the exposure and development processes. It is known that the same phenomenon occurs even when no transfer voltage is applied during non-sheet passing as in this example. In the present embodiment, when the sheet is not passed, the transfer roller is controlled to the transfer roller position immediately below the central axis of the photosensitive drum 5K in FIG. An image defect does not occur.

  In this embodiment, the position of the transfer roller 6K at the time of non-sheet passing is set to the axial center position of the transfer roller 6K immediately below the central axis of the photosensitive drum 5K in FIG. For easy assembling, the axial center position of the transfer roller 6K may be moved further to the upstream side in the recording medium conveyance direction than directly below the photosensitive drum 5K. Conversely, when it is difficult to generate a discharge phenomenon during non-sheet passing, the axial center position of the transfer roller 6K may be moved downstream of the photosensitive drum 5K in the recording medium conveyance direction.

  Similarly, even when the sheet is passed, the axial position of the transfer roller 6K is set at a position L3 in the medium conveyance direction from directly below the central axis of the photosensitive drum 5K in FIG. In the assembling in which the image defect is likely to occur, the axial center position of the transfer roller 6K may be moved further downstream in the recording medium conveyance direction. Conversely, when it is difficult for a discharge phenomenon to occur during paper passing, the axial center position of the transfer roller 6K may be moved to a position close to the position immediately below the 5K center of the photosensitive drum.

  Since the discharge is affected by the gap distance, the position of the transfer roller is optimized by the outer diameter of the photosensitive drum 5K, the outer diameter of the transfer roller 6K, the material of the transfer roller 6K, and the pressing pressure of the transfer roller 6K against the photosensitive drum 5K. There is a need to. In the control of the transfer voltage in this embodiment, the transfer voltage is not applied when the sheet is not passed, but a transfer voltage different from that when the sheet is passed may be applied.

  In the image forming apparatus according to the first embodiment, the transfer roller 6K is moved, and the position of the transfer roller 6K is changed between when paper is not passed and when paper is passed. When the sheet is not fed, discharge after the transfer nip can be prevented. As a result, image defects due to discharge can be prevented. Further, in the present embodiment, the recording medium 17 is conveyed straight without using the guide material of the recording medium 17 and the like, so that image defects due to color misregistration can be prevented.

[Second Embodiment]
The image forming apparatus according to the second embodiment is common in many parts to the configuration of the image forming apparatus according to the first embodiment, and different parts will be described here for simplicity. The second embodiment is different from the first embodiment in the configuration of the transfer roller support portion 23K as shown in FIG.

  As shown in FIG. 6, the transfer roller support portion 23K in the image forming apparatus according to the second embodiment does not press the transfer roller 6K perpendicularly to the direction of the photosensitive drum 5K as in the first embodiment. A1 For example, it has a configuration of pressing at an oblique angle of 45 degrees. That is, the transfer roller support portion 23K is provided with a spring 43 so as to be housed in the housing 44. One end of the spring 43 is fixed to the housing 44, and the other end is attached to the rotating shaft 42 of the transfer roller 6K. Fixed to the bearing member. In the present embodiment, when the recording medium 17 is not between the transfer roller 6K and the photosensitive drum 5K, the axial center of the transfer roller 6K is located immediately below the axial center of the photosensitive drum 5K. That is, the position is the same as the position shown in FIG. 4 of the first embodiment. When the recording medium 17 is between the transfer roller 6K and the photosensitive drum 5K, the axis of the transfer roller 6K is an angle A1 that is an extension direction of the housing 44 against the elastic force from the spring 43, for example. It moves along a line having an angle of 45 degrees.

  The operation of the image forming apparatus according to the second embodiment is common to the operation of the first embodiment in many parts, and different parts will be described here for simplicity. That is, the operation of the transfer roller support portion 23K having a difference will be described. In this embodiment, the black toner transfer roller support 23K is described, but the same operation is performed for other colors.

  The transfer roller 6K is adjusted so that the shaft center of the transfer roller is located immediately below the shaft center of the photosensitive drum in FIG. When printing, when the recording medium 17 enters the transfer nip portion, the transfer roller 6K is pushed down by the thickness of the recording medium 17 as shown in FIG. In the present embodiment, the thickness T4 of the recording medium 17 is, for example, 0.1 mm. When the transfer roller 6K is pushed down by 0.1 mm as the thickness T4 of the recording medium 17, the transfer roller 6K is moved by the movement of the transfer roller support 23K so that the transfer roller shaft position is L4 in the recording medium conveyance direction, for example, 0. Move 1 mm.

  Next, at the same time when the rear end portion of the recording medium 17 passes through the transfer nip portion, the transfer roller 6K moves from the transfer roller position at the time of passing paper in FIG. 7 to the transfer roller position at the time of non-passing paper in FIG. . By operating in this way, the gap portion between the recording medium 17 and the photosensitive drum 5K immediately above where the transfer roller 6K before the transfer nip is in contact with the transfer belt 7 is reduced at the time of sheet passing. Discharge can be suppressed. When paper is not passed, the gap between the recording medium 17 and the photosensitive drum 5K immediately above where the transfer roller 6K after the transfer nip is in contact with the transfer belt 7 decreases.

  By controlling the position of the transfer roller 6K in this way, there is little portion that can be discharged in the gap between the recording medium 17 and the photosensitive drum 5K before the transfer nip when a voltage is applied to the transfer roller 6K. Become. As a result, in this embodiment, image defects due to discharge can be prevented. Further, in this embodiment, the transfer roller 6K moves along an angle of 45 degrees due to the configuration of the transfer roller support portion 23K. By changing this angle, the recording of the transfer roller 6K with respect to the thickness of the recording medium is performed. The moving distance of the medium 17 in the conveyance direction can also be changed.

  In the second embodiment, discharge before the transfer nip is prevented, and discharge after the transfer nip is prevented when the recording medium is not fed. As a result, image defects due to discharge can be prevented. In this embodiment, since the recording medium is conveyed straight, the above-described effects can be obtained without causing image defects due to color misregistration. Furthermore, since these effects can be automatically obtained depending on the thickness of the recording medium, the configuration of the image recording apparatus can be simplified and can be manufactured at low cost.

[Third embodiment]
As shown in FIG. 8, the present embodiment includes a swing arm type transfer roller support 24K instead of the transfer roller support 23K of the image forming apparatus of the second embodiment. The transfer roller support portion 24K has a swing arm 52 that can rotate around a rotation shaft 53, and has a configuration in which the shaft 51 of the transfer roller 6K is pivotally supported at the tip of the swing arm 52. ing. When the recording medium 17 reaches the transfer nip position, the swing arm 52 rotates by an angle A2 as in the transfer roller support portion 23K of the second embodiment, and the gap between the recording medium 17 and the photosensitive drum 5K. The discharge can be suppressed by reducing the portion. Further, by adjusting the mounting angle of the swing arm 52, the moving distance of the transfer roller 6K in the transport direction of the recording medium 17 with respect to the thickness of the recording medium can be changed.

  Although this embodiment has been described using a color image forming apparatus, the present invention can be similarly applied to a monochrome image forming apparatus. (A device without a belt may be used) Therefore, for example, any device having a configuration for developing and transferring a developer by an electric field can be applied to an MFP, a facsimile, and a copying machine. The number of colors is not limited, and the present invention can be similarly applied to a single-color image forming apparatus or a multi-color image forming apparatus.

1 is a configuration diagram illustrating a schematic configuration of an entire apparatus of an image forming apparatus according to a first exemplary embodiment of the present invention. 1 is a schematic diagram of a main part of an image forming apparatus according to a first embodiment of the present invention. 3 is a time chart for explaining the operation of the image forming apparatus according to the first exemplary embodiment of the present invention. FIG. 2 is a schematic diagram of a main part of the image forming apparatus according to the first exemplary embodiment of the present invention, and shows a position of a transfer roller when paper is not passed. FIG. 3 is a schematic diagram of a main part of the image forming apparatus according to the first exemplary embodiment of the present invention, showing a position of a transfer roller during paper feeding. It is a principal part schematic diagram of the image forming apparatus of the 2nd Embodiment of this invention. FIG. 6 is a schematic diagram of a main part of an image forming apparatus according to a second embodiment of the present invention, and is a diagram illustrating a position of a transfer roller during paper feeding. It is a principal part schematic diagram of the image forming apparatus of the 3rd Embodiment of this invention.

Explanation of symbols

1 Image forming apparatus 2, 2K, 2Y, 2M, 2C Charger 3, 3K, 3Y, 3M, 3C Exposure unit 4, 4K, 4Y, 4M, 4C Development unit 5, 5K, 5Y, 5M, 5C Photosensitive drum 6, 6K, 6Y, 6M, 6C Transfer roller 7 Transfer belt 8 Drive roller 9 Idle roller 10 Fixing device 11 First transport roller 12 Second transport roller 13 Discharge roller 14 Hopping roller 15 Write sensor 16 Discharge sensor 17 Recording medium 18 Support plate member 19 Spring 20 Print Control Unit 21K Transfer Roller Support Unit 22K Transfer Roller Moving Unit 31 Rack Gear 32 Shaft 33 Spring 34 Housing 42 Shaft 43 Spring 44 Housing 51 Shaft 52 Swing Arm 53 Rotating Shaft

Claims (8)

An image carrier on which an image is formed by a developer;
A transfer member provided facing the image carrier and transferring the developer formed on the image carrier to a recording medium;
An image forming apparatus comprising: a moving unit configured to move the transfer member in at least a medium conveying direction. When the recording medium is between the image carrier and the transfer member, the transfer member is moved in the medium conveyance direction, and when the recording medium is not between the image carrier and the transfer member, The image forming apparatus according to claim 1, wherein the transfer member is moved in a direction opposite to a medium conveyance direction. The image forming apparatus according to claim 1, wherein the transfer member is a transfer roller that rotates about an axis, and the moving unit is an actuator attached to the shaft of the transfer roller. An image carrier on which an image is formed by a developer;
A transfer member provided facing the image carrier and transferring the developer formed on the image carrier to a recording medium;
An image forming apparatus comprising: a moving unit configured to move the transfer member in at least a medium conveying direction according to a thickness of the recording medium. The image forming apparatus according to claim 4, wherein when the transfer member moves away from the image carrier due to the thickness of the recording medium, the moving unit moves the transfer member obliquely. The transfer member is a transfer roller that rotates about an axis, and the moving means is an actuator that is attached to the shaft of the transfer roller so as to move the transfer roller obliquely with respect to the medium conveyance direction of the recording medium. The image forming apparatus according to claim 5. The image forming apparatus according to claim 4, wherein when the transfer member moves away from the image carrier due to the thickness of the recording medium, the moving unit moves the transfer member in an arc shape. 8. The transfer member according to claim 7, wherein the transfer member is a transfer roller rotating about an axis, and the moving means is an actuator attached to the shaft of the transfer roller so as to move the transfer roller in an arc shape. The image forming apparatus described.