JP4561269B2 - Transfer device and image forming apparatus - Google Patents

Description

  The present invention relates to a transfer device that transfers a toner image formed on an image carrier to a recording medium, and an image forming apparatus including the transfer device.

  2. Description of the Related Art Conventionally, for example, an image forming apparatus shown in FIG. 8 is known as an image forming apparatus such as a copying machine or a printer employing an electrophotographic system.

  In this image forming apparatus 200, the surface of the image carrier 122 such as a photoconductor is uniformly charged by the charging device 204, and then the surface of the image carrier 122 is irradiated with image light 206 to form an electrostatic latent image. The electrostatic latent image is developed by the developing device 208 to form a toner image. This toner image is transferred to a recording medium P such as paper by a transfer roller 210. Further, the surface of the image carrier 122 after the toner image is transferred is cleaned by the cleaning device 212, and the residual toner is removed.

  In such an image forming apparatus 200, the transfer roller 210 is disposed so as to contact the image carrier 122 via the recording medium P, and a high-voltage transfer voltage having a reverse polarity to the toner is applied to the transfer roller 210. The As a result, the toner image on the image carrier 122 is electrically attracted and transferred to the recording medium P (see, for example, Patent Document 1).

However, in such a contact type transfer roller 210, as shown in FIG. 9, when a recording medium P shorter than the maximum width is used, the transfer roller 210 directly contacts the image carrier 122 (C portion). There is a problem in that the transfer voltage is applied from the transfer roller 210 to the image carrier 122 and the image carrier 122 and the like are deteriorated.
Japanese Patent Laid-Open No. 10-063120

  SUMMARY An advantage of some aspects of the invention is that it provides a transfer device and an image forming apparatus that can prevent deterioration of an image carrier even when a recording medium shorter than the maximum width is used. .

In order to solve the above problems, a transfer device according to a first aspect of the present invention is a transfer device that transfers a toner image formed on an image carrier to a recording medium, and is opposed to the image carrier. And a base provided on the conveyance path of the recording medium and having conductive magnetic particles and a protective layer covering the conductive magnetic particles , a magnetic field is generated on the base, and a magnetic field generated by the conductive magnetic particles is generated. Magnetic field generating means for forming a brush and contacting the recording medium via the protective layer , magnetic field control means for generating a magnetic field by the magnetic field generating means in a portion through which the recording medium passes, and transfer voltage to the magnetic brush have a, a power source for applying the said substrate is the inner cylindrical sleeve, said protective layer is extrapolated into said cylindrical sleeve, an outer rotatable electroconductive magnetic particles on the inner peripheral surface side is arranged A cylindrical sleeve, said inner circle The magnetic field generating means inside the sleeve is characterized in that is arranged.

According to the first aspect of the present invention, there is provided a substrate having conductive magnetic particles facing the image carrier and facing the recording medium conveyance path and a protective layer covering the conductive magnetic particles. And a magnetic field is generated on the substrate by the magnetic field generating means. As a result, a magnetic brush made of conductive magnetic particles is formed on the substrate, and this magnetic brush contacts the recording medium via the protective layer . Further, a transfer voltage is applied to the magnetic brush, and the toner image on the image carrier is transferred to the recording medium. At that time, since the magnetic field control means generates a magnetic field in the portion through which the recording medium passes, the magnetic brush is not formed in the portion through which the recording medium does not pass .

According to the first aspect of the present invention, the protective layer provided on the substrate covers the conductive magnetic particles to prevent scattering. This protective layer moves and contacts the recording medium when the magnetic brush is formed. For this reason, the protective layer does not contact the recording medium in a portion where there is no recording medium. Therefore, it is avoided that the transfer voltage is directly applied to the image carrier in a portion where there is no recording medium, and deterioration of the image carrier or the like is prevented.
Furthermore, according to the first aspect of the present invention, the outer cylindrical sleeve rotates as the recording medium moves, so that unnecessary frictional resistance does not occur. Further, a magnetic field is generated by a magnetic field generating means arranged inside the inner cylindrical sleeve. Thus, a magnetic brush made of conductive magnetic particles can be formed between the inner cylindrical sleeve and the outer cylindrical sleeve.

According to a second aspect of the present invention , there is provided an image forming apparatus that faces an image carrier and faces a recording medium conveyance path, and has a base on which conductive magnetic particles are disposed, and generates a magnetic field on the base. Magnetic field generating means for forming a magnetic brush made of the conductive magnetic particles and contacting the recording medium; magnetic field control means for generating a magnetic field by the magnetic field generating means in a portion through which the recording medium passes; and the magnetic brush A transfer device for transferring a toner image formed on the image carrier to the recording medium, and a detection means for detecting the width of the recording medium, and the detection Based on the width of the recording medium detected by the means, a magnetic field of the magnetic field generating means is generated in a portion through which the recording medium passes .

According to the second aspect of the present invention, the substrate having the conductive magnetic particles disposed thereon facing the image carrier and facing the recording medium conveyance path is provided on the substrate by the magnetic field generating means. Is generated. As a result, a magnetic brush made of conductive magnetic particles is formed on the substrate, and the magnetic brush contacts the recording medium. Further, a transfer voltage is applied to the magnetic brush, and the toner image on the image carrier is transferred to the recording medium. At that time, the width of the recording medium is detected by the detecting means, and the control means controls the magnetic field generating means based on the detected width of the recording medium to generate a magnetic field in a portion through which the recording medium passes. For this reason, it can be avoided that the magnetic brush is formed in a portion where the recording medium does not pass. For this reason, it is avoided that the transfer voltage is directly applied to the image carrier in a portion where there is no recording medium, and deterioration of the image carrier or the like is prevented.

The image forming apparatus according to the invention of claim 3, a transfer device according to claim 1, comprising detecting means for detecting a recording medium width, a, based on the detected recording medium width by said detecting means The magnetic field generating means generates a magnetic field in a portion through which the recording medium passes.

According to the third aspect of the present invention, the width of the recording medium is detected by the detecting means, and the control means controls the magnetic field generating means based on the detected width of the recording medium, and the magnetic field is applied to the portion through which the recording medium passes. generate. For this reason, it is possible to avoid the formation of the magnetic brush in a portion where the recording medium does not pass, and it is possible to more reliably prevent the transfer voltage from being directly applied to the image carrier.

  In the transfer device and the image forming apparatus according to the present invention, even when a recording medium shorter than the maximum width is used, the transfer voltage is applied only to a portion through which the recording medium passes, and the transfer voltage is directly applied to the image carrier. Therefore, it is possible to prevent the image carrier from being deteriorated.

  Hereinafter, embodiments of a transfer device and an image forming apparatus according to the present invention will be described with reference to the drawings.

  First, an overview of the overall configuration and printing operation of the image forming apparatus equipped with the transfer apparatus according to the first embodiment of the present invention will be described first, and then the main part of the present invention will be described.

  FIG. 1 shows an image forming apparatus 10 on which a transfer device 20 according to the first embodiment is mounted. This image forming apparatus 10 is an apparatus that forms an image on a recording medium P made of continuous paper in which perforations for cutting are perforated in advance, and the recording medium P inserted into the body of the image forming apparatus 10 is conveyed. The roller 28 is transported at a constant speed along a transport path formed across the body. An image forming unit 30 that forms a toner image is disposed above the conveyance path of the recording medium P. The recording medium P is not limited to continuous paper, and can be applied to cut paper.

  The image forming unit 30 includes an image carrier (photosensitive drum) 12 that is arranged so that the axis is orthogonal to the conveyance direction of the recording medium P. Around the image carrier 12, a charger 14 for charging the image carrier 12, a light beam scanning device 16 that forms an electrostatic latent image by irradiating the charged image carrier 12 with a laser beam, A developing device 18 for forming a toner image of a predetermined color on the image carrier 12 by supplying toner of a predetermined color to the electrostatic latent image forming portion on the image carrier 12 and developing the electrostatic latent image, and a recording medium A transfer device 20 disposed opposite to the image carrier 12 across the conveying path of P, a static eliminator 22 that neutralizes the image carrier 12, a cleaner blade 24 and a cleaner brush for removing residual toner on the image carrier 12 26 is arranged. Although not shown, a flash fixing unit is disposed on the downstream side in the conveyance direction of the recording medium P at a position facing the upper side of the conveyance path.

  In such an image forming apparatus 10, when the image carrier 12 rotates clockwise in FIG. 1, first, the surface of the image carrier 12 is charged almost uniformly to a positive polarity potential by the charger 14. When the image carrier 12 further rotates, the surface of the image carrier 12 is exposed by the light beam scanning device 16, and the potential of the exposed portion of the image carrier 12 is lowered to form an electrostatic latent image. The electrostatic latent image on the image carrier 12 is conveyed to the developing device 18, and the toner charged to the same polarity (plus) as the charging polarity of the image carrier 12 is electrically supplied to the potential lowering portion of the image carrier 12. The electrostatic latent image is developed by adhering to the surface.

  Thereafter, when the image carrier 12 further rotates, the toner is electrically attracted to the transfer device 20 to which a transfer voltage having a reverse polarity (minus) to the toner is applied. As a result, the toner image on the image carrier 12 is transferred to the recording medium P. The toner image transferred to the recording medium P is fixed by flash light emitted from a flash fixing unit (not shown). Further, the surface of the image carrier 12 after the toner image is transferred is neutralized by the static eliminator 22 and cleaned by the cleaner blade 24 and the cleaner brush 26 to remove residual toner.

  Although the number of the image forming units 30 is one in FIG. 1, four image forming units 30 for forming toner images of C (cyan), M (magenta), Y (yellow), and K (black) colors are provided. A color image can be formed by arranging the recording medium P along the conveyance path at substantially equal intervals. That is, a series of processes of charging, exposure (electrostatic latent image formation), development (toner image formation), and transfer in the four image forming units 30 is performed on the recording medium P. It is possible to form a full-color image on the recording medium P by controlling the execution timing so that they overlap each other.

  Next, the transfer device 20 mounted on the image forming apparatus 10 will be described.

  As shown in FIG. 2A, in the transfer device 20, a base 42 made of a nonmagnetic metal plate is disposed on the image carrier 12 at a position facing the image carrier 12 across the conveyance path of the recording medium P. Arranged along the longitudinal direction. The base body 42 is a substantially rectangular metal plate such as aluminum, and is a protruding portion 42A in which four corners of the metal plate protrude upward. A large number of conductive magnetic particles 44 are arranged inside the base 42. As the conductive magnetic particles 44, for example, particles such as magnetite and ferrite having a diameter of 50 μm to 100 μm are used. In order to impart conductivity, the surface of the particles may be coated with a conductive resin. Further, a power source 46 to which a transfer voltage having a polarity opposite to that of toner (minus in the present embodiment) is applied is connected to the base 42.

  As shown in FIG. 3, a magnetic field generator 48 that generates a magnetic field on the base 42 is disposed below the base 42. This magnetic field generator 48 has a plurality of electromagnets 50 arranged at positions corresponding to the maximum width of the recording medium P, and a switch 52 for energizing each electromagnet 50 is provided. The electromagnet 50 is obtained by winding an insulated copper wire around a soft iron core, and the soft iron is magnetized only while the insulated copper wire is energized to become a magnet. The switch 52 is ON / OFF controlled by the control device 54. The control device 54 is set to turn on only the electromagnet 50 corresponding to the portion through which the recording medium P passes in accordance with the width of the recording medium P detected in advance.

  In the present embodiment, the width of the recording medium P is set so that it can be detected when the recording medium P is set on a sprocket (not shown). Note that the present invention is not limited to such a method. For example, the operator may set the width information of the recording medium P to be input, or the recording medium P may be set to be automatically detected by a sensor (not shown). May be.

  Next, the operation of the transfer device 20 according to the first embodiment of the present invention will be described.

  As shown in FIG. 2B, when the recording medium P does not pass, the switches 52 of all the electromagnets 50 are turned off by the control device 54 (see FIG. 3).

  As shown in FIGS. 2A and 3, when the toner image on the image carrier 12 is transferred, a portion through which the recording medium P passes in accordance with the recording medium width detected in advance by the control device 54. The switch 52 of the corresponding electromagnet 50 is turned ON. As a result, a magnetic field is generated on the substrate 42, and the conductive magnetic particles 44 are linked in a spike shape to form a magnetic brush 44A. Then, the magnetic brush 44A comes into contact with the back side of the recording medium P, and the contact state between the recording medium P and the image carrier 12 is maintained. Further, a transfer voltage is applied to the base 42 from a power source 46, and the transfer voltage is applied to the magnetic brush 44 </ b> A (conductive magnetic particles 44) through the base 42 so that the toner image on the image carrier 12 is transferred. Electrically attracted to the recording medium P.

  On the other hand, since the switch 52 of the electromagnet 50 is turned off in the portion where the recording medium P does not pass, the magnetic brush 44A is not formed in this portion. For this reason, the conductive magnetic particles 44 and the image carrier 12 do not come into contact with each other in the portion where the recording medium P does not pass, and the transfer voltage is prevented from being directly applied to the image carrier 12. Therefore, deterioration of the image carrier 12 is prevented.

  Next, a second embodiment of the transfer apparatus according to the present invention will be described with reference to the drawings.

  In addition, the same code | symbol is attached | subjected to the member same as 1st Embodiment, and the overlapping description is abbreviate | omitted.

  As shown in FIGS. 4A and 5, the transfer device 60 is provided with a magnetic field generator 62 that generates a magnetic field on the base 42 at a lower portion of the base 42. In the magnetic field generator 62, a plurality of permanent magnets 64A to 64D are arranged side by side at a position corresponding to the maximum width of the recording medium P. The permanent magnets 64 </ b> A to 64 </ b> D are obtained by magnetizing S poles and N poles at both ends of the plate-like body, and the magnetic flux density on the surface of the base 42 is set to 50 mT to 150 mT. The permanent magnets 64 </ b> A to 64 </ b> D are rotatably supported by a gear 66 around the support shaft 65. The gear 66 meshes with a disc-shaped gear 68, and a motor 70 is connected to the gear 68. The rotation of the motor 70 is controlled by the control device 72. The control device 72 is set to rotate only the motors 70 of the permanent magnets 64A to 64D corresponding to the portion through which the recording medium P passes in accordance with the width of the recording medium P detected in advance.

  In the present embodiment, the permanent magnet 64A is set to a length corresponding to the minimum width of the recording medium P (longitudinal direction of the image carrier 12), and the permanent magnets 64B to 64D are set according to the type (size) of the recording medium P. The structure is such that it is divided. In addition, it is not limited to such a structure, The length and number of permanent magnets can be set suitably.

  Next, the operation of the transfer device 60 will be described.

  As shown in FIGS. 4A and 5, when the toner image on the image carrier 12 is transferred, a portion through which the recording medium P passes in accordance with the recording medium width detected in advance by the control device 72. The motors 70 of the permanent magnets 64A and 64B corresponding to are rotated. As a result, as shown in FIG. 4A, the gear 68 rotates in the direction of arrow A, and with the rotation of the gear 66, the permanent magnets 64A and 64B rotate so as to stand upright. Then, the magnetic brush 44B in which the conductive magnetic particles 44 are arranged in a spike shape is formed by the magnetic force of the permanent magnet 64A, and the magnetic brush 44B contacts the back side of the recording medium P, and the recording medium P and the image carrier 12 contact each other. State is maintained. Further, a transfer voltage is applied to the base 42 from a power source 46, and the transfer voltage is applied to the magnetic brush 44B (conductive magnetic particles 44) through the base 42, whereby the toner image T on the image carrier 12 is applied. Is electrically attracted to the recording medium P.

  On the other hand, as shown in FIGS. 4B and 5, in a portion where the recording medium P does not pass, the control device 72 rotates the motor 70 and rotates the gear 68 in the arrow B direction. As a result, the gear 66 rotates, the permanent magnets 64C and 64D lie down, and the magnetic lines of force are laid down, so that the magnetic brush 44B is not formed. For this reason, the conductive magnetic particles 44 and the image carrier 12 do not come into contact with each other in the portion where the recording medium P does not pass, and the transfer voltage is prevented from being directly applied to the image carrier 12. Therefore, deterioration of the image carrier 12 is prevented.

  Next, a third embodiment of the transfer apparatus according to the present invention will be described with reference to the drawings.

  In addition, the same code | symbol is attached | subjected to the member same as 1st Embodiment or 2nd Embodiment, and the overlapping description is abbreviate | omitted.

As shown in FIG. 6, a protective film 82 is bridged over the transfer device 80 so as to cover the upper surface of the substrate 42 along the circumferential direction of the image carrier 12. The protective film 82 is provided at a position facing the entire length in the longitudinal direction of the image carrier 12, and both end portions 82 A and 82 A of the protective film 82 are attached to the projecting portion 42 A facing the circumferential direction of the image carrier 12. It is worn. For the protective film 82, a conductive or semiconductive resin film is used. For example, polyimide, ETFE, or the like having a thickness of 10 μm to 100 μm and a volume resistance of 10 ⁶ Ω · cm to 10 ¹² Ω · cm is used. The other configuration of the transfer device 80 is the same as that of the transfer device 60 according to the second embodiment.

  Next, the operation of the transfer device 80 will be described.

  In such a transfer device 80, the permanent magnet 64 </ b> A is rotated upright by the rotation of the gears 68 and 66 in the portion through which the recording medium P passes. Then, the magnetic brush 44B in which the conductive magnetic particles 44 are linked in a spike shape is formed by the magnetic force of the permanent magnet 64A, and the magnetic brush 44B pushes up the protective film 82. The protective film 82 contacts the back side of the recording medium P, and the contact state between the recording medium P and the image carrier 12 is maintained. In this state, a transfer voltage is applied from the power source 46 to the base 42, and this transfer voltage is applied to the magnetic brush 44B (conductive magnetic particles 44), whereby the toner image T on the image carrier 12 is transferred to the recording medium P. Is attracted electrically.

  In such a transfer device 80, the upper portion of the base 42 is covered with the protective film 82, so that the conductive magnetic particles 44 can be prevented from scattering. Further, the magnetic brush 44B pushes up the protective film 82, and the protective film 82 comes into contact with the recording medium P, whereby the recording medium P and the image carrier 12 can be maintained in a stable contact state.

  Next, a transfer device according to a fourth embodiment of the invention will be described with reference to the drawings.

  In addition, the same code | symbol is attached | subjected to the member same as 1st-3rd embodiment, and the overlapping description is abbreviate | omitted.

  As shown in FIG. 7, the transfer device 90 is provided with an inner cylindrical base 92 made of a nonmagnetic metal sleeve arranged substantially parallel to the longitudinal direction of the image carrier 12. For the inner cylindrical base 92, aluminum having a thickness of 0.5 mm to 1.5 mm is used. Inside the inner cylindrical base 92, a magnetic field generator 94 that generates a magnetic field at the opposed portion of the inner cylindrical base 92 and the image carrier 12 is disposed. The magnetic field generation device 94 is configured by arranging a plurality of electromagnets 96 at positions corresponding to the maximum width of the recording medium P, and a power supply device 98 for energizing each electromagnet 96 is provided. The power supply device 98 is controlled to be turned on and off by the control device 100, and is set to turn on only the electromagnet 96 corresponding to the portion through which the recording medium P passes in accordance with the width of the recording medium detected in advance. ing.

Further, an outer cylindrical protective film 102 made of a synthetic resin cylindrical sleeve is provided outside the peripheral surface of the inner cylindrical base 92. The inner cylindrical base 92 and the outer cylindrical protective film 102 are mounted on a support member (not shown), and the inner cylindrical base 92 and the outer cylindrical protective film 102 are rotated together by rotation of a support member (not shown). Is configured to do. For the outer cylindrical protective film 102, for example, polyimide, ETFE or the like having a thickness of 10 μm to 100 μm and a volume resistance of 10 ⁶ Ω · cm to 10 ¹² Ω · cm is used. A large number of conductive magnetic particles 44 are filled between the inner cylindrical base 92 and the outer cylindrical protective film 102.

  Next, the operation of the transfer device 90 will be described.

  As shown in FIG. 7, when the toner image on the image carrier 12 is transferred, the control device 100 turns on the power supply device 98 of the electromagnet 96 corresponding to the portion through which the recording medium P passes. As a result, a magnetic field is generated at the facing portion between the inner cylindrical base 92 and the image carrier 12, and a magnetic brush 44 </ b> C in which the conductive magnetic particles 44 are continuous in a spike shape is formed. Then, the magnetic brush 44 </ b> C pushes up the outer cylindrical protective film 102, the outer cylindrical protective film 102 contacts the back side of the recording medium P, and the recording medium P and the image carrier 12 contact each other. A transfer voltage is applied to the inner cylindrical base 92 from the power source 46, and the transfer voltage is applied to the magnetic brush 44C (conductive magnetic particles 44), whereby the toner image T on the image carrier 12 is recorded on the recording medium. Electrically drawn to P.

  In such a transfer device 90, the transfer voltage is applied only to the portion through which the recording medium P passes, and the transfer voltage is not directly applied to the image carrier 12, so that deterioration of the image carrier 12 is prevented. The In addition, since a large number of conductive magnetic particles 44 are filled between the inner cylindrical base 92 and the outer cylindrical protective film 102, scattering of the conductive magnetic particles 44 can be prevented. Further, the magnetic brush 44C pushes up the outer cylindrical protective film 102 and the outer cylindrical protective film 102 comes into contact with the recording medium P, whereby the recording medium P and the image carrier 12 can be maintained in a stable contact state.

1 is a schematic configuration diagram illustrating an image forming apparatus equipped with a transfer device according to a first embodiment of the present invention. FIG. 3 is a configuration diagram in a direction orthogonal to a longitudinal direction of the transfer apparatus according to the first embodiment of the present invention. It is a block diagram in the longitudinal direction of the transfer device according to the first embodiment of the present invention. It is a block diagram in the direction orthogonal to the longitudinal direction of the transfer apparatus which concerns on 2nd Embodiment of this invention. It is a block diagram in the longitudinal direction of the transfer apparatus which concerns on 2nd Embodiment of this invention. It is a block diagram in the direction orthogonal to the longitudinal direction of the transfer apparatus which concerns on 3rd Embodiment of this invention. It is a block diagram in the direction orthogonal to the longitudinal direction of the transfer apparatus which concerns on 4th Embodiment of this invention. It is a schematic block diagram which shows the conventional image forming apparatus. It is a figure explaining the problem in the conventional image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Image carrier 42 Base | substrate 44A Magnetic brush 44B Magnetic brush 44C Magnetic brush 44 Conductive magnetic particle 46 Power supply 48 Magnetic field generator 50 Electromagnet 54 Control apparatus (magnetic field control means)
60 Transfer Device 62 Magnetic Field Generators 64A, 64B Permanent Magnets 64C, 64D Permanent Magnet 70 Motor 72 Controller (Magnetic Field Control Unit)
80 Transfer device 82 Protective film (protective layer)
90 Transfer Device 92 Inner Cylindrical Base (Inner Cylindrical Sleeve)
94 Magnetic field generator 96 Electromagnet 98 Power supply device 100 Control device (magnetic field control means)
102 Outer cylindrical protective film (outer cylindrical sleeve)

Claims (3)

A transfer device for transferring a toner image formed on an image carrier to a recording medium,
A substrate that is provided facing the image carrier and facing the conveyance path of the recording medium, and having conductive magnetic particles and a protective layer covering the conductive magnetic particles ;
A magnetic field generating means for generating a magnetic field on the substrate, forming a magnetic brush made of the conductive magnetic particles, and contacting the recording medium via the protective layer ;
Magnetic field control means for generating a magnetic field by the magnetic field generating means at a portion through which the recording medium passes;
A power source for applying a transfer voltage to the magnetic brush;
I have a,
The base is an inner cylindrical sleeve;
The protective layer is a rotatable outer cylindrical sleeve that is extrapolated to the inner cylindrical sleeve and in which conductive magnetic particles are arranged on the inner peripheral surface side,
The transfer apparatus , wherein the magnetic field generating means is arranged inside the inner cylindrical sleeve . A substrate facing the image carrier and facing the recording medium conveyance path, on which conductive magnetic particles are disposed ;
A magnetic field generating means for generating a magnetic field on the substrate, forming a magnetic brush made of the conductive magnetic particles, and contacting the recording medium;
Magnetic field control means for generating a magnetic field by the magnetic field generating means at a portion through which the recording medium passes;
A power source for applying a transfer voltage to the magnetic brush;
The provided, a transfer device for transferring the toner image formed on the image bearing member to the recording medium,
Detecting means for detecting a recording medium width,
An image forming apparatus, wherein a magnetic field of the magnetic field generating unit is generated in a portion through which the recording medium passes based on a recording medium width detected by the detecting unit. A transfer apparatus according to claim 1;
  Detecting means for detecting a recording medium width,
  An image forming apparatus, wherein a magnetic field of the magnetic field generating unit is generated in a portion through which the recording medium passes based on a recording medium width detected by the detecting unit.

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