JP2006243069A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of satisfactorily preventing an electrostatic offset of a developer from a recording medium to a fixing means. <P>SOLUTION: A charge applying member 60 of an approximately T shape in section is arranged between a transfer section 18 and a fixing section 19 in such a manner that a top surface 63 of the charge applying member 60 intersects with an extension surface 70 extended from the rear end of a support surface 45 of a conveyance belt 38 to a downstream side in the conveyance direction of a paper sheet 3 along the support surface 45. A bias of the polarity reverse to the polarity possessed by the toner image on the paper sheet 3 is applied to the charge applying member 60. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a laser printer.

  An image forming apparatus such as a laser printer includes a photosensitive drum and a transfer device arranged to face the photosensitive drum, and a heating roller and a pressure roller arranged to face each other on the downstream side in the sheet conveying direction. A toner image based on image data is carried on the photosensitive drum, and the toner image carried on the photosensitive drum is transferred to a sheet passing between the photosensitive drum and the transfer roller by applying a transfer bias to the transfer unit. Is done. Thereafter, the sheet on which the toner image is transferred passes between the heating roller and the pressure roller, and at this time, is heated by the heating roller and pressed by the pressure roller. As a result, the toner image is fixed on the paper, and the formation of the image on the paper is achieved.

  The heating roller is disposed on the surface side of the sheet carrying the toner image, and the pressure roller is disposed opposite to the heating roller across the sheet. Therefore, during the fixing process, the toner image carried on the paper comes into contact with the surface of the heating roller, and at this time, unfixed toner on the paper may be transferred to the surface of the heating roller due to electrostatic causes. is there. The toner transferred to the surface of the heating roller is transferred again to the sheet to be fixed next, and the quality of the image formed on the sheet is deteriorated.

In order to prevent such unfixed toner from being transferred due to electrostatic causes (electrostatic offset), for example, a bias having a polarity opposite to that of the toner on the paper is applied downstream of the heating roller and the pressure roller in the paper conveyance direction. It has been proposed to dispose the elastic flexible conductive brush to be in contact with the back surface of the paper held between the heating roller and the pressure roller (see, for example, Patent Document 1). When the sheet comes into contact with the elastic flexible conductive brush, the sheet is charged with a polarity opposite to that of the toner, and the toner on the sheet is attracted to the sheet, thereby preventing electrostatic offset of unfixed toner.
Japanese Patent Publication No. 5-46551

  However, in such a configuration, the electrostatic offset of unfixed toner on the paper cannot be prevented until the paper passing between the heating roller and the pressure roller contacts the elastic flexible conductive brush. For this reason, there is a case where an electrostatic offset of unfixed toner carried at the leading end portion (downstream end portion in the sheet conveying direction) of the sheet occurs. In addition, the elastic flexible conductive brush may cause uneven charging of the paper due to uneven contact of the brush material with the paper, and may cause partial electrostatic offset due to the uneven charging.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of satisfactorily preventing electrostatic offset of a developer from a recording medium to a fixing unit.

  In order to achieve the above object, an invention according to claim 1 is an image forming apparatus that conveys a recording medium and forms an image on the recording medium, an image carrier that carries a developer image, and the image. Transfer means for transferring the developer image carried on the carrier to the recording medium, and the developer image disposed on the downstream side of the transfer means in the transport direction of the recording medium and transferred to the recording medium Fixing means for fixing the toner image on the recording medium, and a smooth surface that is disposed between the transfer means and the fixing means, and that faces the surface of the recording medium opposite to the surface on which the developer image is transferred. And a bias applying unit that applies a bias having a polarity opposite to that of the developer image transferred to the recording medium to the charge applying member.

  According to such a configuration, the recording medium onto which the developer image has been transferred faces the smooth surface of the charge applying member to which a bias is applied from the bias applying unit while being conveyed from the transfer unit to the fixing unit. At this time, a charge having a polarity opposite to that of the developer image is received from the smooth surface. For this reason, when the recording medium enters the fixing unit, the developer image on the recording medium is attracted to the recording medium charged with a polarity opposite to that of the developer image. In addition, the electrostatic offset of the developer to the fixing unit can be prevented. In addition, since the surface of the charge applying member facing the recording medium is a smooth surface, it is possible to achieve uniform charging of the recording medium and prevent partial electrostatic offset due to uneven charging of the recording medium. can do. As a result, the electrostatic offset of the developer from the recording medium to the fixing unit can be satisfactorily prevented, and the deterioration of the image quality due to such an electrostatic offset can be prevented.

The smooth surface may have a surface roughness (ten-point average roughness) Rz of 30 or less, and preferably 12.5 or less.
According to a second aspect of the present invention, in the first aspect of the invention, the charge imparting member guides the recording medium conveyed toward the fixing unit with the smooth surface.

According to such a configuration, the recording medium comes into contact with the smooth surface of the charge applying member, and the conveyance of the recording medium is guided by the smooth surface. Therefore, the recording medium can be charged more uniformly and efficiently. As a result, electrostatic offset of the developer from the recording medium to the fixing unit can be prevented better.
According to a third aspect of the present invention, in the first or second aspect of the present invention, the charge imparting member includes a conductive base and a coating layer formed on the surface of the base and having the smooth surface. And the bias applying means applies a bias to the substrate.

  According to such a configuration, even when the developer is attracted to the smooth surface of the charge imparting member by applying a bias to the charge imparting member (substrate), the developer can be prevented from adhering to the smooth surface. it can. If the developer adheres to the smooth surface of the charge applying member, when the recording medium faces the smooth surface, the developer is opposite to the back surface of the recording medium (the surface opposite to the surface on which the developer image is transferred on the recording medium). The recording medium is constantly soiled, but if the developer can be prevented from adhering to the smooth surface, the recording medium will be charged. Of course, the recording medium faces the smooth surface of the charging member in a non-contact state, and even if the recording medium faces the smooth surface of the charge applying member in a contact state, the back surface of such a recording medium is steadily soiled. Can be prevented.

According to a fourth aspect of the present invention, in the third aspect of the present invention, the coating layer is formed by a coating process using a conductive coating material.
According to such a configuration, the coating layer is formed on the surface of the substrate by the coating process using the conductive coating material. By using a conductive coating material, conductivity can be imparted to the coating layer. Therefore, it can be prevented that the coating layer interferes with the application of electric charge to the recording medium. Therefore, the recording medium can be more efficiently charged.

The invention according to claim 5 is the invention according to claim 3, wherein the coating layer is formed by a coating process using an insulating coating material.
According to such a structure, the coating layer which has insulation is formed in the surface of the base | substrate by the coating process using an insulating coating material. Since the coating layer has an insulating property, even if the recording medium partially contacts the smooth surface (even if the recording medium comes into contact with the smooth surface and a non-contact portion), the recording is possible. Charges do not concentrate on the portion of the medium that contacts the smooth surface. Therefore, the recording medium can be charged more uniformly.

A sixth aspect of the invention is characterized in that, in the invention of any of the third to fifth aspects, the substrate is made of a metal.
According to such a configuration, the electrical resistance of the base can be reduced as compared with the case where the base of the charge imparting member is formed using a conductive resin. Therefore, the bias voltage applied to the charge applying member can be reduced, and the power consumption of the device can be kept low.

According to a seventh aspect of the present invention, in the invention according to any one of the first to sixth aspects, the smooth surface is perpendicular to a conveyance direction of the recording medium, and the recording medium is in a direction along the recording medium. It is characterized by being formed wider than the maximum image that can be formed on the medium.
According to such a configuration, the smooth surface of the charge applying member is formed wider than the maximum image that can be formed on the recording medium. Therefore, while the recording medium is conveyed from the transfer unit to the fixing unit, the entire maximum image forming area of the recording medium can be made to face the smooth surface of the charge applying member, which is opposite to the polarity of the developer image. It can be charged evenly in polarity. As a result, electrostatic offset of the developer from the recording medium to the fixing unit can be prevented even better.

The invention according to claim 8 is the invention according to claim 7, wherein the smooth surface can be conveyed in the image forming apparatus in a direction perpendicular to the conveyance direction of the recording medium and along the recording medium. It is characterized by being formed wider than a maximum size recording medium.
According to such a configuration, the smooth surface of the charge applying member is formed wider than the maximum size recording medium (for example, A4 size recording medium) that can be conveyed in the image forming apparatus. Therefore, even when the recording medium has the maximum size, the entire surface of the recording medium can be opposed to the smooth surface of the charge imparting member while being conveyed from the transfer unit to the fixing unit. Can be charged evenly. As a result, electrostatic offset of the developer from the recording medium to the fixing unit can be prevented even better.

A ninth aspect of the present invention provides the belt according to any one of the first to eighth aspects, wherein the belt has a support surface for supporting the recording medium and conveys the recording medium toward the fixing unit. The charge imparting member is disposed between the belt and the fixing unit.
In such a configuration, the recording medium is conveyed while being electrostatically attracted to the support surface of the belt, and when the recording medium is separated from the support surface of the belt, peeling electrification occurs between the belt and the recording medium In some cases, the developer image is charged to the same polarity as that of the developer image.Because the charge imparting member is disposed between the belt and the fixing unit, the recording medium fed from the belt can reach the fixing unit before reaching the fixing unit. The developer image can be charged to a polarity opposite to that of the developer image. Therefore, electrostatic offset of the developer from the recording medium to the fixing unit can be satisfactorily prevented, and deterioration of image quality due to such electrostatic offset can be prevented.

According to a tenth aspect of the present invention, in the ninth aspect, the smooth surface intersects an extended surface extending along the support surface from the support surface in the conveyance direction of the recording medium. It is characterized by.
According to such a configuration, the smooth surface of the charge applying member intersects with an extended surface that extends from the support surface of the belt along the support surface in the conveyance direction of the recording medium. Therefore, the end (tip) on the downstream side in the transport direction of the recording medium fed from the support surface of the belt can be reliably brought into contact with the smooth surface of the charge applying member. As a result, the recording medium can be reliably brought into contact with the smooth surface of the charge applying member from the front end to the rear end, and the entire surface of the recording medium can be uniformly charged with a polarity opposite to that of the developer image. As a result, electrostatic offset of the developer from the recording medium to the fixing unit can be reliably prevented.

According to an eleventh aspect of the present invention, in the invention according to the tenth aspect, the bias is applied to a portion of the smooth surface that is in contact with at least a downstream end portion of the recording medium conveyed in the conveyance direction. A bias from the means is applied.
According to such a configuration, since the bias is applied to the portion where the end on the downstream side in the conveyance direction of the recording medium contacts on the smooth surface of the charge applying member, the entire surface of the recording medium is reliably charged from the tip. be able to. Therefore, electrostatic offset of the developer from the recording medium to the fixing unit can be more reliably prevented.

According to a twelfth aspect of the present invention, in the invention according to any one of the first to eleventh aspects, the bias applying unit is configured to be able to change a magnitude of a bias to be applied to the charge applying member. It is characterized by being.
According to such a configuration, the magnitude of the bias applied to the charge applying member by the bias applying means can be changed. Therefore, the recording medium is changed by changing the magnitude of the bias applied to the charge applying member in accordance with the type of the recording medium (for example, the thickness and quality of the paper) and / or the environment (for example, temperature and humidity). Can be more reliably charged. As a result, electrostatic offset of the developer from the recording medium to the fixing unit can be more reliably prevented.

According to the first aspect of the present invention, the electrostatic offset of the developer from the recording medium to the fixing unit can be satisfactorily prevented, and the deterioration of the image quality caused by the electrostatic offset is prevented. be able to.
According to the second aspect of the present invention, electrostatic offset of the developer from the recording medium to the fixing unit can be prevented better.

According to the third aspect of the present invention, it is possible to prevent the surface of the recording medium facing the charge applying member from being constantly soiled by the developer.
According to the fourth aspect of the invention, the recording medium can be more efficiently charged.
According to the fifth aspect of the present invention, the recording medium can be more uniformly charged.
According to the sixth aspect of the present invention, the bias voltage applied to the charge applying member can be reduced, and the power consumption of the apparatus can be kept low.

According to the seventh aspect of the present invention, the entire area of the maximum image forming area on the recording medium can be uniformly charged to the polarity opposite to the polarity of the developer image, and the static of the developer from the recording medium to the fixing means can be obtained. Electric offset can be prevented even better.
According to the eighth aspect of the present invention, the entire surface of the maximum size recording medium can be uniformly charged to a polarity opposite to the polarity of the developer image, and the electrostatic offset of the developer from the recording medium to the fixing means. Can be prevented even better.

According to the ninth aspect of the present invention, even when the recording medium is conveyed by the belt, electrostatic offset of the developer from the recording medium to the fixing unit can be satisfactorily prevented. It is possible to prevent deterioration in image quality due to the electric offset.
According to the tenth aspect of the present invention, it is possible to reliably prevent electrostatic offset of the developer from the recording medium to the fixing unit.

According to the eleventh aspect, electrostatic offset of the developer from the recording medium to the fixing unit can be more reliably prevented.
According to the twelfth aspect of the present invention, electrostatic offset of the developer from the recording medium to the fixing unit can be more reliably prevented.

<Overall configuration of color laser printer>
FIG. 1 is a side sectional view showing an embodiment of a color laser printer as an image forming apparatus of the present invention.
The color laser printer 1 is a horizontal type direct tandem type color laser printer in which a plurality of process units 17 are arranged in parallel in a horizontal direction, and includes a sheet 3 as a recording medium in a main body casing 2. A paper feeding unit 4 for feeding the paper, an image forming unit 5 for forming an image on the fed paper 3, and a paper discharge unit 6 for discharging the paper 3 on which the image is formed. .

The main casing 2 has a box-like shape that is substantially rectangular in a side view with the upper side opened, and a top cover 7 is provided on the upper side. The top cover 7 is rotatably supported via a cover shaft provided on the rear side of the main casing 2 (in the following description, the left side in FIG. 1 is the rear side and the right side is the front side). The main casing 2 can be opened and closed.
<Configuration of paper feed unit>
The paper feed unit 4 is provided above the front side of the paper feed roller 11, the paper tray 9 provided at the bottom of the main body casing 2, the pickup roller 10 and the paper feed roller 11 provided above the front side of the paper tray 9. A sheet feeding side U-shaped path 12, a pair of transport rollers 13 and a pair of registration rollers 14 provided in the middle of the sheet feeding side U-shaped path 12 are provided.

In the paper tray 9, the paper 3 is stacked, and the topmost paper 3 is first picked up by the pickup roller 10 and conveyed forward, and then fed by the paper feed roller 11 to the paper feed side U. The paper is fed to the character path 12.
The feed-side U-shaped path 12 has an upstream end adjacent to the feed roller 11 at the lower side so that the paper 3 is fed forward, and a downstream end at the upper side, which will be described later. It is formed as a substantially U-shaped sheet 3 conveying path that is adjacent to the conveying belt 38 and is conveyed rearward.

Then, the sheet 3 fed forward to the upstream end of the sheet feeding side U-shaped path 12 is conveyed by the conveying roller 13 in the sheet feeding side U-shaped path 12, and the conveying direction is reversed in the front-rear direction. After registration by the registration roller 14, the registration roller 14 sends it out backward.
<Configuration of image forming unit>
The image forming unit 5 includes a process unit 17, a transfer unit 18 as a transfer unit, and a fixing unit 19 as a fixing unit.

The process unit 17 is provided for each color of a plurality of colors of toner. That is, the process unit 17 includes four units, that is, a yellow process unit 17Y, a magenta process unit 17M, a cyan process unit 17C, and a black process unit 17K. The process units 17 are sequentially arranged in parallel so as to be overlapped in the horizontal direction at intervals from the front to the rear.
<Process section configuration>
Each process unit 17 includes a scanner unit 20 that is fixedly disposed on each process unit 17 and a process cartridge 21 that is detachably attached to each process unit 17.
<Configuration of scanner unit>
The scanner unit 20 includes a laser light emitting unit (not shown), a polygon mirror 22, a lens 23, and a reflecting mirror 24. In the scanner unit 20, the laser light based on the image data emitted from the laser light emitting unit is reflected by the polygon mirror 22, passes through the lens 23, is reflected by the reflecting mirror 24, and is directed to the photosensitive drum 25 described later. Are emitted.
<Configuration of process cartridge>
Each process cartridge 21 is inclined in the front-rear direction and the vertical direction (the thickness direction of the paper 3), that is, in the direction inclined from the upper side to the lower side toward the rear side (the direction in which the upper side is inclined toward the front side). A photosensitive drum 25 serving as an image carrier, a scorotron charger 26, a developing roller 27, and a supply roller 28 are provided.

  The photosensitive drum 25 has a cylindrical shape, and a drum main body 29 formed by a positively chargeable photosensitive layer whose outermost layer is made of polycarbonate or the like, and an axial center of the drum main body 29 along the axial direction of the drum main body 29. The drum shaft 30 extends. The drum body 29 is provided so as to be rotatable with respect to the drum shaft 30, and the drum shaft 30 rotates on both side walls in the width direction of the casing of the process cartridge 21 (the direction orthogonal to the front-rear direction and the vertical direction, the same applies hereinafter). It is supported impossible. The photosensitive drum 25 is rotationally driven in the same direction (clockwise in the drawing) as the movement direction of the conveyance belt 38 at a contact position (image formation position) with the conveyance belt 38 described later during image formation.

The scorotron charger 26 is a positively charged scorotron charger that includes a wire and a grid, and generates corona discharge. The scorotron charger 26 is disposed behind the photosensitive drum 25 so as not to come into contact with the photosensitive drum 25 with an interval therebetween. Has been.
The developing roller 27 is disposed above the photosensitive drum 25 so as to face the photosensitive drum 25 and is in pressure contact with the photosensitive drum 25. In the developing roller 27, a metal roller shaft 31 is covered with a roller portion 32 made of an elastic member such as a conductive rubber material. More specifically, the roller portion 32 is covered with an elastic roller portion made of conductive urethane rubber, silicone rubber, EPDM rubber, or the like containing carbon fine particles, and the surface of the roller portion. It is formed of a two-layer structure with a coat layer mainly composed of resin, polyimide resin or the like. The roller shaft 31 is rotatably supported on both side walls in the width direction of the process cartridge 21.

The supply roller 28 is disposed above the developing roller 27 so as to face the developing roller 27 and is in pressure contact with the developing roller 27. In the supply roller 28, a metal roller shaft 33 is covered with a roller portion 34 made of a conductive sponge member. The roller shaft 33 is rotatably supported on both side walls in the width direction of the process cartridge 21.
The upper part of the process cartridge 21 is formed as a toner storage chamber 35 for storing toner, and stores toner for each color. That is, in the toner storage chamber 35, for each process unit 17, yellow for the yellow process unit 17Y, magenta for the magenta process unit 17M, cyan for the cyan process unit 17C, and black for the black process unit 17K. A positively chargeable non-magnetic one-component polymerized toner having the following characteristics is accommodated.

  More specifically, as the toner for each color, a substantially spherical polymerized toner obtained by a polymerization method is used. The polymerization toner is a known polymerization such as a suspension polymerization of a styrene monomer such as styrene or an acrylic monomer such as acrylic acid, alkyl (C1 to C4) acrylate, or alkyl (C1 to C4) methacrylate. The main component is a binder resin obtained by copolymerization according to the method, and a toner base particle is formed by adding a colorant, a charge control agent, a wax, etc. to this, and further improves the fluidity. In order to achieve this, an external additive is added.

  As the colorant, the above-mentioned yellow, magenta, cyan and black colorants are blended. Moreover, as a charge control agent, for example, an ionic monomer having an ionic functional group such as an ammonium salt and an ionic monomer such as a styrene monomer or an acrylic monomer can be copolymerized. A charge control resin obtained by copolymerization with a monomer is blended. As external additives, for example, powders of metal oxides such as silica, aluminum oxide, titanium oxide, strontium titanate, cerium oxide and magnesium oxide, inorganic powders such as carbide powders and metal salt powders are blended. Has been.

In each process unit 17, during the image forming operation, the toner for each color stored in each toner storage chamber 35 is supplied to the supply roller 28, and is supplied to the developing roller 27 by the rotation of the supply roller 28. . At this time, the toner is positively frictionally charged between the supply roller 28 and the developing roller 27 to which a developing bias is applied.
On the other hand, the scorotron charger 26 generates a corona discharge by applying a charging bias to uniformly positively charge the surface of the photosensitive drum 25. The surface of the photosensitive drum 25 is uniformly positively charged by the scorotron charger 26 as the photosensitive drum 25 rotates, and then exposed by high-speed scanning of laser light from the scanner unit 20 to form on the paper 3. An electrostatic latent image corresponding to the image to be formed is formed.

  When the photosensitive drum 25 is further rotated, the toner held on the surface of the developing roller 27 and positively charged toner comes into contact with the photosensitive drum 25 by the rotation of the developing roller 27 so as to face the photosensitive drum 25. The electrostatic latent image formed on the surface, that is, the surface of the photosensitive drum 25 that is uniformly positively charged, is supplied to an exposed portion that is exposed to laser light and has a lowered potential. As a result, the electrostatic latent image on the photosensitive drum 25 is visualized, and a toner image by reversal development is carried on the surface of the photosensitive drum 25 for each color.

Further, the process cartridges 21 of the respective colors are mounted at a higher position as the front process unit 17. Specifically, the mounting position of the process cartridge 21 is increased by a predetermined amount between the adjacent process units 17 in the order of the black process unit 17K, the cyan process unit 17C, the magenta process unit 17M, and the yellow process unit 17Y. Is set.
<Configuration of transfer section>
The transfer unit 18 is disposed in the main casing 2 above the sheet feeding unit 4 and below the process unit 17 along the front-rear direction. The transfer unit 18 includes a driving roller 36, a driven roller 37, a transport belt 38 as a belt, and A transfer roller 39 is provided.

The drive roller 36 is disposed on the lower rear side of the photosensitive drum 25 of the process cartridge 21 attached to the black process unit 17K. The drive roller 36 is driven to rotate in the direction opposite to the rotation direction of the photosensitive drum 25 (counterclockwise in the figure) during image formation.
The driven roller 37 is disposed on the lower front side of the photosensitive drum 25 of the process cartridge 21 mounted on the yellow process portion 17Y and slightly above the drive roller 36. The driven roller 37 is driven to rotate in the same direction as the driving roller 36 (counterclockwise in the drawing) when the driving roller 36 is driven to rotate.

  The conveyor belt 38 is made of an endless belt, and is formed of a resin such as conductive polycarbonate or polyimide in which conductive particles such as carbon are dispersed. The conveying belt 38 is wound between a driving roller 36 and a driven roller 37, and an upper portion thereof (the driven roller 37 on the upstream side in the moving direction of the conveying belt 38 is disposed, and the driving roller 36 is disposed on the downstream side. Are extended in an inclined plane shape that becomes higher toward the front side, and are provided so as to face and contact the photosensitive drum 25 of the process cartridge 21 mounted on each process unit 17 from below. The upper surface 45 of the upper portion serves as a support surface for supporting and transporting the paper 3.

Then, the driven roller 37 is driven by the driving roller 36, and the image forming position where the conveying belt 38 contacts the photosensitive drum 25 of each process unit 17 between the driving roller 36 and the driven roller 37. In FIG. 2, it is moved so as to rotate in the same direction as the photosensitive drum 25.
The transfer roller 39 is disposed so as to face the photosensitive drum 25 of each process unit 17 and the conveyor belt 38 in the conveyor belt 38 wound between the driving roller 36 and the driven roller 37. In the transfer roller 39, a metal roller shaft 41 is covered with a roller portion 42 made of an elastic member such as a conductive rubber material. The roller shaft 41 extends in the width direction, and both ends thereof are rotatably supported by a frame 44 that integrally holds the transfer unit 18. Further, the transfer roller 39 is pressed against the photosensitive drum 25 by a spring (not shown), and a nip is formed between the photosensitive drum 25 and the conveyor belt 38. The transfer roller 39 rotates in the same direction (counterclockwise in the figure) as the circumferential movement direction of the conveyor belt 38 at a position where the transfer roller 39 faces and contacts the conveyor belt 38 as the conveyor belt 38 rotates. In addition, a transfer bias having a polarity opposite to that of the toner (toner image) is applied to the transfer roller 39.

  The sheet 3 fed from the sheet feeding unit 4 is supported by the support surface 45 of the conveyor belt 38 that is circulated by the drive of the driving roller 36 and the driven roller 37, and is accompanied by the circumferential movement of the conveyor belt 38. From the front to the rear, the paper is conveyed so as to sequentially pass through the image forming positions between the conveyance belt 38 and the photosensitive drum 25 of each process unit 17. During the conveyance, the toner image for each color carried on the photosensitive drum 25 of each process unit 17 is transferred onto the upper surface of the sheet 3 (the surface facing the photosensitive drum 25) by the transfer bias applied to the transfer roller 39. ) Are sequentially superimposed and transferred, whereby a color image is formed on the paper 3.

  That is, for example, when the yellow toner image carried on the surface of the photosensitive drum 25 of the yellow process unit 17Y is transferred to the paper 3, the magenta toner carried on the surface of the photosensitive drum 25 of the magenta process unit 17M is then transferred. The toner image is transferred onto the sheet 3 on which the yellow toner image has already been transferred, and the cyan toner image carried on the surface of the photosensitive drum 25 of the cyan process unit 17C and the black process unit 17K are operated in the same manner. The black toner image carried on the surface of the photosensitive drum 25 is transferred in a superimposed manner, whereby a color image is formed on the paper 3.

In the formation of such a color image, the color laser printer 1 has a tandem apparatus configuration in which a plurality of process cartridges 21 are provided for each color in each process unit 17, so that the speed for forming a monochrome image is increased. A toner image for each color can be formed at approximately the same speed to achieve rapid color image formation. Therefore, it is possible to form a color image while reducing the size.
<Configuration of fixing unit>
The fixing unit 19 is arranged behind the transfer unit 18 at a predetermined interval. The fixing unit 19 includes a heating roller 48 and a pressure roller 49.

The heating roller 48 is made of a metal base tube having a release layer formed on the surface thereof, and a halogen lamp is provided along the axial direction thereof. Then, the surface of the heating roller 48 is heated to the fixing temperature by the halogen lamp. The pressure roller 49 is provided so as to press the heating roller 48 and forms a nip with the heating roller 48.
The color image transferred onto the paper 3 is then conveyed to the fixing unit 19 and heated and pressed by the heating roller 48 while the paper 3 passes between the heating roller 48 and the pressure roller 49. Under pressure from the roller 49, the toner is fixed on the upper surface of the paper 3.
<Configuration of paper discharge unit>
The paper discharge unit 6 includes a paper discharge side U-shaped path 50, a paper discharge roller 51, and a paper discharge tray 52.

The discharge-side U-shaped path 50 has an upstream end adjacent to the fixing unit 19 at the lower side and a downstream end at the upper side so that the sheet 3 is fed rearward. Is formed as a substantially U-shaped paper 3 conveyance path so that the paper 3 is discharged forward.
The paper discharge roller 51 is provided as a pair of rollers at the downstream end of the paper discharge side U-shaped path 50.

The paper discharge tray 52 is formed on the upper surface of the top cover 7 as an inclined wall that is inclined downward from the front to the rear.
The sheet conveyed from the fixing unit 19 is fed rearward to the upstream end of the discharge side U-shaped path 50, and the conveyance direction is reversed in the discharge side U-shaped path 50. The paper is ejected forward onto a paper ejection tray 52 by a paper ejection roller 51.

FIG. 2 is a plan view of the transfer unit 18 and the fixing unit 19, and FIG. 3 is a side sectional view of the transfer unit 18 and the fixing unit 19. In the color laser printer 1, the sheet 3 conveyed from the conveyance belt 38 toward the fixing unit 19 between the transfer unit 18 and the fixing unit 19, more specifically between the conveyance belt 38 and the fixing unit 19. Is provided with a charge applying member 60 for charging the toner to the polarity opposite to the polarity of the toner image on the paper 3.
<Configuration of Charge Giving Member 60>
The charge applying member 60 is formed using a metal material such as iron, and includes a base 73 having a substantially T-shaped cross section having a flat plate portion 61 and a hanging portion 62 substantially orthogonal to the flat plate portion 61.

  The flat plate portion 61 has a substantially rectangular shape in a plan view, and as shown in FIG. It is provided so as to intersect with the extended extended surface 70. More specifically, the flat plate portion 61 is located near the rear side of the drive roller 36 and below the support surface 45 of the conveyor belt 38, and the nip between the heating roller 48 and the pressure roller 49. It is provided so as to extend slightly upward toward the position.

FIG. 4 is a side sectional view showing a configuration in the vicinity of the charge applying member 60. As shown in FIG. 4, a coating layer 69 for preventing toner adhesion is formed on the entire surface of the flat plate portion 61.
The coating layer 69 is formed, for example, by a coating process using a fluororesin coating material in which conductive carbon is dispersed. The upper surface 63 of the coating layer 69 is a smooth plane having a surface roughness (ten-point average roughness) Rz of 12.5 or less. As shown in FIG. 2, the maximum image on the sheet 3 in the width direction. It is wider than the formation area A, and wider than the maximum size (for example, A4 size) paper 3 that can be used in the color laser printer 1.

The hanging portion 62 is formed in a rectangular plate shape that is long in the width direction, and is provided in a state of hanging from the lower surface of the flat plate portion 61. The rear surface of the lower end portion of the hanging portion 62 is fixed to the housing 71 of the fixing portion 19. On the other hand, a bias terminal 64 for applying a bias to the charge applying member 60 is in contact with the front surface of the lower end portion of the hanging portion 62.
The bias terminal 64 is made of a metal plate (plate spring) having spring properties, one end portion 65 is fixed to the frame 72 of the main casing 2, and the other end portion 66 is elastically attached to the front surface of the lower end portion of the hanging portion 62. It touches. A bias circuit 67 that generates a bias having a polarity opposite to the polarity of the toner image on the paper 3 is connected to one end 65 of the bias terminal 64. Thereby, a bias generated by the bias circuit 67 is applied to the hanging portion 62 via the bias terminal 64. In this embodiment, the bias application means is constituted by the bias terminal 64 and the bias circuit 67.

  Further, the bias circuit 67 has a configuration in which the magnitude of the generated bias can be changed in a plurality of stages, for example. The operation panel (not shown) of the color laser printer 1 is provided with a bias change key 68. When the bias change key 68 is operated by the user, it is applied to the charge applying member 60 (base 73). The magnitude of the bias can be changed according to the type (for example, thickness and quality) of the paper 3 and / or the environment (for example, temperature and humidity).

  According to the above configuration, the charge applying member 60 is disposed between the transfer unit 18 and the fixing unit 19. A bias having a polarity opposite to the polarity of the toner image on the paper 3 is applied to the hanging portion 62 of the base 73 of the charge applying member 60, and the entire base 73 (by the conductivity of the base 73 ( The flat plate portion 61 and the hanging portion are charged with a polarity opposite to that of the toner image.

  The sheet 3 to which the toner image has been transferred is fed out from the transport belt 38 after the toner image is transferred, and is transported toward the fixing unit 19 while being guided by the upper surface 63 of the charge applying member 60 (coating layer 69). . At this time, the sheet 3 receives charges having a polarity opposite to that of the toner image from the upper surface 63 of the charge applying member 60. Therefore, when the paper 3 enters between the heating roller 48 and the pressure roller 49, the toner image on the paper 3 is electrostatically attracted to the paper 3. Therefore, it is possible to prevent a problem that the toner forming the toner image on the paper 3 is transferred to the surface of the heating roller 48 due to an electrostatic cause, that is, a so-called electrostatic offset of the toner during the fixing process. . In addition, since the charging of the paper 3 is achieved before the paper 3 enters between the heating roller 48 and the pressure roller 49, the electrostatic offset of the toner is also reduced at the downstream end in the transport direction of the paper 3. Occurrence can be prevented. Further, since the upper surface 63 that is the surface facing the paper 3 in the charge applying member 60 is a smooth surface, uniform charging of the paper 3 can be achieved, and partial electrostatic charge caused by uneven charging of the paper 3 can be achieved. The occurrence of offset can be prevented. As a result, electrostatic offset of the toner from the paper 3 to the heating roller 48 can be satisfactorily prevented, and deterioration of image quality due to such electrostatic offset can be prevented.

  In particular, in the configuration in which the sheet 3 is conveyed by the conveyance belt 38, the sheet 3 is conveyed while being electrostatically attracted to the support surface 45 of the conveyance belt 38, and the sheet 3 is separated from the support surface 45 of the conveyance belt 38. The sheet 3 may be charged with the same polarity as that of the toner image due to peeling between the transfer belt 38 and the charge belt 60 and the fixing unit 19. As a result, the sheet 3 fed from the conveying belt 38 can be charged to a polarity opposite to that of the toner image before entering between the heating roller 48 and the pressure roller 49. Therefore, even when the paper 3 is transported by the transport belt 38, electrostatic offset of toner from the paper 3 to the heating roller 48 can be satisfactorily prevented, and an image caused by such electrostatic offset can be prevented. Quality deterioration can be prevented.

Furthermore, since the sheet 3 comes into contact with the upper surface 63 of the charge applying member 60 and the conveyance of the sheet 3 is guided by the upper surface 63, the sheet 3 can be more uniformly and efficiently charged. As a result, electrostatic offset of the toner from the sheet 3 to the heating roller 48 can be prevented better.
Further, since the coating layer 69 is formed on the upper surface 63 of the charge applying member 60, even if the toner is attracted to the upper surface 63 by application of a bias to the charge applying member 60, the toner is applied to the upper surface 63. Adhesion can be prevented. When toner is attached to the upper surface 63 of the charge applying member 60, when the paper 3 faces the upper surface 63, the toner is on the back surface of the paper 3 (the surface opposite to the surface on which the toner image is transferred on the paper 3). In other words, the back surface of the sheet 3 is soiled, but if toner adhesion can be prevented, the sheet 3 is in contact with the upper surface 63 of the charge applying member 60. Even with the opposing configuration, such contamination of the back surface of the sheet 3 can be prevented.

In addition, since the coating layer 69 is provided with conductivity by being formed using a conductive coating material, the coating layer 69 is prevented from obstructing the application of charge to the paper 3. be able to. Therefore, the paper 3 can be charged more efficiently.
In addition, since the base 73 of the charge imparting member 60 is formed using a metal, it is applied to the charge imparting member 60 as compared with a case where the base 73 is formed using a conductive resin having an electrical resistivity higher than that of the metal. The bias voltage to be reduced can be reduced, and the power consumption of the apparatus can be kept low.

  Furthermore, the upper surface 63 of the charge applying member 60 is wider than the maximum image forming area A on the paper 3 in the width direction, and wider than the maximum size paper 3 usable in the color laser printer 1. Therefore, not only the entire area of the maximum image forming area A on the sheet 3 but also the entire surface of the maximum size sheet 3 can be made to face the upper surface 63 of the charge applying member 60, and the polarity of the toner image is opposite to that of the toner image. It can be charged evenly. As a result, electrostatic offset of toner from the sheet 3 to the heating roller 48 can be prevented even better.

  In addition, since the upper surface 63 of the charge applying member 60 intersects the extended surface 70, the end (front end) on the downstream side in the transport direction of the sheet 3 fed from the support surface 45 of the transport belt 38 is connected to the charge applying member 60. The upper surface 63 can be reliably brought into contact. Therefore, the sheet 3 can be reliably brought into contact with the upper surface 63 of the charge applying member 60 from the front end to the rear end. In addition, since the bias is applied to the entire upper surface 63 of the charge applying member 60, the entire surface of the sheet 3 can be uniformly charged with a polarity opposite to that of the toner image. As a result, electrostatic offset of toner from the sheet 3 to the heating roller 48 can be reliably prevented.

Furthermore, by operating the bias change key 68, the magnitude of the bias applied to the charge applying member 60 is changed according to the type of the paper 3 (for example, the thickness and quality of the paper) and / or the environment (for example, temperature and humidity). ) And so on. For example, when the paper 3 is thick paper (thick paper than copy paper), the bias applied to the charge applying member 60 is relatively increased, and the paper 3 is thin paper such as copy paper. In this case, the magnitude of the bias applied to the charge applying member 60 can be made relatively small. Therefore, the sheet 3 can be more reliably charged. As a result, electrostatic offset of toner from the paper 3 to the heating roller 48 can be prevented more reliably.
<Modification>
In the above embodiment, the magnitude of the bias applied to the charge applying member 60 can be changed by operating the bias change key 68. For example, the color laser printer 1 is used in a personal computer. When connected, the magnitude of the bias applied to the charge applying member 60 is set based on the paper type information (setting by the printer driver) included in the print command transmitted from the personal computer. You may do it. Further, a temperature sensor and / or a humidity sensor are provided inside the color laser printer 1, and the magnitude of the bias applied to the charge applying member 60 is determined based on the temperature and / or humidity detected by the sensors. It may be set.

  In the above-described embodiment, an example in which the coating layer 69 is formed using a conductive coating material has been described. However, the coating layer 69 may be, for example, PFA (perfluoroalkoxy) or PTFE (polytetrafluoroethylene). And the like may be formed by coating using an insulating coating material. In this case, since the coating layer 69 has an insulating property, a part of the sheet 3 that is in contact with the upper surface 63 of the coating layer 69 and the sheet 3 are partially lifted with respect to the upper surface 63 of the coating layer 69 and the like. Even if a non-contact portion occurs, there is no possibility that charges are concentrated on the portion of the paper 3 that contacts the upper surface 63 of the coating layer 69. Therefore, the paper 3 can be charged with good uniformity.

  Furthermore, in the above embodiment, the direct tandem color laser printer 1 has been described. However, the present invention may be applied to a color laser printer that forms a color image on a sheet by a method other than the direct tandem method. The present invention is not limited to a color laser printer, and may be applied to a monochrome laser printer that forms a monochrome image on paper.

1 is a side sectional view showing an embodiment of a color laser printer as an image forming apparatus of the present invention. FIG. 2 is a plan view of a transfer unit and a fixing unit illustrated in FIG. 1. FIG. 2 is a side sectional view of a transfer unit and a fixing unit shown in FIG. 1. It is a sectional side view of the vicinity of the electric charge provision member shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Color laser printer 3 Paper 18 Transfer part 19 Fixing part 25 Photosensitive drum 38 Conveying belt 45 Support surface 48 Heating roller 49 Pressure roller 60 Charge application member 63 Upper surface 64 Bias terminal 67 Bias circuit 68 Bias change key 69 Coating layer 70 Extension surface A Maximum image forming area

Claims (12)

In an image forming apparatus that conveys a recording medium and forms an image on the recording medium,
An image carrier for carrying a developer image;
Transfer means for transferring a developer image carried on the image carrier to the recording medium;
A fixing unit disposed on the downstream side in the conveyance direction of the recording medium with respect to the transfer unit, and fixing the developer image transferred to the recording medium to the recording medium;
A charge imparting member disposed between the transfer unit and the fixing unit and having a smooth surface facing a surface opposite to a surface on which the developer image on the recording medium is transferred;
An image forming apparatus comprising: a bias applying unit that applies a bias having a polarity opposite to that of the developer image transferred to the recording medium to the charge applying member.   The image forming apparatus according to claim 1, wherein the charge applying member guides the recording medium conveyed toward the fixing unit with the smooth surface. The charge imparting member comprises a conductive substrate and a coating layer formed on the surface of the substrate and having the smooth surface.
The image forming apparatus according to claim 1, wherein the bias applying unit applies a bias to the substrate.   The image forming apparatus according to claim 3, wherein the coating layer is formed by a coating process using a conductive coating material.   The image forming apparatus according to claim 3, wherein the coating layer is formed by a coating process using an insulating coating material.   The image forming apparatus according to claim 3, wherein the base is made of metal.   The smooth surface is formed wider than a maximum image that can be formed on the recording medium in a direction perpendicular to the conveyance direction of the recording medium and along the recording medium. The image forming apparatus according to any one of 1 to 6.   The smooth surface is formed wider than a recording medium of a maximum size that can be transported in the image forming apparatus in a direction perpendicular to the transport direction of the recording medium and along the recording medium. The image forming apparatus according to claim 7. A support surface for supporting the recording medium, and a belt for conveying the recording medium toward the fixing unit;
The image forming apparatus according to claim 1, wherein the charge imparting member is disposed between the belt and the fixing unit.   The image forming apparatus according to claim 9, wherein the smooth surface intersects an extended surface that extends from the support surface along the support surface in a conveyance direction of the recording medium.   The bias from the bias applying unit is applied to at least a portion of the smooth surface that is in contact with an end portion on the downstream side in the conveyance direction of the recording medium conveyed by the belt. The image forming apparatus described in 1. The image forming apparatus according to claim 1, wherein the bias applying unit is configured to be able to change a magnitude of a bias to be applied to the charge applying member.

Images