JP4466295B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms a toner image by selectively attaching toner to a latent image due to a difference in electrostatic potential, transfers the toner image to a recording sheet, and then heats and pressurizes to form a fixed image. It is.

  High-definition images can be obtained by an image forming apparatus using powdered toner, and an image close to a photograph can be output. In such a high-definition image, the demand for gloss is also high, and a high-gloss image is often required. For this reason, a so-called coated paper having a glossy coating layer formed on the surface is used as a recording sheet. After the toner image is transferred to the recording sheet, the toner image is sufficiently heated by a fixing device to obtain a highly glossy toner image. Has been made to form.

  However, when a high-gloss image is formed in this way, a portion where the gloss is uneven, that is, uneven gloss is likely to occur, and the quality of the image may be deteriorated. A typical example of uneven gloss is a so-called roll mark. As shown in FIG. 6, the toner image on the recording sheet P heated and pressed by the fixing device 101 is sandwiched between a pair of rollers 102 for transporting the recording sheet in a state where it is not sufficiently cooled. This occurs when the peripheral surface of the roller contacts the toner image. Many of the roll marks 103 thus generated reduce gloss.

Techniques for reducing such roll marks are disclosed in, for example, Patent Document 1 and Patent Document 2.
The apparatus described in Patent Document 1 blows cooling air to the recording sheet that has passed through the fixing device, and lowers the temperature of the toner image until the recording sheet comes into contact with a conveyance roll provided on the downstream side of the fixing device. It is what.

Further, in the image forming apparatus described in Patent Document 2, leveling means is provided in the area where the toner is in a soft state on the downstream side of the fixing device so that the toner image is pressed against and smoothed. Yes. As a result, even if a roll mark is formed in contact with the paper discharge roll, the surface of the toner image is smoothed by the leveling means and then the roll mark is to be erased. In addition, a leveling means is provided upstream from the paper discharge roll to smooth the surface of the toner image, and contact with the paper discharge roll in a state of being cooled below the glass transition temperature of the toner, thereby generating a roll mark. It has also been proposed to prevent.
Japanese Patent Laid-Open No. 2003-21978 Japanese Patent Laid-Open No. 2003-195663

  In recent years, in order to realize a high gloss color image similar to a photograph or printing, it has been proposed to use a toner having a good melting property encapsulating wax. For example, a high-gloss image can be obtained by transferring such toner onto a coated paper and fixing it while slowing the fixing speed and sufficiently heating. Even in a state where the toner is in high temperature due to contact with the heating roll or the heating belt in the fixing device, the releasability from the heating roll or the like is ensured by the wax.

  However, when a toner containing wax is used to improve the releasability as described above, remarkable gloss unevenness that does not occur when a conventional toner is used may occur. That is, as shown in FIG. 6, when the recording sheet heated and pressed by the fixing device and having the toner image fixed thereon comes into contact with the recording sheet conveyance roll before being sufficiently cooled, the contacted portion becomes highly glossy. This causes uneven gloss. The gloss unevenness is different from the roll mark described in Patent Document 1 in which the gloss of an image using a normal toner is reduced, and is more noticeable than the roll mark in which the gloss is reduced. Noticeable gloss unevenness.

  It has been found that this uneven gloss does not occur if the conveyance roll or the like is not brought into contact until the toner image and the recording sheet are sufficiently cooled. When a recording sheet is used, the cooling rate is slow, and it may be difficult to sufficiently cool the sheet until it contacts the transport roll. Also, it is difficult to reduce the size of a device having a sufficient cooling function.

  The present invention has been made to solve the above-described problems, and its purpose is to reduce gloss unevenness of a toner image using a toner containing a wax for improving releasability, It is to obtain a good image with high gloss.

In order to solve the above-mentioned problems, the invention according to claim 1 includes an image forming unit that forms a toner image by selectively attaching powdery toner to a latent image due to a difference in electrostatic potential, and the toner. Transfer means for transferring an image directly to a recording sheet or to a recording sheet via an intermediate transfer member, and a fixing device for fixing the toner image transferred on the recording sheet by heating and pressurizing the recording sheet The toner includes a resin binder, a color material, and a wax that improves releasability, and contacts the recording sheet on the downstream side of the fixing device. The sheet conveying member to be conveyed is an endless belt that is stretched around a plurality of rolls and is driven around. The recording sheet is placed with the surface on which the toner image is fixed on the upper side, and is conveyed by a circular movement. What to do Ri, the surface layer of the belt or the belt may provide an image forming apparatus thermal conductivity is formed at a lower material.

  It became clear in the process of completing the present invention that the wax contained in the toner is rapidly cooled from the molten state and the wax crystallizes and becomes highly glossy when the temperature falls below the melting point. The present invention has been made based on this fact, and the sheet conveying member that contacts the recording sheet on the downstream side of the fixing device is formed of a material having low thermal conductivity. Little heat transfer from the toner image occurs. For this reason, the temperature of the toner image heated and melted by the fixing device and maintained at a temperature higher than the melting point of the wax hardly decreases, and crystallization due to the rapid cooling of the wax does not occur. Accordingly, it is possible to avoid the contact portion with the sheet conveying member from becoming significantly high gloss and to effectively suppress the occurrence of uneven gloss.

  In this image forming apparatus, the recording sheet discharged from the fixing device is carried by an endless belt that is driven to rotate. At this time, the recording sheet contacts the endless belt, but heat transfer to the endless belt is small, and the temperature of the toner image changes only slightly. For this reason, the toner image does not become remarkably highly glossy due to the rapid cooling of the toner image, and even if the recording sheet and the endless belt are not in uniform contact, the occurrence of noticeable gloss unevenness is suppressed.

The invention according to claim 2, in the image forming apparatus according to claim 1, wherein the belt has a plurality of through to the small holes, and suction from the back side of the belt, the negative pressure on the surface It is assumed that the recording sheet is held.

  In this image forming apparatus, the recording sheet is pressed in a belt shape by negative pressure and conveyed. Thus, even if the recording sheet and the belt are brought into close contact with each other by negative pressure, the heat transfer between them is small. Therefore, even if the close contact portion does not extend over the entire area of the recording sheet, this portion does not become remarkably high gloss and gloss unevenness does not occur.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the sheet conveying member grips and conveys an edge of the recording sheet, and contacts the recording sheet at a gripped portion. The portion to be formed is formed of a material having low thermal conductivity.

  In this apparatus, the edge of the recording sheet is gripped by the sheet conveying member, but there is little heat transfer at the contact portion, and the toner in a state where the wax is melted is not rapidly cooled. Therefore, this portion is prevented from becoming a noticeably high gloss.

The invention according to claim 4 is the image forming apparatus according to any one of claims 1 to 3 , wherein the material having low thermal conductivity has a thermal conductivity of 0.05 w / (m · ° C). Assume that:

  When the thermal conductivity is 0.05 w / (m · ° C.) or less, even if it is in close contact with the recording sheet or toner image, heat transfer is not generated to the extent that they are rapidly cooled. Therefore, even when the temperature of the toner image is equal to or higher than the melting point of the wax, crystallization due to the rapid cooling of the wax does not occur when contacted.

  As described above, in the image forming apparatus according to the present invention, the sheet conveying member that comes into contact with the recording sheet after passing through the fixing device is formed of a material having low thermal conductivity and is heated to have a melting point higher than that of the wax. Even when the toner image is touched, the amount of heat transfer is small and no rapid cooling occurs. Therefore, it is possible to suppress the occurrence of gloss unevenness in which the portion in contact with the sheet conveying member becomes significantly high gloss.

Hereinafter, embodiments of the invention according to the present application will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram showing, as a reference, an image forming apparatus that suppresses uneven gloss of a fixed image as in the present invention . FIG. 2 is a schematic cross-sectional view showing the fixing device and a transport roll provided on the downstream side thereof.
The image forming apparatus includes a cylindrical photosensitive drum 1 on which a latent image is formed on the surface by irradiation with image light after uniform charging, and around the photosensitive drum 1. A charging device 2 for uniformly charging the surface of the photosensitive drum 1, an exposure device 3 for irradiating the photosensitive drum 1 with image light to form a latent image on the surface, and a toner on the latent image on the photosensitive drum. Is transferred to the developing unit 4 to form a toner image, the endless belt-shaped intermediate transfer member 5 that is opposed to the photosensitive drum 1 and is supported so as to be able to circulate on the peripheral surface, and after the toner image is transferred. A cleaning device 6 that removes toner remaining on the photosensitive drum 1 and a static elimination exposure device 7 that neutralizes the surface of the photosensitive drum 1 are provided.

Further, inside the intermediate transfer member 5, a transfer charger 8 for primarily transferring the toner image formed on the photosensitive drum to the intermediate transfer member 5, two support rolls 9a and 9b, and secondary transfer are performed. A transfer counter roll 10 for carrying out is disposed, and the intermediate transfer body 5 is stretched around these rolls so as to be able to go around. A transfer roll 11 for transferring the toner image on the intermediate transfer body to a recording sheet is disposed at a position facing the transfer counter roll 10 via the intermediate transfer body 5. The recording sheet P is fed from a paper tray (not shown) to the pressure contact portion. A fixing device 12 that heats and melts the toner image on the recording sheet and press-bonds it to the recording sheet P is provided on the downstream side of the press contact portion, and the recording sheet that has passed through the fixing device 12 is provided on the downstream side. A guide member 14 that leads to the paper discharge tray 13 is provided. Then, along the guide member 14, conveyance rolls 15 and 16 (sheet conveyance members) convey the recording sheet.

  The photosensitive drum 1 has a metal drum surface with various inorganic photosensitive materials such as Se, a-Si, a-SiC, and Cds, organic photosensitive materials, amorphous selenium photosensitive materials, amorphous silicon photosensitive materials, and the like. What formed the layer can be used.

  The charging device 2 is obtained by coating a metal roll having conductivity such as stainless steel or aluminum with a coating of a high resistance material. The charging device 2 is in contact with the photosensitive drum 1 and is driven to rotate. When a predetermined voltage is applied, a continuous discharge is generated in a minute gap in the vicinity of the contact portion between the roll and the photosensitive drum 1, and the surface of the photosensitive drum 1 is charged almost uniformly. It is.

  The exposure device 3 generates blinking laser light based on the image signal, and scans this in the main scanning direction of the photosensitive drum 1 by a polygon mirror. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 1.

In the developing unit 4, four developing devices 4a to 4d containing yellow, magenta, cyan, and black toners are rotatably supported, and the developing devices 4a to 4d are in close proximity to the photosensitive drum 1 in order. Oppositely, toner is transferred to a latent image corresponding to each color to form a visible image.
The toner used in the developing device is mixed with a magnetic carrier, and is charged negatively mainly by friction with the magnetic carrier, and contains a coloring material, a binder resin, and paraffin wax. The melting point of the binder resin is about 250 ° C., the glass transition temperature is about 100 ° C., and the melting point of the paraffin wax is about 90 ° C.

  The belt-like intermediate transfer member 5 has a two-layer structure of a base layer and a surface layer, and the base layer includes polyimide, polyamide, polyamideimide, etc. in consideration of heat resistance, strength, and surface smoothness. The surface layer is made of silicone rubber, fluorine rubber, or the like which has high heat resistance and high releasability from the toner.

  The transfer charger 8 applies a voltage to the back surface of the intermediate transfer member 5 by applying a voltage so that the toner image on the photosensitive drum 1 is efficiently transferred to the intermediate transfer member 5. The transfer roll 11 is made of a conductive or semiconductive roll-shaped member, and a toner image on the intermediate transfer member 5 is collectively applied to the recording sheet P by applying a voltage between the transfer opposing roll 10. Transcript.

  The fixing device 12 includes a heating roll 12a with a built-in heating source and a pressure roll 12b pressed against the heating roll 12a, and these are arranged in parallel to form a nip portion that is pressed against each other. ing. The recording sheet P to which the toner image has been transferred is fed into the nip portion, and is heated and pressed between the heated roll 12a and the pressure roll 12b that are rotationally driven. It is to be crimped.

  The heating roll 12a is formed by forming an elastic body layer such as heat-resistant rubber on the peripheral surface of a metal core, and forming a release layer on the surface thereof. A halogen heater is incorporated as a heating source. ing. The pressure roll 12b is formed by laminating and covering an elastic body layer and a surface release layer on a metal core as in the case of the heating roll 12a.

As shown in FIG. 2, the conveying rolls 15 and 16 provided as the sheet conveying members are provided so as to face both sides of the recording sheet conveying path, and are respectively rotated around the central axis by metal shafts 20 and 21. It is supported so that it can rotate. The transport rolls 15 and 16 are a plurality of synthetic resin rolls having a small width in the width direction of the transport path, spaced apart from each other, and are respectively paired with metal shafts 20a and 20b and 21a and 21b. The supported conveying rolls 15a, 15b and 16a, 16b are pressed against each other and rotated to sandwich the recording sheet and convey it.
Surface layers 15c and 16c made of polystyrene foam, urethane foam or phenol foam are formed on the peripheral surfaces of the transport rolls 15 and 16 with a thickness of about 2 to 10 mm. The thermal conductivity of the surface layers is It is 0.05 w / (m · ° C.) or less.

  The guide member 14 is a plate-like member disposed along the recording sheet conveyance path, and regulates the recording sheet conveyance direction on both sides of the conveyance path. These guide members 14 are arranged while a plurality of the conveying rolls 15 and 16 are arranged in the width direction, and are continuously provided in the conveying direction beyond the position where the conveying rolls 15 and 16 are arranged. ing. The transport rollers 15 and 16 sandwich the recording sheet between the guide members 14 and transport the recording sheet by rotational driving.

A coating layer made of phenol resin or silicon resin is formed on the surface of the guide member 14 facing the recording sheet, and the thermal conductivity is 0.2 w / (m · ° C.) or less. Yes. As a result, heat transfer from the recording sheet in contact with the guide member 14 to the guide member 14 is reduced, so that the toner image on the recording sheet is not rapidly cooled.
The guide member may have a main portion formed of a material having low thermal conductivity used as the coating layer.

In such an image forming apparatus, the toner image is formed by the following process.
First, the surface of the photosensitive drum 1 is charged almost uniformly by the charging device 2, and subsequently, image light is irradiated from the exposure device 3 to form a latent image on the surface of the photosensitive member 1 due to the difference in electrostatic potential. . Then, the rotation of the photosensitive drum 1 moves to a position facing one developing device 4a of the developing unit 4, and the toner of the first color is transferred from the developing device 4a to form a toner image. This toner image is conveyed to a position facing the intermediate transfer member 5 by the circumferential movement of the photosensitive drum 1 and is electrostatically primary-transferred onto the intermediate transfer member. On the other hand, the toner remaining on the photosensitive drum 1 after the primary transfer is removed by the cleaning device 6, and the surface of the photosensitive drum 1 is potentialally initialized by the static elimination exposure device 7, and again at a position facing the charging device 2. Moving.

  Thereafter, the three developing devices 4b, 4c, and 4d of the developing unit 4 sequentially move to positions facing the photosensitive drum 1, and similarly, second, third, and fourth color toner images are sequentially formed, and intermediate transfer is performed. It is transferred onto the body 5 in a superimposed manner. At this time, the transfer roll 11 for secondary transfer is retracted to a position where it does not come into contact with the intermediate transfer member 5.

  The toner images superimposed on the intermediate transfer member 5 are conveyed to a position where the transfer roller 11 and the transfer counter roller 10 face each other by the circumferential movement of the intermediate transfer member 5, and come into contact with the recording sheet P fed from the paper tray. Is done. A transfer bias voltage is applied between the transfer roll 11 and the intermediate transfer member 5, and the toner image is secondarily transferred electrostatically onto the recording sheet. Then, the recording sheet P to which the toner image is transferred is conveyed to the fixing device 12.

  The fixing device 12 is in a standby state in which the image forming apparatus is turned on and preheated, and the heating roll 12a is heated to a predetermined temperature, and is further heated with the start of the image forming operation. When the recording sheet is conveyed, the temperature is sufficient for fixing the toner image. Then, the recording sheet P guided to the nip portion with the pressure roll 12b is heated and pressurized, and the molten toner is pressed onto the recording sheet to form a fixed image.

  The recording sheet sent out from the fixing device 12 is sandwiched between transport rollers 15a and 15b and 16a and 16b on the downstream side and transported. Since a coating layer having a low thermal conductivity is formed on the portions of the conveying rolls 15 and 16 that come into contact with the recording sheet, even if the recording sheet is sandwiched between the conveying rolls 15 and 16, Hardly moves to the transport rollers 15 and 16, and the temperature of the toner image hardly changes. Therefore, it is possible to prevent the toner image from being rapidly cooled from the melting point of the wax contained in the toner to the melting point or less due to the contact with the conveying rolls 15 and 16. Therefore, occurrence of uneven gloss due to contact with the transport roll is prevented.

  Further, the guide member 14 that regulates the conveyance direction of the recording sheet P in the conveyance path is also covered with a material having a low thermal conductivity in the vicinity of the surface, so that the toner image that has become a high temperature after passing through the fixing device 12. Alternatively, even when the recording sheet comes into contact with the guide member 14, the heat transfer is small and the recording sheet is not rapidly cooled. Therefore, gloss unevenness that becomes significantly high gloss even by contact with the guide member 14 does not occur.

  The recording sheets conveyed by the conveying rollers 15 and 16 and discharged to the paper discharge tray 13 are uniformly and slowly cooled in the paper discharge tray, and a good image can be obtained without causing uneven gloss.

  Further, as shown in FIG. 3, in an image forming apparatus having a function of forming an image on both sides of one recording sheet, the gate 17 in contact with the recording sheet is also covered with a material having low thermal conductivity. Is good. The double-sided conveyance path 18 that reverses the conveyance direction and sends the recording sheet upstream of the secondary transfer portion where the transfer roll 11 faces the intermediate transfer member 5 also has a temperature of the toner image that has been fixed earlier than the melting point of the wax. In the formed area, it is assumed that the transport roll and the guide member that are in contact with the recording sheet are covered with a material having low thermal conductivity.

FIG. 4 is a schematic configuration diagram showing an image forming apparatus which is an embodiment of the invention according to the present application .
In this image forming apparatus, a sheet conveyance member is a conveyance belt 34 that is stretched around two rolls 35 and 36 and moves around, and a recording sheet on which a toner image is fixed is carried on the conveyance belt 34. It is to be transported.
This conveyor belt is formed of a base material made of heat-resistant synthetic resin such as polyimide, and a coating layer made of urethane foam or phenol foam having low thermal conductivity is formed on the outer peripheral surface thereof. The conveyor belt 34 is formed with a large number of small holes distributed almost uniformly, and an air suction device 37 is provided on the back side of the belt. Thereby, the air of the outer peripheral part of a conveyance belt is attracted | sucked through the said small hole, and a negative pressure is formed on the outer peripheral surface of a conveyance belt.
Other configurations are the same as those of the image forming apparatus shown in FIG.
In addition, the said conveyance belt may be formed as a whole with the material with low heat conductivity corresponding to the said coating layer.

  In this image forming apparatus, the recording sheet on which the toner image has been fixed by the fixing device 12 is placed on the transport belt 34 and pressed onto the transport belt 34 by negative pressure by the air suction device 37. Thereby, it hold | maintains firmly and is conveyed. At this time, the recording sheet is in close contact with the conveyance belt 34, but a coating layer having a low thermal conductivity is formed on the surface of the conveyance belt 34, so that rapid cooling due to heat transfer to the conveyance belt 34 does not occur. Accordingly, gloss unevenness that becomes significantly high gloss by rapid cooling is prevented.

FIG. 5 is a schematic configuration diagram showing an image forming apparatus according to another embodiment of the present invention .
As in the image forming apparatus shown in FIG. 4, this image forming apparatus is a transport belt 44 in which a sheet transport member provided on the downstream side of the fixing device 12 is stretched between two rolls 45 and 46. The conveying belt 44 is provided with a gripper 47 for gripping the leading end portion of the recording sheet. The gripper 47 holds the recording sheet on the belt and grips the leading end portion to move around.

  The transport belt 44 also has a coating layer of a low heat conductive material on the surface thereof, similar to that used in the image forming apparatus shown in FIG. 4, so as to prevent the toner image from being rapidly cooled. In addition, the gripper 47 is covered with the same low thermal conductivity material at the portion that contacts the leading end of the recording sheet, so that the portion that contacts the gripper 47 at the leading end is prevented from becoming extremely glossy. .

  In this image forming apparatus, contact with the conveyance belt 44 and rapid cooling of the toner image due to the gripping of the front end portion by the gripper 47 can be prevented, and the portion in contact with the conveyance belt 44 and the gripper 47 is remarkably highly glossy. Thus, uneven gloss can be prevented.

FIG. 2 is a schematic configuration diagram showing, as a reference, an image forming apparatus that suppresses uneven gloss of a fixed image as in the present invention . FIG. 2 is a schematic configuration diagram illustrating a fixing device of the image forming apparatus illustrated in FIG. 1 and a transport roll provided on the downstream side thereof. It is a schematic block diagram of the apparatus which added the function which forms an image on both surfaces to the image forming apparatus shown in FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention . It is a schematic block diagram which shows the image forming apparatus which is other embodiment of the invention which concerns on this application . FIG. 10 is a schematic perspective view for explaining a problem in a conventional image forming apparatus, and shows a fixing device, a transport roll provided on the downstream side thereof, and a roll mark generated on a recording sheet.

1: photosensitive drum, 2: charging device, 3: exposure device, 4: developing unit, 5: intermediate transfer member, 6: cleaning device, 7: static elimination exposure device, 8: transfer charger, 9: support roll, 10 : Transfer opposing roll, 11: Transfer roll, 12: Fixing device, 13: Paper discharge tray, 14: Guide member, 15, 16: Transport roll, 15c, 16c: Cover layer of transport roll, 17: Gate, 18: Both sides Transport path,
20, 21: Metal shaft,
34: Conveying belt, 35, 36: Roll for stretching the belt, 37: Air suction device,
44: Conveying belt, 45, 46: Roll for stretching the baltic, 47: Gripper

Claims (4)

Image forming means for selectively attaching powdery toner to a latent image due to a difference in electrostatic potential to form a toner image;
Transfer means for transferring the toner image directly to the recording sheet or to the recording sheet via an intermediate transfer member;
An image forming apparatus comprising: a fixing device that heats and pressurizes the toner image transferred onto the recording sheet and fixes the toner image on the recording sheet;
The toner includes a resin binder, a coloring material, and a wax that improves releasability,
The sheet conveying member that contacts and conveys the recording sheet on the downstream side of the fixing device is an endless belt that is stretched and driven around a plurality of rolls, and has a surface on which the toner image is fixed on the upper side. The recording sheet is placed and conveyed by a circular movement,
The image forming apparatus, wherein the belt or the surface layer of the belt is formed of a material having low thermal conductivity . The belt has a plurality of through to the small holes, the claims sucked from the rear side of the belt, by applying a negative pressure to the surface side, characterized in that for holding the recording sheet the image forming apparatus according to 1.   The sheet conveying member grips and conveys an edge of the recording sheet, and a portion of the gripping portion that contacts the recording sheet is formed of a material having low thermal conductivity. The image forming apparatus according to claim 1. The image forming apparatus according to claim 1, wherein the material having low thermal conductivity has a thermal conductivity of 0.05 w / (m · ° C.) or less.

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