JP4513619B2 - 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.

  An image forming apparatus using powdered toner can form a high-definition image and can output an image close to a photograph. 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 tends to occur, and the quality of the image may be deteriorated. A typical example of uneven luster 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 rolls 102 for conveying the recording sheet in a state where it is not sufficiently cooled. This occurs when the peripheral surface of the roll comes into contact with 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. This wax is used to improve the releasability from the heating member pressed against the toner image during fixing. Conventionally, oil was applied to the surface of a heating roll or the like in order to maintain releasability, but when heated to a high temperature to obtain a high gloss image, a large amount of oil was applied to maintain good releasability. I need. For this reason, streaks due to uneven application of oil occur, or the oil remains on the surface of the toner image and gets stuck on the hand when touching the fixed image. Further, there arises a problem that the surface of the image cannot be added with a pencil or the like. Using a toner containing wax can solve these problems, and further, an image using such toner is transferred onto a coated paper and fixed in a state where it is sufficiently heated at a low fixing speed. By performing the above, a high gloss image can be obtained. 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, noticeable 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 luster. This unevenness of gloss is different from a roll mark in which the gloss is lowered in an image using a normal toner as described in Patent Document 1, and is different from the roll mark in which the gloss is lowered. Easy and noticeable uneven brightness.

  It has been found that this uneven gloss does not occur if the conveying roll or the like is not brought into contact until the toner image and the recording sheet are sufficiently cooled. However, in order to obtain a high-gloss image, it is as thick as coated paper. 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. Further, it has not been clear to what extent the toner image and the recording sheet can be cooled to prevent uneven gloss where the portion in contact with the roll or the like becomes extremely glossy. For example, Patent Document 2 describes that the toner is cooled to the glass transition temperature of the toner and then brought into contact with a roll or the like. However, when toner containing wax is used, cooling is performed based on the glass transition temperature of the toner. However, the correlation with the occurrence of uneven gloss is not good.

  Further, even when the surface of the toner image is mechanically smoothed using a leveling member as described in Patent Document 2, when toner containing wax is used, gloss unevenness is stably eliminated. I can't. In particular, it is often impossible to eliminate the uneven luster in which the portion in contact with the roll has a significantly high gloss. Then, the leveling member is heated by the repeated contact of the leveling member with the toner image, and thereby the function of eliminating the uneven glossiness may change significantly.

  Further, when the toner containing wax is used, gloss unevenness in which the contact portion with the roll becomes extremely high gloss occurs even if the unevenness state on the surface of the toner image hardly changes. It is not a fundamental solution to make it mechanically smooth.

  The present invention has been made in order to solve the above-described problems, and the object thereof is to reduce uneven gloss 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 via an intermediate transfer member, and fixing for fixing the toner image transferred on the recording sheet by applying pressure and heating to the recording sheet The toner includes a resin binder, a color material, and a wax that improves releasability, and the recording sheet is disposed downstream of the fixing device. sandwiched between two roll-like member having a conveying roller pair for conveying the recording sheet, any of the roll-shaped members constituting the conveying roller pair comprises a uniform cross section in the width direction of the recording sheet The transport roll One or both of the roll-shaped members constituting the pair have an elastic layer formed of a synthetic resin or rubber foam material, and a surface coating layer is formed on the elastic layer. An image forming apparatus is provided.

  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 on the basis of this fact. The conveyance roll is brought into close contact with the entire surface of the toner image heated and pressurized by the fixing device and the wax is melted, and the cooling state of the toner image is changed to the image. Make the surface almost uniform. As a result, no matter how the temperature of the toner image heated by the fixing device changes, the entire image has a substantially uniform gloss, and uneven gloss is suppressed.

  In other words, even when the temperature of the toner image is rapidly cooled from the melting point of the wax to the melting point or less by contact with the transport roll, the whole is remarkably high gloss and does not cause uneven gloss. In addition, if the fixing device is continuously used, the conveyance roll is gradually heated or the ambient temperature rises, and the temperature of the toner image is not rapidly cooled below the melting point of the wax even when contacting the conveyance roll. Even in the state, there is no portion of the entire image that becomes highly glossy, and gloss unevenness does not occur.

  The transport roll pairs are pressed against each other by applying a force to both end portions, but when one of them has an elastic layer, the pressing force can be introduced almost uniformly in the axial direction. That is, although the roll axis is bent, at least one of the rolls has the elastic layer, so that the pressure contact force can be made substantially uniform in the roll axis direction. Therefore, it is possible to effectively suppress the occurrence of uneven gloss due to the difference in the pressure contact force.

By forming the elastic layer from a synthetic resin or rubber foam , a layer having a hardness of 30 degrees or less can be easily obtained. Moreover, by using a foam material, the heat insulation becomes good, and even when it comes into contact with the recording sheet, there is little change in the temperature of the transport roll. Accordingly, it is possible to suppress the change in the temperature of the transport roll due to the heat transfer from the recording sheet, and it is possible to suppress the occurrence of uneven gloss in the image on the recording sheet due to the uneven surface temperature of the transport roll.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the bubbles contained in the elastic layer are not communicated with each other independently .

  The invention according to claim 3 is the image forming apparatus according to claim 1 or 2, wherein both of the roll-shaped members constituting the pair of transport rolls have the elastic layer, and the roll-shaped members are The elastic layer is assumed to have substantially the same hardness.

  If the roll has an elastic layer, and its hardness is low and the amount of deformation is large, the position where both rolls are pressed against each other (nip position) tends to fluctuate. If the position of the pressure contact surface is not set correctly, the recording sheet to be fed may abut against the peripheral surface of the roll at a large angle, and may not be fed to the nip portion, resulting in a paper jam. It is also conceivable to damage the edge of the recording sheet by hitting the recording sheet against the peripheral surface of the roll. However, by setting the hardness of the elastic layer to be substantially the same for both rolls as described above, the adjustment of the nip position can be facilitated, and the above-described obstacles can be avoided.

  The invention according to claim 4 is the image forming apparatus according to claim 3, wherein the elastic layer has a hardness of 30 degrees (JIS A6301) or less.

By setting the hardness of the elastic layer to the above value, the pressure contact force acts almost uniformly in the axial direction of the roll, the amount of heat transferred from the recording sheet is also almost equal, and uneven glossiness occurs in the image on the recording sheet. Can be effectively prevented.
In addition, the lower limit of the hardness of the elastic layer is a value corresponding to the softest state that functions as a transport roll, and the hardness is about 20 degrees.

  As described above, in the image forming apparatus according to the present invention, the toner image is heated and pressed by the fixing device, fixed on the recording sheet, and then conveyed by the pair of conveying rolls. This pair of conveyance rolls is in contact with the surface on which the toner image is formed with the recording sheet sandwiched therebetween, but since the pair of conveyance rolls has a uniform cross section in the width direction of the recording sheet, the conveyance roll pair has almost the same surface. Pressed evenly. Therefore, heat transfer from the toner image to the pair of conveyance rolls can be generated almost evenly, and uneven gloss can be suppressed from occurring in the toner image. Further, since the conveying roll pair is provided with an elastic layer near the surface, the pressing force is almost equal in the axial direction, and uneven gloss due to the distribution of the pressing force can be prevented.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention.
The image forming apparatus includes four image forming units 10 a, 10 b, 10 c, and 10 d that form yellow, magenta, cyan, and black toner images, and has an endless belt shape so as to face these image forming units 10. The intermediate transfer member 11 is supported, and the peripheral surface moves around. On the downstream side of the position where the image forming unit faces the intermediate transfer body 11, a secondary transfer portion is formed in which the transfer roll 12 faces the intermediate transfer body 11. The recording sheet is fed through the conveyance path 14. A fixing device 15 that heats and pressurizes the toner image and presses the toner image onto the recording sheet is provided downstream of the secondary transfer portion in the recording sheet conveyance path, and the toner image is fixed further downstream. A conveying roll pair 16 is provided that conveys the recording sheet and feeds the recording sheet from a sheet discharge port 19 to a discharge tray 18 provided outside the housing 17 of the apparatus.

  Each of the image forming units 10a, 10b, 10c, and 10d has a photosensitive drum 1 (image carrier) on which an electrostatic latent image is formed, and around each photosensitive drum 1, A charging device 2 that charges the surface of the photosensitive drum substantially uniformly, a developing device 3 that forms a toner image by transferring toner to a latent image formed on the photosensitive drum, and a toner on the photosensitive drum 1 A transfer device 4 that primarily transfers an image onto the intermediate transfer member 11 and a cleaning device 5 that collects toner remaining on the photosensitive drum 1 after transfer are provided. An exposure device 6 for writing an electrostatic latent image by irradiating the uniformly charged photosensitive drum 1 with image light based on an image signal is provided.

  The photoconductive drum 1 is obtained by forming an organic photoconductive layer on the peripheral surface of a cylindrical member made of metal, and the metal portion is electrically grounded.

  The charging device 2 includes a roll-shaped member obtained by coating a stainless steel cylindrical cored bar with a medium-resistance rubber. The charging device 2 applies a voltage obtained by superimposing an AC voltage on a DC voltage to apply the surface of the photosensitive drum 1. It is charged to a desired voltage. The DC component Vh of the voltage applied to the charging device 2 is −600 [V], and the superimposed AC peak-to-peak value Vbpp is 1400 [V].

  The exposure device 6 generates a flashing laser beam based on an image signal, and scans this in the main scanning direction (axial direction) of each photosensitive drum 1 by a polygon mirror. As a result, electrostatic latent images corresponding to the images of the respective colors are formed on the surfaces of the respective photosensitive drums 1.

The developing device 3 uses a two-component developer containing toner and a magnetic carrier as a developer, and is magnetically attracted and conveyed to the developing roll 3a. Then, a layer having an appropriate thickness is formed on the developing roll 3 a by a regulating blade (not shown) and supplied to a position facing the photosensitive drum 1. A developing bias voltage Vd = −500 [V] is applied to the developing roll 3 a in order to transfer the toner to the electrostatic latent image on the photosensitive drum 1.
The toner is charged negatively mainly by friction with a 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 intermediate transfer body 11 disposed so as to face the image forming units 10a, 10b, 10c, and 10d is made of a resin film having a thickness of about 10 to 300 μm, and a polyimide film or the like is used. In addition, when the toner image is electrostatically transferred from the photosensitive drum 1 to the intermediate transfer member 11, a powder of a conductive material such as carbon black is mixed in the resin film so as not to disturb the image. The volume resistivity is adjusted to about 10 ¹⁰ Ωcm.

  A driving roll 21 and a counter roll 22 are arranged inside the intermediate transfer body 11, and the intermediate transfer body 11 is stretched around these and moves around in the direction of arrow A shown in the drawing. Yes.

  The transfer roll 12 is provided at a position facing the opposing roll 22 and is pressed against the opposing roll 22 via the intermediate transfer body 11. The transfer roll 12 is a roll formed by forming an outer peripheral portion of a metal core material with a conductive rubber material, and a bias voltage for transfer is applied between the opposite roll 22. .

  The fixing device 15 includes a heating roll 15a having a built-in heating source and a pressure roll 15b pressed against the heating roll 15a, and these are arranged in parallel to form a nip portion that is pressed against each other. ing. The recording sheet on which the toner image has been transferred is fed into the nip portion, and is heated and pressed between the heated roll 15a and the pressure roll 15b that are rotationally driven, and the softened toner is pressed onto the recording sheet. It is supposed to be.

  The heating roll 15a is formed by forming an elastic body layer such as a 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 15b is obtained by coating a metal cored bar with a surface release layer.

  As shown in FIG. 2, the pair of conveyance rolls 16 provided on the downstream side of the fixing device 15 is a pair of rolls having the same dimensions and the same cross-section provided so as to face both sides of the conveyance path of the recording sheet P. It consists of members 16-1 and 16-2, and is supported adjacent to the sheet discharge port 19 through which the recording sheet P is sent to the discharge tray 17. Any one of them is rotationally driven, and the recording sheet P is sandwiched and conveyed between the pair of conveying rolls 16.

  As shown in FIG. 3, each of the roll members 16-1 and 16-2 has an elastic layer 16b made of a flexible foam of synthetic resin formed on the outside of a cylindrical cored bar 16a. A surface layer 16c is formed on the surface. The surface layer 16c has a smooth peripheral surface, is heated and pressed against the soft toner layer, and the surface of the toner layer is finished smoothly. The elastic layer 16b is made of a foam of silicone resin or urethane resin, and it is desirable that the respective bubbles do not communicate with each other independently in order to improve heat insulation. The hardness of the foam is set to 30 degrees or less, and deforms to form a nip when pressed against each other. Even if the axial lines of both the roll members 16-1 and 16-2 are bent, the pressure acting on the nip has almost no difference in the axial direction, and almost equal pressure acts on the nip. .

Next, the operation of the image forming apparatus will be described.
Yellow, magenta, cyan, and black toner images are formed by the four image forming units 10a, 10b, 10c, and 10d provided to face the intermediate transfer member 11, respectively. The toner image is formed by the following process.
Each of the photosensitive drums 1 is charged almost uniformly by the charging device 2, and a laser beam that is turned on / off according to an image signal is emitted from the exposure device 6. As a result, the charge at the position irradiated with light is attenuated, and a latent image is formed on the photosensitive drum 1 due to the difference in electrostatic potential. The electrostatic latent image on each photosensitive drum 1 is developed by toner transfer at a position facing the developing device 3, and a toner image is formed on the photosensitive drum 1.

  The formed color toner images are transferred onto the intermediate transfer body 11 in a superimposed manner by the transfer device 4. As a result, a color image in which a plurality of color toner images are superimposed on the intermediate transfer body 11 is formed. The toner images of a plurality of colors formed on the intermediate transfer body 11 are transferred onto the recording sheet conveyed from the sheet tray 13 at a position where the intermediate transfer body 11 faces the transfer roll 12 and sent to the fixing device 15. . The heating roll 15a of the fixing device is heated to a temperature sufficient to soften the toner image. A recording sheet is sandwiched between the heating roll 15a and the pressure roll 15b, and the toner image is softened and recorded. Crimped onto the sheet.

  The recording sheet that has passed through the fixing device 15 is sent to the downstream conveying roll pair 16 and is sandwiched and conveyed between the pair of roll members 16-1 and 16-2. At this time, if the toner is heated by the fixing device 15 and maintained at a high temperature equal to or higher than the melting point (90 ° C.) of the wax, the toner image is rapidly cooled by contact with the conveying roll pair 16. However, the conveying roll pair 16 is in uniform contact with the entire area where the toner image is formed, and the wax is cooled almost uniformly over the entire area of the image surface, so that the gloss is uniformly high. Therefore, gloss unevenness does not occur in the toner image, and a high-quality toner image can be obtained.

  Further, due to continuous use of the image forming apparatus, heat from the heating source of the fixing device 15 stays above the fixing device, that is, around the path of conveyance from the fixing device 15 to the sheet discharge port 19, and the toner after fixing. There are cases where the temperature of the image does not decrease so much. Then, even after the temperature of the roll members 16-1 and 16-2 also rises and passes through the transport roll pair 16, the temperature of the toner image transitions to a state where it is not cooled below the melting point of the wax. However, even when the conveyance roll pair 16 is heated and the temperature changes in this way, when it comes into contact with the recording sheet P carrying the toner image, the state of heat transfer is almost the entire area where the toner image is formed. Evenly, the gloss of only a part does not change, and a uniform gloss image can be obtained.

  On the other hand, as shown in FIG. 3, when the recording sheet P is fed between the conveying roll pair 16, heat transfer from the roll members 16-1 and 16-2 to the recording sheet occurs, and the temperature of the roll member decreases. there is a possibility. Then, the temperature of the surfaces of the roll members 16-1 and 16-2 fluctuates between when the recording sheet is first contacted with the recording sheet and when the recording sheet is contacted next after one turn, as shown in FIG. The toner image on the recording sheet may have uneven glossiness between the area A that is contacted in the first turn of the roll members 16-1 and 16-2 and the area B that is contacted in the next turn. Conceivable. However, in the above image forming apparatus, the elastic layers of the roll members 16-1 and 16-2 are formed of a foam material having high heat insulating performance, so that the space between the recording sheet and the roll members 16-1 and 16-2 is reduced. Therefore, the heat transfer hardly occurs, and the temperature change of the roll members 16-1 and 16-2 can be suppressed to a slight extent. Therefore, the above-described gloss unevenness does not occur and a good image can be obtained.

  In the above embodiment, only the pair of conveyance rolls 16 are arranged in contact with the recording sheet from the fixing device 15 to the sheet discharge port 19. However, as shown in FIG. A pair of transport rolls 31 and 32 are arranged. At this time, the conveyance roll pair 31 that first contacts the recording sheet on the downstream side of the fixing device 15 and the conveyance roll pair that may have a toner image temperature higher than the melting point of the wax when contacted are described above. A pair of transport rolls having a uniform cross section as in the embodiment is used. Then, as shown in FIG. 5, the conveyance roll pair 32 provided at a position where the temperature of the toner image is below the melting point of the wax under any condition when it comes into contact with the toner image on the recording sheet, A conventionally used roller having a plurality of narrow rollers provided on a single shaft can be used.

1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic perspective view of a main part of the image forming apparatus shown in FIG. 1. FIG. 2 is a schematic cross-sectional view of a transport roll pair used in the image forming apparatus illustrated in FIG. 1. FIG. 2 is a schematic diagram of uneven gloss that is prevented from occurring in the image forming apparatus shown in FIG. 1. It is a schematic perspective view which shows the principal part of the image forming apparatus which is other embodiment of this invention. FIG. 6 is a schematic perspective view for explaining problems in a conventional image forming apparatus, and shows a fixing device, a transport roll provided on the downstream side, and a roll mark generated on a recording sheet.

1: Photosensitive drum, 2: Charging device, 3: Developing device, 3a: Developing roll, 4: Transfer device, 5: Cleaning device, 6: Exposure device, 10: Image forming unit, 11: Intermediate transfer member, 12: Transfer roll, 13: Sheet tray, 14: Recording sheet conveyance path, 15: Fixing device, 15a: Heating roll, 15b: Pressure roll, 16: Conveying roll pair, 16a: Core metal, 16b: Elastic layer, 16c: Surface layer, 17: housing of image forming apparatus main body, 18: paper discharge tray, 19: sheet discharge port,
21: Drive roll, 22: Opposing roll,
31: A pair of conveyance rolls having a uniform cross section, 32: A pair of conveyance rolls generally used from the past

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;
A fixing device that pressurizes and heats 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,
On the downstream side of the fixing device, the recording sheet is sandwiched between two roll-shaped members, and has a pair of conveying rolls for conveying the recording sheet,
Any of the roll-shaped members constituting the conveyance roll pair has a uniform cross section in the width direction of the recording sheet ,
Either one or both of the roll-shaped members constituting the transport roll pair have an elastic layer formed of a synthetic resin or rubber foam material,
An image forming apparatus , wherein a surface coating layer is formed on the elastic layer . The image forming apparatus according to claim 1, wherein the bubbles included in the elastic layer are each independently independent of each other .   The roll-shaped member which comprises the said conveyance roll pair has the said elastic layer, and the said elastic layer of these roll-shaped members is substantially the same hardness, The Claim 1 or Claim 2 characterized by the above-mentioned. Image forming apparatus.   The image forming apparatus according to claim 3, wherein the elastic layer has a hardness of 30 degrees (JIS A6301) or less.

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