JP2009020344A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image forming apparatus and an image forming method with which cracking on the surface of an image is suppressed to obtain a high-gloss image. <P>SOLUTION: The image forming apparatus includes a fixing means 8 heating and fixing a toner image formed on a recording medium 22, and a cooling means 9 rapidly cooling the fixed toner image. The cooling means 9 is equipped with a first cooling roller 91 coming into contact with the toner image and a second cooling roller 92 opposed to the first cooling roller 91 through the recording medium 22. The surface temperature of the first cooling roller 91 is controlled so as to be higher than the surface temperature of the second cooling roller 92. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method for forming a toner image on a recording medium by, for example, electrophotography and fixing the toner to form an image.

In an image forming apparatus based on electrophotography, for example, a toner image formed on a photosensitive member is transferred to an intermediate transfer member, and then transferred to a sheet (recording medium) passed between the intermediate transfer member and a transfer roller. Further, the toner image transferred by heating and melting the toner is heated and fixed on the recording medium.
Methods for heat-fixing a toner image on a recording medium are roughly classified into a contact method and a non-contact method. Heating roller fixing, belt fixing, film fixing and the like are known as contact methods. On the other hand, flash fixing and oven fixing are known as non-contact methods. Among them, the contact method using heating roller fixing is most generalized because the toner image and the heating roller are in direct contact with each other so that the thermal efficiency is high and the apparatus can be miniaturized.

However, in the contact method using heating roller fixing, an offset phenomenon is likely to occur in which molten toner remains on the heating roller when the recording medium passes through the heating roller.
Therefore, as a method of suppressing the offset phenomenon, a method of applying a release agent such as silicone oil having high releasability to the surface of the heating roller, or a method of kneading (dispersing) wax inside the toner and melting the wax during heating. A method for ensuring releasability by covering the toner surface has been proposed. In particular, it is possible to reduce the size of the device and reduce maintenance, and in recent years, the wax dispersion technology has been greatly improved in the production of toner by the polymerization method, so the oilless fixing method in which the wax is dispersed in the toner has become the mainstream. .

By the way, with the widespread use of color images, the demands on image quality for image forming apparatuses are diversified. For example, when creating posters and high-quality magazines, high-quality image quality (which has been difficult with conventional electrophotographic images ( For example, a highly glossy image is required.
However, when the heated toner (molten toner) is cooled to about room temperature, the resin contained in the toner contracts during the cooling process, and shrinkage marks may be formed on the surface of the image. When shrinkage marks are formed, the smoothness of the image surface is lowered, so that an image with low gloss can be obtained.

In recent years, since a highly glossy image can be obtained, there is an increasing demand for papers with reduced surface irregularities and coated papers coated with a material that has high thermal conductivity and fills the surface irregularities and smoothes them.
As a method for obtaining a highly glossy image, for example, in Patent Document 1, in the heating belt fixing method, a non-gloss mode for fixing at a fixing temperature and a gloss mode for fixing at a higher temperature than the non-gloss mode are selected and controlled. A possible image forming apparatus is disclosed.
Further, Patent Document 2 discloses an image forming apparatus including a wax melting point vicinity cooling unit that rapidly cools a toner image in the vicinity of the melting point of the wax included in the toner image. In the image forming apparatus, for example, the toner image is brought into contact with a cooling roller (metal roller) when the surface temperature of the toner image is 88 ° C. with a wax melting point of 80 to 85 ° C. as a reference, and the temperature is rapidly increased from 88 ° C. to 76 ° C. By cooling, the crystallization of the wax is suppressed, the white turbidity of the wax layer covering the image surface is suppressed, and the gloss is improved.
Patent Document 3 discloses an image forming method for rapidly cooling a toner image using a cooling device including a heat pipe, a driven roller, and a heat insulating material. According to the image forming method, the change in volume and area after fixing of the toner image is suppressed, and the roughness (graininess) of the image is improved.
Further, Patent Document 4 discloses an image forming apparatus including a toner image cooling blower having a plurality of air blowing holes between a fixing device and a conveyance roller. According to the image forming apparatus, air is blown to cool the toner image and the recording medium, thereby preventing the occurrence of a low gloss portion.
JP-A-4-195079 JP 2005-250335 A JP 2003-208047 A Japanese Patent Laid-Open No. 2003-21978

However, the image forming apparatus described in Patent Document 1 switches the gloss depending on whether the toner is completely melted or leaves the particle shape, and it is difficult to further increase the gloss after completely melting the toner. Met.
Also, the wax has a function of making the surface of the molten toner a mirror surface by coating it with a very thin surface, thereby improving the gloss. Since the gloss depends on the reflectance of the surface, even if the whitening of the wax layer is suppressed as described in Patent Document 2, it is not always sufficient to improve the gloss.

The image forming method described in Patent Document 3 is a method for preventing the dot image and the line image from being crushed when being nipped by a conveyance roller or the like with a high toner temperature. It was difficult to obtain a sufficient effect. Further, the volume change rate after fixing the toner image is defined as 30% and the area change rate as 20%. These values change depending on heating conditions, fixing pressure, toner loading amount, etc. It is not due to characteristics. Therefore, even if the volume change rate and the surface property change rate are defined, it is difficult to improve the surface properties of the toner such as gloss and gloss of the image.
In the image forming apparatus described in Patent Document 4, the fixing device adjacent to the air blower for cooling the toner image is also easily cooled by the air, so that energy consumption increases or the temperature in the axial direction of the heating roller provided in the fixing device varies. Sometimes occurred. Further, there is a possibility that warm air around the fixing device is scattered in the image forming apparatus, which may cause image deterioration due to a change in the sensitivity of the photosensitive member or a change in the charge amount of the developer.

  Further, when the toner image is cooled, cracks may occur on the image surface. In particular, when the amount of toner on the recording medium increases, the occurrence of cracks is significant.

  The present invention has been made in view of the above circumstances, and an object thereof is to realize an image forming apparatus and an image forming method capable of suppressing cracks generated on the image surface and obtaining a high-gloss image.

An image forming apparatus of the present invention is an image forming apparatus having a fixing unit that heats and fixes a toner image formed on a recording medium, and a cooling unit that rapidly cools the fixed toner image. The cooling means includes a first cooling roller that comes into contact with the toner image, and a second cooling roller that faces the first cooling roller via a recording medium, and the first cooling roller An image forming apparatus, wherein a surface temperature is controlled to be higher than a surface temperature of the second cooling roller.
In addition, the first cooling roller and the second cooling roller are controlled to be in a pressure contact and separation state, and the first cooling roller and the second cooling roller are in a pressure contact state in accordance with the passage of the recording medium, It is preferable to sandwich the recording medium.

The image forming method of the present invention is an image forming method in which a toner image formed on a recording medium is heated and fixed, and then the fixed toner image is rapidly cooled. The first cooling roller that is in contact with the toner image and the second cooling roller that opposes the first cooling roller through the recording medium are cooled, and the first cooling roller The surface temperature is controlled to be higher than the surface temperature of the second cooling roller.
Further, the first cooling roller and the second cooling roller are controlled to be in a pressure contact and separation state, and the first cooling roller and the second cooling roller are brought into a pressure contact state in accordance with the passage of the recording medium, It is preferable to sandwich the recording medium.

According to the image forming apparatus and the image forming method of the present invention, it is possible to obtain a high-quality image in which cracks generated on the image surface are suppressed and gloss is increased.
In addition, according to the present invention, a deep color material original vivid hue can be obtained, which is particularly suitable for forming a color image.
Further, when the toner image is rapidly cooled, the recording medium is also cooled, so that the temperature of the discharged paper can be lowered.

Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic configuration diagram showing an example of an image forming apparatus of the present invention. In the image forming apparatus 100 of this example, the image forming unit 10 is provided substantially at the center. The image forming unit 10 includes a photosensitive drum 1, and includes a charging unit 2, an exposing unit 3, a developing unit 4, a transfer unit 5, a cleaning blade 6, and a roller 7 disposed around the photosensitive drum 1. I have. A fixing unit 8 is disposed downstream of the photosensitive drum 1 in the recording medium (paper) conveyance direction, and a cooling unit 9 is disposed downstream of the fixing unit to cool the toner image. A sheet feeding unit 20 is provided at the lower part of the image forming apparatus 100, and a sheet feeding roller 21 is disposed downstream of the sheet feeding unit 20 in the sheet feeding direction. Further, a paper discharge unit 30 for discharging the recording medium 22 after image formation is provided.

The photosensitive drum 1 has an electrostatic latent image formed on the surface. As the photosensitive drum 1, it is preferable to use an amorphous silicon photosensitive member, and the constitution thereof is a carrier injection blocking layer made of Si: H: B: O or the like on a conductive substrate, and carrier excitation / transport made of Si: H or the like. A layer (photoconductive layer) and a surface protective layer made of SiC: H or the like are sequentially stacked.
The charging unit 2 is installed above the photosensitive drum 1 and uniformly charges the photosensitive drum 1.
The exposure unit 3 forms an electrostatic latent image on the photosensitive drum 1 based on a document image read from an image data input unit (not shown).

  The developing means 4 supplies toner to the surface of the photosensitive drum 1 on which the electrostatic latent image is formed to form a toner image. Here, the developing means 4 includes a rotary rack 40 and a plurality of developing devices 4Y, 4M, 4C, 4K. The rotary rack 40 rotates the plurality of developing devices 4Y, 4M, 4C, and 4K sequentially to the developing positions facing the photosensitive drum while performing rotation by rotating means (not shown) around the rotation shaft 41. Is. Among the plurality of developing devices, the yellow developing device 4Y, the magenta developing device 4M, the cyan developing device 4C, and the black developing device 4K are held side by side in the circumferential direction of the rotary rack 40 in the order of 4Y, 4M, 4C, and 4K. The developing devices are arranged at intervals of about 90 degrees in the circumferential direction.

  The transfer unit 5 transfers the toner image on the photosensitive drum 1 to a recording medium, and includes an intermediate transfer belt 51, primary transfer rollers 52 and 53, a driving roller 55, a secondary transfer counter roller 54, and a secondary transfer roller 56. It has. The intermediate transfer belt 51 is wound endlessly around the primary transfer rollers 52 and 53, the drive roller 55, and the secondary transfer counter roller 54, and is driven by the drive roller 55, and the toner image formed on the photosensitive drum 1. Plays the role of a transcript that is transferred and temporarily held. The secondary transfer roller 56 is disposed at a position facing the secondary transfer counter roller 54 on the outer peripheral surface of the intermediate transfer belt 51, and plays a role of secondary transfer of the toner image to the recording medium.

The cleaning blade 6 is for cleaning deposits such as residual developer remaining on the photosensitive drum 1, and for example, urethane rubber having a hardness of 77 ° is pressed against the photosensitive drum.
The roller 7 is in contact with the surface of the photosensitive drum 1 and has a buffer function for collecting and discharging toner. The roller 7 has a configuration in which a metal shaft is covered with foamed rubber, and is biased to the photosensitive drum 1 by a spring (not shown) at 9.8 N (one side: 4.9 N). Further, the roller 7 rotates with respect to the photosensitive drum 1 at a surface speed at the contact portion that is 1.2 times faster than the surface speed of the drum.

  As shown in FIG. 2, the fixing unit 8 includes a fixing roller (heating roller) 81 that is a rotatable fixing member, and a pressure roller 82 that is a pressure member that rotates while being in pressure contact with the fixing roller 81. Has been. A heater 83 such as a halogen lamp is disposed inside the fixing roller 81. A thermistor 84 is disposed in contact with the fixing roller 81, and the temperature of the surface of the fixing roller 81 is adjusted by controlling the voltage to the heater 83 through a temperature adjustment circuit. In this state, when the recording medium 22 is conveyed, the fixing roller 81 and the pressure roller 82 rotate at a constant speed, and the recording medium 22 passes through between the fixing roller 81 and the pressure roller 82. The unfixed toner image on the surface of the recording medium is melted and fixed by pressing and heating at a constant pressure and temperature from both sides, and a full-color image is formed on the recording medium 22. The recording medium 22 on which the image is fixed is separated 81 from the fixing roller by a separation claw (not shown) and is discharged outside the apparatus.

As the fixing roller 81 used in the fixing unit 8 as described above, for example, a surface of a metal tube such as an aluminum tube has a good releasability such as a fluorine-based resin, and has excellent heat resistance and wear resistance. Are preferably coated to form the outer layer. In the case where the image forming apparatus emphasizes image quality especially like an electrophotographic color copier, it is preferable to use a fixing roller 81 having silicone rubber as an outer layer of the roller. However, silicone rubber is slightly inferior to fluororesin, so it is desirable to apply silicone oil as a release agent to the surface of silicone rubber. In addition, you may use what wound the fluororesin sheet | seat around the silicone rubber.
On the other hand, the pressure roller 82 is not particularly limited. For example, it is preferable to use a silicone rubber roller having a solid iron bar or the like as a core.

As shown in FIG. 2, the cooling means 9 includes a first cooling roller 91 that is rotatably arranged, and a second cooling roller 92 that is arranged to face the first cooling roller 91. The toner image heated by the fixing unit 8 is rapidly cooled.
The first cooling roller 91 and the second cooling roller 92 are paired. The first cooling roller 91 directly contacts the toner image on the recording medium 22 to cool the toner image from the surface layer side of the toner image. To do. On the other hand, the second cooling roller 92 faces the first cooling roller 91 with the recording medium 22 interposed therebetween, and cools the toner image from the lower layer side of the toner image.
Here, the surface temperature of the first cooling roller 91 is controlled to be higher than the surface temperature of the second cooling roller 92.
Hereinafter, the first cooling roller 91 and the second cooling roller 92 may be collectively referred to as a “roller pair”.

When the heated toner image is cooled, the resin contained in the toner shrinks during the cooling process, and shrinkage marks are easily formed on the surface of the image. In particular, when the toner image is gradually cooled (slowly cooled), the resin tends to shrink. When the shrinkage marks are formed, the smoothness of the image surface is lowered, so that it is difficult to obtain a high gloss image.
Further, the toner image is cooled from both the surface layer side and the lower layer side through the recording medium. Usually, the thermal conductivity of the recording medium (paper) is smaller than that of the toner, so the lower layer side of the toner image is cooled. Takes time. Therefore, a difference occurs in the cooling rate between the surface layer side and the lower layer side of the toner image, and the cooling rate on the surface layer side that is directly cooled becomes faster than the cooling rate on the lower layer side that is cooled via the recording medium. As a result, a temperature difference occurs between the surface layer side and the lower layer side of the toner image (that is, the surface layer side that is directly cooled becomes lower in temperature than the lower layer side), and cracks are likely to occur on the image surface. In order to suppress the occurrence of cracks, the cooling rate of the toner image may be reduced. However, as described above, if the toner image is gradually cooled, the resin tends to shrink, and it becomes difficult to obtain a high-gloss image.

However, in the present invention, the shrinkage of the resin contained in the toner can be suppressed by rapidly cooling the toner image heated by the cooling means. Therefore, the smoothness of the image surface is improved, and a high gloss image can be obtained.
Further, the surface temperature of the first cooling roller 91 that directly contacts the toner image is controlled to be higher than the surface temperature of the second cooling roller 92 that contacts the toner image via the recording medium 22. The cooling rate on the lower layer side of the toner image cooled by the second cooling roller 92 via the recording medium 22 and the surface layer side cooled by the first cooling roller 91 are substantially the same. As a result, even if the toner image is cooled from both the surface layer side and the lower layer side of the toner image, a temperature difference between the surface layer side and the lower layer side is less likely to occur, and cracking can be suppressed. Therefore, since it is possible to suppress the occurrence of cracks on the image surface without reducing the cooling rate of the toner image more than necessary, it is possible to achieve both suppression of the occurrence of cracks and high gloss images.

Here, the first cooling roller 91 and the second cooling roller 92 will be described.
As shown in FIG. 3, each of the first cooling roller 91 and the second cooling roller 92 is extended to the outside of the sheet passing area, and one end of the first cooling roller 91, The end portions of the second cooling roller 92 opposite to the end portions are connected to the radiation fins 93 (radiation fins 93a and 93b), respectively. Fans 94 (fans 94 a and 94 b) are provided directly below the heat radiation fins 93. In particular, the second cooling roller 92 is cooled to a constant temperature via the radiation fins 93b because the fan 94b is always operating. Therefore, even when continuous paper is passed, the temperature of the surface of the second cooling roller 92 can be maintained at about atmospheric temperature + 3 ° C.

On the other hand, the first cooling roller 91 includes a heater 95 composed of a heat conducting wire, and a thermistor 96 for measuring the surface temperature of the first cooling roller 91. According to the surface temperature detected by the thermistor 96, ON / OFF of the fan 94a that blows air to the heat dissipating fins 93a at the end can be controlled. For example, when the surface temperature of the first cooling roller 91 detected by the thermistor 96 is lower than a predetermined temperature set to be higher than the surface temperature of the second cooling roller 92, the heater 95 is energized and the first cooling is performed. The roller 91 is heated up to a predetermined temperature. At this time, wind is not applied to the heat dissipating fins 93a connected to the end of the first cooling roller 91. In order to determine whether the heater 94 is energized, a thermistor may be attached to the second cooling roller 92 and the surface temperature of the second cooling roller 92 may be used as a reference.
On the other hand, when the surface temperature of the first cooling roller 91 is equal to or higher than the predetermined temperature, the heater 95 is not energized, and the fan 94a is driven to cool the heat radiating fins 93a to the predetermined temperature.

  With such a configuration, the temperature of the first cooling roller 91 can be adjusted to a predetermined temperature, and the surface temperature of the first cooling roller 91 is always higher than the surface temperature of the second cooling roller 92. To be controlled.

  The surface temperature of the first cooling roller 91 is preferably controlled to be 5 to 35 ° C. higher than the surface temperature of the second cooling roller 92, and more preferably 14 to 22 ° C. If the difference in surface temperature between the first cooling roller 91 and the second cooling roller 92 is less than 5 ° C., a difference occurs in the temperature between the surface layer side and the lower layer side of the toner image, and cracks may occur on the image surface. is there. On the other hand, if the difference in surface temperature between the first cooling roller 91 and the second cooling roller 92 exceeds 35 ° C., the generation of cracks can be suppressed, but the cooling rate of the toner image by the cooling roller 91a becomes slow. Tend to decrease.

  As such first cooling roller 91 and second cooling roller 92, for example, a heat pipe (made of copper) is preferably used. Since the heat pipe is excellent in heat transfer, the toner can be rapidly cooled by directly touching the toner image. However, since the adhesiveness with the toner is lowered, the cooling rate varies, and a non-uniform image is likely to occur. Therefore, for the purpose of ensuring releasability from the molten toner, a PFA (tetrafluoroethylene and perfluoroalkoxyethylene copolymer) sheet adhered to the surface of the heat pipe via a primer may be used. Good. The thickness of the PFA sheet is preferably about 30 μm when the outer diameter φ of the heat pipe is 20 mm. Furthermore, you may use what adhered the silicone rubber instead of the PFA sheet.

  The first cooling roller 91 and the second cooling roller 92 described above are in a separated state when the recording medium 22 is not passed, and are provided in the transfer unit 5 in accordance with the passage of the recording medium 22. At a timing calculated from a paper passing switch (not shown), the recording medium 22 is controlled to be in a press-contact state immediately before reaching the pair of rollers. The pressing force in the pressure contact state is usually 29.4 N (one side).

  As shown in FIGS. 3 and 4, the second cooling roller 92 can be moved up and down by being connected to a solenoid 97d via a push-up bar 97b and an arm 97c connected to each other by a connection bar 97a. The pressure contact state or the separation state can be selected. When the solenoid 97d is lowered in accordance with the passage of the recording medium 22, the end of the arm 97c on the side where the connecting rod 97a is installed rises with the fulcrum 97e as the center, and the second cooling roller 92 also rises in conjunction with it. Then, it is pressed against the first cooling roller 91 via the recording medium 22. When the recording medium 22 is sent out from the roller pair, the solenoid 97d is raised, the end of the arm 97c on the side where the connecting rod 97a is installed is lowered around the fulcrum 97e, and the second cooling roller 92 is also linked. It descends and is separated from the first cooling roller 91.

Usually, during the cooling of the toner image, the recording medium 22 is interposed between the first cooling roller 91 and the second cooling roller 92, so that heat does not easily move between the two. The surface temperature of the roller tends to be kept constant. On the other hand, since the recording medium 22 does not intervene while the toner image is not cooled, heat transfer is likely to occur between the first cooling roller 91 and the second cooling roller 92. When the heat transfer occurs, the surface temperature difference between the two becomes small, so that the surface temperature of the first cooling roller 91 is higher than the second cooling roller 92 in order to maintain the surface temperature difference. In order to keep the surface temperature constant, energy is consumed more than necessary.
However, the toner image is cooled by controlling the first cooling roller 91 and the second cooling roller 92 to be in a pressure contact state and a separated state in this manner (that is, by separating the toner image while the toner image is not cooled). Even during the time period, the movement of heat between the first cooling roller 91 and the second cooling roller 92 can be reduced. Therefore, energy consumption can be suppressed.

The roller pair is preferably arranged so that the distance d from the exit of the fixing nip portion to the entrance of the cooling nip portion is 10 to 50 mm.
Here, as shown in FIG. 2, the fixing nip portion is a contact portion between the fixing roller 81 and the pressure roller 82 in the conveyance direction of the recording medium 22, and the outlet of the fixing nip portion is fixing. It is the end portion 85 on the cooling means 9 side of the nip portion. On the other hand, the cooling nip portion is a contact portion between the first cooling roller 91 and the second cooling roller 92 in the conveyance direction of the recording medium 22, as shown in FIG. Means the end 98 on the fixing means 8 side of the cooling nip portion.

Also, the cooling means 9 can measure the surface temperature of the toner image on the recording medium 22 before and after passing between the first cooling roller 91 and the second cooling roller 92 as shown in FIG. The temperature sensor 99 may be provided before and after the first cooling roller 91.
As the temperature sensor 99, for example, a non-contact type thermometer can be used.

  In the present invention, since the toner image can be cooled without using air or the like by using the cooling roller pair as described above, the adjacent fixing unit 8 is hardly cooled. Accordingly, energy consumption can be increased, and variations in the axial temperature of the fixing roller 81 provided in the fixing unit 8 can be reduced. Further, since warm air around the fixing unit 8 is hardly scattered in the image forming apparatus, image deterioration due to a change in sensitivity of the photosensitive drum 1 or a change in the charge amount of the developer is less likely to occur.

Next, the image forming method of the present invention will be described using the image forming apparatus 100 of FIG.
First, after the photosensitive drum 1 is charged by the charging unit 2, the rotary rack 40 rotates around the rotary shaft 41 provided at the center thereof. Then, the rotary rack 40 stops at the developing position where the developing device 4K corresponding to the first color, black, faces the photosensitive drum 1. In this state, exposure corresponding to black is performed by the exposure unit 3, and an electrostatic latent image corresponding to black is formed on the surface of the photosensitive drum 1. The electrostatic latent image is converted into a toner image by the developing device 4K, and the toner image formed on the surface of the photosensitive drum 1 is transferred onto the transfer belt 51 by a transfer bias applied to the primary transfer rollers 52 and 53. .
When the formation of the black toner image on the transfer belt 51 is completed in this way, the rotary rack 40 rotates around the rotation shaft 41 provided at the center thereof, and the developing device 4C corresponding to cyan develops. Positioned to position. Such an operation is also performed for other colors, cyan, magenta, and yellow, and a full-color toner image is formed on the transfer belt 51.

  As described above, the secondary transfer roller 56 is separated from the transfer belt 51 in the process of primary transfer of the toner image to the intermediate transfer belt 51. On the other hand, when a full-color toner image is formed on the transfer belt 51, the secondary transfer roller 56 is brought into contact with the transfer belt 51. At this time, the transfer belt 51 is formed on the recording medium 22 conveyed from the paper supply unit 20 to the transfer position by the paper supply roller 21 or the like at the same timing by the secondary transfer bias applied to the secondary transfer roller 56. A full color toner image is transferred.

  Next, the full-color toner image transferred to the recording medium is fixed to the recording medium 22 by heating and pressing by the fixing unit 8 as shown in FIG. The heating temperature of the toner may be higher than the softening point of the toner, but is preferably 25 to 40 ° C. higher than the softening point of the toner.

The toner image heated by the fixing unit 8 is rapidly cooled by the cooling unit 9. When the toner image is rapidly cooled, it is preferable that the toner image is rapidly cooled from the high temperature side to the low temperature side from the softening point (that is, in the vicinity of the softening point of the toner). Specifically, the toner image is preferably cooled at a cooling rate of 150 to 250 ° C./second.
The surface temperature of the first cooling roller 91 may be controlled to be higher than the surface temperature of the second cooling roller 92, but is preferably controlled to 30 to 60 ° C. On the other hand, the second cooling roller 92 is preferably maintained at 20 to 30 ° C.

  Note that while the recording medium 22 does not pass the roller pair, the second cooling roller 92 is lowered and separated from the first cooling roller 91 (becomes separated). Then, when the recording medium 22 is conveyed to the cooling unit 9, the second cooling roller 92 rises in accordance with the passage of the recording medium 22 and is pressed against the first cooling roller 91 (becomes a pressed state). ).

The recording medium 22 having the toner image cooled by the cooling means 9 is discharged to the paper discharge unit 30 shown in FIG.
On the other hand, the residual developer remaining on the photosensitive drum 1 is cleaned by the cleaning blade 6 and discarded in a waste toner container (not shown). The toner remaining on the transfer belt 51 is cleaned by bringing a cleaning device (not shown) of the transfer belt 51 into contact with the transfer belt 51 after the secondary transfer, and is discarded in a waste toner container (not shown). The cleaning device for the transfer belt 51 is separated from the transfer belt 51 after cleaning the entire circumference of the transfer belt 51.

As described above, according to the present invention, by rapidly cooling the toner image, the shrinkage of the resin contained in the toner can be suppressed, and the smoothness of the image surface can be improved. Is obtained.
Further, the surface temperature of the first cooling roller 91 that directly contacts the toner image is controlled to be higher than the surface temperature of the second cooling roller 92 that contacts the toner image via the recording medium 22. The cooling rate on the lower layer side of the toner image cooled by the second cooling roller 92 via the recording medium 22 and the surface layer side cooled by the first cooling roller 91 are substantially the same. As a result, even if the toner image is cooled from both the surface layer side and the lower layer side of the toner image, a temperature difference between the surface layer side and the lower layer side is less likely to occur, and the occurrence of cooling spots can be suppressed. Therefore, since it is possible to suppress the occurrence of cracks on the image surface without reducing the cooling rate of the toner image more than necessary, it is possible to achieve both suppression of the occurrence of cracks and high gloss images.

Furthermore, according to the present invention, since a deep and vivid color tone inherent to a color material can be obtained, it is particularly suitable for forming a color image.
Furthermore, since the recording medium (paper) is also cooled when the toner image is rapidly cooled, the temperature of the discharged paper can be lowered.
Further, since the smoothness of the image surface is improved, the stacking property of the discharged paper is also improved.

  In the present invention, since air is used for the cooling means, the fixing means adjacent to the cooling means is not easily cooled. Therefore, energy consumption is less likely to occur and variations in the axial temperature of the fixing roller provided in the fixing unit are less likely to occur. Further, since the warm air around the fixing unit is hardly scattered in the image forming apparatus, image deterioration due to a change in the sensitivity of the photosensitive drum and a change in the charge amount of the developer hardly occurs.

  In addition, if the amount of toner on the recording medium is increased, the glossiness generally tends to increase, but the toner layer becomes thicker, so the temperature difference between the surface layer side and the lower layer side of the toner image increases, and cracks occur. It tends to occur. However, according to the present invention, since cooling spots are hardly generated on the surface layer side and the lower layer side of the toner image, cracks are hardly generated even when the toner amount is increased. Therefore, it is possible to achieve both suppression of cracks and obtaining a high gloss image.

Hereinafter, the present invention will be specifically described with reference to examples.
[Production of toner]
80 parts by mass of styrene, 20 parts by mass of 2-ethylhexyl methacrylate, 5 parts by mass of a cyan pigment (CI Pigment Blue 15: 3) as a colorant, 3 parts by mass of low molecular weight polypropylene, and a quaternary ammonium salt compound ( 2 parts by mass of “P-51” manufactured by Orient Chemical Industries Co., Ltd. and 2 parts by mass of a polymerization initiator 2,2-azobis (2,4-dimethylvaleronitrile) are added to a mixed solution of 1 part by mass of divinylbenzene as a crosslinking agent. These are added to 400 parts by mass of purified water, and 5 parts by mass of tribasic calcium phosphate and 0.1 part by mass of sodium dodecylbenzenesulfonate are added as suspension stabilizers, and a TK homomixer (manufactured by Tokushu Kika Kogyo Co., Ltd.) is used. And stirred for 20 minutes at a rotational speed of 7000 rpm, and further subjected to a polymerization reaction in a nitrogen atmosphere at 70 ° C. and 100 rpm for 10 hours. To obtain a powder having an average particle diameter of 6.3 [mu] m. To this powder, 1.5 parts by mass of hydrophobic silica powder was added and mixed with a Henschel mixer (manufactured by Mitsui Mining Co., Ltd.) to obtain a cyan toner having a volume average particle size of 6.3 μm. The volume average particle size was measured with Multimizer III (Colestar Counter).
The charge amount of the obtained cyan toner was measured with a suction-type charge amount measuring device (Trek), and it was 33 μC / g.

[Examples 1 to 5]
<Configuration of image forming apparatus>
The image forming apparatus having the configuration shown in FIGS. 1 to 4 was used, and the toner obtained previously was accommodated in a developing container. The configurations of the fixing unit 8 and the cooling unit 9 are as follows.
(Fixing means)
As the fixing roller 81, the surface of an aluminum tube having an outer diameter of 30 mm and a wall thickness of 1.0 mm is coated with a silicone rubber having a hardness of 5 degrees (JIS-A) and a thickness of 200 μm, and a PFA having a thickness of 30 μm is applied to the surface. (Ethylene fluoride and perfluoroalkoxyethylene copolymer) tube bonded with a primer was used. A halogen heater lamp 83 is arranged inside the fixing roller 81, and the lamp lighting is controlled by a thermistor 84 in contact with the surface of the fixing roller 81 so that the surface temperature of the fixing roller is always 170 ° C.
As the pressure roller 82, a solid iron rod having a core metal having an outer diameter of 18 mm was used on a silicone rubber core having an outer diameter of 30 mm, a hardness of 5 degrees (JIS-A), and a thickness of 6 mm.

(Cooling means)
The first cooling roller 91 and the second cooling roller 92 have a distance d from the exit of the fixing nip portion to the entrance of the cooling nip portion of 30 mm (that is, the recording medium conveyance speed is 150 mm / second, so in terms of time). 0.2 second downstream).
As the first cooling roller 91 and the second cooling roller 92, a PFA sheet having a thickness of 30 μm was adhered to the surface of a cylindrical heat pipe (made of copper) having an outer diameter φ of 20 mm and an inner diameter φ of 10 mm via a primer. A thing was used.

(First cooling roller temperature control method)
When the surface temperature of the first cooling roller 91 is detected by the thermistor 96 and is lower than a predetermined temperature set to be higher than the surface temperature of the second cooling roller 92, the heater 95 is energized and the first cooling roller 91 is energized. Was heated to a predetermined temperature. On the other hand, when the surface temperature of the first cooling roller 91 is equal to or higher than the predetermined temperature, the heater 95 is not energized, and the fan 94a is driven to blow the radiating fins 93a and cool to the predetermined temperature. Similarly, during the passage of paper and during standby, the energization and drive control of the heater 95 and the fan 94a were performed so as to always maintain a constant temperature while observing the surface temperature of the first cooling roller 91.
The surface temperature of the second cooling roller 92 was detected using a thermistor (not shown) attached to the second cooling roller 92, and was kept at 28 ° C. by the heat radiation fins 93b and the fan 94b.
On the other hand, the surface temperature of the first cooling roller 91 was controlled by the radiating fins 93a, the fans 94a, and the heaters 95 as described above so that the values shown in Table 1 could be maintained.

<Evaluation>
Using the image forming apparatus, the temperature control method described above was evaluated as follows.
In each embodiment, the surface temperature of the first cooling roller 91 is controlled to the value shown in Table 1, the toner amount on the recording medium is set to the value shown in Table 1, and before and after the roller pair passes. An image sample was taken while recording the surface temperature of the toner image (measurement interval was 38 mm = 0.25 seconds later). As a recording medium, “Color Copy paper, basis weight 90 g / m ² ” manufactured by mondi was used. Further, the surface temperature of the toner image before passing through the roller pair was 140 ° C., and the surface temperature of the recording medium where the toner image was not formed was 93 ° C.
Table 1 shows the glossiness of the image surface of the image sample and the presence or absence of cracks when the number of printed sheets is 200. However, the glossiness and the presence or absence of cracks were determined as follows.

(Glossiness)
The glossiness of the image surface of the image sample was measured with a gloss meter (manufactured by Horiba, Ltd., “Handy Gloss Checker IG-331”, incident angle: 60 °).
(With or without cracks)
The presence or absence of cracks on the image surface was visually observed and evaluated according to the following evaluation criteria. In addition, ○ and Δ are acceptable.
○: No crack occurred.
Δ: Slight cracking occurred.
X: Crack occurred.

(Comprehensive evaluation)
Based on the evaluation results of glossiness and presence / absence of cracks, comprehensive evaluation was performed according to the following evaluation criteria. ◎ and ○ are acceptable.
A: Applicable to all of “Evaluation result of presence / absence of crack”, “Glossiness of toner amount 0.5 mg / cm ² is 25 or more”, “Glossiness of toner amount 1.5 mg / cm ² is 35 or more” To do.
○: “Evaluation result of presence or absence of crack is Δ”, “Glossiness of toner amount 0.5 mg / cm ² is 20 or more and less than 25”, “Glossiness of toner amount 1.5 mg / cm ² is 30 or more, 35 Falls under “less than”.
×: “Evaluation result of presence / absence of crack is ×”, “Glossiness of toner amount 0.5 mg / cm ² is less than 20”, “Glossiness of toner amount 1.5 mg / cm ² is less than 30” Applicable.

[Comparative Examples 1 and 2]
Except that the surface temperature of the first cooling roller 91 is controlled to the value shown in Table 1, and the toner amount on the recording medium is set to the value shown in Table 1, the same as in Examples 1-5 Performed and evaluated. The results are shown in Table 1.

As is clear from Table 1, the images obtained in each example had higher glossiness than those of the comparative examples, and cracks did not easily occur. In particular, in Examples 3 to 5 in which the temperature difference between the first cooling roller and the second cooling roller was large, cracks did not easily occur even when the toner amount was increased.
In each embodiment, the surface temperature of the first cooling roller and the second cooling roller is made to correspond to the temperature difference between the surface layer side and the lower layer side of the toner image, so that the cooling rate of the entire toner image is constant. As a result, the occurrence of cracks on the image surface could be suppressed. Although it is difficult to directly measure the temperature of the lower layer side of the toner image, the temperature of the lower layer side is roughly determined from the surface temperature of the toner image and the surface temperature of the recording medium where the toner image is not formed. Can be estimated.

On the other hand, in Comparative Example 1, since the surface temperature difference between the first cooling roller and the second cooling roller was 0 ° C., the surface layer side of the toner image was cooled earlier than the lower layer side, and cracks occurred on the image surface. However, since a rapid cooling effect of the toner image was obtained, a high gloss image could be obtained by visual observation. However, the glossiness was inferior compared to the examples.
In Comparative Example 2, since the first and second cooling rollers were not installed, only the surface layer side of the toner image was not rapidly cooled, so that no crack was generated on the image surface. However, since the toner image was gradually cooled, the resin contained in the toner contracted, and the shrinkage marks were formed on the surface of the image. As a result, the smoothness of the image surface was lost and the glossiness was lowered.

In addition, as a result of observing the image surface of the image sample obtained by each Example and the comparative example with the digital microscope (the Keyence Corporation make, "VHX-600", 1000-2000 times), in each Example, it is slightly As the glossiness increased, the size of the spots became smaller, the interval was narrower, and the depth became shallower.
On the other hand, in each comparative example, spots larger than those of the example, wide in intervals, and large indentations were confirmed.

1 is a schematic configuration diagram illustrating an example of an image forming apparatus of the present invention. It is a schematic block diagram which shows an example of a fixing means and a cooling means. It is a side view of the cooling means shown in FIG. It is a front view of the 2nd cooling roller shown in FIG.

Explanation of symbols

  2: charging means, 3: exposure means, 4: developing means, 5: transfer means, 8: fixing means, 81: fixing roller, 82: pressure roller, 9: cooling means, 91: first cooling roller, 92 : Second cooling roller, 100: image forming apparatus

Claims (4)

An image forming apparatus comprising: a fixing unit that heats and fixes a toner image formed on a recording medium; and a cooling unit that rapidly cools the fixed toner image.
The cooling means includes a first cooling roller that comes into contact with the toner image, and a second cooling roller that faces the first cooling roller via a recording medium, and the first cooling roller An image forming apparatus, wherein a surface temperature is controlled to be higher than a surface temperature of the second cooling roller.   The first cooling roller and the second cooling roller are controlled to be in a pressure contact and separation state, and the first cooling roller and the second cooling roller are in a pressure contact state in accordance with the passage of the recording medium. The image forming apparatus according to claim 1, further comprising: An image forming method in which a toner image formed on a recording medium is heated and fixed, and then the fixed toner image is rapidly cooled.
The toner image is cooled by passing between a first cooling roller in contact with the toner image and a second cooling roller facing the first cooling roller via a recording medium;
An image forming method comprising controlling the surface temperature of the first cooling roller to be higher than the surface temperature of the second cooling roller.   The first cooling roller and the second cooling roller are controlled to be in a pressure contact and separation state, and the first cooling roller and the second cooling roller are brought into a pressure contact state in accordance with the passage of the recording medium. The image forming method according to claim 3, wherein:

Images