JP2007206555A - Fixing mechanism - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing mechanism which is capable of preventing a recording paper from winding around a heat roller. <P>SOLUTION: The fixing mechanism includes a pair of a heat roller and a pressure roller rotating in press contact with each other, and the heat roller and the pressure roller have elastic layers on circumferential surfaces thereof and forms a nip part having a prescribed width where a carried recording paper is heated while being held between them, and a hardness of the elastic layer and that of the heat roller are so set that a surface hardness of the pressure roller is higher than that of the heat roller. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fixing mechanism, and more particularly to a fixing mechanism for fixing a toner image on a recording sheet by applying heat to an unfixed toner image transferred to the recording sheet.

As a prior art related to the present invention, there is a heating roller having a halogen heater lamp inside and a pressure roller in pressure contact with the heating roller, and an unfixed toner image is transferred between the heating roller and the pressure roller. A fixing device that passes the recorded paper and heat-fixes the toner image on the recording paper, and the heating roller is made of either aluminum or iron and has a cylindrical core metal having a thickness of 2.0 mm or less The pressure roller is made of silicon rubber, the surface hardness (Asker C hardness) is 35 to 75 °, the nip pressure between the heating roller and the pressure roller is 0.2 kgf / square cm, and passes through the nip. A fixing device having a time of 20 to 40 msec is known (for example, see Patent Document 1).
JP 2001-42678 A

In recent years, development of a color image forming apparatus having monochrome and color image forming functions and capable of forming an image on one side and both sides of a recording sheet has been progressing.
In such a color image forming apparatus, a color image is formed by transferring toner images of four colors of black (K), cyan (C), magenta (M), and yellow (Y) on a recording sheet. Is done.
Therefore, in the fixing mechanism of the color image forming apparatus, it is necessary to melt and fix the four-layer toner images of K, C, M, and Y transferred onto the recording paper.
That is, the fixing mechanism of the color image forming apparatus is required to apply more heat in fixing the toner image than the fixing mechanism of the conventional monochrome image forming apparatus.

For this reason, in the conventional general color image forming apparatus, in order to apply a large amount of heat without reducing the conveyance speed, the width between which the heating roller and the pressure roller sandwich the recording paper, that is, the nip width Is generally set longer than that of a conventional monochrome image forming apparatus.
In order to set the nip width long, (1) the heating roller is made of a cylindrical metal core made of metal, and the peripheral surface of the pressure roller is made to be a heating roller by lowering the rubber hardness of the pressure roller pressed against the heating roller. In general, the nip width is increased by forming a concave shape along the peripheral surface of the roller, or (2) providing an elastic body having the same hardness on the peripheral surfaces of both the heating roller and the pressure roller.

When the image formed on the recording paper is a monochrome image, the toner image becomes one black layer and the toner layer is thin. Therefore, in any of the above methods (1) and (2), There is little wrapping around the heating roller due to the viscosity of the melted toner, and there is a particular problem because the recording paper is forcibly separated from the heating roller by the peeling claw provided so as to be in contact with the heating roller even if it is wound. Absent.
However, when the image formed on the recording paper is a color image, the toner image becomes four layers of K, C, M, and Y, and the layer thickness is increased. In addition to curling along the peripheral surface of the heating roller, the heated roller is wound due to the viscosity of the melted toner, and the surface of the toner layer remains on the heating roller side when it is peeled off by the peeling claw. The color image is different from the color of the color image that should be output.
Such a problem also occurs depending on the ratio of the image forming area to the area of the recording paper, that is, the printing rate, and the higher the printing rate, the easier the winding occurs due to the viscosity of the melted toner.

  On the other hand, in the method (2), since the nip portion is formed by the elastic body having the same hardness, the nip portion is flat and the recording paper is hardly curled along the peripheral surface of the heating roller. Although the winding around the heating roller is less likely to occur than the method 1), the same problem as described above occurs when the printing rate is high, and the problem cannot be completely solved.

  The present invention has been made in view of the above circumstances, and provides a fixing mechanism capable of preventing a recording sheet from being wound around a heating roller.

  The present invention includes a pair of a heating roller and a pressure roller that rotate while being pressed against each other, and the heating roller and the pressure roller sandwich the recording paper that is conveyed with an elastic layer provided on each peripheral surface thereof. While forming a nip with a predetermined width for heating, the hardness of the elastic layer of the heating roller and the elastic layer of the pressure roller are set so that the surface hardness of the pressure roller is higher than the surface hardness of the heating roller. A fixing mechanism is provided.

According to this invention, the elastic layer of the heating roller and the elastic layer of the pressure roller are set so that the surface hardness of the pressure roller is higher than the surface hardness of the heating roller. The layer is recessed in a concave shape along the peripheral surface of the pressure roller at the nip portion.
Therefore, the recording paper curls along the circumferential surface of the pressure roller when passing through the nip portion, and the leading edge of the recording paper that has passed through the nip portion is conveyed to the pressure roller side. Wrapping around is prevented.

  A fixing mechanism according to the present invention includes a pair of heating rollers and a pressure roller that rotate while being pressed against each other, and the heating roller and the pressure roller are each provided with an elastic layer on a peripheral surface thereof and conveyed. A nip portion having a predetermined width for heating while sandwiching the roller is formed, and the elastic layer of the heating roller and the elastic layer of the pressure roller have a hardness so that the surface hardness of the pressure roller is higher than the surface hardness of the heating roller. Is set.

In the fixing mechanism according to the present invention, the heating roller rotates while being in pressure contact with the heating roller, sandwiches the recording paper conveyed at the press contact portion of both rollers, and is on the recording paper at the nip portion formed at the press contact location. This means that heat is applied to unfixed toner. The heat source for heating the heating roller may be provided either inside or outside the heating roller.
The pressure roller means one that presses against the heating roller and forms a nip portion together with the heating roller. The pressure roller may be provided with a heat source inside or outside so as not to remove heat from the heating roller.
The nip portion means a sandwiched portion of the recording paper formed at the press contact portion between the heating roller and the pressure roller, and the elastic layer of the pressure roller and the heating roller is crushed together so as to have a predetermined width (upstream in the transport direction). And a predetermined length along the downstream side).
The longer the width of the nip portion, the more heat can be applied to the unfixed toner on the recording paper, and the toner can be surely melted. However, if the nip width is too long, the nip portion is heated by the viscosity of the melted toner. This will cause the roller to be wound.
For this reason, the width of the nip portion is set so that the performance required for the fixing mechanism, that is, the heat given to the recording paper becomes an optimal amount of heat.

  For the elastic layer, various rubbers such as silicon rubber can be used, for example, by changing the mixing ratio of rubber materials, extenders and linking agents as raw materials in the manufacturing process, and the processing method in the manufacturing process. A rubber having a desired hardness can be obtained.

In the fixing mechanism according to the present invention, the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller is preferably set within a range of 1 to 10 degrees in terms of Asker C hardness.
This is because when the hardness difference is less than 1 °, the nip portion is flat like the conventional fixing mechanism, and when the printing rate on the heating roller side is high, the viscosity of the melted toner moves to the heating roller. This is because it becomes easy to cause winding.
On the other hand, when the hardness difference exceeds 10 °, the recording paper curls excessively along the circumferential surface of the pressure roller, and when the printing rate on the pressure roller side is high in double-sided printing, heat conduction from the heating roller. This is because there is a possibility that winding around the pressure roller having a certain amount of heat may occur.
The Asker C hardness is a hardness measured with a durometer (spring type hardness meter) defined in SRIS0101 (Japan Rubber Association Standard).

In the fixing mechanism according to the present invention, the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller is preferably set based on the set width of the nip portion.
This is because in the present invention in which the surface hardness of the pressure roller is set higher than the surface hardness of the heating roller, the width of the nip portion is the effect of curling the recording paper along the peripheral surface of the pressure roller. It is because it affects. If the hardness difference between the surface hardness of the heating roller and the pressure roller is set based on the width of the nip as in the above configuration, the recording paper is appropriately curled along the peripheral surface of the pressure roller. It becomes possible.

Further, in the above configuration in which the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller is set based on the width of the nip portion, the difference in hardness between the surface hardness of the heating roller and the surface hardness of the pressure roller is The width of the nip portion is preferably set to be smaller as the width is set longer.
This is because when the width of the nip portion is set long, the distance and time that the recording paper passes through the nip portion naturally increases, and in this state, the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller. This is because, when the value is set large, the action of curling the recording paper along the peripheral surface of the pressure roller may be excessive.
However, if the hardness difference between the surface hardness of the heating roller and the pressure roller is set to be relatively small as the width of the nip portion is set to be long as in the above configuration, the recording paper is placed on the pressure roller. The curling can be appropriately performed along the peripheral surface, and the possibility that the recording paper is wound around the pressure roller can be eliminated as much as possible.

In the fixing mechanism according to the present invention, the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller is preferably set based on the set diameters of the heating roller and the pressure roller.
This is because in the present invention in which the surface hardness of the pressure roller is set to be higher than the surface hardness of the heating roller, the diameter of the heating roller and the pressure roller affects the length of the nip portion, which in turn is the recording paper. This is because it affects the curling action along the circumferential surface of the pressure roller. If the hardness difference between the surface hardness of the heating roller and the pressure roller is set based on the diameters of the heating roller and the pressure roller as in the above configuration, the recording paper is moved along the circumferential surface of the pressure roller. It becomes possible to curl appropriately.

Further, in the above configuration in which the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller is set based on the diameters of the heating roller and the pressure roller, the surface hardness of the heating roller and the surface hardness of the pressure roller The hardness difference is preferably set smaller as the diameters of the heating roller and the pressure roller are set larger.
This is because as the diameters of the heating roller and the pressure roller are set larger, the width of the nip portion becomes longer, and the distance and time that the recording paper passes through the nip portion naturally increases. This is because when the difference in hardness between the surface hardness and the surface hardness of the pressure roller is set large, the action of curling the recording paper along the peripheral surface of the pressure roller may be excessive.
However, if the hardness difference between the surface hardness of the heating roller and the pressure roller is set to be relatively small as the diameters of the heating roller and the pressure roller are set large as in the above configuration, the recording paper is It is possible to curl appropriately along the circumferential surface of the pressure roller, and the possibility that the recording paper is wound around the pressure roller can be eliminated as much as possible.

From another viewpoint, the present invention also provides an image forming apparatus provided with the above-described fixing mechanism according to the present invention.
Here, the image forming apparatus may be of any type that forms an image using toner, and is not particularly limited. Examples thereof include a copying machine, a facsimile machine, and a printer.

From another point of view, the present invention also provides a color image forming apparatus having a double-sided printing function provided with the above-described fixing mechanism according to the present invention.
Here, the color image forming apparatus may be of any type that forms a color image using toner such as black (K), cyan (C), magenta (M), yellow (Y), etc. Although not particularly limited, for example, a color copying machine, a color facsimile, a color printer, and the like can be given.

  Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

Embodiment A color image forming apparatus according to an embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a schematic diagram illustrating an overall configuration of a color image forming apparatus according to an embodiment.

Overall Configuration and Operation of Color Image Forming Apparatus As shown in FIG. 1, a color image forming apparatus 100 incorporating a fixing unit (fixing mechanism) 12 according to an embodiment of the present invention is obtained by scanning a document. According to image data or image data transmitted from the outside, a multicolor (color) or single color image is formed on a predetermined recording sheet.
The color image forming apparatus 100 includes an exposure unit 1, developing units 2a, 2b, 2c, and 2d, photosensitive drums 3a, 3b, 3c, and 3d, charging units 4a, 4b, 4c, and 4d, and cleaner units 5a, 5b, 5c, and the like. 5d, an intermediate transfer belt unit 8, a fixing unit 12, paper transport paths S1, S2, S3, a plurality of paper feed cassettes 10, and a paper discharge tray 15.

The image data handled in the color image forming apparatus 100 corresponds to a color image using each color of black (K), cyan (C), magenta (M), and yellow (Y).
Therefore, the developing units 2a, 2b, 2c, 2d, the photosensitive drums 3a, 3b, 3c, 3d, the chargers 4a, 4b, 4c, 4d, and the cleaner units 5a, 5b, 5c, 5d are four types corresponding to the respective colors. Four each are provided so as to form a latent image.

  That is, the developing device 2a, the photosensitive drum 3a, the charger 4a, and the cleaner unit 5a constitute a black image station, and the developing device 2b, the photosensitive drum 3b, the charger 4b, and the cleaner unit 5b provide a cyan image station. The magenta image station is composed of the developing device 2c, the photosensitive drum 3c, the charger 4c, and the cleaner unit 5c. An image station is configured.

The photosensitive drums 3 a, 3 b, 3 c, and 3 d are respectively disposed on the color image forming apparatus 100.
The chargers 4a, 4b, 4c and 4d are charging means for uniformly charging the surfaces of the corresponding photosensitive drums 3a, 3b, 3c and 3d to predetermined potentials. In this embodiment, the chargers are of the charger type. However, a brush type or contact roller type charger can also be used.
In this embodiment, a laser scanning unit (LSU) including a laser irradiation unit 80 and reflection mirrors 81a, 81b, 81c, 81d is used as the exposure unit 1, but an EL in which light emitting elements are arranged in an array. A write head or LED write head can also be used.

By exposing the photosensitive drums 3a, 3b, 3c, 3d charged by the chargers 4a, 4b, 4c, 4d by the exposure unit 1 according to the input image data, the photosensitive drums 3a, 3b, 3c, An electrostatic latent image corresponding to the image data is formed on the surface of 3d.
The developing units 2a, 2b, 2c, and 2d visualize the electrostatic latent images formed on the respective photosensitive drums 3a, 3b, 3c, and 3d with (K, C, M, Y) toner. It is. The cleaner units 4a, 4b, 4c, and 4d are for removing and collecting toner remaining on the surfaces of the photosensitive drums 3a, 3b, 3c, and 3d after development and image transfer, respectively.

The intermediate transfer belt unit 8 disposed above the photosensitive drums 3a, 3b, 3c, and 3d includes an intermediate transfer belt 7, an intermediate transfer belt driving roller 71, an intermediate transfer belt tension roller 73, an intermediate transfer belt driven roller 72, Intermediate transfer rollers 6a, 6b, 6c and 6d and an intermediate transfer belt cleaning unit 9 are provided.
The intermediate transfer belt drive roller 71, the intermediate transfer belt tension roller 73, the intermediate transfer rollers 6a, 6b, 6c, 6d, the intermediate transfer belt driven roller 72, and the like stretch the intermediate transfer belt 7 and rotate it in the direction of arrow B. Is.

The intermediate transfer rollers 6a, 6b, 6c and 6d are rotatably supported at positions facing the photosensitive drums 3a, 3b, 3c and 3d via the intermediate transfer belt 7, respectively. A transfer bias for transferring the toner images 3c and 3d onto the intermediate transfer belt 7 is provided.
The intermediate transfer belt 7 is provided in contact with the photosensitive drums 3a, 3b, 3c, and 3d, respectively. The color toner images formed on the photosensitive drums 3a, 3b, 3c, and 3d are sequentially superimposed on the intermediate transfer belt 7 and transferred to the color image on the intermediate transfer belt 7 (multicolor toner image). Is formed. The intermediate transfer belt 7 is formed endlessly using a film having a thickness of about 100 to 150 μm.

Transfer of the toner image from the photosensitive drums 3a, 3b, 3c, and 3d to the intermediate transfer belt 7 is performed by intermediate transfer rollers 6a, 6b, 6c, and 6d that are in contact with the back side of the intermediate transfer belt 7. The intermediate transfer rollers 6a, 6b, 6c and 6d have a high voltage transfer bias (a high voltage having a polarity (+) opposite to the toner charging polarity (-)) in order to transfer the toner image to the intermediate transfer belt 7. Is applied.
The intermediate transfer rollers 6a, 6b, 6c, and 6d are based on a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm, and the surface thereof is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). It is. With this conductive elastic material, a high voltage can be uniformly applied to the intermediate transfer belt 7.
In this embodiment, the roller-shaped intermediate transfer rollers 6a, 6b, 6c, and 6d are used as transfer electrodes, but a brush type or the like can also be used.

As described above, the electrostatic images visualized according to the respective hues on the respective photoreceptors 3a, 3b, 3c, and 3d are stacked on the intermediate transfer belt 7, and the recording described later is performed by the rotation of the intermediate transfer belt 7. The image is transferred onto a recording sheet by a transfer belt 11 disposed at a contact position between the sheet and the intermediate transfer belt 7.
At this time, the intermediate transfer belt 7 and the transfer belt 11 are pressed against each other with a predetermined nip width by the transfer roller 11a, and a voltage for transferring the toner onto the recording paper is applied to the transfer roller 11a (toner charging polarity). (-) Is a high voltage with a reverse polarity (+).
Further, the transfer roller 11a and the intermediate transfer belt drive roller 71 have either one of the transfer roller 11a or the intermediate transfer belt drive roller 71 made of a hard material (metal or the like) in order to obtain the above nip width constantly. The other is made of a soft material such as an elastic roller (such as an elastic rubber roller or a foaming resin roller).

Further, as described above, the toner adhered to the intermediate transfer belt 7 due to contact with the photosensitive drums 3a, 3b, 3c, 3d, or the transfer belt 11 is not transferred onto the recording paper and is not transferred onto the intermediate transfer belt 7. The remaining toner is set to be removed and collected by the intermediate transfer belt cleaning unit 9 because it causes toner color mixing in the next process.
The intermediate transfer belt cleaning unit 9 is provided with a cleaning blade 9a as a cleaning member that contacts the intermediate transfer belt 7. The intermediate transfer belt 7 that contacts the cleaning blade 9a is supported by the intermediate transfer belt driven roller 72 from the back side. Has been.

The plurality of paper feed cassettes 10 are openable and closable cassettes for storing recording sheets used for image formation, and are provided below the image forming unit and the exposure unit 1 of the color image forming apparatus 100. .
In addition, a paper discharge tray 15 provided in the upper portion of the color image forming apparatus 100 is a tray for placing recording paper on which an image has been formed face down.

The color image forming apparatus 100 is provided with a substantially vertical sheet conveyance path S1 for sending the recording sheets of each sheet feeding cassette 10 to the sheet discharge tray 15 via the transfer roller 11 and the fixing unit 12. .
Further, a pickup roller 16-1, a conveyance roller 25-1, a registration roller 14, a transfer roller 11, a fixing unit 12, and the like are arranged in the vicinity of the sheet conveyance path S1 from each sheet cassette 10 to the sheet discharge tray 15. ing.

Each conveyance roller 25-1 is a roller for promoting and assisting conveyance of the recording paper, and is provided on the downstream side of each pickup roller 16-1. The pickup roller 16-1 is provided at the end of each paper feed cassette 10, and is a draw-in roller that supplies recording paper from each paper feed cassette 10 to the paper transport path S 1 one by one.
Further, the registration roller 14 temporarily holds the recording paper conveyed through the paper conveyance path S1. The recording paper is transported to the transfer belt 11 at the timing when the leading edge of the multicolor toner image formed on the intermediate transfer belt 7 is aligned with the leading edge of the recording paper.

The fixing unit 12 includes a pair of a heating roller 31 and a pressure roller 32, and the heating roller 31 and the pressure roller 32 rotate with the recording paper interposed therebetween.
The heating roller 31 is set to have a predetermined fixing temperature under the control of a control unit based on a signal from the temperature detector 34 (see FIG. 2), and the recording paper is thermocompression bonded together with the pressure roller 32. As a result, the multi-color toner image transferred to the recording paper is melted, mixed, and pressed to be thermally fixed to the recording paper.
The recording paper after the multicolor toner image is fixed is conveyed to the paper discharge path S3 and is reversed by the paper discharge roller 25-2 (with the multicolor toner image facing downward). 15 is discharged to the top.

Next, the recording paper conveyance path will be described in detail. As described above, the color image forming apparatus 100 is provided with the paper feeding cassette 10 that stores recording paper in advance, and the user does not need to use the recording paper in the paper feeding cassette 10 when printing a small number of sheets. A manual feed portion 20 is provided.
Regardless of the paper feeding method, the sheet transport path is made sheet by sheet by the pickup roller 16-1 disposed at the end of the sheet feeding cassette 10 or the pickup roller 16-2 disposed at the end of the manual feed section 20. It is guided to S1 or S2.

The recording sheet fed from the sheet feeding cassette 10 is conveyed to the registration roller 14 by the conveying roller 25-1 in the sheet conveying path S1, and the leading end of the recording sheet and the leading end of the multicolor toner image on the intermediate transfer belt 7 are aligned. The toner is conveyed to the transfer roller 11 at the matching timing, and the multicolor toner image is transferred onto the recording paper.
Thereafter, the recording paper passes through the fixing unit 12 so that the unfixed toner on the recording paper is melted and fixed by heat and is discharged onto the paper discharge tray 15 by the paper discharge roller 25-2 (in the case of single-sided printing request). ).

  On the other hand, the recording paper supplied to the manual feed section 20 by the user is guided to the paper transport path S2 by the pickup roller 16-2, then transported by the transport roller 25-3, and joined to the paper transport path S1, and then registered. After reaching the roller 14, it is discharged to the paper discharge tray 15 through the same process as the recording paper fed from the paper feed cassette 10 described above (in the case of single-sided printing request).

When double-sided printing is requested, the trailing edge of the recording paper that has finished single-sided printing and has passed through the fixing unit 12 as described above is chucked by the paper discharge roller 25-2, and the paper discharge roller 25-2 rotates reversely. As a result, the paper is guided to the reversing paper transport path S4.
The recording paper guided to the reversing paper transport path S4 joins the paper transport path S1 again, and after printing on the back side through the registration rollers 14, is discharged to the paper discharge tray 15.

Configuration and Operation of Fixing Unit The fixing unit 12 incorporated in the above-described color image forming apparatus will be described with reference to FIGS. FIG. 2 is an explanatory diagram illustrating a schematic configuration of a fixing unit (fixing mechanism) incorporated in a color image forming apparatus according to an embodiment, and FIGS. 3 to 5 are explanatory diagrams illustrating a schematic configuration of a fixing unit according to a comparative example. It is.

As shown in FIG. 2, the fixing unit 12 incorporated in the color image forming apparatus 100 according to the embodiment includes a pair of heating rollers 31 and a pressure roller 32 that rotate while being pressed against each other.
Elastic layers 31a and 32a having a thickness of about 3 mm are provided on the peripheral surfaces of the heating roller 31 and the pressure roller 32, respectively, and conveyed after the multicolor toner image is transferred to the pressure contact portion between the heating roller and the pressure roller. A nip portion N is formed for heating the recording paper P that is being sandwiched.
The elastic layers 31a and 32a provided on the peripheral surfaces of the heating roller 31 and the pressure roller 32 are different in hardness from each other, and the hardness of the elastic layer 32a on the pressure roller 32 side is the elasticity on the heating roller 31 side. It is set higher than the hardness of the layer 31a.

The heating roller 31 is provided with the above-described elastic layer 31a on the peripheral surface of a cylindrical metal core 31b made of stainless steel, aluminum, iron, or the like, and a main heater 31c and a sub heater 31d made of a halogen heater lamp are provided inside the metal core 31b. Built in.
Further, around the heating roller 31, an external heating roller 33 for supplying heat to the heating roller 31 from the outside, a temperature detector 34 comprising a non-contact thermistor for detecting the surface temperature of the heating roller 31, A cleaning unit 35 for removing the toner remaining on the surface of the heating roller 31 and a peeling claw 36 for forcibly separating the recording paper P when the recording paper P is wound around the heating roller 31 by the viscosity of the molten toner are provided. ing.

On the other hand, the pressure roller 32 is provided with the above-described elastic layer 32a on the peripheral surface of a cylindrical metal core 32b made of stainless steel, aluminum, iron, or the like, and the pressure roller heater made of a halogen lamp inside the metal core 32b. A claw 36 for forcibly peeling the recording paper P when the recording paper P is wound is provided on the peripheral surface of the pressure roller 32 in the same manner as the heating roller 31.
The main purpose of the pressure roller 32 is to form a nip portion N that presses against the heating roller 31 and sandwiches the recording paper P. The pressure roller 32 itself also includes a pressure roller heater 32c. By providing, the temperature is maintained at an appropriate temperature so as not to take heat away from the heating roller 31.

The elastic layers 31a and 32a of the heating roller 31 and the pressure roller 32 are each made of silicon rubber, and the hardness is set so that the surface hardness of the pressure roller 32 is higher than the surface hardness of the heating roller 31. In order to keep the releasability of the recording paper P, a thin release layer made of a fluorine resin or the like is formed.
For this reason, as shown in FIG. 2, in the nip portion N, the elastic layer 31 a of the heating roller 31 is depressed in a concave shape along the peripheral surface of the pressure roller 32, and has passed through the nip portion N. The recording paper P is appropriately curled along the peripheral surface of the pressure roller 32, and the leading edge is conveyed not to the heating roller 31 but to the pressure roller 32, and is applied to the heating roller 31 due to the viscosity of the melted toner. Winding of the recording paper P and paper jamming (jamming) due to the winding are prevented.

  Silicon rubber having different hardness can be obtained, for example, by placing an order of a desired hardness according to the specifications from a rubber manufacturer such as Nitto Kogyo Co., Ltd.

  The following table shows the results of an experiment in which the surface hardness of the heating roller 31 and the surface hardness of the pressure roller 32 are intentionally changed and the occurrence of the wrapping of the recording paper P and paper jamming (jam) is examined. It is shown in 1. Of the experimental examples shown in Table 1 below, In the case 1, a heating roller having no elastic layer was used. The heating roller and the pressure roller both have a diameter of 50 mm, and the thickness of the elastic layer is about 3 mm as described above (except for the No. 1 heating roller) in either case. The nip width varies depending on each case, but is about 5 to 8 mm.

  No. in Table 1 above. In the experimental example 1, since the surface hardness of the heating roller 310 is higher than the surface hardness of the pressure roller 320 as shown in FIG. 3, the elastic layer 320 a of the pressure roller 320 is surrounded by the circumference of the heating roller 310 at the nip portion N. It dents in a concave shape along the surface. For this reason, when passing through the nip portion N, the recording paper P curls along the peripheral surface of the heating roller 310, and the viscosity of the melted toner is also helped to cause the recording paper P to be wound around the heating roller 310. .

  No. In the experimental example 2, as shown in FIG. 4, since the surface hardness of the heating roller 311 and the surface hardness of the pressure roller 321 are the same, the nip portion N becomes flat, and the printing rate changes. The recording paper P is wound around the heating roller 311 and the pressure roller 321 due to the viscosity of the melted toner.

No. 3 and no. In the experimental example 4, the hardness difference of 10 ° is set so that the surface hardness of the pressure roller 32 is higher than the surface hardness of the heating roller 31, as shown in FIG. For this reason, the elastic layer 31 a of the heating roller 31 is greatly depressed in the nip portion N along the peripheral surface of the pressure roller 32, and the recording paper P passes along the peripheral surface of the pressure roller 32 when passing through the nip portion N. The tip curls relatively strongly, and the tip is conveyed to the pressure roller 32 side.
When the printing rate on the heating roller 31 side is high and the printing rate on the pressure roller 32 side is low, no wrapping occurs on either the heating roller 31 or the pressure roller 32.
However, since the hardness difference is relatively large at 10 °, the action of curling the recording paper P along the peripheral surface of the pressure roller 32 is strong, and when the printing rate on the pressure roller 32 side is high, the viscosity of the toner There is a possibility that winding around the pressure roller 32 may occur.

No. 5 and no. In the experimental example 6, a hardness difference of 5 ° is set so that the surface hardness of the pressure roller 32 is higher than the surface hardness of the heating roller 31, as shown in FIG. For this reason, the elastic layer 31 a of the heating roller 31 is moderately depressed in the nip portion N along the peripheral surface of the pressure roller 32, and the recording paper P passes through the nip portion N and the peripheral surface of the pressure roller 32. And the tip is conveyed to the pressure roller 32 side.
Therefore, heating is performed both when the printing rate on the heating roller 31 side is high and the printing rate on the pressure roller 32 side is low, and when the printing rate on the heating roller 31 side is low and the printing rate on the pressure roller 32 side is high. Winding around the roller 31 and the pressure roller 32 does not occur.

  As can be seen from the above experimental examples, both the heating roller 31 and the pressure roller 32 are provided with elastic layers 31a and 32a, and the surface hardness of the pressure roller 32 is higher than the surface hardness of the heating roller 31. By appropriately setting the hardness of each of the elastic layers 31a and 32a, the recording paper P can be curled appropriately along the pressure roller 32, and winding around the heating roller 31 and the pressure roller 32 can be prevented. .

  In the experimental example described above, a No. 5 hardness difference between the surface hardness of the heating roller 31 and the surface hardness of the pressure roller 32 was set. 5 and no. Although the 6 experimental examples showed the best results, the hardness difference is optimal depending on the set width of the nip portion N, that is, the thickness of the elastic layers 31a and 32a and the set diameters of the heating roller 31 and the pressure roller 32. Since the value varies, the hardness difference of 5 ° is not necessarily optimal, but is set to an optimal hardness difference based on the set thicknesses of the elastic layers 31a and 32a and the diameters of the heating roller 31 and the pressure roller 32. There is a need to.

  In the above-described embodiments, the color image forming apparatus has been described. Of course, the fixing mechanism according to the present invention is useful for preventing the recording paper from being wound around the heating roller even in a monochrome image forming apparatus.

1 is a schematic diagram showing an overall configuration of a color image forming apparatus according to an embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating a schematic configuration of a fixing unit (fixing mechanism) incorporated in a color image forming apparatus according to an embodiment. FIG. 8 is an explanatory diagram illustrating a schematic configuration of a fixing unit according to a comparative example. FIG. 8 is an explanatory diagram illustrating a schematic configuration of a fixing unit according to a comparative example. FIG. 8 is an explanatory diagram illustrating a schematic configuration of a fixing unit according to a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Exposure unit 2a, 2b, 2c, 2d ... Developing device 3a, 3b, 3c, 3d ... Photosensitive drum 4a, 4b, 4c, 4d ... Charger 5a, 5b, 5c, 5d ... Cleaner units 6a, 6b, 6c, 6d ... Intermediate transfer roller 7 ... Intermediate transfer belt 8 ... Intermediate transfer belt unit 9 ... Intermediate transfer belt cleaning unit 9a ... Cleaning blade 10. ..Paper cassette 11... Transfer belt 11a... Transfer roller 12... Fixing unit 14... Registration roller 15. ... Pickup rollers 25-1, 25-3 ... Conveying rollers 25-2 ... Discharge rollers 31 ... Heating rollers 32 ... Pressure rollers 31a, 32a ... Elastic layers 1b, 32b ... Core metal 31c ... Main heater 31d ... Sub heater 32c ... Pressure roller heater 33 ... External heating roller 34 ... Temperature detector 35 ... Cleaning unit 36 ..Peeling claw 71 ... Intermediate transfer belt drive roller 72 ... Intermediate transfer belt driven roller 80 ... Laser irradiation unit 81a, 81b, 81c, 81d ... Reflective mirror 100 ... Color image forming apparatus N ... Nip part P ... Recording paper S1, S2 ... Paper transport path S3 ... Paper discharge path S4 ... Paper transport path for reversal

Claims (8)

  A pair of heating rollers and a pressure roller that rotate while being pressed against each other are provided, and the heating roller and the pressure roller are provided with elastic layers on their peripheral surfaces, respectively, to heat the recording paper that is being conveyed. A nip portion having a predetermined width is formed, and the hardness of the elastic layer of the heating roller and the elastic layer of the pressure roller are set so that the surface hardness of the pressure roller is higher than the surface hardness of the heating roller. The fixing mechanism.   2. The fixing mechanism according to claim 1, wherein a difference in hardness between the surface hardness of the heating roller and the surface hardness of the pressure roller is set in a range of 1 to 10 degrees in terms of Asker C hardness.   The fixing mechanism according to claim 1, wherein the hardness difference between the surface hardness of the heating roller and the surface hardness of the pressure roller is set based on a set width of the nip portion.   4. The fixing mechanism according to claim 3, wherein the difference in hardness between the surface hardness of the heating roller and the surface hardness of the pressure roller is set smaller as the width of the nip portion is set longer.   The fixing mechanism according to claim 1, wherein the difference in hardness between the surface hardness of the heating roller and the surface hardness of the pressure roller is set based on the set diameters of the heating roller and the pressure roller. .   6. The fixing mechanism according to claim 5, wherein the difference in hardness between the surface hardness of the heating roller and the surface hardness of the pressure roller is set smaller as the diameters of the heating roller and the pressure roller are set larger. .   An image forming apparatus comprising the fixing mechanism according to claim 1.   A color image forming apparatus having a double-sided printing function comprising the fixing mechanism according to claim 1.

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