JP2005189389A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device in which miniaturization is taken into account and a recording medium can naturally be separated from a fixing member securely in comparison to the case of conventional fixing devices, and to provide an image forming apparatus that has the fixing device. <P>SOLUTION: The fixing device has a nip section formed by a fixing roller 32 and a pressure roller 31 opposed in pressure contact with a fixing roller 32, and fixes an unfixed toner image held on a recording medium to the recording medium under heat and pressure in the nip section. In the fixing device, a combination of the fixing roller 32 and the pressure roller 31 is set such that the outside diameters of the fixing roller 32 and pressure roller 31 are 20 to 35 mm , the surface hardness of the fixing roller 32 is 20 to 35 Hs and the surface hardness of the pressure roller 31 is 60 to 90 Hs. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, and a fixing device used therefor. More particularly, the present invention relates to a fixing device that sandwiches a recording body in a nip formed by two surface moving bodies and fixes an unfixed visible image on the recording body, and an image forming apparatus using the same.

  Conventionally, in an image forming apparatus, a latent image on an image carrier is developed with toner supplied from a developing device, and a toner image as a visible image is formed on the image carrier. The toner image on the image carrier is transferred to a recording material as a transfer material by a transfer device and fixed on the recording material by a fixing device. As a fixing means of this fixing device, for example, a heater is incorporated therein, and a fixing roller configured to be rotatable, and a fixing roller configured to be rotatable by contacting the surface of a fixing member having the fixing roller with a predetermined pressure. It consists of a pressure roller. Further, there is known a technique in which a toner image on a recording medium is fixed to the recording medium by heat and pressure at a nip portion formed by a fixing member and a pressure roller.

Along with the recent miniaturization of the image forming apparatus, the fixing device is also required to be downsized, and the fixing roller, the pressure roller, and the like constituting the fixing device are also required to be downsized. However, since the nip width necessary for fixing the toner image on the recording paper on which the toner image is formed does not change even when the apparatus is downsized, the fixing apparatus using a small diameter roller is equivalent to the conventional one. The nip width must be secured.
In a fixing device using a small-diameter roller, a necessary nip width is secured by crushing the roller by applying pressure between the two rollers. However, if an attempt is made to secure the nip width by increasing the pressure between the two rollers, there is a concern that the load applied to the rollers will increase, leading to damage to the rollers. Therefore, the necessary nip width is ensured by adjusting the surface hardness of the roller forming the nip portion. One point that must be taken into consideration at this time is the separation between the recording medium and the fixing member at the nip exit. The recording body fixed at the nip portion is wound around the fixing member on the side that contacts the toner image on the recording body due to the adhesive force of the toner, and is often discharged as it is from the nip portion, resulting in poor separation. To do.

  In order to prevent the occurrence of this separation failure, a configuration is known in which a separation assisting member such as a separation claw is provided between the fixing member and the recording medium. However, since the separation assisting member comes into contact with the surface of the recording medium on which the toner image is fixed, there is a risk of damaging the toner image. Therefore, natural separation without using a separation assisting member is desirable.

  As a fixing device in consideration of this natural separation, there is one disclosed in Patent Document 1. This fixing device has a fixing roller and a heating roller, and an endless fixing belt is stretched between the two rollers to constitute a fixing member. Then, the pressure roller is pressed against the fixing roller via the fixing belt, so that the fixing roller is crushed, and the fixing belt and the pressure roller form a nip portion. Since the surface hardness of the fixing roller is set lower than the surface hardness of the pressure roller, the radius of curvature at the nip exit portion of the fixing roller that bites into the fixing roller and faces the toner image on the recording medium is small. Become. By reducing the radius of curvature at the nip outlet of the fixing roller, the radius of curvature at the outlet of the fixing belt that follows the surface is also reduced. Due to the curvature of the fixing belt and the rigidity of the recording body, the recording body is easily separated naturally, and the recording body is less likely to be wound around the fixing belt.

Japanese Patent Laid-Open No. 2002-268436

However, even if the surface hardness of the fixing roller is lower than the surface hardness of the pressure roller as described above, depending on the value of both hardnesses, the recording body may be wound around the fixing belt and a separation failure may occur. was there.
Such a separation failure is not limited to a fixing member constituted by a fixing roller and a fixing belt, but also in a configuration in which only a fixing roller is used as a fixing member, depending on the surface hardness value of the fixing roller and the pressure roller. May occur.

  The present invention has been made in view of the above problems, and an object of the present invention is a fixing device considering miniaturization, and more naturally separates a recording body from a fixing member more reliably than a conventional one. It is an object of the present invention to provide a fixing device that can perform the above and an image forming apparatus including the same.

In order to achieve the above object, the invention of claim 1 comprises a fixing means for forming a nip portion by a fixing member having a fixing roller having an elastic layer on the surface and a pressure roller facing and pressing the fixing roller. A fixing device for fixing an unfixed toner image carried on a recording medium to a recording medium by heat and pressure at a nip portion of the fixing unit, the outer diameter of the fixing roller and the pressure roller being 20 mm or more, The fixing roller has a surface hardness of 20 Hs or more and 35 Hs or less, and the pressure roller has a surface hardness of 60 Hs or more and 90 Hs or less.
Further, the invention of claim 2, the image forming apparatus according to claim 1, in which the pressure in the nip portion 4.9 N / cm ² or more, and that it has a 19.6 N / cm ² or less.
According to a third aspect of the present invention, there is provided an image carrier, a latent image forming unit for forming a latent image on the image carrier, and a developing unit for developing the latent image on the image carrier to form a toner image. In the image forming apparatus comprising: a transfer unit that transfers the toner image on the image bearing member onto the transfer member; and a fixing unit that fixes the toner image transferred from the transfer unit by the transfer unit. The fixing device according to claim 1 or 2 is used.

In the fixing device according to any one of the first to third aspects, when the surface hardness of each of the fixing roller and the pressure roller having an outer diameter of 35 mm or less is within the range described in claim 1, the following is the experiment. It became clear.
As compared with a fixing device in which the surface hardness of at least one of the fixing roller and the pressure roller is outside the range of claim 1, the recording material is less likely to be wound around the fixing roller.

  According to the first to third aspects of the present invention, since the winding of the recording body is less likely to occur than the roller having a surface hardness outside the range of the first aspect, it is possible to prevent the recording body from being separated from the fixing unit more reliably. There is an excellent effect of being able to.

  Embodiments in which the present invention is applied to an image forming apparatus will be described below. FIG. 1 shows an electrophotographic full-color printer as an image forming apparatus according to this embodiment. In FIG. 1, a plurality of image forming units 2Y, 2M, 2C, and 2Bk as image carrier units in a box-shaped apparatus main body 1 are members for yellow, magenta, cyan, and black, respectively. A transfer belt 3 serving as a recording medium carrier is disposed obliquely in the diagonal direction of the apparatus main body 1 at the center in the apparatus main body 1. The transfer belt 3 is provided so as to be able to be rotationally driven in a direction indicated by an arrow A spanning a plurality of rollers to which rotational driving force is transmitted.

  The transfer belt 3 is stretched around a driving roller 22, a driven roller 23, and tension rollers 24 and 25. On the inner side of the upper running surface of the transfer belt 3, a transfer brush 28 constituting transfer means is provided at positions facing the photosensitive drums 4M, 4C, 4Y, and 4Bk of the color image forming units 2M, 2C, 2Y, and 2Bk, respectively. In contact. A transfer bias having a polarity opposite to the charging polarity of the toner is applied to the transfer brush 28. A paper suction roller 27 is provided above the driven roller 23 with the transfer belt 3 interposed therebetween. The recording paper is fed onto the transfer belt 3 from between the driven roller 23 and the suction roller 27, and is conveyed while being electrostatically attracted onto the transfer belt 3 by a bias voltage applied to the suction roller 27.

  The image forming units 2Y, 2M, 2C, and 2Bk have drum-shaped photoconductors 4Y, 4M, 4C, and 4Bk as image carriers, and the belts are arranged so that the surface of each photoconductor is in contact with the transfer belt 3. It is arrange | positioned above. The photoreceptors 4Y, 4M, 4C, and 4Bk are not limited to drums, and belt-like photoreceptors may be used. The image forming units 2Y, 2M, 2C, and 2Bk are units for forming toner images of colors Y, M, C, and Bk on the photoreceptors 4Y, 4M, 4C, and 4Bk, and are disposed in the apparatus main body 1. The configuration is the same except for the location. Only the yellow image forming unit 2Y will be described below. The developing device 5Y is disposed to face the photoconductor 4Y. The developing device 5Y develops each electrostatic latent image by supplying a two-component developer having yellow toner and a carrier to the electrostatic latent image on the photoreceptor 4Y.

  A writing device 6 as an exposure unit is disposed above the image forming unit 2Y and the like, and paper feeding units 7 and 8 that can store transfer materials of different sizes are disposed below the image forming unit 2Y and the like. Yes. A fixing device 30 is disposed on the downstream side in the moving direction of the transfer belt 3.

  Next, an outline of the image forming process will be described. When image formation is instructed by an operation unit (not shown), the photoconductor 4Y is rotated by a drive source (not shown) and rotates clockwise. A charging roller 11Y as charging means in the image forming unit 2Y is charged with a charging bias from a power source (not shown) to uniformly charge the photoreceptor 4Y. The photosensitive member 4Y is uniformly charged by the charging roller 11Y and then exposed to laser light modulated by Y-color image data by the writing device 6 to form an electrostatic latent image on the surface. The electrostatic latent image on the photoreceptor 4Y is developed by the developing device 5Y to become a Y-color toner image.

On the other hand, a recording sheet as one recording medium is separated from the selected one of the sheet feeding cassettes 7 and 8, and a registration roller disposed on the sheet feeding side from the image forming unit 2Y. It hits 15. Then, the recording paper is conveyed on the transfer belt 3 in synchronization with the transfer timing of each color toner image, and reaches a transfer position facing each of the photoreceptors 4Y, 4M, 4C, and 4Bk. At this transfer position, a transfer electric field is formed by the action of a bias applied to the transfer brush 28 disposed on the back side of the transfer belt 3. By this transfer electric field, the color toner images on the photosensitive drums 4Y, 4M, 4C, and 4Bk are sequentially transferred onto the recording material so as to overlap each other. When a monochrome image is printed, a toner image is formed with black toner only on the photosensitive drum 4Bk of the black image forming unit 2Bk. Then, the recording paper is conveyed by the transfer belt 3 in synchronization with the transfer timing of the toner image, and only the black toner image is transferred.
In this way, the recording paper on which each color toner image is transferred is separated from the transfer belt 3 at the position of the driving roller 22 and sent to the fixing device 30. Then, when passing through the fixing nip of the fixing device 30, each color toner image is fixed on the recording paper by heat and pressure. After the fixing, the recording paper is discharged to a paper discharge tray 10 provided on the upper surface of the apparatus main body.

Next, the fixing device 30 applied to the image forming apparatus of this embodiment will be described. FIG. 2 is a schematic configuration diagram of the fixing device according to the present embodiment. In the present embodiment, a belt fixing method is employed, and an endless fixing belt 34 as a fixing member is wound between the fixing roller 32 and the heating roller 33.
The fixing roller 32 and the pressure roller 31 are in pressure contact with each other via a fixing belt 34 to form a fixing nip portion 35. The heating roller 33 and the pressure roller 31 each have a built-in heater (not shown). The unfixed toner image on the recording medium is fixed to the recording medium by heat and pressure at the fixing nip portion 35.

The fixing roller 32 is formed in a structure having an elastic layer made of liquid silicon, foamed silicon, or the like on the outer periphery of an iron core bar, and is rotatably supported.
The pressure roller 34 is pressed in a direction in which it is pressed against the fixing roller by a pressure contact member (not shown) such as a spring. The pressure roller 31 has a configuration in which an elastic layer made of silicon rubber and a surface layer made of fluororesin are provided on the outer periphery of an aluminum cored bar.
The heating roller 33 is formed of an aluminum pipe. The fixing belt 34 has a multilayer structure including a base material such as metal or polyimide having a low Young's modulus and high tensile strength, an elastic layer such as silicon rubber, and a surface peeling layer such as PFA.

  Further, a temperature detection member (not shown) such as a thermistor for detecting the surface temperature of the heating roller 33 for temperature management of the fixing belt 34, and a guide member for guiding a recording medium to be fixed toward the fixing unit are provided. . Further, on the fixing belt 34, there are provided an application roller (not shown) as means for applying an oil for preventing offset, and a cleaning roller (not shown) in case toner adheres to the belt. In the present embodiment, the fixing belt 34 is stretched around a pair of rollers including the heating roller 33 and the fixing roller 32. However, other rollers may be used and stretched by three or more rollers. . Further, when oilless type toner is used, it is not necessary to provide an application roller as means for applying oil for preventing offset. Further, guide members for regulating the deviation are formed at both ends of the fixing belt 34 to prevent the deviation of the belt.

In order to meet the recent demand for downsizing of the fixing device, the outer diameter D1 of the fixing roller 32 and the outer diameter D2 of the pressure roller 31 are set to 35 mm or less. If the outer diameter is too small, it is difficult to secure a sufficient width for the fixing nip portion 35. Therefore, both D1 and D2 are set to 20 mm or more.
In the fixing process, as the width of the nip portion 35 becomes wider, the amount of heat applied to the recording medium passing therethrough increases, and fixing becomes easier. On the other hand, when the width of the nip portion 35 is narrow, the amount of heat applied is insufficient, and there is a possibility that unfixed toner is not sufficiently fixed on the recording medium, resulting in fixing failure. Therefore, the fixing nip 35 requires a nip width of about 6.0 to 7.0 mm. In order to secure a nip width of 6.0 to 7.0 mm when D1 and D2 are 35 mm or less, it is necessary to largely deform a roller whose cross-sectional shape is originally circular by adjusting the surface hardness. In order to greatly deform the roller, it is necessary to make the surface hardness value of at least one of the fixing roller and the pressure roller sufficiently small so as to be deformable.

Further, after the recording medium passes through the nip portion 35, the recording medium may be wound around the fixing belt 34 that comes into contact with the toner image due to the adhesive force of the toner, and separation failure between the recording medium and the fixing belt 34 may occur.
Conventionally, in order to prevent the occurrence of this separation failure, there has been provided a separation claw or roller. However, since the separating member contacts the image after fixing, there is a risk of damage. Therefore, it is desirable to perform natural separation based on the curvature of the fixing belt at the fixing nip outlet and the rigidity of the recording medium, rather than separation using a separating member. When the recording body is bent along the curvature and the reaction force against the bending of the recording body becomes greater than the adhesive force of the toner, the recording body and the fixing belt can be separated naturally. In order to make the reaction force against the bending of the recording body larger than the adhesive force by the toner, it is necessary to reduce the radius of curvature r of the fixing belt 34 at the nip exit portion.

In order to reduce the radius of curvature r, the fixing roller 32 around which the fixing belt 34 is wound needs to be recessed as shown in FIG. Therefore, it can be said that the magnitude of the curvature radius r depends on the degree of depression of the fixing roller 32. Since the degree of depression of the fixing roller 32 is determined by the surface hardness of the fixing roller 32 and the pressure roller 31, it can be said that the radius of curvature r depends on the surface hardness of the fixing roller 32 and the pressure roller 31.
That is, the larger the dent of the fixing roller 32 is, the smaller the radius of curvature r is. Therefore, the higher the hardness of the pressure roller 31 is, the smaller the radius of curvature r is. it can.
The smaller the radius of curvature r, the better the separation between the fixing belt 34 and the recording body. Therefore, the higher the hardness of the pressure roller 31 compared to the hardness of the fixing roller, the higher the fixing belt 34 and the recording body. Separation is improved.

As described above, it has been clarified that the separability of the natural separation between the recording medium and the fixing belt 34 is deeply related to the surface hardness of the fixing roller 32 and the pressure roller 31. In Tables 1, 2 and 3, the fixing roller hardness and fixing temperature were changed for each pressure roller hardness, and the separability was evaluated under the following experimental conditions. Further, the pressure roller hardness and the fixing roller hardness are the surface hardness of each roller, and the value of the hardness is Asker C.
For evaluation of separability, under each experimental condition, 5 sheets were passed, and if no winding occurred, “○” was marked as acceptable, and if even one sheet was wound, “X” was marked as impossible. .

[Experimental conditions]
・ Pressure roller hardness: 50, 60, 70 (Hs)
Fixing roller hardness: 20, 25, 30, 35, 40 (Hs)
Fixing set temperature: 150, 160, 170, 180, 190 (° C.)
・ Fixing roller outer diameter: 30 mm
・ Pressure roller outer diameter: 30 mm
・ Heating roller outer diameter: 20mm
・ Nip time: 40 ms
-Nip width: 7.0 mm
Surface pressure: 8.82 N / cm ²
-Paper type: NBS45K paper Y eyes-Tip margin: 2.0cm
-Since the adhesive strength of the toner varies greatly depending on the image density, a solid image with the greatest adhesive strength of the toner is obtained.

According to Table 1, when the pressure roller hardness is 50 Hs, the fixing path roller hardness is not 30 Hs or less, so that separation is not possible. Further, when the fixing roller hardness is 30 Hs, the fixing is not performed unless the fixing setting temperature is 180 ° C. or higher. From these facts, it can be seen that when the pressure roller hardness is 50 Hs, other conditions become severe.
From Table 2, it can be seen that when the pressure roller hardness is 60 Hs, separation is possible at all fixing set temperatures if the fixing roller hardness is 35 Hs or less.
From Table 3, it can be seen that separation is possible under almost all conditions when the pressure roller hardness is 70 Hs, but separation is impossible when the fixing roller hardness is 40 ° C. at a fixing setting temperature of 150 ° C.
As a result, as a combination of the surface hardness of the pressure roller and the fixing roller satisfying the separation property, the surface hardness of the pressure roller is 60 Hs or more in Asker C hardness, and the surface hardness of the fixing roller is 35 Hs in Asker C or less. The conclusion was reached.

Further, by setting the surface hardness of the fixing roller to 35 Hs or less in Asker C hardness, the fixing roller 32 is greatly deformed in the nip portion, so that a desired nip width can be ensured.
Considering the separability and the nip width, the surface hardness of the fixing roller 32 is preferably as low as possible. However, if the surface hardness of the fixing roller 32 is too low, if it is left in a pressurized state, the collapsed fixing roller 32 will not return to its original state in its deformed state, and the fixing device will function normally. The problem of disappearing occurs. Accordingly, as the surface hardness necessary for the collapsed fixing roller 32 to return to the original state, the surface hardness of the fixing roller was set to 20 Hs in Asker C hardness.

The larger the value of the pressure applied to the nip portion between the fixing roller 32 and the pressure roller 31 (hereinafter referred to as surface pressure) during nip formation, the easier it is to obtain the nip width, and the fixability is improved. However, when the surface pressure is increased, the load applied to the two rollers also increases, which may lead to plastic deformation of the rollers and damage to the rollers. Therefore, in order to prevent such a problem, the surface pressure that is the pressure applied to the nip portion is set to 19.6 N / cm ² or less.
On the other hand, if the surface pressure is too low, fixing failure may occur. Therefore, the surface pressure is set to 4.9 N / cm ² or more.

  In this embodiment, a heating roller is provided separately from the fixing roller and a fixing belt is stretched between the fixing roller and the heating roller. However, the fixing roller is not limited to this configuration. Absent. For example, a configuration in which a heating unit such as a heater or an electromagnetic induction heating device is provided on the fixing roller itself may be applied as long as the numerical value range of the surface hardness described in claim 1 is satisfied.

As described above, according to the present embodiment, the outer diameters of the fixing roller 32 and the pressure roller 31 are set to 35 mm or less in order to reduce the size of the fixing device 30, and 20 mm or more to ensure the nip width. Further, the fixing roller 31 can be largely recessed by setting the surface hardness of the fixing roller 32 to 35 Hs or less in Asker C hardness and the surface hardness of the pressure roller to 60 Hs or less in Asker C hardness. Since the fixing roller 31 can be greatly recessed, the radius of curvature r at the nip exit portion of the fixing roller can be made small enough that the recording medium can be naturally separated by the rigidity and curvature of the recording medium. Since the radius of curvature r can be made smaller as the recording medium can be naturally separated, the recording body can be more reliably separated from the fixing belt 34 than the conventional fixing device. Furthermore, by setting the surface hardness of the fixing roller 32 to 20 Hs or more in terms of Asker C hardness, the surface of the fixing roller 32 that has passed through the nip portion 35 can be restored to its original shape, and problems due to deformation of the fixing roller can be prevented. be able to.
Further, by setting the surface pressure of the nip portion 35 4.9 N / cm ² or more, it is possible to form a nip width necessary for fixing, by a 19.6 N / cm ² or less, the two It is possible to prevent the roller from being damaged.

1 is a schematic diagram of an image forming apparatus according to an embodiment. 1 is a schematic diagram of a fixing device used in the present embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus main body 2M, 2C, 2Y, 2Bk Each photoconductor unit 4Y, 4M, 4C, 4Bk Each photoconductor drum 5Y, 5M, 5C, 5Bk Each developing device 6 Writing device 7, 8 Paper feed cassette 10 Paper discharge tray 11Y, 11M, 11C, 11Bk Each charging roller 15 Registration roller 22 Driving roller 23 Driven roller 24, 25 Tension roller 27 Paper adsorbing roller 28 Transfer brush 30 Fixing device 31 Pressure roller 32 Fixing roller 33 Heating roller 34 Fixing belt 35 Nip portion

Claims (3)

A fixing member that forms a nip portion by a fixing member having a fixing roller having an elastic layer on the surface and a pressure roller that is in pressure contact with the fixing roller;
In a fixing device for fixing an unfixed toner image carried on a recording medium to a recording medium by heat and pressure at a nip portion of the fixing unit,
The outer diameter of the fixing roller and the pressure roller is 20 mm or more and 35 mm or less,
The fixing roller has a surface hardness of 20 Hs or more and 35 Hs or less,
A fixing device characterized in that the pressure roller has a surface hardness of 60 Hs or more and 90 Hs or less. The image forming apparatus according to claim 1.
The pressure in the nip portion 4.9 N / cm ² or more, the fixing device being characterized in that the 19.6 N / cm ² or less. An image carrier, a latent image forming unit for forming a latent image on the image carrier, a developing unit for developing the latent image on the image carrier to form a toner image, and a toner image on the image carrier An image forming apparatus comprising: a transfer unit that transfers the toner image onto the transfer body; and a fixing unit that fixes the toner image transferred from the transfer body by the transfer unit.
An image forming apparatus using the fixing device according to claim 1 or 2 as the fixing means.

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