JP3818015B2 - Image fixing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image fixing device in which a recording sheet is inserted between a heating roll and a pressure roll that are in pressure contact with each other, and an unfixed toner image transferred onto the recording sheet is heated and pressed to be fixed to the recording sheet. Specifically, the present invention relates to an improvement for uniformly pressing the pressure roll along the axial direction of the heating roll.
[0002]
[Prior art]
Conventionally, as an image fixing apparatus for fixing an unfixed toner image transferred on a recording sheet by an electrophotographic copying machine or a laser beam printer, to the recording sheet, a heating roll incorporating a heater and an additive coated with an elastic layer are used. It is generally known that a pressure roll is brought into pressure contact with each other and a recording sheet is inserted between the heating roll and the pressure roll. In this conventional fixing device, a pressure roll covered with an elastic layer is brought into pressure contact with the heating roll, whereby the elastic layer is crushed to form a nip region that sandwiches a recording sheet, and carries an unfixed toner image. While the recorded sheet passes through the nip area, the toner on the recording sheet is melted by the heat energy of the heater, and the melted toner is pressed against the recording sheet, thereby firmly fixing the toner image to the recording sheet. It has come to be able to do.
[0003]
However, when the heated roll and the pressure roll, which are rotatably supported, are simply pressed against each other, a large pressure can be obtained near both ends in the axial direction of these rolls, but the pressure is near the center in the axial direction. The nip region with a uniform width could not be formed along the axial direction of the heating roll. For this reason, the toner image cannot be fixed on the recording sheet with sufficient strength, and the recording sheet that has passed through the nip region is liable to be wrinkled. In particular, in recent years, in order to shorten the print waiting time at the start of the printer, etc., a device has been devised to reduce the heat capacity by reducing the thickness of the heating roll. Since the heating roll is likely to bend when the two are brought into pressure contact with each other, the pressing force tends to be insufficient in the vicinity of the center in the axial direction of the heating roll.
[0004]
In order to solve such problems, there have been proposed image fixing devices disclosed in JP-A-9-68883 and JP-A-11-38811. In this image fixing apparatus, consideration is given to obtaining a uniform pressure along the axial direction of the heating roll by improving the structure of the pressure roll, and the heating roll has no change from the conventional configuration. The pressure roller has a hollow cylindrical pressure sleeve having an elastic layer formed on the surface and rotated by the heating roll, and is inserted into the hollow portion of the pressure sleeve so that the pressure sleeve is pressed against the heating roll. The pressing shaft is formed such that the outer diameter gradually decreases from the axial center to both ends. When the pressure shaft is pressurized and the heating roll and the pressure sleeve are pressed against each other, the elastic layer on the surface of the pressure sleeve is crushed to form a nip region of the recording sheet, but the outer diameter of the pressure shaft is in the axial direction. Since it is the largest in the central portion, even when the heating roll is bent, a uniform pressing force is obtained along the axial direction of the heating roll, and a nip region having a uniform width along the axial direction is obtained. Considered to be formed.
[0005]
[Problems to be solved by the invention]
However, the pressure shaft of the image fixing device disclosed in these publications is formed by thickening a coating layer made of an elastic body on the surface of a metal shaft having a constant outer diameter and adjusting the thickness of the coating layer as described above. Since the outer shape of the shaft is formed, there are the following problems. That is, since the thickness of the coating layer constituting the pressing shaft is different between the center and the end in the axial direction, the elasticity of the coating layer when the pressing shaft is pressed against the heating roll via the pressing sleeve. As a result, the amount of deformation is different between the center and the end in the axial direction, and it is difficult to form the nip region with a uniform width along the axial direction. Further, when the temperature of the coating layer rises due to the heat energy given from the heating roll, the result is that the amount of expansion of the coating layer differs between the axial center and both ends of the pressing shaft, and a uniform pressure distribution is also achieved along the axial direction. It becomes difficult to obtain. Furthermore, the coating layer made of an elastic body gradually wears due to contact with the inner peripheral surface of the pressure sleeve, but as described above, the width of the nip region is different between the center and both ends in the axial direction. The amount of wear of the coating layer also became non-uniform, and the non-uniform distribution of the applied pressure tended to increase as the fixing device was used over time. Furthermore, the coating layer of the pressing shaft needs to be formed into a convex curved surface that can obtain the desired pressure distribution between the heating roll and the pressing sleeve, but the elastic body is processed into such a shape. However, it was difficult to increase the manufacturing cost.
[0006]
The present invention has been made in view of such problems, and an object of the present invention is to allow a heating roll and a pressure roll to be brought into pressure contact with each other with a uniform applied pressure along the axial direction. An object of the present invention is to provide an image fixing apparatus that can fix a recording sheet with sufficient strength and that does not cause wrinkles on the recording sheet.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the image fixing apparatus of the present invention inserts a recording sheet between a heating roll and a pressure roll that are in pressure contact with each other, and an unfixed toner image transferred to the recording sheet is recorded on the recording sheet. On the premise of an image fixing device that melts and presses against a sheet, the pressurizing roll has a hollow cylindrical pressurizing sleeve having an elastic layer formed on the surface thereof and rotated by the heating roll, and this pressurizing roll. The pressure sleeve is inserted into the hollow portion of the pressure sleeve so that the pressure sleeve is pressed against the heating roll, and the shape of the outer peripheral surface in contact with the inner peripheral surface of the pressure sleeve is gradually narrowed from the center in the axial direction toward both shaft ends. a metal pressing shaft which is formed so as made up of a peripheral surface or an elastic covering layer formed with a uniform thickness on the inner circumferential surface of the pressure sleeve of the pressing shaft, the pressing shaft, A shaft, and is characterized in that it is composed of a plurality of driven disc rotatably arranged around the support shaft.
[0008]
According to such technical means, the shape of the outer peripheral surface of the metal pressing shaft is formed so as to gradually decrease from the center in the axial direction toward both axial ends. Since an elastic coating layer having a uniform thickness is formed on the inner peripheral surface of the pressure sleeve, when the pressing shaft is brought into pressure contact with the inner peripheral surface of the pressure sleeve, the elastic coating layer is in the axial direction of the heating roll. Therefore, the heating roll and the pressure roll can be brought into pressure contact with each other with substantially uniform pressure along the axial direction, and the nip region of the recording sheet can be aligned along the axial direction. Thus, it can be formed with a substantially uniform width. As a result, the toner image on the recording sheet can be reliably fixed to the recording sheet, and troubles such as generation of wrinkles on the recording sheet can be avoided.
[0009]
Here, the covering elastic layer formed on the outer peripheral surface of the pressing shaft or the inner peripheral surface of the pressure sleeve acts so that the metal pressing shaft and the inner peripheral surface of the pressure sleeve are in close contact with each other without a gap. At the same time, it serves to reduce noise generated by the contact between the pressing shaft and the pressing sleeve. For this reason, even if this covering elastic layer is formed in the outer peripheral surface of a press shaft, it may be formed in the inner peripheral surface of a pressurization sleeve.
[0010]
On the other hand, from the viewpoint that the pressing shaft formed so that the outer diameter of the central portion in the axial direction is larger than the outer diameter of both shaft ends presses the pressing sleeve against the heating roll, the pressing sleeve has the shape of the pressing shaft. It is necessary that the pressing shaft can be deformed by copying, and the pressing shaft needs not to be easily deformed by a reaction force obtained by pressing the pressing sleeve against the heating roll. Further, when the pressure sleeve is in pressure contact with the heating roll, if the heating roll is easily deformed, sufficient pressure cannot be applied between the heating roll and the pressure sleeve. Therefore, the rigidity in the direction perpendicular to the axial direction of the heating roll, the pressure sleeve, and the pressing shaft, that is, the radial direction is preferably decreased in the order of the pressing shaft, the heating roll, and the pressing sleeve. With this configuration, the pressure sleeve is easily deformed following the shape of the outer peripheral surface of the pressing shaft when pressed against the heating roll, so that the inner peripheral surface of the pressing sleeve is tightly connected to the outer peripheral surface of the pressing shaft. Accordingly, the pressure sleeve and the heating roll can be pressed more uniformly along the axial direction.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an image fixing device of the present invention will be described in detail with reference to the accompanying drawings.
1 and 2 are a schematic front view and a schematic side view showing an embodiment of an image fixing apparatus to which the present invention is applied. The image fixing apparatus of this embodiment includes a heating roll 1 having a built-in heater 10 and a pressure roll 2 in pressure contact with the heating roll 1, and a recording sheet P onto which an unfixed toner image T has been transferred. By inserting between the heating roll 1 and the pressure roll 2, the toner is melted and pressed on the surface of the recording sheet P, and the toner image T is fixed. The pressure roll 2 includes a hollow cylindrical pressure sleeve 20 that rotates following the heating roll 1, and a pressing shaft 21 inserted into the hollow portion of the pressure sleeve 20. The pressing sleeve 20 is configured to be pressed against the heating roll 1 by pressing the pressing shaft 21 against the heating roll 1.
[0012]
First, the heating roll 1 is obtained by coating the outer peripheral surface of a steel core 11 formed in a cylindrical shape with PFA (perfluoroalkyl vinyl ether copolymer resin) as a release layer 12, and the core 11 has a diameter. The release layer 12 is coated to a thickness of about 30 μm while being formed to have a thickness of 35 mm, a thickness of 0.4 mm, and an effective length of 345.5 mm in the axial direction. Both ends of the core 11 are supported by the fixed frame 3 via bearings 13 and are driven to rotate at a peripheral speed of about 300 mm / s by a drive motor (not shown). A Colts lamp (100 V, 1000 W) as a heater 10 is built in the hollow portion of the core 11, and the heater 10 is fixedly disposed so as to penetrate the core 11. A temperature sensor 14 is in contact with the surface of the heating roll 1, and the heater 10 is turned on / off based on a detection value of the temperature sensor 14, so that the surface temperature of the heating roll 1 is always fixed. (For example, 170 ° C.).
[0013]
On the other hand, the pressure sleeve 20 constituting the pressure roller 2 is formed by uniformly providing an elastic layer 23 made of silicon rubber on the outer peripheral surface of a stainless steel core 22 formed in a hollow cylindrical shape. While the outer diameter is 29 mm, the wall thickness is 0.3 mm, and the effective length is 318 mm, the elastic layer 23 is formed with a thickness of 3 mm and a rubber hardness of 60 ° (JIS A). The pressure sleeve 20 is only suspended by a pressing shaft 21 penetrating through the hollow portion, and is not supported by the fixed frame 3 and the movable frame 4 itself.
[0014]
As shown in FIG. 3, the pressing shaft 21 is a stainless steel member having a shape that gradually narrows from the center in the axial direction to both axial ends, and the outer peripheral surface is formed in a convex curve shape. The effective length of the pressing shaft 21 is 319 mm, the minimum outer diameter within the effective length is 19 mm, and the outer peripheral surface is covered with an elastic coating layer 24 made of silicon rubber having a thickness of 1 mm. Shaft portions 25 are projected at both shaft ends of the pressing shaft 21, and the shaft portions 25 are supported by the movable frame 4 via bearings 26.
[0015]
In order to restrict the movement of the pressure sleeve 20 in the axial direction, a side guide member 27 is fitted to the shaft portion 25 of the pressing shaft 21, and the pressure sleeve 20 abuts against the side surface of the side guide member 27. The movement in the axial direction (thrust direction) is restricted. When the side guide member 27 and the elastic layer 23 of the pressure sleeve 20 come into contact with each other, the rotational speed of the pressure sleeve 20 fluctuates due to the frictional resistance therebetween, and image defects such as rubbing against the toner image T on the recording sheet P occur. Therefore, in the fixing device of this embodiment, the elastic layer 23 of the pressure sleeve 20 is held in non-contact with the side guide member 27. Specifically, as shown in FIG. 4, the core 22 of the pressure sleeve 20 protrudes from the elastic layer 23 at both ends in the axial direction, and only the core 22 contacts the side guide member 27 and the shaft of the pressure sleeve 20. The movement in the direction is restricted. Accordingly, the frictional resistance acting between the side guide member 27 and the pressure sleeve 20 is reduced, and the rotation of the pressure sleeve 20 with respect to the heating roll 1 is smoothly performed.
[0016]
On the other hand, the movable frame 4 is urged toward the fixed frame 3 by a pressing means (not shown), whereby the pressing shaft 21 is pressed against the heating roll 1, and the pressing sleeve 20 is connected to the heating roll 1 and the pressing shaft. 21. As a result, the pressure sleeve 20 comes into pressure contact with the heating roll 1, and when the heating roll 1 rotates, the pressure sleeve 20 rotates with the rotation. Further, when the heating roll 1 and the pressure sleeve 20 are pressed against each other, the elastic layer 23 formed on the outer peripheral surface of the pressure sleeve 20 is crushed, and the recording sheet P is placed between the heating roll 1 and the pressure sleeve 20. A nip region for pinching is formed. While the recording sheet P passes through the nip region, thermal energy is applied from the heating roll 1, and the unfixed toner T transferred to the recording sheet P is melted. Further, the melted toner is crushed by the pressure contact between the heating roll 1 and the pressure sleeve 20 and is pressed onto the recording sheet P. Thus, when the recording sheet P passes through the nip region, the toner image T transferred to the recording sheet P is fixed to the recording sheet P.
[0017]
In order to obtain a good fixed image without poor fixing or rubbing of the toner image T, it is important that the heating roll 1 and the pressure sleeve 20 are in pressure contact with each other with a uniform applied pressure along the axial direction. Further, when the nip region is formed with a non-uniform width along the axial direction, the transportability of the recording sheet P deteriorates, and wrinkles are likely to occur in the recording sheet P that has passed through the fixing device. It is also important that the nip region is formed with a uniform width along the axial direction.
[0018]
However, since the heating roll 1 is not a perfect rigid body, when the pressure roller 20 is pressed against the pressure sleeve 20 with both shaft ends supported by the fixed frame 3, the heating roll 1 is slightly bent in the radial direction as shown in FIG. Arise. Moreover, since the thickness of the core 11 is made thin in order to keep the heat capacity low, the heating roll 1 is crushed in the radial direction by the pressure contact of the pressure sleeve 20. Therefore, in order to generate a substantially uniform pressure distribution in the axial direction in the nip region, it is necessary to determine the shape of the outer peripheral surface of the pressing shaft 21 in consideration of the amount of bending and the amount of crushing of the heating roll 1. is there. For this reason, in the fixing device of this embodiment, the amount of deflection and the amount of crushing of the heating roll 1 generated when a certain load is applied to the heating roll 1 are obtained, and the outer surface of the pressing shaft 21 is determined based on this. The shape is determined. FIG. 5 assumes that a load of 800 N is applied to both shaft ends of the heating roll 1 supported on the fixed frame 3, and the amount of displacement in the radial direction of the nip region resulting from the bending and crushing of the heating roll 1 is shown. The result is obtained by the finite element method, and the result is shown along the axial direction of the heating roll 1. That is, when the load is applied to the heating roll 1 from the radial direction, the nip region formed by the pressure contact between the heating roll 1 and the pressure sleeve 20 is bent as shown by the graph line in FIG. Become. For this reason, if the shape of the outer peripheral surface of the pressing shaft 21 is formed in a convex curved surface shape along this graph line, the distribution of the applied pressure along the axial direction in the nip region can be made substantially uniform.
[0019]
Further, even when the pressing shaft 21 is formed in a specific shape in this way, the pressing sleeve 20 that is actually pressed against the heating roll 1 is deformed following the shape of the pressing shaft 21 and becomes the heating roll 1. If it does not adhere, it will be difficult to generate a uniform pressure distribution in the axial direction in the nip region. For this reason, in the fixing device of this embodiment, the rigidity of the pressure sleeve 20 in the radial direction is set to be smaller than that of the pressing shaft 21 and the heating roll 1. Further, since the heating roll 1 also needs to be deformed following the shape of the pressing shaft 21, the rigidity of the heating roll 1 in the radial direction is set smaller than that of the pressing shaft 21. That is, when comparing the radial rigidity among the three of the heating roll 1, the pressure sleeve 20, and the pressing shaft 21, the rigidity decreases in the order of the pressing shaft 21, the heating roll 1, and the pressing sleeve 20. Accordingly, it is possible to deform the heating roll 1 and the pressure sleeve 20 along a specific shape given to the pressing shaft 21, and to tightly press both of them.
[0020]
Further, the pressing shaft 21 of the present embodiment is formed by processing a stainless steel rod into the above-described shape, and covers the outer peripheral surface pressed against the pressure sleeve 20 with an elastic coating layer 24 having a uniform thickness. Therefore, when a uniform pressure distribution is obtained along the axial direction of the nip region, the amount of deformation of the elastic coating layer 24 is also uniform along the axial direction, and the width of the nip region is reduced in the axial direction. It can be made substantially uniform along. Further, even when the thermal energy of the heating roll 1 flows into the elastic coating layer 24 and the elastic coating layer 24 undergoes thermal expansion, the distribution of the thermal expansion amount of the elastic coating layer 24 is in the axial direction. Therefore, the pressure distribution in the nip region does not become non-uniform due to the thermal expansion of the elastic coating layer 24. Furthermore, since the pressure distribution in the axial direction in the nip region is uniform, even if the elastic coating layer 24 is worn with use of the fixing device over time, the amount of wear is uniform along the axial direction. It is also possible to avoid the inconvenience that the pressure distribution becomes non-uniform over time.
[0021]
FIG. 6 is a graph showing the result of measuring the pressure distribution in the nip region of the fixing device of the present embodiment having the above configuration, and the horizontal axis indicates the position of the heating roll 1 in the axial direction. As shown in this graph, in the fixing device of the present embodiment, the heating roll 1 and the pressure sleeve 20 could be brought into pressure contact with each other with a substantially uniform pressure along the axial direction. Further, when the width of the nip region at this time was measured, it was 4 mm at the center in the axial direction and 4.2 mm at both ends, and a nip region having a substantially uniform width could be formed. Then, when the recording sheet P was actually inserted into the fixing device of this embodiment and the fixing strength of the toner image T and the occurrence of wrinkles on the recording sheet P were observed, the toner image T was observed at both the center and both ends of the recording sheet P. The image was fixed well, and no wrinkles occurred on the recording sheet P.
[0022]
Next, FIG. 7 shows another embodiment of the pressure sleeve 20.
In the above-described embodiment, the elastic coating layer 24 is formed on the outer peripheral surface of the pressing shaft 21. The elastic coating layer 24 is used to tightly press the pressing shaft 21 and the pressure sleeve 20, and Since it is for preventing the generation | occurrence | production of the noise in these contacts, it does not necessarily need to form in the outer peripheral surface of the press shaft 21. FIG. That is, as shown in FIG. 7, an elastic coating layer 28 having a uniform thickness may be provided on the inner peripheral surface of the core 22 constituting the pressure sleeve 20. In this case, it is not necessary to form the elastic coating layer 24 on the outer peripheral surface of the pressing shaft 21.
[0023]
FIG. 8 shows another embodiment of the pressing shaft.
As described above, the outer peripheral surface of the pressing shaft 21 is formed in a convex curved shape, and has a maximum outer diameter at the central portion in the axial direction and a minimum outer diameter at both end portions. The perimeter of the surface is different between the center and both ends. However, since the pressure sleeve 20 with which the pressure shaft 21 is in pressure contact is rotated by the heating roll 1 and rotated at a constant peripheral speed, the pressure shaft in the above embodiment is used to absorb the difference in circumferential length. A slip is generated between the pressure sleeve 21 and the pressure sleeve 20. For this reason, the elastic coating layer 24 or 28 formed on the outer peripheral surface of the pressing shaft 21 or the inner peripheral surface of the pressure sleeve 20 is worn, and the amount of wear differs between the central portion and both end portions in the axial direction. It becomes a thing. Accordingly, in the above embodiment, as the usage time of the fixing device is cumulatively increased, the elastic coating layer 24 or 28 is unevenly worn and the pressure distribution in the nip region changes, and the width of the nip region also changes. There was a concern.
[0024]
Therefore, in the pressing shaft 5 shown in FIG. 8, a plurality of pressing disks 50 having different diameters are arranged with respect to the support shaft 51, and these pressing disks 50 are rotatably supported with respect to the support shaft 51. The rotation speed difference due to the difference is absorbed for each pressing disk 50. In other words, the pressing shaft 21 of the above embodiment is divided into a plurality of disk members, and the individual disk members are arranged so as to be rotatable with respect to the support shaft. If comprised in this way, since each press disk 50 from which circumference differs differs can rotate freely with respect to the spindle 51, when the press shaft 5 is rotated with the pressurization sleeve 20, each press disk 50 is made. There is no slip between the pressure sleeve 20 and the pressure sleeve 20, and the elastic coating layer 24 or 28 is prevented from being worn. As a result, it is possible to prevent the pressure distribution in the nip region from fluctuating due to the use of the fixing device over time, and to avoid the trouble that the width of the nip region changes.
[0025]
Next, FIG. 9 shows another example of the contact structure between the pressure sleeve 20 and the side guide member 27.
In the embodiment shown in FIG. 4, only the core 22 of the pressure sleeve 20 is configured to contact the side guide member, and the frictional resistance between the pressure sleeve 20 and the side guide member 27 is reduced. However, in the example shown in FIG. 9, the surface and side surfaces of the elastic layer of the pressure sleeve 20 are covered with a PFA layer 29 having a thickness of 30 μm, and the PFA layer 29 is in contact with the side guide member 27. Since the frictional resistance of the surface of the PFA layer 29 is lower than that of the elastic layer 23, the frictional resistance acting between the side guide member 27 and the pressure sleeve 20 is also reduced in this example, and the heating roll of the pressure sleeve 20 is reduced. The follow-up with respect to 1 is performed smoothly.
[0026]
FIG. 10 also shows another example of the contact structure between the pressure sleeve 20 and the side guide member 27.
In this example, the side guide member itself is composed of a thrust ball bearing 30, and the side guide member 30 is configured to be able to support its rotation while restricting the axial movement of the pressure sleeve 20. In this example, since frictional resistance hardly acts between the pressure sleeve 20 and the side guide member 30, the pressure sleeve 20 rotates more smoothly than the example shown in FIG. 4 or FIG. Image defects such as T rubbing could be completely eliminated. In the example shown in FIG. 10, the thrust ball bearing itself is used as the side guide member. However, the thrust ball bearing is fixed to both ends of the pressure sleeve 20 and is made to contact the side guide member 27 of a simple plate material. The same result could be obtained even if it was configured.
[0027]
【The invention's effect】
As described above, according to the image fixing device of the present invention, the shape of the outer peripheral surface of the metal pressing shaft is formed so as to gradually decrease from the center in the axial direction toward both ends. An elastic coating layer having a uniform thickness is formed on the outer peripheral surface of the pressing shaft or the inner peripheral surface of the pressure sleeve, and the heating roll and the pressure roll are pressed against each other with substantially uniform pressure along the axial direction, and recording is performed. Since the nip area of the sheet is formed with a substantially uniform width along the axial direction, the toner image can be fixed to the recording sheet with sufficient strength, and wrinkles and the like are generated on the recording sheet. It is possible to avoid troubles.
[Brief description of the drawings]
FIG. 1 is a schematic front view showing an embodiment of an image fixing apparatus of the present invention.
FIG. 2 is a schematic side view showing an embodiment of the image fixing device of the present invention.
FIG. 3 is a cross-sectional view showing a pressing shaft according to an embodiment.
FIG. 4 is an enlarged view showing a contact structure between a pressure sleeve and a side guide member according to the embodiment.
FIG. 5 is a graph showing a result of obtaining a displacement amount of a heating roll of an example by a finite element method.
FIG. 6 is a graph illustrating a pressure distribution in a nip region of the fixing device according to the example.
FIG. 7 is a front cross-sectional view showing another example of a pressure sleeve.
FIG. 8 is a front view showing another example of a pressing shaft.
FIG. 9 is an enlarged view showing a second embodiment of a contact structure between a pressure sleeve and a side guide member.
FIG. 10 is an enlarged view showing a third embodiment of the contact structure between the pressure sleeve and the side guide member.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Heating roll, 20 ... Pressure sleeve, 21 ... Pressing shaft, 23 ... Elastic layer, 24 ... Elastic coating layer, 27 ... Side guide member

Claims (6)

In an image fixing apparatus in which a recording sheet is inserted between a heating roll and a pressure roll that are pressed against each other, and an unfixed toner image transferred to the recording sheet is melted and pressed against the recording sheet.
The pressure roll has a hollow cylindrical pressure sleeve having an elastic layer formed on the surface thereof and rotating following the heating roll,
The pressure sleeve is inserted into the hollow portion of the pressure sleeve to press-contact the pressure sleeve to the heating roll, and the shape of the outer peripheral surface in contact with the inner peripheral surface of the pressure sleeve is gradually increased from the center in the axial direction toward both shaft ends. A metal pressing shaft formed to be thin;
It is composed of an elastic coating layer formed with a uniform thickness on the outer peripheral surface of this pressing shaft or the inner peripheral surface of the pressure sleeve,
2. The image fixing apparatus according to claim 1, wherein the pressing shaft includes a support shaft and a plurality of driven disks arranged around the support shaft so as to be rotatable .   The image fixing device according to claim 1, wherein a shape of an outer peripheral surface of the pressing shaft is formed based on a radial displacement amount of the heating roll in a nip region.   The image fixing apparatus according to claim 1, wherein rigidity in the radial direction of the heating roll, the pressure sleeve, and the pressing shaft decreases in the order of the pressing shaft, the heating roll, and the pressure sleeve.   4. A nip region of a recording sheet formed by pressure contact between the heating roll and a pressure sleeve has a substantially uniform width along the axial direction of the heating roll. An image fixing apparatus according to claim 1.   2. The image according to claim 1, wherein a side guide member for restricting the axial movement of the pressure sleeve is provided, and the side guide member and the elastic layer on the surface of the pressure sleeve are held in a non-contact manner. Fixing device. 6. The image fixing device according to claim 5 , wherein a low friction member is interposed between the side guide member and the elastic layer of the pressure sleeve.

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