JPH05241467A - Fixing device - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain sufficient fixing results when using thick recording paper such as postcards and envelopes without causing wrinkles on thin recording paper. The pressure roller 1 used for the fixing means is formed as a pressure roller 1 having a hardness distribution in the axial direction in which the hardness of the central portion is softer than the hardness of the axial end portions. The size of the nip width W is made uniform so that the fixing result is not affected. [Effect] Even in the case of thick recording paper, the good fixing state can be always maintained, and the life of the fixing device can be extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a toner fixing device used in an image forming apparatus utilizing an electrophotographic process.

[0002]

2. Description of the Related Art An image forming apparatus utilizing an electrophotographic process has a structure in which a toner image formed on a photosensitive drum is transferred to a recording sheet and then the toner is fused to the toner image on the recording sheet. The toner is fixed to the recording paper by a fixing device that applies both heat and pressure for pressing the heat-melted toner onto the recording paper. FIG. 5 is used to explain this fixing device in detail. The fixing device includes a heating means (not shown) and includes bearings 8a and 8b.
A heating roller 7 made of a rigid tube whose both ends are supported by the core member 4, and a surface of a core material 4 covered with an elastic layer 2 and bearings 6a and 6b.
The pressure roller 1 whose both ends are supported by the bearing 6
Pressure means springs 10a and 10 are provided on a and 6b, respectively.
The load is applied by b, and the heating roller 7 of the pressure roller 1
The pressure is applied indirectly to the bearing via the bearing 6. The leading end of the recording sheet (not shown) is sandwiched between the heating roller 7 and the pressure roller 1 which are in pressure contact with each other, the power of the driving means (not shown) is transmitted to the driving means 9, and the heating roller 7 is rotated to convey the recording sheet. It provides the toner with the desired heat and pressure. The pressure roller 1 is generally a driven type having no driving means. Further, in order to improve the releasability from the toner, it is usual to subject each surface of the heating roller 7 and the pressure roller 1 to a fluorine-based surface treatment.

FIG. 6 shows an enlarged cross-sectional view of a portion where the heating roller 7 and the pressure roller 1 are in pressure contact with each other. As described above, since the heating roller 7 that is a rigid body and the pressure roller 1 that is an elastic body are brought into pressure contact with each other, the contact portion of the pressure roller 1 with the heating roller 7 is crushed and follows the surface shape of the heating roller 7. .. Here, the length of the collapsed portion is referred to as "nip width W".

FIG. 7 shows the relationship between the position in the paper width direction and the nip width W. The nip width W is larger at both end portions than at the central portion. The paper width direction here means a direction coinciding with the axial direction of the heating roller 7 and the pressure roller 1. The reason why the nip width W is larger at both end portions is that the pressure roller 1 is configured to apply a load to both end portions, so that the central portion of the pressure roller 1 in the paper width direction is easily bent toward the non-heating roller side, and both end portions are easily bent. This is because the distance between the centers of the heating roller 7 and the pressure roller 1 is longer in the center than in the center, and the elastic layer 2 on the surface of the pressure roller 1 has a smaller crush amount in the center portion.

As a result, both end portions of the recording paper in the paper width direction pass through the large nip width W, a sufficient heating time is given, and a good fixing result is obtained. On the other hand, since the central portion passes through the nip width W which is relatively smaller than the both end portions, the heating time is short and it is difficult to obtain a good fixing result. This is especially noticeable when using thick recording paper such as postcards and envelopes. If the state where the fixing property is insufficient continues to occur, a phenomenon called “offset”, in which fixing failure toner sticks to the surface of the fixing roller 7 and causes paper jam, it is necessary to replace the fixing device.

In order to solve such a problem, (1) a temperature distribution is high only in the central part as means for constructing a heating device sufficient to obtain a good fixing result even in the central part. Use different heating means. (2) The fixing means having a large nip width W is provided so that sufficient heating time can be given even in the central portion. (3) When using thick recording paper, it may be possible to switch to a higher temperature setting by operating the panel or the like. However, in the case of the plan (1), it becomes expensive to manufacture a special heating means having a high temperature distribution only in the central portion. In the case of the plan (2), the nip width W at both end portions becomes too large. As a result, excessive heat is applied to both ends of the recording paper, and paper wrinkles are likely to occur on thin recording paper. In case (3), there are new problems such as complicated operation, etc. Is not suitable.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to produce a thick recording sheet without causing wrinkles on the thin recording sheet. However, it is an object of the present invention to provide a novel fixing device capable of obtaining a good fixing result.

[0008]

In order to solve such a problem, according to the present invention, the pressure roller inside the fixing device for heating and pressing the recording paper discharged from the photoconductor drum has axial end portions. It is characterized in that the hardness of the central portion has a softer hardness distribution in the axial direction than the hardness of.

[0009]

The details of the present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a sectional view of an embodiment of a pressure roller used in the fixing device of the present invention, in which reference numerals 1, 2, and 4 are the same as those in FIG. In this embodiment, the elastic layers 3a and 3b are composed of elastic layers having a second hardness different from that of the elastic layer 2. The second hardness used for the elastic layer 3 is the first one used for the elastic layer 2. Harder than the hardness of. This is because a hard elastic layer 3 is provided at both end portions that are easily crushed, and a soft elastic layer 2 is provided at the central portion that is hard to be crushed. Although not shown, since the surface of the pressure roller 1 is coated with the above-mentioned fluorine-based surface treatment for improving the releasability from the toner, the seam of the elastic layers 2 and 3 is the performance of the fixing device. Does not affect.

FIG. 2 is a diagram showing the relationship between the position in the paper width direction and the nip width W in the fixing device of the present invention shown in FIG. In the fixing device of the present invention, the size of the nip width W between the central portion and both end portions can be made uniform so as not to affect the fixing result. By using the fixing device having such a configuration, it is possible to achieve both good fixing results on thick recording paper and prevention of paper wrinkles on thin recording paper.

FIG. 3 is a sectional view of another embodiment of the pressure roller used in the fixing device of the present invention.
Is similar to FIG. In this embodiment, the diameter of the cored bar 4 is configured such that both end portions are thicker than the central portion. Since the outer diameter of the elastic layer 2 is uniform over the entire area, a difference occurs in the thickness of the elastic layer 2 between the central portion and both end portions. As a result, when a load is applied and the elastic layer 2 is crushed,
Since both ends have a small thickness, the influence of the hardness of the cored bar 4 inside is transmitted to the surface of the elastic layer 2, and a hard property can be imparted as the true hardness. On the other hand, since the central portion has a large thickness, the influence of the hardness of the inner cored bar 4 is less likely to be transmitted to the surface of the elastic layer 2, and the softness can be imparted as the true hardness. As described above, by using the pressure roller having a hardness distribution in the axial direction in which the hardness of the central portion is softer than the hardness of the axial end portions, the central portion and the end portions are the same as in the first embodiment. The size of the nip width W can be made uniform so as not to affect the fixing result.

FIG. 4 is a cross-sectional view of still another embodiment of the pressure roller used in the fixing device of the present invention, in which reference numeral 1 indicates
2 and 4 are the same as in FIG. Reference numeral 5 represents a fluorine-based surface treatment layer that covers the elastic layer 2. In this embodiment, the thickness of the surface treatment layer 5 is configured so that both end portions are thicker than the central portion. Usually, the fluorine-based material used for the surface treatment layer 5 is harder than the material used for the elastic layer 2, and the outer diameter of the pressure roller 1 is uniform over the entire area. As a result, the thick portion of the surface treatment layer 5 and the both end portions shown in the present embodiment can have a relatively hard hardness as the pressure roller 1. On the other hand, the thin portion of the surface treatment layer 5, that is, the central portion shown in the present embodiment, can be softened even if the pressure roller 1 is made of the same elastic material. As described above, by using the pressure roller having a hardness distribution in the axial direction in which the hardness of the central portion is softer than the hardness of the axial end portions, the central portion and the end portions are the same as in the first embodiment. The nip width W can be made uniform to such an extent that the fixing result is not affected.

The elastic layer 2 and the elastic layer 3 used in the present invention.
The material of the above may be general silicone rubber or sponge. The surface treatment layer 5 may be an existing coating form such as a general coating, a tube, a heat shrinkable tube.

[0015]

As described above, in the present invention, since the pressure roller inside the fixing device is configured to have the nip width W of the central portion and both end portions in the axial direction substantially equal to each other, it is expensive. It is possible to obtain a sufficient fixing result when a thick recording paper such as a postcard or an envelope is used without inviting a different heating means or a wrinkle of the thin recording paper. As a result, even in the case of thick recording paper, it is possible to always maintain a good fixing state, to extend the life of the fixing device, reduce the frequency of replacement work of the fixing device, and reduce the running cost. ..

[Brief description of drawings]

FIG. 1 is a sectional view of an example of a pressure roller used in a fixing device of the present invention.

FIG. 2 is a diagram showing the relationship between the position in the paper width direction and the nip width W of the fixing device of the present invention.

FIG. 3 is a sectional view of another embodiment of the pressure roller used in the fixing device of the present invention.

FIG. 4 is a cross-sectional view of another embodiment of a pressure roller used in the fixing device of the present invention.

FIG. 5 is a diagram of a conventional fixing device.

FIG. 6 is an enlarged cross-sectional view of a contact portion between a heating roller and a pressure roller.

FIG. 7 is a position and a nip width W in the paper width direction of a conventional fixing device.
It is a figure showing the relationship of.

[Explanation of symbols]

 1 pressure roller

Claims (1)

[Claims] 1. A fixing device for heating and pressing a recording sheet discharged from a photosensitive drum, wherein a pressure roller inside the fixing device has a hardness in a central portion which is softer than hardness at both end portions in the axial direction. A fixing device characterized by having a distribution.