JPH0950202A - Fixing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing device capable of exhibiting excellent fixing performance by preventing both of wrinkles and abnormal image. SOLUTION: The surface layers of a fixing roller 1 and a pressure roller 2 are of elastic layers. The fixing roller 1 is formed into an inverted hourglass shape and the pressure roller 2 is formed into a straight shape. Then, the shape of the fixing roller 1 is set so that the relation of nip widths (the length in the carrying direction of a paper sheet of a nip) in the central part and end parts in the axial directions of the rollers, of the press-contact part of them is in the range of -5.4 to 0 in an expression: (central part nip width - end part nip width) ÷ central part nip width × 100.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device in an image forming apparatus such as a copying machine or a printer, and more particularly to a fixing device suitable for a full color image forming apparatus.

[0002]

2. Description of the Related Art An unfixed image (an image formed by a toner made of a heat-fusible resin or the like) mounted on an image forming apparatus such as a copying machine, a printer, or a facsimile and transferred onto a recording medium such as a transfer sheet. A fixing roller heated by a heating element (heater) is pressed against a pressure roller, and the recording medium is passed between the two rollers so that heat, pressure and heat are applied to the fixing roller. 2. Description of the Related Art A heat roller type fixing device for fixing an unfixed toner image on a recording medium by using a fixing device is well known.

In the case of a color image forming apparatus which obtains a full-color image by superposing toner images of three or four colors, not only a fixing roller but also a pressure roller is used in order to perform reliable fixing. A heating element is built in, and an elastic layer such as silicone rubber is also provided on the fixing roller in order to increase the nip width (length of the nip in the sheet conveying direction).

By the way, in the heat roller type fixing device, the sheet is sandwiched between the fixing roller and the pressure roller and the sheet is conveyed and processed. At that time, the moisture contained in the sheet is evaporated. Therefore, there is a problem that wrinkles occur due to a change in water content between the fixed portion and the unfixed portion of the sheet.

As a method for preventing wrinkles from being generated on the paper, the roller is formed into a drum shape (inverse crown shape in cross section) so that the central portion of the roller has a minimum diameter and gradually increases toward both ends. There is a method of giving a force (spread effect) for spreading to both ends of the paper by making the paper transport speed at the roller end faster than the center transport speed. FIG. 4 shows a drum-shaped roller and the drum effect thereof. In the roller 20 shown in FIG. 4A, the outer diameter D _{2 at} both ends is larger than the outer diameter D _{1 at the} central portion.
As for the sheet conveyance speed by the roller 20, in FIG. 4B, the conveyance speed V ₂ at the roller end is the conveyance speed V _{1 at the} center.
And a force that pulls both ends of the paper P outward is applied to the paper P.

As a method for preventing the wrinkles of the paper, a paper guide plate (guide plate) for guiding the paper to the nip portion is formed such that the central portion thereof is projected more to the downstream side than the both end portions in the paper conveying direction. In addition, the sheet is arranged so as to be guided above the nip portion so that both end portions of the leading edge of the sheet are separated from the guide plate and enter the nip portion before the central portion. It is disclosed by Japanese Patent Publication No. 00295.

[0007]

However, when the above-mentioned technique for preventing wrinkles is applied to a fixing device for a color image forming apparatus in which an elastic layer is also provided in the fixing roller, It was confirmed that defects other than wrinkles would occur. This is because the drum effect on the paper is disturbed because the drum effect (tensile force) applied to the paper is not appropriate or the paper contacts the fixing roller before the paper enters the nip portion. Is fixed in a scattered state. An example of such an abnormal image is shown in FIG. The abnormal image shown in this figure is generated when a solid image is fixed, and curved abnormal images U are generated on both sides near the trailing edge of the sheet P in the conveying direction (indicated by arrow S). . Note that, in this figure, the negative and the positive are reversed, and the solid portions are shown as a white background and the abnormal image portions are shown as black dots. In an experiment by the inventors of the present application, it is A3 size paper that an abnormal image is particularly likely to occur, and distortion due to fixing occurs at the trailing edge of the paper. Further, when fixing is performed on both sides of a sheet (double-sided image), an abnormal image is more likely to occur than a single-sided image.

It has also been found that such an abnormal image is found when the thickness of the elastic layer provided on the fixing roller is 1.5 mm or more. It is not clear at this stage why an abnormal image occurs when the elastic layer of the fixing roller becomes thick. However, since both the fixing roller and the pressure roller have elastic layers, it is presumed that the movement of the sheet is restricted when the sheet is held in the nip portion. FIG.
The abnormal image as shown in (1) does not occur in a fixing roller device having no elastic layer for a monochrome machine or a fixing roller device having an elastic layer having a thickness of about 0.5 mm.

The present invention solves the problem that an abnormal image occurs when trying to prevent the occurrence of wrinkles in a conventional fixing device for a color image forming apparatus, and prevents both wrinkles and abnormal images. An object of the present invention is to provide a fixing device capable of exhibiting excellent fixing performance.

[0010]

SUMMARY OF THE INVENTION According to the present invention, the above-mentioned problems are solved by arranging a roller surface layer with an elastic body and a fixing roller and a pressure roller having a heating element inside the roller, which are in pressure contact with each other. In a fixing device for fixing an unfixed image on a recording medium by heat and pressure by passing a recording medium carrying an unfixed image between both rollers, a nip formed at a pressure contact portion of the both rollers, The relationship of the nip width between the center and the end in the roller axis direction is as follows: (end nip width-center nip width) / center nip width x 100 within the range of -5.4 to 0. Will be solved by.

Further, it is advantageous that the fixing roller has an inverted drum shape in which both end portions in the roller axial direction have the minimum diameter and gradually increase toward the central portion. Furthermore, it is advantageous that the fixing roller has a drum shape in which the central portion in the axial direction of the roller has a minimum diameter and gradually increases toward both ends.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional configuration diagram showing a color fixing device which is an embodiment of the present invention. This color fixing device 10
The fixing roller 1 and the pressure roller 2 are built in, and the pressure roller 2 is pressed against the fixing roller 1 by a pressure means (not shown). Both the fixing roller 1 and the pressure roller 2 have a heater 3 built therein. Further, the fixing roller 1 is in contact with an oil application member 4 for applying a releasing oil.

The fixing roller 1 comprises an aluminum cored bar having a diameter of 57.0 mm and a HTV (high temperature vulcanizing type) silicone rubber, a fluorinated silicone rubber, and an LTV (low temperature vulcanizing type) silicone rubber, which are coated in this order. An elastic layer is provided and the outer diameter is 60 mm. The thickness of each of the fluorinated silicone rubber and the LTV silicone rubber was set to 60 μm. The shape of the fixing roller 1 is a reverse drum shape (a central portion is thicker than the end portions), a straight shape, or a drum shape (a cross-section reverse crown shape), and the amount of the drum (the center portion and the end portion of the roller). The range of the difference between the outer diameters of the parts is within the range described later. The outer diameter of the fixing roller is 60 mm and represents the maximum diameter of the roller.

The pressure roller 2 is formed to have an outer diameter of 60 mm by providing an iron core having a diameter of 52 mm with an elastic layer similar to that of the fixing roller 1. The pressure roller 2 is not shaped like a drum, but is straight.

FIG. 2 is a graph showing the relationship between wrinkles and the occurrence of abnormal images when the fixing device 10 shown in FIG. 1 changes the shape of the fixing roller (the amount of the drum) to perform the fixing operation. It was Here, the shape of the fixing roller is represented by the drum amount (outer diameter of the central portion-outer diameter of the end portion), and when the value of the drum amount is positive (+), it indicates that the roller has an inverted drum shape. When the drum amount is 0 (zero), the roller is a straight roller, and when the drum amount is negative (-), it is a drum roller.

As shown in this graph, regarding the wrinkles, the hourglass shape and the straight shape are more advantageous and the reverse hourglass shape is more disadvantageous. When the amount of the hourglass exceeds +0.015 mm, the wrinkles gradually increase. Therefore, it can be seen that a smaller image of the amount of heartbeat shown here can provide a good image with less wrinkles.
Also, for abnormal images, the inverted drum shape is more advantageous,
Abnormal images increase as the amount of heartbeat decreases. Therefore, it can be seen that a good image with few abnormal images can be obtained when the amount of the heartbeat is large. That is, it can be understood that the characteristics of wrinkles and the occurrence of an abnormal image are contradictory depending on the amount of the drum.

From the graph of FIG. 2, if the drum amount of the fixing roller is reduced to reduce wrinkles, abnormal images increase,
It can be seen that if the fixing roller is formed in an inverted drum shape in order to reduce abnormal images, wrinkles increase, and both are unsuitable for use. Therefore, in the present invention, attention is paid to the nip width (length in the sheet conveying direction) formed by the fixing roller and the pressure roller, particularly, the difference between the nip width between the center of the roller and both ends. And the occurrence of wrinkles and abnormal images are clarified, and the ratio of the nip width between the center of the roller and both ends is regulated within a predetermined range to prevent wrinkling of the paper at the time of fixing and to prevent toner image formation. It is also possible to prevent the occurrence of abnormal images due to disturbance.

The ratio of the nip width between the center of the roller and both ends of the roller is calculated by the following equation.

[0019]

[Equation 1]

Further, in this embodiment, the ratio of the nip width between the central portion and the end portion of the roller is set by setting the drum amount of the fixing roller (the pressure roller has a straight shape). There is.

When the relationship between the ratio of the nip width at the central portion and the end portion of the roller obtained by the formula and the evaluation for wrinkles and abnormal images was examined, the results shown in Table 1 below were obtained.

[0022]

[Table 1]

As shown in Table 1, when the ratio of the central portion of the nip width to the end portion of the roller is within the range of -5.4 to 0,
Both the abnormal image and the wrinkles were almost satisfactory, and good fixing performance was able to be exhibited. This means that the amount of the fixing roller at the ratio of the nip width (-0.01
5 to +0.060 mm) can be confirmed by fitting it to the graph of FIG. On the other hand, when the ratio is small, the occurrence of wrinkles is small but the number of abnormal images is increased. When the ratio is large, the number of abnormal images is decreased but the generation of wrinkles is increased, and the fixing device is not suitable for use. turn into. Therefore, if the nip width is set so that the above ratio falls within the range of -5.4 to 0, both wrinkles and abnormal images can be prevented. In this embodiment, the fixing roller shape is a reverse drum shape of +0.045 mm.

In addition, in the third row (center row) of Table 1, the amount of drums indicating the shape of the fixing roller is a positive value (here, +
0.045 mm) means that the outer diameter of the central part of the roller is larger than the outer diameter of the end part, but the nip width at this drum amount is larger than that of the central part. Both ends (front side and back side of the device) are larger. This is due to the bending of the roller, and the amount of drumstick is a very small value (0.045 mm).
This is because the deflection of the roller has a greater effect on the nip width than the shape of the roller.

In this embodiment, the fixing roller 1 and the pressure roller 2 both have an outer diameter of 60 mm, but the roller outer diameter is 45 mm.
It can be confirmed that the same evaluation can be obtained even with a fixing device of mm, if the ratio of the center part of the nip width to the end part of the roller is set in the range of -5.4 to 0, and wrinkles and abnormal images are prevented. It was possible to exhibit excellent fixing performance. That is, the present invention is effective regardless of the roller diameter by setting the ratio of the roller central portion and the end portion of the nip width within the above range.

Further, the nip width at the roller ends in the formula is an average value at both ends of the roller, but it is desirable that the nip widths at both ends of the roller are equal in design. The position where the nip width is measured is the center of the roller, that is, the center of the paper passing area, and the end of the roller is located at the position of the roller corresponding to the end of the paper passing area or within 2 to 3 mm from the inside. A good position is desirable. The nip width measurement position is shown in FIG. In this figure, the central nip width is measured at the center C of the fixing roller 1, and the end nip width is measured at both ends of the sheet passing region R or at positions located within 2 to 3 mm inside.

By the way, in the method of measuring the nip width, a sheet carrying an unfixed image is passed between both rollers and is temporarily stopped on the way. Then, more heat is given to the portion stopped by the nip than the other portions. As a result, the reflectance of that portion is different from that of the other portions, so the width (length in the sheet passing direction) may be measured. Alternatively, an OHP film for a copying machine having a surface coated with a resin or the like (only a film that does not carry an image) is temporarily stopped in the same manner, and the film becomes cloudy or conversely transparent (width in the paper transport direction). (Length) may be measured.

As described above, since the influence of the bending of the roller has a greater effect on the nip width than the drum amount of the roller, not only the drum-shaped fixing roller but also the straight-shaped or inverted drum-shaped fixing roller is used. , The above ratio (calculation result of the formula) −
It can be set within the range of 5.4 to 0.

[0029]

As described above, according to the fixing device of the present invention, the nip width at the central portion and the end portion in the roller axial direction of the nip formed by the pressure contact portion of the fixing roller and the pressure roller is reduced. By setting the relationship as follows: (central nip width-end nip width) / central nip width x 100 within a range of -5.4 to 0, both wrinkles and abnormal images are prevented, which is good. It can exhibit excellent fixing performance.

According to the second aspect of the invention, the fixing roller is formed in a reverse drum shape, which is advantageous for preventing the occurrence of an abnormal image.
According to the structure of the third aspect, the fixing roller is shaped like a drum, which is advantageous in preventing wrinkles.

[Brief description of drawings]

FIG. 1 is a cross-sectional configuration diagram illustrating a color fixing device that is an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between wrinkles and an abnormal image occurrence state when a fixing operation is performed by changing a shape (amount of sound) of a fixing roller by the fixing device.

FIG. 3 is an explanatory diagram showing a nip width measurement position in the fixing device.

4A and 4B are diagrams showing a drum effect of a drum-shaped roller in a fixing device, FIG. 4A is a plan view showing the drum-shaped roller, and FIG. 4B shows a sheet conveying speed by the roller and a force acting on the sheet. It is a top view shown.

FIG. 5 is a plan view showing an example of an abnormal image generated in a conventional fixing device.

[Explanation of symbols]

1 Fixing Roller 2 Pressurizing Roller 3 Heater 4 Oil Applying Member 10 Fixing Device 20 Hourglass Roller D ₁ , D ₂ Roller Outer Diameter V ₁ , V ₂ Paper Conveying Speed P Paper R Paper Passing Area S Paper Conveying Direction U abnormal image

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Okamoto 1-3-6 Nakamagome, Ota-ku, Tokyo Stock company Ricoh Co., Ltd. (72) Yasuhiro Kodaira 1-3-6 Nakamagome, Ota-ku, Tokyo Shares Company Ricoh

Claims (3)

[Claims] 1. A fixing roller and a pressure roller, each having a roller surface layer formed of an elastic material and having a heating element inside the roller, are disposed in pressure contact with each other, and an unfixed image is carried between both rollers. In a fixing device for fixing an unfixed image on a recording medium by passing through the recording medium by heat and pressure, a nip width at a central portion and an end portion in the roller axial direction of a nip formed by the pressure contact portions of both rollers. The relationship is as follows: (Center nip width-End nip width) ÷ Center nip width ×
A fixing device having a value of 100 in the range of -5.4 to 0. 2. The fixing roller has an inverted drum shape in which both end portions in the roller axial direction have the smallest diameter and the diameter gradually increases toward the center portion. Fixing device. 3. The fixing roller according to claim 1, wherein the fixing roller has a drum shape in which a central portion in the roller axial direction has a minimum diameter and gradually increases toward both ends. Fixing device.