Electrophotography – Image formation – Fixing
Reexamination Certificate
2001-10-11
2003-11-18
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000, C399S328000, C399S334000
Reexamination Certificate
active
06650863
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the image forming apparatus incorporating a fixing unit, which is includes either a fixing belt or a fixing roller, and effectively used in the image forming apparatus of an electrophotographic copier and printer, and particularly to the fixing unit and image forming apparatus improved to ensure a uniform distribution of the temperature on the surface of the fixing belt or the fixing roller for heating a toner image.
A belt type fixing unit known so far includes the one comprising (1) a backup/heating roller containing a heating means made up of a halogen lamp, (2) a fixing belt applied to two backup rollers consisting of backup/pressure rollers, and (3) a pressure roller which rotates by giving pressure through the aforementioned backup/pressure roller and fixing belt; wherein a toner image is fixed onto the paper with unfixed toner image by the pressure of the pressure roller and heating of the fixing belt.
Such a fixing unit is arranged in such a way that; a temperature sensor as a means of detecting temperature is provided close to or in contact with the external surface of the backup/heating roller on the center in the longitudinal direction, for example, and the output information of the temperature sensor is used to control the power supply to the heating means by a control means, thereby allowing the temperature on the fixing belt surface to be maintained within the range of temperature suited to fixing of toner image.
The above fixing unit is mounted on a wide range of image forming apparatuss for ease of handling. One of the problems with the aforementioned fixing unit, however, is how to reduce the warming up time. Many proposals have been made during the course of a long period of time. One of such proposals is to decrease heat capacity by reducing the thickness of an endless fixing belt.
This is to reduce the thickness of the metallic substrate at the center constituting the fixing belt and the rubber on the outside layer, thereby ensuring the temperature on the fixing belt surface to reach the toner fixing temperature earlier.
It has been found out, however, that another issue occurs even if the thickness of the fixing belt can be reduced to ensure a specified mechanical strength to be maintained, and even if it is possible to reduce the time until stable fixing of the fixing belt surface is ensured after electric power is supplied to the heating means.
For example, when continuous fixing is performing using a small-sized paper with a toner image formed which is fed at the center with reference to the fixing belt, heat on the surface of the fixing belt corresponding to the aforementioned small-sized paper feed area is consumed to heat the paper. However, heat on the surface of the aforementioned fixing belt corresponding to the non-paper feed area on the right and left sides is not consumed. Moreover, heat is stored in the non-paper feed area due to a small shift of heat along the direction of the width of the fixing belt.
Further, electric power is supplied to the heating means to ensure that the temperature on the fixing belt surface in the paper feed area deprived of heat can be kept at a specified fusible temperature. This will increase the amount of heat stored in the portion of the fixing belt corresponding to the aforementioned non-paper feed area, with the result that the temperature range suited for fixing is much exceeded.
When large-sized paper utilizing the non-paper feed area on the right and left sides of the fixing belt is used at the time of the excessive temperature rise mentioned above, and the toner image formed on this paper must be fixed, then irregularity of gloss or high temperature offset occurs to the surface of the paper in conformity to irregular forms of the paper and toner layer caused by the difference of the temperatures heretofore between paper feed area and non-paper feed area. This will reduce the service life of the rubber layer of the fixing belt.
Suck a problem is caused by the aforementioned fixing belt which is made into a substance of low heat capacity through the reduction of the thickness of the fixing belt, with the result that there is a reduction in the function of shifting the heat stored in the non-paper feed area to a place of lower temperature.
Another example of the known image forming apparatus is the one provided with a fixing unit arranged in such a way that paper carrying a toner image is fed between the fixing belt rotatably supported by the backup roller and heated by a proper heating source, and a roller which rotates in contact with this fixing belt, and the aforementioned toner image is fixed to this paper by the pressure and heat produced between the two.
One form of the aforementioned fixing unit conceivable is the one where a heating source consisting of a halogen lamp is installed inside the loop formed by a fixing belt to heat the fixing belt directly, thereby reducing the warming up time.
Such a system, however, is accompanied by the following problem: During the standby period subsequent to the temperature of the fixing belt having been raised to the value which permits fixing, the temperature of the aforementioned fixing belt must be kept at the value allowing fixing or at a proper value lower than the fusible temperature with consideration given to energy saving. This requires the fixing belt to be rotated in order to maintain the temperature of the entire fixing belt, because the heat source is arranged opposite to part of the fixing belt. Further, a greater amount of heat is discharged due to a greater size of the fixing belt, with the result that energy efficiency is poor in such a conventional system.
Further, when paper of a smaller size with respect to the width for fixing and heating on the fixing belt is subjected to continuous fixing in the fixing process for example, there is an excessive rise of temperature in the non-paper feed area (e.g. the portion formed on the right and left sides of the fixing belt which does not contribute to fixing). If large-sized paper is subjected to fixing under this condition, irregular fixing occurs on paper in such a conventional system.
In other words, when the image forming apparatus is assumed to be arranged with reference to the center in such a way that there is an agreement between the center of the fixing belt along the direction of the width and that of the paper to be fixed, for example, the temperature sensor is installed close to or in contact with the center of the fixing belt along the direction of the width. Power supply to the heat source is controlled based on the information detected by this temperature sensor.
Accordingly, the fixing belt of the paper feed area in contact with the small-sized paper is always maintained at a fusible temperature, but the heat of the non-paper feed area is hardly used. Moreover, excessive amount of heat is stored because the heat is replenished by turning on the heat source for keeping the temperature in the paper feed area. This gives rise to the aforementioned problem.
This problem is caused by poor heat transfer capacity along the direction of the width as a result of reducing the heat capacity of the fixing belt.
In a fixing unit used in an image forming apparatus such as a copier, printer and facsimile machine characterized by a high level of technological perfection and high stability, a heating roller fixing method using a rubber roller as a fixing heating roller is employed over an extensive scope ranging from low speed to high speed machines and from monochrome to full-color machines.
In the fixing unit according to the conventional heating roller fixing method, however, a heating roller for fixing with a high heat capacity must be heated when the transfer material or toner is heated, and this is disadvantageous in energy saving. Moreover, this requires much time in warming the fixing unit at the time of printing (longer warming up time).
To solve this problem, a fixing unit according to film fixing method has been proposed and has come
Frishauf Holtz Goodman & Chick P.C.
Konica Corporation
Ngo Hoang
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