Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-09-05
2003-03-04
Grainger, Quana M. (Department: 2852)
Electrophotography
Image formation
Transfer
C219S216000
Reexamination Certificate
active
06529701
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus for forming a visible image by shifting toner to a latent image formed by a difference in electrostatic potential according to an electrophotographic system, electrostatic recording system, ionography or the like, and a fixing device for fixing a toner image, which has been transferred onto a recording medium, on the recording medium by heating and pressurization.
2. Description of the Related Art
A toner image formed by shifting toner to an electrostatic latent image on an image carrier is directly transferred onto a recording medium, or is once primary transferred onto an intermediate transfer member and is further secondary transferred from this intermediate transfer member onto a recording medium. For the transfer of the toner image, there is widely used a method in which a member for carrying an image is brought into contact with or is made to approach a member receiving a transferred image at a transfer portion, and an electric field is formed at this transfer portion to electrostatically shift the toner having an electric charge.
However, in the above method of electrostatically transferring the toner image, there is a case where scattering of the toner occurs, and resolution or dot reproducibility is lowered. Besides, in the case where toner images of plural colors are overlapped with each other and they are transferred at the same time, since transfer efficiency is not sufficient, there is also a case where uneven density or uneven color occurs in an image.
Under the circumstances, there is proposed a technique in which when toner on an intermediate transfer member is transferred onto a recording medium, a toner image on the intermediate transfer member is heated and melted, this is brought into press contact with the recording medium, and transfer and fixation are carried out at the same time.
Like this, the technique for carrying out the transfer and fixation at the same time can be classified according to methods and timing for carrying out heating and pressing as follows:
A first type is such that, for example, like a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 2-106774, before a toner image on an intermediate transfer member is transferred, a recording member is heated, and toner on the intermediate transfer member is melted by heat of the recording member, and is transferred and fixed onto the recording member.
A second type is such that, for example, like a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 9-15933, an endless belt shaped intermediate transfer member is overlapped with a recording medium, these are sent to a nip portion where a heating body and a pressure member are pressed against each other, and heating and pressurization are carried out.
A third type is such that, for example, like a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 11-352804, a toner image is heated on an endless belt shaped intermediate transfer member, and the melted toner image is brought into press contact with a recording medium in a non-heating state.
Among the techniques as described above, in an apparatus of the third type in which after the toner image is heated and melted, it is brought into press contact with the recording medium in the non-heating state, much heat is not taken by a pressure member or the like at the time of heating, so that heating with high efficiency can be performed. Besides, like an apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 11-352804, by using an electromagnetic induction heating device as a heating device, the toner image can be heated up to a predetermined temperature in a short time, and a warm-up time at the time of starting the operation of the device becomes almost unnecessary.
On the other hand, there is widely conventionally used a device which electrostatically performs transfer of a toner image formed on an image carrier onto a recording medium. In such a device, transfer and fixation are not performed at the same time as described above, but after transfer is electrostatically performed, the toner image is fixed by a fixing device. That is, in the case where an image is directly transferred from the image carrier to the recording medium, the transfer is generally electrostatically performed, and thereafter, the toner image is fixed by the fixing device. Also in the case where an intermediate transfer member is used, both primary transfer from the image carrier to the intermediate transfer member and secondary transfer from the intermediate transfer member to the recording medium are electrostatically performed, and a fixing device is provided at the downstream side of the secondary transfer position to obtain a fixed image.
As the fixing device, many devices have been proposed, for example, a device in which a recording medium carrying a toner image is heated and pressed between two rolls having built-in heaters.
As one of them, there is a device in which an endless fixing belt is laid across in a tensioned condition and is circularly driven, and after this belt is heated, it is pressed against a recording medium carrying a toner image by pressure. This device heats the toner image by heat stored by the belt through heating and presses it onto the recording medium, and there is a merit that effective heating can be made by carrying out the heating at the upstream side of a nip portion where pressurization is made.
The above-described image forming apparatus in which secondary transfer and fixation are performed at the same time, and the fixing device in which the heated fixing belt is pressed against the recording medium by pressure have similar problems as described below.
A first problem is such that there is a case where a belt is deformed to corrugate between a position where a belt-like intermediate transfer member or a fixing belt is heated and its downstream nip portion. It appears that this corrugated state occurs since the intermediate transfer member or the fixing belt is heated, so that its portion is expanded, and distortion in the width direction is generated, and tensile force is introduced in the circumferential direction. Especially, in the image forming apparatus in which secondary transfer and fixation are performed at the same time, if the driving speed of the intermediate transfer member is high, heating must be made quickly, so that a low heat capacity thin belt is used, and a corrugated state becomes easy to generate in such a thin belt.
When the belt-like intermediate transfer member comes to have the corrugated state, an image to be transferred is distorted, or permanent wrinkles are produced, and a defect of the image is caused. Also in the case where the corrugated state occurs in the fixing belt, wrinkles which can not be restored are produced on the belt, and poor fixation is caused.
A second problem is such that since heating is not performed at the nip portion after the intermediate transfer member or the fixing belt is heated, poor fixation occurs, or toner remains on the belt, so-called offset can occur.
In general, in a nip portion, at least one of pressed members includes an elastic member, and a predetermined nip length in a circumferential direction is secured by the deformation of the elastic member. Then, the distribution of contact pressure (nip pressure) in this nip is such that the nip pressure is gradually increased from the upstream side in the movement direction of the belt, becomes maximum at a portion near the center, and is gradually decreased toward the nip outlet.
On the other hand, a temperature change when melted toner on the intermediate transfer member passes through the nip is such that when it runs into this nip, heat is absorbed by a recording medium or a pressure member and the temperature is quickly lowered. Thus, a sufficient nip pressure does not act at a portion near the nip inlet where the temperature of the toner is high, and a large nip pressure acts at the ni
Baba Motofumi
Uehara Yasuhiro
Fuji 'Xerox Co., Ltd.
Grainger Quana M.
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