Electrophotography – Image formation – Fixing
Reexamination Certificate
2002-06-04
2004-08-03
Lee, Susan (Department: 2852)
Electrophotography
Image formation
Fixing
Reexamination Certificate
active
06771925
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fixing device, a web differential gear and an image formation apparatus using the fixing device and the web differential gear. More specifically, it relates to an offset preventing structure in a fixing step.
BACKGROUND OF THE INVENTION
In image formation apparatuses such as copying machines, facsimile, printers and printing machines, there are those having a configuration using an electrophotography method as an image formation method. According to the electrophotography method, a visualizing process is executed for an electrostatic latent image supported on a photosensitive member as the latent image supporting member using a developing agent such as a toner.
As the developing agent used for the visualizing process, a one component type developing agent with a toner itself provided with the charging property and the magnetic property so as to enable conveyance or electrostatic adhesion to the electrostatic latent image and a two component type developing agent including a toner and a carrier are included. Recently, in order to meet the demand for the image quality improvement, a small particle size has been adopted in the toners. By using a small particle size toner, the image quality can be improved by improving the dot reproductivity or the sharpness.
According to an image formation apparatus using the electrophotography method, when the transfer step for the toner image as the visual image supported on the photosensitive member as the latent image supporting member is finished, a fixing step is executed for the sheet with the toner image transferred. In the fixing step, for example, a configuration utilizing a thermal roller fixing method having a fixing roller with a heat source stored and a pressuring roller with an elastic layer on the surface facing with each other in a sheet conveyance path for heating and fusing a toner image so as to be permeated in the sheet can be presented. In addition to the thermal roller fixing method, a fixing method using a belt, a fixing method utilizing a film, as to the heating source, in addition to the heater, a method for heating by induction heating, or the like are discussed.
According to the fixing device, the offset phenomenon has been known for a long time, and various countermeasures have been provided therefore. As one of them, a technique of eliminating the offset toner adhered on the fixing member by cleaning and preventing re-adhesion has been proposed. As an embodiment for cleaning, one comprising a web cleaning device which uses a web has been provided.
The web is made of a non-woven fabric produced as a mixture of fibers of an aramide and a polyethylene terephthalate, is soaked in an oil as needed for eliminating the toner or coating the oil while sliding on the fixing roller surface.
However, recently, for the purpose of improvement of the reproductivity and the sharpness, a fixing device which fixes a toner image formed using a small particle size toner has been developed. When a toner formed using a small particle size toner is fixed, it is confirmed that the amount of the offset toner is extremely large compared with the case of a toner of an ordinary particle size (about 20 &mgr;m or more as the volume average particle size).
In order to find the cause thereof, the present inventor has discussed various aspects so as to have the following assumption for the reason. The toner image using a small particle size toner (about 5 &mgr;m to 10 &mgr;m volume average particle size (average value of the particle size of the toner particles contained in a unit volume)) is fused and permeated while being held and conveyed between the fixing roller and the pressuring roller as in the case of a toner image using a non-small size particle size toner (about 20 &mgr;m &mgr;or more volume average particle size conventionally used).
In the sheet with the toner image supported, particularly in a paper such as a transfer paper, ruggedness exists on the surface so that when a toner enters in the ruggedness, contact of the fixing roller and the toner is insufficient so that heat cannot be supplied to the toner so as to have the unfixed toner. As a result, there is a risk of fixation of the unfixed toner with insufficient fusion and permeation can be offset on the fixing roller surface.
FIGS. 6A and 6B
are diagrams which explain the phenomenon. The toner supported on a sheet S passing through a fixing nipper comprising a fixing roller A and a pressuring roller B shown in
FIG. 6A
has insufficient contact with the fixing roller A when it is entered in the ruggedness of the sheet, in particular, in the recess part with a wider facing interval with respect to the fixing roller A as shown in FIG.
6
B.
Particularly in the case of a small particle size toner, the amount entered in the recess part is large so that the toner in the unfixed state due to the insufficient contact is large as well, and thus the toner amount generating the offset due to the counter transfer to the fixing roller is large as well. An offset preventing layer using a mold releasing agent such as a fluorine resin is formed on the fixing roller A surface for the offset prevention. Depending on the material used for the offset preventing layer such as the fluorine resin, cracking, holes or grooves may be generated as time passes by so that the surface can be rugged. Since a small particle size toner of a less than 5 &mgr;m size is used and the size of the cracking, holes or grooves in the offset preventing layer is about 4 to 5 &mgr;m, the unfixed toner can easily enter on the sheet.
From the viewpoint of the environment conservation, papers once used for image formation are reused increasingly at the offices. In the case of such papers, ruggedness exists on the surface due to the toner image so that the surface evenness is poorer than an ordinary paper. Therefore, offset can easily be generated in this kind of papers.
As heretofore explained, although a small particle size toner (about 5 &mgr;m to 10 &mgr;m volume average particle size) provides a higher image quality, it also involves a disadvantage of the offset toner increase. Therefore, at the time of using a small particle size toner, in order to reduce particle size irregularity, a work for further eliminating a small particle size toner is executed in the refining step (one with a less than 5 &mgr;m particle size, hereinafter referred to as fine powdery toner). In order to restrain the influence on the image formation apparatus, the fine powdery toner is adjusted to be about 10 number % as a whole (it referred to the number corresponding to 10% of the total particle number).
Recently, cost reduction and resource conservation are highly demanded for the toner so that it is expected that elimination of the work for reuse from the cost reduction viewpoint and recycling instead of abandonment from the resource conservation viewpoint will be demanded in the future. Therefore, a toner without the need of the elimination work (or toner with a simplified elimination) has about a 5 &mgr;m to 10 &mgr;m volume average particle size, and the fine powdery toner with a less than 5 &mgr;m particle size accounts for about as much as 60 to 80% of the whole.
When such a small particle size toner is used, the offset generation ratio is increased drastically compared not only with a toner with an ordinary particle size but also with a small particle size toner with the irregularity adjustment (toner with about a 5 &mgr;m to 10 &mgr;m volume average particle size, with the irregularity adjustment).
The relationship is shown in the table 1.
TABLE 1
Plain
Recycled
paper
paper
Ordinary particle size
80 sheets
15 sheets
Small particle size (with
15
8
fine powder elimination
process application)
Small particle size (without
10
5
fine powder elimination
process application)
In this experiment, the number of paper passage is counted until the offset toner exceeding the tolerance of the cleaning member pressured against the fixing roller at the time of fixing a solid image on an ordinary
Lee Susan
Ricoh Company Limited
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