Electrophotography – Image formation – Fixing
Reexamination Certificate
2000-07-10
2001-10-16
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000, C399S333000
Reexamination Certificate
active
06304741
ABSTRACT:
The present application claims priority to Japanese Patent Application No. 11-198085 filed Jul. 12, 1999, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image fixing apparatus for use in an image forming apparatus such as a copier machine, a printer, a facsimile machine and a hybrid machine which combines two or more of these three.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic method such as a copier machine and a printer, a surface of a photosensitive member which is an image carrier is electrified, an image is exposed in the electrified area to thereby form an electrostatic latent image, the electrostatic latent image is developed into a visualized toner image, the toner image is transferred onto a recording medium such as a recording paper, and an image fixing apparatus fixes the toner image. Meanwhile, in an image forming apparatus of a direct recording method, a toner is blown in accordance with a received image signal onto a recording medium such as a recording paper from a line of toner supply holes which are formed in a hopper which contains the toner to thereby directly form a visualized toner image without forming an electrostatic latent image, and an image fixing apparatus fixes the visualized toner image. An image fixing apparatus as such usually comprises a fixing roller which contacts an unfixed toner image on a recording medium and a pressure roller which is urged against the fixing roller under pressure and accordingly rotates. The unfixed toner image is subjected to heat and a pressure together with the recording medium while transporting the recording medium carrying the toner image in a nip portion between the two rollers, whereby the toner image is fixed on the recording medium.
Known as a method of heating the fixing roller are a method which requires to heat up the fixing roller from inside by a heater which is disposed inside the fixing roller and a method which requires to heat up the fixing roller from outside by means of a heater which is disposed outside the fixing roller. As compared with the method requiring to heat up the fixing roller from inside, the method requiring to heat up the fixing roller from outside is characterized in that it is possible upon startup of a fixing apparatus to shorten a time which is necessary to increase a temperature of the surface of the fixing roller to an appropriate temperature for fixing and in that when a large number of record images are to be outputted continuously, it is possible to quickly increase a temperature of the surface of the fixing roller which tends to decrease due to heat stolen by the recording medium which moves passing the fixing apparatus. Another characteristic is that it is possible to ensure a wider nip width, as a surface layer of the fixing roller is thickened.
However, the method heating up the fixing roller from outside demands to preliminarily operate the fixing apparatus before a paper arrives, for the purpose of increasing a temperature of the fixing roller to a temperature which is needed for fixing. Hence, when a material whose heat transfer coefficient is large is used for the surface layer of the fixing roller, heat flows into a roller base and escapes, and therefore, a long time is needed until a temperature of the surface of the fixing roller increases to a temperature which enables fixing since an image formation start signal is sent. On the other hand, when a material, such as an expanded rubber, whose thermal conductivity is low is used for the surface layer in an effort to decrease the heat transfer coefficient and accordingly quickly increase a temperature of the fixing roller, a temperature of the surface layer of the fixing roller decreases at a rear end of a recording paper which travels through the fixing apparatus, thereby causing a large difference in gloss of an image within the same recording paper, defective fixing on the rear end side, etc.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image fixing apparatus which shortens a preliminary operation time.
Another object of the present invention is to provide an image forming apparatus which realizes stable and high-quality fixing of images one at a time not to mention, but also even continuously.
These and other objects are achieved by an image fixing apparatus for fixing a toner image on a recording medium, comprising: a fixing member whose heat transfer coefficient in the direction of thickness is 20 W/m
2
·K to 100 W/m
2
·K and which contacts said toner image; and a heater which heats said fixing member from outside.
The fixing member is typically a roller or a belt. That is, examples of the fixing member include a fixing roller and a fixing belt. A fixing roller may be solid or hollow.
Where the fixing member is a solid roller, a heat transfer coefficient of the fixing member in the direction of thickness is a heat transfer coefficient of a portion along the thickness from the center of the roller to a surface taken in the direction of a diameter, and where the fixing member is a hollow roller or a fixing belt, the heat transfer coefficient is a heat transfer coefficient measured at a thick portion of the roller or a thick belt portion. The following relationship holds between the thickness t of the fixing member, a heat transfer coefficient H and a thermal conductivity h:
H=h/t
The objects above are achieved also by an image fixing apparatus for fixing a toner image on a recording medium, comprising: a fixing member which includes an elastic material layer whose heat transfer coefficient is 20 W/m
2
·K to 100 W/m
2
·K and which contacts said toner image; and a heater which heats said fixing member from outside.
The elastic material layer may be formed on a surface of a solid or hollow core roller of metal which has an excellent thermal conductivity, such as iron and aluminum, in a case that the fixing member is of a roller type. Where the fixing member is of a belt type, the elastic material layer may be formed on a surface of a belt of metal which has an excellent thermal conductivity such as iron and aluminum.
The image fixing apparatus according to the present invention is based on the following findings of the inventor. That is, when a toner image is fixed on a recording medium, fixing becomes strong when a temperature of the recording medium after fixing is somewhere around 100° C., and particularly strong when the temperature is on the higher end in that range. Further, when a heat transfer coefficient of the fixing member taken in the thickness direction or a heat transfer coefficient of the elastic material layer which forms a surface layer of the fixing member is 20 W/m
2
·K to 100 W/m
2
·K, a surface temperature of a recording medium immediately after fixing tends to be around 100° C. A surface temperature of a recording medium is particularly high when a heat transfer coefficient is 30 W/m
2
·K to 60 W/m
2
·K. More specifically, an optimal heat transfer coefficient of the fixing member or the elastic material layer is 20 W/m
2
·K to 100 W/m
2
·K, and more preferably, 30 W/m
2
·K to 60 W/m
2
·K. In short, it is desirable to use a material (e.g., a heat transfer coefficient) and a structure (the thickness of the surface layer) which, due to heat from outside, can hold a necessary quantity of heat in the vicinity of the surface layer and do not easily allow diffusion of a large quantity of heat inwardly.
The objects above are achieved alternatively by an image fixing apparatus for fixing a toner image on a recording medium, comprising: a fixing member which includes a first layer having an elasticity whose heat transfer coefficient is 50 W/m
2
·K or smaller and a second layer whose heat transfer coefficient is 60 W/m
2
·K or higher and which is formed on said first layer, said fixing member being in contact with said toner image; and a heater which heats said fixing member from outside.
Using the first and the secon
Chen Sophia S.
McDermott & Will & Emery
Minolta Co. , Ltd.
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