Electrophotography – Image formation – Fixing
Reexamination Certificate
1998-01-20
2002-12-31
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Fixing
C399S333000
Reexamination Certificate
active
06501935
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fixing apparatus used in an image forming apparatus such as a copy machine, a printer or a facsimile machine and, more particularly, to a belt-type fixing apparatus in which a heat generated by a heat roller is transferred to a nip portion between a fixing roller and a pressing roller via a fixing belt.
2. Description of the Related Art
In an image forming apparatus such as a copy machine, a printer or a facsimile machine,:a toner image is formed on a image carrier such as a photosensitive drum or an intermediate transfer belt. The toner image formed on the image carrier is transferred to a print paper or an OHP sheet by a transfer means, and the transferred toner image is fixed by a fixing apparatus. Conventionally, a heat roller fixing apparatus is used for such a fixing apparatus. In the heat roller fixing apparatus, a pressing roller is pressed against a fixing roller which is heated by a heater incorporated in the fixing roller. The pressing roller and the fixing roller are rotated in reverse directions by a drive mechanism. A print paper is conveyed through a nip portion formed between the pressing roller and the fixing roller so that a toner image on the print paper is fixed by a heat provided from the fixing roller and a pressing force applied by the pressing roller.
Japanese Laid-Open Patent Application No. 4-322282 discloses a technique for preventing generation of wrinkles in a print paper when the print paper is subjected to a fixing process in an image forming apparatus. This technique is used in a heat roller fixing apparatus comprising a pair of a fixing roller and a pressing roller. In this technique, generation of wrinkles is prevented by reducing a hardness of a conductive foam layer of the pressing roller.
In the above-mentioned heat roller fixing apparatus, there is a limit to increasing a width of the nip portion between the fixing roller and the pressing roller. In order to fix a toner image on the print paper at a low temperature, a duration (may be referred to as a nip period) of the print paper being positioned in the nip portion should be increased. That is, in order to achieve a complete fixation at a low temperature, a rotational speed of the pressing roller and the fixing roller must be reduced since the width of the nip portion cannot be increased. Thus, it is difficult to achieve a high-speed fixation.
An outer surface of the fixing roller is maintained at a high temperature due to heat provided from a heater incorporated in the fixing roller. Accordingly, a rate of increase in a temperature of the toner is high due to the increase in a nip period during which the fixing belt is located in the nip portion. This results in a decrease in a hot offset temperature which is an upper limit of a fixing temperature of the toner. The hot offset temperature is a temperature at which the hot offset phenomenon occurs. Accordingly, there is a problem in that an allowable range of a temperature for fixing the toner with a good exfoliation is decreased. The allowable range is referred to as an exfoliative fixation temperature range.
Accordingly, in order to eliminate the above-mentioned problem, Japanese Laid-Open Patent Application No.6-318001 discloses a belt-type fixing apparatus for an electrophotographic apparatus using a fixing belt. The belt-type fixing apparatus disclosed in this patent document comprises a fixing roller and a heat roller and an endless fixing belt drivingly engaged with the fixing roller and the heat roller. The fixing roller is provided with an exfoliative layer. A pressing roller is pressed against the fixing roller via the fixing belt interposed therebetween so that a nip portion is formed between the fixing belt and the pressing roller. A heating passage which extends substantially along a straight line is formed between the fixing belt and a recording medium supporting member which is provided under the fixing belt between the heat roller and the nip portion. A specific heat of the fixing belt is set to 0.002 to 0.025 cal/° C./cm
2
.
In the above-mentioned belt-type fixing apparatus, a fixing temperature at the nip portion can be reduced due to a pre-heating effect provided to a toner image by the heating passage. Additionally, a temperature of the toner image is not excessively raised due to a self-cooling effect of the fixing belt having a small heat capacity. Thus, an exfoliation of the toner image from the fixing belt is improved, which results in. a complete fixation of the toner image without an offset even when no oil or a very small amount of oil is applied to the fixing belt.
A description will now be given, with reference to
FIGS. 1 and 2
, of advantages of the belt-type fixing apparatus over the heat roller fixing apparatus.
FIG. 1
is an illustration showing a temperature distribution of each of a fixing roller, a toner image and a print paper. In a heat roller fixing apparatus, a fixing roller
21
comprises a metal core
21
a
and a silicon rubber layer
21
b
provided on the metal core
21
a.
On the assumption that a constant amount of heat is provided from a heater inside the metal core
21
a
in the fixing roller
21
, the fixing roller is initially maintained at a constant temperature T
0
. A print paper
22
and a toner layer
23
forming the toner image on the print paper
22
are also maintained at a temperature Ta which is normally a room temperature (about 25° C.).
Immediately after the print paper
22
and the toner layer
23
enter the nip portion formed between the fixing roller
21
and the pressing roller
24
, a temperature change from the fixing roller
21
to the print paper
22
occurs as indicated by a temperature distribution curve t
1
. As time passes, the distribution of temperature is changed as indicated by curves t
2
and t
3
. At this time, an interface between the silicon rubber layer
21
b
of the fixing roller
21
and the toner layer
23
of the print paper
22
maintains a constant boundary temperature T
1
(corresponding to a temperature of an upper surface of the toner layer
23
).
On the other hand, as time passes, a heat is transferred from the fixing roller
21
to the toner layer
23
. Thus, a boundary temperature Tf (corresponding to a temperature of a lower surface of the toner layer
23
) between the toner layer
23
and the print paper
22
is increased. If the nip period is sufficiently long, the temperature distribution becomes a curve t
4
, and the boundary temperature T
1
is increased and the boundary temperature Tf is also increased.
In the belt-type fixing apparatus, if a heat source is not provided within the nip portion, a temperature of the surface of the fixing belt is decreased as time passes since a heat is removed by the print paper through the surface of the fixing belt. This is referred to as a self-cooling effect of the fixing belt. Accordingly, the boundary temperature Tf is increased as time passes. However, as long as the boundary temperature T
1
is concerned, the boundary temperature T
1
does not sharply increase as is in the conventional heat roller fixing apparatus.
A hot offset phenomenon occurs at the interface between the fixing roller
21
and the toner layer
23
when an interface adhesive force of the toner layer
23
exceeds a cohesive force related to a viscoelasticity when the toner is melted. That is, the hot offset phenomenon is influenced by the level of the boundary temperature T
1
which is a temperature of the interface between the silicon rubber layer
21
b
of the fixing roller
21
and the toner layer
23
. Additionally, a fixation can be achieved when the interface adhesive force exceeds the cohesive force of the toner which is related to the viscoelasticity of the toner when the toner is melted. That is, the fixation is influenced by the level of the boundary temperature Tf which is a temperature of the interface between the toner layer
23
and the print paper
22
.
FIG. 2
is a graph showing fixation temperature characteristics of the heat
Hirai Kazumasa
Kurotaka Shigeo
Brase Sandra
Ricoh & Company, Ltd.
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