Electrophotography – Control of electrophotography process – Control of fixing
Reexamination Certificate
2002-06-28
2004-10-05
Braun, Fred (Department: 2852)
Electrophotography
Control of electrophotography process
Control of fixing
C399S045000, C399S069000
Reexamination Certificate
active
06801729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to imaging apparatuses such as laser beam printers. In particular, the present invention relates to a fixing operation of such an imaging apparatus.
2. Description of the Related Art
Hitherto, in a thermal image-fixing device which is included in an imaging apparatus using a recording method such as an electrophotographic method or an electrostatic recording method, a so-called heat-roller-type heat-fuser has been widely used. In such a device, a toner image is fixed to a sheet as a permanent image when the sheet, which carries the toner image, travels through a pressure-nip region formed between a fusion roller and a pressure roller which rotate while being pressed to each other.
A heat-fuser in which electrical consumption is minimized by suspending electric supply during a standby mode is also known. The heat-fuser uses a film-heat method for thermally fixing toner images on sheets by transferring the sheets between a pressure roller and a heating element (a heater) mounted to a supporting unit via a thin film (a fusing film).
The two types of heat-fusers described above, which form thermal image-fixing devices, each include at least one fusion-temperature-measuring member for use in controlling temperature at which images are thermally fixed. The fusion-temperature-measuring member generally uses a thermistor. The fusion-temperature-measuring member is disposed in contact with the heater or the pressure roller and thereby measures the temperature of the heater or the pressure roller. Recently, a method for measuring the temperature for thermal fusion by measuring radiant heat from the heater or the pressure roller has also been studied.
FIGS. 5A
,
5
B,
5
C, and
5
D show the relationship of positions between a ceramic heater
11
a
serving as a heater element, thermistors
11
d
and
11
e
, and a sheet
501
or
502
. The sheets
501
and
502
are transferred along arrows shown in the drawings. The dimension perpendicular to the transfer direction of each sheet is hereinafter referred to as the width. In
FIG. 5A
, a sheet having a maximum width that can be transferred in the image forming apparatus is sandwiched between the ceramic heater
11
a
and the pressure roller. In
FIG. 5B
, a sheet having a minimum width that can be transferred in the image forming apparatus is sandwiched by the ceramic heater
11
a
and the pressure roller. The ceramic heater
11
a
is fixed to a stay
11
b
which positions the ceramic heater
11
a
, as shown in FIG.
6
.
The ceramic heater
11
a
opposes a pressure roller
10
with a fusing film
11
c
therebetween. When the pressure roller
10
is driven, a sheet which is sandwiched at a position (hereinafter referred to as a nip) at which the pressure roller
10
and the fusing film
11
c
are in contact with each other is conveyed. During this operation, a developer carried on the sheet is fixed onto the sheet as a result of the pressure from the pressure roller
10
and the heat supplied by the ceramic heater
11
a
. The main thermistor
11
d
is disposed in a position where a sheet passes, regardless of size. That is, the main thermistor
11
d
is disposed in a given position in a region
510
shown in FIG.
5
B. The sub-thermistor
11
e
is disposed in a position where a part of a sheet having a maximum width which can be conveyed passes by and which a part of a sheet having a minimum width which can be conveyed does not pass by in a normal state. That is, the sub-thermistor
11
e
is disposed in a given position in region
511
shown in FIG.
5
B.
FIG. 5C
shows an example of temperature distribution in the ceramic heater
11
a
when the sheet having a minimum width is transferred. During the transfer of the sheet having a minimum width, gaps between the ceramic heater
11
a
and the pressure roller
10
are produced immediately outside the sheet because of the thickness of the sheet, and regions in which heat generated by the ceramic heater
11
a
is not transferred either to the sheet or to the pressure roller
10
are thereby produced. Therefore, the temperature in regions A and B of the ceramic heater
11
a
is increased as shown in
FIG. 5C
, when the temperature is set to a target temperature by using the main thermistor
11
d.
The temperature is particularly rapidly increased when a substance having a small width and a large thickness, for example, an envelope is conveyed. For example, when the target temperature is set to 200° C., the temperature in the regions A and B is likely to increase to 300° C.
When the temperature in end portions of the ceramic heater
11
a
is thus increased, the temperature is likely to exceed upper limits of the resistance to heat of the pressure roller
10
and the stay (the heater-supporting member)
11
b
. Therefore, there is a risk of damaging these components. When a plain paper having a large width is printed and the temperature in the end portions of the ceramic heater
11
a
is increased, there is a risk of a hot offset or the like due to an excessively raised temperature.
In order to prevent the temperature of the end portions from increasing when a sheet having a small width is transferred, the throughput is reduced when the temperature measured by the sub-thermistor
11
e
is increased, thereby suppressing the temperature rise in the end portions where no sheet portion passes. The sub-thermistor
11
e
is provided in addition to the main thermistor
11
d
in a position which no sheet portions pass by.
The temperature in the end portions increases at every fusion. Therefore, when the throughput is reduced, the temperature rise in the end portions decreases per unit time, and the components are thereby protected from being damaged.
However, if a user does not correctly set a side-restricting tray when setting envelopes or the like onto a multi-sheet-supply tray, the following problem will occur.
An imaging apparatus, in which a recording sheet having a width smaller than that of a sheet having a maximum width capable of being used in the apparatus is set in a widthwise-intermediate part of a sheet-supply tray (hereinafter referred to as a center-sheet-supply system), uses a side-restricting tray having two side-restricting members provided on the sheet-supply tray when envelopes or the like are supplied. The two side-restricting members are disposed symmetrical with respect to an intermediate part of the side-restricting tray and are movable in conjunction with each other. When a sheet is restricted at both sides thereof, the sheet is positioned at the widthwise-intermediate part (see FIG.
5
B).
However, when envelopes or the like are offset toward one side of the sheet-supply tray as a result of the two side-restricting members being away from each other farther than the width of the envelopes or the like, the envelopes or the like are transferred along the regions at which the main thermistor
11
d
and the sub-thermistor
11
e
are provided, as shown in
FIG. 5D
, and no difference in temperature between the main thermistor
11
d
and the sub-thermistor
11
e
is detected. Consequently, the throughput will not be reduced; as a result, the temperature at positions where no parts of the envelopes or the like pass will increase, and there is a risk of damage to the components such as the pressure roller
10
and the stay
11
b
because the temperature is likely to exceed upper limits of the resistance to heat of these components.
When printing of a plain paper having a larger width is performed when the temperature in the end portions is increased, there is a risk of a hot offset or the like due to an excessively raised temperature.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an image forming apparatus and a method of operating the image forming apparatus, in which a fuser is protected from being damaged even when a user incorrectly sets a side-restricting tray and sheets, such as envelopes, having a small width and a large thickness.
To these ends, according
Nakahara Takashi
Otaka Koichi
Seki Yuichi
Sekiguchi Takashi
Takazawa Hiroshi
Braun Fred
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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