Electrophotography – Image formation – Fixing
Reexamination Certificate
2001-10-24
2003-10-07
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Fixing
C219S216000, C432S060000
Reexamination Certificate
active
06631252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fixing apparatus for fixing an image on a recording material by applying pressure, which is suitably used in image forming apparatuses such as copiers and printers employing electrophotographic and electrostatic recording systems.
2. Related Art
An image forming apparatus employing an electrophotographic system such as a copier, printer or facsimile forms an electrostatic latent image by charging the surface of an electrophotographic photosensitive body by means of a charging apparatus and exposing the surface of the charged electrophotographic photosensitive body by means of an exposing apparatus. The electrostatic latent image is developed by a developing apparatus to form a toner image which is transferred to a recording material such as paper by a transferring apparatus and then fixed on the recording material by a fixing apparatus as a permanent fixed image, and the fixed image is output.
FIG. 8
is a schematic diagram showing an image forming apparatus according to an embodiment of the present invention. The above-mentioned fixing apparatus operates as follows. A recording material P such as paper to which a toner image has been transferred by a transfer nip between a photosensitive drum
2
and a transfer roller
5
as an image carrier is conveyed while being held by a fixing nip formed between a heating roller
12
and a pressure roller
13
of a fixing apparatus
7
through a conveyance guide
9
and the unfixed toner image is thermally fixed on the recording material P as a permanently fixed image by heat from a heater (halogen heater)
14
incorporated in the heating roller
12
and pressure from the fixing nip. Other constituent elements of the image forming apparatus
1
will be described with reference to embodiments of the present invention which will be described below.
When an envelope is conveyed as a recording material in the image forming apparatus constituted as described above, if the envelope is nipped and conveyed while it is heated by the fixing apparatus, there arises a problem that it wrinkles. The envelope is formed by folding a single sheet of paper and pasting the overlapped portions. If there is a difference in shrinkage direction between the front side and the rear side of the sheet, there is no escaping for distortion at the time of shrinkage by heating, with the result that the sheet of paper is apt to wrinkle. To prevent wrinkling, a method making use of pressure means for reducing the contact pressure of the pressure roller is generally employed.
For example,
FIG. 6
shows an example of a pressure mechanism for the fixing apparatus
7
. The pressure mechanism comprises a heating roller
12
including a heater
14
(halogen heater), a pressure roller
13
contacting the heating roller
12
, and a frame
52
supporting these components. Both ends of the heating roller
12
are rotatably supported by the frame
52
through the intermediation of a bearing (not shown). One end of the heating roller
12
is provided with a gear (not shown) to be driven by drive force transmitted from a drive source. Both ends of the pressure roller
13
are rotatably supported by a support member
15
through the intermediation of a bearing (not shown). Reference numeral
16
denotes a rotatable lever which is a contact pressure switching means and movably supported by a shaft
52
a
provided on either side of the frame
52
. The rotatable lever
16
comprises an operation portion
16
a
, a contacting portion
16
b
contacting the support member
15
and a stopping portion
16
c
for stopping the movement of the rotatable lever
16
. Reference numeral
60
denotes an elastic member made of a coil spring for biasing the support member
15
and provided between the frame
52
and the support member
15
. An axis hole
15
a
which serves as a fulcrum is formed in the support member
15
so that the support member
15
is movably supported by a shaft
52
b
provided on either side of the frame
52
. On the free end side of the support member
15
is formed a bending portion
15
b
which serves as a contacting potion with the above-mentioned rotatable lever
16
and the support portion of the above-mentioned elastic member
60
.
When an envelope is to be conveyed, the rotatable lever
16
is first rotated clockwise as shown in FIG.
7
. Since the contacting portion
16
b
of the rotatable lever
16
contacts the bending portion
15
b
of the support member
15
, and the rotatable lever
16
is biased in the direction indicated by an arrow in
FIG. 7
(center direction of the shaft
52
a
) by the support member
15
, the moment generated by the biasing force of the elastic member
60
does not act on the rotatable lever
16
, and the rotatable lever
16
is stopped at a position shown in
FIG. 7
, and the support member
15
is kept down against the biasing force of the elastic member
60
.
Since the rotatable lever
16
has a stopping or detent portion
16
c
, it is prevented from moving more than required. The support member
15
is forced down by the operation of the rotatable lever
16
with the fulcrum
15
a
as the center. The pressure roller
13
supported by the support member
15
is kept at a position at a predetermined distance from the heating roller
12
.
The pressure roller
13
comprises a rubber layer
13
b
formed on the surface of a metal core
13
a
which is in contact with the heating roller
12
to obtain a nip width. Since rubber has a large thermal expansion coefficient and a low heat conductivity, the temperature of the pressure roller
13
gradually rises and the outer diameter of the pressure roller
13
gradually increases (broken line portion in
FIG. 7
) according to the operation time of the image forming apparatus
1
.
Since the biasing force of the elastic member
60
is extremely large, even when the outer diameter of the pressure roller
13
becomes large, the rubber layer
13
b
is deformed and the pressure roller
13
cannot move down easily. Owing to this constitution, the contact pressure to the heating roller
12
of the pressure roller
13
or the nip width increases, thereby causing the problem of the wrinkling of the envelope.
When the amount by which the rotatable lever
16
is forced down (distance between the pressure roller
13
and the heating roller
12
) is set taking into account the change in the outer diameter of the pressure roller
13
by thermal expansion to prevent wrinkling, the unfixed toner image will not be fully melt-fixed until the temperature of the pressure roller
13
is increased to a sufficient degree, a fixed outer diameter thereof is achieved and a predetermined pressure is obtained. In particular, when the thermal expansion of the pressure roller
13
is large, the heating roller
12
and the pressure roller
13
are set in a state that they do not contact each other, so that no heat is transmitted from the heating roller
12
to the pressure roller
13
, making it necessary to provide a heater
14
in the pressure roller
13
as well.
That is, the wrinkling of the envelope and the melt-fixing of the unfixed toner image are contradictory and a long waiting time is required until the fixing apparatus
7
of the image forming apparatus
1
is fully warmed so as to prevent the wrinkling of the envelope and melt-fix the unfixed toner image.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fixing apparatus capable of fixing a toner image on a recording material without wrinkling the recording material.
Another object of the present invention is to provide a fixing apparatus capable of fixing a toner image on an envelope.
Still another object of the present invention is to provide a fixing apparatus capable of changing fixing pressure.
Still another object of the present invention is to provide a fixing apparatus comprising:
a fixing rotator in contact with a recording material supporting an unfixed image;
a pressure rotator in pressure contact with the fixing rotator;
a support member s
Ando Masao
Azeta Takahiro
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