U.S. patent number RE38,810 [Application Number 10/066,035] was granted by the patent office on 2005-10-04 for image heating device and image forming device using the same.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd., Panasonic Communications Co., Ltd.. Invention is credited to Nobuo Genji, Naoaki Ishimaru, Masakazu Naito, Tatsuo Nakatsugawa, Hiroshi Terada, Akinori Toyoda, Yoshihito Urata, Hajime Yamamoto.
United States Patent |
RE38,810 |
Terada , et al. |
October 4, 2005 |
Image heating device and image forming device using the same
Abstract
An image heating device comprises a cylindrical heating roller
with a Curie temperature of 210.degree. C., a magnetization coil
for magnetizing the heating roller with an alternating magnetic
field, which is arranged inside the heating roller, and a nip
portion for heating a recording material that carriers a toner
image with heat from the heating roller, while the recording
material is being conveyed along said nip portion. The ratio
between the amount of heat generated in said heat-generating member
at Curie temperature or higher to the amount of heat generated at
room temperature in said heat-generating member is not more than
1/2. With this configuration, the heating roller can regulate its
own temperature to stabilize at a temperature that is suitable for
fixing, and the problems of partial overheating or underheating,
unstable heat generation, or damage of the device can be
eliminated.
Inventors: |
Terada; Hiroshi (Ikoma,
JP), Toyoda; Akinori (Katano, JP), Urata;
Yoshihito (Kantano, JP), Yamamoto; Hajime (Ikoma,
JP), Genji; Nobuo (Osakasayama, JP),
Ishimaru; Naoaki (Minoo, JP), Nakatsugawa; Tatsuo
(Utsunomiya, JP), Naito; Masakazu (Shioya,
JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JP)
Panasonic Communications Co., Ltd. (Fukuoka,
JP)
|
Family
ID: |
26467429 |
Appl.
No.: |
10/066,035 |
Filed: |
February 1, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
309922 |
May 11, 1999 |
06021303 |
Feb 1, 2000 |
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Foreign Application Priority Data
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May 15, 1998 [JP] |
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10-132984 |
Jul 17, 1998 [JP] |
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10-203005 |
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Current U.S.
Class: |
399/328; 399/329;
399/330; 399/335 |
Current CPC
Class: |
G03G
15/2053 (20130101); G03G 15/2039 (20130101); G03G
2215/2016 (20130101); G03G 2215/2029 (20130101); G03G
2215/2032 (20130101); G03G 2215/2035 (20130101) |
Current International
Class: |
G03G
15/20 (20060101); G03G 015/20 () |
Field of
Search: |
;399/320,328,329,330,333,335,336,338 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Brase; Sandra L.
Attorney, Agent or Firm: Merchant & Gould P.C.
Claims
What is claimed is:
1. An image heating device comprising: a heat-generating member
comprising a magnetic layer with a certain Curie temperature; a
magnetization member for magnetizing said heat-generating member
with an alternating magnetic field, which is arranged in opposition
to said heat-generating member; a nip portion for heating a
recording material that carries a toner image with heat from said
heat-generating member, while the recording material is being
conveyed along said nip portion; wherein a ratio between an amount
of heat generated in said heat-generating member at Curie
temperature or higher to an amount of heat generated at room
temperature in said heat-generating member is not more than
1/2.
2. The image heating device of claim 1, wherein a thickness of said
magnetic layer is at least twice a thickness of a skin depth.
3. The image heating device of claim 1, wherein said
heat-generating member further comprises a conductive layer with
lower resistance than said magnetic layer, which is provided
adjacent to said magnetic layer.
4. The image heating device of claim 3, wherein
wherein .rho.1 is an intrinsic resistance of said magnetic layer,
t1 is a thickness of said magnetic layer, .rho.2 is an intrinsic
resistance of said conductive layer, and t2 is a thickness of said
conductive layer.
5. The image heating device of claim 3, wherein the thickness of
said magnetic layer is equivalent to or greater than the skin
depth.
6. The image heating device of claim 1, wherein said nip portion is
formed by at least a portion of said heat-generating member, and a
pressure member pressed against this portion of said
heat-generating member.
7. The image heating device of claim 6, wherein at least said
magnetic layer of said heat-generating member is a rotatable
roller.
8. The image heating device of claim 6, wherein at least said
magnetic layer of said heat-generating member is a movable
film.
9. The image heating device of claim 8, wherein said film is
loop-shaped.
10. The image heating device of claim 6, wherein at least a
conductive layer of said heat-generating member is a movable
film.
11. The image heating device of claim 1, wherein the nip portion is
formed by a movable film contacting said heat-generating member,
and a pressure member for pressing against said film.
12. The image heating device of claim 11, wherein said
heat-generating member contacts a rear surface of said film.
13. The image heating device of claim 11, wherein said
heat-generating member contacts the rear surface of said film from
a position upstream of said nip portion to a vicinity of said nip
portion, and said magnetization member is provided at the position
upstream of said nip portion.
14. The image heating device of claim 11, wherein said
heat-generating member is provided on the rear side of said film
and contacts a portion of said film, and said magnetization member
is provided on a surface side of said film.
15. The image heating device of claim 11, wherein the pressure
member comprises a roller with low thermal conductivity provided on
the rear surface side of said film and a pressure roller provided
on the front surface side of said film.
16. The image heating device of claim 11, wherein said
heat-generating member comprises a rotatable roller.
17. An image formation device comprising an image formation means
for forming an unfixed image onto a recording material; and a
thermal fixing device for thermally fixing the unfixed image on the
recording material; wherein an image heating device according to
claim 1 is used as the thermal fixing device.
18. An image heating device comprising: a heat-generating member
comprising a magnetic layer with a certain Curie temperature; a
magnetization member for magnetizing said heat-generating member
with an alternating magnetic field, which is arranged in opposition
to said heat-generating member; wherein, when said device is in
operation, a temperature at which said heat-generating member
stabilizes due to a drop of a relative magnetic permeability of
said magnetic layer near said Curie temperature is higher than a
temperature where cold offset begins, and wherein said Curie
temperature is selected such that, when the temperature of said
heat-generating member is stabilized, a temperature of an outgoing
portion of a nip portion is lower than a temperature where hot
offset of the toner begins.
19. The image heating device of claim 18, wherein said
heat-generating member further comprises a conductive layer with
lower resistance than said magnetic layer, which is provided
adjacent to said magnetic layer.
20. The image heating device of claim 19, wherein
wherein .rho.1 is an intrinsic resistance of said magnetic layer,
t1 is a thickness of said magnetic layer, .rho.2 is an intrinsic
resistance of said conductive layer, and t2 is a thickness of said
conductive layer.
21. The image heating device according to claim 18, wherein
wherein Tc is the temperature where cold offset of the toner begins
in said nip portion, Tk is the Curie temperature, and Th is the
temperature where hot offset of the toner begins in an outgoing
portion of said nip portion.
22. The image heating device according to claim 18, wherein
wherein Tk is the Curie temperature.
23. The image heating device of claim 18, wherein said nip portion
is formed by at least a portion of said heat-generating member, and
a pressure member pressed against this portion.
24. The image heating device of claim 23, wherein at least said
magnetic layer of said heat-generating member is a rotatable
roller.
25. The image heating device of claim 23, wherein at least said
magnetic layer of said heat-generating member is a movable
film.
26. The image heating device of claim 23, wherein at least said
conductive layer of said heat-generating member is a movable
film.
27. The image heating device of claim 18, wherein the nip portion
is formed by a movable film contacting said heat-generating
portion, and a pressure member for pressing against said film.
28. The image heating device of claim 27, wherein said
heat-generating member contacts a rear surface of said film.
29. The image heating device of claim 27, wherein said
heat-generating member contacts the rear surface of said film from
a position upstream of said nip portion to a vicinity of said nip
portion, and said magnetization member is provided at the position
upstream of said nip portion.
30. The image heating device of claim 27, wherein said
heat-generating member is provided on the rear side of said film
and contacts a portion of said film, and said magnetization member
is provided on a surface side of said film.
31. The image heating device of claim 27, wherein the pressure
member comprises a roller with low thermal conductivity provided on
the rear surface side of said film and a pressure roller provided
on the front surface side of said film.
32. The image heating device of claim 27, wherein said
heat-generating member comprises a rotatable roller.
33. The image heating device of claim 27, wherein said film is
loop-shaped.
34. An image formation device comprising an image formation means
for forming an unfixed image onto a recording material; and a
thermal fixing device for thermally fixing the unfixed image on the
recording material; wherein an image heating device according to
claim 18 is used as the thermal fixing device. .Iadd.
35. An image heating device comprising: a heat-generating member
comprising a magnetic layer; a magnetization member for magnetizing
said heat-generating member with an alternating magnetic field,
which is arranged in opposition to said heat-generating member; a
nip portion for heating a recording material that carries a toner
image with heat from said heat-generating member, while the
recording material is being conveyed along said nip portion; and a
movable film that is separated from the heat generating member, the
nip portion being formed between a pressure roller and the movable
film; wherein the heat-generating member contacts a contact part of
the movable film and is arranged so as to be opposed to the
magnetization member at a different position from that of the nip
portion, and transmits generated heat to the movable film at the
contact part..Iaddend..Iadd.
36. The image heating device of claim 35, wherein said
heat-generating member contacts a rear surface of said
film..Iaddend..Iadd.
37. The image heating device of claim 35, wherein said
heat-generating member contacts the rear surface of said film from
a position upstream of said nip portion to a vicinity of said nip
portion, and said magnetization member is provided at the position
upstream of said nip portion..Iaddend..Iadd.
38. The image heating device of claim 35, wherein said
heat-generating member is provided on the rear side of said film
and contacts a portion of said film, and said magnetization member
is provided on a surface side of said film..Iaddend..Iadd.
39. The image heating device of claim 35, further comprising a
pressure member with low thermal conductivity provided on the rear
surface side of said film, and wherein the pressure roller is
provided on the front surface side of said film..Iaddend..Iadd.
40. The image heating device of claim 35, wherein said
heat-generating member comprises a rotatable roller..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image heating device using
electromagnetic induction heating and an image forming device using
the same. More specifically, the present invention relates to an
image heating device used in image forming devices, such as
electrophotographical devices or electrostatic recording devices,
that is suitable as a fixing device for thermally fixing unfixed
toner, and to an image forming device using the same.
2. Description of the Prior Art
As image heating devices, for which thermofixing devices are a
typical example, contact-heating devices such as heat-roller
devices and film-heating devices are used conventionally.
In recent years, due to the demand for shorter warming-up periods
and reduced energy consumption, there have been attempts to use
electromagnetic induction heating, which generates heat with high
efficiency and allows concentrated heating, for the heat source of
these contact-heating image heating devices.
FIG. 10 shows an image heating device of the film-heating type,
which is a typical example of a device using electromagnetic
induction heating for the heat source (see Publication of
Unexamined Japanese Patent Application No. Hei 7-114276). As is
shown in FIG. 10, a magnetization coil 203 is wound around a core
material 202 on the inner side of a rotating endless film 201.
Using this coil, an alternating magnetic field can be caused to
penetrate the film 201. Then, this alternating magnetic field
induces an induction current in the film 201, which serves as
heat-generating material and as heating material, and due to the
heat generated by the induction current in the film 201, a toner
image 206 is fixed on a recording material 205, which passes
between the film 201 and a pressure roller 204. Numeral 207 in FIG.
10 denotes a thermistor for detecting the surface temperature of
the pressure roller 204. Depending on the temperature detected by
this thermistor 207, the current applied to the magnetization coil
203 is regulated. In this example, a special layering structure is
devised for the film 201, so that the heat generated by the film
201 does not transmit as easily towards the side of the
magnetization coil 203.
Including this conventional example, image heating devices using
magnetic induction heating generally can heat necessary parts
intensively and with high efficiency, so that they are useful as
one means for reducing warming-up periods and saving energy.
However, in order to effectively reduce warming-up periods and save
energy, it is necessary to reduce the thermal capacity of the
heat-generating member or the heating member in addition to making
the heating means more effective, which brings about new
problems.
When the thermal capacity of the heat-generating member or the
heating member is reduced, the temperature of the heat-generating
member or the heating member reacts with sensitivity to changes in
the generated heat or the escaping heat, which promotes temperature
changes. Moreover, it is useful to reduce their thicknesses in
order to reduce the thermal capacity, but then also their internal
thermal conductivity worsens, so that partial temperature
differences arise easily, and it becomes difficult to regulate the
temperature of the entire heat-generating member or heating member
to a uniform and stable temperature. The above-noted conventional
image heating device using film-heating is an example where this
problem is particularly apparent.
Moreover, in the regular film-heating method, the thermal capacity
of the film is set as small as possible to reduce the warming-up
period, but this gives rise to the problem that the film
temperature partially becomes too high. When the film temperature
becomes too high, the heat generation becomes unstable, and hot
offset may occur, which in turn causes the destruction of the film
and the components around it. Taking the conventional image heating
device in FIG. 10 as an example, this problem is aggravated when a
recording material 205 whose width is smaller than the width of the
image heating device in the depth direction of the drawing is
continuously being transported. This means, heat is dissipated into
the recording material 205 at the portion where the recording
material 205 is transported, so that the heating has to be
performed correspondingly, but if portions where no recording
material 205 is transported are heated simultaneously, the
temperature in these portions will rise, because the thermal
capacity of the film is small and the thermal conductivity in the
width direction is poor. Then, when the temperature of the film
partially becomes excessively high and a recording material 205
with broad width is transported, hot offset occurs, or the overall
amount of heat generated becomes unstable, which in turn may result
in damage of the magnetization coil 203, which provides heat
generation. It is not possible to regulate such a partial
temperature rise by detecting the temperature only in the film
serving as the heat-generating member and heating member or other
members in the above-described conventional example.
On the other hand, when the entire amount of heat generated is
limited to prevent temperature rises, the temperature at the
portions with high temperature absorption will drop, which may
bring about insufficient fixing at these portions.
Not only in the film-heating method, but also when reducing the
thermal capacity in the heat-roller method using a halogen lamp or
magnetic induction by reducing the thickness of the roller in order
to reduce the warming-up time, the same problems arise because of
the instability of the generated heat and because of partial
overheating and underheating. On the other hand, in the above-noted
publication, an attempt was made to achieve temperature
self-regulation using a film whose Curie temperature has been
adjusted, but according to our experiments, it is difficult to
achieve suitable temperature self-regulation using a
heat-generating member (film) with that structure. In other words,
in this example, the electrically conductive film is formed
considerably thinner than the skin depth, and the cross-sectional
area of the path where the induction current flows is the same
above and below the Curie temperature, so that the amount of heat
generated above and below the Curie temperature is almost the same.
Consequently, with this conventional configuration, it is
impossible to perform a suitable temperature regulation for the
image heating device, so that it cannot solve the problem of
partial temperature rises and drops.
One of the results of the research which lead to the present
invention was that to achieve effective temperature self-regulation
applicable for an image heating device, it is necessary that (i)
during start-up, a large amount of heat is generated by letting
almost the entire induction current flow through a highly resistive
portion, (ii) once the Curie temperature is exceeded, the amount of
heat generated is decreased by letting more induction current flow
through a portion with low resistivity, and (iii) certain
conditions should be satisfied so that the differences between
these amounts of heat generated exceeds a certain value.
Furthermore, to achieve optimum fixing, there is a certain range
within which the temperature to be regulated has to be.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the problems
of the prior art. It is a further object of the present invention
to provide an image heating device and an image forming device
using the same, wherein the heating member itself can regulate its
own temperature in a stable manner when an image is heated on a
recording material, and suitable heating conditions can be attained
even without a temperature detecting means, such as the thermistor,
or temperature controlling circuits.
It is a further object of the present invention to provide an image
heating device and an image forming device using the same, wherein,
even when the thermal capacity of heat-generating members or
heating members such as heating rollers or films is reduced,
partial temperature deviations or excessive temperature rises can
be reduced by temperature self-regulation.
It is a further object of the present invention to provide an image
heating device and an image forming device using the same, wherein
even when a recording material of narrow width is transported
continuously, the portion where the recording material does not
pass does not become excessively hot, and there is no hot offset
and no partial under-heating.
It is a further object of the present invention to provide an image
heating device and an image forming device using the same, wherein
the generated heat does not become unstable due to excessively high
temperatures, and where damaging of the magnetization coil, film,
etc. due to heat can be avoided.
It is a further object of the present invention to provide an image
heating device and an image forming device using the same, wherein
the thermal capacity of the heat-generating member and the heating
member can be reduced, and the warming-up period can be
shortened.
In order to achieve these objects, an image heating device in
accordance with a first configuration of the present invention
comprises a heat-generating member comprising a magnetic layer with
a certain Curie temperature; a magnetization member for magnetizing
the heat-generating member with an alternating magnetic field,
which is arranged in opposition to the heat-generating member; and
a nip portion for heating a recording material that carries a toner
image with heat from the heat-generating member, while the
recording material is being conveyed along the nip portion. The
ratio between an amount of heat generated in the heat-generating
member at Curie temperature or higher to an amount of heat
generated at room temperature in the heat-generating member is not
more than 1/2. According to this first configuration of an image
heating device, stable temperature self-regulation can be attained
by the heat-generating member itself when the toner image is heated
on the recording material. Consequently, even without the
temperature detecting means, such as the thermistor, or temperature
controlling circuits, suitable heating conditions can be attained.
Furthermore, as the thermal capacity of the heat-generating member
or the heating member becomes low, a partial temperature difference
in the width direction of the recording material occurs easier, and
the ability of the heat-generating member to regulate its own
temperature also causes a partial difference in the heat
generation, so that even when a recording material of narrow width
is conveyed continuously by the nip portion, the portion where the
recording material does not pass does not become excessively hot,
and when subsequently a recording material of broader width is
conveyed continuously by the nip portion, there is no hot offset.
Consequently, since the thermal capacity of the heat-generating
member or the heating member can be decreased within the scope
where temperature self-regulation is possible, the warming-up time
can be shortened.
In this first configuration of an image heating device according to
the present invention, it is preferable that a thickness of the
magnetic layer is at least twice a thickness of a skin depth. With
this preferable configuration, the ratio of the amount of heat
generated above the Curie temperature to the amount of heat
generated at room temperature can be reduced to less than 1/2, so
that stable temperature regulation becomes possible.
In this first configuration of an image heating device according to
the present invention, it is preferable that the heat-generating
member further comprises a conductive layer with lower resistance
than the magnetic layer, which is provided adjacent to the magnetic
layer. With this preferable configuration, the ratio of the amount
of heat generated above the Curie temperature to the amount of heat
generated at room temperature can be reduced considerably without
increasing the thickness of the layers for the heat-generating
member so much. In this case, it is preferable that
wherein .rho.1 is an intrinsic resistance of the magnetic layer, t1
is a thickness of the magnetic layer, .rho.2 is an intrinsic
resistance of the conductive layer, and t2 is a thickness of the
conductive layer. With this preferable configuration, the ratio of
the amount of heat generated above the Curie temperature to the
amount of heat generated at room temperature can be reduced to less
than 1/2. In this case it is also preferable that the thickness of
the magnetic layer is equivalent or higher than the skin depth.
With this preferable configuration, almost the entire induction
current can be concentrated at the magnetic layer due to the skin
effect.
In this first configuration of an image heating device according to
the present invention, it is preferable that the nip portion is
formed by at least a portion of the heat-generating member, and a
pressure member pressed against this portion of the heat-generating
member. Furthermore, in this case it is preferable that at least
the magnetic layer of the heat-generating member is a rotatable
roller. Furthermore, in this case it is preferable that at least
the magnetic layer of the heat-generating member is a movable film.
Furthermore, in this case, it is preferable that at least the
conductive layer of the heat-generating member is a movable
film.
In this first configuration of an image heating device according to
the present invention, it is preferable that the nip portion is
formed by a movable film contacting the heat-generating portion,
and a pressure member for pressing against the film. Furthermore,
in this case, it is preferable that the heat-generating member
contacts a rear surface of the film. Furthermore, in this case, it
is preferable that the heat-generating member contacts the rear
surface of the film from a position upstream of the nip portion to
a vicinity of the nip portion, and the magnetization member is
provided at the position upstream of the nip portion. According to
these preferable configurations, the amount of heat generated can
be kept stable, because the magnetization member is not heated up
by the temperature of the nip portion. Furthermore, in this case,
it is preferable that the heat-generating member is provided on the
rear side of the film and contacts a portion of the film, and the
magnetization member is provided on a surface side of the film.
With this preferable configuration, the amount of heat generated
can be kept stable, because the magnetization member is not heated
up by the temperature of the heat-generating member. Furthermore,
in this case, it is preferable that the pressure member comprises a
roller with low thermal conductivity provided on the rear surface
side of the film and a pressure roller provided on the front
surface side of the film. With this preferable configuration, the
formation of the nip portion, which requires a strong pressure
force, is performed by the pressure between the roller with low
thermal conductivity and the pressure roller, so that there is no
portion that slides while a large friction force is exerted to form
the nip portion, which is suitable for operation at high speeds
over extended periods of time. Furthermore, in this case, it is
preferable that the heat-generating member comprises a rotatable
roller. Furthermore, in this case, it is preferable that the film
is loop-shaped.
An image heating device in accordance with a first configuration of
the present invention comprises a heat-generating member comprising
a magnetic layer with a certain Curie temperature, and a
magnetization member for magnetizing the heat-generating member
with an alternating magnetic field, which is arranged in opposition
to the heat-generating member. When the device is in operation, a
temperature at which the heat-generating member stabilizes due to a
drop of a relative magnetic permeability of the magnetic layer near
the Curie temperature is higher than a temperature where cold
offset begins. The Curie temperature is selected such that, when
the temperature of the heat-generating member is stabilized, a
temperature of the heat-generating member at an outgoing portion of
the nip portion is lower than a temperature where hot offset of the
toner begins. With this second configuration of an image heating
device according to the present invention, unfixed toner can be
fixed in a stable manner without hot offset.
In this second configuration of an image heating device according
to the present invention, it is preferable that the heat-generating
member further comprises a conductive layer with lower resistance
than the magnetic layer, which is provided adjacent to the magnetic
layer. Furthermore, in this case it is preferable that
wherein .rho.1 is an intrinsic resistance of the magnetic layer, t1
is a thickness of the magnetic layer, .rho.2 is an intrinsic
resistance of the conductive layer, and t2 is a thickness of the
conductive layer.
In this second configuration of an image heating device according
to the present invention, it is preferable that
wherein Tc is the temperature where cold offset of the toner begins
in the nip portion, Tk is the Curie temperature, and Th is the
temperature where hot offset of the toner begins in an outgoing
portion of the nip portion.
In this second configuration of an image heating device according
to the present invention, it is preferable that
wherein Tk is the Curie temperature.
In this second configuration of an image heating device according
to the present invention, it is preferable that the nip portion is
formed by at least a portion of the heat-generating member, and a
pressure member pressed against this portion. Furthermore, in this
case, it is preferable that at least the magnetic layer of the
heat-generating member is a rotatable roller. Furthermore, in this
case, it is preferable that at least the magnetic layer of the
heat-generating member is a movable film. Furthermore, in this
case, it is preferable that at least the conductive layer of the
heat-generating member is a movable film.
In this second configuration of an image heating device according
to the present invention, it is preferable that the nip portion is
formed by a movable film contacting the heat-generating portion,
and a pressure member for pressing against the film. Furthermore,
in this case, it is preferable that the heat-generating member
contacts a rear surface of the film. Furthermore, in this case, it
is preferable that the heat-generating member contacts the rear
surface of the film from a position upstream of the nip portion to
a vicinity of the nip portion, and the magnetization member is
provided at the position upstream of the nip portion. Furthermore,
in this case, it is preferable that the heat-generating member is
provided on the rear side of the film and contacts a portion of the
film, and the magnetization member is provided on a surface side of
the film. Furthermore, in this case, it is preferable that the
pressure member comprises a roller with low thermal conductivity
provided on the rear surface side of the film and a pressure roller
provided on the front surface side of the film. Furthermore, in
this case, it is preferable that the heat-generating member
comprises a rotatable roller. Furthermore, in this case, it is
preferable that the film is loop-shaped.
An image formation device according to the present invention
comprises an image formation system for forming an unfixed image
onto a recording material; and a thermal fixing device for
thermally fixing the unfixed image on the recording material,
wherein an image heating device according to the present invention
used as the thermal fixing device.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating the configuration of an
image heating device according to a first example of the present
invention.
FIGS. 2a and b are diagrams illustrating the temperature
self-regulation of an image heating device according to the first
example of the present invention.
FIG. 3 is a diagram illustrating the relation between the amount of
heat generated by the heating roller and the temperature in an
image heating device according to a first example of the present
invention.
FIGS. 4a and b are diagrams illustrating the temperature
self-regulation of an image heating device according to the second
example of the present invention.
FIG. 5 is a cross-sectional view of the configuration of an image
heating device according to a third example of the present
invention.
FIG. 6 is a perspective view showing a magnetization coil portion
used in an image heating device according to a third example of the
present invention.
FIG. 7 is a cross-sectional view of the configuration of an image
heating device according to a fourth example of the present
invention.
FIG. 8 is a cross-sectional view of the configuration of an image
heating device according to a fifth example of the present
invention.
FIG. 9 is a cross-sectional view showing an image forming device
using an image heating device according to an embodiment of the
present invention as a fixing device.
FIG. 10 is a cross-sectional view showing the configuration of a
conventional image heating device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following is a more detailed description of the present
invention with reference to the accompanying drawings.
FIG. 9 is a cross-sectional drawing showing an image forming device
using an image heating device according to an embodiment of the
present invention as the fixing device.
In FIG. 9, numeral 1 denotes an electrophotographic photoreceptor
(referred to as "photosensitive drum" in the following). While this
photosensitive drum is rotated at a certain velocity in the arrow
direction, its surface is charged evenly to a certain negative dark
potential V.sub.0.
Numeral 3 denotes a laser beam scanner, which outputs a laser beam
that is modulated in accordance with a serial electric digital
image signal of image information that is input from a host device
(not shown in the drawings) such as an image reading device or a
computer. The surface of the photosensitive drum 1, which has been
charged evenly to the dark potential V.sub.0, is scanned and
exposed by the laser beam, and the absolute potential of the
exposed portion is decreased to the light potential V.sub.L. Thus,
a static latent image is formed on the surface of the
photosensitive drum. Then, using a developer 4, this static latent
image is reversely developed with negatively charged powdered toner
and made manifest.
The developer 4 has a rotating developing roller 4a, which is
arranged parallel and in opposition to the photosensitive drum 1.
When a developing bias voltage, whose absolute value is lower than
the dark potential V.sub.0 of the photoelectric drum 1 and higher
than the light potential V.sub.L, is applied to the developing
roller 4a, a negatively charged thin toner layer is formed on the
peripheral surface of the developing roller 4a. The toner on the
developing roller 4a transfers only to the portion of the
photosensitive drum 1 with the light potential V.sub.L, a toner
image is formed, and the static latent image is made manifest.
The recording material 15 is fed one by one from a paper-feed
portion 10, passes a pair of resist rollers 11 and 12, and with a
nip portion consisting of the photosensitive drum and a transfer
roller contacting the same, the recording material 15 is fed with
suitable timing and in synchronization with the rotation of the
photosensitive drum 1. Then, by using transfer roller 13 to which a
transfer bias is applied, the toner image on the photosensitive
drum 1 is sequentially transferred to the recording material 15.
After the recording material 15 has passed between the
photosensitive drum 1 and the transfer roller 13, it is fed into a
fixing device 16, which fixes the transferred toner image. The
recording material 15 onto which the toner image has been fixed is
then delivered into a paper eject tray 17.
After the recording material 15 has passed the photosensitive drum
1, the surface of the photosensitive drum 1 is cleaned with a
cleaning device 5, which removes residual material, such as
remaining toner. By repeating these steps, sequential image
formation is possible.
The following is a more detailed explanation of an image heating
device in accordance with the present invention, with reference to
specific examples.
First Example
FIG. 1 is a perspective view showing an image heating device in
accordance with a first example of the present invention. In this
example, a fixing device using heating rollers made of magnetic
material is used for the image heating device.
As is shown in FIG. 1, a heating roller 21, serving as a
heat-generating member and as a heating member, has a cylindrical
magnetic alloy of 45 mm diameter and 1 mm thickness as a base,
whose composition is adjusted so that the Curie temperature becomes
about 210.degree. C. The surface of the heating roller 21 is coated
with a fluorocarbon resin of 15 .mu.m thickness for aiding the
lubrication of the toner. In this example, an alloy of iron, nickel
and chrome (intrinsic resistivity: 7.2.times.10.sup.-7 .OMEGA.m,
relative magnetic permeability at room temperature: ca. 100,
relative magnetic permeability above the Curie temperature: ca. 1)
was used. The material for the alloy and its composition can be
changed in accordance with the saturation magnetic flux density and
the desired Curie temperature.
The heating roller 21 is supported rotatably against the fixing
device itself by bearings (not shown in the drawings). An induction
heating portion for inductively heating the heating roller 21 is
provided inside the heating roller 21, and fixed with respect to
the fixing device. This induction heating portion comprises a
magnetization coil 23 as a magnetization member, which is wound
around a cylindrical bobbin 22 arranged inside the heating roller
21, and an AC current source 24 for feeding high-frequency
alternating current into the magnetization coil 23. To increase the
heating efficiency, a ferrite 25 is inserted into the bobbin 22 as
a core. For the magnetization coil 23, a litz wire of bundled thin
wires is used.
Numeral 26 denotes a pressure roller whose surface is made of
silicone rubber which is supported rotatably by the main body of
the fixing device by bearings (not shown in the drawings). The
present roller 26 is arranged in parallel to the heating roller 21.
When the silicone rubber of the pressure roller 26 is pressed onto
the heating roller 21, it deforms, so that a nip portion 27, i.e.
an area of certain pressure, is formed between the heating roller
21 and the pressure roller 26. In other words, the heating roller
21 and the pressure roller 26 constitute a nip forming means. The
heating roller 21, onto which the nip portion 27 is formed, is
rotated by a driving system (not shown in the drawing), and the
pressure roller 26 rotates following the heating roller 21. Numeral
28 denotes a thermistor for detecting the temperature on the
surface of the heating roller 21 near an outgoing portion of the
nip portion 27.
The recording material 15, whose surface carries the toner image 31
that has not yet been fixed, is inserted into the fixing device in
arrow direction X, and is heated by the heat of the heating roller
21, while is sandwiched and conveyed by the nip portion 27, thereby
fixing the toner image 31 onto the recording material 15.
Alternating current of 23 kHz frequency from a current source 24 is
fed into the magnetization coil 23 of this fixing device, and a
certain period of time after starting the heating of the heating
roller 21, the heating roller 21 is rotated with a velocity of 200
mm/sec. The surface temperature of the heating roller 21 is
detected by the thermistor 28. It could be established that a
certain period after departing from room temperature, the surface
temperature of the heating roller 21 is stabilized around
190.degree. C.
After the temperature has been stabilized, the recording material
15 is continuously conveyed by the nip portion 27, and the surface
temperature near the outgoing portion of the nip portion 27 of the
heating roller is detected with the thermistor 28. It could be
established that the surface temperature near the outgoing portion
of the nip portion of the heating roller 21 is stabilized around
165.degree. C.
The following is an explanation of the relation between the amount
of heat generated in the heating roller and the regulated
temperature.
First of all, when high-frequency alternating current is supplied
to the magnetization coil 23, a corresponding high-frequency
alternating magnetic field is generated, and this high-frequency
magnetic field interlinks with the heating roller 21. Thus, an
induction current is induced inside the heating roller 21, and the
heating roller 21 is inductively heated. Since the heating roller
is made of a magnetic alloy whose composition is adjusted so that
its Curie temperature becomes about 210.degree. C., there is a
considerable difference between the induction current flowing when
the temperature is below the Curie temperature and when the
temperature is near the Curie temperature or above it. In other
words, the heating roller 21 has the ability of temperature
self-regulation. FIGS. 2(a) and (b) are drawings illustrating this
ability of regulating its own temperature.
In FIG. 2(a), the hatched area corresponds to the area where an
induction current flows when the heating roller 21 is near room
temperature. As is shown in FIG. 2(a), the induction current
concentrates in a portion of a certain thickness on the inner
surface of the heating roller 21, which is due to the skin effect.
The thickness of the portion where most of the induction current
flows, that is, the skin depth .delta.[m] can be expressed
theoretically by the intrinsic resistance .rho.[.OMEGA.m] of the
material, the magnetization frequency f[Hz], and the relative
magnetic permeability .mu. of the material:
In this example, since a magnetic alloy with an intrinsic
resistance of 7.2.times.10.sup.-7 .OMEGA.m and a relative magnetic
permeability at room temperature of about 100 is used for the
heating roller 21, and since the magnetization frequency is about
23 kHz, a skin depth of about 0.28 mm can be calculated. In other
words, near room temperature, almost the entire induction current
is concentrated in a region of about 0.28 mm thickness from the
inner surface of the heating roller 21.
In FIG. 2(b), the hatched area corresponds to the area where an
induction current flows when the heating roller 21 is above the
Curie temperature. In this case, the relative magnetic permeability
of the heating roller 21 becomes about 1, so that the thickness
corresponding to the skin depth .delta. becomes about 10 times the
skin depth at room temperature. Therefore, the induction current
flows over the entire thickness of 1 mm of the heating roller 21,
as is shown in FIG. 2(b).
By changing the induction current, the thickness of the portion
through which an induction current flows at temperatures above the
Curie temperature is about three times higher than at room
temperature, which reduces the total resistance. Consequently, when
magnetization is performed with a constant current, the amount of
heat generated is about one third, since it is proportional to the
resistance.
FIG. 3 shows the generated heat Ba as a function of the temperature
of the material of the heating roller 21. In FIG. 3, the horizontal
axis marks the temperature of the material of the heating roller 21
(assuming that the temperature is distributed evenly across the
heating roller 21), and the vertical axis shows the amount of heat
generated. Actually, the relative magnetic permeability of the
material of the heating roller 21 does not change abruptly from 100
to 1 at the Curie temperature Tk, but rather decreases gradually as
the Curie temperature is approached, so that the amount of heat
generated also decreases gradually as the temperature is increased,
and drops sharply near the Curie temperature Tk. Above, the Curie
temperature Tk, the range in which an induction current flows
becomes the entire thickness of the heating roller 21, and the
amount of generated heat stabilizes at a constant value. In this
example, the ratio between the amount of heat Q1 generated at room
temperature Tn and the amount of heat Q2 generated at temperatures
above the Curie temperature is about 3:1.
The temperature where the heating roller 21 finally stabilizes
(stabilizing temperature) is the temperature where the amount of
heat dissipating away from the heating roller 21 balances against
the amount of heat generated by this magnetic induction heating.
Generally speaking, a certain amount of heat escapes from the
heating roller 21 of the fixing device due to heat transmission
over the supporting bearings or the pressure roller 26, or through
radiation and convection into the atmosphere. This dissipated
amount of heat becomes larger with increasing temperature of the
heating roller 21. When this dissipated amount of heat is expressed
as a thermal dissipation curve, the curve D in FIG. 3 results. The
intersection Ea between the thermal dissipation curve D and the
generated heat curve Ba indicates the stabilization temperature.
The fact that in this example the surface temperature of the heated
heating roller 21 when no recording paper 15 was transported
stabilized at 190.degree. C. means that this intersection Ea is at
190.degree. C. However, if the temperature of the thermal roller 21
were examined in detail, it is possible that there is a temperature
distribution, and that the point where the amount of heat generated
balances the temperature varies slightly between different
portions, but as for the average situation in the entire heating
roller 21, the above consideration can be regarded as valid.
When the recording material 15 is conveyed continuously by the nip
portion 27, the surface temperature of the heating roller 21 near
the outgoing portion of the nip portion 27 stabilizes at
165.degree. C., because the entire thermal load for the heating
roller 21 is increased by the amount of heat that dissipates into
the recording material 15. Since the temperature is measured near
the outgoing portion of the nip portion 27, the somewhat lower
temperature near the surface of the heating roller 21 after heat
has been consumed by the recording material 15 is shown, the
average temperature of the entire heating roller 21 is regulated to
a temperature that is lower than the temperature when no recording
material is being transported. In FIG. 3, F denotes the thermal
dissipation curve when recording material 15 is conveyed
continuously by the nip portion 27, and G denotes the stabilization
point where the heat balance is in equilibrium. The point G
represents the average temperature of the entire heating roller 21,
and is about 175.degree. C., i.e. slightly higher than the
165.degree. C. measured above.
Next, the thermal loss in a typical fixing device was measured.
When the process velocity was 150 mm/sec, and the regulated roller
temperature was 180.degree. C., the total amount of heat was about
490 W. Of these 490 W, about 47% (ca. 230 W) were consumed by the
recording material, and the other 53% were dissipated into the
pressure roller and the supporting portions, or radiated into the
atmosphere. When the process speed was changed, the total amount of
heat changed with the amount of heat consumed by the recording
material, but at the most frequently used process speed of 100-250
mm/sec, when the amount of heat was calculated on the basis of the
heat measured after the recording material has passed by nip
portion, the amount of heat consumed by the recording material per
total amount of heat near the fixing temperature was about 1/2 or
less, and this ratio was fairly stable. Thus, it can be seen that
in most cases, the amount of heat at the stabilization point Ea in
FIG. 3 when no recording material 15 is transported is at least 1/2
of the amount of heat at the stabilization point G when recording
material 15 is continuously conveyed by the nip portion 27.
In order to stabilize the temperature of the heating roller 21
regardless of whether there is a recording material 15, it is
preferable that both stabilization points Ea and G are located in
the portion of the slope of the generated heat curve Ba where the
amount of heat generated near the Curie temperature Tk drops
sharply. In other words, if the ratio between the amount Q2 of heat
generated above the Curie temperature to the amount Q1 of heat
generated at room temperature Tn becomes larger than 1/2 in the
generated heat curve Bb (dashed line) and if the stabilization
point G is placed in the portion where the slope of the amount of
heat generated drops sharply, then the point of stabilization when
no recording material 15 is being transported will correspond to a
certain heat Eb above the Curie temperature Tk, so that the
temperature regulation becomes very unstable if the dissipation
curve is almost horizontal.
Thus, it is necessary to make sure that the ratio between the
amount Q2 of heat generated above the Curie temperature to the
amount Q1 of heat generated at room temperature Tn is less than
1/2. If the ratio between the amount Q2 of heat generated at a
temperature above the Curie temperature Tk to the amount Q1 of heat
generated at room temperature Tn is 1/3 or less, then a very stable
temperature regulation becomes possible, regardless of whether a
recording material 15 is present or not.
Therefore, if the thickness of the magnetic alloy for the heating
roller 21 is at least twice the thickness of the skin depth
corresponding to the magnetization frequency, then the ratio of the
amount of heat generated above the Curie temperature to the amount
of heat generated at room temperature becomes less than 1/2, and a
stable temperature regulation becomes possible.
Next, using a halogen lamp and the thermistor 28, the relation
between the temperature of the heating roller 21 and toner offset
was determined. As a result, it became clear that at the speed set
for the present example, cold offset (toner is not completely
melted and sticks to the heating roller 21) begins when the surface
temperature of the heating roller 21 near the ingoing portion of
the nip portion 27 drops below 160.degree. C., and hot offset
(melted toner sticks to the heating roller 21) begins when the
surface temperature of the heating roller 21 near the outgoing
portion of the nip portion 27 drops exceeds 210.degree. C. Thus, it
was determined that cold offset begins at a temperature Tc of
160.degree. C., and hot offset begins at a temperature Th of
210.degree. C.
In summary, the stabilization temperature at which the heating
roller 21, which is a heat-generating member and a heating member,
stabilizes its own temperature is not the Curie temperature itself,
but is dependent on the heat generation curve and the amount of
dissipated heat, that is, the thermal load. On the other hand, to
fix the unfixed toner image reliably without offset onto the
healing roller 21, the temperature of some portion inside the nip
portion 27 must be higher than Tc, which is the lowest temperature
at which melt-adhesion is possible, and the temperature at the
outgoing portion of the nip portion 27 has to be at most Th, which
is the temperature where hot offset of the toner begins.
First of all, because there is the possibility that the
stabilization temperature in the maximum case becomes close to the
Curie temperature, the Curie temperature Tk has to be at least Tc
or higher.
On the other hand, how high the Curie temperature Tk can be set,
depends on how much the temperature can be regulated away from a
predetermined Curie temperature, or in other words, on how much
lower than the Curie temperature Tk the stabilization point G in
FIG. 3 and the surface temperature of the heating roller 21 at the
outgoing portion of the nip portion 27 can be set. Consequently, a
necessary condition for the Curie temperature Tk is that Tk is not
more than Th plus the largest possible temperature difference
.alpha. that is possible between the surface temperature of the
heating roller 21 at the outgoing portion of the nip portion 27 and
the Curie temperature Tk. This temperature difference .alpha. can
be determined from the form of the generated heat curve Ba and the
dissipation curve, which is dependent on the configuration of the
fixing device and the speed.
Thus, a necessary condition for the Curie temperature Tk is
In this example, the surface temperature of the heating roller 21
near the nip portion 27 stabilizes at about 165.degree. C., which
is about 45.degree. C. lower than the Curie temperature Tk. This
stabilization temperature is sufficiently lower than 210.degree.
C., which is the temperature Th at which hot offset begins, so that
hot offset can be avoided. How high the maximum temperature
difference .alpha. in regular fixing devices can be, is explained
further below.
A fixing device with the above configuration was used in the image
forming device shown in FIG. 9. The recording material 15, onto
which the toner image has been transferred, was inserted into the
fixing device in the arrow direction with the side whereon the
toner 31 has been applied facing the heating roller 21, as shown in
FIG. 1, thereby fixing the toner 31 onto the recording material
15.
According to this example, the heating roller 21 itself, which
serves as a heat-generating member, has the ability to regulate its
own temperature, so that by setting the Curie temperature Tk to a
suitable value with regard to the fixing temperature, the
temperature regulation can be performed automatically.
Consequently, even without a temperature detecting means, such as
the thermistor, or temperature controlling circuits, suitable
heating conditions can be attained. When the thermal capacity of
the heating roller 21, which is also a heating member, is low, a
partial temperature difference in the width direction of the
recording material 15 occurs easier, the ability of the heating
roller 21 to regulate its own temperature also causes a partial
difference in the heat generation, so that even when a recording
material 15 of narrow width is conveyed continuously by the nip
portion 27, the portion where the recording material 15 does not
pass does not become excessively hot, and when subsequently a
recording material 15 of broader width is conveyed continuously by
the nip portion 27, there is no hot offset. Consequently, since the
thermal capacity of the heating roller 21 can be decreased within
the scope where temperature self-regulation is possible, the
warming-up time can be shortened.
Second Example
The following is an explanation of a second example of a fixing
device. The fixing device of this example differs from the fixing
device of the first example only in the configuration of the
heating roller, so that the drawings of the entire configuration
have been omitted, and in the following explanations, structure
elements performing the same function as in the first example are
referred to with the same numerals. FIGS. 4(a) and (b) are
cross-sectional drawings showing the configuration of a heating
roller, which serves as a heat-generating member and a heating
member, according to this example. This drawing illustrates the
temperature self-regulation, which is similar to the first example.
The heating roller 41, which serves as a heat-generating member and
a heating member, is provided on the inside with a magnetic alloy
layer 42 of 0.3 mm thickness, whose composition is adjusted so that
its Curie temperature becomes about 210.degree. C., and on the
outside with an aluminum layer 43 of 0.3 mm thickness, which serves
as a highly conductive layer. The surface of the heating roller 41
is coated with a fluorocarbon resin of 15 .mu.m thickness for
aiding the lubrication of the toner. Also in this example, as in
the first example, an alloy of iron, nickel and chrome (intrinsic
resistivity: 7.2.times.10.sup.-7 .OMEGA.m, relative magnetic
permeability at room temperature: ca. 100, relative magnetic
permeability above the Curie temperature: ca. 1) was used.
Alternating current of 23 kHz frequency from a current source 24 is
fed into the magnetization coil 34 of this fixing device, and a
certain period of time after starting the heating of the heating
roller 41, the heating roller 41 is rotated with a velocity of 200
mm/sec. The surface temperature of the heating roller 41 is
detected by the thermistor 28. It could be established that a
certain period after departing from room temperature, the surface
temperature of the heating roller 41 stabilized around 190.degree.
C.
After the temperature has been stabilized, the recording material
15 is conveyed continuously by the nip portion 27, and the surface
temperature of the heating roller 41 near the outgoing portion of
the nip portion 27 is detected with the thermistor 28. It could be
established that the surface temperature of the heating roller 41
near the outgoing portion of the nip portion 27 stabilized around
175.degree. C. Consequently, in this example, the temperature
difference between the surface temperature of the heating roller 41
at the outgoing portion of the nip portion 27 and the Curie
temperature is about 35.degree. C.
In this example, as in the first example, there is a considerable
difference between the induction current flowing in the heating
roller 41 when the temperature is below the Curie temperature and
when the temperature is near the Curie temperature or above it. In
other words, the heating roller 41 has the ability of regulating
its own temperature.
In FIG. 4(a), the hatched area corresponds to the area where an
induction current flows when the temperature of the heating roller
41 is near room temperature. Since in this example the same
magnetic alloy is used for the magnetic alloy layer 42 as in the
first example, the skin depth 6 becomes about 0.28 mm, which is
roughly the same as the thickness of the magnetic alloy layer 42
(0.3 mm). In other words, as shown in FIG. 4(a), almost the entire
induction current concentrates due to the skin effect and flows
only in the magnetic alloy layer 42. Therefore, the thickness of
the magnetic alloy layer 42 should be at least equal to the skin
depth.
In FIG. 4(b), the hatched area corresponds to the area where an
induction current flows when the temperature of the heating roller
41 is above the Curie temperature. As is shown in FIG. 4(b), almost
the entire induction current flows in the outer aluminum layer 43.
Since in this situation the relative magnetic permeability of the
magnetic alloy layer 42 becomes about 1, the magnetic flux
penetrates the magnetic alloy layer 42, and the induction current
tends to spread out over the entire thickness of the heating roller
41, but because the electrical resistance of the aluminum layer 43
is much smaller than that of the magnetic alloy layer 42, it can be
assumed that almost the entire induction current flows in the
aluminum layer 43.
The magnetic alloy used in this example has an intrinsic resistance
of 7.2.times.10.sup.-7 .OMEGA.m, as in the first example, whereas
the intrinsic resistance of the aluminum is 2.5.times.10.sup.-8
.OMEGA.m, i.e. only 1/29 of the magnetic alloy material. The
thickness of the portion where the induction current flows is in
both layers about 0.3 mm, so that when magnetization is performed
with a constant current, the amount of heat generated above the
Curie temperature is about 1/29 of the amount of heat generated at
room temperature.
As above, for a heating roller 41 with the dual layer configuration
of this example, the amount of heat generated at a temperature near
the Curie temperature or above the Curie temperature can be reduced
considerably compared to the amount of heat generated at room
temperature, without increasing the layer thickness very much. As
explained above, to stabilize the temperature of the heating roller
12 in a regular fixing device regardless of whether there is a
recording material or not, the ratio of the amount of heat
generated above the Curie temperature to the amount of heat
generated at room temperature has to be 1/2 or less. When the
heating roller 41 with dual layer structure of this example is
used, if the electric resistance of the entire highly conductive
layer (in this example, the aluminum layer 43) is not higher than
the electric resistance of the entire magnetic layer, the ratio of
the amount of heat generated above the Curie temperature to the
amount of heat generated at room temperature can be set to 1/2 or
less by adjusting the frequency of the high-frequency current to
set the skin depth to about the thickness of the magnetic layer. In
other words, if
wherein the intrinsic resistance of the magnetic layer is .rho.1
and its thickness is t1, and the intrinsic resistance and the
thickness of the highly conductive layer are .rho.2 and t2, then
the ratio of the amount of heat generated above the Curie
temperature to the amount of heat generated at room temperature can
be set to 1/2 or less. If the intrinsic resistance of the highly
conductive layer is very small, the same effect can be attained
with a very thin layer. This is especially useful when it is
necessary to decrease the thermal capacity of the heat-generating
member or the heating member in order to reduce the warming-up
period.
Using the heating roller 41 with the dual layer configuration of
this example, the ratio between the amount of heat generated at
room temperature to the amount of heat generated above the Curie
temperature can be reduced easily, and since the generated heat
curve drops sharply towards the Curie temperature, the regulated
temperature can be set near the Curie temperature. As pointed out
above, the temperature difference between the surface temperature
of the heating roller 41 at the outgoing portion of the nip portion
27 and the Curie temperature in this example is about 35.degree.
C.
In this example, an aluminum layer 43 was used for the highly
conductive layer, but the same effect also can be attained when
highly conductive material such as copper, nickel etc. is used.
Furthermore, in this example, a heating roller 41 with a dual layer
configuration of a highly conductive layer layered on a magnetic
layer was used, but it is also possible to use a heating roller
comprising only a magnetic layer, and providing a highly conductive
layer adjacent in a non-contacting manner thereto, which surrounds
a periphery of the magnetic layer excluding the nip portion. In
such a non-contact dual layer structure, the thermal capacity of
the heating roller, which serves as a heat-generating member and as
a heating member, can be reduced even further.
Third Example
The following is an explanation of a fixing device according to a
third example of the present invention. FIG. 5 is a cross-sectional
drawing showing the fixing device used as an image heating device
according to the third example of the present invention, and FIG. 6
is a perspective view of the magnetization coil used for this
fixing device.
In FIG. 5, numeral 51 denotes a thin film of 30 mm diameter and 50
.mu.m thickness, which has been formed into a loop-shape by
electroforming with Ni. The surface of the film 51 is coated with a
lubricant layer 52 made of a fluorocarbon resin of 10 .mu.m
thickness, which enhances the lubrication to the toner. As a
material for the film 51, metals such as Fe, Co, Cu, or Cr can be
used alone or in combination. Heat is generated by the
heat-generating member, which is described further below. For the
film 51, a film-shaped heat-resistant non-metallic resin, such as
polyimide resin or fluorocarbon resin can be used. For the
lubricant layer 52, a resin or rubber with good lubrication, such
as PTFE, PFA (tetrafluoroethylene perfluoroalkoxy vinyl ether
copolymer), FEP (tetrafluoroethylene hexafluoropropylene
copolymer), silicone rubber, or fluorocarbon rubber can be used
alone or in combination. If the fixing device is used to fix
monochrome images, only lubrication has to be ensured, but if it is
used to fix color images, it is preferable that it enhances
resilience, and it is necessary to use a little thicker rubber
layer as the lubricant layer 52.
In FIGS. 5 and 6, numeral 53 denotes the magnetization coil serving
as a magnetization member. This magnetization coil 53 is wound
around a core 54 made of a ferrite material. The core 54 is firmly
supported by the main body of the image forming device. An
alternating current of 30 kHz frequency is fed into the
magnetization coil from an AC current source 55, causing the
repeated generation and annihilation of magnetic flux around the
magnetization coil 53 as indicated by arrow H in FIG. 6.
As is shown in FIG. 5, a heat-generating member 56 is provided in
opposition to the magnetization coil 53 and the core 54, separated
by a small gap. When this heat-generating member 56 is biased by a
spring (not shown in the drawings) so that its lower surface
contacts the inner surface (rear surface) of the film 51, it is
supported by the main body of the image forming device. The core 54
is formed and arranged in a manner that the magnetic flux generated
by the magnetization coil 53 penetrates especially the
heat-generating member 56. This is achieved by providing the core
54 with an E-shaped cross-section and letting its opening space
oppose the heat-generating member 56. In the present example, there
is a gap between the magnetization coil 53, the core 54 and the
heat-generating member 56, but it is also possible to fill this gap
with insulating material.
The heat-generating member 56 comprises two metal plates that are
fitted tightly to each other. On the side that is in opposition to
the magnetization coil 53, the heat-generating member 56 has a 0.3
mm thick magnetic plate 57, serving as a magnetic layer, made of an
alloy of iron and nickel and chrome (intrinsic resistance:
7.2.times.10.sup.-7 .OMEGA.m; relative magnetic permeability at
room temperature: ca. 100; relative magnetic permeability at Curie
temperature: ca. 1), whose Curie temperature is set to about
200.degree. C. by adjusting the amount of chrome in the alloy. On
the side that contacts the film 51, the heat-generating member 56
has a 0.4 mm thick conductive plate 58, made of aluminum. The film
51, whose rotation is explained further below, moves while sliding
along the surface of the conductive plate 58 of the heat-generating
member. The heat-generating member 56 is arc-shaped, with a flat
portion 59 at its center portion.
In this example, this configuration of the heat-generating member
56 provides it with the ability to regulate its own temperature. As
in the second example, at room temperature the induction current
concentrates in the magnetic plate 57 due to the skin effect, and
as the temperature of the heat-generating member 56 approaches the
Curie temperature, the magnetism of the magnetic plate 57 is lost,
so that the magnetic flux emanates towards the outer conductive
plate 58, and the induction current flows almost entirely inside
the conductive plate 58 with low electric resistance. In this
situation, the generation of heat decreases considerably, since the
electric resistance of the conductive plate 58 is low. Calculations
show that the depth of the portion where an induction current flows
due to the skin effect at room temperature is about 0.25 mm at 30
kHz frequency. If the thickness of the magnetic plate 57 is the
same as the skin depth or larger, then at low temperatures the
induction current is generated almost entirely inside the magnetic
plate 57. If the frequency of the electric current is raised, the
skin depth decreases gradually, and a thinner magnetic plate 57 can
be used accordingly. However, if the frequency of the magnetization
current is too large, costs will rise, and the noise reaching the
outside will become large.
In FIG. 5, numeral 61 denotes a pressure roller serving as a
pressure member, which is made of resilient silicone rubber of 35
mm diameter and low hardness (25 degrees according to JIS A), which
is formed in one piece with a metal axis 62. The pressure roller 61
is supported rotatably around its axis by the main body of the
image forming device. As is shown in FIG. 5, the pressure roller 61
is pressed against the heat-generating member 56 via the film 51,
while deforming its surface, so that it follows the flat portion 59
of the heat-generating member 56, thereby forming a nip portion 63.
In this situation, the pressure roller 61 is rotated in the arrow
direction Y by a driving system (not shown in the drawings), so
that the film 51 is also rotated following the pressure roller
61.
The pressure roller 61 also can be made of a heat-resistant resin
or rubber, such as fluorocarbon rubber or a fluorocarbon resin.
Further, the surface of the pressure roller 61, can be coated with
a resin or rubber such as PFA, PTFE, or FEP, alone or in
combination, to enhance the abrasion resistance and lubrication of
the pressure roller. Further, to avoid heat radiation, it is
preferable that the pressure roller 61 is made of a material with
low thermal conductivity.
A fixing device as described above was installed in the image
forming device shown in FIG. 9, and toner 31 was fixed on a
recording material 15. For this, the process speed was set to 100
mm/sec, and the recording material, onto which a toner image has
been transferred, was inserted in the arrow direction into the
fixing device with the side carrying the toner 31 facing the
heat-generating member 56, as shown in FIG. 5.
Alternating current of 30 kHz frequency was supplied to the
magnetization coil 53 of the fixing device from an AC current
source 55, and a certain period of time after the heating of the
heat-generating member 56 was started, the pressure roller 61 was
rotated with a peripheral speed of 100 mm/sec. Then, the surface
temperature of the heat-generating member was measured, and it
could be determined that a certain period of time after the surface
temperature of the heat-generating member departed from room
temperature, if stabilized at about 180.degree. C.
After the temperature had stabilized, the recording material 15 was
conveyed continuously by the nip portion 63, and the surface
temperature of the heat-generating member 56 near the outgoing
portion of the nip portion 63 was measured, and it was determined
that the surface temperature of the heat-generating member 56 near
the outgoing portion of the nip portion 63 was about 170.degree. C.
Consequently, in this example, the temperature difference between
the surface temperature of the heat-generating member 56 near the
outgoing portion of the nip portion 63 and the Curie temperature
was about 30.degree. C.
According to this example, the heat-generating member 56 itself has
the ability to regulate its own temperature, so that the
heat-generating member 56 does not become excessively hot, and by
setting the Curie temperature to a suitable value with regard to
the fixing temperature, the temperature regulation near the fixing
temperature can be performed automatically. Consequently, even
without a temperature detecting means, such as the thermistor, or
temperature controlling circuits, suitable heating conditions can
be attained. If a heating member with low thermal capacity such as
the film 51 in this example is used, a partial temperature in the
depth direction of FIG. 5 occurs easily. However, the ability of
the heat-generating member 56 to regulate its own temperature also
causes a partial difference in the heat generation, so that even
when a recording material 15 of narrow width is conveyed
continuously by the nip portion 63, the portion where the recording
material 15 does not pass does not become excessively hot, and when
subsequently a recording material 15 of broader width is conveyed
continuously by the nip portion 63, hot offset does not occur.
Consequently, since the thermal capacity of the heat-generating
member 56 or the film 51 serving as a heating member can be
decreased within the scope where temperature self-regulation is
possible, the warming-up time can be shortened.
Since the material, thickness etc. of the heat-generating material
56 can be chosen independently from the film 51, the material,
thickness and shape most suitable for temperature self-regulation
can be selected, and the thermal capacity of the film 51 also be
selected individually.
In this example, aluminum was used for the conductive plate 58, but
it is also possible to use another metal with high conductivity
such as copper. Furthermore, the same effect can be attained when
another alloy with adjustable Curie temperature is used for the
magnetic plate 57. Moreover, it is also possible to provide a very
thin lubricant layer of fluorocarbon resin, that is thin enough,
perhaps several .mu.m or so, that it hardly influences the thermal
conductivity of the surface that slides against the film 51 of the
conductive plate 58.
Furthermore, in this example, the heat-generating member 56 has a
dual layer structure, but it is also possible to use a
heat-generating member of a single magnetic material that is at
least twice as thick as the skin depth.
By using for the heat-generating member one magnetic plate that is
about as thick as the skin depth, and using for the film 51 for
example a highly conductive material such as copper, it is possible
to reduce the induction current flowing in a portion of the film 51
above the Curie temperature and reduce the generated heat In other
words, if
wherein the intrinsic resistance of the magnetic plate, which
serves as the heat-generating member, is .rho.1 and its thickness
is t1, and the intrinsic resistance and the thickness of the highly
conductive film 51 are .rho.2 and t2, then the ratio of the amount
of heat generated above the Curie temperature to the amount of heat
generated at room temperature can be set to 1/2 or less. For
example, the intrinsic resistance of the film 51 made of copper is
1.7.times.10.sup.-8 .OMEGA.m, and that of the above-noted magnetic
alloy is only 1/42 of that, so that this condition can be met if
the thickness of the film 51 is about 7 .mu.m or more.
By using for the heat-generating member one magnetic plate that is
about as thick as the skin depth, and using a highly conductive
material such as aluminum for the inside portion of the pressure
roller 61 opposing it, an induction current flows in the portion of
the highly conductive material above the Curie point, and it is
possible to reduce the heat generation almost to zero.
Moreover, if the frequency of the magnetization current (AC
current) is increased and a material with large magnetic
permeability or low intrinsic resistance is used, the skin depth
can be reduced, so that it is also possible to use a film
satisfying the above conditions for the heat-generating member.
Fourth Example
Referring to FIG. 7, the following is an explanation of a fourth
example of an image device used for an image forming device, that
is particularly suitable for fixing color images.
In this example, elements having the same structure and performing
the same function as in the fixing device of the third example are
referred to with the same numerals and their further explanation
has been omitted.
With respect to material and thickness, the film 81 of this example
is the same as the film of the third example, but in this example,
the film diameter was set to 80 mm. The surface of the film 81 is
covered with a 50 .mu.m layer of silicone rubber 82 for fixing
color images. Also in this example, the heat generation is
performed with a heat-generating member 89 explained further below,
so that a film-shaped heat-resistant non-metallic resin such as a
polyimide resin or fluorocarbon resin can be used for the film 81.
The film 81 is suspended with a certain tensile force by a first
roller 83 of 30 mm diameter and a second roller 84 of 40 mm
diameter, and is rotatable in arrow direction Z. The first roller
83 is an elastic roller with low thermal conductivity made of
foamed silicone rubber with low hardness (35 degrees according to
ASKER C), which is formed in one piece with a metal axis 85.
Moreover, a second roller 84 is made of silicone rubber with a
hardness of JSI A60 degrees, which is formed in one piece with a
metal axis 86. The metal axis 85 can be driven by a driving system
of the main body to rotate the film 81. A pressure roller 87 is
made of silicone rubber with a hardness of JIS A60 hardness, and
presses against the first roller 83 via the film 81, thereby
forming a nip portion 92. In this situation, the first roller 83 is
rotated, so that the pressure roller 87 with the metal axis 88 at
its center is also rotated following the first roller 83.
On the inner side of the film 81, a heat-generating member 89 is
provided between the first roller 83 and the second roller 84. The
heat-generating member 89 is supported by the main body of the
image forming device, and biased by a spring downwards in FIG. 9,
so that it is pressed against the inner surface (rear surface) of
the film 81. The reason why the heat-generating member 89 is
pressed against the film 81, is to make heat transmission possible,
and since this is unrelated to the formation of the nip portion 92
for fixing the toner, the pressure force can be small. As in the
third example described above, the heat-generating member 89 has a
dual layer structure of a magnetic plate 90 serving as a magnetic
layer on the inside and a conductive plate 91 serving as a highly
conductive layer on the side of the film 81, whose material and
thickness is the same as for the third example. Moreover, a tip
portion 89a, which is located on a side of the conductive plate 91
in film-moving direction, extends to the nip portion 92 formed
between the film 81 and the pressure roller 87. This presses a
portion of the nip portion 92 lightly against the inner surface
(rear surface) of the film 81. On the inside of the film 81, a
magnetization coil portion including a core material 94 made of
ferrite and a magnetization coil 93 serving as a magnetizing member
is provided in opposition to the heat-generating member 89 with a
small gap between the magnetization coil portion and the
heat-generating member 89. The magnetization coil portion is
attached firmly to the main body of the image forming device. The
shape of this magnetization coil portion is basically the same as
the magnetization coil portion of FIG. 6 used in the third
example.
An oil roller 95, which is impregnated with lubricant oil, is
pressed lightly against the outer peripheral surface of the film 81
so that it can be driven and rotated by the film 81. When the film
81 is moved, a certain amount of lubricant oil is supplied to the
surface of the silicone rubber 82 of the film 81.
A fixing device as described above was installed in a color image
forming device (not shown in the drawings), and color toner 95 was
fixed on a recording material 96. For this, the process speed was
set to 150 mm/sec, and the recording material 96, onto which a
toner image has been transferred, was inserted in the arrow
direction into the fixing device with the side carrying the color
toner 95 facing the film 81, as shown in FIG. 7.
The color toner 95 used for this example is a sharp-melting color
toner based on polyester, which has a glass transition point of
58.degree. C. and a softening point of 170.degree. C. For this
color toner 95, it was determined that between the color toner 95
and the film 81 onto which the lubricant oil of this example has
been applied, cold offset occurred when the maximum temperature of
the film 81 at the speed set for this example is less than
150.degree. C., and that hot offset occurred when the temperature
of the film 81 at the outgoing portion of the nip portion 92
exceeded 190.degree. C.
In this example, the Curie temperature of the magnetic plate 90 was
set to 230.degree. C., and the heat-generating member 89 had the
ability to regulate its average temperature and stabilize it at
about 200.degree. C. when recording material 96 was continuously
conveyed by the nip portion 92. Furthermore, it was measured that
the surface temperature of the film 81 near the outgoing portion of
the nip portion 92 stabilized at about 170.degree. C. while
recording material 96 was being transported. In the configuration
of this example, the recording material 96 is passed along the nip
portion 92 while it takes in heat from the film 81, after the film
81 is supplied with heat by the heat-generating member 89. However,
because the thermal capacity of the film 81 is set to a low value,
the surface temperature of the film 81 at the outgoing portion of
the nip portion 92 decreases considerably compared to the surface
temperature of the film 81 at the ingoing portion of the nip
portion 92. Consequently, the temperature difference between the
surface temperature of the film at the outgoing portion of the nip
portion 92 and the Curie temperature becomes 60.degree. C., which
is higher than in the first or second example.
The decrease of the surface temperature of the film 81 at the
outgoing portion of the nip portion 92 becomes larger, the smaller
the thermal capacity of the film 81 is. The film 81 used for this
example comprises a 50 .mu.m thick nickel based, onto which a 50
.mu.m thick silicone rubber has been formed. The thermal capacity
of this film 81 can be calculated to be about 0.005 cal/.degree. C.
per 1 cm.sup.2. In this method of heating the film 81 at the
ingoing portion of the nip portion 92 and performing the fixing
with the retention heat, if the thermal capacity is made even
smaller, the temperature decrease when the film 81 protrudes into
the nip portion 92 becomes even larger and cold offset occurs
easily. Consequently, in this example, the temperature difference
between the surface temperature of the film 81 at the outgoing
portion of the nip portion 92 and the Curie temperature is possibly
the largest of all these fixing methods.
Therefore, in all of the above-noted fixing methods, including this
example, the maximum value for the temperature difference between
the surface temperature of the film at the outgoing portion of the
nip portion and the Curie temperature is 60-70.degree. C.
Thus, it could be determined that a necessary condition for the
Curie temperature Tk in all these fixing methods is
Very often, the temperature Tc at which cold offset between toners,
including color toners, and heating rollers or films including a
lubricant layer of for example a fluorocarbon resin, silicone
rubber, fluorocarbon rubber, etc. sets in and the temperature Th at
which hot offset sets in is at least about 140.degree. C. and at
most about 210.degree. C. Consequently, the above condition can be
written more precisely as
According to this example, as in the third example, the
heat-generating member 89 has due to its configuration the ability
to regulate its own temperature, so that the film 81 does not
become excessively hot and by setting the Curie temperature to a
suitable value with regard to the fixing temperature, the
temperature regulation can be performed automatically at
temperatures near the fixing temperature. Consequently, even
without a temperature detecting means, such as the thermistor, or
temperature controlling circuits, suitable heating conditions can
be attained. If a heating member with low thermal capacity such as
the film 81 in this example is used, a partial temperature
difference in the depth direction of FIG. 7 occurs easily. However,
the ability of the heat-generating member 89 to regulate its own
temperature also causes a partial difference in the heat
generation, so that even when a recording material 96 of narrow
width is conveyed continuously by the nip portion 92, the portion
where the recording material 96 does not pass does not become
excessively hot, and when subsequently a recording material 96 of
broader width is conveyed continuously by the nip portion 92, hot
offset does not occur. Consequently, since the thermal capacity of
the heat-generating member 89 or the film 81 serving as a heating
member can be decreased within the scope where temperature
self-regulation is possible, the warming-up time can be
shortened.
According to this example, the tip portion 89a of the
heat-generating member 89 extends to the vicinity of the nip
portion 92 and supplies the necessary heat at the nip portion 92.
On the other hand, the magnetization coil 93 and the core material
94 can be arranged upstream from the nip portion 92, so that they
do not heat up due to the influence of the nip portion 92. As a
result, the amount of heat generated can be maintained at a stable
level. Furthermore, since the tip portion 89a of the
heat-generating member 89 extends to the vicinity of the nip
portion 92, the temperature at the front half of the nip portion 92
can be controlled precisely. Consequently, it is possible to
perform fixing with sufficient melting and no hotmelt offset, even
in the case of sharp-melting color toner, whose semi-fused state is
comparatively short.
Moreover, according to this example, the forming of the nip portion
92, which requires strong pressures, is performed by pressing it
between the first roller 83 and the pressure roller 87, so that
there is no portion that slides while being subjected to a strong
frictional force due to the forming of the nip portion 92, and a
fixing device can be realized that, in comparison to the third
example, is suitable for operation at higher speeds for longer
times.
According to this example, when the film 81 starts to contact the
recording material 96, heat starts to be transferred to the
recording material 96. And, because the thermal capacity of the
film 81 can be reduced, the temperature of the film 81 decreases
sharply when the film 81 has passed the tip portion 89a of the
heat-generating member 89, so that the toner is not as easily
hot-offset when the recording material 96 passes the nip portion 92
and separates from the film 81. Consequently, hot-offset does not
occur even when the temperature at the ingoing portion of the nip
portion 92 is set relatively high.
The first roller 83 positioned on the inner side (rear surface
side) of the film 81 is made of a foam with low thermal
conductivity, so that due to the voids inside the first roller 83
the heat generated in the film 81 does not escape very easily, and
a fixing device with good thermal efficiency can be attained.
In this example, a heat-generating member 89 with a dual layer
configuration of a highly conductive layer (conductive plate 91)
layered on a magnetic layer (magnetic plate 90) was used, but it is
also possible to use a heat-generating member comprising only a
magnetic layer, and make the film 81 highly conductive by using for
example copper for it, so that above the Curie temperature most of
the induction current flows in the film 81. Also in this case,
if
wherein the intrinsic resistance of the magnetic layer serving as a
heat-generating member is .rho.1 and its thickness is t1, and the
intrinsic resistance and the thickness of the film 81 serving as a
highly conductive layer are .rho.2 and t2, then the ratio of the
amount of heat generated above the Curie temperature to the amount
of heat generated at room temperature can be set to 1/2 or
less.
Furthermore, it is also possible to provide a highly conductive
layer in a non-contacting manner in opposition to the
heat-generating member that comprises a magnetic layer, and that is
adjacent to the outer side of the film 81. If the distance between
the two layers is within a certain distance, temperature
self-regulation can be attained. If such a highly conductive layer
is provided separately to the heat-generating member, the thermal
capacity of the heat-generating member can be reduced even
further.
Fifth Example
Referring to FIG. 8, the following is an explanation of a fifth
example of an image device used for an image forming device.
In this example, elements having the same structure and performing
the same function as in the fixing device of the fourth example are
referred to with the same numerals and their further explanation
has been omitted.
As shown in FIG. 8, in this example, a film 161, which is a
polyimide base of 70 .mu.m thickness and 30 mm diameter, is coated
with a 10 .mu.m fluorocarbon resin serving as a lubricant film 162.
The film 161 is wound around an upper roller 163 of 25 mm diameter,
which is rotatable in the arrow direction. This upper roller 163
has elasticity and low thermal conductivity, and includes foamed
silicone rubber with low hardness (ASKERC 35 degrees), which is
formed in one piece with a metal axis 164. Moreover, a pressure
roller 165 is made of silicone rubber with higher hardness (JIS A60
degrees) than the upper roller 163, and is formed in one piece with
a metal axis 166. The pressure roller 165 is pressed against the
upper roller 163 via the film 161, and due to the hardness
difference, the upper roller 163 is deformed as shown in FIG. 8,
thereby forming a nip portion 167. In this situation, the pressure
roller 165 is rotated by a driving system (not shown in the
drawings) in arrow direction C, followed by the film 161 and the
upper roller 163, which are thus caused to rotate in the arrow
direction, as shown in FIG. 8. A heat-generating member 168 is
provided at the inner side (rear surface side) of the film 161 and
downstream of the nip portion 167. This heat-generating member 168
is supported by the main body of the image forming device, and is
biased by a spring towards the left side in FIG. 8, so as to be
pressed against the film 161. Because the film 161 and the
heat-generating member 168 are pressed against each other, heat
transmission is possible, and because they are not related to the
formation of the nip portion 167 for adhering toner, this pressure
can be small. Therefore, the friction between the film 161 and the
heat-generating member 168 can be small, and the film 161 is not
abraded easily. Other than in the above-noted fourth example, the
heat-generating member 168 comprises a magnetic plate 169 as a
first layer on the outside, sliding in contact against the film
161, and a conducting plate 170 as a second inner layer. The
material and the thickness of these layers can be the same as in
the fourth example. At the position opposing the heat-generating
member 168, a magnetic coil 171 and a core 172 are provided, so
that the heat-generating member 168 and the magnetic coil 171 and
the core 172 sandwich the film 161, with a small gap being provided
between the film 161 and the coil 171 and the core 172.
The recording material 174, onto which a toner image has been
applied, was inserted in the arrow direction into this fixing
device with the surface on which the toner 173 is applied facing
the film 161, as shown in FIG. 8, and the toner 173 was fixed on
the recording material 174.
According to this example, the same temperature self-regulation as
in the fourth example can be attained due to the configuration of
the heat-generating member 168, so that the temperature of the film
161 does not become excessively high and that by setting the Curie
temperature to a suitable value with regard to the fixing
temperature, the temperature regulation to a temperature near the
fixing temperature can be performed automatically. Consequently,
even without a temperature detecting means, such as the thermistor,
or temperature controlling circuits, suitable heating conditions
can be attained. Especially when a heating member with low thermal
capacity such as the film 161 in this example is used, partial
temperature differences in the depth direction in FIG. 8 occur
easily. But since the ability of the heat-generating member 168 to
regulate its own temperature also causes a partial difference in
the heat generation, even when a recording material 174 of narrow
width is conveyed continuously by the nip portion 167, the portion
where the recording material 174 does not pass does not become
excessively hot, and when subsequently a recording material 174 of
broader width is conveyed continuously by the nip portion 167,
there is no hot offset. Consequently, since the thermal capacity of
the heat-generating member 168 and the film 161 serving as the
heating member can be decreased within the scope where temperature
self-regulation is possible, the warming-up time can be
shortened.
Furthermore, according to this example, the formation of the nip
portion 167, which requires a strong pressure force, is performed
by the pressure between the upper roller 163 and the pressure
roller 165, so that there is no portion that slides while a large
friction force is exerted to form the nip portion 167, realizing a
fixing device that is suitable for operation at higher speeds over
extended periods of time compared with the one of the third
example.
Furthermore, according to this example, since the heat-generating
member 168 can be provided on the inner side (rear surface side) of
the film 161, whereas the magnetic coil 171 and the core 172 can be
provided on the outer side of the film 161, the coil 171 etc. is
not subjected to the influence of the temperature of the
heat-generating member 168. As a result, the amount of generated
heat is stabilized.
Furthermore, according to this example, at the nip portion 167, the
film 161 deforms along the outer peripheral surface of the pressure
roller 165, so that when the recording material 174 passes through
the nip portion 167, the direction in which it leaves the nip
portion is the direction in which it also separates from the film
161, so that the defoliation of the recording material 174 from the
film 161 becomes much easier.
Moreover, the upper roller 163 positioned on the inner side (rear
surface side) of the film 161 can be made of a foamed material with
low thermal conductivity, so that due to the voids inside the upper
roller 163 the heat generated in the film 161 does not escape very
easily, and good thermal efficiency can be attained.
In this example, a magnetic plate 169 attached firmly to a
conductive plate 170 is used as the heat-generating member 168, but
the same temperature self-regulation can also be attained when
there is a small air gap between the two. In this case, it is not
necessary to heat the conductive plate 170, so that the thermal
capacity of the heat-generating member can be reduced even
further.
Furthermore, in this example, the magnetic plate 169 is fixed, and
slides along the film 161, but it is also possible to provide a
rotatable cylindrical magnetic roller corresponding to this
magnetic plate 169, and wrap the film 161 around this roller and
the upper roller 163. In this case, the sliding portion can be
reduced further, and an operation at higher speeds over extended
periods of time becomes possible. Furthermore, in this case, if the
portion corresponding to the conducting plate 170 is positioned in
a non-contacting manner inside this magnetic roller the thermal
capacity of the heat-generating member can be reduced even
further.
Moreover, in these examples, the self-regulation temperature of the
heat-generating member is set to the fixing temperature, but the
present invention is not restricted to this configuration, and it
is also possible to perform the control of the fixing temperature
based on the detection of for example a regular thermistor, and to
set the self-regulation temperature higher to prevent an excessive
temperature rise, in order to ensure the protection against damages
due to high temperatures in the device.
The invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof. The
embodiments disclosed in this application are to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description, all changes that come within the meaning and
range of equivalency of the claims are intended to be embraced
therein.
* * * * *