Set temperature changeable image fixing apparatus

An image fixing apparatus includes a heater maintained at a controlled temperature; a film contacted to the heater and movable together with and in sliding contact with a recording material carrying a visualized image, wherein the visualized image is heated by the heater through the film; a temperature detecting element for detecting a temperature of the film or a member contacted to the film; and a controller for changing a level of the controlled temperature on the basis of an output of the temperature detecting element before start of an image fixing operation of the image fixing apparatus.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image heat-fixing apparatus wherein a 
recording material bearing a visualized image is urged through a film 
toward the heater, by which the image is fixed. 
As for the image fixing apparatus used in an image forming apparatus such 
as a copying machine or an electrophotographic printer, a heat-roller type 
image fixing system is widely used. However, this system involves a 
problem in that the waiting period is long for the surface of the heating 
roller to reach a predetermined temperature. 
U.S. applications Ser. Nos. 206,767, 409,341, 435,247, 430,437, 440,380, 
440,678, 444,802 and 446,449 and U.S. Pat. Nos. 4,954,845, 4,998,121, and 
5,026,276 which have been assigned to the assignee of this application 
have proposed an image fixing apparatus comprising a low thermal capacity 
heater and a thin film, wherein the waiting period is significantly 
reduced or eliminated. In this film fixing system, if the temperature of 
the heater is controlled to be constant, the quantity of the heat applied 
to the toner image by the nip varies if the temperature of the fixing film 
varies. 
The inventors have made thermal analysis on the relationship between the 
surface temperature of the fixing film immediately before the nip (the 
temperature on that surface of the fixing film which is contactable to the 
toner image of the recording material) in other words, the initial surface 
temperature and the temperature increase with time after entering the nip. 
The results are shown in FIGS. 9A and 9B, wherein FIG. 9A relates to the 
fixing film having a thickness of 40 microns, and FIG. 9B relates to the 
fixing film having the thickness of 80 microns. In the graphs, 
a curve (1) represents the film surface temperature at which the image 
fixing operation is possible (fixable temperature which is approximately 
200.degree. C. in this Example); 
a curve (2) represents the surface temperature of the fixing film when the 
initial surface temperature is 130.degree. C.; and 
a curve (3) represents the fixing film surface temperature when the initial 
temperature is 20.degree. C. 
As will be understood from the graphs of FIGS. 9A and 9B, when the fixing 
film surface temperature before entering the nip is low, the time required 
for the temperature of the fixing film to reach the fixable temperature 
during the passage of the nip is long, and therefore, the effective toner 
image heating period is short. If the fixing film has a significant 
thickness, it can occur that the fixable temperature is not reached until 
the fixing film has passed through the nip (curve (3) in FIG. 9B). 
Since the temperature of the fixing film is low immediately after the main 
switch is actuated or after the apparatus is left unused, the low 
temperature toner offset can occur due to insufficient fusing of the 
toner. 
If the temperature of the heater is increased in an apparatus wherein the 
recording sheet is separated from the film when the temperature of the 
toner is higher than then glass transition point, the high temperature 
toner offset can occur due to the overfusing of the toner if the 
continuous fixing operation is carried out. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide 
an image fixing apparatus wherein substantially the same quantity of heat 
can be applied to a visualized image irrespective of the initial 
temperature of the fixing film. 
It is another object of the present invention to provide an image fixing 
apparatus wherein the power supply to the heater can be changed, in 
accordance with a temperature of the fixing film. 
It is a further object of the present invention to provide an image fixing 
apparatus wherein the setting temperature for the heater is changed in 
accordance with the temperature of the fixing film. 
It is a further object of the present invention to provide an image fixing 
apparatus wherein the temperature of the fixing film is controlled on the 
basis of an output of a temperature detecting means. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of the preferred embodiments of the present invention taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 8, there is shown an image forming apparatus using an 
image fixing apparatus according to an embodiment of the present 
invention. The image forming apparatus is an image transfer type 
electrophotographic apparatus which comprises a reciprocable original 
supporting carriage and a rotatable drum. 
The apparatus comprises a casing 100, a reciprocable original supporting 
carriage 1 made of transparent material such as glass or the like disposed 
above the top plate 100a of the casing 100. The original carriage 1 
reciprocates above the plate 100a in the left and right directions (a-a'). 
An original G is placed face down on the original carriage 1 in alignment 
with the reference position. Then, the original is covered with an 
original cover 1a. 
A slit opening 100b is formed in the top plate 100a, extending in a 
direction perpendicular to the reciprocal movement direction of the 
original carriage 1 (perpendicular to the sheet of the drawing). Through 
the slit 100b the original is illuminated. 
The surface of the original image of the original G placed on the original 
carriage 1 is passed by the slit opening 100b from the right side to the 
left side during the forward movement (a) of the original carriage 1. 
During this, the original image receives the light L through the slit 100b 
through the transparent original carriage 1, so that the original is 
scanned. The light reflected by the original is imaged on the surface of 
the photosensitive drum 4 through an array of imaging elements 2 having 
short focus and small diameter. 
The photosensitive drum 4 has a photosensitive layer such as a zinc oxide 
photosensitive layer or an organic photosensitive layer, and is rotatable 
in a direction indicated by an arrow b at a predetermined peripheral speed 
about the central axis 4a. During the rotation, the photosensitive member 
is uniformly charged to a positive or negative polarity. The surface 
thereof thus charged is exposed to the image light from the original 
through a slit, so that an electrostatic latent image is formed on the 
photosensitive drum 4. 
The electrostatic latent image is visualized sequentially by a developing 
device 6 with toner made of heat-softenable or -fusible resin material or 
the like. Then, the toner image (visualized image) is conveyed to an image 
transfer station having a transfer discharger 9. 
A cassette S accommodates transfer sheet materials (recording material). 
From the cassette S, a sheet is singled out by rotation of a pick-up 
roller, and is fed to the photosensitive drum 4 by registration rollers 8 
in such a timed relation that when the leading edge of the toner image 
reaches the transfer charger 9 position, the leading edge of the transfer 
sheet P reaches the position between the transfer discharger 9 and the 
photosensitive drum 4. To the surface of the thus fed transfer sheet, the 
toner image is sequentially transferred from the photosensitive drum 4 by 
the transfer discharger 9. 
The transfer sheet having received the transferred image, is separated from 
the surface of the photosensitive drum 4 by an unshown separating means, 
and is conveyed by a conveying device 10 to an image fixing apparatus 11 
where it is subjected to an image fixing operation by heat so that the 
unfixed toner image Ta is fixed. Finally, it is discharged along a guide 
43 and discharging rollers 44 to a discharge tray 12 outside the apparatus 
as a print (copy). 
The surface of the photosensitive drum 4 after the image transfer is 
subjected to a cleaning operation by a cleaning device 13, by which the 
residual toner or contamination is removed, so that it is prepared for a 
repeated image forming operation. 
An image fixing apparatus according to this embodiment will be described. 
FIG. 1 is a sectional view of the fixing apparatus 11. A fixing film 25 in 
the form of an endless belt is stretched around four parallel members, 
namely, a left driving roller 26, a right follower roller 27, a low 
thermal capacity linear heater 20 fixed below a position between the 
rollers 26 and 27 and a guiding roller 26a disposed below the driving 
roller 26. 
The follower roller 27 functions also as a tension roller for the film 25. 
The fixing film 25 is rotated without crease, snaking movement or delay in 
the clockwise direction by the rotation of the driving roller 26 in the 
clockwise direction, at a peripheral speed which is the same as the 
conveying speed of the transfer sheet P (recording material) having the 
unfixed toner image Ta conveyed from the image forming station 9. 
A pressing member 28 in the form of a pressing roller has a rubber elastic 
layer made of silicone rubber or the like having a good parting property. 
It presses the bottom travel of the fixing film 25 to the bottom surface 
of the heater 20 by urging means with a total pressure of 4-7 kg, for 
example. It rotates codirectionally with the transfer sheet P conveyance, 
that is, in the counterclockwise direction. 
Since the fixing film 25 in the form of an endless belt is repeatedly used 
for heating and fixing the toner image, it has a sufficient 
heat-durability, parting property and durability. Generally, the total 
thickness thereof is not more than 100 microns, and preferably not more 
than 40 microns. 
It may a single layer film of a heat resistive resin such as PI 
(polyimide), PEI (polyether imide) or PFA (copolymer of 
tetrafluoroethylene-perfluoroalkylvinylether), or it may be a multi-layer 
film including a thicker film of 20 microns coated with a parting layer of 
10 microns at least on the side contactable to the image, the coating 
being made of PTFE resin (tetrafluoroethylene resin) added by electrically 
conductive material. 
The heater 20 comprises a heater support 21 extended in a lateral direction 
(perpendicular to the fixing film 25 moving direction) and having a high 
rigidity, heat-durability and insulating property. A heater substrate 22 
of good heat conductive material is mounted on the bottom side of the 
support along the length of the support 21. 
A heat generating resistor 23 is mounted on the film side of the heater 
substrate 22, and the heat generating resistor 23 instantaneously 
generates heat upon electric power supply thereto. At the opposite side of 
the heater substrate 22, a temperature detecting element 24 is provided to 
detect the temperature of the heater substrate 22. 
The heater support 21 provides the entire mechanical strength of the heater 
and is made of a heat-durable resin material such as PPS (polyphenylene 
sulfide), PAI (polyamide imide), PI (polyimide), PEEK (polyether 
etherketone) or a liquid crystal polymer material or a compound material 
of such a resin material and a ceramic, metal, glass or the like material. 
An example of the heater substrate 22 has a thickness of 1.0 mm, a width of 
10 mm and a length of 240 mm, made of alumina. 
An example of a heat generating element is in the form of an electric 
resistor material such as Ag/Pt, RuO.sub.2, Ta.sub.2 N or the like applied 
on the bottom surface of the substrate 22 along a substantial center line 
of the substrate 22 with a width of 1.0 mm, by screen printing or the 
like. Thus, the heat generating element 23 is a linear or stripe element 
having a low thermal capacity and activatable by electric power. 
A temperature detecting element 24 is in the form of a temperature sensor 
having a low thermal capacity in the form of a Pt film or thermister. It 
is applied on the top surface of the substrate 22 along a substantially 
central longitudinal line of the top surface (opposite from the heat 
generating element 23 side) In this embodiment, the temperature of the 
substrate 22 having the good thermal conductivity is detected by the 
temperature sensor 24 as the temperature of the heater 20. 
In this embodiment, the heat generating element 23 is connected to a power 
source at the opposite longitudinal ends to generate the heat along the 
entire length of the heat generating element 23. The heat generating 
element is supplied with electric power on the basis of an output of a 
fixing film temperature detecting unit 31 which will be described 
hereinafter and a set (target) temperature so as to compensate the energy 
emission. 
The fixing film 25 is not limited to the form of the endless belt. It may 
be as shown in FIG. 2, in the form of a film rolled on a feeding shaft 41 
and a take-up shaft 42 and stretched therebetween and between the heater 
20 and the pressing roller 28 below a guide roller 26a. The fixing film 25 
in this case is advanced from the feeding shaft 41 to the take-up shaft 42 
at the same speed as the speed of the transfer sheet P. 
In order to prevent the wearing and damage of the heat generating element 
23, the heater 20 has a protection layer made of Ta.sub.2 O.sub.5 or the 
like at the side contactable to the fixing film 25. 
In operation, an unshown copy button is depressed, and when an image 
forming signal is produced, the power supply is effected to reach the set 
temperature of the heater determined in the manner which will be described 
hereinafter. After the set temperature is reached, the power supply is 
controlled to maintain the set temperature. 
The transfer sheet P is conveyed to the fixing apparatus 11 from the 
transfer station 9 by the image forming operation responsive to the image 
formation start, and is conveyed into the nip N (fixing nip) formed 
between the fixing film 25 and a pressing roller 28 and by the heater 20 
and the pressing roller 28, the heater 20 being temperature-controlled. 
The sheet P having the unfixed toner image is passed through the fixing 
film N between the heater 20 and the pressing roller 28 together with the 
fixing film 20 in close contact with the bottom surface of the fixing film 
25 moving at the same speed as the moving speed of the sheet P, without 
surface deviation and without crease. During this, the sheet P is pressed. 
The heat generating element 23 at the bottom of the heater has a width w. 
The heat generating element 23 is within the width of the fixing nip N, 
that is, the contact region between the bottom surface of the heater 20 
and the top surface of the pressing roller 28 through the fixing film 25. 
The toner image bearing surface of the sheet P receives heat through the 
fixing film 25 from the heat generating element 23, while it is passed 
through the fixing nip N with pressure-contact therebetween. The toner 
image is fused by the high temperature and is softened and adhered on the 
surface of the sheet P (Tb). 
In this embodiment, the separation between the sheet P and the fixing film 
25 occurs at the point of time when the sheet P has passed through the 
fixing nip N. 
At this point of separation, the temperature of the toner Tb is still 
higher than the glass transition point of the toner, and therefore, the 
adherence (bonding force) between the sheet P and the fixing film 25 is 
small. Therefore, the sheet P is smoothly separated substantially without 
toner offset to the fixing film 25 surface and substantially without 
wrapping of the sheet P on the film 25 surface due to improper separation. 
Since the toner Tb having the temperature higher than the glass transition 
point has moderate rubber properties, the toner image surface at the time 
of separation does not completely follow the surface property of the 
fixing film so that it has proper roughness. With the surface property 
maintained, the toner image is cooled and solidified, and therefore, the 
toner image surface after the image fixing is not too glossy, and 
therefore, the quality thereof is high. 
After the sheet P is separated from the fixing film 25, it is guided along 
a guide 43 to the discharging roller couple 44. During this, the 
temperature of the toner Tb increases from the temperature above the glass 
transition point decreases by spontaneous cooling down to a point lower 
than the glass transition point, and therefore, it is solidified into a 
toner image Tc. The sheet P having the fixed image is discharged onto the 
tray. 
The temperature control of the heater will be described. 
In FIGS. 1 and 2, there is shown a temperature detecting unit 31 disposed 
in contact with an inside surface of the fixing film 25 upstream of the 
fixing nip N with respect to the movement direction of the fixing film. 
The unit 31 as shown in FIG. 3 comprises a silicone sponge 32, a 
temperature detecting element 33 embedded therein and a PTFE tape 34 
thereon having a good sliding property. The unit 31 is in contact with the 
fixing film to detect the temperature of the inside of the fixing film of 
that portion thereof which is immediately before the fixing nip N, before 
the heater is energized with electric power. 
The information of the detected temperature is fed back to a microcomputer 
M of a heater control system as a controlling factor. 
The inventors' experiments using a commercially available toner for a Canon 
FC copying machine have shown that if the fixing film has a temperature of 
20.degree. C. immediately before the nip, the sufficient image fixing 
operation is not possible unless the temperature of the heater 20 is 
maintained above 190.degree. C. (minimum fixable temperature); and that if 
the temperature is not lower than 210.degree. C., the toner is fused too 
much with the result of toner offset. As shown in FIG. 5, the respective 
temperatures are different depending on the temperature of the fixing film 
immediately before the nip. 
In this embodiment, the set temperature of the heater control system is 
changed using a microcomputer, in accordance with the temperature of the 
fixing film detected by the temperature detecting unit 31 in accordance 
with the table 1 which is determined on the basis of the results shown in 
FIG. 4. 
More particularly, when the temperature of the fixing film upon the start 
of the fixing operation is not higher than 30.degree. C., as in the case 
that the fixing apparatus is left unused for a long period of time, the 
set temperature is selected to maintain 200.degree. C. during the fixing 
operation. When the temperature of the fixing film is increased, and it 
becomes 31.degree.-60.degree. C., the temperature of 195.degree. C. is 
selected, and further when the temperature becomes 61.degree.-100.degree. 
C. the temperature is selected to be 190.degree. C. When the temperature 
of the fixing film is not lower than 101.degree. C., the control 
temperature is lowered to 185.degree. C. 
TABLE 1 
______________________________________ 
Fixing Film Temp. 
Heater Control Temp. 
______________________________________ 
0-30.degree. C. 
200.degree. C. 
31-60.degree. C. 
195.degree. C. 
61-100.degree. C. 
190.degree. C. 
101.degree. C. or higher 
185.degree. C. 
______________________________________ 
Thus, even if the quantity of heat deprived the fixing film changes, the 
good fixed images can be produced without improper image fixing 
attributable to the low temperature of the fixing film and without the 
high temperature toner offset attributable to the too high temperature of 
the fixing film during a long continuous fixing operation. 
Another embodiment will be described. In the foregoing embodiment, the 
temperature of the fixing film is directly detected However, when the 
thickness of the fixing film is very small, not more than 20 microns, for 
example, the fixing film may be damaged by the sliding with the 
temperature detecting unit 31. When the temperature is detected without 
contact, a constant clearance (0.3 mm, for example) is not easily 
maintained in consideration of the waving motion of the fixing film. 
Therefore, the temperature is not always detected correctly. 
In order to solve the problem, this embodiment is intended to particularly 
note that the temperature of the follower roller, the temperature of the 
pressing roller or another member other than the fixing film 25 changes in 
the similar manner as the fixing film temperature. Then, the temperature 
of the member other than the fixing film 25 is detected to switch the 
control temperature for the heater is switched. 
FIG. 5 shows the temperature change of the fixing film 25 and the follower 
roller 27 when the fixing apparatus is stopped after it is operated for 10 
min. As will be understood, the temperature changes have a similarity. In 
the apparatus of this embodiment, as shown in FIG. 6, the temperature 
detecting element 41 is used to detect the follower roller 27 temperature. 
When it detects a temperature not higher than 60.degree. C., the heater 20 
is controlled to be 193.degree. C.; and when it is higher than 60.degree. 
C., the heater 20 is controlled to be 188.degree. C. 
The system in which the temperature of the fixing film is predicted on the 
basis of the detected temperature of the part other than the fixing film 
25 is advantageous in that the contact of the temperature detecting 
element to the fixing film 25 is not required and that the durability of 
the fixing film 25 is increased. 
In this embodiment, the temperature of the follower roller 27 is detected, 
but it is a possible alternative that the temperature in the neighborhood 
of the fixing film 25, such as the temperature of the driving roller 25 or 
the pressing roller 28 is detected. 
In a fixing apparatus in which the movement of the fixing film 25 is so 
stabilized that the gap between the temperature detecting element 31 and 
the fixing film 25 can be maintained constant, a non-contact type 
temperature sensor is effectively usable. 
A further embodiment will be described. When the fixing apparatus is at 
rest, so that the power supply to the heater is not effected, the 
temperature of the heater 20 changes similarly to the fixing film. 
Therefore, in this embodiment, the temperature detecting element 24 for the 
heater 20 is used to detect the temperature of the heater prior to the 
start of the image fixing operation, and on the basis of the detection, 
the set temperature during the fixing operation is changed. 
According to this embodiment, the necessity for the particular temperature 
detecting element is eliminated. 
When a heat fixing toner A (Canon Kabushiki Kaisha) is used, when the 
temperature of the fixing film is not higher than 60.degree. C., the 
optimum temperature of the heater is 190.degree. C., as shown in FIG. 7 
showing the relation between the fixing film temperature and the heater 
temperature. However, when the temperature of the fixing film is not lower 
than 140.degree. C., the temperature 190.degree. C. of the heater is too 
high, and the temperature 185.degree. C. is proper. 
Accordingly, in this embodiment, when the fixing film temperature detected 
by the temperature detecting element 24 for the heater 20 before the start 
of the image fixing operation is not higher than 60.degree. C. the power 
supply is such that the temperature of the heater 20 is 190.degree. C. 
When the temperature of the fixing film is already not lower than 
60.degree. C. before the start of the fixing operation, the heater 20 is 
controlled to be 185.degree. C. from the first copy. By doing so, the 
sufficient image fixing power without toner offset can be provided. 
In the image fixing apparatus of this embodiment, the temperature of the 
fixing film increases by approximately 60.degree. C. by the power supply 
for one minute (which corresponds to 5 sheets processing), until the 
temperature of the fixing film reaches approximately 140.degree. C. In 
consideration of this, the continuous energy supply period is counted from 
the start of the operation by the microcomputer, during the continuous 
operation, on the basis of which the temperature of the film can be 
predicted. When the predicted temperature reaches 60.degree. C., the 
control temperature for the heater 20 is switched to 185.degree. C. 
By predicting the temperature rise of the fixing film 25 in this manner, 
the high temperature offset does not occur even if the control temperature 
is controlled on the basis of the temperature of the heater 20 before the 
start of the fixing operation and even if the fixing operation is carried 
out continuously. 
In this embodiment, the temperature of the fixing film 25 is predicted on 
the basis of the continuous power supply period, but it may be effected on 
the basis of the number of continuously processed sheets. 
As for the control method, the setting temperature of the control system is 
changed in the foregoing embodiment, but it is a possible alternative to 
change the power supply to provide the same effects on the basis of the 
detection of the film temperature or the like. 
While the invention has been described with reference to the structures 
disclosed herein, it is not confined to the details set forth and this 
application is intended to cover such modifications or changes as may come 
within the purposes of the improvements or the scope of the following 
claims.