Image fixing apparatus without crease of fixing film

An image fixing apparatus includes a heater; a movable film in contact with the heater and movable together with a recording material carrying an image to be fixed; a pressing rotatable member for urging the recording material and the film to the heater; and wherein the heater is substantially crowned in a direction perpendicular to a movement direction of the film, and the pressing rotatable member is substantially reversely crowned in the same direction.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image fixing apparatus usable with an 
image forming apparatus such as a copying machine or a printer, more 
particularly to a heat-fixing image fixing apparatus wherein an image is 
fixed by heat from a heater through a film. 
In a widely used conventional image fixing apparatus wherein the toner 
image is fixed on the recording medium supporting an unfixed toner image, 
the recording medium is passed through a nip formed between a heating 
roller maintained at a predetermined temperature and a pressing or back-up 
roller having an elastic layer and press-contacted to the heating roller. 
This system however involves the problem that the warming-up period is 
relatively long until the temperature of the heating roller reaches a 
predetermined level, 
In order to solve the problem, U.S. Ser. No. 206,767, now abandoned in 
favor of U.S. Ser. No. 668,333, U.S. Pat. No. 4,954,845, U.S. Ser. No. 
409,341, now U.S. Pat. No. 5,043,763, 416,539, now U.S. Pat. No. 
4,998,121, 426,082, now U.S. Pat. No. 5,026,276, 435,247, now abandoned in 
favor of U.S. Ser. Nos. 735,709, 430,437, 440,380, now abandoned in favor 
of U.S. Ser. Nos. 751,571, 440,678, 444,802 and 446,449, now U.S. Pat. No. 
5,027,160 which have been assigned to the assignee of this application, 
have proposed a novel fixing apparatus using an instantaneously heatable 
thermal head and a thin film. 
In such a novel fixing apparatus, the use is made with a thin film and a 
driving roller for driving the film under tension. 
In order to maintain uniform tension over the width of the film, the 
members applying the tension to the film is required to be very accurate. 
If it or they involve variation, the film slacks in the middle of the 
width with the result of crease of the film. 
The pressing roller presses the film to the heater to form a nip between 
the film and the pressing roller. When the recording material is in the 
nip, the recording material is creased, or the trailing edge of the 
recording material is unintentionally raised. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide 
an image fixing apparatus wherein the production of the crease of the film 
moving together with the recording material in contact with the heater, is 
effectively prevented. 
It is another object of the present invention to provide an image fixing 
apparatus wherein the production of the crease of the recording material 
passing through the nip formed between the film and the pressing member, 
is effectively prevented. 
It is a further object of the present invention to provide an image fixing 
apparatus wherein the production of the crease in the recording material 
is prevented by which the trailing edge of the recording material is 
prevented from rising when the recording material is in the nip between 
the film and the pressing member. 
It is a yet further object of the present invention to provide an image 
fixing apparatus wherein the heater is crowned in a direction 
substantially perpendicular to the movement direction of the film. 
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 first to FIG. 2, there is shown an image fixing apparatus 
according to an embodiment of the present invention. The image forming 
apparatus comprises an original supporting platen 1 made of transparent 
material such as glass and reciprocable in the direction a to scan an 
original. Right below the original carriage, an array of short focus 
imaging elements is disposed. An original placed on the original carriage 
is illuminated by an illumination lamp 3, and the reflected light image is 
projected through a slit and through said array onto a photosensitive drum 
4. 
The photosensitive drum rotates in the direction b. The photosensitive drum 
4 has a zinc oxide photosensitive layer or an organic photoconductor 
photosensitive layer or the like. A charger 5 uniformly charges the 
surface of the photosensitive drum. The drum 4 thus uniformly charged by 
the charger is exposed to the image light through the array 2, so that an 
electrostatic latent image is formed. The latent image is visualized with 
a toner made of heat-softening or heat-fusible resin by a developing 
device. On the other hand, a sheet P (recording material) accommodated in 
a cassette S is fed to the drum 4 by a pick-up roller 7 and registration 
rollers 8 rotates in synchronism with the image on the photosensitive drum 
4. The toner image formed on the photosensitive drum 4 is transferred onto 
the sheet P by a transfer discharger 9. Thereafter, the sheet P is 
separated from the drum 4 by a known separating means, and is introduced 
along the conveying guide 10 into an image fixing apparatus 11 where it is 
subjected to the heat-fixing operation. Then, the sheet is discharged onto 
the tray 12. After the toner image has been transferred, the residual 
toner remaining on the photosensitive drum 4 is removed by a cleaner 20. 
Referring now to FIG. 1, there is shown an image fixing apparatus 11 
comprising a low thermal capacity linear heat-generating element (heater) 
15, a film 18 in contact with the heater 15, a pressing or back-up roller 
14 for urging the film 18 to the heater 15. The pressing roller rotates 
following the film which is driven. 
FIG. 3 is an enlarged sectional view of the image fixing apparatus 11 shown 
in FIG. 2. 
The heater 15 is fixed at least during the fixing operation and comprises 
an alumina base plate 15b having a thickness of 1.0 mm, a width of 10 mm 
and a length of 240 mm, for example and made of good thermally conductive 
material, a holder 15a made of heat insulative material for supporting the 
alumina base plate 15b, and a heat generating resistor 15c applied on the 
alumina base plate 15b in a width of 1.0 mm. The longitudinal ends of the 
heater 15 are connected with a power source supplying 100 Vdc pulses at 
the frequency of 20 msec. 
A temperature sensor 16 is in the form of a thermister for detecting a 
temperature of the alumina base plate 15b, and the width of the pulse 
applied to the heat generating resistor is controlled generally within the 
range of 0.5-5 msec so as to the temperature detected by the temperature 
sensor 16 is constant. The temperature of the toner at the point where the 
film 18 is separated from the recording material 21 is selected to be 
higher than the glass transition point of the toner, preferably higher 
than a softening temperature of the toner (ball and ring method). 
The fixing film 18 slides on the heater 15 in the direction indicated by an 
arrow c, the heater 15 being maintained at a predetermined temperature. 
The fixing film 18, for example, comprises a heat resistive film of 
polyimide (PI), polyether imide (PEI), polyether sulfone (PES) or 
perfluoroalkoxy (PFA), having a thickness of 20 microns, and a parting 
layer made of polytetrafluoroethylene (PTFE) or another fluorinated resin, 
having a thickness of 10 microns, at least at an image contactable side. 
It is in the form of an endless belt, for example. Generally, the total 
thickness of the film is less than 100 microns, preferably not more than 
40 microns. The fixing film 18 is stretched around a driving roller 19 for 
driving the film 18 and a follower roller 20 driven by the film 18, and by 
the driving force and the tension thereby, it is moved without crease in 
the direction c. 
The pressing roller 19 has a rubber elastic layer made of silicone rubber 
or the like having a good parting property. It applies pressure to the low 
thermal capacity linear heater 15 through the fixing film with the total 
pressure of 4-7 kg. 
The unfixed toner image 22 on the recording material P is introduced along 
an inlet guide 23 into a nip (contact area) between the film 18 and the 
pressing roller 14. The recording material 21 is pressed to the heater 15 
through the film by the pressing roller 14 in the nip. The side of the 
recording material 21 that bears the toner image 22 is contacted to the 
film 18, and the fixed image is provided by the heating. 
In this embodiment, the crease of the film is not produced even if there is 
a slight variation in the parallelism among the driving roller 19, the 
follower roller 20 and the heater 15. 
The reason for this will be described in detail. 
Referring to FIG. 3, the fixing film 18 is tensioned around the driving 
roller 19, the follower roller 20 and the low thermal capacity linear 
heater 15. If the longitudinal surface of the heater 15 which is 
contactable to the film 18 is straight, the tension of the film 18 in the 
middle part of the length of the heater becomes smaller than the tensions 
at the opposite ends, due to the variation of the roller or rollers in the 
cylindricity and straightness and/or due to the central deformation of the 
roller or rollers when the tension is applied. This results in the 
tendency of the lateral ends of the film moving toward the longitudinal 
centers of the heater. Since the film is thin, creases are produced 
adjacent the lateral center of the film. 
As shown in FIG. 1, the bottom surface (near the recording material) of the 
low thermal capacity linear heater 15 is concaved down or crowned in the 
longitudinal direction. By doing so, the weakening of the tension at the 
middle of the film is prevented. The longitudinal crowning of the heater 
15 is such that the holder 15a, the alumina substrate 15b and the heat 
generating resistor material 15c are all crowned toward the recording 
material so that the longitudinal crowned portion of the heater 15 is 
contacted to the film 18. 
By increasing the tension in the middle of the width of the film by the 
crowning configuration of the heater 15, the film is prevented from 
creasing. 
However, when the heater 15 is crowned, the pressure in the middle portion 
becomes larger than the marginal portions. 
The conveying force applied to the recording material is larger at the 
portion where the nip pressure is large than at the portion position where 
it is small. Therefore, when the thickness of the recording material is 
small, the crease may be produced by the force toward the inside in the 
direction of the width of the film, to the recording material P. 
In consideration of this, in addition to the longitudinal crowning of the 
low thermal capacity linear heater 15, the pressing roller 14 is reversely 
crowned in the longitudinal direction to match with the crowning of the 
heater. By the reverse-crowning, the recording material can receive the 
outward forces in the direction of the width of the film, so that the 
recording material is prevented from creasing. 
If the laterally outside conveying force of the recording material is too 
high as compared with that in the middle, the end portions of the 
recording material P are pulled, so that the trailing edge portion of the 
recording material P is raised upwardly, that is, toward the position 
where the film 18 exist beyond a nip line between the film 18 and the 
pressing roller 14. 
If the trailing edge rise of the recording material P is too large, the 
following problem is possible. When a process cartridge including the 
photosensitive drum 4 shown in FIG. 2, for example, is taken out of the 
image forming apparatus, the trailing edge portion of the recording 
material P is rubbed with a member such as a drum shutter 13 for covering 
the photosensitive drum 4 which is disposed between the photosensitive 
drum 4 and the fixing device 11 and which is disposed adjacent the 
recording material passage at the film 18 side beyond the nip line between 
the film 18 and the pressing roller 14. If this occurs, the unfixed toner 
image 22 on the recording material P will be disturbed. 
The rising of the recording material trailing edge is concerned with the 
crowning of the low thermal capacity linear heater and the 
reverse-crowning of the pressing roller, more particularly, the 
configuration and the degree (amount) thereof. 
FIG. 4 is a graph showing the production of the crease of the recording 
material 21 and the disturbance to the image by the trailing edge rising 
in connection with a combination of the crowning of the low thermal 
capacity linear heater 15 and the reverse-crowning of the pressing roller 
14 when the total pressure by the pressing roller 14 is 6 kg, and the 
conveying speed of the recording material is 50 mm/sec, in the experiments 
carried out by the inventor. In FIG. 4, A represents the amount of the 
crowning of the heater, and B-C represent the amount of the 
reverse-crowning of the pressing roller. FIG. 4 shows the case in which a 
thin water-absorbed sheet which is most easily creased and with which the 
trailing edge image disturbance is most easily produced. The region a 
designates by the arrows is the region in which the trailing edge image 
disturbance is not produced, and a region b designates by arrows is the 
region in which the crease is not produced. 
If the amount of the crowning of the heater is too large, the film is 
largely slacked at the ends thereof, and therefore it is preferably not 
more than 0.5 mm. 
If the amount of the reverse-crowning of the pressing roller is too large, 
the pressure at the central portion decreases with the possible result of 
offset, and therefore it is preferably not more than 0.5 mm. 
In view of the results shown in FIG. 4, the amount of the reverse-crowning 
of the pressing roller is selected to be 0.05-0.5 mm from the standpoint 
of preventing the crease of the recording material. 
In this case, in order to prevent the image disturbance at the trailing 
edge of the recording material, the amount of the crowning of the heater 
is set to be 0.2-0.5 mm. 
However, since the heater is fixed by the holder and other metal plates, 
the positioning of the heater is difficult because of the integration of 
the tolerances for the respective elements. Therefore, the tolerable range 
for the amount of the heater crowning is preferably large. 
FIG. 5 shows another embodiment of the present invention, wherein the 
tolerable range is expanded. Spurs 24 are provided in this embodiment at 
an upstream side of the nip between the film 18 and the pressing roller 14 
with respect to the movement direction of the recording material. The 
spurs 24 are disposed near such a side of the recording material P as is 
contactable to the film 18 and away from the passage for the recording 
material P. Therefore, only when the trailing edge portion of the 
recording material P is raised, and therefore, travels upwardly and away 
from the normal conveying passage, does the spur or spurs 24 contact the 
recording material P to suppress the trailing edge rising, thus preventing 
the disturbance of the image adjacent the trailing edge of the recording 
material P. It is preferable that the spur has teeth each of which is 
sharp so that the toner image is not disturbed when they are contacted to 
the toner image. 
By the provision of the rotatable member effective to suppress the rising 
of the trailing edge portions of the recording material, the tolerable 
range for the amount of the heater crowning can be expanded. In this 
embodiment, it is 0.05-0.5 mm. 
It has been confirmed in experiments that when the amount (degree) of the 
heater crowning and the amount (degree) of the reverse-crowning of the 
pressing roller are substantially the same, the crease production of the 
film and the trailing edge disturbance of the recording material are most 
effectively prevented. 
The image forming apparatus to which the image fixing apparatus according 
to the present invention is applicable is not limited to the copying 
machine as shown in FIG. 2, but the present invention is also applicable 
to an electrophotographic printer, electrostatic recording apparatus 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.