Image fixing device for image forming apparatus including means for locally heating inner wall of fixing means at location corresponding to nip

An image fixing device for an image forming apparatus, comprises a hollow cylindrical fixing roller, and cylindrical pressure roller which is kept in contact with the fixing roller so as to rotate in accordance with rotation of the fixing roller. The pressure roller cooperates with the fixing roller to form therebetween a nip extending in a direction of the longitudinal axes thereof and serving to receive a sheet to which a toner image is transferred. The fixing roller is provided with a heater for locally heating the inner wall of the fixing roller at a location corresponding to the nip to fix the toner image to the sheet.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates generally to an image fixing device for an 
image forming apparatus, such as an electrophotographic copying machine or 
a laser-beam printer. More particularly, the invention relates to an image 
fixing device for fixing a toner image to a toner sheet by heat energy. 
2. Description of The Prior Art 
In image forming apparatus, such as copying machines and laser-beam 
printers, a heat-roller type image fixing device is generally used for 
fixing a toner image to a toner sheet. 
Such an image fixing device generally comprises a hollow fixing roller with 
a built-in heater serving as a heating source, and a pressure roller for 
pressing the fixing roller, to fix a toner image to a toner sheet by 
passing the sheet through a nip formed between the heating and pressure 
rollers. 
However, in conventional heat-roller type image fixing devices, there is 
the disadvantage in that heat efficiency is low, since the fixing roller 
is heated in whole by radiation heat. There is also a design restriction 
in that it is necessary to arrange the heater at the center of the fixing 
roller in order to evenly heat the whole fixing roller. 
In addition, when the fixing roller is heated in whole, the toner on the 
sheet which has passed through the nip remains fused by heat of the fixing 
roller around the nip. As a result, the toner fused by heat tends to be 
adhered to the fixing roller, so that there is a disadvantage in that 
so-called offset, in which the toner adhered to the fixing roller is again 
fixed to the sheet, tends to occur. It is also difficult for the sheet to 
be removed from the fixing roller due to adhesion of the toner remaining 
fused by the heat of the fixing roller around the nip, so that there is 
the disadvantage in that the sheet is wound onto the fixing roller causing 
jamming. 
There is also the disadvantage in that electric power is uselessly 
demanded, since it is necessary to maintain the whole fixing roller at a 
predetermined temperature during stand-by time to prevent the offset. 
There is also the disadvantage in that it takes a lot of warm-up time until 
the temperature of the fixing roller reaches a predetermined temperature 
after a main switch is turned on, so that there is a long waiting. If the 
temperature of the heating source is increased in order to decrease the 
warm-up time, there is the disadvantage in that the temperature within the 
image forming apparatus is also increased. 
Furthermore, in some conventional image fixing devices, a temperature 
detector is secured to the heating source for detecting the temperature 
thereof. On the basis of the detected temperature, the temperature of the 
heating source is controlled to be constant, so that the temperature of 
the nip of the fixing roller is maintained within a temperature range 
suitable for the fixing. In such devices, since the heat produced by the 
heating source is transmitted to the inner wall of the fixing roller 
through an air layer, the temperature of the nip tends to differ from the 
temperature of the heating source detected by the detector. As a result, 
there is the disadvantage in that imperfect fixing and offset tends to 
occur, when the variation of temperature of the nip is so large that its 
temperature is out of the temperature range suitable for the fixing. 
SUMMARY OF THE INVENTION 
It is therefore a principal object of the present invention to eliminate 
the aforementioned disadvantages, and to provide an improved image fixing 
device which can decrease the warm-up time, prevent the offset and allow a 
sheet to be easily removed from a fixing roller. 
It is another object of the present invention to decrease the variation of 
temperature of a fixing roller in the image fixing device to prevent 
imperfect fixing and offset. 
It is another object of the present invention to efficiently transmit heat 
produced by a heating source to a nip in the image fixing device to 
prevent imperfect fixing. 
It is further object of the present invention to provide the image fixing 
device which can stably and quietly carry out the fixing without torque 
variation and sliding noise. 
In order to accomplish the aforementioned and other objects, an image 
fixing device has heating means for locally heating the inner wall of a 
fixing means at a location corresponding to a nip to fix a toner image to 
a sheet. 
According to one aspect of the present invention, an image fixing device 
for an image forming apparatus, comprises: rotatable fixing means defining 
therein a space; pressure means for cooperating with the rotatable fixing 
means to form a nip therebetween for receiving a sheet to which a toner 
image is transferred, the pressure means pressing the sheet against the 
rotatable fixing means at the nip; and heating means, arranged in the 
space, for locally heating an inner wall of the fixing means at a location 
corresponding to the nip to fix the toner image to the sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIG. 1, a laser-beam printer 
using an image fixing device, according to the present invention, will be 
described below. 
As shown in FIG. 1, the laser-beam printer comprises a casing 10 in which a 
paper feeding tray 12 for storing sheets is provided. A rotatable feeding 
roller 14 is arranged near the paper feeding tray 12 to pick up the 
sheets. Above the feeding roller 14, a photosensitive drum 16 is arranged 
to cooperate with a transfer roller 18 to form a nip through which the 
sheet passes. A charging brush 20 is arranged near the photosensitive drum 
16 to cause electrostatic charge on the surface of the photosensitive drum 
16. The laser beam printer is also provided with an optical unit 22 which 
radiates a laser beam to the surface of the photosensitive drum 16 to form 
an electrostatic latent image thereon. A developing device 24 is so 
arranged as to contact the photosensitive drum 16. The developing device 
24 serves to cause a toner to adhere to the electrostatic latent image to 
form a toner image thereon. By the transfer roller 18, the toner image 
formed on the photosensitive drum 16 is transferred to the sheet passing 
through the nip. After the toner image is transferred to the sheet, the 
residual toner on the photosensitive drum 16 is raked or cleaned by a 
blade 26 which is so arranged as to contact the photosensitive drum 16. 
Above the photosensitive drum 16, an image fixing device 28 is so arranged 
as to fix the toner to the sheet to which the toner image has been 
transferred. The image fixing device 28 comprises a fixing roller 30 and a 
pressure roller 32 for pressing the sheet onto the fixing roller 30. The 
sheet to which the toner has been fixed is discharged to a discharge tray 
34 through a carrying roller 36 and a discharge roller 38. 
FIG. 2 shows the first preferred embodiment of an image fixing device, 
according to the present invention. 
As mentioned above, the fixing device 28 comprises the fixing roller 30 and 
the pressure roller 32. As shown in FIG. 2, the fixing roller 30 is 
composed of a hollow cylindrical roller, and is rotatably supported on the 
frame of the laser-beam printer. In order to enhance the releasability of 
the fixing roller 30 from the sheet, and in order to prevent offset, the 
peripheral surface of the fixing roller 30 is coated with 
polyfluoroethylene fiber. The pressure roller 32 serving as pressure means 
is also composed of a cylindrical roller. The pressure roller 32 is 
rotatably supported on the frame of the laser-beam printer so as to be 
allowed to rotate while pressing the fixing roller 30. The pressure roller 
32 cooperates with the fixing roller 30 to form a nip 40 therebetween. The 
peripheral surface of the pressure roller 32 is covered with an elastic 
member 32a such as silicon sponge or rubber. 
The fixing roller 30 houses therein a heating element 42, a heat conductive 
member 44 and a supporting member 46. The supporting member 46 is composed 
of an elongated rod having a substantially L-shaped cross-section. The 
supporting member 46 is secured to the frame of the laser-beam printer to 
support thereon the heating element 42. The heating element 42 is composed 
of an elongated strip of a resistance material to produce heat of a high 
temperature when voltage is applied thereto. The heat conductive member 44 
is formed on a metal strip having a high thermal conductivity. The heat 
conductive member 44 comprises a flat upper portion which is secured to 
the upper surface of the heating element 42, and a lower curved portion 
which comes into contact with the inner wall of the fixing roller 30 at a 
portion corresponding to the nip 40 (a portion at which the fixing roller 
30 comes into contact with the pressure roller 32). When the fixing roller 
30 rotates, the curved portion of the heat conductive member slides on the 
inner wall of the fixing roller 30. 
The operation of the laser-beam printer having the aforementioned 
constructions will be described below. 
First, a sheet stored in the paper feeding tray 12 is carried to the nip 
between the photosensitive drum 16 and the transfer roller 18. The toner 
is transferred to the sheet at the nip, and then, the sheet is carried to 
the fixing device 28. The sheet is heated by the fixing roller 30 while it 
is nipped by the fixing roller 30 and the pressure roller 32, so that the 
toner on the sheet is fused by heat and thereby fixed to the surface of 
the sheet. Then, the sheet to which the toner has been fixed is discharged 
to the paper discharge tray 34 via the carrying roller 36 and the 
discharge roller 38. 
Next, the operation of the fixing device 28 will be described below. 
When the power supply of the laser-beam printer is turned on, a given 
voltage is applied to the heating element 42 so that it generates 
high-temperature heat. The heat generated by the heating element 42 is 
transmitted to the whole of the heat conductive member 44. Since the 
thermal conductivity of the heat conductive member 44 is high, the 
temperature of the heat conductive member 44 at a portion at which it is 
in contact with the fixing roller 30, reaches substantially the 
temperature of the heating element 42 in a short time. Since the fixing 
roller 30 is thin and has a relatively low heat capacity, the portion of 
the fixing roller 30 in contact with the heat conductive member 44, i.e. 
the nip 40 of the fixing roller 30, can be heated to a predetermined 
temperature in a short time. Therefore, it is possible to decrease the 
time for the temperature of the nip 49 of the fixing roller 30 to reach a 
temperature suitable for the fixing (150.degree. to 200.degree. C.) after 
the voltage is applied to the heating element 42. 
Since the heat conductive member 44 is kept in contact with the fixing 
roller 30 only at and around the nip 40, the temperature of the fixing 
roller 30 at a portion other than the nip 40 is lower than the temperature 
of the nip 40. When the fixing roller 30 rotates, the portion of the 
fixing roller 30 which has contacted the heat conductive member 44, moves 
away from the heat conductive member 44. Since the heat capacity of the 
fixing roller 30 is low, the temperature of the portion other than the nip 
40 of the fixing roller 30 decreases due to heat radiation. 
The sheet carried to the fixing device 28 is nipped at a portion at which 
the fixing roller 30 is pressed by the pressure roller 32, i.e. at the nip 
40. Since the pressure roller 32 is pressing the fixing roller 30, the 
elastic member formed on the peripheral surface of the pressure roller 32 
is deformed, so that the area of the contact portion at which the fixing 
roller 30 is in contact with the pressure roller 32, i.e. the area of the 
nip 40, increases. When a flexible sheet is introduced into the fixing 
device 28, it comes into contact with the fixing roller 30 at and around 
the nip 40, and the toner on the sheet is fused by the heat of the nip 40 
of the fixing roller 30. As the fixing roller 30 rotates, the portion of 
the sheet at which the toner has been fused by heat moves from the nip 40 
of the fixing roller to a portion neighboring the nip 40. At this time, 
the sheet is in contact with the portion neighboring the nip 40 of the 
fixing roller 30. Since the temperature of the portion neighboring the nip 
40 is lower than the temperature of the nip 40, the temperature of the 
toner fused by heat decreases, so that the toner is fixed to the sheet. 
Therefore, since it is possible to prevent the toner being fused from 
adhering to the portion neighboring the nip 40 of the fixing roller 30, it 
is possible to prevent offset. 
When the fixing roller 30 further rotates, the sheet is intended to be 
removed from the portion neighboring the nip 40 of the fixing roller 30. 
At this time, since the toner has been adhered to the sheet, the adhesive 
strength of the toner has been sufficiently decreased. Therefore, the 
sheet can be easily removed from the fixing roller 30, so that it is 
possible to prevent the sheet from being wound onto the fixing roller 30 
and thereby prevent jamming of the sheet. 
As mentioned above, according to this embodiment, the time necessary to 
heat the fixing roller 30 can be decreased without increase of heating 
value of the heating element 42, so that it is possible to prevent a 
temperature increase within the apparatus, such as a laser-beam printer. 
It is also possible to prevent the demand current to the heating element 
42 from increasing. Furthermore, since the toner is fixed to the sheet 
when the sheet is removed from the fixing roller 30, it is difficult for 
the toner to adhere to the fixing roller 30, so that it is possible to 
prevent offset. At this time, since the adhesive strength of the toner on 
the sheet is decreased, the sheet can be easily removed from the fixing 
roller 30, so that it is possible to prevent jamming of the sheet. 
FIGS. 3(A) and 3(B) show the second preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the first preferred embodiment except that a heat conductive 
member 48 having a shape different to that of the heat conductive member 
44 is substituted therefor. A pair of frame plates 50 are so arranged as 
to be opposite to each other and are fixed to the casing 10 of the 
laser-beam printer (only one frame plate is shown in FIG. 3(B)). The 
respective frame plates 50 are provided with bearings 52 and 54. The 
fixing roller 30 is rotatably supported on the bearing 52, and the 
pressure roller 32 is rotatably supported on the bearing 54 so as to be 
allowed to rotate while pressing the fixing roller 30. 
The fixing roller 30 houses therein a heating element 56, a supporting 
member 58 and the heat conductive member 48. The supporting member 58 is 
composed of an elongated rod, and is secured to the frame of the 
laser-beam printer. The heating element 56 is composed of an elongated 
strip of a resistance material so as to produce heat of a high temperature 
when voltage is applied thereto, and is secured to the lower surface of 
the supporting member 58. The heating element 56 is provided with 
terminals 60 on both ends thereof, so that a given voltage is applied to 
the heating element 56 via the terminals 60. The heat conductive member 48 
is composed of a hollow cylindrical member having a high thermal 
conductivity. Both ends of the heat conductive member 48 are secured to 
the heating element 56 by means of screws 62. In this embodiment, the 
heating device can provide same advantages as that of the first preferred 
embodiment. 
FIGS. 4(A) and 4(B) show the third preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the second preferred embodiment, except that a heat conductive 
member 64 which is not hollow is substituted for the heat conductive 
member 48. 
FIGS. 5(A) and 5(B) show the fourth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the third preferred embodiment, except that a heat conductive 
member 66 having a shape different from that of the heat conductive member 
64 is substituted therefor. The heat conductive member 66 is composed of a 
cylindrical roller, and both ends thereof are rotatably supported on 
bearings 68 which are secured to the supporting member 58. The lower 
portion of the heat conductive member 66 is in contact with the inner wall 
of the fixing roller 30, and the upper portion thereof is in contact with 
the heating element 56. In accordance with the rotation of the fixing 
roller 30, the heat conductive member 66 rotates in a reverse direction to 
the rotational direction of the fixing roller 30 while contacting with the 
inner wall of the fixing roller 30, and the upper portion of the heat 
conductive member 66 slides on the heating element 56. 
FIGS. 6(A) and 6(B) show the fifth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the fourth preferred embodiment, except that a heat conductive 
member 70 is substituted for the heat conductive member 66. As can be 
clearly seen from FIG. 6(B), the heat conductive member 70 has a plurality 
of cutoffs so as to have a comb-shaped cross-section. The heat conductive 
member 70 is secured to the supporting member 58 by means of screws 72. As 
can be clearly seen from FIG. 6(A),the respective portions divided by the 
cutoffs have curved lower end portions which are tightly kept in contact 
with the inner wall of the fixing roller 30. When the fixing roller 30 
rotates, the respective curved lower end portions slide on the inner wall 
of the fixing roller 30. 
If the heat conductive member 70 is not provided with the cutoffs, the 
length of the portion of the heat conductive member 70 which is 
continuously brought into contact with the inner wall of the fixing roller 
30, is long. For that reason, when the manufacturing accuracy of the heat 
conductive member 70 is insufficient, it is difficult to evenly bring the 
heat conductive member 70 into contact with the inner wall of the fixing 
roller 30. In this embodiment, since the heat conductive member 70 has a 
plurality of cutoffs which brings the divided lower portions of the heat 
conductive member 70 into contact with the inner wall of the fixing roller 
30, these portions tend to evenly contact the inner wall thereof. 
Therefore, it is possible to positively transmit heat from the heat 
conductive member 70 to the fixing roller 30, so that it is possible to 
further decrease the time necessary to heat the fixing roller 30. In 
addition, since high manufacturing accuracy of the heat conductive member 
70 is not required, there is the advantage in that it can be easily 
manufactured. 
FIGS. 7(A) and 7(B) show the sixth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the fifth preferred embodiment, except that a heat conductive 
member 74 is substituted for the heat conductive member 70. The heat 
conductive member 47 comprises an elongated shaft 74a, and a heat 
conductive brush 74b having a plurality of bristles radially extending 
from the shaft 74a. Both ends of the shaft 74a are rotatably supported on 
bearings 76 which are secured to the supporting member 58. The brush 74b 
is kept in contact with the heating element 56 and the inner wall of the 
fixing roller 30. In accordance with the rotation of the fixing roller 30, 
the heat conductive member 74 rotates in the reverse direction to the 
rotational direction of the fixing roller 30 while contacting the inner 
wall of the fixing roller 30, and slides on the heating element 56. In 
this embodiment, there is the advantage in that the heat conductive member 
74 can be securely kept in contact with the inner wall of the fixing 
roller 30. 
FIGS. 8(A) and 8(B) show the seventh preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the sixth preferred embodiment, except that a heat conductive 
member 78 is substituted for the heat conductive member 74. The heat 
conductive member 78 comprises a heat conductive plate 78a, and a heat 
conductive brush 78b having a plurality of bristles. The heat conductive 
plate 78a is secured to the supporting member 58 by means of screws 80 so 
as to press the heating element 56. The heat conductive brush 78b extends 
downwards from the lower surface of the heat conductive plate 78a so that 
the lower ends thereof are kept in contact with the inner wall of the 
fixing roller 30. 
FIGS. 9(A) and 9(B) show the eighth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the seventh preferred embodiment, except that a heating element 82 
of an elongated halogen lamp and a heat conductive member 84 are 
respectively substituted for the heating element 78 and the heat 
conductive member 80. The heat conductive member 84 comprises a hollow 
cylinder 84a of a heat conductive material, and a heat conductive brush 
84b having a plurality of bristles. Both ends of the hollow cylinder 84a 
are secured to frames 86. The heat conductive brush 84b extends from the 
hollow cylinder 84a so that the lower ends thereof are kept in contact 
with the inner wall of the fixing roller 30. The heating element 82 is 
housed within the heat conductive member 84, and both ends thereof are 
secured to the frames 86. 
FIGS. 10(A) and 10(B) show the ninth preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the eighth preferred embodiment, except that a heat conductive 
member 88 is substituted for the heat conductive member 84. The heat 
conductive member 88 comprises a heat conductive rod 88a and a heat 
conductive brush 88b having a plurality of bristles. Both ends of the heat 
conductive rod 88a are rotatably supported on bearings 90. 
FIGS. 11(A) and 11(B) show the tenth preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the ninth preferred embodiment, except that a heat conductive 
member 92 is substituted for the heat conductive member 88. The heat 
conductive member 92 is composed of a curved, heat conductive plate, and 
both ends thereof are secured to the frame 86. As can be clearly from FIG. 
11(A), the upper curved portion of the heat conductive member 92 is 
arranged to surround the heating element 82 without contact therewith, so 
that heat produced by the heating element 82 is transmitted to the heat 
conductive member 92 by radiation. The lower curved portion of the heat 
conductive member 92 is kept in contact with the inner surface of the 
fixing roller 30 so as to be capable of sliding thereon. 
According to the aforementioned first to tenth preferred embodiments, it is 
possible to decrease the time necessary for the heating of the fixing 
roller, to prevent the offset, and to allow the sheet from being easily 
removed from the fixing roller. 
FIGS. 12(A) and 12(B) show the eleventh preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the second preferred embodiment, except that a heat conductive 
member 94 is substituted for the heat conductive member 48, and that a 
temperature detector 96 is provided in the fixing roller 30. As can be 
clearly seen from FIG. 12(A), the upper end of the heat conductive member 
94 is supported on the supporting member 58, and the lower end thereof is 
curved. The curved portion of the heat conductive member 94 is kept in 
contact with the inner wall of the fixing roller 30 at a portion 
corresponding to the nip 40 (a portion at which the fixing roller 30 is in 
contact with the pressure roller 32). When the fixing roller 30 rotates, 
the curved portion of the heat conductive member 94 slides on the inner 
wall of the fixing roller 30. 
The temperature detector 96, such as a thermistor, is secured to the heat 
conductive member 94 to detect the temperature thereof. The temperature 
detector 96 is connected to a control unit (not shown) so that the voltage 
(a detection signal) output from the temperature detector 96 is 
transmitted to the control unit. 
The control unit may be, for example, an operational amplifier or a 
microcomputer to carry out a usual PID control. That is, the variation of 
the detected temperature is divided into proportional components, integral 
components and differential components, and the respective components are 
multiplied by a given gain to determine the voltage to be applied to the 
heating element 56. Furthermore, the voltage applied to the heating 
element 56 may be a pulse, and the duty ratio of the pulse may be varied. 
Referring to FIGS. 13(A) and 13(B), the operation of the fixing device 28 
shown in FIGS. 12(A) and 12(B) will be described below. 
FIG. 13(A) is a graph showing the variation of the voltage applied to the 
heating element 56, and FIG. 13(B) is a graph showing the temperature 
variations of the heat conductive member 94 and of the fixing roller 30 at 
the nip 40. In FIG. 13(B), curved line 1 shows the variation of the 
temperature of the heat conductive member 94, curved line 2 shows the 
variation of the temperature of the fixing roller 30 at the nip 40 in this 
embodiment, and curved line 3 shows the variation of the temperature of 
the fixing roller at the nip in a conventional fixing device. In the 
conventional fixing device, the temperature detector is arranged in the 
heating element. 
First, it is assumed that the power supply of the laser beam printer is 
turned on at time t1. At the same time, a target temperature controlled by 
the control unit is set to be a target temperature Tr at a stand-by time. 
For example, the controlled target temperature Tr is set to be 150.degree. 
C. This target temperature Tr at the stand-by time is the controlled 
target temperature of the heat conductive member 94 when the laser-beam 
printer is in a stand-by condition, and this is set to be lower 
temperature than a target temperature Tp in the fixing operation (for 
example, 200.degree. C.). At a time when the power supply is turned on, 
the temperature of the heat conductive member 94 detected by the 
temperature detector 96 is substantially equal to atmospheric temperature, 
which is far lower than the target temperature Tr at the stand-by time. 
The temperature detector 96 detects the temperature of the heat conductive 
member 94 and produces a detection signal to the control unit. The control 
unit determines the voltage which is to be applied to the heating element 
56 in accordance with the difference between the temperature defined by 
the detection signal and the target temperature Tr at the stand-by time. 
When the determined voltage is applied to the heating element 56, the 
heating element 56 produces heat. As the temperature of the heat 
conductive member 94 detected by the temperature detector 96 approaches 
the target temperature Tr at the stand-by time, the voltage applied to the 
heating element 56 decreases. As a result, at time t2, the temperature of 
the heat conductive member 94 reaches the target temperature Tr at the 
stand-by time. 
On the other hand, the nip 40 of the fixing roller 30 is heated by the heat 
conductive member 94, so that the temperature of the nip 40 of the fixing 
roller 30 reaches the temperature of the heat conductive member 94. That 
is, the temperature of the nip 40 of the fixing roller 30 reaches a 
temperature Tr' approximate to the target temperature Tr at the stand-by 
time. Since the temperature of the nip 40 of the fixing roller 30 is not 
directly controlled by a control loop, and since the fixing roller 30 has 
a given heat capacity and heat resistance, there is a temperature 
difference between the nip 40 of the fixing roller 30 and the heat 
conductive member 94. 
In conventional fixing devices, the temperature of the nip of the fixing 
roller is maintained at the target temperature at the stand-by time by 
controlling the heating element in a similar manner to that of this 
embodiment. However, while the temperature detector 96 is provided on the 
heat conductive member 94 in this embodiment, the temperature detector is 
provided on the heating element in a conventional fixing device so that 
only the temperature of the heating element is controlled. Therefore, in 
conventional fixing devices, both of the temperatures of the heat 
conductive member and the nip of the fixing roller can not be controlled 
by the control loop. The heat capacity and the heat resistance from the 
heating element to the nip of the fixing roller, includes the heat 
capacity and the heat resistance of the heat conductive member, so that 
these value become even greater. Therefore, when compared with the 
conventional fixing devices in this embodiment, the temperature of the nip 
40 of the fixing roller 30 can be maintained in a range approximate to the 
target temperature Tr' of the nip 40 at the stand-by time. 
When the laser-beam printer starts a printing operation, the target 
temperature controlled by the control unit is switched from the target 
temperature Tr at the stand-by time to the target temperature Tp at the 
fixing time (at time t3). Since the temperature of the heat conductive 
member 94 is lower than the target temperature Tp at the fixing time, a 
higher voltage is applied to the heating element 56. Thereafter, at time 
t4, the temperature of the heat conductive member 94 reaches the target 
temperature Tp at the fixing time. As a result, heat is transmitted from 
the heat conductive member 94 to the nip 40 of the fixing roller 30, so 
that the temperature of the nip 40 of the fixing roller 30 reaches a value 
approximating the target temperature Tp at the fixing time. Compared with 
the conventional fixing devices in this case, the temperature of the nip 
40 of the fixing roller 30 can be maintained at a value approximating the 
target temperature Tp at the fixing roller 30. 
Assuming that an outside disturbance, such as the variation of surrounding 
temperature, is applied to the fixing roller 40 between time t5 and time 
t6, the nip 40 of the fixing roller 30 is affected by the outside 
disturbance, and its temperature varies. The temperature of the heat 
conductive member 94 is also affected by the variation of the temperature 
of the fixing roller 30 and is inclined to vary. However, since the 
temperature of the heat conductive member 94 is controlled so as to reach 
the target temperature Tp at the fixing time, the variation of the 
temperature of the heat conductive member 94 is restrained. Due to heat 
transfer from the heat conductive member 94, the temperature of the fixing 
roller 30 reaches a value approximating the temperature of the heat 
conductive member 94, so that the temperature variation .DELTA. T3 of the 
fixing roller 30 can be maintained within a given range. Now, assuming 
that the temperature variation of the fixing roller in the conventional 
fixing devices is .DELTA. T4, the temperature variation .DELTA. T3 will be 
compared with the temperature variation .DELTA. T4. In the conventional 
fixing devices, neither the temperature of the fixing roller nor the heat 
conductive member is controlled by the control loop. Therefore, when the 
temperature of the nip of the fixing roller varies due to an outside 
disturbance, the temperature of the heat conductive member also varies 
under the influence of the temperature variation of the fixing roller, so 
that the temperature variation .DELTA. T4 of the nip of the fixing roller 
becomes greater than the temperature variation .DELTA. T3. That is, in 
this embodiment, it is possible to decrease the temperature variation 
.DELTA. T3 of the nip 40 of the fixing roller 30 to less than that of the 
conventional fixing devices. 
When the laser-beam printer moves to stand-by condition at time t7, the 
target temperature controlled by the control unit is set to change from 
the target temperature Tp at fixing time to the target temperature Tr at 
stand-by time, so that the temperature of the heat conductive member 50 is 
controlled to be the target temperature Tr at stand-by time. 
As mentioned above, according to this embodiment, the temperature 
difference between the temperature of the nip 40 of the fixing roller 30 
and the controlled target temperature can be decreased, and the 
temperature variation due to outside disturbance can be also decreased. 
Therefore, it is possible to prevent the temperature of the nip 40 of the 
fixing roller 30 from varying from a temperature which is suitable for the 
fixing, so that it is possible to prevent the imperfect fixing and offset. 
Furthermore, if the heat conductive member 94 is formed of a material 
having a higher heat conductivity, the heat capacity and the heat 
resistance of the heat conductive member 94 can be decreased, so that the 
heat transfer time from the heat conductive member 94 to the nip 40 of the 
fixing roller 30 can be decreased, thereby the control response can be 
enhanced. When the control responsibility is enhanced, it is possible to 
decrease the time for the temperature of the fixing roller 30 to reach a 
temperature suitable for fixing after the power supply has been turned on. 
Therefore, it is possible to decrease the time for the temperature of the 
nip 40 of the fixing roller 30 to reach a temperature suitable for fixing, 
without increasing the heating value of the heating element 56. 
Consequently, it is possible to prevent the demand current from increasing 
and to restrain the temperature increase within the laser beam printer. 
FIGS. 14(A) and 14(B) show the twelfth preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the eleventh preferred embodiment, except that a temperature 
detector 98 substituted for the temperature detector 96 is arranged 
between the heating element 56 and the inner wall of the fixing roller 30 
at a location corresponding to the nip 40, so as to detect the temperature 
of air layer between the heating element 56 and the inner wall of the 
fixing roller 40. Heat produced by the heating element 56 passes through 
the heat conductive member 94 and is transmitted to the inner wall of the 
fixing roller 30 through the air layer. The temperature detector 56 
detects the temperature of the air layer, so that the temperature of the 
air layer is controlled to be substantially constant. 
FIGS. 15(A) and 15(B) show the thirteenth preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
the eleventh preferred embodiment, except that a heat conductive member 
100 is substituted for the heat conductive member 94. The heat conductive 
member 100 comprises an elongated, heat conductive strip 100a and a heat 
conductive brush 100b. The brush 100b is so arranged as to positively 
contact the inner wall of the fixing roller 30. Similar to the eleventh 
preferred embodiment, a temperature detector 102 is secured to the heat 
conductive member 100 to detect temperature thereof. In this embodiment, 
since the heat resistance from the heating element 56 to the heat 
conductive member 100 is small, the deviation and variation of the 
temperature of the nip 40 of the fixing roller can be further decreased, 
so that it is possible to effectively prevent imperfect fixing and offset. 
FIGS. 16(A) and 16(B) show the fourteenth preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the first preferred embodiment, except that a temperature detector 
104 which has the same functions as those of the eleventh preferred 
embodiment, is provided in the fixing roller 30. The heat conductive 
member 44 comprises a flat upper portion which is secured to the upper 
surface of the heating element 42, an intermediate portion which is kept 
in contact with the supporting member 46, and a curved lower portion which 
is kept in contact with the inner wall of the fixing roller 30. The 
temperature detector 104 is secured to the supporting member 46 to detect 
the temperature thereof. Since the supporting member 46 is kept in contact 
with both the heating element 42 and the heat conductive member 44, the 
temperature of the supporting member 46 is substantially equal to the 
temperature of the heat conductive member 44. That is, the temperature of 
the supporting member 46 detected by the temperature detector 104 is 
equivalent to the temperature of the heat conductive member 44. Therefore, 
in this embodiment, it is possible to provide the same advantage as that 
of the eleventh preferred embodiment in which the temperature detector 96 
is secured to the heat conductive member 94. 
According to the aforementioned eleventh to fourteenth preferred 
embodiments, it is possible to prevent imperfect fixing and offset by 
decreasing the temperature variation of the fixing roller. 
FIG. 17 shows the fifteenth preferred embodiment of an image fixing device, 
according to the present invention. 
The image fixing device 28 comprises a fixing roller 110, a pressure roller 
112 and a recording paper guide 114. The fixing roller 110 is composed of 
a hollow cylindrical roller made of stainless steel (24.0 mm in diameter; 
0.5 mm in thickness), which is rotatable in the direction of arrow a. The 
pressure roller 112 is so arranged as to press the fixing roller 110, and 
to be rotatable in the direction of arrow b. The recording paper guide 114 
serves to guide a recording paper. The recording paper to which no toner 
is fixed is carried on the recording paper guide 114 in a direction of the 
arrow c, and then it is introduced into a contact portion between the 
fixing roller 110 and the pressure roller 112. As the recording paper 
passes through the contact portion, the fixing is carried out by heat and 
pressure. The pressure roller 112 comprises a metallic core 116 made of 
stainless steel (12 mm in diameter), and a mold releasing layer 118 made 
of silicone rubber (4 mm in thickness). 
The fixing roller 110 houses therein a magnet 120, a heating source 122 and 
a plurality of metallic beads 124 made of a magnetic material, such as 
stainless steel (1.0 mm in diameter). The heating source 122 comprises a 
ceramic substrate 126 and a resistance heating member 128 baked thereon. 
The magnet 120 is mounted on the surface of the ceramic substrate 126 
opposite to the surface on which the resistance heating member 127 is 
baked, so that the magnet 120 forms a magnetic field in a downwards 
direction in the drawing. The metallic beads 124 are drawn to the heating 
source 122 by the magnet 120, so as to form a magnetic brush MB between 
the heating source 122 and the fixing roller 110. The end of the magnetic 
brush MB is kept in contact with the inner wall of the fixing roller 110 
to transmit heat produced by the heating source 122 to the fixing roller 
110. 
In this embodiment, due to the field of the magnet 120 the metallic beads 
124 remain magnetic, and it is possible to prevent the metallic beads 124 
from diffusing in the direction of arrow a as a result of the rotation of 
the fixing roller 110. In addition, since the metallic beads 124 are 
formed of a high heat conductive material, it is possible to effectively 
transmit heat from the heating source 122 to the contact portion between 
the fixing roller 110 and the pressure roller 112, so as to ensure 
sufficient fixing when the paper passes therethrough. Furthermore, at the 
contact portion where the magnetic brush MB is kept in contact with the 
fixing roller 110, the metallic beads 124 roll in accordance with the 
rotation of the fixing roller 110, thereby making it possible to decrease 
friction between the magnetic brush MB and the fixing roller 110. 
As mentioned above, according to this embodiment, it is possible to stably 
and quietly carry out fixing by local heating without torque variations 
and sliding noises. 
FIG. 18 shows the sixteenth preferred embodiment of an image fixing device, 
according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the fifteenth preferred embodiment, except that the heating source 
122 is arranged on a side of the magnetic brush MB formed of the metallic 
beads 124, which is downstream relative to the direction of rotation of 
the fixing roller 110. That is, the heating source 122 is positioned on a 
side of the magnetic brush MB such that deformation of the brush due to 
rotation of the fixing roller 110 is avoided. In this embodiment, the 
metallic beads 124 of the magnetic brush MB are intended to move in the 
direction of rotation of the fixing roller 110 while being regulated by 
the heating source 122. As a result, it is possible to enhance adhesion of 
the magnetic brush MB to the heating source 122, to more effectively 
transmit heat from the heating source 122 to the magnetic brush MB. 
In the aforementioned fifteenth and sixteenth preferred embodiments, 
although metallic beads 124 are made of stainless steel, in accordance 
with the present invention they may be made of any other magnetic 
materials having a high heat conductivity. 
FIG. 19 shows the seventeenth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, a sheet metal 130 is housed within the fixing roller 
110. The sheet metal 130 has on both ends thereof a pair of regulating 
plate 130a and 130b which are formed by bending the sheet metal 130. The 
ends of the regulating plates 130a and 130b face the inner wall of the 
fixing roller 110 upstream and downstream of the contact portion of the 
fixing roller 110 with the pressure roller 112, so as to form a chamber r 
within the fixing roller 110. In the chamber r, the heating source 122 and 
the metallic beads 124 made of aluminum (1 mm in diameter) are housed. The 
distances d between the ends of the regulating plates 130a and 130b, and 
the inner wall of the fixing roller 110 are smaller than the diameter of 
the metallic beads 124 so that dispersion of the metallic beads 124 is 
prevented. The chamber r extends over the whole length of the fixing 
roller 110 in the direction of the axis thereof, and both ends of the 
chamber r are covered with shielding members (not shown). Furthermore, the 
chamber r may be formed by the ceramic substrate 126 without the sheet 
metal 130. 
In this embodiment, there is the same advantage as that of the fifteenth 
preferred embodiment. In addition, since the regulating plates 130a and 
130b prevent the diffusion of the metallic beads 124, the metallic beads 
124 may also be made of non-magnetic material. In particular, when the 
metallic beads 124 are made of aluminum, it is possible to decrease the 
weight of the image fixing device 28. 
FIG. 20 shows the eighteenth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, a chamber r' for retaining therein the metallic beads 
124 is formed by a pair of magnets 132 and 134. That is, the magnets 132 
and 134 are supported on a sheet metal 136 and arranged at both ends of 
the heating source 122 so as to form a magnetic field toward the fixing 
roller 110. Gaps between the magnets 132 and 134 and the fixing roller 110 
are shielded by the metallic beads 124 made of a magnetic material to form 
the chamber r'. The metallic beads 124 housed within the chamber r' may 
also be made of a non-magnetic material, such as aluminum (1.0 mm in 
diameter). The magnets 132 and 134 extend over the whole length of the 
fixing roller 110 in the direction of the axis thereof, and both ends of 
the magnets 132 and 134 are covered with shielding members (not shown). 
FIG. 21 shows the nineteenth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the seventeenth preferred embodiment, except that a fixing belt 
138 of a heat resistance film having high heat conductivity is substituted 
for the fixing roller 110. The fixing belt 138 is wound onto a pair of 
rollers 140 and 142 which are separated from each other by a predetermined 
distance, and allowed to rotate in accordance with the rotation of the 
rollers 140 and 142. The fixing belt houses therein the heating source 
122, the metallic beads 124 and the sheet metal 130. The metallic beads 
124 are kept in contact with the fixing belt 138 to roll in accordance 
with the movement of the fixing belt 138, such that friction between the 
metallic beads 124 and the fixing belt 138 is decreased. Therefore, the 
abrasion of the fixing belt can be decreased to enhance the durability 
thereof. 
In the aforementioned fifteenth to nineteenth preferred embodiments, the 
metallic beads 124 may be coated with a material, such as a fluororesin, 
to decrease the frictional coefficient of the metallic beads 124. 
As mentioned above, according to the aforementioned fifteenth to nineteenth 
preferred embodiments, heat from the heating source is transmitted to 
heating means, such as the fixing roller or the fixing belt, through heat 
conductive means comprising a plurality of beads. Therefore, it is 
possible to decrease the friction between the heating means and the heat 
conductive means to enable stable and quiet fixing without torque 
variations and sliding noises. In addition, since the heat from the 
heating source is locally transmitted to the contact portion of the 
heating means by a pressure means, such as the pressure roller, it is 
possible to decrease the warm-up time of the image fixing device to 
decrease demand thereof. In a case where the metallic beads are 
magnetically retained by the magnet, between the heating source and the 
contact potion, it is possible to positively bring the heat conductive 
means into contact with the heating means to stably transmit heat thereto 
even if the distance between the heating source and the heating means 
varies due to an error in installation. Furthermore, when the metallic 
beads are retained by means of regulating plates between the heating 
source and the contact portion, it is possible to use a heat conductive 
means of a non-magnetic material with simple constructions. 
Referring to FIGS. 22 to 25, the twentieth preferred embodiment of an image 
fixing device, according to the present invention, will be described 
below. 
The image fixing device 28 comprises a fixing roller 150 (made of 
stainless; diameter 24 mm; thickness 0.3 mm; heat conductivity 16.3 W/m. 
.degree.C.; specific heat 0.46 kJ/kg. .degree.C.; density 
7.8.times.10.sup.3 kg/m.sup.3) serving as a rotating body, a pressure 
roller 152 serving as a pressure body, a pair of guiding plates 154 and 
156, and a removing or cleaning claw 158. The fixing roller 150 is 
designed to rotate in the direction of arrow a in FIG. 22 by means of a 
main motor M by which elements (not shown), such as a sensitizing body and 
a transfer roller, are also driven. The peripheral surface of the fixing 
roller 150 is coated with a fluororesin layer serving as a mold releasing 
layer. The pressure roller 152 is designed to rotate in the direction of 
arrow b in FIG. 22 in accordance with rotation of the fixing roller 150 
while pressing the fixing roller 150. The pressure roller 152 comprises a 
metallic core 160 (made of stainless steel; 12 mm in diameter) and a mold 
releasing layer 162 (made of silicone rubber; thickness 4 mm) formed 
thereon. The pressure roller 152 presses the fixing roller 150 to form a 
nip n (3.6 mm in width tangential to the fixing roller 150). The guiding 
plate 154 serves to guide a recording paper to which a toner image has 
been transferred in a transferring section (not shows), and the guiding 
plate 156 serves to guide the recording paper to which the toner has been 
fixed. The guiding plate 154 has on one end thereof a paper sensor P for 
detecting the presence of a recording paper near the nip n. The cleaning 
claw 158 serves to prevent the recording paper to which the toner has been 
fixed, from being wound onto the fixing roller 150. 
The fixing roller 150 houses therein a heating portion 164 which comprises 
a pivotal shaft 166 arranged concentrically with the fixing roller 150, a 
holder 168 secured to the pivotal shaft 166, a ceramic substrate 170 
supported on the holder 168, a resistance heating element 172 baked on the 
ceramic substrate 170, and a heat conductive member 174 (made of aluminum; 
3.6 mm in width) for transmitting heat energy produced by the resistance 
heating element 172 to the fixing roller 150. The heating portion 164 
extends over the whole length of the fixing roller 150 in the direction of 
the longitudinal axis thereof. The heating portion 164 is designed to 
rotate around the rotation axis of the pivotal shaft 168 while pressing 
the fixing roller 150. 
The pivotal shaft 168 of the heating portion 164 is connected to an 
oscillating portion 176 which comprises a lever 178, a solenoid 180 and a 
spring 182. That is, one end of the pivotal shaft 168 is connected to one 
end of the lever 178, the other end of which is provided with a pin 184. 
The pin 184 engages a flange 186 of the solenoid 180 which causes the 
lever 178 to move in the direction of arrow c in FIG. 22. The pin 184 also 
engages one end of the spring 182 for biasing the lever 178 in the 
direction of arrow c' in FIG. 22. 
With this construction, when the fixing roller 150 and the pressure roller 
152 are stopped, the heating portion 164 is positioned at the nip n as 
shown in FIG. 24. On the other hand, when the fixing roller 150 and the 
pressure roller 152 rotate, the heating portion 164 is moved so that the 
trailing edge 164a thereof is separated from the trailing edge na of the 
nip n by a distance L(mm) upstream relative to the direction of rotation 
of the heating roller 150, as shown in FIG. 25. The reason the heating 
portion 164 is moved by the distance L when the fixing roller 150 rotates, 
is as follows. 
Since the fixing roller 150 has a given thickness, it takes a predetermined 
time .tau. until the heat energy transmitted to the inner wall of the 
fixing roller 150 is transmitted to the outer wall thereof. Assuming that 
the speed of movement of the outer wall of the fixing roller 150 is 
Vs(mm/s) when it rotates, the fixing roller 150 moves by L=Vs.tau. (mm) 
for the predetermined time .tau.. Therefore, as shown in FIG. 26, if the 
heating portion 164 is positioned at a location corresponding to the nip 
when the fixing roller 150 rotates, the heat energy transmitted to a 
region a on the inner wall of the fixing roller 150 is not transmitted to 
a region corresponding to the nip n on the outer wall thereof, but it is 
transmitted to a region b separated from the nip n downstream relative to 
the direction of rotation of the fixing roller 150. For that reason, the 
heat energy transmitted to a region separated from the trailing edge of 
the nip n by a distance L can not be used for the fixing and is radiated 
to atmosphere, so that the temperature within the apparatus is needlessly 
increased. 
Therefore, according to this embodiment, the heating portion 164 is moved 
by the distance L when the fixing roller 150 rotates. The distance L is 
set so that the heat energy transmitted to the inner wall of the fixing 
roller 150 from the heating portion 164 can be assuredly transmitted to 
the nip n without radiation thereof to a region other than the nip n, in 
accordance with the following manner. 
First, since the temperature variation of the fixing roller 150 is 
dependent on time, the conduction of heat energy of the fixing roller 150 
can be defined by the following equation which is well known as a 
differential equation of non-steady heat conduction in a solid. 
EQU (2T/2t)=(K/.rho.c).gradient..sup.2 T (1) 
wherein K is heat conductivity (W/m. .degree.C.), .rho. is density (J/kg. 
.degree.C.) and c is specific heat (J/kg. .degree.C.). 
In addition, it is well known that when a solid of an initial temperature 
T.sub.0 is placed in a surrounding of a temperature T.sub.1, the 
temperature variation at a position P in the solid is dependent on time, 
so that the temperature at the position P asymptotically approaches the 
temperature T.sub.1 as time passes, as shown in FIG. 27. In FIG. 27, the 
temperature T.sub.1 -(T.sub.1 -T.sub.0)/e is a temperature versus time 
constant. It is assumed that the position P is in a steady state at a time 
corresponding to the aforementioned time constant, and that this time is 
defined as a relaxation time .tau.. On the other hand, since the width of 
the heating portion 164 is far smaller than the diameter of the fixing 
roller 150 in this embodiment, the contact portion of the heating portion 
164 with the inner wall of the fixing roller 150 can be considered as a 
plane. Therefore, as shown in FIG. 28, the heat conduction from the inner 
peripheral surface of the fixing roller 150 to the outer peripheral 
surface can be approximated by a non-steady heat conduction in an infinite 
plane of 2X.sub.0 in the thickness and T.sub.0 of initial temperature. 
When the infinite plane is placed in a circumstance of the temperature 
T.sub.1, the relaxation time .tau. can be defined by the following 
equation. 
EQU .tau.=(2X.sub.0 /.pi.h).sup.2 (2) 
wherein h.sup.2 =K/.rho. c is thermal diffusivity. In addition, the 
relaxation time when only one surface of the infinite plane of X.sub.0 in 
thickness and T.sub.0 of initial temperature is heated at the temperature 
of T.sub.1, can be considered similar to equation (2) in view of the 
symmetric property of FIG. 28. The aforementioned equations (1) and (2) 
are cited from "Compact Physics Handbook" (Fuchiro Kobayashi and Tatsunari 
Hirose; published by Maruzen Co., Ltd.) 
In view of the foregoing, in this embodiment, the fixing roller 150 is 
moved by a distance L(mm) obtained by the following equation, for a 
relaxation time .tau. in which the heat energy is transmitted from the 
heating portion 164 to the fixing roller 150. 
EQU L=Vs.tau.=Vs(2X.sub.0 /.pi.h).sup.2 (3) 
wherein Vs(mm/s) is the speed of movement of the fixing roller 150. 
According to this embodiment, the trailing edge 164a of the heating portion 
is moved by the distance L(mm) upstream of the trailing edge na of the nip 
n, so that the heat energy transmitted from the trailing edge 164a of the 
heating portion 150 to the inner peripheral surface of the fixing roller 
150 can be assuredly transmitted to the nip n without radiation to a 
portion other than the nip n. 
In this embodiment, since X.sub.0 =0.3 (mm), c=0.46(kJ/kg. .degree.C.), 
.rho.=7.8(.times.10.sup.3 kg/m.sup.3), K=16.3(W/m. .degree.C.) and 
Vs=50(mm/s), the distance L(mm) may be set to be greater than (0.4(mm). In 
this embodiment, L=1.0(mm) is selected in view of the manufacturing error 
of the fixing roller 150. When the distance L(mm) is set to be greater 
than a theoretical value, the heat energy is transmitted to the outer 
peripheral surface of the fixing roller 150 upstream of the nip n, and 
this heat energy can be used for fixing as preheating energy. Furthermore, 
it is necessary to set an optimum value of the distance L(mm) in 
accordance with the material, thickness and so forth of the fixing roller 
150. 
With this construction, the fixing device 28 is controlled by a CPU 190. 
Input ports A1 and A3 of the CPU 190 are respectively connected to a main 
switch MS and a paper sensor P, and a print signal PS is input from a host 
computer (not shown) to an input port A3. On the other hand, in order to 
control the image fixing device 28, output ports B1, B2 and B4 of the CPU 
190 are respectively connected to a main motor M, a heater switch HS for 
switching the resistance heating element 172 ON and OFF, and the solenoid 
180, and a power switching signal is output from an output port B3. 
When the main switch MS is turned on, a signal for turning ON the heater 
switch HS is output from the output port B2, so that the resistant heating 
element 172 is heated, whereby the surface of the pressure roller 152 is 
heated at a location corresponding to the nip n via the heat conductive 
member 174 and the fixing roller 150. In this condition, the solenoid 180 
is turned off, and the heating portion 164 is stopped by means of the 
regulating spring 182 at a position corresponding to the nip n. 
When the print signal PS is input to the CPU 190, the main motor M is 
turned on, so that the fixing roller 150 rotates in the direction of arrow 
a in FIG. 22 while the solenoid 180 is turned on to move the pin 184 
against the spring force of the regulating spring 182 in the direction of 
arrow c in FIG. 22. As a result, the heating portion 164 rotates around 
the rotating shaft 166 so that the trailing edge 164a of the heating 
portion 164 moves in the direction of arrow a' and is separated from the 
trailing edge na of the nip n by 1 mm upstream relative to the direction 
of rotation of the fixing roller 150. 
Subsequently, when a recording paper to which a toner image has been 
transferred at a transfer portion (not shown), reaches the paper sensor P, 
a detection signal is output from the paper sensor P to the CPU 190. Then, 
the trailing edge of the recording paper finishes passing through the 
paper sensor P, the paper signal is turned OFF, so that the main motor M 
and the solenoid 180 are turned off at a predetermined time after the OFF 
paper signal. In FIG. 30, A is a so-called warm-up region, B is a region 
where the fixing roller 150 rotates, and C is a region where the fixing 
roller 150 is stopped. 
Furthermore, although in this embodiment the fixing roller 150 is stopped 
during the warm-up time, it may rotate during the stand-by time. In this 
embodiment, although the pressure roller 152 is locally heated at the 
portion corresponding to the nip n when the fixing roller 150 is stopped, 
it is not anticipated that uneven fixing occurs, since the pressure roller 
152 is evenly heated by rotation thereof until the recording paper reaches 
the image fixing device 28 after the print signal PS is input. In 
addition, since the pressure roller 152 is locally heated at the portion 
corresponding to the nip n, there is an advantage in that it is possible 
to carry out the fixing immediately after the power is turned on in the 
morning. 
FIGS. 31 and 32 show the twenty-first preferred embodiment of an image 
fixing device, according to the present invention. 
In this embodiment, a fixing belt 192 of a heat resistance film having high 
heat conductivity is substituted for the fixing roller 150. The fixing 
belt 192 is wound onto a pair of rollers 194 and 196 which are separated 
to each other by a predetermined distance, so as to move in the direction 
of arrow d in accordance with rotation of the rollers 194 and 196 which 
are driven by means of a main motor M. On the inner surface side of the 
fixing belt 192, a heating portion 198 and an oscillating portion 200 
which causes the oscillation of the heating portion 198 are provided. The 
heating portion 198 is kept in contact with the inner surface of the 
fixing belt 192. The pressure roller 152 is so arranged as to press the 
outer surface of the fixing belt 192 at a location corresponding to the 
heating portion 198. 
As shown in FIG. 32, the oscillating portion 200 comprises a solenoid 202 
which causes the heating portion 198 to move in the direction of arrow e 
in FIG. 32; a linking rod 204 for connecting the solenoid 202 to the 
heating portion 198; and a regulating spring 206 for biasing the heating 
portion 198 in the direction of the arrow f in FIG. 32 to set the heating 
portion 198 at a position corresponding to the nip n when the fixing belt 
192 is stopped. Furthermore, when the solenoid 202 is turned off, the 
movement of the heating portion 198 in the direction of arrow f is 
restrained by a stopper (not shown) so as to be stopped at the position 
corresponding to the nip n. 
When the fixing belt 192 is stopped, the solenoid 202 is turned off, so 
that the heating portion 198 is stopped at the position corresponding to 
the nip n. On the other hand, when the fixing belt 192 moves, the solenoid 
is turned on, so that the heating portion 198 is moved from the position 
corresponding to the nip n upstream relative to the direction of movement 
of the fixing belt 192 by a predetermined distance L(mm) similar to the 
twentieth preferred embodiment. 
In the aforementioned twentieth and twenty-first preferred embodiments, for 
example, gears driven by a motor may be substituted for the solenoid 202 
serving as moving means. 
As mentioned above, according to the aforementioned twentieth and 
twenty-first preferred embodiments, the heat energy transmitted to the 
inner surface can be assuredly transmitted to the nip without radiation 
thereof to a portion other than the nip, so that an image fixing device 
having very low heat loss can be provided. In addition, it is possible to 
prevent the temperature within the apparatus from being needlessly 
increased, so that parts which have low resistance to heat, such as 
control circuits, can be arranged near the image fixing device, making it 
possible to thereby reduce the size of the image forming apparatus. 
Referring to FIGS. 33(A) and 33(B), and 34(A) and 34(B), the twenty-second 
preferred embodiment of an image fixing device, according to the present 
invention, will be described below. 
In this embodiment, the fixing roller 30 houses therein a supporting member 
210 of sheet metal which has a substantially H-shaped cross-section and 
which extends in the longitudinal direction of the fixing roller 30. Both 
ends of the supporting member 210 are secured to the frame of the 
laser-beam printer. The supporting member 210 is connected to one surface 
of an elongated heat insulating member 212 via springs 214. To the other 
surface of the heat insulating member 212, one surface of a heating 
element 216 is secured. On the other surface of the heating element 216, a 
resistance material is printed. When a voltage is applied to the resistant 
material via a terminal 218, the resistance material produces heat of a 
high temperature. The heat insulating member 212 serves to prevent the 
heat produced by the heating element 216 from being scattered to the 
supporting member 210. 
As can be seen clearly from FIG. 33(A), heat conductive means 220 is 
secured to the heat insulating member 212 so as to surround the heating 
element 216. The heat conductive means 220 comprises a plurality of heat 
conductive blocks 220a which are arranged in series so as to extend in the 
longitudinal direction of the fixing roller 30. Each of the heat 
conductive blocks 220a has a curved surface having substantially the same 
curvature as that of the inner surface of the fixing roller 30. The curved 
surface of the heat conductive blocks 220a is pressed against the inner 
surface of the fixing roller 30 due to the biasing force of the springs 
214, so that it is kept in contact with the inner surface of the fixing 
roller 30 at and around the nip 40 (at the contact portion of the fixing 
roller 30 with the pressure roller 32). When the fixing roller 30 rotates, 
the heat conductive blocks 220a slide on the inner wall of the fixing 
roller 30. 
The operation of the image fixing device 28, according to this embodiment, 
is substantially the same as that of the first preferred embodiment. 
Therefore, as in the case of the first preferred embodiment, in this 
embodiment, the time necessary to heat the fixing roller 30 can be 
decreased without increasing the heating value of the heating element 216, 
so that it is possible to prevent the temperature increasing within an 
apparatus, such as a laser-beam printer. It is also possible to prevent 
the demand current of the heating element 216 from increasing. 
Furthermore, since the toner is fixed to the sheet when the sheet is 
removed from the fixing roller 30, it is difficult for the toner to adhere 
to the fixing roller 30, so that it is possible to prevent offset. Since 
the adhesive strength of the toner on the sheet is decreased, the sheet 
can be easily removed from the fixing roller to prevent the jamming of the 
sheet in the apparatus. 
In this embodiment, the longitudinal length of each of the heat conductive 
blocks 220a is relatively short, so that it is possible to easily 
manufacture straight heat conductive blocks 220a, thereby making it 
possible to improve the manufacturing accuracy of the heat conductive 
blocks 220a. As a result, the heat conductive blocks 220a are pressed 
against the inner wall of the fixing roller 30 to enable tight contact 
therewith, so that the heat transmitted from the heat conductive blocks 
220a is evenly transmitted to the inner wall of the fixing roller 30. 
Even if undesired distortions occur in the heat conductive blocks 220a and 
the inner wall of the fixing roller 30 due to thermal expansion or 
insufficient manufacturing accuracy thereof, at least the distortions in 
the heat conductive blocks 220a can be accommodated by discontinuous 
portions between adjacent heat conductive blocks 220a, since the heat 
conductive blocks 220a are pressed against the inner wall of the fixing 
roller 30 by the biasing force of the spring 214. Therefore, the heat 
conductive blocks 220a are tightly kept in contact with the inner wall of 
the fixing roller 30, so that the heat transmitted from the heat 
conductive blocks 220a is evenly transmitted to the inner wall of the 
fixing roller 30. As a result, the temperature distribution on the inner 
wall of the fixing roller in the direction of the axis thereof can be 
even, so that the toner on the sheet in contact with the nip 40 of the 
fixing roller 30 is fused by heat at an even temperature applied to the 
sheet, thereby preventing the imperfect fixing of the toner. 
FIGS. 35(A) and 35(B) show the twenty-third preferred embodiment of an 
image fixing device, according to the present invention. 
In this embodiment, the image fixing device 28 is substantially the same as 
that of the twenty-second preferred embodiment, except that heat 
conductive blocks 220b in which the surfaces opposed to the adjacent block 
are inclined relative to the longitudinal axis of the fixing roller 30, is 
substituted for the heat conductive blocks 220a. According to this 
embodiment, even if gaps between the adjacent heat conductive blocks 220b 
are present, the heat conductive blocks 220b can evenly slide on the inner 
wall of the fixing roller 30 when the fixing roller 30 rotates. Therefore, 
any portions on the inner wall of the fixing roller 30 can be kept in 
contact with the heat conductive blocks 220b, making it possible to 
prevent the decrease in temperature of the discontinuous portions between 
the adjacent heat conductive blocks 220b. As a result, the temperature 
distribution in the inner wall of the fixing roller 30 in the direction of 
axis thereof is even, and imperfect fixing of the toner to the sheet is 
prevented. 
FIG. 36 shows the twenty-fourth preferred embodiment of an image fixing 
device, according to the present invention. 
In this embodiment, the image fixing device 29 is substantially the same as 
that of the twenty-second preferred embodiment, except that heat 
conductive blocks 220c which have a step which is engageable with the step 
of the adjacent block, are substituted for the heat conductive blocks 
220a. In this embodiment, there is the same advantage as that of the 
twenty-third preferred embodiment. 
In the aforementioned twenty-second to twenty-fourth preferred embodiments, 
grease having high heat conductivity may be applied to the adjacent 
surfaces of the heat conductive blocks 220a to 220c. In this case, it is 
not only possible to prevent gaps from occurring between the adjacent heat 
conductive blocks 220a to 220c, but it is also possible to decrease the 
temperature difference between the respective heat conductive blocks 220a 
to 220c. As a result, the temperature distribution in the longitudinal 
axis of the fixing roller 30 may be more even, so that it is possible to 
prevent imperfect fixing. In addition, when grease is applied to the 
portion between the heat conductive block 220a to 220c and the heating 
element 216 or the inner wall of the fixing roller 30, it is possible to 
prevent a decrease in the heat conductivity from gaps (air layer) 
therebetween. 
As mentioned above, according to the aforementioned twenty-second to 
twenty-fourth preferred embodiments, the heat conductive means can be 
assuredly kept in contact with the inner wall of the fixing roller by 
dividing the heat conductive means into a plurality of heat conductive 
blocks, so that the temperature distribution in the longitudinal direction 
of the fixing roller is even, and it is thereby possible to prevent the 
imperfect fixing. 
Referring to FIGS. 37(A), 37(B) and 38, the twenty-fifth preferred 
embodiment of an image fixing device, according to the present invention, 
will be described below. 
In this embodiment, the fixing roller 30 houses therein a supporting member 
222 which has a substantially H-shaped cross-section and which extends in 
the longitudinal direction of the fixing roller 30. An elongated heat 
insulating member 224 is secured to one surface of the supporting member 
222, and a heating element 226 is secured to the heat insulating member 
224. The heating element 226 is made of an electrical resistant material 
to produce high-temperature heat when voltage is applied thereto. The heat 
insulating member 224 serves to prevent the heat produced by the heating 
element 226 from being scattered to the supporting member 222. A heat 
conductive material 228 is retained by a casing 230 between the heating 
element 226 and the inner wall of the fixing roller 30. 
The heat conductive material 228 is made of an alloy having a low melting 
point such that it is easily softened or fused when heat energy is applied 
thereto. This alloy will be hereinafter referred to as "fusible alloy". 
The fusible alloy may be an alloy which has the same composition as that 
of a three-component eutectic mixture, such as Rose alloy and Newton 
alloy, i.e. [Bi(50%)--Pb(31%)--Sn(19%)], and which is fused at a 
temperature of about 95.degree. C. The fusible alloy may also be an alloy 
which has the same composition as that of a four-component eutectic 
mixture, such as Wood alloy, i.e. [Bi(50%)--Pb(24%)--Sn(14%)--Cd(12%), and 
which is fused at a temperature of about 70.degree. C. When heat energy 
from the heating element 226 is applied to the heat conductive material 
228, it is softened or fused while being retained within the casing 230. 
The casing 230 has on both ends thereof a pair of curved edges having 
substantially the same curvature as that of the inner surface of the 
fixing roller 30, so as to surely retain the heat conductive material 228 
between the casing 230 and the inner wall of the fixing roller 30. As can 
be seen clearly from FIG. 38, the peripheral edge of the casing 230 
contacting with the inner wall of the fixing roller 30 is coated with a 
heat-resistant sealing member 232 so as to prevent the softened or fused, 
heat conductive material 228 from leaking out of the casing 230. In this 
embodiment, since the heat conductive material 228 can be softened or 
fused by the heat energy applied thereto from the heating element 226 
which is evenly kept in contact with the inner wall of the fixing roller 
30, it is possible to evenly heat the outer surface of the fixing roller 
30 corresponding to the nip. 
In this embodiment, although the heat conductive material is made of a 
fusible alloy, it may be made of other metals which are softened or fused 
at a temperature less than the fixing temperature, such as indium, and of 
thermoplastic resins, such as wax. 
According to the present invention, the heat conductive means such as the 
heat conductive member or the heat conductive material, may be made of an 
electric conductive material connected to ground. In this case, in order 
to prevent the heating element from being connected to ground through the 
heat conductive means, it is preferable to insulate the heating element 
from the heat conductive means. With this construction, it is possible to 
effectively remove static electricity produced when the heat conductive 
means slides on the fixing means such as the fixing roller or the fixing 
belt, and thereby prevent the possibility of the toner being adhered to 
the fixing means by Coulomb force.