Fixing roller having low resistance layer and fixing apparatus using same

A fixing apparatus includes a movable member: a rotatable member for forming a nip with the movable member; wherein a recording material carrying an unfixed toner image is fed by the nip therethrough, and the unfixed toner image is fixed while it is being fed thereby; wherein the rotatable member comprises an electroconductive core, a low resistance layer electrically connected with the core and provided outside the core, and an insulation layer outside the low resistance layer; means for applying a bias voltage to the core of the rotatable member; an insulation member for covering an end surface of the low resistance layer of the rotatable member.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to a fixing roller having a low resistance 
layer and a fixing device for fixing an unfixed image on a recording 
material by a nip formed by the fixing roller. 
Heretofore, a heat roller type fixing apparatus is widely used in an image 
forming apparatus such as an electrophotographic apparatus or an 
electrostatic recording apparatus, wherein an unfixed toner image formed 
by image forming process means through a transfer type or direct type 
electrophotographic process, electrostatic recording process, magnetic 
recording process or the like, is heated and fixed, into a permanent fixed 
image, on a recording material (paper) such as transfer material, 
photosensitive paper or electrostatic recording paper. 
The heat roller type fixing apparatus comprises a heating roller (fixing 
roller) maintained at a predetermined temperature and a pressing roller 
having an elastic layer and press-contacted to the fixing roller to form a 
nip therebetween (heating nip namely fixing nip), through which the paper 
bearing the unfixed toner image is passed, by which the unfixed toner 
image is fixed on the paper surface. 
However, this type of the apparatus involves a problem, for example, that 
the warming-up time required for the surface temperature of the fixing 
roller to reach the predetermined fixing temperature is long. 
On the other hand, a so-called film heating type device has been proposed 
with which the warming-up time is reduced, and has been put into practice 
(Japanese Laid Open Patent Application No. SHO-63-313182, Japanese Laid 
Open Patent Application No. HEI-1-263679, Japanese Laid Open Patent 
Application No. HEI-2-157878, Japanese Laid Open Patent Applications Nos. 
HEI-4-44075-44083). 
This type of the apparatus comprises a heating element and an elastic 
pressing roller press-contacted to the heating element to form a heating 
nip with a heat transfer member sandwiched therebetween. The heat transfer 
member is moved in close contact with the heating element in the heating 
nip. The paper carrying the unfixed toner image is introduced into the nip 
between the heat transfer member and the pressing roller and is passed 
therethrough with the heat transfer member, so that it is heated by the 
heating element through the heat transfer member to fix the image on the 
paper. 
The heating element used here is normally a ceramic heater (for example, 
alumina) in the form of a narrow plate, which comprises a heater substrate 
of high electroconductivity material having a heat-resistivity and an 
electric insulation property and an electric heat-generation layer in the 
form of a line or narrow strip formed thereon so that the entire thermal 
capacity is small, and therefore, the temperature rising speed is high. 
The movable heat transfer member is a heat resistive film member (fixing 
film) in the form of a cylindrical (endless belt type) or non-endless 
rolled elongated (non-endless) web having a small thickness, such as a 
polyimide film or the like having a film thickness of several hundreds 
.mu.m approx. 
The heating element is fixed to the supporting member, and the heating 
element and the pressing roller sandwiches the fixing film as the heat 
transfer member therebetween. The heating element is press-contacted 
against the elastic of the pressing roller to form a heating nip (fixing 
nip) having a predetermined width. 
The fixing film is moved or rotated at a predetermined speed in close 
sliding contact with a surface of the heating element in the heating nip 
by a driving means or by rotating the pressing roller, while the heater is 
supplied with electric energy. When the temperature of the heating element 
is raised to a predetermined fixing temperature and is controlled at the 
temperature, the paper carrying the unfixed toner image is introduced into 
between the fixing film and the pressing roller with the toner image 
carrying side thereof contacted to the fixing film. The portion of the 
paper having passed through the heating nip is separated from the surface 
of the fixing film. 
In the film heating type fixing device, the heating nip is concentratedly 
heated, and therefore, the heat efficiency is high. The heating element 
and the fixing film are quickly heated (low thermal capacity), so that the 
temperature at the heating portion (heating nip) is quickly raised to the 
predetermined temperature level. Therefore, a preheating (stand-by 
temperature control) with which electric energization of the heating 
element is continued during the waiting period of the apparatus, is not 
necessary, and the warming-up time can be reduced (quick start feature). 
Additionally, the electric power consumption can be saved, and the 
temperature rise within the apparatus can be reduced. 
The fixing device of the heat roller type and the fixing device of the film 
heating type, involve a problem of so-called "toner offset", that is, a 
part of the toner image is deposited onto the fixing roller or fixing film 
contacted to the toner image. 
Additionally, they involve a problem of "toner scattering", that is, when 
the paper is introduced to the heating nip, the paper is quickly heated in 
the heating nip with the result that the water contained in the paper is 
evaporated, and the vapor blows toward the sheet inlet side of the heating 
nip, and therefore, the part of the unfixed toner image on the paper is 
scattered there. 
The offset and the scattering may be eased by proper selection of the 
surface materials of the fixing film and the fixing roller or by proper 
setting of the heating, but they are still problems. 
It is known that one of the causes is insufficiency of the electrostatic 
attraction force between the toner of the unfixed toner image and the 
paper supporting it. 
In view of this, in a fixing device of the heat roller type, an electric 
field for preventing the offset or scattering has been formed by applying 
a bias to the pressing roller or to the fixing roller. 
For example, when the toner has a positive charging polarity (+), the 
fixing roller contactable with the toner image is supplied with a bias of 
the positive polarity, and the pressing roller contactable to the back 
side of the paper is supplied with the bias of the negative polarity (-), 
so that the toner is subjected to the electric field for the tendency of 
motion toward the paper, by which the electrostatic attraction force 
between the toner and the paper is externally increased, thus preventing 
the offset or scattering of the toner. 
Such an electric field formation is supposed to be theoretically applicable 
to the film heating type heating device. Actually, however, the 
application thereof involves the following problems: 
A) In the case applying the bias to the fixing film: 
In this case, one of the layers constituting the fixing film is to be given 
a low resistance to effectively receive a predetermined voltage. 
On the other hand, the fixing film is contacted to the heating element, 
which has a heat generating resistor as the heat generation source. For 
the heat generation, the heat generating resistor is supplied with a 
relatively large AC or DC current. 
The thickness of the fixing film is several .mu.m--at most several hundreds 
.mu.m approx. from the standpoint of reducing the warming-up time period 
and maintaining the fixing performance. 
Thus, the coexistence of the heat generating resistor receiving a large 
current and the fixing film to be maintained at a predetermined voltage, 
is required in a space less than 1 mm. In order to prevent the offset and 
the scattering of the toner, the fixing film is supplied with at least 
several hundreds V bias voltage, so that electric leakage tends to occur 
between the fixing film and the heat generating resistor of the heating 
element. 
For this reason, the bias application to the fixing film is very difficult, 
practically. 
B) In the case of applying the bias to the pressing roller: 
Normally, by application of a bias to the core metal of the pressing roller 
of aluminum or iron material, the effect is provided to a certain extent, 
but the suppression of the toner offset or scattering is not enough 
despite quite a high voltage is necessary. 
This is because the distance between the toner image and the pressing 
roller core metal portion supplied with the bias is so large that the 
effective electric field formation is difficult. 
Therefore, the effect is enhanced if the resistance of the pressing roller 
surface is decreased, and the bias voltage is applied thereto, but the 
pressing roller surface is so close to the heat generating resistor of the 
heating element that the electric leakage tends to occur between the heat 
generating resistor of the heating element and the low resistance surface 
layer of the pressing roller supplied with the bias voltage, similarly to 
the case a). 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide a 
fixing roller and a fixing apparatus, wherein toner offset or scattering 
can be sufficiently suppressed without occurrence of electric leakage in a 
fixing device having a low resistance layer. 
It is another object of the present invention to provide a fixing roller 
and fixing apparatus, wherein uncontrollable state of a temperature of the 
heating element in a fixing apparatus due to leakage of the current 
applied to an electrode of a heating element, to the pressing roller, is 
prevented. 
According to an aspect of the present invention, there is provided a fixing 
apparatus comprising: a movable member; a rotatable member for forming a 
nip with the movable member; wherein a recording material carrying an 
unfixed toner image is fed by the nip therethrough, and the unfixed toner 
image is fixed while it is being fed thereby; wherein the rotatable member 
comprises an electroconductive core, a low resistance layer electrically 
connected with the core and provided outside the core, and an insulation 
layer outside the low resistance layer; means for applying a bias voltage 
to the core of the rotatable member; an insulation member for covering an 
end surface of the low resistance layer of the rotatable member. 
According to another aspect of the present invention, there is provided a 
fixing apparatus comprising: a heating element having a resistor for 
generating heat upon supply of electric energy thereto; a film in sliding 
contact with the heating element; a rotatable member cooperative with the 
heating element to form a nip therebetween with the film therebetween; 
wherein a recording material carrying an unfixed toner image is fed by the 
nip therethrough, and the unfixed toner image is fixed while it is being 
fed thereby; wherein the rotatable member has an insulation layer, and a 
low resistance layer inside the insulation layer; an insulation member for 
covering an end surface of the low resistance layer of the rotatable 
member. 
According to a further aspect of the present invention, there is provided a 
fixing rotatable member comprising: an insulation layer; an 
electroconductive core; a low resistance layer provided outside the core 
and electrically connected with the core; an insulation member for 
covering an end surface of the low resistance layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The description will be made as to the embodiments of the present invention 
in conjunction with the accompanying drawings. 
Embodiment 1 (FIG. 1-FIG. 4) 
(1) An example image forming apparatus 
FIG. 1 is a schematic view of an example of an image forming apparatus. The 
image forming apparatus of this example is a copying machine or laser beam 
printer using an image transfer type electrophotographic process. 
Designated by 21 is an electrophotographic photosensitive member in the 
form of a drum as an electrostatic image bearing member for carrying an 
electrostatic image, and it is rotated at a predetermined peripheral speed 
(process speed) in the clockwise direction indicated by the arrow. 
The photosensitive member 21 is subjected to a primary charging by which it 
is charged uniformly to a predetermined polarity and potential at its 
peripheral surface by a charging roller 22 as charging means, while it is 
rotated. 
The charged surface of the photosensitive member 21 is exposed to image 
information light L by unshown image information exposure means such as an 
image projection slit exposure means, laser beam scanning exposure means 
or the like, so that an electrostatic latent image is formed in accordance 
with the intended image information. 
Then, the electrostatic latent image is developed into a toner image by a 
developing device 23. The toner image is brought by the continuing 
rotation of the photosensitive member 21 to an image transfer portion 25 
where a nip is formed between the transfer roller 24 as transferring means 
and the photosensitive member 21. 
On the other hand, a transfer sheet P as a recording material is fed out 
one by one from an unshown sheet feeding mechanism, and is introduced to a 
transfer portion 25 at a predetermined timing. While it is passed through 
the nip, the toner image is transferred onto one side of the transfer 
sheet P from the photosensitive member 21. 
The transfer sheet P having passed through the transfer portion 25 is 
separated from the surface of the rotatable photosensitive member 21, and 
is guided by the guides 26, 6 into fixing device A, where the unfixed 
toner image is heat-fixed, and the transfer sheet P is then discharged as 
a print (copy or print). 
After the separation of the transfer sheet, the surface of the rotatable 
photosensitive member 21 is cleaned by a cleaning device 27 so that the 
residual toner or the like is removed, and it is prepared for a repeated 
image forming operation. 
(2) Fixing device A 
1) General arrangement of fixing device A 
FIG. 2 is an enlarged schematic view of the fixing device A. The fixing 
device A of this example is of film heating type using a heat resistive 
film (fixing film) in the form of an endless belt as a heat transfer 
member or movable member. 
Designated by 1 is a fixed heating unit comprising a rigid stay 10, a 
heating element holder 11, a heating element 12 and so on. The rigid stay 
10 is fixed between side plates of the device. A heating element holder 11 
is fixed on a lower surface of the rigid stay 10. The heating element 12 
is fixed by a heat resistive adhesive material in a groove formed in the 
lower surface of the heating element holder 11. The heating unit 1 is 
elongated in a direction perpendicular to the sheet of the drawing. 
The rigid stay 10 is in the form of a channel-like member produced by 
bending a metal plate, for example. 
The heating element holder 11 has a proper rigidity, and high 
heat-resistivity and heat insulation property to provide a strength of the 
entirety of the heating element 12. It is of high heat-resistivity resin 
material such as PPS (polyphenylenesulfide), PAI (polyamideimide), PI 
(polyimide), PEEK (polyetheretherketone) liquid crystal polymer, and a 
compound material thereof and resin material, ceramic, metal glass or the 
like. 
The heating element 12 of this example is a ceramic heater. This will be 
described in detail in Section 2). 
Designated by 3 and 4 are a driving roller and a tension roller, 
respectively, disposed substantially in parallel in the longitudinal 
direction of the heating element unit 1. 
Designated by 2 is a fixing film as a heat resistive film in the form of an 
endless belt type (heat transfer member), and is extended around the lower 
surface of the heating element 12 of the heating unit 1, the driving 
roller 3 and the tension roller 4. The tension roller 4 functions to 
normally apply a proper tension to the fixing film 2. 
The fixing film 2 has a small thickness, for example, several hundreds 
.mu.m at most, and comprises a heat resistive resin material, elastic 
member, metal or the like. More specifically, the fixing film 2 is 
subjected to the heating and fixing for the toner image repeatedly, and 
therefore, it is good in the heat-resistivity, parting property and 
durability, and it has a thickness not more than 500 .mu.m generally, and 
not more than 100 .mu.m preferably. The material thereof is not limiting 
if the above-mentioned properties are satisfactory. Examples thereof 
include a metal film such as aluminum, nickel or stainless steel film, an 
elastomer film such as silicon or fluorine rubber film, a monolayer film 
or complex layer film of a heat resistive resin material such as 
polyimide, polyetherimide, PES, PFA 
(tetrafluoroethyleneperfluoroalkylvinylether copolymer resin material). 
More particularly, it may comprise a 20 .mu.m thickness film and a parting 
property layer thereon at least at a side thereof contactable to the 
image, the parting layer being of fluorine resin material such as PTFE 
(tetrafluoroethylene resin material) or PFA having a thickness of several 
.mu.m--several tens .mu.m added with electroconductive material. 
The said metal, elastomer and resin material may be laminated. For example, 
the use can be made with a SUS film coated with silicone rubber, or with 
fluorine resin material layer. 
Designated by 5 is an elastic pressing roller and is urged by unshown 
urging means to a lower surface of the heating element 1 with the fixing 
film 2 therebetween against the compression elasticity at a total pressure 
of 4-7 kg, for example, to form a heating nip (fixing nip)N having a 
predetermined width. The structure of the pressing roller 5 will be 
described in Section 3). 
The fixing film 2 is rotated in the clockwise direction at a peripheral 
speed which is substantially the same as the feeding speed of the transfer 
sheet P introduced to the fixing device A from the transfer portion 25, by 
rotation of the driving roller 3, using the frictional force between the 
outer surface of the driving roller and the inner surface of the fixing 
film, while the inner surface of the film is in sliding contact with the 
lower surface of the heating element 12 in the heating nip N. 
The pressing roller 5 is driven by the fixing film 2. 
In response to a fixing operation signal, the rotation of the fixing film 2 
is started by the driving roller 3, and simultaneously, the electric 
energy supply to the heat generating resistor 13 of the heating element 12 
is started. When the temperature of heating element 12 reaches a 
predetermined level (predetermined fixing temperature), the temperature 
thereof is maintained by a temperature control, and in this state, the 
transfer sheet P is guided by the guides 26, 6 into the fixing device A, 
and is introduced to the heating nip N (between the fixing film 2 and the 
pressing roller 5) with the toner image surface faced to the fixing film 2 
side. The transfer sheet P is passed through the heating nip N together 
with and in close contact with the fixing film 2. In the process of the 
passing of the transfer sheet P through the heating nip N, the unfixed 
toner image t on the transfer sheet P, is heated by the heat from the 
heating element 12 (mainly the heat of the heat generating resistor 13) 
through the fixing film 2, so that the image is fixed. 
The portion of the transfer sheet having passed through the heating nip N 
is separated from the surface of the fixing film 2, and is discharged from 
the fixing device A by the discharging rollers 7 and 8. When the rear end 
of the transfer sheet P has passed through the heating nip N, the electric 
energization to the heating element 12 is stopped, and the fixing film 2 
continues to rotate for a while, more particularly, until the temperature 
of the temperature lowers to a predetermined level (postrotation), and 
then it is stopped, and placed into a stand-by state. 
2) Heating element 12 
The heating element 12 in this example is a ceramic heater, and FIGS. 3, 
(a) and (b) show the front side and rear side thereof, respectively. 
The heating element 12 of this example is a linear heater having a low 
thermal capacity, comprising: 
A. A substrate (heater substrate) 15 having a heat-resistivity, an electric 
insulation property and a low thermal capacity, more particularly, an 
alumina substrate having a thickness of 1 mm, a width of 10 mm and a 
length of 340 mm, for example: 
B. A heat generating resistor 13 in the form of a fine strip extending 
along a length of the substrate formed on the front side of the substrate 
15, more particularly, a linear or stripe heat generating resistor layer 
having a low thermal capacity and capable of generating heat upon electric 
energy supply thereto, formed by printing (screen printing or the like) a 
width of 2.0 mm of an electric resistance material such as Ta.sub.2 N or 
silver palladium: 
C. First and second electric energization electrode portions 13a, 13b for 
electric connection with ends of the heat generating resistor 13 at 
respective ends of the substrate on the front side thereof, more 
particularly, a pattern electrode portion, formed by printing (screen 
printing or the like) of an electroconductive material such as Ag, for 
example: 
D. A heating element surface protection layer 16 on the substrate surface 
covering the heat generating resistor 13 but not covering the first and 
second electric energization electrode portions 13a, 13b, more 
particularly, a thin heat resistive glass layer having a thickness of 10 
.mu.m approx., for example: and 
E. One or more temperature detecting element 14 in the form of a bead on a 
rear side of the substrate 15, more particularly a thermister, for 
example. 
The heat generating resistor 13 of the heating element 12 is placed in a 
groove formed in the lower surface of the heating element holder 11 and 
extended along the length thereof. It is fixed by a heat resistive 
adhesive material and the bottom surface thereof is exposed. 
The heat generating resistor 13 is supplied with electric energy through 
the first and second electric energization electrode portions 13a, 13b and 
the electric power supply contact springs 17a, 17b from an unshown 
electric energy supply circuit to generate heat, and the temperature 
thereof rises. The temperature of the heating element 12 is monitored by 
the temperature detecting element 14, and the detected temperature 
information is fed back to an unshown control system, and the control 
system controls the electric energy supply to the heat generating resistor 
13 to maintain the temperature of the heating element 12 at the 
predetermined fixing temperature. 
3) Pressing roller 5 
FIG. 4, (a) shows a longitudinal section of the heating unit 1 and the 
pressing roller 5, and (b) shows a lateral side of the pressing roller 5. 
In these Figures, the structural feature is exaggerated, and the 
thicknesses of the constituent layers or the like is not proportional. 
The pressing roller 5 in this example has an outer periphery insulative 
surface layer, and at least one inner layer having a low resistance to 
receive a voltage, and both of the lateral sides are coated with 
insulation members. 
Designated by 51 is an electroconductive roller core metal of aluminum, 
iron or the like; 53 is elastic layer as a low resistance layer comprising 
as a base material silicone rubber, fluorine rubber or the like formed 
coaxially on the roller core metal 51 into a form of a roller; 52 is an 
insulative surface layer (parting layer) outside thereof; and 54a, 54b are 
end insulation members covering the end surfaces of the roller except for 
the core metal portion. 
The elastic layer 53 is a low resistance layer (not more than approx. 
10.sup.10 ohm.cm in volume resistivity), and is produced by mixing low 
resistance powder such as carbon into the silicone rubber, for example. It 
may be a foamed one of such a material (sponge). 
The insulative surface layer 52 comprises an insulative property, 
heat-resistivity and parting property elastic member (for example, 
silicone rubber or fluorine rubber), resin material layer (coating or tube 
of PTFE, PFA or mixture thereof) or a mixed material of the elastic member 
and the resin material (fluorine rubber and fluorine resin material, for 
example), and it has a sufficient withstand voltage (not less than approx. 
10.sup.13 ohm.cm, preferably not less than 10.sup.16 ohm.cm). 
The insulative end surfaces 54a, 54b are electrically insulative elastic 
member (for example, silicone rubber or fluorine rubber), or a deformable 
soft resin material. The insulative end surfaces 54a, 54b have thicknesses 
.alpha..sub.1, .alpha..sub.2 which are covered by extensions 
.alpha..sub.1, .alpha..sub.2 of the insulative surface layer 52 of the 
roller beyond the elastic layer 53 which is a low resistance layer, at the 
opposite ends. A recess formed by the end surface of the elastic layer 53 
and the inside of the expanded portion .alpha..sub.1, .alpha..sub.2 of the 
insulative surface layer 52, is filled with an insulative material to 
cover the entirety of the end portions of the elastic layer 53. Here, the 
end portions are longitudinal end portions of the roller. The our edge 
portion of the insulative end surface 54 and the expanded portion .alpha. 
of the insulative surface layer 52 are firmly close-contacted or 
connected. 
The roller end surface portion insulation members 54a, 54b of deformable 
soft resin material or elastic member, as described hereinbefore, and 
deforms following the elastic layer 53. 
Thus, the pressing roller 5 of this example, has an elastic layer 53 which 
is an inner low resistance layer, and the outer periphery and the end 
portions thereof are coated with the insulative surface layer 53 and the 
insulative end surfaces 54a, 54b, respectively. The roller core metal 51 
and the low resistance elastic layer 53 which is a main body of the roller 
are electrically connected with each other. 
In the pressing roller 5 of this example, the diameter .phi. of the core 
metal 51 is 15 mm; the thickness of the elastic layer 53 is 5 mm; the 
thickness of the surface layer 52 is 0.05 mm; the thickness of the 
insulative end surface 54 is 10 mm; and the hardness 50.degree. is 
50.degree. (Asker C). 
The core metal 51 of such a pressing roller 5 is supplied with a bias 
voltage having a polarity opposite from that of the charge polarity of the 
toner of the toner image t on the transfer sheet P introduced to the 
heating nip N. Designated by E is a bias applying voltage source. In this 
example, the charge polarity of the toner is negative, and therefore, the 
roller core metal 51 is supplied with +1 KV as the bias voltage. 
Since the roller core metal 51 and the low resistance elastic layer 53 are 
electrically connected, the elastic layer 53 also functions as an 
electrode, and therefore, an electric field is formed in such a direction 
that electrostatic attraction force is provided between the elastic layer 
53 of the pressing roller 5 and the toner of the toner image t on transfer 
sheet P introduced to the heating nip N. Namely, the force for directing 
the toner to the low resistance elastic layer 53. 
In this case, the distance between the toner image and the elastic layer 53 
as the electrode supplied with the bias, is so small as corresponds to the 
thickness of the insulative surface layer 52 of the roller plus the 
thickness of the transfer sheet P, and therefore, the formed electric 
field is effective, so that the electrostatic attraction force of the 
toner to the transfer sheet P is increased by the external strong electric 
field tending the toner to be attracted to the transfer sheet P. Thus, the 
toner offset to the fixing film 2 and the toner scattering on the transfer 
sheet P can be effectively suppressed. 
On the other hand, the low resistance elastic layer 53 supplied with the 
bias, is electrically isolated by the insulative surface layer 52 and the 
insulative end surfaces 54a, 54b, so that no electric leakage occurs. The 
low resistance elastic layer 53 is electrically connected with the bias 
voltage source E through the core metal 51, and the current hardly flows 
when the leakage does not occur, and therefore, the potential thereof is 
maintained substantially at the bias potential applied from the voltage 
source E. 
The heat generating resistance layer 13 of the heating element 12 is 
supplied with an AC from an utility voltage source. Although the heat 
generating resistance layer 13 of the heating element 12 and the low 
resistance elastic layer 53 of the pressing roller 5 is very close to each 
other, no leakage occurs therebetween since the low resistance elastic 
layer 53 is coated with the insulative surface layer 52 and the insulative 
end surfaces 54a, 54b. 
Therefore, the toner offset or scattering can be sufficiently suppressed 
without the liability of electric leakage in the film heating type fixing 
device. 
Additionally, the temperature control is possible without the adverse 
influence to the temperature control of the heating element. 
The insulative end surfaces 54a, 54b will be further described. 
1. The material of the insulative end surface 54 may be an elastic member 
or resin material, although the elastic member such as silicone rubber is 
preferable, since the elastic deformation occurs in use. 
2. The nip N is formed by urging the pressing roller 5 to the heating 
element 12 with the fixing film 2 therebetween. In consideration of the 
uniformity within the length of the nip (in the axial direction), it 
preferably has a hardness close to that of the elastic layer 53. 
The inventors have found that if the hardness difference therebetween is 
not less than 10 degrees (JIS A, Asker-C), the nip widths adjacent the 
roller ends are extremely small, and the sheet is creased in some cases. 
If it is less than 10 degrees, the crease is in a tolerable level. 
Therefore, the hardness difference between the elastic layer 53 and the 
insulative end surface member 54 is preferably less than 10.degree.. 
3. When the elastic layer 53 is of sponge material, the insulative end 
surface 54 is preferably of sponge material from the same standpoint. In 
this case, it is desirable that the insulative end surface 54 is of an 
independent pore material (very small number of continuous pore portions 
are permissible). 
Microscopically, the continuous pore portions are, in effect, 
non-insulative, and therefore, the possibility of the leakage between the 
heat generating resistance layer 13 of the heating element 12 and the low 
resistance elastic layer 53 of the pressing roller 5, increases. 
Particularly, the side adjacent to the surface layer 52 of the insulative 
end surface 54 (the side adjacent to the heat generating resistance layer 
13 of the heating element 12) is preferably of independent pore material. 
4. The thickness of the insulative end member 54 (a length measured in the 
axial direction of the pressing roller 5), is preferably large from the 
standpoint of leakage. However, the axial length of the low resistance 
elastic layer 53 has to cover the maximum sheet passing width, and 
therefore, the insulative end member 54 is preferably outside the sheet 
passage area. Particularly, when the insulative end surface member 54 is 
of sponge material, the effective insulative property is lower due the air 
layer contained therein, and therefore, the thickness is preferably as 
large as possible. 
The experiments using silicon sponge as the insulative end surface member 
54 have revealed that when the core metal is supplied with bias of a kV, 
this thickness thereof is preferably not less than 2a mm approx. For 
example, if the applied bias voltage is 1 kV, the thickness is not less 
than 2 mm. If the thickness is smaller than that, the leakage tends to 
occur under low atmospheric pressure conditions. 
An usual pressing roller has a primer (bonding layer) between the core 
metal 51 and the elastic layer 53 and between the elastic layer 53 and the 
surface layer 52, although not particularly described (shown in the 
Figure). 
Embodiment 2 (FIG. 5) 
FIG. 5 shows a pressing roller 5 used in this embodiment. Designated by 51 
is a core metal; 53 is an elastic layer formed coaxially into a roller 
form with a first primer layer 55 (bonding material) outside the roller 
core metal 51; 52 is surface layer formed outside thereof with a second 
primer layer 56 (adhesive material) therebetween. 
The core metal 51, the elastic layer 53 and the surface layer 52 of this 
embodiment are the same as with embodiment 1. Namely, the elastic layer 53 
is a low resistance layer, and the surface layer 52 is of insulative 
material. 
The first primer layer 55 is of low resistance material, for example, 
10.sup.10 ohm.cm or lower, more particularly, a mixture of PI, PES, PAI or 
the like resin material, fluorine resin material such as FEP and low 
resistance material such as carbon, since the bias is applied to the 
elastic layer 53 therethrough from the core metal 51. 
The second primer layer 56, as is different from the first primer layer 55, 
is insulative without, or with very small amount of, low resistance 
material. 
In the pressing roller 5 of this example, the insulative second primer 
layer 56 between the elastic layer 53 and the surface layer 52, as shown 
in the Figure, completely covers the end surface of the elastic layer 53 
at each of the roller ends and is extended to cover a part of the surface 
of the core metal 51. Designated by 56a.sub.1, 56a.sub.2 are extended 
layer portion of the second primer layer 56 adjacent the roller end 
surfaces. Designated by 56b.sub.1, 56b.sub.2 are parts of the second 
primer layer 56 covering the core metal 51. 
In the case of the pressing roller 5 of this of, an inner low resistance 
elastic layer 53 is provided. The pressing roller outer surface 
(circumferential surface and end surfaces) is covered with the insulative 
second primer layer 56, surface layer 52 and extension of the second 
primer layer 56. The elastic layer 53 of low resistance and the roller 
core metal 51 are electrically connected with each other through the low 
resistance first primer layer 55. 
To the core metal 51 of such a pressing roller 5, a bias voltage of the 
opposite polarity from that of the charge polarity of the toner of the 
toner image t on the transfer sheet P introduced to the heating nip N, is 
applied. Then, the elastic layer 53 functions as an electrode since the 
roller core metal 51 is electrically connected with the low resistance 
elastic layer 53 which is a main body of the roller, so that an electric 
field is formed in such a direction that the electrostatic attraction 
force is applied between the toner of the toner image t on the transfer 
sheet P introduced to the heating nip N and the elastic layer 53 of the 
pressing roller 5. 
In this case, the distance between the toner image and the elastic layer 53 
as the electrode supplied with the bias voltage, is as small as the 
thickness of the insulative surface layer 52 including the second primer 
layer 56 plus the thickness of the transfer sheet P, so that effective 
electric field formation is accomplished. So, similarly to the embodiment 
1, a strong electric field is formed to attract the toner onto the 
transfer sheet P, thus externally increasing the electrostatic attraction 
force between the transfer sheet P and the toner. As a result, the offset 
of the toner onto the fixing film 2 and the scattering of the toner on the 
transfer sheet P, are effectively suppressed. 
The outer periphery of the low resistance elastic layer 53 of the pressing 
roller 5 and the end surface portion thereof, namely, the pressing roller 
outer surfaces are coated with the insulative second primer layer 56, the 
surface layer 52, the extensions 56a.sub.1, 56a.sub.2 of the second primer 
layer 56 at the roller end surface, and therefore, no leakage occurs 
between the elastic layer 53 and the heat generating resistance layer 13 
of the pressing roller 5 even if they are close to each other. 
Therefore, the toner offset or scattering can be sufficiently suppressed 
without the liability of electric leakage in the film heating type fixing 
device. 
In the case of the pressing roller 5 of this example, as is different from 
the pressing roller 5 of embodiment 1, does not require additional members 
54a, 54b, and therefore, the structure is simplified. 
The part 56b.sub.1, 56b.sub.2 s of the second primer layer 56 which is 
insulative, covers a part of the core metal 51, and therefore, the leakage 
between the core metal 51 and the member adjacent thereto (side plate of 
the fixing device, for example) can be prevented. 
When this concept is used, the insulative surface layer 52 may be extended 
to the roller end surfaces and to the core metal portion in addition to 
extending the second primer 56, so that the roller end surface is coated 
with two insulative layers to provide further assured structure. 
Embodiment 3 (FIG. 6) 
In the pressing roller 5 of embodiment 1 or 2, the elastic layer 53 has a 
low resistance, but it is not inevitable, and another layer may be used as 
a low resistance layer. 
FIG. 6 shows such an example. An elastic layer 53 of the pressing roller 5 
is of an insulative layer of silicone rubber for example, which is widely 
used conventionally. A second primer layer 56 as an adhesive material 
between the elastic layer 53 and the insulative surface layer 52 outside 
thereof is given a low resistance property. At least part of the second 
primer layer 56 of the low resistance is extended along the 
circumferential surface of the roller (56c.sub.1, 56c.sub.2) and is 
electrically connected with the core metal 51. The insulative surface 
layer 52 is extended to cover the circumferential portion of the roller 
and the opposite end surfaces thereof (52a.sub.1, 52a.sub.2). As shown in 
the Figure, it may be extended to cover a part of the core metal. The 
first primer layer 55 may be insulative. 
In the case of the pressing roller 5 of this structure, the bias voltage 
applied to the core metal 51 is supplied to the low resistance second 
primer layer 56 electrically connected with the core metal 51, and the low 
resistance second primer layer 56 functions as an electrode to form such 
an electric field as to electrostatically attract the toner of the image t 
on the transfer sheet P introduced to the heating nip N toward the second 
primer layer 56 of the pressing roller 5. 
In this case, the distance between the toner image and the second primer 
layer 56 as the electrode supplied with the bias voltage, is as small as 
the thickness of the insulative surface layer 52 of the roller 5 plus the 
thickness of the transfer sheet P, so that an effective electric field 
formation is accomplished. Thus, similarly to embodiment 1 or 2, a strong 
electric field is formed to attract the toner onto the transfer sheet P, 
thus externally increasing the electrostatic attraction force between the 
toner and the transfer sheet P. By this, the offset of the toner to the 
fixing film 2 and the scattering of the toner on the transfer sheet P can 
be effectively suppressed. 
Since the pressing roller outer surface is coated with the insulative 
surface layer 52 and the extension thereof 52a.sub.1, 52a.sub.2, no 
leakage occurs between the low resistance second primer layer 56 of the 
pressing roller 5 and the second primer layer 56 of the heating element 12 
even if they are close to each other. 
Therefore, the toner offset or scattering can be sufficiently suppressed 
without the liability of electric leakage in the film heating type fixing 
device. 
Further, as is different from embodiment 1 and 2, an insulative elastic 
layer 53 is usable so that the hardness of the roller can be decreased. 
When it is necessary to decrease a resistance of a material which is itself 
insulative, a large amount of electroconductive material such as carbon 
has to be mixed thereinto. It is know that such mixing results in increase 
of hardness of the rubber. When the rubber hardness is high, the urging 
force to the fixing film 2 or the heating element 12 is large with the 
result of promoting the wearing of the heating element surface or the 
fixing film, thus decreasing the lifetime thereof. In this sense, this 
embodiment is effective to prevent shortening of the service life of the 
pressing roller. 
The description will be made as to a heating device of a film heating type. 
FIGS. 7, (a), (b), (c) show other embodiments of the heating devices of 
the film heating type. 
In (a), a fixing film 2 in the form of an endless belt type is extended 
between the heating element 12 of a heating unit 1 (10-12) and the driving 
roller 3 and is rotated by the driving roller 3. 
In (b), a cylindrical fixing film 2 is loosely placed around the heating 
unit 1, and is press-contacted to the heating element 12 by the pressing 
roller 5. The fixing film 2 is rotated in close sliding contact with the 
heating element 12 by rotating the pressing roller 5 (pressing roller 
driving and tensionless type type). The heating element holder 11 function 
also as a rotatable guiding member for the cylindrical fixing film 2. 
In (c), the fixing film 2 is a rolled elongated non-endless film rather 
than an endless belt, and is supplied from a supply shaft 31 and is taken 
up by a take-up shaft 32 by way of the heating element 12 at a 
predetermined peripheral speed. 
The pattern of the heat generating resistor 13 of the ceramic heater 12 as 
the heating element and the electric path pattern of the heat generating 
resistor 13, are not limited to the one shown in FIG. 3. Heating element 
surface protection layer 16 may be omitted. 
The heating element is not limited to the ceramic heater, but it may be a 
magnetic metal plate strip 12A capable of electromagnetic induction heat 
generation, as shown in FIG. 8 which is supplied with an alternating 
magnetic field by an excitation coil 18 by which eddy current is produced 
in the magnetic metal plate strip 12A as the heating element to generate 
the joule heat (electromagnetic induction type). 
The fixing device may be used for a temporary fixing process for heating a 
recording material carrying an image to improve the surface property 
thereof (glossiness or the like). 
In the foregoing, the description has been made as to embodiments 1, 2 and 
3, wherein the apparatus is a fixing device, and the pressing roller 
thereof may be used in a heat roller type fixing device, wherein the 
adverse influence due to the leakage can be prevented. 
As described in the foregoing, according to the present invention, there is 
provided a fixing device wherein the offset or the scattering of toner can 
be sufficiently suppressed by formation of an effective electric field 
with a simple structure. Additionally, the hardness of the pressing roller 
can be decreased, thus permitting increase of the service life of the 
fixing device. 
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.