JPH09269687A - Fixing heater, fixing device, and image forming device - Google Patents

Abstract

(57) Abstract: A fixing heater having a high resistance to pressure of a resistance heating element and capable of reducing sliding resistance of a sliding surface on which a fixing target such as paper slides, and a fixing heater including the same. To provide a fixing device and an image forming apparatus. An elongated substrate (1) made of a heat-resistant electrically insulating material
1; an insulating glass lower layer 12 made of an electrically insulating material formed on one surface of the substrate 11; and an insulating glass lower layer 1
A resistance heating element 13 formed on the top surface 15a of the second heating element; and an insulating glass lower layer 1 around the outer surface of the resistance heating element 13 and its periphery.
The insulating glass lower layer 12 has a softening point of the insulating glass lower layer 12
And an insulating glass upper layer 15 made of an electrically insulating material whose main component is the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin fixing heater which is used by being attached to small equipment such as office automation equipment, household electric equipment and precision manufacturing equipment, and a copying machine and a facsimile in which the fixing heater is mounted. The present invention relates to a fixing device used for fixing toner, and an image forming apparatus using the fixing device.

[0002]

2. Description of the Related Art In general, in an electronic copying machine, a copy sheet on which a toner image is formed is passed between a fixing heater and a pressure roller directly or indirectly via a heat-resistant sheet while being pressed. The toner on the copy paper is heated, melted and fixed by the heating.

As this kind of conventional fixing heater, for example, one having a structure as shown in FIG. 6 (a longitudinal sectional view along the width direction) has been put into practical use. The fixing heater 1 is made of alumina (Al ₂ O ₃ ) ceramics or the like, and has a surface 2a of a heat-resistant and electrically insulating substrate 2 elongated in the front and back direction of the paper surface in FIG.
An elongated strip-shaped thick film resistance heating element 3 is formed thereon by, for example, thick film printing or the like, and a pair of electrodes (not shown) electrically connected to the resistance heating element 3 are connected to each other via a connector (not shown). The resistance heating element 3 is energized.

Then, the outer surface of the resistance heating element 3 is covered with a glassy overcoat layer 4 which is an electric insulating layer and a protective layer on the surface 2a of the substrate, so that a machine having abrasion resistance and impact resistance can be obtained. The mechanical strength is improved, corrosion resistance is protected from sulfurization and oxidation, and electrical insulation is provided between the rotatable pressure roller and the like that come into contact with the top surface of the overcoat layer 4.
On the top surface of the overcoat layer 4, for example, a copy paper P on which a toner image is formed is changed from in to o as shown by an arrow in the figure.
The resistance heating element 3 is slid in the width direction toward ut and heated at that time to fix the toner image on the copy paper P.

[0005]

However, in such a conventional fixing heater 1, from the widthwise ends of the resistance heating element 3 along the boundary surface between the substrate surface 2a and the widthwise end of the overcoat layer 4. There is a problem that a minute current may leak, and the pressure resistance is not necessarily high.

Further, in the fixing heater described in JP-A-5-275160, an electrically insulating glass underlayer is formed on a heat resistant and electrically insulating substrate, and a resistance heating element is further formed on the underlayer. And an electrically insulating glass upper layer are sequentially laminated in this order. In this case, since the resistance heating element and the substrate are further electrically insulated by the glass underlayer, the pressure resistance against the resistance heating element can be increased.

However, since the upper layer and the lower layer of the electrically insulating glass are made of the same glass layer having the same softening point, the unevenness of the lower layer is amplified in the upper layer, and the top surface of the upper layer on which the paper to be fixed slides is amplified. There is a problem that the surface roughness becomes large, and the sliding resistance of the copy sheet, which is the fixing target, increases.

Therefore, an object of the present invention is to provide a fixing heater which has a high resistance to pressure by a resistance heating element and which can reduce the sliding resistance of a sliding surface on which an object to be fixed such as copy paper slides.
An object is to provide a fixing device and an image forming apparatus including the same.

[0009]

The present invention is configured as follows in order to solve the above-mentioned problems.

The fixing heater according to claim 1 of the present application includes an elongated substrate made of a heat-resistant electric insulating material; an insulating lower layer made of an electric insulating material formed on one surface of the substrate; and a top surface of the insulating lower layer. And a resistance heating element formed on the outer surface of the resistance heating element and a part of the insulating lower layer around the resistance heating element, the softening point is lower than that of the insulating lower layer, and the main component is the same electrical insulating material. And an insulating upper layer.

In the present and following claims, the substrate made of a heat-resistant electric insulating material is a plate-like body made of alumina ceramics or the like, and the resistance heating element and the upper and lower electric insulating layers are made of a thick film or the like. It is formed. Further, the resistance heating element is formed to have almost the entire width of the substrate, or is formed in a meandering shape or in a zigzag shape, whereby the heating area can be expanded.

Therefore, according to the present invention, the insulating lower layer is interposed between the substrate having heat resistance and electrical insulation and the resistance heating element, and the resistance heating element is formed by the substrate and the insulating lower layer. Since the electric insulation is heavy, it is possible to increase the electric insulation withstanding voltage in the direction parallel to the substrate surface with respect to the resistance heating element.

Further, since the softening point of the insulating upper layer is lower than that of the insulating lower layer, the sliding surface such as the top surface of the upper layer on which the object to be fixed such as copy paper slides is easily formed as a smooth surface. Thus, the sliding resistance can be reduced.

Further, since the insulating lower layer and the upper layer are made of the same borosilicate glass as the main components, the thermal expansion coefficients of these upper and lower layers can be made substantially the same. For this reason, it is possible to reduce or prevent generation of a large stress in the upper layer or the lower layer due to the difference in the coefficient of thermal expansion and the occurrence of cracks.

A fixing heater according to a second aspect of the invention is characterized in that the insulating upper layer and the lower layer are mainly composed of borosilicate glass.

In the present invention, the glass containing borosilicate glass as a main component is, for example, lead borate glass or glass containing an oxide filler and having an excellent withstand voltage.

Therefore, according to the present invention, since the insulating lower layer and the upper layer are made of the same borosilicate glass as the main component, the thermal expansion coefficients of these upper and lower layers can be made substantially the same. For this reason, it is possible to reduce or prevent generation of a large stress in the upper layer or the lower layer due to the difference in the coefficient of thermal expansion and the occurrence of cracks.

According to a third aspect of the present invention, the fixing heater is characterized in that the insulating lower layer is thinner than the insulating upper layer.

Therefore, according to the present invention, even if the thickness of the insulating lower layer is thinned, the heating element is covered with the insulating layer as a result, so that the electric withstand voltage in the direction parallel to the substrate surface is sufficient. In addition to being secured, the lower layer material can be saved.

A fixing heater according to a fourth aspect is characterized in that the width of the insulating lower layer is wider than the width of the insulating upper layer.

Therefore, according to the present invention, the width of the insulating upper layer is narrower than that of the lower layer. Therefore, when manufacturing thick film printing or the like, the end portion in the width direction of the resistance heating element has the width direction of the insulating lower layer. When the protrusion extends outward from the end, the protrusion is exposed to the outside from the insulating upper layer, so that the protrusion can be visually checked.

According to a fifth aspect of the present invention, there is provided a fixing heater in which a space between one end in the width direction of the resistance heating element and one end in the width direction of the insulating lower layer on the same side along the one surface of the substrate is set to a lower layer of the same side. It is characterized in that it is narrower than the distance from the one end in the width direction to the one end in the width direction of the substrate exposed on the outer surface.

Therefore, according to the present invention, since the width of the exposed end of the substrate on which the resistance heating element is not formed is narrowed and the width of the resistance heating element is expanded as much as possible, the substrate having the limited width is formed. The heating area of the resistance heating element can be increased. As a result, it is possible to speed up the heating of the fixing heater and improve the uniformity of the temperature of the entire substrate, and it is possible to reduce heating unevenness, that is, fixing unevenness.

A fixing heater according to a sixth aspect of the invention is characterized in that the top surface of the insulating upper layer is ground to a smooth surface.

Therefore, according to the present invention, since the top surface of the insulating upper layer on which the fixing target such as copy paper is slid can be polished to be a smooth surface, the fixing target has less heating unevenness and the toner Fixing unevenness is reduced.

A fixing heater according to a seventh aspect of the invention is characterized in that the top surface of the insulating upper layer and the other portions have different plane roughness.

Therefore, according to the present invention, since at least only the top surface of the insulating upper layer needs to be formed into a smooth surface, the smooth surface can be easily formed.

In the fixing heater according to the eighth aspect, the resistance heating element is formed so that both ends in the width direction thereof extend within 5 mm from both ends in the width direction of the substrate.

Therefore, according to the present invention, the length in the width direction of the resistance heating element is widened as far as possible to the vicinity of both ends in the width direction of the substrate, so that the width of the resistance heating element on the substrate is limited. The heating area of the resistance heating element can be increased as much as possible, and the same operational effect as the invention according to claim 5 can be obtained.

According to a ninth aspect of the present invention, there is provided a fixing device, wherein the fixing heater according to any one of the first to eighth aspects is arranged so as to face the fixing heater so as to be in pressure contact with the fixing heater. A pressure roller that acts on the toner forming the image of the body and conveys the body to be fixed;
It is characterized by having.

An image forming apparatus according to a tenth aspect of the present invention is the fixing apparatus according to the ninth aspect of the present invention; a toner is attached to an electrostatic latent image formed on a medium to form a reverse image, and the reverse image is fixed on the fixing member. And a means for forming a predetermined image by transferring the image onto the sheet.

Since both the fixing device according to the ninth aspect and the image forming apparatus according to the tenth aspect have the fixing heater according to any one of the first to eighth aspects, they have substantially the same function and effect. You can

[0033]

FIG. 1 is a block diagram showing an embodiment of the present invention.
This will be described with reference to FIG. 1 to 5, the same or corresponding parts are denoted by the same reference numerals, and the dimensions and proportional relationships of the respective members are exaggerated in some cases for convenience of illustration and are not necessarily accurate.

FIG. 1 is a plan view showing a surface of a fixing heater according to a first embodiment of the present invention, and FIG. 2 is an end view of a cross section taken along the line II of FIG. 1. In these drawings, the fixing heater is shown. 1
0 is a heat resistant / electrically insulating material, for example, a length of about 300 mm made of alumina (Al ₂ O ₃ ) ceramics,
A rectangular elongated insulating glass layer 1 is formed on the surface 11a of a rectangular elongated substrate 11 having a width of about 8 mm and a thickness of about 0.6 to about 1 mm.
2 are formed.

The insulating glass layer 12 is mainly composed of borosilicate glass, and is formed, for example, by applying an insulating glass (dielectric) paste 4757 manufactured by DuPont Japan Co., Ltd. by printing and firing to a thickness of about 10 μm. .

On the surface of the insulating glass layer 12, a resistance heating element 13 having a U-shape in plan view, which is a heat source, is formed in the longitudinal direction of the substrate 11.

The resistance heating element 13 is formed along the longitudinal direction of the substrate 11 into a film-like elongated U-shape having a length of about 230 mm and a thickness of about 10 μm. However, the resistance heating element 13 may have a rectangular shape instead of the U-shape.

The resistance heating element 13 is made of silver / palladium (Ag.
Pd) alloy, nickel-tin (Ni-Sn) alloy, or other paste-like paint is formed by printing on the substrate surface 11a and firing, and the palladium contained in this alloy is electrically resistant. It becomes an element, and the resistance value of the heating element is adjusted by the ratio.

The resistance heating element 13 has one end (the right end in FIG. 1) bent in a U shape and folded back to the left in the drawing, and silver or silver / platinum (Ag.Pt) is attached to both left ends in the drawing. ) Alloy, silver / palladium alloy (Ag / Pd), or another film made of a good conductor is formed to partially overlap the pair of electrodes 14a and 14b.

The pair of electrodes 14a and 14b are provided to make the contact electric resistance smaller than that of the resistance heating element 13, and are made of silver (Ag) or silver-platinum alloy (Ag / P).
A metal paste of t), gold (Au), platinum (Pt), or the like is used. That is, after the resistance heating element 13 is formed (after the resistance heating element paste is applied by printing and firing), these metal pastes are applied by printing and firing to form a pair of electrodes 14a and 14b having a thickness of about 10 μm. ing.
The pair of electrodes 14a and 14b are electrically connected to a power supply device through a connector (not shown) made of phosphor bronze or the like, and the resistance heating element 13 is energized and heated through the pair of electrodes 14a and 14b.

The entire outer surface of the resistance heating element 13 is coated with a glassy insulating glass upper layer 15 having high electrical insulation.

The insulating glass upper layer 15 is applied not only to the strip-shaped portion of the resistance heating element 13 but also to the boundary portion between the pair of electrodes 14a and 14b. Since the temperature gradient near the boundary between the strip-shaped portion of the resistance heating element 13 and the electrode formation portion is steep, the resistance heating element 13 in this vicinity is likely to be broken, but this portion is covered with the insulating glass upper layer 15. This reduces this risk. In addition, the vicinity of the boundary between the electrodes 14a and 14b (on the side of the strip portion of the resistance heating element 13) is easily peeled off due to the effect of heat generation, but by covering this portion with the insulating glass upper layer 15, this risk is also reduced.

The insulating glass upper layer 15 is composed mainly of borosilicate glass and is printed and applied and fired after the resistance heating element 13 is formed. The softening point is lower than that of the insulating glass lower layer 12, but the layer thickness thereof is slightly smaller than that of the insulating glass lower layer 12. It is formed thick.

The layer thickness ta of the insulating glass upper layer 15 is formed thicker than the layer thickness of the insulating glass lower layer 12 (ta> tb), while the width Wa of the insulating glass upper layer 15 is larger than the width Wb of the resistance heating element 13. However, the width Wc is smaller than the width Wc of the lower layer 12 (Wb <Wa <Wc). The reason is, for example, as shown in FIG.
If the width Wc of the resistance heating element 13 is narrower than the width Wa of the insulating glass upper layer 15 (Wc <Wa), the widthwise end of the resistance heating element 13 should be the widthwise end of the insulating glass lower layer 12 for some reason. This is because the upper layer 15 conceals the protruding portion 13a even if the defective portion 13a is squeezed out further, so that the defective state cannot be visually checked.

On the other hand, as shown in FIG. 2, if the widths Wa and Wc of the upper and lower layers 15 and 12 on the insulating glass are set to Wa <Wc, the widthwise end portion of the resistance heating element 13 should be formed. Even if the portion of the insulating glass lower layer 12 protrudes outward from the widthwise end of the insulating glass lower layer 12, the protruding portion protrudes outward from the insulating glass upper layer 15, so that the protrusion can be visually checked. . That is, the defective product can be visually checked.

Further, the horizontal gap ga from one end in the width direction of the resistance heating element 13 to one end in the width direction of the insulating glass lower layer 12 on the same side is set to be from the one end in the width direction of the insulating glass lower layer 12 to one end in the width direction of the substrate 11. It is narrower than the horizontal gap gb, that is, the gap at which the substrate surface 11a is exposed to the outside (ga <
gb), and by reducing the exposure interval of the substrate 11 to 5 mm or less, the resistance heating element 1
The width of 3 is expanded as much as possible to increase the heat generation area, the rise of heat generation by the resistance heating element 13 is accelerated, and the substrate 11
The aim is to improve the uniformity of the overall temperature.

The top surface 1 of the insulating glass upper layer 15
5a (FIG. 2), the upper surface is polished to form a smooth surface, and the sliding resistance of copy paper or the like sliding on the top surface 15a in the width direction is reduced.

Therefore, when the resistance heating element 13 is energized via the pair of electrodes 14a and 14b, the resistance heating element 13 generates heat and the upper and lower layers 15 and 12 of the insulating glass and the substrate 1 are insulated.
Heat 1.

Therefore, the top surface 1 of the insulating glass upper layer 15
It is possible to fix the toner image on the copy sheet by heating when the fixing target such as the copy sheet on which the toner image is formed is slid in the width direction on 5a.

According to the fixing heater 10, the substrate 11 having heat resistance and electric insulation and the resistance heating element 1 are used.
3, the resistance heating element 13 is doubly electrically insulated by the substrate 11 and the insulation glass lower layer 12 by interposing the insulation glass lower layer 12 between the resistance heating element 13 and the resistance heating element 13. .

Further, since the softening point of the insulating glass upper layer 15 is lower than the softening point of the insulating glass lower layer 12, the sliding surface of the top surface 15a of the upper layer 15 on which the object to be fixed such as copy paper slides easily. It can be formed on a smooth surface to reduce sliding resistance.

Furthermore, since the insulating glass lower layer 12 and the upper layer 15 are made of the same borosilicate glass, the upper and lower layers 15 and 12 can have substantially the same coefficient of thermal expansion. Therefore, it is possible to reduce or prevent the generation of cracks due to the large stress generated in the upper layer 15 or the lower layer 12 due to the difference in the coefficient of thermal expansion.

Even if the insulating glass lower layer 12 is thinned, the electrically insulating substrate 11 is provided on one surface of the lower layer 12, so that the resistance heating element 13 can have sufficient pressure resistance. The insulating glass underlayer material can be saved.

Further, since the width of the insulating glass upper layer 15 is narrower than the width of the insulating glass lower layer 12, the widthwise end portion of the resistance heating element 13 is bulged outward more than the widthwise end portion of the lower layer. When the protrusion occurs, the protrusion is exposed to the outside from the insulating glass upper layer 15 of the portion 13a, so that the protrusion can be visually checked.

Further, the width of the exposed end of the substrate 11 on which the resistance heating element 13 is not formed is reduced, and the resistance heating element 1 is correspondingly narrowed.
The entire width of 3 can be increased to increase the heat generation area of the resistance heating element 13 on the substrate 11 having a limited width.

Further, since the top surface of the insulating glass upper layer 15 on which the fixing member such as copy paper is slid is polished to be a smooth surface, the sliding resistance of the copy paper can be reduced.

Furthermore, since the length in the width direction of the resistance heating element 13 is expanded as much as possible to both ends in the width direction to the vicinity of both ends in the width direction of the substrate 11, resistance heating on the substrate 11 having a limited width is performed. The heat generation area of the body 13 can be expanded as much as possible. As a result, it is possible to speed up the heating of the fixing heater and improve the uniformity of the temperature of the entire substrate, and it is possible to reduce heating unevenness, that is, fixing unevenness.

FIG. 4 shows the configuration of the second embodiment when the present invention is applied to an electronic copying machine 21 which is a kind of image forming apparatus. The copying machine 21 has a housing 22 and a cassette. An image forming unit 24 that draws in the copy sheet P in the sheet 23 and forms a toner image corresponding to an image of an original document (not shown), and a fixing device 25 that fixes the toner image on the copy sheet P are incorporated.

The fixing device 25 is constructed, for example, as shown in FIG. 5, and faces the pressure roller 26 so as to face the fixing heater 1.
The fixing heater 10 is attached to a substantially cylindrical holder 27.

Then, the holder 2 including the fixing heater 10
A ring-shaped endless heat resistant sheet 28 such as polyimide resin coated with fluorine resin is wound around the outer periphery of 7, and the upper surface of the insulating glass upper layer 15 directly above the fixing heater 10 in the figure is covered with the heat resistant sheet 28. It is elastically in contact with the silicon rubber layer of the pressure roller 26.

The fixing heater 10 includes a pair of electrodes 14
The resistance heating element 13 generates heat by being energized through a connector (not shown) made of a phosphor bronze plate or the like which is in contact with a and 14b and is given elasticity, and this heat generation is applied to the insulating glass upper layer 15. Therefore, the copy sheet P on which the toner image T is formed is heated by the fixing heater 10 between the outer surface of the heat resistant sheet 28 of the insulating glass upper layer 15 and the silicone rubber layer of the pressure roller 26, so that the unfixed toner image is formed. T can be melted and fixed on the copy paper P.

As described above, the fixing heater 10 can increase the heating area of the resistance heating element 12 and improve the pressure resistance, and can reduce the sliding resistance of the copying paper P, so that the electronic copying is performed. The machine 21 has the same effect.

[0063]

As described above, according to the invention of claim 1 of the present application, an insulating lower layer is interposed between a substrate having heat resistance and electrical insulation and a resistance heating element, and the resistance heating element is provided. Is double electrically insulated by the substrate and the insulating lower layer, so that the electric breakdown voltage in the direction parallel to the substrate surface with respect to the resistance heating element can be increased.

Further, since the softening point of the insulating upper layer is lower than that of the insulating lower layer, the sliding surface such as the top surface of the upper layer on which the object to be fixed such as copy paper slides is easily formed as a smooth surface. Thus, the sliding resistance can be reduced.

Since the insulating lower layer and the upper layer are made of the same borosilicate glass as the main components, the thermal expansion coefficients of these upper and lower layers can be made substantially the same. For this reason, it is possible to reduce or prevent generation of a large stress in the upper layer or the lower layer due to the difference in the coefficient of thermal expansion and the occurrence of cracks.

In the fixing heater according to the second aspect, since the insulating lower layer and the upper layer are made of the same borosilicate glass, the upper and lower layers can have substantially the same thermal expansion coefficient. For this reason, it is possible to reduce or prevent generation of a large stress in the upper layer or the lower layer due to the difference in the coefficient of thermal expansion and the occurrence of cracks.

In the fixing heater according to the third aspect, even if the thickness of the insulating lower layer is thinned, the heating element is covered with the insulating layer as a result, so that the electric withstand voltage in the direction parallel to the substrate surface is sufficient. In addition to being secured, the lower layer material can be saved.

In the fixing heater according to the fourth aspect, the width of the insulating upper layer is narrower than that of the lower layer. Therefore, at the time of manufacturing thick film printing or the like, the end portion in the width direction of the resistance heating element is the width of the insulating lower layer. When the protrusion protrudes outward from the end portion in the direction, the protrusion is exposed to the outside from the insulating upper layer, so that the protrusion can be visually checked.

In the fixing heater according to the fifth aspect, the width of the exposed end of the substrate on which the resistance heating element is not formed is narrowed and the width of the resistance heating element is widened as much as possible, so that the width is limited. The heating area of the resistance heating element on the substrate can be increased. As a result, it is possible to speed up the heating of the fixing heater and improve the uniformity of the temperature of the entire substrate, and it is possible to reduce heating unevenness, that is, fixing unevenness.

In the fixing heater according to the sixth aspect, since the top surface of the insulating upper layer on which the fixing target such as copy paper is slid can be ground to be a smooth surface, the fixing target has less heating unevenness and toner. The uneven fixing is reduced.

In the fixing heater according to the seventh aspect, at least only the top surface of the insulating upper layer needs to be formed into a smooth surface, so that the smooth surface can be easily formed.

In the fixing heater according to the eighth aspect, the widthwise length of the resistance heating element is widened as far as possible to the vicinity of both widthwise ends of the substrate, so that the substrate having a limited width is used. The heating area of the resistance heating element can be increased as much as possible, and the same function and effect as the invention according to claim 5 can be obtained.

Since both the fixing device according to the ninth aspect and the image forming apparatus according to the tenth aspect have the fixing heater according to any one of the first to eighth aspects, they have substantially the same operational effect. You can

[Brief description of drawings]

FIG. 1 is a partially cutaway plan view of a fixing heater according to a first embodiment of the present invention.

FIG. 2 is an end view of a section taken along the line II of FIG.

FIG. 3 is a vertical cross-sectional view of a modified example in which a part of the embodiment shown in FIGS. 1 and 2 is modified.

FIG. 4 is an overall configuration diagram of a second embodiment including the fixing heater of the first embodiment shown in FIGS. 1 and 2.

5 is an enlarged vertical sectional view of the fixing device shown in FIG.

FIG. 6 is a vertical sectional view of a conventional fixing heater.

[Explanation of symbols]

 10 Fixing Heater 11 Substrate 11a Substrate Surface 12 Insulating Glass Lower Layer 13 Resistance Heating Element 14a, 14b Pair of Electrodes 15 Insulating Glass Upper Layer 15a Sliding Surface (Top Surface) of Insulating Glass Upper Layer 21 Electronic Copier 22 Housing 23 Cassette 24 Image Forming unit 25 Fixing device 26 Pressure roller 27 Holder 28 Heat-resistant sheet

Claims (10)

[Claims] 1. An elongated substrate made of a heat-resistant electrically insulating material; an insulating lower layer made of an electrically insulating material formed on one surface of the substrate; a resistance heating element formed on a top surface of the insulating lower layer; An insulating upper layer that covers the outer surface of the resistance heating element and a part of the insulating lower layer around the resistance heating element together, and has a softening point lower than that of the insulating lower layer and that is composed of an electrical insulating material having the same main component;
A fixing heater comprising: 2. The fixing heater according to claim 1, wherein a main component of the insulating upper layer and the insulating lower layer is borosilicate glass. 3. The fixing heater according to claim 1, wherein the insulating lower layer is thinner than the insulating upper layer. 4. The fixing heater according to claim 1, wherein the width of the insulating lower layer is wider than the width of the insulating upper layer. 5. The distance along the one surface of the substrate from one end in the width direction of the resistance heating element to one end in the width direction of the insulating lower layer on the same side is defined as the outer surface from the one end in the width direction of the lower layer on the same side. The fixing heater according to any one of claims 1 to 4, characterized in that it is narrower than a space to one side end of the substrate exposed in the width direction. 6. The fixing heater according to claim 1, wherein the top surface of the insulating upper layer is polished to be a smooth surface. 7. The fixing heater according to claim 6, wherein the top surface of the insulating upper layer and the other portions have different plane roughness. 8. The resistance heating element is formed so that both ends in the width direction thereof extend within 5 mm from both ends in the width direction of the substrate. Fixing heater. 9. The fixing heater according to claim 1, which is arranged so as to be in pressure contact with the fixing heater, and heat from the fixing heater forms an image on a fixing target. A fixing device comprising: a pressure roller that acts on the existing toner and that conveys the object to be fixed. 10. The fixing device according to claim 9, wherein toner is attached to the electrostatic latent image formed on the medium to form a reverse image, and the reverse image is transferred to a fixing target to form a predetermined image. An image forming apparatus comprising: a forming unit;