JPH03241379A - Heating device - Google Patents

Abstract

PURPOSE:To prevent a partial failure in fixing and the occurrence of an offset by arranging a safety element as a safety countermeasure when a heating body runs away in the opposite side of the contact surface of the heating body of a film, via a heat insulating member. CONSTITUTION:A transfer material sheet P on which an unfixed toner image Ta is carried at an upper face is guided by a guide 29 and advances between the fixed film 24 of the press-contacting part N of the heating body 20 and a pressure roller 28 and the pressure roller 28. The toner image Ta is heated, turned into a soften and fused image Tb, and then, a toner image Tc completely fixed on a cooled/solidified sheet P before it is ejected. When the safety element 102 as the safety countermeasure at the time of the runaway of the heating body is provided on the heating body 20, it is arranged opposite to the surface where the heating body 20 comes into contact with the film 24 via the heat insulating member 101. Thus, the heat is not takeoff away from the part of the heating body where the safety element 102 is located and the partial reduction of a temperature is prevented, and the partial failure in the fixing and the occurrence of the offset are prevented as well.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a heating device that applies thermal energy to a material to be heated, such as a recording material, through a heat-resistant film.

This device is an image heating fixing device in an image forming apparatus such as an electrophotographic copying machine, a printer, or a fax machine, that is, it is made of heat-melting resin, etc., by an appropriate image forming process means such as electrophotography, electrostatic recording, magnetic recording, etc. Recording materials (electrofax sheets, electrostatic recording sheets,
An unfixed toner image corresponding to the desired image information, formed directly or indirectly (transfer) on the surface of a transfer material sheet, printing paper, etc., is permanently fixed to the surface of the recording material that carries the image. It can be used as an image fixing device that performs heat fixing processing.

Further, the present invention is not limited to an image fixing device, and can be widely used as a means/device for heat-treating a material to be heated such as an image carrier, such as a device for heating a recording material carrying an image to modify its surface properties.

(Prior Art) Conventionally, a heating device for a recording material, for example, for heat-fixing an image, includes a heating roller maintained at a predetermined temperature, and a pressure roller having an elastic layer and pressed against the heating roller. By,
A heated roller system that heats the recording material while nipping and conveying it is often used.

In addition, various methods and configurations such as a flash heating method, an oven heating method, and a hot plate heating method are known and are in practical use.

Heald heating systems are also known, as disclosed in US Pat. No. 3,578,797. This is because: (1) the toner image is brought into contact with the heating web and heated to its melting point, (2) the toner is cooled to a relatively high viscosity after melting, and (2) the toner tends to adhere to the heating web. This is a method of fixing without causing offset by peeling it off from the heating web in a weakened state.

Recently, a heating element (thermal heater, hereinafter referred to as heater) that is fixedly supported, a heat-resistant film (fixing film) that is conveyed while being in pressure contact with the heater, and a recording material closely attached to the heater through the film. An apparatus having a method and configuration that heats and fixes an unfixed image formed and supported on the surface of the recording material by applying heat from a heater to the recording material through the film. heating method) has been devised.

For example, Japanese Patent Application Laid-Open No. 63-3131 related to the applicant's previous proposal
The system, device, etc. disclosed in Publication No. 82 belongs to this category, and includes a thin heat-resistant film (sheet), a means for moving the film, and a fixed support arrangement on one side of the film with the film inside. and a pressure member disposed on the other side facing the heater to bring the image-bearing surface of the recording material to which the image is to be fixed into close contact with the heater through the film, and the film is At least when performing image fixing, the recording material to be image-fixed is conveyed and introduced between the film and the pressure member, and the heater and the pressure member are moved at the same speed in the same direction as the recording material to which the image is to be fixed. The image-bearing surface of the recording material is heated by the heater through the film by passing through a fixing nip section as a fixing section formed by pressure contact with the recording material, and heat energy is applied to the visible image (unfixed toner image). This heating means/device is basically applied to the film, softens and melts it, and then separates the film and recording material after passing through the fixing section at a separation point.

In such film heating type equipment, a low heat capacity heating element can be used as the heater, which results in lower power consumption and less wait time compared to conventional contact heating type devices such as heat roller type or belt heating type. This method is effective because it has advantages such as being able to shorten the time (quick start) and solving various drawbacks of other conventional devices.

The heater as a heating element is formed by coating a low heat capacity current-carrying heating resistor in a linear or band shape on a heat resistant, good thermal conductivity, low heat capacity insulating substrate (base material) such as ceramic. By applying current to the resistor, the heater rapidly raises the temperature of the surface to the required fixing temperature because the heat capacity of the resistor and the substrate is small.

The heat-resistant film in contact with the heater also has a small heat capacity, and thermal energy from the heater side is effectively transmitted via the film to the recording material that is in pressure contact with the film, thereby fixing the image by heating.

The temperature of the heater is controlled using an energization control configuration in which the temperature of the heater is detected by a temperature sensing element, and the energization to the energized heating resistor is controlled based on the temperature detection information to control the temperature of the heater to a predetermined fixing temperature. .

(Problem to be solved by the invention) However, in this film heating type heating device, safety elements such as thermoswitches and temperature fuses are installed on the heating body as a safety measure in case the heating element runs out of control (when the heating element temperature rises excessively). When brought into contact, heat is taken away by the safety element, which has the disadvantage that the temperature of the part of the heating body in contact with the safety element becomes lower.

In other words, if the device is a fixing device, for example, even if the heating element is controlled to be energized to a temperature that allows fixing, the part in contact with the safety element does not reach the temperature that allows fixing, resulting in poor fixing or offset in some parts. was there.

An object of the present invention is to solve the above-mentioned problems in a film heating type heating device.

(Means for Solving the Problems) The present invention provides a heat-resistant film that is driven in close contact with a heating body that is fixedly supported, and a heated recording material or the like is placed on the surface of the heat-resistant film opposite to the heating body side. This is a heating device that applies heat energy to the heated object from the heating element side through a heat-resistant film by passing the heating element in close contact with the heating element, and a heat insulating material is placed on the side opposite to the side of the heating element that comes into contact with the film. This heating device is characterized in that a safety element is disposed through the heating device.

The present invention also provides that the heating device as described above is arranged separately from the safety element or the heat insulating material, the thickness of the heat insulating material is 3 mm or less, and the safety element is arranged spaced apart from the heat insulating material. The thickness of the insulation material is 1mm when
Below, the characteristics include that the separation distance of the safety element from the heat insulating material is 1.5 mm or less, and that the heat capacity per unit length (mm) of the heating element is 2.2X 10-1 J/Kmm or less. It is a heating device that

(Function) When installing a safety element on the heating element as a safety measure in the event of a runaway heating element, it should be installed on the opposite side of the heating element to the surface of the heating element that is in contact with the film, through a heat insulating material as described above. By arranging the safety element on the back side, the temperature of the heating element where the safety element is located is prevented from being absorbed by the safety element and causing a local temperature drop. The occurrence of defects and offsets is prevented, and when the heating element runs out of control, the safety element is activated and the current supply to the heating element is cut off without any problem.

The safety element can also be placed apart from the heat insulating material, and in this case, the contact pressure of the safety element against the heating element will not act, so the safety element can be placed in a configuration where the safety element is placed in contact with the heating element. This eliminates the need for a contact means such as a spring, which makes it possible to simplify and downsize the device configuration, as well as deformation of the heating element due to the contact pressure of the safety element, and the occurrence of waving and wrinkles in the heat-resistant film due to the deformation. Such troubles can also be avoided.

(Example) <Example 1> (1) Example of image forming apparatus (FIG. 2) First, an example of an image forming apparatus using a heating device according to the present invention as a fixing device for an unfixed image will be described.

The apparatus of this example is an electrophotographic copying apparatus of NN4 reciprocating type, rotating drum type, and transfer type.

In FIG. 2, reference numeral 100 denotes an apparatus housing, and numeral 1 denotes a reciprocating document mounting table made of a transparent plate member such as a glass plate, which is disposed on the upper plate 100a of the housing. are driven to reciprocate at predetermined speeds in the right direction a and the left direction a' in the drawing, respectively.

G is a document, which is placed by placing the image side to be copied facing down on the top surface of the document table 1 according to a predetermined placement standard, and placing the document pressure bonding plate 1a on top of it and pressing it down. .

Reference numeral 100b denotes a slit opening, which serves as a document illumination section, and is opened on the surface of the top plate 100a of the machine casing, with its length extending in a direction perpendicular to the direction of reciprocating movement of the document table 1 (direction perpendicular to the plane of the paper). The downward image surface of the document G set on the document platform 1 passes through the position of the slit opening 100b sequentially from the right side to the left side during the forward movement of the document platform 1 to the right side a. During the passage, the light from the lamp 7 is received through the slit opening 100b and the transparent original table 1, and the document is illuminated and scanned. The original M4 surface reflected light of the illumination scanning light is image-formed and exposed on the photosensitive drum 3 surface by the short focus and small diameter imaging element array 2.

The photosensitive tram 3 is coated with a photosensitive layer such as a zinc oxide photosensitive layer, an organic semiconductor photosensitive layer, etc., and is driven to rotate at a predetermined circumferential speed in the clockwise direction shown by the arrow around the central support shaft 3a, and is charged during the rotation process. The image-forming exposed original image is formed on the surface of the photosensitive drum 3 by being subjected to a positive or negative charging process by the charger 4, and the image-forming exposure (slit exposure) of the original image on the charged surface thereof. Electrostatic latent images corresponding to the image are sequentially formed.

This electrostatic latent image is sequentially visualized by a developing device 5 using toner made of a resin or the like that softens and melts when heated, and the developed toner image is transferred to a location where a transfer discharger 8 is provided as a transfer section. I will do it.

S is a cassette loaded with transfer material sheets P as recording materials, and the sheets in the cassette are fed out one by one by the rotation of the feeding roller 6, and then the toner on the tram 3 is fed by the registration roller 9. When the tip of the image forming section reaches the position of the transfer discharger 8, the tip of the transfer material sheet P also reaches the open position of the transfer discharger 8 and the photosensitive tram 3, and is synchronized so that they coincide. be sent. Then, the toner image on the photosensitive tram 3 side is sequentially transferred onto the surface of the fed sheet by the transfer discharger 8.

The sheet to which the toner image has been transferred in the transfer section 8 is sequentially separated from the surface of the photosensitive drum 3 by a separating means (not shown), and is guided by a conveying device 10 to a fixing device 11, which will be described later, to remove the unfixed toner image carried thereon. After the heat-fixing process, the image is discharged onto a paper discharge tray 12 outside the machine as an image-formed product (copy).

On the other hand, the surface of the photosensitive drum 3 after the toner image is transferred is subjected to removal of adhered contaminants such as residual toner by a cleaning device 13, and is used repeatedly for image formation.

(2) Fixing device 11 (Figs. 1 and 2) 24 is an endless belt-shaped fixing film, which is arranged between a drive roller 25 on the left side, a driven roller 26 on the right side, and below between the drive roller 25 and the driven roller 26. A low heat capacity linear heating body 20 as a heating body is suspended between the three members 25, 26, and 20 parallel to each other. The fixing film 24 will be described in detail in section (4) below.

The driven roller 26 is an endless belt-shaped fixing film 2.
The fixing film 24 is fixed clockwise at a predetermined circumferential speed as the drive roller 25 rotates clockwise, that is, the unfixed toner image Ta conveyed from the image forming section 8 side. wrinkles, meandering, etc. at the same peripheral speed as the conveyance speed of the transfer material sheet P carrying the
Rotation is driven without speed delay.

Reference numeral 28 denotes a pressure roller having a rubber elastic layer such as silicone rubber with good mold releasability as a pressure member. The lower surface is pressed against the lower surface by a biasing means (not shown) with a total pressure of, for example, 4 to 7 kg, and is rotated in a forward counterclockwise direction in the conveyance direction of the transfer material sheet P.

The heating element 20 is a low heat capacity linear heating element whose length is in the direction (width direction of the film) that intersects the plane movement direction of the film 24, and includes a heater substrate 21, a current-carrying heating resistor (heating element)
22, a temperature measuring element 23, etc., and is fixedly supported by being attached to a support body 27 made of metal such as SUS through a heat insulating material 101.

The heat insulating material 101 has heat insulating properties and high heat resistance, for example, pp
Highly heat-resistant resins such as s (polyphenylene sulfide), FAI (polyamideimide), PI (polyimide), PEE (polyetheretherketone), liquid crystal polymer, and composite materials of these resins with ceramics, metals, glass, etc. In order to effectively use the heat insulating material 101 to dissipate the heat of the heating body 20, the support body 27 is used to support the heating body 20 so as not to sag in the transverse direction of the fixing film.

The heater board 21 is a heat-resistant, insulating, low heat capacity, and high thermal conductive member, and has a thickness of 1+n+n and a width of 10 m, for example.
It is an alumina substrate with a length of 240 mm.

The heating element 22 is formed of, for example, Ag/P along the longitudinal direction approximately at the center of the lower surface of the heater substrate 21 (the side facing the film 24).
d (silver palladium), Ta2N, RuO2, etc., is applied to a thickness of about 10 μm and a width of 1 to 3 mm by screen printing, etc., and on top of that, a heat-resistant resin such as heat-resistant glass or PTFE is applied to a thickness of about 10 μm as a surface protective layer 21a. It is coated.

For example, the temperature sensing element 23 is mounted on the upper surface of the substrate 21 (the heating element 22
It is a low heat capacity temperature measuring resistor such as a PT film, which is coated by screen printing or the like on the substantially central part of the surface opposite to the surface on which the temperature sensor is provided.

As the temperature measuring element, a thermistor or the like having a low heat capacity may be arranged in contact with the substrate 21.

102 is a safety element disposed between the heat insulating material 101 and the support body 27. This will be explained in detail in section (5) below.

In the case of the heating element 20 of this example, electricity is applied to the heating element 22 which is linear or strip-shaped from both ends in the elongated direction.
2 to generate heat over almost its entire length. AClooV is used for energization.
According to the temperature detected by the temperature measuring element 23, the energization power is controlled by controlling the phase angle of energization by an energization control circuit (not shown) including a triac.

The fixing film 24 is not limited to an endless held type, but also has a third type.
As shown in the figure, the fixing film 24 with an end wound around the feed-out shaft 30 is passed through between the heating body 20 and the pressure roller 28, and is locked to the take-up shaft 31.
A configuration (film winding type) in which the transfer material sheet P is moved at the same speed as the transport speed of the transfer material sheet P from the zero side to the winding shaft 31 side may be used.

(3) Fixing Execution Operation The device performs an image forming operation in response to the image forming start signal, and the transfer material sheet P carrying the unfixed toner image Ta on its upper surface is conveyed from the transfer section 8 to the fixing device 11 by the kite 29.
The pressure contact portion N between the heating body 20 and the pressure roller 28 is guided by
The unfixed toner enters between the fixing film 24 and the pressure roller 28, and the unfixed toner image surface comes into close contact with the lower surface of the fixing film 24, which is rotating in the same direction at the same speed as the conveyance speed of the sheet P, causing surface misalignment. fixing film 24 without causing wrinkles.
The heating body 20 and the pressure roller 28 pass through the mutual pressure contact portion N of the heating body 20 and the pressure roller 28 while being subjected to a clamping force.

Since the heating body 20 is energized and heated at a predetermined timing in response to an image formation start signal, the toner image Ta is heated at the pressure contact portion N.
It is heated to become a softened and melted image Tb.

The fixing film 24 turns at an acute angle (the bending angle θ is approximately 45°) at an edge portion S of the heat insulating material 101 having a large curvature (the radius of curvature is approximately 2 mm). Therefore,
The sheet P, which is conveyed through the pressure contact portion N while overlapping the fixing film 24, is separated from the fixing film 24 by the curvature at the edge portion S, and is then discharged to the paper discharge tray 12.

By the time the sheet is ejected, the toner has sufficiently cooled and solidified, and the sheet P
The toner image is completely fixed (toner image Tc).

The toner used in this example has a sufficiently high viscosity when melted by heating, so even if the toner temperature when separated from the fixing film 24 is higher than the melting point of the toner, the adhesion of the toner to each other and the adhesion of the toner to the fixing film may be insufficient. Therefore, when the fixing film 24 and the sheet P are separated, toner offset with respect to the fixing film 24 does not substantially occur.

In addition, in this example, the heat capacity of the heating element 22 and the substrate 21 of the heating element 20 is small, and since these are supported by the support 27 insulatingly, the surface temperature of the heating element 20 at the pressure welding part N can be reduced in a short time. Toner melting point (or sheet P
There is no need to raise the temperature of the heating element 20 in advance (so-called standby temperature control) by raising the temperature to a sufficiently high temperature relative to the fixing temperature (temperature at which fixing is possible).
Moreover, it also prevents the temperature inside the aircraft from rising.

(4) Regarding the fixing film 24 In order to reduce heat capacity and achieve quick start performance, the fixing film 24 is a single layer or a composite layer with a total thickness of 100 μm or less, preferably 40 μm or less, and has heat resistance, mold releasability, durability, etc. Layer films can be used. FIG. 4 is a schematic diagram of the layer structure of an example of a composite layer film, and this example is a two-layer structure film. 24a is a heat-resistant layer serving as a base layer (base film) of the fixing film, and 24b is a release layer laminated on the outer surface (the surface facing the toner image) of the heat-resistant layer 24a.

The heat-resistant layer 24a is made of, for example, polyimide, polyether ether ketone (PEE), polyether sulfone (PES), etc.
), polyetherimide (PH1), polyparabanic acid (
A film with excellent strength and heat resistance, such as a highly heat-resistant resin film such as PPA), can be used.

The release layer 24b is preferably made of a fluororesin such as PTFE (polytetrafluoroethylene) or PFA-FEP, or a silicone resin.The release layer 24b may be laminated onto the heat-resistant layer 24a by an adhesive laminate of a release layer film or a release layer. Lamination of materials using film-forming techniques such as electrostatic painting (coating), vapor deposition, and CVD,
This can be done by forming a two-layer film by co-extruding the heat-resistant layer material and the release layer material.

Since the thickness of the heat-resistant layer 24a is set to be thicker than the thickness of the release layer 24b, the total thickness of the film can be reduced while maintaining the strength of the film, and the efficiency of heat transfer from the heating body to the recording material is high.

Note that the surface resistance of the release layer 24b is preferably 1010Ω or less. The surface resistance value may be lowered by mixing a conductive agent such as carbon black, graphite, or conductive whiskers. In this case, charging of the toner contacting surface of the fixing film 24 can be prevented. If the toner contact surface of the fixing film 24 is insulative, the surface of the fixing film is electrically charged, which may disturb the toner image on the sheet P or transfer the toner image to the fixing film 24 (so-called charging offset). However, these problems can be avoided by taking the above measures.

(5) Regarding the safety element 102 In this example, the safety element 102 is a temperature fuse with an operating temperature of 160''C, and is arranged in series with the current supply circuit (not shown) to the heating element 20. When the temperature of the heating element 20 reaches 160''C, the power supply to the heating element 22 of the heating element 20 is cut off.

Figure 5 shows the heater unit (heating body 20, heat insulating material 101,
FIG. 3 is an enlarged cross-sectional view of the safety element 102 and support body 27).

The safety element 102 includes a heat insulating material 10 made of pps and having a thickness of 1.5 mm on the back side of the heating element 20 (the opposite side to the side on which the heating element 22 is provided) at approximately the center in the transverse direction of the fixing film.
1 by the pressing force of a spring 103.

According to experiments, the safety element 102, which receives heat from the heating element 20 (temperature controlled at 180''C during use) through the 1.5 mm thick heat insulating material 101, can
Even after continuous operation for more than an hour, the temperature did not exceed 150''C. Therefore, the safety element 102 does not operate in normal use.

On the other hand, if the heating element 20 continues to be heated without controlling the energization, when the heating element 20 reaches approximately 360°C, the safety element 10
2 reaches an operating temperature of 160''C, and the heating element 20 is heated to the transfer material P.
The electricity was cut off before the ignition temperature (approximately 400°C) was reached.

Conventionally, when the safety element 102 was brought into direct contact with the heating element 20 without using the heat insulating material 101, there was a problem in that the temperature of the heating element became lower due to the portion of the heating element in contact with the safety element, resulting in poor fixing. . In order to solve this problem, there are methods such as placing a safety element in contact with the heating element in a direction perpendicular to the transfer material transport direction and outside the transfer material transport position, that is, a part of the heating body that does not affect the fixing process. There were problems such as the length and width of the body had to be increased and the equipment had to be larger.

As explained above, in this example, in order to solve these problems and prevent the heating element 20 from rising in temperature excessively, the safety element 102 is placed through a clean heat insulating material 101. be.

FIG. 6 shows the heating body 20 and safety element 10 in this embodiment.
2, the temperature increase characteristic is Class 7.

The present invention is particularly effective when the heating body 20 has a low heat capacity, for example, the heat capacity per unit length is 2.2 x 10 J/K mm or less, since the temperature drop when the safety element 102 comes into contact with it is large.

The optimum thickness of the heat insulating material 101 varies depending on the thermal conductivity of the heat insulating material 101 used, but in order to insulate the heating element 20, it needs to have a thermal conductivity of about 1.5 W/m k ) or less. . According to experiments, even when a crow is used, the safety element 10 is
2, it is difficult to prevent excessive temperature rise of the heating element, so the thickness of the heat insulating material 101 is preferably 3 mm or less.

<Example 2> In <Example 1>, the safety element 102 is attached to the heat insulating material 101.
However, in this method, the heat insulator 103 such as a spring
However, if the pressure to press the safety element 102 is too strong, the heating element 2o will be deformed, so it was necessary to control the pushing pressure with precision. . In order to prevent the fixing film 24 from becoming corrugated or wrinkled, deformation of the heating element 2° must be avoided at all costs.

Therefore, in this embodiment, as shown in FIG. 7, a thermistor switch 102 as a safety element is arranged on a heat insulating material 101 having a thickness of 0.5 mm at a distance α of 0.3 mm.

By arranging it in this way, while solving the above problems,
The same temperature increase characteristics (FIG. 6) of the heating body and the safety element as in the case of <Example 1> in which the safety element 102 was disposed by being pressed against the heat insulating material 101 were obtained.

Experiments have been conducted using heat insulators made of various materials, and it is preferable that the thickness of the heat insulator is 1 mm or less and the separation distance is 1.5 mm or less.

(Effects of the Invention) As described above, according to the present invention, in a film heating type heating device, when a safety element is provided on the heating body as a safety measure in case of runaway of the heating body, By arranging the safety element on the side opposite to the surface that comes into contact with the film, that is, on the back side of the heating element, the heating element where the safety element is located will absorb heat from the safety element, causing a local temperature drop. Therefore, in the fixing device, partial fixing failures and offsets are prevented from occurring, and furthermore, when the heating element runs out of control, a safety element is activated to cut off the current supply to the heating element without any problem.

In addition, by arranging the safety element away from the heat insulating material, there is no contact pressure of the safety element against the heating element, so there is no contact pressure of the safety element with the heating element, so there is no contact pressure of the safety element with the heating element. This eliminates the need for a contact means, which makes it possible to simplify and downsize the device configuration, and also prevents deformation of the heating element due to the contact pressure of the safety element.
Troubles such as waving and wrinkles of the heat-resistant film caused by the deformation can also be avoided.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the configuration of an embodiment of a heating device (film endless type) according to the present invention, which is utilized as an image fixing device. FIG. 2 is a schematic diagram of the configuration of an example of an image forming apparatus using the apparatus. FIG. 3 is a schematic diagram of the configuration of another image fixing device (film winding type). FIG. 4 is a schematic diagram of the layer structure of the fixing film. FIG. 5 is an enlarged cross-sectional view of the heater unit. Figure 6 is a temperature rise characteristic diagram of the heating element and safety element. FIG. 7 is an enlarged cross-sectional view of a heater unit having another configuration. 11 is an image fixing device (heating device), 20 is a heating element (heater), 21 is a heater substrate, 22 is a heating element, 21a is a surface protective layer (heat-resistant glass layer), 23 is a temperature measuring element, 101 is a heat insulating material, 102 103 is a safety element, 103 is a pressure spring, 27 is a support body, 24 is a heat-resistant fixing film, 25 is a drive roller,
26 is a driven roller, 28 is a pressure roller, and P is a recording material as a heated material. (e) Figure 5+)7

Claims (5)

[Claims] (1) A material to be heated, such as a recording material, is brought into close contact with the surface opposite to the heating body side of a heat-resistant film that is driven to be fed in close contact with a heating body that is placed in fixed support, and passed through the heating body position. This is a heating device that applies thermal energy from the heating body side to the heated body through a heat-resistant film, and a safety element is placed on the side opposite to the side of the heating body that comes into contact with the film through a heat insulating material. A heating device featuring: (2) The heating device according to claim 1, wherein the safety element is arranged apart from the heat insulating material. (3) The heating device according to claim 1, wherein the thickness of the heat insulating material is 3 mm or less. (4) The heating device according to claim 2, wherein the thickness of the heat insulating material is 1 mm or less, and the distance between the safety element and the heat insulating material is 1.5 mm or less. (5) The heat capacity per unit length (mm) of the heating element is 2.2
The heating device according to claim 1, 2, 3, or 4, characterized in that the heating power is not more than ×10^-^1 J/Kmm.