JP2009139822A - Heater, image heating device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent poor heating in an region in contact with a safety element, and to suppress uneven density in the region which is in contact with the safety element. <P>SOLUTION: A heater is configured so that the resistance value per unit length of a first region, as the region in contact with the thermo-switch, of a heat-generating body 31 may become larger than that of a non-reduced region 31a, as the region which is not in contact with the thermo-switch, of the heat-generating body 31. In the heat-generating body 31, a gradient is provided in the resistance value per unit length in a length direction Y, in each boundary region 38 located in between a reduced region 37, showing the highest resistance value among the resistance values of the first region and the non-reduced region 31a. If the length of the reduced region 37 in the length direction Y is (a), the length of the boundary region 38 in the length direction Y is (b), and the relation: 0≤a/(a+2b)<0.8 is satisfied. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer having a function of forming an image on a recording material such as a sheet, and more particularly to a heater and an image heating apparatus provided in these apparatuses. is there.

  Conventionally, in an electrophotographic copying machine, printer, etc., as an image heating apparatus (hereinafter referred to as a heat fixing apparatus) that heats and fixes an unfixed toner image formed and supported on a recording material surface as a permanently fixed image, energy saving with good thermal efficiency A type of film heating method is adopted.

  The film heating type heat fixing apparatus is an apparatus for fixing an unfixed image as a permanent image on a recording material as follows.

  First, a heat-resistant film (fixing film), which is a heating rotator, is brought into close contact with a heating rotator (elastic roller) and is slid and conveyed. Then, a recording material carrying an unfixed image is introduced into a pressure nip formed by a heating body and a pressure rotating body with the heat resistant film interposed therebetween and conveyed together with the heat resistant film. Then, the unfixed image is fixed on the recording material as a permanent image by heat from a heating body made of a ceramic heater applied through a heat resistant film and pressure applied at the pressure nip portion.

  The film heating type heat fixing device can use a low heat capacity linear heating element as the heating element and a thin film with a low heat capacity as the heating element, which can save power and shorten the wait time (quick start). It is.

In this type of fixing method, the following two methods are known.
(1) A driving system for the fixing film, in which a driving roller is provided on the inner peripheral surface of the film, and the film is driven while tension is applied. (2) The film is loosely fitted to the film guide, and the rotary member for pressure is driven. The method of rotating the film following the pressurizing rotator In recent years, since the number of parts is small, the pressurizing rotator driving method (2) is often adopted.

As a ceramic heater used as a heating element, a ceramic substrate having electrical insulation, good thermal conductivity, and low heat capacity such as alumina (Al ₂ O ₃ ) and aluminum nitride (AlN) is used. The ceramic heater is provided with an energization heating resistance layer such as silver palladium (Ag / Pb) formed on the surface of the substrate facing the fixing film along the length of the substrate by screen printing or the like. The surface is covered with a thin glass protective layer.

  In this ceramic heater, when the energization heating resistor layer is energized, the energization heating resistor layer generates heat, and the entire heater composed of the ceramic substrate and the glass protective layer is rapidly heated. The temperature rise of the heater is detected by temperature detection means such as a thermistor disposed on the back surface of the heater, and the temperature is controlled by supplying a power-controlled AC current to the heater heating element.

  In addition, a safety element for preventing an excessive temperature rise is attached to the ceramic heater in preparation for a case where the power control circuit fails in the heater heating element and the heater is fully energized.

The safety element generally uses a thermo switch or a thermal fuse. The safety element is provided in an AC circuit that energizes the heater, and the safety element itself is brought into contact with the ceramic heater. When the temperature rises abnormally, the safety element operates to cut off the AC circuit and supply power to the heater. It is supposed to stop.

  However, the ceramic heater as described above has the following problems.

When the safety element is brought into contact with the back of the heater, the safety element takes heat away from the heater, so that the safety element contact portion may cause poor fixing or uneven fixing. In order to prevent this, conventionally, the heat generation amount per unit length of the safety element contact portion has been made larger than the heat generation amount per unit length other than the safety element contact portion. For example, Patent Document 1 discloses such an example.
JP 09-297478 A

  However, since the heat taken away by the safety element is not constant, there is a concern that further problems may occur.

  That is, the following can be considered in the image forming apparatus. When the heat fixing device is cold, the safety element is also cooled when the heater is heated, so that the amount of heat taken away by the safety element is large. On the other hand, when the heat-fixing apparatus is heated by overlapping the prints, the safety element is also heated, so that the amount of heat taken from the heater to the safety element is reduced.

  In order to make the amount of heat applied to the recording material uniform in the longitudinal direction when the heat fixing device is cold, it is necessary to increase the amount of heat generated per unit length by the amount of heat taken by the safety element. However, if the heat-fixing device becomes hot after repeated printing, the safety element itself is also sufficiently warmed, so the heat from the heating element is less likely to be taken away by the safety element, and an extra amount of heat is given to the recording material. End up.

  Therefore, if the amount of heat locally applied to the recording material in the longitudinal direction is increased, there is a concern that only that portion is overfixed and density unevenness occurs. When such density unevenness occurs, boundary lines with different calorific values become conspicuous.

  The present invention has been made in view of the circumstances as described above, and it is an object of the present invention to prevent defective heating in a region where a safety element contacts and to suppress density unevenness in a region where the safety element contacts.

In order to achieve the above object, the present invention provides:
A heater that configures a nip portion together with a pressure roller via a flexible sleeve and heats the recording material on which the developer image is formed at the nip portion,
A heater body having a heating element on the substrate that generates heat when energized;
A region provided in the heater body, a region where a safety element that cuts off the power supply to the heating element at the time of overheating of the heater body contacts,
With
The resistance value per unit length of the first region of the heating element corresponding to the region where the safety element contacts in the heater body is the second region of the heating element corresponding to the region where the safety element does not contact. In the heater provided to be larger than the resistance value per unit length,
Of the heating element, the resistance per unit length in the axial direction of the pressure roller in the boundary region between the high resistance region showing the highest resistance value in the first region and the second region The value is sloped,
When the axial length of the high resistance region is a, and the axial length of the boundary region is b,
0 ≦ a / (a + 2b) <0.8
It is characterized by satisfying the relationship.

A heater configured to form a nip portion together with a pressure roller via a flexible sleeve and to heat the recording material on which the developer image is formed at the nip portion;
A heater body having a plurality of heating elements that generate heat when energized on the substrate;
A region provided in the heater body, a region where a safety element that cuts off the power supply to the heating element at the time of overheating of the heater body contacts,
With
The sum of the resistance values per unit length of the first region of the plurality of heating elements corresponding to the region where the safety element contacts in the heater body is the plurality of the heat generation corresponding to the region where the safety element does not contact. In the heater provided to be larger than the sum of the resistance values per unit length of the second region of the body,
At least one heating element among the plurality of heating elements has a resistance value per unit length of the first area larger than a resistance value per unit length of the second area,
Of the plurality of heating elements, per unit length in the axial direction of the pressure roller in the boundary region between the high resistance region showing the highest resistance value of the first region and the second region. A slope is provided for the sum of resistance values of
When the axial length of the high resistance region is a, and the axial length of the boundary region is b,
a / (a + 2b) <0.8
It is characterized by satisfying the relationship.

  According to the present invention, it is possible to prevent defective heating in a region where the safety element is in contact and to suppress density unevenness in the region where the safety element is in contact.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the scope to the following embodiments.

(1) Image Forming Apparatus FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus to which the image heating apparatus according to Embodiment 1 of the present invention is applied as a heat fixing apparatus. In this embodiment, a laser beam printer using a transfer type electrophotographic process will be described as an example of an image forming apparatus.

  In FIG. 1, reference numeral 1 denotes a photosensitive drum as an image carrier, which is formed by forming a photosensitive material such as OPC or amorphous silicon on a cylindrical substrate such as aluminum or nickel.

  Hereinafter, the image forming operation will be described.

  First, the surface of the photosensitive drum 1 is uniformly charged by a charging roller 2 as a charging device.

  Next, the laser beam 3 as an exposure unit outputs laser light L that is ON / OFF controlled (modulated) in accordance with the time-series electric digital pixel signal of the image information, and scans and exposes the charged surface of the photosensitive drum 1. . An electrostatic latent image is formed on the photosensitive drum 1 by scanning exposure of the laser beam 3. This electrostatic latent image is developed by the developing device 4 and visualized. As a development method, a jumping development method, a two-component development method, or the like is used, and is often used in combination with image exposure and reversal development.

  The recording material P as the material to be heated is taken out from the storage cassette C by the feeding roller 5 and sent to the registration roller 6. The recording material P is synchronized with the toner image (developer image) formed on the surface of the photosensitive drum 1 by the registration roller 6 and supplied to the transfer nip A formed by the photosensitive drum 1 and the transfer roller 7. In the transfer nip portion A, the toner image on the photosensitive drum 1 is transferred to the recording material P by the action of a transfer bias by a power source (not shown).

  The recording material P holding the toner image is conveyed to a heat fixing device (hereinafter referred to as a fixing device) 8 and heated and pressed at a pressure nip portion ((fixing) nip portion) N of the fixing device 8 to record a toner image. It is fixed on the material P and becomes a permanent image. The recording material P on which the toner image is fixed is discharged out of the apparatus by a discharge mechanism (not shown).

  On the other hand, the transfer residual toner remaining on the photosensitive drum 1 after the transfer is removed from the surface of the photosensitive drum 1 by the cleaning device 9.

  Here, the photosensitive drum 1, the laser beam 3, the developing device 4, and the transfer roller 7 constitute an image forming unit.

(2) Fixing Device FIG. 2 is a cross-sectional view showing a schematic configuration of a film heating fixing device as an example of the fixing device of this embodiment. The configuration and operation will be described below.

(A) Fixing Film In FIG. 2, reference numeral 10 denotes an endless belt-like heat-resistant film (hereinafter referred to as a fixing film) as a flexible sleeve, which has a circumferential length with respect to a semicircular arc-shaped film guide member (stay) 13. It is externally fitted with a margin.

  The fixing film 10 has a single-layer film such as PTFE, PFA, PPS, etc. having heat resistance, releasability, strength, durability, or a release layer coated on the surface of a film such as polyimide, polyamideimide, PEEK, PES, etc. It is a composite layer film. As the release layer, PTFE, PFA, FEP or the like is used. Here, PTFE is polytetrafluoroethylene resin, PFA is purple alloalkoxy resin, PPS is polyphenylene sulfide, PEEK is polyether ether ketone, and PES is polyether sulfone. FEP is a tetrafluoroethylene-hexafluoropropylene resin.

  The fixing film 10 has a total thickness of 100 μm or less, preferably 40 μm or less and 20 μm or more in order to reduce the heat capacity and improve the quick start property.

(B) Pressure roller 11 is a pressure roller as a pressure member, and is an elastic layer 21 in which a heat-resistant rubber such as silicone rubber is molded on a core metal 20, or a sponge elastic layer formed by foaming silicone rubber. 21 is a rotating body. Here, a heat-resistant release layer made of a fluororesin such as PFA, PTFE, or FEP may be formed on the elastic layer 21.

  The pressure roller 11 is arranged so as to be pressed against a ceramic heater (hereinafter referred to as a heater) 12 as a heating member (heater) by a spring (not shown), and is predetermined by the drive system M in a counterclockwise direction indicated by an arrow in FIG. It is rotationally driven at a peripheral speed of. Due to the frictional force between the pressure roller 11 and the outer surface of the fixing film 10 in the pressure nip portion N due to the rotational driving of the pressure roller 11, a clockwise rotational force in FIG. 2 acts on the fixing film 10. In this way, the recording material P introduced into the pressure nip N is sandwiched between the fixing film 10 and the pressure roller 11 and conveyed.

  The unfixed toner image t formed and supported on the recording material P is heated and pressurized in a pressure nip N formed by the heating unit (fixing film 10 and heater 12) of the fixing device 8 and the pressure roller 11. It is fixed on the recording material P.

(C) Ceramic heater FIG. 3 is a diagram for explaining the ceramic heater (heater 12) of the present embodiment.

  The heater 12 includes a heating element 31 in which a heating paste using silver palladium (AgPd) or the like is printed on a ceramic substrate 30 as a substrate, and a glass coating layer 35 for ensuring protection and insulation of the heating element 31. They are formed sequentially. The heater 12 generates heat when an AC current whose power is controlled by the heater driving circuit 25 as a heater driving unit flows to the heating element 31 on the heater 12.

  A chip thermistor 14 as temperature detecting means is provided on the back surface (surface opposite to the fixing film 10) of the ceramic substrate 30 by adhesion.

  Based on the detection result of the chip thermistor 14, the power control circuit 26 as power control means controls the heater drive circuit 25 with respect to a predetermined target temperature of the heater 12, thereby controlling the power to the heater 12. Is going. In this way, the heater temperature of the heater 12 is maintained at the target temperature (printing temperature).

  Further, in preparation for a case where both the power control circuit 26 and / or the heater drive circuit 25 fail, for example, when the heater 12 is constantly energized at the maximum amount, Is equipped with a thermo switch as a safety element to prevent overheating.

  The thermo switch 33 is provided in an AC circuit for energizing the heater 12, and the thermo switch itself is provided so as to contact the heater 12 (heater body). When the temperature of the heater 12 is excessively high, the thermoswitch 33 is activated to cut off the AC circuit and stop the power supply to the heater 12. Here, in FIG. 3, the thermo switch 33 is shown for a portion that contacts the heater 12.

  Aluminum nitride (AlN) is used for the ceramic substrate 30. In this case, since the heat conduction of the ceramic substrate 30 is good, the heating element 31 is formed on the opposite side of the surface of the ceramic substrate 30 that contacts (slids) the fixing film 10 (so-called back surface heating). The heating element 31 is covered with a glass coating layer 35 for insulation and protection. Reference numeral 34 denotes a conductive pattern made of silver platinum (AgPt). Here, the ceramic substrate 30, the heating element 31, the glass coating layer 35, and the conductive pattern 34 constitute a heater body.

(D) Heating element FIG. 4 is a diagram showing dimensions of the heating element in the vicinity of the thermoswitch contact portion. In the following description, the sheet passing direction (conveying direction) of the recording material is the width direction X, the direction substantially orthogonal to the width direction is the length direction Y, and the length in the width direction X is simply referred to as the width. The length in the direction Y is simply called the length. Here, the length direction Y is the axial direction of the pressure roller 11.

  The width of the heating element 31 other than the portion in contact with the thermo switch 33 (second region) is 2.0 mm, and the contact portion (first region) of the thermo switch 33 is provided in a portion of the heating element 31 where the width is narrowed. ing. Since the resistance value per unit volume of the heat generating element 31 is the same, the resistance value per unit length of the portion where the width of the heat generating element 31 is narrowed becomes high, and the amount of heat generation becomes large when energized.

  In the present embodiment, the portion where the width of the heating element 31 is narrowed includes a narrow region (diaphragm region, high resistance region) 37 and a normal heating element width (non-diaphragm region, second region) as shown in FIG. ) And a boundary region 38 in which the heating element width gradually decreases from 31a toward the aperture region 37. Here, in the boundary region 38, the heating element width gradually decreases from the non-throttle region 31 a toward the narrowed region 37, so that a gradient is formed in the resistance value per unit length in the length direction. The Rukoto.

  The length “a” of the aperture region 37 is 4.0 mm, and the length “b” of the boundary region 38 is 2.0 mm. The length of the thermo switch of the present embodiment is 8.0 mm, which is the same as the combined length of the aperture region 37 and the boundary region 38. Further, since the width of the throttle area 37 is 1.8 mm, the resistance value is 2.0 / 1.8 = 1.11 times that of the non-throttle area 31a, and the heat generation amount is 11% larger.

In the image forming apparatus configured as described above, 500 sheets of LTR size paper having a thickness of 105 g / m ² were continuously printed while controlling the heater so that the thermistor temperature was 220 ° C. at a process speed of 310 mm / s. The image pattern was a halftone image and a practical image that was generally printed.

  FIG. 5 is a diagram showing the dimensions of the heating element near the thermoswitch contact portion in the comparative example.

  As a comparative example, the same printing was performed on the heating element shape of the portion in contact with the thermoswitch as shown in FIG.

  The length of the heating element shape of the comparative example is 8.0 mm as in the present embodiment, the combined length of the throttle region 39 and the boundary region 40, but the length of the boundary region 40 is short, each 0.8 mm. . Further, the width of the throttle region 39 is 1.83 mm in order to make the amount of heat generated in the combined portion of the throttle region 39 and the boundary region 40 equal to that of the present embodiment. The resistance value of the stop area with respect to the non-stop area is 2.0 / 1.83 = 1.09 times, and the heat generation amount is increased by 9%.

  The experimental results are shown below.

  FIG. 6 is a diagram showing the temperature transition of the heating element during continuous printing of 500 sheets.

  In this embodiment and the comparative example, since the heat generation amount of the non-restricted region 31a is the same as that of the heating element 31 near the thermistor 14, the value is almost the same as 220 ° C., which controls the measurement temperature through 100 continuous prints. Show.

On the other hand, the aperture region 37 of this embodiment has a heat generation amount 11% higher than that of the non-aperture region 31a. However, since the thermo switch is deprived of heat at the initial stage of printing, the heating element temperature is almost the same value as the non-aperture region 31a. It has become. However, if the temperature of the thermoswitch increases due to continuous printing, the heat is not taken away by the thermoswitch, so the temperature of the heating element gradually increases.
After zero-sheet continuous printing, the temperature is higher by 10 ° C. or more than the non-aperture area 31a.

  Further, since the boundary region 38 has a heat generation amount intermediate between the constricted region 37 and the non-constricted region 31a, the heating element temperature is also approximately intermediate between the constricted region 37 and the non-constricted region 31a.

  On the other hand, the diaphragm area 39 of the comparative example generates 9% more heat than the non-diaphragm area 31a. In the initial stage of printing, heat is taken away by the thermoswitch as in the embodiment, and therefore the temperature of the heating element is almost the same in the throttle region 39 and the non-throttle region 31a. Further, when the thermo switch does not take heat away by continuous printing, the temperature of the heating element gradually increases, but the amount of heat generation is slightly smaller than that of the embodiment, and is about 10 ° C. higher than that of the non-aperture area 31a.

  Table 1 shows the result of comparison for the images.

  In the halftone image, the reflection density in the vicinity of the aperture portion and the visual conspicuousness (visual feeling) when the image is viewed as a whole were compared. In addition, the visual sensations were also compared in practical images. The reflection density was a reflection densitometer RD-918 manufactured by Macbeth.

  FIG. 7 is a diagram illustrating a state of the 500th halftone image.

  In the image of the present embodiment, the portion corresponding to the aperture region 37 is overfixed and the image is darker. However, since the boundary region 38 exists between the apertureless region 31a and the aperture region 37, the image density gradually increases. Therefore, the density unevenness is not conspicuous as a whole.

  On the other hand, in the comparative example, the boundary region 40 is short, and there is almost no effect of gradually increasing the image density as in the present embodiment. Therefore, the density suddenly changes around the boundary between the non-throttle area 31a and the narrowed area 39, and density unevenness is conspicuous.

  In addition, density unevenness tends to be conspicuous in a uniform image such as a halftone image, but in a practical image pattern generally printed on the market, density unevenness could not be confirmed in the configuration of this example. On the other hand, in the configuration of the comparative example, density unevenness was confirmed although it was less noticeable than the halftone image.

  However, the density unevenness of the image cannot be made inconspicuous if the ratio of the length of the aperture region 37 to the combined length of the aperture region 37 and the boundary region 38 is large. End up.

In this embodiment, when the length of the aperture region 37 is a and the length of the boundary region 38 is b,
a / (a + 2b) = 0.5
It was possible to suppress the density unevenness of the image. When the value of a / (a + 2b) is increased, the density unevenness becomes more conspicuous as described above.
a / (a + 2b) = 0.7
Within this range, the density unevenness in the practical image was not visually noticeable.

However, as explained in the comparative example,
a / (a + 2b) = 0.8
In the case of, the density unevenness of the image can be recognized in the practical image.

From the above results, in order to suppress the density unevenness of the image,
0 ≦ a / (a + 2b) <0.8
It is necessary to satisfy the relationship. Here, when 0 = a / (a + 2b), a = 0, that is, only the boundary region 38 and no aperture region 37.

  Also, in the boundary region 38, if the gradient of the resistance value is larger than 10% / mm, it does not play a role as the boundary region 38, and the difference in density between the diaphragm region 37 and the non-diaphragm region 31a becomes easy to see. Can be considered. On the other hand, if the gradient of the resistance value is smaller than 1% / mm, the amount of heat generated at the contact portion of the thermoswitch cannot be sufficiently increased. Therefore, when printing is performed from when the fixing device 8 is cold, the contact of the thermoswitch There is a concern that poor fixing may occur in the portion.

Accordingly, the gradient (rate of change) g of the resistance value in the boundary region 38 is
1% / mm ≦ g ≦ 10% / mm
Is desirable.

  In addition, when the aperture region 37 is outside the contact portion of the thermo switch, heat is not taken away by the thermo switch even when the fixing device 8 is cold. May occur. On the other hand, when the thermoswitch contact portion is in contact with a portion other than the aperture region 37 and the boundary region 38, if the printing is performed when the fixing device 8 is cold, the heat is taken away by the thermoswitch, so that the fixing is performed. Defects may occur.

Therefore, when the length of the region (third region) in contact with the heater 12 in the thermo switch 33 is c,
a ≦ c ≦ a + 2b
In the longitudinal direction, a is preferably inside c, and c is preferably inside a + 2b. That is, in the length direction, the throttle region 37 is inside the region of the thermo switch 33 that contacts the heater 12, and the region of the thermo switch 33 that contacts the heater 12 is a region that combines the throttle region 37 and the boundary region 38. It is desirable to be provided so as to be located inside. In this embodiment, a + 2b can be made larger than that in the comparative example, so that the mounting accuracy of the thermo switch to the heater 12 can be relaxed.

  As described above, in the heater 12 in which the amount of heat generated in the portion of the heating element 31 in contact with the thermo switch 33 is increased, the resistance value of the heating element gradually increases from the end position of the thermo switch to the center position. A boundary region 38 is provided.

  As a result, in the region where the thermo switch 33 of the heater 12 is in contact, even when the fixing device 8 is warmed and the thermo switch 33 does not take heat away, the temperature change in the length direction is gentle, so the density due to overfixing Unevenness can be made inconspicuous. That is, it is possible to suppress the occurrence of image unevenness that is likely to occur when the fixing device 8 is sufficiently warm.

  Further, the thermo switch 33 may be brought into contact with the combined area of the aperture region 37 and the boundary region 38 of the heating element 31, and the amount of heat generated in the boundary region 38 gradually changes. The mounting position accuracy of 33 can be relaxed.

  In the present embodiment, the thermo switch 33 has been described as a safety element. However, the present invention is not limited to this, and for example, the same effect can be obtained with a thermal fuse.

  The second embodiment of the present invention will be described below. In the present embodiment, the overall configuration of the image forming apparatus is the same as that shown in the first embodiment. Therefore, different components from the first embodiment will be described, and the same components as in the first embodiment. The description of is omitted.

  In this embodiment, the heater 12 has two heating elements, and the heating value of the heating element is increased at one thermoswitch contact position.

  FIG. 8 is an enlarged view of the thermoswitch contact portion of the heater 12 of this embodiment. FIG. 9 is a diagram for explaining the heater 12 of this embodiment.

  In the present embodiment, a heating element A 41 having the same amount of heat generation as the non-contacting position at the position in contact with the thermo switch 33 is referred to as a heating element A 41. Further, the heating element B 42 has a width that is narrow at a position in contact with the thermo switch 33 and that has a sufficient boundary region like the heating element 31 of the first embodiment.

  Further, as shown in FIG. 9, the heating element A 41 and the heating element B 42 can be energized independently by the power control circuit 26 as power control means. Then, the power control circuit 26 can control the amount of heat generated at the contact position of the thermo switch 33 by changing the energization ratio (making the input power ratio variable).

  In this embodiment, the energization ratio of the heating element B 42 to the heating element A 41 is set as shown in Table 2 based on the temperature information of the thermistor 14 immediately before the printing operation and the continuous print number information. That is, the higher the thermistor temperature just before printing, the more the thermoswitch is warmed, and the amount of heat generated at the thermoswitch position is reduced. Further, it is assumed that the thermo switch is warmed as the number of continuous prints increases, and the amount of heat generated at the thermo switch position is reduced.

As described above, when the heater 12 has two heating elements, the same effect as that of the first embodiment can be obtained by providing the aperture region and the boundary region at the contact position of the thermoswitch only on one heating element. Can do. Further, by estimating the temperature of the thermo switch from the thermistor temperature and the number of printed sheets information and controlling the energization ratio to each heating element, it is possible to suppress the occurrence of density unevenness in the image at the thermo switch contact position.

In addition, although the present Example demonstrated the case where the heater 12 had two heat generating bodies, it is not restricted to this, The heater 12 may have a several heat generating body. In such a case, the following relationship is established. In the heater body, the sum of the resistance values per unit length of the areas of the plurality of heating elements corresponding to the area where the thermo switch contacts is the unit length of the areas of the plurality of heating elements corresponding to the area where the thermo switch does not contact What is necessary is just to be provided so that it may become larger than the sum of resistance value per hit. And at least 1 should just be a heat generating body like the heat generating body B42 mentioned above among several heat generating bodies. Further, regarding the gradient (rate of change) of the resistance value of the boundary region in this case, when the change rate of the sum of the resistance values of the plurality of heating elements in the boundary region is g,
1% / mm ≦ g ≦ 10% / mm
As long as the relationship is satisfied.

  Further, when the heater 12 has a plurality of heating elements, each heating element has a different amount of heat generation at the contact position of the thermo switch, and a plurality of heating elements according to the warming condition of the thermo switch. It is good to change the energization ratio. This further suppresses uneven density in the image.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to a first embodiment. FIG. 3 is a cross-sectional view illustrating a schematic configuration of a heat fixing apparatus according to the first exemplary embodiment. FIG. 3 is a diagram illustrating a heater in Embodiment 1. FIG. 3 is a diagram for explaining the size and shape of a heating element in Example 1; The figure explaining the dimension and shape of the heat generating body in a comparative example. The figure showing the heater heater temperature transition of Example 1 and a comparative example. The figure showing the image printed by Example 1 and a comparative example. FIG. 6 is a diagram illustrating dimensions and shapes of a heating element in Example 2. FIG. 5 is a diagram illustrating a heater in Embodiment 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fixing film 11 Pressure roller 12 Heater 30 Ceramic substrate 31 Heating element 31a No-diaphragm area 33 Thermo switch 37 Diaphragm area 38 Boundary area N Pressure nip part

Claims (8)

A heater that configures a nip portion together with a pressure roller via a flexible sleeve and heats the recording material on which the developer image is formed at the nip portion,
A heater body having a heating element on the substrate that generates heat when energized;
A region provided in the heater body, a region where a safety element that cuts off the power supply to the heating element at the time of overheating of the heater body contacts,
With
The resistance value per unit length of the first region of the heating element corresponding to the region where the safety element contacts in the heater body is the second region of the heating element corresponding to the region where the safety element does not contact. In the heater provided to be larger than the resistance value per unit length,
Of the heating element, the resistance per unit length in the axial direction of the pressure roller in the boundary region between the high resistance region showing the highest resistance value in the first region and the second region The value is sloped,
When the axial length of the high resistance region is a, and the axial length of the boundary region is b,
0 ≦ a / (a + 2b) <0.8
A heater characterized by satisfying the relationship. When the change rate of the resistance value of the boundary region is g,
1% / mm ≦ g ≦ 10% / mm
The heater according to claim 1, wherein the relationship is satisfied. A heater that configures a nip portion together with a pressure roller via a flexible sleeve and heats the recording material on which the developer image is formed at the nip portion,
A heater body having a plurality of heating elements that generate heat when energized on the substrate;
A region provided in the heater body, a region where a safety element that cuts off the power supply to the heating element at the time of overheating of the heater body contacts,
With
The sum of the resistance values per unit length of the first region of the plurality of heating elements corresponding to the region where the safety element contacts in the heater body is the plurality of the heat generation corresponding to the region where the safety element does not contact. In the heater provided to be larger than the sum of the resistance values per unit length of the second region of the body,
At least one heating element among the plurality of heating elements has a resistance value per unit length of the first area larger than a resistance value per unit length of the second area,
Of the plurality of heating elements, per unit length in the axial direction of the pressure roller in the boundary region between the high resistance region showing the highest resistance value of the first region and the second region. A slope is provided for the sum of resistance values of
When the axial length of the high resistance region is a, and the axial length of the boundary region is b,
a / (a + 2b) <0.8
A heater characterized by satisfying the relationship. When the change rate of the sum of the resistance values of the plurality of heating elements in the boundary region is g,
1% / mm ≦ g ≦ 10% / mm
The heater according to claim 3, wherein the relationship is satisfied. The heater according to any one of claims 1 to 4,
A flexible sleeve that slides on the heater;
A pressure roller that forms a nip portion with the heater via the flexible sleeve;
With
An image heating apparatus, wherein a recording material on which a developer image is formed is heated at the nip portion. When the axial length of the third region of the safety element that contacts the heater body is c,
a ≦ c ≦ (a + 2b)
And satisfy the relationship
The safety element is provided in the axial direction so that the third region includes the high resistance region and is located inside a region including the first region and the boundary region. The image heating apparatus according to claim 5. Image forming means for forming a developer image on the recording material;
An image heating apparatus according to claim 5 or 6,
With
An image forming apparatus, wherein the developer image formed on the recording material by the image forming means is heated by the image heating device.   The image forming apparatus according to claim 7, further comprising a power control unit that changes a ratio of energization to the plurality of heating elements.

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