JP4208587B2 - Fixing device - Google Patents

Description

【００７３】
表１に示すように、本実施例の加熱装置は従来例に比べて大幅に非通紙部温度を下げることができた。
【００７４】
次にＢ５サイズ（小サイズ紙）で坪量が１５７ｇ／ｍ²の厚紙を強制的に重送させて通紙し、何枚重送させると加熱装置の劣化・破損に至るかを試験した。定着温度・入力電圧・プロセススピードはハガキを通紙したときと同条件とした。試験結果を表２に示す。
【００７５】
【表２】

【００７６】
表２に示すように、従来例の加熱装置の場合は、４または５重送で加熱体の破損に至り、ステー・フィルム・加圧ローラ表層の非通紙部に劣化が認められた。一方、本実施例の加熱装置の場合は、２回とも１０重送まで重送枚数を増やしていったが、加熱体は破損せず、ステー・フィルム・加圧ローラにも劣化は認められなかった。
【００７７】
以上説明した通り、ＮＴＣ特性を有するグラファイトを抵抗発熱体に含有させることで、ＮＴＣ特性を有する抵抗発熱体を実現でき、非通紙部昇温を防止することができる。
【００７８】
なお、本実施例では記録材を中央基準で通紙する場合について述べたが、本実施例は記録材を端部基準で通紙する画像形成装置にも適用可能である。
【００７９】
［実施例２］
本実施例では、実施例１で抵抗発熱体の抵抗を調整するために含有させた銀の代わりにパラジウムを抵抗発熱体に含有させる。抵抗発熱体の材料以外の加熱装置及び画像形成装置の構成は実施例１と同じである。
【００８０】
本実施例における加熱体３の抵抗発熱体６は、グラファイト・パラジウム・ガラス粉末（無機結着剤）・有機結着剤を混練して調合したペーストをスクリーン印刷により、加熱体基板７上に幅６．５ｍｍ・長さ２２０ｍｍ・厚さ約１００μｍの線帯状に形成して得たものである。抵抗発熱体６の常温のシート抵抗は実施例１と同じく約９Ω／ｓｑ（厚さ１０μｍ）のものを用い、常温の総抵抗も実施例１と同じく３０Ωとした。
【００８１】
図６に本実施例及び従来例の抵抗発熱体の抵抗温度特性の概略図を示す。横軸は抵抗発熱体の温度を、縦軸は抵抗値を示す。細線は従来例の抵抗発熱体（１４）の抵抗温度特性を示し（図４と同じ）、太線は本実施例の抵抗発熱体６の抵抗温度特性を示す。
【００８２】
本実施例における抵抗発熱体７も実施例１と同じくＮＴＣ特性を示し、抵抗変化率は−４６０ｐｐｍ／℃程度である。よって、本実施例の抵抗発熱体の２００℃における総抵抗は２７．５９Ω、３００℃における総抵抗は２６．２１Ωであり、実施例１の値よりも低い。
【００８３】
パラジウムの抵抗変化率は３０００ｐｐｍ／℃程度であり銀よりも小さい。また、シート抵抗も銀の７倍程度であるので、抵抗発熱体中の割合も銀よりも少なくできる。この２つの効果により、抵抗発熱体全体の抵抗変化率を実施例１よりも小さくでき、非通紙部昇温防止の効果も実施例１よりも大きくなる。
【００８４】
以下に本実施例の加熱装置と従来例の加熱装置との比較を示す。以下の比較は実施例１で述べた比較と同条件で行った。従来例は実施例１で述べた構成と同じであり、結果も同じものである。まず、加熱装置が十分室温（２５℃）になじんだ状態からハガキサイズの記録材を連続で１００枚通紙したときの、非通紙部の最高温度（加熱体裏面を熱電対で測定）を比較した。定着温度・入力電圧・プロセススピードは実施例１と同条件とした。結果を表３に示す。
【００８５】
【表３】

【００８６】
表３に示すように、本実施例は従来例に比べて大幅に温度が下がっているのはもちろん、実施例１の結果（２８８℃）よりも低い値となった。
【００８７】
次にＢ５サイズで坪量が１５７ｇ／ｍ²の厚紙を強制的に重送させて通紙し、何枚重送させると加熱装置の劣化・破損に至るかを試験した。定着温度・入力電圧・プロセススピードは同条件である。試験結果を表４に示す。
【００８８】
【表４】

【００８９】
表４に示すように、本実施例の加熱装置は、２回とも１０重送まで重送枚数を増やしていったが、加熱体の破損及びステー・フィルム・加圧ローラの劣化はなく、実施例１と同じ結果となった。
【００９０】
以上の結果より、抵抗を調整する材料としてパラジウムを用いることで、更に非通紙部昇温防止に有効なＮＴＣ特性を有する抵抗発熱体が実現できることが分かる。
【００９１】
なお、本実施例では記録材を中央基準で通紙する場合について述べたが、本実施例は記録材を端部基準で通紙する画像形成装置にも適用可能である。
【００９５】
【発明の効果】
以上説明したように本発明によれば、フィルム加熱方式の定着装置について、グラファイトの負の抵抗温度特性を利用することによって、装置のスペックを低下させることなくかつ低コストな構成で非通紙部昇温を効果的に抑えることができ、より安全で製品寿命が長く画像も良好なものを提供することが可能になる。
【図面の簡単な説明】
【図１】 実施例１における画像形成装置の要部の概略構成模型図
【図２】 実施例１における加熱装置（画像加熱定着装置）の要部の概略構成模型図
【図３】 加熱体の構成模型図
【図４】 実施例１と従来例とにおける加熱体の抵抗発熱体の抵抗温度特性の概略図
【図５】 抵抗発熱体のモデル図
【図６】 実施例２と従来例とにおける加熱体の抵抗発熱体の抵抗温度特性の概略図
【符号の説明】
１．ステー ２．定着フィルム ３．ヒータ（加熱体） ４．加圧ローラ（加圧体） ４ａ．芯金 ４ｂ．弾性体層 ４ｃ．離形層 ５．検温素子 ６．実施例１の抵抗発熱体 ７．基板 ８．オーバーコート層 ９．１０．給電用電極 １１．ＣＰＵ １２．トライアック １３．ＡＣ電源 １４．従来例の抵抗発熱体 Ｎ．ニップ部 Ｐ．記録材 Ｔ．トナー ａ．記録材搬送方向[0001]
BACKGROUND OF THE INVENTION
  The present invention, for example, in an image forming apparatus such as a copying machine or a laser beam printer, heats a recording material on which an unfixed toner image is formed and supported,SolidifyFix as a ringtone imageFixing deviceAbout.
[0003]
[Prior art]
2. Description of the Related Art Conventionally, in a recording material heating apparatus for image heating and fixing, for example, a material to be heated includes a heating roller maintained at a predetermined temperature and a pressure roller pressed against the heating roller through an elastic layer. In many cases, a heat roller method is used in which the recording material is heated while being nipped and conveyed. In addition, various systems and configurations such as a flash heating system, an open heating system, and a hot plate heating system are known and put into practical use.
[0004]
Recently, instead of such a method, a heating body (heater), a heating body support (stay), a heat-resistant film (fixing film) conveyed while being opposed to the heating body, and a fixing film are provided. It has a pressure body (pressure roller) that attaches the recording material as the material to be heated to the heating body, and forms and supports the recording material surface by applying the heat of the heating body to the recording material through the fixing film. An image heating and fixing method (film heating method heating device) has been devised (for example, see Patent Documents 1 to 4).
[0005]
In such a film heating type heating apparatus or image heating and fixing apparatus, a low heat capacity heating body can be used as the heating body. For this reason, it is possible to save power and shorten the wait time (quick start) as compared with conventional devices such as a heat roller method and a belt heating method.
[0006]
In the above-mentioned film heating type heating apparatus, when a recording material (hereinafter referred to as small size paper) having a width that is somewhat smaller than the maximum size that can be passed (hereinafter referred to as large size paper) is passed, Alternatively, the temperature control of the heated material heating part is often performed based on the output of the temperature measuring element provided in the paper passing part, and the non-paper passing part does not take heat away from the recording material. The temperature rises compared to the paper passing part (non-paper passing part temperature rise).
[0007]
In addition, especially when small recording paper and thick recording materials (thick paper, envelopes, etc.) are multi-fed and passed, a large amount of heat is taken away by the recording material at the paper passing portion. A large amount of electric power is supplied to the non-sheet-passing portion and the temperature rises significantly. Therefore, when the number of double feeds is large, there is a risk that the heating body, the pressure roller and the like are deteriorated or broken. Further, when the temperature rise at the non-sheet passing portion is large, there is a possibility that a high temperature offset may occur at the end when the large size paper is passed immediately after the small size paper is passed.
[0008]
In order to prevent the temperature rise of the non-sheet passing portion, when small-size paper is continuously fed, the throughput is lowered to protect the heating body, the pressure roller, etc. of the non-sheet passing portion (for example, patent Reference 5).
[Patent Document 1]
JP-A-63-313182
[Patent Document 2]
Japanese Patent Laid-Open No. 2-157878
[Patent Document 3]
JP-A-4-44075
[Patent Document 4]
JP-A-4-204980
[Patent Document 5]
Japanese Patent No. 2727899
[0009]
[Problems to be solved by the invention]
However, reducing the throughput reduces the specifications of the image forming apparatus, and providing a separate member increases the cost.
[0010]
  Therefore, the present invention is a film heating method.Fixing deviceIt is an object of the present invention to prevent the above-described temperature rise of the non-sheet passing portion without lowering the specifications of the apparatus with a low cost and simple configuration.
[0011]
[Means for Solving the Problems]
  The present invention is characterized by the following configuration.Fixing deviceIt is.
[0012]
  A heater having a ceramic substrate and a resistance heating element provided along the longitudinal direction of the substrate, a film that moves in a direction perpendicular to the longitudinal direction while being in contact with the heater, and a parallel arrangement with the heater And a pressure roller that forms a fixing nip portion together with the heater through the film, and heats the resistance heating element by energizing the resistance heating element to generate a recording material at the fixing nip portion. In a fixing device that heats and fixes a toner image on a recording material to the recording material while nipping and conveying it,
  The resistance heating element is obtained by baking a paste obtained by mixing graphite, silver or palladium, and a binder.The graphite content is higher than silver or palladium,A fixing device having a negative resistance change rate from 25 ° C. to 300 ° C.
[0027]
In other words, graphite has NTC characteristics (Negative Temperature Coefficient: negative resistance temperature characteristics where resistance decreases as temperature rises) below that temperature, and PTC characteristics (Positive Temperature Coefficient: temperature rises above that temperature). And a positive resistance temperature characteristic in which the resistance increases), and the inflection point temperature is about 700 ° C. In the present invention, by including this graphite in the resistance heating element, the resistance heating element is provided with NTC characteristics below the inflection point temperature, and the heating value of the resistance heating element corresponding to the non-sheet passing portion is suppressed by the NTC characteristics. Thus, prevention of the temperature rise of the non-sheet passing portion was achieved with a low cost and simple configuration without lowering the specifications of the apparatus.
[0028]
  Consists of the above configurationFixing deviceCan prevent the temperature rise in the non-sheet-passing area, and it is safer, has a long product life, and has a good image.Fixing deviceIt becomes possible to provide.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
[Example 1]
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
[0030]
(1) Example of image forming apparatus
FIG. 1 is a schematic configuration model diagram of a main part of an image forming apparatus according to the present embodiment. This image forming apparatus is a laser beam printer using a transfer type electrophotographic process.
[0031]
101 is an organic photosensitive drum as an image carrier, 102 is a charging roller as a charging member, 103 is a laser exposure device, 104 is a developing device including a developing sleeve and a developing blade, and one-component magnetic toner, 105 is a cleaning blade, and 106 is A transfer roller 107 is a heat fixing device. In the image forming apparatus of this embodiment, the maximum sheet passing width is set to A4 size (paper width: 210 mm). In addition, the paper is passed by central reference conveyance.
[0032]
The organic photosensitive drum 101 is rotationally driven at a predetermined peripheral speed and is uniformly charged to a negative predetermined potential in this example by the charging roller 102. Then, scanning exposure of the image information by the laser beam from the laser exposure device 103 is performed on the uniformly charged surface of the organic photosensitive drum 101, and an electrostatic latent image corresponding to the scanning exposure pattern is formed on the organic photosensitive drum 101. .
[0033]
Next, the negative toner charged in the developing device 104 adheres to the exposed bright portion of the electrostatic latent image on the organic photosensitive drum 101, and the electrostatic latent image becomes a visible image as a toner image (reversal development).
[0034]
On the other hand, the sheet feeding roller 108 is driven to rotate at a predetermined sheet feeding control timing, and a sheet of recording material P such as paper is separated and fed from the sheet feeding cassette 109, and includes a transport roller 110, a registration roller 111, and the like. The toner image on the organic photosensitive drum 101 is sequentially applied to the surface of the recording material P through a path 112 and introduced into a transfer nip portion that is a contact portion between the organic photosensitive drum 101 and the transfer roller 106. Transcribed.
[0035]
The recording material P exiting the transfer nip is separated from the surface of the organic photosensitive drum 101, introduced into the image heating and fixing device 107 through the sheet path 113, and subjected to a toner image heating and fixing process. Are discharged onto the paper discharge tray 115.
[0036]
Further, the surface of the organic photosensitive drum 101 after separation of the recording material is cleaned by removing the transfer residual toner by the cleaning blade 105, and is repeatedly used for image formation.
[0037]
(2) Image heating and fixing device 107
FIG. 2 is a schematic configuration model diagram of a main part of an image heating fixing device (hereinafter, referred to as a fixing device) 107 in this embodiment. The fixing device 107 is a tensionless type film heating type heating device disclosed in Japanese Patent Application Laid-Open Nos. 4-44075 to 44083, Japanese Patent Application Laid-Open No. 4-20420 to 204984, and the like.
[0038]
This tensionless type film heating type fixing device (heating device) uses an endless belt-shaped or cylindrical heat-resistant film, and at least part of the circumference of the film is always tension-free (no tension is applied). The film is a device that is rotationally driven by the rotational driving force of the pressure member.
[0039]
Reference numeral 1 denotes a stay, which is a heat resistant and rigid member as a heating body holding member and a film guide member. Reference numeral 3 denotes a ceramic heater as a heating body, which is disposed and held on the lower surface of the stay 1 along the length of the stay. Reference numeral 2 denotes an endless (cylindrical) heat-resistant film that is externally fitted to a stay 1 that is a film guide member including a heating element 3. The inner peripheral length of the endless heat-resistant film 2 and the outer peripheral length of the stay 1 including the heating element 3 are about 3 mm larger than that of the film 2, so that the film 2 is externally fitted with a margin in the peripheral length. ing. In this example, the outer diameter of the film 2 was 18 mm.
[0040]
The stay 1 can be composed of a high heat resistant resin such as polyimide, polyamideimide, PEEK, PPS, or liquid crystal polymer, or a composite material of these resins with ceramics, metal, glass, or the like. In this example, a liquid crystal polymer was used.
[0041]
Film 2 has a film thickness of 100 μm or less, preferably 50 μm or less and 20 μm or more, heat resistant single layer film such as PTFE, PFA, FEP, polyimide, polyamide, etc. in order to reduce heat capacity and improve quick start properties A composite layer film in which PTFE, PFA, FEP or the like is coated on the outer peripheral surface of a film such as imide, PEEK, PES, or PPS can be used. In this example, a polyimide film having a film thickness of about 50 μm coated with PTFE on the outer peripheral surface was used.
[0042]
  4 isArranged parallel to the heating element 3Film 2 between heating element 3With the heating element 3A pressure roller as a film outer surface contact driving unit that forms a pressure nip portion (fixing nip portion) N and rotates the film 2. The pressure roller 4 is composed of a cored bar 4a, an elastic layer 4b, and an outermost release layer 4c. The film 2 is sandwiched by a bearing means / biasing means (not shown) with a predetermined pressing force. It is disposed in pressure contact with the surface. In this embodiment, the cored bar 4a is an aluminum cored bar, the elastic body layer 4b is coated with silicone rubber, and the release layer 4c is coated with PFA. The outer diameter of the pressure roller 4 was 20 mm, and the thickness of the elastic layer 4b was 3 mm.
[0043]
  The pressure roller 4 is rotationally driven by the drive system M in a clockwise direction indicated by an arrow at a predetermined peripheral speed. By the rotational driving of the pressure roller 4, a rotational force acts on the film 2 by the frictional force between the pressure roller and the film outer surface at the pressure nip portion N. The inner surface of the film 2 is heated at the fixing nip portion N. 3, while being in close contact with the surface of 3, the outer periphery of the stay 1 is driven counterclockwise in the counterclockwise direction indicated by the arrow at a rotational speed substantially equal to the rotational speed of the pressure roller 4.That is, the film 2 moves in a direction orthogonal to the longitudinal direction of the heating body 3 while being in contact with the heating body 3.
[0044]
The heating element 3 is a ceramic heater having a resistance heating element including graphite, as will be described in detail in the next section (3). The heating element 3 generates heat by feeding power to the resistance heating element from the power supply circuit units 12 and 13. The temperature rises quickly. The temperature rise of the heating body 3 is detected by the temperature detecting element 5, and the detected temperature information is input to the control circuit unit (CPU) 11. The control circuit unit 11 controls power supply from the power supply circuit units 12 and 13 to the resistance heating element of the heating body 3 so that the heating body temperature detected by the temperature measuring element 5 is maintained at a predetermined temperature (fixing temperature). The temperature of the heating element 3 is adjusted.
[0045]
  Then, the heating body 3 is formed by the heating body 3 and the pressure roller 4 with the film 2 sandwiched in a state where the temperature of the heating body 3 rises to a predetermined level and the rotational peripheral speed of the film 2 is stabilized by the rotation of the pressure roller 4. A recording material P to be image-fixed as a material to be heated is introduced from an image forming portion (transfer portion) into the pressure nip portion N. Then, when the recording material P is nipped and conveyed together with the film 2 through the press nip N, the heat of the heating body 3 is applied to the recording material P via the film 2 and recording is performed.LumberThe unfixed visible image (toner image) T is heat-fixed on the recording material P surface. The recording material P that has passed through the pressure nip N is separated from the surface of the film 2 and conveyed.
[0046]
(3) Heating body 3
FIG. 3A is a partially cutaway plan model diagram on the front side of the heating element 3, a plan model diagram on the back side, and a block circuit diagram of the energization control system. (B) is an enlarged cross-sectional model view of the heating element 3.
[0047]
The heating element 3 of the present embodiment is a heating element having a low heat capacity as a whole, which is composed of the following elements (1) to (5).
[0048]
(1). An elongated heat-resistant / insulating / good thermal conductive substrate 7 whose longitudinal direction is the direction perpendicular to the conveying direction a of the recording material P as a material to be heated.
(2). A resistance heating element 6 formed along the length of the substrate at the center in the lateral direction on the surface side (film sliding surface side) of the substrate 7
(3). The first and second power feeding electrodes 9 and 10 are electrically connected to both ends of the resistance heating element 6 and formed on the surface portions on both ends of the substrate.
(4). The first and second power supply electrodes 9 and 10 are exposed, and the resistance heating element 6 is covered and formed on the surface of the substrate. Coat layer 8
(5). On the back surface side (non-film sliding surface side) of the substrate 7, a thermistor temperature sensing element 5 disposed in contact with the substrate within a substantially central portion (minimum sheet passing width) in the longitudinal direction of the substrate; Conductive path patterns 14 and 14 formed as lead wires.
[0049]
  The heating substrate 7 is a ceramic material such as alumina or aluminum nitride, for example.(Ceramic substrate)In this embodiment, an alumina substrate having a width of 10 mm, a length of 270 mm, and a thickness of 1 mm is used.
  The resistance heating element 6 of the present example has a width of 6.5 mm on the heating element substrate 7 by screen printing a paste prepared by kneading graphite, silver, glass powder (inorganic binder), and organic binder. It is obtained by forming a wire strip having a length of 220 mm and a thickness of about 100 μm. In this embodiment, the resistance heating element 6 has a sheet resistance at room temperature of about 9Ω / sq (thickness 10 μm), and the resistance heating element 6 has a total resistance at room temperature of 30Ω.
[0050]
The first and second power feeding electrodes 9 and 10 used silver palladium screen printing patterns.
[0051]
The overcoat layer 8 is mainly intended to ensure electrical insulation between the resistance heating element 6 and the surface of the heating element 3 and slidability of the film 2. In this example, a heat-resistant glass layer having a thickness of about 50 μm was used as the overcoat layer 8.
[0052]
The temperature measuring element 5 uses a chip thermistor in this embodiment, and is adhered to the back surface of the heating body substrate 7 with an adhesive having excellent heat resistance, conductivity, and thermal conductivity. As the conductive path patterns 14 and 14 formed and provided as lead wires of the temperature measuring element 5, a screen printing pattern of silver palladium was used.
[0053]
The heating body 3 is fixedly disposed by exposing the surface (film sliding surface) side on which the resistance heating element 6 is formed and holding the heating body 3 on the lower surface side of the stay 1.
[0054]
By adopting the above configuration, the entire heating element 3 can be made to have a lower heat capacity compared to the heat roller system, and a quick start becomes possible.
[0055]
The heating element 3 rises in temperature when the resistance heating element 6 generates heat over the entire length by feeding power from the AC power supply 13 to the first and second feeding electrodes 9 and 10 at both longitudinal ends of the resistance heating element 6.
[0056]
The temperature measuring element 5 is electrically connected to the control circuit unit 11 through the conductive path patterns 14 and 14, and the temperature rise of the heating element 3 is detected by the temperature detecting element 5, and the output of the temperature detecting element 5 is A / D converted to the control circuit unit. 11, and the electric power supplied from the AC power supply 13 to the resistance heating element 7 by the triac 12 based on the information is controlled by phase, wave number control, etc., and the temperature of the heating element 3 is controlled.
[0057]
In other words, the heating body 3 is kept at a constant temperature during fixing by controlling energization so that the heating body 3 is heated when the temperature detected by the temperature detecting element 5 is lower than a predetermined set temperature, and is lowered when the temperature is higher than the set temperature. Kept. In this embodiment, the output is changed in 21 steps from 5 to 100% from 0 to 100% by phase control. An output of 100% indicates an output when the heater is fully energized.
[0058]
(4) Resistance heating element containing graphite
Graphite has NTC characteristics at a certain temperature or lower, that is, negative resistance temperature characteristics where the resistance decreases as the temperature rises, and PTC characteristics, that is, positive resistance temperature characteristics where the resistance increases as the temperature rises above that temperature. The inflection point temperature is about 700 ° C.
[0059]
In the present invention, by including this graphite in the resistance heating element, the resistance heating element is provided with NTC characteristics below the inflection point temperature, and the heating value of the resistance heating element corresponding to the non-sheet passing portion is suppressed by the NTC characteristics. Thus, prevention of the temperature rise of the non-sheet passing portion was achieved with a low cost and simple configuration without lowering the specifications of the apparatus.
[0060]
As described above, the resistance heating element 6 in the heating element 3 of the present embodiment is obtained by screen-printing a paste prepared by kneading graphite, silver, glass powder (inorganic binder), and organic binder by screen printing. It is obtained by forming a linear band having a width of 6.5 mm, a length of 220 mm, and a thickness of about 100 μm on the substrate 7. In this embodiment, the resistance heating element 6 has a sheet resistance at room temperature of about 9Ω / sq (thickness 10 μm), and the resistance heating element 6 has a total resistance at room temperature of 30Ω.
[0061]
Since the surface oxidation of graphite starts at about 700 ° C., the firing temperature of the resistance heating element 6 after screen printing was set to about 600 ° C. Moreover, the glass which can be baked at 400-500 degreeC as the material of the glass coat layer (overcoat layer) 8 was selected.
[0062]
As a heating element of a conventional example (comparative example), a paste obtained by mixing silver palladium and glass is formed on a heating element substrate 7 by screen printing into a line band shape having a width of 6.5 mm, a length of 220 mm, and a thickness of about 10 μm. A resistance heating element (14) was obtained. Since this conventional resistance heating element (14) has a sheet resistance at room temperature of about 1 Ω / sq (thickness 10 μm), the thickness is thinner than the resistance heating element 7 of this embodiment. The total resistance was 30Ω at room temperature, as in this example. The heating substrate, glass coat, etc. other than the resistance heating element (14) are the same as in this embodiment.
[0063]
FIG. 4 shows a schematic diagram of resistance temperature characteristics of the resistance heating elements 7 and (14) of this example and the conventional example. The horizontal axis represents the temperature of the resistance heating element, and the vertical axis represents the resistance value. The thin line shows the resistance temperature characteristic of the resistance heating element (14) of the conventional example, and the thick line shows the resistance temperature characteristic of the resistance heating element 7 of this example. As shown in FIG. 4, the conventional example shows a positive resistance temperature characteristic (PTC characteristic) in which the resistance increases as the temperature rises. In this example, the negative resistance temperature characteristic (NTC characteristic) in which the resistance decreases as the temperature rises. Indicates. FIG. 4 shows a temperature region where the inflection point temperature of graphite is 700 ° C. or lower.
[0064]
The resistance change rate of graphite used in this example is about −2000 ppm / ° C. (resistance change rate from 25 ° C. to 300 ° C., and the following values are also the same), and the resistance change rate of silver is about 4000 ppm / ° C. (PTC characteristics). Since the ratio of graphite and silver in this example was about 7: 3, the resistance heating element as a whole showed a resistance change rate of about -240 ppm / ° C. The reason why the resistance heating element 6 contains silver in this embodiment will be described later. The resistance change rate of the conventional example is about 1000 ppm / ° C.
[0065]
When the image forming apparatus is on standby, the temperature of the heating element 3 is close to room temperature, so the resistance of the resistance heating element 7 (14) of the heating element of this embodiment and the conventional example is about 30Ω. When the image forming apparatus receives the print signal, energization is started to the resistance heating element 7 (14) of the heating element 3 and the temperature of the heating element 3 rises to near the fixing temperature. Therefore, in the conventional example, the resistance heating element (14) In this embodiment, the resistance of the resistance heating element 7 is reduced. When the fixing temperature is 200 ° C., the total resistance of the resistance heating element is 35.25Ω in the conventional example, and 28.74Ω in this embodiment. If the temperature of the resistance heating element reaches 300 ° C., the total resistance is 38.25Ω in the conventional example and 28.02Ω in the present example. When passing large size paper (A4 vertical feed: paper width 210 mm), the temperature is almost the same at the center and end of the resistance heating element, so the resistance at the center and end is also the same. When small-size paper is passed, the temperature of the end portion becomes higher than that of the central portion due to the temperature rise of the non-sheet passing portion described above. Therefore, in the conventional example, the resistance of the end portion is higher than that of the central portion, and in this embodiment, the resistance of the end portion is lower than that of the central portion.
[0066]
FIG. 5 is a model diagram of a resistance heating element. When the current flowing through the resistance heating element is I, the resistance at the center is R1, and the resistance at the end is R2, the heating value W1 at the center is I²・ It is R1 and the calorific value W2 at the end is I²R2 (for the sake of simplicity, the region where the resistance is R1 and the length of the region where the resistance is R2 are considered to be the same).
[0067]
Considering the case of passing small-size paper in the conventional example, since R2> R1, the calorific value W2 at the end is larger than the calorific value W1 at the center. When the heat generation amount increases, the temperature rises, so that the resistance further increases, and the circulation of increasing the heat generation amount is repeated.
[0068]
On the other hand, considering the case where small-size paper is passed in this embodiment, since R2 <R1, the heating value W2 at the end is smaller than the heating value W1 at the center. In both the conventional example and the present embodiment, the paper is passed by center reference conveyance, and the temperature detecting element 5 is in the center of the heating body within the minimum paper passing width, and both perform temperature control so that the temperature in the center is constant. Therefore, the calorific value in the central part of the conventional example is substantially the same as the calorific value in the central part of this embodiment. Therefore, the amount of heat generated at the end of this embodiment is smaller than the amount of heat generated at the end of the conventional example, and the temperature of the end of this embodiment can be kept lower than that of the conventional example.
[0069]
Conventionally, a mixture of metal and glass has been used as a resistance heating element, and only those having a PTC characteristic as a whole resistance heating element have been put into practical use due to the PTC characteristic that a metal generally has. In the present invention, by adding graphite having NTC characteristics to the resistance heating element, a resistance heating element having NTC characteristics can be realized, and temperature rise of the non-sheet passing portion can be prevented.
[0070]
In a resistance heating element having NTC characteristics, it is considered that the smaller the resistance change rate, the larger the amount of decrease in resistance at the end portion and the larger the amount of decrease in heating value. The smaller the value of, the smaller the non-sheet passing portion temperature rise). In this embodiment, silver having PTC characteristics is contained in the resistance heating element, but this is intended to reduce the total resistance because of the large sheet resistance of graphite. From the standpoint of preventing the temperature rise at the non-sheet passing portion, it is preferable that silver is not contained because the rate of change in resistance is small, and this is a desirable configuration. If the total resistance is too high, it is not possible to obtain the power necessary for starting up the heating element and maintaining the temperature during paper feeding. Therefore, it is necessary to lower the total resistance to some extent (the value of the total resistance is the image forming apparatus). Determined by the throughput and the configuration of the heating device).
[0071]
A comparison between the heating device of this example and the heating device of the conventional example is shown below. In this example and the conventional example, the configuration other than the heating body is the same, and when 100 sheets of postcard size (small size paper) recording materials are continuously fed from a state where the heating apparatus is sufficiently adapted to room temperature (25 ° C.). The maximum temperature of the non-sheet passing portion (measured on the back surface of the heating body with a thermocouple) was compared. The fixing temperature was 200 ° C. The input voltage is 100 V, and the process speed of the image forming apparatus is 80 mm / sec. It was. The results are shown in Table 1.
[0072]
[Table 1]

[0073]
As shown in Table 1, the heating device of this example was able to significantly reduce the non-sheet passing portion temperature as compared with the conventional example.
[0074]
Next, B5 size (small size paper) with a basis weight of 157 g / m²The paper was forcibly double fed and passed to test how many sheets would cause deterioration and breakage of the heating device. The fixing temperature, input voltage, and process speed were the same as when the postcard was passed. The test results are shown in Table 2.
[0075]
[Table 2]

[0076]
As shown in Table 2, in the case of the heating device of the conventional example, the heating element was damaged by four or five feedings, and deterioration was observed in the non-sheet passing portion of the stay, film, and pressure roller surface layer. On the other hand, in the case of the heating device of this example, the number of double feeds was increased to 10 double feeds in both cases, but the heating body was not damaged, and the stay, film, and pressure roller were not deteriorated. It was.
[0077]
As described above, by adding graphite having NTC characteristics to the resistance heating element, it is possible to realize a resistance heating element having NTC characteristics and to prevent the temperature rise of the non-sheet passing portion.
[0078]
In this embodiment, the case where the recording material is passed on the basis of the center has been described. However, this embodiment can also be applied to an image forming apparatus that passes the recording material on the basis of the end portion.
[0079]
[Example 2]
In this embodiment, palladium is added to the resistance heating element in place of the silver contained in Example 1 in order to adjust the resistance of the resistance heating element. The configurations of the heating device and the image forming apparatus other than the material of the resistance heating element are the same as those in the first embodiment.
[0080]
The resistance heating element 6 of the heating element 3 in this embodiment is obtained by applying a paste prepared by kneading graphite, palladium, glass powder (inorganic binder), and organic binder to the heating element substrate 7 by screen printing. It is obtained by forming into a strip shape of 6.5 mm, length 220 mm, and thickness of about 100 μm. The sheet heating resistance of the resistance heating element 6 was about 9Ω / sq (thickness 10 μm) as in Example 1, and the total resistance at room temperature was 30Ω as in Example 1.
[0081]
FIG. 6 shows a schematic diagram of resistance temperature characteristics of the resistance heating elements of the present embodiment and the conventional example. The horizontal axis represents the temperature of the resistance heating element, and the vertical axis represents the resistance value. The thin line shows the resistance temperature characteristic of the resistance heating element (14) of the conventional example (same as in FIG. 4), and the thick line shows the resistance temperature characteristic of the resistance heating element 6 of this example.
[0082]
The resistance heating element 7 in this example also exhibits NTC characteristics as in Example 1, and the resistance change rate is about -460 ppm / ° C. Therefore, the total resistance at 200 ° C. of the resistance heating element of this example is 27.59Ω, and the total resistance at 300 ° C. is 26.21Ω, which is lower than the value of Example 1.
[0083]
The resistance change rate of palladium is about 3000 ppm / ° C., which is smaller than silver. Further, since the sheet resistance is about 7 times that of silver, the ratio in the resistance heating element can be made smaller than that of silver. Due to these two effects, the resistance change rate of the entire resistance heating element can be made smaller than that of the first embodiment, and the effect of preventing the temperature rise of the non-sheet passing portion is also larger than that of the first embodiment.
[0084]
A comparison between the heating device of this example and the heating device of the conventional example is shown below. The following comparison was performed under the same conditions as the comparison described in Example 1. The conventional example is the same as the configuration described in the first embodiment, and the result is the same. First, the maximum temperature of the non-sheet-passing area (measured by the thermocouple on the back side of the heated body) when 100 sheets of postcard-sized recording material are continuously fed from a state where the heating device is sufficiently accustomed to room temperature (25 ° C). Compared. The fixing temperature, input voltage, and process speed were the same as in Example 1. The results are shown in Table 3.
[0085]
[Table 3]

[0086]
As shown in Table 3, the temperature of the present example was significantly lower than that of the conventional example, and was lower than the result of Example 1 (288 ° C.).
[0087]
Next, basis weight is 157g / m with B5 size²The paper was forcibly double fed and passed to test how many sheets would cause deterioration and breakage of the heating device. Fixing temperature, input voltage, and process speed are the same conditions. The test results are shown in Table 4.
[0088]
[Table 4]

[0089]
As shown in Table 4, the heating device of this example increased the number of double feeds up to 10 double feeds, but there was no breakage of the heating element and deterioration of the stay, film, and pressure roller. The same results as in Example 1 were obtained.
[0090]
From the above results, it can be seen that by using palladium as a material for adjusting resistance, it is possible to realize a resistance heating element having NTC characteristics that are further effective in preventing the temperature rise of the non-sheet passing portion.
[0091]
In this embodiment, the case where the recording material is passed on the basis of the center has been described. However, this embodiment can also be applied to an image forming apparatus that passes the recording material on the basis of the end portion.
[0095]
【The invention's effect】
  As described above, according to the present invention, the fixing device of the film heating system isBy utilizing the negative resistance temperature characteristics of graphite,Non-sheet-passing temperature rise with low-cost configuration without deteriorating device specificationsEffectively suppressIt is possible to provide a safer product with a longer product life and better images.
[Brief description of the drawings]
FIG. 1 is a schematic configuration model diagram of a main part of an image forming apparatus according to a first embodiment.
FIG. 2 is a schematic configuration model diagram of a main part of a heating device (image heating fixing device) in Embodiment 1.
[Fig. 3] Structural model of heating element
FIG. 4 is a schematic diagram of resistance temperature characteristics of a resistance heating element of a heating element in Example 1 and a conventional example.
Fig. 5 Model of resistance heating element
FIG. 6 is a schematic diagram of resistance temperature characteristics of a resistance heating element of a heating element in Example 2 and a conventional example.
[Explanation of symbols]
1. Stay 2. Fixing film 3. 3. Heater (heating body) Pressure roller (pressure body) 4a. Core 4b. Elastic layer 4c. Release layer 5. Temperature sensor 6. 6. Resistance heating element of Example 1 Substrate 8. Overcoat layer 9.10. 10. Power feeding electrode CPU 12. Triac 13. AC power supply 14. Conventional resistance heating element Nip part Recording material Toner a. Recording material conveyance direction

Claims (1)

A heater having a ceramic substrate and a resistance heating element provided along the longitudinal direction of the substrate, a film that moves in a direction orthogonal to the longitudinal direction while being in contact with the heater, and a parallel to the heater And a pressure roller that forms a fixing nip portion together with the heater through the film. In a fixing device that heats and fixes a toner image on a recording material to the recording material while nipping and conveying it,
The resistance heating element is obtained by baking a paste in which graphite, silver or palladium, and a binder are mixed, and the content of graphite is higher than that of silver or palladium . A fixing device having a negative resistance change rate.

Images