JP2004094157A - Transferring roll and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prolong the service life of an image forming apparatus by making a great variation in transferring voltage unnecessary in each environment of use to obtain the best transferring current. <P>SOLUTION: A shaft 31, an elastic layer installed in the surrounding of the shaft 31 and a coating layer which is installed in the surrounding of the elastic layer consisting of a plurality of layers are provided. Then, the resistivity of the layer which is on the outside among each layer of the coating layer is made greater than the resistivity of the inside layer. Transferring current fed toward an image carrier from the shaft 31 is made small before the outer layer with the great resistivity. Therefore, since the proportion of the transferring current taking the outer layer which is more dependent on the environment of use of the image forming apparatus as the path is smaller, it is no longer necessary to greatly vary the transferring voltage in each environment of use to obtain the best transferring current. <P>COPYRIGHT: (C)2004,JPO

Description

また、前記熱可塑性樹脂チューブ３３をシリコーンゴムのスポンジから成る半導電性発泡ゴム３２に被覆すると、被覆前と被覆後とでは、体積抵抗率が表２に示されるように変化し、見かけ上抵抗率は小さくなる。
【００１３】
【表２】

この場合、上層３３ａの表面抵抗率が下層３３ｂの表面抵抗率より小さくされるので、転写電流が、表面抵抗率が大きい下層３３ｂ内において径方向に短く流れ、表面抵抗率が小さい上層３３ａ内において円周方向に長く流れようとする。
したがって、感光体ドラム１１と熱可塑性樹脂チューブ３３とが接触する点を転写部αとすると、転写電流は、図２の矢印で示されるように、前記転写部αを中心として半導電性発泡ゴム３２及び下層３３ｂ内を左右に広がって流れ、上層３３ａ内において中央に集まる。
【００１４】
【特許文献１】
特開平１１−３０５５７２号公報
【００１５】
【発明が解決しようとする課題】
しかしながら、前記従来の転写ローラ２４において、上層３３ａは、イオン伝導層によって形成されるので環境に依存しやすく、温度が高くなると、イオン伝導剤と樹脂を形成している高分子との分子運動が活発になるので、著しく電気伝導度が大きくなり、電気抵抗が小さくなってしまう。
【００１６】
また、イオンになる担体は、電場がない状態においてイオン伝導層を構成する高分子中に取り込まれたときに、正の極性のものと負の極性のものとが対になり、中性で存在することになる。ところが、湿度が高くなると、高分子中の水の分子が多くなり、極性を有する水が各イオン間に入り込んで、イオン同士で引き合う電気力を小さくしてしまう。したがって、電場がない状態でもイオンが解離しやすくなり、電場がある状態では著しくイオンが解離しやすくなってしまう。その結果、電気伝導度が大きくなり、電気抵抗が小さくなってしまう。
【００１７】
このように、転写ローラ２４の抵抗率が、プリンタが置かれた環境に大きく依存するので、トナー像を転写するのに必要とされる最適な転写電流を得るために、各使用環境において転写電圧を大きく変えなくてはならない。
【００１８】
また、低温・低湿の使用環境に加え、経時によって抵抗率が高くなる条件が同時に発生すると、最適な転写電流を発生させるための転写電圧が高くなり、次第に電源電圧が転写電圧に追従することができなくなってしまい、プリンタの寿命が短くなってしまう。
【００１９】
本発明は、前記従来の画像形成装置の問題点を解決して、最適な転写電流を得るために、各使用環境において転写電圧を大きく変える必要がなく、画像形成装置の寿命を長くすることができる転写ローラ及び画像形成装置を提供することを目的とする。
【００２０】
【課題を解決するための手段】
そのために、本発明の転写ローラにおいては、軸と、該軸の周囲に配設された弾性層と、該弾性層の周囲に配設され、複数の層から成る被覆層とを有する。
【００２１】
そして、該被覆層の各層のうちの外側の層の抵抗率が内側の層の抵抗率より大きくされる。
【００２２】
【発明の実施の形態】
以下、本発明の実施の形態について図面を参照しながら詳細に説明する。この場合、画像形成装置としての電子写真式のプリンタについて説明する。
【００２３】
図３は本発明の実施の形態における電子写真式のプリンタを示す概念図である。
【００２４】
図において、１１はドラム状に形成され、図示されない駆動手段によって矢印Ａ方向に回転させられる像担持体としての感光体ドラムであり、該感光体ドラム１１は、導電性支持体１１ａの上に光導電層１１ｂを被覆して形成される。また、１２は、前記感光体ドラム１１の表面に接触又は圧接させられ、矢印Ｂ方向に回転させられる帯電装置としての帯電ローラ、１３は、前記感光体ドラム１１の回転方向における帯電ローラ１２より下流側に、感光体ドラム１１と対向させて配設された露光装置であり、該露光装置１３として、図示されないＬＥＤアレイとロッドアレイレンズとを組み合わせることによって形成されたＬＥＤアレイヘッドを使用することができる。なお、該ＬＥＤアレイヘッドに代えて、レーザと作像光学系とを組み合わせたものを使用することもできる。
【００２５】
また、１４は、前記感光体ドラム１１の回転方向における露光装置１３よりも下流側に、感光体ドラム１１に接触させられて、又は感光体ドラム１１と微小の空間をおいて配設された現像装置であり、該現像装置１４内に現像ローラ１５が配設されるとともに、トナー１６が収容される。前記現像装置１４としては、二成分磁気ブラシ現像器、一成分磁気ブラシ現像器、一成分非磁性現像器等を使用することができる。
【００２６】
前記現像ローラ１５は、トナー１６を付着させながら矢印Ｃ方向に回転させられ、感光体ドラム１１上に形成された静電潜像を現像（反転現像）し、トナー像を形成する。そのために、前記導電性支持体１１ａと現像ローラ１５との間にバイアス電圧を印加すると、前記静電潜像に対応して電気力線が発生する。
【００２７】
本実施の形態においては、感光体ドラム１１が帯電させられる極性が負であるので、トナー１６は、負型の帯電制御剤を添加することによって負帯電型にされる。
【００２８】
そして、１７は用紙カセット、１８は用紙、ＯＨＰ用紙等の印刷媒体、１９は給紙ローラ、２０は送紙ローラ、２４は、前記感光体ドラム１１の回転方向における現像装置１４よりも下流側に、感光体ドラム１１と対向させ、圧接させられ、矢印Ｄ方向に回転させられる転写装置としての転写ローラである。
【００２９】
また、２５は加熱ローラ、２６は加圧ローラ、２７は前記加熱ローラ２５及び加圧ローラ２６から成る定着装置、２８は、前記感光体ドラム１１の回転方向における転写ローラ２４より下流側に、感光体ドラム１１と当接させて配設されたクリーニング装置であり、該クリーニング装置２８は転写が行われた後に感光体ドラム１１に残留したトナー１６を取り除く。
【００３０】
次に、前記構成のプリンタの動作について説明する。
【００３１】
まず、前記帯電ローラ１２は、感光体ドラム１１の表面を一様に、かつ、均一に帯電させ、前記露光装置１３は、画像信号に対応する光を感光体ドラム１１に照射し、感光体ドラム１１の表面に静電潜像を形成する。次に、現像装置１４は、現像ローラ１５において帯電させられたトナー１６を、静電気力によって感光体ドラム１１に付着させ、静電潜像を現像することによってトナー像を形成する。
【００３２】
その後、用紙カセット１７に収容された印刷媒体１８は、給紙ローラ１９によって１枚ずつ繰り出され、停止させられた状態の送紙ローラ２０に送られてスキューが補正される。次に、送紙ローラ２０が矢印方向に回転させられ、印刷媒体１８は感光体ドラム１１と転写ローラ２４との間に形成された転写部αに送られ、転写ローラ２４は、感光体ドラム１１上のトナー像を印刷媒体１８に転写する。
【００３３】
その後、前記トナー像が転写された印刷媒体１８が前記定着装置２７に送られると、加熱ローラ２５の熱によってトナー１６が溶融させられ、加圧ローラ２６による加圧によって印刷媒体１８の繊維間にトナー１６が浸透させられる。このようにして、トナー像が印刷媒体１８に定着される。そして、定着後の印刷媒体１８は、プリンタ外に排出される。
【００３４】
一方、転写後の感光体ドラム１１には微量のトナー１６が残留することがあるが、該トナー１６は前記クリーニング装置２８によって除去される。このようにして、感光体ドラム１１は繰り返し利用される。
【００３５】
ところで、前記印刷媒体１８が搬送されるとき、感光体ドラム１１と転写ローラ２４との間には、数百〜数千〔Ｖ〕の電位差が形成されるが、トナー１６は負の極性に帯電させられているので、感光体ドラム１１に対して転写ローラ２４が高電位になるように前記電位差が形成される。そして、印刷媒体１８の印刷面が正の極性になるように誘電分極が行われると、トナー１６と印刷面とは逆の極性になるので、トナー１６は静電気力によって印刷媒体１８側に移動する。このようにして、トナー像が印刷媒体１８に転写される。
【００３６】
前記トナー像を印刷媒体１８に転写するために必要になる転写電流には最適値があるが、プリンタの使用環境によって、印刷媒体１８の抵抗率、及び転写ローラ２４の抵抗率が変化した場合であっても、転写電流が最適値になるように、転写電圧が変化させられる。
【００３７】
次に、前記転写ローラ２４について説明する。
【００３８】
図１は本発明の実施の形態における電子写真式のプリンタの要部を示す概念図、図４は本発明の実施の形態における転写ローラの断面図である。
【００３９】
図において、２４は転写ローラであり、該転写ローラ２４は、金属製の軸３１、該軸３１の周囲に配設され、シリコーン、ウレタン、ＥＰＤＭ（エチレンプロピレンゴム）、ＮＢＲ（ニトリルゴムブタジエンゴム）等のエラストマーから成る絶縁性ゴム材料を原料とする半導電性の弾性層としての半導電性発泡ゴム３２、及び半導電性発泡ゴム３２の周囲に被覆されて配設され、半導電性の被覆層としての熱可塑性樹脂チューブ５３を備える。
【００４０】
前記半導電性発泡ゴム３２においては、絶縁性ゴム材料に半導電性を与えるために、イオン伝導性を有する伝導剤、又は電子伝導性を有するカーボンブラック等の伝導剤が添加され、軸３１と転写ローラ２４の表面との間の体積抵抗率が１×１０^６〜１×１０^９〔Ω・ｃｍ〕の半導電領域に収められる。
【００４１】
ところで、前記半導電性発泡ゴム３２には、前記伝導剤のほかに加硫剤等が添加されるが、プリンタが温度及び湿度が高い使用環境に放置されて、伝導剤、加硫剤等が滲み出すと、伝導剤、加硫剤等が感光体ドラム１１に浸透して感光体ドラム１１と化学反応し、感光体ドラム１１を汚染したり、感光体ドラム１１の回転の周期で印刷媒体を汚染したりしてしまう。
【００４２】
ところが、前述されたように、半導電性発泡ゴム３２に熱可塑性樹脂チューブ５３が被覆されているので、伝導剤、加硫剤等が滲み出すのが防止され、前述されたような、感光体ドラム１１、印刷媒体が汚染されることがない。
【００４３】
前記熱可塑性樹脂チューブ５３は、複数の層から成る多層構造を有し、本実施の形態においては、外側の層を構成する上層５３ａ及び内側の層を構成する下層５３ｂから成る２層構造を有する。そして、前記上層５３ａは樹脂に有機系イオン伝導剤が添加されたイオン伝導層（ＩＣ層）であり、下層５３ｂは樹脂にカーボンブラックが添加された電子伝導層（ＥＣ層）である。前記イオン伝導層は、電子伝導層と比べて長期間の通電に対して電気伝導度が安定的であることから上層５３ａとして使用され、電子伝導層はイオン伝導層より緻密な構造を有し、前記伝導剤、加硫剤等が滲み出すのを防止するのに適していることから下層５３ｂとして使用される。
【００４４】
このようにして、環境に依存しやすい上層５３ａにおいて、面方向に転写電流が通りにくい構造が実現される。
【００４５】
また、熱可塑性樹脂チューブ５３の全体の厚さ、並びに上層５３ａ及び下層５３ｂの各厚さ及び抵抗率としての表面抵抗率は表３に示されるとおりである。
【００４６】
【表３】

この場合、下層５３ｂの表面抵抗率が上層５３ａの表面抵抗率より小さくされるので、転写電流は、表面抵抗率が小さい下層５３ｂにおいて円周方向に長く流れようとし、表面抵抗率が大きい上層５３ａにおいて径方向に短く流れようとする。したがって、転写電流は、図１の矢印で示されるように、半導電性発泡ゴム３２内においては転写部αを中心として左右に広がって流れ、下層５３ｂ内においては中央に集まるが、上層５３ａ内においては、ほぼ中央だけを流れる。すなわち、軸３１から感光体ドラム１１に向かって流れる転写電流が、表面抵抗率が大きい上層５３ａの手前、すなわち、表面抵抗率が小さい下層５３ｂにおいて絞り込まれることになる。
【００４７】
このように、転写電流が、プリンタの使用環境に依存しやすい上層５３ａを経路に採る割合が少なくなるので、トナー像を転写するのに必要とされる最適な転写電流を得るために、各使用環境において転写電圧を大きく変える必要がなくなる。すなわち、転写電圧を制御するためのテーブルを簡素化することができる。
【００４８】
また、低温・低湿の使用環境に加え、経時によって抵抗率が高くなる条件が同時に発生しても、最適な転写電流を発生させるための転写電圧が高くなるのを抑制することができるので、プリンタの寿命を長くすることができる。
【００４９】
なお、転写電流が流れた部分において、表面抵抗率が小さくなる現象がある（特開２００２−５１５３公報参照）が、本実施の形態においては、転写電流は、主として下層５３ｂ内を円周方向に流れ、下層５３ｂ内において表面抵抗率が小さくなる現象が現れるが、上層５３ａ内においては、表面抵抗率がもともと大きく、転写電流が径方向に流れるので、上層５３ａ内において表面抵抗率が小さくなる現象が現れない。したがって、下層５３ｂ内において表面抵抗率が小さくなることによる影響は極めて小さい。
【００５０】
従来の転写ローラ２４（図２）と本発明の転写ローラ２４とのプリンタの使用環境への依存度を比較すると、表４に示されるようになる。
【００５１】
【表４】

なお、表４において、Ｌ／Ｌは低温・低湿環境（１０〔℃〕、２０〔％〕）を表し、Ｈ／Ｈは高温・高湿環境（３０〔℃〕、８０〔％〕）を表す。低温・低湿環境下の体積抵抗率を高温・高湿環境下の体積抵抗率によって除算したときの値は、従来の転写ローラ２４の場合、６１．１であるのに対して、本発明の転写ローラ２４の場合、２３．８になる。したがって、本発明の転写ローラ２４の体積抵抗率が、プリンタの使用環境に対する依存度が低くなり、従来の転写ローラ２４に対して３９〔％〕になる。
【００５２】
また、上層５３ａの表面抵抗率と下層５３ｂの表面抵抗率との差が大きいと、上層５３ａにおいて転写電流が極端に絞り込まれるので、上層５３ａに局部的な過電流が流れストレスを加えることになる。その場合、上層５３ａが局部的に劣化し、表面抵抗率にばらつきが発生してしまうことがある。そこで、上層５３ａ及び下層５３ｂのうちの少なくとも上層５３ａを、複数の層で形成し、下層５３ｂから上層５３ａに向かって緩やかに表面抵抗率が大きくなっていくようにすると、上層５３ａが局部的に劣化するのを防止することができ、熱可塑性樹脂チューブ５３及び本発明の転写ローラ２４の寿命を長くすることができる。
【００５３】
なお、本発明は前記実施の形態に限定されるものではなく、本発明の趣旨に基づいて種々変形させることが可能であり、それらを本発明の範囲から排除するものではない。
【００５４】
【発明の効果】
以上詳細に説明したように、本発明によれば、転写ローラにおいては、軸と、該軸の周囲に配設された弾性層と、該弾性層の周囲に配設され、複数の層から成る被覆層とを有する。
【００５５】
そして、該被覆層の各層のうちの外側の層の抵抗率が内側の層の抵抗率より大きくされる。
【００５６】
この場合、被覆層の各層のうちの外側の層の抵抗率が内側の層の抵抗率より大きくされるので、軸から像担持体に向かって流れる転写電流が、抵抗率が大きい外側の層の手前で絞り込まれることになる。
【００５７】
したがって、転写電流が、画像形成装置の使用環境に依存しやすい外側の層を経路に採る割合が少なくなるので、トナー像を転写するのに必要とされる最適な転写電流を得るために、各使用環境において転写電圧を大きく変える必要がなくなる。
【００５８】
また、低温・低湿の使用環境に加え、経時によって抵抗率が高くなる条件が同時に発生しても、最適な転写電流を発生させるための転写電圧が高くなるのを抑制することができるので、画像形成装置の寿命を長くすることができる。
【図面の簡単な説明】
【図１】本発明の実施の形態における電子写真式のプリンタの要部を示す概念図である。
【図２】従来の電子写真式のプリンタの要部を示す概念図である。
【図３】本発明の実施の形態における電子写真式のプリンタを示す概念図である。
【図４】本発明の実施の形態における転写ローラの断面図である。
【符号の説明】
１１　　感光体ドラム
２４　　転写ローラ
３１　　軸
３２　　半電導性発泡ゴム
５３　　熱可塑性樹脂チューブ
５３ａ　　上層
５３ｂ　　下層[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a transfer roller and an image forming apparatus.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic printer, a copying machine, and a facsimile apparatus, the surface of a photosensitive drum is uniformly and uniformly charged by a charging roller, and then exposed by an exposure apparatus. Thus, an electrostatic latent image is formed. Then, in a developing device, toner as a developer is attached to the electrostatic latent image to perform development, a toner image is formed, and the toner image is transferred to a printing medium such as paper by a transfer roller. Subsequently, the print medium to which the toner image has been transferred is sent to a fixing device, where the toner image is fixed to the print medium.
[0003]
By the way, when the print medium is conveyed, a potential difference of several hundreds to several thousand [V] is formed between the photosensitive drum and the transfer roller. However, since the toner is charged to a negative polarity, The potential difference is formed such that the transfer roller has a high potential with respect to the photosensitive drum. When the dielectric polarization is performed so that the print surface of the print medium has a positive polarity, the toner and the print surface have opposite polarities, and the toner moves toward the print medium by electrostatic force. Thus, the toner image is transferred to the print medium.
[0004]
The current required to transfer the toner image to the print medium, that is, the transfer current has an optimum value, for example, depending on the use environment of the electrophotographic printer, the resistivity of the print medium, and the transfer roller Even when the resistivity changes, the transfer voltage is changed so that the transfer current becomes an optimum value.
[0005]
FIG. 2 is a conceptual diagram showing a main part of a conventional electrophotographic printer.
[0006]
In the figure, reference numeral 24 denotes a transfer roller. The transfer roller 24 is provided around a metal shaft 31, and is provided with silicone, urethane, EPDM (ethylene propylene rubber), and NBR (nitrile rubber butadiene rubber). A semi-conductive foam rubber 32 made of an insulating rubber material made of an elastomer such as an elastomer, and a thermoplastic resin tube 33 coated around the semi-conductive foam rubber 32 and provided with semi-conductivity (for example, Patent Reference 1).
[0007]
In the semiconductive foamed rubber 32, a conductive agent having ionic conductivity or a conductive agent such as carbon black having electronic conductivity is added to impart semiconductivity to the insulating rubber material. The volume resistivity between the transfer roller 24 and the surface of the transfer roller 24 is set in a semiconductive region of 1 × 10 ⁶ to 1 × 10 ⁹ [Ω · cm].
[0008]
By the way, a vulcanizing agent or the like is added to the semiconductive foamed rubber 32 in addition to the conductive agent. However, when the printer is left in a use environment where the temperature and humidity are high, the conductive agent, the vulcanizing agent and the like are added. When bleeding, a conductive agent, a vulcanizing agent and the like penetrate into the photosensitive drum 11 and chemically react with the photosensitive drum 11 to contaminate the photosensitive drum 11 or to rotate the photosensitive drum 11. The print medium is contaminated periodically.
[0009]
However, as described above, since the thermoplastic resin tube 33 is covered with the semiconductive foamed rubber 32, the conductive agent, the vulcanizing agent, and the like are prevented from oozing out. The drum 11 and the print medium are not contaminated.
[0010]
The thermoplastic resin tube 33 has a two-layer structure including an upper layer 33a and a lower layer 33b. The upper layer 33a is an ion conductive layer (IC layer) obtained by adding an organic ion conductive agent to a resin. It is an electron conductive layer (EC layer) in which carbon black is added to a resin. The ion conductive layer is used as the upper layer 33a because the electrical conductivity is stable with respect to long-term energization as compared with the electron conductive layer, and the electron conductive layer has a denser structure than the ion conductive layer. It is used as the lower layer 33b because it is suitable for preventing the conductive agent, vulcanizing agent, and the like from seeping out.
[0011]
Table 1 shows the overall thickness of the thermoplastic resin tube 33, and the thickness and surface resistivity of the upper layer 33a and the lower layer 33b.
[0012]
[Table 1]

When the thermoplastic resin tube 33 is coated with a semiconductive foamed rubber 32 made of a silicone rubber sponge, the volume resistivity before and after the coating changes as shown in Table 2, and the apparent resistance is changed. The rate will be smaller.
[0013]
[Table 2]

In this case, since the surface resistivity of the upper layer 33a is made smaller than the surface resistivity of the lower layer 33b, the transfer current flows in the lower layer 33b having a large surface resistivity in a short radial direction, and the transfer current flows in the upper layer 33a having a small surface resistivity. Attempts to flow long in the circumferential direction.
Therefore, assuming that the point at which the photosensitive drum 11 and the thermoplastic resin tube 33 come into contact with each other is a transfer portion α, the transfer current is, as shown by an arrow in FIG. It flows right and left within the lower layer 32 and the lower layer 33b, and gathers at the center in the upper layer 33a.
[0014]
[Patent Document 1]
JP-A-11-305572
[Problems to be solved by the invention]
However, in the conventional transfer roller 24, since the upper layer 33a is formed by the ion conductive layer, the upper layer 33a is easily dependent on the environment. When the temperature increases, the molecular motion between the ion conductive agent and the polymer forming the resin is reduced. Since it becomes active, the electric conductivity is significantly increased and the electric resistance is reduced.
[0016]
In addition, when the carrier that becomes an ion is taken into the polymer constituting the ion conductive layer in the absence of an electric field, the carrier having a positive polarity and the carrier having a negative polarity form a pair, and the carrier is neutral. Will do. However, when the humidity increases, the number of water molecules in the polymer increases, and polar water enters between the ions, reducing the electric force attracted by the ions. Therefore, the ions are easily dissociated even in the absence of an electric field, and the ions are significantly easily dissociated in the presence of an electric field. As a result, the electric conductivity increases and the electric resistance decreases.
[0017]
As described above, since the resistivity of the transfer roller 24 greatly depends on the environment in which the printer is placed, in order to obtain the optimum transfer current required for transferring the toner image, the transfer voltage in each use environment is determined. Must be changed drastically.
[0018]
Also, in addition to the use environment of low temperature and low humidity, when the condition that the resistivity increases over time occurs at the same time, the transfer voltage for generating the optimum transfer current increases, and the power supply voltage may gradually follow the transfer voltage. And the life of the printer is shortened.
[0019]
The present invention solves the problems of the conventional image forming apparatus, and in order to obtain an optimum transfer current, it is not necessary to largely change the transfer voltage in each use environment, and it is possible to extend the life of the image forming apparatus. It is an object of the present invention to provide a transfer roller and an image forming apparatus that can perform the transfer.
[0020]
[Means for Solving the Problems]
For this purpose, the transfer roller according to the present invention includes a shaft, an elastic layer disposed around the shaft, and a coating layer disposed around the elastic layer and including a plurality of layers.
[0021]
Then, the resistivity of the outer layer among the layers of the coating layer is made higher than the resistivity of the inner layer.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this case, an electrophotographic printer as an image forming apparatus will be described.
[0023]
FIG. 3 is a conceptual diagram showing an electrophotographic printer according to the embodiment of the present invention.
[0024]
In the figure, reference numeral 11 denotes a photosensitive drum as an image carrier which is formed in a drum shape and is rotated in the direction of arrow A by a driving means (not shown). The photosensitive drum 11 is provided on a conductive support 11a. It is formed by covering the conductive layer 11b. Reference numeral 12 denotes a charging roller serving as a charging device which is brought into contact or pressure contact with the surface of the photoconductor drum 11 and is rotated in the direction of arrow B. Reference numeral 13 denotes a downstream side of the charging roller 12 in the rotation direction of the photoconductor drum 11. The exposure device is disposed on the side opposite to the photoconductor drum 11, and as the exposure device 13, an LED array head formed by combining an LED array (not shown) and a rod array lens may be used. it can. Instead of the LED array head, a combination of a laser and an image forming optical system may be used.
[0025]
A developing device 14 is provided downstream of the exposure device 13 in the rotation direction of the photoconductor drum 11 so as to be in contact with the photoconductor drum 11 or disposed with a small space from the photoconductor drum 11. The developing device 14 includes a developing roller 15 and a toner 16. As the developing device 14, a two-component magnetic brush developing device, a one-component magnetic brush developing device, a one-component non-magnetic developing device, or the like can be used.
[0026]
The developing roller 15 is rotated in the direction of arrow C while attaching the toner 16 to develop (reverse development) the electrostatic latent image formed on the photosensitive drum 11 to form a toner image. Therefore, when a bias voltage is applied between the conductive support 11a and the developing roller 15, lines of electric force are generated corresponding to the electrostatic latent images.
[0027]
In the present embodiment, since the polarity with which the photosensitive drum 11 is charged is negative, the toner 16 is made negatively charged by adding a negative charge control agent.
[0028]
Reference numeral 17 denotes a paper cassette, 18 denotes a print medium such as paper or OHP paper, 19 denotes a paper feed roller, 20 denotes a paper feed roller, and 24 denotes a downstream side of the developing device 14 in the rotation direction of the photosensitive drum 11. And a transfer roller as a transfer device which is opposed to the photosensitive drum 11, pressed against and rotated in the direction of arrow D.
[0029]
Reference numeral 25 denotes a heating roller, reference numeral 26 denotes a pressing roller, reference numeral 27 denotes a fixing device including the heating roller 25 and pressing roller 26, and reference numeral 28 denotes a photosensitive member downstream of the transfer roller 24 in the rotation direction of the photosensitive drum 11. The cleaning device 28 is disposed in contact with the body drum 11, and the cleaning device 28 removes the toner 16 remaining on the photosensitive drum 11 after the transfer is performed.
[0030]
Next, the operation of the printer having the above configuration will be described.
[0031]
First, the charging roller 12 uniformly and uniformly charges the surface of the photoconductor drum 11, and the exposure device 13 irradiates the photoconductor drum 11 with light corresponding to an image signal. An electrostatic latent image is formed on the surface of the surface 11. Next, the developing device 14 forms the toner image by attaching the toner 16 charged by the developing roller 15 to the photosensitive drum 11 by electrostatic force, and developing the electrostatic latent image.
[0032]
Thereafter, the print medium 18 stored in the paper cassette 17 is fed out one by one by a paper feed roller 19 and sent to a stopped paper feed roller 20 to correct the skew. Next, the paper feed roller 20 is rotated in the direction of the arrow, and the print medium 18 is sent to a transfer section α formed between the photosensitive drum 11 and the transfer roller 24. The upper toner image is transferred to the print medium 18.
[0033]
Thereafter, when the print medium 18 to which the toner image has been transferred is sent to the fixing device 27, the toner 16 is melted by the heat of the heating roller 25, and is pressed between the fibers of the print medium 18 by the pressure of the pressure roller 26. The toner 16 is permeated. Thus, the toner image is fixed on the print medium 18. Then, the print medium 18 after fixing is discharged out of the printer.
[0034]
On the other hand, a small amount of toner 16 may remain on the photosensitive drum 11 after the transfer, and the toner 16 is removed by the cleaning device 28. Thus, the photosensitive drum 11 is repeatedly used.
[0035]
When the printing medium 18 is conveyed, a potential difference of several hundreds to several thousand [V] is formed between the photosensitive drum 11 and the transfer roller 24, but the toner 16 is charged to a negative polarity. Therefore, the potential difference is formed such that the transfer roller 24 has a high potential with respect to the photosensitive drum 11. When the dielectric polarization is performed so that the print surface of the print medium 18 has a positive polarity, the toner 16 and the print surface have opposite polarities, and thus the toner 16 moves toward the print medium 18 by electrostatic force. . Thus, the toner image is transferred to the print medium 18.
[0036]
The transfer current required to transfer the toner image to the print medium 18 has an optimum value. However, when the resistivity of the print medium 18 and the resistivity of the transfer roller 24 change depending on the use environment of the printer. Even if there is, the transfer voltage is changed so that the transfer current becomes an optimum value.
[0037]
Next, the transfer roller 24 will be described.
[0038]
FIG. 1 is a conceptual diagram showing a main part of an electrophotographic printer according to an embodiment of the present invention, and FIG. 4 is a sectional view of a transfer roller according to the embodiment of the present invention.
[0039]
In the figure, reference numeral 24 denotes a transfer roller. The transfer roller 24 is provided around a metal shaft 31, and is provided with silicone, urethane, EPDM (ethylene propylene rubber), and NBR (nitrile rubber butadiene rubber). A semiconductive foamed rubber 32 as a semiconductive elastic layer made of an insulating rubber material made of an elastomer such as an elastomer, and a semiconductive foamed rubber that is provided so as to be covered around the semiconductive foamed rubber 32. A thermoplastic resin tube 53 is provided as a layer.
[0040]
In the semiconductive foamed rubber 32, a conductive agent having ionic conductivity or a conductive agent such as carbon black having electronic conductivity is added to impart semiconductivity to the insulating rubber material. The volume resistivity between the transfer roller 24 and the surface of the transfer roller 24 is set in a semiconductive region of 1 × 10 ⁶ to 1 × 10 ⁹ [Ω · cm].
[0041]
By the way, a vulcanizing agent or the like is added to the semiconductive foamed rubber 32 in addition to the conductive agent. However, when the printer is left in a use environment where the temperature and humidity are high, the conductive agent, the vulcanizing agent and the like are added. When oozing out, a conductive agent, a vulcanizing agent, etc., penetrates the photoreceptor drum 11 and chemically reacts with the photoreceptor drum 11, thereby contaminating the photoreceptor drum 11 or cleaning the print medium with the rotation period of the photoreceptor drum 11. Or pollute.
[0042]
However, as described above, since the semiconductive foamed rubber 32 is covered with the thermoplastic resin tube 53, the conductive agent, the vulcanizing agent, and the like are prevented from oozing out. The drum 11 and the print medium are not contaminated.
[0043]
The thermoplastic resin tube 53 has a multilayer structure including a plurality of layers, and in the present embodiment, has a two-layer structure including an upper layer 53a forming an outer layer and a lower layer 53b forming an inner layer. . The upper layer 53a is an ion conductive layer (IC layer) in which an organic ion conductive agent is added to a resin, and the lower layer 53b is an electron conductive layer (EC layer) in which carbon black is added to a resin. The ion conductive layer is used as the upper layer 53a because the electrical conductivity is stable with respect to a long-term energization compared to the electron conductive layer, and the electron conductive layer has a denser structure than the ion conductive layer, It is used as the lower layer 53b because it is suitable for preventing the conductive agent, the vulcanizing agent and the like from seeping out.
[0044]
In this manner, a structure in which the transfer current is difficult to pass in the plane direction is realized in the upper layer 53a that is easily dependent on the environment.
[0045]
Table 3 shows the overall thickness of the thermoplastic resin tube 53, the thickness of each of the upper layer 53a and the lower layer 53b, and the surface resistivity as the resistivity.
[0046]
[Table 3]

In this case, since the surface resistivity of the lower layer 53b is made smaller than the surface resistivity of the upper layer 53a, the transfer current tends to flow longer in the circumferential direction in the lower layer 53b having a smaller surface resistivity, and the upper layer 53a having a larger surface resistivity. Attempts to flow short in the radial direction. Therefore, as shown by the arrow in FIG. 1, the transfer current spreads right and left around the transfer portion α in the semiconductive foamed rubber 32, and converges in the center in the lower layer 53b, but concentrates in the center in the lower layer 53a. Flows almost only in the center. That is, the transfer current flowing from the shaft 31 toward the photosensitive drum 11 is narrowed before the upper layer 53a having a large surface resistivity, that is, in the lower layer 53b having a small surface resistivity.
[0047]
In this manner, the ratio of the transfer current taking the upper layer 53a, which tends to depend on the use environment of the printer, on the path is reduced, and therefore, in order to obtain the optimum transfer current required for transferring the toner image, There is no need to significantly change the transfer voltage in the environment. That is, the table for controlling the transfer voltage can be simplified.
[0048]
In addition, even in a low-temperature, low-humidity use environment, and even when a condition in which the resistivity increases over time occurs at the same time, it is possible to suppress the increase in the transfer voltage for generating the optimum transfer current. Life can be extended.
[0049]
In a portion where the transfer current flows, there is a phenomenon that the surface resistivity decreases (see Japanese Patent Application Laid-Open No. 2002-5153). In the present embodiment, however, the transfer current mainly flows in the lower layer 53b in the circumferential direction. Although a phenomenon in which the surface resistivity decreases in the lower layer 53b appears, the surface resistivity in the upper layer 53a is originally large, and the transfer current flows in the radial direction, so that the surface resistivity decreases in the upper layer 53a. Does not appear. Therefore, the effect of the lower surface resistivity in the lower layer 53b is extremely small.
[0050]
Table 4 shows a comparison between the transfer roller 24 of the related art and the transfer roller 24 of the present invention depending on the use environment of the printer.
[0051]
[Table 4]

In Table 4, L / L represents a low-temperature and low-humidity environment (10 ° C., 20%), and H / H represents a high-temperature and high-humidity environment (30 ° C., 80%). . The value obtained by dividing the volume resistivity under a low-temperature and low-humidity environment by the volume resistivity under a high-temperature and high-humidity environment is 61.1 in the case of the conventional transfer roller 24, whereas the value of the transfer according to the present invention is 61.1. In the case of the roller 24, it becomes 23.8. Accordingly, the volume resistivity of the transfer roller 24 of the present invention is less dependent on the environment in which the printer is used, and is 39% of the conventional transfer roller 24.
[0052]
Further, if the difference between the surface resistivity of the upper layer 53a and the surface resistivity of the lower layer 53b is large, the transfer current is extremely narrowed in the upper layer 53a, so that a local overcurrent flows through the upper layer 53a to apply stress. . In that case, the upper layer 53a may be locally deteriorated and the surface resistivity may vary. Therefore, at least the upper layer 53a of the upper layer 53a and the lower layer 53b is formed of a plurality of layers, and the surface resistivity gradually increases from the lower layer 53b toward the upper layer 53a. Deterioration can be prevented, and the life of the thermoplastic resin tube 53 and the transfer roller 24 of the present invention can be extended.
[0053]
It should be noted that the present invention is not limited to the above-described embodiment, but can be variously modified based on the gist of the present invention, and they are not excluded from the scope of the present invention.
[0054]
【The invention's effect】
As described above in detail, according to the present invention, the transfer roller includes the shaft, the elastic layer disposed around the shaft, and the plurality of layers disposed around the elastic layer. And a coating layer.
[0055]
Then, the resistivity of the outer layer among the layers of the coating layer is made higher than the resistivity of the inner layer.
[0056]
In this case, since the resistivity of the outer layer among the respective layers of the coating layer is made larger than the resistivity of the inner layer, the transfer current flowing from the axis toward the image carrier is reduced by the outer layer having the larger resistivity. It will be narrowed down in the foreground.
[0057]
Therefore, the transfer current is less likely to take the outer layer which is likely to depend on the use environment of the image forming apparatus, so that in order to obtain the optimum transfer current required for transferring the toner image, It is not necessary to largely change the transfer voltage in the use environment.
[0058]
In addition, even in a low-temperature, low-humidity use environment, even when a condition in which the resistivity increases over time occurs at the same time, it is possible to suppress a transfer voltage for generating an optimum transfer current from being increased. The life of the forming apparatus can be extended.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing a main part of an electrophotographic printer according to an embodiment of the present invention.
FIG. 2 is a conceptual diagram showing a main part of a conventional electrophotographic printer.
FIG. 3 is a conceptual diagram illustrating an electrophotographic printer according to an embodiment of the present invention.
FIG. 4 is a cross-sectional view of the transfer roller according to the embodiment of the present invention.
[Explanation of symbols]
11 Photoconductor drum 24 Transfer roller 31 Shaft 32 Semi-conductive foam rubber 53 Thermoplastic resin tube 53a Upper layer 53b Lower layer

Claims (3)

(A) an axis;
(B) an elastic layer disposed around the shaft;
(C) a coating layer disposed around the elastic layer and comprising a plurality of layers;
(D) The transfer roller, wherein the resistivity of the outer layer among the respective layers of the coating layer is made larger than the resistivity of the inner layer. (A) the outer layer is an ion conductive layer;
The transfer roller according to claim 1, wherein (b) the inner layer is an electron conductive layer. (A) an image carrier;
(B) having a transfer roller disposed in contact with the image carrier;
(C) the transfer roller includes a shaft, an elastic layer disposed around the shaft, and a coating layer disposed around the elastic layer and including a plurality of layers;
(D) An image forming apparatus, wherein the resistivity of the outer layer among the respective layers of the coating layer is made higher than the resistivity of the inner layer.

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