JP2004109703A

JP2004109703A - Method and device for transferring, and method and device for forming image

Info

Publication number: JP2004109703A
Application number: JP2002274088A
Authority: JP
Inventors: Tomoko Takahashi; 高橋　朋子
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 2002-09-19
Filing date: 2002-09-19
Publication date: 2004-04-08
Anticipated expiration: 2022-09-19
Also published as: JP4246463B2

Abstract

PROBLEM TO BE SOLVED: To provide an image forming method and an image forming device which prevent pretransferring and do not cause reverse transferring. SOLUTION: In a device configuration for transferring a toner image formed on image carriers 22 (Bk, Y, M and C) onto an intermediate transfer body 31, when a counter bias Vc is applied to the intermediate transfer body 31 of a part corresponding to gaps S1 to S4 formed at the upstream side of the image carriers in a rotating direction with respect to contact parts N (Bk, Y, M and C) between each image carrier and the intermediate transfer body, the counter bias Vc has the same polarity as the charging polarity of the image carriers and also has an absolute value that is a voltage larger than image part voltage VL and controls to satisfy an equation of ¾ Vd - Vc - Vt ¾ < Vth when a surface potential on the intermediate transfer body is defined as Vt, a potential of an image part on the image carriers is as VL, the counter bias is as Vc, a surface potential of a non-image part on the image carriers is as Vd, and discharging start voltage is as Vth. COPYRIGHT: (C)2004,JPO

Description

【０００１】
【産業上の利用分野】
本発明は、像担持体上に形成したトナー像を、中間転写体に転写した後、再び記録材に転写することで画像を得る転写方法や転写装置及び画像形成方法や画像形成装置に関する。
【０００２】
【従来技術】
電子写真プロセスを用いた画像形成装置では、像担持体から中間転写体あるいは記録材へのトナーの転写時、チリによる乱れが従来問題となっている。これは主に、転写領域である転写ニップ部にトナー像が入る以前に像担持体から記録材あるいは中間転写体へ転写される、所謂プレ転写によるものと考えられている。この対策として、例えば特許文献１である特開平８−１６６７２８号公報には、転写部の上流側で像担持体と転写体とを押圧する部材を設け、転写前の空隙を減少させてトナーの飛翔を防止した発明が記載されている。
特許文献２である特開平１０−１８６８７８号公報には、ネガポジの画像形成システムにおいて、転写ニップ部の上流側に像担持体の帯電極性と同極性の電位あるいはゼロ電位となる導電性部材を設け、作像に使用されているトナーと同極性のバイアスを印加することでトナーを積極的に引きつける力を生じにくくした発明が記載されている。
【０００３】
【特許文献１】
特開平８−１６６７２８号公報
【特許文献２】
特開平１０−１８６８７８号公報
【０００４】
【発明が解決しようとする課題】
特許文献１では、押圧部材にトナーの帯電極性と反対の極性を印加しているため、押圧部材の形成する電界がトナーに対し記録材方向に飛翔させる方向に作用し、プレ転写による画像チリは完全には防止できない。
特許文献２では、一般の像担持体において、トナーが付着している画像部電位は、ゼロではなく、幾らかの像担持体の帯電極性と同極性の残留電位があるので、導電性部材の電位がゼロの場合等は、トナー散りが生じてしまう。
【０００５】
中間転写体に複数の色のトナー画像を重ね転写し、記録材に一括転写するカラー画像形成装置において、中間転写体に像担持体の帯電極性と同極性のバイアスを印加すると、そのバイアスの値によっては、２色目以降の画像に対応する像担持体から中間転写体への転写の際、既に中間転写体上に転写されているトナー像のトナーが、中間転写体から像担持体へ逆転写することが懸念される。
【０００６】
このような問題点を省みて、本発明は、プレ転写を防止するとともに逆転写が生じない、転写方法や転写装置及び画像形成方法や画像形成装置を提供することを目的とする。
【０００７】
【課題を解決するための手段】
本発明は、回転可能な像担持体の帯電部に形成された静電潜像にトナーが付着されることで形成されるトナー像を、像担持体に中間転写体を接触させることで中間転写体の表面に転写する第１の転写工程と、中間転写体に転写されたトナー像を記録材に転写する第２の転写工程とを有する転写方法や画像形成方法であって、第１の転写工程において、像担持体と中間転写体との接触部よりも像担持体の回転方向上流側に形成される空隙に対応する部分の中間転写体にカウンターバイアスを印加して、第１の転写工程における接触部よりも上流側の中間転写体上の表面電位をＶｔ、像担持体上の画像部の電位をＶＬ、カウンターバイアスをＶｃ、像担持体上の非画像部の表面電位をＶｄ、放電開始電圧をＶｔｈとした時、カウンターバイアスＶｃが、像担持体の帯電極性と同極性でかつ絶対値が画像部電位ＶＬより大きい電圧であり、｜Ｖｄ―Ｖｃ―Ｖｔ｜＜Ｖｔｈを満たすことで、プレ転写による画像の乱れを防止しながら逆転写発生をも防止するものである。
【０００８】
カラー画像転写方法やカラー画像形成方法とする場合、第１の転写工程では、複数色のトナーに対応する複数の像担持体にそれぞれ形成されたトナー像を前記中間転写体に重ね転写し、第２の転写工程では、重ね転写されたトナー像を前記記録材に転写することになる。
【０００９】
像担持体上の非画像部の表面電位Ｖｄを検知し、検知した値に基づきカウンターバイアスＶｃを決定すると、経時環境等により非画像部電位が変化したときでも、画像の乱れや逆転写を防止することができて好ましい。
【００１０】
中間転写体上の表面電位Ｖｔを検知し、この検知した値に基づきカウンターバイアスＶｃの値を決定すると、常に最新の表面電位に応じたカウンターバイアスが決定され、中間転写体の電位履歴に依らず、画像の乱れを防止して逆転写が確実に防止される。
【００１１】
中間転写体上の表面電位Ｖｔの検知を、中間転写体の表面電位を検知する転写対表面電位検知手段により検出すると、転写前の表面電位の状態を確実に把握でき、安定的で良好な転写が可能となる。
【００１２】
中間転写体の表面電位Ｖｔを、第１の転写工程上流側で中間転写体の表面電位Ｖ０と、この転写工程において接触部よりも上流側で中間転写体上にすでに転写されているトナー層の表面電位Ｖとを加算した値とすると、１色目を転写する以前の中間転写体の表面電位変動に対応したカウンターバイアスの値を決定することができる。
【００１３】
中間転写体上の表面電位Ｖ０を表面電位調整手段に電圧印加することで調整し、調整した値に基づいてカウンターバイアスＶｃの値を決定すると、積極的に転写前の状態をコントロールし、カウンターバイアス値の設定を容易にする。
【００１４】
表面電位調整手段に印加する電圧値を基づき１色目の転写工程時に印加するカウンターバイアス値を決定すると、１色目についての転写前の中間転写体の表面電位検知を行う必要がなく、構成の簡素化を図れる。
【００１５】
第１の転写工程における中間転写体上に存在するトナー層の表面電位Ｖを、該存在するトナー層の像担持体上にあったときの表面電位Ｖｅより推定すると、各色の転写部を構成する接触部毎の表面電位検知手段の設置が不要となり、構成の簡素化を可能とする。
【００１６】
中間転写体上に存在するトナー層が像担持体上にあったときの表面電位Ｖｅの検知を、像担持体の表面電位を検知する表面電位検知手段により検知すると、プロセスの経時、環境変化に依らず、トナー層電位を確実に検知することができ、適切なカウンターバイアスが安定して供給される。
【００１７】
各色の転写時に印加するカウンターバイアス値を同一の電圧とすると、１つの電圧供給手段で各色へのカウンターバイアス供給が行える。１１，３３
各色の決定されたカウンターバイアス値のうち、最も絶対値の小さいカウンターバイアス値を全色のカウンターバイアス値とすると、１つの電圧供給手段でカウンターバイアス値が各カウンターバイアス印加部材にそれぞれ供給されるとともに、確実に逆転写が防止される。
【００１８】
１色目のカウンターバイアス値を基に、他色全てのカウンターバイアス値を決定すると、カウンターバイアス決定の処理を１色についてのみ行うだけでよく、装置の簡素化、判断の効率化を図れる。
【００１９】
最終色目のカウンターバイアス値を、他の色のカウンターバイアスとして用いると、カウンターバイアス決定の処理を１色についてのみ行うだけでよく、装置の簡素化、判断の効率化を図れる。
【００２０】
本発明は、回転可能な像担持体の帯電部に形成された静電潜像にトナーが付着されることで形成されるトナー像を、像担持体に中間転写体を接触させることで中間転写体の表面に転写する第１の転写手段と、中間転写体に転写されたトナー像を記録材に転写する第２の転写手段とを有する転写装置や、帯電手段で帯電された帯電部に形成される静電潜像が現像手段によって現像されることでトナー画像が形成される回転可能な像担持体と、像担持体に中間転写体を接触させることで中間転写体の表面にトナー画像を転写する第１の転写手段と、中間転写体に転写されたトナー像を記録材に転写する第２の転写手段とを有する画像形成装置において、像担持体と中間転写体との接触部よりも像担持体の回転方向上流側に形成される空隙に対応する部分の中間転写体にカウンターバイアスを印加するカウンターバイアス印加部材と、カウンターバイアス印加部材にカウンターバイアスを供給する電圧供給手段とを有し、接触部よりも上流側の中間転写体上の表面電位をＶｔ、像担持体上の画像部の電位をＶＬ、カウンターバイアスをＶｃ、像担持体上の非画像部の表面電位をＶｄ、放電開始電圧をＶｔｈとした時、電源供給手段が、カウンターバイアスＶｃを、像担持体の帯電極性と同極性でかつ絶対値が画像部電位ＶＬより大きい電圧とし、かつ｜Ｖｄ―Ｖｃ―Ｖｔ｜＜Ｖｔｈを満たすように制御して、プレ転写による画像の乱れを防止するとともに、逆転写発生を防止するものである。
【００２１】
本発明に係る転写装置及び画像形成装置において、像担持体上の非画像部の表面電位Ｖｄを検知する非画像部電位検知手段と、非画像部電位検知手段で検知した値に基づきカウンターバイアスの値を決定するカウンターバイアス決定手段とを有し、電源供給手段が、カウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給すると、経時環境等により、非画像部電位が変化したときでも、画像の乱れや逆転写を防止する。
【００２２】
本発明に係る転写装置及び画像形成装置において、中間転写体の表面電位Ｖｔを検出する転写体表面電位検知手段と、転写体表面電位検知手段で検知した値に基づきカウンターバイアスの値を決定するカウンターバイアス決定手段とを有し、電源供給手段が、カウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給すると、中間転写体の電位履歴に依らず、画像の乱れを防止し、逆転写が確実に防止される。
【００２３】
本発明に係る転写装置及び画像形成装置において、転写体表面電位検知手段が、接触部よりも上流側で中間転写体の表面電位Ｖ０を検出する検知手段と、中間転写体上にすでに転写されているトナー層の表面電位Ｖを検知するトナー層電位検知手段と、検知手段及びトナー層電位検知手段で検知した電位を加算して中間転写体上の表面電位Ｖｔを算出する演算部とを有すると、転写前の表面電位の状態を確実に把握でき、安定的で良好な転写が行われ易くなる。
【００２４】
トナー層電位検知手段が、中間転写体上のトナー層が像担持体上にあったときの表面電位Ｖｅを検知する検知手段と、この検知手段で検知した表面電位Ｖｅに基づきトナー層の表面電位Ｖを推定するトナー層表面電位推定手段とを有する場合、１色目を転写する以前の中間転写体の表面電位変動に対応したカウンターバイアスの決定を可能にする。
【００２５】
本発明に係る転写装置及び画像形成装置において、中間転写体の表面電位Ｖ０を調整する表面電位調整手段と、表面電位調整手段により調整された表面電位に基づきカウンターバイアスを決定するカウンターバイアス決定手段とを有し、電源供給手段が、カウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給すると、積極的に転写前の状態をコントロールし、カウンターバイアス値の設定を容易にする。
【００２６】
表面電位調整手段としては、中間転写体に対し非接触状態の除帯電手段を有する場合、中間転写体の表面と非接触となって摺擦せず、中間転写体を劣化させず転写前の電位状態をコントロール可能となる。表面電位調整手段としては、中間転写体に対して接触状態であるとともに、中間転写体からトナーを除去する機能を有する場合、装置の構成の簡素化を図れる。
【００２７】
【発明の実施の形態】
回転可能な像担持体の帯電部に形成された静電潜像にトナーが付着されることで形成されるトナー像を、像担持体に中間転写体を接触させることでその表面に転写する第１の転写工程と、中間転写体に転写されたトナー像を記録材に転写する第２の転写工程とを有する転写方法や画像形成方法では、接触部となる転写部での画像の乱れが問題となっている。これは、像担持体の一例であるドラム状の感光体と中間転写ベルト等の中間転写体が接触してトナーを転写する際、感光体と中間転写体とが接触する以前に、感光体上のトナーが中間転写方向に移動するプレ転写が一因である。これを改善するために、特開平１０−１８６８７８号公報では、感光体と中間転写体との接触部の上流側に転写体裏面と接触し、かつ接地されていない導電部材を設け、転写時のこの導電部材の電位をゼロ又は感光体帯電極性と同極性としている。しかし、この方法では、感光体と中間転写体とが接触する以前にトナーが移動するという現象を完全に防ぐことができなかった。
【００２８】
感光体の帯電電位、露光強度、現像バイアス等のプロセス条件を変化させて実験を行ったところ、トナーの乱れ量は、導電部材に印加する電圧条件だけでは決まらないことがわかった。この結果を考察すると、プロセス条件により形成される感光体の画像部、すなわち、トナーが付着している部分の潜像電位と関係している。感光体上のトナー像に対し、転写部で中間転写体が接触する前の部分での電位の状態を示したものが図３である。図中Ｖｄが感光体上の非画像部電位、ＶＬが画像部電位である。トナーの極性はマイナスであり、ネガポジ方式での画像形成であるので、同極性の潜像のより絶対値が小さい部分に、同極性のトナーが付着している状態になっている。この図は上向きがマイナスとなるように示してある。マイナスのトナーはこの図の下方へ電位的には移動し易い。
【００２９】
ここで、中間転写体の電位として、ＧＮＤの場合を考えると、トナーにとって、ＶＬより安定であり、ＧＮＤ方向にトナーは移動して中間転写体に付着してしまう。理想的な感光体を考え十分に露光した場合、画像部の電位はＧＮＤとなるのだが、現実的には感光体には残留電位があり、十分に露光した場合でも画像部電位ＶＬは−１００Ｖ〜−１５０Ｖであるためである。したがって、像担持体と中間転写体との接触部によりも、感光体の回転方向上流側に形成される空隙に対応する部分の中間転写体に、感光体帯電極性と同極性で、かつ絶対値がＶＬより大きい電圧を印加することが必要である。感光体から中間転写体への転写部となる接触部の、感光体の回転方向上流部の空隙に対応する部分の中間転写体に印加する電圧を「カウンターバイアス」と呼び、符号Ｖｃと示す。具体的にはＶＬに対しＶｃは、等しいか絶対値が大きい同極性かに設定する。したがって、ＶＬ値を予め記憶してある値として動作させることも可能であるが、好ましくは、プリント枚数から予測される経時劣化変化分を加えるか、あるいは残留電位を測定可能な手段を設け、その出力により値を決定することが望ましい。Ｖｃをこのような値に設定し、カウントターバイアスを印加すると、プレ転写を防止でき、良好な転写が可能であった。
【００３０】
転写装置や画像形成装置には、中間転写体上に複数色のトナーを重ね転写し、その後用紙などの記録材に転写することでカラー画像を得るものがある。このような複数色のトナー画像が形成される場合、２色目以降の感光体から中間転写体へのトナー像の転写の際には、既に中間転写体上に転写されたトナーが存在している。２色目以降の転写の際、この既に転写されているトナーが逆に中間転写体から感光体に転写することを逆転写と言うが、この逆転写が発生する場合がある。この逆転写の発生は、観察によれば、カウンターバイアス印加部の空隙における放電の発生と関係がある。
【００３１】
また、逆転写トナーは、感光体上の非画像部に付着しているため、非画像部との放電と逆転写とが関係すると予想される。カウンターバイアスとしてよりマイナス側に大きな電圧を印加して、電位差が大きくなると感光体の非画像部と中間転写体体の間で放電が生じる。放電が生じた場合、放電電荷は、中間転写体上にプラスが、感光体上にマイナスが降る。カラー画像形成可能な装置構成においてネガポジ方式の場合、感光体の帯電極性とトナーの極性は同極性であり、前述のようにカウンターバイアスの印加位置で放電が起きた際、中間転写体上にあるトナー上にはトナー帯電極性と逆極性の電荷が降る。この放電電荷により帯電極性が不安定になったトナーが逆転写すると考えられる。
【００３２】
さらに、放電前の状態で中間転写体の表面電位Ｖｔも、接触部の上流側の空隙の電位差を考える場合、カウンターバイアスと同等であり、放電開始電圧をＶｔｈとすると、後述詳細に説明するが、図４に示すように｜Ｖｄ−Ｖｃ―Ｖｔ｜＜Ｖｔｈの関係を満たすことが放電の生じない条件であり、この条件を満たす値のカウンターバイアスを印加することでプレ転写による画像の乱れを防止するとともに、逆転写発生を防止でき、良好な画像が得られることがわかった。Ｖｔｈとは、カウンターバイアスの印加部における放電開始電圧であり、１気圧におけるパッシェンの放電限界曲線より、近似的に下記の式（１）のように求められる。
【００３３】
Ｖｔｈ＝√７７３７．６Ｄ＋３１２＋６．２Ｄ・・・（１）
Ｄ＝Ｌｐ／εｐ＋Ｌｄ／εｄ
Ｌｐ：感光体の膜厚（μｍ）
εｐ：感光体の誘電率
Ｌｄ：中間転写体の膜厚（μｍ）
εｄ：中間転写ベルトの誘電率
上述した転写装置や画像形成装置において、感光体の非画像部の表面電位を検知し、検知した値に基づきカウンターバイアス値を決定することも考えられる。第１の転写工程の像担持体の回転方向上流部での像担持体の電位状態を検知することで、より確実に放電をしない値にカウンターバイアスを設定することが可能である。
【００３４】
カウンターバイアスの値Ｖｃの非画像部電位Ｖｄ、画像部電位ＶＬとの大小関係を考える（すべての電位はマイナスである）。
｜Ｖｃ｜＞｜Ｖｄ｜＞｜ＶＬ｜の場合、この放電により中間転写体上には正電荷の降る。｜ＶＤ｜＞｜Ｖｃ｜＞｜ＶＬ｜の場合、非画像部と中間転写体との間、中間転写体と画像部との間で放電が生じる可能性がある。非画像部と中間転写体との間で放電が生じた場合には中間転写体上には負電荷が降り、画像部と中間転写体との間で放電が生じた場合には中間転写体上には正電荷が降る。トナーの帯電極性はもともと負であるので、画像部と中間転写体との間での放電によって、すでに中間転写体上に転写されているトナーの帯電が不安定になることがわかる。逆転写の現象は、感光体上の非画像部（負極性）の部分にトナーが付着するものであるので、弱帯電あるいは、正帯電となったトナーは付着し易い。また、逆転写が像担持体の非画像部に付着することから、非画像部の電位を検知し、像担持体非画像部と中間転写体との間の放電を防止することが、逆転写防止に重要である。
【００３５】
上述した転写装置や画像形成装置において、第１の転写工程における感光体と中間転写体との接触部の感光体の回転方向上流側の中間転写体の表面電位を検知し、検知した値に基づきカウンターバイアスの値を決定することも考えられる。第１の転写工程で複数の感光体から複数色のトナー像を中間転写体上に転写する場合、カウンターバイアスの印加部で放電が発生し、すでに中間転写体上に存在するトナー上に正電荷が降ると、正電荷によって帯電が不安定になったトナーが、逆に中間転写体から感光体に逆転写してしまう。中間転写体上に複数色のトナー像を形成するプロセスにおいては、何色目のプロセスかによって中間転写体の電位の状態が異なる。また、中間転写体から記録材へのトナー像の転写（第２の転写工程）を経た後の中間転写体上の電位状態も、二次転写時の条件により異なっている。これらの条件により中間転写体上の電位状態が変化した場合でも、常に適正なカウンターバイアスを印加するためには、中間転写体上の表面電位Ｖｔを検知することが重要である。
【００３６】
中間転写体上の表面電位Ｖｔを検知する場合、中間転写体上の表面電位検知を、転写体表面電位検知手段で検知する。転写前の中間転写体の表面電位は、第２の転写工程での放電履歴、すでに中間転写体上に付着しているトナー像の電位等で変化する。このため、転写前に中間転写体が持っている電位がマイナスの場合にはカウンターバイアスと同様に作用をし、プラスの場合には、うち消す方向の作用をする。したがって、適切なカウンターバイアスを印加するためには、転写体表面電位検知手段にて検知することは表面電位Ｖｔを検出する上で大変有効である。
【００３７】
転写工程における接触部の上流側での中間転写体の表面電位Ｖｔとしては、第１の転写工程における中間転写体の表面電位Ｖ０とこの転写工程の上流側ですでに中間転写体に転写されているトナー層の電位Ｖとを加算した値としても良い。この理由の説明を図５に示す。図５の▲１▼、▲２▼、▲３▼は、１色目、２色目、３色目の転写部となる接触部の上流側での中間転写体の状態を示す。転写３色目の接触部の上流側では、中間転写体上に図中○で示す１色目のトナーと、図中●で示す２色目のトナーとがある。全ての接触部の上流（最上流）側での中間転写体の電位をＶ０とし、各トナーによる電位をＶ１、Ｖ２とすると、３色目の接触部上流側での電位はＶ０＋Ｖ１＋Ｖ２と考えられる。同様に、４色目ではＶ０＋Ｖ１＋Ｖ２＋Ｖ３とする。このとき、１色目の転写に入る以前の中間転写体の電位状態が異なった場合、Ｖ０の値を動かせば良いので、対応が容易である。
【００３８】
トナー層の表面電位Ｖを、該トナー層が感光体上にあった時の表面電位Ｖｅより推定する場合において、誘電体上表面電位Ｖは単位面積あたりの電荷Ｑ、誘電体の厚みｄ、誘電率εとしたとき、Ｑ＝εＶ／ｄで示される。感光体から中間転写体への転写率は略決まっており、感光体と中間転写体との厚み、誘電率が既知であれば、感光体上のトナーの電荷Ｑで、中間転写体上で示す電位を計算することが可能である。検知または測定される感光体上のトナー層の表面電位Ｖｅは、感光体の残留電位も含んでいるので、残留電位分を差し引いた値をトナー層の表面電位Ｖとして計算することで、中間転写体へ特別な装置を設けることなく、トナー層の表面電位を検知することができる。
【００３９】
トナー層の感光体上の表面電位Ｖｅを検知する方法として、表面電位検知手段を用いる。感光体上の画像部の表面電位は、感光体の帯電電位、書き込み光量、感光体の光減衰特性、現像によるトナー付着量等により変化する。さらに、これらのパラメータは、経時、環境等によって変化するため、実際の画像部電位を知るためには、表面電位検知手段が確実である。しかし、表面電位検知手段により、画像部電位を測定する場合、表面電位検知手段の解像度が充分に高くないために、細かな画像を直接測定することは困難である。したがって、実際的には、画像部と同じ作像条件で作像した表面電位検知用のパターンを感光体上の不要部に設け、パターンの測定検知値によって表面電位を検知する。感光体上にトナー像を形成する際のプロセス条件によって、トナー層の持つ電荷量を推定することも可能であるが、表面電位検知手段による場合、最も信頼性が高いものとなる。
【００４０】
転写工程上流側で中間転写体上の表面電位Ｖｔを、中間転写体の表面電位を調整する表面電位調整手段で調整し、調整した値に基づいてカウンターバイアス値を決定するようにしても良い。中間転写体の表面電位は図３に示したＶ０である。Ｖ０は中間転写体から記録材への転写時の放電等の影響を強く受けるため、紙種、環境、経時の抵抗変動等で大きく変化する。このため、カウンターバイアスを常に効果的に作用させ、逆転写を生じさせないためにＶ０をコントロールすることは有効である。
【００４１】
接触部の上流側での中間転写体の表面電位を調整する表面電位調整手段は、中間転写体に対し非接触状態で作用する除帯電手段で構成すると、中間転写体と除帯電手段とが非接触となるので、中間転写体上の状態によらず安定した除帯電が可能である。中間転写体の表面は使用に伴って摺擦による汚れ、傷等が発生する場合がある。また、トナーが付着した状態において除帯電を行いたい場合、接触では不可能であり、非接触状態であれば中間転写体の状態を選ばなくて済む。具体的には、スコロトロンチャージャが最も一般的な非接触の帯電器であり、そのグリッドに印加するＤＣ電圧を決定することで、略グリッドと同電位に中間転写体上をコントロールすることができる。また、非接触部材として帯電ローラ等を用いることが可能である。表面電位のコントロールという点では、定電圧制御とすることが好ましい。
【００４２】
接触部の上流側での中間転写体の表面電位を調整する表面電位調整手段としては、中間転写体に対し接触状態として中間転写体からトナーを除去する機能をも持たせてもよい。所謂、中間転写体からトナーを除去するクリーニング手段は、通常、ブラシ、ローラ、ブレード等のクリーニング部材を中間転写体に接触させるものでクリーニングを効率的に行うために、バイアスを印加する場合もある。このときのバイアスをコントロールし、クリーニング後の中間転写体の表面電位が安定するようにする。例えば、ローラを用いた帯電装置において、ＡＣ＋ＤＣの低電圧制御を行うと、クリーニング後の電位をＤＣ電圧の値とすることができる。また、クリーニング手段とカウンターバイアス印加手段を兼ねることで、構成の簡素化が可能である。
【００４３】
表面電位調整手段に印加する電圧値により、１色目の転写工程時に印加するカウンターバイアスを決定してもよい。カウンターバイアスの決定は、基本的には｜Ｖｄ−Ｖｃ―Ｖｔ｜＜Ｖｔｈを満たすように決定するのであるが、その際における１色目の転写前の中間転写体の表面電位にあたるＶｔの値を、表面電位調整手段に印加する電圧値から決定するようにする。表面電位調整手段では、印加電圧によりその後の中間転写体上の電位は決定されるため、１色目についてこの値をＶｔとして用いることで、判断が不安定になることもなく、また、色毎の表面電位検知手段等の装備も不要となるので、構成の簡素化が可能となる。
【００４４】
転写時のプレ転写、逆転写による画像の乱れを防止できるカウンターバイアスの範囲内において、転写時に印加するカウンターバイアスを、複数色について同電圧とすると、各色について独立した電圧供給手段や電源が不要となり構成の簡素化が可能である。
【００４５】
逆転写による画像の乱れを防止できるカウンターバイアスの範囲内である｜Ｖｄ−Ｖｃ―Ｖｔ｜＜Ｖｔｈによれば、カウンターバイアスＶｃとして設定可能な値の内の絶対値が最大となる値が求まる。求まった値よりも小さい絶対値であれば放電は発生せず、このため逆転写を防止できること。したがって、複数色について前記関係を満たすカウンターバイアス値Ｖｃを求めた後、最も絶対値が小さい値を全色の設定値とすれば、どの色も逆転写を発生することなく、電圧供給手段の共通化が可能である。
【００４６】
通常の複数色のトナー像を中間転写体上に付着させ画像を形成する場合、トナーの帯電極性は同極性であり、感光体の非画像部の電位も略同様である。このような前提で考えると、何色目の転写であるかによって異なるのは、接触部の上流側での中間転写体の表面電位Ｖｔで、色色のトナーが付着した分だけ、中間転主体の表面電位が変化すると考えられる。各色のトナーが最も多く付着した場合の１色についての電位変化をＶｓとすると、色を重ねる毎に印加するカウンターバイアスの絶対値をＶｓだけ小さくすればよい。このような方法でカウンターバイアスを決定することで、カウンターバイアス決定手段が１色目に相当する分の構成だけでよく、構成の簡素化が可能である。
【００４７】
最もカウンターバイアスの絶対値の小さいのは、最終番目の色トナーの転写時であり、イエロー，マゼン，シアン，ブラックの４色トナーを有する一般的な４色カラー画像形成の場合、４色目となる。また、各トナー色に対応する接触部の上流側へ供給するカウンターバイアスを最も絶対値が小さいカウンターバイアスに共通化すれば、全色において放電が発生せず、逆転写防止が可能である。したがって、４色目について決定されたカウンターバイアスを他の色のカウンターバイアスとして使用することで構成の簡素化、処理の効率化に有効に作用する。
【００４８】
このような方法を実現する構成としては次のものがある。
【００４９】
転写装置としては、回転可能な像担持体の帯電部に形成された静電潜像にトナーが付着されることで形成されるトナー像を、像担持体に中間転写体を接触させることで中間転写体の表面に転写する第１の転写手段と、中間転写体に転写されたトナー像を記録材に転写する第２の転写手段とを有することが前提となる。画像形成装置としては、像担持体としての感光体の表面を帯電する帯電手段、この帯電手段で帯電された感光体の帯電部に静電潜像を形成する露光手段、形成された静電潜像に現像剤を供給してトナー画像とする現像手段、像担持体に中間転写体を接触させることで中間転写体の表面にトナー画像を転写する第１の転写手段、中間転写体に転写されたトナー像を記録材に転写する第２の転写手段、記録材に転写されたトナー像を定着する定着装置を備えている。
【００５０】
そして、像担持体と中間転写体との接触部よりも、像担持体の回転方向上流側に形成される空隙に対応する部分の中間転写体にカウンターバイアスを印加するカウンターバイアス印加部材と、カウンターバイアス印加部材にカウンターバイアスを供給する電圧供給手段とを有し、カウンターバイアスを、像担持体の帯電極性と同極性でかつ絶対値が画像部電位より大きい電圧とし、かつ｜Ｖｄ―Ｖｃ―Ｖｔ｜＜Ｖｔｈを満たすように電源供給手段でバイアス制御を行う。
【００５１】
像担持体上の非画像部の表面電位Ｖｄは非画像部電位検知手段で検知し、この非画像部電位検知手段で検知した値に基づきカウンターバイアスの値をカウンターバイアス決定手段で決定し、電源供給手段は、カウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給する。
【００５２】
中間転写体の表面電位Ｖｔは、転写体表面電位検知手段で検知し、この転写体表面電位検知手段で検知した値に基づきカウンターバイアスの値をカウンターバイアス決定手段決定する。そして、電源供給手段は、このカウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給する。
【００５３】
転写体表面電位検知手段は、接触部よりも上流側で中間転写体の表面電位Ｖ０を検出する検知手段と、中間転写体上にすでに転写されているトナー層の表面電位Ｖを検知するトナー層電位検知手段と、検知手段及びトナー層電位検知手段で検知した電位を加算して中間転写体上の表面電位Ｖｔを算出する演算部とで構成する。トナー層電位検知手段は、中間転写体上のトナー層が像担持体上にあったときの表面電位Ｖｅを検知する検知手段と、この検知手段で検知した表面電位Ｖｅに基づきトナー層の表面電位Ｖを推定するトナー層表面電位推定手段とで構成してもよい。
【００５４】
中間転写体の表面電位Ｖ０を調整する表面電位調整手段と、表面電位調整手段により調整された表面電位に基づきカウンターバイアスを決定するカウンターバイアス決定手段とを有する場合、電源供給手段は、このカウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給する。表面電位調整手段としては、中間転写体に対して接触状態でも非接触状態の何れの携帯でも良く。一般に中間転写体の近傍には、クリーニング部材や帯電・除電部材が配設されているので、これら部材らを中間転写体に対するカウンターバイアス印加部材として用いても良い。
【００５５】
本発明が適用される画像形成装置としては、レーザー複写機やレーザープリンター、ファクシミリ、あるいはこれらの複合機が挙げられる。無論各画像形成装置としては所謂白黒画像専用のものであっても、カラー画像形成用のものでもよい。
【００５６】
【実施例】
以下、本発明を適用した画像形成装置として、カラーレーザ複写機（以下、単に「複写機」という）の実施例について説明する。この複写機の基本的な構成について図１を用いて説明する。複写機は、プリンタ部１００、給紙部２００、プリンタ部１００の上部に固定されたスキャナ部３００、これに取り付けされた原稿自動搬送装置（以下、ＡＤＦという）４００等とともに、複写機内の各装置の動作を制御する図示しない制御手段を備えている。
【００５７】
スキャナ部３００は、コンタクトガラス３０１上に載置された原稿の画像情報を読取センサ３０２で読取り、読取った画像情報をこの制御部に送る。図示しない制御手段は、受け取った画像情報に基づいてプリンタ部１００の内部に位置する露光装置１０内の図示しないレーザーやＬＥＤ等を制御して、後述する帯電手段としての帯電器で帯電された像担持体としてのドラム状の感光体（以下「感光体ドラム」と記す）２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃの帯電部に向けてそれぞれ色毎のレーザー光Ｌを照射させる。この照射により、感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃの表面の帯電部には静電潜像がそれぞれ形成され、所定の現像プロセスを経由して各静電潜像に対応する色のトナーが付着することで各色のトナー像に現像される。これら４つの感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃは、プリンタ部１００に設けられたタンデム画像形成部２０内に並列直線状に配設されている。４つの感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃの配置は並列直線状に限定されるものではなく、図１においてタンデム画像形成部２０の左右何れか一方を下げた傾斜配置であっても良い。
【００５８】
プリンタ部１００は、露光装置１０やタンデム画像形成部２０の他、次に説明する装置も備えている。すなわち、第１の転写手段としての中間転写ユニット３０と、第２の転写手段としての２次転写ユニット４０、定着手段として定着ユニット５０、排紙ローラ対８０、図示しないトナー供給装置などである。なお、上記現像プロセスについては後述する。
【００５９】
給紙部２００は、そのペーパーバンク２０１内に多段に設けられた複数の給紙カセット２０２、紙搬送路２０５、これの途中に適宜設けられた複数の搬送ローラ対２０６等を備えている。それぞれの給紙カセット２０２は、カセット内部に収容された記録材としての記録紙を一番上のものから順次送り出す給紙ローラ２０３、給紙ローラ２０３によって重送されてしまった複数の記録紙を個々に分離してから紙搬送路２０５に送り出す分離ローラ２０４を有している。搬送ローラ対２０６は、給紙カセット２０２から受け取った記録紙を後段の搬送ローラ対２０６、あるいはプリンタ部１００内に設けられた給紙路６０に向けて送り出す。本例に係る複写機においては、係る構成の給紙部２００による給紙の他に、手差し給紙も可能となっている。そして、この手差し給紙を実現するための、手差しトレイ７０をプリンタ部１００の一方側面となる右側面に備えている。手差しトレイ７０は給紙ローラ７１と分離ローラ７２を備えており、これらによって手差しトレイ７０にセットされた記録紙をプリンタ部１００内の給紙路６０内に送り出す。
【００６０】
給紙部２００や手差しトレイ７０から給紙路６０内に送り込まれた記録紙は、給紙路６０の途中に設けられたレジストローラ対６１に挟まれる。このレジストローラ対６１は、挟み込んだ記録紙を所定のタイミングで２次転写ニップＮ２に送り込む。この２次転写ニップ部Ｎ２とは、中間転写ユニット３０と２次転写ユニット４０との当接によって形成されるニップ部である。中間転写ユニット３０と２次転写ユニット４０は転写装置を構成している。
【００６１】
装置利用者は、カラーコピーをとるために、まず、原稿をＡＤＦ４００の原稿台４０１上にセットするか、あるいはＡＤＦ４００の開操作によって露出させたスキャナ部３００のコンタクトガラス３０１上にセットする。そして、図示しないスタートスイッチを押す。すると、ＡＤＦ４００からコンタクトガラス３０１上に搬送された原稿、あるいは初めからコンタクトガラス３０１上にセットされた原稿の画像情報を読取るために、スキャナ部３００の駆動が開始される。具体的には、第１走行体３０２の走行を開始してその光源から発した光を原稿面で反射させて第２走行体３０３に向けて送る。そして、同じく走行を開始した第２走行体３０３のミラーによってこの反射光を受けて結像レンズ３０４を通して読取りセンサ３０５に入れて画像情報を読取る。
【００６２】
図示しない制御手段は、スキャナ部３００から画像情報を受け取ると、上述のようなレーザー書込みや現像プロセスによって感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃ上にＢｋ、Ｙ、Ｍ、Ｃ色のトナー像をそれぞれ形成せしめる。なお、記号Ｂｋ、Ｙ、Ｍ、Ｃは、それぞれブラック、イエロー、マゼンタ、シアンの略である。
【００６３】
図２は、プリンタ部１００の一部構成を拡大して示す拡大構成図である。図２において、タンデム画像形成部２０は、４つのプロセスユニット２１Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃを有している。各プロセスユニットは、使用するトナーの色が互いに異なっているが、その他の構成については略同様である。よって、Ｂｋトナーを用いるプロセスユニット２１Ｂｋだけについてその構成を詳述し、他のプロセスユニットの説明については、プロセスユニット２１Ｂｋで使用した数字符号と同一の符号を付すに留め、詳細な説明は省略する。
【００６４】
プロセスユニット２１Ｂｋは、感光体ドラム２２Ｂｋ、帯電器２３Ｂｋ、現像手段２４Ｂｋ、除電ランプ２６Ｂｋ、クリーニング装置２７Ｂｋ等を備えている。感光体ドラム２２Ｂｋは、ユニットの側面に回転可能に支持されていて、図示しない駆動手段によって図中反時計回りに回転駆動されながら、帯電器２３Ｂｋによってその表面が一様帯電せしめられる。そして、一様帯電後の表面が帯電部を構成し、その帯電部に上述のレーザー書込光Ｌが照射されることで静電潜像が形成される。この静電潜像は、像形成物質である、所謂現像剤としてのＢｋトナーを用いる現像器２４ＢｋによってＢｋトナー像に現像される。形成されたＢｋトナー像は、転写ユニット３０の一構成要素を成す中間転写体としての中間転写ベルト１１上に中間転写される。感光体ドラム２２Ｂｋから中間転写ベルト３１上にトナー画像が転写される工程を、ここでは第１の転写工程と称する。
【００６５】
第１の中間転写工程を終えた感光体ドラム２２Ｂｋの表面は、除電ランプ２６Ｂｋによって完全に除電された後、クリーニング装置２７Ｂｋによってその表面の転写残トナーがクリーニングされる。他のプロセスユニット２１Ｙ，２１Ｍ，２１Ｃでも同様のプロセスが実施され、Ｙ，Ｍ，Ｃの各トナー像が形成されて中間転写ベルト３１に準じ重ね転写される。但し、Ｂｋのトナー画像だけを形成する白黒画像形成モードの場合には、プロセスユニット２１Ｙ，２１Ｍ，２１Ｃでの機能は使用されない。
【００６６】
中間転写ユニット３０は、中間転写ベルト３１、複数（３つ）の張架ローラ３２，３３，３４、ベルトクリーニング装置３７を備えている。中間転写ベルト３１は、３つの張架ローラ３２，３３，３４に張架されて各プロセスユニットとの対向側が略水平となるように配置されている。張架ローラ３２，３３，３４の内の１つは、図示しない駆動手段によって回転駆動される駆動ローラであり、他のローラは従動ローラである。中間転写ベルト３１は、駆動ローラが回転駆動されると、図中時計回り方向に無端移動せしめられる。
【００６７】
感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃには、中間転写ベルト３１を介して転写部材としての中間転写ローラ３６Ｂｋ，３６Ｙ，３６Ｍ，３６Ｃがそれぞれ対向配置されている。各中間転写ローラは、それぞれ中間転写ベルト３１の裏面となる内周面に当接するように配設されていて、中間転写ベルト３１を各感光体ドラムの表面に接触させて、各感光体ドラムと中間転写ベルト３１との接触部となる第１の転写部（以下、「中間転写ニップ部」と記す）ＮＢｋ，ＮＹ，ＮＭ，ＮＣを形成している。中間転写ローラ３６Ｂｋ，３６Ｙ，３６Ｍ，３６Ｃよりも中間転写ベルト３１の移動方向の上流側、すなわち、各中間転写ニップ部よりも各感光体ドラムの回転方向上流側には、カンターバイアス印加部材としてのカウンターバイアスローラ３５Ｂｋ，３５Ｙ，３５Ｍ，３５Ｃがそれぞれ設けられている。これらカウンターバイアスローラ３５Ｂｋ，３５Ｙ，３５Ｍ，３５Ｃは、それぞれ中間転写ローラ３６Ｂｋ，３６Ｙ，３６Ｍ，３６Ｃと並列配置されていて、同色の記号が付されたカウンターバイアスローラと中間転写ローラとが一つの対をなしている。例えば、Ｂｋ用においては、カウンターバイアスローラ３５Ｂｋと中間転写ローラ３６Ｂｋとが一つの対を構成している。
【００６８】
カウンターバイアスローラ３５Ｂｋ，３５Ｙ，３５Ｍ，３５Ｃは、感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃの回転方向上流側において中間転写ベルト３１と各感光体ドラムとの間に形成される空隙、すなわち、中間転写ニップＮＢｋ，ＮＹ，ＮＭ，ＮＣよりも上流側に形成される空隙Ｓ１，Ｓ２，Ｓ３，Ｓ４に対応する部分の中間転写ベルト３１にカウンターバイアスＶｔを印加するものである。
【００６９】
中間転写ニップ部ＮＢｋ，ＮＹ，ＮＭ，ＮＣには、図示しない電源によって中間転写ローラ３６Ｂｋ，３６Ｙ，３６Ｍ，３６Ｃにそれぞれ中間転写バイアスが印加されることによって、中間転写電界が作用する。また、カウンターバイアスローラ３５Ｂｋ，３５Ｙ，３５Ｍ，３５Ｃには、後述する電圧供給手段からカウンターバイアスが印加される。これらカウンターバイアスローラにそれぞれ所定の電圧を印加することで、各中間転写ニップ部の上流側において、各感光体ドラムから中間転写ベルト３１へのトナーのプレ転写を防いでいる。
【００７０】
図２に示す感光体ドラム２２Ｂｋ，２２Ｙ，２２Ｍ，２２Ｃ上に形成されたＢｋ，Ｙ，Ｍ，Ｃの各トナー像は、この中間転写電界や中間転写ニップ部でのニップ圧の影響を受けて中間転写ベルト３１上に中間転写（１次転写）される。中間転写ベルト３１への１次転写は、Ｂｋ，Ｙ，Ｍ，Ｃのトナー像という順で、順次重ね合わされるように行われる。これにより、中間転写ベルト３１上には４色の重ね合わせトナー像が形成される。
【００７１】
図２において、符号２８Ｂｋ，２８Ｙ，２８Ｍ，２８Ｃは、各感光体ドラムに対向するように配置され、各感光体ドラム上の表面電位をそれぞれ検知する表面電位検知手段としての表面電位計である。図２おいて、符号２９Ｙ，２９Ｍ，２９Ｃは、中間転写ベルト３１と対向配置され、各中間転写ニップ部よりも上流側の中間転写ベルト３１上の表面電位を検知する転写体表面電位検知手段としての表面電位計である。各表面電位計と対向する中間転写ベルト３１の内側には、図示しないがアースされた電極が配置されている。本実施例では、中間転写ニップＳ１の上流側にスペースが無いため、感光体ドラム２２ＢＫ近傍の中間転写ベルト３１上の表面電位を検知測定する表面電位計は、配置しない構成とした。しかし、感光体ドラム２２ＢＫの近傍には張架ローラ３３が配置される構成となっているため、張架ローラ３３の電位をグランドするととも張架ローラ３３に対向する位置に表面電位計を設置しても良い。
【００７２】
２次転写ユニット４０は、搬送ベルト４１、２つの張架ローラ４２，４３を有している。搬送ベルト４１は、張架ローラ４２，４３に巻きかけられて張架されており、張架ローラ４３が図示しない駆動手段によって回転駆動されることで、図中反時計回りに無端移動せしめられる。張架ローラ４２は、張架ローラ３４と対向配置されていて、搬送ベルト４１と中間転写ベルト３１とが当触するように配置されている。この当接により、中間転写ユニット３０と２次転写ユニット４０との間には、すなわち、中間転写ベルト３１と搬送ベルト４１の当接部には２次転写ニップ部Ｎ２が形成されている。この２次転写ニップ部Ｎ２には、図示しない電源から張架ローラ４２に２次転写バイアスが印加されることで２次転写電界が作用している。
【００７３】
図２に示すように、プリンタ部１００内の給紙路６０に給紙された記録紙は、レジストローラ対６１に挟まれる。このレジストローラ対６１は、挟み込んだ記録紙を中間転写ベルト３１上の４色重ね合わせトナー像に重ね合わせうるタイミングを見計らって２次転写ニップ部Ｎ２に送り出す。２次転写ニップ部Ｎ２においては、中間転写ベルト３１上の４色重ね合わせトナー像が、２次転写電界や２次転写ニップ部Ｎ２に作用するニップ圧の影響を受けて記録紙上に２次転写される。一般に記録紙は白色を呈しているため、４色重ね合わせトナー像が転写されると、これがフルカラー画像となる。このようにしてフルカラー画像が形成された記録紙は、搬送ベルト４１の無端移動に伴って定着装置５０内に送られる。そして、定着装置５０内に設けらたれ加熱ローラ５１と加圧ローラ５２との間に挟まれてフルカラー画像がその表面に定着せしめられた後、排紙ローラ対８０を経由して機外の排紙トレイ９０上に排出される。２次転写後の中間転写ベルト３１上に残留する転写残トナーは、中間転写ユニット３０のベルトクリーニング装置３７によってベルト表面から強制的に除去される。
【００７４】
次に、カウンターバイアスの印加原理について図６を用いて説明する。ここでは、便宜上、７１を感光体ドラム、７２を中間転写ベルト、７３を中間転写ローラ、７４をカウンターバイアスローラとする。図中白○はトナー像を示す。図６において、中間転写ベルト７２は、図中左から右に移動し、感光体ドラム７１は中間転写ベルト７２とその表面速度が略等速になるように反時計回りに回転している。カウンターバイアスローラ７４は感光体ドラム７１と中間転写ベルト７２との接触部Ｎの上流部に位置する。図中、接触部Ｎは、トナーの移動を示すため、感光体ドラム７１と中間転写ベルト７２とを離間させた状態で示している。
【００７５】
カウンターバイアスローラ７４がない一般的な転写装置の場合、中間転写ベルト７２と感光体ドラム７１の接触前の領域（接触部Ｎの上流側）でも中間転写ローラ７３に印加される転写バイアスの影響で、感光体ドラム７１から中間転写ベルト７２へのトナーの移動（プレ転写）が生じてしまう。このプレ転写が生じるのは、感光体ドラム７１と中間転写ベルト７２との空隙が数百μｍ〜接触までと考えられるが、転写バイアスとのバランスを考えると、できるだけ接触部Ｎ１に近い上流側の部分で中間転写ベルト７２にカウンターバイアスを印加することが好ましい。
【００７６】
図７は、中間転写部材として中間転写ブレード７５を、カウンターバイアス印加部材としてローラではなくカウンターバイアスブレード７６をそれぞれ用いるとともに、感光体ドラム７１を中間転写ベルト７２の内側に食い込むように配置し、中間転写ベルト７２と感光体ドラム７１との接触部Ｎ１の幅を確保したものである。この例では、感光体ドラム７１の直径は４０ｍｍ、感光体ドラム７１と中間転写ベルト７２との接触幅Ｎ１（ニップ幅）は約１０ｍｍである。図７において、中間転写ベルト７２は図中左から右に移動する。そして、感光体ドラム７１と中間転写ベルト７２の接触部Ｎ１の最下流側に中間転写ブレード７５を配置し、カウンターバイアスブレード７６との距離を確保している。カウンターバイアスブレード７６は、感光体ドラム７１と中間転写ベルト７２との接触部Ｎ１の最上流部から上流５ｍｍの場所に、中間転写ベルト７２の内面に接触して設けられている。
【００７７】
このような構成をすることで、中間転写ベルト７２と感光体ドラム７１の接触部Ｎ，Ｎ１よりも上流側の空隙に効率的にカウンターバイアスを作用させることができる。また、図７に示す中間転写ブレード７５の代わりに中間転写ベルト７２を内側から支持するローラ７９に転写バイアスを与えるような構成をとることも可能である。この場合、カウンターバイアス印加部と転写バイアス印加部の距離を必要な範囲において離すことができるので、カウンターバイアスを効果的に作用させることが可能である。ローラ７９の配置場所としては、接触部Ｎ１の最下流側よりも幾分下流側の位置であってもよい。
【００７８】
図６において、中間転写ベルト７２の面の内、カウンターバイアスローラ７４が接触する側の抵抗が低いと、カウンターバイアスローラ７４と中間転写ローラ７３との間で大きな電流が流れてしまうため、抵抗値は充分高いことが望ましい。具体的には、１×１０^８Ωｍ〜１×１０^１１Ωｍ程度が使用可能であるが、好ましくは１×１０^９Ωｍが良い。ベルト抵抗が高い場合、自己除電しなくなり、除電装置により中間転写ベルト７２を除電する必要がある。自己除電は、ベルトのバルクを電荷が流れて行われるので、カウンターバイアスローラ７４と中間転写ローラ７３の間には電流を流さず、ベルトバルクは電流を流しやすい構造とすることが可能であれば、本方式に最も適した中間転写ベルトであるといえる。
【００７９】
中間転写ベルト７２を多層構造とすることで、構成の自由度が広がる。例えば、中間転写ベルトを３層構造として、カウンターバイアスローラ７４が接触する内側から感光体ドラム７１が接触する外側に向かって層１、層２、層３とすると、それぞれの体積抵抗率をρ１、ρ２、ρ３としたときρ１＞ρ２＞ρ３、あるいは、ρ１＞ρ２＜ρ３とし、全体の抵抗値を１×１０^８Ωｍ〜１×１０^１０Ωｍとなる様に構成すれば良い。
【００８０】
（実験１）
図１に示した複写機を用い、１色目、２色目（Ｂｋ、Ｙ）の作像で実験を行った。１色目、２色目ともにプロセス条件は、感光体帯電電位−４００Ｖ、レーザーによりライン画像を書き込み出力した。同時にベタ画像を書き込み、潜像の電位を測定したところ、−１００Ｖであった。このライン画像を２成分の現像装置により現像した。現像バイアスは−３５０Ｖで実験を行った。このときの感光体ドラム２２ＢＫ，２２Ｙ上トナー付着量は０．４ｍｇ／ｃｍ^２であった。また、図２に示した中間転写ローラ３６ＢＫ、３６Ｙに転写時に印加する電圧は１色目が８００Ｖ、２色目が９００Ｖである。
【００８１】
図４は、１色目、２色目のカウンターバイアスローラ３５ＢＫ，３５Ｙに印加する電圧を変えて、感光体ドラム２２ＢＫ、２２Ｙから中間転写ベルト３１に転写した後の、中間転写ベルト３１上のトナー像のチリを評価したものである。チリ量の評価は、ライン画像のエッジ部から地肌部へはみ出したトナーの面積を指標としてとらえたもので、値が大きくなる程チリの程度が悪いことを表す。まず、１色目のカウンターバイアスローラ３５ＢＫに印加する値を変えてチリを評価した。図中の実線がその結果である。チリ量２のレベルが、別途行った主観評価により許容値と定めたレベルである。チリ量は、カウンターバイアスローラ３５ＢＫに印加する電圧がマイナス側に大きくなるほど改善の傾向が見られる。カウンターバイアスローラ３５ＢＫに印加する電圧が０の場合、よりプラス側である場合と比較すると、チリは改善しているが充分とはいえない。図４の結果より、−１００Ｖのカウンターバイアスよりマイナス側に大きい値のカウンターバイアスを印加することで、チリ量は許容値を満足することがわかった。
【００８２】
この結果より、１色目のカウンターバイアスローラ３５ＢＫに印加する電圧を−１００Ｖとして２色目のカウンターバイアスを変えて実験を行った。２色目のカウンターバイアスの値を変えて２色目画像のチリ量を、転写後中間転写ベルト３１上で評価したところ、図４の破線に示すようになった。２色目の結果は、１色目の結果を横軸方向にシフトした形であるが、ここでカウンターバイアスをある程度マイナス側に大きくした時、２色目転写時に中間転写ベルト３１上に存在する１色目画像が乱れる現象が観察された。
【００８３】
この現象を詳しく見ると、１色目の画像は、２色目の転写の際に、逆に中間転写ベルト３１から感光体ドラム２２ＢＫに転写する逆転写が起こっていた。逆転写量を、感光体ドラム２２ＢＫ上の逆転写付着量として評価し、その結果を図８に示す。図８において、横軸はカウンターバイアスの値（−Ｖ）を示し、縦軸は逆転写量を示す。カウンターバイアス−１０００Ｖまでは、逆転写は観察されず、−１２００Ｖよりマイナス側に大きいカウンターバイアスでは、絶対値の増加に伴って逆転写量が直線的に増加している。
【００８４】
この実験した系におけるパッシェンの放電限界を計算してみる。計算は前述の式（１）によって行う。像担持体誘電率を３、膜厚を２５μｍ、中間転写体誘電率を９、膜厚を１５０μｍとして計算すると、放電開始電圧は７５０Ｖとなる。逆転写の原因を感光体ドラム２２ＢＫ上の非画像部と中間転写ベルト３１上の画像部との放電であると仮定し、逆転写が発生したカウンターバイアス−１２００Ｖ印加時における、中間転写ベルト３１の接触部の上流側での電位を考察する。非画像部電位は−４００Ｖ、中間転写ベルト３１の画像部の電位は−１００Ｖ、カウンターバイアスの値は−１２００Ｖであるので、感光体ドラム２２ＢＫの非画像部と中間転写ベルト３１の画像部との電位差Ｖｇは、概ね９００Ｖとなる。また逆転写が発生しなかったカウンターバイアス−１０００Ｖでの電位差Ｖｇは７００Ｖである。
【００８５】
実験結果において、−１２００Ｖ印加時より逆転写が発生し、カウンターバイアスの絶対値の増加とともに直線的に逆転写量が増加するのは、カウンターバイアスの絶対値の増加に従って放電電荷量が増加する傾向と一致すると考えられ、放電によって電荷量が不安定になった中間転写ベルト３１上のトナーが感光体ドラム２２ＢＫへ逆転写すると予想される。このような知見に基づいて、電位差Ｖｇが放電開始電圧以下となるカウンターバイアスを印加すれば、逆転写防止が可能であることがわかった。
【００８６】
（実験２）
図２に示す構成を用い、各感光体ドラム上、中間転写ベルト３１上の表面電位を測定しながら、実験を行った。作像の色順は、図示した通りＢＫ、Ｙ、Ｍ、Ｃである。各色の感光体ドラム上の帯電電位を測定し、その測定値をそれぞれＶｐＢｋ、ＶｐＹ、ＶｐＭ、ＶｐＣとする。また、中間転写ベルト３１上の表面電位をそれぞれＶｔＢｋ、ＶｔＹ、ＶｔＭ、ＶｔＣとする。１色目Ｂｋと２色目Ｙを作像し、１色目のＢｋトナーが中間転写ベルト３１上から２色目のＹの感光体ドラム２２Ｙへ逆転写が発生する条件を調べた。２色目の感光体ドラム２２Ｙ上の非画像部電位を固定し、１色目のトナーの作像条件を変化させ中間転写ベルト３１上でのトナー層の電位を変えた。この条件で、カウンターバイアスを１００Ｖづつ変えてプリントを行い、逆転写の発生有無をプロットした。図９はその結果を示す。横軸はカウンターバイアス、縦軸は中間転写体上のトナー層表面電位である。図中のプロットは逆転写が観察された点であり、プロットより右側では逆転写が発生する。非画像部電位−３００Ｖについてのプロットは「○」で示した。中間転写ベルト３１上のトナー像の電位が−１２０Ｖの時はカウンターバイアス−１０００Ｖでは逆転写が見られ、−９００Ｖでは見られなかった。同様に非画像部で−４００Ｖについても実験を行い、逆転写が発生した電圧に「△」を示した。
【００８７】
このような結果に基づき、カウンターバイアスのコントロールを行った。具体的には、各感光体ドラムの非画像部電位の変化範囲と、中間転写ベルト３１上でのトナー層の電位範囲は、ある範囲内を示し、この範囲を超えた場合プロセス異常の可能性があり、通常のコントロールの範囲を外れる。本実施例では、イエローの感光体ドラム２２Ｙの非画像部電位と、中間転写ベルト３１上のトナー付着部の表面電位Ｖｔとの組み合わせによって、カウンターバイアスの値を、テーブルを参照することによって決定した。実験に使用したテーブルを図１０に示す。図１０に示すテーブルに基づくカウンターバイアスとし、２色目の転写を行ったところ、転写チリもなく逆転写も生じない良好な画像が得られた。また、印加可能なカウンターバイアスより絶対値が小さい値でも、感光体ドラムの画像部電位ＶＬより絶対値が大きければ良いので、図１１に示すようなテーブルを用いることも可能であった。この場合もチリも生じず、逆転写も発生しない良好な画像が得られた。
【００８８】
同様に、３色目、最終色目となる４色目についても、各色転写部上流側の中間転写ベルト３１の表面電位、各感光体ドラムの非画像部電位を対応する表面電位計で測定し、プロットしたところ、結果は図９と同じ直線に載った。これにより、何色目の転写についても、感光体ドラム上の非画像部電位が固定の場合、転写前の中間転写ベルト３１の表面電位とカウンターバイアス値の差を一定とするようにコントロールした場合、逆転写を確実に防止できた。
【００８９】
（実験３）
中間転写ベルト３１上の表面電位Ｖｔを測定する代わりに、各感光体ドラム上のトナー付着部の表面電位Ｖｅを測定して、中間転写ベルト３１上の表面電位Ｖの推定を行った。ベタのテストパターンを、最終画像に影響しない感光体ドラム上に形成して現像したトナー像の電位を測定した。このパターンの潜像電位は−１００Ｖであった。測定したトナー層の電位は、トナーの電荷量と感光体ドラムの膜厚、誘電率により決まっている。トナー層は略１００％の転写率で中間転写ベルト３１上に転写されており、また、転写時の画像部に放電は生じていないので電荷量は変化していないものとする。同じ電荷量が、中間転写ベルト３１上に移動した場合、その電位は中間転写ベルト３１の膜厚と誘電率により決まる。本実験の場合、各感光体ドラムから中間転写ベルト３１への移動でｄ／εは約２倍となっているので、中間転写ベルト３１上のトナー層の表面電位Ｖは、感光体ドラム上のトナー層電位の略２倍であると推定できた。
【００９０】
２色目転写時の中間転写ベルト３１の表面電位を、前記推定により決定し、実験２と同様に画像を出したところ、良好な画像が得られた。この実験では１色目の転写を行う以前の中間転写ベルト３１の表面電位は図示しない除電装置により略０Ｖとした。除電を行わない場合、表面電位が本実験時には、約＋３００Ｖとなっており、このまま実験を行ったところ０Ｖの時にプレ転写によるチリが発生しないカウンターバイアスでも多くのプレ転写が発生し、また、逆転写が発生するカウンターバイアスの限界値も高くなっている。おおよそ、＋３００Ｖの帯電だけ、カウンターバイアスの効果が低くなる結果が得られた。
【００９１】
したがって、中間転写ベルト３１の表面電位Ｖｔを推定する際に、転写以前での中間転写ベルト３１の帯電があった場合には考慮する必要がある。転写以前の中間転写ベルト３１の帯電電位Ｖ０、１色目トナー層の推定電位Ｖ１、２色目トナー層の推定電位Ｖ２、３色目トナー層の推定電位Ｖ３とすると、各色の転写部（接触部ＮＢＫ、ＮＹ、ＮＭ、ＮＣ）上流側での中間転写ベルト３１の表面電位は、そのとき中間転写ベルト３１上に転写されているトナー層について加算（４色目ではＶ０＋Ｖ１＋Ｖ２＋Ｖ３）とすることで推定できた。一方、転写前の中間転写ベルト３１の帯電電荷量は、中間転写ベルト３１から紙等の記録材への転写条件により変化するため、安定性の点からは、転写前の中間転写ベルト３１の帯電電荷量を調整することが好ましい。
【００９２】
図１２，図１３は表面電位調整手段１０１、１０２を示す。図１２に示す表面電位調整手段１０１は、中間転写ベルト３１の除電装置であり、交流電流（ＡＣ）を印加したスコロトロン帯電器９１を中間転写ベルト３１に向けて配置した。このときの対向部材となる中間転写ベルト３１を架橋しているローラ３３をアースにしている。グリッドの電位を略０Ｖとし、スコロトロン帯電器９１による除電後の電位を調べたところ略０Ｖで効果的な除電が可能であった。
【００９３】
図１３に示す表面電位調整手段１０２は、クリーニング装置と除帯電器を一体化したものである。図１３において、９２はクリーニング部材としてのブレード、９３はクリーニング部材であり帯電調整部材としても機能するクリーニングローラ、９４がクリーニングローラ９３への電圧供給装置である。クリーニングローラ９３には、ＡＣバイアスを重畳したＤＣバイアスを印加している。中間転写ベルト３１上の転写残トナーをクリーニングローラ９３に付着させる目的であるので、ＤＣバイアスは若干の正極性である必要があり、本実験では＋１００Ｖを印加している。このときの印加バイアスと帯電電位との関係を図１４に示す。図１４において、縦軸は帯電電位、横軸は印加バイアスをそれぞれ示す。図１４に示すように、ＤＣバイアスの値と帯電電位とは比例関係にあり、印加ＤＣバイアスによりクリーニング後の電位を決定することができた。
【００９４】
（実験４）
図１の装置構成に、図１５に示す構成を用いてカウンターバイアスを決定して作像を行った。図１５において、符号５００は、中間転写ベルト３１の表面電位Ｖｔを検出する転写体表面電位検知手段、５０４は感光体ドラムの非画像部の表面電位Ｖｄを検知する非画像部電位検知手段、５０５はカウンターバイアスの値を決定するカウンターバイアス決定手段、５０６はカウンターバイアス決定手段５０５からの信号に基づきカウンターバイアス印加部材であるカウンターバイアスローラやカウンターバイアスブレードにカウンターバイアスＶｃを供給する電源供給手段５０６を示す。
【００９５】
転写体表面電位検知手段５００、接触部よりも上流側で中間転写ベルト３１の表面電位Ｖ０を検出する検知手段５０１と、中間転写ベルト３１上にすでに転写されているトナー層の表面電位Ｖを検知するトナー層電位検知手段５０２と、検知手段５０１及びトナー層電位検知手段５０２で検知した電位を加算して中間転写ベルト３１上の表面電位Ｖｔを算出する演算部５０３とで構成されている。
【００９６】
本発明に係る転写装置及び画像形成装置において、転写体表面電位検知手段で検知した値に基づきカウンターバイアスの値を決定するカウンターバイアス決定手段とを有し、電源供給手段が、カウンターバイアス決定手段からの信号に基づきカウンターバイアス印加部材にカウンターバイアスを供給すると、中間転写体の電位履歴に依らず、画像の乱れを防止し、逆転写が確実に防止される。
【００９７】
図１において、画像形成部２０はタンデム方式であるので、各色のカウンターバイアスローラに対して図１５に示す構成を用いてカウンターバイアスの決定をそれぞれ行った。１色目の転写時には、中間転写ベルト３１上にトナー像は無いので、トナー層電位検知手段５０２から演算部５０３には電位ゼロに相当する信号が送られる。検知手段５０２からは、１色目の転写部上流側での電位に相当する信号が演算部５０３に送られる。演算部５０３では、両信号を処理し、両電位を加算した電位Ｖｔを算出する。このＶｔに相当する信号はカウンターバイアス決定手段５０５に送られる。一方、非画像部電位検知手段５０４では、１色目（Ｂｋ）の感光体ドラムの非画像部の電位を検知し、この電位に相当する信号をカウンターバイアス決定手段５０５に送る。カウンターバイアス決定手段５０５では、送られてきたＶｔに相当する信号と、非画像電位に相当する信号とから、適切な１色目転写部におけるカウンターバイアス、すなわちカウンターバイアスローラ３５Ｂｋに対するカウンターバイアスを決定する。
【００９８】
２色目の場合も同様であるが、２色目の場合トナー層電位としてすでに転写された１色目のトナーによる帯電電位が検知されるので、表面電位Ｖｔ決定に際し、トナー層電位に相当する信号が加えられる。このようにして、４色全てについてカウンターバイアスを決定し、対応する各カウンターバイアスローラにそれぞれ電圧供給手段５０６から供給して作像を行った。各感光体ドラムの非画像部電位を変化させた場合においても、チリ、逆転写の発生を抑え、良好な画像を得ることができた。
【００９９】
（実験５）
図１に示す各色のカウンターバイアスローラに対して図１６に示す構成を用いてカウンターバイアスのそれぞれ決定して作像を行った。図１６において、カウンターバイアス決定手段が、図１０に示すカウンターバイアス決定手段に対応し、各色についての決定手段をそれぞれ５０５Ｂｋ、５０５Ｙ、５０５Ｍ、５０５Ｃと示す。符号５０７は、それぞれのカウンターバイアス決定手段で決定されたカウンターバイアスを比較して最もその絶対値が低いものを選択するカウンターバイアス調整手段を示す。このカウンターバイアス調整手段５０７には、電圧供給手段５０８が接続されている。そして電圧供給手段５０８には、各カウンターバイアスローラがそれぞれ接続されている。
【０１００】
それぞれのカウンターバイアス決定手段では、放電が生じないカウンターバイアスの絶対値の最大が示されるので、示された値より絶対値が小さい値であれば、逆転写は生じない。
【０１０１】
このことから、カウンターバイアス調整手段５０７において、各色のカウンターバイアス調整手段からの出力を比較して最も絶対値が小さい値を選択し、全色に対するカウンターバイアスとして実験を行った。この決定したカウンターバイアスの値が、画像部電位ＶＬより絶対値が小さい値になる色が発生するとプレ転写によるチリの発生が考えられたが、本実験では、そのような色はなく、全色について、逆転写もなくチリも抑えた転写が可能であった。すなわち、カウンターバイアス値を同一電圧とした。このとき、絶対値が最も小さくなる色は、最終色目となる４色目のシアンであった。フルカラー画像の場合、中間転写ベルト３１上には各色幾らかのトナーが上乗せして行くためと考えられる。次に、カウンターバイアスの判断を最終色目となる４色目にしてのみ行い、全色このカウンターバイアス値で転写を行ったところ、同様に良好な画像が得られた。
【０１０２】
１色目の転写上流部での中間転写ベルト３１の表面電位を図１２，図１３に示す表面電位調整手段１０１や１０２により、＋１００Ｖに調整し、実験を行った。図１６におけるカウンターバイアス決定手段：ＢＫのブロック図１０におけるＶｔを、表面電位調整手段により調整した＋１００Ｖの値として処理を行い、ＢＫのカウンターバイアスを決定した。そして、各色のトナーの付着量が最も多い場合の１色あたりの電位Ｖｍａｘを決め、各色は、Ｂｋより下流に行く順にＶｍａｘずつ絶対値が小さい値にカウンターバイアスを設定した。今回は、１５０Ｖずつ絶対値を下げて作像を行ったが良好な画像が得られた。Ｖｍａｘをシステムに適した値とすることで、１色目のカウンターバイアスを基準に各色のカウンターバイアスを決定することが可能である。
【０１０３】
図１５において、トナー層電位検知手段５０２を、図１７に示すように、中間転写ベルト３１上のトナー層が像担持体上にあったときの表面電位Ｖｅを検知する検知手段５０２Ａと、この検知手段で検知した表面電位Ｖｅに基づきトナー層の表面電位Ｖを推定するトナー層表面電位推定手段５０２Ｂで構成すると、１色目を転写する以前の中間転写ベルト３１の表面電位変動に対応してカウンターバイアスを決定することができる。
【０１０４】
なお、本実施例において、各電圧供給手段５０６，５０８においては、基本的には、カウンターバイアスＶｃを、各感光体ドラムの帯電極性と同極性でかつ絶対値が画像部電位ＶＬより大きい電圧とし、｜Ｖｄ―Ｖｃ―Ｖｔ｜＜Ｖｔｈを満たすように制御していることは言うまでもない。また、図１５に示すカウンターバイアス決定手段５０５では、転写体表面電位検知手段５００からの信号と非画像部電位検知手段５０４からの信号を基にカウンターバイアスを決定しているが、転写体表面電位検知手段５００あるいは非画像部電位検知手段５０４からの信号に基づきカウンターバイアスを決定しても良い。
【０１０５】
【発明の効果】
本発明によれば、第１の転写工程において、像担持体と中間転写体との接触部よりも像担持体の回転方向上流側に形成される空隙に対応する部分の中間転写体に印加するカウンターバイアスをＶｃ、第１の転写工程における接触部よりも上流側の中間転写体上の表面電位をＶｔ、像担持体上の画像部の電位をＶＬ、像担持体上の非画像部の表面電位をＶｄ、放電開始電圧をＶｔｈとした時、カウンターバイアスＶｃが、像担持体の帯電極性と同極性でかつ絶対値が画像部電位ＶＬより大きい電圧であり、｜Ｖｄ―Ｖｃ―Ｖｔ｜＜Ｖｔｈを満たすことで、プレ転写による画像の乱れを防止しながら逆転写の発生が極めて少なくなり、良好な画像を得ることができる。
【０１０６】
本発明によれば、像担持体上の非画像部の表面電位Ｖｄを検知し、検知した値に基づきカウンターバイアス値Ｖｃを決定することで、経時環境等により、非画像部電位が変化したときでもプレ転写による画像の乱れを防止しながら逆転写の発生がより少なくなり、良好な画像を得ることができる。
【０１０７】
本発明によれば、中間転写体上の表面電位Ｖｔを検知し、検知した値に基づきカウンターバイアスＶｃを決定することで、中間転写体の電位履歴に依らず、プレ転写による画像の乱れを防止しながら逆転写の発生がより少なくなり、良好な画像を得ることができる。また、中間転写体上の表面電位Ｖｔを、転写体表面検知手段で検知するので、転写前の表面電位の状態を確実に把握でき、より安定した転写が行え、良好な画像を得ることができる。
【０１０８】
中間転写体上の表面電位Ｖｔを、転写工程上流側で中間転写体の表面電位Ｖ０と転写工程の上流側で中間転写体上にすでに転写されているトナー層の表面電位Ｖとを加算した値とすることで、１色目を転写する以前の中間転写体の表面電位変動に対応したカウンターバイアスを決定でき、カラー画像を形成する場合でもプレ転写による画像の乱れを防止しながら逆転写の発生が少なくなり、良好な画像を得ることができる。
【０１０９】
本発明によれば、中間転写体上に存在するトナー層の表面電位を、この存在するトナー層の像担持体上にあったときの表面電位より推定することで、各色転写部に対応して転写体表面検知手段の設置が不要となり、構成の簡素化を図りながら、プレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。
【０１１０】
本発明によれば、トナー層の像担持体上にあったときの表面電位検知を表面電位検知手段で行うと、プロセスの経時や環境変化に依らずトナー層電位を確実に検知できるので、適切なカウンターバイアスを決定でき、プレ転写による画像の乱れを防止しながら逆転写の発生がより少なくなって良好な画像を得ることができる。
【０１１１】
本発明によれば、中間転写体上の表面電位Ｖ０を表面電位調整手段で調整し、調整した値に基づいてカウンターバイアス値を決定することで、積極的に転写前の表面電位の状態を制御できてカウンターバイアスの設定が容易に行え、プレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。表面電位調整手段を中間転写体に対し非接触状態な除帯電手段とすることで、中間転写体の表面との摺擦がなくなって中間転写体の劣化が低減し、転写前の電位状態がコントロールし易くなり、プレ転写による画像の乱れを防止しながら逆転写の発生が少なくなって良好な画像を得ることができる。表面電位調整手段を中間転写体に対し接触状態とすると、中間転写体からトナーを除去するクリーニング機能を持たせることができ、構成の簡素化を図りながら、プレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。
【０１１２】
本発明によれば、表面電位調整手段に印加する電圧値より、１色目の転写工程時に印加するカウンターバイアスを決定することで、１色目について転写前の中間転写体の表面電位検知を行う必要がなく、構成の簡素化を図りながらかプレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。
【０１１３】
本発明によれば、各色の転写時に印加するカウンターバイアスを同電圧とすることで、１つの電圧供給手段で各色へのカウンターバイアスが供給でき、構成の簡素化を図りながらかプレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。特に決定されたカウンターバイアスのうち、最も絶対値の小さいカウンターバイアスに全色のカウンターバイアスを一致させることで、より適切なカウンターバイアスの供給を行え、より確実に逆転写を防止できる。
【０１１４】
本発明によれば、１色目のカウンターバイアス値をもとに全色のカウンターバイアスを決定することで、カウンターバイアス決定の処理を１色についてのみ行えば良く、装置の簡素化、判断の効率化を図りながら、プレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。
【０１１５】
本発明によれば、４色目のカウンターバイアス値を全色に適用することで、カウンターバイアス決定の処理を１色についてのみ行えば良く、装置の簡素化、判断の効率化を図りながら、プレ転写による画像の乱れと逆転写の発生を極めて少なくして良好な画像を得られる。
【図面の簡単な説明】
【図１】本発明の一実施例を示す画像形成装置の全体構成図である。
【図２】図１に示す画像形成装置のプリンタ部の構成を示す拡大図である。
【図３】像担持体表面の画像部と非画像部の電位関係を示すグラムである。
【図４】カウンターバイアス部材に印加するカウンターバイアスと転写チリの発生の関係を示すグラフである。
【図５】中間転写体の表面電位の変化を説明するための模型図である。
【図６】転写ニップ部となる接触部近傍の拡大図である。
【図７】カウンターバイアス部材にブレードを用いた接触部近傍の拡大図である。
【図８】カウンターバイアスと逆転写量の関係を示すグラフである。
【図９】カウンターバイアスと中間転写体の電位の関係を示す図である。
【図１０】カウンターバイアス値を定める際に実験で用いたテーブルの一例である。
【図１１】カウンターバイアス値を定める際に実験で用いた別なテーブルの一例である。
【図１２】接触型の表面電位調整手段の概略構成を示す拡大図である。
【図１３】非接触型の表面電位調整手段の概略構成を示す拡大図である。
【図１４】表面電位調整手段に印加する印加バイアスと中間転写体の帯電電位との関係を示すグラフである。
【図１５】カウンターバイアス印加系の構成を示すブロック図である。
【図１６】カウンターバイアス印加系の別な構成を示すブロック図である。
【図１７】トナー層電位検知手段の一例を示すブロック図である。
【符号の説明】
２２（Ｂｋ，Ｙ，Ｍ，Ｃ）、７１　　像担持体
３０　　第１の転写手段
３１，７２　　中間転写体
４０　　第２の転写手段
３５（Ｂｋ，Ｙ，Ｍ，Ｃ）、７４，７６　　カウンターバイアス印加部材
Ｎ（Ｂｋ，Ｙ，Ｍ，Ｃ）、Ｎ１　　接触部
Ｓ１，Ｓ２、Ｓ３、Ｓ４　　空隙
Ｖ　　トナー層の表面電位
Ｖｔ　　中間転写体の表面電位
ＶＬ　　画像部電位
Ｖｃ　　カウンターバイアス
Ｖｄ　　非画像部表面電位
Ｖｔｈ　　放電開始電圧
１０１，１０２　　表面電位調整手段
５００　　転写体表面電位検知手段
５０１　　表面電位Ｖ０の検知手段
５０２　　トナー層電位検知手段
５０３　　演算部
５０４　　非画像部電位検知手段
５０５（Ｂｋ，Ｙ，Ｍ，Ｃ）　　カウンターバイアス決定手段
５０６，５０８　　電圧供給手段
５０９　　表面電位Ｖｅ検知手段
５１０　　トナー層表面電位推定手段[0001]
[Industrial applications]
The present invention relates to a transfer method and a transfer device, and an image forming method and an image forming apparatus for obtaining an image by transferring a toner image formed on an image carrier to an intermediate transfer member and then transferring the image to a recording material again.
[0002]
[Prior art]
In an image forming apparatus using an electrophotographic process, disturbance due to dust has conventionally been a problem when transferring toner from an image carrier to an intermediate transfer member or a recording material. It is considered that this is mainly due to so-called pre-transfer, in which the toner image is transferred from the image carrier to the recording material or the intermediate transfer member before the toner image enters the transfer nip portion, which is the transfer area. As a countermeasure, for example, Japanese Patent Application Laid-Open No. HEI 8-166728 discloses a member that presses the image carrier and the transfer body on the upstream side of the transfer unit to reduce the gap before the transfer to reduce the toner An invention that prevents flying is described.
Japanese Patent Application Laid-Open No. Hei 10-188688 discloses a negative-positive image forming system, in which a conductive member having the same polarity as the charging polarity of the image carrier or having a zero potential is provided upstream of the transfer nip. Also, there is described an invention in which a bias of the same polarity as that of the toner used for image formation is applied to make it difficult to generate a force for positively attracting the toner.
[0003]
[Patent Document 1]
JP-A-8-166728
[Patent Document 2]
JP-A-10-186878
[0004]
[Problems to be solved by the invention]
In Patent Document 1, since a polarity opposite to the charging polarity of the toner is applied to the pressing member, an electric field formed by the pressing member acts in a direction in which the toner flies toward the recording material toward the recording material. It cannot be completely prevented.
In Patent Literature 2, in a general image carrier, the potential of an image portion to which toner is attached is not zero, and there is a residual potential having the same polarity as the charge polarity of some image carriers. When the potential is zero or the like, toner scattering occurs.
[0005]
When a bias having the same polarity as the charging polarity of the image carrier is applied to the intermediate transfer member in a color image forming apparatus in which toner images of a plurality of colors are overlap-transferred onto the intermediate transfer member and collectively transferred onto a recording material, the bias value is obtained. In some cases, during the transfer from the image carrier corresponding to the second and subsequent images to the intermediate transfer member, the toner of the toner image already transferred on the intermediate transfer member is reverse-transferred from the intermediate transfer member to the image carrier. It is feared that.
[0006]
SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a transfer method and a transfer device, an image forming method and an image forming apparatus that prevent pre-transfer and do not cause reverse transfer.
[0007]
[Means for Solving the Problems]
The present invention is directed to an intermediate transfer method, in which a toner image formed by attaching toner to an electrostatic latent image formed on a charging portion of a rotatable image carrier is brought into contact with an intermediate transfer body on the image carrier. 1. A transfer method and an image forming method, comprising: a first transfer step of transferring a toner image transferred to an intermediate transfer body to a recording material; and a first transfer step of transferring the toner image transferred to the intermediate transfer body to a recording material. In the step, a counter bias is applied to a portion of the intermediate transfer body corresponding to a gap formed in the rotation direction upstream of the image carrier with respect to a contact portion between the image carrier and the intermediate transfer body, and a first transfer step is performed. , The surface potential of the image portion on the image carrier is VL, the counter bias is Vc, the surface potential of the non-image portion on the image carrier is Vd, and the discharge is Vt. When the starting voltage is Vth, the counter bias V Is a voltage having the same polarity as the charging polarity of the image carrier and an absolute value larger than the image portion potential VL, and satisfying | Vd−Vc−Vt | <Vth, thereby preventing image disturbance due to pre-transfer. It also prevents the occurrence of reverse transfer.
[0008]
In the case of a color image transfer method or a color image forming method, in the first transfer step, the toner images respectively formed on the plurality of image carriers corresponding to the toners of a plurality of colors are overlaid and transferred onto the intermediate transfer body. In the transfer step (2), the superimposed toner image is transferred to the recording material.
[0009]
When the surface potential Vd of the non-image portion on the image carrier is detected and the counter bias Vc is determined based on the detected value, even if the non-image portion potential changes due to a temporal environment or the like, the image is prevented from being disturbed and the reverse transfer is prevented. Is preferred.
[0010]
When the surface potential Vt on the intermediate transfer member is detected and the value of the counter bias Vc is determined based on the detected value, the counter bias according to the latest surface potential is always determined, and the counter bias is determined regardless of the potential history of the intermediate transfer member. In addition, the reverse transfer is reliably prevented by preventing the image from being disturbed.
[0011]
When the detection of the surface potential Vt on the intermediate transfer member is detected by the transfer-to-surface potential detecting means for detecting the surface potential of the intermediate transfer member, the state of the surface potential before the transfer can be reliably grasped, and stable and good transfer is achieved. Becomes possible.
[0012]
The surface potential Vt of the intermediate transfer member is determined by comparing the surface potential V0 of the intermediate transfer member upstream of the first transfer step with the surface potential V0 of the toner layer already transferred onto the intermediate transfer member upstream of the contact portion in this transfer step. If a value obtained by adding the surface potential V is used, a counter bias value corresponding to the surface potential fluctuation of the intermediate transfer body before the first color is transferred can be determined.
[0013]
The surface potential V0 on the intermediate transfer member is adjusted by applying a voltage to the surface potential adjusting means, and the value of the counter bias Vc is determined based on the adjusted value. Set values easily.
[0014]
When the counter bias value to be applied in the transfer process of the first color is determined based on the voltage value applied to the surface potential adjusting means, there is no need to detect the surface potential of the intermediate transfer body before the transfer of the first color, thereby simplifying the configuration. Can be achieved.
[0015]
When the surface potential V of the toner layer present on the intermediate transfer member in the first transfer step is estimated from the surface potential Ve of the toner layer present on the image carrier, the transfer portions of the respective colors are formed. It is not necessary to install a surface potential detecting means for each contact portion, and the configuration can be simplified.
[0016]
If the detection of the surface potential Ve when the toner layer present on the intermediate transfer member is on the image carrier is detected by the surface potential detection means for detecting the surface potential of the image carrier, the process potential and environmental change may be reduced. Independently, the toner layer potential can be reliably detected, and an appropriate counter bias is stably supplied.
[0017]
If the counter bias value applied at the time of transfer of each color is the same voltage, the supply of the counter bias to each color can be performed by one voltage supply unit. 11,33
When the counter bias value having the smallest absolute value among the determined counter bias values of the respective colors is set as the counter bias value of all colors, the counter bias value is supplied to each counter bias applying member by one voltage supply means. Thus, reverse transfer is reliably prevented.
[0018]
If the counter bias values of all the other colors are determined based on the counter bias values of the first color, the process of determining the counter bias need only be performed for one color, and the apparatus can be simplified and the determination can be made more efficient.
[0019]
If the counter bias value of the final color is used as the counter bias of another color, the process of determining the counter bias need only be performed for one color, and the apparatus can be simplified and the determination can be made more efficient.
[0020]
The present invention is directed to an intermediate transfer method, in which a toner image formed by attaching toner to an electrostatic latent image formed on a charging portion of a rotatable image carrier is brought into contact with an intermediate transfer body on the image carrier. A transfer device having a first transfer means for transferring the toner image transferred to the intermediate transfer member onto a recording material, and a charging unit charged by the charging means. A rotatable image carrier on which a toner image is formed by developing the electrostatic latent image to be developed by the developing means, and a toner image on the surface of the intermediate transfer body by contacting the intermediate transfer body with the image carrier In an image forming apparatus having a first transfer unit for transferring and a second transfer unit for transferring a toner image transferred to an intermediate transfer member to a recording material, the image forming apparatus has a structure in which a contact portion between the image bearing member and the intermediate transfer member is removed. Corresponding to the gap formed on the upstream side in the rotation direction of the image carrier. A counter bias applying member for applying a counter bias to the intermediate transfer member of the portion, and a voltage supply means for supplying a counter bias to the counter bias applying member; and a surface potential on the intermediate transfer member upstream of the contact portion. Vt, the potential of the image portion on the image carrier is VL, the counter bias is Vc, the surface potential of the non-image portion on the image carrier is Vd, and the discharge start voltage is Vth. Is controlled to satisfy the condition of | Vd−Vc−Vt | <Vth, which is the same polarity as the charging polarity of the image carrier, and whose absolute value is larger than the image portion potential VL. In addition to preventing the occurrence of reverse transfer.
[0021]
In the transfer device and the image forming apparatus according to the present invention, the non-image portion potential detecting means for detecting the surface potential Vd of the non-image portion on the image carrier, and the counter bias based on the value detected by the non-image portion potential detecting means. When the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit, the non-image portion potential changes due to a temporal environment or the like. Even at times, it prevents image disturbance and reverse transfer.
[0022]
In the transfer device and the image forming apparatus according to the present invention, a transfer body surface potential detection unit that detects a surface potential Vt of an intermediate transfer body, and a counter that determines a value of a counter bias based on a value detected by the transfer body surface potential detection unit Having a bias determining unit, the power supply unit, when supplying a counter bias to the counter bias applying member based on a signal from the counter bias determining unit, regardless of the potential history of the intermediate transfer body, to prevent image disturbance, Reverse transfer is reliably prevented.
[0023]
In the transfer device and the image forming apparatus according to the present invention, the transfer body surface potential detection means detects the surface potential V0 of the intermediate transfer body at an upstream side of the contact portion, and the transfer body surface potential detection means has already been transferred onto the intermediate transfer body. A toner layer potential detecting means for detecting the surface potential V of the toner layer present, and an arithmetic unit for calculating the surface potential Vt on the intermediate transfer member by adding the potentials detected by the detecting means and the toner layer potential detecting means. In addition, the state of the surface potential before transfer can be reliably grasped, and stable and favorable transfer is easily performed.
[0024]
A toner layer potential detecting means for detecting a surface potential Ve when the toner layer on the intermediate transfer member is on the image carrier; and a surface potential of the toner layer based on the surface potential Ve detected by the detecting means. When a toner layer surface potential estimating means for estimating V is provided, it is possible to determine a counter bias corresponding to the surface potential fluctuation of the intermediate transfer body before transferring the first color.
[0025]
In the transfer device and the image forming apparatus according to the present invention, a surface potential adjusting unit that adjusts a surface potential V0 of the intermediate transfer body, and a counter bias determining unit that determines a counter bias based on the surface potential adjusted by the surface potential adjusting unit. When the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit, the power supply unit positively controls the state before transfer and facilitates setting of the counter bias value.
[0026]
In the case where the surface potential adjusting means includes a de-charging means in a non-contact state with the intermediate transfer member, the surface potential of the intermediate transfer member is not contacted with the surface of the intermediate transfer member without being rubbed, and the potential before transfer is not deteriorated. The state can be controlled. When the surface potential adjusting means is in contact with the intermediate transfer member and has a function of removing toner from the intermediate transfer member, the configuration of the apparatus can be simplified.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
A toner image formed by attaching toner to an electrostatic latent image formed on a charging section of a rotatable image carrier is transferred to a surface of the intermediate carrier by bringing the intermediate transfer body into contact with the image carrier. In the transfer method and the image forming method including the first transfer step and the second transfer step of transferring the toner image transferred to the intermediate transfer member onto a recording material, the problem of image disturbance at a transfer portion serving as a contact portion is a problem. It has become. This is because when a drum-shaped photosensitive member, which is an example of an image carrier, and an intermediate transfer member such as an intermediate transfer belt come into contact with each other to transfer toner, the photosensitive member is brought into contact with the intermediate transfer member before contacting the intermediate transfer member. The pre-transfer in which the toner moves in the intermediate transfer direction is one factor. In order to improve this, Japanese Patent Application Laid-Open No. Hei 10-188688 discloses that a conductive member that is in contact with the back surface of the transfer member and is not grounded is provided on the upstream side of the contact portion between the photosensitive member and the intermediate transfer member. The potential of this conductive member is set to zero or the same polarity as the photoconductor charging polarity. However, this method cannot completely prevent the toner from moving before the photoconductor and the intermediate transfer body come into contact with each other.
[0028]
An experiment was conducted by changing the process conditions such as the charging potential of the photoconductor, the exposure intensity, and the developing bias. As a result, it was found that the amount of disturbance of the toner was not determined only by the voltage conditions applied to the conductive member. Considering this result, it is related to the latent image potential of the image portion of the photosensitive member formed according to the process conditions, that is, the portion where the toner is adhered. FIG. 3 shows the state of the potential at a portion before the intermediate transfer member contacts the transfer portion with the toner image on the photosensitive member. In the figure, Vd is the non-image portion potential on the photoconductor, and VL is the image portion potential. Since the polarity of the toner is negative and the image is formed by the negative-positive method, the toner of the same polarity is in a state where the latent image of the same polarity has a smaller absolute value. In this figure, the upward direction is shown to be negative. The negative toner is likely to move downward in the figure in terms of potential.
[0029]
Here, when the potential of the intermediate transfer member is considered to be GND, the toner is more stable than VL, and the toner moves in the GND direction and adheres to the intermediate transfer member. When an ideal photoconductor is exposed sufficiently, the potential of the image portion becomes GND. However, in reality, the photoconductor has a residual potential, and even when the exposure is sufficient, the image portion potential VL is -100V. This is because it is -150V. Therefore, even at the contact portion between the image carrier and the intermediate transfer member, the intermediate transfer member corresponding to the gap formed on the upstream side in the rotation direction of the photoconductor has the same polarity as the photoconductor charging polarity and the absolute value. Needs to be applied with a voltage higher than VL. The voltage applied to the intermediate transfer member at the portion corresponding to the gap in the rotation direction upstream portion of the photoconductor in the contact portion serving as the transfer portion from the photoconductor to the intermediate transfer member is referred to as “counter bias” and is denoted by Vc. Specifically, Vc is set to be equal to or equal in polarity to VL. Therefore, it is possible to operate the VL value as a pre-stored value. However, it is preferable to add a time-dependent deterioration change estimated from the number of prints, or to provide a means capable of measuring the residual potential. It is desirable to determine the value based on the output. When Vc was set to such a value and a counter bias was applied, pre-transfer could be prevented and good transfer was possible.
[0030]
2. Description of the Related Art Some transfer devices and image forming devices obtain a color image by superimposing and transferring a plurality of color toners on an intermediate transfer body and then transferring the toner onto a recording material such as paper. In the case where such a plurality of color toner images are formed, when the toner images are transferred from the photoconductors of the second and subsequent colors to the intermediate transfer member, the toner already transferred on the intermediate transfer member exists. . In the transfer of the second and subsequent colors, the reverse transfer of the already transferred toner from the intermediate transfer member to the photosensitive member is called reverse transfer, and this reverse transfer may occur. Observation of this reverse transfer is related to the occurrence of discharge in the gap of the counter bias application section, according to observation.
[0031]
Further, since the reverse transfer toner adheres to the non-image portion on the photoconductor, it is expected that the discharge with the non-image portion and the reverse transfer are related. When a large voltage is applied to the negative side as a counter bias and the potential difference increases, a discharge occurs between the non-image portion of the photosensitive member and the intermediate transfer member. When a discharge occurs, the discharge charge is positive on the intermediate transfer member and negative on the photoreceptor. In the case of a negative-positive system in a device configuration capable of forming a color image, the charging polarity of the photoconductor and the polarity of the toner are the same, and when discharging occurs at the application position of the counter bias as described above, it is on the intermediate transfer member. A charge having a polarity opposite to the charge polarity of the toner falls on the toner. It is considered that the toner whose charge polarity has become unstable due to the discharge charge is reversely transferred.
[0032]
Further, the surface potential Vt of the intermediate transfer member in the state before the discharge is equivalent to the counter bias in consideration of the potential difference of the air gap on the upstream side of the contact portion. As shown in FIG. 4, satisfying the relationship of | Vd−Vc−Vt | <Vth is a condition under which discharge does not occur. By applying a counter bias having a value that satisfies this condition, image disturbance due to pre-transfer is prevented. In addition to preventing the occurrence of reverse transfer, it was found that good images could be obtained. Vth is a discharge starting voltage in a portion to which a counter bias is applied, and is approximately obtained from Paschen's discharge limit curve at 1 atm as approximately the following equation (1).
[0033]
Vth = √7737.6D + 312 + 6.2D (1)
D = Lp / εp + Ld / εd
Lp: photoconductor thickness (μm)
εp: dielectric constant of photoreceptor
Ld: film thickness of intermediate transfer member (μm)
εd: dielectric constant of the intermediate transfer belt
In the transfer device and the image forming apparatus described above, it is also conceivable to detect the surface potential of the non-image portion of the photoconductor and determine the counter bias value based on the detected value. By detecting the potential state of the image carrier at the upstream portion in the rotation direction of the image carrier in the first transfer step, it is possible to more reliably set the counter bias to a value that does not discharge.
[0034]
Consider the magnitude relationship between the counter bias value Vc and the non-image portion potential Vd and the image portion potential VL (all potentials are negative).
In the case of | Vc |> | Vd |> | VL |, this discharge causes positive charges to fall on the intermediate transfer member. In the case of | VD |> | Vc |> | VL |, discharge may occur between the non-image portion and the intermediate transfer member, and between the intermediate transfer member and the image portion. When a discharge occurs between the non-image area and the intermediate transfer body, negative charges fall on the intermediate transfer body, and when a discharge occurs between the image area and the intermediate transfer body, a negative charge is formed on the intermediate transfer body. Has a positive charge. Since the charge polarity of the toner is originally negative, it can be seen that the discharge between the image portion and the intermediate transfer member makes the charge of the toner already transferred on the intermediate transfer member unstable. Since the phenomenon of reverse transfer is that toner adheres to a non-image portion (negative polarity) on the photoconductor, weakly charged or positively charged toner easily adheres. Further, since the reverse transfer adheres to the non-image portion of the image carrier, the potential of the non-image portion is detected, and the discharge between the non-image portion of the image carrier and the intermediate transfer member is prevented. Important for prevention.
[0035]
In the above-described transfer device or image forming apparatus, the surface potential of the intermediate transfer body on the rotation direction upstream side of the photosensitive body at the contact portion between the photosensitive body and the intermediate transfer body in the first transfer step is detected, and based on the detected value, It is also conceivable to determine the value of the counter bias. When a plurality of color toner images are transferred from a plurality of photoconductors onto the intermediate transfer member in the first transfer step, a discharge is generated in a counter bias application portion, and a positive charge is applied to the toner already existing on the intermediate transfer member. Falls, the toner whose charge has become unstable due to the positive charge is reversely transferred from the intermediate transfer member to the photosensitive member. In the process of forming a toner image of a plurality of colors on the intermediate transfer member, the state of the potential of the intermediate transfer member differs depending on the color of the process. Further, the potential state on the intermediate transfer member after the transfer of the toner image from the intermediate transfer member to the recording material (second transfer step) also differs depending on the conditions at the time of the secondary transfer. Even if the potential state on the intermediate transfer member changes due to these conditions, it is important to detect the surface potential Vt on the intermediate transfer member in order to always apply an appropriate counter bias.
[0036]
When the surface potential Vt on the intermediate transfer body is detected, the surface potential on the intermediate transfer body is detected by a transfer body surface potential detection unit. The surface potential of the intermediate transfer member before transfer varies depending on the discharge history in the second transfer step, the potential of the toner image already attached to the intermediate transfer member, and the like. For this reason, if the potential of the intermediate transfer member before transfer is negative, it acts in the same way as the counter bias, and if it is positive, it acts in the direction of erasing. Therefore, it is very effective to detect the surface potential Vt by detecting the surface potential Vt of the transfer member in order to apply an appropriate counter bias.
[0037]
The surface potential Vt of the intermediate transfer member upstream of the contact portion in the transfer process is the same as the surface potential V0 of the intermediate transfer member in the first transfer process and the surface potential Vt of the intermediate transfer member already transferred to the intermediate transfer member upstream of this transfer process. It may be a value obtained by adding the potential V of the existing toner layer. FIG. 5 illustrates the reason for this. (1), (2), and (3) in FIG. 5 show the state of the intermediate transfer member on the upstream side of the contact portion serving as the transfer portion for the first, second, and third colors. On the upstream side of the contact portion of the third transfer color, there are a first color toner indicated by a circle in the figure and a second color toner indicated by a circle in the figure on the intermediate transfer member. Assuming that the potential of the intermediate transfer member on the upstream (upstream) side of all the contact portions is V0, and the potentials of the respective toners are V1 and V2, the potential on the upstream side of the contact portion of the third color is considered to be V0 + V1 + V2. Similarly, for the fourth color, V0 + V1 + V2 + V3. At this time, if the potential state of the intermediate transfer member before the transfer of the first color is different, the value of V0 may be moved, which is easy to cope with.
[0038]
In the case where the surface potential V of the toner layer is estimated from the surface potential Ve when the toner layer is on the photoreceptor, the surface potential V on the dielectric is represented by the charge Q per unit area, the thickness d of the dielectric, When the ratio is ε, it is expressed by Q = εV / d. The transfer rate from the photosensitive member to the intermediate transfer member is substantially determined. If the thickness and the dielectric constant of the photosensitive member and the intermediate transfer member are known, the charge Q of the toner on the photosensitive member is indicated on the intermediate transfer member. It is possible to calculate the potential. Since the detected or measured surface potential Ve of the toner layer on the photoreceptor includes the residual potential of the photoreceptor, the value obtained by subtracting the residual potential is calculated as the surface potential V of the toner layer. The surface potential of the toner layer can be detected without providing a special device on the body.
[0039]
As a method of detecting the surface potential Ve of the toner layer on the photoreceptor, a surface potential detecting unit is used. The surface potential of the image area on the photoconductor changes depending on the charging potential of the photoconductor, the writing light amount, the light attenuation characteristics of the photoconductor, the amount of toner adhered by development, and the like. Furthermore, since these parameters change with time, environment, and the like, the surface potential detecting means is sure to know the actual image portion potential. However, when the image potential is measured by the surface potential detecting means, it is difficult to directly measure a fine image because the resolution of the surface potential detecting means is not sufficiently high. Therefore, in practice, a surface potential detection pattern formed under the same image forming conditions as the image portion is provided in an unnecessary portion on the photoconductor, and the surface potential is detected based on a measured detection value of the pattern. Although it is possible to estimate the charge amount of the toner layer depending on the process conditions when forming the toner image on the photoreceptor, the reliability based on the surface potential detecting means is the highest.
[0040]
The surface potential Vt on the intermediate transfer member may be adjusted on the upstream side of the transfer process by a surface potential adjusting means for adjusting the surface potential of the intermediate transfer member, and the counter bias value may be determined based on the adjusted value. The surface potential of the intermediate transfer member is V0 shown in FIG. V0 is greatly affected by the discharge and the like at the time of transfer from the intermediate transfer member to the recording material, and thus greatly varies depending on the paper type, environment, resistance variation with time, and the like. For this reason, it is effective to always apply the counter bias effectively and control V0 so as not to cause reverse transcription.
[0041]
If the surface potential adjusting means for adjusting the surface potential of the intermediate transfer member on the upstream side of the contact portion is constituted by a discharging member which acts in a non-contact state with respect to the intermediate transfer member, the intermediate transfer member and the discharging member are non-conductive. Because of the contact, stable charge removal is possible regardless of the state on the intermediate transfer member. The surface of the intermediate transfer member may be stained or scratched by rubbing with use. In addition, if it is desired to perform the charge removal in a state where the toner is attached, it is impossible to perform the charge removal by contact, and in the non-contact state, the state of the intermediate transfer member does not need to be selected. Specifically, the scorotron charger is the most common non-contact charger, and by controlling the DC voltage applied to the grid, the potential on the intermediate transfer member can be controlled to approximately the same potential as the grid. . In addition, a charging roller or the like can be used as the non-contact member. In terms of controlling the surface potential, it is preferable to use constant voltage control.
[0042]
The surface potential adjusting means for adjusting the surface potential of the intermediate transfer member on the upstream side of the contact portion may have a function of removing toner from the intermediate transfer member in a state of contact with the intermediate transfer member. The so-called cleaning means for removing toner from the intermediate transfer member is generally a device for bringing a cleaning member such as a brush, a roller, or a blade into contact with the intermediate transfer member, and may apply a bias in order to perform cleaning efficiently. . The bias at this time is controlled so that the surface potential of the intermediate transfer member after cleaning is stabilized. For example, when a low voltage control of AC + DC is performed in a charging device using a roller, the potential after cleaning can be set to a DC voltage value. In addition, the structure can be simplified by using both the cleaning unit and the counter bias applying unit.
[0043]
The counter bias to be applied during the transfer process of the first color may be determined based on the voltage value applied to the surface potential adjusting means. The counter bias is basically determined so as to satisfy | Vd−Vc−Vt | <Vth. At this time, the value of Vt corresponding to the surface potential of the intermediate transfer member before the transfer of the first color is determined by It is determined from the voltage value applied to the surface potential adjusting means. In the surface potential adjusting means, the subsequent potential on the intermediate transfer body is determined by the applied voltage. Therefore, by using this value as Vt for the first color, the determination does not become unstable and Since the equipment such as the surface potential detecting means is not required, the configuration can be simplified.
[0044]
If the counter bias applied during transfer is the same voltage for multiple colors within the range of the counter bias that can prevent image distortion due to pre-transfer and reverse transfer during transfer, independent voltage supply means and power supply for each color are unnecessary. The configuration can be simplified.
[0045]
According to | Vd−Vc−Vt | <Vth, which is within the range of the counter bias that can prevent image disturbance due to reverse transfer, the value that maximizes the absolute value among the values that can be set as the counter bias Vc is obtained. If the absolute value is smaller than the obtained value, no discharge occurs, and therefore reverse transfer can be prevented. Therefore, after the counter bias value Vc that satisfies the above relationship is obtained for a plurality of colors, if the value having the smallest absolute value is set as the set value for all the colors, the reverse transfer does not occur for any color, and the common voltage supply means is used. Is possible.
[0046]
When an image is formed by attaching a normal plurality of color toner images on an intermediate transfer member, the charged polarity of the toner is the same, and the potential of the non-image portion of the photoconductor is substantially the same. Considering such a premise, what is different depending on the color of the transfer is the surface potential Vt of the intermediate transfer member on the upstream side of the contact portion. It is considered that the potential changes. Assuming that the potential change for one color when the toner of each color adheres most is Vs, the absolute value of the counter bias applied every time the colors are overlapped may be reduced by Vs. By determining the counter bias by such a method, the counter bias determining means only needs to have a configuration corresponding to the first color, and the configuration can be simplified.
[0047]
The smallest absolute value of the counter bias is at the time of the transfer of the final color toner. In the case of general four-color image formation having four color toners of yellow, magenta, cyan, and black, the fourth color is formed. . Further, if the counter bias supplied to the upstream side of the contact portion corresponding to each toner color is shared by the counter bias having the smallest absolute value, discharge does not occur in all colors and reverse transfer can be prevented. Therefore, by using the counter bias determined for the fourth color as the counter bias for the other colors, the configuration is simplified and the processing efficiency is effectively improved.
[0048]
The configuration for realizing such a method is as follows.
[0049]
As a transfer device, a toner image formed by attaching toner to an electrostatic latent image formed on a charging portion of a rotatable image carrier is brought into contact with an intermediate transfer member by bringing the intermediate transfer member into contact with the image carrier. It is a prerequisite to have a first transfer means for transferring the toner image transferred to the intermediate transfer body to a recording material, and a first transfer means for transferring the toner image to the surface of the transfer body. The image forming apparatus includes a charging unit that charges a surface of a photoconductor as an image carrier, an exposure unit that forms an electrostatic latent image on a charging unit of the photoconductor charged by the charging unit, Developing means for supplying a developer to an image to form a toner image; first transfer means for transferring the toner image to the surface of the intermediate transfer member by bringing the intermediate transfer member into contact with the image carrier; And a fixing device for fixing the toner image transferred to the recording material.
[0050]
A counter bias applying member for applying a counter bias to a portion of the intermediate transfer body corresponding to a gap formed on the upstream side in the rotation direction of the image carrier from a contact portion between the image carrier and the intermediate transfer body; Voltage supply means for supplying a counter bias to the bias applying member, the counter bias being a voltage having the same polarity as the charging polarity of the image carrier and having an absolute value larger than the image portion potential, and | Vd-Vc-Vt The bias control is performed by the power supply means so as to satisfy | <Vth.
[0051]
The surface potential Vd of the non-image portion on the image carrier is detected by the non-image portion potential detecting means, and the value of the counter bias is determined by the counter bias determining means based on the value detected by the non-image portion potential detecting means. The supply means supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination means.
[0052]
The surface potential Vt of the intermediate transfer body is detected by the transfer body surface potential detecting means, and the value of the counter bias is determined by the counter bias determining means based on the value detected by the transfer body surface potential detecting means. Then, the power supply unit supplies a counter bias to the counter bias applying member based on the signal from the counter bias determination unit.
[0053]
The transfer body surface potential detection means includes a detection means for detecting the surface potential V0 of the intermediate transfer body at an upstream side of the contact portion, and a toner layer for detecting the surface potential V of the toner layer already transferred on the intermediate transfer body. It comprises a potential detecting means, and an arithmetic unit for calculating the surface potential Vt on the intermediate transfer member by adding the potentials detected by the detecting means and the toner layer potential detecting means. The toner layer potential detecting means detects a surface potential Ve when the toner layer on the intermediate transfer member is on the image carrier, and a surface potential of the toner layer based on the surface potential Ve detected by the detecting means. V may be used as the toner layer surface potential estimating means.
[0054]
In the case where there is provided a surface potential adjusting means for adjusting the surface potential V0 of the intermediate transfer member, and a counter bias determining means for determining a counter bias based on the surface potential adjusted by the surface potential adjusting means, the power supply means is provided with the counter bias. A counter bias is supplied to the counter bias applying member based on a signal from the determining means. The surface potential adjusting means may be carried in either a contact state or a non-contact state with the intermediate transfer member. In general, a cleaning member and a charging / discharging member are provided near the intermediate transfer member, and these members may be used as a counter bias applying member for the intermediate transfer member.
[0055]
Examples of the image forming apparatus to which the present invention is applied include a laser copying machine, a laser printer, a facsimile, and a composite machine thereof. Of course, each of the image forming apparatuses may be a so-called monochrome image forming apparatus or a color image forming apparatus.
[0056]
【Example】
Hereinafter, an embodiment of a color laser copying machine (hereinafter, simply referred to as “copier”) will be described as an image forming apparatus to which the present invention is applied. The basic configuration of the copying machine will be described with reference to FIG. The copier includes a printer unit 100, a paper feeding unit 200, a scanner unit 300 fixed above the printer unit 100, an automatic document feeder (ADF) 400 attached to the scanner unit 300, and the like. Control means (not shown) for controlling the operation of.
[0057]
The scanner unit 300 reads image information of a document placed on the contact glass 301 by the reading sensor 302 and sends the read image information to the control unit. A control unit (not shown) controls a laser, an LED, and the like (not shown) in the exposure device 10 located inside the printer unit 100 based on the received image information, and controls an image charged by a charger as a charging unit described later. A laser beam L for each color is irradiated to a charging unit of a drum-shaped photoconductor (hereinafter, referred to as “photoconductor drum”) 22Bk, 22Y, 22M, and 22C as a carrier. By this irradiation, electrostatic latent images are respectively formed on the charged portions on the surfaces of the photosensitive drums 22Bk, 22Y, 22M, and 22C, and a toner of a color corresponding to each electrostatic latent image is passed through a predetermined developing process. The toner images of each color are developed by the attachment. These four photosensitive drums 22Bk, 22Y, 22M, and 22C are arranged in parallel in a tandem image forming unit 20 provided in the printer unit 100. The arrangement of the four photosensitive drums 22Bk, 22Y, 22M, 22C is not limited to a parallel straight line, and may be an inclined arrangement in which one of the right and left sides of the tandem image forming unit 20 is lowered in FIG.
[0058]
The printer unit 100 includes, in addition to the exposure device 10 and the tandem image forming unit 20, an apparatus described below. That is, there are an intermediate transfer unit 30 as a first transfer unit, a secondary transfer unit 40 as a second transfer unit, a fixing unit 50 as a fixing unit, a paper discharge roller pair 80, a toner supply device (not shown), and the like. The developing process will be described later.
[0059]
The paper feed unit 200 includes a plurality of paper feed cassettes 202 provided in a multi-stage manner in the paper bank 201, a paper transport path 205, and a plurality of transport roller pairs 206 provided appropriately along the way. Each paper feed cassette 202 feeds the recording paper as a recording material accommodated in the cassette sequentially from the top, and feeds a plurality of recording papers which have been double fed by the paper feed roller 203. There is a separation roller 204 that separates the paper and then sends the paper to the paper conveyance path 205. The transport roller pair 206 sends out the recording paper received from the paper feed cassette 202 toward the downstream transport roller pair 206 or the paper feed path 60 provided in the printer unit 100. In the copying machine according to the present embodiment, manual paper feeding is possible in addition to the paper feeding by the paper feeding unit 200 having the above configuration. A manual tray 70 is provided on the right side, which is one side of the printer unit 100, for realizing the manual sheet feeding. The manual feed tray 70 includes a paper feed roller 71 and a separation roller 72, and sends out the recording paper set in the manual feed tray 70 into the paper feed path 60 in the printer unit 100.
[0060]
The recording paper fed from the paper feed unit 200 or the manual feed tray 70 into the paper feed path 60 is sandwiched between a pair of registration rollers 61 provided in the middle of the paper feed path 60. The registration roller pair 61 sends the sandwiched recording paper to the secondary transfer nip N2 at a predetermined timing. The secondary transfer nip N2 is a nip formed by the contact between the intermediate transfer unit 30 and the secondary transfer unit 40. The intermediate transfer unit 30 and the secondary transfer unit 40 constitute a transfer device.
[0061]
In order to make a color copy, the apparatus user first sets a document on the document table 401 of the ADF 400 or sets the document on the contact glass 301 of the scanner unit 300 exposed by opening the ADF 400. Then, a start switch (not shown) is pressed. Then, driving of the scanner unit 300 is started in order to read the image information of the original conveyed from the ADF 400 onto the contact glass 301 or the original set on the contact glass 301 from the beginning. Specifically, the traveling of the first traveling body 302 is started, and the light emitted from the light source is reflected on the document surface and sent to the second traveling body 303. Then, the reflected light is received by the mirror of the second traveling body 303 which has also started traveling, and enters the reading sensor 305 through the imaging lens 304 to read image information.
[0062]
Upon receiving the image information from the scanner unit 300, the control unit (not shown) forms the toner images of Bk, Y, M, and C on the photosensitive drums 22Bk, 22Y, 22M, and 22C by the above-described laser writing and developing process. Let each form. Symbols Bk, Y, M, and C are abbreviations of black, yellow, magenta, and cyan, respectively.
[0063]
FIG. 2 is an enlarged configuration diagram illustrating a partial configuration of the printer unit 100 in an enlarged manner. 2, the tandem image forming section 20 has four process units 21Bk, 22Y, 22M, and 22C. Although each process unit uses a different color of toner, the other units are substantially the same. Therefore, only the configuration of the process unit 21Bk using the Bk toner will be described in detail, and the description of the other process units will be given the same reference numerals as those used in the process unit 21Bk, and detailed description will be omitted. .
[0064]
The process unit 21Bk includes a photosensitive drum 22Bk, a charger 23Bk, a developing unit 24Bk, a neutralization lamp 26Bk, a cleaning device 27Bk, and the like. The photosensitive drum 22Bk is rotatably supported on the side surface of the unit, and its surface is uniformly charged by the charger 23Bk while being driven to rotate counterclockwise in the figure by driving means (not shown). Then, the surface after uniform charging constitutes a charging portion, and the charging portion is irradiated with the laser writing light L to form an electrostatic latent image. This electrostatic latent image is developed into a Bk toner image by a developing device 24Bk that uses Bk toner as a developer, which is an image forming substance. The formed Bk toner image is intermediate-transferred onto an intermediate transfer belt 11 as an intermediate transfer body which is a component of the transfer unit 30. The step of transferring the toner image from the photosensitive drum 22Bk onto the intermediate transfer belt 31 is referred to as a first transfer step here.
[0065]
After the surface of the photosensitive drum 22Bk after the first intermediate transfer step is completely discharged by the discharge lamp 26Bk, the transfer residual toner on the surface is cleaned by the cleaning device 27Bk. A similar process is performed in the other process units 21Y, 21M, and 21C, and respective toner images of Y, M, and C are formed and transferred in an overlapping manner on the intermediate transfer belt 31. However, in the monochrome image forming mode for forming only the Bk toner image, the functions of the process units 21Y, 21M, and 21C are not used.
[0066]
The intermediate transfer unit 30 includes an intermediate transfer belt 31, a plurality (three) of stretch rollers 32, 33, and 34, and a belt cleaning device 37. The intermediate transfer belt 31 is stretched by three stretching rollers 32, 33, and 34 and arranged so that the side facing each process unit is substantially horizontal. One of the tension rollers 32, 33, and 34 is a driving roller that is rotationally driven by driving means (not shown), and the other roller is a driven roller. When the driving roller is driven to rotate, the intermediate transfer belt 31 is moved endlessly in the clockwise direction in the figure.
[0067]
Intermediate transfer rollers 36 Bk, 36 Y, 36 M, and 36 C as transfer members are arranged opposite to the photosensitive drums 22 Bk, 22 Y, 22 M, and 22 C via an intermediate transfer belt 31. Each of the intermediate transfer rollers is disposed so as to contact an inner peripheral surface serving as a back surface of the intermediate transfer belt 31, and the intermediate transfer belt 31 is brought into contact with the surface of each of the photosensitive drums, and First transfer portions (hereinafter, referred to as “intermediate transfer nip portions”) NBk, NY, NM, and NC that are contact portions with the intermediate transfer belt 31 are formed. On the upstream side in the moving direction of the intermediate transfer belt 31 from the intermediate transfer rollers 36Bk, 36Y, 36M, and 36C, that is, on the upstream side in the rotation direction of each photosensitive drum from each intermediate transfer nip portion, a counter bias applying member is provided. Counter bias rollers 35Bk, 35Y, 35M, and 35C are provided, respectively. The counter bias rollers 35Bk, 35Y, 35M, and 35C are arranged in parallel with the intermediate transfer rollers 36Bk, 36Y, 36M, and 36C, respectively. Has made. For example, for Bk, the counter bias roller 35Bk and the intermediate transfer roller 36Bk constitute one pair.
[0068]
The counter bias rollers 35Bk, 35Y, 35M, and 35C are formed between the intermediate transfer belt 31 and each of the photosensitive drums at the upstream side in the rotation direction of the photosensitive drums 22Bk, 22Y, 22M, and 22C, that is, the intermediate transfer. The counter bias Vt is applied to the intermediate transfer belt 31 at a portion corresponding to the gaps S1, S2, S3, and S4 formed upstream of the nips NBk, NY, NM, and NC.
[0069]
A power supply (not shown) applies an intermediate transfer bias to the intermediate transfer rollers 36Bk, 36Y, 36M, and 36C to the intermediate transfer nip portions NBk, NY, NM, and NC, thereby applying an intermediate transfer electric field. Further, a counter bias is applied to the counter bias rollers 35Bk, 35Y, 35M, and 35C from a voltage supply unit described later. By applying a predetermined voltage to each of these counter bias rollers, pre-transfer of toner from each photosensitive drum to the intermediate transfer belt 31 is prevented upstream of each intermediate transfer nip.
[0070]
The Bk, Y, M, and C toner images formed on the photosensitive drums 22Bk, 22Y, 22M, and 22C shown in FIG. 2 are affected by the intermediate transfer electric field and the nip pressure at the intermediate transfer nip. Intermediate transfer (primary transfer) is performed on the intermediate transfer belt 31. The primary transfer to the intermediate transfer belt 31 is performed such that the toner images of Bk, Y, M, and C are sequentially superimposed on each other. As a result, a superimposed toner image of four colors is formed on the intermediate transfer belt 31.
[0071]
In FIG. 2, reference numerals 28Bk, 28Y, 28M, and 28C denote surface voltmeters as surface potential detecting means that are arranged to face the respective photosensitive drums and detect the surface potential on the respective photosensitive drums. In FIG. 2, reference numerals 29Y, 29M, and 29C denote transfer body surface potential detection means that is disposed to face the intermediate transfer belt 31 and detects the surface potential on the intermediate transfer belt 31 upstream of each intermediate transfer nip. Is a surface electrometer. A grounded electrode (not shown) is arranged inside the intermediate transfer belt 31 facing each surface voltmeter. In this embodiment, since there is no space upstream of the intermediate transfer nip S1, a surface voltmeter for detecting and measuring the surface potential on the intermediate transfer belt 31 near the photosensitive drum 22BK is not arranged. However, since the stretching roller 33 is arranged near the photosensitive drum 22BK, the potential of the stretching roller 33 is grounded, and a surface voltmeter is installed at a position facing the stretching roller 33. May be.
[0072]
The secondary transfer unit 40 has a transport belt 41 and two stretching rollers 42 and 43. The conveyor belt 41 is wound around and stretched by stretching rollers 42 and 43, and is driven to rotate endlessly by a driving unit (not shown) so that it is endlessly moved counterclockwise in the drawing. The tension roller 42 is arranged to face the tension roller 34, and is arranged such that the transport belt 41 and the intermediate transfer belt 31 are in contact with each other. Due to this contact, a secondary transfer nip N2 is formed between the intermediate transfer unit 30 and the secondary transfer unit 40, that is, at a contact portion between the intermediate transfer belt 31 and the transport belt 41. A secondary transfer electric field is applied to the secondary transfer nip N2 by applying a secondary transfer bias to the stretching roller 42 from a power source (not shown).
[0073]
As shown in FIG. 2, the recording paper fed to the paper feed path 60 in the printer unit 100 is sandwiched between a pair of registration rollers 61. The registration roller pair 61 sends the recording paper to the secondary transfer nip N2 at a timing when the recording paper sandwiched between the registration rollers 61 can be superimposed on the four-color superimposed toner image on the intermediate transfer belt 31. In the secondary transfer nip N2, the four-color superimposed toner image on the intermediate transfer belt 31 is secondarily transferred onto the recording paper under the influence of the secondary transfer electric field and the nip pressure acting on the secondary transfer nip N2. Is done. Generally, the recording paper is white, so that when a four-color superimposed toner image is transferred, it becomes a full-color image. The recording paper on which the full-color image has been formed in this manner is sent into the fixing device 50 with the endless movement of the conveyor belt 41. Then, the full-color image is fixed between the heating roller 51 and the pressure roller 52 provided in the fixing device 50 and fixed on the surface thereof. The paper is discharged onto the paper tray 90. The transfer residual toner remaining on the intermediate transfer belt 31 after the secondary transfer is forcibly removed from the belt surface by the belt cleaning device 37 of the intermediate transfer unit 30.
[0074]
Next, the principle of applying a counter bias will be described with reference to FIG. Here, for convenience, 71 is a photosensitive drum, 72 is an intermediate transfer belt, 73 is an intermediate transfer roller, and 74 is a counter bias roller. In the figure, a white circle indicates a toner image. 6, the intermediate transfer belt 72 moves from left to right in the figure, and the photosensitive drum 71 rotates counterclockwise so that the surface speed of the intermediate transfer belt 72 becomes substantially constant. The counter bias roller 74 is located upstream of a contact portion N between the photosensitive drum 71 and the intermediate transfer belt 72. In the drawing, the contact portion N is shown in a state where the photosensitive drum 71 and the intermediate transfer belt 72 are separated from each other to indicate the movement of the toner.
[0075]
In the case of a general transfer device without the counter bias roller 74, the transfer bias applied to the intermediate transfer roller 73 also affects the area before the contact between the intermediate transfer belt 72 and the photosensitive drum 71 (upstream of the contact portion N). Then, the movement (pre-transfer) of the toner from the photosensitive drum 71 to the intermediate transfer belt 72 occurs. It is considered that the pre-transfer occurs when the gap between the photosensitive drum 71 and the intermediate transfer belt 72 is from several hundred μm to contact. However, considering the balance with the transfer bias, the upstream side is as close as possible to the contact portion N1. It is preferable to apply a counter bias to the intermediate transfer belt 72 at a portion.
[0076]
FIG. 7 shows that an intermediate transfer blade 75 is used as an intermediate transfer member, a counter bias blade 76 is used as a counter bias applying member instead of a roller, and the photosensitive drum 71 is disposed so as to bite into the intermediate transfer belt 72. The width of the contact portion N1 between the transfer belt 72 and the photosensitive drum 71 is ensured. In this example, the diameter of the photosensitive drum 71 is 40 mm, and the contact width N1 (nip width) between the photosensitive drum 71 and the intermediate transfer belt 72 is about 10 mm. 7, the intermediate transfer belt 72 moves from left to right in the figure. Further, an intermediate transfer blade 75 is arranged at the most downstream side of the contact portion N1 between the photosensitive drum 71 and the intermediate transfer belt 72, and a distance from the counter bias blade 76 is secured. The counter bias blade 76 is provided in contact with the inner surface of the intermediate transfer belt 72 at a position 5 mm upstream from the most upstream part of the contact portion N1 between the photosensitive drum 71 and the intermediate transfer belt 72.
[0077]
With such a configuration, it is possible to efficiently apply the counter bias to the gap upstream of the contact portions N, N1 between the intermediate transfer belt 72 and the photosensitive drum 71. Further, it is also possible to adopt a configuration in which a transfer bias is applied to the roller 79 that supports the intermediate transfer belt 72 from inside instead of the intermediate transfer blade 75 shown in FIG. In this case, the distance between the counter bias application unit and the transfer bias application unit can be set within a necessary range, so that the counter bias can be effectively applied. The roller 79 may be arranged at a position slightly downstream of the most downstream side of the contact portion N1.
[0078]
In FIG. 6, when the resistance of the surface of the intermediate transfer belt 72 that contacts the counter bias roller 74 is low, a large current flows between the counter bias roller 74 and the intermediate transfer roller 73. Is desirably sufficiently high. Specifically, 1 × 10 ⁸ Ωm to 1 × 10 ¹¹ Ωm can be used, but preferably 1 × 10 ⁹ Ωm is good. When the belt resistance is high, the static elimination does not occur, and the intermediate transfer belt 72 needs to be neutralized by the neutralization device. Since the self-static elimination is performed by the electric charge flowing through the bulk of the belt, a current does not flow between the counter bias roller 74 and the intermediate transfer roller 73, and the belt bulk can be configured to easily flow the current. It can be said that this is the most suitable intermediate transfer belt for this method.
[0079]
When the intermediate transfer belt 72 has a multilayer structure, the degree of freedom in configuration is increased. For example, assuming that the intermediate transfer belt has a three-layer structure and has layers 1, 2, and 3 from the inside where the counter bias roller 74 contacts to the outside where the photosensitive drum 71 contacts, the volume resistivity of each layer is ρ1, When ρ2 and ρ3, ρ1>ρ2> ρ3 or ρ1> ρ2 <ρ3, and the total resistance value is 1 × 10 ⁸ Ωm to 1 × 10 ¹⁰ Ωm.
[0080]
(Experiment 1)
Using the copying machine shown in FIG. 1, an experiment was conducted with the first color and the second color (Bk, Y). For both the first color and the second color, a line image was written and output by a laser at a photoconductor charging potential of -400 V and a laser. At the same time, a solid image was written, and the potential of the latent image was measured. This line image was developed by a two-component developing device. The experiment was performed at a developing bias of -350V. At this time, the toner adhesion amount on the photosensitive drums 22BK and 22Y is 0.4 mg / cm. ² Met. The voltage applied to the intermediate transfer rollers 36BK and 36Y shown in FIG. 2 during transfer is 800V for the first color and 900V for the second color.
[0081]
FIG. 4 shows the toner images on the intermediate transfer belt 31 after the transfer from the photosensitive drums 22BK and 22Y to the intermediate transfer belt 31 by changing the voltage applied to the first and second color counter bias rollers 35BK and 35Y. Chile was evaluated. The evaluation of the amount of dust is based on the area of the toner that has protruded from the edge portion of the line image to the background portion as an index. The larger the value, the worse the dust is. First, dust was evaluated by changing the value applied to the first color counter bias roller 35BK. The solid line in the figure is the result. The level of the dust amount 2 is a level determined as an allowable value by a subjective evaluation performed separately. The amount of dust tends to improve as the voltage applied to the counter bias roller 35BK increases in the negative direction. When the voltage applied to the counter bias roller 35BK is 0, dust is improved but not sufficient as compared with the case where the voltage is more positive. From the results in FIG. 4, it was found that the amount of dust satisfies the permissible value by applying a counter bias having a larger value on the negative side than the counter bias of -100 V.
[0082]
From these results, an experiment was conducted by changing the voltage applied to the first color counter bias roller 35BK to -100 V and changing the second color counter bias. When the amount of dust of the second color image was evaluated on the intermediate transfer belt 31 after the transfer by changing the value of the counter bias of the second color, the result was as shown by the broken line in FIG. The result of the second color is a form in which the result of the first color is shifted in the horizontal axis direction. Here, when the counter bias is increased to the minus side to some extent, the first color image existing on the intermediate transfer belt 31 at the time of the second color transfer Was observed.
[0083]
Looking at this phenomenon in detail, the reverse transfer in which the image of the first color is transferred from the intermediate transfer belt 31 to the photosensitive drum 22BK on the contrary in the transfer of the second color has occurred. The reverse transfer amount was evaluated as the reverse transfer adhesion amount on the photosensitive drum 22BK, and the result is shown in FIG. In FIG. 8, the horizontal axis indicates the value of the counter bias (−V), and the vertical axis indicates the reverse transfer amount. Up to a counter bias of -1000 V, no reverse transfer is observed. With a counter bias higher than -1200 V on the negative side, the reverse transfer amount increases linearly with an increase in the absolute value.
[0084]
Let us calculate the Paschen's discharge limit in this experimental system. The calculation is performed according to the above equation (1). When the dielectric constant of the image carrier is 3, the film thickness is 25 μm, the dielectric constant of the intermediate transfer member is 9, and the film thickness is 150 μm, the discharge starting voltage is 750 V. It is assumed that the cause of the reverse transfer is a discharge between the non-image portion on the photosensitive drum 22BK and the image portion on the intermediate transfer belt 31. Consider the potential upstream of the contact. Since the non-image portion potential is -400 V, the potential of the image portion of the intermediate transfer belt 31 is -100 V, and the value of the counter bias is -1200 V, the potential of the non-image portion of the photosensitive drum 22BK and the image portion of the intermediate transfer belt 31 are different. The potential difference Vg is approximately 900V. The potential difference Vg at a counter bias of -1000 V at which no reverse transfer occurred was 700 V.
[0085]
In the experimental results, the reverse transfer occurs from -1200 V application and the amount of reverse transfer increases linearly with the increase of the absolute value of the counter bias. The reason for this is that the discharge charge amount tends to increase with the increase of the absolute value of the counter bias. It is considered that the toner on the intermediate transfer belt 31 whose charge amount has become unstable due to the discharge is reversely transferred to the photosensitive drum 22BK. Based on such knowledge, it has been found that reverse transfer can be prevented by applying a counter bias that causes the potential difference Vg to be equal to or lower than the discharge starting voltage.
[0086]
(Experiment 2)
An experiment was performed using the configuration shown in FIG. 2 while measuring the surface potential on each photosensitive drum and on the intermediate transfer belt 31. The color order of the image formation is BK, Y, M, C as shown. The charged potential on the photosensitive drum of each color is measured, and the measured values are defined as VpBk, VpY, VpM, and VpC, respectively. The surface potentials on the intermediate transfer belt 31 are VtBk, VtY, VtM, and VtC, respectively. The first color Bk and the second color Y were formed, and the conditions under which reverse transfer of the first color Bk toner from the intermediate transfer belt 31 to the second color Y photoconductor drum 22Y was examined. The potential of the non-image portion on the photosensitive drum 22Y of the second color was fixed, the image forming condition of the toner of the first color was changed, and the potential of the toner layer on the intermediate transfer belt 31 was changed. Under these conditions, printing was performed while changing the counter bias by 100 V, and the occurrence of reverse transfer was plotted. FIG. 9 shows the result. The horizontal axis represents the counter bias, and the vertical axis represents the toner layer surface potential on the intermediate transfer member. The plot in the figure is the point where reverse transcription was observed, and reverse transcription occurs on the right side of the plot. The plot for the non-image portion potential of -300 V is indicated by “○”. When the potential of the toner image on the intermediate transfer belt 31 was -120 V, reverse transfer was observed at a counter bias of -1000 V, but not at -900 V. Similarly, an experiment was carried out at -400 V in the non-image portion, and "△" was shown for the voltage at which reverse transfer occurred.
[0087]
Based on such a result, the counter bias was controlled. Specifically, the change range of the non-image portion potential of each photoconductor drum and the potential range of the toner layer on the intermediate transfer belt 31 indicate a certain range. There are out of the normal control range. In the present embodiment, the value of the counter bias is determined by referring to the table based on the combination of the non-image portion potential of the yellow photosensitive drum 22Y and the surface potential Vt of the toner attached portion on the intermediate transfer belt 31. . The table used for the experiment is shown in FIG. When the transfer of the second color was performed with the counter bias based on the table shown in FIG. 10, a good image without transfer dust and no reverse transfer was obtained. Further, even if the absolute value is smaller than the applicable counter bias, it is sufficient that the absolute value is larger than the image portion potential VL of the photosensitive drum, so a table as shown in FIG. 11 could be used. In this case as well, a good image was obtained in which no dust and no reverse transfer occurred.
[0088]
Similarly, for the third color and the fourth color, which is the final color, the surface potential of the intermediate transfer belt 31 on the upstream side of each color transfer portion and the non-image portion potential of each photosensitive drum were measured and plotted with the corresponding surface voltmeter. However, the results were on the same straight line as in FIG. Thus, for the transfer of any color, when the non-image portion potential on the photosensitive drum is fixed, when the difference between the surface potential of the intermediate transfer belt 31 before the transfer and the counter bias value is controlled to be constant, Reverse transfer was reliably prevented.
[0089]
(Experiment 3)
Instead of measuring the surface potential Vt on the intermediate transfer belt 31, the surface potential Ve of the toner adhering portion on each photosensitive drum was measured, and the surface potential V on the intermediate transfer belt 31 was estimated. A solid test pattern was formed on a photosensitive drum which did not affect the final image, and the potential of the developed toner image was measured. The latent image potential of this pattern was -100V. The measured potential of the toner layer is determined by the charge amount of the toner, the thickness of the photosensitive drum, and the dielectric constant. It is assumed that the toner layer has been transferred onto the intermediate transfer belt 31 at a transfer rate of about 100%, and that no charge has changed since no discharge has occurred in the image portion during transfer. When the same amount of charge moves onto the intermediate transfer belt 31, the potential is determined by the film thickness and the dielectric constant of the intermediate transfer belt 31. In the case of this experiment, since d / ε is approximately doubled by the movement from each photosensitive drum to the intermediate transfer belt 31, the surface potential V of the toner layer on the intermediate transfer belt 31 is It was estimated that the potential was approximately twice the toner layer potential.
[0090]
The surface potential of the intermediate transfer belt 31 at the time of the second color transfer was determined by the above estimation, and an image was output in the same manner as in Experiment 2. As a result, a good image was obtained. In this experiment, the surface potential of the intermediate transfer belt 31 before the transfer of the first color was set to approximately 0 V by a static eliminator (not shown). When static elimination was not performed, the surface potential was about +300 V during the present experiment. When the experiment was carried out as it was, a large amount of pre-transfer occurred even at a counter bias at which no dust was generated by pre-transfer at 0 V. The limit value of the counter bias at which the photo is generated is also high. The result that the effect of the counter bias was reduced only by the charging of approximately +300 V was obtained.
[0091]
Therefore, when estimating the surface potential Vt of the intermediate transfer belt 31, it is necessary to consider the case where the intermediate transfer belt 31 is charged before the transfer. Assuming that the charged potential V0 of the intermediate transfer belt 31 before transfer is the estimated potential V1 of the first color toner layer, the estimated potential V2 of the second color toner layer, and the estimated potential V3 of the third color toner layer, the transfer portions (contact portions NBK, The surface potential of the intermediate transfer belt 31 on the upstream side (NY, NM, NC) could be estimated by adding (V0 + V1 + V2 + V3 for the fourth color) the toner layer transferred on the intermediate transfer belt 31 at that time. On the other hand, the charge amount of the intermediate transfer belt 31 before the transfer varies depending on the transfer conditions from the intermediate transfer belt 31 to a recording material such as paper. It is preferable to adjust the charge amount.
[0092]
12 and 13 show the surface potential adjusting means 101 and 102. The surface potential adjusting means 101 shown in FIG. 12 is a device for removing electricity from the intermediate transfer belt 31, and a scorotron charger 91 to which an alternating current (AC) is applied is arranged facing the intermediate transfer belt 31. At this time, the roller 33 that bridges the intermediate transfer belt 31 serving as the facing member is grounded. When the potential of the grid was set to about 0 V and the potential after charge removal by the scorotron charger 91 was examined, effective charge removal was possible at about 0 V.
[0093]
The surface potential adjusting means 102 shown in FIG. 13 is obtained by integrating a cleaning device and a static eliminator. In FIG. 13, reference numeral 92 denotes a blade as a cleaning member, 93 denotes a cleaning member, which is a cleaning roller which also functions as a charge adjusting member, and 94 denotes a voltage supply device to the cleaning roller 93. A DC bias on which an AC bias is superimposed is applied to the cleaning roller 93. Since the purpose is to adhere the transfer residual toner on the intermediate transfer belt 31 to the cleaning roller 93, the DC bias needs to have a slight positive polarity, and +100 V is applied in this experiment. FIG. 14 shows the relationship between the applied bias and the charging potential at this time. In FIG. 14, the vertical axis indicates the charging potential, and the horizontal axis indicates the applied bias. As shown in FIG. 14, the value of the DC bias and the charging potential were in a proportional relationship, and the potential after cleaning could be determined by the applied DC bias.
[0094]
(Experiment 4)
An image was formed by determining the counter bias using the configuration shown in FIG. 15 in the apparatus configuration shown in FIG. In FIG. 15, reference numeral 500 denotes a transfer member surface potential detecting unit that detects the surface potential Vt of the intermediate transfer belt 31, reference numeral 504 denotes a non-image portion potential detecting unit that detects the surface potential Vd of the non-image portion of the photosensitive drum, and reference numeral 505. Reference numeral 506 denotes a counter bias determining unit that determines the value of the counter bias. Reference numeral 506 denotes a power supply unit 506 that supplies a counter bias Vc to a counter bias applying member, such as a counter bias roller or a counter bias blade, based on a signal from the counter bias determining unit 505. Show.
[0095]
Transfer body surface potential detecting means 500, detecting means 501 for detecting surface potential V0 of intermediate transfer belt 31 upstream of the contact portion, and detecting surface potential V of the toner layer already transferred on intermediate transfer belt 31 And a calculation unit 503 that calculates the surface potential Vt on the intermediate transfer belt 31 by adding the potentials detected by the detection unit 501 and the toner layer potential detection unit 502.
[0096]
In the transfer device and the image forming apparatus according to the present invention, the transfer device and the image forming device further include a counter bias determination unit that determines a value of a counter bias based on a value detected by the transfer body surface potential detection unit, and the power supply unit includes a counter bias determination unit. When the counter bias is supplied to the counter bias applying member on the basis of the above signal, the image is prevented from being disturbed regardless of the potential history of the intermediate transfer member, and the reverse transfer is reliably prevented.
[0097]
In FIG. 1, since the image forming unit 20 is of a tandem type, the counter bias is determined for each color of the counter bias roller using the configuration shown in FIG. At the time of the transfer of the first color, since there is no toner image on the intermediate transfer belt 31, a signal corresponding to a potential of zero is sent from the toner layer potential detecting means 502 to the arithmetic unit 503. A signal corresponding to the potential on the upstream side of the first color transfer unit is sent from the detection unit 502 to the calculation unit 503. The arithmetic unit 503 processes both signals and calculates a potential Vt obtained by adding both potentials. The signal corresponding to Vt is sent to the counter bias determining means 505. On the other hand, the non-image portion potential detecting means 504 detects the potential of the non-image portion of the photosensitive drum of the first color (Bk) and sends a signal corresponding to this potential to the counter bias determining means 505. The counter bias determining unit 505 determines an appropriate counter bias in the first color transfer unit, that is, a counter bias for the counter bias roller 35Bk, from the signal corresponding to the received Vt and the signal corresponding to the non-image potential.
[0098]
The same applies to the case of the second color. However, in the case of the second color, the charge potential of the toner of the first color already transferred is detected as the toner layer potential, so that a signal corresponding to the toner layer potential is added when determining the surface potential Vt. Can be In this way, the counter bias was determined for all four colors, and supplied to the corresponding counter bias rollers from the voltage supply means 506 to form an image. Even when the non-image portion potential of each photosensitive drum was changed, generation of dust and reverse transfer was suppressed, and a good image was obtained.
[0099]
(Experiment 5)
An image was formed on each of the counter bias rollers of each color shown in FIG. 1 by using the configuration shown in FIG. In FIG. 16, the counter bias determining means corresponds to the counter bias determining means shown in FIG. 10, and the determining means for each color is denoted by 505Bk, 505Y, 505M, and 505C, respectively. Reference numeral 507 denotes a counter bias adjusting unit that compares the counter biases determined by the respective counter bias determining units and selects the one having the lowest absolute value. The voltage supply means 508 is connected to the counter bias adjustment means 507. Each counter bias roller is connected to the voltage supply means 508.
[0100]
Since each counter bias determining means indicates the maximum absolute value of the counter bias at which no discharge occurs, reverse transfer does not occur if the absolute value is smaller than the indicated value.
[0101]
For this reason, in the counter bias adjusting unit 507, the output from the counter bias adjusting unit for each color was compared, the value having the smallest absolute value was selected, and an experiment was performed as the counter bias for all colors. When a color having an absolute value smaller than the image portion potential VL occurs in the determined counter bias value, it is considered that dust occurs due to pre-transfer. However, in this experiment, there is no such color, and all colors are generated. With respect to (2), it was possible to perform transcription with no reverse transcription and with reduced dust. That is, the counter bias value was set to the same voltage. At this time, the color having the smallest absolute value was the fourth color cyan, which is the final color. In the case of a full-color image, it is considered that some toner of each color is added on the intermediate transfer belt 31. Next, the judgment of the counter bias was made only for the fourth color, which is the final color, and all the colors were transferred with this counter bias value. Similarly, a good image was obtained.
[0102]
The surface potential of the intermediate transfer belt 31 at the upstream of the transfer of the first color was adjusted to +100 V by the surface potential adjusting means 101 and 102 shown in FIGS. Counter bias determining means in FIG. 16: BK block Vt in FIG. 10 was processed as a value of +100 V adjusted by the surface potential adjusting means to determine the BK counter bias. Then, the potential Vmax for each color when the amount of toner adhered to each color was the largest was determined, and the counter bias was set to a value having a smaller absolute value for each color by Vmax in order of going downstream from Bk. In this case, the image was formed with the absolute value lowered by 150 V, but a good image was obtained. By setting Vmax to a value suitable for the system, it is possible to determine the counter bias of each color based on the counter bias of the first color.
[0103]
In FIG. 15, the toner layer potential detecting means 502 includes, as shown in FIG. 17, a detecting means 502A for detecting the surface potential Ve when the toner layer on the intermediate transfer belt 31 is on the image carrier, and the detecting means 502A. The toner layer surface potential estimating means 502B for estimating the surface potential V of the toner layer based on the surface potential Ve detected by the means, the counter bias corresponding to the surface potential fluctuation of the intermediate transfer belt 31 before transferring the first color. Can be determined.
[0104]
In this embodiment, in each of the voltage supply means 506 and 508, basically, the counter bias Vc is set to a voltage having the same polarity as the charging polarity of each photosensitive drum and having an absolute value larger than the image portion potential VL. , | Vd-Vc-Vt | <Vth. The counter bias determining unit 505 shown in FIG. 15 determines the counter bias based on the signal from the transfer body surface potential detecting unit 500 and the signal from the non-image portion potential detecting unit 504. The counter bias may be determined based on a signal from the detecting means 500 or the non-image portion potential detecting means 504.
[0105]
【The invention's effect】
According to the present invention, in the first transfer step, the voltage is applied to a portion of the intermediate transfer body corresponding to a gap formed on the upstream side in the rotation direction of the image carrier from the contact portion between the image carrier and the intermediate transfer body. The counter bias is Vc, the surface potential on the intermediate transfer member upstream of the contact portion in the first transfer step is Vt, the potential of the image portion on the image carrier is VL, and the surface of the non-image portion on the image carrier is When the potential is Vd and the discharge start voltage is Vth, the counter bias Vc is a voltage having the same polarity as the charging polarity of the image carrier and having an absolute value larger than the image portion potential VL, and | Vd−Vc−Vt | < By satisfying Vth, occurrence of reverse transfer is extremely reduced while preventing disturbance of the image due to pre-transfer, and a good image can be obtained.
[0106]
According to the present invention, the surface potential Vd of the non-image portion on the image carrier is detected, and the counter bias value Vc is determined based on the detected value. However, the occurrence of reverse transfer is reduced while preventing the image from being disturbed by the pre-transfer, and a good image can be obtained.
[0107]
According to the present invention, the surface potential Vt on the intermediate transfer body is detected, and the counter bias Vc is determined based on the detected value, thereby preventing image disturbance due to pre-transfer regardless of the potential history of the intermediate transfer body. However, the occurrence of reverse transfer is further reduced, and a good image can be obtained. Further, since the surface potential Vt on the intermediate transfer body is detected by the transfer body surface detection means, the state of the surface potential before transfer can be reliably grasped, more stable transfer can be performed, and a good image can be obtained. .
[0108]
A value obtained by adding the surface potential Vt on the intermediate transfer member to the surface potential V0 of the intermediate transfer member on the upstream side of the transfer step and the surface potential V of the toner layer already transferred on the intermediate transfer member on the upstream side of the transfer step. By doing so, the counter bias corresponding to the surface potential fluctuation of the intermediate transfer body before transferring the first color can be determined, and even when forming a color image, the occurrence of reverse transfer can be prevented while preventing image disturbance due to pre-transfer. Less, and a good image can be obtained.
[0109]
According to the present invention, the surface potential of the toner layer present on the intermediate transfer member is estimated from the surface potential of the toner layer present on the image carrier when the toner layer is present on the image carrier. It is not necessary to provide a transfer body surface detecting means, and a good image can be obtained by reducing the occurrence of image disturbance and reverse transfer due to pre-transfer, while simplifying the configuration.
[0110]
According to the present invention, when the surface potential of the toner layer on the image carrier is detected by the surface potential detection means, the toner layer potential can be reliably detected regardless of the aging of the process or environmental change. The counter bias can be determined, and the occurrence of reverse transfer is reduced while preventing the image from being disturbed by the pre-transfer, so that a good image can be obtained.
[0111]
According to the present invention, the surface potential V0 on the intermediate transfer member is adjusted by the surface potential adjusting means, and the counter bias value is determined based on the adjusted value, whereby the state of the surface potential before transfer is positively controlled. As a result, it is possible to easily set the counter bias, and it is possible to obtain a good image by minimizing the occurrence of image disturbance and reverse transfer due to pre-transfer. By making the surface potential adjusting means a de-charging means that is not in contact with the intermediate transfer body, there is no rubbing with the surface of the intermediate transfer body, reducing deterioration of the intermediate transfer body and controlling the potential state before transfer. This makes it easier to perform the transfer, and reduces the occurrence of reverse transfer while preventing the image from being disturbed by the pre-transfer, so that a good image can be obtained. When the surface potential adjusting means is brought into contact with the intermediate transfer member, a cleaning function for removing toner from the intermediate transfer member can be provided, and while simplifying the configuration, image disturbance due to pre-transfer and reverse transfer are prevented. A good image can be obtained with extremely little occurrence.
[0112]
According to the present invention, it is necessary to detect the surface potential of the intermediate transfer body before the transfer for the first color by determining the counter bias to be applied in the transfer process for the first color from the voltage value applied to the surface potential adjusting unit. In addition, it is possible to obtain a good image while simplifying the configuration or by minimizing the occurrence of image disturbance and reverse transfer due to pre-transfer.
[0113]
According to the present invention, by setting the counter bias applied at the time of transfer of each color to the same voltage, the counter bias to each color can be supplied by one voltage supply unit, and the image is formed by pre-transfer while simplifying the configuration. A good image can be obtained by minimizing the occurrence of disturbance and reverse transfer. In particular, by making the counter biases of all colors equal to the counter bias having the smallest absolute value among the determined counter biases, more appropriate counter bias can be supplied, and reverse transfer can be more reliably prevented.
[0114]
According to the present invention, by determining the counter bias of all colors based on the counter bias value of the first color, the process of determining the counter bias needs to be performed for only one color, thereby simplifying the apparatus and increasing the efficiency of the determination. In this way, a good image can be obtained by minimizing the occurrence of image disturbance and reverse transfer due to pre-transfer.
[0115]
According to the present invention, by applying the counter bias value of the fourth color to all the colors, the process of determining the counter bias needs to be performed for only one color, and the pre-transfer is performed while simplifying the apparatus and increasing the efficiency of the determination. And a good image can be obtained by minimizing the occurrence of image disturbance and reverse transfer.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is an enlarged view showing a configuration of a printer unit of the image forming apparatus shown in FIG.
FIG. 3 is a gram showing a potential relationship between an image portion and a non-image portion on the surface of an image carrier.
FIG. 4 is a graph showing a relationship between a counter bias applied to a counter bias member and occurrence of transfer dust.
FIG. 5 is a model diagram for explaining a change in surface potential of the intermediate transfer member.
FIG. 6 is an enlarged view of the vicinity of a contact portion serving as a transfer nip portion.
FIG. 7 is an enlarged view of the vicinity of a contact portion using a blade as a counter bias member.
FIG. 8 is a graph showing a relationship between a counter bias and a reverse transfer amount.
FIG. 9 is a diagram showing the relationship between the counter bias and the potential of the intermediate transfer member.
FIG. 10 is an example of a table used in an experiment when determining a counter bias value.
FIG. 11 is an example of another table used in an experiment when determining a counter bias value.
FIG. 12 is an enlarged view showing a schematic configuration of a contact-type surface potential adjusting means.
FIG. 13 is an enlarged view showing a schematic configuration of a non-contact type surface potential adjusting means.
FIG. 14 is a graph showing a relationship between an applied bias applied to a surface potential adjusting unit and a charging potential of an intermediate transfer member.
FIG. 15 is a block diagram illustrating a configuration of a counter bias application system.
FIG. 16 is a block diagram showing another configuration of the counter bias application system.
FIG. 17 is a block diagram illustrating an example of a toner layer potential detecting unit.
[Explanation of symbols]
22 (Bk, Y, M, C), 71 Image carrier
30 First Transfer Means
31, 72 Intermediate transfer member
40 Second transfer means
35 (Bk, Y, M, C), 74, 76 Counter bias applying member
N (Bk, Y, M, C), N1 contact part
S1, S2, S3, S4 gap
V Surface potential of toner layer
Vt Surface potential of the intermediate transfer member
VL Image section potential
Vc counter bias
Vd Non-image area surface potential
Vth discharge start voltage
101, 102 surface potential adjusting means
500 Transfer body surface potential detecting means
501 Detecting means of surface potential V0
502 Toner layer potential detecting means
503 arithmetic unit
504 Non-image part potential detecting means
505 (Bk, Y, M, C) Counter bias determining means
506,508 Voltage supply means
509 Surface potential Ve detecting means
510 Toner layer surface potential estimating means

Claims

A toner image formed by attaching toner to an electrostatic latent image formed on a charging section of a rotatable image carrier is brought into contact with the intermediate carrier by contacting the intermediate carrier with the image carrier. And a second transfer step of transferring the toner image transferred to the intermediate transfer member to a recording material.
In the first transfer step, a counter bias is applied to a portion of the intermediate transfer body corresponding to a gap formed on the upstream side in the rotation direction of the image carrier from a contact portion between the image carrier and the intermediate transfer body. The surface potential on the intermediate transfer member upstream of the contact portion in the first transfer step is Vt; the potential of the image portion on the image carrier is VL; the counter bias is Vc; When the surface potential of the non-image portion is Vd and the discharge start voltage is Vth,
The counter bias Vc is a voltage having the same polarity as the charging polarity of the image carrier and having an absolute value larger than the image portion potential VL;
| Vd-Vc-Vt | <Vth.

The transfer method according to claim 1,
In the first transfer step, the toner images respectively formed on the plurality of image carriers corresponding to the toners of the plurality of colors are overlaid and transferred to the intermediate transfer body. In the second transfer step, the overlaid and transferred toner images are transferred. A transfer method, wherein the image is transferred to the recording medium.

The transfer method according to claim 2,
A transfer method comprising: detecting a surface potential Vd of the non-image portion; and determining a value of the counter bias Vc based on the detected value.

The transfer method according to claim 2,
A transfer method comprising: detecting a surface potential Vt of the intermediate transfer member; and determining a value of the counter bias Vc based on the detected value.

The transfer method according to claim 4,
A transfer method, wherein the value of the surface potential Vt of the intermediate transfer body is detected by a transfer body surface potential detecting means for detecting the surface potential of the intermediate transfer body.

The transfer method according to claim 4,
The surface potential Vt of the intermediate transfer member is different from the surface potential V0 of the intermediate transfer member upstream of the contact portion in the first transfer step, and the surface potential Vt of the intermediate transfer member upstream of the contact portion in the first transfer step. A transfer method, characterized in that the value is a value obtained by adding the surface potential V of the toner layer which has already been transferred onto the toner layer.

The transfer method according to claim 6,
Adjusting the surface potential V0 of the intermediate transfer member by applying a voltage to a surface potential adjusting means disposed upstream of the contact portion, and determining the value of the counter bias Vc based on the adjusted value. Characteristic transfer method.

The transfer method according to claim 7,
A transfer method, wherein a counter bias value to be applied at the time of transfer of a first color in a first transfer step is determined based on a voltage value applied to the surface potential adjusting means.

The transfer method according to claim 6,
A transfer method, wherein the surface potential V of the toner layer in the first transfer step is estimated from the surface potential Ve when the toner of the toner layer is on the image carrier.

The transfer method according to claim 9,
A transfer method, wherein the surface potential Ve when the toner layer present on the intermediate transfer member is on the image carrier is detected by a surface potential detecting means for detecting the surface potential of the image carrier. .

The transfer method according to claim 2,
A transfer method, wherein a counter bias value applied at the time of transfer of each color is set to the same voltage.

The transfer method according to claim 11,
A transfer method, wherein the counter bias values of all colors are made to coincide with the counter bias value having the smallest absolute value among the counter bias values.

The transfer method according to claim 2,
A transfer method comprising: determining a counter bias value of another color based on a counter bias value of a first color.

The transfer method according to claim 2, wherein
A transfer method, wherein the counter bias value of the final color is set as the counter bias value of another color.

A toner image formed by attaching toner to an electrostatic latent image formed on a charging portion of a rotatable image carrier is brought into contact with the intermediate carrier by contacting the intermediate carrier with the image carrier. And a second transfer unit for transferring the toner image transferred to the intermediate transfer member to a recording material.
A counter bias applying member for applying a counter bias to a portion of the intermediate transfer body corresponding to a gap formed on the rotation direction upstream side of the image carrier from a contact portion between the image carrier and the intermediate transfer body;
Voltage supply means for supplying a counter bias to the counter bias applying member,
The surface potential on the intermediate transfer member upstream of the contact portion is Vt, the potential of the image portion on the image carrier is VL, the counter bias is Vc, and the surface potential of the non-image portion on the image carrier is Vt. Vd, when the discharge starting voltage is Vth,
The power supply means sets the counter bias Vc to a voltage having the same polarity as the charging polarity of the image carrier, an absolute value larger than the image portion potential VL, and satisfying | Vd−Vc−Vt | <Vth. A transfer device characterized by controlling.

The transfer device according to claim 15,
A non-image portion potential detecting means for detecting a surface potential Vd of the non-image portion on the image carrier;
Having a counter bias determining means for determining the value of the counter bias based on the value detected by the non-image portion potential detecting means,
The transfer apparatus, wherein the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit.

The transfer device according to claim 15,
Transfer body surface potential detection means for detecting the surface potential Vt of the intermediate transfer body,
Having a counter bias determining means for determining the value of the counter bias based on the value detected by the transfer body surface potential detecting means,
The transfer apparatus, wherein the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit.

The transfer device according to claim 17,
The transfer body surface potential detecting means detects a surface potential V0 of the intermediate transfer body on the upstream side of the contact portion, and detects a surface potential V of the toner layer already transferred on the intermediate transfer body. A toner layer potential detecting means for calculating the surface potential Vt on the intermediate transfer member by adding the potentials detected by the detecting means and the toner layer potential detecting means.

The transfer device according to claim 18,
The toner layer potential detecting means detects a surface potential Ve when the toner layer on the intermediate transfer member is on the image carrier, and the toner layer potential based on the surface potential Ve detected by the detecting means. And a toner layer surface potential estimating means for estimating the surface potential V of the transfer device.

The transfer device according to claim 15,
Surface potential adjusting means for adjusting the surface potential V0 of the intermediate transfer member;
Having a counter bias determining means for determining a counter bias based on the surface potential adjusted by the surface potential adjusting means,
The transfer apparatus, wherein the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit.

The transfer device according to claim 20,
The transfer device according to claim 1, wherein the surface potential adjusting unit is a discharging unit that is not in contact with the intermediate transfer body.

The transfer device according to claim 20,
The transfer device, wherein the surface potential adjusting means is in contact with the intermediate transfer member and has a function of removing toner from the intermediate transfer member.

A toner image formed by attaching toner to an electrostatic latent image formed on a charging section of a rotatable image carrier is transferred onto the surface of the image carrier by bringing an intermediate transfer member into contact with the image carrier. An image forming method comprising: a first transfer step; and a second transfer step of transferring a toner image transferred to the intermediate transfer member to a recording material.
In the first transfer step, a counter bias is applied to a portion of the intermediate transfer body corresponding to a gap formed on the upstream side in the rotation direction of the image carrier from a contact portion between the image carrier and the intermediate transfer body. The surface potential on the intermediate transfer member upstream of the contact portion in the first transfer step is Vt; the potential of the image portion on the image carrier is VL; the counter bias is Vc; When the surface potential of the non-image portion is Vd and the discharge start voltage is Vth,
The counter bias Vc is a voltage having the same polarity as the charging polarity of the image carrier and having an absolute value larger than the image portion potential VL;
| Vd-Vc-Vt | <Vth.

The image forming method according to claim 23,
In the first transfer step, the toner images respectively formed on the plurality of image carriers corresponding to the toners of the plurality of colors are overlaid and transferred to the intermediate transfer body. In the second transfer step, the overlaid and transferred toner images are transferred. An image forming method, wherein the image is transferred onto the recording medium.

The image forming method according to claim 24,
An image forming method comprising: detecting a surface potential Vd of the non-image portion; and determining a value of the counter bias Vc based on the detected value.

The image forming method according to claim 24,
An image forming method comprising: detecting a surface potential Vt of the intermediate transfer member; and determining a value of the counter bias Vc based on the detected value.

The image forming method according to claim 26,
The image forming method according to claim 1, wherein the value of the surface potential Vt of the intermediate transfer body is detected by a transfer body surface potential detection unit that detects a surface potential of the intermediate transfer body.

The image forming method according to claim 26,
The surface potential Vt of the intermediate transfer member is different from the surface potential V0 of the intermediate transfer member upstream of the contact portion in the first transfer step, and the surface potential Vt of the intermediate transfer member upstream of the contact portion in the first transfer step. An image forming method, characterized in that the value is a value obtained by adding the surface potential V of the toner layer already transferred thereon.

The image forming method according to claim 28,
Adjusting the surface potential V0 of the intermediate transfer member by applying a voltage to a surface potential adjusting means disposed upstream of the contact portion, and determining the value of the counter bias Vc based on the adjusted value. A characteristic image forming method.

The image forming method according to claim 29,
An image forming method comprising: determining a counter bias value to be applied at the time of transfer of a first color in a first transfer step based on a voltage value applied to the surface potential adjusting means.

The image forming method according to claim 28,
An image forming method comprising: estimating a surface potential V of a toner layer in a first transfer step from a surface potential Ve when toner of the toner layer is on the image carrier.

The image forming method according to claim 31,
The image forming apparatus according to claim 1, wherein the surface potential Ve when the toner layer present on the intermediate transfer member is on the image carrier is detected by a surface potential detecting means for detecting the surface potential of the image carrier. Method.

The image forming method according to claim 24,
An image forming method, wherein the same counter bias value is applied during transfer of each color.

The image forming method according to claim 33,
An image forming method, wherein the counter bias values of all colors are made to coincide with the counter bias value having the smallest absolute value among the counter bias values.

The image forming method according to claim 24,
An image forming method, wherein a counter bias value of another color is determined based on a counter bias value of a first color.

The image forming method according to claim 24, wherein
An image forming method, wherein the counter bias value of the final color is a counter bias value of another color.

A rotatable image carrier on which a toner image is formed by developing an electrostatic latent image formed on a charging unit charged by the charging unit with a developing unit, and an intermediate transfer member in contact with the image carrier An image forming apparatus comprising: a first transfer unit configured to transfer the toner image onto the surface of the intermediate transfer body, and a second transfer unit configured to transfer the toner image transferred to the intermediate transfer body to a recording material.
A counter bias applying member for applying a counter bias to a portion of the intermediate transfer body corresponding to a gap formed on the rotation direction upstream side of the image carrier from a contact portion between the image carrier and the intermediate transfer body;
Voltage supply means for supplying a counter bias to the counter bias applying member,
The surface potential on the intermediate transfer member upstream of the contact portion is Vt, the potential of the image portion on the image carrier is VL, the counter bias is Vc, and the surface potential of the non-image portion on the image carrier is Vt. Vd, when the discharge starting voltage is Vth,
The power supply means sets the counter bias Vc to a voltage having the same polarity as the charging polarity of the image carrier, an absolute value larger than the image portion potential VL, and satisfying | Vd−Vc−Vt | <Vth. A transfer device characterized by controlling.

The image forming apparatus according to claim 37,
A non-image portion potential detecting means for detecting a surface potential Vd of the non-image portion on the image carrier;
Having a counter bias determining means for determining the value of the counter bias based on the value detected by the non-image portion potential detecting means,
The image forming apparatus according to claim 1, wherein the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit.

The image forming apparatus according to claim 37,
Transfer body surface potential detection means for detecting the surface potential Vt of the intermediate transfer body,
Having a counter bias determining means for determining the value of the counter bias based on the value detected by the transfer body surface potential detecting means,
The image forming apparatus according to claim 1, wherein the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit.

The image forming apparatus according to claim 39,
The transfer body surface potential detecting means detects a surface potential V0 of the intermediate transfer body on the upstream side of the contact portion, and detects a surface potential V of the toner layer already transferred on the intermediate transfer body. An image forming apparatus, comprising: a toner layer potential detecting means for calculating the surface potential Vt on the intermediate transfer member by adding the potentials detected by the detecting means and the toner layer potential detecting means.

The image forming apparatus according to claim 40,
The toner layer potential detecting means detects a surface potential Ve when the toner layer on the intermediate transfer member is on the image carrier, and the toner layer potential based on the surface potential Ve detected by the detecting means. A toner layer surface potential estimating means for estimating the surface potential V of the image forming apparatus.

The image forming apparatus according to claim 37,
Surface potential adjusting means for adjusting the surface potential V0 of the intermediate transfer member;
Having a counter bias determining means for determining a counter bias based on the surface potential adjusted by the surface potential adjusting means,
The image forming apparatus according to claim 1, wherein the power supply unit supplies a counter bias to the counter bias applying member based on a signal from the counter bias determination unit.

The image forming apparatus according to claim 42,
The image forming apparatus according to claim 1, wherein the surface potential adjusting unit is a discharging unit that is not in contact with the intermediate transfer member.

The image forming apparatus according to claim 42,
The image forming apparatus according to claim 1, wherein said surface potential adjusting means is in contact with said intermediate transfer member and has a function of removing toner from said intermediate transfer member.