JP4557455B2

JP4557455B2 - Image forming apparatus

Info

Publication number: JP4557455B2
Application number: JP2001102641A
Authority: JP
Inventors: 賢一小川
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2001-03-30
Filing date: 2001-03-30
Publication date: 2010-10-06
Anticipated expiration: 2021-03-30
Also published as: JP2002296931A

Description

【０００１】
【発明の属する技術分野】
本発明は、電子写真方式などを利用して画像形成を行う複写機、プリンタ、ファクシミリ等の画像形成装置に関する。
【０００２】
【従来の技術】
図１４は、従来の電子写真方式を利用した複写機、プリンタ、ファクシミリ等の画像形成装置の一例を示す概略構成図である。
【０００３】
この画像形成装置は、感光ドラム１００を矢印方向（時計方向）に回転駆動し、その表面を帯電バイアスが印加された帯電ローラ１０１で均一に帯電した後、露光装置（不図示）によって入力画像信号に応じた画像露光Ｌを行い、静電潜像を形成する。この静電潜像は、トナーを表面に薄層担持した現像装置１０２の現像スリーブ１０２ａによりトナー像として現像される。感光ドラム１００上に形成されたトナー像は、感光ドラム１００と転写ローラ１０３間の転写ニップ部Ｎに搬送される用紙などの転写材Ｐに、転写バイアスが印加された転写ローラ１０３によって転写される。
【０００４】
即ち、転写ニップ部Ｎにて感光ドラム１００に接する転写ローラ１０３には、トナーと逆極性（本例では正極性）の転写バイアスが印加され、転写材Ｐのトナー像が形成されていない背面側からトナーと逆極性の電荷が付与されることにより、感光ドラム１００上のトナー像が転写材Ｐ表面に転写される。
【０００５】
トナー像が転写された転写材Ｐは定着装置（不図示）に搬送され、この定着装置でトナー像を転写材Ｐ表面に熱定着した後、外部に排出される。また、上記転写後に感光ドラム１００上に残留している転写残トナーは、クリーニングブレード１０４で除去されて回収される。
【０００６】
上記の転写ローラ１０３は、図１５に示すように、ＳＵＳ、Ｆｅ等からなる給電電極を兼ねた芯金１０３ａ上にローラ状の導電性ゴムあるいは導電性スポンジの弾性層１０３ｂを設けて構成されている。この弾性体層１０３ｂは、高抵抗から中抵抗のゴムにカーボン等低抵抗の材料を添加させることで１０^６〜１０^１０Ωに抵抗調節されている。
【０００７】
そして、転写時に転写バイアス電源（不図示）から転写ローラ１０３に転写バイアスを印加する際のバイアス制御として、ＡＴＶＣ（ＡｃｔｉｖｅＴｒａｎｓｆｅｒＶｏｌｔａｇｅＣｏｎｔｒｏｌ）方式が提案されている。
【０００８】
このＡＴＶＣ方式は、転写開始前に転写ローラ１０３に所望の定電流を印加（定電流制御）して、そのときに転写ローラ１０３に生じた基準電圧Ｖｏを検出し、転写時に、この基準電圧Ｖｏを基に決定された定電圧の転写バイアスＶｔを転写ローラ１０３に印加するものである。ＡＴＶＣ方式を用いることで、転写ローラ１０３の抵抗値によらず最適な転写バイアスを印加することができる。
【０００９】
図１６は、転写ローラ１０３の抵抗値を変化させた場合における転写電圧（転写バイアス）と出力画像との関係を示した図である。図１６に示すように、低湿環境においては転写電圧がラインＬ１よりも低いと出力画像に爆発飛び散りが発生し、転写電圧がラインＬ２よりも高いと出力画像に砂地画像が発生する。また、高湿環境においては転写電圧がラインＬ３よりも高いと出力画像にゴースト画像する。例えば、抵抗が１．０×１０^９Ωの転写ローラ１０３では転写電圧が２．７ｋＶ以下で出力画像に爆発飛び散りが発生し、転写電圧が３．２ｋＶ以上で出力画像に砂地画像が発生することが分かる。
【００１０】
また、従来では、図１７に示すように、転写ニップ部Ｎにおいて用紙などの転写材Ｐの搬送方向後端Ｐ１から約２．５ｍｍ内側で、転写ローラ１０３に転写電圧（転写バイアス）Ｖｔの印加をＯＦＦし、この後端Ｐ１の約２．５ｍｍ内側から転写電圧Ｖｔよりも低い電圧をしばらく印加する。
【００１１】
そして、紙間時などの転写材Ｐの非通紙時に、転写ローラ１０３にトナーの帯電極性と同極性の負（マイナス）極性の電圧を印加して、転写ローラ１０３上に付着しているトナーを静電的に感光ドラム１００側に転移させ、クリーニングブレード１０４で除去して回収する。これにより、転写ローラ１０３表面がクリーニングされる。
【００１２】
そして、次の給紙される転写材（紙）Ｐが転写ニップ部Ｎに到達する直前から転写ニップ部Ｎに転写材Ｐの先端が突入後約２．５ｍｍまで転写電圧Ｖｔよりも低い正極性の電圧を転写ローラ１０３に印加し、その後通常の転写電圧Ｖｔを転写ローラ１０３に印加して転写を行う。
【００１３】
ところで、上記した転写ローラ１０３を有する画像形成装置が転写材（紙）Ｐの両面に画像形成（両面印字）可能な場合に、低温低湿（例えば温度１５℃、湿度１０％）環境で高抵抗の紙（例えばＫｉｍｂｅｒｌｙ−ＣｌａｒｋＣｏｒｐｏｒａｔｉｏｎ製のＮＥＥＮＡＲＢｏｎｄ：坪量６０ｇ／ｍ^２）を通紙して両面印字を行うと、１面目の印字後に定着装置（不図示）を通過した紙は紙内の水分が蒸発してしまい、紙の抵抗が上昇してしまう。それと同時に紙中の水分の蒸発状態で紙がカールしたり、ウェーブを発生したりする。なお、上記のＮＥＥＮＡＲＢｏｎｄの紙の表面抵抗は通紙前が３×１０^１０Ω／□であり、通紙後の紙の表面抵抗は３．２×１０^１１Ω／□である。
【００１４】
この際、１面目におけるトナー像転写時には、上記したように転写ローラ１０３の抵抗に応じてＡＴＶＣによって決定された転写電圧Ｖｔ（約４．２ｋＶ）を転写ローラ１０３に印加して転写される。そして、２面目におけるトナー像転写時にも、１面目を転写する際に決定された上記転写電圧をそのまま印加するという制御が用いられる。
【００１５】
【発明が解決しようとする課題】
しかしながら、上記の両面印字時もしくは片面印字時において、転写材（紙）Ｐの搬送方向後端領域の約２０ｍｍの範囲では画像不良が発生し、良好な画像を得ることが難しい場合がある。これは、転写材Ｐの後端が転写ニップ部Ｎ通過後に感光ドラム１００と分離する際に、放電という現象（剥離放電）が発生するために、転写材Ｐ上にのったトナーがその放電現象によって転写材Ｐ上で暴れてしまい、不適切な位置に飛び散ってしまうことによる。
【００１６】
このため、従来、転写材（紙）Ｐのカールを補正して、転写時に転写材Ｐと感光ドラム１００との密着性を確保することによって、この剥離放電を防止するようにしていたが、環境（温度、湿度）条件によっては転写材Ｐの分離時に剥離放電が発生する場合があった。
【００１７】
そこで本発明は、環境にかかわらず剥離放電を防止して、転写後の転写材の感光ドラムからの分離時に転写材後端での画像不良の発生を防止することができる画像形成装置を提供することを目的とする。
【００１８】
【課題を解決するための手段】
上記目的を達成するために請求項１記載の発明は、トナー像が形成される像担持体と、前記像担持体に当接して転写ニップ部を形成し、前記転写ニップ部に搬送される転写材の背面に接触して前記像担持体に形成されたトナー像を前記転写材の表面に転写する接触転写部材と、電圧値を任意に可変自在であり定電圧制御された電圧を前記接触転写部材に印加する電圧印加手段と、を備え、前記電圧印加手段から前記トナー像のトナーの帯電極性と逆極性の電圧を前記接触転写部材に印加して、前記転写ニップ部に搬送される転写材に前記像担持体上のトナー像を転写する画像形成装置において、前記電圧印加手段からの前記接触転写部材への電圧印加を制御する制御手段を有し、前記制御手段は、転写時に前記電圧印加手段から前記接触転写部材に所望の電流を流すために、転写動作開始前において、前記電圧印加手段から前記接触転写部材に所定の電圧を印加して出力電流を検出し、所望の出力電流になるようにさらに印加する電圧を変化させて、検出される電圧の値が所定の電圧値より大きいと判断した場合は、前記転写材の搬送方向先端が前記転写ニップ部に入って前記転写材の搬送方向後端から所定距離内側が前記転写ニップ部を通過するまでは、第１の電圧を前記電圧印加手段から前記接触転写部材に印加させ、前記転写材の搬送方向後端から所定距離内側が前記転写ニップ部を通過してから、その後端側が前記転写ニップ部を通過する際には、前記第１の電圧より大きい値の第２の電圧を前記電圧印加手段から前記接触転写部材に印加させるように前記電圧印加手段を制御することを特徴としている。
【００１９】
また、請求項５記載の発明は、トナー像が形成される像担持体と、前記像担持体に当接して転写ニップ部を形成し、前記転写ニップ部に搬送される転写材の背面に接触して前記像担持体に形成されたトナー像を前記転写材の表面に転写する接触転写部材と、電圧値を任意に可変自在であり定電圧制御された電圧を前記接触転写部材に印加する電圧印加手段と、を備え、前記電圧印加手段から前記トナー像のトナーの帯電極性と逆極性の電圧を前記接触転写部材に印加して、前記転写ニップ部に搬送される転写材に前記像担持体上のトナー像を転写する画像形成装置において、画像形成装置内または周囲の温度環境、湿度環境の少なくとも一方を検知する環境検知手段と、前記環境検知手段から入力される検知情報に応じて、前記電圧印加手段からの前記接触転写部材への電圧印加を制御する制御手段と、を有し、前記環境検知手段が低温環境、低湿環境の少なくとも一方を検知した場合には、前記制御手段は、前記転写材の搬送方向先端が前記転写ニップ部に入って前記転写材の搬送方向後端から所定距離内側が前記転写ニップ部を通過するまでは、第１の電圧を前記電圧印加手段から前記接触転写部材に印加させ、前記転写材の搬送方向後端から所定距離内側が前記転写ニップ部を通過してから、その後端側が前記転写ニップ部を通過する際には、前記第１の電圧の値より大きい第２の電圧を前記電圧印加手段から前記接触転写部材に印加させるように前記電圧印加手段を制御することを特徴としている。
【００２０】
また、前記第２の電圧は、前記転写ニップ部に搬送される前記転写材の搬送方向後端の１５ｍｍ内側から印加されることを特徴としている。
【００２１】
また、前記電圧印加手段によって前記接触転写部材を定電圧制御するときの出力電圧を変化させ、変化させた前記出力電圧に応じた出力電流を検知して、前記出力電流が所望の値に達したときの出力電圧を基準電圧とした場合に、前記基準電圧の値に基づいて前記第１の電圧と前記第２の電圧が決定されることを特徴としている。
【００２２】
また、前記転写材の両面にトナー像が転写される場合には、１面目の転写時における前記第２の電圧の値よりも２面目の転写時における前記第２の電圧の値の方を高くすることを特徴としている。
【００２３】
【発明の実施の形態】
以下、本発明を図示の実施の形態に基づいて説明する。
【００２４】
〈実施の形態１〉
図１は、本発明の実施の形態１に係る画像形成装置（本実施の形態では、電子写真方式のレーザプリンタなどの画像形成装置）を示す概略構成図である。
【００２５】
本画像形成装置２０は、像担持体としての感光ドラム１を備えている。感光ドラム１の周囲には感光ドラム１の回転方向に沿って順に、帯電ローラ２、現像装置３、転写ローラ４、クリーニング装置５が配設されており、帯電ローラ２と現像装置３間の上方には露光装置６が配設されている。また、感光ドラム１と転写ローラ４間に形成される転写ニップＮの転写材搬送方向の下流側には、搬送ガイド７と定着装置８が配設されている。なお、転写ニップＮの下流側で搬送ガイド７の上流側には、分離された転写材Ｐの背面の除電を補助するための除電針（不図示）が設置されている。
【００２６】
感光ドラム１は、本実施の形態では負帯電の有機感光ドラムで、アルミニウム製のドラム基体上にＯＰＣ感光層を有しており、駆動手段（不図示）により所定の周速（プロセススピード）で矢印ａ方向（時計方向）に回転駆動され、その回転過程において接触する帯電ローラ２により負極性の一様な帯電を受ける。
【００２７】
接触帯電部材としての帯電ローラ２は、感光ドラム１表面に所定の押圧力で当接して従動回転し、帯電バイアス電源９から印加される帯電バイアスによって感光ドラム１を所定の極性、電位に均一に帯電する。
【００２８】
現像装置３は、現像容器１０の開口部に感光ドラム１表面と略当接する回転自在な現像スリーブ１１を備えており、現像部にて感光ドラム１上の静電潜像にトナーｔを付着させてトナー像として顕像化する。
【００２９】
接触転写部材としての転写ローラ４は、感光ドラム１表面に所定の押圧力で接触して転写ニップ部Ｎを形成し、転写バイアス電源１７から印加される転写バイアスにより、感光ドラム１と転写ローラ４間の転写ニップ部Ｎにて感光ドラム１表面のトナー像を用紙などの転写材Ｐに転写する。
【００３０】
クリーニング装置５は、クリーニングブレード５ａを有しており、転写後に感光ドラム１表面に残留している転写残トナーをクリーニングブレード５ａで除去して回収する。
【００３１】
露光装置６は、パーソナルコンピュータ（不図示）などから入力される画像情報の時系列電気デジタル画像信号に対応して変調されたレーザ光（露光ビーム）をレーザ出力部（不図示）から出力し、帯電された感光ドラム２表面を反射ミラー１２を介して走査露光Ｌすることにより、画像情報に対応した静電潜像を形成する。
【００３２】
定着装置８は、内部にハロゲンヒータ（不図示）を備えた加熱ローラ８ａと加圧ローラ８ｂを有しており、定着ローラ８ａと加圧ローラ８ｂ間の定着ニップにて転写材Ｐを挟持搬送しながら、転写材Ｐの表面に転写されたトナー像を加熱、加圧して熱定着させる。
【００３３】
次に、上記した画像形成装置２０による画像形成動作について説明する。
【００３４】
画像形成時には、感光ドラム１は駆動手段（不図示）により矢印ａ方向（時計方向）に所定の周速で回転駆動され、帯電バイアス電源９から帯電バイアスが印加された帯電ローラ２により一様に負極性の所定電位に帯電される。そして、帯電された感光ドラム１上に露光装置６から反射ミラー１２を介してレーザ光による走査露光Ｌが与えられることにより、感光ドラム１上の電位は走査露光Ｌされた部分の電位が低下して、パーソナルコンピュータ（不図示）などから入力される画像情報に応じた静電潜像が形成される。
【００３５】
そして、現像部にて感光ドラム１の帯電極性（負極性）と同極性の現像バイアスが印加された現像装置３の現像スリーブ１１により、感光ドラム１上に形成された静電潜像にトナーｔを付着させて、反転現像によりトナー像として顕像化する。
【００３６】
一方、カセット１３内の用紙などの転写材Ｐは、ピックアップローラ１４により給紙され、感光ドラム１上でのトナー像の形成と同期してレジストローラ対（不図示）まで搬送される。そして、感光ドラム１上のトナー像が感光ドラム１と転写ローラ４間の転写ニップ部Ｎに到達すると、このタイミングに合わせて転写材Ｐがレジストローラ対（不図示）によって転写前ガイド１５を通して転写ニップ部Ｎに搬送される。
【００３７】
そして、転写バイアス電源１７から前記トナーｔと逆極性（正極性）の転写バイアスが印加された転写ローラ４により、転写ニップ部Ｎに搬送された転写材Ｐに感光ドラム１と転写ローラ４間に発生する静電力によって、感光ドラム１上のトナー像が転写される（本実施の形態における転写ローラ４への転写バイアス印加制御の詳細については後述する）。
【００３８】
そして、トナー像が転写された転写材Ｐは、搬送ガイド７を介して定着装置８に搬送され、加熱ローラ８ａと加圧ローラ８ｂ間の定着ニップにてトナー像を転写材Ｐに加熱、加圧して熱定着する。トナー像が定着された転写材Ｐは、排紙ローラ対１６を介して外部に排紙され、一連の画像形成動作を終了する。
【００３９】
また、上記転写後に感光ドラム１に残留している転写残トナーは、クリーニング装置５のクリーニングブレード５ａによって除去されて回収される。
【００４０】
次に、本実施の形態における転写ローラ４への転写バイアス印加制御について説明する。
【００４１】
本実施の形態で用いた転写ローラ４は、ＳＵＳ、Ｆｅ等からなる給電電極を兼ねた芯金上にローラ状の導電性ゴムあるいは導電性スポンジの弾性層を設けて構成されている。この転写ローラ４の抵抗値は、温度１５℃、湿度１０％のときに１×１０^９Ωｃｍである。なお、高温高湿（温度３０℃、湿度５０％）環境下では９×１０^７Ωｃｍ、低温低湿（温度５℃、湿度１０％）環境下では１×^１０Ωｃｍであり、転写ローラ４の製造上のバラツキによって、それぞれの環境下における抵抗値は±２０％前後のバラツキが生じる。
【００４２】
そして、図１に示した両面画像形成（両面印字）可能な画像形成装置を用い、通紙速度２０ｐｐｍ、プロセススピード１１０ｍｍ／ｓｅｃで転写材（紙）Ｐを連続通紙して片面のみにハーフトーン（グレー色一面の画像）画像を１００枚連続画像形成した。なお、本実施の形態で用いた転写材としての紙は、Ｋｉｍｂｅｒｌｙ−ＣｌａｒｋＣｏｒｐｏｒａｔｉｏｎ製のＮＥＥＮＡＲＢｏｎｄ；坪量６０ｇ／ｍ^２、表面抵抗：３×１０^１０Ω／□（乾燥時：３．２×１０^１１Ω／□）である。
【００４３】
本実施の形態における転写は、上記したＡＴＶＣによって行われる。即ち、制御装置１８の制御により転写動作前に転写ローラ４に一定電流を流してこのときに発生した基準電圧Ｖｏを検出し、図２に示すように、転写時にこの基準電圧Ｖｏに係数を掛けるといった補正を加えた電圧（転写バイアス）Ｖｔを転写ローラ４に印加して、転写ニップ部Ｎにて感光ドラム１上のトナー像を転写材（紙）Ｐに転写する。
【００４４】
そして、本実施の形態では、転写動作前におけるＡＴＶＣによって、転写バイアス電源１７から転写ローラ４に所定の電圧を印加して出力電流を制御装置１８で検出し、所望の出力電流になるようにさらに印加する電圧を変化させて、検出される電圧の値が所定の電圧値より大きいと判断した場合は、転写バイアス電源１７を制御装置１８で制御して、後述するように転写ニップ部Ｎにおいて転写材Ｐの搬送方向後端Ｐ１から約７．５ｍｍ手前で、転写ローラ４に転写バイアスＶｔの印加をＯＦＦし、この後端Ｐ１の約７．５ｍｍ手前から転写バイアスＶｔよりも高い正極性の電圧（以下、この電圧を転写後端バイアスＶｈという）を後端Ｐ１が転写ニップ部Ｎを通過後まで印加する。
【００４５】
つまり、本実施の形態では、低温環境又は／及び低湿環境の場合などにおいて転写ローラ４の抵抗が大きくなったと判断されると、通紙される転写材Ｐの後端Ｐ１の約７．５ｍｍ手前から転写バイアスＶｔよりも高い転写後端バイアスＶｈを印加するようにした。
【００４６】
そして、紙間時などの転写材Ｐの非通紙時に、転写ローラ４にトナーの帯電極性と同極性の負（マイナス）極性の電圧を印加して、転写ローラ４上に付着しているトナーを静電的に感光ドラム１側に転移させ、クリーニングブレード５ａで除去して回収する。
【００４７】
この転写時の転写材Ｐの後端Ｐ１から約７．５ｍｍ手前までは、転写ローラ４の抵抗に応じてＡＴＶＣにより得られた転写バイアス（本実施の形態では、約４．０ｋＶ（９０％の転写効率））Ｖｔを転写ローラ４に印加して、感光ドラム１上のトナー像（ハーフトーン画像）が転写される。
【００４８】
即ち、図３乃至図５に示すように、トナー像（ハーフトーン画像）の転写時には、先ず転写材Ｐの先端が転写前ガイド１５にガイドされて感光ドラム１と転写ローラ４間の転写ニップ部Ｎに入り（図３）、その状態で転写材Ｐの中央付近が転写ニップ部Ｎを通過して（図４）、最後に転写材Ｐの後端が転写ニップ部Ｎを出て（図５）、搬送ガイド７を通して定着装置８に搬送される。
【００４９】
ところで、図５に示したように、転写材Ｐの表面にトナー像を転写した後に転写ニップ部Ｎから転写材Ｐの後端が出た際に、転写材Ｐは感光ドラム１から急激に離間されることにより、感光ドラム１と転写材Ｐとの間で放電現象が起きてしまう（ことがある）。そのため、転写材Ｐの後端側にのみこの放電現象によって画像不良が生じる場合がある。
【００５０】
この１００枚連続画像形成において、転写材Ｐの後端側のどの位置で画像不良が発生しているかを調べたところ、図６に示すような結果が得られた。図６に示す結果から明らかなように、転写材Ｐの後端での画像不良（図７に示すような水玉模様Ａの発生）は、主に転写材Ｐの後端から３．５ｍｍから７．５ｍｍ手前の範囲に８２％が集中している。即ち、１００枚中の８２枚において、転写材Ｐの後端から３．５ｍｍから７．５ｍｍ手前の範囲に画像不良（水玉模様）が発生した。転写材Ｐの後端での画像不良（水玉模様）の発生は、特にハーフトーン画像のようなグラフィック画像の場合に目立つ。
【００５１】
なお、転写材Ｐの後端から３．５ｍｍから７．５ｍｍ手前の範囲における画像不良（水玉模様）の発生は、主に画像形成動作の後半（定着装置８の加圧ローラ８ｂが温まっている状態）で発生している。これは、ハロゲンヒータ（不図示）の発熱により加圧ローラ８ｂに熱膨張が生じて加圧ローラ８ｂの外径が大きくなって、転写材Ｐの先端側が加熱ローラ８ａと加圧ローラ８ｂ間で挟持搬送されることにより、転写ニップ部Ｎでの転写材Ｐの後端側と感光ドラム１との分離が急激に行われてしまうことによるものと考えられる。
【００５２】
そこで、転写材Ｐの後端から３．５ｍｍから７．５ｍｍ手前の範囲における画像不良（水玉模様）の発生を防止するためには、転写材Ｐの後端領域での転写バイアス印加を制御する必要がある。
【００５３】
そこで、転写ニップ部Ｎに搬送される転写材Ｐの後端の８．５ｍｍ手前から印加する転写後端バイアスＶｈを変化させて転写を行い、出力画像の評価を行った。なお、この転写後端バイアスＶｈは、上記したＡＴＶＣにおける基準電圧Ｖｏに係数（０．４、０．６…２．０）を掛けたものである。図８はその評価である。なお、電気回路の関係上、転写材Ｐの後端の７．５ｍｍ手前から効果を得るためには、転写バイアスの立ち上がり時間を考慮して転写材Ｐの後端の８．５ｍｍ手前から転写バイアスの値を変更する必要がある。
【００５４】
図８に示す評価結果から明らかなように、転写後端バイアスＶｈを大きくするにつれて画像状態がよくなり、図７のような水玉画像の発生が防止されるが、後端転写バイアスＶｈが２．０Ｖｏよりも大きくなると、転写材Ｐの後端に突き抜け画像が発生した。このつき抜け画像とは、転写ローラ４への印加バイアスが転写材Ｐを突き抜けて感光ドラム１まで到達し、転写時の電流よりも大きな電流（過電流）が流れて感光ドラム１にプラスメモリーが発生することにより、そのメモリーされている電位の部分が現像されて黒い点々が発生してしまうという現象である。
【００５５】
一方、転写後端バイアスＶｈが０．４Ｖｏよりも小さい場合では、転写材Ｐの後端に爆発画像（と水玉画像）が発生してしまう。この爆発画像は、上記したように感光ドラム１上のトナーが転写ニップ部Ｎで転写バイアスによって転写材Ｐ上に転写される際に、その転写バイアスが低いためにうまく転写されずに転写材Ｐ上で飛び散ってしまうことによって発生する。
【００５６】
また、転写を行う条件として、一番厳しい条件下でも転写を確実に行う必要がある。つまり、この条件下のもとで転写効率をよくするということである。ここで転写効率とは、転写前の感光ドラム１上のトナー量と転写材Ｐ上に転写されたトナー量の比である。即ち、転写効率（％）をη、感光ドラム１上のトナー量をＴ０、転写材Ｐ上のトナー量をＴ１とすると、η＝１００×Ｔ１／Ｔ０のように表される。
【００５７】
しかしながら、実際には感光ドラム１および転写材Ｐ上のトナー量を測定することは難しいため、ここでは、転写効率ηの測定は、「トナー量とトナーの光学式濃度測定値は比例の関係である」ことを利用し、感光ドラム１上と転写材Ｐ上のトナーの光学式濃度測定値から転写効率を求めた。
【００５８】
つまり、転写材Ｐに転写後の感光ドラム１上のトナー濃度（Ｄｄ）、転写材Ｐ上のトナー濃度（Ｔｄ）とすると、転写効率ηは、η＝１００×Ｔｄ／（Ｄｄ＋Ｔｄ）という計算で求められる。一方、転写材Ｐ上に転写されずに残ったトナー（つまり、感光ドラム１上の残トナー）は、現像スリーブ１１によって現像容器１０内に回収される。
【００５９】
図９は、転写効率を変化させた場合における濃度、定着性、画像ムラを調べた結果を示す図である。図９の結果から明らかなように、転写効率が７５％以下においては、濃度が１．２以下となり、見た目に薄く感じ、定着性では、トナー同士の接触／接着力不足のため○から△レベル以下になり、更に濃度のムラにおいても△レベル以下と悪いレベルにある。なお、図９において、◎は最良、○は量、△はギリギリ許容、×は不可を示している。
【００６０】
一方、転写効率が７５％以上であれば、濃度、定着性、画像ムラとも良好であった。つまり、濃度、定着性、画像ムラを良好なレベルとするには、転写効率が７５％以上、好ましくは転写効率を８０％以上とすることが好ましい。そこで、転写時における環境（温度）が、５℃、１０℃、１５℃、２３℃、３０℃の場合において、転写効率を７５％以上に保つ転写バイアスＶｔを求めた。なお、転写バイアスＶｔを決めるために、最悪条件として５℃の低温環境では転写ローラ４の抵抗値を１．２×１０^１０Ωｃｍとなるものを使用し、３０℃の高温環境では７．２×１０^１０Ωｃｍとなるものを使用した。
【００６１】
その結果、転写効率が７５％以上になる条件は、それぞれ５℃時には転写バイアスＶｔ＝５ｋＶ、１０℃時にはＶｔ＝４ｋＶ、１５℃時にはＶｔ＝３．５ｋＶ、２３℃時にはＶｔ＝２ｋＶ、３０℃時にはＶｔ＝１ｋＶという結果になった。
なお、この結果はあくまでも本実施の形態においてのみの値であり、個々の転写ローラ４などによって違うため、７５％以上の転写効率が得られる転写バイアスＶｔの値は違ってくる。
【００６２】
次に、転写ニップ部Ｎに搬送される転写材Ｐの後端の８．５ｍｍ手前から印加する転写後端バイアスＶｈを変化させさせた場合における、環境（温度、湿度）と出力画像の評価を行った。図１０はその評価である。なお、この転写後端バイアスＶｈは、上記したＡＴＶＣにおける基準電圧Ｖｏに係数（０．４、０．６…２．０）を掛けたものであり、図１０におけるＶｏは１ｋＶである。なお、図１０において、◎は最良、○は量、△はギリギリ許容、×は不可を示している。温度が低温環境又は／及び湿度が低湿環境の場合は、転写ローラ４の抵抗が大きくなる。
【００６３】
図１０に示す評価結果から明らかなように、５℃環境ではＶｈは、２．０ｋＶから１０ｋＶの範囲で良好（許容範囲）であり、７．５℃ではＶｈは、１ｋＶから９ｋＶ、１５℃ではＶｈは、１．５ｋＶから５．５ｋＶ、２３℃ではＶｈは、０．１ｋＶから２．０ｋＶ、３０℃ではＶｈは、０．１ｋＶから１．０ｋＶである。このなかでも、それぞれ５℃環境ではＶｈは７ｋＶ、７．５℃では６ｋＶ、１５℃では３．５ｋＶ、２３℃では１．０ｋＶ、３０℃では０．５ｋＶが最も良好であった。
【００６４】
図１０の評価結果からは、転写材Ｐの後端側に印加する転写後端バイアスＶｈの許容範囲は各環境でランダムに見えるが、本画像形成装置において、転写バイアスを決める際の基準電圧（５μＡの定電流を転写ローラ４と感光ドラム１に流す際の電圧値）Ｖｏを基準とした場合、図１１に示すように、各環境におけるＶｏと、Ｖｏに対する転写後端バイアスＶｈとの比率（Ｖｏを１としたとき）との関係は、温度１５℃環境以下の場合には、転写後端バイアスＶｈがＶｏに対して１／２倍から２倍の領域において、図１０の△レベル以上を示しているといった結果が得られた。
【００６５】
また、図１０において、低温環境である７．５℃環境をみると、転写後端バイアスＶｈが９ｋＶを越えたあたりで、画像評価がＮＧ（×）となる領域になっている。これは、高温環境においても言えることだが、あまりにも転写バイアス（この場合は転写後端バイアスＶｈ）が高すぎると転写バイアスが転写材Ｐを突き抜けて感光ドラム１上に転写によるプラスメモリーを発生させてしまうことがある。つまり、この状態では、Ｖｏの２倍（２Ｖｏ）以上の転写後端バイアスＶｈを印加すると、画像に不具合が生じてしまうことが分かる。
【００６６】
一方、温度２３℃以上の高温環境以上の場合には、転写後端バイアスＶｈがＶｏに対して１倍から１／１０倍の領域において、図１０の△レベル以上を示しているといった結果が得られた。
【００６７】
本発明者は、図１０、図１１の評価結果を参考にし、低温環境である温度５℃、湿度１０％環境における転写時の転写バイアスをＶｍａｘ（本実施の形態は５ｋＶ）と定義することで、環境（温度、湿度）と転写後端バイアスＶｈとの間で対応がとれることを見出した。
【００６８】
つまり、低温環境である１５℃以下の場合、即ち、転写ローラ４の抵抗が大きくなった場合には、転写ニップ部Ｎに通紙される転写材Ｐの初期から後端７．５ｍｍ手前までは通常の転写バイアス（規準電圧Ｖｏを基準にして決定されるバイアス）Ｖｔを印加し、転写材Ｐの後端の７．５ｍｍ手前からはＶｏ以上で、かつＶｏの２倍の２Ｖｏを上限とした転写後端バイアスＶｈを印加することで、良好な画像を得ることが可能となる。
【００６９】
更に、条件として、画像形成装置を使用する環境においての転写バイアス（転写効率７５％以上）Ｖｔが、温度５℃、湿度１０％環境下での転写バイアスである上記したＶｍａｘの７０％以上であれば、上記の転写後端バイアスＶｈを転写材Ｐの後端に印加するようにする。
【００７０】
また、この場合の高温環境である２３℃以上では、転写材Ｐの初期から後端７．５ｍｍ手前までは通常の転写バイアス（規準電圧Ｖｏを基準にして決定されるバイアス）Ｖｔを印加し、転写材Ｐの後端の７．５ｍｍ手前からはＶｏの１／１０倍の１／１０Ｖｏを下限とした転写後端バイアスＶｈを印加することで、良好な画像を得ることが可能となる。
【００７１】
上記したように低温環境、即ち、転写ローラ４の抵抗が大きくなった場合において、転写材Ｐの後端側に印加する転写後端バイアスＶｈをそのときに印加される転写電圧Ｖｔよりも高くすることで、転写材Ｐの後端における画像不良の発生が防止されることについて、以下に説明する。
【００７２】
転写材Ｐの後端側で印加バイアスの値を大きくすることで、転写後の転写材Ｐと感光ドラム１の分離が従来よりも早く行われるのである。つまり、低温環境の場合に、転写後端バイアスＶｈの値を転写バイアスＶｔの値より大きくすることにより、本実施の形態では、転写ニップ部Ｎでの感光ドラム１の電位が正極性に帯電される（てしまう）と同時に転写材Ｐの後端電位も正極性に帯電されることで、転写材Ｐと感光ドラム１間に反発力が発生し、転写材Ｐの分離が早く行われるためである。
【００７３】
そして、転写材Ｐの分離がスムーズに行われると、転写材Ｐは転写ニップ部Ｎに近い位置で分離するようになるため、転写ローラ４の下流側に配置している除電針（不図示）による転写ローラ４の背面の除電がより促進され、転写ニップ部Ｎ通過後の転写材Ｐの電位が下がり、感光ドラム１や装置内への不要な放電（リーク）を防止することができる。よって、出力画像の後端に水玉画像などの発生が防止される。
【００７４】
このように本実施の形態では、転写動作前におけるＡＴＶＣによって、転写バイアス電源１７から転写ローラ４に所定の電圧を印加して出力電流を制御装置１８で検出し、所望の出力電流になるようにさらに印加する電圧を変化させて、検出される電圧の値が所定の電圧値より大きいと判断した場合は、転写ニップ部Ｎに通紙される転写材Ｐの先端から後端の７．５ｍｍ手前まで印加する転写バイアスＶｔの値よりも、転写材Ｐの後端手前の７．５ｍｍから印加する転写後端バイアスＶｈの値の方を高くするように、転写バイアス電源１７を制御装置１８で制御することにより、転写材Ｐが転写後に感光ドラム１から良好に分離される。従って、転写材Ｐの分離時における異常な放電現象が抑制されることにより、転写材Ｐの後端での画像不良の発生を防止して良好な画像を得ることができる。
【００７５】
また、本実施の形態では、転写材Ｐの後端の７．５ｍｍ手前から転写後端バイアスＶｈを印加するようにしたが、転写材Ｐの後端の１５ｍｍ手前から転写後端バイアスＶｈを印加するようにしてもよい。
【００７６】
〈実施の形態２〉
図１２は、本発明の実施の形態２に係る画像形成装置を示す概略構成図である。なお、図１に示した実施の形態１の画像形成装置と同一機能を有する部材には同一符号を付し、重複する説明は省略する。
【００７７】
実施の形態１では、転写動作前におけるＡＴＶＣによって、転写バイアス電源１７から転写ローラ４に所定の電圧を印加して出力電流を制御装置１８で検出し、所望の出力電流になるようにさらに印加する電圧を変化させて、検出される電圧の値が所定の電圧値より大きいと判断した場合は、通紙される転写材Ｐの後端Ｐ１の約７．５ｍｍ手前から転写バイアスＶｔよりも高い転写後端バイアスＶｈを印加するようにしたが、本実施の形態では、画像形成装置２０内に設置した温度・湿度を検知する環境センサ１９で画像形成装置２０内が低温及び／又は低湿環境と検知した場合、制御装置１８はこの検知情報に基づいて転写バイアス電源１７を制御して、通紙される転写材Ｐの後端Ｐ１の約７．５ｍｍ手前から転写バイアスＶｔよりも高い転写後端バイアスＶｈを印加するようにした。他の構成及び動作は実施の形態１と同様であり、それらの説明は省略する。
【００７８】
このように本実施の形態では、環境センサ１９によって低温及び／又は低湿環境と検知された場合には、転写ニップ部Ｎに通紙される転写材Ｐの先端から後端の７．５ｍｍ手前まで印加する転写バイアスＶｔの値よりも、転写材Ｐの後端手前の７．５ｍｍから印加する転写後端バイアスＶｈの値の方を高くするように、転写バイアス電源１７を制御装置１８で制御することにより、転写材Ｐが転写後に感光ドラム１から良好に分離される。従って、転写材Ｐの分離時における異常な放電現象が抑制されることにより、転写材Ｐの後端での画像不良の発生を防止して良好な画像を得ることができる。
【００７９】
〈実施の形態３〉
本実施の形態においても図１に示した画像形成装置を用いて説明する。実施の形態１では、転写材の片面のみに画像形成するときの例であったが、本実施の形態では、転写材の両面（１面と２面）に画像形成を行うときの例であり、本実施の形態における転写ローラ４への転写バイアス印加制御についてのみ説明する。
【００８０】
両面画像形成時には、実施の形態１で述べたように転写材Ｐの片面（１面目）にトナー像が転写されて定着された後、排出されることなく反転されたこの転写材Ｐが再び転写ニップ部Ｎに搬送され、同様にして転写材Ｐのもう一方の面（２面目）にトナー像を転写して定着された後に外部に排出される。１面目の画像形成時に定着装置８の加熱によって、転写材（紙）Ｐは乾燥して転写材Ｐ自体の抵抗値が上昇し、２面目の画像形成時には１面目よりも画像不良が転写材Ｐの後端側に発生しやすくなる。
【００８１】
そこで、１００枚連続して両面画像形成を行い、転写材Ｐの２面目の後端側のどの位置で画像不良が発生しているかを調べたところ、図１３に示すような結果が得られた。図１３に示す結果から明らかなように、転写材Ｐの後端での画像不良（図７に示したような水玉模様の発生）は、主に転写材Ｐの後端から３．５ｍｍから７．５ｍｍ手前の範囲に９０％が集中している。即ち、１００枚中の９０枚において、転写材Ｐの後端から３．５ｍｍから７．５ｍｍ手前の範囲に画像不良（水玉模様）が発生した。
【００８２】
よって、２面目の画像形成時においても、実施の形態１と同様に、転写材Ｐの先端から後端の７．５ｍｍ手前まで印加する転写バイアスＶｔの値よりも、転写材Ｐの後端手前の７．５ｍｍから印加する転写後端バイアスＶｈの値の方を高くする。
【００８３】
本実施の形態においても、実施の形態１の図１０、図１１と同様の評価を行った。この評価は、実施の形態１と同様であり、詳細は省略する。
【００８４】
本実施の形態においても、実施の形態１と同様に行った評価結果から低温環境である１５℃以下では、転写ニップ部Ｎに通紙される転写材Ｐの初期から後端７．５ｍｍ手前までは通常の転写バイアス（規準電圧Ｖｏを基準にして決定されるバイアス）Ｖｔを印加し、転写材Ｐの後端の７．５ｍｍ手前からはＶｏ以上で、かつＶｏの５／２倍の５／２Ｖｏを上限とした転写後端バイアスＶｈを印加することで、良好な画像を得ることが可能となる。
【００８５】
また、この場合における高温環境である２３℃以上では、転写材Ｐの初期から後端７．５ｍｍ手前までは通常の転写バイアス（規準電圧Ｖｏを基準にして決定されるバイアス）Ｖｔを印加し、転写材Ｐの後端の７．５ｍｍ手前からはＶｏの１／１０倍の１／１０Ｖｏを下限とした転写後端バイアスＶｈを印加することで、良好な画像を得ることが可能となる。
【００８６】
本実施の形態における低温環境での転写後端バイアスＶｈの値は、実施の形態１における低温環境での転写後端バイアスＶｈよりも大きくなっている。これは、２面目ということで転写材Ｐが一度定着装置８を通過しているために、転写材Ｐ自体の抵抗値が上昇しており、この抵抗上昇分によって生じる電流不足を転写バイアスを大きくすることで、適正な転写バイアスを印加する必要があるのである。
【００８７】
このように本実施の形態においても実施の形態１と同様に、転写動作前におけるＡＴＶＣによって、転写バイアス電源１７から転写ローラ４に所定の電圧を印加して出力電流を制御装置１８で検出し、所望の出力電流になるようにさらに印加する電圧を変化させて、検出される電圧の値が所定の電圧値より大きいと判断した場合は、転写ニップ部Ｎに通紙される転写材Ｐの先端から後端の７．５ｍｍ手前まで印加する転写バイアスＶｔの値よりも、転写材Ｐの後端手前の７．５ｍｍから印加する転写後端バイアスＶｈの値の方を高くするように、転写バイアス電源１７を制御装置１８で制御することにより、転写材Ｐが転写後に感光ドラム１から良好に分離される。従って、転写材Ｐの分離時における異常な放電現象が抑制されることにより、転写材Ｐの後端での画像不良の発生を防止して良好な画像を得ることができる。
【００８８】
【発明の効果】
以上説明したように請求項１記載の発明によれば、転写時に電圧印加手段から接触転写部材に所望の電流を流すために、転写動作開始前において、電圧印加手段から接触転写部材に所定の電圧を印加して出力電流を制御手段で検出し、所望の出力電流になるようにさらに印加する電圧を変化させて、検出される電圧の値が所定の電圧値より大きいと判断した場合は、転写材の搬送方向先端が前記転写ニップ部に入って転写材の搬送方向後端から所定距離内側が転写ニップ部を通過するまでは、第１の電圧を電圧印加手段から接触転写部材に印加させ、転写材の搬送方向後端から所定距離内側が転写ニップ部を通過してから、その後端側が転写ニップ部を通過する際には、第１の電圧より大きい値の第２の電圧を電圧印加手段から接触転写部材に印加させるように電圧印加手段を制御することによって、転写材が転写後に像担持体から良好に分離される。従って、転写材の分離時における異常な放電現象が抑制されることにより、転写材の後端での画像不良の発生を防止して良好な画像を得ることができる。
【００８９】
また、請求項５記載の発明によれば、環境検知手段が低温環境、低湿環境の少なくとも一方を検知した場合には、制御手段は、転写材の搬送方向先端が転写ニップ部に入って転写材の搬送方向後端から所定距離内側が転写ニップ部を通過するまでは、第１の電圧を電圧印加手段から接触転写部材に印加させ、転写材の搬送方向後端から所定距離内側が転写ニップ部を通過してから、その後端側が転写ニップ部を通過する際には、第１の電圧より大きい値の第２の電圧を電圧印加手段から接触転写部材に印加させるように電圧印加手段を制御することによって、転写材が転写後に像担持体から良好に分離される。従って、転写材の分離時における異常な放電現象が抑制されることにより、転写材の後端での画像不良の発生を防止して良好な画像を得ることができる。
【図面の簡単な説明】
【図１】本発明の実施の形態１に係る画像形成装置を示す概略構成図。
【図２】本発明の実施の形態１における転写バイアス制御による転写電圧の印加状態を示す図。
【図３】本発明の実施の形態１における転写ニップ部を転写材先端部が通過した状態を示す図。
【図４】本発明の実施の形態１における転写ニップ部を転写材中央部が通過した状態を示す図。
【図５】本発明の実施の形態１における転写ニップ部を転写材後端が通過した状態を示す図。
【図６】本発明の実施の形態１における転写材の後端での画像不良の発生数を示す図。
【図７】本発明の実施の形態１における転写材の後端での画像不良の発生状態を示す図。
【図８】本発明の実施の形態１における転写後端バイアスを変化させた場合の画像状態の評価結果を示す図。
【図９】本発明の実施の形態１における転写効率を変化させた場合における濃度、定着性、画像ムラの評価結果を示す図。
【図１０】本発明の実施の形態１における環境と転写後端バイアスを変化させた場合の出力画像の評価結果を示す図。
【図１１】本発明の実施の形態１の各環境における基準電圧Ｖｏと基準電圧Ｖｏに対する転写後端バイアスＶｈとの比率との関係を示す図。
【図１２】本発明の実施の形態２に係る画像形成装置を示す概略構成図。
【図１３】本発明の実施の形態２における転写材の後端での画像不良の発生数を示す図。
【図１４】従来例における画像形成装置を示す概略構成図。
【図１５】転写ローラを示す図。
【図１６】転写ローラの抵抗値を変化させた場合における転写電圧と出力画像との関係を示す図。
【図１７】従来例における転写バイアス制御による転写電圧の印加状態を示す図。
【符号の説明】
１感光ドラム（像担持体）
２帯電ローラ
３現像装置
４転写ローラ（接触転写部材）
５クリーニング装置
５ａクリーニングブレード
６露光装置
８定着装置
１７転写バイアス電源（電圧印加手段）
１８制御装置（制御手段）
１９環境センサ（環境検知手段）
２０画像形成装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine that forms an image using an electrophotographic system.
[0002]
[Prior art]
FIG. 14 is a schematic configuration diagram showing an example of an image forming apparatus such as a copying machine, a printer, and a facsimile using a conventional electrophotographic system.
[0003]
In this image forming apparatus, the photosensitive drum 100 is rotationally driven in the direction of an arrow (clockwise), and the surface thereof is uniformly charged by a charging roller 101 to which a charging bias is applied, and then an input image signal is output by an exposure device (not shown). An image exposure L corresponding to is performed to form an electrostatic latent image. This electrostatic latent image is developed as a toner image by the developing sleeve 102a of the developing device 102 carrying a thin layer of toner on the surface. The toner image formed on the photosensitive drum 100 is transferred by a transfer roller 103 to which a transfer bias is applied to a transfer material P such as a sheet conveyed to a transfer nip N between the photosensitive drum 100 and the transfer roller 103. .
[0004]
That is, the transfer roller 103 that contacts the photosensitive drum 100 at the transfer nip N is applied with a transfer bias having a polarity opposite to that of the toner (positive in this example), and the back side of the transfer material P on which the toner image is not formed. Thus, the toner image on the photosensitive drum 100 is transferred onto the surface of the transfer material P by applying a charge having a polarity opposite to that of the toner.
[0005]
The transfer material P onto which the toner image has been transferred is conveyed to a fixing device (not shown), and the toner image is thermally fixed on the surface of the transfer material P by this fixing device and then discharged to the outside. Further, the transfer residual toner remaining on the photosensitive drum 100 after the transfer is removed by the cleaning blade 104 and collected.
[0006]
As shown in FIG. 15, the transfer roller 103 is configured by providing a roller-like conductive rubber or conductive sponge elastic layer 103b on a cored bar 103a also serving as a power supply electrode made of SUS, Fe or the like. Yes. The elastic layer 103b is formed by adding a low resistance material such as carbon to a high resistance to medium resistance rubber. ⁶ -10 ¹⁰ The resistance is adjusted to Ω.
[0007]
An ATVC (Active Transfer Voltage Control) method has been proposed as bias control when a transfer bias is applied to the transfer roller 103 from a transfer bias power source (not shown) during transfer.
[0008]
In this ATVC method, a desired constant current is applied to the transfer roller 103 before starting transfer (constant current control), and a reference voltage Vo generated in the transfer roller 103 at that time is detected. At the time of transfer, this reference voltage Vo is detected. The transfer bias Vt having a constant voltage determined based on the above is applied to the transfer roller 103. By using the ATVC method, an optimum transfer bias can be applied regardless of the resistance value of the transfer roller 103.
[0009]
FIG. 16 is a diagram showing the relationship between the transfer voltage (transfer bias) and the output image when the resistance value of the transfer roller 103 is changed. As shown in FIG. 16, in a low humidity environment, if the transfer voltage is lower than the line L1, explosion scattering occurs in the output image, and if the transfer voltage is higher than the line L2, a sandy image is generated in the output image. In a high humidity environment, if the transfer voltage is higher than the line L3, a ghost image is formed on the output image. For example, the resistance is 1.0 × 10 ⁹ It can be seen that with the transfer roller 103 of Ω, explosion scattering occurs in the output image when the transfer voltage is 2.7 kV or less, and a sand image is generated in the output image when the transfer voltage is 3.2 kV or more.
[0010]
Conventionally, as shown in FIG. 17, a transfer voltage (transfer bias) Vt is applied to the transfer roller 103 about 2.5 mm inside the transfer nip portion N from the rear end P1 in the transport direction of the transfer material P such as paper. And a voltage lower than the transfer voltage Vt is applied for a while from about 2.5 mm inside the rear end P1.
[0011]
Then, when the transfer material P is not passing, such as between papers, a negative (minus) polarity voltage having the same polarity as the charging polarity of the toner is applied to the transfer roller 103 so that the toner adhering to the transfer roller 103 Is electrostatically transferred to the photosensitive drum 100 side, removed by the cleaning blade 104, and collected. As a result, the surface of the transfer roller 103 is cleaned.
[0012]
The positive polarity is lower than the transfer voltage Vt from the time immediately before the next fed transfer material (paper) P reaches the transfer nip portion N to about 2.5 mm after the leading end of the transfer material P enters the transfer nip portion N. Is applied to the transfer roller 103, and then a normal transfer voltage Vt is applied to the transfer roller 103 to perform transfer.
[0013]
By the way, when the image forming apparatus having the transfer roller 103 described above can form images (double-sided printing) on both sides of the transfer material (paper) P, it has high resistance in a low temperature and low humidity (for example, temperature 15 ° C., humidity 10%) environment. Paper (for example, NEENAR Bond manufactured by Kimberly-Clark Corporation: basis weight 60 g / m ² ), When both sides are printed, the moisture in the paper evaporates in the paper that has passed through the fixing device (not shown) after printing the first side, and the resistance of the paper increases. At the same time, the paper curls or generates waves due to the evaporation of moisture in the paper. Note that the surface resistance of the above NENEAR Bond paper is 3 × 10 before paper feeding. ¹⁰ Ω / □, and the surface resistance of the paper after passing through is 3.2 × 10 ¹¹ Ω / □.
[0014]
At this time, when transferring the toner image on the first surface, the transfer voltage Vt (about 4.2 kV) determined by ATVC according to the resistance of the transfer roller 103 as described above is applied to the transfer roller 103 and transferred. Also, when transferring the toner image on the second side, the control of applying the transfer voltage determined at the time of transferring the first side as it is is used.
[0015]
[Problems to be solved by the invention]
However, at the time of the above double-sided printing or single-sided printing, an image defect may occur in the range of about 20 mm in the rear end region in the conveyance direction of the transfer material (paper) P, and it may be difficult to obtain a good image. This is because when the rear end of the transfer material P is separated from the photosensitive drum 100 after passing through the transfer nip portion N, a phenomenon called discharge (peeling discharge) occurs, so that the toner on the transfer material P is discharged. This is due to the fact that the phenomenon is violated on the transfer material P and scattered at an inappropriate position.
[0016]
For this reason, conventionally, the curling of the transfer material (paper) P is corrected to ensure the adhesion between the transfer material P and the photosensitive drum 100 during transfer, thereby preventing this peeling discharge. Depending on the (temperature, humidity) conditions, peeling discharge may occur when the transfer material P is separated.
[0017]
Therefore, the present invention provides an image forming apparatus capable of preventing peeling discharge regardless of the environment and preventing image defects from occurring at the trailing edge of the transfer material when the transfer material after transfer is separated from the photosensitive drum. For the purpose.
[0018]
[Means for Solving the Problems]
In order to achieve the above object, the invention described in claim 1 is directed to an image carrier on which a toner image is formed, and a transfer nip part formed in contact with the image carrier and transferred to the transfer nip part. A contact transfer member for transferring a toner image formed on the image bearing member in contact with the back surface of the material onto the surface of the transfer material; and a voltage whose voltage value is arbitrarily variable and controlled at a constant voltage. A transfer material that is conveyed to the transfer nip portion by applying a voltage having a polarity opposite to the charged polarity of the toner of the toner image from the voltage application unit to the contact transfer member. In the image forming apparatus for transferring the toner image on the image bearing member, the image forming apparatus includes a control unit that controls voltage application from the voltage application unit to the contact transfer member, and the control unit applies the voltage during transfer. Said contact transfer from means In order to flow a desired current through the material, a predetermined voltage is applied from the voltage applying means to the contact transfer member to detect an output current before the transfer operation starts, and further applied so as to obtain a desired output current. When it is determined that the detected voltage value is greater than a predetermined voltage value by changing the voltage, the leading end of the transfer material in the transport direction enters the transfer nip portion and reaches the predetermined position from the trailing end of the transfer material in the transport direction. The first voltage is applied from the voltage applying means to the contact transfer member until the distance inside passes through the transfer nip portion, and the inner side passes through the transfer nip portion by a predetermined distance from the rear end in the transport direction of the transfer material. After that, when the rear end side passes through the transfer nip portion, the voltage applying unit applies a second voltage larger than the first voltage from the voltage applying unit to the contact transfer member. The It is characterized in that Gosuru.
[0019]
According to a fifth aspect of the present invention, there is provided an image carrier on which a toner image is formed, a transfer nip portion in contact with the image carrier, and a back surface of a transfer material conveyed to the transfer nip portion. A contact transfer member for transferring the toner image formed on the image carrier onto the surface of the transfer material, and a voltage for applying a voltage whose voltage value is arbitrarily variable and controlled at a constant voltage to the contact transfer member. And applying a voltage having a polarity opposite to the toner charging polarity of the toner image from the voltage applying unit to the contact transfer member, and transferring the image carrier to the transfer material conveyed to the transfer nip portion. In the image forming apparatus for transferring the toner image on the upper side, an environment detection unit for detecting at least one of a temperature environment and a humidity environment in or around the image forming apparatus and the environment detection unit Detection Control means for controlling the voltage application from the voltage application means to the contact transfer member according to the information, and when the environment detection means detects at least one of a low temperature environment and a low humidity environment, Control means The first voltage is applied from the voltage applying means until the leading end of the transfer material in the transfer direction enters the transfer nip portion and the inner side of the transfer material in the predetermined distance from the trailing end of the transfer material passes through the transfer nip portion. When the rear end side passes through the transfer nip portion after a predetermined distance inside from the rear end in the conveyance direction of the transfer material passes through the transfer nip portion, the first voltage is applied to the contact transfer member. The voltage application unit is controlled to apply a second voltage larger than the value from the voltage application unit to the contact transfer member.
[0020]
Further, the second voltage is applied from the inner side of the rear end of the transfer material conveyed in the transfer nip portion by 15 mm.
[0021]
Further, the output voltage when the contact transfer member is controlled at a constant voltage by the voltage applying means is changed, and the output current corresponding to the changed output voltage is detected, and the output current reaches a desired value. When the output voltage at that time is the reference voltage, the first voltage and the second voltage are determined based on the value of the reference voltage.
[0022]
Further, when the toner image is transferred onto both surfaces of the transfer material, the value of the second voltage when transferring the second surface is higher than the value of the second voltage when transferring the first surface. It is characterized by doing.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described based on the illustrated embodiments.
[0024]
<Embodiment 1>
FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to Embodiment 1 of the present invention (in this embodiment, an image forming apparatus such as an electrophotographic laser printer).
[0025]
The image forming apparatus 20 includes a photosensitive drum 1 as an image carrier. Around the photosensitive drum 1, a charging roller 2, a developing device 3, a transfer roller 4, and a cleaning device 5 are disposed in this order along the rotation direction of the photosensitive drum 1, and above the charging roller 2 and the developing device 3. Is provided with an exposure device 6. Further, a conveyance guide 7 and a fixing device 8 are disposed downstream of the transfer nip N formed between the photosensitive drum 1 and the transfer roller 4 in the transfer material conveyance direction. In addition, on the downstream side of the transfer nip N and on the upstream side of the conveyance guide 7, a static elimination needle (not shown) for assisting in static elimination on the back surface of the separated transfer material P is installed.
[0026]
In this embodiment, the photosensitive drum 1 is a negatively charged organic photosensitive drum, and has an OPC photosensitive layer on an aluminum drum base, and is driven at a predetermined peripheral speed (process speed) by a driving means (not shown). It is rotated in the direction of arrow a (clockwise), and is charged uniformly with negative polarity by the charging roller 2 that contacts in the rotation process.
[0027]
The charging roller 2 as a contact charging member is in contact with the surface of the photosensitive drum 1 with a predetermined pressing force and is driven to rotate, and the photosensitive drum 1 is uniformly made to have a predetermined polarity and potential by a charging bias applied from a charging bias power source 9. Charge.
[0028]
The developing device 3 includes a rotatable developing sleeve 11 that is substantially in contact with the surface of the photosensitive drum 1 at the opening of the developing container 10, and causes the toner t to adhere to the electrostatic latent image on the photosensitive drum 1 at the developing unit. To visualize it as a toner image.
[0029]
The transfer roller 4 as a contact transfer member contacts the surface of the photosensitive drum 1 with a predetermined pressing force to form a transfer nip portion N, and the photosensitive drum 1 and the transfer roller 4 are transferred by a transfer bias applied from a transfer bias power source 17. The toner image on the surface of the photosensitive drum 1 is transferred to a transfer material P such as a sheet at the transfer nip portion N between them.
[0030]
The cleaning device 5 has a cleaning blade 5a, and removes and collects the transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer with the cleaning blade 5a.
[0031]
The exposure apparatus 6 outputs a laser beam (exposure beam) modulated in response to a time-series electrical digital image signal of image information input from a personal computer (not shown) or the like from a laser output unit (not shown), By scanning and exposing the surface of the charged photosensitive drum 2 through the reflection mirror 12, an electrostatic latent image corresponding to the image information is formed.
[0032]
The fixing device 8 includes a heating roller 8a having a halogen heater (not shown) and a pressure roller 8b inside, and the transfer material P is nipped and conveyed at a fixing nip between the fixing roller 8a and the pressure roller 8b. Meanwhile, the toner image transferred to the surface of the transfer material P is heated and pressurized to be thermally fixed.
[0033]
Next, an image forming operation by the image forming apparatus 20 will be described.
[0034]
At the time of image formation, the photosensitive drum 1 is rotationally driven at a predetermined peripheral speed in the direction of arrow a (clockwise) by a driving means (not shown), and is uniformly applied by the charging roller 2 to which the charging bias is applied from the charging bias power source 9. It is charged to a predetermined negative potential. Then, when the scanning exposure L by the laser beam is given from the exposure device 6 to the charged photosensitive drum 1 through the reflection mirror 12, the potential on the photosensitive drum 1 is lowered at the portion exposed to the scanning exposure L. Thus, an electrostatic latent image corresponding to image information input from a personal computer (not shown) or the like is formed.
[0035]
Then, the toner t is formed on the electrostatic latent image formed on the photosensitive drum 1 by the developing sleeve 11 of the developing device 3 to which a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is applied in the developing unit. And is developed as a toner image by reversal development.
[0036]
On the other hand, the transfer material P such as paper in the cassette 13 is fed by the pickup roller 14 and conveyed to a registration roller pair (not shown) in synchronization with the formation of the toner image on the photosensitive drum 1. When the toner image on the photosensitive drum 1 reaches the transfer nip N between the photosensitive drum 1 and the transfer roller 4, the transfer material P is transferred through the pre-transfer guide 15 by a pair of registration rollers (not shown) at this timing. It is conveyed to the nip portion N.
[0037]
Then, the transfer roller 4 to which a transfer bias having a polarity (positive polarity) opposite to that of the toner t is applied from the transfer bias power source 17 is applied to the transfer material P conveyed to the transfer nip N between the photosensitive drum 1 and the transfer roller 4. The toner image on the photosensitive drum 1 is transferred by the generated electrostatic force (details of transfer bias application control to the transfer roller 4 in the present embodiment will be described later).
[0038]
Then, the transfer material P onto which the toner image has been transferred is conveyed to the fixing device 8 via the conveyance guide 7, and the toner image is heated and applied to the transfer material P at the fixing nip between the heating roller 8a and the pressure roller 8b. Press to fix heat. The transfer material P on which the toner image is fixed is discharged to the outside via the discharge roller pair 16 and the series of image forming operations is completed.
[0039]
Further, the transfer residual toner remaining on the photosensitive drum 1 after the transfer is removed and collected by the cleaning blade 5a of the cleaning device 5.
[0040]
Next, transfer bias application control to the transfer roller 4 in the present embodiment will be described.
[0041]
The transfer roller 4 used in the present embodiment is configured by providing a roller-like conductive rubber or conductive sponge elastic layer on a mandrel serving also as a power supply electrode made of SUS, Fe or the like. The resistance value of the transfer roller 4 is 1 × 10 when the temperature is 15 ° C. and the humidity is 10%. ⁹ Ωcm. In a high temperature and high humidity (temperature 30 ° C., humidity 50%) environment, 9 × 10 ⁷ Ωcm, 1 × under low temperature and low humidity (temperature 5 ° C, humidity 10%) ¹⁰ The resistance value in each environment varies about ± 20% due to variations in manufacturing of the transfer roller 4.
[0042]
Then, by using the image forming apparatus capable of double-sided image formation (double-sided printing) shown in FIG. (Gray-colored image) 100 images were continuously formed. In addition, the paper as a transfer material used in this embodiment is NEENAR Bond manufactured by Kimberly-Clark Corporation; basis weight 60 g / m. ² , Surface resistance: 3 × 10 ¹⁰ Ω / □ (when dry: 3.2 × 10 ¹¹ Ω / □).
[0043]
The transfer in the present embodiment is performed by the above-described ATVC. That is, a constant current is passed through the transfer roller 4 before the transfer operation under the control of the control device 18 to detect the reference voltage Vo generated at this time, and a coefficient is multiplied to the reference voltage Vo during transfer as shown in FIG. The corrected voltage (transfer bias) Vt is applied to the transfer roller 4, and the toner image on the photosensitive drum 1 is transferred to the transfer material (paper) P at the transfer nip N.
[0044]
In the present embodiment, a predetermined voltage is applied to the transfer roller 4 from the transfer bias power source 17 by the ATVC before the transfer operation, and the output current is detected by the control device 18 so that the desired output current is obtained. When the applied voltage is changed and it is determined that the detected voltage value is larger than the predetermined voltage value, the transfer bias power supply 17 is controlled by the control device 18 and transferred at the transfer nip N as will be described later. Application of the transfer bias Vt to the transfer roller 4 is turned off approximately 7.5 mm before the rear end P1 in the conveying direction of the material P, and a positive voltage higher than the transfer bias Vt from approximately 7.5 mm before the rear end P1. (This voltage is hereinafter referred to as transfer rear end bias Vh) is applied until the rear end P1 passes through the transfer nip N.
[0045]
In other words, in this embodiment, when it is determined that the resistance of the transfer roller 4 has increased in a low temperature environment and / or a low humidity environment, about 7.5 mm before the rear end P1 of the transfer material P to be passed. The transfer rear end bias Vh higher than the transfer bias Vt is applied.
[0046]
Then, when the transfer material P is not passing, such as between papers, a negative (minus) polarity voltage having the same polarity as the toner charging polarity is applied to the transfer roller 4, and the toner adhering to the transfer roller 4. Is electrostatically transferred to the photosensitive drum 1 side and removed and collected by the cleaning blade 5a.
[0047]
The transfer bias obtained by ATVC in accordance with the resistance of the transfer roller 4 (approximately 4.0 kV (90% in this embodiment) from the rear end P1 of the transfer material P to about 7.5 mm before the transfer. Transfer efficiency)) Vt is applied to the transfer roller 4, and the toner image (halftone image) on the photosensitive drum 1 is transferred.
[0048]
That is, as shown in FIGS. 3 to 5, when transferring a toner image (halftone image), first, the leading end of the transfer material P is guided by the pre-transfer guide 15 and the transfer nip portion between the photosensitive drum 1 and the transfer roller 4 is transferred. N enters the transfer nip portion N (FIG. 4), and finally, the rear end of the transfer material P exits the transfer nip portion N (FIG. 5). ) And conveyed to the fixing device 8 through the conveyance guide 7.
[0049]
Incidentally, as shown in FIG. 5, when the rear end of the transfer material P comes out from the transfer nip portion N after the toner image is transferred to the surface of the transfer material P, the transfer material P is abruptly separated from the photosensitive drum 1. As a result, a discharge phenomenon occurs between the photosensitive drum 1 and the transfer material P (sometimes). For this reason, an image defect may occur only on the rear end side of the transfer material P due to this discharge phenomenon.
[0050]
In this 100-sheet continuous image formation, it was examined at which position on the rear end side of the transfer material P the image defect occurred, and the result shown in FIG. 6 was obtained. As is apparent from the results shown in FIG. 6, image defects (occurrence of a polka dot pattern A as shown in FIG. 7) at the rear end of the transfer material P are mainly from 3.5 mm to 7 from the rear end of the transfer material P. 82% is concentrated in the range of 5mm. That is, in 82 out of 100 sheets, an image defect (polka dot pattern) occurred in a range from 3.5 mm to 7.5 mm before the rear end of the transfer material P. The occurrence of an image defect (polka dot pattern) at the rear end of the transfer material P is particularly noticeable in the case of a graphic image such as a halftone image.
[0051]
The occurrence of an image defect (polka dot pattern) in the range from 3.5 mm to 7.5 mm before the rear end of the transfer material P is mainly caused in the latter half of the image forming operation (the pressure roller 8b of the fixing device 8 is warmed). Status). This is because the thermal expansion of the pressure roller 8b occurs due to the heat generated by the halogen heater (not shown), the outer diameter of the pressure roller 8b increases, and the leading end side of the transfer material P is between the heating roller 8a and the pressure roller 8b. This is considered to be due to abrupt separation of the rear end side of the transfer material P and the photosensitive drum 1 at the transfer nip portion N by being nipped and conveyed.
[0052]
Therefore, in order to prevent the occurrence of an image defect (polka dot pattern) in the range from 3.5 mm to 7.5 mm before the rear end of the transfer material P, the transfer bias application in the rear end region of the transfer material P is controlled. There is a need.
[0053]
Therefore, transfer was performed while changing the transfer rear end bias Vh applied from 8.5 mm before the rear end of the transfer material P conveyed to the transfer nip N, and the output image was evaluated. The rear end transfer bias Vh is obtained by multiplying the reference voltage Vo in the above-described ATVC by a coefficient (0.4, 0.6... 2.0). FIG. 8 shows the evaluation. In order to obtain an effect from 7.5 mm before the rear end of the transfer material P due to the electric circuit, the transfer bias is started from 8.5 mm before the rear end of the transfer material P in consideration of the rise time of the transfer bias. The value of must be changed.
[0054]
As is clear from the evaluation results shown in FIG. 8, the image state improves as the transfer rear end bias Vh is increased, and the generation of a polka dot image as shown in FIG. 7 is prevented, but the rear end transfer bias Vh is 2. When it exceeded 0 Vo, a punch-through image occurred at the rear end of the transfer material P. This slip-through image means that the bias applied to the transfer roller 4 penetrates the transfer material P and reaches the photosensitive drum 1, and a larger current (overcurrent) than the current at the time of transfer flows, so that a plus memory is stored in the photosensitive drum 1. Occurrence is a phenomenon in which the portion of the stored potential is developed and black dots are generated.
[0055]
On the other hand, when the transfer rear end bias Vh is smaller than 0.4 Vo, an explosion image (and a polka dot image) is generated at the rear end of the transfer material P. As described above, when the toner on the photosensitive drum 1 is transferred onto the transfer material P by the transfer bias at the transfer nip portion N, the explosion image is not transferred well because the transfer bias is low. Generated by splattering above.
[0056]
In addition, it is necessary to reliably perform the transfer even under the most severe conditions as a transfer condition. That is, the transfer efficiency is improved under these conditions. Here, the transfer efficiency is the ratio of the amount of toner on the photosensitive drum 1 before transfer and the amount of toner transferred onto the transfer material P. That is, when η is the transfer efficiency (%), T0 is the toner amount on the photosensitive drum 1, and T1 is the toner amount on the transfer material P, η = 100 × T1 / T0.
[0057]
However, since it is actually difficult to measure the toner amount on the photosensitive drum 1 and the transfer material P, here, the measurement of the transfer efficiency η is “the toner amount and the optical density measurement value of the toner are in a proportional relationship. The transfer efficiency was determined from the measured optical density of the toner on the photosensitive drum 1 and the transfer material P.
[0058]
That is, assuming that the toner density (Dd) on the photosensitive drum 1 after being transferred to the transfer material P and the toner density (Td) on the transfer material P are given, the transfer efficiency η is calculated by η = 100 × Td / (Dd + Td). Desired. On the other hand, toner remaining without being transferred onto the transfer material P (that is, residual toner on the photosensitive drum 1) is collected in the developing container 10 by the developing sleeve 11.
[0059]
FIG. 9 is a diagram showing the results of examining the density, fixability, and image unevenness when the transfer efficiency is changed. As is apparent from the results of FIG. 9, when the transfer efficiency is 75% or less, the density is 1.2 or less, and it looks light and the fixing property is from ◯ to △ level due to insufficient contact / adhesion between the toners. In addition, the density unevenness is also in a bad level of Δ level or less. In FIG. 9, “◎” indicates the best, “◯” indicates the amount, “Δ” indicates that the limit is allowed, and “×” indicates that it is not possible.
[0060]
On the other hand, when the transfer efficiency was 75% or more, the density, fixability, and image unevenness were good. In other words, in order to obtain good levels of density, fixability, and image unevenness, the transfer efficiency is preferably 75% or more, and more preferably 80% or more. Therefore, when the environment (temperature) at the time of transfer is 5 ° C., 10 ° C., 15 ° C., 23 ° C., and 30 ° C., the transfer bias Vt that keeps the transfer efficiency at 75% or more was obtained. In order to determine the transfer bias Vt, the resistance value of the transfer roller 4 is set to 1.2 × 10 in a low temperature environment of 5 ° C. as the worst condition. ¹⁰ Use a material with a resistance of Ωcm, and 7.2 × 10 in a high temperature environment of 30 ° C. ¹⁰ What used to become Ωcm was used.
[0061]
As a result, the transfer efficiency becomes 75% or more when the transfer bias Vt = 5 kV at 5 ° C., Vt = 4 kV at 10 ° C., Vt = 3.5 kV at 15 ° C., Vt = 2 kV at 23 ° C., Vt = 2 kV at 30 ° C. The result was Vt = 1 kV.
Note that this result is only a value in the present embodiment, and differs depending on the individual transfer roller 4 and the like, and thus the value of the transfer bias Vt at which a transfer efficiency of 75% or more is obtained is different.
[0062]
Next, the evaluation of the environment (temperature, humidity) and output image when the transfer rear end bias Vh applied from 8.5 mm before the rear end of the transfer material P conveyed to the transfer nip N is changed. went. FIG. 10 shows the evaluation. This transfer rear end bias Vh is obtained by multiplying the reference voltage Vo in the above-described ATVC by a coefficient (0.4, 0.6... 2.0), and Vo in FIG. 10 is 1 kV. In FIG. 10, 最良 indicates the best, ◯ indicates the amount, Δ indicates the permissible limit, and × indicates the impossibility. When the temperature is a low temperature environment and / or the humidity is a low humidity environment, the resistance of the transfer roller 4 is increased.
[0063]
As is apparent from the evaluation results shown in FIG. 10, Vh is good (allowable range) in the range of 2.0 kV to 10 kV in the environment of 5 ° C., and Vh is 1 kV to 9 kV in 15 ° C. Vh is from 1.5 kV to 5.5 kV, and at 23 ° C., Vh is from 0.1 kV to 2.0 kV, and at 30 ° C., Vh is from 0.1 kV to 1.0 kV. Of these, Vh was 7 kV, 7.5 kC, 6 kV at 15 ° C, 3.5 kV at 15 ° C, 1.0 kV at 23 ° C, and 0.5 kV at 30 ° C.
[0064]
From the evaluation result of FIG. 10, the allowable range of the transfer rear end bias Vh applied to the rear end side of the transfer material P appears to be random in each environment, but in this image forming apparatus, the reference voltage ( When a voltage of 5 μA is applied to the transfer roller 4 and the photosensitive drum 1 as a reference, the ratio of Vo in each environment to the rear end transfer bias Vh with respect to Vo (see FIG. 11). When Vo is set to 1), when the temperature is 15 ° C. or lower, the transfer rear end bias Vh is higher than Δ level in FIG. The result is as shown.
[0065]
Further, in FIG. 10, in a 7.5 ° C. environment which is a low temperature environment, the image evaluation is NG (×) when the transfer rear end bias Vh exceeds 9 kV. This can be said even in a high temperature environment, but if the transfer bias (in this case, the transfer rear end bias Vh) is too high, the transfer bias penetrates the transfer material P and generates a positive memory by transfer on the photosensitive drum 1. May end up. That is, in this state, it can be seen that when a transfer rear end bias Vh that is twice (2Vo) or more than Vo is applied, a defect occurs in the image.
[0066]
On the other hand, when the temperature is higher than the high temperature environment of 23 ° C. or higher, the result that the transfer rear end bias Vh shows the Δ level or more in FIG. 10 in the region of 1 to 1/10 times Vo is obtained. It was.
[0067]
The present inventor refers to the evaluation results of FIGS. 10 and 11 and defines the transfer bias at the time of transfer in a low temperature environment of 5 ° C. and humidity of 10% as Vmax (this embodiment is 5 kV). The present inventors have found that correspondence can be taken between the environment (temperature, humidity) and the transfer rear end bias Vh.
[0068]
That is, when the temperature is 15 ° C. or lower, which is a low temperature environment, that is, when the resistance of the transfer roller 4 is increased, from the initial stage of the transfer material P passed through the transfer nip portion N to the front end of 7.5 mm. A normal transfer bias (bias determined based on the reference voltage Vo) Vt is applied, and the upper limit is 2 Vo, which is equal to or higher than Vo and 7.5 times before the rear end of the transfer material P, and twice Vo. By applying the transfer rear end bias Vh, a good image can be obtained.
[0069]
Further, as a condition, a transfer bias (transfer efficiency of 75% or more) Vt in an environment where the image forming apparatus is used is 70% or more of the above-described Vmax which is a transfer bias in an environment of a temperature of 5 ° C. and a humidity of 10%. For example, the transfer rear end bias Vh is applied to the rear end of the transfer material P.
[0070]
In addition, at 23 ° C. or higher, which is a high temperature environment in this case, a normal transfer bias (bias determined based on the reference voltage Vo) Vt is applied from the initial stage of the transfer material P to the front end of 7.5 mm. By applying a transfer rear end bias Vh with a lower limit of 1/10 Vo that is 1/10 times Vo from the rear end of the transfer material P 7.5 mm, it is possible to obtain a good image.
[0071]
As described above, in the low temperature environment, that is, when the resistance of the transfer roller 4 is increased, the transfer rear end bias Vh applied to the rear end side of the transfer material P is set higher than the transfer voltage Vt applied at that time. This will prevent the occurrence of image defects at the rear end of the transfer material P from being described below.
[0072]
By increasing the value of the applied bias on the rear end side of the transfer material P, the transfer material P after transfer and the photosensitive drum 1 are separated earlier than before. That is, in the present embodiment, by setting the value of the transfer rear end bias Vh larger than the value of the transfer bias Vt in the low temperature environment, in this embodiment, the potential of the photosensitive drum 1 at the transfer nip portion N is charged to a positive polarity. At the same time, the trailing edge potential of the transfer material P is also charged positively, so that a repulsive force is generated between the transfer material P and the photosensitive drum 1, and the transfer material P is separated quickly. is there.
[0073]
When the transfer material P is smoothly separated, the transfer material P is separated at a position close to the transfer nip portion N. Therefore, a static elimination needle (not shown) disposed on the downstream side of the transfer roller 4. Is further promoted, the potential of the transfer material P after passing through the transfer nip N is lowered, and unnecessary discharge (leak) into the photosensitive drum 1 and the apparatus can be prevented. Therefore, the occurrence of a polka dot image or the like at the rear end of the output image is prevented.
[0074]
Thus, in the present embodiment, a predetermined voltage is applied from the transfer bias power supply 17 to the transfer roller 4 by the ATVC before the transfer operation, and the output current is detected by the control device 18 so that the desired output current is obtained. Further, when the applied voltage is changed and it is determined that the detected voltage value is larger than the predetermined voltage value, the front end of the transfer material P passed through the transfer nip portion N is 7.5 mm before the rear end. The transfer bias power source 17 is controlled by the controller 18 so that the value of the transfer rear end bias Vh applied from 7.5 mm before the rear end of the transfer material P is higher than the value of the transfer bias Vt applied up to By doing so, the transfer material P is well separated from the photosensitive drum 1 after the transfer. Therefore, the abnormal discharge phenomenon at the time of separation of the transfer material P is suppressed, so that an image defect can be prevented from occurring at the rear end of the transfer material P and a good image can be obtained.
[0075]
In this embodiment, the transfer rear end bias Vh is applied from 7.5 mm before the rear end of the transfer material P. However, the transfer rear end bias Vh is applied from 15 mm before the rear end of the transfer material P. You may make it do.
[0076]
<Embodiment 2>
FIG. 12 is a schematic configuration diagram showing an image forming apparatus according to Embodiment 2 of the present invention. Members having the same functions as those of the image forming apparatus according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted.
[0077]
In the first embodiment, by ATVC before the transfer operation, a predetermined voltage is applied from the transfer bias power supply 17 to the transfer roller 4 and the output current is detected by the control device 18 and further applied so as to obtain a desired output current. When it is determined that the detected voltage value is larger than the predetermined voltage value by changing the voltage, the transfer is higher than the transfer bias Vt from about 7.5 mm before the rear end P1 of the transfer material P to be passed. Although the rear end bias Vh is applied, in this embodiment, the environment sensor 19 that detects the temperature and humidity installed in the image forming apparatus 20 detects that the inside of the image forming apparatus 20 has a low temperature and / or low humidity environment. In this case, the control device 18 controls the transfer bias power source 17 based on this detection information, and is higher than the transfer bias Vt from about 7.5 mm before the rear end P1 of the transfer material P to be passed. It was to apply a copy rear bias Vh. Other configurations and operations are the same as those in the first embodiment, and a description thereof will be omitted.
[0078]
As described above, in the present embodiment, when the environment sensor 19 detects a low temperature and / or low humidity environment, from the front end of the transfer material P to be passed through the transfer nip portion N to 7.5 mm before the rear end. The transfer bias power source 17 is controlled by the controller 18 so that the value of the transfer rear end bias Vh applied from 7.5 mm before the rear end of the transfer material P is higher than the value of the transfer bias Vt to be applied. As a result, the transfer material P is well separated from the photosensitive drum 1 after the transfer. Therefore, the abnormal discharge phenomenon at the time of separation of the transfer material P is suppressed, so that an image defect can be prevented from occurring at the rear end of the transfer material P and a good image can be obtained.
[0079]
<Embodiment 3>
This embodiment will also be described using the image forming apparatus shown in FIG. In the first embodiment, the image is formed on only one side of the transfer material. However, in the present embodiment, the image is formed on both surfaces (one surface and two surfaces) of the transfer material. Only the transfer bias application control to the transfer roller 4 in the present embodiment will be described.
[0080]
At the time of double-sided image formation, after the toner image is transferred and fixed on one side (first side) of the transfer material P as described in the first embodiment, the transfer material P which is reversed without being discharged is transferred again. In the same manner, the toner image is transferred to the other surface (second surface) of the transfer material P, fixed, and discharged to the outside. The transfer material (paper) P is dried by the heating of the fixing device 8 at the time of image formation on the first surface, and the resistance value of the transfer material P itself is increased. It tends to occur on the rear end side.
[0081]
Therefore, 100-sided double-sided image formation was carried out, and it was examined at which position on the rear end side of the second side of the transfer material P that the image defect occurred, and the result shown in FIG. 13 was obtained. . As is apparent from the results shown in FIG. 13, image defects at the rear end of the transfer material P (the occurrence of a polka dot pattern as shown in FIG. 7) are mainly from 3.5 mm to 7 mm from the rear end of the transfer material P. 90% is concentrated in the range of 5mm. That is, in 90 sheets out of 100 sheets, an image defect (polka dot pattern) occurred in a range from 3.5 mm to 7.5 mm before the rear end of the transfer material P.
[0082]
Accordingly, even at the time of image formation on the second surface, in the same manner as in the first embodiment, the transfer material P is closer to the rear end than the transfer bias Vt applied from the front end to the rear end of the transfer material P by 7.5 mm. The value of the transfer rear end bias Vh applied from 7.5 mm is increased.
[0083]
Also in the present embodiment, the same evaluation as that in FIGS. 10 and 11 of the first embodiment was performed. This evaluation is the same as in the first embodiment, and details are omitted.
[0084]
Also in the present embodiment, from the evaluation result performed in the same manner as in the first embodiment, from the initial stage of the transfer material P passed through the transfer nip portion N to the front end of 7.5 mm before the low temperature environment of 15 ° C. Applies a normal transfer bias (bias determined based on the reference voltage Vo) Vt, and from the front end of the transfer material P 7.5 mm before Vo, and 5/5 times Vo By applying the transfer rear end bias Vh with 2Vo as the upper limit, a good image can be obtained.
[0085]
In addition, at 23 ° C. or higher, which is a high temperature environment in this case, a normal transfer bias (bias determined based on the reference voltage Vo) Vt is applied from the initial stage of the transfer material P to the front end of 7.5 mm. By applying a transfer rear end bias Vh with a lower limit of 1/10 Vo that is 1/10 times Vo from the rear end of the transfer material P 7.5 mm, it is possible to obtain a good image.
[0086]
The value of the transfer rear end bias Vh in the low temperature environment in the present embodiment is larger than the value of the transfer rear end bias Vh in the low temperature environment in the first embodiment. This is because the transfer material P once passes through the fixing device 8 because it is the second surface, and thus the resistance value of the transfer material P itself is increased. Thus, it is necessary to apply an appropriate transfer bias.
[0087]
As described above, in the present embodiment, similarly to the first embodiment, the output voltage is detected by the control device 18 by applying a predetermined voltage from the transfer bias power supply 17 to the transfer roller 4 by the ATVC before the transfer operation. When the applied voltage is further changed so as to obtain a desired output current and it is determined that the detected voltage value is larger than the predetermined voltage value, the leading edge of the transfer material P passed through the transfer nip N The transfer bias Vt applied from 7.5 mm before the rear end of the transfer material P is higher than the transfer bias Vt applied from the end of the transfer material P to 7.5 mm before the rear end. By controlling the power supply 17 with the control device 18, the transfer material P is well separated from the photosensitive drum 1 after the transfer. Therefore, the abnormal discharge phenomenon at the time of separation of the transfer material P is suppressed, so that an image defect can be prevented from occurring at the rear end of the transfer material P and a good image can be obtained.
[0088]
【The invention's effect】
As described above, according to the first aspect of the present invention, in order to cause a desired current to flow from the voltage applying unit to the contact transfer member during transfer, a predetermined voltage is applied from the voltage applying unit to the contact transfer member before starting the transfer operation. When the output current is detected by the control means and the applied voltage is further changed so as to obtain the desired output current, and the detected voltage value is determined to be larger than the predetermined voltage value, the transfer is performed. The first voltage is applied from the voltage applying means to the contact transfer member until the leading end of the material in the transfer direction enters the transfer nip portion and the inner side of the transfer material in the predetermined direction from the rear end in the transfer direction passes through the transfer nip portion. When the rear end side passes through the transfer nip after a predetermined distance from the rear end in the conveyance direction of the transfer material passes through the transfer nip, a voltage application unit applies a second voltage greater than the first voltage. From contact transfer member By controlling the voltage applying means so as to apply to the transfer material is well separated from the image carrier after the transfer. Therefore, by suppressing an abnormal discharge phenomenon at the time of separation of the transfer material, it is possible to prevent occurrence of an image defect at the rear end of the transfer material and obtain a good image.
[0089]
According to the fifth aspect of the present invention, when the environment detection unit detects at least one of a low temperature environment and a low humidity environment, the control unit detects that the transfer material conveyance direction front end enters the transfer nip portion and the transfer material. The first voltage is applied from the voltage applying means to the contact transfer member until the inner side of the transfer direction rear end passes through the transfer nip portion, and the inner side of the transfer material at the predetermined distance from the rear end of the transfer direction in the transfer direction. When the rear end side passes through the transfer nip portion after passing through the voltage application means, the voltage application means is controlled so that the second voltage larger than the first voltage is applied from the voltage application means to the contact transfer member. As a result, the transfer material is well separated from the image carrier after the transfer. Therefore, by suppressing an abnormal discharge phenomenon at the time of separation of the transfer material, it is possible to prevent occurrence of an image defect at the rear end of the transfer material and obtain a good image.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a transfer voltage application state by transfer bias control according to Embodiment 1 of the present invention;
FIG. 3 is a diagram showing a state in which a transfer material tip has passed through a transfer nip according to Embodiment 1 of the present invention.
FIG. 4 is a diagram showing a state where a transfer material central portion passes through a transfer nip portion according to the first embodiment of the present invention.
FIG. 5 is a diagram illustrating a state where a rear end of a transfer material has passed through a transfer nip portion according to the first embodiment of the present invention.
FIG. 6 is a diagram showing the number of occurrences of image defects at the trailing edge of the transfer material in Embodiment 1 of the present invention.
FIG. 7 is a diagram showing a state in which an image defect occurs at the trailing edge of the transfer material in Embodiment 1 of the present invention.
FIG. 8 is a diagram showing an evaluation result of an image state when a transfer rear end bias is changed in the first embodiment of the present invention.
FIG. 9 is a diagram showing evaluation results of density, fixability, and image unevenness when the transfer efficiency is changed in Embodiment 1 of the present invention.
FIG. 10 is a diagram illustrating an evaluation result of an output image when the environment and the transfer rear end bias are changed in the first embodiment of the present invention.
11 is a diagram showing the relationship between the reference voltage Vo and the ratio of the transfer rear end bias Vh to the reference voltage Vo in each environment according to the first embodiment of the present invention. FIG.
FIG. 12 is a schematic configuration diagram illustrating an image forming apparatus according to a second embodiment of the present invention.
FIG. 13 is a diagram showing the number of occurrences of image defects at the trailing edge of a transfer material in Embodiment 2 of the present invention.
FIG. 14 is a schematic configuration diagram showing an image forming apparatus in a conventional example.
FIG. 15 is a view showing a transfer roller.
FIG. 16 is a diagram illustrating a relationship between a transfer voltage and an output image when the resistance value of the transfer roller is changed.
FIG. 17 is a diagram illustrating a transfer voltage application state by transfer bias control in a conventional example.
[Explanation of symbols]
1 Photosensitive drum (image carrier)
2 Charging roller
3 Development device
4 Transfer roller (contact transfer member)
5 Cleaning device
5a Cleaning blade
6 Exposure equipment
8 Fixing device
17 Transfer bias power supply (voltage application means)
18 Control device (control means)
19 Environmental sensor (environment detection means)
20 Image forming apparatus

Claims

An image carrier on which a toner image is formed and a transfer nip portion formed in contact with the image carrier, and formed on the image carrier in contact with a back surface of a transfer material conveyed to the transfer nip portion. A contact transfer member that transfers a toner image onto the surface of the transfer material; and a voltage application unit that applies a voltage whose voltage value can be arbitrarily changed and is controlled at a constant voltage to the contact transfer member. An image forming apparatus for transferring a toner image on the image carrier onto a transfer material conveyed to the transfer nip portion by applying a voltage having a polarity opposite to the charging polarity of the toner of the toner image from the means to the contact transfer member In
Control means for controlling voltage application from the voltage application means to the contact transfer member;
The control unit applies a predetermined voltage from the voltage application unit to the contact transfer member before starting a transfer operation so that a desired current flows from the voltage application unit to the contact transfer member during transfer. Is detected, and the applied voltage is further changed to obtain a desired output current. When it is determined that the detected voltage value is greater than a predetermined voltage value, the transfer material front end in the transfer direction is the transfer material. A first voltage is applied from the voltage applying means to the contact transfer member until a predetermined distance inside from the rear end in the conveyance direction of the transfer material passes through the transfer nip portion after entering the nip portion, When the rear end side passes through the transfer nip portion after a predetermined distance inside from the rear end in the transport direction passes through the transfer nip portion, a second voltage greater than the first voltage is applied to the voltage. means Controlling said voltage applying means so as to apply to et the contact transfer member,
An image forming apparatus.

The second voltage is applied from the inner side of the rear end in the transport direction of the transfer material transported to the transfer nip portion by 15 mm.
The image forming apparatus according to claim 1.

When the output voltage at the time of constant voltage control of the contact transfer member by the voltage applying means is changed, an output current corresponding to the changed output voltage is detected, and when the output current reaches a desired value When the output voltage is a reference voltage, the first voltage and the second voltage are determined based on the value of the reference voltage.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

When the toner image is transferred onto both surfaces of the transfer material, the value of the second voltage at the time of transferring the second surface is made higher than the value of the second voltage at the time of transferring the first surface.
The image forming apparatus according to claim 1, 2 or 3.

An image carrier on which a toner image is formed and a transfer nip portion formed in contact with the image carrier, and formed on the image carrier in contact with a back surface of a transfer material conveyed to the transfer nip portion. A contact transfer member that transfers a toner image onto the surface of the transfer material; and a voltage application unit that applies a voltage whose voltage value can be arbitrarily changed and is controlled at a constant voltage to the contact transfer member. An image forming apparatus for transferring a toner image on the image carrier onto a transfer material conveyed to the transfer nip portion by applying a voltage having a polarity opposite to the charging polarity of the toner of the toner image from the means to the contact transfer member In
An environment detection means for detecting at least one of a temperature environment and a humidity environment in or around the image forming apparatus;
Control means for controlling voltage application from the voltage application means to the contact transfer member in accordance with detection information input from the environment detection means,
When the environment detection unit detects at least one of a low temperature environment and a low humidity environment, the control unit determines whether the transfer material conveyance direction front end enters the transfer nip portion and the transfer material conveyance direction rear end is predetermined. The first voltage is applied from the voltage applying means to the contact transfer member until the distance inside passes through the transfer nip portion, and the inner side passes through the transfer nip portion by a predetermined distance from the rear end in the transport direction of the transfer material. Then, when the rear end side passes through the transfer nip portion, the voltage application unit is configured to apply a second voltage larger than the first voltage from the voltage application unit to the contact transfer member. To control the
An image forming apparatus.

The second voltage is applied from the inner side of the rear end in the transport direction of the transfer material transported to the transfer nip portion by 15 mm.
The image forming apparatus according to claim 5.

When the output voltage at the time of constant voltage control of the contact transfer member by the voltage applying means is changed, an output current corresponding to the changed output voltage is detected, and when the output current reaches a desired value When the output voltage is a reference voltage, the first voltage and the second voltage are determined based on the value of the reference voltage.
The image forming apparatus according to claim 5, wherein the image forming apparatus is an image forming apparatus.

When the toner image is transferred onto both surfaces of the transfer material, the value of the second voltage at the time of transferring the second surface is made higher than the value of the second voltage at the time of transferring the first surface.
The image forming apparatus according to claim 5, 6 or 7.

When the environment detection means detects a high temperature environment, the value of the second voltage is made smaller than the value of the first voltage.
The image forming apparatus according to claim 5.