JP4087492B2

JP4087492B2 - Image forming apparatus

Info

Publication number: JP4087492B2
Application number: JP07219998A
Authority: JP
Inventors: 雅司高橋; 稔吉田
Original assignee: Toshiba Corp; Toshiba TEC Corp
Current assignee: Toshiba Corp; Toshiba TEC Corp
Priority date: 1998-03-20
Filing date: 1998-03-20
Publication date: 2008-05-21
Anticipated expiration: 2018-03-20
Also published as: JPH11272098A

Description

【０００１】
【発明の属する技術分野】
本発明は電子写真装置等において、転写材を支持搬送する転写ベルトを介し、転写材裏面から転写電荷を付与して感光体ドラム上のトナー像を接触転写する画像形成装置に関する。
【０００２】
【従来の技術】
プリンタや複写機等画像形成装置において、例えば４連タンデムのカラー画像形成装置にあっては、平行に配置された複数の感光体ドラム上のトナー像に同期して、普通紙やＯＨＰシート等の転写材を各感光体ドラムの転写領域に順次搬送するため転写ベルトを用いている。
【０００３】
また、転写ベルトに搬送される転写材に、感光体ドラム上のトナー像を静電的或いは機械的に引き寄せ転写する転写装置として、特開平６−１１Ｏ３４３号公報には、半導電性の転写ベルト及びこの転写ベルトの背面に設けた転写ローラを用いて転写を行う技術が開示さており、従来のコ口ナ放電に替わり有害なオゾンの発生量が少なく転写効率の改善を実現出来ると言う利点を有する事から、導電性ローラや導電性ブラシ等転写電荷付与部を転写ベルト背面に当接し、転写材裏面から転写電荷を付与し転写を行う接触転写装置が多用されている。
【０００４】
このように転写ベルトを介し感光体と転写電荷付与部との間に形成される転写電界により感光体から転写材へのトナーの移動を静電的力により行う接触転写装置は、画像の印字率により転写電流が変動し、定電流制御電源での転写では印字率により転写効率が変動する事から、従来は転写電荷付与部への転写バイアスの印加を定電圧制御電源により行っていた。又、転写材を先端から確実に転写ベルトに吸着搬送させて転写材先端部での転写抜けを防止し良好な画像を得るために、転写電荷付与部への転写バイアスの印加を、転写材が転写領域に到達するより早めのタイミングで行っていた。更に転写材の材質に拘わらず良好な転写を得るために、転写材の抵抗値が大きくなるにつれ、転写電荷付与部に印加する転写バイアスを増大する等していた。
【０００５】
【発明が解決しようとする課題】
しかしながら、上記転写ベルトを用い定電圧電源により転写バイアスを印加して接触転写を行う装置において、ＯＨＰシートの様な抵抗値の大きい転写材にて良好な転写を得るためには、普通紙に比し転写バイアスを増大しなければならず、このように転写バイアスを増大した場合にも転写材先端における転写抜けを防止するために、転写材の転写領域への到達より早いタイミングで転写電荷付与部に高圧の転写バイアスを印加すると、転写材が無いところでは大電流が流れて、感光体が逆極性に帯電されてしまっていた。そしてこの逆極性の帯電部は、次の画像形成プロセスにおける帯電時に十分な帯電電荷を保持出来ずいわゆる転写メモリとして残ってしまい、これが画像に現れ表示不良を発生するという問題を生じていた。
【０００６】
更に転写材の材質に拘わらず、転写材先端が転写領域に到達するより早いタイミングで転写電荷付与部に転写バイアスを印加する際に、環境湿度が低いと、感光体に流れる電流が小さくても感光体の逆極性帯電を生じてしまい、次の画像形成プロセスに転写メモリを生じ、表示不良を発生するという問題も生じていた。
【０００７】
そこで本発明は上記課題を除去するもので、転写ベルトを用いる接触転写装置にて転写を行う際に、転写材先端での転写抜けの発生を防止し、且つ転写材の材質に拘わらず良好な転写を得ると共に、転写材の材質や環境条件に拘わらず転写メモリ発生による画質低下を来す事が無く、表示品位の向上を図る信頼性の高い画像形成装置を提供する事を目的とする。
【００１０】
【課題を解決するための手段】
又本発明は、上記課題を解決するための手段として、像担持体と、この像担持体に画像を形成する画像形成手段と、転写材を吸着し前記像担持体の転写領域に搬送する転写ベルトと、この転写ベルトを介し前記転写領域にて前記像担持体に対向し、前記転写材の抵抗値が１×１０¹¹Ωｃｍ未満の場合に前記転写材の前記転写領域への到達より早いタイミングで前記転写材裏面から転写電荷を付与する一方、前記転写材の抵抗値が１×１０¹¹Ωｃｍ以上の場合に前記転写材の前記転写領域への到達より遅れるタイミングで前記転写材裏面から前記転写電荷を付与する電荷付与手段とを設けるものである。
【００１１】
又本発明は、上記課題を解決するための手段として、複数の像担持体と、この複数の像担持体の各々に色の異なる画像を形成する複数の画像形成手段と、転写材を吸着し前記複数の像担持体の各転写領域に順次搬送する転写ベルトと、この転写ベルトを介し前記各転写領域にて前記各像担持体に対向し、前記転写材の抵抗値が１×１０¹¹Ωｃｍ未満の場合に前記転写材の前記各転写領域への到達より早いタイミングで前記転写材裏面から転写電荷を付与する一方、前記転写材の抵抗値が１×１０¹¹Ωｃｍ以上の場合に前記転写材の前記各転写領域への到達より遅れるタイミングで前記転写材裏面から前記転写電荷を付与する複数の電荷付与手段とを設けるものである。
【００１４】
又本発明は、上記課題を解決するための手段として、複数の像担持体と、この複数の像担持体の各々に色の異なる画像を形成する複数の画像形成手段と、転写材を吸着し前記複数の像担持体の各転写領域に順次搬送する転写ベルトと、この転写ベルトを介し前記各転写領域にて前記各像担持体に対向し前記転写材裏面から転写電荷を付与する複数の電荷付与手段と、この複数の電荷付与手段に転写バイアスを印加する複数の電源とを具備する画像形成装置において、前記転写ベルトの回転方向上流側における前記各電源は、前記転写材の前記各転写領域への到達より早いタイミングで前記電荷付与手段に前記転写バイアスを印加し、前記転写ベルトの回転方向下流側における前記各電源は、前記転写材の前記各転写領域への到達より遅れるタイミングで前記各電荷付与手段に前記転写バイアスを印加するものである。
【００１５】
又本発明は、上記課題を解決するための手段として、複数の像担持体と、この複数の像担持体の各々に色の異なる画像を形成する複数の画像形成手段と、転写材を吸着し前記複数の像担持体の各転写領域に順次搬送する転写ベルトと、この転写ベルトを介し前記各転写領域にて前記各像担持体に対向し前記転写材裏面から転写電荷を付与する複数の電荷付与手段と、前記転写ベルトの回転方向下流に位置するに従い、前記各電荷付与手段に順次より大きな転写バイアスを夫々印加する複数の電源とを具備する画像形成装置において、前記転写ベルトの回転方向上流側における前記各電源は、前記転写材の前記各転写領域への到達より早いタイミングで前記電荷付与手段に前記転写バイアスを印加し、前記転写ベルトの回転方向下流側における前記各電源は、前記転写材の前記各転写領域への到達より遅れるタイミングで前記各電荷付与手段に前記転写バイアスを印加するものである。
【００１６】
そして上記手段により、転写材の材質や転写領域の環境に拘わらず、転写材先端の転写抜け防止を図ると共に良好な転写を得られ、且つ転写メモリによる表示品位の低下を防止し画像形成装置の信頼性向上を図るものである。
【００１７】
【発明の実施の形態】
以下本発明を図１及び図２に示す第１の実施の形態を参照して説明する。図１は、画像形成装置であるレーザ露光式４連タンデムのデジタルカラー複写機の画像形成部１０を示す構成図であり、転写ベルト１１に沿って、像担持体である感光体ドラム１２ａ〜１２ｄにイエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）の各トナーを用いて画像形成を行う第１乃至第４の画像形成手段１３ａ〜１３ｄが並列配置されている。各感光体ドラム１２ａ〜１２ｄは、直径３０ｍｍの円筒状であり、周囲には感光体ドラム１２ａ〜１２ｄを一様に約−５００Ｖに帯電させる帯電ローラ１４ａ〜１４ｄが感光体ドラム１２ａ〜１２ｄの表面に接触して設けられ、その矢印ｒ方向の回転方向下流には、スキャナー（図示せず）で読み取った原稿画像情報をＣＣＤカメラ（図示せず）により色分解してなるイエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）の各色信号を感光体ドラム１２ａ〜１２ｄにレーザーで露光して静電潜像を形成する露光装置１６ａ〜１６ｄが設けられている。更に感光体ドラム１２ａ〜１２ｄの回転方向下流にはそれぞれイエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）の現像剤にて現像を行う現像装置１７ａ〜１７ｄが設けられている。又現像装置１７ａ〜１７ｄの下流には、転写ベルト１１を介し、電荷付与手段である導電ローラ１８ａ〜１８ｄ、クリ−ニング装置２０ａ〜２０ｄ及び、除電ランプ２２ａ〜２２ｄが設けられている。
【００１８】
搬送ベルト１１は、カーボンが均一に分散された厚さ１２０ｐｍのポリイミドからなり、電気抵抗１０¹²Ωcmの半導電性を示す無端状（シームレス）ベルトであり、転写ベルト１１を所定の速度で矢印ｓ方向に回動させる駆動ローラ１１ａ及び従動ローラ１１ｂに担持されている。駆動ローラ１１ａから従動ローラ１１ｂまでの距離は約３００ｍｍである。従動ローラ１１ｂは、テーパローラになっており、テーパローラの大径側に転写ベルト１１が寄るように成っている。又従動ローラ１１ｂの両端は、スプリング（図示せず）により押圧されており転写ベルト１１にテンシヨンを与えている。尚、１１ｃは転写ベルト１１上に付着したトナーや紙粉を除去するブレードクリーナーである。
【００１９】
画像形成部１０右手には転写材であり、抵抗測定器ハイレスター（三菱油化（株）製）にて２０℃、５０％ＲＨで測定した抵抗値が３×１０¹⁰Ωｃｍであるシート紙Ｐを収容する給紙カセット２３及びこの給紙カセット２３からシート紙Ｐを取り出すピックアップローラ２４が設けられ、給紙カセット装置２３上方には、転写材であり、抵抗測定器ハイレスター（三菱油化（株）製）にて２０℃、５０％ＲＨの環境下で測定した抵抗値が４×１０¹³ΩｃｍであるＯＨＰシートＰ１等を給紙する手差しトレイ２６及び手差しピックアップローラ２７が設けられている。又、各ピックアップローラ２４、２７から転写ベルト１１に達する間には、転写材先端が感光体ドラム１２ａ上に形成されるトナー像先端と一致するようタイミングをとって、転写材を転写ベルト１１側に送り出すレジストローラ２８が設けられている。画像形成装置１０左手には定着ローラ３０、排紙トレイ３１が設けられている。
【００２０】
次に転写ベルト１１を介し、電荷付与手段である導電ローラ１８ａ〜１８ｄにて感光体ドラム１２ａ〜１２ｄに転写電荷を付与する接触転写装置について詳述する。導電ローラ１８ａ〜１８ｄは、φ８ｍｍＳＵＳのシャフト３２ａ〜３２ｄに、カーボンを分散して導電性とした導電性発泡ウレタンからなる外径φ１６ｍｍのゴムローラ３３ａ〜３３ｄを成形している。シャフト３２ａ〜３２ｄとゴムローラ３３ａ〜３３ｄ表面間の電気抵抗は約１０⁶Ωである。そして導電ローラ１８ａ〜１８ｄは、転写ベルト１１を介し、感光体ドラム１２ａ〜１２ｄの中心の鉛直方向真下に位置するよう設置され、シャフト３２ａ〜３２ｄの両端は、導電ローラ１８ａ〜１８ｄを感光体ドラム１２ａ〜１２ｄ側に弾性的に付勢するばね３４により鉛直方向に１０００ｇｆで付勢されている。
【００２１】
シャフト３２ａ〜３２ｄには、カラー複写機の用紙選択モードにより操作されるスイッチ３６ａ〜３６ｄを介し、定電圧電源３７ａ〜３７ｄあるいは定電流電源３８ａ〜３８ｄが接統される様になっている。即ち、用紙選択モードが普通紙であるシート紙Ｐを選択した場合は定電圧電源３７ａ〜３７ｄ側にスイッチされ、ＯＨＰシートＰ１を選択した場合は定電流電源３８ａ〜３８ｄ側にスイッチされる。
【００２２】
定電圧電源３７ａ〜３７ｄは、導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間の抵抗値の変動により変動される電圧値を検知し、フィードバック制御をかけて常に所定の電圧を得る電源であり、転写ベルト１１の下流に向かうに従い順次、導電ローラ１８ａ〜１８ｄに＋１０００Ｖ、＋１２００Ｖ、＋１４００Ｖ、＋１６００Ｖの転写バイアスを印加する様に設定されている。
【００２３】
一方定電流電源３８ａ〜３８ｄは、導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間の抵抗値の変動により変動される電源からの流出電流値を検知し、電流値が一定となるようにフィードバック制御をかける電源であり、電流値２０μＡで、転写バイアスを印加する様に設定されている。
【００２４】
次に作用について述べる。コピー開始時、原稿（図示せず）をセットして操作パネル（図示せず）上よりコピー条件を入力する。この時用紙選択モードにてシート紙Ｐを選択した場合、スイッチ３６ａ〜３６ｄは定電圧電源３７ａ〜３７ｄにスイッチングされ、導電ローラ１８ａ〜１８ｄは定電圧電源３７ａ〜３７ｄにより夫々転写バイアスを印加される。
【００２５】
次いでコピーを開始すると、画像形成手段１３ａ〜１３ｄが駆動され、図示しない照射手段により原稿走査が行われると共に感光体ドラム１２ａ〜１２ｄが矢印ｒ方向に回転され順次画像形成工程が実施される。これにより、各感光体ドラム１２ａ〜１２ｄは、帯電ローラ１４ａ〜１４ｄにより約−５００Ｖに帯電される。次いで各感光体ドラム１２ａ〜１２ｄは、露光装置１６ａ〜１６ｄにより原稿画像を色分解したイエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）のレーザー光を照射され、各色毎の静電潜像を形成される。更に各感光体ドラム１２ａ〜１２ｄは、現像装置１７ａ〜１７ｄにより夫々イエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）の現像剤で現像され、各色のトナー像を形成された後、転写ベルト１１上のシート紙Ｐに順次トナー像を転写する事となる。
【００２６】
一方、給紙カセット装置２３にあっては、ピックアップローラ２４によりシート紙Ｐが取り出され、レジストローラ２８により第１の画像形成手段１３ａの感光体ドラム１２ａ上のトナー像に同期して転写領域である感光体ドラム１２ａと転写ベルト１１とのニップ部に送られる。この時導電ローラ１８ａには、シート紙Ｐがニップ部に到達するより前に定電圧電源３７ａにより＋１０００Ｖの転写バイアスが印加されている。これにより、シート紙Ｐは転写ベルト１１により矢印ｓ方向に搬送される間、転写ベルト１１及びシート紙Ｐ裏側から感光体ドラム１２ａに電荷が付与され、感光体ドラム１２ａ上のイエロー（Ｙ）のトナー像を、シート紙Ｐ先端部から転写抜けを生じる事無く転写される。次いで、同様にして、導電ローラ１８ｂにはシート紙Ｐがニップ部に到達するより前に定電圧電源３７ｂにより＋１２００Ｖの転写バイアスが印加され、第２の画像形成手段１３ｂのニップ部にて感光体ドラムｂ上のマゼンタ（Ｍ）のトナー像を、既にイエロー（Ｙ）のトナー像が形成されるシート紙Ｐに転写させる。更にシート紙Ｐ上には、＋１４００Ｖの転写バイアスが印加される導電ローラ１８ｃにより、第３の画像形成手段１３ｃのニップ部にてシアン（Ｃ）のトナー像を転写され、＋１６００Ｖの転写バイアスが印加される導電ローラ１８ｄにより、第４の画像形成手段１３ｄのニップ部にてブラック（ＢＫ）のトナー像を転写される。これによりシート紙Ｐは、イエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）のトナー像を多重転写される。この後シート紙Ｐは、転写ベルト１１より剥離され、定着ローラ３０に搬送されトナー像を加熱定着され画像を完成された後排紙トレイ３１上に排紙される。
【００２７】
一方、各画像形成手段１３ａ〜１３ｄにあっては、トナー像転写後、クリ−ニング装置２０ａ〜２０ｄ、除電ランプ２２ａ〜２２ｄを経て次のコピー可能状態とされ、転写ベルト１１はブレードクリーナー１１ｃによりクリーニングされる。この後必要枚数に達する迄上記コピー操作を繰返す事となる。
【００２８】
次にコピー条件入力時、用紙選択モードにてＯＨＰシートＰ１を選択した場合、スイッチ３６ａ〜３６ｄは定電流電源３８ａ〜３８ｄにスイッチングされ、導電ローラ１８ａ〜１８ｄは定電流電源３８ａ〜３８ｄにより夫々転写バイアスを印加される。
【００２９】
即ちコピー開始により、各画像形成手段１３ａ〜１３ｄの感光体ドラム１２ａ〜１２ｄ上に夫々イエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）のトナー像が形成される。そしてトナー像は、手差しトレイ２６より供給されたＯＨＰシートＰ１が転写ベルト１１により搬送される間に、定電流電源３８ａ〜３８ｄにより、夫々各画像形成手段において、２０μＡの転写電流となるように転写バイアスを印加される導電ローラ１８ａ〜１８ｄからの電荷付与によりＯＨＰシートＰ１上に順次転写される。又定電流電源３８ａ〜３８ｄによる導電ローラ１８ａ〜１８ｄへの転写バイアスの印加は、ＯＨＰシートＰ１がニップ部に到達するより早いタイミングで行われる。
【００３０】
この定電流電源３８ａ〜３８ｄによる転写バイアス値は、ＯＨＰシート紙Ｐ１の抵抗値が高い事から、通常のシート紙Ｐへの転写時に定電圧電源３７ａ〜３７ｄにより印加される転写バイアス値より高いものの、定電流電源３８ａ〜３８ｄからの流出電流値は、モニタされ、フィードバック制御により一定と成る様制御されており、ＯＨＰシート紙Ｐ１がニップ部に到達するより早いタイミングで電圧印加をしても、導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間に大電流が流れるのを防止している。
【００３１】
この様にして、先端部から転写抜けを生じる事無く順次イエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）のトナー像を多重転写されたＯＨＰシートＰ１は、転写ベルト１１より剥離され、定着ローラ３０に搬送されトナー像を加熱定着され画像を完成された後排紙トレイ３１上に排紙される。又各画像形成手段１３ａ〜１３ｄは、トナー像転写後、クリ−ニング装置２０ａ〜２０ｄ、除電ランプ２２ａ〜２２ｄを経て次のコピー可能状態とされ、転写ベルト１１はブレードクリーナー１１ｃによりクリーニングされる。
【００３２】
この様に構成すれば、普通紙からなるシート紙Ｐへの転写時には定電圧電源３７ａ〜３７ｄにより導電ローラ１８ａ〜１８ｄに転写バイアスを印加する事から、印字率に拘わらず良好な転写を得られ、且つシート紙先端の転写抜けを生じる事も無い。
【００３３】
一方、抵抗値が高く、高い転写バイアスを必要とするＯＨＰシートＰ１への転写時には定電流電源３８ａ〜３８ｄにより導電ローラ１８ａ〜１８ｄに転写バイアスを印加して、先端部の転写抜けを防止するために、ＯＨＰシートＰ１がニップ部に到達するより早くに転写バイアスを印加しても導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間に大電流が流れる事が無い。従ってＯＨＰシートＰ１への転写時に従来生じていた転写開始時の感光体ドラム１２ａ〜１２ｄの逆極性の帯電による転写メモリを防止出来、転写材の材質に拘わらず、画像抜けの無い良好な転写を得られ、転写メモリの無い表示品位の高いカラー画像を実現出来る。
【００３４】
次に本発明を図３乃至図５に示す第２の実施の形態を参照して説明する。本実施の形態は、第１の実施の形態において、転写材の抵抗値が高い場合であっても、導電ローラへの転写バイアスの印加を定電圧電源にて行う一方、高抵抗値の転写材の場合には、ニップ部に転写材が到達した後に導電ローラに転写バイアスを印加するようにしたものであり、第１の実施の形態と同一部分については同一符号を付しその説明を省略する。
【００３５】
本実施の形態の導電ローラ１８ａ〜１８ｄは、スイッチ４６ａ〜４６ｄを介し切り換えられる第１の定電圧電源４７ａ〜４７ｄあるいは第２の定電圧電源４８ａ〜４８ｄに接統される様になっている。即ち、用紙選択モードが普通紙であるシート紙Ｐを選択した場合は第１の定電圧電源４７ａ〜４７ｄ側にスイッチされ、ＯＨＰシートＰ１を選択した場合は第２の定電圧電源４８ａ〜４８ｄ側にスイッチされる。第１の定電圧電源４７ａ〜４７ｄは、夫々＋１０００Ｖ、＋１２００Ｖ、＋１４００Ｖ、＋１６００Ｖの転写バイアスを印加可能であり、第２の定電圧電源４８ａ〜４８ｄは、夫々＋１８００Ｖ、＋２６００Ｖ、＋３４００Ｖ、＋４２００Ｖの転写バイアスを印加可能である。
【００３６】
又導電ローラ１８ａ〜１８ｄへの第１の定電圧電源４７ａ〜４７ｄ或いは第２の定電圧電源による転写バイアスの印加のタイミングは、カラー複写機の用紙選択モードにより切り換えられ、普通紙モードを選択した場合、転写バイアスの印加タイミングを転写材がニップ部に到達する前に印加する一方、ＯＨＰシートＰ１モードを選択した場合は、転写バイアスの印加タイミングを転写材がニップ部に到達してから行われるように制御する。
【００３７】
即ち、導電ローラ１８ａ〜１８ｄへの転写バイアス印加のタイミングは、転写材のニップ部への到達より早いタイミングで転写バイアスを印加する場合は、レジストローラ２８の転写材給紙開始から、（ｔｌ−α）ｓｅｃ経過後に印加する一方、転写材がニップ部に到達した後に転写バイアスを印加する場合は、レジストローラ２８の転写材給紙開始から、（ｔｌ＋α）ｓｅｃ経過後に印加する。
【００３８】
ここで、ｔｌ＝Ｌ１／Ｖ（レジストローラ２８の動作開始による転写材の給紙からタイマーによりカウントを開始し、レジストローラ２８の給紙速度（＝転写ベルト１１の搬送速度）をＶｍｍ／ｓｅｃ、レジストローラ２８のニップから第１の画像形成手段１３ａのニップ部までの距離をＬ１ｍｍとする。）、αは転写材先端がニップ部に到達する迄の時間である。実際にはαは転写ベルト１１の搬送速度（プロセス速度）にもよるが、例えばプロセス速度が１２０ｍｍ／ｓｅｃの本実施の形態の場合、５００ｍｓｅｃとする。即ち、本実施の形態では、転写材の先端がニップ部に到達するより６ｍｍ早くあるいは遅いタイミングで導電ローラ１８ａ〜１８ｄに転写バイアスが印加されることになる。
【００３９】
次に作用について述べる先ず転写材が普通紙であるシート紙Ｐである場合は、用紙選択モードにて普通紙モードを選択することにより、スイッチ４６ａ〜４６ｄは第１の定電圧電源４７ａ〜４７ｄにスイッチングされ、導電ローラ１８ａ〜１８ｄは第１の定電圧電源４７ａ〜４７ｄにより夫々転写バイアスを印加され、更にシート紙Ｐがニップ部に到達するより早く導電ローラ１８ａ〜１８ｄに転写バイアスが印加される様設定される。
【００４０】
そして第１の実施の形態と同様にして、感光体ドラム１２ａ〜１２ｄ上に夫々形成されるイエロー（Ｙ）、マゼンタ（Ｍ）、シアン（Ｃ）、ブラック（ＢＫ）のトナー像が形成され、シート紙Ｐが転写ベルト１１により搬送される間、図４に示すようにシート紙Ｐがニップ部に到達するより６ｍｍ早いタイミングで第１の定電圧電源４７ａ〜４７ｄにより導電ローラ１８ａ〜１８ｄに、夫々＋１０００Ｖ、＋１２００Ｖ、＋１４００Ｖ、＋１６００Ｖの転写バイアスが印加され、シート紙Ｐ上にトナー像が多重転写される。
【００４１】
次に、転写材が高抵抗値のＯＨＰシートＰ１である場合は、用紙選択モードにてＯＨＰシートＰ１モードを選択することにより、スイッチ４６ａ〜４６ｄは第２の定電圧電源４８ａ〜４８ｄにスイッチングされ、導電ローラ１８ａ〜１８ｄは第２の定電圧電源４８ａ〜４８ｄにより夫々転写バイアスを印加される。但し転写バイアスの印加のタイミングは、ＯＨＰシートＰ１がニップ部に到達した後に導電ローラ１８ａ〜１８ｄに転写バイアスが印加される様設定される。
【００４２】
即ち感光体ドラム１２ａ〜１２ｄ上のトナー像は、ＯＨＰシートＰ１が転写ベルト１１により搬送される間、図５に示す様にＯＨＰシートＰ１先端がニップ部から６ｍｍ搬送された後に第２の定電圧電源４８ａ〜４８ｄにより導電ローラ１８ａ〜１８ｄに、夫々＋１８００Ｖ、＋２６００Ｖ、＋３４００Ｖ、＋４２００Ｖの転写バイアスが印加され、ＯＨＰシートＰ１上にトナー像が多重転写される。
【００４３】
この様に構成すれば、シート紙Ｐへの転写時には、シート紙Ｐ先端がニップ部に到達するより早いタイミングで導電ローラ１８ａ〜１８ｄに転写バイアスを印加する事から、ニップ部での微少放電によりシート紙Ｐ表面が帯電して電荷を保持するために、シート紙Ｐは先端から確実に転写ベルト１１上に静電吸着させることができ、転写抜けの無い良好な転写を得られる。一方、ＯＨＰシートＰ１への転写時にはＯＨＰシートＰ１先端がニップ部を通過した後に導電ローラ１８ａ〜１８ｄに転写バイアスを印加することから、導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間にＯＨＰシートＰ１が介在した状態で転写バイアスが印加されており、抵抗値が高く、高い転写バイアスが印加されるにも拘わらず、導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間に大電流が流れる事が無い。従ってＯＨＰシートＰ１への転写時に感光体ドラム１２ａ〜１２ｄが逆極性に帯電され転写メモリを生じる事が無く表示品位の高いカラー画像を得られる。
【００４４】
次に本発明を図６及び図７に示す第３の実施の形態を参照して説明する。本実施の形態は、第１の実施の形態において、導電ローラへの転写バイアスの印加を、転写時の環境（湿度）条件に応じてリミット電流値を切り換える定電圧電源にて行うものであり、第１の実施の形態と同一部分については同一符号を付しその説明を省略する。
【００４５】
環境（湿度）条件における転写メモリの発生電流値の限界を調べた所、表１に示すような結果が得られた。
【００４６】
【表１】

【００４７】
表１より、湿度が２０％ＲＨの場合、導電ローラｄ及び感光体ドラム間の電流が４０μＡ以上になると転写メモリが発生し、２０μＡ以下では転写ができないので、有効電流値は、２０〜４０μＡが望ましい。同様に、５０％ＲＨでは有効電流値２０〜６０μＡ、８５％ＲＨでは１００〜１４０μＡが望ましい。
【００４８】
このような原理を元に本実施の形態にあっては、第１の画像形成手段１３ａ近傍に湿度を検知する環境センサ４０を設け、この環境センサ４０による検知湿度に応じて、導電ローラ１８ａ〜１８ｄに夫々＋１０００Ｖ、＋１２００Ｖ、＋１４００Ｖ、＋１６００Ｖの転写バイアスを印加する定電圧電源４１ａ〜４１ｄのリミット電流値を３段階で切り替える様にする。即ち、環境センサ４０による検知湿度が４０％ＲＨ以下の場合には、定電圧電源４１ａ〜４１ｄのリミット電流値を４０μＡとする。湿度が４０％ＲＨ以上６５％ＲＨ未満の場合には、定電圧電源４１ａ〜４１ｄのリミット電流値を６０μＡとする。湿度が６５％ＲＨ以上の場合には、定電圧電源４１ａ〜４１ｄのリミット電流値を１４０μＡとする。
【００４９】
図７は、定電圧電源４１ａ〜４１ｄの電流値を制御する制御機構を示し、ＣＰＵ４３には環境センサ４０の検知結果が入力され、この結果から得られた湿度に応じて、ＣＰＵ４３はリミット電流記憶手段４２に記憶されるルックアップテーブル４６からリミット電流値を選定し、定電圧電源４１ａ〜４１ｄの電圧可変器４７のリミット電流値を設定する。そしてトナー像転写時、定電圧電源４１ａ〜４１ｄは、シート紙Ｐがニップ部に到達するより早いタイミングで定電圧電源４１ａ〜４１ｄにより導電ローラ１８ａ〜１８ｄに、夫々＋１０００Ｖ、＋１２００Ｖ、＋１４００Ｖ、＋１６００Ｖの転写バイアスを印加して、シート紙Ｐ上にトナー像を多重転写する。尚、定電圧電源４１ａ〜４１ｄの電流値は、定電圧電源４１ａ〜４１ｄの回路を流れる電流値をモニタしてリミット電流値を越えないようフィードバック制御している。
【００５０】
この様に構成すれば、シート紙Ｐを先端から、確実に転写ベルト１１に静電吸着して転写抜けを防止するために、シート紙Ｐ先端がニップ部に到達するより早いタイミングで導電ローラ１８ａ〜１８ｄに転写バイアスを印加する際に、湿度に拘わらず導電ローラ１８ａ〜１８ｄ及び感光体ドラム１２ａ〜１２ｄ間に大電流が流れる事が無く、感光体ドラム１２ａ〜１２ｄの逆極性の帯電による転写メモリを防止出来、画像抜けの無い良好な転写を得られしかも転写メモリを生じる事無く、表示品位の高いカラー画像を実現出来る。
【００５１】
次に本発明を図８乃至図１０に示す第４の実施の形態を参照して説明する。通常４連タンデムのカラー複写機の画像形成部１０では、前段の画像形成手段で、導電ローラからの付与電荷が転写材あるいはトナー上に蓄積され、この電荷が転写ベルトへの静電的吸着力となる一方で、次の画像形成手段での転写電界を弱めてしまう事から、前段の画像形成手段での転写バイアスに比し、後段の転写バイアスを高く設定している。このため、画像形成手段の後段に行くほど、転写メモリの問題が発生し易い。そこで本実施の形態は画像形成手段後段での転写メモリを防止するものである。
【００５２】
本実施の形態は、第１の実施の形態において、導電ローラへの転写バイアスの印加を定電圧電源にて行う一方、転写ベルトの搬送方向上流側の前段に配列される画像形成手段にあっては、ニップ部に転写材が到達するより早いタイミングで導電ローラに転写バイアスを印加し、転写ベルトの搬送方向下流側の後段に配列される画像形成手段にあっては、ニップ部に転写材が到達した後に導電ローラに転写バイアスを印加するようにしたものであり、第１の実施の形態と同一部分については同一符号を付しその説明を省略する。
【００５３】
本実施の形態の導電ローラ１８ａ〜１８ｄは、定電圧電源３７ａ〜３７ｄに接続され、夫々＋１０００Ｖ、＋１２００Ｖ、＋１４００Ｖ、＋１６００Ｖの転写バイアスを印加される様になっている。又導電ローラ１８ａ〜１８ｄへの定電圧電源３７ａ〜３７ｄによる転写バイアスの印加のタイミングは、転写ベルト１１の搬送方向上流側の第１及び第２の画像形成手段１３ａ、１３ｂにあっては、転写材が先端ニップ部に到達する前に印加する一方、転写ベルト１１の搬送方向下流側の第３及び第４の画像形成手段１３ｃ、１３ｄにあっては、転写材先端がニップ部を通過した後に印加する様設定されている。
【００５４】
即ち、転写材先端がニップ部に到達するより早いタイミングで転写バイアスを印加する場合は、レジストローラ２８の転写材給紙開始から、（ｔｌ−α）ｓｅｃ経過後に印加する一方、転写材先端がニップ部を通過した後に転写バイアスを印加する場合は、レジストローラ２８の転写材給紙開始から、（ｔｌ＋α）ｓｅｃ経過後に印加する。
【００５５】
ここで、ｔｌ＝Ｌ１／Ｖ（レジストローラ２８の動作開始による転写材の給紙からタイマーによりカウントを開始し、レジストローラ２８の給紙速度（＝転写ベルト１１の搬送速度）をＶｍｍ／ｓｅｃ、レジストローラ２８のニップから第１の画像形成手段１３ａのニップ部までの距離をＬ１ｍｍとする。）αは、転写バイアス印加後転写材先端がニップ部に到達する迄の時間或いは転写材先端がニップ部通過後、転写バイアスを印加するまでの時間である。実際にはαは転写ベルト１１の搬送速度（プロセス速度）にもよるが、プロセス速度を１２０ｍｍ／ｓｅｃとするの本実施の形態の場合、５００ｍｓｅｃとする。即ち、本実施の形態では、転写材の先端がニップ部に到達するより６ｍｍ早くあるいは遅いタイミングで導電ローラ１８ａ〜１８ｄに転写バイアスが印加されることになる。
【００５６】
そしてトナー像転写時、シート紙Ｐが転写ベルト１１により搬送される間、第１及び第２の画像形成手段１３ａ、１３ｂにあっては図９に示す様にシート紙Ｐ先端がニップ部に到達するより６ｍｍ早いタイミングで定電圧電源３７ａ〜３７ｄにより、導電ローラ１８ａ、１８ｂに夫々＋１０００Ｖ、＋１２００Ｖの転写バイアスが印加される。又第３及び第４の画像形成手段１３ｃ、１３ｄにあっては図１０に示す様にシート紙Ｐ先端がニップ部を６ｍｍ通過した後に定電圧電源３７ａ〜３７ｄにより、導電ローラ１８ａ、１８ｂに夫々＋１４００Ｖ、＋１６００Ｖの転写バイアスが印加され、シート紙Ｐ上にカラートナー像が多重転写される。
【００５７】
この様に構成すれば、４連タンデムのカラー複写機にて同一シート紙ｐ上にトナーを多重転写する際に、転写ベルト１１上流側にあっては、シート紙Ｐ先端がニップ部に到達するより早いタイミングで転写バイアスを印加する事により、シート紙Ｐ先端から確実に転写ベルト１１上に静電吸着させることができ、転写抜けの無い良好な転写を得られる一方、転写ベルト１１下流側にあっては、導電ローラ１８ａ〜１８ｄに印加される転写バイアスが高く成っても、シート紙Ｐ先端がニップ部を通過した後に転写バイアスを印加することから、大電流を生じる事無く、転写メモリを防止出来表示品位の高いカラー画像を得られる。
【００５８】
尚、本発明は上記実施の形態に限られるものでは無くその趣旨を変えない範囲での変更は可能であって、例えば電荷付与手段は、導電ローラに限らず導電性ブラシ、導電性ゴムブレード、ゴムシート等に転写バイアスを印加して像担持体に転写電荷を付与しても良く、導電性シートは、カーボンを分散した体積抵抗値が１０⁵〜１０⁸Ωcm程度の厚さ５ｍｍのシリコーンゴムシートや、ウレタンゴムやＥＰＤＭでも良い。導電ローラも、導電性を有し、像担持体のトナー像に電荷を付与出来る物であれば、その材質及び外径抵抗値等任意であり、印可される転写バイアス値も限定されない。
又転写ベルトも、体積抵抗値が１０⁹〜１０¹³Ωｃｍの半導電性を示す材料であれば良く、カーボンを分散したポリイミドの他に、ポリエチレンテフタレート、ボリカーボネイト、ポリテトラフルオロエチレン、ポリフッ化ビニリデン等にカーボン等の導電粒子を分散させたものでも良い。或いは導電粒子を用いず、組成調整によって電気抵抗を調整した高分子フィルムや、高分子フイルムにイオン導電性物質を混入したもの、比較的電気抵抗が低いシリゴンゴム、ウレタンゴム等のゴム材でも良い。
【００５９】
又第１の実施の形態において定電圧電源にて転写バイアスを印加する転写材の抵抗値或いは、第２の実施の形態において、ニップ部への到達より早くに転写バイアスを印加する転写材の抵抗値は１×１０¹¹Ωｃｍ未満である事が望ましい。一方、第１の実施の形態において定電流電源にて転写バイアスを印加する転写材或いは、第２の実施の形態において、ニップ部への到達後に転写バイアスを印加する転写材は、高抵抗値であれば、ＯＨＰシートで無くても良いが、その抵抗値は１×１０¹¹Ωｃｍ以上である事が望ましい。
【００６０】
第３の実施の形態において、湿度に対するリミット電流値の切り換え段階も限定されず、より適正に対応するよう多段階に切り換えれば、より良好な転写を得られる。更に第４の実施の形態においても、転写材のニップ部への到達より早くに転写バイアスを印加する前段の導電ローラと、転写材のニップ部への到達後に転写バイアスを印加する後段の導電ローラとの区切りは、第１と第２の画像形成手段の間或いは、第３と第４の画像形成手段の間等任意である。
【００６１】
【発明の効果】
以上説明したように本発明によれば、転写ベルトを用いる接触転写装置にて転写を行う際、転写材をその先端から転写ベルトに確実に静電吸着させて転写抜けを防止すると共に、この転写抜け防止策のために転写材の材質或いは環境等により従来生じていた電荷付与手段から像担持体への大電流の発生を防止出来、像担持体が逆極性に帯電するのを防止することにより、次の画像形成プロセスにおける転写メモリの発生を防止出来、転写抜けが無く且つ、転写メモリの無い、表示品位の高い良好な画像を得ることが出来る。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態の画像形成部を示す概略構成図である。
【図２】本発明の第１の実施の形態の導電ローラを示す概略斜視図である。
【図３】本発明の第２の実施の形態の画像形成装置を示す概略構成図である。
【図４】本発明の第２の実施の形態のシート紙Ｐへの転写バイアスの印加タイミングを示す概略説明図である。
【図５】本発明の第２の実施の形態のＯＨＰシートＰ１への転写バイアスの印加タイミングを示す概略説明図である。
【図６】本発明の第３の実施の形態の画像形成装置を示す概略構成図である。
【図７】本発明の第３の実施の形態の定電圧電源のリミット電流値を設置制御する制御機構を示す概略ブロック図である。
【図８】本発明の第４の実施の形態の画像形成装置を示す概略構成図である。
【図９】本発明の第４の実施の形態の第１及び第２の画像形成手段における転写バイアスの印加タイミングを示す概略説明図である。
【図１０】本発明の第４の実施の形態の第３及び第４の画像形成手段における転写バイアスの印加タイミングを示す概略説明図である。
【符号の説明】
１０…画像形成部
１１…転写ベルト
１２ａ〜１２ｄ…感光体ドラム
１３ａ〜１３ｄ…画像形成手段
１７ａ〜１７ｄ…現像装置
１８ａ〜１８ｄ…導電ローラ
３２ａ〜３２ｄ…シャフト
３３ａ〜３３ｄ…ゴムローラ
３６ａ〜３６ｄ…スイッチ
３７ａ〜３７ｄ…定電圧電源
３８ａ〜３８ｄ…定電流電源[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus that applies a transfer charge from the back surface of a transfer material through a transfer belt that supports and conveys the transfer material and contacts and transfers a toner image on a photosensitive drum in an electrophotographic apparatus or the like.
[0002]
[Prior art]
In an image forming apparatus such as a printer or a copying machine, for example, in a quadruple tandem color image forming apparatus, a plain paper, an OHP sheet or the like is synchronized with toner images on a plurality of photoconductor drums arranged in parallel. A transfer belt is used to sequentially convey the transfer material to the transfer area of each photosensitive drum.
[0003]
Japanese Patent Laid-Open No. 6-11O343 discloses a semiconductive transfer belt as a transfer device that electrostatically or mechanically transfers a toner image on a photosensitive drum to a transfer material conveyed to a transfer belt. In addition, a technique for performing transfer using a transfer roller provided on the back surface of the transfer belt is disclosed, and there is an advantage that improvement of transfer efficiency can be realized with less generation of harmful ozone in place of the conventional corner discharge. For this reason, a contact transfer device is often used in which a transfer charge applying portion such as a conductive roller or a conductive brush is brought into contact with the back surface of the transfer belt to transfer the transfer charge from the back surface of the transfer material.
[0004]
In this way, the contact transfer device that moves the toner from the photoconductor to the transfer material by an electrostatic force by the transfer electric field formed between the photoconductor and the transfer charge applying unit via the transfer belt is an image printing rate. The transfer current fluctuates due to the above, and the transfer efficiency varies depending on the printing rate in the transfer with the constant current control power supply. Conventionally, the transfer bias is applied to the transfer charge applying portion by the constant voltage control power supply. In addition, in order to reliably transfer the transfer material from the leading end to the transfer belt to prevent transfer omission at the leading end portion of the transfer material and to obtain a good image, the transfer material is applied to the transfer charge applying portion. It was done at an earlier timing than reaching the transfer area. Further, in order to obtain good transfer regardless of the material of the transfer material, the transfer bias applied to the transfer charge applying portion is increased as the resistance value of the transfer material increases.
[0005]
[Problems to be solved by the invention]
However, in an apparatus for performing contact transfer by applying a transfer bias from a constant voltage power source using the above-described transfer belt, in order to obtain good transfer with a transfer material having a large resistance value such as an OHP sheet, compared with plain paper. The transfer bias must be increased, and even when the transfer bias is increased in this way, in order to prevent transfer omission at the transfer material front end, the transfer charge applying unit is earlier than the transfer material reaches the transfer area. When a high-voltage transfer bias was applied to the photoconductor, a large current flowed in the absence of the transfer material, and the photoconductor was charged with a reverse polarity. This reverse polarity charging portion cannot retain a sufficiently charged charge during charging in the next image forming process, and remains as a so-called transfer memory, which causes a problem in that it appears in an image and causes a display defect.
[0006]
Furthermore, regardless of the material of the transfer material, when the transfer bias is applied to the transfer charge applying portion at an earlier timing than when the leading edge of the transfer material reaches the transfer region, if the environmental humidity is low, even if the current flowing through the photoconductor is small The reverse polarity charging of the photoconductor occurs, and a transfer memory is generated in the next image forming process, resulting in a display defect.
[0007]
Accordingly, the present invention eliminates the above-described problems, and prevents transfer from being lost at the front end of the transfer material when performing transfer with a contact transfer device using a transfer belt, and is excellent regardless of the material of the transfer material. An object of the present invention is to provide a highly reliable image forming apparatus capable of obtaining a transfer and improving display quality without causing image quality deterioration due to generation of a transfer memory regardless of the material of the transfer material and environmental conditions.
[0010]
[Means for Solving the Problems]
Further, according to the present invention, as means for solving the above-mentioned problems, an image carrier, an image forming means for forming an image on the image carrier, and a transfer for adsorbing a transfer material and conveying it to a transfer area of the image carrier A belt and the transfer region are opposed to the image carrier through the transfer belt, and the resistance value of the transfer material is 1 × 10¹¹When the transfer material is less than Ωcm, transfer charge is applied from the back surface of the transfer material at a timing earlier than the transfer material reaches the transfer region, while the resistance value of the transfer material is 1 × 10.¹¹Charge providing means for applying the transfer charge from the back surface of the transfer material at a timing delayed from the arrival of the transfer material to the transfer region when Ωcm or more is provided.
[0011]
The present invention also provides a plurality of image carriers, a plurality of image forming units for forming images of different colors on each of the plurality of image carriers, and a transfer material. A transfer belt that sequentially conveys to each transfer region of the plurality of image carriers, and each transfer region via the transfer belt faces each image carrier, and the resistance value of the transfer material is 1 × 10.¹¹When the transfer material is less than Ωcm, a transfer charge is applied from the back surface of the transfer material at a timing earlier than the arrival of the transfer material to each transfer region, while the resistance value of the transfer material is 1 × 10.¹¹And a plurality of charge applying means for applying the transfer charge from the back surface of the transfer material at a timing delayed from the arrival of the transfer material to the transfer regions in the case of Ωcm or more.
[0014]
The present invention also provides a plurality of image carriers, a plurality of image forming units for forming images of different colors on each of the plurality of image carriers, and a transfer material. A transfer belt that sequentially conveys to each transfer region of the plurality of image carriers, and a plurality of charges that apply transfer charges from the back surface of the transfer material facing the image carriers in the transfer regions via the transfer belt. In the image forming apparatus including a applying unit and a plurality of power sources for applying a transfer bias to the plurality of charge applying units, the power sources on the upstream side in the rotation direction of the transfer belt are the transfer regions of the transfer material. The transfer bias is applied to the charge applying unit at a timing earlier than the arrival of the transfer belt, and the power supplies on the downstream side in the rotation direction of the transfer belt are delayed from the arrival of the transfer material to the transfer regions. Wherein at timing it is to apply the transfer bias to the charging unit.
[0015]
The present invention also provides a plurality of image carriers, a plurality of image forming units for forming images of different colors on each of the plurality of image carriers, and a transfer material. A transfer belt that sequentially conveys to each transfer region of the plurality of image carriers, and a plurality of charges that apply transfer charges from the back surface of the transfer material facing the image carriers in the transfer regions via the transfer belt. An image forming apparatus comprising: an applying unit; and a plurality of power supplies that sequentially apply a larger transfer bias to each of the charge applying units as it is positioned downstream in the rotation direction of the transfer belt. The power sources on the side apply the transfer bias to the charge applying means at a timing earlier than the transfer material reaches the transfer regions, and on the downstream side in the rotation direction of the transfer belt. Serial each power supply is used to apply the transfer bias to the respective charging unit at the timing delayed from reaching the respective transfer area of the transfer material.
[0016]
By the above means, regardless of the material of the transfer material and the environment of the transfer area, the transfer material can be prevented from being transferred at the leading edge and good transfer can be obtained, and the display quality can be prevented from being deteriorated by the transfer memory. It is intended to improve reliability.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described below with reference to the first embodiment shown in FIGS. FIG. 1 is a configuration diagram showing an image forming unit 10 of a laser exposure type quadruple tandem digital color copying machine as an image forming apparatus, and along photosensitive belts 12 a to 12 d as image carriers along a transfer belt 11. In addition, first to fourth image forming units 13a to 13d that perform image formation using toners of yellow (Y), magenta (M), cyan (C), and black (BK) are arranged in parallel. Each of the photoconductive drums 12a to 12d has a cylindrical shape with a diameter of 30 mm, and charging rollers 14a to 14d that uniformly charge the photoconductive drums 12a to 12d to about -500 V are provided around the surface of the photoconductive drums 12a to 12d. In the rotational direction downstream of the arrow r direction, yellow (Y), magenta obtained by color separation of document image information read by a scanner (not shown) by a CCD camera (not shown) is provided. Exposure devices 16a to 16d are provided for forming electrostatic latent images by exposing the photosensitive drums 12a to 12d with laser light on the respective color signals of (M), cyan (C), and black (BK). Further, developing devices 17a to 17d that perform development with yellow (Y), magenta (M), cyan (C), and black (BK) developers are respectively provided downstream in the rotation direction of the photosensitive drums 12a to 12d. Yes. In addition, downstream of the developing devices 17a to 17d, conductive rollers 18a to 18d, cleaning devices 20a to 20d, and discharging lamps 22a to 22d, which are charge applying means, are provided via the transfer belt 11.
[0018]
The conveyor belt 11 is made of polyimide having a thickness of 120 pm in which carbon is uniformly dispersed, and has an electric resistance of 10¹²It is an endless (seamless) belt that exhibits semiconductivity of Ωcm, and is carried by a driving roller 11a and a driven roller 11b that rotate the transfer belt 11 in a direction indicated by an arrow s at a predetermined speed. The distance from the driving roller 11a to the driven roller 11b is about 300 mm. Driven lowLa 11b is a taper roller, and the transfer belt 11 approaches the large-diameter side of the taper roller. Further, both ends of the driven roller 11b are pressed by springs (not shown) to give tension to the transfer belt 11. A blade cleaner 11c removes toner and paper dust adhering to the transfer belt 11.
[0019]
The right side of the image forming unit 10 is a transfer material, and the resistance value measured at 20 ° C. and 50% RH with a resistance measuring instrument Hirestar (Mitsubishi Yuka Co., Ltd.) is 3 × 10.^TenA paper feed cassette 23 for storing the sheet paper P of Ωcm and a pickup roller 24 for taking out the sheet paper P from the paper feed cassette 23 are provided. Above the paper feed cassette device 23 is a transfer material, and a resistance measuring device high The resistance value measured in an environment of 20 ° C. and 50% RH with Leicester (Mitsubishi Yuka Co., Ltd.) is 4 × 10¹³A manual feed tray 26 and a manual pickup roller 27 for feeding an OHP sheet P1 having Ωcm or the like are provided. In addition, while reaching the transfer belt 11 from the

pickup rollers

24 and 27, the transfer material is moved to the transfer belt 11 side at a timing so that the front end of the transfer material coincides with the front end of the toner image formed on the photosensitive drum 12a. A registration roller 28 is provided. A fixing roller 30 and a paper discharge tray 31 are provided on the left hand side of the image forming apparatus 10.
[0020]
Next, a contact transfer device that applies transfer charges to the photosensitive drums 12a to 12d via the transfer belt 11 by the conductive rollers 18a to 18d as charge applying means will be described in detail. The conductive rollers 18a to 18d are formed by forming rubber rollers 33a to 33d having an outer diameter of φ16 mm made of conductive foamed urethane, which is made conductive by dispersing carbon on shafts 32a to 32d of φ8 mmSUS. The electrical resistance between the shafts 32a to 32d and the rubber rollers 33a to 33d is about 10⁶Ω. The conductive rollers 18a to 18d are installed so as to be positioned directly below the center of the photosensitive drums 12a to 12d via the transfer belt 11, and both ends of the shafts 32a to 32d are connected to the conductive rollers 18a to 18d. The spring 34 is elastically biased toward the 12a to 12d side and is biased at 1000 gf in the vertical direction.
[0021]
Constant voltage power supplies 37a to 37d or constant current power supplies 38a to 38d are connected to the shafts 32a to 32d via switches 36a to 36d operated in the paper selection mode of the color copying machine. In other words, when the sheet paper P, which is plain paper, is selected as the paper selection mode, the constant voltage power supplies 37a to 37d are switched, and when the OHP sheet P1 is selected, the constant current power supplies 38a to 38d are switched.
[0022]
The constant voltage power supplies 37a to 37d are power supplies that detect a voltage value that is fluctuated due to a change in the resistance value between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d and always obtain a predetermined voltage by performing feedback control. The transfer biases of +1000 V, +1200 V, +1400 V, and +1600 V are sequentially applied to the conductive rollers 18 a to 18 d toward the downstream side of the transfer belt 11.
[0023]
On the other hand, the constant current power supplies 38a to 38d detect outflow current values from the power supply that are fluctuated by fluctuations in the resistance values between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d, and feed back the current values so that they are constant. It is a power supply for controlling, and is set to apply a transfer bias at a current value of 20 μA.
[0024]
Next, the operation will be described. At the start of copying, a document (not shown) is set and copy conditions are input from an operation panel (not shown). At this time, when the sheet P is selected in the paper selection mode, the switches 36a to 36d are switched to the constant voltage power sources 37a to 37d, and the conductive rollers 18a to 18d are respectively applied with a transfer bias by the constant voltage power sources 37a to 37d. .
[0025]
Next, when copying is started, the image forming units 13a to 13d are driven, the original is scanned by an irradiating unit (not shown), and the photosensitive drums 12a to 12d are rotated in the direction of the arrow r to sequentially perform image forming steps. As a result, the photosensitive drums 12a to 12d are charged to about -500 V by the charging rollers 14a to 14d. Next, the photosensitive drums 12a to 12d are irradiated with yellow (Y), magenta (M), cyan (C), and black (BK) laser beams obtained by color-separating the original image by the exposure devices 16a to 16d. An electrostatic latent image is formed. Further, each of the photosensitive drums 12a to 12d is developed with a developer of yellow (Y), magenta (M), cyan (C), and black (BK) by the developing devices 17a to 17d to form toner images of respective colors. Thereafter, the toner images are sequentially transferred onto the sheet paper P on the transfer belt 11.
[0026]
On the other hand, in the sheet feeding cassette device 23, the sheet paper P is taken out by the pickup roller 24, and in the transfer region in synchronization with the toner image on the photosensitive drum 12a of the first image forming means 13a by the registration roller 28. It is sent to a nip portion between a certain photosensitive drum 12 a and the transfer belt 11. At this time, a transfer bias of +1000 V is applied to the conductive roller 18a by the constant voltage power source 37a before the sheet paper P reaches the nip portion. As a result, while the sheet paper P is conveyed in the direction of the arrow s by the transfer belt 11, a charge is applied to the photosensitive drum 12a from the back side of the transfer belt 11 and the sheet paper P, and yellow (Y) on the photosensitive drum 12a is applied. The toner image is transferred from the front end portion of the sheet paper P without causing transfer omission. Next, similarly, a transfer bias of +1200 V is applied to the conductive roller 18b by the constant voltage power source 37b before the sheet paper P reaches the nip portion, and the photosensitive member is detected at the nip portion of the second image forming means 13b. The magenta (M) toner image on the drum b is transferred to the sheet paper P on which the yellow (Y) toner image has already been formed. Further, a cyan (C) toner image is transferred onto the sheet paper P by the conductive roller 18c to which a transfer bias of +1400 V is applied, at the nip portion of the third image forming unit 13c, and a transfer bias of +1600 V is applied. The black (BK) toner image is transferred at the nip portion of the fourth image forming unit 13d by the conductive roller 18d. Thus, the yellow (Y), magenta (M), cyan (C), and black (BK) toner images are transferred onto the sheet paper P in a multiple manner. Thereafter, the sheet paper P is peeled off from the transfer belt 11, conveyed to the fixing roller 30, the toner image is heated and fixed, and the image is completed, and then discharged onto the paper discharge tray 31.
[0027]
On the other hand, in each of the image forming units 13a to 13d, after the toner image is transferred, the next copying is possible through the cleaning devices 20a to 20d and the neutralizing lamps 22a to 22d. The transfer belt 11 is moved by the blade cleaner 11c. To be cleaned. Thereafter, the above copying operation is repeated until the required number is reached.
[0028]
Next, when the copy condition is input and the OHP sheet P1 is selected in the paper selection mode, the switches 36a to 36d are switched to the constant current power supplies 38a to 38d, and the conductive rollers 18a to 18d are transferred by the constant current power supplies 38a to 38d, respectively. A bias is applied.
[0029]
  That is, by starting copying, yellow (Y), magenta (M), cyan (C), and black (BK) toner images are formed on the photosensitive drums 12a to 12d of the image forming units 13a to 13d, respectively. The toner images are respectively transferred from the constant current power supplies 38a to 38d while the OHP sheet P1 supplied from the manual feed tray 26 is conveyed by the transfer belt 11.In each image forming means,The images are sequentially transferred onto the OHP sheet P1 by applying electric charges from the conductive rollers 18a to 18d to which a transfer bias is applied so as to obtain a transfer current of 20 μA. The application of the transfer bias to the conductive rollers 18a to 18d by the constant current power supplies 38a to 38d is performed at an earlier timing than when the OHP sheet P1 reaches the nip portion.
[0030]
The transfer bias value by the constant current power supplies 38a to 38d is higher than the transfer bias value applied by the constant voltage power supplies 37a to 37d at the time of transfer to the normal sheet paper P because the resistance value of the OHP sheet paper P1 is high. The outflow current values from the constant current power supplies 38a to 38d are monitored and controlled so as to be constant by feedback control, and even when voltage is applied at an earlier timing than when the OHP sheet P1 reaches the nip portion, A large current is prevented from flowing between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d.
[0031]
In this manner, the OHP sheet P1 onto which the toner images of yellow (Y), magenta (M), cyan (C), and black (BK) are sequentially transferred without causing transfer omission from the leading end portion is transferred to the transfer belt 11. The toner image is peeled off, conveyed to the fixing roller 30, the toner image is heated and fixed, and the image is completed and then discharged onto the discharge tray 31. Each of the image forming units 13a to 13d is brought into the next copyable state after the toner image is transferred, and then passed through the cleaning devices 20a to 20d and the charge eliminating lamps 22a to 22d, and the transfer belt 11 is cleaned by the blade cleaner 11c.
[0032]
With this configuration, a transfer bias is applied to the conductive rollers 18a to 18d by the constant voltage power sources 37a to 37d when transferring to the sheet paper P made of plain paper, so that good transfer can be obtained regardless of the printing rate. In addition, there is no occurrence of transfer omission at the front end of the sheet paper.
[0033]
On the other hand, at the time of transfer to the OHP sheet P1, which has a high resistance value and requires a high transfer bias, a transfer bias is applied to the conductive rollers 18a to 18d by the constant current power supplies 38a to 38d so as to prevent a transfer omission at the tip. In addition, even if the transfer bias is applied earlier than the OHP sheet P1 reaches the nip portion, a large current does not flow between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d. Accordingly, it is possible to prevent transfer memory due to reverse polarity charging of the photosensitive drums 12a to 12d at the start of transfer, which has been conventionally generated when transferring to the OHP sheet P1, and to perform good transfer without image omission regardless of the material of the transfer material. As a result, a high-quality color image without a transfer memory can be realized.
[0034]
Next, the present invention will be described with reference to a second embodiment shown in FIGS. In the present embodiment, even if the transfer material has a high resistance value in the first embodiment, a transfer bias is applied to the conductive roller by a constant voltage power source, while a transfer material having a high resistance value is used. In this case, the transfer bias is applied to the conductive roller after the transfer material reaches the nip portion. The same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. .
[0035]
  The conductive rollers 18a to 18d of the present embodiment are connected to the first constant voltage power sources 47a to 47d or the second constant voltage power sources 48a to 48d that are switched via the switches 46a to 46d. That is, when the sheet selection mode is selected as the plain paper PFirst1 is switched to the constant voltage power supply 47a to 47d side, and when the OHP sheet P1 is selected, it is switched to the second constant voltage power supply 48a to 48d side. The first constant voltage power supplies 47a to 47d can apply transfer biases of +1000 V, +1200 V, +1400 V, and +1600 V, respectively, and the second constant voltage power supplies 48 a to 48 d can transfer transfer bias of +1800 V, +2600 V, +3400 V, and +4200 V, respectively. Can be applied.
[0036]
The timing of applying the transfer bias to the conductive rollers 18a to 18d by the first constant voltage power supply 47a to 47d or the second constant voltage power supply is switched by the paper selection mode of the color copying machine, and the plain paper mode is selected. In this case, the transfer bias is applied before the transfer material reaches the nip portion. When the OHP sheet P1 mode is selected, the transfer bias is applied after the transfer material reaches the nip portion. To control.
[0037]
That is, when the transfer bias is applied to the conductive rollers 18a to 18d at an earlier timing than the transfer material reaches the nip portion, the transfer roller starts to feed the transfer material from the registration roller 28 (tl- On the other hand, when the transfer bias is applied after the transfer material reaches the nip portion, the transfer material is applied after (tl + α) sec from the start of the transfer material feeding of the registration roller 28.
[0038]
Here, tl = L1 / V (counting is started by a timer from feeding of the transfer material by the start of operation of the registration roller 28, and the feeding speed of the registration roller 28 (= conveyance speed of the transfer belt 11) is Vmm / sec, The distance from the nip of the registration roller 28 to the nip portion of the first image forming means 13a is L1 mm.), Α is the time until the leading edge of the transfer material reaches the nip portion. Actually, α depends on the conveyance speed (process speed) of the transfer belt 11, but is set to 500 msec in the case of the present embodiment where the process speed is 120 mm / sec, for example. That is, in the present embodiment, the transfer bias is applied to the conductive rollers 18a to 18d at a timing 6 mm earlier or later than the leading edge of the transfer material reaches the nip portion.
[0039]
Next, when the transfer material is sheet paper P, which is plain paper, the switches 46a to 46d are switched to the first constant voltage power supplies 47a to 47d by selecting the plain paper mode in the paper selection mode. The conductive rollers 18a to 18d are applied with the transfer bias by the first constant voltage power supplies 47a to 47d, respectively, and the transfer bias is applied to the conductive rollers 18a to 18d sooner than the sheet P reaches the nip portion. Is set.
[0040]
  Similarly to the first embodiment, toner images of yellow (Y), magenta (M), cyan (C), and black (BK) are formed on the photosensitive drums 12a to 12d, respectively. While the sheet paper P is conveyed by the transfer belt 11, as shown in FIG. 4, at a timing 6 mm earlier than the sheet paper P reaches the nip portion.FirstOne constant voltage power supply 47a to 47d applies + 1000V, + 1200V, + 1400V, and + 1600V transfer bias to the conductive rollers 18a to 18d, respectively, and the toner images are transferred onto the sheet paper P in a multiple manner.
[0041]
Next, when the transfer material is an OHP sheet P1 having a high resistance value, the switches 46a to 46d are switched to the second constant voltage power supplies 48a to 48d by selecting the OHP sheet P1 mode in the paper selection mode. The transfer rollers 18a to 18d are applied with a transfer bias by the second constant voltage power supplies 48a to 48d, respectively. However, the application timing of the transfer bias is set so that the transfer bias is applied to the conductive rollers 18a to 18d after the OHP sheet P1 reaches the nip portion.
[0042]
That is, the toner images on the photosensitive drums 12a to 12d are transferred to the second constant voltage after the leading end of the OHP sheet P1 is conveyed by 6 mm from the nip portion while the OHP sheet P1 is conveyed by the transfer belt 11, as shown in FIG. Transfer biases of + 1800V, + 2600V, + 3400V, and + 4200V are applied to the conductive rollers 18a to 18d by the power supplies 48a to 48d, respectively, and the toner images are transferred onto the OHP sheet P1 in a multiple transfer manner.
[0043]
With this configuration, when transferring to the sheet paper P, a transfer bias is applied to the conductive rollers 18a to 18d at an earlier timing than when the leading edge of the sheet paper P reaches the nip portion. Since the surface of the sheet paper P is charged to hold the electric charge, the sheet paper P can be reliably electrostatically adsorbed from the leading edge onto the transfer belt 11 and good transfer without transfer omission can be obtained. On the other hand, at the time of transfer to the OHP sheet P1, the transfer bias is applied to the conductive rollers 18a to 18d after the leading end of the OHP sheet P1 passes through the nip portion, so that the OHP sheet is interposed between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d. A transfer bias is applied with P1 interposed, and a high current flows between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d even though a high transfer bias is applied. There is no. Accordingly, the photosensitive drums 12a to 12d are charged to the opposite polarity during transfer to the OHP sheet P1, and a transfer memory is not generated, and a color image with high display quality can be obtained.
[0044]
  Next, the present invention is illustrated in FIG.as well asThis will be described with reference to the third embodiment shown in FIG. In the first embodiment, the transfer bias is applied to the conductive roller by the constant voltage power source that switches the limit current value according to the environment (humidity) conditions at the time of transfer in the first embodiment. The same parts as those of the first embodiment are denoted by the same reference numerals, and the description thereof is omitted.
[0045]
When the limit of the generated current value of the transfer memory under the environmental (humidity) conditions was examined, the results shown in Table 1 were obtained.
[0046]
[Table 1]

[0047]
According to Table 1, when the humidity is 20% RH, a transfer memory is generated when the current between the conductive roller d and the photosensitive drum is 40 μA or more, and transfer is not possible when the current is 20 μA or less. Therefore, the effective current value is 20 to 40 μA. desirable. Similarly, an effective current value of 20 to 60 μA is desirable at 50% RH, and 100 to 140 μA is desirable at 85% RH.
[0048]
Based on such a principle, in the present embodiment, an environmental sensor 40 for detecting humidity is provided in the vicinity of the first image forming unit 13a, and the conductive rollers 18a to 18a are arranged in accordance with the humidity detected by the environmental sensor 40. The limit current values of the constant voltage power supplies 41a to 41d that apply the transfer bias of + 1000V, + 1200V, + 1400V, and + 1600V to 18d are switched in three stages. That is, when the humidity detected by the environmental sensor 40 is 40% RH or less, the limit current value of the constant voltage power supplies 41a to 41d is set to 40 μA. When the humidity is 40% RH or more and less than 65% RH, the limit current value of the constant voltage power supplies 41a to 41d is set to 60 μA. When the humidity is 65% RH or more, the limit current value of the constant voltage power supplies 41a to 41d is set to 140 μA.
[0049]
FIG.Is a control mechanism for controlling the current values of the constant voltage power supplies 41a to 41dIndicateThe CPU 43 receives the detection result of the environmental sensor 40, and the CPU 43 stores the limit current according to the humidity obtained from the result.42The limit current value is selected from the look-up table 46 stored in, and the limit current value of the voltage variable device 47 of the constant voltage power supplies 41a to 41d is set. At the time of toner image transfer, the constant voltage power supplies 41a to 41d are supplied to the conductive rollers 18a to 18d by the constant voltage power supplies 41a to 41d at timings earlier than the sheet paper P reaches the nip portion, respectively. A transfer bias is applied, and the toner image is multiplex-transferred onto the sheet paper P. The current values of the constant voltage power supplies 41a to 41d are feedback-controlled so as not to exceed the limit current values by monitoring the current values flowing through the circuits of the constant voltage power supplies 41a to 41d.
[0050]
With this configuration, in order to prevent electrostatic transfer from being reliably electrostatically attracted to the transfer belt 11 from the leading edge of the sheet paper P, the conductive roller 18a is detected at an earlier timing than when the leading edge of the sheet paper P reaches the nip portion. When a transfer bias is applied to .about.18d, a large current does not flow between the conductive rollers 18a to 18d and the photosensitive drums 12a to 12d regardless of the humidity, and transfer due to reverse polarity charging of the photosensitive drums 12a to 12d. Memory can be prevented, good transfer without image omission can be obtained, and color images with high display quality can be realized without generating transfer memory.
[0051]
Next, the present invention will be described with reference to a fourth embodiment shown in FIGS. In the image forming unit 10 of a four-tandem color copying machine, the charge applied from the conductive roller is accumulated on the transfer material or toner in the preceding image forming means, and this charge is electrostatically attracted to the transfer belt. On the other hand, since the transfer electric field in the next image forming means is weakened, the latter transfer bias is set higher than the transfer bias in the preceding image forming means. For this reason, the problem of the transfer memory is more likely to occur as the image processing unit is further downstream. Therefore, the present embodiment prevents a transfer memory in the subsequent stage of the image forming means.
[0052]
In the first embodiment, the transfer bias is applied to the conductive roller by a constant voltage power source in the first embodiment, and the image forming means is arranged upstream of the transfer belt in the transport direction. In the image forming means that applies the transfer bias to the conductive roller at an earlier timing than the transfer material reaches the nip portion and is arranged downstream of the transfer belt in the conveyance direction, the transfer material is placed in the nip portion. The transfer bias is applied to the conductive roller after the arrival, and the same parts as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted.
[0053]
The conductive rollers 18a to 18d of the present embodiment are connected to constant voltage power supplies 37a to 37d, and are applied with transfer biases of + 1000V, + 1200V, + 1400V, and + 1600V, respectively. The timing of applying the transfer bias by the constant voltage power supplies 37a to 37d to the conductive rollers 18a to 18d is the transfer timing for the first and second image forming units 13a and 13b upstream of the transfer belt 11 in the transport direction. In the third and fourth image forming units 13c and 13d on the downstream side in the conveyance direction of the transfer belt 11, the transfer material 11 is applied before reaching the front end nip portion. It is set to apply.
[0054]
That is, when the transfer bias is applied at an earlier timing than the transfer material leading edge reaches the nip portion, the transfer material is applied after (tl-α) sec from the start of feeding the transfer material by the registration roller 28, while the transfer material leading edge is When the transfer bias is applied after passing through the nip portion, the transfer bias is applied after (tl + α) sec from the start of feeding the transfer material of the registration roller 28.
[0055]
Here, tl = L1 / V (counting is started by a timer from feeding of the transfer material by the start of operation of the registration roller 28, and the feeding speed of the registration roller 28 (= conveyance speed of the transfer belt 11) is Vmm / sec, The distance from the nip of the registration roller 28 to the nip portion of the first image forming means 13a is L1 mm.) Α is the time until the transfer material tip reaches the nip portion after the transfer bias is applied or the transfer material tip is nip This is the time until the transfer bias is applied after passing through the part. Actually, α is 500 msec in the present embodiment in which the process speed is 120 mm / sec, although it depends on the conveyance speed (process speed) of the transfer belt 11. That is, in the present embodiment, the transfer bias is applied to the conductive rollers 18a to 18d at a timing 6 mm earlier or later than the leading edge of the transfer material reaches the nip portion.
[0056]
When the toner image is transferred, while the sheet paper P is conveyed by the transfer belt 11, the leading edge of the sheet paper P reaches the nip portion as shown in FIG. 9 in the first and second image forming units 13a and 13b. The transfer bias of +1000 V and +1200 V is applied to the conductive rollers 18 a and 18 b by the constant voltage power supplies 37 a to 37 d at a timing 6 mm earlier than the above. Further, in the third and fourth image forming means 13c and 13d, as shown in FIG. 10, after the leading end of the sheet paper P passes through the nip portion by 6 mm, the constant voltage power sources 37a to 37d are respectively applied to the conductive rollers 18a and 18b. Transfer biases of +1400 V and +1600 V are applied, and the color toner image is transferred onto the sheet paper P in a multiple manner.
[0057]
With this configuration, when the toner is multiplex-transferred onto the same sheet paper p by a four-tandem color copying machine, the leading edge of the sheet paper P reaches the nip portion on the upstream side of the transfer belt 11. By applying the transfer bias at an earlier timing, the sheet can be reliably electrostatically adsorbed onto the transfer belt 11 from the leading edge of the sheet paper P, and good transfer with no transfer omission can be obtained. In this case, even if the transfer bias applied to the conductive rollers 18a to 18d is high, the transfer bias is applied after the leading edge of the sheet P passes through the nip portion, so that the transfer memory is not generated without generating a large current. Color images with high display quality that can be prevented can be obtained.
[0058]
The present invention is not limited to the above-described embodiment, and can be changed without changing the gist thereof. For example, the charge applying means is not limited to a conductive roller, but a conductive brush, a conductive rubber blade, A transfer bias may be applied to the image carrier by applying a transfer bias to a rubber sheet or the like. The conductive sheet has a volume resistance value of 10 in which carbon is dispersed.^Five-10⁸A silicone rubber sheet having a thickness of about 5 Ωcm, urethane rubber, or EPDM may be used. As long as the conductive roller is conductive and can impart a charge to the toner image of the image carrier, the material and the outer diameter resistance value are arbitrary, and the transfer bias value to be applied is not limited.
The transfer belt also has a volume resistance of 10⁹-10¹³Any material that exhibits semiconductivity of Ωcm may be used, and in addition to polyimide in which carbon is dispersed, conductive particles such as carbon are dispersed in polyethylene terephthalate, polycarbonate, polytetrafluoroethylene, polyvinylidene fluoride, and the like. good. Alternatively, a polymer film whose electric resistance is adjusted by adjusting the composition without using conductive particles, a film obtained by mixing an ion conductive substance in a polymer film, or a rubber material such as siligon rubber or urethane rubber having a relatively low electric resistance may be used.
[0059]
In the first embodiment, the resistance value of the transfer material to which the transfer bias is applied with a constant voltage power source, or in the second embodiment, the resistance value of the transfer material to which the transfer bias is applied sooner than reaching the nip portion. The value is 1x10¹¹It is desirable that it is less than Ωcm. On the other hand, the transfer material to which the transfer bias is applied with the constant current power source in the first embodiment or the transfer material to which the transfer bias is applied after reaching the nip portion in the second embodiment has a high resistance value. If it exists, it may not be an OHP sheet, but its resistance value is 1 × 10¹¹It is desirable that it is Ωcm or more.
[0060]
In the third embodiment, the switching stage of the limit current value with respect to the humidity is not limited, and a better transfer can be obtained by switching to multiple stages so as to correspond more appropriately. Further, in the fourth embodiment, a preceding conductive roller that applies a transfer bias earlier than the transfer material reaches the nip portion, and a subsequent conductive roller that applies a transfer bias after the transfer material reaches the nip portion. The first is the firstFirstBetween the two image forming units or between the third and fourth image forming units.
[0061]
【The invention's effect】
As described above, according to the present invention, when transfer is performed by a contact transfer device using a transfer belt, the transfer material is reliably electrostatically adsorbed from the leading end to the transfer belt to prevent transfer loss, and this transfer In order to prevent omission, it is possible to prevent the generation of a large current from the charge applying means to the image carrier, which has been conventionally generated depending on the material or environment of the transfer material, and by preventing the image carrier from being charged to a reverse polarity. Thus, it is possible to prevent the generation of a transfer memory in the next image forming process, and it is possible to obtain a good display quality image with no transfer omission and no transfer memory.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram illustrating an image forming unit according to a first embodiment of the present invention.
FIG. 2 is a schematic perspective view showing a conductive roller according to the first embodiment of the present invention.
FIG. 3 is a schematic configuration diagram illustrating an image forming apparatus according to a second embodiment of the present invention.
FIG. 4 is a schematic explanatory diagram illustrating application timing of a transfer bias to the sheet paper P according to the second embodiment of the present invention.
FIG. 5 is a schematic explanatory diagram showing application timing of a transfer bias to the OHP sheet P1 according to the second embodiment of the present invention.
FIG. 6 is a schematic configuration diagram illustrating an image forming apparatus according to a third embodiment of the present invention.
FIG. 7 is a schematic block diagram showing a control mechanism for installing and controlling a limit current value of a constant voltage power supply according to a third embodiment of the present invention.
FIG. 8 is a schematic configuration diagram illustrating an image forming apparatus according to a fourth embodiment of the present invention.
FIG. 9 is a schematic explanatory diagram illustrating transfer bias application timings in the first and second image forming units according to the fourth exemplary embodiment of the present invention.
FIG. 10 is a schematic explanatory diagram illustrating transfer bias application timings in third and fourth image forming units according to a fourth embodiment of the present invention.
[Explanation of symbols]
10. Image forming unit
11. Transfer belt
12a to 12d ... photosensitive drum
13a to 13d Image forming means
17a to 17d ... developing device
18a to 18d ... conductive roller
32a-32d ... shaft
33a-33d ... Rubber roller
36a-36d ... switch
37a-37d ... constant voltage power supply
38a-38d ... Constant current power supply

Claims

An image carrier, an image forming unit that forms an image on the image carrier, a transfer belt that adsorbs a transfer material and conveys the transfer material to a transfer region of the image carrier, and the image in the transfer region via the transfer belt. facing the carrier, while the resistance value before Symbol transfer material to impart transfer charge from the transfer material back side at a timing earlier than the arrival to the transfer area of the transfer material in the case of less than 1 × 10 ¹¹ Ωcm, the And a charge applying unit that applies the transfer charge from the back surface of the transfer material at a timing delayed from the arrival of the transfer material to the transfer region when the resistance value of the transfer material is 1 × 10 ¹¹ Ωcm or more. An image forming apparatus.

A plurality of image carriers, a plurality of image forming means for forming images of different colors on each of the plurality of image carriers, and a transfer that sucks a transfer material and sequentially conveys it to each transfer region of the plurality of image carriers A belt, and the transfer material that faces the image carrier in each transfer region via the transfer belt, and the transfer material reaches the transfer region when the resistance value of the transfer material is less than 1 × 10 ¹¹ Ωcm. While the transfer charge is applied from the back surface of the transfer material at an earlier timing, the transfer material is delayed at a timing later than the transfer material reaches the transfer regions when the resistance value of the transfer material is 1 × 10 ¹¹ Ωcm or more. An image forming apparatus comprising: a plurality of charge applying units that apply the transfer charge from a back surface.

A plurality of image carriers, a plurality of image forming means for forming images of different colors on each of the plurality of image carriers, and a transfer that sucks a transfer material and sequentially conveys it to each transfer region of the plurality of image carriers A belt, a plurality of charge applying means for applying transfer charges from the back surface of the transfer material facing each image carrier in each transfer region via the transfer belt, and applying a transfer bias to the plurality of charge applying means In an image forming apparatus comprising a plurality of power supplies
The power supplies on the upstream side in the rotation direction of the transfer belt apply the transfer bias to the charge applying unit at a timing earlier than the transfer material reaches the transfer areas, and on the downstream side in the rotation direction of the transfer belt. The image forming apparatus according to claim 1, wherein each of the power supplies applies the transfer bias to the charge applying units at a timing delayed from the arrival of the transfer material to the transfer regions.

A plurality of image carriers, a plurality of image forming means for forming images of different colors on each of the plurality of image carriers, and a transfer that sucks a transfer material and sequentially conveys it to each transfer region of the plurality of image carriers. A belt, a plurality of charge applying means for applying a transfer charge from the back surface of the transfer material facing each image carrier in each transfer region via the transfer belt, and as positioned downstream in the rotation direction of the transfer belt. An image forming apparatus comprising a plurality of power supplies that sequentially apply a larger transfer bias to each of the charge applying units.
The power supplies on the upstream side in the rotation direction of the transfer belt apply the transfer bias to the charge applying unit at a timing earlier than the transfer material reaches the transfer areas, and on the downstream side in the rotation direction of the transfer belt. The image forming apparatus according to claim 1, wherein each of the power supplies applies the transfer bias to the charge applying units at a timing delayed from the arrival of the transfer material to the transfer regions.