JP4261678B2 - Image forming apparatus - Google Patents

Description

第１の実験では、電圧印加部材７は、外径８mmのステンレス製の軸をＥＰＤＭ発泡フォームを覆った外径１６mmの導電性ローラである。電圧印加部材７の抵抗値は、図２に示すような方法で電圧Ｖを測定した。直径３０mmのアルミシリンダ３０を回転区同様として使用し、これに電圧印加部材７を押圧する。このとき電圧印加部材７の軸方向の両端部を、それぞれ５００ｇで加圧する。この状態で、アルミシリンダ３０を回転駆動させて、電圧印加部材７を回転させながら、５０Ｖの電圧を印加する。このときアルミシリンダ３０の電圧ｘを測定用テスタ３１で測定し、次式
抵抗値Ｒ＝５０×１００／ｘ
で抵抗値Ｒを算出する。そして、電圧印加部材７として、その結果が１×１０⁵ Ω以下となるようなローラを使用した。電圧印加部材７の硬度は、ＡｓｋｅｒＣで５０度である。また、電圧印加部材７の押圧については、図１に示すように、この電圧印加部材７を中間転写ベルト６の内側に配置し、５００ｇｆのばねを電圧印加部材７の両端に配置して、中間転写ベルト６を表面側の感光ドラム１に押圧した。
【００４５】
中間転写ベルト６としては、ポリイミドの単層ベルトを使用した。中間転写ベルト７の厚みは７５μｍ、体積抵抗値は、１０¹⁰Ω・cmである。なお、体積抵抗値は、三菱油化株式会社製のHIRESTA （登録商標）を用いて測定し、５００Ｖ印加での測定結果である。この中間転写ベルト６は、カーボンを分散させることにより抵抗調整を行っている。
【００４６】
このような条件の下、感光ドラム１と中間転写ベルト６との接触部分である表面側接触部Ｓ１、また、中間転写ベルト６と電圧印加部材７との接触部分を裏面側接触部Ｓ２とし、これら表面側接触部Ｓ１と裏面側接触部Ｓ２についての中間転写ベルト６の回転方向（矢印Ｒ６方向）についての幅をそれぞれ表面接触幅Ｗ１、裏面接触幅Ｗ２とする。そして、これら表面接触幅Ｗ１と裏面接触幅Ｗ２の測定を行った。まず、染料を電圧印加部材７と感光ドラム１の表面に塗り、電圧印加部材７により、裏面側から中間転写ベルト６を感光ドラム１に押圧させた後、電圧印加部材７を離間させる。この動作により、中間転写ベルト６上に付着した染料の幅を測定し、これを表面側接触幅Ｗ１、裏面側接触幅Ｗ２とした。
【００４７】
測定結果は、
感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１：１mm
中間転写ベルト６と電圧印加部材７との裏面接触幅Ｗ２：１mm
であった。
【００４８】
本出願人らは、このような装置構成条件のもと、以下のような方法で逆転写現象を計測した。
【００４９】
図１を用いて説明すると、まず、第１の画像形成部Ｙにおいて、第１の色成分であるイエローのトナー像を形成し、感光ドラム１上のマイナス極性のイエローのトナー像を、電圧印加部材７に一次転写バイアス電源７ａにてプラス極性のバイアスを印加することで中間転写ベルト６上へ一次転写するプロセスにおいて、一次転写バイアス電源７ａの電圧を変化させることにより、もっとも転写性が良好となるバイアス値を求めた。そのバイアス値は約６００Ｖであった。
【００５０】
同様に、第２の画像形成図Ｍにおいて、第２の色成分であるマゼンタのトナー像についても同様な実験を行い、最適値を求めた。
【００５１】
次に、第１の画像形成図Ｙの感光ドラム１上にイエローのトナー像を形成し、一次転写ニップ部で先に求めた最適バイアス値にて中間転写ベルト６に一次転写し、第２の画像形成部Ｍでは、マゼンタのトナー像は形成せず、一次転写ニップ部では、一次転写バイアス電源７ｂにてバイアスを変化させ、中間転写ベルト６上のイエローのトナーがどの程度、第２の画像形成部Ｍの感光ドラム１上にに逆転写するかを調査した。
【００５２】
以上の結果を図３に示す。一次転写性とは、この場合、第２の画像形成部Ｍで、ＲＥＤ（レッド）のべた画像（Ｙ＋Ｍ）を形成する場合のマゼンタトナーの一次転写性の良否を示したものである。また、逆転写性とは、同じく第２の画像形成部Ｍにおいて、中間転写ベルト６上に転写されているイエローのベた画像が、第２の画像形成部Ｍの感光ドラム１のマゼンタトナー像のない部分にどの程度、逆転写したかについて示したものである。さらに詳述する。
【００５３】
図３の横軸は、一次転写バイアスである。また、縦軸は、感光ドラム１上の一次転写残トナーや感光ドラム１に逆転写されたトナーをテープで採取し、紙上に貼り付け、光学濃度計で測定した値から、テープだけを紙上に貼り付けた場合の光学濃度計の測定値を差し引いたものである。この方法における光学濃度値は、一次転写残トナー重量や逆転写トナー重量とほぼ比例関係を示し、この方法は、直接、一次転写残トナー重量や逆転写トナー重量を化学天秤などで計測する方法と比較すると簡易に微量のトナーを計測できるという利点を持っている。光学濃度値が小さいほど、良好に一次転写されている、あるいは、逆転写が防止されていることを示す。例えば、光学濃度値０．１は、一次転写残トナー、又は逆転写トナーの重量約０．０１mg／cm² に対応している。本出願人らの実験によれば、図３の縦軸の光学濃度が約０．１０以下であれば、Ｙ、Ｍ、Ｃ、Ｂｋの各色５％の印字率で、現像器４の耐久枚数である４００００枚の通紙テスト（以下「耐久テスト」という。）を実施したが、現像器４の汚染は問題とはならなかった。
【００５４】
図３の結果をみると、最適な一次転写を達成するバイアス条件と逆転写を防止するバイアス条件とが一致せず、両立条件がないことがわかる。最適な一次転写を優先させてバイアス条件を決めた場合には、多数の画像形成を行った場合、逆転写量が多いため、帯電ローラ２の汚れやイエロートナーが第２のマゼンタ画像形成部Ｍのマゼンタの現像器４内へ混入してしまい、色変化が発生してしまった。また、逆転写の防止を優先してバイアス条件を決めた場合は、耐久テストにおいて、帯電ローラ２の汚れによる帯電性能の低下が発生した。
【００５５】
第２の実験では、電圧印加部材７として、より軟らかい導電性ローラで同様の実験を実施した。
【００５６】
使用した導電性ローラは、外径８mmのステンレス製の軸をエーテル系の発泡フォームで覆って外径１６mmのローラとした。抵抗値は、図２に示すような方法で電圧ｘを測定し、次式
抵抗値Ｒ＝５０×１００／ｘ
で抵抗値を算出し、その結果が１×１０⁵ Ω以下となるような導電性ローラを使用した。ローラ硬度は、軟らかいため、下記のような方法で測定した（常温、常湿時）。
【００５７】
ローラ表面に、長方形の板材（５０mm×１０mm×１mm）をその長手方向をローラの長手方向と一致させるようにして押し当て、３秒間で１mmローラがつぶれるように侵入させる。そして、６０秒後、板材にかかるローラのフォーム材の反発力を協和株式会社製のひずみゲージで測定した。測定値は、３００ｇｆ前後であった。実装時のローラの押圧に関しては、５００ｇｆのばねをローラの両端に配置させ、中間転写ベルト６を介してその裏面側からローラを感光ドラム１に押圧した。
【００５８】
使用した中間転写ベルト６は、前述の第１の実験と同様の単層のポリイミドのベルトであり、厚み７５μｍ、体積抵抗値１０¹⁰Ω・cmである。なお、体積抵抗値は、三菱油化株式会社製のHIRESTA を用いて測定し、５００Ｖ印加での結果である（常温常湿時）。
【００５９】
このような条件の下、感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１と、中間転写ベルト６と電圧印加部材７との裏面接触幅Ｗ２の測定を行った。まず、染料を電圧印加部材７と感光ドラム１の表面に塗り、電圧印加部材７を中間転写ベルト６を介して感光ドラム１に押圧させ、その後、離間させる。この動作により、中間転写ベルト６上に付着した染料の幅を測定し、表面接触幅Ｗ１、裏面接触幅Ｗ２とした。
【００６０】
測定結果は、
感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１：１mm
中間転写ベルト６と電圧印加部材７との裏面接触幅Ｗ２：４mm
であった。
【００６１】
本出願人らは、このような装置構成条件のもと、再度、以下のような方法で逆転写現象を計測した。
【００６２】
まず、第１の画像形成部Ｙにおいて、イエローのトナー像を感光ドラム１上に形成し、このマイナス極性のイエロートナー像を、一次転写バイアス電源７ａにて電圧印加部材７にプラス極性のバイアスを印加することで、中間転写ベルト６上に一次転写するプロセスにおいて、一次転写バイアス電源７ａの電圧を変化させることにより、最も一次転写性が良好となるバイアス値を求めた。そのバイアス値は約６００Ｖであった。同様に、第２の画像形成部Ｍにおいて、マゼンタのトナー像についても同様な実験を行い、最適値を求めた。
【００６３】
次に、第１の画像形成部Ｙにおいて、感光ドラム１上にイエローのトナー像を形成し、一次転写ニップ部において、先に求めた最適バイアス値を電圧印加部材７に印加して感光ドラム１上のイエローのトナー像を中間転写ベルト６上に一次転写する。第２の画像形成部Ｍにおいては、マゼンタのトナー像は形成せず、一次転写ニップ部におけるバイアスを変化させ、中間転写ベルト６上のイエロートナーがどの程度、第２の画像形成部Ｍの感光ドラム１上に逆転写するかを調査した。以上の結果を図４に示す。
【００６４】
図４の結果を見ると、逆転写性が著しく変化し、この条件では、最適な一次転写条件を達成するバイアス条件と、逆転写を防止するバイアス条件とが両立する領域があることがわかる。この結果から、本出願人らは、感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１と、中間転写ベルト６と電圧印加部材７との裏面接触幅Ｗ２との関係が重要な要因であることを確認した。この条件下で耐久テストを実施したが、イエロートナーが、第２の画像形成部Ｍのマゼンタの現像器４内に混入する現象は問題となるほどは発生せず、色変化も問題とならなかった。また、帯電ローラ２の帯電性能の低下も発生しなかった。
【００６５】
すなわち、感光ドラム１と中間転写ベルト６との表面接触幅をＷ１、中間転写ベルト６とっ電圧印加部材７との裏面接触幅Ｗ２との間に、
Ｗ１＜Ｗ２
の関係がある場合に、逆転写を抑制できることがわかった。
【００６６】
また、電圧印加部材７としては、図５（ａ）に示すように上述の本実験で用いたスポンジ性の導電性ローラ７Ａ以外にも、図５（ｂ）に示すような導電性シート７Ｂやゴム製の導電性ブレードでも同様な効果が得られた。また、図５（ｃ）に示すような導電性化学繊維や金属をブラシ状に形成した導電性ブラシ７Ｃを使用しても同様の効果が得られた。
【００６７】
上述のような効果が得られた理由について、本出願人らは、以下のように考察している。
【００６８】
逆転写のメカニズムは、例えば、第２の画像形成部Ｍで説明すると、感光ドラム１上に形成されたマゼンタトナーを中間転写ベルト６上へ一次転写するプロセスにおいて、感光ドラム１と中間転写ベルト６との分離部近傍の空隙では、感光ドラム１や中間転写ベルト６が移動することにより、静電容量の急激な低下が発生し、剥離放電が発生する。この放電によって発生した電荷は、この空隙にかけられた電界の方向に沿って移動する。本実施の形態の場合は、トナー極性はマイナスであり、電圧印加部材７には、プラスの電圧を印加しているため、放電によって発生したプラス電荷は感光ドラム１表面に移動し、放電によって発生したマイナス電荷は、中間転写ベルト６表面へと移動する。しかし、中間転写ベルト６上に、第１の画像形成部Ｙで転写されたイエロートナーがある場合は、そのトナーはマイナス極性であるため、トナー表面へもわずかに放電によって生じたプラス電荷の移動が生じてしまう。その結果、イエロートナーの極性が一部反転し、第２の画像形成部Ｍの感光ドラム１へと転移してしまう。このように本出願人らは逆転写のメカニズムを考えている。
【００６９】
そして、実施の形態の効果を本出願人らは、以下のように考察している。
【００７０】
本出願人らの解析によれば、中間転写ベルト６と電圧印加部材７との裏面接触幅Ｗ２を、感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１より大きくした場合は、中間転写ベルト６と感光ドラム１との空隙に対し、中間転写ベルト６を介して、裏面側に電圧印加部材７である導電性ローラがあるため、感光ドラム１と中間転写ベルト６で形成される表面側接触部Ｓ１の上流側の空隙で放電が発生した場合、中間転写ベルト６上のトナーは、放電によって発生したマイナス電荷により、マイナス電荷量が増大する。一方、感光ドラム１側は、放電により発生したプラス電荷の影響で帯電電位は低下する。この作用により、感光ドラム１と中間転写ベルト６との表面側接触部Ｓ１や、この表面側接触部Ｓ１の下流側の空隙での放電が抑制され、あるいは、仮に放電が発生しても、中間転写ベルト６上のトナーは、高いマイナスの電荷量を保持していることから、前述したような逆転写が起こりにくいと考えられる。
【００７１】
以上、感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１と、中間転写ベルト６と電圧印加部材７との裏面接触幅Ｗ２との関係において、Ｗ１＜Ｗ２の関係になるように図５（ａ）〜（ｃ）に示した電圧印加部材７（導電性ローラ７Ａ、導電性ブレード７Ｂ、導電性ブラシ７Ｃ）を、図１に示す画像形成装置に組み込み、４色フルカラーモードで耐久テストを実施した結果、現像器４の色味変化や帯電ローラ２の帯電性能の低下は発生せず、良好な画像を得ることができた。
【００７２】
〈実施の形態２〉
図６に、本実施の形態を示す。本実施の形態においては、裏面接触幅Ｗ２を、表面接触幅Ｗ１の中央Ａを基準として、その上流側の裏面接触幅Ｗ３と下流側の裏面接触幅Ｗ４とに分けて、それぞれの作用について以下の方法で解析した。
【００７３】
電圧印加部材７としては、樹脂板（幅２０mm×長さ３００mm×厚み２mm）の上に、導電性レーヨンのブラシ（幅２０mm×長手３００mm）を配置させた導電性ブラシ７Ｃを使用した。ブラシの毛長は５mm、全体の抵抗値は、１０⁵ 以下とした。
【００７４】
使用した中間転写ベルト６は、前述の実施の形態１と同じベルトであり、厚み７５μｍ、体積抵抗率１０¹⁰Ω・cmのポリイミドの無端状のベルトである。なお体積抵抗値は、三菱油化株式会社製のHIRESTA （登録商標）用いて測定し、５００Ｖ印加での結果である。
【００７５】
上述の導電性ブラシ７Ｃの幅方向（中間転写ベルト６の回転方向）の配置については、感光ドラム１の中心を通る鉛直線に対し、左右対称となるようにした。この導電性ブラシ７Ｃによって中間転写ベルト６をその裏面側から押圧して、中間転写ベルト６の表面側を感光ドラム１に押し付けるようにした。この状態で、導電性ブラシ７Ｃのブラシが厚み方向に２mmつぶれるようにした。
【００７６】
この条件で、感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１を測定した。測定方法は、前述の実施の形態１で示した方法と同じである。Ｗ３＋Ｗ４は、裏面接触幅Ｗ２、つまり導電性ブラシ７Ｃの幅に対応している。結果は、Ｗ１：２mm、Ｗ３、Ｗ４：１０mmであった。
【００７７】
次に、図７に示すように、この導電性ブラシ７Ｃを長手方向の中央を中心とし、１０度回転させて配置させた。傾けた目的は、簡易な方法でＷ３、Ｗ４の影響を確認するためである。
【００７８】
本出願人らは、このような装置構成条件のもと、再度、以下のような方法で逆転写現象を計測した。
【００７９】
同様に図１を用いて説明すると、
まず、第１の画像形成部Ｙの感光ドラム１上にイエローのトナー像を形成し、この感光ドラム１上のマイナス極性のイエロートナー像を、導電性ブラシ７ｃに一次転写バイアス電源７ａにてプラス極性のバイアスを印加することで、中間転写ベルト６上に一次転写するプロセスにおいて、一次転写バイアス電源７ａの電圧を変化させることにより、最も転写性が良好となるバイアス値を求めた。そのバイアス値は約５００Ｖであった。前述の実施の形態１では、約６００Ｖで合ったのに対して本実施の形態ではこれよりも１００Ｖ低い５００Ｖである理由は、感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１、及び中間転写ベルト６と導電性ブラシ７Ｃとの裏面接触幅Ｗ２（＝Ｗ３＋Ｗ４）の双方ともが、実施の形態１よりも大きいことに起因していると考えられる。
【００８０】
同様に、第２の画像形成部Ｍにおいてマゼンタのトナー像についても同様な実験を行い、最適値を求めた。
【００８１】
次に、第１の画像形成部Ｙにおいて、感光ドラム１上にイエローのトナー像を形成し、一次転写ニップ部において、先に求めた最適バイアス値にて中間転写ベルト６にイエロートナー像を一次転写し、第２の画像形成部Ｍでは、マゼンタのトナー像は形成せず、一次転写ニップ部ではバイアスを印加し、中間転写ベルト６上のイエロートナーがどのようにして第２の画像形成部Ｍの感光ドラム１に逆転写するかを調査した。
【００８２】
以上の結果を図７で説明する。ここで、同図において、感光ドラム１と中間転写ベルト６との表面側接触部Ｓ１の上流側の境界線をａ₁ 、下流側の境界線をｂ₁ とし、また、中間転写ベルト６と導電性ブラシ７Ｃとの裏面側接触部Ｓ２の上流側の境界線をａ₂ 、下流側の境界線をｂ₂ とする。また、Ｐ１は境界線ａ₁ と境界線ｂ₂ との交点、Ｐ２は境界線ａ₁ と境界線ａ₂ との交点、Ｐ３は境界線ｂ₁ と境界線ａ₂ との交点をそれぞれ示す。
【００８３】
上述のＰ１、Ｐ２、Ｐ３について、Ｐ１とＰ２との間の領域においては、逆転写が非常によく防止され、一方、Ｐ２とＰ３との間の領域においては、逆転写が発生する。
【００８４】
すなわち、必ずしも表面側接触部Ｓ１の全体が裏面側接触部Ｓ２によって覆われている必要はなく、表面側接触部Ｓ１の上流側の境界線ａ₁ が裏面側接触部Ｓ２の範囲内（裏面側接触部Ｓ２の上流側の境界線ａ₂ と下流側の境界線ｂ₂ との間）に配置されてさえいれば、逆転写を抑制する効果があることがわかった。換言すると、裏面側接触部Ｓ２のうち、表面接触部Ｓ１の上流側の境界線ａ₁ を横切る部分については、良好に逆転写を防止することができる。
【００８５】
実際に、導電性ブラシ７Ｃを、図１に示す画像形成装置に組み込み、中間転写ベルト６と導電性ブラシ７Ｃとの裏面側接触部Ｓ２が、感光ドラム１と中間転写ベルト６との表面側接触部Ｓ１の上流側の境界線ａ₁ を跨ぎ、かつ、下流側の境界線ｂ₁ を跨がないように配置した後に、実施の形態１に示した耐久テストを実施したところ、現像器４の色味変化や帯電ローラ２の帯電性能の低下は発生せず、良好な画像を得ることができた。
【００８６】
なお、上述の導電性ブラシ７Ｃに変えて、図５に示すような導電性ローラ７Ａや導電性シート、導電性ブレード７Ｂを使用し、裏面側接触部Ｓ２が、表面側接触部Ｓ１の上流側の境界線ａ₁ のみを覆うように、これらの部材と感光ドラム１との相対位置を設定することにより、上述とほぼ同様の効果を得ることができた。
【００８７】
〈実施の形態３〉
次に、感光ドラム１と中間転写ベルト６との表面側接触部Ｓ１の表面接触幅Ｗ１と、中間転写ベルト６と電圧印加部材７との裏面側接触部Ｓ２の裏面接触幅Ｗ２と、中間転写ベルト６の抵抗値との関係について調べた。
【００８８】
中間転写ベルト６は、厚み７５μｍのポリイミドの単層ベルトを使用した。
【００８９】
中間転写ベルト６の体積抵抗値については、それぞれ値の異なる以下の９種類の中間転写ベルト６を使用した。
【００９０】
体積抵抗値としては、１×１０⁶ 、１×１０⁷ 、１×１０⁸ 、１×１０⁹ 、１×１０¹⁰、１×１０¹¹、１×１０¹²、１×１０¹³、１×１０¹⁴Ω・cmである。
【００９１】
電圧印加部材７としては、導電性ローラ７Ａを使用した。外径８mmのステンレス製の軸を、エーテル系の発泡フォームで覆って外径１６mmのローラを形成した。この導電性ローラ７Ａの体積抵抗値については、図２に示すような方法で電圧ｘを測定し、次式
抵抗値Ｒ＝５０×１００／ｘ
で抵抗値Ｒを算出し、その結果が１×１０⁵ Ω以下となるようなローラを使用した。ローラ硬度は、軟らかいため、下記のような方法で測定した。
【００９２】
ローラ表面に、長方形の板材（２０mm×１０mm×１mm）をその長手方向をローラの長手方向と一致させるようにして押し当て、３秒間で１mmローラがつぶれるように侵入させる。そして、６０秒後、板材にかかるローラのフォーム材の反発力を協和株式会社製のひずみゲージで測定した。測定値は、３００ｇｆ前後であった。実装時のローラの押圧に関しては、５００ｇｆのばねをローラの両端に配置させ、中間転写ベルト６を介してその裏面側からローラを感光ドラム１に押圧した。
【００９３】
感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１と、中間転写ベルト６と導電性ローラ７Ｃとの裏面接触幅Ｗ２とは、前述の実施の形態１と同様な方法で測定し、その結果は、
感光ドラム１と中間転写ベルト６との表面接触幅Ｗ１：１mm
中間転写ベルト６と導電性ローラ７Ｃとの裏面接触幅Ｗ２：４mm
であった。
【００９４】
各体積抵抗値の中間転写ベルト６で、本出願人らは、実施の形態１と同様な方法で逆転写現象を計測するとともに、実施の形態１で示したような耐久テストも実施し、逆転写性の良否について調べた。また、同時に画像品質の評価も行った。それらの結果を図８に示す。
【００９５】
図８に示すように、実験に使用した各中間転写ベルト６で、逆転写性については、良好な結果が得られたが、画像品質については、１×１０¹³Ω・cm以上の抵抗値の中間転写ベルト６では、適正な一次転写バイアス設定は１ｋＶ以上の高電圧になり、感光ドラム１と中間転写ベルト６との表面接触部Ｓ１の上流側や下流側の空隙で転写が開始され、画像にじみや画像の一部が抜けてしまう画像欠陥が発生した。また、１×１０⁶ Ω・cmの中間転写ベルト６では、抵抗値が低いために、図１の第１〜第４の画像形成部Ｙ、Ｍ、Ｃ、Ｂｋの各電圧印加部材７の間で電流の流入、流出が発生し、各色独立で異なる電圧設定ができないという問題が生じ、良好な画像品質を得ることができる電圧に設定できなかった。また、飛び散り（文字などの線画像の周辺にトナーが散る現象）に関しては、１×１０⁹ Ω・cm以上で特に良好であった。
【００９６】
したがって、良好な画像が得られ、かつ、装置構成上問題ない中間転写ベルト６の抵抗値は、１×１０⁷ 〜１×１０¹²Ω・cmの範囲であった。
【００９７】
本発明において、中間転写ベルト６は、これに限定されるものではなく、例えば、ポリスチレン、スチレン系樹脂（クロロポリスチレン、スチレンーブタジエン共重合体等）、メタクリル酸メチル樹脂、アクリル酸エチル樹脂、変性アクリル樹脂（シリコーン変性アクリル樹脂、アクリル・ウレタン樹脂等）、塩化ビニル樹脂、フェノール樹脂、エポキシ樹脂、ポリエステル樹脂、ポリエステルポリウレタン樹脂、ポリエチレン、ポリプロピレン、ポリブタジエン、ポリ塩化ビニリデン、ポリウレタン樹脂、シリコーン樹脂、フッ素樹脂、変性ポリフェニレンオキサイド樹脂、ポリイミド樹脂、ＰＥＳ等が挙げられる。これらの１種類あるいは２種類以上を使用することができる。
【００９８】
被覆層の形成方法は、スプレー塗装、浸漬塗装、静電塗装、押出成形など、任意のものを使用することができる。
【００９９】
中間転写ベルト６の抵抗値を所望値に調節するために、必要に応じ、弾性層や被覆層に導電剤を添加することができる。導電剤としては、特に限定されるものではないが、例えば、カーボン、アルミニウムやニッケル等の金属粉末、酸化チタン等の金属酸化物、四級アンモニウム塩含有ポリメタクリル酸メチル、ポリビニルアニリン、ポリビニルピロール、ポリジアセチレン、ポリエチレンイミン、含硼素高分子化合物及びポリピロール等の導電性高分子化合物等が挙げられる。これらの１種又は２種以上を使用することができる。
【０１００】
また、上記の中間転写ベルト６を用いた場合、逆転写を防止し、現像器４内の現像剤の色味変化を防止し、かつ良好な画像が得られることは言うまでもない。
【０１０１】
なお、上述の実施の形態１〜３では、表面接触幅Ｗ１及び裏面接触幅Ｗ２の測定を、感光ドラム１、中間転写ベルト６、一次帯電ローラ２が停止しているときに行っているが、画像形成のために上述の各部材が動作（回転）している場合においても、表面接触幅Ｗ１が裏面接触幅Ｗ２よりも小さい、という関係は変わらないものとする。
【０１０２】
【発明の効果】
以上説明したように、本発明によると、第２の像担持体の移動方向に沿って複数の画像形成部を並べて構成した画像形成装置において、各画像形成部の第１の像担持体（例えば、感光ドラム）と第２の像担持体（例えば、中間転写ベルト）との接触部分を表面側接触部、また第２の像担持体と電圧印加部材との接触部分を裏面側接触部とし、これら表面側接触部と裏面側接触部についての第２の像担持体の移動方向の幅をそれぞれ表面接触幅、裏面接触幅としたときに、表面接触幅が裏面接触幅よりも小さくなるように表面側接触部と裏面側接触部とを形成することにより、第１の像担持体から第２の像担持体へのトナー像の転写工程において発生する、第２の像担持体から第１の像担持体へのトナーの逆転写を防止して、逆転写されたトナーが現像器内で混色されることによる色味の変化を長期にわたって防止することができる。これにより、さらにクリーナレスの簡単な構成を可能とし、また、回収したトナーを再利用して廃トナーをださない環境対応が可能となる。
【図面の簡単な説明】
【図１】実施の形態１の画像形成装置の概略構成を示す縦断面図。
【図２】導電性ローラの抵抗測定方法を示す図。
【図３】実施の形態１における第１の実験の一次転写性と逆転写性とを示す図。
【図４】実施の形態１における第２の実験の一次転写性と逆転写性とを示す図。
【図５】（ａ）は、電圧印加部材として導電性ローラを使用した例を示す図。
（ｂ）は、電圧印加部材として導電性ブレードを使用した例を示す図。
（ｃ）は、電圧印加部材として導電性ブラシを使用した例を示す図。
【図６】実施の形態２の、表面接触幅、裏面接触幅を説明する図。
【図７】実施の形態２において、表面側接触部と裏面側接触部との位置関係を説明する図。
【図８】実施の形態３において、中間転写ベルトの抵抗値を変化させた場合の、逆転写性や画像品質の変化を示す図。
【図９】従来の画像形成装置の概略構成を示す縦断面図。
【符号の説明】
１ 第１の像担持体（感光ドラム）
６ 第２の像担持体（中間転写ベルト）
７ 電圧印加部材
７Ａ 電圧印加部材（導電性ローラ）
７Ｂ 電圧印加部材（導電性ブレード）
７Ｃ 電圧印加部材（導電性ブラシ）
７ａ〜７ｄ バイアス電源
Ｐ 第３の像担持体（転写材）
Ｓ１ 第１の像担持体と第２の像担持体との表面側接触部
Ｓ２ 第２の像担持体と電圧印加部材との裏面側接触部
Ｗ１ 表面接触幅
Ｗ２ 裏面接触幅
Ｙ、Ｍ、Ｃ、Ｂｋ第１〜第４の画像形成部
ａ₁ 表面側接触部の上流側の境界線[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine or a print using an intermediate transfer member.
[0002]
[Prior art]
FIG. 9 shows an electrophotographic four-color full-color image forming apparatus using an intermediate transfer belt (intermediate transfer member). A medium-resistance elastic body is used as the intermediate transfer belt 20.
[0003]
The image forming apparatus includes a photosensitive drum (first image carrier) 1, and this photosensitive drum 1 is rotationally driven in the direction of arrow R1 at a predetermined peripheral speed (process speed).
[0004]
In this rotation process, the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by the primary charger 2 and then subjected to image exposure by the exposure device 3. Thereby, an electrostatic latent image corresponding to the first color component image (for example, yellow color component image) of the target color image is formed.
[0005]
Next, the electrostatic latent image is developed at the development position by the first developing device (yellow developing device) 41 and visualized as a yellow toner image. At this time, since the second to fourth developing units, that is, the magenta developing unit 42, the cyan developing unit 43, and the black developing unit 44 are not operated and do not act on the photosensitive drum 1, the yellow toner image has the second to fourth. The developing devices 42 to 44 are not affected. The first to fourth developing devices 41 to 44 are mounted on a rotatable rotary 40, and are sequentially moved to a developing position facing the photosensitive drum 1 by the rotation of the rotary 40.
[0006]
The intermediate transfer belt 20 is stretched around rollers 61, 64, and 25, and is driven to rotate at the same direction and the same peripheral speed as the surface of the photosensitive drum 1 at the primary transfer nip portion (contact portion) between the intermediate transfer belt 20 and the photosensitive drum 1. Is done. A primary transfer roller 25 is disposed at an inner position of the intermediate transfer belt 20 in the primary transfer nip portion. The intermediate transfer belt 20 is applied with a primary transfer bias by a bias power source 29 via a primary transfer roller 25. The primary transfer bias has a polarity opposite to that of the toner and is, for example, in the range of +100 V to +2 kV.
[0007]
The yellow toner image formed on the photosensitive drum 1 is formed by the primary transfer bias applied from the primary transfer roller 25 to the intermediate transfer belt 20 in the process of passing through the contact nip portion between the photosensitive drum 1 and the intermediate transfer belt 20. By the primary transfer electric field, the primary transfer is sequentially performed on the outer peripheral surface of the intermediate transfer belt 20.
[0008]
After the transfer of the first color yellow toner image to the intermediate transfer belt 20, the photosensitive drum 1 is cleaned and removed by the cleaning device 13 from the primary transfer residual toner remaining on the surface, and then the next image starting from the primary charging. Subject to the forming process.
[0009]
Similarly, a magenta toner image of the second color, a cyan toner image of the third color, and a black toner image of the fourth color are formed, and are sequentially transferred onto the intermediate transfer belt 20 and primarily transferred to obtain the target color. A composite color image corresponding to the image is obtained.
[0010]
The roller 64 that supports the intermediate transfer belt 20 is a secondary transfer counter roller, and a secondary transfer roller 63 is disposed so as to be able to come into contact with the outer peripheral surface of the intermediate transfer belt 20 where the roller 64 is disposed. Yes. A secondary charging bias is applied to the secondary transfer roller 63 from a bias power source 28. The secondary transfer roller 63 can be separated from the intermediate transfer belt 20 during the primary transfer process of the first to third color toner images.
[0011]
At the timing when the four-color toner images superimposed and transferred on the intermediate transfer belt 20 come close to the secondary transfer nip portion by the rotation of the intermediate transfer belt 20, the secondary transfer bias is applied to the secondary transfer roller 63 from the bias power supply 28. At the same time, the secondary transfer roller 63 is brought into contact with the intermediate transfer belt 20. Further, the transfer material P is fed to the secondary transfer nip portion at a predetermined timing by the paper feed roller 11 and fed through the guide 10.
[0012]
The four color toner images on the intermediate transfer belt 20 are transferred by the secondary transfer bias applied to the intermediate transfer belt 20 from the secondary transfer roller 63 when the transfer material P passes through the secondary transfer nip portion. Secondary transfer is performed collectively on the surface of the transfer material P by the electric field.
[0013]
The transfer material P after the secondary transfer of the toner image is introduced into the fixing device 15 where the four colors of toner are melted and mixed and fixed to the transfer material P by heating and pressurization to form a full color image. The
[0014]
On the other hand, the secondary transfer residual toner remaining on the surface of the intermediate transfer belt 20 after the secondary transfer of the toner image is charged by the belt cleaner 8 with a polarity opposite to that of the photosensitive drum 1. The belt cleaner 8 is a roller that is disposed on the outer peripheral surface of the intermediate transfer belt 20 so as to be able to contact and separate. The belt cleaner 8 has a conductive roller 7 disposed inside the intermediate transfer belt 20 and grounded as a counter electrode, and a cleaning bias having a predetermined polarity is applied by a bias power source 26, whereby secondary transfer residual toner is predetermined. It is charged to the polarity. For example, since the photosensitive drum 1 is negatively charged, the secondary transfer residual toner is charged positively. The belt cleaner 8 can be separated from the intermediate transfer belt 20 during the primary transfer process of the first to third color toner images.
[0015]
The secondary transfer residual toner charged to the opposite polarity on the intermediate transfer belt 20 is electrostatically attracted and transferred to the photosensitive drum 1 at the primary transfer nip portion of the intermediate transfer belt 20 and removed from the intermediate transfer belt 20. Is done.
[0016]
The color image forming apparatus using the intermediate transfer belt 20 described above is, for example, a color image forming apparatus described in Japanese Patent Application Laid-Open No. 63-301960, that is, carrying a transfer material around a transfer drum. Compared to an image forming apparatus that obtains a color image by directly transferring a toner image of each color from a photosensitive drum to a transfer material, no control is performed on the transfer material (for example, control of gripping the transfer material on a gripper of the transfer drum. The toner image can be transferred from the intermediate transfer belt 20 to the transfer material P without performing the control of adsorbing the transfer material on the surface) or giving the transfer material a curvature along the surface of the transfer drum. Therefore, envelopes, postcards, label paper, etc., 40 g / m ² 200 g / m from thin paper ² The advantage that a toner image can be transferred and a color image can be obtained regardless of the thickness, width and length of the paper up to a thick paper, that is, transfer materials having various properties can be used. Has the advantage.
[0017]
However, the above-described image forming apparatus shown in FIG. 9 has a drawback that it is difficult to increase the speed of image formation because four color toner images must be sequentially formed on one photosensitive drum.
[0018]
In order to eliminate this drawback, a plurality of image forming units (first to fourth image forming units in the case of four full colors) are arranged along the intermediate transfer belt, and the first to fourth image forming units are arranged. There has been proposed an image forming apparatus of a system (hereinafter referred to as “in-line system”) in which toner images of the first to fourth colors formed on the respective photosensitive drums are sequentially transferred onto an intermediate transfer belt.
[0019]
[Problems to be solved by the invention]
According to the above-described inline type image forming apparatus, transfer materials having various properties can be used, and there is an advantage that it is possible to cope with higher speed.
[0020]
However, this in-line type image forming apparatus requires a first to fourth image forming unit each having a photosensitive drum, a developing device, a transfer device, a cleaning device, and other process devices, so that the configuration is complicated. There is a drawback that it tends to be.
[0021]
In order to eliminate such drawbacks, a so-called cleanerless image forming apparatus has been proposed in which a cleaning device for removing transfer residual toner remaining on the surface of the photosensitive drum is eliminated. According to the cleanerless image forming apparatus, the collected transfer residual toner can be reused. Therefore, the running cost can be reduced, and waste toner is not generated, which is effective for environmental problems.
[0022]
However, according to the above-described cleanerless image forming apparatus, the first to fourth color toner images formed on the respective photosensitive drums of the first to fourth image forming units are sequentially placed on the intermediate transfer belt. When transferring the toner image from the photosensitive drum onto the intermediate transfer belt, the toner image already transferred onto the intermediate transfer belt is transferred from the intermediate transfer belt onto the photosensitive drum. (Reverse transcription). For example, when the second color toner image formed on the photosensitive drum of the second image forming unit is transferred to the intermediate transfer belt, the first color toner image is already transferred onto the intermediate transfer belt. Therefore, the first color toner is reversely transferred onto the photosensitive drum of the second image forming unit. Similarly, the first color toner and the second color toner are reversely transferred to the photosensitive drum of the third image forming unit, and the first to third color toners are transferred to the photosensitive drum of the fourth image forming unit. The image is reverse transferred. When such reverse transfer occurs, in the other image forming units except the first image forming unit, the reversely transferred toner images of other colors are transferred to the developing device that collects the transfer residual toner on the photosensitive drum. There arises a problem that the color of the toner is changed by mixing.
[0023]
The present invention has been made in view of the above circumstances,
(1) Transfer materials with various properties can be used.
(2) While being an image forming apparatus capable of performing image formation at high speed,
(3) A dedicated cleaning device is not required and the configuration is simple.
(4) The transfer residual toner can be reused to reduce running costs.
(5) No change in color due to toner color mixture,
An object of the present invention is to provide an image forming apparatus.
[0024]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 A plurality of image bearing members that carry toner images; a rotatable intermediate transfer member; and a plurality of voltage application members that form a plurality of primary transfer nip portions together with the image carrier via the intermediate transfer member. The toner image is transferred from the image carrier to the intermediate transfer member at the primary transfer nip portion. In the image forming apparatus The statue Carrier And the intermediate transfer member The contact part with the surface side contact part, Intermediate transfer body And the contact portion between the voltage application member and the back side contact portion, The width of the front surface side contact portion in the rotation direction of the intermediate transfer member is the width of the back surface side contact portion in the rotation direction of the intermediate transfer member. Forming the front surface side contact portion and the back surface side contact portion to be smaller than Then, the boundary line on the upstream side in the rotation direction of the intermediate transfer body of the back surface side contact portion extends over the entire direction orthogonal to the rotation direction of the intermediate transfer body, and the rotation of the intermediate transfer body relative to the front surface side contact portion. Formed on the upstream side It is characterized by that.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0029]
<Embodiment 1>
FIG. 1 shows an example of an image forming apparatus according to the present invention. The image forming apparatus shown in the figure is a four-color full-color laser printer using an electrophotographic process, and the figure is a longitudinal sectional view showing a schematic configuration thereof.
[0030]
First, an outline of the image forming apparatus will be described with reference to FIG.
[0031]
The image forming apparatus shown in FIG. 1 includes four image forming units Y, M, C, and Bk that form toner images of yellow toner, magenta toner, cyan toner, and black toner, each of which has a negative normal charging polarity. These four image forming portions Y, M, C, and Bk are arranged in order from the upstream side in the moving direction (arrow R6 direction) of the intermediate transfer belt 6 as the second image carrier. Has been. That is, the image forming apparatus shown in FIG. 1 is provided on the image carrier of the first (yellow), second (magenta), third (cyan), and fourth (black) image forming units Y, M, C, and Bk. The yellow, magenta, cyan, and black toner images respectively formed on the toner image are sequentially primary-transferred and superimposed on the intermediate transfer belt 6, and then these toner images are transferred onto a transfer material P as a third image carrier. This is a four-drum type (in-line type) printer that obtains a color image by batch transfer.
[0032]
In FIG. 1, an endless intermediate transfer belt 6 is suspended by a driving roller 6a, a tension roller 6b, and a secondary transfer counter roller 6c, and in the direction of arrow R6 as the driving roller 6a rotates in the arrow direction. Is driven to rotate.
[0033]
The first to fourth image forming units Y, M, C, and Bk each include a drum-type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as a first image carrier. These photosensitive drums 1 are arranged on the surface side of the intermediate transfer belt 6 so as to be arranged along the moving direction thereof. Each photosensitive drum 1 is in contact with the surface of the intermediate transfer belt 6 to form a surface side contact portion S1.
[0034]
The photosensitive drum 1 of the first image forming unit Y is uniformly charged to a predetermined polarity and potential by the primary charging roller 2 during the rotation process, and then exposed to the exposure device 3 (color separation / imaging of a color original image). A first color component of a target color image by receiving an image exposure by an exposure optical system, a scanning exposure system using a laser scan that outputs a laser beam modulated in accordance with a time-series electric digital pixel signal of image information, etc. An electrostatic latent image corresponding to the image (yellow component image) is formed.
[0035]
Next, the electrostatic latent image is developed with yellow toner as the first color by the yellow developing device 4.
[0036]
The yellow toner image formed on the photosensitive drum 1 enters a primary transfer nip portion formed between the photosensitive drum 1 and the intermediate transfer belt 6. In the primary transfer nip portion, the voltage applying member 7 is brought into contact with the back surface side of the intermediate transfer belt 6. That is, the primary transfer nip portion is formed by sandwiching the intermediate transfer belt 6 between the photosensitive drum 1 disposed on the front surface side of the intermediate transfer belt 6 and the voltage applying member 7 disposed on the back surface side of the intermediate transfer belt 6. is doing. At this time, the contact portion between the photosensitive drum 1 and the surface of the intermediate transfer belt 6 in the primary transfer nip portion is the surface-side contact portion S1 described above, and the contact portion between the intermediate transfer belt 6 and the voltage applying member 7 is the back surface side. It is contact part S2. Each voltage applying member 7 is connected to a primary transfer bias power source 7a, 7b, 7c, 7d, respectively.
[0037]
At the primary transfer nip portion between the intermediate transfer belt 6 and the first image forming portion Y, first, the yellow toner image on the photosensitive drum is primarily transferred, and then the same process as described above is performed for each color. The toner images of magenta, cyan, and black are primarily transferred from the photosensitive drum 1 sequentially through the primary transfer nip portions so as to overlap each other. At this time, the transfer residual toner remaining on the photosensitive drum 1 is recharged to the normal charging polarity of the toner by the primary charging roller 2, and the recharged transfer residual toner is exposed based on the image information from above. In the developing unit, the toner is electrostatically collected in the developing device 4 simultaneously with the development of the next image, and the toner can be recycled. In addition, the transfer residual toner remaining on the photosensitive drum 1 is once collected by the primary transfer roller 2 and then on the photosensitive drum 1 during a paper interval (between the transfer material P and the transfer material P) or during preliminary running. After that, it is allowed to pass through the developing unit, return to the non-image area on the intermediate transfer belt 6 again, and collect in the cleaner device 9 disposed around the intermediate transfer belt 6. .
[0038]
On the other hand, conventionally, when the amount of retransfer toner is large, when a large number of images are formed (printed), a change in the color of the developer in the developing portion occurs. That is, conventionally, when the retransfer toner passes through the developing portion, the developer in the developing portion is in contact with the photosensitive drum 1, so that it physically contacts the developer in the developing portion regardless of the electric field condition. As a result, it is mixed into the developing section. As a result, if the amount of the retransfer toner is large, the color change of the developer in the developing portion occurs when a large number of prints are performed.
[0039]
The four-color toner images primarily transferred onto the intermediate transfer belt 6 are then collectively transferred to a transfer material P as a third image carrier by the secondary transfer roller 8 and melted by a fixing device (not shown). It is fixed. As a result, a four-color full-color image is formed.
[0040]
This completes the general description of the image forming apparatus.
[0041]
Next, each member of the image forming apparatus having the above configuration will be described in detail.
[0042]
The photosensitive drum 1 is a drum-shaped OPC (organic optical semiconductor) photosensitive member having an outer diameter of 30 mm.
[0043]
The intermediate transfer belt 6 is an endless belt having a circumferential length of 1115 mm, and has a width (width in the left-right direction toward the rotation direction) of 331 mm.
[0044]
The image forming conditions are briefly described below.

In the first experiment, the voltage application member 7 is a conductive roller having an outer diameter of 16 mm and an EPDM foam foam covered with a stainless steel shaft having an outer diameter of 8 mm. As the resistance value of the voltage applying member 7, the voltage V was measured by a method as shown in FIG. An aluminum cylinder 30 with a diameter of 30 mm is used as in the rotation zone, and the voltage application member 7 is pressed against this. At this time, both ends in the axial direction of the voltage applying member 7 are pressurized with 500 g, respectively. In this state, the aluminum cylinder 30 is rotationally driven to apply a voltage of 50 V while rotating the voltage applying member 7. At this time, the voltage x of the aluminum cylinder 30 is measured by the measurement tester 31 and
Resistance value R = 50 × 100 / x
To calculate the resistance value R. And as a voltage application member 7, the result is 1 × 10. ^Five A roller that is Ω or less was used. The hardness of the voltage application member 7 is 50 degrees in AskerC. As for the pressing of the voltage application member 7, as shown in FIG. 1, the voltage application member 7 is arranged inside the intermediate transfer belt 6, and 500 gf springs are arranged at both ends of the voltage application member 7, so that The transfer belt 6 was pressed against the photosensitive drum 1 on the front side.
[0045]
As the intermediate transfer belt 6, a single-layer polyimide belt was used. The intermediate transfer belt 7 has a thickness of 75 μm and a volume resistance value of 10 ^Ten Ω · cm. The volume resistance value is measured using HIRESTA (registered trademark) manufactured by Mitsubishi Yuka Co., Ltd., and is a measurement result when 500 V is applied. The intermediate transfer belt 6 performs resistance adjustment by dispersing carbon.
[0046]
Under such conditions, the front surface side contact portion S1 which is a contact portion between the photosensitive drum 1 and the intermediate transfer belt 6, and the contact portion between the intermediate transfer belt 6 and the voltage application member 7 is referred to as a back surface side contact portion S2. The width in the rotation direction (arrow R6 direction) of the intermediate transfer belt 6 with respect to the front surface side contact portion S1 and the back surface side contact portion S2 is referred to as a front surface contact width W1 and a back surface contact width W2, respectively. And measurement of these surface contact width W1 and back surface contact width W2 was performed. First, the dye is applied to the surface of the voltage applying member 7 and the photosensitive drum 1, the intermediate transfer belt 6 is pressed against the photosensitive drum 1 from the back side by the voltage applying member 7, and then the voltage applying member 7 is separated. By this operation, the width of the dye adhered on the intermediate transfer belt 6 was measured, and this was defined as the front-side contact width W1 and the back-side contact width W2.
[0047]
The measurement result is
Surface contact width W1: 1 mm between the photosensitive drum 1 and the intermediate transfer belt 6
Back surface contact width W2: 1 mm between the intermediate transfer belt 6 and the voltage application member 7
Met.
[0048]
The present applicants measured the reverse transcription phenomenon by the following method under such apparatus configuration conditions.
[0049]
Referring to FIG. 1, first, in the first image forming unit Y, a yellow toner image as the first color component is formed, and a negative polarity yellow toner image on the photosensitive drum 1 is applied with a voltage. In the process of primary transfer onto the intermediate transfer belt 6 by applying a positive polarity bias to the member 7 by the primary transfer bias power source 7a, the voltage of the primary transfer bias power source 7a is changed, so that the transfer property is the best. A bias value was obtained. The bias value was about 600V.
[0050]
Similarly, in the second image formation diagram M, the same experiment was performed for the magenta toner image as the second color component, and the optimum value was obtained.
[0051]
Next, a yellow toner image is formed on the photosensitive drum 1 in the first image forming diagram Y, and is primarily transferred to the intermediate transfer belt 6 at the optimum bias value obtained previously at the primary transfer nip portion, and then the second image is formed. In the image forming unit M, a magenta toner image is not formed, and in the primary transfer nip portion, the bias is changed by the primary transfer bias power source 7b to determine how much yellow toner on the intermediate transfer belt 6 is. It was investigated whether reverse transfer was performed on the photosensitive drum 1 of the forming portion M.
[0052]
The above results are shown in FIG. In this case, the primary transfer property indicates whether the primary transfer property of the magenta toner is good or bad when the second image forming unit M forms a solid image (Y + M) of RED (red). Also, reverse transferability means that in the second image forming unit M, the yellow solid image transferred on the intermediate transfer belt 6 is a magenta toner image on the photosensitive drum 1 of the second image forming unit M. It shows how much reverse transcription was performed on the part without the mark. Further details will be described.
[0053]
The horizontal axis in FIG. 3 is the primary transfer bias. Also, the vertical axis represents the primary transfer residual toner on the photosensitive drum 1 and the toner reversely transferred to the photosensitive drum 1, collected on a tape, pasted on paper, and measured only with an optical densitometer. The measured value of the optical densitometer when pasted is subtracted. The optical density value in this method is substantially proportional to the primary transfer residual toner weight and reverse transfer toner weight, and this method directly measures the primary transfer residual toner weight and reverse transfer toner weight with a chemical balance or the like. In comparison, it has an advantage that a small amount of toner can be measured easily. The smaller the optical density value, the better the primary transfer or the reverse transfer is prevented. For example, the optical density value 0.1 is about 0.01 mg / cm of the weight of the primary transfer residual toner or the reverse transfer toner. ² It corresponds to. According to the experiments by the present applicants, if the optical density on the vertical axis in FIG. 3 is about 0.10 or less, the durable number of the developing device 4 can be obtained at a printing rate of 5% for each of the colors Y, M, C, and Bk. A 40,000 sheet passing test (hereinafter referred to as “endurance test”) was conducted, but contamination of the developing device 4 did not cause a problem.
[0054]
From the results shown in FIG. 3, it can be seen that the bias conditions for achieving optimum primary transfer do not match the bias conditions for preventing reverse transfer, and there is no coexistence condition. In the case where the bias conditions are determined by giving priority to the optimum primary transfer, when a large number of images are formed, the reverse transfer amount is large, so that the contamination of the charging roller 2 and the yellow toner are caused by the second magenta image forming portion M. The magenta developer 4 was mixed in and the color change occurred. In addition, when bias conditions were determined with priority given to prevention of reverse transfer, charging performance deteriorated due to contamination of the charging roller 2 in the durability test.
[0055]
In the second experiment, the same experiment was performed using a softer conductive roller as the voltage application member 7.
[0056]
The conductive roller used was a roller having an outer diameter of 16 mm, with a stainless steel shaft having an outer diameter of 8 mm covered with an ether-based foamed foam. The resistance value is obtained by measuring the voltage x by the method shown in FIG.
Resistance value R = 50 × 100 / x
To calculate the resistance value, and the result is 1 × 10 ^Five A conductive roller having a resistance of Ω or less was used. Since the roller hardness was soft, it was measured by the following method (at normal temperature and normal humidity).
[0057]
A rectangular plate (50 mm × 10 mm × 1 mm) is pressed against the surface of the roller so that the longitudinal direction thereof coincides with the longitudinal direction of the roller, and the 1 mm roller is crushed in 3 seconds. And 60 seconds later, the repulsive force of the foam material of the roller concerning a board | plate material was measured with the strain gauge by Kyowa Co., Ltd. The measured value was around 300 gf. Regarding the pressing of the roller at the time of mounting, 500 gf springs were arranged at both ends of the roller, and the roller was pressed against the photosensitive drum 1 from the back side through the intermediate transfer belt 6.
[0058]
The used intermediate transfer belt 6 is a single-layer polyimide belt similar to that in the first experiment described above, and has a thickness of 75 μm and a volume resistance of 10 ^Ten Ω · cm. The volume resistance value is measured using HIRESTA manufactured by Mitsubishi Oil Chemical Co., Ltd., and is the result when 500 V is applied (at normal temperature and humidity).
[0059]
Under such conditions, the surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6 and the back surface contact width W2 between the intermediate transfer belt 6 and the voltage applying member 7 were measured. First, dye is applied to the surfaces of the voltage applying member 7 and the photosensitive drum 1, the voltage applying member 7 is pressed against the photosensitive drum 1 through the intermediate transfer belt 6, and then separated. By this operation, the width of the dye adhered on the intermediate transfer belt 6 was measured and set as the front surface contact width W1 and the back surface contact width W2.
[0060]
The measurement result is
Surface contact width W1: 1 mm between the photosensitive drum 1 and the intermediate transfer belt 6
Back surface contact width W2 between the intermediate transfer belt 6 and the voltage application member 7: 4 mm
Met.
[0061]
The present applicants measured the reverse transcription phenomenon again by the following method under such apparatus configuration conditions.
[0062]
First, in the first image forming portion Y, a yellow toner image is formed on the photosensitive drum 1, and this negative polarity yellow toner image is applied to the voltage applying member 7 by the primary transfer bias power source 7a. In the process of primary transfer on the intermediate transfer belt 6 by application, the voltage of the primary transfer bias power source 7a was changed to obtain a bias value that provides the best primary transfer performance. The bias value was about 600V. Similarly, in the second image forming unit M, a similar experiment was performed for a magenta toner image to obtain an optimum value.
[0063]
Next, a yellow toner image is formed on the photosensitive drum 1 in the first image forming portion Y, and the optimum bias value obtained previously is applied to the voltage applying member 7 in the primary transfer nip portion to apply the photosensitive drum 1. The upper yellow toner image is primarily transferred onto the intermediate transfer belt 6. In the second image forming unit M, a magenta toner image is not formed, but the bias in the primary transfer nip unit is changed, and the amount of yellow toner on the intermediate transfer belt 6 is determined by the sensitivity of the second image forming unit M. It was investigated whether reverse transfer was performed on the drum 1. The above results are shown in FIG.
[0064]
From the results shown in FIG. 4, it can be seen that the reverse transferability is remarkably changed. Under this condition, there is a region where the bias condition for achieving the optimum primary transfer condition and the bias condition for preventing the reverse transfer are compatible. From this result, the present applicants are concerned with the relationship between the surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6 and the back surface contact width W2 between the intermediate transfer belt 6 and the voltage application member 7. It was confirmed. The durability test was performed under these conditions. However, the phenomenon that yellow toner was mixed in the magenta developing device 4 of the second image forming unit M did not occur so much as to be a problem, and the color change was not a problem. . Further, the charging performance of the charging roller 2 was not deteriorated.
[0065]
That is, the surface contact width between the photosensitive drum 1 and the intermediate transfer belt 6 is W1, and the back surface contact width W2 between the intermediate transfer belt 6 and the voltage applying member 7 is
W1 <W2
It was found that reverse transcription can be suppressed when
[0066]
In addition to the sponge conductive roller 7A used in the above-described experiment as shown in FIG. 5A, as the voltage application member 7, a conductive sheet 7B as shown in FIG. The same effect was obtained with a rubber conductive blade. Moreover, the same effect was acquired even if it used the conductive brush 7C which formed the conductive chemical fiber and the metal as shown in FIG.5 (c) in the shape of a brush.
[0067]
The present applicant considers the reason why the above-described effect is obtained as follows.
[0068]
The reverse transfer mechanism will be described, for example, by the second image forming unit M. In the process of primary transfer of magenta toner formed on the photosensitive drum 1 onto the intermediate transfer belt 6, the photosensitive drum 1 and the intermediate transfer belt 6 are used. When the photosensitive drum 1 and the intermediate transfer belt 6 are moved in the gap near the separation portion, the electrostatic capacity is drastically reduced and peeling discharge is generated. The electric charge generated by this discharge moves along the direction of the electric field applied to this gap. In the present embodiment, the toner polarity is negative, and a positive voltage is applied to the voltage application member 7, so that the positive charge generated by the discharge moves to the surface of the photosensitive drum 1 and is generated by the discharge. The negative charge thus moved moves to the surface of the intermediate transfer belt 6. However, when there is yellow toner transferred by the first image forming unit Y on the intermediate transfer belt 6, the toner has a negative polarity, and the positive charge generated by the discharge is slightly transferred to the toner surface. Will occur. As a result, the polarity of the yellow toner is partially reversed and transferred to the photosensitive drum 1 of the second image forming unit M. Thus, the present applicants consider a reverse transcription mechanism.
[0069]
And the present applicants consider the effect of embodiment as follows.
[0070]
According to the analysis by the present applicants, when the back surface contact width W2 between the intermediate transfer belt 6 and the voltage applying member 7 is larger than the front surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6, the intermediate transfer belt. Since there is a conductive roller as a voltage applying member 7 on the back side through the intermediate transfer belt 6 with respect to the gap between the photosensitive drum 1 and the photosensitive drum 1, the surface side contact formed by the photosensitive drum 1 and the intermediate transfer belt 6 When discharge occurs in the gap on the upstream side of the portion S1, the toner on the intermediate transfer belt 6 has a negative charge amount due to the negative charge generated by the discharge. On the other hand, on the photosensitive drum 1 side, the charging potential is lowered due to the positive charge generated by the discharge. Due to this action, discharge in the surface side contact portion S1 between the photosensitive drum 1 and the intermediate transfer belt 6 and the gap on the downstream side of the surface side contact portion S1 is suppressed, or even if discharge occurs, Since the toner on the transfer belt 6 holds a high negative charge amount, the reverse transfer as described above is unlikely to occur.
[0071]
As described above, the relationship between the surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6 and the back surface contact width W2 between the intermediate transfer belt 6 and the voltage applying member 7 is such that the relationship of W1 <W2 is satisfied (FIG. 5). The voltage application member 7 (conductive roller 7A, conductive blade 7B, conductive brush 7C) shown in a) to (c) is incorporated in the image forming apparatus shown in FIG. 1, and a durability test is performed in a four-color full-color mode. As a result, the color change of the developing device 4 and the charging performance of the charging roller 2 did not occur, and a good image could be obtained.
[0072]
<Embodiment 2>
FIG. 6 shows this embodiment. In the present embodiment, the back surface contact width W2 is divided into an upstream side back surface contact width W3 and a downstream side back surface contact width W4 with reference to the center A of the surface contact width W1. The method was analyzed.
[0073]
As the voltage application member 7, a conductive brush 7C in which a conductive rayon brush (width 20 mm × length 300 mm) is arranged on a resin plate (width 20 mm × length 300 mm × thickness 2 mm) was used. Brush length is 5mm and overall resistance is 10 ^Five It was as follows.
[0074]
The used intermediate transfer belt 6 is the same belt as that of the first embodiment, and has a thickness of 75 μm and a volume resistivity of 10. ^Ten This is an endless belt made of polyimide of Ω · cm. The volume resistance value is measured using HIRESTA (registered trademark) manufactured by Mitsubishi Yuka Co., Ltd., and is the result when 500 V is applied.
[0075]
The arrangement of the conductive brush 7 </ b> C in the width direction (the rotation direction of the intermediate transfer belt 6) is symmetrical with respect to a vertical line passing through the center of the photosensitive drum 1. The intermediate transfer belt 6 was pressed from the back side thereof with the conductive brush 7C, and the front side of the intermediate transfer belt 6 was pressed against the photosensitive drum 1. In this state, the conductive brush 7C was crushed by 2 mm in the thickness direction.
[0076]
Under this condition, the surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6 was measured. The measurement method is the same as the method described in the first embodiment. W3 + W4 corresponds to the back surface contact width W2, that is, the width of the conductive brush 7C. The results were W1: 2 mm, W3, W4: 10 mm.
[0077]
Next, as shown in FIG. 7, the conductive brush 7 </ b> C was rotated 10 degrees around the center in the longitudinal direction. The purpose of tilting is to confirm the influence of W3 and W4 by a simple method.
[0078]
The present applicants measured the reverse transcription phenomenon again by the following method under such apparatus configuration conditions.
[0079]
Similarly, using FIG.
First, a yellow toner image is formed on the photosensitive drum 1 of the first image forming unit Y, and the negative polarity yellow toner image on the photosensitive drum 1 is added to the conductive brush 7c by the primary transfer bias power source 7a. In the process of primary transfer onto the intermediate transfer belt 6 by applying a polar bias, the bias value with the best transferability was obtained by changing the voltage of the primary transfer bias power source 7a. The bias value was about 500V. In the above-described first embodiment, the reason is that the voltage is about 600V, whereas in this embodiment, the voltage is 500V, which is 100V lower than this. The surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6 and the intermediate It is considered that both the back surface contact width W2 (= W3 + W4) between the transfer belt 6 and the conductive brush 7C are larger than those in the first embodiment.
[0080]
Similarly, a similar experiment was performed for a magenta toner image in the second image forming unit M, and an optimum value was obtained.
[0081]
Next, a yellow toner image is formed on the photosensitive drum 1 in the first image forming unit Y, and the yellow toner image is primary-coated on the intermediate transfer belt 6 at the primary transfer nip portion with the optimum bias value obtained previously. In the second image forming unit M, a magenta toner image is not formed, but a bias is applied in the primary transfer nip, and how the yellow toner on the intermediate transfer belt 6 is transferred to the second image forming unit M. It was investigated whether reverse transfer was performed on the M photosensitive drum 1.
[0082]
The above results will be described with reference to FIG. Here, in the figure, the boundary line on the upstream side of the surface side contact portion S1 between the photosensitive drum 1 and the intermediate transfer belt 6 is a. ₁ B for the downstream boundary ₁ In addition, the boundary line on the upstream side of the back surface side contact portion S2 between the intermediate transfer belt 6 and the conductive brush 7C is a. ₂ B for the downstream boundary ₂ And P1 is the boundary line a ₁ And border b ₂ , P2 is the boundary line a ₁ And boundary a ₂ , P3 is the boundary line b ₁ And boundary a ₂ The intersection with is shown.
[0083]
Regarding the above-described P1, P2, and P3, reverse transfer is very well prevented in the region between P1 and P2, while reverse transfer occurs in the region between P2 and P3.
[0084]
That is, the entire front surface side contact portion S1 does not necessarily have to be covered by the back surface side contact portion S2, and the upstream boundary line a of the front surface side contact portion S1. ₁ Is within the range of the back surface side contact portion S2 (the boundary line a on the upstream side of the back surface side contact portion S2 ₂ And downstream boundary b ₂ It was found that the reverse transcription was suppressed as long as it was placed between the two. In other words, the boundary line a on the upstream side of the surface contact portion S1 in the back surface side contact portion S2. ₁ Reverse transfer can be prevented well at the portion crossing the line.
[0085]
Actually, the conductive brush 7C is incorporated in the image forming apparatus shown in FIG. 1, and the back surface side contact portion S2 between the intermediate transfer belt 6 and the conductive brush 7C is in contact with the surface side of the photosensitive drum 1 and the intermediate transfer belt 6. Boundary line a on the upstream side of section S1 ₁ Border line b on the downstream side ₁ When the durability test shown in the first embodiment was performed after the arrangement so as not to straddle the image, no change in the tint of the developing device 4 or a decrease in the charging performance of the charging roller 2 occurred, and a good image was obtained. I was able to.
[0086]
Instead of the conductive brush 7C described above, a conductive roller 7A, a conductive sheet, or a conductive blade 7B as shown in FIG. 5 is used, and the back surface side contact portion S2 is located upstream of the front surface side contact portion S1. Boundary line a ₁ By setting the relative positions of these members and the photosensitive drum 1 so as to cover only the above, it was possible to obtain substantially the same effect as described above.
[0087]
<Embodiment 3>
Next, the surface contact width W1 of the surface side contact portion S1 between the photosensitive drum 1 and the intermediate transfer belt 6, the back surface contact width W2 of the back surface side contact portion S2 between the intermediate transfer belt 6 and the voltage application member 7, and the intermediate transfer. The relationship with the resistance value of the belt 6 was examined.
[0088]
The intermediate transfer belt 6 was a polyimide single layer belt having a thickness of 75 μm.
[0089]
Regarding the volume resistance value of the intermediate transfer belt 6, the following nine types of intermediate transfer belts 6 having different values were used.
[0090]
The volume resistance value is 1 × 10 ⁶ 1 × 10 ⁷ 1 × 10 ⁸ 1 × 10 ⁹ 1 × 10 ^Ten 1 × 10 ¹¹ 1 × 10 ¹² 1 × 10 ¹³ 1 × 10 ¹⁴ Ω · cm.
[0091]
As the voltage application member 7, a conductive roller 7A was used. A stainless steel shaft having an outer diameter of 8 mm was covered with an ether-based foamed foam to form a roller having an outer diameter of 16 mm. For the volume resistance value of the conductive roller 7A, the voltage x is measured by the method shown in FIG.
Resistance value R = 50 × 100 / x
To calculate the resistance value R, and the result is 1 × 10 ^Five A roller that is Ω or less was used. Since the roller hardness was soft, it was measured by the following method.
[0092]
A rectangular plate (20 mm × 10 mm × 1 mm) is pressed against the roller surface so that the longitudinal direction thereof coincides with the longitudinal direction of the roller, and the roller is infiltrated so that the 1 mm roller is crushed in 3 seconds. And 60 seconds later, the repulsive force of the foam material of the roller concerning a board | plate material was measured with the strain gauge by Kyowa Co., Ltd. The measured value was around 300 gf. Regarding the pressing of the roller at the time of mounting, 500 gf springs were arranged at both ends of the roller, and the roller was pressed against the photosensitive drum 1 from the back side through the intermediate transfer belt 6.
[0093]
The surface contact width W1 between the photosensitive drum 1 and the intermediate transfer belt 6 and the back surface contact width W2 between the intermediate transfer belt 6 and the conductive roller 7C are measured by the same method as in the first embodiment, and the result Is
Surface contact width W1: 1 mm between the photosensitive drum 1 and the intermediate transfer belt 6
Back surface contact width W2 of the intermediate transfer belt 6 and the conductive roller 7C: 4 mm
Met.
[0094]
With the intermediate transfer belt 6 of each volume resistance value, the applicants measure the reverse transfer phenomenon by the same method as in the first embodiment, and also perform the endurance test as shown in the first embodiment. The quality of the photo was examined. At the same time, the image quality was evaluated. The results are shown in FIG.
[0095]
As shown in FIG. 8, with each intermediate transfer belt 6 used in the experiment, a good result was obtained for the reverse transfer property, but the image quality was 1 × 10. ¹³ In the intermediate transfer belt 6 having a resistance value of Ω · cm or more, an appropriate primary transfer bias setting is a high voltage of 1 kV or more, and the upstream side or the downstream side of the surface contact portion S1 between the photosensitive drum 1 and the intermediate transfer belt 6. The transfer was started in the gap, and an image defect occurred in which the image was blurred or a part of the image was lost. 1 × 10 ⁶ Since the resistance value of the intermediate transfer belt 6 of Ω · cm is low, inflow and outflow of current between the first to fourth image forming portions Y, M, C, and Bk of FIG. This causes a problem that different voltages cannot be set independently for each color, and it has not been possible to set the voltage to obtain a good image quality. Further, with regard to scattering (a phenomenon in which toner is scattered around the line image of characters and the like), 1 × 10 ⁹ Particularly good at Ω · cm or higher.
[0096]
Therefore, the resistance value of the intermediate transfer belt 6 that provides a good image and has no problem in the apparatus configuration is 1 × 10. ⁷ ~ 1x10 ¹² The range was Ω · cm.
[0097]
In the present invention, the intermediate transfer belt 6 is not limited to this. For example, polystyrene, styrene resin (chloropolystyrene, styrene-butadiene copolymer, etc.), methyl methacrylate resin, ethyl acrylate resin, modified Acrylic resin (silicone-modified acrylic resin, acrylic / urethane resin, etc.), vinyl chloride resin, phenol resin, epoxy resin, polyester resin, polyester polyurethane resin, polyethylene, polypropylene, polybutadiene, polyvinylidene chloride, polyurethane resin, silicone resin, fluorine resin , Modified polyphenylene oxide resin, polyimide resin, PES and the like. One or more of these can be used.
[0098]
As a method for forming the coating layer, any method such as spray coating, dip coating, electrostatic coating, and extrusion molding can be used.
[0099]
In order to adjust the resistance value of the intermediate transfer belt 6 to a desired value, a conductive agent can be added to the elastic layer or the coating layer as necessary. The conductive agent is not particularly limited, but for example, carbon, metal powder such as aluminum and nickel, metal oxide such as titanium oxide, quaternary ammonium salt-containing polymethyl methacrylate, polyvinyl aniline, polyvinyl pyrrole, Examples thereof include conductive polymer compounds such as polydiacetylene, polyethyleneimine, boron-containing polymer compounds, and polypyrrole. These 1 type (s) or 2 or more types can be used.
[0100]
Needless to say, when the above-described intermediate transfer belt 6 is used, reverse transfer is prevented, the color of the developer in the developing device 4 is prevented from changing, and a good image is obtained.
[0101]
In the first to third embodiments, the front surface contact width W1 and the back surface contact width W2 are measured when the photosensitive drum 1, the intermediate transfer belt 6, and the primary charging roller 2 are stopped. Even when the above-described members are operating (rotating) for image formation, the relationship that the front surface contact width W1 is smaller than the back surface contact width W2 is not changed.
[0102]
【The invention's effect】
As described above, according to the present invention, in the image forming apparatus configured by arranging a plurality of image forming units along the moving direction of the second image carrier, the first image carrier (for example, each image forming unit) The contact portion between the photosensitive drum) and the second image carrier (for example, the intermediate transfer belt) is the front surface side contact portion, and the contact portion between the second image carrier and the voltage application member is the back surface side contact portion. When the width in the moving direction of the second image carrier at the front surface side contact portion and the back surface side contact portion is the front surface contact width and the back surface contact width, respectively, the front surface contact width is smaller than the back surface contact width. By forming the front surface side contact portion and the back surface side contact portion, the first image carrier to the first image carrier that is generated in the transfer process of the toner image from the first image carrier to the second image carrier. Toner that has been reversely transferred is prevented by preventing reverse transfer of toner to the image carrier. There it is possible to prevent long-term changes in the color tone to be mixed in the developing device. As a result, a cleaner-less simple configuration is possible, and it is possible to cope with an environment in which the collected toner is reused to prevent waste toner.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a first embodiment.
FIG. 2 is a diagram showing a method for measuring resistance of a conductive roller.
3 is a diagram showing primary transfer properties and reverse transfer properties of a first experiment in Embodiment 1. FIG.
4 is a diagram showing primary transfer properties and reverse transfer properties of a second experiment according to Embodiment 1. FIG.
FIG. 5A is a diagram showing an example in which a conductive roller is used as a voltage application member.
(B) is a figure which shows the example which uses the electroconductive blade as a voltage application member.
(C) is a figure which shows the example which uses the electroconductive brush as a voltage application member.
6A and 6B illustrate a front surface contact width and a back surface contact width according to a second embodiment.
7 is a diagram for explaining the positional relationship between a front surface side contact portion and a back surface side contact portion in Embodiment 2. FIG.
FIG. 8 is a diagram showing changes in reverse transferability and image quality when the resistance value of the intermediate transfer belt is changed in the third embodiment.
FIG. 9 is a longitudinal sectional view showing a schematic configuration of a conventional image forming apparatus.
[Explanation of symbols]
1 First image carrier (photosensitive drum)
6 Second image carrier (intermediate transfer belt)
7 Voltage application member
7A Voltage application member (conductive roller)
7B Voltage application member (conductive blade)
7C Voltage application member (conductive brush)
7a-7d Bias power supply
P Third image carrier (transfer material)
S1 Surface-side contact portion between the first image carrier and the second image carrier
S2 Back side contact portion between second image carrier and voltage application member
W1 Surface contact width
W2 Back contact width
Y, M, C, Bk first to fourth image forming units
a ₁ Boundary on the upstream side of the surface side contact area

Claims (1)

A plurality of image bearing members that carry toner images; a rotatable intermediate transfer member; and a plurality of voltage application members that form a plurality of primary transfer nip portions together with the image carrier via the intermediate transfer member. In the image forming apparatus for transferring a toner image from the image carrier to the intermediate transfer member at the primary transfer nip portion ,
The contact portion between the image carrier and the intermediate transfer member is a surface side contact portion, and the contact portion between the intermediate transfer member and the voltage application member is a back surface contact portion, and the surface side in the rotation direction of the intermediate transfer member Forming the front surface side contact portion and the back surface side contact portion so that the width of the contact portion is smaller than the width of the back surface side contact portion in the rotation direction of the intermediate transfer member ,
The boundary line on the upstream side in the rotation direction of the intermediate transfer body of the back surface side contact portion extends over the whole area in the direction orthogonal to the rotation direction of the intermediate transfer body, and upstream in the rotation direction of the intermediate transfer body from the front surface contact portion. An image forming apparatus formed on the side .

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