JP4410319B2 - Image forming apparatus - Google Patents

Description

また、文字中抜けについては、文字画像を用いて目視で５段階評価を行ったものである。
【００３８】
これらの結果から、ソリッドゴムローラ（あるいは表面にスキン層を持ったローラ）の場合は４５度以下、発泡ゴムローラの場合は４０度以下の硬度で、それぞれ良好な転写性能が得られることが分かった。
【００３９】
一方、転写ローラ２の硬度が低過ぎると永久歪みが発生し易く、長期間放置したあとに印字を行うと、放置時にベルトと当接していた部分が歪んでローラ周期で転写抜けが発生する。
【００４０】
図３（Ａ）（Ｂ）は、硬度が異なる転写ローラを用いて、温度４０℃、湿度８５％の環境下に２４時間放置したあと、ベタ画像の印字を行って転写ローラ周期の転写抜けを測定した結果である。
【００４１】
なお、転写抜け率（％）は以下のように定義される。

上記画像濃度はマクベス濃度を基準としており、また、中抜け部が認識できない場合は０％とする。
【００４２】
この結果から、同図（Ａ）に示すソリッドゴムローラ（あるいは表面にスキン層を持ったローラ）の場合は、２０度以上の硬度であれば永久歪みによる転写抜けが発生しない。同図（Ｂ）に示す発泡ゴムローラを用いた場合は、１５度以上の硬度であれば、永久歪みによる転写抜けが発生しないことが分かった。
【００４３】
実際には、φ８mmの金属シャフト周面に発泡ウレタン（セル径５０μm 前後）材を取着形成し、ローラ直径１３mmφとした転写ローラ２を使用している。この転写ローラ２の硬度は、ＡＳＫＥＲ−Ｃで２３度である。
【００４４】
また、転写ローラ２の加圧力は、ローラの硬度や直径などにより異なるが、線圧で１８〜７５g/cmの範囲が良好である。加圧が弱過ぎると、安定した転写ニップ幅が形成されずに転写不良が発生してしまう。加圧が強過ぎると、文字の中抜けが発生し易い。
【００４５】
図４に、ローラ硬度２８度（ＡＳＫＥＲ−Ｃ）を用いて、ローラ加圧を振ったときの転写効率と中抜けの評価結果を示す。ここでは、感光体ニップ幅、転写ローラニップ幅ともに２．７mm〜３mm程度である。
【００４６】
以上説明したような条件で、感光体ドラム１に対して転写ベルト３と転写ローラ２を用いて用紙Ｐにトナー像の転写を行なうが、転写ローラ２の位置決め手段を何ら採用しない場合は、先に図２５で示したように、感光体ドラム１（Ｓ）の軸心Ｌｓに対して転写ローラ２（Ｒ）の軸心Ｌｒの平行度が出ない。
【００４７】
図５（Ａ）（Ｂ）に、感光体ドラム１の軸心Ｌｓに対して転写ローラ２の軸心Ｌｒの平行度が出ない状態で、転写バイアスを１３００Ｖ、転写ローラ加重線圧を２５〜１５０ｇ／cmの条件で振ったときの、ａ、ｂ、ｃ位置での転写効率と、中抜け率を測定した結果を示す。
【００４８】
ｂ位置は、ａ、ｃ位置より圧力が集中して高くなっているため、軽い荷重で転写が良好になるが、その一方で軽い荷重で圧力過多による中抜けが発生し易く、ａ、ｂ位置と適正条件が大きくシフトしている。よって、ａ、ｂ、ｃすべての位置で、良好な中抜けのない転写画像を得ることはできない。
【００４９】
感光体ニップ幅と転写ローラニップ幅のずれは、感光体ドラム１や転写ローラ２の偏心によっても発生するが、主に感光体ドラム１や転写ローラ２の組立誤差に起因するところが大である。
【００５０】
このような理由から、感光体ドラム１に対する転写ローラ２のベルト移動方向の位置決めをなす。すなわち、感光体ドラム１を装置本体に対して位置決めをする部材と、感光体ドラム１に対して転写ローラ２を位置決め保持する部材を必要とする。
【００５１】
図６（Ａ）（Ｂ）に、その具体的な実施の形態を示す。
感光体ドラム１のシャフト１Ｓは、リア側とフロント側ともに装置本体を構成する支持フレームＫに位置決めブロック２１を介して位置決めのうえ、回転自在に枢支される。
【００５２】
上記転写ローラ２のシャフト２Ｓは、リア側とフロント側ともに軸受け２２に回転自在に枢支される。この軸受２２は、断面コ字状に形成される転写ローラ支持部材２４の下片部上にスプリング２３を介して弾性的に支持される。
【００５３】
すなわち、軸受け２２はスプリング２３によって押し上げ付勢され、上記転写ローラ部材２４の上片部に押し付け保持される。転写ローラ支持部材２４は、転写ローラホルダ２５上に支持され、後述するように位置決めブロック２１に対して位置決めのうえ取付け固定される。
【００５４】
また、転写ローラホルダ２５には位置決め用溝２５Ｍが設けられていて、ここに上記転写ローラ支持部材２４の下片部に一体に突設される凸部２４Ｖが挿入される。
【００５５】
位置決め用溝２５Ｍに対する凸部２４Ｖは、上記転写ベルト３の走行方向（図中、白抜き矢印で示す）のみ余裕があり、この走行方向とは直交する方向では移動可能な状態でタイトである。
【００５６】
上記転写ローラ支持部材２４の上片部には、先端がテーパ状に形成されるガイドピン２４Ｔが突設される一方、位置決めブロック２１の下面部にはテーパ状のガイド孔部２１Ｎが設けられている。上記ガイドピン２４Ｔのテーパ角度は、上記位置決めブロック２１のガイド孔部２１Ｎのテーパ角度よりも小さく形成されている。
【００５７】
このように、転写ローラ支持部材２４にスプリング２３を介して転写ローラ２のシャフト２Ｓを枢支した軸受け２２を弾性的に支持し、転写ローラ支持部材２４を転写ローラホルダ２５上に載置して、転写ユニットＹを形成する。
【００５８】
勿論、転写ローラホルダ２５の位置決め用溝２５Ｍに、転写ローラ支持部材２４の凸部２４Ｖが挿入される。このとき、凸部２４Ｖである転写ローラ支持部材２４の位置は確定されていない。
【００５９】
そして、位置決めブロック２１に対して上記転写ユニットＹを下方から感光体ドラム１側に押し上げる。転写ローラ支持部材２４のガイドピン２４Ｔは、位置決めブロック２１に設けられるガイド孔部２１Ｎ内に挿入され、かつこのテーパに沿ってガイドされる。
【００６０】
そして、転写ローラ支持部材２４の凸部２４Ｖと転写ローラホルダ２５の位置決め用溝２５Ｍとの関係から、転写ローラ支持部材２４は転写ローラ２を弾性的に支持した状態のまま、転写ベルト３の走行方向に沿って移動して位置調整される。
【００６１】
ガイドピン２４Ｔの先端がガイド孔部２１Ｎの頂点部分に係合したとき、感光体ドラム１の軸心Ｌｓに対して転写ローラ２の軸心Ｌｒが完全に一致した状態となり位置決めされる。この位置を確保したうえ、図示しない取付け固定手段によって転写ユニットＹを固定保持する。
【００６２】
なお、ガイドピン２４Ｔの先端がガイド孔部２１Ｎに挿入する以前に、転写ローラ２が転写ベルト３を介して感光体ドラム１に当接すると、ガイドピン２４Ｔを介して転写ローラ支持部材２４の位置決めができない。
【００６３】
また、転写ローラホルダ２５の位置決め用溝２５Ｍに対する転写ローラ支持部材２４の凸部２４Ｖの移動可能範囲が広過ぎると、転写ユニットＹを押し上げた際にガイドピン２４Ｔがガイド孔部２１Ｎに挿入できず、ガイドピン２４Ｔが破損したり、位置決めブロック２１側を損傷する恐れがある。
【００６４】
そこで、以下の条件を満たす必要がある。
１）ローラ支持部材２４の移動可能範囲幅寸法（ｄ１＋ｄ２） ＜ ガイドピン２４Ｔの幅寸法（ｄ３）
２）ガイドピン２４Ｔ先端がガイド孔部２１Ｎに挿入するまでは、転写ローラ２は転写ベルト３を介して感光体ドラム１に当接しない寸法設定。
【００６５】
図７（Ａ）（Ｂ）に、上記感光体ドラム軸心Ｌｓに対する転写ローラ軸心Ｌｒの位置決め手段を構成した上で、転写ローラ２を加重線圧５０ｇ/cm としてバイアスを振ったときと、バイアス１３００Ｖで加圧条件を振ったときの、中抜け評価および転写効率の測定結果を示す。
【００６６】
先に説明したａ、ｂ、ｃ位置での適正電圧条件と、加圧条件は一致しており、ａ、ｂ、ｃ位置の全てで良好な転写画像が得られる広い条件が存在することがわかる。
【００６７】
なお、ここでは感光体ドラム軸心Ｌｓを支持フレームＫに対して位置決め取付けする部材（位置決めブロック２１）の一部（ガイド孔部２１Ｎ）と、転写ローラ支持部材２４の一部（ガイドピン２４Ｔ）を係合したが、感光体ドラムシャフト１Ｓに一体的に取付けた部材と、転写ローラシャフト２Ｓに一体的に取付けた部材相互を係合する構成であってもよい。
【００６８】
図８（Ａ）（Ｂ）に示すような、第２の実施の形態としての転写ローラ２の位置決め手段を採用してもよい。
感光体ドラム１のシャフト１Ｓの両端部は位置決めブロック２１Ａによって枢支されることと、転写ローラ２のシャフト２Ｓの両端部を枢支する軸受け２２はローラ支持部材２４にスプリング２３を介して弾性的に支持されること、およびローラ支持部材２４と、これを支持する転写ローラホルダ２５との構成は先に説明したとおりであるので、ここでは新たな説明は省略する。
【００６９】
上記位置決めブロック２１Ａとローラ支持部材２４との間には、基準ブロック２６が介在される。この基準ブロック２６の一側面には係合腕部２６ａが突設されていて、上記位置決めブロック２１Ａ側面に転写ベルト３の走行方向に沿って設けられるガイド溝２１ｍに移動自在に係合される。
【００７０】
また、基準ブロック２６の上面には、転写ベルト３の走行方向に沿って断面三角状のガイド溝２６ｂが設けられ、下面には円錐状のガイド孔部２６ｃが設けられる。
【００７１】
上記基準ブロック２６上面のガイド溝２６ｂには、位置決めブロック２１Ａに設けられるガイド杆２７が係合される。すなわち、感光体ドラム１のシャフト１Ｓ両端部が支持フレームＫの所定位置に取付けられた位置決めブロック２１に位置決めのうえ枢支されたあと、基準ブロック２６の係合腕部２６ａが上記位置決めブロック２１Ａ側面に設けられるガイド溝２１ｍに係合される。
【００７２】
この状態で、基準ブロック２６上面のガイド溝２６ｂには、位置決めブロック２１Ａに設けられるガイド杆２７が係合される。したがって、感光体ドラム１軸心に対する基準ブロック２６の位置決めがなされる。
【００７３】
つぎに、上記転写ユニットＹを下方から感光体ドラム１側に押し上げる。転写ローラ支持部材２４のガイドピン２４Ｔは、基準ブロック２６に設けられるガイド孔部２６ｃ内に挿入し、ガイドピン２４Ｔの先端がガイド孔部２６ｃの頂点部分に係合したとき、基準ブロック２６に対して転写ローラ２の位置決めがなされる。
【００７４】
結局、感光体ドラム１の軸心Ｌｓに対して転写ローラ２の軸心Ｌｒが基準ブロック２６を介して完全に一致した状態で位置決めされる。この状態を確保したうえ、図示しない取付け固定手段によって転写ユニットＹを固定保持する。
【００７５】
図９に、感光体ニップ幅より転写部材ニップ幅を小さくすべく、デジタルＰＰＣに適用した第３の実施の形態について説明する。基本的な構成および印字作用は、先に図１に示した装置と同様であり、同部品については同番号を付して新たな説明は省略する。
【００７６】
この種装置の傾向として、印字速度が遅く、かつ感光体ドラム１直径が小さい（速度１２５mm/sec ２０ppm 、感光体ドラム径φ４５mm）ので、転写部材として転写ブラシ装置２Ａを使用している。
【００７７】
すなわち、先に説明した装置では転写ローラニップ幅と感光体ニップ幅とがほぼ同じ（２．７〜３．０mm）であったが、より良好な画質が要求される高画質デジタルＰＰＣにおいては、転写部を構成する転写ローラニップ幅が感光体ニップ幅より上流に形成されると、前転写による文字の滲みなどが問題になる。
【００７８】
そこで、感光体ニップ幅より転写部材ニップ幅（先に説明した装置では転写ローラニップ幅として説明）を狭くし、転写部材ニップ幅を感光体ニップ幅内に収めることが要求される。
【００７９】
そこで、たとえば図１０に示すように、ベルト押し上げローラ４０を転写ローラ２の下流側に近接して配置し、転写ベルト３を感光体ドラム１一部に巻き掛けるような構成とすれば、感光体ニップ幅が広くなり、上記転写ローラ２をそのまま用いることができる。
【００８０】
しかしながら、このような構成では，転写ニップ幅前−転写ニップ幅中−転写ニップ幅後の用紙経路がＳ字状に曲がってしまう。そのため、たとえば葉書などの厚紙の場合では通紙性に問題が生じて、円滑な搬送と高精度の転写が損なわれる恐れがある。特に、４連タンデムのカラー印字装置に用いると、用紙経路が複数の複雑湾曲部分を備えることになるので、到底使用に耐えられない。
【００８１】
図１１は、転写部材ニップ幅を感光体ニップ幅中に収めるために、転写部材としてニップ幅を小さくするよう構成する、上記転写ブラシ装置の具体的な構成を示す。
【００８２】
同図（Ａ）に示すように、ブロック状ブラシ２Ａ1 を用意し、この角稜を転写ベルト３を介して感光体ドラム１に当接する。同図（Ｂ）に示すように、所定の硬度を備えたシート２Ａ2 を斜めに配置し、その先端縁を転写ベルト３を介して感光体ドラム１に当接する。同図（Ｃ）に示すように、平板状に成形されたブラシ２Ａ3 を斜めに配置し、その先端縁を転写ベルト３を介して感光体ドラム１に当接する。
【００８３】
上記それぞれの構成では、感光体ドラム１の直径が小径であるため、感光体ニップ幅は２．１〜２．４mmと狭くなる。特に、同図（Ｃ）に示すブラシ２Ａ3 を用いると、ほとんど感光体ドラム１への押圧力がないため、同図（Ａ）（Ｂ）に示すブロック状ブラシ２Ａ1 やシート２Ａ2 で転写ベルト３を押し上げるより感光体ニップ幅を確保できる。
【００８４】
ただし、感光体ニップ幅中に転写ブラシニップ幅を収めるためには、ブラシニップ幅をより狭く、かつ精度よく転写ベルト３を介して感光体ドラム１に当接しなければならない。
【００８５】
図１２に、上記ブラシ２Ａ3 の具体的な構成を示す。すなわち、ブラシ繊維１０２ａをアルミ板１０２ｂで挟み込み、ブラシ繊維１０２ａの広がりを押さえるために両側からマイラシート１０２ｃ、１０２ｄを介在させている。
【００８６】
ブラシ繊維１０２ａは、レーヨン系繊維（太さ６Ｄ）を用いている。抵抗値は５乗〜８乗Ω・cmとし、突き出し長は３〜１０mm程度とすると、良好な転写が行える。ここでは、６乗Ω・cmのブラシ繊維１０２ａで、突き出し長６mmとして使用した。
【００８７】
このブラシ２Ａ3 を傾斜して先端部を転写ベルト３に当接することにより、極力ブラシニップ幅を狭くすることができる。ただし、ブラシ先端の転写ベルトに対する食い込み量は、あまり小さ過ぎると部分的に当接せず転写不良が発生してしまう。食い込ませ過ぎると、ブラシニップ幅が広くなり過ぎてしまい、前転写による転写滲みが発生する。
【００８８】
そこで、転写ブラシ２Ａ3 の転写ベルト３に対する角度を３０度〜８０度、食い込み量を０．５mm〜１．５mmに設定することにより、転写ブラシニップ幅を１〜２mmに規制できる。実際には、ブラシ当接角度６０度、食い込み量を１mmとして、転写ブラシニップ幅を１．３〜１．６mmとした。
【００８９】
図１３（Ａ）に示すように、転写ブラシ２Ａ3 の取付けにあたって、たとえばこの軸心Ｌｂが感光体ドラム１の軸心Ｌｓに対して傾いてしまい、平行度が出ていない場合は、同図（Ｂ）に示すように、転写ブラシニップ幅が感光体ニップ幅の上流側あるいは下流側にはみ出してしまう。
【００９０】
ａ位置と、ｃ位置において、それぞれ上流側と下流側に０．８mmずれているとき、ａ、ｂ、ｃ各位置に対するバイアスを振ったときの転写効率と、文字の滲みを評価した。
【００９１】
図１４に、その結果を示す。転写ブラシニップ幅が感光体ニップ幅からはみ出すと、適正転写バイアスが高圧側にシフトし、ａ、ｂ、ｃすべての位置で転写効率８５％以上の良好な転写が得られない。
【００９２】
そこで、感光体ドラム１に対する転写ブラシ２Ａ3 の転写ベルト３走行方向（すなわち、用紙移動方向）の位置決めをなすために、感光体ドラム１の装置本体に対する位置決めをなす部材と、転写ブラシ２Ａ3 を保持する部材に、互いに係合する部分を設けるようにした。
【００９３】
図１５に、その具体的な構成を示す。
感光体ドラム１のシャフト１Ｓは、リア側とフロント側ともに装置本体を構成する支持フレームＫに位置決めブロック２１を介して位置決めのうえ、回転自在に枢支される。
【００９４】
上記転写ブラシ２Ａ3 の一側端は、断面コ字状に形成される転写ブラシ台座２０４の垂直片部にネジ止め固定されている。上記台座２０４は図示しない転写ベルトの両側端部より外側に位置し、この下面部に突設されるガイド部２０４g がユニット基板２０５に設けられるレール溝２０５Ｌに移動自在に挿入される。
【００９５】
上記レール溝２０５Ｌは転写ベルトの走行方向に沿って設けられていて、転写ブラシ２Ａ3 は上下方向とベルト幅方向には移動できないが、ベルトの走行方向には移動可能になっている。
【００９６】
転写ブラシ台座２０４の上面片部には、先に説明したものと同一形状のガイドピン２０４Ｔが立設されており、位置決めブロック２１に設けられるガイド孔部２１Ｎに係合される。
【００９７】
このように、左右一対の転写ブラシ台座２０４間に転写ブラシ２Ａ3 を架設した状態で転写ブラシ台座２０４上に載置し、転写ユニットＹａを形成する。この状態では、上記ユニット基板２０５のレール溝２０５Ｌに、転写ブラシ台座２０４の凸部２０４ｇが挿入される。このとき、転写ブラシ台座２０４位置は確定されていない。
【００９８】
一方、感光体ドラム１のシャフト１Ｓは、支持フレームＫに位置決めブロック２１を介して位置決めのうえ回転自在に枢支される。この位置決めブロック２１に対して転写ユニットＹａを下方から感光体ドラム１側に押し上げる。
【００９９】
転写ブラシ台座２０４のガイドピン２４Ｔは、位置決めブロック２１に設けられるガイド孔部２１Ｎ内に挿入され、かつこのテーパに沿ってガイドされる。また、転写ブラシ台座２０４の凸部２４Ｖとユニット基板２０５の位置決め用溝２０５Ｍとの関係から、転写ブラシ台座２０４は転写ブラシ２Ａ3 を架設支持した状態のまま、転写ベルト３の走行方向に沿って移動調整される。
【０１００】
ガイドピン２４Ｔの先端がガイド孔部２１Ｎの頂点部分に係合したとき、感光体ドラム１の軸心Ｌｓに対して転写ブラシ２Ａ3 の軸心Ｌｂが完全に一致した状態で位置決めされる。この状態を確保したうえ、図示しない取付け固定手段によって転写ユニットＹａを固定保持する。
【０１０１】
図１６に示すように、感光体ドラム１の軸心Ｌｓに対して転写ブラシ２Ａ3 の軸心Ｌｂが完全に一致した状態では、転写ブラシニップ幅を感光体ニップ幅内に収めることが可能になり、ａ〜ｃ各位置に対応する部分での文字滲みがなく、転写バイアス１０００〜１５００Ｖで良好な転写が行われることが分かった。
【０１０２】
つぎに、第４の実施の形態として、小径の感光体ドラムと、転写部材として転写ローラを用いることを前提とし、感光体ニップ幅より転写ローラニップ幅を大きくし、かつ転写部材の感光体ドラム（転写ベルト）に対する食い込み量を規制する部材を用いた構成について説明する。
【０１０３】
ここでは、感光体ドラムの直径はφ３０mmと小径であり、このような小径ドラムを用いた場合に感光体ニップ幅は非常に狭くなるため、転写部材ニップ幅を感光体ニップ幅内に収めるのは困難である。
【０１０４】
そこで、図１７に示すように、小径の感光体ドラム１Ｂに対して非常に柔らかい転写ローラ２Ｂを用いることとし、感光体ニップ幅Ｎｓに対して転写部材ニップ幅である転写ローラニップ幅Ｎｒが大きくなる構成とする。
【０１０５】
具体的には、上記転写ローラ２Ｂとして、硬度４０度以下のスポンジローラを用いる。感光体ドラム１Ｂに対し加重線圧１８〜７５g/cmで押圧した場合に、感光体ニップ幅は１．５mm以下である。
【０１０６】
図１８に示すように、たとえば転写ローラニップ幅を感光体ニップ幅とほぼ同じ幅に構成しても、感光体ドラム軸心Ｌｓに対する転写ローラ軸心Ｌｒの位置が上流側もしくは下流側にずれる場合がある。
【０１０７】
同図（Ａ）に示すように、感光体ドラム軸心Ｌｓに対する転写ローラ軸心Ｌｒの位置が上流側もしくは下流側へずれ、そのときのずれ量Ｄが０．７５mm以内であれば、感光体ニップ幅と転写ローラニップ幅とが重なる部分がごくわずか確保されて、一応、転写効率が保持される。
【０１０８】
しかしながら、同図（Ｂ）に示すように、感光体ドラム軸心Ｌｓに対する転写ローラ軸心Ｌｒのずれ量Ｄが０．７５mm以上であれば、感光体ニップ幅と転写ローラニップ幅とが全く重ならないので、転写効率が極端に低下してしまう。
【０１０９】
図１９（Ａ）（Ｂ）に、上記転写ローラ２Ｂの硬度を３０度とし、加圧を３０ｇ/cm に設定して、感光体ドラム１Ｂに対する転写ローラ２Ｂの軸心をベルト移動方向の上流側もしくは下流側にずらしたときの転写効率を示した。
【０１１０】
同図（Ａ）は、感光体ドラム軸心Ｌｓより転写ローラ軸心Ｌｒが下流側にずれた場合、同図（Ｂ）は感光体ドラム軸心Ｌｓより転写ローラ軸心Ｌｒが上流側にずれた場合である。転写ローラ２Ｂの直径などの寸法構成は、先に説明したとおりである。
【０１１１】
いずれの場合も、０．７５mm以上ずれると、適正転写バイアスは著しく高い値になり、ずれがない場合の適正バイアスの範囲では良好な転写は行えない。０．５mmのずれでも、さほど良い転写効率は得られない。現実には、部品精度と組立精度を総合すると、０．５mm程度のずれは生じてしまい、問題となる。
【０１１２】
ここでは、先に図１７で説明したように転写ローラニップ幅Ｎｒを感光体ニップ幅Ｎｓより大きくなるように構成し、感光体ドラム軸心Ｌｓと転写ローラ軸心Ｌｒのベルト移動方向の位置が多少ずれても影響が出難い構成としている。
【０１１３】
転写ローラニップ幅Ｎｒを感光体ニップ幅Ｎｓに対して大きくするには、転写ローラ２Ｂの硬度が極力低く、かつ直径が極力大きいものを使用し、しかもローラ荷重を高くしなければならない。
【０１１４】
表１に、転写ローラ２Ｂのスポンジ硬度（ＡＳＫＥＲ−Ｃ）と、転写ローラ径（シャフト径はすべてφ８mm）およびローラ荷重をパラメータにして、転写ローラニップ幅Ｎｒを測定した結果を示す。
【０１１５】
【表１】

【０１１６】
すべての条件において上記感光体ニップ幅Ｎｓは、１．３〜１．６mmである。そして、感光体ニップ幅をパラメータにして文字画像を印字し、そのときの中抜けのレベルを評価した結果を示す。
【０１１７】
ローラ硬度が３０度を越えると、感光体ニップ幅に対して十分に広い転写ローラニップ幅（２mm以上）を形成したうえで文字中抜けが良好となる条件が存在しないことが判明した。そこで、ローラ硬度を３０度以下とし、ローラ径と、転写ローラニップ幅を設定する。
【０１１８】
図２０に、転写ローラ硬度が２０度、ローラ径が１６mm（シャフト径φ８mm）で、ローラ荷重が４５g/cm（感光体ニップ幅１．５mm）の条件において、感光体ドラム軸心Ｌｓに対する転写ローラ軸心Ｌｒのベルト移動方向の位置ずれと、転写効率の関係を示す。
【０１１９】
このときの転写ローラニップ幅は約２．４mm前後であり、０．７５mm程度のずれが生じても、転写効率に大きな影響を与えないことが分かった。すなわち、ローラ硬度が３０度以下で、ローラ荷重を大きく、かつ、ローラ径を大きくすることにより、軸位置のずれの影響を小さくできるが、一方、柔らかい転写ローラを強く当てることにより、このローラの永久歪みが問題になる。
【０１２０】
ローラ硬度２０度、ローラ径１６mm（シャフト径φ８mm）で、ローラ荷重３０gf/cm の条件において、１０日程度高温多湿の環境（３０℃、８５％）下に放置すると、転写ローラは１．５mmも凹んでしまった。この状態で印字を行うと、転写ローラ周期で転写抜けが発生した。
【０１２１】
図２１に、放置による転写ローラの凹み量と、転写抜けの発生状況を示す。放置による凹みが１mmでは、５％を越える転写中抜けが発生する。また、１mmを越える凹みが生じると、復元するのに長い時間がかかることが分かった。
【０１２２】
そこで、図２２（Ａ）に示すように、転写ローラ２Ｂのシャフト２Ｓに金属材などの剛体からなるガイドローラ３０ａ、３０ｂを嵌着して、感光体ドラム１Ｂのリア側端部と、フロント側端部に対向させる。
【０１２３】
同図（Ｂ）に示すように、これらガイドローラ３０ａ、３０ｂ径はφ１４．５mmであり、したがって転写ローラ２Ｂに０．７５mm以上の凹みが生じることがない。転写ローラ２Ｂの実使用条件（転写ローラニップ幅が３mmになる）での食い込み量は０．３mm以下であり、印字動作時にガイドローラ３０ａ、３０ｂは感光体ドラム１Ｂに当接しない。
【０１２４】
このように、転写ローラ２Ｂの硬度を３０度（ＡＳＫＥＲ−Ｃ）とし、転写ローラ径を大きくして加圧を強く設定し、転写ローラニップ幅Ｎｒを感光体ニップ幅Ｎｓより大きく形成する。
【０１２５】
そして、長期放置による転写ローラ２Ｂの凹みを防止するため、印字時には感光体ドラム１Ｂに当接しない径のガイドローラ３０ａ、３０ｂを配置することにより、組み立て誤差などにより感光体ドラム軸心に対して転写ローラ軸心がずれても、文字の中抜けや、転写ローラ周期の転写抜けなどがない良好な転写を得られる。
【０１２６】
図２３に、第５の実施の形態としての４連タンデムカラー印字装置を示し、感光体ドラム軸心に対する転写ローラ軸心の転写ベルト移動方向へのずれ防止手段について後述する。
【０１２７】
図中１ａ〜１ｄは、像担持体としての感光体ドラム（φ６０mmのＯＰＣドラム）である。感光体１ａ〜１ｄはスコロトロン帯電器１１ａ〜１１ｄにより、−５００Ｖ〜−８００Ｖの電位に一様に帯電される。
【０１２８】
そして、感光体ドラム１ａ〜１ｄ上にはレーザスキャナ１２ａ〜１２ｄによって画像データが書き込まれ、静電潜像が形成される。上記静電潜像は、２成分現像器１３ａ〜１３ｄ（１３ａ；イエロー、１３ｂ；マゼンタ、１３ｃ；シアン、１３ｄ；ブラック）により反転現像され、トナー画像が形成される。
【０１２９】
このとき、トナーの帯電極性は感光体１ａ〜１ｄが帯電された極性と同極性の（−）である。用紙Ｐは、図示しない用紙カセットから搬送され、アライニングローラ対１４によって感光体ドラム１ａ上にトナー像を形成するのとタイミングを合わせて、転写ベルト３上に送り出される。
【０１３０】
接触帯電器としての転写ローラ２ａ〜２ｄの転写領域に送られた用紙Ｐには、これらローラの転写電界により感光体ドラム１ａ〜１ｄ上のトナー像が用紙Ｐに多重転写される。
【０１３１】
転写ローラ２ａ〜２ｄには、転写バイアス電源１９ａ〜１９ｄによりトナーの帯電極性とは逆極性の（＋）極性のバイアス（＋８００〜＋４０００v ）が印加されている。
【０１３２】
転写領域でトナー像が用紙Ｐに転写されるとともに、転写ベルト３には転写ローラ２ａ〜２ｄを介して（＋）の電荷が付与され、用紙Ｐは感光体ドラム１ａ〜１ｄとの間の放電により（−）電荷が付与される。
【０１３３】
この（＋）電荷と（−）電荷が引き合うことにより、用紙Ｐは転写ベルト３に静電的に吸着され、転写領域通過後は用紙Ｐはベルト３とともに移動する。そして用紙Ｐは、転写ベルトを駆動する駆動ローラ１５の曲率により剥離される。
【０１３４】
剥離の際の放電を防止するため、駆動ローラ１５は接地されており、かつ駆動ローラ１５の上方には接地された除電ブラシ１７が配置される。より確実に用紙Ｐを剥離するために、ＡＣコロナを利用したコロナ除電器により用紙を除電してもよい。転写ベルト３から剥離された用紙Ｐは定着器２０へと搬送され、熱定着されたあと、装置本体外面に取付けられる排紙トレイへ排出される。
【０１３５】
転写ローラ２ａ〜２ｄの材質、寸法は、先に図１で説明した転写ローラ２と同様である。転写ベルト３の材質、抵抗、肉厚も同様である。
４連タンデムカラー印字装置では、色ズレを防止するために４本の感光体ドラム１ａ〜１ｄの平行度が重要である。そのため、感光体ドラム軸心位置の調整ができるようになっていて、感光体ドラム１ａ〜１ｄ相互間の平行度は高精度で確保されている。
【０１３６】
一方、転写ローラ２ａ〜２ｄは、感光体ドラム１ａ〜１ｄとの接離方向に調整可能に転写ユニットに取り付けられており、互いの軸心平行度は部品精度や組立精度によって決まる。また、転写ユニットは非常に大きく、その自重でたわみや歪みが発生し易く、転写ローラの平行度はあまり良好とは言えない。
【０１３７】
ここでは、第１〜第４感光体ドラム１ａ〜１ｄに対し転写ローラ２ａ〜２ｄの軸心位置を独立して調整できる構成とした。
すなわち、先に図６で説明したような転写ユニットＹを構成し、転写ベルト３が感光体ドラム１に当接して印字可能な状態で押し上げると、転写ローラ支持部材２４のガイドピン２４Ｔが位置決めブロック２１のガイド孔部２１Ｎと係合して、４本の感光体ドラム２ａ〜２ｄそれぞれに対して、４つの転写ローラ２ａ〜２ｄの軸心位置が独立して決まる。
【０１３８】
なお、転写ローラ支持部材２４の移動可能範囲幅＜ガイドピン２４Ｔ幅とするとともに、ガイドピン２４Ｔ先端がガイド孔部２１Ｎに挿入するまでは、転写ローラ２ａ〜２ｄは転写ベルト３を介して感光体ドラム１ａ〜１ｄに当接しないよう設計することは、ここでも同様である。
【０１３９】
このときの転写ローラ２ａ〜２ｄと転写ベルト３の適正バイアス（８５％以上の転写効率が得られる）は、第１ステーションが１０００Ｖ、第２ステーションが１１５０Ｖ、第３ステーションが１３００Ｖ、第４ステーションが１５００Ｖである。
【０１４０】
図２４（Ａ）（Ｂ）に、転写ローラ径φ１４mm、硬度２０度（ＡＳＫＥＲ−Ｃ）の条件下での転写効率（同図Ａ）と、中抜けのレベル（同図Ｂ）を示した。代表値として、第１ステーションはＹ単色、第２ステーションはＹ＋Ｍの重ね、第３ステーションはＹ＋Ｍ＋Ｃの重ね、第４ステーションはＹ＋Ｍ＋Ｃ＋ＢＫの重ね部についてのデータのみを記した。
【０１４１】
ローラ荷重について、第１ステーションに関しては、先に説明した単色機と同様の範囲で良好な結果が得られるが、色重ねの文字やラインなどで中抜けが発生しやすい。よって、後段ほど中抜けが発生する加圧が低くなってくるため、適正加圧のマージンが狭くなっている。
【０１４２】
結果として、第１ステーションは２５〜７５ｇ/cm 、第２ステーションは２５〜７０ｇ/cm 、第３ステーションは２５〜４５ｇ/cm 、第４ステーションは２５〜３０ｇ/cm である。
【０１４３】
そして、第１ステーション〜第３ステーションまで３５ｇ/cm 、第４ステーションのみを２８ｇ/cm と設定し、第１ステーション１０００Ｖ、第２ステーション１１５０Ｖ、第３ステーション１３００Ｖ、第４ステーション１５００Ｖのバイアスを印加し、先に説明したような構成で転写ベルト３の走行方向の感光体ドラム軸心に対する転写ローラ軸心の位置決めをなすことにより、色重ねも良好で文字中抜けのない良好な転写が得られる。
【０１４４】
【発明の効果】
以上説明したように、本発明によれば、転写ベルトおよび接触転写器を用いることを前提として、感光体ニップ幅と転写部材ニップ幅のセンタ位置のずれによる転写ムラを改善し、高画質化を得るという効果を奏する。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態を表す、画像形成装置の概略の構成図。
【図２】転写ローラとして、ゴムローラにした場合と、スポンジローラにした場合の、中抜けの特性を表す図。
【図３】転写ローラとして、ゴムローラにした場合と、スポンジローラにした場合の、転写抜け率の特性を表す図。
【図４】転写ローラとして、所定硬度のローラでローラ加圧を振ったときの、転写効率と中抜けの特性を表す図。
【図５】転写ローラとして、所定転写バイアスで、かつ所定加圧で振ったときの、転写効率と中抜けの特性を表す図。
【図６】感光体ドラムに対する転写ローラの位置決め手段の構成を表す図。
【図７】上記位置決め手段を採用した状態での、ローラ加圧と中抜けの特性図およびローラ加圧と転写効率の特性。
【図８】第２の実施の形態の、感光体ドラムに対する転写ローラの位置決め手段の構成を表す図。
【図９】第３の実施の形態の、画像形成装置の概略の構成図。
【図１０】ベルト押し上げローラを用いた場合の構成図。
【図１１】互いに異なる種類の転写ブラシの構成図。
【図１２】転写部材としての転写ブラシの構成図。
【図１３】転写ブラシを用いた場合の、感光体ドラム軸心に対する転写ブラシ軸心の傾きを表す図。
【図１４】転写ブラシが感光体ドラムに対して傾いた状態での、各位置の転写バイアスに対する転写効率の特性図。
【図１５】感光体ドラム軸心に対して転写ブラシを平行に位置決めする手段を表す図。
【図１６】上記位置決め手段を用いた場合の、各位置の転写バイアスに対する転写効率の特性図。
【図１７】第４の実施の形態の、転写ローラとしてスポンジローラを用いた場合の、感光体ニップ幅と転写ローラニップ幅の相互関係を表す図。
【図１８】転写ローラとしてスポンジローラを用いた場合の、感光体ニップ幅と転写ローラニップ幅のずれ状態を表す図。
【図１９】所定硬度の転写ローラを用いて、感光体ドラム軸心に対して転写ローラ軸心をずらした場合の、転写バイアスに対する転写効率の特性を表す図。
【図２０】所定硬度の転写ローラを用いて、感光体ドラム軸心に対して転写ローラ軸心をずらした場合の、転写バイアスに対する転写効率の特性図。
【図２１】放置による転写ローラの凹み量と転写抜け率の特性図。
【図２２】転写ローラの食い込み量を規制する手段の構成図。
【図２３】第５の実施の形態の、カラー画像形成装置の概略の構成図。
【図２４】各ステーションでのローラ加圧に対する転写効率の特性と、中抜けの特性を表す図。
【図２５】従来の、感光体ドラム軸心に対する転写ローラ軸心のずれを表す図。
【図２６】上記各位置での断面図。
【図２７】転写ベルトを用いない場合の、感光体軸心と転写ローラ軸心がずれた状態を表す図。
【符号の説明】
Ｐ…用紙，
１…像担持体（感光体ドラム）、
３…転写ベルト、
２…転写ローラ、
Ｋ…支持フレーム、
２１…位置決めブロック、
２１Ｎ…ガイド孔部、
２２…軸受、
２４…ローラ支持部材、
２４Ｔ…ガイドピン、
２６…基準ブロック、
Ｙ，Ｙａ…転写ユニット。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus provided in an electrophotographic machine, for example, and more particularly to a positioning structure for an image carrier of a transfer device.
[0002]
[Prior art]
A technique for performing transfer while electrostatically holding a sheet on a transfer belt is disclosed in, for example, Japanese Patent Publication No. 62-24793, and the sheet is held on the transfer belt. There is almost no winding. Further, since the paper path is stabilized, this is an essential technique in a four-tandem color printing apparatus as described in Japanese Patent Publication No. 6-52446.
[0003]
On the other hand, a method using a brush transfer device or a roller transfer device as a transfer charger is a very effective transfer method with less generation of ozone. When this transfer belt is used in combination with a contact transfer device such as a brush transfer device or a roller transfer device, the paper does not easily wrap around the photosensitive drum, the paper path is stable, and ozone is not generated. Good transfer can be achieved.
[0004]
[Problems to be solved by the invention]
However, in a transfer system using both a transfer belt and a contact transfer device, the alignment between the photosensitive drum as an image carrier and the transfer device is an important condition. Naturally, the photoconductor drum and the contact transfer device are designed to be parallel, but in reality, the transfer device cannot be held parallel due to the accuracy of the parts or assembly errors, and the transfer device is not connected to the photoconductor drum axis. It will shift.
[0005]
For example, as shown in FIG. 25, when the transfer roller R is used as a contact transfer device, the axis Lr of the transfer roller R tends to be more or less shifted from the axis Ls of the photosensitive drum S. The white arrow represents the traveling direction of the transfer belt.
[0006]
FIGS. 26A, 26B, and 26C show the photosensitive member nip width (nip width formed by the sheet and the photosensitive drum S) and the transfer roller nip width (transfer belt B and transfer roller R) at this time. The positional relationship with the formed nip width).
[0007]
That is, as shown in FIG. 4B, the centers of both nip widths coincide at the center b position of the photosensitive drum S, but at the position a shown in FIG. On the other hand, the transfer roller nip width is shifted to the upstream side, and the transfer roller nip width is shifted to the downstream side with respect to the photosensitive member nip at the position c shown in FIG.
[0008]
Since the transfer belt B is composed of a rubber belt or a resin belt and is supported with a predetermined tension, in such a state, it is less flexible than a sheet (not shown) and it is difficult to form a uniform transfer electric field. is there.
[0009]
Therefore, by using a photosensitive drum formed with a large diameter as much as possible and a transfer roller similarly formed with a large diameter, by enlarging the photosensitive member nip width and the transfer roller nip width, a part of both nip positions can be shifted. There is an idea that the remaining nip position overlaps to secure a transfer electric field.
[0010]
However, in actuality, it is impossible to increase the diameter of the photosensitive drum from the present situation where the entire apparatus is desired to be reduced in size, which leads to an increase in the size of the apparatus and another adverse effect.
[0011]
When high-performance transfer characteristics are required as in color image forming devices, the width and position of the overlapping area of the nip width will affect the transfer performance even if part of both nip widths overlap. Giving subtle uneven transfer.
[0012]
This situation is not limited to the belt transfer method in which the paper is carried on the belt and transferred, but the intermediate transfer method in which the image is first transferred from the photosensitive drum to the intermediate transfer member and then transferred to the paper. Needless to say, it also occurs in
[0013]
Therefore, in particular, in a color image forming apparatus, it is an essential requirement to make the photoreceptor nip width coincide with the transfer roller nip width.
As shown in FIG. 27, in the transfer system that does not use a transfer belt, the sheet P sandwiched between them is highly flexible, even if the axes Ls and Lr of the photosensitive drum S and the transfer roller R are misaligned. A nip width is formed in a state where the photosensitive drum S, the paper P, and the transfer roller R are in close contact with each other.
[0014]
Therefore, it is possible to form a transfer electric field while pressing the paper P against the photoconductive drum S even if the photoconductive drum S and the transfer roller R are somewhat not parallel depending on the degree.
[0015]
The present invention has been made paying attention to the above circumstances, and the object of the present invention is that the center positions of the photosensitive member nip width and the transfer member nip width are based on the assumption that a transfer belt and a contact transfer device are used. Due to misalignment An object of the present invention is to provide an image forming apparatus which improves transfer unevenness and obtains high image quality.
[0016]
[Means for Solving the Problems]
In order to satisfy the above object, a four-tandem color image forming apparatus according to the present invention holds and conveys a sheet placed on the upper surface, and is in contact with an image carrier on which a toner image is formed, and a transfer belt; A transfer roller that is slidably contacted with the back surface of the transfer belt at a portion facing the image carrier and applies a bias to transfer the toner image on the image carrier to paper;
The transfer roller is made of a material having a hardness of 30 degrees (ASKER-C) or less, and a transfer formed by the transfer roller and the transfer belt rather than a photoconductor nip width formed by the image carrier and paper. The roller nip width is widened, the portion of the transfer roller shaft facing the rear end and the front end of the image carrier is made of a rigid body and has a diameter smaller than the diameter of the transfer roller. When the transfer roller is not in contact with the image carrier under actual use conditions, A guide roller for regulating the amount of biting of the transfer roller with respect to the transfer belt was attached.
[0017]
By adopting the means to solve such problems, it is assumed that the transfer belt and the contact transfer device are used, so that the deviation of the center position between the photosensitive member nip width and the transfer member nip width is reduced. absorption To do.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 schematically shows a configuration of a laser printer as an image forming apparatus according to the first embodiment. The printing speed in this laser printer is 30 sheets per minute with A4 horizontal feed, and the process speed is 175 mm / sec.
[0019]
In the figure, reference numeral 1 denotes a photosensitive drum (an OPC drum having a diameter of 60 mm) which is an image carrier. The photosensitive drum 1 is uniformly charged to a potential of −500 V to −800 V by a scorotron charger 11.
[0020]
On the photosensitive drum 1, image data is written by the laser scanner 12, and an electrostatic latent image is formed. As the photosensitive drum 1 rotates, the electrostatic latent image formed here is reversely developed by the two-component developing device 13 to form a toner image. At this time, the charged polarity of the toner is the same (−) as the charged polarity of the photosensitive drum 1.
[0021]
The paper P is conveyed from a paper cassette (not shown), and is sent out onto the transfer belt 3 in synchronization with the formation of a toner image on the photosensitive drum 1 by the aligning roller pair 14.
[0022]
A toner image on the photosensitive drum 1 is transferred to a sheet P which is arranged via the transfer belt 3 and transferred to a transfer region between the photosensitive drum 1 and the transfer roller 2 as a contact charger by a transfer electric field of the transfer roller 2. Is transcribed.
[0023]
A bias voltage (+800 to +2500 V) having a (+) polarity opposite to the toner charging polarity is applied to the transfer roller 2 by a transfer bias power source 19. The toner image is transferred to the paper P in the transfer area, and (+) polarity charge is applied to the transfer belt 3 from the transfer roller 2.
[0024]
The sheet P is given a (−) charge by the discharge between the photosensitive drum 1 and the (+) charge and the (−) charge attract each other, whereby the sheet P is electrostatically attracted to the transfer belt 3. Is done.
[0025]
After the sheet P passes through the transfer area, the sheet P moves together with the transfer belt 3. Then, the sheet P is peeled from the transfer belt 3 by the curvature of the driving roller 15 that drives the transfer belt 3.
[0026]
The drive roller 15 is grounded to prevent discharge when the paper P is peeled from the transfer belt 3, and a grounded neutralizing brush 17 is disposed above the drive roller 15. (In order to peel the paper P more reliably, a corona static eliminator using an AC corona may be used instead of the static elimination brush 17).
The paper P peeled off from the transfer belt 3 is conveyed to the fixing device 20 and is discharged onto a paper discharge tray (not shown) attached to the outer surface of the apparatus after being thermally fixed. Reference numeral 21 denotes a static eliminator that removes the charge of the toner remaining on the photosensitive drum 1.
[0027]
Next, the transfer belt 3 will be described in detail.
As a material for the transfer belt 3, a rubber material such as urethane, EPDM, or silicon, or a resin material such as polycarbonate, polyimide, polyamide, or PET is used. Here, polycarbonate resin is adopted.
[0028]
If the thickness of the transfer belt 3 is too thin, the transfer belt 3 is liable to be damaged and the life is shortened. From various empirical rules, a suitable belt thickness is in the range of 60 μm to 250 μm.
[0029]
The electric resistance value of the transfer belt 3 is adjusted by dispersing carbon in a polycarbonate resin. Usable resistance values are 5th power Ω · cm to 16th power Ω · cm. If the resistance value is too low, the amount of carbon contained in the belt will be too large, resulting in a decrease in the mechanical strength of the belt or a breakdown of transfer bias due to carbon aggregation.
[0030]
If the resistance value is too high, the transfer bias voltage value becomes abnormally high, or charges accumulate on the belt. If continuous printing is performed in this state, the transfer performance deteriorates, so a device for removing the charge from the transfer belt is required.
[0031]
Therefore, an appropriate belt resistance value when performing transfer with a contact transfer device without a belt static eliminator is in the range of 9th power Ω · cm to 14th power Ω · cm, for example, a resistance value of 12th power Ω · cm. I am using something.
[0032]
The transfer belt 3 is stretched between the driving roller 15 and the driven roller 16, and tension springs (not shown) are brought into contact with both ends of the support shaft of the driven roller 16. By these springs, a tension load of 2.1 kgf is urged to the transfer belt 3.
[0033]
Next, the transfer roller 2 will be described in detail.
The transfer roller 2 is formed by attaching a solid rubber such as urethane, EPDM, or silicon or a foam to the peripheral surface of the metal shaft. If necessary, a skin layer can be formed on the surface of rubber or foam to improve surface properties and voltage resistance.
[0034]
Here, a urethane foam in which carbon having a thickness of 3 mm is dispersed is attached to a metal shaft having a diameter of 8 mm to form a transfer roller 2 having a diameter of 14 mm. And the hardness at the time of using foamed rubber is 15-40 degree | times by ASKER-C. In the case of a transfer roller having a solid rubber or a skin layer on the surface, a hardness of 20 to 45 degrees according to JIS-A is appropriate.
[0035]
If the hardness is too high, a sufficient nip width is not formed between the transfer roller and the transfer belt, and good transfer cannot be performed. At the same time, voids in the character image portion are likely to occur.
[0036]
FIG. 2 (A) shows the relationship between the hollowed out characters for the respective roller hardnesses when a rubber roller is used as the transfer roller 2 and FIG. 2 (B) shows the use of a sponge roller.
[0037]
Here, the transfer efficiency is defined as follows.

In addition, the character dropout is evaluated visually by using a character image in a five-step evaluation.
[0038]
From these results, it was found that good transfer performance can be obtained with a hardness of 45 degrees or less for a solid rubber roller (or a roller having a skin layer on the surface) and a hardness of 40 degrees or less for a foam rubber roller.
[0039]
On the other hand, if the hardness of the transfer roller 2 is too low, permanent distortion is likely to occur. If printing is performed after leaving for a long period of time, the portion that is in contact with the belt at the time of leaving is distorted and transfer omission occurs in the roller cycle.
[0040]
FIGS. 3A and 3B show that a transfer roller having different hardness is used and left in an environment of a temperature of 40 ° C. and a humidity of 85% for 24 hours, and then a solid image is printed to prevent a transfer omission in the transfer roller cycle. It is the result of measurement.
[0041]
The transfer loss rate (%) is defined as follows.

The image density is based on the Macbeth density, and is 0% when a hollow portion cannot be recognized.
[0042]
From this result, in the case of the solid rubber roller (or the roller having a skin layer on the surface) shown in FIG. 4A, transfer omission due to permanent distortion does not occur if the hardness is 20 degrees or more. When the foamed rubber roller shown in FIG. 5B is used, it has been found that if the hardness is 15 degrees or more, transfer omission due to permanent distortion does not occur.
[0043]
Actually, a transfer roller 2 is used in which a foamed urethane (cell diameter of about 50 μm) material is attached to the peripheral surface of a φ8 mm metal shaft, and the roller diameter is 13 mmφ. The hardness of the transfer roller 2 is 23 degrees in ASKER-C.
[0044]
Further, the pressing force of the transfer roller 2 varies depending on the hardness and diameter of the roller, but the linear pressure is preferably in the range of 18 to 75 g / cm. If the pressure is too weak, a stable transfer nip width is not formed, and a transfer failure occurs. If the pressurization is too strong, characters may be easily lost.
[0045]
FIG. 4 shows the evaluation results of the transfer efficiency and the void when the roller pressure is shaken using the roller hardness of 28 degrees (ASKER-C). Here, both the photoconductor nip width and the transfer roller nip width are about 2.7 mm to 3 mm.
[0046]
Under the conditions described above, the toner image is transferred onto the sheet P using the transfer belt 3 and the transfer roller 2 with respect to the photosensitive drum 1, but if no positioning means for the transfer roller 2 is used, As shown in FIG. 25, the parallelism of the axis Lr of the transfer roller 2 (R) with respect to the axis Ls of the photosensitive drum 1 (S) does not occur.
[0047]
5A and 5B, in a state where the parallelism of the axis Lr of the transfer roller 2 with respect to the axis Ls of the photosensitive drum 1 does not occur, the transfer bias is 1300 V, and the transfer roller weighted linear pressure is 25 to 25. The results of measuring the transfer efficiency at the positions a, b, and c and the void ratio when shaken under the condition of 150 g / cm are shown.
[0048]
At the b position, the pressure is concentrated and higher than at the a and c positions, so that the transfer is good with a light load, but on the other hand, a hollow is easily generated due to excessive pressure with a light load. The appropriate conditions have shifted greatly. Therefore, it is not possible to obtain a good transfer image with no void at all positions a, b, and c.
[0049]
The deviation between the photoconductor nip width and the transfer roller nip width also occurs due to the eccentricity of the photoconductor drum 1 and the transfer roller 2, but is largely caused by assembly errors of the photoconductor drum 1 and the transfer roller 2.
[0050]
For this reason, the transfer roller 2 is positioned with respect to the photosensitive drum 1 in the belt moving direction. That is, a member for positioning the photosensitive drum 1 with respect to the apparatus main body and a member for positioning and holding the transfer roller 2 with respect to the photosensitive drum 1 are required.
[0051]
6 (A) and 6 (B) show specific embodiments thereof.
The shaft 1S of the photosensitive drum 1 is rotatably supported after being positioned via a positioning block 21 on a support frame K constituting the apparatus main body on both the rear side and the front side.
[0052]
The shaft 2S of the transfer roller 2 is pivotally supported by the bearing 22 on both the rear side and the front side. The bearing 22 is elastically supported via a spring 23 on a lower piece portion of a transfer roller support member 24 having a U-shaped cross section.
[0053]
That is, the bearing 22 is pushed up and biased by the spring 23 and is pressed and held on the upper piece of the transfer roller member 24. The transfer roller support member 24 is supported on the transfer roller holder 25 and positioned and fixed to the positioning block 21 as will be described later.
[0054]
Further, the transfer roller holder 25 is provided with a positioning groove 25M, into which a convex portion 24V protruding integrally with the lower piece portion of the transfer roller support member 24 is inserted.
[0055]
The convex portion 24V with respect to the positioning groove 25M has a margin only in the traveling direction of the transfer belt 3 (indicated by a white arrow in the figure), and is tight in a movable state in a direction orthogonal to the traveling direction.
[0056]
A guide pin 24T having a tapered tip is provided on the upper piece of the transfer roller support member 24, and a tapered guide hole 21N is provided on the lower surface of the positioning block 21. Yes. The taper angle of the guide pin 24T is smaller than the taper angle of the guide hole 21N of the positioning block 21.
[0057]
In this way, the bearing 22 that pivotally supports the shaft 2S of the transfer roller 2 via the spring 23 is elastically supported by the transfer roller support member 24, and the transfer roller support member 24 is placed on the transfer roller holder 25. The transfer unit Y is formed.
[0058]
Of course, the convex portion 24V of the transfer roller support member 24 is inserted into the positioning groove 25M of the transfer roller holder 25. At this time, the position of the transfer roller support member 24, which is the convex portion 24V, is not fixed.
[0059]
Then, the transfer unit Y is pushed up from the lower side toward the photosensitive drum 1 with respect to the positioning block 21. The guide pin 24T of the transfer roller support member 24 is inserted into a guide hole 21N provided in the positioning block 21 and guided along this taper.
[0060]
Then, due to the relationship between the convex portion 24V of the transfer roller support member 24 and the positioning groove 25M of the transfer roller holder 25, the transfer roller support member 24 runs the transfer belt 3 while elastically supporting the transfer roller 2. The position is adjusted by moving along the direction.
[0061]
When the tip end of the guide pin 24T is engaged with the apex portion of the guide hole portion 21N, the shaft center Lr of the transfer roller 2 is completely aligned with the shaft center Ls of the photosensitive drum 1 and positioned. In addition to securing this position, the transfer unit Y is fixed and held by an attachment fixing means (not shown).
[0062]
When the transfer roller 2 comes into contact with the photosensitive drum 1 via the transfer belt 3 before the tip of the guide pin 24T is inserted into the guide hole 21N, the transfer roller support member 24 is positioned via the guide pin 24T. I can't.
[0063]
If the movable range of the convex portion 24V of the transfer roller support member 24 relative to the positioning groove 25M of the transfer roller holder 25 is too wide, the guide pin 24T cannot be inserted into the guide hole portion 21N when the transfer unit Y is pushed up. The guide pin 24T may be damaged or the positioning block 21 side may be damaged.
[0064]
Therefore, the following conditions must be satisfied.
1) Moveable range width dimension of roller support member 24 (d1 + d2) <Width dimension of guide pin 24T (d3)
2) The dimension is set such that the transfer roller 2 does not contact the photosensitive drum 1 via the transfer belt 3 until the tip of the guide pin 24T is inserted into the guide hole 21N.
[0065]
7 (A) and 7 (B), when the transfer roller axis Lr is positioned with respect to the photosensitive drum axis Ls and the transfer roller 2 is biased with a weighted linear pressure of 50 g / cm 2, The measurement results of the evaluation of hollowing out and the transfer efficiency when the pressure condition is changed with a bias of 1300 V are shown.
[0066]
The appropriate voltage conditions at the a, b, and c positions described above are the same as the pressurizing conditions, and it can be seen that there are a wide range of conditions for obtaining a good transfer image at all the a, b, and c positions. .
[0067]
Here, a part (guide hole 21N) of a member (positioning block 21N) for positioning and mounting the photosensitive drum axis Ls with respect to the support frame K and a part of the transfer roller support member 24 (guide pin 24T). However, a member integrally attached to the photosensitive drum shaft 1S and a member integrally attached to the transfer roller shaft 2S may be engaged with each other.
[0068]
As shown in FIGS. 8A and 8B, a positioning means for the transfer roller 2 as the second embodiment may be employed.
Both ends of the shaft 1S of the photosensitive drum 1 are pivotally supported by a positioning block 21A, and a bearing 22 pivotally supporting both ends of the shaft 2S of the transfer roller 2 is elastically supported by a roller support member 24 via a spring 23. The roller support member 24 and the structure of the transfer roller holder 25 that supports the roller support member 24 are the same as described above, and therefore a new description is omitted here.
[0069]
A reference block 26 is interposed between the positioning block 21 </ b> A and the roller support member 24. An engaging arm portion 26a projects from one side surface of the reference block 26, and is movably engaged with a guide groove 21m provided on the side surface of the positioning block 21A along the running direction of the transfer belt 3.
[0070]
Further, a guide groove 26b having a triangular cross section is provided on the upper surface of the reference block 26 along the running direction of the transfer belt 3, and a conical guide hole portion 26c is provided on the lower surface.
[0071]
A guide rod 27 provided on the positioning block 21A is engaged with the guide groove 26b on the upper surface of the reference block 26. That is, after both ends of the shaft 1S of the photosensitive drum 1 are positioned and pivotally supported by the positioning block 21 attached to a predetermined position of the support frame K, the engaging arm portion 26a of the reference block 26 is positioned on the side surface of the positioning block 21A. It is engaged with a guide groove 21m provided in the.
[0072]
In this state, a guide rod 27 provided on the positioning block 21A is engaged with the guide groove 26b on the upper surface of the reference block 26. Therefore, the reference block 26 is positioned with respect to the photosensitive drum 1 axis.
[0073]
Next, the transfer unit Y is pushed up from below to the photosensitive drum 1 side. The guide pin 24T of the transfer roller support member 24 is inserted into a guide hole 26c provided in the reference block 26. When the tip of the guide pin 24T is engaged with the apex portion of the guide hole 26c, the guide pin 24T is in contact with the reference block 26. Thus, the transfer roller 2 is positioned.
[0074]
Eventually, the shaft center Lr of the transfer roller 2 is positioned with the shaft center Ls of the photoconductive drum 1 completely aligned via the reference block 26. In addition to securing this state, the transfer unit Y is fixed and held by an attachment fixing means (not shown).
[0075]
FIG. 9 illustrates a third embodiment applied to digital PPC in order to make the transfer member nip width smaller than the photoreceptor nip width. The basic configuration and printing operation are the same as those of the apparatus shown in FIG. 1, and the same parts are denoted by the same reference numerals and a new description is omitted.
[0076]
As a tendency of this type of apparatus, since the printing speed is slow and the diameter of the photosensitive drum 1 is small (speed 125 mm / sec 20 ppm, photosensitive drum diameter 45 mm), the transfer brush apparatus 2A is used as a transfer member.
[0077]
That is, in the apparatus described above, the transfer roller nip width and the photoconductor nip width are approximately the same (2.7 to 3.0 mm), but in a high-quality digital PPC that requires better image quality, transfer is performed. If the transfer roller nip width constituting the part is formed upstream of the photoreceptor nip width, character bleeding due to pre-transfer becomes a problem.
[0078]
Therefore, it is required that the transfer member nip width (described as the transfer roller nip width in the apparatus described above) is narrower than the photosensitive member nip width, and the transfer member nip width falls within the photosensitive member nip width.
[0079]
Therefore, for example, as shown in FIG. 10, if the belt push-up roller 40 is arranged close to the downstream side of the transfer roller 2 and the transfer belt 3 is wound around a part of the photoconductor drum 1, the photoconductor The nip width is widened, and the transfer roller 2 can be used as it is.
[0080]
However, in such a configuration, the sheet path before the transfer nip width-in the transfer nip width-after the transfer nip width is bent in an S shape. For this reason, for example, in the case of thick paper such as a postcard, there is a problem in the paper passing property, and smooth conveyance and high-precision transfer may be impaired. In particular, when used in a quadruple tandem color printing apparatus, the paper path is provided with a plurality of complicated curved portions, so that it cannot be used.
[0081]
FIG. 11 shows a specific configuration of the above-described transfer brush device configured to reduce the nip width as a transfer member so that the transfer member nip width falls within the photoreceptor nip width.
[0082]
As shown in FIG. 2A, a block-like brush 2A1 is prepared, and the corner ridge is brought into contact with the photosensitive drum 1 via the transfer belt 3. As shown in FIG. 2B, a sheet 2A2 having a predetermined hardness is disposed obliquely, and its leading edge is brought into contact with the photosensitive drum 1 via the transfer belt 3. As shown in FIG. 2C, a brush 2A3 formed in a flat plate shape is disposed obliquely, and its leading edge is brought into contact with the photosensitive drum 1 via the transfer belt 3.
[0083]
In each of the above configurations, since the diameter of the photosensitive drum 1 is small, the photosensitive nip width is as narrow as 2.1 to 2.4 mm. In particular, when the brush 2A3 shown in FIG. 6C is used, there is almost no pressing force to the photosensitive drum 1, and therefore the transfer belt 3 is attached to the block-like brush 2A1 or sheet 2A2 shown in FIGS. The photoreceptor nip width can be secured by pushing up.
[0084]
However, in order to fit the transfer brush nip width within the photoreceptor nip width, the brush nip width must be narrower and contact the photoreceptor drum 1 via the transfer belt 3 with high accuracy.
[0085]
FIG. 12 shows a specific configuration of the brush 2A3. That is, the brush fibers 102a are sandwiched between the aluminum plates 102b, and the mylar sheets 102c and 102d are interposed from both sides in order to suppress the spread of the brush fibers 102a.
[0086]
The brush fibers 102a are made of rayon fibers (thickness 6D). When the resistance value is 5th to 8th power Ω · cm and the protrusion length is about 3 to 10 mm, good transfer can be performed. Here, the brush fiber 102a of 6 6 Ω · cm was used with a protruding length of 6 mm.
[0087]
The brush nip width can be made as narrow as possible by inclining the brush 2A3 and bringing its tip into contact with the transfer belt 3. However, if the amount of biting on the transfer belt at the tip of the brush is too small, partial contact will not occur and transfer failure will occur. If the bite is too much, the brush nip width becomes too wide, and transfer blur due to pre-transfer occurs.
[0088]
Therefore, by setting the angle of the transfer brush 2A3 with respect to the transfer belt 3 to 30 to 80 degrees and the amount of biting to 0.5 to 1.5 mm, the transfer brush nip width can be regulated to 1 to 2 mm. Actually, the brush contact angle was 60 degrees, the biting amount was 1 mm, and the transfer brush nip width was 1.3 to 1.6 mm.
[0089]
As shown in FIG. 13A, when the transfer brush 2A3 is mounted, for example, when the axis Lb is inclined with respect to the axis Ls of the photosensitive drum 1 and the parallelism is not obtained, As shown in B), the transfer brush nip width protrudes upstream or downstream of the photoreceptor nip width.
[0090]
When the position a and the position c were shifted by 0.8 mm on the upstream side and the downstream side, respectively, the transfer efficiency when the bias was applied to the positions a, b, and c and the bleeding of the characters were evaluated.
[0091]
FIG. 14 shows the result. When the transfer brush nip width protrudes from the photoreceptor nip width, the appropriate transfer bias shifts to the high voltage side, and good transfer with a transfer efficiency of 85% or more cannot be obtained at all positions a, b, and c.
[0092]
Therefore, in order to position the transfer brush 2A3 with respect to the photosensitive drum 1 in the traveling direction of the transfer belt 3 (that is, the sheet moving direction), a member for positioning the photosensitive drum 1 with respect to the apparatus main body and the transfer brush 2A3 are held. The members are provided with portions that engage with each other.
[0093]
FIG. 15 shows a specific configuration thereof.
The shaft 1S of the photosensitive drum 1 is rotatably supported after being positioned via a positioning block 21 on a support frame K constituting the apparatus main body on both the rear side and the front side.
[0094]
One end of the transfer brush 2A3 is fixed to a vertical piece of a transfer brush base 204 having a U-shaped cross section by screws. The pedestal 204 is positioned outside both end portions of the transfer belt (not shown), and a guide portion 204g protruding from the lower surface portion is movably inserted into a rail groove 205L provided on the unit substrate 205.
[0095]
The rail groove 205L is provided along the traveling direction of the transfer belt, and the transfer brush 2A3 cannot move in the vertical direction and the belt width direction, but can move in the belt traveling direction.
[0096]
A guide pin 204T having the same shape as described above is erected on an upper surface portion of the transfer brush base 204 and engaged with a guide hole 21N provided in the positioning block 21.
[0097]
In this way, the transfer brush 2A3 is placed between the pair of left and right transfer brush bases 204 and placed on the transfer brush base 204 to form the transfer unit Ya. In this state, the convex portion 204g of the transfer brush base 204 is inserted into the rail groove 205L of the unit substrate 205. At this time, the position of the transfer brush base 204 has not been determined.
[0098]
On the other hand, the shaft 1S of the photosensitive drum 1 is pivotally supported by the support frame K via the positioning block 21 so as to be rotatable. The transfer unit Ya is pushed upward from the lower side with respect to the positioning block 21 toward the photosensitive drum 1 side.
[0099]
The guide pin 24T of the transfer brush base 204 is inserted into a guide hole 21N provided in the positioning block 21 and is guided along this taper. Further, due to the relationship between the convex portion 24V of the transfer brush pedestal 204 and the positioning groove 205M of the unit substrate 205, the transfer brush pedestal 204 moves along the running direction of the transfer belt 3 while supporting the transfer brush 2A3. Adjusted.
[0100]
When the tip of the guide pin 24T is engaged with the apex portion of the guide hole 21N, the guide pin 24T is positioned in a state where the axis Lb of the transfer brush 2A3 is completely aligned with the axis Ls of the photosensitive drum 1. In addition to securing this state, the transfer unit Ya is fixed and held by an attachment fixing means (not shown).
[0101]
As shown in FIG. 16, when the axis Lb of the transfer brush 2A3 is completely coincident with the axis Ls of the photosensitive drum 1, the transfer brush nip width can be kept within the photosensitive nip width. It was found that there was no blurring of characters at the portions corresponding to the respective positions a to c, and good transfer was performed at a transfer bias of 1000 to 1500V.
[0102]
Next, as a fourth embodiment, on the premise that a small-diameter photosensitive drum and a transfer roller are used as a transfer member, the transfer roller nip width is made larger than the photosensitive nip width, and the transfer member photosensitive drum ( A configuration using a member that regulates the amount of biting into the transfer belt) will be described.
[0103]
Here, the diameter of the photosensitive drum is as small as φ30 mm. When such a small-diameter drum is used, the photosensitive member nip width becomes very narrow. Therefore, the transfer member nip width falls within the photosensitive member nip width. Have difficulty.
[0104]
Therefore, as shown in FIG. 17, a very soft transfer roller 2B is used for the small-diameter photosensitive drum 1B, and the transfer roller nip width Nr, which is the transfer member nip width, becomes larger than the photosensitive member nip width Ns. The configuration.
[0105]
Specifically, a sponge roller having a hardness of 40 degrees or less is used as the transfer roller 2B. When pressed against the photosensitive drum 1B with a weighted linear pressure of 18 to 75 g / cm, the photosensitive member nip width is 1.5 mm or less.
[0106]
As shown in FIG. 18, for example, even if the transfer roller nip width is configured to be approximately the same as the photoreceptor nip width, the position of the transfer roller axis Lr with respect to the photoreceptor drum axis Ls may be shifted to the upstream side or the downstream side. is there.
[0107]
As shown in FIG. 4A, if the position of the transfer roller axis Lr with respect to the photosensitive drum axis Ls is shifted to the upstream side or the downstream side, and the amount of deviation D at that time is within 0.75 mm, A very small portion where the nip width and the transfer roller nip width overlap is ensured, and the transfer efficiency is temporarily maintained.
[0108]
However, as shown in FIG. 5B, if the deviation D of the transfer roller shaft center Lr with respect to the photoreceptor drum shaft center Ls is 0.75 mm or more, the photoreceptor nip width and the transfer roller nip width do not overlap at all. Therefore, the transfer efficiency is extremely lowered.
[0109]
19 (A) and 19 (B), the hardness of the transfer roller 2B is set to 30 degrees, the pressure is set to 30 g / cm 2, and the axis of the transfer roller 2B with respect to the photosensitive drum 1B is located upstream in the belt moving direction. Or, the transfer efficiency when shifted downstream is shown.
[0110]
4A shows the case where the transfer roller axis Lr is shifted downstream from the photosensitive drum axis Ls. FIG. 4B shows the case where the transfer roller axis Lr is shifted upstream from the photosensitive drum axis Ls. This is the case. The dimensional configuration such as the diameter of the transfer roller 2B is as described above.
[0111]
In any case, if the deviation is 0.75 mm or more, the appropriate transfer bias becomes a remarkably high value, and good transfer cannot be performed within the range of the appropriate bias when there is no deviation. Even with a deviation of 0.5 mm, a very good transfer efficiency cannot be obtained. In reality, when component accuracy and assembly accuracy are combined, a deviation of about 0.5 mm occurs, which is a problem.
[0112]
Here, as described above with reference to FIG. 17, the transfer roller nip width Nr is configured to be larger than the photosensitive member nip width Ns, and the positions of the photosensitive drum axis Ls and the transfer roller axis Lr in the belt moving direction are slightly different. Even if it deviates, it is set as the composition which is hard to produce influence.
[0113]
In order to increase the transfer roller nip width Nr with respect to the photoreceptor nip width Ns, it is necessary to use a transfer roller 2B having a hardness as low as possible and a diameter as large as possible, and to increasing the roller load.
[0114]
Table 1 shows the result of measuring the transfer roller nip width Nr using the sponge hardness (ASKER-C) of the transfer roller 2B, the transfer roller diameter (all shaft diameters are φ8 mm), and the roller load as parameters.
[0115]
[Table 1]

[0116]
Under all conditions, the photoreceptor nip width Ns is 1.3 to 1.6 mm. Then, a character image is printed using the photoconductor nip width as a parameter, and the result of evaluating the level of voids at that time is shown.
[0117]
When the roller hardness exceeds 30 degrees, it has been found that there is no condition that the character missing is good after forming a sufficiently wide transfer roller nip width (2 mm or more) with respect to the photoreceptor nip width. Therefore, the roller hardness is set to 30 degrees or less, and the roller diameter and the transfer roller nip width are set.
[0118]
FIG. 20 shows a transfer roller with respect to the photosensitive drum axis Ls under the conditions that the transfer roller hardness is 20 degrees, the roller diameter is 16 mm (shaft diameter φ8 mm), and the roller load is 45 g / cm (photosensitive member nip width 1.5 mm). The relationship between the positional deviation of the axis Lr in the belt moving direction and the transfer efficiency is shown.
[0119]
The transfer roller nip width at this time is about 2.4 mm, and it has been found that even if a deviation of about 0.75 mm occurs, the transfer efficiency is not greatly affected. That is, when the roller hardness is 30 degrees or less, the roller load is increased and the roller diameter is increased, the influence of the deviation of the shaft position can be reduced. On the other hand, by strongly applying a soft transfer roller, Permanent distortion becomes a problem.
[0120]
If the roller hardness is 20 degrees, the roller diameter is 16 mm (shaft diameter φ8 mm), and the roller load is 30 gf / cm 2, the transfer roller will be 1.5 mm if left in a hot and humid environment (30 ° C, 85%) for about 10 days. I was depressed. When printing was performed in this state, transfer omission occurred in the transfer roller cycle.
[0121]
FIG. 21 shows the dent amount of the transfer roller due to neglect and the occurrence of transfer omission. If the dent due to the standing is 1 mm, the transfer dropout exceeding 5% occurs. In addition, it was found that if a dent exceeding 1 mm occurs, it takes a long time to restore.
[0122]
Therefore, as shown in FIG. 22A, a guide roller made of a rigid body such as a metal material on the shaft 2S of the transfer roller 2B. 30a, 30b Is fitted so that the rear side end portion and the front side end portion of the photosensitive drum 1B are opposed to each other.
[0123]
As shown in the figure (B), these guide rollers 30a, 30b The diameter is φ14.5 mm. Therefore, the transfer roller 2B does not have a dent of 0.75 mm or more. The bite amount under the actual usage conditions of the transfer roller 2B (transfer roller nip width is 3 mm) is 0.3 mm or less, and the guide roller during printing operation 30a, 30b Does not contact the photosensitive drum 1B.
[0124]
As described above, the hardness of the transfer roller 2B is set to 30 degrees (ASKER-C), the transfer roller diameter is increased, the pressure is set strongly, and the transfer roller nip width Nr is formed larger than the photosensitive member nip width Ns.
[0125]
In order to prevent the transfer roller 2B from being dented due to long-term standing, a guide roller having a diameter that does not come into contact with the photosensitive drum 1B during printing. 30a, 30b Therefore, even if the transfer roller axis is displaced with respect to the photosensitive drum axis due to an assembly error or the like, it is possible to obtain a good transfer that does not cause a letter omission or transfer roller cycle transfer omission.
[0126]
FIG. 23 shows a quadruple tandem color printing apparatus as a fifth embodiment, and means for preventing the shift of the transfer roller axis with respect to the photosensitive drum axis in the transfer belt moving direction will be described later.
[0127]
In the figure, reference numerals 1a to 1d denote photosensitive drums (φ60 mm OPC drums) as image carriers. The photoreceptors 1a to 1d are uniformly charged to a potential of −500V to −800V by the scorotron chargers 11a to 11d.
[0128]
Then, image data is written on the photosensitive drums 1a to 1d by the laser scanners 12a to 12d, and electrostatic latent images are formed. The electrostatic latent image is reversely developed by two-component developing devices 13a to 13d (13a; yellow, 13b; magenta, 13c; cyan, 13d; black), and a toner image is formed.
[0129]
At this time, the charging polarity of the toner is (−), which is the same polarity as the polarity with which the photoreceptors 1a to 1d are charged. The paper P is conveyed from a paper cassette (not shown), and is sent out onto the transfer belt 3 in synchronization with the formation of a toner image on the photosensitive drum 1a by the aligning roller pair 14.
[0130]
On the paper P sent to the transfer area of the transfer rollers 2a to 2d as contact chargers, the toner images on the photosensitive drums 1a to 1d are multiplex-transferred onto the paper P by the transfer electric field of these rollers.
[0131]
To the transfer rollers 2a to 2d, a bias (+800 to +4000 v) having a (+) polarity opposite to the charging polarity of the toner is applied by transfer bias power sources 19a to 19d.
[0132]
The toner image is transferred onto the paper P in the transfer region, and (+) charge is applied to the transfer belt 3 via the transfer rollers 2a to 2d, and the paper P is discharged between the photosensitive drums 1a to 1d. (-) Charge is given by this.
[0133]
By attracting the (+) charge and the (−) charge, the sheet P is electrostatically attracted to the transfer belt 3, and the sheet P moves together with the belt 3 after passing through the transfer region. The paper P is peeled off by the curvature of the driving roller 15 that drives the transfer belt.
[0134]
In order to prevent discharge at the time of peeling, the driving roller 15 is grounded, and a grounded neutralizing brush 17 is disposed above the driving roller 15. In order to peel the paper P more securely, the paper may be discharged by a corona discharger using an AC corona. The paper P peeled off from the transfer belt 3 is conveyed to the fixing device 20 and thermally fixed, and then discharged to a paper discharge tray attached to the outer surface of the apparatus main body.
[0135]
The materials and dimensions of the transfer rollers 2a to 2d are the same as those of the transfer roller 2 described above with reference to FIG. The same applies to the material, resistance, and thickness of the transfer belt 3.
In the quadruple tandem color printing apparatus, the parallelism of the four photosensitive drums 1a to 1d is important in order to prevent color misregistration. Therefore, the photosensitive drum axis position can be adjusted, and the parallelism between the photosensitive drums 1a to 1d is ensured with high accuracy.
[0136]
On the other hand, the transfer rollers 2a to 2d are attached to the transfer unit so as to be adjustable in the contact / separation direction with respect to the photosensitive drums 1a to 1d, and the parallelism of the axis center is determined by the component accuracy and the assembly accuracy. In addition, the transfer unit is very large, and its weight is likely to cause deflection and distortion, and the parallelism of the transfer roller is not very good.
[0137]
Here, the axial center positions of the transfer rollers 2a to 2d can be independently adjusted with respect to the first to fourth photosensitive drums 1a to 1d.
That is, when the transfer unit Y as described above with reference to FIG. 6 is configured and the transfer belt 3 is brought into contact with the photosensitive drum 1 and pushed up in a printable state, the guide pins 24T of the transfer roller support member 24 are positioned. By engaging with 21 guide hole portions 21N, the axial positions of the four transfer rollers 2a to 2d are independently determined for each of the four photosensitive drums 2a to 2d.
[0138]
The transfer roller support member 24 has a movable range width <the width of the guide pin 24T, and the transfer rollers 2a to 2d pass through the transfer belt 3 until the leading end of the guide pin 24T is inserted into the guide hole 21N. The design so as not to contact the drums 1a to 1d is the same here.
[0139]
At this time, the appropriate bias between the transfer rollers 2a to 2d and the transfer belt 3 (a transfer efficiency of 85% or more can be obtained) is 1000V for the first station, 1150V for the second station, 1300V for the third station, and 1300V for the fourth station. 1500V.
[0140]
FIGS. 24A and 24B show the transfer efficiency under the conditions of a transfer roller diameter of 14 mm and a hardness of 20 degrees (ASKER-C) (FIG. A), and the level of void (FIG. B). As representative values, the data for the first station is Y only, the second station is Y + M, the third station is Y + M + C, and the fourth station is Y + M + C + BK.
[0141]
With regard to the roller load, good results can be obtained in the same range as the previously described monochromatic machine for the first station, but voids are likely to occur in characters and lines of color overlap. Therefore, since the pressurization in which the hollow is generated becomes lower in the later stage, the margin for proper pressurization is narrowed.
[0142]
As a result, the first station is 25-75 g / cm2, the second station is 25-70 g / cm2, the third station is 25-45 g / cm2, and the fourth station is 25-30 g / cm2.
[0143]
The first station to the third station are set to 35 g / cm 2 and only the fourth station is set to 28 g / cm, and the biases of the first station 1000 V, the second station 1150 V, the third station 1300 V, and the fourth station 1500 V are applied. By positioning the transfer roller shaft center with respect to the photosensitive drum shaft center in the running direction of the transfer belt 3 with the configuration as described above, good transfer with good color overlap and no missing characters can be obtained.
[0144]
【The invention's effect】
As described above, according to the present invention, on the assumption that the transfer belt and the contact transfer device are used, the center position deviation between the photosensitive member nip width and the transfer member nip width is different. by There is an effect of improving transfer unevenness and obtaining high image quality.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an image forming apparatus representing a first embodiment of the present invention.
FIGS. 2A and 2B are diagrams showing characteristics of hollows when a transfer roller is a rubber roller and a sponge roller.
FIG. 3 is a diagram illustrating characteristics of a transfer omission rate when a rubber roller is used as a transfer roller and a sponge roller is used.
FIG. 4 is a diagram showing transfer efficiency and hollow characteristics when a roller pressure is applied with a roller having a predetermined hardness as a transfer roller.
FIG. 5 is a diagram illustrating transfer efficiency and hollow characteristics when the transfer roller is shaken with a predetermined transfer bias and with a predetermined pressure.
FIG. 6 is a diagram illustrating a configuration of a transfer roller positioning unit with respect to a photosensitive drum.
FIG. 7 is a characteristic diagram of roller pressing and hollowing out and a characteristic of roller pressing and transfer efficiency in a state where the positioning means is employed.
FIG. 8 is a diagram illustrating a configuration of a transfer roller positioning unit with respect to a photosensitive drum according to a second embodiment.
FIG. 9 is a schematic configuration diagram of an image forming apparatus according to a third embodiment.
FIG. 10 is a configuration diagram when a belt push-up roller is used.
FIG. 11 is a configuration diagram of different types of transfer brushes.
FIG. 12 is a configuration diagram of a transfer brush as a transfer member.
FIG. 13 is a diagram illustrating an inclination of a transfer brush axis with respect to a photosensitive drum axis when a transfer brush is used.
FIG. 14 is a characteristic diagram of transfer efficiency with respect to a transfer bias at each position in a state where the transfer brush is inclined with respect to the photosensitive drum.
FIG. 15 is a view showing a means for positioning the transfer brush in parallel with the photosensitive drum axis.
FIG. 16 is a characteristic diagram of transfer efficiency with respect to a transfer bias at each position when the positioning unit is used.
FIG. 17 is a diagram illustrating a correlation between a photoreceptor nip width and a transfer roller nip width when a sponge roller is used as a transfer roller according to the fourth embodiment.
FIG. 18 is a diagram illustrating a deviation state between the photosensitive member nip width and the transfer roller nip width when a sponge roller is used as the transfer roller.
FIG. 19 is a diagram illustrating a transfer efficiency characteristic with respect to a transfer bias when a transfer roller having a predetermined hardness is used and the transfer roller axis is shifted with respect to the photosensitive drum axis.
FIG. 20 is a characteristic diagram of transfer efficiency with respect to transfer bias when the transfer roller axis is shifted from the photosensitive drum axis using a transfer roller having a predetermined hardness.
FIG. 21 is a characteristic diagram of the dent amount of the transfer roller and the transfer omission rate when left untreated.
FIG. 22 is a configuration diagram of a unit that regulates the amount of biting of the transfer roller.
FIG. 23 is a schematic configuration diagram of a color image forming apparatus according to a fifth embodiment.
FIGS. 24A and 24B are diagrams showing characteristics of transfer efficiency with respect to roller pressurization at each station and characteristics of hollowing out.
FIG. 25 is a diagram illustrating a conventional shift of the transfer roller axis with respect to the photosensitive drum axis.
FIG. 26 is a cross-sectional view at each position.
FIG. 27 is a diagram illustrating a state in which the photosensitive member axial center and the transfer roller axial center are shifted when the transfer belt is not used.
[Explanation of symbols]
P ... paper,
1 Image carrier (photosensitive drum),
3 ... transfer belt,
2 ... transfer roller,
K ... Support frame,
21 ... Positioning block,
21N: Guide hole,
22 ... Bearing,
24 ... roller support member,
24T ... guide pins,
26 ... reference block,
Y, Ya: Transfer unit.

Claims (1)

A transfer belt that holds and conveys the paper placed on the upper surface and is in contact with an image carrier on which a toner image is formed;
A quadruple tandem color system comprising a transfer roller that is slidably in contact with the back surface of the transfer belt and is applied with a bias to transfer a toner image on the image carrier onto a sheet by applying a bias. In the image forming apparatus,
The transfer roller is made of a material having a hardness of 30 degrees (ASKER-C) or less,
The transfer roller nip width formed by the transfer roller and the transfer belt is wider than the photoconductor nip width formed by the image carrier and paper,
The transfer roller shaft is formed of a rigid body at a portion facing the rear side end portion and the front side end portion of the image carrier, and has a diameter smaller than the diameter of the transfer roller. An image forming apparatus, comprising: a guide roller that regulates a biting amount of the transfer roller with respect to the transfer belt when the transfer roller is not in contact and a dent is generated .

Images